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 catalysts was studied with low-energy ion scattering. Using external reference samples, absolute numbers for the atomic densities of both P and V could be determined. VPO catalysts prepared in either aqueous or organic media showed large amounts of carbonaceous surface species. From catalysts prepared in an aqueous medium the carbonaceous species could be removed using a mild oxidation treatment without significantly changing the average valence of the vanadium atoms in the catalyst as evidenced from XPS. After this cleaning treatment the surface concentration of vanadium (atoms/cm2) agrees well with that expected for a vanadyl pyrophosphate structure. However, the surface phosphorus concentration is twice as high as that in vanadyl pyrophosphate, 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 units and the phosphate groups. The catalyst surface is also compared to VOPO4 phases. A significant contribution of a phase such as a -VOPO4 could explain the observed surface P/V ratio of 2.0.