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Development Plus Kinetic and Mechanistic Studies of a Prototype Supported-Nanoparticle Heterogeneous Catalyst Formation System in Contact with Solution: Ir(1,5-COD)Cl/γ-Al2O3 and Its Reduction by H2 to Ir(0)n/γ-Al2O3

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journal contribution
posted on 21.07.2010, 00:00 by Joseph E. Mondloch, Qi Wang, Anatoly I. Frenkel, Richard G. Finke
An important question and hence goal in catalysis is how best to transfer the synthetic and mechanistic insights gained from the modern revolution in nanoparticle synthesis, characterization, and catalysis to prepare the next generation of improved, supported-nanoparticle heterogeneous catalysts. It is precisely this question and to-date somewhat elusive goal which are addressed by the present work. More specifically, the global hypothesis investigated herein is that the use of speciation-controlled, well-characterized, solid oxide supported-organometallic precatalysts in contact with solution will lead to the next generation of better composition, size- and shape-controlled, as well as highly active and reproducible, supported-nanoparticle heterogeneous catalystsones that can also be understood kinetically and mechanistically. Developed herein are eight criteria defining a prototype system for supported-nanoparticle heterogeneous catalyst formation in contact with solution. The initial prototype system explored is the precatalyst, Ir(1,5-COD)Cl/γ-Al2O3 (characterized via ICP, CO adsorption, IR, and XAFS spectroscopies), and the well-defined product, Ir(0)n/γ-Al2O3 (characterized by reaction stoichiometry, TEM, and XAFS). The Ir(0)n/γ-Al2O3 system proved to be a highly active and long-lived catalyst in the simple test reaction of cyclohexene hydrogenation and in comparison to two literature Ir(0)n/Al2O3 heterogeneous catalysts examined under identical conditions. High activity (2.2−4.8-fold higher than that of the literature Ir(0)n/Al2O3 catalysts tested under the same conditions) and good lifetime (≥220 000 total turnovers of cyclohexene hydrogenation) are observed, in part by design since only acetone solvent, cyclohexene, and H2 are possible ligands in the resultant “weakly ligated/labile-ligand” supported nanoclusters. Significantly, the Ir(1,5-COD)Cl/γ-Al2O3 + H2 → Ir(0)n/γ-Al2O3 heterogeneous catalyst formation kinetics were also successfully monitored using the cyclohexene hydrogenation reporter reaction method previously developed and applied to solution-nanoparticle formation. The observed sigmoidal supported-nanoparticle heterogeneous catalyst formation kinetics, starting from the Ir(1,5-COD)Cl/γ-Al2O3 precatalyst, are closely fit by the two-step mechanism of slow continuous nucleation (A → B, rate constant k1 = 1.5(1.1) × 10−3 h−1) followed by fast autocatalytic surface growth (A + B → 2B, rate constant k2 = 1.6(2) × 104 h−1 M−1), where A is the Ir(1,5-COD)Cl/γ-Al2O3 precatalyst and B is the resultant Ir(0)n/γ-Al2O3 catalyst. The kinetics are significant in establishing the ability to monitor the formation of supported-nanoparticle heterogeneous catalysts in contact with solution. They also suggest that the nine synthetic and mechanistic insights from the two-step mechanism of nanoparticle formation in solution should now apply also to the formation of supported-nanoparticle heterogeneous catalysts in contact with solution. The results open the door for new syntheses of supported-nanoparticle heterogeneous catalysts under nontraditional, mild, and flexible conditions where supported organometallics and other precursors are in contact with solution, so that additional variables such as the solvent choice, added ligands, solution temperature, and so on can be used to control the catalyst formation steps and, ideally, the resultant supported-nanoparticle heterogeneous catalyst composition, size, and shape.