Patent Description:
In nature, atmospheric nitrogen (N<NUM>) is biologically fixed into ammonia (NH<NUM>) by nitrogenase enzymes under ambient conditions. Biological nitrogen fixation (BNF) is essential for life forms because such inorganic nitrogen compounds are required for the biosynthesis of basic building blocks, such as DNA and proteins. In consideration of the great importance of nitrogen fixation for human activity, scientists continue to devote substantial efforts in promoting the artificial ammonia production on a large scale.

One of the most important artificial nitrogen fixation (ANF) processes discovered in the last century, the Haber-Bosch (HB) process, has an immense impact on food production globally. Today, more than half of the world's ammonia is obtained through the centralized industrial HB process, in which NH<NUM> is produced via a dissociative pathway involving co-activation of the N<NUM> and H<NUM> over an Fe-based catalyst<NUM>. This process first frees the nitrogen atoms from their triply bonded diatomic form, N=N before they are hydrogenated to ammonia on a catalytic surface. The bond strength can be as high as <NUM> eV to liberate N, thus requiring the HB process to be operated at elevated temperature and pressure (routinely <NUM> and <NUM> bar)<NUM>. In addition, the unfavourable thermodynamic position towards reactants at high temperature makes the process inefficient. More importantly, H<NUM> is obtained from steam reforming of natural gas to combine with N<NUM> from air, which accounts for <NUM>% of the global primary energy demand. The process is therefore extremely carbon intensive and approximately <NUM> CO<NUM> /kg NH<NUM> is released into the atmosphere, representing <NUM> % of global greenhouse gas (GHG)<NUM>. The ammonia transport and distribution from centralized reactors further contribute to CO<NUM> emissions.

In comparison, BNF at small scale is able to overcome these limitations by operating the synthesis at room temperature and pressure (<NUM> and <NUM> bar) via an associative pathway. However, it still requires a large input of chemical energy provided by the hydrolysis of adenosine <NUM>'-triphosphate (ATP)<NUM>.

Iron (or sometimes Mo) sulphur (Fe-S) clusters with tetrahedral Fe/Mo and weak field S ligands are the established molecular electron relay centres for fast redox catalysis in biology<NUM>. In particular, nitrogenase is a multiprotein complex consisting of an Fe-sulphur protein and an associated MoFe-sulphur clusters protein. ATP is consumed at the Fe-sulphur [4Fe-<NUM>] protein which also delivers the generated electrons by a remote outer sphere mechanisms to the catalytic MoFe protein with the iron-molybdenum cofactor (FeMoco) containing [Mo:7Fe:<NUM>:C]<NUM>. The electrons once transferred are believed to finally accumulate at the FeMoco via the molecular electron relay of Fe-S cluster centres and subsequently be utilized for the reduction of N<NUM><NUM>,<NUM>,<NUM>.

Structural and functional mimicking of nitrogenase to produce ammonia is an ongoing scientific endeavour, although most research efforts have to date focussed on enzymatic processes<NUM>,<NUM>,<NUM>. <CIT> discloses a carrier for a synthesis gas production catalyst. Xiaoyan Ma et al discloses ultrathin Co(Ni)-doped MoS<NUM> nanosheets as catalytic promoters enabling efficient solar hydrogen production. Lei Yang et al discloses the optical properties of metal-molybdenum disulfide hybrid nanosheets and their application for enhanced photocatalytic hydrogen evolution. Xinguo Ma et al discloses a DFT study in N<NUM> reduction using single transition-metal atom supported on a defective WS<NUM> monolayer as promising catalysts. Hujiabudula Maimaitizi et al discloses a facile photo-ultrasonic assisted synthesis of flower-like Pt/N-MoS<NUM> microsphere as an efficient sonophotocatalyst for nitrogen fixation. <CIT> discloses a molybdenum disulfide/gold nanorod composite. <CIT> discloses a metal double-hydroxide/molybdenum disulfide nano-composite material. <CIT> discloses a synthesis method of a metal nanoparticle asymmetrical single-face inlayed molybdenum disulfide nanosheet. Xiu Wang et al discloses recyclable nanoscale zero valent iron doped g-C<NUM>N<NUM>/MoS<NUM> for efficient photocatalysis of RhB and Cr(VI) driven by visible light. <CIT> discloses a photocatalyst based on quantum dot/rod and molybdenum disulfide nanosheet. <CIT> discloses a composite photocatalyst. <CIT> discloses an electrocatalytic composition and cathode for the nitrogen reduction reaction.

The present invention was devised with the foregoing in mind.

According to a first aspect of the present invention there is provided a photocatalyst comprising:.

wherein the photocatalyst further comprises <NUM> - <NUM> % by weight, relative to the weight of the base material, of one or more semiconductor materials having an average particle size of <NUM> - <NUM>.

According to a second aspect of the present invention there is provided a process for preparing a photocatalyst as defined herein, the process comprising the steps of:.

wherein the photocatalyst resulting from step b) is contacted with an aqueous solution of one or more semiconductor materials having an average particle size of <NUM> - <NUM>.

According to a third aspect of the present invention there is provided a photocatalyst obtainable, obtained or directly obtained by a process according to the second aspect of the invention.

According to a fourth aspect of the present invention there is provided a use of a photocatalyst as defined by the first or third aspect of the invention in the conversion of molecular nitrogen to ammonia.

According to a fifth aspect of the present invention there is provided a photocatalytic process for the conversion of molecular nitrogen to ammonia, the process comprising the step of:.

wherein step a) is performed under the application of electromagnetic radiation having a wavelength of <NUM> - <NUM>.

In a first aspect, the present invention provides a photocatalyst comprising:.

Through intensive investigations, the inventors have synthesised a catalyst having a core structure mimicking that of biological nitrogenase. The catalyst is highly active at converting molecular nitrogen from air into ammonia in water under visible light illumination without the need for high temperatures and pressure. Moreover, the catalyst exhibits high catalytic efficiency even without the use of a sacrificial agent (e.g. methanol or formaldehyde).

In an embodiment, the layered base material has a trigonal prismatic (<NUM>) or octahedral structure (1T). Suitably, the layered base material has a trigonal prismatic structure.

In an embodiment, the layered base material is molybdenum disulfide. The molybdenum disulfide layered base material may have a trigonal prismatic or octahedral structure. Most suitably, the molybdenum disulfide layered base material has a trigonal prismatic structure.

The layered base material has a maximum of <NUM> layers provided in a stacked arrangement. Alternatively, the layered base material may have fewer layers. In an embodiment, the layered base material (e.g. molybdenum disulfide) comprises <NUM> to <NUM> layers. Suitably, the layered base material comprises <NUM> to <NUM> layers. More suitably, the layered base material comprises <NUM> to <NUM> layers. Most suitably, the layered base material comprises <NUM> to <NUM> layers. The layered base material may be the product of exfoliating a bulk quantity of the base material.

The one or more Group VI, VII, VIII, IX or X transition metals is selected from the group consisting of Fe, Mn, Co, Ni, Ru, Rh, Pd and Pt. Suitably, the one or more transition metals is selected from the group consisting of Fe, Mn, Co, Ni and Ru. More suitably, the one or more transition metals is selected from the group consisting of Fe, Co and Ru. Even more suitably, the one or more transition metals is Fe or Ru. Most suitably, the one or more transition metals is Fe.

In a particular embodiment, the one or more Group VI, VII, VIII, IX or X transition metals is not Co.

In an embodiment, the one or more Group VI, VII, VIII, IX or X transition metals is Fe and optionally one or more selected from the group consisting of Mn, Co, Ni and Ru.

In an embodiment, the photocatalyst comprises <NUM> - <NUM> % by weight, relative to the weight of the base material, of one or more Group VI, VII, VIII, IX or X transition metals. Suitably, the photocatalyst comprises <NUM> - <NUM> % by weight, relative to the weight of the base material, of the one or more transition metals. More suitably, the photocatalyst comprises <NUM> - <NUM> % by weight, relative to the weight of the base material, of the one or more transition metals. Even more suitably, the photocatalyst comprises <NUM> - <NUM> % by weight, relative to the weight of the base material, of the one or more transition metals. Most suitably, the photocatalyst comprises <NUM> - <NUM> % by weight, relative to the weight of the base material, of the one or more transition metals.

In a particular embodiment, the photocatalyst comprises <NUM> - <NUM> % by weight, relative to the weight of the base material, of one or more Group VI, VII, VIII, IX or X transition metals being Fe.

At least a portion of the one or more Group VI, VII, VIII, IX or X transition metals may be provided as single atoms or clusters of single atoms having a maximum diameter of <NUM>. Suitably, ≥<NUM>% of the transition metal is provided as single atoms of the transition metal or clusters of single atoms of the transition metal having a maximum diameter of <NUM>. More suitably, ≥<NUM>% of the transition metal is provided as single atoms of the transition metal or clusters of single atoms of the transition metal having a maximum diameter of <NUM>. Most suitably, ≥<NUM>% of the transition metal is provided as single atoms of the transition metal or clusters of single atoms of the transition metal having a maximum diameter of <NUM>. The amount of transition metal in the photocatalyst can be determined using analytical techniques such as inductively coupled plasma (ICP) and atomic adsorption (AA).

In an embodiment, the one or more Group VI, VII, VIII, IX or X transition metals is provided as single atoms or clusters of single atoms having a maximum diameter of <NUM>. Most suitably, the one or more transition metals is provided as single atoms or clusters of single atoms having a maximum diameter of <NUM>.

The one or more Group VI, VII, VIII, IX or X transition metals may be provided as:.

By way of non-limiting example, when the layered base material is molybdenum disulfide, at least a portion of the one or more Group VI, VII, VIII, IX or X transition metals (e.g. Fe) may be provided as single atoms that are incorporated into the molecular structure of molybdenum disulfide by replacing some of the S or Mo atoms, or both.

When used in relation to the one or more Group VI, VII, VIII, IX or X transition metals, the term "atom" encompasses uncharged (e.g. metallic) and charged (e.g. ionic) forms. For example, when single atoms or clusters of single atoms of the transition metal are provided on and/or throughout the layered base material, they may be present in their metallic form. When single atoms are incorporated into the molecular framework of the layered base material by replacing one or more atoms of the layered base material they may be present in their ionic form. The one or more Group VI, VII, VIII, IX or X transition metals may inter-convert between these different forms within the photocatalyst.

In a particular embodiment, the layered base material is molybdenum disulfide and the one or more Group VI, VII, VIII, IX or X transition metals is Fe. Suitably, the layered base material comprises <NUM> to <NUM> layers. More suitably, the layered base material comprises <NUM> to <NUM> layers and the one or more Group VI, VII, VIII, IX or X transition metals is present in an amount of <NUM> - <NUM> % by weight, relative to the weight of the base material.

The photocatalyst further comprises <NUM> - <NUM> % by weight, relative to the combined weight of the base material and one or more Group VI, VII, VIII, IX or X transition metals, of one or more semiconductor materials having an average particle size of <NUM> - <NUM>. Suitably, the photocatalyst further comprises <NUM> - <NUM> % by weight, relative to the combined weight of the base material and one or more Group VI, VII, VIII, IX or X transition metals, of one or more semiconductor materials having an average particle size of <NUM> - <NUM>. Most suitably, the photocatalyst further comprises <NUM> - <NUM> % (e.g. <NUM> - <NUM> %) by weight, relative to the combined weight of the base material and one or more Group VI, VII, VIII, IX or X transition metals, of one or more semiconductor materials having an average particle size of <NUM> - <NUM>.

The one or more semiconductor materials suitably has an average particle size of <NUM> - <NUM>. Most suitably, the one or more semiconductor materials has an average particle size of <NUM> - <NUM>. The one or more semiconductor materials may be described as quantum dots (e.g. cadmium sulfide quantum dots).

The one or more semiconductor materials may have the compositional formula ABxC<NUM>-x, wherein.

In an embodiment, the one or more semiconductor materials is selected from the group consisting of cadmium sulfide, lead sulfide, cadmium telluride, lead telluride, cadmium selenide and lead selenide. Most suitably, the one or more semiconductor materials is cadmium sulfide.

In an embodiment, the photocatalyst a provided as a plurality of particles having an average particle size of <NUM> - <NUM>. Suitably, the photocatalyst a provided as a plurality of particles having an average particle size of <NUM> - <NUM>. More suitably, the photocatalyst a provided as a plurality of particles having an average particle size of <NUM> - <NUM>. Most suitably, the photocatalyst a provided as a plurality of particles having an average particle size of <NUM> - <NUM>.

In an embodiment, the photocatalyst is fixed to (e.g. immobilized on or supported on/by) a supporting substrate (e.g. as part of a fixed bed apparatus).

In a second aspect, the present invention provides a process for preparing a photocatalyst according to the first aspect of the invention, the process comprising the steps of:.

As described hereinbefore in relation to the first aspect of the invention, photocatalysts obtainable by the process of the second aspect of the invention are highly active at converting molecular nitrogen from air into ammonia in water under visible light illumination without the need for high temperatures and pressure. Moreover, the catalysts exhibit high catalytic efficiency even without the use of a sacrificial agent (e.g. methanol or formaldehyde).

The layered base material and one or more Group VI, VII, VIII, IX or X transition metals may have any of those definitions described hereinbefore in relation to the first aspect of the invention.

In an embodiment, the dispersion of step a) comprises the layered base material dispersed in a liquid. Suitably, the liquid is a mixture of water and isopropyl alcohol. More suitably, the liquid is a mixture of water and isopropyl alcohol in a volume ratio of <NUM>:<NUM>-<NUM> (e.g. <NUM>:<NUM>). Additionally, the dispersion may include a surfactant, a non-limiting example of which is polyvinylpyrrolidone (PVP).

In an embodiment, the layered base material comprising <NUM> to <NUM> layers is prepared by exfoliating the base material in its bulk form. Suitably, the base material in its bulk form is exfoliated by:.

The aqueous mixture may comprise water and optionally an organic solvent. Suitably, the aqueous mixture comprises water and isopropyl alcohol. More suitably, the aqueous mixture comprises water and isopropyl alcohol in a volume ratio of <NUM>:<NUM>-<NUM> (e.g. <NUM>:<NUM>).

The term intercalant is synonymous with an exfoliant. Any suitable intercalant may be used, examples of which include surfactants and solvent molecules. In a particular embodiment, the intercalant is hydrazine or lithium. Suitably, the intercalant is lithium.

In an embodiment, the solution of one or more Group VI, VII, VIII, IX or X transition metals is prepared by dissolving one or more Group VI, VII, VIII, IX or X transition metal precursor compounds in a solvent. For example, when the one or more Group VI, VII, VIII, IX or X transition metal is Fe, the precursor compound Fe nitrate may be dissolved in thiourea solution.

The conditions for carrying out step b) are not particularly limited. In an embodiment, step b) is conducted at a temperature of <NUM> - <NUM>, optionally under hydrothermal conditions (e.g. in a sealed autoclave). Suitably, step b) is conducted at a temperature of <NUM> - <NUM>, optionally under hydrothermal conditions. More suitably, step b) is conducted at a temperature of <NUM> - <NUM>, under hydrothermal conditions. Although step b) can be conducted at room temperature, performing this step at higher temperatures and under hydrothermal conditions may result in fewer agglomerates of the one or more Group VI, VII, VIII, IX or X transition metals.

The process may additionally comprise the following step the process further includes the step:
c) isolating the photocatalyst resulting from step b).

The photocatalyst resulting from step b) (or that isolated from step c)) is contacted with an aqueous solution of one or more semiconductor materials having an average particle size of <NUM> - <NUM>.

The one or more semiconductor materials may have any of those definitions described hereinbefore in relation to the first aspect of the invention.

In an embodiment, the photocatalyst resulting from step b) (or that isolated from step c)) is contacted with an aqueous solution of one or more semiconductor materials by immersing the photocatalyst in the aqueous solution for <NUM> minutes to <NUM> hours (e.g. <NUM> - <NUM> hours).

In an embodiment, the aqueous solution of one or more semiconductor materials comprises <NUM> - <NUM> (e.g. <NUM> - <NUM>) of the one or more semiconductor materials per mL of water.

In a third aspect, the present invention provides a photocatalyst obtainable, obtained or directly obtained by a process according to the second aspect of the invention.

In a fourth aspect, the present invention provides a use of a photocatalyst as defined by the first or third aspect of the invention in the conversion of molecular nitrogen to ammonia.

In a fifth aspect, the present invention provides a photocatalytic process for the conversion of molecular nitrogen to ammonia, the process comprising the step of:.

As described hereinbefore in relation to the first aspect of the invention, the photocatalysts of the invention are highly active at converting molecular nitrogen from air into ammonia in water under visible light illumination without the need for high temperatures and pressure. Moreover, the catalysts exhibit high catalytic efficiency even without the use of a sacrificial agent (e.g. methanol or formaldehyde). When compared with conventional process that rely on the HB process, producing ammonia in this manner offers the flexibility for the decentralisation of ammonia supply to be used as fertiliser in local farmlands.

The electromagnetic radiation applied in step a) may be advantageously in the form of solar radiation (i.e. sunlight). In an embodiment, step a) is performed under the application of electromagnetic radiation having a wavelength of <NUM> - <NUM>. Most suitably, step a) is performed under the application of electromagnetic radiation having a wavelength of <NUM> - <NUM>. The photocatalytic process can be advantageously operated under visible light illumination.

In an embodiment, the electromagnetic radiation is supplied to the mixture of step a) using a solar concentrator.

The temperature at which step a) is performed is not particularly limited. In an embodiment, step a) is conducted at a temperature of <NUM> - <NUM>. Although even greater catalytic activity may be obtained when the process is conducted at high temperature, the inventors have shown that the photocatalyst can achieve impressive quantum efficiency values even when the process is conducted at room temperature, thus presenting obvious industrial advantages. Thus, in an embodiment, step a) is conducted at a temperature of <NUM> - <NUM>. More suitably, step a) is conducted at a temperature of <NUM> - <NUM>. Most suitably, step a) is conducted at a temperature of <NUM> - <NUM>. The thermal energy may be supplied by sunlight. Thus, when step a) is conducted under the application of solar radiation, an additional heat source may not be necessary.

The process may be rendered more performant by using a sacrificial agent in step a), examples of which (e.g. methanol and formaldehyde) will be readily familiar to one of ordinary skill in the art. However, the inventors have shown that the photocatalyst can achieve impressive quantum efficiency values even when the process is conducted in the absence of such a sacrificial agent. In an embodiment, the photocatalyst comprises ≤<NUM> mol% of a sacrificial agent, relative to the number of moles of the layered base material. Suitably, the photocatalyst comprises ≤<NUM> mol% of a sacrificial agent, relative to the number of moles of the layered base material.

There are a number of different ways in which the photocatalytic process may be performed.

In an embodiment, the photocatalyst is provided as a fixed bed or a thin film, over (or through) which water and molecular nitrogen are passed.

In an embodiment, the photocatalyst is provided as a suspension (which is optionally agitated) in water, over (or through) which molecular nitrogen is passed (e.g. bubbled).

The photocatalytic process may be performed in a batch manner (e.g. under agitated or stagnant conditions) or a continuous manner (e.g. where a continuous flow of nitrogen is brought into contact with water and the photocatalyst, or where a continuous flow of nitrogen and water is brought into contact with the photocatalyst).

In an embodiment, the process is decentralised, i.e. it is carried out at, or substantially near to, a location where the produced ammonia is to be consumed (e.g. on a farm or other agricultural site).

One or more examples of the invention will now be described, for the purpose of illustration only, with reference to the accompanying figures, in which:.

Reagents used for synthesis were: MoS<NUM> (Sigma-Aldrich); iron acetate (reagent grade, Alfa Aesar); FeCl<NUM>·<NUM><NUM>O (reagent grade, Alfa Aesar); FeCl<NUM> (reagent grade, Sigma-Aldrich); n-butyllithium/hexane (reagent grade, Sigma-Aldrich); Polyvinylpyrrolidone (PVP, reagent grade, Sigma-Aldrich); Potassium acetate (reagent grade, Sigma-Aldrich); Cd acetate (reagent grade, Sigma-Aldrich); Sodium sulfuride (reagent grade, Sigma-Aldrich); Thioglycolic acid (TGA, anhydrous, ≥ <NUM>%, Sigma-Aldrich); KBr (reagent grade, Sigma-Aldrich); hydrazine (puriss. , absolute ≥ <NUM>% (GC), Sigma-Aldrich); isopropanol (<NUM>%, Sigma-Aldrich); para-(dimethylamino) benzaldehyde (reagent grade, Sigma-Aldrich); H<NUM>SO<NUM> ( ≥ <NUM>%, Sigma-Aldrich); <NUM>N<NUM> (<NUM>%, CK Isotopes).

Few-Layered MoS<NUM>. <NUM> of bulk MoS<NUM> powder was dispersed in <NUM> of Water/Isopropanol (<NUM>:<NUM>, v/v). <NUM> of hydrazine monohydrate was then added. The solution mixture was placed into the sonication bath for <NUM> hours for exfoliation, followed by centrifugation at <NUM> rpm for <NUM> minutes. The supernatant collected was filtered using vacuum filtration, followed by washing with water. The exfoliated product was dried under vacuum for <NUM> hours.

Single-Layered MoS<NUM>. <NUM> of bulk MoS<NUM> powder was soaked in <NUM> of <NUM> n-butyllithium/hexane under nitrogen atmosphere for <NUM> hours. Solid LixMoS<NUM> was then isolated by vacuum filtration, followed by washing with hexane to remove excess n-butyllithium. It was then dried under vacuum for <NUM> hours. The dried product was then immersed into <NUM> of water. The solution was placed into the sonication bath for <NUM> hours and then centrifuged at <NUM> rpm for <NUM> minutes. The supernatant collected was filtered using vacuum filtration, followed by washing with water. The exfoliated product was dried under vacuum for <NUM> hours.

Fe precursor solution was prepared by dissolving <NUM> metal ions into <NUM> of <NUM> thiourea solution and left for overnight to form a metal complex. The metal complex solution was mixed with <NUM> of colloid solution, which was made by dispersing <NUM> of sMoS<NUM> (b MoS<NUM> or fMoS<NUM>) in <NUM> of water/isopropanol (<NUM>:<NUM>, v/v) and <NUM> of PVP (stabiliser). The solution mixture was then transferred to an autoclave and then placed into an oven at <NUM> for <NUM> hours. Afterwards, the precipitate was washed with deionized water and dried under vacuum for <NUM> hours to obtain the solid product.

CdS quantum dots were synthesized according to previous reports with slight modifications<NUM>. Briefly, <NUM> uL of TGA was added into <NUM> of Cd acetate (<NUM>) aqueous solution, and N<NUM> was bubbled throughout the solution to remove O<NUM> at <NUM>. During this period, <NUM> NaOH aqueous solution was slowly added with adjustment to raise the pH to <NUM> gradually. Following this step, <NUM> of <NUM> Na<NUM>S aqueous solution was injected into the CdS quantum dots. The reaction mixture was refluxed under N<NUM> atmosphere for <NUM>. Finally, the desired CdS quantum dots were obtained and stored in a refrigerator at <NUM> for further use. To load the CdS quantum dots onto the basal plane of sMoS<NUM>, <NUM> of Fe-sMoS<NUM> was dipped into <NUM> of CdS quantum dots aqueous solution (<NUM>/mL) for <NUM>.

High-angle annular dark field scanning transition electron microscopy (HAADF-STEM). The finely ground samples were placed onto the holey carbon coated Cu-TEM grid for analysis. The analysis was performed by JEOL-JEM2100 Aberration-Corrected Transmission Electron Microscope in Birmingham. A voltage of <NUM> kV to avoid beam excitation and damage was applied for the imaging. An off-axis annular detector imaging was used for Dark-field (Z-contrast) imaging and atomic-resolution imaging. Compositional analysis by X-ray emission detection was also conducted. For the EDX detector, Bruker <NUM> SDD detector with a window area of <NUM><NUM> was used. All results were then processed with Esprit <NUM> software.

Inductively coupled plasma (ICP). The finely ground samples were dissolved and diluted with <NUM> wt. % HCl for ICP analysis. The analysis was performed by ICP optical emission spectroscopy (Optima2100DV, PerkinElmer). The doped-metal content was controlled at around <NUM> wt. % with error ± <NUM> (Fe <NUM> wt. %, Co <NUM> wt. %, Ni <NUM> wt.

Extended X-ray absorption fine structure (EXAFS). Fe K-edge and Mo K-edge X-ray absorption spectra was conducted in fluorescence mode at the BL07A XAS beamline at NSRRC, Taiwan. To examine the local chemical environment around Fe and Mo atoms, EXAFS data were extracted from XAS spectra. The Demeter ATHENA program was used for XAFS data analysis, where the data were background subtracted, normalised and Fourier transformed. The Demeter ARTEMIS program was used to perform the least-squares curve fitting analysis of the EXAFS χ(k) data. The EXAFS Wavelet analysis was performed following the protocol and calculations developed by Marina Chukalina and Harald Funke, where the backscatter atoms are distinguished within the same atomic shell<NUM>. To confirm the reproducibility of the experimental data, at least <NUM> scan sets were collected and compared for each sample. The spectra were calibrated with Fe and Mo foil as reference. The amplitude reduction factor was obtained from analysis of the Fe and Mo foil, which was used as a fixed input parameter to allow refinement in the coordination number and bond distance of the absorption element.

Time-resolved photoluminescence (TRPL) spectroscopy. Photoluminescence spectra and corresponding lifetime of excitons were obtained from a bespoke micro-photoluminescence setup, in which a Ti-Sapphire laser (λ = <NUM>, pulse duration = <NUM> fs, repetition rate = <NUM>) was directed onto the sample. Time-resolved measurements were performed using the spectrometer as a monochromator before passing the selected signal to a photomultiplier tube (PMT) detector with an instrument response function width of -<NUM> ps connected to a time-correlated single-photon counting module. Parameters describing the photoluminescence were obtained by fitting the background-corrected PL spectra with a monoexponential decay function of the form y = A<NUM>exp(-x/t<NUM>) + y<NUM> for sMoS<NUM>. A double-exponential model using equation of I(t)=A<NUM>exp(-t/ τ<NUM>) + A<NUM> exp(-t/τ<NUM>) when d orbital metal (Mn, Fe, Co, and Ni) was introduced<NUM>,<NUM>.

Attenuated total reflection fourier transform infrared (ATR-FTIR) spectroscopy. In situ ATR-FTIR spectra were collected using a multiple-reflection ATR accessory (PIKE Technologies, custom-modified GladiATR) in a Varian <NUM>-IR spectrometer, controlled by Resolutions Pro software. A trapezoidal Si internal reflection element (IRE, Crystal Gmbh, <NUM> × <NUM> × <NUM><NUM>) with a face angle of <NUM>° was sealed into a polyether ether ketone (PEEK) baseplate using silicone sealant, and a custom cell sealed on top<NUM>. A layer of water molecules, which were necessary to provide protons, was first pre-adsorbed on the surface from a drop of water onto the catalyst. Subsequently, <NUM>/min of N<NUM> saturated with H<NUM>O was passed over the catalyst while the visible light source was turned on and the IR absorption monitored with an MCT detector over the course of the reaction.

Ultraviolet-visible (UV-vis) absorption spectroscopy. UV-vis absorption spectrum was collected using a Varian <NUM> Bio UV-Visible Spectrometer in absorbance mode with a step interval of <NUM>. The solution after reaction overnight was filtered. The obtained <NUM> was mixed with <NUM> of <NUM> para-(dimethylamino) benzaldehyde and <NUM> H<NUM>SO<NUM> solution, finally transferred into an optical glass cuvette for hydrazine measurement. The concentration of ammonia solution is also detected using UV-vis spectrum with Nessler's agent.

DFT Theoretical Calculation. All calculations were performed using the first-principles density of functional theory (DFT) as implemented in Vienna ab initio simulation packages (VASP)<NUM>, the exchange-correlation energy functional described by generalized gradient approximation using Perdew-Burke-Ernzerhof (PBE) functional<NUM>, and the ion-electron interaction was treated using the projector-augmented wave (PAW) method<NUM> with a plane-wave cutoff energy of <NUM> eV. A (<NUM>×<NUM>) supercell of <NUM>-MoS<NUM> was selected to simulate single-layered MoS<NUM> (sMoS<NUM>), periodic boundary conditions were employed and <NUM>Å of vacuum in the z-direction was set to separate neighboring single-layered MoS<NUM>. The Brillouin zone has been sampled using a <NUM>×<NUM>×<NUM> and an <NUM>×<NUM>×<NUM> Monkhorst-Pack<NUM> grid of k-points for geometry optimizations and orbital analysis calculations, respectively. Both lattice constants and atomic positions were relaxed until the forces on atoms were less than <NUM> eV Å-<NUM> and the total energy change was less than <NUM>×<NUM><NUM> eV. To rationalize the different performance of sMoS<NUM> and transition metal doped MoS<NUM> in catalytic ammonia photosynthesis, density of states and frontier orbitals topology analysis were performed at the PBE/PAW level of theory.

All photocatalytic activity experiments were conducted at ambient temperature using a <NUM> W tungsten lamp (Glamox Professional <NUM>) with UV light cut-off to simulate visible light, respectively. For the fixation of molecular nitrogen, <NUM> of photocatalyst was added into <NUM> of double distilled water in a reactor. The reactor was equipped with water circulation in the outer jacket in order to maintain at room temperature of <NUM>. The mixture was continuously stirred in the dark and under visible light with high-purity N<NUM> (<NUM>%) bubbled at a flow rate of <NUM>/min. Five milliliters of the solution was taken out each <NUM> and after filtering to remove the photocatalyst, and the concentration was monitored by colorimetry with the UV-vis spectrometer. For the measurement of ammonia yields, a specialized highly sensitive ammonia detector was used (Thermo Sicentifc™ Orion™ Ammonia Gas Sensing ISE Electrode). Quantum efficiency measurements were carried out under a <NUM> W Xenon lamp through quartz windows using bandpass filters of <NUM>±<NUM>, <NUM>±<NUM>, <NUM>±<NUM>, and <NUM>±<NUM>.

Isotopic N<NUM> was used to prove that the obtained ammonia derives from N<NUM> gas rather than some other sources. Indophenol assays were prepared by adding <NUM> of aliquot solution after <NUM>-h reaction to <NUM> of <NUM>% phenolic solution in <NUM>% ethanol/water. Stepwise, <NUM> of <NUM>% NaClO in alkaline sodium citrate solution and <NUM> of <NUM>% Na[Fe(CN)<NUM>NO] solution were added. The assayed aliquots were aged overnight before analyzing on a Xevo LCMS-ESI system.

The <NUM>-D single molecular layer MoS<NUM> (termed sMoS<NUM>) consisting of three-sub-layers of S-Mo-S in a trigonal prismatic <NUM>-H structure was first synthesized via exfoliation of bulk MoS<NUM> using n-butyllithium. Subsequently Fe atoms were attached to sMoS<NUM> using a hydrothermal method for in-situ formed sulphide species (<FIG> and <FIG>), followed by H<NUM> reduction to afford the molecular Fe-sMoS<NUM> in nanosize. In this synthesis, Fe atoms were atomically dispersed and assembled onto the basal plane of sMoS<NUM>.

Similar transition metal-doped catalysts were prepared in the same manner using Mn, Co or Ni instead of Fe.

<FIG> shows a typical high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of Fe-sMoS<NUM> with corresponding <NUM>-H characteristic pattern. As revealed from the distinctive brighter spots than the surrounding Mo and S<NUM> sites in <NUM>-H arrangement in <FIG> (red circles) and the enlarged image <FIG> show that individual Fe atoms are uniformly dispersed and overlap the position of Mo and S sites in the structural motif of <NUM>-sMoS<NUM>. To further confirm the nature of this adsorbed atom, atomic resolved electron energy loss spectroscopy (EELS) were performed on these brighter spots. The EEL spectra (<FIG>) at the corresponding positions demonstrate the presence of Fe atoms with the characteristic signature L<NUM> edge at <NUM> eV<NUM>. The images for HAADF-STEM and EELS analysis show that isolated Fe atoms are deposited at the two favoured positions in the basal plane of sMoS<NUM>. A typical intensity profile analysis of the HAADF shown in <FIG> demonstrates that a Fe atom commonly takes residence on atop site of Mo of the basal plane. Occasionally, as shown in <FIG>, an Fe atom can be found in the position of S site as substitution. The corresponding structural models shown in <FIG> based on DFT calculations also confirm the existence of these two energetic favoured atomic positions (<FIG>).

As shown in k3-weighted Fourier transformed spectra in the extended X-ray absorption fine structure (EXAFS) for Fe post-k edge analysis of Fe-sMoS<NUM> (<FIG>), there are clearly Fe-S contributions at a distance of <NUM>Å for Fe-sMoS<NUM>, which is distinctively different from the Fe-Fe contributions at a distance of <NUM>Å calibrated by Fe foil. The distance is close to the Fe-CI contributions in FeCl<NUM>, indicating that Fe is atomically isolated as revealed by HAADF-STEM images shown in <FIG>. There is a small but new peak at <NUM>Å attributed to Fe-Mo interaction, which is agreeable to the envisaged bonding environment of Fe atop to the Mo site on <NUM>-H sMoS<NUM> and the theoretical model from the DFT calculations (<FIG>). Similarly, the small peak at <NUM>Å can be attributed to the long Fe-S bonds where Fe substitutes into the S site (<FIG>, see model in <FIG>). The EXAFS curve fit matches with the expected coordination number of the nearest sulfur atoms around the isolated Fe atom of <NUM>± <NUM> with the distance of <NUM>Å, and the nearest Mo atoms around the isolated Fe atom is <NUM>± <NUM> according to the atop model (<FIG> and Table <NUM>).

<FIG> shows the WT-EXAFS wavelet transformed analysis based on Morlet wavelets, which can be used to differentiate closely-related spatial interactions<NUM>. For Fe-sMoS<NUM>, the hot spot of the WT maximum at ~ <NUM>Å-<NUM> is well-resolved at the first coordination shell, which can clearly be related to the Fe-S bond at atop site. In contrast, the WT intensity hot spot at ~ <NUM>Å-<NUM> region corresponding to the Fe-Fe bond was not detected in Fe-sMoS<NUM>, which indicates the sole dispersion of individual Fe atoms in Fe-sMoS<NUM> (<FIG>). In addition, there is an associated weak asymmetry WT intensity area ranging from <NUM>-<NUM>Å for Fe-sMoS<NUM>, which is attributed to the mixed contributions of Fe-Mo bonds and longer Fe-S bonds at the S substitution sites (reference to <FIG>). It is noted that the WT intensity for this Mo atop site is much stronger than that of the S substitution site, demonstrating that this is the principle site for the Fe, as reflected by the HAADF-STEM analysis.

Similar structure was obtained for Co and Ni-doped sMoS<NUM> at comparable doping levels (<FIG>).

X-ray absorption near-edge structure (XANES) analysis was also carried out to better understand the single-atom Fe-sMoS<NUM> catalyst. As shown in <FIG>, the Fe k-edge XANES spectrum of quenched Fe-sMoS<NUM> from photocatalysis is drastically different from that of metallic Fe foil (Fe<NUM>), but its absorption edge in the right-shift position between FeIICl<NUM> and FeIIICl<NUM> implying that the average working oxidation state is between them for the anchored Fe.

The molecular models of FeMoco and Fe-sMoS<NUM> shown in <FIG>, respectively illustrate their similar structural motifs of the four-membered [Fe-S<NUM>-Mo] rings. Interestingly, the derived Fe-S, Mo-S and Fe-Mo bonding lengths of the [Fe-S<NUM>-Mo] in this single layer molecular Fe-sMoS<NUM> catalyst are extremely close to that of the reported molecular [Fe-S<NUM>-Mo] unit in FeMoco from single crystal data within the average deviations of <NUM>%<NUM> (see Table S2(<NUM>), <FIG> and <FIG>).

<FIG> confirms that Fe-sMoS<NUM> with [Fe-S<NUM>-Mo] is active to convert N<NUM> to NH<NUM> via photo-activation to provide excited electrons for the N<NUM> fixation in H<NUM>O at ambient conditions. Bulk MoS<NUM> is shown to be inert for N<NUM> reduction presumably because of its conduction band (CB) is more positive than that of the N<NUM>/NH<NUM> redox couple<NUM> (see <FIG>). By reducing the layers of MoS<NUM>, the activity for nitrogen fixation to ammonia is gradually enhanced. It has been proven that the band gap of MoS<NUM> could be enlarged with a more negative CB edge striding over the N<NUM>/NH<NUM> redox couple<NUM>,<NUM>. In addition, indirect band excitation over few-layer MoS<NUM> can be switched to a more efficient direct band excitation for single layer MoS<NUM><NUM>. While the single layered MoS<NUM> materials display a negligible activity in H<NUM>/O<NUM> splitting from water by visible light, a substantial higher photocatalytic ammonia production rate is recorded. Notably, the introduction of the [Fe-S<NUM>-Mo] motifs into the basal planes of single layered MoS<NUM> displays a far more superior activity for ammonia and (stoichiometric oxygen) production from N<NUM> and H<NUM>O reaction with trace hydrogen gas formation than most recent reported photocatalysts in visible light regime (Table <NUM>), thereby mimicking the FeMoco.

Light driven nitrogen fixation over solid catalysts in aqueous medium has been intensively studied with continual interests. TiO<NUM> has been receiving considerable attention due to its outstanding photochemical properties but the wide band gap of <NUM> eV denies the direct ammonia production by visible light activation. As a result, modified TiO<NUM> materials to reduce the band gap or include promotors to capture visible light have been commonly applied. Despite these attempts, low activities for photo ammonia production using visible light are generally obtained. The low levels of ammonia can be seen from the typical modified TiO<NUM> such as entries <NUM> and <NUM> (Table <NUM>) where a significant quantity of ammonia is produced due the use of unfiltered light source with UV component. Other semi-conductive oxide-based materials such as BiOCI and W<NUM>O<NUM> (entries <NUM> and <NUM> in Table <NUM>) are also known to capture visible light for ammonia production but they show low activities, presumably due to poor charge separation (short lifetime for exciton recombination) from lack of rapid charge separation component for these structures. It should be particularly noted from entry <NUM> (Table <NUM>) that <NUM>% Ru@n-GaN NWs is a promising material, which exhibits higher ammonia production activity by using N<NUM>/H<NUM> at room temperature. In contrast with the recent literature, Fe-sMoS<NUM>with [Fe-S<NUM>-Mo] motifs on <NUM>-D single layered MoS<NUM> show the highest ammonia production activity from direct visible light activation without using sacrificial reagent (see entries <NUM> and <NUM> in Table <NUM>). This indicates its unique structure for the efficient charge separation and activation of N<NUM> in visible light and water for the ammonia production.

Although only a small quantity of photocatalyst was made, it is sufficient to cover more than <NUM><NUM> of farmland per gram of catalyst assuming the leaching rate of <NUM> N ha-<NUM> for a selected crop is used. This value for decentralised photocatalytic ammonia fertilizer production was estimated as follows:.

Isotope labelled <NUM>N<NUM> was used to track the nitrogen source of ammonia, which confirmed that gaseous <NUM>N<NUM> was fixed by this FeMoco-like Fe-sMoS<NUM> (<FIG>). Interestingly, further activity promotion by incorporation of light-captured CdS quantum dots to Fe-sMoS<NUM> can be achieved. The rate for N<NUM> reduction to NH<NUM> maintains for at least <NUM> under constant illumination without showing any obvious attenuation (<FIG> inset). It is expected that the CdS quantum dots can contribute additional electron-hole pairs from visible light illumination, the significant activity enhancement reflects their efficient charge separation by the [Fe-S<NUM>-Mo] motifs in Fe-sMoS<NUM> at the materials interface.

The dynamic N<NUM> reduction to NH<NUM> over Fe-sMoS<NUM> was also studied using in-situ ATR-FTIR with light illumination (<FIG> and <FIG>). The IR absorption bands of <NUM> and <NUM>-<NUM> shown in <FIG> can be attributed to the O-H stretching and H-O-H bending of adsorbed water molecules, respectively on the catalyst structure. Their decreasing signals from background as a result of the consumption of the adsorbed water molecules upon the light illumination in N<NUM>. Simultaneously, four bands at <NUM>, <NUM>, <NUM>, and <NUM>-<NUM> were arisen, which can be attributed to the H-N-H bending, -NH<NUM> wagging, -NH<NUM> twisting, and N-N stretching of adsorbed N<NUM>Hy (<NUM>≤y≤<NUM>) species, respectively<NUM>. Notably, the latter species suggest that the N<NUM> reduction on the Fe-sMoS<NUM> may follow the association pathway under the light illumination. The content of hydrazine was analysed using para-(dimethylamino) benzaldehyde acidic solution, which gave a small but detectable peak at around <NUM> in UV-vis spectroscopy, as shown in <FIG>. This indicates that the formation of N<NUM>H<NUM> from N<NUM> reduction, which forms a complex with the benzaldehyde compound<NUM>. Thus, the Fe-sMoS<NUM> appears to undertake the same association pathway for N<NUM> fixation as that of nitrogenase with both structures containing the common motifs of four membered [Fe-S<NUM>-Mo] rings.

To prove the unique feature of [Fe-S<NUM>-Mo] in photocatalytic ammonia production, the activities of some selected first-row transition metal analogues were compared and are shown in <FIG>. The same volcano activity relationship for typical ammonia production rate from N<NUM> reduction with respect to d orbital filling and position at the optimal value of Fe is presented. Time resolved photoluminescence (TRPL) spectra of sMoS<NUM>, Mn, Fe, Co, and Ni-doped sMoS<NUM> are also shown in <FIG> and <FIG>. As seen from the TRPL spectra, the instantly generated excited electrons and holes in sMoS<NUM> annihilate rapidly within a few nanoseconds. Doping single transition metal atoms onto this structure apparently increases their recombination time, suggesting that the metal exerts an enhanced degree of charge separation by accepting excited electrons. Mn and Ni-doped sMoS<NUM> show a similar exciton lifetime, followed by CosMoS<NUM>. Interestingly, Fe-doped sMoS<NUM> with optimal d-band filling and position also gives the longest excitons lifetime with the slowest PL decay curve. Notably, the rank of their lifetimes (Table <NUM>) shows a strong inverse relationship with photocatalytic activity for ammonia production (<FIG>). It is anticipated that the prolonged excitons lifetime is critical to allow chemical reactions of the excitons to occur before they recombine for relaxation, leading to photocatalytic N<NUM> fixation. Thus, the Fe-doped sMoS<NUM> with the Fe-S<NUM>-Mo motifs displays the best combination of metal site and 'electron relay' components for charge separation analogously to that in the biological system.

For N<NUM> activation over nitrogenase, it was suggested from theoretical calculations that N<NUM> could linearly bind to either the molybdenum atom over the distal pathway (hydrogenation starts at terminal N), or the iron atom over the alternating pathway (hydrogenation starts at N in proximity to Fe) in the FeMoco<NUM>. The electron states of HOMO and LUMO and band structures in Mn, Fe, Co, and Ni-doped sMoS<NUM> were then modelled (<FIG>, <FIG>, and <FIG>).

As shown in <FIG>, the HOMO and LUMO orbitals concentrate on the edge of sMoS<NUM> with relatively low electron delocalization, verifying the highly active edge site of s-MoS<NUM> as that reported in literature. Transition metal atom doped distinctly improves the degree of delocalization of the frontier orbitals, especially to their LUMO, the frontier orbitals delocalization follows the order: Fe > Co > Mn≈ Ni. The higher degree of delocalization indicate the more stable population of photo-excited electrons in LUMO orbitals, thus accounting the longer lifetime for the recombination of excited photo-generated electrons and photo-generated holes. This is in good agreement with the TRPL experimental results. Among them, the LUMO orbital distribution over the Fe atom in Fe doped sMoS<NUM> should be noted (<FIG>). Particularly, they demonstrate that excited electrons could be transferred from valence band to conduction band of sMoS<NUM> via the conductive Fe-S<NUM>-Mo motifs and resided on to the Fe atom during the photo-exciting process to enter to the anti-bonding orbital of absorbed N<NUM> molecule and thus facilitating the hydrogenation reaction of N<NUM> for ammonia production. On the other hand, the density of state calculation indicates a smaller band gap of Fe doping sMoS<NUM> relative to other catalyst materials, which is also favorable for electron transfer from HOMO to LUMO by photo-excitation.

Clearly, excited electrons from CB of sMoS<NUM> after photo-excitation show a strong propensity to transfer and accommodate at Fe<NUM> atom than other transition metals. According to further DFT calculations, it was also found that wherever N<NUM> was placed on Fe<NUM> atom doped sMoS<NUM> slab, the N<NUM> adsorption was always converged onto the Fe<NUM> atom in [Fe-S<NUM>-Mo] as the end on mode spontaneously (<FIG>). Once the electronegative N<NUM> moiety is taken up by the Fe<NUM> atom excited electrons during visible light illumination are expected to retain to further prolong the exciton lifetime for subsequent protons reduction to ammonia on the N<NUM>-Fe<NUM> against the typical fast recombination of excitons from this layer structure, which substantially promotes the N<NUM> to NH<NUM> reaction over H<NUM>O photolysis without in contact with nitrogen gas. In addition, the nitrogen fixation to ammonia on the Fe<NUM> over [Fe-S<NUM>-Mo] appeared to go through the alternating pathway (<FIG> and <FIG>), indicating the similarity in mechanism for both non-biological and biological processes in ammonia synthesis.

It is generally recognized that ammonia synthesis at nitrogenase follows an associative pathway without breaking N=N triple bonds directly in transition state. N<NUM> adsorption and the following first proton and electron reactions of adsorbed N<NUM> (formation of *N<NUM>H) are two key steps in this non-dissociative reduction of N<NUM>. The energy plots in <FIG> and <FIG> then show that the first hydrogenation step by adding hydrogen atom is the most challenging step with the energy going uphill. Hydrogenating the proximal N to Fe is found to be less favourable with a higher energy state whereas, the species from hydrogenating the terminal N is relative stable. The following hydrogenation steps can be separated into two pathways: distal and alternating as shown in the <FIG>. The intermediate species via the latter pathway is more stable compared with that via the former pathway. Based on the energy plots, the nitrogen fixation to ammonia over [Fe-S<NUM>-Mo] sites through the alternating pathway appears to be more energetically favoured despite the higher activation barrier in the first hydrogenation step.

Quantum efficiency (Q. ) for photon to hydrogen in ammonia is the key parameter to evaluate the conversion efficiency of renewable light energy. <FIG> shows that the Q. of this nitrogenase-mimic Fe-sMoS<NUM> can be up to <NUM>% at <NUM>, which is believed to be the highest value reported in photo-ammonia synthesis.

Claim 1:
A photocatalyst comprising:
a layered base material comprising <NUM> to <NUM> layers, the layered base material being selected from the group consisting of molybdenum disulfide, tungsten disulfide, molybdenum telluride, tungsten telluride, molybdenum selenide and tungsten selenide; and
<NUM> - <NUM> % by weight, relative to the weight of the base material, of one or more Group VI, VII, VIII, IX or X transition metals, wherein the one or more Group VI, VII, VIII, IX or X transition metals is selected from the group consisting of Fe, Mn, Co, Ni, Ru, Rh, Pd and Pt,
wherein the photocatalyst further comprises <NUM> - <NUM> % by weight, relative to the weight of the base material, of one or more semiconductor materials having an average particle size of <NUM> - <NUM>.