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Semiconductor photocatalysis: an environmentally acceptable alternative production technique and effluent treatment process
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Semiconductor photocatalysis: an environmentally acceptable alternative production technique and effluent treatment process Peter K.J. Robertson School of Applied Sciences, The Robert Aberdeen, AB25 lHG, Scotland Received
28 June
1996; accepted
Gordon
20 November
University,
St Andrew
Street,
1996
New environmentally acceptable production methods are required to help reduce the environmental impact of many industrial processes. One potential route is the application of photocatalysis using semiconductors. This technique has enabled new environmentally acceptable synthetic routes for organic synthesis which do not require the use of toxic metals as redox reagents. These photocatalysts also have more favourable redox potentials than many traditional reagents. Semiconductor photocatalysis can also be applied to the treatment of polluted effluent or for the destruction of undesirable by-products of reactions. In addition to the clean nature of the process the power requirements of the technique can be relatively low, with some reactions requiring only sunlight. 0 1997 Elsevier Science Ltd Keywords: photocatalysis;
photoelectrochemistry;
Introduction The generation of hazardous industrial effluents is a serious problem experienced by nations throughout the European Union. In recent years there has been an increase in legislation within Europe aimed at protecting the environment. Directives such as the Urban Waste Water Directive, the Fish and Shellfish Directives, the Integrated Pollution Prevention and Control Directive and the Dangerous Substances Directives have all emphasised the importance of developing new effective and environmentally acceptable production techniques and waste treatment systems. The problem of effluent generation may be approached in two manners. Firstly new production technologies could be adopted which could reduce effluent generation by adopting catalytic processes which are non-polluting. The other approach would be the application of novel waste treatment technologies. Techniques such as electrochemistry have been applied to both these requirements with some success.’ There remains, however, a need for new processes to reduce the overall environmental impact of industrial synthetic chemistry. Sectors such as the pharmaceutical, textile, agricultural, food, and chemical industries all produce waste effluent contaminated with organic compounds such as aromatics, haloaromatics, aliphatics, dyes, dioxins and
effluent treatment;
clean technology
a wide range of other polluting materials. Many of these materials are extremely toxic and the waste streams must be treated prior to discharge. Traditional waste treatment systems have involved the use of techniques such as coagulation, chlorination or ozonation which utilise potentially hazardous or polluting materials. Chlorination presents a particular problem since it will often generate mutagenic or carcinogenic by-products when used to treat water contaminated with organic compounds. Techniques such as granular activated carbon adsorption or air stripping processes have drawbacks as the methods are non-destructive.* The adsorbing media loaded with the waste must therefore, also be treated as a hazardous material. Incineration of organic waste is not always effective and can disperse large quantities of toxic emissions, such as products of incomplete combustion or heavy metals, into the atmosphere. An effective treatment system is required which can degrade the polluting materials prior to discharge of the effluent. The photocatalytic process is a technique which could address both these requirements. This process involves the utilisation of a non-toxic semiconductor catalyst such as titanium dioxide (Ti02). When semiconductors are illuminated with light of an appropriate wavelength they become powerful oxidants which will convert most organic materials to carbon dioxide and water. This process has been very effective in
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‘WWhv~Eb
Figure 1 Promotion of an electron from the valence band to the conductance band on illumination of a semiconductor
destroying a wide range of organic materials typical of those generated by the industries mentioned above3. Semiconductor photocatalysis can also be applied to functional group transformations such as oxidations and reductions4 negating the requirement for potentially toxic redox reagents such as chromium or nickel compounds. Following their application for a particular reaction the semiconductor photocatalysts may be recycled for further use. This paper will review some of the various applications of semiconductor photocatalysis to date. Before discussing the applications of semiconductor photocatalysis it will be necessary to describe the basic principles of the photocatalytic process.
The interaction of semiconductors
with light
Semiconductor particles may be photoexcited to form electron donor sites (reducing sites) and electron acceptor sites (oxidising sites), providing great scope as redox reagentP. The molecular orbitals of semiconductors have a band structure. The bands of interest in photocatalysis are the occupied valence band and the unoccupied conductance band. They are separated by an energy distance referred to as the band gap (Ebg). When the semiconductor is illuminated with light of greater energy than that of the band gap, an electron is promoted from the valence band to the conductance band. This leaves a positive hole in the valence band as illustrated in Figure 1. Scheme 1 shows the possible reactions that can occur when a solution containing a semiconductor (SC) absorbs a photon (hv) of a suitable wavelengtl?. The Light Absorption
SC+hu-e-+
Recombiion
a+h+
Oxidation
D +h+ -
Reduction
A + e’-
Photocatalytic conversions using semiconductor materials Semiconductor photocatalysis has already been applied to a wide range of different chemical reactions with both organic and inorganic substrates. Examples of these reactions are oxidations, reductions, rearrangements, isomerisations, cleavages and polymerisations4~8. Unfortunately it is only possible to briefly describe some of these reactions in this review. Other reviews on these reactions are available and further detailed information may be sought there4*7~9*Lo*’ ‘. In addition to the environmental advantages of the photocatalytic synthetic route there are other advantages to this technique4. Firstly by careful selection of the semiconductor the redox chemistry of a particular reaction may be controlled. For example, specific transformations may be achieved by targeting functional groups on molecules. This can enable specialised products to be prepared which may not be accessible by other synthetic routes4. Table 1 displays a list of semiconductor materials which have been used for photocatalytic reactions, together with the valence band and conductance band positions. The final column in the table indicates the wavelength of radiation required to activate the catalysts. h+
J. Cleaner Prod., 1996, Volume 4, Number 3-4
1. 2.
-heat
Scheme 1
204
electron (e-) and hole (h’) pair may recombine with the generation of heat. If they are separated they can become involved in electron transfer reactions with other species in the solution, for example oxidation of species D or reduction of species A. For oxidation to occur (step 3) the valence band must have a higher oxidation potential than the material under consideration. The redox potentials of the valence and conductance bands for different semiconductors varies between +4.0 and -1.5 volts vs Normal Hydrogen Electrode (NHE) respectively’. Therefore by careful selection of the semiconductor photocatalyst a wide range of species can be converted via these processes. The majority of work in this field has involved the use of TiOz. This material has a band gap energy of 3.2 eV and therefore absorbs in the near Ultraviolet light (-380 nm). The valence band and conductance band energies are estimated to be +3.1 and -0.1 volts respectively’. The material is therefore capable of oxidising and reducing a wide range of materials.
+ D’ A’-
3. 4.
Semiconductor photocatalysis: P. K.J. Robertson TiihU
Table 1 The band positions of some common semiconductor photocatalysts in aqueous solution at pH 1’
CH~CO-J~ v
CH3CO2* 328K
Semiconductor
TiOz SnO, ZnO ZnS WOZ CdS CdSe GaAS GaP
Valence Conductance Band band band gap (V vs NHE) (V vs NHB) (eV)
Band gap wavelength (nm)
+3.1 i4.1 +3.0 +1.4 +3.0 i2. I +I.6 +I.0 +I.3
380 318 390 336 443 491 730 887 540
-0.1 +0.3 -0.2 -2.3 +0.2 -0.4 -0.1 -0.4 -1.0
3.0 3.9 3.2 3.7 2.8 2.5 1.7 1.4 2.3
The oxidising and reducing powers of these materials can be compared to redox agents traditionally used by industry with reference to Table 2. A comparison of the strong oxidising potential of the valence band of the semiconductor to the oxidising potential of materials such as CP or Ag*+ suggests that semiconductors should be at least as effective oxidising agents as these traditional reagents. In many cases the semiconductor valence band is even more strongly oxidising than even Ag*+, therefore potentially extending the range of oxidation reactions which may be performed. By modifying the surface of the semiconductor, the types of products generated may be altered. For example, addition of tetra-alkyl ammonium salts to the electrolyte in the photocatalytic reduction of carbon dioxide results in dimeric products being generated while, in the absence of the salts, single carbon products were favoured’*. Other modifications of the surface of the semiconductor can also affect product distribution. For example the deposition of metals over the surface of the photocatalysts has been demonstrated to greatly influence product ratios for a number of reactions13. Control of the intensity of light incident on the reactor vessel can affect the nature of products4. By reducing the light flux to the semiconductor the number of electrons and holes available for reaction may be controlled. This can affect the nature of product distribution by lowering overall reaction rates and thereby Table 2 Oxidation/reduction redox agents
potentials of traditional industrial
Reagent
Redox potential vs NHB
Ag*+ Ce4+ CF+ ClZ Br, Sn2+ Ni2+ Zn2+ AP+ Li’
+I.98 +I.72 +1.33 +I.36 +1.09 -0.14 -0.25 -0.76 -1.66 -3.03
CH3CO2- 2CH3 ‘-
CH3 * C2&
Tiimu
CH3 .-
CH3-
Scheme 2
making some transformations more favourable than others. Examples of such reactions are illustrated below. Most of the reactions considered are usually performed slightly above ambient temperature (3OO-330K), due to heating of the reaction vessel by the irradiating source. The reactions are also performed at atmospheric pressure, therefore reaction design may be cheaper to construct than some traditional reactors. The safety implications of the reactions being performed at atmospheric pressure and near ambient temperature also make the photocatalytic process appear more attractive. One of the first organic photoelectrochemical reactions reported was the decarboxylation of acetic acid using both single crystal n-TiO, and vapour deposited polycrystalline photoanodes in acetonitrile’“‘6. The main products of this photo-Kolbe reaction were ethane and carbon dioxide14. When platinum coated Ti02 powders were used for this reaction the main product was methane15 which was believed to have been generated by the reduction of the intermediate methyl radicals at the platinum sites on the catalyst. Scheme 2 illustrates the proposed mechanism for the process. In this case, the photogenerated hole of the semiconductor abstracts an electron from the acid which subsequently loses carbon dioxide. The resulting methyl radical is either reduced and protonated to form methane or dimerises to form ethane. This system has also been used for the decarboxylation of other saturated carboxylic acidsi6. Yields were maximised using platinised anatase Ti02 together with more intense light sources (450 W producing 10% of the yield obtained with a 1600 W source). The decarboxylation of diacids has been achieved using a platinised Ti02 catalyst17. This reaction is also an example of how product control can be influenced by light flux incident on the reactor. At a low light intensity monodecarboxylation predominated while at higher light intensities complete decarboxylation occurred (Scheme 3). COOH TiQ
COOH
hu COOH Scheme 3
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The oxidation of alcohols to carbonyl compounds (Scheme 4) has been widely studied8*‘*,i9. Irradiation of an isopropanol solution in the presence of ZnO or platinised rutile TiO* and oxygen resulted in the generation of acetone’*. Addition of H202 to the system was found to increase the initial rate of acetone production’*. The anatase form of TiOz was found to be more active for the oxidation of isopropanol”. This oxidation reaction was also studied using TiOz treated with a 0.5% w/w loading of a series of metals. The catalytic activity of these catalysts was reported to decrease in the order Pt!TiOz > Pd/TiO* > Rh/TiOz while catalysis using Au doped materials resulted in no reactioni9. Aromatic compounds such as toluene, benzene and acetophenone have been successfully oxidised using TiOz photocatalysts20*21. The oxidation of toluene resulted in the generation of benzaldehyde and the various isomers of cresol (ratios of 0rtbo:meta:para were 44:40:16), while phenol and biphenyl were produced from the oxidation of benzene. The main product of the photocatalytic oxidation of benzene was C02, resulting from further oxidation of the products. The addition of sulphuric acid to the electrolyte was reported to dramatically increase product yields probably by suppressing CO2 production21. The products from the acetophenone reaction were hydroxy acetophenone, phenol and a dimer product. The oxidation of cyclohexane by Ti02 resulted primarily in the production of cyclohexanone (82.5%) with traces of cyclohexanol (5%), and CO2 (12.5%) also being detected after three hours photolysis22. When the reaction was repeated using a Ti02 catalyst modified with titanium silicate a higher selectivity for cyclohexan01 (49.7%) was observed. It was proposed that the titanium silicate inhibited the further oxidation of cyclohexanol to cyclohexanone. This example displays how modification of the semiconductor surface can influence the selectivity of the photocatalytic reaction. The main products resulting from the oxidation of primary amines by Ti02 were the N-formylation and o-C-N oxidative cleavage products (Scheme 5). The product ratio was found to depend on the initial amine concentration23. The reaction did not proceed in the absence of oxygen as the aminium cation was reduced back to the starting product by electron transfer at the conductance band. Platinisation of the Ti02 did not affect the product distribution. Azo products were reported when aniline and toluidines were illuminated in the presence of zinc oxide semiconductor dispersions24. This is illustrated in Scheme 6. The quantity of azo product generated was found to increase with irradiation time. The reactivity
-
RI-C==0 (278403K)
R2
+ H2
I
R2
Scheme 4
206
a-NHI~ + 0”
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CHO
0
Scheme 5
of the toluidines was found to be of the order mtoluidine > o-toluidine > aniline > p-tolidine. The low reactivity of p-toluidine was believed to be due to a strong methyl-methyl repulsion between two adjacently adsorbed molecules leading to a lower concentration of the compound on the surface of the semiconductor. The oxidation of sulphide materials to sulphoxides has been performed using Ti02 (Scheme 7)25*26.Yields of up to 100% have been reported for the oxidation of sulphides to sulphoxides and sulphones in an acetonitrile solvent, using an anatase Ti02 photocatalyst25. Thiols have been oxidised to disulphides by cadmium sulphide (CdS)27. The conversion of cysteine to cystine was found to be pH dependent with good yields being reported. Other thiols and inorganic sulphides were also successfully oxidised2’. Tokumaru et aL2* reported the conversion of 1: ldiphenylethylene to a mixture of benzophenone, 1: ldiphenyloxirane and 2-metboxy- 1: 1-diphenylethanol using anatase Ti02 and CdS semiconductor suspensions. Other olefins were also examined but their reactivity appeared to diminish with decreasing n-electron donating properties of the double bond. For example 2-methyl- 1: 1-diphenyletbylene was easily oxidised whereas cinnamonitrile was unaffected2*. Fox and Chen29 investigated the oxidative cleavage
NY+
NH2
0 0
ZdMlV -
w+
0 0 0
NH.
40 +0 0ci b+o;
-
0
+H02
NT-5
riollpuhu RI-CH-OH
rH
LCHO
Scheme 6
0 0
NHN
0 x>
Semiconductor
photocatalysis:
P.K.J. Robertson
Scheme 7
TiO
, hv
b 02, CH3cN
Scheme 8
of olefins using platinised and metal free Ti02, ZnO and CdS. By careful selection of the form of the semiconductor powder, the solvent polarity and irradiation time (at =350nm), an efficient oxidative cleavage of a series of olefins was observed with up to 85% conversion of the starting materia129*30.This was proposed to be a cheaper, safer and more selective method for achieving such conversions compared to the use of ozone or conventional oxidants29. Methoxynaphthalene was successfully oxidised in an acetonitrile solvent with a relatively high efficiency, as shown in Scheme 87. This method may provide an alternative synthetic route for preparing ortho-substituted benzenes. The oxidative cleavage of dialkylated naphthalenes resulted in the production of ketones and noncleaved oxidation products7. Amino acids have been prepared using illuminated semiconductors. When a mixture of methane, ammonia (or ammonium chloride) and water were illuminated in the presence of platinised TiO, a mixture of amino acids was produced 31. The products from this reaction included glycine, alanine, serine and aspartic acids. The mechanism for the process was believed to have involved the OH and NH2 radicals. The OH radicals were proposed to have reacted with the methane to generate methyl radicals. Simultaneous reduction at the platinised sites generated hydrogen. No amino acids were detected when unplatinised Ti02 was used. Both amino acids and peptides were produced when methane was replaced by glucose in the previous system3*. Semiconductor particles have also catalysed nucleic acid base formation33. When KCN, NH3 and H20 were irradiated in the presence Ti02 hypoxanthine was produced (Scheme 9). This product was also obtained TiQ KCN+NH,+HP
-
tN+lJH,
when platinised Ti02 and CdS were used as photocatalysts. Relatively few reductions catalysed by irradiated semiconductor materials have been reported. This is due to the position of the conductance band of the semiconductor being too positive to effect an efficient electron transfer, and oxygen strongly competing for the conduction band electrons. In certain cases the use of an electron relay such as methyl viologen can facilitate the electron transfer process. The photoelectrochemical hydrogenation of both double and triple bonds has been reported using CdS and Ti0234,35. The hydrogenation of acetylene catalysed by platinised CdS is displayed in Scheme 10. The reduction catalysed by rhodium loaded TiO, was also investigated however this was found to be a less efficient process. The photoelectrochemical hydrogenation of olefins has been performed using TiO, with ethanol as an electron source. Simple monosubstituted olefins were found to be more reactive than sterically crowded materials. A 63% yield has been reported for the hydrogenation of 2-methyl-pentene after 24 hours irradiation35 (Scheme II). The intensity of light has been found to have an important effect on product distribution in photocatalytic reductions of other organic substrates. Shiragami et ~1.~~investigated the use of CdS as a photocatalyst with substrates such as aromatic ketones, electron deficient alkenes and 1-benzylnicotinamide (BNA). With the ketones and the benzylnicotinamide at lower light intensities the yields of dimer products, such as pinacols and the dimer (BNA)2, were found to increase. At higher light intensities two electron reduction products, i.e. alcohols and 1-benzyl-1:4-dihydronicotinamide, predominated. When alkenes were used as the
llV
-1
Scheme 9
HC==CH + S2- -
CHs-CH3+S
H&&I-I
CHJ--cH3+S
+ S2- -
Scheme IO
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CH3
CI-4 Ti02, hv CH3--C==CH--C~--CH3
-
I CH3-CH-CI+C~~H3
Scheme I1
occurred substrate a cis/trans photoisomerisation regardless of the light flux. Valence isomerisations photoinduced by semiconductors have been described by Al-Ekabi and de Mayo3’. The photoinduced isomerisation of hexamethyl-Dewar benzene to hexamethylbenzene was achieved in methylene chloride using CdS, TiOp and ZnO as photocatalysts. Quantum yields of greater than unity were reported for the reactions using TiOz and ZnO. Cadmium sulphide has also been used for geometric isomerisations. A series of cis-4-substituted stilbenes have been reported to undergo cis/trans isomerisation by photocatalysis with virtually quantitative yields3*. Stereoselective synthesis has been achieved via photomediated semiconductor reactions39. Illumination of a mixture of cis and trans 2:6_diaminopimelic acids in the presence of platinised cadmium sulphide produced piperidine-2:6-dicarboxylic acid. Both the cis/trans ratio and the yield of this product were both found to be dependent on the nature of the catalyst and the platinum loading. The photocatalytic racemization of amino acids by CdS has been reported by Ohtani et ~1.~~.The mechanism for the racemization of I-Lysine was believed to have involved both reduction and oxidation via the photoexcited electrons and positive holes on the semiconductor. The racemization of other amino acids was investigated including 1-leucine, 1-phenylalanine and I-glutamic acid with percentage conversions of 3 l%, 28% and 44% respectively reported after 40 hours photolysis. This process may provide a relatively inexpensive method for converting less valuable enantiomers into more valuable isomers. Cyclisation reactions have been performed using semiconductor photocatalysis4’. When 1: 1-di-p-anisylethylene was illuminated in the presence of CdS two cyclised and two open chain dimeric products were generated. In the presence of oxygen the di-p-anisyl ketone product predominated. The production of I-pipcolinic acid from the amino acid, I-lysine is another example of a photocatalysed cyclisation reaction 42. The number of defects and surface area of the platinised TiO, catalyst was reported to affect the product yield. Table 3 summarises some of the conversions that have been catalysed by illuminated semiconductor materials. As has been illustrated from the preceding paragraphs a wide range of different reactions can be effected using semiconductor photocatalysts. There are, however, certain limitations that need to be considered which have restricted the commercialisation of these processes to date. Firstly, the time taken for several of the conversions
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can be fairly lengthy, greater than 48 hours for some examples. The energy input in these cases could exceed many traditional synthetic methods. There is substantial work being carried out on modification of the catalysts to improve efficiencies so that these difficulties may be surmounted. Another significant problem is encountered when using catalysts such as CdS, which are prone to photodecomposition. This results when the photogenerated holes in the valence band promote the bond cleavage between the surface atoms. These atoms may subsequently dissolve in the electrolyte contaminating it with cadmium, which is a particularly toxic metal itself. This can be minimised by coating with films of inert materials, immobilising the semiconductor in a protective film or by the addition of electron donors to the electrolyte solution4. The problems associated with photodecomposition are, however, not experienced when using materials such as Ti02 or Sn02. Although these materials absorb UV light, and therefore have a higher energy input requirement, modification of these catalysts to enable them to absorb visible light is being investigated. The ability of photocatalysis to promote isomeric transformation is exciting since many traditional methods of promoting such reactions are lengthy and cumbersome. This is of particular importance to the pharmaceutical industry where chiral synthesis is becoming more significant. Semiconductor photocatalysis could provide a route for preparing chiral molecules more cost effectively than many traditional methods. An important difficulty that may be encountered in scaling up many of these processes will be in the reactor design. The basic problem of scale-up of photocatalytic processes is ensuring a uniform distribution of light to a large surface area of the catalyst. Therefore, when designing a new reactor, it is necessary to consider two important parameters; the distribution of light inside the reactor and the provision of a high surface area for the catalyst per unit volume of reactor. There are two different reactor designs which could be considered; a suspended particle catalyst reactors or an immobilised film catalyst reactors. Suspended catalyst reactors may be a more efficient design with respect to quantum yield, and volume efficiency but the separation of the catalyst from the reaction mixture presents difficulties. In the laboratory this stage is easily accomplished by either filtration or centrifugation. However, these options are less feasible on an industrial scale. Alternative separation methods may include flocculation and sedimentation or membrane filtration. The immobilised catalyst film reactor may not have
Semiconductor photocatalysis: P.K.J. Robertson Table3
Examples of organic compounds and substrates which have undergone photocatalytic transformations using semiconductors
Reactant
Product
Reference
Organic Compounds Alcohols, Polyols
Carbonyl compounds, Amines, Amino acids, Peptides
18, 19, 32
Amines
Schiff bases, Azo products
23, 24
Acids
Alkanes
15, 16
Aromatic hydrocarbons
Alcohols, Carbonyl compounds
20, 21
Alkanes
Amino acids
31, 32
Alkenes
Alkanes, Carbonyl compounds
28, 29, 30, 35
Alkynes Ethers
Alkanes, Carbonyl compounds Esters
34, 35
Sulphides
Sulphoxides, Carbonyl compounds
25, 26, 27
Thiols
Sulphides
21
problems of catalyst separation associated with it but, since the reaction occurs at a liquid-solid interface, only a fraction of the catalyst will be in contact with the reactant in solution. Therefore, a reactor which provides a high ratio of illuminated, immobilised catalyst to illuminated surface is required. This problem is being investigated in a number of ways, including immobilisation of the catalyst on materials with large surface areas. In summary, therefore, there are several factors which have so far limited the wide application and commercialisation of this technology in chemical synthesis. As a result of these limitations the immediate application of semiconductor photocatalysis may be restricted to the production of small quantities of high value chemicals for which the technology can compete economically or where there are no alternative synthetic routes. This may be of particular interest to the pharmaceutical or fine chemical industries, particularly where chiral synthesis an expanding interest.
Photocatalytic treatment of waste materials The use of TiOz as a photocatalyst for the destruction of polluting materials has now been demonstrated to be an effective process43. The overall process may be described as follows. A reaction vessel containing the waste material and the semiconductor is illuminated and the waste is converted to a non-toxic form either by an oxidation or reduction process. An example of a typical process would be the destruction of haloaromatic compounds, resulting in the production of CO*, Hz0 and the halide ion. The overall process for the photo-oxidation of waste materials (P), sensitised by TiOz is shown in Scheme 1244. There is a certain amount of debate regarding the mechanism for the destructionU. Some believe that
P + 02w
I
hydroxyl radicals are generated via oxidation at the valence band. These radicals subsequently oxidise the polluting material, while at the conductance band an electron is donated to oxygen thereby generating the superoxide radical anion. This is illustrated in Figure 2. Other workers have, however, suggested that the species being oxidised can transfer an electron directly to the valence band45. In the case of chlorophenol it has been proposed that, at low concentrations, the oxidation occurs via the hydroxyl radicals while at higher concentrations direct valence band oxidation predominates. Regardless of which mechanism for mineralisation predominates, the oxidation potential of either the semiconductor valence band (3.1 volts) or the hydroxyl radical (2.8 volts) is far more oxidising than many materials that are commonly used for waste treatment, including ozone (2.07 volts), hydrogen peroxide (1.78 volts), hypochlorous acid (1.49 volts) and chlorine (1.36 volts). In his study on the kinetics of the oxidation of organic molecules using Ti02, GerischeF6 proposed
I_,-
7Y
-bv
Figure.2 The process of semiconductor purification”
photo-catalysis for water
CO, + Hz0 + Mineral acids
Scheme 12
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Semiconductor photocatalysis: P.K.J. Robertson that the electron transfer from the conduction band to oxygen was the rate determining step of this mechanism. It was also reported that high yields were achieved only when small particles were used. One of the first reported cases of the use of semiconductor photocatalysis in the destruction of organic compounds was reported in 1976 by Carey et al!‘. They reported the successful degradation of biphenyl and chlorobiphenyls in the presence of TiOz. Since then the destruction of many materials using TiOz has been investigated. In 1981 Fox and Chen4* reported that several aromatic hydrocarbons could be oxidatively cleaved when reversibly adsorbed onto the surface of an irradiated suspension of TiOa. This work was subsequently expanded to include a wide range of organic materials including alkanes, alkenes, arenes, acids, esters, amides, amines, ethers, thioethers, organohalides, aldehydes, ketones, organosilanes and phosphonates49-52. Ollis et a1.53*54 subsequently reported the use of Ti02 in the complete destruction of several halogenated hydrocarbons including trichloroethane, methylene chloride, chloroform and carbon tetrachloride. It was found that a simple Langmuirian rate equation represented the destruction of these materials. Matthews55-59 investigated the development of a range of waste treatment cells using a Ti02 photocatalyst including immobilised film and suspended particle reactors. A particularly successful reactor incorporated a thin film coating of Ti02 applied to the internal surface of a spiral glass tube. The solution containing the waste effluent was then pumped through the tube while being illuminated with a UV light. This cell was reasonably efficient in the removal of a variety of organic pollutants with 96% destruction being reported within ten minutes57. A TiO, suspension reactor illuminated with sunlight for the destruction of phenol was also investigated 59. The concentration of this material was reduced from 1Oppm to 1Oppb within 80 minutes, with virtually total mineralisation in 110 minutes. Trinitrotoluene (TNT) contamination is a problem experienced in many former military installations in the United StateP. The destruction of TNT has been achieved in a Ti02 slurry reactor with 90% mineralisation occurring within 120 minutes. These results indicate that this technique is very effective for the destruction of these highly toxic and explosive materials. The destruction of biological materials such as Escherichia Coli by Ti02 has been reported by Ireland et aL6’. They reported a reduction in the concentration of viable organisms of seven orders of magnitude within six minutes photolysis. After nine minutes reaction time the concentration of the E. Coli was below the detection limits of analysis. Eggins et aL6* investigated the potential of the semiconductor photocatalysis for the treatment of potable water. The rapid decomposition of humic substances using a Ti02 photocatalyst was reported. Table 4 displays examples of the waste materials
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Table4 Examples of materials which have undergone photomineralisation using TiO*” Class of compound
Examples
Alkanes
methane; pentane; heptane; n-dodecane; cyclohexane; paraffin
Haloalkanes
mono-, di-, tri-, and tetrachloromethane; dichloroethane; pentachloroethane; di and tribromoethane; I:2-dichloropropane
Aliphatic alcohols
methanol; ethanol; n- and iso-propanol; butanol; penta- I :Cdiol
Aliphatic carboxylic acids
methanoic; ethanoic; trichloroacetic; butyric; oxalic
Alkenes
propene; cyclohexene
Haloalkenes
di-, tri- and tetra-chloroethene; hexafluoropropene
Aromatics
benzene; naphthalene
Haloaromatics
chloro and bromobenzene; chlorobenzenes; halophenols
Phenols
phenol; hydroquinone; catecol; resorcinol; cresol; nitrophenol
Aromatic carboxylic acids
benzoic; phthalic; salicyclic
Polymers
polyethylene; PVC
Surfactants
polyethylene glycol; p-nonyl phenyl ether; sodium dodecyl; benzene sulphonate; paraxon; malathion
Herbicides
methyl viologen; atrazine; simazine; bentazon
Pesticides
DDT; parathion; lindane; monocrotophos
Dyes
methylene blue; rhodamine B; methyl orange; fluorescein
Explosives
trinitrotoluene
Bacteria
E. Coli
that have been successfully treated using Ti02 photocatalystP. Currently there is an extensive interest in this area of research and, although to date only a few systems have been investigated on a pilot plant scale, it is probable that commercially viable treatment systems will result from this work63. Many of the design considerations outlined above for the synthetic reactors also apply for the waste treatment reactor. One system which has received attention was developed by Hamnett et al.&l at the University of Newcastle upon Tyne. This is based on a plate vortex reactor illuminated above by a UV Light source. Bahnemann ef aZ.65 have developed a thin film fixed bed reactor. The Ti02 photocatalyst is immobilised on a sloping plate over which a thin film (~100 km) of the waste water flows while illuminated from above. Matthews% investigated the economics of semiconductor photocatalysis in comparison to activated carbon adsorption. The cost of both techniques were found to be comparable. However, he predicted that, as catalyst efficiencies were improved, photocatalysis could prove to be a cheaper method in the future. Rajeshwd’ determined the cost of treating waste using Ti02 photocatalysis to be $5.22 (US) per 3785 litres of waste
Semiconductor photocatalysis: P.K.J. Robertson compared to a price of $6.20 for activated adsorption.
carbon
Conclusion The use of semiconductors as photocatalysts for synthesis and waste treatment has been discussed. These catalysts represent a class of materials which can be used for more environmentally acceptable production techniques since their application can reduce the amount of harmful materials being emitted to the environment. A wide range of different chemical reactions can be initiated by the photocatalysts, providing in some cases new synthetic routes for generating otherwise inaccessible products. The photocatalysts also have the advantage of providing greater reaction control and thereby allowing product selection. The problems of reactor design for scale-up of the process, improved product yield for some reactions and inhibition of catalyst photocorrosion still have to be addressed. However, these difficulties may be surmountable in the future. The photocatalytic treatment of the waste materials has been shown to be an effective technique for the treatment of a very diverse range of pollutant compounds. Considering the extent of research in this area the development of an effective waste treatment reactor may simply be a matter of time.
18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38
Acknowledgements I would like to thank my colleagues Brian McGaw and Stephanie Rigby of The Robert Gordon University for their useful advice and comments during the preparation of this manuscript.
References
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10 11 12 13 14 15 16 17
Parkes, J., J. Cleaner Prod., 1994, 2, 83. Freeman, H.M., Standard Handbook of Hazardous Waste Treatment and Disposal, McGraw Hill, NY, 1989, Sec. 6.3. Hoffmann, M.R., Martin, ST., Choi, W. and Bahnemann, D.W., Chem. Rev., 1995, 95, 69. Fox, M.A., Topics in Curr. Chem., 1991, 142, 71. Gerischer, H., J. Eiectroanal. Chem., 1975, 58, 263. Bard, A.J., J. Photochem., 1980, 10, 59. Fox, M.A., Act. Chem. Res., 1983, 16, 314. Pichat, P. and Fox, M.A., Photoinduced Electron Transfer, eds M.A. Fox and M. Chanon. Elsevier, Oxford, 1988, Part D, p. 241. Pichat, P., in Photoelectrochemistry, Photocatalysis and Photoreactors, ed. M. Sciavello. D. Reidel. Dordrecht. The Netherlands, 1985, p. 425. Al-Ekabi, H. in Photochemistry in Organised and Constrained Media, ed. V. Ramamurthy. VCH, NY, 1991, p. 495. Fox, M.A., Chem. Rev., 1993, 93, 341. Eggins, B.R., Robertson, P.K.J., Stewart, J.H. and Woods, E., J. Chem. Sot. Chem. Comm. 1993, 349. Shiragami, T., Fukami, S., Pac. C. and Yanagida, S., J. Chem. Sot. Faraday Trans., 1993, 89, 1857. Kraeutler, B. and Bard, A.J., J. Am. Chem. Sot., 1977, 99, 7729. Kmeutler, B. and Bard, A.J., J. Am. Chem. Sot., 1978, 100,2239. Kraeutler, B. and Bard, A.J., J. Am. Chem. Sot., 1978, 100,5985. Fox, M.A., Chen, C.C., Park, K.H. and Younathan, J.N., Amer. Chem. Sot. Sympos. Ser., 1985, 278, 69.
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Cundall. R.B., Rudham, R. and Salim, S., J. Chem. Sot., Far&y Trans. I, 1976, 72, 1642. Hussein. F.H. and Rudham. ’ R..’ J. Chem. Sot.. Far&v , Trans. i, 1984, 80, 2817. Fujihara, M., Satoh, Y. and Osa, T., J. Electroamd. Chem., 1981, 126,277. Fujihara, M., Satoh, Y. and Osa, T., Nature, 1981, 293, 206. Lu, G.X.. Gao. H.X.. Suo. J.H. and Li. S.B.. J. Chem. Sot. Chem. &mm.,’ 1994,. 2423, Fox, M.A. and Chen, M.J., J. Am. Chem. Sot.. 1983, 105, 4497. Kasturirangan, H., Ramakrichnan, V. and Kuriacose, J.C., J. Carat., 1981, 69, 216. Davidson, R.S. and Pratt, J.E. Tetrahedron Lett., 1983, 24, 5903. Hoyer, W. and Asmus, K.D., Sixth International Conference Photochem. Conver. Storage Solar Energy, Abstr C-55, 1986. Spikes, J.D., Photochem. Photobio., 1981, 34, 549. Kanno, T., Oguchi, T., Sakuragi, H. and Tokumura, K., Tetrahedron L&t., 1980, 21, 467. Fox, M.A. and Chen, CC., J. Am. Chem. Sot., 1981, 103, 6757. Fox, M.A. and Chen, C.C., J. Photochem., 1981, 17, 119. Reiche, H. and Bard, A.J., J. Am. Chem. Sot., 1979, 101, 3127. Kawai, T., Fujii, M., Kambe, S. and Kawai, S., Electrochem. Sot. Japan, 1983, At&r E214,211. Onoe, J. and Kawai, T., J. Chem. Sot. Chem. Comm., 1988, 681. Frank, A. J., Goren, Z. and Willner, I., J. Chem. Sot. Chem. Comm., 1985, 1029. Yamataka, H., Seto, N., Ichihara, J., Hanafusa, T. and Teratani, S., J. Chem. Sot. Chem. Comm., 1985, 788. Shiragami, T., Fukami, S., Wada, Y. and Yanagida, S., J. Phys. Chem., 1993, 97, 12882. Al-Ekabi, H. and de-Mayo, P., J. Phys. Chem., 1986, 90, 4075. Hasegawa, T. and de Mayo, P., J. Chem. Sot. Chem. Comm.. 1985: 1534. Ohtani, B., Kusakabe, S., Okada, K., Tsuru, S., Izawa, K., Nishimoto. S.I. and Izawa. K.. Tetrahedron Letters. 1995. 36, 3189.. Ohtani, B., Kawaguchi, J., Kozawa, M., Nishimoto, S., Inui, T. and Izawa, K., J. Chem. Sot. Faraday Trans., 1995, 91, 1103. Al-Ekabi, H. and de-Mayo, P., J. Org. Chem., 1987,52,4756. Ohtani, B., Iwai, K., Kominami, H., Matsuura, T., Kera, Y. and Nishimoto, S., Chem. Phys. Letts., 1995, 242, 315. Ollis. D.F.. Pelizzetti. E. and Semone. N. in Photocatafvsis. Fundamentals and Applications,~ e&l N. Serpone and E. Pelizzetti, John Wiley, New York, 1989, p 603. Mills, A., Davies, R.H. and Worsley, D., Chem Sot. Rev., 1993, 417. Prairie, M.R., Evans, L.R., Stange, B.M. and Martinez, S.L., Environ. Sci. Technoi., 1993, 27, 1776. Gerischer, H., Electrochim Acta, 1993, 38, 3. Carey, J.H., Lawrence, J. and Tosine, H.M. Bull Environ. Contam. Toxicol., 1976, 16, 697. Fox, M.A. and Chen, C.C., J. Am. Chem. Sot., 1981, 103, 6757. Fox, M.A. and Chen, M.J., J. Am. Chem. Sot., 1983, 105, 4497. Fox, M.A., Younathan, J.N. and Sackett, D., Tetrahedron, 1987, 43, 1643. Fox, M.A., Kim, Y-S. and Dulay, M.T., Carat. Lett., 1990, 5, 369. Dulay, M.T., Washington-Dedeaux, D. and Fox, M.A., Photothem and Photobiol., 1991, 61, 153. Hsiao, C-Y., Lee, C-L. and Ollis, D.F., J. Carat., 1983, 82, 418. Turchi, C.S. and Ollis, D.F., J. Carat., 1989, 119, 483. Matthews, R.W., J. Phys. Chem., 1987, 91, 3328. Matthews. R.W.. J. Cataf.. 1988. 113. 549. Matthews, R.W:, J. Chem. Sot: Faraday Trans. I, 1989, 85, 1291. Matthews, R.W., Water Res., 1990, 24, 653. Matthews, R.W., Solar Energy, 1992, 49, 507. Schmelling, DC. and Gray, K.A. War. Res., 1995, 29, 2651. Ireland, J.C., Klosternunn, P., Rice, E.W. and Clark, R.M., App. and Environ. Microbial., 1993, 59, 1668.
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Eggins, B.R., Palmer, F.L. and Btyne, J.A., Water Research, 1997, 31(5), 1223. Leqrini, 0.. Oliveros, E. and Braun, A.M., Chem. Rev., 1993. 93, 671. Christensen, P.A., Hamnett, A., Howarth, C.R., Shaw, K.E., Walker, GM. and Walker, S.A., Electrochem 94 Conference, Edinburgh, Conference Abstract, I2- 16 September 1994. Hilgendoriff, M., Bockelmann, D., Nogueira, R.F.P., Weich-
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66
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grebe, D., Jardim, W.F., Bahnemann, D. and Goslich, R., in Proc. 6th Int. Symp. Thermal Concentrating Technologies, Vol 2, Ciemat, Spain, 1992, p. I 167. Matthews, R.W., in Photocatalytic Purification and Treatment of Water and Air, eds. D.F. Ollis and H. Al-Ekabi, Elsevier, Oxford, 1993, p 121. Rajeshwar, K., Chemistry & Industry, 1996, 454.
Prod.
PII: s0959_652q96j0oo4&3
Vol.
4. No.
0 All
34, pp. 203-2 12, 1996 1997 Elsevier Science Ltd
rights reserved.
ELSEVIER
Printed
in Great
Britain
0959-6526196
$15.00
+ 0.00
Semiconductor photocatalysis: an environmentally acceptable alternative production technique and effluent treatment process Peter K.J. Robertson School of Applied Sciences, The Robert Aberdeen, AB25 lHG, Scotland Received
28 June
1996; accepted
Gordon
20 November
University,
St Andrew
Street,
1996
New environmentally acceptable production methods are required to help reduce the environmental impact of many industrial processes. One potential route is the application of photocatalysis using semiconductors. This technique has enabled new environmentally acceptable synthetic routes for organic synthesis which do not require the use of toxic metals as redox reagents. These photocatalysts also have more favourable redox potentials than many traditional reagents. Semiconductor photocatalysis can also be applied to the treatment of polluted effluent or for the destruction of undesirable by-products of reactions. In addition to the clean nature of the process the power requirements of the technique can be relatively low, with some reactions requiring only sunlight. 0 1997 Elsevier Science Ltd Keywords: photocatalysis;
photoelectrochemistry;
Introduction The generation of hazardous industrial effluents is a serious problem experienced by nations throughout the European Union. In recent years there has been an increase in legislation within Europe aimed at protecting the environment. Directives such as the Urban Waste Water Directive, the Fish and Shellfish Directives, the Integrated Pollution Prevention and Control Directive and the Dangerous Substances Directives have all emphasised the importance of developing new effective and environmentally acceptable production techniques and waste treatment systems. The problem of effluent generation may be approached in two manners. Firstly new production technologies could be adopted which could reduce effluent generation by adopting catalytic processes which are non-polluting. The other approach would be the application of novel waste treatment technologies. Techniques such as electrochemistry have been applied to both these requirements with some success.’ There remains, however, a need for new processes to reduce the overall environmental impact of industrial synthetic chemistry. Sectors such as the pharmaceutical, textile, agricultural, food, and chemical industries all produce waste effluent contaminated with organic compounds such as aromatics, haloaromatics, aliphatics, dyes, dioxins and
effluent treatment;
clean technology
a wide range of other polluting materials. Many of these materials are extremely toxic and the waste streams must be treated prior to discharge. Traditional waste treatment systems have involved the use of techniques such as coagulation, chlorination or ozonation which utilise potentially hazardous or polluting materials. Chlorination presents a particular problem since it will often generate mutagenic or carcinogenic by-products when used to treat water contaminated with organic compounds. Techniques such as granular activated carbon adsorption or air stripping processes have drawbacks as the methods are non-destructive.* The adsorbing media loaded with the waste must therefore, also be treated as a hazardous material. Incineration of organic waste is not always effective and can disperse large quantities of toxic emissions, such as products of incomplete combustion or heavy metals, into the atmosphere. An effective treatment system is required which can degrade the polluting materials prior to discharge of the effluent. The photocatalytic process is a technique which could address both these requirements. This process involves the utilisation of a non-toxic semiconductor catalyst such as titanium dioxide (Ti02). When semiconductors are illuminated with light of an appropriate wavelength they become powerful oxidants which will convert most organic materials to carbon dioxide and water. This process has been very effective in
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‘WWhv~Eb
Figure 1 Promotion of an electron from the valence band to the conductance band on illumination of a semiconductor
destroying a wide range of organic materials typical of those generated by the industries mentioned above3. Semiconductor photocatalysis can also be applied to functional group transformations such as oxidations and reductions4 negating the requirement for potentially toxic redox reagents such as chromium or nickel compounds. Following their application for a particular reaction the semiconductor photocatalysts may be recycled for further use. This paper will review some of the various applications of semiconductor photocatalysis to date. Before discussing the applications of semiconductor photocatalysis it will be necessary to describe the basic principles of the photocatalytic process.
The interaction of semiconductors
with light
Semiconductor particles may be photoexcited to form electron donor sites (reducing sites) and electron acceptor sites (oxidising sites), providing great scope as redox reagentP. The molecular orbitals of semiconductors have a band structure. The bands of interest in photocatalysis are the occupied valence band and the unoccupied conductance band. They are separated by an energy distance referred to as the band gap (Ebg). When the semiconductor is illuminated with light of greater energy than that of the band gap, an electron is promoted from the valence band to the conductance band. This leaves a positive hole in the valence band as illustrated in Figure 1. Scheme 1 shows the possible reactions that can occur when a solution containing a semiconductor (SC) absorbs a photon (hv) of a suitable wavelengtl?. The Light Absorption
SC+hu-e-+
Recombiion
a+h+
Oxidation
D +h+ -
Reduction
A + e’-
Photocatalytic conversions using semiconductor materials Semiconductor photocatalysis has already been applied to a wide range of different chemical reactions with both organic and inorganic substrates. Examples of these reactions are oxidations, reductions, rearrangements, isomerisations, cleavages and polymerisations4~8. Unfortunately it is only possible to briefly describe some of these reactions in this review. Other reviews on these reactions are available and further detailed information may be sought there4*7~9*Lo*’ ‘. In addition to the environmental advantages of the photocatalytic synthetic route there are other advantages to this technique4. Firstly by careful selection of the semiconductor the redox chemistry of a particular reaction may be controlled. For example, specific transformations may be achieved by targeting functional groups on molecules. This can enable specialised products to be prepared which may not be accessible by other synthetic routes4. Table 1 displays a list of semiconductor materials which have been used for photocatalytic reactions, together with the valence band and conductance band positions. The final column in the table indicates the wavelength of radiation required to activate the catalysts. h+
J. Cleaner Prod., 1996, Volume 4, Number 3-4
1. 2.
-heat
Scheme 1
204
electron (e-) and hole (h’) pair may recombine with the generation of heat. If they are separated they can become involved in electron transfer reactions with other species in the solution, for example oxidation of species D or reduction of species A. For oxidation to occur (step 3) the valence band must have a higher oxidation potential than the material under consideration. The redox potentials of the valence and conductance bands for different semiconductors varies between +4.0 and -1.5 volts vs Normal Hydrogen Electrode (NHE) respectively’. Therefore by careful selection of the semiconductor photocatalyst a wide range of species can be converted via these processes. The majority of work in this field has involved the use of TiOz. This material has a band gap energy of 3.2 eV and therefore absorbs in the near Ultraviolet light (-380 nm). The valence band and conductance band energies are estimated to be +3.1 and -0.1 volts respectively’. The material is therefore capable of oxidising and reducing a wide range of materials.
+ D’ A’-
3. 4.
Semiconductor photocatalysis: P. K.J. Robertson TiihU
Table 1 The band positions of some common semiconductor photocatalysts in aqueous solution at pH 1’
CH~CO-J~ v
CH3CO2* 328K
Semiconductor
TiOz SnO, ZnO ZnS WOZ CdS CdSe GaAS GaP
Valence Conductance Band band band gap (V vs NHE) (V vs NHB) (eV)
Band gap wavelength (nm)
+3.1 i4.1 +3.0 +1.4 +3.0 i2. I +I.6 +I.0 +I.3
380 318 390 336 443 491 730 887 540
-0.1 +0.3 -0.2 -2.3 +0.2 -0.4 -0.1 -0.4 -1.0
3.0 3.9 3.2 3.7 2.8 2.5 1.7 1.4 2.3
The oxidising and reducing powers of these materials can be compared to redox agents traditionally used by industry with reference to Table 2. A comparison of the strong oxidising potential of the valence band of the semiconductor to the oxidising potential of materials such as CP or Ag*+ suggests that semiconductors should be at least as effective oxidising agents as these traditional reagents. In many cases the semiconductor valence band is even more strongly oxidising than even Ag*+, therefore potentially extending the range of oxidation reactions which may be performed. By modifying the surface of the semiconductor, the types of products generated may be altered. For example, addition of tetra-alkyl ammonium salts to the electrolyte in the photocatalytic reduction of carbon dioxide results in dimeric products being generated while, in the absence of the salts, single carbon products were favoured’*. Other modifications of the surface of the semiconductor can also affect product distribution. For example the deposition of metals over the surface of the photocatalysts has been demonstrated to greatly influence product ratios for a number of reactions13. Control of the intensity of light incident on the reactor vessel can affect the nature of products4. By reducing the light flux to the semiconductor the number of electrons and holes available for reaction may be controlled. This can affect the nature of product distribution by lowering overall reaction rates and thereby Table 2 Oxidation/reduction redox agents
potentials of traditional industrial
Reagent
Redox potential vs NHB
Ag*+ Ce4+ CF+ ClZ Br, Sn2+ Ni2+ Zn2+ AP+ Li’
+I.98 +I.72 +1.33 +I.36 +1.09 -0.14 -0.25 -0.76 -1.66 -3.03
CH3CO2- 2CH3 ‘-
CH3 * C2&
Tiimu
CH3 .-
CH3-
Scheme 2
making some transformations more favourable than others. Examples of such reactions are illustrated below. Most of the reactions considered are usually performed slightly above ambient temperature (3OO-330K), due to heating of the reaction vessel by the irradiating source. The reactions are also performed at atmospheric pressure, therefore reaction design may be cheaper to construct than some traditional reactors. The safety implications of the reactions being performed at atmospheric pressure and near ambient temperature also make the photocatalytic process appear more attractive. One of the first organic photoelectrochemical reactions reported was the decarboxylation of acetic acid using both single crystal n-TiO, and vapour deposited polycrystalline photoanodes in acetonitrile’“‘6. The main products of this photo-Kolbe reaction were ethane and carbon dioxide14. When platinum coated Ti02 powders were used for this reaction the main product was methane15 which was believed to have been generated by the reduction of the intermediate methyl radicals at the platinum sites on the catalyst. Scheme 2 illustrates the proposed mechanism for the process. In this case, the photogenerated hole of the semiconductor abstracts an electron from the acid which subsequently loses carbon dioxide. The resulting methyl radical is either reduced and protonated to form methane or dimerises to form ethane. This system has also been used for the decarboxylation of other saturated carboxylic acidsi6. Yields were maximised using platinised anatase Ti02 together with more intense light sources (450 W producing 10% of the yield obtained with a 1600 W source). The decarboxylation of diacids has been achieved using a platinised Ti02 catalyst17. This reaction is also an example of how product control can be influenced by light flux incident on the reactor. At a low light intensity monodecarboxylation predominated while at higher light intensities complete decarboxylation occurred (Scheme 3). COOH TiQ
COOH
hu COOH Scheme 3
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The oxidation of alcohols to carbonyl compounds (Scheme 4) has been widely studied8*‘*,i9. Irradiation of an isopropanol solution in the presence of ZnO or platinised rutile TiO* and oxygen resulted in the generation of acetone’*. Addition of H202 to the system was found to increase the initial rate of acetone production’*. The anatase form of TiOz was found to be more active for the oxidation of isopropanol”. This oxidation reaction was also studied using TiOz treated with a 0.5% w/w loading of a series of metals. The catalytic activity of these catalysts was reported to decrease in the order Pt!TiOz > Pd/TiO* > Rh/TiOz while catalysis using Au doped materials resulted in no reactioni9. Aromatic compounds such as toluene, benzene and acetophenone have been successfully oxidised using TiOz photocatalysts20*21. The oxidation of toluene resulted in the generation of benzaldehyde and the various isomers of cresol (ratios of 0rtbo:meta:para were 44:40:16), while phenol and biphenyl were produced from the oxidation of benzene. The main product of the photocatalytic oxidation of benzene was C02, resulting from further oxidation of the products. The addition of sulphuric acid to the electrolyte was reported to dramatically increase product yields probably by suppressing CO2 production21. The products from the acetophenone reaction were hydroxy acetophenone, phenol and a dimer product. The oxidation of cyclohexane by Ti02 resulted primarily in the production of cyclohexanone (82.5%) with traces of cyclohexanol (5%), and CO2 (12.5%) also being detected after three hours photolysis22. When the reaction was repeated using a Ti02 catalyst modified with titanium silicate a higher selectivity for cyclohexan01 (49.7%) was observed. It was proposed that the titanium silicate inhibited the further oxidation of cyclohexanol to cyclohexanone. This example displays how modification of the semiconductor surface can influence the selectivity of the photocatalytic reaction. The main products resulting from the oxidation of primary amines by Ti02 were the N-formylation and o-C-N oxidative cleavage products (Scheme 5). The product ratio was found to depend on the initial amine concentration23. The reaction did not proceed in the absence of oxygen as the aminium cation was reduced back to the starting product by electron transfer at the conductance band. Platinisation of the Ti02 did not affect the product distribution. Azo products were reported when aniline and toluidines were illuminated in the presence of zinc oxide semiconductor dispersions24. This is illustrated in Scheme 6. The quantity of azo product generated was found to increase with irradiation time. The reactivity
-
RI-C==0 (278403K)
R2
+ H2
I
R2
Scheme 4
206
a-NHI~ + 0”
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CHO
0
Scheme 5
of the toluidines was found to be of the order mtoluidine > o-toluidine > aniline > p-tolidine. The low reactivity of p-toluidine was believed to be due to a strong methyl-methyl repulsion between two adjacently adsorbed molecules leading to a lower concentration of the compound on the surface of the semiconductor. The oxidation of sulphide materials to sulphoxides has been performed using Ti02 (Scheme 7)25*26.Yields of up to 100% have been reported for the oxidation of sulphides to sulphoxides and sulphones in an acetonitrile solvent, using an anatase Ti02 photocatalyst25. Thiols have been oxidised to disulphides by cadmium sulphide (CdS)27. The conversion of cysteine to cystine was found to be pH dependent with good yields being reported. Other thiols and inorganic sulphides were also successfully oxidised2’. Tokumaru et aL2* reported the conversion of 1: ldiphenylethylene to a mixture of benzophenone, 1: ldiphenyloxirane and 2-metboxy- 1: 1-diphenylethanol using anatase Ti02 and CdS semiconductor suspensions. Other olefins were also examined but their reactivity appeared to diminish with decreasing n-electron donating properties of the double bond. For example 2-methyl- 1: 1-diphenyletbylene was easily oxidised whereas cinnamonitrile was unaffected2*. Fox and Chen29 investigated the oxidative cleavage
NY+
NH2
0 0
ZdMlV -
w+
0 0 0
NH.
40 +0 0ci b+o;
-
0
+H02
NT-5
riollpuhu RI-CH-OH
rH
LCHO
Scheme 6
0 0
NHN
0 x>
Semiconductor
photocatalysis:
P.K.J. Robertson
Scheme 7
TiO
, hv
b 02, CH3cN
Scheme 8
of olefins using platinised and metal free Ti02, ZnO and CdS. By careful selection of the form of the semiconductor powder, the solvent polarity and irradiation time (at =350nm), an efficient oxidative cleavage of a series of olefins was observed with up to 85% conversion of the starting materia129*30.This was proposed to be a cheaper, safer and more selective method for achieving such conversions compared to the use of ozone or conventional oxidants29. Methoxynaphthalene was successfully oxidised in an acetonitrile solvent with a relatively high efficiency, as shown in Scheme 87. This method may provide an alternative synthetic route for preparing ortho-substituted benzenes. The oxidative cleavage of dialkylated naphthalenes resulted in the production of ketones and noncleaved oxidation products7. Amino acids have been prepared using illuminated semiconductors. When a mixture of methane, ammonia (or ammonium chloride) and water were illuminated in the presence of platinised TiO, a mixture of amino acids was produced 31. The products from this reaction included glycine, alanine, serine and aspartic acids. The mechanism for the process was believed to have involved the OH and NH2 radicals. The OH radicals were proposed to have reacted with the methane to generate methyl radicals. Simultaneous reduction at the platinised sites generated hydrogen. No amino acids were detected when unplatinised Ti02 was used. Both amino acids and peptides were produced when methane was replaced by glucose in the previous system3*. Semiconductor particles have also catalysed nucleic acid base formation33. When KCN, NH3 and H20 were irradiated in the presence Ti02 hypoxanthine was produced (Scheme 9). This product was also obtained TiQ KCN+NH,+HP
-
tN+lJH,
when platinised Ti02 and CdS were used as photocatalysts. Relatively few reductions catalysed by irradiated semiconductor materials have been reported. This is due to the position of the conductance band of the semiconductor being too positive to effect an efficient electron transfer, and oxygen strongly competing for the conduction band electrons. In certain cases the use of an electron relay such as methyl viologen can facilitate the electron transfer process. The photoelectrochemical hydrogenation of both double and triple bonds has been reported using CdS and Ti0234,35. The hydrogenation of acetylene catalysed by platinised CdS is displayed in Scheme 10. The reduction catalysed by rhodium loaded TiO, was also investigated however this was found to be a less efficient process. The photoelectrochemical hydrogenation of olefins has been performed using TiO, with ethanol as an electron source. Simple monosubstituted olefins were found to be more reactive than sterically crowded materials. A 63% yield has been reported for the hydrogenation of 2-methyl-pentene after 24 hours irradiation35 (Scheme II). The intensity of light has been found to have an important effect on product distribution in photocatalytic reductions of other organic substrates. Shiragami et ~1.~~investigated the use of CdS as a photocatalyst with substrates such as aromatic ketones, electron deficient alkenes and 1-benzylnicotinamide (BNA). With the ketones and the benzylnicotinamide at lower light intensities the yields of dimer products, such as pinacols and the dimer (BNA)2, were found to increase. At higher light intensities two electron reduction products, i.e. alcohols and 1-benzyl-1:4-dihydronicotinamide, predominated. When alkenes were used as the
llV
-1
Scheme 9
HC==CH + S2- -
CHs-CH3+S
H&&I-I
CHJ--cH3+S
+ S2- -
Scheme IO
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CH3
CI-4 Ti02, hv CH3--C==CH--C~--CH3
-
I CH3-CH-CI+C~~H3
Scheme I1
occurred substrate a cis/trans photoisomerisation regardless of the light flux. Valence isomerisations photoinduced by semiconductors have been described by Al-Ekabi and de Mayo3’. The photoinduced isomerisation of hexamethyl-Dewar benzene to hexamethylbenzene was achieved in methylene chloride using CdS, TiOp and ZnO as photocatalysts. Quantum yields of greater than unity were reported for the reactions using TiOz and ZnO. Cadmium sulphide has also been used for geometric isomerisations. A series of cis-4-substituted stilbenes have been reported to undergo cis/trans isomerisation by photocatalysis with virtually quantitative yields3*. Stereoselective synthesis has been achieved via photomediated semiconductor reactions39. Illumination of a mixture of cis and trans 2:6_diaminopimelic acids in the presence of platinised cadmium sulphide produced piperidine-2:6-dicarboxylic acid. Both the cis/trans ratio and the yield of this product were both found to be dependent on the nature of the catalyst and the platinum loading. The photocatalytic racemization of amino acids by CdS has been reported by Ohtani et ~1.~~.The mechanism for the racemization of I-Lysine was believed to have involved both reduction and oxidation via the photoexcited electrons and positive holes on the semiconductor. The racemization of other amino acids was investigated including 1-leucine, 1-phenylalanine and I-glutamic acid with percentage conversions of 3 l%, 28% and 44% respectively reported after 40 hours photolysis. This process may provide a relatively inexpensive method for converting less valuable enantiomers into more valuable isomers. Cyclisation reactions have been performed using semiconductor photocatalysis4’. When 1: 1-di-p-anisylethylene was illuminated in the presence of CdS two cyclised and two open chain dimeric products were generated. In the presence of oxygen the di-p-anisyl ketone product predominated. The production of I-pipcolinic acid from the amino acid, I-lysine is another example of a photocatalysed cyclisation reaction 42. The number of defects and surface area of the platinised TiO, catalyst was reported to affect the product yield. Table 3 summarises some of the conversions that have been catalysed by illuminated semiconductor materials. As has been illustrated from the preceding paragraphs a wide range of different reactions can be effected using semiconductor photocatalysts. There are, however, certain limitations that need to be considered which have restricted the commercialisation of these processes to date. Firstly, the time taken for several of the conversions
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can be fairly lengthy, greater than 48 hours for some examples. The energy input in these cases could exceed many traditional synthetic methods. There is substantial work being carried out on modification of the catalysts to improve efficiencies so that these difficulties may be surmounted. Another significant problem is encountered when using catalysts such as CdS, which are prone to photodecomposition. This results when the photogenerated holes in the valence band promote the bond cleavage between the surface atoms. These atoms may subsequently dissolve in the electrolyte contaminating it with cadmium, which is a particularly toxic metal itself. This can be minimised by coating with films of inert materials, immobilising the semiconductor in a protective film or by the addition of electron donors to the electrolyte solution4. The problems associated with photodecomposition are, however, not experienced when using materials such as Ti02 or Sn02. Although these materials absorb UV light, and therefore have a higher energy input requirement, modification of these catalysts to enable them to absorb visible light is being investigated. The ability of photocatalysis to promote isomeric transformation is exciting since many traditional methods of promoting such reactions are lengthy and cumbersome. This is of particular importance to the pharmaceutical industry where chiral synthesis is becoming more significant. Semiconductor photocatalysis could provide a route for preparing chiral molecules more cost effectively than many traditional methods. An important difficulty that may be encountered in scaling up many of these processes will be in the reactor design. The basic problem of scale-up of photocatalytic processes is ensuring a uniform distribution of light to a large surface area of the catalyst. Therefore, when designing a new reactor, it is necessary to consider two important parameters; the distribution of light inside the reactor and the provision of a high surface area for the catalyst per unit volume of reactor. There are two different reactor designs which could be considered; a suspended particle catalyst reactors or an immobilised film catalyst reactors. Suspended catalyst reactors may be a more efficient design with respect to quantum yield, and volume efficiency but the separation of the catalyst from the reaction mixture presents difficulties. In the laboratory this stage is easily accomplished by either filtration or centrifugation. However, these options are less feasible on an industrial scale. Alternative separation methods may include flocculation and sedimentation or membrane filtration. The immobilised catalyst film reactor may not have
Semiconductor photocatalysis: P.K.J. Robertson Table3
Examples of organic compounds and substrates which have undergone photocatalytic transformations using semiconductors
Reactant
Product
Reference
Organic Compounds Alcohols, Polyols
Carbonyl compounds, Amines, Amino acids, Peptides
18, 19, 32
Amines
Schiff bases, Azo products
23, 24
Acids
Alkanes
15, 16
Aromatic hydrocarbons
Alcohols, Carbonyl compounds
20, 21
Alkanes
Amino acids
31, 32
Alkenes
Alkanes, Carbonyl compounds
28, 29, 30, 35
Alkynes Ethers
Alkanes, Carbonyl compounds Esters
34, 35
Sulphides
Sulphoxides, Carbonyl compounds
25, 26, 27
Thiols
Sulphides
21
problems of catalyst separation associated with it but, since the reaction occurs at a liquid-solid interface, only a fraction of the catalyst will be in contact with the reactant in solution. Therefore, a reactor which provides a high ratio of illuminated, immobilised catalyst to illuminated surface is required. This problem is being investigated in a number of ways, including immobilisation of the catalyst on materials with large surface areas. In summary, therefore, there are several factors which have so far limited the wide application and commercialisation of this technology in chemical synthesis. As a result of these limitations the immediate application of semiconductor photocatalysis may be restricted to the production of small quantities of high value chemicals for which the technology can compete economically or where there are no alternative synthetic routes. This may be of particular interest to the pharmaceutical or fine chemical industries, particularly where chiral synthesis an expanding interest.
Photocatalytic treatment of waste materials The use of TiOz as a photocatalyst for the destruction of polluting materials has now been demonstrated to be an effective process43. The overall process may be described as follows. A reaction vessel containing the waste material and the semiconductor is illuminated and the waste is converted to a non-toxic form either by an oxidation or reduction process. An example of a typical process would be the destruction of haloaromatic compounds, resulting in the production of CO*, Hz0 and the halide ion. The overall process for the photo-oxidation of waste materials (P), sensitised by TiOz is shown in Scheme 1244. There is a certain amount of debate regarding the mechanism for the destructionU. Some believe that
P + 02w
I
hydroxyl radicals are generated via oxidation at the valence band. These radicals subsequently oxidise the polluting material, while at the conductance band an electron is donated to oxygen thereby generating the superoxide radical anion. This is illustrated in Figure 2. Other workers have, however, suggested that the species being oxidised can transfer an electron directly to the valence band45. In the case of chlorophenol it has been proposed that, at low concentrations, the oxidation occurs via the hydroxyl radicals while at higher concentrations direct valence band oxidation predominates. Regardless of which mechanism for mineralisation predominates, the oxidation potential of either the semiconductor valence band (3.1 volts) or the hydroxyl radical (2.8 volts) is far more oxidising than many materials that are commonly used for waste treatment, including ozone (2.07 volts), hydrogen peroxide (1.78 volts), hypochlorous acid (1.49 volts) and chlorine (1.36 volts). In his study on the kinetics of the oxidation of organic molecules using Ti02, GerischeF6 proposed
I_,-
7Y
-bv
Figure.2 The process of semiconductor purification”
photo-catalysis for water
CO, + Hz0 + Mineral acids
Scheme 12
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Semiconductor photocatalysis: P.K.J. Robertson that the electron transfer from the conduction band to oxygen was the rate determining step of this mechanism. It was also reported that high yields were achieved only when small particles were used. One of the first reported cases of the use of semiconductor photocatalysis in the destruction of organic compounds was reported in 1976 by Carey et al!‘. They reported the successful degradation of biphenyl and chlorobiphenyls in the presence of TiOz. Since then the destruction of many materials using TiOz has been investigated. In 1981 Fox and Chen4* reported that several aromatic hydrocarbons could be oxidatively cleaved when reversibly adsorbed onto the surface of an irradiated suspension of TiOa. This work was subsequently expanded to include a wide range of organic materials including alkanes, alkenes, arenes, acids, esters, amides, amines, ethers, thioethers, organohalides, aldehydes, ketones, organosilanes and phosphonates49-52. Ollis et a1.53*54 subsequently reported the use of Ti02 in the complete destruction of several halogenated hydrocarbons including trichloroethane, methylene chloride, chloroform and carbon tetrachloride. It was found that a simple Langmuirian rate equation represented the destruction of these materials. Matthews55-59 investigated the development of a range of waste treatment cells using a Ti02 photocatalyst including immobilised film and suspended particle reactors. A particularly successful reactor incorporated a thin film coating of Ti02 applied to the internal surface of a spiral glass tube. The solution containing the waste effluent was then pumped through the tube while being illuminated with a UV light. This cell was reasonably efficient in the removal of a variety of organic pollutants with 96% destruction being reported within ten minutes57. A TiO, suspension reactor illuminated with sunlight for the destruction of phenol was also investigated 59. The concentration of this material was reduced from 1Oppm to 1Oppb within 80 minutes, with virtually total mineralisation in 110 minutes. Trinitrotoluene (TNT) contamination is a problem experienced in many former military installations in the United StateP. The destruction of TNT has been achieved in a Ti02 slurry reactor with 90% mineralisation occurring within 120 minutes. These results indicate that this technique is very effective for the destruction of these highly toxic and explosive materials. The destruction of biological materials such as Escherichia Coli by Ti02 has been reported by Ireland et aL6’. They reported a reduction in the concentration of viable organisms of seven orders of magnitude within six minutes photolysis. After nine minutes reaction time the concentration of the E. Coli was below the detection limits of analysis. Eggins et aL6* investigated the potential of the semiconductor photocatalysis for the treatment of potable water. The rapid decomposition of humic substances using a Ti02 photocatalyst was reported. Table 4 displays examples of the waste materials
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Table4 Examples of materials which have undergone photomineralisation using TiO*” Class of compound
Examples
Alkanes
methane; pentane; heptane; n-dodecane; cyclohexane; paraffin
Haloalkanes
mono-, di-, tri-, and tetrachloromethane; dichloroethane; pentachloroethane; di and tribromoethane; I:2-dichloropropane
Aliphatic alcohols
methanol; ethanol; n- and iso-propanol; butanol; penta- I :Cdiol
Aliphatic carboxylic acids
methanoic; ethanoic; trichloroacetic; butyric; oxalic
Alkenes
propene; cyclohexene
Haloalkenes
di-, tri- and tetra-chloroethene; hexafluoropropene
Aromatics
benzene; naphthalene
Haloaromatics
chloro and bromobenzene; chlorobenzenes; halophenols
Phenols
phenol; hydroquinone; catecol; resorcinol; cresol; nitrophenol
Aromatic carboxylic acids
benzoic; phthalic; salicyclic
Polymers
polyethylene; PVC
Surfactants
polyethylene glycol; p-nonyl phenyl ether; sodium dodecyl; benzene sulphonate; paraxon; malathion
Herbicides
methyl viologen; atrazine; simazine; bentazon
Pesticides
DDT; parathion; lindane; monocrotophos
Dyes
methylene blue; rhodamine B; methyl orange; fluorescein
Explosives
trinitrotoluene
Bacteria
E. Coli
that have been successfully treated using Ti02 photocatalystP. Currently there is an extensive interest in this area of research and, although to date only a few systems have been investigated on a pilot plant scale, it is probable that commercially viable treatment systems will result from this work63. Many of the design considerations outlined above for the synthetic reactors also apply for the waste treatment reactor. One system which has received attention was developed by Hamnett et al.&l at the University of Newcastle upon Tyne. This is based on a plate vortex reactor illuminated above by a UV Light source. Bahnemann ef aZ.65 have developed a thin film fixed bed reactor. The Ti02 photocatalyst is immobilised on a sloping plate over which a thin film (~100 km) of the waste water flows while illuminated from above. Matthews% investigated the economics of semiconductor photocatalysis in comparison to activated carbon adsorption. The cost of both techniques were found to be comparable. However, he predicted that, as catalyst efficiencies were improved, photocatalysis could prove to be a cheaper method in the future. Rajeshwd’ determined the cost of treating waste using Ti02 photocatalysis to be $5.22 (US) per 3785 litres of waste
Semiconductor photocatalysis: P.K.J. Robertson compared to a price of $6.20 for activated adsorption.
carbon
Conclusion The use of semiconductors as photocatalysts for synthesis and waste treatment has been discussed. These catalysts represent a class of materials which can be used for more environmentally acceptable production techniques since their application can reduce the amount of harmful materials being emitted to the environment. A wide range of different chemical reactions can be initiated by the photocatalysts, providing in some cases new synthetic routes for generating otherwise inaccessible products. The photocatalysts also have the advantage of providing greater reaction control and thereby allowing product selection. The problems of reactor design for scale-up of the process, improved product yield for some reactions and inhibition of catalyst photocorrosion still have to be addressed. However, these difficulties may be surmountable in the future. The photocatalytic treatment of the waste materials has been shown to be an effective technique for the treatment of a very diverse range of pollutant compounds. Considering the extent of research in this area the development of an effective waste treatment reactor may simply be a matter of time.
18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38
Acknowledgements I would like to thank my colleagues Brian McGaw and Stephanie Rigby of The Robert Gordon University for their useful advice and comments during the preparation of this manuscript.
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