J. Phys. Chem. 1981, 85, 1627-1628
1627
Photochemical Electron Storage on Colloidal Metals and Hydrogen Formation by Free Radicals A. Hengleln," B. Llndlg, and J. Westerhausen Hahn-Mehner-Institut fur Kernforschung Berlin GmbH, Bereich Strahienchemie, D- 1000 Ber//n 39, West Germany (Received: March 3 1, 198 1)
Photolyticallyproduced 1-hydroxy-1-methylethylradicals were found to form hydrogen in the presence of colloidal platinum, gold, silver, and cadmium. Conductivity measurements showed that electrons stored on the colloidal M silver solution was found to store electrons up particles are intermediates to H2 formation. A 1.25 X to more than 0.3 C/L during illumination.
1-Hydroxy-1-methylethyl and some other short-lived free radicals have recently been shown to form hydrogen in the presence of colloidal silver in aqueous solution: 2(CH3)&OH
-
2(CH3)2CO + H2
(1)
The radicals were radiolytically produced. We report here that reaction 1can as well be initiated with 253.7-nm light from a low-pressure mercury lamp. The absorption of the light occurs via dissolved acetone (0.01-0.02 M) and the radicals are formed via the well-known reaction of the triplet state of acetone (CH3)2CO*
+ (CH3)ZCHOH
-
2(CH3)2COH
(2)
propanol-2 being present at 1M. Experiments were also carried out with solutions of colloidal gold, platinum, and cadmium. The radiolytic preparation of the silver, gold, and cadmium sols has been previously de~cribed.l-~The platinum sol was prepared by reduction of H&C& by citric acid as described by Bonds5 Absorption spectra for each of the colloidal solutions are shown in Figure 1. After addition of acetone to these solutions, approximately 30% of the extinguished light was absorbed by acetone in the silver, gold, and cadmium solutions. In the case of the platinum sol, the acetone accounted for 80% of the total light absorption. Actinometry was performed by optical measurement of the 365-nm product resulting from illumination of an aqueous pyridine solution.6 Quantum yields of hydrogen formation were determined for illumination intensities between 5.5 X lo4 and 8.7 X einstein/(L s). An intensity effect of diminishing yield at high light intensity was observed for all colloidal solutions except platinum. Platinum and gold solutions both exhibited unit efficiency for H2 production (one molecule of H2 produced per quantum absorbed by acetone). The silver and cadmium sols produced H2 with somewhat lower efficiency, @ = 0.7 and 0.4, respectively. The quantum yield did not vary significantly for illumination periods between 4 and 16 h. The high catalytic activities of these metal colloids observed in the photochemical experiments parallel the high G values observed in the y-radiolysis e~periments.l-~ In particular, it should be noted that the platinum sol cata(1) Henglein, A. J.Phys. Chem. 1979, 83, 2209-16. (2) Henglein, A.; Westerhausen, J.; Lilie, J. Ber. Bunsenges. Phys. Chem. 1981,85, 182-9. (3) Henglein, A,; Lilie, J. J.Phys. Chem. 1981, 85, 1246. (4) Henglein, A.;Lilie, J. J. Am. Chem. SOC.1981, 103, 1059-1066. (5) Bond, G. C. Trans. Faraday SOC.1956,52, 1235. (6)Hart, E.J., private communication. 0022-365418 1/2085-1627$0 1.2510
lyses complete conversion of the radicals into hydrogen at M. the relatively low concentration of 1 X In order to extend the analogy between the photochemical and radiation chemical experiments, we measured the electrical conductivity of the solutions during illumination. It has recently been shown, in the case of silver and gold solution^?^ that the increase in conductivity due to H+ ions can be observed when the radicals charge the metal particles by the transfer of electrons:
+
(CH3)2COH M,"-
-
(CH3)2C0+ H+
+ M,(mtl)-
(3)
where n is the agglomeration number and m the number of excess electrons already picked up. After a period of seconds or minutes a stationary H+ concentration is reached, due to the simultaneous discharge of the colloidal particles through the cathodic reduction of water:
M,(mtl)- + H20
-
M,"-
OH- + H+
-
+ 1/2H2+ OH-
(4)
HzO
(5)
After cessation of radical production, the discharge reactions 4 and 5 persist and a decrease in conductivity is observed. Figure 2 shows the temporal changes in the conductivity of a silver solution illuminated at various intensities. As expected, these curves are of the same form as those observed during irradiation with electrons. The charging rate J and the stored charge in the stationary state Q, have been shown to be governed by the relation4 R TC (6) Qs = (YF In J - constant where F is the Faraday constant; C the electrical capacity per liter of the colloid; and a the transfer coefficient of the cathodic reduction of water. The inset of Figure 2 shows a plot of Qs vs. J on a semilogarithmic scale. Assuming a = 0.5, the slope of the straight line gives a capacity C = 0.3 F/L for the storage of excess electrons. It is thus demonstrated that an appreciable amount of electrons can be stored on the colloidal silver particles as a result of a photochemical reaction. The conductometric behavior of gold and cadmium solutions under illumination was also qualitatively the same as observed in the radiolytic experiment^.^^^ The stationary charge Qs in a gold sol is smaller than in a silver sol under identical conditions (equal rates of charging J, equal size of particles). This is understood in terms of a more efficient conversion of stored electrons into absorbed hydrogen atoms at the gold particles. In the case of cadmium solutions, the conductometric signals were much higher than 0 198 1 American Chemical Society
J. Phys. Chem. 1981, 85, 1628-1636
1628
1
1.0 p
I
-200
LOO
300
500
I
600
A [nml
Flgure 1. Optical absorption spectra of various metal colloids. The collolds of Au, Ag, and Cd were prepared by radiolytic reduction of the correspondlng metal ions in the presence of a stabllizer (1 X IO-' basaM sodium polyvinyl sulfate (mol wt = 65 000) for silver and 3 X lo4 base-M for cadmium, 1 X I O 3 base-M polyvlnyl alcohol (mol wt = 60000) for gold. The cadmium solutlon contained 3 X M sodium formate, and the platinum solution contained 4 X lo4 M HCI.
0
1.0 t [rninl
2 .o
Flgure 2. Conductivity increase as a function of time of illumination of a solution containing 1.25 X lo4 M colloidal silver, 1 X I O 4 base-M sodium polyvinyl sulfate, 2 X lo-' M acetone, and 1 M propanol-2. The conductivity increase is caused by the proton-anion pairs formed in reaction 3. I t is expressed as stored charges in C/L. The various curves were obtained at different light intensities or rates of radical formation J. Upper right: Stationary charge Q s plotted vs. the rate of charging J. J was obtained from the initial slope of the curves.
for the silver sols. Residual Cd2+ions are first reduced and deposited on the colloidal particles before excess electrons are stored. No conductivity signal was observed in the case of a Pt sol from which the buffering citrate had been removed with Amberlite. This indicates that the conversion of transferred electrons into adsorbed hydrogen atoms is practically complete and very fast at the Pt particles. The catalysis of Hzformation by photochemically produced free radicals has also been observed for longer-lived radicals such as half-reduced methyl ~ i o l o g e n and ' ~ ~ ben-
z ~ p h e n o n e . ~The present studies show that even the reaction of very short-lived radicals can be catalyzed before the radicals have a chance to deactivate each other and that interesting pool effects may occur. It should be possible to use such electron pools for the initiation of novel photoinduced multielectron reductions as has already been demonstrated in radiation chemical investigations.lOJ1
(7)Lehn, J. M.; Sauvage, J. P. Nouu. J. Chim. 1977,1, 449-51. (8)Meisel, D.;Mulac, W. A.; Matheson, M. S. J.Phys. Chem. 1981, 85,179-87.
(9)Gratzel, C.K.;Gratzel, M. J.Am. Chem. SOC.1979,101,7741-43. (10)Henglein, A. J. Phys. Chem. 1979,83,2858-62. (11)Henglein, A. Ber. Bunsenges. Phys. Chem. 1980,84,253-9.
FEATURE ARTICLE Electron Spin Echo Studies of Solvation Structure Larry Kevan Department of Chemistry, UniversitjJ of Houston, Houston, Texas 77004 (Received: March 24, 198 1)
The development and application of electron spin echo modulation analysis for deducing detailed geometrical information about the solvation shell structure of paramagnetic species is described. This technique makes it possible to measure weaker hyperfine interactions in disordered systems such as frozen solutions and surfaces than is normally possible. The number, distance, and orientation of first solvation shell molecules can then be determined. Applicationsof this technique to silver atom solvation, electron solvation, anion solvation, cation solvation, and cation solvation on surfaces are described.
Introduction The solvation shell or solvation structure of an ion or molecule in the condensed phase is of prime importance for understanding all sorts of chemistry in condensed media. However, little quantitative information about structure especially for reactive intermediates, because there are few techniques. The primary techniques used have been X-ray 0022-3654/81/2085-1628$01.25/0
diffraction,l neutron diffractionl2v3and extended X-ray absorption fine structure (EXAFS).44 These techniques (1) A. Habenschuss and F. H. Spedding, J.Chem. Phys., 70,2797,3758 (1979). (2)(a) R.A. Howe, W. S. Howells, and J. E. Enderby, J. Phys. C, 7, Llll (1974);(b) A. K, Soper, G, W.Neilson, J. E. Enderby, and R,A. Howe, ibid., 10,1793(1977);(c) G. W.Neilson and J. E. Enderby, ibid., 11, L625 (1978).
0 1981 Amerlcan Chemical Society
