Metal Clusters in Catalysis - An Overview

Coordinatively unsaturated clusters can mimic some aspects of the reactivity of metal surfaces in addition to the structure. This area has developed more recently than the above because most of the early clusters, for example of the iron group elements, tended to be coordinatively saturated in contrast to the situation on a clean metal surface. There were, however, some early examples of coordinatively unsaturated clusters, or their equivalents, and one classic example is the 46-electron cluster [Os3H2(CO)io] which readily adds ligands L to give 48-electron [Os3H2(CO)ioL] (Eq. 1). It has now been shown that many such reactions can occur 

In this chemistry, it is natural to focus on models for the catalytic reactions that are most important economically or which are most poorly understood because of difficulties of direct study. One which best fits these criteria is the catalytic hydrotreatment of petroleum feedstocks, which is used to remove sulfur and other heteroatoms, which interfere with subsequent catalytic reactions such as petroleum reforming, from the hydrocarbons. Molybdemun sulfide is the most common metal sulfide used in this catalysis, and hydrogen activation and C-S bond hydrogenolysis are known to be key reactions occurring at the catalyst surface but details are difficult to obtain. Study of model binuclear and cluster complexes has elucidated mechanisms of several of the key reactions and Section 2.6 describes important recent advances in this field, with the focus being on models for hydrodesulfurization catalysts. 

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The term bimetallic was introduced by Sinfelt to account for the fact that a catalyst may contain a muldtude of phases containing the acdve metallic components. Of these many phases, a characteristic is the binary alloy. The term alloy can describe a broad range of situations from well-defined phases or solid solutions to surface alloys in cases where bulk alloys are not thermodynamically favoured but a clearly defined surface local arrangement is obtained. Note that the novel core-shell bimetallic structures are included in this catchall term. An historical overview of the properties of alloys in connection with catalysis has been published by Ponec. At present, a broadly agreed view accepts that alloy components can be chemically recognised and, therefore, supports a somewhat localised interpretation of the alloy nature and properties. Obviously, a delocalised view loses most of its meaning in the case of clusters due to its finite dimensions. 

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