Oxidation of an Automobile Exhaust Gas Mixture by Fiber Catalysts D. M. Nicholas and Y. T. Shah' Department of Chemical and Petroleum Engineering, University of Pittsburgh, Pittsburgh, Pennsylvania 1526 1
1. A. Zlochower PPG Industries, Pittsburgh, Pennsylvania 15230
This paper experimentally analyzes the usefulness of a platinum impregnated porous fiber catalyst for the oxidation of an automobile exhaust mixture. The experimental data for the conversions of carbon monoxide, ethylene, and propane by a reactor packed with the fiber catalysts were obtained at various preheat temperatures and space velocities. The preheat temperature was varied from approximately 180 to 560 OC and the gas hourly space velocity was varied from 20 000 to 70 000 h-'. The results are compared with those obtained from two conventional catalysts, alumina beads and silica gel beads, under similar reaction conditions. It is shown that a lesser amount of fiber catalyst carrier performs better than the two conventional catalysts under all reaction conditions, when the average pore size of the fiber catalyst is large and when the fiber catalyst is tightly packed in the reactor. Both steady state and transient conditions are examined.
Introduction The emission of pollutants from automobiles in the form of exhaust gases is a constant threat to environmental quality. The pollutants from automobiles which contribute to urban air pollution problems are hydrocarbons, carbon monoxide, and oxides of nitrogen. The use of a catalytic converter in the exhaust system helps to eliminate these pollutants by converting them into harmless products. Some improvements may be made on present exhaust catalyst systems. A catalyst that is easily made and readily handled is needed. Also, a better support material is needed for the catalyst. Conventional supports, bead and monolith, present a few problems when used in catalytic converters. Pellet supports, when packed tightly into a converter, tend to raise the back pressure in the exhaust system. Also, the converter requires screens to keep the pellets in place. These screens interfere with exhaust flow. Monoliths permit easier flow of gases, but they are expensive to fabricate and are susceptible to mechanical and thermal damages (Benrey, 1973). A support with a higher surface area to volume ratio than conventional catalysts would be desirable from the point of view of reducing the size of the converter. The large surface area must also be readily accessible to the reacting gases. Finally, a converter must be operable efficiently a t start-up conditions when a significant portion of the pollution from automobile exhaust occurs (Acres and Cooper, 1972). All these aspects discussed above suggest that a new form of support material is needed. Porous glass fibers appear to have some of the qualities that are needed to improve the present technology on catalytic converters. The fiber catalyst is made by first leaching conventional glass fibers and then soaking the fibers in a solution containing the platinum complex. This leached borosilicate fiber catalyst can be easily prepared and has a sufficiently high surface area to volume ratio. The fiber catalyst has very low thermal mass which is needed for the rapid temperature response under the start-up conditions. The low thermal inertia may not, however, dampen out thermal transients causing excessive heating of the converter. This may be harmful to the glass support. If the fibers were loosely
packed in the converter, they could be carried away by the gas flowing a t high velocity. The purpose of this paper is to analyze experimentally the use of a porous fiber catalyst for the oxidation reactions occurring in a catalytic converter for the automobile, and in particular, to compare the performance of a converter packed with fiber catalyst with the performance of a conventional catalyst under steady state and transient conditions.
Previous Work on Oxidation (Laboratory Studies with Conventional Catalyst) In the laboratory oxidation studies a duplication of all the components in the exit stream of an automobile is difficult, but use of the major components of exhaust gas is feasible. Essentially, the general components and compositions (in mol %) of automobile exhaust are (Hurn et al., 1962) as shown in Table I. Some of the important hydrocarbons and their concentrations are shown in Table I1 (Kuo et al., 1971; Innes and Tau, 1963). Most investigators have used only one or two representative hydrocarbons in their studies. Schlatter et al. (1973) used 0.05% propane in their feed gas, Leventhal (1972) used 300 ppm of either ethylene, propane, or n-butane, and Dwyer and Morgan (1973) used 400 ppm propylene. Leventhal (1972) also used 0.05% nitric oxide in his feed gas mixture. The nitric oxide has a sizable retarding effect on the oxidation (Richard et ai., 1972; Snyder et al., 1972) caused by the strong interaction of nitric oxide with platinum (Fogel, 1964). Sourirajan and Accamazzo (1960) studied catalytic oxidation of carbon monoxide in low concentration using CuO-Al203 (1:l) catalyst with a CO-air feed, whereas Schlatter et al. (1973) and Klimisch and Schlatter (1972) studied catalytic oxidation of carbon monoxide using both a base metal catalyst, copper chromite on alumina, and a platinum metal catalyst on alumina. Klimisch and Schlatter (1972) found that increasing the space velocity was detrimental to the performance of the base metal catalyst while a fresh platinum catalyst maintained its efficiency over more than a fivefold increase in gas flow rate. Very recently, Voltz et al. (1973) showed that the oxidation is inInd. Eng. Chem., Prod. Res. Dev., Vol. 15, No. 1, 1976
29
Table I
Table I11
-________
Component
Compositions
Carbon monoxide Carbon dioxide Hydrogen Water Oxygen Nitrogen oxides Hydrocarbons Nitrogen
~-
1.0-8.0%
9.0-13.0% 0.3-3.5% 9.0-11.0% 0.6-1.1% 60-1800 ppm 2260-9910 ppm Balance
Material
Wt %
Silica, SiO, Alumina, A1,0, Boron oxide, B,O, Calcia, CaO Magnesia, MgO
55-56 13 7-8
-_______
Table IV Material ___
Table 11. Exhaust Gas Hydrocarbons Concn, ppm Saturates Methane Ethane Propane Olefins Ethylene Propylene Acetylene Aromatics Benzene Toluene
_________
147.5
Flourine, F, Titania, TiO, Iron oxide, Fe,O, Sodium oxide, Na,O Potassium oxide, K,O
Wt % ____-__ 0.5 0.6
0.3 0.4 0.1
21.4
2.2 160.4
58.4 88.0 21.3 49.3
hibited by carbon monoxide, propylene, and nitric oxide. Similar results were obtained by Shishu and Kowalczyk (1974).
Experimental Section A. Preparation of Fiber Catalysts (Catalyst A). The borosilicate fibers that were used as the support material are a crimped fiber material. A typical composition of glass fibers of type E-621 glass is shown in Table I11 while the composition of the minor ingredients is shown in Table IV. This glass has low alkaline conductivity and high water resistance. It is attacked by very strong acids which leach out the non-silica constituents. A porous high silica material results. Preparation given in brief was as follows. The fibers were soaked in a 5.0 N hydrochloric acid solution for about 1-3 h a t 78-82 O F . They were then rinsed and dried. At this stage a loss in the weight of about 41% was noted. The fibers were then observed under a microscope to examine if they were completely leached. These fibers had extremely fine pores (See Table V, catalyst A). The pore size can be increased by immersion, for example, in a hot dilute NH3 solution. A solution of 0.3% chloroplatinic acid, HzPtClG was prepared. Ammonia was then added to bring the pH to about 7-8. Also, an equal weight of hydroxylamine NHzOH (equal to HzPtC16) was added to the solution. At this stage the H2PtC16-NHzOH-NH3 solution was heated for about 2 h at 80 O F . Fibers were then soaked in the solution for 10 min and were allowed to drip dry before being put in a vacuum oven in a hydrogen atmosphere at 600 OF for 15 min. Preparation of Silica Gel Catalyst Beads (Catalyst B). The 20-40 mesh silica gel beads used were Davison grade 70 silica gel. They were heat treated at 600 OF. Platinum was deposited by soaking the beads into chloroplatinic acid. The beads were then dried a t 250 OF for 3 h and calcined a t 800 O F for 16 h. A BET analysis of these beads is given in Table V. Preparation of Alumina Support Catalyst Beads (Catalyst C). The 10-20 mesh alumina H151 beads, provided by Alcoa, were impregnated with dilute chloroplatinic acid in a hydrogen atmosphere. After the beads were dried, they were put in a vacuum oven in a hydrogen atmo30
21 0.4
Ind. Eng. Chem.. Prod. Res. Dev., Vol. 15, No. 1, 1976
sphere at 600 O F for 15 min. A BET analysis of this catalyst is also given in Table V. B. Reactor Equipment. (a) Description of the Apparatus. The equipment consists of an inlet metering system, a reactor system, and an analytical system (See Figure 1). Inlet Metering System. The inlet metering system consisted of five inlet lines: four feed gas lines and one helium purge line. Each feed gas line was equipped with a properly calibrated rotameter. The first feed line contained a gas mixture of 2.3% CO, 11.7% CO2, and 86% N2 and this mixture was passed through a 500-ml flask water saturator maintained a t 50 OC to provide approximately 11.7% of water on the gas stream. The remaining three feed lines contained mixtures of 20% nitric oxide-80% nitrogen, 4% propane-10% ethylene and 80% nitrogen, and pure oxygen, respectively. The four feed lines were joined a t the mixing tee and are either passed into the reactor system or bypassed around the reactor to the analytical system. Helium was provided after the mixing tee so that the lines may be purged. Reactor System. The reactor system consisted of the reactor itself, the heating system, manometer system, and a reactor bypass line. The reactor was constructed of quartz and served both as the preheater and the catalyst bed (see Figure 2). The preheat section of the reactor, approximately the first 30 cm in depth, was packed with 2-3-mm diameter spheres of high silica glass. The catalyst bed was packed with the catalyst in the next section of the reactor and the remaining length of the reactor was also packed with glass beads. The reactor had an i.d. of 2.0 cm and down the middle of the reactor was a thermocouple well, enabling measurement of temperature at any length down the reactor including the catalyst bed. In most runs performed, three thermocouples were inserted in the catalyst bed. These thermocouples were positioned at the inlet, center, and exit of the bed. Also, a thermocouple was placed 3-4 cm upstream from the bed to measure the temperature of the gas before it enters the bed (preheat temperature). The heating system which enclosed the reactor was a Lindberg tube furnace with a temperature range from 200 to 1200 "C. The manometer system consisted of a mercury manometer and a mercury trap. A reactor bypass line was used to check the composition of the feed entering the reactor. (b) Analytical System-Product Analysis. The analytical system consisted of a Hewlett-Packard Research Chromatograph with a thermal conductivity cell used as a detector. The gas to be analyzed was introduced into the gas chromatograph by use of a gas sampling valve. Separation of the gases was achieved by two columns in series.
Table V. BET Analysis of the Catalysts Used in the Present Study _ _ ----
~
Support Wt % PT BET area, m2/g Average pore radius, .& Total pore volume, cc/g Pore radius distribution Volume, % pore radius,
Catalyst A Glass fibers 0.27 74.7 32.0 0.12 15.817-20 12.2120-30 11.9/30-40 12.1/40-50 48.1 150-1 00 7.71100-200 1.1/200-300 34
a
Platinum crystallite size, A
__
-.--___
~
Catalyst B Silica gel 0.27 317 58 0.92 0/7-20 3.7/20-30 6.5/30-40 10.4/40-50 67.7150-1 00 11.0/100-200 0.6 / 20 0-300 106
~~
Catalyst C Alumina beads 0.43 179.7 23.0 0.37 44.310-19 14.9/19-23 13.1/23-31 13.8/31-43 11.4/43-66 2.3/111-315 2.61315 81
A
ROTAMETER
C3H8 C2H4 & N1 T A N K BUBBLE
OR
WEST TEST METER
NO Ki N 2 T A N K
O2 T A N K U He T A N K
Figure 1. Equipment for the present study.
41
'fa''
KOVAR CONNECTION
/
THERMOCOUPLE WELL
CONNECTION
CAYALYST BED
OUTLET
/ S'' K O V A R CONNECTlON
Figure 2. Schematic of the reactor.
One column consisted of 8 f t of Molecular Sieve 13X while the second column consisted of eight feet of Porapak Q and 8 f t of Porapak R. (c) Experimental Procedure. Before the experimental runs were made, each rotameter was calibrated at 10 psig with a bubble flow meter or a wet-test meter. The procedure for a typical experimental run was as follows. The heating furnace was set at the desired temperature. Helium was introduced at the same GHSV value as that of the reactants and the pressure drop across the bypass line was equalized to that across the reactor. At this stage, the helium flow was stopped and the different components were introduced a t the predecided flow rates but a t equal pressure. Various joints were checked for leaks. After all the flow rates were balanced accurately the composition of the reactant mixture was determined by chromatographic separation. Various flows were checked periodically and the outlet product samples were analyzed a t predetermined time intervals while the different temperatures were recorded at the time of sample injection. When a steady-state conversion was reached the reactants were again switched to the bypass line and then turned off. Helium was passed through the reactor to cool it and to prepare it for the next run. Before any experimental runs were made, the quartz reactor was packed with high silica glass beads ranging between 2 and 4 mm in diameter and tested to see if they or the reactor walls had any catalytic activity. The reactants Ind. Eng. Chem., Prod. Res. Dev., Vol. 15, No. 1, 1976
31
were passed a t the lowest flow rate and up to a preheat temperature of 500 "C. I t was found that neither the beads nor the quartz reactor had any catalytic activity. These glass beads were used in the actual experimental runs to hold the catalyst bed in its place. Experimental Results From the product analysis the conversions of the carbon monoxide, ethylene, and propane oxidation reactions were determined. One of the main purposes of this paper was to illustrate the comparison of the steady-state conversions obtained by fiber catalyst to the ones obtained by the two conventional type catalysts. The second purpose was to compare the transient and steady-state bed temperatures of the different catalysts. The system variables examined were Gas Hourly Space Velocity (GHSV), preheat temperature, and the feed composition. The reactor was operated a t essentially atmospheric pressure. Basically two sets of feed compositions were examined in the present study: one with an excess oxygen concentration and the other one with low oxygen concentration. The details of these feed compositions are illustrated in Table VI. In the following discussion we consider only the conversions of CO, C2H4, and CaH,.j. The detailed product distributions for all the experiments are given by Nicholas (1975). In the present study precautions were taken to eliminate both internal and external mass transfer effects on the performances of various catalyst beds (Nicholas, 1975). When the fiber catalyst was used, the catalyst bed was evenly and tightly packed with the catalyst in order to give a uniform flow distribution within the catalyst bed. The catalyst beds used in the present study were shallow. However, in each experiment the bed length was kept large enough to avoid the backmixing effect. The minimum bed length required to eliminate the backmixing effect was estimated from the analysis presented by Levenspiel (1970). The details are given by Nicholas (1975). For the larger bead catalyst (i.e., catalyst C) the role of intraparticle diffusion effect was evaluated by measuring the conversions for various reactions a t two different particle sizes (and otherwise identical reaction conditions). No significant changes in the conversions were obtained meaning thereby, that the intraparticle diffusion effect was negligible in the experiments with this catalyst. Since the particle size of catalyst B was very small, it was assumed that the intraparticle diffusion effect in the experiments with this catalyst was negligible. Effect of Preheat Temperature. For typical GHSV = 20 000, and feed composition 1 the effects of preheat temperature on the conversions of CO, C2H4, and C3H8 for all three catalyst are illustrated by the solid curves in Figures 3,4, and 5 . The most striking feature of these results is that although a t higher preheat temperatures greater than approximately 235 "C all catalysts perform equally well, the fiber catalyst performs the best a t lower preheat temperature. Of the three reactions studied here, the reaction for the propane is the slowest one. Complete conversion of the propane is achieved a t approximately 330 "C with the fiber catalysts whereas similar results with catalysts B and C were obtained a t approximately 575 "C (extrapolated for catalyst B). The effects of preheat temperature (at GHSV = 20 000) on the conversions of CO and CPH4 for feed composition 2 are illustrated by the dotted curves in Figures 3 and 4. These results indicate that the fiber catalyst once again gave superior results compared to the ones obtained with catalysts B and C, particularly a t the lower preheat temperature. 32
Ind. Eng. Chem., Prod. Res. Dev., Vol. 15,No. 1, 1976
Table VI. Feed Compositions Examined in the Present Study ~~
Composition la Composition 2a Excess oxygen Low oxygen
Component
1.1-1.2% Oxygen 3-4% Nitrogen 73-75??? 73-75% Nitric oxide 0.1% 0% 1.7-2.5% 1.7-2.5% Carbon monoxide 8.5-10% 8.5-10% Carbon dioxide 0.04-0.06% 0.04-0.06% Ethylene 10-1 2% 10-1 2% Water 0.01-0.03% 0.01-0.03% Propane a All compositions are in mole 5%; the detailed feed distributions for all the experiments are given by Nicholas (1975).
195
235
275
315
PREHEAT TEMPERATURE (OCI
Figure 3. Conversion of carbon monoxide vs. preheat temperature a t GHSV = 20 000 h-l.
I 60
/
I
-
I
(%I
/
I
ETHYLENE CONVERSION
I 40
-
//
1
/ 20
-
/
/
4
/
FEEDCOMPOSITION
---
2
-'
SYMBOL
CATALYST
-0 A 0
A 0
/
195
i
.
235
275
PREHEAT TEMPERATURE
315
loci
Figure 4. Conversion of ethylene vs. preheat temperature at GHSV = 20 000 h-l. Effect of GHSV. For a given preheat temperature, an increase in gas hourly space velocity (or a decrease in residence time) should decrease the conversions of all reactants. In the present study GHSV was varied from 20 000 h-' to 70 000 h-l. This range was largely determined by the limitations of the equipment. Some typical results illustrating the effects of variations in GHSV on the conversions of CO and CgH4 under a variety of reaction conditions are shown in Figure 6. These results indicate that the fiber catalyst gave a t least the same or better conversions for both CO and CzH4 than the ones obtained with catalysts B and
415
-
375
-
SYMBOL CATALYST 0 0
A
BED 335 TEMPERATURE
1%) PROPANE CONVERSION
-
loci 295
-
255
"1
-*
J
DIMENSIONLESS BED DEPTH
195
275
355
435
515
595
PREHEAT TEMPERATURE lac)
Figure 8. Typical steady-state temperature distributions in the beds of catalysts A, B, and C a t GHSV = 70 000 h-l; a preheat temperature of 245-255 " C for feed composition 2.
Figure 5. Conversion of propane vs. preheat temperature a t GHSV = 20 000 h-' for feed composition 1. 1
I
b
d
425
- f-
1%) CONVERSION
0
20
40
60
80
100
TIME IMINUTESI
Figure 9. Bed temperature vs. time a t GHSV = 20000 h-l for feed composition 2 . GHSV I H R K 1 I
Figure 6. Conversion of carbon monoxide and ethylene vs. GHSV a t a preheat temperature of 225-255 "C for feed composition 2.
445
405
----_
365 BED TEMPERATURE IOC1 325
I/
I 1
PRE HEAT TEMP
----
-
198 209OC 259%
I
295
I
SYMBOL
CATALYST
1
0
A B
O 245
a
C
-*- - - - - _ _ _ --_ -_ _--p-1_ _ _ - ---- - - - - --4
205
0
05
10
DIMENSIONLESS BED DEPTH
Figure 7. Typical steady-state temperature distributions in the beds of catalysts A, B, and C a t GHSV = 20 000 h-' for feed composition 1.
C at all GHSV considered here. Similar results were obtained for C3H8. Bed Temperatures. The fact that the reactor packed with the fiber catalyst gave better performance under all reaction conditions than the ones packed with catalysts B and C can also be illustrated by observing the steady-state temperature distributions in these three catalyst beds. The steady-state temperature distributions under some typical reaction conditions for these three catalyst beds are shown in Figures 7 and 8. These data indicate that the average temperature of the fiber catalyst bed under all system conditions is much higher than those for catalysts B and C. Since the reactor was operated under nonisothermal conditions, higher conversions of CO, C2H4, and C3H8 should correspond to the higher average temperature of the catalyst bed. The maxima in some bed temperature distributions imply that the maximum reaction rate occurs somewhere within the catalyst bed and that the entire length of the catalyst bed is not necessary to achieve the observed levels of conversion. The fact that the fiber catalyst bed achieves at least equal or higher average steady-state temperatures than the beds of catalysts B or C a t all preheat temperature and GHSV conditions implies that the present fiber catalyst was more active than the conventional bead catalysts. It should be noted that the BET surface area for the fiber catalyst was considerably lower than the ones for catalysts Ind. Eng. Chem., Prod. Res. Dev., Vol. 15, No. 1, 1976
33
B and C (See Table V). Similarly, the platinum w t % for the fiber catalyst was the same as that for catalyst B, both being considerably less than the one for catalyst C. The platinum crystallite sizes for the three catalysts as measured by the hydrogen adsorption method (Nicholas, 1975) are also shown in Table V. These data indicate that the platinum was more uniformly distributed in the fiber catalyst than in other two conventional catalysts. This may explain why in the present study the fiber catalyst was found to be more active than the conventional pellet catalysts.
I
I
0 405
325 BED TEMPERATURE loci
245
B E D DEPTH = 0.5 SYMBOL
165
CATALYST
Unsteady-State Results As mentioned in the Introduction, one of the major advantages of the use of fiber catalysts in an automobile converter lies in this transient operation. Low thermal mass of the fiber should allow quick thermal response and therefore better conversions in the transient state than the conventional bead catalysts. The experimental data described above were normally obtained after 60 to 100 min of operations, when the steady state was definitely established. In order to show the usefulness of the fiber catalysts during the transient conditions, the performances of fiber and bead catalysts in three different types of transient operations were examined. These results are briefly described below. A. Bed Temperature Initially Set at Run Temperature. In this transient operation, the whole system and the catalyst bed were heated to the desired temperature before the feed was introduced. As the feed was introduced and the reaction proceeded, the temperature in the bed increased and it was recorded vs. time. Conversions of CO, CaH4, and C3Ha vs. time were also measured. The temperature responses for various catalyst beds under some typical reaction conditions are compared in Figure 9. These results show that in this type of transient operation the bed temperature (and correspondingly the conversion of carbon monoxide) closely follow the preheat temperature. As expected, the temperature (hence the conversion) of the fiber catalyst bed increases at a faster rate than that of the bed packed with catalyst beads. B. Bed Initially Set at Approximately 60 "C. The transient operation more meaningful to an automobile converter is the one where the catalyst bed is initially cold. As the feed to the reactor is introduced, the bed is heated partly by the heat of reaction and partly due to external heating. In order to simulate this type of transient operation, the beds were initially set at approximately 60 "C. When the feed was introduced into the reactor, the furnace was set and turned on to the desired temperature. Typical temperature responses of the various beds under this type of heating condition are illustrated in Figure 10. These results once again indicate that the fiber catalyst bed responded much faster than the beds packed with the bead catalysts. C. Essentially No External Heat Supplied to the Catalyst Bed. Some transient experiments were performed wherein no external heat was supplied to the catalyst bed. In these experiments, the catalyst beds were kept completely outside the furnace whereas at the beginning of each run the sections of the reactor within the furnace and the preheater were kept at the desired temperature. Thus, the catalyst bed was heated only by the gas and by the heat generated during the reaction. Typical temperature response curves for the various catalyst beds obtained under this type of heating condition are shown in Figure 11. Once again fiber catalyst bed gave faster temperature response and thereby better conversions under transient conditions. 34
Ind. Eng. Chem., Prod. Res. Dev., Vol. 15, No. 1, 1976
a5
20
10
0
30
40
TIME (MINUTES1
Figure 10. Bed temperature vs. time for a bed temperature initially at 85 " C at GHSV = 20 000 h-* for feed composition 1.
505
-
425
-
345
-
BED TEMPERATURE foci
265
-
SYMBOL CATALYST -
BEDDEPTH - 0 5
185 -
105
-
0
20
40
60
TIME IMlNUTESi
Figure 11. Bed temperature vs. time for catalyst beds provided with no external heat initially at GHSV = 20 000 h-' for feed composition 1.
Discussion The results described in Figures 9 to 11 indicate that due to low thermal mass of fibers compare to the conventional pellets, under all conditions fiber temperature increased more rapidly than those of the conventional pellets. The data shown in Figures 9 to 11 cannot be at present quantitatively evaluated because of the lack of accurate knowledge of thermal properties of various catalysts and the reacting mixture under the reaction conditions. It is interesting to note from Figures 9 to 11 that no temperature run away conditions were observed with the fiber catalysts. Furthermore, fibers subjected to high temperature behaved quite normally in the subsequent experiments. Conclusions As a result of the present study the following conclusions are made. (1) At steady-state conditions, a fiber catalyst with a large average pore size provides a higher oxidation of carbon monoxide, ethylene, and propane than conventional alumina and silica gel supported catalysts. (2) The fiber catalyst requires a much lower preheat temperature to initiate the oxidation than that required by the conventional catalysts. (3) In a variety of transient heating conditions, the tem-
perature responses of the fiber catalyst bed are found to be always better than those of beds containing catalyst beads.
Literature Cited Acres, G. J. K., Cooper, E. J.. Platinum Met. Rev., 16, 74 (1972). Benrey. R. M., Search, 3 (1973). Bowdiich, F . W., "A General Motors Publication," 1973. Dwyer, F. G.,Morgan, C. R., Mobil Research and Development Corporation, Research Department, Paulsboro. N.Y.. 1973. Fogel, A,, Kinet. Katal., 5, 496 (1964). Hum. P. W., Dozois, C. L., Chase, J. O., Ellis, C. F.,Ferrin, P. E., Division of Refining, 42, 657, (1962). Innes, W. D., Tau, K., "The Kirk-Othmer Encyclopedia of Chemical Technology," p 814, 1963.
Klimisch, R. L., Schlatter, J. C., presented to American Ceramic Society, Flint, Mich., 1972. Kuo, J. C. W., Morgan, C. R., Lassen, H. G.. SAE Paper No. 710289, 1971. Levenspiel, O., "Chemical Reaction Engineering," 2d ed, Wiley. New York, N.Y., 1972. Leventhal, B., Statement to EPA Hearing on Application for One-Year suspension of Auto Emission Standards, 1972. Nicholas, D.M., Ph.D. Thesis, University of Pittsburgh, 1975. Snyder, P. W., Stover, W. A.. Laseen. G. G., SAE Paper No. 720479, 1972. Schlatter, J. C., Klimisch, R. L., Taylor, K. C., Science, 179, 798 (1973). Shishu, R. C.. Kowalcyzyk, L. S.,Platinum Met. Rev., 18, 58 (1974). Sourirajan. S..Accomazzo, M. A,, Can. J. Chem., 38, 1990 (1960). Voltz, S. E., Morgan, C. R., Liederman. D.. Jacob, S. M., Ind. Eng. Chem.. Prod. Res. Dev., 12, 294 (1973).
Receioed for reuiew February 28,1975 Accepted October 9,1975
Carbon Monoxide Oxidation over a Platinum-Porous Fiber Glass Supported Catalyst D. M. Nicholas and Y. 1.Shah' Department of Chemical Engineering, University of Pinsburgh, Pittsburgh, Pennsylvania 1526 1
This paper presents the kinetics of carbon monoxide oxidation over a platinum-porous fiber glass supported catalyst. In a recent article by Nicholas et al. (1976), the advantages of this catalyst for an automobile exhaust converter were illustrated. We examined the kinetic data illustrating the basic activity of fiber glass supported platinum catalyst for carbon monoxide oxidation and compare the results with the similar results with other platinum catalysts published in the literature. The kinetic data were correlated by three types of model: (a) a powerlaw model, (b) Langmuir-Hinshelwood single site model, and (c) Langmuir-Hinshelwood dual site model. The Langmuir-Hinshelwood dual site model was found to correlate the experimental data best under the entire range of reaction conditions. The experimental data indicated the inhibition effect of carbon monoxide on the rate of oxidation. At low temperatures fiber catalyst appears to be more active than other platinum catalysts investigated in the literature.
Introduction The kinetics of carbon monoxide oxidation have been studied as early as 1922 (Langmuir, 1922). A considerable amount of attention has been, however, given to this reaction in recent years because of its importance in automotive emission control. The kinetics of this reaction has been examined for a variety of catalysts and cat,alyst supports. The most widely used catalysts for the carbon monoxide oxidation have been platinum-supported type catalysts. Langmuir (1922) found that carbon monoxide oxidation over platinum wire could involve the reaction of gaseous carbon monoxide with adsorbed atomic oxygen. Sklyarov (1969), however, found that the oxidation of carbon monoxide involved molecular oxygen and carbon monoxide. With his dual site Langmuir-Hinshelwood model, Shishu and Kowalczyk (1974) also indicated that carbon monoxide reacted with adsorbed molecular oxygen. It is generally agreed that the oxidation of carbon monoxide over a platinum catalyst is inhibited by carbon monoxide a t low conversion and a t low temperatures. Harned (1971) found that a t conversions less than 80%, the rate of carbon monoxide oxidation was inversely proportional to carbon monoxide concentration and directly proportional to oxygen concentration. At conversions greater than 80%, rates were directly proportional to carbon monoxide to the half power. Voltz et al. (1973), in a kinetic study of carbon
monoxide oxidation on a platinum-alumina catalyst, also found the rate of the reaction to be inhibited by carbon monoxide. They proposed a Langmuir-Hinshelwood type of rate model to take into account the inhibition effect of carbon monoxide. Very recently, Shishu and Kowalczyk (1974) also obtained a similar type of Langmuir-Hinshelwood rate model. They also reported a power law-type rate model which indicated the rate of oxidation to be directly proportional to oxygen partial pressure and inversely proportional to the half power of the carbon monoxide partial pressure. All of Shishu's data were, however, for carbon monoxide conversion less than 10%. In the preceding paper, Nicholas et al. (1976) showed that a porous fiber glass-supported platinum catalyst presents some advantages over conventional pellet catalysts for use in an automobile exhaust catalytic converter. Because of the lower thermal mass of fiber catalysts, a converter packed with the fibers has a quicker temperature response than one packed with the conventional pellets. For a simulated automobile exhaust mixture, the fiber catalyst appears to perform better than the conventional pellets a t low temperature. The purpose of the present paper is to examine intrinsic activity of a fiber-supported platinum catalyst for the oxidation of carbon monoxide. The results are compared with the similar results reported in the literature for other platiInd. Eng. Chem., Prod. Res. Dev.. Vol. 15, No. 1, 1976
35
