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: bvnathan@iitm.ac.in

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

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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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155 Yamashita H, Takeuchi M & Anpo M, Encyclop Nanosci

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161 Ogawa M, Ikeue K & Anpo M, Chem Mater, 13 (2001)

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