Title: Interaction of molecular nitrogen with vanadium oxide in the absence and presence of water vapor at room temperature: Near-ambient pressure XPS
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. more »« less
Ashwin Ganesan, Adaeze Osonkie
(, 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.
Chukwunenye, Precious; Ganesan, Ashwin; Gharaee, Mojgan; Balogun, Kabirat; Anwar, Fatima; Adesope, Qasim; Cundari, Thomas R.; D'Souza, Francis; Kelber, Jeffry A.
(, 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.
Chukwunenye, Precious; Ganesan, Ashwin; Gharaee, Mojgan; Balogun, Kabirat; Adesope, Qasim; Amagbor, Stella Chinelo; Golden, Teresa D.; D’Souza, Francis; Cundari, Thomas R.; Kelber, Jeffry A.
(, Physical Chemistry Chemical Physics)
The production of ammonia for agricultural and energy demands has accelerated research for more environmentally-friendly synthesis options, particularly the electrocatalytic reduction of molecular nitrogen (nitrogen reduction reaction, NRR). Catalyst activity for NRR, and selectivity for NRR over the competitive hydrogen evolution reaction (HER), are critical issues for which fundamental knowledge remains scarce. Herein, we present results regarding the NRR activity and selectivity of sputter-deposited titanium nitride and titanium oxynitride films for NRR and HER. Electrochemical, fluorescence and UV absorption measurements show that titanium oxynitride exhibits NRR activity under acidic conditions (pH 1.6, 3.2) but is inactive at pH 7. Ti oxynitride is HER inactive at all these pH values. In contrast, TiN – with no oxygen content upon deposition – is both NRR and HER inactive at all the above pH values. This difference in oxynitride/nitride reactivity is observed despite the fact that both films exhibit very similar surface chemical compositions – predominantly Ti IV oxide – upon exposure to ambient, as determined by ex situ X-ray photoelectron spectroscopy (XPS). XPS, with in situ transfer between electrochemical and UHV environments, however, demonstrates that this Ti IV oxide top layer is unstable under acidic conditions, but stable at pH 7, explaining the inactivity of titanium oxynitride at this pH. The inactivity of TiN at acidic and neutral pH is explained by DFT-based calculations showing that N 2 adsorption at N-ligated Ti centers is energetically significantly less favorable than at O-ligated centers. These calculations also predict that N 2 will not bind to Ti IV centers due to a lack of π-backbonding. Ex situ XPS measurements and electrochemical probe measurements at pH 3.2 demonstrate that Ti oxynitride films undergo gradual dissolution under NRR conditions. The present results demonstrate that the long-term catalyst stability and maintenance of metal cations in intermediate oxidation states for pi-backbonding are critical issues worthy of further examination.
Balogun, Kabirat; Ganesan, Ashwin; Chukwunenye, Precious; Gharaee, Mojgan; Adesope, Qasim; Nemšák, Slavomir; Bagus, Paul S; Cundari, Thomas R; D’Souza, Francis; Kelber, Jeffry A
(, 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.
The activation of reactants by catalytically active metal sites at metal-oxide interfaces is important for understanding the effect of metal-support interactions on nanoparticle catalysts and for tuning activity and selectivity. Using a combined experimental and theoretical approach, we studied the activation of H2 and the effect of CO poisoning on isolated Rh atoms completely or partially covered by a copper oxide (Cu2O) thin film. Temperature programmed desorption (TPD) experiments conducted in ultra-high vacuum (UHV) show that neither a partially nor a fully oxidized Cu2O layer grown on a Rh/Cu(111) single-atom alloy can activate hydrogen in UHV. However, in situ ambient pressure X-ray photoelectron spectroscopy (AP-XPS) experiments performed at elevated H2 pressures reveal that Rh significantly accelerates the reduction of these Cu2O thin films by hydrogen. Remarkably, the fastest reduction rate is observed for the fully oxidized sample with all Rh sites covered by Cu2O. Both TPD and AP-XPS data demonstrate that these covered Rh sites are inaccessible to CO, indicating that Rh under Cu2O is active for H2 dissociation but cannot be poisoned by CO. In contrast, an incomplete oxide film leaves some of the Rh sites exposed and accessible to CO, and hence prone to CO poisoning. Density functional theory calculations demonstrate that unlike many reactions in which hydrogen activation is rate limiting, the rate-determining step in the dissociation of H2 on thin-film Cu2O with Rh underneath is the adsorption of H2 on the buried Rh site, and once adsorbed, the dissociation of H2 is barrierless. These calculations also explain why H2 can only be activated at higher pressures. Together, these results highlight how different the reactivity of atomically dispersed Rh in Cu can be depending on its accessibility through the oxide layer, providing a way to engineer Rh sites that are active for hydrogen activation but resilient to CO poisoning.
Balogun, K., Chukwunenye, P., Anwar, F., Ganesan, A., Adesope, Q., Willadsen, D., Nemšák, S., Cundari, T. R., Bagus, P. S., D’Souza, F., and Kelber, J. A. Interaction of molecular nitrogen with vanadium oxide in the absence and presence of water vapor at room temperature: Near-ambient pressure XPS. The Journal of Chemical Physics 157.10 Web. doi: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., & Kelber, J. A. Interaction of molecular nitrogen with vanadium oxide in the absence and presence of water vapor at room temperature: Near-ambient pressure XPS. The Journal of Chemical Physics, 157 (10). 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., and Kelber, J. A.
"Interaction of molecular nitrogen with vanadium oxide in the absence and presence of water vapor at room temperature: Near-ambient pressure XPS". The Journal of Chemical Physics 157 (10). Country unknown/Code not available: American Institute of Physics. https://doi.org/10.1063/5.0107678.https://par.nsf.gov/biblio/10440384.
@article{osti_10440384,
place = {Country unknown/Code not available},
title = {Interaction of molecular nitrogen with vanadium oxide in the absence and presence of water vapor at room temperature: Near-ambient pressure XPS},
url = {https://par.nsf.gov/biblio/10440384},
DOI = {10.1063/5.0107678},
abstractNote = {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.},
journal = {The Journal of Chemical Physics},
volume = {157},
number = {10},
publisher = {American Institute of Physics},
author = {Balogun, K. and Chukwunenye, P. and Anwar, F. and Ganesan, A. and Adesope, Q. and Willadsen, D. and Nemšák, S. and Cundari, T. R. and Bagus, P. S. and D’Souza, F. and Kelber, J. A.},
}
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