In 2 O 3 /TiO 2 nano photocatalysts for solar hydrogen production from methanol:water mixtures

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1 Indian Journal of Chemistry Vol. 53A, April-May 04, pp In O 3 /TiO nano photocatalysts for solar hydrogen production from methanol:water mixtures K Lalitha, V Durga Kumari* & M Subrahmanyam Inorganic and Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology, Hyderabad , India durgakumari@iict.res.in Received 30 January 04; revised and accepted February 04 In O 3 /nano TiO and In O 3 /TiO (P-5) photocatalysts with different In O 3 loadings have been prepared by solid state dispersion method. These catalysts are characterized by XRD, surface area, TEM, UV-vis DRS and XPS techniques. XRD analysis shows the fine dispersion of In O 3 on the surface of TiO. The photocombinates of In O 3 prepared using the lab-made TiO are nano sized with an average size of 8 0 nm compared to the particle size of In O 3 /TiO (P-5) combinates. The In O 3 /nano TiO photocatalysts show higher surface area as compared to In O 3 /TiO (P-5) photocatalysts due to nanosize of the particles. UV-vis DRS results show the visible light absorption of the catalysts in both the series. XPS analysis shows the interaction between In O 3 and TiO, although the extent of interaction is low. Photocatalytic hydrogen evolution studies have been conducted over these catalysts from water and methanol:water mixtures under solar light irradiation. The wt% In O 3 loaded nano TiO and P-5 catalysts show maximum hydrogen production activity from pure water splitting. A very good improvement in the activity is seen when the reaction is carried out in methanol:water mixtures. Maximum hydrogen production of 4080 µmoles/hour is observed on 5 wt% In O 3 /nano TiO catalyst, which is almost double the activity observed on In O 3 / TiO (P-5) catalyst. Keywords: Catalysts, Photocatalysts, Nanoparticles, Photocatalytic hydrogen evolution, Hydrogen production, Indium oxide, Titania Heterogeneous photocatalysis offers potential solutions for environmental and energy issues affecting our society. The production of renewable and non-polluting fuels by the direct conversion of solar energy into chemical energy is one of the challenging areas of research. Photocatalytic hydrogen production by water splitting is a promising approach since H can be obtained directly from abundant and renewable water and solar light. If successfully developed with economic viability, this could be the ultimate technology that could solve both energy and environmental problems in the future. TiO is considered as the most promising photocatalyst for water treatment and remediation, degradation of organic pollutants and water splitting reactions due to its strong oxidizing and reducing abilities, cost effectiveness, and long-term stability against photocorrosion and chemical corrosion. However, the major drawback of TiO photocatalyst is its large band gap. Titanium dioxide can only be activated upon irradiation with a photon of light <390 nm, limiting its use under solar irradiation. Ultraviolet light makes up 3 5% of the solar spectrum, whereas the spectrum consists of ~40% visible light. Therefore, in order to utilise TiO to its full potential it is necessary to decrease the band gap size facilitating visible light absorption. Furthermore, recombination of photo generated electron and holes is very fast in TiO photocatalyst. Numerous efforts have been made to improve the photocatalytic properties of titanium dioxide. It has been shown that the modification of TiO surface by noble metals, non-metals, transition metals and dyes can enhance the energy range of photoexcitation as well as photocatalytic activity of TiO -7. The incorporation of transition metal ions on TiO surface may result in the formation of new energy levels between VB and CB, inducing a shift of light absorption towards the visible light region. Deposition of noble metals on the surface of TiO enhances the photocatalytic efficiency by acting as an electron trap, promoting interfacial charge transfer and therefore delaying recombination of the electron-hole pair. It has been reported that titania particles coupled with other inorganic oxides and sulfides such as SiO, Al O 3, ZrO, SnO, V O 5, CdS, rare earth oxides and Fe O 3 can change the photocatalytic efficiency 8-5. Several compounds have been prepared by various methods in order to develop materials that can perform the redox reactions in the photocatalysis process efficiently. In addition, these

2 LALITHA et al.: In O 3 /TiO NANO PHOTOCATALYSTS FOR H PRODUCTION FROM CH 3 OH:H O MIXTURES 473 materials must be activated under visible light to have a sustainable process in water splitting reaction. Recently, the class of materials formally having d 0 electronic configurations (In 3+, Ga 3+, Ge 4+, Sn 4+, and Sb 5+ ) have been investigated in an attempt to develop new photocatalysts for water splitting 6-8. Various kinds of indates such as In O 3 (ZnO) m, MIn O 4 (M=Ca, Sr, Ba), Lanthanides doped indates and Sr -x M x In O 4 (M=Ca, Ba), have been reported as photocatalysts for water splitting 9-. In O 3 /TiO photocatalysts have been prepared and studied for various catalytic applications -6. In the present investigation, nano TiO catalyst has been prepared by sol-gel method and the In O 3 /nano TiO, and In O 3 /TiO (P-5) photocatalysts have been prepared by solid state dispersion method. These catalysts are characterized by XRD, BET surface area, TEM, UV-Vis DRS, XPS techniques, and are evaluated for hydrogen production activity from water and methanol:water mixtures under solar light irradiation. Materials and Methods Preparation and characterisation of catalysts TiO P-5 (anatase-80%, rutile-0%, surface area 50 m /g and particle size 7 nm) from Deggussa Corporation, Germany, titanium(iv) isopropoxide from Fluka, In O 3 from Sigma-Aldrich Chemie and methanol from Ranchem Ltd. (India) were procured. Nano TiO catalyst was prepared by sol-gel method. Titanium isopropoxide (6 g) was dissolved in 5 ml of isopropanol solution. This solution was added slowly to deionised water so that titanium isopropoxide was completely hydrolyzed and resulted in the formation of a homogeneous gel. The homogeneous gel obtained was stirred vigorously for half an hour and dried overnight in an oven at 0 C. In O 3 /TiO catalysts were prepared by solid state dispersion method. For this, varying amounts of In O 3 (0.5,,, 5 and 0 wt %) were mixed with nano TiO and also with TiO (P-5) with absolute ethanol in a mortar. After evaporation of the solvent, the samples were dried in an oven and calcined at 450 C for 5 h. The X-ray diffraction patterns of the TiO and In O 3 modified TiO catalysts were recorded with Siemens D-5000 X-ray diffractometer using Ni filtered Cu Kα radiation (λ =.5409 Å) and θ range between 60. The UV vis diffuse reflectance spectra were recorded on a GBC UV Visible Cintra 0e spectrometer, in the wavelength range nm. X-ray photoelectron spectra were recorded on a Kratos Axis 65 equipped with Mg Kα radiation. A Philips Technai G FEI F transmission electron microscope operating at kv was used to record the transmission electron microscopy (TEM) images. The BET surface areas were determined by nitrogen adsorption/desorption isotherms at 77 K using a static volumetric technique (Autosorb Micromeritics). Photocatalytic activity Photocatalytic reactions were carried out under solar light and UV (400 W Hg vapor lamp) light irradiation. The intensity of solar light was measured using a LT Lutron LX-0A digital light meter. The solar light intensity was measured for every hour between 0:00 and 5:00. The average light intensity value observed was around,30,000 lx during the experiments. The reaction was performed in a 50 ml quartz reactor by taking 50 ml of distilled water containing 00 mg of catalyst. The reactions were also carried out in 5% methanol:water mixtures. Prior to the irradiation, the reaction mixture was evacuated for 30 min and also purged with nitrogen gas for 30 min to remove the dissolved oxygen. After irradiation, the evolved gaseous products were analyzed at every h by gas chromatography (Shimadzu GC-04) with molecular sieve 5A column using thermal conductivity detector and N as a carrier gas. Results and Discussion Characterisation of catalysts To investigate the phase structure of In O 3 /nano TiO and In O 3 /TiO (P-5) photocatalysts, XRD patterns of the catalysts were recorded XRD results (Fig. ) clearly show the presence of the characteristic anatase and rutile phases of TiO in both In O 3 /nano TiO and In O 3 /TiO (P-5) photocatalysts without any peaks corresponding to the In O 3 phase. Only at higher dispersion (5 wt%) the In O 3 phase is identified with very low intensity. This may be because of the fine dispersion of In O 3 on the surface of TiO. Furthermore, Fig. (a) shows the XRD patterns of the In O 3 /nano TiO photocatalysts with broad peaks as compared to that of In O 3 /TiO (P-5) photocatalysts. The In O 3 /nano TiO photocatalysts show high surface area as compared to In O 3 /TiO (P-5) photocatalysts due to nanosize of the particles (Table ). With increasing the In O 3 wt% the surface area tends to decrease which may be due to the dispersion of In O 3 on the surface of nano TiO.

3 474 INDIAN J CHEM, SEC A, APRIL-MAY 04 Fig. TEM images of (a) In O 3 /nano TiO, and, (b) In O 3 /TiO (P-5) combinates. Fig. XRD patterns of (a) In O 3 /nano TiO and (b) In O 3 /TiO (P-5) catalysts. [In O 3 (wt%):, 0.5;, ; 3, ; 4, 5]. Table -Surface areas of In O 3 /TiO photocatalysts No. In O 3 (wt %) Surface area (m /g) Nano TiO TiO (P-5) TEM images of In O 3 /nano TiO and In O 3 /TiO (P-5) catalysts are shown in Fig.. The TEM images show that the photo combinates of In O 3 prepared using lab made TiO are highly nano sized with an average size of 8 0 nm as compared to the particle size of In O 3 /TiO (P-5) combinates which is almost equal to the particle size of TiO (P-5), i.e., 7 nm. UV-vis DRS of In O 3 /TiO (P-5) and In O 3 /nano TiO combinates with different In O 3 loadings were recorded to understand the change in the absorption properties of TiO upon In O 3 dispersion and (Fig. 3). Nano TiO shows a red shift of 30 nm in the Fig. 3 UV-vis DRS of (a) In O 3 /Nano TiO and (b) In O 3 /TiO (P-5) catalysts. [In O 3 (wt%):, 0;, 0.5; 3, ; 4, ; 5, 5; 6, 00].

4 LALITHA et al.: In O 3 /TiO NANO PHOTOCATALYSTS FOR H PRODUCTION FROM CH 3 OH:H O MIXTURES 475 absorption as compared to the commercial TiO (P-5). Pure In O 3 shows visible light absorption with a band gap of.53 ev, corresponding to the wavelength of 490 nm. By dispersion of In O 3 on TiO surface, the band edge of TiO is expanded into the visible region and the band gaps of In O 3 /TiO catalysts are in between the band gaps of TiO and In O 3. The red shift in the absorption is more in In O 3 /nano TiO photocatalysts as compared to that in the In O 3 /TiO (P-5) catalysts at lower loadings while it is almost the same at higher dispersions. The band gap energy values of all the catalysts are given in Table. XPS analysis was carried out to understand the chemical environment of TiO after the dispersion of In O 3 (Fig. 4). Characteristic peaks of Ti p 3/ and p / in TiO appear at binding energy values of 458. ev and 464. ev respectively 7. The Ti p 3/ and p / binding energy values in In O 3 dispersed catalysts are slightly shifted to higher binding energy regions and appear at ev and ev. Pure In O 3 shows the characteristic binding energies of In 3d 5/ and 3d 3/ at ev and 45.4 ev respectively 7. The Table Band gap energy values of In O 3 /TiO photocatalysts No In O 3 (wt%) Band gap energy (ev) Nano TiO TiO (P-5) Fig. 4 Effect of In O 3 dispersion on H production activity from water splitting. [, In O 3 /nano TiO ;, In O 3 /TiO (P-5)]. corresponding binding energy values in In O 3 /nano TiO are observed at ev and ev. O s spectra of TiO show the characteristic peaks of Ti-O linkages at binding energy value of 59.9 ev and TiO surface hydroxyl groups at 53.9 ev 7. The characteristic O s binding energies in In O 3 /nano TiO catalyst are slightly lowered and were observed at 59.8 ev and ev. In In O 3 dispersed catalysts the Ti p 3/ and p / binding energy values are slightly shifted to higher binding energy regions and appeared at ev and ev. The binding energies of In 3d 5/ and 3d 3/ in In O 3 /TiO (P-5) are observed at ev and ev respectively. The characteristic O s binding energies in In O 3 /TiO (P-5) catalyst are slightly lowered and observed at ev and ev. In both the In O 3 /TiO series, the binding energy values of Ti p, In 3d, O s are shifted from the characteristic values which is because of the interaction of In O 3 with TiO. Furthermore, the binding energy of Ti is slightly increased from the characteristic values because the electronegativity of indium is more than that of Ti, favoring the charge transfer from Ti to In through Ti-O-In linkages 8. Photocatalytic activity Photocatalytic hydrogen evolution studies were conducted over TiO and In O 3 /TiO catalysts from pure water under solar irradiation. Figure 4 shows the effect of indium oxide dispersion on hydrogen production activity from pure water over In O 3 /nanotio and In O 3 /TiO (P-5) catalysts. Hydrogen production is not observed on nano TiO or P-5 TiO catalysts, whereas high activity is observed on both the catalysts with dispersed In O 3. In both the series, wt% In O 3 loading is optimum for maximum hydrogen production activity. Also, In O 3 /nano TiO shows improved hydrogen production activity as compared to In O 3 /TiO (P-5). This may be due to the high surface area of the nano TiO as well as the electrode positions in the solution, which facilitate the fast charge carrier transfer to the surface which in turn results in electron hole separation 4. TiO and In O 3 /TiO combinate catalysts were also evaluated for hydrogen production from aqueous methanol solution under solar irradiation. Figure 5 shows the hydrogen production as a function of irradiation time over In O 3 /TiO (P-5) combinate catalysts with various In O 3 dispersions. Hydrogen

5 476 INDIAN J CHEM, SEC A, APRIL-MAY 04 Fig. 5 Effect of In O 3 (wt%) on H production activity over In O 3 /TiO (P-5) catalysts in 5% methanol aqueous solution. [In O 3 (wt%):, 0.5;,.0; 3,.0; 4, 5.0]. Fig. 6 Effect of In O 3 (wt%) on H production activity. over In O 3 /nanotio catalysts in 5% methanol aqueous solution. [In O 3 (wt%):, 0.0;, 0.5; 3,.0; 4,.0; 5, 4.0; 6, 5.0; 7, 7.0; 8, 0]. production increased with increasing In O 3 loading up to wt% (75 µmoles/h) and above this loading the activity decreases. Figure 6 shows the hydrogen production over In O 3 /nano TiO catalysts with various In O 3 dispersions. Over these catalysts, the optimum amount of In O 3 dispersion is found to be 5 wt% (4080 µmoles/h) and above this value, the activity is seen to decrease. The difference in the optimum amount of In O 3 in both the series is due to the difference in the surface area of the catalysts. As nano TiO has a higher surface area as compared to the P-5 TiO, it can accommodate more In O 3 with fine dispersion. Furthermore, twice the activity is seen in In O 3 /nanotio catalysts as compared to In O 3 /TiO (P-5) which may be seen because the excess In O 3 is oxidizing the methanol molecules. As 5 wt% In O 3 /nanotio shows maximum activity, effect of methanol concentration was also studied on this catalyst. Hydrogen production increased with increasing methanol concentration and reached optimum at 5% methanol concentration. Above this concentration, the hydrogen production activity is seen to decrease. This may be seen as due to the saturation of methanol on the catalyst surface 9. When In O 3 /TiO systems were studied for water splitting, both In O 3 /nano TiO and In O 3 /TiO (P-5) show wt% In O 3 dispersion as optimum for maximum H production. Furthermore, the activity increase on In O 3 /nano TiO is nominal. The additional hydrogen production activity on In O 3 /nano TiO may be due to the high surface area and oxygen defects in In O 3 /nano TiO which, traps the electrons and minimizes the e - /h + recombination. In methanol:water mixtures, a fold increase in the activity is observed and the optimum amount of In O 3 dispersion is also different. This increase in the catalytic efficiency may be because the excess In O 3 oxidizes the methanol in addition to the high surface area of the catalyst. Conclusions In O 3 TiO combinate photocatalysts show absorption in the visible region and the observed band gaps are in between the values of In O 3 and TiO. In O 3 / nano TiO catalysts have higher surface area as compared to In O 3 / TiO (P-5) catalysts. XPS analysis shows that the interaction between In O 3 and TiO is low. In O 3 /nano TiO catalysts are highly active for photocatalytic hydrogen production from water as well as methanol:water mixtures under solar irradiation as compared to In O 3 /TiO (P-5) catalysts. By correlating the characterization and activity results we can conclude that nano-sized TiO based photocatalytic hydrogen production technology has a high potential for low-cost, environmentally friendly solar-hydrogen production to support the future hydrogen economy. Acknowledgement KL thanks Department of Science & Technology, New Delhi, India, for funding project under Fast Track Young Scientist Scheme (DST No: SR/FT/CS-75/0). The authors thank Ministry of New and Renewable Energy, New Delhi, India, for funding the project. References Fujishima A & Honda K, Nature, 38 (97) 37. Ni M, Leung M K H, Leung D Y C & Sumathy K, Energy Rev, (007) 40.

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