Simultaneous photocatalytic removal of nitrate and oxalic acid over Cu₂O/TiO₂ and Cu₂O/TiO₂-AC composites

Cu₂O/TiO₂ (1–10 wt% Cu₂O) and 2.5% Cu₂O/TiO₂-AC (2.5–20 wt% AC) photocatalyst composites were synthesised by an ethanol reduction method. The materials were characterised by a number of techniques which confirmed the presence of Cu₂O in contact with the TiO₂. Pure TiO₂ alone was not active for the simultaneous photocatalytic removal of nitrate and oxalic acid under conditions employed, however, photocatalytic activity was observed for TiO₂ and TiO₂/AC in the presence of Cu₂O. This may have resulted from suppression of charge recombination via creation of a p-n heterojunction between Cu₂O and TiO₂. Within the series, 2.5% Cu₂O/TiO₂ exhibited the best photocatalytic performance with 57.6 and 99.8% removal of nitrate and oxalic acid, respectively, with selectivities of 45.7, 12.4 and 41.9% to NH₄ ⁺, NO₂ ⁻ and N₂, respectively after 3 h. For the carbon containing photocatalysts, 2.5% Cu₂O/TiO₂-2·5AC displayed the highest activity with 42.5 and 96.6% removal of nitrate and oxalic acid, respectively, with 32.7, 11.6 and 55.7% selectivities to NH₄ ⁺, NO₂ ⁻ and N₂, respectively. The highest AC loading tested resulted in selectivity to NH₄ ⁺ of 21.6 with no NO₂ ⁻ detected, together with an improved N₂ selectivity (78.4%) albeit at lower (12.7%) nitrate conversion. Data suggests that Cu₂O/TiO₂ can be used in the photocatalytic reduction of nitrate and improved selectivity towards N₂ can be attained by influencing factors which control the relative rate of oxalic acid consumption.