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Rate measurements on N + O2(1Δg) → NO + O and H + O2(1Δg) → OH + O
; ~~y+mx17,
; .., ,,:
..number
!. :\
!3
-
--i
.-
__;
CIIEMICAL
.’
PHYSICS LETTERS
15 Ihxember
1970
:.
-
:
i‘
-
..
RATE
.
.’
_-.
:
‘:-.
AND
ON N + Oz(‘a
H + O;(‘ag)
-
F) -
NO+0
OH + 0
J. M. ROSCOE 7 and N. DEHAAS I Tkc J~11n.s flopkim Ut~irersily Spring. Ivffrr_vIrrnd‘70910, L’S4
A. A. WESTENBERG. .
I
:
MEASUREMENTS
:’
AppEed
Phycics Silrcr
I.dm rdo r?
,
blcnsurcmcnts hnvc hccn rnndcon the r:rlc ~6 dw:ty of S :~nd nppcnrnncc of 0 in.thc prcscnco -mixtures of Oz(“SE) :~nd Oz(lA g) :11 1!)5’- XW°K. The results show th:lt the rcportctl quenching by N is not the chcmicnl reaction S +0~(‘4~) -‘SO * 0. Sitnil:lr conclusions :11-cclr:twn for the If .+Oz(lAg) cnsc.
The rate constant of N+O2(‘Ag)
kl - NO+O.
d cxccss of Ob(‘Ag)
this discharge furnished 6 - 10% of the total 02 in 4 state. 0 atoms genemted in the discharge were completely removed immediately downstream by a HgO film deposited on the quartz walls and heated [3] to about SOOC. The absolute concentration of 4 could be determined from the metered flow rates and the decrease in ESR signal height of Z (E-line 141) when the 02 discharge was turned on. The assumption was that all of this decrease was due to formation of A (the 0 atoms having been rccovered), the validity of which was proven by, comparing this result with that obtained from the ratio of the first moment 141 of the lowest field (J=2, dl~=l2) line [51 of 4 to that of 2: (Eline) with the 02 discharge on. The theoretical intensities of these two lines are the same (within 1%) at 3000K. Since the signals from A were quite weak. the use of the E-line sign.1 ‘. height ratio was more reliable and convenienl than the direct measurement of A. Absolute N concentrations (furnished from a trace of N2 diluted with He in the main discharge) were de- _ termined by ESR to be much less than A in all experiments. i.e., [N] i 10m2[AI. Quenching of A by the major species present, Z and He, or the walls is much too slow [S] to cause any loss of A under our experimental conditions. and this was confirmed experimentally over the reactor length. Thus [4 1 was taken as constant throughthe
(1)
been reported [l] to be 1.6 X IO9 cm3 mole-l set-l at 300°K with Eat: 1.2 kcal/ruol& Since the results were obtained by following the decay of 02(lAg) in a flow system with an escess of N. the measured rate could apply to the physical quenching N + 02(‘4,)_ - N +OZ(~ Z;) rather than to the chemical reactIon. although”the authors argued that the former was not the case. We report here experiments on the N + 02(l~g) system following N decay and O(3P) product formation in an excess of ‘Ag. which show that the chemical reaction does not occur appreciably. In what follows. let S and A denote the 3Cg and ‘Ag states of 02, respectively. ,The apparatus was essentially the 2.5 cm diameter (H3B03 coated) discharge flow system with ESR detection described earlier [21. except that the enclosed quartz movable injector was replaced by a teflon injector which could be moved through a simple O-ring sliding joint (kept leak-free by a separate pump). Teflon was used primarily to achieve a smoothly sliding seal through the rubber O-rings without need for grease, although it also provided a surface inactive to atom recombination. The external end of the teflon tobe was connected to a section of $--inch quartz tubiqg carrying a second microwave (2450 MHz) discharge cavity. With pure 02 (or 02 mixed with He) metered into the probe. ._ :: ; has
T-Present address: Department of Chemistry. Acadia University,: Wolfville. Nova Scotia. USA.
out.
At the pressures (1 - 4 torr) and concentrations used three-body gas phase atom recombination was negligible, and with a fixed detectormovable source experiment such as this, any first order atom wall loss cancels (except that 597
Voiume
7.
~ CHEMICAL PHYSICS LETTERS
number 6
due to any possible heterogeneous N +02 reaction). Thus. beside reaction (1) the only other steps of importance were N+02(Z)
k2 - NO+0
(2)
and the occurrence of fast N +NO - N2 +0 after each. Using the subscripts “on’f and “off” to indicate the presence or absence of 3 in the injected 02. the equation governing the N signal at the fixed ESR cavity as the reaction time is v:i ried is In([Ri,,,, ‘[N]off)
= 2[A](+
- kI)f .
At any time I ,[N]on/[N]off~ 1 if kl ‘.. k2. or put in more useful form. the decay slope dlnlNl;i, dl dln[N&lfi’d/. Our esperimental result was qualitatively the opposite. i.e.. kL .. kp. since at any fixed injector position we always found [N]on,,‘[N]off .,l. and the “on” decay slope was always less than the “off” slope. Fig. I illustrates this. To corroborate the literature values for k2 (reviewed in ref. [ 11). we made room temperature measurements (using just the “off” slopes according to our usual technique ]2]), and the result of 22 runs at varying 02 concentrations. pressures, and velocities was k2 = 4.5 * 0.3 x 107. This is in essential agreement with Wilson’s [?I earlier vnlue from this laboratory by the same technique in a different apparatus. and some\ hat lower than Clark and Wayne’s 111 6.5 X 10); . If their :
i
i
DIsrANCE
;
--.
r .._“l--~
Id
Fig. 1.11 and N decay plots in presence (02 discharge
value +1.6:X 16’ ‘represented pl, we would Clearly have kl > k& which is contrary to our finding. More quantit&vely, the difference between “on”.and “off” slopes, ‘a’s in fig. 1, was essentially just that to be expected from the loss of c when the 02 discharge was turned on, i.e., the slope decreased,by the amount ,of A generated. This indiczkes that at room teplperature kl must be negligibly small compared to k2. Measurement of the product 0 appearance by ESR confirmed this, as the energizing of the 02 discharge gave no appreciable increase in 0 over that from the ground state reaction (2). Similar experiments were conducted with the reactor at dry ice temperature (195’K) where the contribution of reaction (2) was entirely negligible (Ea i= 6 - 7 kcal/mole). In this case the ratio [N]on/[N]off did not change measurably from unity over the 70 cm injector range. Conditions were such that if Clark and Wayne’s value 1.1 X 10g at 195OK represented kl the N shouid have decayed by factors of 2 - 10. There was no 0 generation except for the small, constant amount due to reaction (2) occurring in the room temperature region (- 10 cm) between the end of the cooled reactor and the ESR cavity. Thus these results demonstrate conclusively that the A quenching constants reported for N in ref. [l] must be ascribed to physical deactivation and cannot be chemical rate constants and, in fact, that kl a:
N.- T = 29S°K, P = 3.14 tcrr, u = 281 cm/set.
He car-
rier, [3Xgloff = 6.37 x 10m8moles/cm3, r3~Joff/13X,lcn = 1.11. (Slopc),ff/(Slope),, = 1.13. 598
.. :- I5 December 1970
-k3.
-
OH+O,
(3)
might be expected to be slower than (1) on an energy basis alone, since for ground state products it is only 6 kcal/mole exothermic compared to 54 kcal/mole for (1). The reaction
.
volume
7. number
G
CHEhlICAI.
H +‘02(C) + OH + 0 tioes not occur appreciably at ordinary temperatures. but the three-body reac_ tion k4 H+02(3)+M -+ H02+M (4) and its succeeding steps [8] always accompanied the measurement of (3). Thirty room temperaturf% runs on reaction (4) alone were performed in thd present work over the total pressure range 0.5- 2.4 torr. with [Z] .i; [HI. A plot of [Cl-l dln[Hl-‘id/ against pressure was a good straight line through the origin. the slope of which yielded k4 = 7.2 X 1015 cm6 mole-2 see-1 (using the net stoichiometry 1.56 Hi02 of ref. [‘iI).- No difference between Ar and He as third body could be detected. These results are in good agreement
PI-
with the value
8.0
:-: 1015
of
ref.
The experimental res’ltlts on (3) IV -e similar thepiifference in II decay to those on (1). i.e.. slopes between “on” and “off” 02 discharge conditions was just that due to the loss of X during dischargeas shown in fig. 1. It should be noted that even if (3) occurred, it probably would be followed by the very fast OH + 0 - 02( 5) A H thus regenerating the H. so that the net decay of H would be unaffected. Therefore. the lack of apparent result due to A does not prove that (3)
PHYSICS
I.I:TTERS
does not occur. doublet. which
15 December
i97a
The transition state could be :I presumably would be the same a?
normal HO2. so that promotion of an electron would not be necessary. However. the fact that the “on” slope was always less than the “off” slope by the amount of A formed indicates that the A was not participating in a three-body reaction such as (4). Since the first step in the apparent three-body mechanism is presumably H +02(&) - HOi with subsequent HO; collision stabilization. it may be that the HO; complex also shows a similar reluctance to form appreciably in the bimolecular (3). Therefore. the experiments with both N and H tend to bear out the CheIIIiCal intertness of 02(‘ag). REFERENCES [l] I. 13. Clark and II. 1’. \\‘:I\-ne. (1970) 538.
I’roc. 1lo.v. SW. A3 L(i
171 \V. II. \Vilson. .l.Chcm. i’h.~s. -4G (L!JtiS) IOli. [S[31.A. A. Clgnc anti 13.A. Thrush. Proc. Roy. SOC. AZ5
(19Li3) 559.
599
; .., ,,:
..number
!. :\
!3
-
--i
.-
__;
CIIEMICAL
.’
PHYSICS LETTERS
15 Ihxember
1970
:.
-
:
i‘
-
..
RATE
.
.’
_-.
:
‘:-.
AND
ON N + Oz(‘a
H + O;(‘ag)
-
F) -
NO+0
OH + 0
J. M. ROSCOE 7 and N. DEHAAS I Tkc J~11n.s flopkim Ut~irersily Spring. Ivffrr_vIrrnd‘70910, L’S4
A. A. WESTENBERG. .
I
:
MEASUREMENTS
:’
AppEed
Phycics Silrcr
I.dm rdo r?
,
blcnsurcmcnts hnvc hccn rnndcon the r:rlc ~6 dw:ty of S :~nd nppcnrnncc of 0 in.thc prcscnco -mixtures of Oz(“SE) :~nd Oz(lA g) :11 1!)5’- XW°K. The results show th:lt the rcportctl quenching by N is not the chcmicnl reaction S +0~(‘4~) -‘SO * 0. Sitnil:lr conclusions :11-cclr:twn for the If .+Oz(lAg) cnsc.
The rate constant of N+O2(‘Ag)
kl - NO+O.
d cxccss of Ob(‘Ag)
this discharge furnished 6 - 10% of the total 02 in 4 state. 0 atoms genemted in the discharge were completely removed immediately downstream by a HgO film deposited on the quartz walls and heated [3] to about SOOC. The absolute concentration of 4 could be determined from the metered flow rates and the decrease in ESR signal height of Z (E-line 141) when the 02 discharge was turned on. The assumption was that all of this decrease was due to formation of A (the 0 atoms having been rccovered), the validity of which was proven by, comparing this result with that obtained from the ratio of the first moment 141 of the lowest field (J=2, dl~=l2) line [51 of 4 to that of 2: (Eline) with the 02 discharge on. The theoretical intensities of these two lines are the same (within 1%) at 3000K. Since the signals from A were quite weak. the use of the E-line sign.1 ‘. height ratio was more reliable and convenienl than the direct measurement of A. Absolute N concentrations (furnished from a trace of N2 diluted with He in the main discharge) were de- _ termined by ESR to be much less than A in all experiments. i.e., [N] i 10m2[AI. Quenching of A by the major species present, Z and He, or the walls is much too slow [S] to cause any loss of A under our experimental conditions. and this was confirmed experimentally over the reactor length. Thus [4 1 was taken as constant throughthe
(1)
been reported [l] to be 1.6 X IO9 cm3 mole-l set-l at 300°K with Eat: 1.2 kcal/ruol& Since the results were obtained by following the decay of 02(lAg) in a flow system with an escess of N. the measured rate could apply to the physical quenching N + 02(‘4,)_ - N +OZ(~ Z;) rather than to the chemical reactIon. although”the authors argued that the former was not the case. We report here experiments on the N + 02(l~g) system following N decay and O(3P) product formation in an excess of ‘Ag. which show that the chemical reaction does not occur appreciably. In what follows. let S and A denote the 3Cg and ‘Ag states of 02, respectively. ,The apparatus was essentially the 2.5 cm diameter (H3B03 coated) discharge flow system with ESR detection described earlier [21. except that the enclosed quartz movable injector was replaced by a teflon injector which could be moved through a simple O-ring sliding joint (kept leak-free by a separate pump). Teflon was used primarily to achieve a smoothly sliding seal through the rubber O-rings without need for grease, although it also provided a surface inactive to atom recombination. The external end of the teflon tobe was connected to a section of $--inch quartz tubiqg carrying a second microwave (2450 MHz) discharge cavity. With pure 02 (or 02 mixed with He) metered into the probe. ._ :: ; has
T-Present address: Department of Chemistry. Acadia University,: Wolfville. Nova Scotia. USA.
out.
At the pressures (1 - 4 torr) and concentrations used three-body gas phase atom recombination was negligible, and with a fixed detectormovable source experiment such as this, any first order atom wall loss cancels (except that 597
Voiume
7.
~ CHEMICAL PHYSICS LETTERS
number 6
due to any possible heterogeneous N +02 reaction). Thus. beside reaction (1) the only other steps of importance were N+02(Z)
k2 - NO+0
(2)
and the occurrence of fast N +NO - N2 +0 after each. Using the subscripts “on’f and “off” to indicate the presence or absence of 3 in the injected 02. the equation governing the N signal at the fixed ESR cavity as the reaction time is v:i ried is In([Ri,,,, ‘[N]off)
= 2[A](+
- kI)f .
At any time I ,[N]on/[N]off~ 1 if kl ‘.. k2. or put in more useful form. the decay slope dlnlNl;i, dl dln[N&lfi’d/. Our esperimental result was qualitatively the opposite. i.e.. kL .. kp. since at any fixed injector position we always found [N]on,,‘[N]off .,l. and the “on” decay slope was always less than the “off” slope. Fig. I illustrates this. To corroborate the literature values for k2 (reviewed in ref. [ 11). we made room temperature measurements (using just the “off” slopes according to our usual technique ]2]), and the result of 22 runs at varying 02 concentrations. pressures, and velocities was k2 = 4.5 * 0.3 x 107. This is in essential agreement with Wilson’s [?I earlier vnlue from this laboratory by the same technique in a different apparatus. and some\ hat lower than Clark and Wayne’s 111 6.5 X 10); . If their :
i
i
DIsrANCE
;
--.
r .._“l--~
Id
Fig. 1.11 and N decay plots in presence (02 discharge
value +1.6:X 16’ ‘represented pl, we would Clearly have kl > k& which is contrary to our finding. More quantit&vely, the difference between “on”.and “off” slopes, ‘a’s in fig. 1, was essentially just that to be expected from the loss of c when the 02 discharge was turned on, i.e., the slope decreased,by the amount ,of A generated. This indiczkes that at room teplperature kl must be negligibly small compared to k2. Measurement of the product 0 appearance by ESR confirmed this, as the energizing of the 02 discharge gave no appreciable increase in 0 over that from the ground state reaction (2). Similar experiments were conducted with the reactor at dry ice temperature (195’K) where the contribution of reaction (2) was entirely negligible (Ea i= 6 - 7 kcal/mole). In this case the ratio [N]on/[N]off did not change measurably from unity over the 70 cm injector range. Conditions were such that if Clark and Wayne’s value 1.1 X 10g at 195OK represented kl the N shouid have decayed by factors of 2 - 10. There was no 0 generation except for the small, constant amount due to reaction (2) occurring in the room temperature region (- 10 cm) between the end of the cooled reactor and the ESR cavity. Thus these results demonstrate conclusively that the A quenching constants reported for N in ref. [l] must be ascribed to physical deactivation and cannot be chemical rate constants and, in fact, that kl a:
N.- T = 29S°K, P = 3.14 tcrr, u = 281 cm/set.
He car-
rier, [3Xgloff = 6.37 x 10m8moles/cm3, r3~Joff/13X,lcn = 1.11. (Slopc),ff/(Slope),, = 1.13. 598
.. :- I5 December 1970
-k3.
-
OH+O,
(3)
might be expected to be slower than (1) on an energy basis alone, since for ground state products it is only 6 kcal/mole exothermic compared to 54 kcal/mole for (1). The reaction
.
volume
7. number
G
CHEhlICAI.
H +‘02(C) + OH + 0 tioes not occur appreciably at ordinary temperatures. but the three-body reac_ tion k4 H+02(3)+M -+ H02+M (4) and its succeeding steps [8] always accompanied the measurement of (3). Thirty room temperaturf% runs on reaction (4) alone were performed in thd present work over the total pressure range 0.5- 2.4 torr. with [Z] .i; [HI. A plot of [Cl-l dln[Hl-‘id/ against pressure was a good straight line through the origin. the slope of which yielded k4 = 7.2 X 1015 cm6 mole-2 see-1 (using the net stoichiometry 1.56 Hi02 of ref. [‘iI).- No difference between Ar and He as third body could be detected. These results are in good agreement
PI-
with the value
8.0
:-: 1015
of
ref.
The experimental res’ltlts on (3) IV -e similar thepiifference in II decay to those on (1). i.e.. slopes between “on” and “off” 02 discharge conditions was just that due to the loss of X during dischargeas shown in fig. 1. It should be noted that even if (3) occurred, it probably would be followed by the very fast OH + 0 - 02( 5) A H thus regenerating the H. so that the net decay of H would be unaffected. Therefore. the lack of apparent result due to A does not prove that (3)
PHYSICS
I.I:TTERS
does not occur. doublet. which
15 December
i97a
The transition state could be :I presumably would be the same a?
normal HO2. so that promotion of an electron would not be necessary. However. the fact that the “on” slope was always less than the “off” slope by the amount of A formed indicates that the A was not participating in a three-body reaction such as (4). Since the first step in the apparent three-body mechanism is presumably H +02(&) - HOi with subsequent HO; collision stabilization. it may be that the HO; complex also shows a similar reluctance to form appreciably in the bimolecular (3). Therefore. the experiments with both N and H tend to bear out the CheIIIiCal intertness of 02(‘ag). REFERENCES [l] I. 13. Clark and II. 1’. \\‘:I\-ne. (1970) 538.
I’roc. 1lo.v. SW. A3 L(i
171 \V. II. \Vilson. .l.Chcm. i’h.~s. -4G (L!JtiS) IOli. [S[31.A. A. Clgnc anti 13.A. Thrush. Proc. Roy. SOC. AZ5
(19Li3) 559.
599