XPS–SIMS Surface Characterization of Aluminovanadate Oxide Catalyst Precursors Co-Precipitated at Different pH: Effect of Calcination

Abstract

X-ray photoelectron spectroscopy and time-of-flight secondary ion mass spectrometry were employed in a comparative study of the surface physical and chemical state of aluminovanadate oxide catalyst precursors (V–Al–O), which were precipitated in the range of pH from 5.5 to 10, after drying and calcination. Core-level photoelectron spectra, X-ray induced Auger and valence band spectra of the samples were measured so as to quantitatively evaluate the surface concentrations of the catalyst components. The binding energy shifts of the respective O 1s, V 2p and Al 2p lines were determined as a function of pH and analyzed in terms of the initial state effect related to the atomic charge and Madelung potential. The surface of the catalysts was composed of aluminum hydroxide/oxyhydroxide and of dispersed vanadium oxide species. Increasing pH was found to result in a monotonic variation of the elemental surface composition, modification of the valence band, progressive hydroxylation of the surface and increasing dispersion of vanadium oxide species. Increasing pH was also accompanied by an increase in the abundance of V⁴⁺ species, specific surface area and reducibility. Calcination in air at 500 °C gave rise to surface segregation of vanadium, changes in the valence band and partial dehydroxylation. The structural transformations in vanadium oxide species and aluminium hydroxide support and their interaction were accompanied by an increasing abundance of V–O–Al bonds. The net result of the restructuring was a decrease in the specific surface area and reducibility of the calcined catalysts. The enhancement of the catalytic activity in propane oxidative dehydrogenation demonstrated by V–Al–O samples with increasing precipitation pH and after calcination was in good correlation with a growing population of the V⁴⁺ states and increasing nucleophilicity of oxygen sites.