Photocatalytic Process for CO2 Emission Reduction from Industrial

Jan 12, 2006 - proaches: (1) efficient use of carbon-based energy sources, (2) use of alternative or carbon-free energy sources, and (3) use of a post...
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Ind. Eng. Chem. Res. 2006, 45, 2558-2568

Photocatalytic Process for CO2 Emission Reduction from Industrial Flue Gas Streams Phairat Usubharatana, Dena McMartin,* Amornvadee Veawab, and Paitoon Tontiwachwuthikul Faculty of Engineering, UniVersity of Regina, Regina, SK, S4S 0A2 Canada

At present, carbon dioxide (CO2) is the largest contributor among greenhouse gases. This article addresses the potential application of photocatalysis to the reduction of CO2 emissions from industrial flue gas streams. Not only does this process remove CO2, but it can also convert CO2 into other chemical commodities such as methane, methanol, and ethanol. In addition, the photocatalytic process can consume less energy than conventional methods by harnessing solar energy. Given these advantages, photocatalysis is an attractive alternative for CO2 capture. This article reviews the principle of photocatalysis; existing literature related to photocatalytic CO2 reduction; and the effects of important parameters on process performance, including light wavelength and intensity, type of reductant, metal-modified surface, temperature, and pressure. Finally, we discuss various system configurations for UV and solar photocatalytic reactors. The advances in photocatalysis technology indicate a promising application potential for significant reductions of CO2 emissions and a positive impact on climate change effects. 1. Introduction According to the Intergovernmental Panel on Climate Change (IPCC 2001),1 the Earth’s surface temperature has risen by approximately 0.6 K in the past century, with particularly significant warming trends over the past two decades. The primary contributor to this phenomenon is carbon dioxide (CO2) emissions from fossil fuel combustion. A great deal of effort has been expended to reduce CO2 emissions from the industries where the largest percentages of fossil fuels are used. The reduction of CO2 emissions can be achieved by three approaches: (1) efficient use of carbon-based energy sources, (2) use of alternative or carbon-free energy sources, and (3) use of a posttreatment carbon-capture technology.2 With the abundance of fossil fuels, the continued reliance of global markets on this energy source, and the current absence of a cost-effective alternative energy source, posttreatment carbon-capture technology is a viable means of reducing CO2 releases into the atmosphere. Carbon capture refers to the removal of CO2 from industrial flue gas by a gas separation process prior to release to the atmosphere. The captured CO2 can be stored in depleted oil and gas wells, the deep ocean, or aquifers. It can also be utilized in one of two ways. First, the captured CO2 can be used as a chemical commodity for meat freezing, a component of carbonated beverages, or a reactant for methanol production. Second, the captured CO2 can be injected into geological formations for the enhanced recovery of fossil fuel products in processes such as enhanced oil recovery (EOR), enhanced coal bed methane recovery (ECBM), and enhanced gas recovery (EGR). Currently, many technologies are available for the capture of CO2 from flue gas. Such technologies include gas absorption into chemical solvents, permeation through membranes, cryogenic distillation, and gas adsorption onto a solid sorbent (Table 1). Gas absorption into chemical solvents refers primarily to chemical absorption. This is the most promising technology because of its capacity to handle a large volume of flue gas and its efficiency in CO2 capture at atmospheric pressure. * To whom correspondence should be addressed. E-mail: [email protected].

Nevertheless, chemical absorption is costly, with significant energy required for CO2 stripping and solvent regeneration. Gas absorption membranes are also available for flue gas treatment. By combining chemical absorption with membrane separation techniques, the efficiency of CO2 capture can be improved. However, the development of effective membranes with high CO2 selectivity and permeability presents a great challenge. Cryogenic processes, although technically feasible, are not economically viable because of the considerable energy input required for phase transformation from gas to liquid. Furthermore, the cryogenic process must be operated at high CO2 partial pressure, which is not applicable for a typical flue gas. The option of sorption on a solid sorbent is also not currently viable for CO2 capture, and despite its simplicity of operation, the adsorption process is costly. The recent innovations in photocatalytic processes have introduced CO2 capture as a potential application. These processes can be applied for CO2 removal while simultaneously converting CO2 to marketable products such as methane, methanol, and ethanol. Another potential feature of the photocatalytic reduction of CO2 is the use of solar energy for the reaction. This article reviews the principles of the photocatalytic process and its potential application to the capture of CO2 from fossil fuel combustion emission sources. 2. Photocatalytic Method Photocatalysis makes use of semiconductors to promote reactions in the presence of light radiation.8 Unlike metals, which have a continuum of electronic states, semiconductors exhibit a void energy region, or band gap, that extends from the top of the filled valance band to the bottom of the vacant conduction band when exposed to light radiation (Figure 1). The generation of electron-hole pairs (e--h+) and its reverse process are shown in eqs 1 and 2, respectively hV

photocatalyst 98 e- + h+

(1)

e- + h+ f heat

(2)

e-

where hV is the photon energy, represents a conduction band electron, and h+ represents a hole in the valence band.

10.1021/ie0505763 CCC: $33.50 © 2006 American Chemical Society Published on Web 01/12/2006

Ind. Eng. Chem. Res., Vol. 45, No. 8, 2006 2559 Table 1. Advantages and Disadvantages of Alternative CO2-Capture Techniques technique

advantages

disadvantages

chemical absorption

(i) effective for dilute CO2 stream (typically flue gas contains only0-12% CO2 by volume) (ii) commercially available and currently in use (iii) operated at low temperature and pressure (Rao and Rubin3) (i) compact because of a high packing density (ii) operation of the contactor is independent of gas and liquid flow rates (iii) no foaming, channeling, entrainment, or flooding (Falk-Pedersen and Dannstro¨m4) (i) use for high partial pressure of CO2 (ii) use for gas contained more than 90% CO2 (i) uncomplicated technology (ii) relative ease of operation

(i) requires significant energy for regeneration because of the strong bond between CO2 and the absorbent (ii) limited CO2 loadings result from the reaction stoichiometry

membrane gas absorption

cryogenics solid adsorption

The lifetime of this excited electron-hole pair is a few nanoseconds,10 but this is adequate for promoting redox reactions. The initial excitation and electron transfer make chemical reactions in the photocatalytic process possible. Figure 2 illustrates the excitation of an electron from the valence band to the conduction band initiated by light absorption with energy equal to or greater than the band gap of the semiconductor. The separated electron and hole can be followed by one of several pathways. Migration of electrons and holes to the semiconductor surface is followed by transfer of photoinduced electrons to adsorbed molecules or to solvent. The electron-transfer process is more efficient if the species are adsorbed on the surface.11 At the surface, the semiconductor can donate electrons to acceptors (pathway A). In turn, holes can migrate to the surface, where they can combine with electrons from donor species (pathway B). The rate of charge transfer depends on the bandedge position of the band gap and the redox potential of the adsorbate species, respectively. Electron and hole recombination prevents them from transferring to the surface to react with adsorbed molecules. Recombination can occur in the volume of the semiconductor particle (pathway C) or on the surface (pathway D). The optimal characteristics required for photocatalysts include the following:8 (1) The redox potential of the photogenerated valence-band hole must be sufficiently positive for the hole to act as an acceptor. (2) The redox potential of the photogenerated conductance-band electron must be sufficiently negative for the electron to act as a donor. (3) The material must not be prone to photocorrosion or produce toxic byproducts. (4) The material should be commercially and economically available. Table 2lists band-gap energies of semiconductors used for photocatalytic processes.

(i) development of a CO2-selective membrane is challenging because each gas component has its own solubility and permeability through membrane material (Wolsky et al.5)

(i) high energy consumption associated with gas compression and cooling (Plasynski and Chen6) (i) low efficiency (ii) expensive (Meisen et al.7)

Figure 1. Schematic representation of band-gap formation (adapted from Kabra et al.9).

Figure 2. Photoexcitation in solid followed by deexcitation. Table 2. Band-Gap Energies of Semiconductors Used for Photocatalytic Processesa photocatalyst

band-gap energy (eV)

photocatalyst

band-gap energy (eV)

Si WSe2 R-Fe2O3 CdS V2O5 WO3 SiC

1.1 1.2 2.2 2.4 2.7 2.8 3.0

TiO2 rutile Fe2O3 TiO2 anatase ZnO SrTiO3 SnO2 ZnS

3.02 3.1 3.23 3.2 3.4 3.5 3.7

3. Photocatalytic Reduction of CO2 Because CO2 is a relatively inert and stable compound, its reduction is quite challenging. The majority of conversion and removal methods rely on high-energy input for high-temperature and/or -pressure conditions.12 Conversely, photocatalysis occurs under relatively mild conditions with lower energy input, especially where the reaction is activated by solar energy or other easily obtained light sources. The use of solar energy is a particular advantage, as it relies on a continuous and readily available power supply. In addition to reducing CO2 emissions into the atmosphere, photocatalytic methods can also produce valuable chemicals that make such approaches an appealing option to conventional CO2 removal methods. In 1979, Inoue et al.13 first reported the photocatalytic reduction of CO2 in aqueous solution to produce formaldehyde

a

Kabra et al.9

(HCHO), formic acid (HCOOH), methyl alcohol (CH3OH), and trace amounts of methane (CH4) using various semiconductors, such as tungsten trioxide (WO3), titanium dioxide (TiO2), zinc oxide (ZnO), cadmium sulfide (CdS), gallium phosphide (GaP), and silicon carbide (SiC). These semiconductors were activated by both xenon- and mercury-lamp irradiation. In later years, many research groups have studied the mechanism and efficiency of CO2 photocatalytic reduction using a variety of semiconductors.14,15 This literature demonstrates that products

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such as ethyl alcohol (C2H5OH) and ethane (C2H6) can also be obtained through the use of specific semiconductor materials.16 Tennakone et al. conducted many studies of CO2 reduction by photocatalysts over various supported TiO2 materials including platinum (Pt), gold (Au), silver (Ag), cobalt (Co), lead (Pb), and mercury (Hg); reduction with hydrous cuprous oxide (Cu2O‚ H2O); and reduction of carbonate and bicarbonate to form formaldehyde with TiO2 powder.17-19 Investigations related to the photosynthesis reaction of CO2 with water vapor to form CH4 over metal-loaded SrTiO3 have also been conducted.20 Through the development of nanophotocatalysts on SiO2,21,22 Vycor glass, γ-zeolite, and β-zeolite, improved reactivity and selectivity for CO2 reduction to CH4 and CH3OH can be achieved. The use of nanoparticle semiconductors can provide a higher activity for CO2 reduction compared to the corresponding bulk semiconductor.23 When water is used as a reductant, the amount of organic products is very low. Other researchers have observed that the polarity of solvents exerts a significant influence on the reduction pathways of CO2.24 Therefore, improving the efficiency using sacrificial electron donors such as trimethylamine, triethanolamine,25 dimethyformamide,26 and isopropyl alcohol27 has also been extensively studied. Beyond the use of water as a reductant, CO2 photocatalysis can be achieved in the presence of gas-phase H2S,28 H2,29,30 and CH4.31 Furthermore, CO2 photoreduction via the hydrogenation process, using an Fe-based Fischer-Tropsch catalyst, has been used to produce H2 via the water-splitting reaction.32 A chronological compilation of the CO2 photocatalysis literature between 1997 and 2005 is presented in Table 3. 4. Effects of Wavelength, Light Intensity, and Band Gap Semiconductors absorb light radiation with a threshold wavelength that provides sufficient photon energy to overcome the band gap between the valence and conduction bands. The minimum wavelength required to promote the excited state depends on the band-gap energy given by52

λmin )

1240 band-gap energy

(3)

Electron excited states are produced via electronic transitions. The probability of an electronic transition is proportional to the square of the amplitude of the radiation field, E0, and the square of the transition dipole moment, |µif|53,54

P ∝ E0 |µif| 2

2

Although the cadmium sulfide (CdS) semiconductor has a smaller band gap (∼2.4 eV) and works in the visible range, it is not sufficiently positive to act as an acceptor. This causes the photocatalyst to decompose with hole formation.57 5. Effect of Reductant on Pathway Mechanism and Selectivity The photoreduction of CO2 by water is readily available and inexpensive. Two important species involved in CO2 photoreduction are H• (hydrogen atom) and •CO2- (carbon dioxide anion radical) produced by electron transfer from the conduction band as follows:

H+ + e- f H•

(5)

CO2 + e- f •CO2-

(6)

These radicals will also form other stable substances58 (mechanisms and pathways are not indicated):

Carbon monoxide formation CO2 + 2H+ + 2e- f CO + H2O

(7)

Formic acid formation CO2 + 2H+ + 2e- f HCO2H

(8)

Formaldehyde formation CO2 + 4H+ + 4e- f CH2O + H2O

(9)

Methanol formation CO2 + 6H+ + 6e- f CH3OH + H2O

(10)

Methane formation CO2 + 8H+ + 8e- f CH4 + 2H2O

(11)

However, the solubility of CO2 in water is particularly low, and the CO2 photoreduction process is competing with H2 and H2O2 formation, which consumes H+ and e- as follows:

photocatalyst + 4hV T 4e- + 4h+

(12)

Water decomposition 2H2O + 4h+ f O2 + 4H+

(13)

(4)

The amplitude of the radiation field, E0, can be controlled by varying the light intensity. Matthews et al.55 demonstrated that shorter-wavelength (254-nm) radiation is significantly more effective for CO2 degradation using TiO2 than 350-nm radiation. At low light intensities, the degradation rate increases linearly with light intensity;56 at midrange light intensities, the rate is dependent on the square root of intensity; and at high intensities, the degradation rate is independent of intensity. Large-band-gap semiconductors are the most suitable photocatalysts for CO2 reduction, because they provide sufficient negative and positive redox potentials in conductance bands and valence bands, respectively. The disadvantage of using wide band-gap semiconductors is the requirement for high energy input. For example, TiO2 anatase, a stable photocatalyst with a large band-gap energy (∼3.2 eV), is active only in the ultraviolet region of the solar spectrum (Figure 3).

Hydrogen formation 4H+ + 4e- f 2H2

(14)

Hydrogen peroxide formation O2 + 2H+ + 2e- f H2O2

(15)

Because of these limitations, some researchers have attempted to replace water with other reductants. This provides a high reaction yield and high selectivity to desired products by changing the mechanism. Liu et al.24 conducted an experiment with CdS in various solvents including water, methanol, ethanol, and 1-propanol with dielectric constants of 80, 33, 24.3, and 20.1, respectively. The results indicated that, if low-dielectricconstant solvents or low-polarity solvents are used, •CO2- anion radicals can be strongly adsorbed on the surface through the carbon atom of another •CO2- anion radical because these

Ind. Eng. Chem. Res., Vol. 45, No. 8, 2006 2561 Table 3. Summary of the CO2 Photocatalysis Literature photocatalyst used TiO2/zeolite

reductant water

CdS surfaceTEA modified by DMF ZrO2 H2

light source 75-W high-pressure Hg lamp, λ > 280 nm 300-W halogen tungsten lamp, λ > 400 nm 500-W ultrahigh-pressure Hg lamp

primary product(s) CH4 not reported

comments high selectivity to methanol formation

improved photocatalytic activity through the formation of sulfur vacancies CO intermediates exist on the catalyst surface under irradiation, and reaction is expected to proceed to yield CO CdS surfacepropanol 500-W high pressure Hg arc lamp formate, CO, ratio of formate to CO is greater with thiol modified by thiol with 300-nm cutoff filter H2, acetone solution TiO2 (P-25) isopropyl alcolhol 4.2-kW Xe lamp CH4, HCOOH high-pressure CO2 is more efficient in the formation of CH4 water vapor high-pressure Hg lamp, λ > 280 nm CH4, CH3OH catalysts exhibited high efficiency and Ti-MCM-41 and Ti-MCM-48 selectivity for CH3OH formation; larger pore sizes gave higher reactivity and selectivity 500-W high pressure Hg arc lamp formate, CO, reaction of nitrate is rate-determining step in TiO2 nanocrystals lithium nitrate/ 2-propanol with 280-nm band-pass filter NH3, urea in SiO2 photoformation of urea TiO2/zeolite, water vapor high-pressure Hg lamp, λ > 280 nm CH3OH zeolite and molecular sieve catalysts with TiO2 species highly dispersed in their cavities are TiO2/molecular promising as efficient photocatalysts sieves Rh/TiO2 H2 Toshiba UV-29, UV-37 Y-45 CO, CH4 when Rh is fully reduced, the main product changes from CO to CH4 TiO2/Pd/Al2O3, water 250-mW Hg arc lamp acetone, noteworthy selectivity exhibited by ethanol, TiO2/Pd/Al2O3; basic nature of support was TiO2/Pd/SiO2 methanol, more suitable for the activity and selectivity CuO/ZnO and Li2O/TiO2 formaldehyde, of C1-C3 formic acid, supported over MgO, Al2O3 methane, ethane [fac-Re(bpy)TEOA/DMF 500-W Hg lamp with 365-nm CO, H2 indicated the mechanism of these complexes (CO)3(4-Xpy)]+ band-pass filter TiO2 methanol; ethanol; 0.96-kW Xe lamp formic acid reduction of CO2 carried out in supercritical CO2; 2-propanol; protonation after irradiation with acid preferred over pure water nitric, hydrochloric, phosphoric acids (protonation) ZrO2 H2 500-W ultrahigh-pressure Hg lamp CO CO2 adsorbed on surface produces CO2anion radical upon irradiation; hydrogen then reacts with this radical in dark in the rate-determining step ZrO2 CH4 500-W ultrahigh-pressure Hg lamp CO reaction mechanism is similar to that of CO2 photoreduction by H2, except that carbonaceous residue is formed by the reaction of CO2 and CH4 Ti-β zeolite water vapor 100-W high-pressure Hg lamp, CH4, CH3OH H2O molecules can more easily gain access λ > 250 nm to the tetrahedrally coordinated titanium oxide species, Ti-β(OH), but high selectivity for the formation of CH3OH was observed on Ti-β(F). Ti silicalite methanol 266-nm emission of a pulsed HCO2H, CO, formic acid is primary two-electron reduction product; C-H bond formation implied to (TS-1) molecular Nd:YAG laser at 10 Hz HCO2CH3 occur in the initial step sieve Pd/RuO2/TiO2, NaOH, aqueous 450-W Xe short-arc lamp formate activity of CO2 photoreduction improved Na2SO3 Pd/TiO2 after surface noble-metal deposition MgO H2 500-W ultrahigh-pressure Hg lamp CO surface formate acts as a reductant and converts CO2 to CO upon irradiation Ti-containing water vapor 100-W high-pressure Hg lamp CH4, CH3OH Ti-containing mesoporous silica thin films porous silica exhibited significantly higher activity than powdered Ti-MCM-41 and high selectivity for CH3OH thin films formation; high reactivity attributed to their high transparency Cu/TiO2, sol-gel- water, NaOH 8-W Hg lamp methanol, methanol favorably produced in CO2/NaOH prepared Cu/P-25 oxygen [fac-Re(bpy)TEOA/DMF 500-W Hg lamp with 365-nm CO, H2 moderate CO2 pressure (1.36 MPa) desirable for i + efficient CO formation (CO)3P(O Pr)3] band-pass filter Pt/K2Ti6O13 with water 300-W Xe lamp, 150-W Hg lamp H2, CH4, methanol and ethanol produced when hybrid Fe-based catalyst HCHO, catalyst was used HCOOH, CH3OH, C2H5OH transparent Tiwater vapor 100-W highCH4, CH3OH Ti-containing mesoporous silica thin films containing pressure Hg lamp exhibited significantly higher activity than powdered Ti-MCM-41 and high selectivity mesoporous for CH3OH formation silica thin film Co(bpy)32+ DMF/TEOA, system allowed easy handling and recovery of Xe lamp with IR-cut filter CO, H2 sensitized with DMF/H2O/TEOA Ru complex, and CO production was largely 2+ Ru(bpy)3 improved for the partially heterogeneous system ZnO on activated Xe lamp CO, H2 used high temperature (>873 K) via Boudouard carbon reaction; improved gasification occurs by photocatalysis [fac-Re(bpy)use of high CO2 pressure was much more effective CO, H2 Et3N/DMF 500-W Hg lamp with 365-nm (CO)3Cl] band-pass filter than addition of excess Cl- ions; turnover number for CO was 5.1 times that at normal pressure sol-gel-prepared water, NaOH Hg lamp; UV-C (254 nm), methanol higher copper dispersion and smaller copper Cu/TiO2 UV-A (365 nm) particles on the titania surface correspond to a greater improvement in CO2 photoreduction performance

ref Anpo et al.33 Fujiwara et al.26 Kohno et al.29 Liu et al.24 Kaneco et al.27 Anpo et al.21 Liu et al.34 Yamashita et al.35 Kohno et al.12 Subrahmanyam et al.16

Hori et al.36 Kaneco et al.37

Kohno et al.30

Kohno et al.31 Ikeue et al.22

Ulagappan et al.38 Xie et al.39 Kohno et al.40 Ikeue et al.41

Tseng et al.42 Hori et al.43 Guan et al.32

Shioya et al.44

Hirose et al.45

Gokon et al.46 Hori et al.47 Tseng et al.48

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Table 3. (Continued) photocatalyst used

reductant

primary product(s)

light source

MgO

H2, CH4

250-W ultrahigh-pressure Hg lamp

CO

P-25

aqueous NaHCO3

15-W (maximum) source at 365 nm

CH4, CH3OH

Ti-MCM-41

water vapor

266-nm emission of a pulsed Nd:YAG laser at 10 Hz

CO, O2

comments

ref

revealed mechanism of CO2 photocatalytic reduction in the presence of H2 or CH4 adsorption-controlled reaction rate; reaction can be modeled by Langmuir-Hinshelwoodtype mechanism; more effective at higher acidity CO2 reduced to CO as a single-photon twoelectron-transfer product, with O2 as a coproduct.

Teramura et al.49 Ku et al.50 Lin et al.51

radicals are not well dissolved in low-polarity solvents. Here, CO is produced as the major reduction product of CO2. If a high-dielectric-constant solvent is used (e.g., water), the •CO2anion radicals can be greatly stabilized by the solvent, resulting in weak interactions with the photocatalyst surface. Subsequently, the carbon atom of the radical tends to react with a proton to produce formic acid.37

•CO2- + •CO2- f CO + CO32-

(16)

•CO2- + 2H+ + e- f HCO2H

(17)

A study of CO2 photocatalysis in the absence of a reductant made use of TiO2 powder continuously dispersed in supercritical CO2 during irradiation with a Xe lamp.37 Following irradiation, degassed aqueous solution was added to protonate the reaction intermediates on the TiO2 powder. The results showed that CO2 molecules interact with the excited-state photocatalyst surface, resulting in the formation of •CO2- radicals. During irradiation, no gaseous reduction products were identified. It was inferred that the •CO2- anion radicals cannot be adsorbed on another •CO2- anion radical, because the excited surface of the photocatalyst is more active than the •CO2- radical. However, following a washing process with several solvents, formic acid was detected. The amount of formic acid increased with the pH of the solution. From these studies, it was noted that the amount of H+ in the reductant controls the direction and selectivity of the CO2 photoreduction products. Nevertheless, the use of an additional reductant is not commonly employed in practice because it increases the cost of CO2 reduction. Water remains the primary hole scavenger used in most applications. The use of hydrogen as an alternative for CO2 photoreduction results in CO as the major product in a two-step process (Figure 4). Although CO is more toxic than CO2, it is a valuable substrate for many industrial processes, such as FischerTropsch synthesis or methanol synthesis. The first step in CO formation produces formate from CO2 and H2. The second is the reduction of CO2 to CO on the formate radical.

Figure 3. Solar spectrum at sea level with the sun at the zenith.

Figure 4. Mechanism of photocatalytic reduction of CO2 in the presence of H2 on ZrO2 (adapted from Kohno et al.37).

6. Recombination and Effect of Metal-Modified Surface Recombination of the photoexcited electron-hole pair retards an efficient CO2 photoreduction process. Previous photocatalytic studies have been performed on the loading of the photocatalyst with metals that function as “charge-carrier traps”. Chargecarrier traps suppress recombination and increase the lifetime of the separated electrons and holes. An illustration of electron trapping with metal in contact with the semiconductor surface is shown in Figure 5. Several researchers have investigated the efficiency and selectivity of processes by modifying the photocatalyst surface with metal. Hirano et al.59 observed CH4 production from CO2 photoreduction when using a Cu/TiO2 suspension in water. This was confirmed by Tseng et al.48 for Cu/TiO2 prepared by solgel procedures. They noted that a high copper dispersion on the photocatalyst surface corresponds to greater improvement

Figure 5. Metal-modified surface photocatalyst.

in the performance of CO2 photoreduction. The presence of Hg and Pt on TiO2 can accelerate the rate of formation of formaldehyde.17 Another report claims that photoreduction using Pd, Rh, Pt, Au, Cu, and Ru deposited on TiO2 photocatalyst produces methane and acetic acid.60 Pd/TiO2 exhibits a very high selectivity for methane from CO2 photoreduction. The effect of Pt loading on the photocatalytic reactivity of Ticontaining zeolite increased methane and methanol production.61 Metal-loaded surfaces also enhance the efficiency of CO2 photoreduction using hydrogen as a reductant. In 1987, Thampi

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et al.62 reported that Rh/TiO2 resulted in a high activity for CO2 photoreduction using hydrogen. In this experiment, the methane product was selectively obtained, although CO is a more preferable product.12 In the references mentioned above, it was noted that the performance of CO2 photoreduction to produce formaldehyde, methanol, and methane, which consumes more electrons (eq 9-11), was increased. Because the metal contacts the semiconductor surface, the electrons can easily flow from the semiconductor to the metal and distribute on the surface. Additionally, the holes are then free to diffuse to the semiconductor surface, where oxidation of organic species can occur. The metal loading must be optimized and uniformly dispersed over the photocatalyst. An excess metal loading results in a decreased illuminated photocatalyst surface, as photons cannot be absorbed because of reflection. 7. Effect of Temperature Generally, for photocatalysts, photon irradiation is the primary source of energy for electron-hole pair formation at ambient temperature, because the band-gap energy is too high for thermal excitation to overcome. However, at high temperatures, the reaction rate can be increased by raising the collision frequency and diffusion rate. Fox et al.63 stated that, like most photoreactions, photocatalytic reactions are not significantly sensitive to small variations of temperature. Gokon et al.46 studied the effect of ZnO on carbon gasification with CO2 at high temperature and observed that the presence of ZnO enhanced the CO evolution rate at the lower temperature of 873 K. However, the photocatalytic effect cannot be induced at higher temperatures (>1037 K), where the carbothermal reduction of ZnO dominates. This phenomenon occurs because the band-gap energy, chemical potential, and excited electronhole pair lifetime of ZnO decrease with increased temperature. In addition, the luminescence quantum yield of ZnO (ratio of the number of photons emitted to the number of photons absorbed by the substance) is reduced. This translates into lost potential for providing energy to stimulate molecules to the excited state.64 Guan et al.32 attempted to photodegrade CO2 with water under concentrated sunlight using Pt/K2Ti6O13 photocatalyst combined with an Fe-based catalyst at high temperature. In this experiment, the Fe-Cu-k/Day catalyst had no effect on CO2 reduction at room temperature. However, the yields of HCOOH, CH3OH, and C2H5OH significantly increased with temperature between 534 and 590 K. Guan’s research group demonstrated that water decomposition by the photocatalyst is remarkably improved, because high temperatures enhance the rate of hydrogen production. The hydrogen from water decomposition was used in the CO2 hydrogenation process in which Fe-Cu-k/Day acts as the catalyst. Photoreduction at high temperatures also was attempted using TiO2 to photodegrade CO2 with water.65 The products CO, H2, CH4, and some longer-chain hydrocarbons were detected in small quantities at temperatures up to 700 K. This investigation concluded that high temperatures are beneficial to the rate of thermally activated steps such as desorption, which occur after photochemical reactions. Some literature reports state that photocatalytic reactions at high temperature generally suffer from a decreased excited-state lifetime. Kohno et al.30 noted that the photoexcitation step of CO2 with hydrogen over ZrO2 can be deactivated with increased temperature. However, the photoexcitation step does not limit the reaction rate. Many researchers have concluded that the improved reaction rate must be due to the thermal step involved in entire reaction process.

8. Effect of Pressure Increasing the CO2 pressure is one method for increasing the concentration of CO2 in aqueous solvent and improving the reduction selectivity. In fact, investigations have reported the use of high pressure in both water66,67 and other organic media.68,69 Saeki et al.68 studied the electrochemical reduction of CO2 under various pressures galvanostatically at 200 mA/cm2 in a methanol medium. The results show that the current efficiency (ratio of the electrochemical equivalent current density for a specific reaction to the total applied current density) of CO2 reduction increased from 23% at 1 atm (0.1 MPa) to 92% at 20 atm (2 MPa). High pressure enhances the reaction, as reflected in the increased equivalent current density. This effect can be applied to photocatalysis with the same amount of energy supplied to the identical system. The difference between electrochemical reduction and photocatalysis is the source of electrons. Electrons from the electrochemical process are supplied by an applied current; electrons for photocatalysis are supplied by a semiconductor exposed to light radiation. A similar effect was anticipated for the photoreduction of CO2 in aqueous solutions when the Mizuno research group investigated the use of TiO2 powder at high pressure.70,71 Hydrocarbons, such as methane and ethylene, which were not produced at ambient CO2 pressure, were obtained under high CO2 pressure (2.5 MPa). However, a long retention time was required. This can be attributed to the rate of adsorption of CO2 on the surface TiO2 being overcome by the rate of adsorption of H2 with increasing pressure. Hydrogen species then proceed to form lower-mass hydrocarbons, such as methane and ethylene. The effect of pressure on the photocatalytic reduction of CO2 using TiO2 suspension in isopropyl alcohol solution was also investigated.27 The results illustrate that the formation of methane increases with increasing pressure to 2.8 MPa, similarly to the behavior observed in aqueous solution.71 In isopropyl alcohol, ethylene cannot be produced even at relatively high pressure, as no dimerization takes place because of the accelerated formation of methane. Rhenium bipyridine complexes have received a great deal of attention for their photochemical properties. Past researchers have focused primarily on modifying the chemical structure to improve the catalytic performance. Increasing the CO2 pressure is another potential approach. Current researchers have demonstrated that CO2 reduction can be achieved at high pressure using a CO2-soluble rhenium complex without an organic solvent in a one-phase (up to 7.3 MPa) system of liquid CO2.72 However, the one-phase system had no advantage for the production of CO. The presence of the gas phase (at pressures between 1.4 and 5.6 MPa) might be required for efficient CO formation. Therefore, photoreduction of CO2 to CO is conducted in a high-pressure two-phase system using [fac-Re(bpy)(CO)3P(OiPr)3]+ in dimethylfomamide (DMF) solvent with triethanolamine (TEOA) reducing agent (reductant).43 The CO formation rate greatly increased with pressure from 0.10 to 1.36 MPa and gradually increased between 1.36 and 5.57 MPa. The increased CO formation rate is a result of the increase in CO2 solubility with increasing pressure. This result can also be observed with triethylamine (TEA) in place of TEOA, although the CO formation rate is slightly reduced. CO2 is more soluble in DMF/TEA, and TEA is a more efficient electron donor than TEOA. Thus, the lower CO formation rate occurs because the reduction of CO2 to CO requires not only two electrons but also two protons (Figure 6). TEOA can act as both an electron

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Figure 6. CO-to-CO2 photocatalysis at high pressure using [fac-Re(bpy)(CO)3P(OiPr)3]+ (adapted from Hori et al.43).

donor and a proton donor, whereas the stronger base TEA acts only as an electron donor. The same effect of pressure can be seen when [fac-Re(bpy)(CO)3Cl] is used in DMF/trimethylamine (Et3N) under high pressure (2.45 MPa).47 9. Effect of Dispersed Photocatalyst within Zeolites and Mesoporous Molecular Sieve Because of the low water solubility of CO2, some researchers have attempted to perform CO2 photocatalysis in a gas-solid system. Several reports have shown that the photocatalytic reduction of CO2 with gaseous water processed on powdered TiO2 at room temperature forms trace amounts of methane.73-75 The results further show that extremely small TiO2 particles have higher activities.76 Additionally, highly dispersed TiO2 exhibits a higher reactivity than bulk catalyst powder.33 Because zeolite or silicate frameworks offer unique nanoscaled pore reaction fields, unusual internal surface topologies, and ion-exchange capacities, photocatalysts prepared within the zeolite cavity and framework have a unique local structure and high selectivity in photoreduction. The photocatalytic reaction rate and selectivity for the formation of CH3OH depend strongly on the degree of dispersion of titanium species. The distances between the two atom centers (atomic distance) of Ti-MCM48, Ti-MCM-41, and TS-1 (1.88, 1.86, and 1.8 Å, respectively) are much shorter than that of bulk TiO2 (1.96 Å) and show higher dispersion, making them more active in CH3OH formation. Additional distinguishing features of Ti-MCM-48 that produce higher reactivity and higher selectivity are threedimensional channels and a large pore size (>20 Å). Highly dispersed TiO2 catalysts prepared within zeolite cavities as photocatalysts (titanium oxide/Y-zeolite) for CO2 photoreduction increase CH3OH formation.35 Moreover, the effect of catalyst preparation was studied for both the ionexchange and impregnation methods. Those results clearly indicate that the titanium oxide/Y-zeolite prepared by ion exchange consists of highly dispersed isolated tetrahedral titanium oxide species, whereas that prepared by the impregnation method and bulk TiO2 are aggregated octahedral species that tend to form CH4. The conclusion is that, with bulk TiO2 or aggregated octahedral photocatalyst, the holes and electrons rapidly separate and move apart. This prevents the reaction between the carbon radicals and OH• radicals on the same sites and results in the formation of methane through the reaction between the H atoms and carbon radicals at the electron trap.21,33,35,75 Although the synthetic zeolite and mesoporous molecular sieve produce highly efficient photocatalysis, the morphology of these materials in powdered form is difficult to handle in practical applications. In this context, the synthesis of transparent porous silica thin films is a subject of current interest.77,79 Transparent porous silica

two-phase systems

three-phase systems

fluidized bed packed bed liquid or gas catalysts

trickle bed bubble-flow fixed bed high-velocity flow CSTR slurry bubble slurry

thin films have a larger surface area and efficient light absorption. The reactivities of titanium-containing porous silica thin-film mesostructures, Ti-PS(h, 25), Ti-PS(h, 50), and TiPS(c, 50), were studied for CO2 photoreduction by Ikeue et al.41,44 The results indicated that both Ti-PS(h, 50) and its powdered form exhibit higher photocatalytic reactivities than powdered Ti-MCM-41, even with the same pore structure. Unlike Ti-PS(h, 50), which is tetrahedral, Ti-PS(h, 25) has an aggregated octahedral structure similar to that of bulk TiO2. Ti-PS(c, 50) displays a higher selectivity for CH3OH formation, whereas Ti-PS(h, 25) displays a higher selectivity for CH4 formation. High yields observed with these TiO2-containing mesoporous photocatalysts can be attributed to their high transparency and large amount of surface OH•, which lead to a high selectivity for CH3OH formation.44 10. UV Photocatalytic Reactor for CO2 Reduction Both solar and UV radiation can be used for CO2 photocatalytic reduction. Approximately 10% of the solar flux that reaches Earth’s surface is useful for photocatalysis, whereas high-energy UV-light radiation tends to be based on wavelengths that do not reach Earth’s surface (UV-C range). Studies of UV photocatalytic reactor designs for CO2 reduction are limited and have generally been conducted as batch processes. Conversely, heterogeneous catalytic reactor design is well researched within the context of wastewater treatment and air cleansing. Table 4 identifies the variety of heterogeneous catalytic reactors that can be applied for CO2 photocatalysis within two major system types: (1) two-phase and (2) threephase. Two-phase systems can be either gas-catalyst or liquidcatalyst and are typically either fluidized or fixed beds. The chief advantage of the fluidized bed is that vigorous agitation of the solid by passing fluids can reduce temperature gradients within the bed.80 The violent motion of solids also provides high heat- and mass-transfer rates. The disadvantages of fluidized beds are erosion by abrasion of catalyst particles and attrition of the catalyst. Therefore, filters or scrubbers (in gascatalyst systems) are required. For fixed beds, the fluid flow regime approaches plug flow, so high conversion per unit mass of catalyst can be achieved. Another advantage of fixed-bed reactors is the low pressure drop, which enables such systems to be operated at reduced operating costs. Among fixed-bed designs, honeycomb monoliths and annular reactors are the most common because of their higher volumes and lower pressure drops. These designs are required for gassolid systems, which have particularly high gas feed rates.81 Honeycomb monolith reactors have been utilized for automobile exhaust emission control and NOx reduction in power plant gases. The reactor configuration in Figure 7 is an example of a beneficial design for CO2 photocatalytic reduction. Here, the channel cross section is typically square or circular. UV lamps irradiate the monolith from the front and back. Air containing CO2 is forced through the photocatalyst-coated monolith chan-

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Figure 7. Schematic diagram of a scale-up honeycomb photocatalytic reactor for CO2 reduction in the gaseous phase.

Figure 9. Three-phase-system photocatalytic reactor. Figure 8. Schematic diagram of the annular reactor.

nels.82 A prefilter is typically installed upstream to block large particles from entering the system and damaging the UV lamps or reducing the radiation penetration because of turbidity. This filter might or might not be impregnated with the active photocatalyst. An annular reactor is composed of two concentric cylinders that form an annular region. The interior wall of the tube is coated with photocatalyst. The UV light source can be located at the center or around the reactor (Figure 8). In general, the cross section of the reactor is small, inducing a high gas velocity to ensure that products desorbed from the surface are removed.83 In three-phase systems, the reactor is operated in one of two ways (Figure 9): Fixed-bed contactors use large photocatalyst particles, whereas suspended reactors use very fine suspended photocatalyst particles. Because of the higher active surface area and beneficial mass-transfer conditions, suspended systems are the more efficient of the two.84-86 However, separation and retrieval of the catalyst is more complex than in fixed-bed systems, and there is the potential for reduced light penetration into the slurry. In the past decade, low-cost supported catalysts consisting of photoactive materials that deposit on transparent and inert bodies have been developed significantly. This improves not only the surface area of the fixed-bed reactor design but also the UV light penetration (Figure 10).87,88 The proposed design is a bubble-flow fixed-bed column that is cylindrical and contains a bed of catalytic particles and a coaxial UV-lamp radiation source. The catalytic bed is a random packing of relatively small particles made of a UV-transparent material (typically quartz glass) coated with a thin layer of photocatalyst. The reductant flows via gravity countercurrent with the CO2 to enhance mass transfer. The product at the bottom of the reactor is separated for reductant reuse. Separation of the treated gas stream might be necessary to capture beneficial gaseous products. In addition to the above reactors, a new photoelectrochemical cell has been designed that integrates three techniques: pho-

Figure 10. Possible layout of multiple-lamp packed beds for CO2 photoreduction.

tochemical, electrochemical, and membrane processes (Figure 11). One half-cell of this reactor is a proton anode that splits water into one proton (H+) plus diatomic oxygen and consists of sol-gel-deposited TiO2 coated on a Ti metal substrate. Electrons transfer directly to another half-cell, whereas protons are transported through a Nafion membrane. The photons, electrons, and CO2 molecules react at the electrocatalyst (platinum modified by zinc oxide) to form new products. Not only can this reactor convert CO2 into valuable products, but it also produces hydrogen, a carbon-free energy source. This system works with an efficiency of about 12% under UV irradiation, but only 0.3% with solar radiation.89 11. Solar Photocatalytic Reactor for CO2 Reduction Several comprehensive articles have been published discussing the design of solar photocatalytic reactors and wastewater

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Figure 11. Hitachi Research Laboratory schematic for a CO2 photoreduction reactor. Table 5. Reactor Configurations for Solar Photocatalytic Reactionsa suspended photocatalyst

supported photocatalyst

Concentrating Reactors parabolic trough reactor parabolic trough reactor falling-film reactor Nonconcentrating Reactors with Reflectors compound parabolic collecting reactor tubular reactor fiber-optic-cable reactor Nonconcentrating Reactors without Reflectors solar pond flat-plate reactor tubular reactors trickle-down flat-plate reactor inflatable tube reactor thin-film fixed-bed reactor pressurized tube reactor flat-plate reactor trickle-down flat-plate reactor double-skin sheet reactor falling-film reactor a

Adapted from Alfano et al.88

treatment plants. The design concepts for those applications can potentially be applied for CO2 photoreduction (Table 5). Two key design issues for solar photoreactor systems include the options of (i) suspended or supported photocatalysts and (ii) concentrated or nonconcentrated sunlight.90 The advantages and disadvantages of each suspended and supported photoreactors are similar to those under UV photocatalysis. To concentrate solar radiation, light is gathered and concentrated in a photocatalytic reactor by a reflecting surface. The volume of this reactor type is smaller than that of the nonconcentrating type for the same light-harvesting area. Nonconcentrating systems can use more sunlight because they capture diffuse UV light, as well as direct solar beams. They also use light more efficiently than concentrating systems.91 However, nonconcentrating systems function as both solar collectors and photocatalytic reactors, translating into much larger systems than required for the concentrating configuration. The key design differences between wastewater and CO2 treatment systems are that wastewater systems are designed for organic and biological destruction in an aqueous solution, whereas CO2 systems must first dissolve CO2 in water or another solvent prior to the destruction of the gas into beneficial byproducts. Therefore, CO2-capture plants that use chemical absorption can divert CO2-rich solvents through photocatalytic reactors rather than directly to regeneration units. 12. Conclusion The use of photocatalysis for the reduction of CO2 emissions is an innovative alternative technology. Although there are few

current options for the application of this technology, photocatalysis is a potentially economical and environmental CO2 removal process. Understanding the parameters that influence photocatalysis is essential to developing this technology for practical use. There are three general methods for improving the efficiency of a photocatalytic process. The first is choosing semiconductors with appropriate band-gap energies. The proper structure of the photocatalyst can improve the product selectivity and yield and increase the rate of reaction. Therefore, photocatalyst preparation is one of the most important steps that can enhance the capacity of a photoreduction system. The second improvement method involves reductant development. The effectiveness and product selectivity of photocatalytic reactions are obtained using the appropriate reductant. Water is the most common reductant, although the solubility of CO2 in water is particularly low, and the reaction is in competition with H2 and H2O2 formation. By performing the photocatalytic reaction in the gas phase, this difficulty can be eliminated. Hydrogen and some organic solvent are alternative reductants, but they are too costly to replace water. The third and last method is to optimize operating conditions including temperature, pressure, light intensity, and operating wavelength. These will not only provide a high reaction activity but can also improve the selectivity to and yield of products. Photocatalytic systems using solar energy for CO2 reduction ar eone attractive option for CO2 emissions reductions. Further research should focus on the potential and economics of solar reactors and their design. Acknowledgment The Natural Sciences and Engineering Research Council (NSERC) of Canada is gratefully acknowledged for partial financial support of this project. Literature Cited (1) Intergovernmental Panel on Climate Change 2001. Climate Change 2001: The Scientific Basis; Cambridge University Press: Cambridge, U.K., 2001. (2) Kaya, Y. Impact of Carbon Dioxide Emission Control on GNP Growth: Interpretation of Proposed Scenarios; Intergovernmental Panel on Climate Change/Response Strategies Working Group: May 1989. (3) Rao, A. B.; Rubin, E. S. A Technical, Economic, and Environmental Assessment of Amine Based CO2 Capture Technology for Power Plant Greenhouse Gas Control. EnViron. Sci. Technol. 2002, 36, 4467. (4) Falk-Pedersen, O.; Dannstro¨m, H. Separation of Carbon Dioxide from Offshore Gas Turbine Exhaust. Energy ConVers. Manage. 1997, 38, S81. (5) Wolsky, A. M.; Daniels, E. J.; Jody, B. J. CO2 Capture from the Flue Gas of Conventional Fossil-Fuel-Fired Power Plants. EnViron. Prog. 1994, 13 (3), 214. (6) Plasynski, S. I.; Chen, Z.-Y. Review of CO2 Capture Technologies and Some Improvement Opportunities. Prepr. Symp.-Am. Chem. Soc., DiV. Fuel Chem. 2000, 45 (4), 644. (7) Meisen, A.; Shuai, X. Research and Development Issues in CO2 Capture. Energy ConVers. Manage. 1997, 38, S37. (8) Bhatkhande, D. S.; Pangarkar, V. G.; Beenackers, A. ACM. Photocatalytic Degradation for Environmental Application. A Review. J. Chem. Technol. Biotechnol. 2001, 77, 102. (9) Kabra, K.; Chaudhary, R.; Sawhney, R. L. Treatment of Hazardous Organic and Inorganic Compounds through Aqueous-Phase Photocatalysis: A Review. Ind. Eng. Chem. Res. 2004, 43, 7683. (10) Bussi, J.; Ohanian, M.; Vazquez, M.; Dalchiele, D. A. Photocatalytic Removal of Hg from Solid Wastes of Chlor-Alkali Plant. J. EnViron. Eng. 2002, 128 (8), 733. (11) Linsebigler, A. L.; Lu, G.; Yates, J. T., Jr. Photocatalysis on TiO2 Surfaces: Principles, Mechanism, and Selected Results. Chem. ReV. 1995, 95, 735.

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ReceiVed for reView May 16, 2005 ReVised manuscript receiVed November 24, 2005 Accepted November 29, 2005 IE0505763