FTIR and Mass-Spectrometric Measurements of the Rate Constant for the C 6 H 5 + H 2 Reaction

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1 J. Phys. Chem. A 1997, 101, FTIR nd Mss-Spectrometric Mesurements of the Rte Constnt for the C 6 H 5 + H 2 Rection J. Prk, I. V. Dykov, nd M. C. Lin* Deprtment of Chemistry, Emory UniVersity, Atlnt, Georgi ReceiVed: July 3, 1997; In Finl Form: September 9, 1997 X The rte constnt for the C 6 H 5 + H 2 f C 6 H 6 + H rection hs been mesured by pyrolysis/fourier trnsform infrred spectrometry (P/FTIRS) in the temperture rnge of K nd by pulsed-lser photolysis/ mss spectrometry (PLP/MS) in the temperture rnge of K. By P/FTIRS, the rection ws studied by mesuring time-resolved concentrtion profiles of the rectnt (C 6 H 5 NO) nd the product (C 6 H 6 ) using highly diluted mixtures of C 6 H 5 NO in H 2 (with or without Ar dilution). In PLP/MS experiments, the C 6 H 5 rdicl ws generted by the photolysis of C 6 H 5 COCH 3 t 193 nm in the presence of severl Torr of H 2. The C 6 H 5 + H 2 rte constnt ws determined by the bsolute yields of C 6 H 6 nd C 6 H 5 CH 3 products. The results of these two spectrometric mesurements gree closely with our theoreticlly predicted expression, k ) T 2.43 exp (-3159/T) cm 3 /(mol s) nd with tht of shock-tube study by Troe nd co-workers (ref 20) in the temperture rnge of K. Preliminry kinetic dt on the CH 3 + C 6 H 5 ssocition rection re lso presented. I. Introduction Phenyl rdicl hs been considered s one of the most importnt rective species in hydrocrbon combustion chemistry, especilly in the formtion of polycyclic romtic hydrocrbons (PAH s) which re pivotl to soot formtion in its incipient stges. 1-5 For the kinetic studies of phenyl rdicl rections, we hve crried out experiments using the cvity ringdown (CRD) technique, covering typicl temperture rnge of K Temperture brodening of the rovibronic trnsition of C 6 H 5 in the visible region limits its kinetic mesurements by CRD to bout 523 K, bove which its S/N rtio deteriortes rpidly with temperture. This temperture limit (T E 523 K) precludes the studying of slower rections such s C 6 H 5 + H 2 nd CH 4. In order to circumvent the shortcoming, in this work we employed two complementry methods for the kinetic study of the C 6 H 5 + H 2 rection using pyrolysis/fourier trnsform infrred spectrometry (P/FTIRS) nd pulsed-lser photolysis/ mss spectrometry (PLP/MS). The methods effectively extend the kineticlly useful temperture rnge to 1000 K. The ltter method hs been successfully pplied to obtin the rte constnt for the recombintion rection of C 6 H 5 rdicls, 16 nd the totl rte constnts nd product brnching rtios for the rections of NH 2 with NO x. 17,18 The mechnism of the C 6 H 5 + H 2 rection hs been investigted theoreticlly using modified Gussin 2 method (G2M). 19 The rection ws predicted to occur exclusively by direct bstrction with brrier of 8.8 kcl/mol. The trnsition stte (TS) ws found to hve liner C-H-H structure with C 2V symmetry. The rte constnt clculted ccording to the predicted geometries nd moleculr prmeters (frequencies nd moments of inerti) of the TS nd the rectnts using the trnsition-stte theory (TST) with tunneling corrections ws found to be in good ccord with the recent shock tube results of Troe nd co-workers. 20 Comprison of the theory with the present nd other vilble dt in the literture will be discussed. * Corresponding uthor. E-mil: chemmcl@emory.edu. X Abstrct published in AdVnce ACS Abstrcts, November 1, S (97) CCC: $14.00 Figure 1. Typicl FTIR spectr of pyrolyzed nd unpyrolyzed smples. II. Experimentl Section As lluded to bove, two methods complementry to the CRD technique hve been developed for the slow bstrction rection of C 6 H 5 with H 2. The experimentl procedures of the P/FTIRS nd PLP/MS methods hve been given previously, 16-18,25,26 nd brief description of ech is presented below. 1. P/FTIRS. With this method, C 6 H 5 NO ws used s the phenyl source. Highly diluted mixtures of C 6 H 5 NO in H 2 (with or without further dilution with Ar) were pyrolyzed in the K temperture rnge t tmospheric pressure. Pyrolyzed or unpyrolyzed reference smples were expnded into the bsorption cell inside the nlysis chmber for determintion of the concentrtions of the C 6 H 6 product nd the unrected C 6 H 5 NO rectnt. The pressures of the smples fter expnsion re pproximtely 270 Torr. To minimize the effect of pressurebrodening, the pressure of clibrtion mixtures contining vrying mounts of C 6 H 6 nd C 6 H 5 NO ws mintined t 270 ( 10 Torr. Absorption peks t nd cm -1 were employed for the determintion of C 6 H 6 nd C 6 H 5 NO, respectively. Figure 1 presents typicl FTIR spectr for pyrolyzed nd unpyrolyzed smples. This set of spectr illustrtes tht the 1997 Americn Chemicl Society

2 8840 J. Phys. Chem. A, Vol. 101, No. 47, 1997 Prk et l. trce mount of C 6 H 6. Addition of H 2 to the system noticebly reduced the yields of C 12 H 10 nd C 6 H 5 CH 3, with concomitnt increse in the yield of C 6 H 6. Tble 2 summrizes the experimentl conditions employed nd the yields of C 6 H 6 nd C 6 H 5 CH 3 mesured in the PLP/MS experiment. Kinetic modeling of the bsolute concentrtions of these products, determined by creful clibrtions using stndrd mixtures smpled t the sme totl pressure employed in ech run, gve the rte constnts for the formtion of C 6 H 6 nd C 6 H 5 CH 3 by the following respective rection: C 6 H 5 + H 2 f C 6 H 6 + H C 6 H 5 + CH 3 f C 6 H 5 CH 3 Figure 2. Time-resolved C 6H 5NO decy (solid points) nd C 6H 6 formtion (open circles) profiles in P/FTIR experiments t T ) 598 K. Lines re the modeled results. Rection conditions re given in Tble 1. TABLE 1: Experimentl Conditions nd Modeled Rte Constnts in P/FTIR Experiment for the Rection of C 6 H 5 + H 2 t Tempertures Studied T (K) [C 6H 5NO] 0 [Ar] 0 [H 2] 0 k 4 (cm 3 /mol sec) b (1.18 ( 0.66) (1.20 ( 0.18) (1.44 ( 0.15) (1.35 ( 0.14) (1.48 ( 0.20) (1.99 ( 0.13) 10 9 The concentrtion units re in mol/cm 3. b For ech temperture (except 548 nd 563 K), typiclly 3-5 runs were crried out. The uncertinty represents 1σ. mjor bsorption peks of the rectnt nd product re clerly seprted. The C 6 H 6 pek t cm overlps with smll pek of C 6 H 5 NO; the overlp cn be redily corrected. Figure 2 shows typicl set of the concentrtion vs time plots for the formtion of C 6 H 6 nd the decy of C 6 H 6 NO. The corresponding curves represent kineticlly modeled vlues. The vlues of the rte constnt for C 6 H 5 + H 2 obtined by modeling re summrized in Tble 1. The mechnism employed for modeling will be discussed lter. 2. PLP/MS. By the mss-spectrometric method, the pulsed photolysis of C 6 H 5 COCH 3 t 193 nm ws employed s the C 6 H 5 rdicl source. C 6 H 5 COCH 3, crried by n excess mount of He ws premixed in corrugted stinless steel tubing with H 2 before being introduced in the Slfeld-type qurtz rector 27,28 which cn be heted to 1200 K. The rectnts nd the products of the photoinitited rection were supersoniclly smpled nd ionized by electron-impct ioniztion. The mole frction of C 6 H 5 COCH 3 ws typiclly <0.5% nd tht of H 2 >75%, with [H 2 ]/[C 6 H 5 COCH 3 ] > 150. The conversion of C 6 H 5 COCH 3 by the unfocused ArF lser bem rnged from 15% to 40%. The mechnism for the frgmenttion of C 6 H 5 COCH 3 t 193 nm hs been studied by using NO or HBr s the C 6 H 5 rdicl scvengers. 29 The kinetic modeling of mesured yields of C 6 H 5 - NO or C 6 H 6 nd CH 4 under fully inhibited conditions reveled tht 80% of the frgmenttion rection gve rise to C 6 H 5. This result is consistent with the mesured yield of C 6 H 5 CH 3 without NO or HBr. The introduction of n excess mount of NO, for exmple, eliminted the formtion of toluene. 29 In the bsence of H 2, the mjor moleculr products from the C 6 H 5 COCH 3 photolysis were C 6 H 5 CH 3, C 2 H 6 nd C 12 H 10 (which ws not quntittively determined in this study), with CH 4 is present in these experiments in much lower concentrtion thn C 6 H 6 nd ws presumed to be formed by the CH 3 + H 2 f CH 4 + H rection. The comprison of the mesured nd predicted yields will be discussed in the following section. III. Results nd Discussion 1. Kinetics of the C 6 H 5 + H 2 Rection. The rte constnt of the C 6 H 5 + H 2 rection cn be obtined by kinetic modeling in strightforwrd mnner. The mechnisms employed for the modeling re presented in Tbles 3 nd 4 for the pyrolysis of C 6 H 5 NO-H 2 mixtures nd the photolysis of C 6 H 5 COCH 3 - H 2 mixtures, respectively. In the thermlly initited C 6 H 5 NO nd H 2 rection, the formtion of C 6 H 6 ws influenced by the following primry processes: C 6 H 5 NO ) C 6 H 5 + NO (1, -1) C 6 H 5 + C 6 H 5 f C 12 H 10 (2) C 6 H 5 + C 6 H 5 NO f C 12 H 10 NO (3) C 6 H 5 + H 2 f C 6 H 6 + H (4) All but rection 4 hve been determined recently by us. 16,26 The results of our sensitivity nlysis (see Figure 3) ber this out. In order to ccount for the loss of C 6 H 5, severl secondry nd tertiry rections were included in the modeling, employing resonbly ssumed vlues of rdicl-rdicl rection rte constnts (see Tble 3). The vlues of rte constnt for the C 6 H 5 + H 2 rection re presented in Tble 1 nd Figure 5 (vide infr). In the photoinitited rection of C 6 H 5 COCH 3 in the presence of excess mounts of H 2, the production of C 6 H 6 ws found to be influenced by the following primry processes: C 6 H 5 + C 6 H 5 f C 12 H 10 (2) C 6 H 5 + H 2 f C 6 H 6 + H (4) C 6 H 5 + CH 3 f C 6 H 5 CH 3 (5) CH 3 + CH 3 f C 2 H 6 (6) The result of sensitivity nlysis crried out t 887 K (see Figure 4) clerly bers this out. Agin, in order to minimize the loss of C 6 H 5 in our simultion, severl secondry nd tertiry rections were included in the model s shown in Tble 4. A sensitivity nlysis of this system shown in Figure 4 revels tht the yield of C 6 H 6 is most strongly nd positively ffected by rection 4 nd negtively influenced by its competitive rections 2 nd 5.

3 Rte Constnt for the C 6 H 5 + H 2 Rection J. Phys. Chem. A, Vol. 101, No. 47, TABLE 2: Experimentl Conditions nd Product Yields b in the PLP/MS Experiment t the Tempertures Studied [C 6H 6] t [C 6H 5CH 3] t temp (K) P (Torr) [C 6H 5COCH 3] 0 [C 6H 5] 0 [He] 0 [H 2] 0 expt model expt model All concentrtions re in units of mol/cm 3. b Product yields were mesured t t ) 15 ms in their plteu regions. Typiclly 3-5 runs were crried out for ech temperture. To void congestion, the kineticlly modeled vlues of k 4 from the C 6H 6 yields re summrized in Figure 4 nd those of k 5 from the C 7H 8 yields re given in the text. TABLE 3: Rections nd Rte Constnts Used in the Modeling of the C 6 H 5 + H 2 Rection in the P/FTIR Experiment rections A n E ref key rections 1. C 6H 5NO ) C 6H 5 + NO 1.42E+17 b C 6H 5 + C 6H 5 ) C 12H E C 6H 5 + C 6H 5NO f C 12H 10NO 4.90E C 6H 5 + H 2 f C 6H 6 + H 5.72E minor rections 7. C 12H 10NO f C 6H 5NO + C 6H E c 8. C 12H 10NO + C 6H 5 f C 12H 10N + C 6H 5O 1.00E c 9. C 6H 5NO + H f C 6H 6 + NO 4.00E c 10. C 6H 5NO + H f C 6H 5 + HNO 1.00E c 11. C 6H 5 + C 6H 5NO f C 12H 10 + NO 5.00E c 12. C 6H 5 + H f C 6H E C 12H 10N + NO f C 12H 10NNO 1.00E c 14. H + NO + M ) HNO + M 5.40E Rte constnts re defined by k ) AT n exp(-e /RT) nd in units cm 3, mol, nd s; E is in the units of cl/mol. b Red s c Assumed. TABLE 4: Rections nd Rte Constnts Used in the Modeling of the C 6 H 5 + H 2 Rection in the PLP/MS Experiment rections A n E ref key rections 2. C 6H 5 + C 6H 5 ) C 12H E+13 b C 6H 5 + H 2 ) C 6H 6 + H 5.72E C 6H 5 + CH 3 ) C 6H 5CH E CH 3 + CH 3 ) C 2H E minor rections 15. C 6H 5 + H ) C 6H E CH 3 + H 2 ) CH 4 + H 2.89E C 6H 5 + C 6H 5COCH 3 f C 6H 6 + C 6H 5COCH E C 6H 5 + CH 4 f C 6H 6 + CH E CH 3 + C 6H 5COCH 3 f CH 4 + C 6H 5COCH E C 6H 5 + C 6H 5COCH 3 f C 12H 10 + CH 3CO 3.39E C 6H 5 + C 6H 5COCH 3 f C 12H 10COCH E c 22. C 6H 5COCH 2 + CH 3 ) C 6H 5COC 2H E c 23. C 6H 5COCH 2 + C 6H 5 f C 12H 10COCH E c 24. C 6H 5COCH 2 + C 6H 5COCH 2 f (C 6H 5COCH 2) E c 25. C 6H 5COCH 2 f C 6H 5 + CH 2CO 4.00E c 26. C 12H 10COCH 3 f C 12H 10 + CH 3CO 1.00E c 27. CH 3 + CH 3CO ) CH 3COCH E CH 3CO + M ) CH 3 + CO + M 8.74E Rte constnts re defined by k ) AT n exp(-e /RT) nd in units cm 3, mol, nd s; E is in the units of cl/mol. b Red s c Assumed. The kineticlly modeled vlues of k 4 bsed on the bsolute yields of C 6 H 6 re summrized in Figure 5 for comprison with those obtined by FTIR spectrometry nd by Troe nd coworkers using UV bsorption spectrometry crried out in shock tube t tempertures between 1050 nd 1450 K. 20 The three independent, more direct mesurements gree closely with our theoreticlly predicted vlues covering the entire temperture rnge investigted, K. 19 In the figure, we lso compre these three sets of experimentl dt nd the theoreticlly predicted curve with the existing, mostly kineticlly modeled results obtined by shock-heting of C 6 H 6 t high tempertures Among them, the dt of lser schlieren mesurements by Kiefer nd co-workers 23 gree most closely with the theory. Also included in the figure re the dt obtined by Fielding nd Pritchrd 21 with reltive rte method using the C 6 H 5 recombintion rectin s the reference stndrd. The reltive rte dt, fter being converted to bsolute vlues with our C 6 H 5 recombintion rte constnt 17 pper to be much lower thn our FTIR nd theoreticlly predicted results.

4 8842 J. Phys. Chem. A, Vol. 101, No. 47, 1997 Prk et l. Figure 5. Arrhenius plot of the rte constnt for the C 6H 5 + H 2 rection. O, this work by P/FTIRS; 0, this work by PLP/MS; 4, ref 20; 1, theoreticl results of Mebel et l., ref 19; 2, ref 22; 3, ref 23; 3, ref 21 using the rte constnt for the recombintion of C 6H 5 reported by Prk nd Lin (ref 17). Figure 3. Sensitivity nlysis t T ) 589 K for C 6H 6 nd C 6H 5NO in P/FTIRS experiment. gree resonbly well with the expression recommended by Tsng nd Kiefer, k 5 ) (300/T) 0.3 cm 3 /(mol s). 30 A more detiled discussion on k 5 determined by this nd other relted experiments will be presented elsewhere. 29 IV. Concluding Remrks In this investigtion, two complementry experimentl techniques (pyrolysis/ftir nd PLP/MS) hve been developed for the kinetic study of C 6 H 5 rdicl rections. These techniques hve effectively extended the rnge of temperture from the upper limit of the cvity ringdown method (523 K) to 1000 K. The vlues of rte constnt covering the wide temperture rnge of K cn be utilized for relible extrpoltion to the combustion regime ( K), prticulrly with the id of the TST or the RRKM theory using trnsition-stte prmeters obtined by high-level b initio MO clcultions s hs been demonstrted in the present C 6 H 5 + H 2 cse. For the C 6 H 5 + H 2 rection, the results determined with the two techniques covering the temperture rnge K gree closely with the theoreticlly predicted rte constnt, k ) T 2.43 exp(-3159/t)cm 3 /(mol s), by Mebel et l. 19 nd with the results of shock tube study by Troe nd coworkers 20 in the temperture rnge of K using UV bsorption spectroscopy. The bove expression is recommended for combustion modeling. Acknowledgment. The uthors grtefully cknowledge the support of this work by the Division of Chemicl Sciences, Office of Energy Sciences, DOE, under contrct no. DE-FGO5-91ER Figure 4. Sensitivity nlysis t T ) 887 K for C 6H 6 nd C 6H 5CH 3 in PLP/MS experiment. 2. Kinetics of the CH 3 + C 6 H 5 Assocition Rection. The bsolute yield of toluene mesured in the present experiment cn be directly utilized for estimtion of the rte constnt for the ssocition rection: C 6 H 5 + CH 3 f C 6 H 5 CH 3 (5) The vlues of k 5 obtined by kinetic modeling lie in the rnge of (1.8 ( 0.2) cm 3 /(mol s) t K. These vlues References nd Notes (1) Glssmn, I. Combustion, 2nd ed.; Acdemic Press: New York, (2) Bittner, J. D.; Howrd, J. B. 18th Symposium (Interntionl) on Combustion, [Proceedings]; The Combustion Institute: Pittsburgh, 1983; p (3) Brezinsky, K.; Litzinger, T. A.; Glssmn, I. Int. J. Chem. Kinet. 1984, 16, (4) Swyer, R. F. 24th Symposium (Interntionl) on Combustion, [Proceedings]; The Combustion Institute: Pittsburgh, 1992; p (5) Bockhorn, H. Soot Formtion in Combustion; Springer-Verlg: New York (6) Yu, T.; Lin, M. C. J. Am. Chem. Soc. 1993, 115, (7) Lin, M. C.; Yu, T. Int. J. Chem. Kinet. 1993, 25, 875.

5 Rte Constnt for the C 6 H 5 + H 2 Rection J. Phys. Chem. A, Vol. 101, No. 47, (8) Yu, T.; Lin, M. C. J. Phys. Chem. 1994, 98, (9) Yu, T.; Lin, M. C. Int. J. Chem. Kinet. 1994, 26, 771. (10) Yu, T.; Lin, M. C. J. Phys. Chem. 1994, 98, (11) Yu, T.; Lin, M. C. Int. J. Chem. Kinet. 1994, 26, (12) Yu, T.; Lin, M. C. J. Am. Chem. Soc. 1994, 116, (13) Yu, T.; Mebel, A. M.; Lin, M. C. J. Phys. Org. Chem. 1995, 8, 47. (14) Yu, T.; Lin, M. C. Combust. Flme 1995, 100, 169. (15) Yu, T.; Lin, M. C. J. Phys. Chem. 1995, 99, (16) Prk, J.; Lin, M. C. J. Phys. Chem. A 1997, 101, 14. (17) Prk, J.; Lin, M. C. J. Phys. Chem. A 1997, 101, 19. (18) Prk, J.; Lin, M. C. J. Phys. Chem. A 1997, 101, (19) Mebel, A. M.; Lin, M. C.; Yu, T; Morokum, K. J. Phys. Chem. A 1997, 101, (20) Heckmnn, E.; Hippler, H.; Troe, J. 26th Symposium (Interntionl) on Combustion, [Proceedings]; The Combustion Institute: Pittsburgh, PA, 1996; p 543. (21) Fielding, W.; Pritchrd, H. O. J. Phys. Chem. 1962, 66, 821. (22) Asb, T.; Fujii, T. Proc. Int. Shock Tube Symp. 1971, 8, 1. (23) Kiefer, J. H.; Mizerk, L. J.; Ptel, M. R.; Wei, H. C. J. Phys. Chem. 1985, 89, (24) Ro, V. S.; Skinner, G. B. J. Phys. Chem. 1984, 88, (25) Hlbgewchs, M. J.; Diu, E. W. G.; Mebel, A. M.; Lin, M. C.; Melius, C. F. 26th Symposium (Interntionl) on Combustion, [Proceedings]; The Combustion Institute: Pittsburgh, PA, 1996; p (26) Prk, J.; Dykov, I. V.; Mebel, A. M.; Lin, M. C. J. Phys. Chem. A 1997, 101, (27) Wytt, J. R.; DeCorpo, J. J.; McDowell, M. V.; Slfeld, F. E. ReV. Sci. Instrum. 1974, 45, 916. (28) Wytt, J. R.; DeCorpo, J. J.; McDowell, M. V.; Slfeld, F. E. Int. J. Mss Spectrom. Ion Phys. 1975, 16, 33. (29) Prk, J.; Dykov, I. V.; Lin, M. C. To be published. (30) Tsng, W.; Kiefer, J. H. In Chemicl Dynmics nd Kinetics of Smll Rdicls; Liu, K., Wgner, A. F., Eds.; World Scientific: Singpore, 1996; Prt I, p 58. (31) Bulch, D. L.; Cobos, C. J.; Cox, R. A.; Esser, C.; Frnk, P.; Just, Th.; Kerr, J. A.; Pilling, M. J.; Troe, J.; Wlker, R. W.; Wrntz, J. J. Phys. Chem. Ref. Dt 1992, 21, 734. (32) Tsng, W.; Hmpton, R. F. J. Phys. Chem. Ref. Dt 1986, 15, (33) Duncn, F. J.; Trotmn-Dickenson, A. F. J. Chem. Soc. 1962, 4672.

Experimental and Theoretical Studies of the Unimolecular Decomposition of Nitrosobenzene: High-Pressure Rate Constants and the C-N Bond Strength

Experimental and Theoretical Studies of the Unimolecular Decomposition of Nitrosobenzene: High-Pressure Rate Constants and the C-N Bond Strength J. Phys. Chem. A 1997, 101, 6043-6047 6043 Experimentl nd Theoreticl Studies of the Unimoleculr Decomposition of Nitrosobenzene: High-Pressure Rte Constnts nd the C-N Bond Strength J. Prk, I. V. Dykov,