Perovskite membranes, dense ceramic

It is well known that dense ceramic membranes made of the mixture of ionic and electron conductors are permeable to oxygen at elevated temperatures. For example, perovskite-type oxides (e.g., La-Sr-Fe-Co, Sr-Fe-Co, and Ba-Sr-Co-Fe-based mixed oxide systems) are good oxygen-permeable ceramics. Figure 2.11 depicts a conceptual design of an oxygen membrane reactor equipped with an OPM. A detail of the ceramic membrane wall... 

Hydrogen-permselective silica membranes [8,10] can be, as well, synthesized by a particular application of solgel techniques [142,143], Additionally, hydrogen permselective asymmetric membranes composed of a dense ceramic of a proton-conducting perovskite over a porous support have been developed [40], However, some difficulties with respect to their stability is possible in certain reactive environments [121],... 

Dense ceramic ion-conducting membranes (CICMs) are emerging as an important class of inorganic membranes based on fluorite- or perovskite-derived crystalline structures [18]. Most of the ion-conducting ceramics discovered to date exhibit a selective ionic oxygen transport at high temperatures >700°C. Ionic transport in these membranes is based on the following successive mechanisms [25] ... 

The mixed-conducting perovskite oxides have attracted particular interest for use as dense ceramic membrane to control partial oxidation of methane to C2 products or syngas. Such a process bypasses the use of costly oxygen since air can be used as oxidant on the oxygen-rich of the membrane. 

The considerations in this chapter were mainly prompted by the potential application of mixed-conducting perovskite-type oxides to be used as dense ceramic membranes for oxygen delivery applications, and lead to the following general criteria for the selection of materials... 

One category of dense proton conducting membranes that has received considerable attention in the preceding decade is proton conducting perovskite type oxide ceramics [4-6]. The stoichiometric chemical composition of perovskites is represented as ABO3, where A is a divalent ion (A +) such as calcium, magnesium, barium or strontium and B is a tetravalent ion (B +) such as cerium or zirconium. Although simple perovskites such as barium cerate (BaCeOs) and strontium cerate... 

The principles behind this membrane technology originate from solid-state electrochemistry. Conventional electrochemical halfceU reactions can be written for chemical processes occurring on each respective membrane surface. Since the general chemistry under discussion here is thermodynamically downhill, one might view these devices as short-circuited solid oxide fuel cells (SOFCs), although the ceramics used for oxygen transport are often quite different. SOFCs most frequently use fluorite-based solid electrolytes - often yttria stabUized zirco-nia (YSZ) and sometimes ceria. In comparison, dense ceramics for membrane applications most often possess a perovskite-related lattice. The key fundamental... 

Air separation membranes are typically dense ceramic (typically perovskite) membranes, which selectively permeate oxygen in ionic form. Over the past two decades. Air Products (ITMs) and Praxair (oxygen transport membranes [OTMs]) have worked towards the commercial scale-up of these membranes for applications in power generation, gasification, and gas to liquid conversion [94]. Air Products has focused on a planar configuration, whereas Praxair on tubular membranes. 

Concerning non-porous membranes, these are categorized as dense ceramic electrolytes such as yttria-stabilized zirconia (YSZ) and perovskite membranes [16], which allow only the permeation of ionic oxygen. Permeation through metal membranes such as palladium and a palladium alloy is based on the selective dissolution of hydrogen and diffusion through the metal membrane. 

ABO3 perovskite-type oxides with transition-metal ions at the B-site have high ionic and electronic transport in the form of p or n semi-conductivity (mixed ionic and electronic conductivity), caused by different oxidation states of the transition-metal cation. For dense ceramic membranes, perovskite-type oxides with the following cations are preferred A = Ln (lanthanide ion), Ca, Sr, Ba B = Cr, Mn, Fe, Co, Ni, Cu. 

Diethelm, S. and van Herle, J. (2004) Oxygen transport through dense Lao.6Sro.4Feo.8Coo.203 a perovskite-type permeation membranes. /. Eur. Ceram. Soc., 24, 1319-1323. 

A detailed discussion of the mathematical models of oxygen flow in ceramic membranes is given elsewhere. Typical materials employed in dense ceramic membranes have a brownmillerite or perovskite structure. The most commonly studied application for this kind of membrane is the catalytic partial oxidation of methane (POM) to obtain synthesis gas,... 

Most dense ceramic membranes are made from perovskite oxides with general formula ABO3, where the A-site cation is coordinated to 12 oxygen ions forming a cuboctahedral coordination environment while the B-site cation is coordinated to six oxygen ions with... 

It yields an ideal feedstock with H2/CO ratio of 2 1 for methanol synthesis and the Fisher-Tropsch reaction to produce linear hydrocarbons. The use of dense ceramic MRs makes it possible to combine oxygen separation from air, partial oxidation, and reforming of methane in a single step, thus enabling significant reductions of capital investment in the gas-to-liquid industry [40]. Although perovskite membranes may exhibit catalytic activity in the POM reaction, a POM catalyst is usually applied to improve the methane conversion and CO selectivity. Figure 5.7... 

The preparation of composite PCMs starts with the synthesis of porous substrates, followed by the formation of thin PCM films. Detailed procedures are illustrated in Figure 6.3. An exact procedure should be conducted to obtain thin, dense ceramic films [17].The carbon black content for the synthesis of porous substrates can be varied so as to determine conditions that match accurately the shrinkage profile of the porous substrates with that of the deposited perovskite films during the final sintering of the composite membrane. Partial sintering of the green substrates is conducted to match their subsequent shrinkage with that of coated powders so that fissures and cracks can be minimized. 

Abstract Dense ceramic membrane reactors are made from composite oxides, usually having perovskite or fluorite structure with appreciable mixed ionic (oxygen ion and/or proton) and electronic conductivity. They combine the oxygen or hydrogen separation process with the catalytic reactions into a single step at elevated temperatures (>700°C), leading to significantly improved yields, simplified production processes and reduced capital costs. This chapter mainly describes the principles of various types of dense ceramic membrane reactors, and the fabrication of the membranes and membrane reactors. 

Dense ceramic membranes are made from composite oxides usually having perovskite or fluorite crystalline structure (Bouwmeester, 2003 Liu... 

In Chapter 10, the use of membranes for different applications are described. One of the possible membranes for hydrogen cleaning is an asymmetric membrane comprised of the dense end of a proton conduction perovskite such as BaCe0 95 Yb0 05O3 5 and a porous end to bring mechanical stability to the membrane. In this case, it is possible to take from the slurry, obtained by the acetate procedure, several drops to be released over a porous ceramic membrane, located in the spinning bar of a spin-coating machine. Thereafter, the assembly powder, thin film porous membrane is heated from room temperature up to 1573 K at a rate of 2K/min, kept at this temperature for 12 h, and then cooled at the same rate in order to get the perovskite end film over the porous membrane [50],... 

It has been reported [22] that micro/nano surface reactive layers introduced on the surface of oxygen ion transport membranes enhanced the oxygen permeability of dense oxygen-permeable perovskite-type ceramic membranes, Lao.7Sro.3Gao.6Feo.403 6. The results also showed that the oxygen permeation flux is influenced by the surface area of the surface-reactive layer. 

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