Cobalt-containing oxide layers on titanium, their composition, morphology, and catalytic activity in CO oxidation

In this study possibility to form the layered compositions CoOx + SiO2/TiO2/Ti by plasma electrolytic oxidation (PEO) method was shown. Compositions have been obtained by both one-stage PEO method (Method I) with addition of Co(CH3COO)2 into silicate electrolyte and impregnation of preliminary obtained by the PEO method SiO2/TiO2/Ti systems in aqueous solutions containing cobalt salts with their following annealing (Method II). XRD, XPS and SEM/EDX were used to investigate the phase and element composition, microstructure of the coatings prepared by the two various methods. Catalytic activity of the cobalt-containing composites was investigated in the CO oxidation reaction. Under experimental conditions, the structures obtained by impregnation and annealing method were more active, than those obtained by one-stage PEO method. The surface structures of cobalt-containing coatings obtained by the PEO method and by impregnation and annealing differ in both quantitative and qualitative relation. The cobalt content on the surface of impregnating coatings is three times as much as that for those formed by one-stage PEO method. It is found that coatings obtained by the Method II have a more developed surface. The surface of CoOx + SiO2/TiO2/Ti compositions obtained by the PEO method contains, presumably Co(OH)2 and Co3O4. The surface of the similar compositions obtained by the Method II, possibly contains CoO, either Co2O3, or CoOOH. The combination of these factors, perhaps, also provides a higher activity of the compositions formed by the Method II.