CO Oxidation over Nanostructured Pt/ZnO Catalysts

Low-temperature CO poisoning of Pt-based catalysts remains a persistent issue during the start-up of automotive three-way catalysts and at the operation temperatures of polymer electrolyte membrane fuel cells. Hence, we have investigated the low-temperature CO oxidation behavior of Pt/ZnO catalysts composed of size-controlled, 2–4 nm large Pt particles and 8–20 nm wide hexagonal ZnO nanorods. Carbon monoxide conversion abruptly escalated as soon as it reached ∼10%, approaching 100% at temperatures as low as 110 °C in a 1/20/79 CO/O₂/He feed. The onset of CO conversion shifted by ca. 70 °C in a 5/5/90 CO/O₂/He feed, indicating that it is very sensitive to the CO/O₂ reactant ratio. At lower temperatures, the activation of O₂ on the Pt particles was strongly inhibited by CO, pointing to a Langmuir–Hinshelwood-type reaction mechanism. Kinetic analyses of the temperature dependence suggest that the generation of vacant Pt sites for O₂ activation is the oxidation rate-limiting process at the lowest temperatures, while O₂ activation at Pt sites and/or its subsequent reaction with CO becomes rate-determining at higher temperatures. CO turnover frequencies amounted to τ ≈ 0.05 s^–1 at 100 °C in the 1/20/79 feed and τ ≈ 0.29 s^–1 at 190 °C in the 5/5/90 CO/O₂/He feed. Reductive pretreatment of the catalysts increased the metallic character of the Pt particles with a concomitant enhancement of the CO turnover frequencies. Infrared spectroscopic analyses revealed that CO is more strongly activated at metallic than at positively charged Pt sites. Still, the effect of reductive treatment on CO activation is minor in comparison to the impact of CO inhibition on O₂ activation. Therefore, the generation of accessible Pt sites for O₂ activation via reactive removal of CO is a key to low-temperature efficiency improvement of Pt catalysts for CO oxidation.