An official website of the United States government
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.
more »
« less
Electro- and Photocatalytic Conversion of N 2 to NH 3 by Chemically Modified Transition Metal Dichalcogenides, MoS 2 , and WS 2
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 .
more »
« less
- PAR ID:
- 10425289
- Date Published:
- Journal Name:
- Journal of The Electrochemical Society
- Volume:
- 170
- Issue:
- 5
- ISSN:
- 0013-4651
- Page Range / eLocation ID:
- 056501
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Atmospheric nitrogen fixation using a photocatalytic system is a promising approach to produce ammonia. However, most of the recently explored photocatalysts for N 2 fixation are in the powder form, suffering from agglomeration and difficulty in the collection and leading to unsatisfactory conversion efficiency. Developing efficient film catalysts for N 2 photofixation under ambient conditions remains challenging. Herein, we report the efficient photofixation of N 2 over a periodic WS 2 @TiO 2 nanoporous film, which is fabricated through a facile method that combines anodization, E-beam evaporation, and chemical vapor deposition (CVD). Oxygen vacancies are introduced into TiO 2 nanoporous films through Ar annealing treatment, which plays a vital role in N 2 adsorption and activation. The periodic WS 2 @TiO 2 nanoporous film with an optimized WS 2 content shows highly efficient photocatalytic performance for N 2 fixation with an NH 3 evolution rate of 1.39 mmol g −1 h −1 , representing one of the state-of-the-art catalysts.more » « less
-
Electroreduction of N 2 to NH 3 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 V III/IV -oxide films, created by O 2 plasma oxidation of polycrystalline vanadium, exhibiting N 2 reduction at neutral pH with an onset potential of −0.16 V vs Ag/AgCl. DFT calculations indicate that N 2 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.more » « less
-
null (Ed.)Due to its clean and sustainable nature, solar energy has been widely recognized as a green energy source in driving a variety of reactions, ranging from small molecule activation and organic transformation to biomass valorization. Within this context, organic reactions coupled with H 2 evolution via semiconductor-based photocatalytic systems under visible light irradiation have gained increasing attention in recent years, which utilize both excited electrons and holes generated on semiconductors and produce two types of value-added products, organics and H 2 , simultaneously. Based on the nature of the organic reactions, in this review article we classify semiconductor-based photocatalytic organic transformations and H 2 evolution into three categories: (i) photocatalytic organic oxidation reactions coupled with H 2 production, including oxidative upgrading of alcohols and biomass-derived intermediate compounds; (ii) photocatalytic oxidative coupling reactions integrated with H 2 generation, such as C–C, C–N, and S–S coupling reactions; and (iii) photo-reforming reactions together with H 2 formation using organic plastics, pollutants, and biomass as the substrates. Representative heterogeneous photocatalytic systems will be highlighted. Specific emphasis will be placed on their synthesis, characterization, and photocatalytic mechanism, as well as the organic reaction scope and practical application.more » « less
-
The nitrogen electroreduction reaction (NRR) in aqueous solutions under ambient conditions represents an attractive prospect to produce ammonia, but the development of long-term stable and low-cost catalysts with high-efficiency and high-selectivity remains a great challenge. Herein, we investigated the potential of a new class of experimentally available boron-containing materials, i.e. , cubic boron phosphide (BP) and boron arsenide (BAs), as metal-free NRR electrocatalysts by means of density functional theory (DFT) calculations. Our results revealed that gas phase N 2 can be sufficiently activated on the B-terminated (111) polar surfaces of BP and BAs, and effectively reduced to NH 3 via an enzymatic pathway with an extremely low limiting potential (−0.12 V on BP and −0.31 V on BAs, respectively). In particular, the two proposed B-terminated (111) surfaces not only have a large active region for N 2 reduction, but also can significantly inhibit the competitive hydrogen evolution reaction, and thus have rather high efficiency and selectivity for the NRR. Therefore, cubic BP or BAs with mainly exposed (111) facets may serve as promising metal-free NRR catalysts with superior performance.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript
- Journal Article:
- https://doi.org/10.1149/1945-7111/acd02d
