Materials Sciences and Applicatio ns, 2011, 2, 1407-1415
doi:10.4236/msa.2011.210190 Published Online October 2011 (http://www.SciRP.org/journal/msa)
1
Conversion of Carbon Dioxide to Methanol Using
Solar Energy—A Brief Review
Ibram Ganesh
Centre for Photoelectrochemical (PEC) Cells and Advanced Ceramics, International Advanced Research Centre for Powder Metal-
lurgy and New Materials (ARCI), Hyderabad, India.
Received February 3rd, 2011; revised March 23rd, 2011; accepted April 2nd, 2011.
ABSTRACT
This article presents a meticu lous and comprehensive review of literature reported on conversion of carbon dio xide, a
green house gas into methanol or to any other value added chemical following various routes including catalytic, ther-
mal, biological, electrochemical and photoelectrochemical (PEC). More emphasis is given on conversion of carbon
dioxide to methanol using solar energy (i.e., artificial photosynthesis) as this process can tackle the human generated
two pressing problems, i.e., “global warming” and “energy crisis” today world is facing. It also covers information on
various materials required for designing and development of reliable PEC cells for conversion of carbon dioxide to
more value added chemicals including methanol. Finally, it also provides the scope for the future research on this topic
with adequate literature support.
Keywords: Solar Energy, Carbon Dioxide, Methanol, Photoelectrochemical Cells
1. Introduction
Recently, the selective catalytic reduction of carbon di-
oxide (CO2) to value added chemicals using solar energy
has received a great deal of attention from the scientific
community as it can in deed solve two major problems,
i.e., “Global Warming” [1-4] and “Energy Crisis” [5-9]
today world is facing. Atmospheric CO2 resulting from
burning of fossil fuels to meet energy needs of human
beings is mainly responsible for global warming that is
posing severe problems to both ecology and human be-
ings. Recently, the Intergovernmental Panel on Climate
Change (IPCC) has also concluded that the increasing
CO2 concentrations resulting from fossil fuel burning and
deforestation are responsible for most of the observed
global warming [2]. Most national governments have
signed and ratified the Kyoto Protocol [3] of the United
Nations Framework Convention on Climate Change
aimed at reducing greenhouse gas emissions [4]. On the
other hand, today 90% of the world’s primary energy
requirement is met by fossil fuels and for at least next
few decades these fossil fuels are going to be the only
energy resources as alternative reliable renewable energy
resources are yet to be fully realized. The world reserve
of natural gas is estimated as about 1014 m
3; this has a
significant impact on the world’s energy balance. As per
an estimation, at the current rate of use, these reserves
will last for 60 years, compared with 30 years for crude
oil reserves [1-4]. On the other hand, self sufficiency in
energy is a stated national goal for all the countries. Most
of the proposed means to achieve this goal are either en-
vironmentally unacceptable or are not feasible, especially
those not depending on fossil fuel resources [10-20]. Of
the available alternatives, solar energy is the most abun-
dant, inexhaustible single resource available. Solar en-
ergy as available on the surface of the earth constitutes
clean, non-polluting, abundant and relatively “free” re-
source. As much solar energy falls on earth’s surface in a
fortnight as the energy contained in the world’s fossil
fuel resources (1016 kW). The mean solar irradiance at
normal incidence outside the atmosphere is 1360 W/m2
and the total annual incidence of solar energy in India
alone is about 107 kW and for the southern region the
daily average is about 0.4 kW/m2 [11]. However, captur-
ing and utilizing solar energy is not simple. Methods are
being sought to convert solar energy into a concentrated,
storable form of energy. Nature via photosynthesis con-
verts somewhat less than 1% of the sun’s energy at the
earth’s surface to a solid fuel, i.e., plant materials, which
when accumulated and transformed over geologic ages

Conversion of Carbon Dioxide to Methanol Using Solar Energy—A Brief Review 1408
yielded fossil fuels. Current rates of use of these fossil
fuels, and the particular geographic distribution and po-
litical control of major petroleum resources pose prob-
lems for nations that are net petroleum consumers [5-9].
Hence, an alternative method yielding a simple fuel, at a
higher conversion, has long been desired. The conversion
of CO2 into more useful organic fuels (like methanol)
using energy that is not produced from fossil fuels is be-
lieved to be one such alternative method. It is also
strongly believed that the artificial photosynthesis has
tremendous potential, even though it remains to be suc-
cessfully demonstrated on a commercial basis. It is a
scientific challenge, and if this process is successful, the
market will be gigantic. Considering the importance of
solar assisted conversion of carbon dioxide to methanol
from the point of view of global warming and energy
crisis, the complete literature available on this topic has
been reviewed and presented in this article along with the
merits and demerits associated with each reported process.
2. Methods Available for Conversion of
Carbon Dioxide to Value Added
Chemicals
Methanol is the most promising photo-reduced product
of carbon dioxide because it can be transformed into
other useful chemicals such as gasoline (petrol) using
conventional chemical technologies, or easily transported
and used as fuel in the automobile vehicles without major
adjustments [21-28]. There are several other motivations
as listed below for producing methanol or any other use-
ful chemical from carbon dioxide [29-35]. 1) Carbon
dioxide is an inexpensive, nontoxic feedstock that can
frequently replace toxic chemicals such as phosgene of
isocyanates. 2) Carbon dioxide is a renewable feedstock
compared to oil or coal. 3) The production of chemicals
from carbon dioxide can lead to new industrial produc-
tivity. 4) New routes to existing chemical intermediates
and products could be more efficient and economical
than current methods. 5) The production of chemicals
from carbon dioxide could have a small but significant
impact on the global carbon balance. Considering these
benefits, several technologies have been proposed for
utilization of carbon dioxide and for improving the effi-
ciency of energy conversion. The most successful reac-
tion of these types is the Sabatier process [36]. This pro-
cess involves the reaction of hydrogen with carbon diox-
ide at elevated temperatures and pressures in the pres-
ence of a nickel catalyst to produce methane and water
(CO2 + 4H2 → CH4 + 2H2O). It was discovered by the
French chemist Paul Sabatier. It has been proposed as a
key step in reducing the cost of manned exploration of
Mars (Mars Direct) through in-situ resource utilization.
After producing water by combining hydrogen trans-
ported from Earth and carbon dioxide taken from the
atmosphere of Mars, oxygen would be extracted from the
water by electrolysis and used as a rocket propellant. The
stoichiometric propulsion fuel mix ratio is 1:8 between
hydrogen and oxygen by weight (each pound of hydro-
gen requires 8 pound of oxygen to burn), and if only the
light hydrogen has to be transported, and the heavy oxy-
gen extracted locally, that would result in a very consid-
erable weight savings which would have to be trans-
ported to Mars. Nevertheless, the expensive electrolysis
technique employed in the Sabatier process for splitting
water into oxygen and hydrogen is not economical to
practice for vehicles those run on the road.
On the other hand, electrochemical systems developed
for reducing CO2 to methanol or methane involve high
electrode over-potentials, which offer little or no advan-
tage since the quantity of fuel consumed during the syn-
thesis of the reduction products far exceeds the fuel value
of the products [34-36]. Whereas, the catalytic hydro-
genation of CO2 to methanol has also been found to be
not encouraging due to the involvement of high reaction
temperatures [34]. Even, the electrocatalytic reduction of
carbon dioxide to formic acid or carbon monoxide at Pd,
Pt and Hg electrodes were also found to be not commer-
cially viable [37-39]. The production cost of processes
involving reduction of CO2 to methanol such as, high
temperature catalytic-vapour phase reaction of CO2 with
water vapour, thermo-chemical/electro-chemical reduction
of CO2, etc., has been found to be not economical [10].
The main reason for un-economical production of me-
thanol or any other useful chemical from CO2 is the en-
dothermic nature of these reactions (for example, CO2(g)
+ 2H2O(l)  CH3OH(l) + 3/2O2(g); Ho – 727 kJ·mol–1 and
Go – 703 kJ·mol–1).
Given the CO2 reduction process is thermodynami-
cally uphill (CO2 + 6H+ + 6e–  CH3OH + H2O; E˚ =
–0.38 V), economical reduction of CO2 to value added
chemicals is possible only if renewable energy, such as
solar energy, is used as the energy source. In fact, plant
photosynthesis has long been studied with an eye to un-
derstanding its underlying mechanisms and then applying
this knowledge to the production of energy for the needs
of society [40]. Solar energy can be harnessed to drive
CO2 conversion by: 1) artificial photosynthesis using ho-
mogeneous and heterogeneous systems; 2) electroche-
mical reduction using solar electric power; and 3) hy-
drogenation of CO2 using solar-produced hydrogen, i.e.,
using photoelectrochemical (PEC) cells. Though, these
routes exhibited ability to convert CO2 to CH3OH,
HCOOH, HCHO, CH3OCH3, CO, etc., they suffered from
several limitations such as, poor performance, un-sta-
bility of associated catalysts and semi-conductors in wa-
ter, in-efficiency of electrolytic systems, inferior reduc-

Conversion of Carbon Dioxide to Methanol Using Solar Energy—A Brief Review1409
ing cathode materials, etc. Nevertheless, photoelectro-
chemical (PEC) conversion of CO2 to methanol appears
to be more promising among various other routes [41].
3. Photoelectrochemical (PEC) Cells
In a study, Halmon [10] demonstrated that aqueous car-
bon dioxide could be reduced on semiconductor surfaces
such as p-type gallium phosphide to produce formic acid,
formaldehyde and methanol in a photo-assisted electro-
lytic reaction using photoelectrochemical (PEC) cells. In
fact, PEC conversion of visible light to chemical or elec-
trical energy has the potential of being a relatively effi-
cient process. PEC cells enable solar energy utilization to
meet many of the energy needs of the future [42-58].
These cells involve the application of liquid-junction
transducers to solar energy conversion. Semiconductor-
electrolyte junction devices of these cells have an advan-
tage over solid state photo-voltaics in that 1) they can
directly generate hydrogen or carry out other photo-redox
processes, and 2) they can use polycrystalline semicon-
ductor electrodes. The junction is easily prepared merely
by dipping the electrode in the electrolyte, in contrast to
solid state junctions which have to be prepared by diffu-
sion or ion implantation. However, the efficiency of so
far studied PEC cells was found to be low and the semi-
conductor photoelectrodes employed exhibited poor sta-
bility against water reaction under solar irradiation.
Photoelectrolysis of water or reduction of CO2 require
thermodynamic energy inputs of 1.23 and 1.5 eV, re-
spectively. Greater energy input is required to make up
for losses due to band bending (necessary in order to
separate charge at the semiconductor surface), resistance
losses, and overvoltage potentials [42-58]. Furthermore,
when a semiconductor is placed in an electrolyte, partial
differences between the two phases result in charging of
the interface, as in a capacitor. This charging results in a
perturbation of the energy levels of the semiconductor
called “band-bending”. Band-bending is responsible for
separation of electron-hole pairs in photoelectrochemical
processes. Recombination and corrosion processes de-
crease the utilization of the electron-hole pairs generated
on illumination. Photoelectrolysis of water at high effi-
ciency is also hampered by a number of problems. First,
the overvoltages of the hydrogen and oxygen couples at
most moderate band gap (1.0 < Eg < 2.0 eV) semicon-
ductors are substantial. Second, if electron or hole trans-
fer to the substrate does not occur rapidly, recombination
of electron-hole pairs can occur at surface defects or
grain boundaries in polycrystalline samples [42-58].
These two problems arise, at least in part, from one
fundamental deficiency. While the electrolyte-semicon-
ductor interface has excellent characteristics for separa-
tion of charge and generation of high oxidation or reduc-
tion potentials when irradiated, it often has very poor
catalytic properties for reactions with significant activa-
tion energies. Water is particularly an attractive source of
hydrogen for the reduction of CO2 as well as for the di-
rect generation of H2. Water can only be used, however,
if the semiconductor electrodes are stable in its presence.
It has been demonstrated that, the production of en-
ergy-rich materials (e.g., H2, CH3OH, CH2O, CH2O2, and
NH3) is associated with O2 evolution [42-58]. A major
problem in photoelectrochemistry is that the oxidation of
water at the photoanode of non-oxide n-type materials is
thermodynamically and kinetically disfavoured over the
reaction of the valence band holes with the semiconduc-
tor lattice. In fact, all known non-oxide and many oxide
n-type photoanodes are susceptible to photodegradation
in aqueous electrolytes.
In spite of its great economic and social importance,
only a few papers have dealt with selective reduction of
carbon dioxide to value added chemicals using renewable
solar energy as the reaction promoting aid. The first im-
portant step of the photoelectrochemical (PEC) reduction
of CO2 to methanol is the generation of hydrogen ions
and electrons by the solar irradiance of semi-conducting
photoanode. Upon illumination of the semiconductor
with light energy equal to or greater than that of the
semiconductor bandgap, electrons are promoted from the
valence band to the conduction band, creating electron-
hole pairs at or near the interface. The electron-hole pairs
are spatially separated by the semiconductor junction
barrier, and are injected into the electrolyte at the respec-
tive electrodes to produce electrochemical oxidation and
reduction reactions [59-61]. A major impediment to the
exploitation of photoelectrochemical cells in solar energy
conversion and storage is the susceptibility of small band-
gap semiconductor materials to photoanodic and photo-
cathodic degradation. The photoinstability is particularly
severe for n-type semiconductors where the photogener-
ated holes, which reach the interface, can oxidize the
semiconductor itself. In fact, all known semiconducting
materials are predicted to exhibit thermodynamic insta-
bility toward anodic photodegradation. Whether or not an
electrode is photostable then depends on the competitive
rates of the thermodynamically possible reactions, name-
ly, the semiconductor decomposition reaction and the
electrolyte reactions.
4. Transition-Metal Complexes as
Photo-Sensitizers and Catalysts
In a study, Williams and Rembaum have modified the
electrodes surfaces with electron mediators such as, N,N’-
dimethyl, 4,4’-bipyridinium (methyl viologen, MV+2) to
enhance rates of photoelectrochemical reactions and sup-
press photo-corrosion and recombination [52]. MV+2 has

Conversion of Carbon Dioxide to Methanol Using Solar Energy—A Brief Review 1410
been shown to be an efficient electron transfer catalyst
for H2 evolution. The role of the MV+2/MV+1 system is to
provide an oxidized material which is efficiently photo-
reduced and a reduction product which can efficiently
transfer electrons to water or hydrogen ions at metal
(platinum) catalysts to produce H2,
hν, E +2 +1
MV MV
On the surface of p-type semiconductor the following
reaction occurs:

Pt 1+2
22g
MVHOMVOH12 H



The problems with mediation via solutions of methyl
viologen are the intense absorption of visible light by the
MV+1 species, and the relatively low concentration of the
mediator.
Transition-metal complexes have also often been used
as photochemical and thermal catalysts because they can
absorb a significant portion of the solar spectrum, have
long-lived excited states, can promote multielectron
transfer and activate small molecules through binding
[53]. In transition-metal complexes, a central metal has
octahedral, tetrahedral, square planar, square-pyramidal,
or trigonal-pyramidal symmetry depending on the sur-
rounding ligands. Reduced metal centers such as MIL, in
which the oxidation number of the central metal (M) is
plus one and the ligand (L) has four-coordinating atoms,
typically have one or more vacant coordinate sites. These
sites can be used to bind and activate CO2 (or other small
molecules). The oxidative addition of CO2 to MIL to
form a metallocarboxylate, MIIIL( ), stabilizes the
CO2 moiety through two-electron transfer. The MIIIL
() can then react with H+ to form MIIIL, CO and
OH–. The systems that have been used for photochemical
CO2 reduction studies have been divided into the follow-
ing steps:
2
2
CO 
2
2
CO 
1) Ru(bpy)2
3(bpy-2,2”-bipyridine) as both the photo-
sensitizer and the catalyst;

2) Ru(bpy)2
3 as the photosensitizer and another metal
complex as the catalyst;

3) ReX(CO)3(bpy) (X = halide or phosphine-type
ligand) or a similar complex as both the photosensitizer
and the catalyst;
4) Ru(bpy)2
3 or Ru(bpy)2 3-type complex as the pho-
tosensitizer in microheterogeneous systems;
 
5) A metallophorphirin as both the photosensitizer and
the catalyst;
6) Organic photosensitizers with transition-metal com-
plexes as catalysts.
Mechanisms believed to be involved in the above
process are: 1) light absorption by a photosensitizer to
produce the excited state; 2) a quenching reaction be-
tween the excited state and an electron donor to produce
a reduced complex; 3) electron transfer from the reduced
complex to a catalyst; and 4) activation of CO2 by the
reduced catalyst. However, the major products formed in
this process are formic acid (HCOOH) and carbon mon-
oxide.
5. Passivation of Semi-Conductor Surfaces
against Photocorrosion
A great variety of approaches have been made to control
the photo-instability of the semiconductor-electrolyte in-
terfaces using surface coating techniques. For example,
to stabilize semiconductor surfaces from photodecompo-
sition, non-corroding layers of metals or relatively stable
semi-conductor films have been reported. These con-
tinuous metal films which block solvent penetration can
protect n-type GaP electrodes from photo-corrosion.
However, if the films are too thick for the photo-gener-
ated holes to penetrate without being scattered, they as-
sume the Fermi energy of the metal. Then the system is
equivalent to a metal electrolysis electrode in series with
a metal-semi-conductor Schottky barrier. In such a sys-
tem, the metal-semiconductor junction controls the pho-
tovolatge and not the electrolytic reactions. In general, a
bias is required to drive the water oxidation. In other
cases, the metal can form an Ohmic contact that leads to
the loss of the photoactivity of the semiconductor. In
discontinuous metal coatings, the electrolyte contacts the
semiconductor, a situation which can lead to photo-cor-
rosion. For example, discontinuous gold films do not
seem to protect n-type GaP from photo-corrosion [42-
58].
Corrosion-resistant wide-bandgap oxide semiconductor
(TiO2 and titanates mostly) coatings over narrow-band-
gap n-type semiconductors such as GaAs, Si, CdS, GaP,
and InP have been shown to impart protection from pho-
todecomposition. One of two problems is currently associ-
ated with the use of optically transparent, wide-bandgap
semiconducting oxide coatings: either a thick film blocks
charge transmission, or a thin film still allows photocorro-
sion [47,48].
6. The Electro-Active and the Charge
Conductive Polymers
In a study it has been shown that chemical bonding of
electroactive polymers to the semiconductor surface af-
fects the interfacial charge-transfer kinetics such that the
less thermodynamically favoured redox reaction in the
electrolyte predominates over the thermodynamically
favoured semiconductor decomposition reaction [48]. To
date, emphasis has been placed on improving the cata-
lytic properties of p-type electrodes, where photocorro-
sion by reductive processes is not a major problem. The
overvoltage for the evolution of hydrogen from p-type

Conversion of Carbon Dioxide to Methanol Using Solar Energy—A Brief Review1411
electrode surfaces is normally quite large. It has been
demonstrated, however, that the catalytic property of a
p-type Si photocathode is enhanced for hydrogen evolu-
tion when a viologen derivative is chemically bonded to
the electrode surface and Pt particles are dispersed within
the polymer matrix [47]. The viologen mediates the
transfer of the photogenerated electron to H+ by the pla-
tinum to form H2. A thin platinum coating directly on the
p-type silicon surface also improves the catalytic per-
formance of the electrode. Charge conduction is gener-
ally much higher in electrically conductive polymers than
in typical electroactive polymers [42-58].
Accordingly, work on charge conductive polymers in
the filed of photoelectrochemistry has been directed to-
ward stabilization of electrodes against photodegradation
in electricity-generating cells. Charge conductive poly-
mers are known to protect certain semiconductor surfaces
from photodecomposition, by transmitting photogener-
ated holes in the semiconductor to oxidizable species in
the electrolyte at a rate much higher than the thermody-
namically-favoured rate of decomposition of the elec-
trode. In a study it has been demonstrated that coating
n-type silicon semiconductor photoelectrodes with a
charge conductive polymer, such as polypyrrole, enhan-
ces stability against surface oxidation in electricity-ge-
nerating cells. n-type GaAs has also been coated with po-
lypyrrole to reduce photodecomposition in electricity-ge-
nerating cells, although the polymer exhibited poor adhe-
sion in aqueous electrolyte [42-58].
Despite the promising use of polypyrrole on n-type
silicon to suppress photodecomposition, heretofore, whe-
ther or not conductive polymers in general could be used
in conjunction with catalysts was unknown. Moreover, it
can be seen that the discovery of uses for various poly-
mer coatings on photoelectrodes has been on a case by
case basis because of the empirical nature of the effects
on any particular semiconductor and/or the interaction
with a given electrolyte environment [42-70].
7. Catalysts for Carbon Dioxide Reduction
The reduction of carbon dioxide to methanol is repre-
sented as CO2 + 6H+ + 6e– CH3OH + H2O. In a
study, Jr. Frese, et al. [35], carried out this reaction over
the surface of a molybdenum cathode. According to them,
a molybdenum cathode can reduce carbon dioxide to
methanol selectively and with up to 80% to 100% fara-
daic efficiency. Such reductions can occur, for example,
at –0.7 V vs SCE at pH 4.2, only 160 mV negative of the
standard potential corrected for pH. However, this proc-
ess is yet to be tested in a PEC cell.

In another study, Barton et al. [51], conducted a selec-
tive solar-driven reduction of CO2 to methanol using a
catalyzed p-GaP based photoelectrochemical (PEC) cell
in a process called chemical carbon mitigation. The pho-
toinduced conversion of CO2 to a value-added product
without the use of additional CO2 generating power
source is referred to as “chemical carbon mitigation”.
More than 95% conversion and 100% methanol selectiv-
ity was noted in this process. However, the stability of
this semiconductor is to be tested to see the feasibility of
this process running on a commercial basis.
Rather in another study, Xu et al. [13], successfully
reduced CO2 to methanol following a biochemical ap-
proach in which formate dehydrogenase, formaldehyde
dehydrogenase, and alcohol dehydrogenase were used as
catalysts. Prior to reaction, these dehydrogenase catalysts
were encapsulated in an alginate-silica hybrid gel, which
was formed through in situ growth of the silica precursor
within the alginate solution that was followed by Ca2+
cross-linking. The methanol yields noted in this process
were 98.8%, 71.3% and 98.1%, respectively, for reac-
tions catalyzed by free dehydrogenases, dehydrogenases
immobilized in pure alginate and dehydrogenases immo-
bilized in alginate-silica. The latter catalyst showed the
methanol yield of 76.2% even after stored for 60 days.
Nevertheless, these catalysts are yet to be tested in PEC
cells for the same reaction.
8. Water Oxidation Catalysts
Recently, a path breaking observation was made at Law-
rence Berkeley National Laboratory by Heinz Frei and
Feng Jiao [62]. They prepared an aqueous solution con-
taining silica particles that have been embedded with
photooxidizing cobalt oxide nanocrystals plus a sensi-
tizer to allow the water-splitting reaction to be driven by
visible light. When this solution was irradiated by laser
light it turned from gold to blue as the sensitizer ab-
sorbed the light. Bubbles soon begin to form as oxygen
gas was released from the split water molecules. The
idea of Frei and Jiao was to develop artificial leaves in
which light harvesting occurs by capturing of solar pho-
tons by the oxidation of water over a bio-catalyst. Green
plants perform the photooxidation of water molecules
within a complex of proteins called Photosystem II, in
which manganese-containing enzymes serve as the cata-
lyst. Manganese-based organometallic complexes mod-
eled off Photosystem II have been tested by them for the
same reaction. Though these catalysts have shown some
promise as photocatalysts for water oxidation but suf-
fered from being water insoluble and none were very
robust. In looking for purely inorganic catalysts that
would dissolve in water and would be far more robust
than biomimetic materials, Frei and Jiao turned to cobalt
oxide, a highly abundant material that is an important
industrial catalyst. When they have tested micron-sized
particles of cobalt oxide, they found the particles were

Conversion of Carbon Dioxide to Methanol Using Solar Energy—A Brief Review 1412
inefficient and not nearly fast enough to serve as photo-
catalysts. However, when they nano-sized the particles,
it was another story. The yield for clusters of cobalt ox-
ide (Co3O4) nano-sized crystals was about 1600 times
higher than for micron-sized particles and the turnover
frequency (speed) was about 1140 oxygen molecules per
second per cluster, which is commensurate with solar
flux at ground level (approximately 1000 Watts per
square meter). Frei and Jiao used mesoporous silica as
their scaffold, growing their cobalt nanocrystals within
the naturally parallel nanoscale channels of the silica via
wet impregnation technique. The best performers were
rod-shaped crystals measuring 8 nm in diameter and 50
nm in length, which were interconnected by short brid-
ges to form bundled clusters. The bundles were shaped
like a sphere with a diameter of 35 nm. While the cata-
lytic efficiency of the cobalt metal itself was important,
they presumed that the major factor behind the enhanced
efficiency and speed of the bundles was their size.
However, their catalyst is yet to be tested in a reaction of
water oxidation integrated with the carbon dioxide re-
duction step via artificial photosynthesis, which is really
a challenging task.
In another study, Geletti et al. [63], developed a water
soluble new inorganic metal oxide (i.e., a polyoxometa-
late, POM) cluster with a core consisting of four ions of
the rare transition metal ruthenium, which has shown the
fast and effective oxidation of water to oxygen while
remaining stable itself. They say that in contrast to all
other molecular catalysts for water oxidation, their cata-
lyst does not contain any organic components and that is
the reason for its stability in water. However, while Ru
is more abundant and also less expensive than several
other precious metals, its use would nonetheless be
problematical for many large-scale green energy appli-
cations. Given this reality, they turned to the develop-
ment and evaluation of POMs containing multiple
earth-abundant metals as potential water oxidation cata-
lysts (WOCs). Finally, they succeeded in realising a fast
soluble carbon-free molecular water oxidation catalyst
based on abundant metals (Co, W and P) [64,65]. The
POM, [Co4 (H 2O)2(PW9O34)2]10– prepared by them was
found to be comprising a Co4O4 core stabilized by oxi-
datively resistant polytungstate ligands. It was also
found to be hydrolytically and oxidatively stable ho-
mogenous WOC that self assembles in water from salts
of Co, W and P. With [Ru(bpy)3]3+ (bpy is 2,2’-bi-pyri-
dine) as the oxidant, this catalyst showed catalytic turn-
over frequencies for O2 production ≥ 5 s–1 at pH = 8.
According to them, their next challenge is to integrate
these inorganic complexes into photoactive systems,
which efficiently convert solar energy into chemical en-
ergy. So far, energy is still obtained from chemical oxi-
dants in their studies [65].
9. Scope for the Future Research
The above information clearly suggests that among vari-
ous routes proposed for conversion of CO2 to methanol
or to any other value added chemical, the photoelectro-
chemical (PEC) cells have an edge over other process. In
order to realize efficient PEC cells for this reaction, 1) an
n-type semiconductor with desired bade gap and band
edges for photoanode (e.g., pure TiO2, ZnO, CdS, etc.); 2)
a p-type semi-conductor with desired band gap and band
edges for photocathode (e.g., doped TiO2, ZnO, GaP,
etc.); 3) a catalyst for performing water oxidation reac-
tion over or near to the surface of photoanode (e.g.,
polyoxometallates [63-65]), and 4) a catalyst to perform
CO2 reduction reaction over or near to the surface of
photocathode (e.g., pyridinium ions over the surface of
p-GaP [18,41,51]) are essential. Furthermore, these four
systems should exhibit quite stability in water under solar
irradiation. In addition to these, it is also required to un-
derstand the following underlined mechanisms for suc-
cessful realization of PEC cells for converting CO2 to
value added chemicals.
1) Underlined mechanisms in water oxidation and CO2
reduction reactions over the surfaces of photoanode and
photocathode, respectively;
2) The mechanisms involved in the dissolution and
evolution of CO2 in water soluble organic solvents such
as monomethanol amine, etc., to perform CO2 reduction
reactions in PEC cells containing water based electrolyte;
3) Interaction of conducting polymers with both photo-
anode and photo-cathode materials;
4) Underlined mechanisms in the corrosion of non-
oxide (for e.g., GaP and CdS) semi-conducting materials
in water upon solar irradiance and in the passivation of
these semi-conducting materials surfaces against photo-
degradation;
5) Underlined mechanisms in the suppression of H2
formation from H+ ions produced over the surface of
photo-anode upon solar irradiation in PEC cells con-
tained water based electrolyte at the surface of photo-
cathode;
6) The behaviour of electron and ion transporting of
water-soluble organics/catalysts in PEC cells;
7) Finally, integration of various systems required for
conversion of carbon dioxide to methanol or to any value
added chemical in PEC cells.
10. Acknowledgements
Author wishes to thank Dr. G. Sundararajan, Director,
ARCI, Hyderabad for his kind encouragement and for
giving permission to publish this review article.

Conversion of Carbon Dioxide to Methanol Using Solar Energy—A Brief Review1413
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