Studies of carbon monoxide oxidation catalysis using platinum/titanium oxide thin films as supported model catalysts and sensing elements

The priority research areas in heterogeneous catalysis are to study catalyst surfaces while the reaction is taking place and to characterize the actual catalysts at atomic resolution. CO oxidation over Pt/TiOx thin films, as supported model catalysts and sensing elements, are investigated with in situ resistance measurements and ex situ microscopic and spectroscopic characterization. The studies are carried out in a specially designed flow reactor system which can hold thin film model catalysts while simultaneously monitoring the film resistance. The reaction kinetics and the changes of Pt/TiOx thin film resistance during CO oxidation are studied under different O2/CO ratio conditions in a temperature range of 150°C to 375°C. The interaction of CO and O2 with the film surfaces is evaluated by measuring the changes of film resistance and the concentrations of product via on-line gas chromatography. In addition, the effects of the reaction environments on both the composition and microstructure of Pt/TiOx films are characterized with X-ray Photoelectron Spectroscopy (XPS), Rutherford Backscattering Spectrometry (RBS), Transmission Electron Microscopy (TEM), and Atomic Force Microscope (AFM) before and after oxidation. The results show that titanium diffuses through the Pt layer and forms TiOx on the film surface during the oxidation. The surface decoration or metal support interaction behavior on Pt/TiO x thin films for CO oxidation is supported by the kinetic and characterization results. The CO oxidation mechanism on the Pt/TiOx films can not be adequately described by a simple surface reaction model but appears to involve the diffusion of Ti to the film surface. The film resistance serves as an indicator to reveal the dynamic response of the catalyst during the reaction. This study demonstrates that coupling in situ resistance measurements with conventional flow reactor data provides a powerful method for gaining insight into the properties of catalysts under reaction conditions. Also, using thin film model catalysts provides opportunities to fill the gap between single crystal model catalysts and conventional catalysts. The novel approach used in this study is practically applicable and helps to link the macroscopic, practical operation of catalysts with atomic scale structural and mechanistic understanding.