Ind. Eng. Chem. Res. 1993,32, 1790-1794
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RESEARCH NOTES Methane Oxidation over SrCeO3 Catalysts: Effect of Solid-state Reaction Temperature Hidetoshi Nagamoto,' Eiji Shinoda, and Hakuai Inoue Engineering Research Institute, The University of Tokyo, 2-11-16 Yayoi, Bunkyo-ku, Tokyo 113, Japan
SrCeO3 catalysts were prepared by solid-state reaction at 750-1400 "C. The catalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy, and temperatureprogrammed desorption (TPD)of C02. The surface area of the catalysts decreased with increasing solid-state reaction temperature. The rate equation of the methane oxidation was obtained as reo, = kPc&O.sPo,o'6;rcz = k'P~%l.~. The rate constants were approximately proportional to the amount of intermediate basicity of the catalyst measured by TPD of CO2. The active sites for methane oxidation are expected to have intermediate basicity. Introduction
Experimental Section
Oxides of alkaline-earth metals are known to be prominent in selectivity for formation of the C2 compounds C2Hs and C2H4 (e.g., Yamagata et al., 1987). However, they are not stable during the reaction resulting in the formation of the respective hydroxides and carbonates (Carreiro et al., 1989). Therefore, to apply the alkalineearth metals to oxidative coupling of methane, it is necessary to stabilize them. One prominent way is making a complex oxide such as perovskite-type oxides (ABOB). Perovskite-type oxides containing an alkaline-earth metal have been found by the authors (Nagamoto et al., 1988) to have high catalytic activity for oxidative coupling of methane. In this work, cerium was selected as a paired B-site cation, since cerium oxides showed prominent oxidation activity in the preparatory reaction tests. Activity of a catalyst often changes by the preparation methods and conditions. In the case of perovskite-type oxides, Johnson et al. (1976)reported that preparation methods,such as freeze-drying,coprecipitation, and spraydrying methods, affect their oxidation activities. Imai et al. (1987) reported that the crystallinity of LaA103 controlled by changing the solid-statetemperature affected its activity and selectivity. Moriyamaet al. (1986)reported that addition of an alkali metal to MgO decreases the BET surface area and increases the CZ selectivity for oxidative coupling of methane. The first aim of this work is to study the effect of the temperature at which the perovskite-type oxides were prepared by solid-state reaction on their catalytic activities. The temperature of solid-state reaction would alter the BET surface areas of the oxides and change their surface properties. There have been a few works on the spectroscopic analysis of Ce in its oxides (e.g., Nakano et al., 1987; Burroughs et al., 1976). The second aim of this work is to obtain the relationship between physicochemical properties of perovskite-type oxides and their catalytic activities by using X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD),
Solutions of the proper stoichiometry were prepared from preanalyzed single cation nitrate solutions. Nitrates were selected because of their high solubility for the cations. A solution of ammonium oxalate was slowly added into the stirred solution. The precipitates were separated from water in a rotary evaporator and dried. The precipitates were decomposed to oxides at 450-550 "C in air. The oxide mixtures were heated in air from room temperature to a temperature ranging from 750to 1400"C, where solidstate reaction was conducted for 1 h to obtain perovskitetype oxides. These specimens were ground in an agate mortar. All the specimens were characterized by several techniques. Phase identification was performed by X-ray diffraction (XRD) (RAD-B,Rigaku Denki). Surface areas were measured by a single point BET technique using N2 adsorption. XPS was recorded on a JPS-QOSX(JEOL). A conventional gas flowing system was used to test the catalysts. The standard testing conditions were as follows: temperature = 750 "C; Po, = 8 kPa; PCH(= 16 kPa; P H=~77 kPa; total flow rate = 30 mL/min. The catalysts (30-200 mg) without dilutingwithinert bare supports were packed between two flocks of quartz wool and placed in the middle of the quartz reaction tube. A thermocouple was attached on the outer wall of the tube. Reactants and products such as CO, Ha, C02, C2H&C2H4, and C3Hs were analyzed by a gas chromatograph (GC-8A, Shimadzu, 90
* Author to whom correspondence should be addressed. E-mail:
[email protected].
"C). On TPD measurements, a mass spectrometer (TE-600, NEVA) was used to analyze the components in the gas stream. The conditions of TPD were chosen as follows: temperature range = 25-1000 "C; rate of increasing temperature = 10 K/min; flow rate of a carrier gas (He) = 30 mL/min. For TPD of C02, oxides were preheated in a He stream at lo00 "C, followed by COZadsorption at room temperature. Results Effect of Solid-state Reaction Temperature on BET Surface Area. XRD measurements showed that a solid-state reaction temperature of lo00 "C or higher gave a single phase of orthorhombic SrCeOs, and that
0SSS-5SS5/93/2632-1790$04.00/00 1993 American Chemical Society
Ind. Eng. Chem. Res., Vol. 32, No.8,1993 1791
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0.60.5-
0.4I
0.3-800 ' d o 0 ' 1200 ' 1400
537
Solid-state reaction temperature ["c]
.
l
534
I
I
531
I
l
520
.
I
Binding energy [ev]
Figure 1. Change of BET surface area of SrCeOs by the Solid-state reaction temperature.
Figure 3. XPS spectra of O(ls) for SrCeOs.
Binding energy [eV]
Figure 2. XPS spectra of Ce(3d) for SrCeOs.
Table I. Promrtiee of SrCeOa cat. A B C
treatment temp ("(2) 750 (1 h) + lo00 (1 h) 1150(1h) 1400(1 h)
BET surf. area (m2/g)
1.91 0.88 0.39
C02 uptake (rmol/m2) 2.5 7.8 12.5
reaction temperatures of 900 and 750 "C gave a mixture of SrCe03, CeO2, and SrO. The peaks attributed to CeO2 and SrO were as high as the main peak of SrCeO3. However, an additional firing at 1000 "C for 1h enhanced the solid-state reaction to give a phase of orthorhombic SrCeOs and decreased the diffraction peaks of CeO2 and SrO to negligibleheight. The BET surface area of SrCeO3, Sv,was strongly dependent on the solid-state reaction temperature (Figure l),and was expressed as afunctionof the temperature: log S v (m2/g)= 1.27 - 1.19 X 10-32' ("C) for T = 750-1400 "C. The surface area of the powders firedat750"C wasdecreased byonly20% bytheadditional solid-state reaction at lo00 "C. The BET surface areas of SrCeO3 catalysts employed for catalytic reaction are listed in Table I. XPS Analysis of SrCeOs. Powders of catalyst B were pelletized into disks. Diskswere heated in a helium stream containing 10 vol % oxygen at 750 "C for 1h. Figure 2 shows an XPS spectrum of Ce(3d) for SrCeO3 prepared at 1150 "C. In Figure 2, a broken line represents the spectrum of Ce(3d) for CeO2, which shows that Ce is tetravalent. The differencebetween the spectraof SrCeO3 and CeO2 shown by the hatched area implies that there exists Ce3+ in SrCeOs (Nakano et al., 1987). On the other hand, the ratio of Ce/Sr calculated by using their sensitivities was as much as 0.43. Strontium is richer than
1
10
PCH4 [kPa1 Figure 4. CO, formation rate. (a,top)PO,dependence; (b, bottom) PCH,dependence.
cerium on the catalyst surface. Figure 3 shows spectra of O(1s). The peak at binding energy = 531.4 eV was attributed to a carbonate and/or carbon dioxide strongly adsorbed on the catalyst surface by comparing the XPS spectra of O(1s) for SrCOs. Etching the catalyst surface by Ar+ bombardment or heating the catalyst at 600 "C in vacuum led to disappearance of the peak at 531.4 eV. Therefore, if a carbonate is formed, the carbonate exists only in the vicinity of the surface. Reaction Kinetics. Figures 4 and 5 show formation rates of CO, and C2 as a function of partial pressures of oxygen and methane over catalysts A, B, and C shown in
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" 7
s t E
0
Cat. 6
0
m
P
Cat. A
Table 11. Kinetic Parameters for Methane Oxidation SrCeOs formation rate (750 O C ) (pmo1.s-1-m-2) cat. CZ CO, kol@ rnp n1@ kab m2b A 2.84 8.20 0.042 0.02 1.39 0.70 0.55 B 5.08 13.4 0.074 -0.01 1.43 1.09 0.56 C 9.51 22.5 0.132 -0.05 1.58 1.85 0.54 rc, = kOIPO,mlPChnl. b reo, = k&GbPchh.
over
n2* 0.54 0.50 0.48
@
0.1 91
0
1
1
104K [K-'1 Figure 6. Temperature dependence of CZformation over catalyst C. I
1
10
I
PCH4 [kpa] Figure 5. C2 formation rate. (a, top) PO,dependence; (b, bottom) P c dependence. ~
Table I. All the catalysts, irrespective of the solid-state reaction temperature, showed a similar dependenceof the formation rates on Poz and Pc&. Assuming the rate equations expressed by Pi = kiPcbrnjPozn',kinetic parameters were obtained (Table 11). On the whole, the reaction order for CO, formation was approximated to be 0.5 in methane and oxygen and for C2 formation 1.5 in methane and 0 in oxygen. The apparent rate constants ko1 and ko2 in Table 11 were calculated on the basis of these rate equations. The activation energies were obtained by measuring the formation rates under a differential reactor condition. Figures 6 and 7 show the reaction rates over catalyst C as a function of temperature. The calculated values for all the catalysts used were 265 f 10 kJ/mol for C2 formation and 110 f 10 kJ/mol for CO, formation. TPD of C02 and Surface Basicity of SrCeOs. XPS measurement showed that carbon dioxide is strongly adsorbed on the catalyst surface and/or a carbonate exists on the surface,suggestingthat the catalyst surface is basic. Hence, the surface basicity of catalysts was measured by using TPD of C02 adsorbed at room temperature. As shown in Figure 8, three peaks of COzdesorption appeared in the temperature range employed. One is between 100 and 200 "C. Another peak is between 400and 500 "C. The third peak is around 700 "C.
Discussion BET Surface Area of SrCeOs Powders. The BET surface area of complex oxides generally decreases with increasing solid-state reaction temperature, as shown in Figure 1. In the case of the first solid-state reaction temperatures below 900 "C, XRD measurement showed
-
r
1
N
E
0) c
E
104/T [K-'J
Figure 7. Temperature dependence of CO, formation over catalyst C.
that the crystallinity of SrCeOa powders was insufficient and that there were several peaks attributed to CeOz and SrO. However, the second solid-state reaction at lo00 OC for 1h after cooling the powders down to room temperature yielded a phase of SrCeOa of sufficient crystallinity. The BET surface area of the powders to which the second reaction applied was larger than those fired at the same temperature as the first solid-statereaction. For example, when the temperature of the first stage was 750 "C and that of the second was lo00 "C, the surface area was decreased by only 20%, and was by 60% larger than that of the powders fiied at lo00 "C for the first stage. However, the second reaction at 1100 "C or higher yielded almost the same surfacearea as that fired at the same temperature for the first stage. On the basis of the XRD measurements, firing the powders at 900 "C or lower enhanced the crystallization of CeO2 and SrO. The growth of CeO2and SrO crystallites
Ind. Eng. Chem. Res., Vol. 32, No. 8, 1993 1793
Temperature ["C]
Figure 8. TPD spectrum for COz adsorbed on SrCeOa.
might retard the following solid-state reaction to SrCeO3 at lo00 "C, and 1h might be too short to enhance sintering of SrCeO3 powders. In contrast, firing a t 1000 "C for the fist stage is considered to enhance the solid-state reaction sufficiently since the crystallites of CeO2 and SrO are very small. The temperature of the second reaction temperature above 1100 "C might be enough to enhance both the solid-state reaction and sintering. Surface Properties of SrCeOs. XPS analysis of SrCeO3 at room temperature showed that Sr content was approximately twice as large as that of Ce in the vicinity of the surface, and that C02 is strongly bound to the SrCeO3 surface and/or a carbonate forms the surface. The surface composition analyzed by XPS which showed Sr:Ce:C (originated from carbonate) = 1.0:0.43:0.5 implies that the surface of the catalyst consisted of SrCeO3 and SrC03. The amount of SrCO3 which was limited to the vicinity of the surface was so small that XRD analysis showed only a clear perovskite structure. Surface Basicity. The basicity distribution studies by TPD of C02 showed the presence of site energy distributions or a group of sites of different energies on SrCeO3. The basicity is attributed to the anions (Le., 0%) exposed on the surface of the catalyst. The basic strength of the surface sites is considered to be dependent on the effective charge on the anions and/or their coordination on the surface. Surface imperfections such as steps and kinks are expected to be responsible for the presence of sites of different basic strength. The TPD curve shown in Figure 8 suggeststhat the peak of C02 desorption around 700 "C is due to CO2 molecules strongly bound to the surface imperfections since at a higher temperature of the solid-state reaction more surface imperfections are diminished. Reaction Kinetics. The formation of CO, was of 0.5 order in methane and in oxygen. Similar results have been obtained by several researchers (e.g., Lehmann and Baerns, 1992; Lo et al., 1988). Lehmann and Baerns (1992) suggested that methane and oxygen reacting to CO, adsorb dissociatively and that the rate-determining step for CO, formation is the reaction between a CH3 species and an 0 species since no kinetic hydrogen isotope effect was observed. The reaction scheme for CO, formation over SrCeOa is expected to be similar. The formation of Ca compounds was of 1.5 order in methane and of 0 order in oxygen. Ali Emesh and Amenomiya (1986) obtained the dependence of second order in methane and proposed the following reaction scheme:
COP uptake [ CI mol m-'1
Figure 9. Correlation between rate constanta and the amount of COz desorbed from 200 to 600 "C.
The presence of Ce3+in SrCeO3 suggests that the scheme for C2 formation over SrCeO3 is this one. On the other hand, Lehmann and Baerns (1992)obtained the dependence of first order in methane and proposed that the rate-determining step is the formation of methyl radicals by reaction of weakly adsorbed molecular methane with molecularly adsorbed oxygen: CH,(ads) + 02(ads)-CH,'(ads) + H02(ads) (3) On the basis of these two reaction schemes proposed, the 1.5-order dependence suggests that there are two kind of active sites on the catalysts. The dependence of zero order in oxygen suggesta that the oxidation of M,0,1 is sufficiently fast in the case of the reaction scheme proposed by Ali Emesh and Amenomiya (1986) and that, in the case of the scheme proposed by Lehmann and Baerns, the range of oxygen partial pressure employed in this work is within the range where the rate is nearly independent of oxygen partial pressure around its maximum. Active Sites for Methane Oxidation. The studies of the reaction kinetics showed that the rate equations for catalysts A, B, and C are almost the same and that only the rate constant differs. Therefore, the active sites for methane oxidation are expected to have the same properties which are not affected by the solid-state reaction temperature. The sites from which C02 desorbs from 600 to 800 "C are expected to be almost inactive for methane oxidation, since SrC03 were reported to have very little activity (Carreiro et al., 1989). In addition, the density of these sites decreased with increasing solid-state reaction temperature, whereas the reaction rate increased with increasing temperature. C02 uptake, which is defined as the amount of CO2 desorbed from 200 to 600 "C, was correlated with the rate constants (Figure 9). Though the lines determined by the least-squares method do not pass through the origin in Figure 9, the rate constants are quite proportional to the COS uptake. The active sites for methane oxidation are expected to have intermediate basicity . The density of the sites of intermediate basicity increased with the temperature of the solid-state reaction, as shown in Table I. On the other hand, the BET surface area decreased with increasing reaction temperature. The sites expressed as C02 uptake per catalyst weight are 4.8 pmo1.g' (catalyst A), 6.9 pmo1.g' (catalyst B), and 4.9 pmo1.g' (catalyst C). From these results, the number of
1794 Ind. Eng. Chem. Res., Vol. 32, No. 8, 1993
active sites is expected not to be strongly affected by the solid-state reaction temperature. Conclusion Conclusions are summarized as follows: 1. The surface area of SrCeOa catalysts prepared by solid-state reaction decreased with increasing reaction temperature. 2. The rate equation of the methane oxidation was obtained as 0 . 6 ~0.6.
rco, = kPcli,
0,
'
rc, = kfPcH,'.s
3. The rate constants were approximately proportional
to the amount of intermediate basicity of the catalyst measured by TPD of COz. 4. The active sites for methane oxidation are expected to have intermediate basicity. There may be two kinds of active sites for CZformation. Nomenclature C2 = ethane and ethylene CO, = carbon dioxide and monoxide kol = rate constant of C2 formation, pmol min-1 atm-l.s km = rate constant of CO, formation, pmol min-1 atm-1 rn = reaction order in oxygen n = reaction order in methane r = reaction rate Sv = BET surface area of SrCeOs, m2 g-1 Subscripts 1 = C2formation
2 = CO, formation
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Burroughs, P.; Hamnett, A.; Orchard, A. F.;Thorton, G. Satellite Studies in the X-ray Photoelectron Spectra of Some Binary and Mixed Oxides of Lanthanum and Cerium. J. Chem. Soc., Dalton
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Carreiro, J. A. S. P.; Baerns, M. Oxidative coupling I. Alkaline Earth Compound Catalysta. J. Catal. 1989,117,258. Imai, H.; Tagawa, T.; Kamide, N. Oxidative Coupling of Methane over Amorphow Lanthanum Aluminum Oxides. J. Catal. 1987, 106,394.
Johnson, Jr., D. W.; GaUaghar, P. K.; Schrey, F.; Rhodes, W. W. Preparation of High Surface Area Substituted LaMnOsCatalysts. Ceram. Bull. 1976,55,520. Lehmann, L.;B a e m , M. Kinetic Studies of the Oxidative Coupling of Methane over a NaOH/CaO Catalyst. J. Catal. 1992,135,467. Lo, M.-Y.; Agarwal,S. K.; Marcelin,G. OxidativeCouplingof Methane over Antimony-based Catalyst- J. Catal. 1988,112,168. Moriyama, T.; Takaeaki, N.; Iwamateu, E.; Aika, K. Oxidative Dimerization of Methane over Promoted Magnesium Oxide Catalysta. Chem. Lett. 1986, 1165. Nagamoto, H.; Amanwna, K.; Nobutomo, H.; Inoue, H. Methane Oxidation over Perowkitetype Oxide Containing Alkaline-Earth Metal. Chem. Lett. 1988,237. Nakano, T.; Kotani, A.; Parlebas, C. J.Theory of XPS and BIS Spectra in Ce& and CeO2. J. Phys. SOC.Jpn. 1987,56,2201. Yamagata, N.; Tanaka, K.; S d ,S.;Okazaki, S. Oxidative Coupling of Methane over BaO Mixed with CaO and MgO. Chem. Lett. 1987, 81.
Received for review November 3, 1992 Revised manuscript received April 23, 1993 Accepted May 4, 1993
