Preparation of a Pd/ZnO (000 [formula omitted]) model catalyst and its characterization by AES, LEED and UHV reaction studies

Abstract

Supported Pd catalysts are prepared in UHV on ZnO (000 1 ) (oxygen face). By repeated cycles consisting of Pd deposition and an annealing step total amounts from 0.3 to 130 monolayers (ML) are reached. The growth mode of Pd is deduced indirectly from an analysis of Auger peak heights as a function of deposition amount and annealing temperature. At 300 K substrate temperature Pd, deposited with a rate of 1.2 ML/min, weakens the zinc and oxygen Auger yields in such a way that up to a deposit of 10 ML a constant ratio is maintained. This is ascribed to an agglomeration of the Pd deposit into clusters much higher than the escape depth. A critical inititial height of the clusters is derived as 6 ML (14.6 Å). The height increases linearly with Pd deposit. Above a deposit of 10 ML the oxygen Auger yield decreases much faster than the zinc yield. The data can be fitted using the assumption of a homogeneous Pd film on the free areas between the clusters. Annealing of faces with up to 130 ML Pd deposit at temperatures up to 970 K reestablishes free substrate area between the clusters. No evidence is found for zinc or oxygen atoms on top of the clusters. Pd does not desorb but at constant substrate Auger signals the Pd signal is significantly reduced upon annealing above room temperature. Therefore, it is concluded that appreciable amounts of Pd diffuse into the bulk or react with the substrate. After Pd deposition at room temperature only a faint (1 × 1) LEED pattern appears but on annealed faces with less than 35 ML Pd deposit intense (1 × 1) LEED spots of the ZnO lattice prevail indicating epitaxial growth. At higher coverages additional spots with hexagonal symmetry show up, arising from a Pd (111) structure aligned parallel to the ZnO lattice. In addition, TDS results of ethanol decomposition are discussed: The presence of 5.2 ML Pd deposit on ZnO (000 1 ) increases at low temperatures the rates of the products already known from the uncovered face. This implies that a part of the ZnO surface is still accessible for desorption. Above 10 ML Pd deposit also CO from a total oxidation appears