Highly Disperse Metal Catalysts

A radical approach serving to remove all impurities from the electrode surface is that of slicing away a thin layer of metal right during the measurements (i.e., without withdrawing the electrode from the solution or breaking its polarizing circuit). Sometimes, bombardment of the surface with an ion beam is used for a similarly radical surface purification. 

For higher efficiency in catalytic action and smaller quantitative needs, the catalysts often are used in a highly disperse state. An important practical criterion for such catalysts is the specific reaction rate [i.e., the reaction rate per unit mass of the catalyst 

Most often, these disperse metal catalysts are supported by an electronically conducting substrate or carrier that should provide for uniform supply or withdrawal of electrons (current) to or from all catalyst crystallites. The substrate should also serve to stabilize the disperse state of the catalyst and retard any spontaneous coarsening of the catalyst crystallites. Two situations are to be distinguished (1) the disperse metal catalyst is applied to a substrate consisting of the same metal, and (2) it is applied to a chemically different substrate (a foreign substrate). Platinized platinum is a typical example of the former situation. 

1 Methods for Preparing Electrodes with Disperse Catalysts 

Different ways exist to prepare eiectrodes with highiy disperse metal catalysts and lead to the corresponding electrode varieties. 

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Investigations utilizing EXAFS have the very important feature of yielding information in an environment of the kind actually encountered in catalysis. We have recently demonstrated the feasibility of making measurements while a catalytic reaction is actually occurring. One can anticipate that measurements of this type will receive Increased emphasis in the future. For studies of the structures of highly dispersed metal catalysts, EXAFS may well be the most generally applicable physical probe currently available. 

For the present purpose, we take the term ultrathin to refer to an evaporated metal film where the concentration of metal on the substrate is low enough for the film to consist of small isolated metal crystals. If the average concentration of metal atoms on the substrate is of the order of a monolayer or less, the metal crystals are small enough for ultrathin films to serve as models for highly dispersed metal catalysts, but where surface cleanliness and catalyst structure can be better controlled. 

In a more general context, metal carbonyls on zeolites can be a unique way to prepare highly dispersed metal catalysts. In the present work, this is especially the case for iron as no other mild methods are operative. It is expected that the method could be applied to the preparation of bi- and polymetallic catalysts even though the starting material are not bi- or polymetallic clusters, but more conveniently homometallic clusters. 

An efficient, low temperature oxidation catalyst was developed based on highly disperse metal catalyst on nanostructured Ti02 support. Addition of dopants inhibits metal sintering and prevents catalyst deactivation. The nanostructured catalyst was formulated to tolerate common poisons found in environments such as halogen- and sulfur-containing compounds. The nanocatalyst is capable of oxidizing carbon monoxide and common VOCs to carbon dioxide and water at near ambient temperatures (25-50 °C). 

In supported metallic catalysts, the metals are usually from Groups VIII and VB of the Periodic Table. For highly dispersed metallic catalysts, the support or the carrier is usually a ceramic oxide (silica or alumina) or carbon with a high surface area, as described in chapter 2. Supported metallic catalysts can be prepared in a number of ways as described by Anderson (1975). A description of some of the methods used to prepare representative model (thin film) and practical (technological) powder systems follows. 

A MODEL FOR ETHENE HYDROGENATION ON HIGHLY DISPERSED METAL CATALYSTS... 

Various processes taking place in the electrodes and in the electrolyte of a fuel cell also influence the lifetime of SOFCs, in addition to the factors already mentioned. As in other types of fuel cells, the active working surface area of highly disperse metal catalysts has a tendency to shrink gradually with time. This decrease leads to a gradual increase in polarization of the corresponding electrode. 

Metal oxides belong to a class of widely used catalysts. They exhibit acidic or basic properties, which make them appropriate systems to be used as supports for highly dispersed metal catalysts or as precursors of a metal phase or sulfide, chloride, etc. Simple metal oxides range from essentially ionic compounds with the electropositive elements to covalent compounds with the nonmetals. However, taking into account the large variety of metal oxides, the principal objective of this book is to examine only metal oxides that are more attractive from the catalytic point of view, and most specifically transition metal oxides (TMO). In particular, TMO usually exhibit nonstoichiometry as a consequence of the presence of defective structures. The interaction of TMO with surfaces of the appropriate carriers develop monolayer structures of these oxides. The crystal and electronic structure, stoichiometry and composition, redox properties, acid-base character and cation valence sates are major ingredients of the chemistry investigated in the first part of the book. New approaches to the preparation of ordered TMO with extended structure of texturally well defined systems are also included. 

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