Journal of Membrane Science 252 (2005) 215–225

Processing, stability and oxygen permeability of Sr(Fe, Al)O3-based ceramic membranes

V.V. Khartona,b,∗, A.L. Shaulaa, F.M.M. Snijkersc, J.F.C. Cooymansc, J.J. Luytenc,A.A. Yaremchenkoa, A.A. Valented, E.V. Tsipisa, J.R. Fradea, F.M.B. Marquesa, J. Rochad

a Department of Ceramics and Glass Engineering, CICECO, University of Aveiro, 3810-193 Aveiro, Portugalb Institute of Physicochemical Problems, Belarus State University, 14 Leningradskaya Str., 220050 Minsk, Belarus

c Materials Department, Flemish Institute for Technological Research (VITO), 2400 Mol, Belgiumd Department of Chemistry, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal

Received 1 September 2004; received in revised form 13 December 2004; accepted 17 December 2004Available online 2 February 2005

Abstract

inS ability ofS versionr Kw face-limitedo f the samec n fluxes. Fora d9©

K

1

obtcmcri

Gt

ttice.nifi-

lantscy ofnd to

ected

em-dientrons-

n thema-e/gascreas-ce

0d

Minor additions of alumina into perovskite-type SrFe0.7Al 0.3O3−δ, a composition close to the solid solution formation limitsrFe1−xAl xO3−δ system, result in decreasing thermal expansion and increasing oxygen permeability. The improved sinterrFe0.7Al 0.3O3-based composite with 3 wt.% Al2O3 addition enables to fabricate high-quality tubular membranes for the methane con

eactors. No essential degradation in the performance of SrFe0.7Al 0.3O3−δ membranes under air/CH4 or air/H2 H2O gradients at 973–1223as observed during 200–700 h. The stable operation under high oxygen chemical potential gradients is possible due to surxygen transport, indicated by the dependencies of oxygen permeability on the membrane thickness. Applying porous layers oomposition, synthesized via cellulose-precursor technique, onto the permeate-side surface leads to substantially higher oxygemodel reactor with the surface-modified SrFe0.7Al 0.3O3−δ membrane and commercial Ni/Al2O3 catalyst, the CH4 conversion rate achieve0–97% at 1073–1123 K, when the CO selectivity was almost 100%.2005 Elsevier B.V. All rights reserved.

eywords:Ceramic membrane processing; Mixed conductor; Surface activation; Oxygen permeation; Ionic transport; Methane oxidation

. Introduction

Technologies for oxygen separation from air and partialxidation of hydrocarbons, based on dense ceramic mem-ranes with mixed ionic-electronic conductivity, provide po-

ential economic and environmental benefits with respect toonventional industrial processes[1–6]. The advantages ofixed-conductive membranes for natural gas conversion in-

lude the possibility to integrate oxygen separation, methaneeforming and partial oxidation in a single reactor, and alsonfinite theoretical perm-selectivity to oxygen, resulting from

∗ Corresponding author. Present address: Department of Ceramics andlass Engineering, CICECO, University of Aveiro, 3810-193 Aveiro, Por-

ugal. Tel.: +351 234 370263; fax: +351 234 425300.E-mail address:kharton@cv.ua.pt (V.V. Kharton).

the only possible transport mechanism via the crystal laApplications of these technologies are expected to sigcantly decrease capital investments for gas-to-liquid pand for distributed hydrogen, to increase energy efficienoxygen-based combustion processes, and to recover ause remote gases that would otherwise be flared or re-inj[1–4].

Oxygen permeation through a mixed-conducting mbrane placed under an oxygen chemical potential graoccurs due to inducing fluxes of oxygen ions and electflowing in opposite directions[2,6,7]. For given external conditions, the resultant flux of molecular oxygen depends opartial ionic and electronic conductivities of the ceramicterial, and on rates of oxygen exchange at the membranboundaries. Decreasing membrane thickness leads to ining oxygen fluxes, whilst the relative limiting effect of surfa

376-7388/$ – see front matter © 2005 Elsevier B.V. All rights reserved.oi:10.1016/j.memsci.2004.12.018

216 V.V. Kharton et al. / Journal of Membrane Science 252 (2005) 215–225

exchange processes increases. The exchange limitations play,however, a positive role under largep(O2) gradients due tokinetic stabilization of membrane materials thermodynami-cally unstable in reducing environments[6–8]. Namely, whenthe overall oxygen transport is surface-limited, the oxygenchemical potential on the membrane permeate-side surfaceis always higher than that in the gas phase, thus prevent-ing reduction. Most mixed conductors providing commer-cially feasible permeation fluxes, in particular the materialsderived from perovskite-like (Sr,Ln)MO3−δ (M = Fe, Co), areunstable in the mixtures of CH4 conversion products underequilibrium conditions, but enable steady-state operation un-der air/CH4 gradients[7,9]. Furthermore, in most cases ad-ditional activation of the membrane surface is desirable toprovide sufficient permeation fluxes, due to slow kinetics ofthe exchange-related redox reactions in the CH4-containingatmospheres[9–11].

Another important aspect in the membrane developmentsis the dimensional stability of mixed-conducting ceramics.Losses of lattice oxygen under reducing environments and/oron heating result in unfavorable expansion of the lattice,which induces mechanical stresses and may cause fracture[2–4,12]. The thermal and chemical expansion can be sub-stantially decreased substituting iron and cobalt with metalcations having a more stable oxidation state under the reac-tor operation conditions, such as Cr[3,13–15], Ti [13,15,16],G to-i tiono de-c nduc-t ntm ials.T cificd tionwt ility,c f alu-m

tingmw -i n ofm nd tot dingo ility.

2

ep self-c s oft nthe-s con-t theg tric

one, assuming the only gaseous reaction products to be N2,CO2 and H2O. The solutions were heated on a hot plate un-til self-combustion. The resultant powders having a foamedstructure were annealed at 1173 K for 2 h to remove residualorganic substances, and then ball-milled. Gas-tight ceramicswere pressed at 100–600 MPa and sintered at 1490 to 1740 Kin air for 2–8 h; the dependencies of ceramics density on thepressure and sintering conditions are presented below. Aftersintering and polishing, the samples were annealed in air at1170–1270 K for 2–3 h and slowly furnace-cooled to achieveequilibrium with air at low temperatures. The tubular mem-branes were compacted by cold isostatic pressing (CIP) usinga Burton Corblin instrument (France), with pressure vesselof 90 mm in diameter and 500 mm in length.

The materials were characterized by X-ray diffraction(XRD), scanning electron microscopy combined with energydispersive spectroscopy (SEM/EDS), transmission electronmicroscopy (TEM), inductively coupled plasma (ICP) spec-troscopic analysis, standard picnometric technique, dilatome-try, and the measurements of steady-state oxygen permeationfluxes and total conductivity (four-probe dc). Experimentaltechniques and equipment, used for characterization, weredescribed elsewhere (Refs.[7–9,12,15,16,21,22]and refer-ences cited). The studies of oxygen permeation fluxes andmethane conversion under air/H2 H2O N2 and air/CH4 Hegradients were performed as described in Refs.[7,9,21]. Alld orre-s ssuree anest rica was6 -t f thet ther .

3

3S

dw aceg -p n theXci ne-m ForSAt than2 sol-ui the

a [12,15,17], or Zr [18], in order to suppress oxygen schiometry variations. At the same time, the incorporaf tetravalent cations into perovskite lattice significantlyreases oxygen permeation due to decreasing ionic coion [15,19]; in addition, a poor sinterability and insufficieechanical strength are typical for Ti-substituted materhe substitution with gallium is also associated with speisadvantages including possible volatilization, interacith catalysts such as Pt or Ni, and high costs[20]. One in-

eresting alternative, attractive in terms of dopant stabosts and accessibility of the raw materials, is the use oinum.Continuing previous research on the mixed-conduc

embranes for natural gas conversion[7–9,12,15,16], thisork is centered on the studies of Sr(Fe,Al)O3-based ceram

cs. Particular emphasis is focused on the optimizatioembrane compositions and processing conditions, a

he properties determining practical applications, incluxygen permeability, thermal expansion and phase stab

. Experimental

The powders of SrFe1−xAlxO3−δ (x= 0.1–0.5) werrepared via the glycine-nitrate process (GNP), aombustion method using glycine as fuel and nitratehe metal components as oxidant. In the course of syis, glycine was added in an aqueous nitrate solutionaining metal cations in the stoichiometric proportion;lycine/nitrate molar ratio was 1.5–2.0 of the stoichiome

ata on oxygen permeability presented in this paper cpond to the membrane feed-side oxygen partial prequal to 21 kPa (atmospheric air). For all model membr

ested under air/CH4–He gradient, the effective geometrea of the membrane surface exposed to methane flow3± 1 mm2; the results on CH4 oxidation in tubular reac

ors, presentation of which requires detailed analysis oemperature- and chemical potentials distribution alongeactor, will be summarized in subsequent publications

. Results and discussion

.1. Oxygen permeability and thermal expansion ofrFe(Al)O3−δ perovskites

XRD studies of SrFe1−xAlxO3−δ ceramics, equilibrateith air, showed formation of a cubic perovskite lattice (sproupPm3m). The materials atx≤ 0.3 were found singlehase, whereas minor impurity peaks were detected iRD patterns atx= 0.4–0.5. In the case of SrFe0.6Al0.4O3−δ

eramics the secondary phase was identified as Sr3Al2O6;ts amount, estimated from the results of Rietveld refi

ent of the XRD patterns, is approximately 0.85 wt.%.rFe0.5Al0.5O3−δ, the phase impurities include SrAl2O4,l 2O3, and another cubic perovskite marked asP′ in Table 1;

he total amount of the secondary phases is slightly lesswt.%. One may conclude, therefore, that the maximumbility of Al 3+ cations in the iron sublattice of SrFeO3−δ

s close to 35–37%. This estimate is in agreement with

V.V. Kharton et al. / Journal of Membrane Science 252 (2005) 215–225 217

Table 1XRD data and average thermal expansion coefficients of Sr(Fe,Al)O3-based ceramics

Composition Perovskite unit cell parameter (A) Secondary phases Average TECs in air

T (K) � × 106 (K−1)

SrFe0.9Al0.1O3−δ 3.882(6) – 373–823 16.4± 0.3823–1273 31.9± 0.3

SrFe0.8Al0.2O3−δ 3.889(3) – – –SrFe0.7Al0.3O3−δ 3.900(4) – 373–923 15.4± 0.1

923–1273 23.0± 0.1SrFe0.7Al0.3O3−δ + 3 wt.% Al2O3 3.890(1) SrAl2O4 373–923 12.9± 0.1

923–1273 19.8± 0.3SrFe0.6Al0.4O3−δ 3.905(3) Sr3Al2O4 – –SrFe0.5Al0.5O3−δ 3.906(9) SrAl2O4, Al2O3 andP′ 373–923 13.5± 0.1

923–1273 18.6± 0.2

variations of perovskite unit cell parameters and electricalconductivity (Table 1andFig. 1). The lattice parameters in-crease up tox= 0.4 and then become essentially independentof aluminum content. The total conductivity, predominantlyp-type electronic, decreases monotonically with increasingconcentration of insulating Al3+ cations incorporated intoiron sites. Atx= 0.4–0.5, however, the conductivity valuesare quite similar, indicating similar Fe:Al ratios in the ma-jor perovskite phases (Fig. 1). Notice that the solid solutionformation limit in La0.3Sr0.7Fe1−xGaxO3−δ system also cor-responds to approximatelyx≈ 0.35[23].

The segregation of secondary phases in SrFe1−xAlxO3−δ

(x= 0.4–0.5) was confirmed by SEM/EDS analysis; typicalSEM micrographs are presented inFig. 2A–D. Increasingaluminum content leads to a significant decrease in the grainsize, from 5–10�m atx= 0.1 down to 1–2�m atx= 0.5. Sup-pressing grain growth by alumina additions is well knownfor zirconia-based materials and enables 30–50% increasein the mechanical strength of Zr(Y)O2−δ [24]. Furthermore,SrFe(Al)O3−δ ceramics with small Al concentration exhibita tendency to microcrack formation at the grain boundaries,as illustrated byFig. 2A. These microcracks form in thecourse of sintering and subsequent cooling, primarily dueto high thermal expansion (Fig. 3) associated with extensive

Fc

changes of oxygen nonstoichiometry[12]. As a result, sin-tering of gas-tight SrFe1−xAlxO3−δ (x= 0.1–0.3) membraneswas only possible using relatively low heating/cooling rates,0.5–1.5 K/min. The substitution of 30–40% iron for Al3+ hav-

Fig. 2. SEM micrographs of SrFe0.9Al0.1O3−δ (A), SrFe0.7Al0.3O3−δ

(B), SrFe0.7Al0.3O3−δ with 3 wt.% Al2O3 addition (C), SrFe0.5Al0.5O3−δ

ceramics (D), SrFe0.7Al0.3O3−δ oxide fibers (E and F), as-preparedSrFe0.7Al0.3O3-� membrane with porous layer of the same composition(G), SrFe0.7Al0.3O3−δ membrane with porous layer after testing underair/10%H2–90%N2 gradient at 973 K during 715 h (H).

ig. 1. Temperature dependence of total conductivity of SrFe1−xAlxO3−δ

eramics in air.

218 V.V. Kharton et al. / Journal of Membrane Science 252 (2005) 215–225

Fig. 3. Dilatometric curves of SrFe1−xAlxO3−δ ceramics in air.

ing constant oxidation state, makes it possible to reduce oxy-gen content variations, as for (Sr,La)(Fe,Ga)O3−δ [12]. Theaverage thermal expansion coefficients (TECs) decrease withincreasing Al concentration, providing an enhanced stabilitywith respect to thermal cycling. At the same time, the TECsof SrFe1−xAlxO3−δ with x= 0.3–0.5 (18.6–23.0)× 10−6 K−1

at 923–1273 K (Table 1), are still high, which may hampergas-tight connections with other materials of membrane re-actors at elevated temperatures. The tubular-type membranegeometry with hermetization in a low-temperature zone ishence preferable for SrFe(Al)O3-based ceramics.

Single-phase SrFe1−xAlxO3−δ (x= 0.1–0.3) possess quitesimilar levels of the oxygen permeation fluxes (j) under afixed oxygen chemical potential gradient (Fig. 4). Furtherdoping leads to 2–3 times lower oxygen permeablity, thoughSrFe0.5Al0.5O3−δ has considerably lower activation energyfor oxygen transport at temperatures below 1050 K, when thetendency to formation of vacancy-ordered microdomains inthe oxygen sublattice of SrFeO3-based compounds becomes

F roughS ients.

Fig. 5. Comparison of the oxygen permeation fluxes throughSrFe0.7Al0.3O3−δ and other ferrite-based ceramics[15,16,23] underfixed oxygen partial pressure gradient.

critical [16]. The relatively low activation energy may orig-inate from the doping-induced disordering due to statisticaldistribution of Al3+ cations, locally distorting the lattice[23].If compared to other ferrite phases[15,16,23], the oxygen per-meability of SrFe0.7Al0.3O3−δ ceramics is slightly lower thanthe maximum, observed for Co-free perovskites with a highoxygen deficiency and a low degree of vacancy ordering. Thislevel of oxygen transport is, however, 102–103 times higherwith respect to Fe-containing phases with the intergrowth andgarnet-type structures (Fig. 5). Taking into account the dataon thermal expansion and oxygen permeability, perovskite-type SrFe0.7Al0.3O3−δ was selected as a model membranematerial to optimize the ceramic processing techniques andto study the methane oxidation processes.

3.2. Processing of Sr(Fe,Al)O3-based membranes

Fig. 6 illustrates the effects of pressure on the den-sity of green compacts and sintered disk-shaped samples,all made via the uni-axial pressing of GNP-synthesizedSrFe0.7Al0.3O3−δ powder. Although the green density in-creases with pressure, the density of ceramics after thermaltreatment is essentially pressure-independent and is deter-mined by the sintering conditions, including maximum tem-perature, time, heating/cooling rates and other features of thes d at1 u-l stratedp heo-r -i chan-it

ityw ntoS x-ie han-

ig. 4. Temperature dependence of the oxygen permeation fluxes thrFe1−xAlxO3−δ membranes under fixed oxygen partial pressure grad

intering profiles. The highest density could be obtaine473–1523 K (Fig. 7A). However, isostatically pressed tub

ar membranes, sintered at these temperatures, demonoor mechanical properties for a density of 90.5% of the tetical value calculated from the XRD data (Table 1). Increasng the sintering temperature leads to an enhanced mecal strength, allowing easy handling of SrFe0.7Al0.3O3−δ

ubes, but increases porosity.A significant improvement in the sinterabil

as achieved by minor additions of alumina irFe0.7Al0.3O3−δ powder, with an optimum at appro

mately 3 wt.% (Figs. 7B and 8). Adding 3 wt.% Al2O3nables to sinter gas-tight ceramics having sufficient mec

V.V. Kharton et al. / Journal of Membrane Science 252 (2005) 215–225 219

Fig. 6. Pressure dependencies of the density of SrFe0.7Al0.3O3−δ green com-pacts (A) and sintered ceramics (B). Sintering profile #1 corresponds toheating up at 300–973 K, 973–1273 K, and then up to maximum tempera-ture with the rate of 1, 0.5 and 1 K/min, respectively; the sintering time atmaximum temperature was 2 h. For sintering profile #2, the correspondingheating rates were 1, 0.5 and 4 K/min; the sintering time was 4 h. For profile#3, the heating rates were 2, 0.5 and 4 K/min, respectively; the sintering timewas 3 h.

ical strength at 1623 K for 3–4 h. The XRD analysis showedthe presence of a secondary phase, SrAl2O4 (Table 1); exactcalculations of the theoretical density are impossible in thiscase due to unknown distribution of cations between thesetwo phases and also to probable segregation of components,such as alumina, at the grain boundaries. Nonetheless,the density of 3% Al2O3-enriched ceramics was 92% ofthe theoretical value for pure SrFe0.7Al0.3O3−δ and about95% with respect to hypothetical two-phase mixture, where

FA

Fig. 8. Maximum density of SrFe0.7Al0.3O3−δ–Al2O3 composites, obtainedafter uni-axial pressing at 200–300 MPa and sintering at 1473–1673 K.

the perovskite phase and SrAl2O4 are assumed to beSr-deficient and to have nominal composition, respectively.SEM inspections confirmed that the residual porosity of thismaterial is closed and low (Fig. 2C).

One should also note a positive influence of minoralumina additions on the thermal expansion and oxygenpermeability. The average TECs of 3 wt.% Al2O3-dopedSrFe0.7Al0.3O3−δ ceramics decreased down to the level char-acteristic of SrFe0.5Al0.5O3−δ (Fig. 3). On the contrary, theoxygen permeation fluxes were found to increase by ap-proximately 10% if compared to the cation-stoichiometricSrFe0.7Al0.3O3−δ (Fig. 4). The latter effect is similar tothat observed for strontium-deficient Sr1−x(Fe,Ti)O3−δ per-ovskites, where creation of the A-site vacancies promotesdisorder in the oxygen sublattice and thus increases ionicconductivity[16].

On the basis of these results, high-quality tubular mem-branes of SrFe0.7Al0.3O3−δ with 3 wt.% Al2O3 addition wereprepared by CIP at 175 MPa. In the course of processing, theball-milled powder was filled around a steel mandrel with6.35 mm diameter in a flexible latex rubber hose; special carewas taken to distribute the powder symmetrically around themandrel in order to avoid as much as possible eccentricity, i.e.variation of circular wall thickness. The gas-tight tubes withthe inner diameter of 5 mm, wall thickness of 1.0–1.2 mmand length of 170–200 mm (Fig. 9) were sintered at 1623 Kf

3

xest net byF -l

j

w em-bp il-i gent uc-

ig. 7. Density of SrFe0.7Al0.3O3−δ ceramics without (A) and with 3 wt.%l2O3 addition (B) as function of the sintering temperature and time.

or 3–4 h.

.3. Surface modification

Typical relationships between oxygen permeation fluhrough dense SrFe0.7Al0.3O3−δ ceramics and membrahickness (d) under oxidizing conditions are illustratedig. 10. The specific oxygen permeability,J(O2), was calcu

ated as[15,16,23]

= J(O2)

dln

(p2

p1

)(1)

herep1 andp2 are the oxygen partial pressures at the mrane permeate and feed sides, respectively. AsJ(O2) isroportional toj ×d by definition, the specific permeab

ty should be thickness-independent if the overall oxyransport is predominantly limited by the bulk ionic cond

220 V.V. Kharton et al. / Journal of Membrane Science 252 (2005) 215–225

Fig. 9. Dense tubular membranes of SrFe0.7Al0.3O3−δ with 3 wt.% Al2O3

addition.

tion, i.e. the integral form of the Wagner law is observed.This situation indeed takes place, within the limits of ex-perimental uncertainty, at 1223 K. At lower temperatures,however, increasing membrane thickness results in decreas-ing permeation fluxes, while theJ(O2) values increase dueto a decreasing role of the surface exchange kinetics whenthe thickness increases. These tendencies are common foFe-containing perovskites with a high oxygen permeabil-ity [15,16,23] and unambiguously indicate non-negligiblesurface limitations to the oxygen transport, which becomemore pronounced on decreasingp(O2). Note that, if the per-meation would be limited by the exchange only, the fluxeswould be thickness-independent andJ(O2) would increasewith increasingd, according to the definition, Eq.(1). An-other necessary comment is that the decrease of the specificpermeability with reducing permeate-side oxygen pressureis caused by a decrease in both the p-type electronic con-ductivity and oxygen exchange rate, typical for most mixed-conducting perovskites (e.g.[2,25]). Although the influenceof the former factor is small, this effect cannot be neglectedeven for membranes where the partial electron–hole conduc-tivity is 102 to 103 times higher than the ionic conductivity[25].

The permeation-limiting role of the exchange kinetics isexpected to increase dramatically in reducing atmospheres,where the oxygen transport is usually limited by the rateoT e area

of membranes operating under air/CH4 gradient and to in-troduce catalysts, promoting exchange and reforming reac-tions, onto the permeate-side surface[2,5,7,9–11]. On theother hand, the oxidation of methane pulses on the surfaceof SrFe0.7Al0.3O3−δ powder[9] showed that this material isitself active towards the formation of CO and H2. Therefore,in this work the permeate-side surface of SrFe0.7Al0.3O3−δ

membranes was modified applying a porous layer of the samecomposition, synthesized via the cellulose-precursor methodearlier tested for the fabrication of highly active anodes forsolid oxide fuel cells[22]. This technique is based on theuse of structure-modified cellulose containing the metal salts.After reaction with nitric acid and hydration in order to in-crease sorption activity, the cellulose fiber was impregnatedwith an aqueous solution of strontium, iron and aluminumnitrates in the stoichiometric proportion. Then the precur-sor was dried and self-combusted, with subsequent thermaltreatment at 1073 K in air to remove residual carbon. Theoxide material retaining the fiber precursor texture (Fig. 2Eand F) was converted into a powder with submicron parti-cle size (Fig. 11A). The XRD and electron diffraction stud-ies confirmed crystallization of the perovskite-type phase;the porous layers (sheet density of 4–6 mg/cm2) were de-posited onto one side of SrFe0.7Al0.3O3−δ ceramics and thenannealed in air at 1373 K for 1 h. Examples of the electrondiffraction pattern of cellulose precursor-synthesized pow-d difiedm

ilityo mi-c weresm -

F c oxy-g tialp

f oxidation reactions on the membrane surface[2,7,21].his makes it necessary to increase the specific surfac

r

er and the SEM micrograph of as-prepared surface-moembrane are presented inFigs. 11B and 2G, respectively.In order to assess permeation-limiting factors and stab

f SrFe0.7Al0.3O3−δ membranes under high oxygen cheal potential gradients, the oxygen permeation fluxestudied as function of time, supplying dry 10%H2–90%N2ixture with a constant rate of 10.5 cm3/min onto the perme

ig. 10. Dependence of the oxygen permeation fluxes (A) and specifien permeability (B) of SrFe0.7Al0.3O3−δ membranes on the oxygen parressure gradient.

V.V. Kharton et al. / Journal of Membrane Science 252 (2005) 215–225 221

Fig. 11. Bright-field TEM image (A) and electron diffraction pattern (B) of SrFe0.7Al0.3O3−δ powder prepared by the cellulose-precursor technique afterannealing at 1073 K in air.

ate side; the measurement technique was described elsewhere[21]. Although such method has one disadvantage, namelya shift in the permeate-side oxygen chemical potential withtime if the oxygen permeation flux changes, the measure-ments with fixed flow rate of the effluent gas make it possi-ble to model realistic operation conditions of the membranereactors. The results showed that, as for other membrane ma-terials [2,21], the effect of surface exchange processes be-comes indeed critical in reducing atmospheres. In particu-lar, the oxygen permeation flux through a surface-activatedSrFe0.7Al0.3O3−δ membrane at 973 K was 3–5 times higherthan that for ceramics without porous layer on the permeate-side surface, despite that the oxygen partial pressure gradi-ent was essentially lower in the former case (Fig. 12). Asthe modification using the cellulose precursor-synthesizedpowder showed quite attractive results, the same method ofsurface activation was used for the membranes tested in themethane oxidation reactor.

F oughS dera ygenp rmeds owr

3.4. Stability

The time dependencies of the oxygen permeation fluxesunder air/H2–H2O–N2 gradients (Fig. 12) clearly indicatethat no fracture due to the bulk reduction of SrFe0.7Al0.3O3−δ

membranes occurs during 400–700 h. The subsequent mi-crostructural analysis revealed reduction of the surface lay-ers and a possibly important role of hydrogen diffusion inthe pores. Nevertheless, no critical microstructural changeswere observed for a membrane exposed to a 10%H2–90%N2flow during more than 700 h (Fig. 2H) in comparison with as-prepared membrane (Fig. 2G), except for the altered surfacemorphology and traces of cation demixing discussed below.

The phase decomposition boundary of SrFe0.7Al0.3O3−δ

at low oxygen partial pressures, estimated from thep(O2)dependencies of electrical properties as described else-where [7,9,15,21,26], is presented inFig. 13. These re-sults are compared to literature data on undoped SrFeO3−δ

[26] and LaFeO3−δ [27,28]. The anomalous behavior ofSrFe0.7Al0.3O3−δ at temperatures below 1100 K is associatedwith progressive ordering of the oxygen sublattice on cool-

F -df

ig. 12. Time dependence of the oxygen permeation fluxes thrrFe0.7Al0.3O3−δ membranes, with and without surface modification, unir/H2–H2O–N2 gradients at 973 K. The values of permeate-side oxressure (p1) are given in the legend. The measurements were perfoupplying dry 10%H2–90%N2 mixture onto the membrane surface (flate of 10.5 cm3/min).

ig. 13. Phase stability limits of SrFe0.7Al0.3O3−δ under equilibrium conitions. Literature data on SrFeO3−δ [26] and LaFeO3−δ [27,28]are shown

or comparison.

222 V.V. Kharton et al. / Journal of Membrane Science 252 (2005) 215–225

ing and the formation of brownmillerite-like domains. Thistrend is similar to that observed on reduction of the parentcompound, SrFeO3−δ [26]. Such variations of the stabilityboundary can be explained in terms of a lower bonding en-ergy of oxygen ions in the brownmillerite lattice comparedto the FeO2.5 pyramids constituting the structure of the dis-ordered cubic SrFeO2.5±δ [26]. In general, the phase stabilityis quite similar for all perovskite-related ferrites (Fig. 13)as their decomposition boundaries are all determined by theFe O bond strength; the substitution of iron for aluminum inthe lattice of SrFeO3−δ leads to a moderate increase of stabil-ity in reducing atmospheres. For temperatures below 1100 K,the phase stability can be moderately enhanced by partial sub-stitution of Sr2+ with trivalent cations, such as La3+, in orderto suppress brownmillerite formation.

The XRD analysis of SrFe0.7Al0.3O3−δ, annealed in flow-ing H2–H2O–N2 mixtures with the oxygen chemical poten-tials close to the decomposition limit, shows no traces ofmetallic iron and apparently does not reveal even the transi-tion of perovskite into the brownmillerite phase (Fig. 14). Inspite of the low sensitivity of this method to oxygen sublattice,such XRD patterns are indicative for the co-existence of per-ovskite and brownmillerite-like domains at reducedp(O2), inagreement with the Mossbauer spectroscopy data which willbe summarized in a separate publication.

Comparison of the equilibrium stability limits and the test-i m-pmeg corre-sT un-d p-e enc onlyd

F -p rmeate-s Hg

Fig. 15. Cross-section of SrFe0.7Al0.3O3−δ membrane after operation underair/CH4 gradient at 1023–1223 K for 215 h (see text).

Another important factor influencing the long-term sta-bility of ceramic membranes is the kinetic demixing, causedby non-negligible mobility of the constituent cations at op-eration temperatures[2,29]. The SEM/EDS inspection ofSrFe0.7Al0.3O3−δ ceramics, tested under highp(O2) gradi-ents during more than 100–150 h, confirmed relevance of thiseffect. As an illustration,Fig. 15(top) shows a cross-sectionof one membrane, the permeate-side of which was exposed toa flow of dry CH4 during 215 h; the magnified micrographs ofselected zones, marked by the numbers, are given below thecross-section. The depth of the surface layers with clearly vis-ible microstructural changes extends up to 120–150�m. Theinformation on kinetic demixing mechanisms, available inliterature, is still very scarce, although this phenomenon maylead to time degradation of the membrane performance, par-ticularly due to ceramics desintegration and interaction of mi-grating cations with gas species[4,29]. Further optimizationof composition and processing conditions of Sr(Fe,Al)O3-based membranes, aimed to suppress these microstructuralchanges, is now in progress; the basic approaches include:(i) incorporation of small amounts of secondary phases pre-venting grain-boundary diffusion; (ii) use of sintering aids toprovide nearly zero porosity to avoid surface diffusion in thepores; (iii) doping with low-mobility cations to hamper cation

ng results under air/H2–H2O–N2 gradients stresses an iortant role of the kinetic stabilization of SrFe0.7Al0.3O3−δ

embranes due to surface-limited oxygen transport[7]. Forxample, in the course of testing at 973 K (Fig. 12), the oxy-en chemical potential at the membrane permeate sideponded top(O2) values of approximately 10−18 to 10−16 Pa.hese are substantially lower than the stability boundaryer equilibrium conditions (Fig. 13). The steady-state oration of SrFe0.7Al0.3O3−δ membranes under high oxyghemical potential gradients can therefore be achievedue to kinetic reasons discussed above.

ig. 14. XRD patterns of SrFe0.7Al0.3O3−δ ceramics equilibrated at low temeratures in air and annealed at reduced oxygen pressures, and the peide surface of SrFe0.7Al0.3O3−δ membrane after operation under air/C4radient at 1123–1223 K for 140 h.

V.V. Kharton et al. / Journal of Membrane Science 252 (2005) 215–225 223

Fig. 16. Temperature dependence of methane conversion, CO selectivityand calculated oxygen permeation flux density for a model reactor withSrFe0.7Al0.3O3−δ membrane. Experimental conditions are given in the leg-end.

migration in the lattice; and (iv) creation of a compositionalgradient compensating the driving force for cation transportunderp(O2) gradients.

3.5. Methane oxidation

Selected results on the partial oxidation of methane in amodel reactor with surface-modified SrFe0.7Al0.3O3−δ mem-brane are presented inFigs. 16–18. This membrane com-position was selected for the CH4 oxidation tests, since theeffect of minor alumina additions on the electrocatalytic pro-cesses is expected negligible. Taking into account the pre-vious results[7], the reactor was packed with a commercialAl2O3-supported Ni catalyst supplied by the Boreskov In-stitute of Catalysis (Novosibirsk, Russia). The necessity forpacking with an additional reforming catalyst is associated

F on ther alc

Fig. 18. Example of the time dependence of CH4 conversion and CO selec-tivity in a model reactor with SrFe0.7Al0.3O3−δ membrane (see text). Totalflow rates of the influent gas mixture are given at the top.

with gas-phase transport limitations, confining the contactbetween a part of CH4 molecules and the catalytically ac-tive porous layer applied onto the membrane; such effectleads to predominant total combustion of methane on themembrane surface, whilst a significant fraction of suppliedCH4 remains unreacted[7,11,29]. The incorporation of cat-alyst makes it possible to shift the composition of effluentgas mixtures towards thermodynamic equilibrium due to re-forming of residual CH4 with H2O and CO2 formed overceramic membrane. As a result, the CO yield was substan-tially increased (Figs. 16 and 17). For example, when themethane flow rate was approximately 4.7 cm3/min per 1 cm2

membrane area (6 cm3/min for the total influent flow rate),the reactor with SrFe0.7Al0.3O3−δ membrane enables to ob-tain >99% selectivity to CO formation in combination with90–97% conversion rates at 1123–1173 K (Figs. 16 and 17).

Since no traces of O2 were observed at the reactor out-let, the methane consumption is limited by the oxygen per-meation fluxes. Increasing temperature results hence in ahigher CH4 conversion due to increasing oxygen perme-ability of SrFe0.7Al0.3O3−δ (Fig. 16). In all cases, the yieldof C2-hydrocarbons was lower than 0.1%, comparable toexperimental error. As expected from thermodynamics, theCO selectivity decreases when the conversion increases, ei-ther on heating or on decreasing influent flow of methane(Fig. 17). The optimum temperatures, where the CO selec-t rec thec downt ilityp y is9 igh,7

witht aftera , upt them dn mostu ternm ix-i

ig. 17. Dependence of the methane conversion and CO selectivityeactant flow rate supplied onto SrFe0.7Al0.3O3−δ membrane. Experimentonditions are given in the legend.

ivity and CH4 oxidation rate are both sufficiently high, alose to 1123 K. If neglecting a moderate decrease inonversion, the operation temperature can be reducedo 1023 K, which is preferable from the membrane staboint of view. For example, at 1023 K the CO selectivit8%, whereas the methane conversion is still relatively h9%.

No essential degradation in the reactor performanceime was detected, although the relaxation processes

change in the operation conditions were quite slowo 10–25 h (Fig. 18). The subsequent XRD analysis ofembrane permeate-side surface (Fig. 14, bottom) revealeo bulk decomposition; the perovskite phase remains alnchanged. The minor impurity peaks in the XRD patay originate either from surface reduction or cation dem

ng illustrated byFig. 15.

224 V.V. Kharton et al. / Journal of Membrane Science 252 (2005) 215–225

4. Conclusions

Increasing aluminum concentration in perovskite-typeSrFe1−xAlxO3−δ enhances the phase stability at low oxygenchemical potentials, moderately decreasing thermal expan-sion and oxygen permeability within the solid solution forma-tion range,x= 0–0.35. Whilst further increase of thex valuesleads to substantially lower permeation fluxes, small Al2O3additions into SrFe0.7Al0.3O3−δ increase oxygen transport.This type of doping improves also the sinterability and thestability with respect to thermal cycling, which enables tofabricate high-quality tubular membranes for the natural gasconversion reactors. No essential degradation in the perfor-mance of SrFe0.7Al0.3O3−δ membranes placed under air/CH4or air/H2–H2O gradients at 973–1223 K, was observed during200–700 h, except for the surface reduction and temperate ki-netic demixing. The latter phenomenon requires, nonetheless,further optimization of the membrane composition and pro-cessing technologies. The steady-state operation under highoxygen chemical potential gradients is possible due to thesurface exchange limitations to oxygen transport, revealed bythe membrane thickness dependencies of oxygen permeabil-ity and by enhanced oxygen fluxes through the membraneswhere porous layers of the same composition were appliedonto the permeate-side surface. The CH4 conversion rate in amodel reactor with surface-modified SrFeAl O mem-b %a 0%.

A

CTIp andB ram( oce-r d byt

R

portction,

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A.C.ical

ich,lle,tors in995)

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[14] A. Atkinson, R.J. Chater, R. Rudkin, Oxygen diffusion and sur-s,

[ .B.rozau,ixed04)

[ u-bilityc-

[ ed

State

[ m-ne to

[ iya,y of

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[ , p-of

[ ch,alline. 8

[ m-.B.

d150

0.7 0.3 3−δ

rane and commercial Ni/Al2O3 catalyst achieved 90–97t 1073–1123 K, when the CO selectivity was almost 10

cknowledgements

This work was supported by the FCT, Portugal (POrogram and projects BPD/11606/2002, BD/6595/2001D/6827/2001), the NATO Science for Peace prog

project 978002), and the COST 525 “Advanced Electramics: Grain Boundary Engineering” program sponsorehe European Commission.

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