Methane Synthesis from CO2 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.