Transition-Metal Nanocluster Kinetic and Mechanistic Studies Emphasizing Nanocluster Agglomeration: Demonstration of a Kinetic Method That Allows Monitoring of All Three Phases of Nanocluster Formation and Aging

Abstract

A kinetic method following the development, and then the loss, of catalytic activity is used to monitor the nucleation, growth, and then pyridine or acetonitrile-induced agglomeration (also called aggregation, coagulation, or flocculation) of modern, catalytically active Ir(0) nanoclusters stabilized by P2W15Nb3O629- polyoxoanions and Bu4N+ cations in acetone. The agglomeration kinetics of the Ir(0) nanoclusters are shown to fit an often assumed, but rarely experimentally determined, bimolecular aggregation step, B + B → C, rate constant k3, where B is the active catalyst and C is the deactivated catalyst. When this agglomeration step is added to the previously determined slow, continuous nucleation A → B, rate constant k1, and then autocatalytic surface growth, A + B → 2B, rate constant k2, steps (A is the (1,5-COD)Ir(I)+ in the precatalyst), the full steps of nucleation, growth, and then agglomeration are followed and treated kinetically for the first time for a modern, prototype transition-metal nanocl...