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 bar_g 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.