Structured Ni@
 NaA
 zeolite supported on silicon carbide foam catalysts for catalytic carbon dioxide methanation

Funding information EPRSC grant, Grant/Award Number: EP/ R026939/1 EP/R026815/1 EP/R026645/1 EP/R; European Commission Horizon 2020 Marie Skłodowska-Curie, Research and Innovation Staff Exchange (RISE) project, Grant/Award Number: H2020-MSCA-RISEZEOBIOCHEM-872102; European Commission Marie Skłodowska-Curie Individual Fellowship, Grant/Award Number: 748196; European Commission, Grant/Award Number: H2020-MSCA-IF; Horizon 2020; European Union Abstract Carbon dioxide (CO2) conversion is an important yet challenging topic, which helps to address climate change challenge. Catalytic CO2 methanation is one of the methods to convert CO2, however, it is limited by kinetics. This work developed a structured Ni@NaA zeolite supported on silicon carbide (SiC) foam catalyst (i.e., Ni@NaA-SiC), which demonstrated an excellent performance with a CO2 conversion of 82%, being comparable to the corresponding equilibrium conversion, and CH4 selectivity of 95% at 400 C. The activation energy for CO2 conversion over the 15Ni@NaA-SiC catalyst is about 31 kJ mol, being significantly lower than that of the 15Ni@NaA pelletized catalyst (i.e., 84 kJ mol). Additionally, the structured catalyst was highly stable with sustained CO2 conversion at 78.7 ± 1.4% and selectivity to CH4 at 97.7 ± 0.2% over an 80 hr longevity test. In situ diffuse reflectance infrared Fourier transform spectroscopy-mass spectroscopy characterization revealed that catalysis over the structured catalyst proceeded primarily via the CO free mechanism.


| INTRODUCTION
Carbon dioxide (CO 2 ) emissions from many sources such as biogas, natural gas, and coal combustion are the major contributor to the greenhouse gases (ca. 82%), being the primary reasons for the serious global warming and climate change issues. 1 Therefore, many efforts have been made recently to develop highly efficient CO 2 capture and utilization (CCU) technologies for reducing and converting CO 2 emissions, fighting against the urgent environmental issues. 2 The powerto-gas (P2G) process is an energy storage and conversion technology that can convert the surplus sustainable energy into gas fuel (i.e., hydrogen, H 2 ) effectively, which can be subsequently used for CO 2 conversion (to methane). [3][4][5][6][7][8] P2G relies on water electrolysis (for sustainable H 2 production) and CO 2 hydrogenation (the so-called catalytic CO 2 methanation for producing methane, CH 4 , Equation 1).
Therefore, P2G enables the sustainable production of economical viable H 2 to hydrogenate CO 2 into CH 4 which can be either directly injected into the existing gas grid or used as a feedstock for the production of other value-added chemicals (e.g., via methane aromatization into aromatics). 9,10 CO 2 + 4H 2 ! CH 4 + 2H 2 O, Δ r H 0 298 K = −165 kJ mol −1 : Although catalytic CO 2 methanation is thermodynamically favorable at low temperatures, it suffers from the kinetic limitation due to the high stability of CO 2 molecules. Accordingly, elevated temperatures (at ca. 200-750 C) are normally required to activate the catalyst to enable the hydrogenation of CO 2 . 11 In addition, CO 2 methanation is highly exothermic (Equation 1) and the development of local hotspots is common along the catalytic packed bed, being one of the main reasons for catalyst sintering and deactivation. 2 Moreover, conventional fixed bed reactors with packed catalyst pellets normally suffer from poor heat transfer and insufficient heat dissipation, which are likely to result in thermal runaway, and thus associated deactivation and safety issues. 12,13 To overcome the limitation of conventional packed beds, structured catalysts such as monoliths, 14,15 structured fibrous networks, [16][17][18] and structured foams 13,[19][20][21][22][23] have been studied to mitigate the effect by heat/mass transfer on chemical transformation and separation processes, including catalysis such as catalytic foams (with the intrinsically high thermal conductivity) presented the improved heat transfer than alumina foams. In addition, electromagnetic induction heating was applied to regulate heat transfer in structured catalysts (e.g., Ni supported on carbon felt) for catalytic CO 2 methanation process, being able to improve heating/cooling rates with uniform heating environments and high energy efficiency. [24][25][26][27][28] SiC open-cell foams have been demonstrated as ideal candidates to develop structured catalysts for addressing the mass and heat transfer limitations in many challenging exothermic reactions (e.g., methanol-to-propylene (MTP) reaction 21 ), offering many advantages such as high surface-to-volume ratios, improved mass and heat transfer coefficient, as well as low bed pressure-drops. 15 Figure S1. The structured Ni@NaA-SiC catalyst (9 mm O.D. × 25 mm length, volume: 1.6 cm 3 , amount of Ni@NaA phase: 200 mg) was loaded in the middle of a stainless-steel reactor (10 mm O.D. × 120 mm length, as shown in Figure S1 in the SI) which was equipped with a thermocouple to monitor the actual temperature of the catalyst bed. The catalysts were wrapped with graphite tape to avoid the wall effect that is, channeling. Before the reaction, the catalyst was reduced in a 20% : where r CO2 is the reaction rate of CO 2 (mol s −1 g cat −1 ), X CO2 is the conversion of CO 2 , F in CO2 is the molar flow rate of CO 2 in the inlet of the reactor (mol s −1 ), W cat is the mass of the catalyst (g), C Ni is the actual Ni content of the catalyst (by ICP-OES, as shown in Table S2).

| In situ DRIFTS-MS studies of catalytic CO 2 methanation over Ni@NaA-SiC catalyst
In situ DRIFTS characterization of the surface chemistry during the catalysis was performed using a Bruker Vertex 70 FTIR spectrometer equipped with a liquid N 2 -cooled detector, which has been detailed elsewhere. 39 In situ DRIFTS spectra were recorded at every 56 s with a resolution of 4 cm −1 and analyzed with the OPUS software.
Prior to DRIFTS analysis, the 15Ni@NaA-SiC catalyst was ground and loaded into a ceramic crucible in the IR cell, and pretreated at 500 C    Figure S4). Specifically, the averaged pressure drop of the NaA-3-SiC composite (3.5 × 10 5 ± 4.7 × 10 7 Pa m −1 ) increased by 10.1% compared to that of NaA-2-SiC (3.2 × 10 5 ± 2.6 × 10 7 Pa m −1 ). The findings from SEM analysis revealed that the repetitive hydrothermal synthesis (after seeding) could significantly affected the morphology NaA zeolite coating, as well as the amount, which was evidenced by the weight gain of the composites (i.e., 7.5 wt% after one synthesis vs. 16.5 wt% after three synthesis, as shown in Figure S5).
Comparative XRD patterns of NaA-SiC composites with reference to NaA seeds and bare SiC foam are presented in Figure 3a.   Table S1. The BET surface area (S BET ) of bare SiC foam is significantly low (i.e., 0.1 m 2 g −1 ). SiC foams contain macroscopic cellular structure, and their macro-pore properties cannot be probed by N 2 physisorption. NaA seeds (as shown inset in Figure 3c) exhibited the isotherm similar to type II (b) with a hysteresis loop in the p/p 0 range of 0.6-0.8. 40 The NaA-2-SiC composite shows the similar isotherm to that of NaA seeds, but the specific quantity adsorbed is considerably lower (Table S1). The distribution of other elements of Na, Al, and Si in the NaA zeolite phase of 15Ni@NaA-SiC and 15Ni@ catalysts is presented in Figure S6, showing the comparable Si/Al molar ratio of 1.2. However, H 2 pulse chemisorption analysis (as summarized in Table 1) shows that the Ni dispersion and metallic surface area in the 15Ni@NaA-SiC catalysts (i.e., 1.02% and 6.76 m 2 g metal −1 ) were significantly higher than that of 15Ni@NaA (i.e., 0.6% and 3.98 m 2 g metal −1 ).
The findings revealed that, in comparison with the bulk NaA zeolite, the structured NaA supported on SiC foam could possibly promote the metal dispersion, being beneficial to the catalysis. Specifically, in IWI process, the contacting efficiency between the catalyst support and Ni precursor is key to determine the metal dispersion and particle size in the developed catalyst. In this work, for structured NaA-SiC composite supports, thin layer of NaA zeolite coating on the surface of SiC foam has relatively high geometric surface area. Therefore, the contacting efficiency between NaA zeolite coating and Ni precursor in IWI could be higher than that of the conventional NaA zeolite pellet, leading to a good dispersion of Ni species in the zeolite coating.  Figure S7).
The H 2 -TPR profile of the 15Ni@NaA catalyst presented a wide peak in the region of 300-700 C with a sharp peak located at 450-500 C, which can be attributed to the reduction of NiO species on NaA support, being consistent to previous results in the literature. 36 (Table S2), and the 15Ni@NaA-SiC catalyst has a higher CO 2 conversion than 15Ni@NaA, the relatively high specific rate presented by 15Ni@NaA-SiC can be explained. Additionally, the contacting efficiency of the structured catalysts is also expected better than that of the conventional packed bed. 22 Arrhenius plots were used to calculate the activation energy (E A ) for the CO 2 conversion using kinetic data (Table S3), as shown in Figure 5d. The extracted parameters are listed in The long-term stability of the 15Ni@NaA-SiC catalyst for catalytic CO 2 methanation was also evaluated at 400 C, and the results are presented in Figure 6. Over 80 hr TOS, the key indicators of its catalytic performance remained stable as a function of TOS with the CO 2 conversion at 78.7 ± 1.4%, CH 4 selectivity at 97.7 ± 0.2% and CO selectivity at 2.2 ± 0.2%, demonstrating the potential of the developed structured catalysts for practical CO 2 methanation in P2G. Since water is the co-product of the catalysis, at high temperatures, the hydrothermal stability of the zeolite coating needs to be evaluated.
Previously, Ni catalysts supported on zeolite A (pellets) were developed for sorption-enhanced CO 2 methanation due to their high capability of water uptake (e.g., 23 g water kg zeolite −1 at 21 C and 53% relative humidity). 33

| In situ DRIFTS-MS study of catalytic CO 2 methanation over the 15Ni@NaA-SiC catalyst
The mechanisms of catalytic CO 2 methanation over different catalysts were also detected during the catalysis by MS, which could be assigned to species from CO 2 fragmentation and CO from the catalysis, and hence discussion on it was not made). It should be noted that the breakthrough time of water (m/z = 18) in the catalytic system is relatively longer than that of other gases, indicating the sorption enhanced CO 2 methanation due to the good water adsorption capability of NaA zeolite, 46 being in line with the findings reported previously. [33][34][35][36] specifically, previous findings demonstrated that the regeneration of these structured catalysts was possible via a drying step using either reducing (e.g., H 2 ) or oxidizing gas (e.g., air). 36  (1,653 cm −1 ) remained constant. 34,43 As shown in Figure 8, under iso- Based on the in situ DRIFTS-MS studies above, the reaction pathway was proposed for CO 2 methanation over the Ni@NaA-SiC catalyst, which is shown in Figure 10.