Sains Malaysiana 37(2)(2008): 201–203
Synthesis and Characterization of Tris(1-carboxyl-2-phenyl-1,2-ethyleno dithiolenic-S,S’) Tungsten Complex as Photocatalyst for Photolysis
of H
2O Molecules
(Sintesis dan Pencirian Kompleks Tris[1-karboksil-2-fenil-1,2-etilenoditiolenik-S,S’]
Tungsten sebagai Fotomangkin untuk Fotolisis Molekul H2O) FADHLI HADANA RAHMAN, KHUZAIMAH, WAN RAMLI WAN DAUD,
RUSLI DAIK & MOHAMMAD B. KASSIM
ABSTRACT
Tris(1-carboxyl-2-phenyl-1,2-ethylenodithiolenic-S,S’) tungsten complex is one of the most promising photocatalyst to be used in photolysis of water to produce hydrogen. The first step of the synthesis involves a metathesis reaction of tetrapropylammonium bromide [{(C3H7)4N}Br] and ammonium tetrathiotungstate [(NH4)2WS4] to form a tetrapropylammonium tetrathiotungstate [{(C3H7)4N}2WS4] (precursor). Then, the precursor was reacted with phenylacetylenecarboxylic acid (C9H6O2) to form tris(1-carboxyl-2-phenyl-1,2-ethylenodithiolenic-S,S’) tungsten complex (C27H18O2S6W). The infra-red, ultra violet/visible (UV/Vis) spectrum, nuclear magnetic resonance (NMR) and elemental micro-analysis of C, H, N and S agreed with the characteristic of the tris(1-carboxyl-2-phenyl-1,2-ethylenodithiolenic- S,S’) tungsten complex. The (W-S), (C-S) and (C=O) stretching frequencies were detected at 511, (1470 and 1035) and 1655 cm-1, respectively. The 1H NMR spectrum showed six protons in the complex. The 13C NMR showed only 7 signals for carbon atom in the benzene ring, ethylene groups and carboxylic acid pendant group due to the symmetry of the molecules.
The reaction yield was about 50 %. Photolysis of acetone spiked H2O showed that the catalyst was able to produced 1.8µmol/h hydrogen.
Keywords: Dye-sensitised solar cell; hydrogen; photolysis; sunlight; tungsten
ABSTRAK
Kompleks tris(1-karboksil-2-fenil-1,2-etilenoditiolenik-S,S’) tungsten merupakan salah satu daripada kompleks yang berpotensi tinggi untuk digunakan bagi proses fotolisis air untuk menghasilkan gas hidrogen. Langkah pertama sintesis melibatkan tindak balas metatesis di antara tetrapropilammonium bromida [{(C3H7)4N}Br] dan ammonium tetratiotungstat [(NH4)2WS4] untuk menghasilkan tetrapropilammonium tetratiotungstat [{(C3H7)4N}2WS4] (prekursor). Seterusnya prekusor bertindak balas dengan asid fenilasetilenakarboksilik (C9H6O2) dan membentuk kompleks tris(1-karboksil-2- fenil-1,2-etilenoditiolenik-S,S’) tungsten (C27H18O2S6W). Molekul telah diciri dengan kaedah spketroskopi inframerah (IR), ultraviolet/boleh nampak (UV/Vis), resonans magnet nuklear (NMR) dan analisis-mikro unsur C, H, N dan S. Spektrum IR menunjukkan kehadiran frekuensi regangan (W-S), (C-S) dan (C=O) masing-masing pada 511, (1470 dan 1035) dan 1655 cm-1. Spektrum NMR 1H menunjukkan kehadiran 6 proton kumpulan propil dan spektrum NMR13C hanya menunujukkan 7 atom C bagi gelang benzena, etilena dan asid karbosilik. Hanya bilangan setara isyarat resonans 1H dan 13C dikesan kerana struktur mempunyai satah simetri yang tinggi. Peratus hasil kompleks agak rendah (50%) disebabkan kesukaran untuk memisahkan isomer yang terbentuk. Ujian fotolisis air yang dicampur aseton menunjukkan kemampuan fotomangkin untuk menghasilkan hidrogen pada kadar 1.8µmol/j.
Kata kunci: Bahan pewarna-pemeka sel solar; hidrogen; fotolisis; sinaran matahari; tungsten
INTRODUCTION
The shrinking of fossil fuel reserve and the increasing demand for energy has led many researchers to seek for an alternative form of environmentally friendly fuel. One example of clean and environmentally friendly source of energy is hydrogen gas which has been recognized as one of the main energy sources in the future. Conventionally, hydrogen gas is produced by steam reforming of hydrogen- rich fuel as methane gas. It can also be obtained from
electrolysis of water using conventional electricity source as the driving force. Recent reports have suggested that hydrogen gas can also be produced by photolysis of water using sunlight.
Hydrogen is considered the fuel for the future because the product of its energy conversion is pure water. It is a clean and save form of energy carrier for the environment.
There are several ways to produce hydrogen gas namely, steam reforming process (methanol reforming), biological
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process (using algae, enzymes) and photolysis of water (Alonso et al. 2001).
There are a number of catalysts that can be used in water splitting reaction. For instance, titanium dioxide (TiO2), zinc oxide (ZnO), strontium oxide (SnO2), tungsten oxide (WO3) and Fe2O3. The catalyst involved in this process should be stable towards oxidation (corrosion), donates lots of electron to the electrode, have an efficient charge distribution and stable towards heat as well as electromagnetic radiations.
Previous study by Fujishima and Honda (1972) uses TiO2 a catalyst as the anode and platinum black as the cathode to produce hydrogen. Another potential catalyst was tungsten oxide (WO3) which showed very high photocurrent generation efficiencies (Bamwenda et al.
1999). Bearing these qualities in mind and a combination with other substance such as dithiolenes group which has more tendencies to absorb light energy will create a promising photocatalyst for photolysis reaction. The used of tris(dithiolene) tungsten as photocatalyst has been demonstrated in the water splitting reaction (Humhry- Baker et al. 1997). It is a promising candidate to be used as photocatalyst for hydrogen production from water using solar energy. Recent work on this area has shown that heterogenous catalyst is more efficient compared to homogenous catalyst (Serpone 1997).
Significant effort has been undertaken to develop photocatalyst that can catalytically split water molecule with visible light (Samios et al. 1997). A unique and highly desirable characteristic of the tris(dithiolene) tungsten arises from its ability to produce both hydrogen and oxygen from water without a need to add a consumable (sacrificial) donor. Tris(dithiolene) tungsten offers a number of advantages as photocatalyst. It is more active since the dithiolene group absorbs light energy and transfers it to water directly and causes the water molecules to split.
Immobilization of the catalyst onto a photoelectrode produces more efficient charge distribution and hence, a higher rate of reaction. Tris(dithiolene) tungstate is more robust and stable towards heat and electromagnetic radiations.
EXPERIMENTAL
Synthesis of tris(1-carboxyl-2-phenyl-1,2- ethylenodithiolenic-S,S’) tungsten
A solution of the precursor [{(C3H7)4N}2WS4] (Fadhli et al. 2004) was reacted with of phenylacetylenecarboxylate [C9H6O2] in 3 to 1 ratios to produced tris(1-carboxyl-2- phenyl-1,2-ethylenodithiolenic-S,S’) tungsten complex (C27H18O2S6W) in acetonitrile. The mixture was stirred for 10 minutes and all reaction was done under argon atmosphere (Umakoshi et al. 2000). The solution was concentrated to ca. 5 mL under reduced pressure at 20°C.
Then the solution was loaded onto a LH-20 column (25 cm × 2 cm) and eluted with acetonitrile-benzene (10/1) to give several fractions. The first fraction was collected and
re-concentrated again under reduced pressure. It was purified on a second LH-20 column chromatography at the same conditions. Evaporations of the solvent gave a dark brown solid, yield about 50%.
INSTRUMENTATION
The precursor, tetrapropylammonium tetrathiotungstate [{(C3H7)4N}2WS4] has been reported and described in details previously (Fadhli et al. 2004) and the tris(1- carboxyl-2-phenyl-1,2-ethylenodithiolenic-S,S’) tungsten complex (C27H18O2S6W) were characterized by spectroscopic techniques such fourier transform infra-red (FTIR) and ultra violet and visible (UV/Vis). The carbon, hydrogen, nitrogen and sulfur (CHN-S) content of the product were determined by micro-elemental analysis. A suitable crystal was afforded by a slow evaporation of the aqueous solution containing the product molecules.
RESULTS AND DISCUSSION
The infra-red spectrum of tris(1-carboxyl-2-phenyl-1,2- ethylenodithiolenic-S,S’) tungsten complex (C27H18O2S6W) agrees with the expected structure of the complex (Table 1). The structure of C27H18O2S6W complex is shown in Figure 1.
TABLE 1. Selected stretching frequencies of tris(1-carboxyl-2- phenyl-1,2-ethylenodithiolenic-S,S’) tungsten
complex infrared spectrum
Wavenumber (cm-1) Functional Group
511 (W-S) stretching
1470,1035 (C-S)
1655 (C=O) group
1260 C-O-C
2967 C-H aliphatic
3435 O-H group
FIGURE 1. The structure of tris(1-carboxyl-2-phenyl-1,2- ethylenodithiolenic-S,S’) tungsten complex
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The UV/Visible spectrum for (C27H18O2S6W) were recorded in acetonitrile solution. The spectrum showed maximum absorption at 382 nm. The origin of this electronic transition was ascribed to the ! to !* transitions.
The presence of double bonds in the complex, phenyl conjugation and carboxylic groups has been considered as the source of the transitions.
The elemental C, H, N and S data of the complex (C27H18O2S6W) showed the following results; C = 38.22%
(39.80%), H = 3.71% (3.21%) and S = 22.12% (23.58%).
The 13C NMR spectrum for (C27H18O2S6W) showed seven carbons detected at 81.76, 85.99, 120.43, 129.80, 131.72, 133.62 and 154.58 ppm, respectively. The number of signals was reduced due to the high symmetry of the molecules which renders symmetrically equivalent carbon to appear at the same frequencies. There are three peaks of proton detected at 7.46ppm (multiplet, 2 protons), 7.53ppm (multiplet, 1 proton) and 7.61ppm (multiplet, 2 protons). In addition, proton from carboxylic group (COOH) detected at 11.47ppm (singlet, 1 proton). A total of six protons were detected from spectra of the C27H18O2S6W.
Beside that, acetone solvent was detected at 2.04 ppm.
Initial investigation using chronocoulometry shows that the complex produced about 224.68 mC/cm2 during water electrolysis. The current reading declined dramatically, either with and without catalyst (Table 2).
The affect was due to the resistance of catalyst and water during oxidation process. The average current produced for electrolysis process with and without catalyst were about 2.13 × 10-4A and 9.90 × 10-5A, respectively.
produce about 1.8µmol/h hydrogen and 0.9 µmol/h of oxygen.
ACKNOWLEDGEMENT
The authors would like to acknowledge the Malaysian Ministry of Science Technology and Innovation for sponsoring this project under IRPA project 02-02-02-0006 PR0023/11-11.
REFERENCES
Alonso, G., Berhault, G. & Chianelli, R. R. 2001. Synthesis and characterization of tetraalkylammonium thiomolybdates and thiotungstate in aqueous solution. Inorganica Chimica Acta.
316:105-109.
Bamwenda, G. R., Sayama, K. & Arakawa, H. 1999. The effect of selected reaction parameters on the photoproduction of oxygen and hydrogen from a WO3-Fe2+-Fe3+ aqueous suspension. Journal of Photochemistry and Photobiology A:
Chemistry. 122: 175-183.
Fadhli Hadana Rahman, Mohammad Kassim, Rusli Daik & Wan Ramli Wan Daud. 2004. Dithiolene tungsten photocatalyst precursor for water electrolysis. Proceedings of National Chemical Engineering Seminar (Medan, Indonesia). pp: B- 03-1 – B-03-5.
Fujishima, A. & Honda, K. 1972. Electrochemical photolysis of water at a semiconductor electrode. Nature 238: 37-38.
Humphry-Baker, R., Mitsopoulou, C. A., Katakis, D. & Vrachnou, E. 1997. Photophysical study of the decomposition of water using visible light and tungsten tris(dithiolene) as photosensitizers-catalyst. Journal of Photochemistry and Photobiology 114: 137-144.
Samios, J., Katakis, D., Dellis, D., Lyris, E. & Mitsopoulou, C.
A. 1998. Solvation and catalyst-substrate superstructure of a tungsten tris(dithiolene) complex dissolved in water-acetone.
J. Chem. Soc. Faraday Trans. 94: 3169-3175.
Serpone, N. 1997. Relative photonic efficiencies and quantum yields in heterogeneous photocatalysis. Journal of Photochemistry and Photobiology 104: 1-12.
Umakoshi, K., Nishimoto, E., Sokolov, M., Kawano, H., Sasaki, Y. & Onishi, M. 2000. Synthesis, structure and properties of sulfide-bridge dinuclear tungsten(V) complex of dithiolene.
Journal of Organometallic Chemistry 611: 370-375.
Fadhli Hadana Rahman, Rusli Daik & Mohammad Kassim School of Chemical Sciences and Food Technology Faculty of Science and Technology
Universiti Kebangsaan Malaysia 43600 Bangi, Selangor D.E Malaysia
Khuzaimah & Wan Ramli Wan Daud
Department of Chemical and Process Engineering Faculty of Engineering
Universiti Kebangsaan Malaysia 43600 Bangi, Selangor D.E Malaysia
Received: 4 April 2007 Accepted: 13 July 2007
TABLE 2. The current produced during the electrolysis with and without catalyst
Time (s) Current (A), Current (A),
Without catalyst With catalyst
0 1.21 × 10-4 4.90 × 10-4
600 0.73 × 10-4 1.44 × 10-4
1200 0.86 × 10-4 1.25 × 10-4
1800 1.15 × 10-4 0.94 × 10-4
The catalyst reactivity during photolysis process, can be assumed from the number of current produced and convert it to mol of hydrogen produced. Therefore, photolysis of acetone spiked H2O shows that the catalyst was able to produced 1.8µmol/h hydrogen. At the same time, the above process also produced 0.9 µmol/h of oxygen.
CONCLUSION
Tris(1-carboxyl-2-phenyl-1,2-ethylenodithiolenic-S,S’) tungsten complex was successfully synthesized. The characteristics of the molecules were in agreement with previously reported results and other references. Initial investigation showed that this photocatalyst was able to