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

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.

Automated summary

The oxidation state and stability of Pd nanoparticles supported on Co3O4(111) films on Ir(100) were studied under UHV, oxidizing, and humid conditions through STM, XPS, and NAP-XPS. The nanoparticles were predominantly metallic with some Pd2+ species present, leading to oxidation and dissolution under oxidizing and humid conditions, causing significant loss of Pd from the surface. The CoO(111) support remained stable under humid conditions but converted to Co3O4(111) under oxidizing conditions.