Development of a methyl radical source for use in reaction dynamics studies

Chemical Physics North-Holland

168 (1992)

293-300

Development of a methyl radical source for use in reaction dynamics studies H. Heydtmann,

D. Boglu

Instltut ftir Physikalische und Theoretlsche Chemie, Johann- Wolfgang-Goethe-Universitiit, Nlederurseler Hang, W-6000 Frankfurt am Main, Germany

and J.J. Sloan Department of Chemistry, Unwersrty of Waterloo, Waterloo, Ontarro, Canada N2L 3Gl Received

13 July 1992

A source for methyl radicals based on the thermal decompositron of azomethane was subjected to an analysis of products by mass spectrometry. This source was used previously by Schwanke et al. to study the reaction with F atoms and to probe the product mixture by IR chemiluminescence. It was found that the CH3 yield was less than 18%, the exact number depending on the temperature of the tungsten oven. Other products were CH.,, C2H6, C2H4, &Hz and C2H3 radicals. A computer study including 32

elementary reactions suggests that the composition of the source beam can only be explained by heterogeneous processes occurring on the oven walls.

1. Introduction In a previous publication [ 11, we described our initial attempts to develop a source of CH3 radicals for use in reaction dynamics studies. This source was based on the thermal dissociation of azomethane using a tungsten oven capable of achieving very high temperatures. The first methyl radical sources described in the literature use dimethyl metals like (CH, ) zHg as precursors [ 2,3 1. Other laboratories preferred the thermal dissociation of azomethane [ 41 or di-tert-butylperoxide [ 5,6] to produce methyl radicals with varying degree of success. In our initial experiments, azomethane was pyrolized in the oven at a temperature of approximately 2000 K and pressure of about 0.5 Torr. The pyrolysis products effused from a small hole in the oven wall into a vacuum chamber and reacted with F atoms created in a Correspondence to: H. Heydtmann, Institut ftir Physikalische und Theoretische Chemie, Johann-Wolfgang-Goethe-Universitat, Nrederurseler Hang, W-6000 Frankfurt am Main, Germany. 0301-0104/92/$

05.00 0 1992 Elsevrer Science Pubhshers

microwave discharge. The infrared chemiluminescence from the HF produced in this reaction was collected by a multipass optical cell and focussed into a Fourier transform spectrometer. The vibrationally and rotationally resolved spectra recorded in this way were used to calculate the populations of the HF (v’, J’ ) levels created in the reaction. The measurements reported in that first paper showed that the vibrational distribution in the HF produced by the reaction of F atoms with the pyrolysis products was much more excited than that produced by the reaction with the parent azomethane. The reaction with azomethane produced the following distribution: HF (v’=1:2:3:4)= 0.50:0.33: 0.15 :0.02 whereas the reaction with the pyrolysis products gave HF (v’=1:2:3:4)= 0.23:0.51: 0.24 : 0.02. The monotonically decreasing shape of the distribution from the azomethane reaction is similar to that previously observed for the reaction of F atoms with CH&N [ I] and is consistent with our expectations of the dynamics of this kind of reaction. The inverted HF distribution observed for the reac-

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tion with the pyrolysis products is very similar in shape to the distributions observed for the reactions of F atoms with both CH4 and HZ. The major difference between these results and those of the reactions with the pyrolysis products is that four vibrational levels of HF, are created in the latter, whereas both the CH4 and the Hz reactions produce HF in the first three vibrational levels only, even under the high temperature conditions used in these experiments [7,81. Our previous publication also described a computer model which we wrote to assist in the interpretation of the chemiluminescence emission experiments. This model simulated the thermal dissociation of azomethane by integrating the simultaneous rate equations for the processes occurring in the oven. It used a commercial numerical integrator of the RungeKutta type, having variable stepsize and well defined error control procedures. In this first calculation, the rate equations were integrated for a time of 500 us, so that the results would include the residence time in the oven, which was calculated to be 280 ps, based on the known flow rates and pressures of the experiment. In order to simulate the conditions of the gas as it travelled through the oven, the temperature of the simulation was increased from 300 K to the final oven temperature used in the experiments, 2000 K, linearly with the integration time. The integration followed 27 rate processes including the primary thermal dissociation and all important elementary reactions among its products. These calculations indicated that CH3 radicals should be produced abundantly in this thermal dissociation on the timescale assumed by the model. They suggested that during the time from about 250 to about 400 us, CHX should be virtually the only product, and that the gas phase reactions which would lead to the production of stable molecules would not become important until approximately 500 us under the conditions of the simulation. On the basis of these calculations and the fact that the experimental HF vibrational distributions were inverted, we speculated in the previous publication that most of the observed HF was created by the reaction of F with CH3. The chemiluminescence emission results, of course, could not identify the material produced in the pyrolysis directly, so our speculation concerning the presence of CH3 in the oven eMuent could not be verified

Physics 168 (1992) 293-300

at that time. Subsequently, however, we carried out further experiments in order to measure directly the composition of the pyrolysis products. We fitted a mass spectrometer to the apparatus containing the oven, and recorded the mass spectra of the pyrolysis products as a function of the oven temperature. These measurements, which will be reported in detail in a later section of this publication, showed that the oven effluent contained many more chemical compounds than those expected from the primary thermal dissociation of azomethane (CH3 and N2), which were predicted by the computer simulation. In addition to the primary products, the mass spectrometer measurements identified a large number of stable molecules such as CH4, &HZ, C2H6 and C2H4 in the oven effluent. The computer simulation contained a complete description of all possible elementary gas phase reactions which could occur in the thermal dissociation process. It included 27 reactions among the following radicals and molecules: N2 ( CH3)*, N2CHJCHZ, CH3, CJ-L H, Cd-L, Hz, CI-L C&, CH, C2H3, CzH6 and C2Hz. The rates of most of these reactions are well known; the most important of these have been the subject of several determinations. Despite its completeness, however, this model could not reproduce the high concentrations of CH4, C2Hs and CzHz observed in the mass spectrometer experiments. We must conclude, therefore, that in addition to the primary processes considered in our simple model, either additional processes were occurring or the conditions of the simulation were inappropriate, or both. Since the model included all known processes which could occur in the gas phase at the pressures and temperatures of the experiments (0.5 Torr and 300-2000 K) any additional processes should be heterogeneous, occurring (presumably) on the walls of the oven. If we admit the possibility of heterogeneous processes, then recombination of the CH3 radicals becomes possible during the residence time - a process which is negligibly slow in the gas phase at the pressure of the experiment (0.5 Torr). An additional source of uncertainty could arise from our estimates of the residence time of the gas in the oven, since these estimates neglected both the volume change on heating and the true temperature gradient in the oven. If the residence time were longer than we estimated, for example, more of the stable molecular products would

H. Heydtmann et al. / ChemtcalPhysics168 (1992) 293-300

be formed, since these are the predominant end products which would be observed in the equilibrium gas phase system - even at the elevated temperature of the experiment. We note, however, that the estimated residence time is likely an upper limit, and the stable molecular products observed in the mass spectrometer measurements are unlikely to be the result of a very long residence time. They are more likely the result of heterogeneous process, as we shall show later. In the following sections we report the mass spectrometer measurements in detail. In addition, we report several modifications and additions to the computer simulation which show that the observed molecular products of the thermal dissociation could have come from heterogeneous processes on the walls of the oven. The latter conclusion is based on the fact that the model cannot reproduce the observed products if only gas phase processes are taken into account - even for integration time substantially in excess of that expected on the basis of the pressure and flow rate in the experiment. These results suggest how the apparatus may be improved in order to enhance the production of the desired radicals.

2. Experimental Experiments were done to analyze quantitatively the products formed in the thermal decomposition of azomethane. The decomposition itself took place in a tungsten oven similar to that used for the HF chemiluminescence measurements [ 11. The design of the oven and the mode of its operation were as described before [ 93. The heated part consisted of a tungsten tube of 38 mm length and 4.5 mm outer diameter closed at one end; the nozzle consists of a 1.3 mm diameter hole in the middle of the tube. This oven was interfaced to a sector-field mass spectrometer (FINNIGAN MAT CH7a) with a differential pumping unit. From the main gas stream flowing out of the oven nozzle a molecular beam was extracted by a conical nozzle with a central orifice of 1 mm diameter. The beam then entered a small intermediate chamber which was pumped by two 120 Q/s oil diffusion pumps and from this chamber the beam was extracted by a skimmer of 0.8 mm internal diameter into the ion source of the mass spectrometer. The dis-

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tance between the cylindrical oven and the first nozzle was about 20 mm. The first nozzle and the skimmer were 16 mm apart. Azomethane was prepared from an azomethaneCuC& complex by the addition of an aqueous KOH solution and cleaned by trap-to-trap distillation [ 10,111. The azomethane was introduced into the oven through a flowmeter. The oven temperature was varied between 300 and 2200 K. For each temperature mass spectra were obtained using electron bombardement for ionization. Spectra taken at electron energies of 20 eV indicated the formation of methane (m/e= 16) and ethane (m/e= 30) at temperatures above 1200 K. High resolution spectra (R = 4000) were taken to analyze for ethene and nitrogen at m/e=28. This analysis was checked by calibrations for which the pure gases azomethane, N2, ethane, ethene, ethyne and methane were used. After corrections of the raw data for the various stable species, there remained peaks at m/e= 15 and ml e=27. The peak at m/e= 15 clearly indicates the presence of CH3. The peak at m/e=27 consisted of two contributions which were evident at high resolution (R=4000). The contribution at mlez27.01 decreases with temperature and becomes negligible above 1300 K, it is attributed to HCN+ stemming from the azomethane parent ion by fragmentation. The contribution at m/e= 27.02 is very small at 300 K and increases with temperature; it is attributed to C2H: which is formed by parent ion fragmentation from CzH4 and CzH6 but also from the C2H3 radical itself. In summary, we detected two hydrocarbon radical species, CH, and C2H3, which were formed by the azomethane thermolysis under our conditions. By use of the calibrations the raw data were reduced to obtain approximate product compositions for each oven temperature. As a final step in the analysis we corrected for the different ionization cross sections using another series of calibration measurements. The ionization efficiencies for the stable species were determined relative to Nz for each molecule using 1: 1 mixtures. The ionization efficiencies for the two radical species mentioned above could be determined from our experimental data with the use of the mass balance for carbon. Fig. 1 shows the result of our analysis for an azomethane gas flow of 1.6 seem. A

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2d00

TEM%N”RE/ K Fig. 1. Measured concentrations of the indicated species in the oven eflluent as a function of the oven temperature.

qualitatively similar picture was obtained for a flow of 0.4 seem.

3. Computer simulation As in the previous publication, this simulation was carried out in order to determine whether the observed product distribution is a reasonable one based on what is known about the kinetics of the system. The computation has too many variables to have any predictive value; the results have only qualitative significance. If the additions and modifications to be described in this section are included, however, the simulation agrees (qualitatively) with the measurements, whereas without these there is no agreement whatever. We take this to indicate that the processes defined by the model are a possible interpretation for the processes occurring in the oven. The program described in ref. [ 1 ] was used for this calculation. Two modifications were made in order to simulate the present experiments: six additional rate processes were added to the twenty seven in the original model, and the simulation was changed slightly to reproduce the way in which the mass spectrometer experiments were carried out. The additional rate processes included radical recombinations and dissociations. The dissociations would have been negligibly slow under purely gas phase condi-

tions, but they could become significant if heterogeneous processes are possible. The effect of the heterogeneous processes was included in the calculation by assuming that the wall of the oven could supply some of the activation energy for the dissociations. In all cases, the Arrhenius form of the rate expressions was retained. Of course, the published Arrhenius parameters for the gas phase processes are not appropriate for heterogeneous conditions. This aspect was expressed by assuming that the activation energies for the heterogeneous processes are lower than those for the gas phase reactions - the normal catalytic effect. The second modification to the calculation was the use of a procedure in which the temperature was increased linearly from room temperature to the indicated final temperature over a fixed time period of 10e3 s, which was calculated to be the residence time under the conditions of the present experiments. This calculation was carried out for a range of temperatures which matches that of the experiments. The complete set of rate processes used in the present model are listed in table 1. The Arrhenius parameters and rate constants for the homogeneous gas phase processes were all obtained from references in the NIST Standard Reference Database [ 121. The first twenty-seven processes (0 to 26) are the same as those used in our previous calculation, and listed in table 4 of ref. [ 11. These define the strictly homogeneous reaction conditions when the published “gas phase” Arrhenius parameters are used. As reported in ref. [ 11, the results of this calculation show that the most abundant product at the time when the gases effuse from the oven into the vacuum chamber ( and the product distribution is effectively “frozen”) is CHJ, with minor amounts of the various stable molecular products. This does not agree with the mass spectrometer measurements. In order to provide a better description of the system, we added rate processes 27 to 32. The choice of these processes was partially guided by the indentification of their products in the experimental measurements and partly by the necessity to provide a complete description of all processes which could occur in the system. Using the larger set of rate processes, we carried out two calculations: one in which we used the published Arrhenius parameters, appropriate to purely homogeneous conditions, and a second one in

H. Heydtmann et al. /Chemical Physics 168 (1992) 293-300 Table 1 Reactions

used for computer

simulation

No. 0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

of azomethane

Reaction

k (cm’s_‘)

N2(CH3)2+M+N2+2CHs+M CHz+C2H2+CH4+C2H CH3tC2H4+CH4+C2H3 2CH,-+C2HS+H C2HS+H+2CH, CH3+CH3-K2H4+H2 CHS+N2(CHs)+ZH4+R R+M+CH2+CHa+NI+M CH.,+M+CH,+H+M H+CHSN2CH,-tCH,+N2+CHg CH4+‘CH2-+CHa+CHj H+‘CH2+CH+HI C2HS+M-+C2H4+H+M C2HS+H-+C2Ha+H2 CH,+CH,-+CIH,+H2 CIH,tH-+CIH,tH, CHStHI+HtCHI H+CH.+ZHJ+H2 CH,tH+CH2+H2 C2H3+M+C2H2tHtM C2H,+H-X2H2+H2 CH+H,-+CH,+H CH+CH,+Prod CH+CH2-+H2+Prod CH + &H,+Prod “CH2+CH,+C2H4tH ‘CH2 + C2H2-+C3H., C2Hs+CH3+CH, CHI t CH+C,Hs CH,+H--XHd C2H4+CZHZtH2 CH,tC2H6+CH4+C2HS C2H3tCH4+C2H.,tCHS

1.75X10-9exp(-20900/T) 3.0x10-“exp(-8700/T) 1.57~1O-‘~T’~‘exp( -4780/T) 1.33X10-9exp(-13400/T) 6.0x lo-” 1.66x10-*exp(-16117/T) 1.78~10-~~exp(-4908/T) 1.66X 1O-‘O 6.18~10-~exp(-52250/T) 1.465x10-10exp(-1128/T) 2x lo-” exp( -4800/T) 2.7x lo-lo 1.66x10-‘exp(-15636/T) 3.0x lo-‘* 1.66x10-LLexp(-11547/T) 4x10-l2 T*‘exp(-6160/T) 2.5~10-“‘T~‘~exp(-4384/T) 9.8~ lo-” T3 exp( -4406/T) 3.02x10-“‘exp(-7578/T) 2.7~10~‘exp(-17000/T) 3.3x 10-l’ 1.5~ lo-” T18 exp( -840/T) 5.0X 10-‘Lexp(200/T) 1.66x10-‘0 3.49x10-10exp(61/T) 7.0x 10-l’ 5.8x 10-12 1.2X lo-*exp( -37127/T) 4.0X lo-‘* T-0.73exp( +40/T) 2.0X10-‘0T-040 5.0x lo-sexp( -35600/T) 2.5~10-‘~T~~~exp(-3820/T) 1.2x10-‘0exp(-5882/T)

which the activation energies for the dissociations of stable molecular species were reduced to simulate the effect of heterogeneous dissociation on the walls of the oven. Table 2 shows the activation energies before and after this reduction. Table 2 Activation No.

In summary, we assumed that heterogeneous processes in the oven are responsible for the additional species observed in the mass spectrometer measurements, but not predicted by our previous (homogeneous) computer simulation. Also, to provide a more complete description of all processes, we extended the model by adding processes 27 to 32 inclusive.

energy (kcal/mol) Homogeneous conditions

Heterogeneous conditions

4. Results Two separate calculations were done: one in which strictly homogeneous conditions were assumed, and one in which heterogeneous conditions were simulated. Fig, 2 shows the result of the “homogeneous”

20.9

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800 0

1000.0

1200.0

1400.0

1600.0

1800 0

2000.0

TEMPERATURE/K Fig. 2. Calculated concentrations in the oven effluent as a function of oven temperature assuming homogeneous conditions in the oven for a reaction time of 10-s s.

calculation. For this, the activation energies are those of the gas phase processes as listed in table 1. The result differs from that observed by the mass spectrometer measurements (see fig. 1) in several significant respects. First, the azomethane dissociation does not begin until a much higher temperature than that observed in the experiment. Fig. 1 shows that the measured azomethane concentration in the oven effluent is reduced to 97% of its original value at a temperature of 1000 K, whereas fig. 2, the “homogeneous” calculation, indicates that this does not occur until about 1350 K. We interpret this to mean that heterogeneous processes reduce the activation energy for the azomethane decomposition. Furthermore, fig, 2 shows that after the (short) residence time appropriate to the experiments, the oven effluent calculated by the “homogeneous” model contains fewer stable molecular compounds than measured in the experiment. The major products calculated in this case are C2H6 and CH3, with small amounts of CzH4, and CH4, which appear only at higher temperatures. The experiment, on the other hand, shows that CH4, CzHs, C2H2 and C2H4 are major components in the effluent at 1600 K. We interpret this to mean that heterogeneous processes increase the rate with which the radicals are produced and thus enhance the rates of the reactions which form these stable molecules. If the activation energies of the dissociation steps are reduced to simulate reaction on the heated walls, the calculated composition of the oven eflluent as a

function of the oven temperature, is as shown in fig. 3. We refer to these as “homogeneous” conditions, and it is clear that this model not only predicts the major components of the effluent, but also achieves good qualitative agreement with the measured concentrations as well. The calculation predicts that the stable molecules CH4, CzHs, CzH4 and CzHz are contained in the oven eflluent in approximately the same relative concentrations as observed by the measurement. Comparison of fig. 1 and fig. 3 shows that all of the species detected in the mass spectrometer measurements are also obtained in the calculation, and conversely, the calculation does not predict significant concentrations for any products which are not observed in the measurements. We note particularly that the important radicals, CH3 and CzH3, are predicted by this calculation to have significant concentrations, while the concentrations of all other radicals involved in the model are predicted to be very small. It is significant that only CH, and CZH, are detected by the experiments, in agreement with the model. Furthermore, the temperature dependences of the concentrations of the observed radicals are in approximate agreement with those of the calculations. Fig. 3 shows that CHS is produced at lower temperatures (hence earlier during the residence time of the gas in the oven) than CzH3, and the measurements agree, as shown in fig. 1. Agreement with the temperature dependences of

Fig. 3. Calculated concentrations in the oven effluent as a fimction of oven temperature assuming heterogeneous decomposition of stable molecules using the activation energies listed in table 2. The reaction time is lo-‘s.

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the stable molecules is also quite good. Both the experiment and the calculations indicate that CH4 and C2H6 appear at lower temperatures, and their concentrations rise rapidly with increasing temperature, while C2H2, appears at higher temperatures. Despite these points of agreement, we do not feel that the accuracy of the model warrants further attempts to improve the fit to the observations. The calculations cannot be shown to be unique, and therefore they cannot be used to extract rate constants for the heterogeneous processes or even to determine the relative importance of the major processes in the model. The good qualitative agreement between the predictions of the model and the measured results, however, indicates that the model probably contains all of the important reactions occurring in the heated oven, including the heterogeneous processes, and their qualitative temperature dependences. Consequently, we believe the model may be used to predict the likely result of limited modifications to the oven, especially those which change the conditions toward those of a homogeneous system, and reduce the effect of collisions with the surfaces. Following this approach, the model was used to suggest design changes which would enhance the production of the desired radical, CH3 with respect to other species present. Both the model and the measurement show that the CH3 is formed at low temperature and short residence time. The latter, especially, acts to reduce the effect of the secondary processes in which the stable molecules are formed. These observations indicate that the production of CH3 would be enhanced if the oven were made smaller, and the opening through which the dissociation products effuse were made larger. Both of these modifications are more important than maintaining a very high temperature in the oven, since the results show that the parent azomethane is almost completely converted to CH3 at the lower end of the temperature range examined in this study ( 1600- 1800 K). At the upper end of this temperature range, C2H6 will be the major contaminant in the effluent, along with smaller amounts of CH4 and CzH4. A calculation was carried out for which the contact time was reduced to 500 ns, to show the likely effect of the redesign of the oven. The result is shown in fig. 4. This shows that the effluent, at medium tempera-

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8000

10000

1200.0

1400.0

1600.0

moo.0

2000 0

TEMPERATURE/K Fig. 4. Calculated concentrations for the same conditions as those of fig. 3 except the reaction time is 5 x 1Oe4 s.

tures around 1700-l 800 K, is still contaminated by CzH6 and CH,, but the CH, concentration is reasonably high despite this. The extreme thermodynamic stability of CH, and the ease with which C2Hs is formed by radical recombination in the presence of high concentrations of CH3 radicals makes it likely that very little can be done to reduce the concentration of these molecular contaminants as long as a thermal source of this type is used to create radicals. Further calculations and measurements of the kind reported here may make it possible to calculate the relative concentrations of the thermal dissociation products accurately enough to permit their energy distributions to be deconvoluted from measurements of the composite distributions, if experiments are carried out under a range of temperatures and pressures of the thermal source. Further work is under way to discover whether this is a feasible procedure.

Acknowledgement The authors are pleased to acknowledge the assistance of a NATO travel grant and the financial support of the Natural Sciences and Engineering Research Council of Canada, the Deutsche Forschungsgemeinschaft, and the Fonds der Chemischen Industrie.

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References [ 1 ] U. Schwanke, H. Heydtmann and J.J. Sloan, Chem. Phys. 132 (1989) 413. [2] F. Kalos and A.E. Grosser, Rev. Sci. Instr. 40 ( 1969) 804. [ 31 D.L. McFadden, E.A. McCullough, F. Kalos, W. R. Gentry and J. Ross, J. Chem. Phys. 57 ( 1972) 135 1. [ 41 CF. Carter, M.R. Levy and R. Grice, Chem. Phys. Letters 17 (1972) 414; Faraday Discussions Chem. Sot. 55 (1973) 357. [ 51 S.M.A. Hoffmann, D.J. Smith, N. Bradshaw and R. Gtice, Mol. Phys. 57 (1986) 1219.

[ 61 G.N. Robinson, G.M. Nathanson, R.E. Continetti and Y.T. Lee, J. Chem. Phys. 89 (1988) 6744. [7] D.S. Perry and J.C. Polanyi, Chem. Phys. 12 (1976) 419. [ 81 H. Heydtmann and U. Schwanke, unpublished results. [ 91 B. Hildebrandt, H. Vanni and H. Heydtmann, Chem. Phys. 84 (1984) 125. [ lo] M. Remmler, B. Ondruschka and G. Zimmermann, J. Prakt. Chem. 327 (1985) 868. [ 111 S. Zabamik and J. Heiklen, Intern. J. Chem. Kinetics 17 (1985) 455. [ 12 ] NIST Chemical Kinetics Database Version 2.01, Standard Reference Data, NIST, Gaithersburg, MD 20899, USA.