The surface structure and the chemical activity of V2O3(0001) model catalysts

Abstract

Results of a systematic study of various aspects of the well-defined V2O3(0001) model catalyst system are presented. This study deals with the preparation, the characterization and the chemical activity of the V2O3(0001) model catalyst. Experiments are carried out under UHV conditions using a variety of surface sensitive techniques such as low energy electron diffraction (LEED), thermal desorption spectroscopy (TDS), x-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), near-edge x-ray absorption fine structure (NEXAFS), infrared reflection-absorption spectroscopy (IRAS), and high resolution electron energy loss spectroscopy (HREELS). In the present study, the V2O3(0001) model catalyst is prepared as a thin film with a thickness of ~100 A on Au(111) and W(110) substrates. The surface of V2O3(0001) can be terminated by a layer of vanadyl groups or by a layer of vanadium atoms. Characterization studies of these surfaces are performed focusing on their geometric and electronic properties which are correlated with their chemical activities. The chemical activities of the differently terminated V2O3(0001) surfaces are investigated by adsorption of a variety of probe molecules. Chemisorbed species are identified and investigated in terms of stability, geometric and electronic properties as well as reaction paths. The investigated molecules are oxygen, carbon monoxide, carbon dioxide, water, propane and propene. The influence of the surface termination on the adsorption and the reaction of these molecules is addressed. In this way, the structureactivity relationship is established. The vanadyl terminated V2O3(0001) surface is found to be chemically inert towards all the investigated molecules whereas the vanadium terminated V2O3(0001) surface is found to be chemically active. The difference between the chemical activities of the two surface terminations is a result of their differing geometric and electronic properties. These properties permit strong interactions with the vanadium terminated surface only, leading to dissociative adsorption in some cases. Unlike the surface vanadium atoms on the vanadyl terminated surface, the surface vanadium atoms on the vanadium terminated surface are freely accessible to the adsorbed molecules and exhibit a lower oxidation state. The adsorption of O2 on the vanadium terminated surface leads to the formation of vanadyl groups and negatively charged peroxo (O2 ) species. Annealing the peroxo-covered surface restores the vanadyl terminated layer. CO interacts strongly with the vanadium terminated surface and adsorbs in a tilted geometry on the surface. Annealing the CO-covered surface leads to the formation of vanadyl groups, most likely via CO dissociation. The adsorption of CO2 on the vanadium terminated surface induces the formation of strongly bonded, bent CO2 species in addition to weakly bonded, linear CO2 species. CO2 adsorbs in C2v symmetry in which the O–O axis is parallel to the surface. Upon annealing it decomposes to CO and O with the oxygen atoms forming vanadyl groups. H2O dissociates on the vanadium terminated surface and forms OH species which are stable up to ~600 K. The adsorption of C3H8 and C3H6 on the vanadium terminated surface leads to the formation of oxidized hydrocarbon species.