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J . Phys. Chem. 1989, 93, 3600-3605
Gas-Phase Reaction Kinetics of BH Jane K. Rice, N. J. Caldwell,+and H. H. Nelson* Chemistry DivisionlCode 61 1 1 , Naval Research Laboratory, Washington, DC 20375-5000 (Received: July 27, 1988; In Final Form: November 2, 1988)
Ground-state BH, generated from the photodissociation of BH3C0 at 193 nm and monitored by laser-induced fluorescence, is reacted with NO, H20, 02,C02, CO, H2, CHI, C3H8,C2H4, and (CH3)2C=C(CH3)2at 298 K. Values of the absolute rate constants for these reactions were extracted from observed BH concentration decay profiles (units of cm3 s-I): NO, (1.35 f 0.20) X lO-'O; H20, (9.75 A 1.64) X 10-l2;02,(8.08 f 1.20) X C02, (2.64 0.41) X C3Hs, (7.57 A 1.34) X C2H4,(1.17 f 0.18) X lo-''; (CH3)2C=C(CH3)2,(1.87 i 0.30) X The indicated uncertainties are f(2u plus estimated uncertainties in the measurement). An upper limit of cm3 s-' is placed on the rate constant for reaction with CHI. The measured rate constants for the reaction of BH with CO and H2 show a dependence on total pressure. For the reaction BH + H2, we observe a large portion of the falloff region and are able to fit the data using the expression cm3 developed by Troe, leading to an estimated value for the limiting high-pressure rate constant of (3.67 f 0.91) X s-l. For the reaction of BH with CO, measured rate constants could not be extrapolated to the high-pressure limit and therefore were modeled by using a transition-state theory-RRKM approach.
*
I. Introduction Recently, there has been a renewed interest in the reaction chemistry of boron atoms and other small boron-containing species. This interest has been stimulated in part by the possible use of elemental boron and boron-hydrocarbon slurries as fuels. On a volumetric basis, the theoretical energy release of elemental boron combustion exceeds that of hydrocarbon fuels by a factor of 3 and that of aluminum-based fuels by a factor of 2,' but only a fraction of this energy potential has been extracted in practical combustors. The most obvious area for investigation is the oxidation chemistry of boron and boron oxides, but boron hydrides and oxyhydrides are also expected to be of importance to the combustion problem, especially in the combustion of boronhydrocarbon slurries. In addition, boron halides may play an important role in achieving the full energy potential of these fuels in practical systems. On a more fundamental level, the chemical reactivity of these electron-deficient species is poorly understood at present and warrants additional study. Several direct studies of the elementary reactions of small boron-containing species have been reported recently. Reactions of boron atoms with several oxygen-containing species have been studied by Davidovits and co-workers.24 Two groups have studied In addition, kinetic studies the reactions of B and BO with 025-6 of the diatomic radicals BF7 and BC18 and the borane radical, BH3,9have recently been reported. There have also been several studies of the homogeneous chemistry of borane adducts. Anderson and Bauer'O obtained kinetic data on reactions of B2H6and BH3C0 with oxygen and nitrogen atoms and with N20,NO, NO2,and 0,. They concluded that a major pathway involves hydrogen abstraction from the adduct to yield boron hydrides (BH and BH,): O(3P) + H3B.C0 O H + [H2BCO]* OH + BH2 + C O H2O + [HBCO]* ---* H2O + BH + CO
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--
(1)
In two recent papers, Jeffers and Bauer have shown that the reaction rates in such systems are highly dependent on the nature of the adduct]] and that boron hydrides are key intermediates in the oxidation of these species.I2 When any of a number of borane adducts are rapidly mixed with oxygen atoms, BO emission is observed from the a and p systems with emission observed from levels up to v'= 10 in the A state of BO. The only reactions that have sufficient exothermicity to explain the production of such highly excited BO are 0 + BH [HBO] H + BO AH = -1 10 kcal mol-' (2) 0 + BH2 [OBHJ ---* H2 BO AH = -133 kcal mol-' (3)
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'N R C / N R L Postdoctoral Research Associate.
The latter reaction is thought to be a less likely source of the BO emission observed since the H2 product can take some of the available energy as internal energy. Reactions of the BH radical have been included in a kinetic model of the combustion of B2H6I3even though there have been no direct measurements of the rates of these reactions. The model relies on estimated reaction rates that in some cases are based on unproven thermodynamic data. We are attempting to provide experimental data on the reactions of boron-containing intermediates in order to allow more realistic models of boron combustion to be constructed. In this paper we report the results of a study of the gas-phase reactions of BH with several oxygen-containing inorganic species, hydrogen, and several small hydrocarbons using an optical pumpprobe technique. Laser photolysis of B H 3 C 0 at 193 nm is used to produce the BH radical. The major photolysis pathway yields BH3,I4with BH as either a minor, direct photolysis product or an indirect product from the dissociation of vibrationally excited BH3 to BH and H2.I5 The equivalent photolysis of B2H6produces no detectable BH. In any case, BH is observed within 20 ns of the photolysis pulse, which is short compared with the reaction time scales employed in this study. The absorption and emission spectra of BH have been rep~rted.'~,'' Detection of BH is accomplished by laser-induced fluorescence (LIF) on the A'II-X'Z" transition.'* (1) Yetter, R. A.; Rabitz, H.; Dryer, F. L.; Brown, R. C.; Kolb, C. E. Combust. Flame, in press. (2) DeHaven, J.; O'Conner, M. T.; Davidovits, P. J . Chem. Phys. 1981, 75, 1746. ( 3 ) DiGiuseppe, T. G.; Davidovits, P. J . Chem. Phys. 1981, 74, 3287. (4) Tabacco, M. 9.;Stanton, C. T.; Sardella, D. J.; Davidovits, P. J. Chem. Phys. 1985, 83, 5595. (5) Oldenborg, R. C.; Baughcum, S.L.; Winn, K. R. Submitted for publication in J . Phys. Chem. ( 6 ) Llewellyn, I. P.; Fontiin, A.; Clyne, M. A. A. Chem. Phvs. Lett. 1981. 84, 504. (7) Light, G. C.; Herm, R . R.; Matsumoto, J. H. J . Phys. Chem. 1985, 89, 5066. (8) Slavejkov, A. G.; Rogowski, D. F.; Fontijn, A. Chem. Phys. Lett. 1988, 143, 26. (9) Pasternack, L.; Balla, R. J.; Nelson, H. H. J . Phys. Chem. 1988, 92, 1200. Anderson, G. K.; Bauer, S. H. J . Phys. Chem. 1977, 81, 1146. Jeffers, P. M.; Bauer, S. H. Chem. Phys. Lett. 1981, 80, 29. Jeffers, P. M.; Bauer, S . H . J . Phys. Chem. 1984, 88, 5039. Shaub, W. M.; Lin, M. C. NBS Spec. Publ. (US.) 1979, 561, 1249. (14) Pasternack, L. Unpublished results show that $BH, > 0.7. (15) Rice, J. K.; Douglass, C. H.; Nelson, H. H. Nascent Distribution of BH from the Photodissociation of BH$O at 193 nm. Manuscript . in prep. . . aration. (16) Bauer, S. H.; Herzberg, G.; Johns, J. W. C. J . Mol. Spectrosc. 1964, 13, 256. (17) Johns, J. W. C.; Grimm, F. A.; Porter, R. F. J . Mol. Spectrosc. 1967, 22, 435.
This article not subject to U S . Copyright. Published 1989 by the American Chemical Society
The Journal of Physical Chemistry, Vol. 93, No. 9, 1989 3601
Gas-Phase Reaction Kinetics of BH I
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Figure 2. Plot of the pseudo-first-order rate constants versus NO pressure for the reaction of BH with NO. The points correspond to measurements at 5-Torr (A),20-Torr (O), and 100-Torr (B) total pressure with helium as the bath gas.
puter through an IEEE-488 interface (Berkeley Nucleonics Corp. Model 8088). All reactions are carried out under pseudo-first-order conditions with reactant concentrations at least lo2 times larger than the BH precursor, BH3C0. The pseudo-first-order rate constant (k? are obtained from plots of the logarithm of the BH LIF signal versus the delay time between the pump and probe lasers, as shown in Figure 1. As the reactant (NO) pressure is varied, several k’ values are obtained and plotted as a function of reactant concentration yielding a slope that corresponds to the bimolecular rate constant for the reaction. In the case of NO reactant, the reaction is independent of total pressure in the range 5-100 Torr. A typical plot of k’versus reactant concentration showing data at three total pressures for the BH + NO reaction is shown in Figure 2. In the cases where the measured bimolecular rate constant shows a pressure dependence, namely, reactions of BH with H2 and CO, bimolecular rate constants are determined as above over a range of added bath gas pressures. These observed rate constants are fit to the semiempirical expression of Troe1%2’in the case of BH + H2 or are modeled by using a transition-state theoryRRKMZ2approach in the case of BH + CO. The pressure range of 10-700 Torr is examined in the case of BH + Hz and 10-400 Torr for BH CO. The synthesis and purification procedures for BH3C0 are given by Bethke and Wilson.23 Our samples are twice distilled from an isopentane/dry ice slush to an ethanol/dry ice slush and stored in liquid nitrogen. Dilute mixtures (0.1%) of the BH3C0 precursor in helium are made in a 25-L metal cylinder. The practical lifetime of the dilute mixtures is 1-3 days. Precursor, reactant, and bath gases are flowed from storage cylinders through Tylan mass flow meters, needle valves, and a 20-cm length of tubing (to ensure thorough mixing) before entering the reaction cell. Typical flow rates for the B H 3 C 0 precursor mixture are from 1 to 5 sccm (standard cubic centimeter per minute) and for the reactants, 50 to 500 sccm. Total gas flows (of precursor mixture, reactant, and buffer) through the reaction cell are in the “slow flow” range. Flow rates of about 10 L min-l (actual volume displacement) are required to minimize the buildup of reaction products in the probed region. For these flows the reaction cell is “turned over” about 3-4 times per minute. The reactants, NO and C2H4,which have relatively fast reactions with BH, are diluted with bath gas to concentrations of 1.0% and OS%,
+
(18) Dufayard, J.; NtdClec, 0. J . Chem. Phys. 1978, 69, 4708
(19) Troe, J. J . Phys. Chem. 1979, 83, 114. (20) Troe, J. Ber. Bunsen-Ges. Phys. Chem. 1983, 87, 161. (21) Gilbert, R. G.; Luther, K.; Troe, J. Ber. Bunsen-Ges. Phys. Chem. 1983, 87, 169. (22) Berman, M. R.; Lin, M. C . J . Phys. Chem. 1983.87, 3933. ( 2 3 ) Bethke, G. W.; Wilson, M. K.J . Chem. Phys. 1957, 26, 1118.
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The Journal of Physical Chemistry, Vol. 93, No. 9, 1989
TABLE I: AHf(298 K ) for Boron-Containing Species Relevant to This Studv (kcal mol-') JANNAF other JANNAF other B BO BO1 HBO HOBO H2BOH HNB
134 f 3 Of2 -68 f 2 -47.4 f 0.7 134 f 1
BH BH2 BH3 HOB -63d HBOH -70e BHjCO 6 3 . g HBN -2.2b
105.8 f 2 48.0 f 15 25.5 f 2.5
103.5' 73.6' 20.6' -15' -17' -26.5' 75.5'
'Reference 28. bReference 29. cReference 30. dReference 31. CReference 32. /Calculated from the AH, values from ref 24. respectively. Room-temperature liquids, H 2 0 and 2,3-dimethyl-2-butene (tetramethylethylene or TME), are used in concentrations of 0.27% and 0.29%, respectively, in helium. Helium is used as the bath gas for the BH reaction with NO, HzO, 02,c o 2 , N2, CHI, C3Hs, CzH4, and TME. Helium, SF6, and CF4 are the bath gases used for the BH reaction with CO, and SF6 is used for the reaction with H2. Air Products industrial grade O2 (99.5%), N 2 (99.98%), H 2 (99.995%), and C 0 2 (99.8%) and Matheson chemical purity N O (99.0%), C O (99.5%), SF6(99.8%), and C2H4(99.5%) are used without further purification. Research grade CH, (99.998%) and C3Hs (99.998%), for which limits on the ethylene content are quoted as
