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CO separation improvement produced on a ceramic– carbonate dense membrane superficially modified with Au-Pd Oscar Ovalle-Encinia, Heriberto Pfeiffer, and Jose Ortiz-Landeros Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01570 • Publication Date (Web): 18 Jun 2018 Downloaded from http://pubs.acs.org on June 25, 2018

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Industrial & Engineering Chemistry Research

CO2 separation improvement produced on a ceramic–carbonate dense membrane superficially modified with Au-Pd

Oscar Ovalle-Encinia,a Heriberto Pfeiffer,a José Ortiz-Landerosb*

a

Laboratorio de Fisicoquímica y Reactividad de Superficies (LaFReS), Instituto de Investigaciones

en Materiales, Universidad Nacional Autónoma de México, Circuito exterior s/n, Cd Universitaria, Del. Coyoacán, CP 04510, Ciudad de México, Mexico b

Departamento de Ingeniería en Metalurgia y Materiales, Escuela Superior de Ingeniería Química e Industrias Extractivas, Instituto Politécnico Nacional, UPALM, Av. IPN s/n, CP 07738, Ciudad de México, Mexico

*Corresponding author. Phone: +52 5557296000 ext. 54267 e-mail: [email protected]

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract: Ceramic-carbonate membranes have been proposed for the selective separation of CO2. In previous reports, membranes performance has been enhanced through the improvement of microstructural features and conductivity properties. Different to the aforesaid, this paper was focused on modifying the membrane surface by incorporating metallic particles to promote the involved surface reactions. First, a composite made of a Ce0.85Sm0.15O2-δ and Sm0.85Sr0.15Al0.05Fe0.05O3-δ was chemically synthesized, followed by the obtaining of porous supports by pressing of powders and sintering. Then, dense membranes were fabricated by infiltration of the supports with molten carbonates and the subsequent deposition of metallic Au-Pd particles on the membrane feed side surface. Obtained membranes were tested for CO2 separation between 700 and 900°C, using different feed gas mixtures. Membranes show excellent CO2 permeance (1.72 x 10-7 mol·m2 -1

·s ·Pa-1) operating at low CO2 partial pressure in feed side (P = 0.115 atm), wherein

about 24 % of the total permeation values resulted from the surface modification approach. Based on these results, the permeation mechanism is discussed.


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1. Introduction In the last decades, different researches have been conducted to study the Earth’s unusual weather and its negative impact on the ecosystems and human activities.1 These weather anomalies are a direct result of excessive greenhouse gas emissions and global warming phenomena.1,2 In the same way, CO2 is one of the most abundant greenhouse gases, due to increasing industrial activity.3 Therefore, new technologies for CO2 capture,4-8 utilization9-11 and separation12-20 are being developed. Among the potential technologies considered for CO2 mitigation, membrane-based separation processes are a promising alternative from the point of view of their application for selective separation of CO2 from stationary big sources. For instance, in the case of electricity power plants, the application of a high temperature carbon dioxide permselective membrane could offer potential for pre- and post-combustion CO2 separation.21-23 Of course, the potential application of this kind of processes lies on membrane development, which must exhibit thermal and chemical stability, high CO2 selectivity and high permeation flux.17,24-25 Moreover, one of the advantages of membrane-based technologies is that sweep concentrated CO2 can be subsequently used as a carbon feedstock for different applications.26-27 Lin and coworkers developed in 2005, for first time, a dense dual-phase membrane made of steel and molten carbonates for CO2 separation at intermediate temperatures (450750 °C).24 They found out that O2 concentration in the feed gas was required to obtain the best membrane performance for CO2 permeation. The separation mechanism of such metalcarbonate membrane considers the surface reaction between O2 and CO2 from feed gas with electrons from metallic phase to form ions. Afterwards, the produced carbonate ions are diffused to downstream side wherein the backwards surface reaction takes place by -3ACS Paragon Plus Environment


releasing both CO2 and O2 molecular species while the electrons (e-) return to the metal phase (reaction 1).

CO2 + + 2 e- ↔

(1)

Later, in 2006, a novel dense dual-phase membrane based on a mixed ionicelectronic conductor ceramic phase was proposed for CO2 separation.28 The separation mechanism model was explained in 2007 by Wade et. al.29 and 2009 by Rui et.al.30 Briefly, in the absence of O2 in the feed gas, the separation mechanism of such ceramic-carbonate membrane involves the surface reaction between the CO2 with O2- ions from the ionic conductor phase of membrane to form ions, which are subsequently transported through molten carbonates. As the inverse reaction occurs in downstream membrane side, CO2 is released to the permeated side, while oxygen ions (O2-) return to ceramic phase through the oxygen vacancies ( ) (reaction 2). CO2 + O2- ↔ +

(2)

A schematic representation of this separation process is illustrated in figure 1(a).31 Additionally, another CO2 transport mechanism may take place when O2 is in feed gas.30,32 In this case, O2 dissociates into ionized species which are recombined with oxygen vacancies in the ceramic mixed conductor lattice to form atomic oxygens ( ) and electron holes (ℎ∙ ), reacting with CO2 to form . Carbonate ions are transported through molten carbonates according with reactions 3 and 4. Then, in the downstream side CO2 and O2 are released; this mechanism is shown in figure 1(b).32

+ ↔ +2ℎ∙

(3)

CO2 + ↔ +

(4)


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Fig.1

The combination of aforementioned mechanisms was recently observed in the case of ceramic carbonate dual phase membranes made of a fluorite/perovskite composite as the oxide ceramic phase and an eutectic mixture of molten carbonates.33 In this system, the fluorite phase (Ce0.85Sm0.15O1.925) is an excellent oxygen ion conductor; and the perovskite (Sm0.6Sr0.4Al0.3Fe0.7O3) is an ionic/electronic mixed conductor. Moreover, it was concluded that not only ionic conductivity properties of fluorite and perovskite may be involved on the CO2 transport membrane, but also the perovskite electronic conductivity (see reactions 3 and 4). Taking in mind CO2 transport mechanisms through this kind of membranes, studies have been focused on the design and improvement of membrane supports microstructure and conductivity properties.16,32 Hence, works have been devoted for enhancing thermal and chemical stability properties of these membrane systems used in different applications.31 Among different applications, syngas combustion-assisted CO2 capture or syngas steam–methane reformer systems imply CO2 and O2 permeance membranes.34 There is a general agreement that the CO2 transport through the ceramic-carbonate membrane is not limited by the CO32- diffusion itself. Actually, the identified diffusion issues are related with the O2- species when the ceramic phase in the ceramic-carbonate membrane is a pure oxygen ionic conductor (figure 1) Roughly, the ionic conductivity of



CO32- through the molten carbonate phase is one order of magnitude higher than the ionic conductivity of O2- species through the ceramic phase. On the other hand, there are almost no reports regarding the surface reaction issues. Anderson and Lin17 reported the CO2 permeation properties of a perovskite-carbonate membrane. Moreover, they reported the effect of membrane thickness on the permeation flux. They found a significant increase on the permeation flux when the membrane thickness was reduced from 3 mm to 1.5 mm; however, when the thickness was subsequently reduced to 0.75 mm and even more until 0.375 the permeation flux was almost the same. Authors discussed the observed behavior as the surface reaction becomes a factor as thickness decreases. More recently, Marques et. al.37, reported a simple model on the performance of ceramic-carbonate membranes for CO2 separation. Authors discussed the roles of ionic species transport as well as the surface/interface reaction process.

In the present study, it is proposed a new approach to enhance CO2 permeation at high temperatures, by modifying the membrane surface in such a way that the involved surface reactions are promoted. Specifically, it was analyzed the incorporation of gold/palladium (Au-Pd) metallic particles on the membrane surface at feed side to improve CO ion formation. For this purpose, dense membrane Ce0.85Sm0.15O2-δ/Sm0.6Sr0.4Al0.3Fe0.7O3-δ (SDC-SSAF) composite infiltrated with molten carbonate was analyzed.


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2. Experimental procedure The EDTA-citrate complexing method was used to synthesize Ce0.85Sm0.15O2δ/Sm0.6Sr0.4Al0.3Fe0.7O3-δ

method.33,

35

(SDC-SSAF) composite powders by the so called one-pot

Stoichiometric amounts of precursors were calculated to obtain 75:25 wt%

ratio of SDC and SSAF phases, respectively. First, cerium and samarium nitrates (Ce(NO3)3·6H2O, 99.0% Sigma-Aldrich and Sm(NO3)3·6H2O, 99.9% Sigma-Aldrich) were dissolved in deionized water, followed by addition of anhydrous citric acid (99.98% SigmaAldrich) and ethylenediaminetetraacetic acid (EDTA, 98.5% Sigma-Aldrich) previously dissolved in ammonium hydroxide (28.0-30.0%, BAKER ANALYZED®). Then, a second solution was prepared. Nitrate precursors, (Sm(NO3)3·6H2O, 99.9% Sigma-Aldrich, Sr(NO3)2 99.0% Meyer, Al(NO3)3·9H2O, 98% Sigma-Aldrich and Fe(NO3)3·9H2O, 98% Meyer) were dissolved in deionized water, followed by addition of citric acid anhydrous and EDTA as well. An equimolar ratio between citrate and EDTA were added respect to total metal ions. The pH values were adjusted to 6-8, by adding ammonium hydroxide to both solutions. Finally, both solutions were mixed together by stirring and heating up to 90°C. After water evaporation a black paste was produced, which was combusted at 300°C for 3 h. The resultant yellow powders were calcined at 650 °C for 10 h and then at 900 ºC for 10 h. Membrane supports (porous disks) were fabricated by placing 4 g of composite powders with 1 wt% of polyvinyl alcohol (PVA, MP Biomedicals) as binder into a 26 mm diameter stainless steel die and pressed up to 58 kPa in an uniaxial hydraulic press. These supports were sintered at different temperatures (from 900 to 1100 °C) using a heat rate of 1 °C/min



in air for 40 h. Once sintered, disks were infiltrated at 600 ºC with a eutectic molten carbonate mixture composed of Li2CO3/Na2CO3/K2CO3 with ratio of 42.5/32.5/25.0 mol%. After cooling down, excess of carbonates remained on the membrane surface were removed by polishing with sand paper; besides this process was performed until get a membrane thickness of 0.9 mm. Finally, Au-Pd particles were deposited over the membrane feed side surface. The surface modification was performed by physical vapor deposition technique, using a Denton Vaccum Desk II coater. A gold-palladium (46/54 at%) alloy target was used for sputter deposition process and membrane surfaces were exposed for a short time of 20 seconds in order to obtain particles in the nanometric scale instead of a thick film. The structural characterization was evaluated using a diffractometer (Bruker, D8 Advance) with a Cu-Kα (1.54059 Å) radiation source operating at 35 kV and 30 mA. Samples were measured in 2-theta range from 20 to 120° using a step size of 0.02 °. Different phases were identiﬁed using Powder Diffraction File Database (PDF-2). Dilatometric analysis was used to determine the sintering behavior of green disk supports by using a SETSys Evolution TMA apparatus, tests were performed in air between 40 and 1200°C. The disk supports porosity was determined by the Archimedes method using liquid nitrogen36. Porosity tests were performed four times for each sample, in order to calculate average porosity and experimental error. Additionally, the membranes surface morphology was analyzed using a scanning electron microscope JEOL JMS-7600F. High temperature CO2 permeation measurements were conducted between 700 and 900ºC, using a home-made experimental set-up, wherein SDC-SSAF-carbonate membranes were sealed to an inner alumina tube by creating a ceramic sealant paste. The sealant was composed by SDC-SSAF powder sintered at 1000ºC (40 wt%), Pyrex glass powder (50 wt%) and sodium aluminum oxide (Al2O3 Na2O; 10 wt%), mixed with deionized water. -8ACS Paragon Plus Environment

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The system was completely sealed inside a 50 mm outer diameter dense alumina tube and heated at a rate of 3 ºC/min from room temperature to 900 ºC allowing the glass phase to melt and form a gas tight seal. The following feed gas mixtures were used for CO2 permeation experiments; CO2/He (15/15 mL·min-1) and CO2/He/O2 (15/15/6 mL·min-1), N2 balanced to 100 mL·min-1. In all the cases, N2 was used as sweep gas in the downstream side of the membrane array. Helium was used to demonstrate the high density of the membrane by measuring its permeation which always was two order of magnitude lower than carbon dioxide permeation. The permeate gas was analyzed with a GC-2014 gas chromatograph (Shimadzu) equipped with a Carboxen-1000 column.

3. Results and discussion Thermal and chemical stabilities of SDC-SSAF composite under concentrated CO2 atmosphere were analyzed in a previous work with no evidence of secondary phases formation.33 Therefore, here, experiments were conducted straightly to the fabrication and surface modification of dense membranes and their testing for CO2 and O2 permeation at high temperatures. The linear shrinkage curve obtained by dilatometry and its derivative is shown in figure 2. This result reveal that shrinkage process starts at about 850°C. Moreover, the highest dimensional change rate is at 940°C, followed by a slower shrinkage rate observed over 1050 °C. It seems that sintering behavior is due to differences on the melting points of the ceramic composite constituents (fluorite and perovskite phases). Under these non-isothermal tests conditions, total shrinkage up to 1200 °C reached 8 %. Based on dilatometric curve, the obtained porous disks were sintered between 950°C and



1050 °C in order to optimize the open porosity fractions and therefore the volume fraction of infiltrated carbonate of these membranes.

Fig.2 Figure 3 shows XRD patterns of SDC-SSAF sintered at 650, 950, 1000 and 1100 °C. SDC and SSAF phases were successfully identified as fluorite and perovskite phases, corresponding to 43-1002 and 28-1227 PDF files, respectively. Furthermore, this analysis suggests a good thermal stability of this composite material, as XRD patterns did not show neither secondary phases formation nor decomposition of supports when sintered at high temperature.

Fig.3 SEM images of the different sintered supports are shown in Figure 4. Images are the cross sectional views showing the open porous microstructure. It is observed the typical microstructural evolution during densification; wherein the coalescence of particles occurs, and the total porosity decreases as a function of the sintering temperature (figure 4(a)-4(c)). Moreover, the pore size growth is also observed what is an indication of the incipient sintering process that have reached only the early intermediate stage32. General speaking, the membrane supports sintered at 1050 °C shows homogeneous porous distribution and average pore diameter between 0.1 and 1 µm. Total open porosity of these supports was calculated by the Archimedes method (figure 4(d)). As expected, pore volume decreases as a function of sintering temperature. In fact, total porosity varied in 5 vol% between 950 and 1050°C. This result agrees with dilatometric and SEM analyses. As mentioned, shrinkage


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of supports increases while sintering temperature increases, due to pore volume fraction decrement.

Fig.4 Based on the aforementioned results, dense membrane preparation and subsequent CO2 permeation experiments were performed using the support sintered at 1050°C as on that temperature the microstructural characteristics guarantee adequate pores connectivity and pore fraction, and therefore the correct infiltration of the carbonates phase. SEM micrographs of the membrane surface (obtained after infiltration) and cross section are shown in figure 5. As it can be seen, in the cross section the composite (bright phase) and carbonates (dark phase) evidence the formation of a dense membrane with no visible cracks or pin holds revealed (figure 5(a)). Figure 5(b) shows the backscattered electrons image, which confirmed the presence of large (SSAF), and small (SDC) bright particles with some dark zones, corresponding to composite and carbonates phases respectively. Also, figure 5(b) shows the elemental mapping obtained by EDS analysis. It can be seen the presence of big grains composed of Fe, Al, Sr and Sm, which fit to perovskite phase (SSAF). Ce, Sm and O are homogeneously distributed showing the presence of small particles of SDC, however, Ce is not observed in the zone where SSAF is mainly observed. Na and K are distributed on all the scanned area, which means that carbonates are distributed along the membrane. Finally, it can be seen homogeneous dispersion of Au and Pd on feed side membrane surface, distributed as cumulus. The Au-Pd content was estimated by EDS technique; the analyses performed on different areas of the membrane show an Au-Pd concentration of 1-3 %wt.



Fig.5 After membrane characterization, CO2 permeation tests were performed on modify and non-modify membranes at high temperatures (between 700 and 900 °C), using CO2/He/N2 in absence or presence of oxygen (figure 6). Initially, non-modified membrane in absence of oxygen presented the CO2 separation mechanism described above in figure 1(a) and reaction 2. In this case, CO2 permeance depends on ceramic oxygen ions conduction. In that sense, the atomic substitution in fluorite (Ce4+ by Sm3+) and perovskite (Sm3+ by Sr2+) structures produces certain concentration of oxygen vacancies, which increases oxygen diffusion and consequently allows the membrane permeation properties. On the other hand, in presence of oxygen (CO2 permeation schematized in figure 1(b) and represented on reactions 3 and 4), oxygen molecules react with perovskite superficial vacancies increasing the formation of carbonate ions.

Fig.6 The non-modified membranes present a maximum CO2 permeance of 1.17 x 10-7 (figure 6(a)) and 1.38 x 10-7 mol·m-2·s-1·Pa-1

7

(figure 6(b)) at 900°C in absence and

presence of oxygen, respectively. It is clear that O2 addition in feed gas enhanced CO2 permeance by 18%. These results can be explained as it was described in the introduction section (figure 1(b)); wherein CO2 transport mechanism must be favored when O2 is present (reactions 3 and 4). On the contrary, on the modified membrane, CO2 permeance values were 5.72 x10-8 (figure 6(c)) and 1.72 x10-7 mol·m-2·s-1·Pa-1 (figure 6(d)), in absence and presence of O2 respectively. In absence of oxygen, modified membrane presents the poorest performance at any temperature. This behavior can be explained as follows; CO2 -12ACS Paragon Plus Environment

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must react with O2- ions on the surface of ceramic, but metallic particles hinder such reaction because they form cumulus on the upstream side of membrane, obstructing active sites. In counterpart, modified membrane in presence of oxygen shows the best CO2 permeation performance. Here, besides the perovskite electrons, extra electrons provided by metallic particles in surface membrane react with O2 to form O2- ions. Then, these O2ions react with CO2 in the surface membrane to form and increase the amount of CO ions (reaction 1). In fact, the presence of O2 enhanced CO2 permeation in the modified membrane by 24 % more than that in the non-modified membrane. Furthermore, this CO2 permeation was 47 and 201% higher than those obtained for non-modified and modified membranes in absence of O2, respectively. Moreover, figure 6(c) (modified membrane in absence of oxygen) indicates that there is a small oxygen ion contribution, coming from composite, that reacts with CO2 to produce carbonate ions (reaction 2), although metallic particles block the membrane surface. Based on all these results, the proposed CO2 separation mechanism for modified membrane is schematized in figure 7. On the upstream, CO2 reacts with O2- ions of SDC forming CO32-. Moreover, O2 reacts with SSAF oxygen vacancies to form atomic oxygen, which react with CO2 to form more CO32- ions, and finally, O2 and CO2 react with electrons of metallic particles and perovskite to produce additional CO32-. On the downstream, the inverse reactions take place, CO32- reacts with oxygen SDC vacancies to produce O2- and CO2. Thus, O2- returns to upstream side and CO2 is released. In addition, CO32- reacts with SSAF electronic holes forming electrons, CO2

and O2. The electrons return to upstream side, while CO2 and O2 are released as permeated gases. In the proposed model, metallic particles were deposited over the membrane surface but at high temperatures metallic particles must be displaced from the



liquid phase (molten carbonates) towards the interface ceramic-carbonates. From this point of view, this kind of modified membranes would be useful for CO2 separation systems implying the oxygen presence, such as syngas combustion-assisted CO2 capture or syngas steam–methane reformer processes.34

Fig.7 Arrhenius plots for CO2 permeation of modified and non-modified membranes are shown in figure 8. In presence of oxygen, activation energies (Ea) were almost the same, 110.6 and 110.2 kJ·mol-1 for modified and non-modified membrane respectively. However, the modified membrane permeated more CO2 than non-modified. Hence, deposition of Au/Pd particles favored the formation of CO32- and consequently the permeation of CO2, but O2 permeance was not modified significantly (see data below). Therefore, Ea is associated with the limiting process, O2 permeation. Conversely, in absence of O2, Ea depends on crystalline oxygen atoms and active surface sites. Therefore, in these cases Ea values increased, as it is more difficult to take oxygen atoms from SDC-SSAF crystalline structures than from feed gas.

Fig.8 Figure 9 shows permeated O2 by non-modified and modified membranes. In addition, ideal O2 permeations are presented, according to reactions 3 and 4. Based on these reactions, the increase of permeated CO2 (∆(CO2)), after O2 addition, was calculated by subtracting the CO2 permeation in absence of oxygen from CO2 permeated in presence of oxygen. It was observed that the experimental oxygen permeation was higher than the corresponding


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theoretical values (ideal O2 = ∆(CO2)). On non-modified membrane results indicate that

after O2 reacts with vacancies (reaction 3 and figure 9(a)) these oxygen ions are preferentially diffused through the composite to downstream instead of reacting with CO2 on the upstream as reaction 4 suggests. In this sense, the oxygen permeation through a dense membrane made of a mixed ionic-electronic conductor composite, i.e. doped ceria and perovskite composite, has been analyzed in other works, Zhu et. al.35; results show that as higher the total conductivity the higher oxygen permeation flux is. Moreover, in this kind of systems, the distribution of the two phases (SDC and perovskite) in the membrane bulk determines the total conduction across the membranes. Yang and Zhu38, show that the principle of oxygen permeation through these membranes consist in the classical three main steps, 1) interfacial oxygen exchange on feed side, 2) bulk diffusion across grains and grain boundaries, and 3) interfacial oxygen exchange on permeation side. In this case the holes and vacancies are considered as the charge carriers, then the oxygen reacts with the vacancies of perovskite to form atomic oxygens and holes, these atomic oxygens species and holes are diffused to the permeation side to form again molecular oxygen and vacancies. In our case, the incorporation of molten carbonates in the membrane produce that some atomic oxygen reacts with CO2 to form more carbonate ions instead of permeate oxygen. In order to increase CO2 permeation, all the atomic oxygen (formed in reaction 3) must react with carbon dioxide to form additional CO32-. Therefore, surface modification proposed in this work is expected to increase such interaction.

Fig.9 On the other hand, modified membranes show higher ∆(CO2) than non-modified membrane (figure 9(b)). Therefore, electronic density of metal particles and oxygen -15ACS Paragon Plus Environment


enhanced CO2 permeation through reaction 1. In this case, the electrons of metallic conductor react with molecular oxygen (O2) and CO2 to form carbonates ions, the molecular oxygens are dissociated on the surface of metallic particles to form oxygen ions, these ions react with CO2 to form carbonate ions, therefore CO2 permeation is increased. Moreover, ∆(CO2) is higher than CO2 permeation in the modified membrane in absence of oxygen, hence, the mainly involved separation mechanism is associated with the electronic contribution of metallic particles. As in the non-modified case, the permeated O2 is higher than the ideal permeation, but the permeated O2 through the composite is lower which means that more O2 is reacting with CO2 to form CO32-. These results suggest that reaction 3 is controlling O2 permeation through composite. In both cases, experimental O2 permeation values slightly increase with temperature. It is noticeable that oxygen permeation did not experience a significant change after surface membrane modification. This fact could be explained as Au-Pd deposition increases the electron density increasing CO32- ions formation. However, on downstream side there are not as much electron collectors as on the upstream side. Therefore, in the permeate side is more favorable the inverse reaction of CO32- with the vacancies of ceramic to form oxygen ions and CO2 (inverse reaction 2), instead of the inverse coupled reactions 3 and 4. Moreover, vacancies in fluorite and perovskite phases favor the return of oxygen ions through these phases instead of permeate oxygen.


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4. Conclusions A surface-modified dense dual-phase membrane based on a mixed ionic-electronic conductor composite (Ce0.85Sm0.15O2-δ fluorite and Sm0.85Sr0.15Al0.05Fe0.05O3-δ perovskite) was successfully prepared by uniaxial pressing of chemically synthesized powders followed by infiltration of prepared supports with a Li-Na-K molten carbonate phase and finally AuPd metallic particles were effectively deposited on the feed side membrane. From CO2 permeation experiments performed on these membranes, it was clear that both, metallic particle deposition and the oxygen presence (PO2 = 0.046 atm) highly improved CO2 permeation at high temperatures. Based on these results, CO2 and O2 permeation mechanisms were proposed. CO2 permeation at high temperatures involves the reaction between oxygen in the feed gas and electrons coming from metallic phase to form oxygen ions which react with CO2 forming carbonate ions. Additionally, the interaction between lattice oxygen ions and CO2 is preserved, enhancing total carbonate ions formation. Finally, a third mechanism related to oxygen permeation was observed. In this case, oxygen ions were diffused through the perovskite structure instead of reacting with CO2 on the upstream side. Therefore, CO2 and O2 were released on downstream. Nevertheless, the lower electronic density on the sweep side than on feed side disfavored the inverse reaction on downstream. Thus, CO32- mainly decomposes on CO2 and oxygen ions (returning through the composite) instead of releasing O2. Obtained results suggest that surface modification by deposition of metallic particles seems to be an excellent strategy to enhance CO2 permeation properties on this kind of ceramic-carbonate membranes.



Acknowledgments O. Ovalle thanks to CONACYT for financial support. J. Ortiz-Landeros thanks to SIPIPN 20181055, EDI-IPN and SIBE-IPN programs. H. Pfeiffer thanks to SENERCONACYT (251801) project for financial support.

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FIGURE CAPTIONS Figure 1. Schematic representation of CO2 separation mechanism on a dense membrane made of mixed conductor oxide ceramic and molten carbonates, showing two specific cases, where the electronic conductivity of ceramic is zero (a) and where the formation of carbonate ions depends of oxygen presence in the feed gas (b). Figure 2. Dilatometric curve and its derivative of the SDC-SSAF support preheated at 600°C. Figure 3. XRD patterns of SDC-SSAF supports sintered at different temperatures. Figure 4. SEM images of SDC-SSAF supports (cross sectional microstructure) sintered at (a) 950, (b) 1000 and (c) 1050 °C. (d) Total open porosity calculated by Archimedes method using liquid nitrogen for SDC-SSAF supports sintered at different temperatures (experimental error is not larger than 2%). Figure 5. SEM images of a dense SDC-SSAF-carbonates membrane. Cross-section view of the membrane prepared with a disk sintered at 1050°C (a) and surface membrane modified with Au/Pd showing the elemental mapping obtained by EDS (b). Figure 6. CO2 permeance at high temperatures of membranes fabricated with supports sintered at 1050 °C. All these experiments were performed using PCO2 = 0.115 atm in feed gas. Non-modified membrane: (a) PO2=0 atm and (b) PO2=0.046 atm. Modified membrane: (c) PO2=0 atm and (d) PO2=0.046 atm. Curves (a) and (b) were added for comparison purposes.


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Figure 7. Schematic representation of CO2 separation mechanism of SDC-SSAF-carbonates dense membrane modified superficially with Au/Pd particles. The main CO2 separation mechanism follows the Eq. (1) (at the bottom), although reaction represented on Eq. (2) (at the top) is then observed because Au-Pd cumulus did not hinder completely active sites on surface allowing the interaction between oxygen ions and CO2 of feed gas. The same defects help to permeate oxygen from feed gas by reacting as in the Eq. (3) (not shown in this scheme). Figure 8. Arrhenius plots showing the effect of temperature in the CO2 permeance for modified and non-modified membranes. All these experiments were performed using PCO2=0.115 atm in feed gas. Non-modified membranes: (a) PO2=0 atm and (b) PO2=0.046 atm. Modified membranes: (c) PO2=0 atm and (d) PO2=0.046 atm. Curves (a) and (b) were added for comparison purposes. Figure 9. CO2 permeance (∆(CO2)) calculated by subtracting the CO2 permeance in absence of oxygen from the results in presence of oxygen as well as theoretical and experimental oxygen permeations. (a) non-modified and (b) modified membranes.



Figure 1. Schematic representation of CO2 separation mechanism on a dense membrane made of mixed conductor oxide ceramic and molten carbonates, showing two specific cases, where the electronic conductivity of ceramic is zero (a) and where the formation of carbonate ions depends of oxygen presence in the feed gas (b). 139x244mm (300 x 300 DPI)

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Figure 2. Dilatometric curve and its derivative of the SDC-SSAF support preheated at 600°C. 79x79mm (300 x 300 DPI)



Figure 3. XRD patterns of SDC-SSAF supports sintered at different temperatures. 79x99mm (300 x 300 DPI)


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Figure 4. SEM images of SDC-SSAF supports (cross-sectional microstructure) sintered at (a) 950, (b) 1000 and (c) 1050 °C. (d) Total open porosity calculated by Archimedes method using liquid nitrogen for SDCSSAF supports sintered at different temperatures (experimental error is not larger than 2%). 60x45mm (300 x 300 DPI)



Figure 5. SEM images of a dense SDC-SSAF-carbonates membrane. Cross-section view of the membrane prepared with a disk sintered at 1050°C (a) and surface membrane modified with Au/Pd showing the elemental mapping obtained by EDS (b). 160x60mm (300 x 300 DPI)


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Figure 6. CO2 permeance at high temperatures of membranes fabricated with supports sintered at 1050 °C. All these experiments were performed using PCO2 = 0.115 atm in feed gas. Non-modified membrane: (a) PO2=0 atm and (b) PO2=0.046 atm. Modified membrane: (c) PO2=0 atm and (d) PO2=0.046 atm. Curves (a) and (b) were added for comparison purposes. 79x79mm (300 x 300 DPI)



Figure 7. Schematic representation of CO2 separation mechanism of SDC-SSAF-carbonates dense membrane modified superficially with Au/Pd particles. The main CO2 separation mechanism follows the Eq. (1) (at the bottom), although reaction represented on Eq. (2) (at the top) is then observed because Au-Pd cumulus did not hinder completely active sites on surface allowing the interaction between oxygen ions and CO2 of feed gas. The same defects help to permeate oxygen from feed gas by reacting as in the Eq. (3) (not shown in this scheme). 80x80mm (300 x 300 DPI)


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Figure 8. Arrhenius plots showing the effect of temperature in the CO2 permeance for modified and nonmodified membranes. All these experiments were performed using PCO2=0.115 atm in feed gas. Nonmodified membranes: (a) PO2=0 atm and (b) PO2=0.046 atm. Modified membranes: (c) PO2=0 atm and (d) PO2=0.046 atm. Curves (a) and (b) were added for comparison purposes. 79x79mm (300 x 300 DPI)



Figure 9. CO2 permeance (∆(CO2)) calculated by subtracting the CO2 permeance in absence of oxygen from the results in presence of oxygen as well as theoretical and experimental oxygen permeations. (a) nonmodified and (b) modified membranes. 80x70mm (300 x 300 DPI)


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TOC Graphical 299x140mm (300 x 300 DPI)


CO2 Separation Improvement Produced on a ... - ACS Publications

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