Ceria membranes

Table 2.5. Microstructural Characteristics of Titania and Ceria Membranes as a Function of Calcination Time and Temperature (Uhlhom et al. 1988, Bui raaf, Keizer and van Hassel 1989a, b)...

Kaiser, A., Foghmoes, S., Chatzichristodoulou, C. et al. (2011) Evaluation of thin film ceria membranes for syngas membrane reactors-preparation, characterization and tesUn. Journal of Membrane Science, 378, 51-60. 

The performance of acceptor-doped ceria membranes, composite ceria-based membranes, and membranes with redox active dopants was reviewed in Sections 12.6.4.2, 12.6.4.3, 12.6.4.4, and 12.6.4.5, respectively. Variations in sample geometry, membrane thickness, as well as test conditions make comparison of performance somewhat difficult. Also for some studies, catalysts have been added to the surface of the samples to enhance the flux, whereas in other studies the as-prepared surface is used in the experiments. However, in particular, studies of thin-film membranes, both the... 

Kaiser A, Foghmoes S, Chatzichristodoulon C, Spgaard M, Glasscock J A, Frandsen H L and Hendriksen P V (2011), Evalnation of thin fihn ceria membranes for syngas membrane reactors—Preparation, characterization and testing , / Membr Sci,3 7S,51-56. 

Nevertheless, the oxalate coprecipitation method has some problems. For example, this method usually results in rodlike doped ceria particles, which are agglomerations of smaller particles with irregular shapes. Hence, the green density of the compact body is relatively low, so it is difficult to fabricate a dense electrolyte film or membrane. In addition, the poor flow of the rodlike powder makes forming difficult. 

Other evidence for the importance of catalysis in anode performance came from an examination of the products formed by Cu—ceria—YSZ and Cu—molyb-dena—YSZ anodes in membrane—reactor measurements. The anodes in these experiments both had... 

Figure 16. Propylene conversion and product-selectivity results for the membrane-reactor measurements performed at 723 K with pure propylene as the feed. The results in panel a were for the SOFC with a Cu—ceria—YSZ anode, and the results in panel b were for the Cu-molybdena-YSZ anode. In panel a, the points are the rate of CO2 production, and the line was calculated from the current density and eq 8. In panel b, the points show the production of acrolein, and the line was calculated from eq 9. (Reprinted with permission from ref 165. Copyright 2002 Elsevier.)...

Again, points on the curve were the measured acrolein production rates, and the line is the predicted production rate based on the current and the stoichiometry according to eq 9. At higher conversions, we observed significant amounts of CO2 and water, sufficient to explain the difference between the acrolein production and the current. It should be noted that others have also observed the electrochemical production of acrolein in a membrane reactor with molybdena in the anode. The selective oxidation of propylene to acrolein with the Cu—molybdena— YSZ anode can only be explained if molybdena is undergoing a redox reaction, presumably being oxidized by the electrolyte and reduced by the fuel. By inference, ceria is also likely acting as a catalyst, but for total oxidation. 

Zhang et al. reported a nanocomposite membrane of shuttle shaped ceria nanocrystals (Guo et al., 2008), SWNTs, and ILsl-butyl-3-methyli-midazolium hexafluorophosphate (BMIMPFg), which was incorporated on the glassy carbon electrode for electrochemical sensing of the immobilization and hybridization of DNA (Zhang et al., 2009). The electron transfer resistance (Pgt) of the electrode surface increased after the immobilization of probe ssDNA on the Ce02-SWNTs-BMIMPF6 membrane and rose further after the hybridization of the probe ssDNA with its complementary sequence. 

It is generally accepted that thermal and especially hydrothermal treatment of aluminas and other catalytic materials results in deterioration of porous structure, i.e. increase in average pore radius and diminishing in specific surface area [1-4]. It is very important that such alumina materials as some catalyst washcoats and membranes have to be exploited at higher temperatures and at atmosphere of large humidity. Therefore it is necessary to improve their thermal and hydrothermal stabilization by application of new binder materials or additives. Such additives as silica, ceria or zirconia are known as thermal stabilizers. The aim of this work was to determine the influence of addition of the selected stabilizers on hydrothermal stability of alumina material in the temperature range 150 - 225 °C and time up to 72 hours. 

Mixed ion and electronic conducting ceramic membranes (e.g. yttria-stabilized zirconia doped with titania or ceria) can be slip cast into a tubular form from the pastes containing the constituent oxides in an appropriate proportion and other ingredients and the cast tubes are then subject to sintering at 1,200 to 1,500X to render them gas impervious [Hazbun, 1988]. 

An emulsion liquid membrane (ELM) system has been studied for the selective separation of metals. This system is a multiple phase emulsion, water-in-oil-in-water (W/O/W) emulsion. In this system, the metal ions in the external water are moved into the internal water phase, as shown in Fig. 3.4. The property of the ELM system is useful to prepare size-controlled aiKl morphology controlled fine particles such as metals, carbonates/ and oxalates.Rare earth oxalate particles have been prepared using this system, consisting of Span83 (sorbitan sesquioleate) as a surfactant and EHPNA (2-ethyl-hexylphospholic acid mono-2-ethylhexyl ester) as an extractant. In the case of cerium, well-defined and spherical oxalate particles, 20 - 60 nm in size, are obtained. The control of the particle size is feasible by the control of the feed rare earth metal concentration and the size of the internal droplets. Formation of ceria particles are attained by calcination of the oxalate particles at 1073 K, though it brings about some construction of the particles probably caused by carbon dioxide elimination. 

Oxides exhibiting only high ion conductivity are mainly fluorite-related structures based on zirconia or ceria. Zirconia-based electrolytes are currently used in solid oxide fuel cells (SOFCs). The MIEC oxides are more attractive for separative membrane applications, and these oxides mainly belong to the following types fluorite-related oxides doped to improve their electron conduction, - ... 

SEM observations of the surface and the cross-section of the Gd doped ceria coated with YSZ (YSZ GDC) show that the thin YSZ film is porous and the YSZ particles are fairly uniform in size. The thickness of YSZ film was about 20 pm. The conductivity of this composite membrane was 4.39X10 S cm at 500 C under air atmosphere, which was almost same order as that of Sm doped ceria coated with YSZ (YSZISDC) (1.51X10 4 Sxm ). However, either value was lower than that of GDC alone (4.03 X lO Sxm ) or SDC alone (4.66 X lO Sxm ). This may be due to the low conductivity of YSZ film. 

Propene oxidation was carried out by using an electrochemical reactor constructed from a Sm doped ceria electrolyte coated with YSZ (YSZ 1 SDC) as a membrane. In a blank test where nitrogen gas alone was passed over the Au anode instead of the reaction gas at 450 C, it was confirmed that the oxygen pumping was well controlled by the applied current, i.e., the amount of oxygen gas evolved at the anode coincided well the value calculated from the electric current by using Faraday s law. 

The effect of the YSZ coating on the ceria-based solid electrolyte was shown in Table 1. When the YSZ I SDC membrane was used as the solid electrolyte, selectivities to acrylaldehyde (Scho) carbon monoxide (Sco) and carbon dioxide (Scoa) based on converted propene was 13.4%, 25.6% and 61%, respectively. Here, it should be emphasized that the selectivity to acrylaldehyde increased with YSZ coating compared with that (Scho =8.5 %) obtedned by using SDC alone as a solid electrolyte. In addition, it was found that carbon monoxide formation was observed in the present study, although its formation was not detected in the case of SDC alone. The same phenomena were observed, when the Gd doped... 

In order to clarify the reaction site of the partial oxidation of propene using the ceria-based solid electrolyte coated with YSZ as a membrane, we have studied the dependence of the selectivities to oxygenates on the thickness of YSZ. When the Sm doped ceria coated with YSZ (YSZ SDC), each selectivity of the oxidation products did not dependent on the thickness of YSZ, as shown in Figure 4. 

An electrochemical cell system with ceria-based solid electrolyte coated wdth YSZ prepared by the spin coating method showed higher selectivity to acrylsildehyde than that with ceria-based solid electrol5rte alone. This may be due to the fact that a film of YSZ on the ceria-based solid electroljde to suppress the complete oxidation of propene. When the YSZ SDC disk was used as an electrolyte membrane, selectivity of the oxidation products did not depend on the thickness of YSZ. This indicates that the selective oxidation of propene occurred at the Au-YSZ-gas triple phase boundary by the oxygen species pumped electrochemicaUy through the ceria-based solid electrolyte and the YSZ. 

Although the mixed conduction is detrimental to fuel cell application, the high electronic conductivity of doped ceria in low p02 conditions can be exploited in hydrogen separation membrane technology. The chemical, thermal expansion and processing compatibilities of the two oxides in the composite make pervoskite-ceria combination suitable for the mixed conducting membrane application. 

---