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Surface characterization and catalytic properties of La1 − xAxMO3 perovskite oxides. Part II. Studies on La1 − xBaxMnO3 (0 ⩽ x ⩽ 0.2) oxides
cQ& .__ __ @
SOLID STATE
ELSEVIER
IONICS
Solid State Ionics 81 (1995) 243-249
Surface characterization and catalytic properties of La, _XAXMO, perovskite oxides. Part II. Studies on La, _,Ba,MnO, (0 < x < 0.2) oxides N. Gunasekaran
a, S. Rajadurai
a, J.J. Carberry a,*, N. Bakshi b, C.B. Alcock b
a Laboratory of Catalysis, Department of Chemical Engineering, University of Notre Dame, Notre Dame, Indiana-46556, USA ’ Centerfor Sensor Materials, Department of Electrical Engineering, University of Notre Dame, Notre Dame, Indiana-465.56, USA
Received 11 May 1995; accepted for publication 25 July 1995
Abstract Barium substituted lanthanum characterized by X-ray diffraction,
manganite
perovskite-type
oxides
were prepared
by a liquid precursor
method
and
oxygen desorption, and X-ray photoelectron spectroscopic studies. XPS analysis revealed the significant changes in the doublet characteristics of the 01s peak (- 530 eV) resulting from the substitution of Ba in LaMnO,. A well defined Ba3d spectra was seen only with more than 10 mol% substitution of Ba in La site. Methane oxidation reaction was carried out in a flow reactor with a feed containing 0.28% CH,, 15% oxygen and balance helium. The conversion of methane was followed in the temperature range of 200 to 700°C. The light-off temperature for 50% methane conversion was found to be the lowest (490°C) for La,,,Ba,,MnO, composition. Temperature Programmed Desorption (TPD) experiments of oxygen indicate two maxima centered around 320-550°C and 650-760°C respectively. In the low temperature oxygen desorption region, the peak maximum was found to shift with Ba substitution which also corresponded to the activity of methane. The results for oxygen chemisorption and surface composition of these samples are related to the methane oxidation properties. Keywords: Ba
substituted perovskites; XPS studies; 0, properties; Catalysis; CH, oxidation
1. Introduction Substituted perovskite-type oxides have been attempted widely to utilize as an electrode material in SOFC applications, combustion catalysts for hydrocarbon oxidation reactions, etc. [l-3]. Among the various perovskite types, manganese and cobalt-based perovskites have been found to show high catalytic
* Corresponding author.
activities for complete oxidation reactions [4,5]. In these oxides, the partial substitution of cation generally induces the formation of lattice defects which exhibit interesting solid state and catalytic properties. The effects of Sr substitution at the La site in LaMO, oxides (M = Co, Mn or Fe) were extensively studied on the characterization and catalytic activities [6-g]. Seiyama et al. [4] have studied the total oxidation of methane over various mixed oxides including LaO,,Ba,,CoO, and found the activity was strongly influenced by the substitution of the Ba ions. In our previous paper, we have reported that Ba
0167-2738/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0167-2738(95)00189-l
244
N. Gunasekaran et al. /Solid
substituted manganese perovskite show better methane conversion than the Co or Fe based perovskites [lo]. Recently, we have tested the La,_,A,MO, composition (M = Mn, Fe, Co, Cr or Y) for NO, reduction using different hydrocarbons for NO, abatement in lean burn Diesel engines [l 11. In regard to NO, reduction process, we have studied the oxidation properties of methane, propane, propylene, etc., on various types of substituted perovskites. In this paper, the results obtained on methane oxidation over different Ba-substituted LaMnO, perovskites are presented. The preparation, surface characterization, oxygen reactivity and methane oxidation properties of oxides of the formula Lat_,Ba,MO, (0 Q x < 0.2) are investigated and compared with our previous studies. The experimental conditions for the methane oxidation has been selected such that the results would complement the NO, reduction process.
2. Experimental The La, _,Ba,MnO, (0 Q x < 0.2) oxides were prepared by the Pechini method of which the details are described elsewhere [7]. Nitrates of La, Ba and Mn (Aldrich Chemicals, USA) were used in the preparation. The required amounts of nitrate salts are treated with citric acid and ethylene glycol. The gel obtained was then slowly heated to 400°C in air to remove all the organic components. The powder samples were finally sintered at 700°C in vacuum (10 mmHg) for four hours to yield a single phase perovskite oxide and labelled as “fresh” sample. One of the compositions, La,,Ba,,,MnO, was subjected to a further calcination at 1000°C for eight hours (‘sintered’) to study the sintering effect on the methane activity. X-ray diffraction analysis was carried out with Diano 8535D X-ray diffractometer using Cu K (Y radiation at a scan rate of l”(2 f3) per minute to confirm the synthesis. The BET surface area was determined by a single point method using 30% nitrogen in helium at liquid nitrogen temperature on a Quantachrome Monosorp MS16 instrument. The X-ray photoelectron spectra were recorded with Kratos XSAM 800 Auger/ESCA system. Mg K (Y radiation (1253.6 eV) was used as the source
State Ionics 81 (1995) 243-249
and the base pressure in the analyzer chamber was maintained at less than lo-* Torr and the details are given elsewhere [lo]. The resulting binding energy values were corrected using the Cls peak at 284.5 eV [12]. The surface composition was calculated from the peak areas of interest in each spectral region, using the appropriate atomic sensitivity factors as suggested in the instrumentation software. The methane oxidation was carried out in a fixed bed flow reactor [6] by feeding a gas mixture of methane (0.28%), oxygen (15%) and helium (balance) over the catalyst bed containing 0.2 g of sample at a flow rate of 140 cm3/min. The reaction products were analyzed by on-line gas chromatograph, after removal of water vapor, using a carbosphere column (Alltech Associates, USA). The carbon mass balance was > 98% which indicated that CO, and H,O are the major products formed in this reaction under the present experimental conditions. A ratio between methane converted (mole fraction) and W/F (weight/flow) was used to evaluate the methane activity for comparison studies [13]. The reproducibility of the experiments was found to be within +3% in the conversion of methane. Temperature Programmed Desorption (TPD) studies of oxygen over La,_,Ba,MnO, compositions were carried out with 0.1 g of the sample at a heating rate of lo/s with helium flow of 50 cm3/min. Prior to the desorption experiments, the sample was heated in oxygen flow (60 cm3/min) to 700°C for 60 min and cooled to room temperature in oxygen atmosphere. Then, the flow was switched to helium carrier gas and heated at a constant rate upto 800°C. The oxygen desorbed was recorded with a thermal conductivity detector and the TPD curves were reproduced in the repeated runs following the same pretreatment conditions.
3. Results and discussion The X-ray powder diffraction analysis of La, _,Ba,MnO, (0 =Gx < 0.2) indicated the presence of the desired single phase perovskite in all the compositions as shown in Fig. 1. It can be seen that crystalline perovskite-type oxide could be prepared at low temperatures (700°C) than the conventional solid state method which required around 1000°C [6].
N. Gunasekaran et al. / Solid State Ionics 81 (1995) 243-249
0
3b
50
40
60
70
Z-theta(deg)
Fig. 1. X-ray diffraction patterns of La, _,Ba,Mn03 oxides: (a> LaMnO,, (b) La,,Ba,,,,MnO,, (c) La,,Ba,.,MnO,, (d) ~o.xsBao.15MnO, and (e) Lao,,Bao,,Mn03.
The observed d-values were compared with those of the parent unsubstituted LaMO, oxides prepared under the same conditions. Accordingly, the crystal structure was found to conform to a orthorhombic distortion. It is well known that the method of preparation influences the solid state and catalytic properties of the oxide. Therefore, liquid precursor techniques have been widely employed to achieve the single phase perovskite at low calcination temperature with high surface area. For example, in the preparation of substituted perovskites freeze drying
Table 1 XPS data of transition Composition
metal and lanthanum Element level
in La,_,Ba,MO, Position
’ FWHM = full width half maxima.
and citrate or oxalate precursor methods were demonstrated to show superiority over the ceramic method [9,13]. Zhang et al. [14] have showed earlier that manganese-based perovskite could be prepared at low temperature (600°C) with large surface area using the citrate process. The results on the surface area measurements are listed in Table 3. It is interesting to note that the preparation of La,_,Ba,MnO, oxides under vacuum calcination resulted in a high surface area of _ 30 m2/g compared lo-20 m2/g reported for the freeze-drying method [ 131. However, the sample Lao,gBa,,,MnOS when calcined at 1000°C for eight hours, suffered a IO-fold decrease in the surface area due to sintering effect (Table 3). In all the La,_.Ba,MnO, samples, the La3d, Mn2p and 01s level spectra were similar to the previously reported Ba substituted manganese perovskite (10) and the corresponding main peak values and peak areas are presented in Table 1. The observed binding energy (BE) values are N 833 eV and N 641 eV for La3d,,, and Mn2p.p,,z levels, respectively, which are in good agreement with the reported values [ 121. The Ba3d peak at N 779.1 eV (BE) could be seen with 10 mol% substitution as a small peak with noisy background. The other two substitutions (15 and 20%) showed a well defined Ba3d spectra with the core peak binding energy value centered at 779.2 eV which is assigned to the Ba2+ present in mixed oxides [ 121. The 01s level
oxides FWHM a
Area
(eV) 832.8 641.1 833.3 641.2 833.4 778.9 641.5 833.2 779.2 641.1 833.2 779.1 640.9
245
Peak area
(%I 6.4 3.3 6.4 3.6 6.4 1.9 3.7 6.6 3.3 3.7 6.7 3.8 3.7
98353 25290 145473 40465 193417 3751 54049 167469 66983 52409 193235 82475 57786
49.0 51.0 49.4 50.6 48.4 1.02 50.5 37.2 16.2 46.6 38.7 18.0 43.3
246
N.
530
535 Binding
~u~se~r~
525
et al. /Solid
State
520
Energy (eV)
Fig. 2. X-ray photoelectron spectra in the 01s region for La, _,Ba,MnO, oxides: (a) LaMnO,, (b) Lao.,,Bao,O,MnOS, (c) MnO, and(e) Lao,,Bao,,MnO,. ~o,,Bao.,Mn% (d)LaoxBao.,,
ionics
81 (19951243-249
spectra showed significant changes in the doublet peak around 530 eV with the increase in x in La,_,Ba,MnO, compositions and the spectra obtained in the binding energy region of 520-540 eV is given in Fig. 2. The 01s level peak was deconvohued using a Gaussian program included in the instrument software and the corresponding deconvoluted values of BE, peak intensity and area are listed in Table 2. Except for the x = 0 (LaMnO,), the other compositions were all best fitted with three peak positions and the BE values were around 529, 530 and 531 eV respectively. The peak with lower binding energy value of 529.0 eV is generally accepted as the lattice oxygen (metal-oxygen bond). The peak with higher BE value (* 530.0 eV> is attributed to the adsorbed/absorbed oxygen and the 531 eV is due to the moisture. As seen in Fig. 3, the peak N 530 eV was weak for x = 0 and showed an increase in peak intensity with increasing x values. As the Ba substitution increases in La, _,Ba,MnO,, it is expected to create more oxygen vacancy in the lattice and therefore one would also expect an increase in the portion of oxygen. This trend in the change in oxygen is reflected clearly on the 01s peak around 530 eV in the present study and also for other substituted perovskites [9,15].
100
80
300
-
LaMnOB
-
Lao.tx+o.o@“%
-
Lao.Go.1MnO~
-+--
L%.esBao.isM”*3
-
L~o.~B~~M”O~
350
400
450
500
550
600
650
700
Temperature~~) Fig. 3. Methane deep oxidation as a function of temperature over La, _,Ba,MnO, Catalyst weight = 0.2 g.
(0 d x < 0.2) oxides. Flow rate = 140 cm3/min.
N. Gunasekaran et al. /Solid State Ionics 81 (1995) 243-249 Table 2 01s peak deconvoluted La, _,Ba,MO, oxides
and
surface
concentmtion
data
Composition
Position (eV)
FWHM a
Intensity (o/o)
Peak area (%I
LaMnO,
528.9 531.6 528.7 530.2 531.4 528.5 530.2 531.6 ;;;:;
2.1 3.6 1.3 1.8 2.1 1.8 2.0 2.5 1.9 2.5 2.4 1.7 2.0 2.4
84.2 46.3 93.1 33.8 35.2 95.0 46.2 32.5 75.0 40.0 51.3 96.3 73.8 30.0
51.4 48.4 45.1 24.1 27.6 51.0 23.3 24.6 38.4 27.3 33.5 44.0 37.7 18.4
L+r9sBa,r.as MnOs
La,,., Bau.
IMnO,
La cl.ssBaU.r5MnOs
531.3 528.3 530.4 531.8
LaO.xBas.,MnOs
of
’ PWHM = full width half maxima.
The catalytic activity of La,_,Ba,MnO, oxides for the methane oxidation in the temperature range 300-700°C is shown in Fig. 3. A significant change in the conversion of methane was observed only above 300°C and reached a maximum in the temperature around 600 or 650°C. The activity becomes much closer between the compositions at high reaction temperature (600°C). It can be seen that the
0
Fig.
4.
composition with 10 mol% Ba substitution showed the lowest temperature (490°C for 50% conversion) for a given value of methane activity. It has been reported by Arai et al. [16] that the adsorbed oxygen contributes less at high reaction temperature for oxidation and the lattice oxygen is mainly involved in the methane reaction. Also the kinetic study of methane oxidation over Sr-substituted manganese perovskite revealed a positive order dependence on the partial pressure oxygen 1161. Therefore, they concluded that at reaction temperatures less than 600°C the kinetics is mainly controlled by the adsorbed oxygen where the activity due to participation of lattice oxygen is negligible. Similarly, studying the oxidation of propane over La, _ xA .MnO, (A = Sr, Ce, Hf) Nitadori et al. [8] have obtained a correlation between the propane activity and the amount of adsorbed oxygen. The effect of sintering on the methane oxidation was studied with the fresh and sintered samples of Lao,,Ba,,MnC, composition. The plot of methane conversion as a function of reaction temperature is shown in Fig. 4. It can be seen that the temperature for 50% conversion of methane is increased by 160°C (from 490 to 650°C) for sintered La,,Ba,,MnO, oxide which could be primarily due to reduction of surface area (Table 4). This trend in the activity decrease due to sintering has been observed in general on many mixed oxides prepared by differ-
I
300
350
400
450
500
550
Temperature
(“C)
600
650
700
Influence of contact time and sintering temperature on the methane oxidation activity over La,,Ba,,,MnOs composition. (1) and (2) correspond to sintering temperature of 700°C; (3) and (4) represent 1000°C.
241
100
I
200
I
I
300
400
Temperature
I
I
I
500
600
700
6
(“C)
Fig. 5. TPD profiles of oxygen from La,_,Ba,MnO, oxides. Heating rate = lo/s; Helium flow rate = 50 cm3/min: (a) LaMnO,, (b) Lao.9,Bao.05Mn03. (c) Lao.9Bao,,Mn03, (d) Laa.s5Bao.15 MnO, and (e) LaosBao.,Mn03.
248
N. Gunasekaran et al./ Solid State Ionics 81 (1995) 243-249
Table 3 Temperature
programmed
Composition
desorption
data of oxygen over La_,Ba,MnO,
Peak 1 region
LaMnO, La a.9sBaa.asMn0, L+VBa0,1Mn03 L+.ssBaa.&n03 La,.sBaa.,MnO3
(“0
Peak 2 region (“0
330-360 320-360 430-550 390-540 360-540
640-660 630-660 680-710 650-690 640-660
techniques. For methane oxidation, Arai et al. [ 161 have found a temperature difference of 130°C for the La,_,Sr,MnO, calcined at 850°C and 1200°C. The activity decrease due to variations in the surface area has also been reported for CO oxidation over cobalt based perovskites [l&19]. The effect of residence time on the methane activity was examined with W/F values of 0.001 and 0.004 g.min/cm3 on both fresh and sintered La,,Ba,,,MnO, composition and the results are shown in Fig. 4. As expected, an increase in the residence time showed a decrease in the light-off temperature for methane conversion. About 50°C difference in the reaction temperature was observed for the 50% methane conversion on both fresh and sintered sample. Simliar trend in the methane oxidaent
Table 4 Catalytic
activity data on the methane oxidation
J-a, Sr.+.&o.@, La.,$r.34Ni,3Co.,03
(pm/d
(pm/d
156.5 225.9 76.8 111.4 92.1
180.9 172.6 32.5 50.8 26.1
331.4 398.5 109.3 162.2 118.2
tion has been reported by Kirchnerova et al. [13] over Sr substituted cobalt and nickel based perovskites. Temperature programmed desorption experiments were carried out to determine the oxygen adsorption properties of the samples. The TPD oxygen profiles recorded on La, _,Ba,MnO, (0 < x < 0.2) compositions are shown in Fig. 5. It can be seen that two prominent characteristic peaks occur on all samples with a maximum centered around 350°C and 7OO”C, represented as peak-l and peak-2 respectively (Table 3). The desorption temperature maxima indicate the strength with which the oxygen is adsorbed/absorbed in the sample. This desorption peak temperature was found to shift with increase in x values which suggest that the different types of oxygen
and some other reported perovskite
( pm0Vs.g) 0.55 0.47 0.72 0.60 0.65 0.34 0.09 0.16
0.0165 0.0167 0.0234 0.02 0.0211 0.0109 0.028 1 0.0503
this this this this this this this this
1.03 1.84 7.55 c 4.26 ’ 8.25 ’
3.40 6.13
t51 El [31 [31
0.46 0.41 0.44
[lOI [lOI [IO1
47.8 80.3 50.0(535T) 50.0(672OC) -
at 550°C.
at 500°C Ref.
(%I
a Represents W/F = 0.004. h Represents sintered samples. ’ Data corresponds to temperature
compositions
Sp.activity ( pmol/s.m*)
43.2 37.4 54.2 47.1 51.5 71.0 19.0 12.5
0.3 0.3 5.1 0.4 16.5 7.9 18.8
b
( pm/g)
area (m*/g)
L+&brFeO~
La0,8%.2CoO3
Total amount of 0,
33.1 28.3 30.8 30.1 31.0 30.8 3.2 3.2
LaMnO La o.g5Bao.0, MnO3 L%9Ba(j,IMn% La o.ssBao.15Mn0, Lao.sBao.2 MnO3 La,,Ga,,,MnO, a L%.@a,, I M% h La,9Bao.,MnO3 a
L%~Bado03 Lao., Bao.,CoOS
0, amount in region 2
Activity
Surface
Sample weight = 0.1 g
0, amount in region 1
Conversion
Catalyst
~,,.x%zYO3
over La, _ .Ba,MnO,
compounds.
work work work work work work work work
species (O- or 02etc> are formed on La,_,Ba,MnO, compositions. Further studies are needed to identify the oxygen species and relate with the catalytic activity. The amount of oxygen desorbed under the two peak regions was calculated on each composition and the values are given in Table 3. Since the reaction over these oxides was found to reach a maximum methane conversion in the temperature range of 600-650°C it would be more meaningful to correlate oxygen desorbed from the peak-l region (3~-5~OC) with that of methane activity. Among all the compositions, the least amount of was found on desorbed oxygen (- 109 p,moles/g) La,,Ba,,,MnO, which also showed the lowest lightoff temperature for methane activity. Therefore, it is evident that the nature of adsorbed oxygen species is mainly responsible for the difference in the activity displayed in Fig. 3 for La,_,Ba,MnO, oxides. For comparison study, the methane activity on other reported perovskite compounds is included in Table 4. The activity comparison indicates that the difference could be due to the wide variations in the expe~men~l condi~ons employed in the oxidation of methane. Although the surface area seems to be reflect directly to the methane activity as seen in Fig. 4, the oxygen reactivity explains the variations among the group of similar perovskite oxides (La,_,Ba,MnO, oxides). The present study indicates that the perovskite oxide with high surface area (- 30 m’/g) could be prepared by suitably varying the sintering conditions of liquid precursor method. The Ba concentration at the surface appeared only at higher than 10 mol% subs~tution in La, _,Ba,MnO, composition and also found to influence the nature of adsorbed oxygen programmed desorption of species. Temperature oxygen over La,_,Ba,MnO, showed different types
of oxygen adsorbed oxygen species on the surface and correlated well with the methane oxidation activity.
References fll N.Q. Minh, J. Am. Ceram. Sot. 76 (1993) 563. I21 R.J.H. Voorhoeve, Advanced Materials in Catalysis (Academic Press, New York, 1977) p. 129. 131 L.G. Tejuca, J.L.G. Fierro and J.M.D. Tascon, Adv. Catal. 36 (1989) 237. [4] T. Seiyama, Catal. Rev.-&i. Eng. 34 (1992) 281. [51 N. Yamazoe and Y. Teraoka, Catal. Today 8 (1990) 175. (61 R. Doshi, C.B. Alcock, N. Gunasekaran and J.J. Carberry, J. Catal. 140 (1993) 557. [71 C.B. Alcock, R. Doshi, J.J. Carberry and N. Gunasekaran, J. Catal. 143 (1993) 553. [81 T. Nitadori, S. Kurihara and M. Misono, J. Catal. 98 (1986) 221. 191J.J. Liaug and H.S. Weng, Ind. Eng. Chem. Res. 32 (1993) 2563. [lOI N. Gunasekaran, S. Rajadurai, J.J. Carberry, N. Bakshi and C.B. Alcock., Solid State fonics 73 (1994) 289. Ill] N. Gun~ek~~ and J.J. Carberry, unpublished data. [12] CD. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder and G.E. Mullerberg, Handbook of X-ray Photoelectron Spectroscopy (Perkin-Elmer Corp. (Phys. Electron. Div., Eden Prairie, MN 55344, 1979). [13] J. Kirchnerova, D. Klvana, J. Vaillancourt and J. Chnouki, Catal. L&t. 21 (1993) 77. [l4] H.M. Zhang, Y. Teraoka, and N. Yamazoe, Chem. Len. (1987) 665. [I51 N. Yamazoe, Y. Teraoka and T. Seiyama., Chem. Lett. 12 (1981) 1767. 1161H. Anti, T. Yamada, K. Eguchi and T. Seiyama, Appl. Catal. 26 (1986) 265. [I71 T. Nitadori, J. Catal. 121 (1982) 43. 1181 R. Doshi, C.B. Alcock and J.J. Carberry, Catal. Lett. 18 (1993) 337, [19] K.R. Bernard. K. Foger, T.W. Turney and R.D. Williams., J. Catal. 125 265 (1990).
SOLID STATE
ELSEVIER
IONICS
Solid State Ionics 81 (1995) 243-249
Surface characterization and catalytic properties of La, _XAXMO, perovskite oxides. Part II. Studies on La, _,Ba,MnO, (0 < x < 0.2) oxides N. Gunasekaran
a, S. Rajadurai
a, J.J. Carberry a,*, N. Bakshi b, C.B. Alcock b
a Laboratory of Catalysis, Department of Chemical Engineering, University of Notre Dame, Notre Dame, Indiana-46556, USA ’ Centerfor Sensor Materials, Department of Electrical Engineering, University of Notre Dame, Notre Dame, Indiana-465.56, USA
Received 11 May 1995; accepted for publication 25 July 1995
Abstract Barium substituted lanthanum characterized by X-ray diffraction,
manganite
perovskite-type
oxides
were prepared
by a liquid precursor
method
and
oxygen desorption, and X-ray photoelectron spectroscopic studies. XPS analysis revealed the significant changes in the doublet characteristics of the 01s peak (- 530 eV) resulting from the substitution of Ba in LaMnO,. A well defined Ba3d spectra was seen only with more than 10 mol% substitution of Ba in La site. Methane oxidation reaction was carried out in a flow reactor with a feed containing 0.28% CH,, 15% oxygen and balance helium. The conversion of methane was followed in the temperature range of 200 to 700°C. The light-off temperature for 50% methane conversion was found to be the lowest (490°C) for La,,,Ba,,MnO, composition. Temperature Programmed Desorption (TPD) experiments of oxygen indicate two maxima centered around 320-550°C and 650-760°C respectively. In the low temperature oxygen desorption region, the peak maximum was found to shift with Ba substitution which also corresponded to the activity of methane. The results for oxygen chemisorption and surface composition of these samples are related to the methane oxidation properties. Keywords: Ba
substituted perovskites; XPS studies; 0, properties; Catalysis; CH, oxidation
1. Introduction Substituted perovskite-type oxides have been attempted widely to utilize as an electrode material in SOFC applications, combustion catalysts for hydrocarbon oxidation reactions, etc. [l-3]. Among the various perovskite types, manganese and cobalt-based perovskites have been found to show high catalytic
* Corresponding author.
activities for complete oxidation reactions [4,5]. In these oxides, the partial substitution of cation generally induces the formation of lattice defects which exhibit interesting solid state and catalytic properties. The effects of Sr substitution at the La site in LaMO, oxides (M = Co, Mn or Fe) were extensively studied on the characterization and catalytic activities [6-g]. Seiyama et al. [4] have studied the total oxidation of methane over various mixed oxides including LaO,,Ba,,CoO, and found the activity was strongly influenced by the substitution of the Ba ions. In our previous paper, we have reported that Ba
0167-2738/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0167-2738(95)00189-l
244
N. Gunasekaran et al. /Solid
substituted manganese perovskite show better methane conversion than the Co or Fe based perovskites [lo]. Recently, we have tested the La,_,A,MO, composition (M = Mn, Fe, Co, Cr or Y) for NO, reduction using different hydrocarbons for NO, abatement in lean burn Diesel engines [l 11. In regard to NO, reduction process, we have studied the oxidation properties of methane, propane, propylene, etc., on various types of substituted perovskites. In this paper, the results obtained on methane oxidation over different Ba-substituted LaMnO, perovskites are presented. The preparation, surface characterization, oxygen reactivity and methane oxidation properties of oxides of the formula Lat_,Ba,MO, (0 Q x < 0.2) are investigated and compared with our previous studies. The experimental conditions for the methane oxidation has been selected such that the results would complement the NO, reduction process.
2. Experimental The La, _,Ba,MnO, (0 Q x < 0.2) oxides were prepared by the Pechini method of which the details are described elsewhere [7]. Nitrates of La, Ba and Mn (Aldrich Chemicals, USA) were used in the preparation. The required amounts of nitrate salts are treated with citric acid and ethylene glycol. The gel obtained was then slowly heated to 400°C in air to remove all the organic components. The powder samples were finally sintered at 700°C in vacuum (10 mmHg) for four hours to yield a single phase perovskite oxide and labelled as “fresh” sample. One of the compositions, La,,Ba,,,MnO, was subjected to a further calcination at 1000°C for eight hours (‘sintered’) to study the sintering effect on the methane activity. X-ray diffraction analysis was carried out with Diano 8535D X-ray diffractometer using Cu K (Y radiation at a scan rate of l”(2 f3) per minute to confirm the synthesis. The BET surface area was determined by a single point method using 30% nitrogen in helium at liquid nitrogen temperature on a Quantachrome Monosorp MS16 instrument. The X-ray photoelectron spectra were recorded with Kratos XSAM 800 Auger/ESCA system. Mg K (Y radiation (1253.6 eV) was used as the source
State Ionics 81 (1995) 243-249
and the base pressure in the analyzer chamber was maintained at less than lo-* Torr and the details are given elsewhere [lo]. The resulting binding energy values were corrected using the Cls peak at 284.5 eV [12]. The surface composition was calculated from the peak areas of interest in each spectral region, using the appropriate atomic sensitivity factors as suggested in the instrumentation software. The methane oxidation was carried out in a fixed bed flow reactor [6] by feeding a gas mixture of methane (0.28%), oxygen (15%) and helium (balance) over the catalyst bed containing 0.2 g of sample at a flow rate of 140 cm3/min. The reaction products were analyzed by on-line gas chromatograph, after removal of water vapor, using a carbosphere column (Alltech Associates, USA). The carbon mass balance was > 98% which indicated that CO, and H,O are the major products formed in this reaction under the present experimental conditions. A ratio between methane converted (mole fraction) and W/F (weight/flow) was used to evaluate the methane activity for comparison studies [13]. The reproducibility of the experiments was found to be within +3% in the conversion of methane. Temperature Programmed Desorption (TPD) studies of oxygen over La,_,Ba,MnO, compositions were carried out with 0.1 g of the sample at a heating rate of lo/s with helium flow of 50 cm3/min. Prior to the desorption experiments, the sample was heated in oxygen flow (60 cm3/min) to 700°C for 60 min and cooled to room temperature in oxygen atmosphere. Then, the flow was switched to helium carrier gas and heated at a constant rate upto 800°C. The oxygen desorbed was recorded with a thermal conductivity detector and the TPD curves were reproduced in the repeated runs following the same pretreatment conditions.
3. Results and discussion The X-ray powder diffraction analysis of La, _,Ba,MnO, (0 =Gx < 0.2) indicated the presence of the desired single phase perovskite in all the compositions as shown in Fig. 1. It can be seen that crystalline perovskite-type oxide could be prepared at low temperatures (700°C) than the conventional solid state method which required around 1000°C [6].
N. Gunasekaran et al. / Solid State Ionics 81 (1995) 243-249
0
3b
50
40
60
70
Z-theta(deg)
Fig. 1. X-ray diffraction patterns of La, _,Ba,Mn03 oxides: (a> LaMnO,, (b) La,,Ba,,,,MnO,, (c) La,,Ba,.,MnO,, (d) ~o.xsBao.15MnO, and (e) Lao,,Bao,,Mn03.
The observed d-values were compared with those of the parent unsubstituted LaMO, oxides prepared under the same conditions. Accordingly, the crystal structure was found to conform to a orthorhombic distortion. It is well known that the method of preparation influences the solid state and catalytic properties of the oxide. Therefore, liquid precursor techniques have been widely employed to achieve the single phase perovskite at low calcination temperature with high surface area. For example, in the preparation of substituted perovskites freeze drying
Table 1 XPS data of transition Composition
metal and lanthanum Element level
in La,_,Ba,MO, Position
’ FWHM = full width half maxima.
and citrate or oxalate precursor methods were demonstrated to show superiority over the ceramic method [9,13]. Zhang et al. [14] have showed earlier that manganese-based perovskite could be prepared at low temperature (600°C) with large surface area using the citrate process. The results on the surface area measurements are listed in Table 3. It is interesting to note that the preparation of La,_,Ba,MnO, oxides under vacuum calcination resulted in a high surface area of _ 30 m2/g compared lo-20 m2/g reported for the freeze-drying method [ 131. However, the sample Lao,gBa,,,MnOS when calcined at 1000°C for eight hours, suffered a IO-fold decrease in the surface area due to sintering effect (Table 3). In all the La,_.Ba,MnO, samples, the La3d, Mn2p and 01s level spectra were similar to the previously reported Ba substituted manganese perovskite (10) and the corresponding main peak values and peak areas are presented in Table 1. The observed binding energy (BE) values are N 833 eV and N 641 eV for La3d,,, and Mn2p.p,,z levels, respectively, which are in good agreement with the reported values [ 121. The Ba3d peak at N 779.1 eV (BE) could be seen with 10 mol% substitution as a small peak with noisy background. The other two substitutions (15 and 20%) showed a well defined Ba3d spectra with the core peak binding energy value centered at 779.2 eV which is assigned to the Ba2+ present in mixed oxides [ 121. The 01s level
oxides FWHM a
Area
(eV) 832.8 641.1 833.3 641.2 833.4 778.9 641.5 833.2 779.2 641.1 833.2 779.1 640.9
245
Peak area
(%I 6.4 3.3 6.4 3.6 6.4 1.9 3.7 6.6 3.3 3.7 6.7 3.8 3.7
98353 25290 145473 40465 193417 3751 54049 167469 66983 52409 193235 82475 57786
49.0 51.0 49.4 50.6 48.4 1.02 50.5 37.2 16.2 46.6 38.7 18.0 43.3
246
N.
530
535 Binding
~u~se~r~
525
et al. /Solid
State
520
Energy (eV)
Fig. 2. X-ray photoelectron spectra in the 01s region for La, _,Ba,MnO, oxides: (a) LaMnO,, (b) Lao.,,Bao,O,MnOS, (c) MnO, and(e) Lao,,Bao,,MnO,. ~o,,Bao.,Mn% (d)LaoxBao.,,
ionics
81 (19951243-249
spectra showed significant changes in the doublet peak around 530 eV with the increase in x in La,_,Ba,MnO, compositions and the spectra obtained in the binding energy region of 520-540 eV is given in Fig. 2. The 01s level peak was deconvohued using a Gaussian program included in the instrument software and the corresponding deconvoluted values of BE, peak intensity and area are listed in Table 2. Except for the x = 0 (LaMnO,), the other compositions were all best fitted with three peak positions and the BE values were around 529, 530 and 531 eV respectively. The peak with lower binding energy value of 529.0 eV is generally accepted as the lattice oxygen (metal-oxygen bond). The peak with higher BE value (* 530.0 eV> is attributed to the adsorbed/absorbed oxygen and the 531 eV is due to the moisture. As seen in Fig. 3, the peak N 530 eV was weak for x = 0 and showed an increase in peak intensity with increasing x values. As the Ba substitution increases in La, _,Ba,MnO,, it is expected to create more oxygen vacancy in the lattice and therefore one would also expect an increase in the portion of oxygen. This trend in the change in oxygen is reflected clearly on the 01s peak around 530 eV in the present study and also for other substituted perovskites [9,15].
100
80
300
-
LaMnOB
-
Lao.tx+o.o@“%
-
Lao.Go.1MnO~
-+--
L%.esBao.isM”*3
-
L~o.~B~~M”O~
350
400
450
500
550
600
650
700
Temperature~~) Fig. 3. Methane deep oxidation as a function of temperature over La, _,Ba,MnO, Catalyst weight = 0.2 g.
(0 d x < 0.2) oxides. Flow rate = 140 cm3/min.
N. Gunasekaran et al. /Solid State Ionics 81 (1995) 243-249 Table 2 01s peak deconvoluted La, _,Ba,MO, oxides
and
surface
concentmtion
data
Composition
Position (eV)
FWHM a
Intensity (o/o)
Peak area (%I
LaMnO,
528.9 531.6 528.7 530.2 531.4 528.5 530.2 531.6 ;;;:;
2.1 3.6 1.3 1.8 2.1 1.8 2.0 2.5 1.9 2.5 2.4 1.7 2.0 2.4
84.2 46.3 93.1 33.8 35.2 95.0 46.2 32.5 75.0 40.0 51.3 96.3 73.8 30.0
51.4 48.4 45.1 24.1 27.6 51.0 23.3 24.6 38.4 27.3 33.5 44.0 37.7 18.4
L+r9sBa,r.as MnOs
La,,., Bau.
IMnO,
La cl.ssBaU.r5MnOs
531.3 528.3 530.4 531.8
LaO.xBas.,MnOs
of
’ PWHM = full width half maxima.
The catalytic activity of La,_,Ba,MnO, oxides for the methane oxidation in the temperature range 300-700°C is shown in Fig. 3. A significant change in the conversion of methane was observed only above 300°C and reached a maximum in the temperature around 600 or 650°C. The activity becomes much closer between the compositions at high reaction temperature (600°C). It can be seen that the
0
Fig.
4.
composition with 10 mol% Ba substitution showed the lowest temperature (490°C for 50% conversion) for a given value of methane activity. It has been reported by Arai et al. [16] that the adsorbed oxygen contributes less at high reaction temperature for oxidation and the lattice oxygen is mainly involved in the methane reaction. Also the kinetic study of methane oxidation over Sr-substituted manganese perovskite revealed a positive order dependence on the partial pressure oxygen 1161. Therefore, they concluded that at reaction temperatures less than 600°C the kinetics is mainly controlled by the adsorbed oxygen where the activity due to participation of lattice oxygen is negligible. Similarly, studying the oxidation of propane over La, _ xA .MnO, (A = Sr, Ce, Hf) Nitadori et al. [8] have obtained a correlation between the propane activity and the amount of adsorbed oxygen. The effect of sintering on the methane oxidation was studied with the fresh and sintered samples of Lao,,Ba,,MnC, composition. The plot of methane conversion as a function of reaction temperature is shown in Fig. 4. It can be seen that the temperature for 50% conversion of methane is increased by 160°C (from 490 to 650°C) for sintered La,,Ba,,MnO, oxide which could be primarily due to reduction of surface area (Table 4). This trend in the activity decrease due to sintering has been observed in general on many mixed oxides prepared by differ-
I
300
350
400
450
500
550
Temperature
(“C)
600
650
700
Influence of contact time and sintering temperature on the methane oxidation activity over La,,Ba,,,MnOs composition. (1) and (2) correspond to sintering temperature of 700°C; (3) and (4) represent 1000°C.
241
100
I
200
I
I
300
400
Temperature
I
I
I
500
600
700
6
(“C)
Fig. 5. TPD profiles of oxygen from La,_,Ba,MnO, oxides. Heating rate = lo/s; Helium flow rate = 50 cm3/min: (a) LaMnO,, (b) Lao.9,Bao.05Mn03. (c) Lao.9Bao,,Mn03, (d) Laa.s5Bao.15 MnO, and (e) LaosBao.,Mn03.
248
N. Gunasekaran et al./ Solid State Ionics 81 (1995) 243-249
Table 3 Temperature
programmed
Composition
desorption
data of oxygen over La_,Ba,MnO,
Peak 1 region
LaMnO, La a.9sBaa.asMn0, L+VBa0,1Mn03 L+.ssBaa.&n03 La,.sBaa.,MnO3
(“0
Peak 2 region (“0
330-360 320-360 430-550 390-540 360-540
640-660 630-660 680-710 650-690 640-660
techniques. For methane oxidation, Arai et al. [ 161 have found a temperature difference of 130°C for the La,_,Sr,MnO, calcined at 850°C and 1200°C. The activity decrease due to variations in the surface area has also been reported for CO oxidation over cobalt based perovskites [l&19]. The effect of residence time on the methane activity was examined with W/F values of 0.001 and 0.004 g.min/cm3 on both fresh and sintered La,,Ba,,,MnO, composition and the results are shown in Fig. 4. As expected, an increase in the residence time showed a decrease in the light-off temperature for methane conversion. About 50°C difference in the reaction temperature was observed for the 50% methane conversion on both fresh and sintered sample. Simliar trend in the methane oxidaent
Table 4 Catalytic
activity data on the methane oxidation
J-a, Sr.+.&o.@, La.,$r.34Ni,3Co.,03
(pm/d
(pm/d
156.5 225.9 76.8 111.4 92.1
180.9 172.6 32.5 50.8 26.1
331.4 398.5 109.3 162.2 118.2
tion has been reported by Kirchnerova et al. [13] over Sr substituted cobalt and nickel based perovskites. Temperature programmed desorption experiments were carried out to determine the oxygen adsorption properties of the samples. The TPD oxygen profiles recorded on La, _,Ba,MnO, (0 < x < 0.2) compositions are shown in Fig. 5. It can be seen that two prominent characteristic peaks occur on all samples with a maximum centered around 350°C and 7OO”C, represented as peak-l and peak-2 respectively (Table 3). The desorption temperature maxima indicate the strength with which the oxygen is adsorbed/absorbed in the sample. This desorption peak temperature was found to shift with increase in x values which suggest that the different types of oxygen
and some other reported perovskite
( pm0Vs.g) 0.55 0.47 0.72 0.60 0.65 0.34 0.09 0.16
0.0165 0.0167 0.0234 0.02 0.0211 0.0109 0.028 1 0.0503
this this this this this this this this
1.03 1.84 7.55 c 4.26 ’ 8.25 ’
3.40 6.13
t51 El [31 [31
0.46 0.41 0.44
[lOI [lOI [IO1
47.8 80.3 50.0(535T) 50.0(672OC) -
at 550°C.
at 500°C Ref.
(%I
a Represents W/F = 0.004. h Represents sintered samples. ’ Data corresponds to temperature
compositions
Sp.activity ( pmol/s.m*)
43.2 37.4 54.2 47.1 51.5 71.0 19.0 12.5
0.3 0.3 5.1 0.4 16.5 7.9 18.8
b
( pm/g)
area (m*/g)
L+&brFeO~
La0,8%.2CoO3
Total amount of 0,
33.1 28.3 30.8 30.1 31.0 30.8 3.2 3.2
LaMnO La o.g5Bao.0, MnO3 L%9Ba(j,IMn% La o.ssBao.15Mn0, Lao.sBao.2 MnO3 La,,Ga,,,MnO, a L%.@a,, I M% h La,9Bao.,MnO3 a
L%~Bado03 Lao., Bao.,CoOS
0, amount in region 2
Activity
Surface
Sample weight = 0.1 g
0, amount in region 1
Conversion
Catalyst
~,,.x%zYO3
over La, _ .Ba,MnO,
compounds.
work work work work work work work work
species (O- or 02etc> are formed on La,_,Ba,MnO, compositions. Further studies are needed to identify the oxygen species and relate with the catalytic activity. The amount of oxygen desorbed under the two peak regions was calculated on each composition and the values are given in Table 3. Since the reaction over these oxides was found to reach a maximum methane conversion in the temperature range of 600-650°C it would be more meaningful to correlate oxygen desorbed from the peak-l region (3~-5~OC) with that of methane activity. Among all the compositions, the least amount of was found on desorbed oxygen (- 109 p,moles/g) La,,Ba,,,MnO, which also showed the lowest lightoff temperature for methane activity. Therefore, it is evident that the nature of adsorbed oxygen species is mainly responsible for the difference in the activity displayed in Fig. 3 for La,_,Ba,MnO, oxides. For comparison study, the methane activity on other reported perovskite compounds is included in Table 4. The activity comparison indicates that the difference could be due to the wide variations in the expe~men~l condi~ons employed in the oxidation of methane. Although the surface area seems to be reflect directly to the methane activity as seen in Fig. 4, the oxygen reactivity explains the variations among the group of similar perovskite oxides (La,_,Ba,MnO, oxides). The present study indicates that the perovskite oxide with high surface area (- 30 m’/g) could be prepared by suitably varying the sintering conditions of liquid precursor method. The Ba concentration at the surface appeared only at higher than 10 mol% subs~tution in La, _,Ba,MnO, composition and also found to influence the nature of adsorbed oxygen programmed desorption of species. Temperature oxygen over La,_,Ba,MnO, showed different types
of oxygen adsorbed oxygen species on the surface and correlated well with the methane oxidation activity.
References fll N.Q. Minh, J. Am. Ceram. Sot. 76 (1993) 563. I21 R.J.H. Voorhoeve, Advanced Materials in Catalysis (Academic Press, New York, 1977) p. 129. 131 L.G. Tejuca, J.L.G. Fierro and J.M.D. Tascon, Adv. Catal. 36 (1989) 237. [4] T. Seiyama, Catal. Rev.-&i. Eng. 34 (1992) 281. [51 N. Yamazoe and Y. Teraoka, Catal. Today 8 (1990) 175. (61 R. Doshi, C.B. Alcock, N. Gunasekaran and J.J. Carberry, J. Catal. 140 (1993) 557. [71 C.B. Alcock, R. Doshi, J.J. Carberry and N. Gunasekaran, J. Catal. 143 (1993) 553. [81 T. Nitadori, S. Kurihara and M. Misono, J. Catal. 98 (1986) 221. 191J.J. Liaug and H.S. Weng, Ind. Eng. Chem. Res. 32 (1993) 2563. [lOI N. Gunasekaran, S. Rajadurai, J.J. Carberry, N. Bakshi and C.B. Alcock., Solid State fonics 73 (1994) 289. Ill] N. Gun~ek~~ and J.J. Carberry, unpublished data. [12] CD. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder and G.E. Mullerberg, Handbook of X-ray Photoelectron Spectroscopy (Perkin-Elmer Corp. (Phys. Electron. Div., Eden Prairie, MN 55344, 1979). [13] J. Kirchnerova, D. Klvana, J. Vaillancourt and J. Chnouki, Catal. L&t. 21 (1993) 77. [l4] H.M. Zhang, Y. Teraoka, and N. Yamazoe, Chem. Len. (1987) 665. [I51 N. Yamazoe, Y. Teraoka and T. Seiyama., Chem. Lett. 12 (1981) 1767. 1161H. Anti, T. Yamada, K. Eguchi and T. Seiyama, Appl. Catal. 26 (1986) 265. [I71 T. Nitadori, J. Catal. 121 (1982) 43. 1181 R. Doshi, C.B. Alcock and J.J. Carberry, Catal. Lett. 18 (1993) 337, [19] K.R. Bernard. K. Foger, T.W. Turney and R.D. Williams., J. Catal. 125 265 (1990).