Perone et al.
866
Flash Photoelectrochemical Studies of the Photoreduction of C0(NH3)2+ in the Presence of Benzophenone Betty S. Hall,
K. F. Dahnke,+ S. S.
Fratoni, Jr.,$ and S. P. Perone*
Department of Chemistry, Purdue University, West Lafayette, Indiana 47907 (Received November 24, 1975; Revised Manuscript Received February 14, 1977) Publication costs assisted by the National Science Foundation
A photoelectrochemical technique has been applied to the study of the photoreduction of Co(NH3):' in the presence of benzophenone. Currents due to the oxidation of benzophenone ketyl radical and the reduction of the Co(II1) complex were monitored after flash irradiation. In basic alcoholic solution the initial amount
of ketyl radical produced by the flash was significantly diminished in the presence of Co(II1) complex. Also, the aniount of Co(II1) complex seen after the flash was lowered substantially in the presence of benzophenone. The reactivity of Co(II1) and ketyl radical when both are present after a flash appears negligible, failing to account for the significant photoreduction observed immediately after the flash. These data suggest that the primary reaction pathway involves energy transfer from the excited benzophenone triplet to the Co(II1) complex with subsequent decomposition to the Co(I1) product. In acidic solution (pH 5.51, no significant sensitized photoreduction of the Co(1II) complex was observed.
Introduction then undergo internal electron rearrangement to form the Co(I1) product. Photosensitized reactions of coordination compounds All of the previous studies reported have involved the in solution have been studied extensively over the past few use of spectroscopic monitoring techniques. Thus, it was years. While most studies have involved the photothe objective of this work to apply the general techni ues chemistry of Co(1II) other systems studied of flash photoelectrochemistry developed previously to have included Fe(Hz0)2+,l6Cr(II1) and Fe(II1) oxalates,15 the study of Co(II1) photoreduction. It was hoped that PtC1;,6 and Cr(NHJ5XZf (X- = CNS-, Cl-),'l among this approach would provide a better insight into the others.22 The most common sensitizers which have been photoreduction process, particularly with respect to reused include biphen 1: b i a ~ e t y l ben~ophenone,~*~~-'~ ,~*~~~~~~ riboflavin," benzil, naphthalene,2J4 quinoline," transsolving the specific question of whether sensitization occurs by an energy transfer or a chemical mechanism when stilbene-4-carboxylicacid (TSC),l o-phenanthroline," and benzophenone is present in solution. Ru(bpy)?+ 6,9~12-14~1618(where bpy = 2,2'-bipyridine). Some of these s stems undergo sensitized photoaquation reExperimental Section actions,3.'zz1'26 while others undergo photoreduction Instrumentation. All of the instrumentation used in this ~ ~ " 'latter ~ ~ ~ systems ~ are most processes in s ~ l u t i o n . ~ ~These work has been reported el~ewhere.~'The flash apparatus, suitable for photoelectrochemical s t ~ d i e s . ~ ~ ~ ~ ~ optics, electrode and cell design, and data acquisition Photosensitized reduction of Co(II1) complexes has been system were as described in ref 30a; the potentiostat was reported with most of the sensitizers listed above. These as described in ref 30b. The system utilizes a 200-5, 15-p~ complexes include the hexamine,1i5310 aquo entamine,' xenon flash; the monitoring electrode is a hanging mercury a~idopentamine?'~"~ tris(o-phenanthroline),' EDTA,12-14 drop (HMDE); the reference electrode is a saturated bromopentamine," triso~alato,'~ and other species.22 (A calomel electrode (SCE); the potentiostat has a control notable exception is C O ( C N ) ~ - ? ,which ~ ' ~ ~ apparently ~ does unity-gain bandwidth of -900 kHz, with a monitoring not. undergo sensitized photoreduction.) There has been bandwidth of 100 kHz; and the minicomputer data acuncertainty, however, regarding the role of the sensitizer quisition system uses a 50-kHz maximum data rate.31 All in these reactions. For example, Vogler and Adamson' and software is written in Hewlett Packard Basic with funcScandola and Scandola5 interpretted photoreduction of tional subroutine ~ v e r l a y . ~ ' -Cell ~ ~ time constant meaCo(II1)-amine complexes sensitized by various organic surements were made as described compounds (benzophenone, benzil, biacetyl, biphenyl, and Experimental Procedures. The fundamental meaTSC) as involving energy transfer from the triplet state surement in a photoelectrochemical study is the curof the donor to a triplet charge-transfer state of the rent-time curve obtained after the flash at an applied complex. Gafney and Adamson" later added quinoline potential. These are usually obtained potentiostatically to that list of sensitizers, while suggesting that biacetyl (at a constant applied potential). However, if the starting "sensitization" occurred primarily by interaction of triplet material is oxidizable or reducible at the desired monihiacetyl with alcohol solvent to produce ketyl radicals toring potential, the potential must be stepped simultacapable of reducing the Co(II1) complex. They suggested neously with the flash, from a potential where no oxidation a similar mechanism for benzophenone sensitization. Also, or reduction of the starting material occurs, to the final the possibility that the excited benzophenone could abvoltage. If current-time curves are obtained a t several stract a loosely bound hydrogen atom from an ammonia potentials, a current-voltage profile may be obtained. of the complex has been s ~ g g e s t e d The . ~ ~ complex ~~ could Currents at 8: given time after the start of the experiment (the flash) are plotted as a function of the applied poPresent Address: Phillips Research Center, 238 RB-1, Phillips tential. The position of the oxidation or reduction waves Petroleum Company, Bartlesville, Okla. 74004. may then be used to identify the species present. 5 Present Address: Stanford Research Institute, Menlo Park, Calif. (A more detailed description of the methodology of 94025.
x
P
P
The Journal of Physical Chemistry, Vol. 81, No. 9 , 1977
Photoreduction of C0(NH3):+
in Benzophenone Solution
photoelectrochemistry can be found in ref 27.) There are two kinds of background signals for which various chronoamperometric experiments must be corrected. One of these pertains only to potential-step experiments, and this is due to potential-step charging current. This background was measured as described above for potential-step experiments, except the solution contained only the background electrolyte. This blank charging current was then subtracted from the potential-step currents measured with the electroactive species present. The other type of background signal pertains to all chronoamperometric experiments, including the potential-step blank runs. This is the combined background due to electronic disturbances related to the flash discharge and any amplifier drift or offset. A measure of this background was obtained prior to every flash experiment by initiating the flash discharge and measuring the resulting current-time curve, while preventing the light from reaching the cell with a movable shutter. This background signal was then subtracted from a subsequent current-time curve, obtained either potentiostatically or with potential-step, with the shutter open or closed. Further data processing (extraction of Faradaic currents and kinetic information) proceeded only on data corrected for these background signals, At least five experimental curves were averaged a t each potential. Electrode positioning and optical alignment were maintained throughout for all current-time curves measured for each solution used on a particular day. These parameters can greatly affect the photolysis current magnitudes. Following photolysis, solution was aspirated from the cell and replaced by a "fresh" solution. This approach prevented photolytic depletion from affecting subsequent experiments. To eliminate the effects of electrolytic depletion for successive electrochemical experiments the solution was stirred between runs by bubbling nitrogen through. Data Handling. Faradaic Current Extraction Methods. Two methods have been used to obtain purely Faradaic currents from the measured total currents obtained by the above procedure. The first involves the use of the Fratoni-Perone curve-fitting m e t h ~ d . ~If" ~the ~ photolysis mechanism involves dimerization of the electroactive species, this method will correct the chronoamperometric data for Faradaic-induced charging current, while calculating values for k d (the second-order rate constant) and Co (the initial concentration of the species being monitored) using the Birk-Perone second-order plot technique.36 The program iteratively calculates corrected currents, used to obtain k d and Covalues, until they converge to within preset limits. The second method is based on the following relati~nship:~~,~' iF = i, + R,C,,(di,/dt) (11 where i~ is the Faradaic current, iT is the total current, R, is the uncompensated cell resistance, CDL is capacitance of the working electrode double layer, and (diT/dt) is the time derivative of the total current. Following this correction, data are usually smoothed by various te~hniques,3~ since the differentiation method tends to introduce a large amount of noise into the calculated Faradaic current. These two methods produce nearly identical results when used properly on a given data set.37 However, the derivative method is a far more general approach, since the kinetic mechanism need not be known beforehand for its implementation. Therefore, this technique is the one that has been used throughout this study.
867
Extraction of Kinetic Data. In this particular work, the Birk-Perone second-order kinetic plot36has been used to extract information from chronoamperometric data. If the species being monitored is disappearing via a second-order reaction (e.g., ketyl radical dimerization), a plot of l/it1I2 vs. time will produce a straight line having slope proportional to the second-order rate constant (kd) and intercept proportional to the initial species concentration (C").The rigorous mathematical solution to the coupled second-order kinetic-diffusionproblem presented by Britz and Kastening3' predicts that while the intercept of this plot does give a correct initial concentration, the calculated second-order rate constant i s too low by about 27%. Hence, k d values were corrected accordingly. Reagents. Benzophenone (Matheson Coleman and Bell, East Rutherford, N. J.) was recrystallized twice from ethanol. CO("3)6C13 was purchased from Eastman and used without further purification. Its absorption spectrum and electrochemicalhalf-wave reduction potentials agreed well with those in the 1 i t e r a t ~ r e . lBulk ~ ~ ~Reagent ~ Grade 100% ethanol used for solution preparation was first checked for electroactive impurities polarographically, and none were found. Deionized water was purified further by distillation from a Corning AG-16 distillation apparatus (Corning Laboratory Produds, Corning, N.Y.). This water was shown to be electrochemicallypure using polarography and anodic stripping voltammetry. C02 was removed by boiling and storage in polyethylene bottles. KC1, NaOH, Na2B407.10H20,sodium acetate and Analytical Reagent grade glacial acetic acid (Mallinckrodt Chemical Works, St. Louis, Mo.) were used without further purification. For this work, ionic strength was maintained at 0.25 for all solutions. Solutions used in this study were as follows: 0.25 M NaOH, 50% ethanol/H20 by volume (pH 13.2); borax buffer, and 0.20 M KC1, in 20% ethanol/H20 (pH 10.8); 0.25 M sodium acetate, glacial acetic acid in 50% ethanol/H20 (pH 5.5). For convenience, these will be referred to simply as pH 13.2, 10.8, and 5.5 solutions throughout the text. pH measurements were made with a Corning Model 110 expanded scale pH meter (Corning Laboratory Products, Corning, N.Y .). Solutions were deoxygenated with high-purity nitrogen for a minimum of 30 min. To remove traces of oxygen from the nitrogen streams, it was bubbled through two gas-washin bottles containing chromous chloride and zinc amalgam,3fand through two additional bottles containing inert electrolyte solution. Results and Discussion Background. The chemical system chosen for this study was the photoreduction of Co(NH3):+ in the presence of benzophenone. This system has been studied by several w ~ r k e r s ,all ~ ~by~spectroscopic ,~~~~ monitoring methods. The alternative mechanisms suggested for the photoreduction process are shown in Figure 1. Benzophenone absorption at wavelengths less than about 390 nm causes population of its lowest triplet state with unity quantum yield. In the presence of a solvent such as ethanol, this triplet abstracts an a-h drogen atom to form a ketyl radical via pathway I.3 40,41 This radical is in rapid equilibrium with its radical anion (pKA = 9.242). The radicals and/or radical anions may then dimerize by pathway LA to form the benzopinacol product, the rate of this reaction being pH de~endent.~'When the Co(II1) complex is also present, the ketyl radical might react rapidly by pathway IB, reducing Co(II1) to Co(II), with the ketyl radical being reoxidized to benzophenone. This pathway has been suggested by Gafney and Adamson," and is consistent with the measured half-wave potentials
B
The Journal of Physical Chemistry, Vol. 81, No. 9 , 1977
Perone et al.
868
2.01.6-
IC 1.2-
+A) 0.8-
0.4-
CO(UI
++-y+ 0
COlDl (Pholorenriliralionl
/
0 4-'
-=
d C
'A
Figure 1. Possible photoreduction mechanism pathways, benzophenone Co(NH3)2+.
+
of the ketyl radical (E, = -1.6 V vs. SCE, pH 13.2), and the Co(1II) complex = -0.45 V vs. SCE, pH 13.2). Pathway I1 in Figure 1 represents an energy transfer, or photosensitization,mechanism43and has been suggested for this system by Vogler and Adamson' and Scandola and S ~ a n d o l a .Via ~ this path, energy is transferred from the benzophenone triplet state to the Co(II1) complex producing a triplet charge-transfer state of the complex. This species then decomposes to the Co(I1) Benzophenone remains unchanged; no ketyl radical is formed. Pathway I11 represents a possible mechanism for this process suggested by Scandola, Scandola, and Carassiti.2 Because the benzophenone triplet is a voracious hydrogen abstractor, one might expect removal of a hydrogen atom from one of the ammonias bound to Co(III), producing a Co(II1)-radical species plus a benzophenone ketyl radical. This Co(II1)-radical can undergo internal electron rearrangement to produce the Co(I1) product. In order to be significant in our study, however, this pathway would have to be extremely favorable relative to pathway I, as ethanol is about 5 orders of magnitude more concentrated than the Co(II1) complex in solution. Absorption at wavelengths greater than 280 nm in the ligand field bands of the complex results primarily in photoaquation ( C O ( N H ~ ) ~ ( H ~f ~ Or)m~ a+t i o n ) , "accom~~ panied by only a very slight amount of photoreduction.lb However, if light at wavelengths less than 280 nm is used, absorption in the ligand-to-metal charge-transfer bands results primarily in photoreduction. This is not the case here, however, as the flash lamp used in this study produces little light at wavelengths less than 300 nm. General Electrochemical Considerations. Benzophenone has been the subject of man electrochemicalH7 and photoelectrochemical studies.30s36' 3y7 3 ~ 5 1Benzophenone can be reduced at a mercury electrode, and the ketyl radical/radical anion and benzopinacol products can be oxidized at this electrode.36The ketyl radical and radical anion cannot be differentiated electrochemically. Hence, the radical oxidation currents are due to the sum total of these two species present at a given time. Current-voltage profiles for benzophenone with and without flash irradiation in pH 13.2 solution are shown in Figure 2, curves B and C. (These profiles were constructed point-by-point from individual current-time curves, as discussed in the Experimental Procedure section.) Benzophenone is reduced negative of -1.5 V vs. SCE; ketyl radical/radical anion is oxidized positive of -1.5 V; and the benzopinacol product is oxidized positive of -0.7 V. At this pH, the rate constant for ketyl radical dimerization is 9.2 X lo4M-l s-',~ and the half-life (tip = l/lZdCo) is about 120 ms under these experimental conditions. Thus, if ketyl radical The Journal of Physical Chemistry, Vo/. 81, No. 9 , 1977
30. 2 5.
20.
'C 15(/LA)
I 0.
5-
8 0.
:
:
I
P =
'A
51
1.0
C
i
oi
0:3 014
ok
016
o h ob
o:? -E
I:O
I:I
i,z
i.3
114
ik 12
(volts)
Figure 3. Current-voltage profiles, pH 5.5 solutlon, t = 500 ps after M Co(NH3)2+with or without flash; (B) 2.5 X flash: (A) 2.5 X loW4 M benzophenone without flash; (C) 2.5 X M benzophenone with flash.
oxidation currents are measured at times less than about 10 ms after the flash, pinacol formation will be negligible, and will not complicate ketyl radical oxidation current measurements. Therefore, the currents measured at -0.30 V will be purely diffusion controlled, with no contribution due to pinacol oxidation. Measurements at this potential are necessary in the presence of Co(II1) complex, since that species is reduced negative of -0.4 V (Figure 2, curve A). Quantitative measurements of the amount of ketyl radical present after the flash in the presence of Co(II1) complex are important for distinguishing between the various pathways shown in Figure 1. The ketyl radical concentration will be affected by the presence of Co(II1) complex if pathway I or I1 is significant, but not if I11 predominates. At lower pH, the benzophenone reduction wave is shifted in a positive direction, El being -1.4 V in pH 5.5 solution (Figure 3, curves B and Ketyl radical/radical anion is still oxidized at all potentials positive of this potential, and the benzopinacol product is oxidized at Ellz e -0.2 V. Since this is very close to the Hg oxidation potential at this pH, this wave is very difficult to observe quantitatively. As the pH is lowered below 13.2, the rate of the ketyl radical dimerization reaction increases to 9 X 107 M-1 s-1 a t pH 11, and 2.5 X lo8 M-ls-l at pH 5.5.'' Hence, it is not possible to observe the ketyl radical prior to the occurrence of significant photodimerization; mea-
6).
Photoreduction of Co(NH3):+ in Benzophenone Solution
TABLE I: Species Concentrations (X I I1 Initial Initial [benzophenone] [Co(III)] before flash before flash 5.0 5.0 5.0 5.0
0 5.0 0 1.5
889
lo4 M) before and after Flasha I11 IV Initial [ketyl radical] [Co(III)] after flash, CRO after flash 0.9 f 0.1 -0 1.1+ 0.1 0.5 I 0.1
V Change in initial [ketyl radical] with flash, A C R O
VI Change in [Co(III)] with flash
VI1 ACcc0lAC~0
4.0
f
0.1
0.9 I 0.1
1.0
f
0.1
1.1f 0.1
0.9
+
0.1
0.6
0.6
f
0.1
1.0
f
0.1
f
0.2
a All solutions, 0.25 M NaOH, 50% ethanol/H,O by volume. The uncertainties in columns I11 and IV are expressed as +s, the standard deviation, obtained from replicate measurements. The uncertainties in columns V-VI1 are obtained From the usual expressions for the propagation of random errors.
TABLE 11: Species Concentrations (X lo4 M) before and after Flasha 11 I11 I IV Initial Initial Initial [benzophenone] [Co(III)] [ketyl radical] [Co(III)] before flash before flash after Flash, CRO after flash 5.0 5.0 5.0 5.0
0 5.0 0 1.5
3.0 i 0.4 -0 2.6 I 0.4 2.0 0.4
V Change in initial [ketyl radical] with flash, ACRO
VI Change in [Co(III)] with flash
VI1 ACco/AC~O
2.6 f 0.1
3.0
f
0.5
2.4
0.1
-0.8
0.5 I 0 . 1
0.6
+
0.5
1.0 ?: 0 . 1
-1.6
f
a All solutions, pH 10.8, 20%ethanol/H,O by volume. The precision expressed in columns 111-VI was obtained as in Table I. However, the accuracy of values in columns I11 and V (+50%) is much worse than in Table I (see text). Therefore, the ratios in column VI1 can only be considered approximate.
surements on a very long time scale (>5 ms) are also not possible, since no ketyl radical remains significantly in solution. Thus, at lower pH, initial radical concentrations were obtained from kinetic plota, as described below. Ketyl radical oxidation may be observed with minimal contributions from benzopinacol oxidation currents or Co(II1) reduction currents at E = -0.40 V in pH 10.8 solution, and a t E = -0.45 V in pH 5.5 solution. The electrochemistry of Co(NH3)?+ has also been studied e~tensively.~'"~ Current-voltage profiles, obtained for the Co(II1) complex in pH 13.2 and pH 5.5 solutions, are shown in Figures 2 and 3, curves A. There is no significant difference observed with or without the flash entering the cell, indicating that no direct photoreduction occurs. The first wave represents the reduction of Co(II1) to Co(II), and the second wave represents the reduction to Co metal. The product of the first wave may be Co(H20):+ or a mixed complex:53 CO(NH,),~+ + e- t OH- 2 Co(NH,),OH' t 4NH,
Co(NH3)2+is labile and the complex rapidly dissociates. Despite the high OH- concentration in pH 13.2 and pH 10.8 solutions, insoluble CO(OH)~ is not formed because of the above process. At pH 5.5, the strong acidic buffer prevents formation of Co(OH)% In the immediate electrode vicinity, the mixed complex is probably formed. As was mentioned in the Background section, the light from the flash lamp used in this study can produce direct photoaquation of Co(NH3):+. However, the photoaquated complex is, for all practical purposes, indistin uishable from the hexamine complex electrochemically! Hence, the total amount of Co(II1) present in solution may be monitored by measuring the reduction current in the first plateau regions in Figures 2 and 3. It is not possible to monitor the Co(1I) concentration directly electrochemically. At potentials on the second cobalt wave, formation of Co metal causes changes to occur in the electrode surface which make interpretation of current-time curves unreliable, as has been observed in electrochemical studies of Co(SCN)+ solution.54 Also, Co(I1) oxidation was not observed in cyclic voltammograms
obtained at each pH studied in this work. Thus, no interference with measurements of ketyl radical oxidation currents was encountered. In the work reported here, the extent to which photoreduction of Co(II1) complex occurs in the presence of benzophenone can be monitored by measurement of the Co(1II) reduction current plateau. Any Co(II1) disappearance is assumed to be due to photoreduction to Co(I1). Unfortunately, the ketyl radical is oxidized at the same potentials where Co(II1) complex is also reduced. This must be taken into account in the quantitative estimation of Co(II1) concentration in the presence of ketyl radical, as mixed currents may be encountered. The initial amount of the species being monitored is an important parameter for quantitative study of the reaction process. This value may be obtained from purely diffusion-controlledcurrents directly from the Cottrell equation: it = nFADli2C*/(nt)"2
where n is the number of electrons transferred, F is the faraday constant, A is the effective area of the working electrode, D is the diffusion coefficient, and C* is the bulk concentration of electroactive species. Previously, it has been assumed that the effective (illuminated) area of the spherical electrode is about 75% of the total area.36 That assumption was also used in this work for calculating absolute concentrations in Tables I and 11. It can be shown30bthat the ratios of ketyl radical/Co(III) concentration changes are independent of the effective electrode area. These are also reported in Tables I and 11, and represent the most important information. The value for the ketyl radical diffusion coefficient used for quantitative calculations was 3.5 x IO+ c ~ ' / s . Cottrell ~~ plots (it vs. t-''') were used to test data for pure diffusion control and an absence of kinetic effects. This plot should be linear and pass through the origin for a current-time curve under these conditions. A first approximation of the Co(II1) complex concentration, CApp,obtained after a flash could be calculated by taking the ratio of the measured current to that observed at the same time for a potential-step experiment without flash. However, because the electrode surface which was The Journal of Physical Chemistry, Vol. 81, No. 9 , 7977
870
Perone et al.
not exposed to the flash (-25%) was assumed to always be exposed to the initial Co(II1) complex, C",the following formula was used to calculate C,, the Co(1II) concentration in the photolyzed region: Capp= 0.25(C0) + 0.75(Cp) (4)
This relationship was used to calculate the absolute Co(1II) concentrations after the flash reported in Tables I and 11. The error limits stated reflect the deviation in concentration calculated from at least seven different current values from an average i-t curve. When a chemical reaction occurs, in addition to electrolysis, on the time scale that the photolytic species is being monitored, a different theoretical approach must be used. For the stud of benzophenone photodimerization, Birk and Perone39 derived the following approximate relationship for the coupled second-order kinetic-diffusion process describing the electrooxidation of ketyl radical: l / i t ' I 2 = I / K + (k,cRO/K)t (5) where
K = nFAD1I2CRa/?r''2 (6) Hence, if l/itl/' vs. time is plotted, a straight line having slope proportional to the second-order rate constant, kd, and inferce t proportional to the initial radical concentration, CRB, should be obtained. Britz and Kastening's rigorous mathematical solution38has shown that the value of CR' is correct, but k d obtained from this type analysis is about 27% low. Thus, all kinetic data reported here have been corrected accordingly. These kinetic plots can be used to study the effects of the presence of Co(II1) complex on the initial radical concentration, as well as the effects on the second-order radical dimerization kinetics. Electrochemical measurements of this type are very much dependent upon the physical configuration of the electrochemicalcell being used. The position (vertical and horizontal) of the working electrode relative to the incoming light from the flash affects the amount of photolysis observed. Optical alignment and electrode positioning can be maintained constant for a given series of runs. However, from day to day, variations in these positions occur due to cell reassembly, resulting in changes in photolytic efficiency of as much as st15%. Thus,all data reported here for a given series of runs at specific conditions were obtained with fixed alignment. Results from duplicate series of runs with different alignments could not be combined. Experimental Results. Basic Solution. Benzophenone Alone. A current-time curve measured at -0.30 V vs. SCE, after flash irradiation of 5 X lo4 M benzophenone in pH 13.2 solution, is shown in Figure 4,curve A. The initial radical concentration under these conditions was 1.0 X lo4 M, indicating about 20% photolysis. For this same solution, when the potential was stepped from -0.30 to -0.80 V concurrently with the flash, a net anodic current-time curve was obtained, where the current magnitude and initial radical concentration agreed within experimental error with that obtained potentiostatically. Both curves represent purely diffusion-controlled current, since their Cottrell plots are linear and pass through the origin. Co(NH3):' Alone. A potential-step current-time m e obtained a t -0.80 V vs. SCE for 5 X M CO(NH~)~' in pH 13.2 solution, is shown in Figure 5, curve A. The current-time curve for 1.5 X M Co(NH3):' under the same conditions is represented by curve C. These curves are the same whether or not light from the flash is allowed to enter the photolysis cell. The Cottrell plots show the The Journal of Physical Chemistry, Vol. 81, No. 9 , 1977
1
oO
.
TIME (msec) 2 3 4
5
6
' B
".t
1.0-
1.5-
2.0 'A
( pA )
2.5 -
35 3.0
4.04.51
Figure 4. Current-time curves for ketyl radical oxidation, pH 13.2 solution. Flash photolysis with potentiostatic chronoamperometry at E = -0.30 V vs. SCE: (A) 5 X M benzophenone; (6) 5 X M benzophenone 5.0 X M CO(NH~)~~+; (C) 5 X M benzophenone i1.5 X M Co(NH3):+.
+
20 181614(PA),2 I
IC
-
10664-
2110
2:O
310
410 i.0 TIME (m s e d
60
710
Flgure 5. Current-tima curves at plateau potential of first Co(II1) wave, pH 13.2 solution. Flash photolysis with potential-step from -0.30 to M Co(NH3):+ only: (6) 5 X M -0.80 V vs. SCE: (A) 5 X Co(NH,):+ -I-5 X M benzophenone (net current): (C) 1.5 X M C0(NH3)e3+ only; (D) 1.5 X M CO(NH3)8'+ 5 X M benzophenone (net current).
+
reduction to be diffusion controlled at times greater than -5 ma. Estimates of Co(II1) concentration for the studies described below are based on direct comparison with these curves. Co(NH3)63' and Benzophenone. Current-time curves obtained potentiostatidy at -0.30 V for two concentration ratios of Co(II1) complex to benzophenone in pH 13.2 solution are shown in Figure 4,c w e s B and C. The initial ketyl radical concentrations calculated from such a series of runs are listed in Table I, column 111. In all cases, the m o u n t of oxidation current observed due to ketyl radical was significantly diminished in the presence of Co(II1) complex. C w e A was also obtained potentiostatically at -0.30 V; however, no Co(II1) was present in the solution. Both curves A and C in Figure 4 produce linear Cottrell plots which pass through the origin as shown in Figure 6. When the potential was stepped, with the flash, from -0.30 to -0.80 V onto the Co(II1) reduction plateau, anodic or cathodic current-time curves might be obtained, depending on the relative amounts of ketyl radical and Co(1It) remaining after the flash. That is, when both ketyl radical and Co(II1) complex are present in solution after the flash, the observed current at -0.80 V is really a mixed
Photoreduction of CO(NHJ;+
87 1
In Benzophenone Solution
measurements are shown in Table 11, where the initial Co(II1) concentrations after the flash were calculated in 5 . 0 1 the 1 same manner as those in Table I. However, the ketyl
I
4,Ot
J A
// '0
5
IO
, 15
&=
20
25
(seP)
Flgure 8. Cottrell plots for ketyl radical oxidation current, pH 13.2 solution. Potentlostatic measurements at E = -0.30 V vs. SCE: (A) 5X M benzophenone alone; (E) 5 X M benzophenone, 1.5 X M Co(NH,)?+.
current due to ketyl radical oxidation plus Co(II1) reduction. To obtain the net reduction currents due to Co(II1) only, as shown in Figure 5, curves B and D, the observed currents are corrected for the contribution of ketyl oxidation current, as determined from a separate flash experiment monitored at -0.30 V. The Co(II1) complex concentration can be calculated directly from these data; assuming there is no reaction between Co(II1) complex and ketyl radical. This is considered in the Discussion section. When the benzo henone/Co(III) complex ratio was 5 X M/5 X 10-P M, essentially no ketyl radical was observed after the flash, while a considerable amount of Co(II1) remained. When the ratio was 5 X M/1.5 X M, ketyl radical was observed after the flash, as well as a significant amount of Co(II1) complex. The absolute amount of Co(II1) complex disappearing with the flash approximately equaled the amount by which the ketyl radical was diminished in all cases. (See Table I.) Possible reasons for this will be considered in the Discussion section. Experimental Results. Acidic Solution. Nearly identical potentiostatic current-time curves for ketyl radical oxidation are obtained when 5 X loT4M benzophenone is flashed with and without 5 X 10" M Co(II1) complex present at pH 5.5. Also, when second-order kinetic plots3' are made from these data no significant change in the dimerization rate constant is observed. Similar experiments conducted with other concentration ratios provided analogous results. These data demonstrate that there is no significant alteration in the percent photolysis or in the rate of the second-order dimerization reaction in the presence of Co(1IS) complex at pH 5.5. When the potential was stepped from -0.60 to -1.0 V, the Co(II1) complex concentration could be monitored. Current measurements made at times greater than 1ms, when ketyl radical presence could be ignored, showed a negligible difference with and without the flyh when both Co(II1) complex and benzophenone were present. Shorter-time measurements with the flash gave mixed currents, but showed no significant change in the Co(II1) concentration when the oxidation current contribution from the ketyl radical was taken into account. Experimental Results. Intermediate p H . Because of limited buffer solubility, the amount of ethanol presefit in the pH 10.8 buffer solution had to be reduced to 20%. The results are still correlatable with the other pH solutions studied here, as ethanol was present in large excess. As was the case in more basic solutioq, the amount of ketyl radical observ d was diminished in the presence of Co(1II) complex. The ash also diminished the amount of Co(1II) complex in the presence of benzophenone. Quantitative
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radical dimerization reaction occurs significantly even at times
