Photocatalytic reduction of nitrite to dinitrogen in aqueous suspensions of metal-loaded titanium(iv) oxide in the presence of a hole scavenger: an ensemble effect of silver and palladium co-catalysts

Abstract

Nitrite (NO₂⁻) was photocatalytically reduced to dinitrogen (N₂) in an aqueous suspension of two kinds of titanium(IV) oxide particles loaded with palladium and silver (Pd–TiO₂ and Ag–TiO₂) at pH 8 under irradiation of UV light in the presence of sodium oxalate as a hole scavenger. The two metal-loaded TiO₂ photocatalysts had different roles in conversion of NO₂⁻ to N₂ and worked in an effective ensemble without conflict: (1) Pd–TiO₂ induced photocatalytic disproportionation of NO₂⁻ to N₂ and nitrate (NO₃⁻) and (2) Ag–TiO₂ selectively reduced the thus-formed NO₃⁻ back to NO₂⁻ (partially to N₂) with oxalate acting as a hole scavenger. When Pd–TiO₂ was used alone for NO₃⁻ reduction in the presence of sodium oxalate, Pd–TiO₂ induced fruitless photocatalytic decomposition of oxalate to carbon dioxide and dihydrogen. The presence of Ag–TiO₂ suppressed the fruitless decomposition of oxalate by Pd–TiO₂ because Ag–TiO₂ continuously provided NO₂⁻ in the reaction system using oxalate as a hole scavenger and Pd–TiO₂ therefore only worked as a photocatalyst for disproportionation of NO₂⁻ to N₂ and NO₃⁻ as it did when used alone.