Using Atomically Dispersed Bimetallic Active Sites to Probe the Nitrate Reduction Mechanism to Ammonia

Abstract

The production of synthetic ammonia via nitrogen reduction is a critical step toward producing fertilizers, central the supporting global food security. To date, the energy-intensive and greenhouse gas-emitting Haber-Bosch process remains the only way to produce ammonia on industrial scale. Recently, significant research efforts have focused on identifying carbon- neutral and distributed pathways to synthesize ammonia, mainly via the electrochemical reduction of dinitrogen (N2).1 This approach, however, is limited by the low N2 solubility, competing hydrogen evolution reactions, and the stability of the N≡N triple bond. A promising alternative pathway to ammonia production is through the reduction of nitrate (NO3 -), a more kinetically and thermodynamically favorable nitrogen precursor. The reduction of nitrate to ammonia provides dual benefits as both a carbon-neutral pathway to ammonia and as a way of denitrification to remove all-too-prevalent and environmentally harmful nitrate contamination, while creating a useful product.2 Although there exists in the literature several works using metal catalysts for the nitrate reduction reaction (NTRR) to ammonia, only very recently have single atom catalysts (SAC) been applied to nitrate reduction.3 , 4 The NTRR is an 8e- transfer reaction, in which the exact electron transfer pathways, either a direct 8e- transfer or by 2 + 6e- pathway, in which a nitrite (NO2 -) intermediate is desorbed and reabsorbed. These processes are not well-studied on SACs. In this talk, we will present a series of atomically dispersed iron and molybdenum, mono and bi-metallic nitrogen doped carbon (M-N-C, M = Fe, Mo, FeMo) electrocatalysts for the NTRR. Our catalysts show potential for faradic efficiencies over 90%, while exhibiting outstanding stability over 48 hours of operation. Furthermore, our results suggest distinct NTRR mechanisms over Fe and Mo active sites, revealing new insights into preferred NTRR pathways over varying single atom sites. Such a detailed understanding of the NTRR electron transfer pathway will not only advance development of high performance, selective SACs, but stands to provide a new route to environmentally responsible ammonia synthesis. References (1) Choi, J.; Suryanto, B. H. R.; Wang, D.; Du, H. L.; Hodgetts, R. Y.; Ferrero Vallana, F. M.; MacFarlane, D. R.; Simonov, A. N. Identification and Elimination of False Positives in Electrochemical Nitrogen Reduction Studies. Nat. Commun. 2020, 11 (1).(2) van Langevelde, P. H.; Katsounaros, I.; Koper, M. T. M. Electrocatalytic Nitrate Reduction for Sustainable Ammonia Production. Joule 2021, 5 (2),(3) Zhu, T.; Chen, Q.; Liao, P.; Duan, W.; Liang, S.; Yan, Z.; Feng, C. Single-Atom Cu Catalysts for Enhanced Electrocatalytic Nitrate Reduction with Significant Alleviation of Nitrite Production. Small 2020, 16 (49).(4) Niu, H.; Zhang, Z.; Wang, X.; Wan, X.; Shao, C.; Guo, Y. Theoretical Insights into the Mechanism of Selective Nitrate-to-Ammonia Electroreduction on Single-Atom Catalysts. Adv. Funct. Mater. 2020, 2008533 (3), 1–8.. Figure 1