Kinetics of the reaction OH + NO2 + M → HNO3 + M

Volume 16, number 2

1 October 1972

CHEMICAL PHYSICS LETTERS

KINETICS OF THE REACTION

OH + NO, + M + IINOj + I#

J.G. ANDERSON fiepartment

of Physics, University of Pittsbtrrgh, Pittsburgh. Permsylm~ia

152 13, USA

and

Received 3 July 1972

The NO2 + OH + _-Zr-. HNO, + Ar reaction rate constant was mc3sured over the pressure range from 0.5 lo IO torr at 297°K using uItraviolet fluorescent scatterin, n for the detection of OH radicals Preliminary results for the NO + OH f Ar + IfNO -t Ar reaction rate constant are mentioned.

1. Introduction OH+NO, Reactions involving the hydroxyl radical have been studied intensively for the past two decades because of their importance to combustion related processes and to the photochemistry of planetary atmospheres. 1Moreover, it has become increasingly apparent that OH reactions, particularly those involving the oxides of nitrogen, may play a fundamental role in stratospheric poilution processes which are of direct relevance to thd total ozone content of the upper atmosphere [l--4]. Murcray et al. [S] discovered through balloon observations that a significant amount of nitric acid exists in the stratosphere which suggests that reaction mechanisms which convert the nitrogen oxides to nitric acid may constitute a primanl loss mechanism for NO and NO, in the upper atmosphere. A prime candidate for the conversion of nitrogen oxides to nitric acid is the reaction $This research was supported by the advanced Research Pr@ jects Agency, The Department of Defense, and was monitored by U.S. Army Research Office - Durham, Bos CM, Durham, N.C. 27706, under contract No. DA-31-!24-AROD-410, and under grant No. DA-ARO-D-31-124-72689.

+M + HNO, +M,

(1)

for which Mulcahy and Smith [6] first reported a semi-quantitative estimate of ky” from mass spectrometer kinetic studies of OH decay in an H-NO, system. Be’rces and FBrgeteg [7] measured ky” by comparison with the OH + HNO3 rate in a study of nitric acid vapor photolysis. Simonaitis and Heicklen [SJ recently measured the ratio of kye to the OH t CO + CO, + H rate constant, inferring the former from a kno$ edge of the latter. Large discrepancies exist in the rcported values. In the present shady, F$’ is measured directly over a pressure range from 0.5 to 10 torr at 297%. In addition, mole fraction studies with nitrogen replacing argon as the third body aliow direct application of the data to atmospheric considerations. Preliminary results on the reaction @I

OH+NO+M

--f HN02W

(2)

are also mentioned. 375

I October 197:

CHEMICAL PHYSICS LETTERS

Volume 16, number 2,

A8+H,O

RESONANCE

MICROWAVE MICROWAVE

PRESSURE PORT

FIXED

t OUARTZ

LOOP INJECTOR

FLUORESCEiCE

CELL

SLIDING INJECTOR

Fig. 1. Schematic description

of the flow tub e, resonnnce lamp and fluorescence

2. Experimental

The decay of OH in tile absence and presence of added gases was studied in a Pyrex flow tube (fig. 1) 1 m in length and 2.5 cm in diameter using ultraviolet fluorescent scattering to detect OH in a quartz cell at the downstream end of the tube. The fluorescence cell was arranged such that collimated resonance radiation from an Ar f Hz0 microwave discharge lamp illuminated the gas ti a direction perpendicular both to that of the flow and of the observation. A grating monochromator (McPherson, ld ode1 2 18) was used to observe the intensity of the A% + X211 (O-O) band of OH at 3090.9 (see Dieke and Crosswhite [9J), Hydroxyl radicals were produced by the reaction HtN02-+OHtN0.

(3)

Atomic hydrogen was formed by passing molecular hydrogen in an At carrier through a I;yicrowave discharge; NO, (99.5% pure) was drawn directly from a cylinder. Two injectors were used to add gases to the main flow tube: (i) a fixed loop injector, 2 cm in diameter, constructed from 3 mm o.d. teflon tubing was situated at the upstream end of the reaction tube and (ii) a movable 3 mm o.d. polished stainless steel injector, concentric with tb.e main flow tube and per376.

.

cell.

forated radially at the tip, provided for the addition of gas at any point upstream of the fluorescence cell. The flow tube surface was coated with strongly heated syrupy phosphoric acid, aged under vacuum conditions and prepared before each experiment by streaming OH through the tube for an hour or so. Flow rates of gases added through the injectors were determined by measuring the timed pressure change in 3 known volume using presgure transducers (Pace Model P7, indicator CD-Z) calibrated against an oil manometer. The total pressure in the reaction tube was monitored with a transducer caIibrated against a McLeod gauge. The sliding injector was arranged such that it could be connected to the main reaction tube pressure transducer enabling the profile of the viscous pressure drap to be mapped as a function of injector position. A Roots blower (W.C. Heraeus Model RC 350) in combination with a forepump (Stokes, Model 148 H-9) produced linear flow velocities of 2 X IO3 cmlsec in the reaction tube. Pumping rates were selected by bleeding variable amounts of gas into the intake port of the Roots blower.

3. Procedure

Two modes of operation were used: (i) the sliding injector could be used to produce

Volume

16, number

DECAY

,oL-L0

2

CHPXIICAL

CF HYD?OXYL

INTdC

ABSENCE

OFADCED

‘-

IO

23

IhiJECicm

30

40

TIP TO FLlJOREsCENCE

50

GASES

‘-_

60 CELL

70 UISTANCE

80

PHYSICS

I

1

\

90

[CM]

Fig. 2. Hydrosvl decay resulting from homogeneous second order removal and first order heterogeneous wall removal. OH at variable axial positions by adding a small excess ofNO, to the constant H atom flow ([HI = 5 X 10” cn-3) or (ii) OH could be produced upstream by adding small amounts of NO, through the loop and the sliding injector could then be used to add larger amounts of NO2 or NO to study their three body reactions with OH. In the absence of other reactants, OH will decay principally through the reaction series 0H+OH-+H20+0,

(4)

OH+O-+Ht02,

(5)

O+NO,‘NO+O,,

(6)

OH + wall :

(7)

if NO2 is in slight excess with respect to H in reaction (3) [5$ 10- 121. The decay process is dominated by second order removal of OH through reactions (4) and (5) at large OH concentrations (i.e., greater than lOI cm-3) and by the first order loss resulting from collisions with the wall at hydroxyl concentrations

LETTERS

1 Octohcr 1972

below 101’ cmm3. The general features of this decay sequence were verified by injecting NO1 at varying points upstream of the fluorescence cell with the movable injector, i.e., in mode (i) above. Fig. 2 represents a typical decay profile of OH displayed as a function of injector position. A wall decay constant, k,, of 40 i 5 set-j corresponding to a wall recombination coefficient cf 3 X 10m3 for the phosphoric acid coated tube is shown. Early experiments with a clean Pyrex surface showed much larger and less reproducible first order wall recombination rate constants. The significant advantage of using fluorescent scattering as an OH detection technique is apparent from fig. 2. Concentrations of less than lO*O OH molecules cmP3 can easily bs measured and it is therefore possible to carry out kinetic measurements in an GH density regime in which reactions (4) and (5) are negligible during typical residence times in the system (x50 msec). Hydrosyl concentrations between 10” and lOLo molecules cr~i-~ at the fluorescence cell were used. For direct homogeneous OH reaction rate measurements, the system was operated in the second mode whereby hydroxyl was formed at the upstream end of the flow tube by adding NO? through the fLVed loop injector; the gas to be reacted with 0t-I was added through the movable stainless steel injector. Thus, changes in the OH concentration at the fluorescence cell measured as a function of injector position constitute the primary experimental data. Flow rates of gases added through the sliding injector were chosen such that the concentration of the injected gas exceeded the peak OH concentration in the reaction zone by at least two orders of magnitude to insure that the homogeneous rate of reaction was pseudo first order in OH. The ratio of injected gas concentration to carrier gas concentration was characteristically less than one percent so that the addition of gas through the injector did not affect the total flow. Application of rate data to atmospheric modelling requires a knowledge of kr;‘a . Alfltough nitrogen cannot be passed through the discharge with hydrogen because the resulting atomic nitrogen reacts rapidly with NO, and with OH, it can be added downstream of the H-atom source and upstream of the NO2 addition loop (see fig. I) to produce carrier gas mixtures 377

V&me

1 October 1972

CHEXlICAL PHYSICS LETTERS

16. number Z

1

PRESSURE EFFECT: NO#OH+AR+HNO,+AR

ARGON ADDED

T=29i-K

THROUGH INJECTOR

E

z

I

I

2

3

4 TOTAL

5 PRESSURE

6

7

1

8

I

9

IO

(TORR)

Fig. 4. Plot of A.-~’for the pressure range 0.5 to 10 ton.

I

I

I

IO

20

40

30

I

50

60

SLIDING INJECTOR POSITION

[Chq

Fig. 3. Hydrosyl decay through

111s sliding

rcsuhing from the Also displayed

injector.

addition of NO: is the effect

of’

the injector position upon measured OH concentration% with up to SO percent iP,_

4. Results A typical example of OH decay expressed in terms of the injector tip to fluorescence cell distance, with .OH produced at the loop injector at the upstream end of the reaction tube and NO, added through the sliding injector, is sho1i.n in fig. 3. The linearity of the logarithmic decay profile over an OH concentration range of one and a half orders of magnitude verifies the pseudc first order nature of the removal process. At any fked total pressure, the first order rate constant was found to be exactly proportional to the NO2 concentration. Also displayed in fig. 3 is the result of an expgiment wherein a similar few of argon was im jetted in place.of NO, as a verification of the negligible 378

effect of varying rod positions on the OH concentration measured at t!le cell. A plot of the measured first order OH decay rate divided by the injected NO2 concentration versus carrier gas concentration will yield an intercept equal to the second order heterogeneous wall reaction rate constant which was found to be less than 5 X IO- I5 cm3/sec. Fig. 4 is a graph of X-t’ versus total pressure between 0.5 and 10 torr at 797°K. Although the third order rate const2nI is independent of argon pressure up to about 4 torr, it begins to decrease as the pressure is further increased. Appraisal of experimenral scatter in hI’ indicates a standard deviation of 225 percent which is consistent with estimates of calibration uncertainties in the individual experimental parameters (i.e., flow rates, temperature and pressure). A reasonable explanation for the pressure effect displayed in fig. 4 involves the vibrationally excited adduct, HNO;, fomled in the initial OH f NO2 reaction, which undergoes collisional deactivation or unimolecular decomposition. The pressure range from 4 to 10 torr appears to be in the fall off region and it would be of interest for theoretical considerations to extend the pressure range significantly above 10 torr; however radial mking of gases added through the in- jectors is seriously inhibited at higher pressures. As previously mentioned,‘wide discrepancies exist in reported values of kt’. Mulcahy and Smith [6]

Volume 16, number

2

1 October 1972

CHlZ\!ICAL PHYSICS LETTERS

reported a semicyantitative value of 1.7 X 1O-2qcm6/sec for M = He in the pressure range 0.2

of 8 X 10d3r cm5/sec at 5 torr for kye in pulsed vxuurn ultraviolet photochemical studies of OH at

to 1.O torr assuming a homogeneous reaction mechanism. However, as their apparatus was not designed to examine this particular reaction they noted that

room temperature. A study of the temperature dependence of k’;’ and ky is currently in progress in our laboratory in a variety of hi gases as well as preliminary studies of the bimolecular reaction OH + 03 + HO? f 02.

they were unable to distinguish between heterogeneous and homogeneous removal and that therefore their reported rate constant represented an upper limit. Beices and Forgeteg 171 report ky’ = 3.6 X 1O-33 crn6/sec from nitric acid photolysis studies. Simonaitis and Hcicklen [S] studied the decay of OH

radicals in the presence of CO and of NO,, Inferring ky’ = 1.3 X 10e30 cm6/sec at 300°K from knowledge of the OH + CO + CO? + H rate of reaction. Westenberg and coworkers [ 131 are currently studying reaction (1) using ESR to detect OH decay in a flow tube. They report a pressure independent ky” of 1.6 X !O-3o cr&/sec between 1 and 5 torr. Our value at 3 torr For L+’ taken from fig. 4 is 1.0 2 0.3 X 1O-“O cm”,‘sec. Molecular nitrogen was substituted for argon as the diluent gas in varying proportions up to an N2 mo!e fraction of 0.8, yielding an extrapolated value for ky?-

of 2.0 ? 0.5 X 10m30 cm6/sec at a total pressure of 6 torr. A preliminary investigation of kt’ indicates a slower rate constant than for the corresponding NO? reaction. At 5 forr a value of 4 2 2 X tOm31 c&/sec

was meas-

to 3.5 ? 1 X 10-3J cm6/sec at 8 ton-. Stuhl and N&i [ 141 report a third order rate constant

ured decreasing

References [ 11 51. Nicolet, Aeronomico Actr! A79 (1970). F’~~Crutzen, Quart. J. Roy. Meteorol. Sot. 96 (1970)

[ ?]

[ 31 H:S: Johnston,

Scicncc I73 ( 197 I) 5 17. [4 ] hi. Nicolrt, Acronomicn Aotn h96 (1972). [S] D.R. hlurcrsg, T.G. Kyle, F.H. Murcray and W.!. Williams, J. Opt. SOC. Am. 5Y (1969) 113 I. [6] M.F.R. Slulcahy and R.H. Smith. J. Chcm. Phyc 54 (1971) 5215. [ 71 T. Ekes and S. Fb’rgeteg, Trans. Famday Sot. 66 (1970) 640. (81 R. Simonaitis and J. Heicklcn, lonorpheric Research Report 1380 (1972). [9] G.H. Dieke and II.hl. CrosWGte, Bumblebee Ser. Rep. =X7, Johns Hopkins ilniversity, Baltimore, hlarylaod

(1948). [lO[ L.F. Phillips nnd H.I. Schiff, J. Chem. Phys. 37 (1962) 1233. [ 1 I] J.E. Brecn and G.P. Glass, J. Chcm. Phys. .5?. (1970) 1082.

[ 121 F.P. Del Clew

and F. liaufman, Discussions Faraday

Sot. 33 (1962) 128.

[ 131 A. Westznbcrg, J. Chcm. Phys. (1973), to bc published. [ 141 F. Stuhl and 11. Niki, private communication (1972).

379