Activity Boost for Cobalt–Selectivity Shift and Activation During CO$_2$ Methanation by Carbon Nitride‐Modified Al$_2$O$_3$ Support

This research demonstrates an approach to enhance performance during CO$_2$ hydrogenation by using carbon nitride (CN)-coated Al$_2$O$_3$ composites as a support for cobalt-based catalysts. Unlike conventional fine powdered CN with a typically low surface area, the CN coating on Al$_2$O$_3$ enables tuneable porosity and minimizes pressure drop in fixed-bed reactors. Modification of the Al$_2$O$_3$ support with CN induced a significantly improved distribution of separately prepared Co$_3$O$_4$ nanoparticles. The CN layer interacted strongly with the nanoparticles, preventing aggregation. An enhanced stabilization of nanoparticles was also identified when comparing the Co/CN@Al$_2$O$_3$ catalyst with the Co/Al$_2$O$_3$ catalyst post reaction. Catalytic tests for CO$_2$ methanation revealed that Co/CN@Al$_2$O$_3$ yields significantly higher CO$_2$ conversions of up to 63% and a methane selectivity of 94%. In contrast, the Co/Al$_2$O$_3$ catalyst exhibits a lower CO$_2$ conversion of maximum 20% and a methane selectivity of up to 66%. These results illustrate the superior stability and activity of the Co/CN@Al$_2$O$_3$ catalyst, which may be linked to a restructuring of the CN into a N-doped carbon (N─C) layer and potential cobalt carbide formation during activation or CO$_2$ hydrogenation.