EP1646443A2

EP1646443A2 - Composites of zinc phthalocyanine and titanium oxide, for use in photocatalytical processes, and method for their obtention

Info

Publication number: EP1646443A2
Application number: EP04726392A
Authority: EP
Inventors: Antônio Eduardo da Hora MACHADO
Original assignee: Conselho Nacional de Desenvolvimento Cientifico e Tecnologico - CNPQ; Conselho Nacional Desenvolvimento Cientifico e Tecno PROJUR/CNPq; Sattler Christian; Deutsches Zentrum fuer Luft und Raumfahrt eV
Current assignee: CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTIFICO E; DE MIRANDA, JACQUES ANTONIO; Deutsches Zentrum fuer Luft und Raumfahrt eV
Priority date: 2003-04-11
Filing date: 2004-04-08
Publication date: 2006-04-19
Also published as: BR0300920A; WO2004089525A2; EP1646443A4; BR0300920B1; WO2004089525A3

Abstract

The composites object of this invention are catalysts for photochemical processes which aims the environmental decontamination, being their characteristic a very superior photocatalytical efficiency when compared with the observed for pure titanium oxides. The composites, object of the invention, are a combination of TiO2 with a photosensitiser dye, capable to mediate photocatalytical processes using the incident radiation in wavelength ranges incapable to promote the excitation of the pure photocatalyst. This occurs due to the electronic excitation of the dye in these regions of the electromagnetic spectrum. The excited photosensitiser dye promotes electron transfer to the conduction band of the catalyst, potencializing the photocatalytical action. With this, the wastewater treatment with the use of solar radiation becomes interesting, due to the captation of useful photons in a large range of the electromagnetic spectrum and energy conversion from these composites.

Description

COMPOSITES OF ZINC PHTHALOCYANINE AND TITANIUM OXIDE, FOR

USE IN PHOTOCATALYTICAL PROCESSES, AND METHOD FOR THEIR

OBTENTION

Object of the invention Chemical catalysts are known by their application to make feasible alternative routes of synthesis, development of new materials, and environmental remediation.

The present invention consists of composites prepared from a combination of titanium oxide with a photosensitiser dye, capable to mediate electron transfer reactions, potencialising the photocatalyticai action of the titanium oxide.

These composites, obtained as the method described in this document, serve as catalysts for the decontamination, by heterogeneous photocatalysis, of . wastewater (industrial effluents, etc.) that contains substances of low or none biodegradability.

They are also useful in remotion of environmental odour.

Description of the state-of- technique

Between the prospected solutions to approach the environmental contamination, stand out the termed Advanced Oxidative Processes (AOP)_s by the action of the hydroxyl radical as oxidising agent.

Differently from traditional processes, the AOP are capable to introduce profound changes in the chemical structure of the contaminants, resulting in the destruction or inviabilization of the polluting charge.

The AOP have potential application in: • pre-treatment of contaminants resistant to biodegradation;

• treatment of previously treated wastewaters.

The photocatalytical processes are conceived in the class of Advanced

Oxidative Processes.

Semiconductor oxides have been employed as catalysts in photochemical processes aiming the environmental decontamination. These processes can be mediated as by solar radiation as the use of artificial radiation, generally in the ultraviolet region.

Titanium dioxide and zinc oxide have been pointed as the more attractive compounds for this function, by the low cost, environmentally harmless and easily recoverable. They have been used with success in the elimination of non-biodegradable pollutants in aquatic environments, in the reduction of the pollutant charge of industrial effluents, as in the odour elimination in closed places.

Studies involving the elimination of organic compounds point to a substantial reduction of the pollutant charge, obtained in a short time interval, including, in many situations, the complete mineralization of the organic matter. The use of photocatalysis in the remotion of inorganic pollutants is also a fact. On the other hand, titanium dioxide (TiO₂) is recognised as the most efficient photocatalytical agent for environmental applications.

The photocatalytical technology for water detoxification, based on TiO₂, needs to be viable, of electromagnetic radiation with wavelengths lower than 385 nm. The wastewater treatment using photocatalysis and solar radiation under these conditions is limited, however, by the low intensity of the solar radiation in the range between 300 and 385 nm, since that, at the sea level, it corresponds to not more than 5% of the incident solar radiation. On the other hand, the electron transfer from a photosensitiser to a semiconductor has been object of intense investigation due to its potential use in photovoltaic cells.

The solar energy photovoltaic conversion can be considered one of the few sustainable options to provide the demand of electric energy in the future. The technique, based in the combination of titanium dioxide with organic dyes, was developed in Switzerland, and has been explored in laboratory scale, in several countries. In these solar cells, the photosensitiser dye absorbs photons from solar light, injecting electrons into the matrix of titanium oxide. The system can be compared to the natural model of photosynthesis, in which the photosensitiser dye is the chlorophyll. The system consists in a porous structure of titanium oxide nanocrystals, with the dye adsorbed as a monomolecular layer being electronically excited; the dye transfers electrons to the titanium oxide. These electrons diffuse by the matrix, going to the external circuit. Distinction between the invention and the state-of- technique

The composites objects of this invention are catalysts for photochemical processes that aim environmental decontamination, being also possible to extend their application to photovoltaic cells. It is their characteristic a photocatalytic efficiency higher than the observed for pure titanium oxides.

A mixture of anatase and rutile, known as P25, for example, is an excellent photocatalyst for degradation of different types of contaminants, and thus was taken as reference. The composites, object of the invention, are a combination of TiO₂ and a photosensitiser dye.

These combinations make them capable to mediate photocatalytical processes using the incident radiation in wavelength ranges incapable to promote the excitation of the pure photocatalyst. This occurs due to the electronic excitation of the dye in these regions of the electromagnetic spectrum. The excited photosensitiser dye promotes electron transfer to the conduction band of the catalyst, potencializing the photocatalytical action. Thus, the wastewater treatment with the use of solar radiation becomes to be interesting, considering the increased use of photons due to the absorption of solar radiation in an enlarged range by these composites. Ti0₂ absorbs ultraviolet light lower than 385 nm, creating an excess of electrons in its conduction band (e^" _bC) and holes in the valence band (h⁺t_>v). Table 1. Quantum yield of hvdroxy radicals generation for some of the composites

This parameter furnishes an estimate of the efficiency of the photocatalytical process, since that the hydroxy radical can be considered the principal active specie, due to its high reactivity.

Then, if the quantum yield of hydroxy radicals generation shows to be superior to the observed during the action of the pure photocatalyst (between

0,030 and 0,050), must be expected a superior photocatalytic action for the composite, implying in a significative increase of the degradation rate of the contaminants.

Description of the preferred procedure to realise the invention The first produced composites were prepared from dissolution of zinc phthalocyanine in dimethylsulfoxide and posterior addition of titanium dioxide, under stirring and heating. The mixture was maintained under stirring until the partial evaporation of the solvent.

After that, it was maintained in an oven until its complete dryness. However, besides the little uniformity reached, a great loss of material was observed. This procedure was considered inadequate.

The composites invented are prepared, by a very efficient and reproducible way, as the following summarised description:

- The mixture photosensitiser dye/titanium oxide is done after previous dissolution of the photosensitiser (zinc phthalocyanine) in concentrated sulphuric acid; after that the necessary amount of titanium oxide (P25) to prepare the composite in one of the in mass compositions (1.0%, 2.5%, 5.0%,

7.5% and 10.0%) is gradually added to obtain a homogeneous mass; following, distilled water is added; thus, the mass is vigorously stirred until all the solid material be dispersed; the composite is, then, precipitated in the aqueous medium and the excess of acid is neutralised with a 0.100 mol.dm^"3 sodium hydroxide solution.

After that, the mixture rests at 70°C by 24 hours. The water is removed, and the precipitate is washed to remove the salts formed during the neutralisation of the remaining acid. Finally, the composite is dried at 80°C.

The final product is a finely divided bluish powder, insoluble in water, but capable to give sufficiently stable suspensions in this solvent.

The dye maintains its spectroscopic and photophysical properties, adsorbed to the surface of the semiconductor oxide, and indeed after its remotion, which can be done with the aid of certain organic solvents, as dimethylformamide or dimethylsulfoxide. However, the association between the dye and the semiconductor oxide enlarges the photocatalytical action. The diffuse reflectance spectrum of two of these composites shows that the dye absorbs efficiently radiation with wavelength above 500 nm, where the sun presents elevated spectral irradiance.

Figure 1 , annex, exhibits the diffuse reflectance spectra: (a) anatase; (b) P25; (c) composite with 2.5% m/m of dye; (d) composite with 5.0% m/m of dye. The comparison with the anatase or P25 spectra reveals that the composite preserve the spectroscopic characteristics of the photosensitizing dye, favouring the use of radiation with wavelength higher than 500 nm, with the consequent improvement of the photocatalytic activity. The surface area of the composites tends to be lower than the verified for P25, used as reference as previously mentioned. As example, we present the following surface areas: P25/ZnPc 10% m/m: 32,40 m²/g P25: 50 m /g However, this diminish in the surface area is compensated by the action of the photosensitiser.

The manner by which the dye is adsorbed to the surface of the semiconductor oxide is fundamental for stabilising of the better efficiency observed for the composite when compared to the pure semiconductor. Considering the efficiency profile observed for the composites, is very probable that the molecules of zinc phthalocyanine be ordered in an intercalated way on the surface of titanium oxide, do not compromising the usual processes of the photocatalyst.

The combination between photosensitiser and solid semiconductor results in a synergistic effect of the two processes: the trigged by the catalyst (semiconductor oxide) and by the dye.

A view of this can be obtained by the analysis of Table 1 : the saturation of the surface with the dye tends to reduce the capability of the composite in generate hydroxy radicals, although the quantum yields of generation of hydroxy radicals be superior to the observed for the pure semiconductor oxide. In other words, at a certain level of dye adsorption is very probable that the observed effect - the photocatalytical action - occurs almost exclusively by the injection of electrons from the photosensitising dye, to the conduction band of the semiconductor oxide.

Use of the composites in environmental decontamination In the treatment of an effluent model constituted of lignosulfonate fragments in aqueous medium, has been observed a significative increase in the degradation efficiency, as mentioned previously, obtaining degradation rates between 2 and 2.5 times the observed with only the semiconductor oxide.

As the results show, is fundamental that the incident radiation have also components of the visible spectrum: the association P25/photosensitiser (10% (m/m)) made the degradation of the organic matter only about five percent faster, under the action of only UV-A radiation, when compared with the results obtained using pure P25.

Also exemplificative of the obtained results with the invented composites is Figure 2, annex, on the degradation of the organic matter present in the effluent of a paper and cellulose industry, using TiO₂, compared with the degradation promoted by the semiconductor oxide: (a) P25; (b) P25/photosensitiser. Experimental Conditions: 50 mg TiO₂ /L; Initial Chemical Oxygen Demand = 400 ± 50 mg/L; addition of 1 mL H₂O₂/L Temperature = 318 K.

However, using the composite with 5% m/m of the photosensitiser, with the incidence of solar radiation, with a mean UV-A irradiance of 50 W/m², it was observed an increment of about 2 times in the degradation rate, when compared to the performance of P25, and 2.5 times when compared to the 99,9%, in the degradation of a mixture of lignosulfonate fragments, as can be seen in the following figure, demonstrating the superiority and the higher efficiency of the invented composites. Thus, it is demonstrated that the composites are capable to promote an extended degradation of the studied models in the same time interval considered for the two other photocatalysts employed (P25; anatase). This behaviour can be explained by the action of the photosensitiser dye, viabilising the use of other components of the solar spectrum in the activation of the semiconductor oxide.

Figure 3, attached, ilustrates the degradation of lignosulphonate fragments using solar radiation: (a) anatase; (b) P25; (c) composite at 5% m/m. A ₇₆ is the absorbance of the photolysed solution at 276 nm.

Treatment of aqueous suspensions and gaseous effluents The composites can be applied as aqueous suspensions associated to the effluent to be treated, or fixed to the internal surface of the photochemical reactor. In this last application, can be used for the treatment of liquid or gaseous effluents, depending on the geometry of the reactor.

As example, the treatment of considerable amounts of liquid effluents under the action of solar radiation can be done with the use of CPC (Compound Parabolic Collector) - like reactors, using the catalyst in suspension. The aqueous suspensions containing the composite are prepared by addition of a compatible amount to the mixture to be treated. This can be made under vigorous stirring of the suspension, maintained during all photocatalytic process, to warrant a uniform distribution of the catalyst in the medium. Concerning to the fixation of the catalyst over a surface, this can be made by deposition or reaction with the surface (derivatisation). The procedure of deposition, with the formation of a thin film of the composite, preserves its photocatalytical properties. However, this system is more adequate for the treatment of gaseous mixtures, since that for liquid effluents the film tends to shed from surface. For treatment of liquid mixtures, the derivatisation of the composite in the internal surface of the reactor is the alternative.

It is still possible, in the scope of this invention, the use of other photosensitisers associated to the titanium oxide (P25), above all, these as zinc phthalocyanine, that possess low solubility in water, that present photophysical activity near to that observed for zinc phthalocyanine, like as example other metalated phthalocyanines, representing an alternative possibility for the production of new composites in conformity with the invented here.

Claims

CLAIMS 1.- COMPOSITES OF ZINC PHTHALOCYANINE AND TITANIUM OXIDE, FOR USE IN PHOTOCATALYTICAL PROCESSES, AND METHOD FOR THEIR OBTEMTION, characterized by the preparation of the composites from the combination of titanium oxide and a photosensitiser dye capable to mediate electron transfer reactions and potencialise the photocatalytical action of titanium oxide.

2.- COMPOSITES OF ZINC PHTHALOCYANINE AND TITANIUM OXIDE, FOR USE IN PHOTOCATALYTICAL PROCESSES, AND METHOD FOR THEIR OBTENTION, characterized by the electronic excitation of a photosensitising dye, that promotes the electron transfer for the conduction band of the catalyst, potencialysing the photocatalytical action by which the decontamination of wastewaters or the remotion of odour of surroundings can be reached.

3.- COMPOSITES OF ZINC PHTHALOCYANINE AND TITANIUM OXIDE, FOR USE IN PHOTOCATALYTICAL PROCESSES, AND METHOD FOR THEIR OBTENTION, as claims 1 or 2, above, characterized by the photosensitising dye be zinc phthalocyanine.

4.- COMPOSITES OF ZINC PHTHALOCYANINE AND TITANIUM OXIDE, FOR USE IN PHOTOCATALYTICAL PROCESSES, AND METHOD FOR THEIR OBTENTION, as claims 1 or 2, above, characterized by the use of other photosensitisers that possess low solubility in water, exemplified by metalated phthalocyanines.

5.- COMPOSITES OF ZINC PHTHALOCYANINE AND TITANIUM OXIDE, FOR USE IN PHOTOCATALYTICAL PROCESSES, AND METHOD FOR THEIR OBTENTION, characterized by the composites be obtained by the previous dissolution of the photosensitiser in concentrated sulphuric acid, and posterior and gradual addition of the necessary amount of titanium oxide until the obtention of a homogeneous mass; following, distilled water is added, and the mass vigorously stirred with resources and adequate means until the solid material be adequately dispersed; after, the suspension is leave to rest, to precipitate the composite in the aqueous medium; following, the neutralisation of the excess of acid in the composition is done by the use of sodium hydroxide in the proportion of 0.100 mol x dm^'3 ; the mixture is leave to rest for 24 (twenty four) hours, at a constant temperature of about 70^s C; after that, the aqueous phase is removed and the precipitate is washed with distilled water, to remove the salts formed during the neutralisation of the acid and, finally, is leave to dry, at a constant temperature of about 80^s C, until the complete evaporation of the remaining liquid.

6.- COMPOSITES OF ZINC PHTHALOCYANINE AND TITANIUM OXIDE, FOR USE IN PHOTOCATALYTICAL PROCESSES, AND METHOD FOR THEIR OBTENTION, characterized by the composites be used as aqueous suspensions associated to the effluent to be treated.

7.- COMPOSITES OF ZINC PHTHALOCYANINE AND TITANIUM OXIDE, FOR USE IN PHOTOCATALYTICAL PROCESSES, AND METHOD FOR THEIR OBTENTION, characterized by the composites be used fixed to the internal surfaces of photochemical reactors, for treatment of liquid or gaseous effluents.

8.- COMPOSITES OF ZINC PHTHALOCYANINE AND TITANIUM OXIDE, FOR USE IN PHOTOCATALYTICAL PROCESSES, AND METHOD FOR THEIR OBTENTION, characterized by the composites be used as aqueous suspensions, in CPC-like (Compound Parabolic Collector) reactors, added by means of vigorous stirring to the liquid effluents to be treated, maintained during the period in which occurs the photocatalysis.

9.- COMPOSITES OF ZINC PHTHALOCYANINE AND TITANIUM OXIDE, FOR USE IN PHOTOCATALYTICAL PROCESSES, AND METHOD FOR THEIR OBTENTION, as claim 5, above, characterized by the fixation of the catalyst to a surface be made or by deposition to the surface, or by reaction with the surface (derivatisation).

10.- COMPOSITES OF ZINC PHTHALOCYANINE AND TITANIUM OXIDE, FOR USE IN PHOTOCATALYTICAL PROCESSES, AND METHOD FOR THEIR OBTENTION, characterized be used as catalysts for wastewater decontamination and remotion of odour of surroundings, using heterogeneous photocatalysis.