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Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 207-212
CATALYST SECTION Palladium-Tungsten Catalysts for Automotive Exhaust Treatment Karen M. Adams’ and H a m S. Gandhl Engineering and Research Staff, Ford Motor Company, Dearborn, Michigan 48 12 1
Recent efforts to modify palladium catalysts with non-noble metal oxides have resulted in desirable catalytic activiiies for Pd-W03/y-A1203. Hydrocarbon, carbon monoxide, and nitric oxide conversions, as a function of redox ratio of a synthetic exhaust, and the light-off temperatures have been measured. These data demonstrate that the Pd-W03 catalyst can: (1) oxidize saturated hydrocarbons, with higher conversions than Pt when the synthetic exhaust is reducing, and (2) Selectively reduce NO to N, atthough with lower converslons than Rh. A nonaptimized Pd-W03 catalyst, aged to simulate 15 000 miles vehicle durability, maintained saturated hydrocarbon activity at levels at least as high as those of aged production Pt-Rh and Pt-Pd catalysts. Model reactions for propane and sulfur dioxide oxidation have been studied. The volatility of W03/y-AI,03 and the effect of W03 on the y-A1203 surface area have also been investigated.
Introduction Automotive catalysts presently in use contain platinum (Pt),rhodium (Rh), or palladium (Pd) as active components. Pt oxidizes saturated and unsaturated hydrocarbons (HC) and carbon monoxide (CO). In particular, it is currently the only practical catalyst which possesses the high oxidation activity for saturated hydrocarbons (Kummer, 1980; Yao, 1980) required for automotive catalysts. Rh is used in three-way catalysts (TWC) for its activity to selectively reduce nitric oxide (NO) to nitrogen (N,) with low ammonia (NH,) formation (Kummer, 1980; Shelef and Gandhi, 1972; Schlatter and Taylor, 1977). Pd is more active than Pt for CO and unsaturated hydrocarbon oxidation (Kummer, 1980; Yao, 1980) and is used together with Pt in oxidation catalysts. However, it is a poor catalyst for saturated hydrocarbon oxidation and selective NO reduction. The work reported herein is an extension of our ongoing research to modify catalytic properties of Pd by incorporating non-noble metal additives so as to overcome some of the deficiencies of non-modified Pd catalysts. Current efforts have resulted in the development of Pd based catalysts with behavior more characteristic of Rh and Pt. The first development was that of Pt-MOO, and Pd-Moo, catalysts which produced active and selective conversion of NO to N2 analogous to Rh (Gandhi et al., 1980,1982). The next development is this area has been the Pd-W03 catalyst (Adams and Gandhi, 1981; Gandhi and Adams, 1981a,b). (The symbol, Pd-W03, used for the palladiumtungsten catalyst, is not meant to imply any oxidation or chemical state, for either element. These will depend on the thermal and redox conditions to which the catalyst is exposed.) In this report, catalytic activities and other properties relevant to its use in automotive exhaust treatment are examined for Pd-W03 supported on yAlz03. Experimental Section A. Catalysts. Catalysts were prepared on cordierite monoliths with 400 square channel cells per square inch 0196-4321 163/1222-0207$01.50/0
Table I. DescriDtion of Monolithic Catalysts
BET
compn, wt % cat. Pd-W03 Pd
wo3 Pt Pd
Pt
0.22
-
Pd 0.15 0.18
-
0.15
W 4.8
-
4.2
-
y
- Al, 0
a
S(Catapa1) 8 (Catapal) 9 (Catapal) 1 0 (Catapal) 1 6 (Alumina “C”)
area. m2k 20
20 20 20
-
Catapal is made by Conoco Chemicals Co., and Alumina “C” by Degussa. Pd catalyst used for SO, oxidation study. a
and 6 mil wall thickness. The monolith was first coated with a y-A1203washcoat by dipping in an aqueous suspension, then dried and calcined in air at 600 “C for 4 h. Next, the active metals were impregnated using an aqueous solution of PdC12 in 4% (vol) HNO,, a solution of WO,. H 2 0 in concentrated NH40H, and an aqueous solution of H2PtCl,y6H20for Pd, W03, and Pt catalysts, respectively. The catalysts were dried and calcined in air for 4 h at 500 “C for Pd and Pt and a t 300 “C for WO,. The Pd-WO, catalyst was prepared by first impregnating with tungstic solution, dried, and calcined, followed by impregnation with the Pd solution, and again dried and calcined. Table I gives the composition of the catalyst samples. The active metals were analyzed by X-ray fluorescence (XRF);the y-A1203washcoat content, by weight gain. The surface areas were determined by the BET method and are reported per unit weight of the whole monolithic catalyst. Samples used in the flow reactor and pulseflame in. long. reactor studies were in. in diameter by ’I2 B. Flow Reactor Studies. The flow reactor apparatus, testing procedures, and method for gas stream analysis have been described previously (Gandhi et al., 1976). The synthetic exhaust feedgas mixture was composed of 1.5% CO, 0.5% Hz, 1000 ppm C as C3H6,500 ppm C as C3H8, 1000 ppm NO, 0.6% to 3.2% 02,20 ppm SO2 and the balance N2. The propylene (C3H6)and propane (C3H8) 0 1983 American Chemical Society
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Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 2, 1983
were used to represent the fast-burning and slow-burning hydrocarbons, respectively. The oxidizing and reducing characteristics of the feedgas are quantitatively defined by the redox ratio R [PCO + P H Z + (3n)PC,Hz, + (3n + l)PCnHOn+zl R= [ P N O + 2P021 where p is the partial pressure of the indicated gas. For R < 1the feedgas mixture is oxidizing, or lean, and at R > 1, the feedgas is reducing, or rich. Activities and selectivities were reported as percent conversions of HC, CO, gross NO, and net NO (i-e.,NO converted to N2),and NH3 formation as % NO converted. These were measured at 550 "C and 60000/h space velocity. The activity measurement as a function of R is designated as an R-scan. The R is changed by adjusting the oxygen content at the inlet. Light-off temperature is an index of activity measured by raising the temperature, using a gas composition at R = 1, and designated as the temperature needed for 50% CO conversion. Flow reactor data have been collected under steady-state conditions. Propane oxidation was studied with a model feedgas of 500 ppm C as C3Hs, 20 ppm SO2, varied 02,and the balance N2, a t 550 "C and 60000/h space velocity. These activities were also measured over a range of R values by varying the O2 concentration. Sulfur dioxide (SO2) oxidation was measured with a feedgas of 250 ppm SO2,4.5% 02,and with and without 0.2% CO, over the temperature range of 250 to 550 "C, at 60000/h space velocity, and under steady state. The method of gas analysis of SO2 was determined by a microcoulometer (Butler et al., 1980). The method requires sample dilution, which was done with ambient air. C. Pulse-Flame Reactor Studies. The pulse-flame reactor (pulsator) has been used to age catalyst samples for 15000 simulated miles (Gandhi et al., 1976). In order to simulate high temperatures encountered in engine operation, the pulsator durability test cycle was modified such that for 4% of the aging time the exhaust gas is at 950 "C and contains 2% excess O2 (Stepien et al., 1980). A simulated certification fuel was burned which contained 6 mg of Pb, 0.8 mg of P, and 0.03 wt % S per gallon of isooctane. Activities for HC, CO, and gross NO have also been measured on the pulsator after aging. Conditions for these data are dynamic with respect to the redox ratio of the exhaust gas. The pulsator has a natural modulation of the air to fuel ratio (A/F), such that the amplitude is i l A/F, which is equivalent to i 2 . 0 R units, and the frequency is 0.5 Hz. D. Tungsten Oxide Volatility Study. Samples of various concentrations of W03 (1.2% to 5.0% (wt) W) supported on 7-A1203washcoated monoliths were prepared as described above, but these were calcined at 500 "C instead of 300 "C, to ensure complete decomposition and oxidation of ammonium tungstate. These samples were aged in the pulsator to determine if W03 volatilizes under extreme conditions of simulated vehicle durability. The aging was done at 800 "C, for 4 h, and in oxidizing conditions (R = 0.34). Fresh and aged samples were analyzed for W by XRF. A sample with 7-A1203washcoat only accompanied each WO3/7-Al2O3sample during the pulsator aging to determine the effect of W03 on the 7-A1203surface area. The BET surface areas were determined after aging. Results and Discussion A. Activity Measurements. 1. R-Scan. The steady-state activities for HC. CO, and NO are shown as
-ae
100
;80 260 $j 40 20
*
__
OR
17
IF.
REDOX R A T ~ OR,
8
?n
0
12
08
16
20
REDOX RATIO, R
Figure 1. Steady-state conversion efficiencies as a function of redox ratio of synthetic exhaust gas mixture for (a) Pt, (b) Pd, (c) W03, and (d) Pd-W03, at 550 "C and 60000/h space velocity.
0
j
I
I
0
02
04
I W % (
06
-
I
08 10 REDOX RATIO
-
I
12
-
I
I
14
16
I8
Figure 2. Comparison of propane conversion efficiencies, calculated from the total HC conversions shown in Figure 1, as a function of redox ratio of synthetic exhaust gas mixture.
a function of redox ratio of the simulated exhaust feedgas for Pt, Pd, WO,, and Pd-W0, catalysts in Figure 1. A comparison of these activities is discussed below. The total HC conversion in Figure 1 consists of conversion up to 63% from the "easy to oxidize" unsaturated HC, propylene, and the additional conversion, 3770, from the "hard to oxidize" saturated HC, propane. The saturated HC activities have been calculated from the total HC conversions (shown in Figure l), and are compared in Figure 2 as propane conversions. The P d only catalyst shows poor activity for propane oxidation rich of stoichiometry (R 2 l), and the WO, only catalyst is not active for propane oxidation at all. However, the Pd-W03 catalyst has good propane conversion under reducing conditions (R > l),which is better or equal to the Pt catalyst. For example, at R = 1.6 propane conversions are 65% for Pd-W03, 56% for Pt, and 23% for Pd. Under oxidizing conditions the ranking for propane oxidation changes to Pt > P d > Pd-WO3. The CO oxidation activity of the Pd-WO, catalyst is much poorer than for the P d and Pt catalysts (Figure 1). Under reducing conditions a t R = 1.6, Pd-W0, has 24% CO conversion where the P d and P t catalysts have 40% and 41 % , respectively. Even under slightly oxidizing conditions at R = 0.9, the Pd-W0, catalyst has 81% CO conversion compared to 99-100% for the P d and Pt catalysts. The gross NO activity of the Pd-W03 catalyst is 37% at R = 1.6, but this catalyst is very selective for the NO to N, reaction such that the net NO conversion is 35%; Le., less than 5% of the converted NO forms NH3 at R = 1.6. Both the P d and Pt catalysts show higher NO conversions at R = 1.6,71% and 63%, respectively. However, P d and Pt catalysts are known to have poor selectivity for converting NO to N2under reducing conditions (Shelef and Gandhi, 1972), such that NH, formations, as % NO con-
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 2, 1983 209 100
I
'"-
-
I
--# 80 -
$ 80 2
$, 60
z
K W
2 40 0 V
20
0 0.6 0.8 1.0
1.2 1.4 1.6 R E D O X RATIO, R
1.8 2.0
Figure 3. Steady-state conversion efficiencies as a function of redox ratio of synthetic exhaust gas mixture for Pd followed by Pd-W03, at 550 O C and 60000/h space velocity.
verted, are 30% and BO%, and net NO conversions are 49% and 12% for the P d and Pt catalysts, respectively, at R = 1.6. The catalyst with W03 alone shows negligible NO conversion activity. 2. Light-off. The temperature a t 50% CO conversion for Pd-W03 (TM%co= 365 "C) is substantially better than for Pt (TM4bco= 420 "C) but inferior to that for Pd ( T M % ~ o = 310 "C). P d is presently used in oxidation catalysts in combination with Pt to achieve lower light-off temperatures. This lower light-off temperature of P d is observed in a stoichiometric as well as in an oxidizing feedgas. 3. Dual System: Pd followed by Pd-W03. As discussed above, Pd-W03 shows good HC conversion under reducing conditions but is deficient in an oxidizing environment, in addition to its poorer CO oxidation, NO reduction, and light-off activities. In order to overcome these deficiencies, the Pd-W03 catalyst was preceded with the P d catalyst. Since each of the catalysts in this dual system remained in its original dimensions, the constancy of space velocity, 60 000/h, was preserved by doubling the gas flow. The results are shown in Figure 3 as a function of redox ratio. High overall HC, CO, and NO conversions are achieved over a wide range of R values with minimum NH3 formation under reducing conditions. For example, a t R = 1.6 net NO conversion is 64%, and NH3 formation, as percentage of converted NO, is only 6%. Under lean conditions a t R = 0.6 (2% O2 in feedgas), CO conversion is 100% and total HC conversion is 92%. The peak simultaneous conversion of HC, CO, and NO is achieved near R = 1,where HC, CO, and NO conversions are 98%, 99%, and 74%, respectively. The temperature of 50% CO conversion (350 "C) has been lowered by 15 "C as compared with Pd-W03 alone. B. Propane Oxidation. Interesting results were obtained from a study of the propane oxidation over the Pd-W03 and P d catalysts, separately. The propane conversions for a model reaction feedgas of C3H8,02,and SO2, which is described in the experimental section, are compared in Figure 4 for these two catalysts. P d is a much more active catalyst than Pd-W03 for this model mixture over the range of R values (-0.1-1.8). However, as shown earlier (Figure 2), in the presence of the synthetic exhaust, Pd-W03 has much greater propane oxidation activity than P d under reducing conditions. Thus, it is apparent from comparison of Figures 2 and 4 that the propane activity is not only dependent on the catalyst but also on the feedgas composition. In order to determine the source, and possible mechanism, for the effect of feedgas composition on propane oxidation, various components of the synthetic exhaust gas were systematically added to the C3H8-02-S02 mixture. Figure 5 containss the propane conversions as a function of redox ratio measured over the Pd-W03 catalyst for
0
02
04
06
0.8 IO REDOX RATIO
12
14
16
18
Figure 4. Propane conversions compared for Pd-W03 and Pd as a function of redox ratio of feedgas mixture of 500 ppm (0,0)or C as C3H8,20 ppm SOz,varied 02, and Nz balance, at 1650 ppm (0) 550 O C and 60000/h space velocity. '--
I
I-
OL
0
I
I
02
I
04
I
06
1
I
OS IO REDOX RATIO
I
12
I 14
I
16
I IB
Figure 5. Propane conversions for Pd-W03 as a function of redox ratio for the following feedgases: (a) 500 ppm C as C3H8,20 ppm SOz,varied 02, and Nz balance (- -); (b) synthetic exhaust feedgas (-); (c) 1% CO with (a) (d) 0.3% CO with (a) (e) 1% Hz with (a) (--); a t 550 "C and 60000/h.
-
(-e-);
(-.e-);
various feedgas compositions. The conversions for the synthetic exhaust and model reaction gas mixtures over Pd-W03 from Figures 2 and 4, respectively, are replotted in this figure. The other compositions that are compared with these two are (a) 1% CO; (b) 0.3% CO; and (c) 1% H2, each added to the model reaction mixture. The addition of 1% CO improved the propane oxidation activity as compared to the model feedgas to conversions similar to those measured with the synthetic exhaust, which contains 1.5% CO. These data show that CO has a dramatic effect on the propane oxidation activity of Pd-W03, in both the oxidizing and reducing environments. It is postulated that CO in the feedgas may lower the average oxidation state of P d and tungsten oxide (even in oxidizing conditions), and this reduced state is the one more active for the oxidation of propane. The CO, as pointed out earlier, is relatively poorly oxidized over PdW03, and therefore it will remain in the reaction mixture over the length of the catalyst to promote surface reduction. Without the CO, the model feedgas requires a relatively high redox ratio for propane to create the reduced, active surface. The H2, in contrast to CO, is oxidized rapidly a t the front of the catalyst, and under oxidizing conditions there is little or no H2 available for maintaining the catalyst in a reduced state. Under reducing conditions, R > 1.2, there is probably insufficient oxygen left for propane oxidation after rapid H2 oxidation. The propane conversion peaks sharply just rich of stoichiometry, where more oxygen would be available and yet conditions are still reducing. Thus, if one assumes that hydrogen is oxidized much more rapidly than propane, the effective R value may differ from
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 2, 1983
210
Table 11. SO, Oxidation‘ for Pt, Pd, and Pd-WO, Pt, 9%
Pd, %
Pd-WO,, %
T , “C
CO= O
C O = 0.2%
CO= O
250 350 450 550
3 19
0 36 76 79
I;
5
4
17
7
25 32
71 81
28
C O = 0.2%
a Feedgas composition: 250 p p m SO,, 4.5% 0,, 0 or 0.2% CO, balance N,. from “JANAF Thermochemical Tables”, 2nd ed., NSRDS-NBS 37, 1971.
100,
0
0
1
02
04
06
08 IO REDOX RATIO
12
14
16
18
Figure 6. Propane conversion for Pd as a function of redox ratio for the following feedgases: (a) 500 ppm C as C3HB,20 ppm SO2; varied 02,and N2 balance (- - -); (b) synthetic exhaust feedgas (-1; (c) 1.5% CO with (a) (--); at 500 O C and 60000/h.
the nominal one. In mixtures rich of stoichiometry, the oxygen will consume hydrogen preferentially, which starves the oxidation of propane more than would be expected at the nominal R value, and the effective R value will be richer than in the absence of hydrogen. Conversely, in mixtures lean of stoichiometry, the effective R value will be leaner than the nominal one since excess oxygen is left after rapid oxidation of hydrogen, or other easily oxidized components. The P d catalyst, in contrast to Pd-W03, has good propane oxidation activity with the model reaction mixture. When 1.5% CO was added to the model mixture, the propane conversions improved under oxidizing conditions, as shown in Figure 6. This effect is similar but not quite as dramatic as over Pd-W03 under oxidizing conditions, and for both catalysts, the CO probably produces a more active surface by lowering its average oxidation state. However, under reducing conditions, the addition of CO substantially suppressed the propane conversion, quite opposite to its effect on Pd-W03. This suggests that the reduced states of the Pd-W03 and Pd catalysts have quite different affmities for the relative oxidation of propane and CO, such that reduced Pd-W03 is more active for propane than CO oxidation. These results show that the oxidation activities of the reduced state of P d when supported on tungsten oxide, instead of directly on yAlz03,are different. Other modified reaction mixtures which were tested over the Pd catalyst included the addition of 1.5% CO + 0.5% Hz and the addition of 1000 ppm of C3H6 to the model reaction feedgas. Both these mixtures and also the synthetic exhaust showed significantly lower propane conversions under reducing conditions. This suggests that reduced P d supported on 7-AlZO3is active in oxidizing Hz and C3H6,in addition to CO, rather than the saturated hydrocarbon C3Hs. Under oxidizing conditions the use of the gas mixtures with CO + Hzand with C3H6also resulted in lower propane conversions. As discussed above, the effective R value is influenced by the competitive oxidation, which further suppresses the propane conversion. With the synthetic exhaust feedgas, the level of propane
cO=
0 3 0 13
o
C O = 0.2%
equilb %
0 6 23 32
100 100 99.6 98.0
Thermodynamic equilibrium calculated
conversion under the oxidizing conditions, shown in Figure 6, is a balance between enhancement due to a reduced, more active P d surface induced primarily by CO, and suppression due to the oxidizing effect on the catalyst surface of a leaner effective R value. Other more subtle effects associated with the presence of NO in the gas phase may also influence the extent of propane conversion. The above observations indicate that it is the surface composition of the catalyst and its response to the oxyreductive influence of the gaseous reaction mixture in contact with it which determines its relative catalytic activity. The R value, per se, does not uniquely describe this oxyreductive influence since CO is able to maintain the surface of the Pd-W03 catalyst in a more reduced, and more active, state than the equivalent concentrations of propane. Furthermore, the effective R value in itself is shifted from the nominal value by the presence in the gas mixture of reducing components of a widely differing affinity for oxidation. It is apparent from this study that the Pd-W03 differs from the P d catalyst in the activity of the more active, reduced state of the catalysts for propane and CO oxidation. The nature of the interaction of P d and tungsten oxide, or whether the latter is activated by the P d and cannot be ruled out as the active catalyst, is yet to be determined. Propane chemisorption, ESCA, electron microscopy, and other experiments are planned for PdW 0 3 and P d to gain further insight into the mechanism of propane oxidation over these catalysts. Preliminary examination by transmission electron microscopy (TEM) (Plummer et al.) has shown that after a reduction treatment, the P d particles on WO3/-pAlZO3sinter into twodimensional rafts, in contrast to three-dimensional Pd particles which are formed on the y-Alz03support. The two-dimensional rafts provide a larger surface area as well as a different crystal morphology, both of which may be related to increased activity for saturated hydrocarbon oxidation. C. SOz Oxidation. Table I1 contains SOz conversions for Pd-W03, Pt, and Pd, with a feedgas of SOz, Oz, and CO, described in the Experimental Section. The order of SO2oxidation activity with or without CO is Pt >> Pd 2 Pd-W03. However, in the presence of CO the SOz oxidation is enhanced for all three catalysts, although for Pt the enhancement is marginal above 450 “C and still well below thermodynamic equilibrium. Similar results have been reported for Pt and Pd elsewhere, with the exception that CO suppressed the SO2 oxidation over Pt (Gandhi et al., 1977). The reason suggested for the lowered SOz activity caused by CO over P t is competitive adsorption of CO and SOz on Pt where CO is preferred. For the data reported herein the feedgas level of CO is much lower and the level of SO2 much higher (0.2% CO, 250 ppm SOz) than that for the data reported elsewhere (1.0% CO, 20 ppm SOz) (Gandhi et al., 1977). The higher ratio of SO,/CO, used in this study, conceivably favors higher SO2surface coverage and oxidation activity
Ind. Eng. Chem. Prod. Res. Dev., Vol. 22, No. 2, 1983 211
so that the inhibition effect of CO with Pt is not observed under these Conditions. D. Pulse-Flame Reactor Durability. The dual catalyst, Pd followed by Pd-W03, has been aged on the pulsator as described in the Experimental Section and then evaluated both on the pulsator (i.e., under modulation conditions) and the flow reactor (i.e., under steady-state conditions). When evaluated as a TWC (at R = 1.15) on the pulsator, the activities are 70%, 35%, and 42% conversion for HC, CO, and NO, respectively, compared to 65%, 5890, and 60% conversion for an aged standard production Pt-Rh TWC with Pt/Rh = 11 and comparable noble metal content. When evaluated as an oxidation catalyst (at R = 0.34,2% excess 0,) on the pulsator, this catalyst exhibited equivalent HC and CO performance, 75% and 97% conversion, respectively, to an aged production Pt-Pd oxidation catalyst with P t / P d = 2 and comparable noble metal content. The HC activities measured on the pulsator are quite respectable since 25% of the HC in the pulsator feedgas is unreactive methane, and of the remaining 75%, 67% is unsaturated and 8% is saturated HC. A comparison of the steady state total HC conversion activity with that reported elsewhere (Williamson et al., 1980), for a Pt catalyst pulsator aged under normal temperature durability cycle, is given as catalyst Pd t Pd-WO, Pt
R = 1.15
7 5% 4 5%
R = 1.0 80% 65%
R = 0.8 70% 95%
I t should be noted that for these steady-state (flow reactor) activities, 63% of the HC conversion is accounted by propylene oxidation. Thus, the aged Pd plus Pd-W03 catalyst oxidizes propane over the range of R values, and for R 1 1.0 this activity is remarkable. The aged Pt catalyst oxidizes propane only under oxidizing conditions (R < l . O ) , where it is more active than the Pd plus Pd-W03 catalyst. The steady-state, saturated HC activities, presented earlier in this paper for fresh catalysts, also show the PdW03 catalyst to be less active compared to Pt and Pd catalysts under oxidizing conditions. However, the pulsator data exhibit equivalent, if not superior, saturated HC oxidation activity compared to Pt or Pd containing catalysts. The pulsator evaluations, which are modulated by essentially averaging the conversion activity over a range of R values, simulate more closely vehicle feedgas conditions than do steady-state evaluations. Since the Pd-W03 catalyst has such excellent HC oxidation activity for R 1 1.0, the averaging effect of the pulsator evaluations demonstrates the advantage of a Pd-W03 containing catalyst for saturated HC oxidation. One should bear in mind the possible role of the steam-reforming reactions which can take place in the exhaust from the pulsator (or vehicle) due to a high concentration of water vapor (>lo%)which is absent in our flow reactor studies. E. Tungsten Oxide Volatility Study. Samples of W 0 3 supported on 7-A1203, as described in the Experimental Section, were aged on the pulsator in oxidizing conditions a t 800 "C and showed tungsten losses ranging from 11% to 32% (wt) W, as reported in Table 111. However, the W 0 3 loss on samples aged with low concentrations, 1.2 to 3.0% (wt) W/23% 7-A1203,is close to being within the experimental error for the W analysis. This small loss is probably due to strong surface interaction of the dispersed phase of W 0 3 on 7-A1203,which lends to the stabilization of W03 to a large degree. The effect of the redox conditions of the aging atmosphere on the WO, loss is the subject of current investigation.
Table 111. Data from Tungsten Oxide Volatility Study
W concn, fresh,u % 1.2i 0.1 1.8 * 0.2 2.6 * 0.2 3.0 * 0.3 4.3 i 0.4 5.0 r 0.5
r-Al,O concn, %
23 23 23 23 23 23
W concn aged (ox.),% 1.0 i 0.1 1.6 i 0.2 2.2 i 0.2 2.6 r 0.3 3.3 i 0.3 3.4 * 0.3
W
aged surf. area, m21g
w/
loss, %
17 11 15 13 23 32
Al,O, 32 25 22 20 20 24
Concentrations in weight percentage of W. "C" from Degussa.
A1,0,
19 18 19 17 19 18 Alumina
It has been reported (Tittarelli et al., 1981) that there was no W03 loss for samples which were calcined in air at 1050 "C and which contained less than 5.6% (wt) W (7% (wt) W03) on 100% 7-A1203. This is equivalent to 1.3% (wt) W on 23% yAl,03 washcoated monolith in our study. The BET surface area, which is likely to affect volatilization, was -60 m2/g after the 1050 "C treatment for 15 h, compared to 108 m2/g of washcoat (25 m2/g of monolith) after our pulsator treatment for 4 h. It should also be noted that in the pulsator exhaust, increased volatilization of W03 is expected due to the presence of water (Meyer et al., 1959) and the very high flow rates with instantaneaus space velocities approaching 200000/h. The data in Table I11 also show that the 7-A1203surface area is stabilized when W03 is present.
Summary A notable finding for the Pd-W03 catalyst is its activity for saturated HC oxidation in reducing conditions of the synthetic exhaust feedgas. This activity is substantially improved compared to Pd alone such that it is better than or equivalent to that measured for Pt under these same conditions. The model reaction study of propane oxidation shows that in the presence of CO the Pd-W03 catalyst exhibits improved saturated hydrocarbon activity. It is suggested that the role of CO may be to reduce the average oxidation state of Pd and tungsten oxide to one more active for propane oxidation. In fact, this active state shows a greater affinity for propane than CO oxidation, where reduced Pd, supported on 7-A1203,is quite different in that the oxidation is partitioned such that CO is converted more effectively than propane. After pulsator aging, steady-state evaluations show that the Pd-W03 containing catalyst maintains excellent HC activity over the range of oxidizing and reducing conditions. The aged Pt catalyst still has better saturated HC conversion than Pd-W03 under oxidizing conditions; however, under stoichiometric and reducing conditions, the Pt catalyst loses all saturated hydrocarbon activity. When evaluated with A/F modulation, which more closely represent vehicle exhaust conditions, the aged Pd-W03-containing catalyst has HC activity equivalent to that of an aged production Pt-Pd COC and better than that of an aged production Pt-Rh TWC. The Pd-W03 catalyst also selectively reduces NO to N2, but its activity for NO conversion is much lower than an Rh-containing catalyst. The CO oxidation activity is also inferior, but when the Pd-W03 catalyst was combined with a Pd only catalyst, both the CO and NO conversions improved. The Pd-W03 as well as the Pd catalyst exhibit relatively low SO, oxidation activity compared to Pt. The low sulfate formation is important for applications where there is a high sulfur content in the fuel, such as diesel fuel.
Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 212-217
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Most crucial to the practical application of Pd-W03 for automobile exhaust clean-up is the thermal stabilization of WO, supported on yA1203Although the pulsator aged Pd-W03 catalyst maintains excellent saturated HC activity, the potential loss of WO, under operating conditions is not acceptable for toxicological reasons (Clayton and Clayton, 1981). Current efforts are directed to find a thermally stable form of a tungsten oxide which promotes the saturated HC oxidation activity for Pd. Acknowledgment We thank B. Artz and R. Belitz of the Analytical Sciences Department for the XRF analyses, D. Lewis and J. Perry for the pulsator aging and evaluation, W. Watkins for the tungsten oxide volatility data, and W. B. Williamson for providing assitance during the course of this study. We also thank Dr. M. Shelef for his review of the manuscript and helpful suggestions. Registry No. Pd, 7440-05-3; W03, 1314-35-8;carbon monoxide, 630-08-0; nitric oxide, 10102-43-9;propene, 115-07-1;sulfur dioxide, 7446-09-5; propane, 74-98-6.
Literature Cited Adams, K. M.; Gandhi, H. S. US. Patent Application 284759, 1981. Butler, J. W.; Schuetzie, D.; Coivin, A. D.; Korniski, T. J. Dearborn, MI, Jan 1980, EPA Contract 68-02-2787. Clayton, G. D.; Clayton, F. E. “Patty’s Industrial Hygiene and Toxicology”, 2A. 3rd ed.;Wiiey: New York, 1981; pp 1986-95. Gandhi, H. S.;Adams, K. M. US. Patent Application 284762. 1981a. Gandhi, H. S.; Adams, K. M. U S . Patent Application 284763, 1981b. Gandhi, H. S.; Piken, A. G.; Shelef, M.; Deiosh, R. G. SA€ Trans. 1976, 85, 901; SA€ 1978, 760201. Gandhi, H. S.; Piken, A. G.; Stepien, H. K.; Sheief, M.; Deiosh, R . G.; Heyde, M. E. SA€ 1977, 770196. Gandhi, H. S.; Watkins, W. L.; Stepien, H. K. US. Patent 4 192779, 1980. Gandhi, H. S.; Yao, H. C.; Stepien, H. K. ACS Symp. Ser. 1982, 178, 143. Kummer, J. T. Prog. Energy Combust. Sci. 1980, 6 , 177. Meyer, G.; Oosterom, J. F.; Oeveren, W. J. Red. Trav. Chim. 1959, 7 8 , 417; Chem. Abstr. 1960, 5 4 , 23f. Piummer, H. K., Jr.; Shinozaki, S.;Adams, K. M.; Gandhi, H. S., submitted to J. Mol. Catal. Schiatter, J. C.; Taylor, K. C. J. Catal. 1977, 4 9 , 42. Shelef, M.; Gandhi, H. S. Ind. Eflg Chem. Prod. Res. Dev. 1972, 1 1 , 393. Stepien, H. K.;Williamson, W. B.;Gandhi. H. S. SA€ 1980, 800843. Tittareili, P.; Iannibeiio, A.; Villa, P. L. J. SolM State Chem. 1981, 3 7 , 95. Williamson, W. B.;Stepien, H. K.;Gandhi, H. S. Environ. Sd.Techno/. 1980, 14, 319. Yao, Y. F. Y. Ind. Eng. Chem. Prod. Res. Dev. 1980, 19, 293.
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Received for review October 15, 1982 Accepted November 22, 1982
Oxidation of Ethanol and Acetaldehyde over Alumina-Supported Catalysts Robert W. McCabe’ and Patricia J. Mltchell Physical Chemistry Deparlment, General Motors Research Laboratories, Warren, Michigan 48090
The oxidations of ethanol and acetaldehyde were studied in a laboratory flow reactor over alumina-supported catalysts containing 4 wt % Cu-2 wt % Cr, 0.1 wt % Pt, and 4 wt % Mn, respectively. Most experiments were carried out in feedstreams consisting of 0.1 vol % ethanol or 0.025 vol % acetaldehyde and 1% O2 in nitrogen at a space velocity of 5 2 000 (volume feedHvolume catalyst)-‘ h-’ (STP). All three catalysts were found to produce acetaldehyde, carbon monoxide, and carbon dioxide as the major carbon-contalnlng products of ethanol oxidation. CO, was the principal carbon-containing product in the oxidation of acetaldehyde. The steady-state yield of acetaldehyde obtained in the oxidation of ethanol was found to go through a maximum as the temperature was raised over each catalyst. The data suggest that some of the ethanol is oxidized consecutively to acetaldehyde and then to COP over these catalysts. In addition, over Pt, there is also evidence for the direct oxidation of ethanol to co,.
Introduction Ethanol-fueled passenger cars produce high emissions of aldehydes (primarily formaldehyde and acetaldehyde) relative to gasoline-fueled cars (Chui, et al., 1979; Goodrich, 1982). Emissions of unburned ethanol are also significant, particularly during cold-start operation where rich airto-fuel ratios are employed to improve driveability (Chui et al., 1979; Bechtold and Pullman, 1980; Bailey and Edwards, 1980). We have undertaken experiments in the laboratory to assess the potential for applying catalytic converters to the control of emissions of oxygenated hydrocarbons from ethanol-fueled cars. The literature contains little information relating to the oxidation of ethanol and acetaldehyde under conditions similar to those in ethanol-vehicle exhaust. Most studies of ethanol oxidation have been undertaken utilizing catalysts and reaction conditions which favor the production of acetaldehyde (Srihari and Viswanath, 1976; Takezawa et al., 1980; Ganguly et al., 1975; Legendre and Cornet, 1972; Iwasawa et al., 1978). Catalysts with high activity 0196-4321/83/1222-0212$01.50/0
for the partial oxidation of alcohols to aldehydes include silver (Thomas, 1970), copper (Walker, 1964), and mixed oxide catalysts, especially iron-molybdate (Santacesaria and Morbidelli, 1981; Edwards et al., 1977; Pernicone et al., 1969). We are aware of only one study which has been undertaken expressly with the objective of examining catalytic activity and selectivity under conditions which favor total oxidation of ethanol to CO, and water (Ismagilov et al., 1979). Catalysts containing copper and chromium on alumina, and manganese on alumina were used in this study. Both of these catalysts have been found (Klimisch, 1968) to be among the most active base metal formulations for the oxidation of hydrocarbons and CO in the exhaust from gasoline vehicles. Data were also obtained for a platinum on alumina catalyst in order to compare the performance of the base metal catalysts with a noble metal catalyst. Experimental Section Reactor Configuration. The reactor consisted of a 2.5 cm 0.d. quartz tube which was placed in a Lindberg tube 1983 American Chemical Society
