Influence of ZnO Facets on Pd/ZnO Catalysts for Methanol Steam Reforming

A series of Pd/ZnO catalysts with different Pd loadings were prepared using needlelike ZnO crystallites (ZnO-N) with predominant (10–10) nonpolar facets exposed and commercial ZnO (ZnO-P) without any dominant facets. The Pd/ZnO catalysts were characterized using complementary techniques, such as nitrogen physisorption, X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and Fourier transform infrared spectroscopy analysis of adsorbed CO (CO-FTIR). The effect of ZnO crystallite faceting on the activity and selectivity of the Pd/ZnO catalysts was studied in methanol steam reforming (MSR). It was found that the Pd-rich phases (Pd_xZn_y, x > y) are predominantly formed at low Pd loadings on both ZnO supports (ZnO-N and ZnO-P), resulting in high CO selectivity. As Pd loading increases, the x/y ratio in the Pd_xZn_y phases decreases, leading to decreased CO selectivity. At similar Pd loadings, Pd/ZnO-P catalysts are more selective in MSR than Pd/ZnO-N, which is due to more facile formation of the stable PdZn_β phase on polar ZnO (0001) facets than on nonpolar ZnO (10–10) facets. The current study provides insight into the structure–performance relationships in Pd/ZnO catalysts for MSR, helping shed light on the rational design of selective MSR catalysts to minimize CO formation.