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Energy Conversion and Management 76 (2013) 194–214

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Energy Conversion and Management

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Advances in visible light responsive titanium oxide-based photocatalystsfor CO2 conversion to hydrocarbon fuels

0196-8904/$ - see front matter � 2013 All rights reserved.http://dx.doi.org/10.1016/j.enconman.2013.07.046

⇑ Corresponding author. Tel.: +60 7 553 5579; fax: +60 7 5588166.E-mail addresses: [email protected], [email protected] (M. Tahir),

[email protected] (N.A.S. Amin).1 Permanent address: Department of Chemical Engineering, COMSATS Institute of

Information Technology Lahore, Punjab, Pakistan.

Muhammad Tahir 1, NorAishah Saidina Amin ⇑Chemical Reaction Engineering Group (CREG)/Low Carbon Energy Group, Faculty of Chemical Engineering, Universiti Teknologi Malaysia, 81310 UTM, Skudai, Johor Baharu,Johor, Malaysia

a r t i c l e i n f o a b s t r a c t

Article history:Received 16 March 2013Accepted 17 July 2013

Keywords:PhotocatalysisVisible light responsive TiO2

Photocatalytic CO2 reductionSolar hydrocarbon fuels

Solar conversion of CO2 to hydrocarbon fuels seems promising to reduce global warming for improvedsustainability. Solar energy, as direct solar irradiations, is excessively available and it is imperious to uti-lize it for solar fuel production. This review paper is organized to discuss recent innovations and potentialapplications of phototechnology to recycle CO2 via visible light responsive (VLR) TiO2-based photocata-lyst. In this perspective various enhancement methods such as doping with metals and non-metalsand sensitization to expand TiO2 band gap toward visible region are critically discussed. This reviewpaper also presents applications of VLR photocatalysts, advances in photoreactors, and future prospectsof VLR based technology for conversion of CO2 to hydrocarbon fuels. The findings of this study revealedboth metals and non-metals could improve TiO2 photoactivity, but non-metals and especially co-metalswere more efficient. The combination of co-metals with sensitizers exhibited much higher CO, CH4 andCH3OH yield rates. Among photocatalytic reactors, optical fibers and monolith photoreactors are moreefficient because of their efficient light harvesting potential. Although the progress in CO2 reduction tofuels is encouraging, further considerations are required for commercialization purposes.

� 2013 All rights reserved.

1. Introduction

Rapid global energy demand has been driven by a growingworld population. Energy requirements will roughly be doubledby 2050 and tripled by the end of this century [1,2]. The current en-ergy infrastructure is mostly dependent on fossil fuels. Combustionof these fuels generates greenhouse gases (GHG) especially carbondioxide (CO2), the main cause of global warming [3–5]. Exploringnew energy resources are inevitable owing to environmental is-sues, shortage of fossil fuels and continuous increase in energy de-mand [6]. Among energy producing possibilities, one pathway isthe photochemical conversion of CO2 into value-added chemicals[7]. Therefore, hydrocarbons from CO2 reduction using solar energycould be a potential source since CO2 is green, abundant, nontoxicand an inexpensive feedstock [8–10].

CO2 reduction with H2O through photocatalysis is vital in thedevelopment of solar energy based carbon neutral cycle [11]. Thesolar spectrum is the most abundant, permanent and reliablesource of energy worth 100,000 TW; way in excess of the needsand consumptions by living things [12,13]. Solar fuels include

hydrogen (H2), carbon monoxide (CO), methane (CH4), and metha-nol (CH3OH). Furthermore, CO2 conversion to hydrocarbon fuelsunder solar irradiations is an economically viable process withnegligible environmental effects.

Photocatalytic CO2 reduction with H2O to various chemicalswas explored three decades earlier. In this context, in 1978, thepioneering work on the photo-electrochemical reduction of CO2

was reported by Halmann [14] using electrochemical cell. Later, In-oue and Fujishima et al. [15] reported the reduction of CO2 withH2O to CH3OH, formic acid (HCOOH) and formaldehyde (HCHO)under xenon (Xe) and/or mercury (Hg) lamp irradiations. The semi-conductor used were both oxide and non-oxide particles of tita-nium oxide (TiO2), tungsten oxide (WO3), zinc oxide (ZnO),gallium phosphide (GaP), cadmium sulfide (CdS) and silicon car-bide (SiC). Subsequently, CH3OH, CO, H2, HCHO, CH4, and C2H6

were reported by other researchers over various photocatalystsnamely TiO2 [16–18], zirconium oxide (ZrO2) [19], CdS [20], mag-nesium oxide (MgO) [21], and quantized semiconductor (Q-Sc)zinc sulfide (ZnS) [22].

Highly efficient and selective photocatalysts that could functionunder sunlight are utmost important for solar hydrocarbon pro-duction. Among the different semiconductor materials, TiO2 as aphotocatalyst has numerous advantages. Some attractive featuresinclude powerful oxidation properties, good charge transfer poten-tials, low cost and corrosion resistance [23–25]. However, TiO2

displays activity only when irradiated with UV-light (wavelength

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M. Tahir, N.A.S. Amin / Energy Conversion and Management 76 (2013) 194–214 195

less than 380 nm) due to its higher band gap (3.20 eV for anatase)[26]. Therefore, to overcome the strong limitation of UV response,various modification techniques have been employed in the lastfew years. The most common strategies include doping with met-als and non-metals [27,28]; sensitization using dyes, nanocarbons[29], graphene [30], enzymes [31], and novel sensitizers [32].

In addition to visible light responsive (VLR) photocatalysts, effi-cient designs of photoreactors are also eminent. The most widelyused photoreactors under investigations are slurry type, fixedbed and surface coated photoreactors. In general, the photocata-lytic reactors should provide a higher interaction surface area toensure effective harvesting and distribution of solar irradiation tomaximize conversion and yield rates [33–35].

The objective of this review is to describe advances in photocat-alytic CO2 reduction to hydrocarbon fuels using TiO2 based photo-catalysts. In the mainstream, fundamentals of CO2 reduction,thermodynamic analysis, and different modification techniquesto develop VLR TiO2 based photocatalysts are discussed. The appli-cations of VLR photocatalysts and developments in photo-catalyticreaction systems are also deliberated. Finally, future prospects ofphototechnology and recommendations for improvements in thefield are highlighted.

2. Fundamentals and evaluation of CO2 photocatalytic activity

2.1. Principle of photocatalysis

Photocatalysis is a process in which light radiations having en-ergy equal to or greater than the energy band gap (Ebg) of a semi-conductor strike on its surface and generates electron (e�)-hole(h+) pairs. The photogenerated electrons and holes participate invarious oxidation and reduction processes to produce final prod-ucts. However, if the electrons fail to find any trapped species(e.g. CO2) on the semiconductor surface or their energy band gapis too small, then they recombine immediately and release unpro-ductive energy as heat [36]. In particular, the photoactivity ofsemiconductor materials depends on: (a) composition of reactionmedium, (b) adsorption of reactants (e.g. CO2 and H2O) on semi-conductor surface, (c) type of semiconductor and its crystallo-graphic/morphological characteristics, and (d) ability ofsemiconductor to absorb UV or visible light [37,38].

The heterogeneous photocatalysis phenomenon is presented inFig. 1 [39,40]. Basically, photocatalysis is a complex mechanism be-cause many paths are possible to obtain the final product. The yieldrates of the products are also dependent on the life of photogener-ated electrons and holes particles. On the other hand, the rate ofthe charge transfer depends on the positions of the band edges(conduction and valence bands) and redox potentials of adsorbedspecies. In general, during photocatalysis process, if charges havesufficient energy to separate, then the following paths/routes arepossible [36,41]:

(a) In competition with charge transfer to adsorbed species,there are chances of electrons and holes recombination. Sep-arated electron and holes could also recombine in the vol-ume of semiconductor or at the surface.

(b) The surface recombination (Path A) occurs when electronsand holes recombine on the semiconductor surface. How-ever, if they recombine within the volume of the heteroge-neous photocatalyst then this process is known as volumerecombination (Path B).

(c) The photoinduced electrons could move towards the outersurface of the semiconductor and be trapped by theadsorbed species. This electron transfer process will be more

effective if pre-adsorbed species (e.g. CO2 and H2O) alreadyexist at the catalyst surface. At the surface, semiconductorcan donate electron to reduce acceptors (CO2) (Path C), inturn a hole can be transferred to the surface where an elec-tron from donor species can combine with the surface holeto oxidize donor species (H2O) (Path D).

2.2. Thermodynamic analysis

CO2 is thermodynamically a stable compound, which is difficultto be oxidized or reduced to various chemicals and/or fuels at low-er operating temperatures. The standard reduction potentials ofH2O to produce H2 is low (Eo

red ¼ 0 V), which is much lower thanthe standard reduction potential of CO2 to generate�CO�2 ðE

ored ¼ �1:9 VÞ [42]. Thus, in CO2 photocatalysis with H2O,

photoreactions are supposedly more favorable to reduce H2Othrough water splitting instead of CO2. Therefore, during CO2

reduction, it is recommended to measure the volume of hydrogenalong with other products. In this way, if H2 is not produced, then itwill be a good indication that H2O is not competing with CO2. How-ever, if a large volume of H2 is generated, then it should be a con-firmation that water splitting reaction dominates. Under suchcircumstances, photocatalytic activity should be re-evaluatedusing other operating conditions, preferably in the absence of li-quid H2O. However, for CO2 reduction to CH3OH and CH4, a reduc-ing agent that could supply hydrogen is needed. Among differentpossibilities, the more challenging task is to use H2O as a reducingagent for CO2 reduction because of the potential water splittingreaction competing with CO2.

In photocatalyitc CO2 reduction applications, CH3OH is the de-sired product since it can be used directly as liquid fuel. Moreover,to evaluate the possibility of CH3OH or other hydrocarbon produc-tions during CO2 reduction process, it is important to consider ba-sic or fundamental processes that could occur over thesemiconductor. The most probable reactions that could occur dur-ing CO2 reduction with H2O in terms of thermodynamic reductionpotentials vs. normal hydrogen electrode (NHE) are explained byEqs. (1)–(15) and summarized in Table 1 [43–45].

The multielectronic processes in Table 1 (Eqs. (2)–(7)) seem tobe more favorable as these processes require less energy per elec-tron transfer compared to mono-electron process (Eq. (1)). In Eqs.(2)–(7)), each step requires a transfer of additional two electrons.Therefore, to study the feasibility of reactions that could occur inphotocatalytic process, the position of conduction and valenceband edges can be used as a simple tool. Furthermore, photo-ex-cited electrons can be utilized efficiently if the reduction potentialof the reaction is lower than the conductance band of semiconduc-tor [46]. By comparing TiO2 conductance band (�0.50 eV) withthermodynamic potentials in Eqs. (2)–(7) it could be inferred thatreactions in Eqs. (6) and (7) are more favorable to produce CH3OHand CH4. In the CO2 reduction process using liquid H2O, another as-pect that should be considered is the pH of the solution. Usually,photocatalytic CO2 reduction is carried out under basic pH valuesto enhance CO2 solubility in H2O, while there is a possibility forcarbonates and bicarbonates to be produced. But, these speciesare more difficult to reduce than CO2 itself.

On the other hand, carbonates and bicarbonates are good holesquenchers and help to precede oxidation reaction rather thanreduction. The higher proton concentration might produce H2 fas-ter than CO2 reduction, thus favoring water splitting process. How-ever, lower pH could shift the conduction band edge of reactiontowards more positive and thus can reduce CO2 to CH3OH (Eq.(6)). Furthermore, H2CO3 and carbonate ions present in the reac-tion mixture could also lead to different products as illustratedby Eqs. (8)–(13). Molecular species and anions produced during

hv

acceptor

Path C

Path D

Light source

Semiconductor catalyst

CB

VB

hv

e-

(H+ + *OH)

Fig. 1. Mechanism and pathways for photocatalytic oxidation and reduction processes on the surface of heterogeneous photocatalyst.

196 M. Tahir, N.A.S. Amin / Energy Conversion and Management 76 (2013) 194–214

different reactions can also undergo through different reactions(Eqs. (14) and (15) for production of CH3OH.

The possibility of semiconductors to undergo photoinducedelectron transfer to adsorb species depends on the band energy po-sition of semiconductor and redox potentials of the adsorbate (e.g.CO2 and H2O). The band gaps of semiconductors in terms of theirenergy potentials to reduce H2O and CO2 are depicted in Fig. 2[15,43]. In general, the relative potential level of the acceptor isthermodynamically required to be less (more positive) than theconduction band of the adsorbent (semiconductor). Aliwi et al.[47] utilized CdS for photocatalytic reduction of CO2 at pH 6 andreported HCHO and HCOOH as the major products. It is obviousthat the conductance band of CdS is �1.0 eV (Eo (red = �1.0eV)),which is closer to values reported in Eqs. (3) and (4) and thusCdS is favorable for HCHO and HCOOH production. By using ZnS–CdS composites, HCOOH and CO were reported through reactions

Table 1Summary of reduction potentials for half-cell reactions at pH 7 in aqueous solution vs.the normal hydrogen electrode.

Chemical equations (Thermodynamic potential, Vvs. NHE)

CO2 þ e� ! CO�2 �2.0 (1)2Hþ þ 2e� ! H2 �0.41 (2)

CO2 þ 2Hþ þ 2e� ! HCOOH �0.61 (3)

CO2 þ 4Hþ þ 4e� ! HCHOþ H2O �0.52 (4)

CO2 þ 2Hþ þ 2e� ! COþ H2O �0.48 (5)

CO2 þ 6Hþ þ 6e� ! CH3OHþ H2O �0.38 (6)

CO2 þ 8Hþ þ 8e� ! CH4 þ H2O �0.24 (7)

H2CO3 þ 2Hþ þ 2e� ! HCOOHþH2O �0.166 (8)

H2CO3 þ 4Hþ þ 4e� ! HCHOþ 2H2O �0.050 (9)

H2CO3 þ 6Hþ þ 6e� ! CH3OHþ 2H2O +0.044 (10)

2CO2�3 þ 4Hþ þ 2e� ! C2O2�

4 þ 2H2O +0.478 (11)

2CO2�3 þ 3Hþ þ 2e� ! HCOO� þ 2H2O +0.311 (12)

2CO2�3 þ 8Hþ þ 6e� ! CH3OHþ 2H2O +0.209 (13)

2C2O2�4 þ 2Hþ2e� ! 2HCOO� +0.145 (14)

HCOO� þ 5Hþ4e� ! CH3OHþ 2H2O +0.157 (15)

in Eqs. (4) and (5) [48]. Inoue et al. [15] reduced CO2 to CH4, CH3-

OH, HCHO and HCOOH using TiO2, WO3, ZnS and CdS and sug-gested reduction of CO2 to CH4 is a multiple step reductionprocess. It could be concluded that type of semiconductor, thepH of the solution, and thermodynamic reduction potentials ofreactions are the major factors that could influence the productselectivity.

2.3. Progress in CO2 photoreduction and expected challenges

The idea of photocatalytic CO2 reduction to various hydrocar-bons and chemicals was reported three decades earlier. Initially,photo-electrochemical reduction was employed while one of theearliest reports was published by Halmann in 1978 [14] using elec-trochemical cell. The products detected were HCOOH, HCHO andCH3OH when the solution was illuminated with mercury lamp(Hg) and an electric current was supplied. After this breakthrough,significant research activities began to focus toward photochemi-cal and electrochemical CO2 reduction. Canfield et al. [49] em-ployed P-GaAs and P-InP photo-electrodes in a CO2 saturatedaqueous solution of Na2SO4 and successfully converted CO2 to CH3-

OH. Similarly, production of CH3OH, HCHO and HCOOH were re-ported by Blazeni et al. [50] using single crystal P-GaP and P-GaAs photo-anodes. Ichikawa et al. [51] discussed the photo-elec-tro catalysis system comprising a thin film of anatase TiO2 layer onone side of Nafion substrate while ZnO with Cu as electro-catalystson the other end. Under illumination with Hg lamp and sunlight,CO2 was converted to different hydrocarbons.

In heterogeneous photocatalysis, the pioneer work on photocat-alytic reduction of CO2 with H2O to various organic compoundsnamely CH3OH, HCOOH, CH4 and HCHO was reported by Inoueet al. [15] using TiO2, WO3, ZnO, GaP, CdS and SiC semiconductors.In the mainstream, Halmann et al. [52] reported the use of stron-tium titanate powder suspended in aqueous solution throughwhich CO2 was bubbled illuminated by natural sunlight producingHCHO, HCOOH and CH3OH. Anpo et al. [53] reported the use ofhighly dispersed TiO2 on glass for reduction of CO2. Cu loaded

3.0

2.0

1.0

0.0

4.0

-1.0

-2.0

CdS

2.4eV

2.3eV

GaP

WO3

2.8eV

SiC

3.0eV

TiO2

rutile

3.0eV

TiO2

anatase

3.2eV

3.2eV

ZnO

3.8eV

SnO2

4.0eV

Ta2O5

CB

VB VB

VB VB

VBVB

VB VB

VB

CB

CBCB

CB

CB

CB

CB

CB

Higher position

Lower position

2 / HCOO 1C H=O 0.6−

2 / HCHOCO = 0.52−

2OC =OC/ 0.48−

2 3/ CH O 8C H=O 0.3−

2 3H CO / HCOOH= 0.166−

2 3H CO / HCHO= 0.05−

2 3 3H CO / CH OH = 0.044+

Pot

enti

al V

S N

HE

pH=7.0 Redox potentials

4/ CH =CO 0.24−

+2H /H = 0.41−

2 2H O/O = 0.82+

2H O/OH = 2.32+

2

Fig. 2. Schematic representation of conductance band potentials of semiconductors and thermodynamic reduction potentials of various compounds measured at pH 7[15,43,44].


TiO2 powder was dispersed in CO2 pressurized solution at ambienttemperature by Adachi et al. [54]. The products found were CH4

and C2H4 under Xe lamp illumination. Yamashit et al. [11] usedhighly dispersed active titanium oxide catalysts for the photocata-lytic CO2 reduction to CH4, CH3OH, C2-compounds, CO, and O2 un-der UV light irradiations. The yield was strongly dependent on thetypes of catalysts, ratio of CO2/H2O and reaction temperature. Inanother report, photocatalytic reduction of CO2 by H2O vaporswas studied using zeolite modified TiO2 photocatalysts where highCH3OH selectivity in the gas phase was observed [18].

A summary of some important work involving TiO2 semicon-ductor is presented in Table 2. It is evident from this explanationthat different types of TiO2 based photocatalysts have been usedfor CO2 reduction. However, in the majority of the studies, TiO2

was the best performing semiconductor when irradiated with pho-tons of wavelength below 380 nm. Besides, titanium based photo-catalysts have been reported as the most appropriate candidate forthe CO2 photocatalysis applications due to its strong oxidationproperties and numerous advantages.

Although CO2 conversion over TiO2 photocatalyst under UVirradiations is highest among all other types of semiconductors,the product distribution including CO, CH4, CH3OH, HCOOH,CH2O and H2 is complex. Hence, the use of this process even underUV irradiations is limited. On the other hand, UV light that reachesthe earth is about 3–5% of the solar irradiation. The solar energyexists in the visible region is about 50%, and it is vital for the pho-tocatalyst to be sensitive to visible light. Therefore, for the applica-tion of this process to be functional under visible light irradiations,it is necessary to modify TiO2 structure.

3. Developments in visible light responsive TiO2 basedphotocatalysts

Heterogeneous photocatalysts are widely researched in the fieldof solar energy applications. In this section, the main focus is toelaborate technologies that could help to produce VLR TiO2 basedphotocatalysts with enhanced photoactivity and selectivity. Forthis purpose various surface modification techniques includingdoping with metals and non-metals and various sensitizationmethods are discussed.

3.1. Doping/deposition

Doping and/or deposition are effective strategies to increasespectral response of TiO2 through narrowing electronic propertiesand by altering optical responses. The metal ions deposited onthe TiO2 surface usually act as a sink for the photoinduced chargecarriers and thus can improve the interfacial charge transfer pro-cesses. The materials have been under investigation for TiO2 dop-ing/depositions are noble metals, transition metals, rare-earthmetals, and other metal and non-metal ions.

3.1.1. Metal dopingIn doped TiO2, photo-excited electrons are transferred from the

conductance band of TiO2 to metal particles, while holes remainedon its surface [78]. Several metals such as Ag, Au, Rh, Pt, Cu, Sn, andNi have been tested to improve TiO2 photocatalytic activity andselectivity. Among nobel metals, Pt doped into TiO2 can reducetitanium Ti4+ to Ti3+ by transferring electron from TiO2 to Pt. Itcan also help to substitute Pt ions into TiO2 lattice to enable itVLR [79]. In the Pt–Cu doped TiO2 synthesized by Yang et al. [46]a band gap of 3.15 eV and 2.98 eV was measured by Pt and Cuincorporate into TiO2, respectively. Similarly, Adachi et al. [54] re-ported the effectiveness of Cu doped TiO2 under visible light irradi-ations. Furthermore Ag doped TiO2 has excellent photocatalyticproperties which can also reduce TiO2 band gap. In the Ag dopedTiO2 synthesized by Koci et al. [59] a shift in band edge from theUV to the visible was found to be dominated by the amount ofAg incorporation. Similarly, Krejcikova et al. [56] reported thatAg enriched TiO2 with various loadings (0–5.2 wt.%) shifted theband edge into the visible region.

The conventional photocatalysis process can also be improvedusing metal nanoparticle doped TiO2 through plasmon effect. Inthis perspective, gold nanoparticles have strong surface plasmonband in the visible range, and electrons can efficiently be trans-ferred to the TiO2 conductance band under visible light irradiations[80–82]. Similarly, VLR TiO2 base photocatalysts were synthesizedusing Au nanoparticles such as Au–TiO2 by Fazio et al. [83] and Au/TiO2 by Roy et al. [84].

The transition metals are also researched excessively to im-prove TiO2 photoactivity and selectivity under visible light

Table 2Literature summary for progress in CO2 reduction using different types of photocatalysts and reactors.

Year Feed Catalyst Light source Reaction condition Keyproduct(s)

Reactor type Comments Refs.

2013 CO2–H2O

MMT/TiO2 500 W Hg lamp, k = 365 nm 373 K, 0.20 bars, 25 mgcatalyst

CO, CH4 Cell typecatalystsuspendedreactor

The yield rates increased significantly bymodifying TiO2 with montmorillonite. C2–C3

paraffins and olefins products were also observed

[7]

2013 CO2–H2O

CeO2–TiO2,templateSBA-15

300 W Xe arc lamp, similarto sunlight

0.10 g catalyst, 30 �C,110 Kpa, CO2 = 95.5%,H2O = 4.5%,

CH4, CO Stainless steelreactor,volume1500 ml

CO was observed as main product while yieldrate increased significantly over CeO2–TiO2 undervisible light irradiations

[55]

2012 CO2–H2O

Ag–TiO2 8 W Hg lamp (254 or365 nm)

0.1 g catalyst, 100 ml ofNaOH (0.2 M)

CH4,CH3OH

Quartz tubetype slurryreactor

CO2 reduction was enhanced by Ag doping andusing 254 nm

[56]

2011 CO2–H2O

ZnS-MMT 8�W;Hg lamp;k > 254 nm 273 K;1 atm CH3OH,CH4, CO,H2, O2

Quartz tubeslurry reactor

Highest yield was obtained when lamp was nearthe slurry surface

[57]

2010 CO2–H2O

Cu–TiO2–SiO2

Xe lamp(250 nm < k < 400 nm),

Glass fiber filter loaded withcatalyst CO2 and H2O vapor

CO, CH4 Cylindricalcontinuousflow reactor

Methanol, formaldehyde and formic acid were intraces amount. CH4 appeared as a secondaryproduct

[58]

I = 2.4 mW cm�2 273 K;1 atm2010 CO2–

H2OAg/TiO2 8�W;Hg lamp;k > 254 nm 273 K;1 atm CH4,

CH3OHQuartz tubeslurry reactor

Yield of methane and methanol was increasedwith Ag

[59]

2009 CO2–H2O

TiO2 (4.5–29 nm)

8�WHg lamp;k > 254 nm 0.1 g catalyst, 100 ml ofNaOH (0.2 M), PCO2 1.1 atm,273 K, 110 kPa

CH4 andCH3OH

Quartz tubereactor

The yield rate was increased and decreased bychanging particle size. The maximum yield wasobserved at optimum particle size of 14 nm

[60]

2008 CO2–CH4

Ga2O3 300-W, Xe lamp (220–300 nm)

473 K, 1 atm CO, H2,C2H6

Quartz fixedbed reactor

Other products such as C2H4, and C3H8 were alsoobserved. Conversion was about to 1.22%

[61]

2008 CO2–H2O

Cu–Fe/TiO2 150 W high pressure Hglamp

25 �C, 1 atm CH4, C2H4 Catalystcoated opticalfiber reactor

Presence of Fe co-doped on Cu enhanced C2H4

yield. Optical fiber reduction has higher yieldthan glass plate

[62]

2008 CO2–H2O

TiO2 200 W Hg/Xe-lamp – H2, CH4 Slurry reactor Initially, fast cumulative yield of CH4 and after4 h. there was no yield. Maximum yield was21.5 lmole/g.

[63]

2008 CO2–H2O

Ag/TiO2 8 W, Hg lamp – CH4,CH3OH

Batch stirredannularreactor

The yield of methanol was higher at 254 nm than365. At 400 nm, there was very lowphotocatalytic reduction of CO2

[64]

254, 3652007 CO2–

H2O/H2

TiO2, ZrO2 15-W, near-UV lamps,k ¼ 365;254 nm

43 �C, 1 atm CH4 Circulatedreactor

Highest yield was obtained with H2 and ZrO2

catalyst. Traces of CO and C2H6 were alsoobserved

[19]

2006 CO2–H2O

TiO2 UV lamp, 4.8 W 254 nm and3.0 W 365 nm

298 K CH4 Fixed bedquartz reactor

Yield of CH4 was 200 ppm. CO and H2 were alsodetected in the product in small amounts

[65]

2006 CO2–H2O

TiO2 pellets(4 mm)

100 g catalyst, CO2 saturatedwith water, ambientpressure

CH4 Flat bottomreactor withdistributedpellets

TiO2 pellet form could be an alternative forimmobilized catalysts

[65]

2006 CO2/CH4,H2

MgO 500 W, Ultrahigh pressure,Hg lamp

293 K, 1 atm CO, H2 Fixed bed flatbottom reactor

Formaldehyde and acetaldehyde were also usedas reaction substrate. CO of 3.6 lmole wasobserved

[41]

2005 CO2–H2O

Cu/TiO2 75 �C CH3OH yield increased with UV intensity.Maximum yield achieved was0.45 lmole g cat�1 h�1 at 1.29 bars

UV light 1.05–1.4 bar CH3OH Optical fiberphotoreactor

2005 CO2–H2O

Cu dopedTiO2

100 W UV lamp (365 nm), 0.3 g catalyst, 300 ml ofKHCO3 (1 M), 43–100 �C

CH3OH Slurry reactor Copper species determined photocatalyticactivity. The efficiency was in the order of:CuO > Cu2O > Cu

[66]

2005 CO2–H2O

CuO/TiO2,23 nm, 3%CuO,45.8 m2 g�1

UV lamp-10 W, 415–700 nm, 2.78 eV

– CH3OH Pyrex glassslurry reactor

Ethanol, propanol, acetone, and otherhydrocarbons were also produced. CH3OH yieldincreased up to 3% Cu loading

[66]

2004 CO2–CH4

Cu/CdS–TiO2/SiO2

125-W, Ultrahigh pressure,Hg lamp

393 K, 1 atm CH3COCH3 Quartz fixedbed reactor

Conversions of methane and CO2 were 1.47%0.74%, respectively

[67]

CH3CH3,CO

2004 CO2–H2O

TiO2 k ¼ 350 nm – CH4 Slurry reactor Methanol and 2-proponal were used as saturatorin addition to H2O

[68]

2004 CO2–CH4

ZrO2 500-W, Ultrahigh pressure,Hg lamp

293 K, 1 atm CO, H2, Fixed bed flatbottom reactor

The other products like HCHO and CH3CHO werealso observed

2003 CO2–H2O

Ti/Si-h-c(hexagonaland cubic)

100 W high pressure Hglamp

323 K CH3OH,CH4

Quartz cellwith flatbottom(88 cm3)

Minor products like CO and O2 were alsoobserved. The yield was increased linearly withtime

[69]


Table 2 (continued)

Year Feed Catalyst Light source Reaction condition Keyproduct(s)

Reactor type Comments Refs.

2002 CO2–H2O

TiO2/SiO2, Ti-MCM-41

100-W high-pressure Hglamp

50 mg catalyst, CO2

(36 lmol), H2O (180 lmol),50 �C

CH4,CH3OH

Quartz cellwith flatbottom

Traces amount of CO and O2 were also produced.However selectivity of CH3OH was controlled bysurface –OH groups of catalyst

[70]

2001 CO2–H2O

Titaniumbeta-zeolites

100 W Hg lamp(k > 280 nm),I = 265 mW cm�2

50 mg catalyst, CO2

(36 lmol) and H2O(168 lmol), 50 �C

CH4,CH3OH

Quartz cellwith flatbottom

Hydrophilic zeolite enhanced CO2 reductioncompared to hydrophobic zeolite

[71]

1999 CO2–H2

Rh/TiO2 500-W ultrahigh-pressuremercury lamp, k = 290–450

T ¼ 273 K;1 atm CO Fixed bedquartz reactor

CH4 was also produced and its selectivityincreased by increasing Rh loading

[72]

1998 CO2–H2O

TiO2 powder 4.5 kW Xe lamp (>340 nm),I = 0.62 kW m�2

100 mg catalyst, 20 mlwater, 20 �C, PCO2 2–27.6 atm, isopropanol as thescavenger

CH4 Slurry reactor CO2 photoreduction influenced by pressure andisopropanol concentration

[73]

1998 CO2–H2O

Ti-MCM-41and Ti-MCM-48

High pressure mercurylamp, k > 280 nm

328 K, 1 atm CH4,CH3OH

Quartz cellconnectedwith vaccum

TiO2 played an important role for methanolselectivity

[74]

1997 CO2–H2O

Pt/TiO2–Y–zeolite

78-W, high pressure Hglamp, k > 280 nm

328 K, 1 atm CH3OH,CH4

Quartz cell In addition yo CH4 and CH3OH, traces amount ofCO, C2H4, C2H6 and O2 were also observed. Theaddition of Pt increased the selectivity of CH4

[18]

1997 CO2–H2O

TiO2 powder(230 nm)

990 W Xe lamp (>340 nm) 50 mg catalyst, liquid CO2,0–25 �C, PCO2 49.3–79.0 atm

HCOOH Slurry reactor No effect of temperature and pressure [75]

1996 CO2–H2O

TiO2 powder 4.5 kW Xe lamp (>340 nm) 50 mg catalyst, 30 ml ofNaOH (0.2 M), 20 �C, PCO2 1–20 atm

CH4 Slurry reactor CH4 was the main product over TiO2 powder. TheCO2 reduction was also influenced by changingreactor pressure

[76]

1995 CO2–H2O

Cu/TiO2 75-W, High pressuremercury lamp, k > 280 nm

273–323 K, 1 atm CH4,CH3OH,CO

Quartz cell Addition of Cu in TiO2 promoted CH3OHselectivity while TiO2 (110) performed betterthan TiO (100).

[77]


[27].The most widely used transition metals/metal ions include;Co2+, Cu2+, Cr3+, Mo5+, V+4, and Fe3+. Fe–TiO2 photocatalysts withdifferent dopant concentrations were synthesized by Sun et al.[85]. They observed increased visible light band edge of TiO2. Sim-ilarly, Pan et al. [86] investigated the effects of different metals onVLR and photoactivity of TiO2. When V, Fe, Ce, Cu and Cr weredoped in TiO2, it was observed that V and Fe were positioned inthe substitution sites of TiO2, Ce ions were dispersed in the inter-stitial sites, while Cr and Cu aggregated on the surface. The follow-ing was the order of activity: Fe–TiO2 > V-TiO2 > Cr–TiO2 > Ce–TiO2 > TiO2 > Cu–TiO2.

Recently, nano-structured photocatalysts have been under con-siderations because of their numerous advantages which include;(i) high adsorption surface area, (ii) fast diffusion rate (iii) inhibit-ing electron–hole pair recombination, and (iv) higher surface tovolume ratios [87,88]. A variety of TiO2 nanomaterials such asnanotubes [89], nano-membranes, nanofibers, nanowires [90,91]and nano-crystal films [92] etc. have been reported in the lastyears. The addition of metals into TiO2 nano-materials could sepa-rate charge efficiently. Wang et al. [93] investigated Pt–TiO2 nano-wires (NWs) hybrid system in which Pt nanoparticles werehomogeneously deposited over TiO2 NWs. It was found that Pt–TiO2 NWs of 100–200 nm having 5 nm Pt nanoparticles were moreeffective for the separation of electron and hole pairs with en-hanced photoactivity. Furthermore, the ultrafine Pt nanoparticlesin TiO2 single crystal were studied by Wang et al. [94]. The onedimensional (1–D) structure of TiO2 coated with ultrafine Pt nano-particles with size 0.5–2 nm, developed through gas depositionmethod, was very effective in terms of photoactivity andselectivity.

3.1.2. Non-metal dopingSimilar to metals, non-metals doped or deposited on TiO2 struc-

ture could effectively improve TiO2 photoactivity and selectivityunder visible light irradiations [95]. The most commonly usednon-metals so far investigated in different studies are C, N, F, Iand S. [96,97]. When non-metals are doped into TiO2, occupiedorbitals are introduced above the energy of the valence band

(VB), thus narrowing the band gap. Among non-metals, N-dopedTiO2 is the most effective in narrowing the band gap. Vargheseet al. [23] developed higher photoactivity N-doped TiO2 nanotubes(TNTs) for harvesting solar irradiations. In another study, the bandgap for N-doped TiO2 was 2.96 eV [98]. Similarly, Zhang et al. [99]reported photo-active centers of N-doped TiO2 and observed visi-ble light initiated by an excited state of surface (Ti4+–N3�) units.

In addition of nitrogen as an effective dopant, C and S are alsouseful for extending TiO2 band. Wang et al. [100] found a bandgap of 3.0, 2.96 and 2.90 for C–TiO2, S–TiO2 and N–TiO2, respec-tively. In C-doped TiO2, hydrophilic factor was the dominant andvisible response was due to localized C (2p) formed above the va-lence band, known as origin of visible light sensitivity [101]. S-doped TiO2 could narrow band gap, and also prevented anatasetransformation to rutile even at higher temperature [102]. Simi-larly, F was suitable to reduce TiO2 band gap and also enhancedits photoactivity [103].

3.1.3. Co-metal dopingTiO2 doping with both metals and non-metals has been en-

dorsed as an attractive approach to improve VLR TiO2 photoactiv-ity. However, TiO2 photoactivity could be enhanced further usingthe co-doping approach. Co-doping is an important research do-main in the field of titanium photo-activation, even though allthe dopants are not always suitable for this purpose [104].

TiO2 can be co-doped using both metals and non-metals to im-prove VLR and photocatalytic activity [105,106]. Yen et al. [107]synthesized Fe–N doped TiO2 and observed higher VLR than Fe/TiO2 and N/TiO2 samples. The band gap of Fe–N co-depositedTiO2 was 1.97 eV compared to 3.14 eV for Fe-implanted TiO2 and2.16 eV for N-doped TiO2. The effects of Cu–Fe on the VLR ofTiO2–SiO2 were studied by Nguyen et al. [108] for photoreductionof CO2 under solar irradiation. The band edge of TiO2–SiO2 shiftedfrom 3.30 eV to 2.95 eV by doping 0.5 wt.% of Cu and Fe, respec-tively. Similarly, Im et al. [109] developed VLR effective photocat-alysts by doping C, N, B and F into TiO2 under visible lightirradiations. N–Ce co-doped TiO2 was synthesized by Wang et al.[110] and reported enlarged band edge (400–650 nm). In another


study, N–F co-doped TiO2 photocatalysts, synthesized by sol–gelsolvothermal approach, gave visible light absorption edge between400 and 600 nm [111].

All the above results revealed that co-doped systems are veryefficient to improve TiO2 VLR. However, in photocatalytic CO2

reduction, selectivity is also very important and careful consider-ations should be taken during selection of co-doped metals. Thisis because during co-doping, metals are competing with each otheraccording to their individual functions, which can reduceselectivity.

3.2. Photosensitization

Photosensitization is a process in which electrons produced inthe conductance band of a sensitizer during visible light irradiationare transferred to the TiO2 conductance band enabling it to func-tion as a catalyst under visible light [112]. The efficiency of sensi-tized TiO2 photocatalyst depends on sensitizer light interactioncharacteristics and response to transfer electrons toward TiO2 sur-face. However, the key factors that are important in TiO2 sensitiza-tion process include: (a) suitable sensitizer band gap that couldabsorb visible light irradiations; and (b) sensitizer conductanceband needed to be higher (more negative) than TiO2 conductanceband to transfer electrons towards TiO2. In this section, differenttypes of sensitizers such as dyes, coupling semiconductors, and no-vel sensitizers are discussed.

3.2.1. Dye sensitizationDyes are known to be very efficient to extend TiO2 band. How-

ever, VLR of TiO2 could only be possible if the dye is adsorbed on itssurface. Moreover, the transfer of electrons from excited dye toTiO2 conductance band depends on the dye visible light adsorptionefficiency. When visible light strikes on the dye sensitized TiO2,electrons are transferred from the dye to the TiO2 conduction bandand enables it to be active under visible light [78,113]. However,this electron transfer process can be suppressed by competitiveadsorption of other co-existing species in the reaction medium.Therefore, selections of dyes are very important to develop effi-cient dye-TiO2 system for photocatalytic applications under visiblelight. Jiang et al. [114] used chrysoidine G (CG) and tolylene-2, 4-diisocyanate (TDI) dyes to develop VLR TiO2 and observed ex-tended wavelength (400–550 nm).

In addition to dyes, metal complex sensitizers were more effi-cient in terms of both photoactivity and selectivity. For this pur-pose, both metals and non-metals can be used to develop metal-sensitizer complex [115,116]. Le et al. [117] prepared rhodamineB (Rh B) dye sensitized co-doped TiO2 (Co/TiO2) which was func-tional in the range of 450–600 nm. The band gap of Rh B–Co/TiO2

was 2.58 eV, much lower than that of Co/TiO2 (2.75 eV). Similarly,Yuan et al. [118] synthesized copper (I) dyes [Cu (bpy)2]+ sensi-tized TiO2 photocatalyst and found very efficient under direct solarirradiations.

3.2.2. Quantum dots and phthalocyanine oxide derived photocatalystsThe quantum dots (QDs) sensitized TiO2 photocatalysts have

been the growing research area in the field of semiconductor phot-ocatalysts. Photo-excited electrons in semiconductor QDs can beinjected into TiO2 and subsequently would be used to initiate oxi-dation and reduction process. Therefore, these types of TiO2 com-posites utilize visible light irradiation with good chargeseparation. Wang et al. [119] developed CdSe quantum dots(QDs) sensitized Pt/TiO2

. This heterostructured catalyst was func-tional under visible light irradiation (k > 420). In order to furtherinvestigate the effects of quantum dots on visible light response,Wang et al. [120] prepared different sizes PbS QDs and depositedover Cu/TiO2 to develop heterostructured photocatalysts. The

photocatalysts were tested under visible light irradiations (420–610 nm) using 3, 4 and 5 nm sizes of PbS QDs to act as sensitizerfor Cu/TiO2. Higher reduction efficiency was observed when PbSQDs was deposited over Cu/TiO2 catalyst.

Recently, phtalocyanines (Pcs) sensitizers are also found veryuseful in improving VLR TiO2 photoactivity. Pcs are macrocyclicnitrogenated compounds with distinguished characteristics includ-ing excellent semi-conductivity, chemical stability, and higher opti-cal absorption in the visible range [121,122]. In Pc sensitized TiO2,there is possibility of electrons being efficiently injected from the ex-cited state of Pc to a conductance band of TiO2. In this perspective,Wang et al. [123] synthesized novel ZnPc/TiO2 composite usingmicrowave oven assisted thermal process. The band gap of TiO2 ob-served was 2.95 eV at 0.6 wt.% ZnPc and reduced to 2.79 eV by load-ing 5 wt.% ZnPc. Similarly, Yang et al. [124] prepared highly efficientVLR SnSx/TiO2 composites. In another study, CoPc/TiO2 nanocom-posite was synthesized by Zhao et al. [125] through in situ annula-tion of 1,2-dicyanobenzene at the surface of TiO2 using Co2+ as atemplate and observed highly efficient under visible light.

3.2.3. Coupling semiconductorsThe purpose of coupling low band gap semiconductor (more neg-

ative CB level) to TiO2 (high band gap) is the transfer of electronsfrom low band gap semiconductor to high band gap semiconductors.Over the last few years, various researchers have been working inthis research domain. Kezzim et al. [13] discovered Fe2O3 as couplingagent with TiO2 was effective under visible light irradiations. CeO2

coupled with TiO2 was investigated by Wang et al. [55] who reportedhigher activity under visible light irradiations. PdS modified TiO2

nanocomposite, examined in another study, was highly efficient inthe wavelength range of 420–610 nm. Another highly efficient VLRphotoactivity of TiO2 was explored by coupling with CdSe under vis-ible light irradiations (k > 420 nm) [119].

Compared to TiO2, titanium nanotubes (TNTs) are renowned dueto the three dimensional regular structures with higher surface tovolume ratios [126]. By coupling TNTs with lower band gap materi-als redox process under visible light irradiations can be enhanced. Inaddition, co-catalyst nanoparticles on TNTs arrays could further en-hance the redox process. In this perspective Li et al. [87] developedefficient CdS (Bi2S3)/TNTs composites and employed under visiblelight irradiation. TNTs can respond to ultra-violet light of wave-length shorter than 400 nm with estimated band gap of 3.12 eV. Incontrast to TNTs, Bi2S3/TNTs and CdS/TNTs have strong adsorptionability in the visible region. Bi2S3/TNTs absorption spectra appearedmore towards visible domain than CdS/TNTs due to the smaller Bi2S3

band gap than CdS. Therefore, sensitizer/ TNTs composites are capa-ble to utilize visible light for higher photocatalytic activity andselectivity.

Recently, carbonaceous nanomaterials have been considered toproduce VLR TiO2. Some specific features of carbon based materialsinclude higher surface area, adsorption and electron-storage capac-ity. The carbonaceous materials can also hinder photogeneratedcharges recombination [127–129]. The conventional carbonaceousmaterials used in different studies are carbon black, graphite, andother graphitized materials. Among others, carbon nanotubes(CNTs) are particularly interesting because their special electronicproperties are associated with one dimensional (1-D) structure forefficient charge transfer over CNTs/TiO2 nanocomposites. CNTs areclassified as single-wall carbon nanotubes (SWCNTs) and multi-wall carbon nanotubes (MWCNTs). SWCNTs consist of a singlelayer of graphene sheet rolled into cylindrical tube and MWCNTshave multiple concentric tubes [130]. SWCNTs provide higher sur-face area than MWCNTs. The addition of SWCNTs and/or MWCNTsas supports to TiO2 may induce charge transfer properties andcould enhance the photocatalytic activity [127,131]. The effects


of different types of carbon materials on TiO2 visible light behaviorare summarized in Table 3.

3.2.4. Novel sensitizersNovel sensitizers have great importance for improving TiO2

photocatalytic activity under solar irradiations. The conjugatedpolymers (CPs) with extended P-conjugated electron systems canproduce higher photo efficiency due to higher light absorptioncoefficients under visible light spectrum. The CPs also have higherstability and mobility of charge carriers. The chlorophyll extractedfrom cyanobacteria is also considered as an efficient sensitizer. Theinteraction of colloidal TiO2 with chlorophyll could transfer elec-trons from the chlorophyll to the conductance band TiO2, resultingin photosensitized oxidation and reduction processes [132].

Among sensitizers, enzymes are reported as efficient sensitizersfor red shift of TiO2 [32]. TiO2 nanoparticles modified with photosen-sitizers and CO2 reducing enzymes CODHs (carbon monoxide dehy-drogenases) has been reported as an extraordinary photocatalystsfor CO2 reduction under the visible light irradiations. Anaerobicorganism namely carboxydothermus hydro-genoformans (Ch) ex-presses five CODHs; each having active sites of [Ni4Fe-4S]. These en-zymes (Ni-CODHs) are homodimeric containing buried active sitesin each subunit in the [4Fe-4S] cluster. Electrons can enter or leavethe enzyme cluster freely [31,38]. Woolerton et al. [133] developedenzyme modified TiO2 nanoparticles in the presence of solar energy.During this study, TiO2 nanoparticles were modified using the pho-tosensitizer [Ru] and CODH1 enzymes. When the visible light strikeson the sensitizer surface (RuP), excitation occurs. The injection ofelectrons from RuP to TiO2 enables it to be active under solar irradi-ation. A summary of different types of TiO2 modification techniquesvs. enhancement is portrayed in Fig. 3.

It is obvious from VLR technologies that the efforts towarddeveloping VRL TiO2 photocatalysts are appreciable. A variety ofvarious new methods and techniques have been developed andvarious novel sensitizers are also under investigation. In dopingco-loaded TiO2 photocatalyst is more appreciable to enhance visi-ble light response and photoactivity. However, this efficiency couldfurther be improved by using efficient sensitizers that would behelpful to develop economically viable phototechnology.

4. Applications of VLR photocatalysts and advances inphotoreactors

4.1. VLR TiO2 photocatalysts

The concept of CO2 conversion into hydrocarbon fuels usingsunlight is an attractive approach; serving to reduce atmospheric

Table 3Effects of different types of carbon materials to extend TiO2 band gap.

System Measurement techniques/theoretical model

Bandgap(eV)

Refs.

C-doped TiO2 (anatase) UV–Vis absorption edge 2.90 [134]C-doped TiO2 (anatase) Photocurrent density vs.

excitation wavelength2.63 [135]

C-doped TiO2 (rutile) UV–Vis absorption edge 2.32 [136]C-doped nanotubes UV–Vis absorption edge 2.91 [137]C-doped nanowires – 3.05C-doped nanorods UV–Vis absorption edge 3.07N-doped TiO2 – 3.10 [138]C–N doped TiO2 – 2.95Carbon black doped TiO2 UV–Vis absorption edge 1.30 [139]C–V doped TiO2 UV–Vis absorption edge 2.18 [140]Carbon-cation-doped TiO2

(anatase)DFT calculation 2.85 [141]

Carbon-anion-doped TiO2

(rutile 5.2% C)DFT calculation 2.35 [142]

CO2 concentrations and providing renewable energy based hydro-carbon fuels that are compatible with the current energy infra-structure. A number of publications have been released thatdeclare CO2 photoreduction to be an innovative and sustainableprocess for the production of solar hydrocarbon fuels; however,this process is linked to the development of efficient VLR semicon-ductor catalysts. A brief summary of the photocatalysts used forCO2 reduction is portrayed in Fig. 4.

Among metals, copper (Cu) has been used most widely to im-prove TiO2 photoactivity and selectivity in the CO2 photocatalyticapplication. Cu doped TiO2 photocatalysts are very efficient to con-vert CO2 and H2O to CH3OH. Tseng et al. [143] used Cu–TiO2 cata-lyst with efficiency two times higher than Cu-P25 for CO2

reduction to CH3OH. In this study, HCOOH, CH4 and other productswere not produced over Cu loaded TiO2. In the batch type process,there is possibility that CH3OH in the product mixture could reactreversibly during CO2 reduction process. Therefore, continuous re-moval of CH3OH from the system would be of interest to maximizethe productivity. Slamet et al. [66] employed Cu/TiO2 catalyst toinvestigate photocatalytic reduction of CO2 with H2O with CH3OHas the main product. Over 3% Cu loaded and particle size of 23 nm,the maximum CH3OH yield obtained was 442.2 lmole g cat�1 h�1

under visible light irradiation. In another study, Shi et al. [67] usedto couple semiconductor Cu/CdS–TiO2/SiO2 for the photocatalyticreduction of CO2 and CH4 to oxygenated compound. The maximumconversion reported was 1.47% and 0.74% for CH4, and f CO2,respectively while the acetone selectivity was 92.3%.

Besides Cu, other metals were also explored in many studies forthe specific purpose of CO2 reduction with H2O. Ishitani et al. [144]doped Pd, Rh, Pt, Au, Cu, Ru on TiO2 and investigated their effectsfor CO2 reduction to CH4 and CH3CO2H. It was observed that allmetals are favorable for CH4 production and less selective for aceticacid. However, Pd and Rh enhanced the CH4 yield while Rh and Cuproduced more acetic acid. Among transition metals, silver (Ag)has most widely been used to get crystalline titania (Ag/TiO2) forCO2 reduction. Wu et al. [145] reported reduction of CO2 to CH3OHover Ag doped TiO2 and yield rate observed was4.12 lmole g cat�1 h�1 over 1% Ag doped TiO2. With higher Ag con-tent in TiO2, the yield rates of CH4 and CH3OH increased for an opti-mum Ag loading. Similarly, Au doped TiO2 was investigated by Houet al. [146] for photocatalytic CO2 reduction with H2O using greenlaser visible light at 532 nm wavelength. The yield of CH4 achievedwas 0.93 lmole m�2 over TiO2 sheet for 15 h, which increased to22.40 lmole m�2 when Au was doped into TiO2.

In the specific case of photocatalytic CO2 reduction to hydrocar-bon fuels, some metals are more favorable to improve selectivity.Copper doping is most efficient to enhance selectivity toward CH3-

OH and the formation of HCOOH derivatives. In contrast, dopingwith Pt/Pd favors the selectivity toward CH4 and CO production.Therefore, selections of metals are very important to get the de-sired product with appropriate selectivity.

In non-metals, iodine doped TiO2 nanoparticles were synthe-sized by hydrothermal method and tested for the photocatalyticCO2 reduction to CO under visible light. The yield rate of CO wasincreased at higher iodine content. 10 wt.% iodine in TiO2 was opti-mum for maximum CO production under solar irradiations. On theother hand, 5 wt.% I–TiO2 calcined at 375 �C showed reduced activ-ity which was even lower than the pure TiO2 performance [147].Similarly, Yui et al. [148] developed Pd doped TiO2 by photochem-ical deposition method and tested for CO2 reduction. The additionof 1 wt.% Pd in TiO2 gave product distribution from CO over TiO2 toCH4 over Pd–TiO2 with smaller amount of CO and C2H6. The re-useof exhausted Pd–TiO2 was unfavorable to deliver the same resultsas fresh catalyst.

Furthermore, co-doping titanium with either metalnanoparticles and/or quantum dots are also used to improve its

High surface to volume ratioHigher adsorption rateHigher light absorption efficiency Lower band gap energy

TiO2 nanoparticles,TiO2 nanotubesTiO2 nanomembranes

Metals: Ag, Au, Rh, Pt, Pd, Cu, NiNonmetals:C, N, F, S, B, I, Cl, and BrTransition metals:Co, Ru, Pr, Re, La, Ce, Zn, Cr, Ni

Enhance surface electron excitationEnhance photoactivityReduce band gap energyReduce recombination of chargesAct as sink for charge particles

Lower band gap materialsCarbon nanotubesNanoclaysDye

Conjugated polymersCyanobacteria ChlorophyllEnzymes

Higher photocatalytic activityHigher absorption VL Higher mobility of chargesHybrid catalyst for high yield

Fig. 3. Schematic presentation of different modification techniques with their enhancement effects.

Table 4Summary of the various catalytic systems other than TiO2 employed for CO2 photoreduction.

Year Catalyst Light source Conditions Type of reactor Keyproduct

Refs.

2012 ½Zn3AlðOHÞ8�þ2 CO2�

3 ½Zn1:5Cu1:5GaðOHÞ8�þ2 CO2�

3500 W Xe lamp 0.1 g catalyst, 0.02 bar

CO2 (0.177 mmol) + 0.2 barH2 (1.67 mmol), 32–40 �C

Quartz cell(bottom plate23.8 cm2)

CH3OHor CO

[160]

2012 Pt and/or RuO2-Zn1.7GeN1.8O 300 W Xe lamp with cut-off filter(k > 420 nm)

0.1 g catalyst, , 0.4 mL distilled water Glass reactor (area4.2 cm2)

CH4 [161]

2012 W18O49 300 W Xe lamp with or without cut-offfilter (k > 420 nm), 30 mW m�2 lightintensity

0.1 g catalyst, DI water (0.8 mL),70 �C

Glass reactor(bottom plate8.1 cm2)

CH4 [162]

2012 NiO/InNbO4; Co3O4/InNbO4 500 W halogen lamp (k = 500–900 nm),143 mW cm�2

0.14 g catalyst, 50 mL aqueoussolution (0.2 m KHCO3)

Slurry reactor CH3OH [163]

2012 ½Zn3GaðOHÞ8�þ2 [Cu(OH)4]2�H2O 500 W Xe lamp 0.1 g catalyst, , 0.02 bar

CO2 (0.177 mmol) + 0.2 barH2 (1.67 mmol), 32–40 �C

Quartz cell(bottom plate23.8 cm2)

CH3OH [164]

2012 Pt/meso-ZNGO 300 W Xe lamp with cut-off filter(k > 400 nm)

0.2 g catalyst, , 3 mL DI water,CO2 pressure 0.8 bar

Slurry reactor(360 mL reactionvolume)

CH4 [165]

2012 ZnGaNO-modified Pt/ZnAl2O4 300 W Xe lamp with cut-off filter(k P 420 nm)

0.1 g catalyst, 230 mL, 0.4 mL DIwater

Glass-reactor (area4.2 cm2)

CH4 [166]

2011 Bi2S3/CdS:Bi2S3 500 W Xe lamp with cut-off filter(2 m NaNO2, k > 400 nm)

0.2 g catalyst, 200 mL aqueoussolution (0.8 g NaOH and 2.5 gNa2SO3)

Photochemicalreactor

CH3OH [167]

2011 Ni@NiO/N-doped InTaO4 Xe lamp with cut-off filter(390 < k < 770 nm)

0.1 g catalyst, 50 mL DI water, 2.9 barCO2, 25 �C

Slurry typecontinuous flowreactor

CH3OH [154]

2011 Au or Pt/carbon nanoparticles visible light (425–720 nm) aqueous solution, ambientconditions

Optical cell reactor HCOOH [168]

2011 Cu2O/SiC:Cu2O 500 W Xe lamp with cut-off filter(2 m NaNO2; k > 400 nm)

0.2 g catalyst, 200 mL aqueoussolution (0.8 g NaOH and 2.52 gNa2SO3)

Slurry reactorunder magneticstirring

CH3OH [169]

2011 RuO2 or Rh1.32Cr0.66O3/CuxAgyInzZnkSm Xe lamp (1000 W m�2) with cut-off filter(2 m NaNO2, k > 400 nm)

0.05 g catalyst, aqueous solutionwith NaHCO3 (2.5 mmol), 1 bar CO2

Slurry type quartzreactor

CH3OH [170]

2011 Pt/NaNbO3 300 W Xe lamp 0.1 g bulk or 0.03 g nanowirecatalyst, 3 mL H2O

Pyrex glass reactor CH4 [171]

2010 ZnGa2O4; RuO2/ZnGa2O4 300 W Xe lamp 0.1 g catalyst, 1 mL DI water Glass reactor(4.2 cm2, 230 mL)

CH4 [172]


VLR catalyst for CO2 Reduction

Un-modifiedSurface Modified

SensitizedMetal doped

Novel

Dye/TiO2, CdS/ TiO2,Bi2S3/TiO2, Bi2O3/

TiO2, CdSe/TiO2, Pr-CdSe/ TiO2, AgBr/

TiO2, RhB/TiO2, CdS-Pt-Fe-TiO2

Cu-TiO2, Pt/TiO2,Au/TiO2, Rh-TiO2, N-

TiO2, C-TiO2, Ag, Ru, Rh & Pd/TiO2, Br, I, F, S, Ru and Cr/TiO2, C-V-TiO2,

C-N-TiO2

Bi2O3, CdSe, SiC, CdS, Bi2S3, InTaO4,GaP, WO3, In2O3 ,

Fe2O3, W18O49,InNbO4,, NaNbO3,

Conjugate polymer/TiO2,enzymes/TiO2,

chlorophyll/ TiO2, ZnPc/TiO2,

CoPC/TiO2

Capable of utilizing solar energy which is 45% of solar spectrum.Co-metal doped, sensitized and novel sensitizers are very effectual to enhance yield andselectivity. Higher yield and selectivity is possible under mild operating conditions. Methanol ispossible by using co-metal and sensitized catalysts. Cu is good for methanol production.

Surface Modified

TiO2 based catalyst Catalyst other than TiO2

Fig. 4. Schematic presentation of VLR photocatalysts used for photocatalytic CO2 reduction.


photoactivity that could act as an antenna for absorbing visiblelight and transferring it to semiconductor. Wang et al. [119] re-ported the effect of CdSe/TiO2, and Pt/TiO2 on photocatalytic reduc-tion of CO2 using visible light irradiation. With the use of CdSe–Pt–TiO2, the primary reaction product was CH4 while CH3OH, CO andH2 were the secondary. By using only Pt–TiO2, there was no photo-conversion product. However, by replacing Pt with Fe, H2 was pro-duced, suggesting that water splitting was the only photoreactionthat occurred. Typical yields of 48 and 3.30 ppm g cat�1 h�1 of CH4

and CH3OH with trace amounts of CO and H2 were observed. Leet al. [117] modified TiO2 with RhB and Co for CO2 reduction withH2O as reducing agent. H2 was observed as the main product with ayield rate of 98.9 lmole g cat�1 h�1 over RhB-Co/TiO2 under visiblelight irradiation. The significant H2 yield revealed that both RhBand Co were favorable for water splitting instead of CO2 reduction.However, the H2 production rate increased to six times over RhB-Co/TiO2 than Co/TiO2 catalyst. Similarly, Thampi et al. [149] ob-tained selective production of CH4 from a mixture of CO2 and H2

at room temperature and atmospheric pressure using highly dis-persed Ru/RuOx in TiO2. CH4 production rate of 1.16 lL g cat�1 h�1

was obtained using solar simulator.Among dye sensitizers, photocatalytic reduction of CO2 with

H2O into CH4 was reported by Ozcan et al. [150] using dye sensi-tized TiO2 and dye-Pt/TiO2 photocatalysts under visible light irradi-ation. The CH4 yield was increased with dye sensitized Pt dopedTiO2 compared to Pt/TiO2 and pure TiO2 samples. Wang et al.[123] sensitized TiO2 with ZnPc for photocatalytic CO2 reductionwith H2O under visible light irradiations and observed CH3OH asthe main product. CH3OH yield reached to 31 lmole g cat�1 h�1

over 0.6 wt.% ZnPc compared to bare TiO2 having yield rate of1.25 lmole g cat�1 h�1. With the higher amount of ZnPc, the yieldgradually decreased. Much higher enhancement factor i.e. 24.8-fold was achieved using ZnPc as a modifier.

Enzyme (Ch CODH) was investigated to sensitize TiO2 using atungsten halogen lamp, k < 420 nm with UV-light cut filter. Theyield of CO as main product was increased from 0.5 to

58 lmole g cat�1 h�1 using enzyme/TiO2, a 50 times higher thanbare TiO2 [133]. These results confirmed that such types of sensi-tizers are favorable for reduction of CO2 to CO instead of hydrocar-bons. Zhao et al. [125] used CoPc/TiO2 photocatalyst andinvestigated it as an efficient catalyst for CO2 reduction to CH3OHand HCOOH as the main products. The maximum yield rates ofCH3OH and HCOOH were 9.38 and 148.8 lmole g cat�1 h�1

, respec-tively over 0.7% CoPc/TiO2 photocatalyst. Similarly, Tahir et al.[7]described photocatalytic CO2 reduction with H2O vapors usingmontmorillonite (MMT) modified TiO2 nanocomposites. Highestyield rates of 441.5 and 103 lmole g cat�1 h�1 were achieved forCH4 and CO, respectively. The possible products and reactionmechanism over MMT modified TiO2 catalysts are illustrated inFig. 5.

In titanium nanotubes (TNTs), the combination of TNTs andsensitizers were very effective to enhance visible light responseand photoactivity. CdS/Bi2S3 sensitized TiO2 was used for CO2

reduction under visible light irradiations by Li et al. [87]. The yieldof CH3OH achieved was 20.5, 31.9, 44.92 lmole g cat�1 h�1 usingTNTs, TNTs-CdS and TNTs-Bi2O3, respectively. Visible light re-sponse, surface area and CO2 adsorption for Bi2S3/TNTs were higherthan CdS and TNTs. Approximately, 2.2 times higher yield was ob-tained using Bi2O3/ TNTs.

The band gap positions of n-CdS, n-TNT and n-Bi2S3 and rel-evant redox potential of CO2 to CH3OH are depicted in Fig. 6(a).It can be observed that conductance band potentials of Bi2S3

and CdS are more negative than reduction potential of CO2,H2CO3 and CO�2

3 . Thus CO2 in H2O in any form i.e. H2CO3 orCO�2

3 can be ultimately reduced to CH3OH using such photocat-alysts. Furthermore, the schematic presentation of CO2 reduc-tion using CNTs–TiO2 composites is presented in Fig. 6(b).When TiO2 is sensitized with MWCNT, electrons are transferredfrom MWCNT to TiO2 conductance band and then being trans-ferred back to MWCNTs after holes were generated in TiO2.Thus, MWCNTs make TiO2 to be active under visible light irra-diation [129].


For VLR, N-doped TNTs arrays were synthesized by anodizingtitanium foil in an electrolyte consisted of 0.3 M ammonium fluo-ride (NH4F) in 2% water containing ethylene glycol at 55 V. Theband edge shifting of N-doped TNTs arrays were about 540 nm.Dispersed Pt and/or Cu nanoparticles at the top of N-doped TNTshave been used for photocatalytic CO2 reduction with H2O vaporunder natural (outdoor) sunlight. Co-metal deposited N-dopedTNTs arrays for photocatalytic reduction of CO2 and H2O under so-lar irradiation are depicted in Fig. 7(a). Titanium nanotube mem-branes (TNTMs) are also being examined for photocatalyticreduction of CO2 under solar irradiation. The schematic representa-tion of TNTMs for photocatalytic reduction of CO2 and H2O is de-picted in Fig. 7(b). Liu et al. [151] used co-catalysts loadedTNTMs system for CO2 conversion to solar hydrocarbon fuels. TheTNTMs loaded with Cu and Pt was investigated and the hydrocar-bon production rate was 110 ppm cm�2 h�1 under solar spectrum.

Doped TiO2 and/or doped/sensitized TiO2 can be used as effi-cient VLR photocatalysts to develop solar hydrocarbon fuels basedphototechnology. The higher production rate of hydrocarbon fuelslike CH3OH was observed using a hybrid metal/sensitized TiO2

photocatalysts. By investigating the overall trend in CO2 reductionto solar hydrocarbon fuels, an obvious increment trend in CH3OHyield has been reported using VLR TiO2 catalysts. Furthermore, sen-sitizers have a higher yield rate than metal modified TiO2. More-over, metal and sensitized based TiO2 photocatalysts gavemaximum yield and CO2 conversion rate to hydrocarbon fuels.Thus, VLR TiO2 based photocatalysts modified with co-doped met-als and sensitizers would maximize the yield rate of solar hydro-carbon fuels.

4.2. VLR photocatalysts other than TiO2

The demand for clean energy technology has promoted researchin solar energy applications using VLR photocatalysts. TiO2 is themost famous photocatalysts in the history of photocatalysis andserved as a benchmark material for many photoconversion pro-cesses. As an alternative to TiO2 based photocatalysts, many other

(a)

(b)

MMT/TiO2

Fig. 5. (a) Reaction scheme for the production of hydrocarbons during photocatalytic CO2

and possible reaction mechanism for photocatalytic reduction of CO2 with H2O.

types of visible light driven semiconductor with narrow band gaphave been investigated for CO2 photocatalytic reduction.

The photocatalytic reduction of CO2 to CH3OH was reported byLiou et al. [152], with a production rate of 0.16 lmole g cat�1 h�1 inwhich NiO doped InTaO4 photocatalysts were illuminated underthe solar spectrum of wavelength 400 nm. Sato et al. [153] re-ported the p-type InP/Ru complex polymer hybrid photocatalystfor selective reduction of CO2 and the main product observedwas HCOOH with trace amounts of H2 and CO. In another study,Zhou et al. [44] reported ultra-thin and uniform Bi2WO6 squarenanoplates for photocatalytic reduction of CO2. They observed Bi2-

WO6 nanoplates were more efficient for reduction of CO2 to CH4

having a yield rate of 1.1 lmole g cat�1 h�1. Lower concentrationsof CH3OH, O2, H2 and CO as secondary products were also ob-served. Efficient solar driven Ni and NiO modified N doped InTaO4

photocatalysts were investigated by Tsai et al. [154] for efficientCO2 reduction to CH3OH. Xenon lamp was used as light source withPE300BF filters, allowing only visible light (390–770 nm) to pass.CH3OH was the main product with a maximum yield rate of 162,127.5 and 63.5 lmole g cat�1 h�1 for Ni–NiO/InTaO4–N, InTaO4–Nand InTaO4 respectively. Wang et al. [155] reported CO2 photocat-alytic reduction to CH3OH using NiO doped InTaO2 in optical fiberphotoreactor. The rate of CH3OH production was11.1 lmole g cat�1 h�1 using light intensity of 327 mW cm�2 at25 �C. The yield rate was increased to 21.0 lmole g cat�1 h�1 byincreasing the reaction temperature to 75 �C.

Recently, Li et al. [156] investigated the photocatalytic reduc-tion of CO2 to CH4 using Pt loaded SiO2 pillared HNb3O8 photocat-alyst. The maximum yield of CH4 was 3.75 lmole g cat�1 h�1 over0.4 wt.% Pt doped SiO2–HNb3O8. However, no molecular oxygenand hydrogen was detected in this study. Oxygen and hydrogenwere not detected in a product mixture because O2 was physi-sorbed and chemisorbed on the catalyst surface, while H2 was effi-ciently consumed by CO2 to produce CH4. A graphenephotocatalyst-enzyme coupled photosynthesis system for produc-tion of HCOOH from CO2 under solar energy was investigated byYadav et al. [157]. The HCOOH production rate increased linearly

Photocatalyst

MMT layers

reduction and H2O oxidation; (b) schematic structure of MMT/TiO2 nanocomposites

Fig. 6. (a) Band gap position of n-CdS, n-TNT and n-Bi2S3 and relevant redox potential of CO2 for methanol production, (b) mechanism of CO2 photoreduction using MWCNTsensitized TiO2 nanocomposite doped with SWCNT for enhanced photoactivity.


under visible light irradiation and the maximum yield producedwas 55.25 lmole h�1 using CCGCMAQSP photocatalyst, whileW2Fe4Ta2O17 gave a yield rate of 7.125 lmole h�1. Cheng et al.[158] used Bi2WO6 hollow microsphere for reduction of CO2 toCH3OH under visible light energy and maximum yield of CH3OHreported was 17 lmole g cat�1 h�1. Visible light responsive plas-monic shaped Ag X: Ag (X 1=4 Cl, Br) nanoparticles were investigatedby An et al. [159] for reduction of CO2 to CH3OH under visible lightirradiation. It was reported that Ag nanoparticles produced21 lmole g cat�1 h�1 of CH3OH. This study reveals that plasmonicphotocatalysts can be used efficiently for conversion of CO2 intouseful organic compounds. The different types of photocatalystsother than TiO2 along with operating conditions and the majorproducts are listed in Table 4.

It is obvious from the above demonstration that photocatalystscontaining no titanium possess photoactivity for CO2 reductionthat in some cases is higher than the one achieved with TiO2, par-ticularly under visible light irradiations. Therefore, other VLR phot-ocatalysts TiO2 could also be employed to reduce CO2 for valuablechemicals and hydrocarbons using solar energy. However, it is verydifficult to assess the best material because of different experimen-tal conditions. In the few studies higher yield rates were reported,but yet oxidized products such as oxygen was not observed. Byconsidering the productivity and the low selectivity it is obviousthat there is considerable gap available for improvement. There-fore, further efforts are needed to explore different types of mate-rials that could confirm the higher yield rates and selectivity

desirable for industrial applications. This could be the ultimatesolution in developing sustainable phototechnology, which canguarantee the economical production of solar hydrocarbon fuels.

4.3. Advances in photocatalytic reactors

It is broadly accepted that the efficiency of a photocatalytic sys-tem mainly depends on the design of photoreactor and the type ofphotocatalyst. According to the literature, the engineering aspects,yields and selectivity of CO2 reactions were not extensively studieddue to slow reaction rates. However, there is a significant new re-search contribution in the field of phototechnology such as VLRand efficient photocatalysts operating at low temperature. It isdemonstrated that highly efficient photocatalytic materials andintegrated photocatalytic design will be capable to utilize maxi-mum solar spectrum and could result in a higher CO2 conversionrate.

Several innovations pertaining to the design of photoreactorssuggested that the physical geometry of photoreactor is moreimportant to ensure photogenerated photons are collected effec-tively. These types of reactors are less dependent on operationalparameters like temperature and pressures compared to effectiveadsorption of photons and reactants on the catalyst surface. Inaddition, there are several parameters that are important for pho-toreactor design but not included in conventional chemical reac-tors. These include selection of radiant source, source efficiency,spectral distribution, shape and dimensions as well as the design

Fig. 7. Co-metal deposited N-doped NTs for reduction of CO2 with H2O to various hydrocarbon fuels: (a) Schematic of sunlight-driven photocatalytic CO2 reduction using N-doped titania nanotube arrays doped with Cu and/or Pt co-catalyst nanoparticles; (b) depiction of flow-through nanotube array membrane for high rate photocatalyticreduction of CO2.


of irradiation device including mirrors, reflectors, window con-struction materials, and shape. In addition to efficient reactor de-sign strategies and effective catalyst, there are many operatingvariables that can affect the rate and extent of chemical transfor-mation including semiconductor concentration, reactive surfacearea, and particle aggregate size, concentration of electron donorsand acceptors and light intensity [37].

The most common types of photoreactors under investigationsfor CO2 reduction are slurry reactors, fixed bed reactors, annular/bubble flow and surface coated reactors. In 1979, Inoue et al.[15] worked on photocatalytic reduction of CO2 to different prod-ucts using slurry reactor in which catalysts were suspended inwater. During the period of 1980 to 2000, slurry type reactors wereconsidered for reduction of CO2 using UV or visible light irradiation[52,173]. However, such types of reactors provide a less active sur-face area to enhance photoactivity while they bear additional oper-ating cost due to separation of fine catalyst particles.

Furthermore, in the last 10 years, there is gradual developmentin the design of photocatalytic reactors for reduction of CO2 underUV/Visible light irradiation. Some of the reactors being investi-gated include fixed bed, annular type/bubble flow, optical fibersand monolith reactors. In a fixed-bed reactor, the catalyst is coatedor anchored on the reactor wall for efficient light radiation absorp-tion. In such reactors, higher conversion per unit mass of catalyst isachieved for plug flow regime. The fixed-bed reactor also giveslower pressure drop which enables the system to be operated un-der reduced cost and catalyst stability. Shi et al. [67] used fixed bedphotocatalytic reactor for reduction of CO2 with H2O as the reduc-ing agent. CO2 with H2O vapors were continually passed throughthe fixed-bed catalyst for photoreduction process in the quartztubes. Such types of reactors still provide lower contact exposureto light area ratio for maximum conversion and yield rates.

Wang et al. [119] deposited photocatalyst on a glass slide,which was placed inside a quartz cell. The reactor was dosed with3 Torr of H2O vapor and 0.3 Torr of CO2 while 300 W Xe lamp wasused for illumination. The long pass filters were used to remove UVlight, leaving only k > 420 nm. The intensity at the catalysts surface

was about 100 mW cm�2. The yields of the gas phase productsafter visible light illumination were 48 ppm g cat�1 h�1 of CH4,3.3 ppm g cat�1 h�1 of CH3OH with trace amount of CO and H2.

On the other hand, annular type reactors usually have two con-centric cylinders in which catalysts are coated, packed or distrib-uted. The source of light can be located at the center or aroundthe reactor. In general, the cross section of reactor tube is smallindicating higher gas velocity to ensure products desorbed on thesurface are removed effectively. Multitubes reactor consists of acylindrical vessel with number of hollow glass tubes externallycoated with photocatalysts. Such reactor configurations are similarto shell and tube type heat exchangers [174–177]. The schematicrepresentation of annular and cell type photoreactor is shown inFig. 8(a) and (b).

In 2005, Jeffrey et al. [178], for the first time used the optical fi-ber photoreactor for reduction of CO2 and H2O. The schematic rep-resentation of an optical fiber photocatalytic reactor is depicted inFig. 9. Although optical fibers can deliver light efficiently and uni-formly, the reactors have disadvantages including lower adhesionstrength, relatively low surface area and only 20–30% of the totalreactor volume is available for photocatalysis [33]. With theadvancement of technology, monolith reactors have attracted greatinterest due to their advantages compared to commercial reactors.Microchannel monolith reactors have larger light interaction sur-face area, and thus can increase the conversion and yield rates[179]. In monolith type reactors, catalysts are usually coated asvery thin film along the wall of the channels while in seldom caseschannels are filled with catalysts [180–183].

In 2011, photocatalytic reduction of CO2 with H2O to CH3OH ina monolith photoreactor was pioneered by Liou et al. [152]. Theschematic representation of the monolith photoreactor is shownin Fig. 10. High pressure mercury lamp (200 W) and sun simulatorequipped with 300 W xenon lamps were used as light source. Theillumination inside the reactor was promoted by inserting opticalfibers inside the monolith channels. The catalyst was coated insidethe channel by dip coating. The maximum production rate ob-served was 0.16 lmole g catl�1 h�1. Detailed literature about

Table 5Summary of photocatalytic reactors for photoreduction of CO2 to hydrocarbon fuels.

Year Type of reactor Capacity Feed/feed rate Reactionconditions

Light source/ intensity Catalyst dosage Production rate/efficiency Comments Refs.

2012 Pyrex glassreactor(slurry/batchtype)

– CO2 was passedthrough 50 mLwater in cell

Time = 20 h 500 W halogen lamp,I = 143 mW cm�2

0.14 g of catalysts(NiO–InNbO4 orCe3O4–InNbO4) wasdispersed inside water

CH3OH = 1.577 lmole g cat� h�1

with 0.5% NiO-InNbO4 and1.503 lmole g cat�1 h�1 yiewith 1% Ce3O4–InNbO4

0.2 M potassium carbonate wasemployed as an adsorbent of CO2.Catalysts were separated bycentrifuge and liquid sample wasanalyzed with GC-FID

[184]

2012 Pyrex glass cell(slurry/batchtype)

– CO2 was passedthrough watercell forphotoreduction

Time = 8 h 500 W xenon lamp 150 mg of ZnPc/TiO2

catalysts weresuspended in 150 mLof 0.1 M NaOH sol

CH3OH = 248.06 lmole g ca 1

using 0.6% ZnPc/TiO2

The catalyst loaded with ZnPc wasmuch more effective than bareTiO2. The yield increasedsignificantly by using microwave tosynthesize ZnPc/TiO2

[123]

2012 Cylindricalreactor withquartzwindow (batchtype)

– CO2 = 3 ml/minsaturated bypassing throughH2O tank

Time = 5 h 400 W Xe lamp,I = 19.6 mW cm�2,(k = 250–388 nm)

Pt–TiO2 was loadedinside cylindrical tube

CH4 = 1361 lmole g cat�1 h� Fast electron transfer in singlecrystal TiO2 and electron separationby Pt nanoparticle were the mainreasons of this higher yield

[94]

2012 Pyrex glass cell(batch type)

– CO2 = filled thereactor atambient pressure,H2O = 0.4 mL

– 300 W Xe lamp 0.1 g of Zn2SnO4 wasdispersed at thebottom of reactor

CH4 = 20.1 ppm g cat�1 h�1 Gas pump was used to acceleratediffusion. CH4 generation can beincreased to 86.1 ppm g cat�1 h�1

by loading Pt or RuO2 as co-catalysts

[185]

2012 Slurry typecontinuousflow reactor

– CO2 wascontinuouslybubbled throughwater

Time = 5 h 300 W Xe lamp, light withk < 400 nm was removedby 2.0 M NaNO2 sol

200 mg CdS/TNTs orBi2S3/TNTs catalystswere dispersed into200 mL of water

CH3OH = 224.6 lmole g cat� byusing Bi2S3/TNTs

The rate of methanol productionwas higher by using Bi2S3/TNT thanCdS modified TNTs. Needle typevalve was used to collect sample

[87]

2011 Stainless steelreactor withquartzwindow (batchtype)

V = 51.5 mL CO2 and H2O Time = 15 h,T = 75 �C,

UV lightintensity = 20 mW cm�2

(k = 254 nm), visible lightintensity = 350 mW cm�2

(k = 532 nm)

Au/TiO2 catalyst film(10 cm2) was placed onsample holder

CH4 yield was 22.4 lmole m 2

using visible light and�230 lmole m�2 with UV l ht

The irradiated surface area islimited with surface area ofcatalysts. 300 lL was used foranalysis after 15 h. The C2H6,CH3OH, and HCHO were alsoproduced at higher photonintensity

[146]

2011 Pyrex reactor(batch type)

V = 200 mL H2O = 50 mL,CO2 = 3 kg cm�2

T = 25 �C Xenon lamp.I = 100 mW cm�2, filterswere used to pass lightwith wavelength 390–770 nm

0.1 g of N–InTaO4 wasadded to reactantsolution (50 mL H2O).

Methanol was the main pro uctwith is reported to be linea yincreased with time

2 ml of liquid was extracted afterevery 30 min for analysis

[154]

2011 Quartz reactorwith stainlesssteel walls(batch type)

V = 58 cm3 CO2, H2O – 450 W Xe, filters wereused to cut off k < 400 nm

200 mg I–TiO2 wasdispersed on glass fiberfilter and placed at thebottom of reactor,Band gap = 3.0 eV

CO = 2.4 lmole g cat�1 h�1 u ing10% I–TiO2

A gas tight syringe was used toinject sample to GC after every30 min. I–TiO2 calcined at 375 �Chas highest CO2 reduction thancalcined at 450 and 550 �C

[147]

2011 Quartz reactor(batch type)

Area = 4.2 cm2,V = 230 mL

CO2 = filled thevolume ambientpressure,H2O = 1 mL

Time = 5.5 h 300 W Xe, UV cut off filterk > 420

0.1 g Bi2WO6 catalystswas dispersed on glasssurface

CH4 = 6 lmole or1.1 lmole g cat�1 h�1

0.5 mL of sample was continuouslytaken from the reactor for analysis

[44]

2011 Quartz glassreactor (batchtype)

V = 210 mL CO2 was bubbledthrough distilledwater in glass cell

Time = 6 h 250 W high pressure Hg,300 Xe lamp with cut offfilter less than 420 nm

50 mg of CuO/TiO2 wasdispersed in 100 mL ofdistilled watercontaining 0.05%Na2CO3 and0.1%Na2SO3. 50 mg Pt/TiO2 was dispersed inabove filtrate

H2 = 128.2 lmole g cat�1 h� byusing Pt/TiO2 after pre-irrad tionwith CuO/TiO2

After 6 h of irradiation with Hglamp, the solution was filtered andcatalysts were replaced with Pt/TiO2

[46]

2010 Quartz reactor Dead CO2 = 150 lmole, Time = 24 h, 200 W Hg–Xe 2.0 g of ATaO3 (A = Li, CO = 0.42 lmole over LiTaO Li doped TaO2 has higher yield [186]

(continued on next page)

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Table 5 (continued)

Year Type of reactor Capacity Feed/feed rate Reactionconditions

Light source/ intensity Catalyst dosage Production rate/efficiency Comments Refs.

(circulatingtype)

space = 150 mL H2 = 50 lmole, T = 303 K Na, K) was spread overflat bottom

compared to Na, and K. No productwas observed under k > 310 nmusing UV-29 cut filter

2009 Photocatalysiscell

– CO2 = 0.3 Torr,H2O = 3 Torr

– 300 W Xe, UV filters wereused to cut off k < 420 nm,I = 100 mW/cm2

300 mg CdSe/Pt/TiO2

was deposited on glassslide and placed insidecell

CH4 = 48 ppm g cat�1

CH3OH = 3.30 ppm g h�1The trace amounts of CO and H2

were also observed. CdSe sensitizedPt/TiO2 efficiency can further beenhanced by optimizing withquantum dots (QDs)

[119]

2008 Pyrex glassreactor withoptical fibers

V = 216 cm3 CO2 gas and H2Ovapor were wasmaintained at0.72 bar

T = 75 �C, 150 W Hg lamp, solarirradiation

Cu–Fe/TiO2–SiO2

coated fibers wereinsert inside reactor

CH4 = 0.914 lmole g h�1,C2H4 = 0.575 lmole g h�1

using 0.5 wt.% of Cu e

Ethylene and methane were majorproducts while trace amounts ofethane and methanol were alsoobserved

[108]

2008 Quartz reactor(Batch type)

V = 30 cm3,A = 14 cm2,H = 2.14 cm

CH4 = 200 lmole,CO2 = 200 lmole

Time = 3 h,T = 314–673 K

300 W Xe, I = 9 mW cm�2

at k = 220–300 nm0.2 g Ga2O3 spreadover flat bottom

C2H6 = 1.04 lmole,H2 = 2.48 lmole,CO = 1.08 lmole,C2H4 = 0.12 lmole

CH4 was mainly reduced to ethaneand hydrogen at 314 K but withCO2, CO was also produced at 473 K.The traces of other hydrocarbonswere also observed

[61]

2007 Pyrex glass cell(batch type)

V = 75 ml CO2 and H2O Time = 20 h 500 W halogen lamp, 0.14 g NiO/InTaO4 wasdispersed in 50 ml ofsolution

CH3OH = 1.394 lmole t�1 h�1

using 1 wt.% NiOPotassium bicarbonate (0.2 M)solution was used as CO2 absorbent.CH3OH yield was increased withNiO

[187]

2005 Optical fiberreactor

Dia. of glasstube = 3.2 cm,number offiber = 120 fiberdia. = 112 lm,fiberlength = 16 cm

CO2 = 3 ml min�1 T = 75 �C,CO2 = 1.29 bar,H2O = 0.026 bar,mean residencetime = 5000 s

Hg lamp, I = 1–16 W cm�2, k = 365 nm

53 nm film thicknessof 1.2% Cu/TiO2 wascoated on fibers by dipcoating. Bandgap = 3.31 to 3.37 eV

methanolyield = 0.42 lmole g c h�1

Methanol yield increased with lightintensity up to 16 W cm�2. Higherthan 1.2% copper loading gavelower rate of methanol yield

[178]


Deadspace = 18.9 mLIlluminatedarea = 12.6 cm2

CO2 = 150 lmole,CH4 = 50 lmole

Time = 5 h 500 W, ultra-highpressure Hg

0.3 g MgO was spreadover flat bottom

CO = 3.6 lmole, H2 = lmole,CO2 conversion = 9.0% r 30 h

HCHO/CH3CHO (5 lmole) was alsoanchored as reaction substrate onMgO as photoactive species toreduce CO�2 to CO. CH4 consumedup to 12 h irradiation (20 lmole)beyond this it was constant

[21]

2004 Quartz fixedbed reactor(continuousflow)

Dia. oftube = 35 mm

CO2, CH4 T = 373–473 K,CO2/CH4 of 1:1,P = 1 atm, spacevelocity = 200 h�1

125 W ultra-highpressure HgI = 20.0 mW cm�2

10 g of Cu/CdS/TiO2–SiO2 was used in fixedbed

CO2 and CH4 convers f 0.7and 1.47%. Selectivity cetone,ethane and CO was 9 3.1%and 4.6%

The best selectivity of acetone(92.3%) was at 120 K. Attemperature up to 200 �C,conversion of CO2 and CH4

decreases dramatically

[67]

1998 Quartz cell(batch type)

– CO2 = 24 lmole,H2O = 120 lmole

T = 328 K Higher pressure Hg lamp,k > 280 nm

150 mg of TiO2–SiO2

was dispersed overbottom of reactor

CH4 as main product TiO2

and decreases with lo Ticontent

The selectivity of CH3OH stronglydepends on Ti content

[188]


A = 12 cm2 CO2 = 150 lmole,H2 = 50 lmole

Time = 6 h 500 W high pressure Hglamp, glass filters wereused which allowed 50%light at k 290, 370 and450 nm

0.3 g Rh/TiO2 wasdispersed over the flatbottom of reactor

CO = 9.2 lmole, CH4 = mole Other hydrocarbon and oxygenatedcompound were not found. COproduction decreased rapidly byusing filters in order nofilter > 290 nm > 370 nm > 450 nm.Loading of Rh also increasedactivity

[72]

1984 Pyrex flaskreactor

– CO2 = 100 ml/min Time = 0–17 h,T = 60 �C

75 W high pressuremercury lamp,I = 140 W m�2

0.8–1 g of 1% RuO2

TiO2 was dispersed in160 mL of 0.1 M Li2CO3

HCOOH = 1.47 lmoleHCHO = 0.13 lmole,CH3OH = 0.215 lmol

The CO2 was passed through seriesof flasks containing water and thenentered into reaction flask. Theproducts were passed throughseries of traps at 0 �C and samplewas collected after one day

[173]

208M

.Tahir,N.A

.S.Am

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cat�1

cat�1

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withwer

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Fig. 8. (a) Schematic representation of the annular reactor with the lamp outside the shell, (b) schematic of cell type catalyst suspended photoreactor for CO2 reduction.


different types of photocatalytic reactors used for CO2 reduction issummarized in Table 5.

4.4. Future prospects for sustainable phototechnology

In this review, various types of photocatalysts and reactors forphotocatalytic CO2 reduction are discussed in detail. The progresstowards VLR TiO2 based photocatalysts indicates the system ispromising to be used to harvest solar spectrum. In the last fewyears, CO2 has been successfully converted to solar hydrocarbonfuels utilizing VLR photocatalysts containing TiO2 and other typesof catalysts having no titanium. However, the yields rate and selec-tivity of desired products such as CH4, CO and CH3OH from photo-catalytic CO2 reduction were lower than the appreciable target.General observations throughout the literature reveal that no com-plete data are available to evaluate the efficiency of the system.Furthermore, the influence of various operating parameters includ-ing light intensity, CO2 concentration, temperature, reactor pres-sure, solution pH and other experimental conditions on the

Fig. 9. (a) Outlines of gas flow and light reflection in optica

catalytic performance have not been discussed. Besides, most ofthe research work focused on developing new catalysts using slur-ry system (liquid phase), whereas the information for evaluatingthe photoreactor performance are inadequate.

In order to develop efficient CO2 reduction phototechnology,similarly importance should be given to the basic pillars of photo-catalysis i.e. photoreactor and catalyst. The design of photocata-lytic reactor is efficient only if there is good interaction betweenthe four phases (liquid, gas, solid and light) for gas–liquid systemsand three phases (gas, solid and light) for gaseous systems. The dis-tribution of catalyst and light inside the photocatalytic systemplays an important to enhance conversion and yield rates. In thisregard, the monolith photoreactors are considered more efficientas they have higher surface area, more catalyst loading and gooddistribution of light over the catalyst surface due to theirmicrochannels.

In addition to the catalytic activity of fresh materials, importantdata referring to the stability of the photocatalyst and its durabilityshould also be reported. It is also important to consider all

l fibers, (b) images of photoreactor with optical fibers.

Fig. 10. (a) Schematics of the monolith reactor and illumination fibers, (b) theinternally illuminated monolith photoreactor for CO2 reduction with H2O.


operating parameters such as initial CO2 concentration, reactiontime, temperature, light intensity, system pressure and activity de-cay over time. Absolute quantum efficiencies with standard solarlight would be highly desirable to develop standard conditions torank the activity of different materials. These factors would behelpful to evaluate the average productivity that is missing in themajority of reported studies and are important for complete anal-ysis of the CO2 photoreduction process.

5. Conclusions

Numerous scientific research studies have been attempted todevelop economically viable processes for the conversion of CO2

to solar fuels and other value added chemicals. The progress to-ward VLR TiO2 based photocatalysts is appreciable and CO2 hasbeen successfully converted to solar fuels. In this perspective, thefindings from this review are:

(1) The photocatalytic CO2 reduction with H2O into hydrocar-bon fuels is a favorable process for a smooth transitiontoward solar hydrocarbon fuels. For this purpose, photocat-alyst plays an important role to produce specific products.Using both TiO2 based and no titanium photocatalysts dur-ing photocatalytic CO2 reduction, CO, CH4, CH3OH, HCOOHand HCHO have been reported as the most potential prod-ucts. However, the major limitations reported in the major-ity of the previous studies were lower conversion andselectivity. It is difficult to determine the product distribu-tion using a single analytic technique; some of the presenteddata in various studies can be unreliable because the forma-tion of all possible products has not been investigated.

(2) Among doping agents, both metals and non-metals havebeen studied extensively to extend TiO2 spectral responseand to improve its photoactivity, but co-metals are moreefficient in terms of conversion and selectivity. Some metals

are more favorable to improve selectivity and their selectionis very important. However, during selection of the prefera-ble composition of the semiconductor for realistic industrialapplications, the price and toxicity of the elements will beimportant. In this regard, the use of noble metals and toxictransition metals should be avoided. Thus, non-metals aremore preferable for the specific case of cost-effectiveprocess.

(3) Among sensitizers, coupling semiconductors, dyes, carbonnanotubes and novel sensitizers are under investigations toimprove TiO2 photoactivity. The higher efficiency of TiO2

was observed when TiO2 nanocomposites consisting bothdopants and sensitizers were employed in the conversionof CO2 to solar hydrocarbon fuels. As an alternative to TiO2,many other types of visible light driven semiconductor withnarrow band gap are also investigated for CO2 photocatalyticreduction. One key issue that requires conceptual under-standing is the role that a co-catalyst and heterojunctioncan play in increasing the efficiency of the process. By com-paring TiO2 photocatalytic activity with non-titanium phot-ocatalysts, TiO2 was very efficient, while otherphotocatalysts also found effective particularly under visiblelight irradiations. Because of the different experimental con-ditions it was very difficult to assess the best material at thepresent time. However, by considering the productivity andlow product selectivity, it is clear that there is a considerablegap for improvement.

(4) Among photoreactors, optical fibers and monolith photore-actor are more prominent due to the higher illuminated sur-face area and efficient utilization of photon energy.However, majority of reports provide inadequate informa-tions to evaluate the efficiency of photocatalytic system.The efficiency of photocatalysis process could be increasedby operating at optimum process conditions. As future view-point, serious considerations are desirable to design a photo-reactor that could directly utilize direct solar irradiations.

(5) In general, to develop CO2 based solar fuels, three importantaspects have to be considered in the future. The first andmost important is how to select photocatalyst that couldimprove reaction selectivity toward CH3OH (the preferredsolar fuel); the co-catalyst system combined with sensitizerswill play a major role in this process. Next, the selection ofphotoreactor that can efficiently utilize direct solar irradia-tions. Finally, an estimation of the cost, which is importantfor the evaluation of data that can confirm the current sys-tems as the real alternative to fossil fuels.

Acknowledgements

The authors would like to extend their deepest appreciation tothe Ministry of Higher Education (MOHE) and Universiti TeknologiMalaysia for the financial support given to this research underLRGS (Long-term Research Grant Scheme) and RUG (Research Uni-versity Grant) research grants, respectively.

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