M. ANBARAND I. PECHT
352
aliphatic nitro compounds used in this investigation results in the formation of nitrogen dioxide. It is reasonable, therefore, to expect that the primary step
in the photolysis of the more complex aliphatic nitro compounds will also involve carbon-nitrogen bond scission and the formation of SOz.
On the Sonochemical Formation of Hydrogen Peroxide in Water
by M. Anbar and I. Pecht The Weizmann Institute of Science, Rehoooth, Israel
(Received September $0, 1963)
The sonochemical formation of hydrogen peroxide was investigated in HzO1*solutions containing H20P,l6. It was found that nonvolatile OH scavengers do not affect the production of HzOZ,in contrast to volatile organic solutes. It was concluded that HzOzis not formed by recombination of OH radicals in solution. The sonochemical formation of HzOzunder oxygen was studied using HzOls and Oz18r18. It was shown that HzOzis formed by three parallel pathways: from the water, from oxygen atoms, and from oxygen molecules. Oxygen atoms formed on sonolysis were shown to form ozone to a limited extent.
Hydrogen peroxide is known to be formed in water undergoing sonolysis both under oxygen and under other The yield of hydrogen peroxide was shown to be dependent on solutes and on the composition of the gas over the solution. It was suggested that hydrogen peroxide is formed under argon by the recombination of OH The submission of water to the action of ultrasonics under oxygen produces additional pathways for the formation of HzOz. These pathways have been investigated using oxygen-18 as tracer,2 and it was claimed that two-thirds of the HzOzproduced originates from the oxygen gas and that the 0-0 bond is not cleaved during this reaction. On the other hand, it was claimed that ozone is formed sonochemicallyj under similar conditions, which obviously implies t,he cleavage of the 0-0 bond. Thus, further isotope experiments were required to clarify this point. The mechanism of formation of hydrogen peroxide was discussed by Weissler,a who also concluded that HzOz is formed under argon by the recombination of OH radicals. This conclusion was arrived a t in view of the fact that acrylamide, formic acid, and allylthiourea, which are efficientOH scavengers, inhibit the formation The Journal of Physical Chemistry
of HzOz. It has been pointed out that the behavior of the sonolyzed systems, which are not completely homogenous, differs significantly from the same systems undergoing radiolysis. As all three scavengers examined by Weissler are volatile compounds, it remained an open question whether HzOz is formed in the liquid or in the gas phase. It was suggested, therefore, to study the formation of Hz02 in the presence of nonvolatile OH scavengers, e.g., thallous and formate ions. The sonochemical yield of HzOz determined in all previous studies is the net result of the yield of formation and of the presumable decomposition of HzOz. Applying a technique developed in radiation chernistryj6 dilute solutions of H202l6*l6 in H201*were subjected to (1) A. Henglein, Naturwiss., 43, 277 (1956); 44, 179 (1957). (2) M. Del Duca, E. Yeager, M . 0. Davis, and F. Hovorka, J . Acoust. Soc. A m . , 30, 301 (1958). (3) A. Weissler, J . A m . Chem. Soc., 81, 1077 (1959). (4) M.Haissinsky, R. Klein, and P. Rivayrand, J . Chim. Phys., 5 9 , 611 (1962). (5) M. Haissinsky and A. Mangeot, Nuovo Cimento, [ l o ] 4 , 1086 (1954). (6) M . Anbar, S. Guttmann, and G. Stein, J . Chem. Phys., 30, 703 (1961).
SONOCHEMICAL
FORMATION O F HYDROGEN PEROXIDE
IK
ultrasonics. Thus, it was possible to obtain the net yield of formation of hydrogen peroxide. Experimental
Materials. 018-enriched water between 92 and 97 atom yo OI6 was obtained from the isotope distillation plant of the Weizmann Institute. Oxygen-18 gas 97 atom % was prepared by electrolysis, followed by enrichment by thermal diffusion. The oxygen-enriched water contained practically 100 atom % deuterium arid was thus nearly pure D2018. It has been shown that the sonolytic decomposition of water does not involve amy deuterium isotope effect.' In order to simplify formulas, we shall refer to this water as Hz018. The H202 was BDH microanalytical reagent 100 volumes solution. All the water used was triple-distilled. The methanol, T12S04,and NiS04 were BDH Analar and thie sodium formate and &SO4 were Baker analyzed reagents. The gases used were of the highest purity available (Matheson Co. Ltd.). Ultrasonic Irradiation. The ultrasonic genera tor was a Model T 200 (Lehfeldt and Co. GMBH) with a water-cooled quartz tramducer operating at 800 ko. A special coricentrator was adapted to fdcus the ultrasonic energy to a limited volume. The irradiation vessel was thermostated by circulating water a t 20 + 1'. The irradiation vessel used in all experiments was a small Pyrex buIb (volume 7 ml.) connected to an inlet tube. The Pyrex bulb was of minimal thickness in order to reduce sonic absorption, The bulb was fitted a t a constant position in the focus of the ultrasonic concentrator. The energy input was 1.6 X l o 9 ergs/sec./cm.2 in all the experiments. The yield of H2O2, und'er argon, under these conditions was 4.2 X mole/l./min. Analysis. 'The procedure adopted was identical with that used in a previous study.6 After sonolysis, an aliquot of the solution was decomposed over pla tinum black in a vacuum system. The formate-containing H202 solutions were decomposed by cerric ions, as the formate or its decomposition products inhibited the catalytic action of platinum black. The O2 evolved in this procedure originates only from the HzOz and no oxygen exchange between differently labeled HzOzmolecules takes place during the decoxrtposition.8 The oxygen was collected and its isotopic composition was determined by mass spectrometry. The mass spectrometric analysis was carried out with a CEC Model 21401 mass spectromleter. The isotopic abundance was determined by scanning masses 28 to 40. Masses 28 (X2) and 40 (Ar) were determined t o check on the air and argon contamination. Obviously, air causes dilution of the isotopic composition of oxy-
WATER
353
gen, whereas argon introduces an error in the determination of mass 36 (0218118 = Ar3G). In all experiments HzOzof normal isotopic composition was used. Oxygen which originated from the initial Hz02 contained consequently 0.4% 0 1 6 0 1 8 and The H2OI8used contained 3 to 8 virtually no 018018. atom % 0l6and consequently the H202 which originated from water contained 6 to 16% H202.1G,18Allowance for these dilution factors was made in all calculations.6 In the remainder of the sonolyzed solution, the HzOz concentxation was determined spectrophotometrically by either the titanic sulfate9or the triiodide'O methods. Results and Discussion The yield of hydrogen peroxide H20218,18 formed under argon in the presence of different additives is given in Table I. It has been found that H20218,18 is formled under our conditions a t a yield of 4.9 pmoles/l./min., which is independent of the total energy absorbed and of the concentration of HzOZ, within the measured range. Under the same conditions, the yield of Hz02 was determined by nonisotopic methods, in the absence of initial HzOz in solution, and was found to be 4.2 pmoles/l./miu. ; this means that about 0.7 @mole/ l./min. of H2Op are destroyed under the given conditions, irrespective of its concentration. The addition of 7 . 5 X lop2M NiS04to the sonolyzed solution did not affect the yield of Hz0z18,18.Xickel ions were shown to interact with electrons" and with excited water molecules in solution.12 The absence of any effect of nickel ions on the yield of H20z18s18 implies that the latter species is not a precursor of H20218,18 in solution. Thallous ions were found to decrease the yield of H2OZ18s1*to a certain extent and this effect might be attributed to their fast reaction with OH radicals.Ia When, however, the effect of thallous ions is conipared with that of potassium ions, it is found that the latter have even a more conspicuous effect on the yield of H20218,18. It has to be concluded, therefore, that the effect of thallous ions is analogous to that of potassium ions and is due to a change in water structure, which affects the cavitation process.5~'~
( 7 ) M. Anbar and I Pecht, J . Chem. Phys.. in press. (8) M.Anbar, J Am. Chem. Soc., 8 3 , 2031 (1961). (9) P. Bonnet-Maury, Compt. rend., 218, 117 (1944); A. Weissler, Ind. Eng. Chem., A n d . Ed., 17, 695 (1954). (10) A. 0. Allen, C . J. Hochanadel, J. A. Ghormley, and T. 7,V. Davies. J . Phys. Chem., 56, 575 (1952). (11) J. H. Baxendale and R S. Dlxon, Proc. Chem. Soc., 148 (1963). (12) M. Anbar and 11. Meyerstein, Israel AEC Report IA-901 (1963). (13) T.J . Sworsky, Radiation Res., 4, 483 (1956).
Volume 68, Number 2
Februaryq 196.4
M. ANBARAND I. PECHT
354
If H2OZ1*v1* would be formed by recombination of OH radicals in the bulk of the solution, one would exwould strongly depend pect that the yield of H20218,18 on the presence of H20216,16. In other words, the yield of Hz02 would be expected, like under radiolytic conditions,6 to fall below' the yield of H2O2 in the absence of initial hydrogen peroxide. Further, 5 X low2 M Tl+ should bring it down to zero. The experiments with formate ions show that these do hardly affect the yield of Hz02even a t 0.5 M formate. When formate-containing solutions were irradiated for long periods, the yield of Hz0218,16 was found to diminish. This is most probably due to the effect of carbon monoxide formed from formate15 on the production of HzOz. l G The experiments with thallous ions and with formate ions, both of which are efficient OH scavengers, point to the fact that if Hz02is formed by recombination of OH radicals, these are not accessible by scavengers in solution. On the other hand, Weissler has shown that the precursors of HzOz are completely scavengable by formic acid, acrylamide, and allylthiourea, all of which are efficient scavengers of OH radicals. The only difference is that these reagents are volatile, in contrast to formate and thallous ions. Our experiments with methanol, which is a relatively poor scavenger for OH radicals, as compared with acrylaniide, l7 show that the formation of HZOZmay be abolished completely in the presence of any organic volatile compound. The last results are in accord with previous findings on the effect of volatile organic solutes on the sonochemical formation of HzO2.I8 In other words, if HzOz is formed by the recombination of OH radicals, this does not take place in the liquid phase. The effect of oxygen a t small concentrations (2.5%) on the yield of H20218,1* .corroborates the conclusion that hydrogen peroxide is produced in the cavitation and not in the liquid phase. Rloreover, as there is no plausible reaction between oxygen and OH radicals, the latter are unlikely to be the species scavenged by 02. It seems reasonable to suggest excited water molecules as plausible precursors of HzOz in the gas phase. These were previously suggested as the precursors of the "molecular" H202under radiolysis6 and were shown to react both with oxygen and with organic solutes. The formation of excited water molecules in cavitation is not surprising in view of the transient high temperatures i n v ~ l v e d . ~ ~ ~ ~ From the previous results, it is concluded that the yield of H20Z1*,l8 formed from water is considerably diminished under oxygen. On the other hand, it has been shown that the yield of H 2 0 2 under oxygen is a little higher than that under argon.3 This would The Journal of Physical Chemistry
Table I : The Sonochemical Formation of H2021*,1*in H201*under Argon in the Presence of H2O216,16 H20g18'18
Duration of sonolysis. rnin.
20 60 80 120 60 85 30 90
100 93 100 90 85 85 10
100 100 81 a
[Additive], rnoles/l.
[HzOzl, moles/l.
x
x
108
E ormed,la moles/l./ min. X 106
102
..
2.5 2.5 6.3 2 7 2.3 2 .0 2.3 1.9 1.9 1.9 2.5 2.5 2.5 2.5 2.3 2.3 2.3 2.3
4 95 4 95
, .
..
5 05
4 75 5 00
, .
7.5 1.0 2.0
4 80 4 50
2.0 5.0
4 00 3 io 3 30 3 90 2 90 3 80 2 50 4 50 0 58 0 06 3 60
15.0 2 .o
15.0 5.0 20.0 50.0
10.0 50.0 2.5%
Normalized to 100%
0l8.
imply the formation of hydrogen peroxide from the oxygen by different mechanisms. There are two mechanisms for the formation of HzOzfrom 02,which may be distinguished by isotopic tracer experiments. e-
+ 0%+ 0 2
02-;
+2 0 ;
0
202- --+ H20z
+
+ HzO --+HzOz
0 2
(1) (2)
In the first case, the hydrogen peroxide formed will have the isotopic composition of the oxygen without any cleavage of the 0-0 bond, whereas in the second, the hydrogen peroxide formed should have a mixed isotopic composition. Del Duca, et al.,2 have attempted to investigate this problem; however, owing to the unavailability of highly enriched oxygen-18 their results are of low significance. Consequently, we have repeated them using oxygen and water 96-97% in O1*. The results are given in Table 11. It has been found (Table 11) that under 1 atm. of 02'6q16 over HZ018 the yield of Hz0218,18 drops to 1.15 wmoles/l./min. At the same time H20216,18 of mixed isotopic origin is formed a t a yield of 1.25 pmolesjl./min. (14) (15) (16) (17) (18)
M. Anbar and I. Pecht, to be published. A. Weissler, J . Acoust. Soc. Am., 32, 1082 (1960). M. E. Fitzgerald, V. Griffing, and J. Sullivan, J . Chem. Phys., 25, 926 (1955). C. Ferradini, Advan. Inorg. Chem. Radiochem., 3, 171 (1961). A. Henglein and R. Schulz, Z . Naturforsch., SB, 277 (1953).
SONOCHEMICAL FORMATION OF HYDROGEN PEROXIDE IN WATER
355
~
Table 11: The Sonochemical Formation of HzO? under Oxygen in the Presence of H20216a16 HzO+6,18 formed,'
IlnOzlR-18formed,"
Oxygen, atom OrC 0 ' 8
Hz0, atom % 0 ' 8
moles/l./min.
moles/l./min.
0.2 !)7.0
06 . 2 0.2
1.25 1.3
x
106
x
106
1.15 3.3
a Correc-ted for H2021a,1g from 1120216,16 (0.2 atnni yo),for H2O2I6,l8originating from €I2O218m18 (3-4 atom %) and H20217j17 (accompanying the € 1 2 0 2 1 8 , 1 8 ) . Sormalized to 10074 0 1 8 .
On the other hand, under 1 atni. of 0 2 1 * , 1 8 over H2016, the yield of €120218,18 originating from 0t1Rz18 reaches 3.3 pmoles/l./min., whereas the H20216,18 of mixed isotopic origin amounts to 1.3 pmoles/l./min. It may be concluded, therefore, that under oxygen, hydrogen peroxide is produced by three different mechanisms, including both pathways suggested above. A t an O2 pressure of 1 atm., the relative yields of the three pathways are (H20): ( 0 2 ) : (0) = 1: 3 : 1.2. The proportions are expected to change with the partial pressure of oxygen. The over-all yield of T1202under oxygen, 5.8 ~moles/l./min., is about 20% higher than the yield under argon, in accord with previous results.
The formation of H202of mixed isotopic origin suggests the existence of oxygen atoms under cavitation conditions. If oxygen atoms are formed, they may either react with oxygen to form ozone5 or with water to form HzOz. If ozone is formed, it will induce isotopic exchange bctween different oxygen molecules by a chain reaction.lg We have checked 011 this possibility by sonolyzing a gas mixture of 0216.16 and 0218,18 over water and determined the yield of 0 2 1 6 , 1 8 produced. 02'6v18 was produced in this reaction, which was carried out under conditions identical with the previous experiments, with a yield of 6.5 pmoles/l./min., which i s comparable to the yield of H202. This low yield of isot&opic"scran~bling)'does not allow the existence of appreciable amounts of ozone in the sonochemical process. Furthermore, it may be concluded that the rate of reaction of oxygen atoms with water is rather fast and competes effectively with the 0 02 reaction. This result is in accord with former findings that water acts as an inhibitor for radiolytically induced 0 2 l 6 , l 6 O218, is0topic exchange,6
+
Acknowledgment. The authors wish to thank Mrs. V. Fischoff and Mrs. 0. Asher for their devoted help in the mass spectrometric analysis. (19)
R. A. Ogg, Jr., and W. T. Stuppen, Discussions Faraday Soc.,
17, 47 (1954).
Volume 68, .Vumber 2
February, l B G 4
