Effect of X-rays on the surface chemical state of Al2O3, V2O5, and aluminovanadate oxide

The surface composition of Al2O3, V2O5, and alummovanadate oxide, "V-Al-O", was studied by X-ray photoelectron spectroscopy (XPS), using Mg K-alpha to reveal time-dependent irradiation damage of samples. Spectral parameters such as peak intensity and width and absolute and relative peak binding energies were evaluated along with the Auger parameter. Irradiation of Al2O3 was found to cause partial dehydration of the surface hydroxide film, while sputter-cleaned alumina turned out to be resistant to X-rays. In V2O5, a small fraction of V4+ species was seen to form during X-ray exposure. X-ray induced damage in Al2O3 and V2O5 was compared to that caused by bombardment with 500 eV argon ions. The V-Al-O material which is used as a precursor of oxynitride catalysts for ammoxidation turned out to be most susceptible and could be damaged by low X-ray doses. An appreciable reduction from the V5+ to the V4+ formal oxidation state (the latter increases from 20 to 45% after 150 min time of exposure to Mg K-alpha at 150 W) was found along with the decomposition of aluminum hydroxide which is believed to act as an amorphous support in this catalyst. Gas-phase analysis during X irradiation demonstrated desorption of oxygen and water molecules. X-ray induced damage is believed to be caused by electron-hole pair generation and Auger decay rather than by thermal effects since the sample surface temperature increased only slightly.