HAROLD S. JOHNSTON Department of Chemistry and Chemical Engineering, Stanford University, Stanford, Calif.
Photochemical Oxidation of Hydrocarbons Photochemical reactions in smog produce free radicals which constantly recombine through bimolecular collisions. Steady-state partial pressures and half lives are estimated for various degrees of sunlight absorption
IN
the Los -4ngeles atmosphere, Haagen-Smit ( 5 ) and others have demonstrated that photochemical reactions are important in producing eve irritation, plant damage, and ozone. The components of these reactions are regarded as oxygen and organic materials in general, especially hydrocarbons and their oxidation products. .4 prominent suspect as the sensitizer. that is, primary light absorbing material, has been nitrogen dioxide; it is known to absorb stronglv over most of the solar spectrum at sea level; mixtures of nitrogen dioxide, air, and hydrocarbons in sunlight have been shown to yield most of the symptoms of smog. Also, Haagen-Smit has demonstrated that large amounts of biacetyl in sunlight produce ozone among the reaction products. Since the photochemical decomposition of biacetyl yields free radicals, it is strongly implied that free radicals and oxygen interact in some way to yield ozone. Extensive literature on thermal and photochemical oxidation of organic materials gives considerable insight into the mechanisms probably involved in these cases. Scope-limiting Considerations
I n the atmosphere of a large industrial city, the components fall into two classes Middy separated so far as partial pressure is concerned. O n one hand, there are the natural components of the atmosphere (nitrogen, oxygen. water vapor. carbon dioxide, and the noble gases) present in large amounts; on the other there are the contaminant gases (such as hydrocarbons and their oxidation products, oxides and oxy-acids of nitrogen, ozone) present in small amounts, about 10-7 * atm. In most laboratory experiments on photochemical and thermal reactions, all reactants are usually present in comparable amounts, and pressures of to 1 atm. are typical. Thus, mechanism studies carried out under ordinary laboratory conditions
1488
must be re-evaluated in terms of quite different partial pressures. These considerations lead to a considerable simplification of the problem.
radical chain reaction ( J ) , i t is essential to consider these processes in the photochemical oxidation of hydrocarbons in the atmosphere.
Reaction Rates
l i g h t Absorption
An over-all reaction usually consists of a sequence of steps; the order of the over-all reaction may be. for example, 0, 2 ' 3 : 1, 4/3, 3'2, 2>3, or it may have no order at all. HoLvever, the individual steps are either first order, second order,or third order. If a second or higher order reaction occurs at a convenient rateminutes or hours under ordinary laboratory conditions-it Ivould require weeks or more to occur at 10-7 * I atm.; thus such reactions may be dismissed from consideration. Conversely) if a second order reaction has a half life of minutes or hours a t these loiv pressures, its half life at normal laboratory pressures would be a matter of seconds or much less. Thus, only the second or higher order reactions that need to be considered a t reactant pressures of 10-71' atm. are those regarded as very fast or instantaneous at 1 atm. Reactions of interest which may be dismissed as too slow at lois pressures include
The first law of photochemistry is that the only light photochemically active is that Lshich is absorbed. It is intuitively obvious that the primary absorption in the lower atmosphere must be caused either by a very iveak specific ahsorption by a substance present in large amounts (such as oxygen: nitrogen, water vapor), or by an intense specific absorption by a substance present at pressures around 10-7 atm. .I quantitative evaluation of what is meant by very weak and intense will be given. The Beer-Lambert law is -dI'dL = bCL or I = I , exp (-BCL) Ivhere I , is the intensity over a narrow range of wave length falling on a substance of concentration C moles per liter, and I is the intensity after passing through a path length. L . The molar extinction coefficient, E , is equal to b,'2.303. The fraction of I , absorbed per centimeter near the surface of the earth is
',I*,
2N0 0 2
+
0
2
= 2x02
+ hydrocarbons = products
(1)
(2)
.I few fast reactions of possible interest include (3, 6: 7 , 72, 73)
+ SnOa + + NO2 + NO + N20j = 3NO2 KO + NO2 + HzO 2HN02 + olefins = products 2x02
NO
0
3
=
0
0 3 =
0 2
=
0 8
2
(3)
(4)
(5)
(6) (7)
In addition to these over-all reactions there is a large class of fast reactions, the chain-carrying and chain-terminating steps of free radical chain reactions in general ( 7 7 ) . Since it is well established that oxidation of hydrocarbons is a free-
INDUSTRIAL AND ENGINEERING CHEMISTRY
F = -dI/I,dL = SCexp ( - b C L j (8) \There L is the equivalent height of the absorbing substance. The optical density, D:is defined as loglo 13/1 = bCL/2.303
(9)
For each value of L there is a unique relationship ber\\-een F and D . The equivalent height (STP) of the natural atmosphere is 8 X l o 5 cm. I n Figure 1 there is a plot of surface absorption per centimeter us. optical density of the entire atmosphere. The maximum occurs at D = 0.43. and the value of E to give this absorption, based on oxygen as the absorbing substance, is 6 X 1 mole-cm. It can be seen that extremely weak specific absorptions are the most important; strong absorptions lead to removal of virtually all radiation
A I R POLLUTION high in the atmosphere. I n terms of weak absorptions, oxygen, the rare unstable molecule, 0 4 , nitrogen, and water vapor absorb in the visible spectrum. Of these absorptions, the well-known atmospheric bands of oxygen (6) are the most intense, giving a n optical density for the entire atmosphere in the range of 0.1 to 2 or just bracketing the maximum in Figure 1. Contaminants in the atmosphere may have high local concentrations and short optical paths, but the typical case to be considered is that where the contaminant at a pressure of 30 X 10-8 atm. is spread over the entire sub inversion layer in Los Angeles, that is, about 1 X 105 cm. in height. For this situation, the maximum value of F is 3.6 X 10-6 cm.-', and the value of E which gives this maximum is 330 1:mole-cm. In Figure 2. a plot is given of molar extinction coefficient. E, us. the fraction absorbed per centimeter, F , for a substance present a t 30 X 10-8 arm. At the top of the graph, the extinction coefficient for thiee substances, acetone a t 310, acrolein at 385, and nitrogen dioxide at 400 millimicrons. is marked \tith vertical lines. For extinction coefficients below j0. the intensity of absorption is limited by Lieakness of absorption a t these low partial pressures, not by absorption high in the inversion layer. For nitrogen dioxide, this situation no longer applies. A path length of l o 5 cm., or about 3000 feet, cuts out about one third of the incident light a t 400 millimicrons for 30 X l o w satrn of nitrogen dioxide. In other \vords, such pressures of nitrogen dioxide should be clearly visible. A rough comparison of radiation absorbed by atmospheric oxygen and acetone. acrolein, and nitrogen dioxide a t 30 X 10-8 atm. is given in Figure 3
IO6 F cm-1
0
.5
I .o
I.5
2.o
2.5
D Figure 1 . Fraction of incident radiation in a narrow wave-length band absorbed per cm. path length a t sea level, F, as a function of optical density, D, offered a t this wave length by the entire atmosphere
where fraction absorbed per centimeter at sea level is plotted against wave length (the oxygen bands are much more numerous and more narrow than shown in the figure). If each point in Figure 3 in multiplied by I , and by quantum yield, the area under the resulting curves ivould give the rate of primary photochemical processes. If a is defined as the fraction of the total solar energy reaching sea level which is absorbed per centimeter of path length then for the four substances in Figure 3, a is approximately: nitrogen dioxide, 10-7; acrolein, 10-8; acetone, 10-'0; and oxygen, 10-6. For the contaminants spread over a path of 105 cm., these absorptions add
ACETONE (310) ACROLEIN (385)
NO,
(400)
up to the following percentage of total sunlight: nitrogen dioxide, 1;: ; acrolein, 0.1%; and acetone, 0.001%. I n such terms it is easy to see that a maximum possible fraction of sunlight absorbed per centimeter is about 10-5, that is, total absorption of sunlight. A practical maximum figure for cy is about 10-6 per centimeter. Since oxygen itself absorbs to an extent of over 10per centimeter, this number \vi11 be regarded as a good lower limit to the primary absorption. Thus, it is shown that the intensity of sunlight itself sets a n upper limit on the amount of absorption of radiation, absorption by molecular oxygen in the red region of the spectrum sets a lower limit, and certain expected contaminants such as nitrogen dioxide and acrolein (but not acetone) fall in between these t\vo limits. Thus values of a between 10-8 and 10-7 will be regarded as the best estimate for the situation in Los Angeles.
Mechanisms
E (LOGARITHMIC
SCALE )
Figure 2. Fraction of incident radiation in a narrow wave-length band absorbed per cm. path length a t sea level, F, as a function of molar extinctions coefficient, E, for contaminants at 30 p.p.h.m. distributed over lowest kilometer of the atmosphere
Skeleton Mechanism. hlost stable molecules (exceptions include nitric oxide and nitrogen dioxide) contain a n even number of electrons, and such molecules will be abbreviated as M. hlost free radicals contain an odd number of electrons, and any free radical or molecule with a n odd number of clectrons will be abbreviated by X . The symbol R will be reserved for hydrocarbon freeradicals containing a n odd number of electrons, and thus R is a special class of X . It is profitable to write down general or skeleton mechanisms in which especial attention is focused on odd or even numbers of electrons. VOL. 48, NO. 9
SEPTEMBER 1956
1489
R
1
'
30 x 1.0-8 ATM,
+
ROz
=
0 2
(21)
Under exceptional circumstances high yields of peroxides, u p to 86% ( 9 ) , can be obtained
R0z
+ RH = R02H + R
(22)
Usually the products obtained are those expected from oxy-free radicals rather than peroxy-free radicals, and steps such as (7)
02
2R0* = 2 R 0
+
(23)
0 2
or perhaps ( 2 )
ROz
~
8000
00
WAVE LENGTH ANGSTROMS Figure 3. Fraction of incident radiation absorbed per cm., F, for four illustrative substances: acetone acrolein, or nitrogen dioxide at 30 p.p.h.m. and for atmospheric oxygen. The absorption lines of oxygen are more numerous and narrower than those shown, but the approximate relation between wave length and F i s as indicated
Visible or near ultraviolet solar radiations, abbreviated as hv, produce the following types of reactions
+ hu X + .I' M + h, M* M + hv = .W' + M " X + hu = X' + .VI M
=
=
(10)
(12)
(13)
=XI+
(14)
If the product radical reacts to reproduce the original radical, there is a chain
X'+.
=x+
(15)
X" + X"
1 490
X" +
(16)
+
=
M
+
(17)
If two or more free radicals are about equally nonreactive in their particular environment, a recombination process may occur, such as
X'+X''+
= M +
(18)
At very high temperatures or very high energies reactions such as
X
+M
=
X'
+ X" + X"
(19)
may occur, and this process constitutes branching of chains. At atmospheric temperatures, one expects chain branching processes to be rare. Thus, for a typical case, absorption of sunlight leads to two free radicals, a series of secondary reactions may occur between molecules and free radicals which leaves the number of free radicals (odd electrons) unaltered, and finally two relatively nonreactive radicals recombine to end the chain. Oxidation Mechanisms. In the thermal oxidation of hydrocarbons (4, the chain reaction is initiated a t temperatures about 200 to 300' C. by the slow extraction of a hydrogen atom by molecular oxygen
If the product radical is itself highly reactive in its environment, it will undergo a rapid transformation to another free radical
=
The relatively nonreactive radicals (provided their association product is a stable molecule) will recombine
(11)
Reactions such as Equation 12 are rather rare. Excited molecules, M*, can lose their energy by light emission, collisional deactivation, or by chemical reaction. A survey of actual examples shows that in most cases, absorption of sunlight by molecules leads to the production of free radicals; the quantum yields used in the calculation of CY in the preceding section were those for the production of free radicals either by the primary process or a closely coupled secondary process. Subsequent reactions of free radicals can be generalized as follows: The most active radicals will attack molecules or decompose to produce a different radical
x+
X' +
RH
+
0 2
=
R
+ HOz
(20)
T h e hydrocarbon free-radical reacts with oxygen with extreme rapidity
INDUSTRIAL AND ENGINEERING CHEMISTRY
+
= RO
0 2
+
0 3
(24)
have been proposed for the formation of RO. The peroxy-free radical is unusual in that its association product, ( R O Z )is~ : not a stable substance. Recombination can occur only after time-consuming processes such as Equations 23 or 24. Though oxy-free radicals can in principle combine to form peroxide, their usual chain-terminating step is the rather slou~process of hydrogen atom transfer (7). RO
+ RO
=
aldehyde
+ alcohol
(25)
Thus, in the oxidation of hydrocarbons, each avenue which leads to recombination of radicals has some feature which slows it down. Although methyl radicals (77)
CH3
+ CHa
CzHc
=
(26)
recombine upon practically every collision (k = 4 X l O I 3 cc./mole-sec.), homogeneous recombination of radicals produced in oxidation are expected to be much slower than this. If P is defined as the factor by which a given recombination rate constant is lower than that for Equation 26, then for the reaction ( I I ) ,
+ NO = products
CHB
(27)
the value of P is lo4. For oxidation reactions, recombination of radicals is expected to occur much less often than every collision so that values of P, as small as l o 4 are to be expected. Subsequent calculations will be made with values of P taken as 1, and lo4. Somewhat arbitrarily it will be assumed that values near to are the best estimate. Specific Photochemical Mechanisms. When nitrogen dioxide acts as the primary light ab5orber (70). the processes are NO2
+ hv = NO + 0 below 4000.4. 0 + = NO + = SOz + 0 + Hz0 = OH + OH OH + RH = R + H10 0
0
3
2
0
8
0 2
(28)
(29) (30) (31) (32)
Interactions of the free radicals, R, with oxygen follows the sequence given by
A I R POLLUTION Equations 21 through 24. Reaction 30 is so fast (7, 2) that Reactions 28, 29, and 30 alone cannot produce high yields of ozone (about 10 p.p.h.m. if nitrogen dioxide is 100 p.p.h.m.). Considerably larger yields of ozone might be expected if nitric oxide is removed in some other way, for example RO 4- NO ROz
=
RON0
+ NO = RO + KO2
(33) (34)
If an aldehyde or ketone is the primary light absorber, there is a direct split into free radicals. For example ( 7 I ) , (CH3)2CO
+ hu = CHBCO+ CHs
(35)
with subsequent reactions of the free radicals with oxygen. When molecular oxygen absorbs red sunlight, an excited molecule is formed
+
0 % hv =
0 2 *
(36)
Radiation or loss of energy by collision is a forbidden (6) process for the excited oxygen molecule, and its lifetime is lo7 or 108 times as long as is normal for electronically excited molecules. Thus, it is expected to last long enough to react with hydrocarbons a t room temperature (2) Os* R H = R HOz (37)
+
+
This reaction is expected (it has not been observed) by analogy with the reaction given by Equation 20. The important thing to emphasize is that virtually every photochemical process has a high probability of producing free radicals.
Order of Magnitude Calculations The quantity a has been defined as the total fraction of solar energy normally reaching sea level that is absorbed per centimeter of path to produce free radicals directly or indirectly. Previous considerations have set limits on a between 10- and 10-6 cm.-’. The quantity P has been defined as the average fraction of radical-radical collisions which lead to recombination or destruction of free radicals. As noted before, the value P = 1 gives a firm upper limit, but generally one cannot place a firm lower limit. A reasonable estimate for P is to lob2 and calculations will be made with P as low as lo4. With the preceding assumptions and mechanisms, many quantities of interest can be calculated. The intensity of sunlight is taken as 2 cal./sq. cm. per minute or 4.7 X 10-7 einsteins/sq. cm. per second. The rate of production of free radicals is then 4.7 X 10-7 a moles/cc. per second. The rate of destruction of free radicals is kP[X]a
where k is the collision constant. Of course, each of these quantities is a n average, and a n order of magnitude estimate is all that is sought. The maximum rate of any photochemical nonchain branching process is simply Rate of primary photochemical processes = 4.7 X 10-7 01 moles/cc.sec. (38) By making the steady-state approximation and equating rate of production of free radicals with their rate of destruction, one gets the steady-state concentration of free radicals,
[XI= (1.6 X
10-20 a/P)I mole/cc.
(39)
and the time TI12 for the recombination of half these free radicals (since this is a second-order reaction the half life increases as the radical pressure decreases).
T1 2
=
(1.4 X 10’ aP)-’ sec.
(40)
Units are changed to parts per hundred million (atm. X 10-8) for steady-state radical pressures and parts per hundred million per hour for rate of primary photo-chemical processes. There follows a matrix of calculated values of steadystate partial pressure in parts per hundred million of free radicals for various values assumed for cy and P: Steady-State Partial Pressure at c y =
P
10-6
1 0.1 0.01 0.001 0.0001
0.3 1 3 10
30
a = io-:
c y =
0.1 0.3 1 3 10
0 . 0 3 0.01 0.1 0.03 0.3 0.1 1 0.3 3 1
10-8
a =
10-9
Similar data for half-life of radicals in seconds with respect to recombination are : Half time a t
1 0.1 0.01 0.001 0.0001
0.3 1 3 10 30
1 3 10 30 100
3 10 30 100 300
10 30 100 300 1000
The maximum rate of primary photochemical processes in parts per hundred million per hour, which could be regarded as maximum possible rate of production of ozone, is: 01
10-6 10-7 10-8 10-9
Rate, p.p.h.m., hr. 8000 800 80 8
The range of values inside each of these tables is rather large. The value of N is capable of a fairly wide range depending on the identity and partial pressure of the primary radiation absorbers, which in turn depends on meteorological variables as well as source strengths. The value of P is expected to depend on the identity of the hydrocarbon or other organic compound being oxidized. As mentioned before, the order of magnitudes which seem most reasonable for the Los -4ngeles situation is about 10-8 to 10-7 for a and to 10-2for P. With these estimates, the steady-state partial pressure of free radicals is 0.3 to 3 p.p. h.m., or about 1 to 10% of the partial pressure of the molecular contaminants. In view of the great reactivity of free radicals, especially in initiating polymerization chains with olefins, these figures suggest that free radicals themselves are important primary irritants. The half life with respect to recombination is 10 to 100 seconds, or long enough for extensive penetration of free radicals into buildings. (Note added in proof) : Calculations were made for the relative rate of radical-particle collisions for 10 particles per cc., each with a 0.1-micron radius. If every collision of a free radical with a particle results in its removal and eventual destruction, the three entries in the lower right-hand corner of the tables of steady-state pressures and half lives would be excluded.
literature Cited (1) Bell, E. R., Raley, J. H., Rust, F. F.! Seubold, F. H., Vaughn, W. E., Discussions Faraday SOC. 10, 242 (1951). (2) Cadle, R. D., Johnston, H. S., Proc. 2nd Kat. Air Pollution Symposium, Pasadena, Calif., 1952. (3) Cadle, R. D., Schadt, C., J . Am. Chem. Sod. 74, GOO2 (1952). ( 4 ) Discussions Faraday SOC. “Oxidation,’’ 184, 199, 204, 215, 222, 233, 239, 249, 267, 272 (1945). ( 5 ) Haagen-Smit, A. J., IND.ENG.CHEM. 44,1342 (1953). (6) Herzberg, G., “Spectra of Diatomic Molecules,” pp. 167, 278, Van Nostrand, New York, 1950. ( 7 ) Johnston, H. S., Crosby, H. J., J . Chem. Phys. 19, 799 (1951); 22, 689 (19541. \ - - -
I -
Johnston, H. S., Yost, D. M., J . Chem. Phys. 17, 386 (1949). Kooijman, F. L., Rec. trav. chim. 66, 5, 205, 491 (1947). Noyes, Jr., W. A., Leighton, P. 4 . , “Photochemistry of Gases,” Reinhold, New York, 1941. Steacie, E. W. R., “Atomic and Free Radical Reactions,” Reinhold, New York, 1954. Wayne, L. C., Yost, D. M., J . Chem. Phys. 19,41 (1951). Wilson, D. J., Johnston, H. S., J . A m . Chem. SOC.75, 5763 (1953). RECEIVED for review December 22, 1955 ACCEPTEDMarch 5, 1956 VOL. 48, NO. 9
SEPTEMBER 1956
149 1
