PHOTOCATALYTIC REDUCTION OF CO 2 WITH H20 ON TiO 2 AND Cu/TiO2 CATALYSTS

Res. Chem. Intermed. Vol. 20, No. 8, pp. 815-823 (1994) 9 VSP 1994 PHOTOCATALYTIC REDUCTION OF CO 2 WITH H20 ON TiO 2 AND Cu/TiO2 CATALYSTS H. YAMASHITA, H. NISHIGUCHI, N. KAMADA, M. ANPO" Department of Applied Chemistry, College of Engineering, University of Osaka Prefecture, Gakuen-cho. Sakai, Osaka 593, Japan. Y. TERAOKA Department of Applied Chemistry, Faculty of Engineering, Nagasaki University, Nagasaki 852, Japan. H. HATANO Laboratory of Analytical Technology, Osaka Prefectural Industrial Technology Research Institute, Higashi- Osaka, Osaka 577, Japan. S. EI-IARA Ion Engineering Research Institute Corporation, Tsuda, Hirakata, Osaka 573-01, Japan. K. KIKUI FujiMn Incorporated, Nanko-Higashi Suminoeku, Osaka 559, Japan. L. PALMISANO, A. SCLAFANI, M. SCHIAVELLO Dipartimento di lngegneria Chimica dei Processi e dei Materiali, Universita' di Palermo, Italy M. A. FOX Department of Chemistry, The University of Texas at Austin, Texas 78712, U.S.A. Received 12 October 1993; accepted 14 December 1993 Abstraet-Photoinduced reduction of CO 2 by H20 to produce CH 4 and CHaOH has been investigated on wellcharacterized standard TiO 2 catalysts and on a Cu 2 loaded TiO 2 catalyst. The efficiency of this photoreaction depends strongly on the kind of catalyst and the ratio of H20 to CO 2. Anatase TiO2, which has a large band gap and numerous surface OH groups, shows high efficiency for photocatalytic CH 4 formation. Photogenerated Ti 3* ions, H and CH 3 radicals are observed as reactive intermediates, by ESR at 77 K. Cu-loading of the small, powdered TiO 2 catalyst (Cu/TiO2) brings about additional formation ofch3oh. XPS studies suggest that Cu plays a significant role in CH3OH formation. INTRODUCTION Solar utilization for chemical energy storage can be achieved by photocatalytic and/or photoelectrochemical activation of light-sensitive semiconductor surfaces. The photosynthetic reduction of CO 2 by H20 is of special interest and achieving high efficiency

816 M. Anpo et al in this reaction is one of the most desirable objectives in this field [1]. On the other hand, the usefulness of extremely small TiO2 particles as photocatalysts has attracted a great deal of attention [2,3], the actual factors that control the photocatalytic activity of TiO2 particles still being unknown. In the present study, the photosynthetic reduction of CO 2 with HzO to produce CH 4 and CH3OH is investigated on well-characterized standard TiO 2 catalysts in order to clarify the factors that control the photocatalytic activity of these catalysts. The effect of Cu2+-loading of the catalysts is also clarified. EXPERIMENTAL The four types of "standard TiO 2 catalysts" (JRC-TIO-2,3,4,5) supplied by the Catalysis Society of Japan (Table 1) were used in powdered form (grain size: 0.02-1 /xm). Detailed information about these standard TiO2 catalysts [4] is available from the Catalysis Society of Japan (5-21-13 Higashigotannda, Shinagawa, Tokyo, 141, Japan). CuZ+-loaded TiO 2 catalysts were prepared by impregnation of TiO2 (JRC-TiO-4) in an aqueous solution of CuCli2H20. (Nakarai Chem. Ltd.). Both the pretreatment of catalysts and the photocatalytic reactions were carried out in a quartz cell with a flat bottom (60 ml) connected to a conventional vacuum system. Ultimate vacua of 10.6 Tort were attainable. The catalysts were degassed at 725 K for 2 h, calcined in 02 at 725 K for 2 h, and finally degassed at 475 K for 2 h before use. The pretreated catalyst (150 mg) was spread out at the flat bottom of quartz cell. UV irradiation of the catalyst in the presence of CO 2 (124 #mol) and gaseous H20 (372 ~mol) was carried out using a 75-W high-pressure Hg lamp (Toshiba SHL100UV) through water and color filters (X>290 nm) at 275 K. Products were analyzed by gas chromatography and mass spectrometry. ESR spectra were recorded with a JES-RE-2X (X-band) spectrometer at 77 K. XPS spectra were recorded with a Shimadzu ESCA-750 using Mg radiation. Experimental details can be found in our previous papers [2,3]. RESULTS AND DISCUSSION UV irradiation of several TiO2 catalysts in the presence of a mixture of CO2 and H20 led to the evolution of CH 4 in the gas phase at 275 K, whereas no products could be detected in the dark control reaction. Trace amounts of CzH 4 and C2tt 6 were also produced. Figure 1 shows the time profile for the production of CH4 with UV irradiation time. The yield increased with the irradiation time for up to 4 hrs, although a small decrease in the reaction rate then occurred. Probably some reduced carbon intermediate formed and trapped on the surface as one or more stable compounds at 275 K is responsible for this decrease in catalytic activity [1], because graphitic carbon was observed in Cls XPS spectra of the surface of TiO2 after the reaction. Figure 2 shows the effect of the H20/CO2 ratio on the CH 4 yield. The CH4 yield was almost zero in the reaction in the absence of H20 and

Photocatalytic Reduction of CO 2 with He0 817 increased with increasing amounts of H20, suggesting that H20 is a key reductant in this reaction. Figure l. Time profile for CH 4 production in the photocatalytic reduction of CO2 (124/zmol) by H20 (372 /zmol) on TiO 2 (JRC-TIO-4) catalyst. Figure 2. Effect of adsorbed H20 concentration on CH 4 yield in the photocatalytic reduction (reaction time: 4 hrs) of CO 2 (124 #moo by H20 on TiO 2 (JRC-TIO-4) catalyst. These results suggest that the photoinduced reduction of CO2 to CH4 and C2 compounds in the presence of H20 takes place photocatalytically at the solid-gas interface. Photocatalytic production of CH 4 from H20 and CO 2 has also been reported by Hemminger et al [5] on Pt/SrTiO 3 and Inoue et al. [6] have reported that HCOOH, HCHO, and CH3OH

818 M. Anpo et al. are produced upon illumination of aqueous suspensions of a variety of semiconductor powders such as TiO2 and SrTiO3. However, the efficiency of CO2 reduction was low [7,8] when H20 was used as reductant and details of this reaction have not yet been made clear [9]. Recently Anpo et al. [1] have found that photocatalytic reduction of CO2 by H20 to produce CO, CH4 and CH3OH proceeds more efficiently on the highly dispersed anchored titanium oxides and Ishitani et al [ 10] have reported that deposition of a metal (Pd, Pt, Rh, etc) on TiO2 considerably accelerated photocatalytic reduction of CO 2 to CH4 and CHsCO2H. The CH4 yields on four types of TiO 2 catalysts are shown in Table 2. The photocatalytic activity was found to depend on the type of TiO2 catalyst in the order of JRC-TIO-4 > -5 > -2 > -3. These four TiO 2 catalysts were also investigated for photoeatalytic activity in the hydrogenation of methyl acetylene with H20 [2,11 ] and in the isomerization of 2-butene [11]; the results are also shown in Table 2. The same order of catalytic activity was observed for the photocatalytic reduction of CO 2 with H20 as in the hydrogenation of the alkyne and isomerization of the olefin. Because it has already been confirmed that the latter two reactions proceed photocatalytically [2,11 ], this parallel with photocatalytic activity for CO 2 reduction by H20 implies that this reaction also occurs photocatalytically over TiO 2. With all catalysts, the photocatalytic activity for the reduction of CO2 was lower than those for hydrogenation and olefin isomerization. This result indicates the difficulty of photocatalytic reduction of CO 2, because of the highly endothermic property of this reaction. Various chemical and physical properties of these four TiO 2 catalysts are shown in Table 1. From these results, anatase TiO 2, with a large band gap and numerous OH groups, is preferable for efficient photocatalytic reaction. The increased band gap is accompanied by a shift of the conduction band edge to higher energies. This shifts the reductive potential to more negative values and enhances photocatalytic activity. This effect seems to be even more crucial for the difficult reactions such as the reduction of CO2 by H20. It is also often suggested that the surface OH groups and/or physisorbed H20 play a significant role in photocatalytic reactions though the formation of OH radicals and H radicals. Thus, the data in Table 2 can be interpreted as deriving from small changes in the band gap and/or the Table 1 Physical properties of TiO2catalysts Catalyst a Surface area CO z ads. Acid conc. Relative Band (JRC-T10-) (m2/g) (#tool/g) (/Lmol/g) -OH conc. gap (ev) 2 (anat.) 16 1 6 1 3.47 3 (ruti.) 51 17 22 1.6 3.32 4 (anat.) 49 10 5 3.0 3.50 5 (ruti.) 3 0.4 7 3A 3.09 "~ JRC-TIO-2 and -3 samples were prepared by calcination of the residue from the evaporation of aqueous solution of Ti(SO4)2. JRC-TIO-4 and -5 samples were prepared by chemical vapor deposition from TiCI 4.

Photocatalytic Reduction of CO 2 with 1120 819 Table 2 Photocatalytic activities of TiO~ catalysts. Catalyst Reduction" of CO 2 Hydrogenation b of (JRC-TIO-) (#mol/h~ methyl acetylene (#mol/h~ Isomerization r of cis-2-butene (~mol/h,g) 2 (anat.) 0.03 0.20 2.5 3 (ruti.) 0.02 0.12 1.0 4 (anat.) 0.17 8.33 9.4 5 (ruti.) 0.04 0.45 3.8 Reaction: AG%.gs(kJ/mol) a) C02 + 2H20. > CH4 + 202 813 b) CH3C-CH + 3H20 > CH4 + C2H6 + 3/2 02 433 c) cis-2-c4h8 --T---> 1-C4H8 5. trans-2-c4h8-3 concentration of the surface OH groups. At present, we have not yet been successful in determining the actual amount of oxygen, expected as a by-product of the reaction. The difficulty in detecting free 02 seems to be associated with its adsorption onto the TiO 2 surface in the presence of H20 [12]. Figure 3 shows the ESR signals obtained under UV-irradiation of anatase TiO 2 (JRC- TIO-4) in the presence of CO 2 and H20 at 77 K. These signals are attributed to the characteristic photogenerated Ti 3 ions (g l-1.9723 and g I1-1.9628) and H radicals (with 490 G splitting), as well as to CH 3 radicals, which consist of four lines with intensities in a 1:3:3:1 ratio and a hyperfine splitting of Ha=l 9.2 G and g-value of 2.002. The intensity of the signal assigned to CH 3 radicals decreased with increasing amounts of H20, indicating that CH3 radicals react easily to form CH 4 in the presence of sufficient H20. These results clearly suggest that CH 3 radicals are a key intermediate species and react with H radicals formed by the reduction of protons (H supplied from H20 adsorbed on the catalyst. From these results, a reaction scheme for the photocatalytic reduction of CO 2 by H20 on TiO 2 is proposed as shown in Scheme 1. The photo-reduction of CO2 molecules into a carbon allotrope on TiO 2 is attributed to the highly excited electronic state of the catalyst, i.e., to the high reactivity of the charge transfer excited state (Ti3+-O-) 3. species [1], although surface bound carbon atoms were not detected by ESR measurement in the present system. Such high reactivity has been linked with the edge sites of surface TiO 2 [2,11 ]. Major fractions of the reduced bare-carbons may be trapped and remain as graphitic carbons on the surface [1]. These graphitic carbons easily decompose and react with H radicals generated from H20 and/or the surface OH groups to produce CH 3 radicals and then CH 4. The formation of C2H 6 and CzH 4 as minor products presumably result from coupling of the photo-formed CH3 radicals.

820 M. Anpo et al. Figure 3. ESR signals measured at a) 77 K and b) 295K with TiO 2 (JRC-TIO-4) catalyst after irradiation in the presence of CO 2 (124/~mol) and H:O (372 /zmol) for 6 h, and their difference; a)-b). The photoinduced reduction of CO2 by H20 was also investigated on Cu2+/TiO2 catalysts. The effect of Cu 2+ loading on the yields is shown in Fig. 4. Although the adsorption of Cu 2+ to TiO2 produces a catalyst that is less effective for the photocatalytic production of CH4, the formation of CH3OH is more efficient. Cu/TiO2 with Cu2*-loading of 0.3-1.0 wt % produced CH3OH in the yield of 0.006-0.004/xmol/g-cat. On the other hand, the addition of excess Cu 2+ (>3 wt%) to TiO2 is undesirable for the formation of CH3OH. Figure 5 shows the XPS spectra of the Cu2*/TiO2 catalyst after the photocatalytic reaction. The lack of satellite peaks in the Czp spectra and the peak position in Auger CUL3vv spectra indicates that the active species is Cu +. Recently, it has been reported that Cu catalysts play a significant role in the photoelectrochemical production of CH3OH from the CO 2 and H20 system [13]. Although the details for the mechanism of production of CH3OH are not clear the results obtained in the present study show good correspondence with these reports.

Photocatalytic Reduction of C02 with 1120 821 hv TiO2 > e- + hv { (Ti4+~O 2-) > (Ti3+--O-)* } h + (Ti3+mO-)* CO2(ads) > (Ti3+--O-) * CO(ads) + 1/2 02 CO(ads) > C(ads) 1/2 02 H20 e-.> ~ + OH- OHh + > OH h+ > 1/2 02 C(ads) + I~1 > {:CH, :CH2} > CH3 > CH4 Scheme l Reaction scheme for photocatalytic reduction of CO2 by H20 on TiO 2 catalyst. Figure 4. Effect of Cu z* loading on CH 4 and CH3OH yields in photocatalytic reduction (reaction time: 4 hrs) of CO 2 (71 ttmol) by H20 (71 #mol) on Cu/TiO 2 catalyst.

822 M. Anpo et al. Figure 5. XPS signals of Cu2'/TiO2 catalyst after irradiation in the presence of CO 2 (124 #mol) and H20 (372 /.tmol) for 4 h. Acknowledgment The partial financial support of a Grant-in-Aid for Scientific Research (03650659 and 05453123) and that on Priority Areas (03203250) from the Ministry of Education, Science and Culture, Japan, are gratefully acknowledged. This work was also partially supported by the Research and Development Program on "Research on advanced biomaterials" conducted under a program set by New Energy and Industrial Technology Development Organization (NEDO), which is promoted as Industrial Science and Technology Research and Development Program undertaken by Agency of Industrial Science and Technology, Ministry of International Trade and Industry.

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