J. P h p . Chem. 1903, 8 7 , 3807-3808
TABLE 11: Vertical Emission Wavelengths ( A ) between Excited Quartet States of H,a 3pou4x;
uPPe; limit 3dn,
best estimate‘
lower limitd
4nu 6790 6165 6099 18805 16832 16349
3dog 4xu+ 3d6, 4 A u 4sag 4zu+ 4dng 4nu 4dog ‘xu+ 4d6, ‘Au 5sag 4zu+
-
best esti- lower uppe; mate‘ limitd limit
6509 6284 5839 4899 4819 4777 4645 3627
5958 5712 5376 4544 4481 4430 4326 3420
5871 5687 5320 4529 4460 4424 4310 3420
16794
15372
14808
12959 9089 8818
12019 8528 8308
11743 8474 8238
8250
7190
7140
(limit) 5505 5214 5274 a Geometry of H,+X 3 x U t :D,, with R H H = 2.457 a,. Neglecting electron correlation in both the upper and the lower states. ‘ See Table I. Neglecting electron correlation in the upper Rydberg state.
relative to the bond length of H3+as calculated with high precision.s The results are collected in Table I, where vertical electron binding energies for the Rydberg electron are given. In order to obtain total energies they have to be added to the total electronic energy of triplet H3+(-1.11568 a d ) . For the determination of the limits and energies of dissociation we have used the tables published by Huber
3007
and Herzberg.15 Note that the lowest and the third lowest quartet states (both 42,+)are dissociative. Relaxation of the second (411,) and fourth (42,+)states to their equilibrium geometry (see Table I) yields additional stabilization energies of 0.000 72 and 0.000 19 au, respectively. In Table I1 we give estimates for vertical emission wavelengths of electronic transitions to the two lowest bound quartet states. Since as a rule the contribution of electron correlation decreases with increasing excitation level,14upper and lower limits for the emission wavelengths can be estimated, neglecting the correlation energy either in both the upper and the lower states or only in the upper state. We observe that the strong transitions are situated in the infrared whereas higher members of the Rydberg series might be detected in the visible spectrum. Acknowledgment. This work is part of the project Nr. 2.420-0.82 of the Schweizerischer Nationalfonds. We thank the CIBA-Stiftung (Basel) for financial support. K. K. gratefully acknowledges a scholarship from the funds of the chemical industries. We thank Dr. J. K. G. Watson (Ottawa) for drawing our attention to this subject. Registry No. H,,12184-91-7. (15) K.P. Huber and G. Herzberg, “Molecular Spectra and Molecular Structure, IV. Constants of Diatomic Molecules”, Van Nostrand-Reinhold, Princeton, NJ, 1979.
Photocatalytic Hydrogen Production from Solutions of Sulfite Using Platinized Cadmium Sulfide Powder Mlchlo Matsumura, Yuklnarl Saho, and Hlroshl Tsubomura Department of Chemistry, Facub of Engineering Science, Osaka University, Toyonaka. Osaka 560, Japan (Received: July 14, 1983)
Platinized cadmium sulfide powder suspended in a solution of sodium sulfite produces hydrogen efficiently by visible light. Sulfite ions are oxidized to sulfate and dithionate ions.
Introduction Efficient production of hydrogen from water by use of solar energy is one of the most attractive targets for the research of photocatalytic reactions on semiconductors particles. Platinized titanium dioxide (TiOJ has been reported to produce hydrogen from aqueous solutions containing organic materials, such as methanol,’ solid carbon,2etc.,3 and even from water itself though with a low yield.4 Semiconductors having smaller band gaps are more important for solar energy utilization, since TiOp can absorb a very small part of the whole solar radiation. In a previous paper5 we found that methanol, formaldehyde, and formic acid were photocatalytically oxidized on platinized CdS. Hydrogen production was also reported to occur photocatalytically on platinized CdS with the sa(1) Kawai, T.; Sakata, T. J . Chem. Soc., Chem. Commun. 1980,694. ( 2 ) Kawai, T.; Sakata, T. J. Chem. SOC.,Chem. Commun. 1979,1047. (3) Kawai, T.; Sakata, T. Chem. Lett. 1981, 81. (4) Sato, S.;White, J. M. Chem. Phys. Lett. 1980, 72, 83. ( 5 ) Matsumura, M.; Hiramoto, M.; Iehara, T.; Tsubomura, H., submitted to J . Phys. Chem.
crificial oxidation of EDTA,6 cystein: ethan01,~etc. We report here photocatalytic hydrogen production from an aqueous solution of sulfite on platinized CdS. Recently Reber et a1.8 also worked independently on the reactions of sulfite on platinized CdS powder.
Experimental Section In our experiment, the photocatalyst was prepared by mixing and grinding 250 mg of CdS powder (99.999% pure, average size ca. 0.5 km) and 10 mg of platinum powder in an agate mortar. The photocatalyst was added to a 10-mL aqueous solution of sodium sulfite in a glass flask. Then, the solution was deaerated by repeated freeze-pump-thaw cycles. The solution was illuminated by a 500-W Hg lamp combined with a UV cutoff filter (Toshiba, L-39) whose transmittance at 390 nm is 50%. The amount of gas produced in the flask was measured by introducing it into ~
~~
~
(6) Darwent, J. R. J . Chem. Soc., Faraday Trans. 2 1981, 77,1703. Kawai, T. J . Syn. Org. Chem. Jpn. 1981, 39, 589. (7) Sakata, T.;
(8) Reber, J.-F.; Meier, K.; B a l e r , N. Int. Conf. Photochem. Conuers. Storage Sol. Energy, 4th, 1982.
0022-3654/83/2087-3807$01.50/00 1983 American Chemical Society
3808
The Journal of Physical Chemistry, Vol. 87, No. 20, 1983 0.8
1
Letters
i
i I mAcm-'
i
-,
0.1
/' -
,'
-1.0
0 10
6
14
'I I
PH
I I
Figure 1. pH dependence of the amount of hydrogen produced by
illumination for 100 min from the buffered solutions containing 0.5 M Na,SO, and platinized CdS powder.
a vacuum line provided with an oil manometer after the solution was frozen with liquid nitrogen. The mass spectra obtained with an Anelva AGA-100 mass spectrometer showed that the gas produced in the flask by the illumination was hydrogen and a trace of water.
Results and Discussion The products formed in the solution of 0.1 M (mol dm-3) sodium sulfite, which was buffered at pH 8.8 by the addition of boric acid and sodium hydroxide, turned out to be sulfate and dithionate ions with a molar ratio of ca. 4:3. They were quantitatively analyzed as follows: The amount of sulfite which remained in the illuminated solution was determined by titrating a part of the solution (3 mL) with a standardized iodine solution. The total amount of sulfite and sulfate existing in the solution was determined by gravimetric analysis of barium sulfate precipitated by the addition of an aqueous solution of barium chloride to the solution in which iodine had been added. The filtrate was evaporated and the residue was heated to 400 "C so as to decompose the dithionate into sulfate and sulfur dioxide. The amount of sulfate thus produced was analyzed gravimetrically in the form of barium sulfate. The amounts of sulfate and dithionate thus determined were consistent with the decrease of sulfite in the illuminated solution. The reactions occurring at pH higher than 8 can be summarized as follows: CdS + hu e (electron) + h (hole) (1)
-
+ 2e + 2h
2H+
SO3*-+ H,O
2S03*-+ 2h
-
H2
(2)
SO4,- + 2H+
(3)
S2062-
(4)
Conceivably, the hydrogen production (reaction 2) proceeds mostly on platinum, while the oxidation reactions (3 and 4) proceed on the surface of CdS. The rate of hydrogen production decreases slowly with time. This is probably due to the decrease of the concentration of the sulfite ion and the change in pH of the solution. The quantity of hydrogen produced after the illumination for 100 min was measured in buffered solutions of 0.5 M sodium sulfite a t various pH. It showed a maximum a t pH 8 as shown in Figure 1. When CdS powder without platinum was used, the rate of hydrogen production was ca. 1/20 that observed for platinized CdS. Without sulfite in a solution of pH higher than 8 the hydrogen production was negligible. The quantum efficiency of hydrogen production from 0.5 M sodium sulfite (pH 8) was determined to be 8.2% for illumination at a wavelength of 436 nm using a monochromater. The rate of hydrogen production from the same solution under illumination with a solar simulator (AM 1, 100 mW em-? was 0.090 mol m-2 h-l.
I I
I
I
-0.5
U I V vs SCE
- -0.1 --0.2
I
Flgure 2. Current-potential curves of a CdS single crystal electrode measured under illumination in 0.5 M Na,SO, (-) and in the solution without Na,SO, (---), and the current-potential curve of a Pt electrode measured in 0.5 M Na,SO, (- - -). The solutions are buffered at pH 8.2 by the addition of boric acid and sodium hydroxide.
In order to test the durability of the photocatalyst, a solution containing 0.5 M sodium sulfite (pH 8.5) and the photocatalyst was illuminated for 65 h by a 500-W Hg lamp combined with a glass filter (Toshiba, L-391, during which the solution was renewed 9 times so as to maintain the concentration of the sulfite ion and the pH of the solution. The amount of hydrogen produced, 0.050 mol (1.12 L), was 29 times that of CdS added in the solution. Neither weight loss of the photocatalyst nor change of the photocatalytic activity was observed. To clarify the reason for the pH dependence of the rate of hydrogen production depicted in Figure 1,the reactivity of sulfite on a platinum electrode was investigated by using a rotating-rising-disk-electrode technique. It was found that hydrogen generation occurred exclusively on the platinum electrode at a pH higher than 8 and at lower pH the reduction of sulfite took place more easily than hydrogen production. The reduction of sulfite is therefore thought to be the reason for the decrease of hydrogen production by the photocatalyst in acidic solutions. The decrease of the rate of hydrogen production in highly alkaline solutions is explained by the relation between the energy level of the conduction band of CdS and the H+/H2 p ~ t e n t i a l .Namely, ~ as the pH increases, the position of the conduction band becomes less negative than the H+/H2 potential, leading to a decrease in the reactivity of the photocatalyst. The electrochemical properties of a cell composed of a CdS single crystal electrode, a platinum electrode, and an aqueous solution a pH 8.2 under illumination are shown in Figure 2. It is clear from the current-potential curves for the CdS and Pt electrodes that the photolytic hydrogen production and the oxidation of sulfite can occur efficiently in a cell where the platinum electrode and the CdS electrode are short-circuited. As the platinized semiconductor particle is regarded to act like a short-circuited-electrode ~ y s t e mthe , ~ current-potential curves conform to the high activity of the platinized CdS powder for the hydrogen production from aqueous solutions of sulfite. The present result suggests a promising way of solar energy utilization to convert sulfite to hydrogen, a useful energy source. Sulfite is easily produced from sulfur dioxide obtained in large amounts as undesirable industrial waste material. Registry No. Hydrogen, 1333-74-0;sodium sulfite, 7757-83-7; sodium sulfate, 7757-82-6 sodium dithionate, 7631-94-9; cadmium sulfide, 1306-23-6; platinum, 7440-06-4. (9) Izumi, I.; Fan, F. F.; Bard, A. J. J . Phys. Chem. 1981, 85, 218.
