Pressure and temperature dependence of the OH radical reaction with acetylene

Volume

CHEhllCAL

93, numbcr 4

PHYSICS LEI-TXRS

PRESSURE AND TEMPERATURE DEPENDENCE OF THE OH RADICAL

REACTION

WITH ACETYLENE

R A PERRY

Recclvcd I3 August 1982. m final form 72 Scptcmbcr

Absolulc ol’20-403

KIIC constants

for the rcac11on ofOH

1982

WIII acctylcnc

Torr usmg a flash photolysa-rcsonancc

lluorcsccnre

wcrc dctcrmmcd tcchmquc

at 297 and 419 K for thu 101.d pru~rc\

The rate coclTic~cnt k was found to bc pwrurc

dcpcndcnt below 200 Torr at both 297 and 429 K, but \\a m the high-prcssurc hmellc rctxm abolc a tool prcww TOW. TIE best fit to the data over the prcssurc and tcmpcrnrurc r,mgc studred wasgwrn by lhc c\pressron k(T,~)b, + I). ~11crc /J IS the prcssurc anTorr x lo-” C\P[(-327 2 400)/fU~/(3O/p

suits, a stgmlicant zero-prcssurc mterccpt WJS observed in the e\trapolatlon of the low-prcssurc data, m dlsagreement with both hq$-pressure-room tempcraturc results [9) and carhcr low-pressure c\penmcnts [5,7.81.

1. Introduction The hydroxyl radical IS Important m combustion processes [ I,?], dswell as in the chenustry of the troposphere

and stratosphere

constant

deterrrunatlons

reactlon

[4--91

[3].

Whdc

numerous

have been carned

rate

out for the

OH + C,H, _ ” -+ products,

(1)

less effort has gone mto a systemattc study of the pres. sure dependence of this reactton at elevated temperatures [IO] As the modehng of O#etylenc flames becomes more sophlsticatcd, a need ehtsts for detallcd kmetlcs at these higher temperatures. Also, acetylene 1x1s

of X0 = I 18

In tlrc prcscnr study WC seek to resolve

thcsc dlszarcc-

of rmc11011 (I) at higher temperatures and by InvrsGgAting both the 111gl1-and low-prcssurc limlts of tile rcactlon to bcttcr understand the kmetlcs and possible nrcchanisms. Absolute rate constants arc determrncd, using the flash photolysls-rcsonancc fluorcsccncctcchotquc mcnts

[9],

by studying

the pressure

for the reactlon

surcs of 20-403

ofOH

Torr

depcndcncc

with C?Hl

dt total

m the tcmperaturc

prcs-

range 297-

429 K.

bee11 mlplrcated asI sootprecursor in hydrocarbon/

oxygen flames [I I] thus making tts oxidation chenustry of interest to those WIIO wish to understand soot formation. Recently the reaction of OH with acetylene IKIS been studied as a function of :emperature and pressure [lO].The results, using a FP-RF (flash photolynsresonance

fluorescence)

results 191, obtamed ed to account

techmque,

using a slrmlar

Imply that earhcr techmque,

for a strong temperarurc

the high-pressure linut.

0 009-2614/82/0000-0000/S

Also,

had fad-

dependence

in these more

recent

KI

re-

02.; 5 0 1982 North-Holland

2. Experimental

The apparatus scribed

previously

will be given

here

and teclmiqucs Hydrovyl

the pulsed vacuum-ultraviolet

lower-wavelength

used have been dc-

[I 1, hence. only a brlcf summary

hmlts

radicals

for the Hz0

were set by the USCof the following

>1050

were

photolysls

produced of Hz0

by The

photodlssoaal:on cutoff

windows.

W(LIF);>IXO~(C~F~).

331

93. nulniwr 4

Voluns

LODecenlbcr 1982

CHEMICAL PHYSICS EITERS

OH r3dlc31 conccntratlons were momtorcd as a functmn of tnne after the flash by resonance fluorescence using 3 cooled Ehll 9659QB photonultlpher fitled an nitcrfersncc filter lransnuttmg the 3064 A =O-+ X?ll,u” =O).Thc Inter-

wth

hand ofOH(A’S+,$

resonance rxhatlon from a discharge and the dctcction sysrenr dcfincd a fluorescence v~cwng 7onC wl~osc cross section WIS 2 cm In dlamcter TINS rcglon wx well scpsrdtcd from the reaction vessel walls. mmmnzmg wall losses of the OH radicals The rcxtlon vcsscl was enclosed In a furnace which could be held constant IO bcttcr than +5 K The gas temperastm~on of the

Iamp

1UCL WJS

muntcd

rl~.?3SUd

by

3

inside the rcacflon WSSC~.

The flash lamp wds typlcally operated at discharge cncrg~cs

of

I7 5-50

J per flash at rcpctitlon rates of

one flasl~ every 3 s. Signals wcrc obt,nncd

by photon counting in conJunction with multlchnrmcl scllling. Decay curves of OH radmls were accumulated from 10 lo 1GODflashes, depcndmg upon slgnal strengths. Hydrokyl radical hnlf-lwes ranged from 1.72 to I IO ms In order to avojd the accumulation of photolysis or rcdcllon products all eqcrmicnts were carried out under flow condltlons so Ihat Ihc gas IIIIYIUIC in the vessel H;~S rcplcnlshcd cvcry few flosl~rs. Tile partial prcssurc of Hl0 m rlre cell mnged from O,Oi to 0.16 Torr The gases used had the followq purlry levels accordmg IO 111~manufacturer. Ar> 9999S%, C,H, > 99 6% FurIhcr anslys~s of IIIC *I% C,H2 m argon rtukture, aitcr

passtng through Matlrcson model 454 gas purifier

10 rcniovr lhc acetone, gave the foilowing mass spcc-

trA analysts. argon. 98.99, acctylcne, 0 975; nitrogen, 0 0 15, and acetone, 0 017. No further effort MIS made lo purify rhe gas niixturc

4

2

0

8 I

K2H21 molecule cm-36

~~lrO~w~~~~ultle\tltI?rtwxOupk

Tlg I Plotsof the OH decay

rate

agn~nst

tion for the total prcssurcs studied

IO’J

acctylcnc conccnlrs-

at 429 K. Dnra pomls dls-

placed vcrt~cally for clarity (0 s-’ for 20 3 Torr,O,

20 s-’

for

I 4 Torr.o.35 s-’ lor 101 5 Torr. 0, 30 s-* for 201.3 Torr, v, 50 s-’ for 403.0Torr. q

5

spondmg resonance fluorescence intensrtles,liI, IS the first-order rate for removal of OH In the absence of added reactant (pnmanly attributed to diffusion out of the viewing zone and to reactlon wth impuntlcs), and k IS the rate constant for reaction (1). In all cuperimcnts, exponential decrys of the resonance fluorescence s~gnolwere obscrvcd over ;It lcast two half-hves, and the measured rates, defined as R

were found to depend linearly on the concentration of acetylene. Rate constants were measured over a pressure range of 20403 Torr. Fig. I shows typlcd plots of measured rates versus C,H, concentrations. A vanatlon of a factor of 3 in the flash energy produced no vanatlon, wthm experi-

= (f - co) - lo So/S,,

mental

uncertainty,

m the ntc

constants,

mdrcatmg

that secondary reactions were negligible under the ex3. Results and discussion All cxpermlenrs wcrc performed with the acetylene concentrations m great cxess over ~IIC mltlal OH radlcal concentrations The pseudo-first-order decays of the OH radical concmtrat~on, [OH], arc thus given by the inkgrated

rate cvpression

lW,WHll

= SO/s,

= exp {(k(j+ k [reacIanl])

OH mdmls with C2H2 proceeds wa the initial addIllon of OH radical to C,H,, followed by stabilizatm or decomposltron of the OH-C2H, (I - to)},

whcrc [OHIO and [OH Jr arc the concentrations of OH at tmes to and t, rcspcctlvcly.Sg and S, are the corre-

332

perimental condnlons used. Phololysls products orher than OH can be estimated to lead to errors In the measured rate constant of <5- IO% 191. The rate constant k given In table I and plotted qpnst total pressure in fig. 2 shows that reachon (I) is in the fall-off regmn beIow 200 Torr both at 297 and 429 K. This observation mdlcates that rhe reaction of

OH + C,H2 f HOC,H;,

HOC,H;

+ M -+ HOC,H,

+ M.

adduct,

Volume 93, number 4

Ttlblc

Rate

CHEMICAL

PHYSICS LEnERS

I

constantsR(+) for lfw rcxl~on orOH rddrCJk wth CzH2. The mdrcatcd error hmltsarc 111ccslmratcd overall error IIIIIIIS which mcludc the Icasl~u.rcs standard dcwrrmn (2-X%) as ucil as the esttmatcd accuracy limtts of 0111cr psr.mwtcn suclr

a+ prcssurc

IO Dsccmbcr 1982

and calculated Is ketcnc

heats of formatson

lhen estimates

The UI~II~I~~Cproducts

acctylcnc arc uwcrlairt, Prcssurc (Toir)

A-x 10” (cm” molecule-’ 5-l)

(IO

297

25 6 50 7 99 8 707.9 JO? 1

HO f C,H, I_ the

04~

to other

stud~cs of Perry et al. [9] and Michael

hlcrature

val-

and the

ct al [lO\,was

a

pressure dcpcndcnce observed in the remIt

iO90

hlrchael et 31 .I slightly

IIIglIer value fork

fddr

study or 31 thy’ l~gh-

repm was

514

5 07 2 0.5 1

in tk present nlcasurenlet~ts of tltc pressure dcprndtncc of thfs rcactlon at 429 K such a drain3tiL shffl m the hlgfkprcssure hmit was not observed In facf, II appears that the varfatlon m the pressure dcpcndcncc of rlus rcacrlon wrth ternpemturc is sir&r to the fc3cfion 0fOH Wfh

s

6 85 5 0 70

201 3

806+089

403 0

7.89i

to exclude

being present,

I IO

the possibday

of

(2) to zero pressure

lends

to

dercrmincd by M~chacl et al. [IO]. Also, the nalurc of the prcssurc dependence obscrvcd here mdtcntcs kat rcactlon (2) may 197 and 129 K arc below ~bc rnterccpt

nle~atlIet1~~

reaction,

and IIIC

dcpctldcot.

ethylale [I_‘].

I c.

+ H.

of the data

hmrr was found,

found to be hlgltly tcsi~~rature

mdlcatc that these channels are mmor al atmospherrc pressures. In particular. the low.presstrrc vntues at both

not be a simple

value IS Lomparcd

may

The prcsclil

pressure

--* C,H,O _..

e\trapolatlon

that a thrrd body

357~000

WI 11c it is difficult

_hannels

36 kcal, Indicates

cvcn at Hugh tcmpcraturcs

ues In table 2 Note that only rn &is study

5.67 z 0 57 661~066

691

[9,10],

bc involved h&pressure

0 31

488:

adduct

of the re~cttoi~ of OH with

but the stabthty of 111~HOC&

20 3 fOl

other

3.082

of

M~cltacl et al. [lOI for the hat of fortnatlon of frarw tton complex must bc made

and ~~~~tan~ c~n~tnzmtron~

Tcmpenturc

[ 101. If the product

sImIlar fo the calculatlon5

but rather

tts rate may be associated wtth an in~raInolccular arrangement with an appreciable energy barner

The actual tcmpcraturc dcpendcnce of rhc iqltpressure hnfit has niaJor r~~~pl~caIronsrcgxdlng the mlparlance of the terwolecular

renction 31 IuJlcr

Wm.

perrttures. For example, c~trJpolatIorI of the present data tn fig 3 to hrgfi tcnrpcr&urc >I 500 K. results in contribution Fran, this IcnnolCCular rowtioff, as compared to postulated altcmate I~WII~IUSI~~S 1131, SuCll JS

a signikxnt

that rcto

I ,3

hydrogen nagratIon. If the actual product IS the ~somcr HOC =CH. thw bnrrrer can bc esttmnted from known

Table 2 Compwcon of hiphaprcssurr room-lcmperxure X ulth irlcmlurc ~luss _---Total prescurc

10’3 XX

fTorr)

(art3 ,i~‘ll~cul~-~s-9

_-_--.-I (Ar or He)

unk.no\\ n

IO-20 (HC or H2/N20) 1.05-I f(k) ‘O-SOO(Hc) 200-400 (Ar) IN-600(Ar) ?03-J03(Ar)

IOh) 1.92 06

85 20+ 06 1.6.5+015 6 79 ?:0.70 7.76 f 0 73 6.75 ?I0.70

r31c ranslJnl

Teclmlqucaf

_Kcf

__-DI -LSR

IJI 151

FP-RA Dr-RA I‘P-RIr-P-RI-

161 I71

Dt--ESR

t-P-Rr

rp-Rr

198;

1101

nIlr\~orli

3) TOTALPRESSUREkvd rug. 2. Plols of bm~olccular rate conslanlsA w+nst tolid pres-

SW of argon drlucnl a 297 K, 0, and 429 I<. n-

DT-LSR, dtschargc l-lox wrrb ESR detection. DE‘-RA,dwch.trg~~flow WIIII rcson3n~ absorprton, TP-RA. fl.~sh pbotolysls-rcson.mcc absorpllon. TP-RI-, llasb photol)sw. rcsonancc fiuurcscunce b, rrl where PI IS slorcluomctry fxlor, ~lurh MS not rcporlcd.

333

CHEMICAL

Vohimc 9 3, number -I

PHYSICS

LETt-l-ERS

IO Drcrmbcr

cd tn Vandooren

10”.

1982

and vrm Tigpelen’s experimenr.

Work is under way to establtsh the rate of reaction (I)

at hlgbcr temperatures

mlportance

in

order

to further

assess the

of thts reaction at flame temperatures.

From tlte present work we conclude sure hmtt is not stgnificantly at Icast up to 430K.

that the hgh-pres-

temperature

dependent

Also the absence of a stgnificant

zero-pressure lntcrcept IS clgam confirmed.

Acknowledgement 10”

I

0

4

2

5

1000,TK;

t-1:. 3 ~r~hco~~~\ plot 0f log h egalnFt 1000/r(~) data(o), and Jonss to

to1 our

PCTF)CI al (91 (0). M~chxl ct al.1 IO] (x). rcnlmorc [ ‘51 (o),Vandoorcn and vanT~f@cn II-I] (- - -).

and BFO\~I? ~1 aI 1141 (-.

---)

TIIIS work was supported

by the U S Department

of Energy. The asststance of Jon Meek,

The sohd lone is the best fit

and Jim van de Vreugde

IS

greatly

ttonal thanks go to Charles Otto, spMtral

Dean Wdliams,

appreciated LLNL,

Addt-

for the mass

annlysis.

contbtncd datd of Pcrr! ct aI UIIII thr prchcnt datJ

OH + C,H,

By combmmg prcscnt

M,,

+ Hz0 + C2H.

the data of Perry ct al. [9] to descrtbc

e- -WRT

KS. An empmcal

I.lSX

=

IO-12

c\p[(-377+400

TIIIS expresston

Arrltcntus

Further

extrapolation

work IS required

IS

the

Ltndentann

of the data IS wartoJustify

using this

stmplc cmpwcal fit m modeltng combJstlon systems as fix eUrapolatton to htgher temperatures may be SubJ’ct

to

considerable

II should be noted

could reproduce van Ttggclcn

the Arrhcnms

[14] obtained

Torr) O,/acetylene postulated

error.

that usmg this expression

one

plot of Vandooren

from a low-pressure

and

(40

flame wltere tlte reactton (7) was

to explam

the formatton

measured by molecular

of C2H20,

as

beam sdmpltng and mass spec-

trometry analysts Recogmzing the dtfficulty m dtstingutshing between ketene and the isomer HOCCH m a molecular

beam mass spectrometer.

tsts that reactton

0,

+ HOHCCH

(I)

334

e\-

followed by

--c HO2 + HOCCH

may be responstble

the possibility

for the budd-up

References 111 R ht. rrlstrom nod AA Wcstcnbcrg, rlamc structure (McCroa-Hdl, New Yorh, 1965). 12 I A G Gabdon and H G. Wolfhmd. l-lames. thar structure, radtatlon snd tcmpcraturc, 3rd Ed. (Chapman and Hall. London, 1970). 131 R. Alhmson. K.R Darvall, A C. Lloyd, Abl Wmc and

J.N. Pnls Jr., Advan, Photochcm. II (1979) 375 I-I] 1V.C Wilson and A A. Wcstcnbcrg, 1Ilh Symp. (1111.) on

cnl)/RT]

I

best fit IO all t!ie data USIIQ 3 modtficd

ranted

kinet-

expression

30/F, +

ifnn

tile

k = I 55

luglt-pressure

tl~c

where p IS rhc prcssurc tn Torr form.

(3)

fit of the present data over the enttrc

pressure range gives the followng

k(T.P)b,

wtl~

the clpresston

data one can obtain

X IO-”

= 6 kcal

(4) mass 42 as measur-

Combust!on.Thc Combustion Inshtutc (1967) p. 1143 I.5 I JI. Brcun snd CP. Class. Intern J. Chcm. K~net. 3 (1971) I45 161 1.W hl. Smith and R. Zcllncr. J Chcm. Sot roradny Trans.1169 (1973) 1617 17 I A V. pastrclna nnd R.W. Carr Jr., Intern J. Chcm. Kmct. 6 (1974) 587. [“I D D. Davis. S. I-lschcr, R. Schrl-r, R.-l-. Watson and W

Bolhqcr, J. Chcm. Phys. 63 (1975) 1707. 19) R.A Pcny, R Athmson and J N. PII& Jr,

J. Chcm.

Phys

67 (1977) 5577. IO] J V. MIchxI, D r. Nova. R P. Borkowskt, WA. Payne and LJ Sncl, J Chcm. Phys. 73 (1980) 6108 I1 I C Porter. 4th Symp (Int ) on Combustron.Thc tlon Institute (1953) p. 248.

Combus-

121 R Atkmson, RA. Perry and J.N. Pltts Jr., J.Chcm. Phys 66 (1977) 1197. 131 \V.G. Brow, R P. Porter, J.D. Vcrlm and A W. CM, 12th Symp. (lnr.) on Combustion,Thc Combusuon

(1969) p 1035 . _ Inslttule _ . 1141 J. vandooren dttd P.J. vanTtg$len, 16th Symp. (Int.) on Combustlon,Thc Combustion lnstitutc (1976) p. 1133. 115 I C P rcmmore and C.W. Jones, J. Chcm. Phys. 41 (1964) 1887.