and Nickel-Doped Titanium Dioxide Nanoparticles - IJENS

International Journal of Engineering & Technology IJET-IJENS Vol:12 No:06

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Photocatalytic Reforming of Glycerol-Water Over Nitrogen- and Nickel-Doped Titanium Dioxide Nanoparticles Slamet*, Eny Kusrini**, Agus S. Afrozi, Setiadi Department of Chemical Engineering, Faculty of Engineering, Universitas Indonesia, Kampus UI Baru, Dep ok 16424, Indonesia Abstract -- Nitrogen-doped titanium dioxide (N-TiO 2) and nickel-doped nitrogen-titanium dioxide (Ni-N-TiO 2) nanocomposites resulting from various Ni -doping levels were successfully synthesized and characterized. The glycerol has been effectively as sacrificial electron donor for photocatalytic hydrogen production using heteregeneous catalysts of N-TiO 2 and Ni-N-TiO 2. The effects of Ni contents in various concentrations from 0 to 10 wt% as well as the effect of N-doped titanium dioxide in the photocatalysts of Ni-N-TiO 2 nanoparticles were studied. Efficient photocatalytic reforming of glycerol over Ni-doped N-TiO 2 nanocomposite with Ni content of 5 wt% under visible light irradiation was observed. The greatest amount of hydrogen produced (109 µmole) resulted from Ni doping of 5 wt% in a photocatalytic reaction for 4 h. S imultaneous N- and Ni- doping contributed to an eightfold increase in hydrogen production compared to the fourfold increase resulting from N doping. The synthesized Ni-doped NTiO 2 5wt% exhibited much higher hydrogen production activity than the commercially available TiO 2 Degussa P25.

Index Term-- Reforming; Glycerol; Heterogeneous catalyst; Photocatalysis

1. INT RODUCT ION TiO2 is the most promising catalyst for photocatalytic applications [1-3]. Due to its high stability against photocorrosion, chemical stability, environmental friendliness, abundance, cost effectiveness as a photocatalyst, capability to absorb UV light with low quantum efficiency due to its wide energy gap (3.2 eV), and its electronic energy band structure [1,4]. In addition, TiO2 has a highly specific surface area, which affords extremely low photoactivity due to large numbers of bulk and surface defects [2]. However, the efficiency of TiO 2 nanoparticles for photocatalytic hydrogen production is still limited because of large energy bandgap (3.2 eV for anatase), which permits absorption of only a very small part of visible light. Other factors limiting efficiency for photocatalytic hydrogen production are recombination of electron-hole pairs at the surface instead of in bulk, and poor crystallinity of TiO 2 nanoparticles [1,2]. Due to the large energy bandgap of TiO2, input energy greater than 3.2 eV is required to excite an

electron from the valence band to the conduction band. Many efforts to modify the photocatalytic properties of TiO 2 have been initiated to improve stability, reduce the energy bandgap, and enhance the photocatalytic rate. Examples of these efforts include loading the noble or non-noble surface metals, adding a photosensitizer, changing the preparation method, and incorporating nanoparticles into the interlayer of the photocatalyst [1,2,4 -7]. Absorption of visible light in TiO2 nanoparticles for photocatalytic applications can be improved by nitrogen doping [6]. Nitrogen doping (N doping) is a wellknown technique for promoting the absorption of visible light in titanium dioxide (TiO2) nanoparticles for photocatalytic applications. N doping on TiO2 is effectively used to reduce the large energy bandgap [8]. On the other hand, nickel has been found to be an efficient non-noble metal for improving the photocatalytic activity of certain semiconductor photocatalysts used in producing hydrogen from water [9,10]. Jing et al. (2005) reported that a Ni-doped mesoporous TiO2 catalyst plays an important role in improving thermal stability and controlling morphology of the mesoporous photocatalyst [2]; therefore, Ni doping enhances photocatalytic activity. In fact, direct splitting of water to produce hydrogen gas is not efficient due to rapid reverse reactions [1]. These can be overcome by the addition of sacrificial reagents to water, such as alcohol or organic compounds [1,11], which are oxidized to products less reactive to hydrogen gas so that hydrogen production increases. The sacrifical reagents also can contribute to enhancing the catalytic effect of metal particles on the H2 product. In order to improve the photocatalytic activity of TiO 2 under visible light irradiation for renewable hydrogen production from glycerol and water, we doped them with nitrogen and nickel. The effect of simultaneous N- and Ni doping on the photocatalytic performance of H2 production was examined. We also designed the photoreactor for running the photocatalytic production of hydrogen from water by using UV-Vis lamp irradiation in the presence of glycerol as the sacrificial reagent. We used glycerol to increase product value and as an alternative to disposal by incineration [7]. It is noteworthy that glycerol is now available in large amounts as a co-product of oils and fats and biodiesel industries [7,12,13].

128706-6262- IJET-IJENS @ December 2012 IJENS

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International Journal of Engineering & Technology IJET-IJENS Vol:12 No:06 48 2. M AT ERIALS AND M ET HODS dispersive X-ray spectroscopy (EDX). The amounts of 2.1. Materials hydrogen produced were measured by an online Shimadzu 8A All chemicals and solvents were of analytical grade and used gas chromatograph equipped with a thermal control detector without further purification. TiO2 Degussa P-25 was purchased (GC-TCD). from Nippon Aerosil Co., Ltd (Japan). TiO2 Degussa P-25 is a standard material in connection with photocatalytic reactions; 2.5. Photocatalytic activity set up and procedure it contains anatase and rutile phases in a ratio of about 4:1. Photocatalytic hydrogen production was examined using a Precursors of Ni(NO3)2.3H2O were purchased from Sigmabatch photoreactor with an immersed lamp (Fig. 1). The Aldrich (USA). photoreactor was designed from pyrex that was equipped with a magnetic stirrer and hotplate and stored in a container 2.2. Preparation of nitrogen-doped titanium dioxide equipped with a lamp fitting, or buffer, where the lamp 100 watt To 5 g of TiO2 Degussa P-25 was immersed into a 500 mL was used as a source of photons, either UV or visible light. beaker containing 200 mL of 0.5 M NH3 solution. The solution Before initiating the photocatalytic hydrogen evolution was magnetically stirred for 30 min. The beaker was then experiment, the container was purged with argon gas at a opened and the solution was mixed ultrasonically for 10 pressure of 35 torr to remove oxygen gas from the additional min. The mixture was further aged and kept at room photoreactor. The container was placed on a magnetic stirrer temperature for 24 h. Then, the solution was separated by so that the suspension in the photoreactor could be stirred to centrifuge at 9000 rpm for 15 min. The resultant product was promote the reaction. After that, switching on the lamp allowed dried at 80-90C on a hotplate, followed by further drying at visible light to be absorbed by the photocatalysts of N-TiO2 130oC for 1 h in a furnace. The N-TiO2 was finally calcined in and N-Ni-TiO2 nanocomposites prior to running the flow air at 500oC for 1 h. The resulting N-TiO2 was stored in photocatalytic reaction. For each experiment, 0.5 g of bottles for further characterization. photocatalysts was added to a 500 mL mixture of water and glycerol containing glycerol of 10% (v/v). Photocatalytic 2.3. Preparation of Ni- and N-doped TiO2 reaction time for the water-glycerol suspension was 5 h. The Various amounts of Ni(NO3)2.3H2O were added to the N-TiO2 amounts of hydrogen produced were measured with an online solution to achieve the Ni doping level of Ni2+ from 0 to 10% gas chromatography thermal control detector (GC-TCD). (wt). The beaker was closed, and the solution was magnetically Samples of hydrogen produced were taken at intervals of 1 h, stirred for 30 min at room temperature. After that, the beaker and their composition was analyzed by GC-TCD. was opened and ultrasonic mixing was conducted for 10 additional min. The mixture was further aged and retained at room temperature for 24 h. The solid powder that precipitated as a result of the treatment was separated by centrifugation for 15 min at 9000 rpm. The suspension was then heated on a hotplate at 80-90C to evaporate the remaining demineralized water, followed by further heating at 130C for 1 h in a furnace. The Ni-N-TiO2 nanocomposites were finally calcined in flow air at 500oC for 1 h, and various specimens resulting from Ni doping of nanomaterial composites of Ni-N-TiO2 were stored in bottles for further characterization. 2.4. Characterization of photocatalysts Crystallinity of the prepared photocatalysts was investigated using APD PHILIPS 1710 X-ray diffraction (operating at voltages of 40 kV and a current of 20 mA) using a Cu anode tube (λ = 0.154184) and a 2θ scan rate of 0.02°/sec. The energy bandgap of the samples was measured using a Shimadzu UV2450 spectrophotometer and UV-Vis diffuse reflectance spectroscopy. Reflectance spectra were analyzed at ambient conditions in the wavelength ranges of 200-700 nm. The crystallite morphologic micrograph and composition of the nanocomposite results were determined by a JEOL JSM-6510 LA scanning electron microscope equipped with energy

Fig. 1. Sketch of the setup batch photoreactor for photocatalytic hydrogen production


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International Journal of Engineering & Technology IJET-IJENS Vol:12 No:06 49 3. RESULT S AND DISCUSSION subsequent release of Ni doping into the photocatalyst 3.1. Morphology and component analysis of N-doped TiO2 and because of added N doping. Ni-doped N-TiO2 EDX investigation of the N-doped TiO2 (N-TiO2) showed concentrations of N, Ti, and O of 0.79, 61.7, and 37.5%, respectively. Fig. 2a shows the SEM image for the N-TiO2 nanocomposite revealed uniform size, granular texture, and more crystallinity. During impregnation of TiO2 nanoparticles in the ammonia solution, the NH4+ ions were adsorbed into the photocatalytic surface. Due to the calcining of N-TiO2 at 500C, the bonding of NH4+ ions was broken, followed by the N atom replacing the O atom in the lattice of the TiO2 nanoparticles. Thus, the substitution of the O atom by the N atom was incorporated in the TiO2 nanoparticles, allowing covalent bonding of N-Ti to occur [11]. To further investigate the effects of Ni doping on the morphology of Ni-N-TiO2 nanocomposites, the SEM images of various levels of Ni content in photocatalysts were recorded. (a) The SEM image for Ni-N-TiO2 with Ni content of 5% was less smooth than Ni-N-TiO2 with Ni content of 10 wt% (Figs. 2b and 2c). From the surface morphology, the porous structure of NiN-TiO2 nanocomposite containing Ni of 5 wt% appeared to be interconnected and ordered material. The hole or pore structure and interconnectedness is shown in Fig. 2b. Fig. 2c reveals the presence of plain morphology, and no holes are evident. By Ni doping the photocatalysts of Ni-N-TiO2, particle growth was restrained. A significant lattice deformation occurred with increasing Ni content; this development was expected due to an ionic radius of the Ni2+ ion that is similar to the radius of the Ti4+ ion, and which can form an octahedral coordination as the Ti4+ ion does [2]. From the EDX data, we assumed that the Ni doping in nanocomposites resulted in replacement of some of the Ti4+ sites with the Ni2+ ion. The EDX data of the Ni-N-TiO2 nanocomposite with Ni doping of 5 wt% shows the N and Ni contents were 7.30 and 0.94%, (b) (c) respectively (Table I). By contrast, the Ni-N-TiO2 Fig. 2. SEM images for (a) N-TiO2 with magnification of 50,000x, nanocomposites resulting from Ni doping of 10 wt% exhibited and Ni-N-TiO2 containing different Ni contents for Ni-doping N and Ni contents of 4.28 and 2.43%, respectively. The of (b) 5 wt% and (c) 10wt% with magnification of 10,000x success of loading Ni doping in the Ni-N-TiO2 was less than 30% for Ni doping at both 5 and 10 wt%. Thes e results may be due to the Ni doping of TiO2 by impregnation and the T ABLE I EDX DATA FOR THE P HOTOCATALYSTS OF N-T IO2 AND NI-N-T IO2 NANOCOMP OSITES

Element

N-TiO2 (%)

Ni-N-TiO2 with Ni content of 5 wt% Ni-N-TiO2 with Ni content of 10 wt% (%) (%) N 0.79 7.30 4.28 Ni 0.94 2.43 Ti 61.69 41.85 54.90 O 37.52 35.50 29.32 3.2. XRD studies concentrations of Ni doping. All samples show anatase and Fig. 3 presents XRD patterns for titania (TiO2 nanoparticles) rutile phases, regardless of Ni doping. From the XRD analysis, and nickel-doped nitrogen-titania (Ni-N-TiO2) at various it is shown that 2 angles at 25.3, 38.0, 48.3, 54.8, 55.8, 62.58, 128706-6262- IJET-IJENS @ December 2012 IJENS

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International Journal of Engineering & Technology IJET-IJENS Vol:12 No:06 50 67.21, 68.79, 75.18 corresponded to the (101), (004), (200), The mean size of the anatase crystallites in the samples was (105), (211), (204), (116), (220) and (215) reflections from the estimated using the Debye-Scherrer equation [2]. Crystal sizes, anatase phase [7,13,14]. The peaks corresponding to the rutile and anatase phases for TiO2 and Ni-N-TiO2 are crystalline phase of NiO were also observed for Ni doping of summarized in Table II. From the table, it is noted that all 10 wt% (Fig. 3c). According to the literature, the XRD peak crystals were nanosized. The crystal s ize of the TiO2 positions of cubic (fcc) and rhombohedral (hexagonal) nickel nanoparticles (17.61 nm) was smaller than that found in the Nioxide were found at the (111), (200), (220), and (101), (012), N-TiO2 nanocomposites with Ni doping of 5 wt% (20.32 nm) (110), (104) [15] reflections, respectively. In this study, the and Ni doping of 10 wt% (19.33 nm). The crystal size in the small peak resulted from the (200) reflection at 37.5º that was catalyst of Ni-N-TiO2 5 wt% (20.32 nm) is almost similar to that found only for Ni doping of 10 wt%. The presence of a NiO the crystal size of P-25 TiO2 (20.7 nm) [13]. Increasing of peak with a cubic crystalline system was also observed. crystallite size in the presence of Ni2+ was responsible for the high surface area of Ni-doped TiO2. No significant changes in compositions of rutile and anatase fractions were observed (Table II). For TiO2 Degussa P-25, the fraction of anatase crystallites phase was 81%, while the fractions of anatase crystallites for Ni-N-TiO2 with Ni doping of 5 and 10 wt% were 83 and 82%, respectively. We observed that nanoparticles can increase charge-transfer rates in photocatalytic reactions and reduce recombination volume [16,17], thereby enhancing photocatalytic hydrogen production.

Fig. 3. XRD patterns of the P -25 T iO2 (a) and Ni-N-T iO2 nanocomposites with Ni-doping of 5 wt% (b) and 10 wt% (c), where * = anatase, **= rutile and *** = NiO T ABLE II CRYSTAL SIZES, P HASES OF RUTILE AND ANATASE FOR T IO2 NANOP ARTICLES AND NI-N-T IO2 NANOCOMP OSITES CONTAINING DIFFERENT NI CONTENT LEVELS

Photocatalyst Crystal size (nm) Rutile Anatase TiO2 Degussa P-25 17.61 0.19 0.81 Ni-N-TiO2 with Ni content of 5 wt% 20.32 0.17 0.83 Ni-N-TiO2 with Ni content of 10 wt% 19.33 0.18 0.82 3.3. UV-Vis DRS studies nanoparticles, the energy bandgap of the TiO2 nanoparticles The absorption edge of these nanomaterial composites decreased to 2.91 eV. The N atom causes a significant change achieved the visible light region in the ranges of 425–500 nm in the absorption spectrum of TiO2, as reported in previous due to the non-noble metals and non-metals doped into the studies [8,19]. The orbital 2p acceptor states of the N atom give TiO2 nanoparticles. UV-Vis diffuse reflectance absorption rise to a narrowing of the energy bandgap due to mixing with spectra of N-TiO2 and Ni-N-TiO2 contained different the orbital 2p states of the O atom [8]. On the other hand, the compositions of Ni content, as shown in Fig. 4. Upon orbital 2p states of N atom are localized above the top of the 2p increasing the Ni content, the absorption intensities of the valence band of the O atom, known as the interband level [19]. nanocomposites gradually decreased. We also observed that Therefore, the energy bandgap of the resulting introducing N and Ni to the TiO2 nanoparticles could reduce nanocomposites is narrowed because the distance of valence the energy bandgap of TiO2 Degussa P-25. The nanocomposite and conduction band levels also decreases [11]. In this study, results could enhance the visible light absorption ability in the the energy bandgap for the nanocomposite results was closest visible light region at 425 and 500 nm. Absorption at to 2.47 eV within the Ni-N-TiO2 series containing different wavelength greater than 425 nm would affect the formation or levels of Ni content, ranging from 3 to 10 wt% (Fig. 4). A recombination of electron-hole pairs and separation in a beneficial characteristic of photocatalytic applications is the photocatalytic application under light radiation [4, 18]. photocatalyst’s responsiveness to visible light. Following the introduction of the N atom in the TiO 2 128706-6262- IJET-IJENS @ December 2012 IJENS

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Fig. 4. UV-Vis DRS spectra of T iO2 (a), Ni-N-T iO2 nanocomposites with Ni doping of 3 wt% (b), 5 wt% (c), and 10 wt% (d)

3.4. Photocatalytic activity studies Photocatalytic hydrogen production has been investigated over visible light irradiated TiO2, N-TiO2 and Ni-N-TiO2 catalysts both in the absence and in the presence of glycerol in solution. Photocatalytic hydrogen production resulting from the use of various photocatalysts was examined in a batch photoreactor with an immersed lamp that was designed in our laboratory. Each photocatalyst of 0.5 g of N-doped TiO2 and Ni-doped N-TiO2 at 5 and 10 wt% was evaluated for photocatalytic activity. Fig. 5 shows that N doping in a TiO2 nanoparticles played a significant role in enhancing the absorption of visible light irradiation; thus, the amount of hydrogen produced with N doping increased from 10 to 42 mole after 4 h due to direct electron transfer to the conduction band of the TiO2 nanoparticles [1,20]. Upon excitation of N-TiO2 with visible light, the sacrificial reagent for the electron donor acted as a sinkhole to prevent electron-hole recombination [6]. The sacrificial electron donor, glycerol, was employed to search holes formed on the surface or in bulk. The amount of hydrogen produced from the waterglycerol mixture (10% v/v) increased up to four times relative to unmodified TiO2 Degussa P-25. The energy bandgap of the TiO2 nanoparticles was 3.2 eV, with the ability to actively absorp UV-A light with a shorter wavelength. By introducing the N atom to the TiO2 nanoparticles, the photocatalytic activity of TiO2 in the visible light region increased.

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Fig. 5. Photocatalytic hydrogen production for 5 h by using photocatalysts of (a) T iO2 Degussa P-25 and (b) N-T iO2 in mixture of 500 mL water-glycerol (10% v/v) under visible light irradiation with 0.5 g of photocatalysts

The results in Fig. 6 illustrate that hydrogen production was dependent on irradiation time and level of Ni doping. We observed, however, that with increasing Ni doping, the amount of hydrogen produced only changed slightly (Fig. 6). Among the series of Ni-N-TiO2 nanocomposites containing different concentrations of Ni, the greatest amount of hydrogen was produced with Ni doping of 5 wt% (Fig. 7). The maximum amount of hydrogen produced in photocatalytic reaction for 4 h was 109 µmole. As identified by SEM images, ordered photocatalyst materials for Ni-N-TiO2 with Ni contents of 5 and 10 wt% were more beneficial than disorderly material associated with N-TiO2 for promoting diffusion of reactants and products. This facilitated reaching reactive sites of the NiN-TiO2 nanocomposite. By contrast, the disorderly Ni-doped mesoporous TiO2 was favorable for the oxidation of methanol as the sacrificial reagent for a multi-electron process [2].


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Fig. 6. Photocatalytic hydrogen production for 5 h by using photocatalysts of Ni-N-T iO2 containing different Ni-doping levels of (a) 1 wt%, (b) 3 wt%, (c) 5 wt%, and (d) 10 wt% in mixture of 500 mL water-glycerol (10% v/v) under visible light irradiation with 0.5 g o f photocatalysts

Ni doping in the Ni-N-TiO2 nanocomposites resulted in the substitution at some of the Ti4+ sites in the lattice of the TiO2 nanoparticles by the Ni2+ ion, thus shrinking the energy bandgap. UV-Vis DRS analysis showed that the energy bandgap of 2.47 eV is suitable for producing hydrogen. In this study, Ni doping of TiO2 nanoparticles demonstrated that Ni doping is beneficial for inhibiting the quick recombination electron-hole in the surface. According to Zhang et al., the optimum dose for metal doping is obtained when metal doping is incorporated into N-TiO2 at its optimum doping level, and the ionic doping could be central for electron-hole separation and the subsequent increase in photocatalytic activity [21]. By contrast, if metal doping exceeds its optimum dose, the ionic doping would function as the center of recombination, therefore proving disadvantageous for photocatalytic performance. In this study, the optimum dose for Ni doping was only 5 wt% for the photocatalyst of the Ni-N-TiO2 nanocomposite. Simultaneous N- and Ni doping contributed to an eightfold improvement in production of hydrogen via photocatalytic activity compared to the fourfold improvement contributed by N doping. The mechanism of oxidized glycerol to produce hydrogen gas has been previously described by Li et al. [22]. The recombination of hole and electron decreases, and the electron can react with water to finally produce hydrogen gas. Increasing the amount of occupied holes by oxidization of glycerol as a sacrificial reagent enhances the hydrogen produced.

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Fig. 7. T he effect of Ni content in the Ni-N-T iO2 nanomaterials composites for photocatalytic hydrogen production for 5 h in waterglycerol (10% v/v) under visible light irradiation with 0.5 g of each photocatalyst

4. CONCLUSION Results from the present study show that using photocatalysts of N-TiO2 and Ni-N-TiO2 for photocatalytic hydrogen production is environmentally friendly and cost-effective. The glycerol has been effectively as sacrificial electron donor for photocatalytic hydrogen generation. The effects of N- and Ni doping of TiO2 nanoparticles enhance the photocatalytic performance of N-TiO2 and Ni-N-TiO2 nanocomposites were observed. The glycerol promoted photocatalytic hydrogen production and were simultaneously degraded in water on visible light illuminated Ni-N-TiO2 5 wt%. In summary, our catalysts with N and Ni doping could produce the hydrogen by photocatalytic reforming with low cost process, which only used a visible light source. In further studies, more efforts must be put into composition the N and Ni doping of the catalysts in order to have a better photocatalytic performance of TIO 2. In addition, we also need to evaluate in increasing the glycerol volume in photocatalytic reforming of hydrogen production. 5. A CKNOWLEDGEMENT S The authors gratefully acknowledge the support given to this work by Ministry of National Education, Directorate General of Higher Education (DIKTI) for their financial support through research grant Hibah Kompetensi 2012. REFERENCES [1] AA Nada, HA Hamed, MH Barakat, NR Mohamed, T N Veziroglu. Intern J Hydrogen Energy 33 (2008) 3264 –3269. [2] D Jing, Y Zhang, L Guo. Chem Phys Lett 415 (2005) 74–78.


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