202
Ind. Eng. Chem. Prod.
Res. Dev. 1986, 25, 202-208
Palladium-Lanthanum Catalysts for Automotive Emission Control Hldeakl Murakl, Hlrofuml Shlnjoh, Hldeo Sobukawa, Kohjl Yokota, and Yoshlyasu Fujltanl Toyota Central Research and Development Laboratorles, Inc., Aichi-gun, Aichi-ken 480- 1 I, Japan
Recent efforts to provide Rh-free three-way catalysts have resulted in Pd-La,031a-AI,03 with desirable catalytic activities. The performance of the Pd-La203 catalyst was examined in engine exhaust gas and simulated exhaust gas. Also, the acthriies for H,-NO, H,-NO-O,, CO-H,O, and propylene-H,0 reactions, chemisorption,and TPD were measured. A/F dependence of Pd-LazO3 catalyst was almost similar to that of Rh catalyst, and the NO conversion of Pd-LaZO, catalyst during warm-up conditions was much better than that of Pd catalyst in engine exhaust control. These data demonstrate that the presence of L a 2 0 3 on the Pd-LapO:, catalyst has three effects. First, it increases the activity and selecthrity of Pd catalyst for NO reduction by H,. Second, it increases the actiiitiis for CO and propylene reactions with H20. Third, it increases the amount of NO chemisorption. This means that through the three-way catalyst reactions, the role of La,O, is as an excellent NO reserver and an abundant H, supplier.
Introduction The simultaneous control of nitrogen oxides (NO,), carbon monoxide (CO), and hydrocarbon (HC) emissions from automobile exhaust can be achieved by the use of a three-way catalyst (TWC) in connection with an O2sensor and a closed loop feedback control mechanism of A/F ratio (Canale et al., 1978;Engh and Wallman, 1977). Rhodium (Rh) is used in the TWC for its catalytic 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). The standard TWC formulations are those containing both platinum (Pt) and rhodium (Rh). The Rh/Pt ratio in the TWC with sufficient durability is considerably higher than the mine ratio. Therefore, for its extensive use, it is necessary to find ways to minimize the use of the scarce material or provide a Rh-free catalyst. For comparison between Pt and Pd, TWC performance of P d is much similar to that of Rh catalyst (Muraki et al., 1985). Therefore, we selected Pd as the main component of a Rh-free TWC. Although Pd is less resistant to poisoning than Pt (Hegedus et al., 1979), it is more resistant to sintering than Pt (Yao, 1980). In contrast, Pd catalyst forms more NH3 than does Rh catalyst under the stoichiometirc steady feed but less NH3 than Pt catalyst (Schlatter and Taylor, 1977; Taylor, 1975). In order to overcome some of the disadvantages of Pd catalysts, modified Pd catalysts containing base metals were recently reported (Gandhi et al., 1982; Adams and Gandhi, 1983). On the basis of the above requirements, we made an attempt to develop a Pd-based catalyst for automotive use. However, it is well-known that Pd catalyst has poor NO reduction capability and a narrower window gate compared with Rh catalyst. In order to improve the NO reduction activity of Pd catalyst, our efforts were pointed to find more effective additives. As a result, we found that the catalytic behavior of Pd-La203 (Pd-La) catalyst is almost similar to that of Rh catalyst for engine exhaust control and also that the Pd-La catalyst has a peak of NO conversion for temperature below 350 "C. The NO conversion performance of Pd-La catalyst under near-stoichiometric (air-fuel ratio, A/F = 14.6) and reducing (A/F < 14.6) conditions is much better than Pd catalyst. A/F dependence of Pd-La catalyst is similar to that of Rh. The objective was to clarify two behaviors; the Pd-La catalyst has excellent NO conversion during warm-up of the catalyst from room temperature to 350 "C under 0796-4321/86/7225-0202$01.50/0
stoichiometric condition and also under reducing conditions at high temperature. In this paper, we report the catalytic behavior of Pd, Pd-La, and Rh/a-AlzOa catalysts in both simple and complex model gas systems. Also, both the chemisorption and TPD measurements are performed. Experimental Section A. Catalysts. In order to avoid solid-state reactions of La203 with AZO3, chemically inactive a-AlzO3 pellet (0.d. 2-3 mm, BET surface area 10 m2/g, bulk density 0.79 g/cm3) was selected as the support. a-Alz03was obtained by calcination of 6-AlzO3 (Rhcne-Poulenc: SCS-79) at 1200 "C for 3 h. Pd and Rh catalysts were prepared by impregnating a-A1203with aqueous solutions of palladium and rhodium nitrate. The concentrations of respective solutions were adjusted to give 0.05 (low loading) and 2.0 g/L (high loading) metal on the finished catalyst. The impregnated spheres were dried overnight at 110 OC and calcined at 600 "C for 3 h in air. Pd-La catalyst was prepared by first impregnating with an aqueous solution of lanthanum nitrate to give 0.1 mol/L of La in the finished catalyst followed by drying and calcination. It is followed by impregnation with Pd nitrate solution, drying, and calcination under the same abovementioned conditions. High-loading catalysts were used for the chemisorption measurement, TPD, water gas shift reaction, and steam reforming of propylene. In contrast, low-loading catalysts were used for other activity examinations, namely for H2-NO and H2-NO-02 reactions, and both engine exhaust gas and simulated exhaust gas. B. Flow Reactor Studies. Catalytic activity data were obtained by using a conventional fixed-bed flow reactor at atmospheric pressure. A quartz tube with an inner diameter of 18 mm was chosen as the reactor tube. Catalyst (7 cm3, -5.5 g) was placed on a quartz filter at the middle part of the reactor. The upper part of the catalyst bed was packed with 7 cm3of inactive Sic spheres (3 mm 0.d.) for preheating the feed gas. Furnace temperature was controlled with a maximum variation of f1.5 OC by an automatic temperature controller. The gas leaving the reactor was led to a condenser to remove the water vapor. The remaining components were continuously analyzed by nondispersive infrared (CO and C02),flame ionization (HC), magnetic susceptibility (02), chemiluminescence 0 1986 American Chemical Society
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 2, 1986 203 Table I. Compositions of Feedstreams composition of feed streams, vol % expt S-scan (500 "C)
light-off (S = 1.0) HZO-CO H20-CBHB NO-H2 NO-02-HZ (200"C)
0 2
0.6-3.2 0-1.3 0.75 0.75
NO
HZ
0.1
0.5
0.1 0.1
0 0.3
0.1
0
0.5
0.7 0-0.5
co
C3H6
HZO
1.5 1.5 1.0
0.05
3
0
1.3 1.0
0.03
0 3 3 3 3
0.03
0.13 0.13
0.13
(NO,), and gas chromatography analyzer (H2). The stoichiometry number, S, used to identify the redox characteristic of the model gas mixtures is defined as 2w21 + [NO1 S= W 2 1 + [COI + ~ [ C ~ H I ~ I When S < 1.0, S = 1.0, and S > 1.0, the composition of the feedstream is net reducing, stoichiometric, and net oxidizing, respectively. The feed compositions and reaction conditions were as follows: (1) Simulated exhaust gas for S-scan was composed of 1.5% CO, 0.5% H2,1500 ppm C as C3H6, 1000 ppm NO, 0.6%-3.2% 02,3% H20, 12% COO,and the balance N2. Another S-scan experiment was carried out by using the model feed-gas of 1.5% CO, 1000 ppm NO, 0%-1.3% 02, 12% C02, and the balance NP. Catalytic activity and selectivity of the catalysts were expressed as the percent conversions of HC, CO, gross NO, and net NO (i.e., NO converted to N2), and NH3 formation was expressed as percent of NO converted. Conversion data were measured at 500 "C and 30000/h space velocity. The activity measurements as a function of S are designated as S-scan. In the present experiments, S is changed by adjusting the oxygen content a t the inlet of the reactor. (2) Light-off performance was studied with both the simulated exhaust gas of 1.0% CO, 0.3% H2, 1000 ppm C as C3&, lo00 ppm NO, 0.75% 02,3% H20,12% COP,and the balance N2 and the H2-free simulated exhaust gas of 1.3% CO, 1000 ppm C as C3H6, 1000 ppm NO, 0.75% 0 2 , 3% H20, 12% COz, and the balance NO. Either composition gives S = 1.0. In this experiment, the heating rate of the reactor was kept at 6.7 "C/min. (3) Simple gas reaction systems were used. (a) NO reduction by H2was measured with a model feed gas of 0.7% H2, 0.5% NO, and the balance N2 over the temperature range of 100-350 "C a t steady-state conditions. (b) The selectivity for promoting the N&H2 reaction vs. the 02-H2 reaction at 200 "C was measured with a model feed gas of 0.13% NO, 0.13% 02,0%-0.5% H2, and the balance N2. In this case the concentration of the oxidizing gases was fixed and the reducing gas was varied a little above the stoichiometric composition. (c) Water gas shift reaction was studied with model feed gases of 3% H20, 1% CO, and the balance NP. Steam reforming of propylene was studied with the model feed gas of 3% H20,4000 ppm C as C3H6, and the balance N2. The reactor temperature was kept at 600 "C. After the catalyst bed temperature and the outlet concentrations were stabilized, the temperature was cooled down in steps from 600 "C. The space velocity of the above-mentioned reaction gases was kept at 30000/h. Table I lists the composition of the feed gases used in this study. C. Dynamometer Reactor Studies. A 2.0-L L-6 engine (Toyota M-E) equipped with a conventional catalytic converter (1900 cm3) was used in the test reactor experiments. In these experiments, the engine was operated at
COZ 12 12 12 12
N2 bal bal bal bal bal bal bal bal
1600 rpm, -440 mmHg manifold vacuum, and exhaust flow rate of about 10 L/s. After the engine was stabilized, under given values of A/F, the conversion activities of the catalysts for CO, HC, and NO, were measured. The inlet temperature of the catalytic converter was 340-360 "C. The outlet temperatures varied from 380 to 450 "C, depending upon the degree of conversion over the catalysts that determined the amount of heat released through the reactions. In this experiment, the A/F values were varied from 14.1 to 15.1 according to the amount of fuel injected to the engine. Also, in order to examine light-off performance in engine exhaust gas, the feed stream of the exhaust gas under the above-mentioned conditions was switched to the testing catalytic converter at room temperature and the reaction of CO, NO, and HC and the catalyst bed temperature were monitored during warm-up of the catalyst from room temperature to 400 "C. D. Chemisorption Measurements. Chemisorption of CO, H2, 02,C3&, and NO was studied in a flow system similar to that described by Gruber (1962). An amount of 1 cm3 of catalyst was pretreated by (1)increasing the catalyst temperature up to 500 "C at the heating rate of 500 "C/h in 50 mL/min of H2, (2) reducing the catalyst with H2 (50 mL/min) at 500 "C for 1h, (3) changing the H2 steam to 50 mL/min He stream (Ar stream is used in the case of H2 chemisorption) for 1 h at 500 "C, and (4) cooling down to room temperature. After the pretreatments, 0.25 mL of adsorbate gas was injected into the catalyst several times for 3 min intervals at room temperature. In most cases, it was noticed that CO, H2, 02, C3H6,and NO were adsorbed only in the first pulse and not thereafter. The amounts of chemisorption for the respective gases were determined by the difference of peaks. E. TPD Measurements. The reactor used for this study consisted of a 6 mm 0.d. quartz tube with a 12 mm 0.d. bulb-shaped midsection. A quartz fritted disk was fused into the midsection of the reactor. The reactor was heated by a furnace. A temperature programmer was used to control catalyst heating at a linear rate of 40 "C/min up to 500 "C. The carrier gas was helium at atmospheric pressure, which flowed over the catalyst at 50 mL/min. A gas injection valve was connected in series with the line feeding the reactor to permit injection of adsorbate pulses. The effluent from the reactor was analyzed by a quadrupole mass spectrometer. The same reduction and adsorption procedures were used for the TPD of preadsorbed NO in the same manner as for NO chemisorption. Results and Discussion A. TWC Performance in Engine Exhaust. In this first section, TWC performance of Pd-La, Pd, and Rh catalysts will be examined by using the engine exhaust gas. Our experiment was undertaken to determine whether the NO conversion of Pd-La catalyst under reducing conditions might be higher than that of Pd catalyst and then
204
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 2, 1986 100
--9 -c
'ao
0
°
1
0
50 i
al C
0
14.2
14.6 A/F
15.0 Temperature (kj
Figure 2. NO conversion efficiencies as a function of temperature in engine exhaust gas for Pd and Pd-La catalysts at A/F = 14.4.
14.2
14.6 A/F
15.0
14.6 A/F
15.0
100
0
6
1
.'E C
50
>
:
0
Figure 1. Conversion efficiencies as a function of A f F of engine exhaust gas for (a) Pd-La, (b) Pd, and (c) Rh catalysts.
the window gate might be of greater width than that of Pd catalyst. In another set of experiments we wish to determine during warm-up of the catalyst whether the addition of La203to the Pd catalyst might lead to greater conversion than nonmodified Pd catalyst. Figure 1 shows the relation between the conversions of HC, CO, and NO, over Pd-La, Pd, and Rh catalysts and A/F of the engine exhaust gas. It can be seen that the total HC (THC) conversion of Pd-La catalyst near the stoichiometric condition is much lower than that of Pd catalyst. The addition of La to Pd catalyst suppresses the catalytic activity for HC oxidation. However, THC conversion of Pd-La catalyst is similar to that of Rh catalyst. CO conversion of Pd catalyst under reducing conditions is also much lower than that of Rh catalyst. In contrast, CO conversion of Pd-La catalyst is higher than that of Pd catalyst and similar to that of Rh catalyst. It is clear that under reducing conditions, gross NO conversion of Pd-La catalyst is higher than that of Pd catalyst. Under nonstoichiometric conditions, the NO conversion of Pd-La catalyst is lower than that of Rh catalyst. We confirm that the A / F dependence of Pd-La catalyst is almost similar to that of Rh catalyst in the engine exhaust examination.
I
O ' 0'6
'
08
l"0
12
S
Figure 3. Conversion efficiencies as a function of stoichiometric number of simulated exhaust gas for (a) Pd-La, (b) Pd, and (c) Rh catalysts at 500 OC.
Figure 2 shows the light-off performance of Pd-La and Pd catalyst in the engine exhaust gas (A/F = 14.4). The NO conversion of Pd-La catalyst increased with increasing temperature until 300 OC, dropped once, and then decreased with further increase. This phenomenon is the proper nature of Pd-La catalyst. In the next section of this paper we will examine the use of the simulated exhaust gas in order to verify the above-mentioned results. B. Simulated Exhaust Gas Examinations. (1) S Scan. The steady-state conversions of HC, CO, and NO under the simulated exhaust feed gas on Pd, Pd-La, and
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25, No. 2, 1986 205
S
Cat Temperature ( ' C )
Figure 4. Conversion efficiencies as a function of stoichiometric number of N(rCO-02 system for Pd-La and Pd catalysts at 500 OC.
Rh catalysts as a function of S are given in Figure 3. The comparison of these conversions under the reducing conditions were summarized as follows: P d catalyst shows a low conversion for HC, but HC conversion of Pd-La catalyst is similar to that of Rh catalyst. The difference in HC conversion between the engine exhaust and the simulated model gas essentially results from the composition of HC and reaction temperature. The hydrocarbons in the engine exhaust consisted of 40% saturates, 47% olefines, and 13% aromatics (Kuo et al., 1971). In contrast, it was propylene in the case of simulated exhaust gas. CO conversion of the Pd catalyst is lower than that for both Pd-La and Rh catalysts. The gross NO conversion of Pd-La catalyst is 95% at S = 0.8, but this catalyst is very selective for the NO to N2reaction such that the net NO conversion is 86%; Le., less than 10% of the converted NO forms NH, at S = 0.8. Although Rh catalyst shows higher NO conversions, the NH3 formation of Pd-La catalyst gives the same value as that for Rh catalyst. The Pd catalyst shows only poor activity for NO, giving 17% NH, formation and 67% net NO conversion at S = 0.8. In contrast with Pd catalyst, Pd-La catalyst has higher conversions for NO, CO, and HC under reducing conditions. This suggests that under reducing conditions NO may react with Hz produced from water gas shift reaction, steam reforming, and partial oxidation of propylene. In order to verify these hypotheses we will use the model gas mixtures that do not include HzO, Hz, and C3H6. The results are shown in Figure 4 as a function of S. NO conversion over Pd-La catalyst is almost the same as that of Pd catalyst. These data show that NO reacts with H2 produced from water gas shift reaction, steam reforming, and partial oxidation of propylene. The difference in NO conversion between Figures 3 and 4 might result from the self-poisoning effect of propylene. These results will be reported in a subsequent paper. (2) Light-Off Performance. The steady-state conversions for HC, CO, and NO are shown as a function of temperature for Pd-La, Pd, and Rh catalysts in Figure 5. The Pd-La catalyst shows higher NO conversion below 350 "C and lower CO conversion in all temperatures compared with Pd catalyst, while Rh catalyst shows higher NO conversion. The Pd catalyst shows lower catalytic activity for NO reduction compared with CO and HC oxidations. NO reduction on both Pd and Pd-La catalyst proceeds remarkably above 400 "C. In order to understand the reason why the Pd-La catalyst should have a maximum NO conversion below 350 "C, we examined the light-off performance of Pd-La catalyst using the model gas mixture without Hz. The results are shown in Figure 6 as a function of temperature. NO conversion over Pd-La catalyst equals that of the Pd catalyst. This result indicates that NO reacts with H2 on
Cat. Temperature ( ' C )
10 Cat .Temperature ('C )
Figure 5. Conversion efficiencies as a function of temperature in simulated exhaust gas for (a) Pd-La, (b) Pd, and (e) Rh catalysts at s = 1.
Cat Temperature ("C)
Figure 6. Conversion efficiencies as a function of temperature in H2-freesimulated exhaust gas for (a) Pd-La and (b) Pd catalysts at s = 1.
Pd-La catalyst below 350 "C. Also, the data presented in Figure 5 explain that NO r e a d with H2 under the conditions that HC does not react
Ind. Eng. Chem. Prod. Res.
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Dev., Vol. 25, No. 2, 1986
-8
20 C
.-0
z
0 0
0 i
100
Figure 7. Water gas shift reaction over Pd-La, Pd, and Rh catalysts as a function of temperature.
200 Temperature ("C)
300
Figure 9. Conversion curves for NO reduction with H, over Pd, Pd-La, and Rh catalysts. --I
1
0
0-
200
, 400 500 Catalyst Bed Temperature ("C )
300
600
Figure 8. Steam reforming of propylene over Pd-La, Pd, and Rh catalysts as a function of temperature.
with 02.This is, NO can react with H2when O2is present (the oxidizing conditions). It appears that the Pd-La catalyst may have higher selectivity for the NO-H2 reaction than for the 02-H2 reaction. In this section we will confirm two phenomena using the simulated exhaust gas; they are that the Pd-La catalyst shows higher NO conversion under reducing conditions at 500 "C and also during warm-up of the catalyst below 550 "C than the Pd catalyst. The next section of this paper will deal with the catalytic activities for both H2-forming reactions and the H2-NO reaction of the Pd-La catalyst. (3) Water Gas Shift Reaction and Steam Reforming of Propylene. In order to clarify the possible mechanism for the origin of higher NO conversion of Pd-La catalyst under reducing conditions, the hydrogen from reactions is investigated over three catalysts. These reactions are water gas shift reaction and steam reforming of propylene, which should be expected to proceed under the environment of three-way catalyst use (Schlatter, 1978). Figure 7 shows the relative catalytic activities for water gas shift reaction on three catalysts. The addition of La20, to Pd catalyst remarkably promotes CO conversion. In contrast, the catalytic activity for steam reforming of propylene on three catalysts is given in Figure 8. C3H6 conversion of Pd-La catalyst equals that of Rh catalyst in the range of experimental temperatures. The activity of Pd-La catalyst is clearly higher than that for Pd catalyst. Also, partial oxidation of propylene on three catalysts was examined. The relative catalytic activity of partial oxidation was found to be similar to that of the steam reforming of propylene. From the results of the above-mentioned simple reactions, the role of La203 added to Pd catalyst is as an effective H2 supplier. As previously described, under the engine exhaust conditions, CO conversion is improved by addition of La203 to Pd catalyst; on the contrary, HC conversion is not improved. According to the abovementioned reasons, the high catalytic activity of NO re-
'"-i 0
0
(b) Pd
02
0.4
I n i t i a l H2 ( v o i % )
Figure 10. Conversion curves of NO and O2over (a) Pd-La and (b) Pd catalysts at 200 "C.
duction over Pd-La catalyst under reducing conditions is mainly affected by the excellent activity of water gas shift reaction. (4) H2-NO Reaction. In order to clarify the higher NO conversion of Pd-La catalyst below 350 "C, the H2-NO reaction is investigated over three catalysts. The results of NO reduction with hydrogen over these catalysts are compared in Figure 9. Pd catalyst is known to have excellent catalytic activity for NO reduction with hydrogen (Kobylinksi and Taylor, 1974). It is clear that Pd-La catalyst is much more active than Pd catalyst below 300 "C. Next, in order to confirm whether the Pd-La catalyst might be much more selective for the H2-NO reaction than for the H2-02 reaction, the H2-NO-02 reaction is examined. The steady-state conversion of NO and O2 in the net oxidizing feed stream was determined as a function of initial H2 concentration at 200 "C. The results for the Pd and the Pd-La catalysts are presented in Figure 10. The percent conversion of NO exceeds the percent conversion of O2 for all initial H2 concentrations. The activity of Pd-La catalyst for NO reduction is much better than that of the Pd catalyst. The selectivity of these catalysts for promoting the NO-H2 reaction vs. the 02-H2 reaction is compared directly in Figure ll, which is a plot of percent NO conversion vs. percent O2 conversion. Movement along the
Ind. Eng. Chem. Prod. Res. Dev., Vol. 25,
No. 2, 1986 207
-,\"
I
C
.-0
E
> 0 0
0
z
50 1 O2Conversion (yo)
Figure 11. Comparison of the partitioning of Hzbetween NO and O2 over Pd-La and Pd catalysts.
Table 11. ChemisorDtion Data (rtmol/a)" catalvst Pd-La Pd La
Rh
CO 2.7 3.1 0.0 4.7
amount, pmollg NO H, 0, 7.0 7.3 4.3 3.5 4.4 1.6 4.4 0.0 0.0 3.9 7.3 2.1
CQHa 1.1 1.1 0.0 2.3
A
@Noblemetal 2 g/L; La 0.1 mol/L.
curves from left to right is produced by increasing the initial H2 concentration in the feed stream. The diagonal dotted line represents equal percent conversion of NO and 02.For the curves above the diagonal the percent NO reacted at a particular initial H2concentration exceeds that for 02. This comparison illustrates that the Pd-La catalyst has higher selectivity for the NO-H2 reaction than the Pd catalyst. D. Chemisorptions. Chemisorption data obtained from 2.0 g/L Rh, 2.0 g/L Pd, 2.0 g/L Pd-0.l mol/L La, and 0.1 mol/L La are given in Table 11. These data show that the amounts of NO, H2, and 02 chemisorption of Pd-La catalyst are higher than that for the Pd catalyst. Also, NO is chemisorbed on La203 catalyst alone. The amount of CO and C3H6chemisorption on Pd-La catalyst is almost the same as that on Pd catalyst. The amount of NO chemisorption on Pd-La catalyst is nearly equal to the sum of chemistorption on Pd and La catalysts and is also equal to that on Rh catalyst. E. TPD. Three species were detected during the thermal desorption of adsorbed NO: NO, N20, and N2. Figure 12 shows the spectra on Pd, Pd-La, and La catalysts. Above 400 OC, the Pd-La and La catalysts show a second large NO desorption curve. The peak temperatures of N2 and N20 desorption curves on the Pd-La catalyst are lower than that on the Pd catalyst. These results indicate that NO is adsorbed strongly on the La203 and the NO adsorbed on the Pd-La catalyst is dissociated easier than on the Pd catalyst.
Summary A notable finding for the Pd-La catalyst is its activity for NO, reduction both under reducing conditions at relative high temperature and during warm-up conditions of the engine exhaust gas. This activity is substantially improved compared to Pd catalyst alone. Also, HC and CO oxidation activities are equivalent to that measured for Rh. The NO conversion of Pd-La catalyst under oxidizing conditions is poorer than that of Rh catalyst. However, if A/F would be strictly controlled near the stoichiometric point, the Pd-La catalyst can be used for the practical three-way catalyst. It is apparent from these studies that the Pd-La catalyst differs from the Pd catalyst in the activity. The nature
0
100
200
300 400 500 rC)
keep
500
Temperature
Figure 12. TPD spectra of NO, Nz,and NzO, following NO adsorption on (a) Pd-La, (b) Pd, and (c) La catalysts at 25 O C .
of the interaction of Pd and La203 is yet to be determined. ESCA, electron microscopy, and other experiments are planned for Pd-La and Pd to gain further insight into the mechanism of NO reduction over these catalysts. These results will be reported in a subsequent paper. The study of the reaction of simulated exhaust gas shows that in the presence of H20, C3H6, and H2 the Pd-La catalyst exhibits improved NO activity compared to Pd under reducing and stoiochiometric conditions at 500 "C. However, in the absence of H20, C3H6,and H2 the Pd-La catalyst activity is almost equivalent to that for Pd under the same conditions. The model reaction studies of water gas shift and steam reforming to propylene show that Pd-La catalyst exhibits improvement of these two reaction activities. In fact, this H2produced mainly from water gas shift reduces NO under reducing conditions. The light-off study of simulated exhaust gas shows that in the presence of H2 the Pd-La catalyst has peak NO conversion at 300 "C. However, in the absence of H2 the conversions of Pd-La catalyst are almost similar to that of Pd catalyst during warm-up of the catalyst. The study of NO reduction with H2 shows that Pd-La catalyst has higher activity and selectivity for the NO-H2 reaction than the Pd catalyst below 350 "C. The amount of NO and H2chemisorption on the Pd-La catalyst is higher than that on the Pd catalyst. Also, NO chemisorbed on La203alone. It is suggested that the presence of La in the Pd-La catalyst (1) increases the selectivity and the catalytic activity on the Pd catalyst for NO reduction by H2, (2) increases the catalytic activities for CO and propylene reactions by H20, especially under
Ind. Eng. Chem. Prod. Res. Dev.
208
the reducing conditions (S < l), and removes strongly chemisorbed CO and propylene from the catalyst surface, and (3) increases the amount of NO chemisorption. The role of La203on Pd-La catalyst is as an abundant H2 supplier and an excellent NO reserver through the threeway catalyst reactions. We attribute these findings to the strong interaction between Pd and La203on a-A1203. Acknowledgment We express our sincere thanks to Professor Y. Murakami of Nagoya University for his helpful discussion. Registry No. La203,1312-81-8;Pd, 7440-05-3;CO, 630-08-0; NO, 10102-43-9;NO,, 11104-93-1;Rh, 7440-16-6;Hz,1333-74-0; N,, 7727-37-9; NzO, 10024-97-2;propylene, 115-07-1.
1986,2 5 , 208-213
Canale, R. P.; Winegarden, S.R.; Carlson, C. R.; Miles, D. L. Society of Automotive Engineers: Warrendale, PA, 1978; Paper No. 780205. Engh, G. T.; Wallman, S.Soclety of Automotive Englneers: Warrendale, PA, 1977; Paper No. 770295. Gandhi, H. S.;Yao, H. C.; Stepien, H. K. ACS Symp. Ser. 1982, 178, 143. Gruber, H. L. Anal. Chem. 1962, 3 4 , 1828. Hegedus, L. L.; Summers, J. C.; Schlatter, J. C.; Baron, K. J . Catal. 1979, 56, 321. Kobylinski, T. P.; Taylor, 6. W. J. Catal. 1974, 33, 376. Kummer, J. T. Prog. Energy Combust. Sei. 1980, 6 , 177. Kuo, J. C. W.; Morgan, C. R.; Lassen, H. G. Society of Automotive Engineers: Warrendale, PA, 1971; Paper No. 710289. Muraki, H.; Shinjoh, H.; Sobukawa, H.; Yokota, K.; Fujitani, Y. Ind. Eng. Chem. Prod. Res. Dev. 1985. 2 4 , 43. Schlatter, J. C. Society of Automotive Engineers: Warrendale, PA, 1978; Paper No. 780199. Schlatter, J. C.; Taylor, K. C. J. &tal. 1977. 4 9 , 42. Shelef, M.; Gandhi, H. S.Ind. fng. Chem. Prod. Res. Dev. 1972, 1 7 , 393. Taylor, K. C. The Catalflic Chemistry of Nltrogen Oxides; Plenum: New York, 1975; p 173. Yao, Y. F. Y. Ind. Eng. Chem. Prod. Res. Dev. 1980, 79, 293.
Literature Cited Adams, K. M.; Gandhi, H. 207.
Received for review December 5, 1984 Revised manuscript received September 18, 1985 Accepted December 2, 1985
S. Ind. Eng. Chem. Prod. Res. Dev. 1983, 2 2 ,
Selective Methanol Conversion to BTX Grlgore Pop,+ Gavrll Musca,+ Gheorghe Marla,+ Sorln Straja;
and Raul Mlhallt
Chemical and Biochemical Energetics Instnute, Splaiul Independenfei 202, Bucharest, Romania, and Polytechnic Instnute Bucharest, Polizu 1, 78 126 Bucharest, Romania
Methanol conversion to hydrocarbons may be engineered to increase the amount of aromatics, becoming an alternative source of BTX. The high exothermicity of the reaction calls for a fluidized-bed reactor. But industrial fluidized bed reactors exhibit large dynamic fluctuations, strongly screening the catalyst performances. Therefore, among a class of potentially suitable catalysts, two candidates, A and 6, were chosen to be tested on a fixed bed reactor with respect to the BTX yield and the sensitivity to the operating conditions. Both catalysts are of ZSM-5 type, but they have different Si:AI ratios. The BTX amount is comparable with the literature-available data. The curvature radius of the response surface, used as a measure of the sensitivity, shows catalyst B to be more stable with respect to the operating conditions. Moreover, the low durene content may be taken as an advantage in the subsequent separation of aromatics.
Introduction The common raw materials for the synthesis of BTX are the liquid fractions from hydrocarbon cracking and reforming. The total crude oil available by the year 2000, about 2500 million tons with an average naphtha yield of 18%, will have to satisfy the needs of both the chemical industry, taking into account a 5% increase, 300 million tons, and of reforming to get high-octane motor gasoline, which is foreseen at 650 million tons. In other words, some 100 million tons of naphtha equivalent have to be found elsewhere-gas oil, gas coal, or new sources (Taylor, 1979). Synthesis of liquid fuels based on the conversion of coalor methane-derived CO H2 through the methanol-togasoline route emphasizes the shift to an alternative resource. The catalytic conversion of methanol to hydrocarbons has been investigated intensively in recent years, both economically (Harney and Mills, 1980) and scientifically, an extensive review being presented by Liu et al. (1983). Now, it seems that there is some agreement concerning the first carbon-carbon bond formation through carbene
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Chemical and Biochemical Energetics Institute.
* Polytechnic Institute. Deceased.
intermediate (Chang and Silvestri, 1977). The carbon chain proceeds through oligomerization and methylation, their relative contributions being investigated recently by Espinoza (1984). The catalyst structure and composition and the process engineering may induce a wide range of highly selective syntheses of paraffins, olefins, or aromatics, as shown in Table I. To our knowledge, the only industrial unit is a fixed-bed one operating in New Zealand (Fox, 1982). The other data collected in Table I are from laboratory or pilot-scale reactors. The fixed-bed reactors are widely used, but in order to better control the temperature fluidized-bed reactors have been tested, too. The catalysts used differ both in composition and structure, being designed as highly selective for different hydrocarbons. Considering the operating conditions, the temperature is moderate, between 300 and 500 "C, and the pressure is generally the atmospheric one, with some exceptions. Due to the catalyst used, the hydrocarbons distribution is very different: in some cases the process is selective in olefins, in other cases in aromatics. The aromatics are mostly methyl-substituted. Benzene was detected in a small amount only (Liedeman et al., 1978). Toluene and xylenes are paramount (VBdrine et al., 1980). The higher alkylbenzenes are present in smaller quantities. According to Chang and Silvestri (1977), it is because of their lower
0196-4321/86/1225-0208$01.50/0@ 1986 American Chemical Society
