Ag Adsorption on Reduced CeO₂(111) Thin Films

Abstract

The adsorption energies and growth morphology of silver on reduced CeO2(111) thin films at 300 K have been studied using adsorption microcalorimetry in combination with low energy ion scattering (ISS), Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), electron energy loss spectroscopy (EELS), sticking probability measurements, low energy electron diffraction (LEED), and scanning tunneling microscopy (STM). Thin films (4 nm thick) of CeO2−x(111), with x = ∼0.1, were grown on a Pt(111) single-crystal. The AES and ISS signals varied with Ag coverage in a manner indicative of 3D Ag particle growth, with a density of ∼4 × 1012 particles/cm2. The initial heat of adsorption of Ag vapor was ∼200 kJ/mol, which is much lower than the heat of sublimation of Ag (285 kJ/mol). Further reduction of the surface by heating in vacuum (to reach x = ∼0.2) resulted in a slight increase in the initial heat of adsorption to ∼220 kJ/mol; subsequently, higher heats of adsorption were observed until the heat of sublimation of bulk Ag was reached on both films. This increase was attributed to stronger bonding of Ag particles to oxygen vacancies. The initial adsorption energy of Ag on a thinner film (1 nm thick) of CeO1.9(111) was ∼250 kJ/mol, ∼50 kJ/mol higher than the 2, 3, and 4 nm films. This is probably due to an interaction of the Ag particles with the underlying Pt(111) surface. The sticking probability of Ag was measured to be near unity (>0.98) on all these reduced CeO2(111) films between 1−4 nm thick at 300 K and at all Ag coverages.