TRANSITION-METAL NANOCLUSTER FORMATION KINETIC AND MECHANISTIC STUDIES - A NEW MECHANISM WHEN HYDROGEN IS THE REDUCTANT - SLOW, CONTINUOUS NUCLEATION AND FAST AUTOCATALYTIC SURFACE GROWTH

Following an overview of the primitive state of mechanistic studies of the formation of nanoclusters, with a focus on LaMer's classic work o n the formation of sulfur sols, kinetic and mechanistic studies of the formation of our recently reported novel P2W15Nb3O629- polyoxoanion- and Bu4N+ - stabilized Irsimilar to 190-450 (hereafter, lr(0)(similar to 300)) nanoclusters are presented. The work reported consists of the full experimental and other details of the following eight major comp onents: (i) development of an indirect-but easy, continuous, highly qu antitative and thus powerful-method to monitor the formation of the Ir (0) nanoclusters via their catalytic hydrogenation activity and throug h the concept of pseudoelementary reaction steps; (ii) application of the appropriate kinetic equations for nucleation and autocatalysis, an d 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 m ethod in (i) indeed works and does so quantitatively, yielding the sam e 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, anioni c nanocluster stabilizer ([Bu4N](9)P2W15Nb3O62 in the present case), H 2O, 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) disti llation of the results into a minimalistic mechanism consisting of sev eral pseudoelementary steps. Also presented as part of the Discussion are (vii) a concise but comprehensive review of the literature of tran sition metal nanocluster formation under H-2 as the reducing agent, an analysis which provides highly suggestive evidence that the new mecha nism uncovered is a much more general mechanism-if not a new paradigm- for transition metal nanoclusters formed under H? (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 tes ted (four predictions are the expected predominance of magic-number si ze nanoclusters; designed control of nanocluster size via the Living-m etal polymer concept; the synthesis of onion-skin structure bi-, tri-, and higher-metallic nanoclusters; and the use of face-selective cappi ng agents as a way to block the autocatalytic surface growth and, ther eby, to provide designed-shape nanoclusters). Overall, it is hoped tha t the results-the first new mechanism in more than 45 years for transi tion 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, sha pes, and mono-to multimetallic compositions.