Transient behavior and reaction mechanism of CO catalytic ignition over a CuO–CeO₂ mixed oxide

As a key heterogeneous process, the catalytic oxidation of CO is essential not only for practical applications such as automotive exhaust purification and fuel cells but also as a model reaction to study the reaction mechanism and structure-reactivity correlation of catalysts. In this study, the variation in activity-controlling factors during CO catalytic ignition over a CuO-CeO ₂ catalyst was investigated. The activity for CO combustion follows the decreasing order of CuO-CeO ₂ > CuO > CeO ₂. Except for inactive CeO ₂, increasing temperature induces CO ignition to achieve self-sustained combustion over CuO and CuO-CeO ₂. However, CuO provides enough copper sites to adsorb CO, and abundant active lattice oxygen, thus obtaining a higher hot zone temperature (208.3 °C) than that of CuO-CeO ₂ (197.3 °C). Catalytic ignition triggers a kinetic transition from the low-rate steady-state regime to a high-rate steady-state regime. During the induction process, Raman, X-ray photoelectron spectroscopy, CO temperature-programmed desorption and IR spectroscopy results indicated that CO is preferentially adsorbed on oxygen vacancies (Cu ⁺-[Ov]-Ce ³⁺) to yield Cu ⁺-[C≡O]-Ce ³⁺ complexes. Because of the self-poisoning of CO, the adsorbed CO and traces of adsorbed oxygen react at a relative rate, which is entirely governed by the kinetics on the CO-covered surface and the heat transport until the pre-ignition regime. The Cu ⁺-[C≡O]-Ce ³⁺ complex is a major contributor to CO ignition. The step-response runs and kinetic models showed that after ignition, a kinetic phase transition occurs from a CO-covered surface to an active lattice oxygen-covered surface. During CO self-sustained combustion, the rapid gas diffusivity and mass transfer is beneficial for handling the low coverage of CO. The active lattice oxygen of CuO takes part in CO oxidation.