Role of Ensembles in Catalysis by Metals
Among the recent ideas in catalysis by metals, the concept of ensemble appears to be one of the most promising. According to this view the active site is considered as a 2 × D cluster, whose size varies with the reaction considered, in a metallic environment. This model originates from Group VIII-IB alloys experiments, where an active element is diluted in an inactive IB matrix, from kinetics data and from adsorption experiments where the magnetic properties of metals are measured. This is illustrated by Ni and Ni-Cu catalysts and reactions including hydrogenation of benzene, D2-hydrocarbon exchange reaction and hydrogenolysis. The ensemble model allows a description of rates which is quantitative over a wide range of experimental conditions using only one adjustable parameter, which can be identified to the nuclearity (number of atoms) of the ensemble, associated with an energetic term. This model yields a quantitative description, with the very same remarkable few number of adjustable parameters, of apparently different aspects of catalysis such as size-sensitivity and poisoning. Finally, the field of application, the limit and the future of the ensemble model are tentatively outlined.
KeywordsMetal Atom Apparent Activation Energy Landing Site Ensemble Model Bond Number
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