Investigation of V Oxidation States in Reduced V/Al2O3Catalysts by XPS

Abstract

The distribution of vanadium oxidation states in a 5.9 wt% V/Al2O3catalyst reduced in H2or CO was determined from XPS V 2pspectra. Principal component analysis (PCA) was used to statistically determine the number of components (oxidation states) that describe the V 2p3/2envelope of the reduced V/Al2O3catalyst. The peak positions and FWHMs of the components were obtained from iterative target transformation factor analysis (ITTFA). The V 2p3/2envelope is composed of two components (oxidation states), which are centered at 517.2 and 515.5 eV (FWHMs of 2.6 and 2.3 eV, respectively) when H2is the reducing agent and at 517.3 and 515.8 eV (FWHMs 2.7–2.8 eV) when CO is the reducing agent. The components were assigned to V5+and V3+based on the difference in the binding energies and the results of a volumetric study of catalyst reduction in CO. The spectral information gained from PCA and ITTFA was used to curve fit the V 2penvelopes. The extent of reduction of the V phase was estimated from the relative abundances of V5+and V3+determined by curve fitting and corrected for the presumably lower dispersion of the V3+species. The results indicated a decrease in the average oxidation state of V to 3.5 with increasing the reduction temperature in H2up to 662°C. Reduction in the presence of CO was measured volumetrically and showed a decrease in the average oxidation state from 5 to 3.3 over the temperature range 155–510°C. Curve fitting the spectra of the CO-reduced samples using the two spectral components identified through factor analysis yielded an average oxidation state of ca. 3.6 upon reduction at 510°C.