Methane Synthesis from CO ₂ and Ammonia over Supported Ni Catalyst via Isocyanate-Mediated Mechanism: Toward Carbon-Neutral Fuel Production

Methane (CH₄) synthesis using carbon dioxide (CO₂) and green hydrogen (H₂) is a promising means of increasing CO₂ utilization for the development of a more sustainable society. To mitigate the difficulties associated with the handling of H₂, ammonia (NH₃) has been suggested as a suitable hydrogen carrier, meaning highly active catalysts for the one-step synthesis of carbon-neutral CH₄ from CO₂ and green NH₃ (CO₂ + NH₃ methanation) are needed. Here we show that supported Ni catalysts are potential candidates for CO₂ + NH₃ methanation, and this reaction proceeds via an isocyanate species (*NCO) as an intermediate, which differs from the mechanism of CO₂ + H₂ methanation. Operando DRIFTS analyses revealed that destabilization of *NCO over Ni and efficient hydrogen supply to this intermediate via NH₃ decomposition are important for achieving high CH₄ synthesis activity. A detailed investigation of the physicochemical properties of six different oxide-supported Ni catalysts indicated that these two factors are highly dependent on the basic properties of the oxide support, with the destabilization of *NCO and decomposition of NH₃ being promoted by supports with weak and strong basic strength, respectively. The implication is that catalysts with high CH₄ formation activity could be obtained by using mixed supports with two kinds of basicity. Thus, the present study provides design guidelines for the development of highly active catalysts for the one-step synthesis of CH₄ from CO₂ and NH₃.