Low-temperature CO oxidation over Rh/Al2O3 in a stagnation-flow reactor

Abstract

Anthropogenic activities are responsible for nearly half of the total CO emissions in the US. A significant amount of CO is emitted by the transportation sector. Three-way catalytic converters are widely employed to treat CO emissions from gasoline engines; however, current kinetic mechanisms for CO oxidation and the water-gas shift (WGS) reaction on Rh are limited and were built based on data collected over a narrow range of conditions. To fill in this gap, we conducted low-temperature CO oxidation and WGS experiments on 5 wt% Rh/Al₂O₃ in a stagnation-flow reactor, which allows for reducing the problem to one dimension and simplifies the development of accurate kinetic models. We characterized the catalyst via N₂-physisorption, ICP, XRD, H₂-chemisorption, H₂-TPR, STEM and EELS. We studied the effects of pressure, temperature, flowrate, and the presence of H₂O on the conversion of CO to CO₂ and on the WGS reaction over the temperature range relevant to aftertreatment systems. The total operating pressure affected the resolution of the experimental measurements. Higher temperatures resulted in higher CO₂ production due to faster kinetics. Investigating the reaction order with respect to O₂ showed three distinct kinetic regimes, where the order is positive below the stoichiometric ratio, beyond which a negative order was observed which decreased with increasing O₂ content. With respect to CO, the order was positive below the stoichiometric ratio, beyond which the order was negative. When increasing and reducing the O₂ content, we observed bistability manifested as a hysteresis behavior, which is attributed to the oxidation (by O₂) and reduction (by CO) of the metal. This thorough experimental study aids in developing accurate and versatile CO oxidation on Rh kinetic mechanisms that predict reactivity over a wide range of conditions.