CO₂ Hydrogenation on Unpromoted and M-Promoted Co/TiO₂ Catalysts (M = Zr, K, Cs): Effects of Crystal Phase of Supports and Metal-Support Interaction on Tuning Product Distribution

Cobalt catalysts supported on TiO₂ with different crystal forms (anatase and rutile) differ sharply in CO₂ conversion and product selectivity for CO₂ hydrogenation. The Co/rutile-TiO₂ catalyst selectively catalyzed CO₂ hydrogenation to CH₄, while CO is the main product on the Co/anatase-TiO₂ catalyst. In situ DRIFT (diffuse reflectance infrared Fourier transform) results have partially revealed the reaction pathway of CO₂ hydrogenation on these two catalysts. On Co/rutile-TiO₂, the reaction proceeds through the key intermediate formate species, which is further converted to CH₄. Differently, the reaction on Co/anatase-TiO₂ undergoes CO₂ →∗CO, which desorbs to form gas-phase CO instead of subsequent hydrogenation. The strongly bonded∗CO is required to enhance the subsequent hydrogenation. By simply changing the calcination temperature of anatase TiO₂, the product selectivity can be tuned from CO to CH₄ with a significant increase in CO₂ conversion due to the surface phase transition of the anatase to the rutile phase. The addition of Zr, K, and Cs further improves the CO, CO₂, and H₂ adsorption in both the capacity and strength over anatase- and rutile-supported catalysts. The Zr modification makes the reaction pathway over anatase-supported catalyst proceed via the intermediate formate species and enables the subsequent hydrogenation to CH₄. In addition, the surface C/H ratio increases significantly in the presence of promoters (unpromoted < Zr-promoted < K-Zr-promoted ∼ Cs-Zr-promoted), which leads to the highest C₂⁺ selectivity of 17% with 70% CO₂ conversion over K-Zr-Co/anatase-TiO₂ catalyst. These results reveal mechanistic insights into how the product distribution of Co/TiO₂ catalysts can be manipulated through adjusting the adsorption performance and surface C/H ratio.