898
Langmuir 1988,4, 898-903
tures significantly lower than in the absence of the catalyst, resulting in incorrect speciation between the organic and elemental carbon. When a thermal/optical method is used, premature oxidation can be detected and corrections made. Nevertheless, the presence of catalyst in the sample results in unnecessary complication in the analysis and should be avoided. Acknowledgment. This work was supported in part by the National Science Foundation, Grant CPE-81-17288.
Additional support was provided by the National Center for Intermedia Transport Research (EPA Grant CR812771) and the State of California Grant, Engineering and Systems Analysis for the Control of Toxics. S. K. Friedlander wishes to acknowledge a senior Humboldt prize from the German government during the course of this research. We also wish to acknowledge helpful discussions with Jay Turner. Registry No. C, 7440-44-0;Na, 7440-23-5; quartz, 14808-60-7.
Soot Oxidation in Fibrous Filters. 2. Effects of Temperature, Oxygen Partial Pressure, and Sodium Additives Chiao Lint and Sheldon K. Friedlander* Department of Chemical Engineering, University of California, Los Angeles, Los Angeles, California 90024 Received July 10, 1987. I n Final Form: February 12, 1988 Surface reaction rate constants, k , for the oxidation of soot particles in filters were determined as a function of temperature, oxygen partial pressure, and soot and sodium.1oadings. Values of k for glass fiber filters measured by visual observations of samples in a box furnace agreed with values measured by using the optical monitoring method for low loadings. Catalytic quartz filters were prepared by depositing a uniform layer of Na2C03on the fibers of quartz filters. The catalytic behavior of the coated filters was similar to that of the glass fiber filters with the reaction rate depending on the rcltio of sodium to soot loading. At high sodium carbonate to soot mass ratios (>244), values of k for a number of Na-quartz filter samples fall on a single line on the Arrhenius diagram with an apparent activation energy of 22.2 kcal/mol. Since the rate is not limited by sodium supply, it is probable that the reaction is kinetically controlled. For samples with low catalyst to soot ratios, the higher activation energies indicate the reaction is probably influenced not only by the kinetics but also by the catalyst supply and/or diffusion. The reaction rate was proportional to the oxygen partial pressure for both catalyzed and noncatalyzed soot oxidation. The overall rate equation follows the shrinking core model and can be written as dM/dt = -Aond 2 N P ~exp(-E,/RT), , where the preexponential factor, Ao, and the activation energy, E,, are functions ofthe sodium/soot ratio for glass fiber and Na-quartz filters. The catalyst loading of glass fiber filters is equivalent to about 400 pg/cm2 of Na2C03,even though the true Na 0 content is 740 pg/cm2. Values of the preexponential factor range from 8.8 X lo5to 2.35 X lo8 pg/(s.m%-Torr),while the corresponding activation energies range from 21.2 f 0.7 to 32.6 f 1.1 kcal/mol. For noncatalyzed soot oxidation, the apparent activation energy is 32.5 f 1.0 kcal/mol and A. is 3.8 X lo7 pg/(s.m2.Torr).
Introduction Soot is a colloidal form of elemental carbon containing some fraction of organic materials. The ignition temperature of diesel soot, as reported by Enga et al.,l is about 665 "C. Catalysts can be used to oxidize soot a t a lower temperature and a faster rate. Examples are metal salt/oxides of Na, K, Li, Cs, Pb, Pt, Mn, Cr, Co, and Cu, which catalyze the oxidation/gasification of graphite/coal char / ~ o o t . ~ - ~ In a study of the oxidation of soot in fibrous filters,1° we found that alkali metal compounds in glass fibers catalyze the oxidation of deposited soot. The alkali metal compounds appeared to move from the fibers over the soot surface where they catalyze the reaction. As a result, soot particles are rapidly oxidized even though many of them are not in direct contact with the fiber surface. This increases the total oxidation rate of the deposited soot. The rate of reaction depends on temperature, soot loading, and the density of deposition on individual fibers. The de-
* Author to whom correspondence should be addressed. 'Current address: PTD,Intel Corporation, Hillsboro, Oregon 97124. 0743-7463/88/2404-0898$01.50/0
pendence on soot loading probably results from the insufficient supply of catalyst from the smaller, heavily loaded fibers. Since oxygen is the other reactant, its partial pressure must also play a role in the reaction. The object of this study was to gain a better understanding of how these parameters influence the reaction and to derive an expression for the rate of reaction of soot deposited in filters. Experimental Section In a previous study,'O the rate of soot oxidation was measured with an optical measurement system consisting of a He-Ne laser, a photodetector, and a high-temperature tube furnace. The (1) Enga, B. E.; Buchman, Em. F.; Lichtenstein, I. E. SAE Paper 820184 1982.
( 2 ) Wood, B. J.; Sancier, K. M. Catal. Rev.-Sci. Eng. 1984, 26, 233. (3) Otto, K.; Lehman, C.; Bartosiewicz, L.; Shelef, M. Carbon 1982,20, 243. (4) McKee, D. W. Carbon 1970, 8, 623. (5) McKee, D. W.; Chatterji, D. Carbon 1975, 13, 381. (6) McKee, D. W. Fuel 1982, 62, 170. (7) Amariglio, H.; Duval, X.Carbon 1966, 4, 323. (8) Wen, W. Y. Catal. Rev.-Sci. Eng. 1980, 22, 1. (9) Cox, J. L. Clean Fuels from Coal; Symposium 11, 1975; p 271. (10) Lin, C.; Friedlander, S. K. Langmuir, preceding paper in this issue.
0 1988 American Chemical Society
Soot Oxidation in Fibrous Filters residual soot loading was monitored as a function of time. The dependence of the average reaction rate on initial soot loading indicated that reaction takes place on all the surfaces of soot. However, this method is not suited for high soot loadings (>18.7 pg/cm2). Also, to use this technique, a calibration curve relating soot loading to the light attenuation is needed for each type of filter. In the present study, we have used a simpler measurement technique based on the model derived in the earlier study. The method requires only a small piece of sample each time. In this way a set of data can be obtained by using different punches of the same filter sample. Three types of fibrous filters were used. The quartz filters (Pallflex, type 2500QAO) and glass fiber filters (Whatman, EPM 2000) were previously described.*O The third type is the sodium carbonate impregnated quartz filters (Na-quartz filters). The composition of the glass fiber filters is fixed. In the previous study, the dependence of the reaction rate on catalyst loading was implied by the dependence of the reaction rate on the soot loading. To study the effect of sodium loading on the reaction, a technique was developed for uniformly coating the desired amount of sodium on fibrous filters. Sodium carbonate was chosen for this study to permit comparison with results obtained previously with glass fiber filters, for which sodium compounds in the glass are mostly responsible for the catalytic activity. Since quartz filters are relatively inert toward the oxidation of soot and can withstand high temperatures (>lo00 "C), they are potentially useful as substrates for catalyst deposition. The deposition was carried out by immersing a quartz fiber filter in a Na2C03(Mallinckrodt, analytical reagent) solution for about 30 s. The concentration of the solution ranged from 0.01 to 0.165 M. The soaked fiiter was held in the air for a few minutes to allow excess solution to drain. The filter was then placed in a ceramic crucible and dried in a furnace (Thermolyne,Dubuque IV) at 360 "C for 4 h. After cooling, the quartz filter with its fibers coated with sodium carbonate was ready for use. The mass of Na2C03deposited in the filter is calculated by multiplying the solution concentration by the amount of solution absorbed. The amount of solution absorbed by a filter is determined by weighing a solution-soaked filter and subtracting the weight of the dry filter. The Na-quartz fiiters used in this study contained from 45 to 740 pg/cm2 of sodium carbonate. If the deposit is uniform on fibers of average diameter 0.7 pm, the corresponding thickness of the Na2C03layer would range from 10 to 165 8. To determine its temperature resilience, a Na-quartz filter with 450 pg/cm2 of Na2C03was heated at 980 "C for 2 h. The heat-treated sample as well as an unheated piece of the same filter was examined with a scanning electron microscope (SEM) (Cambridge, Stereoscan 250) to see if the high-temperature treatment had brought about any structural change to the filter. The sodium distribution over the fibers before and after the treatment was also studied. The composition of some fibers was analyzed by using the SEM-equipped energy dispersive X-ray spectrometer (EDX). The method of soot generation and deposition in the filters, the determination of the soot loading, and the characterization of the deposition pattern in the filters have been reported.l0 To measure the rate of soot oxidation, pieces of filter were removed with a 5-mm-diameter punch and placed in a preheated crucible which was then placed in a box furnace (Thermolyne, Dubuque IV)set at a constant temperature and static pressure (1atm). The time for the soot to react, tf, was taken to be the time required for the filter sample to turn white. As shown in the following section, tf can be related to the reaction rate constant. Most of the punches were made at the perimeter of the central 35-mm soot-deposited area so each piece tested had both a dark and a white region. This gave the observer a good contrast for comparison purposes. The difference in color between an area with a soot deposit of 0.05 pg/cm2and a clean area can easily be distinguished. With 0.05 pg/cm2 as the lowest distinguishable soot loading by eye, the maximum error in tf due to observation inaccuracy is 8.3% for a 0.6 pg/cm2 sample and 0.1% for a 50 pg/cm2 sample. Each value of tf measured was usually based on the oxidation of several sample pieces. The first run determined the approx-
Langmuir, Vol. 4, No. 4, 1988 899 imate time of reaction, while the subsequent runs served to pinpoint this value and double-check its accuracy. The error associated with the determination of tf is estimated to be about 5% or less. The time of reaction, tf,was measured at temperatures ranging from 266 to 650 "C for glass fiber filter samples with soot loadings between 2.2 and 48.3 pg/cm2. Measurements with quartz filters were made at the soot loading of 0.62 pg/cm2, with reaction temperature ranging from 460 to 700 "C. After a value of tf was determined, the furnace temperature was changed and the process repeated with another piece of the same sample. In this way, the reaction time for a particular sample was measured as a function of temperature. A number of soot samples on quartz, glass, and Na-quartz filters were studied. Details of the soot loading (and catalyst loading for Na-quartz filters) are reported in the next section. The dependence of reaction rate on the partial pressure of oxygen was studied by heating the samples in a quartz tube that was placed in a tube furnace (Lindberg, 54357). Total pressure was 1atm, and totalflow through the quartz tube was maintained at 0.25 L/s. The gas-phase composition in the quartz tube was changed from 9.1 to 159 Torr of oxygen by regulating the feed rates of purified air and nitrogen. Reaction time was monitored by visual observation. The reaction rate of a glass fiber sample with 0.83 pg/cm2soot was studied at 400 "C, and that of a quartz filter sample with 0.62 pg/cm2 soot was studied at 525 "C.
Results and Discussion Calculation of Reaction Rate. Earlier results'O indicate that when sufficient sodium is present reaction takes place on all the surfaces of deposited soot. At sufficiently low soot loadings, the rate of reaction can be assumed to be uniform over all the surfaces of soot and is described by the shrinking core mode1:l' dM/dt = -kTdp2N
(1)
where M is the residual soot loading, d, the particle diameter, N the total number of elementary spheroids (a fixed number for a given sample), and k the surface reaction rate constant. Also
M = (s/6)dP3Np
(2)
where p is particle density, which has a value of 2 g/cm3. Substituting eq 2 into eq 1 gives d(d,)/dt = -2k/p
(3)
Assuming the contact area between particles is negligible, the individual particles will remain spherical during the course of reaction. Integrating with the initial and final conditions, d , = d,, at t = 0 and d, = 0 at t = tf, yields 2ktf d,, = (4) P
tf is the total reaction time and d , is the initial particle diameter, approximately 20 nm as determined by electron microscopy. Rearranging eq 4 yields the rate constant k:
Thus the rate of reaction can be determined by measuring tf, the time it takes for the individual particles to disappear. The value of k is a function of temperature, partial pressure of oxygen, and catalyst loading. These fundamental relationships are explored in this paper. Testing of Na-Quartz Filters. In general, impregnation of fibrous filters with a salt from a solution may (11) Levenspiel, 0. The Chemical Reactor Omnibook; OSU Book Store: Corvallis, OR,1985.
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Figure 2. Same sample as in Figure 1, but heated at 980 O C for 2 h. The fine fibers have disappeared. Remaining fibers were fused together as a result of melting at the fiber surfaces. result in nonuniform deposition on the substrate. Salt tends to concentrate near the junctions where fibers cross because of accumulation of the impregnation solution as a result of surface tension effects. This did not occur in our studies with sodium carbonate solutions. T h e filters containing deposited sodium carbonate (Na-quartz filters) were white and looked similar to a clean, uncoated quartz filter. Observed with a SEM (Figure l),the fibers were round and smooth. The d e m i t formed a film on the fiber surface, and Na2C03particles were not observed. After heating at 980 "C for 2 h, the Na-quartz filter became stiff and brittle. Some melting was observed under an optical microscope (Nikon, Optiphot). The heated sample was examined by using the SEM (Figure 2). Melting was probably caused by the presence of Na2C03 on the fiber surface, which reacted with the silica fibers either directly or through the formation of sodium oxide to form sodium silicate, which has a lower softening temperature. After the fibers cooled they fused together to form a rigid network. Small fihers melted because of the higher Na2C03to silica ratio. Of the remaining fibers, the
smallest was 0.6 pm in diameter compared with 0.2 pm before the sample was heated. Most of the remaining fibers were larger than 1.5 pm, and their shape was irregular. EDX analysis indicated that the distribution of Na2C03from fiber to fiber was uniform. Small fibers are efficient particle collectors but may not be able to provide sufficient catalyst to the surfaces of the deposited soot. This could result in a low soot oxidation rate. Therefore, loss of the small fibers may result in a lower filtration efficiency but an increased capability for oxidizing deposited soot. The magnitude of these effects will depend on the concentration of the impregnation solution and the temperature t~ which the filter was heated. Effect of Temperature on Soot Oxidation: Glass a n d Q u a r t z Fiber Filters. The reaction rate constants, k , for low-loading soot samples on glass and quartz fiber filters are ploted versus the reciprocal temperature in Figure 3. For each sample, k forms a straight line on the Arrhenius plot. For different samples, values of k and the slopes of the lines are different. The rate constants k generally decrease while the activation energy (slope of line) increases with increase in soot loading. The values of k at low soot loadings, measured by Lin and Friedlander,'O ranged from 12.8 to 17.2 ~ g l ( 5 . m a~t) 400 "C and from 101 to 168 pg/(s-m2) at 525 "C. These values are shown in Figure 3 for comparison. The agreement is good, which indicates that the two methods of measuring k are comparable at low soot loadings. The activation energies for samples with initial soot loadings of 2.2 and 3.4 pg/cm2 are 21.2 0.7 and 24.0 0.4 kcal/mol, respectively. Also shown in Figure 3 are the measured k values for soot oxidation on quartz fiber filters. These uncatalyzed rate constants are as much as 2 orders of magnitude lower than the rates of reaction on glass fiber filters under similar conditions. This shows that glass fibers are indeed catalytic for the reaction of soot. The activation energy for the uncatalyzed reaction is 32.5 1.0 kcal/mol. In his summary paper, Walker et al.12 reported activation ener-
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(12) Walker, P. L., Jr.; Rusinko, F.,Jr.; Austin, L. G. Aduonces in Catalysis: Eley e t al.. Eds.;Academic: New York, 1959; Vol. XI.
Langmuir, Vol. 4, No. 4,1988 901
Soot Oxidation in Fibrous Filters
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Lin and Friedlander
902 Langmuir, Vol. 4, No. 4, 1988
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For a given soot loading, the oxidation rate of soot with a 225 kg/cm2 Na2C03loading was 3-6 times that of soot with only 45 pg/cm2 of Na2C03 (Figure 6). The same behavior was observed a t a higher soot loading of 11.4 pg/cm2. The reaction rate of soot with 740 pg/cm2 sodium carbonate is about 1order of magnitude higher than that with 45 pg/cm2 of sodium carbonate. Thus, except for samples with a very high catalyst loading or very low soot loadings, the rate constant is approximately proportional to the amount of sodium deposited, MNa,and inversely proportional to the soot loading, M . Overall Rate Equation. The reaction rate of soot was determined for oxygen partial pressures between 9.1 and 159 Torr. A first-order dependence of the reaction rate on oxygen concentration was found for both the glass and quartz fiber samples (Figure 7 ) . On the basis of the experimental results, the rate constant k is given by
k
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(6)
Therefore, the rate of oxidation of soot is given by dM/dt = -Ao7rd,2NPo, exp(-E,/RT)
(7)
The surface reaction rate constant k is given by
k = A Z O ,exp(-E,/RT)
(8)
For reactions of soot on quartz fiber filters, the value of A , is 3.82 X lo7 kg/(s.m2.Torr). For reactions on glass fiber and Na-quartz filters, A , ranges from 0.88 X lo6 to 5.8 X lo6 pg/(s.m2-Torr)for low soot loading samples. For high-loading samples, the rate of soot oxidation reaction depends not only on the kinetics but also on the catalyst supply and/or its diffusion. The activation energies and the values of A , are higher in this case. E, and A , are plotted as functions of the ratio of catalyst to soot loading for the Na-quartz filter in Figure 8a,b. The catalyst loading for Whatman EPM 2000 glass fiber filters is approximately 400 pg/cm2 of Na2C03. The rate constant, k , is independent of the particle size of the deposited soot. However the fiber size in a filter may influence the value of k . Small fibers are efficient particle intercepters. When the size of the soot particles or clusters is large, the small fibers may collect a disproportionately large amount of soot and decrease the local catalyst to soot ratio. Therefore, for filters composed primarily of fine fibers, a sample with high soot loading
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Figure 8. Variation of activation energy (a, top) and preexponential factors (b, bottom) with mass ratio of sodium carbonate to soot averaged over the filter. The local value of this ratio is the lowest at the front end of the filter where most of the soot deposits. Data are for Na-quartz filters.
may have a lower k value than a low-loading sample with the same catalyst to soot ratio.
Conclusions Surface reaction rate constants for soot deposited in fibrous filters were calculated from the time of reaction measured by visual observation. At low soot loadings, the results compared well with reaction rate constants determined by laser observation.1° The visual observation gave somewhat higher rates for samples with high soot loadings. This may have resulted because the soot in a high-loading sample reacted rapidly near the end, which would shorten the reaction time and increase the rate constant. Soot in quartz filters impregnated with sodium carbonate reacted at a much faster rate than soot on untreated quartz filters. The reaction rate depends on the ratio of sodium to soot loading a t low loadings. For each sodium loading, the temperature dependence of the data followed an Arrhenius plot. When the soot loading is high, the reaction rate may be diminished a t a given temperature because of insufficient supply of catalyst. With high sodium loadings, the same soot oxidation rate is maintained even a t high soot loadings. The soot reaction rate was proportional to the first power of the oxygen partial pressure and, over much of the range studied, to the particle surface area. Data for the oxidation rate of soot could be correlated by an equation of the Arrhenius form. dM/dt = -Ao7rd,2NPo, exp(-E,/RT)
Acknowledgment. This work was supported in part by the National Science Foundation, Grant CPE-81-17288. Additional support was provided by the National Center for Intermedia Transport Research (EPA Grant CR812771) and the State of California Grant, Engineering and
Langmuir 1988,4,903-906 Systems Analysis for the Control of Toxics. S. K. Friedlander wishes to express his appreciation for a senior Humboldt award from the German government during the course of this research.
Nomenclature A0
d,
k k
kB
m
preexponential factor, hg/(s.m2.Torr) diameter of elementary soot spheroids, nm initial diameter of soot spheroids, 20 nm activation energy, kcal/mol surface reaction rate constant, Tg/(s-m2) Boltzmann constant, 1.38 X 10- erg/mol mass of an oxygen molecule, 5.31 X g
903
M
soot loading per cross sectional area of the filter, hg/cm2 catalyst loading per filter cross sectional area, Mcat mg/cm2 molecular weight of carbon, 12 g/mol MWC N number of soot spheroids in filter/cm2 Avogadro number, 6.023 X loz3mol-' partial pressure of oxygen, Torr time, s total reaction time, s temperature, K density of the elementary soot spheroid, 2 g/cm3 Regist;ry No. Na, 7440-23-5;02,7782-44-7; quartz, 14808-60-7.
Study of Photoreaction Processes of PDA Langmuir Films K. Ogawa," H. Tamura, M. Hatada,? and T. Ishihara Semiconductor Research Center Matsushita Electric Ind. Co., Ltd., 3-15, Yagumo-Nakamachi, Moriguchi, Osaka, 570 Japan, and Osaka Lab for Radiation Chemistry, Japan Atomic Energy Research Institute, 25-1, Mii-Minami Machi, Neyagawa, Osaka, 572 Japan Received July 10, 1987. I n Final Form: February 12, 1988 Pentacosadiynoic acid (PDA) monolayers on aqueous subphase containing Ca2+and on water subphase (Langmuir (L) film) were irradiated by UV light, and the reaction of the L films was monitored by surface area change under constanbpressurecondition and by surface pressure change under constant-area condition. The highest rate of reaction was obtained at a surface area of ca. 28 A2/molecule, indicating that the PDA molecules have the most suitable molecular arrangement in the monolayer. The change of the optical absorption spectra of the L films also supports this finding. It was also found that the arrangement of the molecules in the Langmuir-Blodgett (LB) film is related to that of the monolayer from which the LB film was prepared.
Introduction It is known that diacetylene derivatives polymerize to give a polymer having a linear chain of conjugated double bonds when they are exposed to UV light in the solid state. Polymers of this type have attracted interest because they have special properties such as nonlinear optical properties or anisotropic electric conductivity.14 The polymerization of Langmuir-Blodgett films of diacetylene derivatives having a hydrophilic group on one end was also investigated by many researchers to elucidate the reactivity in relation to the arrangement of molecules in the Langmuir-Blodgett (LB) The photoreactivity of diacetylene derivatives whose substituents were replaced with different groups that will affect the molecular arrangement in the LB films has been further studied.*" Studies are also in progress on a chemical change of LB films of different diacetylene derivatives, because the chemical change induced by heat, pressure, ultraviolet (W)light, etc., is accompanied by a color change from blue to red which can be applied to optical recording media.12 It is of interest to study the reactivity of the LB films of these compounds prepared at different surface pressures to the results of the monolayer studies. It is of more interest to know the photochemical reactivity of the monolayers spread on an aqueous subphase as a function of molecular density, which is related to the conformation and arrangement of the molecules in the monolayer, because the molecular density can be measured directly with the surface pressure-area curve of monolayer films on an aqueous subphase (L film). Osaka Labs for Radiation Chemistry. 0743-7463/88/2404-0903$01.50/0
In this paper, we wish to report the change of the reactivity of pentacosadiynoic acid (CH3(CH2)11C=CC=C(CH,),COOH; PDA) L films under irradiation of UV light at different surface pressures and molecular areas and some preliminary results on PDA LB f i i s prepared at two different surface pressures. PDA was selected as film substance in this study, because the photopolymerization of this compound had already been investigated in detail on LB films.
Experimental Section Materials. PDA was obtained from Wako Pure Chemical Industries, Ltd., and used as received. Calcium chloride was also obtained from the same manufacturer and used after removal of (1)Wilson, E. G. J. Phys. C 1974,252, 655. (2) Bloor, D.;Ando, D. J.; Preston, F. H.; Stevens, G. C. Chem. Phys. Lett. 1974, 24, 407. (3) Bloor, D.; Chance, R. R. Polydiacetylenes; Martinus Nijihoff: Boston, MA, 1985. (4) Mehring, H.; Roth, S. Electronic Properties of Polymers and Related Compounds; Springer-Verlag: Berlin, Heidelberg, 1985; p 234. (5) Tieke, B.; Graf, H.-J.; Wegner, G.; Naegele, B.; Ringsdorf, H.; Banerjie, A.; Day, D.; Lando, 3. B. Colloid Polym. Sci. 1977,255(6), 521. ( 6 ) Bubeck.. G.: . Tieke.. B.:. Weaner, . G.Ber. Bunsen-Ges. Phys. Chem. 1982; 86, 495.
(7) Bubeck, C.; Tieke, B.; Wegner, G.Mol. Cryst. Liq. Cryst. 1983,96, 109.
(8) Tieke, B.; Lieser, G.; Weiss, K. Thin Solid Films 1983, 99, 95. (9) Wegner, G.J. Polym. Sci., Polym. Lett. Ed. 1971, 9, 133. (IO) Wegner, G. Pure Appl. Chem. 1977,49,443. (11) Nakanishi, H.; Matsuda, H.; Kato, K.; Thocharis, C.; Jones, W. Polym. Prepr. Jpn. 1984, 33, 2491. (12) Sandaman,D.J.; Tripathy, S. K.; Elman, B. S.; Samuelson, L. A. Synth. Metals 1986, 15, 229.
0 1988 American Chemical Society
