Houeida Issa Hamoud

Valentin Valtchev

Marco Daturi
email: marco.daturi@ensicaen.fr

Selective Catalytic Reduction of NOx over Cu-and Fe-exchanged zeolites and their mechanical mixture

Keywords: NH3-SCR, metal exchanged zeolites, Mordenite, SAPO-34, composite catalysts

SCR-NH3 activities of Cu-SAPO-34 (Cu-CZC) and Fe-Mordenite (Fe-MOR) and their mechanical mixture (50:50) were investigated in the temperature range 150-450 °C using operando FTIR spectroscopy. Structural and textural studies assessed the quality of the material synthesis, while in situ IR characterisation allowed the quantification of the acidic sites at the surface of the zeolites, as well as the cationic distribution inside the pores. As expected, Cu-CZC showed high NOx activity at lower temperatures (75 % of NOx conversion at T < 300 °C), while Fe-MOR was more active (> 85 %) at higher temperatures (300-450 °C). The co-presence of Cu and Fe in the mechanical mixture catalyzes the undesirable oxidation of ammonia and leads to the formation of NO, especially at high temperatures.

However, the mechanical mixture of Cu-CZC and Fe-MOR has been found to efficiently operate in the presence of excess ammonia (NH3/NOx = 1.3) in a much broader temperature range comparing to the catalysts modified with only one transition metal (Fe or Cu) and also respect to a sample containing both cations in a single zeolitic framework, thus opening new opportunities in the catalytic reduction of NOx.

Introduction

Owing to their fuel efficiency and durability, the number of diesel engines has greatly increased worldwide [1]. However, the emission of NOx and particulate matter (PM) from diesel propelled vehicles remains a major constraint for their use. Nitrogen oxides (NOx) are amongst the most serious world ecological problems since they cause irreversible changes in the terrestrial and aquatic ecosystems such as acid rain and photochemical smog [START_REF] Henry | Environmental science and engineering[END_REF], as well as health diseases [START_REF] Adam | Adult lung function and long-term air pollution exposure. ESCAPE: a multicentre cohort study and meta-analysis[END_REF][START_REF] Fiore | Global air quality and climate[END_REF]. In recent years, great efforts have been applied to limit the emission of NOx. The Selective Catalytic Reduction (SCR) of NOx with NH3 is one of the most promising technologies due to their low cost and high efficiency [START_REF] Busca | Chemical and mechanistic aspects of the selective catalytic reduction of NOx by ammonia over oxide catalysts: a review[END_REF][START_REF] Granger | Catalytic NOx abatement systems for mobile sources: from three-way to lean burn after-treatment technologies[END_REF][START_REF]Urea-SCR Technology for deNOx After Treatment of Diesel Exhausts[END_REF]. Among the NH3-SCR catalysts, metal oxides and zeolite-based catalysts promoted by transition metals are the materials of choice for this reaction [START_REF] Busca | Chemical and mechanistic aspects of the selective catalytic reduction of NOx by ammonia over oxide catalysts: a review[END_REF][START_REF] Zhang | Selective transformation of various nitrogen-containing exhaust gases toward N2 over zeolite catalysts[END_REF]. The latter one has attracted much attention due to their superior activity, thermal stability and relatively low cost [START_REF] Zhang | Selective transformation of various nitrogen-containing exhaust gases toward N2 over zeolite catalysts[END_REF]. In general, Fe-containing zeolites were found to be highly active at higher temperatures (> 350 °C), while Cu-zeolites are more active at lower temperatures (< 300 °C) [START_REF] Colombo | A comparative study of the NH3-SCR reactions over a Cu-zeolite and a Fe-zeolite catalyst[END_REF]. Given the difference in activities of Feand Cu-zeolites, it seems plausible that a combination of the two catalysts might achieve higher NOx conversions over a broader temperature range. Such combined Fe-and Cuzeolite systems were thoroughly investigated in the literature [START_REF] Metkar | Experimental and kinetic modeling study of NH3-SCR of NOx on Fe-ZSM-5, Cu-chabazite and combined Fe-and Cu-zeolite monolithic catalysts[END_REF][START_REF] Boron | Influence of Cu on the catalytic activity of FeBEA zeolites in SCR of NO with NH3[END_REF][START_REF] Sultana | Tuning the NOx conversion of Cu-Fe/ZSM-5 catalyst in NH3-SCR[END_REF][START_REF] Gao | A comparative kinetics study between Cu/SSZ-13 and Fe/SSZ-13 SCR catalysts[END_REF][START_REF] Shishkin | Functionalization of SSZ-13 and Fe-Beta with copper by NH3 and NO facilitated solid-state ionexchange[END_REF]. Mektar et al. [START_REF] Metkar | Experimental and kinetic modeling study of NH3-SCR of NOx on Fe-ZSM-5, Cu-chabazite and combined Fe-and Cu-zeolite monolithic catalysts[END_REF] showed that the Fe-ZSM-5 and Cu-Chabazite dual layer catalyst exhibited excellent NOx reduction efficiency over the entire temperature range of 250-550 °C. Various configurations (copper-zeolite layer on top or at the bottom of iron-zeolite layer) and thicknesses of the dual layer were investigated in order to obtain maximum NOx conversion for fast SCR reaction.

Other studies showed improvements in NO conversion over zeolites containing simultaneously copper and iron cations [START_REF] Boron | Influence of Cu on the catalytic activity of FeBEA zeolites in SCR of NO with NH3[END_REF][START_REF] Sultana | Tuning the NOx conversion of Cu-Fe/ZSM-5 catalyst in NH3-SCR[END_REF][START_REF] Gao | A comparative kinetics study between Cu/SSZ-13 and Fe/SSZ-13 SCR catalysts[END_REF][START_REF] Shishkin | Functionalization of SSZ-13 and Fe-Beta with copper by NH3 and NO facilitated solid-state ionexchange[END_REF]. These catalysts were prepared by different methods such as one-pot aqueous ion-exchanged method (AIE) [START_REF] Sultana | Tuning the NOx conversion of Cu-Fe/ZSM-5 catalyst in NH3-SCR[END_REF][START_REF] Gao | A comparative kinetics study between Cu/SSZ-13 and Fe/SSZ-13 SCR catalysts[END_REF], solid state ion-exchange method (SSIE) [START_REF] Shishkin | Functionalization of SSZ-13 and Fe-Beta with copper by NH3 and NO facilitated solid-state ionexchange[END_REF], two-step post synthesis and conventional wet impregnation procedure [START_REF] Boron | Influence of Cu on the catalytic activity of FeBEA zeolites in SCR of NO with NH3[END_REF]. These methods, however, present several drawbacks. For example, the steric hindrance caused by the formation of solvation shells of multivalent metal ions cannot be controlled during the one-pot AIE method. On the other hand, the use of twostep post synthesis needs numerous steps and raises difficulties to overcome the metal leaching during the second ion-exchange step. In the case of conventional SSIE, which requires high temperatures (> 700 °C), the partial destruction of zeolite framework could take place. Even though the previous studies showed a superior performance of Cu-Fe-zeolites for the SCR reaction, the NO conversion over these catalysts did not exceed 30 % at T = 150 °C, a temperature currently reached by the exhaust gases at the cold-start or during low speed driving, as downtown. However, a facile route to prepare catalytic formulations, in which both copper and iron are co-present, is required in order to understand the synergistic effects between the two metals and to control the catalytic performance at low-high temperatures.

Thus, in contrast to the previous studies in which complex systems with dual layer catalysts and Cu-Fe-zeolites were thoroughly investigated, our approach is alternatively based on the complementarity of single copper-and iron-containing zeolites used to prepare mixed catalysts that are expected to cover efficiently a broad temperature range, especially at low temperatures, in the so-called cold-start domain.

In this study, the SCR-NH3 activities of Cu-SAPO-34 (Cu-CZC) and Fe-Mordenite (Fe-MOR) and their mechanical mixture (50:50) were investigated in the temperature range of 150-450 °C using operando FTIR spectroscopy. The impact of mechanical mixing of the two catalysts on the catalytic performance will be discussed and compared with the literature results. In situ IR studies will support the critical discussion on the role of each cation in the reaction pathway and the modification of the porous host properties upon their introduction.

Experimental part

Materials

Two commercial CHA-type zeolites, as protonated (H-CZC) and copper exchanged (Cu-CZC) silicoaluminophosphates (SAPO-34) were directly used as received from Clariant. It should be noted that the incorporation of iron in SAPO-34 catalyst using ion exchange process was very difficult due to the small pore openings in the SAPO chabazite structure.

The parent Mordenite (Si/Al = 6.2, Na + -form, abbreviated as MOR) was supplied by Zeolyst. The Cu or Fe-containing mordenite were prepared by conventional ion exchange procedure and labeled Cu-MOR and Fe-MOR, respectively. In the first step, 1 g of parent zeolite was mixed with 40 mL of aqueous solutions containing 0.05 M of Fe(NO3)3•9H2O or Cu(NO3)2•3H2O and stirred at T = 60 °C for 4 h. The obtained suspensions were centrifuged, washed thoroughly with deionized water, freeze-dried for 2 nights and then calcined at 500°C for 4 h. The ion-exchange procedure was repeated three times. The parent Na-MOR was also calcined under the same conditions and used as a reference.

Techniques

X-Ray Diffraction (XRD) analyses of the parent and metal exchanged catalysts were carried out with a PANalytical X'Pert Pro diffractometer using CuKα radiation (1.5418 Å) and a graphite monochromator at room temperature. Diffraction patterns were recorded between 5 and 50° (2θ) using increments of 0.02°. The relative crystallinity (%) was calculated by comparing the peak intensities of the metal exchanged-zeolites to that of the parent materials, which were considered to be 100 % crystalline.

Nitrogen adsorption/desorption measurements were performed with an ASAP 2020 MP instrument. The specific surface area was calculated with the BET equation while the microporous volumes were determined by the t-plot method. Prior to the measurements, samples were outgassed in situ at 350 ºC for 3 h.

The actual contents of Si, Al, Cu, Fe and P in the samples were analyzed using a Varian ICP-OES 720-ES inductively coupled plasma spectrometry (ICP).

The MAS NMR spectra were recorded on a Bruker Avance 400 spectrometer with rotor diameter of 4 mm. For 29 Si (79.5 MHz), a 10 μs pulse was used with a repetition time of 15 s and 2 s for ion-exchanged zeolites. For 27 Al (104.3 MHz), a 10 μs pulse was used with a repetition time of 1 s. For 31 P (162.0 MHz), a 4 μs pulse was used with a repetition time of 20 s. A mass correction is done for each sample. The ratio of tetrahedral silicon and aluminium in the zeolite framework can be directly calculated from the line intensities in a 29 Si MAS NMR spectrum by the following equation [START_REF] Engelhardt | High-resolution solid-state NMR of silicates and zeolites[END_REF]:

Si/Al = ∑ISi(nAl) / ∑n/4 ISi(nAl) with n = {0;4} (1)
DR-UV-Vis measurements relevant to the oxidation state of iron and copper were carried using a Cary 4000 UV-Vis spectrophotometer and a HARRICK praying Mantis diffuse reflectance accessory. All spectra were recorded between 200-800 nm using an average time of 0.2 s and a scan rate of 300 nm/min.

Hydrogen temperature-programmed reduction (TPR) was carried out in a flow of 5% of H2 in Ar (40 mLmin -1 ). The sample was placed in a quartz microreactor and the quantitative consumption of H2 from 25 to 800 °C (10 degree.min -1 ) was monitored by a TCD detector.

No peaks were observed in the H2-TPR spectrum of Cu-CZC (not shown here) since the loading amount of Cu in the zeolite structure is below the detection limit.

In situ IR adsorption tests were performed in order to study the quantity and the strength of acid sites (Lewis and Brønsted) of parent and metal exchanged-zeolites. The samples were first heated under vacuum (<10 -6 mbar) at 450 °C during 2 h. Then, one equilibrium dose of pyridine (1 mbar) was put in contact with each sample at room temperature. Quantification was performed, after mass correction, by multiplying the area of the peaks corresponding to Lewis (L.A.S: 1450 cm -1 ) and Brønsted (B.A.S: 1545 cm -1 ) acid sites by the molar extinction coefficient values cited in the literature (ƐL.A.S = 2.22 cm.mol -1 ; ƐB.A.S = 1.67 cm.mol -1 ) [START_REF] Travert | Use of pyridine CH(D) vibrations for the study of Lewis acidity of metal oxides[END_REF]. It should be noted that the quantification was based on spectra recorded after pyridine evacuation at increasing temperatures to exclude an overlap of the bands of Py-L and hydrogen physisorbed pyridine. The adsorption step was followed by a Temperature Programmed Desorption study (TPD) between 25 and 450 °C was carried out to study the strength of acid sites. The IR spectra were recorded with a Nicolet 6700 FTIR spectrometer equipped with a DTGS detector at a spectral resolution of 4 cm -1 by accumulating 128 scans.

The interaction of redox sites (Cu or Fe) with carbon monoxide (CO) or nitric oxide (NO) was also studied. Catalysts were always pre-heated under vacuum at 450 °C during 2 h. Small doses of CO or NO (between 2 and 145 μmol/g catalyst) were then put in contact with each sample until saturation. The analysis was followed by a TPD study. For quantification, the integrated areas of characteristic bands were plotted against the adsorption amount of gas (μmol/g). The slope of the initial linear part of the correlation curve and the value at saturation provide the molar extinction coefficient and the amount of Lewis acid sites, respectively.

Catalytic test

The SCR-NH3 activity was tested in an Aabspec CXX 800 °C IR reactor-cell. Prior to the experiment, the catalyst (casted in a disc of ~13.50 mg) was pretreated at 500 °C for 2 h with a raising temperature speed of 5 °C/min under 10 % of O2 in Ar. Then, the reaction was studied in the 450 °C and 150 °C temperature range using a model feed gas composition, which reflects the conditions measured at the exhaust of a diesel car [START_REF] Malpartida | The NO/NOx ratio effect on the NH3-SCR efficiency of a commercial automotive Fe-zeolite catalyst studied by operando IR-MS[END_REF] (see details in Table 1). The gas flow rate in all experiments was controlled at 31 cc.min -1 , to obtain a space velocity of 143 076 h -1 .

All the reactants of the Reaction Flow were analysed by Mass Spectrometry (Quadrupole Pfeiffer Omnistar GSD 301) and stabilized before being sent to the cell. Finally, the composition of the output gas from the IR reactor cell was analyzed simultaneously as follows: NO and NO2 by Chemiluminescence (Thermo-Scientific, model 42 i-HL); CO and NH3 by an IR spectrometer equipped with a gas cell (ThermoNicolet NEXUS 670 FTIR) and MCT with a spectral resolution of 4 cm -1 and accumulating 64 scans. The NOx and NH3 conversions were respectively calculated from the equations ( 2) and ( 3).

NOx conversion [%] = NO x,inlet -NO x,outlet NO x,inlet * 100 [%] (2)
NH 3 conversion [%] = NH 3,inlet -NH 3,outlet NH 3,inlet * 100 [%] (3)

Results and discussion

The main idea of the present paper is to show how the complementary properties of copper and iron cations hosted in specific zeolitic structure can cover efficiently a broad range of temperatures in relevant conditions for the NH3-SCR reaction. Therefore, we will present below the properties of two different families of zeolites (with and without Cu/Fe cations), then we will compare their catalytic performances when both cations will be inserted in the same porous structure, or when the two zeolites will be simply mixed mechanically. The results will be discussed relatively to the chemical-physic properties of each sample, as well as on their supposed influence on the reaction pathway.

Study of H-CZC and Cu-CZC catalysts

Along with the continuous research work on NOx removal by zeolitic catalysts, it was recently reported that extremely high deNOx activities (> 80%) and N2 selectivities (> 90%) across a broad temperature range of 200-400 °C were achieved by introducing Cu in a small pore CHA-type zeolite, as both silicoaluminophosphate (SAPO-34) and aluminosilicate (chabazite or SSZ-13) were employed [START_REF] Ma | In situ DRIFTS and temperature-programmed technology study on NH3-SCR of NOx over Cu-SSZ-13 and Cu-SAPO-34 catalysts[END_REF]. The excellent catalytic behavior and hydrothermal stability of Cu-CHA-type zeolites were attributed to their particular structure combining narrow pore openings of ca. 3.8 Å (8 member ring) with large cavities [START_REF] Zhang | Selective transformation of various nitrogen-containing exhaust gases toward N2 over zeolite catalysts[END_REF].

Therefore, two commercial SAPO-34-type zeolites: H-CZC and Cu-CZC are here employed in the fast SCR-NH3 reaction and studied by different techniques, notably spectroscopic, in order to establish a structure-activity relationship.

Catalytic activity for the SCR-NH3 reaction

The SCR activities of H-CZC and Cu-CZC samples were measured in the range of Importantly, it should be noted that during the SCR experiments, no side products as N2O

were detected in the outlet, witnessing for a 100 % of selectivity for both H-CZC and Cu-CZC samples during the NOx to N2 conversion.

Conversions of CO and hydrocarbons during the SCR-NH3 reaction were also investigated (Fig. S8), being currently present in a typical automotive exhaust. Both conversions increase with increasing the temperature from 150 to 450 °C. This result can be explained by the oxidation of CO and HC in the presence of NO2, as it will be discussed later. It should be noted that a part of CO was adsorbed on Cu-CZC (band at 2153 cm -1 , not shown here) and rapidly consumed at low temperatures (200-250 °C).

Characterization of catalysts

Structural and textural properties

Table 2 summarizes the main structural and textural properties of the samples. Both H-CZC and Cu-CZC show the typical chabazite (CHA) structure for SAPO-34, with a loss of crystallinity after copper exchange (diffraction patterns and further details can be found in Supplementary Information, SI). Cu2O and CuO phases are absent, suggesting that the copper species exist in highly dispersed states or they are beyond the detection limit of XRD [START_REF] Dang | Microwave-assisted synthesis and characterization of Cu-containing AlPO4-5 and SAPO-5[END_REF].

The Cu content of Cu-CZC is 1.56 wt. % with a low copper exchange level (Cu/Al = 0.08).

By comparing the Si/Al and Si/P molar ratios of H-CZC and Cu-CZC, it can be seen that the amount of Al and P gradually decreases after ion exchange, indicating a possible dealumination of the zeolite framework [START_REF] Abu-Zied | Cu 2+ -acetate exchanged X zeolites: Preparation, characterization and N2O decomposition activity[END_REF]. The Cu-CZC catalyst exhibits a significantly decreased specific surface area (568 m 2 .g -1 ) and micropore volume (0.19 cm 3 .g -1 ) as compared with the parent H-CZC catalyst (SBET = 731 m 2 .g -1 ; Vmic = 0.25 cm 3 .g -1 ). This decrease can be attributed to the partial pores blockage by copper species and/or the partial dissolution of the chabazite framework due to the acid attack coming from copper precursor [START_REF] Li | Nature of Cu species in Cu-SAPO-18 Catalyst for NH3-SCR: combination of experiments and DFT calculations[END_REF][START_REF] Pereda-Ayo | Role of the different copper species on the activity of Cu/Zeolite Catalysts for SCR of NOx with NH3[END_REF]. These results are in agreement with the partial loss in crystallinity shown by XRD.

MAS NMR was used to investigate the coordination of framework Al, Si and P of the parent H-CZC and Cu-CZC samples. Results are reported in SI (Fig. S2). They indicate that after copper exchange some Si-O-Al bonds have been broken, causing the migration of tetrahedral Al atoms outside their framework positions to form octahedrally coordinated or extraframework Al species [START_REF] Wang | Improvement of low-temperature hydrothermal stability of Cu/SAPO-34 catalysts by Cu 2+ species[END_REF]. After copper exchange, also the signal corresponding to the tetrahedral silicon becomes broader and less intense, suggesting the formation of amorphous silicon complexes after breakage of Si-O-Al bonds [START_REF] Wang | Improvement of low-temperature hydrothermal stability of Cu/SAPO-34 catalysts by Cu 2+ species[END_REF]. The 31 P MAS NMR signal confirms the formation of extraframework aluminum and suggests that free phosphorous atoms could be coordinated to water molecules to form P(OAl)x(H2O)y clusters.

The DR-UV-vis spectra of the chabazite samples are reported and discussed in SI. They

show the typical features of tetrahedral Al species and isolated Cu 2+ species.

In situ IR analysis

The FTIR-spectroscopic investigation of pyridine adsorption is a widely used method for the characterization of both Brønsted and Lewis acidic sites [START_REF] Knözinger | Infrared spectroscopy for the characterization of surface acidity and basicity[END_REF]. For the sake of brevity, the FTIR spectra of pyridine adsorbed on H-CZC and Cu-CZC samples are not shown here.

The resulting quantitative data are summarized in Table 2, where the amounts of Brønsted CO and NO in-situ chemisorption experiments were conducted to determine the nature and the relative amount of copper sites (Fig. 2(A andB). The H-CZC support does not chemisorb any CO or NO molecules (results not shown here). The IR spectra of CO adsorption on the Cu-CZC samples are shown on Fig. 2

(A).

A main band appears at 2154 cm - 1 and it is due to C-O stretching vibrations for the (Cu + -CO) species [START_REF] Hadjivanov | Characterization of oxide surfaces and zeolites by carbon monoxide as an IR probe molecule[END_REF]. As the equilibrium pressure increases, two other weak bands appear at 2184 and 2109 cm -1 . These bands could be attributed to Cu 2+ -CO and Cu 0 -CO carbonyl species, respectively. The molar extinction coefficient of the Cu + -CO complex, calculated from the band at 2154 cm -1 by introducing increasing probe molecule aliquots on the sample, is 10.81 cm.μmol -1 . This value is in agreement with the literature [START_REF] Hadjivanov | Characterization of oxide surfaces and zeolites by carbon monoxide as an IR probe molecule[END_REF]. The total estimated amount of Cu + sites is thus 71.9

μmol.g -1 with 25 % of cationic dispersion. It is worth noting that this amount is overestimated compared to the L.A.S. number calculated from pyridine-adsorption experiment (Table 2).

This difference is mainly due to the pore diameter of the 8-MR of SAPO-34 (CHA structure, Small doses of NO were progressively added in contact to Cu-CZC (Fig. 2(B)), leading to the development of two strong absorption bands at 1911-1900 and 1800 cm -1 characteristic of Cu 2+ -NO and Cu + -NO cations, respectively [START_REF] Hadjivanov | Characterization of oxide surfaces and zeolites by carbon monoxide as an IR probe molecule[END_REF]. At the higher equilibrium pressures, an additional weak band develops at 1943 cm -1 . This band has been assigned to (Cu 2+ )n slightly associated sites [START_REF] Hadjivanov | Characterization of oxide surfaces and zeolites by carbon monoxide as an IR probe molecule[END_REF]. Therefore, we can conclude that in the conditions of the probe molecule adsorption, both Cu + and Cu 2+ sites are accessible in the substituted zeolite.

d = 3.8 Å),
At the light of these results, we can propose that the observed SCR-activity of Cu-CZC catalyst can be related to the nature of copper species, that predominantly exhibit two types of states: Cu 2+ /Cu + and CuO clusters, as shown by DR-UV-VIS and FTIR in situ analyses. The presence of CuxOy clusters in Cu-CZC sample can catalyze the low-temperature deNOx reaction, while isolated Cu + /Cu 2+ are responsible for the NOx conversion observed at high temperature [START_REF] Zhang | Selective transformation of various nitrogen-containing exhaust gases toward N2 over zeolite catalysts[END_REF].

Study of MOR, Cu-MOR and Fe-MOR catalysts

Among different framework types, the zeolites with large-micropore, such as Mordenite (MOR), show a good catalytic activity in NH3-SCR. Mordenite is a large-pore zeolite with 12-MR windows, that impose less steric hindrance during the metal exchange. For small-and medium-pore zeolites, cations exchanged during the first exchange step may block the pores and thus limit the level of exchange [START_REF] Smeets | Direct NO and N2O decomposition and NO-assisted N2O decomposition over Cu-zeolites: Elucidating the influence of the Cu-Cu distance on oxygen migration[END_REF]. In this part, Cu-and Fe-Mordenites, labeled respectively Cu-MOR and Fe-MOR, were prepared by ion exchanged method, characterized using different techniques, and then evaluated during NH3-SCR process in the range of 150-450 °C.

Catalytic activity in SCR-NH3 reaction

NOx and NH3 conversion profiles during NH3-SCR over Na-MOR, Cu-MOR and Fe-MOR are shown in Figure 3(B). As it can be seen, the catalytic activity of parent zeolite (Na-MOR) is rather good, but only at low temperatures, while ammonia is mainly oxidized at high temperatures. The NOx conversion exceeds 60 % at temperatures between 150 and 350°C. The modification of MOR with copper (4.3 wt% Cu) significantly increases its catalytic activity in the high-temperature SCR-NH3 reaction, without improving the lowtemperature activity of MOR. By the consequence, these evidences clearly show that the presence of copper moieties alone is not a guarantee for obtaining high activity at low temperatures, but the zeolitic structure play a role. These results can be explained by the presence of isolated Cu + /Cu 2+ ions as predominant species in Cu-MOR, which are responsible for the high-temperature SCR activity. The Fe-MOR sample shows lower activity than the other catalysts at low temperatures (150-200°C), while it is the most active catalyst in the 300-450°C temperature range.

CO and HC conversions during NH3-SCR reaction were also investigated (Fig. S8), being species typically present in an automotive exhaust and potentially having an effect on the conversion rates and selectivities [START_REF] Malpartida | The NO/NOx ratio effect on the NH3-SCR efficiency of a commercial automotive Fe-zeolite catalyst studied by operando IR-MS[END_REF][START_REF] Malpartida | An operando IR study of the unburnt HC effect on the activity of a commercial automotive catalyst for NH3-SCR[END_REF]. The Cu-MOR sample shows higher CO and HC conversions than Fe-MOR sample, especially at high temperatures (T > 350 °C).

Characterization of catalysts

Structural and textural properties

The X-ray diffraction patterns of the parent and metal exchanged Mordenite show a preserved structure (with a limited loss of cristallinity) and the absence of extra-framework oxidic species. The main structural and textural properties are resumed in Table 2 and detailed in SI. We can remark that the Si/Al content is similar for all samples, as well as the metal weight percentage. Metal substitution slightly increases the specific surface area and the porosity of the parent compound, due to the replacement of cations with species having a smaller ionic radius, as well as thanks to the removal of residual impurities. MAS NMR spectra show a limited amount of extra-framework aluminum, mainly due to the cation exchange of Na-MOR and namely concerning the Si(1Al) groups. DR-UV-Vis spectra witnesses for the presence of Cu 2+ and Fe 3+ in the related compounds, also confirming a majority of isolated tetrahedral Fe 3+ ions in Fe-MOR, i.e. small oligonuclear FexOy clusters.

In situ IR analysis IR data recorded after exposure of parent and metal-exchanged mordenite to pyridine are shown on Table 2. In particular, the acidity was studied after evacuation at 100 °C, i.e. after the removal of physically adsorbed pyridine. The IR spectrum obtained with the Na-MOR sample (not shown here) evidences mainly adsorbed pyridine on sodium cations. IR data (Table 2) show, as expected, that surface acidity is very low in the Na-MOR sample. Cu-MOR and Fe-MOR possess a significant amount of both Brønsted and Lewis acid sites. The latter is evidenced by the interaction between pyridine and copper/iron species. The increase of Brønsted acidity can be explained by the consequent increase of hydroxyls for cationic and defect compensation [START_REF] Morin | Acidity and catalytic properties of HEMT zeolites dealuminated by steaming[END_REF].

The concentration of Lewis acid sites is 41. andE). At low CO pressures, the IR spectra mainly consist of a single band at 2159 cm -1 for Cu-MOR due to Cu + -CO monocarbonyl species and at 2191 cm -1 for Fe-MOR attributed to Fe 2+ -CO monocarbonyl species [START_REF] Borovkov | Investigation of copper carbonyl species formed upon CO adsorption on copper-exchanged zeolites by diffuse reflectance FTIR[END_REF][START_REF] Ivanova | Evidencing three distinct FeII sites in Fe-FER zeolites by using CO and NO as complementary IR probes[END_REF]. The intensity of these bands is progressively increased with increasing the CO pressure accompanied by the formation of one new band at 2220 cm -1 assigned to the Al 3+ -CO species [START_REF] Borovkov | Investigation of copper carbonyl species formed upon CO adsorption on copper-exchanged zeolites by diffuse reflectance FTIR[END_REF]. At high CO pressure, the band at 2187 cm -1 in the Cu-MOR spectrum can be attributed to the symmetric stretching vibration of Cu + -(CO)2 dicarbonyls. For Fe-MOR, some authors assigned the band at 2158 cm -1 to CO adsorbed on Si-OH groups [START_REF] Hadjivanov | Characterization of oxide surfaces and zeolites by carbon monoxide as an IR probe molecule[END_REF]. On the other hand, it has been claimed that CO does not form complexes with Fe 3+ , but in some cases bands at 2223 and 2158 cm -1 were attributed to Fe 3+ -CO species [START_REF] Zecchina | The vibrational spectroscopy of H2, N2, CO and NO adsorbed on the titanosilicate molecular sieve ETS-10[END_REF]. The assignment of these bands to Fe 3+ -CO species usually lacks from evidence. It should be noted that no changes were observed in the hydroxyl region of zeolites during CO exposure (not shown here). In agreement with the literature [START_REF] Hadjivanov | Characterization of oxide surfaces and zeolites by carbon monoxide as an IR probe molecule[END_REF], the molar extinction coefficients of Cu(I)-CO and Fe 2+ -CO species are 11.29 cm.μmol -1 and 2.46 cm.μmol -1 , respectively. Consequently, the concentration of Cu + and Fe 2+ sites is 219 μmol/g (32 % of dispersion) and 51 μmol/g, respectively, with a very low dispersion (4 %).

Fig. 2(D) shows the IR spectra formed upon NO adsorption. An increase in NO amount results in the formation of distinctly resolved two bands at 1910 and 1812 cm -1 assigned to Cu 2+ -NO and Cu + -NO nitrosyl complexes, respectively. A less intense feature appears at 1952 cm -1 after exposure of 1 mbar equilibrium dose of NO, which was assigned to slightly associated (Cu 2+ )n sites [START_REF] Hadjiivanov | Identification of neutral and charged NxOy surface species by IR spectroscopy[END_REF].

The FTIR spectra of NO adsorption on Fe-MOR (Fig. 2(F)) are comparable with those presented by Decyk et al. [START_REF] Descyk | Studies in surface Science and Catalysis[END_REF] or by Ivanova et al. [START_REF] Ivanova | Evidencing three distinct FeII sites in Fe-FER zeolites by using CO and NO as complementary IR probes[END_REF]. The broad band at 1886 cm -1 is assigned to mononitrosyl complex [Fe 2+ -NO]. It seems to cover another species at lower wavenumber (1813 cm -1 ) suggesting the presence of other type of nitrosyl complexes. The exposure to 1 mbar NO at equilibrium results in the formation of one band at 1765 cm -1 assigned to the Fe 2+ -(NO)2 complex [START_REF] Ivanova | Evidencing three distinct FeII sites in Fe-FER zeolites by using CO and NO as complementary IR probes[END_REF].

Study of Cu-CZC and Fe-MOR mechanical mixture

As we have seen from the previous results, Cu-CZC catalyst shows the best SCR activity at low temperatures (150-350 °C), while Fe-exchanged mordenite is the most active catalyst above 300 °C. In order to increase the NOx conversion temperature window, a mechanical mixture of Cu-CZC (50 %) and Fe-MOR (50 %) was prepared at room temperature and tested in SCR-NH3 reaction from 150 to 450 °C. The SCR activity results for this sample are shown in Figure 3(C). Under stoichiometric conditions (NH3/NOx molar ratio = 1), the mechanical mixture shows lower SCR activity than the individual Cu-CZC and Fe-MOR zeolites in the whole temperature range. About 50-60 % NOx conversion is achieved in the range of 150-450 °C. Comparing with NOx conversion, 30 % overconsumption of ammonia is observed at high temperatures, which is due to the undesired ammonia oxidation reaction.

To compensate the overconsumption of ammonia, an additional SCR test was carried out in the presence of 30 % excess NH3 (NH3/NOx molar ratio = 1.3). The results (Fig. 3

(C)) clearly

show an enhancement in the SCR activity of the mechanical mixture in the whole temperature range. The NOx conversion reaches 60 % at 150 °C, and it exceeds 80 % in the temperature range of 200-450 °C. Comparing with the activities of mono-component catalysts (Cu-CZC in Fig. 3(A) and Fe-MOR in Fig. 3(B)), it can be found that the mechanical mixture exhibits high SCR-NH3 activities in a wide temperature range (150-450 °C), with 100 % of selectivity (no traces of N2O could be detected).

The IR surface spectra of the mechanical mixture recorded during the SCR-NH3 reaction (NH3/NOx = 1.3) at different temperatures are shown in Figure 4. Two significant bands at 1442 and 1626 cm -1 were observed during the low-temperature SCR (150-350 °C). The band at 1442 cm -1 belongs to the asymmetric vibration of NH4 + on the Brønsted acidic sites, while that at 1626 cm -1 is assigned to the NH3 coordinated to Lewis acid sites (copper or iron sites).

The latter can be also overlapped by water adsorption. The bands corresponding to the adsorption of NOx species are located at 1319 cm -1 and 1343 cm -1 . The first band is only observed at 150 °C and it can be assigned to N2O3 species. The latter can be associated to the nitro compound species (M-NO2) that are rapidly formed (in the first 3 min, spectrum not shown here) and decomposed with increasing the temperature. In the 2800-3400 cm -1 region, the observed bands can be assigned to the N-H stretching vibration of ammonium ions and coordinated ammonia. Differently from the former positive N-H bands, the negative peaks detected at 3607 cm -1 should be generated from the occupation of NH3 on structural Si-OH-Al groups. The intensities of all the bands decreased when increasing the temperature from 150 to 450 °C, indicating the desorption of ammonia. The IR bands of the coordinated NH3 (1626 cm -1 ) and NH4 + ions (1442 cm -1 ) almost disappear at 400 °C, although the SCR reaction still persists.

Discussion

The set of experimental data revealed that the Cu-CZC (1.5 wt %) exhibits high SCR activity at lower temperatures, while Fe-MOR sample demonstrated a superior de-NOx activity at high temperatures, with a clear role of the zeolitic structure into promoting the catalytic efficiency. The mechanical mixing of 50 % Cu-CZC and 50 % Fe-MOR shows lower SCR activity than the individual zeolites in the whole temperature range due to the ammonia oxidation under stoichiometric conditions (NH3/NOx = 1). In the presence of excess ammonia (NH3/NOx = 1.3), the mechanical mixture results in highly active catalytic formulation for SCR-NH3 at an extended temperature range (150-450 °C). The question now arises on the role of copper and iron species as well as their co-presence in the mechanical mixture in the SCR-NH3 reaction at 150-450 °C. To clarify the role of these species influencing the catalytic activity, we have correlated the characterization results with the catalytic behavior for NH3-SCR.

It is proposed in the literature that the adsorption of NH3-surface species or NH3intermediates is the key step in the SCR process [START_REF] Yu | Recent NH3-SCR Mechanism Research over Cu/SAPO-34 Catalyst[END_REF]. Thus, the areas of NH3 adsorbed on both L.A.S. (1626 cm -1 ) and B.A.S. (1442 cm -1 ) are calculated for Cu-CZC, Fe-MOR and their mechanical mixture at different temperatures and summarized in Figure 5. By comparing the three catalysts, the smallest amounts of coordinated NH3 and NH4 + were obtained in the presence of Cu-CZC sample, which results from the low concentrations of both Lewis and Brønsted acid sites (Table 2). Therefore, the concentration of adsorbed NH3

species cannot be the only parameter to justify the high de-NOx activity of Cu-CZC at low temperatures (Fig. 3(A)). So that, the reactivity of Cu-CZC can be attributed to the predominance of dispersed (32 % of dispersion) Cu + /Cu 2+ species that are considered as the main active sites for SCR [START_REF] Zhang | Selective transformation of various nitrogen-containing exhaust gases toward N2 over zeolite catalysts[END_REF]. However, the intensity of the NH4 + band declines noticeably with the increasing reaction temperature, in agreement with the acidity results reported in Figure 1. The difficulty to retain ammonia and transform it into ammonium can explain the low SCR activity of Cu-CZC at temperatures above 350 °C as it can be seen in Fig. 3(A).

From Figure 5, we notice that Fe-MOR led to significantly higher amounts of coordinated NH3 and NH4 + that remain adsorbed on catalyst surface even at high temperatures (> 350 °C). This is certainly correlated with the presence of high concentrations of Brønsted and Lewis acid sites, able to retain adsorbed NH3 (Table 2 and Fig. 1(C)), which results in the hightemperature deNOx activity of Fe-MOR (Fig. 3(B)). In fact, we must remember that the reported concentrations for acid sites were calculated upon pyridine desorption, accounting for site strength too, as aforementioned.

Monitoring the NOx signals by chemiluminescence for Cu-CZC sample (see SI, Fig. S9), we observe that the initial amount of NO2 (75 ppm) can be completely suppressed at low temperatures (150-200 °C). This is related to the principal pathway of NO2 transformation into NO. However, a part of NO2 can also be consumed during the oxidation of HC.

Considering the amount of converted HC at 200 o C (Fig. S8 in SI): 25 ppm × 5.4% = 1.3 ppm of C3H6 and 25 ppm× 2.6% = 0.65 ppm of C3H8, this would lead to the consumption of 18.2 ppm of NO2 (C3H6/NO2 = 1/9 and C3H8/NO2 = 1/10). This hypothesis seems to be consistent with the NO2 conversion at 200 °C (Fig. S9) : 75 ppm × 93% = 69.7 ppm, where 51.7 ppm participate to the SCR reaction according to the widely described fast SCR path and 17.9 ppm remaining for HC oxidation. Finally, it can be seen that pure NO2 also contributes to the CO oxidation: the CO conversion is over 10% at 200 • C (Fig. S8). In the region of 300 and 350 °C, an equilibrium can be established between NO and NO2 and followed by NOx consumption (especially NO2). This phenomenon is promoted by the oxidizing power of copper and the temperature. At 400-450°C, we can see a consumption of NOx, especially of NO2. However, a part of NO can be produced at these temperatures during the oxidation of NH3. The same trends were observed over Fe-MOR (not shown here for sake of brevity) and the mechanical mixture (see SI, Fig. S10). However, the transformation of NO2 into NO is enhanced (in time and intensity) in the presence of the mechanical mixture.

The time-evolution of NH4 + (1442 cm -1 ) and NH3 (1626 cm -1 ) band areas during SCR-NH3 reaction at 150-250 °C is shown in Figure 6. Two different phases are observed on this figure. The first one is prominently rapid (in the first 2 minutes), and it is due to the adsorption of NH3 gas molecules on catalyst surface (Fig. 6(B, D and F)). The second phase is ascribed to the consumption of coordinated NH3 to L.A.S. simultaneously accompanied by the formation of NH4 + ions. This result proves that the Lewis acid sites play the role of storing and supplying the NH3 species to the active sites where ammonium is formed during the NH3-SCR process [START_REF] Yu | Recent NH3-SCR Mechanism Research over Cu/SAPO-34 Catalyst[END_REF][START_REF] Rasmussena | Transient operando study on the NH3/NH4 + interplay in V-SCR monolithic catalysts[END_REF]. Compared to other catalysts, the rapid adsorption of NH4 + on Fe-MOR and its accumulation on catalyst surface until saturation can be observed at low temperatures, especially at 150 °C (Fig. 6(A)). Thus it appears that the deNOx activity of the Fe-MOR was hindered by excess ammonia, as already pointed out by other authors [START_REF] Malpartida | CO and NO adsorption for the IR characterization of Fe 2+ cations in ferrierite: An efficient catalyst for NOx SCR with NH3 as studied by operando IR spectroscopy[END_REF]. This behavior was attributed to an ammonia inhibition effect via direct blocking of the active sites, suggesting the presence of an optimal ammonia surface concentration in order to increase NOx conversion. However, the co-presence of iron and copper in the mechanical mixture leads to the formation of an optimal concentrations of coordinated NH3 and NH4 + ions, which is lower than the coverage established on Fe-MOR sample, but higher than that observed for Cu-CZC (Fig. 5 and6). This suggests a synergistic effect between Cu and Fe metals in the mixed catalysts resulting in higher SCR-NH3 (when NOx/NH3 = 1.3) in a wider temperature range. In addition, it was recently shown that the copper of CuO species becomes mobile in the presence of NH3 and NO at medium temperatures (250-350 °C), resulting in the migration of Cu species [START_REF] Shishkin | Functionalization of SSZ-13 and Fe-Beta with copper by NH3 and NO facilitated solid-state ionexchange[END_REF][START_REF] Shwan | Solid-state ion-exchange of copper into zeolites facilitated by ammonia at low temperature[END_REF]. Based on this feature, several composite (Cu+Fe)-zeolite catalysts were prepared starting from the physical mixture of CuO and Fe-exchanged zeolites under NO and NH3 [START_REF] Shwan | Solid-state ion-exchange of copper into zeolites facilitated by ammonia at low temperature[END_REF]. Thus, a possible migration of copper from Cu-CZC into the Fe-MOR structure during SCR-NH3 might be envisaged, especially in the range of 250-450 °C.

However, further studies are needed, such as EXAFS/XANES analyses, in order to confirm or discard this hypothesis.

Despite different reaction conditions, the SCR-NH3 performance of combined (2%)Cu-(2%)Fe-BEA catalyst prepared by solid ion-exchange method ([NH3+NO] SSIE) [START_REF] Shishkin | Functionalization of SSZ-13 and Fe-Beta with copper by NH3 and NO facilitated solid-state ionexchange[END_REF] is similar to our results. Contrary to the results obtained by using the dual-layer samples [START_REF] Metkar | Experimental and kinetic modeling study of NH3-SCR of NOx on Fe-ZSM-5, Cu-chabazite and combined Fe-and Cu-zeolite monolithic catalysts[END_REF],

the activity of the studied mechanical mixture is not decreased by diffusion limitation. Thus, the mechanical mixture Cu-and Fe-containing catalysts is a strong alternative to the Cu-Febased zeolites reported earlier [START_REF] Metkar | Experimental and kinetic modeling study of NH3-SCR of NOx on Fe-ZSM-5, Cu-chabazite and combined Fe-and Cu-zeolite monolithic catalysts[END_REF][START_REF] Sultana | Tuning the NOx conversion of Cu-Fe/ZSM-5 catalyst in NH3-SCR[END_REF][START_REF] Shishkin | Functionalization of SSZ-13 and Fe-Beta with copper by NH3 and NO facilitated solid-state ionexchange[END_REF], thanks to lack of steric restrictions as well as its simplicity and ease of preparation. For instance, varying the proportion of Cu-and Fezeolites in the mechanical mixture may result in even higher NH3-SCR activity.

Conclusions

In this study, commercial SAPO-34 (CZC) and mordenite (MOR) were exchanged with transition metals (Cu, Fe) and studied for SCR-NH3 reaction in the 150-450 °C temperature range. The physicochemical properties of the prepared catalysts were investigated using XRD, N2 physisorption at 77 K, MAS NMR, DR-UV-Vis and in situ FTIR spectroscopies.

Copper exchanged CZC (1.5 wt %) exhibit the best low-temperature SCR activity, while Fe-MOR (7.4 wt. %) sample demonstrated a superior de-NOx activity at high-temperatures.

Highly dispersed isolated lattice mononuclear Cu + /Cu 2+ species are predominant in Cu-CZC sample that are the most active species in the low-temperature SCR. The presence of high concentrations of Brønsted and Lewis acid sites in the Fe-MOR sample, as well as greater strength for NH3 adsorption improves the SCR activity at high temperatures (> 350 °C).

Under an excess of ammonia (NH3/NOx = 1.3), the mechanical mixture of (50 %) Cu-CZC and (50 %) Fe-MOR shows higher NH3-SCR activity over a broader temperature range as compared to the Cu-CZC and Fe-MOR counterparts. The synergistic effects between Cu and Fe metals, as well as their favorable redox properties in the mixture result in higher SCR-NH3 activity. In addition, an optimal amount of coordinated NH3 and surface ammonium was obtained in the presence of this mechanical mixture, which is favorable to the SCR activity.

The mechanical mixing of Cu-and Fe-zeolites is a simple and viable route to prepare highly active catalytic formulations for SCR-NH3 efficient at a wide temperature range.

  150-450 °C (Fig.1(A)). Compared with parent zeolite, Cu-CZC shows superior catalytic activity in the whole temperature range. The NOx conversion exceeds 75 % at 150 °C and it reaches a maximum of 93 % at 350 °C. Meanwhile, the NOx conversion drops from 93 % to 70 % as the reaction temperature increases from 350 to 450 °C. By comparing the NOx and NH3 conversions over Cu-CZC, it is clear that NOx and NH3 conversions are essentially stoichiometric below 350 °C, while 10 % NH3 over consumption is evident for T > 350 °C.This result confirms the non-selective oxidation of ammonia at high temperature. Therefore the decline of SCR activity at high temperature is caused by two reasons: first, part of inlet NOx cannot react due to the insufficient amount of NH3, which is partially consumed in NH3 oxidation. Second, NO is produced during NH3 oxidation reaction, directly decreasing the NOx conversion at high temperature, as shown by chemiluminescence data. Similar trends were observed by Leistner et al.[START_REF] Leistner | Impact of copper loading on NH3-selective catalytic reduction, oxidation reactions and N2O formation over Cu/SAPO-34[END_REF] during SCR-NH3 reaction over 1.6 wt% Cu-SAPO-34, with however, lower NOx conversion (40 %) at 150 °C compared to our catalyst.

and

  Lewis acid sites are referred to the mass of catalyst.A low amount of B.A.S has been detected in H-CZC samples (0.77 μmole/g), possibly resulting from Si-OH-Al groups. The respective Cu-CZC sample shows a relative decrease of B.A.S amount, so that only a marginal Brønsted acidity remained in this sample (0.11 μmol/g). These data point to the replacement of protons by copper during ion exchange. The concentration of Lewis acid sites resulting from Al 3+ of H-CZC sample is 15.8 μmol/g. The quantification data confirms that copper induces additional Lewis acidity for the Cu-CZC sample (17.8 μmol/g). The same behavior was observed by Dang et al.[START_REF] Dang | Microwave-assisted synthesis and characterization of Cu-containing AlPO4-5 and SAPO-5[END_REF] during the adsorption of pyridine on Cu/SAPO-5 catalyst. The adsorption step was followed by a Temperature Programmed Desorption study (TPD) between 25 and 200 °C in order to reveal the strength of Lewis acid sites on both H-CZC and Cu-CZC (Fig.1(A)). The increase of temperature from 25 to 150 °C results in a gradual decrease of L.A.S. by 73 % and 58 % for H-CZC and Cu-CZC, respectively. The amount of L.A.S. remains accountable on Cu-CZC after desorption beyond 150 °C, indicating the presence of strong acidic sites in this sample.It should be noted that the determination of strength of Brønsted acidity was difficult due to the low initial amount of B.A.S. in both samples (in fact the small size of the CZC cavities limits the site accessibility to pyridine). However, a complete desorption of pyridine on B.A.S. is observed at 150 °C for the Cu-CZC sample.

  which is accessible to CO (d = 3.72 Ǻ), but not to pyridine molecules (d = 4.92 Ǻ).

  [START_REF] Fiore | Global air quality and climate[END_REF] and 64.7 μmol/g for Cu-MOR and Fe-MOR, respectively. About 42 % of initial L.A.S. of Cu-MOR is still in interaction with pyridine after thermodesorption at 400 °C, indicating the presence of very strong Lewis acidic sites (Fig.1(B)). The pyridine strongly adsorbed on B.A.S. of Cu-MOR completely disappears after evacuation at 400 °C. A different behavior can be observed for the ironexchanged MOR sample (Fig.1(C)). An increase in the Lewis acidity takes place when the temperature increases from 100 to 350 °C, suggesting that some Lewis acid sites of mediumhigh strength can be recovered during the thermo-decomposition of pyridine.Infrared investigation by CO and NO probe molecule adsorption was performed as well, to have a clearer view of the cationic states. The in situ IR spectra of CO adsorbed on Cu-MOR and Fe-MOR samples at different equilibrium CO pressures are shown in Figure2(C


Houeida Issa Hamoud

Mama Lafjah

Fatima Douma

Oleg I Lebedev

Fatiha Djafri

Valentin Valchev

Marco Daturi

Mohamad El-Roz
email: mohamad.elroz@ensicaen.fr

Photo-assisted SCR over highly dispersed silver sub-nanoparticles in zeolite under visible light: an Operando FTIR study

Keywords: Photo-assisted SCR, Silver clusters, Zeolite, Photocatalysis, Plasmon

In this study, the photo-assisted SCR-NH 3 activities of vanadium containing zeolite X (ZX-V) and Ag@ZX-V catalysts are investigated under visible-light and soft temperature conditions (RT and T = 150 °C). The highly dispersed silver was incorporated into the zeolite structure as Ag n + nano and sub-nano-clusters with small particle size (< 1 nm) using a new photochemical concept. The catalysts were characterized using different techniques such as XRD, N 2 sorption, H-TEM, and DR-UV-Vis. An in situ FTIR study of adsorbed CO was also performed for the characterization of the metal sites and their accessibility. The catalytic reaction was investigated using the operando FTIR technique allowing the study of both Ag@ZX-V and ZX-V catalytic surfaces, and the gas phase of the reaction in real time. Both catalysts are not active in dark and only Ag@ZX-V exhibits a photocatalytic activity under visible light. At RT, the results show that silvers clusters promote the photocatalytic oxidation of NH 3 /NO, and no SCR is observed. This probably due to an energy transfer from the metallic site to the triplet oxygen and the formation of highly oxidant singlet oxygen.

However, at T =150 °C a significant photo-assisted SCR activity is observed under visible light irradiation with more than 18 % of additional NO x removal. The different behavior of the material at 150°C in respect to its activity at RT is mainly attributed to the additional formation of silver nanoparticles and to the possible formation and conversion of intermediates that are not formed and or converted at RT (thermodynamic limitation). Basing on this study, a possible mechanism of the photo-assisted SCR reaction is proposed.

Introduction

Nitrogen oxides (NO x ) from diesel engines contribute substantially to air pollution [START_REF] Larrubia | An FT-IR study of the adsorption and oxidation of Ncontaining compounds over Fe 2 O 3 -TiO 2 SCR catalysts[END_REF]. Selective Catalytic Reduction (SCR) with NH 3 is one of the most effective technologies to control automotive NO x emissions. Most of the SCR catalysts possess a high activity at the intermediate and high temperature ranges (200-500 °C) [START_REF] Gao | Morphology-dependent properties of MnO x /ZrO 2 -CeO 2 nanostructures for the selective catalytic reduction of NO with NH 3[END_REF]. The development of active SCR catalyst at a lower temperature (< 200 °C), during the engine start-up, is very challenging due to the presence of excess H 2 O and adsorption competition between ammonia and combustion residuals on active sites [START_REF] Malpartida | An operando IR study of the unburnt HC effect on the activity of a commercial automotive catalyst for NH 3 -SCR[END_REF]. In this regard, a combined strategy between LNT (Lean NO x Trap) and SCR is necessary, which significantly increases the SCR operational costs and complexity [START_REF] Castoldi | Catalytic behaviour of hybrid LNT/SCR systems: Reactivity and in situ FTIR study[END_REF][START_REF] Xu | LNT+ in situ SCR catalyst system for diesel emissions control[END_REF].

Among the catalysts reported for the low-temperature SCR, single and mixed metal oxides catalysts containing Cr, Mn, Ce, Fe, Co, Ag, V, Ni and/or Cu oxides have been studied, combining the synergetic catalytic effect of the different components [START_REF] Thirupathi | Co-doping a metal (Cr, Fe, Co, Ni, Cu, Zn, Ce, and Zr) on Mn/TiO 2 catalyst and its effect on the selective reduction of NO with NH 3 at low-temperatures[END_REF][START_REF] Park | Effect of sulfur on Mn/Ti catalysts prepared using chemical vapor condensation (CVC) for low-temperature NO reduction[END_REF][START_REF] Huang | Alkali-and sulfur-resistant tungsten-based catalysts for NO x emissions control[END_REF]. For example, silver clusters (Ag n δ+ ) have previously been identified as the prime species for the activity of Ag/Al 2 O 3 in H 2 -assisted NH 3 -SCR (Strömet al., 2018), and it has been shown that the activity is linearly proportional to the relative amount of these clusters. However, these catalysts still present several drawbacks, such as the need of H 2 to form a proton on Al 2 O 3 , which directly participates in the reaction mechanism, the poor N 2 selectivity and low deNO x performance at low temperature (<200 °C) [START_REF] Ström | Surface species and metal oxidation state during H 2 -assisted NH 3 -SCR of NO x over alumina-supported silver and indium[END_REF].

Photo-assisted selective catalytic reduction of NO x under visible light is a promising approach for a significant decrease in the SCR reaction temperature. The active species can be generated at a relatively low temperature ensuring the oxidation-reduction reaction involved in the SCR process. Tanakaet al. have studied the photo-assisted selective catalytic reduction of NO with NH 3 over a serial of TiO 2 based photocatalysts such as: TiO 2 modified by transition metal (i.e, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Ta or W) [START_REF] Yamazoe | Promotion effect of tungsten oxide on photo-assisted selective catalytic reduction of NO with NH 3 over TiO 2[END_REF]Yamamoto et al. 2015) and dye-modified TiO 2 (Yamamoto et al., 2014a(Yamamoto et al., , 2015b)).

In the TiO 2 system, more than 80 % of NO conversion and approximately 100 % of selectivity can be achieved at a gas hourly space velocity (GHSV) of 8000 h -1 at 160 °C.

However, the TiO 2 has a low absorbance and activity in the visible range of light. The modification of the TiO 2 with dyes can enhance significantly its activity under visible light.

On the other hand, these molecules have weak stability under the reaction condition required for photo-SCR reaction (the presence of oxidant species promotes the degradation of the dyes at relatively high temperature). Recently, Lia et al. (2018) have studied the photo-SCR of NO over rare-earth natural clay coupled with reduced graphene oxide under simulated solar-light irradiation. About 70 % of NO conversion and 100% N 2 selectivity were obtained at room temperature. Unfortunately, the complexity of such a system and their high-cost multi-step preparation method limit their practical use in diesel cars. Thus, developing new stable, and low-cost materials which can work at low-temperatures and high-GHSV SCR conditions remains a great challenge. More recently, we have successfully synthesized sub-nanometer silver clusters with enhanced narrow size distribution using the uniform size cages of FAUtype zeolite as a scaffold [START_REF] El-Roz | Uniform Generation of Sub-nanometer Silver Clusters in Zeolite Cages Exhibiting High Photocatalytic Activity under Visible Light[END_REF]. The sub-nanometer Ag clusters show an exceptional photocatalytic activity and selectivity for the reforming of formic acid to H 2 and CO 2 under visible light. The photocatalytic activity was assigned to the plasmonic behavior of the Ag n δ+ clusters. The high performance of these materials in photocatalysis under visible light makes them very promising candidates for the photo-assisted SCR reaction especially at low temperatures, in the so-called cold-start domain.

In this study, sub-nanometer silver cluster in FAU-type zeolite were studied in the photoassisted reaction at room temperature and 150 °C using operando FTIR spectroscopy. The first part of the paper is dedicated to the preparation and the physicochemical and textural characterization of the catalysts. Then, the performance of the prepared catalysts for the SCR-NH 3 reaction under different conditions of temperature and irradiation is reported, and a possible reaction mechanism is discussed.

Experimental part

Materials

Sub-nanometer silver clusters in nanosized zeolite ZX-V (FAU-type, 10-30 nm, Si/Al = 1.63) were prepared in two-steps, as described in reference [START_REF] El-Roz | Uniform Generation of Sub-nanometer Silver Clusters in Zeolite Cages Exhibiting High Photocatalytic Activity under Visible Light[END_REF]. In the first step, the nanosized vanadium-containing zeolite X (ZX-V) was synthesized from a clear precursor suspension with a molar composition: 10 SiO 2 : 1.1 Al 2 O 3 : x V 2 O 5 : 9 Na 2 O : 122 H 2 O. Sodium vanadate was selected as a precursor of the photoactive sites due to the efficiency of the vanadate sites for silver ion reduction versus water oxidation (formation of OH radicals and oxidant species are prevented). The properties of this precursor are fully compatible with the synthesis of the organic-template-free zeolite (same counter ions, low polymerization at high pH, etc.). The initial reactants were mixed to prepare two initial solutions denoted A and B. Solution A was prepared by dissolving 0.296 g aluminum powder (325 mesh, 99.5%, Alfa Aesar) and 2.5 g NaOH (Aldrich, 97%) in 3 g double distilled H 2 O.

Vanadium containing samples were prepared by adding 0.122 g of VO 3 Na (2 wt.%, in respect to the Silica and Alumina content) to this solution and stirring for 3 h. Solution B was prepared by mixing 10 g colloidal silica (Ludox-HS 30, Aldrich) with 1.077 g of NaOH and 1 g double distilled H 2 O. The resulting turbid suspensions were placed in an oven at 100 °C for 5 minutes to obtain water clear suspensions. Solution A was added drop wise under vigorously stirring to the solution B at 4 °C using an ice-bath. The resulting colorless suspensions were stirred 24 h at room temperature followed by dehydration of 6 to 6.5 g H 2 O

(≈ 40 M H 2 O from the general gel composition). Hydrothermal crystallization was conducted at 50 °C for 36 h. The nano-particles of ZX-V were recovered by centrifugation (25 min, 20000 rpm) followed by re-dispersion in double distilled water. The procedure was repeated till reaching pH ~ 8. The second step was devoted to the preparation of Ag@ZX-V by dispersing ZX-V (zeolite concentration = 3 mg/ml) in water solutions containing ethanol as an electron donor (water:ethanol=3:1; with total volume = 20 mL) and silver nitrate as a silver precursor (3 mM). The reduction of silver ions is performed by UV irradiation of the solution for 5 min under stirring using a polychromatic Hg-Xe lamp (200 W). Less than 3 minutes of UV irradiation was enough to change the color of the milky suspension to green.

This typical color modification is due to the formation of reduced silver particles in the solution. The essential role of vanadate and ethanol confirms the photocatalytic behavior of the process. The experiments were repeated many times until obtaining 200 mL of Ag@ZX-V solution. The solid was recovered by centrifugation (25 min, 20000 rpm) followed by redispersion in double distilled water and the procedure was repeated two times in order to remove the excess of free silver and silver precursor. Then, the suspension was dried at 50 °C under atmospheric condition.

Techniques

X-Ray Diffraction (XRD) analyses of the ZX-V and Ag@ZX-V materials were carried out with PANalytical X'Pert Pro diffractometer with CuK α irradiation (λ = 1.5418 Å).

Rietveld refinements were then performed with the JANA2006 software [START_REF] Palatinus | SUPERFLIP-a computer program for the solution of crystal structures by charge flipping in arbitrary dimensions[END_REF]) on PXRD data recorded on a Bruker D8-vario1 diffractometer equipped with a primary germanium (111) Johansson monochromator (λKα1 = 1.5406 Å) and a LynxEye detector.

The PXRD diagrams were recorded at room temperature between 5 and 50° (2θ) with a step size of ~0.014°. Variable divergent slits with a constant illuminated sample length of 6 mm were used. Phase identifications were performed with the PANalytical High Score plus program. The crystal structure of Ag@ZX-V was solved by ab initio method using the SUPERFLIP software [START_REF] El-Roz | Uniform Generation of Sub-nanometer Silver Clusters in Zeolite Cages Exhibiting High Photocatalytic Activity under Visible Light[END_REF].

Nitrogen adsorption/desorption measurements were performed with an ASAP 2020 MP instrument. The specific surface area was calculated with the BET equation while the pore volumes were determined by the BJH method. Prior to the measurements, samples were outgassed in situ at 150 ºC for 3 h.

Advanced transmission electron microscopy (TEM), high-angle annular dark field scanning TEM (HAADF-STEM) together with EDX mapping experiments were carried out on an aberration probe and image corrected JEM ARM200F cold FEG microscope operated at 200 kV equipped with a CENTURIO EDX detector and GIF Quantum spectrometer.

DR-UV-Vis spectra of the dried powder samples were recorded by using a Cary 4000 UV-Vis spectrophotometer and a RSA-CU40 Diffuse Reflectance cell.

In situ FTIR spectroscopy of CO chemisorption was performed in order to determine the nature and the oxidation state of active sites and their accessibility. The samples (pellet of ~20 mg) were first heated under vacuum (<10 -6 mbar) at 150 °C during 2 h. Doses of CO (between 1 and 40 torr) were then put in contact with each sample until saturation. The IR spectra were recorded with a Nicolet 6700 FTIR spectrometer equipped with a DTGS detector at a spectral resolution of 4 cm -1 by accumulating 128 scans.

(Photo)Catalytic tests

For the operando experiments, a 'sandwich like' IR cell-reactor was used to analyze the catalysts surface and measure their SCR activity at the steady-state (El-Roz et al., 2017).

First, the catalyst (shaped as a pellet of ~ 20 mg) was pretreated at room temperature under Ar and irradiation using a Xe-lamp (LC8 Hamamatsu, 200 W) with cut-UV filter (> 390nm). The lamp irradiance was measured using ILT950 spectrilight spectroradiometer from International Light Technologies. Then, the reaction was studied at RT and 150 °C in the dark or under irradiation. The full flow gas composition is reported in Table 1.

Table1.

The total flow rate was fixed at 31 cc.min -1 . The relative concentrations of the effluent gas were continuously measured by Mass Spectrometry (Quadrupole Pfeiffer Omnistar GSD 301) and stabilized before being sent to the cell. Finally, the composition of the output gas from the IR reactor cell, including NO, NO 2 and NO x , was analyzed simultaneously by Chemiluminescence (Thermo-Scientific, model 42 i-HL) and IR spectrometer (ThermoNicolet NEXUS 670 FTIR) equipped with an MCT detector with a spectral resolution of 4 cm -1 and accumulating 64 scans. The NO x conversions were calculated from the equations (1) at the steady-state (after at least 1 hour of reaction). It should be noted that these conversions are corrected by those obtained for the same experiment performed in dark and at RT (no possible reaction is expected in this condition for both catalysts).

(Eq. 1)

Results and discussion

Characterizations of ZX-V and Ag@ZX-Vcatalysts

The X-ray diffraction patterns of the ZX-V and Ag@ZX-V samples are displayed in Figure 1(A).

Figure 1.

The ZX-V sample shows the typical Bragg pattern of FAU-type zeolite. The Rietveld refinements (Table S1 andS2 V. It should be noted that no diffraction peaks of metal oxide (M = V, Ag) phases were detected in the XRD patterns. This results show that the V content (0.25 wt.% V)) is below the detection limit of XRD. The absence of silver peaks is due to the small size of highly dispersed silver phase in the fresh sample.

Figure 1(B) shows that both ZX-V and Ag@ZX-V samples exhibit a combination of Type I and IV isotherms with a H3-type hysteresis. The surface areas and pore volumes of ZX-V and Ag@ZX-V samples are summarized in Table 2.

Table 1.

The Ag@ZX-V catalyst exhibits a significantly decreased specific surface area (369 m 2 /g) and micropore volume (0.091 cm 3 /g) as compared with the parent ZX-V catalyst (S BET = 689 m 2 /g; V mic = 0.185 cm 3 /g). The mesoporosity was also decreased from 1.098 to 0.839 cm 3 /g after Ag loading into ZX-V structure. This decrease can be attributed to the partial pores blockage by Ag species and the higher density of silver particles.

The high resolution HAADF-STEM images of Ag@ZX-V sample are presented in Fig. 2.

The Ag sub-nanoparticles appear as bright white dots within the pores of FAU-type structure.

Ag-free ZX-V crystallites can also be observed in Figure 2(a) with a crystal size lower than 10 nm. The distribution of silver clusters in the zeolite is not perfectly uniform (some pores are empty or not completely filled; Fig. 2c) which in agreement with the loss of external surface area of ZX-V after Ag loading (Table 2). It should be noted that segregation of Ag nanoparticles from the pores into larger Ag nanoparticles can be obtained after the second scan by e-beam inside the microscope during imaging (Fig. S2). The EDS mapping of the Ag@ZX-V is reported in Fig. S3. The result confirms the good dispersion of the silver in the zeolite. However, this result should be interpreted with precaution due to the low stability of the silver particles under the electron beam (Fig. S2). More details concerning this phenomenon are reported in our previous paper [START_REF] El-Roz | Uniform Generation of Sub-nanometer Silver Clusters in Zeolite Cages Exhibiting High Photocatalytic Activity under Visible Light[END_REF].

Figure 2.

The DR-UV-Vis spectra of ZX-V and Ag@ZX-V and activated Ag@ZX-V at 150°C

under Ar are illustrated in Fig. 3. For ZX-V, only one broad band was observed in the region of 250-285 nm, with a maximum at 270 nm. This is attributed to the low-energy charge transfer band (LCT) associated with O to V 4+ electron transfer. This result points out that the amount of polymeric V 5+ oxides is negligible and that namely isolated interstitial V 4+ species are present. The ICP and EDX analyses of the ZX-V sample showed that vanadium content is around 0.25 wt. %. The characterization and localization of vanadate species was difficult due to their relatively low amount in the zeolite. It is clear, however, that oxalate-vanadate species are highly dispersed within the zeolite structure as clusters and no large aggregates are formed.

Figure 3.

The DR-UV-Vis spectrum of Ag@ZX-V powder shows the appearance of new UVvisible bands at 270, 295, 325, 377, 414nm. The bands between 270 and 380 nm are characteristic of the electronic transitions of reduced Ag species (Ag n + ) with sub-nanometer particles size [START_REF] Gurin | Metal clusters and nanoparticles assembled in zeolites: an example of stable materials with controllable particle size[END_REF][START_REF] Severance | Evolution of silver nanoparticles within an aqueous dispersion of nanosized Zeolite Y: mechanism and applications[END_REF]). In addition, new bands at 432 and 562 nm attributed to the formation of silver nano-particles with a particle size >12 nm and >20 nm are also observed [START_REF] Cuong | Theoretical modeling of optical properties of Ag 8 and Ag 14 silver clusters embedded in an LTA sodalite zeolite cavity[END_REF][START_REF] Zou | Controlled growth of silver nanoparticles in a hydrothermal process[END_REF]. The presence of narrow UV bands at specific wavelength indicates that the silver clusters have well-defined size and charge. The visible bands (400-530 nm) can be attributed to the external silver nanoparticles as well as to the connected silver clusters within the network of the zeolite pores.

It should be noted that the same bands are observed in the Ag@ZX-V suspension (Fig. S4). This demonstrates the good stability of the prepared clusters in their solid forms under aerated condition. The shift observed in some absorption band can occur due to a solvatochrom effect. After in-situ activation of the sample at 150°C (under Ar), an increase of the broad plasmonic band has been observed in the visible range. This is most probably due to the migration of the silver clusters and their aggregation promoting the formation of silver nanoparticles. This phenomenon was already observed under the TEM beam (Fig. S2).

The infrared spectroscopy of adsorbed CO was used to determine the oxidation state of silver nanoparticles (Fig. 4). It is well known that surface carbonyls with zero-valent silver atoms cannot be formed at room temperature (Hadjivanov et al., 2002). No characteristic bands of carbonyl groups are observed in the ZX-V spectra, while an intense band at 2185 cm -1 is obtained after CO adsorption on Ag@ZX-V (Fig. 4(A)). This band is attributed to Ag + ions, mainly located inside the pores (Hadjivanov et al., 2002). It is formed by multiple components (Fig. 4

Catalytic activity in the photo-assisted SCR-NH 3 reaction

The physicochemical properties, such as specific surface area, Si/Al ratio, acidity, stability and redox ability of metal-exchanged zeolite catalysts are strongly correlated to the catalytic performances for NO x removal during the SCR process. It is generally recognized that both adsorbed ammonia on Lewis acid sites and ammonium ions associated with Brønsted acid sites are precursors to an activated site that can selectively reduce NO x via different possible mechanisms of low-temperature NH 3 -SCR reaction, combining redox and acid-base steps [START_REF] Gao | A review on selective catalytic reduction of NO x by NH 3 over Mn-based catalysts at low temperatures: catalysts, mechanisms, kinetics and DFT calculations[END_REF][START_REF] Iwasaki | Mechanistic Aspect of NO-NH 3 -O 2 Reacting System[END_REF]I. Nova et al.;[START_REF] Iwasaki | Mechanistic Aspect of NO-NH 3 -O 2 Reacting System[END_REF]. These concepts can be summarized and described through an Eley-Rideal (ER) mechanism, where the gaseous NO first reacts with activated NH 3 -absorbed species to form intermediates and then decomposes into N 2 and H 2 O, or via a Langmuir-Hinshelwood (LH) mechanism, where the gaseous NO is adsorbed on a basic site and further combine with the adjacent activated NH 3 species to form N 2 and H 2 O. According to previous reports regarding the SCR process over metal-exchanged zeolites [START_REF] Deka | Influence of NO 2 on the selective catalytic reduction of NO with ammonia over Fe-ZSM5[END_REF][START_REF] Long | Reaction mechanism of selective catalytic reduction of NO with NH3 over Fe-ZSM-5 catalyst[END_REF], the presence of Brønsted acid sites is essential for achieving a high NO x conversion and high N 2 selectivity. The Brønsted acid sites are involved in the reaction by chemically binding and activating the ammonia to form ammonium ions, which then participates in the SCR reaction.

On the other hand, [START_REF] Bin | Structural characterization and selective catalytic reduction of nitrogen oxides with ammonia: a comparison between Co/ZSM-5 and Co/SBA-15[END_REF] show that the existence of Brønsted acid sites contributes to an excellent dispersion of metal species within the zeolite structure. More recent studies have lowered the importance of these sites in the SCR mechanism [START_REF] Janssens | A consistent reaction scheme for the selective catalytic reduction of nitrogen oxides with ammonia[END_REF]. In our study, the FAU-X based samples do not possess any Brønsted acid site due to its sodium form with a low Si to Al molar ratio (Si/Al = 1.63). This is confirmed by the IR spectra of the ZX, ZX-V and Ag@ZX-V samples (Fig. S5), showing only one band at 3738 cm -1 in the O-H region characteristic of terminal Si-OH group [START_REF] Deka | Influence of NO 2 on the selective catalytic reduction of NO with ammonia over Fe-ZSM5[END_REF]. However, the formation of acid sites during the reaction on Ag@ZX-V cannot be excluded due to the presence of NH 3 and H 2 O in the gas phase. Thus, the monitoring of ZX-V and Ag@ZX-V samples in the photo-assisted SCR-NH 3 reaction at RT and at relatively low temperature (T = 150 °C) is of outmost importance. Then, some possible structure-activity relationships can be discussed by comparing the data obtained during the catalytic test with those relevant to their physico-chemical properties.

NO x conversions during the photo-SCR

Figure 5 shows the conversion of NO, NO 2, and total NO x during the photo-SCR at RT over the ZX-V and Ag@ZX-V catalysts. The values are determined by chemiluminescence at the steady state after 4 hours of visible-light irradiation. The NO x values were subtracted from those obtained at the steady state in dark at RT. Each experiment was repeated at least two to three times, with an estimated uncertainty of about ± 5%. It should be noted that it was difficult to estimate the ammonia conversions during the photo-SCR, due to the low intensity of the NH 3 IR band in the gas phase. There is no significant activity (in the error range) observed over ZX-V, while negative conversions were obtained over Ag@ZX-V catalyst. The negative values obtained over Ag@ZX-V correspond to an increase of the NO, NO 2 and NO x amount in the gas flow due to ammonia photooxidation. It should be noted that Ag + -ZX-V, prepared by a simple ionic exchange (no silver reduction), was also tested under similar previous conditions and no photocatalytic activity was observed (Fig. S6). This result indicates that the highly dispersed Ag n + clusters in the zeolite matrix play a crucial role in the properties of a plasmonic photocatalyst. The significant amount of NO 2 (14.4 % of conversion) detected at the steadystate of the photocatalytic reaction at RT (Fig. 5), indicates possible oxidation of NO into NO 2 over Ag@ZX-V. The capacity of silver nanoparticles in the ammonia oxidation at relatively low temperature is already reported in the literature by using Ag/Al 2 O 3 catalyst [START_REF] Wang | Nanosize effect of Al 2 O 3 in Ag/Al 2 O 3 catalyst for the selective catalytic oxidation of ammonia[END_REF]. The authors show that both the valence state and particle size of Ag species had a significant influence on the catalyst activity for the ammonia oxidation to NO.

Increasing the temperature from RT to 150 °C does not influence the performance of ZX-V in both dark and under visible light irradiation conditions, and no significant activity is observed (Figure 6(A)). This result is due to the absence of active sites for SCR on ZX-V and the lack in the active species with absorption edges in the visible-light region. Same behavior of Ag@ZX-V can be observed at 150°C in dark without any significant catalytic activity. On the contrary, the activity of the Ag@ZX-V sample was radically changed under visible irradiation at 150°C (Fig. 6(B)). Loading of uniform, sub-nanosized and stable silver particles in the ZX-V material improves the NO x conversion from 0.2 ±5 % (in dark) to more than under visible irradiation at RT (spectrum b, Fig. 7(A)). This is in agreement with the absence of any photocatalytic activity of this sample at RT.

In contrast, the intensity of the band at 1642 cm -1 of Ag@ZX-V is significantly decreased when the visible light is switched on (spectrum b, Fig. 7(B)). This is probably related to water desorption and/or NH 3 desorption/conversion. Under visible irradiation, a new broad band appeared at 1400 cm -1 belonging to the asymmetric vibration of adsorbed NH 3 on the Brønsted acid sites (formation of NH 4 + ) (Strömet al. 2018). This is confirmed by the less pronounced absorption bands at 3260 and 3300 cm -1 , assigned to the symmetric and asymmetric N-H stretching vibrations of NH 3 hydrogen bonded to surface OH (Fig. S8).

Thus, the development of the band at 1400 cm -1 indicates the in-situ formation of new B.A.S.

on the zeolite during the reaction, promoted by water dissociation and/or ammonia oxidation.

The in situ formation of B.A.S. has been already reported in the literature [START_REF] Rasmussen | Transient operando study on the NH3 /NH 4 + interplay in V-SCR monolithic catalysts[END_REF][START_REF] Ström | Surface species and metal oxidation state during H 2 -assisted NH 3 -SCR of NO x over alumina-supported silver and indium[END_REF] and authors showed that water can hydrolyze metal sites (Al, Ag n + clusters and/or vanadia species) of the catalyst creating new Brønsted acid sites. Also, the adsorbed ammonia is either directly consumed by the SCR reaction or converted into ammonium, which then participates in the SCR.

As the temperature increased from RT to 150 °C, the intensity of the band at 1642 cm -1 decreased rapidly, indicating the water/ammonia desorption. The other band located at 1400 cm -1 is appeared clearly in the spectra of ZX-V and is become more intense (saturated) in the case of Ag@ZX-V (spectrum (c), Fig. 7). In fact, water molecules might strongly adsorb and fully occupy the adsorption sites of ZX-V at RT, preventing the adsorption of NH 3 on the B.A.S (Fig. S9). However, when the reaction temperature increases to 150 °C, more water molecules would desorb and release the active sites. As a result, the formation of NH 4 + on the Brønsted acid sites of ZX-V is now clearly visible at T = 150 °C. However, the negligible catalytic activity of ZX-V and Ag@ZX-V at 150 °C (in dark) reveals that the formation of NH 4 + is not enough for promoting the deNO x process. Under visible light irradiation at 150 °C, no modification in the IR spectrum of ZX-V surface is observed, as expected. However, a significant decrease of the bands at 1642 cm -1 and 1400 cm -1 are observed on the Ag@ZX-V (spectrum (d), Fig. 7).

The operando FTIR results on Ag@ZX-V reveal more information on the evolution of the surface species as a function of time during the photo-SCR reaction. The results are reported in Fig. 8. The data help to envisage the interplay between the pre-adsorbed species, ammonia and ammonium ions. While the band of NH 3 bending vibrations at Lewis acid sites continuously decreases, a significant increase of the band at 1400 cm -1 , assigned to the vibrations of the ammonium ions at Brønsted acid sites, is observed during the reaction at RT under visible light (Fig. 8(A)). The irradiation of the Ag@ZX-V with visible light at 150°C under reaction flow (Fig. 8(B)) led to a quick decrease of water, ammonium and ammonia bands.

Figure 9.

The time-evolution of NH 4 + (1400 cm -1 ) and NH 3 overlapped by water adsorption (1642 cm -1 ) band areas over Ag@ZX-V during SCR-NH 3 reaction at RT and 150 °C is shown on Fig. 10. The main observation from this experiment is the significant and sudden drop of all species (and especially ammonia) when the visible light is turned on, at all temperatures, except for NH 4 + at RT (Fig. 10(A)). In contrast, no changes in ammonia evolution were observed over ZX-V under similar conditions (supplementary data, Fig. S10).

Otherwise, the NH 4 + ions are continuously formed on ZX-V during the reaction at 150 °C, confirming that the only conversion of ammonia into ammonium cannot promote the SCR at 150 °C. These results with the catalytic activity confirm the photo-assisted SCR reaction over Ag@ZX-V with high selectivity.

Figure 10.

Mechanism

In summary, the set of the previous experimental data revealed that the ZX-V sample was completely inactive during the photo-assisted SCR-reaction at RT and at T = 150 °C. In contrast, the reaction under visible irradiation seems to favor the oxidizing pathway to generate NO and NO 2 at RT over Ag@ZX-V, while the photo-assisted NO x reduction is promoted at T = 150 °C. In this part, the physico-chemical properties of Ag@ZX-V, as well as the reaction mechanism proposed in the literature, can be used to get insights in the photoassisted SCR pathway over Ag@ZX-V at T = 150 °C.

Progressive hydroxylation for supported zeolite with pure Lewis acidity indicates that a significant amount of water molecules will adsorb dissociatively, hydroxylating the zeolite In a classical semiconductor photocatalyst [START_REF] Shishido | A unique photo-activation mechanism by "in situ doping" for photo-assisted selective NO reduction with ammonia over TiO 2 and photooxidation of alcohols over Nb 2 O 5[END_REF][START_REF] Li | Photo-assisted selective catalytic reduction of NO by Z-scheme natural clay based photocatalyst: Insight into the effect of graphene coupling[END_REF], the UV or visible excitation can promote the formation of electron and holes by the excitation of an electron occupying the valence band, transferring it into the conduction band. The produced holes promote the oxidation, and the conduction band electrons promote the reduction of the adsorbed species. However, in the case of metallic clusters, the localized surface plasmon resonance (LSPR) is the unique optical phenomenon that occurs. When the frequency of incident light matches the oscillation frequency of the electrons of the conduction band of the metal, it triggers a collective excitation of the electrons. These excited surface electrons (hot electrons) oscillate coherently with the incident electromagnetic field.

Then, hot electrons relax through a multi-step process initially involving electron-electron scattering, and then on a much slower timescale, transferring energy from the thermalized electron gas to the metal lattice via electron-phonon coupling, causing an interaction between the metal lattice and its surroundings (phonon-phonon interactions) (Liu & Li;2016). This non-radiative LSPR relaxation process converts the kinetic energy of hot electrons into thermal energy heating up quickly the surroundings of a metal surface. The generated heat can promote redox reactions that require a high temperature in dark conditions. One of the possible explanations of the photocatalytic activity of Ag@ZX-V at RT is the formation of oxidant species able to oxidize ammonium into NO, and NO into NO 2 . [START_REF] Planas | Distance-dependent plasmon-enhanced singlet oxygen production and emission for bacterial inactivation[END_REF] demonstrated that silver nanoparticles could enhance the formation of singlet oxygen ( 1 O 2 ). More recently, [START_REF] Macia | Hybrid Silver Nanocubes for Improved Plasmon-Enhanced Singlet Oxygen Production and Inactivation of Bacteria[END_REF] demonstrated an impressive plasmonenhanced singlet oxygen production on hybrid silver nanocubes. However, in the previously cited works, the formation of singlet oxygen through direct sensitization by metal nanoparticles was not investigated, and the role of the plasmon was limited to the enhancement of the singlet oxygen production via a grafted photosensitizer (Rose Bengal).

The unprecedented and well demonstrated observation of singlet oxygen formation through direct sensitization by metal nanoparticles (Ag, Au, Pt) is, however, demonstrated by [START_REF] Vankayala | Metal nanoparticles sensitize the formation of singlet oxygen[END_REF]. The singlet oxygen is well known for his capacity to generate highly oxidant OH and/or peroxide in the presence of labile hydrogen (in our case, the hydrogen of the ammonium) (Insug et al., 2015;[START_REF] Rinalducci | Formation of radicals from singlet oxygen produced during photoinhibition of isolated light-harvesting proteins of photosystem II[END_REF]. Therefore, the active site for the photo-oxidation at RT should be the Lewis acid site on Ag clusters, and the adsorption of NH 3 onto the Lewis acid site would be the first step in the photo-oxidation. The adsorbed NH 3 on Ag clusters (Lewis acid sites) could be activated by singlet oxygen to the NH 2 radical under visible light (Eq. 2). The NH 2 radical further reacts with the singlet oxygen to form NO (Eq. 3). The singlet oxygen can also oxidize NO into NO 2 (Eq. 4).

NH 3 + 1 O 2 → NH 2 • + H + + O 2 -(Eq. 2) NH 2 • + 1 O 2 → NO + H 2 O (Eq. 3) NO + ½ 1 O 2 → NO 2 (Eq. 4)
When the temperature increases from 25 to 150 °C under visible irradiation, the route of NH 3 reduction is more favorable than the ammonia oxidation pathway. The NH 3 reduction is promoted by the local plasmonic heating under visible light, which induces the thermal decomposition of NH 3 -NO x intermediates. In fact, the irradiation by visible light at 150°C leads to significant heating of the catalyst and thus the apparent temperature at the contact point would be higher than 150 °C. Consequently, the SCR can operate at the lowtemperature range in the presence of visible irradiation without increasing the temperature. This is confirmed by comparing the SCR activity of Ag@ZX-V at 150°C under visible light and at 200°C in dark (Fig. S11). The catalyst shows a good SCR activity under both conditions, with however higher NO x conversions at 200 °C. When the lamp is turned on at 150 °C (Fig. 11(B)), the concentration of surface ammonium species is progressively consumed while the chemisorbed ammonia on Lewis sites has completely disappeared. Our results show that the NH 3 adsorbed on Lewis acid sites is preferentially consumed during the photo-SCR reaction, then the NH 4 + species linked to the Brønsted acid site (which react slowly) migrate to the Lewis acid sites and participates in the reaction. The Brønsted sites act primarily as NH 3 storage sites during the photo-assisted SCR reaction. The adsorbed NH 3 on Lewis acid site is most likely activated to the NH 2 radical under visible irradiation . The NO x species could react with NH 2 radical formed under visible irradiation, and N 2 would be selectively produced via nitrosoamide species (NH 2 NO 2 and NH 2 NO) (Eq. 5). Shishido et al.

(2011) confirmed the formation of nitrosamide species (NH 2 NO) by FTIR spectroscopy, with characteristic new bands between 1400 and 1600 cm -1 . Participation of nitrosamide species, as alternative intermediate, in the formation of N 2 and H 2 O (Eq. 6) seems to be doubtful in our case, since no indication of such species was observed in our FTIR spectra. Alternatively, we can assume that they were rapidly decomposed during the reaction. However, it is expected that surface NH 2 NO species are the major intermediates participating in the formation of N 2 over the Ag-based catalyst [START_REF] Kondratenko | Mechanistic aspects of N 2 O and N 2 formation in NO reduction by NH 3 over Ag/Al 2 O 3 : The effect of O 2 and H 2[END_REF]Strom et al., 2018;Shimizu et al,. 2017;[START_REF] Kondratenko | Mechanistic aspects of N 2 O and N 2 formation in NO reduction by NH 3 over Ag/Al 2 O 3 : The effect of O 2 and H 2[END_REF].

NH 2 • + NO (NO 2 ) NH 2 NO (NH 2 NO 2 ) (Eq. 5)

NH 2 NO → N 2 + H 2 O (T>150°C) (Eq. 6)

Based on this information and our previous results, the mechanism of the oxidation of NH 3 and reduction of NO, at room temperature and 150 °C, respectively, can be proposed as follow: at room temperature, the excitation of the silver particle promotes the formation of singlet oxygen, this later oxidize NH 3 first into NH 2 and then to NO. The oxidation of NO by singlet oxygen forming NO 2 can also take place. H + can also be produced from this oxidation process to form the NH 4 + species detected on the surface of the catalyst. At 150 °C, the produced NH 2 under visible light can interact with NO and/or NO 2 to form NH 2 NO and NH 2 NO 2, respectively. The dissociation of these intermediates to N 2 and H 2 O is promoted by the local heating and traduced by a photo-asssisted SCR process. The mechanism of the reaction is illustrated in scheme 1.

Scheme 1.

Conclusions

In summary, sub-nanometer silver clusters in FAU-type zeolite containing vanadate clusters were synthesized and evaluated during the photo-assisted SCR-NH 3 reaction under visible light at RT and 150 °C. The Ag-free sample was completely inactive during the photo-assisted SCR-reaction under studied conditions.,In contrast, Ag@ZX-V revealed a photocatalytic activity under visible light promoted by the high dispersion and the good accessibility of the generated silver nanoparticles. The operando FTIR technique revealed the in-situ formation of Bronsted acid sites on Ag@ZX-V catalyst which play the role of storing and supplying the NH 3 species to the active sites. Further the ammonia activation by singlet oxygen to possibly yield NH 2 radicals is most probably a result of the direct sensitization by Ag n + clusters/nanoparticles under visible irradiation. At room temperature, the singlet oxygen can further oxidize NH 2 radical to NO as well as NO to NO 2 . However, the main reaction pathway leading to N 2 is the thermal decomposition of NH 2 NO intermediates, formed from NO and adsorbed NH 2 fragments. This SCR pathway is probably promoted by the local plasmonic heating of the catalyst at 150 °C and under visible light. However, further experiments are required to investigate the reaction intermediates that are not detected in the present study, likely due to their short lifetime.

  in the supplementary information) show the following chemical formula Na 82 Al 73 Si 119 O 384 (H 2 O) 139 . A drastic change of the relative intensities of the diffraction peaks can be clearly observed after loading Ag into the ZX-V structure. This is mainly related to the change of the cationic composition between the two Faujasites. The deduced chemical formula of the Ag@ZX-Vis Ag 27 Na 55 Al 73 Si 119 O 384 (H 2 O) 120 with the stoichiometric sum of the Ag n + and Na + cations equal to 82, as previously observed for ZX-

  (B)) likely due to different silver clusters inside the zeolite. As the equilibrium pressure increases (Fig. 4(B)), another component appears at ~2135 cm -1 , which is characteristic of physisorbed CO on Ag n + .

  Figure 4.

  Figure 5.

Figure 5

 5 Figure 5 illustrates the activity of both catalysts at RT temperature under visible light.

Figure 8 .

 8 Figure 8.

  support and hydrolyzing the M-O bonds (M = Al or Ag), thus leading to the co-existence of Lewis and Brønsted acid sites. The formation of new Brønsted acid sites by hydroxylation induces the transformation of adsorbed NH 3 into ammonium species. Temperature and presence/absence of visible light conditions modulate the Lewis/Brønsted ratio up to the eventual total solvation of the zeolite by molecular adsorption of water. At RT and in the dark, the water molecules might strongly adsorb and fully occupy the adsorption sites of the catalyst. This can hinder the access of NH 3 molecule to the zeolite and partially inhibits the formation of NH 4 + on the Brønsted acid sites. Under visible irradiation and/or by increasing the temperature, the formation of ammonium species is promoted by partial desorption of water. On the other hand, NH 3 can also be linked to the free Ag Lewis sites. The presence of both adsorbed NH 3 and NH 4 + is indispensable for the photo-SCR reaction.

Acknowledgment

Authors acknowledge the PHC-Tassili project (17MDU983) and the Normady Region (RHIN, RAPHYD project) for supporting this work.

14.7 ±5 % (under visible light). It is to highlight that during the SCR experiments, no side products as N 2 O were detected in the outlet, witnessing for 100 % selectivity to N 2 . The CO and HC conversions were also investigated during the photo-assisted SCR reaction at RT and T = 150 °C (supplementary data, Fig. S7). All conversions are negligible over both ZX-V and Ag@ZX-V samples under the different conditions used in this work.

Figure 6.

Evolution of surface species during the photo-SCR

The FTIR spectra of the catalysts at the steady-state of the SCR-NH 3 reaction are presented in Fig. 7.

Figure 7.

In the absence of irradiations at RT (spectra a, Fig. 7), a band centered at 1642 cm -1 , characteristic of NH 3 coordinated to Lewis acid sites [START_REF] Rasmussen | Transient operando study on the NH3 /NH 4 + interplay in V-SCR monolithic catalysts[END_REF][START_REF] Hadjiivanov | Selective Catalytic Reduction of NO x over Cu-and Feexchanged zeolites and their mechanical mixture[END_REF], is observed for both samples. It can be strongly overlapped with the vibration band of adsorbed water. It is important to note that this band is more intense (saturated) and pronounced in case of ZX-V than for Ag@ZX-V. In addition, traces of band of ammonium cations can be observed, associated with the weak band at 1442 cm -1 (spectra a, Fig. 7), due to the presence of silica impurities that provide weak Brønsted acid sites [START_REF] Gallardo Amores | An FT-IR study of ammonia adsorption and oxidation over anatase-supported metal oxides[END_REF].

Such silica phase (quartz) was detected in the X-ray diffraction patterns of ZX-V and Ag@ZX-V (see part 2.1.). No significant changes were observed in the IR spectra of ZX-V