Time-resolved, high-energy X-ray total scattering measurements, allied to time-resolved Pt L 3 -edge X-ray absorption spectroscopy, show that the ambient temperature oxidation of CO to CO 2 over a commercial Pt/Al 2 O 3 catalyst when conducted in a redox cycling manner is highly correlated to the presence of reactive (toward CO) and extended platinum surface oxides after the oxidizing part of the cycle. This part of the cycle is barely affected by preliminary in situ reduction in H 2 to 573 K. The reducing part of the cycle, wherein the sample is initially covered in molecular CO, is characterized by very different kinetics, which is significantly modified by preliminary in situ reduction, yet the same intermediate visible by infrared spectroscopy appears in both sides of the redox cycle. The steady-state production of CO 2, investigated using diffuse reflectance infrared spectroscopy and mass spectrometry for 4 ≤ O 2 /CO ≤ 333 and 298 ≤ T ≤ 323 K, while comprising only ca. 1% of the post light-off activity of this Pt catalyst (O 2 /CO = 4), is maintained at O 2 /CO = 4 even though the infrared spectroscopy shows that the Pt particles are covered with molecular CO as of O 2 /CO = 20. From the combination of methods used in both transient and steady-state cases, it appears that the Pt nanoparticles behave in a different way to single-crystal surfaces with respect to their response to CO adsorption in comparable pressure regimes, and compression structures are not formed. Instead, increasing fractions of CO are suggested to induce the rearrangement of the nanoparticles to yield vicinal surfaces from exposed (100) facets that are less reactive toward CO oxidation, while Pt (111) facets, which are indicated to support the adsorption of CO, remain intact and capable of turning over CO to CO 2 at ambient temperature. From the sum of these investigations, we suggest possible explanations for this chemistry.
Checchia et al. (Wed,) studied this question.