Plasma electrolytic formation of TiO2-VOx-MoOy-P2O5 coatings on titanium and their application as catalysts for the oxidation of S- and N-containing substances

Abstract

Catalytically active coatings on titanium were formed by plasma electrolytic oxidation (PEO) in the aqueous electrolytes containing Na6P6O18, NaVO3, and/or Na2MoO4. The coatings were investigated by EDS, XPS, and SEM methods. The results show that the replacement of NaVO3 by Na2MoO4 in the electrolyte leads to the substitution of V for Ti in the coatings and the concentrating Mo in the surface layers. In turn, this causes a smoothing of the surface relief, a change in the shape of large pores from oval to round, and a decrease in surface porosity. As to XPS, partially reduced Mo4+ (or Mo5+) and V4+, along with Mo6+, V5+, are present in the surface part of the coatings, facilitating the formation of catalytically active sites on the specimen surfaces. Screening shows that all PEO-coated specimens catalyze the reactions of oxidative denitrogenation (ODN) and oxidative desulfurization (ODS) using hydrogen peroxide as the oxidizing agent. Tendencies were found for a decrease in the conversion of dibezothiophene in 4 h with an increase in the Ti + Mo concentration and for a decrease in the conversion of thiophene, methylphenyl sulfide and pyridine with a decrease in the concentration of Ti + V + Mo. The regularities of simultaneous oxidation of thiophene and pyridine in their mixture were determined for V-containing catalysts and for Mo-containing ones. It was found that V-containing PEO composite accelerates both ODS and ODN reactions, while Mo-containing one accelerates the ODN reaction and slows down the ODS reaction.