泉 康雄

Photocatalytic reduction of CO2 into C2,3 hydrocarbons completes a C‐neutral cycle. The reaction pathways of photocatalytic generation of C2,3 paraffin and C2H4 from CO2 are mostly unclear. Herein, a Co0–ZrO2 photocatalyst converted CO2 into C1–3 paraffin, while selectively converting CO into C2H4 and C3H6 (6.0 ± 0.6 μmol h−1 gcat−1, 70 mol%) only under UV–visible light. The photocatalytic cycle was conducted under 13CO and H2, with subsequent evacuation and flushing with CO. This iterative process led to an increase in the population of C2H4 and C3H6 increased up to 61–87 mol%, attributed to the accumulation of CH2 species at the interface between Co0 nanoparticles and the ZrO2 surface. CO2 adsorbed onto the O vacancies of the ZrO2 surface, with resulting COH species undergoing hydrogenation on the Co0 surface to yield C1–3 paraffin using either H2 or H2O (g, l) as the reductant. In contrast, CO adsorbed on the Co0 surface, converted to HCOH species, and then split into CH and OH species at the Co and O vacancy sites on ZrO2, respectively. This comprehensive study elucidates intricate photocatalytic pathways governing the transformation of CO2 into paraffin and CO to olefins.

<p>The search for novel and low-cost cocatalysts that can achieve high efficiency meanwhile maintain high selectivity in photocatalytic CO2 reduction is highly desirable yet remains challenging. Herein, we demonstrate that...</p>

A newly aqueous colloidal comprised of monodispersed Ni-doped ZnS nanocrystals is reported as excellent visible-light-responsive photocatalysts for CO2 reduction into formate.

Be careful of false “solar fuel” converted from adventitious carbon! Time course of isotope-labeled 13CO2 photoconversion into a 13C-product was successfully monitored using a Ag–Zr oxide photocatalyst.

Layered double hydroxides (LDHs), typically comprising Zn, Cu, and Ga, photoreduce CO2 into methanol and CO, however, selective methanol synthesis using CO2 and heterogeneous photocatalyst is very rare. In this study, the amount of interlayer water molecules is reduced to 31% of that for as-synthesized LDHs by preheating the LDHs at 423 K in vacuum, and the performance for CO2 photoreduction using 0–0.28 MPa of CO2 and 0–0.56 MPa of H2 was investigated. If the LDHs are preheated in vacuum and never in contact with air prior to the photoreduction tests, methanol was produced exclusively in all experiments of this study. LDHs comprising inlayer Cu sites were more active compared to [Zn3Ga(OH)8]2CO3·mH2O, [Zn3Ga(OH)8]2Cu(OH)4·mH2O, and [Zn3Ga(OH)8]2Pd(OH)4·mH2O LDHs. A contour plot for methanol formation rates was drawn for the most active [Zn1.5Cu1.5Ga(OH)8]2CO3·mH2O and the volcano top positioned at 0.12 MPa of CO2 and 0.28 MPa of H2 2.8 μmol-methanol h−1 gcat −1 and the selectivity was &gt 97 mol%-methanol. 13CH3OH formation in the presence of 13CO2 and [Zn1.5Cu1.5Ga(OH)8]2CO3·mH2O confirmed photocatalytic methanol synthesis. Under 0.12 MPa of CO2 and 0.28 MPa of H2, the intensity of the Cu K preedge peak progressively decreased at the rate of 170 μmol-Cu h−1 gcat −1 upon the UV–visible light irradiation for the [Zn1.5Cu1.5Ga(OH)8]2CO3·mH2O LDH, demonstrating photogenerated electron accumulation at the CuII/I sites for subsequent CO2 reduction.

Photocatalyst layers with thicknesses of 0.8−2.1 μm were applied to both electrodes. The primary particle size of TiO2 for the photoanode in solar cell was optimized (15−21 nm), and its performance was further improved by doping with organic dyes, e.g., anthocyanins.

Photocatalytic conversion of CO2 into mainly methane using Pd/TiO2 photocatalyst proceeded faster at 0.80 MPa using water rather than hydrogen as a reductant. The reason of the higher activity using water rather than H2 could be explained owing to the oxygen vacancy sites as confirmed by XAFS.

The reaction pressure in the photocatalytic conversion of CO2 into fuels is optimized between 0 and 0.80 MPa under CO2 and moisture. The higher reactivity of water than H-2 was observed at higher pressure and the reason was clarified using several similar to 10 mu m-thick semiconductor-based photocatalysts. The best Pd/TiO2 photocatalyst produces methane with a reaction order of 0.39. The sum of independent total formation rates of C-containing compounds under UV and visible light does not account for that under UV-visible light, demonstrating synergetic reaction mechanism on Pd for CO2 reduction by excited electrons via surface plasmon resonance and on TiO2 for water oxidation. Active metallic Pd and 0 vacancy sites due to O-2 formation from H2O are confirmed by in situ monitoring of EXAFS [N(Pd-Pd) = 5.9-6.2; N (Ti-0)= 5.2-3.5] and the decrease of the H-bound and bi/tri-coordinated OH peaks in FTIR. Effective redox-site separation explains the higher reactivity of water than H-2. (C) 2017 Elsevier Inc. All rights reserved.

Various layered double hydroxides with the formula [MII3GaIII(OH)8]+2A2−·mH2O {MII = Zn, Cu; A2− = CO32−, [Cu(OH)4]2−} were synthesized and applied to CO2 photoconversion using H2 at the reaction pressure of 0.40 MPa. Exclusive photoconversion into methanol was found.

CO2 photoconversion is a promising method to reduce atmospheric CO2 concentrations and mitigate energy problems simultaneously. Among the various efficient and stable semiconductor photocatalysts used for this purpose, layered double hydroxides (LDHs) have attracted attention as catalysts for CO2 photoconversion into CO and/or methanol. In this study, various LDHs of the formula [MII 3Ga(III)(OH)(8)] 2A center dot mH(2)O (M-II = Zn-II, Cu-II; A(2) = CO32 , [Cu(OH)(4)](2) ) were synthesized and used for CO2 photoconversion at a reaction pressure of 0.40 MPa in the presence of H-2 to result in the exclusive production of methanol. Furthermore, the pretreatment of carbonate-type LDHs at 423 K boosted the reaction rates by a factor of 7.5-20. Interestingly, [Zn3Ga(OH)(8)](2)CO3 center dot mH(2)O was the only LDH that produced methane primarily by an eight-electron reduction (rather than the production of methanol by a six-electron reduction) at a total formation rate of 2.7 mu molh (-1)g(cat) (-1) after it was preheated at 423K and protected by an Ar atmosphere. Conversely, the methanol photogeneration rates of tetrahydroxycuprate- type LDHs were suppressed to less than 0.1 mmolh-1 g(cat) (-1) at 0.40 MPa. In summary, the contribution of the interlayer reaction space created by the partial removal of water molecules and/or carbonate ions of LDHs was suggested.

Fuel cells (FCs) and solar cells (SCs) are indispensable devices for a hydrogen energy society. The voltages obtained are less than 1 V per cell for most FCs and SCs. Herein, we use a recently developed SC comprising two photocatalysts. In principle, both TiO2 and BiOCl are photo-excited, and the energy difference between the conduction band (CB) minimum of TiO2 for excited electrons (-0.11 V) and the valence band (VB) maximum of BiOCl for holes (2.64 V) can provide a theoretical electromotive force of 2.75 V. This SC converts light energy into an electromotive force corresponding to the level difference of the two photocatalysts permanently mediated by the redox of water/O-2. The diffusion overpotential of electrons in the photocatalysts (0.23-0.41 V) and the leakage current (0.38 V) are experimentally evaluated. The contact between the TiO2 film and ITO layer is improved by the better dispersion of the TiO2 suspension at a lower pH than that of the isoelectric point. Cyclic voltammetry data suggest the formation of O/Cl vacancy sites during the SC tests and the superiority of the rear orientation of the BiOCl photocatalyst on the cathode and effective photo-oxidation of water over TiO2, whereas impedance measurements suggest a smaller impedance for the tight and uniform TiO2 film in comparison to the porous BiOCl film. Thus, in the optimized configuration of the electrodes (irradiation from the other side of the photocatalyst), the leakage current and the diffusion overpotential in the catalyst layers are effectively suppressed to realize an open-circuit voltage of 1.91 V and cell output of 55.8 mu W per 1.3 cm(2).

Photocatalytic conversion of CO2 into fuels could mitigate global warming and energy shortage simultaneously. In this study, the reaction pressure was optimized for CO2 reduction by H-2. The major products were methane, CO, and methanol, and the observed catalytic activity order was Cu or Pd on TiO2 &gt;&gt; Ag/ZrO2 similar to g-C3N4 &gt; Ag/Zn3Ga-layered double hydroxide similar to BiOCl. Hot/excited electrons due to surface plasmon resonance could be transferred to CO2-derived species and the remaining positive charge could combine with excited electrons in the semiconductor. As the levels of hot/excited electrons became more negative, the catalysts became more active, except for Ag/ZrO2 and Ag/Zn3Ga-LDH, probably due to lower charge separation efficiency for intrinsic semiconductors or hydroxides. The reaction order was controlled by the partial pressure of H-2, demonstrating preferable adsorption of H on Pd. The photoconversion of CO2 into methane was optimum at P-H2 = 0.28 MPa and P-CO2 = 0.12 MPa, but the rates gradually dropped at higher partial pressures due to adsorption of CO2 being hindered by H. (C) 2016 Elsevier Inc. All rights reserved.

Gold nanoparticles (AuNPs) self-coupled on semiconductors have attracted extensive attentions in the field of catalysis, however, the progress in understanding and optimizing their photocatalytic performance in response to solar light irradiation is limited. In this paper, a series of AuNPs with Zn2Al-layered double hydroxide (LDH) as support was fabricated via self-assembly routes at room temperature and the tuned oxidation state of AuNPs (as Au-0, Au3+ as well as mixed Au-0/Au3+) was revealed to have a crucial effect on establishing their photocatalytic efficiency for the degradation of phenol from aqueous solution under solar irradiation. X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analyses permitted to characterize the specific interactions of Zn2Al-LDH with AuNPs and to verify that the state of Au-0/Au(3+)NPs appears due to the electron transfer from Zn2Al-LDH to AuNPs when Zn2Al-LDH reconstruction in the aqueous solution of Au(O2CCH3)(3) was achieved under solar irradiation. On the basis of these AuNPsZn(2)Al-LDH systems, the possible roles of Au-0, Au3+ or Au-0/Au3+ in establishing synergetic effects with Zn2Al-LDHs supports for enhancing the photocatalytic response induced by the irradiation with solar light, for manipulating the mechanism, and the catalyst stability in phenol degradation process, are critically discussed. (C) 2016 Elsevier B.V. All rights reserved.

Preferential oxidation (PROX) of CO is applicable because of its low cost, ease of implementation, and low loss of H-2 during purification. Mo-SiO2 and Cr-SiO2 photocatalysts utilized charge separation at metal=O bonds, and the PROX selectivity of CO was high. The CO PROX rates using semiconductor-based photocatalysts were comparable to those of photocatalysts (380 mu mol h(-1) g (cat) (-1) ) and were also selective (100 %). These photocatalysts are advantageous because they do not activate H-2 in comparison to noble metal catalysts. Below 473 K using noble metals, Ru, Rh, Pt, or Au supported on reducible metal oxides exhibited excellent CO thermal-PROX rates of similar to 4900 mu mol h(-1) g (cat) (-1) ; however, the CO PROX selectivity of similar to 48 % was insufficient because of H-2 activation on noble metals in nature. CO adsorbed onto TiO2 and O-2 was stabilized at the interface between a Ti site and an Au atom. Weakly adsorbed water increased the effective number of active sites by stabilizing Au-OOH or Au-COOH. The PROX rates of CO using Au/TiO2-based catalysts under dark conditions increased under UV-visible light by the effect of charge separation and surface plasmon resonance and the promoted electron transfer to the adsorbed O-2. In the case of CuO-CeO2 catalysts, CO adsorbed onto CuO and reacted with lattice O atoms at the boundary between CuO and CeO2 to form CO2 at an O vacancy, which was subsequently filled with an O-2 molecule. The combination of Cu or Co with a reducible metal oxides also provided performance comparable to or higher than that of CuO-CeO2 owing to adequate standard reduction potential for Cu2+, Co3+, Mn3+/4+, and Ce4+. Finally, binary metal-organic framework consisting of oxyhydroxide Ti clusters interlinked by organic ligands and Cu oxyhydroxide ligands showed superior CO PROX performance (76 % conversion and 99 % selectivity) to that achieved using CuO-CeO2 owing to effective dispersion of Cu-O-Ti connection in microporous crystallites. Further progress is needed to alleviate the activity loss in the presence of moisture and/or CO2 based on suggestions that steric hindrance of some types of microporous crystallites would suppress the blocking of moisture or CO2.

The importance of metal-support interfaces is widely known in commercial and fundamental heterogeneous catalysis; however, it is difficult to characterize the active interface sites. In this study, we synthesize a new class of compound comprising tetragonal [Ti8O8(OH)(4)](12+) clusters interlinked by terephthalates (bdc) and [Cu-2(OH)(6)](2-) linkers {Ti8O8(OH)(4)center dot(bdc)(2)center dot[Cu-2(OH)(6)](4)}. The crystalline structure was refined for X-ray diffraction and direct links between [Cu-2(OH)(6)](2-) and [Ti8O8(OH)(4)](12+) are confirmed by extended X-ray absorption fine structure. This compound functions very well (k = 0.117 min(-1) in CO 63 Pa + O-2 76 Pa at 323 K) as a catalytic model of interface Cu sites on ultra-dispersed Ti [hydro]oxide for preferential oxidation of CO in predominantly H-2 gas, that is important for the purification of hydrogen used in fuel cells. In comparison, mean 1.7-nm CuO nanoparticles embedded inside the pores of MIL125 were inert (k = 0.0035 min(-1)) because of the absence of links between Cu and [Ti8O8(OH)(4)](12+) clusters. In CO 0.51 kPa + O-2 0.51 kPa at 323 K, the conversion to CO2 and CO PROX selectivity using Ti8O8(OH)(4)center dot(bdc)(2)center dot[Cu-2(OH)(6)](4) (76% and 99%) was significantly higher than that using CuO/CeO2 (28% and 96%, respectively) for 24 h. (C) 2015 Elsevier Inc. All rights reserved.

This study optimized photocatalytic hydrogen purification using the preferential oxidation (PROX) of CO impurities among nanoscale disk-like, spheroidal, and rod-like ZnO promoted by adsorbed Cu ions. Four key factors were examined.

For Ag/[Zn3Ga(OH)8]2CO3·mH2O layered double hydroxide, CO2 photoreduction by H2 under visible light was promoted by the surface plasmon resonance effect of Ag nanoparticles while for Au//[Zn3Ga(OH)8]2CO3·mH2O catalysts Au nanoparticles might act as electron-trapping active sites.

We reported a new PhotoFuel Cell (PFC) comprising two photocatalysts for use of acidic water as a recyclable medium. Nitrogen and oxygen flow was required in the photoanode and photocathode, respectively. In this study, we developed a gas-circulating PFC that needs no gas supply from outside. In the gas-circulating PFC, the reverse reaction of water oxidation at the anode was prevented by the gas flow of photogenerated O-2 from the anode to the cathode inside the PFC. The gas-circulating PFC accommodated an organic solvent layer over the aqueous electrolyte for the anode, and also a vent hole in the upper part of the Proton-Conducting Polymer (PCP) film. O2 transferred from the anode electrolyte to the organic solvent due to the solubility difference between the HCl solution and organic solvent. O-2 transfer from the gas phase in the anode to that in the cathode was achieved by the vent hole in the PCP film due to the pressure difference due to the progress of the reaction. By the addition of a hexane layer to the anode of the PFC, it was demonstrated to achieve a photocurrent value of 69.7 mu A per 1.3 cm(2) of photocatalysts. However, in the stability tests for more than 7 h, the small amount of remaining O-2 in the electrolyte (2.85 mu mol L-1) exhibited serious effects on the PFC performance. The ISC, VOC and PMax values of the gas-circulating PFC were 29.2 mu A, 1.18 V and 6.10 mu W, that were 40%, 74% and 44%, respectively, of those for a N-2 and O-2 flow-type PFC. Apparently, photocurrents were dramatically suppressed by the reverse reaction at the photoanode in the extended tests for the gas-circulating PFC.

Photocatalytic conversion of CO2 into fuels is an attractive option in terms of both reducing the increased concentration of atmospheric CO2 as well as generating renewable hydrocarbon fuels. It is necessary to investigate good catalysts for CO2 conversion and to clarify the mechanism irradiated by natural light. Layered Double Hydroxides (LDH) have been attracting attention for CO2 photoreduction with the expectation of sorption capacity for CO2 in the layered space and tunable semiconductor properties as a result of the choice of metal cations. This study first clarifies the effects of Cu doping to LDH comprising Zn and Al or Ga. Cu could be incorporated in the cationic layers of LDH as divalent metal cations and/or interlayer anions as Cu(OH)4(2-). The formation rates of methanol and CO were optimized for [Zn1.5Cu1.5Ga(OH)8](+)2Cu(OH)4(2-)_mH2O at a total rate of 560 nmol h_1 gcat_1 irradiated by UV–visible light. Cu phthalocyanine tetrasulfonate hydrate (CuPcTs(4-)) and silver were effective as promoters of LDH for CO2 photoreduction. Especially, the total formation rate using CuPcTs-[Zn3Ga(OH)8](+)2CO3(2-)·mH2O irradiated by visible light was 73% of that irradiated by UV–visible light. The promotion was based on HOMO–LUMO excitation of CuPcTs4- by visible light. The LUMO was distributed on N atoms of pyrrole rings bound to central Cu(2+) ions. The photogenerated electrons diffused to the Cu site would photoreduce CO2 progressively in a similar way to inlayer and interlayer Cu sites in the LDH in this study.

The design and synthesis of nanoscale zinc oxides (ZnOs) and their applications to photocatalysis are widely explored. However, the photocatalytic controls needed to design appropriate crystalline faces and promoter sites for each catalytic reaction step using ZnO have been rarely reported. This study optimized photocatalytic hydrogen purification using the preferential oxidation (PROX) of CO impurities among nanoscale disk-like, spheroidal, and rod-like ZnO promoted by adsorbed Cu ions. Four key factors were examined: (1) the diffusion length to the surface of separated electrons and holes induced by light; (2) the crystalline face where formate was formed from CO and its stability; (3) the crystalline face where Cu ions adsorbed for trapping electrons and reducing O-2; and (4) the frequency factor of "charge recombination as cations and anions" derived from photogenerated holes and electrons, respectively, at the boundary of crystalline faces. The optimal photocatalyst for hydrogen purification (CO PROX) was determined to be Cu-spheroidal ZnO. An efficient photocatalytic cycle was obtained by increasing the frequency factor between unstable unidentate formate on the (000 (1) over bar) face and Cu ions adsorbed on neighboring unsaturated {10 (1) over bar1} faces.

総ページ数381. This is the subsequent version of CO2 photoreduction review. The Part I is “Recent advances in the photocatalytic conversion of carbon dioxide to fuels with water and/or hydrogen using solar energy and beyond”, Coor. Chem. Rev., 257, 171(2013).

A photofuel cell comprising two photocatalysts TiO2 and BiOCl that uses acidic water as a fuel is investigated and enabled 1.76 V per cell. The cells were characterized using Bi L3-edge EXAFS, XPS, Raman, and UV-visible spectroscopy.

In polymer electrolyte fuel cells (PEFCs), it is important to secure proximate diffusion paths of reactants and electrons. One approach is to optimize the boundary between polymer electrolyte and Pt nanoparticle surface. Based on synchrotron X-ray absorption fine structure to monitor directly the status of catalysts in PEFCs, it was found that Pt sites were reduced to Pt-0 by alcohols contained in polymer electrolyte dispersion solution during the preparation of cathode of PEFC. As in membrane electrolyte assembly, only the Pt sites not covered by polymer electrolyte re-oxidized to Pt2+/4+. Thus, the interface between Pt and polymer electrolyte was evaluated. The other approach is to functionalize carbon surface with sulfonate/sulfate group to conduct protons. Similar level of proton conductivity was observed in current voltage dependence compared to using polymer electrolyte, but polymer electrolyte was advantageous to lose less voltage for activation. Based on this comparison, optimum catalyst on cathode is proposed comprising surface sulfonate/sulfate group on carbon mixed with polymer electrolyte. Further optimization of cathode catalyst is proposed to functionalize carbon with sulfonate group linked to fluorocarbon branch. (C) 2014 Elsevier B.V. All rights reserved.

Photocatalytic reaction mechanism for the conversion of CO2 into methanol and CO using layered double hydroxides (LDHs) consisting of Zn, Cu, and Ga was investigated. X-ray absorption fine structure was applied to determine the LDH site structures and to monitor the diffusion of photogenerated electrons to Cu-II sites. Electron diffusion to Cu sites was an order of magnitude faster in the direction of the cationic layers (580 mu mol h(-1) g(cat)(-1)) than in the perpendicular direction. According to Fourier-transform infrared spectroscopy, CO2 was in equilibrium with hydrogen carbonate (1629 cm(-1) for (HCO3)-C-13) for the reaction with hydroxy group from the cationic layer or interlayer site and/or with interlayer water. The equilibrium reactions were faster for [Zn3Ga(OH)(8)](+)(2)[Cu(OH)(4)](2-)center dot mH(2)O (400-110 mu mol h(-1) gcat(-1)) than for [Zn1.5Cu1.5Ga(OH)(8)](+)(2)(CO3)(2-)center dot mH(2)O. Furthermore, the reductive decomposition of hydrogen carbonate was suggested in H-2 under UV-visible light, suggesting photocatalytic pathway to methanol/CO. (C) 2013 Elsevier B.V. All rights reserved.

The phenomena of the photocatalytic oxidation of water and photocatalytic reduction of CO2 were combined using reverse photofuel cells, in which the two photocatalysts, WO3 and layered double hydroxide (LDH), were separated by a polymer electrolyte (PE) film. WO3 was used for the photooxidation of water, whereas LDH, comprising Zn, Cu, and Ga, was used for the photoreduction of CO2. For this process, photocatalysts pressed on both sides of the PE film were irradiated with UV-visible light through quartz windows and through the space in carbon electrode plates and water-repellent carbon paper for both gas flow and light transmission. 45% of the photocatalyst area was irradiated through the windows. The protons and electrons, which were formed on WO3 under the flow of helium and moisture, transferred to the LDH via the PE and external circuit, respectively. Methanol was the major product from the LDH under the flow of CO2 and helium. The observed photoreduction rates of CO2 to methanol accounted for 68%-100% of photocurrents. This supports the effectiveness of the combined photooxidation and photoreduction mechanism as a viable strategy to selectively produce methanol. In addition, we tested reverse photofuel cell-2, which consisted of a WO3 film pressed on C paper and LDH film pressed on Cu foil. The photoelectrodes were immersed in acidic solutions of pH 4, with the PE film distinguishing the two compartments. Both the photoelectrodes were completely irradiated by UV-visible light through the quartz windows. Consequently, the photocurrent from the LDH under CO2 flow to WO3 under N-2 flow was increased by 2.4-3.4 times in comparison to photofuel cell-1 tested under similar conditions. However, the major product from the LDH was H-2 rather than methanol using photofuel cell-2. The photogenerated electrons in the irradiated area of the photocatalysts were obliged to diffuse laterally to the unirradiated area of photocatalysts in contact with the C papers in photofuel cell-1. This lateral diffusion reduced the photocatalytic conversion rates of CO2, despite the advantages of photofuel cell-1 in terms of selective formation and easy separation of gas-phase methanol.

A photofuel cell comprising two photocatalysts TiO2 and Ag-TiO2 is demonstrated. The open circuit voltage, short circuit current, and maximum electric power of the PFC were 1.59 V, 74 mu A, and 14 mu W, respectively. The electron flow was rectified due to the Schottky barrier between TiO2 and Ag nanoparticles.

Ir-Sn bimetallic silica-based materials have been prepared via deposition of the molecular organometallic clusters (NEt4)2[Ir 4(CO)10(SnCl3)2] and NEt 4[Ir6(CO)15(SnCl3)] or via deposition of Sn organometallic precursor Sn(n-C4H9) 4 onto pre-formed Ir metal particles. These solids possess promising properties, in terms of selectivity, as catalysts for propane dehydrogenation to propene. Detailed CO-adsorption DRIFTS, XANES and EXAFS characterization studies have been performed on these systems in order to compare the structural and electronic evolution of systems in relation to the nature of the Ir-Sn bonds present in the precursor compounds and to propose a structural model of the Ir-Sn species present at the silica surface of the final catalyst. © 2013 The Royal Society of Chemistry.

Synthesis of dimethyl carbonate (DMC) from CO2 and methanol under milder reaction conditions was performed using reduced cerium oxide catalysts and reduced copper -promoted Ce oxide catalysts. Although the conversion of methanol was low (0.005-0.11%) for 2 h of reaction, DMC was synthesized as low as 353 K and at total pressure of as low as 1.3 MPa using reduced Cu CeO2 catalyst (0.5 wt% of Cu). The apparent activation energy was 120 kJ mol(-1) and the DMC synthesis rates were proportional to the partial pressure of CO2. An optimum amount of Cu addition to CeO2 was 0.1 wt% for DMC synthesis under the conditions at 393 K and total pressure of 1.3 MPa for 2 h (conversion of methanol: 0.15%) due to the compromise of two effects of Cu: the activation of H-2 during reduction prior to the kinetic tests and the block (cover) of the surface active site. The reduction effects in H-2 were monitored through the reduction of Ce4+ sites to Ce3+ based on the shoulder peak intensity at 5727 eV in the Ce L3 -edge X-ray absorption near -edge structure (XANES). The Ce3+ content was 10% for reduced CeO2 catalyst whereas it increased to 15% for reduced Cu CeO2 catalyst (0.5 wt% of Cu). Moreover, the content of reduced Ce3+ sites (10%) associated with the surface 0 vacancy (defect sites) decreased to 5% under CO2 at 290 K for reduced Cu CeO2 catalyst (0.1 wt% of Cu). The adsorption step of CO2 on the defect sites might be the key step in DMC synthesis and thus the DMC synthesis rate dependence on the partial pressure of CO2 was proportional. Subsequent H atom subtraction steps from methanol at the neighboring surface Lewis base sites should combine two methoxy species to the adsorbed CO2 to form DMC, water, and restore the surface O vacancy.

Photocatalytic reduction of carbon dioxide to fuels using solar energy is an attractive option for simultaneously capturing this major greenhouse gas and solving the shortage of sustainable energy. Efforts to demonstrate the photocatalytic reduction of CO2 are reviewed herein. Although the photocatalytic results depended on the reaction conditions, such as the incident/absorbing light intensity from the sun or a simulated solar light source, the performance of different systems is compared. When the reactants included CO2 and water, it was necessary to determine whether the products were derived from CO2 and not from impurities that accumulated on/in the catalysts as a result of washing, calcination, or pretreatment in a moist environment. Isotope labeling of (CO2)-C-13 was effective for this evaluation using Fourier-transform infrared (FTIR) spectroscopy and mass spectrometry (MS). Comparisons are limited to reports in which the reaction route was verified spectroscopically, the C source was traced isotopically, or sufficient kinetic analyses were performed to verify the photocatalytic events. TiO2 photocatalytically produced methane at the rate of similar to 0.1 mu mol h(-1) g(cat)(-1). In aqueous solutions, formic acid, formaldehyde, and methanol were also produced. When TiO2 was atomically dispersed in zeolites or ordered mesoporous SiO2 and doped with Pt, Cu, N, I, CdSe, or PbS, the methane and CO formation rates were greater, reaching 1-10 mu mol h(-1) g(cat)(-1). As for semiconductors other than TiO2, CdS, SiC, InNbO4, HNb3O8, Bi2WO6, promoted NaNbO3, and promoted Zn2GeO4 produced methane or methanol at rates of 1-10 mu mol h(-1)g(cat)(-1), and promoted A(II)La(4)Ti(4)O(15) produced CO at a rate greater than 10 mu mol h(-1) g(cat)(-1), in addition to the historically known ZnO and GaP (formaldehyde and methanol formation). The photocatalytic reduction of CO2 was also surveyed with hydrogen, because hydrogen can be obtained from water photosplitting by utilizing natural light. CO was formed at a rate of similar to 1 mu mol h(-1) g(cat)(-1) using TiO2, ZrO2, MgO. and Ga2O3, whereas both CO and methanol were formed at a rate of 0.1-1 mu mol h(-1) g(cat)(-1) using layered-double hydroxides consisting of Zn, Cu, Al, and Ga. When hydrogen is used, in addition to identifying the origin of the carbon, it is critical to confirm that the products are photocatalytically formed, not thermally produced via CO2 hydrogenation. The feasibility of the strategy involving the recycling of a sacrificial electron donor and the direct supply of protons and electrons released from water oxidation catalysts to photocatalysts for the reduction of CO2 to fuels has been demonstrated. However, based on the results obtained to date, it is clear that the practical use of the photocatalytic reduction of CO2 as one possible solution for global warming and the world's energy problems requires the development of more efficient photocatalysts. (C) 2012 Elsevier B.V. All rights reserved.

Polymer-electrolyte fuel cell supplying CO2 was demonstrated. Using a PEFC, 95 mol% of methanol was selectively formed by supplying H-2 to Pt catalyst and CO2 to Zn-Cu-Ga layered double hydroxide catalyst accompanying protons and electrons transfer, i.e. H-2 -&gt; 2H(+) + 2e(-) at anode followed by CO2 + 6H(+) + 6e(-) -&gt; CH3OH + H2O at cathode. Using PEFC, CO2 reduction with water was also enabled to methanol (100-86 mol%) in contrast that negligible amount of methanol was formed if mixed oxidation and reduction catalysts were under CO2 and moisture. At anode, water was dissociated over Pt catalyst to provide protons (H2O -&gt; OH(adsorbed) + H+ e(-)) that transferred to Zn-Cu-Ga catalysts at cathode on which CO2 was progressively reduced to methanol (CO2 + 6H(+) + 6e(-) -&gt; CH3OH + H2O). (c) 2012 Elsevier B.V. All rights reserved.

The photocatalytic reduction of carbon dioxide into methanol was enabled between the Zn-Ga or Zn-Cu-Ga hydroxide layers using hydrogen and was promoted by the partial desorption of structural water stuffed between the cationic layers. The photoreduction rate obtained using [Zn1.5Cu1.5Ga(OH)(8)](2)(+)(CO3)(2-)center dot mH(2)O was improved by replacing interlayer carbonate anions with [Cu(OH)(4)](2-) to 0.49 mu mol(Methanol) h(-1) g(cat)(-1), and the methanol selectivity was 88 mol%. At the molar level, interlayer Cu species was 5.9 times more active than the octahedral Cu sites in the cationic layers. The bandgap value was evaluated as 3.0 eV for the semiconductor [Zn1.5Cu1.5Ga(OH)(8)](2)(+)[Cu(OH)(4)](2-)center dot mH(2)O. Direct electronic transition from O 2p to metal 3d, 4s, or 4p was responsible for the photocatalysis excited largely by ultraviolet (UV), and to a lesser extent by visible light. (C) 2011 Elsevier B. V. All rights reserved.

Sulfur-doped titanate and Na TNTs were prepared and characterized. S atoms were in the lattice of TNTs as S(2-) anions. TNTs with very low amount of doped S were more active than undoped one for ethanol photo-oxidation.

Preferential oxidation of CO (63 Pa) and O-2 (76 Pa) in H-2 (6.3 kPa) using spheroidal ZnO nanoparticles (22 nm x 47 nm) converted 52% of CO into CO2, and selectivity to CO2 was 92 mol% under UV-visible light for 5 h. When 0.5 wt.% of Cu2+ was adsorbed on ZnO, 91% of CO was converted into CO2 with a selectivity of 99 mol% under UV-visible light for 3 h. CO (63 Pa) was photocatalytically decreased to 2.3 mPa (0.35 ppm) in O-2 (150 Pa) and H-2 (63 kPa) for 5 h with a selectivity of 94 mol%. As evident from a XANES peak at 8983.1 eV, the surface Cu-II sites trapped photogenerated electrons. Furthermore, O-2-derived species were reduced by accepting electrons from Cu-I and protons from the neighboring formate species, as indicated by the FT-IR peaks at 2985, 2879, 1627, 1587, and 1297 cm(-1). (C) 2011 Elsevier Inc. All rights reserved.

Ordered layered double hydroxides (LDHs) consisting of zinc and/or copper hydroxides were synthesized and combined with aluminum or gallium. These LDH compounds were then applied as photocatalysts to convert gaseous CO2 (2.3 kPa) to methanol or CO under UV-visible light using hydrogen. Zn-Al LDH was the most active for CO2 photoreduction and the major product was CO formed at a rate of 620 nmol h(-1) g(cat)(-1), whereas methanol was the major product formed by the inclusion of Cu in the LDH photocatalysts, e.g., at a formation rate of 170 nmol h(-1) g(cat)(-1), using Zn-Cu-Ga photocatalyst. The methanol selectivity improved by the inclusion of Cu from 5.9 to 26 mol% and 39 to 68 mol%, respectively, when Zn-Al (the conversion 0.16-0.11%) and Zn-Ga LDH catalysts were used (the conversion 0.02-0.03%). Specific interaction of Cu sites with CO2 was spectroscopically suggested to enable coupling with protons and photogenerated electrons to form methanol. (C) 2011 Elsevier Inc. All rights reserved.