Combination of Pd–Cu Catalysis and Electrolytic H2 Evolution for Selective Nitrate Reduction Using Protonated Polypyrrole as a Cathode

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.