Kinetic and mechanistic studies of supported-nanoparticle heterogeneous catalyst formation in contact with solution
Date
2011
Authors
Mondloch, Joseph E., author
Finke, Richard G., advisor
Bailey, Travis S., committee member
Ferreira, Eric M., committee member
Prieto, Amy L., committee member
Shores, Matthew P., committee member
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Abstract
This dissertation begins with a comprehensive and critical review of the literature addressing the kinetics and mechanism(s) of supported-nanoparticle heterogeneous catalyst formation. The review chapter that follows makes apparent that routine kinetic monitoring methods, as well as well-defined supported-nanoparticle formation systems, are needed in order to gain fundamental insights into the mechanisms of supported-nanoparticle heterogeneous catalyst formation--a somewhat surprising finding given the long history as well as commercial importance of heterogeneous catalysis. Hence, the research presented within this dissertation is focused on (i) developing a kinetic monitoring method (i.e., in what follows, the cyclohexene reporter reaction method) capable of measuring supported-nanoparticle formation in contact with solution, and (ii) developing a well-defined supported-nanoparticle formation system, also in contact with solution, that is amenable to rigorous mechanistic studies. Development of the cyclohexene reporter reaction has allowed for the rapid and quantitative monitoring of the kinetics of Pt(0)n/Al2O3 and Pt(0)n/TiO2 supported-nanoparticle heterogeneous catalyst formation in contact with solution from H2PtCl6/Al2O3 and H2PtCl6/TiO2 respectively. Importantly, those kinetic studies revealed conditions where the most desirable, chemical-reaction-based, supported-nanoparticle formation conditions are present rather than diffusional-limited kinetic regimes. The largest drawback when utilizing the H2PtCl6 as a supported-precatalyst is its speciation--that is, other solvated Pt-based species form when in contact with solution. Such non-uniform speciation leads to a large variation in the supported-nanoparticle formation kinetics, observations that were obtained through the use of the cyclohexene reporter reaction kinetic monitoring method. Due to the large variability in the formation kinetics associated with the H2PtCl6 precatalyst speciation, synthesized next as a part of this dissertation work was the well-defined, fully characterized, speciation-controlled supported-organometallic precatalyst, Ir(1,5-COD)Cl/γ;-Al2O3. When in contact with acetone, cyclohexene and H2 this supported-precatalyst was found to evolve into a highly active and long-lived Ir(0)~900/γ;-Al2O3 supported-nanoparticle catalyst. The kinetics of Ir(0)~900/γ-Al2O3 formation were successfully followed by the cyclohexene reporter reaction method and found to be well-fit by a two-step mechanism consisting of nucleation (A → B, rate constant k1) followed by autocatalytic surface growth (A + B → 2B, rate constant k2) previously elucidated by Finke and Watzky. More specifically, nucleation was found to occur in solution from Ir(1,5-COD)Cl(solvent), while nanoparticle growth occurs on the γ-Al2O3 support, but in a reaction that involves the Ir(1,5-COD)Cl(solvent) species in solution. Most importantly, the fits to the two-step mechanism suggest that the nine synthetic and mechanistic insights, of nanoparticle formation in solution, should now be applicable to the formation of supported-nanoparticle heterogeneous catalysts in contact with solution. That is, it seems reasonable to expect that these studies will allow a more direct avenue for transferring both the mechanistic and synthetic insights that have resulted from the modern revolution in nanoparticle science to the synthesis of size, shape and compositionally controlled supported-nanoparticle catalysts under the nontraditional, mild and flexible conditions where supported organometallics and other precursors are in contact with solution.
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Subject
catalysis
nanoparticle
mechanism
kinetics
heterogeneous catalyst