Metal-perovskite catalysts particles

Figure 25.7 Suppression of the growth of precious metal particles in intelligent noble metal-perovskite catalysts (Reprinted with permission from Ref. [97]. Copyright 2006, Elsevier.)...

The electrochemical reactions occurring within a SOFC are shown in Eqs. 58 and 59. The anode consists of a porous rrtixture of a Ni or Co catalyst on yttria-stabilized zirconia. Such a mixture of metal and ceramic is referred to as a cermet. The zirconia acts to inhibit grain growth of the catalyst particles of nickel or cobalt and protects against thermal expansion. The cathode is generally a Sr-doped LaMnOs perovskite. The Sr dopant provides for oxygen transfer to the cathode-electrolyte interface. 

CO oxidation, 28 108 iron catalyst, 30 168 kinetics, 28 250-257 complicated, 28 257-263 latest developments in, 5 1 over amorphous metal alloys, 36 372-374 over iron, 36 24-25 on alumina support, 36 47 antipathetic behavior, 36 150, 152 particle size and, 36 131-132 promotion by potassium, 36 36-37 over rhenium. 36 24-25 promotion by potassium, 36 37 photocatalysis over perovskites, 36 304 Anunoxidation, 30 136-137 allyl alcohol, 30 157-158... 

Various metal and metal oxide nanoparticles have been prepared on polymer (sacrificial) templates, with the polymers subsequently removed. Synthesis of nanoparticles inside mesoporus materials such as MCM-41 is an illustrative template synthesis route. In this method, ions adsorbed into the pores can subsequently be oxidized or reduced to nanoparticulate materials (oxides or metals). Such composite materials are particularly attractive as supported catalysts. A classical example of the technique is deposition of 10 nm particles of NiO inside the pore structure of MCM-41 by impregnating the mesoporus material with an aqueous solution of nickel citrate followed by calicination of the composite at 450°C in air [68]. Successful synthesis of nanosized perovskites (ABO3) and spinels (AB2O4), such as LaMnOs and CuMn204, of high surface area have been demonstrated using a porous silica template [69]. 

Perovskite oxides were used as catalysts in other less studied reactions where H20 appears as reactant or as product, such as H20 dissociation, H202 decomposition, and water gas shift. Reduced Lai, M,Co03 (M = Ca, Sr, Ba, Ce) oxides were shown to be active for H20 dissociation at 600°C by Wachowski and Laniecki (282b). Disperse metallic cobalt particles were assumed to be involved in this process. Brookes et al. (282c) reported XPS data indicating SrTiO3(l,0,0) step sites as active centers for... 

As an attempt to simulate real operating conditions of automotive converters, a laboratory bench has been designed and ageing procedures determined to reproduce simultaneous chemical and thermal modifications encountered by catalysts in the exhaust line. Characterization of commercial samples after ageing according to different temperature cycles evidences formation of both platinum/rhodium alloys and cubic perovskite-type compound, CeA103. Simultaneously with the formation of cerium aluminate, a thermal stabilization of catalysts is observed, in terms of mean noble metal particles size and concentration of rhodium in alloyed phases. An interpretation based on the crystallographic adaptation of alumina, cerium aluminate and ceria is proposed. 

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