ArticlePublisher preview available

Efficient Nitrate Synthesis via Ambient Nitrogen Oxidation with Ru‐Doped TiO2/RuO2 Electrocatalysts

Wiley
Advanced Materials
Authors:
Min Kuang
Min Kuang
Min Kuang
  • This person is not on ResearchGate, or hasn't claimed this research yet.
Yu Wang at Chongqing University
Yu Wang
Wei Fang at Leibniz Universität Hannover
Wei Fang
Huiteng Tan
Huiteng Tan
Huiteng Tan
  • This person is not on ResearchGate, or hasn't claimed this research yet.
Show all 10 authorsHide
Read publisher preview
Download citation
To read the full-text of this research, you can request a copy directly from the authors.

Abstract and Figures

A facile pathway of the electrocatalytic nitrogen oxidation reaction (NOR) to nitrate is proposed, and Ru‐doped TiO2/RuO2 (abbreviated as Ru/TiO2) as a proof‐of‐concept catalyst is employed accordingly. Density functional theory (DFT) calculations suggest that Ruδ+ can function as the main active center for the NOR process. Remarkably doping Ru into the TiO2 lattice can induce an upshift of the d‐band center of the Ru site, resulting in enhanced activity for accelerating electrochemical conversion of inert N2 to active NO*. Overdoping of Ru ions will lead to the formation of additional RuO2 on the TiO2 surface, which provides oxygen evolution reaction (OER) active sites for promoting the redox transformation of the NO* intermediate to nitrate. However, too much RuO2 in the catalyst is detrimental to both the selectivity of the NOR and the Faradaic efficiency due to the dominant OER process. Experimentally, a considerable nitrate yield rate of 161.9 µmol h−1 gcat−1 (besides, a total nitrate yield of 47.9 µg during 50 h) and a highest nitrate Faradaic efficiency of 26.1% are achieved by the Ru/TiO2 catalyst (with the hybrid composition of RuxTiyO2 and extra RuO2 by 2.79 wt% Ru addition amount) in 0.1 m Na2SO4 electrolyte. A novel Ru‐doped TiO2/RuO2 composite catalyst is designed and applied to the electrochemical nitrogen oxidation reaction, which enables the efficient conversion of nitrogen gas to aqueous nitrate under ambient condition. The optimized Ru‐doped TiO2/RuO2 catalyst shows a considerable nitrate yield rate of 161.9 µmol h−1 gcat−1 and a highest nitrate Faradaic efficiency of 26.1%.
DFT calculations for electrochemical NOR process. a) Geometric structure of RuxTiyO2. b) NOR intermediates on Ruδ⁺ in doped TiO2. The cyan, green, red, blue, and yellow balls denote Ti, Ru, O, N, and H atoms, respectively. c) Calculated free energy diagram of NOR on RuO2 and Ruδ⁺. d) Partial density of states of Ru site in RuO2 and Ruδ⁺. The red dashed and black solid lines represent the positions of d‐band center (εd) and Fermi level (Ef), respectively. e) Top (upper) and side (bottom) views of deformation electronic density of RuxTiyO2. Cyan and violet refer to electron depletion and accumulation regions, respectively.
DFT calculations for electrochemical NOR process. a) Geometric structure of RuxTiyO2. b) NOR intermediates on Ruδ⁺ in doped TiO2. The cyan, green, red, blue, and yellow balls denote Ti, Ru, O, N, and H atoms, respectively. c) Calculated free energy diagram of NOR on RuO2 and Ruδ⁺. d) Partial density of states of Ru site in RuO2 and Ruδ⁺. The red dashed and black solid lines represent the positions of d‐band center (εd) and Fermi level (Ef), respectively. e) Top (upper) and side (bottom) views of deformation electronic density of RuxTiyO2. Cyan and violet refer to electron depletion and accumulation regions, respectively.
… 
Catalyst characterizations. a–c) XRD patterns of full view (a) and close‐up views (b,c) of as‐prepared Ru/TiO2 catalysts. d) STEM image and e) HRTEM image, along with f) EDXS mapping of Ti, O, and Ru of the 2.79Ru/TiO2 catalyst. g) XPS spectra of Ru 3d of the 2.79Ru/TiO2 catalyst. h,i) XPS spectra of Ti 2p (h) and O 1s (i) of the 2.79Ru/TiO2 catalyst and TiO2.
Catalyst characterizations. a–c) XRD patterns of full view (a) and close‐up views (b,c) of as‐prepared Ru/TiO2 catalysts. d) STEM image and e) HRTEM image, along with f) EDXS mapping of Ti, O, and Ru of the 2.79Ru/TiO2 catalyst. g) XPS spectra of Ru 3d of the 2.79Ru/TiO2 catalyst. h,i) XPS spectra of Ti 2p (h) and O 1s (i) of the 2.79Ru/TiO2 catalyst and TiO2.
… 
a) Nitrate yield rates by NOR for 10 h at 2.2 V versus RHE in 0.1 m Na2SO4 with different Ru addition amounts of doped TiO2 and pure RuO2 and b) their corresponding LSV curves. c) The NOR mechanism of Ru/TiO2 composite electrocatalysts.
a) Nitrate yield rates by NOR for 10 h at 2.2 V versus RHE in 0.1 m Na2SO4 with different Ru addition amounts of doped TiO2 and pure RuO2 and b) their corresponding LSV curves. c) The NOR mechanism of Ru/TiO2 composite electrocatalysts.
… 
Electrochemical NOR performance of the 2.79Ru/TiO2 electrode in 0.1 m Na2SO4. a) Chronoamperometry curves at each given potential. b) Yield rates and c) Faradaic efficiencies of nitrate at the corresponding potentials. d) IC spectra of the products. e) ¹⁵N‐NMR spectra of standard Na¹⁵NO3 and the NOR product.
Electrochemical NOR performance of the 2.79Ru/TiO2 electrode in 0.1 m Na2SO4. a) Chronoamperometry curves at each given potential. b) Yield rates and c) Faradaic efficiencies of nitrate at the corresponding potentials. d) IC spectra of the products. e) ¹⁵N‐NMR spectra of standard Na¹⁵NO3 and the NOR product.
… 
Figures - available from: Advanced Materials
This content is subject to copyright. Terms and conditions apply.
A preview of this full-text is provided by Wiley.
Learn more
Content available from Advanced Materials 
This content is subject to copyright. Terms and conditions apply.
 ( of ) ©  WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.advmat.de
CommuniCation
Ecient Nitrate Synthesis via Ambient Nitrogen Oxidation
with Ru-Doped TiO/RuO Electrocatalysts
Min Kuang, Yu Wang, Wei Fang,* Huiteng Tan, Mengxin Chen, Jiandong Yao,
Chuntai Liu, Jianwei Xu, Kun Zhou,* and Qingyu Yan*
DOI: ./adma.
The exploration of more accessible
approaches to activate and convert the
NN covalent triple bond of dinitrogen
directly into high value-added products
has drawn tremendous interest.[] Hence,
the electrochemical nitrogen (N) fixation
under mild conditions[] represents a prom-
ising alternative to the energy-intensive
industrial processes (such as the Haber–
Bosch[] and Birkeland–Eyde[] processes). To
date, researchers have experimented with
applying various electrocatalysts (including
metal[–] and their derivatives[–] as well
as metal-free alternatives[,]) to N reduc-
tion reaction (NRR) for ambient ammonia
synthesis. However, the electrocatalytic N
oxidation reaction (NOR) under ambient
condition, which could eectively couple
with NRR to construct a full electro-
chemical cell, is still rarely explored. More
importantly, such electrochemical cell only
requires the ambient N and water as the
feedstocks, and thus is very promising as
well as cost-eective. On the other hand, as an aqueous form of
oxidized nitrogen, nitrate is of crucial importance to the produc-
tion of chemical fertilizer, explosives, and gunpowder,[] which
plays a significant role in our modern society.
So far, substantial eorts have been focused on doping[]/
compositing[] engineering or nanostructural tailoring[,] to
improve the NRR rates. With regard to electrochemical NOR,
it is proposed that the conversion of N into nitrate involves
two main steps: the first step is the electrochemical transfor-
mation of inert N into the active NO* intermediate, which is
regarded as the rate-limiting step; the second step is a nonelec-
trochemical step of redox reaction (NO* reacts with HO and
the generated O* from electrocatalytic water splitting to form
nitrate). Therefore, the key factor of this ongoing research is the
rational design of highly ecient and selective electrocatalysts
to catalyze the oxygen-coupled ten-electron NOR process while
suppressing the competing four-electron oxygen evolution
reaction (OER) kinetics at the first step,[] nevertheless, such
NOR electrocatalysts also require reasonable OER activity for
the second step. It is challenging to tailor ideal catalyst that syn-
ergistically possesses N activation with OER suppression, but
still some OER catalytic activity together. It is noted that TiO
particles show very low activity for electrochemical OER,[] but
probably exhibit great potential for NOR through doping. On
the other side, Ru+ oxidate has very high OER activity;[,]
A facile pathway of the electrocatalytic nitrogen oxidation reaction (NOR) to
nitrate is proposed, and Ru-doped TiO/RuO (abbreviated as Ru/TiO) as a
proof-of-concept catalyst is employed accordingly. Density functional theory
(DFT) calculations suggest that Ruδ+ can function as the main active center
for the NOR process. Remarkably doping Ru into the TiO lattice can induce
an upshift of the d-band center of the Ru site, resulting in enhanced activity
for accelerating electrochemical conversion of inert N to active NO*.
Overdoping of Ru ions will lead to the formation of additional RuO on the
TiO surface, which provides oxygen evolution reaction (OER) active sites
for promoting the redox transformation of the NO* intermediate to nitrate.
However, too much RuO in the catalyst is detrimental to both the selectivity
of the NOR and the Faradaic eciency due to the dominant OER process.
Experimentally, a considerable nitrate yield rate of .µmolh−gcat−
(besides, a total nitrate yield of .µg during h) and a highest nitrate
Faradaic eciency of .% are achieved by the Ru/TiO catalyst (with the
hybrid composition of RuxTiyO and extra RuO by .wt% Ru addition
amount) in . NaSO electrolyte.
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/./adma..
Dr. M. Kuang, Dr. W. Fang, Dr. H. Tan, M. Chen, Dr. J. Yao, Prof. Q. Yan
School of Materials Science and Engineering
Nanyang Technological University
 Nanyang Avenue, Singapore , Singapore
E-mail: fangwei@ntu.edu.sg; alexyan@ntu.edu.sg
Dr. Y. Wang, Prof. K. Zhou
Environmental Process Modelling Centre
Nanyang Environment and Water Research Institute
Nanyang Technological University
 CleanTech Loop, Singapore , Singapore
E-mail: kzhou@ntu.edu.sg
Dr. Y. Wang, Prof. K. Zhou
School of Mechanical and Aerospace Engineering
Nanyang Technological University
 Nanyang Avenue, Singapore , Singapore
Prof. C. Liu
Key Laboratory of Materials Processing and Mold
Ministry of Education
Zhengzhou University
Zhengzhou , China
Prof. J. Xu
Institute of Materials Research and Engineering
A*STAR (Agency for Science, Technology and Research)
 Fusionopolis Way, Innovis #-, Singapore , Singapore
Adv. Mater. , 32, 
... Considering the restricted sources of NH 3 and NO, the pathway of direct oxidation of N 2 to form NO 3 − is more attractive [11], mimicking the natural N 2 fixation process with lightning effects [12]. Relevant studies have been reported, and similar to the electrochemical reduction of N 2 to NH 3 , adsorption activation of N 2 and N≡N bond breaking are important challenges in N 2 oxidation reactions [13,14]. ...
... One is the formation of *NO intermediate by electrocatalysis of N 2 , which has been verified as ratedetermination step (RDS). Then HNO 3 /NO 3 − is produced by a chemical (not electrochemical) reaction between *NO and O 2 produced by the oxidation of H 2 O. Herein, the oxygen evolution reaction (OER) process is involved in the N 2 OR process rather than acting as a competing reaction [14,38,39]. Another pathway suggests that N 2 is electrochemically oxidized to form NO 3 − [40,41]. ...
... The Co site also acts as an active site for the OER process to facilitate the formation of *NO to NO 3 − . Ru doped TiO 2 has been found to be active for the N 2 OR, in which the high valence Ru δ+ species is regarded as the active site [14]. It is demonstrated that the upward shift of the d-band center of Ru site promotes the conversion of N 2 molecules into *NO intermediates. ...
Recent Progress in Electrochemical Synthesis and Conversion of Nitrates in Aqueous Electrolyte
Article
Full-text available
  • May 2024
  • TOP CATAL
Nitrate, as an important nitrogen source, plays a crucial role in the fields of energy, chemicals, and the environment. Electrocatalytic nitrogen conversion, powered by renewable energy sources, is a viable and promising strategy for the synthesis and conversion of nitrate. In this work, electrocatalytic synthesis and conversion of nitrates associated with five reactions: ammonia oxidation reaction (NH3OR), nitrogen oxidation reaction (N2OR), NO oxidation reaction (NOOR), nitrate reduction reaction (NO3RR), and the electrochemical C-N coupling of CO2 and NO3⁻ (C–N coupling) were reviewed. At present, it is found that most of these reaction processes still suffer from low Faradaic efficiency and product yield. The underlying mechanisms, key steps, and rational catalyst design for these nitrogen electrochemical reactions were summarized. This review aims to provide insights into the fundamentals in the relevant reactions, the development of efficient catalysts, and the advances in the progress of artificial nitrogen cycling processes. Graphical Abstract
View
... Therefore, in this context, strategic balancing of the interaction between oxygen evolution reaction (OER) and nitrogen oxidation reaction (N 2 OR) with effective utilization of active oxygen species is a highly desired goal. [18] Several approaches have emerged and attracted attention, emphasizing primary doping, [20,21] the implementation of tensile strain techniques, [22] the use of single-atom catalysts (SACs) or clusters, [16,23] and vacancy control, [21] among other techniques [24,25] that have demonstrated their effectiveness in increasing the efficiency and selectivity of N 2 OR, yielding promising results that significantly advance the trajectory of research in this field. One of the major contributions that Prof. Norskov and his research team [26] pioneered the development of a comprehensive theoretical model. ...
... This innovative method has been shown to promote the generation of active sites, resulting in a significant increment of N 2 OR efficiency. Yan and co-workers [20] demonstrated an electrocatalyst (TiO 2 /RuO 2 ) incorporating Ru doping into the TiO 2 lattice. The addition of ruthenium (Ru) to the catalyst has been shown to increase the d-band center of the active site, improving its ability to activate inert N 2 molecules and convert them into reactive NO * intermediates. ...
Boosting Selective Nitrogen Oxidation to Nitric Acid by Synergizing Cobalt Phthalocyanine on Carbon Nitride Surface
Article
Full-text available
The Ostwald process, which is producing HNO3 for commercial use, involves the catalytic oxidation of NH3 and a series of chemical reactions conducted under severe operating conditions. Due to their energy‐intensive nature, these activities play a major role in greenhouse gas emissions and global energy consumption. In response to the urgent requirements of the global energy and environmental sectors, there is an increasingly critical need to develop novel, highly efficient, and environmentally sustainable methods. Herein, CoPc/C3N4 electrocatalyst, integrating CoPc nanotubes with C3N4 nanosheets, is shown. The CoPc/C3N4 electrocatalyst demonstrates yield rate of 871.8 µmol h⁻¹ gcat⁻¹ at 2.2 V, with corresponding Faradaic efficiency (FE) of 46.4% at 2.1 V, which notably surpasses that of CoPc. Through a combination of experimental investigations and density functional theory (DFT) calculations, this study shows that CoPc anchored on C3N4 effectively simplifies the adsorption and activation of chemically inactive nitrogen molecules. The improved catalytic activity for composite system may be the reason of re‐distribution of charges over the CoPc, tuning the valence orbital of Co center due to the presence of 2D layer of C3N4. This mechanism significantly lowers the energy barrier required for critical breaking of inert N2, ultimately leading to a significant improvement in N2 oxidation efficiency.
View
... To realize AMIBs making rapid progress in terms of fast-charging, researchers have been trying to seek for anode materials with high specific capacity, moderate reaction voltages, and fast reaction kinetics. 5,[29][30][31][32][33] As shown in Figure 1, typical anodes can be classified into four categories, which are the intercalation typed (e.g., graphite, hard carbon, graphene, titanate, etc.), [34][35][36][37] alloying typed (e.g., Si, Sn, Sb, Bi, P, etc.), [38][39][40][41][42] conversion typed (e.g., CoP, Co 3 O 4 , NiO, MoP, etc.), 39,43 and conversionalloying typed anodes (e.g., Sn 4 P 3 , SnO 2 , SnS 2 , GeP, etc.). 12,44-46 Among them, as mentioned above, graphite anodes have weaknesses of low capacity, insufficient safety, and poor fast-charging performance. ...
Phosphorus‐based anodes for fast‐charging alkali metal ion batteries
Article
Full-text available
  • May 2024
Advancing fast‐charging technology is an important strategy for the development of alkali metal ion batteries (AMIBs). The exploitation of a new generation of anode material system with high‐rate performance, high capacity, and low risk of lithium/sodium/potassium plating is critical to realize fast‐charging capability of AMIBs while maintaining high energy density and safety. Among them, phosphorus‐based anodes including phosphorus anodes and metal phosphide anodes have attracted wide attention, due to their high theoretical capacities, safe reaction voltages, and natural abundance. In this review, we summarize the research progress of different phosphorus‐based anodes for fast‐charging AMIBs, including material properties, mechanisms for storing alkali metal ions, key challenges and solution strategies for achieving fast‐charging capability. Moreover, the future development directions of phosphorus‐based anodes in fast‐charging AMIBs are highlighted. image
View
... Furthermore, it is apparent that, within the reaction pathway of the subsequent electrochemical reaction of *OH*N 2 O → *O*N 2 O, a maximum potential of ~2.2 V is required to drive this electrochemical transformation. Notably, this value is comparable to or lower than that of previous catalysts such as Ru x Ti y O 2 (2.26 V) 9 , FeN 4 (2.06 V) and FeN 3 (3.13 V) 20 . ...
View
View
Solution plasma-cobalt hydroxide enabled nitrogen fixation
Article
  • Jun 2024
Solution plasma-Co(OH)2 interaction significantly boosts nitrogen fixation and achieves high concentration of NOx- at 9.42 mmol L-1. This surpasses the nitrogen content requirement of 7.67 mmol L-1 for commercial nutrient...
View
Metal vacancies and self-reconstruction of high entropy metal borates to boost the oxygen evolution reaction
Article
  • Jun 2024
  • CHEM ENG J
Exploring highly efficient oxygen evolution reaction (OER) electrocatalysts is important for industrial water electrolysis. Herein, a new high entropy material was reported, i.e., an amorphous high entropy metal borate (CrMnCoNiFe)0.2BOx was synthesized by a new simple solvothermal method. The key of this new synthesis method was to obtain (CrMnCoNiFe)0.2BOx with a high specific surface area (446 m2/g) and porous structure, being expected to promote OER reaction. (CrMnCoNiFe)0.2BOx provided a low overpotential of 236 mV at 10 mA cm− 2, a reduced Tafel slope of 64 mV dec–1, and good stability. The outstanding OER performance of (CrMnCoNiFe)0.2BOx was attributed to the high entropy structure, generation of metal vacancies, and self-reconstruction during OER. The Cr vacancies generated during the OER process have been demonstrated by Cr 2P XPS and EPR experiments. Various in situ and Ex-situ analyses such as In situ Raman, XPS, and TEM were used to investigate the self-reconstruction of the (CrMnCoNiFe)0.2BOx during the OER. The resulting metal (oxy) hydroxide was believed to be the true active center for OER. The DFT calculation showed the high entropy structure can reduce the adsorption and conversion energy barriers of oxygen-containing intermediates, matching well with the catalytic performance results. This study provided a new simple strategy to construct high entropy metal borate (CrMnCoNiFe)0.2BOx and would inspire the design of high-performance OER catalysts.
View
Atomically Dispersed Ru-doped Ti4O7 Electrocatalysts for Chlorine Evolution Reaction with a Universal Activity
Article
  • Apr 2024
  • SMALL
Chlorine has been supplied by the chlor‐alkali process that deploys dimensionally stable anodes (DSAs) for the electrochemical chlorine evolution reaction (ClER). The paramount bottlenecks have been ascribed to an intensive usage of precious elements and inevitable competition with the oxygen evolution reaction. Herein, a unique case of Ru ²⁺ ‐O 4 active motifs anchored on Magnéli Ti 4 O 7 (Ru‐Ti 4 O 7 ) via a straightforward wet impregnation and mild annealing is reported. The Ru‐Ti 4 O 7 performs radically active ClER with minimal deployment of Ru (0.13 wt%), both in 5 m NaCl (pH 2.3) and 0.1 m NaCl (pH 6.5) electrolytes. Scanning electrochemical microscopy demonstrates superior ClER selectivity on Ru‐Ti 4 O 7 compared to the DSA. Operando X‐ray absorption spectroscopy and density functional theory calculations reveal a universally active ClER (over a wide range of pH and [Cl ⁻ ]), through a direct adsorption of Cl ⁻ on Ru ²⁺ ‐O 4 sites as the most plausible pathway, together with stabilized ClO* at low [Cl ⁻ ] and high pH.
View
View
View
Crystal‐Phase‐Engineered PdCu Electrocatalyst for Enhanced Ammonia Synthesis
Article
Full-text available
Crystal phase engineering is a powerful strategy for regulating the performance of electrocatalysts towards many electrocatalytic reactions, while its impact on the nitrogen electroreduction has been largely unexplored. Herein, we demonstrate that structurally ordered body‐centered cubic (BCC) PdCu nanoparticles can be adopted as active, selective, and stable electrocatalysts for ammonia synthesis. Specifically, the BCC PdCu exhibits excellent activity with a high NH3 yield of 35.7 μg h⁻¹ mg⁻¹cat, Faradaic efficiency of 11.5 %, and high selectivity (no N2H4 is detected) at −0.1 V versus reversible hydrogen electrode, outperforming its counterpart, face‐centered cubic (FCC) PdCu, and most reported nitrogen reduction reaction (NRR) electrocatalysts. It also exhibits durable stability for consecutive electrolysis for five cycles. Density functional theory calculation reveals that strong orbital interactions between Pd and neighboring Cu sites in BCC PdCu obtained by structure engineering induces an evident correlation effect for boosting up the Pd 4d electronic activities for efficient NRR catalysis. Our findings open up a new avenue for designing active and stable electrocatalysts towards NRR.
View
Tuning the Electron Localization of Gold Enables the Control of Nitrogen‐to‐Ammonia Fixation
Article
Full-text available
The (photo)electrochemical N2 reduction reaction (NRR) provides a favorable avenue for the production of NH3 using renewable energy in mild operating conditions. Understanding and building an efficient catalyst with high NH3 selectivity represents an area of intense interest for the early stages of development for NRR. Herein, we introduce a CoOx layer to tune the local electronic structure of Au nanoparticles with positive valence sites for boosting conversion of N2 to NH3. The catalysts, possessing high average oxidation states (ca. 40 %), achieve a high NH3 yield rate of 15.1 μg cm⁻² h⁻¹ and a good faradic efficiency of 19 % at −0.5 V versus reversible hydrogen electrode. Experimental results and simulations reveal that the ability to tune the oxidation state of Au enables the control of N2 adsorption and the concomitant energy barrier of NRR. Altering the Au oxidation state provides a unique strategy for control of NRR in the production of valuable NH3.
View
Antimony‐Based Composites Loaded on Phosphorus‐Doped Carbon for Boosting Faradaic Efficiency of the Electrochemical Nitrogen Reduction Reaction
Article
Full-text available
A nanocomposite of PC/Sb/SbPO4 (PC, phosphorus‐doped carbon) exhibits a high activity and an excellent selectivity for efficient electrocatalytic conversion of N2 to NH3 in both acidic and neutral electrolytes under ambient conditions. At a low reductive potential of −0.15 V versus the reversible hydrogen electrode (RHE), the PC/Sb/SbPO4 catalyst achieves a high Faradaic efficiency (FE) of 31 % for ammonia production in 0.1 m HCl under mild conditions. In particular, a remarkably high FE value of 34 % is achieved at a lower reductive potential of −0.1 V (vs. RHE) in a 0.1 m Na2SO4 solution, which is better than most reported electrocatalysts towards the nitrogen reduction reaction (NRR) in neutral electrolyte under mild conditions. The change in surface species and electrocatalytic performance before and after N2 reduction is explored by an ex situ method. PC and SbPO4 are both considered as the active species that enhanced the performance of NRR.
View
Doping strain induced bi-Ti3+ pairs for efficient N2 activation and electrocatalytic fixation
Article
Full-text available
  • Jun 2019
The electrochemical N2 fixation to produce ammonia is attractive but significantly challenging with low yield and poor selectivity. Herein, we first used density function theory calculations to reveal adjacent bi-Ti³⁺ pairs formed on anatase TiO2 as the most active electrocatalytic centers for efficient N2 lying-down chemisorption and activation. Then, by doping of anatase TiO2 with Zr⁴⁺ that has similar d-electron configuration and oxide structure but relatively larger ionic size, the adjacent bi-Ti³⁺ sites were induced and enriched via a strained effect, which in turn enhanced the formation of oxygen vacancies. The Zr⁴⁺-doped anatase TiO2 exhibited excellent electrocatalytic N2 fixation performances, with an ammonia production rate (8.90 µg·h⁻¹·cm⁻²) and a Faradaic efficiency of 17.3% at −0.45 V versus reversible hydrogen electrode under ambient aqueous conditions. Moreover, our work suggests a viewpoint to understand and apply the same-valance dopants in heterogeneous catalysis, which is generally useful but still poorly understood.
View
Nitrogen Vacancies on 2D Layered W2N3: A Stable and Efficient Active Site for Nitrogen Reduction Reaction
Article
Full-text available
Electrochemical nitrogen reduction reaction (NRR) under ambient conditions provides an avenue to produce carbon‐free hydrogen carriers. However, the selectivity and activity of NRR are still hindered by the sluggish reaction kinetics. Nitrogen Vacancies on transition metal nitrides are considered as one of the most ideal active sites for NRR by virtue of their unique vacancy properties such as appropriate adsorption energy to dinitrogen molecule. However, their catalytic performance is usually limited by the unstable feature. Herein, a new 2D layered W2N3 nanosheet is prepared and the nitrogen vacancies are demonstrated to be active for electrochemical NRR with a steady ammonia production rate of 11.66 ± 0.98 µg h−1 mgcata−1 (3.80 ± 0.32 × 10−11 mol cm−2 s−1) and Faradaic efficiency of 11.67 ± 0.93% at −0.2 V versus reversible hydrogen electrode for 12 cycles (24 h). A series of ex situ synchrotron‐based characterizations prove that the nitrogen vacancies on 2D W2N3 are stable by virtue of the high valence state of tungsten atoms and 2D confinement effect. Density function theory calculations suggest that nitrogen vacancies on W2N3 can provide an electron‐deficient environment which not only facilitates nitrogen adsorption, but also lowers the thermodynamic limiting potential of NRR. Nitrogen vacancies on 2D layered W2N3 reveals stable and efficient nitrogen reduction performance. The activity and selectivity of the unique active sites are confirmed by mutually corroborating electrochemical experiments and theoretical computation. The nitrogen vacancies on W2N3 have an electron deficient environment for the acceptance of the lone‐pair electrons of N2, which can facilitate dinitrogen molecule adsorption and activation.
View
Cobalt Modulated Mo-Dinitrogen Interaction in MoS2 for Catalyzing Ammonia Synthesis
Article
  • Nov 2019
Dinitrogen conversion to ammonia via electrochemical reduction with over 10% Faradic efficiency has been demonstrated in this work. Co-doped MoS2-x polycrystalline nanosheets with S vacancies as the catalysts are loaded onto carbon cloth by hydrothermal growth from Mo, Co and S precursors. Sulfur vacancy on MoS2-x basal plane mimicking the natural Mo-nitrogenase active site is modified by Co doping and exhibits superior dinitrogen to ammonia conversion activity. Density-functional simulation reveals that the free energy barrier, which can be compensated by applied over-potential, is reduced from 1.62 eV to 0.59 eV after Co doping. Meanwhile, dinitrogen tends to be chemically adsorbed to defective MoS2-x, which effectively activates the dinitrogen molecule for the dissociation of the N≡N triple bond. This process is further accelerated by Co doping resulted from the modulation of Mo-N bonding configuration.
View
Regulating electrocatalysts via surface/interface engineering for acidic water electro-oxidation
Article
  • Oct 2019
Although proton exchange membrane (PEM) water electrolyzers offer a promising means for generation of hydrogen fuel from solar and wind energy, in acidic environments the corresponding anodic oxygen evolution reaction (OER) remains a bottleneck. Since the activity and stability of electrocatalysts depend significantly on physicochemical properties, material surface/interface engineering can offer a practical way to boost performance. To date, significant advances have been made using a judicious combination of advanced theoretical computations and spectroscopic characterizations. To provide a critical assessment of this field, we focus on the establishment of material property-catalytic activity relationships. We start with a detailed exploration of prevailing OER mechanisms in acid solution through evaluating the role of catalyst lattice oxygen. Then we critically review advances in surface/interface engineering in acidic OER electrocatalysts from both experimental and theoretical perspectives. Finally, a few promising research orientations are proposed to inspire future investigation of high-performance PEM catalysts.
View
Electrochemical Reduction of N2 into NH3 by Donor-Acceptor Couples of Ni and Au Nanoparticles with a 67.8% Faradaic Efficiency
Article
  • Sep 2019
The traditional NH3 production method (Haber-Bosch process) is currently complemented by electrochemical synthesis at ambient conditions, but the rather low selectivity (as indicated by the Faradaic efficiency) for the electrochemical reduction of molecular N2 into NH3 impedes the progress. Here, we present a powerful method to significantly boost the Faradaic efficiency of Au electrocatalysts to 67.8% for the nitrogen reduction reaction (NRR) by increasing their electron density through the construction of inorganic donor-acceptor couples of Ni and Au nanoparticles. The unique role of the electron-rich Au centres in facilitating the fixation and activation of N2 was also been investigated via theoretical simulation methods and then confirmed by experimental results. The highly coupled Au and Ni nanoparticles supported on nitrogen-doped carbon are stable for reuse and long-term performance of the NRR, making the electrochemical process more sustainable for practical application.
View
Quantification of Active Sites and Elucidation of the Reaction Mechanism of the Electrochemical Nitrogen Reduction Reaction on Vanadium Nitride
Article
  • Aug 2019
The distinct isotopic identities of the incoming N2 and the surface N sites on an oxygen‐modified vanadium nitride catalyst enable the differentiation and quantification of active catalytic sites. In their Communication (DOI: 10.1002/anie.201906449), Y. Yan, J. G. Chen, B. Xu and co‐workers used 14N/15N isotopic exchange to determine the density of initial and steady state active sites on vanadium oxynitride in the electrochemical nitrogen reduction reaction.
View
Building Up a Picture of the Electrocatalytic Nitrogen Reduction Activity of Transition Metal Single-Atom Catalysts
Article
  • May 2019
The lack of chemical understanding and efficient catalysts impedes the development of electrocatalytic nitrogen reduction reac-tion (eNRR) for ammonia production. In this work, we employed density functional theory calculations to build up a picture (ac-tivity trends, electronic origins, design strategies) of single atom catalysts (SACs) supported on nitrogen-doped carbons as eNRR electrocatalysts. To construct such a picture, this work presents systematic studies of the eNRR activity of SACs covering twenty different transition metals (TM) centers coordinated by nitrogen atoms contained in three types of nitrogen-doped carbon sub-strates, which gives sixty SACs. Our study shows that the intrinsic activity trends could be established on basis of the nitrogen adatom adsorption energy (∆EN*). Furthermore, the influence of metal and support (ligands) on ∆EN* proved to be related to the bonding/anti-bonding orbital population and regulating the scaling relations for adsorption of intermediates, respectively. Ac-cordingly, a two-step strategy is proposed for improving the eNNR activity of TM-SACs, which involves: (i) selection of the most promising family of SACs (g-C3N4 supported SACs as predicted in this work), and (ii) further improvement of the activity of the best candidate in the aforementioned family via tuning adsorption strength of the key intermediates. Also, the stability of N-doped carbon supports and their selectivity in comparison to the competing hydrogen evolution need to be taken into considera-tion for screening the durable and efficient candidates. Finally, an effective strategy for designing active, stable and selective SACs based on the mechanistic insights is elaborated to guide future eNRR studies.
View