The stability of VPO and different supported VOx catalysts for partial oxidation of small hydrocarbons under the ultra-high vacuum (UHV) conditions of standard X-ray photoelectron spectroscopy (XPS) was studied using a multi-purpose surface analysis apparatus which allows time spans of only a few minutes between the sample transfer into vacuum and the first photoelectron spectrum. The VPO catalysts showed a significant dependence of the vanadium oxidation state upon the time of exposure to UHV, as determined from the shape evolution of the V2p3/2 peak. The initial VV/VIV ratio was found to decrease by a factor of at least ~2 within the first 15 - 25 min of the XPS experiment for fresh as well as used VPO catalysts. From a careful set of experiments in a "stop and go" mode (10 to 20 s) of the X-ray source it can be concluded that the observed changes are predominantly due to the vacuum environment, with a rather minor (if at all) contribution of the X-ray excitation.
In contrast to these results on VPO, the investigated VOx catalysts on various support materials, like Al2O3, TiO2 and SiO2, change under the same conditions to a much lesser degree and on the time scale of hours instead of minutes. The role of the V-O-support bonds has been studied by variation of the support at nearly constant vanadium concentration, and the stabilizing effect of the supporting oxide material on the vanadium oxidation state is discussed.