Zinc Oxide Morphology‐Dependent Pd/ZnO Catalysis in Base‐Free CO 2 Hydrogenation 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, CO 2 ‐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.