Research Adviser: William F. Schneider
Ph.D Thesis: Simulation of NO Oxidation Catalysis over Oxygen-Covered Transition Metal Surfaces
In this work, we apply quantum chemical simulation and thermodynamic and kinetic modeling to elucidate the mechanism of NO oxidation over platinum metal (Pt) catalysts:
NO + 1/2 O2 → NO2
This reaction is interesting for several reasons. From a basic research standpoint, the O--NO bond is weak and chemically labile (ΔHrxn = -60 kJ/mol), so high conversions occur under large O2 potentials (low T, high PO2), and product NO2 readily dissociates on Pt metal. The catalysis thus occurs over large concentrations of surface O. Therefore, it must be studied at realistic reaction conditions. One of the challenges in simulating NO oxidation catalysis is modeling under the appropriate gas conditions. Careful modeling, copious computation, and experimental collaboration are necessary to understand the problem.
From a societal standpoint, catalytic NO oxidation is a key step in NOx remediation processes such as the Lean NOx Trap and the Selective Catalytic Reduction catalyst, so it is an important challenge in environmental catalysis. Platinum is the most common catalyst for the reaction, but it is expensive, and experimental evidence indicates it is prone to oxidative deactivation under NO oxidation conditions. There is a strong motivation to find superior materials.
Experiments show that the NO oxidation reaction exhibits atypical kinetics: The rate is promoted by high O2 pressures and inhibited by product NO2. Further, Pt "lights off" for NO oxidation at 250oC. Coupled with the small heat of reaction, this kinetic feature limits the achievable yield.
Several models of Pt-catalyzed NO oxidation have been reported. They successfully capture some of the observed behavior, such as the catalytic sensitivity to surface O concentration or the Pt light-off temperature. However, none of the models captures all of the observed features. The goal of this work is to develop a model that does in order to develop an understanding of the catalysis and propose superior catalysts.
Our findings are documented in the Journal of Physical Chemistry C, Catalysis Today, Physical Review Letters, and the Journal of Catalysis (please see the Publications subpage.) They focus on calculating thermodynamics and kinetics at catalyst compositions relevant to reaction conditions. Highlights include a Centennial Feature article in the Journal of Physical Chemistry C detailing O chemisorption on Pt(111) under a variety of oxygen potentials, a coupled theoretical/experimental analysis of the relative kinetics of relevant molecular dissociations as a function of surface O concentration (Physical Review Letters), and an O concentration-dependent micro-kinetic model of Pt-catalyzed NO oxidation (Journal of Catalysis). The latter is the only model reported that successfully simulates NO oxidation catalysis under realistic reaction conditions and simultaneously describes several experimentally observed catalytic features.