Date of Award

8-9-2014

Document Type

Open Access Dissertation

Department

Chemical Engineering

First Advisor

Christopher T. Williams

Abstract

This dissertation considers effective bimetallic preparation for supported Ir-based bimetallic catalysts and their catalytic activity. Since bimetallic catalysts exhibit significantly different catalytic and chemical properties than their corresponding monometallic components by providing enhanced selectivity, stability and /or activity, many monometallic catalysts in industrial processes have been replaced by bimetallic catalysts. However, conventional synthetic methods used to produce monometallic and bimetallic catalysts often result in wide particle size distributions and non-uniform materials which can be difficult to characterize on a fundamental level. Two alternate preparation methods, dendrimer templating and electroless deposition, are developed and compared with conventional incipient wetness method. In dendrimer templating method, poly(amidoamine) (PAMAM) dendrimers are used to form and stabilize cluster and nanoparticles in solution generating dendrimer metal nanocomposites (DMN) precursors. On the other hand, suitable reducing agent and metal ion source are selected for electroless deposition method which allows the ability to tailor the catalyst surface sites upon which the secondary metal is deposited. In the present work, the effectiveness of using two distinct method as well as conventional method to prepare Al2O3 supported Ir-Au, Ir-Ag catalysts is reported.

Ir-Au/Al2O3 catalysts were prepared utilizing the DMN approach by four different synthetic routes and allowed for comparison with the conventional counterparts. These catalysts showed different metallic dispersions with various particle sizes and distributions, depending on the preparation method. These properties influence the catalytic performance, dendrimer-derived catalyst with higher dispersion and narrow particle size distribution resulted in enhanced activity toward CO oxidation and higher selectivity towards N2O and a better intrinsic catalytic turnover frequency for reduction of NO by CO and NO decomposition.

On the other hand, a series of alumina supported Ag-Ir and Au-Ir bimetallic catalysts having controlled and incremental coverages of Ag or Au, have been successfully prepared in an optimized electroless deposition bath. The structural and electronic properties of the catalysts were characterized using hydrogen chemisorption, atomic absorption spectroscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The results suggest that Ag and Au metal was deposited on all types of Ir surface sites in a non-discriminatory fashion. However, kinetic studies of CO oxidation revealed different result for Ag-Ir and Au-Ir bimetallic system. That is, higher coverages of Au resulted in lower turnover frequencies (TOFs) indicating that no bimetallic effect between Au and Ir for this reaction. In contrast, the highest TOF was obtained at the half point in Ag-Ir bimetallic catalysts where the Ir-Ag pair sites are maximized. This clearly suggests a bifunctional effect, where the Ag provides a non-competitive source of adsorbed oxygen for reaction with CO adsorbed on Ir. Consistent reaction order studies were obtained. On the other hand, enhanced catalytic activities were found toward NO-CO reaction over both ED-derived bimetallic Au-Ir as well as Ag-Ir catalysts. This can be explained by a probable bifunctional effect, but more kinetic studies need to be done to fully understand the mechanism for this.

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