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Journal of Membrane Science 325 (2008) 704–711

Contents lists available at ScienceDirect

Journal of Membrane Science

journa l homepage: www.e lsev ier .com/ locate /memsci

erovskite membrane reactor for continuous and isothermal redox hydrogenroduction from the dissociation of water

. Evdou, L. Nalbandian, V.T. Zaspalis ∗

enter for Research and Technology-Hellas, Chemical Process Engineering Research Institute, Laboratory of Inorganic Materials, P.O. Box 60361, 57001 Thessaloniki, Greece

r t i c l e i n f o

rticle history:eceived 24 March 2008eceived in revised form 27 August 2008ccepted 28 August 2008vailable online 4 September 2008

eywords:ater splittingydrogenembrane reactor

erovskites

a b s t r a c t

The redox water splitting is one of the most promising routes for sustainable hydrogen production.Towards this goal, serious technological obstacles are set: (i) by the non-isothermal operation of theredox process, that causes serious reactor construction problems, and (ii) by the need for efficient hightemperature oxygen/hydrogen separation technology which is a very challenging development. In thispaper, perovskite materials having the formula La0.3Sr0.7FeO3 were synthesized and subsequently testedfor their high temperature oxidation/reduction behavior. The redox activity of the materials in relationto the water splitting reaction has been also investigated. Dense, disc shaped membranes of the materi-als were synthesized and placed in a membrane reactor. Experiments at 1133 K revealed the possibilityof performing the reduction and oxidation steps simultaneously and isothermally on each side of themembrane reactor. A steady-state situation was thereby achieved where hydrogen was continuously pro-

duced on one side while the material was simultaneously regenerated on the other side. The createdoxygen vacancy gradient formed the driving force for a continuous flux of vacancies from the membranereduction surface to the membrane oxidation surface. The hydrogen production rate under the particularexperimental conditions estimated to be ∼47.5 cm3 H2 (STP) m−2 min−1. It could be increased by a factorof approximately 3, up to ∼145 cm3 H2 (STP) m−2 min−1, if the membrane reduction was enhanced witha reductant such as carbon monoxide. This approach resulted in an efficient execution of the water gas

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. Introduction

The production of hydrogen from water, though an old subject,s still one of the most important long-term goals in fuel produc-ion. The attractiveness of an economy based on hydrogen energyncreased in the past decade due to the confluence of a numberf factors [1–3]. These are related to the need to reduce urban airollution, dependence on foreign oil imports, carbon dioxide emis-ions and climate changes or demand for low- or zero-emissionehicles.

Initially the direct thermochemical water splitting (H2O → H2 +12 O2) received much attention [4–7]. The idea of utilizing solarnergy has been also reported since 1977 [5,7]. The Gibbs freenergy for the direct thermochemical water splitting equals zero

t about 4330 K under a pressure of 1 bar [8]. The main problemsncountered during the various experimental investigations andvaluations of the process, are related to: (i) the very high solar reac-or temperatures need to be achieved, (ii) the reactor construction

∗ Corresponding author. Tel.: +30 2310 498115; fax: +30 2310 498131.E-mail address: zaspalis@cperi.certh.gr (V.T. Zaspalis).

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376-7388/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.memsci.2008.08.042

ity hydrogen production.© 2008 Elsevier B.V. All rights reserved.

roblems associated with this and (iii) the lack of appropriate sta-le membrane materials to perform the required high temperatureydrogen separation.

In order to lower the reaction temperatures and bypass the prod-ct separation problems, several multistep water splitting methodsave been proposed [9–11]. The most interesting, because of theirwo-step simplicity were those involving redox pairs of variousxides like Fe3O4/FeO [5,12,13], Mn3O4/MnO [14], Co3O4/CoO [15],b2O5/NbO2 or ZnO/Zn [16–19]. Although the oxidation tempera-

ures could be reduced down to approximately 1200 K, a problemf the occasionally still high required reduction temperatures (e.g.600 K for Nb2O5) is still left. There are other problems of the lowydrogen yields and the difficult oxygen and zinc vapour high tem-erature separation.

Redox pairs involving solid solutions have been also stud-ed. For example, (Fe1−xMx)3O4/(Fe1−xMx)1−yO redox pairs with

= Mn, Co, Mg appeared in several cases to combine low reduc-

ion temperatures with good hydrogen yields when compared withhe undoped Fe3O4/FeO redox pair [15,20,21]. NiZn- or MnZn-errite systems attracted also considerable interest for the two-stephermochemical water splitting [22–25]. The amount of hydro-en produced appeared to be limited because water splitting

A. Evdou et al. / Journal of Membrane

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as caused by the small magnitude of non-stoichiometry in theerrite.

A common feature of all those processes is that they produceydrogen based only on (renewable) water as hydrogen source.n addition to that, they consist of an oxidation (i.e. water split-ing or hydrogen production) step and a consecutive reduction (i.e.egeneration of the material) step. The temperature required for theegeneration reaction is quite higher than the temperature of theater splitting reaction. This results in complex situations in rela-

ion to reactor construction and oxide material handling betweenhe two stages. The high temperatures may also cause evaporationf volatile compounds present in the redox material (e.g. ZnO) andesult either to reactant loss or to composition changes and activityeduction.

In order to overcome those problems a continuous and isother-al process is proposed based on a dense perovskite membrane

eactor. It is quite similar to that used for electrochemicalells. Similar experiments have been reported in literature usingeramic–metal composite mixed conducting membranes. Theembranes consisted of gadolinium doped cerium oxide withetal nickel as the electronic conductor [26,27]. As suggested,

he membrane acts as separator of the oxygen produced dur-ng water splitting that takes place in the gas phase. Despite theery interesting results, the experiments were performed in a wayhat, although of academic interest, was of limited interest fromhe application point of view. Mainly hydrogen and occasionally

ethane were used as reductants on the membrane reductionide. The oxidation of hydrogen produced water; the substancehose dissociation was taking place on the oxidation side of theembrane. The purpose of this article is the experimental veri-

cation and evaluation of a perovskite redox membrane reactorowards high purity hydrogen production, preferably based onlyn renewable water source. The basic background, the experi-ents and the results of this work are presented in the following

aragraphs.

. Theoretical background

.1. Reactor principle

A schematic diagram of the membrane reactor is shown in Fig. 1.he reactor consists of two compartments (indicated as 1 and 2 inhe figure). They are separated by a dense membrane made of anxidic material that is able to perform the redox operation. Reac-

ant water vapour is fed to the active and regenerated membraneurface in compartment 1. The water splitting reaction takes placeroducing hydrogen and lattice oxygen according to the reaction:

Ored + H2O → MOox + H2↑ (1)

Atfiop

Science 325 (2008) 704–711 705

here MOred and MOox stand for the reduced (active) and oxi-ized (non-active) membrane material states. Eq. (1) can be writtenccording to the Kröger–Vinck notation as:

2O + Vo•• + 2e′ → Oo + H2↑ (1a)

here Vo•• denotes a positively charged oxygen vacancy, e′ is a

egatively charged electron and Oo is lattice oxygen.Eqs. (1) and (1a) indicate that an oxygen vacancy containing

aterial reacts with water vapour in order to fill the vacancies withxygen. This leads to the simultaneous production of hydrogen.

The lattice oxygens are transported through the membrane tohe opposite side, at compartment 2, that is kept at very low or zeroartial pressure of oxygen. They desorb to produce gaseous oxygennd oxygen vacancies according to the reactions:

Oox → MOred + 12 O2 ↑ (2)

r

o → 12 O2 + Vo

•• + 2e′ (2a)

Alternatively, an oxidizable compound (CX) can be fed in com-artment 2 in order to facilitate the membrane regeneration andacancy creation process according to the reaction:

o + CX → CXO + Vo•• + 2e′ (3)

At steady-state conditions there is a concentration gradient andux of oxygen from compartment 1 to compartment 2, with a con-inuous flux of stoichiometric amount of oxygen vacancies andlectrons to the opposite direction. The membrane is oxidized inompartment 1 and simultaneously regenerated in compartment 2,iving rise to a continuous and isothermal process. When the mate-ial reduction in compartment 2 occurs through Eq. (2) or (2a), theet effect is the production of hydrogen by water dissociation. Alter-atively, when the reduction in compartment 2 proceeds throughq. (3) (e.g. where CX is CO or CH4), the net effect is water gashift (WGS) reaction or steam reforming reaction. In addition, partf the hydrogen being produced in high-purity form in compart-ent 1. The first mode of operation is certainly preferred since it is

ntirely based on a renewable hydrogen source. It provides an alter-ative to the redox two-step water splitting process performed onxidic materials. As probably obvious, the reactor has a lot in com-on with a conventional electrochemical electrolyser or reverse

uel cell.

.2. Membrane material

There are certain demands to be fulfilled by the materials to besed for the membrane synthesis. First, they must be able to keep

arge concentrations of oxygen vacancies in their structure withaintenance of their basic crystallographic stability. In addition,

hey must be able to abstract oxygen from water vapour in ordero fill their vacancies while they must be able to desorb easily thisxygen at identical temperatures. Finally, they must show good lat-ice oxygen and vacancy permeability for being able to effectivelyransport lattice oxygen and electrons through their body.

The great majority of the single- and mixed-oxide or ferriteedox pair materials does not fulfill the material requirements formembrane reactor operation. The regeneration (reduction) tem-eratures are significantly, usually some hundreds degrees, higherhan the temperatures of the water splitting reaction [19,28,29].

ttempts are being undertaken to reduce the oxide regeneration

emperatures [25,28]. The oxygen or anion vacancy diffusion coef-cient in spinel ferrites is strongly affected by the partial pressuref oxygen. They exhibit values that are considered small when com-ared to the vacancy creation and annihilation rates caused by the

7 brane Science 325 (2008) 704–711

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06 A. Evdou et al. / Journal of Mem

eduction and oxidation (water splitting) reactions, respectively.or example, cation diffusion coefficients in non-stoichiometricpinel oxides are found to be even smaller than 10−10 cm2 s−1

30,31]. The diffusivity of lattice oxygen or oxygen vacancies is con-idered to be even smaller and their mobility is proven to be theontrolling step of the material sintering process [32].

An interesting class of materials that possesses high oxygen con-uctivity and is expected to fulfill the oxygen and vacancy fluxequirements is the perovskite type materials. They have beenxtensively used as oxygen permeation membranes in fuel cells33]. Their oxygen transport and exchange properties have beenxtensively studied [34–37]. However, their activity in relation toater splitting, to the authors’ knowledge, has not attracted much

ttention yet. In this work La0.3Sr0.7FeO3−ı materials in the formf powdered particles or self-supported membrane pellets werenvestigated in relation to their activity for the water splitting reac-ion and to their performance in a membrane reactor.

. Materials and methods

.1. Powdered material synthesis

Materials with the general formula La0.3Sr0.7FeO3 were syn-hesized according to the following process [38]: the appropriatemounts of the metal precursors were initially mixed and subse-uently diluted in water with the additions of hydrogen peroxideH2O2) and aqueous ammonia solution, stirred and left to dry untilomplete solvent evaporation. The metal precursors used were:a(CH3COO)3 (Aldrich, Cat: 30,633-9, 99.9% purity) or La2O3 (Sigma,at: L4000, 99.9% purity) for La, Sr(CH3COO)2 (Aldrich, Cat: 38,854-) or SrCO3 (Aldrich, Cat: 472018, 99.9+% purity) for Sr, Fe2O3,Merck, A218224 119) for Fe. In a typical synthesis of La0.3Sr0.7FeO3

aterial, 1.63 g La2O3, 3.45 g SrCO3 and 2.66 g Fe2O3 were diluted in4 ml double distilled water with the addition of 25 ml H2O2 solu-ion of 30 wt.% (Merck 822287) and 1 ml NH3 solution of 25 wt.%Merck 105432). The solid product was then dried at 90 ◦C for 16 h,omogenized and finally fired in air atmosphere to a temperaturef 1000 ◦C.

.2. Membrane preparation

The dried powdered sample was initially ball-milled down ton average particle size of 0.5 �m and mixed with 10 wt.% aque-us solution of polyvinylalcohol binder (Fluka, molecular weight2,000) with a concentration of 2 wt.%. Subsequently it was dried,oll-granulated and uniaxially pressed in the form of cylindricalellets with a diameter of 10 mm and height of 15 mm. The com-acted specimens were finally sintered to 1300 ◦C for 40 h in air.hin membrane slices with a thickness of 3 mm were cut fromhe fired cylindrical specimens, with a microtome using a diamondheel.

.3. Testing

The oxidation/reduction behavior of the powdered samples wasested in a multicomponent mixture thermogravimetric analyzerith simultaneous chemical analysis of the exit stream by mass

pectrometry (Hiden-Isochema, IGA). The absolute measurementrror of the thermogravimetric analyzer was 0.001 mg. Initially,

he sample was brought to a constant temperature of 1133 Knder helium atmosphere. The total pressure was kept constant at050 mbar (∼1 atm). While keeping the total pressure and the totalow constant at 200 ml/min, reductant methane and oxidant oxy-en were successively, at periodic time intervals, introduced into

sc(wc

Fig. 2. Schematic diagram of the membrane reactor.

he gas mixture in order to reduce and oxidize the sample, respec-ively. Relative weight differences were calculated to the materialeight that corresponds to equilibrium with helium atmosphere

t 1133 K.The activity of the powdered samples (200 ± 5 mg) was tested

n a U-type reactor. For a better quantitative approach the reac-ants were fed to the reactor via constant and known volumeulses. The reactor outlet stream was directed to a quadrupole masspectrometer (Balzers-Omnistar) where it was quantitatively anal-sed. The mass fractions corresponding to: water (m/e: 18, 17, 16),ydrogen (m/e: 2), nitrogen (m/e: 28, 14), oxygen (m/e: 32, 16),arbon monoxide (m/e: 28, 12), carbon dioxide (m/e: 44, 28, 12)nd nitrogen oxides (m/e: 30) were monitored during each exper-ment. The ratios of the different m/e fragment signals for eachomponent were found to be constant during the experiments. Theuantities of unconverted water and of produced hydrogen werealculated, based on the areas of the corresponding MS injectioneaks, m/e = 18 was used for water and m/e = 2 for hydrogen. Theontribution of water to the m/e = 2 peak was found to be negligi-ly small. It was however subtracted from the total signal in ordero obtain the pure hydrogen signal. The reaction products were

easured during each pulse and integrated for the whole pulseuration. The mass-spectrometer response to each of the reactantsnd products was calibrated by using pulse injections from either ofoth pure substances and calibration mixtures using a 4-way valvequipped with a 100 �l or 1000 �l loop. A typical experimental pro-edure proceeds as follows: The powdered perovskite material waslaced in a U-type reactor and heated in helium atmosphere at60 ◦C. The material was initially inactive for the water splittingeaction. Each experiment started therefore with the activation ofhe redox material with methane. Methane was fed pulse-wise, inonstant volume pulses of 100 �l, to the reactor, until reduction ofhe material was completed. No conversion of methane to total orartial oxidation products was observed at the end point. Wateras then fed to the reactor also pulse-wise, in constant 15 �molulses until the catalyst was deactivated and no water conversion orydrogen production was observed. The entire reduction/oxidationycle could be eventually repeated several times. The material inquilibrium with helium is considered to have an oxygen deficiencyref (gram-atoms O/mol solid, which is unknown). From total oxy-en mass balances during the activation step (the material is thenly oxygen source) the oxygen deficiency �ı = (ı − ıref) can be cal-ulated as a function of the injected methane, up to the final �ımax

here activation practically stops [39]. At this point the methaneonversion becomes zero. Subsequently, during oxidation, the �ıs a function of the injected water can be also calculated from theater conversion.

The membrane reactor consists of two co-axial quartz tubes ashown in Fig. 2. An inlet and an outlet stream are attached to each

ompartment, all connected individually to a mass spectrometerBalzers-Omnistar) for chemical analysis. The membrane specimenas gas tightly sealed at the end of the inner tube with spe-

ially developed ceramic sealing mixtures based on commercially

A. Evdou et al. / Journal of Membrane Science 325 (2008) 704–711 707

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ig. 3. X-ray diagram of the La0.3Sr0.7FeO3 membrane (Tsint = 1300 ◦C) indicating thexistence of the single perovskites phase (* indicates the peaks due to the back-round holder, Ref. data are from JCPDS 35-1480 [40]).

vailable ceramic kits. To control leakage free operation during thentire duration of the experiment, different inert carrier gases weresed; helium was used in compartment 1 and argon in compart-ent 2.

. Results and discussion

.1. General characterization

An X-ray analysis of the powdered samples calcined at 1000 ◦Chowed the existence only of the pure perovskites phase. Nitrogendsorption/desorption measurements indicated a BET surface areaf ∼2 m2 g−1. X-ray diffraction and scanning electron microscopenvestigations of cross sections of membrane specimens revealedhe existence of the pure perovskite structure [40] (Fig. 3) and aense polycrystalline microstructure consisting of grains with a rel-tively uniform grain size varying between 2 �m and 5 �m (Fig. 4).

.2. Oxygen adsorption/desorption behavior

The weight changes of the powdered sample are shown inig. 5a. The sequence of the changes of the sample atmosphere ishown in Fig. 5b. The sample is initially considered to be in equi-

ig. 4. Scanning electron microscope cross-section image of a dense membranepecimen prepared from perovskite La0.3Sr0.7FeO3.

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ig. 5. The sample weight (a) and the partial pressure sequence of helium, oxy-en and methane (b) as a function of time during a three-cycle oxidation reductionxperiment.

ibrium with helium at 1133 K. Subsequently methane entered inhe gas mixture at a partial pressure of 50% (i.e. PCH4 ∼ 525 mbar,alance helium). The sample started losing weight and stabilizeshen methane was removed from the gas mixture. This weight

oss was probably due to removal of its lattice oxygen that reactedith methane. Subsequently, oxygen entered the gas mixture at

133 K and a partial pressure of 2.5% (i.e. PO2 ∼ 26 mbar, balanceelium). The sample gained weight since oxygen vacancies wereow filled up with oxygen. The final oxygen content was thatorresponding to the equilibrium with this particular partial pres-ure of oxygen at 1133 K. When oxygen was removed and replacedith helium the sample lost weight because its oxygen contentad to attain a new equilibrium situation that corresponded to anlmost zero partial pressure of oxygen. As shown in Fig. 5a, thisehavior was reversible and could be repeated during successiveeduction–oxidation cycles. The weight of the sample after reduc-ion with methane or after oxidation with oxygen or in equilibriumith helium after three successive cycles, showed a remarkable

ccuracy. The relative error was 0.1%.The previous results led to the conclusion that the synthesized

a0.3Sr0.7FeO3−ı material was able to reversibly absorb and des-rb oxygen at the temperature of 1133 K and the total pressure ofbar. From the gravimetric data, it can be calculated that a forced

eduction with methane resulted in weight difference between thexidized and the reduced state of at least 0.014 g oxygen g−1 ofaterial (indicated as i in Fig. 5a). A reduction caused only by oxy-

en removal resulted in weight variations of 0.005 g oxygen g−1 ofaterial (indicated as ii in Fig. 5a). The ratio of the oxygen vacan-

ies created in the material by reduction with methane to thoseorresponding to equilibrium with helium atmosphere is at least2.75. This is expected qualitatively to reflect on the ratio of the

oncentration gradients within the membrane, between a forcednd an unforced reduction operation in compartment 2 (Fig. 1).

708 A. Evdou et al. / Journal of Membrane

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Ftctapartment 1 was led through a saturator and water was introducedto the system. This is obvious from the water signal increase atthe exit of compartment 1 and the expected corresponding changein the helium signal. The introduction of water vapour causes asmall increase in the m/e = 2 mass signal, as indicated in Fig. 7a

ig. 6. (a) Product yields during the activation step with 100 CH4 injection pulses. (b)ater pulse conversion and cumulative hydrogen production during the oxidation

tep as a function of the total amount of water injected to the reactor and the materialxygen deficiency �ı (gram-atoms O/mol solid).

.3. Water splitting activity

Initial blank experiments with an empty reactor or a reactorlled with equal amount of �-Al2O3 powder indicated the absencef any water splitting or methane oxidation activity. Any furtheresults are therefore attributed to the perovskite material.

The results of an activation/oxidation cycle are shown in Fig. 6a.ince the sample was initially inactive, the first step was activationith methane. In Fig. 6a the products of the methane oxidation

eactions during the activation step are shown as a function of thenjected amount of methane by the first 100 injections. Initially, theotal oxidation of methane took place leading to the formation ofarbon dioxide and water. Subsequently, and as the oxygen defi-iency of the material increased, the partial oxidation of methaneo carbon monoxide and hydrogen took place mainly.

The water conversion and hydrogen production results duringhe oxidation step are shown in Fig. 6b. On top of Fig. 6b the cor-esponding oxygen deficiency �ı = (ı − ıref) is also shown. Initially,he very high activity of the oxygen deficient material (�ı ∼ 0.15)esulted in water conversions as high as approximately 80%. Due tohe gradual material deactivation, the water conversion per pulseecreased down to the point of complete deactivation (�ı ∼ 0.07).ach pulse contained a constant small volume of water that wased in a constant small time. The time for the water to flow throughhe reactor was probably constant. Under these conditions, conver-ion is considered proportional to the reaction rate. The splittingeaction rate was probably proportional to the concentration of

vailable oxygen vacancies as the result of a detailed investiga-ion [39]. The cumulative hydrogen production during oxidations also shown in Fig. 6b. The material appeared to exhibit stableerformance after eight successive cycles. Considering the mate-ial as deactivated after the passage of 1300 �mol H2O g−1, where

Fn

Science 325 (2008) 704–711

he conversion per pulse dropped to below 10%, a cumulativeydrogen production of 286 �mol was measured. Theoretically, theembrane reactor operation has an advantage when the oxidation

urface is continuously kept at relatively high vacancy concentra-ion by continuous removal of oxygen. This gives rise to a highctivity, as indicated by the conversion line shown in Fig. 6b.

.4. Membrane reactor operation

Prior to the membrane reactor experiments blank runs wereerformed using either a quartz dead-end inner reactor tube orn identically sealed dense �-Al2O3 membrane. In both cases noeaction was detected in either compartment. The conclusion isherefore drawn that the reactor and the sealing material have nonfluence on the process.

The results of the membrane reactor operation are shown inigs. 7 and 8. Initially only inert helium and argon were flowinghrough compartments 1 and 2, respectively. All other componenturrent signals (H2O, H2) at the outlet of compartment 1 are atheir background level (Fig. 7a). At a certain moment, indicated withrrow on the x-axis of Fig. 7a and b, the inert helium line of com-

ig. 7. Water, hydrogen and inert carrier gas initial characteristic mass current sig-als in compartment 1 (a) and compartment 2 (b).

A. Evdou et al. / Journal of Membrane

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ig. 8. Hydrogen characteristic mass current signals at the outlet stream of com-artment 1 for the case where (a) carbon monoxide is simultaneously supplied inompartment 2 and (b) only inert argon is supplied in compartment 2.

i.e. from 3 × 10−9 A to 3.5 × 10−9 A) which is attributed to someater vapour fragmentation in the mass-spectrometer chamber.

he introduction of water in compartment 1 leaves all current sig-als in compartment 2 unaffected at their background level, as

ndicated in Fig. 7b. The signal stability in compartment 2 as wells the absence of argon in compartment 1 and helium in compart-ent 2 confirm that the membrane reactor system was leakage

ree. A totally stable system in respect to all signals was achievedfter about 15–30 min from the introduction of water which was theomponent with the longest response time. The system’s responseime for all other components was in the order of 100 s. Theseesults also indicate that no water splitting took place. Thus, at theeginning of the experiment the membrane material was not active.his is in agreement with similar observations made during waterplitting activity investigations on powdered materials, describedn the previous section.

After the achievement of a stable situation, carbon monoxideas injected in compartment 2 (Fig. 8a). This resulted in a sig-ificant increase in the hydrogen signal in compartment 1. Theost probable explanation is this provided by Eqs. (3) and (1a).

he metal oxide was reduced by carbon monoxide that picked upaterial lattice oxygen. Oxygen vacancies were therefore created

n the membrane material, diffused to the opposite side surfacend the water splitting took place, producing hydrogen in com-

artment 1. The entire system has been left to run for more than anour under these conditions having a constant level of hydrogen asmain product in compartment 1 and a constant level of carbonioxide in compartment 2.

TT4c

Science 325 (2008) 704–711 709

Subsequently, as shown in Fig. 8b, the introduction of carbononoxide was stopped. The hydrogen signal followed, as expected,decreasing trend and stabilized after about 6–7 h at a lower level,hich was higher than this corresponding to the totally inactive

ituation (indicated by the dashed line in Fig. 8b). Oxygen was des-rbed with driving force by difference between the concentrationf oxygen on the membrane surface and the equilibrium concen-ration with an inert atmosphere having zero partial pressure ofxygen (Eq. (2) or (2a)). Under these conditions hydrogen coulde produced by water splitting without the assisting action of aeductant. During the experiment shown in Fig. 8b the stable hydro-en production at this stage could be maintained for more than0 h. Complementary experiments towards signal quantificationhat involved periodical water feed shut downs (i.e. loop injections)t various stages of membrane surface activity excluded the possi-ility of base-line shifting and confirmed the fact that the observedignal intensity corresponded to real hydrogen production. This islso shown in Fig. 8b where after about 10 h of stable operation, ahut down of the water feed resulted in reduction of the hydrogenignal down to its original background level. When the water feedupply is restored the hydrogen production is also restored. The dif-erence in the hydrogen signal detected at the exit of compartmentin the case of the presence of carbon monoxide is approximately

hree times higher than that in the case of oxygen desorption dueo partial pressure reduction (indicated as i′ and ii′ in Fig. 8a and b).t seems that under forced oxygen removal a higher vacancy con-entration and consequently a higher activity was maintained athe membrane surface at compartment 1. This gave rise to moreydrogen production.

. Discussion

From the environmental point of view the most interestingode of operation is the one that does not make use of a car-

on containing reductant (i.e. carbon monoxide or methane in thexperiments reported in this article). In this case the entire processroduces hydrogen without carbon dioxide emissions, based onlyn renewable raw material source. The overall reaction is the onebtained by combining Eq. (1) or (1a) with Eq. (2) or (2a):

2O → 12 O2 + H2 (4)

The advantage in relation to the conventional redox processes ishe continuous and isothermal operation of the membrane reac-or with simultaneous bypassing of the extremely difficult highemperature hydrogen/oxygen separation.

The hydrogen production rate depends on whether reaction (2a)r reaction (3) takes place in compartment 2 (e.g. Fig. 8a and b).he main difference between those two modes of operation is theate of oxygen removal from the material in compartment 2. Noonditional changes occur in compartment 1. The hydrogen pro-uction rate is clearly influenced by a factor ∼3. This shows that theater dissociation in compartment 1 is a fast process step under the

xperimental conditions used in this work. The rate-determiningtep must be sought either in the bulk diffusion through the mem-rane or in the surface exchange reactions in compartment 2.

The hydrogen production rate under unforced reduction isstimated from the loop pulsing calibration experiments in theembrane reactor (for accurate baseline determination) to be2.24 �l H2 (STP) min−1 and corresponds to 10% conversion of

he volumetric rate of inert helium was 80 cm3 He (STP) min−1.he membrane specimen surface area was 7.85 × 10−5 m2 about0% of which was covered with sealing material during theonstruction of the membrane reactor and did not take place

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10 A. Evdou et al. / Journal of Mem

n the reaction. Consequently, a hydrogen production rate of47.5 cm3 H2 (STP) m−2 min−1 can be calculated through compart-ent 1. As found by experiments with other membrane specimens

f identical thickness cut from the same cylindrical sample,he hydrogen production rate is subjected to a relative error of10%. The rate becomes about three times higher and reaches45 cm3 H2 (STP) m−2 min−1 if metal oxide reduction in compart-ent 2 takes place by carbon monoxide. This is quite comparableith hydrogen production from water splitting rates reported in

iterature, with the oxidation of methane as the reduction reaction.nder a water partial pressure of 0.03 bar a hydrogen produc-

ion rate of 1300 cm3 H2 (STP) m−2 min−1 is reported at 1173 K foradolinium doped cerium oxide membrane with nickel [26]. How-ver, taking into account the much thinner membrane (0.46 mm)s well as the much higher water partial pressures used and thelightly higher temperature, a normalized comparable hydrogenroduction rate of 200 cm3 H2 (STP) m−2 min−1 can be calculated.he rate increased almost 10 times when a stronger reductantuch as hydrogen was used. No data, however, are reported forembrane reactor operation on partial pressure based desorptionithout the presence of a reductant.

Taking into account that the purpose of the present work is thexperimental demonstration of a principle and that the system isot optimized neither on kinetics nor on material compositions,e believe that this reactor is worth more accurate quantified

pproach and further optimization on material, membrane andeactor design.

As argued in the first paragraph of this section the use of a carbonontaining reductant such as carbon monoxide is basically unde-irable. The overall reaction in this case is obtained by combiningeactions (1a) and (3):

2O + CO → H2 + CO2 (5)

q. (5) represents the well-known water gas shift reaction (WGS)hich is an important step for hydrogen production in industrial

ycles for upgrading H2 rich streams by CO conversion present inyngas mixtures [41]. Membrane reactor technology based mainlyn palladium membranes [42,43] is being extensively studied forhis reaction. The experiments described in the previous para-raphs indicate that the WGS reaction can also take place inembrane reactors utilizing perovskite oxygen conducting mem-

ranes. An additional advantage is the elimination of separationrocesses and the ability for high purity hydrogen production suit-ble to feed fuel cell units.

The initial introduction of water to the membrane (or to theowdered material) did not lead to water splitting and hydrogenroduction (Fig. 7a). The situation changed if the water introduc-ion was preceded by material reduction with carbon monoxideFig. 8a). It seems that a certain activation process is necessary inrder to bring the material in operation towards continuous pro-uction of hydrogen [39]. The exact mechanism of this activationrocess is not known and will be the subject of further research inhe laboratory.

In this study the stability of the membrane reactor system underoth forced and unforced diffusion conditions was verified for aaximum period of 24 h. Stability experiments of much longer time

ntervals and microstructural investigations on possible effects onhe activity and mechanical integrity of the membrane are required.

. Conclusions

Perovskite materials with the composition La0.3Sr0.7FeO3−ı

ere synthesized in powder and in membrane pellet form, by wethemical methods.

[

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Science 325 (2008) 704–711

The powdered samples reversibly adsorbed and desorbed oxy-en isothermally at high temperatures (e.g. 1133 K). After completexidation oxygen release of 0.014 g oxygen g−1 of material wasound during reduction with methane and oxygen release of.005 g oxygen g−1 of material was found during equilibration withelium.

The powdered samples exhibited remarkable activity for thewo-step thermochemical water splitting reaction. After completeegeneration, ∼370 �mol H2 g−1 of material was produced untilomplete deactivation. The water pulse conversion was graduallyecreasing and depended on the concentration of oxygen vacancies

n the lattice.Disc shaped membrane specimens were utilized for the con-

truction of a membrane reactor consisting of two compartments.embrane oxidation (hydrogen production) in compartment 1

ould proceed simultaneously with membrane reduction (oxy-en desorption or carbon monoxide oxidation) in compartment. This gave rise to a continuous and isothermal water split-ing process. Under unforced reduction conditions a hydrogenroduction rate of 47.5 cm3 H2 (STP) m−2 min−1, was measuredy the result in compartment 1. When the reduction wasnhanced with carbon monoxide the hydrogen production was45 cm3 H2 (STP) m−2 min−1.

cknowledgements

T. Tsilipiras from CERTH’s mechanical workshop is gratefullycknowledged for the construction of the membrane reactor. N.eorgiou and A. Moudiotis, from the laboratory of environmen-

al fuels and hydrocarbons at CERTH, are gratefully acknowledgedor their support in relation to the construction and maintenancef the experimental setup.

The European Network of Excellence “Inside Pores” (NoE00895-2), and the Greek Ministry of Industrial Developmentcontract nr. 05MOXLOS03) are gratefully acknowledged for theirnancial assistance.

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