Ind. Eng. Chem. Res. 1993,32, 2563-2512
2563
Catalytic Properties of Lal-BrxB03 (B = Mn, Fe, Co, Ni) for Toluene Oxidation Jeng-Jong Liangt and Hung-Shan Weng' Department of Chemical Engineering, National Cheng Kung University, Tainan, Taiwan, 70101, R.O.C.
The alkaline coprecipitation method was employed to prepare Lal,Sr,B03 ( x = 0-1 and B = Mn, Fe, Co, Ni). The activities of the resulting catalysts for oxidizing toluene were measured by a differential flow reactor, and their physical properties were characterized by BET, XRD,AES, SIMS, XPS, SEM, TGA/DTGA, and stepwise oxygen desorption. The results indicate that substitution of La by Sr affects activity due to the oxygen vacancy. Among the Co and Fe series catalysts, those for which x = 0.3 demonstrate the highest activity. The catalytic activity of the Mn series catalysts was found to increase not only with the quantity of Mn atoms but also with the sum of Mn and 0 atoms. In this series, the amount of chemically adsorbed oxygen, instead of the lattice oxygen, plays an important role in the catalytic activity, especially in the temperature range of toluene oxidation, with the catalytic activity for toluene oxidation increasing with the amount of oxygen chemically adsorbed. The activity of the Ni series catalysts does not increase with increasing amounts of NiO and LazNiOr; the highest activity occurs at x = 0.5. The experimental results are discussed in terms of the role played by various B-site cations, the influence of catalytic surface composition on catalytic activity, and the effect of lattice and chemically adsorbed oxygen. Four influencing factors on the catalytic activity of these four series catalysts were discriminated by analysis of the results of kinetic measurements and by surface characterization.
Introduction Perovskite-type mixed oxides have the general formula AB03 and the ability to change and/or substitute cation composition at sites A and B.2738 Recently, oxides doped with A', having the formula AA'B03 have attacted a lot of attention due to their unique but excellent catalytic activity. These substances catalyze many reactions including the reduction of nitric oxide to nitrogen gas and the oxidation of carbon monoxide and hydrocarbons to carbon dioxide and ~ater,2,13,19,21,26,29,30,34 The effect on catalytic activity of partially substituting La with Sr has been well documented by many auth0rs.7~g~l6~18~~2~23132133 However, informtion regarding catalytic activity when the B-site cation in Lal,Sr,BO3 catalysts is changed is not at all comprehensive. Moreover, the roles that various B-site cations play in catalytic activity, as well as the influence of catalytic surface composition, are also not clear. The catalytic activities of oxides of the Lal-,Sr,BO3 type (where x = 0-1 and B = Mn, Fe, Co, Ni) are discussed in this paper based on the experimental results of toluene oxidation. The effects of substituting various amounts of strontium for lanthanum in perovskite catalysts containing various B cations on the catalytic activities are comparatively examined. Moreover, the way in which catalytic activity is influence by various B-site cations and catalytic surface composition has been investigated by the techniques of AES, SIMS, and XPS. The effect of desorbed oxygen on catalytic activity has also been studied in detail.
Experimental Methods Catalyst Preparation. All Lal,Sr,BO3 catalysts (where x = 0,0.3,0.5,0.7,1 and B = Mn, Fe, Co, Ni) were prepared by the alkaline coprecipitation method using K2CO3 solution (0.181 M) as the precipitant. The precipitates were filtered, dried (at 120"C), and then calcined + On leave from the Department of Environmental Science, Feng Chia University, Taichung, Taiwan. * To whom all correspondence should be addressed.
0888-5885193/2632-2563$04.O0/0
in air for 6 h at the maximum calcination temperatures (see Table I) determined from TGA thermograms. Characterization Measurements. Powder X-ray diffraction (XRD) patterns were obtained from an X-ray diffractometer (Rigaku D/MAX 111.V XRD) using Cu Ka radiation. The BET surface properties were acquired using an accelerated surface area and porosimetry system (Micromeritics ASAP 2000). An Auger electron spectrometer (Anelva AAS 200) was used to measure the quantitative composition of the catalytic surface, while a secondary ion mass spectrometer (Anelva AGA 360) was employed to measure the surface chemical composition and an X-ray photoelectron spectrometer (VG Micro Lab. MK 111)was used to determine the chemical state of the catalytic surface oxygen atom. Micrographs of the catalysts were obtained from a scanning electron microscope (JEOLJSM-35 SEM). Oxygen Adsorption. Oxygen adsorption of various perovskite catalysts was probed with an apparatus for temperature-programmed desorption, utilizing the technique of stepwise desorption of oxygen. In addition, the derivative thermal gravimetric analyzer (DuPont Thermal Analyst 2000)was also used to measure desorbed oxygen. Reaction Apparatus and Procedures. Catalytic activity data were obtained using a vertical differential flow reactor at atmospheric pressure. A quartz tube with an inner diameter of 0.5 in. was chosen as the reactor. Catalyst powder (35-60 mesh, 0.2 g) mixed with quartz powder (35-60 mesh, 0.4 g) was placed on a 100-mesh stainless steel screen in the middle of the tube. The upper and lower p& of the tube were packed with quartz powder (24-32 mesh, 0.8 g) to disperse the reaction gas. The reactor was heated by a furnace with a PID temperature controller (Shinko MCD-150). Prior toeach experimental run, the catalysts were pretreated with a mixed gas ( 0 2 33.3 % , N2 balance) stream for 1 h at 400 OC. After this, the temperature of the reactor was decreased to the preset value. Then the gaseous mixture of toluene (0.042 96, except in the experimental run for obtaining the rate expression), 0 2 (33.3961, and N2 (balance) at a flow rate of 120 mL min-1 was fed into the reactor. The gas at 0 1993 American Chemical Society
2564 Ind. Ens. Chem. Res., Vol. 32,No. 11, 1993 Table 11. Crystalline Structure of Catalysts.
Table I. Maximum Calcination Temperature. ("C) for the Catalysts
1:
X
catalyst 0 0.3 0.5 0.7 0.8 0.9 1 1020 980 980 1020 1050 Lal,Sr,MnOs 780 950 1050 Lal,Sr,FeOs 780 lo00 lo00 1020 950 Lal,Sr,CoOs 780 980 980 1050 1050 850 1020 1020 1020 Lal,Sr,NiOs The catalysts were calcined at the maximum calcination temperature in air for 6 h determined from TGA thermograms. (I
I
catalyst Lal,Sr,MnO3 Lal,Sr,FeO3 Lal,Sr,CoO3 Lal,Sr,NiO3
0
0.3
0.5
P P P P
P P P P+N
P+H P P P+N
0.8 0.9 P+H P+H P+H P P P+N 0.7
1
H P DP N+S
P = perovskite Structure. H = hexagonal SrMnOs (ABAC-type) structure. DP = distorted perovskite structure. N = nickel oxide, NiO. S = strontium peroxide, SrOz.
1
Figure 1. X-ray diffraction patterm of perovskite catalysts prepared in this study: (a) Lal,Sr,MnOs, (b)Lal,Sr,FeOs, (c) Lal-,SrXCoO3, (d) Lal,Sr,NiO3.
the inlet and outlet to the reactor was analyzed by a gas chromatograph (Shimadzu GC 9A-FID) using a 3-m Silicone OV-17 column (50%phenyl and methyl) kept at 120 "C. Each experimental run was kept running until steady state was attained.
Results and Discussion An examination of the X-ray diffraction patterns revealed that the employment of the alkaline coprecipitation method, utilizing potassium carbonate as the precipitant, successfully prepared all the perovskite catalysts in this study wihout loss of cations. Structures and Surface Properties of Catalysts, Structures. The X-ray diffraction patterns of the catalysts prepared in this study are shown in Figure 1, while their crystalline structures (examined with reference to ASTM cards) are summarized in Table 11. In the range 0 Ix I0.3, Lal-,Sr,MnOa have the perovskite structure, while the XRD pattern of SrMnO3 is consistent with that of the four-layer hexagonal SrMnO3 (ABAC-type) structure reported by Negas and Roth.l'J8 In the range 0.5 I x I0.9, the Lal,Sr,MnOscatalysts consist of aperovskitetype structure together with a small amount of hexagonal SrMnO3, whose proportion increases with increasing x .
Both Lal,Sr,FeO3 and Lal-,Sr,CoO3 possess perovskite structures in the range 0 I x I 1 and 0 I x I0.9, respectively. Note that while LaNiOs's structure is perovskite, that of SrCoO3 is the distorted one. The NiO and LanNiO4 phases appeared together with the perovskite phase in Lal,Sr,NiO3 (0.3 Ix I0.71, where the XRD peak intensity of the NiO phase increases with increasing x . Due to the NiO phase, these catalysts exhibit XRD patterns similar to those of LaNiO3, which was prepared Note that SrNiO3 and treated with H2 by Fierro et catalyst possesses both NiO- and SrO2-phase structures. Specific Surface Area. The specificsurface area, total pore volume, average pore diameter, and surface atomic elemental composition of the prepared catalysts are listed in Table 111. I t can be seen from this table that the specific surface area generally decreases with increasing x . As the maximum calcination temperature tends to increase with increasing x (except for La1-,SrXCoO3, where x = l),we conclude that the higher the calcination temperature, the lower the surface area. Note that the specific surface area of the catalyst with x = 0.5 in each of the Mn, Fe, and Co series is larger than that with x = 0.3. Similar results were obtained by other a ~ t h o r s . ~ ~ Table J ~ t ~I11 ~ also J ~ reveals that the specific surface area of perovskite catalyst with a higher value of x is lowered when the B-site atoms change from Mn to Fe, Co, and Ni. Figure 1 also reveals that the Fe series of catalysts retain their perovskite structure no matter what the extent of Sr substitution, while the substitution of La by Sr in other series often results in a change from perovskite to another structure. Surface Morphology. The scanning electron micrographs of the various catalysts prepared are shown in Figure 2. The crystal sizes of the catalysts were found to increase with increasing x , being particularly large (approximately 2.0 pm) for L~o.3Sr0.7C003,SrCoO3, and SrFeO3. Those catalysts possessing a large crystal size have correspondingly lower specific surface areas (Table 111) and catalytic activities, while those crystals smaller than 0.4 pm possess accordingly larger specific surface areas. Surface Atomic Elemental Percentage. The surface atomic elemental percentages for each catalyst under investigation were determined by dividing the Auger peak areas by their corresponding sensitivity factors in order to obtain the relative atomic ratios, which were then further normalized as percentages. Each catalytic surface atomic percentage was an average value of 10 measurements at random surface position. An examination of Table I11reveals that when x is increased the surface atomic percentage of Sr is enhanced, while that of La is gradually lowered. Although an increase in the amount of La substituted by Sr does not result in a big change in the surface atomic percentages of both B and 0,the catalytic activities of the parent compounds were significantly affected.5s6J0 The data in Table I11 also reveal that the sum of the percentages of A atoms is on the high side while
Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993 2666 Table 111. Surface Characterization for L a l - S r S O s (B = Mn, Fe, Co, Ni) Catalysts catalyst LalA,MnOa x=o 0.3 0.5 0.7 0.8 0.9 1 Lal,Sr,FeOs x=o
0.3 0.5 0.7 1 Lal,SrxCoOs x-0
0.3 0.5 0.7 1 Lal,Sr,NiOs x=o
0.3 0.5 0.7 1
BET surface area (m2/g)'
total pore volume (cmVg)b
avg pore diameter (A).
18.55 3.96 5.67 4.24 3.67 3.99 3.17
0.1224 0.0175 0.0169 0.0131 0.0105 0.0142 0.0091
264.1 176.8 112.1 124.1 114.2 142.7 114.2
11.43 5.04 5.38 3.77 0.54
0.0429 0.0107 0.0130 0.0093 0.0010
150.3 85.2 96.8 99.0 75.8
15.82 3.00 3.16 0.62 0.49
0.0817 0.0092 0.0068 0.0012 0.0010
206.5 123.4 85.7 77.4 83.4
3.04 3.10 2.48 2.69 2.63
0.0101 0.0114 0.0057 0.0110 0.0108
132.3 147.2 91.8 163.6 163.4
La 46.88 32.52 16.74 13.69 9.88 9.01
-
43.11 31.97 22.88 16.40
-
surface atomic percentaged Sr B
18.02 24.30 26.06 29.68 30.50 e
16.44 25.42 24.46 39.44
50.22 28.21 22.76 12.06
-
49.06 31.08 29.94 18.99
-
-
-
18.94 22.11 21.03 43.58 21.26 26.58 28.18 51.29
0
18.56 18.51 27.49 25.08 25.43 30.04 e
34.56 30.95 31.47 35.17 35.01 30.45 e
26.59 23.05 19.60 23.41 24.22
30.30 28.54 32.10 35.73 36.34
16.94 16.88 19.48 22.11 18.58
32.84 35.97 35.65 44.80 37.84
16.67 16.56 14.62 20.55 14.21
34.27 31.10 28.86 32.28 34.50
a Surfaceareas were measured by a BET technique using Na adsorption. Single point total pore volume of pores less than 1360-A diameter at PIP0 = 0.9860. Average pore diameter for 4VIA by BET. Divide the peak areas of Auger peaks by their corresponding sensitivity factors to obtain relative atomic ratios, which on normalizing give the surface atomic percentages. Each catalytic surface atomic percentage is an average value of 10 rneasurementa (10 random surface positions) by AES. e The conductivity of Srh4nOa is very low, so ita surface elements cannot be measured by AES.
the sum of the percentage of oxygen is on the low side-a Also, O(1s) shifted by Co atom in the Co series catalysts similar result was obtained by Tabata and M a t s ~ m o t o . ~ ~ is smaller than that shifted by Sr atom but larger than This behavior may be attributable to the fact that A and that shifted by La atom. This information suggests that 0 atoms easily escape from the lattice to make a vacancy, the quantities of surface lattice oxygen atom bonded to but the A atom always accumulates on the catalytic surface metal atoms vary with the type of metal atoms in the and the oxygen always leaves the structure. following order: Mn > Sr > La on the Mn series catalysts and Sr > Co > La on the Co series catalysts. A comparison Surface Chemical Composition. The secondary ion of the O(ls), Mn, and Co series spectra revealed that the mass spectra of the various catalysts prepared are shown amount of lattice oxygen bonded to Mn is larger than that in Figure 3. Note that those spectra belonging to catalysts bonded to Co on the catalytic surface. The quantity of B of the same series show similar features. Also note that bonded to the lattice oxygen will also be used in the the peak intensities of Sr+ and SrO+ decrease when B sites are changed from Mn to Fe, Co, and Ni and are larger explanation of catalytic activity later. Oxygen Desorption of Catalysts. Quantity of than those of BO2+and B03+ at lower x values but smaller than those of BO2+ and BOs+ a t higher x values. In Desorbed Oxygen. The stepwise oxygen desorption experiments of the various catalysts prepared were carried addition, the peak intensities of B+and BO+increase when x 5 0.5 and decrease when x > 0.5. Those of BO2+ and out at temperatures from 50 to 600 "C, using steps of 50 BO3+are clearly enhanced with increasing x but tend to "C. The amounts of oxygen desorbed a t each temperature decrease as x nears a value of 1. The sum of the intensities step are listed in Table IV. Note that the amounts of of BO,+ (y = 0, 1,2,3) is equivalent to the quantity known oxygen desorbed in the Mn series catalysts (less than 178 as the (BO#- cluster; B atoms bonded with 0 atoms on Nmol g-l) are smaller than those desorbed from the other the surface will later be used to help explain the catalytic series. Due to the hexagonal closest packing in the SrMnO3 activity. (ABAC-type) structure of Lal,Sr,MnO3 (where x > 0), the lattice oxygen is difficult to desorb. Except for the Chemical State of the Surface Oxygen Atom. An Mn series catalysts, the amounts of oxygen released from examination of the X-ray photoelectron spectroscopy the catalysts increase with increasing x and especially (XPS) spectra (Figure 4) of the catalytic surface oxygen increase sharply when x 1 0.5 and above 300 "C. In the ions revealed that multiple splitting exists a t the 1s level Mn series catalysts, the amount of oxygen desorbed of 02- ions. More than one O(1s) peak appeared in each increases with increasing x from 0.3 to 0.7 ( x = 0.7 giving spectrum because of the different electronegativity of the the maximum amount) and then decreases. different cations causing the chemical shifts. Note that the O(1s) spectra of La0.3Sro.7MnO3 (curve 4) and La0.7Types of Desorbed Oxygen. The stepwise oxygen desorption profiles of the various catalysts are shown in Sr0.3CoO3 (curve 2) exhibit triplets and not doublets as reported by Yamazoe et al.31 This is a result of the fact Figure 5; their TGA/DTGA thermograms (heating rate: that the peak of O(1s) electron binding energy, which is 20 "C/min) are shown in Figure 6. Note that the results of Figures 5 and 6 are essentially similar. Many aushifted by the La atom, is easily hidden by other peaks. The area of the peak corresponding to the electron binding thors'6J8~~~%3 have reported that the amount of lattice energy of O(1s) shifted by Mn atom in the Mn series oxygen desorbed from the perovskite-type catalysts will Catalysts is larger than those shifted by La and Sr atoms. increase sharply with temperature, while the amount of
2566 Ind. Eng. Chem. Res., Val. 32, No. 11,1993
Figure 2. Scanning electron micrograph of the catalysts prepared - 1.5 Irm (a*),
chemisorbed oxygen is small hecause its absorption on the surface is as a monolayer. In view of the fact that most of the chemisorbed oxygen desorbs before the lattice oxygen in the course of desorption, the stepwise oxygen desorption diagrams (Figure 5 ) and TGA/DTGA thermograms (Figure 6) can show the approximate amounts of the chemisorbed oxygen and lattice oxygen desorbed a t the corresponding temperature ranges. In the intermediate temperature range, both Chemisorbed oxygen and lattice oxygen might desorb simultaneously. As can be seen from these desorption diagrams and thermograms, the Mn series performed differently from the other catalysts due to the fact that the lattice oxygen does not desorb below 600 O C , while the lattice oxygen for the Fe, Co, and Ni series desorbs a t temperatures as low as 300 "C when x > 0.5. The replacement of La3+by Sr2+ in LaBO3 perovskite-type catalysts changes the oxidation states of the B cation from 3+ to 4+.'.4~8J43The Mn
= 2.0 wn
(k,o),EE 3.0 pm (f-i1-m
cation, unlike other B-site cations, exists in the 4+ state. Thus, incombination with Sr2+,overall charge equilibrium can more easily be affected through bonding with three 02-ions. As a result, the bonding of oxygen with Mn in the Mn series catalysts is more ionic, and a higher temperature is required for desorption in order to create an oxygen vacancy.I8 Therefore, the amount of chemisorbed oxygen, instead of the lattice oxygen, plays an important role in the activity of Mn series catalysis for toluene oxidation,especially in the low-temperaturerange. Usually, the Chemisorbed oxygen takes part in the reaction in the low-temperature range while the lattice oxygen will he involved in the reaction in the hightemperature range. When both chemisorbed oxygen and lattice oxygen take part in the reaction a t the same time, the former might be more important than the latter because the empty sites which originally absorb the Chemisorbed oxygen can adsorb oxygen more easily and
Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993 2667
jl
15
II
h‘ I6
li
2 B
c
I
10
111
1
1
0.1 0
100
200
100
900
200
300
100
200 300
100
200
900
100
200 300
w e Figure 3. Secondary ion maas spectra of the catalysts prepared (1)0+,(2)B+, (3)BO+, (4)Sr+, (5)BOz+, (6)SrO+, (7) BOs+, (8) La+, (9) Lao+, (10)SrBO+, (11) LaOz+, (12)SrB02+, (13)SrBOs+, (14)SrBO,+, (15)Laos’, (16)LaSrOz+, where B = Mn, Fe,Co, Ni.
of lattice oxygen does not necessarily have a higher activity. Besides, the amount of BO,+ also affects the activity, as will be discussed later. Tajima et al.25reported that the bonding in Lac003 is covalent and thus not affected by Sr substitution. However, in the case of LaFeO3, the bonding is ionic rather than covalent,and the substitution of La by Sr will increase the degree of covalency in this material. A similar result was obtained by Roberts.20 Activities of Catalysts. In this study, the activities of catalysts were represented by the rate constants of the toluene oxidation. The reaction rate can be fit by the Langmuir-Hinshelwood model in the form oP1
E
-f
6
6,O.l I
bzo
I
625
I
I
I
I
I
530
I
I
I
I
I
636
I
I
~Ko~KTCO~CT (1+ KozCoJ(l + KTCT)
(1)
When Cozis in large excess, the above expression can be simplified to
L I
=
I
6
Electron binding energy (eV)
Figure 4. XPS spectra of O(1s) for catalysts: (1) LaMn03, (2) Lao.aSro..rMnOs, (3) LaCoOs, (4)Lao.7Sro.&oOs. (a) Chemical shift due to Co atom, (b)chemical shift due to La atom, (c) chemical shift due to Sr atom, (d) chemical shift due to Mn atom.
faster than those sites which originally exist as the lattice oxygen. Thus, the catalyst which desorbs a larger amount
where k’ = kKo$TCoz/(l + Ko,Co,). Here, k’ can be considered as an apparent reaction rate constant, and its value represents the relative magnitude of catalyst activity. When an isothermal differential reactor is employed for kinetic study, the average reaction rate can be evaluated
2668 Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993 Table IV. Amounts of Oxygen Desorbed from Lal-BrBOa (B = Mn, Fe, Co, Ni) Catalysts by the Stepwise Desorption Technique in Each Temperature Range (pmol gl) temp (OC) catalyst 50-100 100-150 150-200 200-250 250-300 300-350 350-400 400-450 450-500 5 W 5 5 0 550-600 La&,MnOs 96 177 76 74 81 90 127 22 34 x=o 17 17 64 106 22 36 41 52 81 17 16 15 15 0.3 60 113 34 34 40 53 83 24 39 17 15 0.5 77 41 134 37 39 61 103 15 23 41 16 0.7 58 105 35 32 36 44 73 17 23 32 15 0.8 85 61 33 28 36 44 92 14 27 31 0.9 15 40 19 69 23 22 26 51 14 20 20 1 23 Lal-&FeO3 29 24 22 22 23 57 48 38 36 x=o 14 23 300 511 207 224 380 34 119 162 100 14 23 0.3 1145 1032 1239 1086 1353 950 75 485 1560 14 0.5 16 1740 1774 2581 2274 1646 2059 36 561 2687 14 0.7 15 4315 3524 3266 3054 201 3472 15 2753 25 14 1 14 Lal,Sr,CoO3 47 74 99 60 44 68 89 x=o 21 55 61 55 440 260 345 425 659 490 348 45 136 0.3 9 16 1224 1341 1407 1198 1686 2272 1398 9 38 838 10 0.5 2112 2041 2449 2207 10 272 3852 959 2206 0.7 8 9 79 3712 2705 1712 2776 12 14 724 3748 9 1 11 La1,SrZNiO3 131 79 50 59 64 79 103 x=O 15 105 68 34 69 27 23 35 56 87 97 30 14 16 15 0.3 241 34 478 497 534 22 31 371 103 16 16 0.5 681 888 1027 1120 1129 1026 236 570 17 64 0.7 30 2220 479 545 419 1114 2239 1411 134 16 31 1 17
from the equation -fa
= FTXT -
W
(3)
where the concentration dependence for the average reaction rate should be the average value of the inlet and outlet concentration, CTa, i.e. (4)
The rearrangement of the above equation gives the following equation, which can be used to evaluate the kinetic parameters k' and KT: 1 KT -=-+-
1
k'cTa Figure 7 shows the plot of l / ( - f a ) versus 1/CTa for the oxidation of toluene over Lao.5Sro.5BO3 (B = Mn, Fe, and Co) catalysts. It is obvious that the experimental data can be well fitted by eq 5. Because the value of CTa multiplied by KT is very low (Figure 7), eq 2 might be further simplified to -fa
k'
-fa= k'c,, The apparent rate constants of toluene oxidation over the Mn, Fe, and Co series catalysts at 250 "C are shown in Figure 8. Note that the apparent rate constants of the Ni series catalysts are not shown in this figure. When the Lal,SrxNi03 ( x > 0) catalysts were used, rearrangement and disproportionation reactions take place (see the explanation described below) and play more important roles than the oxidation reaction. Thus, it is improper to evaulate the apparent rate constants by using the above equation. For LaNiO3, the apparent rate constant obtained at 250 OC is 0.1150 L g-l min-' (or 0.038 L m-2 min-1). Figure 9 presents the Arrhenius plots of the experimental data of Mn series catalysts. Both Figures 8 and 9 show for toluene oxidation over the Lao.&0.7that k' and EnCt
Mn03 catalyst are higher than those over other catalysts in the same series. Mn Series Catalysts. The apparent rate constants of the Mn series catalysts were found to be higher than those of Fe series catalysts and parts of the Co series catalysts (Figure 8), though the amounts of oxygen desorbed from the Fe series catalysts are larger (Table IV). This might be due to the fact that most of the oxygen desorbed from the Fe series catalysts is lattice oxygen. In addition, the BET surface areas and the pore diameters of the Mn series catalysts are larger than those of other series (Table 111). These two favorable physical properties usually give a higher overall oxygen adsorption rate and hence result in a higher activity. However, the ability to adsorb (also desorb) oxygen, surface area, and the amount of hexagonal SrMnOa are three factors that affect the activity represented by the apparent rate constant per weight of catalyst. When x increases, the first two quantities will generally decrease (except the amounts of oxygen desorbed at x = 0.3 and 0.7 and the surface area at x = 0.5) while the last one increases. These three contradictory factors result in a maximum activity at x = 0.7. When the activity is represented by the apparent rate constant per surface area, the profile is also concave downward, having a local mimimum at x = 0.5. The lower value at x = 0.5 is due to an exceptionally large surface area and a small amount of desorbed oxygen. A similar result was obtained by Nitadori et a1.18 The activities of Mn series catalysts were affected by the crystalline structure, surface atomic percentages of both the B and 0 atoms, total quantities of BO,+ (y = 0, 1 , 2 , 3) ions on the surface, the amount of lattice oxygen bonded with B atom, and the extent of oxygen chemisorption: 1. Effect of Crystalline Structure. In the Mn series catalysts, the hexagonal SrMnO3(ABAC-type)crystalline structure was found to increase with increasing x , enhancing the surface atomic percentages of both the Mn and 0 atoms, which in turn enhanced the total amount of
Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993 2569 I
I
A
I 200 S k W 5 0
350 500 00
Temperature ("C) Figure 5. Stepwise oxygen desorption profiles of the Catalystsprepared. Note that the scale of the desorption rate for the Mn series is different from that for the others.
MnO,+ (y = 0, 1,2,3) ions, as well as the surface oxygen chemisorption. 2. Effect of Surface Atomic Percentages. Tabata and MatsumotoZ4reported that the catalytic activity of La&3r,CoOa increased with an increase in the quantity of cobalt atoms on the surface. However, in this study, the catalytic activity of the Mn series catalysts was found to increase not only with the quantity of Mn atoms but also with the sum of Mn and 0 atoms (see Table 111and Figure 8). 3. Effect of Total Quantities of BOy+. The catalytic activity of Mn series catalysts was also found to increase with increasing total quantities of BO,+ (y = 0, 1,2,3)ions (see Figure 3) on the surface. Kojima et alS1O reported that catalytic behavior of LaCoO3, LaFeO3, and LaA103 was affected by the electron transfer between a surface cation and an interacting molecule and that (BOsI7clusters are located on the catalyst surface, whereas the (BO#- clusters are embedded in the lattice structure. Thus, catalytic activity will increase with the amount of (BOa)7- clusters which amplify the buildup of surface charge, enhancing the electron transfer between a surface cation and an interacting molecule. Note that the higher quantities of B and 0 atoms on the catalyst surface, together with a high affinity of the B site cation for oxygen
(see Figure 41, increase the number of (BOs)7- clusters (the sum of the BOy+),so as to enhance the activity. Mn series catalytic activity follows this principle in that Lm.3Sr0.7MnO3, which has the greatest sum of the total quantities of BOy+,as well as both B and 0 atoms on the surface, also possesses the highest activity. 4. Effect of the Amount of Lattice Oxygen Bonded with B Atom. From the XPS spectra of LaMnO3 and Lm.&o.,Mn03, it is obvious that the amount of lattice oxygen bonded to Mn atoms is larger than those bonded to La and Sr. This fact might be the reason that the Mn series catalysts have higher activity than the other series. 5. Effect of Desorbed Oxygen. As stated previously, the amount of chemically adsorbed oxygen, instead of the lattice oxygen, plays an important role in the catalytic activity of the Mn series catalysts, especially in the temperature range of toluene oxidation, with the catalytic activity for toluene oxidation increasing with the amount of oxygen chemically adsorbed. Thus, Lm.3Sro.,MnOa, which has the largest amount of this kind of oxygen, also exhibits the highest catalytic activity. A similar result was obtained by Zhang et al.33 Fe and Co Series Catalysts. Nakamura et al.'6 reported that when values of x were low the catalytic activities of the perovskite catalysts increased propor-
2570 Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993
99.5
-
-
99.0
-
97.5
-
ourw
x
Temperature CC)
Temperature ('C)
(a>
w.5
-
99.0
-
Lal-Sr.MnOa
(b) Lal-&BeOa
r
0-
Temperature ( 'C) (c) Lal-=Sr.CoOa
Temperature
CC)
(4 Lal-8rsNiOa
Figure 6. TGA and DTGA thermograms of the catalysts prepared (a) La&3rxMn03, (b) Lal,Sr,FeO3, Note that the ordinate scale of each figure is different.
(c) Lal,Sr,CoOa,
(d)
0.08 I
2.00
1.80
-
11.20
-
Y rc, --
catnlpt
symbol
-
0.16
&$ran% k&r&e%
0
4.l4X1O4 0.1255 0.1014 6.5QX104 0.1~2 0.12~10~
&$rucoo,
o
h '4
0
0: 0:
0:
"'I
0.00 I 0.00
I
0.04
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0.08
I
0.12
I 0.18
I / C ~ . (L @mol-')
Figure 7. Plot of 1/r' versus for the oxidation of toluene over Lm.&o.aBOa (B= Mn, Fe, Co) catalysts from which k' (L/gmin) and KT were calculated.
tionally with x but decreased at higher values of x . In this study, similar results were obtained for both the Co and Fe series catalysts (Figure 8): the maximum activity (represented by the apparent rate constant per weight of catalyst) occurred at x = 0.3. As for the Mn series catalysts, when x 2 0.3 in the Co series, it has the lowest value of the apparent rate constant per surface area at x = 0.5.
B = Un B = Fe B = Co
-I t
0.0
0.2
0.4 0.6 x in Lal-&.BOS
0.8
1 .o
Figure 8. Effects of the Sr substitution for La in the catalytic oxidation of toluene at 250 O C (flow method).
This is also due to a larger surface area. The apparent rate constant per surface area has a second highest value at r: = 0.7 due to an unusually low surface area (see Table 111). A similar result was also obtainedby some authors.16* The catalytic activity of the Fe series catalysts is lower than that of the other series, and the profile of the apparent
Ind. Eng. Chem. Res., Vol. 32,No. 11,1993 2571 -&.-a
-
cume
-1.6
-
1 2 3
4
-
h
I
-1.8
d
.I
E
-2.0
-
d
I
M
5-2.2
-
catalyat MnOa La&r&O.
h&r&O.
Table V. Relative Peak Height of Products from the Rearrangement and Disproportionation of Toluene Over LaodSro.~NiOsCatalyst Measured by GC/MS
,.,E 9.66 9.76 10.04
product relative peak height toluene 2-propenylidenecyclobutene bicyclo[2.2.llhepta-2,5-diene xylene + ethylbenzene
3
5 2 6 1
7
h C
4
-2.4
-
-2.6
-
-2.8
I
I
I
1
rate constant per surface area vs x is also different from this profile for other series. Note that the variations in the activity are not as large as those in the other series catalysts. It might be due to the fact that the perovskite structure can be retained and no other solid phase appears no matter what the extent of Sr substitution, as has been mentioned above. The catalytic activity of the Fe series, is similar to that of the Mn series in that their activity was also enhanced by the total quantity of BO,+ on the surface. The sum of B and 0 atoms on the catalyst surface, the total quantity of BO,+, and the adsorbed oxygen all influenced the activity of the Co series catalysts in the same way they affected the Fe series (see Figures 3 and 8). The XPS information reveals that the amount of lattice oxygen bonded to Mn (largest area for O(1s) electron binding energypeak shift by Mn) is larger than that bonded to Co (lower area for O(1s) electron binding energy peak shift by Co) and that the amount of oxygen bonded to Co on the catalyst surface is also correspondingly lower. Although the total amount of oxygen (expressed as the surface atomic percentage composition of oxygen atoms) is relatively small, it is larger than that of the other series (Table 1111, and, as a result, the Co series therefore possesses the highest catalytic activity (Figure 8). The stepwise oxygen desorption profiles for the Fe and Co series catalysts show that lattice oxygen can be desorbed when x > 0 (a result that has been confirmed by Tajima et al.25)and the greater the degree of Sr substitution, the higher the degree of covalency and the larger the amount of the desorbable lattice oxygen. The results of experiment show that the catalytic activity increases with increasing amount of chemisorbed oxygen (see Table IV, Figure 5-f0, and Figure 8) but does not always increase with increasing amount of lattice oxygen. The other factors mentioned above also affect the activity. Ni Series Catalysts. The oxidation of toluene over the Lal-,Sr,NiOa ( x > 0) catalysts was found to be different that that over the LaNiOs catalyst and other series catalysts. Some byproducts, which were formed by the rearrangement and disproportionation reactions of toluene, were detected. In addition, the conversion curves (conversionversus reaction temperature) are not as smooth as those obtained by using the other series catalysts. The NiO and La2NiO4 phases are responsible for these side
226 1 0.709 0.716 0.027
temp ("0 302 340 1 1 1.178 1.020 1.140 0.996 0.056 0.084
reactions. The amount of NiO and LazNiO4 phases in the Ni series catalysts was found to increase with increasing x , leading to the rearrangement and disproportionation of toluene. Bicyclo[2.2.llhepta-2,5-diene and 2-propenylidenecyclobutene were formed by the rearrangement of toluene, while o-xylene, m-xylene, p-xylene, and ethylbenzene were produced by the disproportionation of toluene. The relative amounts of products from the rearrangement and disproportionation of toluene over La0.3Sr0.7Ni03catalyst measured by GC/MS are shown in Table V, the products of rearrangement predominating. An analysis of the GUMS results confirmed that oxidation of toluene did not lead to the rearrangement and disproportionation when Mn, Fe, and Co series catalysts were used. A further investigation of the rearrangement and disproportionation of toluene by Ni series catalysts was published. l2
Conclusions An alkaline coprecipitation method, using potassium carbonate as the precipitation agent, was successfully employed to prepare Lal,Sr,B03 ( x = 0-1 and B = Mn, Fe, Co, Ni) catalysts, with no loss of cations. The Mn and Ni series catalysts were found to differ from those of both the Co and Fe series. The hexagonal SrMnO3 (ABAC-type)structure enhances oxygen adsorption. The number of Mn, as well as 0,atoms on the surface of these Mn series catalysts was found to increase with increasingx , enhancing the catalytic activity. The catalytic activity of the Mn series catalysts was found to increase not only with the quantity of Mn atoms but also with the sum of Mn and 0 atoms. The amount of chemically adsorbed oxygen, instead of the lattice oxygen, plays an important role in the catalytic activity of the Mn series catalysts, especially in the temperature range of toluene oxidation. The NiO and La2NiOr phases of the Ni series were found to increase with increasing x , causing rearrangement and disproportionation of toluene oxidation. The following four factors affect perovskite catalytic activity: (1) substitution of La by Sr, which causes a structural change in the Mn and Ni series catalysts in addition to changing the oxygen vacancy; (2)the relative amount of lattice oxygen bonded with the B-site cation on the catalyst surface; (3) the amount of (BO5)'- clusters on the catalyst surface, which controls electron transfer between a surface cation and an interacting moelcule; and (4) the desorption of lattice oxygen due to a change in bond type.
Acknowledgment We express our sincere thanks to Professor Barry Yang of the National Central University for allowing us to use his temperature-programmed desorption apparatus. Nomenclature CO, = concentration of oxygen (mol L-1) CT = concentration of toluene (mol L-I)
2572 Ind. Eng. Chem. Res., Vol. 32, No. 11, 1993
CT*3 averagevalue of inlet and outlet concentration of toluene (mol L-1)
Ea& = apparent activation energy (kcal mol-’)
FT = molar feed rate of toluene (mol min-1)
k = surface reaction constant k’ = apparent reaction rate constant (Lg1 min-1) KO#= adsorption equilibrium constant of oxygen (atm-9 KT = adsorption equilibrium constant of toluene (atm-1) J = reaction rate of toluene (mol g-1 min-1) r‘, = average reaction rate of toluene (mol g-1 min-1) W = weight of catalyst (g) XT = fractional conversion of toluene
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1, 1993.
published in Advance ACS Abstracts, September
