Heterogeneous photocatalyst for partial oxidation of (c1-c2) alkanes into value added products in visible light

Abstract

The present invention relates to a heterogeneous photocatalyst system for partial oxidation of C1-C2 alkanes into value added products, comprising visible-light active semiconductor material (M1VO4) assembled and integrated onto a mesoporous metal oxide supported onto support, wherein said M1 has different or interchangeable mixed oxidation states. Further, the present invention relates to a process of preparation of said heterogeneous photocatalyst system by doctor blade method for preparation of mesoporous metal oxide supported onto support, followed by SILAR method for assembling and integrating visible-light active semiconductor material (M1VO4). Furthermore, the present invention relates to a process for conversion of lower alkane (e.g. C1-C2, and a mixture thereof) into value added products by reacting saturated aqueous alkane solution in presence of said heterogeneous photocatalyst system and simulated solar light condition or direct sunlight in a photoreactor.

Description

[0001]HETEROGENEOUS PHOTOCATALYST FOR PARTIAL OXIDATION OF (C1-C2) ALKANES INTO VALUE ADDED PRODUCTS IN VISIBLE LIGHT FIELD OF THE INVENTION The present invention relates to a heterogeneous photocatalyst system for partial oxidation of C1-C2 alkanes into value added products, comprising visible-light active semiconductor material (M1VO4) assembled and integrated into a mesoporous metal oxide supported onto support, wherein said M1 has different and / or interchangeable mixed oxidation states. Particularly, the present invention also relates to a process for the preparation of said heterogeneous photocatalyst system by doctor blade method for preparation of mesoporous metal oxide supported onto support, followed by SILAR method (Successive Ionic Layer Adsorption and Reaction) for assembling and integrating visible-light active semiconductor material (M1VO4) into the pores of former. Moreover, the present invention also relates to a process for conversion of lower alkane (e.g. C1-C2, and a mixture thereof) into value added products by reacting saturated aqueous alkane solution in presence of said heterogeneous catalyst system in visible light condition or direct sunlight in a photoreactor for time period of up to 3 hrs to obtain said value added products. BACKGROUND AND PRIOR ART OF THE INVENTION The C1 (or C2) chemistry remains a perplexing reaction mechanism in the scientific community. This is attributed to the robust chemical inertness of the tetrahedral methane molecule. Partial oxidation of methane (POM) to industrially acclaimed products like methanol, formaldehyde, formic acid, etc. and their derived oxygenates has been explored for over a decade for wide chemical applications. Currently, ethanol has emerged as a cornerstone in industrial and technological aspects. The global ethanol industrial market demand is estimated to reach 135.5 billion liters by the year 2026. The major source of ethanol production remains the sugar industry which is tainting / littering the air with carbon dioxide / carbon superfluity. The need to develop a propitious methodology to utilize the ubiquitous methane (or ethane) source to directly produce higher alcohols like ethanol or propanol or value added gaseous products under ambient or near-ambient conditions holds a pivot position in the field of catalysis. The said methodology seeks higher energy and infrastructural input in the currently available industrial processes that employ noble metal-based catalytic systems majorly. With regard to this, one of published patents US5504262A of Vanderspurt et.al disclosed a technology to directly convert methane to a mixture of methanol, ethanol, propanol, acetaldehyde, acetic acid, and CO2, preferentially ethanol, by reacting methane with water and using acidic aqueous solution of an electron acceptor selected from the group consisting of Fe2(SO4)3and Fe(ClO4)3 having a pH of less than 3, in the presence of noble metal or metal- containing alloy catalyst of Group VIII specifically noble metal catalyst selected from the group consisting of platinum and palladium having a particle diameter of at least about 100 Å. The operational pressure and temperature regime that facilitates chemisorbed methyl species to desorb from the catalyst surface and form the coupled C2 product was claimed to lie between 30-200 atmospheres and 60-100 degrees, respectively. The yield reported is 0.406 mole of ethanol per gram Pt-catalyst, which is very less. Pt black used therein, which is economically non-viable, considering the employment of noble metals as well as keeping in view the operational conditions which are very harsh to make it viable for industrial production. The electro-oxidation of methane towards ethanol remains a topic of industrial and fundamental significance but, the results as stated in the aforementioned reports do not still meet the desired value. The numbers still stay far from being at par with the expected outcome from electrochemical technology that consumes electrical energy, demanding the use of precious metal-based electrodes and continuous supply of electrolytes for enhanced and sustainable productivity. Photocatalytic conversion of methane (or propane) is an efficient and promising alternative that catalyzes methane activation under ambient operational conditions. Though photocatalytic methane (or propane) conversion is an effective alternative, however designing a photocatalyst to result in remarkable activity remains a scarcely explored option compared to the available conventional methods. Amongst the very few countable reports on photocatalytic methane oxidation, Xiaoyu Du, et.al., article entitled “Efficient Photocatalytic Conversion of Methane into Ethanol over P Doped g C3N4under Ambient Conditions”, Energy Fuels, Vol 36, 2022, 3929-3937, reported ethanol formation under solar irradiation using the optimised photocatalyst, i.e., P- doped g-C3N4 (CNPx, where x= 50 mg of NaH2PO2.H2O) exhibited an ethanol production rate of 51 micromoles / g.h at 25°C, 1 atm under illumination in the presence of water and O2. Xiyi Li, et.al., reported in their article “PdCu nanoalloy decorated photocatalysts for efficient and selective oxidative coupling of methane in flow reactors”, Nature Communications, Vol 14, 2023, 6343, high conversion of methane, i.e., conversion rate of 2480 μmol g−1h−1to C2 products (ethane and ethylene) with PdCu nano-alloy decorated photo-catalyst in a flow reactor using a 365 nm (UV) LED under room temperature and atmospheric pressure. Despite good yield obtained in this case, the catalyst suffers deactivation due to the formation of coke after certain loading% and duration of reaction time; adding to the issue of accelerated cost of the catalyst. Further it works only in UV, and hence the concept of harvesting solar energy is not present. In another article “Direct Conversion of Methane to Ethylene and Acetylene over an Iron-Based Metal−Organic Framework”, J. Am. Chem. Soc., Vol 145, 2023, 20792-20800, Yujie Ma, et. al., reported direct conversion of methane to C2 and C2+ products with an Iron based metal organic framework [MFM-300(Fe)] compound. The high time yields are reported as 334 μmol gcat−1h−1(C2) and 2056 μmol gcat−1h−1(C2+) in a flow reactor driven by non-thermal plasma under ambient conditions. Though the yields are high, they demand higher cost of separation on industrial scale. The observed ethanol (or ethylene) yield is at par with the reported yields from electrochemical methods and thus, assures a scope of improvement and enhancement in the yield and selectivity of C2 and / or higher alcohols and oxygenates of industrial value through photo-irradiation. Nonetheless, a big quantum jump of several orders of increase in ethanol (ethylene) yield as well as selectivity is desired, preferably in the mmol / g.h and possibly higher yield. Another article by Yingjie Li et al entitled “TiO2-modified CeVO4catalyst for the selective catalytic reduction of NOxwith NH3” in Catal. Sci. Technol.2022, 12, 4884, reported Ti / Ce or Ti / V ratio varied between 2.5 and 20. While this catalyst was prepared by homogeneous precipitation method from precursors, it was not evaluated as photocatalyst, rather for nitric oxide reduction with ammonia. Another article by Linbing Yao et al entitled “One-dimensional hierarchial CeVO4 / TiO2 heterostructure with enhanced photocatalytic performance” in J. Nanopart. Res. 2019, 21, #140, reported TiO2nanofibers were made by electrospinning method followed by CeVO4 / TiO2preparation by hydrothermal method. This catalyst has been evaluated for Rhodamine B and nitrobenzene degradation by photocatalysis with light source of wavelength between 200 and 1200 nm. It is to be noted that when UV light is present it will be dominantly absorbed. Further organic compound degradation is simple, as it goes on breaking all the bonds and ends up in CO2 and water. Similarly, yet another article by Cong Sen Wang et al entitled “Microwave synthesis and photocatalytic activity of CeVO4 Nanocrystalline” Adv. Mater. Res.1081, 146-150 (2014), reported photocatalytic degradation of methyl orange dye in visible light. Current report is focused on the most difficult C-H activation of simplest alkanes, namely, methane and ethane by visible light driven photocatalysis. Further the material integration aspect is unique in the present findings, which is the critical reason for the high activity observed with the most difficult reaction. All these three reports disclose catalysts which are very different from the present application which pertain to the synthesis method and the type of binary semiconductor construction. Inventors of present invention provide incorporation of semiconductor material, by assembling the ionic precursors and integrating into the nanopores of wide band gap semiconductor (TiO2) that modifies the intrinsic band structure of the integrated photocatalyst or in other words, re-construction of the band structure occurs. This synthesized photocatalyst thus can be modulated to be compatible with redox reactions. Also, the quantum confinement effect comes in play that controls the growth of the desired particle in the nano-regime. The other reported systems are generally a category of composites or mixed metal oxides that have different properties. Hence, without the use of noble metals or any co-catalyst, the catalyst system of the present invention shows an impressive yield and is setting a new benchmark in this area.. Thus, it is necessary to develop a more efficient and better catalyst system to directly convert the lower alkanes (methane or ethane) into value added products at ambient or near to ambient conditions with better yield, selectivity and conversion. OBJECTIVE OF THE INVENTION The main objective of the present invention is to provide a heterogeneous photocatalyst system for partial oxidation of C1-C2 alkanes into value added products, comprising visible-light active semiconductor material (M1VO4) assembled and integrated onto a mesoporous metal oxide supported onto support. Another objective of the present invention relates to a process of preparation of said heterogeneous catalyst system by doctor blade method for preparation of mesoporous metal oxide supported onto support, followed by SILAR method for assembling and integrating visible-light active semiconductor material (M1VO4). Yet another objective of the present invention relates to a process for conversion of lower alkane (e.g. C1-C2, and a mixture thereof) into value added products by reacting saturated aqueous alkane solution in presence of said heterogeneous catalyst system and under visible and UV+Visible light condition or direct sunlight. Yet another objective of the present invention relates to production of ethanol and / or ethylene as primary value-added products in higher amounts by reacting saturated aqueous lower alkane solution in presence of said heterogeneous photocatalyst system and under visible and UV+Visible light condition or direct sunlight. Yet another objective of the present invention relates to an efficient photocatalyst by employing a binary semiconductor material for the direct conversion of methane to C- C coupled alcohol, especially ethanol, under visible and UV+Visible light or direct sunlight conditions. Yet another objective of the present invention relates to an efficient photocatalytic system for conversion of methane by a photocatalytic device, wherein the creation of heterojunction at the interface of the semiconductor materials selectively enhances the ethanol production. Yet another objective of the present invention relates to methodically manipulating the band gap of the photocatalyst to enhance visible light absorption and simultaneous lowering of charge recombination rate. Still another objective of the present invention relates to coat the photocatalyst as a thin film on support (e.g. FTO plate, glass slide, etc.) to make a photocatalytic device with improved light absorption capacity with large and equally exposed surface area of the entire device. Still another objective of the present invention relates to make photocatalyst thin films of the photo-active materials by employing SILAR method under application of required temperature for adequate ion diffusion into the thin film. Yet another objective of the present invention is to tune the selectivity of the primary product (ethanol or ethylene) with and without using a suitable co-catalyst with M1VO4photocatalyst. SUMMARY OF THE INVENTION Accordingly, the present invention relates to a heterogeneous photocatalyst system for partial oxidation of C1-C2 alkanes into value added products, comprising: a) a visible-light active semiconductor material (M1VO4), b) a mesoporous metal oxide, and c) a support; wherein, said visible-light active semiconductor material (M1VO4) is assembled and integrated into the nanopores of a mesoporous metal oxide supported onto support, and wherein said M1 metal has mixed oxidation states with value in the range of 2 to 4. In an embodiment, present invention provides a heterogeneous catalyst system for partial oxidation of C1-C2 alkanes into value added products, comprising: a) a visible-light active semiconductor material (M1VO4); b) a mesoporous metal oxide; c) a support; and d) optionally, a co-catalyst; wherein the M1 is selected from transition series metal and Lanthanides series metal or combinations thereof; wherein a particle size of M1-VO4nanoparticles is in the range of 1.5-8 nm inside the metal oxide mesopores and wherein a particle size of the mesoporous metal oxide nanoparticles is in range of 15 to 25 nm; and wherein the heterogeneous catalyst has a thickness in the range of 5-15 microns. In an embodiment of the present invention, M1 metal in visible-light active semiconductor material (M1VO4) is selected from the group consisting of cerium, praseodymium, terbium, palladium; the mesoporous metal oxide is selected from mesoporous TiO2 and mesoporous ZnO; the support is selected from the group consisting of a fluorine-doped tin oxide (FTO) glass plate, glass vessels / glass containers, indium tin oxide (ITO) glass plate, glass plate / slide, silicon wafer, glass plate with thin layer of conducting material coated on it, and co-catalyst is selected from Pt, Ni-Cu alloy, Pt-coated Ni-Cu alloy. In another embodiment of the present invention, said praseodymium metal has 3+ and / or 4+ stable oxidation states, terbium has 3+ and / or 4+ stable oxidation states, cerium has 3+ and / or 4+ stable oxidation states, and palladium has 2+ and / or 4+ stable oxidation states. In another embodiment, the present invention provides a process of preparation of said heterogeneous catalyst system, wherein the process comprising the steps of: a) preparing a mesoporous metal oxide by Doctor blade method and fabricating onto a support or cleaned support to obtain a thin film, followed by calcining the thin film at a temperature in the range of 350 to 450 °C for time period in the range of 45 to 90 minutes; and b) incorporating or fabricating visible-light active semiconductor material (M1VO4) into said thin film of step a) by SILAR method wherein the SILAR method comprising: (i) preparing two solutions A and B wherein solution A is made of M1-metal ion precursor solution, and solution B is made of vanadate precursor salt; wherein the concentration of the precursor solutions (A & B) is maintained at a ratio of 1:1 ratio; (ii) dipping said thin film of step a) into said solutions of A and B consecutively at temperature in the range of 65 to 80 ºC for residence time of dipping in the range of 25 to 40 seconds, followed by washing the dipped film with water or DI (deionised) water; and (iii) drying the washed film of step ii) at temperature in the range of 50 to 70 °C for time period of 45 to 75 minutes and calcining at temperature in the range of 400 to 480 °C for time period of 1 to 4.5 hrs at a ramping rate of 5 degrees per minute in a muffle furnace to obtain visible-light active semiconductor material (M1VO4) integrated / fabricated thin film as the heterogeneous catalyst system or photocatalytic thin film. In yet another embodiment of the present invention, the vanadate precursor salt is selected from ammonium metavanadate (NH4VO3), sodium orthovanadate, ammonium pyro-vanadate, and ammonium decavanadate. In yet another embodiment of the present invention, the thin film of step a) is fabricated with visible light-absorbing material M1-VO4 deposited in the supported mesoporous metal oxide thin film using the respective ionic components diffused into the pores. In yet another embodiment of the present invention, the mesoporous metal oxide is prepared by a Doctor blade method, wherein the process comprising the steps of: (a) adding metal oxide in solvent or mixture of solvents selected from methanol, ethanol and glacial acetic acid or mixture thereof, followed by stirring for 5 to 15 minutes and 5 to 15 minutes of sonication to obtain dispersion of metal oxide; (b) adding ethyl cellulose as binder into dispersion as obtained in step a) followed by stirring and sonication for 5 to 15 minutes to obtain a solution mixture; (c) providing viscosity to the solution mixture as obtained in step b) by adding terpinol under stirring for 20 to 40 minutes followed by 20-40 minutes of sonication to obtain wet paste; and d) evaporating the paste as obtained in step c) at temperature in the range of 40 to 60 C to remove the solvents in order to obtain the desired viscose thick paste of mesoporous metal oxide. In yet another embodiment of the present invention, the support is fluorine-doped tin oxide (FTO) glass plate and is cleaned by washing steps comprising of: a) washing support (in the form of plate, slide, etc.) first with isopropyl alcohol (IPA) followed by cleaning with soap solution and thrice with deionised (DI) water using sonication; b) subsequent washing of support of step a) with ethanol, and drying at 60 °C; c) dipping dried support of step b) in TiCl3 solution kept inside an oven for time period of 45 to 75 minutes followed by washing with IPA and DI water; Or wherein the support is glass slides / plates and is cleaned by washing steps comprising of: a) washing of support using acetone under 5 to 15 minutes of sonication followed by cleaning with soap solution and DI water at least three times; and b) again, washing with acetone followed by drying at temperature of 50 to 70 °C to obtain cleaned support. In yet another embodiment, present invention provides a process for conversion of C1- C3 lower alkane into value added products, wherein the process comprises of reacting saturated aqueous alkane solution in the presence of said heterogeneous catalyst system under simulated solar light condition or direct sunlight in a photoreactor under atmospheric pressure and temperature reaction conditions for time period of 1 to 3 hrs to obtain said value added products; wherein the saturated aqueous alkane solution is prepared by purging alkane in gas form in the water or DI water until saturation for time period of 10 to 40 minutes at a temperature in the range of 0-10°C. In yet another embodiment of the present invention, lower alkane is selected from methane or ethane; and wherein said value added products are selected from methanol, ethanol, propanol, iso-propanol, n-butanol, isobutanol, tert-butanol, n- or tert-pentanol, ethylene, H2, methyl hydrogen peroxide, H2O2, formic acid, formaldehyde, CO, and acetic acid. In yet another embodiment of the present invention, external addition of intermediate product selected from methanol and isopropanol enhances the production rate of ethanol and tert-butanol by 17-18 times, respectively by expediting the consumption of starting lower alkane. In yet another embodiment of the present invention, in the one sun condition, the photo thin film is exposed to illumination for at least 3 h in static condition and wherein in the direct sunlight condition, the temperature is in the range of 35 to 40 °C for a time period in the range of 1-6 h, with solar radiation flux of 50-80 mW / cm2. In yet another embodiment, the present invention also relates to a process of preparation of said heterogeneous catalyst system comprising: a) preparing mesoporous metal oxide supported onto support by a doctor blade method, and b) assembling and integrating visible-light active semiconductor material (M1VO4) from the ionic precursors onto said mesoporous metal oxide supported onto support by SILAR (Successive Ionic Layer Adsorption and Reaction) method. In yet another embodiment, the present invention relates to a process for conversion of lower alkane into value added products, comprising: reacting saturated aqueous alkane solution in presence of said heterogeneous photocatalyst system under visible light condition or direct sunlight in a photoreactor under specific reaction conditions for time period of 1 to 6 h to obtain said value added products. In yet another embodiment, the present invention relate to a methodical strategy for the selective oxidation of methane to ethanol in an aqueous medium by a propitious binary photocatalytic system that harnesses the surplus solar energy to propel the oxidation reaction efficiently under ambient conditions. In yet another embodiment, the present invention provides heterogeneous catalyst system comprises a heterojunction between a transition metal-based oxide (M1-oxide) and a visible-light active semiconductor material, put together in the form of a thin film. In yet another embodiment, the present invention provides a provision of converting a UV active mesoporous metal oxide material into a visible-light active composite by combining with said visible-light active semiconductor material (M1VO4). BRIEF DESCRIPTION OF THE DRAWINGS Figure 1: shows illustration of process of preparation of the heterogeneous catalyst system by SILAR method (M1VO4 = CeVO4, mesoporous metal oxide is TiO2 and support is FTO plate) where SILAR initiates the seeding and growth of CeVO4 nanoparticles (1.5 - 8 nm) preferentially in the pores of TiO2(20±2 nm). Solvent rinse step is to remove any physically adsorbed or weakly / loosely held precursor ions. Figure 2a and 2b: shows HRTEM images of the heterogeneous catalyst system wherein 2A confirms the presence of visible-light active semiconductor material (CeVO4) assembled and integrated into a mesoporous metal oxide (pre-coated TiO2- FTO plate), and 2B confirms the formation of heterojunction sites which promotes e- -h+(or charge) separation and enhance the redox activity. Figure 3: Panel (a) shows stack plot of XRD spectra of the heterogeneous photocatalyst system (CeVO4 / TiO2) along-with bulk CeVO4 and TiO2; where diffraction pattern for CeVO4(▼) observed on TiO2that matches well to JCPDS files 79-1065 (tetragonal CeVO4) and 21-1272 (anatase TiO2). Solid inverted triangle represents integrated CeVO4 in the nanopores of TiO2. Panel b shows the enlarged patterns depicting the shift in the positions or 2θ values for comparison. Figure 4a & 4b shows optical properties of the heterogeneous photocatalyst system CeVO4 / TiO2 (CVT) based on Tauc plot obtained from UV-Vis spectra, and these Tauc plots show that the band gap of the materials are in the order: TiO2> CeVO4-TiO2(CVT)> CeVO4 (CV), where it confirm incorporation of CeVO4 nanoparticles inside the pores of TiO2. Figure 5a-e: show high-resolution TEM images of the CeVO4 / TiO2integrated photocatalyst at different resolutions varying from 50- 2 nm scale bars. It clearly depicts the incorporation of CeVO4 nanoparticles (with dark contrast and small particles) in the mesoporous TiO2 (grey contrast and bigger nanoparticles) thin film in a size range of 6-8 nm. The interface of the two semiconductor materials forms a heterojunction that promotes the efficiency of the developed photocatalyst. The catalyst was scratched off the thin films and subjected to sonication for nearly 30 minutes for sample preparation. The deposited CeVO4 nanoparticles staying intact with TiO2 suggests the integrated nature of the CeVO4 / TiO2 photocatalyst. Figure 6: shows the X-ray valence band spectra of the synthesised materials [CeVO4(CV), CeVO4 / TiO2 (CVT) and TiO2]. The calculated valence band maximum (VBmax) helps in depicting the conduction band minimum (CBmin) of the materials and thus, elucidates the pathway of flow of photo-generated electrons from the QDs into TiO2in the CeVO4 / TiO2 demonstrating type-II heterojunctions. The Ce-4f band also shifts for CeVO4 / TiO2 (compared to CeVO4), which indicates the highly reactive nature of Ce in CeVO4-TiO2. Figure 7: shows1H NMR spectrum of methane partial oxidation reaction confirming formation of ethanol as value added product, where 1 cm2dimension of CeVO4-TiO2 film and 10 mL water (saturated with methane for 1 h), exposed to the solar simulator (one sun or visible light condition) for 3 h. Figure 8: shows1H NMR data of partial oxidation reaction using bare CeVO4 on FTO plate with 1 cm2dimension and 10 mL water (saturated with methane for 1 h), exposed to the solar simulator (one sun or visible light condition) for 3 h; where it failed to form ethanol or any other value added products, which confirms that the heterojunctions between CeVO4 and TiO2 nanoparticles are important and crucial for formation of value added products. Bare CeVO4does not satisfy the band edge positions required to drive the photo-oxidation of methane. Figure 9: shows1H NMR data of partial oxidation reaction using 4 cm2dimension of CeVO4-TiO2 film and 10 mL water (saturated with methane for 1 h), exposed to the solar simulator (visible light or one sun condition) for 3 h. Compared to direct sunlight (provided in figure 10), the yield is higher in the simulated reaction because of the effect of ambient temperature. As the solubility of methane is quite low in water, at high temperatures methane escapes out of the medium. The one sun condition (simulated condition) is carried out in an air-conditioned laboratory where the temperature is maintained at ~ 25 degrees whereas under direct sunlight, during the experiment, the temperature rose to ~ 40-50 degrees. Thus, inventors attribute slightly diminished yield under direct sunlight to this temperature difference; however, there is considerable and effective yield in both conditions. Figure 10: shows1H NMR data of partial oxidation reaction using 1 cm2dimension and 10 mL water (saturated with methane for 1 h), exposed to direct sunlight for 3 h. Figure 11: shows the gas chromatogram pattern confirming the selective formation of ethylene as the major (C2) gaseous product using 1 cm2dimension of CeVO4 / TiO2 film coated with Pt co-catalyst (~ 6 mg) and 10 mL water (saturated with methane for 1 h), exposed to direct sunlight for 3 h. Figure 12: shows (a) photocurrent response and (b) IPCE (incident photon-to-current efficiency) of CeVO4 / TiO2(CVT) film over CeVO4(CV) alone or TiO2alone. Figure 13: shows GC pattern confirming no over-oxidation of CH4 to CO and / or CO2. Figure 14: illustrates photocatalytic reactor setup employed to measure the activity for 1 cm2device under one sun conditions. Same device can be used for direct sunlight measurements too. DETAILED DESCRIPTION OF THE INVENTION The term “alkane” or “lower alkane” used herein refers to the methane, ethane or mixture thereof. The term “value added products” or “essential / commercially important products” used herein refers to the chemicals or materials selected from but not limited to said value added products are selected from methanol, ethanol, propanol, iso-propanol, n-butanol, isobutanol, tert-butanol, n- or tert-pentanol, ethylene, H2, methyl hydrogen peroxide, H2O2, formic acid, formaldehyde, CO, and acetic acid, etc. The terms “heterogeneous photocatalyst system”, “photocatalyst”, “photocatalytic device” “photoanode” and “photocatalytic thin film” disclosed herein are used interchangeably with the same meaning and scope. The terms “visible-light active semiconductor material”, “integrated / composite semiconductor material” and “small band-gap semiconductor material” disclosed herein are used interchangeably with the same meaning and scope, with the meaning that two different semiconductor materials with representative band gaps and band edge positions, are combined in such a way that they form into one integrated system of photo-catalyst with rearranged band edge positions that align in a way to satisfy / facilitate the redox reaction. The terms “wide band-gap semiconductor material” and “mesoporous metal oxide” disclosed herein are used interchangeably with the same meaning and scope, with the meaning of the semiconductor material with which band modulation can be done by incorporating another semiconductor into its pores. To accommodate the 2nd semiconductor, the pores should be in the mesoporous regime that facilitates better diffusion of the 2nd semiconductor component ions and eventually form the 2nd semiconductor in the nano pores. Further, CeVO4 / TiO2(CVT) indicates the CeVO4quantum dots integrated in the nanopores of TiO2; CeVO4 (CV) indicates the bulk form of CeVO4 and the particle size is in the range of ~500 nm. The present invention relates to a heterogeneous catalyst system for partial oxidation of C1-C2 alkanes into value added products, comprising: a) visible-light active semiconductor material (M1VO4), b) mesoporous metal oxide, c) support; and d) with and without employing a co-catalyst to tune the product selectivity. wherein said visible-light active semiconductor material (M1VO4) is assembled and integrated onto a mesoporous metal oxide supported onto support, and wherein said M1 metal has mixed oxidation states with value in the range of 2 to 4. Co-catalyst can be Pt, Ni-Cu alloy, Pt-coated Ni-Cu alloy. In another embodiment, M1 metal in said visible-light active semiconductor material (M1VO4) is selected from transition series metal and Lanthanides series metal or combinations thereof. In specific embodiment, the M1 metal in visible-light active semiconductor material (M1VO4) is selected from but not limited to cerium, praseodymium, terbium, palladium, and so on. In another specific embodiment, the M1 metal in said visible-light active semiconductor material (M1VO4) has combination of stable oxidation states ranging between 2 to 4 wherein the praseodymium metal has 3+ and / or 4+ stable oxidation states, terbium has 3+ and / or 4+ stable oxidation states, cerium has 3+ and / or 4+ stable oxidation states, and palladium has 2+ and / or 4+ stable oxidation states. In another embodiment, the mesoporous metal oxide is selected from but not limited to mesoporous TiO2, mesoporous ZnO and so on. In another embodiment, the support is a fluorine-doped tin oxide (FTO) glass plate, glass vessels / glass containers, indium tin oxide (ITO) glass plate, (commercially available) glass plate / slide, silicon wafer, any glass plate with thin layer of conducting material coated on it, and so on. In another embodiment, the heterogeneous catalyst or photocatalytic thin film has a thickness in the range of 5-15 microns, and this is measured using a surface profilometer. In another embodiment, the size of the mesoporous metal oxide nanoparticles is in range of 15 to 25 nm. Specifically, the size of the mesoporous metal oxide nanoparticles is 20 nm. In another embodiment, the size of M1-VO4 nanoparticles is in the range of 1.5-8 nm inside the metal oxide mesopores. In another embodiment, the M1 metal is cerium, mesoporous metal oxide is TiO2, co- catalyst is Pt, and support is glass slide or fluorine-doped tin oxide (FTO) glass plate. In another embodiment, the heterogeneous catalyst system or photocatalytic thin film comprises one or more of: i) interface structure formation between visible-light active semiconductor material with a mesoporous metal oxide supported onto support thin film; ii) normally, the TiO2 is active under UV radiation and not in visible light, hence, by making combination / interface with said visible light active semiconductor material, making it active in visible light with efficient charge separation at the interface and charge utilization at redox sites; iii) higher stability due to visible light absorbing semiconductor oxide into mesoporous oxide, meaning oxide in oxide system; iv) M1 metal ions reside in different / multiple / interchangeable stable oxidation states as confirmed by XPS; v) formation of heterojunctions and pores in the catalyst system where the mesoporous oxide has larger pore size than the semiconductor material i.e., 1.5 nm to 8 nm, along with very small amount of much larger pores; and vi) the catalyst system can be prepared by a combination of doctor blade and SILAR methods and used with dimensions of 1x1 cm2which goes up to multiples up to 25 cm2and possibly bigger. In another embodiment, the present invention also relates to a process of preparation of said heterogeneous catalyst system comprising: a) preparing mesoporous metal oxide supported onto support by a doctor blade method; and b) assembling and integrating visible-light active semiconductor material (M1VO4) onto said mesoporous metal oxide supported onto support by SILAR (Successive Ionic Layer Adsorption and Reaction) method. In another embodiment, the present invention also relates to a process of preparation of said heterogeneous photocatalyst system, comprising steps of: a) preparing and fabricating a mesoporous metal oxide onto a support or cleaned support to obtain a thin film, followed by calcining the thin film at a temperature in the range of 350 to 450 °C for time period in the range of 45 to 90 minutes; and b) incorporating or fabricating visible-light active semiconductor material (M1VO4) into said thin film of step a) by SILAR method wherein the SILAR method comprising: (i) preparing two solutions A and B wherein solution A is made of M1-metal ion precursor solution, and solution B is made of vanadate precursor salt; (ii) dipping said thin film of step a) into said solutions of A and B consecutively at temperature in the range of 65 to 80 ºC for residence time of dipping in the range of 25 to 40 seconds, followed by washing the dipped film; and (iii) drying and calcining the washed film of step ii) to obtain visible-light active semiconductor material (M1VO4) integrated / fabricated thin film as the heterogeneous catalyst system or photocatalytic thin film. In another embodiment, the vanadate precursor salt is selected from but limited to ammonium metavanadate (NH4VO3), sodium orthovanadate, ammonium pyrovanadate, ammonium decavanadate and so on. In another embodiment, the thin film of step a) is fabricated with visible light- absorbing material M1-VO4 deposited in the supported mesoporous metal oxide thin film using the respective ionic components diffused into the pores. The gradual seeding and growth of M1-VO4nanoparticles in the fixed pore size range of supported mesoporous metal oxide is maintained and is ensured by the SILAR method. In another embodiment, the mesoporous metal oxide is prepared by: (a) adding metal oxide in solvent or mixture of solvents selected from methanol, ethanol and glacial acetic acid or mixture thereof, followed by stirring for 5 to 15 minutes and 5 to 15 minutes of sonication to obtain dispersion of metal oxide; (b) adding ethyl cellulose as binder into dispersion of step a) followed by stirring and sonication for 5 to 15 minutes; (c) providing viscosity to the solution mixture of step b) by adding terpineol under stirring for 20 to 40 minutes followed by 20-40 minutes of sonication to obtain wet paste; and d) evaporating the paste of c) at temperature in the range of 40 to 60 ºC to remove the solvents in order to obtain the desired viscose thick paste of mesoporous metal oxide. In another embodiment, the cleaned support is obtained by washing steps comprise of: a) washing support (in the form of plate, slide, etc.) first with isopropyl alcohol (IPA) followed by scrupulous cleaning with soap solution and thrice with deionised (DI) water using sonication; b) subsequent washing of support of step a) with ethanol, the plates were dried at 60 ºC; c) dipping dried support of step b) in TiCl3solution kept inside an oven for time period of 45 to 75 minutes followed by washing with IPA and DI water, which results in the formation of a pale layer of TiCl3 over the conductive side of the support plates that could promote the sticking coefficient of mesoporous metal oxide. Preferably, this process of cleaning is applicable to FTO plate as support. In another embodiment, the cleaned support is obtained by washing of support using acetone under 5 to 15 minutes of sonication followed by scrupulous cleaning with soap solution and DI water at least three times; and again, washing with acetone followed by drying at temperature of 50 to 70 ºC to obtain cleaned support. Preferably, this process of cleaning is applicable to glass slides / plates as support. In another embodiment, the dipping step b) is done once or repeatedly for 2 to 10 times. In another embodiment, the concentration of the precursor solutions (A & B) is maintained at a ratio of 1:1 ratio. In another embodiment, the dipping time in each precursor solutions (A & B) is kept at 30 seconds. In another embodiment, the temperature of the precursor solutions was also optimized at 75 ºC to facilitate the diffusion of the ions, especially the anions (VO4 ions) into the porous metal oxide. In another embodiment, the drying of step b) iii) is done at temperature in the range of 50 to 70 ºC for time period of 45 to 75 minutes. In another embodiment, the calcination of step b) iii) is done at temperature in the range of 400 to 480 ºC for time period of 1 to 4.5 hrs at a ramping rate of 5 degrees per minute in a muffle furnace. In another embodiment, the washing of step b) iii) is done using water or DI water at least 1 to 3 times. Specifically, the TiO2 thin film as mesoporous metal oxide film was coated using the standard Doctor Blade method that enables the formation of a uniform layer of 5-15 micron thickness. SILAR was employed to incorporate the visible light-absorbing material CeVO4 into the thin film. For the said process, Ce3+and the metavanadate species containing precursor solutions were used, to which, the TiO2 thin films prepared by the doctor blade method, were dipped in sequential cycles. Calcination of the TiO2 thin film after the Doctor Blade method at 400 ºC and the final calcination of the CeVO4 / TiO2 thin films at 450 ºC for the required duration ensures the high crystallinity of the photocatalyst material. SILAR facilitates the homogeneous distribution of CeVO4nanoparticles in the pores of TiO2thin film with a size range of 1.5-8 nm. The incorporation of the CeVO4 nanoparticles by SILAR establishes the formation of heterojunction between the two semiconductor materials that enhances the photo response by reducing the charge recombination rates. The interfacial active sites formed, provide surface sites for adsorption and generation of intermediate active radical species that tune the selectivity of methane oxidation. In yet another embodiment, the present invention relates to a process for conversion of lower alkane into value added products, comprising: reacting saturated aqueous alkane solution in presence of said heterogeneous catalyst system under simulated solar light condition or direct sunlight in a photoreactor under ambient reaction conditions for time period of 1 to 3 hrs to obtain said value added products. In some embodiment, the conversion of lower alkane into value added products is done using a photocatalytic setup (Figure 14) covering, where it comprises heterogeneous catalyst system as photocatalytic thin film of 1*1 cm2dimension or any other dimension as per the requirement or need, quartz reactor, holder, solar simulator and one sun condition. Here, the ambient conditions have been referred to the atmospheric temperature and pressure conditions. For one sun or visible light condition, the temperature was maintained between 24-28 degrees celsius while under direct sunlight, the temperature over the period of time during the summer of year 2022 ranged from 35-50 ºC. In another embodiment, the lower alkane is selected from methane or ethane. In another embodiment, the value added products are selected from but not limited to methanol, ethanol, ethylene, H2, methyl hydrogen peroxide, H2O2, formic acid, formaldehyde, CO, acetic acid and so on. In another embodiment, the saturated aqueous alkane solution is prepared by purging alkane in gas form in the water or DI water until saturation for time period of 10 to 40 minutes, under ice-cold condition which increases the solubility of alkane in water. In another embodiment, in the one sun condition, the photo thin film is exposed to illumination for at least 3 h in static condition. In another embodiment, in the direct sunlight condition, the temperature kept nearly 35 to 40 ºC for time period of at least 3 hrs, with solar radiation flux of 50-80 mW / cm2. In another embodiment, the source of simulated sunlight irradiation was used with filters for UV, UV+ Visible, and Visible sources as required and known in the art. In another embodiment, the direct sunlight absorption process was conducted without use of any filters. In specific embodiment, the photo-oxidation of CH4is carried out by purging deionized water with CH4through an injection until saturation and placing the photo- device in the quartz photo reactor that is then exposed to solar irradiation or a suitable solar simulator at room temperature and / or +10 degrees when kept under direct sunlight. In specific embodiment, the purging of CH4 is done under ice-cold condition increases the solubility of CH4 in water. The said CH4 saturated aqueous solution was irradiated under solar spectrum to yield the desired products like ethanol along with minor product like methanol as gaseous product catalysed by the developed photo-catalyst. In some embodiments of the present invention relate to a methodical strategy for the selective oxidation of methane to ethanol in an aqueous medium by a propitious integrated photocatalytic system that harnesses the surplus solar energy to propel the oxidation reaction efficiently under ambient conditions. In some embodiments, an external addition of intermediate product selected from methanol and ethanol enhances the rate of product formation by 17-18 times by expediting the consumption of starting lower alkane. Similarly, deliberate addition of isopropanol leads to tert-butanol as the end product. In some embodiments of the present invention provide a heterogeneous catalyst system comprising of a heterojunction between a transition metal-based oxide (M1- oxide) and a visible-light active semiconductor material, put together in the form of a thin film. In some embodiments of the present invention provide a provision of converting a UV active mesoporous metal oxide material into a visible-light active material by combing with said visible-light active semiconductor material (M1VO4). In some embodiments of the present invention provide a methodology for the activation of the most stable C1 species- methane, and direct formation of coupled product- ethanol which is of great industrial value. The said methodology involves the development of a photo device coated with wisely chosen / rationally chosen binary semiconductor material. The said integrated system comprises of a wide band gap semiconductor material and a visible light-absorbing semiconductor material which is incorporated into the mesopores of the wide band gap material. This results in the formation of active sites at the interface that promotes coupling reactions yielding ethanol. In some embodiments of the present invention provide a methodology for the activation of the most stable C1 species- methane, and direct formation of coupled product- ethylene which is of great industrial value. The said methodology involves the use of co-catalyst coated with wisely chosen / rationally chosen binary semiconductor material. The said co-catalyst is chosen as a noble metal catalyst coated on the periphery of the previously said photocatalytic thin film. This results in the formation of active sites that drive faster electron consumption that promotes coupling reactions yielding selectively ethylene gas. In some embodiments of the present invention, TiO2 is used as the wide band gap semiconductor material. It’s photochemical and photophysical properties have been widely exploited in various photocatalytic reactions. The said semiconductor has been greatly explored for the oxidation of organic substances to harmless compounds under UV irradiation. The mesoporous nature of the said semiconductor allows its modifications with other photo-active materials to fine-tune the photoactivity by enhancing photo-response; lowering toxicity in some cases. One such efficient tailoring way is the creation of hetero-active sites for complex reactions to initiate effortlessly. The inventors have thus incorporated CeVO4nano-particles into the TiO2thin film to enhance the photo-oxidation of methane. In the said invention, the authors have chosen a visible-light-absorbing material chosen from 3d and 5d transition metal oxide. The incorporation of the CeVO4nanoparticles is carried out by SILAR which allows a systematic growth of CeVO4in the nanopores of TiO2thin film with near- uniform size distribution. This, in turn, results in the formation of an interface between the two semiconductor materials that are capable of tuning the selectivity of partial oxidation of methane to ethanol by photocatalysis. In some embodiments of the present invention provide a method of photocatalytic formation of liquid products and / or fuel that are of high industrial value, using rationally developed photocatalyst, by illuminating a mixture of CH4 and water under solar spectrum wherein the said photocatalyst is in the form of a thin film device. In another embodiment, the heterogeneous catalyst and a process of formation of value added products do not form CO and / or CO2 confirming no run-away reactions. In another specific embodiment, the present invention provides said photocatalyst / photoanode as a thin film device, which is more efficient for photocatalytic activation involving gaseous molecules and in solution phase, than its powder form as suspension in solution. The photo-active material when devised as a thin film, exposes a larger surface area for adsorption of the gaseous molecules, as like methane, and also has a more exposed area available for illumination conditions. The better the absorption of photo energy, the higher the chances of generation of photo- generated charge carriers, i.e., the electron-hole pairs. In some embodiments of the present invention relate to photocatalysis which harvests energy from sunlight that activates the semiconductor material to produce photogenerated charge carriers, i.e., electron-hole pairs initiating the redox reactions. Thus, photocatalysis promises to be an advantageous method to drive an energy- intensive, uphill, complex reaction as methane oxidation under ambient conditions when coupled with a rationally designed semiconductor material. In the present invention, a methodology is developed to enhance the selective oxidation of methane to ethanol by tactfully creating a heterojunction between a visible light absorbing transition metal-based oxide (M1-oxide) and a wide band gap semiconductor material, put together in the form of a thin film. Herein, a semiconductor material with a wide band gap is used as the substrate and a transition metal-based metal-oxide is incorporated in it to create heterojunctions at the interface that promotes complex multi-step reaction like methane oxidation to the C-C coupled product under ambient conditions upon exposure to solar irradiation. Furthermore, the use of noble metal-based co-catalysts can promote the efficient photo-generated electrons utilization towards improving selectivity for ethylene over ethanol. Herein, TiO2 is used as the wide band-gap semiconductor material in the anatase phase and CeVO4 is incorporated / integrated in it, in order to enhance the visible light absorption. TiO2is well exploited in photocatalytic reactions because of its robust chemical and physical properties. The band gap of TiO2 is theoretically 3.2 eV which makes it a UV-active material. In order to enhance catalytic performance, modifications with TiO2have been done to improve the photo quantum efficiency, wherein it is combined with a visible light active material with a shorter band gap such as CeVO4. Hereby, the formation of heterojunction between both the semiconductor materials at the interface serves as the ground for the catalytic activation of methane and tunes the subsequent C-C coupling of generated methyl radical species leading to improved ethanol selectivity under ambient conditions. This can be attributed to better charge separation, i.e., lower charge recombination and consumption for efficient activity. Thus, the rational tunability of appropriate semiconductor materials to create active sites at the interface promotes selective oxidation of methane under photo- irradiation. The said photocatalytic reaction was carried out in a batch-type photoreactor made of quartz having a flat face, with a liquid holding capacity of 15 mL. The said photocatalyst is coated on 1x1 cm2FTO plate is inserted in the quartz reactor and 10 mL water is added. After sealing the mouth of the reactor with a septum, the solution is saturated with high pure (99.999 %) methane gas under ice-cold conditions. The film is then exposed to solar simulation for 3 h to evaluate for methane oxidation. The photosystem is also studied under direct sunlight to compare the results. The gaseous and liquid product analysis is carried out by GC and1H-NMR, respectively to identify and quantify the products. A good selectivity towards ethanol with a yield of 4.36 micromoles / h.cm2is obtained w / o co-catalyst, while high selectivity towards ethylene with a yield of 39.54 micromoles / h.cm2is obtained with co-catalyst, and thus provides scope for improved conversion and yield with a proper combination of photo-active materials and co-catalyst systems. EXAMPLES The following examples are given by way of illustration and therefore should not be construed to limit the scope of the invention. Materials used: Titatium dioxide- P25, terpineol, ethanol, ethyl cellulose, glacial acetic acid, titanium chloride, cerium nitrate hexahydrate, ammonium metavanadate, ammonium orthovanadte, PT-1 Platinum paste, Methane gas (99.99% purity) procured from Vadilal gases, deuterium oxide, potassium hydrogen phthalate. Example 1: Preparation of TiO2 paste The first layer of the said photo device is a coat of TiO2paste that is coated by the Doctor blade method. The paste is prepared by the following steps. 1 g of TiO2 (Degussa P25) powder was weighed and taken in a R.B to which 33 mL of ethanol and 0.33 mL of glacial acetic acid were added. After proper dispersion of TiO2in the solvents by 10 minutes of stirring followed by 10 minutes of sonication, 0.5 g of ethyl cellulose was added as binder, followed by stirring and sonication for 10 minutes each. To provide viscosity to the solution mixture, 3 mL of terpinol was added and the solution mixture was kept for 30 minutes of stirring followed by 30 minutes of sonication. The solution was evaporated by rota vapor at around 50 degrees to remove the solvents in order to obtain the desired viscose thick paste. Example 2. Cleaning of the substrate / support To coat the photocatalyst, the inventors chose two different scrupulously cleaned support that are commercially available glass slides and FTO (fluorine doped Tin oxide) plates. To begin with, both the substrates / supports were cut into pieces of required dimensions to provide a 1x1 cm2area for coating the film. The cleaning of the FTO plates involves washing them first with isopropyl alcohol (IPA) followed by scrupulous cleaning with soap solution and thrice with D.I water using sonication. After a final wash with ethanol, the plates were dried at 60 degrees. A 100 mL solution of TiCl3 was prepared separately. After the plates were dry, they were dipped in the prepared TiCl3 solution and kept in the oven for 1 h and then washed properly with IPA and D.I water. This results in the formation of a pale layer of TiCl3over the conductive side of the FTO plates that could promote the sticking coefficient of TiO2. The plates were properly dried before coating. The commercial glass slides were cleaned using 10 minutes of sonication in acetone followed by scrupulous cleaning with soap solution and D.I water 3 times. After a final wash with acetone, they were dried at 60 degrees and used as such. Depending on the observations obtained from the use of both substrates, the inventors found the FTO to be more suitable for the photoreaction and thus, all activity reported is carried out using the photo device made of FTO thin films. Example 3: Fabrication of TiO2 thin films The FTO plates were coated with the as-prepared TiO2 paste by the Doctor blade method. For the same, the scrupulously cleaned plates were coated with the prepared TiO2 paste in example 1 using a glass slide as a slider over an area of 1x1 cm2, whereby the borders of the plate were tapped with scotch tape. This resulted in the uniform coating of the TiO2paste over a 1x1 cm2area with uniform thickness. The films were calcined at 450 degrees for 1 h. The thickness of the films was measured to be in the range of 5-15 microns using a surface profilometer and the size of the TiO2 nanoparticles was around 18-20 nm. Example 4: Integrating visible-light active semiconductor material into TiO2 For improving the light-absorbing capacity of the catalyst so as to work efficiently under solar spectrum, a transition metal oxide semiconductor material was chosen that shows the absorption of light in the visible range. The visible light-absorbing material is CeVO4 which was introduced into the mesopores of TiO2 by SILAR method. In the said procedure, the required cationic and anionic solutions were taken in two clean beakers. The precursors taken for the cationic and anionic solutions are Ce(NO3)3.6H2O and NH4VO3. 40 mL of 25 mM solutions of each were taken in separate beakers and kept at a temperature of 75 degrees. The as-prepared TiO2 films were dipped into the solutions one after the other followed by washing in D.I water. The films were then dried in an oven at 60 ºC for 1 h followed by calcination at 450 degrees for 4 h at a ramping rate of 5 degrees per minute in a muffle furnace. The method facilitates the deposition of CeVO4 nanoparticles in the size range of 6-8 nm in the TiO2mesopores. After optimization of the number of cycles, dipping time, and concentration of precursor solutions, the photocatalytic methane activation experiment was evaluated. Example 5: Optimization of Synthesis parameters The inventors have optimized the parameters for the synthesis of the photocatalyst which remains a crucial part of the invented photo device. The concentration of the precursor solutions is maintained at a 1:1 ratio at a concentration of 25 mM. The number of SILAR cycles was optimized to 10 and the dipping time in each precursor solution at 30 seconds. The temperature of the precursor solutions was also optimized at 75 ºC to facilitate the diffusion of the ions, especially the anions into the porous TiO2. However, the number of SILAR cycles varies to some extent and it depends on the thickness of TiO2 layers. Example 6: Loading of co-catalyst As mentioned earlier, the use of co-catalyst especially noble metals like Pd, Pt, NiCu alloy, Ag, Pt-coated NiCu alloy etc. can promote the effective utilization of the photo- generated electrons. Though the yield and / or activity can be promoted by the use of noble metal co-catalyst, it is important to take into consideration the cost of production. So, herein, the inventors have loaded the Pt co-catalyst in such a way that minimises the amount of co-catalyst requirement or rather needs very low amount for the photocatalytic oxidation of methane. This was done by coating / loading very low amount (~6 mg) of the co-catalyst (Pt paste) on one side of the device 1*1 cm2thin film over FTO. This leads to better transfer of the charge carriers from the CeVO4 / TiO2 thin film to the co-catalyst via the FTO substrate. Example 7: Characterization of synthesized heterogeneous catalyst / photocatalytic thin film 1) Morphology: Figures 2a and 2b show the high-resolution TEM images of the CeVO4 / TiO2binary photocatalyst at different resolutions with 20 nm and 5 nm scale bars. It clearly depicts the incorporation of a little darker CeVO4nanoparticles in the mesoporous TiO2 (grey bigger nanoparticles) thin film in a size range of 6-8 nm. The interface of the two semiconductor materials forms a heterojunction that promotes the efficiency of the developed photocatalyst. The catalyst was scratched off the thin films and subjected to sonication for nearly 30 minutes for sample preparation. The deposited CeVO4 nanoparticles staying intact with TiO2 suggests the formation of a good interaction / binding of the CeVO4species. 2) XRD analysis: Figure 3 shows XRD pattern of the heterogeneous photocatalyst system (CeVO4-TiO2), where diffraction pattern corresponding to CeVO4 (CV) component observed features are denoted by solid-inverted triangle (▼) on TiO2, that also matches well to JCPDS files 79-1065 (tetragonal CV shown as stick pattern) and 21-1272 (anatase TiO2). Solid inverted triangle represents integrated CeVO4 in the nanopores of TiO2 (CVT). Interestingly, CeVO4 also shows shift towards lower angle with (200) and (112) features as shown in Figure 3b, after integration with TiO2. Shift to lower angle suggests an increasing d-value and possibly an elongation of associated bond lengths. Shift in XRD features and hence d-values of both TiO2and CeVO4underscores the structural integration of the components into one system. Although TiO2 amount is more than 90%, the parent (101) feature from TiO2 denoted with (*) and (200) feature of CeVO4 shows comparable intensity in the integrated system fully supporting the structure integration aspect. It is also to be noted that those three high-intensity diffraction features of CV show much smaller or negligible intensity after integration with TiO2 in CVT reiterates that selected facets are interacting with TiO2. Structural integration aspects demonstrated by XRD pattern (Fig. 3), and hence inevitable electronic integration among the components of photocatalyst, is unique with the present system, and not reported in any prior arts. 3) Light absorption. Figures 4a and 4b depict the UV-Vis absorption spectra of said photocatalytic system. For comparison purposes, the absorption spectra of bare TiO2 and CV are also displayed. From the stacked plots depicted in the figure 4a, and extended visible light absorption property of the CVT is observed as compared to the TiO2base material. The appearance of indirect band gap also suggests interfacial interaction between the two semiconductors. The Tauc plot helps in acquiring the band gap values of the materials as shown in Figure 4b. The decrease in band gap value from 3.13 eV of TiO2semiconductor to 2.65 eV upon CeVO4incorporation is a clear indication of improved photon absorption in the visible region making the catalytic system efficient under a wider spectrum, i.e., UV as well as UV-Vis irradiation. Example 8: X-ray Valence Band (XVB) Spectroscopy The XVB plot gives the VB-Maximum (VBMax) energy that helps in the elucidation of CB-Minimum (CBMin). It is observed that VBMax-CV and VBMax-CVT at 0.75 eV demonstrate a vivid shift (by 2.21 eV) than TiO2, which appears at 2.96 eV. The CBMin-CVTcalculated to be at -1.90 eV is at high potential would facilitate injection of photoexcited electrons from QDs into TiO2 in the CeVO4-TiO2 demonstrating type-II heterojunctions. The Ce-4f band shifts from 2.45 eV (CV) to 2.0 eV for CVT (CeVO4- TiO2), which indicates highly reactive nature of Ce in CeVO4-TiO2. The said observation demonstrates a rich electronic and structural integration of QDs with TiO2. Example 9: Photocatalytic set-up The photocatalytic methane oxidation reaction was carried out using the photo device in a custom-made quartz photoreactor. The said photoreactor was made of quartz with a flat surface that can expose the photo device to solar irradiation. The said photo reactor holds a liquid volume capacity of 15 mL, with a diameter of 28 mm and a height of 50 mm. The mouth of the photoreactor is fitted with airtight rubber septum, just before the reaction. Example 10: Photocatalytic oxidation of methane under solar simulator The photocatalytic methane oxidation reaction was carried out with the fabricated photo device having the photocatalyst coated on 1x1 cm2over the conductive side of an FTO thin film. The photocatalyst is placed inside the scrupulously cleaned photoreactor containing 10 mL of D.I water at pH 7. It is sealed adequately ensuring no leakage. The photoreactor is placed in an ice bath and purged with methane (99.999% purity) for about 30 minutes for saturation. The low temperature facilitates a higher dissolution of methane in water. The photo device is then exposed to illumination under one sun condition for at least 3 h. The said reaction was carried out in ambient conditions. The photo-oxidation was evaluated by analyzing the liquid and gaseous samples collected at steady intervals with a tightly closed syringe. The gaseous samples were analyzed by Gas chromatography and the liquid samples were analyzed by1H-NMR (refer, figures 7 and 9). To elucidate the reaction mechanism and identify the intermediate, methanol was added deliberately to the reaction mixture and illuminated under one-sun condition. A huge enhancement observed in ethanol yield confirms the oxidation of methane to methanol, which further couples with methyl species to yield ethanol (Table 1). Figure 7 depicts the 1H-NMR spectra of liquid products analysed and quantified. The spectra include the results of three reactions carried out under the same reaction conditions, i.e., 1 cm2of CeVO4-TiO2film, 10 mL water (saturated with methane for 1 h), exposed to the solar simulator (one sun condition) for 3 h. The quantification was done by using KHP (potassium hydrogen phthalate) as an internal standard. Example 11: Photocatalytic methane oxidation under direct sunlight The photo-oxidation of methane was evaluated under direct sunlight following the rest of the procedure as such as mentioned in example 10. The average temperature recorded over the period was nearly 39 degrees with solar radiation flux of nearly 65 mW. The gaseous samples were analyzed by Gas chromatograph and the liquid samples were analyzed by1H-NMR (refer, figure 10). Example 12: Photocatalytic methane oxidation under solar simulator with co- catalyst loaded thin film device The photo-oxidation of methane was evaluated under one sun condition with the co- catalyst loaded thin film photocatalyst following the rest of the procedure as such as mentioned in example 10. The gaseous samples were analyzed by Gas chromatograph and the liquid samples were analyzed by1H-NMR (refer, figure 11). Example 13: Analysis of liquid product The formation of liquid products was identified and quantified using the proton nuclear magnetic resonance (1H-NMR) technique. For the analysis of the liquid sample collected after the reaction, a scrupulously cleaned Norelle XR55 NMR tube was taken to which, 100 microlitres of D2O as solvent was added, followed by 500 microlitres of the reaction sample and mixed thoroughly. This sums up to 600 microlitres of total liquid sample that is maintained as the minimum requirement for analysis. The instrumental details are as stated: Bruker BioSpin AG with Magnetic resonance of 400-500 MHz; AV-Neo 400 and 500, available in CSIR-NCL, Pune, MH, India. The liquid product identified as ethanol along with a minor amount of methanol is observed as referred in Figures 7-10. The liquid products were further verified by spiking experiments with standard samples of the pure components. Example 14: Quantification of liquid product The identified liquid products from1H-NMR were quantified using an internal standard. For the same, potassium hydrogen phthalate (KHP) was identified as a suitable internal standard and a calculated solution of the same is prepared that constitutes 1 mM strength when 20 microlitres of it is present in 600 microlitres of liquid sample. Thus, the liquid composition for the quantification experiment now becomes 20 microlitres of KHP, 80 microlitres D2O, and 500 microlitres of reaction sample. The concentration of liquid products was calculated by applying the formula: Here, nx represents the molar concentration of KHP in the reaction sample, i.e., 1 mM, Ix represents the integral area in 1H NMR spectra for KHP, and Nx is the number of nuclei (4 equivalent protons for KHP appearing at 7.25 ppm). Similarly, ny is calculated from the above formula for the desired liquid product while Iy represents the integral area of the product formed, and Ny is the number of nuclei associated with the peak. Table 1 outlines the products formed with yield using synthesized 1 cm2CeVO4 / TiO2 FTO film under simulated one sun condition or direct sunlight Table 1: Catalytic Oxidation of methane to ethanol S.No. Conditions Yield of ethanol (µmol / cm2.h)#,%1. 10 mL H2O+ 4.36#CH4 with 1 cm2CeVO4 / TiO2 FTO film under one sun condition 2.10 mL H2O+ 1.2 CH4 with 1 cm2CeVO4 / TiO2glass film under one sun condition 3. 10 mL H2O+ 1.49 CH4with 1 cm2CeVO4 / TiO2 FTO film under direct sunlight 4. 10 mL H2O+ No yield CH4 with 1 cm2CeVO4FTO film under one sun condition5.9.98 mL 75 H2O+ 0.020 mL MeOH+ CH4with 1 cm2CeVO4 / TiO2 FTO film under one sun condition 6. 9.94 mL 73.5&H2O+ 0.060 mL iso- propanol+ CH4 with 1 cm2CeVO4 / TiO2FTO film under one sun condition#Amount of CeVO4 / TiO2 catalyst is ~ 1 mg / cm2and amount of light absorbing CeVO4 is ~4-5 wt %.&Experiment with deliberate addition of isopropanol and the product is tert-butanol. Four important points are worth noting here on scale-up aspects: (a) 1 mg of CeVO4 / TiO2is demonstrated to spread over 1 cm2area, and hence normalization to 1 g of CeVO4 / TiO2 spread over 1000 cm2should produce 4.36 mmol / h.g ethanol yield. (b) It has been observed that deliberate addition of methanol (0.02 ml), at the beginning of the experiment (results shown in row 5), enhances the ethanol yield by 17.2 times to 75 µmol / cm2.h. Similarly, deliberate addition of iso-propanol (0.06 ml) leads to a new product, namely, tert-butanol (data shown in row 6). (c) It is also observed that the weight ratio of CeVO4quantum dots (QDs) and TiO2 is 1:24 in the above devices. Hence, normalization to 1 g of CeVO4 QDs contained in 24 g TiO2 spread over 25000 cm2(or 2.5 m2) is expected to yield 109 mmol / h.gCeVO4-QDs ethanol; similarly, the ethanol yield expected to reach 1874.8 mmol / h.gCeVO4-QDswith deliberately added methanol. It is to be underscored that 50% carbon in 1874.8 mmol ethanol produced would have come from deliberately added methanol, and the rest from methane. (d) These experiments support the formation of methanol as an intermediate reacting with methyl species to form ethanol and deliberately added methanol enhances the reaction rate by 17-18 times.%Results reported is reproduced within 10% error value. For improving the activity under sunlight, the methodology can be modified with engineering aspects. Under continuous flow conditions with temperature control (using water jacket, etc.) may address the concerned issue. Example 15: Quantification of Gaseous product Hydrogen evolution from water splitting alongside generation of the oxidizing species like hydroxyl and / or superoxide species, is observed and detected from GC analysis. The gas sample collected from the head space of the photo reactor after completion of the photo-oxidation reactions were injected using a gas tight injection.500 µL of the gas sample was introduced to the GC equipped with TCD detector and Carbosieve SII column to detect some of the gases. O2gas was observed along with unreacted CH4gases for the CeVO4 / TiO2thin film photocatalytic device. No significant amount of H2 was detected indicating its formation is largely prevented. Similarly, 500 µL of the gas sample was introduced to the same GC equipped with FID detector and Plot Q column to detect higher hydro-carbons. Ethylene gas was confirmed with high yield in case of co-catalyst loaded photocatalytic thin film device (refer, figure_11) Table 2 outlines the gaseous product formed with yield using synthesized co-catalyst loaded CeVO4 / TiO2FTO film under simulated one sun condition S.No. Conditions Yield of ethylene (µmol / cm2.h) 1. 10mL H2O + Negligible CH4 with 1 cm2CeVO4 / TiO2FTO film under one sun condition 2.10mL H2O + 39.54#CH4 with 1 cm2Pt- CeVO4 / TiO2FTO film under one sun condition 3. 10mL H2O + 96.95 CH4with 4 cm2Pt- CeVO4 / TiO2 FTO film under one sun condition#Amount of CeVO4 / TiO2 catalyst is ~ 1 mg / cm2and amount of light absorbing CeVO4 is ~4-5 wt %. Normalizing ethylene formation to 1 g CeVO4 / TiO2spread over 1000 cm2reaches a value of 39.54 mmol / h.g; however, normalization to 1 g of CeVO4QDs in 24 g TiO2spread over 2.5 m2area reaches a value of 988.5 mmol / h.gCeVO4-QDs Example 16: No over-oxidation of CH4 to CO and / or CO2 A major limitation of methane partial oxidation remains the uncontrolled over- oxidation of the desired oxygenates to undesirable CO and / or CO2. Thus, the gas samples were collected after the completion of the reaction duration and injected into the Agilent Gas Chromatogram equipped with TCD detector, Carbosieve S II column, and Helium as carrier gas, to confirm no run-away reaction of methane to CO and CO2 formation (refer, figure 13). The observation remains the same even after 7 hours of reaction duration. The inventors thus, have successfully designed a catalyst that avoids over-oxidation of desired oxygenates. The oxygen observed is attributed to dissolved oxygen. Example 17: Electrochemical measurements The photo response of the developed binary photocatalyst was measured. The said measurement was done using a light chopping experiment under an electrochemical environment. For the said experiment, GAMRY 3000 potentiostat instrument was used wherein, a 1 cm2film was used as the working electrode, and the Saturated Calomel Electrode (SCE) was employed as the reference electrode while the graphite electrode was the counter electrode. The measurements were carried out in a 0.5 M Na2SO4 electrolyte solution maintaining the pH at 7. For comparison, 1 cm2films of TiO2 and CeVO4 were also taken. The solar simulator was used for the said experiment where the electrodes dipped in the electrolyte were exposed to the irradiation of 1 sun condition. The irradiation was chopped off at regular intervals to collect the required data set. As shown in the figure 12a, CeVO4 / TiO2 binary photocatalyst shows the highest photo response. Said figure 12a shows the photocurrent response of the synthesized catalyst compared with its bulk counterparts. The study was carried out at an applied potential of 0.4 V Vs. SCE under light chopping conditions to acquire the data. CeVO4 / TiO2 shows a higher current as compared to the bare TiO2as well as CeVO4films. The observed higher value at 0 minute is the inherent current density @0.4 V of each system. λ- dependant IPCE (incident photon to current efficiency) measurements were carried out at 298 K (Fig.12b). Expectedly, TiO2shows poor IPCE response in visible light. An enhanced IPCE over the entire visible light range is observed with CeVO4 / TiO2, compared to CV. CV shows 24% IPCE at λ=420 nm, which increases to 58% with CeVO4 / TiO2. Inset in Fig.12b shows the integrated current density as a function of λ; 3-times higher current density observed with CeVO4 / TiO2than CeVO4underscores an efficient light absorption and charge separation. High current generation between 400-460 nm demonstrate an efficient electron injection at significantly higher photovoltage from QDs into TiO2. Example 18: Calculation of number of heterojunctions Enhancement in photoactivity is attributed to the synthetic artefacts of SILAR method rendering the formation of heterostructures between CV-QDs and TiO2, a quasi- quantitative look at the number of heterojunctions is proposed based on some valid assumptions derived from TEM and BET analyses. From prior reports it can be safely assumed that TiO2has two major pore sizes, i.e., with diameter 2.5 nm and 4 nm wherein the CV-QDs are assembled into. Next, assuming that they are filled in the 1:4 weight ratio in those pores; 40 µg CV-QDs are filled in as 10 µg: 30 µg in the 2.5 and 4 nm mesopores, respectively, of TiO2. The number of unit cells (both complete and partial unit cells of an extended lattice) of CeVO4 required to fill the 2.5 and 4 nm mesopores is calculated. This is followed by the number of mesopores required to accommodate 10 and 30 µg in 2.5 and 4 nm pores, respectively. The volume (V of one 4 nm spherical pore is calculated to be 33.51 nm3. One tetragonal CeVO4occupies a volume of 0.35 nm3(a=b= 0.738 nm, c= 0.648 nm; volume= a2c). So, number of CeVO4 unit cells that can occupy a 4 nm pore is ~95. One unit cell of CeVO4 contains 6 molecules and thus, 570 molecules are present in one 4 nm mesopore. Similarly, it is calculated for 2.5 nm pore diameter. 1 mole of CeVO4 (6.02x1023molecules) weighs 255.05 g, and hence, 570 molecules weigh 24.14x10-20g. It is assumed that a single TiO2 mesopore is integrated with one CV-QD, and consequently one heterojunction forms. No. of heterojunctions by 4 nm pore= (30x10-6) / 24.14 x10-20= 124.27 x1012. Similarly, no. of heterojunctions by 2.5 nm pore= (10 x10-6) / 5.939 x10-20= 168.4 x1012Approximately 124 trillion heterojunctions are formed by 124 trillions of 4 nm mesopores of TiO2 integrated with 30 µg of CVQDs; 1 mg of CVT photocatalyst in 1 cm2area could form nearly a total of 293 trillion heterojunctions assuming 20% error in these calculations. ADVANTAGES OF INVENTION The present invention provides a heterogeneous photocatalyst system for partial oxidation of C1-C2 alkanes into value added products, specifically ethanol as one of value added products in higher amounts / yield by reacting saturated aqueous lower alkane solution in presence of said heterogeneous photocatalyst system and under visible light condition or direct sunlight. The selectivity is tuned towards ethylene with the use of noble metals as co-catalyst. It also provides an efficient heterogeneous catalyst system for conversion of alkane, where the creation of heterojunction at the interface of the semiconductor materials selectively enhances the ethanol production. It also provides a way of methodically manipulating the band gap of the photocatalyst to accelerate visible light absorption and simultaneous lowering of charge recombination rate. It also fabricates a photocatalytic device with improved light absorption capacity and higher exposed surface area. It further provides photocatalyst 5 thin films of the photo-active materials by employing SILAR method under application of required temperature for adequate ion diffusion into the thin film.

Claims

We Claim 1. A heterogeneous catalyst system for partial oxidation of C1-C2 alkanes into value added products, comprising: a) a visible-light active semiconductor material M1VO4; b) a mesoporous metal oxide; c) a support; and d) optionally, a co-catalyst; wherein the M1 is selected from transition series metal and lanthanides series metal or combinations thereof; wherein a particle size of M1-VO4nanoparticles is in the range of 1.5-8 nm inside the metal oxide mesopores and a particle size of the mesoporous metal oxide nanoparticles is in the range of 15 to 25 nm; and wherein the thickness of heterogeneous catalyst is in the range of 5-15 microns.

2. The catalyst system as claimed in claim 1, wherein the M1 in visible-light active semiconductor material M1VO4 is selected from the group consisting of cerium, praseodymium, terbium, palladium; the mesoporous metal oxide is selected from mesoporous TiO2 and mesoporous ZnO; the support is selected from the group consisting of a fluorine-doped tin oxide FTO glass plate, glass vessels / glass containers, indium tin oxide ITO glass plate, glass plate / slide, silicon wafer, glass plate with thin layer of conducting material coated on it, and the co-catalyst is selected from the group consisting of Pt, Ni-Cu alloy, Pt-coated Ni-Cu alloy.

3. The catalyst system as claimed in claim 2, wherein the stable oxidation states of the praseodymium metal is 3+ and / or 4+ , the stable oxidation states of terbium is 3+ and / or 4+, the stable oxidation states of cerium is 3+ and / or 4+, and the stable oxidation states of palladium is 2+ and / or 4+.

4. A process for preparation of the heterogeneous catalyst system as claimed in claim 1, comprising the steps of: a) preparing a mesoporous metal oxide by Doctor blade method and fabricating onto a support or cleaned support to obtain a thin film, followed by calcining the thin film at a temperature in the range of 350 to 450 °C for time period in the range of 45 to 90 minutes; andb) incorporating or fabricating visible-light active semiconductor material (M1VO4) into said thin film of step a) by SILAR method wherein the SILAR method comprising: (i) preparing two solutions A and B wherein solution A is made of M1-metal ion precursor solution, and solution B is made of vanadate precursor salt; wherein the concentration of the precursor solutions A & B is maintained at a ratio of 1:1 ratio; (ii) dipping the thin film of step (a) into the solutions of A and B consecutively at temperature in the range of 65 to 80 ºC for residence time of dipping in the range of 25 to 40 seconds, followed by washing the dipped film with water or deionised water; and (iii) drying the washed film of step (ii) at temperature in the range of 50 to 70 °C for time period of 45 to 75 minutes and calcining at temperature in the range of 400 to 480 °C for time period of 1 to 4.5 hrs at a ramping rate of 5 degrees per minute in a muffle furnace to obtain visible-light active semiconductor material (M1VO4) integrated / fabricated thin film as the heterogeneous catalyst system or photocatalytic thin film.

5. The process as claimed in claim 4, wherein the vanadate precursor salt is selected from ammonium metavanadate (NH4VO3), sodium orthovanadate, ammonium pyro- vanadate, and ammonium decavanadate.

6. The process as claimed in claim 4, wherein, the thin film of step a) is fabricated with visible light-absorbing material M1-VO4 deposited in the supported mesoporous metal oxide thin film using the respective ionic components diffused into the pores.

7. The process as claimed in claim 6, wherein the mesoporous metal oxide is prepared by a Doctor blade method, wherein the process comprising the steps of: (a) adding metal oxide in solvent or mixture of solvents selected from methanol, ethanol and glacial acetic acid or mixture thereof, followed by stirring for 5 to 15 minutes and 5 to 15 minutes of sonication to obtain dispersion of metal oxide; (b) adding ethyl cellulose as binder into dispersion as obtained in step a) followed by stirring and sonication for 5 to 15 minutes to obtain a solution mixture; (c) providing viscosity to the solution mixture as obtained in step b) by adding terpinol under stirring for 20 to 40 minutes followed by 20-40 minutes of sonication to obtain wet paste; andd) evaporating the paste as obtained in step c) at temperature in the range of 40 to 60 C to remove the solvents in order to obtain the desired viscose thick paste of mesoporous metal oxide.

8. The process as claimed in claim 4, wherein the support is fluorine-doped tin oxide (FTO) glass plate and is cleaned by washing steps comprising of: a) washing support (in the form of plate, slide, etc.) first with isopropyl alcohol (IPA) followed by cleaning with soap solution and thrice with deionised water using sonication; b) subsequent washing of support of step a) with ethanol, and drying at 60 °C; c) dipping dried support of step b) in TiCl3solution kept inside an oven for time period of 45 to 75 minutes followed by washing with IPA and deionised water; or wherein the support is glass slides / plates and is cleaned by washing steps comprising of: a) washing of support using acetone under 5 to 15 minutes of sonication followed by cleaning with soap solution and deionised water at least three times; and b) again, washing with acetone followed by drying at temperature of 50 to 70 °C to obtain cleaned support.

9. A process for conversion of C1-C3 lower alkane into value added products, wherein the process comprising of reacting saturated aqueous alkane solution in the presence of said heterogeneous catalyst system as claimed in claims 1 under simulated solar light condition or direct sunlight in a photoreactor under atmospheric pressure and temperature reaction conditions for time period of 1 to 3 hrs to obtain said value added products; wherein the saturated aqueous alkane solution is prepared by purging alkane in gas form in the water or deionised water until saturation for time period of 10 to 40 minutes at a temperature in the range of 0-10°C.

10. The process as claimed in claim 9, wherein lower alkane is selected from methane or ethane; and wherein said value added products are selected from methanol, ethanol, propanol, iso-propanol, n-butanol, isobutanol, tert-butanol, n- or tert-pentanol, ethylene, H2, methyl hydrogen peroxide, H2O2, formic acid, formaldehyde, CO, and acetic acid.

11. The process as claimed in claim 9, wherein external addition of intermediate product selected from methanol and isopropanol enhances the production rate of ethanol and tert-butanol by 17-18 times, respectively by expediting the consumption of starting lower alkane.

12. The process as claimed in claim 11, wherein in the one sun condition, the photo thin film is exposed to illumination for at least 3 h in static condition and wherein in the direct sunlight condition, the temperature is in the range of 35 to 40 °C for a time period in the range of 1-6 h, with solar radiation flux of 50-80 mW / cm2.

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