The Four-Step, Double-Autocatalytic Mechanism for Transition-Metal Nanocluster Nucleation, Growth, and Then Agglomeration: Metal, Ligand, Concentration, Temperature, and Solvent Dependency Studies

The four-step, double-autocatalytic mechanism for transition-metal nanocluster nucleation, growth, and agglomeration described previously has been found to be more general; specifically, the four-step mechanism is shown to apply to the reduction of five transition metals that were examined—Pt, Ru, Ir, Rh, and Pd—under conditions where a high enough concentration of a strongly coordinating ligand is also present. Reaction temperature, metal concentration, solvent, and stirring rate do not change the reduction mechanism within experimental error and within the range of conditions studied herein. Changing the solvent from acetone to the higher dielectric constant solvent propylene carbonate does, however, change the product of metal reduction from solid bulk metal to soluble metal particles. Three alternative mechanisms (in addition to the 15 tested already) are ruled out, leaving the four-step double autocatalytic mechanism as the only mechanism to date that is able to fit the observed kinetic data. Additional support for the generality of the four-step mechanism is given by the identification of three additional literature systems for which the four-step mechanism is observed. Multiple other experiments and results are also described and then summarized in a Conclusions section.