CO Gas Induced Phase Separation in PtPb@Pt Catalyst and Formation of Ultrathin Pb Nanosheets Probed by In Situ Transmission Electron Microscopy.

PtPb@Pt catalysts are very useful and widely applied in various industrial reactions. Here, the phase stability of such catalysts is compared in both CO gas and vacuum conditions at elevated temperatures using aberration-corrected in situ transmission electron microscopy (TEM). A Pt aggregation process takes place affected by CO gas, which results in direct exposure of the PtPb core to CO. A phase separation process, in which Pb atoms are stripped off the original PtPb@Pt nanoparticles, is unambiguously identified in CO gas. At initial stages, the as nucleated Pb islands are amorphous. Once the ultrathin Pb islands reach ≈3.5 nm or higher, they suddenly became crystalline. The interaction between Pb and CO gas stabilizes the ultrathin Pb nanosheets, resulting in the formation of a large quantity of Pb nanosheets and Pb-depleted PtPb nanoparticles. In sharp contrast, when heated up in a vacuum, the PtPb@Pt catalyst remains intact. The results of this study shine light on the "toxic" effect of CO that results in failures of many Pt-based catalysts and discloses formation mechanism of ultrathin Pb nanosheets.