Zinc Oxide Morphology-Dependent Pd/ZnO Catalysis in Base-Free CO2Hydrogenation into Formic Acid

Pd/ZnO catalysts with ca. 2 wt.% Pd loading employing ZnO nanocrystals with various morphologies as supports, including ZnO nanoplates (p-ZnO) predominantly exposing polar {002} facets, ZnO nanorods (r-ZnO) predominantly exposing nonpolar {100} and {101} facets, and commercial ZnO (c-ZnO) exposing random facets, have been prepared as catalysts for the catalytic reaction of CO(2)hydrogenation into formic acid (FA) without the addition of base under mild conditions. A remarkable morphology-dependence of zinc oxide in the Pd/ZnO catalysts was observed. Catalytic activities of various Pd/ZnO catalysts in CO(2)hydrogenation into FA follow an order of Pd/r-ZnO>Pd/p-ZnO>Pd/c-ZnO, and all the catalysts are stable. Kinetic study, CO2-TPD analysis, and in situ DRIFTS spectra provide evidence that all the Pd/ZnO catalysts follow a similar catalytic mechanism that CO(2)activation is the rate-determining step. Consequently, r-ZnO, possessing a higher density of weak basic sites than p-ZnO and c-ZnO, can act as a superb support to prepare Pd/r-ZnO catalyst, which is more active than Pd/p-ZnO and Pd/c-ZnO catalysts for the production of FA. These findings nicely demonstrate the crystal-plane engineering of the metal oxide support in tuning the catalytic performance of Pd-based catalysts for CO(2)hydrogenation into FA.