Journal of Photochemistry and Photobiology C: Photochemistry Reviews 10 (2009) 96–110

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Journal of Photochemistry and Photobiology C:Photochemistry Reviews

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Review

Photochemistry with microwavesCatalysts and environmental applications

Satoshi Horikoshia,∗, Nick Serponeb

a Research Institute for Science and Technology, Tokyo University of Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japanb Gruppo Fotochimico, Dipartimento di Chimica Organica, Universita di Pavia, Via Taramelli 10, Pavia, 27100, Italy

a r t i c l e i n f o

Article history:Received 5 May 2009Received in revised form 12 June 2009Accepted 12 June 2009Available online 30 June 2009

Keywords:PhotochemistryMicrowavesCatalystsEnvironmental applicationsElectrodeless lampsThermal and non-thermal microwaveeffectsVacuum-UV, UV and visible radiation

a b s t r a c t

Microwave radiation has recently become an active source of thermal energy in numerous chemical reac-tions. As such, the microwave energy is not ordinarily and is not likely to be used to drive photochemicalreactions. Accordingly, is the role of microwaves then relegated solely to be a source of heat? They do nothave to be since photochemical reactions can be activated indirectly by microwaves using the UV lightemitted from certain gas-fills excited by microwave radiation. This article examines the microwave radi-ation not only as a dielectric heat source but also a source of vacuum-UV radiation and UV light throughmicrowave discharge electrodeless lamp devices, which in some cases (depending on design) can alsoserve as photoreactors.

© 2009 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 971.1. Microwaves in chemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97

2. Influence of microwaves on photochemical reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 972.1. Mechanisms of converting microwave energy into thermal (caloric) energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97

2.1.1. Dielectric loss heating. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 972.1.2. Conduction loss heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 972.1.3. Magnetic loss heating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 972.1.4. Penetration depth of microwaves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98

2.2. Microwave-assisted processes in environmental remediation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 982.3. Thermal effect and specific non-thermal (non-caloric) effect in catalyzed heterogeneous reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 982.4. Generation of light with microwave energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 992.5. Mysterious effects of the microwave energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

3. Reactor configurations and applications of microwave-/photo-assisted processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1003.1. Experimental setup of an integrated microwave/photoreactor (MPR) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

3.2. Reason for using the microwave technique to effect TiO2-photo-assisted reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1013.3. Microwave effects on various TiO2 particle specimens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1023.4. Relationship(s) between heating effectiveness and dielectric properties of various TiO2 samples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1033.5. Effects of microwaves in the mechanism of the photodegradation of organic substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1033.6. Probing the non-caloric microwave effect(s) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104

∗ Corresponding author.E-mail address: horikosi@rs.noda.tus.ac.jp (S. Horikoshi).

1389-5567/$20.00 © 2009 Elsevier B.V. All rights reserved.doi:10.1016/j.jphotochemrev.2009.06.001

4.3. Air purification using a MDEL system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1075. Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109. . . . . .

1

1

ntrTeptnh(t

toaw[coniedtmTts

2

mnsitd

2(

mtiieph

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

. Introduction

.1. Microwaves in chemistry

Microwave radiation has become one of the more popular tech-ologies both domestically and industrially used and describeshe low-energy electromagnetic radiation that spans the frequencyange 30 GHz to 300 MHz, i.e. the wavelengths from 100 cm to 1 cm.wo rather familiar devices that make extensive use of this low-nergy radiation are the microwave cooking oven and the cellularhone. Early (since 1949) industrial examples may be found in thehermal molding of wood and plastics, and in the drying of medici-al products, fibers, teas, and cigarettes. In recent years, microwavesave been used in the sintering of ceramics, in cancer treatmentshyperthermia), in the drying and sterilization of foodstuffs, and inhe vulcanization of rubber, among others.

In the chemical and materials sciences, early (1975) fundamen-al research into microwaves focused on the microwave heatingf alumina (1968 [1]) and in the desulfurization of coke used ascarbon fuel [2]. In the inorganic chemistry area, active researchas directed at the microwave sintering of ceramics (early 1980s)

3] in which the principal feature was the formation of compactrystal grains in short time at relatively low temperatures. In therganic field, microwave irradiation to drive organic syntheses wasot explored until the mid-1980s, i.e. not until the first two reports

n 1986 by Gedye et al. [4] and Giguere et al. [5] on microwave-nhanced organic chemistry. Since then, many organic chemistsiscovered the benefits of microwaves to drive synthetic reac-ions, as a consequence of which industries began to manufacture

icrowave ovens specifically designed for research laboratories.he number of reports on the use of microwaves as an energy sourceo drive chemical reactions has witnessed an astronomical growthince [6].

. Influence of microwaves on photochemical reactions

An important question in photochemistry is the role thaticrowaves can play in driving photochemical reactions. In a large

umber of cases, microwaves are simply a heat source. To be morepecific, the role of microwaves in chemical reactions can be classedn terms of thermal energy and some other form of energy. This sec-ion first explores the mechanisms of generation of heat and thenescribes other possible roles of microwaves in chemical reactions.

.1. Mechanisms of converting microwave energy into thermalcaloric) energy

An important process of energy conversion involves convertingicrowave energy into thermal energy [7,8]. Generally speaking,

here exist three types of heating by microwaves: (i) dielectric heat-

ng, (ii) conduction loss heating, and (iii) magnetic loss heating. Fornstance, induction heating by the microwaves’ magnetic field isxpected to occur in catalyzed reactions involving solids, an exam-le being the rapid induction heating of magnetite (Fe3O4) but notematite (Fe2O3) as the latter is not a magnetic material [9].

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109

The thermal energy P produced per unit volume originating frommicrowave radiation is given by Eq. (1),

P = �fε0ε′′|E|2 + 12 �|E|2 + �f�0�′′|H|2 (1)

where |E| and |H| denote the strength of the electric field andmagnetic field of the microwaves, respectively; � is the electri-cal conductivity; f is the frequency of the microwaves; ε0 is thepermittivity in vacuum; ε′′ is the dielectric loss factor; �0 is themagnetic permeability in vacuum; and �′′ is the magnetic loss. Thefirst term in Eq. (1) expresses to dielectric loss heating; the sec-ond term denotes conduction loss heating, whereas magnetic lossheating is given by the third term. Microwave heating of solutionsis ruled mostly by dielectric loss heating, whereas conduction lossheating involves mostly solid materials. Generation of heat by mag-netic loss heating is expected only in magnetic (solid) materials.A catalyzed reaction occurring on a solid catalyst is expected toshow characteristic differences with the solution bulk because thereactant substrates in the liquid are adsorbed on the solid surface.Temperature rise in systems involving solely gaseous molecules isnegligible because the molecular density is small.

2.1.1. Dielectric loss heatingA mechanism by which a non-conductive material can be heated

by the microwaves’ electric and magnetic fields involves molecu-lar rotation of the molecules’ electric dipole moment, which alignsitself with the field. As the field alternates, the molecules reversedirection and accelerate the motion of individual molecules oratoms, thereby generating heat through friction of the moleculesrotating against each other. Dielectric loss heating can therefore beapplied to the heating of solutions. By contrast, heating of electro-conductive solids, such as metals, implicate ohmic losses in thematerial in virtue of the current flow induced by the oscillatingelectric field.

2.1.2. Conduction loss heatingThe hydroxide ion is a typical ionic species with both ionic and

dipolar characteristics. For solutions containing large amounts ofionic salts, the conductive loss effect can become larger than dipo-lar relaxation. For solids, conduction losses tend to be slight atambient temperature but change substantially with temperature.A typical example is alumina (Al2O3) for which dielectric losses arenegligibly small (∼10−3) at room temperature but can reach fusionin very short time (minutes) in the microwave cavity. This effectarises from a strong increase of conduction losses associated withthe thermal activation of electrons which migrate from the oxygen2p valence band to the 3s3p conduction band of alumina. Moreover,conduction losses in solids are usually enhanced by defects in thematerials which sharply reduce the energy gap between the valenceand conduction bands. To the extent that conduction losses are highfor carbon black powder, it is often used as added loss impuritiesor additives to induce losses within the solids for which dielectric

S. Horikoshi, N. Serpone / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 10 (2009) 96–110 97

4. Microwave discharge electrodeless lamp (MDEL) devices . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1054.1. Wastewater treatment using MDEL system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1054.2. Purification of a dioxin-contaminated solid using MDEL and pyrolysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106

losses are too small.

2.1.3. Magnetic loss heatingFor metal oxides such as ferrites and other magnetic materi-

als, magnetic losses occur in the microwave region. These losses

9 d Photobiology C: Photochemistry Reviews 10 (2009) 96–110

aamimmal

2

tεrE

D

wNmdrioahi

2

awts[asotte

tCutowwtghafosoItbi

n

Table 1Extraction of contaminant by using the microwave extraction method [24].

Year Sample Pollutants Reference

PAH (polycyclic aromatic hydrocarbon)1986 Soil and food PAH 251994 Soil, sediment PAH 261995 Soil PAH 271996 Polyurethane foam PAH 28

PCBs (polychlorinated biphenyls)1995 Soil, sediment PCB 291996 Marine PCB 301997 Sediment PCB 311997 Sediment PCDD/F and PCB 321999 Fly ash PCB 24

PCDDs (polychlorinated dibenzo-p-dioxins)1995 Sewage sludge, fly ash,

sediment, soilPCDD/F 33

1996 Soil, sediment, urban dust PCDD/F 34

Pesticides and other toxins1995 Soil, compost Phenol, pesticide/Cl 351996 Soil Pesticide/Cl 361997 Soil Phenoric compounds 371998 Soil Atrazine 381999 Soil, water, organic solvent Triazine 392000 Soil Mono-phenols 402001 Soil Pesticide 412002 Soil Metribuzin 422002 Soil N-Methylcarbamates 432003 Soil Carbamate pesticides 442003 Soil Chlorophenol 452005 Soil Organophosphorous

pesticides46

8 S. Horikoshi, N. Serpone / Journal of Photochemistry an

re different from hysteresis or eddy current losses because theyre induced by domain wall and electron-spin resonance. Suchaterials are typically placed at positions of magnetic field max-

ma for optimal absorption of the microwave energy. Transitionetal oxides such as those of iron, nickel, and cobalt possess highagnetic losses and can thus be used as added loss impurities or

dditives to induce losses within those solids for which dielectricoss is too small.

.1.4. Penetration depth of microwavesMicrowaves can penetrate deeply into dielectric loss liquid sys-

ems. For instance, in low loss dielectrics (i.e. for ε′′/ε′ � 1; where′ is the dielectric constant), the depth (Dp, in cm) that microwaveadiation penetrates the absorbing media can be estimated fromq. (2) [10],

p =(

�0

2�

)(√ε′

ε′′

)(2)

here �0 is the wavelength of the irradiation (�0(2.45 GHz) = 0.122 m).ote that Dp denotes the depth at which the power density of theicrowaves is reduced to 1/e of its initial value, and is strongly

ependent on temperature and the frequency of the microwaveadiation. The penetration depth can increase 3.7-fold by anncrease in temperature from ambient to 90 ◦C owing to a decreasef microwave absorption by the medium [11,12]. In other words, thebsorbing media become more transparent to microwaves at theigher temperature. The penetration depth is an important factor

n microwave heating.

.2. Microwave-assisted processes in environmental remediation

The history of microwave sterilization has been used primarilys a disinfection technology of food in an industrial scale [13]. It isell known that microwave-sterilized food keeps for longer periods

han does thermally sterilized food. This observation points to somepecial effects of the microwaves that have yet to be clarified. Sato14] carried out detailed experiments at the laboratory scale and onlarger scale using a one-way flow-through method for bacterial

terilization. Subsequent experiments on lake waters carried outn-site using a flow-through method with a microwave heating sys-em demonstrated the effective killing of Microcystis aeruginosa. Inhis case, bacterial sterilization was caused by a microwave thermalffect.

An environmental treatment process involving catalyzed reac-ions driven by microwave heating was first reported by Wan [15].hlorinated and brominated volatile organics were decomposedsing metallic reduction catalysts such as Ni, Fe, and Co subjectedo microwave irradiation [16]. Continuous and pulsed irradiationf catalyzed reaction by microwaves have been examined togetherith the frequency effect [17]. Dehydration of halogenated gasesith metallic catalysts under microwave radiation has been inves-

igated actively [18,19], as have the decomposition of other toxicases [20,21]. Wada et al. [22] reported the microwave-assistedydrogenation of chlorinated phenols using platinum supported onn activated carbon catalyst. Phenol was the only organic productormed in the process through prompt microwave-induced heatingf the activated carbon with dechlorination taking place at the Pturface. Kobayashi et al. [23] examined the controlled adsorptionf substrates on a solid catalyst surface by the effect of microwaves.ndeed, these authors demonstrated that adsorption and desorp-

ion on activated carbon and on zeolitic surfaces can be controlledy the microwave energy for mass processing, which in all casesnvolved prompt heating by the microwaves.

An extraction technique of pollutants using microwave tech-iques was reported in the early 1980s, and only in the 1990s

2006 Soil, sediment Aliphatichydrocarbons

47

were research studies undertaken actively in this area. Examples ofmicrowave extraction of organic toxins from natural environmentsare listed in Table 1 [24–47]. The degradation of organic toxins insoils has been effected using microwave heating. In this regard,Abramovitch et al. treated PCBs (polychlorinated biphenyls) [48]and PAHs (polycyclic aromatic hydrocarbons) [49] by microwavepyrolysis. It was suggested that iron in the soil was a principalinfluential factor of the degradation. Iron oxide [50] and active car-bon [51] can be mixed in the soil to improve absorption of themicrowaves.

2.3. Thermal effect and specific non-thermal (non-caloric) effectin catalyzed heterogeneous reactions

As a dielectric loss heating source, microwave radiation has ledto some remarkable enhancements of reaction dynamics in vari-ous organic reactions. Heating effects in such processes have beendelineated from conventional heating by considering that catalyzedreactions could be influenced by temperature gradients (hot-spots)created within the catalyst beds [52]. In this regard, although reac-tion dynamics and selectivity of various solid catalysts are affectedby specific non-thermal (non-caloric) effects when subjected tomicrowave radiation fields (dielectric loss heating), the origin ofsuch effects is likely due to the impact of the microwave fields onthe catalysts.

As an example of this supposition, the hydrogenation of ace-tophenone to phenylethanol in 2-propanol solvent as the hydrogensource with various nickel catalysts (Urushibara nickel: Ni/Zn

[53,54] and Raney nickel: Ni/Al catalyst) was recently examined[55] to assess how microwave dielectric loss heating, in contrastto conventional heating, affects the surface composition of thesecatalysts as well as the product yields.

S. Horikoshi, N. Serpone / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 10 (2009) 96–110 99

Table 2Content ratios of the elements (by Electron Dispersive X-ray techniques) for severalNi catalyst specimens [55].

Content ratio (%)

Ni Zn Al

U-Ni0 min 10.9 89.1 –MW – 150 min 7.8 92.2 –Oil bath – 150 min 9.9 90.1 –

U-Ni-A0 min 81.4 18.6 –MW – 150 min 82.8 17.2 –Oil bath – 150 min 78.1 21.9 –

U-Ni-B0 min 22.7 77.3 –MW – 150 min 17.7 82.3 –Oil bath – 150 min 12.4 87.6 –

U-Ni-N0 min 1.1 98.9 –MW – 150 min 0.2 99.8 –Oil bath – 150 min 1.1 98.9 –

R-W-60 min 88.3 – 11.7MW – 150 min 79.5 – 20.5Oil bath – 150 min 80.3 – 19.7

R-W-2

UtSra(mcatBm

a

0 min 87.9 – 12.1MW – 150 min 91.2 – 8.8Oil bath – 150 min 93.3 – 6.7

Four different Urushibara catalyst specimens (the U-Ni, U-Ni-A,-Ni-B and U-Ni-N) and two specimens of aggregated particles of

he Raney-Ni catalyst were examined to drive reaction (3) (Table 2).EM images of U-Ni-B showed the particles to be somewhat porouselative to U-Ni, U-Ni-A and U-Ni-N samples, confirmed by surfacerea measurements. Hydrogenation caused no changes in the shapespherical for the Urushibara and platelets for the Rainey speci-

ens) of the catalysts. The initial elemental compositions of theatalysts and the compositions subsequent to 150 min of microwavend conventional heating are given in Table 2. Catalysts containinghe greater quantity of Ni in the Ni/Zn systems were U-Ni-A > U-Ni-> U-Ni > U-Ni-N; no significant correlation existed between theicrowave effect and the Ni content ratio.

(3)

The % yields of 1-phenylethanol produced with each of the cat-lyst specimens (reaction (3)) are illustrated in the histograms of

Fig. 2. (a) A doughnut style MDEL on the dish, and (b) under

Fig. 1. Synthesis yields of 1-phenylethanol in the presence of Urushibara catalystsU-Ni, U-Ni-A, U-Ni-B, U-Ni-N, and the Rainey R-W-6 and R-W-2 hydrogenation cata-lysts when subjected to microwave dielectric loss heating and conventional heating.From ref. [55]-Reproduced by permission of The Royal Society of Chemistry.

Fig. 1 at various time intervals up to 150 min. The microwave effectwas clearly evident for the U-Ni-B catalyst: note the large dif-ferences in yields between conventional heating and microwavedielectric loss heating, such as for instance the ∼52% yield byconventional heating with the ∼92% yield of phenylethanol afteronly 30 min using U-Ni-B, a nearly 70% increase in yield with themicrowaves. At 150 min into the reaction, the yield was ca. 95% withmicrowave heating relative to ca. 68% under conventional heating.The worst performing catalyst was the acid-washed Urushibara Nispecimen U-Ni-A regardless of heating methods, whereas the bestperforming catalysts were the alkali-washed U-Ni-B sample and theRaney Ni specimen R-W-6.

2.4. Generation of light with microwave energy

As alluded to above, the typical use of microwave radiation isas a heat source since microwave energy is too weak to be useddirectly to drive chemical reactions as the wavelength and thephoton energy of the 2.45-GHz microwaves (most often used fre-quency) is only ca. 12.24 cm and 1 × 10−5 eV, respectively, withthe latter being about five orders of magnitude smaller than typ-

ical chemical bond energies. Chemical reactions cannot be drivenby microwave energy. However, the role of microwaves in chem-ical reactions might be attractive if it were possible to convertmicrowave energy into some other form of energy that could thenbe used as a new tool in the chemist’s repertoire.

microwave irradiation in a microwave domestic oven.

100 S. Horikoshi, N. Serpone / Journal of Photochemistry and Phot

Fb

rdvtoHtudoibr

dfibosttolttfl

e

e

e

A

H

dtdtWtadcal

ig. 3. Spectrum of vacuum-UV (183 nm) and UVC (252 nm) wavelengths emittedy the mercury/argon gas-fill in the MDEL lamp. Reproduced from ref. [58].

The microwave energy can be used indirectly to produce UVadiation by means of microwave discharge electrodeless lampevices (MDEL) that convert the energy of the microwaves intoacuum-UV (VUV) and UV light. One such lamp design is illus-rated in Fig. 2a with the device shaped as a doughnut. A featuref this lamp is that it consists of a quartz envelope containing ag/Ar gas-fill; note that the device has no electrodes and no elec-

rical wires. As such the MDEL can be freely shaped depending onsage. When the doughnut-shaped MDEL is placed in a microwaveomestic oven and is microwave-irradiated, it causes the activationf the gas-fill that subsequently emits UV radiation (Fig. 2b) of var-ous wavelengths that depend only on the nature of the gas-fill (seeelow). The fabrication of MDELs has been described in an earliereport [56].

A brief sequence of the events occurring in the Hg/Ar gas-fillevice is summarized by reactions (3)–(7) [57]. Free electrons in thell (i.e. electrons that have become separated from the environmentecause of the ambient energy) accelerate as a result of the energyf the electromagnetic field of the microwave (MW) radiation, sub-equent to which they collide with the Ar atoms thereby ionizinghem to Ar+ ions and releasing additional electrons. Repetition ofhese steps causes the number of electrons to increase significantlyver a short time period (effect is referred to as an avalanche). Theowest energy level of the Ar atom is excited predominantly relativeo the Hg atom to yield the excited Ar* atom which in turn activateshe Hg atom through energy transfer. Emission of UV light occursrom the excited Hg* atom, which then returns to its ground stateevel.

− + MW → e−∗(accelerated) (3)

−∗ + Ar → Ar+ + 2e− (4)

−∗ + Ar+ → Ar∗ (5)

r∗ + Hg → Hg∗ + Ar (6)

g∗ → Hg + h� (7)

A spectral analysis of the bluish white light emitted by the MDELevice is displayed in Fig. 3. The 252-nm UVC light together withhe 183-nm vacuum-UV wavelength are generated by the MDELevice [58]. The vacuum-UV photon energy at 183 nm, correspondso an energy of ca. 6.8 eV, enough to cleave most chemical bonds.

hen subjected to VUV radiation, oxygen and water are convertedo reactive oxygen species such as ozone (O3), singlet oxygen (1O2)

nd •OH radicals [59], which can be put to great use in various oxi-ation processes [60]. With the 252-nm UV light the MDEL devicean be used as a germicidal lamp. Examples of microwave-/photo-ssisted reactions using MDEL devices/photoreactors are describedater.

obiology C: Photochemistry Reviews 10 (2009) 96–110

2.5. Mysterious effects of the microwave energy

Currently, microwave dielectric loss heating is a natural conse-quence of microwave radiation in both domestic applications and inparticularly in the field of microwave-assisted chemical reactions.The effect of microwave irradiation in chemical reactions can beattributed to a combination of thermal (caloric) and to such specificnon-thermal (non-caloric) effects as overheating, hot-spots, andselective heating, together with non-thermal effects of the highlypolarizing field, in addition to effects on the mobility and diffu-sion of molecules or atoms that may increase the probabilities ofeffective collisions [61].

In organic syntheses, the rates of reactions affected by conven-tional heating methods are enhanced significantly by microwaveradiation as a result of some specific non-thermal microwave effects[62–64]. Other studies have examined the microwave effects. Localheating effects (i.e., hot-spots) in heterogeneous catalyzed reac-tions have also been investigated [65,66]. A more mysteriousnature, however, is the dramatic enhancement of the reactiondynamics observed in organic syntheses induced by microwaveradiation under cooled ambient temperature conditions [67,68].It appears that although chemical reactions are enhanced bymicrowave irradiation, reaction efficiency is improved further atambient temperature when reactions take place under microwaveheating while simultaneously cooled. Nonetheless, despite certainclaims, the essential features of microwave effects have yet to befully clarified and fully understood. Accordingly, further researchis necessary to clarify some of these effects through establishinga library of observed data, particularly as they may be relevant inapplications to environmental remediation processes.

3. Reactor configurations and applications ofmicrowave-/photo-assisted processes

3.1. Experimental setup of an integrated microwave/photoreactor(MPR)

Continuous microwave irradiation of a wastewater samplecan be achieved in a single mode applicator using a 2.45-GHzmicrowave generator, a power monitor (to assess incident andreflected microwave power), a three stub tuner and an isolator (anair cooling device) such as fabricated by the Hitachi Kyowa Engi-neering Co. Ltd. (Fig. 4a). A typical reactor set-up would containa toxin in aqueous media (30 mL) containing TiO2 particles (load-ing, 60 mg) introduced into the closed (with two Byton O-rings anda stainless steel cap) high-pressure 150-mL Pyrex glass cylindri-cal reactor from the top side that is subsequently irradiated with alow-pressure Hg lamp through a fiber optic. The solution tempera-ture is typically measured with a K-type thermocouple. A pressuregauge and a release bulb are connected to the cover of the reactor.The reaction mixture can be stirred continually during the irradia-tion using a magnetic bar whether under batch or reflux conditions.Other microwave frequencies (e.g. 5.8 GHz) and a multimode appli-cator can also be used to effect the reaction [11]. A characteristic ofthe single-mode applicator is that it can efficiently irradiate thesample with the microwaves, although the irradiation area of thereactor could be limited somewhat. By contrast, a multimode appli-cator (Fig. 4b) can microwave irradiate a large size photoreactor,but the irradiation efficiency tends to be low in comparison with asingle mode applicator.

With such reactor configurations as those of Fig. 4, four different

methodologies can be used to examine the photodecompositionof aqueous samples of toxins in aqueous TiO2 dispersions. Thefirst is the microwave-/photo-assisted method using UV light andmicrowave irradiation (TiO2/UV/MW). The second method entailsUV irradiation alone (TiO2/UV), whereas the third method involves

S. Horikoshi, N. Serpone / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 10 (2009) 96–110 101

or hav

altoopoTtmee

3T

ttToabptoohssiiTtna

iabTtht[sotl

combination of microwaves and TiO2 photo-assisted technologiesis certainly conceivable. The photo-assisted degradation by thismetal oxide is unaffected by conventional heating (CH) – comparefor example the TiO2/UV and the TiO2/UV/CH methods in Fig. 5.

Fig. 4. (a) Photograph of an integrated microwave/photoreact

thermally assisted photodegradation of the dispersions using UVight and externally applied conventional heat (TiO2/UV/CH). Forhe latter method, the external heat is usually supplied by coatingne part of the cylindrical photoreactor with a thin metallic film onne side and at the bottom of the reactor, whereas the uncoated sideermits UV radiation to reach the dispersion. The rate of increasef temperature (error typically ≤ ±1 ◦C) and the pressure in theiO2/UV/CH method are maintained at levels otherwise identicalo those used for the TiO2/UV/MW procedure. The fourth and last

ethod involves irradiation with microwaves only (MW). No differ-nces in the temperature profiles should be observed when usingither microwave or conventional heating.

.2. Reason for using the microwave technique to effectiO2-photo-assisted reactions

Studies on environmental remediation using TiO2 specimens upo 1995 have been reviewed [69,70]. Applications of photo-assistedreatments of air pollution have been developed by immobilizingiO2 on suitable supports such as on air conditioner filters, amongthers [71]. However, such photo-assisted degradation processesre not suitable for large-scale wastewater treatment systemsecause of the slow rates of degradation of the organic compoundsresent in the wastewaters. Among the various factors that affecthe processes, three are particularly significant in the constructionf a practical water treatment plant: (i) the extent of adsorptionf the organic contaminants onto the TiO2 particle surface at theigh concentrations typically encountered in highly loaded wastetreams; (ii) the relatively slow permeation of the pollutants in thetream; and (iii) the limitation of the UV light to penetrate andrradiate the metal oxide owing to other extraneous componentsnherently present in a muddy stream, along with its effect on theiO2 particle surface in the bulk water as the degradation processakes place without significant movement of the organic contami-ants. Disposal of polluted wastewaters and the need for drainagere also important considerations.

Wastewater treatment by the TiO2-photo-assisted methods improved by combining the methods from the variousdvanced oxidation technologies (AOT): for instance, the com-ined TiO2/ozone [72], TiO2/sonochemical cavitations [73] andiO2/supercritical water [74]. Although the rates of wastewaterreatment are improved by such combinations, the activity of TiO2as not been altered by the combined methods. Novel degrada-ion techniques have been proposed to circumvent this problem

75]. The notion of irradiating TiO2 with microwaves may appeartrange because the photon energy (1 × 10−5 eV) of the microwavesf frequency 2.45 GHz is several orders of magnitude lower thanhe band-gap energy (3.2 eV) of the TiO2 semiconductor. Nonethe-ess, the photo-assisted degradation process can be enhanced with

ing a single mode applicator, and (b) multi-mode applicator.

the assistance of microwave radiation to degrade wastewaters andsoil pollutants such as dyes, polymers and surfactants [76], as wellas herbicides (e.g. 2,4-dichlorophenoxy acetic acid; 2,4-D) [77],and endocrine disruptors (e.g. bisphenol-A; BPA) [78] even underinferior photodecomposition conditions of small quantities of TiO2used, low concentration of dissolved oxygen and low light irradi-ation [76]. Thus, most of the problems encountered in wastewatertreatment by the TiO2-photo-assisted process can be resolved by anintegrated microwave-/photo-assisted methodology. In this tech-nique, a characteristic feature of the reaction on the TiO2 surfaceinvolves thermal and specific non-thermal effects originating fromabsorption of microwave radiation [77–82].

In some early studies we explored the photo-assisted degrada-tion of the cationic dye rhodamine-B (RhB) to probe the effectsof microwaves on the process [76,79]. The photodegradation rateof RhB is slow in acidic media because of the positive TiO2 sur-face charge (Ti–OH2

+). Changes (or lack thereof) in the intensityof the color of the RhB dye are illustrated in Fig. 5, in whichit is evident that the degradation dynamics are enhanced sig-nificantly when aqueous TiO2 dispersions are exposed to bothmicrowave and UV light irradiation for the same time periods(compare for example the TiO2/UV/MW with the other meth-ods). These observations demonstrate that a method that cantreat large quantities of pollutants in wastewaters by a hybrid

Fig. 5. Visual comparison of color fading in the degradation of RhB solutions(0.05 mM) subsequent to being subjected to various degradation methods for150 min. From left to right: initial RhB solution; TiO2/UV, photo-assisted degrada-tion with TiO2; TiO2/UV/MW, integrated microwave-/photo-assisted degradation;TiO2/UV/CH, thermal-/photo-assisted degradation.

1 d Photobiology C: Photochemistry Reviews 10 (2009) 96–110

Ce

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T

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e

O

•

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R

R

T

O

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Table 3Average disappearance kinetics of RhB at the UV–vis spectral wavelengths (197 nm,256 nm, 354 nm and 550 nm) and rate constants for the decrease of TOC in the degra-dation of the rhodamine-B dye under UV–vis irradiation with suitable light cutofffilters in combination with microwave radiation [79].

Degradation method kTOC (10−3 min−1) kdeg (10−3 min−1)

TiO2/UV–vis/MW 43 115TiO2/UV–vis 8.8 24TiO2/UV–vis/CH 13 42TiO2/UV/MW 14 58TiO2/UV 0.6 6.9TiO2/UV/CH 4.7 26

TNTo

PARMU

02 S. Horikoshi, N. Serpone / Journal of Photochemistry an

learly, the effect of microwave irradiation is not a mere thermalffect.

Based on past experience, the degradation of RhB can occur inwo similar but distinct stages, one involving the activation of TiO2y absorption of UV light at energies greater than the band-gapnergy 3.2 eV (i.e. at wavelengths below ∼387 nm) through reac-ions (8)–(13) [83–85] via first formation of electron–hole pairsreaction (8)) followed by formation of the reactive oxygen species•OH, O2

•− and HO2• radicals) that react with RhB to yield interme-

iates and ultimately the final products.

iO2 + h�(� < 387.4 nm) → TiO2(e−CB + h+

VB) (8)

+VB + OH−(H2O) → •OH + H+ (9)

−CB + O2 → O2

•− (10)

2•− + H+ → •OOH (11)

OH(orHO2•) + RhB → → mineralizedproducts (12)

+VB + RhB → RhB•+ → → mineralized products (13)

In the other stage (reactions (14)–(20)), chemisorbed RhB isxcited at wavelengths longer than 480 nm to produce singlet andriplet excited states of the dye (reaction (14); RhB∗

ads) followed bylectron transfer from these states to the semiconductor particles86], causing the dye to be converted to other organic substances87] through formation of RhB•+

ads radical cations, and througheactions (16)–(19) that are similar to reactions (8)–(13) above toenerate the oxidizing radicals.

hBads + h� (� ≥ 480 nm) → RhB∗ads (14)

hB∗ads + TiO2 → RhB•+

ads + TiO2(e−CB) (15)

iO2(e−CB) + O2 → O2

•−X (16)

2•− + H+ → •OOH (17)

OOH + O2•− + H+ → O2 + H2O2 (18)

2O2 + O2•− → OH− + •OH + O2 (19)

hB•+ads + O2 (and/or O2

•−, and/or •OH) → intermediates

→ → → mineralized products (20)

The influence of the microwaves on both reactions sequencesas examined [79] using either or both in combination UV–vis

nd microwave (MW) radiations: (i) the TiO2/UV–vis/MW),iO2/UV/MW (with a � < 400 nm filter), and TiO2/vis/MW (using

� > 400 nm filter) techniques; (ii) the TiO2/UV–vis, TiO2/UV

� < 400 nm filter), and TiO2/vis (� > 400 nm filter) methods witho microwave radiation; and (iii) the thermally TiO2-photo-assistedegradation methods under UV–vis radiation (TiO2/UV–vis/CH), UV

rradiation only (TiO2/UV/CH; � < 400 nm filter), and visible irra-iation only (TiO2/vis/CH; � > 400 nm filter) procedures. Results

able 4umber of DMPO–•OH spin-adducts (relative to a Mn2+ standard) produced in an aqueouiO2 dispersion simultaneously subjected to both 2.45 GHz microwaves and UV radiationf 0.025 mM 4-CP using the several protocols are also reported [12].

Relative number of DMPO−•OH kdega (TiO2/MW)

MW UV UV/MW

-25 25 182 259 ∼0.07natase 11 110 92 ∼0.08utile 5 110 76 ∼0.02AR 2 21 35 ∼0.05V-100 4 45 51 0.21

a Zero-order rates of degradation of 4-CP given as 10−4 mM min−1. No •OH radicals wer

TiO2/vis/MW 0.4 1.1TiO2/vis 1.6 2.4TiO2/vis/CH 0.6 1.5

summarized in Table 3 showed that the degradation involvingmicrowave radiation was especially efficient when coupled to UVirradiation (reactions (8)–(13)), whereas the extent of degradationof RhB involving suitable excited states of the dye through vis irra-diation (reactions (14)–(20)) of the dye was rather inefficient whencoupled to microwave radiation. Evidently, microwaves have animpact on the UV photoactivity of TiO2. Accordingly, these effectswere probed and are described below.

3.3. Microwave effects on various TiO2 particle specimens

The question of how to achieve the highest photoactivity of TiO2particles has intrigued researchers for some time from both theoret-ical considerations and model experiments. Unfortunately theoryhas not kept pace or accorded with model experimental data and assuch how to revitalize the metal oxide is decidedly not predictable.Degussa P-25 TiO2 has proven to be an excellent overall materialin hundreds (if not thousands) of research studies. Examination ofthe influence of microwaves on various titanium dioxide specimens(e.g. P-25 TiO2, Hombikat UV-100 TiO2, and Wako anatase and rutileTiO2) was carried out by the photodegradation of 4-chlorophenol(4-CP) (see Table 4) [12]. The Wako anatase and rutile specimenswere also mixed in a ratio of 80:20 (MAR) so as to mimic the mixtureratio present in the P-25 TiO2 specimen.

For P-25 TiO2, the TiO2/UV/MW method was 1.7-fold more effec-tive under identical temperature conditions than the TiO2/UV/CHprocedure, whereas microwave irradiation using the TiO2/UV/MWprotocol was 2.5-fold more effective than the photo-assisted pro-cess with TiO2/UV. Pure anatase particles crystal shape (UV-100and Anatase TiO2) also showed greater efficiency with microwaves(TiO2/UV/MW protocol) relative to the TiO2/UV method, but not sorelative to TiO2/UV/CH. Evidently, the thermal effect was a signifi-cant factor in the photoactivity of the Hombicat UV-100 specimen,

and specific non-thermal (non-caloric) effects of the microwaveradiation had no influence on this metal-oxide specimen. Withrutile TiO2, the dynamics of the photodegradation of 4-CP wereone to two orders of magnitude slower than for the anatase and theP-25 TiO2 specimens, which inferred that microwave irradiation

s microwave-irradiated TiO2 dispersion (TiO2/MW), UV-irradiated (TiO2/UV), and a(TiO2/UV/MW) for various TiO2 specimens; the zero-order rates for the degradation

kdega (TiO2/UV) kdeg

a (TiO2/UV/MW) kdega (TiO2/UV/CH)

1.0 2.5 1.71.5 1.7 1.8

∼0.02 0.05 0.171.1 1.3 1.250.6 1.37 1.54

e produced in H2O under any condition.

d Photobiology C: Photochemistry Reviews 10 (2009) 96–110 103

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rpfartoTcco

t≈Ttrrotna•

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S. Horikoshi, N. Serpone / Journal of Photochemistry an

ad no identifiable effect on the rutile phase of titanium dioxide.or the anatase/rutile TiO2 mixture the degradation tendency ofach of the methods was somewhat less than for the anatase and-25 specimens, except for the TiO2/UV/MW method for whichhe rate was two-fold smaller. No microwave specific effects werevident in many of the photo-assisted reactions expressed by theata of Table 4.

DMPO spin-trap EPR spectroscopy showed the number of •OHadicals photogenerated in microwave-irradiated TiO2 aqueous dis-ersions (against a Mn2+ standard) [88] was considerably less thanor the other two methods used (TiO2/UV and TiO2/UV/MW) bys much as a factor of 10. For the two, the largest number of

•OH

adicals occurred for the P-25 TiO2 and the smallest number forhe mixed anatase/rutile MAR particulates, even though the ratiof anatase to rutile was identical to the anatase/rutile ratio in P-25iO2, which infers that P-25 TiO2 is more than a simple mechani-al mixture of anatase and rutile. A better anatase–rutile interfacialontact is likely formed during the high-temperature preparationf P-25 TiO2.

The decreasing number of •OH radicals formed on TiO2 byhe UV and UV/MW irradiation was (Table 4): P-25 > > anataserutile > UV-100 > MAR. This order correlated neither with the

iO2/UV protocol nor with the TiO2/UV/MW protocol in the rates ofhe photo-assisted degradation of 4-CP. For the latter method, theates decreased as P-25 > anatase > UV-100 ≈ MAR »> rutile. Mostemarkable about the data of Table 4 is that even though the numberf •OH radicals formed for the rutile system was 1–3 times smallerhan for P-25 and anatase TiO2, the rates of degradation of 4-CP wereearly 50- and 34-fold smaller, respectively, with a similar trendlso observed for the TiO2/UV procedure. Interestingly, the sum ofOH radicals produced for the TiO2/MW and TiO2/UV methods wasmaller (207) than the number of •OH radicals (259) produced byhe TiO2/UV/MW system, suggesting some sort of synergistic effecthen both UV and MW radiations were brought to bear on theetal-oxide particulates. Finally, factors other than the number of

OH radicals formed impact negatively on the photodegradations inhe case of rutile TiO2, one of which may be the differences in thextent of adsorption of the organic substrate on the rutile particleurface (also see below).

.4. Relationship(s) between heating effectiveness and dielectricroperties of various TiO2 samples

The dielectric loss factors (ε′′), dielectric constants (ε′) andhe dielectric loss tangents (tan ı = ε′′/ε′; also referred to as theissipation factor) for various TiO2 specimens in pellet form are

isted in Table 5 [12]. The dielectric loss factor, which reflectshe heating efficiency of the TiO2 pellets by the microwaveadiation, varied in the order: rutile > P-25 > UV-100 ≈ anatase cor-elating with the extent of anatase/rutile (i.e. crystalline phase)ontent in the various specimens. Strangely, however, the dielec-

ric constant (ε′) of the P-25 TiO2 sample, which contains ca.8% rutile, was smaller than that of anatase, whereas the dielec-ric loss tangent (tan ı) of P-25 was nearly identical to thatf rutile. A high loss tangent (tan ı) infers a large internal

able 5ielectric properties at 2.45 GHz for the P-25, anatase, rutile and UV-100 TiO2 inellet form [12].

Dielectric lossfactor (ε′′)

Dielectricconstant (ε′)

Dielectric losstangent (tan ı)

-25 0.181 5.626 0.0322natase 0.142 6.525 0.0218utile 0.317 9.550 0.0332V-100 0.150 6.728 0.0223

Fig. 6. Temperature-time profiles of the absorption of microwave radiation by thepowdered TiO2 systems packed in a tube. From ref. [12]. Copyright by the AmericanChemical Society.

attenuation of the microwave radiation in the P-25 TiO2 parti-cles.

The extent of absorption of microwave radiation has beeninferred from the temperature increase (Fig. 6) of each naked TiO2powdered sample (300 mg) packed in a tube and measured at thecenter of the tube using a thermocouple. The rates of tempera-ture increase were P-25 (92.5 ◦C min−1) > Rutile (58.9 ◦C min−1)> TiO2/Al2O3 (28.6 ◦C min−1) > UV-100 (6.3 ◦C min−1) ≈ MAR(4.7 ◦C min−1) ≈ Anatase (2.2 ◦C min−1). Fig. 6 demonstrates thatthe efficiency of absorption of microwave radiation by the P-25 andrutile powders was greater than those of the other TiO2 powderedspecimens.

The rapid increase of temperature for the rutile TiO2 is reason-able because of the relatively high dielectric loss factor (ε′′) (Table 5)compared to the anatase TiO2 particles. However, the dielectric lossfactor (�′′) of P-25 is much smaller than that of rutile. Evidently,the efficiency of heat generated by the absorption of microwaveradiation by TiO2 must involve factors other than the crystallinestructures. In contrast, there was no difference between the dielec-tric loss factor and the rate of temperature increase between theUV-100 and anatase specimens as the former is also 100% anatase.Consequently, the behavior of P-25 must be seen as unusual. Therate of microwave absorption by the bulk solutions in aqueous TiO2dispersions showed no dependence on the nature of the TiO2 sam-ples, at least at ambient temperature.

3.5. Effects of microwaves in the mechanism of thephotodegradation of organic substrates

The initially formed intermediates following irradiation of anaqueous TiO2 dispersion can play a significant role in the extentof adsorption of organic substances on the TiO2 surface and there-fore can impact on the overall dynamics of the photomineralizationof a toxin with significant differences observed whether the dis-persion is exposed to UV radiation alone or to both MW and UVradiations. Hence, it was important to probe the microwave effect(s)that might enhance the photoactivity of the metal-oxide specimenthrough changes in surface affinity, electric surface charge, and sizeof aggregated TiO2 particles by in situ observation techniques [82].In this regard, microwave effects can have an impact on the relation-ship between the TiO2 particle surface and the organic substrates.This is one special consequence of the effects of microwave radia-tion because no similar effects have been observed by conventional

heating under otherwise similar temperature conditions.

In TiO2-photo-assisted degradations of RhB [79] and dimethylphthalate [80], UV irradiation alone produced intermediates dif-ferent from those observed when the aqueous TiO2 dispersion

104 S. Horikoshi, N. Serpone / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 10 (2009) 96–110

S /UV an

wmpmoorpiit

oittemtitidad

3

ap2o

FTi

The microwave thermal effect in the photo-assisted degradationof BPA was also examined using the conventional heating method(TiO2/UV/CH protocol) at temperatures of 20, 50 and 80 ◦C by cir-culating hot water in the double-layered photoreactor of Fig. 8.

cheme 1. Proposed initial mechanistic steps of the degradation of RhB by the TiO2

as simultaneously UV and microwave-irradiated (TiO2/UVMWethod). The latter method proved more efficient than the TiO2/UV

rocedure. Scheme 1 illustrates the differences between the twoethodologies used to degrade/mineralize the RhB dye. These

bservations must be due to some characteristic changes occurringn the TiO2 surface as a result of it being exposed to microwaveadiation. Thus, the decomposition of organic pollutants can (inrinciple) be optimized by changes in some physical and/or chem-

cal property of the TiO2 surface that results from the microwaverradiation. As a case of such an enhancement, we examined [79]he fate of the nitrogen functions in RhB dye.

Formation of NH4+ ions in the degradation of RhB in aque-

us TiO2 dispersions by several degradation methods is reportedn Fig. 7. No NO3

− ions formed, or at least were detected for allhe methods used. Formation kinetics of NH4

+ ions decreased inhe order TiO2/UV/MW > > TiO2/UV/CH > TiO2/UV, with the gen-ration of NH4

+ ions enhanced 10-fold with the TiO2/UV/MWethod compared to the TiO2/UV procedure. On the other hand,

he TiO2/UV/CH method yielded a quantity of NH4+ ions exper-

mentally very different from the TiO2/UV/MW procedure evenhough the temperatures at which the processes occurred weredentical in both cases. Formation of NH4

+ ions takes place by priore-ethylation of the RhB structure subsequent to microwave irradi-tion. Prior demethylation was also observed in the degradation ofimethylphthalate by the integrated UV/MW irradiation method.

.6. Probing the non-caloric microwave effect(s)

The microwave effect(s) has also been examined from anotherpproach. To probe this inferred effect, the microwave-assistedhotodegradation of bisphenol-A (BPA) in the presence of TiO2 (P-5) was examined at ambient temperature to assess the importancef the microwave specific non-thermal effect [89]. The integrated

ig. 7. Formation of NH4+ ion in the photodecomposition of RhB (0.05 mM) by the

iO2/UV/MW, TiO2/UV and TiO2/UV/CH protocols. From ref. [79]. Copyright by Amer-can Chemical Society.

d TiO2/UV/MW protocols. From ref. [79]. Copyright by American Chemical Society.

microwave/photoreactor (MPR) was equipped with a cooling sys-tem (Fig. 8). Specifically, for these experiments the Pyrex reactorconsisted of a double-layered structure with internal and exter-nal diameters 75 and 100 mm, respectively. Silicone oil was thecoolant kept at −20 ◦C and circulated through the inner part ofthis double-layered arrangement. The UV light source was a superhigh-pressure 150-W mercury lamp.

Normally BPA photodegrades under microwave irradiationthrough microwave dielectric loss heating, as evidenced by therise in temperature (the thermal effect). However, cooling themetal-oxide dispersion so that the temperature of the dispersionwas constant at ca. 21 ◦C led to a two-fold faster (Fig. 9) rateof the microwave-/photo-assisted degradation of BPA under con-trolled ambient conditions (TiO2/UV/MW-Cool protocol) relativeto the degradation of BPA by the TiO2/UV/MW method otherwiseoccurring at a higher temperature (6.5 × 10−4 mM min−1 versus3.3 × 10−4 mM min−1, respectively). The extent of degradation ofBPA followed the order: TiO2/UV/MW-Cool (78%) > TiO2/UV/MW(42%) > TiO2/UV (21%) after a 60-min irradiation period by themicrowaves and/or UV irradiation alone.

Fig. 8. Schematic illustration of the microwave-assisted photoreactor with a coolingsystem in the microwave multimode applicator.

d Phot

T(BriBdteicttnm

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S. Horikoshi, N. Serpone / Journal of Photochemistry an

he ratio of the relevant degradation rates were 1.0 (20 ◦C):1.450 ◦C):1.8 (80 ◦C). Under these conditions, the degradation rate ofPA at the lower temperature never exceeded the correspondingate at the higher temperatures, a result in line with the find-ngs of Kaneco et al. [90]. The photo-assisted decomposition ofPA increased with heating the dispersion, whether by microwaveielectric loss heating or by conventional heating methods, in con-rast to earlier reports [90,91], which noted that although thextent of degradation of bisphenol-A in aqueous TiO2 dispersionsncreased gradually with temperature from ca. 60 to 70%, no appre-iable change was observed in the temperature range 10–70 ◦C, andhat the photo-assisted degradation of BPA is not very sensitive toemperature fluctuations. Also evident in Fig. 9 is that the specificon-thermal (non-caloric) effect in the degradation of BPA is clearlyore important than the thermal effect.

. Microwave discharge electrodeless lamp (MDEL) devices

Ever since the first reports on the use of a microwave dischargelectrodeless lamp (MDEL) in organic reactions by Ward and Wish-ok [92] in 1968 such lamps have failed, except in the last decade,o become the light sources of choice in photochemical organic syn-heses [93,94]. Yet various types of MDEL light sources had foundide applications at the industrial level: for example, in paint hard-

ning (dryness) equipment [95], a technology developed in the U.S.n the early 1970s to cast a polymer rapidly through a UV hardening

ethod starting from a monomeric material. Another applicationnvolved MDELs filled with sulfur gas for illumination purposes asonceived by Dolan et al. [96] and Turner et al. [97] in the 1990s.owever, to the extent that sulfur can corrode various electrodeaterials, it was unsuitable for conventional electrode lamps. This

s not a distraction in electrodeless lamps because corrosion is of noonsequence owing to the nature of the MDELs. This particular typef MDELs generate light principally in the vis spectral region (wave-engths, 370–840 nm) and are currently used to illuminate variousmithsonian Institution museums in Washington, D.C. Develop-ent in this area has led to MDEL lamps that can simulate sunlight.The combined activation of chemical reactions by two different

ypes of electromagnetic radiation (namely, microwave and UV–visadiations) now covers the field that is broadly referred to asicrowave-assisted photochemical organic synthesis. Synergistic

ffects of MDELs have been demonstrated by comparing synthesisethods using light alone and/or microwave radiation [93]. MDEL

pparatuses used in organic synthesis have been examined inome detail [98,99]. Additionally, the concentration and naturef the gas-fill compound in MDELs have been optimized for

ig. 9. Temporal decrease of concentration of bisphenol-A (0.05 mM) during itsecomposition in aqueous media by the photo-assisted oxidation (TiO2/UV proto-ol), by the microwave-/photo-assisted oxidation (TiO2/UV/MW protocol) method,nd by the integrated microwave-/photo-assisted degradation under cooling condi-ions (TiO2/UV/MW-Cool protocol). From ref. [89]. Copyright by Elsevier B.V.

obiology C: Photochemistry Reviews 10 (2009) 96–110 105

photochemical reactions [100] and MDEL light sources have beenevaluated in various organic solvents [101].

Application features of an MDEL light source in chemical reac-tions are many: (i) microwaves can be used separately for providinglight and microwave irradiation; (ii) the lifetime of electrodelesslamps is long because there is no deterioration of the electrodeas might occur in a traditional Hg electrode lamp; and (iii) theMDEL lamp shape presents no complications because the lampsare electrodeless. MDEL devices are ideal in chemical reactionsbecause of the synergistic effect of irradiating simultaneouslywith both UV–vis light and microwaves. Photo-assisted processesoccurring in aqueous TiO2 dispersions are well in the applicationof such MDEL devices.

4.1. Wastewater treatment using MDEL system

A cylindrically shaped MDEL device (Fig. 10a) containing Hg/Argases has been fabricated as the light source to drive a photochem-ical water treatment process, whereas Fig. 10b shows a photographof the microwave-activated MDEL device placed in a water beakerinside a microwave oven. While such MDEL devices are ideal inapplications of wastewater treatments, they nonetheless have somepositive and negative attributes.

Positive attributes: The vacuum-UV and UV light from the MDELand the microwaves can simultaneously irradiate an aqueous solu-tion when placed directly in the wastewater without any electricshield (Faraday cage). Moreover, the simplification of the deviceand the highly effective UV light emitted present considerableadvantages in placing this MDEL radiation source directly into thewastewaters. In this regard, the vacuum-UV light (100–200 nm[102]) emitted by the MDEL device in water is not attenuatedby the oxygen dissolved in the aqueous media [59], whereas ifthe device irradiated a reactor externally, the radiation reach-ing the reactor would be attenuated significantly by atmosphericoxygen.Negative attributes: If the MDEL device is immersed in water itwould nonetheless present some albeit non-insurmountable prob-lems such as the continuity of the emitted UV light and theself-ignition of the electrodeless lamp by the microwave radiation,and not least the high dielectric constant of water would suppressthe supply of continuous microwave energy in the self-ignitionof the lamp at low microwave power levels. For an MDEL deviceimmersed in wastewaters, significant high applied voltages areneeded to achieve the microwave power levels for self-ignition.Accordingly, self-ignition of MDEL light sources in an aqueousmedium at low microwave power levels needed improvement thatcould be achieved by the use of a tungsten wire antenna deviceembedded in a synthetic quartz tube and attached to the (Hg/Argas-fill) MDEL system [58]. This antenna acts as a trigger caus-ing a decrease of the otherwise need for high-power microwaveradiation levels. In actual experiments, positioning the tungsten-triggered W-MDEL device in aqueous media needed 200 W forself-ignition, but once ignited the light emission from the W-MDELsource could be maintained by lowering the microwave power to70 W, even in the presence of a high dielectric solvent such aswater (Fig. 11a). By contrast, a comparable naked MDEL immersedin aqueous solution did not self-ignite even at 800 W of microwaveradiation.

Moreover, the temperature on the surface of the MDEL device in

the circulating solution was not uniform, indicating that the smallamount of evaporated mercury was inefficient under unstable tem-perature situations, and thus continuous VUV/UV light emissionwith small mercury levels could not be maintained under suchconditions, contrary to stable temperatures maintained with the

106 S. Horikoshi, N. Serpone / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 10 (2009) 96–110

Fig. 10. (a) Cylindrical shaped microwave discharge electrodeless lamp (MDEL) and (b) appearance of MDEL lighting in microwave oven.

F r micrl

is

awpeeasttttr22weogat

TKW

DD

ig. 11. (a) Bluish white light emitted by the W-MDEL device in aqueous media undeight source device in aqueous media under microwave irradiation.

ncorporation of the tungsten trigger. The W-MDEL device is alsouitable in circulation-type reactors.

An example application of the W-MDEL device in a wastew-ter treatment process is the photooxidative conversion of theidely used and highly toxic synthetic phytohormone 2,4-dichloro-henoxyacetic acid (2,4-D) herbicide [81] using both a conventionallectrode Hg lamp and the electrodeless Hg-fill W-MDEL pow-red by microwave radiation. The VUV/UV light from the W-MDELt low microwave power levels, using a circulation flow-throughystem coupled to a cooling apparatus (Fig. 11b), led to the effec-ive decomposition of 2,4-D in aqueous media. For comparison,he photooxidation of 2,4-D was also examined using a conven-ional Hg electrode lamp at a power level otherwise identical tohe power used for the W-MDEL device. Table 6 summarizes theelevant dynamics of the photodegradation and dechlorination of,4-D under various experimental conditions. The degradation of,4-D was complete after 25 min irradiation by the microwaves,ith the degradation and dechlorination being somewhat more

fficient by a factor of 1.3 in oxygen-saturated media as a resultf the presence of oxidative species (e.g. singlet oxygen and ozone)enerated by vacuum-UV light and dissolved oxygen. Degradationnd dechlorination processes occurring with the W-MDEL/TiO2 sys-em were 2–3-fold faster than with the W-MDEL method alone,

able 6inetics of degradation and dechlorination of 2,4-D under VUV and UVC irradiation in air--MDEL device and an electrode low-pressure Hg lamp [58].

W-MDEL W-MDEL/O

egradation of 2,4-D (10−2 min−1) 7.4 9.3echlorination (10−2 min−1) 2.5 3.3

owave irradiation (200 W) and (b) sketch of the tungsten-triggered MDEL (W-MDEL)

and more so in comparison with the Hg electrode lamp/TiO2 sys-tem.

Remediation of dye pollutants using either the TiO2/MDEL/MWprocedure or a naked MDEL light source alone has been carried outfor such substrates as methylene blue [103], acid orange 7 [104,105],bromophenol blue [106], and brilliant red X-3B [107]. Církva et al.[108] have examined the photodegradation of monochloroaceticacid using a TiO2-coated MDEL system depicted in Fig. 12. Theacid was totally decomposed to HCl, CO2, and H2O with the reac-tor arrangement. Their study showed that the reaction efficiencydepended on light irradiance and on the initial pH of the aqueousmedium. The degradation was enhanced in alkaline media in thepresence of H2O2, and significantly enhanced on increasing lightirradiance. However, the efficiency of UV irradiation was not influ-enced by the number of TiO2-coating cycles or by air bubbling.

4.2. Purification of a dioxin-contaminated solid using MDEL andpyrolysis

Purification of a contaminated soil has been examined at both aprocessing plant and on-site using a microwave irradiation methodthat led to efficient pyrolysis of soil contaminants (microwave ther-mal effect) relative to the more conventional heating methods. This

equilibrated and oxygen-saturated aqueous media with and without TiO2 using the

2 bubbling W-MDEL/TiO2 Electrode Hg-lamp/TiO2

18.1 0.217.3 0.30

S. Horikoshi, N. Serpone / Journal of Photochemistry and Phot

Faf

radhbecnseid

afldfipdtiad

Fe

ig. 12. The microwave photocatalysis arrangement on microwave multimodepplicator with TiO2 thin film coated electrodeless discharged lamp (EDL). Adaptedrom ref. [108]. Copyright by Elsevier B.V.

epresents a good example of an application of microwave pyrolysiss a degradation method to remediate generated fly-ash and metalust from scrap factories that contain high concentrations of theighly toxic dioxins. Halogenated organic substrates in soils cane disposed by microwave heating under alkali conditions. How-ver, the process also causes gasification of a fraction of the dioxinontent and intermediates so that after microwave pyrolysis it isecessary under these conditions to collect the gasified dioxin onome filter, without which the dioxins and intermediates in thexhaust gases can cause additional environment problems. Hence,t is important to develop a method with which to treat the gasifiedioxins and the corresponding intermediates.

A set-up that combines microwave pyrolysis and an MDEL-ssisted photolysis is herein proposed to degrade dioxins present iny-ashes and soils, a set-up that can simultaneously pyrolyze theioxins in the fly-ash and soils and photolyze any escaping gasi-ed dioxins. The dioxin-contaminated fly-ash was placed in thehotoreactor and subsequently covered by the lid-shaped MDEL

evice (Fig. 13a). The fly-ash was mixed with a propeller-type stirrerhroughout the period in which the MDEL device was microwave-rradiated with a multimode applicator. Exposure to vacuum-UVnd UV light together with microwave heating led to completeecomposition of the dioxin and the related intermediates. Fig. 13b

ig. 13. Experimental set-up used in the degradation of dioxin in actual ash by micrxperimental schematic image and (b) photograph of the actual experimental set-up duri

obiology C: Photochemistry Reviews 10 (2009) 96–110 107

illustrates the actual operational system during the decompositionprocess taking place by microwave pyrolysis coupled to the MDEL-assisted photolysis. Clearly, microwave energy should be exploredand exploited further for both pyrolytic and photolytic processes inthe remediation of contaminated soils.

4.3. Air purification using a MDEL system

The TiO2-photo-assisted destruction of air pollutants currentlyfinds practical application in various commercially available air fil-ters, self-cleaning surfaces and photocatalytic concrete and paints[71]. However, the method is not suitable in air purification whentreating high concentrations of pollutants unless the photoactivityof the metal-oxide solid is significantly improved. An early studyby Kataoka et al. [109] demonstrated the photo-assisted degrada-tion of the VOC ethylene using microwaves and a UV light sourceto irradiate the TiO2/ZrO2 fixed bed reactor. The photo-assistedair purification was achieved using the UV light emanating froma microwave-activated MDEL device.

The microwave-assisted photodegradation of acetaldehyde, amodel pollutant widely used to compare the activities of differ-ent metal-oxide specimens [110], was examined at the surfaceof TiO2 pellets subjected to UV radiation emitted from an MDELlight source [111]. Results showed that the combination of TiO2pellets, microwave radiation and the MDEL-emitted UV light isa suitable treatment method against air pollutants. Such treat-ment displayed significant advantages when compared to otherphoto-assisted methods. The experimental set-up used in degrad-ing CH3CHO accorded with the standardized photodegradationmethod for NOx gases as expressed by the Japanese IndustrialStandard JIS R 1701-1:2004. The official protocol involved dryair (500 mL min−1), wet air (500 mL min−1), and an acetalde-hyde/nitrogen gas mixture (5 mL min−1) accurately controlled by amass flow controller (Fig. 14a). The concentration of acetaldehydein the feed CH3CHO/N2 gas mixture was adjusted to 1 ppm andintroduced through a Teflon tube into the cylindrical Pyrex reac-tor containing TiO2 pellets and the Hg/Ar gas-filled MDEL device(length, 130 mm; diameter, 13 mm). The reactor was positionedinside the multimode microwave applicator. The concentration ofacetaldehyde in the gas stream exiting the microwave Pyrex reactorwas monitored by gas chromatography. In the dark, more than 90%of the initial incoming acetaldehyde was adsorbed onto the TiO2pellet’s surface. Fig. 14b shows photographs of the MDEL inside the

reactor containing TiO2 pellets and subjected to microwave irradi-ation.

The degradation of acetaldehyde in the humidified air streamover TiO2 pellets under the combined microwave and VUV/UV lightirradiation from the MDEL is reported in Fig. 15a as a function

owave pyrolysis and MDEL-assisted photolysis under microwave irradiation: (a)ng the degradation process.

108 S. Horikoshi, N. Serpone / Journal of Photochemistry and Photobiology C: Photochemistry Reviews 10 (2009) 96–110

Fig. 14. (a) Schematic presentation of the experimental set-up illustrating some of the details of the reactor, (b) TiO2 pellets irradiated with light emitted from the MDELsource. From ref. [111]. Copyright by Elsevier B.V.

S n the Tb

ooi6

Fur

cheme 2. Proposed mechanism of the microwave desorption process of CH3CHO oy the microwave-irradiated MDEL device.

f reaction time. After the microwave irradiation was switchedn, the concentration of acetaldehyde in the outlet gas streammmediately increased and reached a maximum of 8.7 ppm aftermin indicating desorption of CH3CHO from the TiO2 surface

ig. 15. Time-dependent change of concentration of acetaldehyde when degradedsing either (a) the MW/MDEL/TiO2 method or (b) the TiO2/black light method. Fromef. [111]. Copyright by Elsevier B.V.

iO2 surface and its degradation on the TiO2 pellets under VUV/UV radiation emitted

owing to an increase in temperature to ca. 207 ◦C within 5 minof the microwave-irradiated TiO2 pellets. Nearly 95% of the initialacetaldehyde decomposed after 20 min of microwave irradiation.Substitution of the microwave-irradiated MDEL device by a stan-dard conventional low-pressure Hg light source (black light) causedonly ca. 80% of the initial substrate to decompose after 200 min ofirradiation period (Fig. 15b).

The different reaction steps proposed for the degrada-tion of acetaldehyde under the experimental conditions ofTiO2/MDEL/MW method are presented in Scheme 2. The initiallyadsorbed acetaldehyde is removed from the surface of the TiO2pellets by microwave heating. Photodegradation of acetaldehyde issubsequently initiated by the VUV/UV light emitted from the MDELdevice by direct photolysis and through the mediated action of thephotogenerated reactive oxygen species O3 detected within 1 minof irradiation. Adsorbed acetaldehyde was also degraded at thesurface of the TiO2 pellets by the TiO2-mediated photo-oxidativeprocess and to a minor extent by secondary thermal reactions.

In summary, the effect of microwave irradiation combinedwith the MDEL device and TiO2 pellets on the overall decrease ofacetaldehyde can be rationalized by various physical and chemicalprocesses. For example, desorption, pyrolysis, photolysis, photo-assisted degradation and ozonation all played a role. This proposednovel method suggests that microwave energy can be used effec-tively for the degradation of organic compounds in polluted air.

5. Concluding remarks

The microwave effect on heterogeneous catalysts in polarsolvents is mostly a thermal effect that causes a dramatic enhance-

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S. Horikoshi, N. Serpone / Journal of Photochemistry an

ent of the reaction dynamics owing to prompt heating by theicrowaves. Nonetheless, the microwave specific effect (non-

aloric) in the absence of the thermal factor also affects in someay heterogeneous catalysts. Catalyzed heterogeneous reactions

aking place in polar solvents are subjected to peculiar effects ofhe microwaves, which may involve some special thermal and/orpecific non-thermal factors (non-conventional thermal effect).he existence of a “microwave-catalyst” will be elucidated in theear future. In the mean time, general chemical reactions can beccelerated by peculiar thermal factors when using microwavenergy. When the microwave energy is converted into a moreonventional form of energy the usage of microwaves will bextended.

cknowledgements

We are grateful to the several academic and industrial collabo-ators and students noted in the reference section without whosenput many of the studies would not have seen the light of day.

eferences

[1] (a) N. Taniguchi, T. Osada, Sci. Pap. IPCR 105 (1968) 62–63;(b) K. Shimomura, T. Miyazaki, A. Tutiya, Annual Report, RIKEN, Japan, 1972,p. 4811.

[2] N. Miyoshi, S. Tsutum, N. Sonoda, Technical Report, Osaka University, 1975, p.25.

[3] H.M. Kingston, S.J. Haswell (Eds.), Microwave-Enhanced Chemistry, AmericanChemical Society, Washington, DC, 1997.

[4] R. Gedye, F. Smith, K. Westaway, H. Ali, L. Baldisera, L. Laberge, J.R. Rousell,Tetrahedron Lett. 27 (1986) 279–282.

[5] R.J. Giguere, T.L. Bray, S.M. Duncan, G. Majetich, Tetrahedron Lett. 27 (1986)4945–4948.

[6] A. Loupy, R.S. Varma, Chem. Today 24 (2006) 36–39.[7] S. Yanagida, Y. Wada, T. Yamamoto, Chem. Indust. 53 (2002) 729–734.[8] D. Stuerga, M. Delmotte, in: A. Loupy (ed.), Microwaves in Organic Synthesis,

Wiley-VCH Verlag, Weinheim, Germany, 2006, Chapter 1, pp. 7–18.[9] R. Roy, D. Agrawal, J. Cheng, S. Gedevanishvil, Nature 399 (1999) 668–770.

[10] D. Bogdal, Microwave-assisted Organic Synthesis – One Hundred Reaction Pro-cedures, Tetrahedron Organic Chemistry Series, Elsevier, vol. 25, 2005, pp.9–11.

[11] S. Horikoshi, F. Sakai, M. Kajitani, M. Abe, N. Serpone, Chem. Phys. Lett. 470(2009) 304–307.

[12] S. Horikoshi, F. Sakai, M. Kajitani, M. Abe, A.V. Emeline, J. Phys. Chem. C 113(2009) 5649–5657.

[13] T. Oguro, T. Ogura, M. Kuga, Y. Furutaki, T. Moriike, Food Indust. 43 (2000)40–47.

[14] Y. Sato, New Industrial Technology Using Microwaves, NTS, Tokyo, Japan, Chap-ter 9.

[15] J.K.S. Wan, US patent 1 159 010 (1983).[16] T.R.J. Dinesen, M.Y. Tse, M.C. Depew, J.K.S. Wan, Res. Chem. Inter. 15 (1991)

113–127.[17] J.K.S. Wan, Y.G. Chen, Y.J. Lee, M.C. Depew, Res. Chem. Intermed. 26 (2000)

599–619.[18] M.T. Radoiu, I. Calinescu, D.I. Martin, R. Calinescu, Res. Chem. Intermed. 29

(2003) 71–81.[19] H. Takashima, M. Karches, Y. Kanno, Appl. Surf. Sci. 254 (2008) 2023–2030.[20] X. Zhang, D.O. Hayward, Inorg. Chim. Acta 359 (2006) 3421–3433.[21] J. Tang, T. Zhang, D. Liang, H. Yang, N. Li, L. Lin, Appl. Catal. B: Environ. 36 (2002)

1–7.[22] Y. Wada, H. Yin, T. Kitamura, S. Yanagida, Chem. Lett. 29 (2000) 632–633.[23] S. Kobayashi, Y.-K. Kim, C. Kenmizaki, S. Kushiyama, K. Mizuno, Chem. Lett. 25

(1996) 769–770.[24] M. Takaoka, J. Takada, N. Takeda, T. Fujiwara, J. Jpn. Soc. Waste Manage. Experts

10 (1999) 331–340.[25] K. Ganzler, J. Bati, K. Valko, J. Chromatogr. 371 (1986) 299–306.[26] V. Lopez-Avila, R. Young, W. Beckert, Anal. Chem. 66 (1994) 1097–1106.[27] J. Dean, I. Barnabas, I. Fowlis, Anal. Comm. 32 (1995) 305–308.[28] R. Lao, Y. Shu, J. Holmes, C. Chiu, Microchem. J. 53 (1996) 99–108.[29] V. Lopez-Avila, J. Benedicto, C. Charan, R. Young, Environ. Sci. Technol. 29 (1995)

2709–2712.[30] K. Hummert, W. Vetter, B. Luckas, Organohalogen Compd. 27 (1996) 360–363.[31] Y. Shu, C. Chiu, R. Turle, T. Yang, R. Lao, Organohalogen Compd. 31 (1997) 9–13.

[32] C. Chiu, G. Poole, M. Tardif, W. Miles, R. Turle, Organohalogen Compd. 31 (1997)

175–180.[33] M. Schlabach, A. Biseth, H. Glmdersen, M. Oehme, Organohalogen Compd. 23

(1995) 105–108.[34] C. Chiu, G. Poole, Y. Shu, R. Thomas, R. Turle, Organohalogen Compd. 27 (1996)

333–338.

obiology C: Photochemistry Reviews 10 (2009) 96–110 109

[35] V. Lopez-Avila, R. Young, R. Kim, W. Beckert, J. Chromatogr. Sci. 33 (1995)481–484.

[36] J. Fish, R. Revesz, LC-GC 14 (1996) 230–234.[37] M.P. Liompart, R.A. Lorenzo, R. Cela, K. Li, J.M.R. Bélanger, J.R.J. Peré, J. Chro-

matogr. A 774 (1997) 243–251.[38] G. Xiong, J. Liang, S. Zou, Z. Zhang, Anal. Chim. Acta 371 (1998) 97–103.[39] G. Xiong, B. Tang, X. He, M. Zhao, Z. Zhang, Z. Zhang, Talanta 48 (1999)

333–339.[40] M.A. Crespín, M. Gallego, M. Valcárcel, J. Chromatogr. A 897 (2000) 279–293.[41] E.A. Hogendoorn, R. Huls, E. Dijkman, R. Hoogerbrugge, J. Chromatogr. A 938

(2001) 23–33.[42] E.N. Papadakis, E. Papadopoulou-Mourkidou, J. Chromatogr. A 962 (2002)

9–20.[43] R.C. Prados-Rosales, M.C. Herrera, J.L. Luque-Garcí a, M.D.L.d. Castro, J. Chro-

matogr. A 953 (2002) 133–140.[44] L. Sun, H. Lee, J. Chromatogr. A 1014 (2003) 165–177.[45] M.-C. Wei, J.-F. Jen, J. Chromatogr. A 1012 (2003) 111–118.[46] A. Serrano, M. Gallego, J. Chromatogr. A 1104 (2006) 323–330.[47] C. Padrón-Sanz, R. Halko, Z. Sosa-Ferrera, J.J. Santana-Rodríguez, J. Chromatogr.

A 1078 (2005) 13–21.[48] R.A. Abramovitch, H. Bangzhou, D.A. Abramovitch, S. Jiangao, Chemosphere 38

(1999) 2227–2236.[49] R.A. Abramovitch, H. Bangzhou, D.A. Abramovitch, S. Jiangao, Chemosphere 39

(1999) 81–87.[50] G. Cravotto, S.D. Carlo, B. Ondruschka, V. Tumiatti, C.M. Roggero, Chemosphere

69 (2007) 1326–1329.[51] X. Liu, G. Yu, Chemosphere 63 (2006) 228–235.[52] M. Hájek, Microwaves catalysis in organic synthesis, in: A. Loupy (ed.),

Microwaves in Organic Synthesis, Wiley-VCH Verlag, Weinheim, Germany,2006, Chapter 13, pp. 615–652.

[53] Y. Urushibara, Bull. Chem. Soc. Jpn. 25 (1952) 280.[54] K. Sakai, M. Ishige, H. Kono, I. Motoyama, K. Watanabe, K. Hata, Bull. Chem.

Soc. Jpn. 41 (1968) 1902–1908.[55] S. Horikoshi, J. Tsuzuki, F. Sakai, M. Kajitani, N. Serpone, Chem. Commun. (2008)

4501–4503.[56] S. Horikoshi, M. Kajitani, S. Sato, N. Serpone, J. Photochem. Photobiol. A: Chem.

189 (2007) 355–363.[57] P. Klán, V. Církva, Microwaves in photochemistry, in: A. Loupy (Ed.),

Microwaves in Organic Synthesis, Wiley-VCH Verlag, Weinheim, Germany,2006, Chapter 19, pp. 860–897.

[58] S. Horikoshi, T. Miura, M. Kajitani, N. Serpone, Photochem. Photobiol. Sci. 7(2008) 303–310.

[59] M.G. Gonzalez, E. Oliveros, M. Wörner, A.M. Braun, J. Photochem. Photobiol. C:Rev. 5 (2004) 225–246.

[60] See e.g. EPA’s Handbook on Advanced Oxidation Processes, Report No.EPA/625/R-98/004, December 1998 in http://www.epa.gov/ord/NRMRL/pubs/625r98004/625r98004.pdf (accessed May 2009).

[61] A. Loupy, Nonthermal effect of microwaves in organic chemistry, in: A. Loupy(Ed.), Microwaves in Organic Synthesis, Wiley-VCH Verlag, Weinheim, Ger-many, 2006, Chapter 4, pp. 134–218.

[62] K.C. Westaway, R. Gedye, J. Microwave Power 30 (1995) 219–230.[63] L. Perreux, A. Loupy, Tetrahedron 57 (2001) 9199–9223.[64] N. Kuhnert, Angew. Chem. 41 (2002) 1863–1866.[65] X. Zhang, D.O. Hayward, D.M.P. Mingos, Chem. Commun. (1999) 975–976.[66] Y. Zhang-Steenwinkel, H.L. Castricum, J. Beckers, E. Eiser, A. Bliek, J. Catal. 221

(2004) 523–531.[67] S. Horikoshi, M. Ohmori, M. Kajitani, N. Serpone, J. Photochem. Photobiol. A:

Chem. 189 (2007) 374–379.[68] S. Horikoshi, J. Tsuzuki, M. Kajitani, M. Abe, N. Serpone, New J. Chem. 32 (2008)

2257–2262.[69] N. Serpone, E. Pelizzetti (Eds.), Photocatalysis – Fundamentals and Applica-

tions, Wiley, New York, 1989.[70] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995)

69–96.[71] A. Fujishima, K. Hashimoto, T. Watanabe, TiO2 Photocatalysis Fundamentals

and Applications, BKC Publ., Tokyo, Japan, 1999.[72] See e.g. K. Tanaka, K. Abe, C.Y. Sheng, T. Hisanaga, Environ. Sci. Technol. 26

(1992) 2534–2536.[73] N.L. Stock, J. Peller, K. Vinodgopal, P.V. Kamats, Environ. Sci. Technol. 34 (2000)

1747–1750.[74] S. Horikoshi, H. Hidaka, Chemosphere 51 (2003) 139–142.[75] S. Horikoshi, N. Serpone, H. Hidaka, in: Proceedings of the 7th International

Conference on TiO2 Photocatalytic Purification Treatment of Water and Air,London, ON, Canada, 2000, p. 102–103.

[76] S. Horikoshi, H. Hidaka, N. Serpone, Environ. Sci. Technol. 36 (2002)1357–1366.

[77] S. Horikoshi, H. Hidaka, N. Serpone, J. Photochem. Photobiol. A: Chem. 159(2003) 289–300.

[78] S. Horikoshi, A. Tokunaga, H. Hidaka, N. Serpone, J. Photochem. Photobiol. A:Chem. 162 (2004) 33–40.

[79] S. Horikoshi, A. Saitou, H. Hidaka, N. Serpone, Environ. Sci. Technol. 37 (2003)5813–5822.

[80] S. Horikoshi, F. Hojo, H. Hidaka, N. Serpone, Environ. Sci. Technol. 38 (2004)2198–2208.

[81] S. Horikoshi, A. Tokunaga, N. Watanabe, H. Hidaka, N. Serpone, J. Photochem.Photobiol. A: Chem. 177 (2006) 129–143.

1 d Phot

[

[

[

[

[

[[[[

10 S. Horikoshi, N. Serpone / Journal of Photochemistry an

[82] S. Horikoshi, M. Kajitani, H. Hidaka, N. Serpone, J. Photochem. Photobiol. A:Chem. 196 (2008) 159–164.

[83] N. Serpone, D. Lawless, R.F. Khairutdinov, J. Phys. Chem. 99 (1995)16646–16654.

[84] N. Serpone, D. Lawless, R.F. Khairutdinov, E. Pelizzetti, J. Phys. Chem. 99 (1995)16655–16661.

[85] D. Lawless, N. Serpone, D. Meisel, J. Phys. Chem. 95 (1991) 5166–5170.[86] R. Rossetti, L.E. Brus, J. Am. Chem. Soc. 106 (1984) 4336–4340.[87] G. Liu, X. Li, J. Zhao, S. Horikoshi, H. Hidaka, J. Mol. Catal. A 153 (2000) 221–229.[88] S. Horikoshi, H. Hidaka, N. Serpone, Chem. Phys. Lett. 376 (2003) 475–480.[89] S. Horikoshi, M. Kajitani, N. Serpone, J. Photochem. Photobiol. A: Chem. 188

(2007) 1–4.[90] S. Kaneco, M.A. Rahman, T. Suzuki, H. Katsumata, K. Ohta, J. Photochem. Pho-

tobiol. A: Chem. 163 (2004) 419–424.[91] A.E.H. Machado, J.A. de Miranda, R.F. de Freitas, E.T.F.M. Duarte, L.F. Ferreira,

Y.D.T. Albuquerque, R. Ruggiero, C. Sattler, L. de Oliverira, J. Photochem. Pho-tobiol. A: Chem. 155 (2003) 231–241.

[92] H.R. Ward, J.S. Wishnok, J. Am. Chem. Soc. 90 (1968) 5353–5357.[93] V. Církva, M. Hájek, J. Photochem. Photobiol. A: Chem. 123 (1999) 21–23.

[94] P. Klán, J. Literák, M. Hájek, J. Photochem. Photobiol. A: Chem. 128 (1999)

145–149.[95] K. Ashikaga, K. Kawamura, Ind. Coat. 201 (2006) 40–44.[96] J.T. Dolan, G.M. Ury, C.H.A. Wood, Sixth International Symposium on the Sci-

ence and Technology of Light Sources, Technical University of Budapest, 1992.[97] B.P. Turner, M.G. Ury, Y. Leng, W.G. Love, J. Illuminat. Eng. Soc. (1997) 10–16.

[

[

obiology C: Photochemistry Reviews 10 (2009) 96–110

[98] P. Klán, M. Hájek, V. Církva, J. Photochem. Photobiol. A: Chem. 140 (2001)185–189.

[99] V. Církva, L. Vlková, S. Relich, M. Hájek, J. Photochem. Photobiol. A: Chem. 179(2006) 229–233.

100] P. Müller, P. Klán, V. Církva, J. Photochem. Photobiol. A: Chem. 171 (2005)51–57.

101] V. Církva, J. Kurfürstová, J. Karban, M. Hájek, J. Photochem. Photobiol. A: Chem.168 (2004) 197–204.

102] As defined by the International Ultraviolet Association (IUVA), seehttp://www.iuva.org/what is uv (accessed July 2009).

103] J. Hong, C. Sun, S.-G. Yang, Y.-Z. Liu, J. Hazard. Mater. B133 (2006)162–166.

104] X. Zhang, G. Li, Y. Wang, J. Qu, J. Photochem. Photobiol. A: Chem. 184 (2006)26–33.

105] X. Zhang, Y. Wang, G. Li, J. Qu, J. Hazard. Mater. B134 (2006) 183–189.106] J. Hong, N. Ta, S.-G. Yang, Y.-Z. Liu, C. Sun, Desalination 214 (2007) 62–69.107] X. Zhang, G. Li, Y. Wang, Dyes Pigment. 74 (2007) 536–544.108] V. Církva, H. Zabová, M. Hájek, J. Photochem. Photobiol. A: Chem. 198 (2008)

13–17.

109] S. Kataoka, D.T. Tompkins, W.A. Zeltner, M.A. Anderson, J. Photochem. Photo-

biol. A: Chem. 148 (2002) 323–330.110] L. Sopyan, S. Murasawa, K. Hashimoto, A. Fujishima, Chem. Lett. 4 (1994)

723–726.[111] S. Horikoshi, M. Kajitani, N. Horikoshi, R. Dillert, D.W. Bahnemann, J. Pho-

tochem. Photobiol. A: Chem. 193 (2008) 284–287.