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Methane oxidation over perovskite-related ferrites: Effects of oxygen nonstoichiometry
Solid State Sciences 7 (2005) 1344–1352 www.elsevier.com/locate/ssscie
Methane oxidation over perovskite-related ferrites: Effects of oxygen nonstoichiometry V.V. Kharton a,b,∗ , M.V. Patrakeev a,c , J.C. Waerenborgh d , V.A. Sobyanin e , S.A. Veniaminov e , A.A. Yaremchenko a , P. Gaczy´nski d , V.D. Belyaev e , G.L. Semin e , J.R. Frade a a Department of Ceramics and Glass Engineering, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal b Institute of Physicochemical Problems, Belarus State University, 14 Leningradskaya Str., 220050 Minsk, Belarus c Institute of Solid State Chemistry, UD RAS, 91 Pervomaiskaya Str., Ekaterinburg 620219, Russia d Chemistry Department, ITN/CFMC-UL, Estrada Nacional 10, P-2686-953 Sacavém, Portugal e Boreskov Institute of Catalysis, SB RAS, 5 pr. Akademika Lavrentieva, Novosibirsk 630090, Russia
Received 30 April 2004; received in revised form 9 August 2005; accepted 28 August 2005 Available online 14 October 2005
Abstract The oxidation of CH4 pulses supplied in helium flow over perovskite-related La0.3 Sr0.7 Fe0.8 M0.2 O3−δ (M = Ga, Al) and SrFe0.7 Al0.3 O3−δ leads to significant yields of CO and H2 after achieving a critical level of oxygen deficiency in the ferrite-based mixed conductors. This effect, reproducible under steady-state conditions in the membrane reactors for methane conversion, may be of interest for the development of monolithic ceramic reactors where the dense membrane and porous catalyst at the permeate-side surface are made of similar compositions. The Mössbauer spectroscopy and coulometric titration studies show that the presence of metallic Fe under typical operation conditions can be neglected, whilst most oxygen vacancies in the ferrite lattices are ordered. Increasing selectivity towards the partial oxidation of methane is observed in the vicinity of the state where the iron cations are predominantly trivalent and massive ordering processes in the oxygen sublattice start. The catalytic activity of ferrite-based materials may hence result from the lattice instability characteristic of morphotropic phase transformations. The correlations between catalytic behavior and oxygen ionic transport are briefly discussed. 2005 Elsevier SAS. All rights reserved. Keywords: Perovskite; Ferrite; Methane oxidation; Mixed conductor; Mössbauer spectroscopy; Oxygen nonstoichiometry; Catalytic activity
1. Introduction Synthesis gas (syngas), a mixture of carbon monoxide and hydrogen, is an important feedstock for commercial Fischer– Tropsch synthesis [1,2]. To date, a large-scale industrial route for syngas production comprises steam reforming of methane, a capital- and energy-intensive process. Alternative technologies are based on the catalytic partial oxidation of methane (POM), where the major confinement relates to the high costs of oxygen plants, and the use of dense ceramic membranes with mixed oxygen-ionic and electronic conductivity [1–5]. The lat* Corresponding author. Present address: Department of Ceramics and Glass Engineering, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal. Fax: +351-234-425300; Tel.: +351-234-370263. E-mail address: [email protected] (V.V. Kharton).
1293-2558/$ – see front matter 2005 Elsevier SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2005.08.004
ter combines oxygen separation from air, POM and reforming, thus enabling to significantly reduce capital investments [4,5]. At the same time, the development of commercially feasible membrane technologies is essentially limited due to materials problems, particularly as a result of rigid requirements to physicochemical, thermomechanical and transport properties of mixed-conducting ceramics. One important aspect in these developments relates to the catalytic activity of membrane materials. Most promising mixed conductors are based on perovskite-related phases containing transition metal cations [4–8]. As a general rule, the transition metal oxides are active for complete oxidation rather than for the POM; a high activity towards POM is typical for metal oxide–metal mixtures, where the steam and/or CO2 reforming on the metal surface follows the total CH4 oxidation reaction on the oxide surface [1]. This makes it necessary to
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incorporate reforming catalysts in the membrane reactors. On the other hand, significant technological and economic advantages could be expected for the monolithic reactors where a thick-film membrane is applied onto porous ceramics, the latter simultaneously acting as a catalyst support, providing necessary mechanical strength and enhancing the surface area of the membrane [5,6]. Due to high operation temperatures, 970–1170 K, using of similar compositions for the membrane and for the support is preferable in order to minimize degradation caused by cation interdiffusion. The choice of such compositions should obviously account their catalytic behavior. An attractive combination of stability, oxygen transport and catalytic properties is known for perovskite-related systems based on (Sr,La)FeO3−δ [5,7–12]. The substitution of iron for cations having a stable oxidation state, such as Al3+ or Ga3+ , decreases oxygen nonstoichiometry variations and, thus, suppresses thermal and chemical expansion under the operation conditions [8–12]. Also, statistical distribution of the B site cations may partially prevent oxygen-vacancy ordering which is typical for most ferrites on reduction, thus increasing ionic transport [11]. Contrary to other transition metal oxidecontaining systems, several examples of a high catalytic activity of perovskite-related ferrites with respect to POM at temperatures above 970 K were reported in the literature [13–17]. The nature of this phenomenon is still unclear. Possible hypotheses ascribed selectivity towards POM to the strongly-bonded lattice oxygen having a low mobility, to a presence of carbonate species on the surface, and to formation of catalytically active centers such as iron carbide or metallic Fe [13–16]. Comparative analysis of the data on catalytic behavior and thermodynamic stability [12,16–19] shows, however, that a critical role of metallic iron is very unlikely. Another assumption requiring further verification is that syngas generation occurs at the regular surface sites, whilst oxygen supplied via extended defects (e.g., vacancy-ordered microdomain boundaries) is mainly consumed for the total oxidation reaction [15,19]. The present work is focused on the analysis of the oxygen nonstoichiometry variations and catalytic behavior of ironcontaining mixed conductors, using La0.3 Sr0.7 Fe0.8 M0.2 O3−δ (M = Ga, Al) and SrFe0.7 Al0.3 O3−δ as model compositions. The data on transport and physicochemical properties of these materials were published elsewhere [9–12,17]. 2. Experimental The synthesis and ceramic processing conditions of La0.3 Sr0.7 Fe0.8 Al0.2 O3−δ , La0.3 Sr0.7 Fe0.8 Ga0.2 O3−δ and SrFe0.7 Al0.3 O3−δ were described in previous publications [9–12]. After sintering in air, the formation of single phases with cubic perovskite structure was verified by X-ray diffraction (XRD); the structural parameters were reported earlier [9–12,20]. For all ferrites, the density of gas-tight ceramics was higher than 92% of the theoretical. The powdered samples, used to assess the behavior in reducing atmospheres by XRD and Mössbauer spectroscopy, were obtained by grinding of dense ceramics annealed at 1023 K in flowing H2 –H2 O–N2 mixtures for 5–25 h with subsequent fast cooling. The oxygen partial pressure in
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H2 -containing gas flows was fixed maintaining a given H2 /H2 O ratio and controlled continuously by an electrochemical oxygen sensor. These p(O2 ) values, in the range 1 × 10−17 to 2 × 10−16 atm, were selected as typical for non-equilibrium mixtures of CH4 conversion products in the membrane reactors where the mixed-conducting ceramics possess moderate oxygen permeability, close to that of La0.3 Sr0.7 Fe0.8 M0.2 O3−δ and SrFe0.7 Al0.3 O3−δ [21]. The XRD patterns of reduced samples were obtained using a Rigaku D/MAX-B diffractometer (CuKα radiation, 2Θ = 20–80◦ , step 0.02◦ , 1 s/step). The Mössbauer spectra were collected between room temperature and 10 K in transmission mode using a conventional constant-acceleration spectrometer and a 25 mCi 57 Co source in a Rh matrix; the low-temperature measurements were performed using a liquidhelium flow cryostat with a temperature stability of ±0.5 K. The velocity scale was calibrated using an α-Fe foil. The absorbers were prepared by pressing the powdered samples (5 mg of natural Fe/cm2 ) into perspex holders. The spectra were fitted to Lorentzian lines using a non-linear least-square method [22] and to distributions of magnetic splittings according to the histogram method [23]. Isomer shifts (IS, Tables 1 and 2) are given relative to metallic α-Fe at room temperature. The coulometric titration technique [11] was used to determine oxygen content as a function of oxygen partial pressure and temperature at 923–1223 K. For oxygen chemical potential values far from the order–disorder transitions discusses below, the reproducibility error of the oxygen nonstoichiometry (δ) values was less than 0.001. In the vicinity of phase changes this error was larger, up to 0.003, due to significant hysteresis phenomena. The values of oxygen ionic conductivity in air Table 1 Parameters estimated from the Mössbauer spectra of reduceda La0.3 Sr0.7 Fe0.8 M0.2 O3−δ , taken at 10 K M
Iron state
IS, mm/s
ε, mm/s
Bhf , T
I, %
Al
CN6-Fe3+ CN4-Fe3+ CN6-Fe3+ CN4-Fe3+
0.45 0.22 0.47 0.27
−0.01 0.28 −0.07 0.44
54.3 45.6 54.1 44.9
94b 6 93b 7
Ga
CN6-Fe3+ and CN4-Fe3+ correspond to octahedrally and tetrahedrally coordinated Fe3+ , respectively. Bhf is the average hyperfine field and IS is the average isomer shift of the magnetic splitting distributions. IS is given relative to metallic α-Fe at 295 K. ε and I are the quadrupole shift and relative area, correspondingly. Estimated standard deviations are < 2% for I , < 0.4 T for Bhf and < 0.02 mm/s for the other parameters. a Reduction was performed at 1023 K and p(O ) = 2 × 10−16 atm during 2 30 h. b A very small contribution of pentacoordinated Fe3+ to these intensity values cannot be excluded. Table 2 Parameters estimated from the Mössbauer spectra of reduceda SrFe0.7 Al0.3 O3−δ , taken at 15 K Domains
Iron state
IS, mm/s
ε, mm/s
Bhf , T
I, %
Perovskite Brownmillerite
Fe3+
0.44 0.47 0.22
−0.28 −0.01 0.16
46.7 53.1 45.6
7 80 13
CN6-Fe3+ CN4-Fe3+
a The material was reduced at 1023 K and p(O ) = 1 × 10−17 atm. 2
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and in reducing atmospheres were calculated from the data on oxygen permeability and faradaic efficiency under oxidizing conditions, and analyzing the oxygen pressure dependencies of the total conductivity at p(O2 ) < 10−9 atm, respectively [9,10, 12,20]. A description of the experimental procedures used for the studies of a model membrane reactor, comprising a dense mixed-conducting membrane and a powdered ferrite catalyst, is found in Ref. [17]. The interaction of ferrite powders with CH4 pulses was examined using a microcatalytic setup [24] in flowing helium; the conditions were analogous to those used in a previous work [17]. As these experiments were aimed to analyze processes occurring on the membrane surface, the powdered samples for catalytic tests were prepared by grinding of dense ceramics in order to provide a relatively low specific surface area, 0.03– 0.06 m2 /g. Prior to the pulse experiments, the samples (0.40 g) were annealed in oxygen flow at 1023 K for 1.5 h. Then a He flow (0.69 cm3 /s) started to pass through the reactor and the amount of desorbed oxygen was determined as a function of time. After annealing in flowing helium, a pulse CH4 probe (1 cm3 STP) was injected during 1 s into the flow. The reaction products were frozen by liquid nitrogen in two sorbent-filled traps, one of which was used to collect CO2 , H2 O and C2 hydrocarbons and another collected CO, H2 , CH4 , N2 and O2 . After a pulse, the traps were rapidly heated and the products were analyzed by gas chromatograph; then this procedure was repeated. The reaction selectivity was calculated as the concentration ratio between a given product and the sum of all detected carbon-containing products, namely CO, CO2 and C2 hydrocarbons (C2 H2 , C2 H4 and C2 H6 ). 3. Results and discussion
Fig. 1. Amount of products per 1 cm3 CH4 pulse supplied in flowing helium onto the La0.3 Sr0.7 Fe0.8 Al0.2 O3−δ powder at 1023 K. The inset shows approximate relationships between oxygen nonstoichiometry, CH4 conversion and CO selectivity (see text).
3.1. Catalytic behavior The interaction of pre-oxidized La0.3 Sr0.7 Fe0.8 Al0.2 O3−δ with CH4 pulses results in predominant CO2 formation, suggesting a major role of total combustion process at the initial stage of ferrite reduction (Fig. 1). For the first 10 pulses, the selectivity towards CO and C2 -hydrocarbon formation varies in the ranges 1.2–2.5% and 9.5–11.0%, respectively; the methane conversion and the yields of all products decrease with the pulse number. A qualitatively similar behavior was also observed in the case of La0.3 Sr0.7 Fe0.8 Ga0.2 O3−δ , but only for the first 4 pulses (Fig. 2). Further reduction of the Ga-substituted ferrite is accompanied with a drastic increase of syngas yield, whilst CO2 selectivity decreases from approximately 98% down to 56%. Contrary to La0.3 Sr0.7 Fe0.8 M0.2 O3−δ , the oxidation of the first CH4 pulses over SrFe0.7 Al0.3 O3−δ surface yields mainly syngas with a H2 : CO ratio of approximately 2, characteristic of the POM process (Fig. 3). The selectivity to CO2 and C2 -hydrocarbon formation is 7–12 times lower than that of carbon monoxide. For comparison, Fig. 3 presents also the data on cobaltite analogue of the Ga-containing ferrite, La0.3 Sr0.7 Co0.8 Ga0.2 O3−δ , characterized in previous works [17, 25]. In the latter case, a relatively smaller and time-independent
CO2 yield accompanied with progressive evolution of molecular oxygen is observed; due to oxidative dimerization the presence of gaseous O2 may cause formation of the significant amounts of C2 -hydrocarbons. These results unambiguously show that catalytic activity of perovskite-like ferrites towards the total and partial oxidation of methane is directly related to the oxygen nonstoichiometry variations. After pre-oxidation and treatment in a He flow under similar conditions, one may expect similar values of the oxygen chemical potential at the surface of La0.3 Sr0.7 Fe0.8 M0.2 O3−δ and SrFe0.7 Al0.3 O3−δ . In this state, CH4 oxidation over La0.3 Sr0.7 Fe0.8 M0.2 O3−δ , where the oxygen deficiency is lower than that in SrFe0.7 Al0.3 O3−δ due to a higher average charge of the A-site cations, leads primarily to CO2 formation; for SrFe0.7 Al0.3 O3−δ the yield of CO is considerably higher with respect to carbon dioxide. Increasing the oxygen vacancy concentration in La0.3 Sr0.7 Fe0.8 M0.2 O3−δ increases the tendency to the POM reaction. Although the latter is also contributed by shifting CO/CO2 thermodynamic equilibrium ratio due to progressive decrease of the oxygen chemical potential, this trend and the difference in initial behavior of La0.3 Sr0.7 Fe0.8 M0.2 O3−δ and SrFe0.7 Al0.3 O3−δ confirm the relevance of oxygen stoichiometry as the POM
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Fig. 3. Product concentrations in 1 cm3 CH4 pulses supplied in flowing He onto the powders of SrFe0.7 Al0.3 O3−δ and La0.3 Sr0.7 Co0.8 Ga0.2 O3−δ at 1023 K. Fig. 2. Amount of products per 1 cm3 CH4 pulse supplied in flowing helium onto the La0.3 Sr0.7 Fe0.8 Ga0.2 O3−δ powder at 1023 K. The inset shows approximate relationships between oxygen nonstoichiometry, CH4 conversion and CO selectivity (see text).
activity-determining factor. Moreover, as shown by coulometric titration (Fig. 4), the differences in catalytic behavior of La0.3 Sr0.7 Fe0.8 Al0.2 O3−δ and La0.3 Sr0.7 Fe0.8 Ga0.2 O3−δ (Figs. 1 and 2) also correlate with the variations of δ values. The equilibrium oxygen content in La0.3 Sr0.7 Fe0.8 Al0.2 O3−δ is very similar to that in La0.3 Sr0.7 Fe0.8 Ga0.2 O3−δ under oxidizing conditions, but becomes higher with respect to the Gasubstituted ferrite on reduction. As a result, oxygen losses from the lattice of La0.3 Sr0.7 Fe0.8 Ga0.2 O3−δ in the course of CH4 oxidation are larger compared to La0.3 Sr0.7 Fe0.8 Al0.2 O3−δ ; the former composition faster achieves a critical level of the oxygen deficiency necessary to promote POM. These tendencies are in excellent agreement with the data on oxygen desorption (Fig. 5). The insets in Figs. 1 and 2 show the overall changes of oxygen deficiency of the ferrite-based phases in the course of pulse experiments. The nonstoichiometry estimations were made from the molar ratios between catalyst, injected methane and products, assuming that the oxygen content in ferrites prior to CH4 oxidation tests is equilibrium (Fig. 4) and that the carbon and hydrogen balances are 100%. For both La-containing compositions, the values of δ are close to 0.35, the stoichiometric state where the average oxidation state of iron cations
is 3+. Taking into account the relatively high oxygen content and significant oxygen-ion mobility in the title materials [9, 10,12], formation of metallic iron on their surface during the catalytic experiments seems very unlikely. Furthermore, under similar experimental conditions no increase in the CO yield due to possible separation of Co metal was observed in the case of La0.3 Sr0.7 Co0.8 Ga0.2 O3−δ (Fig. 3), despite higher reducibility of the cobaltite compound. Another necessary comment is that general trends of catalytic behavior, identified in the course of pulse experiments, are also observed when using ferrite-based materials as catalysts in the membrane reactors for CH4 oxidation. As an illustration, Fig. 6 presents the time dependencies of CH4 conversion rate and CO selectivity for a model reactor comprising one dense membrane and a powdered catalyst, both made of SrFe0.7 Al0.3 O3−δ ; the reactor configuration and the conditions of materials processing and testing were described elsewhere [17]. Since no deep optimization of the reactor and its operation regime was performed, the selectivity to CO formation in steady-state conditions was slightly lower compared to the pulse experiments, achieving 63% at 1223 K. At the same time, the performance was found essentially time-independent, corroborating the absence of bulk reduction of the ferrite catalyst. This conclusion was confirmed by XRD (inset in Fig. 6). Note that XRD analysis did not reveal apparently the transition of perovskite into vacancy-ordered phases, characteristic of strontium ferrite-based materials ([26,27] and references cited); most
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Fig. 6. Time dependence of CH4 conversion and CO selectivity in a reactor with dense membrane and powdered catalysts both made of SrFe0.7 Al0.3 O3−δ . The operation conditions are given in the legend. The inset shows XRD pattern of SrFe0.7 Al0.3 O3−δ after operation during approximately 100 h.
Fig. 4. Oxygen partial pressure dependencies of the oxygen content in La0.3 Sr0.7 Fe0.8 M0.2 O3−δ , determined by coulometric titration. Solid lines are for visual guidance only. Arrows show approximate p(O2 ) values corresponding to the start of disorder-order transition.
Fig. 5. Time dependence of oxygen amount desorbed from the pre-oxidized samples into He flow at 1023 K.
likely, the ordering processes cannot be distinguished by XRD due to low sensitivity of this method to the oxygen sublattice. In order to evaluate the exact phase composition and the states of iron relevant to the catalytic behavior, the Mössbauer spectroscopy was used. 3.2. Mössbauer spectra The Mössbauer spectra of La0.3 Sr0.7 Fe0.8 M0.2 O3−δ reduced under typical conditions for non-equilibrium mixtures of CH4 conversion products in the membrane reactors [21], consist mainly of a six-peak pattern (Figs. 7 and 8). At room temperature the peaks are asymmetrically broadened, while at 10 K it is possible to distinguish the presence of a second sextet. The relatively poor resolution at 295 K results probably from the decrease in the magnetic sextets splittings with increasing temperature and/or from peak broadening due to a faster relaxation of the magnetic moments. Even at 10 K the peaks are broad and the spectra may only be adequately fitted by two distributions of magnetic hyperfine fields. A dependence of the isomer shift (IS) on the magnetic hyperfine field Bhf was also considered. The average values estimated for IS and Bhf are given in Table 1. The results show that all the iron, within detection limits, is in the Fe3+ state. A similar conclusion was reached analyzing the 15 K Mössbauer spectrum of SrFe0.7 Al0.3 O3−δ , reduced at 1023 K and p(O2 ) = 10−17 atm. As for the other ferrites, no accurate information in the latter case might be obtained from the room-temperature data due to peak broadening. At 15 K, SrFe0.7 Al0.3 O3−δ is close to magnetic saturation and the resolution of the spectrum (Fig. 8(C)) is as good as that of the 10 K spectra of La0.3 Sr0.7 Fe0.8 M0.2 O3−δ . The spectrum of SrFe0.7 Al0.3 O3−δ may be described using two distributions of
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Fig. 7. Room-temperature Mössbauer spectra of La0.3 Sr0.7 Fe0.8 Al0.2 O3−δ (A) and La0.3 Sr0.7 Fe0.8 Ga0.2 O3−δ (B) quenched after annealing at 1023 K and p(O2 ) = 2 × 10−16 atm during 25 h.
Fig. 8. Mössbauer spectra of reduced La0.3 Sr0.7 Fe0.8 Al0.2 O3−δ at 10 K (A), La0.3 Sr0.7 Fe0.8 Ga0.2 O3−δ at 10 K (B) and SrFe0.7 Al0.3 O3−δ at 15 K (C). The La0.3 Sr0.7 Fe0.8 M0.2 O3−δ samples were quenched after annealing at 1023 K and p(O2 ) = 2 × 10−16 atm during 25 h. SrFe0.7 Al0.3 O3−δ was reduced at 1023 K and p(O2 ) = 1 × 10−17 atm. The line on the experimental points is the sum of two distributions of magnetic splittings attributed to tetrahedrally and octahedrally coordinated Fe3+ , and for (C) an additional sextet assigned to Fe3+ in perovskite-like domains, shown slightly shifted, for clarity. The probability distributions (P ) of the magnetic hyperfine fields (Bhf ) are shown on the right-hand side of the corresponding spectra.
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magnetic splittings, with the average parameters consistent with those of 6- and 4-coordinated Fe3+ in a brownmillerite-type lattice [28,29]. The estimated relative areas I (Table 2) indicate that Al3+ prefers tetrahedral coordination, whilst Fe3+ tends to occupy octahedrally-coordinated sites, as in brownmillerite Ca2 FeAlO5+δ [28,29]. No magnetic splittings with Bhf lower than 40 T are observed. Therefore, neither traces of metallic iron nor splittings typical for Fe4+ can be detected. On the other hand, a significant improvement in the fitting quality was further achieved considering a third contribution, with the parameters typical for Fe3+ in a perovskite-type lattice such as those estimated for SrFe0.7 Al0.3 O3−δ equilibrated in air [30]; the perovskite phase contains approximately 7% of the total iron amount. Reduction of SrFe0.7 Al0.3 O3−δ at 1023 K occurs, therefore, via the formation of brownmillerite-like domains coexisting with perovskite domains down to relatively low oxygen chemical potentials. Such a behavior is typical for perovskitelike ferrites (for instance, [5,11,15,26–28,31] and references therein); analysis of the literature data shows that this local ordering can be expected up to, at least, 1200 K. On the contrary, if considering a third sextet describing the possible presence of perovskite-like domains in La0.3 Sr0.7 Fe0.8 M0.2 O3−δ , there is no improvement in the fitting quality and the relative area of this additional sextet consistently refines to zero. The average IS and Bhf values (Table 1), falling in the range characteristic of 6- and 4-coordinated Fe3+ at 10 K [30,31], are similar to those for orthorhombic Sr2 LaFe3 O8 [32]. This indicates that vacancy-ordered domains prevail in La0.3 Sr0.7 Fe0.8 M0.2 O3−δ . As for SrFe0.7 Al0.3 O3−δ , such ordering cannot be detected from the XRD data (Fig. 9). While the tetrahedral Fe3+ Bhf distributions turned out to be symmetric, with coincident average and most probable IS and Bhf values, the octahedral Fe3+ distributions in the La0.3 Sr0.7 Fe0.8 M0.2 O3−δ spectra exhibit a tail in the low Bhf range which is more conspicuous for the Ga-containing compound (Fig. 8). Furthermore, as the best adequacy was achieved for a positive dependence of IS on Bhf , the contributions with the lowest Bhf for these distributions, in the range 46–49 T for both spectra, also have the lowest IS, in the range 0.39– 0.41 mm/s. The tails of the octahedral Fe3+ distributions seem, hence, contributed by the presence of low amounts of pentacoordinated Fe3+ [31], existing in microdomains where the oxygen vacancies are still disordered, at least in part. For these tails, the relative value of accumulated probabilities for Bhf up to 49 T is higher for La0.3 Sr0.7 Fe0.8 Ga0.2 O3−δ , 7.1%, as compared to 2.6% for La0.3 Sr0.7 Fe0.8 Al0.2 O3−δ (Fig. 8). Such difference is larger than the experimental uncertainties, suggesting a higher degree of local disorder in the oxygen sublattice of the Ga-substituted ferrite. The broadening of Fe3+ absorption peaks in the spectra of La0.3 Sr0.7 Fe0.8 M0.2 O3−δ , described by sextet distributions instead of single sextets as in orthorhombic Sr2 LaFe3 O8 [32], may arise from several factors. These include local lattice distortions near M 3+ cations having a strong influence on the oxygen sublattice [11], and the presence of pentacoordinated Fe3+ and different environments due to random distribution of the B-site dopant cations in each sublattice. In orthorhombic
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Fig. 10. The equilibrium p(O2 )–T –δ diagram of SrFe0.7 Al0.3 O3−δ , determined by coulometric titration. The inset shows XRD patterns of oxidized and reduced SrFe0.7 Al0.3 O3−δ . Arrows show approximate start of the disorder-order transition.
Fig. 9. XRD patterns of La0.3 Sr0.7 Fe0.8 M0.2 O3−δ equilibrated at low temperatures in air (A) and reduced at 1023 K and p(O2 ) = 2 × 10−16 atm (B).
Sr2 LaFe3 O8 where the oxygen vacancy concentration is very similar to that in reduced La0.3 Sr0.7 Fe0.8 M0.2 O3−δ (Fig. 4), the ratio of octahedral to tetrahedral iron sites is 2 : 1 [32]. The data summarized in Table 1 show, hence, that Fe3+ cations in La0.3 Sr0.7 Fe0.8 M0.2 O3−δ possess a strong preference to octahedral coordination as compared to either Ga3+ or Al3+ . This tendency is even more pronounced than that in brownmilleritelike SrFe0.7 Al0.3 O2.5 phase (Table 2). 3.3. Relationships between oxygen stoichiometry, catalytic activity and ionic transport The estimations made from the Mössbauer spectroscopy results were verified by the coulometric titration method. As an example, the values of total oxygen content in SrFe0.7 Al0.3 O3−δ at 1023 K and p(O2 ) = 1 × 10−17 atm, determined by these two techniques, are 2.499 and 2.500 per formula unit, respectively. For La0.3 Sr0.7 Fe0.8 M0.2 O3−δ reduced at p(O2 ) = 2 × 10−16 atm, the corresponding values are 2.646 and 2.650. This difference is within the limits of experimental uncertainty. The equilibrium p(O2 )–T –δ diagrams presented in Figs. 4 and 10, all exhibit a stepwise decrease of the oxygen content at oxygen chemical potentials where a massive transfor-
mation into vacancy-ordered modifications occurs, i.e., the content of ordered domains becomes critical. In the case of SrFe0.7 Al0.3 O3−δ , this transition starts at oxygen partial pressures below 10−5 atm when the average oxidation state of iron cations is slightly higher than 3+. For La0.3 Sr0.7 Fe0.8 M0.2 O3−δ , extensive ordering processes are observed at considerably lower oxygen chemical potentials; decreasing temperature leads to a shift of the p(O2 ) values characteristic of “disorder–order” transition towards more reducing conditions, suggesting possible stabilization of the domain microstructure at low temperatures. The latter phenomenon may result from the interfacial contribution to the thermodynamic functions of partially ordered materials, caused by the space charge effects at the microdomain boundaries [33]. If comparing the data on equilibrium oxygen nonstoichiometry and oxygen desorption kinetics (Figs. 4, 5 and 10), one may conclude that the disorder–order transition in the oxygen sublattice of SrFe0.7 Al0.3 O3−δ should, at least, start after the treatment in a He flow; in contrast, oxygen vacancies in the lattices of La0.3 Sr0.7 Fe0.8 M0.2 O3−δ (M = Ga, Al) should mainly remain disordered. This seems responsible for the difference in the initial catalytic behavior (Figs. 1–3). In fact, the tendency to POM increases when the oxygen content in La0.3 Sr0.7 Fe0.8 M0.2 O3−δ becomes close to 2.65 and extensive ordering processes start (Fig. 4 and inset in Fig. 2). At the same time, literature data [15,19] show that the catalytic activity of brownmillerite-like ferrite phases with a completely ordered oxygen sublattice is relatively low; for the perovskite-related ferrites with ordered microdomains, the highest activity is observed at elevated temperatures, 1050– 1170 K, when the perovskite-brownmillerite phase boundary
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Fig. 11. Partial molar enthalpy of oxygen in the ferrite-based materials, calculated from the oxygen stoichiometry data (see text). Solid lines are for visual guidance only.
is mobile due to thermally-induced disordering [13–16]. Also, the maximum syngas yields over ferrite-based catalysts are achieved supplying alternate CH4 and O2 pulses [14,16]. The high catalytic activity of perovskite-related ferrites, such as La0.3 Sr0.7 Fe0.8 M0.2 O3−δ and SrFe0.7 Al0.3 O3−δ , towards syngas generation may therefore originate from the lattice instability due to morphotropic phase transformations on reduction. In order to illustrate these statements, Fig. 11 presents the values of the partial molar enthalpy of oxygen (HO ), calculated using the well-known equation [11,34,35] 1 µO (δ, T ) = RT · ln p(O2 ) = HO (δ) − T SO (δ) (1) 2 where SO is the partial molar entropy of oxygen, the temperature dependence of the oxygen chemical potential variations with respect to a standard state in the gas phase (µO ) at δ = const is approximated by a linear function, and all thermodynamic quantities are related to one mole of oxygen atoms at 923–1223 K. Under oxidizing conditions, the oxygennonstoichiometry dependence of the partial oxygen enthalpy is rather weak, suggesting that oxygen intercalation into the disordered perovskite phases can be described by the ideal solution model [11]. Increasing vacancy concentration leads to anomalous behavior in the vicinity of order–disorder transition due to perovskite lattice instability; the approximate δ ranges, where the validity of Eq. (1) is very limited, are marked by open squares in Fig. 11. On further reduction the values of HO drastically decrease since the oxygen sublattice of ferrites is predominantly ordered and the vacancy formation process becomes energetically less favorable. The ionic conductivity of all studied materials increases with decreasing p(O2 ), despite the progressive vacancy ordering (Fig. 12). This suggests that a significant part of oxygen vacancies are trapped even under oxidizing conditions, prob-
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Fig. 12. Temperature dependence of oxygen ionic conductivity in the ferrite-based ceramics at atmospheric oxygen pressure (open symbols) and under reducing conditions (closed symbols). Solid lines are for visual guidance only.
ably in the form of defect associates. As the radius of Al3+ is substantially smaller compared to Fe3+ and Ga3+ [36], the substitution of iron for aluminum promotes defect clustering [10, 37], thus decreasing ionic transport at oxygen pressures close to atmospheric. The level of ionic conduction correlates with the rates of oxygen desorption and total CH4 oxidation at the initial stage of catalytic experiments (Figs. 1–3 and 5), because the latter processes are both governed by the bonding energy of oxygen in the perovskite-related lattice and oxygen supply rate on the ferrite surface. On the other hand, minimum CO2 yield was found for SrFe0.7 Al0.3 O3−δ where the ionic conduction is relatively low, but the concentration of ordered domain boundaries should be higher with respect to La0.3 Sr0.7 Fe0.8 M0.2 O3−δ . It seems quite obvious that oxygen transport via the boundaries cannot be considered as a factor having key influence on the total oxidation rate, which is determined by the oxygen chemical potential at the ferrite surface and, thus, influenced by the overall level of ionic conductivity comprising both bulk and boundary contributions. Under reducing conditions another situation is observed. Namely, the ionic conduction increases as La0.3 Sr0.7 Fe0.8 Al0.2 O3−δ < La0.3 Sr0.7 Fe0.8 Ga0.2 O3−δ < SrFe0.7 Al0.3 O3−δ , which corresponds to decreasing total oxygen content and increasing lattice disorder detected by the Mössbauer spectroscopy. In principle, such tendency might indeed suggest a significant contribution of oxygen diffusion along domain boundaries as assumed in Refs. [15,19]; verification of the latter hypothesis requires, however, additional investigations since the overall boundary concentration is expected to be rather low. Whatever the mechanism of oxygen transport, the results clearly show that the catalytic activity towards syngas formation correlates with the level of ionic conductivity, as these properties are both
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dependent on the fraction of disordered oxygen vacancies and the metal-oxygen bond energy. 4. Conclusions The comparative analysis of equilibrium p(O2 )–T –δ diagrams, Mössbauer spectra and interaction of CH4 pulses with the surface of perovskite-related SrFe0.7 Al0.3 O3−δ , La0.3 Sr0.7 Fe0.8 Ga0.2 O3−δ and La0.3 Sr0.7 Fe0.8 Al0.2 O3−δ reveals qualitatively similar trends in the catalytic behavior. Namely, the selectivity towards the total and partial oxidation of methane was found determined by the oxygen nonstoichiometry level. Increasing CO yield is observed when the dominant oxidation state of iron cations is 3+. This state is characterized with lattice instability due to massive ordering processes in the oxygen sublattice. The catalytic activity towards POM correlates with the degree of oxygen-vacancy disorder and, hence, with the level of oxygen ionic conductivity, which also has a primary importance for membrane applications. Although one cannot completely exclude the presence of trace amounts of metallic iron on the ferrite surface and its contribution to the catalytic processes, the Mössbauer spectroscopy data suggest that this effect, if any, should be minor. The optimization of Fe-containing mixed conductors for the monolithic membrane reactors requires, therefore, extensive studies of oxygen thermodynamics in order to identify factors, enabling to shift order-disorder transition towards the oxygen chemical potentials typical for the operation conditions. Acknowledgements This work was supported by the NATO Science for Peace program (project 978002), the FCT, Portugal (projects POCTI/ CTM/58570/2004, SFRH/BPD/11606/2002 and SFRH/BPD/ 17649/2004), and the Siberian Branch of the Russian Academy of Sciences (Integration project No 42). Helpful discussions and experimental contributions, made by A.A. Valente, A.L. Shaula and A.P. Viskup, are gratefully acknowledged. References [1] A.P.E. York, T. Xiao, M.L.H. Green, Top. Catal. 22 (2003) 345. [2] D.J. Wilhelm, D.R. Simbeck, A.D. Karp, R.L. Dickenson, Fuel Process. Technol. 71 (2001) 139. [3] K. Aasberg-Petersen, J.-H.B. Hansen, T.S. Christensen, I. Dybkjaer, P.S. Christensen, C.S. Nielsen, S.E.L.W. Madsen, J.R. Rostrup-Nielsen, Appl. Catal. A 221 (2001) 379. [4] T.J. Mazanec, R. Prasad, R. Odegard, C. Steyn, E.T. Robinson, Stud. Surf. Sci. Catal. 136 (2001) 147. [5] H.J.M. Bouwmeester, A.J. Burggraaf, in: A.J. Burggraaf, L. Cot (Eds.), Fundamentals of Inorganic Membrane Science and Technology, Elsevier, Amsterdam, 1996, pp. 435–528.
[6] M.F. Carolan, P.N. Dyer, US Patent 5569633 (1996). [7] T.J. Mazanec, T.L. Cable, J.G. Frye, W.R. Kliewer, US Patent 5591315 (1997). [8] M. Schwartz, J.H. White, A.F. Sammells, US Patent 6214757 (2001). [9] V.V. Kharton, A.A. Yaremchenko, M.V. Patrakeev, E.N. Naumovich, F.M.B. Marques, J. Europ. Ceram. Soc. 23 (2003) 1417. [10] A.A. Yaremchenko, M.V. Patrakeev, V.V. Kharton, F.M.B. Marques, I.A. Leonidov, V.L. Kozhevnikov, Solid State Sci. 6 (2004) 357. [11] M.V. Patrakeev, E.B. Mitberg, A.A. Lakhtin, V.L. Kozhevnikov, V.V. Kharton, M.Yu. Avdeev, F.M.B. Marques, J. Solid State Chem. 167 (2002) 203. [12] A.L. Shaula, V.V. Kharton, M.V. Patrakeev, J.C. Waerenborgh, D.P. Rojas, F.M.B. Marques, Ionics 10 (2004) 378. [13] A.G. Andersen, T. Hayakawa, M. Shimizu, K. Suzuki, K. Tokehira, Catal. Lett. 23 (1994) 59. [14] R. Li, C. Yu, S. Shen, J. Natural Gas Chem. 11 (2002) 137. [15] V.A. Sadykov, L.A. Isupova, I.S. Yakovleva, G.M. Alikina, A.I. Lukashevich, S. Neophytides, React. Kinet. Catal. Lett. 81 (2004) 393. [16] F. Martinez-Ortega, C. Batiot-Dupeyrat, G. Valderrama, J.-M. Tatibouët, C. R. Acad. Sci. IIC 4 (2001) 49. [17] V.V. Kharton, A.A. Yaremchenko, A.A. Valente, V.A. Sobyanin, V.D. Belyaev, G.L. Semin, S.A. Veniaminov, E.V. Tsipis, A.L. Shaula, J.R. Frade, J. Rocha, Solid State Ionics 176 (2005) 781. [18] A. Slagtern, H.M. Swaan, U. Olsbye, I.M. Dahl, C. Mirodatos, Catal. Today 46 (1998) 107. [19] L.A. Isupova, I.S. Yakovleva, V.A. Rogov, G.M. Alikina, V.A. Sadykov, Kinet. Catal. 45 (2004) 446. [20] V.V. Kharton, A.L. Shaulo, A.P. Viskup, M.Yu. Avdeev, A.A. Yaremchenko, M.V. Patrakeev, A.I. Kurbakov, E.N. Naumovich, F.M.B. Marques, Solid State Ionics 150 (2002) 229. [21] A.A. Yaremchenko, A.A. Valente, V.V. Kharton, E.V. Tsipis, J.R. Frade, E.N. Naumovich, J. Rocha, F.M.B. Marques, Catal. Lett. 91 (2003) 169. [22] J.C. Waerenborgh, M.O. Figueiredo, J.M.P. Cabral, L.C.J. Pereira, Phys. Chem. Min. 21 (1994) 460. [23] J. Hesse, A. Rübartsch, J. Phys. E 7 (1974) 526. [24] V.P. Shchukin, S.A. Veniaminov, G.K. Boreskov, Kinet. Catal. 12 (1971) 621. [25] 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. 9 (2005) 10. [26] J.P. Hodges, S. Short, J.D. Jorgensen, X. Xiong, B. Dabrowski, S.M. Mini, C.W. Kimball, J. Solid State Chem. 151 (2000) 190. [27] A. Lebon, P. Adler, C. Bernhard, A.V. Boris, A.V. Pimenov, A. Maljuk, C.T. Lin, C. Ulrich, B. Keimer, Phys. Rev. Lett. 92 (2004) 037202. [28] R.W. Grant, J. Chem. Phys. 51 (1969) 1156. [29] J.C. Waerenborgh, D.P. Rojas, N.P. Vyshatko, A.L. Shaula, V.V. Kharton, I.P. Marozau, E.N. Naumovich, Mater. Lett. 57 (2003) 4388. [30] J.C. Waerenborgh, D.P. Rojas, A.L. Shaula, G.C. Mather, M.V. Patrakeev, V.V. Kharton, J.R. Frade, Mater. Lett. 59 (2005) 1644. [31] J.C. Waerenborgh, F.M. Figueiredo, J.R. Frade, M.T. Colomer, J.R. Jurado, J. Phys.: Condens. Mat. 13 (2001) 8171. [32] P.D. Battle, T.C. Gibb, S. Nixon, J. Solid State Chem. 79 (1989) 75. [33] J. Maier, Solid State Ionics 157 (2003) 327. [34] L.-M. Liu, T.H. Lee, L. Qiu, Y.L. Yang, A.J. Jacobson, Mater. Res. Bull. 31 (1996) 29. [35] M.H.R. Lankhorst, PhD Thesis, University of Twente, Enschede, Netherlands (1997). [36] R.D. Shannon, Acta Cryst. A 32 (1976) 751. [37] J.A. Kilner, Solid State Ionics 129 (2000) 13.
Methane oxidation over perovskite-related ferrites: Effects of oxygen nonstoichiometry V.V. Kharton a,b,∗ , M.V. Patrakeev a,c , J.C. Waerenborgh d , V.A. Sobyanin e , S.A. Veniaminov e , A.A. Yaremchenko a , P. Gaczy´nski d , V.D. Belyaev e , G.L. Semin e , J.R. Frade a a Department of Ceramics and Glass Engineering, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal b Institute of Physicochemical Problems, Belarus State University, 14 Leningradskaya Str., 220050 Minsk, Belarus c Institute of Solid State Chemistry, UD RAS, 91 Pervomaiskaya Str., Ekaterinburg 620219, Russia d Chemistry Department, ITN/CFMC-UL, Estrada Nacional 10, P-2686-953 Sacavém, Portugal e Boreskov Institute of Catalysis, SB RAS, 5 pr. Akademika Lavrentieva, Novosibirsk 630090, Russia
Received 30 April 2004; received in revised form 9 August 2005; accepted 28 August 2005 Available online 14 October 2005
Abstract The oxidation of CH4 pulses supplied in helium flow over perovskite-related La0.3 Sr0.7 Fe0.8 M0.2 O3−δ (M = Ga, Al) and SrFe0.7 Al0.3 O3−δ leads to significant yields of CO and H2 after achieving a critical level of oxygen deficiency in the ferrite-based mixed conductors. This effect, reproducible under steady-state conditions in the membrane reactors for methane conversion, may be of interest for the development of monolithic ceramic reactors where the dense membrane and porous catalyst at the permeate-side surface are made of similar compositions. The Mössbauer spectroscopy and coulometric titration studies show that the presence of metallic Fe under typical operation conditions can be neglected, whilst most oxygen vacancies in the ferrite lattices are ordered. Increasing selectivity towards the partial oxidation of methane is observed in the vicinity of the state where the iron cations are predominantly trivalent and massive ordering processes in the oxygen sublattice start. The catalytic activity of ferrite-based materials may hence result from the lattice instability characteristic of morphotropic phase transformations. The correlations between catalytic behavior and oxygen ionic transport are briefly discussed. 2005 Elsevier SAS. All rights reserved. Keywords: Perovskite; Ferrite; Methane oxidation; Mixed conductor; Mössbauer spectroscopy; Oxygen nonstoichiometry; Catalytic activity
1. Introduction Synthesis gas (syngas), a mixture of carbon monoxide and hydrogen, is an important feedstock for commercial Fischer– Tropsch synthesis [1,2]. To date, a large-scale industrial route for syngas production comprises steam reforming of methane, a capital- and energy-intensive process. Alternative technologies are based on the catalytic partial oxidation of methane (POM), where the major confinement relates to the high costs of oxygen plants, and the use of dense ceramic membranes with mixed oxygen-ionic and electronic conductivity [1–5]. The lat* Corresponding author. Present address: Department of Ceramics and Glass Engineering, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal. Fax: +351-234-425300; Tel.: +351-234-370263. E-mail address: [email protected] (V.V. Kharton).
1293-2558/$ – see front matter 2005 Elsevier SAS. All rights reserved. doi:10.1016/j.solidstatesciences.2005.08.004
ter combines oxygen separation from air, POM and reforming, thus enabling to significantly reduce capital investments [4,5]. At the same time, the development of commercially feasible membrane technologies is essentially limited due to materials problems, particularly as a result of rigid requirements to physicochemical, thermomechanical and transport properties of mixed-conducting ceramics. One important aspect in these developments relates to the catalytic activity of membrane materials. Most promising mixed conductors are based on perovskite-related phases containing transition metal cations [4–8]. As a general rule, the transition metal oxides are active for complete oxidation rather than for the POM; a high activity towards POM is typical for metal oxide–metal mixtures, where the steam and/or CO2 reforming on the metal surface follows the total CH4 oxidation reaction on the oxide surface [1]. This makes it necessary to
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incorporate reforming catalysts in the membrane reactors. On the other hand, significant technological and economic advantages could be expected for the monolithic reactors where a thick-film membrane is applied onto porous ceramics, the latter simultaneously acting as a catalyst support, providing necessary mechanical strength and enhancing the surface area of the membrane [5,6]. Due to high operation temperatures, 970–1170 K, using of similar compositions for the membrane and for the support is preferable in order to minimize degradation caused by cation interdiffusion. The choice of such compositions should obviously account their catalytic behavior. An attractive combination of stability, oxygen transport and catalytic properties is known for perovskite-related systems based on (Sr,La)FeO3−δ [5,7–12]. The substitution of iron for cations having a stable oxidation state, such as Al3+ or Ga3+ , decreases oxygen nonstoichiometry variations and, thus, suppresses thermal and chemical expansion under the operation conditions [8–12]. Also, statistical distribution of the B site cations may partially prevent oxygen-vacancy ordering which is typical for most ferrites on reduction, thus increasing ionic transport [11]. Contrary to other transition metal oxidecontaining systems, several examples of a high catalytic activity of perovskite-related ferrites with respect to POM at temperatures above 970 K were reported in the literature [13–17]. The nature of this phenomenon is still unclear. Possible hypotheses ascribed selectivity towards POM to the strongly-bonded lattice oxygen having a low mobility, to a presence of carbonate species on the surface, and to formation of catalytically active centers such as iron carbide or metallic Fe [13–16]. Comparative analysis of the data on catalytic behavior and thermodynamic stability [12,16–19] shows, however, that a critical role of metallic iron is very unlikely. Another assumption requiring further verification is that syngas generation occurs at the regular surface sites, whilst oxygen supplied via extended defects (e.g., vacancy-ordered microdomain boundaries) is mainly consumed for the total oxidation reaction [15,19]. The present work is focused on the analysis of the oxygen nonstoichiometry variations and catalytic behavior of ironcontaining mixed conductors, using La0.3 Sr0.7 Fe0.8 M0.2 O3−δ (M = Ga, Al) and SrFe0.7 Al0.3 O3−δ as model compositions. The data on transport and physicochemical properties of these materials were published elsewhere [9–12,17]. 2. Experimental The synthesis and ceramic processing conditions of La0.3 Sr0.7 Fe0.8 Al0.2 O3−δ , La0.3 Sr0.7 Fe0.8 Ga0.2 O3−δ and SrFe0.7 Al0.3 O3−δ were described in previous publications [9–12]. After sintering in air, the formation of single phases with cubic perovskite structure was verified by X-ray diffraction (XRD); the structural parameters were reported earlier [9–12,20]. For all ferrites, the density of gas-tight ceramics was higher than 92% of the theoretical. The powdered samples, used to assess the behavior in reducing atmospheres by XRD and Mössbauer spectroscopy, were obtained by grinding of dense ceramics annealed at 1023 K in flowing H2 –H2 O–N2 mixtures for 5–25 h with subsequent fast cooling. The oxygen partial pressure in
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H2 -containing gas flows was fixed maintaining a given H2 /H2 O ratio and controlled continuously by an electrochemical oxygen sensor. These p(O2 ) values, in the range 1 × 10−17 to 2 × 10−16 atm, were selected as typical for non-equilibrium mixtures of CH4 conversion products in the membrane reactors where the mixed-conducting ceramics possess moderate oxygen permeability, close to that of La0.3 Sr0.7 Fe0.8 M0.2 O3−δ and SrFe0.7 Al0.3 O3−δ [21]. The XRD patterns of reduced samples were obtained using a Rigaku D/MAX-B diffractometer (CuKα radiation, 2Θ = 20–80◦ , step 0.02◦ , 1 s/step). The Mössbauer spectra were collected between room temperature and 10 K in transmission mode using a conventional constant-acceleration spectrometer and a 25 mCi 57 Co source in a Rh matrix; the low-temperature measurements were performed using a liquidhelium flow cryostat with a temperature stability of ±0.5 K. The velocity scale was calibrated using an α-Fe foil. The absorbers were prepared by pressing the powdered samples (5 mg of natural Fe/cm2 ) into perspex holders. The spectra were fitted to Lorentzian lines using a non-linear least-square method [22] and to distributions of magnetic splittings according to the histogram method [23]. Isomer shifts (IS, Tables 1 and 2) are given relative to metallic α-Fe at room temperature. The coulometric titration technique [11] was used to determine oxygen content as a function of oxygen partial pressure and temperature at 923–1223 K. For oxygen chemical potential values far from the order–disorder transitions discusses below, the reproducibility error of the oxygen nonstoichiometry (δ) values was less than 0.001. In the vicinity of phase changes this error was larger, up to 0.003, due to significant hysteresis phenomena. The values of oxygen ionic conductivity in air Table 1 Parameters estimated from the Mössbauer spectra of reduceda La0.3 Sr0.7 Fe0.8 M0.2 O3−δ , taken at 10 K M
Iron state
IS, mm/s
ε, mm/s
Bhf , T
I, %
Al
CN6-Fe3+ CN4-Fe3+ CN6-Fe3+ CN4-Fe3+
0.45 0.22 0.47 0.27
−0.01 0.28 −0.07 0.44
54.3 45.6 54.1 44.9
94b 6 93b 7
Ga
CN6-Fe3+ and CN4-Fe3+ correspond to octahedrally and tetrahedrally coordinated Fe3+ , respectively. Bhf is the average hyperfine field and IS is the average isomer shift of the magnetic splitting distributions. IS is given relative to metallic α-Fe at 295 K. ε and I are the quadrupole shift and relative area, correspondingly. Estimated standard deviations are < 2% for I , < 0.4 T for Bhf and < 0.02 mm/s for the other parameters. a Reduction was performed at 1023 K and p(O ) = 2 × 10−16 atm during 2 30 h. b A very small contribution of pentacoordinated Fe3+ to these intensity values cannot be excluded. Table 2 Parameters estimated from the Mössbauer spectra of reduceda SrFe0.7 Al0.3 O3−δ , taken at 15 K Domains
Iron state
IS, mm/s
ε, mm/s
Bhf , T
I, %
Perovskite Brownmillerite
Fe3+
0.44 0.47 0.22
−0.28 −0.01 0.16
46.7 53.1 45.6
7 80 13
CN6-Fe3+ CN4-Fe3+
a The material was reduced at 1023 K and p(O ) = 1 × 10−17 atm. 2
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and in reducing atmospheres were calculated from the data on oxygen permeability and faradaic efficiency under oxidizing conditions, and analyzing the oxygen pressure dependencies of the total conductivity at p(O2 ) < 10−9 atm, respectively [9,10, 12,20]. A description of the experimental procedures used for the studies of a model membrane reactor, comprising a dense mixed-conducting membrane and a powdered ferrite catalyst, is found in Ref. [17]. The interaction of ferrite powders with CH4 pulses was examined using a microcatalytic setup [24] in flowing helium; the conditions were analogous to those used in a previous work [17]. As these experiments were aimed to analyze processes occurring on the membrane surface, the powdered samples for catalytic tests were prepared by grinding of dense ceramics in order to provide a relatively low specific surface area, 0.03– 0.06 m2 /g. Prior to the pulse experiments, the samples (0.40 g) were annealed in oxygen flow at 1023 K for 1.5 h. Then a He flow (0.69 cm3 /s) started to pass through the reactor and the amount of desorbed oxygen was determined as a function of time. After annealing in flowing helium, a pulse CH4 probe (1 cm3 STP) was injected during 1 s into the flow. The reaction products were frozen by liquid nitrogen in two sorbent-filled traps, one of which was used to collect CO2 , H2 O and C2 hydrocarbons and another collected CO, H2 , CH4 , N2 and O2 . After a pulse, the traps were rapidly heated and the products were analyzed by gas chromatograph; then this procedure was repeated. The reaction selectivity was calculated as the concentration ratio between a given product and the sum of all detected carbon-containing products, namely CO, CO2 and C2 hydrocarbons (C2 H2 , C2 H4 and C2 H6 ). 3. Results and discussion
Fig. 1. Amount of products per 1 cm3 CH4 pulse supplied in flowing helium onto the La0.3 Sr0.7 Fe0.8 Al0.2 O3−δ powder at 1023 K. The inset shows approximate relationships between oxygen nonstoichiometry, CH4 conversion and CO selectivity (see text).
3.1. Catalytic behavior The interaction of pre-oxidized La0.3 Sr0.7 Fe0.8 Al0.2 O3−δ with CH4 pulses results in predominant CO2 formation, suggesting a major role of total combustion process at the initial stage of ferrite reduction (Fig. 1). For the first 10 pulses, the selectivity towards CO and C2 -hydrocarbon formation varies in the ranges 1.2–2.5% and 9.5–11.0%, respectively; the methane conversion and the yields of all products decrease with the pulse number. A qualitatively similar behavior was also observed in the case of La0.3 Sr0.7 Fe0.8 Ga0.2 O3−δ , but only for the first 4 pulses (Fig. 2). Further reduction of the Ga-substituted ferrite is accompanied with a drastic increase of syngas yield, whilst CO2 selectivity decreases from approximately 98% down to 56%. Contrary to La0.3 Sr0.7 Fe0.8 M0.2 O3−δ , the oxidation of the first CH4 pulses over SrFe0.7 Al0.3 O3−δ surface yields mainly syngas with a H2 : CO ratio of approximately 2, characteristic of the POM process (Fig. 3). The selectivity to CO2 and C2 -hydrocarbon formation is 7–12 times lower than that of carbon monoxide. For comparison, Fig. 3 presents also the data on cobaltite analogue of the Ga-containing ferrite, La0.3 Sr0.7 Co0.8 Ga0.2 O3−δ , characterized in previous works [17, 25]. In the latter case, a relatively smaller and time-independent
CO2 yield accompanied with progressive evolution of molecular oxygen is observed; due to oxidative dimerization the presence of gaseous O2 may cause formation of the significant amounts of C2 -hydrocarbons. These results unambiguously show that catalytic activity of perovskite-like ferrites towards the total and partial oxidation of methane is directly related to the oxygen nonstoichiometry variations. After pre-oxidation and treatment in a He flow under similar conditions, one may expect similar values of the oxygen chemical potential at the surface of La0.3 Sr0.7 Fe0.8 M0.2 O3−δ and SrFe0.7 Al0.3 O3−δ . In this state, CH4 oxidation over La0.3 Sr0.7 Fe0.8 M0.2 O3−δ , where the oxygen deficiency is lower than that in SrFe0.7 Al0.3 O3−δ due to a higher average charge of the A-site cations, leads primarily to CO2 formation; for SrFe0.7 Al0.3 O3−δ the yield of CO is considerably higher with respect to carbon dioxide. Increasing the oxygen vacancy concentration in La0.3 Sr0.7 Fe0.8 M0.2 O3−δ increases the tendency to the POM reaction. Although the latter is also contributed by shifting CO/CO2 thermodynamic equilibrium ratio due to progressive decrease of the oxygen chemical potential, this trend and the difference in initial behavior of La0.3 Sr0.7 Fe0.8 M0.2 O3−δ and SrFe0.7 Al0.3 O3−δ confirm the relevance of oxygen stoichiometry as the POM
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Fig. 3. Product concentrations in 1 cm3 CH4 pulses supplied in flowing He onto the powders of SrFe0.7 Al0.3 O3−δ and La0.3 Sr0.7 Co0.8 Ga0.2 O3−δ at 1023 K. Fig. 2. Amount of products per 1 cm3 CH4 pulse supplied in flowing helium onto the La0.3 Sr0.7 Fe0.8 Ga0.2 O3−δ powder at 1023 K. The inset shows approximate relationships between oxygen nonstoichiometry, CH4 conversion and CO selectivity (see text).
activity-determining factor. Moreover, as shown by coulometric titration (Fig. 4), the differences in catalytic behavior of La0.3 Sr0.7 Fe0.8 Al0.2 O3−δ and La0.3 Sr0.7 Fe0.8 Ga0.2 O3−δ (Figs. 1 and 2) also correlate with the variations of δ values. The equilibrium oxygen content in La0.3 Sr0.7 Fe0.8 Al0.2 O3−δ is very similar to that in La0.3 Sr0.7 Fe0.8 Ga0.2 O3−δ under oxidizing conditions, but becomes higher with respect to the Gasubstituted ferrite on reduction. As a result, oxygen losses from the lattice of La0.3 Sr0.7 Fe0.8 Ga0.2 O3−δ in the course of CH4 oxidation are larger compared to La0.3 Sr0.7 Fe0.8 Al0.2 O3−δ ; the former composition faster achieves a critical level of the oxygen deficiency necessary to promote POM. These tendencies are in excellent agreement with the data on oxygen desorption (Fig. 5). The insets in Figs. 1 and 2 show the overall changes of oxygen deficiency of the ferrite-based phases in the course of pulse experiments. The nonstoichiometry estimations were made from the molar ratios between catalyst, injected methane and products, assuming that the oxygen content in ferrites prior to CH4 oxidation tests is equilibrium (Fig. 4) and that the carbon and hydrogen balances are 100%. For both La-containing compositions, the values of δ are close to 0.35, the stoichiometric state where the average oxidation state of iron cations
is 3+. Taking into account the relatively high oxygen content and significant oxygen-ion mobility in the title materials [9, 10,12], formation of metallic iron on their surface during the catalytic experiments seems very unlikely. Furthermore, under similar experimental conditions no increase in the CO yield due to possible separation of Co metal was observed in the case of La0.3 Sr0.7 Co0.8 Ga0.2 O3−δ (Fig. 3), despite higher reducibility of the cobaltite compound. Another necessary comment is that general trends of catalytic behavior, identified in the course of pulse experiments, are also observed when using ferrite-based materials as catalysts in the membrane reactors for CH4 oxidation. As an illustration, Fig. 6 presents the time dependencies of CH4 conversion rate and CO selectivity for a model reactor comprising one dense membrane and a powdered catalyst, both made of SrFe0.7 Al0.3 O3−δ ; the reactor configuration and the conditions of materials processing and testing were described elsewhere [17]. Since no deep optimization of the reactor and its operation regime was performed, the selectivity to CO formation in steady-state conditions was slightly lower compared to the pulse experiments, achieving 63% at 1223 K. At the same time, the performance was found essentially time-independent, corroborating the absence of bulk reduction of the ferrite catalyst. This conclusion was confirmed by XRD (inset in Fig. 6). Note that XRD analysis did not reveal apparently the transition of perovskite into vacancy-ordered phases, characteristic of strontium ferrite-based materials ([26,27] and references cited); most
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Fig. 6. Time dependence of CH4 conversion and CO selectivity in a reactor with dense membrane and powdered catalysts both made of SrFe0.7 Al0.3 O3−δ . The operation conditions are given in the legend. The inset shows XRD pattern of SrFe0.7 Al0.3 O3−δ after operation during approximately 100 h.
Fig. 4. Oxygen partial pressure dependencies of the oxygen content in La0.3 Sr0.7 Fe0.8 M0.2 O3−δ , determined by coulometric titration. Solid lines are for visual guidance only. Arrows show approximate p(O2 ) values corresponding to the start of disorder-order transition.
Fig. 5. Time dependence of oxygen amount desorbed from the pre-oxidized samples into He flow at 1023 K.
likely, the ordering processes cannot be distinguished by XRD due to low sensitivity of this method to the oxygen sublattice. In order to evaluate the exact phase composition and the states of iron relevant to the catalytic behavior, the Mössbauer spectroscopy was used. 3.2. Mössbauer spectra The Mössbauer spectra of La0.3 Sr0.7 Fe0.8 M0.2 O3−δ reduced under typical conditions for non-equilibrium mixtures of CH4 conversion products in the membrane reactors [21], consist mainly of a six-peak pattern (Figs. 7 and 8). At room temperature the peaks are asymmetrically broadened, while at 10 K it is possible to distinguish the presence of a second sextet. The relatively poor resolution at 295 K results probably from the decrease in the magnetic sextets splittings with increasing temperature and/or from peak broadening due to a faster relaxation of the magnetic moments. Even at 10 K the peaks are broad and the spectra may only be adequately fitted by two distributions of magnetic hyperfine fields. A dependence of the isomer shift (IS) on the magnetic hyperfine field Bhf was also considered. The average values estimated for IS and Bhf are given in Table 1. The results show that all the iron, within detection limits, is in the Fe3+ state. A similar conclusion was reached analyzing the 15 K Mössbauer spectrum of SrFe0.7 Al0.3 O3−δ , reduced at 1023 K and p(O2 ) = 10−17 atm. As for the other ferrites, no accurate information in the latter case might be obtained from the room-temperature data due to peak broadening. At 15 K, SrFe0.7 Al0.3 O3−δ is close to magnetic saturation and the resolution of the spectrum (Fig. 8(C)) is as good as that of the 10 K spectra of La0.3 Sr0.7 Fe0.8 M0.2 O3−δ . The spectrum of SrFe0.7 Al0.3 O3−δ may be described using two distributions of
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Fig. 7. Room-temperature Mössbauer spectra of La0.3 Sr0.7 Fe0.8 Al0.2 O3−δ (A) and La0.3 Sr0.7 Fe0.8 Ga0.2 O3−δ (B) quenched after annealing at 1023 K and p(O2 ) = 2 × 10−16 atm during 25 h.
Fig. 8. Mössbauer spectra of reduced La0.3 Sr0.7 Fe0.8 Al0.2 O3−δ at 10 K (A), La0.3 Sr0.7 Fe0.8 Ga0.2 O3−δ at 10 K (B) and SrFe0.7 Al0.3 O3−δ at 15 K (C). The La0.3 Sr0.7 Fe0.8 M0.2 O3−δ samples were quenched after annealing at 1023 K and p(O2 ) = 2 × 10−16 atm during 25 h. SrFe0.7 Al0.3 O3−δ was reduced at 1023 K and p(O2 ) = 1 × 10−17 atm. The line on the experimental points is the sum of two distributions of magnetic splittings attributed to tetrahedrally and octahedrally coordinated Fe3+ , and for (C) an additional sextet assigned to Fe3+ in perovskite-like domains, shown slightly shifted, for clarity. The probability distributions (P ) of the magnetic hyperfine fields (Bhf ) are shown on the right-hand side of the corresponding spectra.
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magnetic splittings, with the average parameters consistent with those of 6- and 4-coordinated Fe3+ in a brownmillerite-type lattice [28,29]. The estimated relative areas I (Table 2) indicate that Al3+ prefers tetrahedral coordination, whilst Fe3+ tends to occupy octahedrally-coordinated sites, as in brownmillerite Ca2 FeAlO5+δ [28,29]. No magnetic splittings with Bhf lower than 40 T are observed. Therefore, neither traces of metallic iron nor splittings typical for Fe4+ can be detected. On the other hand, a significant improvement in the fitting quality was further achieved considering a third contribution, with the parameters typical for Fe3+ in a perovskite-type lattice such as those estimated for SrFe0.7 Al0.3 O3−δ equilibrated in air [30]; the perovskite phase contains approximately 7% of the total iron amount. Reduction of SrFe0.7 Al0.3 O3−δ at 1023 K occurs, therefore, via the formation of brownmillerite-like domains coexisting with perovskite domains down to relatively low oxygen chemical potentials. Such a behavior is typical for perovskitelike ferrites (for instance, [5,11,15,26–28,31] and references therein); analysis of the literature data shows that this local ordering can be expected up to, at least, 1200 K. On the contrary, if considering a third sextet describing the possible presence of perovskite-like domains in La0.3 Sr0.7 Fe0.8 M0.2 O3−δ , there is no improvement in the fitting quality and the relative area of this additional sextet consistently refines to zero. The average IS and Bhf values (Table 1), falling in the range characteristic of 6- and 4-coordinated Fe3+ at 10 K [30,31], are similar to those for orthorhombic Sr2 LaFe3 O8 [32]. This indicates that vacancy-ordered domains prevail in La0.3 Sr0.7 Fe0.8 M0.2 O3−δ . As for SrFe0.7 Al0.3 O3−δ , such ordering cannot be detected from the XRD data (Fig. 9). While the tetrahedral Fe3+ Bhf distributions turned out to be symmetric, with coincident average and most probable IS and Bhf values, the octahedral Fe3+ distributions in the La0.3 Sr0.7 Fe0.8 M0.2 O3−δ spectra exhibit a tail in the low Bhf range which is more conspicuous for the Ga-containing compound (Fig. 8). Furthermore, as the best adequacy was achieved for a positive dependence of IS on Bhf , the contributions with the lowest Bhf for these distributions, in the range 46–49 T for both spectra, also have the lowest IS, in the range 0.39– 0.41 mm/s. The tails of the octahedral Fe3+ distributions seem, hence, contributed by the presence of low amounts of pentacoordinated Fe3+ [31], existing in microdomains where the oxygen vacancies are still disordered, at least in part. For these tails, the relative value of accumulated probabilities for Bhf up to 49 T is higher for La0.3 Sr0.7 Fe0.8 Ga0.2 O3−δ , 7.1%, as compared to 2.6% for La0.3 Sr0.7 Fe0.8 Al0.2 O3−δ (Fig. 8). Such difference is larger than the experimental uncertainties, suggesting a higher degree of local disorder in the oxygen sublattice of the Ga-substituted ferrite. The broadening of Fe3+ absorption peaks in the spectra of La0.3 Sr0.7 Fe0.8 M0.2 O3−δ , described by sextet distributions instead of single sextets as in orthorhombic Sr2 LaFe3 O8 [32], may arise from several factors. These include local lattice distortions near M 3+ cations having a strong influence on the oxygen sublattice [11], and the presence of pentacoordinated Fe3+ and different environments due to random distribution of the B-site dopant cations in each sublattice. In orthorhombic
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Fig. 10. The equilibrium p(O2 )–T –δ diagram of SrFe0.7 Al0.3 O3−δ , determined by coulometric titration. The inset shows XRD patterns of oxidized and reduced SrFe0.7 Al0.3 O3−δ . Arrows show approximate start of the disorder-order transition.
Fig. 9. XRD patterns of La0.3 Sr0.7 Fe0.8 M0.2 O3−δ equilibrated at low temperatures in air (A) and reduced at 1023 K and p(O2 ) = 2 × 10−16 atm (B).
Sr2 LaFe3 O8 where the oxygen vacancy concentration is very similar to that in reduced La0.3 Sr0.7 Fe0.8 M0.2 O3−δ (Fig. 4), the ratio of octahedral to tetrahedral iron sites is 2 : 1 [32]. The data summarized in Table 1 show, hence, that Fe3+ cations in La0.3 Sr0.7 Fe0.8 M0.2 O3−δ possess a strong preference to octahedral coordination as compared to either Ga3+ or Al3+ . This tendency is even more pronounced than that in brownmilleritelike SrFe0.7 Al0.3 O2.5 phase (Table 2). 3.3. Relationships between oxygen stoichiometry, catalytic activity and ionic transport The estimations made from the Mössbauer spectroscopy results were verified by the coulometric titration method. As an example, the values of total oxygen content in SrFe0.7 Al0.3 O3−δ at 1023 K and p(O2 ) = 1 × 10−17 atm, determined by these two techniques, are 2.499 and 2.500 per formula unit, respectively. For La0.3 Sr0.7 Fe0.8 M0.2 O3−δ reduced at p(O2 ) = 2 × 10−16 atm, the corresponding values are 2.646 and 2.650. This difference is within the limits of experimental uncertainty. The equilibrium p(O2 )–T –δ diagrams presented in Figs. 4 and 10, all exhibit a stepwise decrease of the oxygen content at oxygen chemical potentials where a massive transfor-
mation into vacancy-ordered modifications occurs, i.e., the content of ordered domains becomes critical. In the case of SrFe0.7 Al0.3 O3−δ , this transition starts at oxygen partial pressures below 10−5 atm when the average oxidation state of iron cations is slightly higher than 3+. For La0.3 Sr0.7 Fe0.8 M0.2 O3−δ , extensive ordering processes are observed at considerably lower oxygen chemical potentials; decreasing temperature leads to a shift of the p(O2 ) values characteristic of “disorder–order” transition towards more reducing conditions, suggesting possible stabilization of the domain microstructure at low temperatures. The latter phenomenon may result from the interfacial contribution to the thermodynamic functions of partially ordered materials, caused by the space charge effects at the microdomain boundaries [33]. If comparing the data on equilibrium oxygen nonstoichiometry and oxygen desorption kinetics (Figs. 4, 5 and 10), one may conclude that the disorder–order transition in the oxygen sublattice of SrFe0.7 Al0.3 O3−δ should, at least, start after the treatment in a He flow; in contrast, oxygen vacancies in the lattices of La0.3 Sr0.7 Fe0.8 M0.2 O3−δ (M = Ga, Al) should mainly remain disordered. This seems responsible for the difference in the initial catalytic behavior (Figs. 1–3). In fact, the tendency to POM increases when the oxygen content in La0.3 Sr0.7 Fe0.8 M0.2 O3−δ becomes close to 2.65 and extensive ordering processes start (Fig. 4 and inset in Fig. 2). At the same time, literature data [15,19] show that the catalytic activity of brownmillerite-like ferrite phases with a completely ordered oxygen sublattice is relatively low; for the perovskite-related ferrites with ordered microdomains, the highest activity is observed at elevated temperatures, 1050– 1170 K, when the perovskite-brownmillerite phase boundary
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Fig. 11. Partial molar enthalpy of oxygen in the ferrite-based materials, calculated from the oxygen stoichiometry data (see text). Solid lines are for visual guidance only.
is mobile due to thermally-induced disordering [13–16]. Also, the maximum syngas yields over ferrite-based catalysts are achieved supplying alternate CH4 and O2 pulses [14,16]. The high catalytic activity of perovskite-related ferrites, such as La0.3 Sr0.7 Fe0.8 M0.2 O3−δ and SrFe0.7 Al0.3 O3−δ , towards syngas generation may therefore originate from the lattice instability due to morphotropic phase transformations on reduction. In order to illustrate these statements, Fig. 11 presents the values of the partial molar enthalpy of oxygen (HO ), calculated using the well-known equation [11,34,35] 1 µO (δ, T ) = RT · ln p(O2 ) = HO (δ) − T SO (δ) (1) 2 where SO is the partial molar entropy of oxygen, the temperature dependence of the oxygen chemical potential variations with respect to a standard state in the gas phase (µO ) at δ = const is approximated by a linear function, and all thermodynamic quantities are related to one mole of oxygen atoms at 923–1223 K. Under oxidizing conditions, the oxygennonstoichiometry dependence of the partial oxygen enthalpy is rather weak, suggesting that oxygen intercalation into the disordered perovskite phases can be described by the ideal solution model [11]. Increasing vacancy concentration leads to anomalous behavior in the vicinity of order–disorder transition due to perovskite lattice instability; the approximate δ ranges, where the validity of Eq. (1) is very limited, are marked by open squares in Fig. 11. On further reduction the values of HO drastically decrease since the oxygen sublattice of ferrites is predominantly ordered and the vacancy formation process becomes energetically less favorable. The ionic conductivity of all studied materials increases with decreasing p(O2 ), despite the progressive vacancy ordering (Fig. 12). This suggests that a significant part of oxygen vacancies are trapped even under oxidizing conditions, prob-
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Fig. 12. Temperature dependence of oxygen ionic conductivity in the ferrite-based ceramics at atmospheric oxygen pressure (open symbols) and under reducing conditions (closed symbols). Solid lines are for visual guidance only.
ably in the form of defect associates. As the radius of Al3+ is substantially smaller compared to Fe3+ and Ga3+ [36], the substitution of iron for aluminum promotes defect clustering [10, 37], thus decreasing ionic transport at oxygen pressures close to atmospheric. The level of ionic conduction correlates with the rates of oxygen desorption and total CH4 oxidation at the initial stage of catalytic experiments (Figs. 1–3 and 5), because the latter processes are both governed by the bonding energy of oxygen in the perovskite-related lattice and oxygen supply rate on the ferrite surface. On the other hand, minimum CO2 yield was found for SrFe0.7 Al0.3 O3−δ where the ionic conduction is relatively low, but the concentration of ordered domain boundaries should be higher with respect to La0.3 Sr0.7 Fe0.8 M0.2 O3−δ . It seems quite obvious that oxygen transport via the boundaries cannot be considered as a factor having key influence on the total oxidation rate, which is determined by the oxygen chemical potential at the ferrite surface and, thus, influenced by the overall level of ionic conductivity comprising both bulk and boundary contributions. Under reducing conditions another situation is observed. Namely, the ionic conduction increases as La0.3 Sr0.7 Fe0.8 Al0.2 O3−δ < La0.3 Sr0.7 Fe0.8 Ga0.2 O3−δ < SrFe0.7 Al0.3 O3−δ , which corresponds to decreasing total oxygen content and increasing lattice disorder detected by the Mössbauer spectroscopy. In principle, such tendency might indeed suggest a significant contribution of oxygen diffusion along domain boundaries as assumed in Refs. [15,19]; verification of the latter hypothesis requires, however, additional investigations since the overall boundary concentration is expected to be rather low. Whatever the mechanism of oxygen transport, the results clearly show that the catalytic activity towards syngas formation correlates with the level of ionic conductivity, as these properties are both
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dependent on the fraction of disordered oxygen vacancies and the metal-oxygen bond energy. 4. Conclusions The comparative analysis of equilibrium p(O2 )–T –δ diagrams, Mössbauer spectra and interaction of CH4 pulses with the surface of perovskite-related SrFe0.7 Al0.3 O3−δ , La0.3 Sr0.7 Fe0.8 Ga0.2 O3−δ and La0.3 Sr0.7 Fe0.8 Al0.2 O3−δ reveals qualitatively similar trends in the catalytic behavior. Namely, the selectivity towards the total and partial oxidation of methane was found determined by the oxygen nonstoichiometry level. Increasing CO yield is observed when the dominant oxidation state of iron cations is 3+. This state is characterized with lattice instability due to massive ordering processes in the oxygen sublattice. The catalytic activity towards POM correlates with the degree of oxygen-vacancy disorder and, hence, with the level of oxygen ionic conductivity, which also has a primary importance for membrane applications. Although one cannot completely exclude the presence of trace amounts of metallic iron on the ferrite surface and its contribution to the catalytic processes, the Mössbauer spectroscopy data suggest that this effect, if any, should be minor. The optimization of Fe-containing mixed conductors for the monolithic membrane reactors requires, therefore, extensive studies of oxygen thermodynamics in order to identify factors, enabling to shift order-disorder transition towards the oxygen chemical potentials typical for the operation conditions. Acknowledgements This work was supported by the NATO Science for Peace program (project 978002), the FCT, Portugal (projects POCTI/ CTM/58570/2004, SFRH/BPD/11606/2002 and SFRH/BPD/ 17649/2004), and the Siberian Branch of the Russian Academy of Sciences (Integration project No 42). Helpful discussions and experimental contributions, made by A.A. Valente, A.L. Shaula and A.P. Viskup, are gratefully acknowledged. References [1] A.P.E. York, T. Xiao, M.L.H. Green, Top. Catal. 22 (2003) 345. [2] D.J. Wilhelm, D.R. Simbeck, A.D. Karp, R.L. Dickenson, Fuel Process. Technol. 71 (2001) 139. [3] K. Aasberg-Petersen, J.-H.B. Hansen, T.S. Christensen, I. Dybkjaer, P.S. Christensen, C.S. Nielsen, S.E.L.W. Madsen, J.R. Rostrup-Nielsen, Appl. Catal. A 221 (2001) 379. [4] T.J. Mazanec, R. Prasad, R. Odegard, C. Steyn, E.T. Robinson, Stud. Surf. Sci. Catal. 136 (2001) 147. [5] H.J.M. Bouwmeester, A.J. Burggraaf, in: A.J. Burggraaf, L. Cot (Eds.), Fundamentals of Inorganic Membrane Science and Technology, Elsevier, Amsterdam, 1996, pp. 435–528.
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