Studies show that the rate of CO oxidation on Pt-group metals at temperatures between 450 and 600 K and pressures between 1 and 300 Torr increases markedly with an increase in the O2/CO ratio above 0.5. The catalytic surfaces, formed at discrete O2/CO ratios >0.5, exhibit rates 2–3 orders of magnitude greater than those rates observed for stoichiometric reaction conditions and similar reactant pressures or previously in ultrahigh vacuum studies at any reactant conditions and extrapolate to the collision limit of CO in the absence of mass transfer limitations. The O2/CO ratios required to achieve these so-called "hyperactive" states (where the reaction probabilities of CO are thought to approach unity) for Rh, Pd, and Pt relate directly to the adsorption energies of oxygen, the heats of formation of the bulk oxides, and the metal particle sizes. Auger spectroscopy and X-ray photoemission spectroscopy reveal that the hyperactive surfaces consist of approximate 1 ML of surface oxygen. In situ polarization modulation reflectance absorption infrared spectroscopy measurements coupled with no detectable adsorbed CO. In contrast, under stoichiometric O2/CO conditions and similar temperatures and pressures, Rh, Pd, and Pt are essentially saturated with chemisorbed CO and exhibit far less activity for CO oxidation.
