Particle growth in model supported metal catalysts—I. Theory
Abstract
A theoretical study of capillarity driven growth has been performed for several regimes of importance to the ripening of noble metal particles supported on flat, partially wetted, metal oxide substrates. Three related models are presented, and in each case two modes of interparticle transport are considered, namely: vapor phase transport and substrate surface diffusion. The first model is analogous to classical Ostwald ripening. The second is a simple extension of the first but allows for interparticle transport via a volatile oxide species. The third model is applicable to the growth of faceted particles and incorporates the concepts of “pill box” nucleation as a growth inhibiting factor. The growth kinetics predicted by the third model are quite different from those predicted by classical ripening models. In particular, changes in the initial particle size distribution, which cause only short-lived transients in the case of classical ripening, are found to have a profound effect on the growth kinetics as well as other characteristics of nucleation inhibited growth.
Résumé
On a effectué une étude théorique de la croissance par capillarité, pour divers régimes de maturation de particules de métaux nobles sur des supports d'oxydes métalliques plats, partiellement humides. On présente trois modèles et, dans chaque cas, on tient compte de deux modes de transport entre particules, à savoir: le transport en phase vapeur et la diffusion à la surface du substrat. Le premier modèle est analogue à la maturation d'Ostwald classique. Le second est simplement l'extension du premier, mais il permet un transport entre particules par un oxyde volatil. Le troisième modèle s'applique à la croissance de particules à facettes et il utilise le concept de la germination par “boite à pilules” comme facteur d'inhibition de la croissance. La cinétique de croissance prévue par le troisième modèle est très différente de celles que prévoient les modèles classiques de maturation. En particulier, des changements dans la répartition initiale de la taille des particules, qui ne provoquent que des perturbations transitoires de courte durée dans la maturation classique, ont un effet très marqué sur la cinétique et sur les autres caractéristiques de la croissance inhibée par la germination.
Zusammenfassung
Eine theoretische Untersuchung des kapillaritätsgetriebenen Wachsens wurde durchgeführt für mehrere Bereiche, die wichtig sind beim Reifen von Partikeln edler Metalle, die auf flachen, teilweise angefeuchteten Metalloxidsubstraten liegen. Drei verwandte Modelle werden vorgestellt; in jedem dieser Fälle werden zwei verschiedene Transportprozesse zwischen den Partikeln betrachtet: Transport in der Gasphase und Oberflächendiffusion über das Substrat. Das erste Modell ist analog der klassischen Ostwald-Reifung. Das zweite stellt eine einfache Erweiterung des ersten dar. berücksichtigt aber den Transport zwischen den Partikeln über flüchtige Oxide. Das dritte Modell läβt sich auf das Wachsen facettierter Teilchen anwenden und enthält die Konzepte der “pill-box”-Keimbildung als wachstumshindernden Faktor. Die von dem dritten Modell vorausgesagte Wachstumskinetik unterscheidet sich stark von derjenigen klassischer Reifungsmodelle. Insbesondere haben Änderungen der Ausgangsgröβenverteilung der Partikel, die bei der klassischen Reifung nur kurzlebige Übergänge verursachen, einen ausgeprägten Einfluβ auf die Wachstumskinetik und auch andere Charakteristiken des durch Keimbildung behinderten Wachstums.
References (21)
- E. Ruckenstein et al.
J. Catal.
(1973) - P. Wynblatt et al.
Prog. Solid State Chem.
(1975) - B.K. Chakraverty
J. Phys. Chem. Solids
(1967) - I.M. Lifshitz et al.
J. Phys. Chem. Solids
(1961) - P. Wynblatt et al.
Scripta Met.
(1973) - K.Y. Chen et al.
J. Nucl. Mater.
(1974) - P. Wynblatt
Acta Met.
(1976) - J.P. Hirth et al.
- E. Ruckenstein et al.
AIChE J.
(1973) - P. Wynblatt et al.
The Physical Basis for Heterogeneous Catalysis
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