Characterization of catalyst surfaces by STM image calculations

Abstract

ZnO and Cu/ZnO are important industrial catalysts for many hydrogenation reactions, for example, the methanol synthesis from synthesis gas. The identification of the dominant surface structures and surface defects under reaction conditions is essential for a microscopic understanding of the activity of a catalyst. Using density functional theory (DFT) we have calculated the formation energy-for the most important atomic defects on the ZnO(101̄0) surface as a function of the redox properties of a surrounding gas phase. To give guidelines on how these defects may appear in a scanning tunneling microscopy (STM) experiment, STM images have been calculated using our recent implementation of Bardeen's tunneling formula into the Car-Parrinello Molecular Dynamics (CPMD) code. We find significant differences in the tunneling properties between the ideal surface and O, Zn, and ZnO vacancies, which may allow to identify these defects in STM measurements. As a first step to study the morphological changes of the Cu particles in the binary Cu/ZnO catalyst when exposed to an oxidizing environment we have studied the stability of various copper oxide surfaces by combining DFT calculations with a thermodynamic formalism. STM images of the most stable surface structures were calculated to be compared with experimental results.