Oxidation Kinetics over Rh/Al2O3 in a Stagnation-Flow Reactor

Abstract

Transportation produces about 25% of the global CO2 emissions, with gasoline and diesel light-duty vehicles being responsible for nearly half the transportation sector energy use. The reduction of emissions from transportation is an enormous challenge. While electrification is on the horizon, around 70% of the world’s vehicles are expected to be non-electric by 2050. One pragmatic way of tackling the emissions is to optimize the systems employed in the over 1 billion vehicles on the road today. In this dissertation, the issue of reducing the transportation emissions is tackled from experimental and simulation points of view. This entails building a stagnation-flow reactor, which reduces the problem to one dimensional and helps attain accurate kinetic data, and employing the state-of-the-art catalytic microkinetic modeling techniques.

Gasoline vehicles utilize three-way catalyst systems, where CO, NO and unburned hydrocarbons are simultaneously converted to CO2, N2 and H2O. These systems are based on platinum (Pt), rhodium (Rh) and palladium (Pd) catalysts. Rh is the most expensive metal and estimated to only be 0.0002 ppm of the Earth's crust. Therefore, it is important to understand the detailed chemistry on Rh to better utilize it. First, thorough characterization experiments of a commercial 5 wt. % Rh/Al2O3 catalyst were performed via N2-physisorption, ICP-OES, XRD, H2-TPR, H2--chemisorption, STEM, and EELS.

Additionally, the current microkinetic mechanisms for CO oxidation to CO2 over Rh/Al2O3 are limited to stoichiometric conditions and cannot predict the behavior at lean conditions, where gasoline engines are more efficient. An improved understanding of CO oxidation was attained by performing experiments at low-temperature in the stagnation-flow reactor. This includes the effects of temperature, pressure, inlet composition, and flow rate.

Then, a microkinetic mechanism that is DFT-parametrized and captures the CO oxidation behavior was developed. The mechanism is versatile and accurately predicts the observed behavior at vastly different temperatures, inlet compositions, and flow rates. The detailed chemistry was examined by performing sensitivity analysis at different compositions. Also, the surface coverage was investigated, and the surface behavior was explained based on thermodynamics.

Lastly, the oxidation of dimethyl ether, a potential alternative fuel for diesel engines, was studied at low temperatures in the stagnation-flow reactor. In addition to testing total oxidation, partial oxidation was examined, where the oxidation zone was isolated from the reforming zone. This is of relevance to the after-treatment of DME-powered engines. Additionally, intrinsic activation energy values for DME oxidation are reported for the first time for this catalyst system.

Overall, the work advances the current knowledge of existing and alternative transportation systems. It paves the way for accurate modeling of these catalytic process as well as rational design for cheaper and more effective catalysts.