The nature of thermal hysteresis during CO oxidation over Pt catalysts

The origins of activity hysteresis during heating and cooling stage in CO oxidation are still in heavy debate. Pt-CeO₂ and Pt-Al₂O₃ were observed to display completely different hysteresis phenomena; Pt-Al₂O₃ exhibited normal behavior while Pt-CeO₂ showed inverse hysteresis. The affecting issues of initial Pt nanoparticle size, mass and heat transfer limitations, and local overheating were all excluded for both catalysts. By using operando and in situ characterization technologies, kinetics analyses, density functional theory (DFT) calculations as well as ab initio molecular dynamics (AIMD) simulations, the nature of thermal hysteresis was revealed. The different interactions between the metal and supports induced the different dynamic oxidation behaviors and morphology evolutions of Pt during the heating and cooling reactions are the root causes. Pt in Pt-CeO₂ suffered irreversible oxidation mainly raised by reverse oxygen spillover and formed Pt_xO_y ensemble which displays poorer activity. In contrast, Pt in Pt-Al₂O₃ underwent structural collapse and dispersion on Al₂O₃ surface at high temperature, and oxygen spillover to support preserved the initial chemical state of Pt.