cobalt oxide silica membranes

Separation and Purification Technology 171 (2016) 248–255

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Gas permeation redox effect of binary iron oxide/cobalt oxide silica membranes Adi Darmawan a,b, Julius Motuzas a, Simon Smart a, Anne Julbe c, João C. Diniz da Costa a,⇑ a

The University of Queensland, FIM2Lab – Functional Interfacial Materials and Membranes Laboratory, School of Chemical Engineering, Brisbane, Qld 4072, Australia Diponegoro University, Department of Chemistry, Semarang 50275, Indonesia c Institut Européen des Membranes, UMR5635-CNRS-UM-ENSCM, Université de Montpellier-cc047, Place Eugène Bataillon, 34095 Montpellier cedex 5, France b

a r t i c l e

i n f o

Article history: Received 7 July 2016 Received in revised form 20 July 2016 Accepted 20 July 2016 Available online 22 July 2016 Keywords: Iron oxide Cobalt oxide Redox Silica membranes Gas permeance

a b s t r a c t This work investigates the redox cycling effect on the physicochemical characteristics and gas permeation behaviour of binary metal oxide (iron/cobalt) silica membranes prepared by sol–gel method from tetraethyl orthosilicate, cobalt and iron nitrates, peroxide and water. A Fe/Co ratio of 10/90 conferred more stable silica microstructure under redox cycling, contrary to Fe/Co ratios P25/75 which led to structural densification after the first redox cycle. The gas permeance redox effect of Fe/Co = 10/90 silica membranes resulted in a switchable permeance increase and decrease as the membrane was reduced and oxidised, respectively. However, the extent of the switchable changes were not constant and tended to decrease at each cycle, as evidenced by the He/CO2 permselectivity shift from a higher value of 130 (first oxidation cycle) down to 80 (third oxidation cycle). It was found that a shift in temperature corresponding to the exothermic oxidation peak, attributed to the progressive changes in the formation of FexCo1 xOy mixed oxide embedded in the silica matrix at each redox cycle. This was further supported by the sequential disappearance of the infrared absorbance peak at 565 cm 1 upon redox cycling, thus demonstrating structural re-arrangement of the membrane. The permselectivity of He and H2 over CO2 sequentially decreased at a higher ratio for after the oxidation cycle, as a result of the silica matrix densification at high temperatures (400 °C). Ó 2016 Elsevier B.V. All rights reserved.

1. Introduction Switchable gas flow membranes were first reported in early 2000s by Julbe and co-workers [1–3]. The membranes were based on the dispersion of vanadium oxide in an aluminium phosphate matrix formed in the pores of a macroporous alumina support. The switch between high and low gas flow rates was achieved by simply exposing the membrane to reducing and oxidizing (redox) conditions in the temperature range 400–500 °C. This redox conditioning approach was attractive to tailor pore size distributions of ceramic membranes in the macro/mesoporous range, for potential application as a safe oxygen distributor for partial oxidation of alkanes [3]. Further development of this concept came about in 2012 for metal oxide silica membranes. Miller et al. [4] showed that cobalt oxide silica membranes repeatedly switched the flux of He, CO2 and N2 by exposing the membrane to redox conditioning. An interesting aspect of this work was that the metal oxide was constrained by an amorphous silica matrix, though the switch⇑ Corresponding author. E-mail address: [email protected] (J.C. Diniz da Costa). http://dx.doi.org/10.1016/j.seppur.2016.07.030 1383-5866/Ó 2016 Elsevier B.V. All rights reserved.

ing redox effect has not been previously observed for metal oxide silica membranes which underwent extensive research since early 2005. A major reason for this switching effect was attributed to replacing the conventional TEOS silica precursor with ES40, which proved to have superior properties to cope with the switching associated with cobalt phase change in the silica matrix under redox cycling [4]. The sol-gel synthesis technique is a very versatile method, accommodating the incorporation of a number of metals or metal oxides in the preparation of silica derived membranes such as nickel [5], cobalt [6–8], palladium [9], alumina [10], niobia [11], zirconia [12] and binary metals such as iron cobalt oxide [13]. By embedding metal oxides into silica films, the pore tailoring from the reducing step has been mainly attributed to loss of oxygen from the metal oxide, thus opening a molecular gap and generally increasing the flux for the larger gas molecules. Very recently, Ji and co-workers [14] reported an anomalous 170% increase in H2 permeance from H2/Ar gas mixture to single H2 gas using cobalt oxide membranes. This was attributed to the shrinking core model, causing the larger metal oxide particles to crackle as they break down in a gas and solid reaction, and leading to the formation of

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smaller particles with inter-particle porous structures accessible for gas transport [15,16]. Therefore, improved pore interconnectivity was proposed by the crackling core effect of the particles, and improving gas diffusion. Another approach recently reported includes the effect of using binary metal oxides in silica membranes. Ballinger and co-workers [17] prepared silica based membranes containing binary palladium and cobalt oxides. They showed that palladium oxide preferentially reduced to palladium metal at 500 °C, as compared to cobalt oxide which remained as an oxide phase during reduction at this temperature. Hence, a molecular gap was created by the loss of oxygen in palladium reduction, thus increasing the permeation of both N2 and CO2 whilst maintaining the permeation of the smaller gases He and H2 at a similar level during the redox conditions. In another case, novel lanthanum silicate cobalt oxide silica membranes showed that the redox effect led to the slight reduction of gas permeation, which was more pronounced for the larger gases. As a result, the He/N2 permselectivity increased from 80 to 160 after 3 redox cycles and was attributed to the silicate role, opposing the densification of small pores [18]. The separation of H2 from industrial gas streams using H2 selective metal oxide silica membranes is likely to be affected by, and subsequently affect the H2 permeation through the membrane. The impact of the redox effect on the permeation behaviour of binary metal oxide silica membranes is not well understood, particularly that there is an array of possible metal oxide combinations and this warrants further research. In this work, we investigate the redox permeation effect of iron and cobalt oxides in silica membranes, a metal oxide combination which differs from previous reported palladium/cobalt [17] and lanthanum/cobalt [18] membranes. Iron oxide is a common and cheap metal oxide precursor, thus reducing the cost of silica based membranes in comparison with the formulations using more expensive oxide precursors such as lanthanum and palladium. The redox effect of iron cobalt oxide membranes is therefore studied at high temperatures up to 400 °C. The structural effect of the redox conditions on xerogels are fully characterised using spectroscopy, nitrogen adsorption and thermogravimetry. The membranes were tested for various gases (He, H2, CO2 and N2) to determine how the redox structural changes affect the transport phenomena of gases through the membranes.

2. Experimental 2.1. Synthesis and characterisation A series of samples were prepared by changing the iron oxide to cobalt oxide ratio, though the total metal oxide to silica ratio was kept constant at 1:4 as described elsewhere [13]. Briefly, iron/ cobalt oxide sols were synthesised by the hydrolysis and condensation of tetraethyl orthosilicate (TEOS) in ethanol and 30% aqueous H2O2 with iron nitrate nonahydrate (Fe(NO3)3
9H2O) and cobalt nitrate hexahydrate (Co(NO3)2
6H2O). An initial sol with molar composition 255 EtOH:4 TEOS:1 Metal Nitrate Hydrate: 9H2O2:zH2O was mixed and vigorously stirred for 3 h in an icecooled bath. The molar concentration of the iron and cobalt (x:y) oxides was varied between 0:1 (0/100) and 1:0 (100/0). Hence, the total molar concentration of the binary metal was always 1 mol. Blank samples of iron oxide silica (x = 1 and y = 0) and cobalt oxide silica (x = 0 and y = 1) were prepared for comparison purposes. The value of z related to the H2O molar content was varied to maintain a water molar ratio constant at 40. Sol samples were dried in a temperature controlled oven at 60 °C and atmospheric conditions to form a xerogel. The xerogel samples were crushed finely and calcined at ramp rates 1.5 °C min 1 to

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500 °C and 600 °C in an oxidizing (air) atmosphere with a 2.5 h holding time at the desired temperature. Nitrogen adsorption was studied at 77 K using a Micromeritics Tristar 3020 to determine the main sample microstructure characteristics (e.g. specific surface area and average pore sizes). The samples were initially degassed for 24 h at pressures of 2 Pa at 200 °C. The XRD patterns were obtained using a Rigaku Smartlab X-ray diffractometer at 45 kV and 200 mA with a step size of 2h = 0.02° and speed of 5° min 1 using a filtered Cu Ka radiation (k = 1.5418 Å). The high resolution transmission electron microscopy (HRTEM) was performed on a JEOL 2010 apparatus operating at 200 kV. Samples were prepared by placing a drop of diluted sample dispersion in ethanol onto a carbon-coated copper grid and air-dried prior to examination. The binary iron cobalt oxide silica xerogels were characterized by Fourier transform infrared (FTIR) spectroscopy by Shimadzu IRAffinity-1 with a Pike MIRacle diamond attenuated total reflectance (ATR) attachment across a 4000– 600 cm 1 wavenumbers. The TGA/DSC analysis of all xerogel samples was carried out using a thermo-gravimetric differential scanning colorimetric analysis TGA-DSC1 (Mettler Toledo) at air flow rate of 60 ml min 1 and dwell time of 60 min at 400 °C at aramping rate of 5 °C min 1. 2.2. Single gas permeation test Iron cobalt oxide silica thin membrane films were coated on ceramic tubular supports 10 cm in length and 1.4 cm external diameter provided by the Energy Centre of the Netherlands (ECN). Top film formation was carried out via dip-coating of the ceramic tube in a binary iron cobalt oxide silica sol with a dwell time of 2 min and a dipping and withdrawal rate of 10 cm min 1. Each layer was dip-coated and calcined separately at 600 °C in a temperature controlled furnace, with a hold time of 2.5 h and a ramp rate of 1 °C min 1. The dip-coating and calcination procedure were repeated four times to reduce the probability of thin film defects. The performance of the iron oxide silica membrane and iron cobalt oxide silica membrane were measured by single gas permeance using a custom-made setup (Fig. 1) with a membrane housing module. The membrane was sealed with graphite ferrules before being placed inside the permeation module. During the permeation measurements, one end of the membrane tube was plugged and the pressure in the permeate side was set at atmospheric pressure. No sweep gas was used. The membrane was then heated and tested for single gas permeance for helium (He), hydrogen (H2), and carbon dioxide (CO2) upon decreasing temperatures in the range from 500 °C to 100 °C at 50 °C increments with a transmembrane pressure of 400 kPa. Gas permeance was measured after successive reducing and oxidizing treatments at 400 °C overnight. The measurements were performed at 400 kPa in the following order: H2, He, CO2 for the reduced membranes, and He, CO2, H2 for the oxidised membranes to avoid membrane being initially reduced at the oxidation cycle. A maximum experimental error of ±10% was determined based on the measurements of flow rates, the length and diameter of the membrane tube to calculate

Fig. 1. Schematic of the membrane testing system.

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Fig. 2. Effect of the reducing or oxidizing treatment on the (a) specific surface area and (b) average pore sizes for iron cobalt oxide silica xerogels.

area, as well as instrumental errors from pressure transducers to determine pressure values. All the results are based on average values of up to five measurements.

3. Results and discussion Fe/Co oxide xerogels were initially exposed to redox conditions to determine structural changes, and the xerogel powders were analysed by N2 adsorption. The specific surface area and pore sizes for the oxidised and reduced xerogel samples are shown in Fig. 2a and b. All xerogels underwent some degree of densification during the redox cycles. The Fe/Co = 25/75 and Fe/Co = 50/50 porous structure collapsed after the first redox cycle, as revealed by the significant decrease of the surface area and associated widening of the average pore size. Contrary to silica xerogels containing the structurally superior cobalt oxide and given high surface areas [19], iron oxide silica structures have shown lower surface areas [20] which decreased as a function of the iron molar content [21]. In addition, previous iron cobalt oxide silica xerogels have also shown that surface area decreased with the increase of iron molar content [22]. The results in Fig. 2 are in line with previous

reports in the literature, and the redox effect is consistent with conventional thermal densification of porous silica, as the smallest pores tend to close. This thermal densification process, coupled with the large coefficient of expansion mismatch between iron and silica [23], and the stronger iron oxide to silica matrix interactions [24], caused microstructural rearrangement and yielded pore size enlargement. The Fe/Co = 10/90 xerogel exhibited a lower degree of structural collapse, whilst average pore sizes remained almost constant, as the higher molar ratio of the embedded cobalt oxide produced more stable xerogel structures. The reduction of specific surface area for the Fe/Co = 10/90 xerogel was not as abrupt as those observed for the Fe/Co = 25/75 and 50/50 xerogels, and tended to stabilise after the first redox cycle, although a slight decrease was observed after each cycle. The specific surface areas measured for the Fe/Co = 10/90 xerogel during the reduction condition were always higher than that under the oxidation step. As the Fe/Co = 10/90 xerogel gave the most attractive microstructural reversibility upon redox cycling, Fe/Co = 10/90 membranes were prepared and initial permeation results measurements were performed between 100 and 400 °C as presented in Fig. 3a. All gases tested showed temperature activated transport.

Fig. 3. Single gas permeances measured for the Fe/Co = 10/90 (a) as a function of temperature and (b) gas kinetic diameters (dk) at 400 °C.

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Fig. 4. Redox cycles for Fe/Co = 10/90 silica membranes for single gas permeation at 400 °C.

In this case, the smaller gases He and H2 displayed positive apparent activation of energy as permeance increased with temperature. Contrary to this, the larger CO2 molecule gave a negative apparent energy of activation, as the permeance decreased with temperature, as conventionally reported for silica derived membranes [25,26]. This is attributed to CO2 having a strong heat of adsorption [12,27], thus reducing permeance as the temperature increases. It is also observed that permeance values are directly related to the kinetic diameters of gas molecules, with the following evolution: permeance of He (2.6 Å) > H2 (2.89 Å) > CO2 (3.3 Å). These results indicate that gas permeation is controlled by the molecular sieving regime, also known as activated transport [28] derived from Barrer’s model [29] for gas diffusion through inter-crystalline micropores. Based on molecular probing of gases with different kinetic diameters as shown in Fig. 3b, the average membrane pore size could be estimated to be around 3 Å, between the kinetic diameters of He and CO2. These results are consistent to those classically reported for molecular sieve silica membranes published elsewhere [19,30], and with pore size distribution of amorphous silica measured by positron annihilation spectroscopy [31].

Fig. 5. Gas permselectivity for redox cycles for Fe/Co = 10/90 silica membranes at 400 °C. Blue and red lines refer to He/CO2 and H2/CO2 permselectivity, respectively. Closed and open symbols refer to the oxidation (Ox) and reduction (Red) cycles, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

After the initial gas permeation measurements, the Fe/ Co = 10/90 membrane was disassembled and kept in a dry box for 3 weeks. Subsequently, the Fe/Co = 10/90 membrane was reassembled into the membrane module and exposed to several reduction and oxidation cycles for a period of ten days and permeation results measured at 400 °C are shown in Fig. 4. An evolution was observed in the membrane performance was observed by switching from the reduced to oxidised state and vice versa. This is clearly observed for the permeance results for all gases tested after the first redox cycle. For instance, H2 permeance at the reduced state of 1.1  10 8 decreased to 7.5  10 9 mol m 2 s 1 Pa 1 after the first oxidizing treatment. This permeation behaviour of H2 was almost entirely reversible and reproducible over the three redox cycles, giving permeance variations of 31% for the first cycle, and 14% and 15% for the subsequent redox cycles. The permeation of He after the 2nd redox cycle varied only slightly and

Fig. 6. Oxidation sequential cycles (1st, 2nd and 3rd treatments) of a Fe/Co = 10/90 silica xerogel sample reduced in H2 (a) TGA mass loss and (b) DSC.

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was within experimental error (±10%). The variation of CO2 permeation was also significant after the first redox cycle, though it tended to reduce after subsequent cycles. The CO2 permeance was higher for the reduced membrane and lower for the oxidised one. This switchable effect is in line with the redox treatment effect on the specific surface area values, as shown in Fig. 2a. The effect of the redox treatment (Fig. 4) suggests that the reduction step has caused a structural re-arrangement of the Fe/ Co = 10/90 silica matrix. Ji and co-workers [14] attributed the increased permeance on the reduction cycle to the crackling core effect of metal oxide particles, associated with H2 reaction with solid particles and leading to formation of smaller particles with porous structures [15,16]. However, the results observed for a silica membrane prepared with binary metal (FeCo) are transient in this work and they differ from the consistent switching redox effect for single metal (Co) oxide silica membranes reported by Miller et al. [4]. A closer inspection of the permselectivity results given in Fig. 5 reveals interesting trends. The primary and most important trend is that the permselectivity values reduced after each redox cycle. This result implies that the permeance of He and H2 were decreasing more rapidly than that of CO2. Consequently, a proportion of the pores with sizes accessible to both He (dk = 2.6 Å) and H2 (dk = 2.89 Å) were closing or becoming smaller so He and H2 could no longer diffuse. Physically this means that there is an ongoing densification process of the porous matrix. A second interesting trend is that the permselectivity decreased faster for the oxidation treatment than the reduction treatment (Fig. 5). This result indicates that the densification phenomenon altered mainly the porous silica matrix at each redox cycle, as metal oxide particles are non-porous except during the reduction cycle when particle crackling core effect occurred. The densification of the Fe/Co = 10/90 silica membrane can be further observed

by comparing the initial gas permeation measurements in Fig. 3 to those obtained three weeks after the first oxidation treatment in Fig. 4, which revealed a degradation ratio of 95% for single gas permeances values tested at 400 °C. For instance, He permeance reduced from 2.7  10 7 (Fig. 3) to 1.6  10 8 (Fig. 4) mol m 2 s 1 Pa 1. The densification process also tallies well with the slightly reduction of specific surface area measured by N2 physisorption after each redox cycle (Fig. 2a). In order to better understand the densification phenomenon and loss of membrane permselectivity during redox cycles, xerogels samples were prepared and exposed to the same redox treatments as the membranes. The xerogel samples prepared with Fe/ Co = 10/90 ratio were first reduced with H2 in a furnace and then oxidised in air in a TGA apparatus. This operation (redox cycling) was repeated 3 times. Fig. 6a shows that there was less mass loss for each subsequent oxidation cycle treatment, where the first and last cycle gave the highest and lowest mass loss, respectively. In principle this could indicate that the reduction was not fully completed at each cycle and a small fraction of oxygen remained in the binary FeCo metal. However, Fig. 6b shows an interesting trend related to heat flows. Although the initial endothermic dip at 70 °C is similar for all samples and associated with water desorption from the xerogel porous structure [32], a progressive shift appears at exothermic peaks around 300 °C, assigned to oxidation reactions. The peak shifted to higher temperatures sequentially for each oxidation cycle namely: first (285 °C), second (299 °C) and third (308 °C) cycles. These results strongly suggest that the oxidation reactions correspond to slightly different compositions of the FeCo oxides at each redox cycle. The FTIR spectra of the xerogel samples are exhibited in Fig. 7a. The as-prepared sample prior to any redox cycling displayed the characteristic absorption bands of silica which were assigned to

Fig. 7. Analysis of Fe/Co = 10/90 silica xerogel samples (a) FTIR spectra and (b) XRD patterns of the as-prepared sample and after sequential redox cycles (1st, 2nd and 3rd) of a Fe/Co = 10/90 silica xerogel.

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Fig. 8. TEM images of the Fe/Co = 10/90 oxide silica xerogel calcined at 500 °C (a) and (b) as-prepared, (c) first oxidation, (d) second oxidation, (e) third oxidation, and (f) d111spacing evolution with each cycle where zero value is for as-prepared sample.

siloxane bonds (SiAOASi) at 800 and 1080 cm 1, and a shoulder at 950 cm 1 to silanol bonds (SiAOAH) [33–35]. It is also observed bands at 660 and 565 cm 1. These bands are composed of peaks at 570 and 670 cm 1 assigned to Co3O4 [36,37] and at 585 and 632 cm 1 to Fe3O4 [38]. Reduction of both Co3O4 and Fe3O4 oxides after H2 treatment was evidenced by the disappearance of these peaks. The bands below 650 cm 1 were very noisy for the reduced samples and were not included in Fig. 7a. These results are in line with reduction of Co3O4 to CoO, as the peak at 650 cm 1 disappeared [39]. However, it is worth noticing the significant variation in the absorption peaks shape and intensity when comparing the as-prepared to the reduced samples from the first redox cycle. The well-defined peak at 565 cm 1 completely disappeared and became very noisy, whilst the peak at 660 cm 1 reduced in intensity. These results strongly suggest the potential formation of mixed oxides such as FexCo1 xOy, which could explain the shift of the exothermic DSC peak from 285 °C

to 299 °C and 308 °C in Fig. 6b. It is also important to notice that the FeAOASi peak at 584 cm 1 [40] could be affected by the reactions occurring between the metal oxides during the redox cycles. Fig. 7b displays the XRD patterns of the xerogels, where it is observed for all samples a broad hump at 2h 23°, a characteristic of amorphous silica structures. The XRD patterns were compared to a PDF2 XRD pattern data basis. The peaks at 2h = 18.94°, 36.84°, 44.36°, 55.54°, 59.18°, 65.08° and 77.44° were assigned to cobalt (II, III) oxide with cubic phase Fd 3m (227) (file number #00-043-1003) in the as-prepared samples. These peaks were replicated after each sequential oxidation treatment, though with lower sharpness as compared to the as-prepared sample. For the reduced samples, peaks attributed to the cobalt (II) oxide with a cubic phase Fm 3m (225) (file number #00-043-1004) emerged at 2h = 36.54°, 42.58°, 61.50°, 73.84°. It is worth noting a shift to lower angle for all existing diffraction peaks (Fig. 7b) compared to the pure cobalt (II, III) oxide. This shift was attributed to the

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presence of iron in the structure of a FexCo1 xOy mixed oxide. Other characteristic peaks associated with iron oxides were not observed in the diffraction patterns as they tended to be masked by the diffraction peaks of cobalt oxides occurring at similar 2h angles. The intensity of the broad silica hump at 2h 23° was found to increase for all reduced or oxidised samples in comparison with the as-prepared sample. The slight shift of 2h angles coupled with the higher intensity of the broad silica hump strongly suggest structural re-arrangements in the materials upon redox cycling, in line with the FTIR (Fig. 7a) and DSC results (Fig. 6b). TEM images in Fig. 8a reveal that iron cobalt oxide silica samples consist of a rather disordered (amorphous) structure, containing both large (10–30 nm) and small (2–4 nm) particles heterogeneously distributed in the silica matrix. A closer visualisation of the particles for the as-prepared sample is given in the TEM image in Fig. 8b. It is observed a highly ordered hexagonal particle measuring 24 nm across the longest side assigned to cobalt oxide. The d-spacing of the crystal lattice was also analysed and found to be 0.465 nm which corresponds to Co3O4 d-111 as reported elsewhere [41–43]. The distribution of the Co3O4 phase is similar to that reported earlier for cobalt oxide prepared with silica carriers [44,45]. The smaller particles (below 5 nm in size) embedded in the silica matrix were assigned to crystalline iron oxide. This was confirmed by the d-spacing of the crystal lattice (0.25 nm) which correlated well with d-211 of maghemite [46]. The TEM images in Fig. 8c–e are representative for each sequential oxidation cycle. The lattice parameters for the iron oxide particles slightly changed as the d-spacing increased from 0.24 to 0.25 nm after the second cycle, and reverted to 0.24 nm after the third cycle. However, it is interesting to observe that the d-spacing values for the cobalt oxide slightly reduced in a sequential manner, from 0.456 nm for the as-prepared sample to 0.434 at the third oxidation cycle. These results further confirm that iron insertion during redox cycling affected the crystallinity of the cobalt oxide, in line with observed variations in the FTIR spectra (Fig. 7a) and XRD patterns (Fig. 7b). In terms of real applications for gas streams containing H2, all the results discussed above suggest that H2 is likely to affect the overall performance of the membrane. First, the reduction effect caused by H2 decreased the H2/CO2 permselectivity. Second, real industrial gas separation processes are likely to undergo start-up and shut-down cycles, which could be similar to the redox cycles investigated in this work. These industrial cycles are likely to also reduce H2/CO2 permselectivity.

4. Conclusion This work investigates for the first time the effect of redox cycling on the physico-chemical characteristics of silica derived membranes embedded with a binary metal (Fe and Co) oxide particles. The reduction step resulted in higher gas permeance attributed to the crackling core effect of FeCo mixed oxide particles, which caused an increase in both porosity and pore connectivity via smaller cracked particles. These inter-particle pores increased the CO2 permeance and decreased both He/CO2 and H2/CO2 permselectivities. Contrary to this, the oxidation step closed the interparticles porosity, thus decreasing CO2 permeance and increasing He/CO2 and H2/CO2 permselectivities. Therefore, the redox effect caused a switchable gas permeance, though not constant. It was found that the properties of the binary FeCo oxide particles were affected by the redox conditions, and likewise by the amorphous silica microstructure. As a consequence, structural rearrangement occurred at each sequential redox cycle, leading to slight closure of the small pore sizes accessible by the smaller kinetic diameter gas molecules (He and H2), which translate in

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