Dr Hab Costa

Elena Marie

Galvez

Christophe Dujardin

François Garin

Sandrine Liu

Armando Duong

Radosław Izquierdo

Dębek

Prof Stéphane Philippe Zaleski

Prof Guibert

Maya Guil Lau Me Le Gr Os

Ev Daou

Ann El Yn E M Ignon

M Arc

Ren Aud

J A La

J Ean -M Ari E C

Iter Ne

Jérôme Bonnety

Hugo Dutilleul

Christian Ollivon

Dominique Busquet

Jêrome Péquin

Frédéric Segretain

Elza Rodrigues

Achraf Hamsi Elza

year study at UPMC. It has been an unforgettable, rewarding, interesting, and sometimes even challenging experience pursuing this Ph.D.

characterizations as part of my thesis was indispensable to its completion. I am sincerely grateful to both of them.

Prior to my arrival at UPMC, I was inspired by many devoted teachers, friends and classmates. Particularly, I would like to use this opportunity to show my gratitude to Prof. Haochun ZHANG for acting as my guarantor without hesitation when I asked.

Undoubtedly, I am indebted to all the teachers, friends and classmates who witnessed my personal growth and even contributed to it. ZNN has, for over two years, chosen to enhance the cohesion of our coterie, sharing with me her life and her knowledge, for which I am exceptionally grateful. She has been a supportive person throughout my late 20s, ensuring that the stress was never overwhelming and walking with me through up and down. In addition, she has patiently dealt with my awful interaction in communication, even occasionally having to bear it. I look forward to journeying with you as a member of a "big" group.

I would be incredibly remiss if I did not acknowledge the immense love and support from my family. I feel blessed to have you as a strong backing for my pursuing greater success and impenitent life. I dedicate this thesis to you.

II

Résumé

Résumé

En raison des de l'augmentation de la pollution à l'échelle mondial du notamment aux

VII

Résumé

Nous avons également étudié sur le DOC commercial sous forme de carotte de monolithe, l'influence de l'addition de différentes espèces de métaux alcalins (K et Na) par des essais catalytiques réalisés avec des mélange complets (C 3 H 6 / CO / NO / NO 2 ). Les conditions d'opératoires ont été également été étudiées. Les résultats du dopage des métaux alcalins sur un DOC commercial montrent que l'introduction de dopants entraîne des impacts négatifs ou positifs sur l'oxydation catalytique selon la réaction étudiée ou l'alcalin utilisé. L'addition K conduit à une promotion des réactions d'oxydation tandis que l'addition de Na inhibe les mêmes réactions. Il n'y a aucun impact de la composition des gaz d'échappement sur le classement des catalyseurs. Cependant, il a été démontré que la présence de NO 2 favorise la réaction d'oxydation alors qu'en présence de NO, la concurrence en termes d'oxydation dans les mêmes sites catalytiques conduit à un décalage de la température de la lumière pour CO et C 3 H 6 à des températures plus élevées.

Dans le but d'étudier les effets de la présence de métaux alcalins sur le DOC au niveau de la phase active, un DOC bimétallique a été préparé sous forme de poudre, puis différentes espèces de métaux alcalins sont incorporées. Les résultats des tests d'activité catalytique du dopage des métaux alcalins sur un modèle DOC démontrent que l'addition de composé alcalin (K ou / et Na) conduit à une promotion des réactions d'oxydation de C 3 H 6 tout en entraînant une inhibition de la conversion du NO. À partir de l'analyse de la modification des polluants dans l'alimentation, on conclut que C 3 H 6 et NO sont mutuellement inhibiteurs l'un de l'autre en ce qui concerne leur oxydation.

Enfin, encouragés par l'influence évidente des composés alcalins sur les DOC, leurs impacts sur les catalyseurs SCR en aval sont également étudiés dans cette thèse. Un catalyseur SCR commercial à base de V 2 O 5 fourni par Umicore a également été étudié. Les résultats du dopage de métaux alcalins sur un catalyseur SCR commercial indiquent que l'introduction de dopants conduit à des impacts négatifs sur la performance catalytique des catalyseurs usés. En termes d'effet d'empoisonnement, il semble que Na présente une inhibition plus forte que K sur la même base de masse.

En outre, il est constaté que leur combinaison entraîne généralement la perte complète d'activités, que ce soit en respectant SCR standard ou Fast SCR. In its latest report on world energy, petroleum industry company BP projected the world's population would reach 8.8 billion by 2035. Meanwhile, global Gross Domestic Product (GDP) is expected to almost double over the next 20 years [START_REF]BP Energy Outlook[END_REF] . As known to all of us, the global economy is powered by energy. Growth in the world economy requires more energy consumption. As expected, the global energy consumption is always increasing.

VIII

Résumé

Along with ever increasing energy consumption, its pressure on natural resources and the damage to the environment is causing growing concern. Growth in the global energy consumption means tons of pollutants. Pollutants mainly come from the energy consumption process among which transportation plays a major part. It takes up around one quarter of total energy consumption (Fig. 1.1) [2] . Fig. 1.1 World delivered energy consumption by end-use sector (quadrillion Btu), 2012-2040 [2] .

In 1886, Karl Benz invented a machine the likes of which the world had never seen before: the automobile [3] . The invention of the modern car became the departure point into the future of mobility.

The automotive industry begun in the 1890s brought far-reaching implications for the nature of societies. For decades, the U.S. took the lead in total automobile production. The output of global automobile production, which has been increasing rapidly, stood at 90.8 million units in 2015. With 24.5 million units manufactured in 2015, China doubled the U.S. production (12.1 million units), while the EU-27 member countries in the second place with a fleet of 18.2 million [4] .

Total global registrations or sales of new vehicles were around 89.7 million in 2015 [ 5] , making the number of vehicles in use worldwide surpass the 1.2 billion-unit mark, but Navigant Research suggests that annual sales could jump to 127 million by 2035, bringing the world vehicle population to more than 2 billion [START_REF] Shepard | Transportation Forecast: Light Duty Vehicles[END_REF] .

Most automobiles in operation today are powered by an internal combustion engine, with gasoline or diesel as the fuel. The use of fossil fuels for internal combustion engines inherently gives out emissions that have environmental and health implications [START_REF] Webster | 25 years of catalytic automotive pollution control: a collaborative effort[END_REF] . Their removal from exhaust gases is desired, if not mandated [START_REF] Sanchez | Estimated cost of emission reduction technologies for light-duty vehicles[END_REF] .

Europe is heavily dependent on diesel, unlike the US, China and Japan (Fig. 1.2) [START_REF] Holloway | UK under pressure from European air quality ruling[END_REF] . Europe's vehicle fleets have been persistently transformed from being petrol-powered to dieselpowered over the last 26 years (Fig. 1.3) [START_REF] Cames | Critical evaluation of the European diesel car boom-global comparison, environmental effects and various national strategies[END_REF][START_REF]Share of diesel in new passenger cars in Western Europe[END_REF] . In Western Europe, diesel vehicles are a popular choice for consumers, with almost half now opting for this type of powertrain [START_REF]Share of diesel in new passenger cars in Western Europe[END_REF] .

Overall in 2016, 49.5% of all new passenger car registrations in Western Europe were powered by diesel and 45.8% by petrol [START_REF]Trends in fuel type of new cars between 2015 and 2016, by country[END_REF] . Fig. 1.2 Powertrain map of major markets [START_REF] Holloway | UK under pressure from European air quality ruling[END_REF] . 3 Share of diesel in new passenger car registrations in Western Europe, 1990-2016 [START_REF]Share of diesel in new passenger cars in Western Europe[END_REF] .

Emissions are formed in numerous ways. Incomplete combustion of fossil fuels generates carbon monoxide (CO), unburned hydrocarbons (HCs), and soot as well. Other hydrocarbon reactions can form polynuclear aromatic components and other volatile organic compounds (VOCs), which can themselves be emissions or further react with nitrogen oxides (NO x ) in sunlight. NO x forms from the N 2 in air during combustion, and to a much lesser extent from fuel-bound nitrogen [START_REF] Majewski | Diesel emissions and their control: Society of Automotive Engineers[END_REF] . While NO x emissions are primary pollutants, they also undergo photochemical reactions to generate ozone (O 3 ), a strong oxidizer, irritant, and smog component [START_REF] Russell | Diesel Oxidation Catalysts[END_REF] .

Long-term compliance with automotive exhaust gas emission regulations is one of the challenges confronting the automotive industry. The EU emissions standards Euro 5 took effect on September 1st 2009. At the same time, the automotive industry has already been informed of the proposed limits of the Euro 6 standard by the EU (effective from 2014) [START_REF] Ngk | EURO standards[END_REF] .

Increasingly stringent emission standards (Table 1.1) and concern on the depletion of the fossil fuels propel work on seeking improvements in automotive emission control systems including new engine designs, novel fuel concepts, and in the design of exhaust catalysis [8, 16] . Positioned in the exhaust pipe, catalytic converts are emission control devices through a combination of catalysts. The catalytic converts employed in gasoline vehicles include oxidation (two-way) catalysts and oxidation-reduction (three-way) catalysts. Emission control catalysts were first introduced on gasoline-fueled vehicles in the mid-1970s [START_REF] Farrauto | Automotive Emission Control: Past, Present and Future[END_REF] . Three-way catalyst (TWC) technology was introduced in the 1980s and later became an integral part of the stoichiometric gasoline-fueled vehicles [START_REF] Khajepour | Electric and Hybrid Vehicles: Technologies, Modeling and Control-A Mechatronic Approach[END_REF] .

The TWC is capable of oxidizing hydrocarbons (HCs) and CO, and simultaneously reducing NO x on the principle of non-selective catalytic reduction by CO and HC if the engine is operated at a nearly stoichiometric air-to-fuel (A/F) ratio [START_REF] Eastwood | Critical Topics in Exhaust Gas Aftertreatment[END_REF] . The latest systems can simultaneously reduce all three pollutants by more than 99% after the catalyst has reached normal operating temperature if the air-fuel ratio is controlled pretty precisely at stoichiometry [START_REF] Sanchez | Estimated cost of emission reduction technologies for light-duty vehicles[END_REF] .

Lean-burn (fuel-lean, oxygen-rich) gasoline and diesel vehicles are arousing more and more interest, especially in Europe, because of their better fuel economy than stoichiometric gasoline counterparts. Lean-burn engines, particularly diesels, have higher thermal efficiency and can help reduce carbon dioxide emissions [START_REF] Twigg | Progress and future challenges in controlling automotive exhaust gas emissions[END_REF] . Higher thermal efficiency contributes to reducing the environmental impact of road transport and to helping the transition towards a low-carbon economy. Other benefits brought by lean-burn technique include, for diesel engines particularly, higher power production and superior durability [ 21 , 22 ] . These characteristics have promoted the widespread use of diesel engine in the passenger vehicle market [START_REF] Haaß | Structural characterization of automotive catalysts[END_REF] .

Diesel exhaust is lean because the engine injects only the required amount of fuel into compressed hot air to generate the desired power [START_REF] Majewski | Diesel emissions and their control: Society of Automotive Engineers[END_REF] . Partly due to the excess air input, diesel exhaust is also typically lower in temperature than gasoline counterparts. The abatement of both NO x and particulate matter (PM) emissions is more difficult result from the low temperature of diesel exhaust, generally between 200-300 • C (light-duty engine) or 300-450

• C (heavy-duty engine) [START_REF] Russell | Diesel Oxidation Catalysts[END_REF] .

Due to the lean combustion of diesel engines, the TWC used for stoichiometric-burn gasoline engines fails to efficiently operate in diesel exhaust. The presence of excess oxygen in the exhaust makes it much less effective in reducing NO x to N 2 [ 24 ] . To achieve the same emissions targets, diesel exhaust after-treatment systems require multiple strategies and system integration, encompassing a series of catalytic units [START_REF] Auvray | Fundamental studies of catalytic systems for diesel emission control[END_REF] . Typically, the first component following the engine is the diesel oxidation catalyst (DOC). As the name suggests, oxidation reactions of CO, hydrocarbon and NO readily occur over the DOC, although total NO x emissions are not necessarily altered.

NO x and PM emission control from diesel vehicles is far more complicated and requires the implementation of relatively innovative technologies (Fig. 1.4) [START_REF] Posada | Estimated Cost of Emission Control Technologies for Light-Duty Vehicles Part 2 -Diesel[END_REF] . NO x emission levels complemented in emission standards Euro 6 are reduced by 56% from Euro 5, requiring NO x abatement devices [START_REF] Sanchez | Estimated cost of emission reduction technologies for light-duty vehicles[END_REF][START_REF] Tourlonias | Model-based comparative study of Euro 6 diesel aftertreatment concepts, focusing on fuel consumption[END_REF] . Meeting this target is estimated to increase health benefits by 60-90 percent over Euro 5 [START_REF]Euro 5 and 6 will reduce emissions from cars[END_REF] . The NO x abatement systems for diesel emission control, such as the lean NO x trap (LNT) or NO x storage and reduction (NSR) [ 29 -32] and selective catalytic reduction (SCR) catalysts [START_REF] Busca | Chemical and mechanistic aspects of the selective catalytic reduction of NO x by ammonia over oxide catalysts: A review[END_REF] , are placed downstream of the DOC [START_REF] Katare | Diesel Aftertreatment Modeling: A Systems Approach to NOx Control[END_REF] . In order to comply with particle number standards in Euro 6, diesel vehicles will need diesel particulate filter (DPF) technology [START_REF]Euro 5 and 6 will reduce emissions from cars[END_REF] .

Advanced catalyst-based technologies will play a central role in the emission control system of the future [START_REF] Bertelsen | Future US motor vehicle emission standards and the role of advanced emission control technology in meeting those standards[END_REF] . The diesel exhaust after-treatment line is now composed by a DOC, a DPF and a NO x abatement system [START_REF] Sanchez | Estimated cost of emission reduction technologies for light-duty vehicles[END_REF] . A typical diesel exhaust after-treatment line is illustrated in Technologies required for compliance: Diesel [START_REF] Posada | Estimated Cost of Emission Control Technologies for Light-Duty Vehicles Part 2 -Diesel[END_REF] .

ASC: ammonia slip catalyst Fig. 1.5 Combination of catalysts to control diesel exhaust emissions [START_REF] Auvray | Fundamental studies of catalytic systems for diesel emission control[END_REF] .

In the present work, two of these catalytic systems were studied. Oxidation catalysts in charge of the oxidation of CO, hydrocarbons and NO were examined in monolith and flow reactor experiments. A V 2 O 5 -based SCR catalyst, devoted to the catalytic NO x reduction by ammonia was also investigated. Particular attention was paid to the influence of the introduction of alkali compounds resulting from the use of biofuel on the performance of the exhaust aftertreatment systems.

Objectives of the current work

With the oil resources depletion and the always-increasing oil demand, the choice of future engines should not rely only on oil-based fuels; rather several feasible fuels should be considered and at least regionally offered in the market place. As such, the EU has suggested a substitution of approximately 20% of oil-based fuel for road vehicles by natural gas, hydrogen or biofuel [START_REF] Knecht | Diesel engine development in view of reduced emission standards[END_REF] .

The use of renewable biofuels in transport vehicles has been considered in the last decades as a way of stabilizing CO 2 anthropogenic emissions, inherent to fossil-fuel utilization [START_REF] Demirbas | Progress and recent trends in biofuels[END_REF]37] .

Among them, biodiesel is one of the main options considered. However, its real impact on emissions (NO x , soot, unburnt hydrocarbons, CO) and engine performance are not yet quite well known [START_REF] Xue | Effect of biodiesel on engine performances and emissions[END_REF][START_REF] Zheng | Biodiesel engine performance and emissions in low temperature combustion[END_REF] . Moreover, the presence of alkali compounds coming out of the engine together with soot, soluble organic fraction (SOF: unburned fuel and lubricating oil) or directly in the gas stream, will have an impact on the efficiency of the after-treatment technologies used for exhaust cleaning.

The objective of the present study is to investigate the influence of the presence of different alkali metal species (K and Na) on catalytic after-treatment systems: Oxidation and SCR catalysts. In general, it can be divided into two main parts. The first part is on the DOCs in both the monolith and powder form. Monolithic DOCs are from the commercial source while powder DOCs self-made. The second part is concerned with commercial V 2 O 5 -based SCR catalyst in the monolith form. The method of interest is involved with catalytic tests of the target catalysts employing synthetic gas bench (SGB) plants.

After a literature review, the first part of this thesis deals with the study of the performance of oxidation catalysts in the presence of alkali compounds representative of biofuel contaminants. Systematic methods were employed to get insights into the influence of the presence of different alkali metal species (K and Na): from the commercial catalysts to the active phase, from the activity tests to building the structure-activity relationship.

The second part of this thesis focuses on the NH 3 -SCR after-treatment system. Particularly, the performances of automotive SCR catalysts in the presence of alkali compounds representative of biofuel contaminants were investigated utilizing the same systematic methods as in the first part.

CHAPTER 2: LITERATURE REVIEW 2. Literature Review

In this chapter, the widely-used catalytic converters for pollutant abatement will be first mentioned and reviewed. This section is followed by a review on the deactivation of the catalysts. Finally, the effects of biodiesel on engine-out emissions and on catalytic aftertreatment systems were addressed.

Catalytic converters for pollutant abatement

The environmental disaster of the industrialized Meuse Valley, Belgium [START_REF] Nemery | The Meuse Valley fog of 1930: an air pollution disaster[END_REF] , proved for the first time that air contamination in an industrial community can actually cause acute disabling diseases. The link between health and air pollution during the episode was evident since a soaring air pollution levels was immediately followed by sharp increases in morbidity and mortality.

No major incident has since occurred in the Meuse Valley, but the warnings did not prevent similar events from taking place around the globe, toxic and lethal smog in Donora claiming 20 lives while the Great Smog of London causing as many as 12,000 deaths [41 -43] .

In 1954, Los Angeles experienced a series of severe photochemical smog episodes, leading to the closure of schools and industry for almost a month [START_REF] Butcher | In the Fog about Smog: Solving the Smog Puzzle on Earth and from Space[END_REF] . It prompted concerns about possible serious health implications, particularly in light of the recent Donora and London disasters [START_REF] Bachmann | Will the circle be unbroken: a history of the US National Ambient Air Quality Standards[END_REF] .

Unlike the sulfurous smog that claimed lives in Donora and London, the smog in Los Angeles had a "bleach-like" odor, destroying rubber tires and damaging crops. It would take years for scientific community to solve the puzzle of the Los Angeles smog and decades to implement policies to improve the air quality [START_REF] Butcher | In the Fog about Smog: Solving the Smog Puzzle on Earth and from Space[END_REF] .

It was Arie Jan Haagen-Smit who made the discovery of the chemical nature and source of smog and linked smog to automobile [START_REF] Aj | Chemistry and Physiology of Los Angeles Smog[END_REF] . Moreover, he convinced politicians, regulators and industry that automobiles were the principal smog culprit in Los Angeles and had to be regulated. The three main exhaust gas pollutants are hydrocarbons (including partially oxidized organic compounds), carbon monoxide and nitrogen oxides [START_REF] Twigg | Progress and future challenges in controlling automotive exhaust gas emissions[END_REF] .

By the 1960s, automobiles had been in large-scale mass production for decades. Increased use of automobiles had resulted in serious concerns about urban air quality caused by engine exhaust gas emissions themselves, and by the more harmful species derived from them via photochemical reactions [START_REF] Twigg | Progress and future challenges in controlling automotive exhaust gas emissions[END_REF] . The photochemical reactions is initiated by the dissociation of NO 2 into NO and atomic oxygen under the influence of sunlight [START_REF] Twigg | Controlling automotive exhaust emissions: successes and underlying science[END_REF] . This reactive oxygen attacks organic material, i.e., hydrocarbons, resulting in the formation of ozone and various oxidation products presented in the following steps. where the add-in type catalytic converter comes in.

The catalytic converter was invented by Eugene Houdry, a French American engineer who moved to the US in 1930 [START_REF] Bera | Recent advances in auto exhaust catalysis[END_REF][START_REF] Hall | A Biographical Dictionary of People in Engineering: From the Earliest Records Until[END_REF] . Houdry first developed catalytic converters for smoke stacks and for indoor forklifts using low grade non-leaded gasoline.

About 1950, when the results of early studies of smog in Los Angeles were published,

Houdry became concerned about the role of automobile exhaust in air pollution and founded a company, Oxy-Catalyst, to develop oxidizing catalysts to clean up exhaust gases from gasoline engines [START_REF] Pardiwala | Review paper on Catalytic Converter for Automotive Exhaust Emission[END_REF] . His generic catalytic converter, which makes a drastic reduction in CO emissions and unburned HCs in automobile exhausts, was awarded U.S. Patent in 1956 [START_REF] Houdry | Oxy Catalyst Inc, assignee. Catalytic structure and composition[END_REF] .

Catalytic converters were further developed by a series of engineers including Carl Keith and John Mooney at the Engelhard Corporation (now BASF), Rodney Bagley and Irwin Lachman at Corning Glass Works, creating the first production of catalytic converter in 1973 [START_REF] Bera | Recent advances in auto exhaust catalysis[END_REF][START_REF] Vessel | Corning Researchers Honored for Pollution-Control Invention[END_REF] . It was widely introduced in automobiles in the US market in 1975 to comply with EPA regulations on automobile exhaust emissions. Today, this invention comes standard on every automobile in production for controlling emission. The automobile equipped with catalytic converters will cease to be a significant part of the air pollution problem in most areas of the world.

Widespread adoption of the lead-intolerant catalytic converters, however, didn't occur until tightening emission regulations forced the elimination of the extremely effective anti-knock agent tetra-ethyl lead (lead was introduced in the 1920s to raise octane levels) from most gasoline, because the Pb poisons the catalyst by forming a coating on the catalyst's surface and would makes the converter function poorly, if at all [START_REF] Nriagu | The rise and fall of leaded gasoline[END_REF] .

As the best effective way to control automobile exhaust emission among all the types of technologies developed so far, catalytic converters with different kind of catalysts at the core that converts NO x , CO and unburned HCs into benign gases have undergone several stages of development over the years [START_REF] Bera | Recent advances in auto exhaust catalysis[END_REF] .

Types of catalytic converters for automotive exhaust emission

Catalytic oxidation was initially associated with the early development of catalysis and it subsequently became a part of many chemical processes, so it is not surprising it was exploited to oxidize hydrocarbons and carbon monoxide in the post-engine exhaust when it became necessary to control these emissions from automobiles in 1975 [START_REF] Twigg | Rôles of catalytic oxidation in control of vehicle exhaust emissions[END_REF] .

The oxidizing catalytic converter is one of the pronounced environmental success stories of the last century. It evolved in two directions due to the differences of the exhaust environment between gasoline and diesel: (a) from two-way converters to three-way converters for gasoline applications; (b) from oxidizing converters to diesel oxidation converters, with other pollution-control devices downstream responsible for the abatement of regulated emissions for diesel counterparts.

The earliest converters dating back to 1970s were "oxidizing" or "two-way" converters because only the oxidations of HCs and CO in the exhaust happened over the catalyst. These older converters did nothing to control NO x in the exhaust. NO x emissions standards were left out of the requirements by the U.S. EPA at that time so researchers could focus on HCs and CO. But when they took effect in 1981, the same catalytic system would have to convert NO x to N 2 [START_REF] Klepetar | Technology-Forcing and Law-Forcing: The California Effect in Environmental Regulatory Policy[END_REF] . That's how three-way catalyst converters came into the scenario (Fig. 2.1).

Oxidizing converters were first used on gasoline vehicles to cut tailpipe emissions. Diesel vehicles did not need such technology at that moment thanks to their already low HC and CO emissions [START_REF] Auvray | Fundamental studies of catalytic systems for diesel emission control[END_REF] . However with increasingly tightening emission standards, the utilization of oxidation catalyst was extended to diesel vehicles, with the title diesel oxidation catalysts (DOC) [START_REF] Zelenka | Worldwide diesel emission standards, current experiences and future needs[END_REF] . Over DOC, oxidation of hydrocarbons and carbon monoxide is plainly evident, except that it has the additional task to convert NO into NO 2 , which participates in particulate filter regeneration and improves the downstream NO x abatement process.

The Two-way catalyst

Catalytic emissions control was first introduced in the form of platinum-based oxidation catalysts that resulted in drastic reduction in hydrocarbons and CO emissions [START_REF] Twigg | Progress and future challenges in controlling automotive exhaust gas emissions[END_REF] . The first catalysts used a simple formulation of platinum (Pt) deposited on aluminium oxide which in turn was coated onto a support material so that it could be placed in the exhaust clean-up line of the vehicle [START_REF] Matthey | Palladium use in diesel oxidation catalysts[END_REF] .

These designs were essentially two-way oxidation catalysts since they aided in both reactions of the hydrocarbons and CO with the remaining oxygen in the exhaust. It has two simultaneous tasks: a. oxidation of CO to CO Two-way catalytic oxidizing converters are widely used on gasoline vehicles in US through 1981 when they were superseded by three-way catalysts due to their inability to control nitrogen oxides [START_REF] Khajepour | Electric and Hybrid Vehicles: Technologies, Modeling and Control-A Mechatronic Approach[END_REF] .

The Three-way catalyst

When NO x emissions standards required by the EPA took effect in 1981, the same catalyst system that added oxygen to hydrocarbons and CO would have to remove oxygen from NO x , to give out nitrogen (N 2 ).

Reduction of NO x to N 2 was initially achieved over a platinum/rhodium catalyst in fuel-rich exhaust gas before air was added to permit oxidation of HC and CO over an oxidation catalyst [START_REF] Twigg | Progress and future challenges in controlling automotive exhaust gas emissions[END_REF] . Two catalytic converters were used in series, one for reducing, the other oxidizing. Some of the older three-way converters have an air pipe connected to an air pump or aspirator valve to supply air between the oxidation and reduction catalysts. This approach, however, was not technical and economical satisfactory. It would add significantly to the bulk and cost of the device.

It was observed a platinum/rhodium catalyst could, under appropriate air-fuel conditions, simultaneously convert CO and HC and reduce NO x with high efficiency. This catalyst concept became known as a three-way catalyst (TWC) [START_REF] Twigg | Progress and future challenges in controlling automotive exhaust gas emissions[END_REF] . Simultaneous conversion of all three pollutants over a single TWC to harmless products became possible when the composition of the exhaust gas could be maintained close to the stoichiometric point. Such three-way catalysts are still being used and improved on. Today, modern gasoline vehicles with three-way catalysts can achieve almost complete removal of all three exhaust pollutants over the lifetimes of the vehicles [START_REF] Twigg | Progress and future challenges in controlling automotive exhaust gas emissions[END_REF] .

Diesel oxidation catalyst (DOC)

The diesel oxidation catalyst (DOC) has been part of diesel exhaust systems since regulations were introduced to limit the amount of harmful emissions released to the environment from diesel engines [START_REF] Russell | Diesel Oxidation Catalysts[END_REF] . The first commercial DOC application on a new vehicle was unveiled by automaker Volkswagen in 1989 when it added platinum-based diesel oxidation catalysts, similar to the first conventional two-way gasoline catalysts, to its diesel-powered Golf "Umwelt" model [START_REF] Zelenka | Reduction of Diesel Exhaust Emissions by Using Oxidation Catalysts[END_REF] . This application was, however, voluntary. The DOC didn't spread out on a large scale until the introduction of the Euro 2 standard in 1996. It was stricter Euro 3 standard (2000) which made them standard devices on all diesel-powered passenger vehicles [START_REF] Majewski | Commercial DOC Technologies[END_REF] .

With excess oxygen in the diesel exhaust, the operating requirements for DOC are different from those for TWC. The temperature of diesel exhaust is typically lower than that of gasoline counterparts partly due to the excess air input. Operated under lower temperature conditions, DOCs have 2~3 times the amount of catalytic material as a TWC for the same quantity of exhaust clean-up [START_REF] Russell | Diesel Oxidation Catalysts[END_REF][START_REF] Haaß | Structural characterization of automotive catalysts[END_REF] .

Enclosed by a highly oxidizing environment, the primary functions of DOC are oxidation of CO, unburned hydrocarbons and NO. Undesirable oxidation of SO 2 also occurs over DOC.

The oxidation of hydrocarbons over DOC can be used to generate exotherms required for downstream pollution-control devices. Moreover, converted NO 2 can be exploited to improve the performance of some particulate filters and NO x abatement systems downstream DOC [START_REF] Russell | Diesel Oxidation Catalysts[END_REF] .

Selective catalytic reduction (SCR)

NO x is effectively controlled from gasoline exhaust by a TWC under stoichiometric conditions [START_REF] Nova | Urea-SCR technology for deNOx after treatment of diesel exhausts[END_REF] . However, it fails to remove NO x in the case of lean-burn gasoline and diesel engines [START_REF] Skalska | Trends in NOx abatement: A review[END_REF] . For lean diesel conditions, selective catalytic reduction (SCR) is currently the leading technique of remediation [START_REF] Nova | Urea-SCR technology for deNOx after treatment of diesel exhausts[END_REF] .

The SCR reaction requires reductant for the control of nitrogen oxides. The reductant, such as ammonia (NH 3 ), which needs to be introduced to the exhaust, selectively reduces the NO x with the aid of specific catalysts, rather than being oxidized by the excess oxygen [START_REF] Nova | Urea-SCR technology for deNOx after treatment of diesel exhausts[END_REF] .

While compressed or liquefied ammonia is appropriate for the stationary SCR applications, the odorous ammonia gas had to be replaced in mobile applications by an ammonia precursor such as urea which can be safely stored and reliably release ammonia [START_REF] Nova | Urea-SCR technology for deNOx after treatment of diesel exhausts[END_REF] . Other reducing agents were also proposed to avoid handling NH 3 .

Based on the utilized reductants, major SCR techniques can be categorized into three groups [START_REF] Roy | Catalysis for NOx abatement[END_REF] : a), HC-SCR [START_REF] Iwamoto | Removal of nitrogen monoxide from exhaust gases through novel catalytic processes[END_REF] , b), H 2 -SCR [64 -66] , and c), NH 3 -SCR [START_REF] Liu | Recent Advances in Catalytic DeNOX Science and Technology[END_REF] . Among them, NH 3 -SCR is considered as the top choice.

The novel idea employing urea to reduce NO x sources for diesel engines dated back to 1988 [START_REF] Held | Catalytic NOx Reduction in Net Oxidizing Exhaust Gas[END_REF][START_REF] Held | Verfahren zur Reduktion von in Abgasen enthaltenen Stickoxiden mittels eines zeolithhaltigen Katalysators[END_REF] . Not surprisingly, urea was already exploited as an NH 3 precursor for stationary SCR applications at 1985 [START_REF] Bowers | Reduction of nitrogen-based pollutants through the use of urea solutions containing oxygenated hydrocarbon solvents[END_REF] . Down to the core, urea-SCR is NH 3 -SCR. First results on mobile urea-SCR were publically presented in 1990, showing remarkable NO x conversions [START_REF] Held | Catalytic NOx Reduction in Net Oxidizing Exhaust Gas[END_REF] . In addition, urea is neither toxic nor corrosive, and can easily be handled as aqueous solutions, such as a urea-water-solutions (UWS) with a concentration of 32.5 wt.%, bearing the trade name AdBlue® [START_REF] Nova | Urea-SCR technology for deNOx after treatment of diesel exhausts[END_REF] .

Since the introduction of AdBlue® in 2004, it has been widely adopted for heavy-duty diesel applications [START_REF] Nova | Urea-SCR technology for deNOx after treatment of diesel exhausts[END_REF][START_REF] Gabrielsson | Urea-SCR in Automotive Applications[END_REF][START_REF] Granger | Catalytic NOx Abatement Systems for Mobile Sources: From Three-Way to Lean Burn after-Treatment Technologies[END_REF] . In some countries, the solution became known as Diesel Exhaust Fluid (DEF) or as Aqueous Urea Solution (AUS) [START_REF] Nova | Urea-SCR technology for deNOx after treatment of diesel exhausts[END_REF] .

In practice, urea solution is stored onboard in an additional tank (Fig. 2.2) and this solution is atomized into the main exhaust pipe where it decomposes due to the hot exhaust gas stream and on the SCR catalyst to yield the actual reducing agent NH 3 [60, 73 -75] .

Fig. 2.2 Schematic diagram of a urea-SCR system [START_REF] Koebel | Urea-SCR: a promising technique to reduce NOx emissions from automotive diesel engines[END_REF] .

Focus on Diesel Oxidation Catalysts (DOC)

DOCs are precious metal based catalysts. Platinum group metals (PGMs), such as platinum (Pt) and palladium (Pd), showed high oxidation performance as well as pronounced thermal durability, proving their cost-effectiveness [START_REF] Twigg | Rôles of catalytic oxidation in control of vehicle exhaust emissions[END_REF] . By contrast, base metals such as copper and nickel were too sensitive to fuel impurities and lacked sufficient thermal durability, making them improper candidates though able to perform the job at low cost [START_REF] Yao | A study of high temperature treated supported metal oxide catalysts[END_REF] . Pt and Pd, as catalytic species, are typically dispersed on high surface area carriers like alumina which are generally called washcoat. The most common practice for dispersing the catalytic components within the carrier is by impregnating an aqueous solution containing a salt of the catalytic element(s). Following impregnating, drying and calcination of the catalyzed carrier are needed to obtain the finished catalyst in the powder form. Monolithic catalysts, however, are usually prepared in this way: the catalyzed carrier is made into an acidified aqueous slurry; the honeycomb monolith is dipped into the slurry which contains the washcoat; the excess mixture is air blown to clear the channels and dried at about 110℃; the final step is calcination, performed in air at temperatures between 300 and 500℃. Commercial catalysts often contain additives as stabilizers or promoters that improve their thermal stability and durability or provide a promoting function such as oxygen storage capacity [START_REF] Auvray | Fundamental studies of catalytic systems for diesel emission control[END_REF] .

The catalytically active metal(s) is usually deposited on a support or carrier. The role of the support is to provide a matrix that stabilizes the catalytically active component in small particles, hence, increasing the availability of the catalytically active surfaces to the reactants.

It also increases the thermal stability of the active component. The properties of the support material used, is of great importance in determining the performance of the final catalyst.

These properties include the support surface area, pore volume, surface acidity and reactivity, thermal conductivity, and thermal expansion [START_REF] Neyestanaki | Deactivation of postcombustion catalysts, a review[END_REF] .

Great efforts have been devoted to most effectively employing the precious metal components in automotive depollution since their inception in 1970s in recognition of their remarkable thermal stability, their lower tendency (compared to the base metals) to react with support materials and less sensitivity to fuel contaminants [START_REF] Gandhi | Automotive exhaust catalysis[END_REF][START_REF] Gandhi | Optimization of ruthenium-containing, stabilized, nitric oxide reduction catalysts[END_REF] . Distilled to its core, the story of diesel exhaust cleaning revolves around three precious metals-platinum (Pt), palladium (Pd), and rhodium (Rh) [START_REF] Webster | 25 years of catalytic automotive pollution control: a collaborative effort[END_REF][START_REF] Twigg | Rôles of catalytic oxidation in control of vehicle exhaust emissions[END_REF][START_REF] Heck | Automobile exhaust catalysts[END_REF] . Pt and Pd have been considered as clear choices for the commercial oxidation catalysts, especially since Rh is relatively scarce compared to Pt and Pd, and shows lower activity for olefin conversion under oxidizing conditions [START_REF] Gandhi | Automotive exhaust catalysis[END_REF][START_REF] Kummer | Use of noble metals in automobile exhaust catalysts[END_REF] .

Platinum has historically been favored for use in diesel aftertreatment because the diesel exhaust stream is highly oxidizing and, under these conditions, palladium is readily converted to its metal oxides form which is less catalytically-active, whereas platinum remains in its metallic form [START_REF] Matthey | Palladium use in diesel oxidation catalysts[END_REF][START_REF] Wang | NO Oxidation on Platinum Group Metals Oxides: First Principles Calculations Combined with Microkinetic Analysis[END_REF] . Palladium is not an equally effective catalyst for each of the oxidation reactions it is required to perform. It is not particularly effective at converting some of the hydrocarbons as well as NO present in diesel exhaust gas while platinum is a better catalyst for these reactions. NO oxidation to NO 2 was found to always depend directly on the content of platinum [START_REF] Kim | The effect of Pt-Pd ratio on oxidation catalysts under simulated diesel exhaust[END_REF][START_REF] Kaneeda | Improvement of thermal stability of NO oxidation Pt/Al2O3 catalyst by addition of Pd[END_REF] .

However, palladium is effective for the CO oxidation, particularly at high concentrations of this gas [START_REF] Matthey | Palladium use in diesel oxidation catalysts[END_REF][START_REF] Shakya | The Effect of Pt:Pd Ratio on Heavy-Duty Diesel Oxidation Catalyst Performance: An Experimental and Modeling Study[END_REF] . Skoglundh et al. found that for the oxidation of CO, the light-off temperature over monometallic palladium catalyst was much lower than that of platinum [START_REF] Skoglundh | Combinations of platinum and palladium on alumina supports as oxidation catalysts[END_REF] . The addition of a small amount of Pt (Pd:Pt = 80:20), however, slightly decreased the light-off temperature [14, 86] . In this sense, Platinum acts a promotor for palladium concerning the CO oxidation though not to a large degree.

For control of sulfates, palladium is preferred over platinum since Pd catalysts are less likely to form sulfates by direct oxidation [START_REF] Stein | Diesel oxidation catalysts for commercial vehicle engines: strategies on their application for controlling particulate emissions[END_REF][START_REF] Henk | Diesel catalysts for low particulate and low sulfate emissions[END_REF] . Meanwhile, the scarcity and high cost of Pt makes it desirable to attempt to use palladium alongside platinum in diesel oxidation catalysts.

Moreover, the presence of palladium can provide much-needed thermal stability in a catalyst followed by a DPF [START_REF] Kaneeda | Improvement of thermal stability of NO oxidation Pt/Al2O3 catalyst by addition of Pd[END_REF] . The use of platinum, however, remains important in providing high catalytic activity [START_REF] Matthey | Palladium use in diesel oxidation catalysts[END_REF][START_REF] Skoglundh | Combinations of platinum and palladium on alumina supports as oxidation catalysts[END_REF] . Palladium does not promote the oxidation of some hydrocarbons and NO over platinum [START_REF] Skoglundh | Combinations of platinum and palladium on alumina supports as oxidation catalysts[END_REF] .

The high temperatures experienced by the catalyst during the regeneration events resulting from the regeneration process of the DPF mean that the thermal stability of the catalyst becomes more desirable where a filter is fitted, making the use of platinum/palladium technology more attractive [START_REF] Matthey | Palladium use in diesel oxidation catalysts[END_REF] .

The first public announcement of the fabrication of such a bimetallic platinum/palladium catalyst was made in 2004 and the very first commercial catalysts were fitted to vehicles one year later [START_REF] Matthey | Palladium use in diesel oxidation catalysts[END_REF] . With a wider range of possible roles for the DOC, no single catalyst formulation can be universally applied [START_REF] Matthey | Palladium use in diesel oxidation catalysts[END_REF] .

A typical DOC formulation currently in use might have a Pt/Pd ratio of 2:1 on the basis of mass (or approximately 1.1:1 on the basis of atom). Raising the share of palladium among the certain total amount of metals without sacrificing the catalytic performance is highly desirable from the economic point of view. The performance and properties of bimetallic catalysts depend on the total amount of metals, composition, method of catalyst preparation, thermal history, etc [START_REF] Skoglundh | Combinations of platinum and palladium on alumina supports as oxidation catalysts[END_REF] .

The performance of DOC is generally evaluated in terms of the oxidation reactions that occur over it. The oxygen-rich environment of diesel exhaust makes it remarkably important the interaction of oxygen with the catalyst and other reactants.

Gland et al. found that during the interaction of oxygen with the Pt (111) surface (low index plane) over the temperature range between 100 and 1400 K, three states of oxygen have been characterized: molecular adsorption predominates below 120 K; adsorbed atomic oxygen predominates in the 150 to 500 K temperature range; subsurface "oxide" formation may occur in the 1000 to 1200 K temperature range [START_REF] Gland | Oxygen interactions with the Pt(111) surface[END_REF] . Imbihl et al. investigated the oxygen adsorption on a Pd (111) surface in the temperature range from 30 K to 300 K, concluding that the dissociation process was completed around 200 K and the dissociative chemisorption of oxygen on Pd (111) does not proceed in a single step but through a sequence of several well defined molecular precursor states [START_REF] Imbihl | Adsorption of oxygen on a Pd(111) surface studied by high resolution electron energy loss spectroscopy (EELS)[END_REF] .

The chemisorption of oxygen over Pt metals as well as Pd metals is dissociative: O 2 adsorbs onto the metal surface and quickly dissociates well below practical temperatures [START_REF] Nolan | Molecularly chemisorbed intermediates to oxygen adsorption on Pt(111): A molecular beam and electron energy-loss spectroscopy study[END_REF][START_REF] Artsyukhovich | Low temperature sticking and desorption dynamics of oxygen on Pt(111)[END_REF] . It is only possible at special surface sites which are capable of breaking the bond of the O 2 molecule. These sites are believed to be linked with some type of structure defects such as atomic step. It follows that the minimum energy pathway leading to thermal dissociation of adsorbed O 2 lies below the energetic barrier for thermal desorption [START_REF] Artsyukhovich | Low temperature sticking and desorption dynamics of oxygen on Pt(111)[END_REF] . The associative desorption of adsorbed O atoms as O 2 occurs at high temperatures (above 445 • C) [92 -94] , such that for the typical DOC operating range, oxygen adsorption is irreversible.

CO oxidation

Due to its strong toxicity, CO must be strictly controlled in the automotive exhaust. One of the primary functions of the DOC is to oxidize carbon monoxide into carbon dioxide.

Mechanism

The whole process of CO oxidation over DOC is complicated though it seems that it's just simple catalytic reaction that readily occurs.

The catalytic reaction of CO oxidation relies on the interaction between the active sites and the reactants, manifesting as a series of chemisorption-desorption and surface reaction steps [START_REF] Salomons | On the use of mechanistic CO oxidation models with a platinum monolith catalyst[END_REF] . Both reactants adsorb on the catalyst surface prior to reacting to form CO 2 .

The kinetics of this catalytic oxidation reaction has been the subject of intensive studies [START_REF] Langmuir | The mechanism of the catalytic action of platinum in the reactions 2CO+ O2= 2CO2 and 2H2+ O2= 2H2O[END_REF][START_REF] Mcclure | New insights into catalytic CO oxidation on Pt-group metals at elevated pressures[END_REF].

It is currently widely accepted that the dominant mechanism for this reaction over a wide range of conditions is the Langmuir-Hinshelwood dual-site mechanism [95, 97 -99] . It is well established and proceeds via the following schematic steps:

CO(g) CO ad + ⊗  (6) 2 O (g) 2O ad + ⊗ -⊗ → (7) 2 CO
O CO (g)

ad ad + → + ⊗ -⊗ (8)
Where ⊗ denotes a free adsorption site on the catalyst surface, ⊗ -⊗ two adjacent free sites.

Eq. ( 6) corresponds to molecular adsorption/desorption of reactant CO, Eq. ( 7) the dissociative adsorption of O 2 , while Eq. ( 8) the surface reaction by the diffusion of a CO towards an adsorbed atomic oxygen due to its higher mobility on the surface, forming CO 2 , which immediately leaves the surface.

It is well known that the coadsorption characteristics of CO and O 2 are quite different, relying on the sequence of the adsorption. The inhibition of adsorption of CO and O 2 is asymmetric as preadsorbed CO blocks oxygen adsorption due to the lack of available pairs of neighbor sites while at all oxygen coverages there is still enough place to accommodate CO molecules, a phenomenon named asymmetric inhibition [START_REF] Krischer | Oscillatory CO oxidation on Pt (110): Modeling of temporal self-organization[END_REF] .

Bonzel et al. explored the kinetics of the carbon monoxide oxidation on a clean Pt (110) crystal in an UHV (ultra-high vacuum) system by utilizing Auger electron spectroscopy (AES), low-energy electron diffraction (LEED) and residual gas analysis [START_REF] Bonzel | Mechanisms of the catalytic carbon monoxide oxidation on Pt (110)[END_REF] . the Pt(100)-(5×20) surface using flash desorption spectroscopy, pointing out the phenomenon of reconstruction [START_REF] Barteau | The adsorption of CO, O2, and H2 on Pt(100)-(5×20)[END_REF] . Then they studied the kinetics of the CO oxidation reaction on the Pt( 100)-(5×20) surface under UHV conditions, confirming the applicability of Langmuir-Hinshelwood kinetics for both the reaction of CO with preadsorbed oxygen atoms and the oxidation of preadsorbed CO [START_REF] Barteau | The oxidation of CO on the Pt(100)-(5 × 20) surface[END_REF] . The reaction kinetics in both cases changed dramatically below 300 K due to phase separation at low temperature. What's more, they found that oxygen atoms were adsorbed in islands on the metal surface. Strong attractive interactions within the oxygen islands were at least partially responsible for this difference, the activation energy calculated for the reaction of CO with preadsorbed oxygen being 30 kcal/mol greater than the activation energy measured for the reaction of O 2 with preadsorbed CO.

Goodman et al. found that at near atmospheric pressures, the reaction kinetics fall into three regimes: i, a CO-inhibited regime at low temperatures where CO desorption is the ratedetermining step; ii, a mass transfer limited regime at high temperatures; iii, a transient, highrate regime which lies in between the other two regimes [START_REF] Gao | CO oxidation trends on Pt-group metals from ultrahigh vacuum to near atmospheric pressures: A combined in situ PM-IRAS and reaction kinetics study[END_REF] . With the aid of surface-sensitive techniques amenable to high pressure environments (e.g. PM-IRAS), they also focused on producing a definitive picture of the CO oxidation reaction on PGMs (Rh, Pd, Pt, and Ru) across the 'pressure gap'. Their studies of both low and high pressures demonstrated that: (a)

Langmuir-Hinshelwood kinetics adequately described CO oxidation kinetics on PGMs (Pt, Pd and Rh) (i.e. there is no pressure gap) and (b) the most active surface was one containing minimal coverage of CO plus a chemisorbed atomic oxygen [START_REF] Mcclure | New insights into catalytic CO oxidation on Pt-group metals at elevated pressures[END_REF] .

Carlsson et al. experimentally investigated the oxidation of CO over supported Pt/Al 2 O 3 catalysts operating in oxygen excess at atmospheric pressure, stressing that the conventional three-step L.H. scheme used to interpret CO oxidation on Pt surfaces must be complemented by a Pt oxidation and reduction mechanism during transient conditions [START_REF] Carlsson | A transient in situ FTIR and XANES study of CO oxidation over Pt/Al2O3 catalysts[END_REF] . Sales et al.

developed a simple physical model to explain the oscillatory oxidation of CO over PGMs-based (Pt, Pd and Ir) catalysts, assigning the oscillations to a cycle of slow oxidation and reduction of the metal surface layer inducing transitions between two branches of a L.H. scheme [START_REF] Sales | Oscillatory oxidation of CO over Pt, Pd and Ir catalysts: Theory[END_REF] .

Salomons et al. presented experiments and model predictions for the oxidation of CO over a monolithic platinum catalyst [START_REF] Salomons | On the use of mechanistic CO oxidation models with a platinum monolith catalyst[END_REF] . A hysteresis effect demonstrating the presence of multiple steady states was observed from the light-off curves. The two branches corresponded to the two states of predominantly CO covered or oxygen covered. The model assuming dissociative chemisorption of oxygen on two surface sites based on adsorption and surface reaction using the L.H. approach best reproduced the light-off behavior.

Enhancement factors

As far as the simple two involving reactants are concerned, it is widely accepted that O 2 promotes while CO inhibits the reaction in the temperature range 225-425 • C. A state with high O coverage exhibits high rate of CO 2 production [START_REF] Krischer | Oscillatory CO oxidation on Pt (110): Modeling of temporal self-organization[END_REF] . Rise in temperatures leads to the drop of the strong self-inhibition by CO since CO desorption increases significantly which leads to more empty sites allowing a higher reaction rate [START_REF] Krischer | Oscillatory CO oxidation on Pt (110): Modeling of temporal self-organization[END_REF] .

Studies on the effect of different Pt particles sizes illustrated that the specific reaction rate was higher for the large particles [START_REF] Briot | Effect of particle size on the reactivity of oxygen-adsorbed platinum supported on alumina[END_REF] . It is possibly because the larger Pt particles result in weaker bonds between the metal surface and the oxygen [START_REF] Russell | Diesel Oxidation Catalysts[END_REF][START_REF] Briot | Effect of particle size on the reactivity of oxygen-adsorbed platinum supported on alumina[END_REF][START_REF] Putna | Oxygen desorption from α-Al2O3 (0001) supported Rh, Pt and Pd particles[END_REF] or due to the easier desorption of CO from the surface [START_REF] Lee | Effect of Pt dispersion on the reduction of NO by propene over alumina-supported Pt catalysts under lean-burn conditions[END_REF] .

Inhibition factors

When it comes to the factors inhibiting the CO oxidation, the mention of strong self-inhibition by CO is inevitable. The chemisorption of CO blocks oxygen adsorption to the surface sites and thus inhibits the reaction, exhibiting low rate of CO 2 production [START_REF] Salomons | On the use of mechanistic CO oxidation models with a platinum monolith catalyst[END_REF][START_REF] Carlsson | A transient in situ FTIR and XANES study of CO oxidation over Pt/Al2O3 catalysts[END_REF] .

Goodman et al. classified the reaction limitations according to the parameters pressure and temperature: CO inhibition dominated the low temperature region irrespective of pressure and it's dictated by CO desorption; oxygen inhibition prevailed under the condition of high temperature but low pressure; both oxygen inhibition (oxidation at sufficiently high O 2 /CO ratios) and mass transfer limitations must be considered under the condition of high temperature plus high pressure [START_REF] Langmuir | The mechanism of the catalytic action of platinum in the reactions 2CO+ O2= 2CO2 and 2H2+ O2= 2H2O[END_REF][START_REF] Gao | CO oxidation trends on Pt-group metals from ultrahigh vacuum to near atmospheric pressures: A combined in situ PM-IRAS and reaction kinetics study[END_REF][START_REF] Yao | Oxidation of Alkanes over Noble Metal Catalysts[END_REF] .

Hydrocarbon oxidation

Due to its environmental and health implications, hydrocarbons must be strictly reduced to its minimum in the automotive exhaust. One of the primary functions of the DOC is to oxidize hydrocarbons into carbon dioxide and water.

In diesel exhaust there exist different kinds of HCs, including saturated and unsaturated components. They represent a large group of chemical compounds exhibiting different stability. Saturated HCs (alkanes or paraffins) with short carbon chain, such as methane, are usually stable because its C-H bonds are difficult to break up. As the carbon chain length increases, the adsorption of the alkanes gets harder because more adjacent sites are required, resulting in their slow reaction rates [START_REF] Yao | Oxidation of Alkanes over Noble Metal Catalysts[END_REF] . Unsaturated HCs (alkenes or olefins), such as propene (also known as propylene, C 3 H 6 ), are characterized by a C=C double bond that offers them a higher reactivity than their saturated counterparts, namely alkanes.

In general, propane and propene were used to represent saturated HCs and unsaturated HCs, respectively, to test HCs oxidation over catalysts [START_REF] Patterson | The effect of carbon monoxide on the oxidation of four C6 to C8 hydrocarbons over platinum, palladium and rhodium[END_REF] . ), illustrating that the high electron density of propene favors the interaction and adsorption on a catalytic surface [START_REF] Auvray | Fundamental studies of catalytic systems for diesel emission control[END_REF][START_REF] Yao | The effects of SO2 on the oxidation of hydrocarbons and carbon monoxide over Ptγ-Al2O3 catalysts[END_REF] . They also showed that over the same Pt/Al 2 O 3 catalysts the light-off temperature of propene was lower than that of propane.

In this thesis, propene was exploited as a representative hydrocarbon in the diesel exhaust.

Mechanism of propene oxidation

As with CO, the Langmuir-Hinshelwood dual-site mechanism is the currently preferred mechanism to describe the complete oxidation of propene [START_REF] Russell | Diesel Oxidation Catalysts[END_REF] .

The total oxidation of propene tends to proceed via the following schematic steps [START_REF] Barresi | Deep Catalytic Oxidation of Aromatic Hydrocarbon Mixtures: Reciprocal Inhibition Effects and Kinetics[END_REF] :

HC [] [HC] + ↔ (9) 2 2 O () (O ) 2(O) + → → (10) 2 [HC] (O) CO [] () + → + + (11)
Where [] and () denote free adsorption sites on the catalyst surface. Eq. ( 9) corresponds to molecular adsorption/desorption of reactant HC, Eq. ( 10) the dissociative adsorption of O 2 , while Eq. ( 11) the surface reaction, currently accepted as the rate-determining step [START_REF] Russell | Diesel Oxidation Catalysts[END_REF] . At temperatures below light-off, HC adsorption is stronger than that of oxygen, thus inhibiting the process indicated as Eq. ( 10) [START_REF] Benard | Kinetics of the propene oxidation over a Pt/alumina catalyst[END_REF] .

Benard et al. conducted the kinetic study of propene total oxidation on a Pt/Al 2 O 3 catalyst in excess of oxygen, concluding propene was adsorbed more easily and strongly than oxygen on the Pt surface [START_REF] Benard | Kinetics of the propene oxidation over a Pt/alumina catalyst[END_REF] . Also, their TPD results of propene and oxygen revealed that propene was associatively adsorbed on the Pt surface.

In terms of activation energies, they varied in the literature: 16-29 kJ/mol [START_REF] Cant | Catalytic oxidation: II. Silica supported noble metals for the oxidation of ethylene and propylene[END_REF] , 20-50 kJ/mol [START_REF] Baylet | Parametric study of propene oxidation over Pt and Au catalysts supported on sulphated and unsulphated titania[END_REF] , 41.9 kJ/mol [START_REF] Carballo | Crystallite size effects during the catalytic oxidation of propylene on Ptγ-Al2O3[END_REF] and 85.8 kJ/mol [START_REF] Benard | Kinetics of the propene oxidation over a Pt/alumina catalyst[END_REF] .

Enhancement factors

When examining the influence of Pt crystallite size on the catalytic oxidation of propylene, Carballo and Wolf found that larger crystallites effectively favored the oxidation of propylene [START_REF] Carballo | Crystallite size effects during the catalytic oxidation of propylene on Ptγ-Al2O3[END_REF] .

Among the parameters investigated by Denton et al., they concluded only the Pt dispersion was a major factor affecting the intrinsic activity [START_REF] Denton | Role of the Nature of the Support (Alumina or Silica), of the Support Porosity, and of the Pt Dispersion in the Selective Reduction of NO by C3H6 under Lean-Burn Conditions[END_REF] . The intrinsic activity increased with decreasing dispersion. Other factors such as the nature of the support and the nature of the platinum precursor were not easily discernable and less important.

By contrast, regarding the Pt dispersion, Haneda et al. observed the enhancement of C 3 H 6 oxidation with the increasing dispersion [START_REF] Haneda | Effect of platinum dispersion on the catalytic activity of Pt/Al2O3 for the oxidation of carbon monoxide and propene[END_REF] . They found that the TOF values for C 3 H 6 oxidation over Pt/Al 2 O 3 catalysts rose with the increasing Pt dispersion. In other words, the C 3 H 6 oxidation was promoted over high-dispersion catalysts.

Inhibition factors

As with CO, propene is self-inhibiting at high concentration by covering the platinum sites and blocking oxygen adsorption to the surface sites and thus inhibits the reaction. In their work, Carballo and Wolf concluded that at low C 3 H 6 concentration, the oxidation reaction rate increased with the C 3 H 6 concentration until it reached a maximum. A further increase in the concentration after that led to a decrease in reaction rate, demonstrating negative-order behavior [START_REF] Carballo | Crystallite size effects during the catalytic oxidation of propylene on Ptγ-Al2O3[END_REF] .

In the presence of other gases such as NO, the catalytic oxidation of C 3 H 6 is strongly influenced by surface interaction among O 2 , C 3 H 6 and other gases. The presence of NO would result in the increase of the light-off temperature and it's attributed to the occupation of active sites by NO [START_REF] Oh | NO Oxidation Inhibition by Hydrocarbons over a Diesel Oxidation Catalyst: Reaction Between Surface Nitrates and Hydrocarbons[END_REF] .

NO oxidation

The oxidation of NO is an exothermic reaction, with the standard enthalpy being -57.19 kJ/mol [START_REF] Auvray | Fundamental studies of catalytic systems for diesel emission control[END_REF] . The equation of the global reaction is:

2 2 1 NO O = NO 2 + ( 12
)
The forward reaction of the NO oxidation is favored at low temperature since the oxidation of NO is exothermic, as shown in Fig. 2.3, with NO 2 predominating at lower temperatures. In other words, the oxidation of NO is thermodynamically limited at higher temperatures [START_REF] Russell | Diesel Oxidation Catalysts[END_REF] . It therefore requires the DOC to work pronouncedly at relatively low temperature. On the other hand, it's noteworthy that NO oxidation is kinetically limited at low temperatures. Thus, these two constraints narrow the temperature range which is appropriate for DOC.

Mechanism

The vital schematic steps of NO oxidation are listed as follows [START_REF] Russell | Diesel Oxidation Catalysts[END_REF][START_REF] Weiss | NO Oxidation Catalysis on Pt Clusters: Elementary Steps, Structural Requirements, and Synergistic Effects of NO2 Adsorption Sites[END_REF] :

*
Where * denotes the Pt surface site. Eq. ( 13) corresponds to molecular (associative) adsorption of O 2 . Eq. ( 14) corresponds to the dissociation of adsorbed O 2 via reaction with a vicinal site to form chemisorbed atomic oxygen. Eq. ( 15) corresponds to the reaction of adsorbed O 2 with chemisorbed NO to form adsorbed NO 2 and O*. Eq. ( 16) corresponds to the adsorption of NO. Eq. ( 17) corresponds to the surface reaction. Eq. ( 18) does not actually represent an elementary reaction as written, reflecting instead a sequence of quasi-equilibrated steps [START_REF] Weiss | NO Oxidation Catalysis on Pt Clusters: Elementary Steps, Structural Requirements, and Synergistic Effects of NO2 Adsorption Sites[END_REF] . In the quasi-equilibrated steps of NO reacting with O*, NO 2 as well as vacancies is formed.

It's generally found that the forward rate of NO oxidation reaction was nearly first order in both NO and O 2 and nearly negative first order in NO 2 [24, 121 -124] .

When investigating the inhibition effect of NO 2 on the NO oxidation reaction, Mulla et al.

proposed the associative adsorption of O 2 as the rate-determining step (RDS, the rate-limiting step is assumed to be the step/steps that consumes the most free energy.), adsorbed atomic oxygen as the most abundant surface intermediate [START_REF] Mulla | NO2 inhibits the catalytic reaction of NO and O2 over Pt[END_REF][START_REF] Boudart | Kinetics of heterogeneous catalytic reactions[END_REF] . These were later verified by Bhatia et al. in their microkinetic analysis of the NO oxidation reaction [START_REF] Bhatia | Experimental and kinetic study of NO oxidation on model Pt catalysts[END_REF] . Moreover, adsorbed NO was the other of the two predominant surface species [START_REF] Bhatia | Experimental and kinetic study of NO oxidation on model Pt catalysts[END_REF] .

Based on their calculations of the Gibbs free energy, , since this step got the largest value of G ∆ [START_REF] Olsson | A Kinetic Study of Oxygen Adsorption/Desorption and NO Oxidation over Pt/Al2O3 Catalysts[END_REF] . These authors also found that either mechanism (Langmuir-Hinshelwood, Eley-Rideal, or their combination) is most appropriate for NO oxidation as all of them seem to describe the experiments equally satisfactorily [START_REF] Mulla | NO2 inhibits the catalytic reaction of NO and O2 over Pt[END_REF][START_REF] Olsson | A Kinetic Study of NO Oxidation and NOx Storage on Pt/Al2O3 and Pt/BaO/Al2O3[END_REF] . However, to describe NO oxidation as well as NO 2 decomposition to NO and O resulting in a surface mostly covered by oxygen, the Langmuir-Hinshelwood model was suggested.

Meanwhile, the capacity of the Langmuir-Hinshelwood model to predict the NO conversion dependence on NO concentration at low NO concentrations which was experimentally observed by Despres et al. showed its applicability [START_REF] Russell | Diesel Oxidation Catalysts[END_REF][START_REF] Després | Catalytic oxidation of nitrogen monoxide over Pt/SiO2[END_REF] . [START_REF] Auvray | Fundamental studies of catalytic systems for diesel emission control[END_REF][START_REF] Olsson | A Kinetic Study of NO Oxidation and NOx Storage on Pt/Al2O3 and Pt/BaO/Al2O3[END_REF][START_REF] Crocoll | Mean field modeling of NO oxidation over Pt/Al2O3 catalyst under oxygen-rich conditions[END_REF] .

Enhancement factors

In a whole scenario, the parameters involving the materials and operation can impact the performance of NO oxidation.

It was suggested that larger particles have a higher intrinsic activity, supported by a higher turn-over rate (rate of moles of NO converted per Pt site) [START_REF] Auvray | Fundamental studies of catalytic systems for diesel emission control[END_REF] . The reason behind this is the greater resistance towards oxidation of the larger particles.

In their investigation, Denton et al. concluded only the Pt dispersion (particle size) was a major factor affecting the intrinsic activity [START_REF] Denton | Role of the Nature of the Support (Alumina or Silica), of the Support Porosity, and of the Pt Dispersion in the Selective Reduction of NO by C3H6 under Lean-Burn Conditions[END_REF] . The intrinsic activity increased with decreasing dispersion. Other factors such as the nature of the support and the nature of the platinum precursor were not easily discernable and less important. There existed a lower limit on Pt dispersion to improve the NO oxidation activity which is discussed by Clayton et al [START_REF] Clayton | Pt dispersion effects during NOx storage and reduction on Pt/BaO/Al2O3 catalysts[END_REF] .

Matam et al. suggested that particle morphology played an even more important role than particle size in the NO oxidation activity [START_REF] Matam | The impact of aging environment on the evolution of Al2O3 supported Pt nanoparticles and their NO oxidation activity[END_REF] . They found that the one with higher dispersion presented the higher normalized rate constant, owing to more active particle morphology.

They concluded that cuboctahedral morphology was the best configuration compared to truncated cubic and spherical Pt particles [START_REF] Auvray | Fundamental studies of catalytic systems for diesel emission control[END_REF] .

Among the three types of platinum catalysts over different supports examined by Xue et al.

for the oxidation of NO to NO 2 , the activity was found to be in the order Pt/SiO 2 >Pt/γ-Al 2 O 3 >Pt/ZrO 2 [START_REF] Xue | Roles of supports, Pt loading and Pt dispersion in the oxidation of NO to NO2 and of SO2 to SO3[END_REF] .

Inhibition factors

As kinetic studies showed, the forward rate of NO oxidation reaction was nearly negative first order for NO 2 , indicating inhibition by NO 2 [24, 121 -124] . In their study, Mulla et al. found that the apparent activation energy of NO oxidation in the presence of NO 2 was twice as that of without NO 2 . The increase of the apparent activation energy clearly demonstrated the negative effect of NO 2 .

The formation of an inverse hysteresis when the catalysts were successively heated up and cooled down under NO oxidation conditions were found by multiple investigators: The NO conversion was higher during catalyst heat-up than cool-down [14, 25, 132 -134] . It's said to be caused by the reversible formation of platinum oxide [START_REF] Hauptmann | Inverse hysteresis during the NO oxidation on Pt under lean conditions[END_REF] . As a strong oxidant, NO 2 played a major role in reducing the catalyst activity [START_REF] Bhatia | Experimental and kinetic study of NO oxidation on model Pt catalysts[END_REF] .

At highly lean conditions, as Hauff et al. discovered, the oxidizing effect of NO 2 was drastically reduced [START_REF] Hauff | Platinum oxide formation and reduction during NO oxidation on a diesel oxidation catalyst -Experimental results[END_REF] . Oxygen, instead, was responsible for the formation of platinum oxide and complete oxidation of the particle surface led to catalyst deactivation [START_REF] Auvray | Fundamental studies of catalytic systems for diesel emission control[END_REF][START_REF] Mulla | Reaction of NO and O2 to NO2 on Pt: Kinetics and catalyst deactivation[END_REF][START_REF] Smeltz | Coupled theoretical and experimental analysis of surface coverage effects in Pt-catalyzed NO and O2 reaction to NO2 on Pt(1 1 1)[END_REF] .

Selective catalytic reduction of NOx with ammonia (NH 3 -SCR)

Since the first commercial installations in Japan around 1975, SCR system has evolved in response to increasingly strict regulations and changing application conditions [ 135 , 136 ] .

Currently, the most commonly used NO x abatement technique is NH 3 -SCR providing high NO x removal efficiency [START_REF] Skalska | Trends in NOx abatement: A review[END_REF] .

The catalytic reduction of NO x in exhaust gases from stationary sources and mobile sources can be carried out selectively using ammonia or ammonia precursor such as urea [START_REF] Pârvulescu | Catalytic removal of NO[END_REF] . In typical exhaust gases, NO x is composed mainly of NO (>90%) [START_REF] Schnelle | Air pollution control technology handbook: CRC press Boca Raton[END_REF] , which reacts with NH 3 according to the Standard SCR reaction [START_REF] Madia | The Effect of an Oxidation Precatalyst on the NOx Reduction by Ammonia SCR[END_REF][START_REF] Gómez-García | Pollution by nitrogen oxides: an approach to NOx abatement by using sorbing catalytic materials[END_REF] :

4NH 3 + 4NO + O 2 = 4N 2 +6H 2 O ( 19
)
Under the usual operating conditions of diesel exhaust, the standard SCR reaction has a high potential of NO x reduction at temperatures above 300°C [START_REF] Madia | The Effect of an Oxidation Precatalyst on the NOx Reduction by Ammonia SCR[END_REF] . On the other hand, the DOC upstream of the SCR catalyst can produce great amount of NO 2 . When both NO and NO 2 are present in the exhaust, the two following reactions can occur:

4NH 3 + 2NO + 2NO 2 = 4N 2 + 6H 2 O (20) 4NH 3 + 3NO 2 = 3.5N 2 + 6H 2 O (21)
The Eq. ( 20) is referred to as "Fast SCR" because it proceeds faster than the Standard SCR reaction and shows a maximum rate with an equimolar mixture of NO and NO 2 [141 -144] . The Eq. ( 21) describes the reaction between NO 2 and NH 3 , denoted as "NO 2 SCR". However, its reaction rate is much slower than the Fast SCR [START_REF] Koebel | Reaction Pathways in the Selective Catalytic Reduction Process with NO and NO2 at Low Temperatures[END_REF] . Compared with Standard SCR, its relative rate depends on the catalyst and the conditions under which it is used. The NO 2 SCR can be faster over a Fe-zeolite catalyst [START_REF] Luo | Spatially resolving SCR reactions over a Fe/zeolite catalyst[END_REF] while slower over a vanadia-based catalyst [START_REF] Koebel | Selective catalytic reduction of NO and NO2 at low temperatures[END_REF] .

In an NH 3 -SCR system, vaporized ammonia is injected into the flue-gas at about 300-400℃, which is then passed over a catalyst. The catalyst promotes reactions between NO x and NH 3 to form molecular nitrogen and water vapor.

Catalysts

Though there exists many types of catalysts used in SCR process and many new ones are being developed, they can be grouped into three main categories [START_REF] Skalska | Trends in NOx abatement: A review[END_REF] :

i. Noble metal-based catalysts [START_REF] Pârvulescu | Catalytic removal of NO[END_REF][START_REF] Burch | Selective reduction of nitrogen oxides by hydrocarbons under lean-burn conditions using supported platinum group metal catalysts[END_REF]148] , e.g. Pt/Al 2 O 3

ii. Metal oxide-based catalysts [START_REF] Nova | NH3-NO/NO2 chemistry over V-based catalysts and its role in the mechanism of the Fast SCR reaction[END_REF][START_REF] Koebel | Selective catalytic reduction of NO and NO2 at low temperatures[END_REF] , e.g. V 2 O 5 -WO 3 /TiO 2 iii. Zeolites-based catalysts [149 -154] , e.g. Fe-ZSM5.

Among the extensively investigated catalysts for NH 3 -SCR, it's concluded the use of Pt-based catalysts has been rather limited due to its narrow optimal temperature range and that the most effective ones are metal oxides-based [START_REF] Pârvulescu | Catalytic removal of NO[END_REF][START_REF] Yun | Modeling the selective catalytic reduction of NOx by ammonia over a Vanadia-based catalyst from heavy duty diesel exhaust gases[END_REF] .

Metal oxides-based catalysts such as V 2 O 5 -WO 3 /TiO 2 have been extensively studied due to their excellent activity, selectivity and stability [START_REF] Pârvulescu | Catalytic removal of NO[END_REF][START_REF] Nova | NH3-NO/NO2 chemistry over V-based catalysts and its role in the mechanism of the Fast SCR reaction[END_REF][START_REF] Koebel | Selective catalytic reduction of NO and NO2 at low temperatures[END_REF] . Their major drawbacks are high activity for the undesired oxidation of SO 2 , the high toxicity of vanadia and a lack of efficiency at low and high temperatures [START_REF] Auvray | Fundamental studies of catalytic systems for diesel emission control[END_REF][START_REF] Moliner | Cu-SSZ-39, an active and hydrothermally stable catalyst for the selective catalytic reduction of NOx[END_REF][START_REF] Long | Catalytic Performance of Fe-ZSM-5 Catalysts for Selective Catalytic Reduction of Nitric Oxide by Ammonia[END_REF] . Nonetheless, their strengths give them an edge for commercialization. The most widely used catalyst for the SCR process is V 2 O 5 dispersed over the TiO 2 anatase support promoted by WO 3 (and/or MoO 3 ). In general, the overall surface area of the catalysts is 50-100 m 2 /g, with V 2 O 5 virtual contents of 0.5-3%

(w/w) and MoO 3 or WO 3 contents of 5-10% (w/w). Infrared and Raman studies show that vanadyl, wolframyl and molybdenyl species are present on the dry surfaces of these catalysts [START_REF] Busca | Catalytic abatement of NOx: Chemical and mechanistic aspects[END_REF] . Due to its unrivalled price/performance ratio, it will continue to be a major catalyst in the SCR industry [START_REF] Arnarson | The reaction mechanism for the SCR process on monomer V5+ sites and the effect of modified Bronsted acidity[END_REF] .

Recently, zeolites-based catalysts are attracting more and more attention, especially metal ion exchanged zeolites-based catalysts such as Fe-ZSM-5 and Cu-ZSM-5 [START_REF] Kato | Reaction between nitrogen oxide (NOx) and ammonia on iron oxide-titanium oxide catalyst[END_REF][START_REF] Komatsu | Kinetic Studies of Reduction of Nitric Oxide with Ammonia on Cu2+-Exchanged Zeolites[END_REF] . The reason behind this is that ion-exchanged zeolite catalysts feature in high activity in a wide temperature window of operation and high durability [START_REF] Skalska | Trends in NOx abatement: A review[END_REF][START_REF] Qi | Ultra-active Fe/ZSM-5 catalyst for selective catalytic reduction of nitric oxide with ammonia[END_REF][START_REF] Iwasaki | Characterization of Fe/ZSM-5 DeNOx catalysts prepared by different methods: Relationships between active Fe sites and NH3-SCR performance[END_REF] .

The ideal catalyst for an automotive application should cover a wide range of temperature since it is low during the cold start of the engine and high during the exothermic regeneration of the particulate filter upstream of SCR system [START_REF] Auvray | Fundamental studies of catalytic systems for diesel emission control[END_REF][START_REF] Skalska | Trends in NOx abatement: A review[END_REF] .

Reaction mechanism of NH 3 -SCR over V-based catalysts

To improve the performance of the commercially available catalysts used today and design more effective catalysts for the future, it is vital to understand the underlying mechanism governing the SCR chemistry. Actually, this endeavor has been underway since the 1970s, with a number of reaction mechanism proposed [33, 144, 161 -167] .

Inomata et al. proposed that ammonia is first adsorbed as NH 4 + at a Brønsted acid site (V-OH) adjacent to a V(5+)= O site and then reacts with gas-phase NO according to an Eley-Rideal mechanism to form N 2 and H 2 O, along with the formation of V(4+)-OH. Later, V(4+)-OH species are re-oxidized to V(5+)=O by gaseous oxygen [START_REF] Inomata | Mechanism of the reaction of NO and NH3 on vanadium oxide catalyst in the presence of oxygen under the dilute gas condition[END_REF] .

The mechanism was confirmed and slightly modified by Janssen et al. who proposed that V= O Lewis acid sites are the active centers for ammonia adsorption and demonstrated that one N atom of the N 2 product comes from NH 3 and the other from NO [START_REF] Janssen | Mechanism of the reaction of nitric oxide, ammonia, and oxygen over vanadia catalysts. 2. Isotopic transient studies with oxygen-18 and nitrogen-15[END_REF] .

Based primarily on FT-IR studies, Ramis et al. proposed the ''amide-nitrosamide'' mechanism, as shown in Fig. 2.4 [START_REF] Ramis | Fourier transform-infrared study of the adsorption and coadsorption of nitric oxide, nitrogen dioxide and ammonia on vanadia-titania and mechanism of selective catalytic reduction[END_REF] . This mechanism consists of the following steps: NH 3 is adsorbed over a Lewis acid site where it is activated to form an amide NH 2 species; this amide species then reacts with gas-phase NO via a radical coupling, giving rise to a nitrosamide intermediate, which then decomposes into N 2 and H 2 O; the catalytic cycle is closed by reoxidation of the reduced catalyst by gaseous oxygen [164, 168 -170] . This mechanism has been successfully realized by computer modelling by Jug et al. [START_REF] Jug | Reaction Mechanism of the Selective Catalytic Reduction of NO with NH3 and O2 to N2 and H2O[END_REF] . presence of oxygen [START_REF] Ramis | Fourier transform-infrared study of the adsorption and coadsorption of nitric oxide, nitrogen dioxide and ammonia on vanadia-titania and mechanism of selective catalytic reduction[END_REF] .

On the basis of in situ FTIR studies under steady-state conditions, Topsøe et al. came up with the mechanistic scheme shown in Fig. 2.5 [START_REF] Topsoe | Vanadia-titania catalysts for selective catalytic reduction of nitric-oxide by ammonia: II Studies of active sites and formulation of catalytic cycles[END_REF] . The proposed catalytic cycle consists of both acid-base and redox functions. In this mechanism, the catalytic activity is found to be related to the ammonia adsorbed on the Brønsted acid sites (V-OH). Also, V(5+)= O groups are involved in the reaction, specifically in the activation of adsorbed ammonia. Ammonia is adsorbed over a V(5+)-OH Brønsted acid site and is activated by an adjacent V(5+)=O group.

This activation process involves the transfer or partial transfer of a hydrogen from the NH 3 molecule and accordingly leaves behind a reduced V(4+)-OH site. Once ammonia has been activated, NO reacts from the gas-phase with the activated ammonia complex leading to the formation of an intermediate which finally decomposes to N 2 and H 2 O. The catalytic cycle is closed by the re-oxidation of V(4+)-OH to V(5+)=O by gaseous oxygen. This mechanism can be regarded as a modification of the mechanism by Ramis et al., with the Brønsted acid sites instead of the Lewis acid sites serving as the ammonia adsorption sites [START_REF] Ramis | Fourier transform-infrared study of the adsorption and coadsorption of nitric oxide, nitrogen dioxide and ammonia on vanadia-titania and mechanism of selective catalytic reduction[END_REF][START_REF] Topsoe | Vanadia-titania catalysts for selective catalytic reduction of nitric-oxide by ammonia: II Studies of active sites and formulation of catalytic cycles[END_REF][START_REF] Topsoe | Vanadia/titania catalysts for selective catalytic reduction (SCR) of nitric-oxide by ammonia: I. Combined temperature-programmed in-situ FTIR and on-line mass-spectroscopy studies[END_REF] . It is worth noting Topsøe et al. failed to follow the behavior of coordinated NH 3 because the spectral region below 1300 cm -1 was cut in their spectra [START_REF] Busca | Chemical and mechanistic aspects of the selective catalytic reduction of NO x by ammonia over oxide catalysts: A review[END_REF][START_REF] Topsoe | Vanadia-titania catalysts for selective catalytic reduction of nitric-oxide by ammonia: II Studies of active sites and formulation of catalytic cycles[END_REF] .

The NH 3 adsorption mode on the active site has been a topic of intensive debate: whether it is found to be coordinated on a Lewis acid site or protonated to NH 4 + on a Brønsted acid site (V-OH). Whoever play(s) the role in adsorbing and activating ammonia, the role of nitrosamidelike intermediates which decomposes selectively to N 2 and H 2 O is widely recognized [START_REF] Busca | Catalytic abatement of NOx: Chemical and mechanistic aspects[END_REF][START_REF] Forzatti | Present status and perspectives in de-NOx SCR catalysis[END_REF] . Fig. 2.5 Scheme illustrating the catalytic cycle of the SCR reaction over vanadia/titania catalyst in the presence of oxygen proposed by Topsoe et al [START_REF] Topsoe | Vanadia-titania catalysts for selective catalytic reduction of nitric-oxide by ammonia: II Studies of active sites and formulation of catalytic cycles[END_REF] .

Also, it is worth mentioning, particularly for commercial catalysts (low V loading, high W or

Mo and Ti coverage), that other catalyst components (W or Mo, and Ti surface sites) can adsorb ammonia and participate in the reaction as ''reservoir'' of adsorbed ammonia species [169, 173] . Accordingly, a distinction should be made between the ''adsorption'' sites and the ''reaction'' sites of ammonia [START_REF] Forzatti | Present status and perspectives in de-NOx SCR catalysis[END_REF] .

In spite of the ongoing disagreement on whether coordinated or protonated ammonia play(s) the role of reactive intermediate species, new relevant contribution appeared in the open literature, in particular concerning the reaction mechanism and the role of preoxidation of NO to NO 2 [144, 157, 166, 174] .

With the aid of IR spectra data, Busca et al. found that the coordinated ammonia disappears upon reaction with NO and water is formed. On the contrary, protonated ammonia grows more than disappear, particularly at the first stages of reaction. They also proposed that NO is the actual reactant but the presence of NO 2 enhances the reaction rate, possibly because of its role in reoxidizing the catalyst [START_REF] Busca | Catalytic abatement of NOx: Chemical and mechanistic aspects[END_REF] .

Janssens et al., for the first time, constructed a consistent NH 3 -SCR reaction mechanism combining the standard and fast SCR in a complete catalytic cycle capable of producing the correct stoichiometry while allowing adsorption and desorption of stable molecules only [START_REF] Janssens | A Consistent Reaction Scheme for the Selective Catalytic Reduction of Nitrogen Oxides with Ammonia[END_REF] .

A consequence of the reaction scheme is that all intermediates in fast SCR are also part of the standard SCR cycle. They also pointed out that the oxidation of an NO molecule by O 2 to a bidentate nitrate ligand is the rate-determining step for standard SCR by the use of density functional theory (DFT) calculations. It worth mentioning, however, the new reaction scheme presented was based on a Cu-CHA catalyst.

Following the same guidelines, Arnarson et al. presented a complete catalytic mechanism describing both the Standard and the Fast SCR reactions in their correct stoichiometric form on a monomeric VO 3 H/TiO 2 (001) catalytic model [START_REF] Arnarson | A complete reaction mechanism for standard and fast selective catalytic reduction of nitrogen oxides on low coverage VOx/TiO2(0 0 1) catalysts[END_REF] . As they previously reported, for the particular monomeric site that the most stable structure of adsorbed NH 3 was at the Brønsted acid site, protonated as NH 4 + [158] . The complete mechanism is presented in Fig. [START_REF] Arnarson | A complete reaction mechanism for standard and fast selective catalytic reduction of nitrogen oxides on low coverage VOx/TiO2(0 0 1) catalysts[END_REF] .

Moreover, they found at low temperatures the H 2 O formation and desorption is the RDS for the Standard SCR reaction. For Fast SCR, NO 2 reacts directly with the reduced site resulting in higher rate. The rate for the two reactions is the same at higher temperatures as the RDS is in the reduction part which is common to both reactions [START_REF] Arnarson | A complete reaction mechanism for standard and fast selective catalytic reduction of nitrogen oxides on low coverage VOx/TiO2(0 0 1) catalysts[END_REF] . Now it's quite established that the SCR reaction vanadium-based catalysts is a redox process that occurs with a redox or Mars-van Krevelen-type mechanism [START_REF] Roy | Catalysis for NOx abatement[END_REF] . Irrespective of the nature of the catalyst and the local environment of the active sites, the NH 3 -SCR reaction can be divided into two parts: a reduction part where NO and NH 3 cooperate, and an oxidation part [62, 166, 167, 175] .

Catalyst deactivation

It is inevitable that all catalysts will deactivate with time [START_REF] Bartholomew | Mechanisms of catalyst deactivation[END_REF] . Catalyst deactivation, a drop of catalyst activity, can occur with its origin in the nature of physical and chemical aspects [START_REF] Forzatti | Catalyst deactivation[END_REF] .

Three particular causes of deactivation are briefly defined in Table 2.1 and reviewed in some detail in the following subsections with an emphasis on the last one.

Catalytic converters with the catalysts at the center to reduce automotive exhaust emissions may be fouled or poisoned by fuel or lubricant additives and/or engine corrosion products. If the reaction occurred at high temperatures, thermal degradation may occur in the form of active phase crystallite growth, collapse of the support pore structure, and/or solid-state reactions of the active phase with the support [START_REF] Argyle | Heterogeneous Catalyst Deactivation and Regeneration: A Review[END_REF] .

Table 2.1 Mechanisms of catalyst deactivation [START_REF] Bartholomew | Mechanisms of catalyst deactivation[END_REF] Mechanism Type Brief Description / Definition

Mechanical deactivation (Fouling)

Fouling is the physical (mechanical) deposition of species from the fluid phase onto the catalyst surface, which results in activity loss due to blockage of sites and/or pores [START_REF] Argyle | Heterogeneous Catalyst Deactivation and Regeneration: A Review[END_REF] .

This mechanism of deactivation is common to reactions involving hydrocarbons. It results from a carbonaceous (coke) material being deposited on the surface of a porous catalyst although carbon-and coke-forming processes also involve chemisorption of different kinds of carbons or condensed hydrocarbons that may act as catalyst poisons [START_REF] Argyle | Heterogeneous Catalyst Deactivation and Regeneration: A Review[END_REF] . The schematic of deactivation by coke forming is depicted in Fig. 2.7.

Chemical deactivation (Poisoning)

Poisoning is the loss of activity due to the strong chemisorption of reactants, products, or impurities on sites otherwise available for catalysis [START_REF] Argyle | Heterogeneous Catalyst Deactivation and Regeneration: A Review[END_REF] . Thus, poisoning has operational meaning: whether a species acts as a poison depends upon its adsorption strength relative to the other species competing for catalytic sites [START_REF] Argyle | Heterogeneous Catalyst Deactivation and Regeneration: A Review[END_REF] .

In addition to acting simply by physically blocking adsorption sites, adsorbed poisons may Moreover, a poison may. Poisons can also.

Adsorbed poisons can bring following effects:

i.

Inducing changes in the electronic or geometric structure of the surface [START_REF] Argyle | Heterogeneous Catalyst Deactivation and Regeneration: A Review[END_REF] ;

ii. Altering the adsorptivity of other species essentially by an electronic effect [START_REF] Forzatti | Catalyst deactivation[END_REF] ;

iii. Modifying the chemical nature of the active sites or result in the formation of new compounds (reconstruction) so that the catalyst performance is undoubtedly changed [START_REF] Forzatti | Catalyst deactivation[END_REF] .

It's noteworthy to make a distinction between poisons and inhibitors: the interaction of poisons with the active sites is usually strong and irreversible, whereas the adsorption of inhibitors onto the catalyst surface is generally weak and reversible [START_REF] Forzatti | Catalyst deactivation[END_REF] .

Actually, poisoning itself may be reversible or irreversible [START_REF] Argyle | Heterogeneous Catalyst Deactivation and Regeneration: A Review[END_REF] . In the first case, the poison is not too strongly adsorbed and accordingly reactivation of the catalyst is usually achieved by simply removing the poison from the feed. This is the case, for instance, of oxygen-containing compounds (e.g. H 2 O and CO x ) impacting the catalysts for ammonia synthesis. These compounds can hinder nitrogen adsorption, thus diminishing the catalyst activity, but elimination of these compounds from the feed and reduction with hydrogen to remove the adsorbed oxygen can restore the iron surface [START_REF] Forzatti | Catalyst deactivation[END_REF] . Thus, these gaseous catalyst poisons are called temporary poisons which lower the activity only while present in the synthesis gas [START_REF] Jennings | Catalytic ammonia synthesis: fundamentals and practice[END_REF] .

However, gross oxidation with oxygen results in bulk changes which cannot be readily reversed, causing an irreversible loss of catalyst activity. In other cases, poisons such as sulfur accumulate upon the catalyst surface, resulting in irreversible poisoning and may be detected by chemical and spectroscopic analysis. They are permanent poisons.

The poisoning reaction should be viewed like any other chemical reaction between a gas phase reactant and the solid surface, where the poisoned sites are distributed throughout the catalyst pore structure as a function of poison diffusion into the catalyst and the rate of the poisoning reaction [START_REF] Walke | Catalyst Deactivation and Regeneration[END_REF] .

Thermal deactivation (Sintering / Aging)

Thermal deactivation usually refers to a physical process where the active surface area of the catalyst shrinks via structural changes [START_REF] Russell | Diesel Oxidation Catalysts[END_REF] . This process is often designated as sintering, and is physical in nature [START_REF] Forzatti | Catalyst deactivation[END_REF] . It occurs, unequally, to both the catalyst support and active phase, based on exhaust gas conditions [START_REF] Russell | Diesel Oxidation Catalysts[END_REF][START_REF] Argyle | Heterogeneous Catalyst Deactivation and Regeneration: A Review[END_REF] .

Support sintering

Heterogeneous catalysis is an interface phenomenum and it is advantageous to prepare catalysts with the highest possible surface area with the aid of support [START_REF] Trimm | Thermal Stability of Catalyst Supports[END_REF] .

Thermodynamically, this is an unfavourable state since minimal surface free energy is associated with material coalesced into a sphere. Thus, there is always a tendency for catalysts to sinter and the driving force for sintering is to lower the surface energy [START_REF] Neyestanaki | Deactivation of postcombustion catalysts, a review[END_REF][START_REF] Trimm | Thermal Stability of Catalyst Supports[END_REF] .

When sintering occurs over supported metal catalysts, the loss of support area due to support collapse can happen [START_REF] Bartholomew | Mechanisms of catalyst deactivation[END_REF] . The transport of material leads to coalescence of particles, particle growth and closure of the pores [START_REF] Neyestanaki | Deactivation of postcombustion catalysts, a review[END_REF] . When it occurs, surface area shrinks, porosity alters and the activity of catalysts drops [START_REF] Neyestanaki | Deactivation of postcombustion catalysts, a review[END_REF] .

Temperature, particle size and the atmosphere are three main parameters determining the sintering of a given support [START_REF] Neyestanaki | Deactivation of postcombustion catalysts, a review[END_REF] . Among them, studies showed that temperature and the atmosphere were two factors mostly affecting sintering rates of supported metal catalysts [START_REF] Forzatti | Catalyst deactivation[END_REF] .

It is useful to consider the behavior of alumina as a particular example when examining the support materials sintering since this support is one of the most common materials as support for in oxidizing atmospheres and exhibits many of the sintering characteristics.

In the case of alumina, temperature plays a major part in its sintering which is accompanied by phase transformation (Fig. 2.8) [START_REF] Neyestanaki | Deactivation of postcombustion catalysts, a review[END_REF][START_REF] Argyle | Heterogeneous Catalyst Deactivation and Regeneration: A Review[END_REF][START_REF] Arai | Thermal stabilization of catalyst supports and their application to high-temperature catalytic combustion[END_REF] . Obviously, sintering becomes much more significant at higher temperatures. The reaction atmosphere also affects the sintering: water vapor accelerates support sintering by forming mobile surface hydroxyl groups that are subsequently removed at higher temperatures (Fig. 2.9) [START_REF] Neyestanaki | Deactivation of postcombustion catalysts, a review[END_REF][START_REF] Argyle | Heterogeneous Catalyst Deactivation and Regeneration: A Review[END_REF][START_REF] Trimm | Thermal Stability of Catalyst Supports[END_REF] .

Fig. 2.9 Schematic of surface dehydroxylation from contact area of two adjacent particles.

Active phase sintering

Sintering occurs both in supported metal catalysts and unsupported catalysts [START_REF] Forzatti | Catalyst deactivation[END_REF] . Most of the previous sintering studies concentrate on supported metals: the active metals are usually deposited on a carrier in a form of small particles (crystallites) [START_REF] Neyestanaki | Deactivation of postcombustion catalysts, a review[END_REF][START_REF] Argyle | Heterogeneous Catalyst Deactivation and Regeneration: A Review[END_REF] .

Active phase sintering generally occurs via agglomeration and coalescence of small metallic particles into larger ones [START_REF] Neyestanaki | Deactivation of postcombustion catalysts, a review[END_REF] . This leads to a drop of the surface-to-volume ratio and a decrease in precious metal dispersion [START_REF] Russell | Diesel Oxidation Catalysts[END_REF] . As a result, there is less surface exposed to the reactants for catalysis.

Two sintering models have been proposed for supported metal catalysts: a), atomic migration, and b), crystallite migration [START_REF] Neyestanaki | Deactivation of postcombustion catalysts, a review[END_REF][START_REF] Forzatti | Catalyst deactivation[END_REF] . Atomic migration involves detachment of metal atoms or molecular metal clusters from crystallites, migration of these atoms over the support surface, and ultimately, capture by larger crystallites because the larger crystallites require a low concentration of migrating metal atoms to reach equilibrium [START_REF] Russell | Diesel Oxidation Catalysts[END_REF][START_REF] Argyle | Heterogeneous Catalyst Deactivation and Regeneration: A Review[END_REF][START_REF] Flynn | Experimental studies of sintering of supported platinum catalysts[END_REF] . Crystallite migration model suggests that the metal crystallites themselves travel along the surface, followed by collision and coalescence [START_REF] Russell | Diesel Oxidation Catalysts[END_REF][START_REF] Argyle | Heterogeneous Catalyst Deactivation and Regeneration: A Review[END_REF] . Wanke et al. suggested that crystallite migration may be feasible in the early stages of sintering, but it is unlikely to be the prevailing mechanism throughout the entire process [START_REF] Russell | Diesel Oxidation Catalysts[END_REF][START_REF] Wanke | The Sintering of Supported Metal Catalysts[END_REF] . The processes of atomic and crystallite migration are vividly illustrated in Fig. 2.10 [START_REF] Argyle | Heterogeneous Catalyst Deactivation and Regeneration: A Review[END_REF] . (B), crystallite migration [START_REF] Argyle | Heterogeneous Catalyst Deactivation and Regeneration: A Review[END_REF] .

The nature of metal and the carrier would definitely affect the metal sintering. Also, the temperature and to a less degree the reaction atmosphere controls the metal sintering [START_REF] Neyestanaki | Deactivation of postcombustion catalysts, a review[END_REF] .

Experimental observations proved that temperature had remarkable effect on the extent of sintering: the higher the temperature, the higher the sintering; and the longer the high temperature exposure, the higher the sintering [START_REF] Russell | Diesel Oxidation Catalysts[END_REF][START_REF] Argyle | Heterogeneous Catalyst Deactivation and Regeneration: A Review[END_REF][START_REF] Lassi | Deactivation correlations of Pd/Rh three-way catalysts designed for Euro IV Emission Limits: effect of ageing atmosphere, temperature and time[END_REF] .

When investigating the sintering-induced redispersion of supported platinum in oxygen, Fiedorow et al. found that at higher temperatures, rapid, time-dependent decreases in dispersion occurred [START_REF] Fiedorow | The sintering of supported metal catalysts[END_REF] . At high temperatures, Pt activity decreases due to thermal degradation. In the same way, Pd-based catalysts are also deactivated by high temperature exposure, which can cause sintering and decomposition of the active PdO to Pd, thereby decreasing catalyst activity [START_REF] Neyestanaki | Deactivation of postcombustion catalysts, a review[END_REF] . The sintering of Pt and other precious metals generally occurs at high temperatures (more than 600 °C) [START_REF] Russell | Diesel Oxidation Catalysts[END_REF] .

In terms of atmosphere, oxidizing environment tends to cause more rapid sintering compared to sub-stoichiometric conditions, especially at high temperatures [START_REF] Russell | Diesel Oxidation Catalysts[END_REF][START_REF] Flynn | Experimental studies of sintering of supported platinum catalysts[END_REF][START_REF] Chen | Morphology and composition of PtPd alloy crystallites on SiO2 in reactive atmospheres[END_REF][START_REF] Harris | Growth and structure of supported metal catalyst particles[END_REF] . For example, the sintering of platinum is reported to be higher in oxidizing environment compared to neutral or reducing environments [START_REF] Flynn | Experimental studies of sintering of supported platinum catalysts[END_REF] .

Water vapor also increases the sintering rate of supported metals, likely through chemicalassisted sintering effects [START_REF] Argyle | Heterogeneous Catalyst Deactivation and Regeneration: A Review[END_REF] .

Effects of biodiesel on engine-out emissions and on catalytic after-treatment systems

Biodiesel is a renewable, alternative diesel fuel of domestic origin derived from a variety of fats and oils by a transesterification reaction [START_REF] Knothe | Exhaust Emissions of Biodiesel, Petrodiesel, Neat Methyl Esters, and Alkanes in a New Technology Engine[END_REF] . It is defined by the American Society for Testing and Materials (ASTM) as "a fuel comprised of mono-alkyl esters of long-chain fatty acids derived from vegetable oils or animal fats, designated B100" [START_REF] Hoekman | Review of the effects of biodiesel on NOx emissions[END_REF] .

Pure biodiesel contains approximately 10% oxygen on the basis of mass. Nevertheless, many of these fuels were not as pure as was originally intended because of high levels of impurities in the feedstocks [START_REF] Graboski | The effect of biodiesel composition on engine emissions from a DDC series 60 diesel engine[END_REF] . These fuels covered a very wide range of feedstocks as well as systematically varying chemical properties such as fatty acid chain length and number of double bonds in the fatty acid chain [START_REF] Graboski | The effect of biodiesel composition on engine emissions from a DDC series 60 diesel engine[END_REF] .

Besides being renewable and of domestic origin, advantages of biodiesel include biodegradability, reduction of most regulated exhaust emissions, miscibility in all ratios with conventional diesel, and compatibility with the existing fuel distribution infrastructure [START_REF] Knothe | Designer" biodiesel: optimizing fatty ester composition to improve fuel properties[END_REF] .

In practical use, it is typically blended with petroleum diesel at levels up to 20% (B20),

bearing the potential to replace some of the conventional diesel market [START_REF] Mccormick | Effects of biodiesel blends on vehicle emissions: fiscal year 2006 annual operating plan milestone 10[END_REF] . The high presence of oxygen in the fuel leads to a reduction in emissions of hydrocarbons (HC) and toxic compounds, CO, and PM when biodiesel blends are burned in diesel engines [START_REF] Knothe | Exhaust Emissions of Biodiesel, Petrodiesel, Neat Methyl Esters, and Alkanes in a New Technology Engine[END_REF][START_REF] Mccormick | Effects of biodiesel blends on vehicle emissions: fiscal year 2006 annual operating plan milestone 10[END_REF] .

These reductions are robust and have been observed in numerous engine and vehicle testing studies [START_REF] Mccormick | Effects of biodiesel blends on vehicle emissions: fiscal year 2006 annual operating plan milestone 10[END_REF] .

Technical problems with biodiesel include oxidative stability, cold flow, and increased NO x exhaust emissions [START_REF] Knothe | Designer" biodiesel: optimizing fatty ester composition to improve fuel properties[END_REF][START_REF] Lapuerta | Effect of biodiesel fuels on diesel engine emissions[END_REF] . Engine dynamometer studies reviewed in a 2002 report from EPA show a 2% increase in oxides of nitrogen (NO x ) emissions for B20 [START_REF] Mccormick | Effects of biodiesel blends on vehicle emissions: fiscal year 2006 annual operating plan milestone 10[END_REF] .

Another problem associated with biodiesel is the possible introduction of metallic contaminants from the production process [START_REF] Meher | Technical aspects of biodiesel production by transesterification-a review[END_REF] . The biodiesel processing flow diagram was illustrated in Fig. 2.11 [START_REF] Demirbas | Importance of biodiesel as transportation fuel[END_REF] . The transesterification process occurs in the presence of a catalyst, typically sodium hydroxide or potassium hydroxide. Following transesterification and separation of the glycerin, the biodiesel must go through the purification process to get rid of the impurities. In this process, residual amounts of sodium (Na) or potassium (K) from the catalyst can remain, though [START_REF] Williams | Impact of Biodiesel Impurities on the Performance and Durability of DOC, DPF and SCR Technologies: Preprint[END_REF] . Meanwhile, small amounts of calcium (Ca) or magnesium (Mg) can be added to the fuel during the purification process [START_REF] Williams | Impact of Biodiesel Impurities on the Performance and Durability of DOC, DPF and SCR Technologies: Preprint[END_REF] . These metallic fuel contaminants can be converted in the combustion process and form an inorganic ash that can be deposited onto the diesel exhaust emission control devices, causing negative impact [START_REF] Williams | Impact of Biodiesel Impurities on the Performance and Durability of DOC, DPF and SCR Technologies: Preprint[END_REF] .

Fig. 2.11 Biodiesel processing flow diagram [START_REF] Demirbas | Importance of biodiesel as transportation fuel[END_REF] .

Effects of biodiesel on engine-out regulated emissions

The emission impacts of biodiesel in different kinds of diesel engine have been investigated by a number of studies [198 -204] .

A major review was conducted by the U.S. Environmental Protection Agency (EPA) and the average emission changes for B20 from statistical analysis were shown in Table 2.2 [START_REF] Mccormick | Effects of biodiesel blends on vehicle emissions: fiscal year 2006 annual operating plan milestone 10[END_REF][START_REF] Korotney | A comprehensive analysis of biodiesel impacts on exhaust emissions[END_REF] . Fig. 2.12, taken from the same EPA report, shows the overall trends as a function of biodiesel content for all four regulated pollutants [START_REF] Mccormick | Effects of biodiesel blends on vehicle emissions: fiscal year 2006 annual operating plan milestone 10[END_REF][START_REF] Korotney | A comprehensive analysis of biodiesel impacts on exhaust emissions[END_REF] . Compared to conventional diesel, use of biodiesel is generally found to result in reduction of emissions of HC, CO and PM, with the exception of increased NO x emissions [START_REF] Mccormick | Effects of biodiesel blends on vehicle emissions: fiscal year 2006 annual operating plan milestone 10[END_REF] . Fig. 2.12 Average emission impacts of biodiesel content as estimated from published engine dynamometer data in the EPA study [START_REF] Korotney | A comprehensive analysis of biodiesel impacts on exhaust emissions[END_REF] .

In their study, McCormick et al. reviewed and summarized relevant literature concerning biodiesel NO x effect, and pertinent theories to explain this effect [START_REF] Hoekman | Review of the effects of biodiesel on NOx emissions[END_REF] . In modern diesel engines, several factors related to fuel composition and engine control strategies are important, though no single theory provides an adequate explanation of the biodiesel NO x effect under all conditions [START_REF] Hoekman | Review of the effects of biodiesel on NOx emissions[END_REF] .

Effects of biodiesel on the DOC

When it comes to the effects of biodiesel on the DOC, it is mainly about the impacts brought by the contaminants from biodiesel. The main contaminants are K, Na, Mg, Ca, P and S.

Alkali metals are well known poisons for catalysts and have been shown to negatively impact the mechanical properties of ceramic substrates [START_REF] Dou | Impact of Alkali Metals on the Performance and Mechanical Properties of NOx Adsorber Catalysts[END_REF] .

Cavataio et al. exploited a laboratory flow reactor study to determine the durability impact of alkali metal (K and Na) exposure on three Pt/Pd-based DOCs. By evaluating for changes in CO and HC light-off to acquire the activity impact on the contaminated DOCs, they found that contamination levels of 3.0 wt% Na had a higher negative impact on Pt-based DOC for T-50 CO and HC light-off [START_REF] Cavataio | Laboratory Study to Determine Impact of Na and K Exposure on the Durability of DOC and SCR Catalyst Formulations[END_REF] .

Williams et al. conducted a study to determine if a fuel containing metals (K, Na, Ca and Mg) at the ASTM limits could cause adverse impacts on the performance and durability of diesel emission control systems. Their results demonstrated that a decrease in DOC activity was seen after exposure to 150,000 mile equivalent aging, resulting in higher HC slip and a reduction in NO 2 formation [START_REF] Williams | Impact of Biodiesel Impurities on the Performance and Durability of DOC, DPF and SCR Technologies: Preprint[END_REF] .

In another study, Williams et al. investigated the metal exposure of alkali and alkaline earth metal impurities (K, Na and Ca) on diesel exhaust catalysts [START_REF] Williams | Impact of Fuel Metal Impurities on the Durability of a Light-Duty Diesel Aftertreatment System[END_REF] . The studied exhaust systems included a diesel oxidation catalyst (DOC), selective catalytic reduction (SCR) catalyst, and diesel particulate filter (DPF). The electron probe microanalysis (EPMA) imaging of aged catalyst parts found that both the K and Na penetrated into the washcoat of the DOC, while Ca remained on the surface of the washcoat.

Lance et al. aimed to elucidate the potential of alkali and alkaline earth metal impurities (K, Na, Ca and Mg) to degrade diesel emissions control systems [START_REF] Lance | Evaluation of Fuel-Borne Sodium Effects on a DOC-DPF-SCR Heavy-Duty Engine Emission Control System: Simulation of Full-Useful Life[END_REF] . Na, in particularly, was found to have minimal impact on DOC activity.

The sulfur originated from the fuel and lubricants is oxidized in the engine, predominantly forming SO 2 . When the flue gases are passing over an oxidizing catalyst, a significant part of SO 2 is oxidized to SO 3 . Once it is formed it is stable until T>700 °C [START_REF] Neyestanaki | Deactivation of postcombustion catalysts, a review[END_REF] . It can act as a catalyst poison or interact with water vapor to form sulfuric acid and contribute to additional problems/emissions [START_REF] Neyestanaki | Deactivation of postcombustion catalysts, a review[END_REF] . The presence of sulfur on the DOC results in the deterioration of its CO oxidation performance, and more significantly affects the oxidation of HC [START_REF] Russell | Diesel Oxidation Catalysts[END_REF][START_REF] Corro | Sulfur impact on diesel emission control-A review[END_REF] .

Effects of biodiesel on the SCR

Commercial V/Ti-based SCR catalysts generally have to be cleaned or substituted due to the accumulated deactivation by alkaline metals that exist in exhaust gases [START_REF] Kristensen | High performance vanadia-anatase nanoparticle catalysts for the Selective Catalytic Reduction of NO by ammonia[END_REF] . This deteriorates when it comes to the utilization of biomass (straw, wood) because the biomass-fired flue gas becomes highly poisonous due to increased contents of potassium and sodium (almost up to 2 wt.%) [211 -213] . Thus, in the case of biodiesel, its impact on SCR system, particularly the effects from the metal impurities, should be examined.

The study conducted by Lisi et al. offered us some general idea on the effect of deposition of alkali metals on SCR catalysts, though the catalyst was targeted for the stationary applications [START_REF] Lisi | Single and combined deactivating effect of alkali metals and HCl on commercial SCR catalysts[END_REF] . The negative impact of alkali metals (K and Na) on vanadia-based SCR catalysts was illustrated. The explanation for this was thought to be a significant loss of surface acidity which itself is result from the alkali metals deposited on the catalysts surface. It occurs in the way that acid sites were neutralized by alkali metals. In terms of neutralizing effect, potassium seems stronger than sodium. Specifically, the catalyst exposed to 0.18 wt.% of Na resulted in a 40% reduction in NO conversion [START_REF] Williams | Impact of Biodiesel Impurities on the Performance and Durability of DOC, DPF and SCR Technologies: Preprint[END_REF] .

Cavataio et al. employed a laboratory flow reactor study to determine the durability impact of alkali metal (Na and K) exposure on vanadium-based SCR catalysts [START_REF] Cavataio | Laboratory Study to Determine Impact of Na and K Exposure on the Durability of DOC and SCR Catalyst Formulations[END_REF] . They concluded that SCR catalysts showed significant irreversible deactivation in the NO x activity and NH 3 storage capacity due to the alkali metal (Na and K) exposure. Specifically, increased amounts of K contamination led to the linear decline of BET surface area, NH 3 storage capacity, and the subsequent NO x conversion [START_REF] Cavataio | Laboratory Study to Determine Impact of Na and K Exposure on the Durability of DOC and SCR Catalyst Formulations[END_REF] .

Williams et al. conducted a study to determine if a fuel containing metals (K, Na, Ca and Mg) at the ASTM limits could cause adverse impacts on the performance and durability of diesel emission control systems [START_REF] Williams | Impact of Biodiesel Impurities on the Performance and Durability of DOC, DPF and SCR Technologies[END_REF] . It turned out that the metal-zeolite SCR catalyst experienced a slight loss in activity after exposure to 435,000 mile equivalent aging.

In another study, Williams et al. investigated the metal exposure of alkali and alkaline earth metal impurities (Na, K and Ca) on SCR catalyst [START_REF] Williams | Impact of Fuel Metal Impurities on the Durability of a Light-Duty Diesel Aftertreatment System[END_REF] . The electron probe microanalysis (EPMA) imaging of aged catalyst parts found that both the Na and K penetrated into the washcoat of the SCR catalysts, while Ca remained on the surface of the washcoat. They also showed that the first inch of the SCR catalysts exposed to Na and K had reduced NO x conversion and NH 3 storage capacity. The SCR catalyst exposed to Ca had similar NO x conversion and NH 3 storage performance compared to the catalyst aged with ultra-low sulfur diesel (ULSD).

CHAPTER 3: EXPERIMENTAL

Experimental

In this chapter, the lab-scale devices, devoted to studies on both monolith and powder catalysts for diesel exhaust cleaning-up, will be described first. Subsequently, the characterization techniques pertinent to this thesis are detailed.

Description of lab-scale devices

Experimental device for monolithic catalysts

Experimental device for monolithic DOC

The apparatus for the catalytic activities tests of DOC, shown schematically in Fig. 3.1, mainly comprised three parts: gas-supply system (see 1 in Fig. 3.2), catalytic reactor (see 3 in As the name synthetic gas bench (SGB) suggests, several gases were used in order to obtain the desired reaction mixtures. All bottles of gas were supplied by Air Liquide (France). The description of supplied gas cylinders can be found in Table 3.1. The resulting stream was then redirected to a 3-way valve which offered two options: a), guiding the stream through the pre-heater and then through heated lines to the reactor; b), bypassing the reactor.

With the aid of the pre-heater as well as the heated lines, a proper heating of the stream before entering the reactor was ensured and made the gap between the temperatures upstream and downstream the monolith as small as possible. As such, the temperature field of the catalyst could be homogeneous.

The detectors exploited were listed in Table 3.2. For the purpose of illustration, the pictures of the experimental setup were presented in Fig.

3.2.

Experimental device for monolithic SCR catalysts

The apparatus for the catalytic activities tests of SCR catalysts, shown schematically in Fig. 3.3, mainly included three parts: gas-supply system, catalytic reactor and analytical instrumentation (Detectors). As the name synthetic gas bench (SGB) suggests, several gases were used in order to obtain the desired reaction mixtures. All bottles of gases were supplied by Air Liquide (France). The description of supplied gas cylinders can be found in Table 3.3. The detectors employed in this plant were presented in Table 3.4.

Experimental device for DOC study in powder form

The apparatus for the catalytic activities tests of DOC in the form of powder, was shown schematically in Fig. 3.4. It is mainly composed of three parts: gas-supply system, a U-shaped reactor and analytical instrumentation (Detectors). As the name synthetic gas bench (SGB) suggests, several gases were used in order to obtain the desired reaction mixtures. All bottles of gas were supplied by Air Liquide (France). The description of supplied gases can be found in Table 3.5. The stream containing the gases previously mixed, joined the stream coming from the water saturator. The resulting stream will be directed to a 6-way valve. The 6-way valve was fed by three inlet streams and connected to three outlet streams. It determined whether the resulting stream would go to the reactor or otherwise to the by-pass.

The reactors used in this set-up were Pyrex U-shaped tubes (L: 23 cm; ID: 2 cm). It had a reactional chamber where a porous quartz filter (porous type nr. 3) holds the catalyst.

As far as the detectors were concerned, they were detailed in Table 3.6. For the sake of illustration, the picture of the experimental setup was shown as Fig. 3.5. .

Characterization techniques

To get an idea of the feature of the catalyst, physiochemical characterization techniques are necessary. The techniques used in this thesis are:

-X-Ray Diffraction (XRD), in order to identify the crystalline phase of the supports and some metal species, particularly metal oxides and metallic particles in the study;

-Transmission Electron Microscopy (TEM), in order to identify palladium, platinum particles and to estimate the particle size;

-BET (Brunauer-Emmett-Teller) surface measurements, in order to follow the accessible surface for chemical reaction;

-H 2 -Temperature Programmed Reduction, in order to know the oxide phases presented in the catalysts.

XRD

XRD can be used to get an idea of the crystallinity of the material. Also, it allows the identification of the different crystallographic phases presented in the catalyst, particularly some metal oxides and metallic particles.

The diffraction of a monochromatic beam of X-ray by a crystalline compound is the basis of this characterization technique: The monochromatic beam of X-Ray source (Cu-Kα radiation, λ=0.15418 nm) is focused on the sample with different incident angles; at certain angles, if the sample is composed by one or more crystalline phases, the incident radiation is diffracted to the X-Ray detector. This phenomenon is described by Bragg's Law [START_REF] Bartolomeu | Nox selective catalytic reduction in biofuel powered vehicles[END_REF] :

2 sin θ nλ d =
Where, n is the positive integer number, λ wavelength of incident radiation, d interplanar distance (nm), θ Bragg's Angle at which the diffraction phenomenon occurs (degrees).

Diverse crystallographic phases are characterized by diffraction peaks at certain angles, with a given intensity that is up to the interplanar distance of the crystalline phase.

Transmission electron microscope (TEM)

The TEM uses a high-energy electron beam transmitted through a very thin sample to image and is capable of conducting the atomic level structural analysis. Combining different modes of observation on the same catalyst sample with the aid of high-performance detectors, precise information about the size, the morphology and the arrangement of the particles, crystal phases and composition can be obtained.

The high-energy electron beam is emitted through an electron gun and transmitted through the sample, indicating that TEM micrographs represent a plane projection of the tridimensional sample. High vacuum is required for the operation of the electron microscope, which means that TEM micrographs are obtained when the sample is under high vacuum conditions. This fact can be extremely important because some samples are vacuum-sensitive, suffering variations under these conditions.

If the electron microscope is sufficiently potent, a diffraction phenomenon occurs and the inter-reticular planes of a crystalline sample can be observed (e.g., metallic particles). Thus, in addition to the morphological information that TEM provides, by measuring the distances between the inter-reticular planes of a sample and using the same database used for XRD, information on the nature of the crystalline sample can be obtained. However, it should be noted that some samples are sensitive to the electron beam. Taking into account that the higher the magnification, the higher the amount of energy of the electron beam, this limits the upper magnification range that can be attained on certain samples (e.g., zeolites) [START_REF] Diaz | TEM studies of zeolites and ordered mesoporous materials[END_REF] .

Temperature programmed reduction (TPR)

TPR is highly useful in providing a quick characterization of metallic catalysts. This technique is capable of determining the reducibility of the catalysts: the reducible species (phases) present in the catalyst and the temperatures needed for the reduction of each species.

It also can provide information on the eventual degree of reduction. For bimetallic catalysts, TPR patterns often show whether the two components are mixed or not [START_REF] Niemantsverdriet | Spectroscopy in catalysis[END_REF] .

In this thesis, H 2 -TPR measurements were conducted using a BELCAT-M-183 facility under 5 vol.% H 2 in a flow of Ar. The reason for the argon being chosen as a carrier gas for hydrogen is that it allows an optimization of the thermal conductivity discrepancy between the carrier gas and the reactant. ii. Reduction: The reduction process was carried out under a stream of 5 vol.% H 2 /Ar with the flow rate of 50 mL/min, from 100°C to 900°C, at a heating rate of 7.5°C/min, while the evolution of H 2 was measured using the TCD. It can be divided into four main sections. In the first place, the performance of a commercial oxidation catalyst using different aging characteristics was examined. The influence of the presence of alkali compounds on the commercial honeycomb oxidation catalyst comes after it. Later, the influence of the presence of alkali compounds on a model oxidation catalyst in the form of powder was investigated. Finally, the comparison between the commercial honeycomb oxidation catalyst and the model oxidation catalyst in the form of powder was carried out.

Influence of different aging characteristics on the performance of a commercial Pt-

Pd/Al 2 O 3 diesel oxidation catalysts

To reduce the impact from vehicle emissions on the environment and to reach the super strict EURO VI standards, better understandings of the after-treatment systems is required.

Increasingly stringed regulations on diesel engine emissions have a significant impact on the required efficiency of Diesel Oxidation Catalysts (DOC) since a complete after-treatment line is now composed by a DOC, a DPF and a NO x abatement system (NO x trap or NH 3 -SCR) [START_REF] Russell | Diesel Oxidation Catalysts[END_REF] .

As the first pollution control device following the engine, lowered DOC oxidation efficiency due to thermal aging effects influences the efficiency of the exhaust cleaning-up systems downstream of the DOC and mostly on SCR systems [START_REF] Manigrasso | Hysteresis effect study on diesel oxidation catalyst for a better efficiency of SCR systems[END_REF] . It is already known that the control of the NO 2 concentration before the SCR system is pretty important, due to the so-called "FAST SCR" reactions (Eq. ( 20), Section 2.1.3, pg 28). This section focuses on the catalytic behavior of the DOC using different aging characteristics of road mileage. The effect of hydrothermal aging on the reactivity and structure of a commercial DOC is investigated as a function of temperature. The goal of this study is to understand the evolution of the active phase with the aging and to correlate these aging results with the corresponding overall catalytic activity and their impact on the reactions involved in the DOC with respect to CO, hydrocarbons and NO.

Experimental

Catalysts samples

The catalyst used for this study was a commercial DOC catalyst provided by Continental AG, Germany. The catalyst was composed by a bimetallic Pt/Pd (3/1 on mass basis) phase supported on alumina with a metallic loading of 80g/ft 3 , 400 channels per square inches (CPSI). It was cut into cylindrical "carrots" (d=2.54 cm; L=2.6 cm).

Four cylindrical "carrots" were hydrothermally aged in a laboratory box furnace (Rohde Ecotop 50, Germany) in flowing air with 10 vol.% H 2 O (total flow of 19 NL.min -1 ) under the conditions presented in Table 4.1 [START_REF] Wiebenga | Deactivation mechanisms of Pt/Pd-based diesel oxidation catalysts[END_REF] . "F" stands for fresh, indicating no manipulation at all.

The aged catalysts are denoted as "A750", "A850_10H", "A850_24H" and "A950" respectively, because the catalysts have been aged at 750°C, 850°C and 950°C under different durations. The first two aging temperatures were chosen because they were representative of real driving cycles and the last one was selected in order to cause a severe aging of the DOC [START_REF] Xie | Aging of Commercial Diesel Oxidation Catalysts: A preliminary Structure/Reactivity Study[END_REF] . As the aging severity increases, however, sintering occurs, resulting in a drop of the specific activity of the catalyst (A950), reflected by the light-off temperature of C 3 H 6 shifting to higher part. The mild hydrothermal aging of the catalyst optimizes its structure and its associated performance while further severe aging leads to the sintering of the catalyst, engendering negative influence on the conversion of C 3 H 6 . Similar results are obtained for CO conversion over studied catalysts, which is consistent with other study [START_REF] Wiebenga | Deactivation mechanisms of Pt/Pd-based diesel oxidation catalysts[END_REF] .

Effects of different aging characteristics on the performance of oxidation catalysts

As for the evolution of NO x , the stabilized catalyst (A750) stands out with more conversion of NO 2 and more formation of NO. From Fig. 4.1b and c, the conversions of NO 2 and CO can be considered as peaking simultaneously, indicating a reaction between NO 2 and CO. The NO 2 conversion after the peak is then reduced due to the increasing importance of O 2 as the experiments. However, it is known that this reaction is possible when C 3 H 6 is not burned up over the catalyst before it can selectively reduce NO 2 [START_REF] Bamwenda | The role of the metal during NO2 reduction by C3H6 over alumina and silica-supported catalysts[END_REF] . of the specific activity of the catalyst. As expected, the higher the aging temperature, the worse the performance of the resulted catalyst: A750 > A850_10H > A950.

Similar results are obtained for CO conversion over studied catalysts. It is worth noting that the oxidation of C 3 H 6 can be considered as no occurring until the complete oxidation of CO. This is attributed to the strong adsorption of CO.

As for the evolution of NO, the positive effect of aging on NO oxidation in representative mixture is quite evident: the A950 catalyst distinguishes itself from others with more conversion of NO, followed by the A850 catalyst. This might be principally due to the improved NO oxidation by O 2 , favored by bigger Pt/Pd particles. The cause of this NO oxidation activity enhancement with increased Pt particle size can be attributed to the greater resistance of larger Pt particles against forming Pt oxides, or in other words, the large Pt particles adsorb oxygen more weakly [START_REF] Russell | Diesel Oxidation Catalysts[END_REF][START_REF] Xue | Roles of supports, Pt loading and Pt dispersion in the oxidation of NO to NO2 and of SO2 to SO3[END_REF][START_REF] Benard | Supported platinum catalysts for nitrogen oxide sensors[END_REF] . Another possible reason responsible for this could be larger particles enabling stronger metal-NO bonds as reported by Benard et al. [START_REF] Benard | Supported platinum catalysts for nitrogen oxide sensors[END_REF] . Thus, it can be concluded that aging effect, which causes sintering of the particles, improves the NO conversion. It is interesting to observe that the NO conversion deteriorates with respect to the A750 catalyst, which has the optimal activities regarding the oxidations of C 3 H 6 and CO. It demonstrates that the change of the catalyst to homogeneity is not conductive to the oxidation of NO and implies different mechanisms in terms of the oxidations of these three reactants (C 3 H 6 , CO and NO).

Moreover, for both fresh and aged catalysts, NO 2 is not detected at low temperature until CO is oxidized. Schmeisser et al. found that reducing agents in the lean feed such as CO and C 3 H 6 hindered the formation of NO 2 [START_REF] Schmeißer | Experimental results concerning the role of Pt, Rh, Ba, Ce and Al2O3 on NO x -storage catalyst behaviour[END_REF] . Similar results have been also found in a DPNR (Diesel particulate NO x reduction) in lean gas conditions [START_REF] Millet | Synthetic gas bench study of a 4-way catalytic converter: Catalytic oxidation, NOx storage/reduction and impact of soot loading and regeneration[END_REF] .

Effect of the gas composition

In order to illustrate the effect of gas compositions on the catalytic performance of studied catalysts, different experimental conditions Mix 1 to Mix 5 were compared (Table 4.2). NO 2 is removed from the feed to form Mix 2. In Mix 3, NO is removed while CO is removed regarding Mix 4. Finally, both NO and NO 2 are removed from the feed in Mix 5. be considered as no occurring until the almost complete oxidation of CO, which is consistent with other studies [START_REF] Khosravi | Kinetic modelling of Pt and Pt:Pd diesel oxidation catalysts[END_REF] . The oxidation of CO is enhanced when NO 2 is present in the feed gas (Mix 3 and Mix 1) due to the CO/NO 2 interaction already described elsewhere [START_REF] Millet | Synthetic gas bench study of a 4-way catalytic converter: Catalytic oxidation, NOx storage/reduction and impact of soot loading and regeneration[END_REF] . This is also supported by the simultaneously increasing conversion of NO 2 in this study (Fig. 4.4c).

Moreover, the NO 2 conversion in Mix 3 is stronger than that of Mix 1, indicating the absence of NO is beneficial to the reaction between NO 2 and CO, which gives out NO. It can be ascribed to less competition for the active sites when NO is absent. Thus, the order for CO oxidation over fresh catalyst is Mix 3 > Mix 1 > Mix 2.

As expected, in terms of C 3 H 6 oxidation performance, the sequence is Mix 3 > Mix 2 > Mix 1. i. The comparison of the NO evolution profiles between Mix 1 and Mix 2 (mixture with and without NO 2 , respectively) suggests that the presence of NO 2 contributes to the formation of NO, by reacting with CO [START_REF] Xie | Aging of Commercial Diesel Oxidation Catalysts: A preliminary Structure/Reactivity Study[END_REF] .

The conversions of C

ii. The comparison of NOx evolution profiles (NO 2 conversion and NO transformation)

between Mix 1 and Mix 4 (mixtures with and without CO, respectively), supports the assertion that NO 2 reacts with CO below 150°C: When both NO 2 and CO are present as in the case of Mix 1, the conversion of NO 2 begins much earlier, enhancing the NO formation by reacting with CO at low temperature.

The reaction, however, is limited by the concentration of NO 2 . Taking the Mix 1 as an example, NO 2 is approximatively totally converted (refer to the formation of NO in Fig. 4.5c)

when about 30% of CO has reacted (i.e., 100 ppm of reactant transformed). Actually, the comparison of the CO evolution profile between Mix 1 and Mix 2 illustrates that Mix 2

overtakes Mix 1 when about 20% of CO is reacted, suggesting the role of O 2 as the oxidizer instead of NO 2 in the oxidation of CO thereafter. At low temperature, the conversion of NO leading to NO 2 also favors the CO oxidation [START_REF] Irani | Effect of hydrocarbon species on no oxidation over diesel oxidation catalysts[END_REF] . In other words, the oxidation of CO can be enhanced by the converted NO 2 serving as an intermediate. The CO oxidation enhanced by NO 2 is also embodied in the relatively high conversions of CO and NO 2 for Mix 3 and Mix 1 at low temperature (Fig. 4.5b and c).

As temperature rises, in terms of CO oxidation, with the mixture Mix 5, the conversion is higher since the absence of other reactants pays off. It is followed by Mix 2, and then Mix 3

with Mix 1 being the last, namely Mix 5 > Mix 2 > Mix 3 > Mix 1. Similar trend is applied to the conversions of C 3 H 6 . At temperature above 200℃ where CO oxidation is almost completed, NO oxidation, however, competes for the oxidations sites against C 3 H 6 . It was shown that the absence of other gas leads to an increase of C 3 H 6 oxidation since there is no reaction competition between NO and CO, C 3 H 6 oxidation [START_REF] Irani | Effect of hydrocarbon species on no oxidation over diesel oxidation catalysts[END_REF][START_REF] Auvray | Stability and activity of Pd-, Pt-and Pd-Pt catalysts supported on alumina for NO oxidation[END_REF] . Actually, over this stabilized catalyst, it is hard to tell the contribution to the C 3 H 6 conversion from the absence of CO at low temperature though this favor is evident over the fresh catalyst as mentioned can be considered as no occurring until the almost complete oxidation of CO as noticed before [START_REF] Millet | Synthetic gas bench study of a 4-way catalytic converter: Catalytic oxidation, NOx storage/reduction and impact of soot loading and regeneration[END_REF] . What's also evident is that at low temperature, NO 2 reacts with CO to give out NO and enhance the oxidation of CO.

The comparison of evolution profiles of CO and NO 2 between Mix 1 and Mix 3 illustrates that the absence of NO is beneficial to the reaction between NO 2 and CO which gives out NO.

As temperature increases, in terms of CO oxidation, Mix 5 takes the lead since the absence of other reactants offers it an edge. Thus, there comes the order Mix 5 > Mix 1/3 > Mix 2. Mix 2 being the last can be ascribed to the lack of contribution from NO 2 (Mix 1 and Mix 2) as well as the disadvantage of NO (Mix 2 and Mix 5).

With respect to the conversions of C 3 H 6 , the lead of Mix 4 at low temperature evidences that NO 2 acts as a strong oxidizer in the oxidation of C 3 H 6 when there exists no CO in the feed.

Concluding remarks

A structure-reactivity study of fresh, mildly thermal aged and severely thermal aged DOC samples in changing the pollutants in feed is presented. It is clearly showed that thermal aging significantly affects the overall oxidation activities of the catalysts. It turns out that mild thermal aging actually activates the catalyst. Mild aging on the catalyst improves the oxidation performance of C 3 H 6 as well as CO to a large extent while severe aging improves a little bit. NO oxidation over severely aged catalyst gets better.

Over these DOC samples, fresh and aged ones, the CO oxidation takes place at lower temperature than C 3 H 6 oxidation. Meanwhile, the NO 2 is a strong oxidant which reacts with CO if present to give out NO, enhancing the CO oxidation and formation of NO at low temperature. Moreover, the NO competes for the oxidations sites against C 3 H 6 .

In the next section, the influence of the presence of alkali compounds on the performance of a commercial Pt-Pd/Al 2 O 3 diesel oxidation catalysts will be experimentally investigated.

Influence of the presence of alkali compounds on the performance of a commercial

Pt-Pd/Al 2 O 3 diesel oxidation catalyst

It is generally accepted that alkali and alkali earth compounds are able to adsorb N-species. In this way, Milt et al. reported the formation of KNO 3 species as intermediates leading to an enhancement of the performance of K/CeO 2 traps in soot oxidation [START_REF] Milt | Soot oxidation on a catalytic NOx trap: Beneficial effect of the Ba-K interaction on the sulfated Ba,K/CeO2 catalyst[END_REF] . These results were confirmed on structured catalysts for the simultaneous removal of soot and nitrogen oxides prepared by means of coating cordierite monoliths with alumina-based suspensions containing Cu, Co or V and K as the catalytically active phase. The enhancement of activity was thus ascribed to higher ability to generate NO 2 [START_REF] Gálvez | Me (Cu, Co, V)-K/Al2O3 supported catalysts for the simultaneous removal of soot and nitrogen oxides from diesel exhausts[END_REF] .

More recently, it was shown by Chen et al. that formaldehyde oxidation over Pt/MnO 2 catalysts is enhanced by the presence of alkali metal salts [START_REF] Chen | Enhanced formaldehyde oxidation on Pt/MnO2 catalysts modified with alkali metal salts[END_REF] . However, the effect of alkali metals on the DOCs has never been clearly studied, particularly the effects of the presence of alkali metals from biofuels on the performance of this commercial catalytic after-treatment system.

On the other hand, few studies were devoted to the investigation of the catalytic activity of precious-metal-based catalysts under multicomponent co-feeding conditions. Lang et al.

conducted the study on the light-off and steady-state behavior for individual oxidation and cooxidation of CO and C 3 H 6 under near-stoichiometric conditions (based on three-way catalyst) using Pd/Al 2 O 3 and Pd/CeO 2 -ZrO 2 monolith catalysts [START_REF] Lang | Co-oxidation of CO and propylene on Pd/CeO2-ZrO2 and Pd/Al2O3 monolith catalysts: A light-off, kinetics, and mechanistic study[END_REF] . The effect of C 3 H 6 , H 2 O and their combination on NO oxidation activity over different powder catalysts (for diesel oxidation catalyst application) was also investigated by Auvray et al. [START_REF] Auvray | Stability and activity of Pd-, Pt-and Pd-Pt catalysts supported on alumina for NO oxidation[END_REF] . However, to the best of authors' knowledge, there is no study concerning the interaction of multi-species when they are co-fed in the simulated gas mixture over monolith bimetallic diesel oxidation catalyst.

In this sense, the objective of the present section is to investigate the influence of the addition of different alkali metal species (K and Na) on a commercial monolith Pt-Pd/Al 2 O 3 DOC, through catalytic tests performed on this structured catalyst under multicomponent (C 3 H 6 / CO / NO / NO 2 ) co-feeding conditions. Their effect was evaluated by examining the evolution profiles of C 3 H 6 , CO and NO x .

Experimental

Catalysts samples

The catalyst used for this study was taken from the same untreated brick of the commercial DOC catalyst provided by Continental. Again, it was cut into cylindrical "carrots" (d=2.54 cm; L=2.54 cm).

Alkali metals such as potassium (K) or/and sodium (Na) were introduced to the fresh DOC monolith catalyst by impregnation from the potassium and sodium nitrate solutions with 1 wt. % loading. After drying at 110°C overnight, the samples were calcined at 550°C for 4 h.

Catalysts prepared for this study were detailed in Table 4.3. It's worth noting that the prepared catalyst named K-Na contains 0.5 wt. % K and 0.5 wt. % Na, resulting in 1 wt. % loading in total.

Characterization of catalysts

Several characterization techniques were employed to identify the physicochemical composition of the studied catalysts. XRD patterns of studied catalysts were recorded on a Bruker Advanced D8 diffractometer equipped with a graphite monochromator using Cu-Kα radiation. XRD measurements were performed from 5 to 70° in an interval of 0.02° with a count time of 12s for each point. High resolution TEM (HRTEM) coupled with EDS was performed on a JEOL-JEM 2011 HR (LaB) microscope operating at 200 kV. The specific surface area was calculated by using the Brunauer-Emmett-Teller (BET) method. N 2 adsorption-desorption isotherms were acquired at liquid nitrogen temperature by a Micromeritics ASAP 2010. Before the measurements, the samples were evacuated at 200ºC for 2 hours under vacuum (0.2666Pa) to remove gases and vapors that might have been physically adsorbed.

The prepared structured catalysts were also characterized by means of Scanning electron microscopy (SEM, Hitachi S-3400 N coupled with EDS analysis, Röntec XFlash).

These techniques can be referred to in Chapter 3.

Catalytic activity studies

The same synthetic gas bench installation as for the previous aging study was used to investigate the catalytic activities of these four doped catalysts.

As mentioned in the previous aging study, a typical reactant gas mixture representative of the exhaust gases from diesel engines consists of C 3 H 6 , CO, NO, NO

Effects of the presence of alkali compounds on the performance of a commercial oxidation catalyst

Effect of the gas compositions

In order to illustrate the effect of gas compositions on the catalytic performance of studied catalysts, different experimental conditions Mix 1 to Mix 5 were compared (Table 4.4). NO 2 is removed from the feed to form Mix 2. In Mix 3, NO is removed while CO is removed regarding Mix 4. Finally both NO and NO 2 were removed from the feed in Mix 5. that the absence of other gas leads to an increase of C 3 H 6 oxidation since there is no reaction competition between NO and CO, C 3 H 6 oxidation [START_REF] Irani | Effect of hydrocarbon species on no oxidation over diesel oxidation catalysts[END_REF][START_REF] Auvray | Stability and activity of Pd-, Pt-and Pd-Pt catalysts supported on alumina for NO oxidation[END_REF] . As expected, in terms of C 3 H 6 oxidation performance, the ranking is then Mix 3 > Mix 2 > Mix 1. The conversions of C 3 H 6

for Mix 1 and Mix 4 can be regarded as almost the same except that the absence of CO favors the C 3 H 6 conversion at temperature lower than 200℃ for the Fresh and K catalysts while lower than 250℃ for Na-associated catalysts. The absence of CO favoring the C 3 H 6 conversion is due to the lack of inhibition from CO as observed by Lang et al. [START_REF] Lang | Co-oxidation of CO and propylene on Pd/CeO2-ZrO2 and Pd/Al2O3 monolith catalysts: A light-off, kinetics, and mechanistic study[END_REF] . When CO adsorbs onto surface active sites, it can result in the decrease of C 3 H 6 conversion in two ways: a), occupy the active sites which otherwise can be available for the adsorption of Over these DOC samples, the CO oxidation (Fig. 4.8) takes place at lower temperature than C 3 H 6 oxidation by comparing corresponding light-off curves. This is also consistent with the findings of Lang et al. [START_REF] Lang | Co-oxidation of CO and propylene on Pd/CeO2-ZrO2 and Pd/Al2O3 monolith catalysts: A light-off, kinetics, and mechanistic study[END_REF] . In general, similar to that of the C 3 H 6 oxidation, Mix 5 distinguishes itself from others with high CO oxidation performance.

Na-doped catalyst and (d). K/Na-doped catalyst

At low temperature, the oxidation of CO is enhanced when NO 2 (strong oxidant) is present in the feed gas (Mix 3 and Mix 5, difference in the presence of NO 2 ) due to the CO/NO 2 interaction already described elsewhere [START_REF] Millet | Synthetic gas bench study of a 4-way catalytic converter: Catalytic oxidation, NOx storage/reduction and impact of soot loading and regeneration[END_REF] . As temperature rises, in terms of CO oxidation, Mix 5 takes the lead since the absence of other reactants pays off. The absence of NO in Mix 3 offers it an edge in terms of CO oxidation, compared with Mix 1. The presence of NO 2 in Mix 1, however, contributes to the oxidation of CO, compared to Mix 2. Thus, the order for CO oxidation in general is Mix 5 > Mix 3 > Mix 1 > Mix 2.

The NO 2 conversion in Mix 3 is stronger than that of Mix 1, supporting that the absence of NO is beneficial to the reaction between NO 2 and CO which gives out NO (Fig. 4.9). It can be ascribed to less competition for the active sites when NO is absent. For better understanding of how multi-species interact when they are co-fed in the simulated gas mixture over studied catalysts, their behavior over the fresh catalyst was analyzed as an example. As observed from Fig. 4.10, the oxidation of C 3 H 6 can be considered as no occurring until the almost complete oxidation of CO, which is consistent with other study [START_REF] Khosravi | Kinetic modelling of Pt and Pt:Pd diesel oxidation catalysts[END_REF] .

Moreover, the oxidation of CO is enhanced when NO 2 is present in the feed gas (Mix 3 and Mix 1) due to the CO/NO 2 interaction [START_REF] Millet | Synthetic gas bench study of a 4-way catalytic converter: Catalytic oxidation, NOx storage/reduction and impact of soot loading and regeneration[END_REF] . The reaction between NO 2 and CO at low temperature in present study is evidenced by following observations: i.

The comparison of the NO x evolution profiles between Mix 1 and Mix 2 (mixture with and without NO 2 , respectively) suggests that the presence of NO 2 contributes to the formation of NO, by reacting with CO.

ii. The comparison of NO x evolution profiles (NO 2 conversion and NO transformation) between Mix 1 and Mix 4 (mixtures with and without CO, respectively), supports this assertion that NO 2 reacts with CO below 150°C: When both NO 2 and CO are present as in the case of Mix 1, the conversion of NO 2 begins much earlier, enhancing the NO formation by reacting with CO at low temperature.

iii. Increasing conversion of CO is accompanied by that of NO 2 in this study (Fig.

4.10c).

NO 2 , however, is less reactive towards the hydrocarbon according to the study of Yentekakis et al. [START_REF] Yentekakis | A comparative study of the C3H6 + NO + O2, C3H6 + O2 and NO + O2 reactions in excess oxygen over Namodified Pt/γ-Al2O3 catalysts[END_REF] . The comparison of conversion profiles of in the chemisorption of adsorbates [START_REF] Yentekakis | A comparative study of the C3H6 + NO + O2, C3H6 + O2 and NO + O2 reactions in excess oxygen over Namodified Pt/γ-Al2O3 catalysts[END_REF] . Mix the Na-doped catalyst shifts to the higher part taking fresh one as the reference, suggesting the negative influence from the introduction of Na. Moreover, the combination of K and Na lead to the decrease of the activity. Similar results are obtained for the conversions of CO. As for the evolution of NO, there are more conversions of NO over K-or Na-doped ones, as already reported in other studies [229 -231] . Thus, the addition of single dopant can be regarded as beneficial to the NO conversion. However, the combination of K and Na leads to a decrease of conversion. Again, the introduction of K obviously promotes the oxidation reaction while Na brings the opposite effect. Also, the combination of these two stands out as the worst. Thus, the absence of NO/NO 2 or CO has no effect regarding the promotion of K addition or inhibition of Na addition to the studied catalysts.

Apparently, the promotion of K addition or inhibition of Na addition to the studied catalysts is also reflected in the conversion of NO 2 . It is commonly accepted that alkali and alkali earth compounds are able to adsorb N-species.

In their study, Milt and co-workers reported the KNO 3 species formed in this way as intermediates led to the enhanced performance of K/CeO 2 traps in soot oxidation [START_REF] Milt | Soot oxidation on a catalytic NOx trap: Beneficial effect of the Ba-K interaction on the sulfated Ba,K/CeO2 catalyst[END_REF] . These results were confirmed on structured catalysts for the simultaneous removal of soot and nitrogen oxides prepared by means of coating cordierite monoliths with alumina-based suspensions containing Cu, Co or V and K as the catalytically active phase [START_REF] Gálvez | Me (Cu, Co, V)-K/Al2O3 supported catalysts for the simultaneous removal of soot and nitrogen oxides from diesel exhausts[END_REF] . The mechanism leading to the enhancement of the activity due to the presence of alkali metals might go through the formation of adsorbed N-species (nitrites/nitrates) on the catalyst surface, favored by the presence of K or Na as proposed by Galvez et al. [START_REF] Gálvez | Influence of the surface potassium species in Fe-K/Al2O3 catalysts on the soot oxidation activity in the presence of NOx[END_REF] . The enhancement of activity was thus ascribed to higher ability to generate NO 2 .

According to the reported spectroscopic study, Na exists in the form of chemical compounds such as Na 2 CO 3 or NaNO 3 under C 3 H 6 /NO/O 2 atmosphere [START_REF] Yentekakis | A comparative study of the C3H6 + NO + O2, C3H6 + O2 and NO + O2 reactions in excess oxygen over Namodified Pt/γ-Al2O3 catalysts[END_REF][START_REF] Filkin | A Kinetic and Spectroscopic Study of the in Situ Electrochemical Promotion by Sodium of the Platinum-Catalyzed Combustion of Propene[END_REF] . Both promotional and poisoning effects induced by Na in de-NO x catalytic chemistry of platinum group metals are possible when Na is present at the catalyst surface. These effects depend on the amount of Na supplied to the catalyst surface as well as a notable dependence on reactant gas phase composition (the relative molar ratio of the electron-donating to electron-accepting adsorbates imposed to the surface) [START_REF] Yentekakis | A comparative study of the C3H6 + NO + O2, C3H6 + O2 and NO + O2 reactions in excess oxygen over Namodified Pt/γ-Al2O3 catalysts[END_REF][START_REF] Filkin | A Kinetic and Spectroscopic Study of the in Situ Electrochemical Promotion by Sodium of the Platinum-Catalyzed Combustion of Propene[END_REF] . It's well established that Na-induced changes in the relative adsorption strengths of the various reactants are the main cause [233, 236 -238] .

Yentekakis et al. showed that remarkable beneficial effects on both de-NO x efficiency and N 2 selectivity were realized with Na-doped Pt/γ-Al 2 O 3 catalysts operated under lean burn conditions [START_REF] Yentekakis | A comparative study of the C3H6 + NO + O2, C3H6 + O2 and NO + O2 reactions in excess oxygen over Namodified Pt/γ-Al2O3 catalysts[END_REF] . They proposed that the observed alkali promotion of activity and N 2 selectivity in de-NO x catalysis were due to alkali-induced strengthening of the metal-NO bond (thus increasing NO coverage) and the consequent dissociation of NO molecules adsorbed on the platinum metal component of the catalyst. Later, their spectroscopic evidence for weakening of the N-O bond by means of in situ DRIFTS justified the hypothesis [START_REF] Koukiou | Spectroscopic evidence for the mode of action of alkali promoters in Pt-catalyzed de-NOx chemistry[END_REF] .

Vernoux et al. found that the addition of Na can lead to a strengthening of the Pt-electronacceptor (O and NO) bonds and a weakening of the Pt-electron-donor (C 3 H 6 ) bonds [START_REF] Vernoux | Effect of the addition of Na to Pt/Al2O3 catalysts for the reduction of NO by C3H8 and C3H6 under lean-burn conditions[END_REF] .

Another point of interest is that the strengthening of the Pt-O bond is much more pronounced than that of the Pt-NO bond because O is a much stronger electron-acceptor than NO [START_REF] Vernoux | Effect of the addition of Na to Pt/Al2O3 catalysts for the reduction of NO by C3H8 and C3H6 under lean-burn conditions[END_REF] . An excessive promotion of oxygen adsorption due to the presence of Na, however, can cause severe inhibition in both C 3 H 6 and NO conversions [START_REF] Yentekakis | A comparative study of the C3H6 + NO + O2, C3H6 + O2 and NO + O2 reactions in excess oxygen over Namodified Pt/γ-Al2O3 catalysts[END_REF][START_REF] Vernoux | Effect of the addition of Na to Pt/Al2O3 catalysts for the reduction of NO by C3H8 and C3H6 under lean-burn conditions[END_REF] . In terms of inhibition, thick layers consisting of sodium carbonate which accounts for catalyst poisoning can be another possibility [START_REF] Filkin | A Kinetic and Spectroscopic Study of the in Situ Electrochemical Promotion by Sodium of the Platinum-Catalyzed Combustion of Propene[END_REF] .

To sum up, the involved environment would determine whether Na acts as a promoter or an inhibitor. This is particularly reflected in Fig. 4.12 of present study: the presence of Na can enhance the conversions of CO and NO while inhibit the conversion of C 3 H 6 . The prepared structured catalysts were also characterized by means of Scanning electron microscopy (SEM, Hitachi S-3400 N coupled with EDS analysis, Röntec XFlash).

These techniques can be referred to in Chapter 3.

Catalytic activity studies

The same synthetic gas bench installation as for the previous aging study was used to investigate the catalytic activities of these catalysts.

As mentioned in the previous aging study, a typical reactant gas mixture representative of the exhaust gases from diesel engines consists of C 3 H 6 , CO, NO, NO 2 , O 2 , CO 2 , H 2 O and N 2 , as Mix 1 in Table 4.6. The total gas flow rate was 11.8 L•min -1 referenced to 293K and 1 atm, resulting in a gas hourly space velocity (GHSV) of 55,000 h -1 . Gas mixtures without NO 2 , NO, CO or NO x were setup to explore the influence of each gas component on the catalytic process. Table 4.6 summarized the conditions of the different experiments in terms of gas composition.

Effects of the dual deactivation (hydrothermal and chemical) on the performance of a commercial oxidation catalyst

Effect of the dual deactivation on a DOC under mixtures representative of diesel exhaust

Fig. 4.17 presents the evolution profiles of reactants of interest over studied catalysts, fresh and conditioned ones, under the simulated gas condition Mix 1, which is representative of a real exhaust mixture. Compared with the Fresh catalyst, the light-off temperatures of C 3 H 6 over both the A750 and the K-doped one shift to the lower temperatures, indicating positive influence brought by the mild aging or by the introduction of K. By contrast, the light-off temperatures of C 3 H 6 over the dual deactivation sample shifts to the higher temperatures, demonstrating negative influence of this manipulation.

Evidently, compared to the Fresh catalyst, the mild aging on the catalyst can promote the oxidation activity of C 3 H 6 , so can the addition of K, while other case (their combination) engenders negative influence on the conversion of C 3 H 6 . Similar results are obtained for CO conversion over studied catalysts. As for the evolution of NO x , the stabilized catalyst (A750) stands out with more conversion of NO 2 and more formation of NO. From Fig. 4.17b and c, the conversions of NO 2 and CO can be considered as peaking simultaneously, indicating the reaction between NO 2 and CO. The NO 2 conversion after the peak is then reduced due to the increasing importance of O 2 as the oxidizer in CO oxidation. Reactions between NO 2 and C 3 H 6 are not clearly identified in these experiments. Again, the performance of the K-A750 catalyst can be considered as the worst particularly from the viewpoint that the NO 2 conversion light-off occurs at much higher temperature compared to the other catalysts. Fig. 4.18 shows the evolution profiles of reactants of interest over studied catalysts, fresh and three other manipulated ones, under the simulated gas condition Mix 2, thus in the absence of NO 2 in the feed. It's not hard to find that the absence of NO 2 does not affect the promotion by mild aging or the addition of K on both the C 3 H 6 and CO oxidation activities. The inhibition influence by the manipulation of combination between the aging and doping is not affected, either. However, it's worth noting that after mild aging, the C 3 H 6 oxidation activity is significantly improved. In this sense, mild aging actually activates the catalyst.

As for the evolution of NO, the effect of doping of K on NO oxidation in representative mixture is positive and strong. This promotion is even more evident when comparing the NO conversion profiles between A750 and K-A750 catalysts. The mechanism of resulted enhancement of the activity from the presence of K will most probably be linked to the formation of adsorbed N-species (nitrites/nitrates) on the catalyst surface, as proposed by Galvez et al. [START_REF] Gálvez | Influence of the surface potassium species in Fe-K/Al2O3 catalysts on the soot oxidation activity in the presence of NOx[END_REF] .

Moreover, for both Fresh and other manipulated catalysts, NO 2 is not detected at low temperature until CO is oxidized. Schmeisser et al. found that reducing agents in the lean feed such as CO and C 3 H 6 hindered the formation of NO 2 [START_REF] Schmeißer | Experimental results concerning the role of Pt, Rh, Ba, Ce and Al2O3 on NO x -storage catalyst behaviour[END_REF] . Similar results have been also found in a DPNR (Diesel particulate NO x reduction) in lean gas conditions [START_REF] Millet | Synthetic gas bench study of a 4-way catalytic converter: Catalytic oxidation, NOx storage/reduction and impact of soot loading and regeneration[END_REF] . Again, the introduction of K or mild aging obviously promotes the oxidation reaction while their combination brings the opposite effect. Thus, the absence of NO/NO 2 or CO has no effect regarding the promotion or inhibition resulted from these manipulations to the studied catalysts.

Apparently, the promotion or inhibition resulted from these manipulations to the studied catalysts is also reflected in the conversion of NO 2 when it plays a major part as a strong oxidizer in the oxidation of C 3 H 6 .

Effect of the gas compositions

In order to illustrate the effect of gas compositions on the catalytic performance of studied catalysts, different experimental conditions Mix 1 to Mix 5 were compared (Table 4.6). NO 2 is removed from the feed to form Mix 2. In Mix 3, NO is removed while CO is removed regarding Mix 4. Finally both NO and NO 2 were removed from the feed in Mix 5.

Fig. 4.22 depicts the evolution profiles of C 3 H 6 as a function of temperature under different mixtures over the studied catalysts. The evolution profile of C 3 H 6 under the gas condition Mix 5 is not easy to neglect due to its remarkable oxidation performance. It was shown that the absence of other gas leads to an increase of C 3 H 6 oxidation since there exists no reaction competition between NO and CO, C 3 H 6 oxidation [START_REF] Irani | Effect of hydrocarbon species on no oxidation over diesel oxidation catalysts[END_REF][START_REF] Auvray | Stability and activity of Pd-, Pt-and Pd-Pt catalysts supported on alumina for NO oxidation[END_REF] . As expected, in terms of

K catalyst and (d). K-A750 catalyst

Over these DOC samples, the CO oxidation takes place at lower temperature than C 3 H 6 oxidation by comparing corresponding light-off curves. In general, similar to that of the C3H6 oxidation, Mix 5 separates itself from others with high CO oxidation performance.

At low temperature, the oxidation of CO is enhanced when NO 2 (strong oxidant) is present in the feed gas (Mix 3 and Mix 5, difference in the presence of NO 2 or not) due to the CO/NO 2 interaction already described elsewhere [START_REF] Millet | Synthetic gas bench study of a 4-way catalytic converter: Catalytic oxidation, NOx storage/reduction and impact of soot loading and regeneration[END_REF] . The absence of NO in Mix 3 offers it an edge in terms of CO oxidation, compared with Mix 1. The comparison of NO 2 conversion profiles between Mix 3 and Mix 1 justifies this (Fig. 4.24). The comparison on the evolution of NOx between Mix 1 and Mix 2 (mixture with and without NO 2 , respectively) suggests that the presence of NO 2 contributes to the formation of NO and oxidation of CO by reacting with CO [START_REF] Xie | Aging of Commercial Diesel Oxidation Catalysts: A preliminary Structure/Reactivity Study[END_REF] . As temperature rises, in terms of CO oxidation, Mix 5 takes the lead since the absence of other reactants pays off. The raising momentum acquired by Mix 2 with increasing temperature is also observed, which overtakes Mix 1 finally. Thus, in general, the order for CO oxidation in general is Mix 5 > Mix 3 > Mix 2 > Mix 1.

The comparison on the evolution of NO between Mix 1 and Mix 4 (mixture with and without CO, respectively) also confirms that NO formation from reduction of NO 2 is enhanced when NO 2 reacts with CO at low temperature [START_REF] Xie | Aging of Commercial Diesel Oxidation Catalysts: A preliminary Structure/Reactivity Study[END_REF] . K catalyst and (d). K-A750 catalyst

Concluding remarks

On a commercial Diesel Oxidation Catalyst, studied in the monolithic form, under representative exhaust gases mixtures, it is clearly showed that the mild thermal aging leads to positive impacts on the catalytic oxidation, so does the introduction of K. Their combination, however, results in the drastic reduction of the oxidation performance.

Over these DOC samples, Fresh and manipulated ones, the CO oxidation takes place at lower temperature than C 3 H 6 oxidation. Meanwhile, the NO 2 is a strong oxidant which reacts with CO if present to give out NO, enhancing the CO oxidation and formation of NO at low temperature.

There is no impact of feed gas composition on the ranking of catalysts in terms of catalytic performance. However, it was shown that the presence of NO 2 promotes the oxidation reaction while the presence of NO competes for the same catalytic sites in terms of oxidation, leading to a shift of light-off temperature for CO and C 3 H 6 to higher temperatures.

In the next section, the influence of the presence of alkali compounds on the performance of lab-made Pt-Pd/Al 2 O 3 diesel oxidation catalysts in the powder form will be explored.

Influence of the presence of alkali compounds on the performance of a lab-made Pt-

Pd/Al 2 O 3 diesel oxidation catalyst in the powder form

Experimental

Catalysts preparation

Catalysts powder samples were prepared with Alumina as the support while Pt and Pd as the active phase whose ratio was 3:1 on mass basis. Alkali metals (K or/and Na, 1 wt.%) were subsequently introduced to the DOC powdered catalysts through a conventional wet impregnation method. The catalyst named K-A750 in Table 4.7 was aged for 4 hours at 750℃

prior to the doping of K.

Characterization of catalysts

The acquired catalysts were characterized by XRD. XRD patterns of studied catalysts were acquired on a PANalyical-Empyrean diffractometer with monochromatic beam of X-Ray source (Cu-Kα radiation, λ=0.15406 nm). XRD measurements were performed from 3 to 90° in an interval of 0.02° (2θ) with a step time of 60s for each point during data acquisition.

Catalytic activity studies

A synthetic gas bench was used, the total flow rate being 250 mL•min -1 (GHSV=150,000 h -1 ).

A typical reactant gas mixture (Mix 1, see

Effects of presence of alkali compounds on the performance of oxidation catalysts

Effect of alkali metals on a DOC under gaseous mixtures representative of diesel exhaust

Fig. 4.25 presents the evolution profiles of reactants of interest over studied catalysts, Fresh and conditioned ones, under the simulated gas condition Mix 1, which is representative of a real exhaust mixture. As far as the conversions of C 3 H 6 are concerned, the addition of dopants can be generally considered as beneficial to the catalytic performance. Among them, the presence of Na alone exerts the most significant impact. The mechanism of resulted enhancement of the activity from the presence of Na will most probably be associated with the formation of adsorbed N-species (nitrites/nitrates) on the catalyst surface, as proposed by Galvez et al. [START_REF] Gálvez | Influence of the surface potassium species in Fe-K/Al2O3 catalysts on the soot oxidation activity in the presence of NOx[END_REF] . Particular attention should be paid to the conversions of NO. The presence of dopants, either K/Na alone or it their combination, inhibits the NO conversion at temperatures below 350°C.

It's obvious that the maximal conversion of NO is reached by the Fresh catalyst in the first place, followed by the ones with dopants but without Na, while the doping of Na drastically postpones it. The plausible explanation for the postponement can be attributed to the overpromotion after the addition of alkali compounds [START_REF] Yentekakis | A comparative study of the C3H6 + NO + O2, C3H6 + O2 and NO + O2 reactions in excess oxygen over Namodified Pt/γ-Al2O3 catalysts[END_REF] .

The comparison of the promotion on C 3 H 6 oxidation reaction by the alkali compound (K or/and Na) to their respective inhibition on NO oxidation reaction sheds lights on different mechanism associated with the oxidation reactions of reactants of interest. Again, it's difficult to notice the differences among the CO conversion profiles for studied catalysts. The NO conversion profiles for studied catalysts, on the contrary, exhibits remarkable differences: in terms of the NO conversion performance, the order is Na > F > K.

In other words, Na acts as a promoter compared to the Fresh catalyst while K an inhibiter.

The resulted enhancement of the activity due to the presence of Na is in accord with other reported study [START_REF] Gálvez | Me (Cu, Co, V)-K/Al2O3 supported catalysts for the simultaneous removal of soot and nitrogen oxides from diesel exhausts[END_REF][START_REF] Chen | Enhanced formaldehyde oxidation on Pt/MnO2 catalysts modified with alkali metal salts[END_REF] . As for K acting as an inhibiter, it can be ascribed to the decrease of the active sites of the targeted catalyst after the introduction of K. As can be noticed, the introduction of alkali compound (K or Na) obviously promotes the C 3 H 6 oxidation reaction. Thus, the absence of NO or CO has no effect regarding the promotion on C 3 H 6 oxidation of K addition or Na addition to the studied catalysts.

The introduction of alkali compound (K or Na), however, significantly inhibits the NO oxidation reaction. As pointed out before, this can be caused by the decrease of the actives sites for NO oxidation reaction after the addition of K or Na.

The comparison of the promotion on

Effect of the gas composition

In order to illustrate the effect of gas compositions on the catalytic performance of studied catalysts, different experimental conditions Mix 1 to Mix 4 were compared (Table 4 oxidation since there exists no reaction competition between NO and CO, C 3 H 6 oxidation [START_REF] Irani | Effect of hydrocarbon species on no oxidation over diesel oxidation catalysts[END_REF][START_REF] Auvray | Stability and activity of Pd-, Pt-and Pd-Pt catalysts supported on alumina for NO oxidation[END_REF] . As expected, in terms of C 3 H 6 oxidation performance, the ranking in general is then Mix

4 > Mix 3 > Mix 1.

Concluding remarks

On the lab-made diesel oxidation catalyst, studied in the powder form, under representative exhaust gases mixtures, it is clearly showed that the introduction of dopants leads to either negative or positive impacts on the catalytic oxidation.

In general, the addition of alkali compound (K or/and Na) leads to a promotion of the C 3 H 6 oxidation reactions while incurs an inhibition on the NO conversion. From the analysis of changing the pollutants in the feed, it's concluded that C 3 H 6 and NO are mutually inhibiter to each other when it comes to their oxidation. Moreover, the influence of CO on NO conversion is negligible though the presence of CO brings negative impact to the C 3 H 6 oxidation. There is no impact of exhaust gas composition on the ranking of catalysts.

In the next section, the influence of the presence of alkali compounds on monolith DOC and lab-made DOC performance will be compared.

Comparison of lab-based research work on monolith DOC to model DOC in the presence of alkali compounds

As presented in previous sections, the research on the diesel oxidation catalyst varies in a broad spectra: from using different aging characteristics to the doping of alkali compounds, from employing the monolith form of catalyst to the powder form of lab-made catalyst.

In this section, the comparison of lab-based research work on monolith DOC to model DOC in the presence of alkali compounds will be performed.

Experimental

First, some description about the studied catalysts will be provided. The monolith DOC is a commercial catalyst, with bimetallic Pt/Pd (3/1 on mass basis) phase supported on alumina with a metallic loading of 80g/ft 3 , 400 channels per square inches (CPSI). It was cut into cylindrical "carrots". The used lab-made DOC also has Pt and Pd as the active phase and their mass ratio is Pt/Pd=3. Then, related experimental conditions will be detailed, see Table 4.9 and Table 4.10.

Results and discussion

In this section, the results of their catalytic activities tests will be presented and compared in order to shed light on the role of the catalyst form in the reaction process mainly involving C 3 H 6 , CO and NO.

Concluding remarks

The comparison of lab-based research work on monolith DOC to powder DOC in the presence of alkali compounds is performed taking a typical mixture representative of diesel exhaust as an example.

It's found that the introduction of K strongly promotes the C 3 H 6 oxidation reaction over the catalyst in the monolith form while the introduction of double dopants (K and Na) strongly affects the CO oxidation reaction over the catalyst in the monolith form in a negative way.

The introduction of Na strongly promotes the NO conversion over the catalyst in the monolith form while it acts as an inhibitor when it affects the powder catalyst. exhaust gases from for municipal solid waste (MSW) combustion [START_REF] Lisi | Single and combined deactivating effect of alkali metals and HCl on commercial SCR catalysts[END_REF] . They found that doping agents do not cause loss of surface area nor pore occlusion but a significant loss of surface acidity was observed upon alkali metals doping. They ascribed the significant loss of surface acidity to the decrease in the number of acid sites neutralized by alkali metals adsorbing ammonia in the temperature range typical of SCR process. Moreover, HCl promotes the formation of new acid sites, restoring to some extent surface acidity of the codoped catalyst (alkaline metal + HCl) though showing a lower activity compared to the undoped one.

Tang et al. investigated the deactivation of lab-prepared V 2 O 5 /TiO 2 powder catalysts due to deposition of alkali and alkaline earth metal salts in fly ashes, finding that Na + ions exhibited stronger poisoning effect than Ca 2+ ions. The surface acidity of catalyst, the reducibility of dispersed vanadia species as well, accounts for the different poisoning degree of Na + and Ca 2+ ions [START_REF] Tang | The poisoning effect of Na+ and Ca2+ ions doped on the V2O5/TiO2 catalysts for selective catalytic reduction of NO by NH3[END_REF] . Klimczak et al. studied the influence of inorganic components (K/Na from biodiesel impurities or urea solution) on the catalytic activity and selectivity of honeycomb catalysts coated with an industrial V 2 O 5 -WO 3 /TiO 2 powder for NH 3 -SCR mobile application. Doping of alkaline metals results in a strong catalyst deactivation, evidenced by a reduced ammonia storage capacity. The single poisoning effect of the additives is related to the basicity of the inorganic components and decreases in the order K > Na. Furthermore, the influence of combinations of poisons using Design of Experiments (DOE) was determined by them.

However, there was no evidence showing the experimental investigation into the co-doping effect of K and Na in their study [START_REF] Klimczak | High-throughput study of the effects of inorganic additives and poisons on NH3-SCR catalysts-Part I: V2O5-WO3/TiO2 catalysts[END_REF] .

More recently, the deactivation mechanisms by the alkali metals on the V 2 O 5 -WO 3 /TiO 2 catalyst were extensively studied by Peng et al. with both the experimental and DFT theoretical approaches [START_REF] Peng | Design strategies for development of SCR catalyst: improvement of alkali poisoning resistance and novel regeneration method[END_REF] . They asserted that alkali atom mainly influences the active site V species rather than W oxides; Alkali metals atoms can directly bonded to the active site of vanadia for the V 2 O 5 -WO 3 /TiO 2 catalyst and lower the surface acidity and reducibility. The resulted decrease of reducibility by dopant atoms, however, is not the main reason responsible for the catalysts' deactivation. Instead, the lowered surface acidity, resulting in reduced amount of NH 3 adsorption, mainly accounts for the catalysts' deactivation. So far, the combined effect of addition of potassium/sodium on vanadia-SCR for mobile applications has been less investigated, especially so on commercial V 2 O 5 -based SCR monolith catalyst. Partially based on this, single and combined effect of addition of 1 wt.% of potassium or sodium on the SCR activity of one commercial V 2 O 5 -based catalysts in monolith form has been investigated in this thesis.

Influence of the presence of alkali compounds on the performance of a commercial V 2 O 5 -based SCR catalyst in the monolith form

Experimental

Catalysts samples

The catalyst used for this study was a commercial SCR catalyst provided by Umicore, Belgium. The active material of the catalyst was mainly composed by Vanadia. In terms of its substrate, it's 400 channels per square inches (CPSI). It was cut into cylindrical "carrots" (d=2.54 cm; L=2.54 cm). One of these untreated carrots would be taken as the reference one, Fresh.

Alkali metals such as potassium (K) or/and sodium (Na) were introduced to other untreated SCR monolith catalysts by impregnation from the potassium and sodium nitrate solutions with 1 wt. % loading. After drying at 110°C overnight, the samples were calcined at 400°C for 4 h in a tubular electrical furnace under a stream of dry air. Catalysts prepared for this study were presented in Table 5.1. It is worth noting that the prepared catalyst named K-Na contains 0.5 wt. % K and 0.5 wt. % Na.

Catalytic activity studies

A synthetic gas bench installation was used in order to investigate the catalytic activities of these catalysts. The carrot was put inside a stainless steel reactor, which itself was placed inside an oven reaching temperatures up to over 400ºC. To ensure that the temperature field of the catalyst was homogeneous, two thermocouples were available to follow the reaction temperature at the inlet and the outlet of the catalyst.

A model lean-burn diesel exhaust together with the injected reductant NH 3 reaching the SCR system consists of NH Catalysts were examined in terms of oxidation capacity and NO x reduction for temperatures ranging from 50°C to 400°C. Exhaust gas compositions before and after the reactor were characterized by an Environnement S.A. analyzer composed of a set of modules. Among them, the TOPAZE 32M module allowed NO, NO 2 and total NO x measurement thanks to a chemiluminescence analyzer. A MIR2M module was composed of an IR spectrogram, for the measurement of H 2 O molar fraction, and a magnetic sensor, for the O 2 molar fraction measurement.

Focus on the Fresh commercial V 2 O 5 -based SCR catalyst in monolith form

NH 3 oxidation capability of the Fresh commercial V 2 O 5 -based SCR catalyst in the monolith form

To have an idea on the NH 3 oxidation capability of the Fresh V 2 O 5 -based SCR catalyst in the monolith form, activity tests were conducted over the used catalyst under NH 3 /O 2 atmosphere.

Moreover, the effect of steam on the NH 3 oxidation was explored. The related results were presented in Fig. 5.1. As can be seen, the Fresh V 2 O 5 -based SCR catalyst can be considered as inactive regarding NH 3 oxidation when temperature is below 300°C. With increasing temperature, the conversions of NH 3 speed up for both cases while NH 3 oxidation in the absence of steam exhibits higher activity. In their study, Li et al. found that water had a dramatic negative effect on the NH 3 conversion over the V 2 O 5 /TiO 2 catalyst since water is an excellent Lewis base, typically acting as an inhibitor for a catalytic reaction over metals and oxides [START_REF] Li | Selective NH3 oxidation to N2 in a wet stream[END_REF] . In parallel, water inhibition was observed by Lietti et al. when they explored the reactivity of two commercial TiO 2 -supported V 2 O 5 -MoO 3 and V 2 O 5 -WO 3 based catalysts in the NH 3 oxidation reaction under dry and wet conditions [START_REF] Lietti | Selective catalytic reduction (SCR) of NO by NH3 over TiO2-supported V2O5-WO3 and V2O5-MoO3 catalysts[END_REF] . governing the catalyst reactivity in the high-temperature region [START_REF] Lietti | Selective catalytic reduction (SCR) of NO by NH3 over TiO2-supported V2O5-WO3 and V2O5-MoO3 catalysts[END_REF] . It is also noticeable that the feeding of steam in the first place, imposes negative influence on the reaction involving NH 3 and NO. In their review, Busca et al. stated that it's generally agreed that water hinders the SCR reaction [START_REF] Busca | Chemical and mechanistic aspects of the selective catalytic reduction of NO x by ammonia over oxide catalysts: A review[END_REF] . The inhibiting effect of water has been explained in terms of competition against NH 3 for the adsorption onto the active sites or as modification of the structure of the active sites, e.g., transformation of Lewis acid sites into Brønsted acid sites [START_REF] Busca | Chemical and mechanistic aspects of the selective catalytic reduction of NO x by ammonia over oxide catalysts: A review[END_REF][START_REF] Lietti | Selective catalytic reduction (SCR) of NO by NH3 over TiO2-supported V2O5-WO3 and V2O5-MoO3 catalysts[END_REF] .

Another point of interest is that the presence of steam brings different effects on the conversions of NO and NH 3 , particularly at relatively high temperatures (> 300°C). It is evidenced by the diversion of NH 3 conversions (Fig. 5.3b). It can be rationalized by the inhibition of water on NH 3 oxidation (Fig. 5.1) while part of loss in the SCR reactivity for NO is compensated by the promotion of water on NO oxidation (Fig. 5.2). This is also in line with reported literature [START_REF] Lietti | Selective catalytic reduction (SCR) of NO by NH3 over TiO2-supported V2O5-WO3 and V2O5-MoO3 catalysts[END_REF] . As can be seen, the activities of the catalysts drastically decrease when alkali metals are doped onto the catalysts surface. According to reported literature, the deposition of alkali metals can decrease both the number and the strength of Brønsted acid sites, resulting in a reduced ammonia storage capacity and thus lower SCR activity [214, 241 -243] . The resulted reducibility of dispersed vanadia species due to the interaction between vanadia and doped alkali metal was also thought to play some role, not a major part though [START_REF] Chen | Deactivation of the Vanadia Catalyst in the Selective Catalytic Reduction Process[END_REF] . In contrast to most studies reporting that in terms of poisoning effect, it decreases in the order K > Na on the base of the same molar concentration, the present study seems to show that Na exhibits stronger poisoning effect than K on the same mass basis [START_REF] Lisi | Single and combined deactivating effect of alkali metals and HCl on commercial SCR catalysts[END_REF][START_REF] Klimczak | High-throughput study of the effects of inorganic additives and poisons on NH3-SCR catalysts-Part I: V2O5-WO3/TiO2 catalysts[END_REF] . It may be explained by higher molar concentration of Na than K when their mass is the same. Similar to the case of Standard SCR, the activities of the catalysts pronouncedly decrease with doping of alkali metals. In terms of performance, it generally follows the order Fresh > K >

Effects of the presence of alkali compounds on the performance of a commercial V 2 O 5 -based SCR catalyst in monolith form

Concluding remarks

On a commercial SCR catalyst, studied in the monolithic form, under model exhaust gases mixtures, it was clearly showed that the introduction of dopants leads to either negative impacts on the catalytic performance of the used catalysts. In terms of poisoning effect, it seems that Na exhibits stronger inhibition than K on the same mass basis which is possibly due to higher molar concentration of Na than K resulting in drastic reduction of active sites.

Additionally, it is found that their combination usually leads to the complete loss of activities whether it's with respects to Standard SCR or Fast SCR.

CHAPTER 6: CONCLUSIONS AND PERSPECTIVES

Conclusions and perspectives

To explore what it means for the diesel exhaust after-treatment systems when bio-diesel is utilized as the alternative fuel, experimental investigations were conducted over two systems, Diesel Oxidation Catalysts and SCR catalysts.

The aging study on DOC shows that thermal aging significantly affects the overall oxidation activities of the catalysts. It turns out that mild thermal aging actually activates the catalyst.

Mild thermal aging on the catalyst improves the oxidation performance of C 3 H 6 as well as CO to a large extent while severe aging improves a little bit. NO oxidation over severely aged catalyst gets better. Over these DOC samples, fresh and aged ones, the CO oxidation takes place at lower temperature than C 3 H 6 oxidation. Meanwhile, the NO 2 is a strong oxidant which reacts with CO if present to give out NO, enhancing the CO oxidation and formation of NO at low temperature. Moreover, the NO competes for the oxidations sites against C 3 H 6 .

The results of doping of alkali metals on the same type of commercial DOC in the monolithic form illustrate that the introduction of dopants leads to either negative or positive impacts on the catalytic oxidation. The K addition leads to a promotion of oxidation reactions while the Na addition inhibits the same reactions. There is no impact of exhaust gas composition on the ranking of catalysts. However, it was shown that the presence of NO 2 promotes the oxidation reaction whereas in the presence of NO, the competition in terms of oxidation in the same catalytic sites leads to a shift of light off temperature for CO and C 3 H 6 to higher temperatures.

Also studied on a commercial Diesel Oxidation Catalyst, in the monolithic form, under representative exhaust gases mixtures, it is clearly showed that the mild thermal aging leads to positive impacts on the catalytic oxidation, so does the introduction of K. Their combination, however, results in the drastic reduction of the oxidation performance.

The results of doping of alkali metals on a home-made model DOC in the powder form demonstrate that the addition of alkali compound (K or/and Na) leads to a promotion of the C 3 H 6 oxidation reactions while incurs an inhibition on the NO conversion. From the analysis of changing the pollutants in the feed, it's concluded that C 3 H 6 and NO are mutually inhibiter to each other when it comes to their oxidation. Moreover, the influence of CO on NO conversion is negligible though the presence of CO brings negative impact to the C 3 H 6

List of Figures

  gaz d'échappement des automobiles, de nouvelles réglementations d'émissions ont été mises en place depuis les années 1990. Ces règlementations ont conduit à une évolution des carburants traditionnels vers les biocarburants et à des systèmes de posttraitement des gaz d'échappement, notamment pour les moteurs diesel, de plus en plus sophistiqués. Ils comprennent à ce jour une série d'unités catalytiques, contenant un filtre à particules, un catalyseur d'oxydation pour traiter le monoxyde de carbone et les hydrocarbures et enfin un catalyseur pour réduire les oxydes d'azote. Dans ce travail de thèse, deux de ces systèmes catalytiques industriels ont été étudiés, les catalyseurs à l'oxydation diesel (DOC) et les catalyseurs de réduction catalytique sélective (SCR). Une attention particulière est accordée à leur performance en présence de composés métaux alcalins. En effet, les alcalins sont présent dans les biocarburants et lorsque le biodiesel est utilisé comme carburant de remplacement, ils vont donc être présents dans les gaz d'échappement et à priori dans les systèmes de post-traitement. Tout d'abord, cette thèse se concentre sur le comportement catalytique du Catalyseur d'Oxydation Diesel en utilisant différentes caractéristiques de vieillissement simulant le kilométrage routier. Le catalyseur étudié est un catalyseur commercial d'oxydation diesel (Pt / Pd / Al 2 O 3 ) fourni par Continental AG. Le vieillissement hydrothermal dans des différentes conditions sur les carottes de monolithe a également été effectué. Les résultats de l'étude du vieillissement sur le DOC révèlent que le vieillissement affecte de manière significative les activités globales d'oxydation des catalyseurs. Il s'avère que le vieillissement à basse température (750°C) active le catalyseur. Le vieillissement hydrothermal léger sur le catalyseur améliore les performances d'oxydation de C 3 H 6 ainsi que le CO dans une large mesure, tandis que le vieillissement sévère contribue un peu. L'oxydation des catalyseurs sévèrement vieillis, cependant, s'améliore.

Mots clés:

  Systèmes de post-traitement des échappements; DOC; SCR; Bio-diesel; Composés alcalins; Vieillissement hydrothermal

  Fig.1.3 Share of diesel in new passenger car registrations in Western Europe, 1990-2016[START_REF]Share of diesel in new passenger cars in Western Europe[END_REF] .

Fig. 1

 1 Fig. 1.5.

Fig. 1 . 4

 14 Fig.1.4 Technologies required for compliance: Diesel[START_REF] Posada | Estimated Cost of Emission Control Technologies for Light-Duty Vehicles Part 2 -Diesel[END_REF] .

  of their environmental and health implications engine exhaust emissions have to be strictly controlled. With the passage of the Clean Air Act of 1970, engine modifications alone were not sufficient enough to get the automobile out of the local air pollution problem. That's

Fig. 2 . 1

 21 Fig. 2.1 Schematic diagram of a three-way catalyst converter (Image courtesy of ClearMechanic.com)

Fig. 2 .

 2 Fig. 2.3 NO oxidation thermodynamic equilibrium curve for a stream initially of 300 ppm NO and 10% O 2 .

Fig. 2 . 4

 24 Fig. 2.4 Mechanism of the NO-NH 3 reaction over vanadia-based catalysts proposed by Ramis et al. in the

2 . 6 .

 26 It can be regarded as the simplest possible reaction path for the SCR reaction. It consists of two cycles, a NO-activation cycle (outer) and a Fast SCR cycle (inner) that share the same reduction part but use NO + O 2 and NO 2 respectively for the reoxidation. The reaction steps in the Fast SCR cycle are all inherently part of the Standard SCR reaction since NO 2 is formed in the NOactivation cycle and is consumed in the Fast SCR cycle. In excess of NO 2 , the Fast SCR cycle can run independently, which is the Fast SCR reaction. The Standard SCR reaction is the sum of the black and the blue cycle.

Fig. 2 .

 2 Fig.2.6 A reaction mechanism for the SCR process proposed by Arnarson et al[START_REF] Arnarson | A complete reaction mechanism for standard and fast selective catalytic reduction of nitrogen oxides on low coverage VOx/TiO2(0 0 1) catalysts[END_REF] .

Fouling

  from fluid phase onto the catalytic surface and blocking of surface Poisoning Chemical Strong chemisorption of species on catalytic sites, thereby blocking sites for catalytic reaction Sintering(Aging) Thermal Thermally induced loss of catalytic surface area, support area and active phase-support reactions

Fig. 2 . 7

 27 Fig. 2.7 Schematic of deactivation by coke forming.

Fig. 2 . 8

 28 Fig. 2.8 Phase transformation and specific surface area of alumina.

Fig. 2 .

 2 Fig. 2.10 Two principal mechanisms of metal crystallite growth due to sintering: (A), atomic migration, and

Fig. 3 .Fig. 3 . 1

 331 Fig. 3.2) and analytical instrumentation (Detectors, see 6 in Fig. 3.2).

1 -

 1 Fig. 3.2 Pictures of synthetic gas bench (SGB) plant for monolithic DOCs study.

N2Fig. 3 . 3

 33 Fig. 3.3 Schematic overview of synthetic gas bench plant for monolithic SCR catalysts study.

Fig. 3 . 4

 34 Fig. 3.4 Schematic overview of synthetic gas bench (SGB) plant for powdered DOCs study.

Fig. 3 . 5

 35 Fig.3.5 Picture of synthetic gas bench (SGB) plant for powdered DOCs study[75]

Fig. 3 .

 3 Fig. 3.6 XRD patterns obtained from the PDF-2 Release 2005 database for PdO and Pt 0 .

Fig. 3 . 7

 37 Fig. 3.7 The temperature program for TPR test.

4 .

 4 On the performance of oxidation catalysts using different aging characteristics or/and in the presence of alkali compounds representative of biofuel contaminantsThis chapter is the first nucleus of this double-core thesis and it's mainly about the research conducted on the oxidation catalysts.

4. 1 . 2 . 1

 121 Fig. 4.1 presents the evolution profiles of reactants of interest over studied catalysts, fresh and two aged ones, under the simulated gas condition Mix 1, which is representative of a real exhaust mixture. Compared with the fresh one, the light-off temperature of C 3 H 6 over the aged ones shift to the lower temperatures, suggesting positive influence brought by the aging.In this regard, aging actually activates the catalyst and improves its performance when the aging condition is mild.

Fig. 4 . 1

 41 Fig. 4.1 Evolution profiles of C 3 H 6 , CO and NO x for Mix 1 versus temperature for reference and aged catalysts: (a). Conversions of C 3 H 6 , (b). Conversions of CO and (c). Transformations of NO x

Fig. 4 . 3

 43 Fig. 4.3 Evolution profiles of C 3 H 6 , CO and NO for Mix 2 versus temperature for reference and aged catalysts: (a). Conversions of C 3 H 6 , (b). Conversions of CO and (c). Conversions of NO

Fig. 4 .

 4 Fig. 4.4 depicts the evolution profiles concerning C 3 H 6 , CO and NO x of fresh catalyst under different mixtures as a function of temperature. It is noteworthy that the oxidation of C 3 H 6 can

3 H 6 forFig. 4 . 4

 3644 Fig. 4.4 Effect of gas compositions on the evolution profiles of fresh catalyst: (a). C 3 H 6 , (b). CO, and (c). NO x

Fig. 4 . 5 Fig. 4 .

 454 Fig. 4.5 Effect of gas compositions on the evolution profiles of A750 catalyst: (a). C 3 H 6 , (b). CO, and

Fig. 4 . 6

 46 Fig. 4.6 Effect of gas compositions on the evolution profiles of A950 catalyst: (a). C 3 H 6 , (b). CO, and

Fig. 4 .

 4 Fig. 4.7 depicts the evolution profiles of C 3 H 6 for different mixtures over the studied catalysts. The evolution profile of C 3 H 6 under the gas condition Mix 5 with remarkable oxidation performance is different from those obtained under other mixtures. It was shown

Fig. 4 . 7

 47 Fig. 4.7 Effect of gas compositions on the evolution profiles of C 3 H 6 : (a). Fresh catalyst, (b). K-doped catalyst, (c). Na-doped catalyst and (d). K/Na-doped catalyst

Fig. 4 . 8

 48 Fig. 4.8 Effect of gas compositions on the evolution profiles of CO: (a). Fresh catalyst, (b). K-doped catalyst, (c).

Fig. 4 . 9

 49 Fig. 4.9 Effect of gas compositions on the evolution profiles of NO x : (a). Fresh catalyst, (b). K-doped catalyst, (c). Na-doped catalyst and (d). K/Na-doped catalyst

  C 3 H 6 over doped catalysts under Mix 3 and Mix 5 (mixture with and without NO 2 , respectively) in present study actually demonstrates that the presence of NO 2 inhibits C 3 H 6 oxidation. The NO 2 inhibition of C 3 H 6 oxidation was also found by us when comparing the conversion profiles of C 3 H 6 over studied catalysts when CO was further removed from the Mixes 3 and 5 (Figure not shown). In their comparative study of the C 3 H 6 + NO + O 2 , C 3 H 6 + O 2 and NO + O 2 reactions, Yentekakis et al. found the NO inhibition of propene combustion as well as propene acting as an inhibitor to NO + O 2 → NO 2 reaction, and they attributed the cause to the resulted change

Fig. 4 . 4 . 2 . 2 . 2 Fig. 4 .

 442224 Fig. 4.10 Effect of gas compositions on the evolution profiles of Fresh catalyst: (a). C 3 H 6 , (b). CO, and

Fig. 4 .

 4 Fig.4.12 displays the evolution profiles of reactants of interest over studied catalysts under the simulated gas condition Mix 2, thus in the absence of NO 2 in the feed. The light-off temperature of C 3 H 6 over the K-doped catalyst shifting to the lower part evidences that the addition of K improves again the catalyst performance, namely K acts as a promoter compared to the fresh one. It is not hard to observe that the light-off temperature of C 3 H 6 over

Fig. 4 .

 4 Fig.4.13 presents the evolution profiles of reactants of interest over studied catalysts under the simulated gas condition Mix 3, in the absence of NO in the feed. As can be seen, the presence of K improves the oxidation performance of the catalyst while Na brings the contrary effect. Also, the combination of these two stands out as the worst. Thus the absence of both NO or NO 2 does not affect the promotion of K addition or inhibition of Na addition to DO Catalysts.

Fig. 4 .

 4 Fig. 4.14 displays the evolution profiles of reactants of interest over studied catalysts under the simulated gas condition Mix 4, in absence of CO in the feed.

Fig. 4 .

 4 Fig. 4.15 exhibits the evolution profiles of reactants of interest over conditioned catalysts under the simulated gas condition Mix 5 in absence of both NO and NO 2 in the feed. The conversion profiles of C 3 H 6 and CO in terms of performance lead to the following ranking: K>Na>K-Na. The conclusion of the effect of the alkali addition on DOC can be draw: the presence of K promotes the oxidation reaction whereas the addition of Na or K-Na inhibits the oxidation reaction.

Fig. 4 .Fig. 4 .Fig. 4 .FFig. 4 .Fig. 4 .Fig. 4 . 3 4. 3 . 1 . 2

 4444443312 Fig. 4.11 Evolution profiles of C 3 H 6 , CO and NO x for Mix 1 versus temperature for reference and conditioned catalysts: (a). Conversions of C 3 H 6 , (b). Conversions of CO and (c). Transformations of NO x

Fig. 4 .

 4 Fig. 4.17 Evolution profiles of C 3 H 6 , CO and NO x for Mix 1 versus temperature for reference and conditioned catalysts: (a). Conversions of C 3 H 6 , (b). Conversions of CO and (c). Transformations of NO x

Fig. 4 .Fig. 4 .Fig. 4 .Fig. 4 .

 4444 Fig.4.19 presents the evolution profiles of reactants of interest over studied catalysts under the simulated gas condition Mix 3, in the absence of NO in the feed. As can be seen, the presence of K or mild aging improves the oxidation performance of the catalyst while their combination brings the contrary effect. Thus the absence of either NO or NO 2 does not affect the promotion by mild aging or the addition of K on both the C 3 H 6 and CO oxidation activities. The worst scenario incurred by the manipulation of combining the aging and doping is not affected, either.

Fig. 4 .Fig. 4 .Fig. 4 .

 444 Fig. 4.20 Evolution profiles of C 3 H 6 and NO x for Mix 4 versus temperature for reference and conditioned catalysts: (a). Conversions of C 3 H 6 and (b). Transformations of NO x

  Fig. 4.22 Effect of gas compositions on the evolution profiles of C 3 H 6 : (a). Fresh catalyst, (b). A750 catalyst, (c). K catalyst and (d). K-A750 catalyst

Fig. 4 .Fig. 4 .

 44 Fig. 4.23 depicts the evolution profiles of CO as a function of temperature under different mixtures over the studied catalysts.

Fig. 4 .

 4 Fig. 4.24 Effect of gas compositions on the evolution profiles of NO x : (a). Fresh catalyst, (b). A750 catalyst, (c).

Fig. 4 .

 4 Fig. 4.25 Evolution profiles of C 3 H 6 , CO and NO for Mix 1 versus temperature for reference and conditioned catalysts: (a). Conversions of C 3 H 6 , (b). Conversions of CO and (c). Conversions of NO

Fig. 4 .

 4 Fig. 4.26 displays the evolution profiles of reactants of interest over studied catalysts under the simulated gas condition Mix 2, thus in the absence of C 3 H 6 in the feed.

Fig. 4 .Fig. 4 .Fig. 4 .Fig. 4 .

 4444 Fig.4.27 presents the evolution profiles of reactants of interest over studied catalysts under the simulated gas condition Mix 3, in the absence of NO in the feed. As can be seen, the presence of alkali compound (Na or K) improves the oxidation performance of the catalyst compared to the Fresh catalyst. In terms of enhancement degree, the presence of Na engenders stronger impact than that of K.

Fig. 4 .

 4 Fig. 4.28 Evolution profiles of C 3 H 6 and NO for Mix 4 versus temperature for reference and conditioned catalysts: (a). Conversions of C 3 H 6 and (b). Conversions of NO

Fig. 4 .Fig. 4 .Fig. 4 .Fig. 4 .

 4444 Fig. 4.29 Effect of gas compositions on the evolution profiles of C 3 H 6 : (a). Fresh, (b). K and (c). Na

Fig. 4 .

 4 Fig.4.32 shows the activity profiles of C 3 H 6 , CO and NO as a function of temperature under Mix 1 over Fresh monolith and powder catalysts. Generally speaking, powder catalyst exhibits better performance than that of monolith DOC.

Fig. 4 .

 4 Fig. 4.33 displays the activity profiles of C 3 H 6 , CO and NO as a function of temperature under Mix 1 over K-doped monolith and powder catalysts. Since the CO oxidation can be considered as almost completed, attention should be focused on the conversions of C 3 H 6 andNO. In terms of C 3 H 6 oxidation, K-doped monolith takes the lead. Even at low temperature, it exhibits better performance than that of powder DOC. As temperature rises, it's overtaken by powder DOC as far as the NO conversion is concerned.

Fig. 4 .Fig. 4 .Fig. 4 .Fig. 4 .Fig. 4 .Fig. 4 .Fig. 4 .Fig. 4 .Fig. 4 .Fig. 4 .Fig. 4 .

 44444444444 Fig.4.34 presents the activity profiles of C 3 H 6 , CO and NO as a function of temperature under Mix 1 over Na-doped monolith and powder catalysts. Generally speaking, powder catalyst exhibits better performance than that of monolith DOC when taking the oxidations of C 3 H 6 and CO as the reference. In terms of NO conversion, however, K-doped monolith takes the lead.

Fig. 4 .Fig. 4 .Fig. 4 .

 444 Fig. 4.38 Conversion profiles of CO as a function of temperature under Mix 1 over Fresh and doped monolith and powder catalysts: (a). Fresh, (b). K, (c). Na, and (d) K-Na

Fig. 5 . 1 Fig. 5 . 2

 5152 Fig. 5.1 Conversion profiles of NH 3 oxidation over Fresh SCR monolith catalyst without/with H 2 O

Fig. 5 . 3

 53 Fig. 5.3 Conversion profiles of reactants of interest over Fresh catalyst without/with H 2 O:

Fig. 5 .

 5 Fig.5.4 displays the conversion profiles of NO over different SCR catalysts, Fresh and doped ones, under model gas condition Mix 3. In general, it seems that all studied SCR catalyst can be considered as ineffective concerning NO transformation in the whole temperature range if the intrinsic error of the system is taken into account. Still, it's worth pointing out that the NO conversion over the Na-doped SCR catalyst is enhanced. It can be rationalized by the strengthening of metal-NO bond due to the addition of Na over the catalyst surface.

Fig. 5 .Fig. 5 . 4 Fig. 5 . 5 4 Fig. 5 .

 5545545 Fig. 5.5 presents the conversion profiles of NO over different SCR catalysts under simulating gas condition Mix 4. As a whole, there exists no major difference with increasing temperature or among varied catalysts since the NO conversions can be generally regarded as negligible.

Fig. 5 . 6

 56 Fig. 5.6 Conversion profiles of reactants of interest over different SCR catalysts under Mix 5: a), Conversions of NO; and b), Conversions of NH 3 .

Fig. 5 .Fig. 5 . 7

 557 Fig.5.7 depicts the conversion profiles of NO and NH 3 over different SCR catalysts under simulating gas condition Mix 6, corresponding to Standard SCR with feeding steam in the first place. Obviously, the activities of the catalysts drastically decrease when alkali metals are doped onto the catalysts surface. A point of interest is that in presence of steam, the activity of the used catalysts is completely lost due to the introduction of Na either in single form or in conjunction with K.

Fig. 5 .

 5 Fig. 5.8 depicts the conversion profiles of NO, NO 2 and NH 3 over different SCR catalysts under simulating gas condition Mix 8, corresponding to Fast SCR under wet conditions.

Fig. 1 . 1 51 Fig. 3 . 5

 115135 Fig. 1.1 World delivered energy consumption by end-use sector (quadrillion Btu), 2012-2040. .................................................................................................................................................... Fig. 1.2 Powertrain map of major markets. ................................................................................ Fig. 1.3 Share of diesel in new passenger car registrations in Western Europe, 1990-2016. .... Fig. 1.4 Technologies required for compliance: Diesel. ............................................................ Fig. 1.5 Combination of catalysts to control diesel exhaust emissions. ..................................... Fig. 2.1 Schematic diagram of a three-way catalyst converter (Image courtesy of ClearMechanic.com) ................................................................................................................ Fig. 2.2 Schematic diagram of a urea-SCR system. ................................................................. Fig. 2.3 NO oxidation thermodynamic equilibrium curve for a stream initially of 300 ppm NO and 10% O 2 . .............................................................................................................................. Fig. 2.4 Mechanism of the NO-NH 3 reaction over vanadia-based catalysts proposed by Ramis et al. in the presence of oxygen. ............................................................................................... Fig. 2.5 Scheme illustrating the catalytic cycle of the SCR reaction over vanadia/titania catalyst in the presence of oxygen proposed by Topsoe et al. ................................................. Fig. 2.6 A reaction mechanism for the SCR process proposed by Arnarson et al. .................. Fig. 2.7 Schematic of deactivation by coke forming. ............................................................... Fig. 2.8 Phase transformation and specific surface area of alumina. ......................................... Fig. 2.9 Schematic of surface dehydroxylation from contact area of two adjacent particles. ... Fig. 2.10 Two principal mechanisms of metal crystallite growth due to sintering: (A), atomic migration, and (B), crystallite migration. ................................................................................. Fig. 2.11 Biodiesel processing flow diagram. .......................................................................... Fig. 2.12 Average emission impacts of biodiesel content as estimated from published engine dynamometer data in the EPA study. ....................................................................................... Fig. 3.1 Schematic overview of synthetic gas bench (SGB) plant for monolithic DOCs study. .................................................................................................................................................. Fig. 3.2 Pictures of synthetic gas bench (SGB) plant for monolithic DOCs study. ................. Fig. 3.3 Schematic overview of synthetic gas bench plant for monolithic SCR catalysts study. .................................................................................................................................................. Fig. 3.4 Schematic overview of synthetic gas bench (SGB) plant for powdered DOCs study.51 Fig. 3.5 Picture of synthetic gas bench (SGB) plant for powdered DOCs study. .................... Fig. 3.6 XRD patterns obtained from the PDF-2 Release 2005 database for PdO and Pt 0 . .... Fig. 3.7 The temperature program for TPR test. ...................................................................... Fig. 4.1 Evolution profiles of C 3 H 6 , CO and NO x for Mix 1 versus temperature for reference and aged catalysts: (a). Conversions of C 3 H 6 , (b). Conversions of CO and (c). Transformations of NO x ........................................................................................................... Fig. 4.2 TEM analysis of aged catalysts ...................................................................................

Fig. 4 . 3

 43 Fig. 4.3 Evolution profiles of C 3 H 6 , CO and NO for Mix 2 versus temperature for reference and aged catalysts: (a). Conversions of C 3 H 6 , (b). Conversions of CO and (c). Conversions of NO ............................................................................................................................................ Fig. 4.4 Effect of gas compositions on the evolution profiles of fresh catalyst: (a). C 3 H 6 , (b). CO, and (c). NO x ...................................................................................................................... Fig. 4.5 Effect of gas compositions on the evolution profiles of A750 catalyst: (a). C 3 H 6 , (b). CO, and (c). NO x ...................................................................................................................... Fig. 4.6 Effect of gas compositions on the evolution profiles of A950 catalyst: (a). C 3 H 6 , (b). CO, and (c). NO x ...................................................................................................................... Fig. 4.7 Effect of gas compositions on the evolution profiles of C 3 H 6 : (a). Fresh catalyst, (b). K-doped catalyst, (c). Na-doped catalyst and (d). K/Na-doped catalyst .................................. Fig. 4.8 Effect of gas compositions on the evolution profiles of CO: (a). Fresh catalyst, (b). Kdoped catalyst, (c). Na-doped catalyst and (d). K/Na-doped catalyst ...................................... Fig. 4.9 Effect of gas compositions on the evolution profiles of NO x : (a). Fresh catalyst, (b). K-doped catalyst, (c). Na-doped catalyst and (d). K/Na-doped catalyst .................................. Fig. 4.10 Effect of gas compositions on the evolution profiles of Fresh catalyst: (a). C 3 H 6 , (b). CO, and ..................................................................................................................................... Fig. 4.11 Evolution profiles of C 3 H 6 , CO and NO x for Mix 1 versus temperature for reference and conditioned catalysts: (a). Conversions of C 3 H 6 , (b). Conversions of CO and (c). Transformations of NO x ........................................................................................................... Fig. 4.12 Evolution profiles of C 3 H 6 , CO and NO for Mix 2 versus temperature for reference and conditioned catalysts: (a). Conversions of C 3 H 6 , (b). Conversions of CO and (c). Conversions of NO ................................................................................................................... Fig. 4.13 Evolution profiles of C 3 H 6 , CO and NO 2 for Mix 3 versus temperature for reference and conditioned catalysts: (a). Conversions of C 3 H 6 , (b). Conversions of CO and (c). Conversions of NO 2 ................................................................................................................. Fig. 4.14 Evolution profiles of C 3 H 6 and NO x for Mix 4 versus temperature for reference and conditioned catalysts: (a). Conversions of C 3 H 6 , and (b). Transformations of NO x ................ Fig. 4.15 Evolution profiles of C 3 H 6 and CO for Mix 5 versus temperature for reference and conditioned catalysts: (a). Conversions of C 3 H 6 , and (b). Conversions of CO ........................ Fig. 4.16 Effect of gas compositions on the evolution profiles of Fresh catalyst: (a). C 3 H 6 , (b). CO, and ..................................................................................................................................... Fig. 4.17 Evolution profiles of C 3 H 6 , CO and NO x for Mix 1 versus temperature for reference and conditioned catalysts: (a). Conversions of C 3 H 6 , (b). Conversions of CO and (c). Transformations of NO x ........................................................................................................... Fig. 4.18 Evolution profiles of C 3 H 6 , CO and NO for Mix 2 versus temperature for reference and conditioned catalysts: (a). Conversions of C 3 H 6 , (b). Conversions of CO and (c). Transformations of NO ............................................................................................................

Fig. 4 .

 4 Fig. 4.19 Evolution profiles of C 3 H 6 , CO and NO 2 for Mix 3 versus temperature for reference and conditioned catalysts: (a). Conversions of C 3 H 6 , (b). Conversions of CO and (c). Conversions of NO 2 ................................................................................................................. Fig. 4.20 Evolution profiles of C 3 H 6 and NO x for Mix 4 versus temperature for reference and conditioned catalysts: (a). Conversions of C 3 H 6 and (b). Transformations of NO x ................. Fig. 4.21 Evolution profiles of C 3 H 6 and CO for Mix 5 versus temperature for reference and conditioned catalysts: (a). Conversions of C 3 H 6 , and (b). Conversions of CO ........................ Fig. 4.22 Effect of gas compositions on the evolution profiles of C 3 H 6 : (a). Fresh catalyst, (b). A750 catalyst, ........................................................................................................................... Fig. 4.23 Effect of gas compositions on the evolution profiles of CO: (a). Fresh catalyst, (b). A750 catalyst, (c). K catalyst and (d). K-A750 catalyst ........................................................ Fig. 4.24 Effect of gas compositions on the evolution profiles of NO x : (a). Fresh catalyst, (b). A750 catalyst, (c). K catalyst and (d). K-A750 catalyst ........................................................ Fig. 4.25 Evolution profiles of C 3 H 6 , CO and NO for Mix 1 versus temperature for reference and conditioned catalysts: (a). Conversions of C 3 H 6 , (b). Conversions of CO and (c). Conversions of NO ................................................................................................................. Fig. 4.26 Evolution profiles of CO and NO for Mix 2 versus temperature for reference and conditioned catalysts: (a). Conversions of CO and (b). Conversions of NO ......................... Fig. 4.27 Evolution profiles of C 3 H 6 and CO for Mix 3 versus temperature for reference and conditioned catalysts: (a). Conversions of C 3 H 6 and (b). Conversions of CO ....................... Fig. 4.28 Evolution profiles of C 3 H 6 and NO for Mix 4 versus temperature for reference and conditioned catalysts: (a). Conversions of C 3 H 6 and (b). Conversions of NO ....................... Fig. 4.29 Effect of gas compositions on the evolution profiles of C3H6: (a). Fresh, (b). K and (c). Na ..................................................................................................................................... Fig. 4.30 Effect of gas compositions on the evolution profiles of CO: (a). Fresh, (b). K and (c). Na ..................................................................................................................................... Fig. 4.31 Effect of gas compositions on the evolution profiles of NO: (a). Fresh, (b). K and (c). Na ..................................................................................................................................... Fig. 4.32 The activity profiles of C 3 H 6 , CO and NO as a function of temperature under Mix 1 over Fresh monolith and powder catalysts: (a). Conversions of C 3 H 6 , (b). Conversions of CO and (c). Conversions of NO .................................................................................................... Fig. 4.33 The activity profiles of C 3 H 6 , CO and NO as a function of temperature under Mix 1 over K-doped monolith and powder catalysts: (a). Conversions of C 3 H 6 , (b). Conversions of CO and (c). Conversions of NO ............................................................................................. Fig. 4.34 The activity profiles of C 3 H 6 , CO and NO as a function of temperature under Mix 1 over Na-doped monolith and powder catalysts: (a). Conversions of C 3 H 6 , (b). Conversions of CO and .................................................................................................................................... Fig. 4.35 The activity profiles of C 3 H 6 , CO and NO as a function of temperature under Mix 1 over K-Na monolith and powder catalysts: (a). Conversions of C 3 H 6 , (b). Conversions of CO and (c). Conversions of NO ....................................................................................................

Fig. 4 .

 4 Fig. 4.36 The activity profiles of C 3 H 6 , CO and NO as a function of temperature under Mix 1 over K-A750 monolith and powder catalysts: (a). Conversions of C 3 H 6 , (b). Conversions of CO and (c). Conversions of NO ............................................................................................. Fig. 4.37 Conversion profiles of C 3 H 6 as a function of temperature under Mix 1 over Fresh and doped monolith and powder catalysts: (a). Fresh, (b). K, (c). Na, and (d) K-Na ............ Fig. 4.38 Conversion profiles of CO as a function of temperature under Mix 1 over Fresh and doped monolith and powder catalysts: (a). Fresh, (b). K, (c). Na, and (d) K-Na ................... Fig. 4.39 Conversion profiles of NO as a function of temperature under Mix 1 over Fresh and doped monolith and powder catalysts: (a). Fresh, (b). K, (c). Na, and (d) K-Na ................... Fig. 5.1 Conversion profiles of NH 3 oxidation over Fresh SCR monolith catalyst without/with H 2 O ......................................................................................................................................... Fig. 5.2 Conversion profiles of NO oxidation over Fresh SCR monolith catalyst without/with H 2 O ......................................................................................................................................... Fig. 5.3 Conversion profiles of reactants of interest over Fresh catalyst without/with H 2 O: Fig. 5.4 Conversion profiles of NO over different SCR catalysts under Mix 3 ..................... Fig. 5.5 Conversion profiles of NO over different SCR catalysts under Mix 4 ..................... Fig. 5.6 Conversion profiles of reactants of interest over different SCR catalysts under Mix 5: a), Conversions of NO; and b), Conversions of NH 3 . ............................................................ Fig. 5.7 Conversion profiles of reactants of interest over different SCR catalysts under Mix 6: a), Conversions of NO; and b), Conversions of NH 3 . ............................................................ Fig. 5.8 Conversion profiles of reactants of interest over different SCR catalysts under Mix 8: a), Conversions of NO; b), Conversions of NO 2 ; and c), Conversions of NH 3 . .................... Fig. A.1 Results of Mix 6_TPD tests for home-made DOC: a), Concentrations of NO; b), Concentrations of NO 2 ; and c), Concentrations of NO x . ........................................................

Table 1 .

 1 1 EU emissions standards for diesel passenger cars, g/km

	Tier	Date	CO	NO x	HC+NO x	PM	PN (#/km)
	Euro 1	1992.07	2.72	-	0.97	0.14	-
	Euro 2	1996.01	1.0	-	0.7	0.08	-
	Euro 3	2000.01	0.64	0.50	0.56	0.05	-
	Euro 4	2005.01	0.50	0.25	0.30	0.025	-
	Euro 5a	2009.09	0.50	0.180	0.230	0.005	-
	Euro 5b	2011.09	0.50	0.180	0.230	0.005	6×10 11
	Euro 6	2014.09	0.50	0.080	0.170	0.005	6×10 11

Table 2 .

 2 2 Average emissions change for B20 from published engine dynamometer data in the EPA review

Table 3 .

 3 1 Description of the gases used for monolithic DOCs study

	Gas Cylinder	Composition of the gas cylinder (vol.%)
	N 2	100% N 2
	O 2	100% O 2
	CO 2	100% CO 2
	CO	0.5% CO + 99.5% N 2
	NO	0.8% NO + 99.2% N 2
	NO 2	1.5% NO 2 + 98.5 % N 2
	C 3 H 6	5.0% C 3 H 6 + 95.0% N 2
	The desired concentrations of the reactant gases were obtained by blending C 3 H 6 (as a
	representative hydrocarbon), NO

2 , NO, CO, CO 2 , O 2 and N 2 . The synthetic gas mixtures were controlled by mass flowmeters (Brooks). Water vapor (up to 5 vol.%) was added to the mixture when it passed through a water saturator.

Table 3 .

 3 2 Detectors of synthetic gas bench (SGB) plant for monolithic DOCs study

	Detector	Component
	Flame ionisation detector (Environnement SA GRAPHITE 52M)	HCs
	Chemiluminescence analyzer (Environnement SA TOPAZE 32M)	NO/NO 2
	Infrared analyzer (Environnement SA MIR 2M)	CO/CO 2 / O 2 / N 2 O

Table 3 .

 3 3 Description of the gases used for monolithic SCR catalysts study The desired concentrations of the reactant gases were obtained by blending NH 3 , NO 2 , NO, O 2 and N 2 . The synthetic gas mixtures containing N 2 and O 2 , controlled by mass flowmeters (Brooks), were passed through a water saturator (to provide 5 vol.% H 2 O).

	Gas Cylinder	Composition of the gas cylinder (vol.%)
	N 2	100% N 2
	O 2	100% O 2
	NO	0.8% NO + 99.2% N 2
	NO 2	1.5% NO 2 + 98.5 % N 2
	NH 3	1.0% NH 3 + 99.0% N 2

Table 3 .

 3 4 Detectors of synthetic gas bench (SGB) plant for monolithic SCR catalysts study

	Detector	Component
	Fourier Transform UV analyzer (Environnement SA UV-TF)	NH 3
	Chemiluminescence analyzer (Environnement SA TOPAZE 32M)	NO/NO 2
	Infrared analyzer (Environnement SA MIR 2M)	CO/CO 2 / O 2 / N 2 O

Table 3 .

 3 5 Description of the gases used for powdered DOCs study

	Gas Cylinder	Composition of the gas cylinder (vol.%)
	Ar	100% Ar
	O 2	100% O 2
	CO	0.972% CO + 99.028% Ar
	NO	0.977% NO + 99.023% Ar
	NO 2	0.9% NO 2 + 99.1 % Ar
	C 3 H 6	0.5% C 3 H 6 + 99.5% Ar
	The desired concentrations of the reactant gases were obtained by blending C 3 H 6 (as a
	representative hydrocarbon), O

2 , CO, NO and Ar. The streams were controlled by mass flowmeters (Brooks Series 5850E). P art o f Ar will be fed to the wat er saturator inside the thermostatic bath 1 (to provide 3 vol.% H 2 O in the system).

Table 3 .

 3 6 Detectors of synthetic gas bench (SGB) plant for powdered DOCs study

	Detector	Component
	Flame ionisation detector (Siemens FIDAMAT 5E)	HCs
	Infrared analyzer (Siemens Ultramat 9)	CO/CO 2
	Infrared analyzer (Siemens Ultramat 9)	N 2 O
	Chemiluminescence analyzer (Eco Physics CLD 700 AL)	NO/NO 2

Table 4 .

 4 1 Different aging methods dealing with commercial monolithic DOC Characterization of catalysts XRD data were recorded on a Bruker Advanced D8 using CuKα radiation. PXRD measurements were performed from 5 to 90° in an interval of 1° with a count time of 6s for each point. The specific surface area was obtained by using the Brunauer-Emmett-Teller (BET) method. N 2 adsorption-desorption isotherms were measured at liquid nitrogen temperature by a Micromeritics ASAP 2010. Before the measurements, the samples were Catalytic activity studies A synthetic gas bench installation was used in order to investigate the catalytic activities of these catalysts. The carrot was put inside a stainless steel reactor, which itself was placed inside an oven reaching temperatures up to over 500ºC. Two thermocouples were available to follow the reaction temperature at the inlet and the outlet of the catalyst.A typical reactant gas mixture representative of the exhaust gases from diesel engines consists of hydrocarbon (C 3 H 6 in this study), CO, NO, NO 2 , O 2 , CO 2 , H 2 O and N 2 , as Mix 1 in Table4.2. The total gas flow rate was 11.8 L•min -1 resulting in a gas hourly space velocity (GHSV) of 55,000 h -1 . Gas mixtures without NO 2 , NO, CO or NO x were setup to investigate the influence of each gas component on the catalytic process. Table4.2 summarized the conditions of the different experiments in terms of gas composition.Catalysts were evaluated in terms of oxidation capacity and NO conversion for temperatures ranging from 50°C to 500°C. Exhaust gas compositions before and after the reactor were characterized by an Environnement S.A. analyzer composed of a set of modules. Among them, the TOPAZE 32M module allowed NO, NO 2 and total NO x measurement thanks to a chemiluminescence analyzer. A MIR2M module was composed of an IR spectrogram, for the measurement of H 2 O, CO and CO 2 molar fractions, and a magnetic sensor, for the O 2 molar fraction measurement. At last, a flame ionization detector (FID), present in the GRAPHITE 52M module was used for measuring total HC.

	Entry	Temp. (°C)	Aging conditions Duration (h)	Water (vol.%)
	F	-	-	-
	A750	750	4	10
	A850_10H	850	10	10
	A850_24H	850	24	10
	A950	950	4	10
	4.1.1.2

Table 4 .

 4

				2 Synthetic gas bench experimental conditions
		C 3 H 6	NO 2	NO	CO	CO 2	O 2	H 2 O	N 2
		ppm	ppm	ppm	ppm	(%)	(%)	(%)	(%)
	Mix 1	200	100	300	300	5	10	5	Balance
	Mix 2	200	-	300	300	5	10	5	Balance
	Mix 3	200	100	-	300	5	10	5	Balance
	Mix 4	200	100	300	-	5	10	5	Balance
	Mix 5	200	-	-	300	5	10	5	Balance

Table 4

 4

		.3 Different doping methods dealing with DOC
	Entry	Catalyst condition
	F	Fresh
	K	1% KNO 3
	Na	1% NaNO 3
	K-Na	1% K+Na NO 3

  Exhaust gas compositions before and after the reactor were characterized by an Environnement S.A. analyzer composed of a set of modules. Among them, the TOPAZE 32M module allowed NO, NO 2 and total NO x measurement thanks to a chemiluminescence analyzer. A MIR2M module was composed of an IR spectrogram, for the measurement of H 2 O, CO and CO 2 molar fractions, and a magnetic sensor, for the O 2 molar fraction measurement. At last, a flame ionization detector (FID), present in the GRAPHITE 52M module was used for measuring total HC.

	2 , O 2 , CO 2 , H 2 O and N 2 , as
	Mix 1 in Table 4.4. For every test (Mix 1-Mix 5), the total gas flow rate was 11.8 L•min -1
	referenced to 293K and 1 atm, resulting in a gas hourly space velocity (GHSV) of 55,000 h -1 .
	Gas mixtures without NO 2 , NO, CO or NO x were setup to explore the influence of each gas
	component on the catalytic process. NO 2 is removed from the feed to form Mix 2. In Mix 3,
	NO is removed while CO is removed regarding Mix 4. Finally both NO and NO 2 were
	removed from the feed, obtaining Mix 5. Table 4.4 summarized the conditions of the different
	experiments in terms of gas composition.

Catalysts were evaluated in terms of oxidation capacity and NO conversion for temperatures ranging from 50°C to 500°C.

Table 4

 4

		.4 Synthetic gas bench experimental conditions for alkali-as-dopant DOC study
		C 3 H 6	NO 2	NO	CO	CO 2	O 2	H 2 O	N 2
		ppm	ppm	ppm	ppm	(%)	(%)	(%)	(%)
	Mix 1	200	100	300	300	5	10	5	Balance
	Mix 2	200	-	300	300	5	10	5	Balance
	Mix 3	200	100	-	300	5	10	5	Balance
	Mix 4	200	100	300	-	5	10	5	Balance
	Mix 5	200	-	-	300	5	10	5	Balance

Table 4

 4

		.6 Synthetic gas bench experimental conditions for DOC dual deactivation study
		C 3 H 6	NO 2	NO	CO	CO 2	O 2	H 2 O	N 2
		ppm	ppm	ppm	ppm	(%)	(%)	(%)	(%)
	Mix 1	200	100	300	300	5	10	5	Balance
	Mix 2	200	-	300	300	5	10	5	Balance
	Mix 3	200	100	-	300	5	10	5	Balance
	Mix 4	200	100	300	-	5	10	5	Balance
	Mix 5	200	-	-	300	5	10	5	Balance

Table 4 .

 4

		7 Different methods dealing with powder DOC
	Entry	Catalyst condition
	F	Fresh
	K	1% KNO 3
	Na	1% NaNO 3
	K-Na	1% K+Na NO 3
	K-A750	750_4H + KNO 3

Table 4

 4

	.8) representative of the exhaust gases from

Table 4 .

 4

		8 Synthetic gas bench experimental conditions for powder DOCs
		C 3 H 6	NO	CO	O 2	H 2 O	Ar
		ppm	ppm	ppm	(%)	(%)	(%)
	Mix 1	200	300	300	8	3	Balance
	Mix 2	-	300	300	8	3	Balance
	Mix 3	200	-	300	8	3	Balance
	Mix 4	200	300	-	8	3	Balance
	Mix 5	200	-	-	8	3	Balance
	Mix 6	-	300	-	8	3	Balance
	Mix 7	-	-	300	8	3	Balance
	Mix 6_TPD	-	300	-	8	3	Balance

  The comparison of C 3 H 6 conversion profiles between Mix 1 and Mix 4 (mixture with and without CO, respectively) evidences that the absence of CO is beneficial to the C 3 H 6 oxidation. Additionally, the absence of NO has the same effect in some way from the comparison of C 3 H 6 conversion profiles between Mix 1 and Mix 3 (mixture with and without NO, respectively). It was shown that the absence of other gas leads to an increase of C 3 H 6

	.8). C 3 H 6
	is removed from the feed to form Mix 2. In Mix 3, NO is removed while CO is removed
	regarding Mix 4.
	Fig. 4.29 depicts the evolution profiles of C 3 H 6 for different mixtures over the studied
	catalysts.

Table 4 .

 4 9 Experimental setup of monolith catalysts for the comparison study

	Entry	C 3 H 6 ppm	NO 2 ppm ppm NO	CO ppm	CO 2 (%)	O 2 (%)	H 2 O (%)	N 2 (%)
	Fresh monolith Mix 1	200	-	300	300	5	10	5	Balance
	KNO 3 monolith Mix 1	200	-	300	300	5	10	5	Balance
	NaNO 3 monolith Mix 1	200	-	300	300	5	10	5	Balance
	K+Na NO 3 monolith Mix 1	200	-	300	300	5	10	5	Balance
	750+KNO 3 monolith Mix 1	200	-	300	300	5	10	5	Balance
	Table 4.10 Experimental setup of powder catalysts for the comparison study
		C 3 H 6	NO 2	NO	CO	CO 2	O 2	H 2 O	Ar
	Entry	ppm	ppm	ppm	ppm	(%)	(%)	(%)	(%)
	Fresh powder Mix 1	200	-	300	300	-	8	3	Balance
	KNO 3 powder Mix 1	200	-	300	300	-	8	3	Balance
	NaNO 3 powder Mix 1	200	-	300	300	-	8	3	Balance
	K+Na NO 3 powder Mix 1	200	-	300	300	-	8	3	Balance
	750+KNO 3 powder Mix 1	200	-	300	300	-	8	3	Balance

Table 5 .

 5 1 Different methods dealing with monolithic SCR catalysts

	Entry	Catalyst condition
	F	Fresh
	K	1% KNO 3
	Na	1% NaNO 3
	K-Na	1% K+Na NO 3

Table 5 .

 5 3 , NO, NO 2 , O 2 , H 2 O and N 2 , as Mix 8 in Table5.2. The total gas flow rate was 7.51 L•min -1 resulting in a gas hourly space velocity (GHSV) of 35,000 h -1 . Gas mixtures without NO, NO 2 , NO x or NH 3 were setup to investigate the influence of each gas component on the catalytic process. The effects of steam on the NH 3 oxidation capability, NO oxidation capability and standard SCR reaction of studied catalysts were also investigated. 2 summarized the conditions of the different experiments in terms of gas composition.

Table 5 .

 5 2 Synthetic gas bench experimental conditions for monolithic SCR catalysts

	Entry	NH 3 ppm	NO ppm	NO 2 ppm	O 2 (%)	H 2 O (%)	N 2 (%)
	Mix 1	400	-	-	8	-	Balance
	Mix 2	400	-	-	8	5	Balance
	Mix 3	-	400	-	8	-	Balance
	Mix 4	-	400	-	8	5	Balance
	Mix 5	400	400	-	8	-	Balance
	Mix 6	400	400	-	8	5	Balance
	Mix 7	400	-	400	8	5	Balance
	Mix 8	400	200	200	8	5	Balance
	Mix 3-TPD	-	400	-	8	-	Balance

Table A

 A

	Table A.1 Continued
	TPD	Temperature Programmed Desorption
	RDS	Rate-Determining Step
	FT-IR	.1 List of abbreviations used in this thesis Fourier Transform Infrared
	DFT	Density Functional Theory
	UPMC ASTM	Université Pierre et Marie Curie American Society for Testing and Materials
	FRT EPMA	Fluides Réactifs et Turbulence Electron Probe Microanalysis
	CSC ULSD	China Scholarship Council Ultra-Low Sulfur Diesel
	DOC SGB	Diesel Oxidation Catalysts Synthetic Gas Bench
	SCR ID	Selective Catalytic Reduction Inner Diameter
	DPF FID	Diesel Particulate Filter Flame Ionization Detector
	GDP XRD	Gross Domestic Product X-Ray Diffraction
	VOC TEM	Volatile Organic Compound Transmission Electron Microscopy
	TWC BET	Three-way Catalyst Brunauer-Emmett-Teller
	HC ICDD	Hydrocarbon International Center for Diffraction Data
	A/F HRTEM	Air-to-Fuel High Resolution TEM
	PM EDS	Particulate Matter Energy-dispersive X-ray Spectroscopy
	LNT BJH	Lean NO x Trap Barrett-Joyner-Halenda
	SOF TCD	Soluble Organic Fraction Thermal Conductivity Detector
	EPA TPR	Environmental Protection Agency Temperature Programmed Reduction
	UWS pg	Urea-Water-Solutions Page
	DEF CPSI	Diesel Exhaust Fluid Channels Per Square Inches
	AUS PXRD	Aqueous Urea Solution Powder XRD
	UHV GHSV	Ultra-High Vacuum Gas Hourly Space Velocity
	AES DPNR	Auger Electron Spectroscopy Diesel Particulate NO x Reduction
	LEED wt.	Low-Energy Electron Diffraction Weight
	L.H. SEM	Langmuir-Hinshelwood Scanning Electron Microscopy
	E.R. EHMO	Eley-Rideal Extended Hückel Molecular Orbital
	PM-IRAS MSW	Polarization-Modulated Infrared Absorbance Spectroscopy Municipal Solid Waste
	PGM DOE	Platinum Group Metal Design of Experiments

Acknowledgments

Acknowledgments

Institutional acknowledgements belong to the two providers for catalysts, Continental AG for the commercial diesel oxidation catalysts, Umicore for the commercial

In this thesis, high resolution TEM (HRTEM) coupled with EDS was performed on a JEOL JEM-2011 transmission electron microscope (LaB6 cannon) with a spatial resolution of 0.16 nm operating at 200 kV. Prior to TEM, the sample was crushed and then dispersed with solvent ethanol on a carbon-coated copper TEM grid. Ethanol is added to the grid for the purpose of dispersing the crushed sample and improving its contact with the grid. Energydispersive X-ray spectroscopy (EDS) was performed with the probe PGT-Bruker.

Nitrogen adsorption measurements at 77 K

Surface area and porosity are two important textural characteristics of solid materials used for catalyst. They highly determine the properties and performance of the catalyst. Differences in the surface area and porosity of particles within a solid material can greatly influence its reactivity and thus can be useful in building the structure-reactivity relationship.

In general, the specific surface area and porosity of each catalyst can be determined by physical adsorption of nitrogen on the surface of the solid to obtain adsorption and desorption isotherms. The determination is usually carried out at the temperature of liquid nitrogen.

In this thesis, the adsorption and desorption isotherms of nitrogen were measured using a Micromeritics ASAP 2010 apparatus. It performs surface area analysis besides pore size and pore volume distributions. The calculations were carried out applying the BET (Brunauer-Emett-Teller) method, to acquire the surface area, the average and total pore volume, the BJH (Barrett-Joyner-Halenda) pore size distribution and to conduct micro-pore analysis.

Before the measurements, the samples were evacuated at 200ºC for 2 hours under vacuum (0.2666Pa) to remove gases and vapors that might have been physically adsorbed. This step it's fundamental, because if outgassing is not achieved, the specific surface area may be less or may be variable, since part of the surface area was already covered with molecules that were previously adsorbed.

Concluding remarks

On a commercial Diesel Oxidation Catalyst, studied in the monolithic form, under representative exhaust gases mixtures, it was clearly showed that the introduction of dopants leads to either negative or positive impacts on the catalytic oxidation. The K addition leads to a promotion of oxidation reactions while the Na addition inhibits the same reactions. There is no impact of exhaust gas composition on the ranking of catalysts. However, it was shown that the presence of NO 2 promotes the oxidation reaction whereas in the presence of NO, the competition in terms of oxidation in the same catalytic sites leads to a shift of light off temperature for CO and C 3 H 6 to higher temperatures.

In the next section, the influence of the dual aging (hydrothermal and chemical) on the performance of a commercial Pt-Pd/Al 2 O 3 diesel oxidation catalysts will be studied.

Influence of the dual aging (hydrothermal and chemical) on the performance of a commercial Pt-Pd/Al 2 O 3 diesel oxidation catalysts

Experimental

Catalysts samples

The catalyst used for this study was taken from the same untreated brick of the commercial DOC catalyst provided by Continental. Again, four cylindrical "carrots" (d=2.54 cm; L=2.54 cm) were cut out of it.

Two carrots were hydrothermally aged in a laboratory box furnace (Rohde Ecotop 50, Germany) in flowing air with 10 vol.% H 2 O. Alkali metal (K) was introduced to one of the aged DOC monolith catalysts by impregnation from the potassium nitrate solutions with 1 wt. % loading, obtaining the dual deactivation sample. Also, alkali metal (K) was introduced to the fresh DOC monolith catalyst using the same method of wet impregnation. After drying at 110°C overnight, the samples were calcined at 550°C for 4 h. Catalysts prepared for this study were detailed in Table 4.5.

CHAPTER 5: ON THE PERFORMANCE OF SCR CATALYSTS IN THE PRESENCE OF ALKALI COMPOUNDS

REPRESENTATIVE OF BIOFUEL CONTAMINANTS

On the performance of SCR catalysts in the presence of alkali compounds representative of biofuel contaminants

This chapter is the second nucleus of this double-core thesis and is mainly about the research conducted on the SCR catalysts.

Emissions from the biodiesel-powered engine can contain alkali metals due to the release of the remained alkali compounds which is used in the biodiesel production [START_REF] Jeong | Emission profile of rapeseed methyl ester and its blend in a diesel engine[END_REF] . Definitely, the presence of alkali compounds coming out of the engine together with soot, soluble organic fraction such as unburned fuel and lubricating oil, or directly in the gas stream, will have an impact on the efficiency of the after-treatment technologies used for exhaust cleaning. In this sense, as a continuous effort of investigating their influence on the diesel exhaust aftertreatment system such as the DOC and SCR, the impact of elements such as K and Na on the performance of SCR catalyst will be explored.

As an effective way of controlling the NO x , SCR succeeded in establishing its existence both in the stationary and mobile applications. It has been reported that the deposition of alkali metals can decrease both the number and the strength of Brønsted acid sites vital for SCR activity, resulting in a reduced ammonia storage capacity [214, 241 -243] . Later, the resulted reducibility of dispersed vanadia species due to the interaction between vanadia and doped alkali metal was also demonstrated as a cause, not a major one though [START_REF] Tang | The poisoning effect of Na+ and Ca2+ ions doped on the V2O5/TiO2 catalysts for selective catalytic reduction of NO by NH3[END_REF] .

Chen et al. conducted a comprehensive study of the effects of major possible poisons on the SCR process over a V 2 O 5 /TiO 2 catalyst for stationary application, concluding that the effect of the additive on the catalyst activity is directly related to the basicity of the additive; deactivation is caused by the weakening or destruction of Bronsted acid sites [START_REF] Chen | Deactivation of the Vanadia Catalyst in the Selective Catalytic Reduction Process[END_REF] . Their Extended Hückel molecular orbital (EHMO) calculation also showed decreases in the Brønsted acidity (The extraction energy for proton from the V-OH group and the net charge of H in the V-OH group were used as indicators for Brønsted acidity) by the addition of alkali metals, and the order of the decrease follows the order of the basicity of the alkali metal [START_REF] Chen | Mechanism of poisoning of the V2O5/TiO2 catalyst for the reduction of NO by NH3[END_REF] .

Lisi et al. evaluated the performance of two commercial SCR catalysts (with varied V loading) in the form of crushed powder, after being contacted with different amounts of Na and K and with HCl vapors, to simulate poisoning by species more specifically contained in Na > K-Na. Again, as long as Na is present (alone or in combination with K), the activities of the catalysts are significantly suppressed. oxidation. There is no impact of exhaust gas composition on the ranking of catalysts in terms of activity.

The performed comparison of lab-based research work on monolith DOC to powder DOC in the presence of alkali compounds under a typical mixture representative of diesel exhaust justifies that the catalyst form is of great importance. It is concluded that the introduction of K strongly promotes the C 3 H 6 oxidation reaction over the catalyst in the monolith form while the introduction of double dopants (K and Na) strongly affects the CO oxidation reaction over the catalyst in the monolith form in a negative way. The introduction of Na strongly promotes the NO conversion over the catalyst in the monolith form while it acts as an inhibitor when it affects the powder catalyst.

Finally, the results of doping of alkali metals on a commercial SCR catalyst in the monolithic form under varied model exhaust gases mixtures indicate that the introduction of dopants leads to either negative impacts on the catalytic performance of the used catalysts. In terms of poisoning effect, it seems that Na exhibits stronger inhibition than K on the same mass basis which is possibly due to higher molar concentration of Na than K resulting in drastic reduction of active sites. What's more, it is found that their combination usually leads to the complete loss of activities whether it's with respects to Standard SCR or Fast SCR.

As for further study based on this thesis, varied operating parameters as well as advanced characterization techniques can be employed to get insights into the reaction mechanism. In particular, operando spectroscopy techniques should offer huge amount of information regarding the evolutions of active components of the catalyst, evolutions of reactions, and evolutions of varied interactions in heterogeneous catalysis.

List of Tables