Probing Photocatalytic Nitrogen Reduction to Ammonia with Water on the Rutile TiO2 (110) Surface by First-Principles Calculations

Reactions of fluid and lattice oxygen mediated by interstitial atoms at the TiO2(110)-water interface

O2 interacts with TiO2 surfaces in numerous aqueous reactions for clean hydrogen production, wastewater cleanup, reduction of CO2 and N2, and O2 sensing. In many cases, these reactions involve reversible exchange of O with the solid, whose participation is usually thought to require oxygen vacancies (VO). Based on measurements of oxygen isotopic self-diffusion in rutile TiO2 under water, this work proposes a different perspective centered on O interstitial atoms (Oi). Experiments with varying concentrations of O2 and mole fractions of 18O show that the (110) surface facilitates O exchange with both the H2O liquid and its dissolved O2. First-principles calculations indicate that on-top and "surface Oi" configurations of adsorbed O participate sequentially in the exchange process. Adsorbed OH appears to provide a single pathway for H2O and O2 to contribute oxygen, although fitting the diffusion data to simple models indicates that H2O contributes more. Because rutile TiO2 is a prototypical metal oxide, this picture based on Oi probably generalizes in many cases to other oxides - explaining important aspects of their thermal, electrochemical, and photochemical reactions with dissolved O2.

Glycerol Adsorption on TiO2 Surfaces: A Systematic Periodic DFT Study

Conversion of glycerol to added-value products is desirable due to its surplus during biodiesel synthesis. TiO2 has been the most explored catalyst. We performed a systematic study of glycerol adsorption on anatase (101), anatase (001), and rutile (110) TiO2 at the Density Functional Theory level. We found several adsorption modes on these surfaces, with anatase (101) being the less reactive one, leading to adsorption energies between -0.8 and -0.4 eV, with all adsorptions molecular in nature. On the contrary, anatase (001) is the most reactive surface, leading to both molecular and dissociative adsorption modes, with energies ranging from -4 to -1 eV and undergoing severe surface reconstructions in some cases. Rutile (110) also shows both molecular and dissociative adsorptions, but it is less reactive than anatase (001). Surfaces with oxygen vacancies affects the adsorbed states and energies. The electronic structure analysis reveals that glycerol adsorption mainly affects the band gap of the material and not the individual contributions to the valence and conduction band. Bader charge analysis shows that strong adsorption modes on anatase (001) and rutile (110) are associated with large charge transfer from glycerol to the surface, while weak and molecular adsorption modes involve low charge transfer.

Advances in electrocatalytic nitrate reduction to ammonia over Cu-based catalysts

Ammonia (NH3) is a critical basic material for both the agricultural and pharmaceutical industries. Traditionally, NH3 synthesis has relied on the Haber-Bosch process, which is characterized by high greenhouse gas emissions and stringent reaction conditions. As a more sustainable and cost-effective alternative, electrocatalytic NH3 synthesis has gained increasing attention. Nitrate (NO3-), a common pollutant in water and soil, is considered a promising nitrogen source for NH3 production due to its high solubility and relatively low N=O bond dissociation energy. This makes it particularly suitable for electrocatalytic nitrate reduction to ammonia (NRA), a process with significant potential for addressing nitrate pollution while contributing to NH3 production. However, challenges such as slow reaction kinetics and poor product selectivity persist in the NRA process. To overcome these challenges, the selection and optimization of catalysts are crucial for improving NRA performance. Among the various catalysts explored, copper-based (Cu) catalysts have attracted widespread attention due to their unique electronic structure and outstanding catalytic performance. This review provides a comprehensive analysis of the application and reaction mechanisms of Cu-based catalysts in NRA, along with an overview of testing systems and evaluation metrics used in the field. Additionally, it highlights current challenges and outlines future research directions to support the continued development of Cu-based materials for NRA applications.

Unveiling the potential of step-scheme and Type II photocatalysts in dinitrogen reduction to ammonia

Innovative photocatalytic systems designed to enhance efficiency of nitrogen fixation processes, specifically focusing on sustainable ammonia (NH3) production strategies via dinitrogen (N2) reduction into ammonia (NH3). This process is critical for sustainable agriculture and energy production. To improve photocatalyst activity, catalyst stability and reusability, reduction efficiency due to electron/hole recombination, and light-absorption efficiency has drawn extensive attention. Herein, a broad range of research progress and comprehensive overview of step-scheme/type-II heterojunctions focusing on dinitrogen (N2) reduction are reviewed with focus on general synthesis, characterization by their unique charge separation mechanisms that improve light absorption and electron-hole pair utilization. The review highlights recent advancements in material design, which have shown promising results in enhancing photocatalytic activity under visible light irradiation. A significant portion of the review delves into the underlying mechanisms which these heterojunctions operate. Despite the promising literature results, several challenges facing this field, such as scalability, stability of photocatalysts, and environmental impact under operational conditions were also discussed. In summary, this review provides valuable insights into the potential of step-scheme/type-II photocatalysts for dinitrogen reduction to ammonia. The need for interdisciplinary approaches to overcome existing challenges such as incorporation of piezoelectric biomaterials and unlocking the full potential of these materials in addressing global nitrogen demands sustainably are highlighted, outlining future directions for further research and innovations.

Effect of Facet on Local Electron Density of Oxygen Vacancy in Catalysts for Nitrogen Electro-Reduction

Oxygen-vacancy (Ov) engineering is an effective strategy to manipulate the electronic configuration of catalysts for electrochemical nitrogen reduction reaction (eNRR). The influence of the stable facet on the electronic configuration of Ov is widely studied, however, the effect of the reactive facet on the local electron density of Ov is unveiled. In this work, an eNRR electrode R(111)-TiO2/HGO is provided with a high proportion exposed reactive facet (111) of rutile-TiO2 (denoted as R(111)-TiO2) nanocrystals with Ov anchored in hierarchically porous graphite oxide (HGO) nanofilms. The R(111)-TiO2/HGO exhibits excellent eNRR performance with an NH3 yield rate of 20.68 µg h-1 cm-2, which is ≈20 times the control electrode with the most stable facet (110) exposed (R(110)-TiO2/HGO). The experimental data and theoretical simulations reveal that the crystal facet (111) has a positive effect on regulating the local electron density around the oxygen vacancy and the two adjacent Ti-sites, promoting the π-back-donation, minimizing the eNRR barrier, and transforming the rate determination step to *NNH→*NNHH. This work illuminates the effect of crystal facet on the performance of eNRR, and offers a novel strategy to design efficient eNRR catalysts.

Assessing Exchange-Correlation Functionals for Heterogeneous Catalysis of Nitrogen Species

Increasing interest in the sustainable synthesis of ammonia, nitrates, and urea has led to an increase in studies of catalytic conversion between nitrogen-containing compounds using heterogeneous catalysts. Density functional theory (DFT) is commonly employed to obtain molecular-scale insight into these reactions, but there have been relatively few assessments of the exchange-correlation functionals that are best suited for heterogeneous catalysis of nitrogen compounds. Here, we assess a range of functionals ranging from the generalized gradient approximation (GGA) to the random phase approximation (RPA) for the formation energies of gas-phase nitrogen species, the lattice constants of representative solids from several common classes of catalysts (metals, oxides, and metal-organic frameworks (MOFs)), and the adsorption energies of a range of nitrogen-containing intermediates on these materials. The results reveal that the choice of exchange-correlation functional and van der Waals correction can have a surprisingly large effect and that increasing the level of theory does not always improve the accuracy for nitrogen-containing compounds. This suggests that the selection of functionals should be carefully evaluated on the basis of the specific reaction and material being studied.

Tunable optical properties of isoreticular UiO-67 MOFs for photocatalysis: a theoretical study

A theoretical study of the reported photocatalytic systems based on Zr-based MOF (UiO-67) with biphenyl-4,4'-dicarboxylic acid (bpdc) and 2,2'-bipyridine-5,5'-dicarboxylic acid (bpydc) as linkers was performed. Quantum chemical calculations were carried out to understand the optical properties of the materials and to facilitate the rational design of new UiO-67 derivatives with potentially improved features as photocatalysts under ambient conditions. Hence, the effect of the structural modifications on the optical properties was studied considering different designs based on the nature of the linkers: in 1 only the bpdc linker was considered, or the mixture 1 : 1 between bpdc and bpydc linkers (labeled as 1A). Also, substituents R, -NH2, and -SH, were included in the 1A MOF only over the bpdc linker (labeled as 1A-bpdc-R) and on both bpdc and bpydc linkers (labeled as 1A-R). Thus a family of six isoreticular UiO-67 derivatives was theoretically characterized using Density Functional Theory (DFT) calculations on the ground singlet (S0) and first excited states (singlet and triplet) using Time-Dependent Density Functional Theory (TD-DFT), multiconfigurational post-Hartree-Fock method via Complete Active Space Self-Consistent Field (CASSCF). In addition, the use of periodic DFT calculations suggest that the energy transfer (ET) channel between bpdc and bpydc linkers might generate more luminescence quenching of 1A when compare to 1. Besides, the results suggest that the 1A-R (R: -SH and NH2) can be used under ambient conditions; however, the ET exhibited by 1A, cannot take place in the same magnitude in these systems. These ET can favor the photocatalytic reduction of a potential metal ion, that can coordinate with the bpydc ligand, via LMCT transition. Consequently, the MOF might be photocatalytically active against molecules of interest (such as H2, N2, CO2, among others) with photo-reduced metal ions. These theoretical results serve as a useful tool to guide experimental efforts in the design of new photocatalytic MOF-based systems.

Modelling Photocatalytic N2 Reduction to Ammonia: Where We Stand and Where We Are Going

Artificial ammonia synthesis via the Haber-Bosch process is environmentally problematic due to the high energy consumption and corresponding CO2 ${_2 }$ emissions, produced during the reaction and before hand in hydrogen production upon methane steam reforming. Photocatalytic nitrogen fixation as a greener alternative to the conventional Haber-Bosch process enables us to perform nitrogen reduction reaction (NRR) under mild conditions, harnessing light as the energy source. Herein, we systematically review first-principles calculations used to determine the electronic/optical properties of photocatalysts, N2 adsorption and to expound possible NRR mechanisms. The most commonly studied photocatalysts for nitrogen fixation are usually modified with dopants, defects, co-catalysts and Z-scheme heterojunctions to prevent charge carrier recombination, improve charge separation efficiency and adjust a band gap to for utilizing a broader light spectrum. Most studies at the atomistic level of modeling are grounded upon density functional theory (DFT) calculations, wholly foregoing excitation effects paramount in photocatalysis. Hence, there is a dire need to consider methods beyond DFT to study the excited state properties more accurately. Furthermore, a few studies have been examined, which include higher level kinetics and macroscale simulations. Ultimately, we show there is still ample room for improvement with regard to first principles calculations and their integration in multiscale models.

Highly efficient nitrogen fixation over S-scheme heterojunction photocatalysts with enhanced active hydrogen supply

Photocatalytic N2 fixation is a promising strategy for ammonia (NH3) synthesis; however, it suffers from relatively low ammonia yield due to the difficulty in the design of photocatalysts with both high charge transfer efficiency and desirable N2 adsorption/activation capability. Herein, an S-scheme CoSx/ZnS heterojunction with dual active sites is designed as an efficient N2 fixation photocatalyst. The CoSx/ZnS heterojunction exhibits a unique pocket-like nanostructure with small ZnS nanocrystals adhered on a single-hole CoSx hollow dodecahedron. Within the heterojunction, the electronic interaction between ZnS and CoSx creates electron-deficient Zn sites with enhanced N2 chemisorption and electron-sufficient Co sites with active hydrogen supply for N2 hydrogenation, cooperatively reducing the energy barrier for N2 activation. In combination with the promoted photogenerated electron-hole separation of the S-scheme heterojunction and facilitated mass transfer by the pocket-like nanostructure, an excellent N2 fixation performance with a high NH3 yield of 1175.37 μmol g-1 h-1 is achieved. This study provides new insights into the design of heterojunction photocatalysts for N2 fixation.

Formation of Carbon-Induced Nitrogen-Centered Radicals on Titanium Dioxide under Illumination

Titanium dioxide is the most studied photocatalytic material and has been reported to be active for a wide range of reactions, including the oxidation of hydrocarbons and the reduction of nitrogen. However, the molecular-scale interactions between the titania photocatalyst and dinitrogen are still debated, particularly in the presence of hydrocarbons. Here, we used several spectroscopic and computational techniques to identify interactions among nitrogen, methanol, and titania under illumination. Electron paramagnetic resonance spectroscopy (EPR) allowed us to observe the formation of carbon radicals upon exposure to ultraviolet radiation. These carbon radicals are observed to transform into diazo- and nitrogen-centered radicals (e.g., CHxN2• and CHxNHy•) during photoreaction in nitrogen environment. In situ infrared (IR) spectroscopy under the same conditions revealed C-N stretching on titania. Furthermore, density functional theory (DFT) calculations revealed that nitrogen adsorption and the thermodynamic barrier to photocatalytic nitrogen fixation are significantly more favorable in the presence of hydroxymethyl or surface carbon. These results provide compelling evidence that carbon radicals formed from the photooxidation of hydrocarbons interact with dinitrogen and suggest that the role of carbon-based "hole scavengers" and the inertness of nitrogen atmospheres should be reevaluated in the field of photocatalysis.

Screening and Discovery of Metal Compound Active Sites for Strong and Selective Adsorption of N2 in Air

Photocatalytic nitrogen fixation has the potential to provide a greener route for producing nitrogen-based fertilizers under ambient conditions. Computational screening is a promising route to discover new materials for the nitrogen fixation process, but requires identifying "descriptors" that can be efficiently computed. In this work, we argue that selectivity toward the adsorption of molecular nitrogen and oxygen can act as a key descriptor. A catalyst that can selectively adsorb nitrogen and resist poisoning of oxygen and other molecules present in air has the potential to facilitate the nitrogen fixation process under ambient conditions. We provide a framework for active site screening based on multifidelity density functional theory (DFT) calculations for a range of metal oxides, oxyborides, and oxyphosphides. The screening methodology consists of initial low-fidelity fixed geometry calculations and a second screening in which more expensive geometry optimizations were performed. The approach identifies promising active sites on several TiO2 polymorph surfaces and a VBO4 surface, and the full nitrogen reduction pathway is studied with the BEEF-vdW and HSE06 functionals on two active sites. The findings suggest that metastable TiO2 polymorphs may play a role in photocatalytic nitrogen fixation, and that VBO4 may be an interesting material for further studies.

Progress in Photochemical and Electrochemical C-N Bond Formation for Urea Synthesis

ConspectusHere, we discuss recent advances and pressing challenges in achieving sustainable urea synthesis. Urea stands out as the most prevalent nitrogen-based fertilizer used across the globe, making up over 50% of all manufactured fertilizers. Historically, the Bosch-Meiser process has been the go-to chemical manufacturing method for urea production. This procedure, characterized by its high-temperature and high-pressure conditions, reacts ammonia with carbon dioxide to form ammonium carbamate. Subsequently, this ammonium carbamate undergoes dehydration, facilitated by heat, producing solid urea. A concerning aspect of this method is its dependency on fossil fuels, as nearly all the process heat comes from nonrenewable sources. Consequently, the Bosch-Meiser process leaves behind a considerable carbon footprint. Current estimates predict that unchecked, carbon emissions from urea production alone might skyrocket, reaching a staggering 286 MtCO2,eq/yr by 2050. Such projections paint a clear picture regarding the necessity for more eco-friendly, sustainable urea production methods. Recently, the scientific community has shown growing interest in forming C-N bonds using alternative methods. Shifting toward photochemical or electrochemical processes, as opposed to traditional thermal-based processes, promises the potential for complete electrification of urea synthesis. This shift toward process electrification is not just an incremental change; it represents a groundbreaking advancement, the first of many steps, toward achieving deep decarbonization in the chemical manufacturing sector. Since the turn of 2020, there has been a surge in research focusing on photochemical and electrochemical urea synthesis. These methods capitalize on co-reduction of carbon dioxide with nitrogenous reactants like NOx and N2. Despite the progress, there are significant challenges that hinder these processes from reaching their full potential. In this comprehensive review, we shed light on the advances made in electrified C-N bond formation. More importantly, we focus on the invaluable insights gathered over the years, especially concerning catalytic reaction mechanisms. We have dedicated a section to underline key focal areas for up-and-coming research, emphasizing catalyst, electrolyte, and reactor design. It is undeniable that catalyst design remains at the heart of the matter, as managing the co-reduction of two distinct reactants (CO2 and nitrogenous species) is complex. This process results in a myriad of intermediates, which must be adeptly managed to both maintain catalyst activity and avoid catalyst deactivation. Moreover, the electrolytes play a pivotal role, essentially dictating the creation of optimal microenvironments that drive reaction selectivity. Finally, reactor engineering stands out as crucial to ensure optimal mass transport for all involved reactants and subsequent products. We touch upon the broader environmental ramifications of urea production and bring to light potential obstacles for alternative synthesis routes. A notable mention is the urgency of accelerating the uptake and large-scale implementation of renewable energy sources.

Adsorption-Controlled Wettability and Self-Cleaning of TiO2

Molecular adsorption on solids is inevitable and has significant influences on the wettability of materials, while the tuning mechanism of the wettability from molecular adsorption is yet to be understood. Using molecular dynamics (MD) simulations, we comprehensively studied the relation between the wettability of the TiO₂ surface and the adsorption of water and carboxylic acid molecules. Our results reveal that the increasing amount of surface hydroxyl groups from the decomposition adsorption of H₂O increases the hydrophilicity of TiO₂, providing molecular-level evidence for the previously proposed mechanism of photo-induced hydrophilicity. By contrast, the surface wettability becomes tunable with water contact angles changing from 0 to ∼130° through length adjustment of the adsorbed carboxylic acids. The TiO₂ surface is hydrophilic with the adsorption of short-alkyl-chain carboxylic acids (e.g., HCOOH) and becomes hydrophobic when longer-alkyl-chain carboxylic acids (H(CH₂)_nCOOH, n > 2) are present. Furthermore, long-alkyl-chain acids also increase surface oleophilicity, while the adsorption of HCOOH and CH₃COOH significantly enhances the oleophobicity of TiO₂. Water molecules can also more easily penetrate the space between oily contaminants and adsorbed short acid molecules, thereby further increasing its self-cleaning capacity. The present simulations not only reveal the mechanism of wettability caused by molecular adsorption but also provide a promising method to create materials with controllable wettability and high self-cleaning efficiency.

Unifying the Nitrogen Reduction Activity of Anatase and Rutile TiO2 Surfaces

TiO₂ is a model transition metal oxide that has been applied frequently in both photocatalytic and electrocatalytic nitrogen reduction reactions (NRR). However, the phase which is more NRR active still remains a puzzle. This work presents a theoretical study on the NRR activity of the (001), (100), (101), and (110) surfaces of both anatase and rutile TiO₂ . We found that perfect surfaces are not active for NRR, while the oxygen vacancy can promote the reaction by providing excess electrons and low-coordinated Ti atoms that enhance the binding of the key intermediate (HNN*). The NRR activity of the eight facets can be unified into a single scaling line. The anatase TiO₂ (101) and rutile TiO₂ (101) surfaces were found to be the most and the second most active surfaces with a limiting potential of -0.91 V and -0.95 V respectively, suggesting that the TiO₂ NRR activity is not very phase-sensitive. For photocatalytic NRR, the results suggest that the anatase TiO₂ (101) surface is still the most active facet. We further found that the binding strength of key intermediates scale well with the formation energy of oxygen vacancy, which is determined by the oxygen coordination number and the degree of relaxation of the surface after the creation of oxygen vacancy. This work provides a comprehensive understanding of the activity of TiO₂ surfaces. The results should be helpful for the design of more efficient TiO₂ -based NRR catalysts.

Prospects and good experimental practices for photocatalytic ammonia synthesis

The development of photocatalysts is greatly hindered by false positives or non-reproducible data. Here, The authors describe the current known causes of non-reproducible results in the literature and present solutions to mitigate these false positive results.

Identifying Photocatalytic Active Sites of C2H6 C-H Bond Activation on TiO2 via Combining First-Principles Ground-State and Excited-State Electronic Structure Calculations

The activation of C-H bonds at low temperatures has attracted widespread interest in heterogeneous catalysis, which involves complex thermocatalytic and photocatalytic reaction processes. Herein, we systematically investigate the photothermal catalytic process of C-H bond activation in C₂H₆ dehydrogenation on rutile TiO₂(110). We demonstrate that the photochemical activity of the C₂H₆ molecule adsorbed on TiO₂(110) is site-sensitive and that C₂H₆ is more easily adsorbed at the Ti_5c site with a lower dehydrogenation energy barrier. The first C-H bond activation of the C₂H₆ adsorbed at the Ti_5c site tends to occur in the ground state, whereas O_br-adsorbed C₂H₆ is more photoactive during the initial adsorption. During the dehydrogenation of C₂H₆, the photogenerated electrons are always located at the Ti⁴⁺ sites of the TiO₂ substrate while the photogenerated holes can be captured by C₂H₆ to activate the C-H bond.

Anchoring boron on a covalent organic framework as an efficient single atom metal-free photo-electrocatalyst for nitrogen fixation: a first-principles analysis

The production of ammonia in a sustainable cost-effective manner and ambient conditions is a very challenging task. Photo-/electrocatalytic nitrogen reduction (NRR) is a convenient way to produce NH₃ for industrial applications. In this work, anchoring B atoms in Tp-bpy-COF is shown to effectively reduce N₂ to NH₃. By employing density functional theory, we demonstrated that N₂ can be efficiently activated on the B center due to the synergistic effect of B-N. Meanwhile, we found that the NRR happens predominantly by the alternating path with a small limiting potential of 0.13 V. Moreover, the suitable band edge positions and broad visible light absorption zone result in B@Tp-bpy-COF acting as a promising photocatalyst. Our proposed catalytic system exhibits favorable formation energy and excellent structural stability during AIMD simulations, which suggest the feasibility of experimental synthesis. The system turns out to be highly selective toward the NRR compared to other competitive reactions. These findings may pave a new way for designing SACs on COFs for N₂ fixation with high activity, which may also apply to other reactions.

CO Adsorbate Promotes Polaron Photoactivity on the Reduced Rutile TiO2(110) Surface

Polarons play a major role in determining the chemical properties of transition-metal oxides. Recent experiments show that adsorbates can attract inner polarons to surface sites. These findings require an atomistic understanding of the adsorbate influence on polaron dynamics and lifetime. We consider reduced rutile TiO₂(110) with an oxygen vacancy as a prototypical surface and a CO molecule as a classic probe and perform ab initio adiabatic molecular dynamics, time-domain density functional theory, and nonadiabatic molecular dynamics simulations. The simulations show that subsurface polarons have little influence on CO adsorption and CO can desorb easily. On the contrary, surface polarons strongly enhance CO adsorption. At the same time, the adsorbed CO attracts polarons to the surface, allowing them to participate in catalytic processes with CO. The CO interaction with polarons changes their orbital origin, suppresses polaron hopping, and stabilizes them at surface sites. Partial delocalization of polarons onto CO decouples them from free holes, decreasing the nonadiabatic coupling and shortening the quantum coherence time, thereby reducing charge recombination. The calculations demonstrate that CO prefers to adsorb at the next-nearest-neighbor five-coordinated Ti³⁺ surface electron polaron sites. The reported results provide a fundamental understanding of the influence of electron polarons on the initial stage of reactant adsorption and the effect of the adsorbate-polaron interaction on the polaron dynamics and lifetime. The study demonstrates how charge and polaron properties can be controlled by adsorbed species, allowing one to design high-performance transition-metal oxide catalysts.

Shedding Light on the Role of Chemical Bond in Catalysis of Nitrogen Fixation

Ammonia (NH₃ ) and nitrates are essential for human society because of their widespread utilization for producing medicines, fibers, fertilizers, etc. In recent years, the development on nitrogen fixation under mild reaction conditions has attracted much attention. However, the very low conversion efficiency and ambiguous catalytic mechanism remain the major hurdles for the research of nitrogen fixation. This review aims to clarify the role of chemical bond in catalytic nitrogen fixation by summarizing and analyzing the recent development of nitrogen fixation research. In detail, the atomic-scale mechanism of nitrogen fixation reaction, the various methods to improve the nitrogen fixation performance, and the computational investigation of nitrogen fixation are discussed, all from a chemical bond perspective. It is hoped that this review could trigger more profound pondering and deeper exploration in the field of catalytic nitrogen fixation.

Piezo-enhanced activation of dinitrogen for room temperature production of ammonia

The catalytic conversion of nitrogen to ammonia remains an energy-intensive process, demanding advanced concepts for nitrogen fixation. The major obstacle of nitrogen fixation lies in the intrinsically high bond energy (941 kJ mol^-1) of the N≡N molecule and the absence of a permanent dipole in N₂. This kinetic barrier is addressed in this study by an efficient piezo-enhanced gold catalysis as demonstrated by the room temperature reduction of dinitrogen into ammonia. Au nanostructures were immobilized on thin film piezoelectric support of potassium sodium niobate (K_0.5Na_0.5NbO₃, KNN) by chemical vapor deposition of a new Au(III) precursor [Me₂Au(PyTFP)(H₂O)]1(PyTFP = (Z)-3,3,3-trifluoro-1-(pyridin-2-yl)-prop-1-en-2-olate) that exhibited high volatility (60 °C, 10^-3mbar) and clean decomposition mechanism to produce well adherent elemental gold films on KNN and Ti substrates. The gold-functionalized KNN films served as an efficient catalytic system for ammonia production with a Faradaic efficiency of 18.9% achieved upon ultrasonic actuation. Our results show that the spontaneous polarization of piezoelectric materials under external electrical fields augments the sluggish electron transfer kinetics by creating instant dipoles in adsorbed N₂molecules to deliver a piezo-enhanced catalytic system promising for sustained activation of dinitrogen molecules.

Recent Advances in Nonprecious Metal Oxide Electrocatalysts and Photocatalysts for N2 Reduction Reaction under Ambient Condition

NH3 plays an indispensable role in agriculture, fertilizer production as well as in the chemical industry. However, its large-scale production still relies deeply on the century-old Haber-Bosch process under high temperature and pressure along with greenhouse gas emission and fossil fuel consumption. The electrocatalytic and photocatalytic N2 reduction reactions (NRRs) for NH3 production are favorable approaches to avoid these issues because they are carbon-neutral and energy-saving. Recently, the nonprecious metal oxides (NPMO) have gathered the most attention due to their ease of synthesis, controllable stability, lower cost, and environmental friendliness. Herein, the recent advances in NPMO electrocatalysts and photocatalysts for the NRR are narrated and the strategies to improve the poor NRR activity of pristine NPMOs by heteroatom doping to engineer their surface-active sites and introduction of oxygen vacancies are highlighted. A brief summary of and the future perspective on this research field are also presented.

Recent advances in photocatalytic nitrogen fixation: from active sites to ammonia quantification methods

Photocatalytic nitrogen fixation has become a hot topic in recent years due to its mild and sustainable advantages. While modifying the photocatalyst to enhance its electron separation, light absorption and nitrogen reduction abilities, the role of the active sites in the catalytic reaction cannot be ignored because the N[triple bond, length as m-dash]N nitrogen bond is too strong to activate. This review summarizes the recent research on nitrogen fixation, focusing on the active sites for N2 on the catalyst surface, classifying common active sites, explaining the main role and additional role of the active sites in catalytic reactions, and discussing the methods to increase the number of active sites and their activation ability. Finally, the outlook for future research is presented. It is hoped this review could help researchers understand more about the activation of the nitrogen molecules and lead more efforts into research on nitrogen fixation photocatalysts.

Near-Infrared-Responsive Photo-Driven Nitrogen Fixation Enabled by Oxygen Vacancies and Sulfur Doping in Black TiO2-xSy Nanoplatelets

Solar-driven nitrogen fixation is a promising clean and mild approach for ammonia synthesis beyond the conventional energy-intensive Haber-Bosch process. However, it is still challenging to design highly active, stable, and low-cost photocatalysts for activating inert N₂ molecules. Herein, we report the synthesis of anatase-phase black TiO_2-xS_y nanoplatelets enriched with abundant oxygen vacancies and sulfur anion dopants (V_O-S-rich TiO_2-xS_y) by ion exchange method at gentle conditions. The V_O-S-rich TiO_2-xS_y nanoplatelets display a narrowed bandgap of 1.18 eV and much stronger light absorption that extends to the near-infrared (NIR) region. The co-presence of oxygen vacancies and sulfur dopants facilitates the adsorption of N₂ molecules, promoting the reaction rate of N₂ photofixation. Theoretical calculations reveal the synergistic effect of oxygen vacancies and sulfur dopants on visible-NIR light adsorption and photoexcited carrier transfer/separation. The V_O-S-rich TiO_2-xS_y exhibits improved ammonia yield rates of 114.1 μmol g^-1 h^-1 under full-spectrum irradiation and 86.2 μmol g^-1 h^-1 under visible-NIR irradiation, respectively. Notably, even under only NIR irradiation (800-1100 nm), the V_O-S-rich TiO_2-xS_y can still deliver an ammonia yield rate of 14.1 μmol g^-1 h^-1. This study presents the great potential to regulate the activity of photocatalysts by rationally engineering the defect sites and dopant species for room-temperature N₂ reduction.

A boron-decorated melon-based carbon nitride as a metal-free photocatalyst for N2 fixation: a DFT study

On the basis of the electron "acceptance-donation" concept, a boron decorated melon-based carbon nitride (CN) is studied as a metal-free photocatalyst to efficiently reduce N2 to NH3 under visible light irradiation. The results revealed that a boron-interstitial (Bint)-decorated melon-based CN has an outstanding N2 reduction capacity through the enzymatic mechanism with a rather low overpotential (0.32 V). The excellent efficiency and selectivity of Bint-decorated melon-based CN in N2 reduction reaction (NRR) are attributed to the concentrated spin polarization on the B atom, the significant enhancement of visible and infrared light absorption, and the effective inhibition of the competitive hydrogen evolution reaction (HER). Importantly, B-doped melon-based CN has been successfully synthesized in the experiments, so obtaining Bint-decorated melon is promising, while proton transfer from the -NH2 group in CN to the B atom surely will affect the functionality of the catalyst through deactivation of the N2 adsorption site. Our study provides a novel single atom metal-free photocatalyst with high efficiency for NRR, which is conducive to the sustainable synthesis of ammonia.

A Dopant Replacement-Driven Molten Salt Method toward the Synthesis of Sub-5-nm-Sized Ultrathin Nanowires

The high-temperature molten-salt method is an important inorganic synthetic route to a wide variety of morphological phenotypes. However, its utility is limited by the fact that it is typically incapable of producing ultrathin (<5 nm diameter) nanowires, which have a crucial role in novel nanotechnology applications. Herein, a rapid molten salt-based synthesis of sub-5-nm-sized nanowires of hexagonal tungsten oxide (h-WO₃ ) that is critically dependent on a substantial proportion of molybdenum (Mo) dopant is described. This dopant-driven morphological transition in tungsten oxide (WO₃ ) may be attributable to the collapse of layered structure, followed by nanocluster aggregation, coalescence, and recrystallization to form ultrathin nanowires. Interestingly, due to the structural properties of h-WO₃ , the thus-formed ultrathin nanowires are demonstrated to be excellent photocatalysts for the production of ammonia (NH₃ ) from nitrogen (N₂ ) and water. The ultrathin nanowires exhibit a high photocatalytic NH₃ -production activity with a rate of 370 µmol g^-1 h^-1 and an apparent quantum efficiency of 0.84% at 420 nm, which is more than twice that obtained from the best-performing Mo-doped W₁₈ O₄₉ nanowire catalysts. It is envisaged that the dopant replacement-driven synthetic protocol will allow for rapid access to a series of ultrathin nanostructures with intriguing properties and increase potential applications.

A DFT screening of single transition atoms supported on MoS2 as highly efficient electrocatalysts for the nitrogen reduction reaction

The development of low-cost and highly efficient materials for the electrocatalytic nitrogen reduction reaction (NRR) under ambient conditions is an attractive and challenging topic in chemistry. In this study, the electrocatalytic performance of a series of transition metal (TM) atoms supported on MoS2 nanosheets (TM@MoS2) was systematically investigated using density functional theory (DFT) calculations. It was found that Re supported on MoS2 (Re@MoS2) has the best NRR catalytic activity with a limiting potential of -0.43 V, along with high selectivity over the competing hydrogen evolution reaction (HER). Moreover, the ab initio molecular dynamics (AIMD) simulations at 500 K and density of states (DOS) calculations indicated the high thermodynamic stability and excellent electrical conductivity of Re@MoS2. A linear trend between several parameters of single atom catalysts (SACs) and the adsorption Gibbs free energy change of the NH species (ΔG*NH) was observed, indicating the later as a simple descriptor for the facilitated screening of novel SACs. These results pave the way for exploring novel, highly efficient electrocatalysts for the electrochemical NRR under ambient conditions.