Catalysis at the NSLS: Importance of Oxygen Vacancies in the Behavior of Oxide Catalysts

Introduction Metal oxides are widely used as catalysts in environmental chemistry and commercial processes that deal with the conversion of hydrocarbons [1]. Thus, oxide catalysts are useful in the destruction of the SO 2 and NO x species produced during the combustion of fuels in automobiles, factories and power plants. By preventing the emission of SO 2 and NO x into the atmosphere, they help to minimize the negative effects of acid rain on the environment [1,2]. Selective oxidation, ammoxidation, and dehydrogenation probably constitute the most important industrial applications of oxide catalysts active for the conversion of hydrocarbons [1]. Each year these processes produce millions of dollars in revenues. Over the years there has been a considerable interest in obtaining a fundamental understanding of phenomena responsible for the good performance of oxide catalysts [1,3,4]. Part of the problem in explaining the behavior of these systems arises from the fact that they are complex and very difficult to characterize, in many cases containing several interacting phases and a small fraction of active sites [1]. Useful knowledge in this subject can be obtained through synchrotron based techniques, which nowadays allow the detailed study of the interaction of molecules with surfaces (photoemission, x-ray absorption spectroscopy, infrared spectroscopy, etc) or the evolution of catalytic materials under reaction conditions (x-ray diffraction and scattering, extended x-ray absorption fine structure, etc) [5-8]. These techniques become particularly powerful when combined with state-of-the-art density function (DF) calculations [9]. Using such an approach, recent studies carried out at the NSLS (U7A, X7B, X16C, X19A beamlines) have shown the importance of oxygen vacancies in the behavior of oxide catalysts [10-14].