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Ind. Eng. Chem. Res. 1992,31, 246U-2465
Removal of Methane from Compressed Natural Gas Fueled Vehicle Exhaust 5. Subramanian,*R. J. Kudla, and M. S. Chattha M D 3179 SRL,Chemical Engineering Department, Ford Motor Company, P.O.Box 2053, 20,000 Rotunda Drive, Dearborn, Michigan 48121-2053
The objective of this study is to investigate the modes of methane (CH$ removal from simulated compressed natural gas (CNG) fueled vehicle exhaust under net oxidizing, net reducing, and stoichiometric conditions. Model reaction studies were conducted. The results suggest that the oxidation of methane with oxygen contributes to the removal of methane under net oxidizing conditions. In contrast, the oxidation of methane with oxygen as well as nitric oxide contributes to ita removal under net reducing conditions. The steam reforming reaction does not eignificantly contribute to the removal of methane. The methane conversions under net reducing conditiona are higher than those observed under net oxidizing conditions. The study shows that the presence of carbon monoxide in the feed gas lea& to a gradual decrease in the methane conversion with increasing redox ratio, under net oxidizing conditions. A minimum in methane conversion is observed a t a redox ratio of 0.8. The higher activity for the methane-oxygen reaction resulting from a lowering in the overall oxidation state of palladium and the contribution of the methane-nitric oxide reaction toward the removal of CH4 appear to account for the higher CHI conversions observed under net reducing conditions.
Introduction Natural gas is an attractive source of fuel for vehicles. The advantagea provided by natural gas include lower fuel cost, longer engine life, lower maintenance, and reduced oil consumption. Methane (CHJ is the major constituent of natural gas. The development of catalysts for oxidizing the unburnt methane in the exhaust stream of natural gas fueled vehiclea is of importance. Several inveatigatorshave reported studies on noble metal and base metal catalysts for methane oxidation (Yu Yao, 1980; Firth and Holland, 1969a,b; Anderson et al., 1961; Stein et al., 1963; Drozdov et al., 1985). Recently, the activity of 403-supported Pt, Pd, and Rh catalysts for methane oxidation has been investigated using CH4-CO-02 and CH4-02 reaction mixtures (Oh et al., 1991). These results show that the CHI oxidation activity under net oxidizing conditions varies as Pd > Rh > Pt. The oxidation of methane over supported Pd catalysts has been widely investigated. The study of support effects (Cullis et al., 1972; Cullis and Willat, 1983), catalyst deactivation mechanism (Cullis and Nevell, 19761, reaction mechanism (Mezaki and Watson, 1966; Seimanides and Stoukides, 1986; Cullis et al., 1971; Ahuja and Mathur, 1967),kinetica (Cullis et aL, 19711, and structure sensitivity effects (Hicks et al., 1990a,b) has attracted attention. It has been shown that the order of the reaction with respect to oxygen and methane is 0 and 0.5, respectively (Cullis et al., 1971). Investigators have reported that methane oxidation is a structure-sensitive reaction. These investigators conclude that the crystalline form of Pd (large particles) is more active than the dispersed form (small particles) (Hicks et al., 199oa,b). It has been observed that the Pd/A1203catalyst shows an increase in activity when pretreated in oxygen at 550 *C (Igarashi and Sakurai, 1989). Inveatigators have also reported that the activity of the Pd/A1203catalyst increases with the time it is on stream in the CH4-02-N2 reaction mixture (Briot and Primet, 1991; Baldwin and Burch, 199oa-c). These investigators speculate that the reconstruction of the palladium oxide crystallite may account for the observed catalyst activation. Recently the
* To whom correspondence should be addressed. 0888-5885/92/2631-2460$03.00/0
effect of chlorine poisoning on the methane oxidation activity of Pd/Al2O3 catalysts has been investigated (Simone et al., 1991). These investigators have shown that palladium nitrate based catalysts are more active than those prepared from palladium chloride. All investigations reported to date focue on the removal of methane from CH4-02 mixtures. The typical compressed natural gas vehicle exhaust mixture con& significant amounts of carbon monoxide, hydrogen,and nitric oxide in addition to methane and oxygen. No attempt has been made to systematically study the removal of methane from such exhaust mixtures. This investigation attempts to fill this void. The objedive of this investigation is to study the modea of methane removal from shulated CNG vehicle exhaust mixtures under net oxidizing, net reducing, and stoichiometric conditions. Model reaction studiea were conducted. The study suggests that the CH4-02 reaction contributes to the removal of methane under net oxidizing conditions. Two reactions contribute to the removal of CHI under net reducing conditions-the CHI-02 reaction and the CHI-NO reaction. The steam reforming of CH4doea not appear to significantly contribute to the removal of CH,. The study ale0 shows that the presence of carbon monoxide in the gas mixture gradually lowers the methane conversion when the redox ratio increases under net oxidizing conditions. The higher activity for the CH4-02 reaction resulting from a lowering in the overall oxidation state of palladium and the contribution of the CH4-NO reaction toward the removal of CHI appear to account for the relatively high CHI conversions observed under net reducing conditions.
Experimental Section Catalyst Preparation. The incipient wetness procedure was used to prepare the catalyst investigated in this study. Five grams of modified y-alumina powder (7-403 from Degussa Corp, BET surface area 100 m2/g, particle size 20-40 mesh) was contacted with 6 cm3of Pd(N03)2 (Johnson Matthey) solution of desired concentration to obtain a 1%Pd catalyst precursor. This precursor waa dried at 100 O C for 1h and later calcined at 600 OC for 6 h. Q 1992 American Chemical Society
Ind. Eng.Chem. Res., Vol. 31, No. 11,1992 2461
Flow Reactor Studies. A sample of 0.2 g of the 1% Pd/modified 7-A1203 catalyst was used in the flow reactor (integral)experiments. A description of such an apparatus is given by Gandhi et al. (1976). Rsaction studies were conducted using the following feed gas mixtures: (a) CH4-NWO-HZ-Oz; (b) CH4-NoCo-02; (c) CH,-N(F H2-02; (d) CH&; (e) C H d 2 4 0 ; ( f ) CH4-NO; (g) CK-NO-Oa (h)CH4-HzO. Methane (0-5000 ppm), nitric oxide (0-1200 ppm), carbon monoxide (0-3000 ppm), Hz (0-3000ppm), oxygen (U-100000 pprn), and water vapor (0-23000 ppm) were used in the feed gas mixtures. Nitrogen was used as the carrier gas, and the total feed gas flow rate was kept constant a t 3000 cm3/min for all the experiments. The experimental conditions provide a space velocity of WOO0 h-' based on the packed density of the catalyst sample (0.6 g/cm3). The feed gas concentrations used in the individual experiments are shown in the figurea. The conversions reported in the present study have an experimental error of &3%. The carbon monoxide, nitric oxide, and methane concentrations were measured using the Beckman 868 (IR analyzer), B951A (chemiluminescence), and 400 (flame ionization detector) analyzers, respectively. The oxygen concentration was measured using a Beckman OM 11-EA analyzer. Typically, the feed gas oxygen and nitric oxide concentrations were varied to change the redox ratio. The redox ratio, R, is a measure of the gas mixture stoichiometry and its related to the air-fuel ratio. It is the ratio of reducing to oxidizing components and is calculated from the partial pressures of the components in the feed gas: R = [CO + H2 + 4CHJ/[NO + 2021 (1) R > 1refers to a net reducing gas mixture, R = 1 refers to the stoichiometric gas mixture, and R < 1refers to a net oxidizing gas mixture. The generalized form of the redox ratio (eq 1)may be simplified on the basis of the components in the feed gas. For example, in the case of the CH4-CO-02 reaction, the above ratio reduces to R [CO + 4CH4]/202 (2) Studies conducted at different redox ratios are called R-scan experiments. It hae been o b e e ~ e dthat the CHI conversion increases when a Pd/A&03catalyst is aged in a CH4-02-Nz reaction mixture (Baldwin and Burch, 199Oa-c). The Pd/Al2O3 catalyst is known to show hysteresis effects; that is, different h e l a of CHI conversion are obtained depending on whether the catalyst is being heated or cooled (Farrauto et al., 1992). It has been o b e e ~ e din this laboratory that the activity of the modified Pd/A&03catalyst depends on the compoeition of the feed gas mixture (net oxidizing or net reducing) in which the catalyst temperature is increased from room temperature to 550 "C. The methane conversion ala0 depends on whether the gas composition during the R-scan experiment is being made progressively oxidizing or progressively reducing. Hence all the experimenta in the present study were conducted such that the catalyst temperature was raised to 550 OC under oxidizing conditions (at the lowest value of R reported) and the R-scan data were obtained by making the feed gas progressively reducing. Rerults and Diecursion Figure 1 show a typical plot of the steady-state conversions for methane, carbon monoxide, and nitric oxide for the modified Pd/Al2O3 catalyst as a function of the redox ratio. The methane, carbon monoxide, and nitric oxide concentrations in the feed gas mixture are repre-
80
z a
E
I
u
20 0 0
0.5
1.5
2.0
R Value Figure 1. Typical redox data for a 1%P d / m d i e d y-AlzO, catalyst using a simulated compressed natural gas fueled vehicle exhaust mixture. The feed gas containa CHI (300 ppm), CO (22M) ppm), Hz (750pprn), O2(varied between 0.08 and 2.0%), NO (425 pprn), and Nz(carrier) a t 550 "C.
sentative of the exhaust from a compressed natural gas fueled engine operating at an +fuel ratio of 13 (Thomas et d,1986). The salient featurea of the methane removal curve shown in Figure 1 are (a) a decrease in the CHI conversion under oxidizing conditions leading to a minimum in the methane conversion slightly lean (or net oxidizing) of stoichiometry (at a redox ratio of 0.8), (b) a increase in methane conversion around the stoichiometric point, and (c) relatively high methane conversions under net reducing conditions (with respect to those observed under net oxidizing conditions). The objective of the present investigation is to identify the modes of CHI removal under net reducing and net oxidizing regions and thereby identify the factors responsible for the observed CH4conversions as a function of the oxygen concentration as shown in Figure 1. Model reaction studies are conducted. The results of the reaction studies are discussed in the following sections. Ekperimenta were conducted with feed gas mixtures containing CH4-H2-NO-02 and CH4-CONO-02. The results of these studies are presented first (Figures2 and 3). The results of the CH4-02 reaction are presented next (Figures 5 and 6). The results observed during the CH4-NO (Figure 91, CH4-NO-02 (Figure l l ) , and CH4-H20model reaction studies are discussed later. CH4-NO-H2-02 and CH4-NOxO-O2 Reactions. Experimentswere conducted to first identify the feed gas component accounting for the observed decrease in CHI conversion under net oxidizing conditions. Carbon monoxide (2250 ppm) in the simulated CNG vehicle exhaust mixture (Figure 1)was replaced with an equivalent amount of Hz (2250 ppm) to obtain a CH4-H2-NO-02-contn;nin9 feed gas mixture. (Note that this feed gas mixture contains no carbon monoxide and the CH4and NO concentrations are identical to those used to simulate the CNG exhaust mixture.) The results of this model reaction study are shown in Figure 2. It is observed that the methane conversion remains constant (within experimental error of *3%) under net oxidizing conditions when carbon monoxide is not present in the feed gas mixture. Likewise, H2 (750 ppm) in the simulated CNG vehicle exhaust mixture was replaced with an equivalent amount of CO (750 ppm) to obtain a CH4-CO-No-02-containing feed gas mixture. The resulta for this model reaction are shown in Figure 3. The methane conversion decreases from 25 to 10% when the redox ratio changes from 0.1 to 0.8. Figure 4 shows the methane conversion as a function of the redox ratio for varying CO/H2 ratios. (It may be noted that the CHI conversions shown in Figure 4 have been taken from the redox data reported in Figures 1-3.)
2462 Ind. Eng. Chem. Res., Vol. 31, No. 11, 1992
f I
U
60
l---k--20
0
0 0
0.5
1.a R Value
I .5
2.0
Figure 2. Redox data for the model reaction conducted with CHI (300 ppm), Hz (3000 ppm), O2 (varied between 0.08 and 2.0%), NO (425pprn), and N2 (carrier) at 550 "C. I
'"I
o CHI Converltd to CO
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2o12--#ed.o 00
0-41 D
'
0.5
1.a
1
'
1
'
1
1.5
'
1
'
1
J 2.0
R Value
Figure 3. Redox data for the model reaction conducted with CHI (300 ppm), CO (3000 ppm), O2(varied between 0.08 and 2.0%),NO (425ppm), and N2 (carrier) at 550 OC.
t
I
A
", 25 u I
s
0
0.5
1 .o R Value
I .5
2.0
Figure 4. Methane conversion as a function of the redox ratio with CO/H2 ratio as the parameter.
The CHI conversion decreases when a CO-containing feed gas becomes progressively reducing and reaches a minimum value at R equal to 0.8. These investigations show that the presence of CO in the feed gas mixture leads to a decrease in the methane conversion under net oxidizing conditions. In a study conducted on supported Pd catalysts, it was found that the adsorption capacity of Pd decreases over several cycles of CO adsorption (at 25 "C) and reduction (at 400 "C) (Chou and Vannice, 1987). These investigators suggest that CO adsorption alters the structure of Pd crystallites. Recently, it has been proposed that cycles of CO exposure and reduction render the Pd crystallite surface smooth whereas cycles of oxidation and reduction roughen the surface (Hicks et al., 1990~). In the present study, a decrease in methane conversion is observed under net oxidizing conditions only when CO is present in the feed gas. The CO/O2 ratio varies from
0.5
1
1.5
2.0
R Value Figure 6. Redox data for the model reaction conducted using CH4 (5000 ppm), O2 (varied between 0.5 and 4%), and N2 (carrier) at 550 "C.
0.11 to 2.69 as the redox ratio changes from 0.1 to 2.0. As shown in Figure 1, the minimum in CH4 conversion is observed at a redox ratio of 0.8 where the CO/02 ratio is 0.93. The CO-induced decrease in CH4conversion is significant even at space velocities as low as 10000 h-' (Subramanian et al., 1992). It is tempting to speculate that the observed decrease in CHI conversion under net oxidizing conditions may result from the restructuring of Pd crystallites due to changes in the CO/O2ratio. However such an explanation is not plausible based on the following considerations: 1. Surface restructuring is in general a slow process and requires several heating cycles (Hicks et al., 1990~). 2. The concentration of CO in the feed gas is quite low (2250 ppm). The CH4:NO-Hz-O2 and CH4-NO-CO-OZ model reaction studies suggest that the presence o€CO in the fed gas leads to a gradual decrease in CHI conversion with increasing R when conditions are net oxidizing. In the following section, the increase in CHI conversion around the stoichiometric point is examined in relation to the results observed during the CH4-O2 and CH4-02-C0 model reaction studies. CH4-Oz and CH4-02-C0 Reactions. Figure 5 shows the transient behavior in CHI conversion over a modified Pd/A1203catalyst at 550 OC. This shows that the catalyst achieves steady state only after 60 min of reaction time. This observation is consistent with those reported in the literature (Baldwin and Burch, 1990a,b). These investigators have reported the increase in CH4oxidation activity with time when a Pd/A1203 catalyst is placed in a oxygen-rich CH4-02 mixture. They speculate that the Pd crystallites in the Pd/AlZO3catalyst restructure in the CH4-02 mixture; the restructuring apparently accounts for the increase in CH4 conversion with time.
Ind. Eng. Chem. Res., Vol. 31, No. 11, 1992 2463 The results for the CH4-O2 reaction with O2 concentration as the variable are shown in Figure 6. The data points at different R values were obtained by changing the oxygen concentration. Once steady state was achieved at the lowest R value reported (Figure 61, steady state at higher R values was attained within 30 min. The data points reported in Figure 6 are steady-state conversions. A survey of the literature shows that there is no information available on the speciationof the products observed during the oxidation of CHI over the catalyst used in this investigation. It has been reported that partial oxidation products are not observed during the oxidation of CHI over bulk noble metals (Yu Yao, 1980; Firth and Holland, 1969a). However, the formation of HCHO, CO, and H2 during the oxidation of CHI over Pd/A1203catalysts at temperatures lower than 400 "C has also been reported (Cullis et al., 1971). CO is the only partial oxidation product at temperatures greater than 400 "C. In the present investigation,the formation of CO was monitored; CO formation was observed at redox ratios higher than 1.3. The fraction of CHI converted to CO (i.e., ratio of CO concentration at exit to the CHI concentration at inlet) is shown in Figure 6. The data show that approximately 10% of CHI in the feed is converted to CO at R equal to 2. Auxiliary experiments were conducted where Fourier transform infrared spectroscopy was used to perform the product analysis. These investigations did not show the formation of partially oxygenated products such as formaldehyde. The above studies suggest that C02and H20 are the only oxidation products at redox ratios lower than 1.3 and CO is observed at redox ratios higher than 1.3. The results presented in Figure 6 show that the methane conversion increases around the stoichiometricpoint. This observation is consistent with that reported recently on Pd/A1203catalysts (Oh et al., 1991). When the redox ratio is greater than unity, the amount of oxygen in the feed gas is less than that required for the complete (100% conversion) stoichiometric removal of CHI as shown in eq 3. CH4 + 202 --* COP + 2H20 (3) The amount of unconverted O2is lower when the feed gas mixture is net reducing or rich in CHI. Previous studies conducted on Pd wires and supported Pd catalysts have shown that Pd metal is more active than Pd oxide for methane oxidation (Yu Yao, 1980). It haa also been shown that PdO dispersed over A1203 is much less active than the oxide covering the face centered cubic (fcc) Pd crystallites (Hicks et al., 199Ob). The existence of species such as PdO, has been proposed (Farrauto et al., 1992). It has been speculated on the basis of thermogravimetry measurements that A120,-supported PdO forms PdOJPd (i.e., PdO, covers Pd metal) and Pd under oxidizing conditions at temperatures less than 570 OC. These considerations suggest that the "effective" or overall oxidation state of Pd may be lower when the feed gas changes from net oxidizing to net reducing. The lowering in the oxidation state of Pd may account for the observed increase in CHI oxidation activity around the stoichiometric point. The CH4-02 reaction was conducted in the absence and presence of CO in order to examine the effect of CO on the CHI-O2 reaction. The results for the CH4-02-C0 reaction are shown in Figure 7. The CHI concentrations wed for the CHI-O2 (Figure 6) and CH4-CO-02 (Figure 7) model reaction studies were different since the detection limits of the analyzers had to be accommodated. Quantitative comparison between the data reported in Figures 6 and 7 is not feasible. However a qualitative comparison can be made to investigate the trends. Such a comparison shows that a gradual decrease in CHI conversion is ob-
f
0
--
I
0.5
1.o R Value
I .5
2.0
Figure 7. Redox data for the model reaction conducted using CH, (300 ppm), CO (3000 ppm), O2(varied between 0.1 and 2.85%),and N, (carrier) at 550 OC. 100
80
.-I
$
60
I
V
40 20
0
0.5
1.o R Value
1.5
2.0
Figure 8. Redox data for the model reaction conducted using CHI (150 ppm), NO (varied between 300 and 1500 ppm), and N2(carrier) at 550 O C .
served with increasing R under oxidizing conditions only in the presence of CO. One may speculate since CO is more readily oxidized than CHI, most of the available oxygen is consumed by CO and this may lead to a lowering in CHI conversion. However this argument is inconsistent with the proposal that the oxidation of CH4is zero order in oxygen (Cullis et al., 1971). Recall that the CHI conversion gradually decreases under net oxidizing conditions with increasing R when CO is included in the H2- and NO-containing feed gas mixture (Figure 4). On the basis of the above considerations we speculate that the lowering in CHI conversion under oxidizing conditions may result either (i) from the changes in the interactions between Pd and 0 in the presence of CO or (ii) from the alterations in the chemisorption patterns (competitivechemisorption) of CHI, 02,and C02when CO is present in the feed gas. CH4-N0 Reaction. Figure 8 shows the results observed for the oxidation of methane with nitric oxide in the absence of oxygen. In this experiment, the NO concentration was varied to change the redox ratio. NO and CH4react under net oxidizing conditions as well as under net reducing conditions. The CHI and NO conversions under net reducing conditions are higher than those observed under net oxidizing conditions. As in the case of the CH4-O, reaction, the increase in CH4 conversion occurs around the stoichiometric point. The overall oxidation state of palladium may be lower when the feed gas composition changes from net oxidizing to net reducing. The considerations discussed in conjunction with the CH4-02 reaction results apply here; the lowering in the overall oxidation state of Pd may account for the increase in activity around the stoichiometric point. The ratio of the amount of nitric oxide converted to the amount of methane removed as a function of the redox
2464 Ind. Eng. Chem. Res., Vol. 31, No. 11, 1992
0
1 .o
0.5
1.5
2.0
R Value
Figure 9. Stoichiometry of the CH4-N0 reaction as a function of the redox ratio.
ratio is shown in Figure 9. It is observed that the above ratio varies from 4.3 to 2.8 when the redox ratio increases from 0.5 to 2.0. This suggests that the product distribution varies with the feed gas composition. On the basis of the reaction stoichiometry 4N0 + CH4 -w 2Nz + COP + 2H20 (4) the ratio of NO to CHI consumed should be 4. Values higher than 4 can in principle be explained by the formation of NO2 and NzO. Several investigators have investigated the CH,-NO reaction over supported Rh and Pt catalysts (Hardee and Hightower, 1984; Hu and Hightower, 1976). These investigators show that N20 is formed as a reaction product only at temperatures lower than 400 "C. Above 400 OC, no N20 is formed. In the present study, the formation of N20can be ruled out on the basis of the temperature at which the CHI-NO reaction was carried out, i.e., 550 O C . NO2was not monitored separately. The NO, concentrations were determined using an analyzer (chemiluminescence). We speculate that the NO/CH4 consumption ratios exceeding 4 result from formation of NO2. Values lower than 4 may be explained by CO formation. If the reaction proceeds as shown in eq 5, the NO/CH4 consumption ratio should be 3. These consid6N0 + 2CH4 3N2 + 2c0 + 4H20 (5) erations suggest that, under reducing conditions ( R = 21, catalyst coking may occur to some extent and this may deactivate the catalyst. CH4-N0-O2 Reaction. The removal of methane in the ternary system, CH4-N0-02, was studied. These results are shown in Figure 10. Here the oxygen concentration was varied to change the redox ratio. Qualitatively,in contrast to the CH,-NO reaction, NO conversion is observed only under net reducing conditions in the CH,-NO-O, system. This suggests that, in the presence of 02,nitric oxide reacts with CH, only under reducing conditions. The CH, conversion increases around the stoichiometric point. The CHI conversion under reducing conditions is higher than that observed under oxidizing conditions. It may be noted that the CHI concentration in the feed gas for the CH4-NO-02 study is different from that used for the CH4-02 and CH4-NO reactions. The CHI feed gas concentration was changed to accommodate the detection limits of the analyzers. It is not appropriate to compare the conversions observed on a one-to-one basis. However, on the basis of the similarities in the shapes of the NO and CHI removal curves in these systems, we speculate that the removal of CHI occurs by two parallel reactions under net reducing conditions: the CH4-02 reaction and the CH4-NO reaction. In contrast, the oxidation of CHI under net oxidizing conditions occurs by the CH4-02 reaction.
-
00 .
0.5
1 .o
1.5
0
0
R Vilue
Figure 10. Redox data for the model reaction conducted using CHI (3000 ppm), NO (1000 ppm), O2 (varied between 0.25 and 5.95%), and N4 (carrier) at 550 "C.
CH,-H20 Reaction. It is known that the higher hydrocarbons such as propane and propylene react with water formed from H2 present in the feed gas in what is called the steam reforming reaction (Muraki et al., 1986; Schlatter, 1978). It has been reported that a part of the propane (or propylene) present in the simulated gasoline car exhaust is removed by the steam reforming reaction when the feed gas is net reducing (Muraki et al., 1986). It has been reported that the steam reforming of CH4occurs at 800 "C over a K20-promoted NiO/ar-A1203catalyst (Pitchai and Klier, 1986). Studies were conducted to ascertain the reaction between CH, and H20. Nitrogen was bubbled through water to generate a feed gas containing 23 OOO ppm H20. The methane conversion was monitored as a function of temperature. The methane conversion at 550 OC was lower than 5%. These results indicate that negligible amounts of methane react with water at temperatures lower than 550 OC. Consequently, steam reforming of CH, may not significantly contribute to the removal of CH, from CNG vehicle exhaust mixtures. Summary The results from the CH4-NO-Oz, CH4-02, and CHINO model reaction experiments conducted during this study on the modified Pd/Al2O3 catalyst show that the removal of CHI under net oxidizing conditions occurs by the oxidation of CHI with 0% In contrast, the removal of CH, under net reducing conditions occurs by two reactions, the CH4-02 reaction and the CH,-NO reaction. A part of the CH, is oxidized to CO under reducing conditions (R > 1.3). One has to be careful while extending the interpretations of the results observed from the model reaction studies to those occurring during the removal of CH, from the simulated CNG-fueled vehicle exhaust. The presence of species such as CO and H2may alter the reaction modes. It has been observed that the CH4 conversion gradually decreases with increasing R under net oxidizing conditions when CO is present in the feed gas. A minimum in the CHI conversion is observed when the redox ratio is 0.8. It suffices to say that the CH4-02 reaction contributes to the removal of CHI from CNG vehicle exhaust under net oxidizing conditions and the CH4-02 reaction and the CH,-NO reaction contribute to its removal under net reducing conditions. The methane conversions observed under net reducing conditions are higher than those observed under net oxidizing conditions during all the reaction studies conducted. The higher activity for the CH4-02 reaction resulting from a lowering in the overall oxidation state of Pd and the contribution of the CHI-NO reaction toward the removal
Ind. Eng. Chem. Res., Vol. 31,No. 11, 1992 2465
of CHI appear to account for the higher CHI conversions observed under net reducing conditions. Steam reforming of CHI may not significantly contribute to the removal of CHI.
Acknowledgment The authors thank Drs.R. W. McCabe and K. Otto for critically reading the manuscript and Mr. W. L. H. Watkine for determining the product speciation using Fourier transform infrared spectroscopy. &&&,try NO.CHI, 7482-8;CO, 630-080;NO,10102-43-9;Pd, 1440-05-3;HP, 1333-14-0;02,7182-44-7.
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Received f o r review March 10, 1992 Revised manuscript received June 23, 1992 Accepted July 31, 1992
