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

Abstract

Cobalt catalysts supported on TiO 2 with different crystal forms (anatase and rutile) differ sharply in CO 2 conversion and product selectivity for CO 2 hydrogenation. The Co/rutile-TiO 2 catalyst selectively catalyzed CO 2 hydrogenation to CH 4, while CO is the main product on the Co/anatase-TiO 2 catalyst. In situ DRIFT (diffuse reflectance infrared Fourier transform) results have partially revealed the reaction pathway of CO 2 hydrogenation on these two catalysts. On Co/rutile-TiO 2, the reaction proceeds through the key intermediate formate species, which is further converted to CH 4 . Differently, the reaction on Co/anatase-TiO 2 undergoes CO 2 → *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 TiO 2, the product selectivity can be tuned from CO to CH 4 with a significant increase in CO 2 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, CO 2, 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 CH 4 . 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 C 2 + selectivity of 17% with 70% CO 2 conversion over K-Zr-Co/anatase-TiO 2 catalyst. These results reveal mechanistic insights into how the product distribution of Co/TiO 2 catalysts can be manipulated through adjusting the adsorption performance and surface C/H ratio.