Stability of the Pd/Co₃O₄(111) Model Catalysts in Oxidizing and Humid Environments

Abstract

The oxidation state and stability of Pd nanoparticles supported on well-ordered Co3O4(111) films prepared on Ir(100) have been investigated in UHV and under both oxidizing and humid conditions by means of scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and near ambient X-ray photoelectron spectroscopy (NAP–XPS). After preparation, the supported Pd nanoparticles (size 4 nm) were found to be predominantly metallic. Small amounts of Pd2+, resulting from the electronic metal support interaction (EMSI), were found in the form of PdO and as ionic species dissolved in Co3O4(111). Annealing of the Pd/Co3O4(111) model catalyst in UHV triggers sintering of the Pd nanoparticles but leaves the oxidation states of Pd and the substrate largely unaffected. The oxidation of Pd/Co3O4(111) is coupled with dissolution of Pd2+ species into Co3O4(111) and underlying Ir(100) resulting in a significant loss of Pd from the surface. The corresponding phenomenon occurs largely under oxidizing and, to a minor extent, under humid conditions. The reverse oxygen spillover is facilitated in the presence of Pd2+ species dissolved in Co3O4(111) yielding Pd nanoparticles supported on CoO(111). The CoO(111) support remains stable under humid conditions but is reversibly converted to Co3O4(111) under oxidizing conditions.