J . Phys. Chem. 1985, 89, 553-554
553
COMMENTS Comment on "Factors Affecting the Efficiency of Photochemical Water Cleavage Systems. The Reaction between 0, and the Reduced Electron Acceptor" Sir: Recently, Kozak et al.' have examined factors affecting the efficiency of photochemical water cleavage. I wish to draw attention to a number of conceptual ambiguities in their paper. The three main points of contention are as follows: (i) The limitations imposed on the system by thermodynamics are not taken into account. (ii) The pH of the solution does not appear in their treatment while it plays an essential role. (iii) The reaction scheme proposed allows the buildup of superoxide ions 02;to levels which are unrealistically high in view of the known reactivity2 of this radical ion. These considerations have already been taken into account in a 1982 publication by myself3 and are further developed below. A meaningful modeling of a water cleavage system producing hydrogen and oxygen must take into account the thermodynamic limitations imposed via the two equilibria (reactions 10 and 11 of ref 3) A-
+ H 2 0 + A + 1/2H2+ OH-
(2) One assumes the presence of two catalysts each with perfect specificity toward one or the other of the above reactions. Catalyst 1 is totally inactive toward the A-/A and H 2 / H 2 0redox systems, while similarly catalyst 2 is totally inactive toward the S+/S and H 2 0 / 0 2 redox systems. When under steady irradiation, such a system exchanging no matter with the outside will eventually come to a stationary state. The oxygen concentration cannot increase beyond a certain value dictated by the redox potentials of the S+/S and H 2 0 / 0 2system and the pH of the solution. The same type of consideration applies to the hydrogen. In the absence of reaction g of Kozak et al., i.e. the reduction of O2 by A-, the concentrations of O2 and H2 would increase steadily as long as the system is irradiated, disregarding the thermodynamic limitations just mentioned. In addition, the redox potentials of the constituents of the system and the pH of the solution do not appear in ref 1, although these properties must play a very important role in the process described. As an example, at pH 7.5, if the hydrogen pressure is 1 atm ([HJ E 7 X M) a t room temperature, then thermodynamics indicate that at equilibrium [MV+]/[MV2+] = 1. Thus, further production of hydrogen could not occur unless this ratio increases. Each decrease of the pH by one unit would also decrease the necessary [MV+]/[MV2+]by a factor of 10. The highest concentration of H2calculated in ref 1 for such a system (0.12 X M after lo4 s) used in conjunction with the corresponding ratio of [A-],/[A], requires thermodynamically a solution pH not higher than -2.65 ([H+] = 455 M). It is clear, however, that if O2 reacts with the A- radical (and this is known to be the case for the methylviologen radical MV+)2 such a reaction must be included in the scheme. Including this reaction under the form A-
+0 2
k3 +
A
+ 02-*
(3)
reaction g in ref 1 implies the possibility of an accumulation of (1) T.W. Ebbesen, B. L. Tembe, and J. J. Kozak, J . Phys. Chem., 88,683 (1984). (2) J. A. Farrington, M. Ebert, and E. J. Land, J . Chem. SOC.,Faraday Trans 2, 665 (1978). (3) P. P. Infelta, J. Photochem., 20, 181 (1982).
0022-3654/85/2089-0553!$01 .50/0
the superoxide ion 02-to unrealistically high concentrations about equal to that of hydrogen, when thermodynamic limitations are again not considered. It is known2 that 02-.is a very reactive species which may lead to a variety of products and intermediates, for example, 02- and H202.Typically, the reaction rate for O p with MV' is 6.5 X lo* M-' s-', almost identical with the reaction rate of O2with MV' (7.7 X lo8 M-' s-'), and it is difficult for its concentration to build up to the levels calculated in ref 1. In my study of such a ~ y s t e m I, ~have also considered reaction 3, but it is clear that, as the equation stands, it cannot be satisfactorily used for the modeling. To circumvent this problem, I have replaced reaction 3 by the one-electron stoichiometric redox reaction A-
+ 1/402 + H+
-
A
+ '/2H20
(4)
By doing so, I provide the loss channel whereby products from the desired reaction are consumed to give back some of the initial species. We make the simplifying assumption (for modeling or products formed from it do not consume purposes) that 02-reducing species other than A- (such as Hz). In a system which has reached a photostationary state, this scheme implies that each three more time an A- reacts with an O2 molecule to give 02-., or products formed from it; the rate for A- will react with 02-. reaction 4 is simply taken as 4k3[A-] [O,]. Henry's law is used as the simplest available means for a description of the gas evolution from the solution. Bearing in mind the limitations due to the various assumptions in my study allows an illustration of the effect of the redox potentials of the systems involved (A-/A, S+/S, H 2 0 / H 2 ,H 2 0 / 0 2 ) ,the importance of the O2 and H2 pressures, the existence of an irradiation intensity threshold below which water cleavage will not take place, the relevance of the pH of the solution, the existence of some optimal concentration for the electron relay, A, and, of course, the relevance of the catalyst efficiency. The very presence of O2 in the solution may not simply slow down such a system, but prevent it from achieving water cleavage altogether. Registry No. H,O, 7732-18-5.
Institut de Chimie Physique Ecole Polytechnique FZdZrale CH- 101 5 Lausanne, Switzerland
Pierre P. Infelta
Received: May 5, 1984: In Final Form: July 24, 1984
Reply to the Comment on "Factors Affecting the Efficiency of Photochemical Water Cleavage Systems. The Reaction between 0, and the Reduced Electron Acceptor" Sir: There are two aspects that we have to address in answering the comments of Infelta on our paper: what was the aim of our study and what are the problems and errors in his points of contention? We first give our answers to his points of contention: (i) and (ii). In our model we assumed that equilibrium 1 and 2 were completely displaced to the right, for reasons of simplicity, thereby excluding any back-reactions. In other words, we did not take into account the redox potential of the acceptor as Infelta did. However, in his illustration he makes a serious confusion between equilibrium and photostationary states. Thereby he calculates a H + concentration assuming the system to be at equilibrium! (iii). Infelta must not have read our paper very carefully since 0 1985 American Chemical Society
