Christopher S. Polster and Chelsey D. Baertsch. School of Chemical Engineering, Purdue University, 480 Stadium Mall Drive, West Lafayette, IN 47907-2100
Elucidating the mechanism and active species responsible for CO preferential oxidation (PROX) catalysts can be useful in designing catalysts which are better suited for PROX processes. Towards this aim, detailed studies of CO oxidation in excess H2 have been conducted. A comparison of independent H2 and CO oxidation to preferential oxidation (PROX) of CO in the presence of H2 is made. High ideal selectivities calculated from independent experiments indicate a kinetic preference for CO oxidation, while relatively higher PROX selectivities indicate other factors in addition to kinetic differences are important to achieve high selectivity. Isothermal selectivity versus conversion plots are used to show that the main factor is the competition between CO and H2 for adsorption/reaction sites. CO coverage is quantified and its relationship to selectivity is discussed. However, determining the precise sites responsible for selective reactions remains the key challenge in elucidating catalytic pathways and comparing catalyst activity over CuOx-CeO2 catalysts. A unique approach combining high resolution transmission electron microscopy (HR-TEM) with anaerobic titrations has been used to identify as well as quantify the active sites on a series of CuOx-CeO2 catalysts. Removal of oxygen under steady state conditions provides for a precise count of “active oxygen” species that are available for either CO or H2 oxidation. HR-TEM images allow for the identification of sites by correlating particular surface structures with the amount of active oxygen. These techniques also enable the measurement of true turnover frequencies, which further allows for catalyst comparison and understanding of the high selectivity toward CO2. In light of these techniques, a detailed kinetic model is therefore proposed and a modified Mars and van Krevelen rate expression describing competitive oxidation pathways is derived and fit to kinetic data. This approach can be generalized to other metal oxide catalysts, thus determining the active species involved in CO and H2 oxidation could provide for catalyst design parameters for designing more effective sensor substrates.