-
Journal of Photochemistry and Photobiology C: Photochemistry
Reviews 9 (2008) 112
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology C:Photochemistry
Reviews
journa l homepage: www.e lsev ier .com/ locate /
jphotochemrev
Review
Heterogeneous photocatalytic degradation of organic contaminants
overtitanium dioxide: A review of fundamentals, progress and
problems
Umar Ibrahim Gayaa, Abdul Halim Abdullaha,b,
a Department of Chemistry, Faculty of Science, Universiti Putra
Malaysia, 43400 Serdang, Selangor D.E., Malaysiab Advanced
Materials and Nanotechnology Laboratory, Institute of Advanced
Technology, Universiti Putra Malaysia, 43400 Serdang, Selangor
D.E., Malaysia
a r t i c l e i n f o
Article history:Received 6 August 2007Received in revised form27
September 2007Accepted 15 December 2007Available online 18 March
2008
Keywords:SemiconductorTitaniaDegradationPhotocatalysisEcotoxicity
a b s t r a c t
Even though heterogeneous photocatalysis appeared in many forms,
photodegradation of organic pol-lutants has recently been the most
widely investigated. By far, titania has played a much larger
rolein this scenario compared to other semiconductor photocatalysts
due to its cost effectiveness, inertnature and photostability.
Extensive literature analysis has shown many possibilities of
improving the
Contents
1. Introduction . . . . . . . . . . . . . . . . . . .2. Basic
principles of photocataly3. Mechanism of titania-assisted4. Titania
versus existing photoc5. Effect of operational paramete
5.1. Light intensity . . . . . . . . .5.2. Nature and
concentrati5.3. Nature of the photocat5.4. Photocatalyst
concentr5.5. pH . . . . . . . . . . . . . . . . . . . . .5.6.
Reaction temperature .
6. Methods of utilization of titania photocatalyst . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . 77. Trends
in improving the activity of titania . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Novel photocatalyst preparations . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77.2.
Combined operations . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
8. Miscellaneous . . . . . . . . . . . . . . . .8.1. Ecotoxicity
of titania ph8.2. Future trends . . . . . . . . . .
9. Conclusion . . . . . . . . . . . . . . . . . . .
.Acknowledgement . . . . . . . . . . .References . . . . . . . . .
. . . . . . . . . . .
Corresponding author at: Departmenfax: +60 389435380.
E-mail address: [email protected]
1389-5567/$20.00 2008 Elsevier B.V.
Adoi:10.1016/j.jphotochemrev.2007.12.003. . . . . . . . . . . . . .
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t of Chemistry, Faculty of Science, Universiti Putra Malaysia,
43400 Serdang, Selangor D.E., Malaysia. Tel.: +60 389466777;
du.my (A.H. Abdullah).
ll rights reserved.efciency of photodecomposition over titania
by combining the photoprocess with either physical orchemical
operations. The resulting combined processes revealed a exible line
of action for wastew-ater treatment technologies. The choice of
treatment method usually depends upon the compositionof the
wastewater. However, a lot more is needed from engineering design
and modelling for success-ful application of the laboratory scale
techniques to large-scale operation. The present review paperseeks
to offer an overview of the dramatic trend in the use of the TiO2
photocatalyst for remediationand decontamination of wastewater,
report the recent work done, important achievements and
prob-lems.
2008 Elsevier B.V. All rights reserved.
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. . . . . . . . . . . . . . . . . . . . . 2sis . . . . . . . . . .
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. . . . . 2photocatalytic degradation . . . . . . . . . . . . . . .
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. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . 2atalysts . . . . . . . . . . . . . . . . . . . . .
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. . . . . . . . . . . . . . . . . . . . . . . . . . . . 5on of the
substrate . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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. . . 5alyst . . . . . . . . . . . . . . . . . . . . . . . . . . .
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. . . . . . . . . . . . . . . . . . . . . 6ation . . . . . . . . .
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2 U.I. Gaya, A.H. Abdullah / Journal of Photochemistry and
Photobiology C: Photochemistry Reviews 9 (2008) 112
1. Introduction
As recalcitrant organic pollutants continue to increase in air
andwastewater streams, environmental laws and regulations
becomemore stringent [1,2]. As a response, the development of
newereco-friendly methods of destroying these pollutants became
animperative task.Ultimately, researchactivities
centredonadvancedoxidation processes (AOPs) for the destruction of
synthetic organicspecies resistant to conventionalmethods. AOPs
rely on in situ gen-eration of highly reactive radical
species,mainlyHO by using solar,chemical or other forms of energy
[3,4]. Themost attractive featureof AOPs is that this highly potent
and strongly oxidizing radicalallows thedestructionof awide rangeof
organic chemical substratewith no selectivity.
Among AOPs, heterogeneous photocatalysis has proved to beof real
interest as efcient tool for degrading both aquatic andatmospheric
organic contaminants [5]. Heterogeneous photocatal-ysis involve the
acceleration of photoreaction in presence ofsemiconductor
photocatalyst. One of themajor applications of het-erogeneous
catalysis is photocapartial or total mineralisation of ginants to
benign substances [6].with a partial degradation, the tusually
refers to complete photoeralisation, essentially to CO2, H[7].
Titania photocatalysis also refeeffect was rst unfolded by
theandHonda [8]. Theseworkers revetingbyphotoelectrochemical
celltitania anode. Consequently, theysis extended to
environmentalthe rst time reported the apploxidation of CN and SO32
in aSubsequent reports of photocatalal. [10] attracted more
interest to
As part of the dawn, the photbutane reported by Izuml et al.
[1tion mechanism based on hydronew chapter in organic
synthesistodecomposition of organic comreaction parameters was
reported
Heterogeneous photocatalysissince its infancy considering the
hand books devoted bymany reseacations the basic photophysical
prphotocatalysis is largely the same.the fundamentals of the
heterogeof organic contaminants and repments. Although
photocatalytic dfor destruction of both organic anof this paper is
on organic contam
2. Basic principles of photocata
Heterogeneous photocatalysislarge variety of reactions:
organitoreduction, hydrogen transfer, Oisotopic exchange, metal
deposittherapy,water detoxication, gaseAmong these appearances
titanicatalytic oxidation has received malternative method for
puricatio
The basic photophysical and ping photocatalysis are already
est
in many literatures [15,16]. Vinodgopal and Kamat [17]
reportedthat the dependence of the rate of
1,3-diphenylisobenzofuran pho-todegradation on the surface
coverage. In other words, only themolecules that are in direct
contact with the catalyst surfaceundergo photocatalytic
degradation.
Photocatalytic reaction is initiatedwhenaphotoexcitedelectronis
promoted from the lled valence band of semiconductor photo-catalyst
(SC) to the empty conduction band as the absorbed photonenergy, h,
equals or exceeds the band gap of the semiconductorphotocatalyst
leaving behind a hole in the valence band. Thus inconcert, electron
and hole pair (eh+) is generated. The followingchain reactions have
been widely postulated.
Photoexcitation : TiO2/SC+h e +h+ (1)Oxygen ionosorption :
(O2)ads + e O2 (2)Ionizationof water : H2O OH +H+ (3)Protonationof
superoxides : O2 +H+ HOO (4)
l forrolo
ctionctor ple oc
rm sual (H
siste
seding rfor tto adinguct:
nistiationme feratuiIIIOHurfacce-buivaer a
nd Seare inationd the lasbed bctroemp
iO2/Sd drers what ftalytic oxidation (PCO) to effectas phase or
liquid phase contam-Even though degradation beginserm
photocatalytic degradationcatalytic oxidation or photomin-2O, NO3,
PO43 and halide ions
rred to as the HondaFujishimapioneering research of
Fujishimaaled thepossibilityofwater split-having an inert cathode
and rutileapplication of titania photocatal-frontiers. Frank and
Bard [9] forication of TiO2 in photocatalyticqueous medium under
sunlight.ytic reduction of CO2 by Inoue ettitania
photocatalysis.o-kolbe decarboxylation route to1] and the
suggestion of its reac-xyl radical generation opened a. The
earliest description of pho-pounds and studies of effects ofby
Kraeutler and Bard [12].has attracted constant researchigh number
of excellent reviewsrchers [7,13]. Despitemany appli-inciple and
physical chemistry inThepresent reviewsheds lightonneous
photocatalytic degradationorts the important accomplish-egradation
has broad generalityd inorganic compounds the focusinants.
lysis
is a discipline which includes ac synthesis, water splitting,
pho-218O216 and deuteriumalkane
ion, disinfection and anti-cancerous pollutant removal, etc.
[7,14].a-assisted heterogeneous photo-ore attention for many years
as
n of both air and water streams.hotochemical principles
underly-ablished and have been reported
The hydroperoxyl radicaproperty as O2 thus doubly p
HOO + e HO2
HOO +H+ H2O2Both the oxidation and reduthe photoexcited
semicondution between electron and hoscavenge the electrons to
foform the hydroperoxyl radic
3. Mechanism of titania-asdegradation
Titania has been widely ucharge carriers thereby
inducrespectively [18]. Generally,alytic reaction as
opposednegative [7]. The corresponstituent is formed as by prod
Many elementary mechain the photocatalytic degradsurface. The
characteristic tibeen reported in previous lit
The {>TiIVOH+} and {>Tvalence band electron and strons,
respectively. The surfa{>TiIVOH+} is chemically eqallowing the
use of the form[20]. According to Lawless aa surface-bound OH
radical
There exist a good correlics, their surface densities
andegradation over TiO2. In thsions of TiO2 have been
profemtosecond absorption speelectron scavenger has beenond
spectroscopic study of Tand Bowman [23] indicatetion of trapped
charge carriThe results also conrmed tmed in (4) also has
scavengingnging the lifetime of photohole:
(5)
(6)
can take place at the surface ofhotocatalyst (Fig. 1).
Recombina-curs unless oxygen is available toperoxides (O2), its
protonatedO2) and subsequently H2O2.
d photocatalytic
as a photocatalyst for generatingeductive and oxidative
processes,itania-assisted aerobic photocat-photosynthetic reaction
G isacid HA of the non-metal sub-
(7)
c processes have been describedof organic compounds over TiO2or
each elementary reaction hasre (Table 1).} represent the
surface-trappede-trapped conduction band elec-ound OH radical
represented bylent to the surface-trapped holend latter terms
interchangeablyrpone [22] the trapped hole anddistinguishable
species.between charge carrier dynam-e efciency of the
photocatalytict two decades, aqueous suspen-y picosecond and more
recentlyscopies [23,24]. Traditionally, anloyed in such study. A
femtosec-CN aqueous system by Colomboamatic increase in the
popula-ithin the rst few picoseconds.or species adsorbed to TiO2,
the
-
U.I. Gaya, A.H. Abdullah / Journal of Photochemistry and
Photobiology C: Photochemistry Reviews 9 (2008) 112 3
Fig. 1. Schematic photophysical and photochemical processes over
photon activated semiconductor cluster (p) photogeneration onation,
(r) recombination in the bulk, (s) diffusion of acceptor and
reduction on the surface of SC, and (t) oxidation of donor on
the
hole-transfer reaction can successfully compete with the
picosec-ond electronhole recombination process. The following
interfacialphotochemical reactions were described:
Photoexcitation : TiO2 +h eCB +h+VB (8)Charge carrier trapping :
eCB eTR (9)Charge carrier trapping : h+VB h+TR (10)
Electronhole recombination :
eTR +h+VB(h+TR) eCB +heBahnemann et al. [25] provide
carriers using ash radiolysis. Porganic molecules is due to
the(Ti3+) and h+TR (presumably OHIn agreement with the
foregoingtrapped carriers mainly exist nea
Table 1Primary processes and time domains in tipollutants
[1921]
Primary process
Charge carrier generationTiO2 +h e +h+
Charge carrier trappingh+ + >TiIVOH {>TiIVOH+}e
+>TiIVOH>TiOHIII}
e +>TiIV TiIII
Charge carrier recombinatione + {>TiIVOH+}>TiIVOHh+
+>TiIIIOHTiIVOH
Interfacial charge transfer{>TiIVOH+}+organic molecule
>TiIVOH+oxidized molecule{>TiIIIOH}+O2 >TiIVOH+O2
undergo rapid (1ps) recombinattation. The important
consequencand electrons (eTR) to the phothave been highlighted by
Serpon
In most applications, photocacarried out in presence of waterthe
photocatalyst. The presencephotocatalysis. Earlier work oncated
that the reaction did not p[29]. Fig. 2 shows the stages in
thphotomineralisation of organic co
otenno
with
onp
dativtro-leasuH [3bsen
ons (holeadicaat (11)
d evidence for the trapped chargerimarily, the ssion of bonds
ininteraction of the trapped eTR) pairs near TiO2 particle
[26].
, Furube et al. [27] observed thatr the particle surface and do
not
tania-catalyzed mineralisation of organic
Characteristic time
fs (very fast)
10ns (fast)100ps (shallowtrap; dynamicequilibrium)
Photoholes have great pdirectly (although mechanismrectly via
the combinationsolution [15,30]:
H2O + h+ OH + H+
RH + OH R + H2O
R + h+ R+ Degradati
The mediation of radical oxievidencedbyphoto- andelectrode in
aqueous solutions mpotential and the solution poxidative species
originally aunder anodic bias.
The primary photoreactiof charge carriers (electrontion.
Essentially, hydroxyl r10ns (deep trap)
100ns (slow)10ns (fast)
100ns (slow)
ms (very slow)
ions (O2) and hydroperoxyl radintermediates that will act
concety of organic pollutants includ(VOCs) and bioaerosols [32,33].
Itally that the oxidative reactionoccurs mainly via the formationof
5.7102) not hydroxyl rad7105) [34].f electron/hole pair, (q)
surface recombi-surface of SC particle.
ion immediately after photoexci-es of surface trapped holes
(h+TR)oxidation of organic compoundse et al. [28].talytic
degradation reactions are, air, the target contaminant andof water
is indispensable in TiO22-propanol photooxidation indi-roceed in
the absence of watere photoinduced processes of thentaminant in
presence of TiO2.tial to oxidize organic speciest proven
conclusively [6]) or indi-
OH predominant in aqueous
(12)
(13)
roducts (14)
e species in photooxidation wasuminescence spectraof TiO2
elec-red as functions of the electrode1]. It was found that the
radicalt accumulated after illumination
1)(11) indicate the critical rolepair) in photooxidative
degrada-ls (OH), holes (h+), superoxide
icals (OOH) are highly reactiveomitantly to oxidize large
vari-ing volatile organic compoundst is however argued experimen-on
titania photocatalyst surfaceof holes (with quantum yield
icals formation (quantum yield
-
4 U.I. Gaya, A.H. Abdullah / Journal of Photochemistry and
Photobiology C: Photochemistry Reviews 9 (2008) 112
Fig. 2. Conceptual diagram for the primary processes involved in
photomineralisation of organic compounds.
As aphotochemical application, TiO2 photocatalysis is
invariablyaffected by the surface properties of the TiO2 particle.
The photoin-duced phenomenon is affected by quantum size. Anpo et
al. [35]observed blue shift and increase in reaction yield and
photocat-alytic activity as the diameter of the TiO2 particles
becomes smallerespecially below 100 A. This observation was
attributed to the sup-pression of radiationless transfer and the
concurrent enhancementof the activities of the charge carriers.
The reaction mechanism for the photooxidative degradation ofmany
organic pollutants over titania particle has been
extensivelyreviewed [19,20,36]. The number of intermediates in the
reac-tion and ease of decomposition depends upon the nature
organiccontaminant studied. The photocatalytic degradation of
methanaland phenol are interesting mechanistic examples on the role
ofhole, superoxide and hydroxyl radicals in titania-assisted
pho-tomineralisation of aliphatic and aromatic organic templates.
Themechanisms are illustrated in Figs. 3 and 4, respectively.
In the degradation of phenol,reported (Fig. 4). The OH
radicaphenol (a), yielding catechol (b), r(d) andhydroquinone (e),
then thebreak up to givemaleic acid (f), thas, 3-hydroxy propyl
carboxylic2-hydroxy-ethanoic acid glycol athoughH produced during
the at
Fig. 3. Photocatalytic oxidation o
in the process, it is scavenged bwhich nally convert to OH
radi
The kinetics of photocatalytpounds usually follows the
L[16,19,20].
r = dCdt
= kKC1 + KC
where r represents the initial ratetration of the reactant, t
the irraof the reaction and K is the adsorAt mM concentrations C1,
theapparent rate order equation [39,
ln C0C
= kKt = Kapptor
rstC vellutanationrst o
deraoved]. Ator thd nondedound
hoto
photes [7several intermediates have beenl attacks the phenyl
ring of theesorcinol (c), benzene-1,2,3-triolphenyl rings in
thesecompoundsen short-chain organic acids suchacid (g), 2-hydroxy
propanal (i),cid (j), nally CO2 and H2O. Eventack of bonds by OH
participates
Ct = C0 eKapp t
where Kapp is the apparentslope of the graph of lnC0/centration
of the organic pocondition, the initial degradconforming to the
apparent
r0 = KappCA quasi-exhaustive consi
of organic contaminants prtion above holds true [4146model
serves as a basis fcompounds even if it coul[47]. Nevertheless, for
suspedegradationof organic comp[48].
4. Titania versus existing p
An ideal photocatalyst forized by the following attribu
(1) Photo-stability.f formaldehyde over titania [37].
(2) Chemically and biologically in(3) Availability and low
cost.y oxygen to form HO2 radicals,cals.ic degradation of organic
com-angmuirHinshelwood scheme
(15)
of photooxidation, C the concen-diation time, k the rate
constantption coefcient of the reactant.equation can be simplied to
the40]:
(16)
order rate constant given by thersus t and C0 is the initial
con-t. Consequently under the samerate could be written in a
formrder rate law:
(17)
tion of photodestruction studiesthat the rst order rate
equa-
any rate, LangmuirHinshelwoode photodegradation of organict
directly give adequate ttingtitania-mediated photocatalytics
pseudo-zerothorder is reported
catalysts
tocatalytic oxidation is character-]:ert nature.
-
U.I. Gaya, A.H. Abdullah / Journal of Photochemistry and
Photobiology C: Photochemistry Reviews 9 (2008) 112 5
(4) Capability to adsorb reactanttion (h Eg).
Many chalcogenide semicondCdS, MoS2, Fe2O3 and WO3
havetocatalysts for the degradation ominimum band gap energy
requireration of charge carriers over TiOis 3.2 eV corresponding
towavelenTiO2, photoactivation takes place
The photoinduced transfer oadsorbed speciesover
semiconduband-edge position of the semicoof the adsorbates
[30].
In spite of the constant vigordecades in search for an ideal
phmodication has remained a bencing material candidate will be
metitania is reported to give the beand photostability [6]. Nearly
alltalline forms of titania namely anand Maggard [52] reported
theamorphous titania with wider basignicant photocatalytic
activitypositions for various semiconduc
5. Effect of operational parame
5.1. Light intensity
Photocatalytic reaction rate dabsorption of the photocatalyst
[5in the degradation rate with incretocatalytic degradation. The
natuaffect the reaction pathway [56sensitization mechanism does
nodation.
Unfortunately, only5%of the tosufcient energy to cause
effectiveenergy loss due to light reectionheat is inevitable in the
photoproinvited more research in the app
ght athe q
actioof
2 iniationdeteid r
ic efle [6
eld orevio
terna3] beld cheredisag De
of t
can are suradatted tphenoroapheenerbenzdark
idatiFig. 4. Degradation of phenol in wastewater over
nanomaterial titania [38].
s under efcient photonic activa-
uctors such as TiO2, ZnO, ZrO2,been examined and used as pho-f
organic contaminants [49]. Theed for photon to cause photogen-2
semiconductor (anatase form)gth of 388nm [50]. Actuallywithin the
range 300388nm.f electrons that take place withctorphotocatalyst
dependson thenductor and the redox potentials
ous research activities over twootocatalyst, titania in its
anatasehmark against which any emerg-asured [51]. The anatase form
ofst combination of photoactivity
studies have focused on the crys-atase and rutile. However,
Zhangpreparation of hydrated form ofnd energy gap than anatase and.
The schematic diagram of bandtors is shown in Fig. 5.
ters
epends largely on the radiation3]. Refs. [54,55] revealed
increasease in light intensity during pho-re or form of the light
does not]. In other words, the band-gapt matter in photocatalytic
degra-
tal irradiatednatural sunlighthas
tion. The overall quanta of lireactant is given by overall,
overall =rate of re
rate of absorption
as metal oxide such as TiOabsorb all the incident
radexperimentally difcult tolight scattering in solidliquAnother
factor limits photontion between electron and hothat reference to
quantum yitem is ill-advised despite preferences [61,62].
A practical and simple alciencies was suggested [6efciency r. A
quantum yifrom r, as = r phenol, wthe photocatalyzed
oxidativesecondary actinometer) usincatalyst material.
5.2. Nature and concentration
Organicmoleculeswhichthe photocatalyst will be moThus the
photocatalytic degsubstituent group. It is reporan adsorbing
substrate than[65]. In thedegradationof chlout that
mono-chlorinatedtri-chlorinated member. In gdrawing group such as
nitroto adsorb signicantly in thedonating groups [67].
During photocatalytic ox
photosensitization [57]. Besides,, transmission and energy loss
ascess [58]. This limitation largelylication of TiO2 to
decontamina-
substrate over time is dependenAt high-substrate
concentrationsdiminishes and the titanium dioleading to catalyst
deactivation [6bsorbed by any photocatalyst oruantum yield:
nradiation
(18)
a heterogeneous regime cannotdue to refraction, it has been
rmine quantum yield [59]. Theegime particularly is
signicant.ciency is the thermal recombina-0]. For these reasons, it
is arguedr efciency in heterogeneous sys-us use of the term by
previous
tive for comparing process ef-y dening a relative photonican
subsequently be determinedphenol is the quantum yield forppearance
of phenol (a standardgussa P-25 TiO2 as the standard
he substrate
dhere effectively to the surface ofsceptible to direct oxidation
[64].ion of aromatics depends on thehat nitrophenol is much
strongerol and therefore degrades faster
romatics,Huqulet al. [66]pointednol degrades faster than di-
oral, molecules with electron with-ene and benzoic acid were
foundcompared to those with electron
on the concentration of organic
t upon photonic efciency [68].however, the photonic efciencyxide
surface becomes saturated9].
-
6 U.I. Gaya, A.H. Abdullah / Journal of Photochemistry and
Photobiology C: Photochemistry Reviews 9 (2008) 112
Fig. 5. The conduction and valence band sentsThe right hand
scale is the normal hydrog
5.3. Nature of the photocatalyst
There is direct correlation betwface coverage of TiO2
photocatalythat the number of photons strcontrols the rate of the
reactionthe reaction takes place only inconductor particle. A very
impoperformance of photocatalyst in pface morphology, namely the
pa[72].
Numerous forms of TiO2 havmethods to arrive at a
photocataproperties, activity and stability foEvidently, there is a
clear connecties, the rational development of ipossible usefulness
of the materiaFor instance, smaller nano-particconversion in
gaseous phase phopounds over nano-sized titanium
5.4. Photocatalyst concentration
The rate of photocatalytic reconcentration of the
photocatalyreactions are known to showtodegradation with catalyst
loadphotocatalytic application, themust be determined, in order
tototal absorption of efcient phunfavourable light scatteringandthe
solution is observed with exc
in this thpertIn tulskhaveia cafollo
ll remativeis reesspositions of selected metal oxide
semiconductors at pH 0. The left hand scale repreen electrode scale
which allows predictions based on reduction and oxidation.
een of organic pollutant and sur-st [70]. Kogo et al. [71]
reportediking the photocatalyst actually. The latter is an
indication thatthe adsorbed phase of the semi-rtant parameter
inuencing thehotocatalytic oxidation is the sur-rticle size and
agglomerate size
e been synthesized by differentlyst exhibiting desirable
physicalr photocatalytic application [73].tion between the surface
proper-mproved synthesis routes and thel prepared in application
[74,75].le size is reported to give highertomineralisation of
organic com-
5.5. pH
An important parameterplace on particulate surfacestates the
surface charge proof aggregates it forms [80].of zero charge (pzc)
by Kosmand 20% rutile is reported tocondition the surface of
titanrespectively according to the
TiOH + H+ TiOH2+
TiOH + OH TiO +H2OThus, that titania surface wimedium (pH6.9).
Titanium dioxideactivity at lower pH but excreaction rate
[82].dioxide [76].
action is strongly inuenced byst. Heterogeneous
photocatalyticproportional increase in pho-ing [77]. Generally, in
any givenoptimum catalyst concentrationavoid excess catalyst and
ensureotons [78]. This is because anreductionof lightpenetration
intoess photocatalyst loading [79].
The effect of pH on the phocompounds and adsorption on Tstudied
[83,84]. Change in pH caefciency of photoremoval of
organiumdioxidewithout affecting thconditions better degradation
ha
5.6. Reaction temperature
Experimental studies on theof degradation of organic
compocarried out since 1970s [12]. Manimental evidence for the
depenon temperature [8791]. Generathe internal energies to the
vacuum level.
e photocatalytic reactions takinge pH of the solution, since it
dic-ies of the photocatalyst and sizehe current update of the
pointsi [81] Degussa P-25, 80% anatasepzc 6.9. Under acidic or
alkalinen be protonated or deprotonatedwing reactions:
(19)
(20)
ain positively charged in acidicly charged in alkaline
mediumported to have higher oxidizingH+ at very low pH can
decreasetocatalytic reactions of organiciO2 surface has been
extensivelyn result in enhancement of thenic pollutants in presence
of tita-e rate equation [85]. At optimizeds been reported [86].
dependence of the reaction rateunds on temperature have beeny
researchers established exper-dence of photocatalytic activitylly,
the increase in temperature
-
U.I. Gaya, A.H. Abdullah / Journal of Photochemistry and
Photobiology C: Photochemistry Reviews 9 (2008) 112 7
enhances recombination of charge carriers and desorption
processof adsorbed reactant species, resulting in decrease of
photocatalyticactivity. This is in conformity with Arrhenius
equation, for whichthe apparent rst order rate constant Kapp should
increase linearlywith exp(1/T).
6. Methods of utilization of titania photocatalyst
Titania photocatalyst can be used either as free-standing
par-ticulate or as coating on a substrate. Most experiments
utilizednely powdered TiO2 particles suspended in contaminated
water,which provides large surface area and makes recovery easy
aftertreatment [92]. Larger particulates may prove useful even in
thecase of gaseous organic contaminants but are rather
commerciallyunavailable and may be costly [6].
Coated catalyst congurations, on the other hand, eliminate
theneed for catalyst ltration and centrifugation but generally
result ina signicant reduction in system efciency. A reduction of
6070%reduction inperformance is reporbilized TiO2 as compared to
the u
Many kinds of support have balyst which include soda lime
glatiles [95] and coated glass [96].actual active surface area of
the phall volume is low. Despite aforemephotocatalysts and
immobilisatioIn many of these cases TiO2 coateciency in organic
compound rem
7. Trends in improving the activ
Despite drawbacks of titania ppounds have been
successfullyviewpoint of air and water puriccontaminants have been
studiedcompounds (EDCs) [97]. By far, tconstantly explored mainly
to sution and to enhance the photoseapplication [58].
7.1. Novel photocatalyst preparatio
Due to the constraints involvetivation, there has been
growingwavelengthof 388nmwhich correThe principal foci of these
activiti
incorporation of energy levels i changing the life time of
charge substitution of the Ti4+ with cat shifting the conduction
band an
photoexcitation at lower energthe preparation method.
The major practices involvemetal coating, surface
sensitizatidesign and development of secoface sensitization has
been inteRecent studies indicate enhancetion of gaseous organic
contamiphenol over uorinated titania susubstrate and reaction
conditioncan be positive or negative [99].
Fig. 6. Modication of titania photocatalyst by metal doping.
Since 1980s TiO2 has been modied mainly by metal loadingor
platinization to achieve better photocatalytic activity
[100102].Successful doping can be achieved with either transition
metal ionor with non-metal resulting in enhanced efcacy of the
photocata-lyst system (Fig. 6). The last 4 years has attracted
growing interestin doping of titania with Pt due to promising
improvement in pho-tooxidation rate especially in gas phase. PtTiO2
has been foundto improve the photooxidation rate of ethanol,
acetaldehyde andacetone in gaseous phase [103]. Nitrogen doped into
substitutional
eforte
hephut thTi4+
typed Ni,um dcasee ba
sts Tandt beylighas t
hes a9,11aredshowcenthotolyticTiO2in thiO2 he phoy
crylyticensiinteescrto de
h+)T
)
iO2 c136]ted in aqueous systems for immo-nsupported catalyst
[39].een explored for TiO2 photocat-ss [93], aluminium [94],
ceramicSince coatings are very thin, theotoreactor compared to the
over-ntioned drawbacks, more coatedn techniques are still
investigated.d on support assumed more ef-oval than uncoated TiO2
[18].
ity of titania
hotocatalysis many organic com-investigated largely from the
ation. A large number of organicincluding endocrine
disrupting
he following avenues have beenppress electronhole
recombina-nsitivity of titania for successful
ns
d in ensuring effective photoac-quest to go beyond the
thresholdsponds to thebandgapof titania.es include [57,98]
nto the band gap of the titania,carriers,ion of the same size
andd/or valence band so as to enableies; success of which depends
on
catalyst modication by doping,on, increase in surface area or
byndary titania photocatalyst. Sur-nsively reviewed elsewhere
[7].ment of photocatalytic degrada-nants such as acetaldehyde
andrface. Depending on the kind ofthe effect of surface
uorination
sites of TiO2 has proven verydoped zirconia has been repeven
titania P-25 [105,106]. TTiO2 is not yet understood bvalencies
lower than that ofcentres as opposed to the p-ions such as V, Cr,
Mn, Fe anthe absorption band of titaniThere is no red shift in
the[107] which indicates that thimplantation itself.
Unlike many photocatalyform of sun or room lightbe improved to
absorb lighThe assertion to use visibleNozik [108] has been as
oldsis itself. Numerous researcphotocatalytic oxidation
[10photocatalyst have beenprepmetal or non-metal. Table
2photocatalyst made in the re
Inmixed semiconductor phas marked effect on the catalyst as
reported in the case ofoxides have great potentialcase of TiO2ZrO2
andTiO2Sture has marked effect on threcombination is increased
bachieved folds of photocatadegradation withmixed suspprimary
processes leading to(Fig. 7) were conjectured to dtheory has
beenwidely usedphotocatalysis [135].
CdS+h CdS(h+ + e)TiO2 +h TiO2(h+ + e)CdS(h+ + e) + TiO2 CdS(
CdS(h+ + e) + TiO2(h+ + e
The excess electrons on the Tdiatomic molecular oxygen [cient
for photocatalysis [104]. Fed to show lower efciency
thanotophysicalmechanismof dopede p-type metal ion dopants (with)
are believed to act as acceptor[7]. Metal ion implantation withwas
found to cause large shift inioxide towards the visible region.of
TiO2 implanted with Ar or Tithocromic shift is not due to the
iO2 can absorb UV-A light in theas reported in this review
canond the wavelength of 388nm.t in photoinduced processes byhe
phenomenon of photocataly-imed at investigating solar TiO20] and
novel visible responsivebydoping the photocatalystwiths promising
preparation of TiO2years.catalysts the synthesis procedureactivity
of the hybrid photocata-
SiO2 [130,131]. TiO2SiO2 binarye removal of VOCs [132]. In
theybrid oxides the crystallite struc-toactivity [133] perhaps
becausestallite defects. Doong et al. [134]performance in
2-chlorophenolon of CdS and TiO2. The followingr-particle electron
transfer (IPET)ibe the phenomenon. Today, IPETscribe promotion
effect in similar
(21)
(22)
iO2(e) (23)
CdS(h+ +h+) + TiO2(e + e)(24)
an be scavenged by chemisorbed.
-
8 U.I. Gaya, A.H. Abdullah / Journal of Photochemistry and
Photobiology C: Photochemistry Reviews 9 (2008) 112
Table 2Some novel preparations of UV and visible light
responsive titania photocatalysts
Photocatalyst Highlights on synthesis and comments on
performance Reference
Nd3+TiO2 sol The sol prepared by coprecipitation had anatase
crystalline structure and showed higherphotocatalytic activity with
reactive dye X-3B than titania under visible light illumination
[111]
N-doped TiO2 Nitridation of TiO2 extended absorption to visible
region. The addition of PdCl2 further extendedabsorption to near IR
with high-photocatalytic activity
[112]
TiO2xNy The photocatalyst was prepared by low-temperature
process involving mechanical doping and oxygenplasma treatment
[113]
Highly active bicrystalline TiO2 (anatase-brookite)
Nanocrystalline TiO2 was prepared at 100 C by hydrolysis of
Ti(C3H5O)4. High specic area of750m2/g and higher photoactivity
over Degussa P-25 was obtained
[114]
Mesoporous TiO2 Titanium isopropoxide was the Ti source in the
solgel method. Co-polymer surfactant was used todirect structure.
The polymeric template was removed by solvent extraction
[115]
Cr-doped TiO2 Cr doped anatase TiO2 was prepared by the
combination of solgel and hydrothermal methods. Crdoping improved
photocatalytic activity for the degradation of XRG dye
[116]
Ce-doped mesophorous TiO2 Doping inhibited mesophores collapse
and anataserutile phase transformation. Doped mesophorousanatase
nanoparticles exhibited higher photocatalytic activity than
commercial Degussa P-25
[117]
Ag-doped nanocrystalline TiO2 order Photocatalyst was prepared
by solgel method and subsequent ultrasonication. Photocalalytic
activitywas evaluated with methylene blue in presence and in
absence of NO32 , SO42 and CH3COO . Theanions caused signicant
increase in the photocatalytic degradation of the dye which
followedpseudo-rst order
[118]
C-modied nano-TiO2 The photocatalyst was prepared by heating
TiO2 at high temperatures in an atmosphere of hexane.without C
deposition the latter
[119]
Fe3+-, Cr3+- and Co2+-doped nano-crystall increasing dopant
[120]
Titanium oxocluster-derived nanocrystall than the one obtained
from [121]
Zn2+-doped TiO2 ethods and evaluated withsulted
[122]
NF codoped TiO2 tetrabutyl titanate precursor.ced
p-chlorophenol
[123]
TiO2/WO3 multilayer thin lm tocatalytic activity towardsn
[124]
V-doped TiO2 tocatalytic activity in the [125]
TiO2SiO2 beads te by dip coating hollow glassf silica inhibited
anataserutile
[126]
TiO2ZnO ing citric acid as complexingards methyl orange
improved
[127]
TiO2-carbide is loaded in the former. Thisncentration
[128]
Ball-milled TiO2/SnO2 O was used as disperser. Thend by
[129]
7.2. Combined operations
For deriving an effective decocost theUV/TiO2 version of
photoobeen combined with either phyThe rise in the hyphenated
metnumber of novel water treatmening high-treatment costs. The
chocomposition of water in terms ofpollutant level [38].
In combined TiO2/inorganicscavengers such as O3 [137], Fe2+
and BrO3 [139] are added to trap[50].
H2O2 + e OH+ OHS2O82 + e SO42 + SO4
SO4 +H2O SO42 + OH + HBrO3 +2H+ + e BrO2 + H2BrO3 +6H+ +6e
[BrO2,HOO3 + e O3Even though lower photocatalytic activity was
obtained than the TiO2showed less turbidity after photocatalyst
sedimentation
ine TiO2 Under UVvis excitation anataserutile transformation
increased withconcentration
ine TiO2 Photocatalyst sourced from Ti7O4(OEt)20 precursor had
higher activitytitanium tetraisopropoxideThe photocatalyst was
prepared by solgel and solid phase reaction mRhodamine B. Signicant
enhancement of the photoactivity of TiO2 reAnatase NF doped TiO2
was prepared by solvothermal method usingThe photocatalyst showed
very high activity towards visible light induphotooxidative
degradationMultilayer prepared by pulse laser deposition (PLD)
showed high-phophotodecomposition of methylene blue under visible
light illuminatioPhotocatalyst prepared by modied solgel method
showed high-phodegradation of crystal violet and methylene blue
under visible lightThe photocatalyst was prepared from
[Ti(iso-OC3H7)4] and ethyl silicamicrobeads and calcination
preferably at 650 C for 5h. The addition otransitionThe novel
photocatalyst was prepared by modied solgel method usagent. By
sulfating, the degradation efciency of the photocatalyst
towsignicantlyCorrugated shapes of carbide were prepared on metal
mesh and TiO2preparation has commercial potentials for
photodegradation at low coPhotocatalyst was prepared by ball
milling through doping of TiO2. H2crystal faces of TiO2 were not
changed and new crystal faces were fou
O3 + H+ HO3ntamination at relatively lowerxidative degradation
process has
sical or chemical operation [13].hods paved way to the growingt
technologies thereby overcom-ice of method depends upon theclass of
the organics [1] and the
additive photoprocess electron/Fe3+ +H2O2 [138], H2O2, S2O82
electrons and generatemore OH
(25)
(26)
+ (27)
O (28)
Br] Br +3H2O (29)(30)
HO3 HO + O2
Fig. 7. Band diagram illustrating charge caball milling
(31)(32)
rrier transfer in coupled semiconductors.
-
U.I.G
aya,A.H
.Abdullah
/JournalofPhotochemistry
andPhotobiology
C:Photochemistry
Reviews9(2008)
1129
Table 3Photocatalytic degradation of organic compounds by TiO2
under UV irradiation
Class of organic contaminants Target compound Highlights on
experimentals and ndings Reference
Aldehydes Acetaldehyde Photomineralisation efciency over TiO2 lm
exceeds that over F-TiO2. Surface uorination causes
adsorptioninhibition to a large extent
[99]
Formaldehyde 30W UV lamp (297) and silica support were used in
the study. CO and CO2, found as product of degradationadsorb on the
TiO2 photocatalyst. This could cause catalyst deactivation
Carboxylic acids Phenoxy acetic acid and 2,4,5-phenoxyacetic
acid The effect of pH, catalyst, BrO3 and H2O2 to degradation was
signicant in all cases. Degussa P-25 was moreefcient photocatalyst
than Hombicat UV 100, Millenium Inorganic PC500 and Travancore
[145]
Oxalic acid Synergetic effect of combined sonolysis
photocatalysis was conrmed in Ar atmosphere. H2 and CO wereobtained
in addition to CO2. H2O2 played signicant role in the process
[146]
Chloroanilines 2-Chloroaniline Slower degradation resulted at
low pH in the UV/TiO2/H2O2 system. Excess H2O2 retarded the
degradation rate [147]Chlorocarboxylic acids Monochloro-acetic acid
In the ozonation-photocatalysis, O3 showed high-electron afnity
thereby improving the removal rate. O3 was
not decomposed by UV light in the system[137]
Chlorophenols 2-Chlorophenol Photoelectrocatalytic degradation
carried out in a batch reactor using TiO2 coated Ti sheets as anode
and Ptcathode was more effective in acidic medium and rate
increases linearly with light intensity
[148]
4-Chlorophenol Photomineralisation studied with different
samples of TiO2. Degussa P-25 proved more effective aphotocatalyst.
Both solar pilot plant and laboratory experiment indicated apparent
rst order kinetics. Fewerintermediates and faster TOC disappearance
was observed in the solar pilot plant which worked with
smalleroptimum titania concentration
[5]
2,4-Dichlorophenol Two kinetic models for photocatalytic
degradation of 2,4-dichlorophenol over Degussa P-25TiO2
suspensionwere proposed based on the inuence of different
variables; pH, radiation and TiO2 concentration
[149]
Mixture of 4-chlorophenol,
2,4-dichlorophenol,2,4,6-trichlorophenol and pentachlorophenol
Sequential photochemical-biological degradation proved useful.
There was no removal of chlorophenol withH2O2 or TiO2 alone
[150]
Dyes Acid orange 8 and Acid red 1 Sonophotocatalytic degradation
was faster than photocatalytic degradation followed by sonolysis
[151]Chrysoidine Y Degussa P-25 was found to be more effective than
ZnO in photomineralisation of the dye at laboratory scale.
UV analysis was used in studies[152]
Acridine orange and Ethidium bromide Degussa P-25 showed
superior photocatalytic activity than PC300. Degradation rate was
affected by inorganicadditives
[153]
Methylene blue, methyl orange, indigo carmine, Chicagosky blue,
mixed dye (mixture of the four dyes)
TiO2 photocatalyst was immobilised on glass and used for dye
removal. Chicago sky blue was the mostresistant to the
photodegradation. Methyl orange with t1/2 85.6min was removed
faster
[154]
Ethers Methyl tert-butyl ether (MBTE) H2O2-enhanced
photocatalysis had additive effects apart from synergetic effect
but hydrogen peroxidephotolysis had higher degradation rate.
Acetone, tert-butyl formate and tert-butyl alcohol were determined
asintermediates
[155]
Flourophenols 4-Fluorophenol TiO2-P25 was found to be more
efcient than ZnO under the study conditions. The efciency of anion
oxidantsand cations is respectively in the following order IO4
>BrO3 >S2O82 >H2O2 >ClO3 andMg2+ > Fe3+ > Fe2+
>Cu2+
[156]
Fungicides Fenamidone Coated optical bre photoreactor was used
in the study. Slow photocatalytic degradation of fenamidone
overTiO2 was observed. COO and SO42 were identied in the
reactor
[157]
Herbicides Isoproturon Degradation rate over Degussa TiO2 was
faster than Hombicat 100 and was increased by the addition
ofelectron acceptors. Degradation was slower under solar
illumination
[158]
Ketones Acetone Vibrouidized and multiple xed bed photoreactors
were compared. The comparison was based on thequantum efciency for
the photooxidation of acetone using TiO2 (Hombicat UV 100).
Vibrouidized-bedshowed higher activity for photooxidation.
Application of ultrasound does not inuenced the rate
ofphotooxidation of acetone
[159]
Perouroaliphatics Triouroacetic acid, sulfonic acid of
nonauorobutane andheptadecaourooctane
Peruorosulphonic acids were persistent under the experimental
conditions studied. However,phosphotungstic acid enhanced
mineralisation at extreme acidic pH. CO2 evolution was dependent
uponmolecular oxygen availability for the process
[160]
Phenolics Phenol TiO2 does not favour degradation at
concentrations higher than 100ppm. Active radicals in mechanism
wereconrmed to be H and HO
[36]
Pharmaceuticals Tetracycline Compound was resistant to
photolysis. Photocatalysis over 0.5 g/l TiO2 suspension showed
rapid rate ofdegradation. The irradiated solution inhibits the
activity of microorganisms
[161]
Lincomycin Solar photocatalysis and membrane separation was used
to study the degradation of the antibiotic.Photocatalysis was
coupled with membrane separation to remove catalyst. The
photooxidation of lincomycinfollowed pseudo-rst order rate
kinetics
[162]
Polymers Polyvinylpyrollidone (PVP) Three major steps were
identied in the photomineralisation of PVP; Adsorption, cleavage of
main ring andlactam ring caused by OH and OOH attack and nally the
conversion of ammine to NH4+ and NO3 . DegussaP-25 with particle
size 2030nm was used for the study. The higher the intensity from 1
to 4mWcm2 thehigher the CO2 yield resulting from mineralisation
[163]
-
10 U.I. Gaya, A.H. Abdullah / Journal of Photochemistry and
Photobiology C: Photochemistry Reviews 9 (2008) 112
Fig. 8. Photocatalytic system development cycle.
The UV/TiO2/O3 version of the TiO2/inorganic additive
pho-toprocess is based on attractive features of ozone for
organiccontaminant removal which include relatively higher
scaveng-ing effect provided by ozone, ability to decrease the rate
ofde-excitation of electrons and elimination of residues
[133,140].The combined ozononation-phott shared in the scenario of
disinforganic contaminants. Applicatiocial to remedy the failureof
theozof recalcitrant N-nitrosodimethyl
Coupled methods of operationand water purication.
Suryamanbiological treatment with photocthe removal of phenol to
reducetocatalytic process. The mineralito the single biological
treatmecompared to the photocatalysis. Isonophotocatalytic
degradationmineralisation of the popular heapplication of
photocatalytic membine slurry with membrane ltraof separation of
photocatalyst ingives an overview of pollutantsover titania.
8. Miscellaneous
The stages involved in the devform. The cycle, shown in Fig.
8sis of needs by the end-user. Eacis critical for successful
operatioproblem identied with stage (a)treated are in a form of
mixturelaboratory studies have been carryears [164168].
Additionally, labon degradation kinetics of targeting the toxicity
of degradation inthe intermediate of photodegradcoli than the
starting material. Mfor successful industrial applicattor
conguration is mainly depeas much catalyst per unit volumfor
scale-up [170]. Due to the higduced during the
photocatalyticcontaminants it is difcult tomakwork is needed in the
fundamentation of heterogeneous photoreactout the development of
faulty moreactors by many researchers [1[140,149] stressed the need
to dethis promising method of decont
8.1. Ecotoxicity of titania photoprocesses
The nal products of photocatalytic degradationmay not purelybe
innocuous substances. Harmful by-productsmay cause decreasein
reaction rate and secondary pollution [35]. There has been
scantyliterature on the toxicity of the photocatalyst or the
overall photo-catalytic process. Although TiO2 is found in
toothpaste due to itssafety nano-scale TiO2 water suspension has
been reported toxic toE. coli and antibacterial inhibition
increased with particle concen-tration during photocatalytic
experiment [174]. Maness et al. [175]attributed this toxicity to
the attack of polysaturated phospholipidsin the E. coli by radical
oxidizing species.
8.2. Future trends
At the present infancy stage of new century future trends
fordevelopment would include:
the preparation of photocatalyst material capable of selectiveof
ory m
ationies botocar bot
tic dativeth aprobon-pcicotocobictes stoden-exganirincip
Uniahim
nvirotandar
e, Res77 (20rrman
shraeEnginr, Proe (Lonhem.shi, K.. Physhem.lmisa
53 (19ocatalysis provides double bene-ection and removal of
recalcitrantn of this method could be bene-onation-photolysis in
the removalamine (NDMA) from water [141].have proven their efcacy
in airet al. [142] considered couplingatalysis degradation method
foroperational cost on single pho-
sation time shortened comparednt and the electrical cost savedt
is reported that H2O2-enhancedmethod is efcient in completerbicide
propyzamide [143]. Thebrane reactors designed to com-tion is a
solution to the problemaqueous systems [144]. Table 3successfully
photodecomposed
elopment of AOPs assume cyclicstarts and ends with the analy-h
stage of the development cyclen of the next immediate step. Aand
(b) is most of the pollutants[1] and only a limited number ofied on
mixtures mainly in recentoratory studies concentrate onlyorganic
contaminants disregard-termediates. Ref. [169] reportedation more
toxic to Escherichiaodelling and design are criticalion. Design
efciency of a reac-ndent upon its ability to installe as possible a
factor necessaryh number of intermediates pro-oxidation of even
simple organice complete kinetic analysis. Morels of reactor design
andoptimiza-ors [171]. Several reports pointeddels for
photocatalytic oxidation72,173]. Hernandez-Alonso et al.velop
accurate kinetic models foramination.
photocatalytic degradation novel preparation of terna
idative degradation; novelphotocatalystprepar
more member of the famil design of more reliable ph
by visible and solar light o
9. Conclusion
Heterogeneous photocatalylyst remains a viable alternorganic
contaminants in boing technology solves theorganic contaminants to
n
Despite its moderate speremains a very efcient ph
Environmental friendly aerproduce fewer intermediaaccurate
development of m
Though this review is nocatalytic degradation of orregarding
fundamental paddressed.
Acknowledgement
The authors acknowledgelowship granted to Umar Ibr
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Heterogeneous photocatalytic degradation of organic contaminants
over titanium dioxide: A review of fundamentals, progress and
problemsIntroductionBasic principles of photocatalysisMechanism of
titania-assisted photocatalytic degradationTitania versus existing
photocatalystsEffect of operational parametersLight intensityNature
and concentration of the substrateNature of the
photocatalystPhotocatalyst concentrationpHReaction temperature
Methods of utilization of titania photocatalystTrends in
improving the activity of titaniaNovel photocatalyst
preparationsCombined operations
MiscellaneousEcotoxicity of titania photoprocessesFuture
trends
ConclusionAcknowledgementReferences