Enhanced photocatalytic selectivity of noble metallized TiO2 nanoparticles for the reduction of selenate in water: tunable Se reduction product H2Se(g)vs. Se(s)†
Selenium (Se) contamination as a result of anthropogenic activity (i.e. mining, power generation, and oil & gas refining) is becoming a global concern due to its associated aquatic toxicity concerns. Herein, heterogeneous nanoscale photocatalysts were synthesized by depositing noble metal nanoparticles (gold (Au), silver (Ag), platinum (Pt) and palladium (Pd)) onto titanium dioxide (TiO2), which demonstrated work-function dependent bimodal selectivity of final products during the photocatalytic reduction of selenate to elemental Se (Se0) or hydrogen selenide gas (H2Se). The Se-noble metal-TiO2 (Se-NM-TiO2) photocatalytic system is structured in a direct Z-scheme arrangement when Au, Ag or Pt are used, allowing for high selectivity towards H2Se. In contrast, Pd acted as an electron sink which decreased reducibility of the photogenerated electrons, ultimately causing a higher selectivity towards Se0. Au–TiO2 offers the largest H2Se selectivity of all catalysts tested, while Pd–TiO2 offers the highest selectivity to solid Se0 generation. This study elucidates electron transport mechanisms and Fermi level equilibration via quantized double-layer charging effects of the Se-NM-TiO2 system and sheds light on advanced reduction processes using nanoscale heterogeneous catalysts. Finally, tunability of the Se reduction product is key to designing a sustainable treatment approach with a potential for Se capture and reuse.