Selenate Se(VI) reduction to elemental selenium on heterojunctioned rutile/brookite nano-photocatalysts with enhanced charge utilization

Abstract

Newly synthesized nanocrystalline TiO2 mixed-phase photocatalysts are shown to surpass in Selenium (VI) reduction effectiveness the commercial P25 nanoTiO2 photocatalyst Heterojunctioned rutile (63 %)/brookite (37 %) (designated as R/b) nanoTiO2 is found to be the most effective photocatalyst achieving 95 % photo-assisted reductive deposition vs. 25 % only by the benchmark P25 composed of anatase (80 %) and rutile (A/r). This is attained at very high photocharge utilization with at least 5-fold less formic acid as hole scavenger. The superior performance of R/b is attributed to a favorable band alignment at the heterojunction and its 1D-dominated nanocrystal orientation enabling efficient co-adsorption and enhanced charge separation. The deposition of elemental Se further “auto-catalyzes” the photocatalytic process by enabling visible light activity with simultaneous suppression of charge recombination. In-depth probing of the R/b photocatalyst after selenium removal provides evidence of a 2-step mechanism involving (a) reductive adsorption of Se(VI)aq → Se(IV)surf with the participation of co-adsorbed formate and Ti3+ defects, and photoreduction of Se(IV)surf → Se(0)surf.