DESC:FIELD OF INVENTION
The subject matter described herein in general relates to layered oxide catalyst composites for photo catalytic reduction of carbon dioxide and the process for preparing the catalyst composites. In particular, the present disclosure relates to catalyst composites comprising of strontium titanate with one or more modifying agents, for producing lower hydrocarbons and hydrocarbon oxygenates by the photo catalytic reduction of carbon dioxide in the presence of water.

BACKGROUND
The traditional fossil fuels, i.e., crude oil, gas and coal continue to be the major sources of energy in spite of the global efforts for alternative and renewable resources. Carbon dioxide (CO2) is one of the gases emitted when fossil fuels are burned. CO2 traps heat in the earth atmosphere but is not as potent a greenhouse gas (GHG) as oxides of nitrogen, methane, and fluorinated gases. However, continued usage of fossil fuels has resulted in a drastic increase in atmospheric CO2 levels over the past few decades. This is a matter of great concern since increasing levels of CO2 emissions are related to global warming. Hence mitigation of CO2 is the key challenge to contain global warming.
Efforts are being made worldwide to develop effective technologies to capture and utilize abundant CO2. Conversion or recycling of CO2 into high-energy content or value added fuels/chemicals, also known as chemical carbon mitigation, is an attractive avenue that is currently receiving world-wide attention. A wide range of CO2 conversion techniques are under investigation, which include, chemical, photo-chemical, bio-chemical, bio-photochemical, radio-chemical, electro-chemical, electro-photochemical, bio-photo-electrochemical routes (Scibioh et al., Proc. Indn. Natl. Acad. Sci., 2004, 70 A(3), 407).
Conventional catalytic reduction of CO2 to chemicals such as formic acid, methanol, methane etc. with external hydrogen source is feasible (Nam et al.,Appl. Catalysis A. Gen., 1999, 179, 155). However, conventional routes for catalytic reduction of CO2 are expensive. In order to make CO2 reduction economical and sustainable, production of hydrogen has to be through sustainable routes.
Mitsui Chemicals, Japan, developed a process for methanol synthesis using a highly active catalyst formulation, CO2 (released from a petrochemical plant), and hydrogen obtained by photo catalytic splitting of water (http://www.mitsui.chem.co.jp.e.dt, accessed August 2008). However, large scale production of hydrogen by photo catalytic or photo electro catalytic (PEC) routes is at its infancy.
Titania, modified titania catalysts, layered titania catalysts and many other mixed oxide catalysts have been used for photo catalytic reduction of CO2 (Mori et al., RSC Advances, 2012, 2, 3165). JP 54.112813A discloses a process for photochemical reduction of CO2 to formic acid using perylene or triphenyl amine as a donor and an aromatic hydrocarbon having electron withdrawing group like benzoquinone as an acceptor. NiO loaded NaTaO3 doped with lanthanum has been used as a photo catalyst for water splitting into hydrogen and oxygen in stoichiometric amount under UV irradiation (Kudo et al., J. Am. Chem. Soc., 2003, 125, 3082; Tanaka et al., Applied Catalysis B: Environmental, 2010, 96, 565; Yamakataet al., J. Phys. Chem. B, 2003, 107, 14383).
Similarly, layered perovskite type titanates like Sr3Ti2O7, with co-catalysts like NiO or even without co-catalysts are active for photo catalytic splitting of water to yield hydrogen and oxygen (Int. J. Hydrogen Energy, 31,1142,2006; A.Kudo and Y.Miscki, Chem.Soc.Rev.,38,253,2009). Layered structure facilitates the transfer of charge carriers and the conduction band and valence band energy levels are suitable for the reduction of CO2 and water respectively. Hence Sr3Ti2O7 in unmodified form and with suitable modifications are expected to be good catalysts for CO2 photo reduction.
CO2 is a highly stable molecule and therefore its activation and conversion are highly energy intensive processes. A combination of activation procedures, catalytic/bio process, aided by photo and/or electro chemical activation is needed to achieve the desired conversion. Equally difficult is the reduction/splitting of water to yield hydrogen and hence requires similar combination of activation steps.
Therefore there exists a need to provide improved catalysts that produce lower hydrocarbons and hydrocarbon oxygenates from carbon dioxide and water in a photo catalytic reduction reaction.

SUMMARY OF THE INVENTION
The present invention provides catalyst composites comprising: strontium titanate, (SrTiO3, Sr3Ti2O7 or Sr4Ti3O10) as a base catalyst, with different modifiers like N, S, Fe2O3, MgO, Al2O3, added separately or two or more together. Modifiers or modifying agent(s) are in the range of 0.5 to 25% w/w of the base catalyst; and one or more modifying agent(s) are added to the base catalyst.
Another aspect of the present invention provides a process for producing strontium titanate by adopting modified polymer complex method (PCM) (Int. J. Hydrogen Energy, 31, 1142, 2006). The process comprises of a) hydrolysing 2 moles of titanium tetra iso-propoxide (TTIP) in citric acid medium in presence of appropriate quantity of Sr(NO3)2 (3 moles, as per the stoichiometry), ethylene glycol and methanol. b) Adding methanol, ethylene glycol (EG) and citric acid(CA) in mole ratio of 2:1;0.5c)Heating this mixture at 130°C for 20 hrs turns the initial clear solution in to a gel due to polyesterification. d) Then pyrolyzing the gel at 350°C, followed by calcination at 900°C for 2 hrs.
The modified catalyst composites are obtained by adding 2 moles of urea (precursor for modification by nitrogen) or 2 moles of thio-urea (precursor for modification by nitrogen & sulfur) or 3 wt% Fe2O3 or MgO 3wt% or Al2O3 3wt% to the mixture of TTIP, methanol, ethylene glycol, citric acid and Sr(NO3)2 prior to heating at 130°C, to effectively incorporate the modifiers and then subjecting to further processing at 350°C. One or more modifiers were added together to obtain co-modified catalyst composites.
Yet another aspect of the present disclosure provides a process for producing lower hydrocarbons and hydrocarbon oxygenates, the process comprising: suspending a catalyst composite in a solution of NaOH in water with stirring in a reactor to obtain a first mixture (pH-13); passing carbon dioxide through the first mixture to obtain a second mixture with pH in the range of 8-12; and exposing the second mixture to electromagnetic radiation with wavelength in the range of 300-700 nm to produce lower hydrocarbons and hydrocarbon oxygenates.
Accordingly, the present invention provides a catalyst composite comprising strontium titanate as a base catalyst; and at least one modifying agent incorporated into the base catalyst; characterized in that the modifying agent is selected from the group comprising of nitrogen, sulphur, ferric oxide, magnesium oxide, aluminium oxide and mixtures thereof. The amount of the modifying agent is in the range of 0.5 to 25% w/w of the base catalyst.
In an embodiment, the catalyst composite comprises strontium titanate in the form of SrTiO3, Sr3Ti2O7 or Sr4Ti3O10 as the base catalyst and 0.5 to 25% w/w of nitrogen as the modifying agent.
In another embodiment, the catalyst composite comprises strontium titanate in the form of SrTiO3, Sr3Ti2O7 or Sr4Ti3O10 as the base catalyst and a mixture of nitrogen and sulphur as the modifying agent. In a preferred aspect, the amount of nitrogen in the mixture is in the range of 0.5 to 15% w/w of the base catalyst and the amount of sulphur in the mixture is in the range of 0.5 to 5% w/w of the base catalyst.
In yet another embodiment, the catalyst composite comprises strontium titanate in the form of SrTiO3, Sr3Ti2O7 or Sr4Ti3O10 as the base catalyst and 0.5 to 5% w/w of ferric oxide as the modifying agent.
In still another embodiment, the catalyst composite comprises strontium titanate in the form of SrTiO3, Sr3Ti2O7 or Sr4Ti3O10 as the base catalyst and a mixture of nitrogen and ferric oxide as the modifying agent. In a preferred aspect, the amount of nitrogen in the mixture is in the range of 0.5 to 15% w/w of the base catalyst and the amount of ferric oxide in the mixture is in the range of 0.5 to 5% w/w of the base catalyst.
In a further embodiment, the catalyst composite comprises strontium titanate in the form of SrTiO3, Sr3Ti2O7 or Sr4Ti3O10 as the base catalyst and a mixture of nitrogen, sulphur and ferric oxide as the modifying agent. In a preferred aspect, the amount of nitrogen in the mixture is in the range of 0.5 to 15% w/w of the base catalyst, the amount of sulphur in the mixture is in the range of 0.5 to 5% w/w of the base catalyst and the amount of ferric oxide in the mixture is in the range of 0.5 to 5% w/w of the base catalyst.
In a further more embodiment, the catalyst composite comprises strontium titanate in the form of SrTiO3, Sr3Ti2O7 or Sr4Ti3O10 as the base catalyst and 0.5 to 5% w/w of magnesium oxide as the modifying agent.
In yet another embodiment, the catalyst composite comprises strontium titanate in the form of SrTiO3, Sr3Ti2O7 or Sr4Ti3O10 as the base catalyst and 0.5 to 5% w/w of aluminium oxide as the modifying agent.
The present invention also provides a process for preparing a catalyst composite comprising strontium titanate as a base catalyst and at least one modifying agent incorporated into the base catalyst. The process comprises the steps of:
mixing a source of titanium, a source of strontium, at least one source of modifying agent, methanol, ethylene glycol and citric acid to obtain a reaction mixture;
heating the reaction mixture to obtain a polymerized complex gel; and
pyrolyzing and calcining the polymerized complex gel to obtain the catalyst composite.
The modifying agent is selected from the group comprising of nitrogen, sulphur, ferric oxide, magnesium oxide, aluminium oxide and mixtures thereof and the amount of the modifying agent is in the range of 0.5 to 25% w/w of the base catalyst.
In an embodiment of the present invention, the source of titanium is titanium (IV) iso-proproxide.
In another embodiment of the present invention, the source of strontium is strontium nitrate.
In yet another embodiment of the present invention, the source of the modifying agent is selected from the group comprising of urea, thio urea, dimethyl urea, mercaptan, thiomalic acid, ferric oxide, ferric nitrate, ferric acetate, magnesium oxide, aluminium oxide and mixtures thereof.
In still another embodiment of the present invention, wherein methanol, ethylene glycol and citric acid are present in a mole ratio of about 2:1:0.5.
In a further embodiment of the present invention, the reaction mixture is heated at about 120ºC for a time period of about 20 hours to obtain the polymerized complex gel.
In a furthermore aspect of the present invention, the polymerized complex gel is pyrolyzed at a temperature of about 350ºC for about 4 hours to obtain a pyrolyzed gel.
In another embodiment of the present invention, the pyrolyzed gel is calcined at a temperature of about 900ºC for about 2 hours to obtain the catalyst composite.
The present invention further provides a process for producing lower hydrocarbons and hydrocarbon oxygenates. The process comprises:
suspending a catalyst composite comprising strontium titanate as a base catalyst and at least one modifying agent incorporated into the base catalyst, in alkaline solution to obtain a first mixture with pH-13;
passing carbon dioxide through the first mixture to obtain a second mixture having a pH in the range of 8 to 12; and
irradiating the second mixture with an electromagnetic radiation having wavelength in the range of 300 to 700 nm to produce lower hydrocarbons and hydrocarbon oxygenates.
The modifying agent for the catalyst composite is selected from the group comprising of nitrogen, sulphur, ferric oxide, magnesium oxide, aluminium oxide and mixtures thereof and the amount of the modifying agent is in the range of 0.5 to 25% w/w of the base catalyst.
In an embodiment of the present invention, the steps of passing and irradiating are performed in an all glass thermostatic photo-catalyst reactor provided with a quartz window for irradiation of the second mixture.
In another embodiment of the present invention, the second mixture is irradiated for a time period in the range of 0.1 to 20 hours at a temperature in the range of 20 to 40ºC.
These and other features, aspects, and advantages of the present invention 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 invention.

BRIEF DESCRIPTION OF THE DRAWINGS
The detailed description is described with reference to the accompanying figures. In the figures, the left-most digit(s) of a reference number identifies the figure in which the reference number first appears. The same numbers are used throughout the drawings to reference like features and components.
Figure 1 graphically illustrates a photo catalytic reactor for CO2 reduction. Where (1) is a Hg lamp source; (2) is a quartz window; (3) is a magnetic stirrer; (4) is an outlet for cooling water; (5) is a magnetic pellet; (6) is an inlet for cooling water; (7) is a CO2 gas outlet; (8) thermo well; (9) pressure gauge; (10) CO2 gas inlet.
Figure 2 graphically illustrates the X-ray diffractogram of Sr3Ti2O7.
Figure 3 graphically illustrates the morphology of neat Sr3Ti2O7 in Fig 3 (a) and differently modified Sr3Ti2O7 (Fe/ N S/Sr3Ti2O7) prepared by polymer complex method in Fig 3(b). Figure 4 graphically illustrates the electronic spectra of neat Sr3Ti2O7 and catalyst composites modified using N, S and Fe2O3 and mixtures thereof.
Figure 5 graphically illustrates the electronic spectra of neat Sr3Ti2O7 and catalyst composites modified using MgO and Al2O3.
Figure 6 graphically illustrates time on stream data for unmodified (a) Sr3Ti2O7, (b) Sr3Ti2O7-xNx.
Figure 7 graphically illustrates the time on stream data for neat Sr3Ti2O7 and modified Sr3Ti2O7 catalyst composites using N, S and Fe2O3 and mixtures thereof (Sr3Ti2-x-yFexSyO7-zNz).
Figure 8 shows XRD patterns for neat and doped Sr titanates. The effect of doping, especially Fe2O3 is clearly indicated in the shift in d-lines- due to the location of Fe3+ ions in Ti4+ sites.
Figure 9 shows DR spectra for neat and doped Sr titanates which illustrates that the band gap values decrease when Light absorption edge is shifted to visible region.
Figure 10 shows photo luminescence spectra of neat and doped Sr titanates. The spectra show that the dopants trap photo electrons and holes and retard recombination, thus increasing their lifetime.
Figure 11 shows activity pattern for CO2 photo reduction on neat and doped Sr titanates. Amongst Sr titanates studied, Sr3Ti2O7 when doped with N, S & Fe2O3 displays maximum activity.
It should be appreciated by those skilled in the art that any block diagrams herein represent conceptual views of illustrative systems embodying the principles of the present subject matter.

DETAILED DESCRIPTION OF THE INVENTION
The present invention now will be described more fully hereinafter. Indeed, the invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will satisfy applicable legal requirements. As used in the specification, and in the appended claims, the singular forms “a”, “an”, “the”, include plural referents unless the context clearly dictates otherwise.
The invention disclosed herein relates to catalyst composites for photo catalytic reduction of carbon dioxide. Sr3Ti2O7 based catalyst composites have not been exploited for photo catalytic reduction of CO2 by water. It is the main object of the present invention to provide a catalyst composite comprising of Strontium titanates as a base catalyst along with several modifying agents like, N, S & Fe2O3, MgO and Al2O3 added either individually or two or more together. The metal in the catalyst composite may be present in their elemental form or as metal oxide or as metal salt or mixtures thereof.
An embodiment of the present invention relates to a catalyst composite comprising of Strontium titanate, selected from SrTiO3, Sr3Ti2O7 or Sr4Ti3O10, as a base catalyst with nitrogen as modifying agent in the range of 0.5 to 15% w/w of the base catalyst.
Another embodiment of the present invention provides a catalyst composite, wherein two different modifying agents, N & S are incorporated together.
An embodiment of the present invention relates to a catalyst composite comprising of Strontium titanate (SrTiO3, Sr3Ti2O7 or Sr4Ti3O10) as a base catalyst with nitrogen as modifying agent in the range of 0.5 to 15% w/w and sulfur (S) as additional modifier in the range 0.5 to 5% w/w of the base catalyst.
An embodiment of the present invention relates to a catalyst composite comprising of Strontium titanate (SrTiO3, Sr3Ti2O7 or Sr4Ti3O10) as a base catalyst with iron (Fe) as modifying agent in the range of 0.5 to 5 w/w % of the base catalyst.
Another embodiment of the present invention provides a catalyst composite, wherein two different modifying agents, N & Fe are incorporated together.
An embodiment of the present invention relates to a catalyst composite comprising of Strontium titanate (SrTiO3, Sr3Ti2O7 or Sr4Ti3O10) as a base catalyst with nitrogen as modifying agent in the range of 0.5 to 15% w/w and iron (Fe) as additional modifier in the range 0.5 to 5% w/w of the base catalyst.
Another embodiment of the present invention provides a catalyst composite, wherein three different modifying agents, N, S & Fe are incorporated together.
Another embodiment of the present invention provides a catalyst composite, wherein three different modifying agents, N in the range of 0.5 to 15% w/w, S in the range 0.5 to 5% w/w & Fe in the range 0.5 to 5% w/w, of the base catalyst are incorporated together.
An embodiment of the present invention relates to a catalyst composite comprising of Strontium titanate (SrTiO3, Sr3Ti2O7 or Sr4Ti3O10) as a base catalyst with magnesium in the form of MgO as modifying agent in the range of 0.5 to 5% w/w of the base catalyst.
An embodiment of the present invention relates to a catalyst composite comprising of Strontium titanate (Sr3Ti2O7) as a base catalyst with Aluminium in the form of Al2O3 as modifying agent in the range of 0.5 to 5% w/w of the base catalyst.
Yet another embodiment of the present invention provides a catalyst composite, wherein the modifying agents are incorporated, either individually or together, on to SrTiO3, Sr3Ti2O7 or Sr4Ti3O10 during the formation stage to evolve active composites.
The present invention relates to a catalyst composite, comprising of Strontium titanate as a base catalyst and one or more than one modifying agent (s) in the range of to 0.5 to 25% w/w of the base catalyst.
In yet another embodiment, the present invention provides a catalyst composite, wherein the base catalyst SrTiO3, Sr3Ti2O7 or Sr4Ti3O10 is incorporated with modifiers selected from the group comprising of 0.5-15.0% w/w of N with respect to the base catalyst, 0.5-5.0% w/w of S with respect to the base catalyst, and 0.5-5.0% w/w of Fe with respect to the base catalyst.
The invention described herein also relates to photo catalytic reduction of carbon dioxide in presence of alkaline water to produce lower hydrocarbons and hydrocarbon oxygenates. The present invention relates to different catalyst composites, wherein the catalyst composite is used for photo catalytic reduction of carbon dioxide in presence of alkaline water to produce lower hydrocarbons and hydrocarbon oxygenates.
The present invention further relates to a process for producing a base catalyst, the process comprising of hydrolysis of 2 moles of titanium tetra iso-propoxide (TTIP) in citric acid medium in presence of appropriate quantity of Sr(NO3)2 (3 moles, as per the stoichiometry), ethylene glycol and methanol. Methanol, ethylene glycol and citric acid are added in mole ration of 2:1:0.5. Heating this mixture at 130ºC for 20 hrs turns the initial clear solution in to a gel due to polyesterification.
An embodiment of the present invention relates to a process, wherein the precursor gel obtained due to polyesterification is pyrolyzed at 350°C for 4 hours.
Another embodiment of the present invention relates to a process, wherein the pyrolyzed gel is further calcined at 900°C for 4 hours to obtain the base catalyst, Sr3Ti2O7.
An embodiment of the present invention relates to a process, wherein the precursor for the modifier is selected from the group comprising of urea, thio-urea, dimethyl urea, ammonium salt, ferric oxide, ferric tri nitrate nano hydrate.
Precursor for nitrogen is selected from urea, dimethyl urea, and ammonium salt. Any Nitrogen containing organic or inorganic precursor soluble in the medium is used. An embodiment of the present invention relates to a process, wherein the precursor for nitrogen is urea.
The present invention further relates to a process, wherein any precursor containing sulfur, thio-urea, mercaptan, thiomalic acid or sulfur containing inorganic or organic precursor is selected for incorporation of sulfur as modifying agent. An embodiment of the present invention relates to a process, wherein thio-urea is used as source for sulfur.
The salts of iron for modifying agent are selected from the group comprising of ferric nitrate, ferric chloride, ferric acetate and ferric oxide. Salts of iron can be simply any organic or inorganic metal salts containing iron. An embodiment of the present invention relates to a process, wherein the salt of iron is ferric oxide.
The present invention further relates to a process, wherein water is distilled and deionized. Any other purified form of water preferably non-ionic can also be used.
The present invention further relates to a process for producing lower hydrocarbons and hydrocarbon oxygenates, the process comprising: suspending a catalyst composite in a solution of NaOH in water with stirring in a reactor to obtain a first mixture; passing carbon dioxide through the first mixture to obtain a second mixture with pH in the range of 8-12; and exposing the second mixture to electromagnetic radiation with the wavelength in the range of 300-700 nm to produce lower hydrocarbons and hydrocarbon oxygenates.
The reactor used in the present disclosure is an all-glass thermostatic photo-catalytic reactor provided with a quartz window for irradiation of the catalyst suspension.
An embodiment of the present invention relates to a process, wherein carbon dioxide gas is pure and dried before use. Carbon dioxide is preferably purified by passing through hydrocarbon and moisture traps. The present invention describes a process, wherein the second mixture is exposed to radiation for 0.1 to 20 h at a temperature range of 20-40°C. The present disclosure further relates to a process, wherein the second mixture is exposed to radiation under ambient conditions.
In another embodiment, the present invention provides a process, wherein the lower hydrocarbon is selected from the group comprising of methane, ethane, and mixtures thereof. In another embodiment of the present invention, the hydrocarbon oxygenate is selected from the group comprising of methanol, ethanol, acetaldehyde, and mixtures thereof. The present invention relates to a process for photo catalytic transformation of carbon dioxide to a mixture of light hydrocarbons and hydrocarbon oxygenates which includes alcohols and aldehydes by reaction with water. The present invention further relates to a process for producing light hydrocarbons and hydrocarbon oxygenates including but not limited to methane, methanol, ethane, ethanol, acetone, formaldehyde, and free hydrogen.
Yet another embodiment of the present invention relates to a process, wherein the catalyst composite is used for photo catalytic reduction of carbon dioxide in presence of alkaline water to produce methanol selectively among other hydrocarbon oxygenates and lower hydrocarbons.
Another embodiment of the present invention relates to a process, wherein water is the hydrogen source for photo-catalytic reduction of carbon dioxide. The present invention also relates to a process wherein photons from visible light are used as source of energy and water as hydrogen (H2) source for photo catalytic transformation of carbon dioxide to a mixture of light hydrocarbons and hydrocarbon oxygenates.
The present invention relates to a process, wherein the catalyst composite is dispersed in slurry state in aqueous alkaline solution, within a jacketed all glass reactor provided with a quartz window for irradiation of the dispersed medium. The present invention further relates to a process, wherein the catalyst composite is dispersed in alkaline solution and saturated with CO2 before irradiating with visible light to facilitate the photo reduction of dissolved CO2. The present invention relates to a process, wherein the alkaline solution increases the solubility of carbon dioxide. Yet another embodiment of the present invention relates to a process, wherein higher carbon dioxide concentration leads to higher yields of lower hydrocarbon and hydrocarbon oxygenates.
Another embodiment of the present invention relates to a process, wherein the light source is 250 W Hg lamp covering both UV & VIS region of light with wavelength in the range of 300-700 nm.
An embodiment of the present invention relates to a process for producing light hydrocarbons and hydrocarbon oxygenates from carbon dioxide by photo catalytic reduction of carbon dioxide at ambient temperature and atmospheric pressure. Catalyst composites prepared and characterized for structural and photo physical properties exhibited significant and stable activity for photo reduction of CO2 with water to yield a range of useful hydrocarbons and hydrocarbon oxygenates. Thus Sr3Ti2O7 based catalysts hold promise as potentially effective candidates for CO2 photo reduction. It is observed that CO2 photo reduction activity is closely related to the activity for photo catalytic splitting of water. Sr3Ti2O7 modified with NiO displays high activity for water splitting and hence expected to be active for CO2 photo reduction as well.
In the present invention, hydrogen is generated in-situ by photo catalytic splitting/oxidation of water and a range of hydrocarbons are formed. It is observed that Sr3Ti2O7 based catalysts exhibit exceptionally stable activity with methanol and ethanol as the major products. N, S & Fe2O3 act as efficient modifiers of the band gap of Sr3Ti2O7 and help in extending light absorption edge to visible region, thus increasing photo catalytic reduction of CO2 to hydrocarbons.
Another embodiment in the present invention is the synergistic effect due to co-doping of more than one modifier at a time. Co-doping of N-S, N-Fe2O3 and N-S-Fe2O3 improve the photo catalytic activity.
Although the invention has been described in considerable detail with reference to certain preferred embodiments thereof, other embodiments are possible. As such, the spirit and scope of the appended claims should not be limited to the description of the preferred embodiment contained therein. Typical applications of the catalyst composites that constitute part of the present invention are given below in the form of examples.
The invention will now be illustrated with working examples, which is intended to illustrate the working of invention and not intended to take restrictively to imply any limitations on the scope of the present invention. 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 invention belongs. Although methods and materials similar or equivalent to those described herein can be used in the practice of the disclosed methods and composites, the exemplary methods, devices and materials are described herein.

Example 1: Photo catalytic reaction
The CO2 photo reduction process on Sr3Ti2O7 based catalyst composites were conducted in slurry phase, in batch mode. An Hg lamp source (1) with a quartz window (2) (5 cm diameter) for light source as shown in Figure 1 was used for studying the photo catalytic reduction of CO2 by water. 250 W Hg lamp source (1) covering both UV & VIS region of light (300-700 nm) was used. 0.4 g of catalyst was dispersed in 400 ml of 0.2M aqueous NaOH solution to increase the solubility of CO2 and act as a hole scavenger. Catalyst remained in a suspended state with continuous stirring with the help of a magnetic stirrer (3). Pure and dry CO2 gas (purified by passing through hydrocarbon & moisture traps) was bubbled for 30 minutes to remove oxygen. When the aqueous alkaline solution was completely saturated with CO2, pH of the medium was decreased from 13 to 8. Under these conditions, CO2 available in the medium was estimated to be 71000 µ moles. Reactor inlet (10) and outlet (7) were closed tightly with trapped CO2 gas in contact with water. Photo chemical reaction in batch mode was initiated by switching on the irradiation. Gas and liquid samples were taken out at regular intervals & analyzed by GC. Reactions were carried out for 20 h. The temperature of the reaction medium was maintained at 25°C by circulation of water (4, 6) in the jacket. Amount of CO2 consumed in the formation of all types of hydrocarbons and hydrocarbon oxygenates were observed in the GC analysis. Accordingly, conversion of CO2 was calculated based on the stoichiometry.

Example 2: Control experiment
No reaction could be observed without the catalyst and with catalyst in dark. When the aqueous alkaline medium with dispersed catalyst was purged, saturated with nitrogen and irradiated, very small quantities of hydrocarbons, possibly due to the conversion of residual carbon on catalyst surface, was observed up to six hours after which no product in measurable quantities could be detected. However, on purging and saturation with CO2, hydrocarbons or hydrocarbon oxygenates in increasing amounts up to 20 h and beyond could be observed, thus establishing that the products are actually due to photo catalytic reduction of CO2.
Based on these observations, with each catalyst composite, alkaline solution saturated with nitrogen was irradiated for 12 hrs to remove hydrocarbons formed from carbon residues on catalyst surface. The solution was then saturated with CO2 on clean catalyst surface and CO2 photo reduction was carried out from then on up to 20 hrs.

Example 3: Base catalyst- Sr3Ti2O7
The base catalyst Sr3Ti2O7 was prepared by adopting modified polymer complex method (PCM) (Int. J. Hydrogen Energy, 31,1142, 2006). The method comprises of hydrolysis of 2 moles of titanium tetra iso-propoxide (TTIP) in citric acid medium in presence of appropriate quantity of Sr(NO3)2 (3 moles, as per the stoichiometry), ethylene glycol and methanol. Methanol, ethylene glycol and citric acid are added in mole ratio of 2:1:0.5. Changes in the molar ratio resulted in the formation of SrTiO3 and not Sr3Ti2O7. On heating this mixture at 130 °C for 20 hrs initially clear solution turns into a gel due to polyesterification. The gel is first pyrolyzed at 350°C followed by calcination at 900°C for 2 hrs.
The base catalyst prepared showed characteristic XRD pattern as indicated in Figure 2. Plate like morphology displayed by the catalyst was revealed in Figure 3 and typical electronic spectrum in Figure 4, which gave band gap value of 3.14 e V. Crystallite size and band gap values for neat and modified Sr3Ti2O7 catalyst composites are given in Table.1. The CO2 conversion realized along with product profile are given in Table 2. The base catalyst showed moderate activity for CO2 photo reduction. The base catalyst was active for both reactions, i.e., splitting of water to yield hydrogen and reduction of CO2 under irradiation.

Table.1 Crystallite size and band gap values for neat and modified Sr3Ti2O7 composites
Catalyst Crystalline Size (nm) Band Gap (eV)
Sr3Ti2O7 46.3 3.14
N/Sr3Ti2O7 37.2 2.99
N-S/Sr3Ti2O7 34.3 2.85
Fe/Sr3Ti2O7 33.5 2.73
Fe/N/Sr3Ti2O7 24.9 2.57
Fe/N,S/Sr3Ti2O7 21.9 2.39
Al2O3/ Sr3Ti2O7 - 3.14
MgO/ Sr3Ti2O7 - 3.09
SrTiO3 28.9 3.05
Fe/N,S/SrTiO3 33 3
Sr3Ti2O7 46 3.14
Fe/N,S/Sr3Ti2O7 39 2.4
Sr4Ti3O10 52.7 3.12
Fe/N,S/Sr4Ti3O10 46.7 2.9

Table 2: CO2 photo reduction on neat Sr3Ti2O7 and different modified formulations

Table 3: Photo reduction of CO2- Cumulative conversion
Catalyst Products formed after 20 hrs of irradiation (µmol/g) Conversion (%)
CH4 C2H4 C2H6 CH3OH C2H4O C2H5OH C3H6O C3H6 Total CO2 consumed
SrTiO3 0.42 0.3 0.14 212 0 89.2 0 0 0.55
Fe/N,S/SrTiO3 0.5 0.6 0 498.9 0 66.4 0 0.2 567.8 0.79
Sr3Ti2O7 0.5 0.2 0.2 248.8 7.6 53.4 0 0.2 373 0.5
Fe/N,S/Sr3Ti2O7 0.1 5.4 0.2 561 1.6 284 25 0.9 1221.7 1.7
Sr4Ti3O10 0.22 0.8 0.2 222.3 0 129. 0 0.8 485.4 0.68
Fe/N,S/Sr4Ti3O10 0.28 0 0.2 409.6 1.8 123.6 0 0 661.1 0.93

Example 4: Sr3Ti2O7 modified with nitrogen (N)
N modified Sr3Ti2O7was prepared by the same procedure as described above, by adding 2 moles of urea along TTIP, citric acid, methanol and ethylene glycol. After polyesterification, the sample was pyrolyzed at 350°C followed by calcination at 900°C for 2 hrs. as described in Example 3.
Addition of nitrogen to the base catalyst as described in Example 3results in decrease crystallite size as well as band gap energy (Table1) due to structural changes as well as changes in photo physical properties (Figure 4). These changes result in marginal increase in CO2 conversion as indicated in Table 2.

Example 5: Sr3Ti2O7 modified by nitrogen (N) and sulfur (S)
Both N & S were incorporated together into Sr3Ti2O7by the same procedure as described in Example 3, by adding 2 moles of thio-urea along TTIP, citric acid, methanol and ethylene glycol. After polyesterification, the sample was pyrolyzed at 350°C followed by calcination at 900°C for 2 hrs as described in Example 3.
Incorporation of N &S together into Sr3Ti2O7 resulted in further decrease in crystallite size and band gap energy (Table1). These changes lead to significant increase in the CO2 photo conversion, as presented in Table 2.

Example 6: Sr3Ti2O7 modified by ferric oxide (Fe2O3)
Fe2O3 incorporated together into Sr3Ti2O7by the same procedure as described in Example 3, by adding 3 wt % Fe2O3 powder along with TTIP, citric acid, methanol and ethylene glycol. After polyesterification, the sample was pyrolyzed at 350°C followed by calcination at 900°C for 2 hours as described in Example 3.
Incorporation of Fe as Fe2O3 together into Sr3Ti2O7 resulted in further decrease in crystallite size and band gap energy (Table1). These changes lead to significant increase in the CO2 photo conversion, as presented in Table 2.

Example 7: Sr3Ti2O7 modified by nitrogen and ferric oxide (Fe2O3)
Both N & S were incorporated together into Sr3Ti2O7by the same procedure as described in Example 3, by adding 2 moles of urea and 3 wt% Fe2O3 along TTIP, citric acid, methanol and ethylene glycol. After polyesterification, the sample was pyrolyzed at 350°C followed by calcination at 900°C for 2 hrs. as described in Example 3.
As per the data presented in Table1 modifications by adding N & Fe2O3 together lead to decrease in crystallite size and band gap energy. Table 2 shows that addition of N & Fe2O3 together results in increase of photo catalytic reduction of CO2.

Example 8: Simultaneous incorporation of N, S and Fe2O3 into Sr3Ti2O7
N, S and Fe2O3 were incorporated together into Sr3Ti2O7 by the same procedure as described in Example 3, by adding 2 moles of thio urea and 3 wt% Fe2O3 along TTIP, citric acid, methanol and ethylene glycol. After polyesterification, the sample was pyrolyzed at 350°C followed by calcination at 900°C for 2 hrs as described in Example 3.
As per the data presented in Table1, modifications by adding N, S & Fe2O3 together lead to further decrease in crystallite size and band gap energy and correspondingly, the activity for photo catalytic reduction of CO2 increases (Table 2).

Example 9: Modification of Sr3Ti2O7 by MgO
MgO incorporated together into Sr3Ti2O7 by the same procedure as described in Example 3, by adding 3 wt % MgO powder along with TTIP, citric acid, methanol and ethylene glycol. After polyesterification, the sample was pyrolyzed at 350°C followed by calcination at 900°C for 2 hrs. Figure 5 illustrates electronic spectra of neat Sr3Ti2O7 and catalyst composites modified using MgO. Table 1 provides the modifications attained in the band gap energy by addition of MgO.

Example 10: Modification of Sr3Ti2O7 by Al2O3
Al2O3 incorporated together into Sr3Ti2O7 by the same procedure as described in Example 3, by adding 3 wt % Al2O3 powder along with TTIP, citric acid, methanol and ethylene glycol. After polyesterification, the sample was pyrolyzed at 350°C followed by calcination at 900°C for 2 hrs. Figure 5 illustrates electronic spectra of neat Sr3Ti2O7 and catalyst composites modified using Al2O3. Table 1 provides the modifications attained in the band gap energy by addition of Al2O3.
Example 11: Modification of Sr Titanates, SrTiO3 and Sr4Ti3O10
Neat SrTiO3 and Sr4Ti3O10 samples were prepared as per the modified polymer complexes method described earlier. Simultaneous incorporation of N, S & Fe2O3 in both Sr titantes were performed adopting the procedure described earlier. Fig. 8 gives the changes in the XRD patterns for the three Sr titanates after doping with N, S & Fe2O3. Corresponding changes in the electronic spectra and photo luminescence spectra of multiply doped Sr titanates are given in Fig.9 and Fig.10 respectively. Table 3 and Fig.11 show comparison of the activity of three Sr titanates, in neat as well as in modified forms. While all the modified Sr titanates display higher activity vis-a vis neat ones, N, S and Fe2O3 modified Sr3Ti2O7 shows maximum activity for CO2 photo reduction.
,CLAIMS:WE CLAIM:
1. A catalyst composite comprising:
Strontium titanate as a base catalyst; and
at least one modifying agent incorporated into the base catalyst;
characterized in that:
the modifying agent is selected from the group comprising of nitrogen, sulphur, ferric oxide, magnesium oxide, aluminium oxide and mixtures thereof; and
the amount of the modifying agent is in the range of 0.5 to 15% w/w of the base catalyst.
2. The catalyst composite as claimed in claim 1, wherein the catalyst composite comprises strontium titanate in the form of SrTiO3, Sr3Ti2O7 or Sr4Ti3O10 as the base catalyst and 0.5 to 15% w/w of nitrogen as the modifying agent.
3. The catalyst composite as claimed in claim 1, wherein the catalyst composite comprises strontium titanate in the form of SrTiO3, Sr3Ti2O7 or Sr4Ti3O10 as the base catalyst and a mixture of nitrogen and sulphur as the modifying agent.
4. The catalyst composite as claimed in claim 3, wherein the amount of nitrogen in the mixture is in the range of 0.5 to 15 % w/w and the amount of sulphur in the composite is in the range of 0.5 to 5 % w/w.
5. The catalyst composite as claimed in claim 1, wherein the catalyst composite comprises strontium titanate in the form of SrTiO3, Sr3Ti2O7 or Sr4Ti3O10 as the base catalyst and 0.5 to 5% w/w, based on the base catalyst, of ferric oxide as the modifying agent.
6. The catalyst composite as claimed in claim 1, wherein the catalyst composite comprises strontium titanate in the form of SrTiO3, Sr3Ti2O7 or Sr4Ti3O10 as the base catalyst and a mixture of nitrogen and ferric oxide as the modifying agent.
7. The catalyst composite as claimed in claim 6, wherein the amount of nitrogen in the composite is in the range of 0.5 to 15 % w/w and the amount of ferric oxide in the composite is in the range of 0.5 to 5% w/w of the base catalyst.
8. The catalyst composite as claimed in claim 1, wherein the catalyst composite comprises strontium titanate in the form of SrTiO3, Sr3Ti2O7 or Sr4Ti3O10 as the base catalyst and a mixture of nitrogen, sulphur and ferric oxide as the modifying agent.
9. The catalyst composite as claimed in claim 8, wherein the amount of nitrogen in the composite is in the range of 0.5 to 15 % w/w, the amount of sulphur in the composite is in the range of 0.5 to 5 % w/wand the amount of ferric oxide in the composite is in the range of 0.5 to 5%w/w of the base catalyst.
10. A process for preparing a catalyst composite comprising strontium titanate as a base catalyst and at least one modifying agent incorporated into the base catalyst, said process comprising the steps of:
(a) mixing a source of titanium, a source of strontium, at least one source of modifying agent, methanol, ethylene glycol and citric acid to obtain a reaction mixture;
(b) heating the reaction mixture to obtain a polymerized complex gel; and
(c) pyrolyzing and calcining the polymerized complex gel to obtain the catalyst composite;
characterized in that:
the modifying agent is selected from the group comprising of nitrogen, sulphur, ferric oxide, magnesium oxide, aluminium oxide; and the amount of the modifying agent is in the range of 0.5 to 15% w/w of the base catalyst.
11. The process as claimed in claim 10, wherein the source of titanium is titanium (IV) iso-proproxide.
12. The process as claimed in claim 10, wherein the source of strontium is strontium nitrate.
13. The process as claimed in claim 10, wherein the source of the modifying agent is selected from the group comprising of urea, thio urea, dimethyl urea, mercaptan, thiomalic acid, ferric oxide, ferric nitrate, ferric acetate and mixtures thereof.
14. The method as claimed in claim 10, wherein methanol, ethylene glycol and citric acid are present in a mole ratio of about 2:1: 0.5.
15. The method as claimed in claim 10, wherein the reaction mixture is heated at about 130ºC for a time period of about 20 hours to obtain the polymerized complex gel.
16. The method as claimed in claim 10, wherein the polymerized complex gel is pyrolyzed at a temperature of about 350ºC for about 4 hours to obtain a pyrolyzed gel.
17. The method as claimed in claim 16, wherein the pyrolyzed gel is calcined at a temperature of about 900ºC for about 2 hours to obtain the catalyst composite.
18. A process for producing lower hydrocarbons and hydrocarbon oxygenates, the process comprising:
(a) suspending a catalyst composite comprising strontium titanate as a base catalyst and at least one modifying agent incorporated into the base catalyst, in alkaline solution to obtain a first mixture;
(b) passing carbon dioxide through the first mixture to obtain a second mixture having a pH in the range of 8 to 12; and
(c) irradiating the second mixture with an electromagnetic radiation having wavelength in the range of 300 to 700 nm to produce lower hydrocarbons and hydrocarbon oxygenates;
characterized in that:
the modifying agent is selected from the group comprising of nitrogen, sulphur, ferric oxide, magnesium oxide, aluminium oxide; and the amount of the modifying agent is in the range of 0.5 to 25% w/w of the base catalyst.
19. The process as claimed in claim 18, wherein steps (b) and (c) are performed in an all glass thermostatic photo-catalyst reactor provided with a quartz window for irradiation of the second mixture.
20. The process as claimed in claim 18, wherein the second mixture is irradiated for a time period in the range of 0.1 to 20 hours at a temperature in the range of 20 to 40ºC.