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 H 2 evolution for NO 3 – reduction with protonated polypyrrole (PPy) as a cathode. The surface of PPy is not only beneficial for H 2 evolution, but exclusive for NO 3 – adsorption, and thus inhibits NO 3 – reduction. Meanwhile, the in situ H 2 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 NO 3 – –N removal (100%) and N 2 selectivity (93–95%) after 90 min. In comparison with the results obtained with other cathode materials (Ti, Cu, Co 3 O 4, and Fe 2 O 3 ) and obtained by other researchers, the new process shows a faster NO 3 – –N reduction rate and much higher N 2 selectivity. However, the O 2 generated on the anode can oxidize Cu to Cu 2 O that may work as the catalyst for NO 3 – –N reduction to NH 4 + –N by H 2, resulting in more than 60% NH 4 + –N generated without a proton exchange membrane. Both the PPy film and Pd–Cu catalyst exhibit good stability and there is no Cu 2+ or Pd 2+ in solution after reaction. Real industrial wastewater is further treated in this system, the NO 3 – –N is reduced from 670 mg L –1 to less than 100 mg L –1 in 90 min, and only little amount of NH 4 + –N is generated.