Process for synthesizing ammonia with semiconductor nanoparticles and visible light

Abstract

A process for synthesizing ammonia (NH3) with semiconductor nanoparticles is provided. The process involves production of ammonia (NH3) through photocatalytic reduction of nitrate (NO3 -) and nitrite (NO2 -) ions using indium phosphide (InP) quantum dots (QDs) under visible light irradiation. The process utilizes InP QDs as an effective photocatalyst, 5 achieving NH3 yields ~ 94% within 2 hours at room temperature. The process provides selective NH3 production with minimal by-products, effective operation at low substrate concentrations (down to 250 μM), and reduced toxicity compared to traditional quantum dot materials. The process not only addresses the environmental concerns associated with nitrate pollutants but also contributes to sustainable ammonia production, making it suitable for 0 applications in energy and water remediation sectors.

Description

PROCESS FOR SYNTHESIZING AMMONIA WITH SEMICONDUCTOR NANOPARTICLES AND VISIBLE LIGHTFIELD OF INVENTION

[0001] The present disclosure relates generally to the field of chemical synthesis and renewable energy, specifically to methods and systems for producing ammonia (NH3). In particular, the present disclosure pertains to a process for synthesizing ammonia with semiconductor nanoparticles and visible light.BACKGROUND

[0002] Ammonia (NH3) is one of the largest produced chemicals in the world, the majority being incorporated into fertilizers. Alongside, NH3is being unlocked for its use in the energy sector as well. However, a significant proportion of NH3is produced via the Haber-Bosch process.

[0003] Smil V, 1999, Nature, 400, 415 discloses that the Haber-Bosch process is responsible for the majority of NH3produced in the world. Appl, M., 2006, Ullmann's Encyclopedia of Industrial Chemistry, discloses that in a typical Haber-Bosch process, pure H2and N2are reacted at high pressure (100-200 atm) and temperature (400-500 °C) over a Fe-based catalyst promoted with A12O3and potassium. The overall process is highly energy intensive and consumes almost 2% of the global energy. This industrial ammonia production is not just energy consuming but is associated with a large carbon footprint as it releases ~ 1.9 metric tons CO2per metric ton of NH3produced. One of the main reasons for enormous CO2emission is the requirement of pure H2Jwhich is produced via natural gas reforming (CH4 + 2H2O 4H2+ CO2) or coal gasification (C + 2H2O 2H2+ CO2). As a result, alternate ways to synthesize NH3in a sustainable way are required.

[0004] Ye L et al., 2017, Chem, 3, 709-714 discloses that currently, three different methods to decarbonize the Haber-Bosch process are under consideration, 1) Sequestration of carbon dioxide emitted during the process (blue NH3), b) production of H2via water electrolysis from renewable energy sources for small-scale Haber-Bosch plants (green NH3), and 3) development of alternate ways for NH3production, including photochemical and electrochemical methods. Utilization of abundant solar energy is a promising approach to produce NH3and relies on clean and readily available raw materials, sunlight, water, and air.Photochemical NH3synthesis usually requires a photocatalyst with appropriate catalytic sites and band positions. Consequently, a large number of photocatalysts (semiconductors, plasmon-based catalysts, carbon materials, etc.) are reported to drive N2 reduction to ammonia. However, one of the fundamental limitations in the majority of the reports is the poor efficiency of the overall process due to high bond dissociation energy (931 kJ mol'1) and low solubility of N2.

[0005] Keeping in mind the limitations of photocatalytic N2 reduction to NH3, NCh’ / NC ’ as N-sources are gaining attention because of their high solubility and lower bond dissociation energy. Furthermore, NO37NO2' ions are ubiquitous contaminants in water bodies, which need urgent attention to reduce their detrimental effects. In this regard, a few photocatalysts emerged for NCh' / NC ' reduction. However, photocatalytic reduction of NO3' to NH3is often been achieved with UV-active photocatalysts. Additionally, the differences in kinetics and thermodynamics of multistep reaction pathways involved, result in the accumulation of undesired by-products (majorly N2 and NOXas the end-product), instead of NH3. Thus, these limitations of existing approaches call for rationally designed visible-light photocatalysts for the efficient and selective NH3 formation from NO3' and NO2' sources.

[0006] Therefore, there is a need to overcome the limitations of existing approaches through a rational design of visible-light photocatalyst for the efficient and selective NH3formation from NO3' and NO2’.OBJECTS OF THE PRESENT DISCLOSURE

[0007] Some of the objects of the present disclosure, which at least one embodiment herein satisfies are as listed herein below.

[0008] An object of the present invention is to reduce the carbon footprint associated with the traditional Haber-Bosch process by utilizing visible light and indium phosphide semiconductor nanoparticles or quantum dots (InP QDs) as photocatalysts.

[0009] An object of the present invention is to provide a process for synthesizing ammonia with semiconductor nanoparticles and visible light.

[0010] Another object of the present invention is to harness visible light, including natural sunlight, to drive the ammonia production process, making it more accessible and practical for large-scale and outdoor applications.SUMMARY

[0011] This summary is provided to introduce a selection of concepts in a simplified form that is further described below in the detailed description section. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.

[0012] Aspects of the present disclosure relates to a selective visible-light-driven ammonia production from nitrate and nitrite ions with indium phosphide quantum dots (InP QDs) as the photocatalyst.

[0013] Accordingly, in an aspect the present disclosure provides Indium phosphide (InP) semiconductor nanoparticles or quantum dots (QDs) as photocatalyst for visible light- mediated NH3formation. The presence of catalytic indium atoms and the introduction of favorable catalyst-reactant interactions between positively charged InP QDs and NO3' anions results in a selective and efficient NH3formation within 2 hours of visible light irradiation.

[0014] In certain embodiments, ammonia is produced in a yield of ~94 % in both aqueous and gaseous phases within 2 hour of visible-light irradiation at room temperature, using the synthesized InP QDs as the photocatalysts.

[0015] In various embodiments, the InP -based photocatalyst is suitable to produce NH3even with lower NO / ZNCh' concentrations such as micromolar concentration and hence, can be used effectively for water remediation purposes.

[0016] In certain embodiments, the process is devoid of hole scavengers.

[0017] In another aspect, the present disclosure provides a method for ligand place exchanging of InP QDs, comprising steps of: i) dissolving N, N, N-trimethyl(l 1 -mercaptoundecyl) ammonium chloride (TMA) in water to obtain a TMA solution and impart a positive charge on the InP QDs surface; ii) adding as-synthesized oleylamine-capped InP QDs to the TMA solution under vigorous shaking, followed by the addition of toluene to obtain a mixture; iii) stirring the mixture continuously for 4 hours to achieve complete phase transfer of InP QDs from organic to an aqueous layer; and iv) separating the aqueous layer and precipitating the InP QDs using isopropanol.

[0018] In various embodiments, the precipitated InP QDs are redispersed in water and stored in the dark at a room temperature.

[0019] In certain embodiments, the similar procedure is carried out to prepare 11- mercaptoundecanoic acid (MUA) functionalized InP QDs to attain a negative surface charge and water solubility.

[0020] In yet another aspect, the present disclosure provides a method for producing ammonium (NH3) using InP QDs as a photocatalyst, comprising steps of:I) dispersing InP QDs in a solution containing NO3' or NO2' ions; andII) irradiating the solution with visible light to obtain more than 90%, particularly, ~ 94% of ammonium within 2 hours at room temperature.

[0021] In various embodiments, the visible light is provided by low-power light-emitting diodes (LEDs) or natural sunlight.

[0022] In certain embodiments, the ammonia formation was detected in aqueous or gaseous phases.

[0023] In certain embodiments, the NO3' or NO2' ions are present at micromolar concentrations, suitable for water remediation purposes.

[0024] In various embodiments, the InP QDs are surface-engineered to facilitate favourable catalyst-reactant interactions through electrostatic attraction with NO3' anions.

[0025] The interference of various commonly present ions in ammonia formation was carried out. Anions such as sulfate, carbonate, and phosphate and cations such as sodium, and magnesium were added to the photocatalytic reaction mixture consisting of [+] InP QDs and KNO3 in equimolar amounts. Ammonia formation was still observed in the presence of different ions, confirming the minimal interference from the commonly found ions in real water samples.

[0026] Introduction of a positively charged metal-solvent complex, [In(DMF)e]3+, referred as MSC, as a ligand on InP QD surface boosted ammonia synthesis with ~16 fold enhancement in reaction rate compared to positively charged TMA functionalized InP QDs. Within 30 min of visible light irradiation, -100 % NO3' was converted to NH3. This clearlyproves the effect of favourable catalyst-reactant interactions between positively charged surface ligands and negatively charged NO3' ions for efficient NO3' to NH3conversion.

[0027] Various objects, features, aspects and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments.BRIEF DESCRIPTION OF DRAWINGS

[0028] The accompanying drawings are included to provide a further understanding of the present disclosure and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the present disclosure and, together with the description, serve to explain the principles of the present disclosure.

[0029] FIG. 1 illustrates characterization of InP QDs a) Tauc plot of red-emitting core- only InP QDs, clearly showing the broad absorption in the visible region; b) Schematics showing the ligand exchange of oleylamine-capped InP QDs with positively charged N,N,N- trimethyl(l l-mercaptoundecyl)ammonium chloride (TMA) and negatively charged 11- mercaptoundecanoic acid (MUA) thiolated ligands; c) Zeta potential plot confirming the successful functionalization of InP QDs with [+] and [-] thiolated ligands; and d) A representative TEM image of core-only [+] InP QDs. Inset shows the corresponding size distribution histogram (collected from ~ 300 QDs), in accordance with an embodiment of the present disclosure.

[0030] FIG. 2 illustrates optimization of reaction parameters for photocatalytic reduction of NO3' to NH4+with visible light a) Schematics showing the regulation of catalyst (InP QD)- reactant (NO3') interactions through electrostatic forces emanating from QD surface ligands. A favorable catalyst-reactant interaction between [+] InP and [-] NO3' ions enable efficient and selective NHV formation; b) Variation in NHV concentration and total NO3' conversion as a function of [+] InP amount. [KNO3] = 0.5 mM, t = 2 h; c) Bar diagram summarizing the effect of KNO3 (reactant) concentration on NH4+with 0.5 mol % [+] InP QDs under 2 h visible-light irradiation; d) Progress of [+] InP QD photocatalysed reduction of NO3' to NH4+monitored through IC. NH4+concentration increases gradually and attains saturation after 2 hour of visible-light irradiation with 0.5 mol % [+] InP QDs and 0.5 mM KNO3. The dotted lines are only to guide the eyes, in accordance with an embodiment of the present disclosure.

[0031] FIG. 3 illustrates bar diagram showing the amount of NH4+formed under different control experiments, revealing the necessity of InP QDs and light for selective NO3' to NH3conversion, in accordance with an embodiment of the present disclosure.

[0032] FIG. 4 illustrates a) 'H-NMR spectrum and b) bar diagram comparing14NH4+and15NH4+formed froml4N0f and isotope labelledl5NOf. respectively, in accordance with an embodiment of the present disclosure.

[0033] FIG. 5 illustrates photocatalytic NH3synthesis under different light excitations (450 nm, 532 nm, and 580 nm). The inset shows the feasibility of NH3production under natural sunlight, in accordance with an embodiment of the present disclosure.

[0034] FIG. 6 illustrates photocatalytic NH3 formation from nitrate ions in the presence of different a) anions and b) cations, commonly found in real water samples.

[0035] FIG. 7 illustrates a) NO3' conversion to NH3with InP QDs functionalized with different positively charged TMA and MSC surface ligands under visible light irradiation for 30 min and b) photocatalytic NO3' conversion to NH3as a function of time performed with InP QDs functionalized with [+] MSC and [+] TMA ligands. MSC refers to metal-solvent complex, [In(DMF)e]3+.DETAILED DESCRIPTION OF THE INVENTION

[0036] The following is a detailed description of embodiments of the disclosure. The embodiments are in such detail as to clearly communicate the disclosure. However, the amount of detail offered is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present invention.

[0037] All publications herein are incorporated by reference to the same extent as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.

[0038] Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.

[0039] In some embodiments, numbers have been used for quantifying weight percentages, ratios, and so forth, to describe and claim certain embodiments of the invention and are to be understood as being modified in some instances by the term “about.” Accordingly, in some embodiments, the numerical parameters set forth in the written description and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by a particular embodiment. In some embodiments, the numerical parameters should be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of some embodiments of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as practicable. The numerical values presented in some embodiments of the invention may contain certain errors necessarily resulting from the standard deviation found in their respective testing measurements.

[0040] Various terms as used herein are shown below. To the extent a term used in a claim is not defined below, it should be given the broadest definition persons in the pertinent art have given that term as reflected in printed publications and issued patents at the time of filing.

[0041] As used in the description herein and throughout the claims that follow, the meaning of “a,” “an,” and “the” includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise.

[0042] Unless the context requires otherwise, throughout the specification which follow, the word “comprise” and variations thereof, such as, “comprises” and “comprising” are to be construed in an open, inclusive sense that is as “including, but not limited to.”

[0043] The recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range. Unless otherwise indicated herein, each individual value is incorporated into the specification as if it were individually recited herein.

[0044] All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g. “such as”) provided with respect to certain embodiments herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the invention.

[0045] Groupings of alternative elements or embodiments of the invention disclosed herein are not to be construed as limitations. Each group member can be referred to and claimed individually or in any combination with other members of the group or other elements found herein. One or more members of a group can be included in, or deleted from, a group for reasons of convenience and / or patentability. When any such inclusion or deletion occurs, the specification is herein deemed to contain the group as modified.

[0046] The description that follows, and the embodiments described therein, is provided by way of illustration of an example, or examples, of particular embodiments of the principles and aspects of the present invention. These examples are provided for the purposes of explanation, and not of limitation, of those principles and of the disclosure.

[0047] It should also be appreciated that the present invention can be implemented in numerous ways, including as a system, a method or a device. In this specification, these implementations, or any other form that the invention may take, may be referred to as processes. In general, the order of the steps of the disclosed processes may be altered within the scope of the invention.

[0048] The headings and abstract of the invention provided herein are for convenience only and do not interpret the scope or meaning of the embodiments.

[0049] The following discussion provides many example embodiments of the inventive subject matter. Although each embodiment represents a single combination of inventiveelements, the inventive subject matter is considered to include all possible combinations of the disclosed elements. Thus if one embodiment comprises elements a, b, and c, and a second embodiment comprises elements b and d, then the inventive subject matter is also considered to include other remaining combinations of a, b, c, or d, even if not explicitly disclosed.

[0050] Aspects of the present disclosure relates to a selective visible-light-driven ammonia production from nitrate and nitrite ions with indium phosphide quantum dots (InP QDs) as the photocatalyst.

[0051] Accordingly, in an aspect the present disclosure provides Indium phosphide (InP) semiconductor nanoparticles or quantum dots (QDs) as photocatalyst for visible light- mediated NH3formation. The presence of catalytic indium atoms and the introduction of favorable catalyst-reactant interactions between positively charged InP QDs and NO3' anions results in a selective and efficient NH3 formation within 2 hours of visible light irradiation.

[0052] In certain embodiments, ammonia is produced in a yield of ~94 % in both aqueous and gaseous phases within 2 hour of visible-light irradiation at room temperature, using the synthesized InP QDs as the photocatalysts.

[0053] In various embodiments, the InP -based photocatalyst is suitable to produce NH3 even with lower NCh’ / NC ’ concentrations such as micromolar concentration and hence, can be used effectively for water remediation purposes.

[0054] In certain embodiments, the process is devoid of hole scavengers.

[0055] In another aspect, the present disclosure provides a method for ligand place exchanging of InP QDs, comprising steps of: i) dissolving N, N, N-trimethyl(l 1 -mercaptoundecyl) ammonium chloride (TMA) in water to obtain a TMA solution and impart a positive charge on the InP QDs surface; ii) adding as-synthesized oleylamine-capped InP QDs to the TMA solution under vigorous shaking, followed by the addition of toluene to obtain a mixture; iii) stirring the mixture continuously for 4 hours to achieve complete phase transfer of InP QDs from organic to an aqueous layer; and iv) separating the aqueous layer and precipitating the InP QDs using isopropanol.

[0056] In various embodiments, the precipitated InP QDs are redispered in water and stored in the dark at a room temperature.

[0057] In certain embodiments, the similar procedure is carried out to prepare 11- mercaptoundecanoic acid (MUA) functionalized InP QDs to attain a negative surface charge and water solubility.

[0058] In certain embodiments, as-synthesize InP QDs were functionalized with inorganic metal-solvent complex, comprising of following steps: i) 50 mg InCL was dissolved in 2 mb of DMF to form [In(DMF)6]3+complex ([+] MSC); ii) Adding 800 pL of parent oleylamine-capped InP QDs in hexane, followed by vigorous shaking for 12 h to achieve complete phase transfer from hexane to DMF, iii) Finally, purification of [+] MSC QDs with hexane, followed by precipitation using toluene, and redispersing in water.

[0059] In yet another aspect, the present disclosure provides a method for producing ammonium (NH3) using InP QDs as a photocatalyst, comprising steps of:I) dispersing InP QDs in a solution containing NO3' or NO2' ions; andII) irradiating the solution with visible light to obtain ~ 94% of ammonium within 2 hours at room temperature.

[0060] In various embodiments, the visible light is provided by low-power light-emitting diodes (LEDs) or natural sunlight.

[0061] In certain embodiments, the ammonia formation was detected in aqueous or gaseous phases.

[0062] In certain embodiments, the NO?' or NO?' ions are present at micromolar concentrations, suitable for water remediation purposes.

[0063] In various embodiments, the InP QDs are surface-engineered to facilitate favourable catalyst-reactant interactions through electrostatic attraction with NO3' anions.

[0064] In an embodiment, the photocatalytic reduction of nitrate (NO3') to ammonia (NH3) using indium phosphide (InP) quantum dots (QDs) involves several key reaction parameters. The concentration of the InP QD photocatalyst was systematically varied from 0.1 mol % (0.5 pM) to 1 mol % (5 pM) to optimize photocatalytic efficiency. The substrate concentration of nitrate was adjusted between 250 pM and 1 mM to assess its impact on ammonia yield. Reaction times were explored from 15 minutes to 4 hours, with the highest ammonia production observed at the 2-hour mark. The pH of the reaction was maintained within a range of 2 to 11 to determine its influence on the process. Additionally, various light sources, including wavelengths of 450 nm, 532 nm, 580 nm, and natural sunlight, were employed to evaluate their effectiveness in driving the photocatalytic reaction. This comprehensive approach to optimizing these parameters is essential for maximizing ammonia yield in a sustainable manner.

[0065] In an embodiment, the electrostatic attraction arising from the ligands on the surface of InP QD photocatalysts was the key in the selective and efficient production of NH3.

[0066] Introduction of a positively charged metal-solvent complex, [In(DMF)6]3+referred as MSC, as a ligand on InP QD surface boosted ammonia synthesis with ~16 fold enhancement in reaction rate compared to positively charged TMA functionalized InP QDs. Within 30 min of visible light irradiation, -100 % NO3' was converted to NH3. This clearly proves the effect of favourable catalyst-reactant interactions between positively charged surface ligands and negatively charged NOf ions for efficient NOf to NH3conversion.

[0067] In an embodiment, InP QDs exhibited excellent photocatalytic activity even at lower concentrations of NO3'. Typical nitrate levels in natural environment or wastewater are less than 700 micromolar.

[0068] In an exemplary embodiment, surface-engineered InP QDs dispersed in water were mixed with NO3' solution in a quartz / glass reactor. The reaction mixture was irradiated with low-power LEDs or exposed to sunlight to produce NH3. No specially designed reactors or high-intensity light sources are required to perform photocatalytic reduction of NO3'.

[0069] While the foregoing description discloses various embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope of the disclosure. The invention is not limited to the described embodiments,versions or examples, which are included to enable a person having ordinary skill in the art to make and use the invention when combined with information and knowledge available to the person having ordinary skill in the art.EXAMPLES

[0070] The present invention is further explained in the form of following examples. However, it is to be understood that the foregoing examples are merely illustrative and are not to be taken as limitations upon the scope of the invention. Various changes and modifications to the disclosed embodiments will be apparent to those skilled in the art. Such changes and modifications may be made without departing from the scope of the invention.Example 1: Synthesis of InP Quantum Dots

[0071] The synthesis of core-only InP QDs was carried out using a modified literature procedure (Tessier M et al., Chem. Mater. 2015, 27, 4893-4898) In a typical synthesis, 0.45 mmol indium chloride (100 mg), 2.2 mmol zinc chloride (300 mg), and oleylamine were taken in a 100 mL three-neck flask. This was heated at 120 °C under an inert atmosphere with continuous stirring, till a clear solution was obtained. The solution was degassed for 1 hour at the same temperature to remove traces of moisture. The temperature was then raised to 180 °C, followed by a quick injection of 1.93 mmol tris-(dimethylamino)phosphine (350 pL) into the reaction mixture. The temperature was then raised to 200 °C, and the reaction mixture was kept for 1 hour for the complete growth of the InP QDs. The growth was arrested by quenching the reaction mixture with a water bath and adding 5 mL of hexane. The QDs were purified by centrifugation at 7000 rpm to remove excess oleylamine and unreacted reagents. The QDs were finally redispersed in toluene and stored in the dark at 4 °C for further use.Example 2: Ligand Place Exchange of InP Quantum Dots

[0072] As synthesized oleylamine capped QDs were place-exchanged with positively and negatively charged ligands using the protocol developed in our group. To impart the positive charge on the QD surface, we chose N, N, / V-trimcthyl( 11 -mercaptoundecyl) ammonium chloride (TMA) as the ligand to impart water solubility. In a typical procedure, ~50 mg TMA was dissolved in water, and 800 pL of as-synthesized QDs was added under vigorous shaking, followed by toluene. Continuous stirring for ~4 hour ensured a complete phase transfer of QDs from the organic to the aqueous layer. The aqueous layer was separated and precipitated with isopropanol, followed by redispersing QDs in water. Similarly, 11-mercaptoundecanoic acid (MUA) functionalized QDs were prepared to attain negative surface charge and water solubility. All the QDs were stored in the dark at room temperature. The QDs were well characterized with various spectroscopic and microscopic techniques before using them in the photocatalytic reactions.

[0073] A modified literature protocol was used to functionalize metal-solvent complex on the InP QD surface (Hai, Y. et.al., J. Am. Chem. Soc. 2024, 146, 12808-12818). Firstly, 50 mg InCE was dissolved in 2 mb of DMF to form [In(DMF)e]3+complex. To this solution, 800 pL of parent OAm-capped InP QDs in hexane were added. The reaction mixture was stirred vigorously for 12 h to facilitate the transfer of InP QDs from hexane to DMF. The place-exchanged QDs were purified twice with hexane, followed by precipitation using toluene, and redispersed in water for further photocatalytic reactions.Example 3: Characterization of InP QDs

[0074] The as-synthesized core-only InP QDs possess appropriate band positions to facilitate photocatalytic reduction of NO+ZNCE- to NH4+with visible-light irradiation (valence and conduction bands were positioned at -5.84 eV and -3.82 eV vs vacuum, respectively). The band gap was calculated to be 2.02 eV from Tauc plot, which clearly shows excellent light absorption in the visible region (Figure la). The native OAm ligands on InP QDs were place -exchanged with cationic / V, / V, / V-trimcthyl( l I - mercaptoundecyl)ammonium chloride (TMA, [+]) ligand to achieve colloidal stability in aqueous medium as well as to install favourable catalyst-reactant interactions (Figure lb). The complete transfer of InP QDs from toluene to the aqueous phase indicates the successful ligand exchange with TMA and MUA ligands, respectively. The thiol groups of TMA and MUA bind to the QD surface, while the terminal positively charged quaternary ammonium group in TMA and negatively charged carboxylic acid group in MUA provide dispersion in the aqueous medium. The zeta potential values of +24.0 ± 8.5 mV and -37.0 ± 6.9 mV confirm the successful functionalization of TMA and MUA ligands on the surface of InP QDs, respectively (Figure 1c). Transmission electron microscopy (TEM) studies confirm the formation of uniform-sized [+] InP QDs having an average diameter of 3.5 ± 0.4 nm (Figure Id). After the successful synthesis, characterization, and ligand place exchange, the core-only InP QDs were employed for photocatalytic ammonia synthesis with visible light.Photocatalytic reduction of nitrate ions (NOT) to ammoniaExample 4: General procedure for the photocatalytic N( to NH3conversion

[0075] All the photocatalytic experiments were performed in a 4 mL long-neck quartz cuvette. In a typical procedure, [+] InP QDs (2.5 pM, 0.5 mol %) and KNO3 (0.5 mM) in water were mixed, purged with Ar, and irradiated with 2 x 10 W 450 nm LEDs for 2 hour. Subsequently, the reaction mixture was analyzed for NH4+formation using ion chromatography by injecting 500 pL of the reaction mixture. The area under the peak at retention time ~8.5 min was used to calculate the concentration of NH4+formed from the calibration curve. A similar analysis was carried out for NO3' and NO2' conversion as well to calculate the total conversion yields.Example 5: Optimization of reaction parameters

[0076] The permanent positive charge of the quaternary ammonium group modifies the local environment of [+] InP QD by electrostatically channeling NO3' and NO2' reactants towards the QD photocatalysts (Figure 2a). A series of experiments were performed to optimize the reaction condition with respect to [+] InP QD photocatalyst, reactant, and time of irradiation. With the increase in the concentration of [+] InP QDs, there was a gradual increase in the NH4+formation as well as NO3' conversion within 2 hour of visible-light irradiation (Figure 2b). Even though the maximum NH4+formation was obtained with 5.0 pM (1.0 mol %) [+] InP QDs, 2.5 pM (0.5 mol %) of photocatalyst was used in further studies to avoid the excessive use of QDs. The NH4+production increased by ~48 %, with an increase in KNO3 from 0.25 mM to 0.5 mM (Figure 2c). A further increase in KNO3 concentration did not increase NH4+production, which may be due to the saturation of all active sites on [+] InP QD photocatalyst. Likewise, the NH4+production was saturated after 2 hour of visible-light irradiation of the reaction mixture containing 2.5 pM [+] InP QDs and 0.5 mM KN03(Figure 2d).Example 6: NH3synthesis under optimized conditions

[0077] Under the optimized condition (2.5 pM, 0.5 mol % [+] InP QDs, 0.5 mM KNO3, and 2 hour visible light irradiation), the favorable catalyst-reactant interactions accelerated the charge transfer and extraction steps, resulting in the efficient and selective formation of NH4+within 2 hour of visible-light irradiation (57.5 ± 9.3% NO3' to NH4+and 42.9 ± 8.9% NO3' to NH4+in aqueous phase, with internal quantum efficiency @450 nm = 0.15% and 0.12%, respectively). The sole presence of NH3was detected in the gaseous phase as well,further adding to the total NH3yield of the photocatalytic reaction. The yield of NH4+was estimated to be 57.5 ± 9.3 % in the aqueous phase with respect to NO3' conversion. A series of control experiments revealed that a negligible amount of NH4+was formed in the absence of either light or [+] InP QDs, confirming their necessity for the photocatalytic reduction of NO3' to NH4+(Figure 3). Further, a characteristic triplet (J = 52 Hz) corresponding to NH4+was observed in the 'H-NMR study (Figure 4). More importantly, the isotope studies withl5NOf provided the ultimate proof for NO3' as the sole N-source in the [+] InP QD photocatalysed synthesis of NH4+(a characteristic doublet, J = 73 Hz, corresponding to15NH4+was observed in the 'H-NMR spectrum. Figure 4a. Quantitatively similar amounts of14NH4+and15NH4+were formed froml4NOf andl 5NOf reactants, respectively, which further validates the claim of NO3' as the sole N-source for NH4+synthesis (Figure 4b). Interestingly, the gas phase mass analysis revealed the sole presence of NH3in the headspace as well. The presence of NH3in the headspace was further validated by performing isotope studies with labeled15NC>3' where mass signals corresponding to15NH3were observed.

[0078] Ammonia synthesis was feasible with other positively charged ligands on InP QDs. Introduction of a positively charged metal-solvent complex, [In(DMF)e]3+referred as MSC, as a ligand on InP QD surface boosted ammonia synthesis with ~16 fold enhancement in reaction rate compared to positively charged TMA functionalized InP QDs. Within 30 min of visible light irradiation, -100 % NO3' was converted to NH3. This clearly proves the effect of favourable catalyst-reactant interactions between positively charged surface ligands and negatively charged NO3' ions for efficient NO3' to NH3conversion.Example 7: NH3 synthesis under real world conditions

[0079] The solar spectrum extends from ultraviolet to the infrared region. As can be seen from Figure la, InP QDs has broad absorption in UV-visible region. The action spectrum constructed using the IQY at different excitation wavelengths (580 nm, 532 nm, and 450 nm) followed the ground state absorption profile of [+] InP QDs (Figure 5), which again confirmed the active participation of InP QDs in the photocatalytic NH4+synthesis. A decent NH4+production across broad visible-light spectrum motivated us to perform the photocatalytic reaction under direct sunlight. Notably, we observed 91 ± 30 pM of NH4+(NO3' conversion = 26.5 ± 7.7 %) within 2 hour of exposure of the reaction mixture to natural sunlight (Inset to Figure 5). All these experiments conclusively prove the excellent potential of [+] InP QDs for efficient and sustainable NH3synthesis with visible light. NOf reductionwas conducted in the presence of other commonly found ions in water bodies such as sulfates, carbonates, phosphates, sodium, and magnesium. A decent ammonia production was observed even in the presence of these ions (Figure 6 and 7), suggesting the suitability of [+] InP QDs for ammonia synthesis from NCLTNCL' present in real water samples. ADVANTAGES OF THE PRESENT DISCLOSURE

[0080] The invention provides photocatalytic reduction of NO3' with InP QDs yielding solely NH3 as the product in both the aqueous and gas phases.

[0081] InP QDs exhibited excellent photocatalytic activity under visible light irradiation, both indoor lights as well as natural sunlight. A majority of the previously reported photocatalytic systems for NO3' to NH3conversion use UV light irradiation, which is not sustainable as well as results in undesired side reactions.

[0082] InP QDs exhibited excellent photocatalytic activity even at lower concentrations ofNO3’.

[0083] InP QDs are less toxic than conventional cadmium (Cd) and lead (Pb) based QD, which strengthens the sustainability aspect for real-world applications.

Claims

We Claim:1] A process for producing ammonia comprising dispersing surface-modified indium phosphide (InP) semiconductor quantum dots (QDs) in an aqueous solution comprising nitrate (NCh ) and / or nitrite (NO2 ) ions; and irradiating the solution with visible light, characterized in that the surface of the InP QDs is functionalized with a positively charged ligand / s.2] The process as claimed in claim 1 wherein the positively charged ligand is N,N,N- trimethyl(l l-mercaptoundecyl)ammonium chloride (TMA), and / or a metal-solvent complex of the formula [In(DMF)6]3+.3] The process as claimed in claim 2 wherein a positive charge is imparted to indium phosphide quantum dots by ligand exchange method comprising:-dissolving N,N,N-trimethyl(l l-mercaptoundecyl)ammonium chloride (TMA) in water;-adding oleylamine-capped InP QDs to the solution with vigorous shaking;-adding toluene to form a biphasic mixture;-stirring for 4 hours to allow phase transfer of QDs to the aqueous layer; and -separating and precipitating the InP QDs using isopropanol.4] The process as claimed in claim 1 wherein the visible light is provided by low-power LEDs emitting at wavelengths between 450-580 nm or natural sunlight.5] The process as claimed in claim 1 wherein the surface-engineered InP QDs have an average size of 3.5 ± 0.4 nm.6] The process as claimed in claim 1 wherein the quantum dots are in a concentration of 0. 1 mol% to 1 mol% relative to the nitrate / nitrite concentration.7] The process as claimed in claim Iwherein the reaction is conducted in a simple quartz or glass reactor under atmospheric pressure.8] The process as claimed in claim 1 wherein the process is carried out at room temperature resulting in at least 90% conversion of NOs and / or NO2 to ammonia within 2 hours.9] The process as claimed in claim 1 wherein ammonia is produced in both aqueous and gaseous phases.10] The process as claimed in claim 1 wherein a contaminated aqueous source comprising micromolar concentrations of nitrate (NOs ) and / or nitrite (NO2 ) ions is contacted with positively charged InP QDs, irradiating the mixture with visible light and reducing the nitrate and / or nitrite to ammonia with a yield greater than 90% within 2 hours.

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