Enhanced photocatalytic oxidation of free cyanide using hydrogen-treated TiO₂: effect of reduction temperature

Abstract

Hydrogen-treated titanium dioxide (grey TiO2) crystals were synthesized through a temperature-programmed reduction of commercial TiO2 under an H2/Ar mixed gas flow at elevated temperatures ranging from 600 to 800 °C. Analytical techniques, including Raman spectroscopy and x-ray Photoelectron Spectroscopy (XPS), were employed to probe the presence of oxygen vacancies in the grey TiO2 and to track the variation in Ti3+ species concentration as a function of temperature. The reduced samples obtained at different temperatures were used as photocatalysts to oxidize free cyanide ions under UV light illumination. Among these, the photocatalysts reduced at 600 °C demonstrated superior activity, converting 30 ppm of free cyanide to cyanate ions within 120 min. This reaction time represents a significant enhancement in efficiency as compared to untreated TiO2, which necessitated over 240 min to accomplish the same conversion. The improved performance of grey TiO2 is ascribed to its elevated photocurrent and the positive shift in the flat band energy, which together accelerate electron transfer and limit electron–hole recombination rates.