Catalytic hydrogenation of CO2 into CH4 is an effective method to convert waste CO2 and green hydrogen into clean fuel on a large scale. However, the viability of such process largely relies on the development of highly active heterogeneous catalysts. Here, a tailored methanation catalyst, Co nanoparticles immobilized into a highly porous N‐doped carbon matrix, is prepared by the carbonization of a cobalt‐based layered zeolitic imidazolate framework (ZIF−L) material under an argon atmosphere. This catalyst displays a specific activity of 22.3 molCH4/gcat.min at 350 °C, significantly outperforming a similar catalyst derived from the more conventional ZIF‐67 (11.7 molCH4/gcat.min). This is explained by the stabilization of small Co nanoparticles (∼20 nm) and by the presence of abundant medium‐strength basic sites related to the nitrogen doping in the catalyst prepared from ZIF−L. Notably, the new catalyst shows high stability; no deactivation is observed up to 60 hours on stream.

No abstract

No abstract

In the up-and-coming power-to-gas scenario (PtG), surplus of renewable electricity is stored in the form of methane, by reacting green hydrogen with waste CO2 through the Sabatier reaction (CO2 methanation). While the catalytic hydrogenation of CO2 to methane has already attracted much attention, the development of catalysts that feature a high specific activity at low temperature and a reasonable cost remains challenging but is needed in the perspective of industrial deployment. Concomitantly, the mechanism of CO2 methanation remains debated, and its elucidation would drive further progress. Herein, we disclose the preparation of a series of high-loading Ni/SiO2 catalysts via sol-gel method. Through (HR)-TEM, XRD, N2 physisorption, and H2 chemisorption, we show that small Ni particles (<5 nm, high Ni dispersion) could be obtained in a highly porous silica matrix, even at loading up to 50 wt%. The most active catalyst reached a high specific activity of 10.2 µmolCH4.g-1.s-1 at 300 °C (96% selectivity to CH4 with 79% CO2 conversion). Being based on inert silica, these catalysts are idea model materials to study the reaction mechanism. Combining XPS, CO2-TPD, in-situ CO2-DRIFTS, and TPSR on the one hand, and theoretical calculations (DFT) on the other hand, we show that CO2 methanation follows mostly the RWGS+CO-hydrogenation and the formate pathways, the former being dominant at low temperature. Upon CO2 adsorption on Ni/SiO2, the carbonyl species formed from the adsorbed bicarbonates react with H2 to form CH4 via the RWGS+CO-hydrogenation pathway, while the adsorbed monodentate carbonates are hydrogenated to CH4 via the formate pathway.

CO2 methanation is effectively catalyzed by Ni-based catalysts, and reactivity can be further tuned by the addition of promoters. Deciphering the relationship between the promoter in Ni-based catalysts and the corresponding catalytic performance in CO2 methanation mechanism is of great meaning for the development of highly active catalysts. Herein, a series of model bimetallic catalysts were prepared by sol-gel chemistry to address this fundamental challenge. Compared to Ni/SiO2 catalyst, the Mn-doped and Co-doped catalysts showed a higher methanation activity, with the former showing better performance below 250 °C and the latter showing better performance over 300 °C. On the contrary, the Cu-promoted catalyst showed a lower CO2 conversion with a lower CH4 selectivity in the whole temperature range. A comprehensive characterization study (TEM, XRD, XPS, H2-TPR, CO2-TPD, in situ DRIFTS, and TPSR analyses) suggests that the effect of promoters is not directly related to improvement of dispersion, reducibility, or basicity. Instead, we show that the promoters orient the reaction mechanism and favor the conversion of key intermediates. Mn addition has the highest promoting effect on the hydrogenation of formaldehyde intermediate (*OCH2) to methoxy intermediate (*OCH3), i.e. the rate determining step of the “RWGS+CO hydrogenation” pathway which is shown to predominate at low reaction temperature. Co addition facilitates the formation of formate species, i.e. the rate determining step of the formate pathway which is also active at high reaction temperature. Cu addition has a negative effect on the rate determining step of those two pathways, resulting a lower performance of Ni-Cu/SiO2.