Mechanistic studies of nanocluster nucleation, growth, and agglomeration

Abstract

Following a critical review of the relevant literature, the research presented herein focuses on the mechanisms by which transition-metal nanoclusters nucleate, grow, and agglomerate. The studies include: (i) the generality of the recently uncovered, 4-step mechanism for nanocluster formation and agglomeration; (ii) a study addressing the question of whether the hydrogenation of olefins using (1,5-COD)PtII complexes proceeds via homogeneous or heterogeneous catalysis; and (iii) a comparison of the kinetics of transition-metal nanocluster formation and the kinetics of solid-state reactions. Recently, a new, 4-step mechanism was discovered for the nucleation, growth, and agglomeration of transition-metal nanoclusters, using a single Pt complex in the system under study. Herein, the mechanism is shown to be general to the formation of nanoclusters of at least four other metals. In addition, the effects of ligands, concentration, temperature, solvent, and stirring on the mechanism are examined. Several alternative mechanisms are ruled out, leaving the 4-step mechanism as the only one to date that can account for the observed kinetics. The question of "is it homogeneous or heterogeneous catalysis?" is addressed with respect to the hydrogenation of olefins using (1,5-COD)Pt II complexes. The data presented herein provide compelling evidence that these complexes are first reduced to Pt0 nanoclusters and/or bulk metal, which are the true hydrogenation catalysts. Included herein is a brief overview of the literature of Pt-catalyzed hydrosilylation reaction, with respect to the "is it homogeneous or heterogeneous catalysis?" question. The literature is replete with mechanisms describing solid-state phase transitions, the kinetics of which appear similar to the kinetics of transition-metal nanocluster formation. It is found that the solid-state equation can fit nanocluster formation kinetic data, and vice versa. This finding leads into a comparison of solid-state reaction mechanisms and the Finke-Watzky mechanism, with a focus on the strengths and limitations of each.