Perovskite oxides, 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... 

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... 

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... 

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. 

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... 

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... 

Clearly, not all these perovskite compositions are useful for oxygen delivery applications. For example, ceramics based on Laj.xAxCrOj (x == Sr, Ba, Ca), Lai, rxCri yMny03.5 and Lai xCaxCri yCOy03 have been proposed for use as interconnection material (separator) in solid oxide fuel cells (SOFC), and therefore should be dense and impermeable in order to prevent burning off of the fuel without generating electricity [137,138]. 

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... 

Numerous studies have investigated coating the interconnect with a dense protective oxide layer to reduce surface oxidation and chromium contamination of other key components [124, 125]. Perovskite materials such as LaCrOs typically used for higher temperature ceramic interconnects are ideal as their conductivity and thermal expansion coefficient can be tailored through doping to ensure compatibility and optimise performance. 

Mtxed-ronducting dense (nonporous) solid oxide membranes prepared out of appropriate perovskite ceramic materials are such that both ion induction and eleirtron transport can occur through the membrane at h temperatures the material is otherwise impermeable to gases. No voltage is applied across the membrane. 

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