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

Abstract

The present investigation is development of the TiO.sub.2 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 (H.sub.2=99,823 μmol.Math.h.sup.−1.Math.g.sup.−1 catalyst) from glycerol-water mixtures under solar light irradiation. A method is disclosed to produce CuO/TiO.sub.2 nanotubes with high sustainability and recyclable activity for hydrogen production.

Claims

1. A process for H.sub.2 production from glycerol-water mixture using a CuO—TiO.sub.2 nanocomposite photocatalyst comprising TiO.sub.2 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—TiO.sub.2 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 N.sub.2 gas subsequently stirring the solution under solar light for a period ranging between 1 to 4 h to obtain H.sub.2 gas.

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

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is an X-ray diffraction pattern of pristine and CuO modified TiO.sub.2 nanotube. FIG. 1 shows X-ray diffraction patterns of TNP, TNT and CuTNT-4 materials that reveal the anatase-rutile mixed phase in TiO.sub.2 with characteristic diffraction peaks (101) and (110) at 2θ=25.4 and 27.5 respectively. The CuO/TiO.sub.2 photocatalyst showed intense peaks due to improved crystallinity and no characteristic peak exists for copper species due to fine dispersion of quantum sized particles.

(2) FIG. 2 is a TEM image of (a) TiO.sub.2 nanotubes and (b) CuO/TiO.sub.2 nanotubes (c) TiO.sub.2 (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.

(3) 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, E.sub.g=3.05 eV due to CuO content. The observed shift is likely due to nanoheterojunction formed between CuO and TNT interface.

(4) 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 TiO.sub.2. These binding energies indicate that the oxidation state of copper present on the surface of TiO.sub.2 is +2. This is also in agreement with previous reports

(5) 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 TiO.sub.2 nanoparticles (TNP) under solar light irradiation. FIG. 5B is evaluation of photocatalytic activity for hydrogen generation using TiO.sub.2 tubes (TNT) under solar light irradiation. FIGS. 5A and 5B display that on both TNP&TNT volume of H.sub.2 generated with irradiation time and the best results are observed with 5 mg catalyst, where H.sub.2 production is maximum 4625 μmoles h.sup.−1 g.sup.−1.sub.cat was observed on TNT. The present results reveal that ˜10% of catalyst amount i.e. 0.1 g L.sup.−1 is sufficient for efficient H.sub.2 production rates under solar light irradiation. This is the best active photocatalyst system for H.sub.2 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 H.sub.2 and O.sub.2 over Lanthanum-Doped NaTaO.sub.3 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.

(6) FIG. 6 is effect of copper loading on photocatalytic activity of CuO/TiO.sub.2 nanotubes for hydrogen generation under solar light irradiation. FIG. 6 displays optimization of copper with TNT catalysts for efficient H.sub.2 generation. Amount of catalyst taken 100 mg. It is observed that increase in copper loading results in higher H.sub.2 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 H.sub.2 production besides light screening effect. At optimized conditions H.sub.2 production rate was found to be 9389 μmoles h.sup.−1g.sup.−1.sub.cat.

(7) FIG. 7 is effect of copper loading on photocatalytic activity of CuO/TiO.sub.2 nanotubes for hydrogen generation under solar light irradiation. FIG. 7 displays optimization of copper with TNT catalysts for efficient H.sub.2 generation. Amount of catalyst taken 0.005 g. It is observed that increase in copper loading results in higher H.sub.2 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 H.sub.2 production besides light screening effect. At optimized conditions H.sub.2 production rate was found to be 99, 823 μmoles h.sup.−1g.sup.−1.sub.cat.

(8) FIG. 8 is comparison of photocatalytic activity for hydrogen generation over TiO.sub.2 nanoparticles, TiO.sub.2 nanotubes and CuO/TiO.sub.2 nanotubes under solar light irradiation. FIG. 8 displays comparison of different photocatalysts for H.sub.2 generation under solar irradiation. The efficiency of photocatalytic H.sub.2 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 H.sub.2 generation. The observed significant improvement in H.sub.2 evolution is likely due to delocalization of electrons along the axis of nanotube. CuTNT-4 exhibited largest H.sub.2 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 H.sub.2 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).

(9) 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 Cu.sub.2O/TiO.sub.2 composite is due to nanosized effects.

(10) 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 H.sub.2 production under solar irradiation using CuTNT-4 photocatalysts is as follows: The band potential of CuTNT-4 itself sufficient for protons reduction to H.sub.2 generation. The photogenerated electrons are transferred from CB of TiO.sub.2 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 Cu.sub.2O formation. This facilitated interfacial electron transfers from Cu.sub.2O to H.sup.+ in solution resulting H.sub.2 production. Meanwhile accumulation of holes in the VB of CuO/Cu.sub.2O and TiO.sub.2 could be consumed by the sacrificial agent glycerol or generating hydroxyl radical (.OH) reaction with H.sub.2O molecules. Consequently, this process reduced the recombination of photogenerated electron-hole pairs and facilitated enhanced rate of H.sub.2 production.

DETAILED DESCRIPTION OF THE INVENTION

(11) The present invention relates to CuO—TiO.sub.2 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/TiO.sub.2 nanotubes photocatalyst. Further, the invention is directed to synthesize nanostructured TiO.sub.2 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.

(12) The present invention relates to synthesis of one dimensional TiO.sub.2 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 TiO.sub.2 nanotube exhibits improved photocatalytic efficiency for hydrogen generation than TiO.sub.2 nanoparticles.

(13) 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 Cu.sub.2O under band gap irradiation.

(14) 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.

(15) 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.

(16) Another aspect of the invention is a method of synthesis of the photocatalyst and it includes the steps: (a) The use of micron-sized TiO.sub.2 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

(17) Another aspect of the invention is use of amount of photocatalyst for enhanced hydrogen production using TiO.sub.2 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.

(18) 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 TiO.sub.2 nanotubes best photocatalytic activity for hydrogen generation was observed at optimal catalyst amount.

(19) Yet another aspect of the invention is deposition of copper oxide on photocatalyst surface by wet impregnation method using Cu(NO.sub.3).sub.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.

(20) Yet another aspect of the invention is characterization of photocatalyst for structure-activity relationship using different techniques such as XRD, TEM and XPS spectra.

(21) 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.

(22) Yet another aspect of the invention is that among the reported TiO.sub.2-based photocatalysts as well as solar light active photocatalysts, CuO/TiO.sub.2 catalyst excited under UV-Visible band of solar light reports largest volume of hydrogen production H.sub.2=99,823 μmol.Math.h.sup.−.Math.g.sup.−1 catalyst.

(23) 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 2.sup.nd day. The catalyst showed reproducible results for hydrogen production.

EXAMPLES

(24) 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)

(25) In a typical synthesis process, TiO.sub.2 (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.

(26) The X-ray diffraction (XRD) patterns (FIG. 1) of Cu-modified TiO.sub.2 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 10.sub.e 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

(27) Wet impregnation method of preparation was used for CuO dispersion on TiO.sub.2 NT (CuTNT-4). For each Cu modified sample, required amount of TiO.sub.2 nanotube (0.5 g) was dispersed into Cu(NO.sub.3).sub.2.3H.sub.2O (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.

(28) The X-ray diffraction (XRD) patterns (FIG. 1) of Cu-modified TiO.sub.2 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 10.sub.e 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

(29) The photocatalytic H.sub.2 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. TiO.sub.2 nanoparticles and TiO.sub.2 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 N.sub.2 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 N.sub.2 as a carrier gas. (FIG. 5A and FIG. 5B)

Example-4

Photo Catalytic Hydrogen Production from CuO Dispersed on TiO2 NT (Cu-TNT-4)

(30) The photocatalytic H.sub.2 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.

(31) 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 N.sub.2 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 H.sub.2 generation. It is observed that increase in copper loading results in higher H.sub.2 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 H.sub.2 production besides light screening effect. At optimized conditions H.sub.2 production rate was found to be 99, 823 μmoles h.sup.−1 g.sup.−1.sub.cat. 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 N.sub.2 as a carrier gas.

Example-5

Photo Catalytic Hydrogen Production Using Hole Scavenger Over CuO TiO2 NT (Cu-TNT-4) for Under Solar Irradiation

(32) The photo catalytic H.sub.2 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.

(33) 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 H.sub.2 generation. It is observed that increase in copper loading results in higher H.sub.2 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 H.sub.2 production besides light screening effect. At optimized conditions H.sub.2 production rate was found to be 9,389 μmoles h.sup.−1 g.sup.−1.sub.cat. 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 N.sub.2 as a carrier gas.

(34) Table 1 below is comparison of H.sub.2 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/TiO.sub.2 nanotubes (Cu-TNT-4) exhibited nearly 5 times higher efficiency in comparison with Cu.sub.2O/TiO.sub.2 nanoparticles.

(35) TABLE-US-00001 TABLE 1 Sl. Light Rate of Hydrogen No Photocatalyst Scavenger Source μmol .Math. h.sup.−1 .Math. g.sup.−1.sub.cat 1. NiO/NaTaO.sub.3:La Methanol Hg 38400 2. Pt/TiO.sub.2 Methanol Hg 21350 3. CuO/TiO.sub.2 nanotube Methanol Hg 71600 4. Ag.sub.2O/TiO.sub.2 Methanol Solar 3350 5. Cu.sub.2O/TiO.sub.2 Glycerol Solar 20060 6. CoO/TiO.sub.2 Glycerol Solar 11021 7. CuO/TiO.sub.2 nanotube Glycerol Solar 99823 (Cu-TNT-4)

ADVANTAGES OF THE INVENTION

(36) A novel nano composite photocatalyst composed of cheap, earth abundant and eco-friendly materials such as TiO.sub.2 and CuO. The reaction conditions and experimental procedure for synthesis and processing of TiO.sub.2 nanotube, CuO/TiO.sub.2 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 TiO.sub.2 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 H.sub.2 production.