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 (PdxZny, 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 PdxZny 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 PdZnβ 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.