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Efficient homogeneous photochemical hydrogen generation using rhodium compounds as catalyst precursors
Journal of Molecular
Catalysis,
37 (1986)
297 - 307
297
EFFICIENT HOMOGENEOUS PHOTOCHEMICAL HYDROGEN GENERATION USING RHODIUM COMPOUNDS AS CATALYST PRECURSORS HANNU HUKKANEN and TUULA T. PAKKANEN University (Finland)
of Joensuu,
Department
(Received January 8,1986;
of Chemistry,
P.O. BOX Ill,
SF-801 01 Joensuu
10
accepted May 28,1986)
Summary A homogeneous catalytic system for efficient generation of hydrogen by visible light irradiation is described, using Ru(bipy)& as a photosensitizer, Rh carbonyls as catalysts and triethanolamine as an electron donor. The hydrogen generation probably involves reductive quenching of the Ru( bipy):2 excited triplet state, reduction of Rh compounds by the Ru(bipy)s+ species formed, and hydrogen generation by the reduced catalysts. The system is highly active only in organic medium. The activity of the system decreases linearly with the concentration of water when water is added to the solution. The long-term stability of the system is poor due to decomposition of the photosensitizer in organic medium.
Introduction In recent years, extensive work has been performed on hydrogen generation in artificial water-splitting systems using visible light, photosensitizers, homogeneous or heterogeneous catalysts and organic or inorganic electron donors [ 1 -51. We describe here an efficient photochemical hydrogen generation by a homogeneous system using rhodium compounds as catalysts. There have been reports on photochemical hydrogen generation using bipyridine complexes of rhodium as homogeneous catalysts [2c] and metallic rhodium as a heterogeneous catalyst [ 31. The investiiation of rhodium compounds was motivated by these earlier observations on photochemical hydrogen generation.
Results and discussion The effect of the gas phase composition Hydrogen generation was achieved by visible light irradiation of a photosensitizer/relay-catalyst/donor system, Ru(bipy),Cl,/Rh carbonyl or 0304-5102/86/$3.50
@ Elsevier Sequoia/Printed in The Netherlands
298
TABLE 1 Generation of hydrogen by visible light irradiation of solutions containing Ru(bipy)sCla, tested catalysts and electron donor Catalyst
Solvent system
Apparent pH
Volume Ha a Irradiation time (ml) (h)
RhCls RhC13 cis-Rh(cyclam)C12 c&r-Rh(cyclam)Cla HRh(CO)z(PPhs), HRh(CO)a(PPhs)s
TEOA:DMF 1:5 TEOA:DMF 1:5 TEOA:DMF 1:5 TEOA:DMF 1:5 TEOA:DMF 1:5 TEOA:DMF 1:5 TEOA:DMF 1:2 TEOA:DMF 1:5 TEOA:DMF 1:5 TEOA:DMF 1:2 TEOA:DMF 1:5 TEOA:DMF 1:5 TEOA:DMF 1:5 TEOA:DMF 1:5 TEOA:DMF 1:5 TEOA:DMF 1:5 Ascorbic acidlascorbate Ascorbic acid/ascorbate Ascorbic acidlascorbate 2 ml 0.3 M S2-/28 ml DMF 2 ml 0.3 M S2-/28 ml DMF TEOA:DMF 1:5 TEOA:DMF 1:5 20% H20/TEOA:DMF 1:5 33% H20/TEOA:DMF 1:5
11 11 11 11 11 11 11 11 11 11 11 11 11 11 7 11 4 4 4 13 13 11 11 11 11
12.01 20.08 1.31 2.32 7.24 13.80 6.87b 14.43 18.03 6.65 31.70 -c _d 12.77 0.82e 4.90 0.06 1.28 0.52 4.33 10.80b 4.15 1.62 0.76
IRh(COWl12
~4(co)12 ~4(co)12 =6(co)16
Rhb(CO)le Rhe(CO)re Rh6(CO):6 (PPN)2Rh12(CO)se (PPN)2Rh12(CO), co3=(coh2
RhCl, cis-Rh(cyclam)Cla Rh6(CO)16 Rh6(CO)16 Rh6(CO)re Co(dmgH2) Cu(cyclam)Cl2 Cu(cyclam)Clz Cu(cyclam)Cl2
2.0 6.0 2.5 4.5 2.0 6.5 1.0 3.2 6.0 1.0 7.3 1.5 9.0 2.0 2.0 4.3 2.5 2.5 7.0 2.0 14.0 1.0 4.6 4.6 4.6
aVolume of hydrogen at 298 K and 1 atm pressures. bCOa 4.2 mmol. CNo photosensitizer in the solution. din dark. eThe pH of the solution was adjusted with acetic acid.
RhCl&iethanolamine. The experiments were performed in a carbon dioxide gas phase or in uucuo. The hydrogen production was found to be independent of the composition of the gas phase (Table 1). In the absence of light or the photosensitizer, no hydrogen was produced (Table 1). Differences between the catalyst precursors In short-term photolysis, the hydrogen generation rate was similar for almost all the Rh compounds studied. The maximum hydrogen production rate of 7 ml h-l with a turnover number of 4200 (mol of Hz produced (mol catalyst)-’ (24 h)-‘) was measured for Rh6(C0)16 (Table 1). The various Rh carbonyls and RhCls used as catalyst precursors show very similar activities (Table 2) in the present system. A considerably lower rate was observed for
299
TABLE 2 Activities of some studied Rh compounds DMF (1:5)
in 3 - 7 h photolysis in Ru(bipy)s+2/TEOA:
Compound
Photolysis time (h)
Turnover numbers/day
RhCls cis-Rh(cyclam)Clz HRh(CO)a(PPhs)s Rh4(CO)12
4.0 4.5 6.0 3.2 7.3 4.5
316 36 184 264 704 315
~6@0)16
(PPWRhls(CO)se
aTumover number = mole Hz (mol Rh atoms)-‘.
the macrocyclic complex cis-Rh( cyclam)C12, probably due to electrochemical and steric reasons. In long-term photolysis, the main differences appeared between the catalysts. The hydrogen generation ability decreased in the order Rh6-?I -I
E
T NM I
R 5
R LD-
N-
m-
cis-RhlcycLomlCL2
0
5
I 40
I
15
I
I
20
25 time/h
Fig. 1. Hydrogen generation by various Rh compounds (1:5) system in a long-term photolysis.
in the Ru(bipy)s+2/TEOA:DMF
300
> RhC13 > Rh4(CO)12 > HRh(C0)2(PPh3)3 > > PW&MC%o cis-Rh(cyclam)Cl, (Fig. 1.). In addition, a qualitative experiment was done using the mixed metal cluster, CosRh( CO) 12, as a catalyst precursor. This compound was less active than the monometallic Rh carbonyls. In the Co3Rh(CO)i2 experiment, a small amount of carbon monoxide was detected due to the decomposition of the catalyst. In the same manner [ Rh(CO),Cl] 2 was qualitatively tested in a short-term photolysis under CO2 atmosphere. Its hydrogen generation ability was similar to that of the other monometallic Rh carbonyl catalysts tested in vacuum conditions. (co),,
Composition of the catalyst precursors and effect of the solubility
Although there are reports [6] that the metal carbonyl compounds are photolabile against ligand dissociation and metal-metal bond breaking, it was observed that no carbon monoxide was generated during photolysis when monometallic Rh carbonyls were used as catalyst precursors. The chemical composition of the active carbonyl catalysts used was difficult to determine because of their low concentrations. For example, the IR spectra of 1.66 X 10e3 M (PPN)2Rh,2(C0)30 solutions in DMF and TEOA:DMF (1: 5) recorded after 2 h photolysis showed in the metal carbonyl region several absorption bands, the assignment of which turned out to be difficult. In the vacuum experiments, a small amount of carbon dioxide was detected in the gas phase which may have been formed by the oxidation of a carbonyl ligand or decomposition of organic material. The solubility of the catalyst precursors affected the hydrogen generation rate. If the catalyst was not fully dissolved, the rate was lower than that with a dissolved catalyst. Only Rh6(CO)16 has a poor solubility in the dimethylformamide solvent; it was dissolved by refluxing in DMF for 1 h. Some carbon monoxide was detected in the gas phase after solvation, probably due to the dissociation of carbonyl ligands from Rh6 cluster. Nevertheless, the best Rh CatdySt was Rh6(CO)i6, which operated more efficiently than the other Rh compounds studied. Long term stability and temperature effects
The stability of the Ru(bipy)3’2/Rh catalyst/TEOA:DMF system was examined in a long-term photolysis experiment. The hydrogen production rate was observed to decrease as a function of time, and cease after 10 h of photolysis. When a fresh amount of photosensitizer was added, the hydrogen production rate rose to the initial level (Fig. 2.). This suggests that the main reason for the instability of the present system is decomposition of the photosensitizer. The reaction temperature had a moderate effect on the decomposition of Ru(bipy)3+2. The stability of the photosensitizer decreased when the temperature was raised from 30 to 50 “C. These results conform with the earlier observations concerning the photolability of the Ru(bipy)3’2 in organic medium [ 8, 91.
301
0
I
0
IO
I
20
I
30
I
40
I
50
1
60
I
10
I
60
I
90
1
100
Tcme/h
Fig. 2. Hydrogen generation in the Ru(bipy)3+2/Rh6(C0)16/TEOA:DMF (15) system. After 48 h photolysis 1 ml of Ha0 and after 68 h photolysis fresh Ru(bipy)$lz were added.
Solvent effects Addition of water to the solvent system caused a linear decrease in the rate of the hydrogen production (Fig. 3). A similar effect was observed if the electron donor/solvent system was diluted with water. The hydrogen generation rate was insensitive to the concentration of the electron donor. The observed rate was identical at donor/solvent ratios of 1: 2 and 1:5. Most of the experiments were performed with an electron donor/solvent ratio of 1:5. However, the hydrogen ion activity of the solution had a remarkable effect on the hydrogen generation. If the ‘pH’ of the TEOA: DMF solution (1:5) was adjusted to 7, the hydrogen production rate was l/15 of the rate in the pure TEOA:DMF mixture (1:5, ‘pH 11’) when using (PPN)2Rh1,(C0)s0 as a catalyst. In this experiment, precipitation of the carbonyl catalyst was observed (Table 1). The Rh compounds were also tested in other solvent systems (Fig. 4). Ascorbic acid-sodium ascorbate-water solution was used as an electron donor and a buffer [7b] when Rh6(C0)16, (PPN),Rh12(C0)s,, HRh(CO)*(PPh&, RhC13 and cis-Rh(cyclam)Cl, were tested as catalysts. Only cis-Rh-
302
0
I
0
10
I
20
I
30
,
I
50
40
H20/
CotoC
vol_x
Fig. 3. The effect of dimethylformamide substitution by water on the hydrogen generation in Ru(bipy)3+2/(PPN)2Rh12iC0)30/TEOA:DMF (15) system in 2.5 h photolysis (PPN = bis(triphenylphosphoranylidene)ammonium cation).
(cyclam)Cl, and Rh,(CO)i6 generated a small amount of hydrogen. The slow hydrogen generation in aqueous solutions may be due to the different solute-solvent effects in water compared to aprotic organic medium, which change the electron-transfer kinetics of the photosensitizer-solvent system and cause side- and back-reactions [lld], or the pH of the ascorbic acidascorbate solution is undesirable for efficient hydrogen generation by Rh compounds (Table 1). The poor solubility of the carbonyl catalysts in aqueous solutions also has a considerable effect on the hydrogen generation. The Rh compounds were also tested using sulphide ion as electron donor [7a] and changing the composition of the solvent. The best results were obtained using DMF as a solvent. In aqueous solution no hydrogen was generated. Both the photosensitizer and the catalyst decomposed when Szwas used as an electron donor. This may be due to the reactions of the catalyst precursors and the sensitizer with the S2- ion. The UV-Vis spectra of the resulting mixtures supported this assumption. The source of hydrogen and possible reaction mechanisms 13C NMR spectroscopy was used for the determination of the decomposition products of the electron donor TEOA. In long-term photolysis using an NMR tube as the reaction vessel, triethanolamine was found to decompose to diethanolamine and glycolaldehyde. This indicates that water must have been present as an impurity in TEOA (Pro Analysis grade was used
303
^ r-I
, TEOWDHF
E ;: l?W
VI-
P-
,H~O/ASCORBIC
ACID
TIME/(h)
Fig. 4. Hydrogen generation by Rhs(C!O) 16 in various solvent systems with Ru(bipy)J’* as a photosensitizer (2 ml 0.3 M Na2S:Hz0/28 ml DMF; TEOA:DMF 1:5 and ascorbic acid-sodium ascorbate (both 0.5 M) in HzO).
without further purification). The net light-driven reaction is probably the same as found earlier [7a], the catalysts in this study being Ru( bipy)3’2 and Rh compounds: N(CH2CH20H)3 + H20 p
hv + catalysts
HN( CH,CH,OH), + CHO-CH20H + Hz
UV-Vis spectra of the reaction solutions were recorded after photolysis. Absorption bands of Ru(bipy), in the oxidation states I and II were usually found [ll]. In some experiments, bands of Ru(bipy),LL’ (LL’ = solvent molecule or an anion) were observed [lo]. According to Vis spectra, the possible reaction mechanism of the photosensitizer Ru(bipy)3’2 involves the reductive quenching of the excited triplet state. The existence of Ru(bipy)3+ was also found by diluting the reaction solution with water after the reaction. A change in the colour of the solution from red-orange to yellow was observed due to oxidation of the Ru(bipy)3+ to Ru(bipy)3+2. This behaviour has been noted earlier [ llc]. Decomposition of Ru( bipy)3+2 to Ru( bipy)2 LL’ was observed to occur in long-term photolysis of the Rh catalyst solutions and in short-term photolysis of the tested reference catalyst compounds. The formation of Ru(bipy)*LL’ caused broadening of the bands in the Vis spectra.
304
Comparative experiments Comparative experiments were performed in order to relate the hydrogen generation efficiency of the present system to the previously studied ones. Using cobaloxime [7a] as a catalyst with a carbon dioxide gas phase in TEOA:DMF solvent, a 10 ml h-’ hydrogen production rate was obtained in our experimental conditions. This refers to 7 - 8% quantum yields when the monometallic Rh carbonyls are used as catalyst precursors, since the quantum yield of hydrogen by the cobaloxime catalyst has been noted to be 13% [7a] (Table 1). When using the cobalt dimethylglyoxime complex as a catalyst in an ascorbic acid-ascorbate medium, it was observed that at least a part of the hydrogen generated came from decomposition of the dimethylglyoxime ligand. This phenomenon was noted in long-term photolysis (Fig. 5). Using the macrocyclic complex Cu(cyclam)Cl, as a catalyst, we examined the effect of water on the activity of the TEOA:DMF solvent system. The results were similar to the Rh experiments (Table 1).
T Lme/h Fig. 5. Hydrogen generation in a long-term photolysis of the Ru(bipy)3+*/Co(dmgH2)/ ascorbic acid-sodium ascorbate system. After 2.5 h and 22 h photolysis, fresh dmgH2 was added (dmgH2 = dimethylglyoxime).
305
Conclusions The system described in this work achieves efficient photochemical hydrogen generation by the homogeneous rhodium catalysts. The reaction mechanisms are difficult to detail, due to the complex nature of the catalysis system. The hydrogen generation probably involves formation of Rh hydride complexes, as with the other catalysts described in the literature. The practical difficulties of the present system are the use of a sacrificial organic electron donor and a weak long-term stability. Nevertheless, use of an organic electron donor-solvent system for photochemical hydrogen generation serves as a rapid method for measuring the maximum hydrogen generation rates of possible catalysts for artificial water-splitting.
Experimental Ru(bipy)&*5H20 and macrocyclic complexes were synthesised according to literature methods [12, 131. Cyclam and 2,2’-bipyridine were commercially available compounds (Strem and Aldrich). Solvents and triethanolamine were used without further purification. Reagent grade NazS.9Hz0, CoCl,, dmgHz and Pro Analysis grade ascorbic acid and sodium ascorbate were used. All these compounds were stored in air. The Rh compounds were commercially available (Aldrich, Johnson-Matthey), except DWCOWll,, RMCOh, Co$WW,z and (PPN)2Rh12(C0)30 which were synthesised by literature methods [14,15]. The Rh compounds, except RhC13, HRh( CO) *(PPh3)3 and Rh& CO) 16, were stored under nitrogen. Irradiation experiments were performed on a 30 ml solution contained in a round-bottom flask equipped with a Teflon stopcock (115 ml capacity) and stirred magnetically at 500 - 1000 r.p.m. The initial concentration of the photosensitizer in the TEOA:DMF experiments was 10 mg/0.0136 mmol and in the other experiments 20 mg/0.0272 mmol. The concentration of the catalysts varied, and usually was chosen so that the Rh-atom/Ru-atom ratio was 1: 1. The concentration of Co(dmgH,) was the same as that used in the literature experiments [7a]. In the Cu(cyclam)Cl, studies, the concentrations were similar to those in the Rh experiments. The reactor was evacuated before the experiment using a pump-freezethaw cycle. The coolant was liquid nitrogen and the evacuation was performed for about 2 min. A measured amount of carbon dioxide was added to the reactor. The reactor was re-evacuated when a fresh amount of photosensitizer or ligand was added to the reaction solution during the long-term experiments. Three light sources were employed: a 250 W halogen lamp in a slideprojector, fitted with a 400 nm cutoff filter (SRI Filter), and two 1000 W halogen lamps equipped with 400 nm cut-off filters and self-made infrared water filters (60 cm length). Different reaction temperatures were produced by the IR radiation of the light sources. The temperatures achieved with the
306
lamps were: 30 “C by the 250 W lamp, 35 - 40 “C by the 1000 W lamp with 11 cm diameter IR filter and 45 - 50 “C by the 1000 W lamp with 14 cm diameter IR filter. The 62.9 MHz 13CNMR spectra were recorded on a Bruker AM 250 FT spectrometer and UV-Vis spectra on a Beckman Model 25 spectrophotometer. IR spectra were recorded on a Perkin-Elmer 297 IR spectrometer using 0.5 mm NaCl solution cells. The experiments where a NMR tube was used as a reactor were carried out in a manner similar to the flask experiments. The qualitative UV-Vis spectra of the reaction solutions were obtained by diluting the solutions after experiment by DMF or water before measuring the UV-Vis spectra. TEOA:DMF, TEOA:DMF/H,O or DMF/H*O was used as reference. After irradiation, the gas phase of the reactor was analysed by a Carlo Erba 4200 multicolumn gas chromatograph attached to a manometer/ vacuum pump system and equipped with a sample loop, a column switching valve, and a thermal conductivity detector. The light components of the gas phase, H2 and CO, were separated on a Molecular Sieve 5 A column and the heavy component, COz, on a Chromosorb 102 column. The carrier gas was 99.995% helium. The gas chromatograph was calibrated with the measured gases (H,, CO, COz) and the amounts of the gases were calculated by a computer program. The error limits of the gas phase analysis are within + 10%. Acknowledgement We thank the Neste Ltd Foundation
for financial support.
References 1 K. Kalyanasundaram, Coord. Chem. Rev., 46 (1982) 161. 2 (a) J. M. Lehn, J. P. Sauvage and M. Kirch, Helu. Chim. Acta, 62 (1979) 1345; (b) C. V. Krishnan, C. Creutz, D. Mahajan, H. A. Schwarz and N. Sutin, Zsr. J. Chem., 22 (1982) 98; (c) S. F. Chan, M. Chou, C. Creutz, T. Matsubara and N. Sutin, J. Am. Chem. Sot., 103 (1981) 369; (d) F. Pina, Q. G. Mulazzani, M. Venturi, M. Ciano and V. Balzani, Znorg. Chem., 24 (1985) 848. 3 J. M. Lehn, in J. S. Connolly (ed.), Photochemical Conversion of Solar Energy, Academic Press, New York, 1981. 4 M. Griitzel, K. Kalyanasundaram and J. Kiwi, Struct. Bonding, 49 (1982) 37. 5 G. McLendon, in M. Griitzel (ed.), Energy Resources through Photochemistry and Catalysis, Academic Press, New York, 1983, p. 99. 6 (a) G. L. Geoffroy and M. S. Wrighton, Organometallic Photochemistry, Academic Press, New York, 1979; (b) M. S. Wrighton, J. L. Graff, J. C. Luong, C. L. Reichel and J. L. Robbins, ACS Symp. Ser., 155 (1981) 132. 7 (a) J. Hawecker, J. M. Lehn and R. Ziessel, Nouv. J. Chim., 7 (1983) 271; (b) C. V. Krishnan and N. Sutin, J. Am. Chem. Sot., 103 (1981) 2141. 8 D. V. Pinnick and B. Durham, Znorg. Chem., 22 (1984) 1440. 9 P. E. Hoggard and G. B. Porter, J. Am. Chem. Sot., ZOO (1978) 1457.
10 (a) P. Belser and A. von Zelewsky, Helu. Chim. Acta, 63 (1980) 1675; (b) N. R. Davies and T. L. Mullins, AZ&. J. Chem., 20 (1967) 657. 11 (a) Q. G. Mulazzani, S. Emmi, P. G. Fuochi, M. Z. Hoffmann and M. Venturi, J. Am. Chem. Sot., 100 (1978) 981; (b) G. M. Heath, L. J. Yellowlees and P. S. Braterman, J. Chem. Sot., Chem. Commun., (1981) 287; (c) J. Hawecker, J. M. Lehn and R. Ziessel, J. Chem. Sot., Chem. Commun., (1984) 328; (d) V. Balzani, F. Bolletta, M. Gandolfi and M. Maestri, Top. Curr. Chem. 75 (1978) 1. 12 J. N. Braddock and T. J. Meyer, J. Am. Chem. Sot., 95 (1973) 3158. 13 (a) E. J. Bounsall and S. R. Koprich, Can. J. Chem., 48 (1970) 1481; (b) B. Bosnich, M. L. Tobe and G. A. Webb, Inorg. Chem., 4 (1965) 1109. 14 S. Martinengo, P. Chini, V. G. Albano, F. Cariati and T. Salvatori, J. Organometall. Chem., 59 (1973) 379. 15 (a) P. Chini and S. Martinengo, J. Chem. Sot. Chem. Commun., (1969) 1092; (b) P. Chini and S. Martinengo, Znorg. Chim. Acta, 3 (1969) 299.
Catalysis,
37 (1986)
297 - 307
297
EFFICIENT HOMOGENEOUS PHOTOCHEMICAL HYDROGEN GENERATION USING RHODIUM COMPOUNDS AS CATALYST PRECURSORS HANNU HUKKANEN and TUULA T. PAKKANEN University (Finland)
of Joensuu,
Department
(Received January 8,1986;
of Chemistry,
P.O. BOX Ill,
SF-801 01 Joensuu
10
accepted May 28,1986)
Summary A homogeneous catalytic system for efficient generation of hydrogen by visible light irradiation is described, using Ru(bipy)& as a photosensitizer, Rh carbonyls as catalysts and triethanolamine as an electron donor. The hydrogen generation probably involves reductive quenching of the Ru( bipy):2 excited triplet state, reduction of Rh compounds by the Ru(bipy)s+ species formed, and hydrogen generation by the reduced catalysts. The system is highly active only in organic medium. The activity of the system decreases linearly with the concentration of water when water is added to the solution. The long-term stability of the system is poor due to decomposition of the photosensitizer in organic medium.
Introduction In recent years, extensive work has been performed on hydrogen generation in artificial water-splitting systems using visible light, photosensitizers, homogeneous or heterogeneous catalysts and organic or inorganic electron donors [ 1 -51. We describe here an efficient photochemical hydrogen generation by a homogeneous system using rhodium compounds as catalysts. There have been reports on photochemical hydrogen generation using bipyridine complexes of rhodium as homogeneous catalysts [2c] and metallic rhodium as a heterogeneous catalyst [ 31. The investiiation of rhodium compounds was motivated by these earlier observations on photochemical hydrogen generation.
Results and discussion The effect of the gas phase composition Hydrogen generation was achieved by visible light irradiation of a photosensitizer/relay-catalyst/donor system, Ru(bipy),Cl,/Rh carbonyl or 0304-5102/86/$3.50
@ Elsevier Sequoia/Printed in The Netherlands
298
TABLE 1 Generation of hydrogen by visible light irradiation of solutions containing Ru(bipy)sCla, tested catalysts and electron donor Catalyst
Solvent system
Apparent pH
Volume Ha a Irradiation time (ml) (h)
RhCls RhC13 cis-Rh(cyclam)C12 c&r-Rh(cyclam)Cla HRh(CO)z(PPhs), HRh(CO)a(PPhs)s
TEOA:DMF 1:5 TEOA:DMF 1:5 TEOA:DMF 1:5 TEOA:DMF 1:5 TEOA:DMF 1:5 TEOA:DMF 1:5 TEOA:DMF 1:2 TEOA:DMF 1:5 TEOA:DMF 1:5 TEOA:DMF 1:2 TEOA:DMF 1:5 TEOA:DMF 1:5 TEOA:DMF 1:5 TEOA:DMF 1:5 TEOA:DMF 1:5 TEOA:DMF 1:5 Ascorbic acidlascorbate Ascorbic acid/ascorbate Ascorbic acidlascorbate 2 ml 0.3 M S2-/28 ml DMF 2 ml 0.3 M S2-/28 ml DMF TEOA:DMF 1:5 TEOA:DMF 1:5 20% H20/TEOA:DMF 1:5 33% H20/TEOA:DMF 1:5
11 11 11 11 11 11 11 11 11 11 11 11 11 11 7 11 4 4 4 13 13 11 11 11 11
12.01 20.08 1.31 2.32 7.24 13.80 6.87b 14.43 18.03 6.65 31.70 -c _d 12.77 0.82e 4.90 0.06 1.28 0.52 4.33 10.80b 4.15 1.62 0.76
IRh(COWl12
~4(co)12 ~4(co)12 =6(co)16
Rhb(CO)le Rhe(CO)re Rh6(CO):6 (PPN)2Rh12(CO)se (PPN)2Rh12(CO), co3=(coh2
RhCl, cis-Rh(cyclam)Cla Rh6(CO)16 Rh6(CO)16 Rh6(CO)re Co(dmgH2) Cu(cyclam)Cl2 Cu(cyclam)Clz Cu(cyclam)Cl2
2.0 6.0 2.5 4.5 2.0 6.5 1.0 3.2 6.0 1.0 7.3 1.5 9.0 2.0 2.0 4.3 2.5 2.5 7.0 2.0 14.0 1.0 4.6 4.6 4.6
aVolume of hydrogen at 298 K and 1 atm pressures. bCOa 4.2 mmol. CNo photosensitizer in the solution. din dark. eThe pH of the solution was adjusted with acetic acid.
RhCl&iethanolamine. The experiments were performed in a carbon dioxide gas phase or in uucuo. The hydrogen production was found to be independent of the composition of the gas phase (Table 1). In the absence of light or the photosensitizer, no hydrogen was produced (Table 1). Differences between the catalyst precursors In short-term photolysis, the hydrogen generation rate was similar for almost all the Rh compounds studied. The maximum hydrogen production rate of 7 ml h-l with a turnover number of 4200 (mol of Hz produced (mol catalyst)-’ (24 h)-‘) was measured for Rh6(C0)16 (Table 1). The various Rh carbonyls and RhCls used as catalyst precursors show very similar activities (Table 2) in the present system. A considerably lower rate was observed for
299
TABLE 2 Activities of some studied Rh compounds DMF (1:5)
in 3 - 7 h photolysis in Ru(bipy)s+2/TEOA:
Compound
Photolysis time (h)
Turnover numbers/day
RhCls cis-Rh(cyclam)Clz HRh(CO)a(PPhs)s Rh4(CO)12
4.0 4.5 6.0 3.2 7.3 4.5
316 36 184 264 704 315
~6@0)16
(PPWRhls(CO)se
aTumover number = mole Hz (mol Rh atoms)-‘.
the macrocyclic complex cis-Rh( cyclam)C12, probably due to electrochemical and steric reasons. In long-term photolysis, the main differences appeared between the catalysts. The hydrogen generation ability decreased in the order Rh6-?I -I
E
T NM I
R 5
R LD-
N-
m-
cis-RhlcycLomlCL2
0
5
I 40
I
15
I
I
20
25 time/h
Fig. 1. Hydrogen generation by various Rh compounds (1:5) system in a long-term photolysis.
in the Ru(bipy)s+2/TEOA:DMF
300
> RhC13 > Rh4(CO)12 > HRh(C0)2(PPh3)3 > > PW&MC%o cis-Rh(cyclam)Cl, (Fig. 1.). In addition, a qualitative experiment was done using the mixed metal cluster, CosRh( CO) 12, as a catalyst precursor. This compound was less active than the monometallic Rh carbonyls. In the Co3Rh(CO)i2 experiment, a small amount of carbon monoxide was detected due to the decomposition of the catalyst. In the same manner [ Rh(CO),Cl] 2 was qualitatively tested in a short-term photolysis under CO2 atmosphere. Its hydrogen generation ability was similar to that of the other monometallic Rh carbonyl catalysts tested in vacuum conditions. (co),,
Composition of the catalyst precursors and effect of the solubility
Although there are reports [6] that the metal carbonyl compounds are photolabile against ligand dissociation and metal-metal bond breaking, it was observed that no carbon monoxide was generated during photolysis when monometallic Rh carbonyls were used as catalyst precursors. The chemical composition of the active carbonyl catalysts used was difficult to determine because of their low concentrations. For example, the IR spectra of 1.66 X 10e3 M (PPN)2Rh,2(C0)30 solutions in DMF and TEOA:DMF (1: 5) recorded after 2 h photolysis showed in the metal carbonyl region several absorption bands, the assignment of which turned out to be difficult. In the vacuum experiments, a small amount of carbon dioxide was detected in the gas phase which may have been formed by the oxidation of a carbonyl ligand or decomposition of organic material. The solubility of the catalyst precursors affected the hydrogen generation rate. If the catalyst was not fully dissolved, the rate was lower than that with a dissolved catalyst. Only Rh6(CO)16 has a poor solubility in the dimethylformamide solvent; it was dissolved by refluxing in DMF for 1 h. Some carbon monoxide was detected in the gas phase after solvation, probably due to the dissociation of carbonyl ligands from Rh6 cluster. Nevertheless, the best Rh CatdySt was Rh6(CO)i6, which operated more efficiently than the other Rh compounds studied. Long term stability and temperature effects
The stability of the Ru(bipy)3’2/Rh catalyst/TEOA:DMF system was examined in a long-term photolysis experiment. The hydrogen production rate was observed to decrease as a function of time, and cease after 10 h of photolysis. When a fresh amount of photosensitizer was added, the hydrogen production rate rose to the initial level (Fig. 2.). This suggests that the main reason for the instability of the present system is decomposition of the photosensitizer. The reaction temperature had a moderate effect on the decomposition of Ru(bipy)3+2. The stability of the photosensitizer decreased when the temperature was raised from 30 to 50 “C. These results conform with the earlier observations concerning the photolability of the Ru(bipy)3’2 in organic medium [ 8, 91.
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0
I
0
IO
I
20
I
30
I
40
I
50
1
60
I
10
I
60
I
90
1
100
Tcme/h
Fig. 2. Hydrogen generation in the Ru(bipy)3+2/Rh6(C0)16/TEOA:DMF (15) system. After 48 h photolysis 1 ml of Ha0 and after 68 h photolysis fresh Ru(bipy)$lz were added.
Solvent effects Addition of water to the solvent system caused a linear decrease in the rate of the hydrogen production (Fig. 3). A similar effect was observed if the electron donor/solvent system was diluted with water. The hydrogen generation rate was insensitive to the concentration of the electron donor. The observed rate was identical at donor/solvent ratios of 1: 2 and 1:5. Most of the experiments were performed with an electron donor/solvent ratio of 1:5. However, the hydrogen ion activity of the solution had a remarkable effect on the hydrogen generation. If the ‘pH’ of the TEOA: DMF solution (1:5) was adjusted to 7, the hydrogen production rate was l/15 of the rate in the pure TEOA:DMF mixture (1:5, ‘pH 11’) when using (PPN)2Rh1,(C0)s0 as a catalyst. In this experiment, precipitation of the carbonyl catalyst was observed (Table 1). The Rh compounds were also tested in other solvent systems (Fig. 4). Ascorbic acid-sodium ascorbate-water solution was used as an electron donor and a buffer [7b] when Rh6(C0)16, (PPN),Rh12(C0)s,, HRh(CO)*(PPh&, RhC13 and cis-Rh(cyclam)Cl, were tested as catalysts. Only cis-Rh-
302
0
I
0
10
I
20
I
30
,
I
50
40
H20/
CotoC
vol_x
Fig. 3. The effect of dimethylformamide substitution by water on the hydrogen generation in Ru(bipy)3+2/(PPN)2Rh12iC0)30/TEOA:DMF (15) system in 2.5 h photolysis (PPN = bis(triphenylphosphoranylidene)ammonium cation).
(cyclam)Cl, and Rh,(CO)i6 generated a small amount of hydrogen. The slow hydrogen generation in aqueous solutions may be due to the different solute-solvent effects in water compared to aprotic organic medium, which change the electron-transfer kinetics of the photosensitizer-solvent system and cause side- and back-reactions [lld], or the pH of the ascorbic acidascorbate solution is undesirable for efficient hydrogen generation by Rh compounds (Table 1). The poor solubility of the carbonyl catalysts in aqueous solutions also has a considerable effect on the hydrogen generation. The Rh compounds were also tested using sulphide ion as electron donor [7a] and changing the composition of the solvent. The best results were obtained using DMF as a solvent. In aqueous solution no hydrogen was generated. Both the photosensitizer and the catalyst decomposed when Szwas used as an electron donor. This may be due to the reactions of the catalyst precursors and the sensitizer with the S2- ion. The UV-Vis spectra of the resulting mixtures supported this assumption. The source of hydrogen and possible reaction mechanisms 13C NMR spectroscopy was used for the determination of the decomposition products of the electron donor TEOA. In long-term photolysis using an NMR tube as the reaction vessel, triethanolamine was found to decompose to diethanolamine and glycolaldehyde. This indicates that water must have been present as an impurity in TEOA (Pro Analysis grade was used
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^ r-I
, TEOWDHF
E ;: l?W
VI-
P-
,H~O/ASCORBIC
ACID
TIME/(h)
Fig. 4. Hydrogen generation by Rhs(C!O) 16 in various solvent systems with Ru(bipy)J’* as a photosensitizer (2 ml 0.3 M Na2S:Hz0/28 ml DMF; TEOA:DMF 1:5 and ascorbic acid-sodium ascorbate (both 0.5 M) in HzO).
without further purification). The net light-driven reaction is probably the same as found earlier [7a], the catalysts in this study being Ru( bipy)3’2 and Rh compounds: N(CH2CH20H)3 + H20 p
hv + catalysts
HN( CH,CH,OH), + CHO-CH20H + Hz
UV-Vis spectra of the reaction solutions were recorded after photolysis. Absorption bands of Ru(bipy), in the oxidation states I and II were usually found [ll]. In some experiments, bands of Ru(bipy),LL’ (LL’ = solvent molecule or an anion) were observed [lo]. According to Vis spectra, the possible reaction mechanism of the photosensitizer Ru(bipy)3’2 involves the reductive quenching of the excited triplet state. The existence of Ru(bipy)3+ was also found by diluting the reaction solution with water after the reaction. A change in the colour of the solution from red-orange to yellow was observed due to oxidation of the Ru(bipy)3+ to Ru(bipy)3+2. This behaviour has been noted earlier [ llc]. Decomposition of Ru( bipy)3+2 to Ru( bipy)2 LL’ was observed to occur in long-term photolysis of the Rh catalyst solutions and in short-term photolysis of the tested reference catalyst compounds. The formation of Ru(bipy)*LL’ caused broadening of the bands in the Vis spectra.
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Comparative experiments Comparative experiments were performed in order to relate the hydrogen generation efficiency of the present system to the previously studied ones. Using cobaloxime [7a] as a catalyst with a carbon dioxide gas phase in TEOA:DMF solvent, a 10 ml h-’ hydrogen production rate was obtained in our experimental conditions. This refers to 7 - 8% quantum yields when the monometallic Rh carbonyls are used as catalyst precursors, since the quantum yield of hydrogen by the cobaloxime catalyst has been noted to be 13% [7a] (Table 1). When using the cobalt dimethylglyoxime complex as a catalyst in an ascorbic acid-ascorbate medium, it was observed that at least a part of the hydrogen generated came from decomposition of the dimethylglyoxime ligand. This phenomenon was noted in long-term photolysis (Fig. 5). Using the macrocyclic complex Cu(cyclam)Cl, as a catalyst, we examined the effect of water on the activity of the TEOA:DMF solvent system. The results were similar to the Rh experiments (Table 1).
T Lme/h Fig. 5. Hydrogen generation in a long-term photolysis of the Ru(bipy)3+*/Co(dmgH2)/ ascorbic acid-sodium ascorbate system. After 2.5 h and 22 h photolysis, fresh dmgH2 was added (dmgH2 = dimethylglyoxime).
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Conclusions The system described in this work achieves efficient photochemical hydrogen generation by the homogeneous rhodium catalysts. The reaction mechanisms are difficult to detail, due to the complex nature of the catalysis system. The hydrogen generation probably involves formation of Rh hydride complexes, as with the other catalysts described in the literature. The practical difficulties of the present system are the use of a sacrificial organic electron donor and a weak long-term stability. Nevertheless, use of an organic electron donor-solvent system for photochemical hydrogen generation serves as a rapid method for measuring the maximum hydrogen generation rates of possible catalysts for artificial water-splitting.
Experimental Ru(bipy)&*5H20 and macrocyclic complexes were synthesised according to literature methods [12, 131. Cyclam and 2,2’-bipyridine were commercially available compounds (Strem and Aldrich). Solvents and triethanolamine were used without further purification. Reagent grade NazS.9Hz0, CoCl,, dmgHz and Pro Analysis grade ascorbic acid and sodium ascorbate were used. All these compounds were stored in air. The Rh compounds were commercially available (Aldrich, Johnson-Matthey), except DWCOWll,, RMCOh, Co$WW,z and (PPN)2Rh12(C0)30 which were synthesised by literature methods [14,15]. The Rh compounds, except RhC13, HRh( CO) *(PPh3)3 and Rh& CO) 16, were stored under nitrogen. Irradiation experiments were performed on a 30 ml solution contained in a round-bottom flask equipped with a Teflon stopcock (115 ml capacity) and stirred magnetically at 500 - 1000 r.p.m. The initial concentration of the photosensitizer in the TEOA:DMF experiments was 10 mg/0.0136 mmol and in the other experiments 20 mg/0.0272 mmol. The concentration of the catalysts varied, and usually was chosen so that the Rh-atom/Ru-atom ratio was 1: 1. The concentration of Co(dmgH,) was the same as that used in the literature experiments [7a]. In the Cu(cyclam)Cl, studies, the concentrations were similar to those in the Rh experiments. The reactor was evacuated before the experiment using a pump-freezethaw cycle. The coolant was liquid nitrogen and the evacuation was performed for about 2 min. A measured amount of carbon dioxide was added to the reactor. The reactor was re-evacuated when a fresh amount of photosensitizer or ligand was added to the reaction solution during the long-term experiments. Three light sources were employed: a 250 W halogen lamp in a slideprojector, fitted with a 400 nm cutoff filter (SRI Filter), and two 1000 W halogen lamps equipped with 400 nm cut-off filters and self-made infrared water filters (60 cm length). Different reaction temperatures were produced by the IR radiation of the light sources. The temperatures achieved with the
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lamps were: 30 “C by the 250 W lamp, 35 - 40 “C by the 1000 W lamp with 11 cm diameter IR filter and 45 - 50 “C by the 1000 W lamp with 14 cm diameter IR filter. The 62.9 MHz 13CNMR spectra were recorded on a Bruker AM 250 FT spectrometer and UV-Vis spectra on a Beckman Model 25 spectrophotometer. IR spectra were recorded on a Perkin-Elmer 297 IR spectrometer using 0.5 mm NaCl solution cells. The experiments where a NMR tube was used as a reactor were carried out in a manner similar to the flask experiments. The qualitative UV-Vis spectra of the reaction solutions were obtained by diluting the solutions after experiment by DMF or water before measuring the UV-Vis spectra. TEOA:DMF, TEOA:DMF/H,O or DMF/H*O was used as reference. After irradiation, the gas phase of the reactor was analysed by a Carlo Erba 4200 multicolumn gas chromatograph attached to a manometer/ vacuum pump system and equipped with a sample loop, a column switching valve, and a thermal conductivity detector. The light components of the gas phase, H2 and CO, were separated on a Molecular Sieve 5 A column and the heavy component, COz, on a Chromosorb 102 column. The carrier gas was 99.995% helium. The gas chromatograph was calibrated with the measured gases (H,, CO, COz) and the amounts of the gases were calculated by a computer program. The error limits of the gas phase analysis are within + 10%. Acknowledgement We thank the Neste Ltd Foundation
for financial support.
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