30
G. P. Glass and R. E. Quy
The Journal of Physical Chemistry, Vol. 83, No. 1, 1979
(49) J. 0. Anderson, J. J. Margitan, and F. Kaufman, J . Chem. Phys., 60, 3310 (1974). (50) M. A. A. Clyne in "Physical Chemistry of Fast Reactions", Vol. 1, B. P. Levitt, Ed., Plenum Press, New York, N.Y., 1973, Chapter 4. (51) C. J. Howard, H. W. Rundle, and F. Kaufman, J . Chem. Phys., 53, 3745 (1970). (52) D.L. Baulch and D.C. Montague, J . Phys. Chern. (this symposium), in this issue.
(44) A. Fontijn, W. Felder, and J. J. Houghton, Symp. (Int.) Combust., [Proc.], 15th, 775 (1975). (45) R. J. Cvetanovic, R. P. Overend, and G. Paraskevopoulos, Int. J . Chem. Kinet., Symp., 1, 249 (1975). (46) R. J. Cvetanovic and D.L. Singleton, Int. J . Chem. Kinet., 9, 481, 1007 (1977). (47) A. A. Westenberg and N. deHaas, J. Chem. Phys., 58, 4061 (1973). (46) A, A. Westenberg and N. deHaas, J. Chem. Phys., 47, 1393 (1967).
Measurement of High Temperature Rate Constants Using a Discharge Flow Shock Tube G. P. Glass" and R. B. Quy Department of Chemistry, Rice University, Houston, Texas 7700 1 (Received June 28, 1978) Publication costs assisted by the Robert A. Welch Foundation
A discharge flow shock tube has been constructed, and has been used to study the reaction of atomic hydrogen with NzO at temperatures between 1473 and 2710 K. Over this temperature range, the rate constant for reaction 1,H + NzO N2 + OH, has been measured as hl = (6.35 f 1.10) X 1O-I' exp(-16900 f 750 cal/RT) cm3molecule-l s-'. The apparatus should allow many other elementary free-radical reactions to be studied for the first time in isolation, at flame temperatures. The results so far obtained show that reaction 1could serve as a suitable source for the study of high temperature reactions of OH. -+
Introduction Over the past several years the rates of many elementary gas phase reactions have been measured at room temperature with great precision. Much of this precision has resulted from the use of discharge flow and flash photolysis systems that have allowed individual elementary reactions to be studied in isolation, free from the complexities that are inherent in studies of overall transformations that involve a large number of sequential elementary steps. At flame temperatures the situation is somewhat different. In the past most high temperature rate constants have been obtained either from flame studies or from shock tube experiments. Each one of these types of experiment offers its own particular advantage. The advantage associated with the study of steady state flames is that measurements can be taken over a long time period. In shock tube experiments the advantage is associated with the high degree of spatial homogeneity that can be obtained. Unfortunately, both types of study suffer because measurements are usually made on complex reaction systems that involve many different elementary steps, and sorting out the contribution of any one step to the overall transformation is often difficult. Therefore, only a few rate constants of elementary free-radical reactions are known a t high temperature with any degree of certainty. This is particularly unfortunate because many practically important processes, for example, combustion and pyrolysis, involve such reactions. Recently several attempts have been made to rectify this situation by designing experiments so as to isolate, at high temperatures, particular free-radical reactions. For example, Fontijn, Kurzius, and Houghtonl have developed the high temperature fast flow reactor (HTFFR), and have used it to investigate metal atom and metal monoxide reactions over the temperature range 300-1700 K.' Other workers3 have attempted to monitor individual free-radical reactions in shock tubes by preforming the radical of interest in a fast preliminary process. An alternative approach is presented in this paper. An apparatus is described that allows many individual elementary reactions to be studied in isolation a t flame temperatures. This 0022-3654/79/2083-0030$0 1.OO/O
apparatus, the discharge flow shock tube, combines the specificity of the discharge flow technique with the temperature range of the normal shock tube. A similar apparatus4has been used previously by Gross and Cohen5 to study the temperature dependence of several chemiluminescent afterglows. Here its application to the study of reaction kinetics is illustrated in an investigation of the reaction H NZO Nz + OH (1)
+
+
a t temperatures between 1473 and 2710 K.
Experimental Section The apparatus consisted of a conventional shock tube modified so that the experimental section formed part of a discharge flow system. In operation, the ordinary discharge flow situation appropriate to the particular process under study was first established, and then, a shock wave was propagated into the predissociated gas from a driver section located upstream of the discharge section. An earlier version of the apparatus has been described in detail elsewheree6 The present apparatus is shown schematically in Figure 1. The experimental section was 12 ft long, 1.5 in. i.d, and was constructed in three parts. The first two sections consisted of 5-ft lengths of Corning Double Tough Pyrex Pipe connected to each other and to the diaphragm section by the multiholed gas inlet flanges B and A, respectively. The final section of the tube, a 2-ft section containing several sapphire windows through which infrared emission could be monitored, was connected to the other sections by a third gas inlet flange, C. The driver section, which consisted of a 4-ft length of 4-in. i.d. brass pipe, was separated from the experimental section by an aluminum diaphragm. This diaphragm could be ruptured using a solenoid driven plunger, although, in practice, it was usually pressure burst. A 70-L dump tank prevented reflected shock waves from propagating back through the experimental section. Continuous pumping through the experimental section was achieved using a 1000 L/min rotary pump. At typical preshock pressures of 5-20 torr, linear flow speeds of 0 1979 American
Chemical Society
The Journal of Physical Chemistry, Vol. 83, No. 1,
Measurement of High Ternperature Rate Constants
DIAPHRAGM
Figure 1. Schematic diagram of the apparatus. Shown are the shock tube driver section (D), three multihoied gas inlet flanges (A, B and C), the microwave discharge (M), and four beam splitters (Si)that divert the He-Ne laser beam onto the photomultiplier tubes (P)via the knife edges (K).
1000-1500 cm/s were obtained. Nitrous oxide (0.1-0.2%) was mixed into the main portion of the argon carrier gas, and introduced into the shock tube experimental section a t flange A. Hydrogen atoms were injected into the main flowstream a t flange B. They were generated by microwave discharge (50-100 W) of dilute H,/Ar mixtures in a piece of 9-mm i.d. quartz tubing that terminated, flush with the shock tube wall, a t flange B. The flow rates of all gaseous components were measured using calibrated rotameters. The pressure in the shock tube was measured using a Wallace and Tiernan Model FA160 pressure gauge. The 300-W, 13.6-MHz radio-frequency generator, which was inductively coupled to the shock tube as shown in Figure 1, was not used in the experiments described in this paper. Before each experiment the concentration of atomic hydrogen was measured a t flange C using a chemiluminescent titration reaction. Molecular chlorine was added as the titrating gas, and the progress of the titration was monitored by observing the chlorine afterglow using a red sensitive photomultiplier (RCA C7151W) in conjunction with a long pass colored glass filter (Schott 715). With this combination, radiation between 7100 and 9000 A could be detected. The titration itself was based on the reaction sequence H + Clp ----* HCl C1 (2) C1 + C1 “chlorine afterglow” (3) Reaction 2 is extremely rapid,’ and the afterglow intensity is a function of the chlorine atom concentration.8 Thus, as C12 was added to atomic hydrogen in the shock tube, chlorine atoms were produced, and the afterglow intensity increased. The end point was reached when all of the atomic hydrogen wa13 consumed. Further addition of CI2 beyond this point produced no further change in the chlorine atom concentration, and therefore no further increase in the afterglow intensity. In fact, the afterglow intensity tended to decrease very slightly, probably due to the occurrence of the reaction c 1 c1 Cl, c1, C12 (4)
+
-
+ +
-
+
In order to prevent, wall recombination of chlorine atoms between their point of formation and the point a t which the afterglow was detected (7 cm), the wall in this region was coated with an inert halocarbon wax.g Other possible wall effects were partially eliminated by injecting the Clz flow into the center of the shock tube through a 3-mm 0.d. glass retractable probe which, a t the end of the titration procedure, could be pulled back into flange C. In a typical experiment the preshock concentration of atomic hydrogen a t flange C was measured as 1-2 x atom cm-’. This corresponded to 25% dissociation of H,.
1979 31
The decay of atomic hydrogen along the experimental section of the shock tube was occasionally monitored by adding a small amount of NO molecule cm”) at flange A, and observing the emission from HNO (IA” I,’) a t several points along the tube using the photomultiplier/ filter combination described above. The atom concentration never decayed more than 10% over a 20 cm length of the shock tube. This rate of decay corresponded to a wall decay constant, h,, of 6 s-’, and a surface recombination coefficient, y,of Shock wave velocities were estimated from the time taken for the shcck front to traverse four laser schlieren stations placed approximately 30 cm apart. Details concerning the laiser schlieren stations and the measurement procedure have been published previously.6 In these experiments the shock wave velocity was typically observed to decrease between the various laser schlieren stations a t a rate corresponding to an attenuation of 1-2% /m. Infrared emission from shock heated N 2 0 was detected, after passage through a suitable interference filter, by a liquid nitrogen cooled indium antimonide photovoltaic cell. The detector (Texas Instruments ISV-329) had an active area 2 mm in diameter, and a value,of D* (500, 1000, 1) of 1.5 X 1O1O cm Hz112W l. Radiation from the shock tube was focused onto the detector, through a 3-mm diameter collimating hole, using a 5.0-cm focal length CaFz lens. The interference filter used in this study had a maximum transmission of 56% a t 2120 cm-l, and a half-power bandwidth of 200 cm-l. The response time of the detector and its associated amplifier was measured, by observing shock waves in rnore concentrated N,O/Ar mixtures, as less than 2 ,us.
-
Results The reaction of NzO with atomic hydrogen was studied a t temperatures between 1473 and 2710 K in the presence of an excess of both atomic and molecular hydrogen. Under these conlditions only the following reactions were of major importance: H + N 2 0 Nz+ OH = -62.4 kcal/mol (1) OH + Hz H2O + H A H 0 2 9 8 = -15.1 kcal/mol (5) Now, in this temperature range k5 is a t least a factor of 10 greater than and in our experiments the concentration of Hz ‘was greater than that of atomic hydrogen, which in turn was greater than that of NzO. Therefore, the concentration of OH rapidly reached its steady state level (typically, within 10% in less than 5-ps laboratory time), reaction 5 maintained the concentration of atomic hydrogen a t its original value, and the concentration of NzO decayed exponentially as shown in Figure 2. A number of measurements were made of the exponential decay of N 2 0 in the presence of varying amounts of atomic hydrogen. The results of these measurements are shown in Table I. In these experiments the initial concentration of NzO was varied by a factor of 3, and that of atomic hydrogen by a factor of 2, without any systematic effect on the measured rate constant. The ratio (H)/ (NZO), varied from 2.2 to 7.3. Addition of an especially large excess of €1, (mixture F) had no effect on the rate of removal of NzO. Two corrections were made to the original experimental data in order to obtain the rate constants shown in Table I. These corrections were applied in order to take account of thermal decomposition of N 2 0 , and of shock wave nonideality.
+
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The Journal of Physical Chemistry, Vol. 83, No. 1,
50
t
I
20c
tv)
z
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W
IO
20
G. P. Glass and R. 8.Quy
1979
TABLE I: Measured Rate Constants for the Reaction H + N,O -+ N, + ONa
mixture
1831 2141 2193 2482
7.64 7.94 7.92 7.87
1.12 1.13 1.13 1.36
6.07 11.60 13.90 23.10
( B ) 98.75%Ar O.ll%N,O 0.45% H 0.68% H,
1493 1766 180'7 1845 2094 2150 2170 2188 2221 2565 2710
7.94 8.59 8.07 8.09 8.22 8.84 8.18 7.71 7.69 7.94 8,13
2.01 2.10 2.09 1.95 1.98 2.16 2.14 1.97 1.97 2.01 2.03
2.79 4.82 5.76 5.65 10.20 7.84 11.30 15.20 13.80 26.40 29.10
(C) 98.8% Ar O.ll%N,O 0.38% H 0.72% H,
1473 1611 1855 1894 1948 1981 2118 2202 2412 2465 2510
7.20 7.09 7.17 7.04 5.86 7.08 7.04 6.57 7.31 6.19 7.64
1.87 1.84 2.10 2.12 1.89 1.94 1.93 1.96 1.98 1.97 2.03
2.12 4.10 6.21 7.31 7.05 7.24 14,90 11.90 15.90 17.40 27.10
( D ) 99.4% Ar 0.11% N,O 0.27% H 0.22% H, ( E ) 98.7% Ar O.l7%N,O 0.39% I4 0.72% H,
2166 2196 2277
5.37 4.84 4.17
1.88 1.90 1.94
9.98 10.80 30.80
1894 1998 2057 2170 2218 2411 2598 2625
7.16 6.84 7.47 8.14 6.67 6,95 6.82 6.41
2.97 3.00 3.03 3.10 3.05 3.13 3.16 3.02
8.15 9.25 8.67 13.20 14.00 17.20 28.80 26.60
2496
4.12
1.91
22.20
30
Figure 2. The decay of infrared emission from N20 behind the shock
In the absence of atomic and molecular hydrogen, N 2 0 decomposes thermally via a unimolecular reaction N20 Ar N2 $. 0 4- Ar (6) The rate constant for this reaction has been measured by a number of workers,11J2 and the values that they have obtained range around those given by Olschewski, Troe, and Wagner,12 namely, K , = 8.32 X exp(-58000 cal/RT) cm3 molecule-' s Values approximately 20% below these were obtained in this laboratory in a number of experiments performed in the absence of H and HP, but otherwise under conditions identical with those shown in Table I. These experiments were peformed in order to allow us to subtract out from the decay of NzO, measured in the presence of atomic hydrogen, that contribution that arose from the occurrence of reaction 6. The amount subtracted varied from less than 1% at 1500 K, to as much as 25% a t the highest temperature studied (2710 K). The experimental measurements were corrected for shock wave nonideality using the equations of Belford and S t r e h l ~ wwhich , ~ ~ were derived from the original analysis of Mirels.14 According to this analysis, the formation of a boundary layer causes the temperature and density behind the incident shock to drift upward during the observation period, and the particle testing time to increase beyond that estimated using ideal shock wave theory. A simple correction for nonideality was applied to every experiment shown in Table I by assuming each experiment to have been peformed under the conditions that pertained a t the midpoint of the observation period. A typical correction caused the rate constant to decrease by 109'0, and the experimental temperature to increase by 36 K over that calculated using ideal theory. The corrections are smaller than those normally encountered in shock tube experiments because almost all measurements were made a t laboratory times of less than 50 ps. The data shown in Table I were not corrected for the effects of reactive heating caused by the occurrence of the exothermic reactions 1and 5 , because in these experiments very dilute mixtures were used, and the temperature rise due to reactive heating never amounted to more than 6 E( a t the half-reaction point. Furthermore, no account was taken of the decay of atomic hydrogen that occurred along the shock tube due to wall recombination. The error introduced by neglecting this latter effect must be rela-
+
molecule-'
( A ) 98.8% Ar 0.06%N20 0.44% H 0.69% H,
TIME (ysec) front. The shock temperature was 2510 K. Post-shock concentrations were as follows (molecule ~ m - ~ )[HI : = 7.64 X 10i5,ieH2] = 11.5 X IOi5, [N,O] = 2.03 X IOi5, and [Ar] = 1.81 X 10 .
10-'5 molecule
temp, 15.
-
( F ) 98.1% Ar 0.10% N,O 0.22% H 1.6% H,
cmS- ' --___-_
a The preshock pressure was approximately 1 6 torr in all experiments.
tively small. The decay of N 2 0 was seldom observed for more than 50 ps, and gas arriving a t the detector in this time interval originated from points within 15 cm of the detector, Hydrogen atom decay over this length never amounted to more than 7 % . Discussion Several previous measurements of the rate of reaction 1 have been p ~ b l i s h e d . l ~However, -~~ only one other direct study of the isolated reaction has been made. Albers et al.19 investigated the reaction at temperatures between 718 and 1111K in an isothermal flow reactor using quantitative ESR detection of atoms and radicals. The results of that study are compared with the results of the present study in Figure 3. As can be seen, a reasonable overlap occurs between the two sets of rate constants. However, a closer inspection reveals that an extrapolation of the least-squares line through our data hl = (6.35 f 1.1) X exp(-16900 f 750 cal/RT) cm3 molecule s-l
The Journal of Physical Chemistry, Vol. 83, No. 1, 1979 33
Measurement of High Temperature Rate Constants
I
International Shock Tube Symposium",Stanford University Press,
0 0
I
1
II
I
I
I x l i
TI
0.4
I .2
0.8
IOYT
(3) See, for example, S. C. Baber and A. M. Dean, Int. J. Chem. Kinet., 7, 381 (1975). (4) R. A. Hartunion, W. P. Thompson, and E. W. Hewitt, J. Chem. Phys., 44, 1765 (1966). (5) (a) R. W. F. Gross, J . Chem. Phys., 48, 1302 (1968); (b) R. W. F. Gross and N. Cohen, ibid., 48, 2582 (1968); (c) N. Cohen and R. W. F. Gross, ibtf., 50, 3119 (1969). (6) J. E. Breen, R. 13. Quy, and G. P. Glass, "Proceedings of the Ninth
(
K-I)
Figure 3. Arrhenius plot of k , . Points marked with circles are from this study. Those marked X or with error bars are from ref 15. The error bars represent uncertainties associated with a knowledge of the
reaction stoichiometry.
lies a factor of 2-3 below that drawnlg through the data of Albers et al. No clear reason for this discrepancy is apparent. Therefore, since activation energies derived from data spanning limited temperature ranges have, in the past, proved to be less accurate than the individual rate constants themselves, a "best" estimate of the activation energy for reaction 1 might be obtained by combining the two sets of measurements. If this is done, an activation energy of approximately 14 kcal/mol is obtained. The results of these experiments confirm that reaction 1 is sufficiently fast that it can be used as a clean source of the hydroxyl radical. Using this source, further studies are planned of several reactions of OH whose temperature dependence appears to exhibit non-Arrhenius behavior.20 The discharge flow shock tube (DFST) offers a number of advantages in the study of high temperature reactions. Using this apparatus, individual free-radical reactions can be studied in isolation, free from the complex reaction sequences that occur in flames and other overall combustion systems. In addition, some of the inherent disadvantages of conventional shock tube studies can be avoided when using the DFST. Since free-radical reactions are extremely rapid, they can be studied a t very short reaction times before boundary layer development and the problems of shock wave nonideality become severe. The rapidity of these reactions also lessens the effect on the overall kinetics of any trace impurities present in the reaction system. Furthermore, flowing preshock systems offer advantages whenever one of the reactants is unstable, or whenever adsorption of any component on the shock tube wail (e.g., HSO or HC1) is troublesome.
Acknowledgment. Acknowledgment is made to the Robert A. Welch Foundation of Houston, Texas (Grant C 312) for support of this work. References and Notes (1) A. Fontijn, S. C. Kurzius, and J. J. Houghton, Rev. Sci. Instrum., 43, 726 (1972). (2) A. Fontijn, W. Felder, and J. J. Houghton, Symp. ( I n t . )Combust., [Proc.]. 76th, 871 (1977).
1975, p 375. (7) (a) H. G. Wagner, U. Welzbacher,and R. Zellner, Ber. Bunsenges. Phys. Chem., 80, 902 (1976); (b) P. F. Ambidge. J. N. Bradley, and D. A. Whvtock. J . Chem. Soc.. Faradav Trans. 7. 72, 1157 (1976); (c)P. P. Bemandl and M. A. A. Clyne, J. Chem. Soc., Faraday Trans. 2 , 73, 394 (1977). (8) M. A. A. Clyne and D. H. Stedman, Trans. Faraday Soc., 64, 1816 (1968). (9) R. B. Badachhape, P. Kamarchik, A. P. Conroy, G. P. Glass, and J. L. Margrave, Ifit. J . Chem. Kinef., 8, 23 (1975). (10) D. L. Baulch, D. D. Drysdaie, D. G. Home, and A. C. Lloyd, "Evaluated Kinetic Data for High Temperature Reactions", Vol. 1, Butterworths, London, 1972. (11) See, for example, S. C. Baber and A. M. Dean, Int. J. Chern. Kinet., 7, 381 (1975); J. E. Dove, W. S. Nip, and H. Teitelbaum, Symp. (Int.) Combust. [ P r o c . ] , 75th, 903 (1975), and references therein. (12) H. A. Olschewski,J. Troe, and H. G. Wagner, Ber. Bunsenges. Phys. Chem., 70, 450 (1966). (13) R. L. Belford and R. Strehlow, Annu. Rev. Phys. Chem., 20, 247 (1969). (14) H. Mirels, Phys. Fluids. 6, 1201 (1963). (15) C. P. Fenimore and G. W. Jones, J . Phys. Chem., 63, 1154 (1959). (16) G. Dixon-Lewis, M. M. Sutton, and A. Williams, Symp. (Int.)Combust. [ P r o c . ] , 70th, 495 (1965). (17) H. Henrici and S. H. Bauer, J . Chem. Phys., 50, 1333 (1969). (18) R. W. Walker, Symp. (Int.)Combust. [ P r o c . ] , 74th, 117 (1973). (19) E. A. Albers, K:. Hoyermann, H. Schacke, K. J. Schmatjko, H. G. Wagner, and J. Wolfrum, Symp. (Int.) Combust., [Proc.], 75th, 765 (1975). (20) W. C. Gardiner, Jr., Acc. Chern. Res., 10, 326 (1977).
Discussion WIKC TSANC (National Bureau of Standards). (a) To what extent are the uncertainties of the thermochemical and physical properties of the (discharge reflected in the determinations of conditions in the post-shock region? (b) Labile species can be generated in situ in shock tube studies by pyrolytic decomposition of suitable precursors. 0 atoms for example have been generated from N 2 0and 03,CH3 radicals from (CH3)2N2.We have found hexamethylethane to be a very satisfactory H-atom source. Further work in this direction, by selection of the proper precursors, would be extremely worthwhile.
G. P. GLASS.(a) Our measurements are taken on gas samples that have traveled, prior to shock heating, at least 1 m from the discharge. Thus, tlhe samples contain negligible quantities of ions and other short-lived intermediates, and their physical and thermodynamic properties are usually well known. (b) Experimentation using labile species generated in situ from suitable precursors is very rewarding. Reference 3 provides a nice example of such work. However, the requirement that the precursor decomposes in a time interval short compared to the half-life of the reaction of interest can sometimes place limitations on the temperature range that can be studied using this technique. J. TROE(Institut fur Physikalische Chemie der Universitat Gottingen). Have you determined whether your corrections for shock wave nonidealities in terms of Mirel's theory are correct, either by using a tube with a larger diameter or by measuring a test reaction which was studied before in a larger diameter tube? G. P. GLASS.Yes, we have studied the thermal decomposition of N 2 0 dilute in argon. When corrections for shock wave nonideality are included. our measurements on this system are in good agreement with those obtained in previous studies (see, for example, ref 11 and 12). The agreement is less good if shock wave nonideality is ignored.
