Pd Clusters Loaded with Multivalent Cu Foam for Superior Electrochemical Nitrate Reduction and Selective N≡N Bond Formation

The electrochemical denitrification of nitrate (NO₃ ^-) in actual wastewater to nitrogen (N₂) is an effective approach to reversing the current imbalance of the nitrogen cycle and the eutrophication of water. However, electrostatic repulsion between NO₃ ^- and the cathode results in the low efficiency of NO₃ ^- reduction reaction (NO₃RR). Here, density functional theory (DFT) calculations are used as a theoretical guide to design a Pd cluster-loaded multivalent Cu foam (Pd/Cu₂O-CF) electrocatalyst, which achieves a splendid 97.8% NO₃ ^- removal rate, 97.9% N₂ selectivity, 695.5 mg N g^-1 _Pd h^-1 reduction efficiency, and 60.0% Faradaic efficiency at -1.3 V versus SCE. The projected density of states (pDOS) indicates that NO₃ ^- and Pd/Cu₂O-CF are bonded via strong complexation between the O 2p (in NO₃ ^-) and Cu 3d (in Cu₂O) with the input of voltage, which reduces the electrostatic repulsion and enhances the enrichment of NO₃ ^- on the cathode. In-situ characterizations demonstrate that Pd[H] can reduce Cu₂O to Cu, and subsequently Cu reduces NO₃ ^- to nitrite (NO₂ ^-) accompanied by in situ reconfiguration of multivalent Cu foam. NO₂ ^- is then transferred to the surface of Pd clusters by the cascade catalysis and accelerates the breaking of N─O bonds to form Pd─N, and eventually achieves the N≡N bond formation.