PHOTOCHEMICAL DECOMPOSITION OF HYDROGEN PEROXIDE
Dec. 5, 1932
[CONTRIBUTIONFROM THE GEORGEHERBERTJONES LABORATORIES, CSIVERSITY
The Photochemical Decomposition of Hydrogen Peroxide. and Fractionation Effects
3999
OF CHICAGO]
Quantum Yields, Tracer
BY JOHN P. HUNTAND HENRYTAUBE RECEIVED MAY3, 1952 The quantum yield of the photochemical decomposition of hydrogen peroxide a t relatively high light inteiisity is independent of the concentration of hydrogen peroxide, of acidity and of the presence in the solution of Br-, C1-, S H 4 +or M n + + . At 25", the limiting quantun yield a t X 2537 A. io 0.98 =t0.05 and a t 0 " it is 0.76 fk 0.05. The primary efficiencies are taken as '/z of the limiting quantum yields. Tracer experiments show that the oxygen formed in the photodecomposition originates entirely in the hydrogen peroxide. The exchange between 0 2 and H20s during thc photodecomposition is a t most very slight. The fractionation effects associated with the non-chain process for decomposition have been determined. They do not appear to be compatible with hydroxyl radicals as the sole net products of the primary act.
The original purpose of the work described in this paper was to measure the isotope fractionation effects in the reaction
+ H~01---+ Ha0 + E102 + HpOz +HO + HzO + 0.
HO 1102
using light acting on hydrogen peroxide to generate the radicals. The fractionation factors for these reactions are of interest in comparison with the factors which have been measured for a number of catalysts acting on hydrogen peroxide. I n order to determine the fractionation factors in a simple way from measurement on the photodecomposition, i t is necessary to choose conditions so that the total decomposition produced by the chain process be large compared to that by the separate path consisting of the chain-initiating and -terminating steps, i.e., the chains must be long. The work of others has demonstrated, 2 - 4 and our experience has confirmed, the great difficulty that arises in purifying the solvent and the reagents and otherwise conducting the experiments so that the chain decomposition is restricted to some intrinsic path for the systein such as is represented above. The original goal has been set aside at least for the present and, in the work reported here, we have limited our studies to conditions under which the chain decomposition is eliminated. This is achieved by using light of sufficiently high intensity5 and results in a system which is much less sensitive to impurities. Our results on the kinetics of the photodecomposition in general confirm those obtained by Lea,6 and considerably amplify his. In addition to experiments on the quantum yield of the reaction, results obtained in tracer experiments and in the study of fractionation effects are presented and discussed with reference to mechanisnis for the primary act and subsequent processes.
Experimental The light sotirce was :I G.E. -!-watt gcrrnicitl:rl lnnip opcrating from it Sola constant voltagc transformer. The light cinittctl i i i thc ~vavelength region affcctiiig thc uranyl oxal,ttc actinoinrtcr is almost entirely 25.37 A . Shortrr wixvc lengths are efliciently removed by the glass walls of the lamp. When a Pyrex glass filter was interposed, the effect (1) A. E. Cahill and H . Taube, THISJOURNAL, 74, 2312 (195%). (2) F. 0 Rice and 11. I.. Kilpatrick, J. P h y s . C h e m . , 81, 1607 ( 1827).
1 3 ) A. J. Allmand and D . W. G. Style, J . Chcm. Soc., 5!lG (lY30).
(4) L. J. Heidt, THISJOURNAL, 64, 2840 (1932). ( 3 ) D. E. T,ea. T w n s . Fiirririay Soc., 4 6 , 81 (1949).
on thc uranyl oxalate actinometer mas observed to be only 370 of the effect when it was omitted, thus showing that mercury lines of longer wave lengths contribute little to the light energy. The lamp was placed in a fused quartz tube closed : I t oiie end, and this end was lowered into the neck of the reaction flask. Provision was made for sealing the quartz tube to the mouth of the flask. The flask was fitted with an entry tube for adding solutions, and an exit tube, which could be used to collect evolved gases. I n most of the experiments, the light was used without collimation, the lamp being held several centimeters above the surface of the solution. Thelight flus for each particular geometry was determined by replacing the hydrogen peroxide solution with an equal volume of actinometer solution .6 The geometry described required correction of the results for incomplete absorption of light when the hydrogen peroxide was a t low concentration. I n the worst case this correction was estimated to be 22%. Since the correction cannot be made accurately, a series of quantum yield determinations were made using a collimated light beam which was confined to the central portion of the solution. For this geometry the maximum correction amounted to only 6% and the results agreed with those obtained using full illumination. The solutions were stirred by rocking the reaction cell assembly, or by inserting :I glass stirring blade. No difference in results was noted for the two methods. Except where the influence of tap distilled water was studied, redistilled water was used. The hydrogen peroxide was Merck and Co., Inc., 30%, inhibitor free. The results a t high light intensity were found to be independent of lot iiuxnber of the hydrogen peroxide, of the perchloric acid and of the redistilled water, and as the results will showe; : remarkably independent of the concentration of certain catalysts." Analyses for hydrogen peroxide were made by titration with standardized ceric sulfate solution. This was added in excess and the excess determined by titration with standardized ferrous sulfate solution using the iron orthophenanthroline indicator.
Results Table I contains a summary of the experiments devoted to a study of the kinetics of the photoreaction. No special effort was made to vary the intensity of the light. However, during the course of the work on changing the geometry, it varied ca. 2-fold, but no effect on 4 was observed. The observation that the quantum yield is substantially independent of (HzOZ)further supports the conclusion that it is also independent of the rate of absorption of light since the number of quanta absorbed per unit volume changes. The rate of absorption of light was < 5 X 10'' quanta 1.-1 set.-' for all the experiments. I n some experiments, the effect of passing gases through the hydrogen peroxide solution was tested. When COZ, A or 02 was swept through, the rate of decomposition was U I I affected. The tests were made wit: H z O at ~ 8 X lo-* M , with acid at M and a t 1 M a t 0 . Under the same conditions, H1 does affect the rate, enhancing it when the gas is passed at a high rate. The gases were scrubbed by a solution of potassium iodide after leaving the reaction vessel. Only with 0 2 was a noticeable amount of oxidizing agent carried (6) W. G. Leighton and G. S. Forbes, THIS JOURNAL, 62, 3134 il030).
6000
JOHN
h i . 74
P. RUNTAND E~ENRYTAUBE
TABLE I QUAXTUMYIELD OF THE PHOTODECOMPOSITIOV OF HYDROGEN PEROXIDE AT HIGHLIGHTINTEN5ITY Concentrations in moles litrr; time of insolation, 4 to 6 hr.
fo
=
fr
=
THE
No.
1 2 3
T , " C . (HCIOII
25& 23 & 25 & 25 +
I
1.0
Orhcr suhs.
(HzO?)
6
0 017
0.99 1 1.0 ,048 1.06 1 1.0 1.29 .1b6 4 I 1.0 186 1 .:30 .i (J 1 .0 0.77 030 ti (1 1 .0 ,020 .:ti 7 0 1.0 .84 ,047 8" 0 1.0 047 .92 !I 0 1.0 0 . 0 2 .\I sac1 047 ,236 04 7 .80 IO o i . ( ~ 2 x 10-.3 AI SH,CI I1 (1 1.0 2 X l W 3 .if S a B r .XI) .047 .S.5 I2 0 1 .ti 0 , O t 5 .lI Mn( C l 0 , ) ~ .047 13 0 1 . 0 0 . 0 1 .lf I~e(C104),j ,048 .49 1.1 0 10-1 .. ,021 .8U I3" 0 1.@ ...,...... ,0078 .ii liih o 1.0 , . . . ..... ,020 .T9 1Th 0 1.0 . . . .., .. ,054 .83 IS5 0 10-4 .......... 022 .84 Tap distilled, rather than redistilled water \\-as used. h Collimated light beam. ... . .. . , . .., .... , . , . .... . , , . ..., . .,. . .... , , . ., . ... ... . ,....
--
over. The amount, which represented only a small fraction of the hydrogen peroxide decomposed was very nearly the same even when hydrogen peroxide is omitted from the system, and is attributed to the formation of 0 3 in the oxygen. The effect of Ce(CIOl)a was also studied, as part of the scries in Table 11. The presence of C e + + + a t 0.011 -11 caused a decrease in 4 of c a ~10%. A tracer experiment showed that in the photodeconiposition under the conditions we have employed, the oxygeii comes solely from the hydrogen peroxide. Therefore, the intermediates which yield 0 2 cannot undergo appreciable exchange with the solvent, nor can the hydrogen peroxide undergo appreciable exchange with water during the reaction. The experiment was conducted by dissolving hydrogen peroxide of normal isotopic composition in water %fold enriched in 0l8, without added acid. The mole fraction of 0 ' 8 in the initial hydrogen peroxide (N,O) was 2.123 X 10-8, in the oxygen liberated after decomposition of 0.4 of and in the rethe hydrogen peroxide, (N,) 2.117 X The sidual hydrogen peroxide (iVp) was 2.148 X deviations of N8 and Npfrom ~vp"are real. Within experiniental error they are the same as those observed when the environment has the same isotopic composition as the peroxide, and are attributed to isotope fractionation. The second type of tracer experiment was perforrnctl i n which 0 2 of normal isotopic composition was passed through :i solution of hydrogen peroxide fa. %fold enriched i t i OL8it: water of normal isotopic composition. After a consitlcrahlc fraction of the hydrogen peroxide had decomposed photochemically, its isotopic composition was determined. 1 1 1 an experiment with the concentration of H202 initially a t 0.037 M and no acid added to solution, analysis of the oxygen released from the residual hydrogen peroxide (17%) on oxidation with Ce(1V) showed it to be enriched i n O'* by 1.8%. This increase is about what would be expected of normal isotope fractionation in the photodecomposition ( v i d e infra) and is opposite in direction from changes expected for exchange. We conclude that exchange betweeii oxygen and H202, direct and catalyzed by the photoprocesses is very slight, at any rate not great enough to invalidate the measurements of fractionation fa.ctors which are rcported. A slight exchange effect (1% change in Np produced by thephotolysis) would however have escaped notice. The method of determining fractionation factors for the isotope effects in the decomposition of hydrogen peroxide, and the significance of data of this type, were outlined in an earlier paper.' Three fractionation factors are defined' ( 7 ) T h e definition of t h e fractionation factors differs from t h a t i i i reference 1. Although t h e present form is more correct, tlic \.dtics yielded are identical a t low e n r i c h m e n t levels.
+
dOgls dow'' (OPl6) (OPl8) dOg16 dOWL6 The subscripts g, p and w refer to oxygen, hydrogen peroxide and water, respectively. fo and fi describe the fractionation for the production of oxygen and water, respectively, andf', the fractionation on formation of products, irrespective of their identity. With sufficient accuracy for our purposes
It
+
=
ft
=
+f* 2
fo
f,, is obtained by comparing the isotopic composition of the residual hydrogen peroxide after a measured fractional decomposition ( x ) with that of the hydrogen peroxide initially
The ratiof,/f, is given by
.q-
X
-
1 1Vg,2N,0 - xN, - 2Np(1 - X ) The sum and ratio of fo and fr being known, each factor can be calculated. The highest precision in measuringf, can be obtained by comparing No after very small fractional decomposition with N p o . I n the limit as x -+ 0, f,, = N , / N P o . This procedure was adopted in a number of the experiments, the value being corrected for the slight change in Npduring decomposition. The results obtained in a study of the fractionation effectr are presented in Table Il. Measurements of the quantum yield were made for some of the experiments, more as a check on the process being studied than as quantum yield determinations. I t is of interest to note that a t 25' without added acid, substituting tap distilled water for the redistilled water introduces a chain decomposition. At 0" with added acid, the corresponding substitution caused only a slight increase in 4 (cf. experiments 7 and 8, Table I ) .
Discussion Lea6 has shown that a t sufficiently high light intensity (> 2.4 X 10'' quanta 1.-' set.-' when (H202) is cu. 0.01 M ) the quantum yield for the decomposition of hydrogen peroxide approaches a limit which is independent of acidity and of the concentration of hydrogen peroxide. Our observations made in the range of high light intensity confirm these conclusions. However, the limiting value of 4 (0.9s f 0.05) a t 2.5" we have measured is lower than that obtained by Lea, 1.30 rt 0.11. \Ye are unable to account for the diff erencc. The observation that 4 decreases as markedly as i t does when the teinperature falls proves that 1.0 a t 25" cannot be the limiting value of Q attributed to a primary efficiency of unity. Two mechanisms which are consistent with the requirement on the primary efficiency, with the kinetics of the change and which otherwise appear reasonable are
-
I
0
+ k~
I1
H20 + 0 + Hz02 = HO + HO:
HzOn
--f
(1') (2)
(1') and ( 2 ) followed by ( 3 ) and (4) as above.
A reaction scheme in which the products of the primary act are H and HOz has been considered but rejected because changing the concentration
Dec. 5, 19.52
PHOTOCHEMICAL
6001
DECOMPOSITION OF HYDROGEN PEROXIDE TABLE I1
ISOTOPE FRACTIONATION EFFECTS IN THE PHOTODECOMPOSITON OF
f1202
AT =
2537 A.
Concentrations in moles /liter (HC104) (HiOz)a
added
Temp., "C.
X
fo
fr
0.1 .1 .1
None None None None None None
25 f 1 25 f 1 25 f 1 0.5f0.5 25 f 1 25f 1 25f 1 25 f 1 25 f 1 25 f 1 25dz 1 25 f 1 25 f 1
0.378 .097 .204 .162 .05 .985 .436 .815 .679 697 .740 .67 ,687
0,990 .991 .990 .991 .989 .989
0.976
,988 .989 .989
.950 ,995 .995
... ...
... ...
.981
.951
.1 .1 -05 .025
.1 .05 .05 .05 .05
.05
None None 1.00
1.oo 1.00 1.00 1.00
.
...
of oxygen over a aide range has no effect on the rate. Oxygen competes effectively against Hz02 for hydrogen atoms,8 and would therefore cause a decrease in 4 if hydrogen atoms were present. Both mechanisms I and I1 are consistent with the tracer experiments. By each mechanism a limiting value of 4 is reached which is twice the primary efficiency. The primary process can be considered in terms of the scheme H202 + hv +H202* H2O2* --f HsOz, k d €LO2*-+ dissociation, k,
ZIzOz*represents the activated entity, whether an electronically excited species or radicals trapped in a solvent cage. Using the limits of 4 as establishing the primary efficiencies as 0.49 and 0.38 a t 25 and O", respectively, k r / k d a t the two temperatures is 0.96 and 0.61. If the temperature coefficient of k d is assumed to be unity, the activation energy corresponding to dissociation to effective products is 2.9 kcal. This value seems reasonable in comparison with cu. 7 kcal. for the corresponding process for bromine and the value for chlorine (wave length dependent) of cu. 3 kcal.9 In all cases, the values represent lower limits for the activation energies of the dissociation steps. Even if hydroxyl radicals are assumed to be formed as primary products of the absorption of light (mechanism I), i t is not unreasonable to suppose that they can undergo reaction in the 0 solvent cage to form the products HzO (mechanism 11). "Escape from the solvent cage" on this interpretation means formation of these products in competition with reunion and deactivation. Good evidence that the reaction of hydroxyls 0 takes place has been presented to form HzO by Hardwick.lo The fact that substanccs such as Br- and C1which arc known to react with HOII do not affect the quantum yield does not prove that HO is absent. The effect of the reaction
+
+
HO
+ Br-
--f
OH-
+ Br
( 8 ) H. Pricke, J . Chcm. Phys., I), 349 (1934). (9) A. C. Rutenberg and H. Taube, THISJOURNAL,
73, 4426
(1951).
(10) T. J. Hardwick, Can. J . Chcm., SO, 23 (1952). (11) H.Taube and W. C. Bray, TXISJOURNAL, 69, 3357 (1940).
ft 0.983
... ...
... ... ... ...
...
,984 .9sg ,969 .992 .992 * 993 .986 .966
...
@
..
......... ......... .........
...
.978
Remarks
......... .........
.........
......... Tap distilled HzO
......... ......... .........
6 X
MFe+++
M Fe+++
0.99 1.00
.. .. 1.19
..
3.8 1.00 0.99 .96 .91 .88
for example, may merely be to replace reaction 3 by Br
+ HpOz+H + + Br- + HOz
Similarly with Mn++, the formation of M n + + + would be followed by Mn+++
+
H202
--+ M n + + + H0z + H +
without necessarily producing a change in quantum yield. Evidence that Fe+++ does participate, although affecting $I only slightly a t low concentration is that f,,and fr change when Fe+++ is added. The changes in the fractionation factors prove that new substances are reacting with H z 0 ~ when Fe+++ is present. When tap distilled water replaces rcdistilled water, a chain decomposition of hydrogen peroxide is initiated. The values off are different from those observed when the limiting quantum yield is observed, and show that new intermediates are involved. It is very doubtful that these are HO and HOz, and they are presumably formed from some impurity, dissolved or solid, in the tap distilled water. The experiments with Fe+++ added are difficult to evaluate quantitatively. At 10-2M Fe+++ only 3% of the light is absorbed by HzOz; however, the quantum yield is decreased by only 40%. A question of interest which will require more data to be settled is whether light absorbed by Fe+++ is also photochemically effective. If Fe+++ acts as an efficient internal filter, i t must act also to increase the chain length since i t produces a relatively slight change in 4 when present in sufficient amount to absorb nearly all the light. The decrease in 4 produced by Ce+++ may be due to an internal filter action. It is of interest to consider the measured values of f, andf, in relation to mechanism I and 11. For mechanism I,fo = fs.f4 and fr = fi. f i describes the discrimination between isotopic forms of HzO2 in rcaction (l),fa in reaction (3) and f4 describes the isotope discrimination when HOz forms 0 2 . On the basis of mechanism I , the fractionation of isotopes in forming water takes place only in the primary act (HO once formed yields water with no further isotope discrimination). The observations on fi.are inconsistent with this mechanism since it provides no way of explaining the change off, with acidity. Further, the magnitude of the fractionation effect in low acid cfr = 0.977) seems too great to be attri-
buted to a primary act of etliciency nearly 0.3. llechanisni I1 as written similarly givesj; = .II’ and meets the same difficulty. However, if it is supposed that at low acid, reaction 2 takes place as
chain lengths are long makes i t questionable to ;issume that the chain carriers are these radicals i n an!. particular case. The fractionation factors .fo andJ should be useful in characterizing the intermediates in future work. They have the advanO* -+ OOH- = 0 ’ 0 oir the necessary degree of ireedom to account for the tage over rate measurements that they are indechange in f r is introduced. -In analogous change pendent of chain length, and are affected only by in mechanism does not appear as reasonable with changing the identity of the intermediates. Thus HO as the reactant. The fractionation data teiid d n accidental inhibitor that acts only by breaking to disqualify mechanism I, and favor T I a i a iiicans chains will affect the quantum yield but not the \.alues of i o aiid J , . I t IS assumed in these remarks of explaining all the observations. The transition from the limiting deconiposition to that the chains are long enough so that the princithe chain decomposition has been discussed by pal path lor deconiposition of HLO? is by the chain Lea.5 The slight trend in I#I with concentration of mechanism Acknowledgment.- -This work was supported by hydrogen peroxide (Table I, experiments 1-4 and 5-7) is presumably due to a residual chain decom- the Oftice of Kava1 Research under contract NBposition, which diminishes as the peroxide cuncen- ori-020%. The funds for the purchase of the mass tration decreases. I t is by n u means certain that spectrometer used in the research were supplied by the chain decomposition which sets in is carried b>- the -1toiiiic. Energy Commission under contract HO and HOz. The difficulty that has been exper- - \ t i l l -1)-!12. ienced in obtaining reproducible data when the CHICA(>O 37, 1I,I,I’i
