Author's School

School of Engineering & Applied Science

Author's Department/Program

Energy, Environmental and Chemical Engineering

Language

English (en)

Date of Award

1-1-2011

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Chair and Committee

Palghat Ramachandran

Abstract

Rational catalyst design must be advanced beyond its state-of-the-art given the significant economic and environmental relevance of catalytic technologies. To address this challenge, precise kinetic characterization of industrial catalysts is required for elucidating complex reaction mechanisms, establishing structure-activity relationships, and building scientifically-sound microkinetic models of catalytic processes. In this thesis, a novel framework for non-steady-state high-throughput kinetic characterization of complex catalytic reactions is theoretically developed, experimentally validated, and applied to a catalytic reaction of considerable interest. This novel framework of catalyst characterization is based on Temporal Analysis of Products: TAP) experiments. These pulse-response experiments employ Knudsen diffusion as a reliable standard process of gas transport to measure intrinsic rates of chemical transformations on catalysts. Specifically, this work focuses on the Thin- Zone: TZ) configuration of the TAP microreactor which allows monitoring of reaction kinetics for a well-defined and spatially uniform catalyst state with resolution on the millisecond scale. In the TZ TAP reactor, a narrow catalytic sample is packed between two inert zones in order to minimize macroscopic concentration and temperature non-uniformities across the catalyst. Unlike traditional kinetic testing devices such as well-mixed or tubular reactors, the TZ TAP reactor maintains the catalyst in a highly uniform state for an extended range of reactant conversions. One of the main implications of maintaining TZ uniformity is the ability to effectively decouple reaction kinetics from external transport in the microreactor. Typically, TAP data analysis relies on a set of mechanistic assumptions about a reaction in order to obtain kinetic information from coupled reaction-diffusion data. In our framework, intra-pulse kinetic characteristics in the TZ including reaction rates, gaseous concentrations, and surface concentrations are reconstructed from exit flows via the 'model free' Y-Procedure and then used for elucidating the reaction mechanism and estimating kinetic parameters. The core idea of the data analysis framework developed in this thesis is that the network of elementary steps behind a catalytic reaction can be revealed by examining how reconstructed kinetic characteristics evolve in relation to each other during a pulse-response experiment. Our results suggest, for example, that the temporal coherence between reactant consumption and product generation rates can provide compelling arguments in favor of one potential reaction mechanism over another. The analysis of rate-concentration data can also be used to estimate intrinsic kinetic parameters once the network of reaction steps is identified. These theoretical developments have been translated into a viable experimental methodology which has been validated using well-characterized oxygen uptake on polycrystalline platinum as a benchmark problem. Finally, the Y-Procedure was applied to study CO oxidation and oxygen storage on the Au/SiO2 catalyst prepared by magnetron sputtering. Oxygen was introduced to the catalyst during ow pretreatments under elevated pressure and then titrated o the catalyst by multi-pulse CO sequences under TAP vacuum conditions. The data indicate that oxygen is stored on the catalyst in two kinetically distinct reservoirs. Both reservoirs get filled with oxygen under ow pretreatment, but only one of them directly contributes oxygen for CO oxidation under vacuum. The two reservoirs exchange oxygen between each other and after one of them is depleted by the oxidation reaction during a CO pulse, the second reservoir resupplies oxygen before the next CO pulse arrives. Further research is needed to identify the chemical nature of the second oxygen reservoir. However, our findings testify to the utility of the Y-Procedure as an advanced tool for mechanistic research in catalysis. The thesis outlook section suggests several research directions which will be facilitated by the systematic application of the Y-Procedure.

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Permanent URL: http://dx.doi.org/10.7936/K7JD4TV0

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