J. Phys. Chem. 1993,97, 9673-9676
9673
Kinetics of the Reaction Al(2Po) + H20 over an Extended Temperature Range Roy E. McCleant and H. H. Nelson' Chemistry Division/Code 6111. Naval Research Laboratory, Washington, D.C. 20375
Mark L. Campbell Chemistry Department, United States Naval Academy, Annapolis, Maryland 21 402 Received: April 20, 1993; In Final Form: June 28, 1993'
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The temperature dependence of the reaction A1(2pO) H2O has been investigated over the temperature range 298-1 174 K. Aluminum atoms were produced by photodissociation of Al(C2Hs)s and were detected by laserinduced fluorescence. Nonlinear Arrhenius behavior is observed, and the measured rate constants can be described by the expression k(7') = (1.9 f 1.5) X exp[-(0.88 f 0.44 kcal mol-')/RT] (1.6 f 0.7) X 10-10 exp[-(5.7 f 0.9 kcal m o P ) / R T ] cm3 s-l, where the uncertainties represent f2a. At room temperature the rate constant is pressure independent between 10 and 110 Torr total pressure (Ar buffer gas). Results are interpreted in terms of two metathesis reactions with different Arrhenius parameters, one yielding A10 and the other AlOH.
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Introduction The reaction kinetics and dynamics of aluminum atoms have been investigated extensively over the years.' This is due, in part, to the use of aluminum in liquid propellant slurries, in solid propellants, and in explosives. Visible chemiluminescence (CL) is observed when trimethylaluminum (TMA) or the products of aluminized grenades are released into the a t m ~ s p h e r e .This ~ ~ ~CL is dominated by a broad continuum which peaks in the 500-620-nm region. The molecular identity of the chemiluminescent species is unknown, but it is believed to be an aluminum-containing species. In 1963, Rosenburget al.4suggested that the CL results from the radiative recombination of A10 and 0. In 1975, Kolb et al.2 suggested that excited states of A10 are sources of the CL. (Blue-green A10 fluorescencewith an underlying continuum has been observed in twilight releases of TMA.5) Oblath and Gole6 suggested in 1980 that the CL is due, at least in part, to an aluminum hydrate, HAIOH. They observed a similar CL continuum after reacting A1 with H20 under both single- and multiple-collision conditions. Fluorescence ascribableto A10 was also observed under multiplecollision conditions. Recently, the exploding wire technique was used to investigate the combustion of AI + H20e7The spectral intensity emitted from the A1 + H20 reaction in the 300-800-nm region was found to have an underlying continuum which was plausibly attributed to A1203 particles. Hauge et a1.*investigated the interaction of several metal atoms with water using matrix isolation/vibrational spectroscopy methods. They found that A1 + H2O reacts to form the divalent HAlOH species and that photolysis of HAlOH yields AlOH. Electron spin resonance investigations also found that HAlOH is formed when AI and H20 are allowed to react in matrices.gJ0 Irradiation of the matrices produced H atoms and decreased the amount of HAlOH ~ r e s e n t .Theoretical ~ calculations indicate that HAlOH is stable by -44 kcal mol-' with respect to A1 H20,11-13 and the adduct Al.OH2 is bound by -8 kcal mo1-1.11-13J4 This calculated exothermicityfor the formation of HAlOH is not large enough to explain the short wavelength onset of atmospheric chemiluminescence discussed above and is therefore inconsistent with the suggestion by Gole and Kolb3that HAlOH is responsible for the observed emission. This laboratory has been concerned with the oxidation kinetics of aluminum atoms and aluminum oxides. The reaction kinetics
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NRC/NRL Postdoctoral Research Asaociate. Abstract published in Aduance ACS Absmcrs, August 15, 1993.
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of A1 0 2 1 5 and A1 + CO2l6 have been reported. Here, we report kinetic results of the reaction of ground-state aluminum atoms, Al(2P), with H2O(g) over an extended temperature range (298-1 174K) andlimitedpressurerange(10-110Torr). Results are based on the temporal disappearance of A1 atoms. This investigation allows us to test the likelihood of adduct formation (A10H2), insertion (HAlOH), and metathesis reaction(s). We are unaware of any gas-phase kinetic studies on this reaction.
Experimental Section Apparatus and Procedure. Kinetic experiments were carried out in a high-temperaturereactor (HTR) using a laser photolysis/ laser-induced fluorescence technique. Details of the reactor have been described previ0us1y.l~ Aluminum atoms were produced by the 248-nm photodissociation of triethylaluminum (TEAL). A Lambda Physics EMG 201 excimer laser operating on KrF was used as the photolysis source. Detection wasvia laser-induced fluorescence (LIF) using an excimer-pumped dye laser (Lambda Physics EMG102/FL2002). The photolysis laser and dye laser beams counterpropagated through the reactor. Ground-state aluminum atoms were excited and detected on the 394.4-nm (zSl/9P1/2) and 396-nm (2S1/r2P3/2)'8lines using PBBO dye and a Corning 4002 narrow pass filter. LIF was monitored perpendicular to the laser beams by a RCA C31000M photomultiplier tube (PMT). The output of the PMT was sent to a gated boxcar sampling module (Stanford Research Systems SR250), digitized, stored, and analyzed by a computer. The sampling module was triggered by scattered laser light incident upon a fast photodiode. A digital delay generator (Stanford Research SystemsDG535) controlledby the computer was used to trigger the photolysis and dye laser systems at a repetition rate of 21 Hz. Reaction time was determined by the delay between the photolysis laser pulse and the dye laser pulse. LIF decay profiles were obtained by increasingthe delay time until A1 atoms were no longer detected. The minimum delay time was 1 ps in order to avoid collecting prompt emission and to allow thermalization of A1 atoms. A base line was obtained by triggering the probe laser before the photolysis laser. LIF decay profiles consisted of 500 points, each averaged over 2-10 laser shots. Tylan mass flow meters (FM-360) and flow controllers (FC260) were used to monitor the flow of gases into the reactor. TEAL vapor was entrained in a 1-2 sccm flow of argon and
0022-3654/93/2Q97-9673S04.00/0Q 1993 American Chemical Society
9674
The Journal of Physical Chemistry, Vol. 97, No. 38, 195'3
McClean et al.
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/
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: .
c
0.02 -
0.01
ot-
0
o.oo[,
i
I
100
200
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0.0
300
,
,
,
,
,
0.2
0.1
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0.3
, 0.4
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H20 Pressure (Torr)
Time (ps)
Figure 1. Typical A1 atom decay profile. T = 741 K, Ptol = 21 Torr, PH@ = 0.19Torr. Thesolidcurvethrough thedatapointsisanexponential fit over 3 half-lives. T = 80.2 i 0.7 ps.
Figure 2. Linear dependence of 1 / as ~ a function of HzO pressure at different temperatures. Ptot= 21 Torr. The point at 741 K and 0.19 Torr is that obtained from Figure 1. The second-orderrate constants are obtained from the slopes. See Table I.
introducedintothereactor. Watervapor wascarried to thereactor in a flow of argon carrier gas. The argon passed over liquid water at =38 OC where it entrained the water vapor. The H20/Ar flow then passed through a coil (= 5-mm i.d.) of Pyrex tubing at 22.5 OC, after which the mixture passed through a flow meter. The temperature differential caused a relatively high water number density to enter the coil, thus ensuring that the carrier gas leaving thecoil was saturated with water vapor. The coil was maintained at a temperature lower than room temperature to prevent condensation of water vapor in the lines going to the HTR. The pressure of the H20/Ar mixture was measured just prior to the flow meter in order to determine the partial pressure of H20. The amount of H2O entering the HTR was varied by changing the H20/Ar flow rate and backing pressure of the argon carrier gas. We were convinced that saturation conditions existed in our experimental runs since the measured decay rates for a given H20 partial pressurewere identicalunder variousbacking pressure and flow conditions. Pressure measurements were made with MKS Baratron manometers. The windows on the HTR were purged with a slow flow of argon to prevent deposition of TEAL and photodissociation products. Total flows were =300-1000 sccm (linear flow rate in the reactor tube of 4.5-14 mm shot-') in order to obtain fresh reactants between laser pulses, to minimize product buildup, and to allow time for temperature equilibration. Argon buffer gas, after passing through a Tylan mass flow controller, combined with the H20/Ar mixture and entered the HTR. Reaction zone temperatures were determined from measured AlO(B22+-X22+) rotational spectra. The experimental procedure has been described elsewhere.16 In brief, A10 rotational spectra were recorded at the completionof each kinetic experiment from which rotational temperatures were determined. A10 was produced from the reaction A1 0 2 . The delay between the photolysis and dye laser pulses was typically 50 ~s for these spectra. Materials. TEAL (Texas Alkyl, 90%), argon (Air Products Industrial Grade, 99.997%), and oxygen (Matheson, 99.6%) were used as received. Distilled water was used.
TABLE I: Measured Reaction Rate Constants for AI(2P) +
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Results and Discussion
Experiments were conducted by monitoring the temporal behavior of Al(2PO). Temporal profiles of [AI] were measured under pseudo-first-order conditions with [HzO] in excess. A typical decay plot is shown in Figure 1. The pretrigger base line is shown at f < 0. The solid line through the data is an exponential fit which covers three lifetimes, T . In all experiments, exponential behavior was observed over a decay time spanning 3-4 lifetimes. Except for the pressure dependence studies, all experiments were carried out at =21 Torr total pressure. The water accounted for
H4-P
298 331 f 8 456 f 14 459 i 8 544 i 22 641 f 26
(4.6 i 0.8) X l@l3 (5.9 f 1.0) X lW3 (1.0 f 0.2) X 10-lz (1.1 f 0.2) X 10-12 (1.5 i 0.2) X (2.6 f 0.4) X
672 f 28 741 i 18 751 f 42 1004 & 66 1007 f 74 1174 i 100
(3.1 f 0.5) X (4.4 f 0.7)X 1&12 (5.3 f 0.9) X (9.9 -+ 1.7) X 10-12 (1.6 f 0.3) X (1.1 i 0.3) X IO-"
a Uncertainties are i 2 u . Rate constants were measured at 21 Torr total pressure.
no more than 4% of this pressure, and the TEAL'S concentration was only a fraction of a percent. Lifetime measurements were independent of probe and photolysis laser beam intensities. Second-order rate constants were obtained from plots of 1 / ~ vs H20 pressure such as those shown in Figure 2. The intercept represents diffusion of A1 atoms out of the detection zone and reaction of A1 atoms with TEAL, photofragments, and any impurities that may have been present. Second-order rate constants are calculated from the slopes. Observed second-order rate constants measured at -21 Torr total pressure and at the various temperatures investigated are listed in Table 1. The uncertainties represent two standard deviations and take into account the sum of experimental scatter in the data, uncertainties in flow meter readings (5%), the time base of the digital delay generator (2%), pressure readings (5%), and temperature measurements (15%). Weobservednodifferencewithinexperimental uncertainty in the rates of reaction of the higher spin-orbit state of Al(*PO3,2) at room temperature, suggesting either interconversion among the spin-orbit states is fast or the rates of the two states are the same under these experimental conditions. The reaction rates at room temperature were investigated over the total pressure range 10-1 10Torr; a pressure dependence was not observed. Theobservedrateconstantsat 21 Torr areplottedin Arrhenius form in Figure 3. Note the nonlinear Arrhenius behavior. The observed rate constants can be described by the expression k(T) =
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(1.9 f 1.5) X exp[-(0.88 f 0.44 kcal mol-')/Rr] (1.6 f 0.7) X lo-'' exp[-(5.7 f 0.9 kcal mol-')/RT] (1) where k(T) is in units of cm3 s-1 and the uncertainties represent f2a. Reaction channels and their enthalpies of reaction that are thermodynamically accessible at our experimental temperatures are
Kinetics of the Reaction Al(2PO) 1000
lo'lo
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The Journal of Physical Chemistry, Vol. 97, No. 38, 1993 9675
600
400
I
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ki 300
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I
10-11:
r
dl
v)
m
-6 Y
H+AIOH
--10-12:
1 0 - l ~ '
'
a
'
'
'
.
I
---
I
Figure 4. Potential energy diagram for A1 + H20. Energies are in kcal mol-'. (V)WOH = 958 cm-I.l2 A1 = 2.5 X cm3s-l; El = 1.0 kcal mol-'; A-1 = 1.0 X loL2s-l; E-I = 45.0 kcal mol-'; A2 = 2.0 X 1012 s-1;
+ H,O
AI(2Pol/2)
A1(ZPol/,)
+ H,O
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AlOH
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+H AHr,, = -12 kcal mol-'
A10
+ H, AHm = -5 kcal mol-'
''
(4)
'' ( 5 )
The enthalpy change of reaction 2 is an average from theoretical calculations. The enthalpy change of reaction 3 is obtained from theoretical calculationsand matrix studies and could be uncertain by as much as f l S kcal Since the reaction rates were pressure independent, any adduct formation or insertion (cf. reactions 2 and 3) would have to be in or near its high-pressure limit or have a low-energy exit channel to other products. The observed nonlinear Arrhenius behavior and the quality of the biexponential fit suggest two reaction channels with different temperature dependencies. For argument, we take one reaction channel as A1 H2O HAlOH* H AlOH and the other reaction channel as A1 + H20 A10 H2. The small activation energy in expression 1 is assigned to the A1 H20 H AlOH channel since there is indirect evidence that A1 inserts into H2O with little or no barrier.8J1 The second part of expression 1 is assigned to A1 H20 A10 + H2. Possible reaction paths are shown in Figure 4. It is conceivable that A1 + H20 H + AlOH may not go through an intermediate. However, since HAlOH is bound by 44 kcal mol-' relative to A1 H20, we treat Figure 4 as a possible reaction route. Approximate QRRK2@22calculations were performed to test the pressure and temperature dependence of reaction 3 with a subsequent decomposition pathway to H AlOH. Experimental rate constants for the formation of stable intermediate(s) are required in QRRK calculations. Since experimental rate constants are not available for the Al/H20 system, estimated values were used and are listed in the Figure 4 caption. Reaction enthalpies of reactions 3-5, activation energies from expression 1, and transition-state theory were used in estimating the rate constants. Note that in QRRK calculations the input rate
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E2 = 32.0 kcal mol-'. parameters are for the formation and decomposition of stabilized adducts. El, the barrier to formation of HAlOH, is assumed to be 1 kcal mol-l, the smaller of the two activation energies in expression 1. E-1, the barrier for decomposition of HAlOH back to reactants, isobtainedfrom A H d +El. An order-of-magnitude starting estimate of A-1 and A2 was obtained from transitionstate theory, and A I was calculated via microscopic reversibility. Theentropies of the transition states are not known, and therefore the A factors are necessarily approximate and were adjustable in the calculations. The ratio of kl:k-1was fixed at the equilibrium constant value which was calculated from statistical mechanic^.^' E2, the barrier for decomposition of HAlOH to H AlOH, is E2 is necessarily less than E-' obtained from AH-4 - AH,,3. because we did not observe a rate constant pressure dependence. For E2 L E-', calculationsyield a significant rate constantpressure dependence. The interaction potential of HAlOH, CWOH, is estimated at 600 f 400 K.24 (Results were insensitive to this quantity.) The collisional efficiency, (3, was calculated by the CHEMACT programzl according to Troe's formalism. Separately, values of j3 from 0.1 to 0.9 were used with no noticeable effect. This is not surprising since, as will be shown below, collisional stabilization is unimportant at these pressures. The dashed curve in Figure 3 is the computed result at 21 Torr total pressure. The calculated results do not depend on pressure from 10 to 110 Torr. Addition of the Arrhenius parameters for the A1 H20 A10 + H2 reaction channel to the calculated curve results in a curve that approximately overlaps the biexponential fit. It is believed that Al.OH2 may be a local minimum on the potential energy surface of A1 H2O HAlOH, but the barrier for Al.OH2 HAlOH could be small.'' Our proposed mechanism is consistent with previous A1 H2O work. We infer reaction channels that produce A10 and AlOH, products that have been observed elsewhere.6J.g AlOH is photolytically produced from HAlOH in matrices. At the temperatures and pressure in this work, it is proposed that HAlOH* falls apart to H AlOH. Lower temperatures and/or higher pressures would be required before any termolecular process becomes important. If reaction 3 were moreexothermic,wemight expect to observe some pressure dependence of our observed rate constant. If the exothermicitywere as large as 75-80 kcal mol-' as suggested by Gole and Kolb,3 our simple calculations predict a substantialamount of adduct stabilization which would manifest itself in the temperature dependenceof the product distribution. Detection of reaction products and trajectory calculations would aid in the elucidation of the reaction mechanism. We are unable at present to detect any A10 products from this reaction due to interferences from the rapid oxidation of A1 by 0 2 and other oxidants trapped in the insulation in the reactor. We are constructing a new reactor and plan to pursue a measurement of
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9676 The Journal of Physical Chemistry, Vol. 97, No. 38, 1993
the yield of A10 as a function of temperature which we hope will add substantial insight into the mechanism. In summary, the reaction rate of AI(?) with HlO(g) has been measured. Room temperature pressuredependence studies and QRRK estimates indicate termolecular processes would be important only at relatively low temperatures and/or high pressures. Temperature dependence studies suggest the presence of two reaction channels with different activation energies. We propose one channel gives AlOH via the intermediate HAlOH*, and the other channel yields A10. Lower temperature kinetic studies and product identification could yield valuable information on the onset of adduct and/or insertion reaction channels.
Acknowledgment. M.L.C. gratefully acknowledges salary support through the Naval Research Laboratory and Naval Reserve Scienceand Technology Program of the Office of Naval Research. We thank L. J. Medhurst for helpful discussions and the Office of Naval Research for funding this work through the Naval Research Laboratory. References and Notes (1) Gas-PhaseMetal Reactions; Fontijn, A., Ed.; Elsevier Science: New York, 1992. (2) Kolb, C. E.; Gersh, M. E.; Herschbach, D. R.Combust.Flame. 1975, 25, 3 1 and reference therein. (3) Gole, J.L.;Kolb,C.E.J. Geophys.Res.1981,86,9125andreferenccs therein. (4) Rosenberg, N. W.; Golomb, D.; Allen, E. F., Jr. J. Geophys. Res. 1963,68,3328. (5) Rosenberg, N. W. Science 1966,152, 1017.
McClean et al. (6) Oblath, S.B.; Gole, J. L. Combust. Flame 1980,37, 293. (7) Jones, M. R.; Brewster, M. Q.J. Quant. Specrrosc.Radiat. Transfer 1991, 46, 109. ( 8 ) Hauge, F. H.; Kauffman, J. W.; Margrave, J. L. J. Am. Chem. Soc. 1980,102,6005. (9) Knight, L.B., Jr.; Woodward, J. R.;Kirk, T. J.; Anington, C. A. J. Phys. Chem. 1993,97, 1304. (10) Knight, L. B., Jr.;Gregory, B.;Cleveland, J.; Arrington,C. A. Chem. Phys. Lett. 1993,204, 168. (11) Kurtz, H. A.; Jordan, K. D. J. Am. Chem. Soc. 1980,102, 1177. (12) Sakai, S.;Jordan, K. D. Chem. Phys. Lett. 1986,130,103. (13) Sakai, S. J. Phys. Chem. 1992,96,8369. (14) Trenary, M.; Schaefer, H. F. 111; Kollman, P. A. J . Chem. Phys. 1978,68,4047. (15) Garland, N. L.;Nelson, H. H. Chem. Phys. Lett. 1992,191,269. (16) Garland, N. L.; Douglass, C. H.; Nelson, H. H.J. Phys. Chem. 1992, 96,8390. (17) Garland, N. L.;Stanton, C. T.; Fleming, J. W.; Baronavski, A. P.; Nelson, H. H. J. Phys. Chem. 1990,94,4952. (18) Moore, C. E. Atomic Energy Levels as Derived from the Analysis of Optical Spectra, Vol 111; Natl. Stand. Ref Data Ser. (US.Narl. Bur. Stand.) 1971,NSRDS-NBS 35. (19) Chase, M. W.,Jr., Davies, C. A., Downey, J. R., Jr., Frurip, D. J., McDonald, R. A,, Syverud, A. N., Eds. J . Phys. Chem. Re/. Data 1985,I4 (Suppl. 1). (20) Dean, A. M. J. Phys. Chem. 1985,89,4600. (21) Dean, A. M.; Bozzelli, J. W.; Ritter, E. R. Combust. Sci. Technol. 1991, 80, 63. (22) The computer d e for the computations was provided by J. W. Bozzelli. (23) Geometry and vibrational frequencies of 1-HAIOH were obtained from ref 12, and rotational constants of t-HAlOH were calculated using a computer code from ref 25. Other parameters were taken from ref 24. (24) Reid, R. C.; Prausnitz, J. M.; Poling, B. E. The Properties ofGases and Liquids, 4th ed.; McGraw-Hill: New York, 1987. (25) Pitt, I. G.;Greenhill, P.G. PROGRAM GEOM, 1990.
