Monitoring supported-nanocluster heterogeneous catalyst formation: product and kinetic evidence for a 2-step, nucleation and autocatalytic growth mechanism of Pt(0)n formation from H2PtCl6 on Al2O3 or TiO2.

ABSTRACT

A pressing problem in supported-metal-nanoparticle heterogeneous catalysis--despite the long history and considerable fundamental as well as industrial importance of such heterogeneous catalysts--is how to monitor such catalysts' formation more routinely, rapidly, and in real time. Such information is needed to better control the size, shape, composition, and thus resultant catalytic activity, selectivity, and lifetime of these important catalysts. To this end, a study is reported of the formation of supported Pt(0)(n) nanoparticles by H(2) reduction of H(2)PtCl(6) on Al(2)O(3) (or TiO(2)) to give 6 equivalents of HCl plus supported Pt(0)(n)/Al(2)O(3) (or Pt(0)(n)/TiO(2)), all while in contact with a solution of EtOH and cyclohexene. The HCl and Pt(0)(n) products were confirmed, respectively, by the stoichiometry of HCl formation using pH(apparent) measurements, appropriate standards, and by TEM and EDX measurements. The hypothesis of this research is that the kinetics of formation of this supported heterogeneous catalyst could be successfully monitored by a fast cyclohexene hydrogenation catalytic reporter reaction method first worked out for monitoring transition-metal nanoparticle formation in solution (Watzky, M. A. and Finke, R. G. J. Am. Chem. Soc. 1997, 119, 10382-10400). Significantly, sigmoidal kinetics of Pt(0)(n)/Al(2)O(3) catalyst formation were in fact successfully monitored by the catalytic hydrogenation reporter reaction method and then found to be well fit to the Finke-Watzky (hereafter F-W) 2-step, slow continuous nucleation and then autocatalytic surface growth mechanism, A --> B (rate constant k(1)) and A + B --> 2B (rate constant k(2)), respectively, in which A is the H(2)PtCl(6) and B is the growing, catalytically active Pt(0) nanoparticle surface. The finding that the F-W mechanism is applicable is significant in that it, in turn, suggests that the > or = 8 insights from studies of the mechanisms of soluble nanocluster formation can likely also be applied to supported heterogeneous catalyst synthesis, including a recent equation that gives nanocluster size vs time in terms of k(1), k(2), [A](o) and other parameters (Watzky, M. A., Finney, E. E. and Finke, R. G. J. Am. Chem. Soc. 2008, 130, 11959-11969 ). Also presented are the use of the catalytic reporter reaction to reveal H(2) gas to-solution mass-transfer-limitations (MTL) in the system of H(2)PtCl(6) on TiO(2), results relevant to a recent communication in this journal. The use of the F-W 2-step nucleation and autocatalytic growth kinetic model to fit 3 literature examples of heterogeneous catalyst formation, involving H(2) reduction of both supported or bulk M(x)O(y) (i.e., and in gas-solid reactions), are also presented as part of the Supporting Information. A conclusion section is then provided summarizing the insights and caveats from the present work, as well as some needed future studies.