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Chapter 1

Recent Advances (2012–2015) in the
Photocatalytic Conversion of
Carbon Dioxide to Fuels Using Solar Energy:
Feasibilty for a New Energy
Yasuo Izumi*
Department of Chemistry, Graduate School of Science, Chiba University,
Yayoi 1-33, Inage-ku, Chiba 263-8522, Japan*E-mail: yizumi@faculty.chiba-u.jp

In this chapter, recent advances in photocatalytic CO2
conversion with water and/or other reductants are reviewed
for the publications between 2012 and 2015. Quantitative
comparisons were made for the reaction rates in μmol h−1
gcat−1 to acertain the progress of this field although the rates
depends on photocatalyst conditions and reaction conditions(temperature, pressure, and photon wavelength and flux). TiO2
photoproduced methane or CO from CO2 and water at rates
of 0.1–17 μmol h−1 gcat−1 depending on the crystalline phase,crystalline face, and the defects. By depositing as minimal
thin TiO2 film, the rates increased to 50–240 μmol h−1 gcat−1.
Gaseous water was preferred rather than liquid water formethane/CO formation as compared to water photoreduction to
H2. Pt, Pd, Au, Rh, Ag, Ni, Cu, Au3Cu alloy, I, MgO, RuO2,
graphene, g-C3N4, Cu-containing dyes, and Cu-containing
metal-organic frameworks (MOFs) were effective to assist theCO2 photoreduction using TiO2 to methane (or CO, methanol,
ethane) at rates of 1.4–160 μmol h−1 gcat−1. Metals of greater
work function were preferred. By depositing as minimal thinphotocatalyst film, the rates increased to 32–2200 μmol h−1
gcat−1. The importance of crystal face of TiO2 nanofiber was
suggested. As for semiconductors other than TiO2, ZnO,
Zn6Ti layered double hydroxide (LDH), Mg3In LDH, KTaO3,
In(OH)3, graphene, graphene oxide, g-C3N4, CoTe, ZnO,
© 2015 American Chemical Society
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ZnTe, SrTiO3, ZnGa2O4, Zn2GeO4, Zr–Co–Ir oxides, Nb2O5,
HNbO3, NaNbO3, InNbO4, NiO, Co3O4, Cu2O, AgBr, carbon
nanotube, and the composites of these were reported to form
methane, CO, methanol, acetaldehyde from CO2 and waterat rates of 0.15–300 μmol h−1 gcat−1 that were comparable
to rates using promoted TiO2. The band energy designs
comprising appropriate conduction band for CO2 reduction
and valence band for water oxidation were made progresses
in these semiconductors and semiconductor junctions in thethree years. If H2 was used as a reductant, Ni/SiO2-Al2O3
formed methane at 423 K under pressurized CO2 + H2 at a rateof 55 mmol h−1 gcat−1. This rate was not enabled by heating
the system under dark, suggesting photoactivated reaction
followed by thermally-assisted reaction(s) via Ni–H species.
As pure photocatalytic reactions from CO2 + H2, methanol
formation rates were improved up to 0.30 μmol h−1 gcat−1 by
the doping of Ag/Au nanoparticles, [Cu(OH)4]2− anions, andCu-containing dyes to Zn–Ga LDH. Furthermore, sacrificial
reductants, e.g. hydrazine, Na2SO3, methanol, triethanol
amine, and triethyamine, were also utilized to form CO,formate, and methanol at rates of 20–2400 μmol h−1 gcat−1
using semiconductor or MOF photocatalysts. Finally, similar
to the integrated system of semiconductor photocatalyst forwater oxidation and metal complex/enzyme catalyst for CO2(photo)reduction, two semiconductors (WO3, Zn–Cu–Ga LDH)
were combined on both side of proton-conducting polymer
to form methanol at a rate of 0.05 μmol h−1 gcat−1 from CO2
and moisture. These promotion of photoconversion rates of
CO2 and new photocatalysts found in these three years haveindicated the way beyond for a new energy.

Introduction
To enable carbon neutral cycle in modern industrial society, CO2 conversion
into fuels utilizing natural (sustainable) energy is one of the ideal methods.
Carbon monoxide, methane, methanol, acetaldehyde, and ethane obtained from
CO2 are attractive products because they can be easily intergrated into the existing
fuel and chemical technology. Although various review articles have been
published dealing with the conversion of solar energy into fuels (1, 2) and theconversion of CO2 into fuels (3–8) until 2012, the researches and developments
are more intensive during 2012 and 2015 for the target of CO2 reduction into
fuels that enables carbon neutral and suppresses the anthropogenic increase of
CO2 concentrations in the atmosphere.
In this chapter, recent advance based on various intensive researches between
2012 and 2015 in the CO2 photoconversion into fuels is reviewed in hope for a newenergy. Similar to previous review for studies before early 2012 (4), the scope2
In Advances in CO2 Capture, Sequestration, and Conversion; He, et al.;ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

of this chapter is limited to semiconductor photocatalysis for the conversion of
CO2. Especially, some orders of increase for the CO2 conversion rates and new
photocatalysts are focused.

Photon Energy Conversion of CO2 to Fuels with Water
1. TiO2 Photocatalysts

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1.1. Difference of Crystal Phases
The CH4 formation rate was 0.19 μmol h−1 gcat−1 in atomospheric pressure of
CO2 and moisture using TiO2 (9). Under similar reaction conditions using somecrystal phases of TiO2, the CO formation rates were in the order

If the TiO2 photocatalysts were preheated at 483 K in a flow of helium to makedefects, the CO formation rates were in the order

The order did not change if minor methane formation was added to the COformation (Table 1). The binding of CO2 on Ti3+ sites and the reduction of thebound CO2− species by O-defect sites were suggested (10).

The activity order reported in reference (11) was consistent with equation 1.
Furthermore, the anatase:brookite phase ratio was varied by changing the amoutof urea used during the TiO2 synthesis. The improved photoconversion of CO2
suggested heterojunction effects of two crystalline phases.

3
In Advances in CO2 Capture, Sequestration, and Conversion; He, et al.;ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

Table 1. Reported CO2 Photoreduction Catalysts, Reaction Conditions, and the Formation Rates in Water/with Moisture Using TiO2
T (K)Light source

Reactor

Major product{formation rate
(μmol h−1 gcat−1)}

Ref

Room T

100W Hg

ClosedCH4 (0.19)(9)150 W solar
simulator

Stainless steel,Flow

CO (3.3)(10)

TiO2 (rutile)

CO (0.82)(10)

TiO2 (brookite)

CO (1.5)(10)

Defective TiO2
(anatase)aCO (10.4), CH4
(2.5)

(10)

Defective TiO2
(rutile)aCO (4.0)(10)

Defective TiO2
(brookite)aCO (17.0), CH4
(1.9)

(10)

CO (1.2)

(11)

TiO2
(atatase:brookite=3:1

CO (2.1)(11)

TiO2 (brookite)

CO (0.7)(11)

CH4 (1.2)(12)

Reactants

4

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Photocatalyst
Brand nameAmount (mg)

CO2

H2O

P25

10

Atmosph. P

Atmosph. P

TiO2 (anatase)100

99 kPa2.3 kPa

TiO2 (anatase)

TiO2 (anatase)
{010}

100

100

Atmosph. P

101 kPa

Saturated

Liq (1 mL)

150 W solar
simulator

293

300 W Hg

Stainless steel

Pyrex

In Advances in CO2 Capture, Sequestration, and Conversion; He, et al.;ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

Reactants

Ref

TiO2 (anatase){101}

CH4 (0.74)(12)

TiO2 (anatase){001}

CH4 (0.19)(12)

CH4 (0.15)(13)

TiO2 (anatase){001}:{101}=58:42CH4 (1.4)

(13)

TiO2
(anatase){001}:{101}=83:17

CH4 (0.55)(13)

Brand nameTiO2 (anatase){001}:{101}=11:89Amount (mg)

CO2

T (K)

Light source

H2O

100

5

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Reactor

Major product{formation rate
(μmol h−1 gcat−1)}

Photocatalyst

P25

20

200 kPa12.3 kPa

323

100 W Xe

Stainless steel

CO (1.2), CH4(0.38)

(15)

P25

20

200 kPa

Liq (4 mL)

323

100 W Xe

Stainless steel

CO (0.80), CH4(0.11)(15)

TiO2 (anatase)

500

Saturated

Liq (1 L)

303

UV lamp

Pyrex

CH4 (11)(18)

TiO2 nanofiber

5

Atmosph. P

Satur. P

CO (12), CH4 (6)(20)

TiO2 (anatase)

Film

Atmosph. P

Satur. P

CO (240)(26)

500 W Xe
Room T

400 W Xe

Stainless steel,FlowContinued on next page.

In Advances in CO2 Capture, Sequestration, and Conversion; He, et al.;ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

Reactants

Photocatalyst
Brand nameAmount (mg)P25

1.25 cm2
Film

TiO2

100

CO2

H2O

190 kPa

Satur. P

T (K)Light source

Reactor

Major product{formation rate
(μmol h−1 gcat−1)}

Ref

<333

1000W Xe

Aluminum

CH4 (50)(27)

CH4 (0.52)(29)

a Heated at 493 K for 90 min in a flow of He.

6

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Table 1. (Continued). Reported CO2 Photoreduction Catalysts, Reaction Conditions, and the Formation Rates in Water/with
Moisture Using TiO2

In Advances in CO2 Capture, Sequestration, and Conversion; He, et al.;ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

1.2. Difference of Crystal Facets

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The reactivity order for facets in the CO2 photoreduction to methane using
anatase TiO2 was reported

The ratio of exposed {0 0 1} face versus thermodynamically stable {1 0 1}
face was progressively varied between 11:89 and 83:17 for anatase TiO2 utilizing
the F− as stabilizing agent for the {0 0 1} face (13). The conduction band (CB)and valence band (VB) for {0 0 1} face were calculated to position at slightly
negative energy, thereby photoexcited electrons and holes move to {1 0 1} and{0 0 1} faces, respectively, similar to normal heterojunction of two kinds ofsemiconductors (Figure 1). The rates of CO2 photoreduction to methane reachedthe maxium when the exposed face ratio of 58:41 (1.4 μmol h−1 gcat−1; Table 1).
Related to the study, hollow anatase TiO2 dominating {1 0 1} face wassynthesized and the methane formation rate using TiO2 doped with 1 wt% RuO2
was by a factor of 1.5 higher than general hollow anatase TiO2 (Table 2) (14).

1.3. Difference of Reactant Water Phases
The photoreduction CO2 (200 kPa) with gaseous water (12.3 kPa) and liquidwater was compared (Figure 2) (15). Total formation rates of CO and methane
using TiO2 were improved by a factor of 1.7 with gaseous water (1.6 μmol h−1
gcat−1) rather than with liquid water.

Figure 1. The control of {001} and {101} exposed face ratio for anatase TiO2
for photocatalytic CO2 reduction (13). Reproduced from reference listing (13).
Copyright 2014, ACS.

7In Advances in CO2 Capture, Sequestration, and Conversion; He, et al.;ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

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Figure 2. Reactors used for photocatalytic reduction of CO2 with H2O. Left: TiO2
under moisture, right: TiO2 immersed in liquid water. The distance between the
Xe lamp and the catalyst was adjusted to be the same for the two reaction modes(15). Reproduced from reference listing (15). Copyright 2014, ACS.

2. Metal-Loaded TiO2 Photocatalysts
2.1. Comparisions of Metal
The photoreduction CO2 (200 kPa) with gaseous water (12.3 kPa) and liquidwater was compared (Figure 2) (15). The improvement of total photoreduction
rates of CO2 using Pt-TiO2 was by a factor of 2.9 (6.3 μmol h−1 gcat−1) (Table2). This improvement was greater compared to the case for TiO2 (Table 1). The
comparison between solid–gas and solid–liquid interface reactions was also madefor CO2 photoreduction over layered double hydroxide (LDH) photocatalyst (16).
The major pathways were CO2 reduction to methanol and proton reductions to H2,
respectively.
Total photo-formation rates of methane and CO from CO2 and gaseous water
were in the order:

following the order of work function (WF)

8In Advances in CO2 Capture, Sequestration, and Conversion; He, et al.;ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

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The selectivity for CO2 photoreduction as compared to H2O photoreduction
to H2 was 56% using TiO2, but it decreased to 39–45% by the addition of Pt, Pd,
Au, Rh, and Ag because the trapped electrons on these metals thermodynamically
preferred to transfer to protons [E° = 0 – 0.0591 × pH V, versus standard hydrogenelectrode (SHE)] rather than both CO2 and protons (E° = −0.32 – 0.0591 × pH V,
versus SHE) (16).
Solvothermal synthesis of anatase TiO2 with Ni ions was done and the band
gap slightly decreased from 3.16 eV for TiO2 to 3.00 eV for Ni-TiO2. Accordingly,
photocatalytic production rate of methane from CO2 increased from 11 (TiO2) to14 μmol h−1 gcat−1 (Ni-TiO2; Table 2) (18). Ni-TiO2 prepared via sol–gel method,coated on monoliths, and threaded with quartz plate photocatalytically converted
CO2 into methanol at a rate of 20 μmol h−1 gcat−1 (Table 2) (19). The reductionof CO2 on TiO2 and oxidation of water on NiO were assumed utilizing the p–nheterojunction.
The effects of Au and Pt loading on TiO2 were also tested by preparing
nanofiber TiO2 by electrospinning by appling 15 kV between needle for sample
precursors and collector (20). Under atmospheric pressure of CO2 saturated
with moisture irradiated by UV-visible light, the TiO2 nanofiber formed COand minor methane (Table 1). The selectivity changed to form major methaneby the loading of Au (62 μmol h−1 gcat−1) and Pt (84 μmol h−1 gcat−1). The
generation rate of mehane further improved to 110 μmol h−1 gcat−1 using Au(0.25
atomic%)-Pt(0.75%)-TiO2 nanofiber (Table 2). Au was metallic state whereas Pt
was Pt0, Pt2+, and Pt4+ states in these photocatalysts. Electron trap effect due togreater WF for Pt and surface plasmon resonance (SPR) effect at 540–590 nm for
Au worked synergetically in the Au(0.25 atomic%)-Pt(0.75%)-TiO2 nanofiber.
The relatively higher formation rates compared to reference (15) would be thedifference of amount of photocatalyst used (5 mg (20) versus 20 mg (15)).
Doubly doping of Cu(0.1%) and I(10%) to TiO2 was effective to photoconvert
CO2 into CO at a rate of 12 μmol h−1 gcat−1 in comparison to that (0.7 μmol h−1
gcat−1) using undoped TiO2 (Table 2) (21). Double-walled TiO2 nanotube promoted
Cu and Pt with the ratio 1:2 showed good hydrocarbon generation rates of 154–164μmol h−1 gcat−1 in pressurized CO2 conditions (Table 2) (22).
The combination of graphene and TiO2 was reported to effectively form
ethane at a formation rate of 17 μmol h−1 gcat−1 (23) and CO at a formation rate of
8.9 μmol h−1 gcat−1 (24) from CO2 and water (Table 2). Nitrogen-doped TiO2 was
combined with graphitic carbon nitride (g-C3N4) (25). The assembly preparedfrom urea and Ti(OH)4 with the molar ratio of 7:3 at 853 K was the best to form
CO at a rate of 12 μmol h−1 gcat−1 (Table 2). While graphene is regarded as amedia to shuttle electrons from TiO2 (23), g-C3N4 (CB minimum: −1.12 V versus
SHE) was considered as electron donor to TiO2 (25).

9In Advances in CO2 Capture, Sequestration, and Conversion; He, et al.;ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

10

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Table 2. Reported CO2 Photoreduction Catalysts, Reaction Conditions, and the Formation Rates in Water/with Moisture Using
Metal, Metal Oxide, or Other Additive-Loaded TiO2
T (K)Light source

Reactor

Major product{formation rate
(μmol h−1 gcat−1)}

Ref

Satur. P

300 W Xe

Teflon lined
StainlessCH4 (1.7)

(14)

323

100 W Xe

Stainless steel

CH4 (5.2), CO (1.1)(15)

Pd(0.5%)-P25

CH4 (4.3), CO (1.1)(15)

Au(0.5%)-P25

CH4 (3.1), CO (1.5)(15)

Rh(0.5%)-P25

CH4 (3.5), CO(0.62)(15)

Ag(0.5%)-P25

CH4 (2.1), CO (1.7)(15)

MgO(1.0%)Pt(0.5%)-P25

CH4 (11), CO(0.03)(15)

Reactants

Photocatalyst
Brand namea

Amount (mg)

CO2

RuO2-hollow
TiO2 {101}
major

100

60 kPaPt(0.5%)-P25

20

200 kPa

H2O

12.3 kPa

Pt(0.5%)-P25

20

200 kPa

Liq (4 mL)

323

100 W Xe

Stainless steel

CH4 (1.4), CO(0.76)(15)

Ni-TiO2(anatase)

500

Saturated

Liq (1 L)

303

UV lamp

Pyrex

CH4 (14)(18)

Ni(1.5%)TiO2 monolith

200

Atmosph. P

Satur. P

200 W Hg

Pyrex

CH3OH (20)(19)

Au(1%b)-TiO2
nanofiber

5

Atmosph. P

Satur. P

500 W Xe

CH4 (62), CO (40)(20)

In Advances in CO2 Capture, Sequestration, and Conversion; He, et al.;ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

ReactantsRef

5

CH4 (84), CO (16)(20)

Au(0.25%b)-Pt(0.75%b)TiO2 nanofiber5

CH4 (110), CO (54)(20)

Cu(0.1%)I(10%)-TiO2

100

Atmosph. P

Satur. P

CO (12)(21)

Cu-Pt2/TiO2
Nanotube

6.7

99.9%

Satur. P

CO (12)

(22)

1.0%

Satur. P

CH4 (160)(22)

C2H6 (17), CH4
(8.0)

(23)

CO (8.9)(24)

Brand namea

Amount (mg)Pt(1%b)-TiO2
nanofiber

11

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Reactor

Major product{formation rate
(μmol h−1 gcat−1)}

Photocatalyst

CO2

T (K)

Light source

H2O

450 W Xe
(λ>400 nm)
Solar
Simulator,
AM1.5High P cell

CH4 (120)graphene(2%)TiO2

100

Atmosph. P

Liq (0.4 mL)

300 W Xe arc

graphene-TiO2

10

Atmosph. P

Liq (0.4 mL)

300 W Xe arc

g-C3N4-NTiO2

100

Atmosph. P

Satur. P

303

300 W Xe arc

TeflonCO (12)(25)

Pt(0.21%b)TiO2

Film

Atmosph. P

Satur. P

Room T

400 W Xe

Stainless steel,Flow

CH4 (420)(26)

CH4 (1400)

(26)

Glass

Pt(0.94%b)TiO2

Continued on next page.

In Advances in CO2 Capture, Sequestration, and Conversion; He, et al.;ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

12

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Table 2. (Continued). Reported CO2 Photoreduction Catalysts, Reaction Conditions, and the Formation Rates in Water/with
Moisture Using Metal, Metal Oxide, or Other Additive-Loaded TiO2
Reactants

Reactor

Major product{formation rate
(μmol h−1 gcat−1)}

Ref

Pt(1.3%b)TiO2

CH4 (630)(26)

Pt(2.5%b)TiO2

CH4 (63)(26)

CH4 (32)(27)

CH4 (44)(27)

Photocatalyst
Brand namea

Amount (mg)Au(1.5%)-P25

50

Au(0.5%)Cu(1%)-P25

50

CO2

T (K)

Light source

H2O

190 kPa

Satur. P<333

1000W Xe

Aluminum25

125 W HP Hg

none

(27)

25

150 W Xe
(λ>400 nm)

CH4 (0.89)(27)

Cu(1.5%)-P2550

CH4 (40)(27)

Au(1.5%)-P25

0.6 (1.25 cm2)CH4 (210)(27)

Au(0.5%)Cu(1%)-P25

CH4 (2200)(27)

Cu(1.5%)-P25

CH4 (280)(27)

CH4 (0.3)(28)

Cu(bpy)2-P25

100

Gas

Gas

λ>400 nmIn Advances in CO2 Capture, Sequestration, and Conversion; He, et al.;ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

Reactants

Photocatalyst
Amount (mg)Cu3(btc)2core|TiO2shell300

Cu3(btc)2

200

CO2

H2O

150 kPa

Liq (5 mL) not
immerseda The loading ratio is weight % except for b.

T (K)Light source

Reactor

Major product{formation rate
(μmol h−1 gcat−1)}

Ref

313

300 W Xe arc(λ>400 nm)

Stainless steel

CH4 (2.6)

(29)

none

(29)

b Atomic %.

13

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Brand namea

In Advances in CO2 Capture, Sequestration, and Conversion; He, et al.;ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

2.2. Effective Thin Layer Photocatalysts

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The CO2 reduction rates using moisture and Pt-supported TiO2 were improvedby the deposition of photocatalyst thin layer (26). Ti(i-OC3H7)4 was bubbled byN2 gas and deposited on indium tin oxide (ITO)-coated glass maintained at 773
K. Then, platinum was sputtered on the TiO2 film. Based on the images of fieldemission scanning electron microscopy (FE-SEM), the TiO2 film comprised single
crystal rods grown on the glass with a typical column diameter of 250 nm andheight of 1.5 μm and possessing an orientation of anatase {1 1 2} plane (Figure 3).

Figure 3. FE-SEM image of thin Pt-TiO2 film deposited from Ti(i-OC3H7)4 and Pt
sputtering (26). Reproduced from reference listing (26). Copyright 2012, ACS.

Due to the high dispersion, the TiO2 film produced CO at higher rate per unit
amout of photocatalyst: 240 μmol h−1 gcat−1 from CO2 and moisture (Table 1).
When the Pt atomic % was 0.21, 0.94, 1.3, and 2.5, the average Pt particle size
was 0.63, 1.0, 1.3, and 1.9 nm, respectively. By the doping of Pt, the selectivity
changed from CO to CH4. This selectivity change was ascribed that sufficientelectrons were accumulated in Pt and thermodynamically stable CH4 was produced
rather than CO. The CH4 formation rates were maximal at 0.94 atomic % of Pt. By
checking the TiO2 surface coverage of Pt nanoparticles, the increase of methane
formation rates until 0.94 atomic % of Pt was ascribed to the increased number ofeffective Pt sites. The decrease above 0.94 atomic % of Pt was ascribed to particle
size effects of Pt. Namely, the reduction potential for Pt nanoparticles is similar
to the CB of TiO2 (≈0 V versus SHE) and electrons excited in CB of TiO2 would
shift to Pt (Figure 4, middle). In contrast, when the Pt nanoparticle size increaseat the higher loading, the potential shift to +1.21 V versus SHE as the WF of Pt is5.65 eV, thereby both electrons and holes tend to shift to Pt and recombine (Figure
4, right) (26).

14In Advances in CO2 Capture, Sequestration, and Conversion; He, et al.;ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

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The CO2 reduction using moisture and metal-supported TiO2 was further
improved irradiated by 1000 W-Xe lamp, pressurized reactor cell (190 kPa of
CO2), and also the deposition of photocatalyst thin layer (0.6 mg per 1.25 cm2)(27). Using sequentially-deposited and hydrogen-reduced (@673 K) Au(0.5wt%)-Cu(1.0 wt%)-TiO2 photocatalyst, methane was formed at a rate of 2200
μmol h−1 gcat−1 (Table 2). In comparison to the performance of Au(1.5 wt%)-TiO2
and Cu(1.5 wt%)-TiO2 photocatalysts (210–280 μmol h−1 gcat−1), Au–Cu alloy
phase detected in high-resolution transmission electron microscopy (HR-TEM)
seemed to be specifically effective. The role of hydrogen-reduced Ti3+ sites was
also suggested to donate electrons to CO2.

Figure 4. CO2 photoreduciton mechanism using Pt-TiO2 nanostructured film.
The photogenerated electrons move fast inside the highly-oriented TiO2 single
crystals and flow to the Pt nanoparticles, where the reduction reaction occurs toconvert CO2 into CO or CH4 (middle) (26). Reproduced from reference listing(26). Copyright 2012, ACS.

Using 125-W high-pressure Hg and 150-W Xe lamps (with cutoff filter of λ> 400 nm) for ultraviolet (UV) and visible irradiations, respectively, for Au-CuTiO2 photocatalyst, no product and methane only were formed, respectively (Table2) (27). Thus, SPR (the wavelength 570–580 nm) effect of Au in visible lightregion was suggested to transfer the hot electrons to Cu sites (Figure 5). Carbene
pathway was proposed based on Fourier transform infrared (FTIR) rather thanformaldehyde pathway (Scheme 1) (5).

15In Advances in CO2 Capture, Sequestration, and Conversion; He, et al.;ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

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Figure 5. Proposed photocatalytic reaction mechanism of CO2 reduction withmoisture using Au(0.5 wt%)-Cu(1.0 wt%)-TiO2 photocatalyst (27). Reproduced
from reference listing (27). Copyright 2014, ACS.

16In Advances in CO2 Capture, Sequestration, and Conversion; He, et al.;ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

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Scheme 1. Two proposed mechanisms for the photoreduction of CO2 to methane:
formaldehyde (left) and carbine pathways (right) (5). Reproduced from reference
listing (15). Copyright 2013, John Wiley and Sons.

17
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3. Metal Oxide or Metal-Organic Framework (MOF)-Loaded TiO2
Photocatalysts
The Pt-TiO2 photocatalyst in previous section was further doped with MgO(15). It became methane selective and total formation rate of methane and CO (11μmol h−1 gcat−1) increased by a factor of 1.8 compared to that using Pt-TiO2 (Table2). Doped MgO layer would help CO2 chemisorption over the catalyst and improve
the selectivity for CO2 reduction versus proton reduction. The Cu bipyridyl (bpy)dye to TiO2 also promoted the photoreduction of CO2 to methane at a rate of 0.3μmol h−1 gcat−1 (Table 2) (28).
The assembly of TiO2 with MOF was also reported (29). Cu3(btc)2 (btc
= benzene-1,3,5-tricarboxylate) microcrystals synthesized in the presence of
polyvinylpyrrolidone were mixed with tetrabutyl titanate in ethanol and then in
HF aqueous solution. By heating at 453 K for 12 h, anatase-phase TiO2 shells ofmean thickness of ~200 nm over the Cu3(btc)2 cores were formed (Figure 6).
Methane was formed using Cu3(btc)2-core|TiO2-shell from CO2 and water
irradiated by visible light (λ > 400 nm) at a rate of 2.6 μmol h−1 gcat−1, muchhigher than than the sum of rates using each component (0.52 and ~0 μmol h−1
gcat−1; Tables 1, 2). Exclusive selectivity to methane rather than H2 was also
claimed, probably due to high electron density in the core-shell structure forthe eight electron reduction to methane. A 1-ps build-up time in the transientabsorption (TA) spectroscopy for the Cu3(btc)2-core|TiO2-shell suggested electrontransfer/relaxation from CB of TiO2 to the interface state with Cu3(btc)2 (29).

Figure 6. (a) Core–shell structure, (b) TEM, and (c) SEM images ofCu3(btc)2@TiO2 (29). Reproduced from reference listing (29). Copyright 2014,
John Wiley and Sons.
4. Semiconductor Photocatalysts Other Than TiO2
4.1. Photocatalysts Comprising Single Compound

4.1.1. Metal Oxides and Hydroxides
By the irradiation of deep UV light at 185 nm from Hg lamp and using thesynthetic quartz windows for the reactor (Figure 7), CO2 (9.6 kPa) was converted
into CH4 with saturated moisture at a rate of 2.1 μmol h−1 for 13.9 h in the absenceof catalyst (Table 3). It was suggested that water photolysis was the exclusive
18
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hydrogen source for the CO2 conversion (see Photon energy conversion of CO2
to fuels with hydrogen or sacrificial reducing agents, section 1). The conversion
of CO2 was accelerated (7.7 μmol h−1; Table 3) in the presence of Zn6Ti LDH(100 mg), but the addition of MgO, TiO2, CeO2, or HY-zeolite rather reduced thephotocomversion rates of CO2 (30).
A
seriesof
LDHsof[Ni3MIII(OH)8]2CO3·mH2O,
III
III[Mg3M (OH)8]2CO3·mH2O, and [Zn3M (OH)8]2CO3·mH2O (MIII = Al, Ga, andIn) was tested for CO2 photoreduction in water (31). [Mg3MIII(OH)8]2CO3·mH2O
was relatively active, e. g. [Mg3In(OH)8]2CO3·mH2O produced CO andO2 at the formation rates of 3.2 and 17 μmol h−1 gcat−1, respectively (Table
3).
[Mg3MIII(OH)8]2CO3·mH2O was selective to form hydrogen whereas
III[Ni3M (OH)8]2CO3·mH2O was relatively selective to CO formation. The affinityof the surface with CO2 may be related.
Hexagonal nanoplate-textured micro-octahedron Zn2SnO4 as large as a few
microns was synthesized and tested for CO2 photoreduciton with 0.4 mL of water(32). Due to its small (quantum) size, the band gap was relative wide (3.87 eV)in which the VB maximum at 2.7 eV and CB minimum at −1.17 eV. The methane
formation rate increased from 4.8 to 35 ppm h−1 gcat−1 by loading of 1 wt% of
Pt and 1 wt% of RuO2 to nanoplate-textured micro-octahedron Zn2SnO4. KTaO3
photocatalytically reduced CO2 with water irradiated by Xe arc lamp to form CO
at a rate of 0.34 μmol h−1 gcat−1 (Table 3) (33).
Mesoporous In(OH)3 photoproduced methane from CO2 and water at a rateof 0.8 μmol h−1 gcat−1 (Table 3). The rate was by 20 times greater thant that using
In(OH)3 without mesoporous structure due to the difference of specific surface
area and pore volume (34). Monoclinic Bi6Mo2O15 sub-nanowires photocatalyzed
CO2 to methane and the rate was maximal by the preheating at 1073 K. Surface
oxygen vacancy created by the heating was considered as an electron trap (35).

Figure 7. Left: Photoreactor composed by 185 nm-Hg lamp (1), synthetic quartz
windows (2), and gas chamber (3). Right: Upper view showing the shallow bedof the photocatalyst placed in the photoreactor (30). Reproduced from reference
listing (30). Copyright 2012, ACS.
19In Advances in CO2 Capture, Sequestration, and Conversion; He, et al.;ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

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Table 3. Reported CO2 Photoreduction Catalysts, Reaction Conditions, and the Formation Rates in Water/with Moisture Using
Semiconductor Photocatalysts Other Than TiO2
Reactants

Photocatalyst

T (K)Light source

Reactor

Major product{formation rate
(μmol h−1 gcat−1)}

Ref

338

Hg (185, 254
nm)

QuartzCH4 (2.1a)(30)

Amount (mg)

CO2

H2O

No catalyst–—

9.6 kPa

Satur. P

KY-zeolite

500

CH4 (3.1)(30)

Zn6Ce LDH

500

CH4 (4.3)(30)

Zn6Ti LDH

500

CH4 (3.3)(30)

Zn6Ti LDH

100

CH4 (77)(30)

Mg3In LDH

100

500 μmol

Liq (4 mL)

(31)

KTaO3

100

Atmosph. P

Liq (6 mL)

Pt(0.5%)-In(OH)3200

80 kPaLiq (2–3 mL)

Graphene
Oxide

200

101 kPa

Satur. P

g-C3N4 (bulk)

20

60 kPaLiq (0.1 mL)

Brand name

300 W Xe arc

298

200 W Hg-Xe

QuartzCO (3.2)

Borosilicate

CO (0.34)(33)

300 W Xe arc

Circulated

CH4 (0.8)(34)

300 W halogen

Stainless

CH3OH (0.17)(36)

300 W Xe

Teflon-lined

CH3CHO (3.9)(37)

CH4 (4.8)(37)

CH4 (5.0)(38)

g-C3N4(nanosheet)
CoTe

–—

124 kPa

Satur. P

288

300 W Xe (λ >420 nm)QuartzIn Advances in CO2 Capture, Sequestration, and Conversion; He, et al.;ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

Reactants

21

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Photocatalyst

T (K)Light source

Reactor

Major product{formation rate
(μmol h−1 gcat−1)}

Ref

288

300 W Xe arc(λ > 420 nm)

QuartzCH4 (44)

(39)

CO2

H2O

10

Saturated

Liq (80 mL)

ZnTe(25%)SrTiO3

20

Atmosph. P

Satur. P

300 W Xe arc(λ > 420 nm)

QuartzCH4 (2.4)(40)

ZnGa2O4

100

Atmosph. P

Liq (0.4 mL)

300 W Xe arc

Glass

CH4 (0.26)(41)

ZnGa2O4:Zn2GeO4(4.5:1)100

CH4 (3.9)(41)

Zr-Co-IrSBA-15

5

101 kPa

13 Pa

355 nm laser

CO (1.7)(45)

Pt(0.5%)-cNaNbO3

100

80 kPaSatur. P

300 W Xe arc

CH4 (4.9), CO(0.82)(46)

Pt(0.5%)-oNaNbO3100

CH4 (2.5)(46)

Pt(0.4%)Nb2O5

100

CH4 (0.15)(47)

Pt(0.4%)HNb3O8

100

CH4 (0.47)(47)

Pt(0.4%)SiO2-HNb3O8

100

CH4 (2.9)(47)

Pt(0.4%)-SiO2
pillar-HNb3O8

100

CH4 (3.8)(47)

Brand nameAmount (mg)

ZnTe(3.4%)ZnO

89 kPa81 kPa

12 kPa333

350 W Xe

Pyrex

Quartz

20 kPaContinued on next page.

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Table 3. (Continued). Reported CO2 Photoreduction Catalysts, Reaction Conditions, and the Formation Rates in Water/with
Moisture Using Semiconductor Photocatalysts Other Than TiO2
Reactants

Photocatalyst
Brand nameAmount (mg)

T (K)

CO2

H2O

Saturated

0.2M KHCO3

Light source

Reactor

Major product{formation rate
(μmol h−1 gcat−1)}

Ref

500 W halogen

Pyrex

CH3OH (1.4)(48)

InNbO4140

NiO(0.5%)InNbO4

140

CH3OH (1.6)(48)

Co3O4(1.0%)InNbO4

140

CH3OH (1.5)(48)

Pt-o-NaNbO3

50

none

(49)

Pt-g-C3N4

CH4 (0.8)(49)

Pt-g-C3N4/oNaNbO3

CH4 (6.4)(49)

CO (29)(50)

CH4 (300)(51)

g-C3N4CH4 (0.26)(52)

graphene(15%)-g-C3N4

CH4 (0.59)(52)

Atmosph. P

300 W Xe arc(λ > 420 nm)

Liq (4 mL)

ZnO-g-C3N4

10

400 kPa

Liq (4 mL)

Pt(0.5%)-red
P-g-C3N4

20

Atmosph. P

Liq (0.2 mL)

353

500 W Xe (λ >420 nm)Pyrex vessel

Stainless steel

500 W Xe arc

In Advances in CO2 Capture, Sequestration, and Conversion; He, et al.;ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

Reactants

a

T (K)

Light source

Reactor

Major product{formation rate
(μmol h−1 gcat−1)}

Ref

CO2

H2O

500

Atmosph. P

Liq (3 mL)

150 W Xe

Glass

CO (0.43)

(53)

500

7.5 MPa

0.2M KHCO3(100 mL)

150 W Xe (λ >420 nm)Stainless steel

CH4 (30),
CH3OH (18),
CO (8.0)(54)

Brand nameAmount (mg)

RGO(0.5%)Cu2O
Ag/AgBr/
CNT
μmol of CH4 h−1.

23

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Photocatalyst

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4.1.2. Carbon-Related Materials
Graphene oxide (GO) with lateral dimension of several micrometers andthickness of 1–3 nm was exfoliated from graphite in acids and tested for CO2
reduction with moisture irradiated by UV–visible light (36). The methanol
formation rate (0.17 μmol h−1 gcat−1) was by a factor of 5.8 higher compared to one
using TiO2 under the reaction conditions (Table 3). The CB minimum estimatedfor GO (−0.79 V versus SHE) would bend to nearly close to reduction potentialfrom CO2 to methanol (−0.38 V versus SHE) if the GO with electron-drawing
oxygenated functional groups was p-type semiconductor.
Bulk g-C3N4 with the Bruanuer–Emmett–Teller surface area (SBET) of 50
m2 g−1 was compared to g-C3N4 nanosheet with SBET of 306 m2 g−1 (37). Clear
difference of selectivity to acetaldehyde and methane (−0.24 V) was observed
and one of the major reasons was the difference of band gap (BG), 2.77 and2.97 eV, respectively. More negative electron potential for the latter sample wasconsidered to effectively reduce CO2 via glyoxal to acetaldehyde and then tomethane (5).

4.1.3. Other Materials
As one of the metal calcogenide compounds, the CB of hydrothermallysynthesized CoTe positioned at relatively negative potential and the BG was 2.05
eV. The methane formation rate using CoTe was 5.0 μmol h−1 gcat−1 (Table 3) (38).

4.2. Assembled Photocatalysts

4.2.1. Zn, Cu-Containing Compounds
The doping of high-potential ZnTe to ZnO was reported (39). The BG of ZnTe
was 2.2 eV and the CB minimum was at −1.8 V versus SHE. Due to the excitedelectrons to CB of ZnO and holes remained at VB of ZnTe, methane was formed
irradiated by visible light (λ > 420 nm). ZnTe was a sensitizer for visible light, but
the reason of quite high formation rate (44 μmol h−1 gcat−1; Table 3) from CO2 tomethane via electron transfer from CB of ZnO is not known and control reaction
tests are needed. High-potential ZnTe was also combined with SrTiO3 and formed
methane from CO2 and moisture (Table 3) (40).
The performance of CO2 photoreduction to methane using cubic spinel
ZnGa2O4 was improved by mixing pseudo cubic inverse spinel Zn2GeO4 from
0.26 to 3.9 μmol h−1 gcat−1 (Table 3) (41). One of the reasons was suggested tobe the reduction of band gap from 4.45 to 4.18 eV to utilize wider spectrum oflight (42).
Zn-doped (100)-oriented p-GaP was utilized for CO2 photoreduction withwater irradiated by a laser light at 532 nm. To prevent the photocorrosion of
GaP, atomic layer deposition of Ti choloride and water vapor was performed to24
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create 1–10 nm-thick TiO2 layers over GaP (43). Methanol was formed on the
GaP photoelectrode covered with 5 nm-thick TiO2 at a rate of 1.2 μmol h−1 cmcat−2
at the applied voltage of −0.5 V (versus SHE) in 0.5 M NaCl and 10 mM pyridine
solution. Thin TiO2 layer seemed not just the passivation layer to transmit the lightat 532 nm, but also to serve to form p–n junction for charge separation (43).
Cu2O electrodeposited on CuO nanorods perpendicularly grown on Cu foil
photoreduced CO2 in CO2-saturated 0.1 M Na2SO4 aqueous solution at the electric
potential smaller than 0.37 V (versus SHE) irradiated by simulated AM 1.5 light.
The geometry of CuO nanorod and Cu2O shell should facilitate charge separation
irradiated by light and CO2 reduction on CuO (44).

4.2.2. Zr, Co, Ir-Containing Compounds
Stepwise synthesis of photoreduction sites of CO2 and photooxidation sitesof water in single sample was reported. Zirconocene dichloride was reactedwith an ordered mesoporous SiO2, SBA-15, to form Zr-SBA-15 (45). The Co
precursor preferably reacted with Zr–OH group to form links of ZrIV–O–CoII in
Zr(1.1 mol%)-Co(0.7 mol%)-SBA-15. IrIII acetylacetonate was photodepositedon Zr-Co-SBA-15, but not on Zr-SBA-15 and Co-SBA-15, suggesting electrontransfer by the irradiation of 355 nm laser light from Co to Zr (ZrIII–O–CoIII)
coupled with the electron transfer from Ir to Co (IrIII···O–CoIII → IrIV–O–CoII).
Though the IrIV–O–CoII link may be lost upon calcination later than theelectrodeposition, ZrIV–O–CoII and IrOx nanocluster were formed in the vicinity
over SBA-15.
The CO formation at a rate of 1.7 μmol h−1 gcat−1 under 101 kPa of CO2
and 13 Pa of water irradiated by laser at 355 nm (127 mW cm−2; Table 3) was
ascribed to water oxidation on IrOx nanocluster and the resultant electron transfer
to neighboring CoII and then to ZrIV (45). Spectroscopic insight for the interactionof ZrIII and CO2 would be expected.

4.2.3 Nb-Containing Compounds
Two phases of perovskite-type NaNbO3 doped with 0.5 wt% of Pt were tested(Table 3) (46). One is cubic NaNbO3 (c-NaNbO3) and the other is more common
orthorhombic NaNbO3 (o-NaNbO3). The Pt-c-NaNbO3 photocatalyst produced
major methane (4.9 μmol h−1 gcat−1) and minor CO (0.82 μmol h−1 gcat−1) fromgaseous CO2 and moisture, and the methane formation rate was by a factor 2.0
greater compared to that using Pt-o-NaNbO3. Two reasons of the difference wassuggested to be (1) minor difference of band gap value and (2) homogeneous
frontier orbital [highest occupied molecular orbital (MO), lowest unoccupied MO]
distribution in x, y, z-direction for c-NaNbO3 versus the distorted distribution in
z-direction for o-NaNbO3.
Pt-photodeposited layered HNb3O8 was superior to Pt-photodeposited layered
KNb3O8 for CO2 photoreduction at 333 K (47). The methane formation rates were
improved to 3.8 μmol h−1 gcat−1 when the moisture pressure increased from 12 to25
In Advances in CO2 Capture, Sequestration, and Conversion; He, et al.;ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

20 kPa and also SiO2 pillared between the layers of HNb3O8 (Table 3). However,not O2 or H2 was detected in the study, suggesting incomplete photocatalytic cycle.
For InNbO4, the doping of NiO (0.5%) or Co3O4 (1.0%) slightly promoted
the methanol photoformation (1.6–1.5 μmol h−1 gcat−1) from CO2 compared tounpromoted one (1.4 μmol h−1 gcat−1; Table 3) (48).

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4.2.4 C-Containing Compounds
o-NaNbO3 nanowire was combined with polymeric g-C3N4 (49). Methane
was photogenerated using Pt-g-C3N4/NaNbO3 at a rate of 6.4 μmol h−1 gcat−1 higher
by a factor of 8 compard to that using Pt-g-C3N4 (Table 3). Pt-o-NaNbO3 wasinert in the conditions. The advantage of Pt-g-C3N4/NaNbO3 was ascribed to thechage separation at the heterojunction of p-type g-C3N4 to attract holes and n-type
NaNbO3 to attract excited electrons.
g-C3N4 was also combined with ZnO (50) and red-P (51) to form
heterojunction and converted CO2 into major CO and major methane at rates of 29
and 300 μmol h−1 gcat−1, respectively. The reason of quite high methane formation
rate using red-P-g-C3N4 is unclear and control reaction tests are needed.
The disadvantages, low electric conductivity and rapid recombinationof photogenerated electrons and holes, of 2-dimensional(D) g-C3N4 forphotocatalysis were solved by assembling graphene with g-C3N4 (52). The
electrons at N 2p was photoexcited to C 2p (CB: −1.42 V versus SHE) in g-C3N4
and could move to conducting network graphene (Fermi level: −0.08 V versus
SHE). Thus, CO2 photoreduction rate into methane was boosted from 0.26 μmol
h−1 gcat−1 (g-C3N4) to 0.59 μmol h−1 gcat−1 for graphene (15 wt%)-g-C3N4 assembly(Table 3).
Cu2O assembled with 0.5 wt% of reduced graphene oxide (RGO) was also
reported. CO was produced from CO2 and water at a rate of 0.43 μmol h−1 gcat−1(Table 3) (53). Similar to reference 44, negative potential of CB for Cu2O (−1.44
V versus SHE, pH 0) should be advantageous for electron donation toward CO2derived species.
A part of Ag+ of AgBr/carbon nanotube (CNT) was photoreduced to form
Ag/AgBr/CNT (54). The photocatalyst was immersed in weak alkaline solutionpressurized with 7.5 MPa of CO2. Methane, methanol, and CO were formed at a
total formation rate of 56 μmol h−1 gcat−1 (Table 3). Longer CNT was preferable
than shorter one probably due to better charge separation efficiency.

Photon Energy Conversion of CO2 to Fuels with Hydrogen or
Sacrificial Reducing Agents
1. Photon Energy Conversion of CO2 to Fuels with Hydrogen
If oxidation of water produces O2, protons, and electrons, utilizing naturallight, photon energy conversion of CO2 to fuels with hydrogen (H atom equivalent
to a proton and an electron) is also important as a latter half cycle of artificial
photosynthesis (4).
26In Advances in CO2 Capture, Sequestration, and Conversion; He, et al.;ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

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By the irradiation of deep UV light at 185 nm from Hg lamp and usingsynthetic quartz windows for the reactor, CO2 (9.6 kPa) was converted into
major CH4 and minor CO with H2 (45 kPa) at a rate of 4.5 μmol h−1 for 82 h in
the absence of catalyst (Table 4) (30). This rate was faster than the photolysis
of CO2 with water (see Photon energy conversion of CO2 to fuels with water,
section 4.1.1; Table 3). Light of 185 nm was confirmed to be responsible for thephotolysis of CO2 rather than light of 254 nm from the Hg lamp.
High conversion of CO2 with H2 was reported using Ni/SiO2-Al2O3 in similar
reactor to Figure 7 at a methane formation rate of 55 mmol h−1 gcat−1 (Table4) (55). By the effects of high-power solar simulator and heat of reaction forCO2 hydrogenation to methane (165 kJ mol−1), the temperature reached 423 K
and gas pressure would be some hundreds kPa. Control experiment at 453 K
in dark formed methane at significantly lower rate of 8.9 μmol h−1 gcat−1. Thus,
the photocatalytic reduction was photoactivated and accompanied by thermal
catalytic step(s). Ni/SiO2-Al2O3 when UV light was filtered showed similar
activity to NiO irradiated by UV-visible light (13 mmol h−1 gcat−1; Table 4). The
performance irradiated under UV-visible light was in the order

Scheme 2. Proposed reaction mechanism of photocatalytic CO2 reduction with
H2 presumably assisted by later thermal catalytic step(s) using Ni/SiO2-Al2O3(55). Reproduced from reference listing (55). Copyright 2014, ACS.

It was difficult to find the direct relationship between the reactivity and BG
values or the conduction band level. Instead, formation of active Ni–H species
was proposed by reducing/oxidizing H2 to H−/H+ and reacting with Ni+/Ni−,respectively (Scheme 2) (55).

27
In Advances in CO2 Capture, Sequestration, and Conversion; He, et al.;ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

Reactants

Photocatalyst

T (K)Light source

Reactor

Major product{formation rate
(μmol h−1 gcat−1)}

Ref

Amount (mg)

CO2

H2

No catalyst–—

9.6 kPa

45 kPa338 K

Hg (185, 254 nm)

QuartzCH4 (4.5a)(30)

Ni/SiO2-Al2O3

250

3.7 mmol

17 mmol<423

Solar simulator(AM1.5 filter)

Aluminum

CH4 (55 000)

(55)

NiO

CH4 (13 000)

(55)

Fe2O3

CO (7 200)

(55)

CoO

CO (2 500), CH4(1 600)(55)

CH4 (13 000)(55)

CH4 (210), CO
(170)

(55)

Brand name28

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Table 4. Reported CO2 Photoreduction Catalysts, Reaction Conditions, and the Formation Rates Using Various Semiconductors
with Hydrogen

Ni/SiO2-Al2O3

250

3.7 mmol

17 mmol

Solar simulator (λ> 420 nm)

Aluminum

NiO
Zn3Ga|CO3LDH

100

2.3 kPa

21.7 kPa<313

500 W Xe arc

Pyrex

CO (0.08), CH3OH(0.05)(56)

Ag(0.36%)Zn3Ga|CO3
LDH

100

2.3 kPa

21.7 kPa<313

500 W Xe arc

Pyrex

CH3OH (0.12),
CO (0.10)(57)

Au(4.1%)Zn3Ga|CO3
LDH

100

2.3 kPa

21.7 kPa<313

500 W Xe arc

Pyrex

CO (0.20), CH3OH(0.03)(57)

In Advances in CO2 Capture, Sequestration, and Conversion; He, et al.;ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

Reactants

Photocatalyst

Light source

Reactor

Major product{formation rate
(μmol h−1 gcat−1)}

Ref

Amount (mg)

CO2

H2

CuPcTsZn3Ga|CO3
LDH

100

2.3 kPa

21.7 kPa<313

500 W Xe arc

Pyrex

CO (0.11), CH3OH(0.096)(58)

Zn3Ga|Cu(OH)4
LDH

100

2.3 kPa

21.7 kPa<313

500 W Xe arc

Pyrex

CH3OH (0.30),
CO (0.13)(60)

Brand namea μmol of CH4 h−1.

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A LDH [Zn3Ga(OH)8]2CO3·mH2O (m~4; Zn3Ga) exhibited direct electronictransition and the BG value was 5.6 eV (56). This wide-BG LDH was combinedwith SPR of Ag and Au nanoparticles (Scheme 3) (57). The SPR peaks appeared invisible light region centered at 411 and 555 nm, respectively. By the irradiation of
UV-visible light, the total formation rates of CO and methanol using Zn3Ga LDH(0.13 μmol h−1 gcat−1) increased to 0.22 μmol h−1 gcat−1 using Ag/Zn3Ga LDH andto 0.23 μmol h−1 gcat−1 using Au/Zn3Ga LDH. The selectivity to methanol was also
improved from 39 to 54 mol% (Table 4) by the doping of Ag, whereas the Au dope
led to selective CO formation (13 mol%).
Ag/Zn3Ga LDH and Cu phthalocyanine tetrasulfonate (PcTs)-doped Zn3Ga
LDH were active irradiated by visible light (λ > 420 nm; total formation rate 0.12
and 0.15 μmol h−1 gcat−1, respectively) (57, 58). The mechanism could be explainedby the electron shift due to SPR in Ag to wide-BG Zn3Ga LDH then to CO2-derived
species (Scheme 3, right) and electron shift from wide-band gap Zn3Ga LDH to
LUMO of CuPcTs then to CO2-derived species (Scheme 4). Conversely, as theWF of Au was larger than that for Ag (17), the electrons could not be supplied
to the CB of LDH (Scheme 3, left). Thus, Au worked as electron trap from LDH
irradiated by UV light and SPR in Au was ineffective for CO2 photoreduction (57).
The electron flows originating from SPR were in consistent with charge transfer
mechanism reviewed previously (59).

Scheme 3. The energy diagram and proposed electron flows in Ag/Zn3Ga LDH(right) and Au/Zn3Ga LDH (left) (57). Reproduced from reference listing (57).
Copyright 2015, Elsevier.

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The CO2 photoreduction ability of [Zn1.5Cu1.5Ga(OH)8]2CO3·mH2O (Figure8A) was also improved by the interlayer anion substitution by [Cu(OH)4]2−forCO32− (Figure 8B) (60). Especially, methanol formation rates increased
from 0.05 to 0.30 μmol h−1 gcat−1 by the anion substitution (Table 4). X-ray
absorption near-edge structure (XANES) was applied to monitor the diffusion ofphotogenerated electrons to active CuII sites as the 1s-3d pre-edge peak intensity(61). Electron diffusion to Cu sites was an order of magnitude faster in the
direction of the cationic layers of [Zn1.5Cu1.5Ga(OH)8]2CO3·mH2O (580 μmol h−1
gcat−1; Figure 8A) than in the perpendicular direction to interlayer [Cu(OH)4]2− in[Zn3Ga(OH)8]2[Cu(OH)4]·mH2O (36 μmol h−1 gcat−1; Figure 8B) (61, 62). With
the information of FTIR, selective methanol formation mechanism via hydrogencarbonate (bicarbonate) and step-by-step reduction with proton and electron was
proposed using [Zn3Ga(OH)8]2[Cu(OH)4]·mH2O LDH (Scheme 5).

Scheme 4. The energy diagram and proposed electron flows in CuPcTs/Zn3Ga
LDH.

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Figure 8. Structure of [Zn1.5Cu1.5Ga(OH)8]2CO3·mH2O (A) and structure of[Zn3Ga(OH)8]2[Cu(OH)4]·mH2O (B). Interlayer carbonates are not drawn in(A) for clarity (62). Reproduced from reference listing (62). Copyright 2014,
High-Energy Accelerator Research Organization.

Scheme 5. Proposed photocatalytic cycle of CO2 reduction to methanol or
CO using LDH catalysts comprising Zn, Ga, and interlayer Cu sites (61).
Reproduced from reference listing (61). Copyright 2014, Elsevier.
2. Photon Energy Conversion of CO2 to Fuels with Other Sacrificial
Reducing Agents than H2
CO2 photoreduction into major CO using Au3Cu alloy on SrTiO3/TiO2
nanotubes and hydrous hydrazine was reported at a formation rate of 730 μmol
h−1 gcat−1 (Table 5) (63). Cuboid Cu2O powder was superior (20 ppm h−1 gcat−1)
to octahedral Cu2O for CO2 photoreduction to CO in water and the rate was byseveral times enhanced by the addition of 0.25 wt% of RuOx using Na2SO3 as
sacrificial reducing agent (64).
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Na2SO3 was also used as sacrificial reducing agent, and the CO2 pressure
to water suspension of Au-TiO2 was raised to 1.0 MPa at 298 K. The methane
generation rate increased to 20 μmol h−1 gcat−1 (Table 5) (65). Methanol was usedas sacrificial reducing agent, and the generation rates of methyl formate from CO2
was in the order using hexagonal ZnIn2S4 > cubic ZnIn2S4 > microspheric ZnIn2S4(Table 5) (66).
The progress of MOF as photocatalysts for CO2 reduction was fast in thesethree years. MIL-101(Fe) comprising Fe3O clusters linked by terephthalates toform quasi-spherical cages produced formate at a rate of 150 μmol h−1 gcat−1
in the presence of triethanol amine (TEOA) as reducing agent from dissolved
CO2 in acetonitrile irradiated by visible light (420 < λ < 800 nm; Table 5) (67).
MIL-53(Fe) comprising chains of hydroxy corner-sharing FeO6 octahedra linked
by terephthalates to form 1D pores and MIL-88(Fe) comprising oxo-centered
Fe3O clusters linked by terephthalates to form 3D porous network were lessactive (74–23 μmol h−1 gcat−1) compared to MIL-101(Fe) (Table 5). The electrontransfer from O2− to Fe3+ irradiated by visible light was proposed as the first stepfor the CO2 reduction.
When 2-aminoterephthalic acid was used for the syntheses of MOF’s above,
the generation rates of formate were improved to 450–75 μmol h−1 gcat−1 comparedto MIL’s unfunctionalized by the amine group (150–23 μmol h−1 gcat−1; Table
5). Amine group significantly extended the UV absorption edge toward visible
region, and proposed to create second electron transfer pass from NH2 to Fe-O
center. The flat band position for MIL-101(Fe), MIL-53(Fe), and MIL-88(Fe)were −0.52, −0.70, and −0.48 V, respectively, versus SHE. The values were clearly
more negative than the reduction potential of CO2 to formate (−0.28 V), however,
the order of photocatalysis

was not correlated to the flat band potential (67).
A combination of Zn-containing MOF (called ZIF-8) and Zn2GeO4 wasreported (68). In aqueous solution with 0.1 M Na2SO3 aqueous solution saturated
with CO2, methanol was photogenerated at a rate of 0.22 μmol h−1 gcat−1 (Table 5).
A Co-containing MOF, Co-ZIF-9, was tested with the aid of CdS photocatalyst
and sacrificial reductant, TEOA irradiated by visible light (69). CO was formed
at a rate of 2400 μmol h−1 gcat−1 (Table 5). Co ions in ZIF-9 was suggested toreceived excited electrons in CdS.
5,10,15,20-Tetrakis(4-carboxyphenyl)porphyrin coordinated to Al3+ to form
MOF. The methanol formation rate from CO2 utilizing triethylamine (TEA; 38
ppm h−1 gcat−1) was improved by introducing Cu2+ to the porphyrin in the MOF(260 ppm h−1 gcat−1; Table 5) (70). The surface of GO was carboxylated andwas functionalized with –CH2–COCl and then reacted with Co phthalocyanine
tetrasulfonamide (71). Sensitization by CoPc and transferred electrons to GO weresuggested to reduce CO2 into methanol at a rate of 79 μmol h−1 gcat−1 compared
to 39 μmol h−1 gcat−1 using unsensitized GO by using TEA as sacrificial reductant(Table 5).
33
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Table 5. Reported CO2 Photoreduction Catalysts, Reaction Conditions, and the Formation Rates with Sacrificial Reducing Agents
Reactants

34

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Photocatalyst

T (K)Light source

Reactor

Major product{formation rate
(μmol h−1 gcat−1)}

Ref

CO (730)

(63)

CO2

Reducing
agent

5

33.7 kPa

N2H4·H2O

300 W Xe

Cu2O

500

Saturated

0.7 M Na2SO3

300 W Xe (λ >350 nm)

Glass

CO

(64)

Au(0.1%)TiO2500

1.0 MPa

Na2SO3298

125 W Hg

High P cell

CH4 (20)(65)

hex-ZnIn2S4

10Saturated

CH3OH

298

250 W Hg

MF (190)(66)

Brand nameAmount (mg)

Au3Cu/
SrTiO3/TiO2

cub-ZnIn2S4

MF (160)

sph-ZnIn2S4

MF (50)
HCO2− (150)(67)

MIL-53(Fe)HCO2− (74)(67)

MIL-88(Fe)HCO2− (23)(67)

NH2-MIL101(Fe)HCO2− (450)(67)

NH2-MIL53(Fe)HCO2− (120)(67)

NH2-MIL88(Fe)HCO2− (75)(67)

MIL-101(Fe)50Saturated in
CH3CN

TEOA

300 W Xe (420< λ < 800 nm)

Flask

In Advances in CO2 Capture, Sequestration, and Conversion; He, et al.;ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

Reactants

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Photocatalyst
Brand nameAmount (mg)

Zn2GeO4ZIF-8

200

CdS-CoZIF-9 20, 1
Al–porphyrin
MOF

30

T (K)

Reactor

Major product{formation rate
(μmol h−1 gcat−1)}

Ref

500 W Xe arc

CH3OH (0.22)(68)Light source

CO2

Reducing
agent

Saturated

Liq (100 mL),
0.1 M Na2SO3

101 kPaTEOA (1 mL)

303

300 W Xe (λ >420 nm)CO (2400)(69)

Atmosph. P

TEA (1 mL)

278

300 W Xe (λ >420 nm)

CH3OH

(70)

CH3OH

(70)

CH3OH (39)(71)

CH3OH (79)(71)

Al–Cuporphyrin
MOF
GO

100

TEA (10 mL)

20 W whitecold LED

Borosil

CoPc-GOIn Advances in CO2 Capture, Sequestration, and Conversion; He, et al.;ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

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Photon Energy Conversion of CO2 to Fuels Using Combination
System with Photooxidation Catalysts
Although the homogeneous photocatalysts and enzymes for CO2 reduction areout of scope of this chapter that is limiting to semiconductor photocatalysts, several
studies were reported/reviewed to combine homogeneous complex or enzyme forCO2 photoreduction with semiconductor photocatalyst, e.g. Ru bpy-like complex
with Ag/TaON (72, 73).
The phenomena of photocatalytic oxidation of water and photocatalytic
reduction of CO2 were combined using a cell (Figure 9A), in which the two
photocatalysts, WO3 and [Zn1.5Cu1.5Ga(OH)8]2[Cu(OH)4]·mH2O LDH, wereseparated by a polymer electrolyte (PE) film (Figure 9B) (16). WO3 was used
for the photooxidation of water, whereas Zn–Cu–Ga LDH was used for thephotoreduction of CO2. Protons and electrons, which were formed on WO3
under the flow of moisture (solid–gas interface mode; Figure 9B), were used on
Zn–Cu–Ga LDH instead of reactant H2 in Photon energy conversion of CO2 tofuels with hydrogen or sacrificial reducing agent, section 1 (56–58, 60). For
this process, photocatalysts pressed on both sides of the PE film were irradiatedby UV–visible light through quartz windows and through the space in carbon
electrode plates set for both gas flow and light transmission. Methanol was the
major product on LDH at a formation rate of 0.045 μmol h−1 gcat−1 under the flowof CO2 (Table 6). This rate accounted for 68–100% of photocurrents between thetwo redox photoelectrodes.
In comparison, liquid-type another cell, which consisted of WO3 andLDH immersed in acidic solutions, with the PE film distinguishing the twocompartments was reported (Figure 9C). The photocurrent from LDH to WO3
was increased by 2.4–3.4 times in comparison to gas–solid mode cell (Figure 9B)
tested under similar conditions. However, major product from LDH was H2 at aformation rate of 0.67 μmol h−1 gcat−1. The difference of phase of water (moisturein panel B, liquid in panel C; Figure 9) directed the selectivity to methanol(equation 12) versus H2 (equation 13) (16). This trend of phase difference was inconsistent with that found for metal-loaded TiO2 (Photon energy conversion of
CO2 to fuels with water, section 2.1) (15).

Using a similar tandem twin reactor separated by PE film, CO2 conversion
to methanol was also reported at rates of 1.6–1.8 μmol h−1 gcat−1 using WO3
on photoanode and Pt(1 wt%)-CuAlGaO4 and/or Pt(0.8 %)-SrTiO3:Rh on
photocathode (74). A clear difference to reference 16 was the photocatalysts were
not mounted on electrode and redox mediator (Fe3+ in anode and Fe2+ in cathode)was used. However, it is not certain whether electrons or Fe2+ would transfer
through the PE film.
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Figure 9. The reaction path in photofuel cell (A), cell-1 to flow moisture to WO3(B, front) and to circulate CO2 to LDH (B, back), and cell-2 to flow N2 to WO3(C, right) and CO2 to LDH (C, left) immersed in HCl solutions (16). Reproduced
from reference listing (16). Copyright, 2014, The Royal Society of Chemistry.

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Table 6. Reported CO2 Photoreduction Catalysts, Reaction Conditions, and the Formation Rates Combined with Photooxidation
Catalysts
Reactants

Photocatalyst
Brand name

CO2

H2O

95

(Ne/N2)

Satur. P

45

3.5 kPa

300(Ar)

225 mL (Fe3+:
2mM)

300

Saturated(Fe2+: 2mM)

Amount (mg)T (K)Light source

Reactor< 315

500 W Xe arc

PEFC

Major product{formation rate
(μmol h−1 gcat−1)}

Ref

anode:
WO3(16)

cathode:Zn1.5Cu1.5Ga|Cu(OH)4

CH3OH (0.045)

anode:WO3

300 W Xe

Twin R separated
by PE

(74)

cathode:Pt(1%)-CuAlGaO4

CH3OH (1.8)

anode:
WO3

300

(74)

cathode:Pt(1 %)-CuAlGaO4,

150

Pt(0.8%)-SrTiO3:Rh

150

CH3OH (1.6)

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Concluding Remarks
Recent advances in photocatalytic CO2 conversion with water and/or otherreductants in these three years were reviewed in comparison to classic studied prior
to 2012 (4) using TiO2 (75–81), ZnO (81), CdS (81), GaP (81), SiC (81), WO3 (81),Zn2GeO4 (82), Bi2WO6 (83), HNb3O8 (84), CuGa1−xFexO2 (85), AIILa4Ti4O15 (86),[Zn1.5Cu1.5Ga(OH)8]2CO3·mH2O (56, 60), ZrO2 (87), GaP (81, 88), GaAs (88, 89),and InP (89). Starting from CO2 and water, methane or CO formation using TiO2
was improved to 0.1–17 μmol h−1 gcat−1 by choosing approperiate crystalline phase(brookite or anatase), crystalline face, and the defects. Metals, e.g. Pt, Pd, Au,
Rh, Ag, Ni, Cu, Au3Cu alloy, I, MgO, RuO2, graphene, g-C3N4, Cu-containingdyes, and Cu-containing MOFs were effective to assist TiO2 to form methane,
CO, methanol, or ethane at rates of 1.4–160 μmol h−1 gcat−1 and further to 32–2200
μmol h−1 gcat−1 if minimal thin photocatalyst film was well deposited.
Semiconductors, e.g. ZnO, Zn6Ti LDH, Mg3In LDH, KTaO3, graphene,
GO, g-C3N4, CoTe, TiO2-coated p-GaP, Zn2SnO4, ZnO, ZnTe, SrTiO3, ZnGa2O4,
Zn2GeO4, Zr–Co–Ir oxides, Nb2O5, HNbO3, NaNbO3, KNb3O8, InNbO4, NiO,
Co3O4, Cu2O, and their assemblies were effective to photogenerate methane, CO,methanol, acetaldehyde at rates of 0.15–300 μmol h−1 gcat−1 in good competition
with promoted TiO2.
If H2 was used, new photoactivation of CO2 followed by thermal assisted
reaction(s) was reported using Ni/SiO2-Al2O3 to produce methane at 423 K underpressurized CO2 + H2 at a rate of 55 mmol h−1 gcat−1 presumably via Ni–H species.
As pure photocatalytic reaction, methanol formation rates were improved up to
0.30 μmol h−1 gcat−1 by the doping of Ag/Au nanoparticles, [Cu(OH)4]2− anions,and Cu-containing dyes to Zn–Ga LDH. The combination of two semiconductors(WO3, Zn–Cu–Ga LDH) were reported to from methanol at a rate of 0.05 μmol
h−1 gcat−1 from CO2 and gaseous water.
In this chapter, the improvement of CO2 conversion and new materials
for the photoreduction were focused, and the importance of wavelength (deepUV, UV, and visible light) and flux of incident light, temperature, and form
(powder, nanofiber, and/or film) of photocatalysts, pressure of reactants, werecritical to determine the photoconversion of CO2 for the stage of new energy
application levels. To enable the optimum reaction and photocatalyst conditions,the importance of reactor cell (30, 55) was also suggested.
However, reaction mechanisms of CO2 activation, e.g. via glyoxal (9), via
O-defect to form CO (equations 3–5) (10), multiple-step formaldehyde pathway(5), mutiple-step carbene pathway (5, 27), via metal–H active species (55), andvia hydrogen carbonate (bicarbonate) (56, 61), are still in debate. To clarify themechanism and responsible sites, spectroscopic studies, e.g. FTIR (27, 45, 61,
67, 70), diffuse reflectance infrared Fourier transform (10), electron paramagnetic
resonance (9, 67), FE-SEM (22, 26, 49, 57), HR-TEM (20, 26, 27, 46, 47, 49, 54),
photoluminescence (15), action (in-profile) spectrum (55, 57, 58), time-resolved
TA (26, 29, 55), valence X-ray photoelectron (35, 39, 49), XANES (57, 61),
EXAFS (90), and density functional theory calculations (9, 13, 29, 34, 46) are
inevitable.
39
In Advances in CO2 Capture, Sequestration, and Conversion; He, et al.;ACS Symposium Series; American Chemical Society: Washington, DC, 2015.

Acknowledgments

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The authors are grateful for the financial supports from the Grant-in-Aid
for Scientific Research C (26410204, 22550117) from Japan Science Promotion
Agency, the Feasibility Study Stage of A-STEP (Proposal Nos. AS262Z00159L,
AS251Z00906L, AS231Z01459C) from the Japan Science and Technology
Agency, the Iwatani Naoji Foundation, Asahi Glass Foundation, the Promotion
Section of Technology Innovation, Daikin Industries, and approval of the Photon
Factory Proposal Review Committee for X-ray measurements (2014G631,
2011G033, 2009G552) for research related to this chapter.

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