An experimental and theoretical investigation into nonlinear phenomena during carbon monoxide oxidation on platinum(100)

Tanmay Prakash Lele, Purdue University

Abstract

This thesis seeks to further the fundamental understanding of spatio-temporal pattern formation during chemical reactions on transforming single crystal surfaces. The research philosophy appeals to a variety of tools and approaches consisting broadly of utilizing sophisticated experimentation coupled with rigorous mathematical modeling. Photoelectron emission microscopy and imaging ellipsometry were employed to image spatio-temporal pattern formation coupled with mass spectrometry during CO oxidation on the Pt (1 0 0) surface at low and intermediate pressures. A fresh perspective was provided on various aspects of turbulent patterns via detailed qualitative analysis of the development and self-sustenance of patterns. On the basis of analysis of recent experimental observations, sub-surface oxygen was established to be an active participant during pattern formation at intermediate pressures. Sub-surface oxygen was proposed to be responsible for non-linear phenomena at low pressures as opposed to the traditional surface phase transformation mechanism. The interpretation of the large (and ever expanding) body of experimental data for reactions on transforming surfaces has been sorely limited by the quality of theoretical models in the literature. The modeling approaches in the literature were not based on proper identification of a differential area for writing differential mass balances, causing important effects to be neglected. This work is a significant step toward identifying and developing a rigorous theoretical approach to modeling reactions on transforming surfaces. The basic postulates of the theory are the availability of a length scale over which the local infinitesimal area is of one surface type or another and a time scale in which changes in coverage occur on this scale, deterministically describable by continuous variables. A combination of probability averaging and area averaging is employed to arrive at a deterministic set of partial differential equations for locally averaged surface concentrations. The equations include reaction and surface diffusion and terms associated with dilution/augmentation of surface concentration of species brought about by phase transformation. Dilution and augmentation terms are of significance to predicting the nonlinear behavior of the system and in extracting the kinetics of surface reactions from dynamic data, which was established by computation for the CO oxidation reaction on Pt.

Degree

Ph.D.

Advisors

Ramkrishna, Purdue University.

Subject Area

Chemical engineering

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