X-ray absorption spectroscopy under reaction conditions: suitability of different reaction cells for combined catalyst characterization and time-resolved studies

Abstract

Structure–activity correlations of solid catalysts and time-resolved studies generally require that the structure within a solid catalyst is probed simultaneously and at the same location where the catalysis and the structural changes occur. These requirements lead to a compromise between the spectroscopic arrangement and the optimum design for an in situ reactor cell. Opportunities and limitations of in situ and time-resolved X-ray absorption spectroscopy (XAS) combined with gas analysis are critically analysed with the help of two different cell designs, an in situ EXAFS cell designed for solids in the form of pressed wafers and a capillary cell where the catalyst is packed similarly to a plug flow reactor. On the basis of three examples, the reduction of CuO/ZnO, the reduction of PdO/ZrO₂ and methane oxidation over PdO_x/ZrO₂, criteria are developed which allow to judge the appropriate cell design in solid–fluid reactions. The prerequisites for the design of an in situ cell including the catalyst shape strongly depend on the time resolution required. Important issues embrace the type of reaction to be investigated (slow vs. fast), the reaction medium, and the porosity of the catalyst material. Criteria for assessing the role of pore and film diffusion in in situ studies are of paramount importance for a proper experimental design.