The Important Role of Alloy–Oxide Interfaces in Controlling Methanol Formation in CO ₂ Hydrogenation

A series of PdZn/TiO2 catalysts prepared by chemical vapor impregnation (CVI) were tested for CO2 hydrogenation at 20 bar pressure and at temperatures of 230–270 °C. Changing the Pd and Zn molar ratio (Zn:Pd = 0–20) in a PdZn/TiO2 catalyst has a dramatic effect on selectivity for the CO2 hydrogenation reaction. Pd alone shows three main products: methanol, CO, and methane. Addition of small quantities of Zn results in the formation of a PdZn alloy, preventing methanation. At equimolar ratios of Pd and Zn, a 1:1 β-PdZn alloy is formed and a reverse water gas shift catalyst is produced. Adding Zn in excess relative to the Pd loading results in the formation of ZnO on the TiO2 surface in addition to the PdZn alloy, dramatically increasing methanol selectivity from 5% at Zn:Pd = 1 to 55% for Zn:Pd = 2. Through a combination of theory and experiment, the active site for methanol synthesis is concluded to be the interface between PdZn nanoparticles and the ZnO overlayer on the TiO2, where interfacial formate can react with hydrogen dissociated by the metal nanoparticle.