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 CO, 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 H-2 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 similar to Cs-Zr-promoted), which leads to the highest C-2(+) selectivity of 17% with 70% CO, 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.