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1. Electrochemical Reduction of N2 to Ammonia by Vanadium Oxide Thin Films at Neutral pH: Oxophilicity and the NRR Reaction

   https://doi.org/https://doi.org/10.1149/1945-7111/abde7f  

   Ashwin Ganesan, Adaeze Osonkie (February 2021, Journal of the Electrochemical Society)

   null (Ed.)

   Electroreduction of N2 to NH3 is an energy- and environmentally-friendly alternative to the Haber-Bosch process. Little is known, however, about reactive sites for electrochemical nitrogen reduction reaction (NRR) at Earth-abundant oxide or oxynitride surfaces. Here, we report N-free VIII/IV-oxide films, created by O2 plasma oxidation of polycrystalline vanadium, exhibiting N2 reduction at neutral pH with an onset potential of −0.16 V vs Ag/AgCl. DFT calculations indicate that N2 scission from O-supported V-centers is energetically favorable by ~18 kcal mol−1 compared to N-supported sites. Theory and experiment yield fundamental insights concerning the effect of metal oxophilicity towards design of earth-abundant NRR electrocatalysts. 

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2. 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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3. Vanadium oxide, vanadium oxynitride, and cobalt oxynitride as electrocatalysts for the nitrogen reduction reaction: a review of recent developments

   https://doi.org/10.1088/1361-648X/acd49d  

   Balogun, Kabirat; Ganesan, Ashwin; Chukwunenye, Precious; Gharaee, Mojgan; Adesope, Qasim; Nemšák, Slavomir; Bagus, Paul S; Cundari, Thomas R; D’Souza, Francis; Kelber, Jeffry A (May 2023, Journal of Physics: Condensed Matter)

   Abstract The electrocatalytic reduction of molecular nitrogen to ammonia—the nitrogen reduction reaction (NRR)—is of broad interest as an environmentally- and energy-friendly alternative to the Haber–Bosch process for agricultural and emerging energy applications. Herein, we review our recent findings from collaborative electrochemistry/surface science/theoretical studies that counter several commonly held assumptions regarding transition metal oxynitrides and oxides as NRR catalysts. Specifically, we find that for the vanadium oxide, vanadium oxynitride, and cobalt oxynitride systems, (a) there is no Mars–van Krevelen mechanism and that the reduction of lattice nitrogen and N₂to NH₃occurs by parallel reaction mechanisms at O-ligated metal sites without incorporation of N into the oxide lattice; and (b) that NRR and the hydrogen evolution reaction do occur in concert under the conditions studied for Co oxynitride, but not for V oxynitride. Additionally, these results highlight the importance of both O-ligation of the V or Co center for metal-binding of dinitrogen, and the importance of N in stabilizing the transition metal cation in an intermediate oxidation state, for effective N≡N bond activation. This review also highlights the importance and limitations ofex situandin situphotoemission—involving controlled transfer between ultra-high vacuum and electrochemistry environments, and ofoperandonear ambient pressure photoemission coupled within situstudies, in elucidating the complex chemistry relevant to the electrolyte/solid interface. 

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4. 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, Kabirat Yetunde; Chukwunenye, Precious; Anwar, Fatima; Ganesan, Ashwin; Adesope, Qasim; Willadsen, Dominic; Nemsak, Slavomir; Cundari, Thomas R; Bagus, Paul S.; D'Souza, Francis; et al (August 2022, The Journal of Chemical Physics)

   Interactions of N2 at oxide surfaces are important for understanding electrocatalytic nitrogen reduction reaction 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 401.0 eV and 398.7 eV binding energy, with relative intensities of ~ 3:1, respectively. These features remained upon subsequent pump down to 10^-9 Torr. Observed maximum N surface coverage was ~ 1.5 x 10^13 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 calculated shake-up and normal, final ionic configurations, for N2 end-on bonding to V(III) sites, but not V(IV) sites. 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 electrolyte under NRR-relevant conditions. 

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5. Electro- and Photocatalytic Conversion of N ₂ to NH ₃ by Chemically Modified Transition Metal Dichalcogenides, MoS ₂ , and WS ₂

   https://doi.org/10.1149/1945-7111/acd02d  

   Ganesan, Ashwin; Alhowity, Samar; Alsaleh, Ajyal Z.; Guragain, Manan; Omolere, Olatomide; Cundari, Thomas R.; Kelber, Jeffry; D’Souza, Francis (May 2023, Journal of The Electrochemical Society)

   Electro- and photocatalytic reduction of N 2 to NH 3 —the nitrogen reduction reaction (NRR)—is an environmentally- and energy-friendly alternative to the Haber-Bosch process for ammonia production. There is a great demand for the development of novel semiconductor-based electrocatalysts with high efficiency and stability for the direct conversion of inert substrates—including N 2 to ammonia—using visible light irradiation under ambient conditions. Herein we report electro-, and photocatalytic NRR with transition metal dichalcogenides (TMDCs), viz MoS 2 and WS 2 . Improved acid treatment of bulk TMDCs yields exfoliated TMDCs (exTMDCs) only a few layers thick with ∼10% S vacancies. Linear scan voltammograms on exMoS 2 and exWS 2 electrodes reveal significant NRR activity for exTMDC-modified electrodes, which is greatly enhanced by visible light illumination. Spectral measurements confirm ammonia as the main reaction product of electrocatalytic and photocatalytic NRR, and the absence of hydrazine byproduct. Femtosecond-resolved transient absorption studies provide direct evidence of interaction between photo-generated excitons/trions with N 2 adsorbed at S vacancies. DFT calculations corroborate N 2 binding to exMoS 2 at S-vacancies, with substantial π -backbonding to activate dinitrogen. Our findings suggest that chemically functionalized exTMDC materials could fulfill the need for highly-desired, inexpensive catalysts for the sustainable production of NH 3 using Sunlight under neutral pH conditions without appreciable competing production of H 2 . 

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