Catalysis of NO Decomposition by Mn304 Harry W. Edwards" Department of Mechanical Engineering, Colorado State University, Fort Collins, Colo. 80523
Roy M. Harrison Department of Environmental Sciences, University of Lancaster, Lancaster LA1 4YQ, England
A flow reactor and chemiluminescent NO, analyzer were used t o measure the rate of decomposition of NO catalyzed by Mn:jOl in purified air a t atmospheric pressure. The decomposition is first order in NO with Arrhenius parameters of Eapp= 12.4 f 0.2 kcal mol-' and In A ( s ) = 9.29 f 0.24. Because M n 9 0 4 is the primary Mn-containing combustion product of the fuel additive methylcyclopentadienylmanganese tricarbonyl (MMT), the catalytic properties of MnsO4 should be considered in evaluating the potential air quality problem posed by MMT.
T h e effects of methylcyclopentadienylmanganese tricarbony1 ( M M T ) on automotive emissions and air quality have attracted recent attention (1-5). M M T has been used commercially as an antiknock additive for unleaded gasoline. In limited amounts, M M T may be compatible with lead-sensitive automotive emission control devices. Additionally, M M T is currently used as a ccmbustion improver and smoke inhibitor in fuel oil. Because of the toxic properties of manganese compounds, a National Academy of Sciences panel recommended objective evaluation of the potential air quality problem associated with increasing use of Mn-containing fuel additives ( I ). T h e present investigation is concerned with the heterogeneous catalysis of NO decomposition by Mn:j04, the primary Mn-containing cornbustion product of M M T ( 2 , 4 ) .Tests with both simulated ( 4j and actual ( 5 )automotive exhaust indicate that M M T can profoundly affect emissions. Several mechanisms have been suggested for observed decreases in NO emissions when M M T is added to gasoline. These include suppression of NO formation during combustion, catalysis of NO reduction by CO, and catalysis of NO decomposition. The purpose of this preliminary communication is to report the rapid catalytic decomposition of NO by Mn:jOl under conditions which exclude potentially confounding effects due t o other reactions. Despite favorable thermodynamic considerations, the rate of the homogeneous decomposition of NO to NZ is very slow below 1000 "C (61. Activation energies reported for the homogeneous decomposition range from 64 to 86 kcal mol-'. Although the study did not include Mni304,Shelef e t al. (7) identified several metal oxides as catalysts for the heterogeneous decomposition with apparent activation energies ranging from 11 to 33 kcal mol-'. Due to very low preexponential factors, however, these catalysts were judged unsuitable for practical decomposition of NO formed in combustion 0013-936X/79/0~1:~-0673$01 .OO/O
processes. Moreover, inhibition by 0 2 was observed in some cases. The catalytic activity of Mn:j04 toward NO in the presence of 0 2 is therefore of potential interest in air pollution control, in addition to the need for information on the effects of MMT. Experimental The flow reactor shown schematically in Figure 1 was used to measure the rate of catalyzed NO decomposition in purified air as a function of temperature. NO was introduced into the air carrier gas a t atmospheric pressure from a cylinder of 6.2 f 0.1 ppm of NO and 0.3 f 0.1 ppm of NO2 in Nz supplied by B.O.C. Special Gases (U.K.). Introduction of atmospheric constituents which might confound the experiment was prevented by passing laboratory air through a five-canister purification train followed by a particulate filter (Figure 1).The NO flow rate was varied from 102 to 620 cmJ min-' in approximately 100 cm3 min-l increments to give 6 inlet NO concentrations ranging from approximately 0.2 to 1.2 ppm a t each of 4 temperatures. A mechanical pump was used to draw the gases through the reactor a t a constant combined flow rate of 3.2 L min-I. Flow meters were calibrated with air a t 25 "C. T h e reactor was a 2.5-cm i.d. L-shaped Pyrex tube fitted with high-vacuum O-ring seals. This configuration facilitated removal of the reactor for charging with M n j 0 4 following measurements in the absence of sample, e.g., blank determinations. After initial pumpdown with oil diffusion and mechanical vacuum pumps, the system was found capable of maintaining a steady pressure of approximately 1 X 1O-l Torr, the lower detection limit of the Pirani gauge. The temperature in the center of the 0.262-L reaction zone containing M n j 0 4 was maintained within k0.5 "C of each setting by a Pye Ether 1793B/10 electronic controller and insulated heating tape. The controller was calibrated with a Comark electronic thermometer with the probe placed inside the reactor with air flowing a t 3.2 L min-l. Measurement of the temperature a t various positions within the reaction zone showed that ternperature differences did not exceed 2 "C. Gas mixtures entering and leaving the reactor were analyzed for NO and NO:! with a Thermo Electron Model 14D chemiluminescent NO-NOp-NO, analyzer. The analyzer was calibrated with a standard sample of NO and NO? in N2 and cross-checked for NO:! with a gas permeation tube apparatus previously calibrated by a gravimetric technique. The instrumental calibration was rechecked before and after concentration measurements a t each temperature. The calibration was found essentially drift-free; concentration mea-
@ 1979 American Chemical Society
Volume 13, Number 6, June 1979 673
2 TO VACUUM PUMPS
3 5
TO AIR PUMP, DISCHARGE
5
I/
E
M
I 7
C
H I-
-C
/.
--E
d
E D + = E q
, I
TO At- LYZER C
P
INLET A A . NO-in-Na
Cylinder 8.Regulator and Valve C. Teflon Tubing D. Rotometar E. Swagelok Coupling F. Pyrax Mixing Chamber G. Pyrax Tubing H. Glass Wool I. 0-Ring Coupling J. 2.5cm ID Reactor K. Mn3 04 L. Fa/Con Tharmocouple Robe M.Pye Contrdkc
N. Asbestos Insulation 0.Heating Tape P. Pirani Gauge, Indicator Q. Desiccant R,S. Activated Charcoal U. 13x Mol. Sieve V. 5 A Mol. Slave W Glass Fibre Filter X. T w n Tubing 1-9. Vacuum Stopcocks
Figure 1. Schematic diagram of
surements were reproducible within 0.01 ppm. Mn;{O4 was prepared by heating reagent-grade MnOz (B.D.H. Chemicals Ltd.) in air for 3 days a t 1000 "C (8).X-ray diffraction analysis confirmed the formation of Mn304 and the absence of any other crystalline phases; both d spacings and relative intensities were in good agreement with accepted values (9).The material produced by this method is heterogeneous in appearance and consists of predominantly coarse, dark brown particles flecked with smaller purplish-black particles. The powder patterns for the two types of particles
apparatus
were indistinguishable, however. A very crude estimate of the specific surface area was made by sieving the sample into four size categories. Assuming cubic geometry and using the bulk density of 4.86 g cm-3 yield the estimate of 0.005 m2 g-1, probably a minimum value. Following determination of the system blanks, the reactor was charged with 4.03 g of Mn304 distributed evenly throughout the reaction zone. Blank determinations in the absence of Mn:3O4were made at temperatures from 30 to 243 "C over the NO concentration range of 0.2-1.2 ppm. NO? concentrations ranged from 0.04
Table 1. Kinetic Data 243 " C
234 OC
0.23 0.45 0.59 0.76 0.86 1.09
674
0.20 0.37 0.51 0.67 0.75 0.92
0.24 0.46 0.62 0.76 0.92 1.10
Environmental Science & Technology
0.20 0.38 0.50 0.63 0.77 0.95
CI, ppm
Ceg ppm
0.23 0.42 0.58 0.78 0.90 1.11
0.19 0.32 0.46 0.60 0.70 0.85
CI, ppm
0.21 0.41 0.57 0.78 0.90 1.06
Ce, ppm
0.16 0.30 0.42 0.57 0.65 0.80
1.0
0.9
00 0.7
os E, n 05
si
n
8
u
0.4
0.3 Q2
_.v
1.90
200 lO'oc/T,
1.95
2.05
210
Figure 3. Arrhenius plot Figure 2. Comparison of inlet and exit NO concentrations at 243 OC
Table It. Temperature Dependence of Rate Constant
to 0.10 ppm. For each flow setting, inlet and outlet NO concentrations were identical a t all temperatures. NO2 concentrations were similarly unchanged. Observed NO and NOn concentrations could be computed solely on the basis of dilution of the NO-NO2 source with the carrier gas. These observations demonstrate that within the temperature range studied, surfaces of the apparatus in contact with the gas mixture were inactive toward NO and N02. T h e reactor was then charged with Mn304and pumped down with heating for 5 days to degas the system. The determinations were repeated with Mn304 in the reactor. The steady-state NO concentrations are given in Table I and shown graphically in Figure 2 for 243 "C.Outlet NO concentrations are lower than inlet NO concentrations; the difference increases with increasing temperature. T h e linear relationship between the logarithms of the inlet and outlet NO concentrations is significant in terms of subsequent analysis of the kinetic data. At each temperature, inlet and outlet NO2 concentrations were identical a t each flow setting and unchanged with respect to values measured in the blank determinations.
Data Analysis Evaluation of the kinetic parameters for the heterogeneous decomposition of NO according to: NO(g) = 0.5Nz(g)
+ 0.502(g)
is based upon application of the Arrhenius equation to the integrated rate law for a plug flow reactor (10).In general, the NO decomposition rate is given by:
where C is the NO concentration, t is time, k is the heterogeneous rate constant, and n is the reaction order. T h e order n can be obtained from a plot of log (dC/dt) as a function of log C for various inlet concentrations of NO. In this case, n = 1 and the integrated rate law for the flow reactor becomes:
where u is the volume flow rate, Vis the reactor volume, and C, and Ci are the steady-state exit and inlet NO concentrations, respectively. The linearity of log C, as a function of log Ci a t each temperature confirms the applicability of the plug flow reactor model for the temperature range of this study. In each case, the slope is unity, a further confirmation of the
r, oc 215 223
1, oc
k , s-'
234 243
0.0309 0.0373
k , s-I
0.0499 0.0613
applicability of the first-order integrated rate law for the flow reactor. Values of k given in Table I1 were computed from Equation 2 using the data in Table I. T h e Arrhenius parameters were evaluated on the basis Of
-d=InE akp p dT
RT2
(3)
where EaPP is the apparent activation energy, T is the absolute temperature, and R is the gas constant. In integrated form, the Arrhenius equation may be written: k = A exp(-E,,,/RT)
(4)
where A is the temperature-independent preexponential factor for the temperature range of this study. As shown in Figure 3, the Arrhenius plot of In k as a function of 1/T is linear with Eapp= 12.4 f 0.2 kcal mol-l and In A ( s )= 9.29 f 0.24. The uncertainties are given in terms of the 90% confidence intervals (11).
Discussion The results demonstrate that Mn304 can accelerate the decomposition of NO in purified air a t moderate temperatures. The observed reaction order of 1is consistent with many previous studies of the catalyzed decomposition of NO. The apparent activation energy of 12.4 kcal mol-' is lower than most previously reported values for other catalysts but comparable to values of 10.6 and 13.2 kcal mol-l for two catalysts studied by Shelef et al. (7). The preexponential factor for the Mn:3O4-catalyzed decomposition of NO, however, is much larger than values reported for other catalysts by Shelef et al., who compared NO decomposition rates on the basis of mol m-2 min-l a t 500 "C and 1 atm. Extrapolation of the present data to these conditions results in an NO decomposition rate several orders of magnitude faster than rates reported previously for other catalysts. The possible practical application of this finding is a subject for continuing study. With regard to the M M T issue, these findings suggest the possibility of a beneficial effect, namely, a decrease in NO, emissions, and may help to explain such effects in tests with M M T under simulated automotive exhaust conditions ( 4 ) . However, extrapolation to either actual automotive exhaust Volume 13,Number 6,June 1979 675
or the atmosphere would be premature in the absence of more extensive tests. Nevertheless, these findings indicate that in evaluating the potential air quality problem posed by MMT, the catalytic activity of M M T combustion products should be considered, in addition to increases in atmospheric Mn concentrations.
Literature Cited
(4) Otto, K., Sulak, R. J., Enciron. Sci. Techno/., 12, 181-4 (1978). ( ! 5 ) Holiday, E. P., Parkinson, M. C., “Another Look a t the Effects of Manganese Fuel Additive ( M M T ) on Automobile Emissions”, Preprint 78-54.2,71st Annual Meeting of the Air Pollution Control Association, Houston, Tex., J u n e 25-30, 1978. (6) Shelef, M., Kummer, J. T., Chem. Eng. Prog. Symp. Ser., 67 ( l l j ) , 74-92 (1971). ( 7 ) Shelef, M., Otto, K., Gandhi, H., Atmos. Enuiron., 3, 107-22 (1969). (8)National Bureau of Standards Monograph 25, Section 10, 1972, p 38. (9) Joint Committee on Powder Diffraction Standards, Card No. 24-734. (10) Hougen, 0. A., Watson, K. M., “Chemical Process Principles”, P a r t 3 , Wiley, New York, 1947. ( 11) Swinbourne, E. S., “Analysis of Kinetic Data”, Nelson, London, 1971, p p 40-2.
(1) “Manganese”, National Academy of Sciences, Washington, D.C., 1973. ( 2 ) T e r Haar, G. L., Griffing, M. E., Brandt, M.. Oberding, D. G., Kapron, M., J . Air Pollut. Control Assoc., 25, 858-60 (1975). ( 3 ) Calabrese, E. J., Sorensen, A,, J . Air Pollut. Control Assoc., 25, 1254-5 (1975).
t l w r i t ~ e d/or recieii, August 28, 1978. Accepted December 27, 1978. This incestigation u’as supported b,y the Department fi/ Enuironmental Sciences, University of Lancaster, England, where H . W. Edwards spent the 1977-78 academic year on sabbatical leave from ‘olora do St a t e L‘niuersi ti..
Acknowledgments The authors are grateful for the technical assistance provided by P. D. E. Biggins and H. A. McCartney with the X-ray diffraction analyses and calibration of the NO, analyzer, respectively.
Rearrangement of Poly(dimethylsiloxane) Fluids on Soil Robert R. Buch” and Donald N. lngebrigtson Dow Corning Corporation, Midland, Mich. 48640
Poly(dimethylsi1oxane) (PDMS) fluids in intimate contact with many soils undergo siloxane bond redistribution and hydrolysis, resulting in the formation of low molecular weight cyclic and linear oligomers. Low molecular weight hydroxyfunctional hydrolysis products are water soluble, and the cyclics and trimethylsiloxy-end-blocked oligomers are volatile, thus providing materials which can partition from the soil t o the water and atmospheric environmental compartments. Clays were shown to be the catalytic soil component responsible for the above oligomerization and hydrolysis reactions. Although the activity of all types of clays was shown to be inversely related to the level of clay hydration, some were more susceptible to attenuation than others; e.g., montmorillonite was deactivated by hydration much more readily than kaolinite or halloysite. Furthermore, montmorillonite attack is preferentially upon trimethylsilicon sites, leading primarily t o linear oligomerization, whereas kaolinite attack, which is less discriminating, leads predominantly to cyclic oligomers. Silicones encompass an array of materials including resins, elastomers, and fluids of wide viscosity range. The fluids command a major portion of the silicone market. More specifically, a single type of fluid, poly(dimethylsiloxane), produced in a range of viscosities 0.65 to 60 000 cSt, is the dominant product of the silicone industry. These fluids have a unique combination of useful physical and chemical properties (low surface tension, thermal and oxidative stability, high dielectric constant), while manifesting virtually no biological activity or toxicity. Their exceptionally diverse spectrum of applications ranges from food and cosmetic additives to hydraulic fluids and electrical transformer fluids. Consequently, these fluids have many means of entry t o the environment. Estimated world production of PDMS fluids for 1978 is 100 million pounds. Emerging new markets are expected to increase this figure substantially over the next several years. In view of steadily increasing annual production and the absence of definitive information regarding the environmental 676
Environmental Science 8, Technology
persistence of PDMS, concern has developed over its possible adverse ecological impact ( I , 2). This concern stems from several well-known facts: (a) PDMS fluids are essentially nonreactive, except when catalyzed by strong acids and bases; (b) PDMS based elastomers are uniquely resistant to weathering (Le., hydrolysis and oxidation); (c) no evidence has been found for biodegradation of these polymeric fluids. An earlier report ( 3 ) from our laboratories was directed toward effective containment and cleanup procedures for spills of silicone fluids on water, roadways, and soils. The present paper describes the chemical effects of various soils on PDMS.
Experimental Materials. Solvents used in this study were of reagent grade quality. Activated charcoal was commercial BPL grade, obtained from Pittsburgh Activated Carbon. Silicone fluids used i n this study included commercial grade Dow Corning 200 Fluids (MD,M, where M = (CHI)‘$i0,p2 and D = (CHI).Si01 of 10, 50, lo’, lo4, and loficSt viscosity; hydroxyl endblocked PDMS (HOD,H); linear decamethyltetrasiloxane, L4 (MD2M); linear dodecamethylpentasiloxane, L5 (MD3M); and octamethylcyclotetrasiloxane (D4). The 14C-tagged PDMS fluid as well as the dimethylsilanediol were specially prepared. Various local Michigan soil types representative of the local region’s surface and subsoils were obtained with the assistance of a local USDA soil advisor. Others, as well as pure clays, were obtained from Wards Mineral Supply House (Rochester, N.Y.). Procedures. Preparation of soils consisted of grinding (mortar and pestle) and sifting through an 80-mesh screen. Soils were dried by two techniques. For thorough drying, soils were placed in an air-circulating oven a t 80 “C for 7 days. Early work was done on samples dried for 2 h a t 105 “C. Less stringent drying was accomplished by spreading soil on Teflon sheets in the laboratory a t ambient conditions for 14 days. Soils having controlled levels of moisture were obtained by storing thoroughly dried soils in desiccators having constant 0013-936X/79/0913-0676$01.00/0
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1979 American Chemical Society
