XPS-SIMS surface characterization of aluminovanadate oxide catalyst precursors: Effect of precipitation pH

X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (TOF SIMS) are used to study the surface physical and chemical state of aluminovanadate oxide catalyst precursors (V-Al-O) precipitated at different pH within the range from 5.5 to 10. For comparison, the reference V 2 O 5 and Al 2 O 3 oxides are also investigated. Using XPS, the core-level photoelectron spectra, x-ray induced Auger spectra, and the valence- band spectra of the samples are obtained, and the surface concentration of components of the catalyst is quantitatively evaluated. Auger parameters for Al and O in V-Al-O, V 2 O 5 and Al 2 O 3 are measured, and the chemical Wagner diagrams are plotted. The O 2p-2p hole repulsion energy for all the samples is estimated. The binding-energy shifts of the O I s , V 2p and Al 2p lines are determined as a function of pH and analysed in terms of the initial-state effect related to the atomic charge as well as the Madelung potential and of the final-state relaxation effect. Increasing of pH is found to result in the monotonic variation of the surface composition with the V/Al ratio decreasing by a factor of ≅ 2.6, in the modification of the valence band, progressive hydration of the surface, and increasing dispersion of vanadium-oxide particles. In V-Al-O catalysts, the fraction of reduced V 4+ ions revealed by XPS is increasing with increasing pH, so, the average vanadium oxidation state is changed from ≅+4.8 to +4.6. The enhancement of catalytic activity for propane oxidative dehydrogenation demonstrated by V-Al-O at high precipitation pH is in a good correlation with decreasing surface V/Al ratio, growing population of the V 4+ states, and increasing nucleophilicity of oxygen atoms.