Indian Journal of Chemistry
Vol. 51A, Sept-Oct 2012, pp. 1263-1283
Titania based catalysts for photoreduction of carbon dioxide: Role of modifiers
V Jeyalakshmia, b
, R Mahalakshmya, K R Krishnamurthy
b & B Viswanathan
b, *
aDepartment of Chemistry, Thiagarajar College, Madurai Kamaraj University, Madurai 625 021, Tamil Nadu, India
bNational Centre for Catalysis Research, Indian Institute of Technology Madras, Chennai 600 036, Tamil Nadu, India
Email: [email protected]
Received 24 May 2012; revised and accepted 22 June 2012
Photocatalytic conversions on titania utilizing sunlight as the energy source have been studied extensively for a variety of
processes/ synthesis, like removal of pollutants in air and liquid streams, self-cleaning, anti-fogging and anti-bacterial
applications, splitting of water into hydrogen and oxygen and photoreduction of CO2 by water to yield hydrocarbons. These
processes are receiving global attention as an off-shoot of the frantic search for alternative energy sources. Though titania
continues to be the preferred catalyst in view of its low toxicity, ability to resist photo-corrosion, versatility, and abundant
availability at low cost, critical limitations do exist in terms of its inability to get activated with visible light and in achieving
high conversion efficiency and quantum yield. Several techniques of modifying titania to improve its performance have
evolved over the years resulting in correlations and concepts on structure-property-activity and the role of preparation
methods. Such modifications have lead to changes in light absorption efficiency, electronic structure, energy levels,
morphology, phase composition and other photophysical properties with moderate improvements in the performance.
Efforts to understand the mode of action of the modifiers in terms of the first principles, i.e., rationalization of the activity in
terms of electronic and structural properties and establishing theoretical basis for the photocatalytic action, have met with
only partial success, due to conflicting observations/results.
The objectives towards modifications, namely, extending the light absorption range, retarding charge carrier
re-combination, facilitating their fast transport to the active sites on titania surface and incorporation of active elements
suitable for redox reactions, have been achieved to a reasonable level. However, commensurate improvement in
activity/CO2 conversion has not been observed. Maximization of selectivity (to methane or methanol) and arresting catalyst
deactivation are the two major issues yet to be understood in clear terms. An in-depth study to understand the surface
transformations at molecular level under activation by light energy, is needed to achieve further improvements in the activity
of the catalysts and the process. This review brings forth an account of the investigations on modified titania, capturing
some significant and selected contributions out of the vast literature available, with an emphasis on application for
photocatalytic reduction of CO2 with water.
Keywords: Photocatalysts, Photoreduction, Carbon dioxide photoreduction, Photophysical properties, Titania, Solar energy
Energy and environment are the two most critical
issues that the modern society is seriously concerned
about. Photocatalysis is emerging as one of the possible
means that can provide viable solutions to the
challenges from these two areas. Solutions in the form
of different photocatalytic conversions/processes are at
various stages of development. The key applications on
the energy front utilizing the abundantly available
sunlight are production of hydrogen by splitting of
water using a photocatalyst, generation of electricity
using solar cells (photovoltaic cells) and photocatalytic
production of methane and other hydrocarbons (solar
fuels) from CO2 and water (artificial photosynthesis).
Removal of pollutants from the industrial effluents and
drinking water (Advanced Oxidation Processes),
purification of air, self-cleaning, anti-fogging and
anti-bacterial applications on different materials are
the typical applications of photocatalysis towards
environmental protection. Some like self-cleaning,
anti-fogging and anti-bacterial applications are already
being practiced on large scale.1-4
Historically, these processes center around titania
and titania-based catalysts. Efforts to develop
alternative catalysts, based on various metal
oxides/sulphides/nitrides/phosphides, layered titanates,
binary and ternary oxides of Nb, Ta, Ga and In in
conjunction with alkaline, alkaline earth and rare
earth oxides and with a host of co-catalysts and
sensitizers, constitute the central theme of current
research in this area.5-6
INDIAN J CHEM, SEC A, SEPT-OCT 2012
1264
Titania continues to be the preferred and hence
extensively studied catalyst due to its low toxicity,
ability to resist photo-corrosion, versatility in terms of
wide spread applications and above all, abundant
availability at low cost. In spite of these virtues,
titania fails to fulfil some of the essential criteria for
an ideal photocatalyst,7 like, stable and sustained
photocatalytic activity, good overlap of absorption
cross-section with solar spectrum, high conversion
efficiency and quantum yield. Two major drawbacks
with titania, that affect its overall efficiency as
photocatalyst are:
(i) Absorption of only a small fraction (< 5 %) in
the UV region of the solar spectrum, in
accordance with its band-gap of 3.0 – 3.2 eV.
No absorption is observed in the visible region
(λ > 400 nm) which constitutes the major part
of the solar spectrum, and,
(ii) Low interfacial charge-transfer rates of
photo-generated charge carriers and
consequently high recombination rate.8
Hence, intensive research efforts9-17
are being
pursued to suitably modify the photophysical
properties of titania and circumvent these
shortcomings.
The Need for Modifications
The photocatalytic activity of a typical
semiconductor like titania is initiated by the
absorption of light energy corresponding to/or higher
than the band gap energy, resulting in the generation
of electrons and holes. The electron-hole pair, on
migration to the semiconductor surface, interacts with
the adsorbed reactants to facilitate reduction and
oxidation process respectively. In the absence of such
an energy transfer, the pairs recombine with the loss
of energy. Electron-hole recombination process,
which is two to three orders of magnitude faster,
competes with the desirable redox processes.
Therefore, the recombination process needs to be
minimized by suitable modifications to increase the
efficiency. Hence, modifications in TiO2 are aimed at
dealing with the two major issues, namely, extending
light absorption range beyond UV region, and,
arresting the recombination of charge carriers. Several
other modifications as detailed in the review have
been attempted to improve the activity further.
By suitable modification of the band gap,
i.e., reduction in band gap by creation of
additional/impurity energy levels, light absorption
range/wavelength could be increased. In the case of
titania, light absorption range is increased to cover
a part of the visible region.8-9
Arresting the
recombination rate increases the life time of
photo-electrons and holes, thus leading to a
corresponding increase in photocatalytic activity.
Highly conducting materials like carbon nanotubes
(CNT) and graphene, when added to titania facilitate
faster transport of the photo-generated electrons,
minimizing further the charge carrier recombination
(vide infra).
It is observed that similar approaches for
modification of titania have been adopted in all the
three major applications, namely, removal of
pollutants, splitting of water and reduction of CO2
with water. The reaction pathways and hence the
required photophysical characteristics being different
for each process, the types of modifications required
could as well be different. Amongst the three major
processes, photocatalytic reduction of CO2 involving
multi-electron transfer steps, is highly complex and
difficult and its reaction mechanism is yet to be
established.5b,16b
The objective of this review is to
critically analyze the general approaches adopted for
the modifications of titania for all the processes and
then examine the type of modifications and
characteristics that are essential for photocatalytic
reduction of CO2 with water.
Modifications in Titania – General Approaches
Implications of modifications on the electronic structure of
titania
A number of strategies, as listed below have
been devised to bring about the modifications in
the electronic structure of titania, with a view
to overcome the drawbacks indicated earlier. These
include:
(i) Doping with metals, cations and anions
(ii) Coupling with other semiconducting oxides
(iii) Sensitization using light harvesting compounds/
dye molecules
(iv) Plasmon resonance induced by specific metals
Excellent and exhaustive reviews have been
compiled describing the changes brought about by the
above strategies in the electronic and photophysical
properties of titania and the consequent improvements
observed in the photocatalytic activity for various
reactions.9-17
The main features of these modifications
and the implications in the properties of titania are
summarized in the Table 1. Schematic representation
JEYALAKSHMI et al.: ROLE OF MODIFIERS IN PHOTOREDUCTION OF CO2 BY TiO2-BASED CATALYSTS
1265
of these modifications and the implications in each
case are illustrated in the Fig. 1.
Doped metals act as electron traps, facilitating
charge separation and preventing recombination.
Experimental evidence towards this effect was
presented by Anpo et al.13
in their studies on Pt
loaded on TiO2. As shown in Fig. 2, when irradiated
with UV light, pure titania exhibits ESR signals
attributed to Ti3+
formed by the photo-generated
electrons. On loading the TiO2 with Pt, the intensity
of the ESR signal due to Ti3+
falls sharply due to
the transfer of electrons from titania to Pt, while the
holes remain in TiO2, resulting in charge separation.
Introduction of anions like N, S in TiO2 results in
narrowing of band gap due to mixing of p states of
dopants (N,S) with oxygen 2p states in the valance
band of TiO2 and creation of impurity states above
the valence band of titania.8, 9, 12
Such changes in
the electronic structure of TiO2 facilitate visible
light absorption.
Another strategy to extend the light absorption
range by TiO2 is to couple it with a narrow
Table 1 – Implications of different modifications in titania
Modifications Implications/Mode of action
Doping of metals/metal ions • Acts as electron traps and facilitate charge carrier separation
• Introduces impurity states and induce visible light absorption
• Absorbs visible light via surface plasmon resonance
Doping of anions • Narrowing of band gap due to mixing of p states of dopants (N,S) with O 2p states in the
valance band of TiO2
• Introduces impurity states above the valence band of titania
• Both states induce visible light absorption
Coupling with semi-conductors • A narrow band gap semiconductor, with appropriate energy levels, absorbs visible light and
transfers excited electrons into the conduction band of titania. UV light source not needed
• Besides visible light activity, effective separation of charge carriers is achieved
Sensitization with light harvesting
components/dyes • Light absorbing components can absorb visible light and inject photo-excited electrons into
the conduction band of titania
• Besides visible light activity, effective separation of charge carriers is achieved
Fig. 1 – Implications of modifiers on the electronic structure of titania.
INDIAN J CHEM, SEC A, SEPT-OCT 2012
1266
band semiconductor (for example CuO with TiO2)
which can be activated by visible light and injecting
the photo-generated electron directly into the
conduction band of titania.8, 9
Sensitization by light
harvesting dyes/macrocyclic ligands (phthalocyanins,
porphyrins) that transfer photo-generated electrons
directly into the conduction band of TiO2, is another
means of using the abundantly available visible light
region in sunlight.8,9
Both strategies have inherent limitations. Coupling
is restricted to those narrow band semiconductors
with band energy levels appropriate for electron
transfer to conduction band of TiO2. In the case of dye
sensitization, stability of dyes and ease of
regenerability are the concerns. In spite of such
limitations, both strategies, especially dye sensitization,
are utilized for several photocatalysts, including TiO2.
Addition of methanol or SO32-
as hole scavengers or
Ag+, Fe
3+ as electron scavengers is another means of
restricting charge recombination and improving the
quantum efficiency. Examples that illustrate the effect
of the modifications in titania are presented in the
following sections.
Doping of metals
Choice of metals/metal ions for doping depends on
several criteria like17
:
• ionic radii to be closer to that of Ti4+
,
• propensity to exhibit multiple oxidation states,
• Mn+
/M(n+1)
energy levels should lie closer to
Ti3+
/Ti4+
,
• possess higher electronegativity than Ti,
partially filled electronic configuration, and,
• capacity to trap and de-trap both e- /h
+.
Effect of metal ion doping on the characteristics of
titania and the improvements observed in its
photocatalytic activity for a variety of reactions has
been studied extensively.3,9,11-14,18-22
A comprehensive
study on the effects of the doping of 21 metal ions
on titania has been reported by Choi et al.17
, who
observed that doping significantly influences
photoreactivity for CHCl3 degradation, charge carrier
recombination rates and interfacial electron-transfer
rates. Doping titania with Fe3+
, Mo5+
, Ru3+
,Os3+
,
Re5+
, V4+
and Rh3+
at 0.1–0.5 at.% significantly
increases the photocatalytic activity for both oxidation
and reduction reactions, while Co3+
and Al3+
doping
decreases the activity. Photoreactivity increases with
the relative concentration of trapped charge carriers
and appears to be a complex function of the dopant
concentration, the energy level of dopants within
the TiO2 lattice, their d electronic configuration,
the distribution of dopants, the electron donor
concentration and the light intensity.
Doping of anions
Though a number of anions from C, N, F, P and
S have been used for doping titania, studies on doping
with nitrogen are predominant.9-16
According to
Asahi et al.22,23
substitutional type of doping is
possible by mixing of the 2p states of oxygen from
titania with the 2p states of N resulting in intra-band
states, which in turn can effectively narrow down the
band gap, thereby facilitating visible light absorption.
Interstitial type doping is found to be ineffective.
In contrast, Valentin et al.24,25
have proposed that
absorption of nitrogen in substitutional state is more
effective in the formation of localized levels within
the band gap and concluded that both substitutional
and interstitial nitrogen may exist with energy levels
as shown in Fig. 3, depending on the preparation
conditions. Two N1s XPS peaks observed for
N-doped titania, one at 396 eV for substitutional and
the other at 400 eV for interstitial nitrogen species,
lend credence to this theory.13
While the assignment
of the XPS peak at 396 eV for susbstitutional
N species has been corroborated by other
researchers,26,27
there is a raging controversy on the
proposed peak due to interstitial N species at 400 eV
and has been attributed to different species like
N-O-Ti-O or O-N-Ti-O.28-29
Fig. 2 – Growth of the ESR signal intensity of the photoformed
Ti3+ active site on Pt-loaded and unloaded on TiO2 catalysts
(record at 77 K). [Reproduced from Ref. 13 with permission from
Elsevier, Amsterdam, The Netherlands].
JEYALAKSHMI et al.: ROLE OF MODIFIERS IN PHOTOREDUCTION OF CO2 BY TiO2-BASED CATALYSTS
1267
It is well-known that the preparation history of
doped titania (type of precursors, level of doping,
method of preparation and pre-treatment) profoundly
influences the type of nitrogen species, characteristics
and activity in visible light and these observations
hold good for titania doped with S,30,31
C,32,33
F34,35
and B36,37
as well. Conflicting views emerge from
different studies on the mode of action, enhancements
in visible light absorption and photocatalytic activity.
Co-doping
An interesting and useful outcome of the extensive
studies on the effects of doping on titania is the
process of co-doping and the synergistic effects
observed therein. Existence of synergistic effects in
metal-non-metal co-doped systems has been
established with sound theoretical basis.38
Long
and English38
have investigated the co-doping effect
of TM metal (TM = W, Re, Os) and X non-metal
(X = N, C) on titania from first principles
calculations. They found that co-doping indeed
narrows the band gap of titania and that (N+ W)
co-doping is highly effective amongst the other
co-doping combinations studied. This theoretical
deduction is in line with the experimental
observations on W doping on N-doped titania.39,40
Co-doping is possible with (metal-metal), (metal-
non-metal) and (non-metal-non-metal) pairs. In
particular, metal and non-metal co-doping could lead
to two distinct effects; first, narrowing of band gap by
forming acceptor and donor levels, and, second,
emergence of two different hetero structures with
respect to titania. While the non-metal could enhance
visible light absorption, the metal can facilitate
the transfer of charge carriers thus retarding
recombination. The critical factors are the choice of
the pair for co-doping, level of doping and effective
method of introducing dopants. Several pairs of
co-dopants, metal-metal,41-43
metal-non-metal44
as
well as non-metal- non-metal45-47
combinations have
been reported to be highly effective due to the
synergistic effects.
Driven by the enormous application potential of
titania based photocatalytic processes for hydrogen
generation by splitting of water and removal of
pollutants in air and water, there has been an
explosive growth of publications dealing with various
strategies for modification, covering comprehensively
the experimental as well as theoretical approaches. A
special issue48
on “Doping and functionalization of
photoactive semiconducting metal oxides” focusing
on major scientific pursuits on modifications in titania
has been published.
Coupling with semiconductors
One of the elegant examples of coupled
semiconductors is the heterostructured Cu2O-TiO2
system illustrated (Fig. 4) by Huang et al.49
Cu2O with
a band gap of 2.0 eV is a typical narrow band
semiconductor with both conduction and valence
bands located above those of TiO2, a state which
thermodynamically favours the transfer of excited
electrons and holes between them. This state of
Fig. 3 – Electronic band structure for (a) substitutional and
(b) interstitial N doped anatase TiO2. [Reproduced from Ref. 24 with
permission from American Chemical Society, Washington, USA].
Fig. 4 – Photogeneration and separation of charge carriers in
Cu2O-TiO2 hetero-structure in UV and visible regions.
[Reproduced from Ref. 49 with permission from Elsevier,
Amsterdam, The Netherlands].
INDIAN J CHEM, SEC A, SEPT-OCT 2012
1268
energy levels facilitates all the four charge transfer
processes indicated in Fig. 4, in UV as well as in
visible regions. While processes 1 and 2 can proceed
with UV as well as visible radiation, processes 3 and
4 require UV radiation. All four processes together
constitute a highly effective coupled semiconductor
system, with very high efficiency. Heterostructures
with 30 % and 70 % Cu2O, both under UV and vis
radiation display 6 times and 27 times higher activity
respectively, compared to the base material, P-25.49
Examples wherein narrow band semiconductors
like PbS, CdS and CdSe are used to form effective
heterostructures with TiO2 are known.50-52
Inherent
drawbacks with such systems are photo-corrosion
and charge carrier recombination. In such cases
electron donors like sulfide or sulfite are added as
hole scavengers. Surface sensitization with dyes
Certain organic dyes can harvest visible light and
inject photo-generated electrons into the conduction
band of titania. Various dyes such as erythrosine B,
eosin, rose bengal, rhodamine B, crestalviolet,
thionine, chlorophyllin, porphyrins, phthalocyanines,
and carbocyanines have been used as sensitizers.53-55
Since the dyes undergo oxidative degradation during
the process, redox couples are often employed to
regenerate the sensitizer. Principles of dye
sensitization have been extensively applied in the
development of dye sensitized solar cells (DSSC).56,57
Plasmonic photocatalysis
Increase in photocatalytic activity for
decomposition of methyl blue under UV radiation by
silver nanoparticles embedded in titania was first
noticed by Awazu et al.58
It was attributed to the
Surface Plasmon Resonance (SPR) displayed by the
Ag nano particles and hence, was termed as
plasmonic catalysis. Besides UV radiation, SPR
provides additional means of excitation in the
visible region. Resonance wavelength depends on
the nanoparticle size and shape.
Nanoparticles of gold also display SPR. Silva et al.59
applied this concept in their studies on photocatalytic
splitting of water using Au nanoparticles supported
on P-25 titania in UV as well as visible light regions
(> 400 nm). It was observed that Au nanoparticles
play a dual role (Fig. 5). With UV light, Au
nanoparticles act as electron traps and facilitate H2
gas evolution. In the visible region, plasmonic
photocatalysis takes place by injection of electrons
into the conduction band of titania.
It was observed that the gold loading, catalyst
preparation and pre-treatment conditions influence the
Au nanoparticle size and activity. Maximum activity
was displayed by 0.2 wt% Au (Au crystallite
size-1.87 nm) loaded on titania. The catalyst was
prepared by precipitation of Au as hydroxide from
HAuCl4 solution using 0.2 M NaOH at pH 9, followed
by calcination at 200 °C.
Chen et al.60
elucidated the effect of SPR (Fig. 6)
by carrying out similar studies on Au/TiO2
using radiation of different wavelengths. Maximum
Fig. 5 – Photocatalytic activity of Au/ TiO2 under (a) UV radiation, and, (b) in visible region due to excitation by surface plasmon band.
[Reproduced from Ref. 59 with permission from American Chemical Society, Washington, USA].
Fig. 6 – Photocatalytic splitting of water by Au/TiO2 enhanced by
SPR effect. [Reproduced from Ref. 60 with permission from
American Chemical Society, Washington, USA].
JEYALAKSHMI et al.: ROLE OF MODIFIERS IN PHOTOREDUCTION OF CO2 BY TiO2-BASED CATALYSTS
1269
hydrogen and oxygen evolution rates (53.75 and
25.87 µM/g of catalyst respectively) were observed
while using UV and visible radiations together
whereas the corresponding yields were less (35.04
and 17.52 µM/g of catalyst) when only UV light
was used. No activity was observed with only
visible light. These observations clearly bring out
the role of SPR in utilization of the full spectrum
of sunlight. Au nanoparticles act as electron traps
in UV region and promote excitation through SPR with
visible light.
Hu et al.61
have added another dimension to the
effect of SPR. With Au nano particles deposited on
TiO2, plasmonic enhancement of the photocatalytic
reduction of CO2 with H2O is observed under visible
light (λ-532 nm). With the same catalyst, when
irradiated with high energy UV light (λ-254 nm),
inter-band (d-band) transitions occur within Au,
resulting in excitation to a level higher than the
conduction band of TiO2. This set of excitons provide
additional means of photocatalytic conversions and
products, thus maximizing overall activity.
Modifications in Physicochemical Properties
Besides the electronic properties, a number of other
characteristics of titania like particle size, bulk and
surface crystal structure, morphology, textural
properties and nano size characteristics affect its
performance. Many of these characteristics are
acquired during the preparation and pre-treatment
stages and this aspect explains the importance of
preparation conditions.
Lattice defects and impurities
Lattice defects and trace impurities can provide
intermediate energy levels within the band gap, thus
facilitating excitations from valence band to
intermediate levels and then from that level to
conduction band, besides direct valence band to
conduction band transitions.62
Pre-treatment processes
adopted for titania e.g., oxidation/reduction, may
induce lattice defects like oxygen vacancies, electron
deficient oxygen, Ti vacancies and interstitial Ti.63-65
Oxygen vacancies and Ti interstitials are known
to generate donor defect levels at 0.75–1.18 and
1.23–1.56 eV below the conduction band of TiO2,
respectively, while Ti vacancies lead to the acceptor
defect levels located above the valence band of
titania.66,67
Oxygen vacancies due to doping can
effectively capture electrons.63-65
Quantum size effects
Photocatalytic reactions include the following major
steps:
• Photo-generation of charge carriers by
excitation,
• Bulk diffusion and interfacial transfer of charge
carriers to surface species, and,
• Reduction/oxidation and further conversion of
surface species
Effective charge transfer to the surface species with
minimum loss due to recombination (life-time of
charge carriers) is an important parameter. While
metal ion doping could reduce the degree of
recombination by electron trapping, titania particle
size could play a different role. Quantum size effect,
applicable to particles < 100 nm, leads to an increase
in the band-gap (resulting in blue-shift), thus
improving the photocatalytic activity. In addition, this
effect induces specific electronic modifications within
TiO2 nanoparticles that ensure close existence of
charge carriers and effective transfer to surface
species.14,68
However, according to Serpone et al.69
size quantization effect may not be observed below a
threshold value.
Surface structure
Observations on quantum size effect on nano sized
titania opened up several approaches towards design
of highly active and dispersed titania catalysts.14
Anchoring of highly dispersed titania on various
inert supports, incorporation within zeolite networks,
supporting on high surface area binary oxides and
incorporation of titania in the zeolite framework
provided the means of exposing titania in different
structural environments on the surface (isolated titania
clusters in tetrahedral/octahedral environments).
Anpo et al.70,71a
observed that at lower Ti content,
small/fine crystallites of TiO2 displayed high activity
for photocatalytic hydrogenolysis of unsaturated
hydrocarbons with water, decomposition of NO into
N2 and O2 and reduction of CO2 with water. Highly
dispersed and isolated TiO2 within the zeolite
framework, exposed in tetrahedral coordination, were
found to be selective for methanol formation during
reduction of CO2 with water. Correlation established
between co-ordination number of titania in zeolites,
(determined by EXAFS analysis) and the selectivity
for methanol formation is shown in Fig. 7.
Influence of surface structure of titania on activity and selectivity during CO2 photoreduction was
INDIAN J CHEM, SEC A, SEPT-OCT 2012
1270
observed earlier by Anpo et al.70,71a
in their studies on single crystal surfaces (100) and (110) of TiO2. While the formation of both methanol and methane could be observed on TiO2 (100), only methanol could be observed on TiO2 (110). Surface density and coordination environment of Ti ions strongly influence the activity/selectivity. Besides, carbon dioxide is known to get adsorbed preferentially on four and five coordinated surface sites.
71a
Our recent investigations on CO2 photoreduction
on different titania catalysts have shown that methane
and methanol are formed on five co-ordinated surface
Ti4+
while on four co-ordinated surface Ti4+
ethanol is
formed by dimerization of methyl and methylene
species.71b
It is pertinent to mention that the kind of
structural features discussed above are acquired
during the preparation of titania,72-75
and hence,
preparation methods/techniques play a vital role in
evolving the surface structures that direct the
transformation of the transient species on the surface
to various products.
Photocatalytic Reduction of CO2 by Water
Process steps
Photocatalytic reduction of carbon dioxide with
water on semiconductor oxide catalyst surfaces using
solar energy to yield fuels/chemicals (methane,
methanol, etc.,) has the potential to become a viable
and sustainable alternative energy source to fossil
fuels.76-79
The process involves two major steps,
(i) splitting of water to yield hydrogen, which in turn
helps in the (ii) reduction of carbon dioxide to
different hydrocarbon products in the second step.
Design of effective catalysts for such a complex
process involving multi-electron transfer steps, holds
the key for the viability of the process.
Ideal catalysts are expected to display maximum
efficiency towards solar energy absorption and
possess requisite band energy levels to drive the redox
reactions on the semi-conducting catalyst surface
(Fig. 8). Functionally, the catalysts should have
valence band top energy level suitable for splitting of
water to generate hydrogen, which is the primary step.
The second and the most crucial step is the
reduction of CO2 to hydrocarbons, which requires the
bottom energy level of the conduction band to be
more negative with respect to reduction potential for
CO2. It is essential that the photo-generated electrons
and holes are spatially separated so that they have a
sufficient life time to initiate redox reactions
on titania surface. In this respect, life time of the
photo-generated electrons is a crucial factor for the
photoreduction of water as well as CO2. A complex
sequence of process steps involving two, four, six
or eight electrons for reduction, lead to the formation
of formic acid/CO, formaldehyde, methanol and
methane respectively,80-81
depending on the type of
the catalyst and reaction conditions.
Thermodynamics and kinetics
Reduction potentials for water splitting and
reduction of CO2 to various reduced products with
Fig. 7 – Correlation between selectivity for methanol formation
and coordination number of titania. [Reproduced from Ref. 13
with permission from Elsevier, Amsterdam, The Netherlands].
Fig. 8 – Conduction band and valence band potentials for
photocatalysts versus energy levels of redox couples in water.
[Reproduced from Ref. 79 from Nature Publishing Group,
London, UK].
JEYALAKSHMI et al.: ROLE OF MODIFIERS IN PHOTOREDUCTION OF CO2 BY TiO2-BASED CATALYSTS
1271
reference to NHE at pH 7 are given below.
Conduction band electrons of titania at Ecb = – 0.5 V
have sufficient energy to reduce CO2 to CH4
(E0
– (CO2/CH4) = –0.24 V). In fact, based on the
reduction potentials, methane formation is a
thermodynamically facile process vis-a-vis hydrogen
evolution E0–(H
+/H2) = –0.41 V. However, hydrogen
evolution is kinetically faster than methane formation,
since eight electrons are involved in the latter process.
Hence, facile generation of photoelectrons with longer
life cycles is the primary criterion.
CO2 + 2H+ + 2e- CO + H2O
2H+ + 2e- H2
CO2 + 1e- CO2.-
CO2 + 6H+ + 6e- CH3OH + H2O
CO2 + 8H+ + 8e- CH4 + 2H2O
CO2 + 4H+ + 4e- HCHO + H2O
CO2 + 4H+ + 4e- C + 2H2O
CO2 + 2H+ + 2e- HCOOH
Eo = -0.41V
Eo = -2 V
Eo = -0.52 V
Eo = -0.20 V
Eo = -0.61 V
Eo = -0.48 V
Eo = -0.38 V
Eo = -0.24 V
Unique features of CO2 photoreduction
Activity for hydrogen generation from water is a
necessary but is not sufficient condition for a catalyst
to be active for CO2 reduction. Conduction band
bottom energy level has to be more negative vis-à-vis
CO2 reduction potential. Besides, metal, metal oxide,
and anionic dopants, structural and textural features,
morphology and addition of sensitizers have different
roles to play in the modification of photophysical
properties and photocatalytic activity. Several
examples to this effect are enumerated in the
following sections.
Role of metal dopants in TiO2 for CO2 photoreduction
Doping of titania with different metals enhances
the activity by minimization of electron-hole
recombination since they act as electron traps and
facilitate CO2 reduction process by photo-generated
electrons. Besides improvement in activity, distinct
selectivity patterns with respect to different metals
have been observed. While the incorporation of
metals like Pd, Pt on TiO2 results in preferential
formation of methane, methanol is the preferred
product on titania doped with Cu oxide. Among the
series of metal doped titania (metal loading of 2 %
w/w on P-25), Ishitani et al.82a
have observed that the
rate of methane formation (values within brackets,
expressed as µM/g cat./h) during CO2 photoreduction
in water decreases in the following order: Pd (32.9) >
Rh (13.3) > Pt (6.7) >Au (4.4) > Cu (2.5) > Ru (0.8).
However, the exact mode of action of these metals/
co-catalysts in terms of their electronic, structural
and surface reactivity characteristics in promoting
methane or methanol formation is yet to be
understood.
Observations by Dzhabiev et al.82b
on CO2
photoreduction (Table 2) on different metals loaded
on TiO2 and SrTiO3 supports prove this point further.
Significant variations in products selectivity is
observed, thereby unravelling the crucial role played
by metals in directing the surface reaction pathways.
Considering the fact that CO2 photoreduction
involves two processes, splitting of water to yield
hydrogen and reduction of CO2 to hydrocarbons, it is
seen from the rate data in Table 2, the rate for
H2 formation is 6-7 times faster on Pt/ TiO2 vis-à-vis
Pt/ SrTiO3. Rh/SrTiO3 is more selective towards H2
formation than Pt/SrTiO3. Other metals like Ag and
Pt-Ru supported on SrTiO3 form CO indicating that
reduction of CO2 is partial, up to CO only. Possibly
the differences in the mode of activation of CO2 on
these metals, modulated with the nature of the
supports, could explain the rate data for various
products. Studies in this direction could lead to
catalysts with higher selectivity for methane.
A series of publications by Solymosi et al.83-85
on the photo-assisted conversion of CO2 + H2O
brings out the effect of modification of TiO2
with WO3, loading of Rh and of oxidation state of Rh
on product formation. Pure TiO2 at 333 K yielded
0.283 µmoles/h of HCOOH and 0.594 µmoles/h of
HCHO with total conversion of 0.877 µmoles/h.
Doping with 0.1 wt% WO3 brings down the
activity (0.404 µmoles/h) with respect to pure TiO2
(0.877 µmoles/h), but selectivity shifts totally to
formaldehyde (0.404 µmoles/h). Increasing WO3
loading to 2 % does not change the activity or
selectivity. Introduction of 1 % Rh on TiO2 lowers
total activity (0.479 µmoles/h), but methanol
Table 2 − Product patterns for photocatalytic reduction of CO2 by
water on metal doped semiconductor oxidesa
Catalyst Ratio of initial rates of product
formation CH4:H2:CO
Pt/TiO2 1.2 : 8.0 : 0.0
Pt/SrTiO3 1.0 :1.0: 0.0
Rh/SrTiO3 1.0 : 250.0 : 0.0
Ag/SrTiO3 1.0 : 7.0:15.0
Pt-Ru/SrTiO3 1.0 : 6.0 : 1.2
aRef. 82b.
INDIAN J CHEM, SEC A, SEPT-OCT 2012
1272
appears as additional product. On TiO2+2WO3
modified support, overall conversion increases to
0.669 µmoles/h. On reduction of Rh to metallic state,
both formaldehyde and formic acid are converted to
methanol, with an associated increase in total
conversion to 0.7µmoles/h. However, when external
hydrogen is used for photo-assisted conversion of
CO2 on Rh/TiO2, CO is formed as the main product in
the oxidized state and methane in the reduced state86
.
Thus the nature of the support and the oxidation state
of metal affect overall conversion and selectivity.
Rh in metallic state facilitates the supply of
photoelectrons towards deeper reduction products.
Methane and methanol are formed when Rh is
in the metallic state and HCOOH and HCHO in
the oxide form.
Ru and Os colloids, in homogeneous phase, with
Ru (bpy)32+
as photo-sensitizer, tri-ethanolamine as electron donor, and N,N'-dimethyl 2,2'-bipyridinium
as charge relay, lead to the conversion of CO2 to
methane, ethylene and hydrogen. However, changing over to Ru(II) tris(bipyrazine) as sensitizer results in
methane, ethylene and ethane but with no evolution of hydrogen.
87 Ligand modifications direct the
selectivity in homogeneous phase.
In the heterogeneous phase, Ru/TiO2 follows a different pathway. Formaldehyde, formic acid and
methanol were observed for Ru loaded on TiO2.88
Ru supported on titania displays higher conversion of
CO2 to methane and methanol compared to bare titania support. However, when Ru is supported on
same titania (10 % w/w) in the dispersed state on high surface area silica, CO2 conversion decreases,
possibly due to the inhibition of Ti-O-Si bonds
formation by Ru.89
Earlier studies on Pt or Pd supported titania were
carried out with aqueous carbonate/bicarbonate media
which resulted in formaldehyde and formic acid
respectively.90, 91
Anpo and co-workers92
established
the crucial role of Pt in increasing CO2 conversion
and selectivity towards methane formation (Fig. 9).
Dispersion of titania on zeolite-Y resulted in the
formation of isolated TiO2 with Ti in tetrahedral
coordination which facilitates selective formation of
methanol, while Ti in octahedral coordination leads to
methane formation. Doping with Pt invariably results
in methane formation. Zhang et al.93
also observed
that Pt loaded on to nano sized titania powder and
titania nanotubes yielded exclusively methane (Fig. 10).
Pd loading (0.5 wt%) on titania, which has been made
free of any adsorbed hydrocarbon impurities, leads to
mainly methane, with a little CO. Prolonged
irradiation caused deactivation of the catalyst due to
oxidation of Pd to PdO.94
Nature of the support influences surface reaction
pathways of Pd catalysts. Pd when supported on
titania, whose acid-base properties are modified by
addition of Al2O3, SiO2 and MgO, yields different
products.95
Acidic supports yield C1 products, (CH4 and
CH3OH), while basic supports yield C2 hydrocarbons
(C2H6, C2H5OH, CH3COCH3). Sasikala et al.96
have
also reported that titania when dispersed on zeolites
(NaY) and zirconia displays better photocatalytic
activity (for hydrogen generation from water) as
Fig. 9 – Products distribution of the photocatalytic reduction of
CO2 with H2O on the (a) anatase TiO2 powder, (b) the imp-Ti-
oxide/Y-zeolite (10.0 wt% as TiO2), (c) the imp-Ti-oxide/
Y-zeolite (1.0wt % as TiO2), (d) the ex-Ti-oxide/Y-zeolite,
(e) the Pt-loaded ex-Ti-oxide/Y zeolite catalysts. [Reproduced
from Ref. 92 with permission from American Chemical Society,
Washington, USA].
Fig. 10 – Effect of Pt metal content in Pt/TO-NP on methane yield
for photocatalytic reduction of CO2 after 7 h irradiation at 323 K.
(H2O/CO2= 0.02). [Reproduced from Ref. 93 with permission
from Elsevier, Amsterdam, The Netherlands].
JEYALAKSHMI et al.: ROLE OF MODIFIERS IN PHOTOREDUCTION OF CO2 BY TiO2-BASED CATALYSTS
1273
compared to dispersion on ceria or alumina. The
acidity of the support also plays a key role.
Promotion with metals like Pt,93
Ru,89
Rh,27
and
Ag97
vastly enhances the rate in several ways, e.g.,
charge separation, retarding re-combination and
trapping of charge carriers, besides activation of CO2
and water reduction, thus facilitating further surface
transformations leading to hydrocarbon products.
Doping of titania (prepared by sol-gel inverse micelle
method) with silver increases CO2 photoreduction
(Fig. 11), leading to increase in the formation of
methane and methanol.97
Upto Ag loading of 5 %,
impurity levels formed within the band gap of
titania extend the light absorption range8, and beyond
this level, Ag forms metal clusters that retard
electron-hole recombination via electron trapping.
Electron transfer from conduction band of TiO2 to the
metallic silver particles could take place at the
interface since the Fermi level of TiO2 is higher than
that of silver metal.98
Such a phenomenon results in
the formation of Schottky barrier at the Ag-TiO2
contact region, which increases the life of charge
carriers and hence improvement in the photocatalytic
activity.99
Similarly, AgBr/TiO2 nanocomposites also
display very high activity for CO2 photoreduction
under visible light.100
A unique feature with AgBr
promoted titania catalyst is that besides methane and
methanol, ethanol is also observed in measurable
quantities. While ethanol is one of the minor products
with the titania support, loading with Ag significantly
increases the formation ethanol. Formation of ethanol
is expected to proceed by dimerization of transient
surface C1 species, but mechanistic pathways to
this end is yet to emerge. Using optical fibre
photo-reactor, Wu101
could give a methanol yield
of 4.12 µmoles/g cat./h, with 1 % Ag/TiO2 catalyst
under UV light intensity of 10 W/cm2.
Modifications with metal oxides
As discussed vide supra, coupling of metal oxides
that possess electronic band energy levels suitable for
charge transfer, with TiO2 is one of the options
available for increasing its photocatalytic activity.
Copper oxide for coupling with TiO2 has been
investigated by several authors. Use of Cu (as oxides)
as a co-catalyst was reported by Adachi et al.102
wherein Cu (≤ 5 wt%) loaded TiO2 powder was
suspended in a CO2 pressurized (to 50 kg/cm2
initially) aqueous solution at ambient temperature.
Under Xe lamp illumination, methane, ethylene and
ethane are formed with attendant decrease in pressure
to 28 kg/cm2. Dimerization of surface
·CH2 species
was proposed as the route for the formation of
ethylene. However other investigators observed
methanol as the major product when the reaction was
carried out at atmospheric pressure.
Tseng et al.103-105
studied the effect of copper
loading on titania prepared by sol-gel technique. With
an optimum Cu loading of 2 wt%, methanol yield of
118 µmoles/g cat. was observed after 6 h of UV
illumination. It was proposed that the redistribution of
the electric charge and the Schotky barrier of Cu and
TiO2 facilitates electron trapping via supported Cu.
Lowering of the combination probability for hole-
electron pairs helped in increasing the activity of
Cu/TiO2 (quantum efficiency of 10 % and energy
efficiency of 2.5 %) versus bare TiO2. Based on XPS
analysis, Cu2O was identified as the active phase and
reduction of the oxide to metallic Cu affected the
activity adversely. Slamet et al.106
in their studies of
Cu/TiO2(P-25) observed 3 wt% Cu loading to be
optimum and could achieve very high activity, with
2655 µ mol/g of methanol after 6 h of irradiation
compared to 809 µmol/g of methanol on bare
P-25 TiO2. While Tseng et al.103-105
used TiO2
(anatase) prepared by sol-gel route and 0.2 N aqueous
NaOH as the reaction medium, Slamet et al.106
employed P-25 titania (mixture of anatase and rutile)
Fig. 11 – Dependence of product yields on Ag content in Ag/TiO2
(after 24 hrs). [Reproduced from Ref. 97 with permission from
Elsevier, Amsterdam].
INDIAN J CHEM, SEC A, SEPT-OCT 2012
1274
and 1 M KHCO3 as the medium. To what extent these
factors affect the CO2 photoreduction is not clear.
Besides, Slamet et al.103-106
suggest that CuO may be
the more active dopant as compared to Cu2O, based
on the redox values for Cu2+
and Cu+ and the
interaction of Cu species with TiO2 lattice.
Cu2+
+2e- Cu
0 E
0 = 0.34 V
Cu2+
+ e- Cu
+ E
0 = 0.17 V
Cu+ + e
- Cu
0 E
0 = 0.52 V
With the highest value of redox potential, Cu+
should be more effective for trapping of electrons so
as to avoid recombination. For the same reason,
interaction of Cu+
with TiO2 lattice would be stronger
and, hence, electrons are not easily transferred to the
conduction band of TiO2. Therefore, Cu2O in TiO2
could become a centre for recombination and CuO
with lower redox potential could become a more
effective dopant.
According to Roy et al.76
location of the band edge
levels of the oxides TiO2, Cu2O and CuO, could
promote highly facile charge transfer between the
oxide phases. Figure 12 shows the band edge positions
of Cu oxides and that of TiO2 in contact with water
and CO2. Both CuO (1.6 eV) and Cu2O (2.4 eV) could
couple with TiO2 and transfer photo-electrons
with UV as well as visible radiations, in line with
coupling of the electronic energy levels envisaged by
Huang et al.49
as shown in Fig. 4.
During CO2 photoreduction, due to the presence of
H2 it is possible that CuO gets reduced, though the
extent of reduction is not known. In fact, Li et al.107
in
their investigations on Cu/TiO2 dispersed on
mesoporous silica have observed that during CO2
photoreduction the colour of the catalyst changes
from very light green to dark grey, possibly due to
reduction which also signified the onset of
deactivation. On exposure to atmospheric oxygen the
catalyst readily undergoes re-oxidation. Thus, the
exact nature Cu oxide phase during reaction needs to
be identified by suitable experimental methods. Synergistic effects due to co-doping
Doping of two metals on titania results in higher
hydrocarbon yield as well as changes in product
patterns. Varghese et al.108
observed that on nitrogen
doped titania nanotube arrays, introduction of Cu
yields C1-C4 hydrocarbons up to 104 ppm/cm2/h,
and addition of Pt increases the hydrocarbon yields to
116 ppm/cm2/h. However, the product patterns for
individual doping were different. Hydrocarbons and
CO were predominant with Cu, while with Pt,
hydrogen yields were higher.
With Cu and Fe (at −0.5 wt% each) dispersed on
TiO2-SiO2, CO2 photoreduction with UV radiation
yields methane and ethylene, while the bare support
TiO2-SiO2 yields only methane.101
This could be due
to the fact that the reduction potential for methane is
lower than that of ethylene. Synergistic effect of Cu
and Fe brings out the formation of methane and
ethylene. Wavelength of the radiation is an additional
factor that affects the product pattern. Only methane
is formed with the same Cu-Fe bimetallic system
under natural sunlight (with negligible UV light).
Thus the effect of metal doping on titania for CO2
photoreduction appears to be complex, with no clear
correlations established between the properties of the
dopants and activity/selectivity/product patterns.
Variations in metal dopant compositions, (mono,
bi-metallic, metal-non-metal) type of radiations used
and other experimental conditions adopted make it
difficult to arrive at meaningful correlations. Detailed
understanding of the activation of CO2 and reaction
pathways with different metals could help in arriving
at the most efficient metal/pair of metals or metal-
non-metal combinations. Modes of activation of CO2
by different metals could play a key role in
activity/selectivity improvements.
Anions as dopants on TiO2 for CO2 photoreduction
Nitrogen and carbon constitute the most effective
anion dopants on titania for CO2 photoreduction.
Fig. 12 – Band-edge positions of CuO, Cu2O and anatase TiO2 in
contact with water vapor and CO2. [Reproduced from Ref. 76 with
permission from American Chemical Society, Washington, USA].
JEYALAKSHMI et al.: ROLE OF MODIFIERS IN PHOTOREDUCTION OF CO2 BY TiO2-BASED CATALYSTS
1275
Varghese et al.108
introduced nitrogen into titania
nanotube arrays during its preparation by anodic
oxidation process and subsequent annealing at
600 °C. Nitrogen doping with N2p states located
above the valence band of titania shifted light
adsorption end into the visible range, to 540 nm.
Fan et al.109
introduced nitrogen at various levels of
doping using urea as the nitrogen precursor by sol-gel
route, during controlled hydrolysis of titanium
tetrabutoxide in n-butanol medium. Ni2+
, with nickel
nitrate as precursor, was incorporated as co-dopant
during the sol-gel process itself. Synergistic effect of
Ni and N co-doping shifted the light absorption edge
to 430 nm. With a light source at 365 nm, doping with
4 wt% N and 6 % Ni was found to be optimum
as revealed by the methanol yield. Methanol yield of
3 µl/g with bare titania increased to 167.6 µl/g with
4 wt% N doping and further to 253.5 µl/g with
co-doping of 6 wt% Ni. While it is clear that
N doping does help in extending the light absorption
edge, it is also expected that in the process, trap states,
i.e., Ti3+
, could be created leading to charge
recombination.110,111
Sulfur (4−5 wt%) doped titania samples with
particle size in the range 8−12 nm have been reported
to be very active for the formation of methanol,
ethanol and propanol.112
Increase in S doping from
4 to 5.2 % decreases the band gap from 3.31 to 3.25 eV.
XPS peak at 160−161 eV indicates the presence of
S2−
. It is suggested that S-doping and the optimum
particle size enhance the photocatalytic activity.
Enhancement of photocatalytic activity of titania
by carbon based nanomaterials has been studied
extensively.113,114
The survey by Leary and
Westwood115
covers comprehensively the details and
recent progress in the application of TiO2-carbon
nanocomposites, using carbon in different forms, e.g.,
activated carbon, carbon doping, carbon nanotubes,
fullerenes, graphene, thin layer carbon coating,
nanometric carbon black and carbon in different
morphologies. Of the many non-metal dopants,
carbon, especially in the nanosize, has the unique
ability to circumvent the two major drawbacks with
titania, by extending the light absorption edge to
visible range and facilitating charge carrier separation.
Several other benefits could be realized115
,
e.g., synthesis of carbon based nanomaterials with
well-defined and controllable structure and electrical
properties for facile charge transport. Their high
surface area ensures availability of a large number of
active sites for adsorption of reactants and promotion
of forward transformation of surface species. It is
possibile to incorporate different functional groups on
carbon, adding a new dimension to its application. An
intimate contact between titania and carbon
nanomaterials leads to creation of a number of hetero
junctions, ensuring efficient charge transfer to the
surface species.
While the band gap tuning by carbon doping in
titania has been established by several experimental
observations as well as theoretical calculations
(Table 2 in Ref. 115), it is not clear whether it exists
in the lattice as substitutional anion or interstitial
cation116
. DFT calculations by Valentin et al.117
have
shown that at low oxygen concentration, carbon
substitutes for oxygen in TiO2 lattice with the
formation of oxygen vacancies, while under oxygen
rich conditions C atoms occupy the interstitial sites.
Along with variations in band gap, several
intermediate states are induced.
Most of the applications of C-doped titania
pertain to removal of pollutants, as exemplified by
its activity for degradation of model pollutant
compounds/dyes/indicators.115
Results on the effects
of co-doping of other anions (N and S) with carbon
are controversial. Increase in methylene blue
degradation has been observed for C and N co-doped
titania, which is attributed to a synergistic effect
resulting from band-gap narrowing caused by
N substituting for oxygen and surface carbon species
acting as a photo-sensitizer.118
With C-S co-doping,
no improvement in activity vis-à-vis TiO2 for
methanol degradation could be observed by
Sun et al.119
though earlier investigations by
Tachikawa et al.120
showed a negative effect.
Carbon nanotubes (CNTs) possess several
characteristics essential for photocatalytic activity,
like high-surface area, high concentration of active
sites, facile transport of photo-generated electrons
leading to minimization of electron-hole recombination
and simultaneously facilitating visible light catalysis
by modification of band-gap and/or sensitization.121,122
These functions arise through the effective synergistic
interactions between the electronic levels of titania
and CNT. CNT can (a) act as a sink for electrons
(storage capacity of one electron for every 32 C
atoms) from the conduction band of titania and inhibit
recombination, (b) function as a photo-sensitizer by
injecting electrons/holes into conduction/valence band
of titania, depending upon the respective energy
INDIAN J CHEM, SEC A, SEPT-OCT 2012
1276
levels, and, (c) create impurity levels thus promoting
activation by visible light.
Morphology of CNT, especially the aspect ratio,
loading of CNT and the degree of dispersion of TiO2
on CNT, as governed by the method of preparation of
TiO2-CNT composites, are the factors that define the
mode and extent of activity enhancement.
Combined effect of carbon doping and its
morphology in TiO2-xCx carbon nanotube array in
improving the activity for photochemical splitting of
water was established by Park et al.123
Similar
improvements in activity of nanocomposites of
multi-walled carbon nanotubes (MWCNT) supported
on titania have been observed for water splitting
as well as CO2 photoreduction with water.124-126
[60]-Fullerenes are well-known for their unique
electronic properties, which make them interesting
materials for applications in photocatalysis.127-130
Fullerenes absorb light in the visible as well as UV
regions. In their excited state, they are good electron
donors as well as good electron acceptors, capable of
accepting as many as six electrons.131-132
Since the
excited state of fullerene lies at −0.2 eV (vs NHE) and
the conduction band of titania at −0.5 eV, electrons
may be transferred from titania to fullerene, leading
to the formation of radical [60]-fullerene anion
C60−. This anion may then react with species adsorbed
at an interface between [60]-fullerene and TiO2,
resulting in reduced products. Under certain conditions,
fullerene can also act as acceptor as observed by
Kamat et al.133,134
and Krishna et al.135, 136
Water
soluble polyhydoxy fullerenes (PHF) adsorbed on
titania was found to be very effective by Krishna et al.
in terms of electron transfer between PHF and
TiO2 and consequent enhancement of activity for
degradation of procion red dye and degradation of
E. coli. Several other examples of enhancement of
activity by TiO2-fullerene[60] composites have been
cited by Leary and Westwood115
in their review,
which pertain to degradation of dyes/pollutants. Some
of the unique characteristics of TiO2-fullerene
[60]composites (fullerene acting as donor or acceptor
of electrons) are yet to be explored for hydrogen
generation from water and CO2 photoreduction.
Graphene is emerging as a material with vast
application potential due to its strikingly different
properties like very high surface area (~ 2600 m2/g for
exfoliated graphene) on which the electrons can
travel without scattering at mobilities exceeding
~15,000 m2 V
-1 s
-1 at room temperature. In this sense
it is potentially an ideal electron sink and acts as
electron transfer-bridge. Graphene has a two
dimensional one-atom-thick structure and its surface
properties can be tuned via chemical modification.
Graphene oxides (GO) are derived by partial
chemical oxidation of graphene which expose
carboxylic groups on the surface. Titania nanoparticles
can be readily anchored to GO by the carboxylic
groups. GO containing anchored TiO2 can be
dispersed in ethanol. Upon UV irradiation, a part of
the photoelectrons generated reduce GO to some
extent to yield reduced graphene oxide (RGO) and the
remaining electrons are retained by GO matrix.
Removal of solvent yields photoreduced graphene-
TiO2 composite, which is useful for photocatalytic
processes. Kamat et al.137-138
have demonstrated
anchoring of TiO2 and Pt separately on RGO
nanosheet which acts as a conduit for shuttling of
electrons from titania to Pt site where gas evolution
could take place. Such a scheme (Fig. 13) would be
ideal for solar water splitting process.
Many examples of graphene-TiO2 composites
displaying better activity as compared to bare TiO2
for photocatalytic degradation of organic pollutants
are known115
. Graphene obtained by solvent
exfoliation (SEG) is expected to be relatively free
from structural as well as chemical defects compared
to RGO.139-141
Liang et al.142
have prepared two
different TiO2-graphene nanocomposites, one by
solvent exfoliation in DMF (SEG) and the other by
solvent reduced graphene oxide (SRGO) by thermal
reduction process. Both nanocomposites have been
characterized by Raman spectroscopy, four point
probe sheet resistance measurement and UV-vis-NIR
optical absorbance spectroscopy. Sheet resistance of
SRGO films was ~ 2.4 times higher than that of SEG
films, indicating higher electrical mobility in SEG
which is expected to facilitate the diffusion of
photoelectrons. Both composites were stabilized by
ethyl cellulose and then studied for photocatalytic
Fig. 13 – Shuttling of electrons from TiO2 site to Pt site via
reduced grapheme nanomat. [Reproduced from Ref. 137 with
permission from American Chemical Society, Washington, USA].
JEYALAKSHMI et al.: ROLE OF MODIFIERS IN PHOTOREDUCTION OF CO2 BY TiO2-BASED CATALYSTS
1277
activity for oxidation of acetaldehyde and reduction of
CO2 with water. SEG composite films exhibited
around 2-fold and 7-fold increase in activity for
photo-oxidation of acetaldehyde and photoreduction
of CO2 respectively as compared to TiO2 alone under
visible radiation. No significant activity enhancement
was observed with SRGO-TiO2 nanocomposite. Higher
activity with SEG nanocomposite is attributed to the
higher electrical mobility and facile photoelectron
transport on less defective SEG surface. Applications
of graphene based composites as photoactive supports
are worthy of further investigation.
Functionalized carbon nanoparticles could be used
for harvesting photons from visible light. Cao et al.143
have observed that a two-step surface functionalization
of carbon nanoparticles (< 10 nm) by first refluxing
with aqueous nitric acid (2.6 M) followed by
treatment with oligomeric poly (ethylene glycol)
diamine yields finely dispersed carbon nanoparticles
in water.
When Au is deposited on suspended nanoparticles
by photolysis under UV radiation, UV absorption
maxima around 550 nm is observed. Similarly,
Pt could be deposited by photolysis. Under UV
radiation carbon nanoparticles act as electron
donors facilitating the reduction of Au/Pt salts.
Functionalized carbon nanoparticles with Pt/Au
could then be used as photocatalysts for reduction
of CO2 in the most desired visible region (425-720 nm)
yielding formic acid as the reduction product. Besides,
these catalysts were also active for generation of
hydrogen from water. Thus, titania-carbon composites
in several configurations continue to receive attention
as catalysts for photoreduction of CO2.
Modifications in physical properties of titania
The methods/approaches described above aim to
improve photocatalytic activity of titania through
chemical methods. Besides the control of particle size,
several other physical properties like morphology
(nanosheets, nanobelts, foams) and pore structure
(ordered mesoporous sturcture) and spatial structure
(nano tubes) offer several advantages in terms of
high surface area, easy transport of charge carriers
and effective charge separation which ultimately
contribute towards improving photocatalytic activity.
Aprile et al.144
have reviewed the effect of “spatial
structuring” or controlling nano particles on length
scale and encapsulation in nano size cavities of
zeolites on photo catalytic activity.
Encapsulation of titania nano clusters within the
zeolite cavities with precise size control leads to
significant improvements in photocatalytic activity
and these aspects have been extensively covered by
Anpo and co-workers145-153
for NOx and CO2 fixation.
Further improvements in catalyst design has been
reported by Bossmann and co-workers,154
who have
encapsulated light harvesting component, Ru(bipy)22+
,
along with titania so that photoelectrons from the
complex could be transferred to the conduction
band of TiO2. In this case TiO2 acts as relay,
transferring electron to the external Co macrocyclic
Co (dphen)33+
unit.
Titania based catalysts systems with both chemical
as well as physical modifications, i.e., encapsulation
and doping of anions/metals have been designed and
explored for different photocatalytic applications.155-157
Ordered meso-porous titania with a large surface
area and pore volume and nano-sized titania dispersed
on mesoporous silica matrix have been studied
extensively as catalysts for photocatalytic reduction of
CO2 with H2O.89,107,158
The mesoporous structure is
expected to facilitate fast electron transfer within the
matrix and retard charge recombination, both of
which lead to increased photocatalytic
activity.145,147,159-161
One-pot sol-gel method107
and
evaporation driven self-assembly route in a furnace
aerosol reactor (FuAR)158
have been adopted to
prepare Cu-TiO2-SiO2 catalysts for photoreduction of
CO2 with methane and CO as products. Catalyst
preparation technique adopted has resulted in very
high dispersion as well as synergy between the active
phases, TiO2 and Cu oxide on mesoporous silica
matrix, thereby increasing the rate for photocatalytic
reduction of CO2 to CO and methane.
As shown in Table 3, Li et al.107
observed a distinct
synergy effect between the components when the
Cu-TiO2-SiO2 prepared by the one-pot sol-gel method is
used for CO2 photoreduction. While the nominal effects
of dispersion of TiO2 on SiO2 (increase in CO formed
from 8.1 to 22.7 µmol/g TiO2/h) and Cu loading on TiO2
Table 3 − CO and CH4 formation on Cu/TiO2 supported on
mesoprous silica catalysts – Synergy effecta
Catalyst CO formation
(µmol/g TiO2/hr)
CH4 formation
(µmol/g TiO2/hr)
TiO2 8.1 -
TiO2-SiO2 22.7 -
4% Cu/TiO2 11.8 1.8
0.5% Cu/TiO2-SiO2 60.0 10 aRef. 107.
INDIAN J CHEM, SEC A, SEPT-OCT 2012
1278
(increase in CO formed from 8.1 to 11.8 µmol/g TiO2/h
and additional methane formation of 1.8µmol/g TiO2/h)
could be observed separately, synergistic effect in the
composite catalyst is observed by significant increase in
CO (60 µmol/gTiO2/h) and methane formation
(10 µmol/gTiO2/h). Specific catalyst preparation
techniques thus facilitate the positive interaction
between the components of the composite leading to
improved performance.
Titania nanotubes possess several features that
make them interesting materials for photocatalysis.162
Compared to spherical particles, one-dimensional
nanotubes provide a high surface area and a high
interfacial charge transfer rate. Free movement of
charge carriers across the length of the nanotube is
expected to reduce the e−/h
+ recombination probability.
Time-resolved diffuse-reflectance spectroscopy163
has
shown that charge recombination is disfavored by
the tubular morphology of the titania.
It is observed that the half-life of photo-generated
hole, τ1/2, as measured by laser flash photolysis is the
longest with titania nanotube, at 3.5 ± 0.4 µs, as
compared to 0.6−1.0 µs observed for spherical
nanoparticles of TiO2. Nanotubes with a higher aspect
ratio facilitate longer diffusion length of charge
carriers and better charge separation.
According to Roy et al.76
in vertically oriented TiO2 nanotubes, both inner and outer walls consist of anatase crystallites that are made available to the reactants which results in enhanced activity. The hole diffusion length in titania
164 is reported to be ~10 nm,
while electron diffusion length165
is ~100 µm and half of nanotube wall thickness is ~10 nm. Such dimensional features ensure facile contact between the surface species and charge carriers. Since half of nanotube wall thickness is less than or comparable to minority charge carrier diffusion length, both electrons and holes are generated very near to the interface between tube surface-reactant-gas. Thus, all charge carriers formed deep inside the nanotube walls reach the interface by diffusion and those created near the interface are readily available for the reactants. Such dimensional advantages at nanoscale could accelerate CO2 conversion. Similar advantages for Pt supported on titania nanotubes has been reported by Zhang et al.
93 and by Xia et al.
125 for titania-MWCNT
composites for CO2 photoreduction with water. Nano sized hollow titania spheres with large surface area and porosity could be another potentially useful material for different photocatalytic conversions.
166
Methods for synthesis of nano sized titania in various
morphological modifications have been covered extensively by Chen and Mao.
167
Sensitization of titania by macrocyclic ligands
Macrocyclic ligands like phthalocyanins (Pc) and
porphyrins (Pr) are good candidates for sensitization
of titania because of their high absorption coefficient
in the solar spectrum, especially in visible region
and good chemical stability. (Pr)-titania composites
display excellent activity for photocatalytic
degradation of Rhodamin-B under visible light, while
titania incorporated with Fe-porphyrin are active in
the UV range.168
Since photoelectrons are directly
transferred to the conduction band of titania, charge
carrier separation is very effective. Zhihuan et al.169
and Shaohua et al.170
observed that phthalocyanins
containing Zn and Co prepared by sol-gel technique
were active for photocatalytic reduction of CO2, but
the yields were low.169,170
Zhao et al.171
reported that
in situ synthesis of CoPc/TiO2 (Fig. 14) results in
highly efficient photocatalysts for CO2 reduction
yielding formic acid, methanol and formaldehyde
in an aqueous solution using visible light irradiation.
In situ synthesis route results in the formation of
isolated CoPc species (within the confined space of
mesoporous titania), which effectively absorbs visible
light enabling ultrafast injection of electrons from the
excited state into the conduction band of the titania
support. This helps charge carrier separation and
increase in photo-conversion efficiency.171
Since the
reported band gap for TiO2 is 3.22 eV versus 2.14 eV
for CoPc, such a sensitization process is highly
feasible. CO2 photoreduction data presented in
Table 4 show that in situ CoPc/TiO2 catalyst is more
Fig. 14 – Flow diagram for the preparation of in situ CoPc/ TiO2.
[Reproduced from Ref. 171 with permission from Elsevier,
Amsterdam, The Netherlands].
JEYALAKSHMI et al.: ROLE OF MODIFIERS IN PHOTOREDUCTION OF CO2 BY TiO2-BASED CATALYSTS
1279
active than a simple mechanical mixture of CoPc
and TiO2, indicating a cooperative effect between
dispersed CoPc and titania surface for the effective
transfer of photogenerated electrons. Compositing
titania with macrocyclic ligands has emerged as one
of the effective means of increasing photocatalytic
reduction of CO2 on titania surface.
Future Trends
Titania and modified titania formulations continue
to be the preferred catalysts for photoreduction of CO2
and other photocatalytic applications. Though the
improvements achieved by various modifications are
moderate, several useful concepts and correlations have
emerged. Photophysical properties of titania originate
during the preparation/pre-treatment conditions and
hence preparation methods have attained tremendous
importance.71b
When used as support, properties of
titania affect the reducibility and dispersion of the
active phase like CuO, induce electronic level changes
and affect the catalytic performance.172
Limitations in
light absorption efficiency of titania could be overcome
by the choice of visible light active mixed oxides of In,
Nb and Ta,173,174
besides the modifications described
in this review. Alkali metal tantalates like NaTaO3
promoted by La with NiO as co-catalysts, display
significant activity for CO2 photoreduction with
water, forming methanol and ethanol as major
products.175
Detailed investigations in the following
aspects would be helpful in arriving at catalysts with
improved performance:
1 Modes of adsorption and activation of CO2 on
metals, metal oxides, which act as co-catalysts–
experimental as well as theoretical approaches;
2 Use of different co-catalysts, single as well as
bi-component systems;
3 Exploring the application of TiO2-nanocarbon
composites with graphene, fullerene and carbon
nanotubes, etc.;
4 Elucidation of the mechanistic pathways for the
transformation of transient surface species and
formation of C2 and higher carbon number
products;
5 Modes of deactivation of catalysts and
possibilities for regeneration.
It is expected that the use of photoelectrochemical
cells (PEC) for conversion of CO2 to methanol176, 177
would add another dimension to the process to get
improved yields of hydrocarbons.
Recent advances in these directions on
photocatalytic conversion of CO2 to fuels have been
compiled by Izumi178
, wherein the necessity of
ensuring that the source of carbon for the
hydrocarbons formed originates from the CO2 used
and not from the carbon containing impurities
gathered on the catalyst surface by various means
like residual organic templates, and solvents used
during preparation is highlighted and strategies for
recycling of sacrificial donor and direct supply of
protons electrons released from water oxidation
photocatalysts have been demonstrated for the
photoreduction of CO2.
Application of 13
CO2 isotopic labelling and
in situ FT-IR spectroscopic studies on surface
transformations, backed by mass spectroscopic
analysis of the intermediates and products to this end,
have helped to unravel the reaction path for CO2
photoreduction. This has established unequivocally
that the hydrocarbons do originate from the reactant
CO2. However, according to Izumi178
the mechanism
for the preferred formation of methane on TiO2 from
CO2 + H2O is yet unclear, though pathways based on
surface carbon species179
, viz., CO and formyl group
species, have been proposed101,180,181
backed by
several in situ surface spectroscopic techniques.
Summary and Conclusions
As enumerated above, titania, subjected to a wide
range of modifications, in the form of titania based
composites and in different morphological forms has
been explored to enhance the activity for
photocatalytic reduction of CO2 with water with
respect to pristine titania. Primary objectives behind
the modifications, namely, promoting visible light
activity, retarding the recombination of charge
carriers by effective physical separation (by doping
with metals, anions and cations), facilitating their
transport through titania surface, isolation of titania
sites by dispersion on high surface area supports
Table 4 − Product profile for photoreduction of CO2 on
TiO2-CoPc systema
Catalyst Product yield (µmoles/g catalyst)
HCOOH CH3OH HCHO Total organic
carbon
TiO2 221 - - 221
1wt%CoPc/TiO2 450.6 12.1 38.5 501.2
0.7% in situ
CoPc/TiO2
1487 93 134.3 1714
aData taken from Ref. 171 with permission from Elsevier,
Amsterdam, The Netherlands.
INDIAN J CHEM, SEC A, SEPT-OCT 2012
1280
and incorporation of suitable active elements to
bring about the required redox reactions, have been
realized to a significant extent. However, the
expected improvement in activity has been moderate
(an increase by 2-4 times), which is too low for any
possible practical applications. Multiple and complex
surface reaction pathways that involve several carbon
containing ion radicals, render the selective formation
of methane or methanol a difficult task. Further,
decomposition of the products and promotion of
backward reactions also contribute towards lower
yields. Catalyst deactivation proceeds through the
formation of carbonaceous species on the surface.
This implies that the metal function responsible for
the hydrogenation of carbonaceous species needs to
be improved though availability of hydrogen via
water splitting may not be the issue. Indepth
investigations on the surface reaction pathways by
in situ spectroscopic methods, supported by sound
theoretical studies on the activation and surface
transformations of CO2 and other aspects for
investigation, as detailed in the earlier section, will be
helpful in controlling deactivation and achieving
higher conversions.
Acknowledgement
The authors gratefully acknowledge the
Department of Science and Technology, New Delhi,
Govt. of India, for the grant towards establishing
NCCR at IIT Madras, Chennai, India and
M/s Hindustan Petroleum Corporation Limited,
Mumbai, India, for funding the project on
photocatalytic conversion of CO2.
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