Determination of the distribution of chromium oxidation states in reduced Cr/Al 2 O 3 catalysts from XPS by factor analysis and curve fitting

Principal component analysis (PCA) and target testing (TT) were performed on a series of reduced 2.5 wt.% Cr/Al 2 O 3 catalysts to determine the number, positions, shapes and relative spectral compositions (i.e. Cr oxidation states) in Cr 2p spectra as monitored by ESCA. The results from PCA indicated the presence of only two abstract components. The abstract factors were transformed through iterative TT into real spectroscopic components that were attributed to Cr 6+ and Cr 3+ . These factor analysis results were used in addition to non‐linear least‐squares curve fitting to provide a detailed description of changes in the Cr 2p envelope during reduction of the Cr/Al 2 O 3 catalysts. Only two components were necessary to reproduce the original data.