In-Situ Extended X-Ray Absorption Fine Structure Study of Chemical Promoters in Catalysis

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The interaction of alkali-metal promoters with supported rhodium catalysts is studied using extended X-ray absorption fine structure (EXAFS). By use of an in-situ EXAFS cell, it is possible to observe the nature and diversity of the alkali-metal and transition-metal atoms both in the reduced catalysts and under reaction conditions. These data are combined with results of catalyst characterization by atomic absorption, X-ray diffraction, gas adsorption, infrared spectroscopy, and thermal desorption, as well as transient kinetic studies, on the same catalysts to define the bonding, electronic state, nearest neighbors, and local structure of both the promoter and the catalytic metal. Chemical modification of catalysts by the addition of small quantities of a promoter can greatly affect catalyst activity and selectivity. Many fundamental questions about how promoters function remain unanswered, including where they are located on the surface, what relationship they have with the support, and what interactions they have with the reactants. These questions are addressed using a combination of EXAFS, transient kinetics, and more traditional techniques. This could lead to a general theory of surface chemical promotion.