J . Phys. Chem. 1990. 94. 4 9 5 2 4 9 5 6
4952
BH Reaction Kinetics Studied with a High-Temperature Reactor Nancy L. Garland,* C. T. Stanton,+ James W. Fleming, A. P. Baronavski, and H. H . Nelson Chemistry DicisionlCode 61 10, Naval Research Laboratory, Washington, D.C.20375-5000 (Receiced: November 2, 1989; In Final Form: February 8, 1990)
The rate coefficients of BH radical reactions with 02,CH4, C3H8,C2H4,NO, and H 2 0 were determined over the temperature range 298-750 K in a high-temperature reactor. The BH radicals were generated by the photolysis of BH3C0 at 193 nm and monitored by laser-induced fluorescence. The absolute bimolecular rate coefficients were obtained from measurement of the BH chemical lifetime as a function of reactant number density. For the reaction of BH with 02,a fit of the data to the standard Arrhenius expression &( T ) = A exp[-E,/RT] yielded a preexponential factor of (4.9 f 0.6) X IO-" cm3/s cm3/s and an activation energy of 2.4 f 0.1 kcal/mol over the range 298-750 K. A rate coefficient of (1.9 f 0.2) X was found for the reaction of BH with CHI at 675 K. For the reaction of BH with C3H8over the range 400-650 K, the data can be fit to the expression k ( T ) = (7.8 f 3.7) X cm3/s exp[(-9.5 f 0.4 kcal/mol)/RT]. The reactions of BH with NO and with C2H4both show a slight negative temperature dependence over the range 298-650 K . For reaction with NO, the data can be fit to the expression k ( T ) = (4.5 f 0.8) X IO-" cm3/s exp[(0.48 i 0.14 kcal/mol)/RT], while for the reaction with C2H4, the data can be fit to the expression k ( T ) = (4.0 f 0.7) X IO-" cm3/s exp[(0.50 f 0.14 kcal/mol)/RU. A linear fit of the data for the reaction of BH with H 2 0 yielded a preexponential factor of (5.0 f 1.0) X IO-'* cm3/s and an activation energy of 0.38 f 0.16 kcal/mol over the range 298-650 K.
Introduction The gas-phase reactions of boron and boron-containing compounds are of interest, in part due to the potential of very large energy release during boron combustion.',2 This potential energy release has led to the proposal for addition of elemental boron particles to liquid hydrocarbon fuels in the form of slurries or to solid propellant grains for combustion in air-breathing applications.* One difficulty in the practical development of these fuels has been the presence of a boron oxide coating on the surface of the unburned boron particles which inhibits the transport of oxygen to the elemental boron surface.l,2 This oxide coating could be removed by vaporization at very high temperatures or by chemical reactions with water vapor and other hydrocarbon combustion intermediates. Yetter et al.2 predict that an increase in the concentration of hydrogen in a reacting system containing boron/oxygen/hydrogen/carbon compounds will significantly increase the burning rate, but unfortunately, it also leads to a buildup of species such as HOBO and HBO. Under these conditions, elemental boron would not oxidize completely to the thermodynamically favored product, B203, and hence the maximum release of energy would not be achieved. The mechanism of the conversion of boron to B203 including steps involving the bottleneck intermediates HOBO and HBO is not well characterized; knowledge of the kinetics and thermodynamics involving key species in the combustion of boron is needed. A number of kinetic studies of boron-containing species have been carried out. These studies include reactions of B BCI,S BF,6 BH,C0,9 and B2H6.9 Anderson and Bauer9 and later Jeffers and BauerIo found that boron hydrides are key intermediates in the oxidation of BH,CO and H3BN(CH3)3. Very recently, the reactions of the boron hydrides BH311and BHI2-l4 have been studied near room temperature. These boron hydrides could be very important in fuel-rich environments, e g , near the surface of a boron particle during boron-assisted hydrocarbon combustion. The experimental determination of BH rate coefficients at higher temperatures will help us understand the chemical behavior of boron-containing species at combustion temperatures. We report here a determination of the rate coefficients of BH radical reactions with 02,CHI, C3H8,NO, C2H,, and H,O over the temperature range 298-750 K . We studied the temperature dependence of these reactions to determine the Arrhenius parameters for modeling purposes and to discover new product channels which can open up at elevated temperatures. The ex-
periments were carried out in a high-temperature reactor which can operate at temperatures up to 1500 K; however, thermal instability of the BH precursor precluded kinetics experiments above 750 K. The reactor is similar to that described by Fontijn.ls The reactants chosen for study are typically those encountered in the combustion of boron/hydrocarbon fuels. Experimental Section A mixture of 0.1% BH3CO/He, helium buffer gas, and reactant gas was slowly flowed through a resistively heated reactor. BH radicals were generated by photolysis of B H 3 C 0 at 193 nm and were monitored by laser-induced fluorescence (LIF) via the A'II-X'Z (1,O) band near 398 nm. A plot of the reciprocal BH chemical lifetime (disappearance rate) vs reactant partial pressure yielded the bimolecular rate coefficient. The total pressure in the reactor was typically 20 Torr; in some experiments, the total pressure was varied to investigate possible pressure dependence of the reaction rate coefficients. In the following sections, we describe the high-temperature reactor and give experimental details concerning the optical setup, the electronics, the temperature determination, the gases used, and calibration of the reactor. Reactor. The 8'/,-in.-long, 8I/,-in.-wide, and l%-in.-high stainless steel reactor was modeled after that of FontijnIs and is illustrated in Figure 1. Four ports on the sides of the vacuum housing provide optical access for collinear laser beams and for fluorescence collection at right angles. Gases flow into the reactor ( I ) Brown, R. C.; Kolb, C. E.; Yetter, R. A,: Dryer, F. L.; Rabitz, H. R Aerodyne Report ARI-RR-580. 1987. ( 2 ) Yetter. R. A.; Rabitz, H.: Dryer, F. L.: Brown. R. C.: Kolb. C. E. Combust. Flume, in press. (3) DiGiuseppe, T. G.; Davidovits, P. J . Chem. Phys. 1981, 74, 3287. (4) iMcKenzie, S. M.; Stanton, C. T.; Tabacco, M. B.; Sardella, D. J.: Davidovits, P. J . Phys. Chem. 1987, 91, 6563, and references therein. (5) Slavejkov. A. G.: Rogowski, D. F.; Fontijn. A . Chem. Phys. Lett. 1988, / 4 3 , 26. (6) Light, G. C.: Herm, R. R.; Matsumoto, J . H. J . Phys. Chem. 1985.
89. 5066. ( 7 ) Llewellyn, 1. P.; Fontijn, A,; Clyne, M.A . A . Chem. Phys. Lerr. 1981, 84. 504.
( 8 ) Oldenborg, R. C.; Baughcum, S. L.; Winn, K , R. Submitted for publication in J. Phys. Chem. (9) Anderson, G. K.; Bauer, S. H. J. Phys. Chem. 1977, 8 / , 1146. ( I O ) Jeffers. P. M.: Bauer, S. H . Chem. Phys. Lett. 1981, 80, 29. ( I I ) Pasternack, L.: Balla, R. J.; Nelson, H. H. J . Phys. Chem. 1988, 92, 1200.
(12) Rice. .I. K.: Caldwell. N . J.: Nelson, H. H. J. Phys. Chem. 1989, 93, 3600. (13) Harrison, J . A,; Meads, R. F.; Phillips. L. F. Chem. P ~ J JLett. S . 1988. 150. 299.
(14) Caldwell, N. J.: Rice, J. K.; Nelson, H. H.; Adams, G , F.; Page, M +OYT/NRL Postdoctoral Research Associate
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0022-3654/90/2094-4952$02.50/0
J . Chem. Phys., in press. ( 1 5 ) Liarshall. P.: Fontijn. A. J. Chem. Phys. 1986. 85. 2637
1990 American Chemical Society
The Journal of Physical Chemistry, Vol. 94, No. 12, 1990 4953
BH Reaction Kinetics
-+Baratron
Pump t
Y
Buffer Gas /Reactant ,--Inlet 'Cooled
Precursor Figure 1. Schematic diagram of high-temperature reactor apparatus. Double-lined arrows show the path of the laser beams. \
either directly into the base or through an inlet tube. The inlet tube is a 161/2-in.-long, 1/4-in.-diameter stainless steel tube surrounded by a I-in.-diameter, 91/2-in.-long water-cooled stainless steel jacket. The inlet tube is vertically translatable and is cooled so we can use photolytic precursors which may be thermally unstable at high reactor temperatures. A four-way cross on the top of the reactor provides ports for pumping, for pressure measurements, and access for thermocouples. A 21/4-in.-o.d.,2-in.-i.d., 1 1 '/,-in.-long AI2O3/SiO2(mullite) reaction tube is situated inside the reactor. Four holes in the middle of the tube (two with '/2-in. diameter, two with 3/4-in. diameter) provided optical access. The reaction tube is surrounded by four vertical columns of Sic resistively heated rods (ISquared-R) and Al2O3 insulation material. The rods were connected via vacuum electrical feedthroughs (MDC MCT-600) to autotransformers (Variac W20) which provided the current to heat the rods. Three banks of four rods and one bank with five rods heated the cell. The arms of the reactor which house the ends of the heating rods are sealed by flanges with O-rings and are water-cooled as is the top plate of the reactor. Opfical Setup. The BH radicals were generated by the photolysis of B H 3 C 0 with the 193-nm output of a Lumonics TE861 laser operating on ArF. The nature of the photodissociation process producing BH is not clear.16 It is, however, known that BH3 is formed in this photodissociation with a quantum yield of 10.7.I' The photolysis beam was collimated by an SI, 5-cmdiameter, 2-m focal length lens, passed through a 6-mm-diameter iris, and directed through the reactor. The BH chemical lifetime had no dependence on the photolysis pulse energy which varied from 3 to 8 mJ/pulse. The BH radicals were probed with the output of a Lambda Physik excimer pumped dye laser (EMG102/FL2002) operating with PBBO dye in dioxane. The dye laser and photolysis laser beams were collinear and counterpropagated through the reactor. For most experiments, we used the Q 4 line in the (1,O) band, but for the higher temperature experiments we used Q5 or 4 6 . The fluorescence was focused by a two-lens telescope through an iris onto a filtered photomultiplier tube (PMT) (RCA 31000AN). We used a Corion 4315 band-pass filter which transmitted fluorescence in the BH A'n-XIZ ( 1 , l ) band and discriminated against visible emission from the rods. A liquid acetone filter discriminated against 193-nm light from the photolysis laser. Intense emission from the rods, particularly at reactor temperatures above 500 K, precluded placing the filtered P M T directly a t the optical port of the reactor. Electronics. A programmable digital delay generator (DDG) (SRS DG-535) controlled the timing for the kinetics experiments. (16) Rice, J . K.; Douglass. C. H.; Nelson, H. H. Chem. Phys. 1990, 109, 143. (17) Weiner, B. R.; Pasternack, L.; Nelson, H. H.; Prather, K. A,; Ro-
senfeld, R. N. J . Phys. Chem., in press.
The DDG provided a trigger pulse at a fixed delay to the photolysis laser and trigger pulses at variable delay simultaneously to the probe laser and the boxcar integrator. A gated boxcar integrator captured the BH L I F signal and directed it to a computer. The BH chemical lifetime was obtained by sequentially increasing the delay between the two laser trigger pulses. All kinetics scans were initiated 10 ~s after the photolysis laser trigger, when the prompt emission from electronically excited A-state BH generated by the photolysis laser had decayed away and when the ground-state BH radicals had rotationally and vibrationally thermalized. The bimolecular rate coefficient was obtained by measuring the lifetime as a function of reactant partial pressure. In the absence of added reactant, the BH disappearance was primarily due to the rapid reaction with the precursor with a small contribution (510%) from diffusion. The experimental procedure is described in greater detail elsewhere. I* Temperature Determination. The gas temperatures were obtained by using a combination of shielded and unshielded thermocouples. MoffatI9 has pointed out that the radiation error of a thermocouple measurement can be estimated by the use of shielded thermocouples; the radiation correction using shields is reduced by a factor of n + 1 where n is the number of shields. For a probe with two thermocouples, one of which has two shields and one of which is unshielded, the gas temperature can be calculated as
where Tunshielded and Tshie]d4are the temperature readings of the unshielded and shielded thermocouples, respectively. This correction to the unshielded thermocouple reading can be as large as 75 K at 1000 K. We used thermocouple readings obtained with the inlet tube I O cm below the unshielded thermocouple as close proximity of the inlet tube to the unshielded thermocouple may contribute to radiation error.20 The leads of two 24-in.-long, 0.01-in.-diameter Pt/l3% Pt-Rh thermocouples were electrically isolated by 1/16-in.diameter ceramic tubes which in turn were housed in a 42-cm-long, '/4-in.-diameter ceramic tube. The shielded thermocouple bead was recessed several millimeters from the end of the larger ceramic tube. A second shield was created by attaching a 1/2-in.-diameter, 2-in.-long ceramic tube to the end of the large ceramic tube with a high-temperature ceramic adhesive. The unshielded thermocouple extended several millimeters beyond the end of the second shield. The gas temperatures were compared to O H rotational temperatures obtained in separate experiments using LIF. OH was generated by photolysis of H N 0 3 in H e at 193 nm and probed with the frequency-doubled output of the excimer pumped dye laser operating with Rhodamine 610 dye. Transitions in the A2Z+-X2ni (0,O) band near 309 nm were excited, and care was taken to ensure that the transitions were not optically saturated. A Corning 7-37 filter was used to transmit OH A2Z+-X211, (0,l) emission at 343 nm. The gas temperatures from the thermocouple readings were higher than the corresponding OH LIF rotational temperatures because eq 1 only corrects for radiation and not for convection or conduction by the flowing gases. The latter two effects change the temperature of the reactor tube as well as the shields depending on the flow rate and composition of the gases. For the conditions employed in this study, the O H L I F rotational temperatures were as much as 200 K lower than the corrected thermocouple temperatures a t 1100 K. A calibration plot correlating the gas temperatures from the thermocouple readings and the OH L I F rotational temperatures is linear, and it was used to calculate the reaction temperature in all kinetic experiments. Uncertainties in the temperature are derived from statistical uncertainties in the calibration. Gases. The flow rates of the gases are measured with calibrated mass flow meters (Tylan). Typically, 30 sccm of an 0.1% (18) Balla, R. J.; Weiner, B. R.; Nelson, H. H. J . Am. Chem. SOC.1987, 109, 4804. (19) Moffat, R. J. Temp. Meas. Control Sei. Ind., Proc. Symp. 4th, 1961, 1962, 3, Part 2, Chapter 52. (20) Fleming, J. W. Unpublished results.
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The Journal of Physical Chemistry, Vol. 94, No. 12, 1990 1 (K) 900
700
'
I
10-1'
500
TABLE I: Summary of BH Rate Coefficients reactant T, K P, Torr
300
'
Garland et al.
I
E
Y
298 298 347 355 430 553 625 740
20 20 20 50 20 20 20 20
k," IO-'* cm3/s 0.9 (0.1) 0.88 (0.09) 1.4 (0.1) 1.5 (0.2) 3.3 (0.3) 7.0 (0.7) 7.1 (1.0) 8.1 (0.8)
CH4
355 647 695
50 50 50
