Transition Metal Nanocluster Formation Kinetic and Mechanistic Studies. A New Mechanism When Hydrogen Is the Reductant: Slow, Continuous Nucleation and Fast Autocatalytic Surface Growth

Abstract

Following an overview of the primitive state of mechanistic studies of the formation of nanoclusters, with a focus on LaMer's classic work on the formation of sulfur sols, kinetic and mechanistic studies of the formation of our recently reported novel P 2 W 15 Nb 3 O 62 9 - polyoxoanion- and Bu 4 N + - stabilized Ir ∼ 190 - 450 (hereafter, Ir(0) ∼300 ) nanoclusters are presented. The work reported consists of the full experimental and other details of the following eight major components: (i) development of an indirect but easy, continuous, highly quantitative and thus powerful method to monitor the formation of the Ir(0) nanoclusters via their catalytic hydrogenation activity and through the concept of pseudoelementary reaction steps; (ii) application of the appropriate kinetic equations for nucleation and autocatalysis, and then demonstration that these equations fit the observed, sigmoidal-shaped kinetic curves quantitatively with resultant rate constants k 1 and k 2; (iii) confirmation by a more direct, GLC method that the method in (i) indeed works and does so quantitatively, yielding the same k 1 and k 2 values within experimental error; (iv) collection of a wealth of previously unavailable kinetic and mechanistic data on the effects on nanocluster formation of added olefin, H 2 pressure, anionic nanocluster stabilizer ([Bu 4 N] 9 P 2 W 15 Nb 3 O 62 in the present case), H 2 O, HOAc, and temperature; (v) careful consideration and ruling out of other hypotheses, notably that particle-size rate effects alone might account for the observed sigmoidal shaped curves; and then (vi) distillation of the results into a minimalistic mechanism consisting of several pseudoelementary steps. Also presented as part of the Discussion are (vii) a concise but comprehensive review of the literature of transition metal nanocluster formation under H 2 as the reducing agent, an analysis which provides highly suggestive evidence that the new mechanism uncovered is a much more general mechanism if not a new paradigm for transition metal nanoclusters formed under H 2 (and, the data argue, probably also for related reducing agents); and (viii) a summary of the seven key predictions of this new mechanism which remain to be tested (four predictions are the expected predominance of magic-number size nanoclusters; designed control of nanocluster size via the living-metal polymer concept; the synthesis of onion-skin structure bi-, tri-, and higher-metallic nanoclusters; and the use of face-selective capping agents as a way to block the autocatalytic surface growth and, thereby, to provide designed-shape nanoclusters). Overall, it is hoped that the results the first new mechanism in more than 45 years for transition metal nanocluster formation will go far toward providing a firmer mechanistic basis, and perhaps even a new paradigm, for the designed synthesis of new transition metal nanoclusters of prechosen sizes, shapes, and mono- to multimetallic compositions.