Methane oxidation on the surface of mixed-conducting La0.3Sr0.7Co0.8Ga0.2O3-δ

Catalysis Communications 5 (2004) 311–316 www.elsevier.com/locate/catcom

Methane oxidation on the surface of mixed-conducting La0:3Sr0:7Co0:8Ga0:2O3-d V.V. Kharton a,b,*, V.A. Sobyanin c, V.D. Belyaev c, G.L. Semin c, S.A. Veniaminov c, E.V. Tsipis a, A.A. Yaremchenko a, A.A. Valente d, I.P. Marozau b, J.R. Frade a, J. Rocha d b

a Department of Ceramics and Glass Engineering, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal Institute of Physicochemical Problems, Belarus State University, 14 Leningradskaya Str., 220050 Minsk, Belarus c Boreskov Institute of Catalysis RAS, 5 Av. Akademika Lavrentieva, Novosibirsk 630090, Russia d Department of Chemistry, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal

Received 10 December 2003; received in revised form 29 March 2004; accepted 29 March 2004 Available online 30 April 2004

Abstract Mixed-conducting La0:3 Sr0:7 Co0:8 Ga0:2 O3-d (LSCG) possesses substantial oxygen permeability, but exhibits a high activity to complete CH4 oxidation, thus making it necessary to incorporate reforming catalysts in the membrane reactors for methane conversion. Dominant CO2 formation is observed for the steady-state conversion of CH4 by atmospheric oxygen (methane/air ratio of 30:70) in a fixed bed reactor with LSCG as catalyst, and for the oxidation of CH4 pulses supplied in helium flow over LSCG powder. The conversion of dry CH4 by oxygen permeating through dense LSCG ceramics, stable operation of which under the air/ CH4 gradient is possible due to the surface-limited oxygen transport, yields CO2 concentrations higher than 90%. The prevailing mechanism of total methane combustion is probably associated with weak Co–O bonding in the perovskite-related LSCG lattice, in correlation with data on oxygen desorption, phase stability and ionic transport. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Methane oxidation; Ceramic membrane; Mixed conductor; Carbon dioxide selectivity; Perovskite

1. Introduction Dense ceramic membranes with mixed oxygen-ionic and electronic conductivity are of great interest for the conversion of natural to synthesis gas (syngas), a mixture of CO and H2 [1–4]. Conventional technologies for methane conversion are based on steam reforming, which is energy-intensive due to the highly endothermic nature of the reaction, and/or on partial oxidation, which requires significant capital investments for an oxygen plant. On the contrary, the membrane electrocatalytic reactors combine oxygen separation, partial oxidation and reforming in one single step. The mate*

Corresponding author. Tel.: +351-234-370263; fax: +351-234425300. E-mail address: [email protected] (V.V. Kharton). 1566-7367/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2004.03.010

rials of ceramic membranes should satisfy numerous requirements, including high oxygen permeability, stability under operation conditions, moderate thermal and chemically-induced expansion, and high catalytic activity with respect to the partial oxidation and reforming reactions [1,3,4]. One promising candidate is the perovskite-type La0:3 Sr0:7 CoO3-d phase exhibiting a very high level of oxygen transport compared to other mixedconducting oxides [5,6], although the thermodynamic stability of LaCoO3 -based solid solutions at reduced p(O2 ) is limited [7]. In fact, higher oxygen permeation was reported only for Bi2 O3 -based composite materials and several perovskite-related compounds derived from A(Co,Fe)O3-d (A ¼ Sr, Ba), which possess, however, poor mechanical properties and are even less stable than La0:3 Sr0:7 CoO3-d [1,5]. Doping of (La,Sr)MO3-d (M ¼ Fe, Co) perovskites with metal cations having a constant

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oxidation state, such as Ga3þ or Al3þ , is known to suppress oxygen nonstoichiometry variations and, thus, lattice expansion induced by changing the temperature and oxygen chemical potentials [3,8,9]. Continuing our research on electrocatalytic and electrochemical phenomena in the ion-conducting membrane reactors [10–12], the present work is centered on the study of methane oxidation processes on the surface of La0:3 Sr0:7 Co0:8 Ga0:2 O3-d (LSCG) ceramics. The results of detailed physicochemical characterization of the title material were reported elsewhere [9]. In this paper the properties of La0:3 Sr0:7 Co0:8 Ga0:2 O3-d are compared to those of SrFe0:7 Al0:3 O3-d , another prospective membrane material where doping with Al is used to stabilize strontium ferrite lattice, similar to the Ga incorporation into La0:3 Sr0:7 CoO3-d .

2. Experimental The submicron powder of perovskite-type La0:3 Sr0:7 Co0:8 Ga0:2 O3-d was synthesized via the glycine-nitrate process (GNP), a self-combustion technique using glycine as fuel and nitrates of the metal components as oxidant [13]. Glycine was added to an aqueous solution containing metal nitrates in the stoichiometric proportion (glycine/nitrate molar ratio of 2.0). After drying and self-combustion, the powder was annealed in air at 1073 K for 2 h and then ball-milled; the disc-shaped samples were pressed at 250–300 MPa. The single-phase ceramics with 96.5% density were sintered in air at 1533 K for 2 h with subsequent slow cooling to achieve equilibrium oxygen nonstoichiometry. Dense membranes of SrFe0:7 Al0:3 O3-d (SFA) were prepared by similar technique with sintering at 1623 K. The X-ray diffraction (XRD) analysis showed formation of single cubic perovskite phases in both cases; the unit cell parameters are given in Table 1. The grain size estimated by the scanning electron microscopy (SEM), was 30–70 lm for LSCG and 15–20 lm for SFA ceramics. As the methane oxidation studies on the powders were aimed to analyze processes occurring on the surface of dense ceramic membranes, the powdered samples for catalytic tests were prepared by grinding of ceramic samples, thus

providing a relatively low surface area (Table 1). The cation composition of ceramics and powders was verified by inductively-coupled plasma (ICP) spectroscopic analysis and energy-dispersive spectroscopy (EDS). Materials characterization included also the measurements of steady-state oxygen permeation fluxes, determination of total conductivity and Seebeck coefficient, and dilatometry. The description of experimental techniques and equipment, used for characterization, was published elsewhere [6,8–12]. The catalytic activity of powdered samples in the process of steady-state methane oxidation by atmospheric oxygen was assessed in a conventional quartz fixed-bed flow reactor at 973–1073 K. A mixture of CH4 and air (30:70 vol%), where the methane/oxygen ratio was close to the stoichiometric value for methane-tosyngas reaction, was supplied into the reactor (catalyst weight of 0.50 g; flow rate of 1 cm3 /s). The composition of the influent and effluent gas mixtures was analyzed by gas chromatographs with molecular-sieve and PorapakQ columns. For all temperatures, the measurements were continuously performed until achieving a steadystate; the results obtained on heating and on cooling were identical within the limits of experimental error. The interaction of powders with methane at reduced oxygen chemical potentials was studied using a pulse microcatalytic set-up [14] in flowing helium. Prior to the experiments, the samples (0.40 g) were annealed in oxygen at 1023 K for 1.5 h. Then a helium flow (0.69 cm3 / s) started to pass through the reactor; the amount of desorbed oxygen was determined as function of time. After annealing in flowing helium during approximately 1 h, a pulse CH4 probe (1 cm3 STP) was injected during 1 s into the flow. The reaction products were frozen by liquid nitrogen during 3 min in two sorbent-filled traps, one of which was used to collect CO2 , H2 O and C2hydrocarbons and another collected, CO, H2 , CH4 , N2 and O2 . After the pulse, the traps were rapidly heated and the products were analyzed; then next CH4 pulse was injected and the procedure was repeated. For each pulse, the methane conversion was lower than 2%. The studies of dry methane oxidation on dense diskshaped membranes were performed using a set-up described earlier [9,11], where a LSCG membrane

Table 1 Properties of mixed-conducting membrane materials Composition

Structure

Unit cell  parameter (A)

Surface area of powders used for catalytic experiments (m2 /g)

Relative density of ceramic membranes (%)

Average thermal expansion coefficients in air T (K)

a
106 (K1 Þ

La0:3 Sr0:7 Co0:8 Ga0:2 O3-d

Cubic perovskite

3.871(1)

0.035

96.5

360–710 710–1030

15.9  0.5 27.9  0.4

SrFe0:7 Al0:3 O3-d

Cubic perovskite

3.901(4)

0.030

99.8

370–920 920–1220

15.39  0.07 23.95  0.08

V.V. Kharton et al. / Catalysis Communications 5 (2004) 311–316

30 25 20

La0.3Sr0.7Co0.8Ga0.2O3-δ

CO2 O2 C2

T = 1023 K

5 0 2

3

4

5 3

3

1

60 50

SrFe0.7Al0.3O3-δ T = 1023 K

40

Oxygen amount, cm

3

10

6

7

8

La0.3Sr0.7Co0.8Ga0.2O3-δ

1.5

T = 1023 K m = 0.4 g

1.0 0.5 0.0

SrFe0.7Al0.3O3-δ 0

40

80

30

(b)

20

H2 CO CO2 C2

10 2

3

4

5

Number of pulse

Table 2 Methane conversion by atmospheric oxygen and selectivities to Ccontaining products formed on the surface of La0:3 Sr0:7 Co0:8 Ga0:2 O3-d powder in steady-state regime

973 1023 1073

120

time, min

1

The steady-state methane oxidation by atmospheric oxygen on the surface of La0:3 Sr0:7 Co0:8 Ga0:2 O3-d powder was found to result in a dominant CO2 formation (Table 2). The CO selectivity and methane conversion both increase with temperature, achieving 5.5% and 6.3% at 1073 K, respectively. As the conversion efficiency is relatively low, these results show a major role of complete CH4 oxidation reaction on LSCG surface. A very similar trend was observed in the pulse reactor where the main products include CO2 and C2-hydrocarbons, the concentrations of which are essentially independent of time (Fig. 1). In addition to these products, the methane oxidation on LSCG powder was accompanied with progressive evolution of molecular oxygen. Contrary, the interaction of CH4 pulses with SrFe0:7 Al0:3 O3-d yields mainly syngas with H2 :CO ratio close to 2, the optimum value required for further synthesis of hydrocarbons or methanol; no traces of O2 were detected (Fig. 1(b)).

T (K)

9

2.0

0

3. Results and discussion

(a)

15

Amount in 1 cm pulse, mm

(effective surface area of about 60 mm2 ; thickness of 0.95 mm) was hermetically sealed onto one yttria-stabilized zirconia (YSZ) tube. The feed- and permeate-sides were exposed to atmospheric air and flowing CH4 –He (50:50 vol%) mixture, respectively. In order to increase CO selectivity [10] and to facilitate surface processes which become oxygen flux-limiting in reducing atmospheres [1,15], porous Pt layers (sheet density of 8.6 mg
cm2 ) were applied onto the permeate-side surface. The influent and effluent gas mixtures were analyzed by a Varian CP-3800 gas chromatograph with a semi-capillary CarboPLOT P7 column; the oxygen chemical potential at the reactor outlet was determined by an electrochemical sensor. No impurities in the initial gas mixture and no leaking of air into the cell were detected. The carbon imbalance between the gas flows at the inlet and outlet, monitored in the course of experiments, was within the limits of experimental uncertainty (5%). The reaction selectivity was calculated as the concentration ratio between a given product and the sum of all detected Ccontaining products, namely CO, CO2 , C2 H6 , C2 H4 and C 2 H2 .

313

CH4 conversion (%)

Selectivity (%) CO

CO2

C2-hydrocarbons

2.42 3.41 6.27

1.47 2.11 5.47

97.06 92.83 83.89

1.47 5.06 10.64

Fig. 1. Product concentrations in 1 cm3 CH4 pulses supplied in flowing He onto the powders of La0:3 Sr0:7 Co0:8 Ga0:2 O3-d (a) and SrFe0:7 Al0:3 O3-d (b) at 1023 K. Inset shows time dependencies of the oxygen amount desorbed from the samples, pre-oxidized at 1023 K in air.

Such a difference in the catalytic behavior indicates the presence of large amounts of weakly-bonded oxygen both on the surface and in the lattice of La0:3 Sr0:7 Co0:8 Ga0:2 O3-d , and also a relatively fast diffusion of oxygen ions from the bulk lattice of this material. Indeed, the amount of oxygen evolved in helium flow from the pre-oxidized LSCG is 3–4
larger than that desorbed by SrFe0:7 Al0:3 O3-d , as illustrated by the inset in Fig. 1. Analogously, the oxygen permeation fluxes through dense LSCG ceramics are 3–5
higher than that for SFA (Fig. 2(a)). For both materials, the oxygen permeability is determined by bulk ionic conduction and surface exchange rates at the membrane/gas interface (e.g. [9]). Since the oxygen vacancy concentration in these perovskites is high enough, the ionic conductivity is primarily affected by ion mobility, a function of the bond energy between oxygen and B-site cations in the ABO3 perovskite lattice [1,5,8]. The Co–O bonds are weaker with respect to Fe–O, leading to faster surface exchange and higher oxygen-ion mobility in LSCG compared to doped strontium ferrite. Fig. 2(b) presents approximate stability boundaries of La0:3 Sr0:7 Co0:8 Ga0:2 O3-d at reduced oxygen partial pressures, estimated from results on the total conductivity (r) and Seebeck coefficient (a) vs. p(O2 ) in

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La0.3Sr0.7Co0.8Ga0.2O3-δ , 1023 K

-2

La0.3Sr0.7Co0.8Ga0.2O3-δ

-7.5

5

La0.3Sr0.7Co0.8Ga0.2O3-δ perovskite

(b)

log σ (S/cm)

d = 1.0 mm p2 = 21.2 kPa p1 = 2.7 kPa

SrFe0.7Al0.3O3-δ

2

phase mixture

log p(O2) (Pa)

brownmillerite

perovskite: T=300 K, p(O2)=21 kPa

1

0

brownmillerite: T=1123 K, p(O2)=4.5 Pa 20

40

60

2Θ, La0.3Sr0.7Co0.8Ga0.2Ox brownmillerite

CH4-conversion

-10 CoO/Co

-15 -20 7.5

SrFe0.7Al0.3O3-δ "FeO"/Fe

8.0

8.5

9.0

9.5

4

-1

10.0

10.5

-1 -15

-50

<

0 -5

0

perovskite

-6.9 -7.2

>

3

-10

-5

0

o

80

-100

-α , µV/K

-1

-6.6

Intensity

log j (mol × s × cm )

4

(a)

-6.3

-150

100

5

-200

log p(O2) (atm) Fig. 3. Example of the oxygen partial pressure dependence of total conductivity and Seebeck coefficient of La0:3 Sr0:7 Co0:8 Ga0:2 O3-d at 1023 K. Dashed lines indicate approximate phase stability boundaries. Inset shows XRD patterns of La0:3 Sr0:7 Co0:8 Ga0:2 O3-d ceramics after oxidation in air (top) and after annealing at p(O2 ) corresponding to the brownmillerite phase domain (bottom).

10 /T, K

Fig. 2. Temperature dependence of the oxygen permeation fluxes through La0:3 Sr0:7 Co0:8 Ga0:2 O3-d and SrFe0:7 Al0:3 O3-d membranes under a fixed oxygen partial pressure gradient (a), and the low-p(O2 ) stability limits of La0:3 Sr0:7 Co0:8 Ga0:2 O3-d and SrFe0:7 Al0:3 O3-d phases (b) compared to typical p(O2 ) values in the membrane reactor for dry CH4 oxidation. Literature data on the stability limits of CoO and Fe1-x O [16] are shown for comparison.

combination with XRD. As an example, Fig. 3 illustrates the determination of stability limits from the data on electrical properties at 1023 K. The oxygen pressure, at which the slope of log r  log pðO2 Þ and a  log pðO2 Þ dependencies started to change, was considered as a phase stability boundary at a given temperature. The XRD analysis of LSCG samples annealed at oxygen partial pressures around these boundaries confirms the presence of distinct phase domains. The cubic perovskite phase of La0:3 Sr0:7 Co0:8 Ga0:2 O3-d transforms on reduction into a brownmillerite-type polymorph isostructural to Sr2 Co2 O5 (PDF card 34-1475); representative examples of the XRD patterns are shown in the inset of Fig. 3. Further reduction leads to the formation of several phases including (Sr,La)3 (Co,Ga)2 O6 - and (La,Sr)2 (Co,Ga)O4 -based solid solutions and CoO; then, at p(O2 ) values close to Co/CoO boundary (Fig. 2(b)), the oxide phase mixture is reduced to metallic cobalt and binary metal oxides. This decomposition mechanism is quite similar to that reported for perovskite-type La0:3 Sr0:7 CoO3-d at 1373 K [7]. Compared to the brownmillerite LSCG polymorph, SrFe0:7 Al0:3 O3-d decomposes at oxygen pressures 107 – 108
lower. Again, the difference in the phase stability

reflects different B–O bond energy, which is higher for the Fe-containing material. Another factor reflecting the metal-oxygen bond strength in perovskite-related compounds is thermal expansion [8]. Table 1 lists average thermal expansion coefficients (TECs) calculated from dilatometric data in air. The apparent increase of thermal expansion on heating is mainly associated with progressive oxygen losses, which cause additional lattice expansion due to decreasing oxidation state of B-site cations and the corresponding increase in their radii. In the low-temperature range the TECs of LSCG and SFA are similar, whilst higher thermal expansion of the Co-containing ceramics at temperatures above 900 K correlates with greater oxygen losses (inset in Fig. 1). In spite of the expansion mechanism, the high-temperature TECs of both materials are incompatible with those of common construction materials such as alumina or stainless steels; their use is hence possible only in tubular reactors, with hermetization in a low-temperature zone. It should be separately mentioned that the perovskite and brownmillerite modifications of LSCG are both thermodynamically unstable at the oxygen partial pressures characteristic of the effluent gas mixtures of membrane reactors for dry methane conversion; these non-equilibrium p(O2 ) values estimated using the electrochemical sensor are shown in Fig. 2(b) by black squares. Under these conditions, if the oxygen chemical potential on the membrane permeate-side surface would be equal to that in the gas phase, the LSCG membrane should decompose. However, no time degradation in the performance of Co-containing membranes exposed to

V.V. Kharton et al. / Catalysis Communications 5 (2004) 311–316

flowing dry methane during more than 50 h was observed; the subsequent SEM/EDS analysis revealed no traces of bulk reduction. Moreover, the XRD pattern of the membrane permeate-side surface after electrocatalytic experiments (inset in Fig. 5) showed that the perovskite-type phase was not even reduced into brownmillerite. Only minor peaks of Pt catalyst and La2 O3 , marked by asterisks in the XRD pattern, were observed in addition to the major perovskite reflections; the formation of lanthanum oxide is caused, most likely, by cation demixing under oxygen chemical potential gradient (e.g. [1]). Such a kinetic stabilization is possible when the steady-state oxygen permeation is controlled by kinetics of surface processes, either recombination of lattice oxygen into gaseous O2 or oxidation reactions on the membrane surface. In these conditions, the oxygen chemical potential on the membrane permeate-side should be considerably higher than that in the gas environment, preventing reduction of the mixed-conducting material. Indeed, data on the oxygen fluxes through La0:3 Sr0:7 Co0:8 Ga0:2 O3-d membranes with various thickness confirm a significant permeation-limiting role of surface exchange processes; selected examples are presented in Fig. 4. The values of specific oxygen permeability J(O2 ), given in Fig. 4(b), are related to the permeation flux density j as [6,12]   J ðO2 Þ p2  ln j¼ ; ð1Þ d p1

315

where d is the membrane thickness, p1 and p2 are the oxygen partial pressures at the membrane permeate- and feed-sides, respectively. Since the quantity J(O2 ) is, by definition, proportional to j
d, the specific permeability would be thickness-independent in the case of negligible effect of the processes at the membrane/gas boundaries. However, the data on LSCG membranes unambiguously indicate that oxygen transport is determined by both bulk ambipolar conduction and oxygen surface exchange. The permeation fluxes decrease with increasing d, while J(O2 ) increases due to a decreasing role of the exchange rate. In addition, inspection of the data (Fig. 4) shows that limiting effect of the exchange processes increases with increasing temperature and reducing oxygen pressure. At 1223 K the permeation fluxes are almost completely controlled by the surface, being almost thickness-independent. This phenomenon enables stable operation of LSCG membranes under air/ CH4 gradient. The general trends, observed in a model membrane reactor for dry methane conversion (Fig. 5), are similar to those exhibited by LSCG powder in the course of catalytic experiments (Fig. 1 and Table 2). Increasing temperature leads to higher CH4 conversion and CO selectivity, but in all cases the formation of carbon dioxide is predominant. At 1123–1223 K the selectivity to CO and to C2-hydrocarbons was found lower than 6% 5 60

La0.3Sr0.7Co0.8Ga0.2O3-δ

4

50

× -6.6

40

1123 K

30

-6.9 La0.3Sr0.7Co0.8Ga0.2O3-δ p2 = 21 kPa d = 1.00 mm d = 1.40 mm

-7.2

log J(O2) (mol×s-1×cm-1)

1223 K

-7.4

50% CH4 - 50% He

20

1223 K

6

after membrane operation

30

5 25

*

* *

40

2Θ,

20

CO o

60

80

4 C2 3

50% CH4 - 50% He 3

flow rate 4.4 cm /min -8.0

(b)

0.0

1123 K 0.4

0.8

1.2

log p2/p1 Fig. 4. Oxygen permeation fluxes (a) and specific oxygen permeability (b) of La0:3 Sr0:7 Co0:8 Ga0:2 O3-d membranes as functions of the oxygen partial pressure gradient, temperature and membrane thickness.

1

3

flow rate 2.1 cm /min

15

-7.8

3 2

C2

20

-7.6

CO

(a)

Selectivity, %

(a)

CH4 conversion, %

log j (mol×s-1 cm-2)

-6.3

1110

1140

1170

(b)

1200

1230

2

T, K Fig. 5. Temperature dependencies of the methane conversion efficiency and the selectivities to CO and C2-hydrocarbons in the reactor with La0:3 Sr0:7 Co0:8 Ga0:2 O3-d membrane modified with porous Pt layer on the permeate-side surface. Inset shows XRD pattern recorded at the permeate-side surface after the operation under air/CH4 gradient.

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V.V. Kharton et al. / Catalysis Communications 5 (2004) 311–316

and 5%, respectively. As expected from thermodynamics, the yields of carbon monoxide and C2 products increase with increasing methane flow, when the oxygen chemical potential and CH4 conversion decrease. The observed behavior may indicate that the oxidation reactions occur primarily on the LSCG membrane surface; the effect of porous platinum, which is a wellknown reforming catalyst [10], seems rather minor. Such assumption was supported by the data on a LSCG membrane without Pt layer, where the conversion rate was 1.5–3
lower than that for the surface-modified ceramics. At the same time, no substantial changes in the CO selectivity were observed for the unmodified membrane. This suggests that platinum layers increase oxygen permeation due to enhancement of the surface exchange, but have no essential effect on mechanism of the oxidation processes. The methane oxidation processes on the surface of LSCG powder and ceramics seem to occur via similar mechanisms. Most likely, these reactions mainly involve total CH4 oxidation on the membrane surface and subsequent reforming of residual methane by CO2 and steam, in agreement with the so-called CRR (combustion and reforming reaction) mechanism. The absence of O2 traces in the gas mixtures at the membrane reactor outlet suggests that all permeated oxygen is consumed at the first stage of reaction, full combustion; the second stage, reforming, occurs then in the gas phase and might be stagnated. Although more detailed studies are necessary to reveal roles of each reaction stage, all data on CH4 conversion rate and CO/CO2 ratio as functions of the influent gas flow and methane partial pressure, obtained in this work, were found to support CRR mechanism. In fact, CO2 formation dominated within all range of experimental conditions, where the LSCG membranes remain stable and sufficient oxygen fluxes can be obtained. In summary, La0:3 Sr0:7 Co0:8 Ga0:2 O3-d membranes possess relatively high oxygen permeability and are kinetically stable under air/CH4 gradient due to surfacelimited oxygen transport; their use in a model reactor resulted in CH4 conversion efficiency up to 57%. However, the methane oxidation on LSCG surface, either by oxygen supplied from the crystal lattice or by gaseous O2 , leads to high yields of carbon dioxide. This makes it necessary to develop thick porous layers and/or reactor

packing with reforming catalysts in order to increase CO selectivity.

Acknowledgements This work was supported by the NATO Science for Peace program (Project 978002), the FCT, Portugal (POCTI program and Projects BD/6827/2001 and BPD/ 11606/2002), and the INTAS (Project 00276). Helpful discussions and experimental contributions, made by A. Shaula, A. Viskup and N. Vyshatko, are gratefully acknowledged. References [1] H.J.M. Bouwmeester, A.J. Burggraaf, in: A.J. Burggraaf, L. Cot (Eds.), Fundametals of Inorganic Membrane Science and Technology, Elsevier, Amsterdam, 1996, pp. 435–528. [2] T.J. Mazanec, R. Prasad, R. Odegard, C. Steyn, E.T. Robinson, Stud. Surf. Sci. Catal. 136 (2001) 147. [3] M. Schwartz, J.H. White, A.F. Sammells, US Patent 6214757, 2001. [4] P.N. Dyer, R.E. Richards, S.L. Russek, D.M. Taylor, Solid State Ionics 134 (2000) 21. [5] V.V. Kharton, A.A. Yaremchenko, E.N. Naumovich, J. Solid State Electrochem. 3 (1999) 303. [6] V.V. Kharton, A.V. Kovalevsky, A.A. Yaremchenko, F.M. Figueiredo, E.N. Naumovich, A.L. Shaulo, F.M.B. Marques, J. Membrane Sci. 195 (2002) 277. [7] V.A. Cherepanov, L.Ya. Gavrilova, L.Yu. Barkhatova, V.I. Voronin, M.V. Trifonova, O.A. Bukhner, Ionics 4 (1998) 309. [8] V.V. Kharton, A.A. Yaremchenko, M.V. Patrakeev, E.N. Naumovich, F.M.B. Marques, J. Eur. Ceram. Soc. 23 (2003) 1417. [9] V.V. Kharton, E.V. Tsipis, I.P. Marozau, A.A. Yaremchenko, A.A. Valente, A.P. Viskup, J.R. Frade, E.N. Naumovich, J. Rocha, J. Solid State Electrochem., 2004 (accepted for publication). [10] V.A. Sobyanin, V.D. Belyaev, V.V. Gal’vita, Catal. Today 42 (1998) 337. [11] A.A. Yaremchenko, A.A. Valente, V.V. Kharton, I.A. Bashmakov, J. Rocha, F.M.B. Marques, Catal. Commun. 4 (2003) 477. [12] A.A. Yaremchenko, V.V. Kharton, M.V. Patrakeev, J.R. Frade, J. Mater. Chem. 13 (2003) 1136. [13] L.A. Chick, L.R. Pederson, G.D. Maupin, J.L. Bates, L.E. Thomas, G.J. Exarhos, Mater. Lett. 10 (1990) 6. [14] V.P. Shchukin, S.A. Veniaminov, G.K. Boreskov, Kinet. Catal. 12 (1971) 621. [15] S.J. Xu, W.J. Thomson, AICHE J. 43 (1997) 2731. [16] E. Jacobsson, E. Rosen, Scand. J. Metall. 10 (1981) 39.