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Kinetics of the gas-phase oxidation of formaldehyde
Kinetics of Combustion Reactions 40
KINETICS OF THE GAS-PHASE OXIDATION OF FORMALDEHYDE a By MILTON D. SCHEER Spence 6 showed that the oxidation over extensive surfaces of powdered glass occurred slowly and almost exclusively according to the overall reaction: CH20 -k 02 = COs -k H20. In unpacked Pyrex vessels, he found that the variation of initial rates of pressure rise with temperature yielded an overall activation energy of 17.6 kcal/mol. He suggested a reaction chain analogous to that given by Biickstrom 7 for the oxidation of aldehydes in the liquid phase. This scheme involved PA, activated CH20, and activated HCOOH molecules. The assumption of a steady state for these intermediate species yielded the following equation for the initial rate of disappearance of formaldehyde:
Introduction
The importance of formaldehyde as the ratedetermining intermediate in methane oxidation has been conclusively demonstrated 1, 2. Even in the oxidation of the higher hydrocarbons, formaldehyde has been shown to play an important role s, 4. Therefore, in order to be able to solve the general problem of hydrocarbon oxidation, the kinetics of the oxidation of formaldehyde must first be understood. In the past two decades, many investigators have been engaged in the study of formaldehyde oxidation. Fort and Hinshelwood 5 showed that the rate of pressure rise was independent of oxygen concentration and strongly dependent upon the formaldehyde concentration (F). They also found that E~ct = 20 kcal/mol, as determined from the times of half-reaction at different temperatures. Bone and Gardner 1 studied the reaction rates of 2:1 and 1 : 1 mixtures of CH20--02 at pressures (~1 atm) considerably higher than thoseemployed byother investigators. They found that the rate of pressure rise was retarded by an increase in the surface-to-volume ratio. By an extensive analysis of the reaction products, they found that CO and H20 were the major products, whereas C02, H2 and HCOOH were formed in smaller amounts. They also found that relatively large quantities of performic acid (PA) and dioxymethyl peroxide (ItOCH2--O--O--CH2OH) were formed during the early stages of reaction. These two peroxidic substances were shown to disappear with time, while the more stable products and the total pressure increased appreciably. This research is a part of the work by the Bureau of Mines on Order No. CS-670-54-9, supported by Headquarters, Air Research and Development Command, Office of Scientific Research, through Project SQUID.
( d-F- -)~ , = k ' ( F ) ~ - k " ( F ) , + k ' " . This equation fits his data well, if his observed (dp/dt)~ are assumed to be proportional to (--dF/dt)~. Snowden and Style s made both gas analytical and manometric measurements. In agreement with previous investigators, they found that CO, H20, H2 and CO2 were the'reaction products. From stoichiometric considerations, neglecting peroxide formation, they obtained evidence for the presence of HCOOH. In several experiments they made use of a spectrophotometric method for determining CH20 (Dband absorption in the near ultraviolet using a hydrogen discharge tube as the source of light). Except for a short initial period of acceleration, they found that their results could be expressed by the rate law: - d F / d t = KF(F - C), where K and C were shown to vary erratically with surface condition. This equation was shown to be a consequence of a modified form of Spence's reaction scheme. Treatment of the reaction-vessel surface with hydrofluoric acid resulted in a
435
436
KINETICS OF COMBUSTION REACTIONS
very slow rate of pressure rise after an appreciable induction period. The presence of Hg vapor increased the rate of pressure rise, whereas washing the surface with HNOa gave the most reproducible results. Axford and Norrish 9, on the other hand, found, with the exception of a few rare experiments, no convexity toward the time axis in their observed pressure-time curves. Indeed, no observable diameter effect upon the rate of pressure rise was noted in their experiments. From the analytical results of a course-of-reaction series o f experiments, they showed that ( - dF/dt) i = 2 ( d p / d t ) i . In addition, the simple rate law, (-dF/dt)~ = KF~, was found to fit their data quite well, and an overall activation energy of 21 kcal/mol was' observed. They also found that CO and'H20 were the major products, whereas C02 and H2, in almost equal amounts, were minor products. No evidence for the presence of HCOOH or peroxides was found. The addition of N2 as an inert diluent inhibited the rate of pressure rise. Small quantities of oxygen were found to induce a slow decomposition of formaldehyde at temperatures where it is normally quite stable. These observations were successfully accounted for by these authors by means of a simple reaction scheme involving H atoms as the predominant chain carrier. More recently, Vanp6e 1~ studied this reaction at considerably higher temperatures (370-550~ and lower pressures (2080 mm Hg) than had been done previously. He used a fine tungsten filament (40~) as a temperature-sensing element in a thermal method for following the rate of reaction. He was able to demonstrate that the reaction followed a (-dF/dt)~ = KF~ rate law with an overall activation energy of 29.4 kcal/mol. He showed that an active intermediate, which he assumed to be PA, builds up to a steady state during an initia] induction period and is destroyed by a bimolecular process at the vessel wall. The mechanism suggested is a further modification of the B~ckstrom type of scheme first used by Spence. In a recent review of the available literature on formaldehyde oxidation, Lewis and von Elbe H came to the conclusion that PA is formed by a chain reaction in which HCO is the predominant chain carrier b. The PA thus formed is then asb They cite the work of Gorin ~, ~3and Style and Summers 14as evidence for the stability of the HCO radical (i.e., HCO = H -t- C O - - > 26 kcal). The photolysis of CH~O in the presence of D2 has been studied recently in this laboratory by Dr. L. J.
sumed to diffuse to the vessel wall and decompose into the products CO, H~O, C02, H2 and HCOOH. If this process is rapid enough, further gas-phase reactions of the intermediate performie acid with the chain carrier CHO are neglected, and the process becomes a simple steady-state reaction. If the vessel surface, on the other hand, is inactive toward peroxide destruction, gasphase reactions of the PA then play a role, and the kinetics become more complex. These authors thus attribute the simple kinetics observed by Axford and Norrish to use of a system which rapidly decomposed the intermediate peroxides. In particular, they suggest that the Hg vapor introduced into the Axford and Norrish reaction vessel, as a result of the use of heated mercury cutoffs, could have caused the rapid removal of PA. The more complicated kinetic data obtained by Spence, Snowden and Style, and Bone and Gardner was attributed to the use of systems in which the intermediate PA was not rapidly destroyed. The purpose of this present investigation is primarily to resolve the difference between the kinetic results obtained by Axford and Norrish on the one hand, and Spence, Snowden and Style, and Bone and Gardner on the other; second, further information is sought concerning the nature of, and the role played by, the intermediate peroxides; and finally, a mechanism is arrived at which is capable of explaining the main features of this reaction as observed to date.
Experimental MATERIALS
Formaldehyde: Monomeric liquid formaldehyde was prepared by the method of Spence and Wild 15 and stored at - 8 0 ~ Alpha-polyoxymethylene was used as the starting material instead of paraformaldehyde, since the former polymer contains less water than the latter. Oxygen: Oxygen, 99.6 per cent pure, was taken from a cylinder and condensed at liquid-nitrogen temperatures. The middle fraction was evaporated into a large evacuated bulb up to a pressure of about 1000 mm Hg and stored there until ready for use. APPARATUS
The reaction vessels used were 6-cm and 4-cm silica bulbs. The reaction vessel, wrapped in Schoen. His data provide further independent evidence for the stability of HCO (to be published).
437
GAS-PHASE OXIDATION OF FORMALDEHYDE
aluminum foil, was immersed in an electrically heated, thermocouple-controlled (•176 furnace. A slow stream of air was blown over the reaction vessel to keep the temperature gradient from one side of the bulb to the other within • ~~ Temperatures were measured with ironconstantan thermocouples, which were calibrated at the ice point and boiling point of water, and the melting points of Sn, Pb, and Zn. Pressures were measured by means of a Bodenstein quartz spiral manometer, which had a galvanometer mirror mounted on it. A conventional projection lamp and scale assembly could then be arranged to give an optical lever which resulted in an instrument sensitivity of 2.5 mm of scale deflection per 1.0 mm Hg pressure differential. The entire system could be evacuated to <10 -e mm Hg by means of a suitable combination of diffusion and mechanical pumps. Octoil-S was used as the diffusion-pump fluid. To avoid polymerization of formaldehyde, all lines and stopcocks in contact with formaldehyde were wrapped with nichrome wire and electrically heated to 110~ Dow-Corning high-vacuum silicone grease was found to be suitable as a stopcock lubricant under these conditions. Figure 1 is a schematic diagram of the apparatus. PROCEDURE
Mixtures of formaldehyde and oxygen were made up in the thermostated (110~ mixing vessel. The contents of the mixing vessel was then rapidly shared with the thermostated, thoroughly evacuated, reaction vessel, and the pressure-time dependence was measured with the quartz spiral manometer. At the end of each reaction, or after any period of reaction, the hot gases were quickly shared with, and isolated in, the thermostated (ll0~ sampling vessel. The pressure of this sample was quickly measured and then rapidly admitted into the freeze-out trap, which was immersed in liquid nitrogen. The permanent gases were then circulated through the sampling vessel, freezing trap system by means of the Toepler pump, to assure complete removal of the condensable gases. The noncondensable gases were then pumped back into the sampling vessel, and their pressure was measured. An ethyl iodide slurry (--ll0~ was then used as a refrigerant in place of the liquid nitrogen. Thus, a rough separation of C02 and unreacted formaldehyde was made possible 9. The C02 and traces of formaldehyde were then pumped back into the sampling vessel
and the pressure measured once more. The gases now in the sampling vessel were then pumped into the mass spectrometer sample bulb (SB). The sample bulb (SB) was then isolated, removed from the system, and analyzed mass spectrometrically for CO, C02, 02, H2, and traces of CH~O. In experiments 10A and 18A, the condensable products were scanned with the mass spectrometer. This was accomplished by opening the stopcock communicating between the freezing trap and the sample bulb (SB), removing the ethyl iodide slurry surrounding freezing trap, and immersing the sample bulb in liquid nitrogen. The sample bulb was again isolated and removed High vacuum pumps
t~
~ Monomer~c li:4uid CH 20reservoir LH"~-~0 2reservoir q- 7 . . . . . . . I
~F2 I I
)S.B
IR. V. - ~
l t_ . . . . FI
2
FIG. 1. Schematic diagram of apparatus. R.V., reaction vessel; S.M., quartz spiral manometer; MI., galvanometer mirror; M., mercury manometer; M.V., mixing vessel; S.V., sampling vessel; Fz, main furnace; F2 secondary furnace thermostatically controlled at 110~ F.T. freeze-out trap; T.P., toepler pump; S.B., mass spectrometer sample bulb. from the system, but this time it was kept immersed in the refrigerant until ready to be Opened to the evacuated mass-spectrometer inlet manifold and ionization chamber. In this way the possibility of detecting and identifying the unstable peroxides was enhanced. For those experiments where a fresh, clean silica surface was desired, the reaction vessel was removed from the apparatus, washed with concentrated HN03, thoroughly rinsed with distilled H20, placed back in the apparatus, and thoroughly evacuated for 24 hr at 350~ In all other experiments, the vessel was evacuated at 350~ for ~i-hr before each run. To reproduce the experimental conditions of Axford and Norrish, who used heated mercury cutoffs, mercury was added to the mixing vessel.
438
KINETICS OF COMBUSTION REACTIONS
Since the mixing vessel was thermostated at l l 0 ~ mercury vapor at a partial pressure of ~ 1 m m tIg was thereby introduced into the reaction vessel with the reaction mixture. Stoichiometry
With the above apparatus and procedure it was possible to measure the following parameters for this reaction system: PCH2O~ = the initial partial pressure of formaldehyde; Ap = the change in total pressure; t = time ; Po2~ = the initial partial pressure of oxygen; Po2 = the partial pressure of oxygen at time t; where &PO2 = P021 -- e 0 2 Pco = the partial pressure of CO at time t; PH2 = the partial pressure of H2 at time t; Pco~ = the partial pressure of CO2 at time t; and Peond = the partial pressure of the remaining constituents at time t, which are condensable at - 110~ The analytical d a t a of Bone and Gardner, as well as the results of this research, indicate t h a t in general the following overall reactions are required to account for the products of this reaction: CH20 + ~ 02 = CO + H20,
(a)
CH20 + O2 = CO2 + H~O,
ides remaining is v e r y small and (e) and (f) m a y be neglected. Case II(b): Same as II(a) above, but where the reaction is stopped sufficiently short of completion, so that the a m o u n t of peroxides present cannot be neglected. All of the six overall reactions must then be considered. F r o m a straightforward stoichiometric consideration of the overall reactions, unmeasured parameters, such a s A p c H ~ o ---- PcH2o~ - - PCH20 , PH20, PHCOOH, etc., can be calculated from the measured quantities in the following fashion:
Case I: ApCH2O = P c o + P c o 2 , and Pa2o = Pco
As a check upon the consistency of these results, one can compare the experimentally measured values with those calculated as follows: Apealc
=
~ (Pco +
Pcondcalc =
P H 2 0 + P C H 2 0 = PCH2Oi -- PH2 9
Case II(a): Ap CH20 = P c o + Pco~ + PHCOOH
(c) =
CH20 + ~ 02 = HCOOH (formic acid),
(d)
CH20 + 02 (e)
= HCOOOH (performic acid
PH2),
and
(b)
CH~O = Hz + CO,
+ P c o 2 -- PH2 9
or
PA),
PH2
+ 2Apo2 -- PC02 ;
PH20 = PCo + Pc02 -- PH2 ; and PHCOOH = PH2 + 2Apo2 -- PCO -- 2Pc02,
and
3CH20 +
O2
CO + (CH2OH)202 (dioxymethyl peroxide).
(f)
I t has been found necessary to assume two types of stoichiometry. T h e seemingly conflicting results of Axford and Norrish, as opposed to those of Spence, and Snowden and Style, etc., suggest just such a treatment. Case I: Peroxides are decomposed rapidly into CO, H20, C 0 2 , and H2, and no H C O O H is formed. Reactions (d), (e) and (f) can then be neglected (the conditions of Axford and Norrish). Case II(a): The peroxides decompose more slowly into CO, H20, CO2, H2 and H C O O H . If the reaction is allowed to go far (90-100 per cent) toward completion, then the q u a n t i t y of perox-
Once more, as a check upon the consistency of the scheme, one can calculate: Apcale = PCo + Pc02 -- Apo2, and Pcondc~lr = P H 2 0 , + PHCOOH +
PCH20 = PCH20~ -- PH~ .
Case lI(b): ApCH20
= P c o + Pco2 + PHCOOH + PPA + 2P(cH2OH)2Oz = 1/~ (3Apo2 + P c o -- Pco2 -- &p) + PH~ -- PPA; PH20 = ~/~ (Pco + Pco., + Apo2 + Ap) -- PH2 ; P(CH2OH)202 = 1~
( P c o + Pc02 -- :XP02 -- Ap);
439
GAS-PHASE OXIDATION OF FORMALDEHYDE
and P H C O O H -~- 2 P p A
= 1/~ (5Apo2 -- 3 P c o
-- 5 P c 0 2
a n d again, to verify t h e scheme, one can calculate: Peondcalc =
+ Ap) -4- P H
2
;
consistency of t h e
PH20 -4- PCH2O A- PHCOOH -I- PPA -~- P ( C H 2 O H ) 202
= PCH20~ + Apo2 + Ap -- PH2 -- P c o -- P c o 2 .
lar results were o b t a i n e d in e x p e r i m e n t s carried o u t in t h e presence of m e r c u r y vapor, which was i n t r o d u c e d in t h e m a n n e r described above. C u r v e 2 in Figure 2 is t y p i c a l of t h o s e e x p e r i m e n t s carried out in a silica vessel a f t e r making several successive runs w i t h o u t cleaning (hereafter referred to as a n aged vessel). T h e time i n t e r v a l for t h e initial convexity t o w a r d t h e time axis t e n d e d to increase s o m e w h a t w i t h increased vessel aging. T h i s effect is strongly r e m i n i s c e n t of t h e induction period p h e n o m e n a e n c o u n t e r e d so frequently in h y d r o c a r b o n oxidation. As will be shown below, this similarity is only superficial in t h a t
TABLE 1 (All Quantities in m m Hg at 337~ Experiment No . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Stoichiometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Vessel and surface condition . . . . . . . . . . . . . . . .
Observed q u a n t i t i e s At (minutes) . . . . . . . . . . . . . . . . . . . PCH2Oi . . . . . . . . . . . . . . . . . . . . . . . . Apo2 . . . . . . . . . . . . . . . . . . . . . . . . . . P co ........................... Pco2 .......................... PH2 . . . . . . . . . . . . . . . . . . . . . . . . . . . Apobs ..........................
Peondob, - . . . . . . . . . . . . . . . . . . . . . . Calculated quantities Apealc . . . . . . . . . . . . . . . . . . . . . . . . .
7A*
17E*
30t
8B:~
I
I
II(a)
II(b)
6-cm silica bulb, freshly cleaned with HNOa
4-cm silica bulb, contaminated with Hg vapor
6-cm silica bulb, aged surface
4-cm silica bulb, aged surface
32 127.4 50.7 84.0 23.2 13.6
20 97.5 30.3 56.8 8.0 2.8 28.8 89.5
35 93.4 45.3 62.1 12.4 16.6 29.0 76.4
3 161.6 34.3 29.5 26.6 38.0 11.9 113.7
29.8 95.6 62.0 64.8 ---
29.2 76.8 57.9 93.4 -20.3
-113.2 13.1 -85.0
45.8
103.0 48.8
Peondcalr . . . . . . . . . . . . . . . . . . . . . . .
113.8
.......................... Ap CH20 . . . . . . . . . . . . . . . . . . . . . . . . Apc~20 + P P n . . . . . . . . . . . . . . . . P~COOH . . . . . . . . . . . . . . . . . . . . . . . PHCOOH + 2 PPA . . . . . . . . . . . . . . P (CH2OH) 202 . . . . . . . . . . . . . . . . . . .
93.6 107.2 ----
PH2O
--
--
--
--
18.9
--
--
5.0
~
* Exp. 7A, 17E: (dp/dt)max = ( d p / d t ) ~ . t Exp. 3C: ( d p / d l ) m ~ = ( d p / d t ) t = 3 minutes. :~ Exp. 8B: (dp/dt)m~x = ( d p / d t ) t = 2 minutes. T a b l e 1 gives t h e a n a l y t i c a l results of four experim e n t s which are typical of t h e different stoichiometric conditions described above. Results
t h e r a t e a t which f o r m a l d e h y d e disappears is n o t slow d u r i n g this t i m e b u t is a c t u a l l y g r e a t e r t h a n in those e x p e r i m e n t s p e r f o r m e d in a clean vessel, or w i t h m i x t u r e s c o n t a m i n a t e d with m e r c u r y vapor.
THE PRESSURE-TIME DEPENDENCE
T w o t y p e s of p r e s s u r e - t i m e c u r v e s h a v e been o b s e r v e d . C u r v e 1 in F i g u r e 2 gives a typical e x a m p l e of those e x p e r i m e n t s carried o u t in a silica vessel freshly cleaned with nitric acid. Simi-
PRODUCTS CONDENSABLE AT -- 1 1 0 ~
T o o b t a i n specific i n f o r m a t i o n a b o u t t h e n a t u r e of t h e p r o d u c t s o t h e r t h a n CO, CO2, a n d H2, a s a m p l e of t h e p r o d u c t s c o n d e n s a b l e a t - l l 0 ~
440
KINETICS O F C O M B U S T I O N
w a s taken as described above and scanned by the mass spectrometer in the 0-100 mass range. The results for samples taken from an aged vessel and
/
y o
v """
REACTIONS
Gardner ~ and of Snowden and Styl@. Its comparative absence in the mercury-contaminated system agrees with the results of Axford and Norrish. The presence of 02 and CO2 in the sample taken from the aged vessel can probably be attributed to the breakdown of peroxides in the ionization chamber of the mass spectrometer. The unidentified species exhibited a mass spectrum extending over the 30-95 mass range. Small peaks at 62, 61 and 60 were observed and tentatively assigned to PA (with the loss of one and two hydrogen atoms). Since calibrating spectra for peroxides are not available, no particular peroxide species could be positively identified. The above facts, together with the previous observations of Bone and Gardner l, Snowden and Style s, and Axford and Norrish 9, must be taken as strong evidence for the peroxide destroying property of mercury vapor. COURSE-OF-REACTION
j IO
20 TIME,MINUTES
~
40
FIG. 2. Curve 1: Exp. ~7A; HNOs cleaned vessel; Po82o~ = 127.4 mm Hg and Po2~ = 79.3 mm Hg; T = 337~ 6-era silica bulb. Curve 2: Exp. $3A; aged vessel; PCH2Or = 127.6 mm Hg and Po2i -- 68.6 mm Hg; T -- 337~ 6-cm silica bulb. T,ABLE 2
Aged Vessel
Reaction Mixture Contaminated with Mercury Vapor
Experiment no . . . . . . . . . 10A 18A Temperature, ~ . . . . . . . 337 337 Time, rain . . . . . . . . . . . . . 5 5 PcH2o~ , m m Hg . . . . . . 175.1 176.0 Po2~, mm Hg . . . . . . . . . 80.9 80.7 Analysis, tool per cent CH20 (unreacted) . . . . . 11.7 5.5 H20 . . . . . . . . . . . . . . . . . . . 61.2 92.1 HCOOH . . . . . . . . . . . . . . . 3.2 Trace (<0.1) 02 . . . . . . . . . . . . . . . . . . . . . 10.0 0.0 COs . . . . . . . . . . . . . . . . . . . 1.4 0.0 Unidentified . . . . . . . . . . . 12.5 2.4
from a reaction mixture contaminated with mercury vapor are given in Table 2. In agreement with all previous investigators, it is found t h a t H~O is a major product. The presence of HCOOH in the aged vessel is in complete agreement with the observations of Bone and
EXPERIMENTS
As shown in Table 1, consistent analyses result when Case I stoichiometry is applied to a system with a vessel freshly cleaned with HN03 or to a mercury contaminated mixture. Case I I stoichiometry, on the other hand, yields consistent results when applied to a system consisting of an aged silica vessel. The significance of these facts is demonstrated by Tables 3 and 4. Table 3 gives the results of a course-of-reaction series of experiments for an aged vessel, whereas Table 4 gives a similar set of d a t a for a vessel contaminated with mercury vapor. For the vessel contaminated with mercury vapor, dp/dt reaches a maximum at the start of the reaction. Since Case I stoichiometry can be used successfully, peroxides and HCOOH are not present in significant quantities. Also, it is seen that (dp/dt)~ = ( ~ ) ( - - d F / d t ) ~ . These facts are in complete agreement with the data reported by Axford and Norrish. For the aged vessel, dp/dt does not reach a maximum until about 2.5 minutes after the start of the reaction. Also, note that (dp/dt)~ << ( ~ ) IdF/dtl~. Since Case I I stoichiometry must be applied to yield consistent results, the presence of HCOOH, PA and H O C H 2 - - O - - O - - C H 2 0 H is strongly indicated and their partial pressures are calculated in the manner described above. These calculations show t h a t during the early stages of reaction the concentration of (CH~OH)20~ builds up faster than either PA or HCOOH. During the first minute of reaction, (PHcooH T 2PrA) is shown to be very small.
441
GAS-PHASE OXIDATION OF FORMALDEHYDE
TABLE 3. VESSEL: AGED 4 CM SILICA BULB (All Q u a n t i t i e s in m m H g at 337~ Experiment No . . . . . . . . . . . . . . . . . Time, min
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
9B
9C
9D
1
3
.5
Observed quantities Z~po2 . . . . . . . . . . . . . . . . . Pco ................. Pco2 . . . . . . . . . . . . . . . . . PH2 . . . . . . . . . . . . . . . . . .
6.7 14.2 2.2 4.4 Apob . . . . . . . . . . . . . . . . . . . 2 dp/dt, m m H g / m i n . . . . . 8 Pcondobs 9. . . . . . . . . . . . . 140.6 Calculated quantities Pcondcalc 9 . . . . . . . . . . . . . 140.9 PH20 . . . . . . . . . . . . . . . . . 7.4 A p C H 2 0 -I- P P A . . . . . . . 25.9 PHCOOH q- 2PPA . . . . . . 0 P(CH2OH)20~ . . . . . . . . . . 4.8
9E
9F
10
20
35.9 55.8 5.3 11.4 5.4 3,6 124.9
51.3 73.5 7.5 15.3 I1,7 3.6 119.9
62.0 94.1 14.5 19.1 30.9 2,5 122,0
61.9 95.0 18.2 18.2 41,8 ,5 129,3
125.2 34.8 87.8 6.9 9.9
119.6 56.7 119.4 20.4 9.0
122.5 81.6 136.5 12.1 7.9
130.0 90.2 128.6 5.8 4.8
T = 337~ 155 m m Hg. Po:~ = 75 m m Hg, (dp/dt)m~x = 3,6 m m H g / m i n (between 2.5 a n d 8.5 rain a f t e r t h e s t a r t of t h e r e a c t i o n ) . PCH2Oi =
TABLE 4. V E S S E L : 4 CM S I L I C A B U L B . R E A C T I O N M I X T U R E C O N T A M I N A T E D W I T H M E R C U R Y V A P O R AT .4. P A R T I A L P R E S S U R E OF ABOUT 1 MM H G
(All Q u a n t i t i e s in m m H g a t 337~ Experiment No . . . . . . . . . . . . . . . . . . . . . . .
17A
17B
Time, min . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1
3
1.6 5.3 1.5 1.5 3.3 3.3 93.5 3.4 95.3 5.3 6.8
Observed quantities Apo2 . . . . . . . . . . . . . . . . . . . . . . Pco ....................... Pco2 ...................... PH2 . . . . . . . . . . . . . . . . . . . . . . . Apobs ......................
dp/dt, ram H g / r a i n . . . . . . . . . P c o n d o b B. . . . . . . . . . . . . . . . . . .
Calculated quantities Apcalr . . . . . . . . . . . . . . . . . . . . . . Pcondcalc 9 . . . . . . . . . . . . . . . . . . PH20 . . . . . . . . . . . . . . . . . . . . . . ApCH20 . . . . . . . . . . . . . . . . . . .
17C
17D
17E
;
10
20
9.2 19.2 3.6 2.3 9.6 3.2 90.1
17.0 31.7 4.4 1.8 16.5 2.4 94.0
24.2 44.0 6.6 3.1 24.0 1.0 91.3
30.3 56.8 8.0 2.8 28.8 .3 89.5
10.7 94.4 20.5 22.8
16.8 97.0 34.3 36.1
23.6 94.1 47.5 50.6
29.8 95.6 62.0 64.8
T = 337~ 97.5 ram Hg. P021 = 100 ram Hg. PCH20~ =
(dp/dt)m~, = (dp/dt)~ = (1/2) ( - d F / d t ) ~ . 2
THE DEPENDENCE OF ( - d F / d t ) ~ UPON Po2~ AND PcH2O~
and
A x f o r d a n d N o r r i s h 9 h a v e s h o w n t h a t , for a m e r c u r y - c o n t a m i n a t e d s y s t e m (where P A is rapidly destroyed and never achieves significant* c o n c e n t r a t i o n s ) , (dp/dt)~ is i n d e p e n d e n t of P o ~
b e e n s h o w n h e r e t h a t (dp/dt)~ is n o t s i m p l y rel a t e d to ( - dF/dt)~. C o n s e q u e n t l y , simple m a n o m e t r i c m e a s u r e m e n t s do n o t suffice for a d e t e r m i n a t i o n of t h e initial rate. T o m e a s u r e
is p r o p o r t i o n a l to PcH20~ a n d = ( ~ ) ( - d F / d t ) ~ . F o r a n a g e d vessel, however, it h a s
442
KINETICS OF COMBUSTION REACTIONS
(-dF/dt)i in an aged vessel, it has been necessary to resort to the use of analytical data for shortduration experiments. Whereas this technique TABLE 5. VESSEL: AGED 4-CM S I L I C A BULB (All quantities in mm Hg at. 350~ Experiment No . . . . . . . . . . . . .
12B
Observed quantities PCH~O~. . . . . . . . . . . . 112.3 Po2~ . . . . . . . . . . . . . . . 59.7 Apo2. . . . . . . . . . . . . . 11.0 Pco . . . . . . . . . . . . . . . 17.4 Pco2 . . . . . . . . . . . . . . 2.7 PH2 . . . . . . . . . . . . . . . 4.4 Poo~dob,. . . . . . . . . . . 101.0 Calculated quantities Peo~a~l~ . . . . . . . . . . . 101.5 PH2O. . . . . . . . . . . . . . 9.1 P(CH~OH) 202. . . . . . . 3.3 ~pcH20 . . . . . . . . . . . . 26.7
12C
12D
112.2 lll.8 11.7 19.4 2.3 7.1 96.3
111.7 169.3 12.5 17.3 2.6 7.0 99.9
97.0 9.9 4.7 30.0
99.0 10.2 2.7 25.2
THE OVERALL ACTIVATION ENERGY
T = 350~ Run duration: 1.0 min (PHcooH = PeA = 0). 60 [
"
'
LEGENO '
'
/I
.
.o
'
Table 5 indicates that (--dF/dt)~ is independent of Po~ in an aged vessel. Previous investigators, working under various experimental conditions, are in unanimous agreement with this fact. Figure 3 gives the data for the dependence of (--dF/dt)~ upon PCH20~ 9 The results of Axford and Norrish 9 and the data obtained in this reseaxch for both mercury contaminated and freshly cleaned vessels are shown to be entirely in agreement. The data obtained for aged vessels, on the other hand, definitely show somewhat larger values for (-dF/dt)i but still exhibit the simple square law dependence upon PeH2O~, as shown in Figure 3.
,"
(-dF/dt)~ in an aged vessel was calculated from analytical data for 1-minute experiments for constant composition mixtures of CH20 and 02 in the temperature range 317 ~ to 377~ ([CH20]~ = [02]~ = 2.63 • 10-6 mol/cc). An Arrhenius plot of In (dF/dtl~ versus 1/T is given in Figure 4. The activation energy is 27.4 kcal/mol. Table 6 gives the activation-energy measurements of previous investigators. Only the Axford and Norrish ~ value of 21 kcal/mol is for the well-defihed condition of rapid peroxide destruction. The rather large spread among the other values can probably best be accounted for, at this time, by the use of different types of vessels and surfaces, resulting in different rates of PA destruction and different initial steady-state concentrations of PA. Mechanism
and Discussion
SUMMARY OF THE KINETIC
.
Z/zi
/A
I
L 3O
I~
,.
40
50
F 2 x 10 3 ( M M . H g } 2
(T
upon PcH2O, FIG. 3. Dependence of - (dF) ~ 3T7~ =
has several experimental shortcomings, such as the difficulty of obtaining precise determinations of At, as well as the inherent inaccuracy of measuring small quantities of products, the data given in Table 5 and Figure 3 do show consistent results.
DATA
The results of previous investigations, as well as those of this research, indicate that the following pertinent facts must be taken into account in any final choice of a mechanism for this reaction. (1) Mercury vapor and surfaces freshly cleaned with HN03 result in simple p-t curves where (dp/dt)ma~ = (dp/dt)~. The products are CO, H20, C02 and It2, and the available analytical data show that (dp/dt)~ = (~)(-dF/dt)~. The governing rate law is given by (-dF/dt)~ = K1F~, and the overall activation energy is about 21 kcal/mol. Inert diluents such as N2 are found to decrease the rate of reaction9. (2) For other systems (such as the aged vessel reported in this paper), the p-t curves can show a decided initial convexity toward the time axis, so that (dp/dt)ma~ # (dp/dt)~. During this initial
GAS-PHASE
OXIDATION
period of small dp/dt, formaldehyde reacts at a rate even faster than that observed in systems where (dp/dt)m~x = (dp/dt)i. In addition to the products CO, H20, CO2, H2, significant amounts of HCOOH, PA and (CH~OH)202 are detected, where (CH~OH):O~ is found in measurable quantities earlier than either HCOOH or PA. The - 16
d In Eact.= - R
-17
tually build up to relatively large concentrations. If the vessel conditions allow the second alternative, the PA initially reacts rapidly with CH20 to form (CH2OH)20~ ~, which can heterogeneously decompose to form HCOOH, C02 and 2H2 (reactions (8) and (9) below). In either case, however, (PA) is small initially, so that a simple stationary state may be assumed during the early stages of the reaction. The details of such a mechanism are given in the following section. PROPOSED MECHANISM AND ITS CONSEQUENCES
dF (~-tF)i = 27.4 T M
I. Main chain:
d/§)
1. F + O 2 --* HCO + ttO2 k~ chain inflating
\
2. HCO + 05 -~ HCO3 ks 3. HCO3 + F ~ chain continuing - , P A + HCO k31
\
_r
4'. HCO~ + 05 --* CO + HO2 + 02 k4,fchain terminating 4. HCO, + M CO +HO2 + M k.~J wall
5. HOs - 20 1.5
443
OF FORMALDEHYDE
1.6
1.7
1.8
10 3 T~
9 destruction k5
II. For the conditions of rapid PA destruction, the main chain is followed by: 6. 2PA
FIG. 4. Overall activation energy measurements, temperature range: 317-377~
active surface or Hg vapor
TABLE 6
9 2CO
+ 2H20 + 05
k61 t
Investigators
Activation energy kcal/mol
Fort and Hinshelwood . . . . . . . . . . . . . . Spence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Snowden and Style . . . . . . . . . . . . . . . . . Axford and Norrish . . . . . . . . . . . . . . . . . Vanp6e . . . . . . . . . . . . . . . . . . . . . . . . . . . . . This research (for an aged vessel)...
20.0 17.6 25.0 21.0 29.4 27.4
7. 2PA
~where k7 < ks
wall
" 2CO2 + 2H2 + O2 k7
III. For the aged vessel condition, the main chain is followed by: 8. PA + 2F ---* (CH2OH)2Os + CO 9. (CH2OH)sOs
observed rate law is again found to be of the form ( - d F / d t ) ~ = K2F2~ (where Ks = 1.4 K~ at 337~ with an overall activation energy of 27.4 kcal/mol. These experimental facts are readily interpreted if one assumes that PA is formed by a simple, homogeneous, free radical chain involving CHO and CHO3 as the major chain carriers. The PA thus formed is then either rapidly destroyed at the wall or by mercury vapor to form the products CO, H20, COs and H2 ; or it can even-
J ks
wall
9 HCOOH + COs + 2Ha ks
6'. 2PA inactive surface
9 2 C O + 2 H 2 0 + 02
wall
7'. 2PA
1
kv wherek7
9 2CO2 + 2H2 + O2 k7
[
)
< k~p < k6
c Evidence for the reaction between Ctt20 and peroxides to form hydroxyperoxides ;s given by G. H. N. Chamberlain and A. D. Wal:~h, Third Symposium on Combustion, Flame and Explosion Phenomena, The Williams & Wilkins Co., Baltimore, 1948, pp. 375-382.
444
KINETICS OF COMBUSTION REACTIONS
For the conditions of rapid destruction of PA (I and II) the rate of disappearance of CH~O is given by
dF dt =
---
equation (6) becomes
(dp)
dF dt
---=
k,(HCO,)(F).
(1)
Now, with the usual steady-state assumptions for (HCO) and (HCO3), it is readily shown that (HCO,) =
kl(O~) (F)
(2)
k4'(02) --~ k 4 ( M ) '
and equation (1) becomes --d-t = k4,(O2) + k,(M) (F)~
klk3 (M) (03)
(F)%
(3)
For initial rates in the absence of inert diluents, equation (3) becomes
klk~
- - - ~ , = 7 4 , (F)~
(4)
which is just the type of rate dependence experimentally observed. Therefore, E~:t = E~ -4- Ea E4' = 21 kcal/mol. This activation energy is consistent with an approximate bond energy estimate of AHl -{- AH3 - AH4; = 10 kcal exothermic~, ~7 Applying steady-state considerations to (PA), it follows that 2(k6 + kT)(PA) ~ = b3(HCO3)(F).
Comparison of equation (8) with equation (4) yields
which again is seen to be in agreement with the result given by experiment. For the aged vessel conditions (I and I I I ) the rate of disappearance of CH~O is given by
dF = dt
---
( d- ~F/)
k3(HCO3)(F) + 2ks(PA)(F) 2.
(5)
(dP) =3(ks+k~)(PA)2+k,,(HC03)(02) ,
(6)
- k~(HCO)(O~). From the steady-state condition for (HCO3)
(10)
2k,(PA),(F)~.
(11)
k,k3(F)~
2kr(k~,+ ~) O. (12)
For (PA)~ small, it follows that, to a second-order approximation, 2ks(PA),(F)~ = ~
(F)~ (13)
[2
k~k3 k4, 4(ks, ~ + ~k,) + ks(F)~
"'"
1
Substituting equation (13) into equation (11), the initial rate for the aged-vessel conditions becomes
( d- ~F ) , = ~klb3 (F)~ (14)
[ 3 - k,k3 k,, 4(k., 2 + 2kT) -tka(F)~
1 ""
From the experimental results given in Figure 3, the quantity in the brackets is seen to be approximately equal to 1.4 for the vessel conditions reported here. DISCUSSION
k4(HCO~) (O~) -- k~(HCO) (O2) =--k3(HCO.~)(F),
klk3 (F)~ +
, = ~
ks(F)~ (PA)~ + 2(ks, + k,) (PA),
In the absence of inert diluents, the initial rate of pressure rise is given by
and from equation (5)
(8)
The stationary state condition for (PA)~ is given by
k4' + k4 - -
(dF)
(F)~.
In the absence of inert diluents, the initial rate is given by using equation (2):
klk3(O~) -
klka
~-[ , = ~k3(HCOa)(F), = ~ ~
k~(F)(02) + k3(HCO3)(F).
Assuming long chains, and since 1 is chaininitiating, kl(F)(O2) can be neglected relative to the chain continuing step 3. Therefore,
dF
3(k6 -I- kT)(PA) 2 = ~ ks(HCO~)(F),
(7)
Whereas the above mechanism provides a satisfactory explanation for most of the observations encountered for this reaction, there are
GAS-PHASE OXIDATION OF FORMALDEHYDE
several results reported by previous investigators which do not fit directly into the scheme. Spence noted that over extensive surfaces of powdered glass, the reaction occurred slowly and almost exclusively to form C02 and H20. This fact can be understood in light of the suggested mechanism, if one assumes that in the presence of extremely large glass surface to volume ratios, the main chain I is followed predominantly by 10.
PA
extensive surface
~ CO2 ~ H20
k~0
wall
HCO
9 destruction
k~
or
HCO3
12.
wall
* destruction
k12 ,
thereby resulting in a slower rate. Another significant observation that should be amenable to interpretation in the light of the proposed mechanism is that a slow, low-temperature decomposition of CH20 to CO and H2 can be induced by traces of oxygen. 9 The following reaction sequence can qualitatively account for this observation: 1.
13. 11.
F -~- 02 ~ H C O -~- HO2
HCO ~- F ~ CO -t- H~ ~ HC0 HCO
wall
* destruction
kl
k13 kll
and 5.
HO2
wall
, destruction
suggesting this problem and, second, with Drs. R. Klein and L. Schoen, for participating in the stimulating discussions that provided an ideal atmosphere for this work. Also, the author would like to express his appreciation to A. G. Sharkey and the mass-spectrometer group at the Bureau of Mines, without whose cooperation this research could hardly have been carried out in its present form. REFERENCES 1. BONE, W. A., AND GARDNER, J . B. : Proc. Roy.
in lieu of 6 and 7, or 8 and 9. Also, the chain lengths are considerably decreased by the increased likelihood of 11.
445
k5
where 13 is assumed to be much slower than 2 but, in the absence of appreciable quantities of oxygen, becomes rate determining. Thus, the suggested reaction scheme provides a consistent framework from which the diverse and seemingly conflicting observations encountered in the formaldehyde oxidation reaction can be understood. A more complete understanding of the problem, however, must wait upon an accurate knowledge of the heterogeneous decompositions of (PA) and (CH2OH)~O2, as well as further information concerning the detailed kinetics of the reaction between PA and CH20 to form the hydroxyperoxide.
Acknowledgments The author wouid like to take this opportunity to thank Drs. B. Lewis and G. von Elbe, first, for
Soc. (London), A15~, 297 (1936). 2. NORRISH, R. G. W., AND FOORD, S. G. : I b i d , A157, 503 (1936). 3. BONE W. A., AND HILL, S. G . : Ibid., A12g, 434 (1930). 4. PEASE, R. N.: Chem. Rev., 21,279 (1937). 5. FORT, R., AND HINSHELWOOD, C. N . : Proc. Roy. Soc. (London), A129, 284 (1930). 6. SPENCE, a . : J. Chem. Soc. 649 (1936). 7. BXCKSTROM, H. L. J.: Z. Physik. Chem., B25, 99 (1934). 8. SNOWDEN, F . F . , AND STYLE, D. W. G. : T r a n s . Faraday Soc., 85, 426 (1939). 9. AXFORD, D. W. E., AND NORRISH, R. G. W.: Proc. Roy. Soc. (London), A192, 518 (1948). 10. VANP~.E, M.: Bull. Soc. Chim. Belg., 62, 285 (1953). 11. LEwis, B., AND VON ELBE, G.: Combustion, Flames and Explosions of Gases, pp. 94-106. New York, 1951. Academic Press, Inc. 12. GORIN, E.: Acta Physicochim. (U.R.S.S.), 9, 681 (1938). 13. GOR1N, E.: J. Chem. Phys., 7, 256 (1939). 14. STYLE, D. W. G., AND SUMMERS, D.: Trans. Faraday Soc., 42, 388 (1946). 15. SPENCE, R., AND WILD, W. : J. Chem. Soc., 338 (1935). 16. PAULING, L.: Nature of the Chemical Bond. Cornell Univ. Press, 1945. 17. GLOCKLER,G.: J. Chem. Phys., 19, 124 (1951). DISCUSSION BY M. VANP]~E
In the mechanism proposed by Dr. Scheer, the initiating step is attributed to the reaction: HCHO + O2 = HO~ + CHO If this reaction is the only initiating step, then there is no induction period. The author claims that this is the case in the presence of mercury vapor. However, it will be seen from Figure 1, which represents the combustion of formaldehyde in the presence of mercury vapor, that the use of single pressure-time curves is not sufficient to establish conclusively the absence of an induction period.
446
KINETICS OF COMBUSTION REACTIONS
I t is this which has led us to develop a more sensitive technique* in which t h e reaction velocity is measured directly b y t h e small t e m p e r a t u r e rise due to the heat l i b e r a t e d b y the reaction. T h e AT curve t h u s o b t a i n e d u n d o u b t e d l y has its origin a t zero, (dP/dt)o = O, a l t h o u g h the AP curve shows only a smooth inflexion when AT passes t h r o u g h a well characterized m a x i m u m . T h e r e is thus no d o u b t as to the existence of a definite induction period, b o t h in the presence and in the absence of m e r c u r y vapor. We must conclude therefore t h a t the reaction is catalyzed b y a long-lived i n t e r m e d i a t e . T h e obvious compound for this role is performic acid. The slightly modified symbolic m e c h a n i s m previously proposed b y Snowden and Style: X + F = 2X
(1)
2 X = p r o d u e t s reaction
(2)
is in perfect agreement with this conclusion a n d we have shown* t h a t it also fits r e m a r k a b l y well all the q u a n t i t a t i v e kinetie data. For step (1), we proposed the following scheme: PA --, HCO -{- HO~ HCO + O~ -~ HCO~ HCO + F -~ HCO -t- PA HCO3 --~ wall
initiation ["~
;propagation termination
AUTHOR'S REPLY The t h e r m a l m e t h o d of following the rate of reaction devised by Dr. Vanpde is a r a t h e r interesting approach to the ever present problem of kinetic m e a s u r e m e n t s in complex reaction systems. However, this novel technique is open to question on the following grounds. Using the formaldehyde-oxygen reaction as an example, it has been shown, not only in this research b u t in the work of Bone and G a r d n e r , Spence, and Snowden and Style, t h a t , in the absence of mercury vapor, spurious surface catalytic effects are always present. Consequently, the use of a fine t u n g s t e n wire to follow small local t e m p e r a ture changes, which in t u r n are i n t e r p r e t e d as i n s t a n t a n e o u s reaction rates, is questionable unless it can be conclusively shown t h a t the t u n g s t e n surface c o n t r i b u t e s no catalytic a c t i v i t y to any phase of the reaction. Evidence in t h e literature* shows t h a t oxygen is strongly chemisorbed upon t u n g s t e n (probably as O atoms). Could not some of t h e AT observed by Dr. Vanp~e be a t t r i b u t e d to a surface reaction between the adsorbed oxygen a t o m s and the s u b s t r a t e formaldehyde? The question of t h e existence of an i n d u c t i o n period for this reaction m u s t u l t i m a t e l y be dedF t e r m i n e d from knowledge of - ~ during t h e
With regard to the decomposition of performic acid, it is i n t e r e s t i n g to consider the combustion of a mixture c o n t a i n i n g a great excess of formaldehyde (Fig. 2). After all the oxygen is consumed, the formation of performic acid is a u t o m a t i c a l l y suppressed and the only possible reaction is the decomposition of the peroxide. The curve representing this decomposition definitely corresponds to a bimolecular process a n d may t h u s be considered as direct e x p e r i m e n t a l evidence of step (2). T h e absence of any vertical fall at point A shows f u r t h e r t h a t the f o r m a t i o n of performic acid from formaldehyde and oxygen (step (1)) is thermoneutral, and t h a t the h e a t generated by the reaction is due only to the decomposition of the peroxide (step (2)).
initial stages of the reaction. F r o m the analytical m e a s u r e m e n t s given in this m a n u s c r i p t as well as those obtained by Axford and Norrish, no such effect has been observed, e i t h e r for clean vessels or in the presence of mercury vapor. In the aged vessel, a true i n d u c t i o n period m a y indeed be present (see Table 3), since the PA c o n c e n t r a t i o n can build up and react with F by something like reaction (8). In agreement with Dr. Vanp~e, it should be emphasized t h a t the S-shape of the 5p-t curve is not conclusive evidence for the existence of an i n d u c t i o n period, since it has been shown here not to be simply related to the true course of reaction u n d e r conditions inactive to rapid PA d e s t r u c t i o n (i.e., in an aged vessel).
* M. Vanpde: Bull. Soc. Chim. Belg., 62, 285, (1953); 62, 661 (1953).
* I. L a n g m u i r and D. S. Villars: J. Am. Chem. Soc., 53, 486 (1931).
KINETICS OF THE GAS-PHASE OXIDATION OF FORMALDEHYDE a By MILTON D. SCHEER Spence 6 showed that the oxidation over extensive surfaces of powdered glass occurred slowly and almost exclusively according to the overall reaction: CH20 -k 02 = COs -k H20. In unpacked Pyrex vessels, he found that the variation of initial rates of pressure rise with temperature yielded an overall activation energy of 17.6 kcal/mol. He suggested a reaction chain analogous to that given by Biickstrom 7 for the oxidation of aldehydes in the liquid phase. This scheme involved PA, activated CH20, and activated HCOOH molecules. The assumption of a steady state for these intermediate species yielded the following equation for the initial rate of disappearance of formaldehyde:
Introduction
The importance of formaldehyde as the ratedetermining intermediate in methane oxidation has been conclusively demonstrated 1, 2. Even in the oxidation of the higher hydrocarbons, formaldehyde has been shown to play an important role s, 4. Therefore, in order to be able to solve the general problem of hydrocarbon oxidation, the kinetics of the oxidation of formaldehyde must first be understood. In the past two decades, many investigators have been engaged in the study of formaldehyde oxidation. Fort and Hinshelwood 5 showed that the rate of pressure rise was independent of oxygen concentration and strongly dependent upon the formaldehyde concentration (F). They also found that E~ct = 20 kcal/mol, as determined from the times of half-reaction at different temperatures. Bone and Gardner 1 studied the reaction rates of 2:1 and 1 : 1 mixtures of CH20--02 at pressures (~1 atm) considerably higher than thoseemployed byother investigators. They found that the rate of pressure rise was retarded by an increase in the surface-to-volume ratio. By an extensive analysis of the reaction products, they found that CO and H20 were the major products, whereas C02, H2 and HCOOH were formed in smaller amounts. They also found that relatively large quantities of performic acid (PA) and dioxymethyl peroxide (ItOCH2--O--O--CH2OH) were formed during the early stages of reaction. These two peroxidic substances were shown to disappear with time, while the more stable products and the total pressure increased appreciably. This research is a part of the work by the Bureau of Mines on Order No. CS-670-54-9, supported by Headquarters, Air Research and Development Command, Office of Scientific Research, through Project SQUID.
( d-F- -)~ , = k ' ( F ) ~ - k " ( F ) , + k ' " . This equation fits his data well, if his observed (dp/dt)~ are assumed to be proportional to (--dF/dt)~. Snowden and Style s made both gas analytical and manometric measurements. In agreement with previous investigators, they found that CO, H20, H2 and CO2 were the'reaction products. From stoichiometric considerations, neglecting peroxide formation, they obtained evidence for the presence of HCOOH. In several experiments they made use of a spectrophotometric method for determining CH20 (Dband absorption in the near ultraviolet using a hydrogen discharge tube as the source of light). Except for a short initial period of acceleration, they found that their results could be expressed by the rate law: - d F / d t = KF(F - C), where K and C were shown to vary erratically with surface condition. This equation was shown to be a consequence of a modified form of Spence's reaction scheme. Treatment of the reaction-vessel surface with hydrofluoric acid resulted in a
435
436
KINETICS OF COMBUSTION REACTIONS
very slow rate of pressure rise after an appreciable induction period. The presence of Hg vapor increased the rate of pressure rise, whereas washing the surface with HNOa gave the most reproducible results. Axford and Norrish 9, on the other hand, found, with the exception of a few rare experiments, no convexity toward the time axis in their observed pressure-time curves. Indeed, no observable diameter effect upon the rate of pressure rise was noted in their experiments. From the analytical results of a course-of-reaction series o f experiments, they showed that ( - dF/dt) i = 2 ( d p / d t ) i . In addition, the simple rate law, (-dF/dt)~ = KF~, was found to fit their data quite well, and an overall activation energy of 21 kcal/mol was' observed. They also found that CO and'H20 were the major products, whereas C02 and H2, in almost equal amounts, were minor products. No evidence for the presence of HCOOH or peroxides was found. The addition of N2 as an inert diluent inhibited the rate of pressure rise. Small quantities of oxygen were found to induce a slow decomposition of formaldehyde at temperatures where it is normally quite stable. These observations were successfully accounted for by these authors by means of a simple reaction scheme involving H atoms as the predominant chain carrier. More recently, Vanp6e 1~ studied this reaction at considerably higher temperatures (370-550~ and lower pressures (2080 mm Hg) than had been done previously. He used a fine tungsten filament (40~) as a temperature-sensing element in a thermal method for following the rate of reaction. He was able to demonstrate that the reaction followed a (-dF/dt)~ = KF~ rate law with an overall activation energy of 29.4 kcal/mol. He showed that an active intermediate, which he assumed to be PA, builds up to a steady state during an initia] induction period and is destroyed by a bimolecular process at the vessel wall. The mechanism suggested is a further modification of the B~ckstrom type of scheme first used by Spence. In a recent review of the available literature on formaldehyde oxidation, Lewis and von Elbe H came to the conclusion that PA is formed by a chain reaction in which HCO is the predominant chain carrier b. The PA thus formed is then asb They cite the work of Gorin ~, ~3and Style and Summers 14as evidence for the stability of the HCO radical (i.e., HCO = H -t- C O - - > 26 kcal). The photolysis of CH~O in the presence of D2 has been studied recently in this laboratory by Dr. L. J.
sumed to diffuse to the vessel wall and decompose into the products CO, H~O, C02, H2 and HCOOH. If this process is rapid enough, further gas-phase reactions of the intermediate performie acid with the chain carrier CHO are neglected, and the process becomes a simple steady-state reaction. If the vessel surface, on the other hand, is inactive toward peroxide destruction, gasphase reactions of the PA then play a role, and the kinetics become more complex. These authors thus attribute the simple kinetics observed by Axford and Norrish to use of a system which rapidly decomposed the intermediate peroxides. In particular, they suggest that the Hg vapor introduced into the Axford and Norrish reaction vessel, as a result of the use of heated mercury cutoffs, could have caused the rapid removal of PA. The more complicated kinetic data obtained by Spence, Snowden and Style, and Bone and Gardner was attributed to the use of systems in which the intermediate PA was not rapidly destroyed. The purpose of this present investigation is primarily to resolve the difference between the kinetic results obtained by Axford and Norrish on the one hand, and Spence, Snowden and Style, and Bone and Gardner on the other; second, further information is sought concerning the nature of, and the role played by, the intermediate peroxides; and finally, a mechanism is arrived at which is capable of explaining the main features of this reaction as observed to date.
Experimental MATERIALS
Formaldehyde: Monomeric liquid formaldehyde was prepared by the method of Spence and Wild 15 and stored at - 8 0 ~ Alpha-polyoxymethylene was used as the starting material instead of paraformaldehyde, since the former polymer contains less water than the latter. Oxygen: Oxygen, 99.6 per cent pure, was taken from a cylinder and condensed at liquid-nitrogen temperatures. The middle fraction was evaporated into a large evacuated bulb up to a pressure of about 1000 mm Hg and stored there until ready for use. APPARATUS
The reaction vessels used were 6-cm and 4-cm silica bulbs. The reaction vessel, wrapped in Schoen. His data provide further independent evidence for the stability of HCO (to be published).
437
GAS-PHASE OXIDATION OF FORMALDEHYDE
aluminum foil, was immersed in an electrically heated, thermocouple-controlled (•176 furnace. A slow stream of air was blown over the reaction vessel to keep the temperature gradient from one side of the bulb to the other within • ~~ Temperatures were measured with ironconstantan thermocouples, which were calibrated at the ice point and boiling point of water, and the melting points of Sn, Pb, and Zn. Pressures were measured by means of a Bodenstein quartz spiral manometer, which had a galvanometer mirror mounted on it. A conventional projection lamp and scale assembly could then be arranged to give an optical lever which resulted in an instrument sensitivity of 2.5 mm of scale deflection per 1.0 mm Hg pressure differential. The entire system could be evacuated to <10 -e mm Hg by means of a suitable combination of diffusion and mechanical pumps. Octoil-S was used as the diffusion-pump fluid. To avoid polymerization of formaldehyde, all lines and stopcocks in contact with formaldehyde were wrapped with nichrome wire and electrically heated to 110~ Dow-Corning high-vacuum silicone grease was found to be suitable as a stopcock lubricant under these conditions. Figure 1 is a schematic diagram of the apparatus. PROCEDURE
Mixtures of formaldehyde and oxygen were made up in the thermostated (110~ mixing vessel. The contents of the mixing vessel was then rapidly shared with the thermostated, thoroughly evacuated, reaction vessel, and the pressure-time dependence was measured with the quartz spiral manometer. At the end of each reaction, or after any period of reaction, the hot gases were quickly shared with, and isolated in, the thermostated (ll0~ sampling vessel. The pressure of this sample was quickly measured and then rapidly admitted into the freeze-out trap, which was immersed in liquid nitrogen. The permanent gases were then circulated through the sampling vessel, freezing trap system by means of the Toepler pump, to assure complete removal of the condensable gases. The noncondensable gases were then pumped back into the sampling vessel, and their pressure was measured. An ethyl iodide slurry (--ll0~ was then used as a refrigerant in place of the liquid nitrogen. Thus, a rough separation of C02 and unreacted formaldehyde was made possible 9. The C02 and traces of formaldehyde were then pumped back into the sampling vessel
and the pressure measured once more. The gases now in the sampling vessel were then pumped into the mass spectrometer sample bulb (SB). The sample bulb (SB) was then isolated, removed from the system, and analyzed mass spectrometrically for CO, C02, 02, H2, and traces of CH~O. In experiments 10A and 18A, the condensable products were scanned with the mass spectrometer. This was accomplished by opening the stopcock communicating between the freezing trap and the sample bulb (SB), removing the ethyl iodide slurry surrounding freezing trap, and immersing the sample bulb in liquid nitrogen. The sample bulb was again isolated and removed High vacuum pumps
t~
~ Monomer~c li:4uid CH 20reservoir LH"~-~0 2reservoir q- 7 . . . . . . . I
~F2 I I
)S.B
IR. V. - ~
l t_ . . . . FI
2
FIG. 1. Schematic diagram of apparatus. R.V., reaction vessel; S.M., quartz spiral manometer; MI., galvanometer mirror; M., mercury manometer; M.V., mixing vessel; S.V., sampling vessel; Fz, main furnace; F2 secondary furnace thermostatically controlled at 110~ F.T. freeze-out trap; T.P., toepler pump; S.B., mass spectrometer sample bulb. from the system, but this time it was kept immersed in the refrigerant until ready to be Opened to the evacuated mass-spectrometer inlet manifold and ionization chamber. In this way the possibility of detecting and identifying the unstable peroxides was enhanced. For those experiments where a fresh, clean silica surface was desired, the reaction vessel was removed from the apparatus, washed with concentrated HN03, thoroughly rinsed with distilled H20, placed back in the apparatus, and thoroughly evacuated for 24 hr at 350~ In all other experiments, the vessel was evacuated at 350~ for ~i-hr before each run. To reproduce the experimental conditions of Axford and Norrish, who used heated mercury cutoffs, mercury was added to the mixing vessel.
438
KINETICS OF COMBUSTION REACTIONS
Since the mixing vessel was thermostated at l l 0 ~ mercury vapor at a partial pressure of ~ 1 m m tIg was thereby introduced into the reaction vessel with the reaction mixture. Stoichiometry
With the above apparatus and procedure it was possible to measure the following parameters for this reaction system: PCH2O~ = the initial partial pressure of formaldehyde; Ap = the change in total pressure; t = time ; Po2~ = the initial partial pressure of oxygen; Po2 = the partial pressure of oxygen at time t; where &PO2 = P021 -- e 0 2 Pco = the partial pressure of CO at time t; PH2 = the partial pressure of H2 at time t; Pco~ = the partial pressure of CO2 at time t; and Peond = the partial pressure of the remaining constituents at time t, which are condensable at - 110~ The analytical d a t a of Bone and Gardner, as well as the results of this research, indicate t h a t in general the following overall reactions are required to account for the products of this reaction: CH20 + ~ 02 = CO + H20,
(a)
CH20 + O2 = CO2 + H~O,
ides remaining is v e r y small and (e) and (f) m a y be neglected. Case II(b): Same as II(a) above, but where the reaction is stopped sufficiently short of completion, so that the a m o u n t of peroxides present cannot be neglected. All of the six overall reactions must then be considered. F r o m a straightforward stoichiometric consideration of the overall reactions, unmeasured parameters, such a s A p c H ~ o ---- PcH2o~ - - PCH20 , PH20, PHCOOH, etc., can be calculated from the measured quantities in the following fashion:
Case I: ApCH2O = P c o + P c o 2 , and Pa2o = Pco
As a check upon the consistency of these results, one can compare the experimentally measured values with those calculated as follows: Apealc
=
~ (Pco +
Pcondcalc =
P H 2 0 + P C H 2 0 = PCH2Oi -- PH2 9
Case II(a): Ap CH20 = P c o + Pco~ + PHCOOH
(c) =
CH20 + ~ 02 = HCOOH (formic acid),
(d)
CH20 + 02 (e)
= HCOOOH (performic acid
PH2),
and
(b)
CH~O = Hz + CO,
+ P c o 2 -- PH2 9
or
PA),
PH2
+ 2Apo2 -- PC02 ;
PH20 = PCo + Pc02 -- PH2 ; and PHCOOH = PH2 + 2Apo2 -- PCO -- 2Pc02,
and
3CH20 +
O2
CO + (CH2OH)202 (dioxymethyl peroxide).
(f)
I t has been found necessary to assume two types of stoichiometry. T h e seemingly conflicting results of Axford and Norrish, as opposed to those of Spence, and Snowden and Style, etc., suggest just such a treatment. Case I: Peroxides are decomposed rapidly into CO, H20, C 0 2 , and H2, and no H C O O H is formed. Reactions (d), (e) and (f) can then be neglected (the conditions of Axford and Norrish). Case II(a): The peroxides decompose more slowly into CO, H20, CO2, H2 and H C O O H . If the reaction is allowed to go far (90-100 per cent) toward completion, then the q u a n t i t y of perox-
Once more, as a check upon the consistency of the scheme, one can calculate: Apcale = PCo + Pc02 -- Apo2, and Pcondc~lr = P H 2 0 , + PHCOOH +
PCH20 = PCH20~ -- PH~ .
Case lI(b): ApCH20
= P c o + Pco2 + PHCOOH + PPA + 2P(cH2OH)2Oz = 1/~ (3Apo2 + P c o -- Pco2 -- &p) + PH~ -- PPA; PH20 = ~/~ (Pco + Pco., + Apo2 + Ap) -- PH2 ; P(CH2OH)202 = 1~
( P c o + Pc02 -- :XP02 -- Ap);
439
GAS-PHASE OXIDATION OF FORMALDEHYDE
and P H C O O H -~- 2 P p A
= 1/~ (5Apo2 -- 3 P c o
-- 5 P c 0 2
a n d again, to verify t h e scheme, one can calculate: Peondcalc =
+ Ap) -4- P H
2
;
consistency of t h e
PH20 -4- PCH2O A- PHCOOH -I- PPA -~- P ( C H 2 O H ) 202
= PCH20~ + Apo2 + Ap -- PH2 -- P c o -- P c o 2 .
lar results were o b t a i n e d in e x p e r i m e n t s carried o u t in t h e presence of m e r c u r y vapor, which was i n t r o d u c e d in t h e m a n n e r described above. C u r v e 2 in Figure 2 is t y p i c a l of t h o s e e x p e r i m e n t s carried out in a silica vessel a f t e r making several successive runs w i t h o u t cleaning (hereafter referred to as a n aged vessel). T h e time i n t e r v a l for t h e initial convexity t o w a r d t h e time axis t e n d e d to increase s o m e w h a t w i t h increased vessel aging. T h i s effect is strongly r e m i n i s c e n t of t h e induction period p h e n o m e n a e n c o u n t e r e d so frequently in h y d r o c a r b o n oxidation. As will be shown below, this similarity is only superficial in t h a t
TABLE 1 (All Quantities in m m Hg at 337~ Experiment No . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Stoichiometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Vessel and surface condition . . . . . . . . . . . . . . . .
Observed q u a n t i t i e s At (minutes) . . . . . . . . . . . . . . . . . . . PCH2Oi . . . . . . . . . . . . . . . . . . . . . . . . Apo2 . . . . . . . . . . . . . . . . . . . . . . . . . . P co ........................... Pco2 .......................... PH2 . . . . . . . . . . . . . . . . . . . . . . . . . . . Apobs ..........................
Peondob, - . . . . . . . . . . . . . . . . . . . . . . Calculated quantities Apealc . . . . . . . . . . . . . . . . . . . . . . . . .
7A*
17E*
30t
8B:~
I
I
II(a)
II(b)
6-cm silica bulb, freshly cleaned with HNOa
4-cm silica bulb, contaminated with Hg vapor
6-cm silica bulb, aged surface
4-cm silica bulb, aged surface
32 127.4 50.7 84.0 23.2 13.6
20 97.5 30.3 56.8 8.0 2.8 28.8 89.5
35 93.4 45.3 62.1 12.4 16.6 29.0 76.4
3 161.6 34.3 29.5 26.6 38.0 11.9 113.7
29.8 95.6 62.0 64.8 ---
29.2 76.8 57.9 93.4 -20.3
-113.2 13.1 -85.0
45.8
103.0 48.8
Peondcalr . . . . . . . . . . . . . . . . . . . . . . .
113.8
.......................... Ap CH20 . . . . . . . . . . . . . . . . . . . . . . . . Apc~20 + P P n . . . . . . . . . . . . . . . . P~COOH . . . . . . . . . . . . . . . . . . . . . . . PHCOOH + 2 PPA . . . . . . . . . . . . . . P (CH2OH) 202 . . . . . . . . . . . . . . . . . . .
93.6 107.2 ----
PH2O
--
--
--
--
18.9
--
--
5.0
~
* Exp. 7A, 17E: (dp/dt)max = ( d p / d t ) ~ . t Exp. 3C: ( d p / d l ) m ~ = ( d p / d t ) t = 3 minutes. :~ Exp. 8B: (dp/dt)m~x = ( d p / d t ) t = 2 minutes. T a b l e 1 gives t h e a n a l y t i c a l results of four experim e n t s which are typical of t h e different stoichiometric conditions described above. Results
t h e r a t e a t which f o r m a l d e h y d e disappears is n o t slow d u r i n g this t i m e b u t is a c t u a l l y g r e a t e r t h a n in those e x p e r i m e n t s p e r f o r m e d in a clean vessel, or w i t h m i x t u r e s c o n t a m i n a t e d with m e r c u r y vapor.
THE PRESSURE-TIME DEPENDENCE
T w o t y p e s of p r e s s u r e - t i m e c u r v e s h a v e been o b s e r v e d . C u r v e 1 in F i g u r e 2 gives a typical e x a m p l e of those e x p e r i m e n t s carried o u t in a silica vessel freshly cleaned with nitric acid. Simi-
PRODUCTS CONDENSABLE AT -- 1 1 0 ~
T o o b t a i n specific i n f o r m a t i o n a b o u t t h e n a t u r e of t h e p r o d u c t s o t h e r t h a n CO, CO2, a n d H2, a s a m p l e of t h e p r o d u c t s c o n d e n s a b l e a t - l l 0 ~
440
KINETICS O F C O M B U S T I O N
w a s taken as described above and scanned by the mass spectrometer in the 0-100 mass range. The results for samples taken from an aged vessel and
/
y o
v """
REACTIONS
Gardner ~ and of Snowden and Styl@. Its comparative absence in the mercury-contaminated system agrees with the results of Axford and Norrish. The presence of 02 and CO2 in the sample taken from the aged vessel can probably be attributed to the breakdown of peroxides in the ionization chamber of the mass spectrometer. The unidentified species exhibited a mass spectrum extending over the 30-95 mass range. Small peaks at 62, 61 and 60 were observed and tentatively assigned to PA (with the loss of one and two hydrogen atoms). Since calibrating spectra for peroxides are not available, no particular peroxide species could be positively identified. The above facts, together with the previous observations of Bone and Gardner l, Snowden and Style s, and Axford and Norrish 9, must be taken as strong evidence for the peroxide destroying property of mercury vapor. COURSE-OF-REACTION
j IO
20 TIME,MINUTES
~
40
FIG. 2. Curve 1: Exp. ~7A; HNOs cleaned vessel; Po82o~ = 127.4 mm Hg and Po2~ = 79.3 mm Hg; T = 337~ 6-era silica bulb. Curve 2: Exp. $3A; aged vessel; PCH2Or = 127.6 mm Hg and Po2i -- 68.6 mm Hg; T -- 337~ 6-cm silica bulb. T,ABLE 2
Aged Vessel
Reaction Mixture Contaminated with Mercury Vapor
Experiment no . . . . . . . . . 10A 18A Temperature, ~ . . . . . . . 337 337 Time, rain . . . . . . . . . . . . . 5 5 PcH2o~ , m m Hg . . . . . . 175.1 176.0 Po2~, mm Hg . . . . . . . . . 80.9 80.7 Analysis, tool per cent CH20 (unreacted) . . . . . 11.7 5.5 H20 . . . . . . . . . . . . . . . . . . . 61.2 92.1 HCOOH . . . . . . . . . . . . . . . 3.2 Trace (<0.1) 02 . . . . . . . . . . . . . . . . . . . . . 10.0 0.0 COs . . . . . . . . . . . . . . . . . . . 1.4 0.0 Unidentified . . . . . . . . . . . 12.5 2.4
from a reaction mixture contaminated with mercury vapor are given in Table 2. In agreement with all previous investigators, it is found t h a t H~O is a major product. The presence of HCOOH in the aged vessel is in complete agreement with the observations of Bone and
EXPERIMENTS
As shown in Table 1, consistent analyses result when Case I stoichiometry is applied to a system with a vessel freshly cleaned with HN03 or to a mercury contaminated mixture. Case I I stoichiometry, on the other hand, yields consistent results when applied to a system consisting of an aged silica vessel. The significance of these facts is demonstrated by Tables 3 and 4. Table 3 gives the results of a course-of-reaction series of experiments for an aged vessel, whereas Table 4 gives a similar set of d a t a for a vessel contaminated with mercury vapor. For the vessel contaminated with mercury vapor, dp/dt reaches a maximum at the start of the reaction. Since Case I stoichiometry can be used successfully, peroxides and HCOOH are not present in significant quantities. Also, it is seen that (dp/dt)~ = ( ~ ) ( - - d F / d t ) ~ . These facts are in complete agreement with the data reported by Axford and Norrish. For the aged vessel, dp/dt does not reach a maximum until about 2.5 minutes after the start of the reaction. Also, note that (dp/dt)~ << ( ~ ) IdF/dtl~. Since Case I I stoichiometry must be applied to yield consistent results, the presence of HCOOH, PA and H O C H 2 - - O - - O - - C H 2 0 H is strongly indicated and their partial pressures are calculated in the manner described above. These calculations show t h a t during the early stages of reaction the concentration of (CH~OH)20~ builds up faster than either PA or HCOOH. During the first minute of reaction, (PHcooH T 2PrA) is shown to be very small.
441
GAS-PHASE OXIDATION OF FORMALDEHYDE
TABLE 3. VESSEL: AGED 4 CM SILICA BULB (All Q u a n t i t i e s in m m H g at 337~ Experiment No . . . . . . . . . . . . . . . . . Time, min
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
9B
9C
9D
1
3
.5
Observed quantities Z~po2 . . . . . . . . . . . . . . . . . Pco ................. Pco2 . . . . . . . . . . . . . . . . . PH2 . . . . . . . . . . . . . . . . . .
6.7 14.2 2.2 4.4 Apob . . . . . . . . . . . . . . . . . . . 2 dp/dt, m m H g / m i n . . . . . 8 Pcondobs 9. . . . . . . . . . . . . 140.6 Calculated quantities Pcondcalc 9 . . . . . . . . . . . . . 140.9 PH20 . . . . . . . . . . . . . . . . . 7.4 A p C H 2 0 -I- P P A . . . . . . . 25.9 PHCOOH q- 2PPA . . . . . . 0 P(CH2OH)20~ . . . . . . . . . . 4.8
9E
9F
10
20
35.9 55.8 5.3 11.4 5.4 3,6 124.9
51.3 73.5 7.5 15.3 I1,7 3.6 119.9
62.0 94.1 14.5 19.1 30.9 2,5 122,0
61.9 95.0 18.2 18.2 41,8 ,5 129,3
125.2 34.8 87.8 6.9 9.9
119.6 56.7 119.4 20.4 9.0
122.5 81.6 136.5 12.1 7.9
130.0 90.2 128.6 5.8 4.8
T = 337~ 155 m m Hg. Po:~ = 75 m m Hg, (dp/dt)m~x = 3,6 m m H g / m i n (between 2.5 a n d 8.5 rain a f t e r t h e s t a r t of t h e r e a c t i o n ) . PCH2Oi =
TABLE 4. V E S S E L : 4 CM S I L I C A B U L B . R E A C T I O N M I X T U R E C O N T A M I N A T E D W I T H M E R C U R Y V A P O R AT .4. P A R T I A L P R E S S U R E OF ABOUT 1 MM H G
(All Q u a n t i t i e s in m m H g a t 337~ Experiment No . . . . . . . . . . . . . . . . . . . . . . .
17A
17B
Time, min . . . . . . . . . . . . . . . . . . . . . . . . . . . .
1
3
1.6 5.3 1.5 1.5 3.3 3.3 93.5 3.4 95.3 5.3 6.8
Observed quantities Apo2 . . . . . . . . . . . . . . . . . . . . . . Pco ....................... Pco2 ...................... PH2 . . . . . . . . . . . . . . . . . . . . . . . Apobs ......................
dp/dt, ram H g / r a i n . . . . . . . . . P c o n d o b B. . . . . . . . . . . . . . . . . . .
Calculated quantities Apcalr . . . . . . . . . . . . . . . . . . . . . . Pcondcalc 9 . . . . . . . . . . . . . . . . . . PH20 . . . . . . . . . . . . . . . . . . . . . . ApCH20 . . . . . . . . . . . . . . . . . . .
17C
17D
17E
;
10
20
9.2 19.2 3.6 2.3 9.6 3.2 90.1
17.0 31.7 4.4 1.8 16.5 2.4 94.0
24.2 44.0 6.6 3.1 24.0 1.0 91.3
30.3 56.8 8.0 2.8 28.8 .3 89.5
10.7 94.4 20.5 22.8
16.8 97.0 34.3 36.1
23.6 94.1 47.5 50.6
29.8 95.6 62.0 64.8
T = 337~ 97.5 ram Hg. P021 = 100 ram Hg. PCH20~ =
(dp/dt)m~, = (dp/dt)~ = (1/2) ( - d F / d t ) ~ . 2
THE DEPENDENCE OF ( - d F / d t ) ~ UPON Po2~ AND PcH2O~
and
A x f o r d a n d N o r r i s h 9 h a v e s h o w n t h a t , for a m e r c u r y - c o n t a m i n a t e d s y s t e m (where P A is rapidly destroyed and never achieves significant* c o n c e n t r a t i o n s ) , (dp/dt)~ is i n d e p e n d e n t of P o ~
b e e n s h o w n h e r e t h a t (dp/dt)~ is n o t s i m p l y rel a t e d to ( - dF/dt)~. C o n s e q u e n t l y , simple m a n o m e t r i c m e a s u r e m e n t s do n o t suffice for a d e t e r m i n a t i o n of t h e initial rate. T o m e a s u r e
is p r o p o r t i o n a l to PcH20~ a n d = ( ~ ) ( - d F / d t ) ~ . F o r a n a g e d vessel, however, it h a s
442
KINETICS OF COMBUSTION REACTIONS
(-dF/dt)i in an aged vessel, it has been necessary to resort to the use of analytical data for shortduration experiments. Whereas this technique TABLE 5. VESSEL: AGED 4-CM S I L I C A BULB (All quantities in mm Hg at. 350~ Experiment No . . . . . . . . . . . . .
12B
Observed quantities PCH~O~. . . . . . . . . . . . 112.3 Po2~ . . . . . . . . . . . . . . . 59.7 Apo2. . . . . . . . . . . . . . 11.0 Pco . . . . . . . . . . . . . . . 17.4 Pco2 . . . . . . . . . . . . . . 2.7 PH2 . . . . . . . . . . . . . . . 4.4 Poo~dob,. . . . . . . . . . . 101.0 Calculated quantities Peo~a~l~ . . . . . . . . . . . 101.5 PH2O. . . . . . . . . . . . . . 9.1 P(CH~OH) 202. . . . . . . 3.3 ~pcH20 . . . . . . . . . . . . 26.7
12C
12D
112.2 lll.8 11.7 19.4 2.3 7.1 96.3
111.7 169.3 12.5 17.3 2.6 7.0 99.9
97.0 9.9 4.7 30.0
99.0 10.2 2.7 25.2
THE OVERALL ACTIVATION ENERGY
T = 350~ Run duration: 1.0 min (PHcooH = PeA = 0). 60 [
"
'
LEGENO '
'
/I
.
.o
'
Table 5 indicates that (--dF/dt)~ is independent of Po~ in an aged vessel. Previous investigators, working under various experimental conditions, are in unanimous agreement with this fact. Figure 3 gives the data for the dependence of (--dF/dt)~ upon PCH20~ 9 The results of Axford and Norrish 9 and the data obtained in this reseaxch for both mercury contaminated and freshly cleaned vessels are shown to be entirely in agreement. The data obtained for aged vessels, on the other hand, definitely show somewhat larger values for (-dF/dt)i but still exhibit the simple square law dependence upon PeH2O~, as shown in Figure 3.
,"
(-dF/dt)~ in an aged vessel was calculated from analytical data for 1-minute experiments for constant composition mixtures of CH20 and 02 in the temperature range 317 ~ to 377~ ([CH20]~ = [02]~ = 2.63 • 10-6 mol/cc). An Arrhenius plot of In (dF/dtl~ versus 1/T is given in Figure 4. The activation energy is 27.4 kcal/mol. Table 6 gives the activation-energy measurements of previous investigators. Only the Axford and Norrish ~ value of 21 kcal/mol is for the well-defihed condition of rapid peroxide destruction. The rather large spread among the other values can probably best be accounted for, at this time, by the use of different types of vessels and surfaces, resulting in different rates of PA destruction and different initial steady-state concentrations of PA. Mechanism
and Discussion
SUMMARY OF THE KINETIC
.
Z/zi
/A
I
L 3O
I~
,.
40
50
F 2 x 10 3 ( M M . H g } 2
(T
upon PcH2O, FIG. 3. Dependence of - (dF) ~ 3T7~ =
has several experimental shortcomings, such as the difficulty of obtaining precise determinations of At, as well as the inherent inaccuracy of measuring small quantities of products, the data given in Table 5 and Figure 3 do show consistent results.
DATA
The results of previous investigations, as well as those of this research, indicate that the following pertinent facts must be taken into account in any final choice of a mechanism for this reaction. (1) Mercury vapor and surfaces freshly cleaned with HN03 result in simple p-t curves where (dp/dt)ma~ = (dp/dt)~. The products are CO, H20, C02 and It2, and the available analytical data show that (dp/dt)~ = (~)(-dF/dt)~. The governing rate law is given by (-dF/dt)~ = K1F~, and the overall activation energy is about 21 kcal/mol. Inert diluents such as N2 are found to decrease the rate of reaction9. (2) For other systems (such as the aged vessel reported in this paper), the p-t curves can show a decided initial convexity toward the time axis, so that (dp/dt)ma~ # (dp/dt)~. During this initial
GAS-PHASE
OXIDATION
period of small dp/dt, formaldehyde reacts at a rate even faster than that observed in systems where (dp/dt)m~x = (dp/dt)i. In addition to the products CO, H20, CO2, H2, significant amounts of HCOOH, PA and (CH~OH)202 are detected, where (CH~OH):O~ is found in measurable quantities earlier than either HCOOH or PA. The - 16
d In Eact.= - R
-17
tually build up to relatively large concentrations. If the vessel conditions allow the second alternative, the PA initially reacts rapidly with CH20 to form (CH2OH)20~ ~, which can heterogeneously decompose to form HCOOH, C02 and 2H2 (reactions (8) and (9) below). In either case, however, (PA) is small initially, so that a simple stationary state may be assumed during the early stages of the reaction. The details of such a mechanism are given in the following section. PROPOSED MECHANISM AND ITS CONSEQUENCES
dF (~-tF)i = 27.4 T M
I. Main chain:
d/§)
1. F + O 2 --* HCO + ttO2 k~ chain inflating
\
2. HCO + 05 -~ HCO3 ks 3. HCO3 + F ~ chain continuing - , P A + HCO k31
\
_r
4'. HCO~ + 05 --* CO + HO2 + 02 k4,fchain terminating 4. HCO, + M CO +HO2 + M k.~J wall
5. HOs - 20 1.5
443
OF FORMALDEHYDE
1.6
1.7
1.8
10 3 T~
9 destruction k5
II. For the conditions of rapid PA destruction, the main chain is followed by: 6. 2PA
FIG. 4. Overall activation energy measurements, temperature range: 317-377~
active surface or Hg vapor
TABLE 6
9 2CO
+ 2H20 + 05
k61 t
Investigators
Activation energy kcal/mol
Fort and Hinshelwood . . . . . . . . . . . . . . Spence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Snowden and Style . . . . . . . . . . . . . . . . . Axford and Norrish . . . . . . . . . . . . . . . . . Vanp6e . . . . . . . . . . . . . . . . . . . . . . . . . . . . . This research (for an aged vessel)...
20.0 17.6 25.0 21.0 29.4 27.4
7. 2PA
~where k7 < ks
wall
" 2CO2 + 2H2 + O2 k7
III. For the aged vessel condition, the main chain is followed by: 8. PA + 2F ---* (CH2OH)2Os + CO 9. (CH2OH)sOs
observed rate law is again found to be of the form ( - d F / d t ) ~ = K2F2~ (where Ks = 1.4 K~ at 337~ with an overall activation energy of 27.4 kcal/mol. These experimental facts are readily interpreted if one assumes that PA is formed by a simple, homogeneous, free radical chain involving CHO and CHO3 as the major chain carriers. The PA thus formed is then either rapidly destroyed at the wall or by mercury vapor to form the products CO, H20, COs and H2 ; or it can even-
J ks
wall
9 HCOOH + COs + 2Ha ks
6'. 2PA inactive surface
9 2 C O + 2 H 2 0 + 02
wall
7'. 2PA
1
kv wherek7
9 2CO2 + 2H2 + O2 k7
[
)
< k~p < k6
c Evidence for the reaction between Ctt20 and peroxides to form hydroxyperoxides ;s given by G. H. N. Chamberlain and A. D. Wal:~h, Third Symposium on Combustion, Flame and Explosion Phenomena, The Williams & Wilkins Co., Baltimore, 1948, pp. 375-382.
444
KINETICS OF COMBUSTION REACTIONS
For the conditions of rapid destruction of PA (I and II) the rate of disappearance of CH~O is given by
dF dt =
---
equation (6) becomes
(dp)
dF dt
---=
k,(HCO,)(F).
(1)
Now, with the usual steady-state assumptions for (HCO) and (HCO3), it is readily shown that (HCO,) =
kl(O~) (F)
(2)
k4'(02) --~ k 4 ( M ) '
and equation (1) becomes --d-t = k4,(O2) + k,(M) (F)~
klk3 (M) (03)
(F)%
(3)
For initial rates in the absence of inert diluents, equation (3) becomes
klk~
- - - ~ , = 7 4 , (F)~
(4)
which is just the type of rate dependence experimentally observed. Therefore, E~:t = E~ -4- Ea E4' = 21 kcal/mol. This activation energy is consistent with an approximate bond energy estimate of AHl -{- AH3 - AH4; = 10 kcal exothermic~, ~7 Applying steady-state considerations to (PA), it follows that 2(k6 + kT)(PA) ~ = b3(HCO3)(F).
Comparison of equation (8) with equation (4) yields
which again is seen to be in agreement with the result given by experiment. For the aged vessel conditions (I and I I I ) the rate of disappearance of CH~O is given by
dF = dt
---
( d- ~F/)
k3(HCO3)(F) + 2ks(PA)(F) 2.
(5)
(dP) =3(ks+k~)(PA)2+k,,(HC03)(02) ,
(6)
- k~(HCO)(O~). From the steady-state condition for (HCO3)
(10)
2k,(PA),(F)~.
(11)
k,k3(F)~
2kr(k~,+ ~) O. (12)
For (PA)~ small, it follows that, to a second-order approximation, 2ks(PA),(F)~ = ~
(F)~ (13)
[2
k~k3 k4, 4(ks, ~ + ~k,) + ks(F)~
"'"
1
Substituting equation (13) into equation (11), the initial rate for the aged-vessel conditions becomes
( d- ~F ) , = ~klb3 (F)~ (14)
[ 3 - k,k3 k,, 4(k., 2 + 2kT) -tka(F)~
1 ""
From the experimental results given in Figure 3, the quantity in the brackets is seen to be approximately equal to 1.4 for the vessel conditions reported here. DISCUSSION
k4(HCO~) (O~) -- k~(HCO) (O2) =--k3(HCO.~)(F),
klk3 (F)~ +
, = ~
ks(F)~ (PA)~ + 2(ks, + k,) (PA),
In the absence of inert diluents, the initial rate of pressure rise is given by
and from equation (5)
(8)
The stationary state condition for (PA)~ is given by
k4' + k4 - -
(dF)
(F)~.
In the absence of inert diluents, the initial rate is given by using equation (2):
klk3(O~) -
klka
~-[ , = ~k3(HCOa)(F), = ~ ~
k~(F)(02) + k3(HCO3)(F).
Assuming long chains, and since 1 is chaininitiating, kl(F)(O2) can be neglected relative to the chain continuing step 3. Therefore,
dF
3(k6 -I- kT)(PA) 2 = ~ ks(HCO~)(F),
(7)
Whereas the above mechanism provides a satisfactory explanation for most of the observations encountered for this reaction, there are
GAS-PHASE OXIDATION OF FORMALDEHYDE
several results reported by previous investigators which do not fit directly into the scheme. Spence noted that over extensive surfaces of powdered glass, the reaction occurred slowly and almost exclusively to form C02 and H20. This fact can be understood in light of the suggested mechanism, if one assumes that in the presence of extremely large glass surface to volume ratios, the main chain I is followed predominantly by 10.
PA
extensive surface
~ CO2 ~ H20
k~0
wall
HCO
9 destruction
k~
or
HCO3
12.
wall
* destruction
k12 ,
thereby resulting in a slower rate. Another significant observation that should be amenable to interpretation in the light of the proposed mechanism is that a slow, low-temperature decomposition of CH20 to CO and H2 can be induced by traces of oxygen. 9 The following reaction sequence can qualitatively account for this observation: 1.
13. 11.
F -~- 02 ~ H C O -~- HO2
HCO ~- F ~ CO -t- H~ ~ HC0 HCO
wall
* destruction
kl
k13 kll
and 5.
HO2
wall
, destruction
suggesting this problem and, second, with Drs. R. Klein and L. Schoen, for participating in the stimulating discussions that provided an ideal atmosphere for this work. Also, the author would like to express his appreciation to A. G. Sharkey and the mass-spectrometer group at the Bureau of Mines, without whose cooperation this research could hardly have been carried out in its present form. REFERENCES 1. BONE, W. A., AND GARDNER, J . B. : Proc. Roy.
in lieu of 6 and 7, or 8 and 9. Also, the chain lengths are considerably decreased by the increased likelihood of 11.
445
k5
where 13 is assumed to be much slower than 2 but, in the absence of appreciable quantities of oxygen, becomes rate determining. Thus, the suggested reaction scheme provides a consistent framework from which the diverse and seemingly conflicting observations encountered in the formaldehyde oxidation reaction can be understood. A more complete understanding of the problem, however, must wait upon an accurate knowledge of the heterogeneous decompositions of (PA) and (CH2OH)~O2, as well as further information concerning the detailed kinetics of the reaction between PA and CH20 to form the hydroxyperoxide.
Acknowledgments The author wouid like to take this opportunity to thank Drs. B. Lewis and G. von Elbe, first, for
Soc. (London), A15~, 297 (1936). 2. NORRISH, R. G. W., AND FOORD, S. G. : I b i d , A157, 503 (1936). 3. BONE W. A., AND HILL, S. G . : Ibid., A12g, 434 (1930). 4. PEASE, R. N.: Chem. Rev., 21,279 (1937). 5. FORT, R., AND HINSHELWOOD, C. N . : Proc. Roy. Soc. (London), A129, 284 (1930). 6. SPENCE, a . : J. Chem. Soc. 649 (1936). 7. BXCKSTROM, H. L. J.: Z. Physik. Chem., B25, 99 (1934). 8. SNOWDEN, F . F . , AND STYLE, D. W. G. : T r a n s . Faraday Soc., 85, 426 (1939). 9. AXFORD, D. W. E., AND NORRISH, R. G. W.: Proc. Roy. Soc. (London), A192, 518 (1948). 10. VANP~.E, M.: Bull. Soc. Chim. Belg., 62, 285 (1953). 11. LEwis, B., AND VON ELBE, G.: Combustion, Flames and Explosions of Gases, pp. 94-106. New York, 1951. Academic Press, Inc. 12. GORIN, E.: Acta Physicochim. (U.R.S.S.), 9, 681 (1938). 13. GOR1N, E.: J. Chem. Phys., 7, 256 (1939). 14. STYLE, D. W. G., AND SUMMERS, D.: Trans. Faraday Soc., 42, 388 (1946). 15. SPENCE, R., AND WILD, W. : J. Chem. Soc., 338 (1935). 16. PAULING, L.: Nature of the Chemical Bond. Cornell Univ. Press, 1945. 17. GLOCKLER,G.: J. Chem. Phys., 19, 124 (1951). DISCUSSION BY M. VANP]~E
In the mechanism proposed by Dr. Scheer, the initiating step is attributed to the reaction: HCHO + O2 = HO~ + CHO If this reaction is the only initiating step, then there is no induction period. The author claims that this is the case in the presence of mercury vapor. However, it will be seen from Figure 1, which represents the combustion of formaldehyde in the presence of mercury vapor, that the use of single pressure-time curves is not sufficient to establish conclusively the absence of an induction period.
446
KINETICS OF COMBUSTION REACTIONS
I t is this which has led us to develop a more sensitive technique* in which t h e reaction velocity is measured directly b y t h e small t e m p e r a t u r e rise due to the heat l i b e r a t e d b y the reaction. T h e AT curve t h u s o b t a i n e d u n d o u b t e d l y has its origin a t zero, (dP/dt)o = O, a l t h o u g h the AP curve shows only a smooth inflexion when AT passes t h r o u g h a well characterized m a x i m u m . T h e r e is thus no d o u b t as to the existence of a definite induction period, b o t h in the presence and in the absence of m e r c u r y vapor. We must conclude therefore t h a t the reaction is catalyzed b y a long-lived i n t e r m e d i a t e . T h e obvious compound for this role is performic acid. The slightly modified symbolic m e c h a n i s m previously proposed b y Snowden and Style: X + F = 2X
(1)
2 X = p r o d u e t s reaction
(2)
is in perfect agreement with this conclusion a n d we have shown* t h a t it also fits r e m a r k a b l y well all the q u a n t i t a t i v e kinetie data. For step (1), we proposed the following scheme: PA --, HCO -{- HO~ HCO + O~ -~ HCO~ HCO + F -~ HCO -t- PA HCO3 --~ wall
initiation ["~
;propagation termination
AUTHOR'S REPLY The t h e r m a l m e t h o d of following the rate of reaction devised by Dr. Vanpde is a r a t h e r interesting approach to the ever present problem of kinetic m e a s u r e m e n t s in complex reaction systems. However, this novel technique is open to question on the following grounds. Using the formaldehyde-oxygen reaction as an example, it has been shown, not only in this research b u t in the work of Bone and G a r d n e r , Spence, and Snowden and Style, t h a t , in the absence of mercury vapor, spurious surface catalytic effects are always present. Consequently, the use of a fine t u n g s t e n wire to follow small local t e m p e r a ture changes, which in t u r n are i n t e r p r e t e d as i n s t a n t a n e o u s reaction rates, is questionable unless it can be conclusively shown t h a t the t u n g s t e n surface c o n t r i b u t e s no catalytic a c t i v i t y to any phase of the reaction. Evidence in t h e literature* shows t h a t oxygen is strongly chemisorbed upon t u n g s t e n (probably as O atoms). Could not some of t h e AT observed by Dr. Vanp~e be a t t r i b u t e d to a surface reaction between the adsorbed oxygen a t o m s and the s u b s t r a t e formaldehyde? The question of t h e existence of an i n d u c t i o n period for this reaction m u s t u l t i m a t e l y be dedF t e r m i n e d from knowledge of - ~ during t h e
With regard to the decomposition of performic acid, it is i n t e r e s t i n g to consider the combustion of a mixture c o n t a i n i n g a great excess of formaldehyde (Fig. 2). After all the oxygen is consumed, the formation of performic acid is a u t o m a t i c a l l y suppressed and the only possible reaction is the decomposition of the peroxide. The curve representing this decomposition definitely corresponds to a bimolecular process a n d may t h u s be considered as direct e x p e r i m e n t a l evidence of step (2). T h e absence of any vertical fall at point A shows f u r t h e r t h a t the f o r m a t i o n of performic acid from formaldehyde and oxygen (step (1)) is thermoneutral, and t h a t the h e a t generated by the reaction is due only to the decomposition of the peroxide (step (2)).
initial stages of the reaction. F r o m the analytical m e a s u r e m e n t s given in this m a n u s c r i p t as well as those obtained by Axford and Norrish, no such effect has been observed, e i t h e r for clean vessels or in the presence of mercury vapor. In the aged vessel, a true i n d u c t i o n period m a y indeed be present (see Table 3), since the PA c o n c e n t r a t i o n can build up and react with F by something like reaction (8). In agreement with Dr. Vanp~e, it should be emphasized t h a t the S-shape of the 5p-t curve is not conclusive evidence for the existence of an i n d u c t i o n period, since it has been shown here not to be simply related to the true course of reaction u n d e r conditions inactive to rapid PA d e s t r u c t i o n (i.e., in an aged vessel).
* M. Vanpde: Bull. Soc. Chim. Belg., 62, 285, (1953); 62, 661 (1953).
* I. L a n g m u i r and D. S. Villars: J. Am. Chem. Soc., 53, 486 (1931).