# Patent

## Title
CuO—TiO2 nanocomposite photocatalyst for hydrogen production, process for the preparation thereof

## Abstract

The present investigation is development of the TiO2 nanotubes concept of preparation of and their composite with fine dispersion of copper. The inventions also relates to identify a method for optimum amount of photocatalyst required for efficient and maximum hydrogen production reported than earlier (H2=99,823 μmol·h−1·g−1 catalyst) from glycerol-water mixtures under solar light irradiation. A method is disclosed to produce CuO/TiO2 nanotubes with high sustainability and recyclable activity for hydrogen production.

## Background
FIELD OF THE INVENTION
The present invention relates to CuO—TiO2 nanocomposite photocatalyst for hydrogen production, process for the preparation thereof. Particularly, the present invention provides a process for largest volume of hydrogen production in high yield than reported till date under solar light irradiation using CuO/TiO2 nanotubes photocatalyst. More particularly, the present invention relates to the hydrogen production under solar light irradiation using CuO/TiO2 nanotubes photocatalyst from Glycerol:water mixture.
BACKGROUND OF THE INVENTION
The demand for a large volume of fuel for transportation sector, for industrial use and increasing modern societal life-style is mainly supplied by petroleum based derivatives. Both industrial processing of fossil resources and its commercial utilization damaged the environment. Since the steady increase in fossil fuel requirements and decrease in natural fossil resources are at alarming rate that have urged to go for alternative and renewable energy resources. To full-fill the future energy requirements (at the same time) development of eco-friendly process for hydrogen production occupies first place as it has proven as cleaner resource and significant improvement in energy efficiencies up to 60% compared to petroleum derived fuels (30-35%) in internal combustion engine.
Several methods are existing for hydrogen production like steam reforming of natural gas, partial oxidation of hydrocarbons, auto thermal reforming of glycerol, gasification of coal and biomass, photo biological production and photocatalytic water splitting [Acta Geodyn. Geomater, “The resources and methods of hydrogen production”. Vol. 7, No. 2,158 (2010) pp 175-188, Chi-Hung Liao, Chao-Wei Huang and Jeffrey C. S. Wu, “Hydrogen Production from Semiconductor-based Photocatalysis via Water Splitting”. Catalysts Vol. 2 (2012) pp. 490-516].
Some of the above methods require higher temperature and pressure the resources are non-renewable in nature and potential to damage the environment. Photocatalytic water splitting process demonstrated as promising one for hydrogen production as it works well under ambient conditions utilizing renewable resources like water, sunlight in the presence of semiconductor photocatalysts [M. Anpo and P. V. Kamat (Eds), “Environmentally Benign Photocatalysts: Applications of Titanium-oxide based Materials” Springer, New York (2010)]. Among the important criteria identified for efficient photocatalytic water splitting are: (i) energy band configuration and (ii) surface properties of semiconductor photocatalyst.
The energy bands were systematically tuned to their desired energy level by modulation of valence band and conduction band. The surface properties were controlled by facilitating charge-carriers transfer to the surface by using hybrid catalysts, composite catalysts, using noble metal loaded catalysts and nanostructured catalysts. They greatly influence the optical and surface properties of the photocatalyst.
Nano structured photocatalysts exhibited improved performance than nanoparticles in water splitting process [A. Kudo and Y. Miseki, “Heterogeneous Photocatalyst Materials for Water Splitting” Chem. Soc. Rev., Vol. 38 (2009) pp. 253-278; X. Chen, S. Shen, L. Guo and S. S. Mao, “Semiconductor-based Photocatalytic Hydrogen Generation” Chem. Rev., Vol. 110, (2010) pp. 6503-6570; H. Tong, S. Ouyang, Y. Bi, N. Umezawa, M. Oshikiri and J. Ye, “Nano-photocatalytic Materials: Possibilities and Challenges” Adv. Mater., Vol. 24 (2012) pp. 229-251; Y. Quab and X. F. Duan, “Progress, Challenge and Perspective of heterogeneous photocatalysts”, Chem. Soc. Rev., Vol. 42 (2013) pp. 2568-2580; C. Huang, W. Yao, A. T. Raissi and N. Muradov, Development of efficient photoreactors for solar hydrogen production, Solar Energy, Vol. 85 (2011) pp. 19-27]. Particularly, 1-D TiO2 nanostructure with tubular and hollow space inside (porous) is of great potential for photocatalysis applications [D. V. Bavykin, V. N. Parmon, A. A. Lapkin and F. C. Walsh, “The effect of hydrothermal conditions on the mesoporous structure of TiO2 nanotubes”, J. Mater. Chem., Vol. 14 (2004) pp. 3370-337]. Such catalysts showed a large surface area, extended energy band potential and electron delocalization along the uni-directional axis. Enhancement of H2 production rate with TiO2 nanostructures modified with dopents, sensitizers, co-catalysts and scavengers were reported [M. V. Shankar and J. Ye, “Inorganic alkaline-sols as precursors for rapid synthesis of ETS-10 microporous titanosilicates and their photocatalytic reforming of methanol under visible-light irradiation” Cat. Comm., Vol. 11 (2009) pp. 261-265; J. Krishna Reddy, G. Suresh, C. H. Hymavathi, V. Durga Kumari and M. Subrahmanyam, “Ce(III) species supported zeolites as novel photocatalysts for hydrogen production from water”, Cat. Today. Vol. 141 (2009) pp. 89-93; Z. Jin, X. Zhang, Y. Li, S. Li and G. Lu, “5.1% Apparent quantum efficiency for stable hydrogen generation over eosin-sensitized CuO/TiO2 photocatalyst under visible light irradiation” Catal. Commun., Vol. 8 (2007) pp. 1267-1273; L. Zhang, B. Tian, F. Chen and J. Zhang, “Nickel sulfide as co-catalyst on nanostructured TiO2 for photocatalytic hydrogen evolution”, Int. J. Hydrogen Energy, Vol. 37 (2012) pp. 17060-17067; F. Guzman, S. S. C. Chuang and C. Yang, “Role of Methanol Sacrificing Reagent in the Photocatalytic Evolution of Hydrogen”, Ind. Eng. Chem. Res. Vol 52 (2013) pp. 61-65]. Copper based TiO2 photocatalysts showed efficient H2 generation superior to several noble-metal loaded TiO2 systems [X. Qiu, M. Miyauchi, H. Yu, H. Irie and K. Hashimoto, “Visible-Light-Driven Cu(II)-(Sr1-yNay)(Ti1-xMox)O3 Photocatalysts Based on Conduction Band Control and Surface Ion Modification”, J. Am. Chem. Soc., Vol. 132 (2010) pp. 15259-15267; H. Yu, H. Irie and K. Hashimoto, “Conduction Band Energy Level Control of Titanium Dioxide: Toward an Efficient Visible-Light-Sensitive Photocatalyst” J. Am. Chem. Soc., Vol. 132 (2010) pp. 6898-6899; S. Xu and D. D. Sun, “Significant improvement of photocatalytic hydrogen generation rate over TiO2 with deposited CuO”, Int. J. Hydrogen Energy Vol. 34 (2009) pp. 6096-6104; L. S. Yoong, F. K. Chong and B. K. Dutta, “Development of copper-doped TiO2 photocatalyst for hydrogen production under visible light”, Energy. Vol. 34 (2009) pp. 1652-1661; S. Xu, J. Ng, X. Zhang, X. Zhang, H. Bai and D. D. Sun, “Fabrication and comparison of highly efficient Cu incorporated TiO2 photocatalyst for hydrogen generation from water”, Int. J. Hydrogen Energy., Vol. 35 (2010) pp. 5254-5261; W. J. Foo, C. Zhang and G. W. Ho, “Non-noble metal Cu-loaded TiO2 for enhanced photocatalytic H2 production” Nanoscale, Vol. 5 (2013) pp. 759-764; W. Fan, Q. Zhang and Y. Wang, “Semiconductor-based nanocomposites for photocatalytic H2 production and CO2 conversion”, Phys. Chem. Chem. Phys., Vol. 15 (2013) pp. 2632-2649. a) S. Xu, J. Ng, A. J. Du, J. Liu and D. D. Sun, “Highly efficient TiO2 nanotube photocatalyst for simultaneous hydrogen production and copper removal from water”, Int. J. Hydrogen Energy, Vol. 36 (2011) pp. 6538-6545; H. Dang, X. Dong, Y. Dong, Y. Zhang and S. Hampshire, “TiO2 nanotubes coupled with nano-Cu(OH)2 for highly efficient photocatalytic hydrogen production”, Int. J. Hydrogen Energy, Vol. 38 (2013) pp. 2126-2135; S. S. Lee, H. Bai, Z. Liu and D. D. Sun, “Novel-structured electrospun TiO2/CuO composite nanofibers for high efficient photocatalytic cogeneration of clean water and energy from dye wastewater”, Wat. Res. Vol. 47 (2013) pp. 4059-4073]. They possess significant advantages in photo conversion efficiency and promote electron-hole separation via interfacial charge transfer process. Copper oxide has several benefits such as narrow band-gap, stability, affordability and abundantly available in nature. Notably few publications on copper oxide based nano-TiO2 exhibited high photocatalytic activity under solar light and UV light irradiation [K. Lalitha, G. Sadanandam, V. Durga Kumari, M. Subrahmanyam, B. Sreedhar and N.Y. Hebalkar, “Highly Stabilized and Finely Dispersed Cu2O/TiO2: A Promising Visible Sensitive Photocatalyst for Continuous Production of Hydrogen from Glycerol:Water Mixtures”, J. Phys. Chem. C Vol. 114 (2010) pp. 22181-22189; S. Xu, A. J. Du, J. Liu, J. Ng and D. D. Sun, “Highly efficient CuO incorporated TiO2 nanotube photocatalyst for hydrogen production from water”, Int. J. Hydrogen Energy Vol. 36 (2011) pp. 6560-6568].
Disadvantage of the prior art reported here, the excitation source of catalyst is non-renewable that is Ultra-Violet (UV) light emitted by Hg lamp, which is non-renewable, total process is expensive due to various factors viz., lamps cost, its recycling, electricity, and to maintain reaction temperature, besides its deleterious effects such as carcinogenic and environmental concerns. [H. Kato, K. Asakura, and A. Kudo, “Highly Efficient Water Splitting into H2 and O2 over Lanthanum-Doped NaTaO3 Photocatalysts with High Crystallinity and Surface Nanostructure”, J. Am. Chem. Soc., Vol. 125 (2003) pp. 3082-3089; A. Naldoni, M. D. Arienzo, M. Altomare, M. Marelli, R. Scotti, F. Morazzoni, E. Selli and V. D. Santo, “Pt and Au/TiO2 photocatalysts for methanol reforming: Role of metal nanoparticles in tuning charge trapping properties and photoefficiency” Appl. Catal. B, Vol. 130-131 (2013) pp. 239-248; K. Lalitha, J. Krishna Reddy, M. V. P. Sharma, V. Durga Kumari and M. Subrahmanyam, “Continuous hydrogen production activity over finely dispersed Ag2O/TiO2 catalysts from methanol:water mixtures under solar irradiation: A structure-activity correlation”, Int. J. Hydrogen Energy Vol. 35 (2010) pp. 3991-4001; G. Sadanandam, K. Lalitha, V. Durga Kumari, M. V. Shankar and M. Subrahmanyam, “Cobalt doped TiO2: A stable and efficient photocatalyst for continuous hydrogen production from glycerol: Water mixtures under solar light irradiation”, Int. J. Hydrogen Energy Vol. 38 (2013) pp. 9655-9664., D. Praveen Kumar, M. V. Shankar, M. Mamatha Kumari, G. Sadanandam, B. Srinivas, V. Durga Kumari, “Nano-size effects on CuO/TiO2 catalysts for highly efficient H2 production under solar light irradiation” Chem. Commun., Vol. 49 (2013) pp. 9443-9446].
The catalyst synthesis involves harsh conditions, wherein TiO2 (Degussa p-25) as precursor in 10M NaOH solution and heated at 150° C. for 48 h, with post-synthesis modification and 400° C. calcination temperature. The above TiO2 precursor reported particle size 25 nm and is widely reported catalyst for various photocatalytic applications. TiO2 (DegussP-25) cost INR 1500-1800 per kg, whereas TiO2 μm-size (Merck, India) cost only INR 700-800 per kg. Hence use of TiO2 (DegussP-25) is expensive one. For CuO/TiNT, they have reported 10 wt % of Cu/Ti ratio, that is very high compared to our report, 10% Methanol is used to improve photocatalytic performance. Moreover, methanol can be directly used as fuel in Direct Methanol Fuel Cell, which is more beneficial. Methanol is a well-known carcinogenic chemical and its usage is often restricted due to clean environmental concerns. The earlier report does not claim stability of the photocatalyst. In our report, we have used ambient conditions and natural solar (renewable energy) light as light source. We have synthesized TiO2 micron-size particles (Merck) in 10 M NaOH solution and hydrothermally heated at 130° C. for 20 h & it was post-synthesis calcined at 350° C. and on the whole overall less time and lower temperature compared to previous report, that saves considerable amount of electrical energy, environment and cost as well. We have used industrial by-product glycerol as hole scavenger for enhanced H2 production. Need of use of UV lamps which are cost effective for the process of hydrogen generation and their preparation involving adsorption and calcination method is also cost effective and reproducibility of catalyst and the recycle activity and leaching of copper are not mentioned. Whereas, the advantage of the present investigation is that metal loading by simple incipient wet impregnation and utilization of renewable source of energy i.e. natural Solar light. More over higher amount of hydrogen is produced than that of earlier report. No leaching of copper and 2 times recycle activity is observed.
SUMMARY OF THE INVENTION
The main objective of the present invention is to provide CuO—TiO2 nanocomposite photocatalyst for hydrogen production, process for the preparation thereof.
Another objective of the present invention is to provide a process for hydrogen production under solar light irradiation using CuO/TiO2 nanotubes photocatalyst.
Another objective of the present invention is to provide hydrogen production under solar light irradiation using CuO/TiO2 nanotubes photocatalyst from Glycerol: water mixture.
Yet another objective of the present invention is to synthesis of highly efficient nanostructured photocatalysts for hydrogen production under solar light irradiation using glycerol-water mixtures.
Yet another objective of the present invention is to utilize renewable solar light as excitation source for semiconductor photocatalyst.
Yet another objective of the present invention is to produce largest volume of hydrogen.
Accordingly, the present invention provides CuO—TiO2 nanocomposite photocatalyst which comprises of TiO2 nanotubes in the range of 98-99.9 wt % and CuO in the range of 0.1 to 2 wt %.
In an embodiment of the present invention TiO2 nanotube composed of bicrystalline anatase-rutile phase with tube length 300 to 400 nm and diameter 8-12 nm.
In one embodiment of the present invention CuO is deposited on TiO2 nanotubes surface in the form of quantum dots.
In another embodiment of the present invention size of CuO quantum dots is less than 10 nm.
In another embodiment of the present invention a method for the preparation of CuO—TiO2 nanocomposite photocatalyst, wherein the said process comprising the steps of;

    
    
        a) dispersing TiO2 μm-sized particles (TMP) into NaOH aqueous solution under magnetic stirring at temperature ranging between 25 to 35° C. for a period ranging between 0.5 to 2 h to obtain homogeneous suspension;
        b) heating homogeneous suspension as obtained in step (a) into an autoclave for a period ranging between 6 to 72 h at temperature ranging between 120 to 150° C. to obtain precipitate of TiO2 nanotube followed by washing with water, dilute HCl and ethanol in steps subsequently drying the precipitate at temperature ranging between 60 to 100° C. for a period ranging between 8 to 24 h then calcining TiO2 nanotube at temperature ranging between 300 to 400° C. for a period ranging 2 to 7 h to obtain calcined TiO2 nanotube;
        c) dispersing calcined TiO2 nanotube as obtained in step (b) into Cu(NO3)2.3H2O aqueous solution followed by evaporating excess water to dryness with slow heating and constant magnetic stirring subsequently drying at 100-150° C. for 8 to 16 h and calcining at 300-400° C. for 3-6 h range to obtain CuO—TiO2 nanocomposite photocatalyst.
    
    


Still in another embodiment of the present invention mol ratio of TiO2 nanotube and Cu(NO3)2.3H2O in step (c) used is in the range of 1.5 wt %.
Still in another embodiment of the present invention a process for H2 production from glycerol-water mixture using CuO—TiO2 nanocomposite photocatalyst as claimed in claim 1, wherein the said process comprises stirring CuO—TiO2 nanocomposite photocatalyst with glycerol-water mixture (5 vol. % (3 to 7 vol. %)) for a period ranging between 0.5 to 2 h under dark condition at temperature ranging between 28-34° C. followed by evacuation and purged with N2 gas subsequently stirring the solution under solar light for a period ranging between 1 to 4 h to obtain H2 gas.
Still in another embodiment of the present invention H2 production rate is in the range of 82,746 to 99,823 μmol·h−1·g−1 catalyst



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an X-ray diffraction pattern of pristine and CuO modified TiO2 nanotube. FIG. 1 shows X-ray diffraction patterns of TNP, TNT and CuTNT-4 materials that reveal the anatase-rutile mixed phase in TiO2 with characteristic diffraction peaks (101) and (110) at 2θ=25.4 and 27.5 respectively. The CuO/TiO2 photocatalyst showed intense peaks due to improved crystallinity and no characteristic peak exists for copper species due to fine dispersion of quantum sized particles.
FIG. 2 is a TEM image of (a) TiO2 nanotubes and (b) CuO/TiO2 nanotubes (c) TiO2 (Merck) particles with average particle size 01 μm (TMP) FIG. 2 (a) TEM image showing TNT hollow nature and open-ends on both sides. The lengths and diameters of tube are 300 to 400 nm and 8-12 nm respectively. TEM image (b) of CuTNT-4 showed clear evidence for CuO quantum dots (<10 nm) deposited on surface of nanotubes without damaging the tubular structure.
FIG. 3 is a DRS UV-Vis spectrum of TNP, TNT and CuTNT-4 photocatalysts. FIG. 3 shows diffuse reflectance UV-Vis spectrum of photocatalysts that reveals TNT absorption which is slightly blue shifted compared with TNP due to quantum size effects. This has pushed the band edges and increased effective band gaps. The CuTNT-4 catalyst appears pale yellow-blue colour and showed extended absorption towards visible region, Eg=3.05 eV due to CuO content. The observed shift is likely due to nanoheterojunction formed between CuO and TNT interface.
FIG. 4 is XPS spectrum of CuTNT-4 photocatalysts. In FIG. 4. The spectrum indicates the presence of Ti, O, Cu and C elements. The observed photoelectron peaks are at binding energies of 459.4 (Ti2p), 530.6 (O1s), 933.7 (Cu2p) and 284.6 eV (C1s). The observed atomic ratio of Ti to O is about 1:2, which is in good agreement with the nominal molecular composition of TiO2. These binding energies indicate that the oxidation state of copper present on the surface of TiO2 is +2. This is also in agreement with previous reports
FIGS. 5A and 5B are evaluations of photocatalytic activity for hydrogen generation using solar light irradiation. Catalyst amount from 0.003 to 0.100 g. FIG. 5A is evaluation of photocatalytic activity for hydrogen generation using TiO2 nanoparticles (TNP) under solar light irradiation. FIG. 5B is evaluation of photocatalytic activity for hydrogen generation using TiO2 tubes (TNT) under solar light irradiation. FIGS. 5A and 5B display that on both TNP&TNT volume of H2 generated with irradiation time and the best results are observed with 5 mg catalyst, where H2 production is maximum 4625 μmoles h−1 g−1cat was observed on TNT. The present results reveal that  ̃10% of catalyst amount i.e. 0.1 g L−1 is sufficient for efficient H2 production rates under solar light irradiation. This is the best active photocatalyst system for H2 production rate ever obtained using the smallest amount of photocatalyst. Similarly, earlier report demonstrates that doubled quantum yield at 0.5 to 0.05 g catalyst for highly efficient photocatalyst [H. Kato, K. Asakura, and A. Kudo, “Highly Efficient Water Splitting into H2 and O2 over Lanthanum-Doped NaTaO3 Photocatalysts with High Crystallinity and Surface Nanostructure”, J. Am. Chem. Soc., Vol. 125 (2003) pp. 3082-3089]. Thus, our invention indicates that our catalyst is well-dispersed at optimal catalyst amount resulting in effective light utilization for oxidation-reduction reactions.
FIG. 6 is effect of copper loading on photocatalytic activity of CuO/TiO2 nanotubes for hydrogen generation under solar light irradiation. FIG. 6 displays optimization of copper with TNT catalysts for efficient H2 generation. Amount of catalyst taken 100 mg. It is observed that increase in copper loading results in higher H2 production up to 1 wt %, beyond which the opposite effect was observed. Above the monolayer dispersion, agglomeration of copper species on nanotubes surface may produce large size CuO nanoparticles having low band potential, which is inefficient for H2 production besides light screening effect. At optimized conditions H2 production rate was found to be 9389 μmoles h−1g−1cat.
FIG. 7 is effect of copper loading on photocatalytic activity of CuO/TiO2 nanotubes for hydrogen generation under solar light irradiation. FIG. 7 displays optimization of copper with TNT catalysts for efficient H2 generation. Amount of catalyst taken 0.005 g. It is observed that increase in copper loading results in higher H2 production up to 1.5 wt %, beyond which the opposite effect was observed. Above the monolayer dispersion, agglomeration of copper species on nanotubes surface may produce large size CuO nanoparticles having low band potential, which is inefficient for H2 production besides light screening effect. At optimized conditions H2 production rate was found to be 99, 823 μmoles h−1g−1cat.
FIG. 8 is comparison of photocatalytic activity for hydrogen generation over TiO2 nanoparticles, TiO2 nanotubes and CuO/TiO2 nanotubes under solar light irradiation. FIG. 8 displays comparison of different photocatalysts for H2 generation under solar irradiation. The efficiency of photocatalytic H2 generation is in the order of CuTNT-4>TNT>TNP. In TNP photocatalyst, majority of the generated charge carriers undergo fast recombination and only a fraction of them utilized for H2 generation. The observed significant improvement in H2 evolution is likely due to delocalization of electrons along the axis of nanotube. CuTNT-4 exhibited largest H2 production as a cumulative effect of photogenerated electrons and efficient transfer of electrons to CuO sites followed by proton reduction. It is the best photocatalytic H2 evolution rate ever obtained on Ti-based catalysts which is about five times larger than the best reported values so far using glycerol as sacrificial agent. This has favourable band-edges that straddle the redox potential of water photo electrolysis and also exhibit stability and recyclability in the literature compared with similar earlier materials (Table 1).
FIG. 9 is sustainability of the photocatalyst for repeated use. FIG. 9 shows evaluation and sustainability of the photocatalysts tests which were also conducted by evacuating the produced gases at regular intervals (4 h). A continuous and stable photocatalytic activity is observed. The greater activity of CuTNT-4 is seen, compared to our earlier results on Cu2O/TiO2 composite is due to nanosized effects.
FIG. 10 is a pictorial illustration of band gap excitation and charge transfer processes in solar photocatalytic hydrogen generation. FIG. 10 depicts possible mechanism for highly efficient H2 production under solar irradiation using CuTNT-4 photocatalysts is as follows: The band potential of CuTNT-4 itself sufficient for protons reduction to H2 generation. The photogenerated electrons are transferred from CB of TiO2 nanotube to CuO nanoparticles, due to lower potential of the CB energy level. Accumulation of excess electrons at CuO causes negative shift in the Fermi level that leads to Cu2O formation. This facilitated interfacial electron transfers from Cu2O to H+ in solution resulting H2 production. Meanwhile accumulation of holes in the VB of CuO/Cu2O and TiO2 could be consumed by the sacrificial agent glycerol or generating hydroxyl radical (.OH) reaction with H2O molecules. Consequently, this process reduced the recombination of photogenerated electron-hole pairs and facilitated enhanced rate of H2 production.



DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to CuO—TiO2 nanocomposite photocatalyst for hydrogen production, process for the preparation thereof. Further, the present invention provides a process for hydrogen production in high yield than reported earlier under solar light irradiation using CuO/TiO2 nanotubes photocatalyst. Further, the invention is directed to synthesize nanostructured TiO2 based photocatalysts with desired morphology that are exhibiting improved conducting properties in overcome the rapid recombination of photogenerated charge carriers and their effective utilization in water splitting using glycerol as scavenger.
The present invention relates to synthesis of one dimensional TiO2 nanotube having tubular structure and hollow space having great potential in photocatalysis due to a large surface area, extended energy band potential and fast electron delocalization along the uni-directional axis which exhibits higher hydrogen production efficiency. Yet another aspect is TiO2 nanotube exhibits improved photocatalytic efficiency for hydrogen generation than TiO2 nanoparticles.
Another aspect of the invention is to use of non-noble metal as inorganic sensitizer as well as co-catalyst (dual role) for efficient solar light harvesting and also for enhanced hydrogen production. The non-noble metal changes its oxidation state from CuO to Cu2O under band gap irradiation.
Yet another aspect of the invention is to improve the photocatalytic efficiency of semiconductor nanocomposites with appropriate band potential in-turn to improve the oxidation-reduction reactions with glycerol-water mixture to generate hydrogen. The method involves non-noble metal as co-catalyst to enhance the charge transfer properties and increase the hydrogen production rate efficiency.
The other aspect of the photocatalytic efficiency improvement is band-gap tuning with narrow and wide band gap semiconductors nanocomposite that facilitates utilization of solar light with low energy photon harvesting. Copper plays dual role both as visible light sensitizer and as co-catalyst for enhanced hydrogen production.
Another aspect of the invention is a method of synthesis of the photocatalyst and it includes the steps: (a) The use of micron-sized TiO2 as precursor that do not exhibit any photocatalytic activity under solar light irradiation, (b) use of aqueous NaOH solution used as mineralizing agent, (c) stainless steel autoclave for hydrothermal synthesis conditions, (d) Thus the as synthesized material consists of amorphous and crystalline phases and on further calcination improves crystallinity
Another aspect of the invention is use of amount of photocatalyst for enhanced hydrogen production using TiO2 nanoparticles from 3 to 100 mg. The enhanced hydrogen production efficiency observed at lower quantities whereas, with increasing in amount of catalyst the hydrogen generation drastically affected.
Yet another aspect of the invention is that when the amount of the catalyst was varied from 0.003 to 0.100 g using calcined TiO2 nanotubes best photocatalytic activity for hydrogen generation was observed at optimal catalyst amount.
Yet another aspect of the invention is deposition of copper oxide on photocatalyst surface by wet impregnation method using Cu(NO3)2 from 0.1 to 5 wt %. The copper concentration (effect of copper loading) and its fine dispersion over titania nanotubes and its interaction with nanotube along the axis and inside the nanotube for enhanced hydrogen production. Beyond the optimized amount particle size of copper oxide increases bigger and that modified the band potential for hydrogen evolution.
Yet another aspect of the invention is characterization of photocatalyst for structure-activity relationship using different techniques such as XRD, TEM and XPS spectra.
Another aspect of the invention is photocatalytic experiments that include irradiating glycerol-water mixture under solar light irradiation and the hydrogen produced is analyzed off-line using gas chromatograph.
Yet another aspect of the invention is that among the reported TiO2-based photocatalysts as well as solar light active photocatalysts, CuO/TiO2 catalyst excited under UV-Visible band of solar light reports largest volume of hydrogen production H2=99,823 μmol·h−·g−1 catalyst.
Yet another aspect of the invention is to verify the stability and sustainability of the photocatalyst for hydrogen production. After photocatalytic activity test under solar light irradiation, the same system was kept under dark and degassed and purged with nitrogen before solar experiment for the 2nd day. The catalyst showed reproducible results for hydrogen production.
EXAMPLES
Following are the examples given to further illustrate the invention and should not be construed to limit the scope of the present invention.
Example-1
Synthesizing TiO2 Nano Tube (TNT)
In a typical synthesis process, TiO2 (Merck) 0.1 μm average sized particles (TMP) (2.5 g) was dispersed into 10 M NaOH (200 mL) aqueous solution under magnetic stirring for 1 h at 32±2° C. The homogeneous suspension was transferred into 250 mL teflon-lined autoclave and fitted with nuts and bolts. Then, the autoclave was kept in hot air oven at 130° C. for 20 h. The autoclave was removed from the oven and cooled-down to room temperature under tap water flow. At room temperature the autoclave was opened, discarded supernatant solution from white precipitate. Thus obtained white precipitate was subjected to washing in 3 steps under magnetic stirring for 2 hours and each step repeated twice: the precipitate was washed in distilled water, followed by 0.1 M HCl and absolute ethanol. Further, washed precipitate was subjected to drying in oven at 80° C. for 12 h and calcined at 350° C. for 5 h @ 2° C./min.
The X-ray diffraction (XRD) patterns (FIG. 1) of Cu-modified TiO2 nanotube catalysts (Cu-TNT-4) were recorded with Siemens D-5000 X-ray diffract meter using Cu Kα radiation. A Philips Technai G2 FEI F12 transmission electron microscope operating at 80-100 kV was used to record the transmission electron microscopy (TEM) patterns (FIG. 2). The Diffuse Reflectance UV-Visible spectra (FIG. 3) were recorded on a GBC UV-visible Cintra 10e spectrometer, in the wavelength 200-800 nm range. X-ray photoelectron spectra (FIG. 4) (XPS) were recorded on a KRATOS AXIC 165 equipped with Mg Kα radiation. All binding energies were referenced to C1s at 284.8 eV. BET surface area and pore size distribution were determined at liquid nitrogen temperature (77 K) using a Micromeritics ASAP 2010 system.
Example-2
Preparation of CuO Dispersed TiO2 NT
Wet impregnation method of preparation was used for CuO dispersion on TiO2 NT (CuTNT-4). For each Cu modified sample, required amount of TiO2 nanotube (0.5 g) was dispersed into Cu(NO3)2.3H2O (0.028 g, 1.5 wt %, 10 mL water) concentration aqueous solution for 1 h at 110±2° C. Excess water was evaporated to dryness with slow heating and constant magnetic stirring. The sample was dried at 110° C. for at least 12 h and calcined at 350° C. for 5 h.
The X-ray diffraction (XRD) patterns (FIG. 1) of Cu-modified TiO2 nanotube catalysts were recorded with Siemens D-5000 X-ray diffract meter using Cu Kα radiation. A Philips Technai G2 FEI F12 transmission electron microscope operating at 80-100 kV was used to record the transmission electron microscopy (TEM) patterns (FIG. 2). The Diffuse Reflectance UV-Visible spectra (FIG. 3) were recorded on a GBC UV-visible Cintra 10e spectrometer, in the wavelength 200-800 nm range. X-ray photoelectron spectra (FIG. 4) (XPS) were recorded on a KRATOS AXIC 165 equipped with Mg Kα radiation. All binding energies were referenced to C1s at 284.8 eV. BET surface area and pore size distribution were determined at liquid nitrogen temperature (77 K) using a Micromeritics ASAP 2010 system.
Example-3
Photocatalytic Hydrogen Production from TiO2 NP (TNP) and TiO2 NT (TNT) Using Hole Scavenger Under Solar Irradiation
The photocatalytic H2 production experiments were carried out in a sealed quartz reactor (volume: 150 ml) using industrial by-product as scavenger at ambient temperature and pressure under natural solar irradiation on the terrace of Nanocatalysis Research Lab YVU Kadapa. TiO2 nanoparticles and TiO2 nanotubes were used separately as photocatalysts in this study. Powdered photocatalyst was suspended in 5 vol. % glycerol-water mixture (50 mL). In order to have better adsorption by the reaction mixture, it was magnetically stirred for 1 h at 32±2° C. under dark condition by covering with aluminium foil followed by evacuation and purged with N2 gas Further, solar Photocatalytic experiments were conducted after removal of aluminium foil and kept on a four point magnetic stirrer (up to 4 quartz reactors can be accommodated for solar experiments) to ensure homogeneity of the suspension during reaction. Thus produced gases were collected at fixed intervals (every 1 h) and analysed using an off-line Gas Chromatograph with TCD detector (Shimadzu GC-2014 with Molecular Sieve/5 A) using N2 as a carrier gas. (FIG. 5A and FIG. 5B)
Example-4
Photo Catalytic Hydrogen Production from CuO Dispersed on TiO2 NT (Cu-TNT-4)
The photocatalytic H2 production experiments (FIG. 6) were carried out in a sealed quartz reactor (volume: 150 ml) using industrial waste glycerol as scavenger at ambient temperature and pressure under natural solar irradiation on the terrace of Nanocatalysis Research Lab YVU Kadapa.
Powdered photocatalyst (0.005 g) was suspended in 5 vol. % glycerol water mixture (50 mL). In order to have better adsorption by the reaction mixture, it was magnetically stirred for 1 h at 32±2° C. under dark condition by covering with aluminium foil followed by evacuation and purged with N2 gas. Further, solar Photocatalytic experiments were conducted after removal of aluminium foil and kept on a four point magnetic stirrer (up to 4 quartz reactors can be accommodated for solar experiments) to ensure homogeneity of the suspension during reaction. FIG. 6 displays optimization of copper with TNT catalysts for efficient H2 generation. It is observed that increase in copper loading results in higher H2 production up to 1.5 wt %, beyond which the opposite effect was observed. Above the monolayer dispersion, agglomeration of copper species on nanotubes surface may produce large size CuO nanoparticles having low band potential, which is inefficient for H2 production besides light screening effect. At optimized conditions H2 production rate was found to be 99, 823 μmoles h−1 g−1cat. Thus produced gases were collected at fixed intervals (every 1 h) and analysed using an off-line Gas Chromatograph with TCD detector (Shimadzu GC-2014 with Molecular Sieve/5 A) using N2 as a carrier gas.
Example-5
Photo Catalytic Hydrogen Production Using Hole Scavenger Over CuO TiO2 NT (Cu-TNT-4) for Under Solar Irradiation
The photo catalytic H2 production experiments (FIG. 7) were carried out in a sealed quartz reactor (volume: 150 ml) using industrial waste glycerol as scavenger at ambient temperature and pressure under natural solar irradiation on the terrace of Nanocatalysis Research Lab YVU Kadapa.
Powdered photocatalyst (0.1 g) was suspended in 5 vol. % glycerol-water mixture (50 mL). In order to have better adsorption by the reaction mixture, it was magnetically stirred for 1 h at 32±2° C. under dark condition by covering with aluminium foil followed by evacuation and purged with N2 gas. Further, solar Photocatalytic experiments were conducted after removal of aluminium foil and kept on a four point magnetic stirrer (up to 4 quartz reactors can be accommodated for solar experiments) to ensure homogeneity of the suspension during reaction. FIG. 7 displays optimization of copper with TNT catalysts for efficient H2 generation. It is observed that increase in copper loading results in higher H2 production up to 1 wt %, beyond which the opposite effect was observed. Above the monolayer dispersion, agglomeration of copper species on nanotubes surface may produce large size CuO nanoparticles having low band potential, which is inefficient for H2 production besides light screening effect. At optimized conditions H2 production rate was found to be 9,389 μmoles h−1 g−1cat. Thus produced gases were collected at fixed intervals (every 1 h) and analysed using an off-line Gas Chromatograph with TCD detector (Shimadzu GC-2014 with Molecular Sieve/5 A) using N2 as a carrier gas.
Table 1 below is comparison of H2 production rates. Table. 1 shows comparison of hydrogen production rates with reported photocatalyst. It is clear that highest amount of hydrogen is reported in this invention under solar light irradiation compared to all the reports. Further, under similar conditions, the CuO/TiO2 nanotubes (Cu-TNT-4) exhibited nearly 5 times higher efficiency in comparison with Cu2O/TiO2 nanoparticles.











TABLE 1





Sl.


Light
Rate of Hydrogen


No
Photocatalyst
Scavenger
Source
μmol · h−1 · g−1cat



















1.
NiO/NaTaO3:La
Methanol
Hg
38400


2.
Pt/TiO2
Methanol
Hg
21350


3.
CuO/TiO2 nanotube
Methanol
Hg
71600


4.
Ag2O/TiO2
Methanol
Solar
3350


5.
Cu2O/TiO2
Glycerol
Solar
20060


6.
CoO/TiO2
Glycerol
Solar
11021


7.
CuO/TiO2 nanotube
Glycerol
Solar
99823



(Cu-TNT-4)









ADVANTAGES OF THE INVENTION


    
    
        A novel nano composite photocatalyst composed of cheap, earth abundant and eco-friendly materials such as TiO2 and CuO.
        The reaction conditions and experimental procedure for synthesis and processing of TiO2 nanotube, CuO/TiO2 nanocomposites are novel in the present investigation.
        The solar photocatalytic activity measurements involve optimization of catalyst amount for highly efficient hydrogen production is novelty in the present investigation.
        Utilization of bio-diesel industry by-product (about 10 wt %) glycerol as a cheap hole scavenger, for environment friendly potentially economical process.
        The bi-crystalline nature of TiO2 nanotube (Anatase+Rutile mixture), its one dimensional morphology, influence of CuO as co-catalyst and solar light harvesting from UV-A and Visible light showed synergetic effects for enhanced H2 production.

## Claims

1. A process for H2 production from glycerol-water mixture using a CuO—TiO2 nanocomposite photocatalyst comprising TiO2 nanotubes in the range of 98-99.9 wt % and CuO in the range of 0.1 to 2 wt %, wherein the said process comprises stirring the CuO—TiO2 nanocomposite photocatalyst with a glycerol-water mixture (3 to 7 vol. %) for a period ranging between 0.5 to 2 h under dark conditions at a temperature ranging between 28-34° C. followed by evacuation and purged with N2 gas subsequently stirring the solution under solar light for a period ranging between 1 to 4 h to obtain H2 gas.

2. The process as claimed in claim 1, wherein H2 production rate is in the range of 82,746 to 99,823 μmol·h−1·g−1 catalyst.