854
The Journal of Physical Chemistty, Vol. 82, No. 8, 1978
Eu3+(H,0),
2 Euz+(H,O),-, + H+ + .OH
K. Hasegawa and P. Neta
(1)
would be readily reversed through reoxidation of Eu2+by OH radical. This we prevent by scavenging OH with the organic solute (2-propanol or formate) RH2: .OH t RH,
H,O t .RH
-+
(2)
Now Eu2+ becomes the light absorber: E u 2 + ~ H , Eu3+ O ~ t OH' t H-
(3)
RH2 scavenges H. atoms too and gives H2 evolution: H.
+ RH,
+
H,t
+ *RH
(4)
-RH radicals from (2) and (4) reduce Eu3+: Eu3++ *RH Eu'+ -+
+ H+ t R
(5)
Reactions 3 and 5 carry the photocatalytic cycle yielding H2 and R (e.g., acetone). H2 originates partly in the water and partly in the organic solute. H loss from water is replenished from RH as in ( 5 ) . The net result is RH,
hv
(Eu")
R t H,
with Eu2+ the photosensitizer. In the absence of the organic scavenger radical back reactions with Eu2+and Eu3+ decrease the quantum yield so that a smaller production
of H2 is observed (for 254-nm irradiation only). The quantum yield a t X >300 nm in the long wavelength band of Eu2+ is lower than a t X 254 nm, in the short wavelength band. One explanation of this would be that the more energetic quanta absorbed in the short wavelength band of Eu2+ cause a transition in which the electron in the expanded orbital overlaps the solvent to a greater extent with greater efficiency in producing the separated H atom in reaction 3. To sum up, the present work establishes, in agreement with the suggestion of Marcus, that Eu2+in the presence of suitable scavengers can mediate the photocatalytic evolution of hydrogen from water up to X 400 nm. The wavelength region utilizable in the solar spectrum limits the usefulness of the system correspondingly.
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References and Notes (1) R. J. Marcus, Science, 123, 399 (1956). (2) V. Balzani, L. Moggi, M. P. Manfrin, F. Bolletta, and M. Glerii, Science, 189, 852 (1975). (3) G. Stein and A. Zeichner, to be published (Casali Institute Report, 1975). (4) Y. Hass, G. Stein, and M. Tomkiewicz, J . Phys. Chem., 74, 2558 (1970). ( 5 ) C. K. Jorgensen, Mol. Phys., 5 , 271 (1962). (6) Y. Haas, G. Stein, and R. Tenne, Isr. J . Chem., 10, 529 (1972). (7) D. D. Davis, K. L. Stevenson, and G. K. King, Inorg. Chem., 16, 670 (1977). (8) R. A. Cooley and D. M. Yost, Inorg. Synfh., 2 , 7 (1946). (9) F. D. S . Butement, Trans. Faraday SOC.,44, 617 (1948). (10) S. P. Sinha, Z . Anorg. Allg. Chem., 8 (1967).
Rate Constants and Mechanisms of Reaction of Cia- Radicals' K. Hasegawa and P. Neta" Radiation Laboratory, University of Notre Dame, Notre Dame, Indiana 46556 (Received December 27, 7977) Publication costs assisted by the Office of Basic Energy Sciences, U S . Department of Energy
The rate constants for reaction of C12-with various organic and inorganic compounds were determined by kinetic spectrophotometric pulse radiolysis. The Clf radical can abstract hydrogen from aliphatic compounds with rate constants that vary from 7 X lo2 M-' s-l for t-BuOH to 1.9 X lo6 M-' s-l for HC02-. The reaction with olefinic compounds results in C1 atom addition and takes place with generally higher rate constants, ranging from -lo6 M-l s-l for double bonds deactivated by adjacent cyano or carboxyl groups to -6 X lo8 M-I s-l for allyl alcohol and sorbic acid, for example. Addition to an aromatic ring can also take place with k 5 10' M-' but the important reaction of Clz-with the aromatic compounds is the direct oxidation by electron transfer. Such oxidation is possible when the ring is substituted with OH, OCH,, NH2, and related groups. The cation radical produced from a methoxy substituted compound was identified by both optical and ESR spectra. The cation radicals from hydroxy or amino substituted compounds undergo deprotonation to produce the phenoxy1 or anilino radicals. The reaction of Clz-with inorganic compounds was also found to involve either hydrogen abstraction or one-electron oxidation for the different compounds studied.
Introduction The oxidation of chloride ions in aqueous solutions is known to produce the Clz- radical as an intermediate. Oxidation can be carried out directly, for example, by SO, radicah2
so; Cl
t c1-
-+
S0,Z- t c1
+ c1- * c1,-
(1) (2)
It is clear, however, that the reaction of OH with C1- in acid solution also leads to rapid formation of C12-. In a simplified way we can formulate the reactions as follows: OH
+ C1- P
HOCI-
HOCl- t H+ + H,O
(3)
+ Cl
61 t Cl- 2 c1,-
(4)
(2)
The C12- radical is a reactive species which may oxidize many inorganic compounds and may react with organic While OH oxidizes chloride in a similar way, the mechanism involves various equilibria and is fairly ~ o m p l e x . ~ compounds by various mechanisms. I t may abstract 0022-3654/78/2082-0854$01 .OO/O
0 1978 American Chemical Society
The Journal of Physical Chemistry, Vol. 82, No. 8, 1978 855
Reaction Mechanism of Clp- Radicals
hydrogen, add to unsaturated compounds or oxidize by electron transfer. The reactions of C12- were previously studied with only a limited number of organic compounds (see, e.g., ref 4-6), so that the relative importance of the various modes of reaction has been unclear. In an attempt to shed light on the behavior of C1, we have investigated the rate constants and the mechanism of its reaction with a wide variety of compounds. It is found that hydrogen abstraction by Clz- is very slow, while addition to double bonds can be rapid if not inhibited by neighboring groups. Addition to an aromatic ring takes place relatively slowly but various substituted aromatics can be efficiently oxidized by Clz-. The reactions with several inorganic compounds were also studied.
Experimental Section The Clz- radicals were produced by the two methods mentioned in the Introduction, and their reactions were followed by kinetic spectrophotometric pulse radiolysis. Experiments at pH 1-3 utilized OH radicals for the oxidation of C1- while experiments at pH 3-10 utilized SO4-, which was produced by reaction of ea; with S202-. In both cases the solutions contained 1 M C1- in order to minimize the competition by the substrates for the OH and SO4-. The intense absorption of Clf at 340 nm (t 8800 M-l ~ m - ' ) ~ was observed immediately after the pulse. The decay of this absorption was monitored in the absence and presence of various concentrations of the substrates, S, and from the plot of the pseudo-first-order rate constants for the decay of the C12- signal vs. [SI, the second-order rate constants for the reaction C12- + S were derived. In the case of hydroxy and methoxy substitued aromatics, the radical produced exhibited optical absorption in the 400-nm region and thus allowed the observation of the kinetic of its buildup as another measure for the rate constant of C12- + S. Solutions were prepared using Reagent Grade water from a Millipore Milli-Q system. Sodium chloride was Baker Analyzed Reagent and sodium persulfate was obtained from Sigma. The organic and inorganic substrates used were of the highest purity commercially available and were used without further purification. The sources of the various compounds are indicated in Tables 1-111. The solutions were deoxygenated by bubbling with pure nitrogen (or nitrous oxide when necessary) and were irradiated by 5-11s electron pulses from an ARC0 LP-7 linear accelerator. The dose per pulse was such that it produced only -1 pM of radicals in order to minimize the second-order decay of C12-. The computer-controlled pulse radiolysis apparatus7 allowed averaging of many kinetic traces taken consecutively while the solution flowed through the irradiation cell. At least four concentrations of each substrate were used to obtain the second-order rate constants. The overall uncertainty in these rate constants is between f10% for k lo8 M-l and &20% for the very low rates, unless otherwise indicated. No correction was applied for ionic strength effects because of the nonlinearity of these effects in the 1M region. However, the ionic strength was very similar in all experiments.
-
-
Results and Discussion The rate constants for the reactions of Clz- radicals determined in the present study are summarized in Tables 1-111. The first portion of Table I lists saturated alcohols and acids which can react with Clz- to undergo hydrogen abstraction as the main process. Rate constants for hy(e 6, drogen abstraction by C12- range from
