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Catalysis is an area of research which stands to benefit greatly from the development of surface analysis techniques. X-ray photoelectron spectroscopy (XPS), or electron spectroscopy for chemical analysis (ESCA), is one of these techniques and is already providing valuable information even though its limitations have not yet fully been established. This paper describes several applications of XPS to research in catalysis.
Two catalyst systems are discussed in terms of the type of information which can be derived by using XPS. The first one is the hydrodesulfurization (HDS) catalyst, which consists of oxides of cobalt and molybdenum on an alumina support. It will be shown how XPS studies can be used to elucidate the surface chemistry of activated HDS catalysts. The study demonstrates that both binding energy and intensity are required to describe the working catalyst fully. The HDS system is also used to demonstrate the use of XPS for quantitative surface analysis of catalysts.
The second catalyst system, rhodium on carbon, is first discussed in terms of catalyst deactivation. Changes in the intensity ratio of rhodium to carbon are used to monitor changes in the catalyst surface produced by hydrogen (H2) reduction at elevated temperatures and are related to catalyst performance. The results reinforce the importance of intensity ratios as a measure of catalyst dispersion. It is also shown, using rhodium on carbon catalysts with carbon supports, that observed intensity ratios are affected by properties of the support and therefore can result in significant errors in quantitative measurements.
Other aspects of the application of XPS to catalyst studies discussed are catalyst poisoning (auto exhaust catalyst) and the distribution of active components on catalyst surfaces (supported silver catalysts).
surface analysis, X-ray photoelectron spectroscopy (XPS), electron spectroscopy for chemical analysis (ESCA), catalysis
Principal research chemist, American Cyanamid Co., Stamford, Conn.