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Development and potential of new generation photocatalytic systems for air pollution abatement an overview.

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ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING
Asia-Pac. J. Chem. Eng. 2009; 4: 387–402
Published online 25 June 2009 in Wiley InterScience
(www.interscience.wiley.com) DOI:10.1002/apj.321
Review Article
Development and potential of new generation
photocatalytic systems for air pollution abatement: an
overview
Melvin Lim,1 * Yan Zhou,2 Lianzhou Wang,1,3 Victor Rudolph3 and Gao Qing (Max) Lu1
1
Australian Research Council Centre of Excellence for Functional Nanomaterials, University of Queensland, Brisbane, Australia
Advance Water Management Centre, University of Queensland, Brisbane, Australia
3
Division of Chemical Engineering, University of Queensland, Brisbane, Australia
2
Received 4 December 2008; Revised 3 March 2009; Accepted 5 March 2009
ABSTRACT: Photocatalysis is the process by which various undesired substrates are reduced or oxidised on the
surface of a photoresponsive material when exposed to a sufficiently energetic irradiation source. Together with other
processes like photolysis and ozonation, photocatalysis forms a larger, important group of technologies known as
Advanced Oxidation Processes or AOPs.
This short review begins with an introduction to the fundamental processes and entities involved in general
semiconductor photocatalysis. Various major air pollutants are considered, along with their health effects and traditional
means of abatement. Recent advances in photocatalytic materials (including the use of novel materials other then titania),
together with heterogeneous photoreactor design (in particular, of the flow-type) are then described. Concluding remarks
are included, along with some recommendations for possible future work.  2009 Curtin University of Technology
and John Wiley & Sons, Ltd.
KEYWORDS: photocatalysis; air pollutants; TiO2 ; review; reactor; visible light; air purification; volatile organic
compounds; fluidised bed; AOP; titanium dioxide
INTRODUCTION
Photocatalysis refers to the process by which various
undesired substrates are reduced or oxidised on the
surface of a photoresponsive material (i.e. a photocatalyst) when exposed to a sufficiently energetic radiation
source. Together with other processes like photolysis
and ozonation, photocatalysis forms a larger, important
group of technologies known as Advanced Oxidation
Processes (AOPs).[1]
This treatise begins with an introduction to the fundamental processes and entities involved in general semiconductor photocatalysis. Various major air pollutants
are considered, along with their health effects and traditional means of abatement. Recent advances in photocatalytic materials (including the use of novel materials
other then titania), together with heterogeneous photoreactor design (in particular, of the flow-type) are then
described. Finally, concluding remarks are included,
*Correspondence to: Melvin Lim, Division of Environmental &
Water Resources Engineering, BlK N1-B4b-07, Nanyang Avenue,
Singapore 639798. E-mail: melvin.lim@ntu.edu.sg
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
along with some recommendations for possible future
work.
BACKGROUND TO PHOTOCATALYSIS
In 1972,[2] the rutile form of TiO2 was used in an electrochemical photocell as an anode; in the presence of
ultraviolet (UV) irradiation and a platinum counter electrode, water was split into hydrogen and oxygen. In
1977,[3] Frank and Bard demonstrated the use of both
anatase and rutile TiO2 in the photocatalytic oxidation
of aqueous cyanide ions, using both solar and artificial
light sources. In view of these two breakthroughs, and
till today, the most widely investigated photocatalyst
undoubtedly is titanium dioxide (TiO2 ). This semiconducting material is a relatively cheap and generally
harmless (possibly unless inhaled or ingested in a fugitive nanometric form[4] ) material with ores found in
the major exporting countries of Australia, India, Norway, Malaysia and China.[5] Even so, the most often
described disadvantage of neat TiO2 is that of its rather
wide bandgap. A semiconductor is often characterised
using two energy bands – the valence band (VB) and
388
M. LIM ET AL.
Asia-Pacific Journal of Chemical Engineering
conduction band (CB). The energies between the VB
and CB of semiconductors are approximately in the
range from 0 to 4.0 eV,[6] and this difference is termed
the bandgap (Eg ).
The importance of the bandgap, when discussed in
semiconductor-based photocatalysis, arises when one
considers the minimum photonic energy level (hv ≥
Eg ) that is able to excite VB electrons (e− VB ) to the
CB, in the process, leaving behind positively charged
holes (h+ ). Various excitation energies, corresponding
to different wavelengths in the electromagnetic (EM)
spectrum, can be used (depending on the semiconductor
of concern) for this purpose, and are listed in Table 1. A
schematic illustration of this process is shown in Fig. 1.
The bandgap of several semiconductors are illustrated
in Table 2. Two crystal structures of TiO2 (rutile and
anatase) have bandgaps of 3.1 and 3.2 eV and appear
white. The most often-reported, commercially available
titania material is Degussa P25, consisting of about 78%
anatase, 14% rutile and 8% amorphous material.[7] Its
primary particle size is about 25–30 nm with a specific
surface area of 60 m2 /g,[8] and is generally considered
one of the best commercially available titania products
for photocatalysis under UV irradiation.
At 3.2 eV, the bandgap of anatase titania requires
excitation using at least UV-A (i.e. a photonic source
in the range of wavelengths 315–380 nm). This also
restricts the use of irradiation sources of lower frequencies or longer wavelengths. Also, with the solar
spectrum containing about 3–6%[7] UV and the rest as
infrared or visible light, there is hence a discrepancy in
attempting to use the solar spectrum to excite such widebandgap materials. Attempts have been made, through
bandgap engineering, to either introduce localised electronic (both donor or acceptor) levels in the forbidden
gap of a semiconductor, or to extend the CB or VB via
doping the neat semiconductor, such that the bandgap
decreases and enables the semiconductor to be excited
via the use of shorter wavelengths. An example of the
introduction of localised states in TiO2 occurs in its
less than ideal crystal-lattice (which should only consist of Ti4+ ) containing Ti3+ cations, thereby forming
donor states near to the CB and supplying the band with
electrons. This is depicted generally in Fig. 1(b).
In photocatalysis, bandgap engineering through doping a neat semiconducting material is in general, a
method to make a semiconductor function similarly
with a decreased bandgap, when exposed to less energetic photons. The redox reactions which take place on
the surface of the semiconducting photocatalyst must
also be taken into consideration. Figure 2 depicts an
example of reduction–oxidation processes occurring in
a photocatalytic process, in the presence of water and
oxygen. The formation of active species responsible
Table 1. Types of excitation energy used in photoprocesses[1] .
Table 2. Bandgap energies of several semiconductors
and appearance in terms of colour.
Type of EM
radiation
Semiconductor
Vacuum UV (VUV)
UV-C
UV-B
UV-A
Visible light
(a)
Wavelengths
(nm)
Energy
(eV)
100–200
200–280
280–315
315–380
380–780
12.4–6.2
6.2–4.4
4.4–3.9
3.9–3.3
3.3–1.6
Cu2 O
Bi2 O3
TiO2 (rutile)
TiO2 (anatase)
ZnO
SnO2
Eg (eV)
[9]
2.2
2.7[10]
3.1[11]
3.2[12]
3.2[13]
3.8[14]
Colour
Red
Yellow
White
White
White
Grey
(b)
Figure 1.
(a) Photoexcitation of a semiconductor by
incoming photonic energy (hv) and subsequent formation
of VB hole. (b) Introduction of impurity states in the same
semiconductor.
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Figure 2. Redox processes occurring on the surface of an
illuminated semiconductor in the presence of water and
oxygen. Note increasing electron energy indicated.
Asia-Pac. J. Chem. Eng. 2009; 4: 387–402
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
NEW GENERATION PHOTOCATALYTIC SYSTEMS FOR AIR POLLUTION
Table 3. Selected reduction potentials of general
interest in AOPs[1] .
Redox
couple
−
aq/eaq
Br• /Br−
Br2 •− /2Br−
Br2 /Br2 •−
CN• /CN−
CO2 /CO2 •−
CO3 •− /CO3 2−
CO2 •− , H+ /HCO2 −
Cl• /Cl−
ClO2 • /ClO2 −
ClO2 • , H+ /HClO2
Cl2 •− /2Cl−
Cl2 /Cl2 •−
F• /F−
I• /I−
I2 •− /2I−
I2 /I2 •−
N3 • /N3 −
•
NO2 /NO2 −
•
NO3 /NO3 −
•
OH, H+ /H2 O
•
OH/OH−
O•− , H2 O/2OH−
O2 , O2 −
O2 , H+ /HO2
O2 (1 g )/O2 −
O2 − , H+ /HO2 −
O2 •− , 2H+ /H2 O2
H2 O2 , H+ /2H2 O
HO2 • , H+ /H2 O2
HO2 • /HO2 −
H2 O2 , H+ /H2 O, • OH
O3 , H+ /O2 , H2 O
O3 /O3 •−
O3 , H+ /HO3 •
HS• /HS−
SCN• /SCN−
•
SO3 − /SO3 2−
•
SO3 − /HSO3 −
SO4 2− , H2 O/• SO3 − , 2OH−
SO4 •− /SO4 2−
Reduction
potentiala
E(V)
Remarks
−2.870
2.000
1.660
0.300
1.900
−1.900
1.500
1.070
2.200–2.600
0.934
1.277
2.300
0.420–0.600
3.600
1.270–1.420
1.000–1.130
0.060–0.300
1.330
0.870–1.040
2.300–2.600
2.730
2.590–2.850
1.800–2.180
1.900
1.780–1.870
−0.330
−0.037
0.650
1.000
0.940
1.77
1.420
0.790
0.800
2.07
1.190–1.600
1.800
1.150
1.620
0.630
0.840
−2.470
2.430
–
–
–
–
–
–
–
pH = 7
–
pH = 4–6
pH = 0
–
–
–
–
–
–
–
–
–
–
pH = 0
pH = 7
–
–
–
pH = 0
pH = 7
–
pH = 7
–
pH = 0
–
pH = 7
–
pH ≥ 11
pH = 7
–
pH = 1.3
pH ≥ 8
pH = 3.6
–
–
a
Standard Hydrogen Electrode (SHE). Dispersion of values due to
varying calculation methods.
for both oxidation [e.g. h+ and captured holes in the
form of hydroxyl radicals (OH)] and reduction [e− and
superoxide anions (O2 •− )] are indicated. S1 and S2 are
substrates which have potential to be reduced and oxidised to S1 − and S2 + , respectively. In more complex
systems, the formation of other active species or radicals
with different reduction potentials is possible. Table 3
provides a selected list of reduction potentials that are
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
generally of interest in AOPs. It should be realised that
the single step involving the promotion of an electron
from the VB to the CB (and hence leaving behind an
oxidising hole) is the initial step to the formation of
other active species on the surface of the semiconductor.
In heterogeneous photocatalysis, the availability of
reactive oxygen species is one of the most important
factors to be considered, even though it is commonly
assumed that hydroxyl radicals and superoxide anions
are the primary species formed upon the excitation of
a semiconductor in the presence of water and air. Secondary reactive oxygen species may also take part in
further redox reactions, and is depicted in an idealised
scheme in Fig. 3.
As such, the possible recombination of these entities
would have to be prevented from occurring if an active
photocatalyst is desired. A summary of the fundamental photophysical/photochemical processes occurring in
and on a semiconducting material is provided in Fig. 4.
Again, S1 and S2 are substrates which have potential
to be reduced and oxidised to S1 − and S2 + , respectively. Such substrates can be for example, undesired,
volatile organic compounds (VOCs) in the atmospheric
air (where moisture is also present) or water bodies.
In the case of organic compounds being destroyed
through photocatalysis, the desired end-products, under
ideal conditions, are carbon dioxide and water (Eqn 1).
It is common for reaction intermediates to form[15,16]
and limit the reaction from proceeding to completion
(Eqn 2). This is due to the intermediates themselves
interacting with the active species formed on the illuminated semiconductor.
hv ,photocatalyst
y
y
−−→
x CO2 + H2 O(1)
Cx Hy + (x + )O2
4
2
hv ,photocatalyst
Cx Hy + O2
−−→
Intermediates + CO2 + H2 O
(2)
AIR POLLUTANTS AND EFFECTS OF
EXPOSURE
In modern living, increases in standards of living
generally result in the demand for better and more
variety of goods. This converts to increased industrial
activity which contributes to an increasing emissions
inventory of undesired pollutants in the atmospheric
environment. In the United States, under the Clean Air
Act 1971,[17] six criteria pollutants are particles with
aerodynamic diameters under 10 and 2.5 µm, ozone,
sulphur dioxide, nitrogen dioxide, carbon monoxide
and lead. In addition to these, 189 other potentially
harmful air pollutants are listed as toxic or hazardous
air pollutants (Table 4). In Australia, a list of priority
Asia-Pac. J. Chem. Eng. 2009; 4: 387–402
DOI: 10.1002/apj
389
390
M. LIM ET AL.
Asia-Pacific Journal of Chemical Engineering
Figure 3. Oxygen-derived reactive species in AOPs.[1] Note: sens: photosensitiser, e.g.
light-absorbing dyes.
encountered. The effects on flora and fauna are also of
concern, as compounds like mercury and arsenic are
highly persistent[18] in water, and are of acute toxicity
to aquatic life, birds and other animals.
CONVENTIONAL METHODS FOR THE
ABATEMENT OF AIR POLLUTANTS
Traditionally, to meet air emission standards based on
regulation guidelines, industries commonly employ one
or more of the following technologies.[19]
Combustion
Figure 4. Summary of fundamental photo-physical/chemical
processes in and on a semiconducting particle. Note: LT,
lattice trap; ST, surface trap; hv’, resultant photon due to
relaxation of electron in LT.[1] .
toxic air pollutants[18] have also been identified and
while including some chemical substances listed in
Table 4, also contain acetaldehyde, arsenic compounds,
dichloromethane and fluoride compounds.
The human health effects of the prioritised air pollutants have been studied to some extent and include
negative consequences on the human respiratory, central
nervous and reproductive systems. Some of these compounds bioaccumulate[18] and are also known to transmit a risk of developing cancer (or are known human
carcinogens) and tumours when prolonged exposure is
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Generally, combustion devices are fundamentally classified as thermal or catalytic, and thereafter further classified according to the method of heat recovery (recuperative or regenerative) being used. Combustion occurs
when an exhaust stream of combustible VOCs are
present at adequate temperature and in a sufficient residence time to allow oxidation of these compounds into
carbon dioxide and water vapour according to Eqn 3:
y
y
O2 →x CO2 + H2 O
(3)
Cx Hy + x +
4
2
Combustion generally requires high temperatures
(typically 538 ◦ C and 1093 ◦ C)[19] to destroy VOCs.
Because high energy inputs are required, thermal recovery is frequently employed to lower the operating fuel
costs of such plants.
Asia-Pac. J. Chem. Eng. 2009; 4: 387–402
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
NEW GENERATION PHOTOCATALYTIC SYSTEMS FOR AIR POLLUTION
Table 4. Clean Air Act designated hazardous air pollutants[17] .
Chemical name
Acetamide
Acetonitrile
Acetophenone
2-Acetylaminofluorene
Acrolein
Acrylamide
Acrylic acid
Acrylonitrile
Allyl chloride
4-Aminobiphenyl
Aniline
σ -Aniside
Asbestos
Benzene
Benzidine
Benzotrichloride
Benzyl chloride
Biphenyl
Di(2ethylhexyl)phthalate
Bis(chloromethyl)ether
Bromoform
1,3-Butadiene
Calcium cyanamide
Caprolactum
Captan
Carbaryl
Carbon disulphide
Carbon tetrachloride
Carbonyl sulphide
Catechol
Chloramben
Chlordane
Chlorine
Chloroacetic acid
2-Chloroacetophenone
Chlorobenzene
Chlorobenzilate
Chloroform
Chloromethyl methyl
ether
Chloroprene
Cresols/Cresylic acid
o-Cresol, m-Cresol
p-Cresol
Cumene
2,4-Dichlorophenoxyacetic
acids, salts & esters
Lead compounds
DDE
Diazomethane
Dibenzofurans
1,2-Dobromo-3-chloropopane
Dibutylphthalate
1,4-Dichlorobenzene(p)
3,3-Dichlorobenzidene
Dichloroethyl ether
1,3-Dichloropropene
Dichlorvos
Diethanolamine
N ,N -Diethyl aniline
Diethyl sulphate
3,3-Dimethoxybenzidine
Dimethyl aminoazobenzene
3,3-Dimethyl benzidine
Dimethyl carbamoyl chloride
Dimethyl formamide
1,1-Dimethyl hydrazine
Hexamethylphosphoramide
Hexane
Hydrazine
Hydrochloric acid
Hydrogen fluoride
Hydrogen sulfide
Hydroquinone
Isophorone
Lindane (all isomers)
Maleic anhydride
Methanol
Methoxychlor
Methyl bromide
Methyl chloride
Methyl chloroform
Methyl ethyl ketone
Methyl hydrazine
Methyl iodide
Methyl isobutyl ketone
Propionaldehyde
Propoxur (Baygon)
Propylene dichloride
Propylene oxide
1,2-Propylenimine
Quinoline
Quinone
Styrene
Styrene oxide
2,3,7,8-Tetrachlorodibenzop-dioxin
1,1,2,2-Tetrachloroethane
Tetrachloroethylene
Titanium tetrachloride
Toluene
2,4-Toluene diamine
2,4-Toluene diisocyanate
o-Toluidine
Toxaphene
Dimethyl phthalate
Dimethyl sulphate
4,6-Dinitro-o-cresol and salts
2,4-Dinitrophenol
2,4-Dinitrotoluene
1,4-Dioxane
1,2-Diphenylhydrazine
Epichlorohydrin
1,2-Epoxybutane
Ethyl acrylate
Ethyl benzene
Ethyl carbamate
Ethyl chloride
Ethylene dibromide
Ethylene dichloride
Ethylene glycol
Ethylene imine
Ethylene oxide
Ethylene thiourea
Ethylene dichloride
Methyl isocyanate
Methyl methacrylate
Methyl tert butyl ether
4,4-Methylene bis(2-chloroaniline)
Methylene diphenyl diisocyanate
4,4 -Methylenedianiline
Naphthalene
Nitrobenzene
4-Nitrobiphenyl
4-Nitrophenol
2-Nitropropane
N -Nitroso-N -methylurea
N -Nitrosodimethylamine
N -Nitrosomorpholine
Parathion
Pentachloronitobenzene
Pentachlorophenol
Phenol
p-Phenylenediamine
Phosgene
1,2,4-Trichlorobenzene
1,1,2-Trichloroethylene
Trichloroethylene
2,4,5-Trichlorophenol
2,4,6-Trichlorophenol
Triethylamine
Trifluralin
2,2,4-Trimethylpentane
Vinyl acetate
Vinyl bromide
Vinyl chloride
Vinylidene chloride
Xylenes (isomers, mixture)
o-Xylenes, m-Xylenes
p-Xylenes
Antimony compounds
Beryllium compounds
Cadmium compounds
Chromium compounds
Cobalt compounds
Formaldehyde
Heptachlor
Hexachlorobenzene
Hexachlorobutadiene
Hexachlorocyclopentadiene
Hexachloroethane
Phosphine
Phosphorus
Phthalic anhydride
Polychlorinated biphenyls
1,3-Propane sultone
β-Propiolactone
Coke oven emissions
Cyanide compoundsa
Glycol ethersb
Manganese compounds
Mercury compounds
Fine mineral fibresc
Hexamethylene-1,6-diisocynate Nickel compounds
Selenium compounds
Polycyclic organic matterd
Radionuclides radon includede
Note: Listings containing the word ‘compounds’ and for glycol ethers, unless otherwise specified, include the unique chemical substance that
contains the named chemical as part of its infrastructure.
a X CN where X H or a group where a formal dissociation may occur.
b
Mono- and di-ethers of ethylene, diethylene and triethylene glycol R-(OCH2 CH2 )n -OR where n = 1,2 or 3; R = alkyl or aryl groups;
R = R, H or groups which, when removed, yield glycol ethers: R-(OCH2 CH)n -OH. Polymers are excluded from the glycol category.
c
Mineral fibre emissions from facilities manufacturing or processing glass, rock, and slag fibres of average diameter 1 µm or less.
d
Organic compounds with more than one benzene ring which have a boiling point greater than or equal to 100 ◦ C.
e
A type of atom which spontaneously undergoes radioactive decay.
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pac. J. Chem. Eng. 2009; 4: 387–402
DOI: 10.1002/apj
391
392
M. LIM ET AL.
Conversely, catalytic oxidisers enable relatively lower
(27 to 527 ◦ C) temperature reactions to occur[20] by
introducing VOC-laden streams through catalytic beds
(usually containing noble metals such as platinum,
or other metals like chromium, manganese, copper,
cobalt and nickel deposited on an alumina honeycombed
support).
Regenerative oxidisers contain beds of ceramic material through which the hot exhaust products or cool
inlet process (VOC-laden) gas stream flows at a single time, resulting in a ‘flip-flopping’ procedure. The
heat-resistant ceramic material stores heat from the hot
exhaust gas stream while another pre-heated regenerator releases the heat to the cold inlet pollutant stream.
Due to the way gases are switched, the overall oxidation efficiency of regenerative systems can reach a
maximum of ca. 95%.[20] In recuperative oxidisers, high
efficiency heat-exchangers are used to recover energy
by preheating incoming combustion air. In general, the
overall oxidation efficiency of the recuperating oxidiser
is higher than that of the regenerative oxidiser. In contrast, the efficiency of heat recovery in recuperative
oxidisers is generally lower amongst the two recovery
systems described.
Adsorption technology
Adsorption uses a mass transfer process involving
the capture of gas molecules on solid phases by
physisorption or chemisorption. Activated carbon is
used most frequently in adsorption systems.[19] Other
adsorbents like silica gel, polymeric materials, alumina,
zeolites, or natural materials like clays can also be
used. Generally, undesired pollutant molecules[21] are
captured on the surface area present in pore spaces in
the carbon entities.
Activated carbon is well suited for adsorbing higher
molecular weight (e.g. more than 40) and nonpolar
chemical substances; compounds with boiling points
greater than 150 ◦ C do not desorb well.[19] Three
kinds of carbon adsorption systems are commonly
used, namely fixed bed carbon adsorption, fluidised
bed carbon adsorption and activated carbon filter panels. Used carbon adsorbents in the earlier two systems are usually regenerated by means of desorption
via thermal methods, while activated carbon filter panels are usually disposed of after near-breakthrough is
achieved.
Condensation
Condensation (refrigeration) involves lowering the temperature of a VOC-laden exhaust stream below the saturation temperature of the VOC to be condensed, and
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pacific Journal of Chemical Engineering
subsequently removing the relevant VOC. Single stage
refrigeration units achieve typical temperatures between
4 and −29 ◦ C, with others reaching −51 ◦ C. Multistage
units operate between −23 and −73 ◦ C.
Biological treatment
Bioreactors generally include three types of reactors
(bioscrubbers, biofilters and the trickle-bed reactors)
employing microorganisms to metabolise pollutants.
Microorganisms that have been used in the degradation
of aromatic VOCs like styrene include Cladosporium
sphaeraspermum, Exophiala lecanii-corni ;[22] Tsukamurella, Pseudomonas, Sphingomononas, Xanthomonas [23] and Exophiala jeanselmei.[24] Generally, such
reactors can be classified under the categories of suspended growth (bioscrubbers) and fixed-film bioreactors
(biofilters and trickle-bed reactors). The main attractions of using biological processes are that they proceed at ambient temperature and pressure (hence having
lower energy requirements) and also produce no nitrogen oxides. The basis of biological treatment technology
is biodegradation. This process encompasses the oxidation of hydrocarbons to produce alcohols, which then
react to form aldehydes, followed by organic acids and
finally, CO2 and H2 O.
Bioscrubbers are generally used, when biological
degradation products such as acids, hydrogen sulphide
(H2 S) and ammonia may harm a biofilter bed. Biofilters are most effective when hydrocarbon concentrations are below 1 mg/m3 ; at concentrations greater
than 10 mg/m3 , biofilters should be followed by charcoal adsorbers.[20] In practical operations, flow rate,
humidity, pH, pressure drop, temperature, growth of
biomass and bacteria count are critical operating parameters. Trickle-bed reactors typically consist of structured or randomly packed synthetic materials designed
not to retard flows and to ensure relatively more uniform gas distribution and biological contact. This is as
opposed to biofilters, in which organic matter (compost
and/or wood chips) liable to compaction or swelling is
used.
Membrane technology
The use of membrane technology is based on selectively
permeable membranes to separate organic compounds
in preference to air. Basic membrane systems for
organic emissions control generally consist of two steps,
namely, the conventional compression–condensation
process and the membrane separation stage. Figure 5
illustrates a typical membrane system.[19]
Other than the inherent characteristics[26] of membranes used, other factors have to be considered in the
Asia-Pac. J. Chem. Eng. 2009; 4: 387–402
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
VOC in Air
Condenser
NEW GENERATION PHOTOCATALYTIC SYSTEMS FOR AIR POLLUTION
Membrane
Modules
Vent
RECENT DEVELOPMENT OF
PHOTOCATALYSTS FOR AIR POLLUTION
ABATEMENT
Cation–anion dopants and defect-induced
sensitisation
Compressor
Liquid VOC
Permeate
Figure 5. Typical membrane setup with recycling system[25] .
use of membrane technologies for the control of organic
emissions. Transport through the membranes is caused
by maintaining correct pressure differences;[27,28] the
pressure ratio is equivalent to the feed pressure divided
by the permeate ratio. Vapour pressure on the permeate side of the membrane is lower, compared to that
on the feed side to provide the driving force for permeation. Another operating condition to be considered
is stage cut.[25,29] This refers to the fraction of total
flow that permeates the membrane, and is equivalent to
the permeate flow rate divided by the feed flow rate.
It is often impossible to separate two components sufficiently in a single pass through a membrane system.
Better separation can be achieved using a multistage,[30]
multistep,[30] or single membrane unit with a recycling
system.[19,30]
ALTERNATIVE TO TRADITIONAL METHODS
OF AIR POLLUTION ABATEMENT
It is noted that the conventional methods of air pollution
require intensive energy inputs (e.g. thermal combustion
or condensation), or present larger land requirements
(biological treatment). Adsorption technologies simply
present a temporary method of containing the pollutant
constituents, finally requiring either the regeneration
or replacement of adsorbents. Photocatalysis presents
an alternative to the traditional methods of air pollution abatement. Inarguably, one of the most attractive
features of this technology is that it is a potentially sustainable method, if the solar spectrum (comprising both
UV and visible light) can be utilised as a source of
energy to enable the reduction or oxidation of pollutant
molecules or ions. The development of such ‘second
generation’ visible light-responsive photocatalysts will
hence require synthesis strategies which either extend
the absorbance spectra of mainly UV-absorbing materials (e.g. neat TiO2 ) into the visible range (and at the
same time maintaining the ability of the material to
absorb in the UV range), or the creation of novel materials with improved band structures.
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Different methods have been used in trying to develop
photocatalysts which are able to work under both
UV and visible light to degrade air pollutants. Doping of neat TiO2 by cations or anions has been used
extensively, with studies reporting the introduction of
cations like Fe,[31,32] Co,[33] Nb,[34] Cr,[35 – 37] Cu[38]
and V;[39,40] or anions like N,[41] I,[42,43] S,[44] C,[45 – 47]
F[48,49] and B.[50] Co-doping of titania by a mixture of ions has also been reported, including that of
N/S,[51] B/N,[52] or Bi/S.[53] Many studies entailing the
use of doped-photocatalysts also provide computational
details[41,54,55] (via the calculation of density of states)
of the possible electronic bandgap structures resulting from the doping procedure. The reader is directed
to a more specialised text[56] for details of the fundamental theory behind such techniques. Serpone[57]
argues, through the use of peak-deconvolution techniques, that attempting to dope a semiconductor in
trying to extend either the CB or VB and consequently decrease the bandgap would require relatively
larger amounts of the dopant to be introduced, such
that the original semiconducting material may be transformed into a different material possessing different
band structures. Furthermore, the presence of lattice
defects (e.g. due to oxygen vacancies) can produce
colour-centres (generically termed F+ and F− centres,
which are associated with one or two electrons, respectively) which act as photosensitisers in the degradation of substrates. Consequently, it is possible that
attempts to dope semiconducting materials only result
in the introduction of mainly localised impurity states.
In many cases involving doping procedures, tailing
absorbance shoulders appear in the absorbance spectra
of the doped catalysts. Studies reporting the formation
of such absorbance shoulders in materials developed
and still possessing the ability to degrade targeted substrates could indicate localised states-to-CB electronic
transitions (Fig. 6).
In metal-doped TiO2 , this hints at the pronounced
presence of the dopant at an impurity level and could
affect the performance of a photocatalyst negatively,
(Anpo M, personal communication) due to the possible introduction of additional recombination sites. An
attempt[37] to dope titania was carried out with highenergy RF implantation of a low atomic percentage of
metal ions in a titania film. The resulting absorbance
spectra of the titania film (Fig. 7) after the sputtering is
observed to undergo a relatively large band-to-band displacement, exhibiting a red-shift into the visible range;
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Figure 6.
Localised dopant
states and electronic transitions
to the CB.
Figure 8. (a) UV–Vis absorption spectra of
titanium dioxide and (b –d ) Cr ion-doped
titanium dioxide photocatalyst materials
prepared by an impregnation method.
Amount of doped Cr ions (wt. %):
(a) 0; (b ) 0.01; (c ) 0.1; (d ) 0.5 and (e )
1. Note: (0.1wt.% = 4.9 µmol/g TiO2 ).[37] .
This figure is available in colour online at
www.apjChemEng.com.
(a) UV–Vis absorption spectra
titanium dioxide and (b–d) Cr ion-implanted
titanium dioxide photocatalyst materials.
Amount of implanted Cr ions (µmol/g):
(a) 0; (b) 0.22; (c) 0.66 and (d) 1.3.[37] .
This figure is available in colour online at
www.apjChemEng.com.
Figure 7.
this is in contrast with titania that has been doped with
chromium via an impregnation method[37] (Fig. 8).
Surface functionalities
Another strategy employed to develop efficient catalysts is the functionalisation of the surface of TiO2 .
Such attempts include the introduction of electron
accepting/donating entities like light-sensitive transitional metals salts[58] or other forms of transitional
metals[59 – 61] on the surface of amorphous or crystalline
semiconducting materials, which is viewed to serve the
purpose of trapping or injecting free electrons, and in
so doing, creating redox sites for further oxidation reactions to occur. It is important that such redox sites are
themselves not a target of photocorrosion,[62] a problem
commonly encountered in dye-sensitised titania-based
materials.[63] Abe et al .[64] reported that the uniform
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
deposition of ca. 5 nm platinum particles (via a phož
todeposition using H2 PtCl6 6H2 O) on a pristine tungsten oxide material efficiently pooled electrons on these
sites, enabling multielectron reductions in O2 to occur
on the loaded material (Eqns 4 and 5), consequently
allowing photooxidation to occur effectively via remaining holes. It has been a traditional view that such pristine oxide materials which absorb in the visible range
have CB levels which are generally more positive than
the reduction potentials of oxygen (due to the deeply
positive level of VB, mainly consisting of O 2p electrons), and therefore should not be able to cause electron
trapping via the reduction of O2 .
O2 + 2H+ + 2e− → H2 O2 (aq)
(4)
O2 + 4H+ + 4e− → 2H2 O
(5)
Lim et al .[65] introduced C- and/or F-entities into
titania, resulting in the creation of fluorocarbon groups
CFx on the surface of well-crystallised, microspherical
titania (Fig. 9). Such fluorocarbon groups are also
known to be strongly electron withdrawing, and the
resulting photocatalysts have been shown to degrade
airborne styrene well under visible light (>400 nm) and
20% relative humidity.
Novel structures/composites
Metal cation graft on silica surfaces
Originally conceptualised by the Frei group,[66] Nakamura et al .[67] developed ‘grafted’ visible light harvesting bimetallic assemblies of Ti(IV)–O–Ce(III) on
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NEW GENERATION PHOTOCATALYTIC SYSTEMS FOR AIR POLLUTION
(a)
(b)
Figure 9. (a) SEM images of fluorine-doped TiO2 microspheres and (b)
Fluorocarbon-containing TiO2 microspheres.
mesoporous silica (Fig. 10), in which metal-to-metal
charge transfers (MMCT) occurred in the system and
effective in the decomposition of 2-propanol into acetone and CO2 . This method utilises the strategy of
combining two metal ions (with selected redox potentials) using oxo-bridge assemblies, to drive the transfer
of electrons in the presence of oxygen. Some of the
merits of this system include that of directed electron
transfer (i.e. recombination of charges now demands
less attention) and the included flexibility in designing
molecular-based inorganic photocatalysts under mild
conditions.
Solid state solutions
Solid state chemistry has increasingly been used to
develop materials for use as photoresponsive materials. Because of the resulting nonporous solid state
mixes (usually after firing at relatively high temperatures of 700–1200 ◦ C), such materials have not been
evaluated as much for the purpose of degradation of
Figure 10. Ti(IV)–O–Ce(III) bimetallic
assembly on porous MCM41 and MMCT
process.[67] .
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
organic compounds as compared to, for instance, harvesting hydrogen via water-splitting. Perhaps, this is
because it is usually deemed important to have reasonably large specific surface areas/volumes, in addition to well-crystalline structures for photocatalysts to
perform effectively in the degradation of pollutant substrates. Even so, reports encompassing such processes
are now increasingly surfacing and include that of
the author’s and co-worker’s recent effort in developing visible light-responsive (Nb–Ti–Si)–(Bi/Bi–O)
heterojunctions.[68]
Wang et al .[69] developed a (Ag1−x Srx )(Nb1−x Tix )O3
perovskite-type photocatalyst having a bandgap of ca.
2.8 eV and capable of degrading acetaldehyde under
visible light (>440 nm). The enhanced photocatalytic
activity of the material is attributed to the modulated
band structure formed by a hybridised CB of empty
(Ti 3d and Nb 4d) orbitals and overall VB of the
occupied (O 2p and Ag 4d) orbitals. In a similar
context, (AgNbO3 )1−x (NaNbO3 )x [70] was also developed. Maruyama et al . synthesised a delafossite-type αAgGaO2 [71] through cation ion exchange, and utilising
NaGaO2 as a precursor (prepared through a solid-state
mix) demonstrated the decomposition of 2-propanol
using the photocatalyst which possessed a bandgap of
2.4 eV.
Solid states mixes involving other metals like lead,
niobium, tungsten, or indium have also been reported.
For example, lead niobates such as Pb3 Nb2 O9 ,[72]
Pb3 Nb4 O13 [72] and InWo6 [73] (experimental bandgap
of 3.3 eV) have been prepared and studied using
spectroscopic methods like XPS and UPS, coupled
with computational methods. As opposed to relatively higher temperatures used to prepare conventional
solid state solutions, In(OHy )Sz [74] was prepared by a
facile method by which S− ions were homogeneously
introduced into the lattice structure of In(OH)3 using
thiourea as a precursor. Increases in S/In ratios up
to two not only increase the intensity of absorbance,
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Figure 11. SEM images of TiO2 inverse opals obtained from (a) 210 nm
polystyrene spheres as a template and (b) 240 nm spheres at higher magnification
showing the nanocrystallites within the inverse opal framework. (c) TEM image
of sample from b. (d) X-ray diffraction pattern of anatase titania inverse opals
calcined at 450 ◦ C.[76] .
but also shifts the adsorption edge into the visible
region.
Highly thermal-stable titania
The photoactive anatase form of titania is known to
undergo a phase transition to the rutile form at temperatures of about 700–850 ◦ C. This implies that calcination
processes which may be required to form anatase should
not exceed this phase transition temperature. Lim et al .
have recently demonstrated[75] that a layered composite of anatase titania-clay has the ability to withstand
temperatures up to 1200 ◦ C, maintaining the anatase
structure completely.
Inverse opals
Photonic crystals[76] are also known as inverse opals
or photonic band stop materials. They are spatially
periodic structures (3DOM, or 3-dimensionally ordered
materials) fabricated from materials having different
dielectric constants, and are capable of influencing
electromagnetic waves in a similar manner to electrons
in a semiconductor. These photonic crystals exclude the
passage of photons of a selected range of frequencies,
therefore aiding in manipulation of photons in all three
dimensions and for the case of such photonic crystals
made of titania (Fig. 11), are believed to be highly
efficient in transmitting wavelengths (for purpose of
photocatalytic processes) according to selected design
parameters. By controlling the pore size in a photonic
crystal, the bandgap can be fixed at a given wavelength.
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
The relationship between the pore size and band gap is
described by the following Eqns,[77] where Eqn 6 is a
modified form of Bragg’s law.
2
− sin2 θ
(6)
λmax = 2d111 neff
neff = nTiO2 f + nair (1 − f )
2
d111 =
D
3
(7)
(8)
λmax indicates the maximum absorption wavelength
of the material; nTiO2 , nair and neff refer to the refractive
indices of TiO2 , air and an overall composite of TiO2
and air, respectively. θ denotes the Bragg angle. d111
is the inter-planar distance along the (111) direction,
whilstDis the distance between neighbouring air spheres
which is related to the size of the template (commonly
uniform polystyrene beads) used in the development of
the photonic crystal. f is the phase volume percentage
of titania.
In general, the use of inverse opals in air-based
photocatalysis is a rather novel application and has been
demonstrated by Ren et al .[77] in the degradation of 1,2dichlorobenzene vapour, which the photonic efficiency
of the dimensionally ordered material showed a 248%
improvement over the use of P25.
Nanotubes/nanowires
Physical 1D structures, including that of nanotubes,
nanowires and nanofibres, have also been used for the
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NEW GENERATION PHOTOCATALYTIC SYSTEMS FOR AIR POLLUTION
(b)
(a)
5nm
50nm
10nm
Figure 12. [78] (a) TEM images of titania nanotubes and (b) Scanning TEM
dark-field image of the same nanotubes loaded with gold particles.
Figure 13. SEM images of nanofibres resulting from post-calcination of titania
nanotubes.[8] .
purpose of air pollution control through photocatalysis. Xu et al . applied multiwalled titania nanotubes[78]
coated with gold or platinum nanoparticles (Fig. 12),
in a flow-type photoreactor, and attributed the highperformance of the system to the bending/curving of the
Ti octahedral sheets which results in electrical polarity on the outer and inner surfaces of the nanotubes
and consequently enhancing charge separation. Chen
et al .[79] extended the concept of using titania nanotubes
for air purification and introduced platinum into the core
of titanium nanotubes by means of a novel templating
method. Through this process, p–n junctions are created
on the boundary where the platinum cores and titanium
nanotubes meet and prove to be efficient in degrading
airborne toluene.
Yu et al .[8] developed anatase nanofibres of titania
from P25 as a precursor (Fig. 13) by subjecting titanate
nanotubes to hydrothermal treatment at 200 ◦ C for up
to 24 h. The fibres were found to be able to degrade
acetone more effectively than neat P25, and this is
accounted for by the relatively smaller crystallite sizes
and better porosimetric (surface area and pore volume)
characteristics of the nanofibres.
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Apart from reports involving the use of titania-based
1D-nanostructures, Zhou et al .[80] synthesised β-SiC
nanowires via the high-frequency induction heating of
SiO. With a bandgap of ca. 2.5 eV, these nanowires
(generally investigated for their electrical and physicochemical properties), were found to be photoactive in
degrading acetaldehyde under UV irradiation.
Table 5 summarises selected recent reports of photocatalysts used in the degradation of air pollutants. For
a review of relatively earlier work in the photocatalytic
degradation of airborne pollutants, the reader is directed
to that of a comprehensive treatise by Demeestere
et al .[16]
RECENT DEVELOPMENT IN PHOTOREACTOR
ENGINEERING FOR AIR PURIFICATION
Photocatalytic destruction of airborne pollutants
involves different processes. These generally include
diffusion in the bulk gas phase, physisorption (adsorption–desorption), charge separation on the surface of
the photocatalysts and chemical conversions. Consequently, suitable reactors (either batch, semi-batch, or
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Table 5. Selected list of recent studies in photodegradation of various air pollutants.
Targeted pollutant
Acetaldehyde
Acetaldehyde
Acetaldehyde
Acetaldehyde
Acetaldehyde
Acetone
Acetone
Ammonia
Ammonia
Ammonia
Benzene
Benzene
Benzene
Benzene
Benzene
Benzene
Benzene
Benzene
Cyclohexane
Cyclohexane
Dichloromethane
Ethanol
Ethanol
Ethylbenzene
Ethylbenzene
Ethylbenzene
Ethylene
Ethylene
Formaldehyde
NO
NOx
NOx
Perchloroethylene
Styrene
Styrene
Styrene
Toluene
Toluene
Tetrachloroethylene
Trichloroethene
Trichloroethylene
Hydrogen sulphide
Hydrogen sulphide
Hydrogen sulphide
Hydrogen sulphide
Hydrogen sulphide
Xylene
2-Propanol
Photocatalyst
Dopant/sensitiser
Excitation energy (nm)
Ref.
TiO2
TiO2
TiO2
TiO2
TiO2
TiO2
VOx
TiO2
TiO2
TiO2
AgAlO2 , AgCrO2 , Ag2 CrO4
Ga2 O3
InOOH
Sr2 Sb2 O7
TiO2
TiO2
TiO2
TiO2
TiO2
TiO2
TiO2
TiO2
TiO2
Ga2 O3
TiO2
TiO2
TiO2
TiO2
TiO2
TiO2
TiO2
TiO2
TiO2
TiO2 -SiO2
TiO2 -CFx
(Ti-Nb-Si)O-(Bi/BiO)
Ga2 O3
TiO2
TiO2
SnO2 /ZnO
TiO2
TiO2
ZnFe2 Ta2 O9
CuGaO2 , CuGa1−x Inx O2
TiO2
TiO2
TiO2
TiO2
Co
C
V, C
–
–
InVO4
MgF2
V, Pt
–
–
–
–
–
–
–
InVO4
–
N, Pt
S, Mo
InVO4
Fe
V
V
–
–
InVO4
N
N, Pt
–
CdS
C, Pt
C
–
–
–
–
–
InVO4
–
–
–
–
–
–
–
–
–
Ce, Co, Eu, Sm, W, Yb
>400
>420
>420
UV-A
UV-A
450–900
>320, >380, >520, >560
280–700
365
UV-A
300–650
254
300
254
365
450–900
352
>420
Unknown
450–900
310–385
>420
396–450
254
352
450–900
420–800
>420
365
>400
365, 400, 435, 500, 546
Solar spectrum
352
248
>400
>400
254
450–900
UV-A
365
352
UV-A
>420
>420
>330
UV-A
352
>400
[33]
[81]
[81]
[82]
[83]
[84]
[85]
[39]
[86]
[82]
[87]
[88]
[89]
[90]
[91]
[84]
[92]
[93]
[94]
[84]
[95]
[96]
[40]
[88]
[92]
[84]
[97]
[93]
[98]
[99]
[100]
[101]
[92]
,[15][75]
[65]
[68]
[88]
[84]
[83]
[102]
[92]
[103]
[104]
[105]
[106]
[82]
[92]
[107]
flow-type reactors) have to be utilised, taking into
account physical parameters like flow characteristics (in
the case of flow reactors and with implication on flow
velocities, density of fluids/particulate entities involved
pressure drops) and excitation energy type (UV/visible
light/solar irradiation). Overall, mass transfer effects
and chemical reaction kinetics affect the destruction of the targeted compounds, with heat-transfer
considerations taking secondary importance (unless
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
strongly exothermic/endothermic reactions are of concern). For a comprehensive coverage on photoreactor
modelling, the reader is directed to the work of Cassano
et al .[108] or De Lasa et al .[109]
From an industrial/practical point of view, the flow
reactor is generally considered to be most suitably
appropriate where large volumetric flows of gases
are encountered (with the exception of photocatalytic
microreactors[110 – 112] ). As such, this section introduces
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NEW GENERATION PHOTOCATALYTIC SYSTEMS FOR AIR POLLUTION
Figure 14. Reactors: (a) single-annular reactor with high residence time; (b and c)
single-annular reactor with low residence time; (d) multiannular parallel flow reactor;
(e) multiannular series flow reactor (each annular channel wall coated with TiO2 film
of uniform thickness) and (f) multiannular series flow reactor (superficial rate of photon
absorption uniformly distributed on active surfaces and achieved by selecting different
thicknesses of TiO2 films). Note: Photonic source at central axis of each reactor; arrows
indicate flow of gases.[113] .
photoreactors which are of the flow-type. A simple
strategy used commonly to improve the performance
of this type of reactor is by proper design consideration
for geometrical and flow patterns. Several annular flow
type photoreactors employing coatings of titanium as
the active photocatalyst are illustrated in Fig. 14.
The concept of an annular flow photoreactor has also
been integrated with that of particle-fluidisation,[15,114]
whereby a bed of photocatalysts is fluidised by an inlet
flow stream. Such a configuration not only encourages
intimate contact of the photocatalyst and reactant gas,
but also allows the interior of the photocatalyst bed
to be exposed to photonic energy.[114] At relatively
higher flow rates, mass transfer limitations may be
negligible, provided an optimum flow velocity is not
exceeded. The optimum flow velocity (for maximum
degradation of targeted compounds) in a well-behaved
fluidised bed is usually expressed as a multiple of
the minimum fluidisation velocity (Umf ) of the bed
of particles, and Umf is usually calculated using the
correlation between various parameters (including the
dimensionless Archimedes’ number), as proposed by
Wen and Yu.[115] With this taken into consideration, the
annular fluidised-bed photoreactor can be designed to
provide higher reactant through-put and lower pressuredrops in the bed.[114]
Where fluidisation is employed, the physical characteristics of the photocatalysts (particle size and shape
factor) are important parameters to ensure proper fluidisation. For instance, the use of a support[15,116] (usually
silica) for the active photocatalyst (e.g. nanoparticulate
P25) is common to allow fluidisation and prevent the
entrainment of particle fines in the outlet gas stream.
Particles of Geldart class[117,118] B are considered most
suitable for fluidisation (note that P25 particles are
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Figure 15. Fluidised-bed photoreactor.[15] .
classified Geldart class C). An example of a fluidisedbed photoreactor is depicted in Fig. 15.
Monolith photoreactors have also been designed as
flow-type reactors. Figure 16 illustrates such a reactor.
Such reactors provide a high surface-to-volume ratio
(typically 10–100 times more than bead or plate substrates with the same outer dimensions).
Some of the disadvantages faced by the monolith
reactor include the decrease of the light flux by about
50% due to a shadowing effect on the incoming diffuse
light at the channel entrance by the wall. At a distance
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M. LIM ET AL.
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Fluid in
Fluid out
Photonic
Source
Honeycomb structure with channels
coated with photocatalyst
Figure 16. Monolith photoreactor.[110] .
CONCLUSIONS
The fundamental processes behind semiconductor photocatalysis and recent advances in the development
of photocatalytic materials/photoreactors have been
described in this short review. Whilst the reported efficiencies of photocatalysts used in air pollution control vary depending on the type of photocatalyst used,
reactor design and targeted compounds to be degraded;
there is a trend towards the development of novel photocatalysts based on unique material structures or optical
properties. In addition, there is an ongoing effort to
better understand photocatalytic systems through, for
instance, computational methods.
Future investigation involving the photocatalytic
degradation of airborne pollutants should take its cue
from priority pollutants, and along with advances in
developing materials/reactors for such photocatalytic
processes, should also strive to contribute to fundamental knowledge behind the basic mechanisms of photocatalysis.
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Figure 17. Multiannular type
photoreactor as an improvement to the conventional
monolith
photoreactor.[119]
Note keys: 1 is gas outlet; 2
is gas outlet; 3 is distribution
head; 4 is borosilicate glass
tubes and 5 is UV source.
of 1–2 aspect ratios (i.e. selected length per channel
width) in the square channels, only 10% of the initial
light flux remains and at a distance of 3–4 aspect ratios
the light flux decreases to 1% of the initial flux. As
an improvement over this design, Imoberdorf et al .[113]
proposed a multiannular photoreactor using concentric
walls which are transparent to UV irradiation (depicted
earlier in Fig. 14) and detailed in Fig. 17.
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
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