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1. Electrocatalytic selectivity for nitrogen reduction vs. hydrogen evolution: a comparison of vanadium and cobalt oxynitrides at different pH values

   https://doi.org/10.1039/D2TA05180J  

   Chukwunenye, Precious ; Ganesan, Ashwin ; Gharaee, Mojgan ; Balogun, Kabirat ; Anwar, Fatima ; Adesope, Qasim ; Cundari, Thomas R. ; D'Souza, Francis ; Kelber, Jeffry A. ( October 2022 , Journal of Materials Chemistry A)

   The electrocatalytic nitrogen reduction reaction (NRR) is of significant interest as an environmentally friendly method for NH 3 production for agricultural and clean energy applications. Selectivity of NRR vis-à-vis the hydrogen evolution reaction (HER), however, is thought to adversely impact many potential catalysts, including Earth-abundant transition metal oxynitrides. Relative HER/NRR selectivities are therefore directly compared for two transition metal oxynitrides with different metal oxophilicities—Co and V. Electrocatalytic current–potential measurements, operando fluorescence, absorption, and GC measurements of H 2 and NH 3 production, ex situ X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations are combined to directly compare NRR and HER activities under identical reaction conditions. Results show that cobalt oxynitrides – with Co primarily in the Co( ii ) oxidation state – are NRR active at pH 10, with electrochemical reduction of both lattice nitrogen and dissolved N 2 , the latter occurring without N incorporation into the lattice. Removal of lattice N then yields Co( ii ) oxide, which is still NRR active. These results are complemented by calculations showing that N 2 binding at Co( ii ) sites is energetically favored over binding at Co( iii ) sites. GC analysis demonstrates that H 2 production occurs in concert with ammonia production but at a far greater rate. In contrast, vanadium oxynitride films are HER inactive under the same (pH 10) conditions, as well as at pH 7, but are NRR active at pH 7. DFT calculations indicate that a major difference in the two materials is hindered O–H dissociation of H 2 O adsorbed at O-ligated Co vs. V cation centers. The combined studies indicate significant variation in HER vs. NRR selectivity as a function of employed transition metal oxynitrides, as well as different HER mechanisms in V and Co oxynitrides. 

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2. Interaction of molecular nitrogen with vanadium oxide in the absence and presence of water vapor at room temperature: Near-ambient pressure XPS

   https://doi.org/10.1063/5.0107678  

   Balogun, K. ; Chukwunenye, P. ; Anwar, F. ; Ganesan, A. ; Adesope, Q. ; Willadsen, D. ; Nemšák, S. ; Cundari, T. R. ; Bagus, P. S. ; D’Souza, F. ; et al ( September 2022 , The Journal of Chemical Physics)

   Interactions of N2 at oxide surfaces are important for understanding electrocatalytic nitrogen reduction reaction (NRR) mechanisms. Interactions of N2 at the polycrystalline vanadium oxide/vapor interface were monitored at room temperature and total pressures up to 10−1 Torr using Near-Ambient Pressure X-ray Photoelectron Spectroscopy (NAP-XPS). The oxide film was predominantly V(IV), with V(III) and V(V) components. XPS spectra were acquired in environments of both pure N2 and equal pressures of N2 and H2O vapor. In pure N2, broad, partially resolved N1s features were observed at binding energies of 401.0 and 398.7 eV, with a relative intensity of ∼3:1, respectively. These features remained upon subsequent pumpdown to 10−9 Torr. The observed maximum N surface coverage was ∼1.5 × 1013 cm−2—a fraction of a monolayer. In the presence of equal pressures of H2O, the adsorbed N intensity at 10−1 Torr is ∼25% of that observed in the absence of H2O. The formation of molecularly adsorbed H2O was also observed. Density functional theory-based calculations suggest favorable absorption energies for N2 bonding to both V(IV) and V(III) cation sites but less so for V(V) sites. Hartree–Fock-based cluster calculations for N2–V end-on adsorption show that experimental XPS doublet features are consistent with the calculated shake-up and normal, final ionic configurations for N2 end-on bonding to V(III) sites but not V(IV) sites. The XPS spectra of vanadium oxide transferred in situ between electrochemical and UHV environments indicate that the oxide surfaces studied here are stable upon exposure to the electrolyte under NRR-relevant conditions.

    

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3. Advanced Reactor Design for Ammonia Production from Electrochemical Nitrogen and Nitrate Reduction

   Yifu Chen, Wenzhen Li ( November 2021 , AIChE 2021 Annual Conference)

   Motivated by the increasing demand for flexible and sustainable routes of ammonia (NH3) production, the electrochemical nitrogen (N2) and nitrate reduction reaction (NRR and NO3RR) have attracted intense research interest in the past few years1,2. Compared to the centralized Haber-Bosch process that operates at elevated temperature and pressure, the electrochemical pathway features mild operating conditions but high input energy density, allowing for distributed and on-site generation of NH3 with water as the proton source, thereby reducing the transportation and storage costs of NH3 and H23. Besides N2 which is highly abundant in the atmosphere, nitrate-N exists widely in agricultural and industrial wastewaters, and its presence has raised severe concerns due to its known impacts on the environment and human health4,5. In this regard, NO3RR provides a promising strategy of simultaneously removing the harmful nitrate-N and generating NH3 as a useful product from those wastewater streams. While research activities on both NRR and NO3RR are blooming with substantial progress in the field of electrocatalysis, some major challenges remain unnoticed or unresolved so far. Due to the wide existence of reactive N-containing species in laboratory environments, the source of NH3 in NRR measurements is sometimes elusive and requires rigorous examination by control experiments with costly 15N26,7. On the other hand, while the electro-reduction of nitrate is much more facile, additional costs arising from the enrichment and purification of nitrate in contaminated waste resources have challenged the practical feasibility of NO3RR both technically and economically2. In this talk, we will present our latest research progress as part of the solutions to these challenges in state-of-the-art NRR and NO3RR studies, from the perspective of reactor design. By taking advantage of the prior developments in 15N2 control experiments, here we suggest an improved 15N2 circulation system that is effective and affordable for NRR research, allowing for more accurate and economized quantitative assessment of NH3 origins, so that false positives and subtle catalytic activities can be identified more reliably. For NO3RR, we developed a compact reactor system for rapid and efficient electrochemical conversion of nitrate to NH3 from real nitrate-containing waste sources, accompanied by the concurrent separation and enrichment of the produced NH3 in a trapping solution to yield pure ammonium compounds. Our work highlights the importance of advanced reactor design in N-related electrochemistry research, which will facilitate the transformation of the current N-centric chemical industries towards a sustainable future. 

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4. Computational Screening of Efficient Single‐Atom Catalysts Based on Graphitic Carbon Nitride (g‐C 3 N 4 ) for Nitrogen Electroreduction

   https://doi.org/10.1002/smtd.201800368  

   Chen, Zhe ; Zhao, Jingxiang ; Cabrera, Carlos R. ; Chen, Zhongfang ( December 2018 , Small Methods)

   The development of low‐cost and efficient electrocatalysts for nitrogen reduction reaction (NRR) at ambient conditions is crucial for NH₃synthesis and provides an alternative to the traditional Harber‐Bosch process. Herein, by means of density functional theory (DFT) computations, the catalytic performance of a series of single metal atoms supported on graphitic carbon nitride (g‐C₃N₄) for NRR is evaluated. Among all the candidates, the Gibbs free energy change of the potential‐determining step for five single‐atom catalysts (SACs), namely Ti, Co, Mo, W, and Pt atoms supported on g‐C₃N₄monolayer, is lower than that on the Ru(0001) stepped surface. In particular, the single tungsten (W) atom anchored on g‐C₃N₄(W@g‐C₃N₄) exhibits the highest catalytic activity toward NRR with a limiting potential of −0.35 V via associative enzymatic pathway, and can well suppress the competing hydrogen evolution reaction. The high NRR activity and selectivity of W@g‐C₃N₄are attributed to its inherent properties, such as significant positive charge and large spin moment on the W atom, excellent electrical conductivity, and moderate adsorption strength with NRR intermediates. This work opens up a new avenue of N₂reduction for renewable energy supplies and helps guide future development of single‐atom catalysts for NRR and other related electrochemical process.

    

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5. Atomically Dispersed Single Ni Site Catalysts for Nitrogen Reduction toward Electrochemical Ammonia Synthesis Using N 2 and H 2 O

   https://doi.org/10.1002/smtd.201900821  

   Mukherjee, Shreya ; Yang, Xiaoxuan ; Shan, Weitao ; Samarakoon, Widitha ; Karakalos, Stavros ; Cullen, David A. ; More, Karren ; Wang, Maoyu ; Feng, Zhenxing ; Wang, Guofeng ; et al ( February 2020 , Small Methods)

   Ammonia (NH₃) electrosynthesis gains significant attention as NH₃is essentially important for fertilizer production and fuel utilization. However, electrochemical nitrogen reduction reaction (NRR) remains a great challenge because of low activity and poor selectivity. Herein, a new class of atomically dispersed Ni site electrocatalyst is reported, which exhibits the optimal NH₃yield of 115 µg cm⁻²h⁻¹at –0.8 V versus reversible hydrogen electrode (RHE) under neutral conditions. High faradic efficiency of 21 ± 1.9% is achieved at ‐0.2 V versus RHE under alkaline conditions, although the ammonia yield is lower. The Ni sites are stabilized with nitrogen, which is verified by advanced X‐ray absorption spectroscopy and electron microscopy. Density functional theory calculations provide insightful understanding on the possible structure of active sites, relevant reaction pathways, and confirm that the Ni‐N₃sites are responsible for the experimentally observed activity and selectivity. Extensive controls strongly suggest that the atomically dispersed NiN₃site‐rich catalyst provides more intrinsically active sites than those in N‐doped carbon, instead of possible environmental contamination. This work further indicates that single‐metal site catalysts with optimal nitrogen coordination is very promising for NRR and indeed improves the scaling relationship of transition metals.

    

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