QUANTITATION OF SPECIES ON CATALYST SURFACES

Methodology for the quantitation of the distribution of oxidation states in reduced catalysts is reviewed. The adopted approach is based on curve fitting the x-ray photoelectron spectroscopy (ESCA, XPS) envelope using a non-linear least squares curve fitting (NLLSCF) routine. The procedure was applied to the study of the reduction of Mo/TiO2 and Mo/Al2O3 catalysts. The distribution of Mo oxidation states obtained by ESCA was corroborated by chemical measurements and correlated with catalytic activity. The limitations of the NLLSCF approach are discussed. It is shown that the uncertainties inherent in curve fitting methods can be minimized by the use of factor analysis. This data analysis technique provides an independent estimate of the number, positions, and shapes of components required to describe the Mo 3d envelopes in the reduced catalysts.