Origin of the Normal and Inverse Hysteresis Behavior during CO Oxidation over Pt/Al2O3

The hysteresis in the CO oxidation profile over Pt/Al2O3 catalysts has been intensively studied, but its origin is still controversial. In this work, the influence of the Pt particle size and size distribution was systematically investigated. By the application of conventional and advanced preparation methods, such as flame spray pyrolysis and supercritical fluid reactive deposition, a series of catalysts containing homogeneous distributions of Pt particles were obtained. An optimal Pt particle size of 2-3 nm was identified for the CO oxidation light-off on Pt/Al2O3 catalysts. The CO oxidation results show a clear correlation between the Pt nanoparticle size and the ignition/extinction profile, including the switch of the hysteresis loop. On the basis of high-angle annular dark-field scanning transmission electron microscopy characterization combined with in situ diffuse reflectance infrared Fourier transform spectroscopy and operando X-ray absorption spectroscopy measurements, the appearance of inverse hysteresis for catalysts containing very small Pt nanoparticles (<2 nm) and typical hysteresis for larger Pt nanoparticles could be related to the different CO adsorption strengthd, to surface/bulk oxidation of Pt particles, and to the ability of the catalyst to regenerate the active sites, considering also the exothermicity of the CO oxidation reaction.