Pd Nanoparticles on Cu(111)-Supported Ultrathin Ceria Films: Sintering, Alloying, Encapsulation, and Catalytic Activity

Metal nanoparticles (NP) on metal-supported cerium oxide (ceria) ultrathin films are important model catalysts for the study of various catalytic reactions. The thermal stability of such systems is of crucial importance as the catalytic NP activity can change due to thermally induced sintering, alloying, and oxide encapsulation of the NPs. Here, it is shown that ultrahigh vacuum (UHV) annealing of PdNPs on ceria/Cu(111) leads to sintering and Pd–Cu alloying, as observed by scanning tunneling microscopy (STM), noncontact atomic force microscopy (nc-AFM), Kelvin probe force microscopy (KPFM), and low energy electron diffraction (LEED): the PdNPs originally grown at room temperature sinter up to a temperature of 673 K to form larger PdNPs. At 773 and 853 K, the NPs increase their volume due to the diffusion of Cu from the Cu(111) support onto the ceria film and into the PdNPs forming bimetallic PdCuNPs. Due to alloying, the catalytic NP activity changes, as observed by KPFM during the water-forming reaction between atomic O and H on the NPs. Especially after annealing at 853 K, the high Cu content deactivates the NPs. Apart from sintering and alloying, annealing between 500 and 700 K leads to a possible encapsulation of the PdNPs with ceria.