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Palladium surface modified La0.6Sr0.4Co0.2Fe0.8O3−δ hollow fibres for oxygen separation
Journal of Membrane Science 380 (2011) 223–231
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Palladium surface modified La0.6 Sr0.4 Co0.2 Fe0.8 O3−ı hollow fibres for oxygen separation Christelle Yacou a , Jaka Sunarso a , Chun X.C. Lin a , Simon Smart a , Shaomin Liu b , João C. Diniz da Costa a,∗ a b
The University of Queensland FIMLab – Films and Inorganic Membrane Laboratory, School of Chemical Engineering, Brisbane, Qld 4072, Australia Department of Chemical Engineering, Curtin University of Technology, Perth WA 6845, Australia
a r t i c l e
i n f o
Article history: Received 29 April 2011 Received in revised form 30 June 2011 Accepted 2 July 2011 Available online 8 July 2011 Keywords: Oxygen separation Hollow fibres Perovskites LSCF Pd surface modification
a b s t r a c t La0.6 Sr0.4 Co0.2 Fe0.8 O3−ı (LSCF) hollow fibres were prepared by a phase inversion/sintering method using polyetherimide as a binder. In order to overcome surface exchange kinetics limitation, LSCF hollow fibres were coated with ∼200 nm palladium (Pd) nanoparticles. The O2 flux of best performing membranes increased by up to 350% in comparison to unmodified LSCF hollow fibres. Optimal enhancement was achieved with a single Pd coating. Additional coatings resulted in reduced O2 fluxes, thus counter acting the beneficial spill-over effect of the catalyst. Long term stability testing in atmospheric air at 850 ◦ C showed that a LSCF membrane modified with a single Pd coating continually outperformed a pure LSCF hollow fibre for over 400 h, though the level of enhancement was reduced over time. A dramatic reduction in performance of more than 45% occurred within the first 24 h of testing, which was attributed to the coalescence and aggregation of Pd catalyst particles to ∼1000 nm size at the LSCF grain boundaries. This greatly reduced the available area for the oxygen species to spill-over onto the LSCF surface and thus reduced the overall O2 flux. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Separation of oxygen (O2 ) from air for industrial use is a big business producing nearly 100 million tonnes of O2 each year [1] and this market is predicted to expand rapidly in the near future because virtually all large scale carbon feedstock clean energy technologies will require O2 as a feed [2]. Currently, cryogenic distillation is the process of choice for air separation, as it offers the minimal deployment risk of those technologies currently employed on an industrial scale. However, the cryogenic process has a high energy cost as it operates at very low temperatures and high pressures. With the recent developments in ceramic ionic transport membranes, it is envisaged that this technology can replace cryogenic air separation on a commercial scale. In this technological race to commercialise ceramic membranes for air separation, an array of ceramic materials, based on perovskites (ABO3 ), fluorites (AO2 ), brownmillerites (A2 B2 O5 ), the Ruddlesden–Popper series (An+1 Bn O3n+1 ), and Sr4 Fe6−x Cox O13 compounds, have been studied by the research community over the last 20 years. The parameters of principal importance in technology delivery are production and quality capabilities. In principle, defect-free ceramic membranes allow for O2 transport via ionic diffusion only, thus delivering 100% purity. Production is related to oxygen fluxes
∗ Corresponding author. Tel.: +61 7 3365 6960; fax: +61 7 3365 4199. E-mail address: [email protected] (J.C. Diniz da Costa). 0376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2011.07.008
and perovskite membranes have shown great promise, particularly barium strontium cobalt iron oxides (BSCF) and lanthanum strontium cobalt iron oxides (LSCF) which have been extensively studied. Major improvements were realised by the reduction of the membrane thickness from discs (1 mm) to hollow fibres (0.25 mm). The major advantage here is that the O2 flux is inversely proportional to the membrane thickness. Hence, reducing the thickness of the membrane translates to a reduction in the oxygen ionic bulk diffusion resistance. As a result, BSCF hollow fibre geometries operating at high temperatures in excess of 900 ◦ C have demonstrated large O2 fluxes of 5 ml cm−2 min−1 [3,4]. Recently Leo et al. [5] reported even higher O2 fluxes of 9.5 ml cm−2 min−1 by changing the conventional polyethersulfone binder in the hollow fibre synthesis process to a sulfur-free polymer. Variations in the dopants and composition have also led to high membrane permeance values such as BaCoFeZr [6] and BaBiScCo [7] which delivered oxygen fluxes of 7.3 and 11.4 ml cm−2 min−1 , respectively. Most available literature on LSCF hollow fibres have been largely devoted to its preparation and performance aspects [8–11] including the transport layer modification via acid etching or methane activation. Nevertheless, LSCF O2 fluxes remain low compared to BSCF fluxes, generally in the range of 0.1–2.5 ml cm−2 min−1 depending upon membrane synthesis, surface modification and experimental conditions as listed in Table 1. Secondary parameters for consideration deal with the process efficiencies and operating costs associated with the technology. Ceramic membranes operate efficiently at high temperatures,
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Table 1 Comparison of reported oxygen fluxes through La0.6 Sr0.4 Co0.2 Fe0.8 O3−ı hollow fibres membranes. Hollow fibre thickness (cm) 0.2 0.3 0.2 0.2 0.6 0.6 0.6 0.8
Transport layer modification
Operating temperature range (◦ C)
Atmosphere used low pO2 /high pO2
O2 flux (ml cm−2 min−1 )
Ref.
CH4 activation CH4 activation Acid modification Acid etching Porous LSCF modified
540–950 800–900 700–900 858 800–1000 800–1050 800–1050 700–1000
He/air Ar/air Ar/air CH4 /air He/air He/air He/air He/air
0.09–0.04 0.1–1.2 0.1–0.8 0.26–0.8 0.06–0.725 0.35–2.5 0.5–2.5 0.08–1.48
[15] [14] [13] [15] [12] [16] [17] [18]
traditionally in excess of 900 ◦ C, and it is questionable as to whether efficiency improvements can be realised at these extreme operating conditions [19]. Indeed heating air to high temperatures, where the recovery rate for oxygen may be 10% or lower compared to the total air volume, will attract considerable energy costs. Hence, process analysis dictates the optimal design will be realised by reducing the operating temperature whilst maintaining reasonably high O2 fluxes. This can be achieved by anchoring catalysts to the membrane surface. Improvements of additional O2 flux were reported for BSCF hollow fibres, surface modified with platinum (Pt) [20] and silver (Ag) [21] catalysts, as well as LSCF hollow fibres, modified with Pt [22] and Ag [18] particles. Recently, Leo et al. [5] also demonstrated that BSCF coated with palladium (Pd) nanoparticles allowed O2 fluxes to significantly increase, by a factor of 10 below 700 ◦ C, thus reducing operating temperature requirements by 250 ◦ C. The major advantage here is the spill-over effect of the catalyst which allows for a higher O2 absorption and faster surface diffusion of atomic oxygen species to the membrane surface, thus overcoming the surface exchange kinetics limitation and translating into higher O2 fluxes. In addition, palladium has superior thermal stability than silver as its melting point of 1554.9 ◦ C is higher than silver at 961.8 ◦ C [23]. A final, but no less important, parameter for technology delivery is the long-term operational performance, which in this case is related to the thermal stability of the ceramic membrane material. This is an area of weakness for BSCF membranes, as prolonged exposure to temperatures below 900 ◦ C results in the cubic BSCF structure transforming into the hexagonal phase, thus reducing oxygen permeation [24]. By contrast, LSCF membranes have been proven to be stable for operation for over 3000 h at 800 ◦ C [25]. Although LSCF oxygen fluxes are much lower that BSCF, the technological race may point to the former as the preferred option, particularly when operating and replacement costs are imperative to predict the commercialisation prospects of the technology. In this work, we investigate the performance of LSCF hollow fibre membranes coated with Pd nanoparticles. LSCF was chosen due to its well-publicised thermal stability, whilst Pd has delivered the best shift in O2 production of those catalysts investigated. We also study how Pd loading on the LSCF surface aids O2 flux improvement at temperatures ranging from 700 to 950 ◦ C, and with several driving forces created by varying the sweep gas flow rates in the permeate stream. We are also interested in the long term operational stability of Pd coated LSCF hollow fibres. Hence, the membranes are further tested for over 400 h using atmospheric air rather than synthetic air, to emulate commercial air separation process units. 2. Experimental 2.1. Preparation methods and characterisation La0.6 Sr0.4 Co0.2 Fe0.8 O3−ı (herein after called LSCF) powders were prepared by a combined EDTA–citrate complexing sol–gel
process. Stoichiometric quantities of La(NO3 )3 ·6H2 O (99.0+%, Ajax Finechem, Australia), Sr(NO3 )2 (99.0+%, Sigma–Aldrich), Co(NO3 )2 ·6H2 O (98.0+%, Sigma–Aldrich) and Fe(NO3 )2 ·9H2O (99.0+%, Ajax Finechem) were added to the EDTA (99.4+%, Ajax Finechem, Australia), anhydrous citric acid (99.5%, Fluka) and NH3 aqueous solution (28%, Ajax Finechem, Australia) to control the pH at 6 (avoiding precipitation) under heating and stirring followed by the addition of anhydrous citric acid resulting in a violet aqueous solution. The molar ratio of all metal nitrates:EDTA:citric acid is kept at 1:1.1:2. The solution was heated at 80 ◦ C for 6–8 h under continuous stirring to remove water until a viscous gel was formed. The gel was heated further by a hot plate until they dried. The dried gel was then pretreated on the furnace at 700 ◦ C for 12 h (heating rate = 5 ◦ C min−1 ) to remove residual carbon and form a solid precursor. The powders were then ball-milled in a planetary mill (Pulverisette 5, Fritsch) at 500 rpm for 2 h to obtain particle size of less than 100 m. A phase inversion, wet spinning technique was employed to fabricate green LSCF hollow fibres as described elsewhere [26–28], though modified with the use of a sulfur-free polymeric binder [5]. Briefly, the milled LSCF powder was added to a mixture of 1-methyl-2-pyrrolidinone (NMP) [synthesis grade, Sigma Life Science] and polyetherimide (PEI) [(SABIC Innovative Plastics, Saudi Arabia)] in a mass ratio 6.71:4:1 and stirred at high speed for 2 h to ensure a uniform mixture. A small amount, 0.5–1% by mass, of polyvinylpyrrolidone (PVP) [M.W. 1,300,000; Alfa Aesar] was added to adjust the viscosity of the mixture. To form LSCF–NMP–PEI mixture into the required hollow fibre geometry, a tube-in-orifice spinneret with orifice diameter/inner diameter of 2.5 mm/0.8 mm was used. Tap water was employed as the internal and external coagulant. The extruded hollow fibres were dried and cut onto short lengths and sintered at 1350 ◦ C for 12 h (heating rate = 1 ◦ C min−1 ) to obtain gas tight membranes. The surface modification process of the sintered hollow fibres was adapted from a previous work published by Leo et al. [5]. A palladium precursor solution (PdNO3 at 0.1 M) was firstly carefully coated on both inner and outer surfaces of the membrane followed by a drying step at 60 ◦ C for 2 h. An equivalent amount of N2 H4 (7% by volume in water), was then applied to the inner and outer surfaces of the membrane to induce the chemical reduction of Pd2+ to Pd0 particles based on the following equation 2Pd2+ + N2 H4 → 2Pd0 + 4H+ + N2 [29,30]. The fibres were then immediately heat treated in air at 700 ◦ C for 10 min to remove residual N2 H4 and other by-products. The fibres were then cooled to room temperature at 1 ◦ C min−1 . This Pd surface modification process was repeated up to three times to establish optimal coverage. Regarding the nomenclature used in this work, Pd modified hollow fibres were labelled LSCF–Pd × y, where y represents the number of catalyst coatings. For comparison purposes the original La0.6 Sr0.4 Co0.8 Fe0.2 O3−ı membranes without catalyst deposition were also investigated and simply labelled LSCF. The morphological features of the prepared hollow fibres were examined by a Philips XL-30 Scanning Electron Microscope (SEM).
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Fig. 1. Hollow fibre permeation testing setup.
Energy Dispersive X-ray (EDX) and quantitative composition analysis were performed on a JEOL JSM-6460 LA low vacuum analytical SEM equipped with an integrated JEOL Hyper mini-cup, 133 eV resolution, SiLi crystal, ultra thin window (UTW) and EDX Spectrometer. All samples were coated with carbon to reduce charging. Additional structural data were collected from X-ray diffraction (XRD) patterns of the equivalent membranes powders using a
Bruker D8 Advance diffractometer with a Cu K␣ radiation (40 kV, ˚ 20 mA, = 1.5409 A). 2.2. Oxygen permeation measurements The performance of the LSCF hollow fibre membranes was investigated in an oxygen permeation cell schematically shown in Fig. 1.
Fig. 2. SEM micrographs of the (a) inner, (b) outer surface of the original LSCF, and (c) inner surface, (d) outer surface, (e) cross-section of LSCF–Pd × 1 membrane and (f) EDX spectra of identified areas.
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Fig. 3. XRD patterns of the equivalent membranes powder corresponding to LSCF–Pd × 0 to LSCF–Pd × 3.
Membranes, approximately 80 mm in length, were suspended in a split hinge tube carbolite furnace with a constant temperature zone of 10 cm. Quartz tubes (diameter of 18 mm) were attached to both ends of the LSCF fibres and sealed with a silver-based sealant. The seals were verified as gas-tight if nitrogen was not detected when the permeate stream was tested at room temperature. Nevertheless, a very small amount of nitrogen was detected by gas chromatography for temperatures in excess of 600 ◦ C, probably as a result of thermal expansion of the hollow fibres causing negligible leaks at the silver paste and membrane interface. For instance, the oxygen leak rate was 12% at 950 ◦ C relative to the total oxygen flow rate. Permeation testing was conducted by passing argon (BOC, purity > 99.998%) as a sweep gas through the permeate side of the membrane (inner shell of the hollow fibre) and by varying the temperature of the furnace. The argon gas flow rate was varied from 50 to 200 ml min−1 and the operating temperature between 700 and 950 ◦ C. The permeate stream was fed directly to the gas chromatograph (Shimadzu GC-2014 equipped with a 5 A˚ molecular sieve column) for analysis. The total flow rate of the permeate stream was measured using a bubble flow meter. In this work, we report only the oxygen flux derived from ionic transport. Therefore, the contribution of molecular oxygen flux from leakage was subtracted from the overall flux values by measuring the nitrogen concentration. The effective membrane area (A) and oxygen permeation flux (JO2 ) was calculated using Eqs. (1) and (2), where L, do and di is the length, outside and inside diameter of fibre (mm), respectively; FT is the total flow rate of permeate stream (ml min−1 ). A=
L(do − di ) ln(do /di )
(1)
JO2 =
% O2 −
21 % N2 79
F T ·
A
(2)
3. Results and discussion 3.1. Microstructure and morphology analysis SEM micrographs of the synthesized and subsequently Pd modified hollow fibre membranes are depicted in Fig. 2. The inner and outer surfaces of the LSCF material display a homogenous microstructure with no apparent pores or cavities. The LSCF particles in the size range of ∼5–10 m were tightly connected to each other, thus indicating that the grains coalesced during the sintering process and gave rise to a dense and smooth surface layer. White spherical spots assigned to the Pd particles in Fig. 2c and d were detected on both the inner and outer surfaces of the modified membrane. These figures demonstrate that the Pd particles, which had an average size of approximately 200 nm, were relatively well dispersed over the surface without any obvious aggregation. However, the Pd deposition was not homogeneous over the entire membrane surface, with several areas containing lower than average numbers of Pd particles identified and confirmed by EDX analysis (example spectra are shown in Fig. 2f). Of particular interest, increasing the number of Pd coatings did little to overcome the heterogeneous deposition of catalyst particles. Finally, the external and internal diameters of the modified fibres were measured from the SEM micrographs at 2030 ± 1 m and 1815 ± 1 m, respectively (Fig. 2e). It must be emphasized that the entire cross-section of the fibre was not fully dense, as the presence of pores are apparent. However, these pores are not interconnected and do not provide
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Fig. 4. Oxygen flux isotherms of LSCF and Pd modified hollow fibres as a function sweep gas flow rate.
a percolation pathway, as the hollow fibres proved to be gas-tight during air leak measurements performed at room temperature. In order to confirm that the perovskite membranes employed in this work had retained their original crystalline structure after the catalyst deposition, XRD patterns were collected from the equivalent membrane powders (Fig. 3). Strong diffraction peaks assigned to the rhombohedral phase of La0.6 Sr0.4 Co0.2 Fe0.8 O3−ı (2 1 1 0 = 33◦ , 2 2 0 2 = 40.64◦ , 2 0 2 4 = 47.07◦ , 2 2 1 4 = 58.5◦ , 2 2 0 8 = 68.5◦ , 2 2 1 8 = 78.03◦ ) were detected as reported elsewhere [31], showing the structural homogeneity of the prepared materials. The XRD patterns also reveal a broad weak peak at 2 = 34.2◦ which can be attributed to the main reflection (1 1 0) of PdO [32]. This peak was only detectable in the LSCF–Pd × 2 and LSCF–Pd × 3 samples due to the low Pd content in the LSCF–Pd × 1 sample. The intensity of this peak is emphasized for the LSCF–Pd × 3 sample, indicating that sequential deposition of catalyst particles did not damage or change the LSCF phase, whilst also revealing their nanocrystalline nature. The presence of this phase is in agreement with the experimental procedure used in this work and attributed to the oxidation of Pd◦ to PdO in the temperature range of 500–600 ◦ C [33,34]. 3.2. Oxygen permeation measurements Fig. 4 displays the O2 fluxes isotherms as a function of the sweep gas rate for the LSCF and Pd modified hollow fibres. The O2 fluxes increased with temperature for all membranes, indicating the effect of temperature in enhancing ionic transport and surface exchange kinetics. For instance, the unmodified hollow fibre exhibited O2 fluxes between 0.11 and 0.50 ml cm−2 min−1 as the
temperature was raised from 700 ◦ C to 950 ◦ C, respectively, thus agreeing with previously published work [8,10]. By comparison, all Pd modified LSCF hollow fibres exhibited higher O2 fluxes over the entire temperature range, reaching a maximum value close to 2.0 ml cm−2 min−1 at 950 ◦ C. These results clearly indicate that the Pd catalyst coating procedure used in this work was beneficial in improving O2 fluxes. Further, these significant enhancements in O2 fluxes clearly demonstrate that the kinetics of the surface reaction is the limiting factor in oxygen transport through the LSCF hollow fibre. The Pd catalyst particles acted as O2 adsorption promoters, and helped facilitate O2 transport to the membrane surface. Fig. 4 also shows that the sweep gas flow rate plays an important role in the ability of the membrane system to separate O2 from air. In principle, increasing the sweep gas flow rate decreases the O2 partial pressure in the permeate stream. As a result, the driving force associated with the O2 partial pressure difference between the feed and permeate stream should increase, thus translating in improved O2 fluxes. However, this is not entirely the case here as there are two distinct O2 transport regimes apparent in Fig. 4. For temperatures equal to or below 800 ◦ C, the O2 flux remained almost constant for all membranes, suggesting that bulk ionic diffusion and/or surface exchange kinetics limited O2 transport independent of the sweep gas flow rate used. It is possible that at these relatively lower temperatures, the major transport resistance is associated with the surface exchange kinetics at the air feed side of the membrane, responsible for transferring gaseous O2 into ionic oxygen in the solid perovskite lattice. The change of O2 partial pressure in the permeate stream by the sweep gas was therefore not affected by the gas atmosphere surrounding the membrane surface in the
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feed stream. However, the effect of the sweep gas flow rate was clearly noticeable for temperatures above 800 ◦ C, similar to findings published elsewhere [18]. Surface reactions begin enhancing the overall O2 permeation rate above 800 ◦ C, and the rate of oxygen ions recombining into O2 molecules at the inner surface of the hollow fibre became similar to the transport rate of the O2 molecules into the convective sweep gas flow [7]. Thus, increasing the sweep gas flow rate also increased the O2 partial pressure difference across the membrane and the O2 flux accordingly. Of course, once the transport of the O2 molecules into the convective sweep gas flow exceeded the rate of ionic recombination, or the rate of oxygen ion supply prior to ionic recombination, then further increases in the sweep gas flow rate were ineffectual, as evidenced by the plateauing of the O2 flux in Fig. 4 for the Pd modified membranes at higher sweep gas flow rates. Fig. 5a shows the relative improvement in O2 flux as a function of the number of Pd coatings. Optimal improvements were observed for a single Pd coating which led to a significant increase of O2 fluxes by two orders of magnitude, up to 350% as compared to the pure LSCF hollow fibre. On the other hand, it was unusual to observe that the relative improvement in O2 flux decreased as the number of catalyst loadings increased. It would be anticipated that by increasing the catalyst loading on the membrane surface, O2 fluxes would increase accordingly. One major distinction here is the non-homogeneity of the catalyst coverage of the LSCF surface (Fig. 2). In addition, Fig. 6 exhibits the SEM micrographs, clearly showing a process leading to the aggregation of Pd particles as the number of Pd coats increase from rather than dispersing on the LSCF membrane surface. This aggregation and growth of catalyst particles was detrimental as it reduced the surface area available for the oxygen surface exchange reactions per Pd particle volume. Nevertheless, the remarkable relative improvement in O2 flux observed in this work using a Pd catalyst is significantly higher than previous work, which included flux increases for surface modified LSCF membranes of 128% (Ag catalyst) [18], and for surface modified BSCF membranes of 25% (Pt catalyst) [20], 93% (Ag catalyst) [21] and 105% (Pd catalyst) [5]. Interestingly, the relative improvement in O2 flux peaked at different temperatures depending upon the number of Pd coatings. For instance, Fig. 5a shows the optimal improvement was reached at 850 ◦ C (LSCF–Pd × 1), 900 ◦ C (LSCF–Pd × 2) and 950 ◦ C (LSCF–Pd × 3). In principle, the Pd particle operates as a catalyst which overcomes the rate determining surface reaction exchange as depicted in the Arrhenius diagram in Fig. 5b. Therefore, the oxygen fluxes significantly increase for a single Pd coated membrane, aided by the spill-over of oxygen from Pd particle to the surface of the LSCF hollow fibre. Hence, the rate of oxygen permeation is determined by the rate of the supply of oxygen ions to the membrane surface in tandem with diffusion rates, and the rate of oxygen ion recombination at the permeate side. The positive effect of the Pd catalyst is observed as the apparent energy of activation
Fig. 5. (a) Percentage improvement of the O2 flux through LSCF–Pd × 1, LSCF–Pd × 2 and LSCF–Pd × 3 compare to the original LSCF membrane, as a function of the operating temperature, and (b) Arrhenius diagram of oxygen fluxes. Sweep gas flow rates at 200 ml min−1 .
for the oxygen fluxes reduced from 87 to 75 kJ mol−1 , from pure to a single Pd coated LSCF hollow fibre, respectively. For temperatures in excess of 850 ◦ C when a single Pd coat is applied, the relative increment of oxygen flux to the base case (e.g. pure LSCF) observed in Fig. 5a is no longer significant, and actually reduces at 950 ◦ C. These results suggest that the activity of oxygen chemisorption on Pd particle started decaying at high temperatures in excess of 800 ◦ C, thus reducing the oxygen spill-over effect of the Pd catalyst.
Fig. 6. SEM micrographs of the surface of Pd modified LSCF membranes.
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Time (hours) 0
100
200
300
LSCF-0×Pd LSCF-1×Pd
0.4 -2 -1 Oxygen flux (ml.cm .min )
400
0.3
0.2
Fig. 7. Schematic of the effect of Pd coated LSCF membranes.
A similar trend was also observed for silver coated BSCF hollow fibres reported elsewhere [21]. The increase in the number of coats was accompanied by Pd particle aggregation shown in Fig. 6, which led to a diminished ratio of the available spill-over area to the surface area occupied by the catalyst itself. Consequently, the diffusion of oxygen from the catalyst particles to the LSCF surface, particularly at low temperatures, affected the overall oxygen flux as observed in Fig. 5b. Hence, the reduction of effective area for oxygen surface access resulted in an increase of the apparent energy of activation for oxygen fluxes to values ∼120 kJ mol−1 , much higher than those calculated for the pure and single Pd coat LSCF hollow fibres. The optimal improvement of O2 fluxes observed at 900 and 950 ◦ C for the two and three catalyst coatings in Fig. 5a respectively, could then be explained by the combination of the increased surface exchange kinetics of the LSCF surface at higher temperatures together with improved bulk diffusion, though accompanied by the reduced spill-over effect from the larger Pd catalyst particles. This is schematically shown in Fig. 7 and based on the SEM micrographs in Fig. 6. As the Pd particles increase in size, they reduce the LSCF surface area accessible to ionic diffusion via spill-over, or via direct access of O2 molecules from the gas phase to the surface of the membrane. Thus, whilst the supply of oxygen ions from the LSCF surface exchange reaction is generally lower than from the catalytic Pd particles, the faster kinetics of this reaction counteracts the diminishing spill-over effect of the larger Pd particles at higher temperatures. It must be borne in mind that the improvements in O2 fluxes in Fig. 5a are relative to
Log O2 flux improvement (%)
0.1
O2 flux improvement (%)
100
10
0
100
200
300
400
Time (hours) Fig. 8. Time-dependence of O2 flux through original LSCF and LSCF–Pd × 1 hollow fibres (T = 850 ◦ C, sweep gas flow rate = 100 ml min−1 ).
the performance of a pure LSCF membrane. In principle, the ionic flux in LSCF materials will increase as a function of temperature. The relative increments observed should be considered in the light of trade-offs between the catalyst activity, catalyst particle size, particle distribution on the membrane surface, and accessible area for surface ionic diffusion. 3.3. Long term oxygen permeation testing In order to evaluate the long term operational stability of the prepared hollow fibres, the O2 flux was evaluated over 410 h of operation at 850 ◦ C for the pure LSCF hollow fibre (LSCF–Pd × 0) and the surface modified (LSCF–Pd × 1), as shown in Fig. 8. During the first 24 h of operation, LSCF–Pd × 0 exhibits an initial reduction in O2 flux of 5.8%. After 24 h, the change in O2 flux stabilized, resulting in a constant rate of reduction of approximately 0.05% per hour. These long term results are in good agreement with Schlehuber et al. [25] who reported a fairly stable performance for a similar LSCF
Fig. 9. SEM micrographs of the LSCF–Pd × 1 membrane surfaces (a) before and (b) after O2 permeation measurement (c) EDX spectrum of identified area.
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membrane tested for 3000 h of operation at 800 ◦ C. In contrast, the LSCF–Pd × 1 membrane displayed a significant loss in O2 flux in the first 24 h. For instance, the initial oxygen flux of 0.4 ml min−1 cm−2 underwent a considerable reduction of 45.2 ± 0.1%; equivalent to a rate of reduction of 0.2% per hour. From thereon, the Pd surface modified membrane became more stable and much slower decline of 0.047% per hour was observed. Comparatively the Pd surface modified LSCF membrane gave superior performance throughout the entire 410 h. However, the improvement in O2 flux offered by the Pd surface modification reduced from 134% at 0 h to 32% within the first 24 h, followed by further reduction to 24% after 410 h. As the long term testing was carried out using atmospheric air, the small loss of performance observed within the first 24 h of operation for the pure LSCF hollow fibre could be associated with the effect of the small concentration of CO2 present. However, degradation as a result of CO2 exposure would not explain the significant drop in O2 flux measured for the Pd surface modified membrane. To further investigate this problem, SEM micrographs were taken after O2 permeation testing and compared with those taken prior to testing as depicted in Fig. 9. It is observed that the Pd particles (confirmed through EDX analysis – Fig. 9c) coalesced and increased in size from ∼200 nm to ∼1000 nm during testing. These results suggest that the Pd particles did not adhere strongly to the LSCF surface and were free to coalesce and aggregate at elevated temperatures. Similar non-adherence effects were also reported for Ag catalysts on BSCF electrodes [35]. Furthermore, prolonged testing at high temperatures resulted in the thermodynamically favoured movement of the Pd particles towards the perovskite grain boundaries. This coalescence and subsequent increased size of the Pd particles served to counteract their beneficial spill-over effect on the LSCF surface. 4. Conclusions This work demonstrates that the O2 flux of LSCF hollow fibres can be dramatically enhanced through Pd surface modification. The coating of Pd nanoparticles onto the surface of the LSCF hollow fibres overcame the surface exchange reaction kinetics and enhanced the O2 flux by up to 350%, in comparison to unmodified LSCF hollow fibres. This represents the largest performance increase of an LSCF hollow fibre by catalytic surface modification in the published literature to date. Optimal enhancement was achieved with a single Pd coating. Further coatings were shown to be ineffectual at increasing the O2 flux due to the nonhomogeneous distribution and aggregation of Pd nanoparticles, thus reducing the spill-over effect of the catalyst. The flow rate of the sweep gas in the permeate stream was found to improve O2 fluxes only at temperatures above 850 ◦ C, where the surface reactions were dominant. Long term stability testing showed that whilst the Pd modified LSCF membrane continually outperformed the pure LSCF hollow fibre for over 400 h, the level of enhancement was reduced over time. The reduction in performance was non-linear with the most dramatic reduction occurring within the first 24 h, attributed to the coalescence and aggregation of the Pd nanoparticles at the LSCF grain boundaries. Acknowledgments This work was financially supported by the Australian Research Council (ARC) through Discovery Project program (DP0878849). The authors gratefully acknowledge Ms. Anya Yago (Centre for Microscopy and Microanalysis), Mr. Hong-yi Xu (Centre for Microscopy and Microanalysis) for their contribution in the XRD and the SEM-EDX analysis, respectively.
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Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Palladium surface modified La0.6 Sr0.4 Co0.2 Fe0.8 O3−ı hollow fibres for oxygen separation Christelle Yacou a , Jaka Sunarso a , Chun X.C. Lin a , Simon Smart a , Shaomin Liu b , João C. Diniz da Costa a,∗ a b
The University of Queensland FIMLab – Films and Inorganic Membrane Laboratory, School of Chemical Engineering, Brisbane, Qld 4072, Australia Department of Chemical Engineering, Curtin University of Technology, Perth WA 6845, Australia
a r t i c l e
i n f o
Article history: Received 29 April 2011 Received in revised form 30 June 2011 Accepted 2 July 2011 Available online 8 July 2011 Keywords: Oxygen separation Hollow fibres Perovskites LSCF Pd surface modification
a b s t r a c t La0.6 Sr0.4 Co0.2 Fe0.8 O3−ı (LSCF) hollow fibres were prepared by a phase inversion/sintering method using polyetherimide as a binder. In order to overcome surface exchange kinetics limitation, LSCF hollow fibres were coated with ∼200 nm palladium (Pd) nanoparticles. The O2 flux of best performing membranes increased by up to 350% in comparison to unmodified LSCF hollow fibres. Optimal enhancement was achieved with a single Pd coating. Additional coatings resulted in reduced O2 fluxes, thus counter acting the beneficial spill-over effect of the catalyst. Long term stability testing in atmospheric air at 850 ◦ C showed that a LSCF membrane modified with a single Pd coating continually outperformed a pure LSCF hollow fibre for over 400 h, though the level of enhancement was reduced over time. A dramatic reduction in performance of more than 45% occurred within the first 24 h of testing, which was attributed to the coalescence and aggregation of Pd catalyst particles to ∼1000 nm size at the LSCF grain boundaries. This greatly reduced the available area for the oxygen species to spill-over onto the LSCF surface and thus reduced the overall O2 flux. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Separation of oxygen (O2 ) from air for industrial use is a big business producing nearly 100 million tonnes of O2 each year [1] and this market is predicted to expand rapidly in the near future because virtually all large scale carbon feedstock clean energy technologies will require O2 as a feed [2]. Currently, cryogenic distillation is the process of choice for air separation, as it offers the minimal deployment risk of those technologies currently employed on an industrial scale. However, the cryogenic process has a high energy cost as it operates at very low temperatures and high pressures. With the recent developments in ceramic ionic transport membranes, it is envisaged that this technology can replace cryogenic air separation on a commercial scale. In this technological race to commercialise ceramic membranes for air separation, an array of ceramic materials, based on perovskites (ABO3 ), fluorites (AO2 ), brownmillerites (A2 B2 O5 ), the Ruddlesden–Popper series (An+1 Bn O3n+1 ), and Sr4 Fe6−x Cox O13 compounds, have been studied by the research community over the last 20 years. The parameters of principal importance in technology delivery are production and quality capabilities. In principle, defect-free ceramic membranes allow for O2 transport via ionic diffusion only, thus delivering 100% purity. Production is related to oxygen fluxes
∗ Corresponding author. Tel.: +61 7 3365 6960; fax: +61 7 3365 4199. E-mail address: [email protected] (J.C. Diniz da Costa). 0376-7388/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2011.07.008
and perovskite membranes have shown great promise, particularly barium strontium cobalt iron oxides (BSCF) and lanthanum strontium cobalt iron oxides (LSCF) which have been extensively studied. Major improvements were realised by the reduction of the membrane thickness from discs (1 mm) to hollow fibres (0.25 mm). The major advantage here is that the O2 flux is inversely proportional to the membrane thickness. Hence, reducing the thickness of the membrane translates to a reduction in the oxygen ionic bulk diffusion resistance. As a result, BSCF hollow fibre geometries operating at high temperatures in excess of 900 ◦ C have demonstrated large O2 fluxes of 5 ml cm−2 min−1 [3,4]. Recently Leo et al. [5] reported even higher O2 fluxes of 9.5 ml cm−2 min−1 by changing the conventional polyethersulfone binder in the hollow fibre synthesis process to a sulfur-free polymer. Variations in the dopants and composition have also led to high membrane permeance values such as BaCoFeZr [6] and BaBiScCo [7] which delivered oxygen fluxes of 7.3 and 11.4 ml cm−2 min−1 , respectively. Most available literature on LSCF hollow fibres have been largely devoted to its preparation and performance aspects [8–11] including the transport layer modification via acid etching or methane activation. Nevertheless, LSCF O2 fluxes remain low compared to BSCF fluxes, generally in the range of 0.1–2.5 ml cm−2 min−1 depending upon membrane synthesis, surface modification and experimental conditions as listed in Table 1. Secondary parameters for consideration deal with the process efficiencies and operating costs associated with the technology. Ceramic membranes operate efficiently at high temperatures,
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Table 1 Comparison of reported oxygen fluxes through La0.6 Sr0.4 Co0.2 Fe0.8 O3−ı hollow fibres membranes. Hollow fibre thickness (cm) 0.2 0.3 0.2 0.2 0.6 0.6 0.6 0.8
Transport layer modification
Operating temperature range (◦ C)
Atmosphere used low pO2 /high pO2
O2 flux (ml cm−2 min−1 )
Ref.
CH4 activation CH4 activation Acid modification Acid etching Porous LSCF modified
540–950 800–900 700–900 858 800–1000 800–1050 800–1050 700–1000
He/air Ar/air Ar/air CH4 /air He/air He/air He/air He/air
0.09–0.04 0.1–1.2 0.1–0.8 0.26–0.8 0.06–0.725 0.35–2.5 0.5–2.5 0.08–1.48
[15] [14] [13] [15] [12] [16] [17] [18]
traditionally in excess of 900 ◦ C, and it is questionable as to whether efficiency improvements can be realised at these extreme operating conditions [19]. Indeed heating air to high temperatures, where the recovery rate for oxygen may be 10% or lower compared to the total air volume, will attract considerable energy costs. Hence, process analysis dictates the optimal design will be realised by reducing the operating temperature whilst maintaining reasonably high O2 fluxes. This can be achieved by anchoring catalysts to the membrane surface. Improvements of additional O2 flux were reported for BSCF hollow fibres, surface modified with platinum (Pt) [20] and silver (Ag) [21] catalysts, as well as LSCF hollow fibres, modified with Pt [22] and Ag [18] particles. Recently, Leo et al. [5] also demonstrated that BSCF coated with palladium (Pd) nanoparticles allowed O2 fluxes to significantly increase, by a factor of 10 below 700 ◦ C, thus reducing operating temperature requirements by 250 ◦ C. The major advantage here is the spill-over effect of the catalyst which allows for a higher O2 absorption and faster surface diffusion of atomic oxygen species to the membrane surface, thus overcoming the surface exchange kinetics limitation and translating into higher O2 fluxes. In addition, palladium has superior thermal stability than silver as its melting point of 1554.9 ◦ C is higher than silver at 961.8 ◦ C [23]. A final, but no less important, parameter for technology delivery is the long-term operational performance, which in this case is related to the thermal stability of the ceramic membrane material. This is an area of weakness for BSCF membranes, as prolonged exposure to temperatures below 900 ◦ C results in the cubic BSCF structure transforming into the hexagonal phase, thus reducing oxygen permeation [24]. By contrast, LSCF membranes have been proven to be stable for operation for over 3000 h at 800 ◦ C [25]. Although LSCF oxygen fluxes are much lower that BSCF, the technological race may point to the former as the preferred option, particularly when operating and replacement costs are imperative to predict the commercialisation prospects of the technology. In this work, we investigate the performance of LSCF hollow fibre membranes coated with Pd nanoparticles. LSCF was chosen due to its well-publicised thermal stability, whilst Pd has delivered the best shift in O2 production of those catalysts investigated. We also study how Pd loading on the LSCF surface aids O2 flux improvement at temperatures ranging from 700 to 950 ◦ C, and with several driving forces created by varying the sweep gas flow rates in the permeate stream. We are also interested in the long term operational stability of Pd coated LSCF hollow fibres. Hence, the membranes are further tested for over 400 h using atmospheric air rather than synthetic air, to emulate commercial air separation process units. 2. Experimental 2.1. Preparation methods and characterisation La0.6 Sr0.4 Co0.2 Fe0.8 O3−ı (herein after called LSCF) powders were prepared by a combined EDTA–citrate complexing sol–gel
process. Stoichiometric quantities of La(NO3 )3 ·6H2 O (99.0+%, Ajax Finechem, Australia), Sr(NO3 )2 (99.0+%, Sigma–Aldrich), Co(NO3 )2 ·6H2 O (98.0+%, Sigma–Aldrich) and Fe(NO3 )2 ·9H2O (99.0+%, Ajax Finechem) were added to the EDTA (99.4+%, Ajax Finechem, Australia), anhydrous citric acid (99.5%, Fluka) and NH3 aqueous solution (28%, Ajax Finechem, Australia) to control the pH at 6 (avoiding precipitation) under heating and stirring followed by the addition of anhydrous citric acid resulting in a violet aqueous solution. The molar ratio of all metal nitrates:EDTA:citric acid is kept at 1:1.1:2. The solution was heated at 80 ◦ C for 6–8 h under continuous stirring to remove water until a viscous gel was formed. The gel was heated further by a hot plate until they dried. The dried gel was then pretreated on the furnace at 700 ◦ C for 12 h (heating rate = 5 ◦ C min−1 ) to remove residual carbon and form a solid precursor. The powders were then ball-milled in a planetary mill (Pulverisette 5, Fritsch) at 500 rpm for 2 h to obtain particle size of less than 100 m. A phase inversion, wet spinning technique was employed to fabricate green LSCF hollow fibres as described elsewhere [26–28], though modified with the use of a sulfur-free polymeric binder [5]. Briefly, the milled LSCF powder was added to a mixture of 1-methyl-2-pyrrolidinone (NMP) [synthesis grade, Sigma Life Science] and polyetherimide (PEI) [(SABIC Innovative Plastics, Saudi Arabia)] in a mass ratio 6.71:4:1 and stirred at high speed for 2 h to ensure a uniform mixture. A small amount, 0.5–1% by mass, of polyvinylpyrrolidone (PVP) [M.W. 1,300,000; Alfa Aesar] was added to adjust the viscosity of the mixture. To form LSCF–NMP–PEI mixture into the required hollow fibre geometry, a tube-in-orifice spinneret with orifice diameter/inner diameter of 2.5 mm/0.8 mm was used. Tap water was employed as the internal and external coagulant. The extruded hollow fibres were dried and cut onto short lengths and sintered at 1350 ◦ C for 12 h (heating rate = 1 ◦ C min−1 ) to obtain gas tight membranes. The surface modification process of the sintered hollow fibres was adapted from a previous work published by Leo et al. [5]. A palladium precursor solution (PdNO3 at 0.1 M) was firstly carefully coated on both inner and outer surfaces of the membrane followed by a drying step at 60 ◦ C for 2 h. An equivalent amount of N2 H4 (7% by volume in water), was then applied to the inner and outer surfaces of the membrane to induce the chemical reduction of Pd2+ to Pd0 particles based on the following equation 2Pd2+ + N2 H4 → 2Pd0 + 4H+ + N2 [29,30]. The fibres were then immediately heat treated in air at 700 ◦ C for 10 min to remove residual N2 H4 and other by-products. The fibres were then cooled to room temperature at 1 ◦ C min−1 . This Pd surface modification process was repeated up to three times to establish optimal coverage. Regarding the nomenclature used in this work, Pd modified hollow fibres were labelled LSCF–Pd × y, where y represents the number of catalyst coatings. For comparison purposes the original La0.6 Sr0.4 Co0.8 Fe0.2 O3−ı membranes without catalyst deposition were also investigated and simply labelled LSCF. The morphological features of the prepared hollow fibres were examined by a Philips XL-30 Scanning Electron Microscope (SEM).
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Fig. 1. Hollow fibre permeation testing setup.
Energy Dispersive X-ray (EDX) and quantitative composition analysis were performed on a JEOL JSM-6460 LA low vacuum analytical SEM equipped with an integrated JEOL Hyper mini-cup, 133 eV resolution, SiLi crystal, ultra thin window (UTW) and EDX Spectrometer. All samples were coated with carbon to reduce charging. Additional structural data were collected from X-ray diffraction (XRD) patterns of the equivalent membranes powders using a
Bruker D8 Advance diffractometer with a Cu K␣ radiation (40 kV, ˚ 20 mA, = 1.5409 A). 2.2. Oxygen permeation measurements The performance of the LSCF hollow fibre membranes was investigated in an oxygen permeation cell schematically shown in Fig. 1.
Fig. 2. SEM micrographs of the (a) inner, (b) outer surface of the original LSCF, and (c) inner surface, (d) outer surface, (e) cross-section of LSCF–Pd × 1 membrane and (f) EDX spectra of identified areas.
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Fig. 3. XRD patterns of the equivalent membranes powder corresponding to LSCF–Pd × 0 to LSCF–Pd × 3.
Membranes, approximately 80 mm in length, were suspended in a split hinge tube carbolite furnace with a constant temperature zone of 10 cm. Quartz tubes (diameter of 18 mm) were attached to both ends of the LSCF fibres and sealed with a silver-based sealant. The seals were verified as gas-tight if nitrogen was not detected when the permeate stream was tested at room temperature. Nevertheless, a very small amount of nitrogen was detected by gas chromatography for temperatures in excess of 600 ◦ C, probably as a result of thermal expansion of the hollow fibres causing negligible leaks at the silver paste and membrane interface. For instance, the oxygen leak rate was 12% at 950 ◦ C relative to the total oxygen flow rate. Permeation testing was conducted by passing argon (BOC, purity > 99.998%) as a sweep gas through the permeate side of the membrane (inner shell of the hollow fibre) and by varying the temperature of the furnace. The argon gas flow rate was varied from 50 to 200 ml min−1 and the operating temperature between 700 and 950 ◦ C. The permeate stream was fed directly to the gas chromatograph (Shimadzu GC-2014 equipped with a 5 A˚ molecular sieve column) for analysis. The total flow rate of the permeate stream was measured using a bubble flow meter. In this work, we report only the oxygen flux derived from ionic transport. Therefore, the contribution of molecular oxygen flux from leakage was subtracted from the overall flux values by measuring the nitrogen concentration. The effective membrane area (A) and oxygen permeation flux (JO2 ) was calculated using Eqs. (1) and (2), where L, do and di is the length, outside and inside diameter of fibre (mm), respectively; FT is the total flow rate of permeate stream (ml min−1 ). A=
L(do − di ) ln(do /di )
(1)
JO2 =
% O2 −
21 % N2 79
F T ·
A
(2)
3. Results and discussion 3.1. Microstructure and morphology analysis SEM micrographs of the synthesized and subsequently Pd modified hollow fibre membranes are depicted in Fig. 2. The inner and outer surfaces of the LSCF material display a homogenous microstructure with no apparent pores or cavities. The LSCF particles in the size range of ∼5–10 m were tightly connected to each other, thus indicating that the grains coalesced during the sintering process and gave rise to a dense and smooth surface layer. White spherical spots assigned to the Pd particles in Fig. 2c and d were detected on both the inner and outer surfaces of the modified membrane. These figures demonstrate that the Pd particles, which had an average size of approximately 200 nm, were relatively well dispersed over the surface without any obvious aggregation. However, the Pd deposition was not homogeneous over the entire membrane surface, with several areas containing lower than average numbers of Pd particles identified and confirmed by EDX analysis (example spectra are shown in Fig. 2f). Of particular interest, increasing the number of Pd coatings did little to overcome the heterogeneous deposition of catalyst particles. Finally, the external and internal diameters of the modified fibres were measured from the SEM micrographs at 2030 ± 1 m and 1815 ± 1 m, respectively (Fig. 2e). It must be emphasized that the entire cross-section of the fibre was not fully dense, as the presence of pores are apparent. However, these pores are not interconnected and do not provide
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Fig. 4. Oxygen flux isotherms of LSCF and Pd modified hollow fibres as a function sweep gas flow rate.
a percolation pathway, as the hollow fibres proved to be gas-tight during air leak measurements performed at room temperature. In order to confirm that the perovskite membranes employed in this work had retained their original crystalline structure after the catalyst deposition, XRD patterns were collected from the equivalent membrane powders (Fig. 3). Strong diffraction peaks assigned to the rhombohedral phase of La0.6 Sr0.4 Co0.2 Fe0.8 O3−ı (2 1 1 0 = 33◦ , 2 2 0 2 = 40.64◦ , 2 0 2 4 = 47.07◦ , 2 2 1 4 = 58.5◦ , 2 2 0 8 = 68.5◦ , 2 2 1 8 = 78.03◦ ) were detected as reported elsewhere [31], showing the structural homogeneity of the prepared materials. The XRD patterns also reveal a broad weak peak at 2 = 34.2◦ which can be attributed to the main reflection (1 1 0) of PdO [32]. This peak was only detectable in the LSCF–Pd × 2 and LSCF–Pd × 3 samples due to the low Pd content in the LSCF–Pd × 1 sample. The intensity of this peak is emphasized for the LSCF–Pd × 3 sample, indicating that sequential deposition of catalyst particles did not damage or change the LSCF phase, whilst also revealing their nanocrystalline nature. The presence of this phase is in agreement with the experimental procedure used in this work and attributed to the oxidation of Pd◦ to PdO in the temperature range of 500–600 ◦ C [33,34]. 3.2. Oxygen permeation measurements Fig. 4 displays the O2 fluxes isotherms as a function of the sweep gas rate for the LSCF and Pd modified hollow fibres. The O2 fluxes increased with temperature for all membranes, indicating the effect of temperature in enhancing ionic transport and surface exchange kinetics. For instance, the unmodified hollow fibre exhibited O2 fluxes between 0.11 and 0.50 ml cm−2 min−1 as the
temperature was raised from 700 ◦ C to 950 ◦ C, respectively, thus agreeing with previously published work [8,10]. By comparison, all Pd modified LSCF hollow fibres exhibited higher O2 fluxes over the entire temperature range, reaching a maximum value close to 2.0 ml cm−2 min−1 at 950 ◦ C. These results clearly indicate that the Pd catalyst coating procedure used in this work was beneficial in improving O2 fluxes. Further, these significant enhancements in O2 fluxes clearly demonstrate that the kinetics of the surface reaction is the limiting factor in oxygen transport through the LSCF hollow fibre. The Pd catalyst particles acted as O2 adsorption promoters, and helped facilitate O2 transport to the membrane surface. Fig. 4 also shows that the sweep gas flow rate plays an important role in the ability of the membrane system to separate O2 from air. In principle, increasing the sweep gas flow rate decreases the O2 partial pressure in the permeate stream. As a result, the driving force associated with the O2 partial pressure difference between the feed and permeate stream should increase, thus translating in improved O2 fluxes. However, this is not entirely the case here as there are two distinct O2 transport regimes apparent in Fig. 4. For temperatures equal to or below 800 ◦ C, the O2 flux remained almost constant for all membranes, suggesting that bulk ionic diffusion and/or surface exchange kinetics limited O2 transport independent of the sweep gas flow rate used. It is possible that at these relatively lower temperatures, the major transport resistance is associated with the surface exchange kinetics at the air feed side of the membrane, responsible for transferring gaseous O2 into ionic oxygen in the solid perovskite lattice. The change of O2 partial pressure in the permeate stream by the sweep gas was therefore not affected by the gas atmosphere surrounding the membrane surface in the
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feed stream. However, the effect of the sweep gas flow rate was clearly noticeable for temperatures above 800 ◦ C, similar to findings published elsewhere [18]. Surface reactions begin enhancing the overall O2 permeation rate above 800 ◦ C, and the rate of oxygen ions recombining into O2 molecules at the inner surface of the hollow fibre became similar to the transport rate of the O2 molecules into the convective sweep gas flow [7]. Thus, increasing the sweep gas flow rate also increased the O2 partial pressure difference across the membrane and the O2 flux accordingly. Of course, once the transport of the O2 molecules into the convective sweep gas flow exceeded the rate of ionic recombination, or the rate of oxygen ion supply prior to ionic recombination, then further increases in the sweep gas flow rate were ineffectual, as evidenced by the plateauing of the O2 flux in Fig. 4 for the Pd modified membranes at higher sweep gas flow rates. Fig. 5a shows the relative improvement in O2 flux as a function of the number of Pd coatings. Optimal improvements were observed for a single Pd coating which led to a significant increase of O2 fluxes by two orders of magnitude, up to 350% as compared to the pure LSCF hollow fibre. On the other hand, it was unusual to observe that the relative improvement in O2 flux decreased as the number of catalyst loadings increased. It would be anticipated that by increasing the catalyst loading on the membrane surface, O2 fluxes would increase accordingly. One major distinction here is the non-homogeneity of the catalyst coverage of the LSCF surface (Fig. 2). In addition, Fig. 6 exhibits the SEM micrographs, clearly showing a process leading to the aggregation of Pd particles as the number of Pd coats increase from rather than dispersing on the LSCF membrane surface. This aggregation and growth of catalyst particles was detrimental as it reduced the surface area available for the oxygen surface exchange reactions per Pd particle volume. Nevertheless, the remarkable relative improvement in O2 flux observed in this work using a Pd catalyst is significantly higher than previous work, which included flux increases for surface modified LSCF membranes of 128% (Ag catalyst) [18], and for surface modified BSCF membranes of 25% (Pt catalyst) [20], 93% (Ag catalyst) [21] and 105% (Pd catalyst) [5]. Interestingly, the relative improvement in O2 flux peaked at different temperatures depending upon the number of Pd coatings. For instance, Fig. 5a shows the optimal improvement was reached at 850 ◦ C (LSCF–Pd × 1), 900 ◦ C (LSCF–Pd × 2) and 950 ◦ C (LSCF–Pd × 3). In principle, the Pd particle operates as a catalyst which overcomes the rate determining surface reaction exchange as depicted in the Arrhenius diagram in Fig. 5b. Therefore, the oxygen fluxes significantly increase for a single Pd coated membrane, aided by the spill-over of oxygen from Pd particle to the surface of the LSCF hollow fibre. Hence, the rate of oxygen permeation is determined by the rate of the supply of oxygen ions to the membrane surface in tandem with diffusion rates, and the rate of oxygen ion recombination at the permeate side. The positive effect of the Pd catalyst is observed as the apparent energy of activation
Fig. 5. (a) Percentage improvement of the O2 flux through LSCF–Pd × 1, LSCF–Pd × 2 and LSCF–Pd × 3 compare to the original LSCF membrane, as a function of the operating temperature, and (b) Arrhenius diagram of oxygen fluxes. Sweep gas flow rates at 200 ml min−1 .
for the oxygen fluxes reduced from 87 to 75 kJ mol−1 , from pure to a single Pd coated LSCF hollow fibre, respectively. For temperatures in excess of 850 ◦ C when a single Pd coat is applied, the relative increment of oxygen flux to the base case (e.g. pure LSCF) observed in Fig. 5a is no longer significant, and actually reduces at 950 ◦ C. These results suggest that the activity of oxygen chemisorption on Pd particle started decaying at high temperatures in excess of 800 ◦ C, thus reducing the oxygen spill-over effect of the Pd catalyst.
Fig. 6. SEM micrographs of the surface of Pd modified LSCF membranes.
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Time (hours) 0
100
200
300
LSCF-0×Pd LSCF-1×Pd
0.4 -2 -1 Oxygen flux (ml.cm .min )
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Fig. 7. Schematic of the effect of Pd coated LSCF membranes.
A similar trend was also observed for silver coated BSCF hollow fibres reported elsewhere [21]. The increase in the number of coats was accompanied by Pd particle aggregation shown in Fig. 6, which led to a diminished ratio of the available spill-over area to the surface area occupied by the catalyst itself. Consequently, the diffusion of oxygen from the catalyst particles to the LSCF surface, particularly at low temperatures, affected the overall oxygen flux as observed in Fig. 5b. Hence, the reduction of effective area for oxygen surface access resulted in an increase of the apparent energy of activation for oxygen fluxes to values ∼120 kJ mol−1 , much higher than those calculated for the pure and single Pd coat LSCF hollow fibres. The optimal improvement of O2 fluxes observed at 900 and 950 ◦ C for the two and three catalyst coatings in Fig. 5a respectively, could then be explained by the combination of the increased surface exchange kinetics of the LSCF surface at higher temperatures together with improved bulk diffusion, though accompanied by the reduced spill-over effect from the larger Pd catalyst particles. This is schematically shown in Fig. 7 and based on the SEM micrographs in Fig. 6. As the Pd particles increase in size, they reduce the LSCF surface area accessible to ionic diffusion via spill-over, or via direct access of O2 molecules from the gas phase to the surface of the membrane. Thus, whilst the supply of oxygen ions from the LSCF surface exchange reaction is generally lower than from the catalytic Pd particles, the faster kinetics of this reaction counteracts the diminishing spill-over effect of the larger Pd particles at higher temperatures. It must be borne in mind that the improvements in O2 fluxes in Fig. 5a are relative to
Log O2 flux improvement (%)
0.1
O2 flux improvement (%)
100
10
0
100
200
300
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Time (hours) Fig. 8. Time-dependence of O2 flux through original LSCF and LSCF–Pd × 1 hollow fibres (T = 850 ◦ C, sweep gas flow rate = 100 ml min−1 ).
the performance of a pure LSCF membrane. In principle, the ionic flux in LSCF materials will increase as a function of temperature. The relative increments observed should be considered in the light of trade-offs between the catalyst activity, catalyst particle size, particle distribution on the membrane surface, and accessible area for surface ionic diffusion. 3.3. Long term oxygen permeation testing In order to evaluate the long term operational stability of the prepared hollow fibres, the O2 flux was evaluated over 410 h of operation at 850 ◦ C for the pure LSCF hollow fibre (LSCF–Pd × 0) and the surface modified (LSCF–Pd × 1), as shown in Fig. 8. During the first 24 h of operation, LSCF–Pd × 0 exhibits an initial reduction in O2 flux of 5.8%. After 24 h, the change in O2 flux stabilized, resulting in a constant rate of reduction of approximately 0.05% per hour. These long term results are in good agreement with Schlehuber et al. [25] who reported a fairly stable performance for a similar LSCF
Fig. 9. SEM micrographs of the LSCF–Pd × 1 membrane surfaces (a) before and (b) after O2 permeation measurement (c) EDX spectrum of identified area.
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membrane tested for 3000 h of operation at 800 ◦ C. In contrast, the LSCF–Pd × 1 membrane displayed a significant loss in O2 flux in the first 24 h. For instance, the initial oxygen flux of 0.4 ml min−1 cm−2 underwent a considerable reduction of 45.2 ± 0.1%; equivalent to a rate of reduction of 0.2% per hour. From thereon, the Pd surface modified membrane became more stable and much slower decline of 0.047% per hour was observed. Comparatively the Pd surface modified LSCF membrane gave superior performance throughout the entire 410 h. However, the improvement in O2 flux offered by the Pd surface modification reduced from 134% at 0 h to 32% within the first 24 h, followed by further reduction to 24% after 410 h. As the long term testing was carried out using atmospheric air, the small loss of performance observed within the first 24 h of operation for the pure LSCF hollow fibre could be associated with the effect of the small concentration of CO2 present. However, degradation as a result of CO2 exposure would not explain the significant drop in O2 flux measured for the Pd surface modified membrane. To further investigate this problem, SEM micrographs were taken after O2 permeation testing and compared with those taken prior to testing as depicted in Fig. 9. It is observed that the Pd particles (confirmed through EDX analysis – Fig. 9c) coalesced and increased in size from ∼200 nm to ∼1000 nm during testing. These results suggest that the Pd particles did not adhere strongly to the LSCF surface and were free to coalesce and aggregate at elevated temperatures. Similar non-adherence effects were also reported for Ag catalysts on BSCF electrodes [35]. Furthermore, prolonged testing at high temperatures resulted in the thermodynamically favoured movement of the Pd particles towards the perovskite grain boundaries. This coalescence and subsequent increased size of the Pd particles served to counteract their beneficial spill-over effect on the LSCF surface. 4. Conclusions This work demonstrates that the O2 flux of LSCF hollow fibres can be dramatically enhanced through Pd surface modification. The coating of Pd nanoparticles onto the surface of the LSCF hollow fibres overcame the surface exchange reaction kinetics and enhanced the O2 flux by up to 350%, in comparison to unmodified LSCF hollow fibres. This represents the largest performance increase of an LSCF hollow fibre by catalytic surface modification in the published literature to date. Optimal enhancement was achieved with a single Pd coating. Further coatings were shown to be ineffectual at increasing the O2 flux due to the nonhomogeneous distribution and aggregation of Pd nanoparticles, thus reducing the spill-over effect of the catalyst. The flow rate of the sweep gas in the permeate stream was found to improve O2 fluxes only at temperatures above 850 ◦ C, where the surface reactions were dominant. Long term stability testing showed that whilst the Pd modified LSCF membrane continually outperformed the pure LSCF hollow fibre for over 400 h, the level of enhancement was reduced over time. The reduction in performance was non-linear with the most dramatic reduction occurring within the first 24 h, attributed to the coalescence and aggregation of the Pd nanoparticles at the LSCF grain boundaries. Acknowledgments This work was financially supported by the Australian Research Council (ARC) through Discovery Project program (DP0878849). The authors gratefully acknowledge Ms. Anya Yago (Centre for Microscopy and Microanalysis), Mr. Hong-yi Xu (Centre for Microscopy and Microanalysis) for their contribution in the XRD and the SEM-EDX analysis, respectively.
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