Three-part nano-catalyst and use thereof for photocatalysis

Abstract

Disclosed is a nanocatalyst-type nanoscale composition including a nanoparticle semiconductor, plasmonic metal nanoparticles and an organic photosensitiser of the carbo-mer type. Also disclosed is a method for producing such a nano-catalyst. Also disclosed is use of the nanocatalyst for photoelectrolysis, in particular, for the photoelectrolysis of water, as well as to a power source including the nanocatalyst.

Claims

1. A three part nano-catalyst comprising: a semiconductor in nanoparticulate or nanorod form; nanoparticles of plasmonic metal; and an organic photo-sensitizer that is a carbo-mer.

2. The nano-catalyst according to claim 1, wherein the semiconductor in nanoparticulate or nanorod form is a metal oxide.

3. The nano-catalyst according to claim 1, wherein the plasmonic metal is gold, silver, copper, aluminium or platinum.

4. The nano-catalyst according to claim 1, wherein the carbo-mer is a carbo-benzene.

5. The nano-catalyst according to claim 1, wherein the nanoparticles of plasmonic metal are located on the surface of the semiconductor in nanoparticulate or nanorod form.

6. The nano-catalyst according to claim 1, wherein the semiconductor in nanoparticulate or nanorod form, and/or the nanoparticles of plasmonic metal are coated with the photosensitizer.

7. A method for fabricating a three-part nano-catalyst according to claim 1 comprising the following steps: (1a) mixing a semiconductor in nanoparticulate or nanorod form with an organic photosensitizer; (1b) mixing the composition obtained at step (1a) with a complex comprising an ion of a plasmonic metal; (2) irradiating the composition obtained at step (1b) under electromagnetic radiation.

8. The method according to claim 7, wherein the complex comprising an ion of the plasmonic metal is an amidinate or carboxylate of silver, gold, copper, aluminium or platinum.

9. A process for producing hydrogen comprising applying an effective amount of the three-part nano-catalyst according to claim 1.

10. A power supply device, comprising a three-part nano-catalyst according to claim 1.

11. The three-part nano-catalyst of claim 1, wherein the organic photo-sensitizer is a carbo-benzene or carbo-n-butadiene.

12. The three-part nano-catalyst of claim 2, wherein the semiconductor in nanoparticulate or nanorod form is selected from the group consisting of: tin oxide, indium oxide, gallium oxide, tungsten oxide, copper oxide, nickel oxide, cobalt oxide, iron oxide, zinc oxide and titanium oxide.

13. The method of claim 7, wherein the step (1b) is followed by an agitation step (1c).

14. The method of claim 7, wherein the electromagnetic radiation of step (2) is sunlight.

15. The nano-catalyst according to claim 1, wherein the carbo-mer is selected from the group consisting of 4-[10-(4-aminophenyl)-4,7,13,16-tetraphenylcyclooctadeca-1,2,3,7,8,9,13,14,15-nonaen-5,11,17-triyn-1-yl]aniline or 4,4′ ((4,7,13,16-tetraphenylcyclooctadeca-1,2,3,7,8,9,13,14,15-nonaen-5,11,17-triyne-1,10-diyl)bis(ethyne-2,1-diyl))dianiline.

16. The three-part nano-catalyst of claim 11, wherein the semiconductor in nanoparticulate or nanorod form is selected from the group consisting of: tin oxide, indium oxide, gallium oxide, tungsten oxide, copper oxide, nickel oxide, cobalt oxide, iron oxide, zinc oxide and titanium oxide.

17. The nano-catalyst according to claim 11, wherein the carbo-mer is selected from the group consisting of 4-[10-(4-aminophenyl)-4,7,13,16-tetraphenylcyclooctadeca-1,2,3,7,8,9,13,14,15-nonaen-5,11,17-triyn-1-yl]aniline or 4,4′ ((4,7,13,16-tetraphenylcyclooctadeca-1,2,3,7,8,9,13,14,15-nonaen-5,11,17-triyne-1,10-diyl)bis(ethyne-2,1-diyl))dianiline.

18. The nano-catalyst according to claim 11, wherein the plasmonic metal is gold, silver, copper, aluminium or platinum.

19. The nano-catalyst according to claim 11, wherein the nanoparticles of plasmonic metal are located on the surface of the semiconductor in nanoparticulate or nanorod form.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1 is a schematic showing the general UV-Visible absorption spectrum of the molecule of carbo-benzene type, illustrated in the schematic.

(2) FIG. 2 is a photograph showing TEM observations of nano-objects formed after irradiation under UV only (Example 2a-).

(3) FIG. 3 is a photograph showing TEM observations of nano-objects formed after irradiation under UV only at D+134 (Example 2a-).

(4) FIG. 4 is a photograph showing TEM observations of nano-objects formed after irradiation in the UV+Visible ranges (Example 2b-).

(5) FIG. 5 is a photograph showing TEM observations of nano-objects formed after irradiation in the UV+Visible ranges at D+134 (Example 2b-).

(6) FIG. 6 is a photograph showing TEM observations of nano-objects formed after irradiation in the Visible range only (Example 2c-).

(7) FIG. 7 is a photograph showing TEM observations of nano-objects formed after irradiation in the Visible range only at D+134 (Example 2c-).

(8) FIG. 8 is a photograph showing TEM observations on the formation of nano-objects after 3-hour irradiation in the Visible range only (Example 4).

(9) FIG. 9 is a photograph showing the .sup.1H NMR spectrum with T2 filter of the gaseous phase after 45-min irradiation in the UV+Visible ranges (Example 4).

(10) FIG. 10 is a photograph showing observation under HRTEM of the nano-objects at D+20 (Example 4).

(11) FIG. 11 is a photograph showing EDX analysis performed under HRTEM of the nano-objects at D+20 (Example 4).

(12) FIG. 12 is a photograph showing analysis of the diffraction configuration of a nanoparticle (NP) of Ag deposited on the surface of a NP of ZnO derived from the nano-objects at D+20 (Example 4).

(13) FIG. 13 is a graph giving the trend in the production of dihydrogen in gaseous phase as a function of irradiation time, when photo-reducing water, the reaction being catalyzed by nanorods of zinc oxide/1% carbo-benzene/3% silver; by particles of Aeroxide P25 titanium oxide particles/2% carbo-benzene/1% silver; by particles of Aeroxide P25 titanium oxide/2% carbo-benzene/2% silver; by particles of Aeroxide P25 titanium oxide/1% carbo-benzene/1% silver; by nanorods of titanium oxide/1% carbo-benzene/1% silver, or by particles of Degussa P25 titanium oxide/2% carbo-benzene/1% silver.

EXAMPLES

(14) The present invention will be better understood on reading the examples below illustrating but not limiting the invention.

(15) Abbreviations

(16) NP: nanoparticle;

(17) F-P: Fisher-Porter;

(18) BAG: Glove-box;

(19) TEM: Transmission Electron Microscopy;

(20) HRTEM: High Resolution Transmission Electron Microscopy;

(21) PS: photosensitizer.

(22) Material

(23) The semiconductor in nanoparticulate state used was composed of nanoparticles (NPs) of commercial ZnO (nano-powder, size<100 nm, Sigma-Aldrich).

(24) The plasmonic nanoparticles used were NPs of silver derived from photo-reduction of a silver amidinate complex, silver (N,N′-diisopropylacetamidinate), obtained with the method developed by Gordon [Lim, B. S.; Rahtu, A.; Park, J.-S.; Gordon, R. G., Inorg. Chem., 2003, 42(24), 7951-7958].

(25) The organic photosensitizer used (PS), of carbo-benzene type, was the compound «4,4′((4,7,13,16-tetraphenylcyclooctadeca-1,2,3,7,8,9,13,14,15-nonaen-5,11,17-triyne-1,10-diyl)bis(ethyne-2,1-diyl))dianiline», of formula:

(26) ##STR00004##

(27) obtained following the synthesis method developed by the team led by R. Chauvin for a similar compound [Rives, A.; Baglai, I; Malytskyi, V.; Maraval, V.; Saffon-Merceron, N.; Voitenko, Z.; Chauvin, R. Chem. Commun., 2012, 48, 8763-8765].

(28) The UV-Visible absorption spectrum of this compound is given in FIG. 1.

Example 1: Fabrication of a Two-Part Nano-Catalyst, Without Organic Photosensitizer Method

(29) 1/60 mg of commercial ZnO were degassed in a small Fisher-Porter (F-P) bottle, then placed in a glovebox (BAG).

(30) 2/20 mg of silver amidinate complex in solution in 5 mL of dry, degassed toluene were added. This amount corresponds to 20 atomic % of Ag relative to the atoms of Zn in ZnO.

(31) 3/A white precipitate was observed in the solution, which was subjected to UV radiation (100 W Mercury lamp) for 2 h.

(32) 4/After manual agitation, occurrence of a yellow suspension. Observation under TEM.

(33) At a second test, procedure was carried out with an exposure time to UV radiation of 30 min and under agitation. A yellow supernatant was also observed in this case.

(34) Results

(35) The TEM images of the complex obtained at steps 1/ to 4/show NPs of Ag distributed over the carbon film of the microscope screen, indicating that there remains silver amidinate complex in the reaction medium that has not reacted. Also, observation of the yellow supernatant indicates that NPs of Ag have been formed in solution and not on the surface of ZnO.

(36) Direct photo-reduction of the silver amidinate complex via UV radiation of the NPs of ZnO does not therefore allow growth of the NPs of Ag in the absence of a ligand.

(37) An organic molecule («ligand») acting as stabilizing agent is needed to stabilize the formed NPs of Ag. In Examples 2 to 4, an organic photosensitizer (of carbo-benzene type) was used which fulfils this role of ligand in the fabrication method of the nano-catalyst.

Example 2: Fabrication of a Three-Part Nano-Catalyst in the Presence of UV and/or Visible Radiation (5% Aci, 5% carbo-benzene)

(38) Method

(39) 1/2.15 mg of commercial ZnO were degassed in a small F-P bottle, then placed in a glovebox BAG.

(40) 2/1.0 mL of carbo-benzene in solution in dry, degassed toluene (1.0 mg/mL) were added. This amount of carbo-benzene (1.0 mg) corresponds to 5 mole % of carbo-benzene relative to the molar amount of Zn in ZnO.

(41) 3/The resulting mixture was left under agitation at ambient temperature in the glovebox BAG for 1 h.

(42) 4/5 mL of dry, degassed toluene were added.

(43) 5/In parallel a 0.36 mg/mL solution of silver amidinate was prepared from 18 mg solubilized in 50 mL of dry, degassed toluene. This amount corresponds to 5 atomic % of Ag relative to the Zn atoms in ZnO.

(44) 6/6 mL of this solution were placed in a Schlenk tube containing the previously prepared solution of NPs of ZnO+carbo-benzene.

(45) 7/a—The solution obtained after step 5/was illuminated under UV for 1 h (100 W Mercury lamp).

(46) 8/a—TEM analysis was performed immediately after this operation (FIG. 2) then at D+134 (FIG. 3).

(47) Steps 1/to 6/were repeated after which:

(48) 7/b—The solution obtained after step 5/was placed in sunlight (UV+Visible ranges) for several hours.

(49) 8/b—TEM analysis was performed immediately after this operation (FIG. 4) then at D+134 (FIG. 5).

(50) Steps 1/to 6/were repeated, after which:

(51) 7/c—The solution obtained after step 5/was placed in sunlight in a UV-filtered clean room (Visible range only) for several hours.

(52) 8/c—TEM analysis was performed immediately after this operation (FIG. 6) then at D+134 (FIG. 7).

(53) Results

(54) TEM images show that, irrespective of the irradiation source (Visible range only FIGS. 6 and 7; UV range only FIGS. 2 and 3; UV+Visible range, FIGS. 4 and 5), a deposit of Ag NPs occurs on the surface of the NPs of ZnO. These NPs of Ag have a size of approximately 8 nm±1 nm. The TEM images also show that carbo-benzene organizes itself into the form of an organic layer which can be seen on the surface of the NPs of Ag and on the surface of the NPs of ZnO.

(55) It is known that ZnO in the nanoparticulate state and under UV radiation (□≈350 nm) produces electron-hole pairs. The electron and hole migrate towards the surface of ZnO to be used in reduction and oxidation reactions respectively.

(56) This experiment showed four effects of the carbo-benzene used as photosensitizer (PS): (1) formation of a colloidal solution of the NPs of ZnO in the solvent used (here dry, degassed toluene). In the absence of carbo-benzene, the NPs of ZnO are found in the form of a suspension in the solvent used and not in colloidal form; (2) stabilization of the NPs of Ag formed after photo-reducing the silver amidinate complex. Contrary to the observation made in the absence of carbo-benzene (Example 1), here all the NPs of Ag are in contact with the surface of the NPs of ZnO and not isolated; (3) formation of a protective organic layer on the surface of the NPs of ZnO and of the NPs of Ag; (4) generation of electron/hole pairs within the ZnO after Visible radiation (whether or not associated with UV radiation). Procedures 2a-, 2b- and 2c-, which were performed under different radiation conditions (UV only, Visible only, and UV+Visible), did not exhibit any notable difference regarding the formation of NPs of Ag via photo-reduction of silver amidinate. Conversely, in the absence of photosensitizer, the NPs of ZnO only directly create electron/hole pairs under UV radiation.

Example 3: Fabrication of a Three-Part Nano-Catalyst in the Absence of UV Radiation (1% Ag, 1% Carbo-Benzene)

(57) Method

(58) 1/2.1 mg of commercial ZnO were degassed in a small F-P bottle and placed in a glovebox BAG. 2/0.2 mL of carbo-benzene solution in solution in dry, degassed toluene (1.0 mg/mL) with 1 mL of dry, degassed toluene were added. This amount of carbo-benzene (0.2 mg) corresponds to 1 mole % of carbo-benzene relative to the molar amount of Zn in ZnO.

(59) 3/In parallel, a 0.36 mg/mL solution of silver amidinate was prepared from 18 mg in 50 mL of dry, degassed toluene. This amount corresponds to 1 atomic % of Ag relative to the Zn atoms in ZnO.

(60) 4/0.4 mL of this solution were placed in the small F-P bottle containing the previously prepared solution of NPs of ZnO+carbo-benzene, followed by the addition of 1 mL of dry, degassed toluene.

(61) 5/A septum was placed on the small F-P bottle for later sampling and the bottle wrapped in inactinic paper (to allow filtering of UV radiation).

(62) 6/The solution was exposed to the luminosity of a clean room (UV-filtered room). A sample of the solution was taken at regular time intervals for TEM observation: 30 min, 1 h, 3 h, 20 h.

(63) Results

(64) The TEM images of the different samples of solution show that on and after 30 min, there occurs formation of the NPs of Ag via photo-reduction of the silver amidinate complex. Nevertheless, NPs of Ag alone can also be seen on the carbon film of the TEM screen indicating that one portion of the silver amidinate complex has not reacted.

(65) Observations at 1 h and 3 h show identical results.

(66) On the other hand, after 20-hour irradiation in the Visible range only, there are no longer any NPs of Ag alone on the carbon film of the microscope screen, indicating that the entirety of the silver amidinate has been photo-reduced.

(67) As in Example 2, all the NPs of Ag observed are located on the surface of the NPs of ZnO, and the carbo-benzene organizes itself in the form of an organic layer visible on the surface of the NPs of Ag and on the surface of the NPs of ZnO.

(68) These results show that carbo-benzene indeed acts as photosensitizer for ZnO under Visible radiation. Since the NPs of ZnO are unable directly to produce electron/hole pairs (since the UV range has been filtered), it is necessarily the carbo-benzene which absorbs

(69) Visible radiation (having regard to its profile under UV-Visible spectroscopy, the absorption maximum lies at λ.sub.max=493 nm) and which transfers radiation energy to the NPs of ZnO so that the latter produce electron/hole pairs which will participate in the photo-reduction of the silver amidinate complex.

(70) Contrary to the results in Example 2, irradiation lasting between 3 h and 20 h is needed for complete formation of the NPs of Ag. Several reasons can explain this difference: procedure 3 was performed in winter, at a time of year when sunshine hours are considerably reduced and the intensity of solar radiation is low; there remained free amidinate ligand in the sample of silver amidinate, which could have been oxidized or reduced instead of the complex itself; there was no «impregnation» step of the carbo-benzene around NPs of ZnO, i.e. agitation for 1 h between the NPs of ZnO alone and the carbo-benzene did not take place, contrary to the method in Example 2.

Example 4: Fabrication and Use of a Three-Part Nano-Catalyst (1% Aq, 1% carbo-benzene) in the Absence of UV Radiation for the Photocatalytic Production of Hydrogen

(71) Method

(72) 1/2.15 mg of commercial ZnO were degassed in a small F-P bottle and placed in a glovebox BAG.

(73) 2/0.20 mL of carbo-benzene solution in dry, degassed toluene (1.0 mg/mL) were added with 0.8 mL of dry, degassed toluene. This quantity of carbo-benzene (0.20 mg) corresponds to 1 mole % of carbo-benzene relative to the molar amount of Zn in ZnO.

(74) 3/The mixture was agitated at ambient temperature in the glovebox BAG for 15 min.

(75) 4/In parallel, a 0.36 mg/mL solution of silver amidinate was prepared from 18 mg in 50 mL of dry, degassed toluene. This quantity corresponds to 1 atomic % of Ag relative to the atoms of Zn in ZnO.

(76) 5/0.4 mL of this solution were placed in the small F-P bottle containing the previously prepared solution of NP ZnO+carbo-benzene, to which were added 3.6 mL of dry, degassed toluene (total volume of the solution: 5 mL).

(77) 6/The F-P bottle was wrapped in inactinic paper (to allow filtering of UV radiation).

(78) 7/The solution was irradiated with a light source in the Visible range only (Xenon lamp, 100 W equipped with a filter blocking out solely UV radiation) under magnetic stirring for 3 h.

(79) 8/Observation under TEM was carried out to monitor the formation of the nano-objects and complete photo-reduction of the silver amidinate (FIG. 8).

(80) 9/The solution was then concentrated and transferred to pressure-resistant NMR tube.

(81) 10/The solvent was entirely evaporated in the NMR tube which was placed in an inert atmosphere with 3 vacuum/argon cycles. 11/200 microlitres of distilled water degassed under a stream of argon were added, the tube placed under argon pressure (100 mbar) and irradiated with a light source emitting in the UV+Visible ranges (Xenon lamp, 100 W) for 45 min.

(82) 12/The presence of dihydrogen H.sub.2 was monitored by gas phase .sup.1H NMR spectroscopy with application of a T2 relaxation filter T2 (FIG. 9).

(83) 13/Observation was carried out under HRTEM (High Resolution TEM, FIG. 10), followed by EDX analysis at D+20 (FIG. 11) and representation of the diffraction configuration of a NP of Ag deposited on the surface of a NP of ZnO at D+20 of the nano-objects formed (FIG. 12).

(84) Results

(85) Monitoring of irradiation for the formation of NPs of Ag (FIG. 8) shows that the photo-reduction of the silver amidinate complex takes place after 3-hour irradiation.

(86) Photo-reduction of water was followed by gas phase .sup.1H NMR with application of a T2 relaxation filter allowing cancellation of the water signal in vapour phase (saturation vapour pressure). Two narrow signals were observed after application of this T2 filter: one corresponding to dihydrogen H.sub.2 (δ≈5 ppm) and the other corresponding to a non-identified, mobile protonated gas species (δ≈4 ppm) (FIG. 9).

(87) Observations under HRTEM of the nano-objects at D+20 showed that the NPs of Ag are deposited either directly on the NPs of ZnO, or on the organic layer of carbo-benzene (FIG. 10).

(88) EDX analyses allowed evidencing of the formation of NPs of Ag on the surface of ZnO (FIG. 11).

(89) Analyses of the diffraction configuration of a NP of Ag deposited on the surface of a NP of ZnO (FIG. 12) show that the deposited nanoparticles of silver are not oxidized in contact with water. This observation allows envisaging of overall photoelectrolysis of water i.e. reduction of water (formation of gaseous dihydrogen H.sub.2) and oxidation of water (formation of gaseous dioxygen O.sub.2) at the same time. Here by «overall photoelectrolysis of water» is meant the chemical breakdown of water, leading to the simultaneous formation of gaseous dihydrogen H.sub.2 and dioxygen O.sub.2.

Example 5: Fabrication and Use of Three-Part Nano-Catalysts in the Absence of UV Radiation for the Photocatalytic Production of Hydrogen

(90) The Applicant has synthesized several three-part nano-catalysts following the protocol described in Example 2, by adapting the quantities of carbo-benzene and silver and/or by substituting the particles of zinc oxide by other metal oxides. The compositions of these three-part nano-catalysts are given in the following table.

(91) TABLE-US-00002 Ref. Metal oxide Nano- Size Quantity mole % carbo- mole % catalyst Type Form (nm) (mg) benzene (Cb) silver N1 ZnO Particulate <100 100 1 0 N2 ZnO Particulate <100 100 1 1 N3 ZnO Particulate <100 100 1 3 N4 ZnO Particulate <100 100 1 5 N5 ZnO Particulate <50 100 1 3 N6 ZnO Rod diameter × 100 1 3 length: 50 nm × 300 nm N7 TiO.sub.2 Particulate 23 100 1 0 N8 TiO.sub.2 Particulate 23 100 1 1 N9 TiO.sub.2 Particulate 23 100 1 3 N10 TiO.sub.2 Particulate 23 100 1 5 N11 TiO.sub.2 Particulate 23 100 2 1 N12 TiO.sub.2 Particulate 23 100 2 3 N13 TiO.sub.2 Particulate 23 100 3 3 N14 TiO.sub.2 Rod diameter × 100 1 3 length: 10 nm × 10 μm N15 TiO.sub.2 Particulate <50 100 2 3 N16 CuO Particulate <50 100 1 3 N17 Fe.sub.2O.sub.3 Particulate <50 100 1 3 N18 NiO Particulate <50 100 1 3 N19 WO.sub.3 Particulate <50 100 1 3

(92) These three-part nano-catalysts were characterized before and after use thereof as catalyst, with one or more of the following techniques: transmission electron microscopy (TEM), high resolution TEM (HRTEM), solid phase UV/Visible spectroscopy, fluorescent X-ray spectroscopy (FluoX), X photoelectronic spectroscopy «XPS», infrared spectroscopy «IR», Raman spectroscopy or nuclear magnetic resonance «NMR».

(93) The three-part nano-catalysts were employed for catalysis with the following protocol. In a quartz rector of 135 ml capacity, 30 ml of distilled water and 30 mg of nano-catalyst were mixed and agitated at ambient temperature. The volume of the gas phase was 105 ml. Irradiation was performed with a Xenon UV/Visible lamp of 300-Watt power equipped with an optical fibre.

(94) Monitoring of catalysis (rate of hydrogen production as a function of irradiation time) was carried out by sampling the gas phase every 6 hours.

(95) The results (FIG. 13) show that the best results are obtained with the three-part nano-catalysts TiO.sub.2/2% carbo-benzene/3% silver.

(96) In addition, the Applicant measured the production rate of dihydrogen in gas phase using different catalysts after 84-hour irradiation. The results are given in the following table:

(97) TABLE-US-00003 Ref. Rate of Nano- production of H.sub.2 catalyst Type of nano-catalyst (μmol .Math. h.sup.−1 .Math. g.sup.−1) — ZnO (<100 nm) — N1 ZnO (<100 nm)/1% Cb 5 .Math. 10.sup.−3 N2 ZnO (<100 nm)/1% Cb/1% Ag 12.2 × 10.sup.−3 N3 ZnO (<100 nm)/1% Cb/3% Ag 17.2 × 10.sup.−3 N4 ZnO (<100 nm)/1% Cb/5% Ag 6 .Math. 10.sup.−3 N6 ZnO nanorods (diameter × length: 50 nm × 0.029 300 nm)/1% Cb/3% Ag — TiO.sub.2 P25 Aeroxide ®  7.9 × 10.sup.−3 N7 TiO.sub.2 P25 Aeroxide ®/1% Cb 0.015 N8 TiO.sub.2 P25 Aeroxide ®/1% Cb/1% Ag 0.085 N9 TiO.sub.2 P25 Aeroxide ®/1% Cb/3% Ag 0.41 N10 TiO.sub.2 P25 Aeroxide ®/1% Cb/5% Ag — N11 TiO.sub.2 P25 Aeroxide ®/2% Cb/1% Ag 0.5 N12 TiO.sub.2 P25 Aeroxide ®/2% Cb/3% Ag 2.2 N13 TiO.sub.2 P25 Aeroxide ®/3% Cb/3% Ag 1.4 N14 TiO.sub.2 P25 Degussa/2% Cb/3% Ag 1.34 N15 TiO.sub.2 nanorods (diameter × length: 10 nm × 2.45 10 μm)/1% Cb/3% Ag

(98) The results show that, even after 84-hour irradiation, the nano-catalysts of the invention still remain active.

(99) In general: the nano-catalysts comprising titanium oxide are more active than those comprising zinc oxide; the nano-catalysts in nanorod form are more active than those in nanoparticulate form; when particulate nano-catalysts are employed, the best results are obtained with nano-catalysts comprising 2% carbo-benzene; the most active three-part nano-catalyst is composed of TiO.sub.2 nanorods/1% carbo-benzene/3% silver.