COMBUSTION AND FLAME 21, 261-264 (1973) 261

Hydrogen Peroxide as a Source of OH Radicals

at 340 to 380 °C

DAVID BOOTH* and D. J. HUCKNALLt Department o f Chemistry, Newcastle upon Tyne Polytechnic, Newcastle upon 2~ne, NEI 8ST, England

and

R. J. SAMPSON I.C.I. Ltd., Petrochemicals Division, Billingham, Teesside, TS23 IJB., England

Pyrex reactors, thinly coated with boric acid and aged by repeated H 2 - -02 SlOW tcactions 'and explo- sions at 500°C, have been asea in a study of tile competitive oxidation of pairs of aikaues in the pres- ence of excess H202, 02 and N 2 at 340 ° and 380°C. Under these conditions, the homogeneous decomposition of H202 can be used effectively as a source of OH radicals and reproducible results can readily be obtained. Complications can arise, however, ff thick coatings of boric acid are used. Such coatings can be aged but adsorption of H202 and/or H20 on to the thick coating is appreciable.

Hydrogen peroxide is a product of, or inter- mediate in, numerous combustion reactions. Also, the radicals formed from hydrogen peroxide (HOa and OH), and even H202 itself, are frequently important chain centres in such reactions [1 ] .

Because H20~ is sensitive to heterogeneous de- composition, its use as a thermal source of HOa or OH radicals in studies related to combustion chemistry has been of limited value below 400°C. Irreproducible behaviour which frequently ham- pers the study of combustion reactions is due, at least in part, to the erratic surface destruction of Ha O2 and HOa.

An important advance in combustion chemistry was made when Baldwin and on.workers [2] dis- covered that repeated H2--O2 slow reactions and explosions resulted in the "ageing" to a reproduc. ible condition of reaction vessels coated with boric acid. Using such "aged" reactors kinetic

*Correspondence to this author at: Department of Chemistry, Newcastle upon Tyne Polytechnic, Ellison Building. Newcastle upon Tyne, NE1 8 ST.

"t~Present address: Department of Chemistry. The City University, St. John Street, London, ECIV 4PB, England.

data were obtained from which a detailed de- scription of the hydrogen-oxygen reaction and precise values of the rate.constants for the indi- vidual reaction steps were deduced [3]. By in- clusion of small quantities of alkanes, and from studies of aldehyde oxidation [4], a comprehen- sive set of Arrhenius parameters for reactions in- volving the species H, OH, HOa, R, 02 a,~d RH were obtamed [5]. Reproducible behaviour for the reaction between butane-2-one and oxygen in vessels of this type has also been obtained [6]. The success of all these studies, carried out at various temperatures in the region 450 to 600°C, has been attributed [2, 7] to the inertness of the reactors towards destruction of H2 02 an J HOa.

Below this temperature range, the high activation energy homogeneous decomposition of H202 is so slow that it has been assumed [8] that hetero- geneous decomposition would be predominant. The present work shows that this not need be so: aged boric acid coated vessels yield .reproducible results down to at least 340°C, for even at this temperature the homogeneous decomposition of H2 O2 is significant compared with its surface destruction.

Copyxight © 1973 by The Combustion Institute Published by American Elsevier Puhlishin~ Company, Inc.

262 DAVID BOOTH and D. J, HUCKNALL

TABLE 1 Maximum rates of pressure change for Ha/Oa/N a mixtures a at 500°C, R m, and alkene production for alkane/HaOa/Oa/N2 mixtures ~ at 340* and 3800C in various reactors

R m (torr rain -1) tdkene c (tort, after 2 rains) Reactor History at 500°C C2H4 C3H6

(i) At 380"C I Thickly-coated 5.9(9) 0.072 ± 0.005 0.099 ± 0.005 2 Packed 3.8(2) 0.017 d 0.028 d 3 Thinly-coated 6.0(3) 0.098 ± 0.004 0.1 ]t9 ± 0.006 4 Reactor 3,

thinly-coated 5.6(9) 0.096 ± 0.006 0.I I l ± 0.005 6 Reactor 3,

thinly-mcoatcd 6.0(2) 0.099 ± 0.01 0.110 ± 0.007 7 New, thinly-

coated 5.7(6) 0.102 ± 0.01 0.109 ± 0.004 Many, 34 mm i.d., References 2, 3 5.87 ± 0.15

(ii) At 340"C 7 New, thinly-

coated 5.7(6) 0.011 ± 0.001 0.017 ± 0.001

at40 tort H2, 70 torr 02,290 tort" N2 b2 tort C2H6 + CsHs (1:1), 10 tort H202, 60 torr 02,228 tort N 2 CAlkanes measured by g~s chromatography; 6' Porapak Q, 520C, 10 psig N2 carrier

± refers to maximum for a particular reactor (mean of 8-25 determinations) aTypical remits (large scatter)

The experimental evidence for this conclusion [9] comes from competitive oxidations of pairs of alkanes, at 340 ° and 380°C, in the presence of excess I t 202 ,02 and N2, in reactors, 36 m m i.d. × 220 ram long (S/V, 0.083 mm "t ), which had been thinly coated with boric acid and aged by repeated H2--O2 slow reactions and explosions at 500°C, using Baldwin's procedure [7]. Thin coatings were achieved by introducing a saturated solution (25-30 cm 3) of AnalaR boric acid in aqueous ethanol (50% v/v), heating the solution in the reactor for about 5 minutes, and then pour- ing the majority away. The residual solution (about 5 cm 3) was then allowed to evaporate quickly from the hot reactor surface, under re- duced pressure, wlfile the reactor was rotated con- tinuously. Before insertion in the reaction fitrnaee, the reactor was evacuated for about 45 minutes, using a rotary pump, when a fine white coating was produced.

In these thinly-coated vessels the consumption of alkanes and the production of the major organic products (the conjugate alkenes) were

extremely reproducible, not only from day to day, but over several months and from reactor to reactor. The table shows the maximum rate of pressure change, Rm (corrected for dead volume, but uncorrected for consumption of reactants'), for 'standard' Ha--On--N2 mixtures at 500°C. lit also shows the quantities of alkenes produced after 2 minutes from standard mixtures of C2 ]~16, CsH6, H 2 0 2 , 0 2 , and N2 at 340 ° and 380°C. Yields of C2 H4 and C3 Ha were almost 100% (e.g. 97 ± 3% for C2H6), for up to 20% conversion of C ~ H6 and for up to 2.5% conversion of C 3H6. Other products found will be reported in a later paper [10].

Detailed discussion of the nature of the reactions occurring in the paraffin-containing system, and the roles of HO2 and OH radicals, will also appear in the forthcoming paper [10]. It appears that there is minimal chain-character, and that llhe main reactions which occur are the tbllowing:

H20~ +M ~ 2 O H + M (1)

H202 ~ surface destruction (2)

HYDROGEN PEROXIDE AS A SOURCE OF OH RADICALS 263

OH + H202 --~ H20 + HO2 (3)

C.H2.÷e + OH ~ C.H2.÷, + H20 (4)

C.H2..t + O2 --" C.H2. + HO2 (5)

2HO2 ~ H202 + 0'2 (6)

HO2 --~ surface destruction. (7)

Abstraction by HO2, reaction (8), is of little importance.

HO2 + C.H2n+2 ~ H:O2 + C.H~.÷ I. (8)

Clearly, it is of interest to establish the relative importance of the two modes, (I) and (2), by which H20~ is removed. Attempts were made to establish the H2 O2 concentration in the alkane- containing system, by allowing the reaction to proceed at 380 ° for a specified time, and then pumping the reactor contents through a Volman trap at -78.50C. Though the method gave some. what erratic results [3, 11], it indicated that after 6 minutes the H~ O2 pressure exceeded 20% of its initial value. The data were used to obtain the numerical value of k2, the rate constant for sur- face destruction, by two methods. Firstly, the measured initial rate of disappearance of H,. O~ was equated to [H20~] z=o {Z km [M] + k: }. Literature values of k~ n for M = N2, O2 and H202 [8, 12] were used, and decomposition brought about by the paraffins was neglected (the partial pressure of the paraffins was very much smaller than ofNz, 02 or H202). Secondly, if it is assumed that some OH radicals are converted to H02 by reaction (3), and the remainder by reac- tion (4) followed by (5), and that all the H02 radicals disproportionate to form H~02 by reaction (6), the integrated form of the rate equation becomes

£n ([H2Oz] t-a/lH202 It=t) = (2Y~k~n [M] + k=)t.

Then, using established values [8, 12], for 2Zk~ n [M] as before, k2 was obtained. The second method, which does not depend upon evaluation of a slope at zero time is probably :he more reliable, and gave k2 = 8.(5) X I0-* see- ' . This value showed that the ratio of the initial rate of homogeneous decomposition to heterogeneous decomposition was approximately unity.

Thus, at temperatures at least as low as those of these experiments, and in suitably prepared vessels, hydrogen peroxide decomposes reproducibly with comparable extents of homogeneous breakdown and heterogeneous destruction. These vessels should thus prove of value for studies utilizing the controlled homogeneous decomposition of hydro- gen peroxide. They may also prove valuable for achieving reproducibility in studies of combustion reactions.

Certain complications arose in the use of aged boric acid coated vessels in this lower temperature range, as at higher temperatures [2, 7]. Firstly, "de.aging" eventually occurred; its onset was very marked, reproducibility being completely lost, the amounts of alk~nes fluctuated markedly from run to run and ultimately fell to very low values. The inclusion of standard runs in a series readily established whether "de-aging" had occurred. Secondly, " thin" boric acid c~_atings were neces- sary: if appreciably more than 5 cm 3 of the boric acid solution were evaporated, "thick" coatings re- sulted. Results obtair.,ed with "thick" coatings, and also for a reactor packed with glass tubes (S/V 0.49 ram- ' ) before thinly coating, are in- eluded in the table. Alkane consumption in the packed vessel was too small to be detected, how- ever, small irreproducible quantities of alkenes were produced. The thickly.coated reactor, which took much longer than usual to age (3 to 4 weeks instead of 8-10 days) exhibited behaviour inter- mediate between that of a thinly.coated vessel and the packed reactor: alkane consumption and alkene production were reproducible, but less than in tile thinly-coated reactors. In the thickly coated reactor the pressure of H2 O2 dropped to about 20% of its initial value in 2 minutes. Pressure' ob- servations on the system H202 (16 torr) and N2 (84 torr) at 300-500°C in thickly coated vessels were compatible with appreciable adsorption of H20~ and/or H20 on to the thick coating. [cf 13, 141.

Thanks are due to the Newcastle upon Tyne Education Committee for a maintenance grant (D.J.H.), to the Royal Society for a grant-in-aid, to L CL Ltd., Petrochemicals (formerly tI.O.C } Division, for the loan o f equipment and provision o f hydrocarbons attd to Laporte Industries Ltd., for tt, e gifts e~f 97-99~ hydrogen peroxide.

264

References 1. Sampson, R. J., J. Chem. Soc., 5095 (1963). 2. Baldwin, R. R. and Mayor, L., Rev. Inst. Franc.

P~trole. 13, 397 (1958), Seventh Syrup. (Intern./ Combus., p. 8. Butterworths, London (1958).

3. Baldwin, R. R. and Mayor, L., Trans. Faraday Sot.. 56, 80, 93, 163, (1960).

4. Baldwin, R. R., Langford, Do H., and Walker, R. W., Trans. Faraday Soc., 65,792, 806, (1969).

5. Baldwin, R. R., Everett, C. J., Hopkins, D. E., and Walker, B. W.,Advances in Chem., No. 76, VoL II, "Oxidation of Organic Compounds', p. 124, Amer. Chem. Soe., Washington, D.C. (1968). Bakes, R. R., Baldwin, R. R., and Walker, R. W., Thirteenth Syrup. (Intern.) Combust.. p. 291. Combostio~ Institute, Pittsburgh (1971).

6. Boodl, D., and Lever, G., to be published. Booth, D., and Young, R. A., to be published.

7. Bartett, J. M., and Walker, R. W., Combust. and Flame 12,501 (1968).

DAVID BOOTH and D. I. HUCKNALL

8. Baldwin, R. R., and Brattan, D., Eighth Symp. (Intern.) Combust., p. 110, Williams and Wflkios, Baltimo~, (1962).

9. Huekann, D. J., Some reactions concerning the gas phase oxidation of hydrocarbons, Ph.D., thesis, C.N.A.A., 1971.

10. Booth, D., Hucknall, D. J., and Sampson, R. J., to bc published.

11. Baldwin, R. R., Rossiter, B. N., and Walker, R. W., Trans. Faraday Soe. 65, 1044 (1969).

12. Baldwin, R. R., .lackson, D., Walker, R. W., and Webster, S. L, Trans. Faraday Soc., 63, 1665, 1676, (1967).

13. Edgerton, Sir A. C., Minkoff, G. J., and Salooja, K. C., Combust. and Flame, 1, 25 (1957).

14. Burgess, A. R., and Cullis, C. F.,J. Chem. Soe., 3041 (1961).

(Received May 12, 1972; revised M~rch 9, 1973)