New insights into the nature of the active phase of VPO catalysts - A quantitative static LEIS study

The atomic composition of the outermost atomic layer of equilibrated VPO ca talysts was studied with low-energy ion scattering, Using external referenc e samples, absolute numbers for the atomic densities of both P and V could be determined. VPO catalysts prepared in either aqueous or organic media sh owed large amounts of carbonaceous surface species, From catalysts prepared in an aqueous medium the carbonaceous species could be removed using a mil d oxidation treatment without significantly changing the average valence of the vanadium atoms in the catalyst as evidenced from XPS. After this clean ing treatment the surface concentration of vanadium (atoms/cm(2)) agrees we ll with that expected for a vanadyl pyrophosphate structure. However, the s urface phosphorus concentration is twice as high as that in vanadyl pyropho sphate, leading to a surface P/V ratio of 2.0 +/- 0.2. This shows that VPO catalysts may be terminated by a distorted vanadyl pyrophosphate structure, where the excess amount of phosphorus is positioned between the vanadyl un its and the phosphate groups. The catalyst surface is also compared to VOPO 4 phases. A significant contribution of a phase such as alpha (II)-VOPO4 co uld explain the observed surface PN ratio Of 2.0. (C) 2000 Academic Press.