Methodical aspects in the surface analysis of supported molybdena catalysts

Abstract

Supported molybdena catalysts, with TiO2, CeO2 and Al2O3 supports, were studied by XPS and ISS. It was found that reliable results are obtained only when samples are calcined and transferred into the ultrahigh vacuum system without further contact with the ambient atmosphere ('in situ calcination'). This applies also to catalysts that were previously calcined but had been stored in the ambient atmosphere. Supported Mo oxide becomes reduced under x-ray irradiation during extended XPS data acquisition. A slight decrease of the Mo/support cation intensity ratio as a consequence of this reduction was detected by ISS in MoO3/TiO 2 and MoO3/CeO2, therefore ISS analysis should be performed on freshly calcined samples without prior extended exposure to x-rays. Because ISS spectra change rapidly due to sputtering, a correct analysis of the surface properties of the supported Mo catalyst requires extrapolation of the trend to the start of the experiment. It was established by this methodology that the surface of a 7% MoO3/TiO2 catalyst (5.3 Mo nm-2) is completely covered by a monolayer of Mo oxide species, and no Ti cation is exposed. In a submonolayer MoO3/CeO 2 catalyst the exposed support could be detected, as expected, whereas in an MoO3/Al2O3 catalyst with an Mo oxide loading equal to the monolayer coverage a slight exposure of the Al support cation also was noted probably because of the high curvature of the smaller Al2O3 particles. Copyright © 2004 John Wiley & Sons, Ltd.