CO/FTIR Spectroscopic Characterization of Pd/ZnO/Al2O3 Catalysts for Methanol Steam Reforming

Abstract

An as-synthesized 8.8wt% Pd/ZnO/Al2O3 catalyst was either pretreated under O2 at 773 K followed by H2 at 293 K or under H2 at 773 K to obtain, respectively, a supported metallic Pd° catalyst (Pd°/ZnO/Al2O3) or a supported PdZn alloy catalyst (PdZn/ZnO/Al2O3). Both catalysts were studied by CO adsorption using FTIR spectroscopy. For the supported PdZn alloy catalyst (PdZn/ZnO/Al2O3), exposure to a mixture of methanol and steam, simulating methanol steam reforming reaction conditions, does not change the catalyst surface composition. This implies that the active sites are PdZn alloy like structures. The exposure of the catalyst to an oxidizing environment (O2 at 623 K) results in the break up of PdZn alloy, forming a readily reducible PdO with its metallic form being known as much less active and selective for methanol steam reforming. However, for the metallic Pd°/ZnO/Al2O3 catalyst, FTIR results indicate that metallic Pd° can transform to PdZn alloy under methanol steam reforming conditions. These results suggest that PdZn alloy, even after an accidental exposure to oxygen, can self repair to form the active PdZn alloy phase under methanol steam reforming conditions. Catalytic behavior of the PdZn/ZnO/Al2O3 catalyst also correlates well with the surface composition characterizations by FTIR/CO spectroscopy.