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Ind. Eng. Chem. Res. 2004, 43, 5670-5680
Methane Oxidation over Nanocrystalline LaCo1-XFeXO3: Resistance to SO2 Poisoning S. Royer, A. Van Neste,† R. Davidson,‡ S. McIntyre,‡ and S. Kaliaguine* Department of Chemical Engineering, Laval University, Ste Foy, Quebec G1K 7P4, Canada
Catalytic activity for CH4 oxidation and resistance to SO2 poisoning were tested for three LaCo1-XFeXO3 prepared by reactive grinding. The catalysts prepared by this method showed well-crystallized perovskite structure and various specific surface areas. The activity in CH4 oxidation correlates well with BET specific surface area. TPDO experiments showed very large quantities of desorbed β-oxygen that could not be correlated to the activity for CH4 oxidation. Resistance to sulfur poisoning in the same reaction was tested at 500 °C with 25 ppmv SO2 added to the feed. Two steps were distinguished during the poisoning reaction. The first one followed an exponential decrease of conversion versus time. The time to reach the end of this step was shown to be directly proportional to the specific surface area. In this step, deactivation was found to be reversible. During regeneration, some sulfur species were found to desorb from the catalyst. The second step was found to be a linear diminution of conversion with time. The slopes of these lines were found to be the same for the three catalysts. A total of 860 h is necessary to deactive the catalyst with the higher specific surface area, and the quantity of sulfur accumulated in the catalyst was found to vary linearly with the poisoning time. XRD showed significant conversion of the catalyst into La2(SO4)3, Co3O4, and iron oxide. The complex poisoning behavior is discussed and associated to the observed differences in the catalyst morphology. Introduction Perovskites are mixed oxides of ABO3 formula with different cations in position A and B. Many compositions are possible for A and B cations provided that the tolerance factor, t [defined as t ) (rA + rO)/x2‚(rB + rO)], is in the range between 0.8 and 1 and electroneutrality is respected.1 Since the beginning of the 1970s, some perovskites have been known to be good catalysts for gas-phase oxidation reactions.2-7 One of the major problems is the low specific surface area developed by these materials. Ceramic methods8 produce perovskites with specific surface area of less than 2 m2/g. The development of new methods of preparation (coprecipitation,9 citrate,10,11 and freezedrying12 and recently flame-hydrolysis13 and reactive grinding14-16) in the last 30 years permitted one to obtain perovskites with larger specific surface areas. These, however, rarely exceed 10-15 m2/g. Generally, specific surface area decreases with a raise in calcination temperature (Tc). High Tc values are, however, necessary for complete perovskite crystallization in most of the usual synthesis techniques. With the use of reactive grinding, no thermal treatment is necessary to obtain crystallization,14 as high grinding energy permits crystallization at nearly ambient temperature with formation of nanometric crystals. The second important problem is the easy poisoning of these catalysts, especially by sulfur compounds. In the case of an oxidation reaction in the presence of * To whom correspondence should be addressed. Tel.: +1418-656-2708. Fax: +1-418-656-3810. E-mail: kaliagui@ gch.ulaval.ca. † Nanox Inc., 4975 rue Rideau, local 100, Que´bec G2E 5H5, Canada. ‡ Western Science Centre, Room G-1, The University of Western Ontario, London, Ontario N6A 5B7, Canada.
sulfur species, SO2 is preferentially adsorbed on the surface of the perovskite, because of the basicity of these compounds. Then, SO2 is oxidized in SO3, because of the oxidizing atmosphere and the strong oxidizing activity of the transition metal, such as Co or Mn, in the B-position of the perovskite. SO3 is a precursor of sulfate formation on the perovskites. This perovskite poisoning process is assessed by the appearance of sulfate, observed by FT-IR in some cases and by XRD when amount of trapped sulfur is significant. The sulfate formation modifies the catalysts structure (destruction of the perovskite phase and formation of oxide and sulfate phases with a diminution in specific surface area) and suppresses quickly their activity. Yao17 studied the poisoning effect of SO2 on some manganates and cobaltites in the oxidation reactions of CO and C2H4. The compounds tested showed a quick and drastic diminution in activity that was not restored when SO2 was suppressed from the feed. Different substitutions in both the A and B positions were tested to find better resistance, but no significant gain was obtained.18 Rosso et al. studied the sulfur poisoning of LaMn1-XMgXO319 and LaMn1-XMgXO3 + yMgO.20,21 The extent of poisoning was found to vary with the catalyst composition and conditions of aging. After aging at 800 °C for 24 h in the presence of 200 ppmv SO2 in CH4 oxidation reaction, the authors observed an important diminution in activity, whereas no significant decrease was observed when aging is performed at 400 and 600 °C. Sulfate is preferentially formed on Mg as observed by FT-IR analysis. Dispersing the perovskite in a matrix of magnesium oxide permitted improvement of its sulfur resistance.20 Studies on LaMn1-XCrXMg0.5O3 + yMgO22 showed an enhanced resistance to SO2 with the substitution of Mn by Cr. The compounds studied in the present work were prepared by reactive grinding. By this method, per-
10.1021/ie030775r CCC: $27.50 © 2004 American Chemical Society Published on Web 06/25/2004
Ind. Eng. Chem. Res., Vol. 43, No. 18, 2004 5671 Table 1. Properties of Perovskite Samples Co1, Co2, and Co3 after Grinding and Calcinationa catalyst
Co1
Co2
Co3
calcin temp (°C) crystalline phaseb elem comp SBET (m2/g) D1 (nm) STh (m2/g) D2 (nm) SBET/STh
550 P, C La0.94Co0.97Fe0.03O3-δ 4.2 9.37 87.8 196 0.048
550 P La0.93Co0.90Fe0.10O3-δ 10.9 10.10 81.5 75.5 0.134
550 P La1.00Co0.80Fe0.20O3-δ 17.2 9.40 87.6 47.6 0.196
a D , particle size calculated from the Sherrer equation; S , specific surface area calculated assuming cubic particles and a density of 1 Th LaCoO3 equal to 7.29; D2, equivalent cubic particle size calculated from BET surface area. b P, perovskite; C, cobalt oxide.
ovskites were crystallized at ambient temperature and the nanometric structure of the crystals confers to these materials a high activity in oxidation reactions, especially for the low-temperature oxidation reactions.23,24 The objective of this work is therefore to study the poisoning behavior of this new materials. To this end, three LaCo1-XFeXO3 catalysts with different specific surface areas were prepared. Prior to the deactivation studies, the kinetics of catalytic methane oxidation over these solids was established. Experimental Section Catalyst Preparation. Three LaCoO3 samples (Co1, Co2, and Co3) with different specific surface areas were prepared by reactive grinding. Lanthanum oxide (La2O3, Alpha 99.99%) and cobalt oxide (Co3O4, Baker and Adamson 97.49%) were used as precursors for the synthesis. La2O3 was previously calcined at 600 °C to transform any hydroxide into oxide. Grinding was performed in a laboratory SPEX shaker mill with an agitation speed of 1040 rpm. Tempered steel vials and three balls were used for the synthesis. The characteristics of the vials are described in ref 14. The mass of precursors was adjusted to obtain 5 g of perovskite. Milling was performed in an O2 atmosphere at ambient temperature. In the case of Co1, only one step of grinding was performed. This step permitted us to obtain crystalline LaCoO3 phase. Co2 and Co3 were prepared in two grinding steps: a first step to obtain perovskite crystallization and a second step with additive to enhance specific surface area. The obtained compounds (perovskite + additive) were washed repeatedly (with water or solvent) to free samples from any trace of additive. The physical properties of the three catalysts are presented in Table 1. Co1 was designed to present a very low specific surface area for comparison purposes. NaCl used as the additive in the case of Co2 led to a lower surface area than ZnO used for Co3, even if the crystallite size calculated with the Sherrer equation led to similar values for the three catalysts. Catalyst Characterization. XRD spectra of fresh catalysts after grinding and before calcinations were collected on a SIEMENS D5000 diffractometer using Cu KR (λ ) 1.54184 Å) to verify perovskite crystallization. Spectra were recorded for 2θ values from 10 to 80° by 0.05° step with a step duration of 2.4s. The Sherrer equation was used to calculate the crystallite size. XRD spectra of poisoned catalysts were also recorded. Specific surface area was measured by N2 adsorption at 77 K (BET method) on an OMNISORP 100 apparatus. About 0.5 g of catalyst was evacuated for 6 h at 200 °C. The specific surface area was determined from the linear part of the BET curve. Sample composition (Fe, La, Co,
and S) was determined by ICP on a P40 from PerkinElmer. The catalysts were first dissolved in dilute HCl and aged at 60 °C to ensure a complete dissolution. SEM micrographs were obtained on a JEOL JSM-840. Acceleration voltage was 15 kV. The samples were first dispersed on an aluminum stub and coated with a Au/ Pd film. The solids were analyzed by XPS using a Kratos AXIS Ultra spectrometer. The XPS spectra were obtained using a monochromatized Al KR source (15 W, 14 kV), for a small area (700 × 300 micron). For the survey spectra, the pass energy was 160 eV, and for high-resolution spectra, the pass energy was 20 eV. The spectra were obtained with a 90° takeoff angle and the analysis chamber vacuum was about 1 × 10-9 Torr. All samples were mounted on carbon adhesive and were assumed to be insulating and thus analyzed using the special Kratos charge-neutralizing system. The C1s signal of adventitious carbon was positioned at 285.02 eV for charge correction. TPD of oxygen was performed on an RXM 100 (Advanced Scientific Designs Inc). The system was equipped with both a mass spectrometer (UTI 100) and a TCD. Catalyst (0.1 g) was calcined in situ (ramp ) 5 °C/min, 4 h at 550 °C) in a flow of 20% O2 in He (total flow rate ) 20 mL/min). Oxygen desorptions were performed from ambient temperature to 900 °C under flowing helium (10 mL/min). Currents for masses 32, 28, and 44 were recorded as a function of time. Catalytic Tests. CH4 activity measurements were performed on catalysts calcined at 550 °C. Perovskite (0.2 g) was weighed and enclosed in a U quartz tube (i.d. ) 5 mm). The reactor was placed in a furnace and the temperature was controlled by 2 K-type thermocouples (on the top and under the catalytic bed). The catalyst was first purged under He (20 mL/min) for 1 h. Thereafter, a flow composed of 0.25% CH4-1% He in O2 passed through the reactor and the temperature was raised. The total flow rate was adjusted at 15, 22.5, 30, and 40 NmL/min, which corresponds to a VHSV between 5625 and 13 875 h-1. Conversions were measured in the steady-state regime for temperature between calcination temperature and zero conversion temperature. Reactants and products were analyzed on a gas chromatograph (HP 6890 series) equipped with a TCD. A HayeSep DB column (i.d. ) 1 mm, L ) 2 × 5 m) was used for separation. Typical S curves were obtained. Calculation of activation energy and preexponential factor were made by assuming first-order reaction with respect to CH4: r ) kPCH4. Details concerning kinetic data treatment can be found in ref 25. Poisoning tests were performed on the same system. The flow was composed by 0.25% CH4-10% Ne-12.5 ppm SO2 in O2. Conditions of reaction were Qv ) 22.5 NmL/min and T ) 500 °C.
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Figure 1. X-ray diffraction patterns of catalysts Co1, Co2, and Co3 after milling. *, Co3O4.
Results and Discussion Physical Characterization. The three catalysts prepared were perovskites having specific surface areas between 4.2 and 17.2 m2/g after calcination at 550 °C. The X-ray patterns of the compounds obtained after grinding are reported in Figure 1. Co1 presents a strong pattern of rhomboedral LaCoO3 (JCPDS card 09-0358), but weak lines for Co3O4 are also found. Co2 and Co3, which were milled for 24 h, show complete conversion to LaCoO3. SEM photographs of the calcined compounds are presented in Figure 2. They are consistent with the three catalysts being constituted of agglomerates of elementary nanometric particles of perovskites with diameters in the range 10-15 nm, as shown in ref 14. Similar structures were already observed by Szabo15 and Zhang and Saito16 for nanocrystalline perovskites prepared by the same technique. Figure 2C, recorded at lower magnification, allows us to show that in sample Co3 (representative of all these samples) the agglomerate diameter varies from 500 nm to several microns. No
differences in particle size can be observed between the three catalysts. Zhang and Saito16 reported aggregates of several microns formed by fine particles with size less than 100 nm. Rougier et al.26 also observed that the augmentation of shock number allowed an augmentation of the fracture number and a diminution of the mean aggregate particle size. As mentioned above, Co1 was prepared in one grinding step, whereas for the preparation of Co2 and Co3, a second milling step was performed in the presence of an additive, yielding an enhanced specific surface area. The use of a pure oxygen mill atmosphere increases significantly the rate of perovskite formation. Table 1 presents some properties of the obtained compounds. The particle diameters calculated by the Sherrer equation from XRD lines are also reported. These diameters are about 9-10 nm for the three catalysts. It seems thus that the second step of grinding has little effect on the nanoparticle size. The large differences between the specific surface areas measured by BET and those calculated from diameters evaluated from the X-ray line broadening indicate that the nanoparticles are not separated and that there is essentially little or no voidage between these elementary particles. The equivalent diameters calculated from BET values give particle sizes between 48 and 196 nm, but no real differences in size can be observed by SEM and XRD. It seems thus that the second grinding step affects the specific surface area by changing the internal porosity of the agglomerates. The SBET/STh ratio allows us to quantify this effect. Catalytic Activity for CH4 Oxidation. Figure 3 presents results of steady-state methane conversions obtained in the reaction of CH4 oxidation by oxygen at various temperatures and feed flow rates. The curves have the usual S shape, observed in many other studies.7,27-29 Significant differences in activity between the three catalysts can be observed. Co3, which is the catalyst with the largest specific surface area, presents the highest activity, as also shown in Table 3. At 550 °C and a flow rate of 15 mL/min (VHSV ) 5625 h-1), Co1 showed only 60.3% conversion, which is very low
Figure 2. SEM photographs obtained for catalysts (a) Co1, (b) Co2, (c and d) Co3 after calcination at 550 °C.
Ind. Eng. Chem. Res., Vol. 43, No. 18, 2004 5673
Figure 4. Arrhenius plots of the first-order rate constant k for the three catalysts: O, Co1; 0, Co2; ∆, Co3. Two parameters (Ea and A0) are linear; Dotted line: linear regression with Ea ) 22.4 kcal/mol.
Figure 3. Conversions obtained for the oxidation of 0.25% CH4 as a function of the temperature for catalysts Co1, Co2, and Co3. VHSV ) (]) 5625 h-1, (4) 8437 h-1, (0) 11 250 h-1, (O) 13 875 h-1. Table 2. Characteristic Conversion Temperatures Obtained at VHSV of 5625 h-1 and First-Order Rate Constant Parameters exptl temp (°C)
kinetic parameters
E A0 A0 cora catalyst T5% T50% T95% (kcal/mol) (mol/g s atm) (mol/g s atm) Co1 Co2 Co3 a
411 522 311 410 307 402
503 478
24.4 21.0 21.7
211 186 461
56.4 498.4 750.8
A0 cor: calculated with E ) 22.4 kcal/mol.
Table 3. Amounts of Oxygen Desorbed as r- and β-Oxygen in TPDO Experiments O desorbed (µmol/g)
no. of monolayers desorbeda
sample
R-O2
β-O2
R-O2
β-O2
Co1 Co2 Co3
9.2 26.8 74.5
35.8 41.8 278.9
0.55 0.61 1.08
2.13 0.96 4.05
a
Calculated by assuming 4 µmol/m2 for one monolayer.
compared to the conversions obtained for the other two catalysts. The curves in Figure 3 are obtained by fitting the first-order rate constant k values numerically (r ) kPCH4) through a numerical integration of the rate equation over the length of the isothermal tubular reactor as described in refs 14, 23-25. These values are plotted as Arrhenius curves in Figure 4. The preexponential factor A0 and activation energy Ea obtained by linear regression of these curves are summarized in Table 2. Figure 4 illustrates the differences in activity among the three catalysts. The calculated activation energies vary between 21.0 and 24.4 kcal/mol. In a
Figure 5. Corrected preexponential factor as a function of the specific surface area.
previous work, we reported for LaCo1-XFeXO3 activation energies between 22.4 and 26.3 for five catalysts with different values of X and different calcinations temperatures.25 Baiker et al.28 reported 24.88 kcal/mol for the same reaction over LaCoO3. Linear regression of the Arrhenius plots performed using a fixed value of Ea ) 22.4 kcal/mol (chosen as the average Ea from Table 2) yields acceptable fits, as seen in Figure 4 (dotted lines). The values of the preexponential factor (A0 cor) obtained from these fits allow a straightforward comparison of the respective activities of the catalysts (Table 2). The values of A0 cor are plotted as a function of specific surface area in Figure 5. It is seen from this figure that catalysts Co2 and Co3, which are prepared at the same grinding time, display the same value of the specific activity per unit surface area, whereas the specific activity of Co1 is significantly lower. This is likely associated with the shorter grinding time (and lower crystallinity) of the sample. Figure 6 reports TPDO results for the three catalysts (integrated number of moles of desorbed oxygen are given in Table 3). As largely discussed in the literature, these curves allow us to distinguish two types of desorbing oxygen. R-O2, which desorbs below 700 °C, is attributed by some authors to oxygen chemisorbed on surface anionic vacancies.29,30 The desorption temperatures agreed well with those previously reported for catalysts prepared in our group by reactive grinding at
5674 Ind. Eng. Chem. Res., Vol. 43, No. 18, 2004
Figure 6. Normalized TPD-O2 spectra (per gram of catalyst) for the three LaCoO3 calcined at 550 °C (ramp ) 5 °C/min under He from 25 to 900 °C).
the same pretreatment temperature14 and are similar for the three catalysts. A minor desorption peak is also observed at T ) 350 °C, but the corresponding amount of oxygen is too low to be quantified. In our case, it is found that the amount of R-O2 desorbed rises with the specific surface area of the catalysts, but remains lower than one monolayer, except for Co3, which desorbed about 1.08 monolayer (see Table 3). The latter value must be considered equal to one within experimental and estimation errors. Previously, Nitadori et al.30 also reported values less than one monolayer for R-desorption. The amounts of β-oxygen desorbed (T ) 840-860 °C) are larger than those of R. β-oxygen originates from the bulk of the catalyst and is associated with the reduction of some Co3+ into Co2+.8,29,30 The amounts of β-oxygen are much higher that those reported for the same compounds LaCoO3 prepared by the citrate method.31 This is likely due to the lower calcination temperature and higher defect density of the materials prepared by grinding, as it was previously shown that the quantity of β-oxygen desorbed is dependent on the calcination temperature.14 XRD analysis of the samples after the TPDO experiments revealed the presence of brownmillerite (LaCoO2.5). Obviously, however, this conversion is incomplete, since even Co3, which desorbed the higher amount of oxygen, shows a δ (defined as the oxygen deviation to stoichiometry LaCoO3-δ) value of 0.14 after desorption. The high value of the number of monolayers represented by the desorbed β-oxygen confirms that this oxygen cannot originate only from the surface, which means that a large fraction of it is extracted from the bulk. This is indeed associated with a reduction of some Co3+ cations in the bulk. It is important to note that the amounts of desorbed R- and β-oxygen are not simply related to the specific surface area, as shown in Figure 7, and therefore cannot be related to the activity of the catalyst. Iron contamination, which differs for the three catalysts, must have an effect on the amount of R-O2 desorbed. Teraoka et al.8 indeed observed an increase in oxygen desorbed in the low-temperature region with the substitution of Co by Fe, but they observed the progressive disappearance of the sharp β-peak with substitution of Co by Fe, which we did not observe. A quick comparison between the results in Figures 5 and 7 allows us to conclude that
Figure 7. Amounts of R- and β-oxygen desorbed as a function of the specific surface area: O, R-oxygen; 4, β-oxygen.
Figure 8. Effect of SO2 on the activity of the catalysts for the reaction of CH4 oxidation (500 °C, VHSV ) 8437 h-1, 12.5 ppm SO2, 0.25% CH4, 10% Ne, balance O2): O, Co1; 0, Co2; 4, Co3; 2, Co3 reaction with SO2 during 130 h, end of the reaction without SO2.
the activity for CH4 oxidation is not directly related to the quantity of β-oxygen desorbed. Poisoning Mechanism. Figure 8 shows the effect of injecting 12.5 ppm of SO2 in the feed on the activity of the catalysts for the CH4 oxidation reaction. Two regimes of poisoning can be distinguished for the three catalysts. In the first one, the conversion followed an exponential decrease to a plateau, which represents about 10-15% diminution of the conversion. During the second poisoning regime, the conversion is decreasing linearly with the reaction time until essentially complete deactivation. Poisoning Regime 1. The duration of this first stage is directly proportional to the specific surface area of the catalyst, as shown in Figure 9. At this stage, deactivation is reversible. After 130 h of poisoning, Co3 recovered all its activity in 210 h after removal of SO2 from the feed. Results of analysis of the solids after poisoning and regeneration are presented in Table 4. At the end of the first poisoning stage, the specific surface area of Co3 drops from 17.2 to 4.8 m2/g and 2.03 wt % sulfur is accumulated on the catalyst. The sulfur content measured by ICP is nearly the sulfur fed over the catalyst, as shown in Figure 12. The XRD pattern
Ind. Eng. Chem. Res., Vol. 43, No. 18, 2004 5675
Figure 9. Duration of the first stage vs specific surface area of the fresh catalyst. Table 4. Physical Properties of the Three Catalysts after Poisoning poisoning regeneration SBET sample time (h) time (h) (m2/g)a Co1 Co2 Co3 Co3 Co3
187 333 859 130 130
207
2.5 2.9 2.8 4.8 12.3
D1 (nm)b
D2 sulfur (nm)c (wt %)
100-1000 100-1000 ∼100 15.9
