- Email: [email protected]
La0.6Sr0.4Co0.2Fe0.8O3 as an anode for direct methane activation in SOFCS
Solid State Ionics 113–115 (1998) 247–251
La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 as an anode for direct methane activation in SOFCS M. Weston*, I.S. Metcalfe Department of Chemical Engineering, University of Edinburgh, Mayfield Road, Edinburgh, EH9 3 JL, UK Received 3 September 1998; accepted 14 September 1998
Abstract This study aims to investigate the use of La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 as a prospective anode material that can operate directly off methane fuel. The reaction of methane over the material has been investigated using a novel measurement system to obtain simultaneous catalytic and gravimetric information. This gravimetric system relies on sensing the change in the natural frequency of a quartz tube that houses the catalyst sample and allows a wide range of in-situ gravimetric experiments to be performed. The weight loss suffered by the catalyst indicates some potential degree of thermal instability with the reduction of oxide occurring as vacancies are produced. The results effectively show how increasing defect concentration effects catalytic activity and selectivity during reaction over a LSCFO oxide surface. 1998 Published by Elsevier Science B.V. All rights reserved. Keywords: Perovskites; LSCFO; SOFC; Thermogravimetric; Anodes
1. Introduction Several metal oxides with the perovskite structure and of the form ABO 3 (A 5 lanthanide or alkaline earth metal, B 5 transition metal) have been shown to be good catalysts for oxygen reduction and hydrocarbon combustion [1]. They have also received much interest as the perovskite lattice allows a wide variation in cation substitution with only minimal changes in structure, so is ideal for the examination of composition-activity relationships. As well as catalytic performance changing the cation can also effect the conductivity (both electrical and ionic) and the sintering behaviour. This allows
*Corresponding author. E-mail: [email protected]
materials to be developed with optimal properties for many different applications. Our interest in these perovskite materials is as prospective candidates for anodes in solid oxide fuel cells (SOFCs), in particular materials with mixed conduction that can operate effectively using methane at reduced temperatures without a degradation in performance. The advantage of a mixed conducting material is that it may extend the available three phase boundary (gas / electrode / electrolyte) over more of the electrode surface. It is important that this material is also sufficiently catalytically active towards the reaction proceeding on the electrode surface to minimise polarisation losses while being both thermally stable and chemically compatible with the electrolyte. These stringent requirements combined with fabrication considera-
0167-2738 / 98 / $ – see front matter 1998 Published by Elsevier Science B.V. All rights reserved. PII: S0167-2738( 98 )00377-4
248
M. Weston, I.S. Metcalfe / Solid State Ionics 113 – 115 (1998) 247 – 251
tions make the development of suitable new materials a challenging task, however the range of perovskite materials that are available for consideration and the continuing interest in these materials currently makes them ideal candidates for further research. To effectively investigate the range of possibilities requires a demanding test regime that must cover all aspects of the anode performance. This regime is typically comprised of a number of experiments, such as temperature programmed analysis of reaction kinetics and thermogravimetric analysis of the reduction / oxidation of the material. We have gained improvement to the effectiveness of our current catalyst testing procedure by using a highly accurate microbalance which operates on an unique principle based on the change in the resonant frequency of a tapered element. This principle eliminates the problems associated with conventional gravimetric setups which are not suitable for kinetic studies on catalytic systems because of problems with buoyancy effects in flow systems, which severely limit the resolution of such equipment. This new piece of apparatus allows us to combine these steps to allow a more robust and versatile system without a loss in accuracy or performance.
1.1. Operating principle of microbalance As a result of research and development by NASA for orbiting laboratory experiment, a new method of performing gravimetric measurements has been developed [2]. This method is capable of picogram resolution in flowing gases at a range of pressures from vacuum up to 70 atmospheres. The method is based upon an oscillating tubular sample holder (see Fig. 1) which operates in a clamped-free mode similar to that of a tuning fork. The system determines the change in mass of the material between times 0 and 1 according to the following equation: Dm 5 K0 [1 /f 12 2 1 /f 02 ]
(1)
Where K0 is a unique calibration constant of the tapered element and f0 and f1 are the natural oscillating frequencies at time 0 and a later time 1, respectively. It is easy to see that this equation has been derived from the equation of a cantilever beam, m5k /f 2 where m is the total oscillating mass, k is
Fig. 1. Principle of microbalance operation.
the spring constant and f is the natural frequency of the system. In this case the spring constant k is fixed by the geometry of the design and so masses can be determined from measurement of the natural vibrational frequency, since as the mass increases the oscillating frequency will decrease. The natural frequency is measured optically by a pair of sensors located on either side of the tapered element. This method reduces the effects of external influences such as vibration, change in room temperature and static electricity. The standard configuration of the microbalance is analogous to a packed bed reactor (see Fig. 1). Catalyst particles are held in the vibrating quartz tube and reactant gases are passed through the bed and a porous supporting membrane and the products of any reactions analysed downstream by a mass spectrometer. This set-up has the added advantage that the gases contact with the solid in a much more characterisable way than in other alternative gravimetric systems. It is our aim to investigate the state of catalysts under real operating conditions. Since the equipment can be operated under flow conditions accurate
M. Weston, I.S. Metcalfe / Solid State Ionics 113 – 115 (1998) 247 – 251
kinetic measurements can be simultaneously obtained with gravimetric measurements. This equipment is therefore ideal for the study of catalyst systems where the state of the catalyst is known to affect the kinetic behaviour. Such catalyst systems are often poorly understood because of the problems associated with obtaining a truly in-situ indication of the catalyst state. Such an approach can be extended to investigate the behaviour of oxide electrodes and catalysts. In a working electrochemical cell the emf is related to the defect concentrations. Gravimetric work provides complimentary data on defect behaviour. These techniques are combined with kinetic measurements which are sensitive to the defect concentration at the surface of the catalyst / electrode. The apparatus has a fast time response which allows transient behaviour to be observed from experiments such as potential pulse and gas switching. These experiments are particularly relevant to the understanding of oxide electrodes in solid oxide fuel cells, ceramic membrane reactors and gas sensing devices. This paper will focus mainly on the establishment and refinement of the necessary techniques required by this new system. For this study we have chosen a perovskite of the form La–Sr–Co–Fe–O which has been shown to exhibit many of the required characteristics in the desired temperature range. Work by Teraoka et al. [3] reported notable mixed conductivities and significant oxygen permeabilities at 8008C, while studies on methane oxidation by Balachandran and associates [4] illustrated its catalytic activity. The thermal stability in the anodic atmosphere is likely to be poor at higher temperatures [5] however we are interested in studying the reaction kinetics at lower temperatures since it is here that the kinetics of the reaction become increasingly important. It is also possible that the reduction of the catalyst may lead to the production of different catalytic sites, through the removal of lattice oxygen, which may subsequently lead to enhanced surface activity. A mechanism for the cycle of generation and destruction of these sites (V ??o ) is shown by the ¨ respective Kroger–Vink equations (Eqs. (2) and (3)) below, where O xo represents the lattice oxygen. CH 4 1 O o x → CO 1 2H 2 1V o?? 1 2e 2 1 ] 2
O 2 1V o?? 1 2e 2 → O ox
(2) (3)
249
2. Experimental
2.1. Fabrication and characterisation The La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 powder was produced by the combination of the relevant metal salts (La, Sr, Co acetate and Fe nitrate) at a high temperature (8508C) thus forming the perovskite via a solid state reaction. The composition of the material was confirmed by X-ray diffraction analysis and the surface area determined by a standard BET method at liquid N 2 temperature.
2.2. Sample preparation Each sample (|50 mg) of the catalyst was loaded onto the quartz element of the microbalance and tightly packed with quartz wool to restrict movement in order to minimise unwanted vibrations. The sample was then heated at 1508C for 30 min in a stream of helium (60 ml / min) to desorb any water present.
2.3. Combustion of methane A simple TPRx (temperature programmed reaction) experiment was carried out, where a mixture of methane (3%) and oxygen (6%) was passed over the catalyst and the extent of reaction as the temperature was increased (linearly at 108C / min) observed using a mass spectrometer (Leda-Mass).
2.4. Oxidation and reduction of catalyst A fresh catalyst sample was exposed to a reducing atmosphere (5% hydrogen) at 6008C for 30 min. The hydrogen was then quickly flushed out of the system by a stream of dry nitrogen. When the levels of hydrogen had fallen below the detection range of the mass spectrometer a flow of 5% oxygen was introduced. This was passed through the bed for a further 20 min at which point the oxygen was flushed out in an identical fashion. Methane (5%) was then fed across the bed for another 20 min before the sample was allowed to cool to room temperature in a stream of helium.
250
M. Weston, I.S. Metcalfe / Solid State Ionics 113 – 115 (1998) 247 – 251
2.5. Decomposition of methane over catalyst The sample from the previous experiment was exposed to a stream containing methane (5%) and heated to 6008C at a constant rate (108C / min) where it was maintained for several hours. This allowed the study of the long term effects of the reaction of the dry fuel gas over the catalyst surface. Any changes in the reaction during this time were examined by a mass spectrometer. The catalyst was then exposed to a flow of 5% oxygen to oxidise any decomposition products.
Fig. 3. Oxidation / Reduction of LSFCO in 5% CH 4 at 6008C.
3.2. Oxidation and reduction of catalyst 3. Results and discussion
3.1. Combustion of methane The data obtained from the temperature programmed experiments is shown in Fig. 2 and shows that the reaction to form CO 2 and water commenced at above 2008C but did not proceed rapidly until temperatures of over 5008C were reached. This is in good accordance with previous studies carried out using our dedicated TP apparatus and studies by other groups [6]. In general the temperature programming capabilities of the system were excellent, with good monitoring and control, however it proved difficult to analyse the gravimetric data produced during any significant change in temperature. This was due to the thermal expansion of the quartz element resulting in large frequency changes causing an apparent change in mass of several milligrams. Fortunately this change was found to be reproducible and progress is being made in resolving data produced under these conditions by removal of a baseline produced by control experiments.
In Fig. 3 the results acquired from a series of experimental runs are represented. As might be expected the catalyst gained mass as oxygen was adsorbed onto the surface and oxygen vacancies were filled. While in the nitrogen atmosphere an overall slight increase in mass is observed which has been attributed to an electrical drift resulting from imperfect tuning of the sensors. On addition of methane the mass quickly dropped as any absorbed and adsorbed oxygen was consumed and regeneration of oxygen vacancies occurred. The mass spectrometer detected a slight increase in the CO 2 signal during the reduction however it was too close to the background level to gain conclusive data and perform an accurate carbon balance. These results illustrate the general accuracy of the system since mass changes of 10 26 g and below were readily quantifiable. Kinetic information can be inferred from these changes in mass, such as rates of absorption and reaction. In this case the filling of the oxygen vacancies appears to proceed at a much greater rate than their generation during reduction. This suggests that during fuel cell operation the material may remain stable in the anodic atmosphere if sufficient oxygen is supplied through the electrolyte.
3.3. Decomposition of methane over catalyst
Fig. 2. TPRx between CH 4 and O 2 (1:2) over LSFCO.
Initial investigations showed that reduction of the catalyst was occurring as indicated by a small quantity of CO 2 being produced alongside a corresponding decrease in mass (Fig. 4). As the reduction continued at a constant 6008C this loss of mass was
M. Weston, I.S. Metcalfe / Solid State Ionics 113 – 115 (1998) 247 – 251
Fig. 4. Continued reduction of LSFCO in 5% CH 4 at 6008C.
seen to reverse. This reversal can be explained by considering the changes that are occurring on or near the oxide surface. Initially the reaction of the lattice oxygen with methane is favoured, this produces oxygen vacancies which subsequently may act as catalytic sites for competing reactions. One such reaction is the decomposition of methane to form solid carbon on the catalyst surface and hydrogen. It can be proposed that as the number of oxygen vacancies increases that the methane decomposition becomes more favoured until it dominates the surface reaction. Thus over time the catalytic activity of the perovskite will be seen to change. This opinion is supported by an increase in the amount of hydrogen being detected however a subsequent oxidation step did not produce a noticeable amount of CO 2 or give rise to a drop in mass, which suggests that any carbon deposited may be of a form which is stable at the temperatures used (up to 6008C). This observed reduction may prove beneficial since this could lead to the formation of a high dispersion of catalytically active sites over the material, with different phases possibly providing both mechanical and catalytic support.
4. Conclusions The combined temperature programming and gravimetric system has shown itself to be easy to use and accurate, however the technique still requires
251
significant refinement, particularly in the measurement of mass change during temperature ramping. The temperature range of operation is currently limited by the materials of construction of the system, however this temperature ceiling can be elevated and steps have been taken to achieve this. The weight loss suffered by the catalyst indicates some potential degree of thermal instability with the reduction of oxide occurring as vacancies are produced. Therefore LSCFO is unlikely to remain as a stable perovskite at higher temperatures except under tightly controlled conditions. Reduction and reoxidation studies indicate that the reoxidation process is relatively fast implying that such conditions may be feasible to maintain in practice. Furthermore the results obtained effectively show how increasing defect concentration effects catalytic activity and selectivity during reaction over a LSCFO oxide surface.
Acknowledgements MW would like to thank the EPSRC for financial assistance.
References [1] H. Arai, T. Yamada, K. Eguchi, T. Seiyama, Appl. Catal. 26 (1986) 265–276. [2] F. Hershkowitz, P.D. Madiara, Ind. Eng. Chem. Res. 32 (1993) 2969–2974. [3] Y. Terakoa, H. Zhang, S. Furukawa, N. Yamazoe, Oxygen Permeation Through Perovskite Type Oxides, Chem. Lett. (1985) 1743–1746. [4] U. Balachandran, J.T. Dusck, R.L. Mieville, R.B. Poeppel, M.S. Kleefisch, S. Pei, T.P. Kobylinski, C.A. Udovich, A.C. Bose, Appl. Catal. A 133 (1995) 19–29. [5] B. Steele, P.H. Middleton, B.A. Rudkin, Solid State Ionics 40–41 (1990) 388. [6] H. Zhang, Y. Shimiza, Y. Teraoka, N. Miura, N. Yamazoe, J. Catal. 121 (1990) 432–440.
La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 as an anode for direct methane activation in SOFCS M. Weston*, I.S. Metcalfe Department of Chemical Engineering, University of Edinburgh, Mayfield Road, Edinburgh, EH9 3 JL, UK Received 3 September 1998; accepted 14 September 1998
Abstract This study aims to investigate the use of La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 as a prospective anode material that can operate directly off methane fuel. The reaction of methane over the material has been investigated using a novel measurement system to obtain simultaneous catalytic and gravimetric information. This gravimetric system relies on sensing the change in the natural frequency of a quartz tube that houses the catalyst sample and allows a wide range of in-situ gravimetric experiments to be performed. The weight loss suffered by the catalyst indicates some potential degree of thermal instability with the reduction of oxide occurring as vacancies are produced. The results effectively show how increasing defect concentration effects catalytic activity and selectivity during reaction over a LSCFO oxide surface. 1998 Published by Elsevier Science B.V. All rights reserved. Keywords: Perovskites; LSCFO; SOFC; Thermogravimetric; Anodes
1. Introduction Several metal oxides with the perovskite structure and of the form ABO 3 (A 5 lanthanide or alkaline earth metal, B 5 transition metal) have been shown to be good catalysts for oxygen reduction and hydrocarbon combustion [1]. They have also received much interest as the perovskite lattice allows a wide variation in cation substitution with only minimal changes in structure, so is ideal for the examination of composition-activity relationships. As well as catalytic performance changing the cation can also effect the conductivity (both electrical and ionic) and the sintering behaviour. This allows
*Corresponding author. E-mail: [email protected]
materials to be developed with optimal properties for many different applications. Our interest in these perovskite materials is as prospective candidates for anodes in solid oxide fuel cells (SOFCs), in particular materials with mixed conduction that can operate effectively using methane at reduced temperatures without a degradation in performance. The advantage of a mixed conducting material is that it may extend the available three phase boundary (gas / electrode / electrolyte) over more of the electrode surface. It is important that this material is also sufficiently catalytically active towards the reaction proceeding on the electrode surface to minimise polarisation losses while being both thermally stable and chemically compatible with the electrolyte. These stringent requirements combined with fabrication considera-
0167-2738 / 98 / $ – see front matter 1998 Published by Elsevier Science B.V. All rights reserved. PII: S0167-2738( 98 )00377-4
248
M. Weston, I.S. Metcalfe / Solid State Ionics 113 – 115 (1998) 247 – 251
tions make the development of suitable new materials a challenging task, however the range of perovskite materials that are available for consideration and the continuing interest in these materials currently makes them ideal candidates for further research. To effectively investigate the range of possibilities requires a demanding test regime that must cover all aspects of the anode performance. This regime is typically comprised of a number of experiments, such as temperature programmed analysis of reaction kinetics and thermogravimetric analysis of the reduction / oxidation of the material. We have gained improvement to the effectiveness of our current catalyst testing procedure by using a highly accurate microbalance which operates on an unique principle based on the change in the resonant frequency of a tapered element. This principle eliminates the problems associated with conventional gravimetric setups which are not suitable for kinetic studies on catalytic systems because of problems with buoyancy effects in flow systems, which severely limit the resolution of such equipment. This new piece of apparatus allows us to combine these steps to allow a more robust and versatile system without a loss in accuracy or performance.
1.1. Operating principle of microbalance As a result of research and development by NASA for orbiting laboratory experiment, a new method of performing gravimetric measurements has been developed [2]. This method is capable of picogram resolution in flowing gases at a range of pressures from vacuum up to 70 atmospheres. The method is based upon an oscillating tubular sample holder (see Fig. 1) which operates in a clamped-free mode similar to that of a tuning fork. The system determines the change in mass of the material between times 0 and 1 according to the following equation: Dm 5 K0 [1 /f 12 2 1 /f 02 ]
(1)
Where K0 is a unique calibration constant of the tapered element and f0 and f1 are the natural oscillating frequencies at time 0 and a later time 1, respectively. It is easy to see that this equation has been derived from the equation of a cantilever beam, m5k /f 2 where m is the total oscillating mass, k is
Fig. 1. Principle of microbalance operation.
the spring constant and f is the natural frequency of the system. In this case the spring constant k is fixed by the geometry of the design and so masses can be determined from measurement of the natural vibrational frequency, since as the mass increases the oscillating frequency will decrease. The natural frequency is measured optically by a pair of sensors located on either side of the tapered element. This method reduces the effects of external influences such as vibration, change in room temperature and static electricity. The standard configuration of the microbalance is analogous to a packed bed reactor (see Fig. 1). Catalyst particles are held in the vibrating quartz tube and reactant gases are passed through the bed and a porous supporting membrane and the products of any reactions analysed downstream by a mass spectrometer. This set-up has the added advantage that the gases contact with the solid in a much more characterisable way than in other alternative gravimetric systems. It is our aim to investigate the state of catalysts under real operating conditions. Since the equipment can be operated under flow conditions accurate
M. Weston, I.S. Metcalfe / Solid State Ionics 113 – 115 (1998) 247 – 251
kinetic measurements can be simultaneously obtained with gravimetric measurements. This equipment is therefore ideal for the study of catalyst systems where the state of the catalyst is known to affect the kinetic behaviour. Such catalyst systems are often poorly understood because of the problems associated with obtaining a truly in-situ indication of the catalyst state. Such an approach can be extended to investigate the behaviour of oxide electrodes and catalysts. In a working electrochemical cell the emf is related to the defect concentrations. Gravimetric work provides complimentary data on defect behaviour. These techniques are combined with kinetic measurements which are sensitive to the defect concentration at the surface of the catalyst / electrode. The apparatus has a fast time response which allows transient behaviour to be observed from experiments such as potential pulse and gas switching. These experiments are particularly relevant to the understanding of oxide electrodes in solid oxide fuel cells, ceramic membrane reactors and gas sensing devices. This paper will focus mainly on the establishment and refinement of the necessary techniques required by this new system. For this study we have chosen a perovskite of the form La–Sr–Co–Fe–O which has been shown to exhibit many of the required characteristics in the desired temperature range. Work by Teraoka et al. [3] reported notable mixed conductivities and significant oxygen permeabilities at 8008C, while studies on methane oxidation by Balachandran and associates [4] illustrated its catalytic activity. The thermal stability in the anodic atmosphere is likely to be poor at higher temperatures [5] however we are interested in studying the reaction kinetics at lower temperatures since it is here that the kinetics of the reaction become increasingly important. It is also possible that the reduction of the catalyst may lead to the production of different catalytic sites, through the removal of lattice oxygen, which may subsequently lead to enhanced surface activity. A mechanism for the cycle of generation and destruction of these sites (V ??o ) is shown by the ¨ respective Kroger–Vink equations (Eqs. (2) and (3)) below, where O xo represents the lattice oxygen. CH 4 1 O o x → CO 1 2H 2 1V o?? 1 2e 2 1 ] 2
O 2 1V o?? 1 2e 2 → O ox
(2) (3)
249
2. Experimental
2.1. Fabrication and characterisation The La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 powder was produced by the combination of the relevant metal salts (La, Sr, Co acetate and Fe nitrate) at a high temperature (8508C) thus forming the perovskite via a solid state reaction. The composition of the material was confirmed by X-ray diffraction analysis and the surface area determined by a standard BET method at liquid N 2 temperature.
2.2. Sample preparation Each sample (|50 mg) of the catalyst was loaded onto the quartz element of the microbalance and tightly packed with quartz wool to restrict movement in order to minimise unwanted vibrations. The sample was then heated at 1508C for 30 min in a stream of helium (60 ml / min) to desorb any water present.
2.3. Combustion of methane A simple TPRx (temperature programmed reaction) experiment was carried out, where a mixture of methane (3%) and oxygen (6%) was passed over the catalyst and the extent of reaction as the temperature was increased (linearly at 108C / min) observed using a mass spectrometer (Leda-Mass).
2.4. Oxidation and reduction of catalyst A fresh catalyst sample was exposed to a reducing atmosphere (5% hydrogen) at 6008C for 30 min. The hydrogen was then quickly flushed out of the system by a stream of dry nitrogen. When the levels of hydrogen had fallen below the detection range of the mass spectrometer a flow of 5% oxygen was introduced. This was passed through the bed for a further 20 min at which point the oxygen was flushed out in an identical fashion. Methane (5%) was then fed across the bed for another 20 min before the sample was allowed to cool to room temperature in a stream of helium.
250
M. Weston, I.S. Metcalfe / Solid State Ionics 113 – 115 (1998) 247 – 251
2.5. Decomposition of methane over catalyst The sample from the previous experiment was exposed to a stream containing methane (5%) and heated to 6008C at a constant rate (108C / min) where it was maintained for several hours. This allowed the study of the long term effects of the reaction of the dry fuel gas over the catalyst surface. Any changes in the reaction during this time were examined by a mass spectrometer. The catalyst was then exposed to a flow of 5% oxygen to oxidise any decomposition products.
Fig. 3. Oxidation / Reduction of LSFCO in 5% CH 4 at 6008C.
3.2. Oxidation and reduction of catalyst 3. Results and discussion
3.1. Combustion of methane The data obtained from the temperature programmed experiments is shown in Fig. 2 and shows that the reaction to form CO 2 and water commenced at above 2008C but did not proceed rapidly until temperatures of over 5008C were reached. This is in good accordance with previous studies carried out using our dedicated TP apparatus and studies by other groups [6]. In general the temperature programming capabilities of the system were excellent, with good monitoring and control, however it proved difficult to analyse the gravimetric data produced during any significant change in temperature. This was due to the thermal expansion of the quartz element resulting in large frequency changes causing an apparent change in mass of several milligrams. Fortunately this change was found to be reproducible and progress is being made in resolving data produced under these conditions by removal of a baseline produced by control experiments.
In Fig. 3 the results acquired from a series of experimental runs are represented. As might be expected the catalyst gained mass as oxygen was adsorbed onto the surface and oxygen vacancies were filled. While in the nitrogen atmosphere an overall slight increase in mass is observed which has been attributed to an electrical drift resulting from imperfect tuning of the sensors. On addition of methane the mass quickly dropped as any absorbed and adsorbed oxygen was consumed and regeneration of oxygen vacancies occurred. The mass spectrometer detected a slight increase in the CO 2 signal during the reduction however it was too close to the background level to gain conclusive data and perform an accurate carbon balance. These results illustrate the general accuracy of the system since mass changes of 10 26 g and below were readily quantifiable. Kinetic information can be inferred from these changes in mass, such as rates of absorption and reaction. In this case the filling of the oxygen vacancies appears to proceed at a much greater rate than their generation during reduction. This suggests that during fuel cell operation the material may remain stable in the anodic atmosphere if sufficient oxygen is supplied through the electrolyte.
3.3. Decomposition of methane over catalyst
Fig. 2. TPRx between CH 4 and O 2 (1:2) over LSFCO.
Initial investigations showed that reduction of the catalyst was occurring as indicated by a small quantity of CO 2 being produced alongside a corresponding decrease in mass (Fig. 4). As the reduction continued at a constant 6008C this loss of mass was
M. Weston, I.S. Metcalfe / Solid State Ionics 113 – 115 (1998) 247 – 251
Fig. 4. Continued reduction of LSFCO in 5% CH 4 at 6008C.
seen to reverse. This reversal can be explained by considering the changes that are occurring on or near the oxide surface. Initially the reaction of the lattice oxygen with methane is favoured, this produces oxygen vacancies which subsequently may act as catalytic sites for competing reactions. One such reaction is the decomposition of methane to form solid carbon on the catalyst surface and hydrogen. It can be proposed that as the number of oxygen vacancies increases that the methane decomposition becomes more favoured until it dominates the surface reaction. Thus over time the catalytic activity of the perovskite will be seen to change. This opinion is supported by an increase in the amount of hydrogen being detected however a subsequent oxidation step did not produce a noticeable amount of CO 2 or give rise to a drop in mass, which suggests that any carbon deposited may be of a form which is stable at the temperatures used (up to 6008C). This observed reduction may prove beneficial since this could lead to the formation of a high dispersion of catalytically active sites over the material, with different phases possibly providing both mechanical and catalytic support.
4. Conclusions The combined temperature programming and gravimetric system has shown itself to be easy to use and accurate, however the technique still requires
251
significant refinement, particularly in the measurement of mass change during temperature ramping. The temperature range of operation is currently limited by the materials of construction of the system, however this temperature ceiling can be elevated and steps have been taken to achieve this. The weight loss suffered by the catalyst indicates some potential degree of thermal instability with the reduction of oxide occurring as vacancies are produced. Therefore LSCFO is unlikely to remain as a stable perovskite at higher temperatures except under tightly controlled conditions. Reduction and reoxidation studies indicate that the reoxidation process is relatively fast implying that such conditions may be feasible to maintain in practice. Furthermore the results obtained effectively show how increasing defect concentration effects catalytic activity and selectivity during reaction over a LSCFO oxide surface.
Acknowledgements MW would like to thank the EPSRC for financial assistance.
References [1] H. Arai, T. Yamada, K. Eguchi, T. Seiyama, Appl. Catal. 26 (1986) 265–276. [2] F. Hershkowitz, P.D. Madiara, Ind. Eng. Chem. Res. 32 (1993) 2969–2974. [3] Y. Terakoa, H. Zhang, S. Furukawa, N. Yamazoe, Oxygen Permeation Through Perovskite Type Oxides, Chem. Lett. (1985) 1743–1746. [4] U. Balachandran, J.T. Dusck, R.L. Mieville, R.B. Poeppel, M.S. Kleefisch, S. Pei, T.P. Kobylinski, C.A. Udovich, A.C. Bose, Appl. Catal. A 133 (1995) 19–29. [5] B. Steele, P.H. Middleton, B.A. Rudkin, Solid State Ionics 40–41 (1990) 388. [6] H. Zhang, Y. Shimiza, Y. Teraoka, N. Miura, N. Yamazoe, J. Catal. 121 (1990) 432–440.