10497
J. Phys. Chem. 1994, 98, 10497-10503
Mechanistic Study of Photoelectrochemical Reactions: Phototransient Experiments Richard G. Compton,*Robert A. W. Dryfe, and Judy Erst Physical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QZ, United Kingdom Received: February 4, 1994; In Final Form: June 24, 1994@
A novel channel electrode phototransient experiment for the mechanistic study of photoelectrochemical reactions is described in which the evolution of the photocurrent in time is monitored after the stepwise application of light to the system once steady-state transport-limited currents have been established in the dark. It is shown that the phototransient data in combination with steady-state photocurrent/flow rate data can accomplish mechanistic discriminations which may be impossible using the latter data alone. The theory of the experiment is given and a working surface presented which allows the analysis of experimental transients regardless of the cell geometry or solution flow rate used in their measurement. The approach is applied to the photoelectrochemical reduction of p-bromonitrobenzene in acetonitrile solution at a platinum electrode. The process, in the presence of light of wavelengths near 330 nm, is shown to be of the photo-ECE type.
Introduction Recent work has hinted that the sequential use of electrochemical and photochemical activation of organic and organometallic molecules may lead to a wealth of hitherto unsuspected chemistry including (i) novel mechanistic pathways, (ii) the discovery of new compounds associated with unusual reactive intermediates, and (iii) new synthetic routes; the area has been reviewed.' In recent publications we have introduced a general methodology, based on channel electrodes, for the mechanistic interrogation of such photoelectrochemical The merits of this approach are first that it is directly amenable to simultaneous study with a variety of spectroscopic techniques, including ESR and luminescence spectroscopies, and second that quantitative mechanistic deductions are possible via photocurrent measurements made as a function of the rate of mass transport (solution flow rate) to the electrode. The latter attribute has proved exceptionally powerful and has permitted the study of a wide diversity of systems. In particular, photochemical analogs have been discovered of many of the familiar mechanisms of dark electrochemistry, including "photo"-EC, -CE, -ECE, and -DISP pathways.' However, the use of such a protocol for kinetic and mechanistic diagnosis is indirect in nature and cannot necessarily be guaranteed to provide unambiguous answers. The aim of the present paper is to introduce a novel phototransient experiment in which the photocurrent is followed after the sudden exposure of the electrode to light once transport-limited steady-state currents have been established in the dark. It is shown that the combination of phototransient data with steady-state photocurrent/flow rate data can accomplish mechanistic discriminations which may be impossible using steady-state data alone. The joint approach is illustrated with reference to the photoelectrochemical reduction of p-bromonitrobenzene in acetonitrile solution at platinum electrodes. In a preliminary note6 we have reported that, in the absence of light, one-electron reduction leads to the formation of the radical anion, [Br-+-N02]'-, which is kinetically stable on the voltammetric time scale. However, when the electrode is illuminated with light of wavelengths around 330 nm, efficient dehalogenation of the radical anion takes place through the expulsion of bromide ion.6 Various candidate mechanistic schemes can be proposed for the overall reaction. In particular, the isomeric species @Abstractpublished in Advance ACS Absrrucrs, September 15, 1994.
0022-3654/94/2098- 10497$04.50/0
o-bromonitrobenzene undergoes reductive dehalogenation without the assistance of light on reduction in acetonitrile, and two alternative mechanisms have been suggested for this reaction. These form an obvious basis with which to account for the photoelectrochemical reduction of p-bromonitrobenzene. Conventional wisdom7-' suggests a "photo-ECE' process.
+ e- - [p-Br-$-NO,]'[p-Br-$-NO,]'- + hv - Br- + 'C6H4N0, 'C,H,NO, + HS - PhNO, + S' PhNO, + e- - [PhNO,I'p-Br-$-NO,
(4 (b) (c) (4
where HS is a solvent molecule. However, the above type of description has been recently questioned by Montenegro, Pletcher, and co-workers'2 in the context of the dark reduction of o-bromonitrobenzene. They have suggested the further step
-
+
S' eS(e> may operate so that the overall transformation should be regarded as an ECEE process. In the following, phototransient experiments are used to prove conclusively that the photoelectrochemical reduction of p-bromonitrobenzene proceeds via a "photo-ECE', rather than "photo-ECEE', mechanism, perhaps suggesting the insensitivity of microelectrode experiments in achieving this discrimination.12
Theory We consider the following generalized electrode reaction mechanism
BLX+Y X f e-
-
products
(ii) (iii)
Y f eproducts (iv) where the decomposition of B only takes place in the presence 0 1994 American Chemical Society
Compton et al.
10498 J. Phys. Chem., Vol. 98, No. 41, 1994 Electrode I
J.
cover plate
Y
/
/ a&--/
-2.-
.*?‘
Pt wire contact Neff
.
ABOVE
\\
Flow
channel unit
Figure 1. Schematic diagram of a channel electrode which defines the coordinate system used.
m
of light. If all four processes i-iv operate, then the mechanism is of the photo-ECEE type, whereas if Y is not electroactive, then step iv does not occur at the electrode and the mechanism is photo-ECE. The convective-diffusion equations describing the distributions of A, B, X, and Y in time (t) and space (x,y) are
35700 3.4000 - 3.5700 3.0600 - 3.4000 2.7200 - 3.0600 2.3800 - 2.7200 20400 - 23800 1.7000 - 2.0400 13600 - 1.7000 1.0200 - 13600 BELOW 1.0200
Figure 2. Working surface for a photo-ECE process showing how N,ff depends on K and z. Light is applied to the system at z = 0.313.
Neff ABOVE 4.73 4.20 3.67 3.13 2.60 -
(3)
5.00
5.00
4.73 4.20 3.67 3.13 2.07 - 2.60 1.53 - 2.07 1.00 - 1.53 mow 1.00
Figure 3. Working surface for a photo-ECEE process showing how N,ff depends on K and z. Light is applied to the system at z = 0.3 13.
(4) where DLis the diffusion coefficients of species L (= A, B, X, or Y) and the Cartesian coordinates x and y can be understood with reference to Figure 1. v, is the solution velocity in the x direction; the components in the y and z directions are zero. Given laminar flow conditions and that a sufficiently long leadin length exists upstream of the electrodes so as to allow the full development of Poiseuille flow, then v, is parabolic
transport-limited electrolysis of A and that this “switches on” the kinetic decomposition of B . Accordingly, the concentration profiles of B, X, and Y change with time in accordance with eqs 2-4. The following new boundary conditions apply: t > t*, ally,
X
< 0, [A] = [AIbulk, [B] = 0
t > t*, ally, x < 0, [XI = 0, [Y] = 0
(9) (10)
t > t*, y = 0, 0 < x < xe, [A] =0, (5) where h is the half-height of the cell and vo is the solution velocity at the center of the channel. Equations 1-4 assume that axial diffusion effects may be neglected; this is valid provided the electrodes considered are not of microelectrode dimen~i0ns.l~ We suppose that before any light is applied to the system (t < t*) the electrode potential has been adjusted to a value corresponding to the steady-state transport-limiteddischarge of A so that the following boundary conditions apply to eqs 1 and 2.
t < t*, y = 0, 0 < x < xe, [A] = O ,
t > t*, y = o , o < x < x e , [X]=O
(12)
A final boundary condition is specific as to whether the photoECEE or the photo-ECE mechanism operates: photo-ECEE mechanism t > t*, y = o , 0 < x < x e , [Y] = o
(15)
photo-ECE mechanism
where x e is the length of the electrode and 2h is the cell depth. The concentrations of X and Y are zero throughout all space for t < t* since no decomposition of B has yet occurred. We next assume that light is applied to the system at time t = t* while the electrode is maintained at a potential for the
The mass transport equations (1)-(4) can be readily solved under both sets of the above-specified boundary conditions by direct application of an implicit finite-difference method previously developed by the authors for the solution of time-
J. Phys. Chem., Vol. 98, No. 41, 1994 10499
Mechanistic Study of Photoelectrochemical Reactions Species B
Species B
k = 1.6
k = 1.6
20ms
700m5
After light
After light
Vf = 6.849e-3
Vf = 6.849e-3
= m = _
