Rotating Photoelectrode for Electrochemical Study of the Products of Photochemical Reactions photolytic reactions. Our simulation programs are nearly completed. We report here the design of the apparatus and results of some preliminary experiments which verify the potential utility of the photoelectrode.
SIR: Electroanalytical techniques can be useful for kinetic studies of photolytically induced reactions in liquid solvents when the product of the photochemical reaction is electroactive. Berg (1-4) and Pitts ( 5 ) employed conventional voltammetric techniques for the detection of products and unstable intermediates of photochemical reactions. Perone and Birk (6,7)described a time-delayed, potentiostatic technique using a hanging, mercury-drop electrode for determining the rate of chemical reactions following flash photolysis of solution in the vicinity of the electrode. These authors encountered several difficulties in mathematically interpreting their results. First, the solution near the surface of the electrode was not fully illuminated since the drop shadowed the side opposite the flash lamp. Second, the intensity-profile of the light beam was difficult to describe mathematically since the light was incident on the solution at a finite distance from the electrode and the absorption path-length was variable because of the curvature of the drop. As a result, the concentration of photoproduct at the surface of the electrode was not uniform and an accurate description of the concentration profile was not possible. We have modified the design of the rotating ring-disk electrode (RRDE) to make it applicable to studies of photochemical reactions. We believe the profiles of concentration and light intensity can easily and accurately be described at this electrode configuration. The inert-metal disk electrode was replaced with an optically transparent disk. In use, a collimated beam of light is passed through the transparent disk of the rotating photoelectrode. Products of photochemical reactions occurring in the vicinity of the disk are transported by convective-diffusional processes to the surface of the ring electrode where they can undergo electrochemical reaction. Albery and Bruckenstein in a series of papers (8) solved the equation of convective diffusion for the RRDE under conditions of pseudo-first and second order kinetics. It is unlikely that their results are applicable to the photoelectrode since for usual concentrations and molar absorptivities of absorbing species (0.1M and 104M-' cm-I) the light beam will penetrate the solution to a distance much greater than the diffusion layer at the RRDE. Therefore, the usual assumption that only the first terms of the series expansions describing the axial and radial components of fluid flow are needed in solving the equation of convective diffusion is not valid. Prater and Bard ( 9 ) used digital simulation of the mass transport processes at the RRDE. Their results are in agreement with those of Albery and Bruckenstein. The success of the simulation procedure led us to be optimistic that the current in the ring electrode of the rotating photoelectrode can be quantitatively related to the rate of chemical reactions involving products of (1) (2) (3) (4) (5)
EXPERIMENTAL
H. Berg, Collect. Czech. Chem. Cammwz., 25, 3404 (1960). H. Berg and H. Schweiss, Nature, 191, 1270 (1961). H. Berg and H. Schweiss, Electrochim. Acra, 9,425 (1964). H. Berg, Z . A m / . Chem., 216, 165 (1966). J. N. Pitts, Jr., H. W. Johnson, Jr., and T. Kuwana, J . Plzys.
Cliem., 66,2456(1962). (6) S . P. Perone and J. R. Birk, ANAL.CHEM., 38, 1589 (1966). (7) J. R. Birk and S . P. Perone, ibid.,40,496 (1968). (8) W. J. Albery and Stanley Bruckenstein, Trans. Faraday SOC., 62,1915,1920,1932,1938,1946,2584,2596(1966). (9) K. B. Prater and A. J. Bard, J. Electrocliem. SOC.,117, 207, 335, 1517 (1970).
Instrumentation. The photoelectrode and variable speed rotator used in this study were constructed by Pine Instrument Company of Grove City, Pa. 16127. Figure 1 contains schematic diagrams showing the design of the photoelectrode. In constructing the photoelectrode, one end of a stainless steel cylinder was machined so it was compatible with the chucking mechanism of the variable speed rotator. A platinum annulus was silver-soldered to the opposite end of the cylinder. The inner and outer surfaces of the annulus and cylinder were machined to the dimension given in Figure 1. A quartz rod 2 cm long with a diameter slightly less than the inside diameter of the annulus was inserted into a thin cylinder of Teflon (Du Pont). The Teflon-covered quartz was then pressed into the stainless steel cylinder until the end surface of the quartz rod was in the same plane as the flat surface of the platinum annulus. The quartz was Suprasil from Amersil Inc. of Hillside, N.J. 07205. A shroud of Teflon was placed about the outer surface of the cylinder and machined so the end surface was in the plane of the quartz rod and platinum annulus. The end surfaces were then polished using mechanical procedures with the final step using Buehler 1.O-micron diamond on nylon lubricated by Buehler METADI fluid. The photoelectrode was rotated about an axis perpendicular to and passing through the center of the quartz disk. A collimated beam of light parallel to and symmetrical about the axis of rotation was passed through the quartz disk of the photoelectrode. A schematic diagram of the optical system is shown in Figure 2 and Figure 3 is a photograph of the apparatus without the photoelectrolysis cell and the necessary electronics. The light source was an Osram, high pressure, 500-watt mercury arc (Model HBO-500 w/2) powered by a dc power supply from George W. Gates and Co. (Model P-520-DV). The lamp was enclosed in a housing from Oriel Optics Corp. (Model C-60-51). The lamp housing had a f U . 5 , 1.375inch diameter, collimating lens (C-60-31) made of fused silica which was mounted in a focusing sleeve and an aluminized, spherical, rear reflector mirror (C-60-50-04) with a 2-inch diameter and a 2-inch radius of curvature. A filter holder (C-61-36) and a 90" light tube (C-61-35) were used which were also from Oriel Optics Corp. The light tube had an adjustable, aluminized, plane mirror. The transmission filter used for isolating the desired region of the electromagnetic spectrum was from Corning Glass Works (6-56-20). It had a maximum transmission at 360 nm and a bandpass of about 50 nm. A flat piece of metal served as a shutter when placed in the filter holder. Exposure times were less than 1 minute and were followed by a period during which the filter was allowed to cool. The potentiostat was constructed according to the conventional design (10). A Hewlett-Packard x-y recorder (Model 7035B) was used to record current-potential data. The design of the all-glass cell was described (11) previously. The
(10) W. L. IJnderkofler and Irving Shain, ANAL.CHEM.,35, 1778 (1963). (11) D. C. Johnson, Ph.D. Dissertation, University of Minnesota, Minneapolis, Minn., 1967.
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M O D E L D P PHOTO
ELECTRODE
Stainless Steel
Quartz Window
CUT AWAY VIEW OF DP PHOTO E L E C T R O D E Figure 1. Schematic diagram of the photoelectrode
I l l
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spherical
collimating
transmission
mirror
lens
filter
pulleys
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mercury-arc lamp
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photoapprox. 5 crn
electrode
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Figure 2. Schematic diagram of the optical system associated with use of the photoelectrode 638
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