Minh Nguyen-Quang

Federico Azzolina-Jury
email: federico.azzolina-jury@ensicaen.fr

Frédéric Thibault-Starzyk

Arnaud Travert

Magdalena Ziąbka

Bogdan Samojeden

Monika Motak

Patrick Da Costa

Federico Azzolina

Unveiling the potential of surfactant Pluronic-P123 application during the synthesis of Ni-hydrotalcite-derived catalysts for low-temperature CO2 methanation: A novel approach

Keywords: low-temperature CO2 methanation, surfactant Pluronic P123, xP-NMA, HCO-intermediates, in-situ, Operando FTIR

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Nowadays, severe environmental challenges such as global warming caused by the accumulation of the greenhouse gas CO2 in the biosphere due to the world population expansion and massive energy consumption are becoming urgent. The CO2 concentration in the air reached 418 ppm of threshold during December 2022 [1] and gestured a nonstoppable rise. It is thus indispensable to create breakthrough sustainable-green technologies to convert and ideally utilize CO2 for long-term modern life evolution. Many efforts to lower CO2 concentration in the biosphere have been engaged for directly capturing carbon dioxide from the atmosphere or the flue gas of fossil-fuel power plants and industrial processes [START_REF] D'alessandro | Carbon Dioxide Capture: Prospects for New Materials[END_REF][START_REF] Nema | An overview of global climate changing in current scenario and mitigation action[END_REF].

Carbon Capture and Storage processes (CCS) have been developed to capture and store CO2 [START_REF] Nema | An overview of global climate changing in current scenario and mitigation action[END_REF][START_REF] Xiaoding | Mitigation of CO2 by Chemical Conversion: Plausible Chemical Reactions and Promising Products[END_REF]. CSS, however, can be hard to implement on an industrial scale for a number of reasons, including cost, transportation owing to the distance from locations, and the lack of knowledge in terms of long-term environmental safety [START_REF] Centi | Opportunities and prospects in the chemical recycling of carbon dioxide to fuels[END_REF]. One way to overcome this barrier would be CO2 utilization, namely Carbon Capture and Utilization (CCU) [START_REF] Gabrielli | The Role of Carbon Capture and Utilization, Carbon Capture and Storage, and Biomass to Enable a Net-Zero-CO2 Emissions Chemical Industry[END_REF][START_REF]Carbon capture, utilisation and storage[END_REF], a family of processes in which CO2 extracted from human-induced processes can be converted into a variety of valuable-added products with the addition of a secondary substance and energy from renewable sources. Currently, carbon dioxide utilization following the CCU approach can be listed as a chemical feedstock or intermediate in just a few processes, including the manufacturing of urea, salicylic acid as a medicinal component, and polycarbonates.

CO2 conversion into fuels such as methane or methanol has recently drawn much attention [START_REF] Song | Global challenges and strategies for control, conversion and utilization of CO2 for sustainable development involving energy, catalysis, adsorption and chemical processing[END_REF]. The Sabatier reaction [START_REF] Müller | Sabatier based CO2-methanation of Flue Gas Emitted by Conventional Power Plants[END_REF] (Eq. 1), also known as the methanation of CO2, is a promising strategy for converting CO2 into useful substances [START_REF] Götz | Renewable Power-to-Gas: A technological and economic review[END_REF][START_REF] Götz | State of the Art and Perspectives of CO2 Methanation Process Concepts for Power-to-Gas Applications[END_REF]. The National Aeronautics and Space Administration (NASA) has launched a significant effort in CO2 methanation applications in spaceflight habitation on Mars, in which earth hydrogen is sent to Mars, allowing carbon dioxide in the Martian atmosphere to be converted into CH4 and H2O for energy and astronaut-life infrastructures, respectively [START_REF] Van Der Wiel | Carbon Dioxide Conversions in Microreactors, Conference: 4th International Conference on Microreaction Technology[END_REF].

CO2 methanation is the reaction in which carbon dioxide and hydrogen are used as reactants in the catalytic generation of methane and water at temperatures ranging from 250°C to 550°C and pressure between 1 and 100 bar [START_REF] Wei | Methanation of carbon dioxide: an overview[END_REF]. The reaction is thermodynamically favorable, with a negative Gibbs free energy (∆G298K = -130.8 kJ/mol); nevertheless, it is highly exothermic; thus, a complete reduction of carbon dioxide to methane is favored by low temperature and high pressure. Under mild reaction conditions, the equilibrium shifts towards the product side, according to Le Chatelier's principle [START_REF] Wei | Methanation of carbon dioxide: an overview[END_REF][START_REF] Aziz | CO2 methanation over heterogeneous catalysts: recent progress and future prospects[END_REF].

Because the methanation reaction is an eight-electron process (the carbon oxidation number changing from +4 in CO2 to -4 in CH4) with considerable kinetic limitations, the requirement of a catalyst is mandatory. The most challenging task is developing a thermally stable catalyst with good activity and selectivity towards methane at moderate temperatures and pressures [START_REF] Su | Catalytic carbon dioxide hydrogenation to methane: A review of recent studies[END_REF]. Various metals, principally based on Fe, Co, Ni, Rh, Ru, and Pt, among others, have been investigated as methanation catalysts, with Rh, Ru, and Ni being the most active for CO2 methanation [START_REF] Garbarino | Methanation of carbon dioxide on Ru/Al2O3 and Ni/Al2O3 catalysts at atmospheric pressure: Catalysts activation, behaviour and stability[END_REF][START_REF] Weatherbee | Hydrogenation of CO2 on group VIII metals: IV. Specific activities and selectivities of silica-supported Co, Fe, and Ru[END_REF][START_REF] Vesselli | Catalytic CO2 Hydrogenation on Nickel: Novel Insight by Chemical Transient Kinetics[END_REF][START_REF] Weatherbee | Hydrogenation of CO2 on group VIII metals: I. Specific activity of NiSiO2[END_REF][START_REF] Rönsch | Review on methanation -From fundamentals to current projects[END_REF]. Nickel is particularly appealing for several reasons, including the fact that it is significantly less expensive than ruthenium [START_REF] Garbarino | Methanation of carbon dioxide on Ru/Al2O3 and Ni/Al2O3 catalysts at atmospheric pressure: Catalysts activation, behaviour and stability[END_REF] and rhodium [START_REF] Beuls | Methanation of CO2: Further insight into the mechanism over Rh/γ-Al2O3 catalyst[END_REF], even though both are highly active and still being investigated.

Furthermore, the availability of Ni in comparison to Ru and Rh is an essential factor in the industrial applications of Ni-based catalysts.

Aside from active metals, support substantially impacts the activity and stability of a catalyst [START_REF] Gac | Effects of support composition on the performance of nickel catalysts in CO2 methanation reaction[END_REF][START_REF] Frontera | Supported Catalysts for CO2 Methanation: A Review[END_REF][START_REF] Martínez | High Selectivity and Stability of Nickel Catalysts for CO2 Methanation: Support Effects[END_REF]. Aluminum oxide, for example, is well-known for its large specific surface area (200-380 m 2 /gcat) [START_REF] Garbarino | Methanation of carbon dioxide on Ru/Al2O3 and Ni/Al2O3 catalysts at atmospheric pressure: Catalysts activation, behaviour and stability[END_REF][START_REF] Russell | Alumina Surface Area Measurements[END_REF], whereas magnesium oxide, despite its moderate surface area, exhibits Lewis basicity -a property that enhances CO2 adsorption capacity [START_REF] Cosimo | Apestegu a, Structure and Surface and Catalytic Properties of Mg-Al Basic Oxides[END_REF] and promotes coking resistance [START_REF] Hu | Enhanced Investigation of CO Methanation over Ni/Al2O3 Catalysts for Synthetic Natural Gas Production[END_REF]. Designing Ni-based CO2 methanation catalysts with improved low-temperature activity is, thereby, a continuing endeavor. Studies have been carried out with different supports and promoters, aiming to assist support-metal interactions, enhance the number and type of acidic and basic sites, tailor the particle size and dispersion to avoid sintering of Ni on the support [START_REF] Pandey | Effect of support on the catalytic activity of supported Ni-Fe catalysts for the CO2 methanation reaction[END_REF][START_REF] Park | A highly dispersed Pd-Mg/SiO2 catalyst active for methanation of CO2[END_REF][START_REF] Pan | Insight into the reaction route of CO2 methanation: Promotion effect of medium basic sites[END_REF][START_REF] Sholeha | Enhanced CO2 methanation at mild temperature on Ni/zeolite from kaolin: effect of metal-support interface[END_REF] or understand and mitigate the deactivation [START_REF] Lorito | Understanding deactivation processes during bio-syngas methanation: DRIFTS and SSITKA experiments and kinetic modeling over Ni/Al2O3 catalysts[END_REF]. Some could be listed, such as zeolites [START_REF] Sholeha | Enhanced CO2 methanation at mild temperature on Ni/zeolite from kaolin: effect of metal-support interface[END_REF], La2O3 [START_REF] Tang | Highly Loaded Mesoporous Ni-La2O3 Catalyst Prepared by Colloidal Solution Combustion Method for CO2 Methanation[END_REF], CeO2, Al2O3, TiO2, MgO [START_REF] Tada | Ni/CeO2 catalysts with high CO2 methanation activity and high CH4 selectivity at low temperatures[END_REF][START_REF] Razzaq | Catalytic Methanation of CO and CO2 in Coke Oven Gas over Ni-Co/ZrO2-CeO2[END_REF], ZrO2 [START_REF] Jia | Structural effect of Ni/ZrO2 catalyst on CO2 methanation with enhanced activity[END_REF], MOF [START_REF] Zhen | Enhancing catalytic activity and stability for CO2 methanation on Ni@MOF-5 via control of active species dispersion[END_REF], CNT [START_REF] Feng | A Study of CO2 Methanation over Ni-Based Catalysts Supported by CNTs with Various Textural Characteristics[END_REF], ... Hydrotalcite-derived catalysts containing transition metals have lately stimulated the interest of researchers due to their (1) CO2 adsorption capacity; (2) brucite layer cationexchange ability; (3) interlayer space anion-exchange ability; and (4) adjustable surface basicity [START_REF] Nishimura | Characterization, synthesis and catalysis of hydrotalcite-related materials for highly efficient materials transformations[END_REF]. After calcination, hydrotalcite is converted into mixed oxides, which possess basic properties that facilitate CO2 transformation [START_REF] Pan | Insight into the reaction route of CO2 methanation: Promotion effect of medium basic sites[END_REF][START_REF] Zhao | Hydrogenation of CO2 to Formate with H2: Transition Metal Free Catalyst Based on a Lewis Pair[END_REF]. Upon H2-reduction, active components with extensively dispersed metallic particles are frequently formed [START_REF] Zhao | Hydrogenation of CO2 to Formate with H2: Transition Metal Free Catalyst Based on a Lewis Pair[END_REF][START_REF] Li | Ni-based catalysts derived from layered-doublehydroxide nanosheets for efficient photothermal CO2 reduction under flow-type system[END_REF][START_REF] Feng | Supported catalysts based on layered double hydroxides for catalytic oxidation and hydrogenation: general functionality and promising application prospects[END_REF][START_REF] Wang | Recent Advances in the Synthesis and Application of Layered Double Hydroxide (LDH) Nanosheets[END_REF].

Furthermore, the versatility, simplicity, and low cost of this material are key advantages of its application [START_REF] Vaccari | Preparation and catalytic properties of cationic and anionic clays[END_REF].

Hydrotalcite-like compounds (HLCs) are characterized by the general formula (Eq. 2)

[45]:

[ Where M 2+ and M 3+ are different divalent and trivalent metal ions with representing the layer, and indicating the interlayer composition. The interlayer anion, A, and the M 3+ /(M 2+ +M 3+ ) ratio, x, can vary between 0.20 and 0.40 [START_REF] Cavani | Hydrotalcite-type anionic clays: Preparation, properties and applications[END_REF][START_REF] Forano | Chapter 14.1 -Layered Double Hydroxides (LDH)[END_REF]. Only for the values of x between 0.2-0.33 crystalline hydrotalcitelike compounds can be obtained, with the value of x in natural compounds is generally 0.25. The kind of anion species that can be incorporated into the LDH structure has no practical limitations, that could be either mono-or di-valent anions; however, to synthesize pure or well-crystalline materials, it is a challenge to avoid contamination by CO2 in the aqueous solution when synthesizing hydrotalcite-like compounds (HTlcs) with anions other than carbonate, for example. Moreover, incorporating interlayer anions into the HTlc is also a significant aspect. Shigeo Miyata et al. [START_REF] Miyata | Anion-Exchange Properties of Hydrotalcite-Like Compounds[END_REF] have established the affinity of interlayer anions as follows: C >>S >>O > > > >N > . Concerning synthesis routes, coprecipitation-based strategies at constant pH have been discovered to be the most often employed approach to this problem; where a solution comprising divalent and trivalent cation precursors is introduced to an alkaline mixture containing the anion, which will be intercalated in a certain-pH alkaline medium.

So far, to control such characteristic features of the Ni-HT, synthetic approaches have mainly dealt with incorporating a second metal, leading to bi-metallic catalysts [START_REF] Asmelash | Hydrotalcite based Ni-Fe/(Mg, Al)O:X catalysts for CO2 methanation-tailoring Fe content for improved CO dissociation, basicity, and particle size[END_REF][START_REF] Chatla | Highly stable and coke-resistant Zn-modified Ni-Mg-Al hydrotalcite derived catalyst for dry reforming of methane: Synergistic effect of Ni and Zn[END_REF][START_REF] Summa | Co-Precipitated Ni-Mg-Al Hydrotalcite-Derived Catalyst Promoted with Vanadium for CO2 Methanation[END_REF][START_REF] Tsiotsias | Bimetallic Ni-Based Catalysts for CO2 Methanation: A Review[END_REF].

Promoters, such as Cu [START_REF] Summa | Investigation of Cu promotion effect on hydrotalcite-based nickel catalyst for CO2 methanation[END_REF], Y [START_REF] Sun | On the effect of yttrium promotion on Ni-layered double hydroxides-derived catalysts for hydrogenation of CO2 to methane[END_REF], La [START_REF] Wierzbicki | Novel Ni-La-hydrotalcite derived catalysts for CO2 methanation[END_REF], Fe [START_REF] Yin | Insight into the role of Fe on catalytic performance over the hydrotalcite-derived Ni-based catalysts for CO2 methanation reaction[END_REF], Ru [START_REF] Martins | CO2 Methanation over Hydrotalcite-Derived Nickel/Ruthenium and Supported Ruthenium Catalysts[END_REF], and V [START_REF] Świrk | anadium promoted Ni(Mg,Al)O hydrotalcite-derived catalysts for CO2 methanation[END_REF], have been demonstrated as highly effective Ni-based catalysts towards CO2 methanation. However, in addition to the support materials' chemical bulk characteristics, modifying the morphology and metals-HT framework interaction are critical factors but less studied. In this regard, our work has been dedicated to changing the properties of HT materials in terms of the surface, basicity, and stability in one-step synthesis.

On the other hand, in many contexts, SBA-15-supported catalysts exhibit several superior characteristics that aid chemical conversion, particularly a large surface area, highly uniform and orderly pore, and a very high stability due to the peculiarity of structural template Pluronic P123 [START_REF] Wang | Synthesis and characteristics of SBA-15 with thick pore wall and high hydrothermal stability[END_REF]. However, in the present case, because of the basicity requirement for CO2 methanation, the acidic-surface hydroxyl groups limit the application of such support for this reaction [START_REF] Hongmanorom | Enhanced performance and selectivity of CO2 methanation over phyllosilicate structure derived Ni-Mg/SBA-15 catalysts[END_REF]. Still, the preparation method of SBA-15a hydrothermal synthesis with the triblock copolymer Pluronic P123 as a templating agent can be a source of inspiration for developing mesoporous-basic supports.

Figure 1. P123 structure and self-assembling mechanism of P123 in water

The Pluronic structure (P123) includes hydrophilic and hydrophobic heads, which will self-assemble to generate channels in the material. After thermal treatment, the structural template was removed, leaving porosity in the catalyst. Figure 1 depicts the selfassembling mechanism of P123 in water.

Following this line and for the first time, the present study used the Pluronic P123 to alter NiMgAl-hydrotalcite-derived catalysts. The structural template allows the basic properties of hydrotalcite material to be preserved while the surface area, the stability, on the other hand, may most undoubtedly be enhanced. NiMgAl-mixed-oxide derived from C hydrotalcite with x = 0.25 was initially synthesized as the parent catalyst, using the pH-constant technique in implementation. P123 was added in 1wt.%, 3wt.%, and 5wt.% as a modifying agent. All catalysts were tested under CO2 methanation from 250 to 450°C.

Experimental

Development of the novel synthesis

In order to synthesize mixed-oxides derived from hydrotalcite catalysts, the coprecipitation method was followed, in which an aqueous solution of metallic cations (solution A) was introduced dropwise into a basic carbonate solution (solution B) at 65°C

with stirring, as described elsewhere [START_REF] He | Unique catalysis of Ni-Al hydrotalcite derived catalyst in CO2 methanation: cooperative effect between Ni nanoparticles and a basic support[END_REF][START_REF] Cavani | Hydrotalcite-type anionic clays: Preparation[END_REF]. As described earlier [START_REF] He | Unique catalysis of Ni-Al hydrotalcite derived catalyst in CO2 methanation: cooperative effect between Ni nanoparticles and a basic support[END_REF][START_REF] Cavani | Hydrotalcite-type anionic clays: Preparation[END_REF], solution A was dropped into solution B in a beaker while vigorously stirring, keeping the temperature constant at 65°C. Following the coprecipitation step, the residual slurry was aged for 1 hour under the same conditions, keeping the pH unchanged. The obtained solid from vacuum filtering and washing with deionized water (65-80°C) till pH = 7, was then dried overnight at 80°C and followed the thermal treatment at 550°C for 5 hours with a 5°C/min heating ramp. After the calcination step, the catalysts containing NiMgAl were transformed into mixed-oxides form and denoted as NMA, 1P-NMA, 3P-NMA, and 5P-NMA.

Characterizations of the catalysts

Physicochemical characterizations

All catalysts were characterized via XRD, SEM-TEM, BET, H2-TPR and CO2-TPD, and TG techniques. Detailed information can be found in Supplementary notes.

IR characterization of CO2 Sorption (in-situ measurements)

In-situ CO2 adsorption and desorption experiments under static conditions at working temperature (250°C-450°C, 50°C/step) were carried out to first identify the possible surface species correlated to basic sites from CO2-TPD measurements. Secondly, provide a clearer view of the stability of species formed on the catalyst surface. Spectra were collected by a Nicolet iS-5 spectrophotometer with a mercury-cadmium-telluride (MCT) detector, in the 4000-400 cm -1 range, at a resolution of 2 cm -1 and using 64 scans.

The self-supporting wafer (ca. 20 mg, 18 mm in diameter) was initially activated at 450⁰ C under flowing H2/Ar (5% vol./vol.), then evacuated in Ar. The system is equipped with an automatic mechanism PELICAEN (the scheme and device can be found in Supplementary Figure 1), that automatically lifts and lowers the catalyst position to the IR-beam zone. The CO2 sorption protocol can be described as follows: first, the catalyst wafer was exposed to a pure CO2 gas at static conditions, 10 torr, from room temperature to working temperatures (remained 10 min at each temperature) for adsorption experiments.

After finishing the adsorption experiment at 450°C, the system was vacuuming flushed in order to remove weakly adsorbed species, and the temperature was lowered to 250°C before the desorption experiments. Spectra were collected after 10 min at each temperature for adsorption and desorption experiments. Before each measurement, the background was collected and used to subtract the absorbance from the other spectra.

Catalytic evaluation in CO2 methanation as a function of temperature and Time

On Stream (TOS run)

The catalytic activity was examined in a fixed-bed U-shaped reactor with a K-type thermocouple at temperatures ranging from 250°C to 450°C. Two layers of quartz wool were installed to cover the catalyst.

Before the reaction, the catalyst was reduced in-situ at 900℃ for 1 hour at a flow rate of 100 ml/min with a mixture of H2 and Ar (5% vol./vol.). After the temperature had been decreased to 150℃, the gas was switched to the reactant flow, with H2/CO2/Ar = 12/3/5 in a total flow rate of 100 ml/min controlled by 3 mass flow controllers (H2/CO2 = 4).

Catalytic performances were measured from 250°C to 450°C in steps of 50°C, with the steady-state kept for 30 minutes at each temperature and a ramp at 10°C/min between each step. In our investigations, the mass of the catalyst was calculated based on its own density in order to achieve 12,000 h -1 of gas hourly space velocity (GHSV).

A gas chromatograph (490Varian Micro-GC) with a thermal conductivity detector was used to analyze the outlet. The catalytic activity was evaluated via CO2 conversion and CH4 selectivity, which were defined as follows (Eq. 3 and 4):

where in-and out-terms correspond to the input and the output, and n-is the molar concentration of CO2, CH4, and CO measured by GC, respectively. F is represented to the total flow rate. Assuming that CH4 and CO are the main-, and by-products of CO2 methanation, Eq. 4 could be used to calculate the CH4 selectivity.

Site-Time-Yield [START_REF] Janz | Space-Time Yield and Reaction Rate[END_REF] of CH4 (Eq. 5) was monitored, with nCH4 being the flow of CH4 in the outlet [mmol.h -1 ] and mcat is the sample weight (gcat) [START_REF] Boudart | Surface, catalytic and magnetic properties of small iron particles: I. Preparation and characterization of samples[END_REF], respectively.

The definition of the Site-time-yield can be stated as the rate of the catalytic reaction over the total number of accessible sites present in the reactor.

The stability of catalysts was examined by keeping track of the conversion and selectivity evolution by time-on-stream run over a considerable period of 16 hours at 300°C.

Operando FTIR Experiments

To get a deeper insight into the behavior of the parent catalyst (NMA) and catalyst modified by P123 (3P-NMA), Operando FT-IR CO2 adsorption and methanation measurements were carried out at 300°C.

The Operando setup includes connections, and the analysis methodology was previously described in [START_REF] Lesage | Studying the NO-trap mechanism over a Pt-Rh/Ba/Al2O3 catalyst by operando FT-IR spectroscopy[END_REF] (the system layout are presented in Supplementary Figure 2), where the measurements were performed in transmission mode with a Nicolet 6700 FT-IR spectrometer coupled with an MCT detector). FTIR spectra with a resolution of 4 cm -1

were acquired using 64 scans of each spectrum. The catalysts were positioned in the reactor cell between two KBr windows in self-supporting wafers of 20 mg (the same as insitu CO2 sorption measurements). In this scenario, the system setup allows for distinct diagnostics of gas-phase and surface evolutions by switching between two-mode measurements.

Following the same reduction procedure as the reaction, catalysts were reduced at 900°C. In order to remove contaminates and weakly pre-adsorbed species surely, the wafer was reduced again at 450°C for 1 hour in the flow of H2/Ar, 10 ml/min (5% vol./vol.), and Ar purged the system prior to measurements.

CO2 adsorption: CO2/Ar (10% vol./vol.) total flowrate of 10 ml/min, CO2 adsorption was kept for 30 min at 300°C. After that, the flow was switched to 10 ml/min of Ar to flush the weakly adsorbed species on the catalyst surface before methanation.

CO2 methanation: H2/CO2/Ar = 4/1/5 ml/min at a total flowrate of 10 ml/min; methanation was carried out for 30 min at 300°C.

In both cases of in-situ and Operando experiments, spectra were collected and treated using OMNIC ® software.

Spent catalyst evaluations

In order to evaluate the catalysts after the reaction, wide Angle X-ray Diffraction, Thermogravimetric analysis, and Scanning electron microscopy-energy dispersive X-ray spectrometry were carried out. The configurations and program for measuring can be revised in Supplementary notes.

Results and discussion

Structural properties of NMA catalysts

Textural properties and morphology of catalysts

Micrographs of the P123-NMA series and element mapping on NMA and 3P-NMA are shown in Figure 2. As can be observed, all catalysts were formed as nanoparticle aggregates [START_REF] Wiyantoko | Synthesis and Characterization of Hydrotalcite at Different Mg/Al Molar Ratios[END_REF]. The parent NMA generally formed particles with irregular grains (Figure 2, top-left), which agreed with observed results in earlier studies [START_REF] Nguyen-Quang | On the influence of the preparation routes of NiMgAl-mixed oxides derived from hydrotalcite on their CO2 methanation catalytic activities[END_REF][START_REF] Török | Mixed Oxides Without Added Noble Metals Derived from Layered Double Hydroxides of the Hydrotalcite Type in the Hydrodechlorination Reaction of Trichloroethylene[END_REF][START_REF] Taherian | Anti-coking freezedried NiMgAl catalysts for dry and steam reforming of methane[END_REF]. The particles of mixed oxides with incorporated P123 had a more irregular appearance, with smaller particle sizes. Amazingly, while NMA displayed a solid structure, all catalysts prepared with P123 demonstrated soft, porous/spongy-like structures. The element mapping, furthermore, indicated a homogeneous distribution of Ni (green), Mg (blue), and Al (red) on the entire NMA and 3P-NMA catalysts after reduction. Additionally, EDX analysis (Supplementary Table 1) revealed an increase in oxygen content concurrent with a decrease in Ni concentration in all catalysts prepared with P123.

These observations visually suggested a total change in the morphology of materials modified by P123 that has never been reported before in the literature.

Additionally, in order to clarify the critical influences of P123 on catalyst structure, TEM measurements were carried out for the reduced NMA and 3P-NMA catalysts. The catalysts were reduced under the same conditions as the reaction, TEM on the reduced catalysts, therefore, can offer information on the active form of the catalysts. Interestingly, the changes in the P123-modified catalyst isotherms were clearly seen. More specifically, these isotherms exhibited an H3-hysteresis loop at a higher volume at STP (ml/gcat) than the parent NMA, which displayed an H1-type hysteresis loop. This latter result, together with the changes in morphological structure revealed by SEM and TEM, confirmed that surfactant P123 completely changed the textural properties of the hydrotalcite materials. As discussed, the reorientation of HT layering by P123 was the reason why all catalysts containing P123 became more layered compared to the parent NMA.

The measured mean pore diameter (Table 1) clearly showed a pore enlargement after modification by P123. As its value for NMA was about 4.6 nm, while for 1P-NMA, 3P-NMA, and 5P-NMA, the values were ca. 5.1 nm, 8.0 nm, and 10.7 nm, respectively. The specific surface area (SSA, calculated from the BET method) and pore volume (from the BJH method) are summarized in Table 1. In terms of texture and morphology, the preliminary effects of employing P123 during NMA synthesis were obvious. P123 not only rearranged the catalyst layers but also generated the hexagonal channel and expanded the pore without affecting the inherent hydrotalcite properties. Mean pore diameter (rp).

Table 1 showed that the surface area increased from 165 m 2 /g (NMA) to 177 m 2 /g and 178 m 2 /g, respectively, once 1wt% and 3wt%. P123 was introduced. It is worth noticing that the surface areas of 1P-NMA and 3P-NMA were identical, even though adding 5wt% P123 lowered the total surface area to 152 m 2 /g. Since this surface area was smaller (8% less) than that of parent NMA, it is reasonable to assume that adding 3wt% was the maximum amount that could drastically improve HT surface area. A larger quantity might likely overstretch the pores, causing displacement of the HT structure. Additionally, adding 5 %wt. P123 caused the increase in the surface NiO particle size from 2.55 nm (NMA) to 4.49 nm of 5P-NMA, which was nearly double the value (this will be addressed in the next section 3.1.3. Crystallinity state of catalysts). These factors together logically contributed to the partial blocking of the smaller pore, resulting in a loss in SSA. The surface area in the 5P-NMA catalyst was possibly contributed by the larger pores. It explained why 5P-NMA had a larger pore size (consequently, a higher pore volume) yet a lower SSA.

Crystallinity state of catalysts

Figure 5 shows the diffractograms of calcined at 550°C and non-calcined samples.

There were some noticeable changes in XRD profiles between calcined and dried samples; however, distinctive peaks indicating the presence of structures, that had previously been documented for hydrotalcite materials [START_REF] Forano | Chapter 14.1 -Layered Double Hydroxides (LDH)[END_REF], were confirmed.

For dried samples (represented by the dashed lines), the basal (003), (006), and (009) planes reflected narrow, symmetric, and intense reflections at lower 2θ angle regions, while the non-basal (015), (018) (JCPDF file 00-15-0087) planes had broader, more minor reflections at higher angles [START_REF] Wiyantoko | Synthesis and Characterization of Hydrotalcite at Different Mg/Al Molar Ratios[END_REF][START_REF] Świrk | Effect of low loading of yttrium on Ni-based layered double hydroxides in CO2 reforming of CH4[END_REF][START_REF] Świrk | a Costa, ry reforming of methane over Zr-and Y-modified Ni/Mg/Al double-layered hydroxides[END_REF]. These diffraction patterns indicated the good crystallinity of the prepared catalysts. Al 3+ in the brucite-like layers [START_REF] Rodrigues | Influence of Ni content on physico-chemical characteristics of Ni, Mg, Al-Hydrotalcite like compounds[END_REF].

Impact of the preparation on the bulk parameters of the hydrotalcite-based catalysts was evaluated via lattice parameters a and c (Supplementary Eq. 2), and the values are reported in Table 2. A deeper look at the temperature range of 100-550°C indicated that the rapid reduction of NiO species was linked to the first two peaks that emerged at around 185°C-205°C and ca. 294°C. These low-temperature peaks accounted for most reductions of the isolated NiO [START_REF] Perez-Lopez | Effect of composition and thermal pretreatment on properties of Ni-Mg-Al catalysts for CO2 reforming of methane[END_REF] and NiO non-uniformly dispersed on the support surface. When compared to NMA, only the first peak at ca. 200°C was visible, indicating that more easily reduced NiO species had been produced by adding P123. Nevertheless, the reduction peaks of 5P-NMA were hardly seen in this temperature range. The more intense peaks that appeared at a maximum temperature of around 396°C and 506°C were due to a reduction in free-bulk NiO/well-dispersed NiO and bulk NiO weakly interacting with the Mg(Al)O phase [START_REF] Tichit | Activation under oxidizing and reducing atmospheres of Ni-containing layered double hydroxides[END_REF], respectively.

The addition of P123 enabled this interaction to be altered, resulting in lower reduction temperatures (maxima at ca. 380°C and 485°C for 1P-NMA 3P-NMA vs. 396°C and 506°C for NMA). This is also linked to a dilution of Ni when P123 was added, as evidenced by the EDX data in Supplementary Table 1, in which it has been reported in the literature [START_REF] Fornasari | Structure and reactivity of high-surface-area Ni/Mg/Al mixed oxides[END_REF] and was thought to be caused by an increase in foreign ions (Mg 2+ and/or primarily Al 3+ ) within the NiO phase. A high-intensity reduction peak at 379°C together with a broad base at a high-temperature range for the 3P-NMA profile demonstrated that the NiO species of this catalyst was easily reduced and uniformly distributed. The corresponding reduction peak of 5P-NMA shifted towards higher temperatures with decreasing intensities and became wider, indicating that the proper P123 concentration was favorable to the generation of small and uniformly distributed-Ni particles (c.f. Table 2).

As a result, the catalytic activity of CO2 methanation can be significantly influenced.

Overall, Eq. 7 could be used to illustrate such reduction in the 250-550°C range:

NiO + H2 = Ni 0 + H2O (Eq. 7)

Figure 6. H2-TPR profiles of xP-NMA series At a higher reduction temperature, the shoulder peak of about 622°C was attributed to the reduction of non-stoichiometric amorphous nickel aluminate [START_REF] Ichikuni | Promoting effect of NiAl2O4 for supported Ni particles on sprayed Ni/Al2O3 catalysts[END_REF] (Eq. 8):

NixAl2O3+x + xH2 = xNi 0 + xH2O + Al2O3 (Eq. 8)
Finally, the reduction of stoichiometric NiAl2O4 species was registered as the highest hydrogen consumption for the most prominent peak observed on the isothermal stage at 700-800°C [START_REF] Hu | Enhanced Investigation of CO Methanation over Ni/Al2O3 Catalysts for Synthetic Natural Gas Production[END_REF][START_REF] Ichikuni | Promoting effect of NiAl2O4 for supported Ni particles on sprayed Ni/Al2O3 catalysts[END_REF]:

NiAl2O4 + H2 = Ni 0 + H2O + Al2O3 (Eq. 9)

Compared to NMA, such band position did not change much for 1P-NMA (763°C vs.

767°C), slightly shifted toward a higher temperature for 3P-NMA (795°C) and shifted backward to a much lower temperature for 5P-NMA (704°C). This showed that adding 3 wt.% P123 to the catalyst potentially enhanced the interaction between NiO and the metal oxide support, while this was not the case with the 5P-NMA catalyst, as seen by the position of the reduction peak, which consumed 99.6% of the hydrogen (Table 3). The results from Table 3 suggested that while adding 5 wt.% P123 has a substantial impact on reduction profiles, it had a minor influence on overall H2 consumption, which ranged from 3.53 to 4.34 mmol/gcat for all catalysts. Herein, we believed that the use of P123 during catalyst synthesis enabled Ni 2+ , Mg 2+ , and Al 3+ to be co-precipitated in different manners, resulting in the of the Ni-matrix interactions. Importantly, adding 1 and 3 %wt. P123 enhanced the incorporation of Ni 2+ into the framework while weakening the reducibility, which was evidenced by the shift of the reduction peaks to higher temperatures and the lower H2 uptake, respectively. The overstretch by adding 5 %wt. P123 made the reconstruction of the catalyst to be inefficient. Furthermore, the interaction of cations found on 5P-NMA mainly to be NiAl2O4 spinel with 99.6% hydrogen consumption; it explained the bigger particle size found by XRD analysis. desorbed from strong-basic sites at higher temperatures, as observed for hydrotalcitederived catalysts via the CO2-bond on the low oxygen co-ordination anion [START_REF] Kalai | The effect of La on the hydrotalcite derived Ni catalysts for dry reforming of methane[END_REF][START_REF] Ębek | Ni-containing Ce-promoted hydrotalcite derived materials as catalysts for methane reforming with carbon dioxide at low temperature -On the effect of basicity[END_REF][START_REF] Wierzbicki | Examination of the influence of La promotion on Ni state in hydrotalcitederived catalysts under CO2 methanation reaction conditions: Operando X-ray absorption and emission spectroscopy investigation[END_REF][START_REF] Corma | Determination of base properties of hydrotalcites: Condensation of benzaldehyde with ethyl acetoacetate[END_REF][START_REF] Prinetto | Synthesis and characterization of sol-gel Mg/Al and Ni/Al layered double hydroxides and comparison with co-precipitated samples[END_REF].

In order to identify the carbonate species-related basicity, infrared spectroscopy examination of the surfaces of the mixed oxides upon CO2 adsorption near room temperatures was executed to further describe the basic sites of the catalysts. Figure 7 shows the FTIR spectra of the previously selected mixed oxides (3P-NMA) after CO2 adsorption at room temperature in the 1900-950 cm -1 range. species. The structure of these species can be reviewed in Figure 7b.

In-situ CO2 sorption measurements at static conditions under working temperatures (250-450°C):

In order to interpret the formation and the thermal stability of carbonate species, the temperature dependence of these bands was investigated by undertaking CO2 adsorption and desorption experiments at static temperatures of 250°C, 300°C, 350°C, 400°C, and 450°C.

The initial presence of carbonate species on the parent NMA catalyst after calcination at 550°C, after reduction at 900°C, and re-reduced at 450°C before the experiment was first determined. The results are plotted in Figure 8, where carbonate species (ranging from 1595 cm -1 to 950 cm -1 ) were found at all conditions, with the most significant number belonging to calcined samples along the absorbed water (broad τ(H2O) stretching band from 3000 cm -1 to 3800 cm -1 .

Figure 8. Infrared spectra of NMA after calcination, reduction at 900℃ and re-reduction at

450℃

After reducing at 900°C (red), absorbed water and carbonate species vastly decreased.

The stretching vibration of OH from water in the interlayer space τ(H2O), which was accountable for a band at 3721 cm -1, was observed. The band at 1470 cm -1 was associated with the hydrogen carbonate species [START_REF] Cosimo | Apestegu a, Structure and Surface and Catalytic Properties of Mg-Al Basic Oxides[END_REF][START_REF] Busca | Infrared spectroscopic identification of species arising from reactive adsorption of carbon oxides on metal oxide surfaces[END_REF] and has vanished by re-reduction at 450°C (blue). In this sense, it is necessary to remove all adsorbed species before CO2 sorption experiments. In the following experiments, therefore, all catalysts were re-reduced at 450℃ before conducting experiments. The CO2 sorption results are reported in Figure 9, with corresponding adsorption behaviors (left) and desorption behaviors (right). Interestingly, in the case of 3P-NMA, those carbonyl band intensities were observed at a much lower intensity. In addition to the bands at 1552 cm -1 and 1410 cm -1 (which shifted from 1393 cm -1 when temperature up to 350°C-green, blue, and pink lines and with higher intensity than NMA), a novel IR band at ca. 1795 cm -1 could be observed. This IR band was ascribed to the polydentate bidentate (νas mode) species, according to [START_REF] Coenen | An in-situ IR study on the adsorption of CO2 and H2O on hydrotalcites[END_REF], while the others belong to symmetric and antisymmetric (COO) vibration of unidentate carbonate, found at slightly different positions with respect to NMA catalyst. The total basicity could be the reason for this different behavior, as CO2-TPD indicated that NMA modified by P123 led to a double amount of basicity. It was, therefore, suggested that preparing NMA with P123 seems to function for CO2 to be directly absorbed on the catalyst surface, resulting in the formation of various carbonate species. While CO2 tends to be firstly dissociated into CO and O2, CO was then adsorbed into NMA surface to generate more carbonyl species.

In terms of desorption behavior -thermal stabilization (Figure 9, right), desorption at 250°C first resulted in the elimination of all carbonyl groups and all hydrogen carbonate species, whereas a proportion of unidentate carbonate remained. Precisely, the collapse of the unidentate species and the drop of the bands at 1685, 1646, and 1221 cm -1 [START_REF] Coenen | An in-situ IR study on the adsorption of CO2 and H2O on hydrotalcites[END_REF] corresponding to unstable hydrogen carbonate species were seen for NMA at the starting temperature of 250°C. Unidentate carbonate (1410 cm -1 and 1530 cm -1 ) [START_REF] Coenen | An in-situ IR study on the adsorption of CO2 and H2O on hydrotalcites[END_REF] persisted at lower intensities with the increase in temperature, and the 1530 cm -1 band [START_REF] Coenen | An in-situ IR study on the adsorption of CO2 and H2O on hydrotalcites[END_REF][START_REF] Ramis | Low-temperature CO2 adsorption on metal oxides: spectroscopic characterization of some weakly adsorbed species[END_REF] (asymmetric COO stretching) vanished at increasing temperatures up to 450°C.

The persistence of unidentate carbonate on 3P-NMA could be noted; nevertheless, the bicarbonate species corresponding to those bands at 1410 cm -1 and 1551 cm -1 (unidentate carbonate [START_REF] Coenen | An in-situ IR study on the adsorption of CO2 and H2O on hydrotalcites[END_REF]) did not disappear at 450°C. Interestingly, the band around 1797 cm -1 [START_REF] Roiaz | Reverse Water-Gas Shift or Sabatier Methanation on Ni(110)? Stable Surface Species at Near-Ambient Pressure[END_REF] corresponding to polydentate species appeared to be stable for all conditions, with lower intensity at the higher temperature. The sum of measurements and correlation between the basic sites and the IR bands are presented for the first time in Table 4. Following the TPD results, the desorption peaks of all P123-containing catalysts showed a shift toward higher temperatures but remained at lower temperatures than for NMA, implying the incorporation of P123 into NiMgAl assisted the formation of weaker basic sites. Furthermore, the CO2-TPD profiles obtained for P123-modified solids could be divided into two groups with similar profiles: (1) NMA, 5P-NMA, and (2) 1P123 and 3P-NMA, the primary difference being greater CO2 desorption from strong basic-sites.

In the literature, weak and medium basic sites were reported to lead to CO2 adsorption and activation [START_REF] Pan | Insight into the reaction route of CO2 methanation: Promotion effect of medium basic sites[END_REF], actively contributing to CO2 methanation. The CO2 molecules would be adsorbed and dissociated on the support surface to form various carbonate species (the formation, identification, and stabilization of carbonate species could be reviewed in the upper part), then combined with the active H generated on the Ni-metallic site to produce the final product methane [START_REF] Pan | Insight into the reaction route of CO2 methanation: Promotion effect of medium basic sites[END_REF][START_REF] Lee | Reaction Mechanism and Catalytic Impact of Ni/CeO2-x Catalyst for Low-Temperature CO2 Methanation[END_REF]. Therefore, alongside validating carbonate species (basicity) on catalysts, the total amount of basic sites was calculated by integrating the CO2-TPD peaks. Data are presented in Table 5. Overall, all catalysts containing P123 exhibited a higher total number of basic sites than the parent NMA. Among the studied catalysts, the 1P123 and 3P123 revealed the highest total number of basic sites of ca. 115-121 µmole/g.cat. On the other hand, it was found to be substantially lower for the 5P-NMA catalyst, dropping to 79 µmole/g.cat. The fact that adding P123 into HT-material could lower the amount of strong-basic site and increase the weak-basic site at the same time while the medium-basic site witnessed around 53-60%. It seems that P123 concentrations mainly affected weak and strong sites' distribution.

The total amount of weak and medium sites calculated can be placed in the order of NMA (47 µmol/g.cat) < 5P-NMA (69 µmol/g.cat) < 1P-NMA (74 µmol/g.cat) < 3P-NMA (95 µmole/g.cat).

Catalytic performance in CO2 methanation

The CO2 conversions (a), CH4 selectivity (b) of the catalysts as a function of the reaction temperature alongside the TOS assessment and TOF values (c) are presented in Figure 10. The CO2 methanation was initiated at 250°C and reached 46% CO2 conversion at 300°C for the non-modified HT-derived catalyst (NMA), with ca. 97% selectivity of CH4.

When the temperature increased to 350°C, the CO2 conversion reached a maximum of 82% maximum, with 99.6% CH4 selectivity. Both CO2 conversion and CH4 selectivity remained as high as 81% and above 97%, respectively, up to 450°C.

Catalysts containing 1 wt%. P123 exhibited better performance. As demonstrated by Figure 10a, CO2 conversion reached 79% at 300℃. This value increased further and peaked at 400°C with a maximum conversion of 86% and 99.7% CH4 selectivity.

In our study, the best performance was observed for the 3P-NMA catalyst with ca.

83.5% and nearly 90% CO2 conversion at 300℃ and 350℃, respectively. Figure 10b also showed a remarkable CH4 selectivity for this catalyst at medium temperature (ca. 99.5%).

However, a slight decrease could be observed for temperatures higher than 450℃, still with a selectivity of around 98%. 6 shows the comparison of 3P-NMA to available data in the literature, with different supports and with the same NMA-type but different ways of modification.

Among all, 3P-NMA seemed to be the best candidate for CO2 methanation in terms of CO2 conversion and CH4 selectivity.

TOS run was performed at 300°C for 3P-NMA (Figure 10c). CO2 conversion and CH4 selectivity are in line with the catalytic test with ca. 81-82% and 99%, respectively. After a 16-hour test, the CH4 selectivity decreased by ca. 1% while CO2 conversion remained constant. This excellent stability of the 3P-NMA catalyst provides considerable potential for industrial applications. Moreover, the STY calculations on all catalysts, mmol/gcat.h, led to the following ranking: STY 5P-NMA (0.3) < STY NMA (3.4) < 1P-NMA (9.6) < 3P-NMA (10.4), which was also in accordance with the catalytic test. The highest value was seen on 3P-NMA catalyst. The result of the stability test and the outstanding performance of 3P-NMA at 300°C allows us to point out that such material is a promising catalyst for methanation at a low temperature.

On the other hand, the 5P-NMA clearly showed inferior conversion in all temperature ranges. One can note that CO2 conversion could be observed at only 60% at 450°C, and its performance was separated from the others (Figure 10a,b). The different behavior may be attributed to the high amount of P123 introduced even though the morphology structure of HT remains intact, as pointed out by the SEM, BET, and XRD results.

Why does the 3P-NMA be the best catalyst system? To understand the better catalytic activity of 3P-NMA, the relationships between CO2 conversion versus catalyst features will be discussed in this following part of this paper. The correlations are presented in Figures 11a andb.

*Calculated from the Scherrer equation at 2θ = 51° on reduced catalysts (Supplementary Table 2). (a)-The catalytic performance in relation to BET surface area and pore size is plotted in Figure 11a to illustrate how structural characteristics impact catalytic performances. It was discovered that adding 1-3wt% resulted in an increase in surface area (from 165 m 2 /g to 177 m 2 /g and 178 m 2 /g, respectively); however, 5P-NMA exhibited a significant decline even when compared to the original NMA. It is required to have a large surface area where the CO2 molecule could be absorbed and dissociated into intermediates that can subsequently be hydrogenated to yield CH4. The catalytic tests from our study were found to be in good agreement with measured surface area, 1P123, 3P-NMA having a greater conversion and selectivity than that of NMA, which possess the largest surface area, while 5 wt% P123 was introduced, a substantial decrease in the surface area could be noted, resulted in low performance. Meanwhile, the pore size of the series had been gradually increased in the order of NMA < 1P-NMA < 3P-NMA < 5P-NMA catalysts. As previously discussed, this is most likely owing to a partial blockage of a smaller pore caused by the high concentration of surfactant P123.

(b)-The influences of Ni dispersion and basicity toward CO2 methanation have been investigated. As shown in Figure 11b, some remarks could be made. In terms of Ni dispersion, an examination of Ni 0 particle size calculated from reduced samples (Supplementary Eq. 1) shows that: 5P-NMA possesses the largest Ni 0 size (LNi0 = 21.08 nm), followed by a smaller size of NMA (LNi0 = 18.75 nm). 1P-NMA and 3P-NMA own relatively small Ni metal particles, more than a half at 8.55 nm and 8.49 nm, respectively. This was demonstrated by TEM results, in which the metallic Ni 0 particle size could be seen at a smaller value (Figure 3, 1B and 2B). Separately, smaller particle size had been reported as a critical parameter to achieve a high catalytic activity toward CO2 methanation [START_REF] Asmelash | Hydrotalcite based Ni-Fe/(Mg, Al)O:X catalysts for CO2 methanation-tailoring Fe content for improved CO dissociation, basicity, and particle size[END_REF][START_REF] Tao | CO2 methanation over NiCe/Al2O3 catalysts: effect of nickel loading and particle size on catalytic performance[END_REF][START_REF] Kopula Kesavan | Nickel supported on YSZ: The effect of Ni particle size on the catalytic activity for CO2 methanation[END_REF]. The results show that the smallest Ni-particle was found on 3P-NMA (see the black line).

Regarding the influence of basicity on catalytic performances, the prevailing opinion was that basic sites facilitate acidic-CO2 activation and restrict carbon deposition on the catalyst surface, hence enhancing activity and stability. Our results revealed that all P123containing catalysts had a higher number of basic sites than non-modified hydrotalcite (blue-continuous line), implying that incorporating P123 into the HT-derived framework will generate more basic sites: NMA < 5P-NMA < 1P-NMA < 3P-NMA. The proportions of the medium site (blue-dashed line) on each catalyst, on the other hand, were influenced by the concentration of P123, even though total basicity ranged between 53 and 60%.

Although NMA had the least total basicity but the highest share of medium sites, 3P-NMA owned the highest number of basicity with roughly the same percentage of medium sites.

As a consequence, the medium site proportions increased in the order of NMA (34 µmol/g.cat) < 5P-NMA (47 µmol/g.cat) < 1P-NMA (61 µmol/g.cat) < 3P-NMA (71 µmol/g.cat).

Overall, 5P-NMA possessed a modest number of medium sites, with surface area typically low along with a large crystallite size; this explained the poor performance compared to the others. 3P-NMA, on the contrary, exhibited the best catalytic activity with the highest surface area, the smallest Ni 0 particle size and the highest amount of mediumbasic site properties (see Table 7), in particular the presence of bidentate species that was found to be relatively huge on 3P-NMA at 250-350°C.

Operando FTIR measurement results at 300°C

With regarding the catalytic activity of all catalysts in the previous section, it was discovered that the best candidate at 300°C is 3P-NMA; hence, Operando FTIR tests were carried out at the same temperature for the parent-NMA and 3P-NMA to further understand the rationale. This section will discuss the Operando FTIR observations of NMA and 3P-NMA in gas-phase and catalyst surface, sequentially.

Gas-phase diagnostic

Conventional Operando FT-IR spectroscopy analysis in the range of 4000-950 cm -1 was implemented to identify the gas-phase compounds during the CO2 hydrogenation reaction. The results of CO2 adsorption and methanation at 300°C are illustrated in Figure 12.

Figure 12. Operando FTIR gas-phase on NMA and 3PNMA catalysts (full range)

Figure 12 depicts the emergence of all bands in the gas phase during CO2 adsorption and CO2 methanation studies (CO2(g), CO(g) [START_REF] Roiaz | Reverse Water-Gas Shift or Sabatier Methanation on Ni(110)? Stable Surface Species at Near-Ambient Pressure[END_REF][START_REF] Thibault-Starzyk | Infrared Spectroscopy[END_REF][START_REF] Haq | Adsorption and decomposition of formic acid on Ni{110}[END_REF][START_REF] Coenen | An in-situ IR study on the adsorption of CO2 and H2O on hydrotalcites[END_REF], H2O(g) [START_REF] Coenen | An in-situ IR study on the adsorption of CO2 and H2O on hydrotalcites[END_REF][START_REF] Hadjiivanov | Chapter Two -Identification and Characterization of Surface Hydroxyl Groups by Infrared Spectroscopy[END_REF], and CH4(g) [START_REF] Zarfl | DRIFTS study of a commercial Ni/γ-Al2O3 CO methanation catalyst[END_REF][START_REF] Völs | Methanation of CO2 and CO by (Ni,Mg,Al)-Hydrotalcite-Derived and Related Catalysts with Varied Magnesium Aluminum Oxide Contents[END_REF]).

CO2 was dissociated into CO in both conditions owing to the presence of Ni as active sites. During methanation monitoring, a CO signal (ground state) was found between 2165 and 2120 cm -1 [START_REF] Coenen | An in-situ IR study on the adsorption of CO2 and H2O on hydrotalcites[END_REF], along with methane [START_REF] Völs | Methanation of CO2 and CO by (Ni,Mg,Al)-Hydrotalcite-Derived and Related Catalysts with Varied Magnesium Aluminum Oxide Contents[END_REF] (3015 cm -1 ) and water generated. The results also showed that such bands were visible at higher intensities on 3P-NMA than on NMA, implying that methane production was stimulated more effectively on 3P-NMA.

Catalyst surface diagnostic

As illustrated in Figure 13 and 14, the subtracted spectra on the surface of catalysts are given in two wavenumber ranges: 4000-2450 cm -1 and 2450-950 cm -1, respectively. All band intensities increased immediately after reactant injection and became stable after ca.

min.

In the range of 4000-2450 cm -1 wavenumbers (Figure 13): The CO2 adsorption spectra on both catalysts differed in appearance and band intensity, as seen in Figure 13.

The identifications of all bands during CO2 adsorption and CO2 methanation were also made [START_REF] Hadjiivanov | Chapter Two -Identification and Characterization of Surface Hydroxyl Groups by Infrared Spectroscopy[END_REF][START_REF] Ito | The vibrational spectra of the formate, acetate, and oxalate ions[END_REF][START_REF] Raskó | Adsorption and surface reactions of acetaldehyde on aluminasupported noble metal catalysts[END_REF][START_REF] Miao | Catalysis mechanisms of CO2 and CO methanation[END_REF][START_REF] Huynh | Promoting effect of Fe on supported Ni catalysts in CO2 methanation by in situ DRIFTS and DFT study[END_REF].  Under methanation conditions, the C-H stretching vibrations of *CH4 were observed at 2995 cm -1 and 3015 cm -1 [START_REF] Völs | Methanation of CO2 and CO by (Ni,Mg,Al)-Hydrotalcite-Derived and Related Catalysts with Varied Magnesium Aluminum Oxide Contents[END_REF][START_REF] Ito | The vibrational spectra of the formate, acetate, and oxalate ions[END_REF], respectively, while the C-H stretching vibration from *HCOO (CHx species) was attributed to small bands at 2897 cm -1 and 2855 cm -1 [START_REF] Ito | The vibrational spectra of the formate, acetate, and oxalate ions[END_REF] was seen on both catalyst surfaces.



Furthermore, the 2735 cm -1 IR band combined with the 1763 cm -1 band might be ascribed to aldehyde hydrogen *HCO species (formyl) [START_REF] Raskó | Adsorption and surface reactions of acetaldehyde on aluminasupported noble metal catalysts[END_REF][START_REF] Miao | Catalysis mechanisms of CO2 and CO methanation[END_REF], which has received less recognition in the literature in terms of IR measurement

In terms of differences in catalytic behaviors, while absorbed H2O-CO2 (∇) was detected at a greater intensity on 3P-NMA with three-adsorbed species, the parent-NMA clearly displayed only two equivalent bands at a lower intensity during CO2 adsorption studies. Besides this, all band intensities were seen at a higher level on 3P-NMA under both adsorption and methanation conditions. Overall, the Operando adsorption and methanation data validated the promotional effect of adding P123 to the NMA catalyst by enhancing adsorption capacity, which coincided with the CO2-TPD results and in-situ CO2 adsorption investigations.

In the 2450-950 cm -1 range (Figure 14): To be able to get insight into species diversification, arrows with distinct meanings were used, with the direction of the arrow symbolizing the increase or collapse of bands, and bolder and thinner arrows suggesting stronger and weaker changes, respectively. The various tints, on the other hand, reflected various species (the blue arrow -formate, methane, and CHx species, the black arrowwater, dark-green arrows -formyl, and the pink arrow -all carbonate species).

But first and foremost, it should be emphasized that the operando measurement was carried out with flow settings, unlike in-situ measurements at static conditions, which led to certain band position shifts. Two bands, for example, were visible in Operando but not in the in-situ analysis (CO(ad) 1992 and 1811 cm -1 ) (cf. Figure 9



The appearance of the bands at 2052 cm -1 , 2026 cm -1 , and 1905 cm -1 corresponded to the carbonyl species at 2086 cm -1 , 2067 cm -1 , and 1903 cm -1 from in-situ CO2 sorption measurements, which were displaced under the flowing situation as discussed.



While hydrogen-carbonate species were thought to originate from the surface-OH (*OH) group, where it could function as weak-basic sites, Di Cosimo et al. [START_REF] Cosimo | Apestegu a, Structure and Surface and Catalytic Properties of Mg-Al Basic Oxides[END_REF] proposed that oxygen from M n+ -O 2-pairs (medium basic site) was interacted with CO2 to generate *CO3 species. Carbonate species would be further hydrogenated on the catalyst surface to produce methane.

Additionally, the emergence of the band at 1763 cm -1 (formyl, HCO - (ad)) during methanation (which was not found during Operando CO2 adsorption) suggests that the mechanism may follow a pathway that involves formyl as an intermediate [START_REF] Miao | Catalysis mechanisms of CO2 and CO methanation[END_REF][START_REF] Akamaru | Density functional theory analysis of methanation reaction of CO2 on Ru nanoparticle supported on TiO2 (101)[END_REF]. The mechanism involving formyl, therefore, could be described as follows: carbonate/hydrogen carbonate  formyl  formate  methane.

Note that carbonate and formate species were observed with considerably higher intensity on 3P-NMA than on NMA. On 3P-NMA, CO2 adsorption and conversion were accelerated, allowing for the formation of formate intermediates as well as an increase in CO2 methanation yield under the same working conditions.

Based on in-situ, operando, and CO2-TPD measurements, we preliminary proposed a mechanism of CO2 methanation on the 3P-NMA catalyst with regard to [START_REF] Miao | Catalysis mechanisms of CO2 and CO methanation[END_REF][START_REF] Akamaru | Density functional theory analysis of methanation reaction of CO2 on Ru nanoparticle supported on TiO2 (101)[END_REF]. CO2

has been considered as a Lewis acid, with carbon atom lingering in the center due to its low electronegativity ( . As a result, the transformation of CO2

necessitates the use of a catalyst having basic sites where it can be absorbed and subsequently transformed (see Figure 15). Hydrogen was spent in four-consecutive phases to hydrogenate such carbonate species to formyl species (detected by Operando FTIR measurements under methanation conditions) and further to CHx and lastly CH4, completing one cycle. One molecule of CO2 and four molecules of H2 were consumed (presented in red) to produce 1 molecule of CH4, and 2H2O molecules (in green) are the total count for one cycle.

Evaluations of spent catalysts

X-ray diffraction results

The WAXD results of all catalysts after the thermal reaction are shown in Figure 16.

As presented, there was no diffraction peak detected at 2θ = 26.6°, which is associated with graphitic carbon formation [START_REF] Han | Uncoupling the size and support effects of Ni catalysts for dry reforming of methane[END_REF]; therefore, we can rule out C-deposition on all of our catalysts.

Figure Diffractograms of spent catalysts.

The reflections confirmed the presence of metallic nickel phase (α-phase) at 2θ ca.

44.5°, 51°, and 76°, corresponding to crystal planes of ( 111), ( 200) and (220), respectively.

Meanwhile, the diffractions at 2θ ca. 36.7°, 43° and 62.5° were observed (β-phase), which were also reported elsewhere on the same NiMgAl-HT materials [START_REF] Świrk | Effect of low loading of yttrium on Ni-based layered double hydroxides in CO2 reforming of CH4[END_REF][START_REF] Świrk | a Costa, ry reforming of methane over Zr-and Y-modified Ni/Mg/Al double-layered hydroxides[END_REF]. They correspond to the mixed oxides obtained after the thermal treatment of double-layered hydroxides.

Interestingly, the intensities of β-phase were sharper and more intense for 5P-NMA, suggesting that the existence of periclase-like structure was more intact in 5P-NMA; in other words, stronger Ni-HT matrix interactions were proved, in accordance with H2-TPR results. The latter showed lower intensities of diffraction peaks and broader bands.

The size of Ni metal crystallites was determined using the Scherrer equation from the 2θ diffraction peak at approx. 51° corresponding to the (200) crystal plane of Ni 0 , data can be found in Supplementary Table 2. Nickel crystallite size decreased from ca. 18.75 nm for the NMA to ca. 8.55 nm and 8.49 nm for 1P-NMA and 3P-NMA due to the promotion of P123. However, the value obtained for 5P-NMA was higher than a non-modified catalyst, i.e., 21.08 nm, respectively. It seems that 5 wt% P123 negatively affected the size of Nimetal size.

The Ni metal particle did not increase after the test, but decreased for all catalysts except 1P-NMA, which showed a minor change. The possibility of re-dispersed 0 occurring after the reaction that resulted in a decrease in Ni 0 after the experiments could be pointed out. In this case, the sintering of active sites after the thermal reaction could be suppressed. Possibly, the sintering of active sites was then re-oxidized by highly active oxygen on the framework of mix-oxides, resulting in the re-dispersion of metals on the surface under operating temperature conditions.

TG measurements

In our results, two regions of weight loss could be seen: the first occurred near room temperature in the flow of synthetic air, which was due to the drying step with about 0.7-1.4% weight loss. The negative peaks could clearly see with a loss of ca. 4.1%, 4.8%, 3.9% and 3.6 wt% corresponding to NMA, 1P-NMA, 3P-NMA and 5P-NMA, respectively, from 180 to 250℃ due to the removal of water in the framework. Besides, the color of catalysts had been turned into light green (precursor NiO color)

during the TG measurement. These revealed that catalysts were re-oxidized during the methanation reaction. In the latter case, the removal rate of water was slower than the reoxidation of Ni in mixed-oxide catalysts. This suggested us the positive peaks obtained at 350-460℃ have mainly belonged to the transformation of Ni into NiO. This was also stated by Dias [START_REF] Dias | Carbon dioxide methanation over Ni-Cu/SiO2 catalysts[END_REF], where the spent catalysts were examined after CO2 methanation was performed over Ni-Cu/SiO2 catalyst. Overall, our catalysts demonstrated a high Cdeposition tolerance.

Conclusions

In this study, hydrotalcite-derived catalysts were modified by a novel method with surfactant Pluronic P123-the structural template of SBA-15: Incorporating P123 to modify the morphology and properties of hydrotalcite-derived materials, instead of adding a second metal -which is currently the most common way. The results turned out to be promising, with excellent stability and selectivity towards CH4 at a lower temperature.

After calcination, P123 was removed, leaving the modification in the structure of catalysts while maintaining the intrinsic characteristics of the catalyst. The SEM images indicated a complete transformation in the morphology of catalysts containing P123 from solid into soft and porous phases. The N2-sorption revealed a change in isotherm plots in which more layers were observed with catalysts containing surfactant; the highest surface area of 178 m 2 /g belonging to 3P-NMA against 165 m 2 /g of non-modified catalyst.

Furthermore, we observed that adding P123 also enlarged the pore size of parent hydrotalcite. Wide-angle X-ray diffraction, together with H2-TPR and CO2-TPD, the structure of HT-derived catalysts was carried out without distortion, the layer structure after the modification has been preserved. The existence of readily and hardly reducible Nikel-species also affected the catalytic performance of the catalysts. It can be said that using P123 during the synthesis step rebuilt the NiMgAlOx catalysts.

In-situ CO2 investigation at working temperatures revealed the presence of unidentate, bidentate (chelating and bridging), and tridentate carbonate on the catalyst surface after CO2 adsorption. The relationship between these species and related weak, medium, and strong basic sites in CO2-TPD was proposed.

By adjusting the chemical portion of P123 at 3wt%, the catalyst displayed a remarkable low-temperature performance in CO2 conversion at 83.5% with almost 99.5%

CH4 selectivity at 300°C. The appropriate combination of high surface area, small Ni 0 particle size, well-dispersed Ni, and a large amount of weak-medium basic site was found on 3P-NMA.

Operando experiments at 300°C demonstrated that 3P-NMA could facilitate CO2 adsorption in the metal-oxide pair more than the generation of surface OH. As a result, the basicity of NiMgAl-oxide was enhanced. Experimentally, it was a strong indication that the mechanism might concern *HCO as a key intermediate following CHx and CH4 formation.

Finally, there is a potential for applying the same methodology with various metals to synthesize a proper catalyst for particular reaction and to scale up the procedure for industrial application in environmental catalysis.

Figure 2 .

 2 Figure 2. SEM images of calcined and reduced catalysts.

Figure 3 .

 3 depicts the acquired results, with (a), (b) present NMA and (c), (d) on 3P-NMA. When (a) and (c) were compared, it was obvious that 3P-NMA had substantially smaller Ni 0 particle sizes. Metallic Ni 0 seemed to be aggregated in the NMA sample rather than well distributed on 3P-NMA, which resulted in the formation of clusters in the NMA catalyst.

Figure 3 .

 3 Figure 3. TEM images of reduced NMA (a, b) and 3P-NMA (c, d).

  c

Figure 5 .

 5 Figure 5. WAXD results of dried (dashed lines) and calcined (continuous lines) catalysts.

*

  Calculated from d-spacing of (110) plane; a = 2.d(110) **Calculated from appropriate d-spacings of (003), (006) and (009) planes: c = d(003) + 2.d(006) + 3.d(009)The a parameter was identical for all materials at 3.05 Å and close to the theoretical values for brucite layers[START_REF] Vaccari | Layered double hydroxides: present and future[END_REF], implying the absence of lattice distortions. Furthermore, in contrast to the NMA material, the obtained c values, which corresponded to the triple thickness between brucite layers in hydrotalcite structure, somewhat decreased for samples prepared with P123. These values indicated the modification of NMA by P123. All calcined catalysts had a related-metallic NiO phase with particle sizes ranging from 2.5 nm in NMA to 4.5 nm in 5P-NMA, suggesting that the Ni particle size in the modified materials was proportional to the P123 content. It is worth noting that smaller nickel particles correlate with higher metal dispersion, and the smaller nickel particles assist in hydrogenating carbon dioxide to methane. Consequently, 1P123 and 3P-NMA could be expected to deliver high catalytic performance.3.1.4. On the reducibility of Ni-speciesThe H2-TPR was investigated to understand better the reducibility of Ni in the xP-NMA catalyst series. Multiple peaks could be seen in the TPR profiles in the temperature range of 100-900°C. Peaks at approximately 200°C, 300°C, 400°C, 500°C, 620°C, as well as a massive reduction peak around 700-800°C could be identified, which corresponded to Ni-species in various phases of interaction with hydrotalcite-derived materials. These reduction peaks might be separated into two groups: one for lower reduction temperatures (100-550°C) and another for higher reduction temperatures (600-800°C).

3.1. 5 .

 5 Effect of P123 concentration on the basic properties of catalysts CO2-temperature programmed desorption had used to explore the basicity of the catalysts further. CO2-TPD profiles of catalysts can be found in Supplementary Figure 3, in which three peaks denoting weak, medium, and strong sites were recognized at 120-150⁰ C, 270-340⁰ C, and around 550⁰ C for all xP-NMA series. No CO2 desorption peak was found above 600⁰ C. The first desorption peak was assigned to CO2 desorption from the surface basic OHgroup (hydrogen carbonate species), whereas the medium-strength basic sites were centered around 270-340⁰ C, corresponding to CO2 desorption from metal-oxygen pairs (Mg 2+ -O 2-, Al 3+ -O 2-pairs) which were known as carbonate species [80, 81]. Finally, CO2

Figure 7 .

 7 Figure 7. (a) Infrared spectra of CO2 adsorbed on 3P-NMA after 30min and (b) Possible species appeared in the CO2 adsorption experiment.

Regarding

  CO2 adsorption results, in detail, CO2 adsorption on the NMA at high temperatures resulted in CO forming, leading to bands around 1930 cm -1 , 2067 cm -1 , 2081 cm -1 , and 2283 cm -1[START_REF] Roiaz | Reverse Water-Gas Shift or Sabatier Methanation on Ni(110)? Stable Surface Species at Near-Ambient Pressure[END_REF]. The IR band at 1930 cm -1 was attributed to the mono carbonyl formed on the Ni site (C=O stretching mode). The two later bands at 2067 cm -1 and 2086 cm -1 were due to the di-carbonyl on the surface[START_REF] Roiaz | Reverse Water-Gas Shift or Sabatier Methanation on Ni(110)? Stable Surface Species at Near-Ambient Pressure[END_REF][START_REF] Thibault-Starzyk | Infrared Spectroscopy[END_REF][START_REF] Haq | Adsorption and decomposition of formic acid on Ni{110}[END_REF].

Figure 9 .

 9 Figure 9. Collected spectra of the catalysts during the CO2 adsorption (left) and CO2 desorption (right) experiment at different temperatures. Spectra are recorded at room temperature.

Table 4 .

 4 Correlation between CO2-TPD and in-situ CO2 sorption experiments Basic sites (CO2-TPD) Alongside the deactivation of Ni-catalyst and reverse water gas shift (RWGS) reaction occurring at high temperatures, the stabilization of those carbonate species could negatively contribute to the overall activity.

Figure 10 .

 10 Figure 10. (a) CO2 conversion and (b) CH4 selectivity of all catalysts under CO2 methanation tests, and (c) TOS run on 3P-NMA catalyst and STYCH4 calculation at 300°C (conditions: GHSV = 12,000 h -1 , F = 100 ml/min, H2/CO2/Ar = 60/15/25). The dashed lines reflect the thermodynamic equilibrium of CO2 conversion and CH4 selectivity.

Figure 11 .

 11 Figure 11. The correlations of CO2 conversion (%) at 300°C versus (a) BET surface area (m 2 /g) and rp (nm) and (b) Ni 0 particle size (nm), total basicity (mmol/g.cat), and medium basic site distribution (%).

Figure 13 .

 13 Figure 13. Surface Operando-FTIR results of NMA and 3P-NMA catalysts in the 4000-2450 cm -1 range during I. CO2 adsorption and II. CO2 methanation

  left, CO2 adsorption behaviors). This event occurred because such species were adsorbed and maintained on the catalyst surface under static conditions; nevertheless, they were peeled/flushed from the surface under flowing conditions, resulting in a change of such positions. A similar situation was observed for carbonate species, with a little change in band location in the 1670-950 cm -1 wavenumbers. Figure14depicts the results, which include positions and assignments for all bands[26, 87-90, 103, 104].

Figure 14 .

 14 Figure 14. Surface Operando-FTIR results of NMA and 3P-NMA catalysts in the 2450-950 cm -1 range during I. CO2 adsorption, and II. CO2 methanation

Figure

  Figure Proposed mechanism of CO2 methanation over 3P-NMA

Figure 17 .

 17 Figure 17. TG profiles of catalyst series

Table 1

 1

	NMA	165	0.36	4.6
	1P-NMA	177	0.62	5.1
	3P-NMA	178	0.73	8.0
	5P-NMA	152	0.97	10.7

. Physicochemical properties of calcined sample: BET surface area, total pore volume, and pore sizes.

Catalysts

SBET (m 2 /g) a Pore volume (cm 3 /g) b rp, (nm) c a Specific surface area from BET method.

b Pore volume determined from the adsorption isotherms by the BJH method.

Table 2 .

 2 Structural parameters calculated from XRD measurements

	Catalysts	Structural parameters NiO Particle size (nm) a* (Å)	c** (Å)
	NMA	2.55	3.05	23.33
	1P-NMA	2.45	3.05	23.14
	3P-NMA	2.56	3.05	23.22
	5P-NMA	4.49	3.05	23.16

Table 3 .

 3 H2 consumption calculation and distribution of reduction peaks

	Catalysts	Total H2 consumption (mmol/gcat)	1 st peak	% Peak consumption 2 nd peak 3 rd peak	4 th peak
	NMA	4.34	0.07	0.07	63.15	36.71
	1P-NMA	3.53	0.70	0.30	61.04	37.96
	3P-NMA	3.62	1.32	0.20	63.34	35.14
	5P-NMA	4.11	0.36	0.01	99.63

Table 5 .

 5 Calculated basicity for xP-NMA series

	Catalysts	Total basicity (µmol/g.cat)		% Basic sites
			Weak	Medium	Strong
	NMA	57	22.5	60.0	17.5
	1P-NMA	115	11.9	52.8	35.3
	3P-NMA	121	20.2	58.2	21.6
	5P-NMA	79	29.8	58.5	11.7

Table 6 .

 6 Comparison with available data with different types of catalysts over CO2

			methanation at 300℃
	Catalysts	Composition*	Reaction conditions H2/CO2 GHSV/WHSV	CO2 Conv.(%)	CH4 Sel.(%)	Ref.
	SBA-15	Ni15	4	10,000 mL/gcat‧ h	15	62	[94]
	MCM-41	Ni15	4	43,000 mL/gcat‧ h	21	91
	MCM-41 Ni15/Ce15	4	43,000 mL/gcat‧ h	40	98	[95]
	USY	Ni15	4	43,000 mL/gcat‧ h	20	93
	ZrO2	Ni15	4	n.m	39	59	[96]

**NMA-NiMgAl-mixed oxides derived from hydrotalcite catalysts Table

Table 7 .

 7 Summarized properties of xP-NMA catalysts dedicated to different behaviors

	3P-NMA	5P-NMA
	smallest Ni 0 particle size	largest Ni 0 particle size
	largest surface area	lowest surface area
	the highest number of medium basic site	low amount of medium basicity
	best catalytic performance	poor catalytic activity

adsorption-desorption and (b) pore size distributions of all calcined catalysts.

Acknowledgments

This work has received support from the European Research Framework Program: H2020 | Marie Skłodowska-Curie Action (MSCA), PIONEER project: plasma catalysis for CO2 recycling (Grant no. 813393). The part of research results presented in this paper has been developed with the use of equipment financed from the funds of the "Excellence Initiative -Research University" program at AGH University of Science and Technology. The authors would like to thank Prof. Świerczek for evaluating TG on spent catalysts (AGH University of Science and Technology, Poland), and Prof. Jaafar (LCS-CNRS, France) for the help with the SEM-EDX operation.