Dual-site Electrocatalytic Nitrate Reduction to Ammonia on Oxygen Vacancy-enriched and Pd-decorated MnO Nanosheets

Electrocatalytic nitrate reduction (NRR) represents one promising alternative to the Haber-Bosch process for NH production due to the lower reaction energy barrier compared to N reduction and the potential recycling of nitrogen source from nitrate wastewater. The metal oxides with oxygen vacancy (O) display high NH selectivities in NRR (NO/N as side products), but the complexity in O enrichment and the inferior hydrogen adsorption on oxides make NRR an inefficient process. Herein, one superior dual-site NRR electrocatalyst that is composed of O-enriched MnO nanosheets (MnO-O) and Pd nanoparticles (deposited on MnO) is constructed over the three-dimensional porous nickel foam (Pd-MnO-O/Ni foam). In a continuous-flow reaction cell, this electrode delivers a NO-N conversion rate of 642 mg N m h and a NH selectivity of 87.64% at -0.85 V Ag/AgCl when feeding 22.5 mg L of NO-N (0.875 mL min), outperforming the Pd/Ni foam (369 mg N m h, 85.02%) and MnO-O/Ni foam (118 mg N m h, 32.25%). Increasing the feeding NO-N concentration and flow rate to 180.0 mg L and 2.81 mL min can further lift the conversion rate to 1933 and 1171 mg N m h, respectively. The combination of experimental characterizations and theoretical calculations reveal that the MnO-O adsorbs, immobilizes, and activates the NO and N-intermediates, while the Pd supplies the O sites with sufficient adsorbed hydrogen (H*) for both the NRR and O refreshment. Our work presents a good example of utilizing dual-site catalysis in the highly selective conversion of NO to NH that is important for nitrate pollution abatement, nitrogen resource recycling, as well as sustainable NH production.