Designing carbon nitride modifications for tuneable CN@Al2O3 supports in cobalt-catalysed methanation of CO2.

Support materials are often considered as simple stabilising structures for the active phase, yet their physicochemical properties decisively modulate catalyst performance. Carbon nitride (CN) has frequently been combined with metal oxides, predominantly in photocatalysis, where typically only a single bulk-type CN material is employed. Consequently, the influence of different CN morphologies on the structure and catalytic behaviour of CN-modified oxide supports remains largely unexplored. Herein, we establish CN as a tuneable support modification for Co-catalysed CO2 methanation by systematically integrating distinct CN morphologies, namely sponge-like (spCN), bulk (bCN), highly porous (hpCN), self-assembled (saCN), and triazine-based (tbCN), onto a conventional Al2O3 support via a solution-deposition strategy. This approach yields CN@Al2O3 composites with comparable CN loadings (27-31 wt%) and distinct textural and chemical properties. Comprehensive characterisation confirms that the surface coverage of Al2O3 depends on the morphology of CN, which, in turn, dictates the distribution of Co3O4 during decoration with nanoparticles from a separate synthesis. Catalytic testing at 325 °C and 5 barg reveals a strong morphology-performance relationship. Co/spCN@Al2O3 exhibits the highest productivity with 60% CO2 conversion and 97% CH4 selectivity, outperforming composites based on bCN and hpCN or unmodified Al2O3 as the support material. In contrast, saCN and tbCN modifications resulted in poor performance. Post-run analyses indicate that CN modification may enhance interaction with cobalt and mitigate sintering, with the sponge-like morphology providing the most favourable metal-support interactions. These findings demonstrate that CN morphology represents a key design parameter for tailoring metal-support interactions and catalytic performance in thermocatalytic CO2 hydrogenation.