Why Should Spinel Formation be Controlled in Promoted Ni–Mg–Al Mixed Oxide Catalyst for Ambient Pressure CO 2 Hydrogenation?

Ni–Mg–Al catalysts promoted with Fe, Co, or Y were synthesized via thermal and microwave‐assisted solution combustion synthesis (SCS) to investigate the influence of synthesis routes and promoter chemistry on structural properties and CO 2 methanation performance. Thermal SCS produced catalysts with higher surface area, well‐developed mesoporosity, greater basicity, and improved Ni reducibility. In contrast, microwave SCS resulted in lower surface area, limited basicity, and increased formation of MgAl 2 O 4 and NiAl 2 O 4 spinel phases. Promoter chemistry significantly influenced Ni reducibility and its local coordination environment. Notably, yttrium incorporation suppressed excessive spinel formation, enabling MW_NiY to retain a higher fraction of readily reducible Ni species and a metallic‐like local structure comparable to thermally synthesized catalysts. Post‐reaction characterization revealed Ni lattice contraction and reduced crystallite size across all catalysts, suggesting promoter migration and Ni redistribution under methanation conditions. Catalytic evaluation demonstrated consistently high activity and selectivity for thermal SCS catalysts, whereas MW_NiY substantially outperformed MW_NiFe and MW_NiCo despite its less favorable textural properties. These findings establish the critical interplay between synthesis methodology, promoter chemistry, and structural evolution, highlighting yttrium promotion as an effective strategy to overcome microwave SCS limitations and enhance CO 2 methanation performance in multicomponent catalytic systems.