Investigating the Thermal and Environmental Causes of Particle Migration in Pt/Al₂O₃ Catalysts

Emission control catalysts are an important environmental protection technology that eliminate harmful gases from hydrocarbon combustion reactions. Pt nanoparticles supported on alumina (Pt/Al₂O₃) are among the most common catalysts for this purpose. However, catalytic activity is lost over time to sintering processes facilitated by a combination of particle migration and coalescence, and also Ostwald ripening, that are driven by chemical and thermal interactions. These interactions are quite complicated and can produce substantially different dominant sintering mechanisms across the typical range of operating temperatures. Investigating these transient effects at atomic resolution under realistic environmental conditions requires the use of in situ atmospheric scanning transmission electron microscopy (STEM).<br/>We have performed a series of in situ atmospheric and vacuum STEM experiments to understand the origin of particle migration in Pt/Al₂O₃ catalysts. Within a vacuum environment, Pt nanoparticles become mobile at 700 °C. However, the presence of oxygen and water vapor enhance the mobility of Pt nanoparticles so that motion becomes possible at temperatures as low as 500 °C. This motion is undulating in nature, wherein the nanoparticles extend and contract as they move across the Al₂O₃ surface. Through density functional theory calculations, it has been determined that enhanced particle migration in Pt is driven by nanoparticle-oxygen interactions. The adsorption of oxygen atoms increases the stability of anisotropic morphologies, permitting nanoparticle extension, while the release of PtO₂ molecules results in the contraction of the nanoparticles. Experimentally observed particle migration at 500 °C is not immediately followed by coalescence in neighboring nanoparticles. Instead, coalescence is gradually facilitated at higher temperatures. This delayed and temperature specific sintering behavior demonstrates the importance of understanding the complex environmental and thermal interactions that exist under realistic operating conditions.