267
Ind. Eng. Chem. Prod. Res. Dev. 1082, 21, 267-274
CATALYST SECTION
Ceria-Promoted Three-way Catalysts for Auto Exhaust Emission Control Gwan Klm W. R. &ace & Company, Davison Chemlcal Division, Columbla, Maryland 21044
In an attempt to improve the three-way catalyst (TWC) performance for CO removal under 0,deficient conditions, a laboratory study was conducted to select a non-noble metal oxide promoter for a typical Pt-Pd-Rh TWC supported on alumina. Ceria was found to be the best promoter largely because it enhances the water-gas shift reaction (CO H20 = COP H2), and possibly due, in part, to the additional oxygen storage it provides to the TWC. The compatibility at high temperatures with alumina as well as Pd is also a desirable property of ceria.
+
+
Introduction Using the single bed approach, limited success has been achieved (Canale et al., 1978) over the past few years in meeting the US. federal auto emission standards for the 1981 model year, 0.41 hydrocarbons (HC), 3.4 CO, and 1.0 nitrogen oxides (NO,), g/mile, respectively. In this approach, all three pollutants-HC, CO, and NO,-are simultaneously removed over a single bed of three-way catalysts (TWCs), converting HC and CO to C02and water by oxidation and NO, to largely N2 by reduction. Thus, to achieve optimal simultaneous redox reactions requires the air to fuel ratio (A/F) to be in the vicinity of the stoichiometric air to fuel ratio, (A/F),, since the immediate vicinity of the (A/F), forms the TWC "window" where the catalyst efficiency is high for the conversion of all three pollutants. It is therefore essential for the TWCs to have a large (tall and wide) TWC window even after repeated e x p u r e s to an actual auto exhaust environment. The size of the TWC window is determined largely by the NO, conversion on the lean (0, excess) side and the CO conversion on the rich (0, deficient) side, since the HC conversion efficiency of a typical noble metal TWC varies relatively little with the A/F in the vicinity of the (A/F),. It has been discovered (Adawi et al., 1977) that the TWC window could be significantly widened on both sides of the (A/F), as a result of the A/F oscillation, which was caused by the lag between the oxygen sensor and the carburetor in a closed-loop control system. The efforts in the past to chemically improve the TWC window on the lean side led to the now well-established Pt-(Pd)-Rh formulation, by virtue of the benefit Rh offers, Le., its ability to allow NO, conversion to N2 in the presence of a trace of oxygen (Taylor, 1975) or of SO2 (Summers and Baron, 1979) better than Pt and Pd. Iridium has been found (Lester et al., 1978) to be even more effective than Rh for the NO, conversion to N2 on the lean side, but only when present in an unaffordably high loading. Ruthenium has now been ruled out because of the toxicity and the oxide volatility. This study was aimed at improving the TWC window on the slightly rich side by selecting a suitable non-noble metal oxide as a promoter primarily for the CO conversion. 0196-4321/82/1221-0267$01.25/0
Anticipating the surface acidity or the basicity to be one of the factors controlling catalytic activity for the CO oxidation on the rich side, the oxides of two different metals, Ce and W, were chosen as the model non-noble metal oxides-one intrinsically basic, the other acidic. Ceria was chosen because it is compatible with both Pd (Sanchez et al., 1974) as well as alumina, and improves the performance of Pd-containing catalysts at low as well as high temperatures (Sergeys et al., 1975; Hindin and Dettling, 1976). Tungsten oxide was also chosen because it adds not only the intrinsic acidity, but it also may allow a greater chance for the nitrogen containing molecules to interact with the catalyst surface. In the initial phase of this study, the model TWCs were tested for converting the reactants (italicized) in the following CO reaction runs as a function of bed temperature. CO HC O2 (1)
+
+
+ HzO CO + NO CO + NO + H2O CO + HC + NO + Hz + O2 + H i 0 + COP CO
(4) (5)
Since the reactions involving NO are also of interest not only for the single bed TWCs, but also in connection with the TWCs for the dual bed system as well, the following three reaction runs were also included in this study.
NO + H2 NO + NH,
(6)
NH3 The TWC samples selected for the above study were based on a fixed formulation consisting of Pt-Pd-Rh, all supported on monolithic substrates. Thus the metals formulation as well as the penetration depth are outside the scope of this study. In the subsequent phase of this study, a variety of non-noble metal oxides were examined using alumina spheres rather than monolithic substrates. As a result of 0 1982 American Chemical Society
288
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 2, 1982
Table I. Monolithic Pt-Pd-Rh TWC washcoat compn (wt %)
washcoat BET ( N,) S.A., m'/g
amt of washcoat, g/L monolith
TWC prepared thereon
CeO, ( 4 % ) /
110 101
83 74
M-1 M-2
95
111
M-3
86
112
M-4
wo, ( 7 % ) / wo, (ll%)/ A1203
this study, ceria was found to be the best promoter not only for the CO conversion of a fresh TWC on the rich side, but also for the TWC durability for the CO conversion. Experimental Section Monolithic TWC. The monolithic TWC samples were prepared on cordierite substrates with 45 cells/cm2by first washcoating alumina to approximately 15 w t % , followed by sequential dipping into the solutions containing Pt, Pd, and Rh, with gradual air drying at 75-160 "C and decomposition in 350 "C air after each dipping. The washcoat with or without an additive such as ceria or tungsten oxide was prepared from transitional alumina powder having 110 m2/g BET (N,) surface area. The washcoating technique has already been described elsewhere (Dolbear and Kim, 1975). Cerous nitrate and ammonium metatungstate were the sources for the additive oxides, while chloroplatinic acid, Pd nitrate, and Rh trichloride were the noble metal sources. Each TWC shown in Table I was formulated to have the following metals loading (g of metal/L of catalyst): 0.847 Pt, 0.339 Pd, and 0.0498 Rh. Spheroidal TWC. Spheroidal alumina (3.2 mm diameter) having 0.47 g/cm3 bulk density and 118 m2/g BET (Nd surface area was modified to include oxides of Ba, Mg, Cr, Mn, Co, Ni, Zn,and Ce, either singly or in combination, using the usual impregnation procedure. It was followed by 135 OC air drying and the air calcination indicated in Table 11. The spheres were then sequentially impregnated to incipient wetness with the solutions bearing Pt, Pd, and Rh, followed by the usual drying and decomposition in 350 "C air. An identical metals formulation (g of metal/L of catalyst) was applied to each of the first 13 TWCs (S-1 through S-13) in Table I1 as follows: 0.402 Pt, 0.161 Pd, and 0.0297 Rh. Two samples of the spheroidal TWCs, S-1R and S-2R, are identical with S-1 and S-2, respectively, except for the way they were activated prior to evaluation. These two
samples were reduced in flowing Nz containing 5% H2for 1 h at 560 "C, whereas all the rest of the TWCs were activated simply by heating in 350 "C air. The last two spheroidal TWCs (S-14R and S-15R) were prepared in the same manner as S-2R and S-3R, but using a different metals formulation (g of metal/L of catalyst) as follows: 0.659 Pt, 0.264 Pd, and 0.0659 Rh. TWC Evaluation. The three different procedures employed in this study include chronologically (1)warm-up test, (2) sweep test, and (3) perturbed sweep test. The warm-up test-often called dynamic heat-up testsimulates the test for the engine start-up period. Thus the catalyst sample at room temperature was exposed to a preheated (371 "C) gas mixture shown in Table 111. The details of this test have already been described (Briggs and Graham, 1973; Ernest and Kim, 1980). In the sweep test, a TWC sample, either a 1.27 cm long monolith with 2.54 cm diameter or 8 cm3 of spheres was evaluated in a Vycor glass reactor with 2.65 cm i.d., using the simulated exhaust gas mixture shown in Table 111. The TWC efficiency for the HC, CO, and the gross as well as the net NO conversions were determined as the feed composition was varied stepwise from a rich condition to a lean condition by increasing the O2 concentration in the feed. The net NO conversion represents, as usual, the difference between the gross NO conversion and the NO conversion to NH,. The oxidant to fuel ratio R, defined (Kim and Maselli, 1977; Schlatter, 1978) as a measure of the A/F, represents the molar ratio of the total oxidants (0, and NO) to the total reducing agents (HC, CO, and H,) in the feed, with each entity expressed in terms of 0, equivalent. The samples in this test were pretreated in situ at R = 0.77 (i.e., 23% 0, deficient) for 30 min before starting the evaluation. The TWCs were evaluated at several different levels of inlet temperatures, including 471 "C. The perturbed sweep test approximates the TWC test on an engine dynamometer with a closed-loop control system. This test is similar to the cycled TWC test (Schlatter, 1978; Hegedus et al., 1979), but differs in the way the test feed composition is varied while introducing perturbations in the A/F. In this test, the feed composition (feed 3 in Table 111) was varied by increasing the CO + Hz concentration in the main feed while simultaneously decreasing the O2concentration independent of perturbation. Perturbations with a fixed amplitude were introduced into the main feed at 1 Hz through two auxiliary lines by alternately injecting 1/2-spulses of CO + H2
Table 11. Pt-Pd-Rh TWCs Supported o n Spheroidal Alumina with Additivesn TWC efficiency, % additive oxide (wt %) nil CeO, (3%) azo, ( 3 % )
ZnO (3%) Cr,O, ( 3 % ) ;ZnO (3%)
COO (6%) COO (3%) Cr,O, ( 3 % ) BaO ( 3 % ) MgO (3%) CeO, (3%);MnO, (1.5%) CeO, ( 3 % ) ;NiO ( 1 . 5 % ) CeO, (1.5%) CeO, (6%) CeO, ( 3 % ) Cr,O, ( 3 % )
TWC no.
HC
co
s-1
91 85 94 87 94 86 90 76 86 76 79 86 81
64 99 72 99 63 99 61 99 64 99 64 98 67 99 67 99 55 98 70 99 69 99 69 99 74 99 not measured not measured
s-2c s-3c 5-4 c
s-5c S-6e S-7e S-ad
S-9d s-1oc
s-11
S-12C
S-13 S-14e S-15 e
NO (gross)
NO (net) 78 71 73 78
74 63 56 65 80 73 76 73 68
All the modified alumina supports had BET (N,) S.A. in the 106-123 m'/g range. At approximately 6 5 000 GHSV, 0.77, and 471 "C inlet. Air calcined with the additive(s) at 1 0 3 0 "C. Air calcined with the additive(s) at 980 "C. e Air calcined with the additive(s) a t 760 "C. a
R
=
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 2, 1982 209 Table 111. Simulated Auto Exhaust Gas Mixture (Vol %) for the Laboratory Evaluation of TWCs feed 1 feed 3 warm-up feed 2 perturbed gas test sweep test sweep test HCC
0.0567u 3.0 0 0 4.5 0 10 balance
co
H, NO 0,
co,
H,O N,
0.0225 1.00 0.34 0.10 0.4-0.9 12.0 13.0 balance
0.040 0.225-0.7 56 0.075-0.252 0.185 0.245-0.725 14.5 10.0 balance
A pora Either C,H, or C,H, rather than a mixture. s each tion each of these was turned o n and off for alternating between CO + H, and 0, through the auxiliary lines. C,H,/C,H, = 3/1.
Figure 2. HC and CO conversions over fresh Pt-Pd-Rh TWCs in the CO + HC + O2 reaction run using feed 1 in Table V (cf. Figure 1 for symbols). 1001
1
I
200
300
I
I
I
I
Table IV. Feed Compositions (Vol %) Used in the Test for Assessing the Relative Oxygen Storability lean ( R = 1.5) 0.04 0.342 0.114 0.185 0.524 14.5 10 balance
gas HCa
co
% 0 2
CO,
H,O NZ C,H,/C,H, '""I
rich (R = 0.5) 0.04 0.756 0.252 0.185 0.252 14.5 10 balance
40
>
82 0
100
1
I
I
I I 400 500 BED TEMPERATURE ("C) C
600
Figure 3. CO conversion over fresh Pt-Pd-Rh TWCs in the CO + H 2 0reaction run using feed 2 in Table V (cf. Figure 1for symbols).
= 3/1. I
400 500 BED TEMPERATURE('C) C
I
I
I
I
600
,
I
I
I
I
1
Figure 4. CO conversion over fresh Pt-Pd-Rh TWCs in the CO + NO reaction run using feed 4 in Table V (cf. Figure 1for symbols).
Figure 1. CO conversion over fresh Pt-Pd-Rh TWCs in the routine sweep test at R = 0.77 using feed 5 in Table V. Symbols represent different washcoata: M-1, AlzO3; M-2, CeOz (4%)/A1203;M-3, W03 (7%)/AlZO3;M-4, W 0 3 (ll%)/A1203 (cf. Table I).
and O2 directly into the inlet gas near the TWC bed. Sweeping from a lean (R = 2.5) side to a rich (R = 0.5) side, this test was carried out after a 10-min exposure of each sample to the lean feed, maintaining the inlet gas temperature at 482 "C. The test unit was essentially identical with the one described in the sweep test. The sample size was 17 cm3 for the low metal TWCs (e.g., S-1R) and 8.5 cm3 for the high metal TWCs (e.g., S-14R). The analytical equipment employed includes a flame ionization detector for HC, a nondispersive infrared analyzer for CO, a chemiluminescent NO, analyzer with a built-in NH3 to NO converter for NO as well as NH3, and an oxygen analyzer based on paramagnetism for OF No analysis was performed for N20. Oxygen Storability. A TWC sample in a Vycor glass reactor was subjected to R = 1.5 for 10 min, and then to R = 0.5, by changing the feed composition as shown in Table IV. This test was carried out to see if the CO conversion observed under the lean condition could be sustained over a prolonged period of time even after switching to the rich condition possibly due to the oxygen stored in the catalyst during the period of 10 min exposure
Figure 5. Gross and net (bold symbols) NO conversions over fresh Pt-Pd-Rh TWCs in the routine sweep test at R = 0.77 using feed 5 in Table V (cf. Figure 1 for symbols).
to the lean condition. The gas inlet temperature in this test was also 482 OC. Results and Discussion CO Conversion Efficiency under Slightly Rich Conditions. The four samples of fresh monolithic TWCs (Table I) were evaluated in the eight separate reaction runs described earlier (Introduction), using the sweep test procedure and the different feeds shown in Table V. The HC, CO, the gross as well as the net NO, and the NH3 conversions observed in these runs are presented in Figures
270
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 2, 1982
Table V. Feed Composition (Vol %) for the Reaction Runs 1-8 gas HCa
2
1
co
HZ NO NH,
0.0225 1.00 0 0
0 1.00 0 0
0
0
0 12.0 13.0
0.37 12.0 0
0 2
CO, HZO N*
3
4
0 1.00 0 0.100 0 0 12.0 0
C,H,/C,H, = 3/1. Identical with feed 2 in Table I11 a t R = 0.77. maintain approximately 80 000 GHSV in all the reaction runs.
r
-1"e60-
z 0
iT 40-
w
z
-
-
,s
I
1
,
400 500 BED TEMPERATURE (*CIC
300
I
0.050 0 12.0 13.0
3
CO-NO
4
CO-NO+H20
5
SWEEPTEST NO-H,
6 7
8 20
NOtNH,
I I I 300 400 500 BED TEMPERATURE ("C) c
1
I 600
Figure 9. Gross NO conversions over a fresh monolithic Pt-PdRh/A1,03 TWC (M-1) in five different NO reaction runs using feeds 3-7 in Table V.
600 BED TEMPERATURE ( ' ' C I C
Figure 10. NO and/or NH3 conversions over fresh Pt-Pd-Rh TWCs in the NO + NH3 reaction and in the NH3 decomposition runs using feeds 7 and 8 in Table V, respectively (cf. Figure 1 for symbols).
JZh-%=-O%T-q
260 W
-
z
820 0
0
200
Figure 7. Gross and net (bold symbols) NO conversions over fresh Pt-Pd-Rh TWCs in the NO + H2 reaction run using feed 6 in Table V (cf. Figure 1 for symbols). I00
0
RUN NO. REACTION
0
+
200
0 0 0
The total gaseous feed rate is 8.67 L (STP)/min t o
1
Figure 6. Gross and net NO conversions over fresh Pt-Pd-Rh TWCs in the CO NO reaction run using feed 3 in Table V (cf. Figure 1 for symbols).
0
7 0 0 0 0.050 0.050 0 12.0 13.0
100'
@
8 20-
8
6
0 0 0.34 0.100 0 0 12.0 13.0
5b
0.0225 1.00 0.34 0.100 0 0 0 0.54 12.0 12.0 13.0 13.0 balance 0 1.00 0 0.100
I
I
I
400
500
600
BED TEMPERATURE ("Cl --C
Figure 8. Gross and net (bold symbols) NO conversions over fresh Pt-Pd-Rh TWCs in the CO + NO + H20 reaction run using feed 4 in Table V (cf. Figure 1 for symbols).
'1 e 60
0
0
0
M-3
0
M-4
vi
g4 0
M-l M-2
8 2
iw
I
O L 200 & d S 6 &
400
1 500
BED TEMPERATURE ( " C I C
I 600
Figure 11. HC conversion over fresh Pt-Pd-Rh TWCs in the routine sweep test at R = 0.77 using feed 5 in Table V (cf. Figure 1 for symbols).
1-11 as a function of bed temperature.
It is immediately apparent from the data shown in Figure 1 that, below about 450 O C , the efficiency of the supported Pt-Pd-Rh TWCs cannot be improved for the CO conversion under the oxygen-deficientconditions simply by modifying the alumina support with either ceria or tungsten oxide, whereas above about 450 "C, an appreciable improvement can be made when the support is modified with ceria. The data presented in Figures 1-4 clearly indicated the ceria promotion of the water-gas shift reaction (CO + H20)to be largely responsible for the
enhanced CO conversion observed in the routine sweep test run (run no. 5) at R = 0.77 and above 450 "C. Ceria-alumina by itself is nearly inactive for the water-gas shift reaction under the experimental conditions of this study, exhibiting only a few percent CO conversion. While it is reasonable to postulate a formic acid or formate intermediate (Sachtler and Fahrenfort, 1960) for the water-gas shift reaction and to expect a faster decomposition of such an intermediate over the surface of somewhat basic oxides (Ai, 1977) than over the acidic oxides, precisely
how ceria promotes the noble metal catalysts for this reaction remains to be elucidated. A recent study on the alumina-supported noble metal catalysts (Grenoble et al., 1981) explains the water-gas shift reaction by postulating a formic acid intermediate and the role of alumina as a Lewis acid, which facilitates chemisorption of water. It is interesting to note that ceria is ranked among the highest, and higher than alumina in Lewis acidity (Krylov, 1970). It is also evident from Figures 1-4 that, below about 400 "C, the CO conversion observed in the routine sweep test at R = 0.77 is mainly contributed by the CO + O2 reaction. Above about 400 "C, however, as shown in Figures 3 and 4,the water-gas shift reaction rate increases significantly with increasing temperature, although the increase is thermodynamically limited. In the case of the ceria-promoted TWC at about 550 "C, the water-gas shift reaction appears to be as efficient in CO removal as the CO O2 reaction in the CO HC O2run at R = 0.77 (run no. 1). Figures 1 and 2 show that the CO conversions observed in the CO HC O2 reaction run (run no. 1) as well as in the sweep test (run no. 5) at R = 0.77 over the alumina-supported and the ceria-alumina-supported TWCs reach maxima in the vicinity of about 360 "C in both reaction runs. Over the same samples in the CO HC O2 reaction run, a further increase in temperature results in a steady decrease in the CO conversion (Figure 2), while a similar decrease is also observed up to about 450 "C in the sweep test at R = 0.77 (Figure 1). Such a pattern in the CO conversion vs. temperature can be explained in terms of the competition between HC and CO for the surface oxygen, judging from the data in Figure 2, which indicates the HC oxidation rate in the CO + HC + O2 reaction run to be substantially higher in this temperature region than the CO oxidation rate in the same reaction run. In the absence of HC, no such a decrease in the CO conversion was observed with increasing temperature. NO Conversion Efficiency under Slightly Rich Conditions. The data presented in Figures 5-8 reveal that all four samples of Pt-Pd-Rh TWCs (Table I), regardless of the nature of the support, are about equivalent in the fresh efficiency for the gross NO conversion. However, the ranking of the supports based on the fresh efficiency of a TWC for the net NO conversion appears to be tungsten oxide-alumina > alumina > ceria-alumina. It is evident from the data that the fresh efficiency for the net NO conversion above about 400 "C is inversely related to the TWC efficiency for the water-gas shift reaction, which in turn is dependent upon the nature of the TWC support. In general, the selectivity of NO toward N2 in all the reactions involving NO (run no. 3-6) and the efficiency for the CO conversion in the water-gas shift reaction are effected in an exactly opposite manner by the support modification with the oxides of non-noble metals. This is consistent with the view (Klimisch and Barnes, 1972) that the surface hydrogen resulting from the water-gas shift reaction is readily available for the reaction with NO. Mechanistic Implications on NO Removal. Figure 9 compares the gross NO conversions observed over a fresh sample of alumina-supported TWC (M-1 in Table I) in the five different reaction runs involving NO (Introduction). Similar results were obtained for the remaining three monolithic TWCs (Table I). The data clearly establish the NO + H2 reaction to be the fastest (Otto et al., 1970) of the three different reactions examined, NO + CO, NO + H2, and NO + NH,. I t is obvious that the CO + NO + H 2 0 reaction run (run no. 4) represents a combination of
+
+
+
+
+
+
+
P
5
3-
u w
a -
9
I$I,2 -
a m
? X P I
-
P
40
\p
.: w%>eS&
\&
- --...________.._..____ .____._..... ...
I
272
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 2, 1982
-1
\
\.
I
0
I
I
40
I I I 80 TIME (SEC)-
I < $ t 120
c
*io 1
I
S-l4R
g ‘ L h i = z _ i 240
TIME (SEC ) --C
Figure 13. CO conversions over fresh spheroidal TWCs, Pt-PdRh/Al209 (S-1R) and Pt-Pd-Rh/CeO, (3 wt %)-A1203 (S-2R),at R = 0.5 and approximately NO00 GHSV, with a 10-min prior exposure to R = 1.5, using feeds in Table IV.
Figure 14. CO conversions over fresh spheroidal TWCs, Pt-PdRh/Ce02 (3 wt %)-A1203 (S-14R) and Pt-Pd-Rh/Crz03 (3 wt %)-Alz03 (S-l5R),at R = 0.5 and approximately 60000 GHSV, with a 10-min prior exposure to R = 1.5, using feeds in Table IV.
Table VI. CO Conversion Delay Time as a Measure of the Oxygen Storability of R-Pd-Rh TWCs
dotted lines in Figure 13 correspond to the blank runs representing the change in the CO concentration in the outlet when the inlet CO concentration over a charge of dummy alumina spheres was changed from zero (i.e., 100% CO conversion) to the levels attained by the TWC samples under the steady-state conditions. These blank run data indicate that at least 30 s is required for flushing the entire flow system between the gas mixing chamber and the CO detector. Thus the shape of these curves after deviating from the initial CO conversion (the straight line portion) appears to be largely determined by the difference between the initial and the final steady-state CO conversions. According to a recent study by others (Schlatter and Mitchell, 1980) in a similar experiment, the change in the CO conversion efficiency during the transient periodabout 6 min in this study-must be largely due to the gradually diminishing rate of the water-gas shift reaction. This has been well confirmed by a study on the effect of pretreatment of the ceria-promoted TWCs with steam (Kim, 1981). However, the initial CO conversion (-100%) delay cannot be accounted for simply by the water-gas shift reaction alone, because the CO conversion attainable via the water-gas shift reaction cannot exceed approximately 67% under the experimental condition. This means at least 33% of the initial CO conversion must have come from the CO + 0 (surface) reaction. It is therefore reasonable to take the observed CO conversion delay time as a measure of oxygen storability-the ability of the catalyst to allow some oxygen storage (Yao, 1979; Schlatter and Mitchell, 1980). The data presented in Figure 13 show that the presence of 3 wt % CeOz in alumina resulted in a 13-s additional delay in the initial CO conversion of the Pt-Pd-Rh TWC. The difference in the oxygen storability between the alumina-supported and the ceria-alumina-supported TWCs (Figure 13) should roughly correspond to the amount of CO represented by the shaded area between the two curves. This amounts to approximately 60 pmol of CO. This means about 30 pmol of O2 is required from the surface or “stored”oxygen in order for the CO + 0 (stored) reaction to be primarily responsible for the additional 13-s CO conversion delay time. Assuming at least half of the oxygen stored in the ceria to be available for the CO oxidation, the amount of oxygen that can be supplied from about 8.5 g (17 cm3) of ceria (3 wt % Ce02)-alumina is estimated at about 125 pmol of 02,based on the data recently reported (Schlatter and Mitchell, 1980). This is much greater than 30 pmol of O2 required for the CO + 0 (stored) reaction to account for the 13 s additional delay time. It should be noted, however, while this estimation
co TWC S-lR‘ S-2R“ S-14Rb A-15Rb
tested a t GHSVC
conv. delay time, s
%)/
30 000 30 000
8.8 21.7
A1;0, G O , ( 3 wt %)/
60 000
9.8
60 000
7.9
support A1,0,
G O , ( 3 wt A1203
O,O, ( 3 wt %)/
A1203
‘
Using 8.5 g sample. Using 4.25 g sample. gaseous flow rate a t 8.5 y S T P ) / m i n .
Total
of the support, reaching approximately 3 over the ceriaalumina-supported TWC. The reason for the presence of such a maximum in the ratio cannot be readily explained based on the reaction mechanism proposed in the past (Otto et al., 1970), unless a trace (10 ppm in this study) of oxygen is also involved in the low-temperature (>290 “C) reaction such as 2N0 + 2NH3 + ‘ / 2 0 2 = 2Nz
+ 3H20
(7-3)
This remains to be confirmed, however. Effect of Non-Noble Metal Oxide Additives. The sweep test data on the fresh samples of spheroidal TWCs (S-1through S-12) summarized in Table I1 reveal that ceria is the more effective promoter for the CO conversion under the oxygen deficient conditions than any other oxides of non-noble metals examined in this study, including some that are known to be the ingredients of the catalysts for the water-gas shift reaction. In general, the data confirm that those with a relatively poor efficiency for the CO conversion on the rich side usually have a better NO selectivity toward N2 than those exhibiting a high efficiency for the CO conversion. The data also reveal that those exhibiting an exceptionally high efficiency for the HC conversion on the rich side are often rather low in the CO conversion efficiency under the rich conditions, in agreement with the data presented in Figures 1, 2, and 11. Oxygen Storability. In Figures 13 and 14 as well as in Table VI are compared three different types of supports, alumina, ceria (3 wt % CeO&alumina, and chromia (3 wt % Cr203)-alumina for their ability to help the Pt-Pd-Rh TWCs sustain the initial CO conversion efficiency under a rich condition ( R = 0.5) after being switched from a 10-min exposure to a lean ( R = 1.5) environment. The experimental conditions are described in Table IV. The
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 2, 1982 273 o lo&=&=
NO (GROSS1
I
1
I 1
i
i 4
6 0
0
0
WEIGHT % COOz
C
Figure 15. Efficiency of fresh spheroidal Pt-Pd-Rh TWCs in the routine sweep test at R = 0.77 and approximately 30000 GHSV as a function of weight percent CeOz in the ceria-alumina support.
Table VII. Warm-up Performancea of Thermally Aged TWC
TWC
S-1R S-2R
fresh or agedb fresh aged fresh aged
tso-
#so-
CO, AtCO C3H,
CO eff,
C3H, eff,
sc
sd
sc
%
%
41 69 42 56
19 26 19 31
84 269 82 343
100 99 100 100
83 54 81 52
Figure 16. Performance in the perturbed sweep test of spheroidal TWCs, Pt-Pd-Rh/Alz03 (S-1R) and Pt-Pd-Rh/CeOz (3 wt %)A1203(S-2R, bold lines), fresh (a), after 24 h exposure to 932 "C air (b), after l ' h reduction (c) in Hz (5%)/Nz at 560 OC. Dotted lines represent the net conversion of NO.
W
a Based o n the warm-up test a t approximately 35 000 GHSV. Thermally aged in 982 "C air for 24 h. Time required to attain 50% conversion. Time required to attain 90%conversion from 10%conversion.
+
suggests a possibility of the CO 0 (stored) reaction in contributing at least in part to the CO conversion observed under a rich condition in a TWC test with an oscillating A/F, it does not necessarily rule out the possible contribution from the water-gas shift reaction during the ' I zs swing to a rich condition in this study. It is also interesting to point out that, while both ceria-alumina and chromia-alumina appear to be almost equally effective in delaying the initial CO conversion (Figure 14), only the former is successful in promoting the CO conversion. Effect of Ceria Level. Figure 15 shows the sweep test data on the performance of the fresh spheroidal TWCs under a slightly rich condition (R = 0.77) as a function of wt % Ce02 in the ceria-alumina support. With increasing ceria level, the CO conversion efficiency at R = 0.77 increases moderately up to about 3 wt % CeOz, and then slowly thereafter. It is also apparent from the data that such a gain in the CO conversion efficiency on the rich side can be made only at the sacrifice of the HC and the net NO conversion efficiencies on the rich side. Thus, with an emphasis on the CO conversion efficiency on the rich side, it appears appropriate to look for an optimal ceria level in the vicinity of 3 wt % Ce02 for the single bed TWC. Admittedly, however, the optimal ceria level should be located based on the TWC testing with the oscillating A/F of the properly aged samples. In addition, the ceria level required to adequately promote the CO conversion efficiency was found to be roughly proportional to the BET surface area of the support, indicating the role of ceria to be primarily of surface modification. Thermal Stability and Durability. Two samples of the spheroidal TWCs (Table I1 and Experimental Section), one (S-1R) without ceria promotion, the other (S-2R) promoted with 3 wt % CeOz, were evaluated before and after 24 h exposure to 928 OC air, and then with 1 h reduction treatment in flowing N2 containing 5% H2 at 560 "C, using the warm-up as well as the perturbed sweep test procedures. The results presented in Figure 16 and Table
-1
P
IO
20
R
IO
C
R
2.0 C
Figure 17. Performance in the perturbed sweep test of spheroidal TWCs, Pt-Pd-Rh/AlzOB (S-1R) and Pt-Pd-Rh/CeOz (3 wt %)Alz03 (S-2R, bold lines), after dynamometer aging (a) of S-1R to 15 100 mile equivalent and of S-2R to 50000 mile equivalent, after 1h reduction (b) in Hz (5%)/Nz at 560 "C. Dotted lines represent the net conversion of NO. Table VIII. Change in TWC Properties after Dynamometer Aging properties aged t o
S-1R before
after
S-2R before
after
mile equivalent 15 100 50 000 Bet (N,) S.A., m'/g 113 107 119 100 walumina content 0 O+ 0 ot poison pick-up, wt % Pb 0 0.041 0 0.079 P 0 0.21 0 0.54 S 0.1 0.1 0.1 0.3
VI1 indicate, as expected, the ceria-promoted TWC (S-2R) to be appreciably better for the CO conversion than the unpromoted (S-1R) in the overall three-way as well as in the warm-up performances, despite the fact that the TWCs must have suffered thermal deactivation due to Pt sintering, and Pt and Rh interactions with the support (Summers and Ausen, 1979; Yao et al., 1977). A separate sample each of the alumina-supported and the ceria-alumina-supported TWCs after dynamometer aging was also evaluated in the same manner. Both samples were reevaluated by the perturbed sweep test after 1 h reduction treatment in flowing N2 containing 5 % Hz at 560 "C. The results are presented in Figure 17. Post-mortem data on the dynamometer aged TWCs are compared with those on the fresh samples in Table VIII. The data reveal that the dynamometer aged samples are permanently deactivated, while some of the efficiencies lost in the thermal aging in air can be regained by a simple reduction treatment at sufficiently high temperature. Overall, the data led to the conclusion that the ceriapromoted Pt-Pd-Rh TWC is superior to the unpromoted
274
Ind. Eng. Chem. Prod. Res. Dev. 1982, 21, 274-278
in the TWC durability, especially for the CO conversion efficiency. Acknowledgment The author wishes to thank D. E. Achey and W. M. Wang of AC Spark Plug Division of General Motors Corporation for their suggestions on TWC evaluation by the perturbed sweep test procedure. R. Maher conducted all the TWC testing, except the warm-up test, which was done by J. Manr. Discussions with Professor P. H. Emmett of Portland State University are also gratefully acknowledged. Literature Cited
Kim, G. W. R. Grace 8 Co., Davison Chemical Division, unpublished data, 1981. Kim, G.; Maselli, J. M. SOC.Automot. Eng. Mtg., Warrendale, PA, 1977; Paper No. 770368. Kiimisch, R. L.; Barnes, G. J. Environ. Sci. Techno/. 1972, 6 . 543. Krylov, 0. V. "Catalysis By Nonmetals"; Academlc Press New York, 1970; p 88. Lester, G. R.; Joy, G. C.; Brennan, J. F. Soc. Automot. Eng. Mtg., Warrendale, PA, 1978; Paper No. 780202. Otto, K.; Shelef, M.; Kummer, J. T. J. phvs. Chem. 1970, 7 4 , 2690. Sachtier, W. M. H.; Fahrenfort, J. Acres Congr. Int. Catal. 2nd 1981. 7 , 831. Sanchez, M. G.; Maseili, J. M.; Graham, J. R. U.S. Patent 3830756, 1974. Schlatter, J. C. Soc. Automot. Eng. Mtg., Warrendale, PA, 1978; Paper No. 780199. Schlatter, J. C.; Mitchell, P. J. Ind. Eng. Chem. Prod. Res. Dev. 1980, 19. 288. Sergeys, F. J.; Maselli, J. M.; Ernest, M. V. U.S. Patent 3903020, 1975. Summers, J. C.; Ausen, S. A. J. Catal. 1979, 58, 131. Summers, J. C.; Baron, K. J. Catal. 1979, 57, 380. Taylor, K. C. "The Catalytic Chemistry of Nitrogen Oxides"; Kiimisch, R. L.. Larson, J. G., Ed.; Plenum Press; New York, 1975; p 173. Yao. Y.-F. Yu "The Redox Capacity of CeO, Containing Noble Metal Catalysts", presented at the 6th North American Meeting of the Catalysis Society, Chicago, IL, March 1979.
Adawi, M. K.; Brlggs, A. D.; Delosh, R. G.; Smith, C. S. U S . Patent 4 024 706, 1977. Ai, M. J. Catal. 1977. 50, 291. Briggs, W. S.;Graham, J. R. Soc.Automot. Eng. Mtg., Warrendale, PA, 1973; Paper No. 730275. Canale, R. P.; Winegarden, S. R.; Carlson, C. R.; Mlles, D. L. SOC.Automot. Eng. Mtg.. Warrendale, PA, 1973; Paper No. 760205. Dolbear, G. E.; Kim, G. A&. Chem. Ser. 1875, No. 143, 32. Ernest, M. V.; Kim, G. Soc. Automot. Eng. Mtg., Warrendale, PA, 1980; Paper No. 800083. Grenobie, D. C.; Estadt, M. M.; Ollls. D. F. J. Catal. 1981, 67, 90. Hegedus, L. L.; Summers, J. C.; Schlatter, J. C.; Baron, K. J. Catal. 1979. 56, 321. Hindln. S. G.; Dettling, J. C. U.S. Patent 3993572, 1976.
Received for review June 3 , 1981 Accepted December 23, 1981
Paper presented at the 178th National Meeting of the American Chemical Society, Washington, DC, September 1979, INDE 50.
Fischer-Tropsch Studies over Well-Characterized Silica-Supported Pt-Ru Bimetallic Clusters Hlroshl Mlura' and Rlchard D. Gonzalez" Department of Chemistry, university of Rhode Island, Kingston, Rhode Island 0288 1
series of silica-supported Pt-Ru bimetallic clusters has been completed. The effect of increasing the surface concentration of Pt has a marked effect on methane selectivity. Higher hydrocarbon products are observable in significant yields only for catalysts which have a surface concentration of less than 50% Pt. However, higher hydrocarbon product distributions are not sensitive to changes in surface compositiin. The Fscher-Tropsch reaction was shown to be structure Sensitive occurring predominantly on Ru surface ensembles. A temperature-programmedreaction (TPR) study using "in situ" infrared spectroscopy suggests that CO adsorbed on Pt surface sites undergoes desorption followed by readsorption on Ru surface sites prior to reaction. From these results R is concluded that Pt surface sites are inactive in the CO-H, reaction under the conditions of this study. The role of Pt is, therefore, reduced to that of a surface diluent. A Fischer-Tropsch study over a well-characterized
Introduction In a previous paper we reported on the formation of methane from synthesis gas over well-characterized silica-supported Pt-Ru bimetallic clusters (Miura and Gonzalez, 1981a). The results of these studies suggest that the methanation reaction is structure sensitive and occurs primarily on Ru surface ensembles consisting of several adjacent Ru surface atoms. When the surface composition of Ru is high, the observed turnover number for CHI formation is in good agreement with the sum total of the corresponding turnover numbers for methanation on the separate meMs. For low surface concentrations of Ru, the reverse is true. Pt adsorbs CO associatively at methanation reaction process temperatures and is, therefore, a poor catalyst for 'On leave from Saitama University, Japan. 0196-4321 18211221-0274$01.25/0
this reaction. Ru, on the other hand, is a good catalyst for both the Fischel-Tropsch and methanation reaction (Araki and Ponec, 1976; King, 1978a,b; Rabo et al., 1978; Ramamoorthy and Gonzalez, 1979b). On the other hand, Pt is an excellent hydrogenation catalyst. Because Pt and Ru form stable bimetallic clusters (Brown and Gonzalez, 1977; Ramamoorthy and Gonzalez, 1978a; Miura and Gonzalez, 1981a), it is possible that the surface concentration of hydrogen atoms could increase due to the presence of Pt atoms on the surface of the Pt-Ru bimetallic clusters. This increase in H atom concentration might be expected to alter the product distribution of the Fischer-Tropsch reaction. As the surface concentration of Pt is increased, a higher surface concentration of H atoms should result in a decrease in the olefin-paraffin ratio. Because of these considerations, and due to the growing industrial interest in maximizing the concentration of olefins in synthesis gas processing, we felt that a study of 0 1982 American Chemical Society
