Combination of Pd-Cu Catalysis and Electrolytic H₂ Evolution for Selective Nitrate Reduction Using Protonated...

Pd-Cu catalysis is combined with in situ electrolytic H₂ evolution for NO₃^- reduction with protonated polypyrrole (PPy) as a cathode. The surface of PPy is not only beneficial for H₂ evolution, but exclusive for NO₃^- adsorption, and thus inhibits NO₃^- reduction. Meanwhile, the in situ H₂ generation exhibits a much higher utilization efficiency because of the smaller bubble size and higher dispersion. The Pd-Cu catalysts with the ratios of 6:1 and 4:1 exhibit the highest NO₃^--N removal (100%) and N₂ selectivity (93-95%) after 90 min. In comparison with the results obtained with other cathode materials (Ti, Cu, Co₃O₄, and Fe₂O₃) and obtained by other researchers, the new process shows a faster NO₃^--N reduction rate and much higher N₂ selectivity. However, the O₂ generated on the anode can oxidize Cu to Cu₂O that may work as the catalyst for NO₃^--N reduction to NH₄⁺-N by H₂, resulting in more than 60% NH₄⁺-N generated without a proton exchange membrane. Both the PPy film and Pd-Cu catalyst exhibit good stability and there is no Cu²⁺ or Pd²⁺ in solution after reaction. Real industrial wastewater is further treated in this system, the NO₃^--N is reduced from 670 mg L^-1 to less than 100 mg L^-1 in 90 min, and only little amount of NH₄⁺-N is generated.