Plasma electrolytic synthesis and characterization of oxide coatings with MWO4 (M = Co, Ni, Cu) as photo-Fenton heterogeneous catalysts

Abstract Titanium-supported oxide coatings with MWO4 (M = Cu, Ni, or Co) were formed by the single-stage plasma electrolytic oxidation of titanium in alkaline tungstate-phosphate electrolytes with M(II)–EDTA complex ions. All formed coatings exhibited photocatalytic activity in the reaction of degradation of methyl orange (10 mg/L MO, pH = 6.8) in the presence of 10 mmol/L hydrogen peroxide under UV and Vis irradiation. The Ti/TiO2-CoWO4 composite was most effective catalyst: the degree of MO azo dye degradation reached 85 and 55% after 180 min irradiation under UV and Vis light, respectively. The bandgap values determined by the Tauc's method for the direct allowed transition were 2.2, 2.45 and 2.5 eV for Co-, Cu- and Ni-containing samples, correspondingly. The leaching of cobalt, copper and nickel from catalysts during the degradation process was less than 0.88, 0.52 and 1.47 mg/L per cycle, but this fact did not lead to a decrease in MO degradation with the number of cycles. The degradation reaction under UV and Vis irradiation has a first order rate coefficient of (8.9–11.0) × 10−3 and (2.0–8.3) × 10−3 min−1, respectively.