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

Abstract

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 (O_v) display high NH₃ selectivities in NRR (NO₂⁻/N₂ as side products), but the complexity in O_v 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_v-enriched MnO₂ nanosheets (MnO₂-O_v) and Pd nanoparticles (deposited on MnO₂) is constructed over the three-dimensional porous nickel foam (Pd–MnO₂-O_v/Ni foam). In a continuous-flow reaction cell, this electrode delivers a NO₃⁻-N conversion rate of 642 mg N m⁻²_electrode 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⁻²_electrode h⁻¹, 85.02%) and MnO₂-O_v/Ni foam (118 mg N m⁻²_electrode 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⁻²_electrode h⁻¹, respectively. The combination of experimental characterizations and theoretical calculations reveal that the MnO₂-O_v adsorbs, immobilizes, and activates the NO₃⁻ and N-intermediates, while the Pd supplies the O_v sites with sufficient adsorbed hydrogen (H*) for both the NRR and O_v 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.