Lanthanum Lead Manganite Catalyst for Carbon Monoxide and Propylene Oxidation Seymour Katr;
John J. Croat, and Joseph V. Laukonls
General Motors Research LaboraMes. Warren, Michigan 48090
The activity of single crystal lanthanum lead manganite (La0,~Pb~.3Mn03) was compared to Pt/A1203, Co304, and CuO under conditions important to automotive emission control. In simulated automotive exhaust ambients, the observed order of activity of fresh catalyts toward both CO and C3H6 oxidation was found in general to be: Pt/Al2O3 Co304 > La0.7Pb0.3Mn03 CuO. Pt/AIzO3 remained the most active of the materials throughout tests in which the susceptibility of catalysts to various modes of deactivation, namely, high temperature exposure, SOz poisoning and reaction with y-Alz03, was measured. The results for the single crystal manganite catalyst showed it to be susceptible to thermal deactivation but resistant to SOz poisoning. Manganite was also found to react extensively with yA1203 at 1000°C suggesting potential difficulties with supported catalysts. Basic differences were found between the properties of single crystal and polycrystalline forms of La0,~Pb0,3Mn03. They showed different oxidation kinetics and the specific catalytic activity of the single crystals was found to be roughly 100 times higher than the polycrystalline materials. The data suggest that the enhanced activity might be due to the presence of small quantities of Pt.
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Introduction Voorhoeve et al. (1972, 1973) have reported that, based on the same surface area, La0.7Pb0.3Mn03was catalytically as active as Pt for the oxidation of CO. They furthermore suggested this material for treatment of CO in automotive exhaust. Since an automotive catalyst for CO must also effectively oxidize hydrocarbons and, in addition, must resist deactivation by high-temperature and gas-phase poisons, a study was undertaken to extend the comparison of the specific activity of manganite and Pt/A1203 into these areas. A second aspect of this work was a reexamination of the relative CO oxidation activity of Lao.7Pb0.3Mn03 and Pt/ A1203made under conditions that simulate automotive exhaust. This was considered necessary because the applicability of the original work to the automotive operation had been questioned (Schlatter et al., 1973) on the basis that the determination was made under unrepresentative conditions without the availability of the kinetic information needed for accurate extension. In addition to single crystal Lao.7Pbo.3Mn03 and Pt/ A1203, two base metal catalysts, Cos04 and CuO, were included in the study to broaden the scope by which the activity of manganite catalysts could be assessed. The activity of polycrystalline La0.7Pbo.3Mn03materials was also investigated. Experimental Section Materials. A total of four manganite samples were investigated. These included two samples of crushed Lao,7Pbo,3Mn03 single crystals (37-250 pm) which were grown from a lead borate flux in a Pt crucible and subsequently etched 3-5 min in hot (SOOC) 20% nitric acid (Voorhoeve et al., 1973). The two samples were prepared at different laboratories. The first (sample A) was obtained from J. P. Remeika (Bell Telephone Laboratories), and the second (sample B) was prepared by the authors. The remaining samples were polycrystalline. Sample C was prepared from coprecipitated hydroxides which were calcined in A1203 crucibles for 24 hr a t 600°C and etched 5 min in hot (5OOC) 15% nitric acid (Voorhoeve et al., 1973). Testing was conducted on particles in the 149-250-pm range. Samples A-C were found by X-ray diffraction analysis to be single phase material with well defined perovskite structures. 274
Ind. Eng. Chem., Prod. Res. Dev.. Vol. 14, No. 4, 1975
The final sample (D)was prepared by sintering intimate mixtures of oxides and carbonates in A1203 crucibles a t 1000°C (72 hr) followed by etching in 5% nitric acid at 65OC for 5 min (Voorhoeve et al., 1973). X-Ray diffraction measurements indicated the presence of small amounts of secondary phases. Sample D was too fine to be accommodated in the flow reactor. As a result, the material was combined with 10 wt % y-A1203(Alon, Cabot Corporation), and a limited amount of water was added with stirring to form small agglomerates. The mixture was dried, and the particles in the 150-420-pm range were separated and fired at 55OOC for 1hr. Because the single crystal materials were grown in platinum crucibles, emission spectrographic analysis for platinum was performed on all samples. Catalysts A and B, prepared in platinum ware, were found to contain Pt (0.020.03 wt %). As expected, no Pt was detected in samples C and D. Platinum catalysts were prepared by impregnating crushed (250-420 pm) commercial y-Al.203 beads (KaiserKCBAS) with an aqueous solution of chloroplatinic acid, drying in air at 150°C, and then firing for 4 hr at 54OOC in a 4.0% hydrogen, balance nitrogen atmosphere. The Pt contents of the two catalysts studied were 0.75 and 10.3 wt %. The Cos04 catalyst was prepared by precipitation of cobalt hydroxide by the addition of ammonia to a cobalt nitrate solution. The precipitated material was calcined at 540°C for 24 hr and the 149-250-pm particle size range was separated. The CuO catalyst was 149-250 pm fraction of crushed Harshaw Cu-307 catalyst pellets. The reported composition was 99% CuO. The compositions of both base metal oxides were verified by X-ray diffraction analysis. Specific Surface Area Measurements. The specific surface area of the oxide catalysts was obtained from fourpoint nitrogen isotherms obtained with a modified PerkinElmer Sorptometer. The surface area of platinum in the Pt/A1203 catalysts was determined by both flow (Roca et al., 1968) and static (Spenadel and Boudart, 1960) hydrogen chemisorption techniques. Activity Measurements. Activity measurements on the catalyst powders were made in the integral flow reactor system shown in Figure 1. A flowing gas stream, blended from compressed gases to simulate automotive exhaust,
.
400 mesh screen
flow
valve
,jump (c:
' g rotameters a s 'blending'
bath
filterJ
gas analyzers
Figure 1. Schematic of integral flow reactor system.
was passed through the powders which were supported in the externally heated reactor tube (3.75 cm i.d.). Water was introduced into the feed stream at the reactor inlet by controlled injection into a heated bed of Sic. The composition of the gas stream at the inlet and outlet to the reactor was determined by alternately directing a small fraction of the gas flow through a series of gas analyzers (nondispersive infrared for CO and NO, flame ionization for hydrocarbons and paramagnetic analysis for 02). The space velocity of the gas flow through the catalyst was comparable to that obtained in an automobile converter. Catalyst temperature was measured with two thermocouples located in the mid-plane of the catalyst sample. One thermocouple was located near the wall of the reactor and the other on its centerline. Agreement between thermocouples was generally 5-10°C. Where agreement exceeded 10°C, data were not reported. Average readings were used in reporting data. Activity measurements were made by monitoring gas inlet and outlet compositions as a function of catalyst temperature which was dropped from 500 to 100°C over a period of 3 hr. The raw data were converted to plots of percent CO or hydrocarbon converted as a function of catalyst tem-
perature. Two basic gas compositions were used to assess the catalysts: A. 0.5% c o , 0.025% C3H6, 3% 0 2 , 10% H20, balance N2, and B. 2% CO, 0.05% C3H6, 2% 0 2 , 10%H20, balance N2. The first represents automotive exhaust under hot engine operation; the second represents engine warm-up conditions. Carbon dioxide is the only major ingredient of exhaust gas absent from the above test compositions. It was omitted because it was found not to significantly affect the rate of the reactions. The use of propylene to represent exhaust hydrocarbons has been demonstrated to be a reasonably valid simplification (Kuo et al., 1971; Voltz et al., 1973). To further simplify the interpretation of data, the effect of NO (present in highest concentrations during automobile acceleration) was evaluated separately using gas composition A with 0.1% NO added. Composition A was also used to evaluate effects of thermal aging and SO2 poisoning on the catalysts. Stability Determinations. The susceptibility of the catalysts to poisoning by SO2 was determined by both activity measurements and X-ray diffraction analysis. The activity test samples were poisoned by heating at 55OoC for 24 hr in a flowing gas stream containing 0.01% SO2, 2% 0 2 , balance N2. For the X-ray studies, the catalysts were heated for 60 hr at both 550 and 65OOC in a gas stream containing 0.09% S02, 2% 02, balance N2. The reaction between the catalysts and alumina support material was measured by intimately mixing the catalytic materials with 50 mol % 7-A1203 (Alon, Cabot Corporation) pressing into briquettes, and heating at 600, 800, and 1000°C for from 24 to 250 hr. The extent of reaction was determined by X-ray diffraction analysis. The effects of thermal aging were determined by activity and surface area measurements on the catalysts subsequent to a 24-hr, 1000°C heat treatment in air. Although catalysts normally operate at 600 to 75OoC, temperatures on the order of 1000°C can occur during periods of engine malfunction, e.g., spark plug failure. In fact, even considerably higher temperatures have been recorded (Morgan et al., 1973). Results Reactivity Data. The weight, specific surface area, and total surface area of the catalysts used in the activity tests are given in Table I. Sample volume in all cases was 11.0
Table I. Catalyst Sample Weights and Surface Areas Catalyst
Wt, g
Specific surface area, m2/g
Sample surface area, m2
Series I
CUO Pt (10.3 wt 0/O)/A1203
40.0 36.4 9.1 13.8 4.8b
La, ,Pb,.,MnO, La,, Pb MnO, La,.,Pb,.,MnO, La, ,Pb,,MnO, Lao,,Pbo.,MnO, Lao.,Pbo.,MnO,
36.4 15.5 16.2 21.9 19.7b 22.6'
La,.,Pb,,MnO, (A)" Lao.,Pb,,Mn03 (B)" c0304
1 .o 1.4 8.6 5 .O 9 .0-13.1'
40 51 78 69 43-63'
Series I1
, ,
(B-T)" (C -U) (C)
(C-T) (D-U)
(D)
0.31 8.4 40.6 0.75 0.26" 2.7'
11 130 664 16 5" 61'
Series I11 Pt (10.3 wt %)/A1203 (TI Pt (0.75 wt %)/Al,O, a
Contains 0.02-0.03 w t 70Pt. b Weight of catalyst
4.8b 5.1b
+ A1203.
0.13-0.14' 0.55'
0 52-0.68' 2.8'
Catalyst surface area only. Ind. Eng. Chem., Prod. Res. Dev., Vol. 14, No. 4, 1975
275
d
80-
CUO
60-
:! 0
100 Temperature, C '
Temperature,
OC
Figure 2. CO conversion vs. temperature for (a) Pt, (b) Lm.7Pbo.3Mn03, (c) C0304, and (d) CuO under conditions listed in Table 11.
cm3,-The surface areas for the Pt catalysts were determined by hydrogen chemisorption and, therefore, represent only the exposed surface of noble metal. The data are divided into three series. The first comprises samples used to compare the intrinsic activities of the catalysts. In this series, sample surface areas differ by less than a factor of 2, a small difference in terms of its effect on catalytic properties. Series I1 compares properties of Lm.7Pbo.3MnO3 samples produced under a variety of conditions. This includes the three preparative techniques in etched, unetched (U), and thermally aged (T) condition. The last series presents data for Pt/A1203 catalysts which were used for activity comparisons. Manganite sample B was used for all evaluation studies requiring single crystal material. However, to be assured that the materials produced here had comparable properties to those used by Voorhoeve et al. (1972) (sample A), a comparison of the activities of A and B materials were made using gas composition A. Sample B was found slightly more active as it attained a given level of CO and propylene conversion about 15OC below that for A. Conversion-temperature data for the CO to C02 oxidation are given in Figure 2 for the series I catalysts listed in Table I. The activity of each catalyst was tested under six standard conditions which are listed in Table 11. Conditions 1-4 collectively were taken to represent the environmental conditions to which a catalyst is exposed in normal automotive operation. Conditions S and T represent the activity of catalysts subsequent to deactivation by SO2 (S) and thermal aging (T). The degree of deactivation can be determined by comparing curves S and T with curve 2, all of which were tested under the same ambient conditions. Figure 3 contains analogous conversion data for the catalytic oxidation of propylene to CO2 and H2O. The large amount of data presented in Figures 2 and 3 makes com276
Ind. Eng. Chem., Prod. Res. Dev., Vol. 14, No. 4, 1975
Figure 3. Propylene conversion vs. temperature for (a) Pt, (b) La0.7Pbo.3Mn03, (c) c0304, and (d) CuO under conditions listed in Table 11.
Table 11. Feedstream Conditions for Activity Testsa Condition identiGas fication composition
Flow rate, Catalyst 1. /min preconditioning
1 A 5.5b ... 2 A 16.5' ... 3 A + O.l%NO 16.5 ... 4 B 16.5 ... S A 16.5 SO, poisoning T A 16.5 Thermal aging Note: Gas composition A = 0.5% CO, 0.025% (C3H6), 3% 0 2 , 10% HzO, balance Nz; Gas composition B = 2.0% CO, 0.05% (C3H6), 2% 0 2 , 10% H20, balance Nz. bApproximate space velocity: 30,000hr-1, Approximate space velocity: 90,000 hr-l. parisons of the data difficult. T o facilitate subsequent discussions, the 80% conversion temperature for each curve was tabulated (Table 111).The particular choice of 80% was based on the need for high conversion in automotive emission control. Figures 4 and 5 illustrate the results obtained for condition 2 with La0.7Pbo.3Mn03 catalysts prepared by the three techniques discussed previously. The activity of sample C is shown before (C-U) and after etching (C), and after thermal aging (C-T). For sample D, only data before (D-U) and after etching (D) are given. All polycrystalline samples were also tested under condition 4 (higher CO and C3& and lower oxygen than condition 2), and sample C was further tested under condition 3 (condition 2 NO). In contrast to the considerably reduced CO and propylene oxidation rates exhibited by the single crystal La0.7Pbo.3Mn03 catalysts when tested under condition 4, all of the polycrystalline activity results for conditions 2 and 4 were nearly identical indicating that the CO and C3H6 oxidation rates could be expressed in irreversible first-order form. In addition, the conversion curve for sample C obtained with NO
+
Table 111.8Wo CO and C3H6 Conversion Temperatures ("C) from Figures 2 and 3
Feedstream conditions Catalyst
Carbon monoxide data 150 Pt/Al2O3 190 7pb0.3MnO, (B) 190 c0304 245 CUO Propylene data 155 Pt/ f w 3 285 La0.7Pb0.3MnO, (B) 220 c0304 425 cuo
T
4
S
210 240 260 275
315 300
222 310
278 340
227 245 290 315
305 310
> 500 > 500
285 >>500
212 245 325 450
315 425
225 360
280 500
312 285 335 500 475 >500
> 500 > 500
460 >>500
1
2
3
Temperature, OC
Temperature,
Figure 5. Propylene conversion vs. temperature for single crystal and polycrystalline Lao.7Pbo.3Mn03. Tested under condition 2, Table 11.
"C
Figure 4. CO conversion vs. temperature for single-crystal and polycrystalline Lm.yPbo.3Mn03. Tested under condition 2, Table
11.
in the feed-stream (condition 3) was also identical to the condition 2 curve indicating the absence of inhibition by
2 ' 7 2 ' 5 2 ' 3 2'1 1'9 1'7 1'5 -1430 - 6
NO.
I/T x 103
Based on the foregoing evidence, first order rate constants, k,, were calculated by applying the integral-plug flow reactor equation
k, = nI:-
(1 - X )
(1)
to the portion of the polycrystalline Lao.7Pb0.3Mn03 data between 5% and 50% conversion in Figures 4 and 5. The data are plotted in Figure 6 as a function of 1/T. Included also are three CO data points calculated from polycrystalline Lao.7Pbo.3Mn03 data reported by Voorhoeve et al. (1973). Activation energies obtained from a least-squares fit of the data were 13 kcal/mol for CO and 24 kcal/mol for CsHs. Yao (1975) obtained an activation energy of 10 kcal/ mol for CO oxidation using a sample equivalent to sample C. Based on the correlation of the first-order rate constants in Figure 6, it is evident that the difference in activity observed for the various polycrystalline samples in Figures 4 and 5 are almost entirely accounted for by differences in their surface areas. This conclusion is further supported by evidence reported by Johnson and Gallagher (1973). Figure 7 plots the specific reaction rates (r,) for all Lao.7Pbo.3Mn03 and Pt catalysts shown in Table I vs. reciprocal temperature. Data were obtained from low conversion (
