Pd/Co<sub>3</sub>O<sub>4</sub>(111) Interface Formation
Maximilian Kastenmeier, Lukáš Fusek, Fatema Mohamed, Christian Schuschke, Michal Ronovský, Tomáš Škála, Matteo Farnesi Camellone, Nataliya Tsud, Viktor Johánek, Stefano Fabris, Jörg Libuda, Simone Piccinin, Yaroslava Lykhach, Josef Mysliveček, Olaf Brummel
Abstract
The formation of the metal–oxide interface in the Pd/Co 3 O 4 (111) model catalyst was investigated by means of density functional theory (DFT), synchrotron radiation photoelectron spectroscopy (SRPES), and scanning tunneling microscopy (STM). The electronic metal–support interaction results in a substantial charge transfer at the interface yielding atomically dispersed Pd 2+ species and partially oxidized Pd δ+ aggregates coupled with a partial reduction of Co 3 O 4 (111). Atomically dispersed Pd 2+ species at the fcc site on the Co 3 O 4 (111) surface were found to be the most energetically favorable configuration. In comparison to the dispersed Pd 2+ species, the formation of Pd dimers, trimers, and tetramers was found to be less favorable. The analysis of the Bader charges revealed a substantial net positive charge on Pd atoms in dimers, trimers, and tetramers which is consistent with the formation of partially oxidized Pd δ+ aggregates detected by SRPES. The analysis of the charge distribution in Co 3 O 4 (111) revealed a partial reduction of Co 3+ to Co 2+ cations in the first and second Co layers. According to DFT, Pd δ+ aggregates are prone to oxidation to PdO in the presence of O 2 and H 2 O. The partially oxidized Pd δ+ and Pd 4 O x aggregates form 1 to 2 monolayer thick clusters which serve as nuclei for the growth of metallic Pd 0 nanoparticles. At high Pd coverage, Pd nanoparticles coalesce resulting in the growth of two-dimensional islands that densely cover the Co 3 O 4 (111) substrate.