Dual-site electrocatalytic nitrate reduction to ammonia on oxygen vacancy-enriched and Pd-decorated MnO2 nanosheets

Electrocatalytic nitrate reduction (NRR) represents one promising alternative to the Haber-Bosch process for NH3 production due to the lower reaction energy barrier compared to N2 reduction and the potential recycling of nitrogen source from nitrate wastewater. The metal oxides with oxygen vacancy (Ov) display high NH3 selectivities in NRR (NO2-/N2 as side products), but the complexity in Ov enrichment and the inferior hydrogen adsorption on oxides make NRR an inefficient process. Herein, one superior dual-site NRR electrocatalyst that is composed of Ov-enriched MnO2 nanosheets (MnO2-Ov) and Pd nanoparticles (deposited on MnO2) is constructed over the three-dimensional porous nickel foam (Pd-MnO2-Ov/Ni foam). In a continuous-flow reaction cell, this electrode delivers a NO3--N conversion rate of 642 mg N m-2electrode h-1 and a NH3 selectivity of 87.64% at -0.85 V vs. Ag/AgCl when feeding 22.5 mg L-1 of NO3--N (0.875 mL min-1), outperforming the Pd/Ni foam (369 mg N m-2electrode h-1, 85.02%) and MnO2-Ov/Ni foam (118 mg N m-2electrode h-1, 32.25%). Increasing the feeding NO3--N concentration and flow rate to 180.0 mg L-1 and 2.81 mL min-1 can further lift the conversion rate to 1933 and 1171 mg N m-2electrode h-1, respectively. The combination of experimental characterizations and theoretical calculations reveal that the MnO2-Ov adsorbs, immobilizes, and activates the NO3- and N-intermediates, while the Pd supplies the Ov sites with sufficient adsorbed hydrogen (H*) for both the NRR and Ov refreshment. Our work presents a good example of utilizing dual-site catalysis in the highly selective conversion of NO3- to NH3 that is important for nitrate pollution abatement, nitrogen resource recycling, as well as sustainable NH3 production.