Photocatalytic Hydrogen Generation from Glycerol and Water using Pt loaded N-doped TiO 2 Nanotube

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1 International Journal of Engineering & Technology IJET-IJENS Vol: 11 No: 3 91 Photocatalytic Hydrogen Generation from Glycerol and Water using Pt loaded N-doped TiO 2 Nanotube Slamet, Anny, Setiadi Department of Chemical Engineering, Faculty of Engineering, University of Indonesia Kampus UI Depok, Depok 16424, Indonesia slamet@che.ui.ac.id Abstract - Photocatalytic hydrogen generation from glycerol and water was investigated using the Pt loaded N-doped TiO 2 Nanotube (Pt-N-). TiO 2 nanotube () was prepared via hydrothermal and calcination post-treatments. The doped by nitrogen and platinum by impregnation and photoassisted deposition (PAD) methods, respectively. The effects of glycerol concentration, morphology of TiO 2 as well as type of dopants were inspected. The results showed that modified TiO 2 photocatalyst () could produce more hydrogen compare to Degussa P-25 (TiO 2 nanoparticle). UV-vis Diffuse Reflectance Spectroscopy (DRS) analysis also shows that N- TiO 2 nanotube was more responsive in visible light, thus drastically more active in producing hydrogen by visible light. In other hand, Pt-N-TiO 2 has a superior activity in producing hydrogen if using a UV light. Comparing with other results, these achievements are quite superior, especially for the N- doped in visible light photo-producing hydrogen from glycerol and water. nanotube to increase its specific surface area [8]. Addition of metal dopant like platinum to the photocatalyst can also increases the hydrogen production by increasing rate of electron transfer to photocatalyst surface [2,9]. However, the modification of TiO 2 with metal and non-metal dopants simultaneously such as Pt and N is still rarely studied, especially those applied to the production of hydrogen from glycerol. Accordingly, this study is intended to modify TiO 2 photocatalyst for hydrogen production from glycerol and water. The modification of the photocatalyst is studied by reforming the TiO 2 nanoparticle to the form of nanotube as well as doping it with nitrogen and platinum through impregnation and PAD methods, respectively. The effect of glycerol concentration on hydrogen production is also observed. Keywords: photocatalysis, TiO 2, platinum, nitrogen, hydrogen, water splitting, glycerol. 1. INTRODUCTION The fossil hydrocarbons as the main energy source today, are not environmentally friendly and non renewable, so that alternative energy sources must be sought. An alternative energy, hydrogen is a quite ideal for environmentally friendly and renewability resource. However, so far about 95% of hydrogen consumed in the world is still produced from fossil fuels mainly by catalytic thermal and gasification processes at high temperature [1]. Photocatalytic hydrogen generation from glycerol and water using TiO 2 semiconductor as photocatalyst has been regarded as one of the most promising inexpensive method. Glycerol is one of the biomass derivatives and a side product in biodiesel production. As the production of biodiesel is increasing, the amount of glycerol production is also increasing to the point of wasteful because of the lack of supporting capacity in glycerol-utilize industries [2-3]. To fully utilize its potential, glycerol is can be used as sacrificial agent in photocatalytic water splitting which reduces recombination of hole and electron, and also as a reactant which can light-induced oxidized to produce hydrogen at room temperature and atmospheric pressure [4]. Although the oxide of TiO 2 is famous for its stability [5], it doesn t response well for photocatalytic activity in visible light due to its wide energy bandgap. Hence, a modification by doping nitrogen to TiO 2 photocatayst is needed in order to increase its response in visible light [6-7]. Furthermore, the morphology of the photocatalyst is also needed to be reformed such as 2. EXPERIMENTAL 2.1. Catalyst Preparation The TiO 2 Degussa P-25 photocatalyst (TP25) was reformed into nanotube by disolving of 3 g TiO 2 powder into 15 ml of 1M NaOH, and ultrasonicated for 15 min prior to hydrothermal treatment at 13 C for 12 hours using an autoclave. The drying then was carried out at 15ºC for 2 hours. The preparation of nitrogen-doped TiO 2 nanotube was carried out by submerging of 2.5 g TiO 2 nanotube powder in a 2 ml of.5 M NH 3 aqueous solution during 24 hours, followed by drying (2ºC, 2 hr) and calcination (5ºC, 1 hr). The resulted solid subsequently was grounded to fine powders. The Platinum doping photocatalyst was prepared by PAD method for 6 hours under UV-irradiation. 2 g of the nitrogen-doped TiO 2 nanotube was submerged into 9 ml demineralized water containing.5 g H 2 PtCl 6.6H 2 O salt. Further step, 1 ml methanol was added to the solution to decrease the recombination rate during photodeposition. After 6 hours irradiation, the mixture was centrifuged and the solid obtained was dried for 3 hours at 15ºC and grounded into fine powders Photocatalyst Activity Test Photocatalytic activity test has been carried using a Pyrex reactor with 2.5 L capacity that equipped with 6 pieces of 12 W UV-A lamps or 6 pieces of 12 W Phillips visible light lamps as a photon sources. The reactor was equipped with bubbler for oxygen purging and the injection port for introducing of 5 ml of aqueous glycerol solution and.5 g catalyst. Prior to irradiation, argon (Ar) was bubbled through the mixture for 2 minutes to remove

2 International Journal of Engineering & Technology IJET-IJENS Vol: 11 No: 3 92 oxygen. The gas product hydrogen was analyzed by a Shimadzu TCD gas chromatograph (Ar as gas carrier). Glycerol adsorbed on the catalyst can react with the formed hydroxyl radical, as follow: 2.3. Catalyst Characterization The surface morphologies of the catalysts were carried out using a scanning electron microscope (SEM) JEOL JSM-639A equipped with energy dispersive X-ray spectroscopy (EDS). A UV-Vis 245 (UV Probe Shimadzu) was used to record the diffuse reflectance spectra (DRS) of the samples. Reflectance spectra were analyzed under ambient condition in the wavelength range of 3-6 nm. The specific surface area of catalysts was measured by BET of N 2 adsorption in a Quantachrome Autosorb RESULTS AND DISCUSSION 3.1. Effect of glycerol concentration Fig. 1 describes the effect of glycerol concentration on hydrogen production. In the absence of glycerol, no hydrogen was produced during 5 hours of irradiation, whereas in the presence of 2%v glycerol, the amount of the hydrogen produced is the best among all variations, which was 317 µmole. The absence of hydrogen in % glycerol concentration is maybe resulted from recombination reaction of holes and electrons. In the absence of glycerol, there is no sacrificial agent presents, thus the holes were free to react with electrons and the water couldn t be reduced into hydrogen [1]. e Fig Glycerol %v Glycerol 1 %v Glycerol 15 %v Glycerol 2 %v Effect of glycerol concentration (V solution = 5 ml, catalyst =.5 g TiO 2 P25, light source: UV-A) As the increasing of the glycerol amount, the number of occupied holes by oxidizing the glycerol as sacrificial agent is also increasing. Thus the recombination of hole and electron is reduced and the electron could react with water to produce hydrogen. Furthermore, the hydrogen was produced from oxidized glycerol which reacted with the mechanism proposed by Li et al [11] that can be seen below. (1) (2) (3) The formed CH 2 OHCHOHCHOH can react with water to form CH 2 OHCHOHCH(OH) 2 and H which can transform into H 2 further. The formed CH 2 OHCHOHCH(OH) 2 is unstable, which transforms into aldehyde further: OH can react continuously with the formed aldehyde: (4) (5) (6) (7) (8) The formed CH 2 OHCHOHCOOH can react directly with photoinduced hole (h + ) so that decarboxylation takes place. The formed CH 2 OHCHOH repeats the reactions (5), (6), (7), (8) and (9) to form water and carbon dioxide at last. CH 2 OHCHOH CH 2 OHCH(OH) 2 (CH 2 OHCHO) CH 2 OHCO CH 2 OHCOOH CH 2 OH HCH(OH) 2 (HCHO) HCO HCOOH CO 2 + H 2 O 3.2. Effect of TiO 2 morphology The effect of TiO 2 morphology was investigated by comparing the photocatalytic activity in producing hydrogen between TiO 2 nanotube () and TiO 2 nanoparticle (TP25). Fig. 2 shows that in the form of nanotube, TiO 2 is more active in producing hydrogen compare to nanoparticle form, whereas the hydrogen production of and TP25 are 356 µmole and 316 µmole, respectively TP25 (9) Fig. 2 Effect of photocatalyst morphology (V solution = 5 ml, catalyst =.5 g, light source: UV-A, glycerol concentration: 2%v)

3 International Journal of Engineering & Technology IJET-IJENS Vol: 11 No: 3 93 Nanotube form of TiO 2 resulted in larger specific surface area compare to nanoparticle form, which can be observed from BET characterization results of both morphology, i.e 116 m 2 /g and 54 m 2 /g, respectively. Fig. 3 shows the SEM characterization result for which was calcinated in 5ºC atmospheric furnace for 1 hour. TiO 2 reformation by 15 minutes ultrasonication and 12 hours hydrothermal method resulted in nanotube with diameter range of 9 12 nm. The large specific area could accommodate more reaction and also, reducing the chance of recombination. Hence, the photocatalytic activity of TiO 2 nanotube is better. and replacing O to bond with titanate forming Ti-N bond [12]. Fig. 4 shows the comparison of photocatalytic activity in visible light between N-doped TiO 2 nanotube (N- ), TiO 2 nanotube () and Pt/N-doped TiO 2 nanotube (PT-N-). The figure shows that the N- gave the best response in visible light for hydrogen production, as it could produce 312 µmole of hydrogen in 4 hours irradiation compare to and Pt-N- which only produced 71 µmole and 27 µmole, respectively. To observe more detail of this phenomenon, DRS characterization was conducted to obtain the energy bandgap of the photocatalyst. a N- Pt-N b Fig. 3 SEM Image of TiO 2 nanotube via 12 hours hydrothermal treatment and 15 minutes ultrasonication: (a) x; (b) 5x 3.3. Effect of nitrogen doping N-doped TiO 2 nanotube was prepare by submerging TiO 2 nanotube powder in 2 ml.5 M NH 3 solution for 24 hour, followed by 2 hour drying in 2ºC and 1 hour calcinations in 5ºC with atmospheric furnace. During submerging, NH + 4 ions were adsorbed into the + catalyst surface. Because of calcinations, the bonds in NH 4 ions were broken, and the N atom goes to the lattice of TiO 2 Fig. 3 Effect of nitrogen doping (V solution = 5 ml, catalyst =.5 g, light source: visible, glycerol concentration: 2%v) Fig. 5 shows the energy bandgap of, N- and Pt-N-. By extrapolating the absorbance trend, it could be achieved the relation of wavelength and energy bandgap of each photocatalyst, as shown in Table 1. It is clearly shown that N-TiO 2 nanotube is the most responsive photocatalyst as it responded to light with 46 nm wavelength and its energy bandgap is 2,69 ev. This wavelength is still in the range of visible light wavelength. Hence, N-TiO 2 could be activated in the irradiation of visible light. By comparing the energy bandgap of TiO 2 degussa P-25 and TiO 2 nanotube, it could be concluded that the reforming of TiO 2 into nanotube could lower the energy bandgap of TiO 2. Liu et.al. (27) investigated that the surface area of photocatalyst affects the energy bandgap in a photocatayst. The total energy of the TiO 2 is inversely proportional to the molecular size, largely due to the steric strains introduced to the structure when the size is smaller [13]. Thus, as the TiO 2 Degussa P-25 is in the form of nanoparticle which is smaller in size, then the steric strain in TiO 2 Degussa P-25 is larger than TiO 2 nanotube. It means that the TiO 2 energy bandgap in the form of nanoparticle is larger than in the nanotube form. As the nitrogen introduced to the TiO 2, the energy bandgap of TiO 2 decrease in drastic measure until 2,69 ev. As the nitrogen atom went through the lattice of TiO 2, the mixture of 2p states of N and p states of O would move the

4 International Journal of Engineering & Technology IJET-IJENS Vol: 11 No: 3 94 valence band of TiO 2 as the conduction band of TiO 2 didn t move. Hence the energy bandgap of TiO 2 decrease because the distance of valence band and conduction band decreased [4]. Absorbance, % N- Pt-N Wavelength, nm Fig. 4 DRS Characterization of TiO 2 Nanotube, N-TiO 2 Nanotube and Pt-N-TiO 2 Nanotube The unresponsive behavior of nanotube Pt-N-TiO 2 photocatalyst in visible light irradiation could be explained by the concentration of Nitrogen in the photocatalyst, which were examined by EDS characterization (nitrogen concentration data based on EDS characterization). It was predicted that the doped-nitrogen had mostly gone from the photocatalyst during the doping of platinum procedures. As the photodeposition of platinum was prepared by dissolving and stirring of nanotube N-TiO 2 photocatalyst in H 2 PtCl 6 solution, the nitrogen escaped from the TiO 2 lattice and dissolved into the solution. Moreover, the washing of photocatalyst after photodeposition also contributed to the loss of nitrogen. Hence, this photocatalyst did not respond well in visible light. Therefore, further development concerning with TiO 2 -modification techniques with two doped N and Pt needs to be investigated. The study of the technique is still in progress to elucidate the Pt loaded N-doped TiO 2 catalyst. Table 1 Correspondent wavelength and energy bandgap for TiO 2 Degussa P-25, TiO 2 nanotube, N-TiO 2 nanotube and Pt-N-TiO 2 nanotube Photocatalyst TP25 N- Pt-N- Wavelength absorbance, nm Bandgap, ev 3,28 2,88 2,69 2, Effect of platinum doping Fig. 6 shows that the introduction of platinum into the lattice of TiO 2 could increase photocatalytic activity in drastic measure. For 5 hours of UV-A irradiation, the hydrogen production of Pt-N-TiO 2 reached the point of about 6 µmole. This production is much higher than the production of TiO 2 nanotube on the same reaction condition, which is only 356 µmole. Daskalaki et al (29) investigated that the introduction of platinum into TiO 2 structure resulted in the increasing of the rate of electrons transfer to the TiO 2 surface [2]. This behavior plays a significant role of increasing hydrogen production. As the photo excited electrons were accumulated in the surface of TiO 2, the fermi level shifted nearer to the conduction band, resulting on energy moved into more negative level. Moreover, the surface-accumulated electrons would be transferred into the protons which were adsorbed to the TiO 2 surface. Hence the protons were reduced into hydrogen Pt-N- vis Pt-N- UV UV Fig. 5 Effect of platinum doping (V solution = 5 ml, catalyst =.5 g, light source: visible/uv-a, glycerol concentration: 2%v) Fig. 6 also shows that the Pt-N-TiO 2 did not respond well in visible light radiation, as mentioned before in section 3.3. This response behaviors of nanotube Pt-N-TiO 2 in visible and UV-A light explain that the doping of platinum into TiO 2 structure does not lower TiO 2 bandgap. It means that in this research, only nitrogen played the main role to improve TiO 2 nanotube activity in visible light 4. CONCLUSIONS The presence of Glycerol in the photocatalytic water splitting reaction could significantly increase the hydrogen production. The nanotube form of TiO 2 plays a significant role in increasing the specific surface area of TiO 2, and hence could improve the hydrogen production at about 2%. Nitrogen doping could increase TiO 2 photocatalitic activity in visible light significantly. The introduction of platinum doping into TiO 2 nanotube structure could increase the photocatalitic hydrogen production was about twice compared to the un-doped TiO 2. ACKNOWLEDGEMENT The authors would like to thank Directorate of Research and Community Services (DRPM UI), University of Indonesia and Directorate General of Higher Education (DGHE), Indonesian ministry of National Education for the financial support on this research.

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