Supported-Nanoparticle Heterogeneous Catalyst Formation in Contact with Solution: Kinetics and Proposed Mechanism for the Conversion of lr(1,5-COD)CI/γ-AI_2O_3 to lr(0)_(~900)/γ-AI_20_3

摘要

A current goal in heterogeneous catalysis is to transfer the synthetic, as well as developing mechanistic, insights from the modern revolution in nanoparticle science to the synthesis of supported-nanopartide heterogeneous catalysts. In a recent study (Mondloch, J. E.; Wang, Q.; Frenkel, A. I.; Finke, R. G.J. Am. Chem. Soc. 2010,132, 9701-9714), we initialized tests of the global hypothesis that quantitative kinetic and mechanistic studies, of supported-nanopartide heterogeneous catalyst formation in contact with solution, can provide synthetic and mechanistic insights that can eventually drive improved syntheses of composition-, size-, and possibly shape-controlled catalysts. That study relied on the development of a well-characterized Ir(l,5-COD)Cl/γ-Al_2O_3 precatalyst, which, when in contact with solution and H_2 turns into a nonaggregated Ir(0)_(~900)/γ-Al_2O_3 supported-nanopartide heterogeneous catalyst. The kinetics of the Ir(l,5-COD)Cl/γ-Al_2O_3 to Ir(0)_(~900)/γ-Al_2O_3 conversion were followed and fit by a two-step mechanism consisting of nucleation (A-* B, rate constant k_1) followed by autocatalytic surface growth (A + B -* 2B, rate constant k_2). However, a crucial, but previously unanswered question is whether the nucleation and growth steps occur primarily in solution, on the support, or possibly in both phases for one or more of the catalyst-formation steps. The present work investigates this central question for the prototype Ir(l,5-COD)Cl/y-Al_2O_3 to Ir(0)_(~900)/γ-Al_2O_3 system. Solvent variation-, y-Al_2O_3-, and acetone-dependent kinetic data, along with UV-vis spectroscopic and gas-liquid-chromatography (GLC) data, are consistent with and strongly supportive of a supported-nanopartide formation mechanism consisting of Ir(l,5-COD)Cl(solvent) dissociation from the γ-Al_2O_3 support (i.e., from Ir(l,5-COD)Cl/γ-Al_2O_3), solution-based nudeation from that dissociated Ir(l,5-COD)Cl(solvent) species, fast Ir(0)_n nanopartide capture by y-Al_2O_3, and then subsequent solid-oxide-based nanopartide growth from Ir(0)_n/γ-Al_2O_3 and with Ir(l,5-COD)Cl(solvent), the first kinetically documented mechanism of this type. Those data disprove a solid-oxide-based nudeation and growth pathway involving only Ir(l,5-COD)Cl/y-Al_2O_3 and also disprove a solution-based nanoparticle growth pathway involving Ir(l,5-COD)Cl(soIvent) and Ir(0)_n in solution. The present mechanistic studies allow comparisons of the Ir(l,5-COD)Cl/y-Al_2O_3 to Ir(0)~9oo/y-Al_2O_3 supported-nanopartide formation system to the kinetically and mechanistically well-studied, Ir(l,5-COD) • P_2WisNb_3O_(62)~(8-) to Ir(0)_(~300)· (P_2W_(15)Nb_3O_(60)~(8-))_n~(-8n) solution-based, polyoxoanion-stabilized nanopartide formation and stabilization system. That comparison reveals dosely analogous, solution Ir(l,5-COD)~+ or Ir(l,5-COD)Cl-mediated, medianisms of nanopartide formation. Overall, the hypothesis supported by this work is that these and analogous studies hold promise of providing a way to transfer the synthetic and mechanistic insights, from the modern revolution in nanopartide synthesis and characterization in solution, to the rational, mechanism-directed syntheses of solid oxide-supported nanopartide heterogeneous catalysts, also in contact with solution.