Elsevier

Nano Energy

Volume 103, Part A, 1 December 2022, 107705
Nano Energy

Boosting electrochemical nitrate-ammonia conversion via organic ligands-tuned proton transfer

https://doi.org/10.1016/j.nanoen.2022.107705Get rights and content

Highlights

  • A partial decomposition route is developed to craft copper nanoparticles inlaid in MOF with uncoordinated carboxylate ligands.

  • The obtained copper-based nanocomposites display excellent performance for the electrochemical reduction of nitrate to ammonia.

  • Identifying that the poor water dissociation ability of copper greatly limits the proton transfer during the nitrate reduction.

  • Nearby uncoordinated carboxylate ligands enhance the water dissociation ability of copper for boosting the nitrate reduction.

Abstract

Electrochemical nitrate (NO3-) reduction reaction (NO3-RR) offers an ideal route to harvest ammonia (NH3) under ambient conditions. Despite recent advances in Cu-based NO3-RR electrocatalysts, their synthesis heavily relies on the regulation of adsorption strength towards nitrogen-containing intermediates, and other important factors are ignored (i.e., the proton transfer rate). Here, we select Cu nanoparticles (NPs) as model catalysts to investigate whether and how the proton transfer rate impacts the NO3-RR kinetics. The results indicate that the proton transfer is involved in the rate-determining step (RDS) of NO3-RR, and the weak water dissociation ability of Cu leads to slow proton transfer rate and consequently sluggish NO3-RR kinetics. To this end, we enhance the water dissociation ability of Cu NPs by incorporating uncoordinated carboxylate ligands to enable rapid proton transfer, which in turn boosts the hydrogenation of key intermediates for reducing the overall energy barrier of NO3-RR. As a result, Cu NPs with the ligands display a maximum NH3 yield rate of 496.4 mmol h−1 gcat−1, outperforming counterpart without ligands. This work not only deepens our knowledge on the NO3-RR mechanism, but also offers new guidelines for the smart design of efficient electrocatalysts.

Introduction

With world-wide application of nitrogen-containing fertilizers and other chemicals (i.e., textiles), the nitrate (NO3-) is rapidly accumulating on the surface and underground water; this has led to sever environmental issues including photochemical smog and acid rain [1], [2]. Moreover, the NO3- also poses a great threat to human health because it can be in vivo converted into carcinogenic nitrite (NO2) [3]. Thus, it is highly desirable to adopt the NO3- as raw sources to generate harmless or even value-added products, thereby closing the nitrogen cycles. As NH3 is an important nitrogen sources for the production of fertilizers and also a promising clean fuel carrier [4], [5], [6], the electrochemical nitrate (NO3-) reduction reaction (NO3-RR) to ammonia (NH3) under ambient conditions has been regarded as a promising route to remove NO3- pollutants while produce NH3 [7], [8], [9]. Recently, copper (Cu)-based electrocatalysts have demonstrated high selectivity for NO3-RR with Faradaic efficiencies towards NH3 (FENH3) over 90% [10], [11], [12]. However, the activity of NO3-RR is still greatly limited by the poor understanding of reaction mechanism, and the current densities for NO3-RR is usually less than 50 mA cm−2, [13] which renders the NH3 production rate well below the industrial Haber-Bosch. Therefore, more efforts should be concentrated on the understanding of the reaction mechanism, so as to develop efficient strategies for boosting the NO3-RR activity of Cu.

Due to the strong corrosion effect of acid solution on Cu, the Cu-catalyzed NO3-RR is usually conducted in the neutral or alkaline media. In the above two media, the NO3-RR primarily proceeds according to the previously reported eight-electron transfer process (NO3- + 6 H2O + 8e- → NH3 + 9OH-, E0 = 0.69 vs reversible hydrogen electrode (RHE), pH = 14) [1], [14]. This process requires the generation of nine protons to react with the nitrogen-containing intermediates, so-called a proton-coupled electron transfer (PCET) [15], [16]. In both neutral and alkaline media, protons are intrinsically produced via the dissociation of water molecules (H2O → H* + OH*) [17], [18]. Accordingly, the energy barrier to dissociate water undoubtedly governs the proton transfer rate and thus greatly impacts the reaction kinetics of NO3-RR. We note that such energy barrier on metallic Cu is substantially high [19], most likely leading to the slow proton transfer and limited activity for NO3-RR. However, most attention for boosting NO3-RR are paid on tuning adsorption strength of nitrogen-containing intermediates (i.e., *NO3-, *NO2, and *NH2) [9], [20], and little has been done work to enhance the water dissociation ability of Cu and thus promote the sluggish proton transfer of NO3-RR.

Herein, we sought to enhance the water dissociation ability of Cu by organic ligands with the goal of boosting proton transfer and thus reaction kinetics of NO3-RR. We started with DFT calculations to search suitable ligands and found that the uncoordinated carboxylate ligands could significantly reduce the energy barrier of water dissociation on Cu. To this end, we developed a partial decomposition route to craft Cu NPs inlaid in uncoordinated carboxylate ligands-rich metal organic framework (MOF). Deuterium kinetic isotope effects and proton inventory studies uncovered that the uncoordinated carboxylate ligands markedly accelerated the proton transfer and reaction kinetics of NO3-RR by promoting the water dissociation processes. More importantly, operando Raman spectra uncovered that the accelerated proton transfer could facilitate the hydrogenation of key intermediates (i.e., *NO and *NOH), thereby reducing the energy barrier of RDS. Consequently, the Cu-based electrocatalysts with the ligands displayed excellent performance for NO3-RR, such as an ultrahigh NH3 yield rate of 496.4 mmol h−1 gcat−1 at a small potential of − 0.2 V vs RHE, and long-term stability of 20 h.

Section snippets

Results and discussion

In the cytochrome c nitrite reductase enzyme, the carboxylate groups play important roles in promoting the proton transfer and thus the reaction kinetics of nitrite reduction [21], [22]. Inspired by this, we conceived that the uncoordinated carboxylate ligands could enhance the adsorption strength of water on Cu and favor the dissociation of water. To examine it, we first performed DFT calculations on structural models of bare Cu(111) and Cu(111) with the benzene-1,3,5-tricarboxylic acid (BTC)

Conclusions

In summary, we have demonstrated a facile yet robust approach to boost the NO3--NH3 conversion by using uncoordinated carboxylate ligands to promote the water dissociation process of Cu. In order to create an uncoordinated carboxylate ligands-rich environment for Cu NPs, a partial decomposition route was developed to synthesize Cu NPs-embed into hierarchical brush-like MOF with abundant uncoordinated carboxylate groups. Benefiting from the unique brush-like architecture and the uncoordinated

Synthesis of Cu@Cu-BTC-MOF

The rod-like Cu-BTC-MOF precursor was synthesized through a coprecipitation method based on a previous literature [62]. The Cu-BTC-MOF was then calcinated at 250 °C in air for 2 h to achieve the partial decomposition of MOF structure, and the obtained product was denoted CuO@Cu-BTC-MOF. To covert the CuO to Cu NPs, the CuO@Cu-BTC-MOF was subjected to an electrochemical reduction at the potential of − 0.4 V vs RHE in the electrolyte of 1 M KOH for 1 h, and the resulting product was denoted

CRediT authorship contribution statement

Jiaying Yu, Yongjie Qin, Hongju Zheng, Keru Gao: Carried out the synthesis, materials characterizations and electrochemical measurements. Xiaodeng Wang: Carried out DFT calculations and analyzed date from calculations. Hengpan Yang, Laiyong Xie: Analyzed the data from experiments. Qi Hu: Designed the experiments and wrote the manuscript. Chuanxin He: Conceived the project and ideal.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

We appreciate the financial support of the National Natural Science Foundation (NNSF) of China (21975162, 51902208) and Shenzhen Government’s Plan of Science and Technology (JCYJ20200109105803806, JCYJ20190808142219049, and JCYJ20180507182057026). We also acknowledged the Instrumental Analysis Centre of Shenzhen University for testing the TEM and H NMR.

References (68)

  • Q. Hu et al.

    Crafting MoC2-doped bimetallic alloy nanoparticles encapsulated within N-doped graphene as roust bifunctional electrocatalysts for overall water splitting

    Nano Energy

    (2018)
  • Y. Liu et al.

    Modulating the mechanism of electrocatalytic CO2 reduction by cobalt phthalocyanine through polymer coordination and encapsulation

    Nat. Commun.

    (2019)
  • D.P. Butcher et al.

    Nitrate reduction pathways on Cu single crystal surfaces: effect of oxide and Cl

    Nano Energy

    (2016)
  • Y. Wang et al.

    First-principles mechanistic study on nitrate reduction reactions on copper surfaces: effects of crystal facets and pH

    J. Catal.

    (2021)
  • G. Dima et al.

    Electrocatalytic reduction of nitrate at low concentration on coinage and transition-metal electrodes in acid solutions

    J. Electroanal. Chem.

    (2003)
  • S. Garcia-Segura et al.

    Electrocatalytic reduction of nitrate: fundamentals to full-scale water treatment applications

    Appl. Catal. B Environ.

    (2018)
  • L.C. Green et al.

    Analysis of nitrate, nitrite, and [15N] nitrate in biological fluids

    Anal. Biochem.

    (1982)
  • Y. Wang et al.

    Enhanced nitrate-to-ammonia activity on copper–nickel alloys via tuning of intermediate adsorption

    J. Am. Chem. Soc.

    (2020)
  • B.T. Nolan et al.

    Probability of nitrate contamination of recently recharged groundwaters in the conterminous United States

    Environ. Sci. Technol.

    (2002)
  • S. Yao et al.

    Robust route to photocatalytic nitrogen fixation mediated by capitalizing on defect-tailored InVO4 Nanosheets

    Environ. Sci. Nano

    (2022)
  • D. Qi et al.

    High-efficiency electrocatalytic NO reduction to NH3 by nanoporous VN

    Nano Res. Energy

    (2022)
  • J.-D. Liu et al.

    Ru-doped phosphorene for electrochemical ammonia synthesis, Ru-doped phosphorene for electrochemical ammonia synthesis

    Rare Met.

    (2020)
  • J.G. Chen et al.

    Beyond fossil fuel–driven nitrogen transformations

    Science

    (2018)
  • G.-F. Chen et al.

    Electrochemical reduction of nitrate to ammonia via direct eight-electron transfer using a copper–molecular solid catalyst

    Nat. Energy

    (2020)
  • Y. Wang et al.

    Unveiling the activity origin of a copper‐based electrocatalyst for selective nitrate reduction to ammonia

    Angew. Chem. Int. Ed.

    (2020)
  • J.-X. Liu et al.

    Activity and selectivity trends in electrocatalytic nitrate reduction on transition metals

    ACS Catal.

    (2019)
  • Z.W. Seh et al.

    Combining theory and experiment in electrocatalysis: insights into materials design

    Science

    (2017)
  • G. Chen et al.

    Promoted oxygen reduction kinetics on nitrogen-doped hierarchically porous carbon by engineering proton-feeding centers

    Energy Environ. Sci.

    (2020)
  • Y. Zheng et al.

    The hydrogen evolution reaction in alkaline solution: from theory, single crystal models, to practical electrocatalysts

    Angew. Chem. Int. Ed.

    (2018)
  • R. Subbaraman et al.

    Trends in activity for the water electrolyser reactions on 3d M (Ni, Co, Fe, Mn) hydr (oxy) oxide catalysts

    Nat. Mater.

    (2012)
  • M. Luo et al.

    Hydroxide promotes carbon dioxide electroreduction to ethanol on copper via tuning of adsorbed hydrogen

    Nat. Commun.

    (2019)
  • R. Jia et al.

    Boosting selective nitrate electroreduction to ammonium by constructing oxygen vacancies in TiO2

    ACS Catal.

    (2020)
  • D. Bykov et al.

    Six-electron reduction of nitrite to ammonia by cytochrome c nitrite reductase: insights from density functional theory studies

    Inorg. Chem.

    (2015)
  • E.T. Judd et al.

    Hydrogen bonding networks tune proton-coupled redox steps during the enzymatic six-electron conversion of nitrite to ammonia

    Biochemistry

    (2014)
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