CO₂methanation over γ-Al₂O₃nanosheets-stabilized Ni catalysts: Effects of MnO_xand MoO_xadditives on catalytic performance and reaction pathway

With 5 nm Ni nanoparticles anchored by the nanosheet-shaped γ-Al2O3 supports, a ~10 wt% Ni/Al2O3 catalyst displayed ca. 45% conversion and >98% CH4 selectivity in CO2 methanation reaction at 300 °C for 450 h. Benefited from stable interfaces between Ni sites and highly dispersed MnOx promoters on Al2O3, a modified Ni-Mn/Al2O3 catalyst exhibited 93-88% conversions, >99.9% selectivity and great stability in 1100 h reaction. As a contrast, a Mo-modified Ni-Mo/Al2O3 catalyst showed severe deactivation with declined conversions from >80 to 50% within initial 20 h. The high-dispersion MoOx species in the fresh catalyst occurred aggregation and reduction during the reaction, reconstructing the Ni-support interfaces. As clarified by in-situ DRIFT spectra, the COads-mediated hydrogenation reaction pathway was followed on these Ni catalysts via sequential steps, CO2 → bicarbonates → bidentate carbonates → bidentate formates → COads → CH4. Ni-Mn/Al2O3 catalyst exhibited significantly high and well-matched reaction rates in overall steps, whereas Ni/Al2O3 catalyst showed low activities in key steps of formate decomposition and COads methanation. Ni-Mo/Al2O3 catalyst displayed superior ability for CO2 dissociation into bicarbonates, and nonetheless, it could not steadily achieve the formation and hydrogenation of COads species, allowing the inactive surface species to be accumulated and to induce the catalyst structural change for deactivation.