Novel synthesis of ZrxTi1-xOn mixed oxides using titanyl sulphate and pressurized hot and supercritical fluids, and their photocatalytic comparison with sol-gel prepared equivalents

ZrxTi1-xOn mixed oxides with five various molar ratios of Zr:Ti and their parent oxides were prepared, combining thermal hydrolysis (H) with pressurized hot water crystallization and supercritical methanol drying. Their photoactivity was compared to equivalent materials prepared by sol-gel (G) method. Zr0.1Ti0.9On-G was the most photoactive mixed oxide, which showed significantly higher photoactivity than Zr0.1Ti0.9On-H. Higher photoactivity of Zr0.1Ti0.9On-G can be attributed to phase composition (anatase-brookite phase mixture) and smaller TiO2 anatase crystallite-size. The Zr dopation in TiO2 did not improve the photoactivity neither of sol-gel-synthetized, nor thermal hydrolysis-synthetized mixed oxides in AO7 photodegradation. Zr in higher loadings (>10 mol%) formed separate monoclinic and tetragonal ZrO2 and monoclinic zirconium titanates. Unlike Zr4+ ions in Zr doped TiO2 the separate monoclinic and tetragonal ZrO2 and monoclinic zirconium titanates did not serve as electron trapping centres but as electron-hole recombination centres. The Zr loading in TiO2 should be below 10 mol%.