A- and B-Site Substituted Lanthanum Cobaltite Perovskite as High

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A- and B-Site Substituted Lanthanum Cobaltite Perovskite as High Temperature Oxygen Sorbent. 1. Thermogravimetric Analysis of Equilibrium and Kinetics S. Guntuka, S. Banerjee, S. Farooq,* and M. P. Srinivasan Department of Chemical & Biomolecular Engineering, National UniVersity of Singapore, 4 Engineering DriVe 4, Singapore 117576

Perovskite samples of the general formula La0.1A0.9CoyFe1-yO3-δ (where A ) Ca, Sr, Ba; y ) 0.1, 0.5, 0.9) were synthesized in our laboratory. Further substitution of Sr to a small extent by Ag and complete substitution of La by Sr were also studied. For a fixed perovskite composition (SrCo0.5Fe0.5O3-δ), samples obtained by carbonate coprecipitation and citrate methods of synthesis were compared. Use of helium as the carrier gas produced more weight loss (i.e., higher oxygen vacancy) than nitrogen in all the perovskite samples. Oxygen sorption equilibrium and sorption kinetics were thermogravimetrically studied in the temperature range 500800 °C at atmospheric pressure using oxygen-nitrogen mixtures in which the oxygen fraction ranged from ∼5 to 50%. Desorption kinetics were studied by allowing the equilibrated sample to desorb in pure nitrogen. For a fixed B-site substitution, oxygen capacity varied with A-site substitution in the order Sr > Ba > Ca. Consideringbothequilibriumcapacityandsorption-desorptionkinetics,SrCo0.5Fe0.5O3-δ andLa0.1Sr0.8Ag0.1Co0.5Fe0.9O3-δ were found to be the more promising candidates for further investigation. Introduction With the current oil price at a record high, the importance of conserving nonrenewable energy sources and harnessing energy from renewable alternatives cannot be overemphasized. Most industrial heating processes generate required energy by combusting hydrocarbon fuel, such as oil or natural gas, with air as the oxidant. In many cases, there is room to enhance the combustion process by using either a pure or an enriched oxygen stream. High-purity oxygen is also an essential requirement in many of the emerging technologies such as solid oxide fuel cells (SOFCs) and methane to syngas production. Hence, the economics of oxygen generation technologies is intimately related to the optimum utilization of fossil fuels in an environmentally friendly way. Air separation for the production of oxygen and nitrogen is one of the major industrial applications of pressure swing adsorption (PSA) technology. In this regard, PSA enjoys a significant economic advantage over the competing technologies, such as cryogenic distillation and membrane separation, for medium-scale operations.1 PSA air separation processes are either equilibrium or kinetically controlled. Stronger equilibrium affinity for nitrogen in zeolite adsorbents is used to selectively remove this component over oxygen from air to produce oxygen enriched raffinate product. On the other hand, slower diffusion of nitrogen molecules in the micropores of carbon molecular sieve (CMS) adsorbent makes oxygen preferentially adsorbed. This produces a raffinate stream enriched in nitrogen. For the separation of oxygen from air, zeolite 5A and alkali or rare earth metal cation exchanged zeolite X have been reported in the literature.2 Many innovative cycles have also been proposed for improving energy efficiency.3,4 Despite very aggressive research efforts directed toward developing adsorbents with higher selectivity and cycles with improved energy efficiency for both classes of air separation, the industrial processes still suffer from several limitations. For example, zeolite adsorbents are nonselective to oxygen and argon, which * To whom correspondence should be addressed. Tel.: (65)-65166545. Fax: (65)-6779-1936. E-mail: [email protected].

limits the oxygen purity and necessitates secondary treatment to achieve higher purity at the expense of further loss in recovery. Moreover, the best achieved adsorbent selectivity (∼10) between oxygen and nitrogen is still far from what would make a PSA air separation process competitive with cryogenic distillation beyond medium-scale operation. Furthermore, both classes of adsorbents are susceptible to the presence of trace impurities in the feed, such as moisture, carbon dioxide, etc. Hence the development of an adsorbent that will exclusively adsorb only oxygen (i.e., infinite selectivity) has the potential to significantly improve the economics of adsorption-based air separation. Perovskites are known as ABO3-type mixed metal oxides, where A ion can be an alkali, rare earth, alkaline earth, or other large ion that can be accommodated in the dodecahedral framework, and B ion can be a 3d, 4d, or 5d transitional metal ion occupying the octrahedral site. The relative ease with which the A-site and/or B-site can be partially or fully substituted without disturbing its cubic structure makes the perovskite suitable for diverse applications as a superconductor, multilayer capacitor, refractory electrode, ferromagnetic material, and oxidation catalyst.5 Apart from the relative size factor of the anion and cation required to stabilize its cubic structure, the charge neutrality condition must also be fulfilled. For example, if A- and/or B-sites are substituted with lower valent cations, then to maintain charge neutrality some oxygen ions are removed from the lattice thereby giving rise to oxygen nonstoichiometry. Formation of nonstoichiometry in perovskites has been discussed in detail in several reviews.5,6 Oxygen vacancies in perovskite oxides lead to oxygen ion conductivity. At high temperatures, oxygen ions are transported by a vacancy diffusion mechanism driven chemically by the oxygen chemical potential difference. Selective transport of oxygen ion across perovskite oxide crystals makes them ideal candidates for preparing highly oxygen selective membrane. Teraoka et al.7 first reported high oxygen flux through various partially substituted lanthanum cobaltite structures at elevated temperatures. Since then several studies have been carried out to understand perovskite oxides as an oxygen selective membrane, which may find direct application in some of the

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emerging technologies such as methane to syngas production, oxidative coupling of methane (OCM), and solid oxide fuel cells (SOFCs). Several technological challenges still remain unresolved. Thermochemical stability vis-a`-vis mechanical stability of a perovskite membrane under oxygen-rich and oxygen-lean conditions, formation of hot spots during exothermic reaction and subsequent reaction runaway due to higher oxygen diffusion through those spots, attaining film uniformity when synthesizing for actual commercial applications, etc., are some of the major issues that need to be addressed before perovskite oxides find applications as high temperature membranes in industrial processes. Some of the aforementioned difficulties may be easily overcome if perovskite oxides can be used as a high temperature adsorbent instead. Though use of an adsorbent in a high temperature process may at first seem unsuitable, it should be noted that several applications require use of pure or enriched oxygen at high temperature and a high temperature adsorbent should, in fact, bring synergism for these applications. The idea of an efficient high temperature oxygen sorption process based on perovskite-type oxygen-deficient ceramic adsorbent has been discussed in a series of papers by Lin and co-workers.8-11 It utilizes the fact that, at elevated temperatures, some oxygen-deficient perovskites can adsorb only oxygen (over nitrogen) at an appreciable amount, thereby giving practically infinite selectivity for oxygen. The oxygen deficiency or nonstoichiometry within the crystal lattice can be varied by changing temperature and/or oxygen partial pressure. Two perovskite structures, viz., La0.1Sr0.9Co0.5Fe0.5O3-δ and La0.1Sr0.9Co0.9Fe0.1O3-δ, were studied, and were found to have reasonable adsorption capacity and fast uptake for oxygen.9 Kusaba et al.12,13 also investigated a group of (partially) A- and B-site substituted strontium cobaltite oxides for oxygen sorption properties under temperature swing conditions in air and found that SrCo0.4Fe0.6O3-δ gave the best oxygen sorption-desorption properties under a temperature swing between 300 and 500 °C. The oxygen production rate was 25 m3 (STP)/ton of oxide/h. Similar high temperature air separation processes utilizing this unique property of perovskite oxide based sorbents have also been reported in other studies. Stevens et al.14 have patented a pressure swing adsorption system to extract oxygen from an oxygen-rich feed gas by using lanthanum calcium cobalt ferrites at elevated temperatures. Yang and Lin15 also studied the fixed bed dynamics with the two aforementioned compositions and confirmed that the oxygen nonstoichiometry was reversible and within a particular range of temperature and oxygen partial pressure the perovskite structures were stable despite high oxygen nonstoichiometry. While the adsorption breakthrough was very sharp, as expected for a material showing fast oxygen uptake, the desorption step was rather slow with a long tail in column breakthrough response. A slower desorption process can severely hinder the prospect of these perovskites as potential sorbent materials. Hence, oxygen transport in a perovskite adsorbent is an important area for further investigation. There are several reports in the literature on the synthesis and characterization of perovskites and their uses as catalysts.7 However, dedicated reports on the use of perovskites as adsorbents are very few. This provides additional incentive for carrying out a systematic study of gas adsorption and transport in these materials. The effects of A and B substitution on oxygen sorption and transport in perovskites were examined in the present study. Perovskite samples of the general formula La0.1A0.9CoyFe1-yO3-δ

Figure 1. XRD patterns of perovskite samples differing in composition.

Figure 2. FESEM photographs of SrCo0.5Fe0.5O3-δ synthesized by the (a) carbonate method and (b) citrate method.

(where A ) Ca, Sr, Ba; y ) 0.1, 0.5, 0.9) were synthesized. Thermogravimetric analysis (TGA) and analysis of column dynamics were carried out on the synthesized samples over a range of temperatures and oxygen partial pressures in order to assess their oxygen capacities and uptake rates, which are essential for assessing the potential for process development. The TGA results are detailed in this paper, part 1 of a two-part series. The results from the column dynamics study are presented in part 2. Experimental Section Synthesis Procedure. Perovskite samples used in this investigation were synthesized by the carbonate coprecipitation method (henceforth called the carbonate process). Some representative perovskite compositions were also synthesized by the citrate process for comparison. In the carbonate process, stoichiometric amounts of lanthanum acetate and nitrates of cobalt and iron together with nitrate of strontium, calcium, or barium were dissolved together in deionized water and an aqueous solution of Na2CO3 was added dropwise to the solution of metal salts, under constant stirring at room temperature, until complete precipitation of the metal carbonates. The precipitate was aged in the mother liquor for another 30 min under stirring and then allowed to settle for 1 h. The precipitate was then filtered, washed thoroughly with deionized water until the pH of the wash liquid was between 7 and 8, and then dried at 110 °C for 12-18 h. The dried mass

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Table 1. Summary of Different Perovskite Sorbents Used in This Study stoichiometric formula of perovskite

method of synthesis

phase(s) present

BET surface area (m2/g)

densitya (g/cm3)

La0.1Sr0.9Co0.5Fe0.5O3-δ La0.1Sr0.9Co0.5Fe0.5O3-δ La0.1Sr0.9Co0.1Fe0.9O3-δ La0.1Sr0.9Co0.9Fe0.1O3-δ La0.1Ba0.9Co0.5Fe0.5O3-δ La0.1Ca0.9Co0.5Fe0.5O3-δ La0.1Sr0.8Ag0.1Co0.5Fe0.5O3-δ SrCo0.5Fe0.5O3-δ

carbonate citrate carbonate carbonate carbonate carbonate carbonate carbonate

perovskite perovskite perovskite perovskite perovskite perovskite perovskite + Ag2O perovskite

3.30 2.98 2.28 2.53 2.21 1.81 2.59 2.94

2.43 2.78 2.67 3.04 2.99 1.96 2.46 3.16

a Perovskite sample of known weight was taken in a system of predetermined constant (empty) volume, and the system volume was determined again using helium (inert) gas. The difference in system volume with and without perovskite gave the sample volume from which the density was calculated.

Figure 3. Thermogravimetric response of SrCo0.5Fe0.5O3-δ showing the difference of heating in helium and nitrogen on oxygen vacancy creation.

was decomposed at 500 °C for 5 h. It was then finely ground and pelletized under a pressure of 5-6 tons that was held for 5-7 min. The pellets were calcined at 925 °C for 6 h in static air. In the citrate process, stoichiometric amounts of the aforementioned metal salts were dissolved in deionized water and aqueous citric acid solution was added slowly under stirring. The mole ratio of total metal ions to citric acid was maintained at 1.0. The mixture was stirred for 8-10 h at a temperature of 80 °C. It was then heated for 2 h at 120 °C in air, followed by decomposition of the dry mass at 500 °C for 5 h. The remaining treatments, namely, grinding, pelletization, and calcination, were the same as those described under the carbonate process. The product obtained was dried and pelletized under pressure using a pellet press (no external binder material was added) with a 13 mm diameter pellet die. A few calcined pellets, synthesized by both processes, were finely ground and subjected to X-ray diffraction (XRD) study to confirm perovskite structure. Grinding, pelletization, and calcination steps were repeated until perovskite structure was achieved. These steps helped in physically bringing the different metal oxides closer to facilitate the formation of the perovskite phase. In the following sections, unless otherwise mentioned, preparation by the carbonate process will be implied. Structural Confirmation. XRD analysis was carried out using a Shimadzu X-ray diffractometer (Model Lab-X 6000) and Cu KR radiation. The crystal morphology and particle size of the samples were determined by field emission scanning electron microscopy (FESEM) using a JEOL JSM-6700F instrument. Powder XRD of the synthesized perovskite samples, shown in Figure 1, confirms that perovskite structure was achieved in all cases. However, the characteristic peak intensity was

Figure 4. Thermogravimetric response of SrCo0.5Fe0.5O3-δ upon cycling between pure nitrogen and 50:50 oxygen-nitrogen mixture at 500 °C at 1 atm.

rather low in the cases of La0.1Ba0.9Co0.5Fe0.5O3-δ and La0.1Ca0.9Co0.5Fe0.5O3-δ. These samples had to be calcined again to achieve the desired structure. High temperature calcination for a longer period compared to that for the other samples most likely led to more sintering, thus somewhat diminishing the peak intensity. For La0.1Sr0.8Ag0.1Co0.5Fe0.5O3-δ, an extra low intensity peak appeared around 38.2°, which can be attributed to Ag2O phase (the characteristic XRD peaks of Ag2O appear at 32.8 and 38.2°). This suggests that Ag ion was not fully incorporated within the A-site of the crystal lattice of perovskite, and a separate phase of Ag2O has been formed. Since the amount of Ag precursor used was small and the XRD peak of Ag2O was weak, a reliable quantitative estimation of the extent of Ag ion incorporated within the perovskite lattice was not possible. The FESEM pictures showing the crystal morphologies of two perovskite samples having the same composition but synthesized by two different methods are given in Figure 2. The samples had similar general morphology, which is expected since both samples were calcined at the same high temperature for the same duration after pelletizing at high pressure. BET surface areas of the powdered perovskite samples were measured by a Quantachrome Surface Area Analyser at liquid nitrogen temperature. These areas and some other physical properties are summarized in Table 1. The table shows that all the synthesized sorbent materials had a very low surface area (∼2 m2 g-1). Changing the synthesis procedure, for example, in the case of La0.1Sr0.9Co0.5Fe0.5O3-δ, did not change the surface area to any significant extent, probably due to the same calcination temperature and duration of calcination used in both processes. Adsorption Isotherm and Uptake Measurements. The perovskite samples used in this study were screened for oxygen capacity and sorption rate based on gravimetric measurements using a thermogravimetric analyzer (TA Instruments, TGA 2050). The procedure is detailed in the following sections.

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Figure 5. Comparison of (a) oxygen uptake (open symbols) and desorption (closed symbols) and (b) oxygen capacity at 500 °C in SrCo0.5Fe0.5O3-δ prepared by carbonate and citrate methods. 21% oxygen in nitrogen was used for uptake, while pure nitrogen was used for desorption.

Sample Preparation. In preparation for capacity measurement, each synthesized sample was first heated externally in a furnace at 900 °C for 10 h. The sample was then allowed to cool to room temperature before putting it on the sample pan of the TGA. Adsorbent Regeneration. Powdered samples of 5-15 mg (mesh size La0.1Ba0.9Co0.5Fe0.5O3-δ > La0.1Ca0.9Co0.5Fe0.5O3-δ. It has already been mentioned that, to achieve perovskite structure from the latter two compositions, calcination had to be repeated. This may have led to sintering and probably ordering of oxygen vacancy to some extent, thus reducing

oxygen capacity. Generally, high temperature desorption of oxygen (or β-peak) is more specifically related to the B-site substitution, though it is also affected by A-site substitution.21 The low sorption capacity for Ca-substituted perovskite may be related to the smaller ionic radius (1.08 nm) of Ca2+, its lower electronegativity, and higher Ca-O interaction which results in low desorption of oxygen from crystal lattice. Substitution of La3+ by Ba2+ yields lower sorption capacity than substitution by Sr2+. Hence, Sr2+ was chosen from among Sr2+, Ca2+, and Ba2+ as a suitable A-site substituent. Complete substitution of La3+ by Sr2+ was also carried out (SrCo0.5Fe0.5O3-δ). Though it did not increase the sorption capacity compared to La0.1Sr0.9Co0.5Fe0.5O3-δ, it led to improved sorption-desorption behavior, which will be discussed later. Introduction of Ag+ in the A-site by substituting Sr2+ in La0.1Sr0.9Co0.5Fe0.5O3-δ did not seem to have any impact on the sorption capacity over La0.1Sr0.9Co0.5Fe0.5O3-δ, which is not surprising. It should be noted that the expectation from Ag substitution was an increase in the sorption-desorption rate, which will be discussed in the next section. Three perovskite samples having the general formula La0.1Sr0.9CoyFe1-yO3-δ were synthesized by substituting Co ion in the B-site with Fe ion, where the y values were 0.1, 0.5, and 0.9. From the results presented in Figure 6, it is clear that oxygen capacity increased with increasing substitution of Co by Fe. In the temperature range covered in this study, oxygen capacity decreased with increasing temperature and the capacity

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was highest at 500 °C (see Figure 7, for example). Of course, the extent of decrease with increasing temperature varied from sample to sample. Representative results are shown in Figure 8. There are two perovskite compositions, La0.1Sr0.9Co0.5Fe0.5O3-δ and La0.1Sr0.9Co0.9Fe0.1O3-δ, in the present study that are in common with the compositions studied by Yang et al.9 The reported oxygen sorption capacities in these two samples are compared with those obtained in the present study at two temperatures in Figure 9. It is important to note that, while the samples in the present study were prepared by the carbonate method, Yang et al.9 used the citrate method of perovskite synthesis in their study. However, La0.1Sr0.9Co0.5Fe0.5O3-δ sample from the present study gave a somewhat higher oxygen capacity, whereas the capacity was lower for La0.1Sr0.9Co0.9Fe0.1O3-δ. The opposing trends at different compositions precludes the possibility of the difference being due to different synthesis methods. Sorption-Desorption Kinetics. In view of the very low oxygen vacancy created by Ba and Ca substitution, these two samples were not considered for the investigation of sorptiondesorption kinetics. For the remaining five perovskite samples (SrCo0.5Fe0.5O3-δ, La0.1Sr0.8Ag0.1Co0.5Fe0.5O3-δ, La0.1Sr0.9Co0.1Fe0.9O3-δ, La0.1Sr0.9Co0.5Fe0.5O3-δ, La0.1Sr0.9Co0.9Fe0.1O3-δ), the sorptionesorption measurements were repeated many times to ensure reproducibility. Representative repeat runs are shown in Figure 10, which shows that the degree of reproducibility achieved in the sorption-desorption measurements was indeed satisfactory. The kinetic responses at 500 °C for three different oxygen fractions are shown in Figure 11, and the important trends are discussed in the following paragraphs. Except in La0.1Sr0.9Co0.9Fe0.1O3-δ, oxygen uptake was over 95% complete in less than 100 s in all the remaining four samples at 500 °C for all three concentration levels studied. Uptake of oxygen in La0.1Sr0.9Co0.9Fe0.1O3-δ was marginally slower, reaching about 90% in 100 s. Since the uptake was very fast, it may appear that oxygen concentration did not have any further effect on adsorption kinetics. However, a close look at the uptake profile of La0.1Sr0.9Co0.9Fe0.1O3-δ, which was the slowest among the samples studied, would reveal that adsorption kinetics indeed became faster as the oxygen fraction was increased from ∼5 to 50%. The effect was just the opposite in the case of desorption, with the rate decreasing with increasing oxygen fraction in the gas with which the sample was equilibrated prior to switching to pure nitrogen. These behaviors are consistent with the expectations for strongly curved isotherm systems. Generally, oxygen sorption on perovskite is preceded by a somewhat slower, activated adsorption kinetics involving the formation of O2- species from molecular O2.21 It has been reported that the presence of Ag+ in optimum amounts, either in the A-site or in a dissolved state within the perovskite matrix, or both, considerably increases the surface exchange rate of oxygen between the bulk and the crystal surface. The presence of a small amount of Ag+ also does not significantly alter the phase stability of the perovskite.20 As expected, incorporation of Ag+ enhanced desorption to some extent. Most noticeable was, however, the effect of complete La substitution by Sr, which significantly enhanced desorption kinetics, although at the expense of reduced equilibrium capacity. It was shown in Figure 6 that oxygen capacity increased with increasing substitution of Co with Fe. To the contrary, there was a general decline in desorption kinetics with increasing Fe,

which got worse with increasing the partial pressure of oxygen at which the sample was equilibrated before desorbing in nitrogen. Considering both uptake and desorption, the performance of SrCo0.5Fe0.5O3-δ was the best followed by La0.1Sr0.8Ag0.1Co0.5Fe0.5O3-δ. A close look at the isotherms and desorption plots seems to suggest a correspondence between the two. For example, the oxygen isotherm on SrCo0.5Fe0.5O3-δ has a modest curvature compared to the nearly rectangular isotherms in the Fe-rich samples, which is consistent with the faster desorption kinetics in the former. The effect of temperature on sorptiondesorption kinetics at three levels of oxygen partial pressure is compared for the two better performing samples and a third sample with poor desorption kinetics, especially at high partial pressure of oxygen, in Figure 12. High oxygen partial pressure is expected at the product-end of an oxygen enrichment process. There was generally a decrease in sorption rate and an increase in the desorption rate with an increase in temperature. Decrease in sorption rate with increasing temperature is a clear indication that the process was not diffusion controlled. A likely explanation is that oxygen sorption-desorption in the perovskite samples was equilibrium controlled and the equilibrium shifted to the left with increasing temperature. Concentration dependence of oxygen uptake and desorption at different temperatures in the better performing samples is shown in Figure 13, which suggests similar trends as discussed earlier in relation to Figure 11. The effect of concentration was not very pronounced for either uptake or desorption, except at 800 °C. At 800 °C, uptake for the 5% case was very slow but became very fast at 21 and 50%. Lack of significant sensitivity of desorption responses to oxygen concentration is another proof that the process was not diffusion controlled. Conclusion Some very recent literature has reported the feasibility of using perovskite-based sorbents as high temperature highly oxygen selective sorbents. Despite showing high potential, little has been studied of the equilibrium and kinetic aspects which can significantly affect the sorption process. The fact that the perovskite structure can accommodate almost 90% of the metal ions in the periodic table makes the proper selection/optimization of the material composition very difficult. Moreover, the method of synthesis and various synthesis parameters make the effort more complex. In this study, the equilibrium and kinetics of oxygen sorption in seven perovskite compositions comprising lanthanum cobaltite structures with substitutions in A- and B-sites have been investigated. The following conclusions can be drawn from this study: (a) The study reveals that when helium is used to generate oxygen vacancy instead of nitrogen, the former results in a greater weight loss than the latter, indicating more oxygen desorption from the crystal lattice in a helium atmosphere for all the perovskite compositions used in the study. To the best of our knowledge, no other literature has reported this observation, which is important for the appropriate choice of a carrier in equilibrium and kinetic studies. (b) For a particular perovskite composition (La0.1Sr0.9Co0.5Fe0.5O3-δ), the change in synthesis process from the citrate process to the carbonate process did not significantly affect the equilibrium capacity for oxygen uptake, but desorption kinetics became faster. Among the different divalent A-site substituents, Sr2+ produced the best results, the sorption capacity being in the order Sr2+ > Ba2+ > Ca2+ for the temperature range

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covered. Introduction of small amounts of Ag giving the composition La0.1Sr0.8Ag0.1Co0.5Fe0.5O3-δ improved the desorption kinetics over the base composition La0.1Sr0.9Co0.5Fe0.5O3-δ. The complete substitution of La3+ by Sr2+ decreased the sorption capacity, but the sorption-desorption kinetics became remarkably faster. For a fixed A-site substitution of La3+ by Sr2+ (90%), the increasing substitution of Co ion (B-site) by Fe ion monotonically increased the oxygen sorption capacity, but adversely affected the desorption kinetics. (c) Considering both uptake and desorption, the performance of SrCo0.5Fe0.5O3-δ was the best followed by La0.1Sr0.8Ag0.1Co0.5Fe0.5O3-δ. Based on the above findings, it can be further concluded that perovskite sorbent has great potential for use in many applications, for example, air separation, trace oxygen removal, coal gasification, and oxygen storage and transport. Further studies are necessary to fully exploit this new type of oxygen sorbent. Dynamics of oxygen sorption in columns packed with the aforementioned promising samples will be discussed in part 2 of this series. Literature Cited (1) Ruthven, D. M.; Farooq, S.; Knaebel, K. S. Pressure Swing Adsorption; VCH Publishers: New York, 1994. (2) Yang, R. T. Adsorbents: Fundamentals and Applications; WileyInterscience: New York, 2003. (3) Keller, G. E. Separations: New Directions for an Old Field. AIChE Monogr. Ser. 1987, 83, 54. (4) Monreau, C. The Air Liquide compact VSA: a view on an innovative radial adsorber. Munich Meeting on Air Separation Technology, Munich; Oct 10-12, 1996; p 147. (5) Twu, J.; Gallagher, P. K. Properties of Bulk and Supported Perovskite. In Properties and Applications of PeroVskites-type Oxides; Tejuca, L. G., Fierro, J. L. G., Eds.; Marcel Dekker Inc.: New York, 1993 pp 1-24. (6) Smyth, D. M. Oxidative Nonstoichiometry in Perovskite Oxides. In Properties and Applications of PeroVskites-type Oxides; Tejuca, L. G., Fierro, J. L. G., Eds.; Marcel Dekker Inc.: New York, 1993; pp 4772. (7) Teraoka, Y.; Yoshimatsu, M.; Yamazoe, N.; Seiyama, T. OxygenSorptive Properties and Defect Structure of Perovskites-Type Oxides. Chem. Lett. 1984, 13 (6), 893-896.

(8) Yang, Z. H.; Lin, Y. S. A transient TGA Study on Oxygen Permeation Properties of Perovskite-Type Ceramic Membrane. Solid State Ionics 1998, 110 (3-4), 209-221. (9) Yang, Z. H.; Lin, Y. S.; Zeng, Y. High-Temperature Sorption Process for Air Separation and Oxygen Removal. Ind. Eng. Chem. Res. 2002, 41 (11), 2775-2784. (10) Yang, Z. H.; Lin, Y. S. Equilibrium of Oxygen Sorption on Perovskite-Type Lanthanum Cobaltite Sorbent. AIChE J. 2003, 49 (3), 793798. (11) Yang, Z. H.; Lin, Y. S. Synergetic Thermal Effects for Oxygen Sorption and Order-Disorder Transition on Perovskite-Type Oxides. Solid State Ionics 2005, 176 (1-2), 89-96. (12) Kusaba, H.; Sakai, G.; Shimanoe, K.; Miura, N.; Yamazoe, N. Oxygen-Sorptive and -Desorptive Properties of Perovskites related Oxides under Temperature-Swing Condition for Oxygen Enrichment. Solid State Ionics 2002, 152, 689-694. (13) Kusaba, H.; Sakai, G.; Shimanoe, K.; Miura, N.; Yamazoe, N. Temperature Swing based Oxygen Enrichment by using Perovskites-Type oxide. J. Mater. Sci. Lett. 2002, 21 (5), 407-409. (14) Stevens, W. C.; Cummings, D.; Chen, P. High Temperature Pressure Swing Adsorption System for Separation of Oxygen Containing Gas Mixtures. U.S. Patent 6,361,584, 2002. (15) Yang, Z. H.; Lin, Y. S. High-Temperature Oxygen Sorption in a Fixed Bed Packed with Perovskite-Type Ceramic Sorbents. Ind. Eng. Chem. Res. 2003, 42 (19), 4376-4381. (16) Pena, M. A.; Tascon, J. M. D.; Tejuca, L. G. Surface interaction of NO with LaFeO3. NouV. J. Chim. 1985, 9, 591. (17) Tascon, J. M. D.; Fierro, J. L. G.; Tejuca, L. G. Infrared Spectroscopy and Temperature-Programmed Desorption of Perovskite Surfaces. In Properties and Applications of PeroVskites-type Oxides; Tejuca, L. G., Fierro, J. L. G., Eds.; Marcel Dekker Inc.: New York, 1993; pp 171-194. (18) Nakamura, T.; Misono, N.; Yoneda, Y. Reduction-Oxidation and Catalytic Properties of Perovskite-Type Mixed Oxide Catalysts (La1-xSrxCoO3). Chem. Lett. 1981, 10 (11), 1589-1592. (19) Choudhary, T. V.; Banerjee, S.; Choudhary, V. R. Catalysts for Combustion of Methane and Lower Alkanes. Appl. Catal., A: Gen. 2002, 234, 1-23. (20) Tan, L.; Yang, L.; Gu, X.; Jin, W.; Zhang, L.; Xu, N. Structure and Oxygen Permeability of Ag-Doped SrCo0.8Fe0.2O3-δ. AIChE J. 2004, 50 (3), 701-707. (21) Pena, M. A.; Fierro, J. L. Chemical Structures and Performance of Perovskite Oxides. Chem. ReV. 2001, 101 (7), 1981-2017.

ReceiVed for reView June 22, 2007 ReVised manuscript receiVed September 28, 2007 Accepted October 2, 2007 IE070859Q