Dynamic surface behaviour of VPO catalysts under reactive and non-reactive gas compositions: an in-situ XAS study

The surface of an activated vanadium phosphorus oxide (VPO) catalyst was investigated by means of in situ X-ray absorption spectroscopy in the total electron yield mode. We observed significant changes of the V L3-near edge X-ray absorption fine structure (NEXAFS) when the material was transferred from room temperature to working conditions at 400°C in the reaction atmosphere. We studied the same VPO material under different gas compositions comprising the reaction mixture of n-butane and oxygen, pure oxygen and vacuum to elucidate the influence of the gas-surface interaction and the effect of the temperature. The results of this extensive study indicate a dynamic response of the catalyst surface to the applied conditions

The Influence of the Chemical Potential on Defects and Function of Perovskites in Catalysis

A Sm-deficient Sm0.96MnO3 perovskite was prepared on a gram scale to investigate the influence of the chemical potential of the gas phase on the defect concentration, the oxidation states of the metals and the nature of the oxygen species at the surface. The oxide was treated at 450°C in nitrogen, synthetic air, oxygen, water vapor or CO and investigated for its properties as a catalyst in the oxidative dehydrogenation of propane both before and after treatment. After treatment in water vapor, but especially after treatment with CO, increased selectivity to propene was observed, but only when water vapor was added to the reaction gas. As shown by XRD, SEM, EDX and XRF, the bulk structure of the oxide remained stable under all conditions. In contrast, the surface underwent strong changes. This was shown by AP-XPS and AP-NEXAFS measurements in the presence of the different gas atmospheres at elevated temperatures. The treatment with CO caused a partial reduction of the metals at the surface, leading to changes in the charge of the cations, which was compensated by an increased concentration of oxygen defects. Based on the present experiments, the influence of defects and concentration of electrophilic oxygen species at the catalyst surface on the selectivity in propane oxidation is discussed

The role of carbon species in heterogeneous catalytic processes: an in situ soft x-ray photoelectron spectroscopy study

High pressure X-ray photoelectron spectroscopy (XPS) is used to characterize heterogeneous catalytic processes. The success of the new technique based on the possibility to correlate the catalytic activity and the electronic structure of an active surface. The dynamic character of a catalyst surface can be demonstrated impressively by this technique. In this contribution the basics of high pressure XPS will be discussed. Three examples of heterogeneous catalytic reactions are presented in this contribution. The selective hydrogenation of 1-pentyne over Pd based catalysts and the dehydrogenation of n-butane and the oxidation of ethylene over V based catalysts. It is shown, that the formation of subsurface carbons plays an important role in all the examples. The incorporated carbon changes the electronic structure of the surface and so controls the selectivity of the reaction. A change of the educts in the reaction atmosphere induces modifications of the electronic surface structure of the operation catalysts