Boosting electrochemical nitrate-ammonia conversion via organic ligands-tuned proton transfer
Graphical Abstract
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)
- et al.
Flower-like open-structured polycrystalline copper with synergistic multi-crystal plane for efficient electrocatalytic reduction of nitrate to ammonia
Nano Energy
(2022) - et al.
Electrocatalytic nitrate/nitrite reduction to ammonia synthesis using metal nanocatalysts and bio-inspired metalloenzymes
Nano Energy
(2021) - et al.
Electrocatalytic nitrate reduction for sustainable ammonia production
Joule
(2021) - et al.
Electrochemical reduction of nitrate and nitrite in alkaline media at CuNi alloy electrodes
Electrochim. Acta
(2013) - et al.
Electrodeposition of Cu–Rh alloys and their use as cathodes for nitrate reduction
Electrochem. Commun.
(2012) - et al.
Hydrogenation of biomass-derived compounds containing a carbonyl group over a copper-based nanocatalyst: insight into the origin and influence of surface oxygen vacancies
J. Catal.
(2016) - et al.
Electrocatalytic reduction of nitrate on copper electrodes prepared by high-energy ball milling
J. Electroanal. Chem.
(2006) - et al.
Hydrogen evolution assisted electrodeposition of porous Cu-Ni alloy electrodes and their use for nitrate reduction in alkali
Electrochim. Acta
(2014) - et al.
Electroreduction of nitrate to ammonia in alkaline solutions using hydrogen storage alloy cathodes
Electrochim. Acta
(1999) - et al.
Alternative route for electrochemical ammonia synthesis by reduction of nitrate on copper nanosheets
Appl. Mater. Today
(2020)
Crafting MoC2-doped bimetallic alloy nanoparticles encapsulated within N-doped graphene as roust bifunctional electrocatalysts for overall water splitting
Nano Energy
Modulating the mechanism of electrocatalytic CO2 reduction by cobalt phthalocyanine through polymer coordination and encapsulation
Nat. Commun.
Nitrate reduction pathways on Cu single crystal surfaces: effect of oxide and Cl−
Nano Energy
First-principles mechanistic study on nitrate reduction reactions on copper surfaces: effects of crystal facets and pH
J. Catal.
Electrocatalytic reduction of nitrate at low concentration on coinage and transition-metal electrodes in acid solutions
J. Electroanal. Chem.
Electrocatalytic reduction of nitrate: fundamentals to full-scale water treatment applications
Appl. Catal. B Environ.
Analysis of nitrate, nitrite, and [15N] nitrate in biological fluids
Anal. Biochem.
Enhanced nitrate-to-ammonia activity on copper–nickel alloys via tuning of intermediate adsorption
J. Am. Chem. Soc.
Probability of nitrate contamination of recently recharged groundwaters in the conterminous United States
Environ. Sci. Technol.
Robust route to photocatalytic nitrogen fixation mediated by capitalizing on defect-tailored InVO4 Nanosheets
Environ. Sci. Nano
High-efficiency electrocatalytic NO reduction to NH3 by nanoporous VN
Nano Res. Energy
Ru-doped phosphorene for electrochemical ammonia synthesis, Ru-doped phosphorene for electrochemical ammonia synthesis
Rare Met.
Beyond fossil fuel–driven nitrogen transformations
Science
Electrochemical reduction of nitrate to ammonia via direct eight-electron transfer using a copper–molecular solid catalyst
Nat. Energy
Unveiling the activity origin of a copper‐based electrocatalyst for selective nitrate reduction to ammonia
Angew. Chem. Int. Ed.
Activity and selectivity trends in electrocatalytic nitrate reduction on transition metals
ACS Catal.
Combining theory and experiment in electrocatalysis: insights into materials design
Science
Promoted oxygen reduction kinetics on nitrogen-doped hierarchically porous carbon by engineering proton-feeding centers
Energy Environ. Sci.
The hydrogen evolution reaction in alkaline solution: from theory, single crystal models, to practical electrocatalysts
Angew. Chem. Int. Ed.
Trends in activity for the water electrolyser reactions on 3d M (Ni, Co, Fe, Mn) hydr (oxy) oxide catalysts
Nat. Mater.
Hydroxide promotes carbon dioxide electroreduction to ethanol on copper via tuning of adsorbed hydrogen
Nat. Commun.
Boosting selective nitrate electroreduction to ammonium by constructing oxygen vacancies in TiO2
ACS Catal.
Six-electron reduction of nitrite to ammonia by cytochrome c nitrite reductase: insights from density functional theory studies
Inorg. Chem.
Hydrogen bonding networks tune proton-coupled redox steps during the enzymatic six-electron conversion of nitrite to ammonia
Biochemistry
Cited by (21)
Hollow mesoporous carbon supported Co-modified Cu/Cu<inf>2</inf>O electrocatalyst for nitrate reduction reaction
2024, Journal of Colloid and Interface ScienceTuning work function difference of copper/cobalt oxides heterointerfaces enables efficient electrochemical nitrate reduction
2024, Applied Catalysis B: EnvironmentalHigh-valent cobalt active sites derived from electrochemical activation of metal-organic frameworks for efficient nitrate reduction to ammonia
2024, Applied Catalysis B: EnvironmentalInterfacial engineering of CoMn<inf>2</inf>O<inf>4</inf>/NC induced electronic delocalization boosts electrocatalytic nitrogen oxyanions reduction to ammonia
2023, Applied Catalysis B: EnvironmentalCitation Excerpt :The peaks at 323, 378 and 664 cm−1 were assigned to CoMn2O4 and did not change significantly during the reaction [49], indicating good structural stability. Besides, the Raman peak at 982 and 1049 cm−1 was referenced as adsorbed sulfate species (SO42−) and NO3− on the surface of catalysts [50,51], respectively. It is worth noting that a new peak appears around 1591 cm−1, which is assigned to *NH3 [51,52].
Tailored electronic structure by sulfur filling oxygen vacancies boosts electrocatalytic nitrogen oxyanions reduction to ammonia
2023, Chemical Engineering JournalCitation Excerpt :Raman spectra indicated that S5-Co3O4 retained the initial structure. The characteristic peaks of NO3− and SO42− ions are located around 1050 and 982 cm−1 (Fig. S17a) [47,48], respectively. Moreover, another group of peaks appeared at about 3500 cm−1, which belonged to coordinated hydrogen-bonded water molecules (2-HB·H2O and 4-HB·H2O) (Fig. S17b) [49].