Modifications of the AgMo₃P₂O₁₄ Catalyst in the Oxidation of Propane

Abstract
The catalytic performances in the oxidation of propane of the AgMo₃P₂O₁₄ catalyst are quite good, leading to 70% of propene selectivity at 7% of propane conversion. We have checked that, before reaction, results of XANES and EXAFS obtained at the Mo K-edge (19999 eV) confirm those obtained by X-ray spectroscopy (oxidation state and Mo number of oxygen neighbour of 5.66, Mo=O bond length=1.72Å). After reaction, we have shown that the catalyst is partially reduced [5.66 → 5.2], the number of oxygen atoms increases to 6 and the mean molybdenyl bond length decreases (1.68Å) which is in agreement with the variation of this bond length with the oxidation state of the molybdenum in molybdenum phosphates. XPS measurements also confirmed a partial reduction of molybdenum species and showed a migration of the silver cation to the surface, certainly participating to the surface stabilization. Moreover, a correlation between the catalytic activity and the modification of the catalyst is made showing that an equilibrium between Mo^V and Mo^VI species in the catalyst leading to the avrerage oxidation state of molybdenum of 5.2 seems to be very important.