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Recent Developments in Photocatalysis.

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Dev. Chem.Eng. Mineral Process., 6(1R),pp.55-84, 1998.
Recent Developments in Photocatalysis
R.F. Howe
Dept. of Physical Chemistry,University of New South Wales, Sydney,
New South Wales 2052, AUSTRALIA
This article reviews some recent developments in the area of photocatalysis. These
include the development of metho& for preparing nanoctystallme (Q-sized) Ti02, the
doping and promotion of titania to improve its photoreactivity, recent studies
gf
reaction mechanisms, the introduction of other oxide photocatalysts, and recent
advances in the design of reactorsfor photocatalysis. It is concluded that practical
implementation of photocatalysis, particularly in environmental applications, is now
close to realitv.
Introduction
The subject of photocatalysis has been addressed in the literature for more than 25
years. Strictly speaking, heterogeneous photocatalysis involves absorbtion of light by
a solid photocatalyst which then modifies the state of activation of at least one
reactant adsorbed on the catalyst surface, thereby producing a new reaction path with
a lower activation barrier than that for the thermal reaction proceeding in the absence
of light [l]. This process does not involve direct absorption of light by reactant
molecules, but resembles the homogeneous process of photo-sensitization in which a
reactant molecule is electronically excited through interaction with a sensitizer which
strongly absorbs the incident light. A distinction can also be made between the
photocatalytic reaction, where the reaction in question does not proceed at all in the
absence
of
light,
and
a
photo-assisted
catalytic
reaction
in
which
55
R.F. Howe
exposure to light enhances the rate of catalytic reaction which does proceed albeit
slowly, in the dark. A further development is the closely related topic of
photoelectrochemistry, where the combination of light and an electric field is used to
drive a desired process.
In all of the above, the central aspect is the use of light energy to promote or
enhance an effective chemical reaction in the presence of a solid surface. This paper
will outline some of the more recent developments in this general field. Other more
comprehensive reviews, particularly of Ti02 photocatalysis, are available [2-91.
Photocatalysis with Ti@: Some General Aspects
Ti02 has received by far the most attention as a semiconducting oxide photocatalyst.
The 1972 discovery of photocatalytic splitting of water over Ti02 electrodes by
Fujishima and Honda [lo] prompted a vast research effort investigating the
photoreactivity of titania. Titania is readily available, robust, and has a band-gap in
the near ultraviolet region of the spectrum (3.2 eV or 387 nm for anatase). The
anatase form of Ti02 is regarded as more photoactive than the mile form [l I],
although that does depend on the precise preparation conditions of the titania and the
particular reaction involved [6]. The mechanism of photocatalysis over Ti02 is
understood in broad outline, although as discussed further below mechanistic details
of prospects for optimizing photocatalytic performance are still being hotly pursued.
The initiating act in photocatalysis over a semiconductor oxide such as Ti02 is
absorbtion of a photon with energy equal to or greater than the band gap of the
semiconductor, producing holes and electrons. Figure 1, from reference [9], shows
this process schematically. Once the holes and electrons are produced (corresponding
to an excited state in the photocatalyst), there are several de-excitation events which
may occur. The holes and electrons may recombine, either in the interior of the
semiconductor or after migration to the surface. Various hole or electron trap sites
(e.g. bulk impurity or dopant ions, surface defect states, etc.) may mediate the
recombination process (not shown in Figure 1). Migration of holes andor electrons
to the surface followed by interfacial electron transfer from/to adsorbed donor or
acceptor
56
molecules
respectively
are
the
events
leading
to
reaction.
Recent developments in photocatalysis
Figure 1. Schematic of phot-citation
and de-excitation processes in an oxide
semiconductor (Reproduced with permission@om reference [9]).
The overall efficiency (quantum yield) of a photocatalytic process is then determined
by the competition between interfacial electron transfer and ho1e:electron
recombination. Interfacial electron transfer is in turn controlled by the band energy
positions of the semiconductor relative to the redox potentials of the adsorbates: the
acceptor species must lie below the bottom of the conduction band and the donor
species above the top of the valence band.
There is now a vast body of literature describing the use of Ti02 as a
photocatalyst, particularly in recent times for environmental applications [ 6 ] . Table 1
provides a summary of different reaction types investigated.
A much more
comprehensive list is provided by Hofiinann et al. [6].
The photodecompositionof water first reported by Fujushima and Honda [ 101
employed a Ti02 electrode in a photoelectrochemicalceli; separation of holes and
57
R.F. Howe
Table 1. Reactions Photocatalyzed over Ti02.
Reaction type
1. Decomposition
[Go2+ 'So2+ 2 '60180
H20
+ H2+f02
2. Photooxidation of Inorganic Species
co+fo2+ CO2
N2+ + 022NO
+NZ+ 3H20
CN-taq3 +: 0 2 + OCN-Iaq]
2NH3 + 0 2
2HX[aq] +
02
+ X2 + H2O
3. Photo-reduction of Inorganic Species
+ 2NH3 + 3 O2
C02 + 2H20 + CH30H + O2
N2 + 3H20
~~
4. Photooxidation of Organic Species in the Gas Phase
+ acetone
trichloroethylene+ 0 2 + C02, HCI,CO, Clz
4chlorophenol+ 0 2 + oxidation products
all<ane+02
5. Photooxidation of Organic Contaminants in the Aqueous Phase
chlorinated aromatics
chlorinated aliphatics
chlorinated olefins
nitro compounds
CFCs and HCFCs
6. Photodestruction of Malignant Cells
Generation of H202 and OH radicals in vitro and in vivo
58
Reference
Recent developments in photocatalysis
electrons was achieved through an external circuit allowing electrons to flow to a
platinum electrode where they reduce water to hydrogen, while the holes at the Ti02
surface oxidize water to oxygen. Water decomposition does not occur on irradiation
of Ti02 alone; although the reaction is energetically favourable it is inhibited by the
large overpotentials for evolution of H2 and 0 2 [9]. Water decomposition can be
achieved over Ti02 containing added metal and metal oxide components. Figure 2
shows for example a schematic of a Ti02 particle containing added Pt and RuO2.
This functions as a short-circuited micro photoelectrochemicalcell in which the Pt is
the cathode and Ru02 the anode; the role of the added components is to effectively
lower the overpotential for evolution of H2 at the Pt surface and 0 2 at the RuO2
surface respectively [16]. An alternative approach has been to provide a sacrificial
species to remove either holes or electrons, allowing the other to react with water.
For example, sustained production of H2 has been observed when aqueous
suspensions of Ti02 are irradiated in the presence of methanol [17]. Methanol
scavenges positive holes from the structure of the Ti02 (being in the process
oxidized to COz), allowing electrons to reduce water to H2.
>O'"0 2
Figure 2. Schematic of water dissociation over a Ti02 photocatalyst contairiing
added Pt and Ru02 (after reference [I 61).
59
R.F.Howe
Photooxidation of inorganic species over Ti02 has been undertaken both in the
gas phase and in aqueous solution. For example, Bickley et al. [19] detected NO by
thermal desorption after irradiation of an N2, 0 2 mixture in the presence of Ti02 at
room temperature, while Fitzmaurice et al. [35] observed 12- and I2 formation on
irradiation of colloidal sols of Ti02 in the presence of iodide ions. Photoreduction
of nitrogen over an iron-doped Ti02 catalyst was reported by Schrauzer et al. [22]
to yield ammonia and hydrazine. Photoreduction of C02 has attracted some
attention. In this case optimum conversion of C02 to hydrocarbon products is
achieved by adding a metal component to the T i 0 3 for example Pd-Ti02 shows
very high selectivity for C b formation [36]. Presumably, the Pd functions in the
same way as the metal component in water decomposition photocatalysis, as a
cathode.
A thin-film photoelectrocatalytic cell for reduction of carbon dioxide has
recently been described by Ichikawa and Doi [37]. This combines a Ti02 film on a
conducting substrate functioning as the anode with a metal (e.g. Pt or Cu) cathode,
separated by a Nafion polymer layer. Water is oxidized to 0 2 at the photoanode,
while electrons flowing through the external circuit to the metal cathode reduce
carbon dioxide. Addition of a small bias voltage enhanced the C02 conversion, and
in a very interesting observation operating the bias voltage in a pulsed mode was
found to inhibit catalyst deactivation.
Photooxidation of hydrocarbons in the gas phase over Ti02 catalysts was the
subject of intensive study in the 1970s by the Teichner group [l]. Products obtained
were primarily ketones and aldehydres; it was pointed out that the low temperatures
at which photocatalysis is conducted avoided the gas phase free radical reactions
leading to complete combustion in the thermal catalytic oxidation. More recently,
the possibility of using photocatalysis to destroy organic compounds present as
environmental impurities in air has been realised.
Miller and Fox [38] have indicated that from an economic viewpoint
photocatalytic treatment of contaminated air is commercially viable for high
quantum efficiency reactants such as trichloroethylene (TCE). Anderson et al. [39]
have described, for example, complete conversion of TCE to C02, HCl and H20 in
60
Recent developments in photocatalysis
a single pass through a reactor containing Ti02 photocatalyst at TCE levels of 450
ppm.
Other hydrocarbons, particularly aromatics, are much less reactive, and
photo-oxidation compares unfavourably with incineration or carbon absorption
separation as means of removal.
011;s et al. [29] have recently reported that photocatalyzed destruction rates of
low quantum efficiency contaminant compounds in air can be dramatically
promoted by adding a high quantum efficiency promoter such as TCE,
perchloroethylene or trichloropropene. Addition of TCE was found, for example, to
produce high quantum efficiency conversion at contaminant levels of 50 mg m-3 for
several aromatics and volatile oxygenates. The authors propose that adsorbed
chlorine radicals play a key role in the enhancements observed, although they note
that not all hydrocarbon contaminants can be influenced in this way, and the
reaction mechanisms remain uncertaiti.
The comprehensive review by Hoffmann et al. [6]on environmental applications
of semiconductor photocatalysis lists more than 400 references to published work on
photooxidation of organic compounds in water over titania photocatalysts. The
catalysts used include solid powders, colloidal sols, and thin films. The potential of
photocatalysis for wastewater treatment is clearly established, but to date no
commercially available process uses W irradiation of Ti02 photocatalysts for
remediation of contaminated waters. However, a commercial unit for analysis of
total organic carbon in water based on photochemical conversion of organics to C02
over Ti02 has been developed from the work of Mathews et al. [40].
In reviewing the existing literature on both gas-phase and liquid-phase
photocatalysis over Ti02, there are several reasons which can be identified as to
why photocatalytic processes have not so far become commercially viable, and why
the ultimate objective of utilizing sunlight to drive chemical reactions has not been
attained.
First, intrinsic activity of many photocatalysts reported in the literature is low,
and variable. A commercial anatase (Degussa P25, which is actually a rutileanatase mixture) is frequently used as a standard catalyst, but has a relatively low
surface area (approx. 55 m2g-l). Tanaka et al. [41]have reported wide variations in
61
R.F. Howe
photocatalytic activity over different crystalline forms of Ti02 having different
surface areas and calcined at different temperatures. More recently, sol-gel methods
are being used to produce higher surface area forms of titania (with in principle at
least higher reactivity); the properties of nanocrystalline Ti02 are discussed further
below.
Enhancement of activity by doping the Ti02 photocatalyst with transition metal
ions has been widely attempted, but with variable success. Another motivation for
modifying the Ti02 with dopants is to shift the absorbtion spectrum from the near
ultra-violet into the visible, in order to match more closely the solar spectrum.
These issues are considered below.
Although the general features of photocatalytic reaction mechanisms are
understood, the detailed events following creation of the hole and the electron and in
particular the interfacial electron and hole transfers to adsorbed species are not well
characterized. As in other types of catalysis, an improved understanding of reaction
mechanisms is essential if catalyst performance is to be improved. The complexity
of the solid-liquid interface, particularly where the solid surface is not well defined,
contributes to the mechanistic uncertainty. Recent approaches to this problem
include the application of surface science techniques to well defined single crystal
Ti02 surfaces [9], and the use of time-resolved spectroscopic methods to probe the
kinetics of fundamental events [6]. These and other developments in mechanistic
studies are discussed further below.
Another crucially important aspect of photocatalysis is reactor design. Early
studies of photocatalysis were not particularly concerned with reactor efficiency,
and used typically a fixed bed of Ti02 powder or an aqueous slurry. The absorbtion
coefficient for UV radiation of Ti02 is however extremely high, such that 99% of
light absorbtion occurs within a 5 pm depth of Ti02 powder [42]. The typical
particle sizes of 30 to 100 nm for commercial anatase powder mean that light
scattering from fixed beds or powder slurries will act to reduce photochemical
efficiency. Some of the recent activities in reactor design intended to overcome
these problems are described below.
62
Recent developments in photocatalysis
Nanocrystalline Ti02
The surface area of a catalyst can be dramatically increased by reducing the particle
size into the nanometre range (e.g. 10 nm = 100 A). The photoreactivity of colloidal
dispersions of Ti02 in this size range was first investigated by Gratzel et al. [43].
The advantages of such colloidal dispersions were pointed out to be the optical
transparency (minimal light scattering), rapid carrier diffusion to the interface, and
high surface to volume ratio. Since that time the chemistry involved in producing
colloidal sols has been investigated by many groups [44] and methods for producing
large quantities of material developed [45].
The reduction of particle size below 10 nm has other important consequences for
photocatalytic reactivity.
The spatial confinement of charge carriers within a
nanometre size range produces so-called size quantization effects. The band gap of
the semiconductor increases and the band edges shift to yield larger redox potentials,
thus increasing rate constants for charge transfer at the surface. For systems in
which interfacial charge transfer is the rate limiting step, size-quantized (Q-sized)
Ti02 particles may show enhanced photoefficiency [46].
Three methods have been used to study the photoreactivity of Q-sized Ti02
particles.
Gratzel et al. [43] initially studied aqueous colloidal dispersions as
prepared.
Such dispersions, although optically transparent and convenient for
spectroscopic investigation [47-49] are metastable, and do not allow convenient
recovery of catalyst. Q-sized Ti02 powder can be recovered from colloidal sols by,
for example, freeze drying [50]. Alternatively, films of Q-sized particles can be
formed by dip-coating or spin-coating suitable substrates with the sol solutions.
Figure 3 shows for example a scanning electron micrograph of a Q-sized
Ti02
film prepared by dip-coating a quartz slide with a colloidal Ti02 sol [51]. The
particle sizes evident in the micrograph are remarkably uniform, in the range 5-10
nm, and are unchanged when the film is fired at temperatures up to 800°C. Grazing
incidence X-ray diffraction and EXAFS measurements have revealed however that
the average crystalline domain size increases progressively on firing. The films as
prepared comprise poorly crystalline anatase, as judged by X-ray diffraction,
EXAFS and Raman spectroscopy. The crystallinity improves on firing but even
63
R.F. Howe
after firing at 800°C the films remain predominantly anatase (Q-size anatase bulk
powders convert to rutile at about 500°C).
The dramatic effect of crystalline domain size on the band gap of Q-size anatase
films is illustrated in Figure 4 [52]. This shows the uv-visible transmission spectra
of films after firing to various temperatures, compared with the diffuse reflectance
spectrum of bulk anatase. The increased band gap associated with Q-size particles
is clearly evident; the difference between the film and bulk anatase decreases as the
firing temperature increases, but the band gap is still blue shifted even after firing at
800°C.
Figure 3.
SEM image of Q-size Ti02 film prepared by dip coatirig a quartz
substrate with colloidal sol Cfrom reference [51] ).
The films prepared by dip-coating illustrated in Figures 3 and 4 are typically 50
nm thick.
Thicker films can be prepared by multiple coating, although the
properties of such thicker films have not yet been investigated in detail. The films
are extremely porous and may be expected to show high photoreactivity. Anderson
64
Recent developments in photocatalysis
et al. [39] have reported the activity of similar films (prepared by spin-coating) for
photodegradation of chlorophenols, and show that the optimum performance is
obtained with films coated three times in succession. They point out however that
the optimum film thickness will be dependent on reactor design. Likewise, Cui
et al. [53] report that the photocatalytic activity of nanocrystalline Ti02 films for
degradation of salicyclic acid does not increase with increasing film thickness
beyond about 500 nm. Their data show in fact that there is only a small increase in
activity beyond a film thickness of 200 nm.
200
400
300
Nanometers
Figure 4. UV-visible spectra of I96 Fe doped Q-size Ti02 film after firing to (a)
125OC. (b)350°C,(c) SOOOC, ( d ) Bulk anatase.
65
R.F. Howe
Kato et al. [54] have examined the performance of nanocrystalline Ti02 films
prepared by dip-coating from colloidal sols for the photodegradation of acetic acid,
as a function of film pretreatment.
They find that films prepared to be
nanocrystalline anatase show good activity for acetic acid photodegradation,
whereas films pretrated (by rapid heating cycles) to contain a mixture of anatase and
rutile are less active, and show an approximately linear correlation between activity
and the fraction of anatase in the films.
Much thicker nanocrystalline Ti02 films have been used extensively by Gratzel
et al. [43] in photoelectrochemical and photovoltaic cells. Films of 3-10 Frn
thickness are prepared by spreading a paste of nanocrystalline colloidal Ti02
particles on to a conducting glass support then sintering; the properties of the
resulting films depend very much on the prior history of the colloidal sol
(particularly particle size and particle size distribution) as well as the sintering
conditions used. [7,55,56]
The dynamics of light induced charge separation in thick nanocrystalline titania
films have been studied by ORegan et a]. [57], following injection of electrons from
the photo-excited state of a ruthenium complex into the conduction band of the
titania. Two important findings from this work were that the colloidal particles
constituting the film are in electronic contact, forming a three dimensional array,
and that the films contain a high concentration of electron trap sites which exert a
strong influence on hole : electron recombination processes. The nature of these
electron trap sites was not determined, although earlier EPR studies of colloidal
Ti02 sols [48] indicated that interstitial Ti4+ sites at grain boundaries may be
important.
Characterization of bulk nanocrystalline Ti02 powders prepared by sol-gel
methods has been described by Terwillinger and Chiang [58].
The initially
amorphous freeze-dried sols crystallize first to anatase, but nucleation to rutile
occurs soon after. (300-4Oo0C)The addition of tin (1 mole %) was found to
promote the crystallization of rutile, such that nanocrystalline mile with particle
sizes less than 20 nm could be prepared. This approach offers the possibility of
preparing well defined nanocrystalline rutile for comparitive studies with the
66
Recent developments in photocatalysis
corresponding anatase material. It is also clear that the issue of phase transitions in
nanocrystalline Ti02 is an important one; the role of a film substrate in stabilizing
the anatase phase and inhibiting particle growth is one which has important
consequences for photocatalysis.
Another new approach to producing highly dispersed titania photocatalysts is the
anchoring of Ti02 onto high surface area supports such as Vycor glass. Anpo et al.
[59] describe the reaction of T i c 4 with hydroxyl groups on porous Vycor glass,
followed by hydrolysis of the anchored compounds, to produce supported Ti02.
EXAFS and photoluminescence measurements indicate that, at least at low titanium
loadings, isolated Ti02 species are produced. The resulting photocatalysts showed
high activity for oxidative decomposition of l-octanol (up to 1000 times higher
activity per unit mass of Ti02 powders).
The conventional description of
photocatalysis in terms of semiconductor band structure is clearly inappropriate for
such catalysts, and Anpo et al. [59] prefer to describe the activity in terms of a
charge transfer complex :
Ti"02-+
hv 4 [Ti3+@]*
...(1)
Doped and Promoted Titania
There have been many reports of influencing the activity of titania photocatalysts by
incorporating transition metal dopants into the titania, but also widespread
disagreement as to sign and magnitude of the effects. For example, Mu et al. [60]
reported that doping with trivalent andor pentavalent metal ions was detrimental to
the photocatalytic activity, whereas others [61] showed that activity was enhanced
by doping with pentavalent ions. Fe3+ enhances the photoreduction of N2 [22], but
has little influence on the photodegradation of phenol [62].
Photoactivity of Ti02 doped with Mo and V is reported to be lower than that of
the parent oxide 1631. There is disagreement however as to whether Cr3+ enhances
or inhibits the photoreactivity of titania [60,64].
67
R.F. Howe
Doping with transition metal ions is also known to influence the dynamics of
ho1e:electron recombination and interfacial charge transfer in Ti02.
Flash
photolysis experiments on aqueous colloidal sols showed that doping Ti02 with
Fe3+ or V4+ dramatically augmented the lifetime of the hole electron pairs created
by band gap irradiation [47]. It was further shown by EPR spectroscopy that
irradiation of such doped colloids causes the EPR signal of Fe3+ to fade and new
Ti3+ signals to appear. However in the case of V4+, a new V4+ signal first appears
and then fades as a Ti3+ signal develops [65]. These observations were attributed to
the ability of Fe3+ and V& to function as both electron and hole traps; hole trapping
at the transition metal site allowed the accumulation of trapped electrons at Ti4+
sites, accompanied by partial dissolution of the colloids.
More recently, Hoffman et al. [66] have undertaken a systematic study of
nanocrystalline Ti02 doped with a range of different metal ions, using transient
spectroscopy to measure charge carrier recombination dynamics and correlating
these with photoreactivity for oxidation of CHC13 and reduction of CCl4. Figure 5
from reference [66] shows a periodic table summarizing the effects of different
dopant ions on the photoreactivity of Ti02. Relative to Ti4+ (data for the undoped
TiO2), the largest enhancement effects are seen with Fe3+, Mo5+, Ru3+, V4+ and
V3+, while A13+ and
appear to cause some inhibition of the photooxidation
activity at least. The quantum yields in Figure 5 could be quantitatively correlated
with the transient recombination dynamics. According to Hoffman et al. [66],
dopant ions can function as both hole and electron traps, and as mediators of
interfacial charge transfer. To be photo-active, a dopant should act as both an
electron trap and a hole trap. Thus ions such as Fe3+, Mo5+, Ru3+, V4+ and V3+ are
most effective, being capable of both oxidation (hole trapping) or reduction
(electron trapping). Ions such as V5+ which can function only as electron traps do
not appreciably inhibit hole-electron recombination, since the trapped electrons can
readily combine with the still mobile holes. However, at high light intensity levels
when the trap sites are saturated, they may then begin to function as recombination
centres.
68
Recent developments in photocatalysis
Dopant ions also mediate interfacial charge transfer. This was shown in
reference [66] by the variation in photoactivity enhancement by different metal ions
as a function of particle size of the Ti02. The largest enhancements were found in
nano-sized particles, in which all dopant ions are located within 1-2 nm of the
surface. As the particle size increased, the number of near-surface dopant ions
decreased, and the enhancement effects lessened (undoped Ti02 showed no
variation of activity with particle size over the same range). Hoffmann et al.
suggested that enhancement of interfacial charge transfer is in fact the single most
important factor in enhancement of photoreactivity of doped Ti02, at least for
nanocrystalline materials.
(0.08
0.141
4.08
I: I 1
051
0.66
Ni2+
050
0.09
4.08
027
1.20
1.60
4.08
0.80
0.84
(0.08
Figure 5. Periodic table of the photocatalytic effects of various metal ion dopants in
Ti02 (reproduced with permission from reference [66] ). The upper numbers are
quantum yields for CHClj oxidation, and the lower for CCl4 reduction.
69
R.F. Howe
Undoubtedly one reason for variations in dopant effects reported in the literature
is differences in the location and coordination of the dopant ions, which depend
critically on the methods of sample preparation and pretreatment. In preparing
nanocrystalline Ti02 for example by sol-gel methods, dopant ions may be
introduced during the initial hydrolysis step, during peptization of the gel produced
by hydrolysis, or following dispersal of the sol. Dopant ions initially adsorbed on
the surface of sol particles may be incorporated into the particles on firing, or may
form separate metal oxide phases. Dopant ions incorporated into the interior of the
Ti02 may occupy either lattice (substitutional)or interstitial sites. The ability of the
dopant ions to function as trap sites and/or to mediate interfacial charge transfer will
depend on all of these factors. Characterizationof the doped catalysts is thus crucial
if their performance is to be understood.
A detailed characterization of vanadium doped nanocrystalline Ti02 has been
reported by Hoffmann et al. [67]. They show evidence for the presence of surface
bound V02+, interstitial V4+, surface V2O5, solid solutions of V, Ti1-~02and
lattice substituted V4+, depending on the exact conditions of sample preparation and
treatment. In all cases, photoreactivity of the doped catalysts was reported to be
lowered, implying that vanadium centres both at the surface and in the bulk act as
ho1e:electron recombination centres. The complexity of these materials (and the
resulting difficulty in interpreting photoreactivity data) has been confirmed in a later
EPR and 5IV N M R study by Luca et al. [68].
In contrast, recent studies of Fe3+ doped nanocrystalline Ti02 films prepared by
dip coating from a colloid sol where Fe3+ was added during the peptization step
have shown that Fe3+ (at levels of 1% or less) is incorporated completely into
substitutional sites in the anatase lattice upon firing at elevated temperatures [69].
Figure 6 shows Fourier transforms of Fe K-edge E M S data for 1% Fe3+ doped
titania films after firing at different temperatures. The film dried at 150°C shows
Fe3+ with a single oxygen coordination shell, corresponding to isolated Fe3+
adsorbed on the surface of the Ti02 particles. After firing at 800°C. on the other
hand, the coordination environment is identical to that of Ti4+ in anatase: a distorted
octahedral oxygen first shell, and a titanium second shell, proving that upon firing
70
Recent developments in photocatalysis
Fe3+ migrates into substitutional lattice sites in the Ti02 (anatase) structure.
Preliminary EXAFS analysis indicates that this does not happen in the corresponding
films doped with Cr3+ or MoG.
-'I
'T
LF
r........ ....
.
.
j
5
.
...
4 t
......-<.
.
1
Figure 6. Fourier transfonns of Fe K-edge EXAFS front 1%
F2+
doped titania
f i l m fired at (A) 150°C; (B) 800°C. Solid curves are experimental data and dotted
curves simulations.
The addition of a second transition metal (or non-metal) oxide to Ti02 at high
concentrations must be described as promotion rather than doping. There have been
71
R.F. Howe
a number of studies of such promoted Ti02 photocatalysts. For example, Do et al.
[70] report that addition of WO3 to Ti02 greatly enhanced its photocatalytic activity
for degradation of 1,4 dichlorobenzene. Their data show a factor of 2 increase in
rate of degradation at an optimum W03 content of 3 mole %. The enhancement is
attributed to enhanced electron transfer from Ti02 to the promoter. The flat-band
potential of WO3 is more positive than that of TiO-2, thus photoelectrons will
transfer to the W03 conduction band, and holes will accumulate in the Ti02 (and
thus be available for oxidation of dichlorobenzene).
A similar enhancement has been described on addition of Nb2O5 to Ti02
photocatalysts. Cui et al. [71] point out that the 2-3 mole % promoter found to be
optimum corresponds to the theoretical monolayer capacity of P25 anatase, although
no direct evidence for monolayer dispersion of the promoter has been presented.
The increase in photocatalytic activity could be correlated with an increase in
surface acidity of the catalysts as the Nb2O5 content increased, suggesting that
surface acidity may play an important role in the photocatalytic reactivity.
Surface acidity was also considered by Anderson et al. [72] as a possible
explanation for the enhanced performance of TiOz/Si02 and Ti02/ZrO2 catalysts
for the photooxidation of ethylene in the gas phase. They concluded however that
the up to 3-fold enhancement observed must be due to the increased surface area of
the promoted catalysts and the inhibition of the anatase to rutile phase transition.
Weller et al. [73] have considered the sensitization of Ti02 (and other wide
band-gap oxide semiconductors) photocatalysts by addition of quantum-sized
narrow band-gap semiconductors. This concept follows from the use of organic
dyes to sensitize Ti02 electrodes [7]. The sensitizer has a high cross-section for
absorption of visible light, and injects electrons into the conduction band of the
underlying semiconductor. The advantages of using quantum sized semiconductors
as sensitizers are that the band gap can be adjusted by varying the particle size and
the materials are in principle more stable than organic dye sensitized Ti02. In
practice, the measurements of Weller et al. [73] indicate that photocorrosion or
other processes causing loss of efficiency are still a problem with the dual
semiconductor systems. For example, the photocurrent quantum yield for a PbS
72
Recent developments in photocatalysis
coated nanocrystalline Ti02 photoelectrode decreased from 65%to 25% after 4 days
of illumination with 460 nm light.
Photocorrosion in aqueous media is a
characteristic probIem with narrow band-gap semiconductors such as PbS or CdS.
The possibility of using such semiconductor sensitized nanocrystalline Ti02 for gasphase photocatalysis may however be attractive.
Studies of Reaction Mechanisms
As noted above, the mechanistic events occumng in photocatalysis following
creation of holes and electrons in the oxide semiconductor remain a matter of
speculation. There have however been several approaches taken recently to provide
more definitive mechanistic information, particularly for Ti02 photocatalysts.
The interfacial processes of electron and hole transfer from the semiconductor to
adsorbed molecules are crucial steps in any photocatalytic mechanism. The surface
structure of micro or nanocrystalline Ti02 is however difficult to study and define.
There have been a number of surface science studies made of single crystals of Ti02
which attempt to relate chemistry observed on the well-defined single crystal
surfaces to that occumng on photocatalysts. These surface science studies have
been reviewed in detail by Yates et al. [9]. Most authors have studied single crystals
of rutile. Photoemission spectroscopy shows that the electronic structure of nearly
perfect rutile surfaces is closely similar to that of the bulk [74], but the introduction
of surface defects (Ti3+ sites) introduces additional states in the band-gap around
0.8 eV below the F e d level. One of the few studies on anatase single crystals
reports similar electronic structure to rutile surfaces [75].
The chemisorption properties of single crystal Ti02 surfaces are dominated by
defect sites. Dissociation of water fills surface oxygen vacancies to produce
hydroxyl groups [76], while coordinatively unsaturated Ti cations are highly reactive
towards adsorbed organic molecules. To date, only a single surface science study of
a photooxidation process on a well defined single crystal surface has been reported.
Lu et al. [77] examined the photodegradation of methyl chloride on a rutile (1 10)
surface, and showed that the active sites for this reaction are surface Ti3+ sites
73
R.F. Howe
(oxygen vacancies). Interaction of oxygen with these sites produced a species
activated by electron transfer from the conduction band of electrons generated by
band-gap irradiation, and this activated oxygen species then reacted with methyl
chloride.
It should be emphasized that the circumstances under which the experiments of
Lu et al. were undertaken (low pressure gas phase reaction) are quite different from
those of normal photocatalysis. Nevertheless, the demonstration that catalyst
mediated photoactivation of oxygen is the crucial step in photodegradation of
methyl chloride will undoubtedly have bearing on mechanistic considerations in
photocatalysis (and finds some support from EPR studies which failed to detect
surface hydroxyl radicals commonly postulated as the reactive species [781 ).
The photoelectrochemical properties of single crystals of anatase were recently
reported by Gratzel et al. [79]. This work confirmed that the main difference
between anatase and rutile surfaces is the position of the conduction band edge; the
flat band potential of anatase (101) surfaces is shifted by 0.2 eV relative to that of
rutile (001). Interestingly, the photosensitized electron injection from an adsorbed
ruthenium complex dye into the conduction band of the anatase single crystal had a
quantum efficiency 3-fold less than that of the corresponding nanocrystalline Ti02
thin film electrode, suggesting that surface texture plays an important role.
The other new development in studying mechanisms of photocatalytic reactions
is the adoption of laser flash photolysis techniques to quantify the kinetics of various
mechanistic steps. Early studies on aqueous colloidal sols by Rothenberger et al.
[941 used pico- and nano-second transient absorbtion measurements to monitor the
fate of photo-produced holes and electrons. Electron trapping was reported to occur
within 30 ps, the time resolution of the system used, while the trapping time for
holes was estimated to be <250 ns.
Second order kinetics were observed for
e1ectron:hole recombination, and a mean lifetime for the e1ectron:hole pair of 30 ns
was estimated. Subsequent studies with femto-second time resolution on similar
aqueous colloidal sols revealed that e1ectron:hole recombination occurred on a picosecond time scale rather than nano-second, and the electron trapping time was
determined to be approx. 200 femto-seconds [95].
74
Recent developments in photocatalysis
Bowman et al. [97, 981 have recently applied for the first time the technique of
time-resolved diffuse reflectance spectroscopy to observe transient events in solid
Ti02 powders.Comparison of electron trapping in quantum sized Ti02 colloids, wet
and dry P-25 anatase powder revealed that in all 3 cases electron trapping occurred
in less than 200 femto-seconds.
The resulting decay of the trapped electron
absorbtion signal due to recombination occurred within 50 ps, and was also
independent of the nature of the sample. Addition of SCN- as a hole scavenger
profoundly lengthened the lifetime of the trapped electrons, indicating that there is
efficient competition of charge transfer of holes to the adsorbed SCN- with
recombination. Kinetic data of this kind are essential to the development of detailed
mechanistic models of photocatalysis.
Other Oxide Photocatalysts
Although Ti02 has received by far the most attention in the photocatalysis literature,
other oxide semiconductors are attracting some interest (as noted above, non-oxide
binary semiconductors such as CdS, CdSe or PbS are regarded as insufficiently
stable for catalysis, at least in aqueous media). The band gap of ZnO (3.2 eV) is
identical to that of anatase, and some early reports describe photoactivity of ZnO for
CO oxidation, for example [go]. ZnO is however also unstable in water, leading to
catalyst deactivation [81]. Other candidate oxides include W 0 3 (2.8 eV) and Sr
Ti03 (3.2 eV). SrTi0-j modified with NiO has been shown to decompose water into
H2 and 0 2 catalytically [82], and evidence has been presented for the presence of
NiO, Ni and Ni(OH)2 phases in the active photocatalysts [83].
W03 and doped W03 catalysts have been shown [84] to be active for the
photochemical reaction of methane with water to form methanol :
C&
+
H20
+ CH30H +
H2
...( 2)
75
R.F. Howe
This reaction occurs homogeneously in the gas phase under irradiation with 185
nm light [ 8 5 ] , through a mechanism involving photolysis of water to produce OH
radicals which abstract a hydrogen atom from CH4. Taylor et al. [84] found that
adding a La promoted W 0 3 catalyst to this reaction promoted the rate of methanol
production, as illustrated in Figure 7. Cu doping inhibited methanol formation,
whereas doping with Pt or Cu + La had no effect on the homogeneous (no catalyst)
reaction. Addition of hydrogen peroxide to the reaction enhanced 1O-fold the yield of
methanol in the presence of the WOg catalysts, supporting the suggestion that OH
radicals play an important role. There remain many unanswered questions about this
reaction, the exact role of the catalyst, and why La doping is important, but it does
represent an interesting new development.
TiME (HOURS)
Figure 7. Methanol production @om reaction of CH4 with H 2 0 over various
catalysts under w illumination (reproduced with permission@om reference f84J).
No catalyst (-El-);
W O r L a (-); WOyPt (-0-);
WOrLa-Cu (*);
and
woj-cu (+).
Other non-conventional oxide photocatalysts have also been reported recently
[86, 871. K4Nb6017 shows activity for the water decomposition reaction with a
76
Recent developments in photocatalysis
noticeably high quantum efficiency. The band-gap of K4Nb6017 is 3.5 eV, which
is also in the useful range for practical photocatalysis.
Domen et al. [87]
investigated the dependence of reaction rate on light intensity, and concluded that
the reaction mechanism involves competition between hole and electron reaction
with H20 and recombination.
They suggest an upper limit for the quantum
efficiency of 20%. Other ternary oxide systems based on perovskite structures are
referred to in the patent literature [88].
Reactor Design
Oxide photocatalysis has been traditionally practiced with slurries of Ti02 dispersed
in water (for aqueous reactions), or with a thin-layer fixed bed of Ti02 powder for
gas phase reactions. For aqueous systems, the problem of catalyst recovery prevents
the use of this method for large scale water treatment. For both liquid and gas phase
reactions, powdered catalyst beds are inefficient from the aspect of irradiation.
A number of groups have therefore begun investigating reactors in which thin
coatings of the photocatalyst are employed, such that all Ti02 particles receive in
principle uniform irradiation. The important issues then become preparation of the
coating (and its adherence to the substrate) and design of the reactor to achieve
optimum contact between reactants and the irradiated photocatalyst. [89]
Anderson et al. [39] have described 3 different types of reactor design for the
specific task of photodecomposing organic contaminants in water. These are shown
in Figures 8-10. The annular photoreactor illustrated in Figure 8 houses a mercury
vapour lamp which irradiates a dipcoated titania sol film; reaction solution suitably
oxygenated is pumped through the annular space between the film and the outer
cooling jacket. Problems encountered with this reactor include difficulties with
temperature control, transmission of radiation through the catalyst film inducing
homogeneous photochemistry in the annular space, and perhaps most seriously, the
limited amount of catalyst that is in the reactor (the catalyst film thickness is
typically less than 1 micron).
77
R.F. Howe
Figure 9 shows an alternative flat plate design in which a glass slide coated with
Ti02 is sandwiched between two halves of a teflon block. In this case sample
heating is not a problem, and the light intensity is readily controlled with an external
lamp (it is also possible to place an array of flat plate reactors under a single lamp.)
This design still suffers however from the same process capacity limitation as the
annular reactor.
Inner
tube
I
Catalyst
/ coating
Cooling chamber/
Reaction chamber /
02/Air
Cooling entry
Figure 8. Annular plzotoreacrorfor water purification (reproduced with pernrission
from reference 139j ).
A third design, which allows more catalyst to be incorporated into the reactor
without losing irradiation efficiency is shown in Figure 10. In this design, described
78
Recent developments in photocatalysis
in detail in reference [90], light is coupled into the reactor via optical fibres coated
with titania thin films. Coating of a bundle of e.g. 200 fibres with Ti02 gives a
much higher loading of catalyst in the reactor, and has given conversions and
apparent quantum efficiencies comparable with those of a slurry reactor [91]. A
practical difficulty referred to in [39] is the fragility of the Ti02 coated optical
fibres; improving the durability of the nanocrystalline Ti02 coatings is an obvious
objective.
Nevertheless, the fibre optic reactor concept offers many attractive
advantages, including the possibilities of use in remote locations.
Figure 9. Flat plate reactur for water purification (reproduced with permission
from reference [39] ).
Catalyst recovery is not an issue in performing photocatalysis on gas phase
reactions. The optical efficiencies of the flat plate and fibre optic designs would
79
R.F. Howe
also make them attractive however for gas phase reactions. A flat plate reactor has
been used by Obee [92] to remove formaldehyde and toluene from the gas phase.
Nimlos et al. [93] used small diameter coated tubes in a reactor design otherwise not
dissimilar to that in Figure 9 for oxidation of ethanol in the gas phase, while
Anderson et al. [39] refer to use of a packed (coated) capillary tube reactor in their
gas phase studies.
,
Optical fibers coated
with anatase membrane
Contaminated
water in
Figure 10. Opticalfibre reactor (reproduced wit11permissionfrom refereme [39] ).
Conclusions and Outlook
After 20 years of largely empirical and often contradictory studies, heterogeneous
photocatalysis is now approaching a level of understanding where practical
implementation on a large scale, particularly for environmental applications, can be
contemplated. Some particular issues which still confront the field are :
80
Recent developments in photocatalysis
*The apparent improvements in catalytic performance of Ti02 that can be
achieved by doping with transition metal ions must be understood more clearly
if this method is to be used in practice.
The sensitization of Ti02 with a second component to enhance activity and
shift the wavelength of irradiation into the visible region is a goal which
should be pursued further. Organic and organometallic dyes of the type used to
sensitize Ti02 electrodes are less likely to succeed in photocatalysis, but the
concept of photosensitizing with narrow band-gap
semiconductors is an
attractive one which warrants investigation.
*The use of more novel semiconductors (WO3, KqNb6017, perovskite mixed
oxides) in practical reactors will require methods for preparing high surface area
thin films of these materials to be developed.
As in all other forms of catalysis, improved understanding of reaction
mechanisms in photocatalysis is another important objective. Time resolved
optical spectroscopy and EPR spectroscopy will play important roles.
Optimization of reactor design. Further improvements in reactor design for
both liquid phase and gas phase reactions may be anticipated.
Environmental catalysis has become a major theme of the 1990s. Heterogeneous
photocatalysis is now poised to play a pivotal role in environmental control, and
may contribute also to new pathways to chemical products.
Acknowledgements
The author's current research on titania sol-gel films is supported by the
Australian Research Council as part of a joint project with the Advanced Materials
Group of ANSTO (Drs J. Bartlett and J. Woolfrey).
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Received:
84
3 'February 1997; Accepted
after revision:
3
March
1997.
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