Stability of La0.6Sr0.4Co0.2Fe0.8O3-δ Perovskite ... - ACS Publications

Mar 19, 1998 - Stability of La0.6Sr0.4Co0.2Fe0.8O3-δ Perovskite Membranes in Reducing and .... Shiguang Li, Wanqin Jin, Pei Huang, Nanping Xu, and Ju...
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Ind. Eng. Chem. Res. 1998, 37, 1290-1299

MATERIALS AND INTERFACES Stability of La0.6Sr0.4Co0.2Fe0.8O3-δ Perovskite Membranes in Reducing and Nonreducing Environments Sherman J. Xu and William J. Thomson* Department of Chemical Engineering, Washington State University, P.O. Box 2710, Pullman, Washington 99164-2710

The chemical stability of perovskite La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF-6428) has been investigated in connection with its potential use as a catalytic membrane for the oxidative coupling of methane (OCM). Once a steady-state oxygen-defect gradient is established (15-20 h), these membranes are found to be very stable under air/nitrogen gradients at temperatures up to 960 °C and they respond instantaneously to temperature changes with an apparent activation energy of 159 kJ/ mol. However, serious near-surface etching occurs when one side of the membrane is exposed to 100% CH4 at 850 °C and atmospheric pressure, which resulted in dramatic increases in oxygen flux (a factor of 5). While this also caused an increase in the OCM reaction rate, the selectivity of C2+ hydrocarbons fell from 40 to 10%. It is also shown that stable operation under OCM conditions is possible if CH4 pressures are reduced to 0.1 atm but at a cost of C2+ production rate. 1. Introduction Because of their oxygen semipermeability and catalytic activity, ion-conducting perovskites have a great potential for use as catalytic membrane reactors for methane partial oxidation reactions such as the oxidative coupling of methane (OCM) to higher hydrocarbons (Xu and Thomson, 1997a; Xu et al., 1996; ten Elshof et al., 1995; Andersen et al., 1994) or the partial oxidation of methane to synthesis gas (Tsai et al., 1996; van Hassel et al., 1994). The two primary advantages of membrane reactors for these applications are the use of air instead of oxygen and the avoidance of extensive contact of oxygen with the hydrocarbon products which can dramatically reduce selectivity. Earlier work in our laboratory demonstrated that both selectivity and activity for the OCM reaction could be raised by factors of 4-7 with either a La0.4Sr0.6Co0.2Fe0.8O3-δ (LSCF-4628) or La0.2Ba0.8Co0.2Fe0.8O3-δ (LBCF-2828) membrane reactor when compared with a fixed-bed reactor under comparable reaction conditions (Xu and Thomson, 1997a; Xu, 1994). However, for commercial utilization, perovskite membranes must have long-term stability under the reducing environments characteristic of partial oxidation conditions. In general, ABO3 perovskites can decompose by either of the following B site reduction reactions (Katsura et al., 1975, 1978; Kamata et al., 1978):

1 1 ABO3 f A2O3 + BO + O2 2 4

(A)

* Corresponding author. E-mail: [email protected]. Telephone: (509) 335-8580. Fax: (509) 335-4806.

1 3 ABO3 f A2O3 + B + O2 2 4

(B)

Factors that influence these reactions include the bulk structure and composition of ABO3, temperature, oxygen partial pressure, and the reducing capability of the environment. To improve both oxygen permeation and catalytic activity, mixed A and B sites in the perovskite lattice structure have been previously studied (Teraoka et al., 1985, 1988; Hayakawa et al., 1992). It was found that different substitutions and oxygen nonstoichiometry are also important factors that influence the perovskite chemical stability. From a crystal structure point of view, the stability of these materials depends on the combination of stable A and B cations in dodecahedral and octahedral coordinations, respectively (Voorhoeve, 1977). At the same time, the radii of A and B cations must be restricted by eq 1, where rO is the

0.75 < (rA + rO)/x2(rB + rO) < 1.00

(1)

radius of lattice oxygen. It has been determined that stoichiometric LaCoO3 and LaFeO3 satisfy these requirements (Voorhoeve, 1977). The motivation for the stability study of nonstoichiometric La1-xSrxCo1-yFeyO3-δ is that A and B site substitution tend to provide oxygen vacancies (van Hassel et al., 1993; Ling et al., 1993; Kro¨ger, 1964) which tend to increase both oxygen flux and catalytic properties (Teraoka et al., 1985, 1988), as was evident in our previous work (Xu and Thomson, 1997a; Xu, 1994). The stability of LaCoO3 and LaFeO3 in H2/O2 atmospheres at 1000 °C was studied by Nakamura et al. (1979), using TGA measurements. Their results, listed in Table 1, showed that the equilibrium oxygen partial pressure of LaCoO3 reduction to Co (reaction B) is much lower than that of LaCoO3

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Ind. Eng. Chem. Res., Vol. 37, No. 4, 1998 1291 Table 1. Equilibrium PO2 for the Reduction of LaCoO3 and LaFeO3 at 1000 °C (Nakamura et al., 1979) ABO3 perovskite

reduction reaction

PO2,eq. [Pa]

LaCoO3 LaCoO3 LaFeO3

A B B

1.0 × 10-2 2.0 × 10-15 8.9 × 10-11

reduction to CoO (reaction A). This indicates that the complete reduction of B cations is very difficult even with a reducing agent as strong as H2. In addition, they also concluded that the perovskite with Co at the B site was more stable than that with Fe at the B site, since the equilibrium PO2 for LaCoO3 reduction to Co was dramatically lower than that for the complete reduction of the Fe cation. In a membrane configuration, the stability of oxygen permeation under a nonreducing atmosphere is primarily influenced by the oxygen nonstoichiometry and the order-disordered structure of oxygen vacancies. During these phase transitions, temperature plays the most important role and highly mobile oxygen vacancies can only be obtained with the disordered vacancy states at elevated temperatures (Steele, 1992). In a study of oxygen permeation over SrCo0.8Fe0.2O3-δ and La0.6Sr0.4CoO3-δ materials, Kruidhof et al. (1993) found that the ordering of oxygen vacancies could be gradually established in these perovskites at intermediate temperatures (650-750 °C), which caused a decrease in oxygen flux. However, a sharp increase in the flux was also observed at elevated temperatures (790-940 °C), which was attributed to the transformation from an ordered to a disordered structure as temperature was increased. When methane is introduced into the membrane configuration (as in an OCM application), surface reduction, methane activation, and ion exchange will all influence the stability of oxygen permeation and eventually influence the OCM reaction. ten Elshof et al. (1995) have used a similar perovskite membrane, LSCF-6428, in an OCM application and observed that strontium segregation occurred after the membrane was treated in an air/CH4 gradient at 880 °C. We have also shown that the reduction of La0.6Sr0.4Co0.2Fe0.8O3-δ, La0.4Sr0.6Co0.2Fe0.8O3-δ, and La0.2Ba0.8Co0.2Fe0.8O3-δ perovskites in pure methane or ethane (101 kPa) takes place at temperatures above 750 °C (Xu and Thomson, 1997a). These two previous observations of the stability of LSCF perovskites have only limited application to their stability under OCM conditions. That is, low concentrations of O2 in the presence of CH4 are known to stabilize LSCF-6428 (Xu and Thomson, 1997a). Thus, the primary objective of this paper is to investigate and compare the oxygen permeation stability of the La0.6Sr0.4Co0.2Fe0.8O3-δ membrane under nonreducing (O2/N2 gradient) and reducing (OCM) environments and to determine the longer-term effects of OCM conditions on both perovskite stability and OCM catalytic activity and selectivity. 2. Experimental Section 2.1. Preparation of Perovskite Membranes. The perovskite-type oxide membranes were prepared at Battelle Pacific Northwest National Laboratory (PNNL), and detailed procedures for this preparation have been given previously (Xu and Thomson, 1997a). After calcining the synthesized powder of La0.6Sr0.4Co0.2Fe0.8O3-δ (LSCF-6428) in air at 850 °C for 12 h, they were compressed into disks, 17 mm in diameter and

1-3.5 mm in thickness, using uniaxial pressure (55 MPa) followed by isostatic pressure (138 MPa). The pressed compacts were then sintered in MoSi2 furnaces at 1200 °C for 2 h in air using heating and cooling rates of 5 °C/min. The phase development of calcined powder and sintered disks was determined by X-ray diffraction analysis and the relative densities of sintered disks were measured by the Archimedes method using ethanol. According to these analytical results, the synthesized disks were fully densified with a 100% perovskite phase. 2.2. Membrane Reactor Setup. A “disk membrane” configuration was used for the measurements of oxygen permeation and OCM activity, and the schematic diagram of the complete reactor setup is shown in Figure 1. The membrane reactor assembly consisted of two sets of mullite tubes surrounding a quartz tube and separated by the perovskite membrane. Gold gaskets were used to obtain effective seals between the disk and the walls of the mullite tubing at high temperatures by placing the assembly in compression with the use of spring clamps. The inlet gas flows were controlled by Matheson mass flow controllers (models 8274 and 8270) which maintain stable and constant flow, and the reactor working pressures were maintained constant by two HP backpressure regulators. According to the requirements for specific runs, either oxygen or air flows (2-150 mL/min (STP)) were introduced into the upper chamber, just above the catalytic disk in order to minimize gas-solid mass transport effects. Methane or nitrogen was fed to the lower chamber in the same fashion (5-50 mL/min (STP)). Streams exiting from both chambers were analyzed by means of a gas chromatograph (HP model 1800A GCD) which was equipped with a HP-1 capillary column and a mass spectrograph as the detector. The column was operated at 35 °C, and the actual detection between gas components was achieved by using ion extraction from mass spectra. During the OCM activity tests, water in the exiting methane stream was condensed and separated with an ice condenser. The reactor assembly was surrounded by a vertical tube furnace, and temperatures were measured by Type K thermocouples encased in thin quartz tubes. A Eurotherm microprocessor temperature controller (model 810) was used to control the temperatures to within (1 °C of the set points. Since hydrogen was out of the mass range of the GCMS, the possible existence of hydrogen in the effluent stream was determined by a Carle model 111-H analytical gas chromatograph (GC) equipped with a 1.83 m molecular sieve 5A, a 80/100 mesh column, and a hydrogen transfer tube (HTT). The HTT was operated at 600 °C with N2 (50 mL/min (STP)) and He (27 mL/ min (STP)) used as carrier gases. It was found that, under the reaction conditions used in this work, hydrogen was not detected to within the highest resolution of the GC (∼0.3%). 2.3. Procedures for Permeation Measurements and Activity Tests. After the membrane disk was loaded, the reactor was heated to 850 °C at 15 °C/min, with helium and argon flowing through either chamber of the reactor at the same rate of 50 mL/min (STP). The effectiveness of the gold gasket seal was evaluated by holding the temperature at 850 °C for 20 min and then comparing the inlet and exit flows in both chambers. During the course of an experimental run, the disk integrity was continuously verified by periodically introducing at least 10% argon in an oxygen source on

1292 Ind. Eng. Chem. Res., Vol. 37, No. 4, 1998

Figure 1. Membrane reactor setup.

one side and then analyzing for Ar in the exit stream at the other side. The measurements of oxygen permeation in the nonreducing environment were conducted under various gradients of oxygen vs nitrogen across the membrane and at temperatures of 760-950 °C. The oxygen flux was determined using eq 2, where Fbtm is the total flow

JO2 [mL/min/cm2 (STP)] ) Fbtm [mL/min (STP)]yO2 [%] A [cm2]

(2)

rate of the bottom stream in which the oxygen concentration is yO2 and A is the effective membrane surface area. The OCM activity tests were carried out at 850 °C under various oxygen/methane gradients. In this case, carbon balances were used to determine the validity of the activity data, which were required to be within (2%. The oxygen permeation rates were calculated based on both hydrogen and oxygen balances. The catalytic performances were evaluated in terms of the production rate of C2+ hydrocarbons, which is defined in eq 3, and

rC2+ )

outlet flow rate of C2+ hydrocarbons membrane disk surface area [mL/min/cm2 (STP)] (3)

in terms of its selectivity to C2+ hydrocarbons, which is defined in eq 4. The flow rates of methane and oxygen

SC2+ ) moles of CH4 converted to C2+ hydrocarbons × moles of CH4 converted to all carbon products 100% (4) were both varied to ensure that reaction rates were not influenced by gas-solid mass-transfer rates, and all experiments were conducted with flow rates of 33.4 mL/ min/cm2 (STP). At these flow rates, methane conversions were low and thus the methane pressures at the

methane side of the membrane were essentially constant. Methane conversions were kept below 3% for all experiments, thus allowing for accurate and repeatable measurements of C2 production rates. 2.4. Perovskite Characterization. In situ “dynamic X-ray diffraction” (DXRD) was used to examine the phase developments occurring in perovskite powder samples under various temperature-composition conditions. Postreacted membrane samples were also analyzed by XRD at room temperature in air. The equipment consisted of a Siemens D-500 θ - 2θ powder diffractometer equipped with a Co KR1 (1.7890 Å) source of radiation, a flow-through Anton-Parr hot stage, and a position-sensitive detector capable of rapid scanning (60°/min) at high resolutions (0.01°). In the case of powder samples, a thin layer (less than 0.2 mm) of the perovskite was placed on the top of a platinum strip which was electrically heated at a controlled rate of 2 °C/min. Membrane disk samples were fixed on a special strip to set the top surface of the membrane at the same position as powder samples. A type S thermocouple attached to the strip is used to monitor and control the temperature by means of Microstar temperature controller model 828D. Changes in membrane surface morphology were characterized with a high-resolution JEOL JSM-6400 scanning electron microscope (SEM), which was equipped with secondary and backscattered electron detectors and a high-performance, low-noise image processing system. Larger surface etchings were also characterized with a Nikon Microflex HFX-II optical microscope. 3. Results and Discussion 3.1. Powder Sample Stability in Reducing and Nonreducing Environments. Oxygen partial pressure is one of the major factors that influences the chemical stability of nonstoichiometric perovskites. Taking LSCF-6428 as an example, it tends to lose its lattice oxygen at high temperatures and low oxygen partial pressures (Stevenson et al., 1997) according to reaction C: Consequently, “oxygen-free” environments created during reactor operations could force reaction C to the

Ind. Eng. Chem. Res., Vol. 37, No. 4, 1998 1293 k1/k2

L0.6Sr0.4Co0.2Fe0.8O3 798 δ L0.6Sr0.4Co0.2Fe0.8O3-δ + O2 (C) 2 right-hand side to an extreme extent and cause excessive oxygen loss from the perovskite structure. Although the reaction paths could be complex (Pei et al., 1995; Mizusaki et al., 1991; Nakamura et al., 1979), the eventual result of this process would be the following reduction reactions:

La0.6Sr0.4Co0.2Fe0.8O3-δ f 0.3La2O3 + 0.4SrO + 0.2CoO + 0.4Fe2O3 + (0.15-0.5δ)O2 (D) La0.6Sr0.4Co0.2Fe0.8O3-δ f 0.3La2O3 + 0.4SrO + 0.2CoO + 0.8Fe + (0.75-0.5δ)O2 (E)

Figure 2. DXRD results on LSCF-6428 powder in different gases. L ) La2O3, P ) La0.6Sr0.4Co0.2Fe0.8O3-δ perovskite. The sample was (1) fresh powder at room temperature, (2) calcined in He at 850 °C for 8 h, (3) calcined in Ar at 850 °C for 8 h, (4) calcined in N2 at 850 °C for 12 h, and (5) calcined in CH4 at 760 °C for 0.5 h.

which could eventually cause fractures in the disk mechanical structure. The purpose of this phase of the investigation was to investigate the stability of LSCF6428 powder in oxygen-free environments such as pure He, Ar, or N2 at high temperatures by means of DXRD. By using TGA, Stevenson et al. (1997) studied the influence of oxygen partial pressure on the oxygen nonstoichiometry of LSCF-6428 and did not observe any reduction reactions or phase changes at temperatures as high as 1250 °C and under oxygen pressures as low as 10 Pa. However, a distinctive cubic-to-orthorhombic phase transition was observed by Pei et al. (1995) with the Sr(Co,Fe)Ox system at 800 °C in pure N2. Since thermodynamics predicts that an oxygen partial pressure of 10 Pa would be sufficiently high so as to prevent phase changes at high temperatures, it is of interest to find out if a similar phase transition will happen with LSCF-6428 under an oxygen-free environment. The experiments were conducted by heating the sample strip at a rate of 1.2 °C/min after the DXRD reaction chamber was purged with a specific gas (150 mL/min (STP)) for 1.5 h at room temperature. The temperature was then kept constant at 850 °C for 8-12 h, depending on the specific gas being used. The DXRD results obtained in pure He, Ar, and N2 at the end of this experiment are shown as curves 2-4 in Figure 2, respectively. For comparison purposes, the results of fresh powder scanned at room temperature in air and a sample calcined in CH4 at 760 °C for 30 min are also shown in the figure as curves 1 and 5, respectively. As can be seen, no phase changes were observed when the LSCF-6428 powder was heated at 850 °C in He or Ar for 8 h or in N2 for 12 h. This demonstrates that partial substitution of the A site (La3+) with Sr2+ in LSCF-6428 can maintain its chemical stability in an inert gas at 850 °C, whereas complete substitution in the Sr(Co, Fe)Ox system is not stable under similar conditions, as observed by Pei et al. (1995). On the other hand, when the sample was exposed to methane at 760 °C for 30 min, a phase transformation (curve 5 in Figure 2) occurred. The new peaks formed in the 2θ region from 31° to 40° were identified as La2O3, indicating that reduction reactions (D or both D and E) took place. 3.2. Oxygen Permeation under O2 vs N2 Gradient. 3.2.1. Transition toward Steady-State Oxygen Permeation. The study of the initial transient phenomenon of oxygen permeation over oxygen-defect perovskite membranes is important to understanding their permeation mechanisms. During this transient

period, processes such as facilitation of oxygen vacancies, movement of their concentration front, and orderdisorder phase transitions will take place and are influential to the long-term stability of these perovskite materials. However, literature reports are currently very limited on the duration of this period and how the defect structure and oxygen flux would change during this period. In this work, the initial and steady-state oxygen permeations were studied under air (top) vs N2 (bottom) gradient over three fresh LSCF-6428 membrane disks. Since the ultimate purpose of this study is to understand the permeation behavior of perovskite membranes for the OCM reaction, a base temperature of 850 °C was selected, close to the optimum temperature for these materials in terms of C2+ selectivity (Xu and Thomson, 1997b). The thicknesses and on-stream times of these disks are listed in Table 2. According to the results plotted in Figure 3, the three disks took 15-22 h to reach their steady-state oxygen permeation and, during this period of time, oxygen permeation rates increased from their initial values by 47-82%. The variations in the steady-state fluxes between the three disks are attributed to surface morphology, as will be discussed below. The increase in oxygen flux during the transient period is due to the gradual facilitation and movement of an oxygen-defect structure from the oxygen-lean side of the membrane to the oxygen-rich side. As the oxygendefect zone spreads toward the oxygen-rich surface, the bulk oxygen permeation resistance gradually decreases with on-stream time, until a steady-state oxygen-defect (δ) gradient is established. Consistent with this phenomenon, it took 22 h for the thicker disk (no. 2) to reach a steady-state defect structure, while steady state was established in 15 h in both disk nos. 1 and 3 which were thinner. During a study of La0.6Sr0.4CoO3-δ, Kruidhof et al. (1993) observed that the oxygen flux decreased by a factor of 8 within 100 h at 750 °C. This observation is not contradictory to our results considering the thermal history of their membrane. Prior to these measurements, they had used the same disk at temperatures as high as 925 °C for a series of permeation and thermodynamic experiments, which would be expected to create a fully developed, high-temperature, oxygendefect structure inside the disk. At the lower temperature of 750 °C, this structure could become more ordered and give rise to the dramatic decrease in oxygen

1294 Ind. Eng. Chem. Res., Vol. 37, No. 4, 1998 Table 2. List of Disks Studied under an Air/N2 Gradient at 850 °C disk no.

L [mm]

on-streama [h]

unstable periodb [h]

PO2 (N2 side) [Pa]

JO2 (steady state) [mL/min/cm2]

1 2 3

1.57 1.88 1.65

79.4 101.9 41.6

15 22 15

229-414 207-365 216-318

0.137 ( 0.003 0.121 ( 0.005 0.105 ( 0.005

a The total length of time in which the disk was studied under an air/N gradient at 850 °C. After this study, disk no. 2 was tested 2 under an air/CH4 gradient and disk no. 3 was tested at different temperatures in an air/N2 gradient. b The length of time for the oxygen flux to reach 98% of its steady-state value.

Figure 3. Oxygen flux under an air/N2 gradient at 850 °C.

Figure 4. Surface defects on a fresh LSCF-6428 disk: (a) regular surface; (b) surface defects; (c) surface hole.

flux. However, as we will discuss later, the same process over LSCF-6428 occurs very rapidly, unlike the long times reported for La0.6Sr0.4CoO3-δ. 3.2.2. Influence of Surface Defects. As shown in Figure 3 and in Table 2, the steady-state oxygen permeation rates under an air/N2 gradient at 850 °C differed by about 30% in the three disks. One possible explanation would be that the variation in the thicknesses of the disks could result in differing defect gradients in the disks. However, as can be seen from Table 2, there is no systematic dependence of the steadystate oxygen flux on disk thickness. Furthermore, the results of our studies (Xu and Thomson, 1997a,b) have demonstrated that the oxygen flux over LSCF-6428 perovskite membranes at 850 °C is primarily controlled by surface ion-exchange rates and is almost independent of membrane thickness. Similar conclusions were reached by ten Elshof et al. (1995) and van Hassel et al. (1994) for La0.6Sr0.4Co0.8Fe0.2O3-δ and La1-ySryFeO3-δ perovskites, respectively. Consequently, surface morphology and, in particular, surface oxygen defects would be expected to control the oxygen flux. Figure 4 shows an electron micrograph of a typical surface texture for an LSCF-6428 disk membrane which clearly indicates the difference between (a) regular

surface, (b) surface defects, and even (c) surface holes. On the basis of more extensive SEM studies, we found that the total amount and the distribution of surface defects varied from disk to disk. Because of the irregularity of surface defect locations, direct and quantitative comparison of surface morphology for these three particular disks is not available. Nonetheless, based on the differences in the steady-state oxygen fluxes shown in Figure 3, it is likely that the differences are due to variations in surface morphology during synthesis of the membrane disks. 3.2.3. Irreversibility of a Fully Developed, Oxygen-Defect Structure. As discussed above, LSCF perovskites needed at least 15 h to establish its steadystate oxygen nonstoichiometric chemical structure. To determine the stability and reversibility of the defect structure, a series of treatments on disk no. 2 were conducted after its steady state was established at 850 °C as indicated by the letters, A-D, in Figure 3. After the disk had been used under air/N2 at 850 °C for 68 h, it was cooled to 25 °C and then was heated back to 850 °C under the same gradient with both cooling and heating rates of 15 °C/min (“A” in Figure 3). As can been seen, the oxygen permeation immediately reached steady state following this treatment and had the same oxygen permeation rate as before the treatment. This indicates that once the oxygen-defect structure is established at high temperatures, it remains stable, at least during relatively fast temperature changes (15 °C/min). During experiments B-D, the disk was treated under different combinations of cooling and heating gradients including an air/N2 gradient and an oxygen-free Ar/N2 gradient. As shown in Figure 3, at the end of these treatments there was still no change in the oxygen flux. From a thermodynamic point of view, changes between oxygen stoichiometric and nonstoichiometric structures of LSCF perovskites are reversible, as indicated in reaction C. Even though temperature changes would be expected to influence the oxygen-defect structure, the relatively rapid heating and cooling cycles associated with experiments A-D did not provide sufficient time for the structure to change. That is, the kinetics of oxygen-defect activation at temperatures below 850 °C was too slow in these experiments. 3.2.4. Effect of Temperature. It is well-known that, with LSCF-6428, different oxygen-defect structures are established at different temperatures (Stevenson et al., 1997). However, as the data in Figure 3 show, the development of oxygen-defect structures inside LSCF-6428 membranes at 850 °C is a slow process. Obviously, if the oxygen permeation process is controlled by surface ion exchange, the response of flux to temperature should be almost instantaneous since surface reaction kinetics are very sensitive to temperature. Consequently, a series of experiments to determine the response of oxygen flux with temperature were conducted on disk no. 3.

Ind. Eng. Chem. Res., Vol. 37, No. 4, 1998 1295

Figure 5. Effect of temperature on oxygen permeation over disk no. 3 (heating/cooling rate ) 15 °C/min).

As shown in Figure 5, after being tested under air/ N2 at 850 °C for 42 h (step 1), the disk was heated to 963 °C (step 2) and kept at this point for 44 h (step 3). It was then cooled to 701 °C (step 4), followed by heating to 850 °C (step 5) under the same gradient. The temperatures were maintained at each set point between 701 and 963 °C for 1.5 h, before it was changed to the next set point at a heating/cooling rate of 15 °C/ min. According to the results obtained in step 3, a new steady-state oxygen flux was achieved instantly, even as the temperature was raised to 963 °C. It appears that, once a fully defect structure is established at high temperatures (850 °C), the structure is flexible and able to keep pace with rapid temperature changes (15 °C/ min) at higher temperatures (850-963 °C). At these higher temperatures, an instant response of oxygen flux to temperature change would also be observed whether the permeation process is rate-limited by bulk diffusion or under mixed control of both diffusion and surface reactions. In fact, analysis by Xu and Thomson (1997b) have demonstrated that the oxygen permeation is under mixed control at high temperatures (>900 °C), while surface exchange exerts dominant control at temperatures below 850 °C. Additionally, the fact that identical flux curves are obtained for both cooling (step 4) and heating (steps 2 and 5) demonstrates the reversibility and chemical stability of the membrane against temperature change. The response of oxygen flux to temperature changes yielded an apparent activation energy of 159 kJ/mol over the temperature range from 700 to 960 °C. According to the results shown in Figure 5 (step 4), a sharp decrease of oxygen flux with decreasing temperature was observed between 750 and 960 °C. However, after the temperature decreased below 750 °C, the oxygen flux leveled off to an extremely low value of 0.005 mL/min/cm2 (STP). Since, as Xu and Thomson (1997b) have shown, the activation energy for surface exchange (∼235 kJ/mol) is relatively high compared to that for bulk diffusion (74 kJ/mol), it is likely that the surface

ion-exchange rate decreases dramatically at temperatures below 750 °C. After completion of the oxygen permeation experiments, disks nos. 1 and 3 were unloaded from the membrane reactor and subjected to SEM analysis. With both disks and on either side of the membrane, there were no significant differences between the used membrane surfaces and the fresh disks. Since there were also no disk cracks detected with argon throughout the permeation tests, it was confirmed that the LSCF-6428 perovskite membranes were chemically and mechanically stable under air/N2 gradients at temperatures below 960 °C. 3.3. Oxygen Permeation under O2 vs CH4 Gradient. In the course of applying a Sr(Co,Fe)Ox membrane for methane conversion to synthesis gas, serious reduction of the perovskite membrane was observed by Pei et al. (1995) at high temperatures (800-950 °C). This was attributed to the strong reducibility of the synthesis gas (CO + H2) generated in the reaction. Another reason for this instability was the absence of La3+ at the A site in the Sr(Co,Fe)Ox system, since, as pointed out by ten Elshof et al. (1995), La3+ at the A site promotes the chemical stability of the perovskite structure. Thus, even though partial substitution of La3+ with Sr2+ is able to create more oxygen vacancies (Teraoka et al., 1985, 1988), 60% of La3+ was maintained at the A site of LSCF-6428 in this work. To determine its surface stability in a methane-reducing environment, experiments were conducted on three disks at 850 °C under different O2/CH4 gradients as indicated in Table 3. Before these disks were exposed to the reducing environment, disk no. 2 had been used at 850 °C under an air/N2 gradient for 102 h while disk no. 4 and 5 were fresh disks. 3.3.1. Effect of Methane Presence on Oxygen Permeation. If methane is present at the oxygen-lean side of the membrane, two surface reactions will competitively consume oxygen ions transported from the air side of the membrane-methane activation (reaction F) and recombination of oxygen ions (reaction G) (Xu and Thomson, 1997a) where h• represents electron holes; • •• 2CH4 + 2h• + O× O f 2CH3 + H2O + VO

(F)

•• • 2O× O + 4h f O2 + 2VO

(G)

•• O× O, lattice oxygen; and VO, oxygen vacancies. If surface reactions on the oxygen-lean side of the membrane are the rate-limiting steps for oxygen permeation, this will result in an increase in oxygen flux. A comparison of oxygen flux with and without methane on the oxygenlean side of the membrane is shown in Figure 6, for disk no. 2. As shown in this figure, when methane was first introduced into the reactor, the oxygen flux instantly jumped from 0.12 mL/min/cm2 (STP) under an air/N2 gradient to 0.26 mL/min/cm2 (STP), indicating that the oxygen flux was increased by the addition of reaction F. This fact also demonstrates that the permeation

Table 3. List of Disks Studied under a Reducing Environment of CH4 at 850 °C disk no.

thickness [mm]

PO2 (top) [atm]

PCH4 (bottom) [atm]

time before CH4 [h]

time with CH4 [h]

2 4 5

1.88 1.73 1.77

0.21 0.21 0.80

1.0 1.0 0.1

101.9 0 0

77.0 51.8 48.5a

a

Total time period under an O2/CH4 gradient, in which the time for the initial permeation stability test was 25.2 h.

1296 Ind. Eng. Chem. Res., Vol. 37, No. 4, 1998

Figure 6. Effect of the presence of methane on oxygen flux at 850 °C over disk no. 2.

process under an air/CH4 gradient at 850 °C is controlled by surface reaction rates. However, with the presence of methane, the oxygen flux started to increase with on-stream time and reached a steady-state value of 1.29 mL/min/cm2 (STP), which was almost 5 times higher than the initial flux value, about 20 h later. This dramatic increase in oxygen flux is much higher than the increase observed due to the development of the oxygen-defect structure (Figure 3). Although some of this increase could be due to a further increase in the oxygen deficiency, δ, this would be small compared to the measured increase exhibited in Figure 6. Consequently, since the data indicate that surface reactions seem to be the controlling step, a detailed examination of the membrane surfaces was conducted. 3.3.2. Mechanism of Surface Etching. After being used at 850 °C under an air/CH4 gradient for 77 h, disk no. 2 was unloaded and analyzed with an optical microscope. It was found that more than 50% of the membrane surface on the methane side was significantly etched (Figure 7b) as compared to the fresh surface (Figure 7a). However, the surface on the oxygen side of the membrane was not significantly different from the fresh surface. A more detailed analysis with SEM showed that those etched areas consisted of piles of loose particles attached to the surface, as shown in Figure 8a,b. Interestingly, the individual particle sizes (2-4 µm) were in the same range as that of the LSCF6428 perovskite powder before being compressed into membrane disks. The SEM results also clearly demonstrate the multilayered nature of the etched surface, which was as thick as 10 µm. This thickness was suitable for X-ray analysis, and the XRD results on the used disk no. 2 at the methane side showed no evidence of phase change (Figure 9). Thus, the individual particles shown in Figure 8 appear to be single-phase perovskite powders. Note also that after the loose particles are removed, slightly fused particles still remain (left-hand side of Figure 8c). The right-hand side of Figure 8c compares this with an adjacent surface, which was not etched. Given the fact that the etching phenomenon occurs only at the methane side of the membrane and produces particles which are single-phase perovskites of the same size as the original powder, we hypothesize that undetectable surface reduction takes place at points of localized stress which causes the surface to “de-sinter”. That is, under the pressure of thermal expansion at high temperatures, the surface gradually becomes de-sin-

Figure 7. Different extents of surface etching compared with fresh surface: (A) fresh surface; (B) disk no. 2; (C) disk no. 4.

tered until all focal points of stress are broken and a new surface stress balance is established. Since a steady state appears to be reached (Figure 6), either the process ceases after relieving the surface stresses or continual de-sintering and attrition of particles maintain an etched surface of approximately 10 µm. Since our experiments were not conducted for a sufficiently long time, this question is unresolved at this point. The etched surface on the methane side of the membrane produces a higher surface area and more surface defects than a regular surface, which dramatically increases both methane activation (reaction F) and oxygen ion recombination (reaction G), and consequently the oxygen permeation rate. The behavior of the oxygen flux with time, as shown in Figure 6, corresponds to the initial expansion of surface etching and the eventual establishment of a new surface stress balance which results in a higher, stable oxygen flux. Surface stability and oxygen permeation tests were also conducted on another disk (disk no. 4) which was exposed only to an O2/CH4 gradient under the same conditions (850 °C, O2/CH4 gradient, PCH4 ) 1 atm) as for disk no. 2. The transient behaviors of the oxygen flux for these two disks are compared in Figure 10, where it can be seen that disk no. 4 has a steady-state flux which is about 50% higher than disk no. 2. Furthermore, it took 36 h for disk no. 4 to reach its steady-state value, while it only took 20 h for disk no. 2 to do so. The postreacted surface morphologies of

Ind. Eng. Chem. Res., Vol. 37, No. 4, 1998 1297

Figure 9. DXRD results on disk no. 2 before and after it was used under an air/CH4 gradient: (A) fresh surface; (B) used surface at the O2 side; (C) used surface at the CH4 side; (Au) gold gaskets attached to disks; (P) perovskite.

Figure 10. Comparison of oxygen permeation over disk nos. 2 and 4.

Figure 8. SEM results on the membrane surface exposed to methane at 850 °C. (A) loose surface and surface particles; (B) surface with particles; (C) surface after removing particles.

these two disks are also compared in Figure 7b,c. As can be seen, the surface of disk no. 4 at the methane side was almost completely etched after being used for 52 h, while the etching process only occurred on half the surface of disk no. 2 during 77 h on stream. By correlating these observations to the oxygen flux results shown in Figure 10, we can infer that disk no. 4 had more surface stress spots which were distributed more evenly and were more difficult to be break as compared with disk no. 2. It should be noted that, as indicated in Table 3, disk no. 2 had been annealed at 850 °C in nonreducing environments for 102 h before being exposed to the O2/CH4 gradient. Although this process did not cause surface etching, it is possible that the extensive exposure to high temperatures results in an “annealing” effect which reduces the strength and quantity of focal stress points on the membrane surfaces. This is probably why the etching process on the

nonpretreated disk no. 4 was slower but more extensive than that on the annealed disk no. 2. 3.3.3. Improvement on Surface Stability. As already discussed, surface etching under O2/CH4 gradients may be due to slight methane reduction on the very surface of the membranes. If this hypothesis is true, lower methane partial pressure should prevent or, at least, delay the etching process. This hypothesis was confirmed by an experiment conducted on disk no. 5 which was exposed only to an O2/CH4 gradient at 850 °C with PCH4 ) 0.1 atm for 48 h. At the conclusion of this experiment, SEM and optical microscope analysis indicated that the surface of the used disk was very similar to that of a fresh disk as shown in Figures 4 and 7a, indicating that very little surface etching took place. The oxygen flux through disk no. 5 is plotted in Figure 11 and shows the same transition process toward steady-state oxygen permeation with increasing oxygen deficiency (δ) as was seen in Figure 3, where the disks were under O2/N2 gradients from time ) 0. However, it only took about 7 h for disk no. 5 to reach its steadystate flux, implying that surface reaction control took over earlier under an O2/CH4 gradient than under an O2/N2 gradient. However, because of the low methane partial pressure, the steady-state oxygen flux caused by the methane activation reaction on the nonetched surface is essentially the same as that observed with an O2/N2 gradient (Figure 3) and 30 times lower than the measured value at PCH4 ) 1 atm (Figure 6).

1298 Ind. Eng. Chem. Res., Vol. 37, No. 4, 1998

Figure 11. Oxygen flux over disk no. 5 at 850 °C with PCH4 ) 0.1 atm.

structure. Although the transition periods were longer for thicker disks, systematic dependence of the steadystate oxygen flux on the membrane thickness was not observed, implying that the steady-state oxygen permeation across LSCF-6428 at 850 °C was probably controlled by surface reactions taking place on surface defects. It has also been demonstrated that, once the oxygen-defect structure is fully developed, the magnitude of the oxygen flux is able to respond quickly to further temperature changes with an apparent activation energy of 159 kJ/mol. Different extents of surface etching (from 50% to almost 100%) have been observed on the methane side of LSCF-6428 membranes after being exposed to O2/CH4 gradients at 850 °C for over 50 h. On the basis of XRD and SEM results together with the data on oxygen permeation, we hypothesize that the surface etching results from the breakup of focal points of stress on the membrane surface which is probably initiated by undetectable degrees of reduction of the perovskite with methane. Experiments at PCH4 ) 0.1 atm demonstrate that a lower methane partial pressure is able to delay or possibly prevent the etching process. The highly etched membrane surface at PCH4 ) 1 atm appears to increase both methane activation and oxygen ion recombination rates and consequently leads to dramatic increases in oxygen flux (a factor of 5 for disk no. 2), a further indication of the surface nature of the controlling step in oxygen permeation at 850 °C. Since the increase in C2+ production rate from 0.02 to 0.05 mL/min/cm2 (STP) is accompanied by a decrease in C2+ selectivity from 40 to 10%, it can be inferred that surface etching increases the oxygen recombination rate even more than the OCM rate. The nonetched surface at lower methane pressures can provide higher and more stable C2+ selectivity but at the cost of C2+ production rate. Acknowledgment

Figure 12. OCM activity at 850 °C with disk nos. 4 and 5.

The production rate of C2+ hydrocarbons (RC2+) and the selectivities of the OCM reaction are compared for disk nos. 4 and 5 in Figure 12. With respect to OCM activity, the steady-state rate of C2+ formation is 5 times higher at the higher methane pressure but the C2+ selectivity drops continuously from 40 to 10% over a 40-h period. On the other hand, the C2+ selectivity at PCH4 ) 0.1 atm remains stable in disk no. 5 at about 45%. Thus, since the rate of C2+ production increases at the same time that selectivity decreases, it can be concluded that surface etching increases the oxygen ion recombination rate more than the OCM rate. Therefore, lower methane pressures provide surface stability and higher C2+ selectivities but at the cost of C2+ production rates. 4. Conclusions It has been experimentally observed that oxygen permeation over LSCF-6428 perovskite membranes under air/N2 gradients at 850 °C requires 15-22 h to reach steady state. During this period of time, the oxygen flux increases 47-82% from its initial value due to the gradual development of an oxygen-deficient

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Received for review October 29, 1997 Revised manuscript received January 20, 1998 Accepted January 26, 1998 IE970761J