WO2020178862A1

WO2020178862A1 - Photocatalytic nano-composite and applications thereof

Info

Publication number: WO2020178862A1
Application number: PCT/IN2020/050210
Authority: WO
Inventors: Kiran Prakash SHEJALE; Devika LAISHRAM; Rakesh Kumar SHARMA; Arun Kumar
Original assignee: Indian Institute Of Technology, Jodhpur
Priority date: 2019-03-06
Filing date: 2020-03-06
Publication date: 2020-09-10

Abstract

The present disclosure discloses a photocatalytic nano-composite comprising: (a) at least one metal (M) selected from La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu; (b) at least one metal oxide (MO); (c) at least one semi-conductor substrate co-catalyst (ABO3); and (d) at least one semi-conductor substrate. Additionally, a process of preparation of the nano-composite is also disclosed. Further, a process for H2 production from water is also disclosed.

Description

PHOTOCATALYTIC NANO-COMPOSITE AND APPLICATIONS THEREOF

FIELD OF INVENTION

[001] The present disclosure relates to the field of photo-catalytically active nano-composites and to the process of preparing the nano-composite. In particular, it discloses the photocatalytic H₂ production under natural sunlight from water using the nano-composite of the present disclosure.

BACKGROUND OF THE INVENTION

[002] A large amount of chemical energy is stored in terms of H₂ and can be easily harvested by water splitting. Sunlight driven water splitting in the presence of a catalyst provides a simple and easy way for H₂ evolution.

[003] The solar-driven H₂ production efficiency would be dependent on the water splitting principle and photoactive composite material properties. This emphasizes strong absorption over the full light spectrum, suitable band alignment to perform both oxidation and reduction of water, high stability, charge transport, and low potential for both reactions.

[004] Theoretically, the water splitting reaction can only be performed if the photon energy is more than 1.23 eV which is equivalent to the photon energy having a wavelength of around 1010 nm. This amount of energy is enough for oxidation and reduction of water. Also, the photon energy should be sufficiently high enough to provide activation energy to cross the barrier between semi- conductors and water molecules. Thermodynamically, the potential of band structure by nano- composite should be linearly matched. The charge separation, mobility, transport, electron, and hole’s lifetime also play key roles in efficient photocatalytic activity. The electron-hole generation and migration strongly depend on the co-catalyst presence and have a high impact over catalyst type, structural and electronic properties. Loading oxidation and reduction type co-catalyst facilitates faster redox reaction and reduces the recombination of species with the reduction in the activation energy for H₂ evolution.

[005] The conventional nanosized photocatalyst utilizes only a small fraction of the photoexcited charge carriers due to the smaller particle size and the space charge layer. The photo-corrosion stability of the catalyst restricts the use of photocatalysts for water splitting. Mostly, wide band gap metal oxide semi-conductors are often stable against photo-corrosion, but there are issues with small bandgap semi-conductors. For example, TiO₂ has 3.2 eV bandgap with excellent stability over different pH values, potential, and light exposure but is able to harvest light only from the UV region.

[006] High improvement in charge separation can be targeted by semi-conductor-metal and semi- conductor-semi-conductor nano-composite. The anchored metal and metal oxide acts as a reservoir for photogenerated electrons and promotes an interfacial charge-transfer process in the water splitting. However, limitations in the catalyst, such as bandgap with its valence and conduction band position and stability of catalyst are the main issues that act as a deterrent for these catalysts to be used as a photocatalyst in an industrial scale.

[007] An efficient photocatalyst is expected to have a large surface area, superior sensitivity to the visible region of the solar spectrum, appropriate band energetics, and agile carrier transport to inhibit recombination processes .

[008] Metal oxide photocatalysts are being studied extensively for their enhanced catalytic abilities supplemented by highly stable nature under the electrochemical reaction conditions. Most often bandgap with its valence and conduction band position and stability of catalyst are the main issues. There are limitations for material to meet both demands to act as a photocatalyst. The major issue is the photo-corrosion stability that limits its usage as an efficient photocatalyst for H₂ production by water splitting. Thus, photocatalyst nano-composite having active sites for both oxidation and reduction of water with excellent stability is a challenge and has not been overcome. SUMMARY OF THE INVENTION

[009] In an aspect of the present disclosure, there is provided a photocatalytic nano-composite comprising: (a) at least one metal (M) selected from La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu; (b) at least one metal oxide (MO); (c) at least one semi-conductor substrate co- catalyst (ABO₃); and (d) at least one semi-conductor substrate.

[0010] In another aspect of the present disclosure, there is provided a process of preparation of the photocatalytic nano-composite comprising: (a) at least one metal (M) selected from La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu; (b) at least one metal oxide (MO); (c) at least one semi-conductor substrate co-catalyst (ABO₃); and (d) at least one semi-conductor substrate (CdS), the process comprising: (i) contacting at least one metal salt of a metal (M) selected from La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu, at least one semi-conductor substrate co- catalyst (ABO₃), and at least one solvent to obtain a solution; (ii) drying the solution to obtain a reaction mixture; (iii) annealing the reaction mixture to obtain a first mixture; (iv) contacting the first mixture and the at least one semi-conductor substrate to obtain the pre-composite; and (v) annealing the pre-composite to obtain the nano-composite.

[0011] In yet another aspect of the present disclosure, there is provided a process for H₂ production from water, the process comprising: (i) contacting water and the photocatalytic nano-composite comprising: (a) at least one metal (M) selected from La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu; (b) at least one metal oxide (MO); (c) at least one semi-conductor substrate co- catalyst (ABO₃); and (d) at least one semi-conductor substrate, to obtain a dispersion; and (ii) degassing the dispersion followed by stirring the solution under solar light for a period in the range of 1 to 7 hours at least 1 sun solar irradiation to obtain H₂ gas.

[0012] In a further aspect of the present disclosure, there is provided a water-based dispersion comprising the photocatalytic nano-composite comprising: (a) at least one metal (M) selected from La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu; (b) at least one metal oxide (MO); (c) at least one semi-conductor substrate co-catalyst (ABO₃); and (d) at least one semi-conductor substrate (CdS), wherein the nano-composite photo-catalytically splits water.

[0013] These and other features, aspects, and advantages of the present subject matter will be better understood with reference to the following description and appended claims. This summary is provided to introduce a selection of concepts in a simplified form. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURES

[0014] In order that the disclosure may be readily understood and put into practical effect, reference will now be made to exemplary embodiments as illustrated with reference to the accompanying figures. The figures together with a detailed description below, are incorporated in and form part of the specification, and serve to further illustrate the embodiments and explain various principles and advantages, in accordance with the present disclosure wherein:

[0015] Figure 1 illustrates XRD data of photocatalytic nano-composites (M/MO/ABO₃/CdS), in accordance with an embodiment of the present disclosure.

[0016] Figure 2 illustrates the H₂ production rate of the photocatalytic nano-composite (M/MO/ABO₃/CdS), in accordance with an embodiment of the present disclosure. [0017] Figure 3 illustrates the H₂ production rate of the photocatalytic nano-composite (Sm/Sm₂O₃/KNbO₃/CdS), in accordance with an embodiment of the present disclosure.

DETAILED DESCRIPTION OF THE INVENTION

[0018] Those skilled in the art will be aware that the present disclosure is subject to variations and modifications other than those specifically described. It is to be understood that the present disclosure includes all such variations and modifications. The disclosure also includes all such steps, features, compositions, and compounds referred to or indicated in this specification, individually or collectively, and any and all combinations of any or more of such steps or features. Definitions

[0019] For convenience, before further description of the present disclosure, certain terms employed in the specification, and examples are collected here. These definitions should be read in the light of the remainder of the disclosure and understood as by a person of skill in the art. The terms used herein have the meanings recognized and known to those of skill in the art, however, for convenience and completeness, particular terms and their meanings are set forth below.

[0020] The articles“a”,“an” and“the” are used to refer to one or to more than one (i.e., to at least one) of the grammatical object of the article.

[0021] The terms“comprise” and“comprising” are used in the inclusive, open sense, meaning that additional elements may be included. It is not intended to be construed as“consists of only”.

[0022] Throughout this specification, unless the context requires otherwise the word“comprise”, and variations such as“comprises” and“comprising”, will be understood to imply the inclusion of a stated element or step or group of elements or steps but not the exclusion of any other element or step or group of elements or steps.

[0023] The term“including” is used to mean“including but not limited to”.“Including” and “including but not limited to” are used interchangeably.

[0024] Ratios, concentrations, amounts, and other numerical data may be presented herein in a range format. It is to be understood that such range format is used merely for convenience and brevity and should be interpreted flexibly to include not only the numerical values explicitly recited as the limits of the range, but also to include all the individual numerical values or sub- ranges encompassed within that range as if each numerical value and sub-range is explicitly recited. For example, a concentration range of about 2 - 100 ppm should be interpreted to include not only the explicitly recited limits of about 2 ppm to about 100 ppm, but also to include sub- ranges, such as 10 ppm, 500 ppm, 75 ppm, and so forth, as well as individual amounts, including fractional amounts, within the specified ranges, such as 10.5 ppm, and 25.7 ppm, for example.

[0025] Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the disclosure, the preferred methods, and materials are now described. All publications mentioned herein are incorporated herein by reference.

[0026] The formula M/M O/ ABO₃/CdS as represented herein is a general formula of the nano- composite of the present dislcosure, wherein M represents a metal selected from La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb or Lu; MO represents a metal oxide, i.e., metal oxides of lanthanides, ABO₃ represents a mixed metal oxide, and CdS represents at least one semi-conductor substrate. Instead of CdS other kinds of semi-conductor substrates, such as CdSe, ZnS, or ZnSe may also be used.

[0027] The term“at least one metal salt of a metal (M)” in the present disclosure refers to nitrate salts of lanthanides (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb or Lu) in the hexahydrate form.

[0028] The present disclosure is not to be limited in scope by the specific implementations described herein, which are intended for the purposes of exemplification only. Functionally- equivalent products, compositions, and methods are clearly within the scope of the disclosure, as described herein.

[0029] As discussed in the background section most semi-conductors and nano-composite are not able to fulfill the requirement for efficient water splitting, wherein bandgap alignment must be done according to the oxidation and reduction potential of water. The crucial part of the photocatalytic nano-composite is a catalyst exhibiting properties, such as photo-functionally active, wide band semi-conductor, charge separation, and diffusion length for the transport. Subsequently, the charge carriers produced are utilized in surface chemical redox reactions for the desired purposes, such as the conversion of water molecules into H₂ and oxygen. The nano- composite of the present disclosure provides a facile heterostructure, i.e., photocatalytic nano- composite comprising: (a) at least one metal; (b) at least one metal oxide (MO); (c) at least one semi-conductor substrate co-catalyst ( ABO₃); and (d) at least one semi-conductor substrate (CdS), wherein both reduction and oxidation co-catalysts are present, assisting semi-conductor for highly efficient water splitting system under direct sunlight. No sacrificial agents, i.e., electron donors have been added in the photocatalytic nano-composite of the present disclosure.

[0030] The objective of present invention is to provide a simple and a bulk synthetic process for highly active and high surface area M/MO/ A BO 3/CdS nano-composite series and employ them into sunlight driven water splitting for highly pure H₂ production in high yield.

[0031] In an embodiment of the present disclosure, there is provided a photocatalytic nano- composite comprising: (a) at least one metal (M) selected from La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu; (b) at least one metal oxide (MO); (c) at least one semi-conductor substrate co-catalyst (ABO₃); and (d) at least one semi-conductor substrate.

[0032] In an embodiment of the present disclosure, there is provided a photocatalytic nano- composite comprising: (a) at least one metal (M) selected from La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu; (b) at least one metal oxide (MO) selected from La₂O₃, Ce0₂, Pr₂O₃, Nd₂O₃, Sm₂O₃, EU₂O₃, Gd₂O₃, Tb₂O₃, Dy₂O₃, Ho₂O₃, Er₂O₃, Tm₂O₃, Yb₂O₃, or LU₂O₃; (c) at least one semi-conductor substrate co-catalyst (ABO₃); and (d) at least one semi-conductor substrate.

[0033] In an embodiment of the present disclosure, there is provided a photocatalytic nano- composite comprising: (a) at least one metal (M) selected from La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu; (b) at least one metal oxide (MO); (c) at least one semi-conductor substrate co-catalyst (ABO₃) selected from alkali metal niobate or alkali metal tantalate; and (d) at least one semi-conductor substrate.

[0034] In an embodiment of the present disclosure, there is provided a photocatalytic nano- composite comprising: (a) at least one metal (M) selected from La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu; (b) at least one metal oxide (MO); (c) at least one semi-conductor substrate co-catalyst (ABO₃) selected from alkali metal niobate or alkali metal tantalate; and (d) at least one semi-conductor substrate, wherein the alkali metal niobate or alkali metal tantalate has a perovskite structure.

[0035] In an embodiment of the present disclosure, there is provided a photocatalytic nano- composite comprising: (a) at least one metal (M) selected from La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu; (b) at least one metal oxide (MO); (c) at least one semi-conductor substrate co-catalyst (ABO₃); and (d) at least one semi-conductor substrate is selected from CdS, CdSe, ZnS, or ZnSe. [0036] In an embodiment of the present disclosure, there is provided a photocatalytic nano- composite comprising: (a) at least one metal selected from La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu; (b) at least one metal oxide (MO) selected from La₂O₃, Ce0₂, Pr₂O₃, Nd₂O₃, Sm₂O₃, EU₂O₃, Gd₂O₃, Tb₂O₃, Dy₂O₃, H0₂O₃, Er₂O₃, Tm₂O₃ , Yb₂O₃, or LU₂O₃; (c) at least one semi-conductor substrate co-catalyst (ABO₃) selected from alkali metal niobate or alkali metal tantalate; and (d) at least one semi-conductor substrate selected from CdS, CdSe, ZnS, or ZnSe.

[0037] In an embodiment of the present disclosure, there is provided a photocatalytic nano- composite comprising: (a) at least one metal selected from La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu; (b) at least one metal oxide (MO) selected from La₂O₃, Ce02, Pr₂O₃, Nd₂O₃, Sm₂O₃, EU₂O₃, Gd₂O₃, Tb₂O₃, Dy₂O₃, H0₂O₃, Er₂O₃, Tm₂O₃ , Yb₂O₃, or LU₂O₃; (c) at least one semi-conductor substrate co-catalyst (ABO₃) selected from alkali metal niobate or alkali metal tantalate with a perovskite structure; and (d) at least one semi-conductor substrate is selected from CdS, CdSe, ZnS, or ZnSe.

[0038] In an embodiment of the present disclosure, there is provided a photocatalytic nano- composite comprising: (a) at least one metal selected from La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu; (b) at least one metal oxide (MO) selected from La₂O₃, Ce0₂, Pr₂O₃, Nd₂O₃, Sm₂O₃, EU₂O₃, Gd₂O₃, Tb₂O₃, Dy₂O₃, H0₂O₃, Er₂O₃, Tm₂O₃ , Yb₂O₃, or LU₂O₃; (c) at least one semi-conductor substrate co-catalyst (ABO₃) selected from alkali metal tantalate having a perovskite structure; and (d) at least one semi-conductor substrate selected from CdS, CdSe, ZnS, or ZnSe.

[0039] In an embodiment of the present disclosure, there is provided a photocatalytic nano- composite comprising: (a) at least one metal selected from La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu; (b) at least one metal oxide (MO) selected from La₂O₃, Ce0₂, Pr₂O₃, Nd₂O₃, Sm₂O₃, EU₂O₃, Gd₂O₃, Tb₂O₃, Dy₂O₃, H0₂O₃, Er₂O₃, Tm₂O₃ , Yb₂O₃, or LU₂O₃; (c) at least one semi-conductor substrate co-catalyst (ABO₃) selected from alkali metal niobite having a perovskite structure; and (d) at least one semi-conductor substrate selected from CdS, CdSe, ZnS, or ZnSe.

[0040] In an embodiment of the present disclosure, there is provided a photocatalytic nano- composite comprising: (a) at least one metal selected from La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu; (b) at least one metal oxide (MO) selected from La₂O₃, Ce0₂, Pr₂O₃, Nd₂O₃, Sm₂O₃, EU₂O₃, Gd₂O₃, Tb₂O₃, Dy₂O₃, H0₂O₃, Er₂O₃, Tm₂O₃ , Yb₂O₃, or LU₂O₃; (c) at least one semi-conductor substrate co-catalyst (ABO₃) selected from alkali metal tantalate having a perovskite structure; and (d) at least one semi-conductor substrate selected from CdS, CdSe, ZnS, or ZnSe.

[0041] In an embodiment of the present disclosure, there is provided a photocatalytic nano- composite as described herein, wherein the at least one metal has a weight percentage in the range of 0.21% to 0.65 % with respect to the photocatalytic nano-composite; the at least one metal oxide (MO) has a weight percentage in the range of 21.71% to 39.56 % with respect to the photocatalytic nano-composite; the at least one semi-conductor substrate co-catalyst has a weight percentage in the range of 28.91% to 60.54 % with respect to the photocatalytic nano-composite; the at least one semi-conductor substrate has a weight percentage in the range of 2.23 % to 15.51% with respect to the photocatalytic nano-composite.

[0042] In an embodiment of the present disclosure, there is provided a photocatalytic nano- composite as described herein, wherein the nano-composite has a bandgap energy in the range of 2.1 - 2.7 eV at a pH of 6.8 - 7.2.

[0043] In an embodiment of the present disclosure, there is provided a photocatalytic nano- composite as described herein, wherein the nano-composite has a catalytic efficiency in the range of 76-99 %.

[0044] In an embodiment of the present disclosure, there is provided a process of preparation of the photocatalytic nano-composite comprising: (a) at least one metal (M) selected from La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu; (b) at least one metal oxide (MO); (c) at least one semi-conductor substrate co-catalyst (ABO₃); and (d) at least one semi-conductor substrate, the process comprising: (i) contacting at least one metal salt of a metal (M), at least one semi- conductor substrate co-catalyst (ABO₃) selected from alkali metal niobate, or alkali metal tantalate, and at least one solvent to obtain a solution; (ii) drying the solution to obtain a reaction mixture; (iii) annealing the reaction mixture to obtain a first mixture; (iv) contacting the first mixture and the at least one semi-conductor substrate to obtain the pre-composite; and (v) annealing the pre- composite to obtain the nano-composite.

[0045] In an embodiment of the present disclosure, there is provided a process of preparation of the photocatalytic nano-composite as described herein, wherein the at least metal salt to the at least one semi-conductor substrate co-catalyst (ABO₃) to the at least one solvent weight ratio is in the range of 1:0.1 : 10 - 9: 1.0:250. [0046] In an embodiment of the present disclosure, there is provided a process of preparation of the photocatalytic nano-composite as described herein, wherein the at least metal salt to the at least one semi-conductor substrate co-catalyst to the at least one solvent weight ratio is in the range ofl :0.2:50 - 9: 1.0:200.

[0047] In an embodiment of the present disclosure, there is provided a process of preparation of the photocatalytic nano-composite as described herein, wherein the at least metal salt to the at least one semi-conductor substrate co-catalyst to the at least one solvent weight ratio is in the range of. 2:0.2:50 - 8:0.9:200

[0048] In an embodiment of the present disclosure there is provided a process of preparation of the photocatalytic nano-composite as described herein, wherein the at least metal salt to the at least one semi-conductor substrate co-catalyst to the at least one solvent weight ratio is in the range of 1:0.1: 10 - 7:0.7:200.

[0049] In an embodiment of the present disclosure, there is provided a process of preparation of the photocatalytic nano-composite as described herein, wherein the at least metal salt to the at least one semi-conductor substrate co-catalyst to the at least one solvent weight ratio is in the range of 4:0.4:90 - 6:0.6: 110

[0050] In an embodiment of the present disclosure, there is provided a process of preparation of the photocatalytic nano-composite as described herein, wherein the at least metal salt to the at least one semi-conductor substrate co-catalyst to the at least one solvent weight ratio is 5:0.5: 100.

[0051] In an embodiment of the present disclosure, there is provided a process of preparation of the photocatalytic nano-composite as described herein, wherein (a) contacting at least one metal salt of a metal (M), at least one semi-conductor substrate co-catalyst selected from alkali metal niobate, or alkali metal tantalate, and at least one solvent is carried out at a temperature in the range of 25 - 40 °C for a period in the range of at a stirring speed in the range of 10-1000 RPM to obtain a solution; (b) drying the solution at a temperature in the range of 100 - 120 °C for a period in the range of 10 hours - 14 hours to obtain a reaction mixture; (c) annealing the reaction mixture in H₂ atmosphere at a temperature in the range of 450 °C - 550 °C for a period in the range of 1.5 to 3 hours, followed by oxidation at a temperature in the range of 700 °C - 900 °C for a period in the range of 1.5 hours to 3 hours to obtain a first mixture; (d) contacting the first mixture and the at least one semi-conductor substrate at a temperature in the range of 25 °C - 100 °C for a period in the range of 5 hours - 10 hours to obtain the pre-composite; (e) annealing the pre-composite at a temperature in the range of 650 °C - 800 °C for a period in the range of 4-6 hours to obtain the nano-composite.

[0052] In an embodiment of the present disclosure, there is provided a process of preparation of the photocatalytic nano-composite as described herein, wherein the at least one solvent is selected from the group consisting of deionized water, isopropanol, glycerol, phenol, and combinations thereof.

[0053] In an embodiment of the present disclosure, there is provided a process for H₂ production from water, the process comprising: (a) contacting water and the photocatalytic nano -composite comprising: (i) at least one metal (M) is selected from La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu; (ii) at least one metal oxide (MO); (iii) at least one semi-conductor substrate co-catalyst (ABO₃); and (iv) at least one semi-conductor substrate to obtain a dispersion; and (b) degassing the dispersion followed by stirring the solution under solar light to obtain H₂ gas.

[0054] In an embodiment of the present disclosure, there is provided a process for H₂ production from water, the process comprising: (a) contacting water and the photocatalytic nano-composite comprising: (i) at least one metal (M) selected from La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu; (ii) at least one metal oxide (MO); (iii) at least one semi-conductor substrate co- catalyst (ABO₃); and (iv) at least one semi-conductor substrate to obtain a dispersion; and (b) degassing the dispersion followed by stirring the solution under solar light for a period in the range of 1 to 7 hours at least 1 sun solar irradiation to obtain H₂ gas.

[0055] In an embodiment of the present disclosure, there is provided a process for H₂ production from water as described herein, wherein contacting water and the photocatalytic nano-composite as claimed any one of the claims 1-9 for a period ranging between 0.5 to 2 h under dark conditions at a temperature ranging between 28-34° C to obtain a dispersion.

[0056] In an embodiment of the present disclosure, there is provided a process for H₂ production from water as described herein, wherein degassing the dispersion followed by stirring the solution under solar light for a period in the range of 1 to 7 hours at 1 sun solar irradiation is carried out at a pH in the range of 7.7 - 8.2 to obtain H₂ gas.

[0057] In an embodiment of the present disclosure, there is provided a process for H₂ production from water as described herein, wherein H₂ production rate is in the range of 2262 - 11428 mmol h^_1g^_1 of the nano-composite. [0058] In an embodiment of the present disclosure there is provided a process for H₂ production from water as described herein, wherein H₂ production rate is in the range of 2362 - 11400 mmol h^_1g^_1 of the nano-composite.

[0059] In an embodiment of the present disclosure, there is provided a water-based dispersion comprising the photocatalytic nano-composite comprising: (i) at least one metal (M); (ii) at least one metal oxide (MO); (iii) at least one semi-conductor substrate co-catalyst (ABO₃); and (iv) at least one semi-conductor substrate, wherein the nano-composite photo-catalytically splits water.

EXAMPLES

[0060] The disclosure will now be illustrated with working examples, which is intended to illustrate the working of disclosure and not intended to take restrictively to imply any limitations on the scope of the present disclosure. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood to one of ordinary skill in the art to which this disclosure belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice of the disclosed methods and compositions, the exemplary methods, devices, and materials are described herein. It is to be understood that this disclosure is not limited to particular methods, and experimental conditions described, as such methods and conditions may apply.

[0061] The working examples described herein clearly depict a series of photocatalyst having four component photocatalytic nano-composite system which were prepared by the solid-state reaction method. In the present disclosure, nano-composites of potassium niobate (KNbO₃) and CdS by solid-state reactions were synthesized and investigated for their properties and photoactivity for H₂ production under visible light irradiation (~1000W/m² or 1 Sun) in the presence of water as an electron donor. Moreover, the efficiency of H₂ production is significantly enhanced by loading lanthanides on KNbO₃. Accordingly, the present disclosure discloses a photocatalytic nano-composite comprising: (a) at least one metal; (b) at least one metal oxide (MO); (c) at least one semi-conductor substrate co-catalyst (ABO₃); and (d) at least one semi- conductor substrate.

[0062] In aqueous media, specific metal nitrate and potassium nitrate salts were used as a precursor. These catalysts are comprising of M/MO/ABO₃/CdS structure. The resulting solution was then stirred and dried subsequently followed by sulfurization and calcination process. Materials and Methods

[0063] Potassium niobate, iron (III) nitrate nonahydrate, cobalt (II) nitrate hexahydrate, nickel (II) nitrate hexahydrate, copper (II) nitrate trihydrate, zinc nitrate hexahydrate, palladium (II) nitrate, cadmium acetate dihydrate, lanthanide (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb or Lu) nitrates in their hexahydrate form, sodium sulfide, absolute ethanol, and propanol were procured from Alfa Aeser. All the chemicals were analytical reagent (AR) grade used without further purification.

[0064] The nano-composite of present disclosure, as synthesized in Example 1, were characterized by scanning electron microscopy (EV018 special edition Carl Zeiss) with an accelerating voltage of 20 kV, X-ray diffractometer (Bruker D8 advance) with Cu Ka as X-ray source (l = 1.54056A0) at 40 Kv and 40 mA, UV-visible diffuse reflectance spectroscopy (Cary 4000 scan Varian UV- vis system), Fourier-transform infrared spectra (Vertex 70v spectrometer - Bruker) in 400-4000 cm^-1 range with KBr as a reference sample and TGA-6000 thermal analyzer (Perkin Elmer) under a N2 atmosphere at heating rate of 100 °C min^-1. All measurements were done at room temperature.

EXAMPLE 1

General process of preparation of photocatalytic nano-composite of the present disclosure

(M/MO/ABOs/CdS structure)

[0065] A series of photocatalyst having four component nano-composite system were prepared by solid-state reaction method. All used reagents were high purity AR grade and used without further purification. Iron (III) nitrate nonahydrate, cobalt (II) nitrate hexahydrate, nickel (II) nitrate hexahydrate, copper (II) nitrate trihydrate, zinc nitrate hexahydrate, palladium (II) nitrate, cadmium acetate dihydrate, lanthanide (La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb or Lu) nitrates in their hexahydrate form were used as a metal source in the preparation of the nano- composite. The process of preparation of the photocatalytic nano-composite is discussed as below:

[0066] At least one metal salt, at least one semi-conductor substrate co-catalyst selected from alkali metal niobate, or alkali metal tantalate, and at least one solvent was contacted at a temperature in the range of 25 °C - 40 °C or a period in the range of at a stirring speed in the range of 10-1000 rpm to obtain a solution. The solution was then dried at a temperature in the range of 100 °C- 120 °C for a period in the range of 10 hours- 14 hours to obtain a reaction mixture. Further, the reaction mixture was annealed in H₂ atmosphere at a temperature in the range of 450 °C to 550 °C for a period in the range of 1.5 to 3 hours, followed by oxidation at a temperature in the range of 700 °C- 900 °C for a period in the range of 1.5 hours to 3 hours to obtain a first mixture. The first mixture and the at least one semi-conductor substrate were contacted at a temperature in the range of 25 °C - 100 °C for a period in the range of 5 hours to 10 hours to obtain the pre-composite. Further to this, the pre-composite was annealed at a temperature in the range of 650 °C- 800 °C for a period in the range of 4 hours to 6 hours to obtain the nano-composite.

[0067] In aqueous media, specific metal salt, KNbO₃ and deionized water were mixed together in the weight ratio of 5:0.5: 100 under vigorously stirring condition. The same procedure was repeated with every single reported metal salt and with KTaO₃. The M/MO/ABO₃ solution was further dried by evaporating solvent by thermal treatment. Then the prepared reaction mixture was annealed twice, firstly reduction in H₂ atmosphere at 500°C for 2 hours then in an oxygen atmosphere 800°C for 2 hours. CdS was loaded in M/MO/ABO₃ by adding 0.5 M cadmium acetate dihydrate in 50 ml, ethanol with continuously stirring for 24 hours followed by the sulfurization with 0.5 g sodium sulfide. The prepared nano-composite (M/MO/ABO₃/CdS) was annealed at 700°C for 5 hours.

EXAMPLE 2

Characterization of the nano-composite

[0068] The nano-composites of present disclosure were characterized by X-ray diffractometer, the analysis shows the diffraction peaks were intensive and narrow. It suggests that the nano- composites are highly crystalline with large particle size. As these nano-composites have been firstly synthesized and are virtually absent in the present literature, so there is no standard X-ray diffraction pattern so far in the ICDD-PDF (International Centre for Diffraction Data, Powder Diffraction File) database. The peaks at 21.940, 22.290, 31.220, 31.500, 44.840, 45.580, 50.620, 51.180, 55.890, 56.330, 65.440, 65.900, 70.370, 74.340 and 75.450 are attributed to the (3 1 0), (0 0 1), (4 2 0), (3 1 1), (6 2 0), (0 0 2), (5 5 0), (3 1 2), (5 5 1), (4 2 2), (5 4 2), (9 1 0), (8 5 0), (8 5 1) and (10 0 ) plane reflections respectively of the prepared M/MO/ABO₃/CdS (ABX3 = KNbO₃/KTaO₃) nano-composites. (Figure 1)

EXAMPLE 3

Photocatalytic H2 production under natural sunlight from water [0069] Substantial amount of H₂ gas was produced readily using natural sunlight as the irradiation source. The 4-component nano-composite was exposed to solar light during day (~1000W/m² or 1 Sun) time at 10:30 am to 4:30 pm at Jodhpur (Coordinates: 26.28°N 73.02°E) on August 13, 2006.

[0070] In 250 ml air-tight reaction vessel, 0.05 gm of nano-composite (Sm/Sm₂O₃/KNbO₃/CdS) was mixed in aqueous solution with subsequent degassing by nitrogen gas for 3 min and pH was maintained in between 7.7- 8.2. Then it was exposed to direct sunlight and solar simulator irradiation (1 Sun) for 4 hours. A YL 6500 gas chromatography system incorporating a 2mm~4mm (I.D) stainless steel packed column was used for thermal analysis of evolved H₂ during photolysis. Specific metal salt of metal (M), KNbO₃ and deionized water were mixed together in the weight ratio of 5:0.5: 100 for the preparation of the nano-composite of the present disclosure. As is clear from Table 1 and Figure 2 various photocatalytic nano-composite were tried and tested. The highest H₂ production was realized in the case of Sm/Sm₂O₃/KNbO₃/CdS, i.e., 11428 mmol h 'g ¹ for almost 7 hours. The same is reflected in Figure 3. The other nano-composites reflect a H₂ production rate in the range of 2262 - 11428 mmol h^-1g^-1 of the nano-composite.

Table 1 : Performance of the photocatalytic nano-composites of the present disclosure

EXAMPLE 5

Catalytic efficiency of the photocatalvtic nano-composite of the present disclosure

[0071] The photocatalytic activity of the nano-composites of the present disclosure, the rate of H₂ evaluation under sunlight and solar simulator irradiation were simultaneously carried out in Jodhpur, India at 10:30 am -16:30 pm every day. A YL 6500 gas chromatography system incorporating a 2mm~4mm (I.D) stainless steel packed column was used for thermal analysis of evolved thduring photolysis. Data were acquired and integrated using YL clarity software. Analytical conditions unless otherwise noted, are given in Table 2.

Table 2. Analytical conditions for H₂ analysis

EXAMPLE 6

Comparative example

[0072] The H₂ production rate under sunlight (mmol h^-1g^-1) for the nano-composite Co/CoO/KNbCL/CdS, was found to be 1964.28 mmol h^-1g^-1 which is very low in comparison to the nano-composites of the present disclosure. This can be attributed to the less surface area and active sites.

[0073] The naked KNbCL or KTaCL, Ln-doped KNbCL, Ln/LnO/KNbCL, samples showed no activity for H₂ production under visible light. Only the CdS nanoparticle suspensions produced H₂, but at very low rates.

ADVANTAGES OF THE PRESENT DISCLOSURE

[0074] The present disclosure provides a photocatalytic nano-composite comprising: (a) at least one metal; (b) at least one metal oxide (MO); (c) at least one semi-conductor substrate co-catalyst (ABO₃); and (d) at least one semi-conductor substrate. The applications of the nano-composite of the present disclosure includes in employing them into sunlight driven water splitting for highly pure H₂ production in high yield. The combination of the components, i.e., lanthanides and lanthanide oxides on the semi-conductor substrate, such as KNbO₃ co-catalyst embedded on CdS photocatalyst enhances the H₂ production by splitting water under direct sunlight. It is possible to produce highly pure H₂by involving full solar spectrum irradiation.

Claims

I/We Claim:

1) A photocatalytic nano-composite comprising:

(a) at least one metal (M) selected from La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu;

(b) at least one metal oxide (MO);

(c) at least one semi-conductor substrate co-catalyst (ABO₃); and

(d) at least one semi-conductor substrate.

2) The photocatalytic nano-composite as claimed in claim 1, wherein the at least one metal oxide (MO) is selected from La₂O₃, Ce02, Pr₂O₃, Nd₂O₃, Sm₂O₃, EU₂O₃, Gd₂O₃, Tb₂O₃, Dy₂O₃, H0₂O₃, Er₂O₃, Tm₂O₃, Yb₂O₃, or LU₂O₃.

3) The photocatalytic nano-composite as claimed in claim 1, wherein the at least one semi- conductor substrate co-catalyst (ABO₃) is selected from alkali metal niobate or alkali metal tantalate.

4) The photocatalytic nano-composite as claimed in claim 3, wherein the alkali metal niobate or the alkali metal tantalate has a perovskite structure.

5) The photocatalytic nano-composite as claimed in claim 1, wherein the at least one semi- conductor substrate is selected from CdS, CdSe, ZnS, or ZnSe.

6) The photocatalytic nano-composite as claimed in claim 1 , wherein the at least one metal has a weight percentage in the range of 0.21% to 0.65 % with respect to the photocatalytic nano-composite; the at least one metal oxide (MO) has a weight percentage in the range of 21.71% to 39.56 % with respect to the photocatalytic nano-composite; the at least one semi- conductor substrate co-catalyst (ABO₃) has a weight percentage in the range of 28.91% to 60.54 % with respect to the photocatalytic nano-composite; the at least one semi-conductor substrate has a weight percentage in the range of 2.23 % to 15.51% with respect to the photocatalytic nano-composite.

7) The photocatalytic nano-composite as claimed in claim 1 , wherein the nano-composite has bandgap energy in the range of 2.1 - 2.7 eV at a pH of 6.8 - 7.2.

8) The photocatalytic nano-composite as claimed in claim 1 , wherein the nano-composite has a catalytic efficiency in the range of 76-99 %.

9) A process of preparation of the photocatalytic nano-composite as claimed in claim 1 , the process comprising: (a) contacting at least one metal salt of a metal (M) selected from La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, or Lu, at least one semi-conductor substrate co-catalyst (ABO₃), and at least one solvent to obtain a solution;

(b) drying the solution to obtain a reaction mixture;

(c) annealing the reaction mixture to obtain a first mixture;

(d) contacting the first mixture and the at least one semi-conductor substrate to obtain the pre-composite; and

(e) annealing the pre-composite to obtain the nano-composite.

10) The process as claimed in claim 9, wherein the at least one metal salt to the at least one semi-conductor substrate co-catalyst to the at least one solvent weight ratio is in the range of 1 :0.1 : 10 to 9: 1.0:250.

11) The process as claimed in claim 9, wherein (a) contacting at least one metal salt, at least one semi-conductor substrate co-catalyst (ABO₃) selected from alkali metal niobate or alkali metal tantalate, and at least one solvent is carried out at a temperature in the range of 25 °C - 40 °C for a period in the range of at a stirring speed in the range of 10 - 1000 RPM to obtain a solution; (b) drying the solution at a temperature in the range of 100 - 120 °C for a period in the range of 10 hours -14 hours to obtain a reaction mixture; (c) annealing the reaction mixture in H₂ atmosphere at a temperature in the range of 450 °C to 550 °C for a period in the range of 1.5 to 3 hours, followed by oxidation at a temperature in the range of 700 °C - 900 °C for a period in the range of 1.5 hours to 3 hours to obtain a first mixture; (d) contacting the first mixture and the at least one semi-conductor substrate at a temperature in the range of 25 - 100 °C for a period in the range of 5 hours to 10 hours to obtain the pre-composite; (e) annealing the pre-composite at a temperature in the range of 650 °C to 800 °C for a period in the range of 4-6 hours to obtain the nano-composite.

12) The process as claimed in claim 9, wherein the at least one solvent is selected from the group consisting of deionized water, isopropanol, glycerol, phenol, and combinations thereof.

13) A process for H₂ production from water, the process comprising:

(a) contacting water and the photocatalytic nano-composite as claimed in any one of the claims 1-8 to obtain a dispersion; and (b) degassing the dispersion followed by stirring the solution under solar light for a period in the range of 1 to 7 hours at least 1 sun solar irradiation to obtain H₂ gas.

14) The process as claimed in claim 13, wherein contacting water and the photocatalytic nano- composite as claimed in any one of the claims 1-9 for a period ranging between 0.5 to 2 h under dark conditions at a temperature ranging between 28-34° C to obtain a dispersion.

15) The process as claimed in claim 13, wherein degassing the dispersion followed by stirring the solution under solar light for a period in the range of 1 to 7 hours at 1 sun solar irradiation is carried out at a pH in the range of 7.7 - 8.2 to obtain H₂ gas.

16) The process as claimed in claim 13, wherein the H₂ production rate is in the range of 2262 - 11428 mmol h^-1g^-1 of the nano-composite.

17) A water-based dispersion comprising the photocatalytic nano-composite as claimed in any one of the claims 1-8, wherein the nano-composite photo-catalytically splits water.