Variation of the vanadium oxidation state within a VPO catalyst layer in a membrane reactor: XPS mapping and modelling

Recently, the feasibility of butane oxidation in an electrochemical membrane reactor (EMR) using a vanadium phosphorus oxide (VPO) catalyst layer on a tubular anodic electrode has been reported. This novel application of EMR gives rise to questions about the vanadium oxidation state (Vox) under working conditions and about its spatial distribution in the catalyst layer. It has now been determined by means of position-resolved XPS measurements. In addition, model calculations on the spatial Vox distribution have been performed for the first time. The simulations reveal a non-uniform 3D distribution of Vox due to the relative rate of reduction and re-oxidation processes in the catalyst layer, in good agreement with the experimental XPS data.