Support-dependent rate-determining step of CO2 hydrogenation to formic acid on metal oxide supported Pd catalysts

Investigating the mechanism of CO2 transformation into value-added chemicals is crucial for developing improved heterogeneous catalysts. In the present study, two common metal oxide (CeO2 and ZnO) supported Pd catalysts were studied in detail. Basic sites on the support promote CO2 activation to form surface adsorbed intermediates that further react with hydrogen atoms on metallic palladium to form formic acid. As such, both the support basicity and the (electronic) structure of Pd are critical parameters determining the catalytic performance, where the relative reaction rate for CO2 activation and hydrogenation decides which step is rate-determining. Pd/CeO2 with a higher density of basic sites readily activates CO2 and therefore the hydrogenation is the limiting step governed by the nature of the Pd species on the support. For Pd/ZnO, however, the rate-determining step is the formation of adsorbed carbonaceous intermediates due to the low density of basic sites and thus, the structure of Pd has little influence on the catalytic performance. These insights are further validated on a Pd/TiO2 catalyst, in which the optimized basic site density and Pd structure enhanced CO2 hydrogenation activity of up to an order of magnitude. (C) 2019 Elsevier Inc. All rights reserved.