Adsorption and Adhesion of Au on Reduced CeO2(111) Surfaces at 300 and 100 K

The adsorption of vapor-deposited Au onto CeO2‑x(111) thin films (x = 0.05 and 0.2) at 300 and 100 K was studied using single crystal adsorption calorimetry (SCAC). The morphology of Au on these films was investigated using He+ low-energy ion scattering spectroscopy (LEIS) and X-ray photoelectron spectroscopy (XPS) by monitoring the changes in substrate and adsorbate signals with Au coverage. Both techniques indicate that Au grows on CeO1.95(111) as three-dimensional particles in the approximate shape of hemispherical caps with a density of 2.8 × 1012 particles/cm2 at 300 K and 7.8 × 1012 particles/cm2 at 100 K. At 300 K, Au initially grows on CeO1.80(111) with a shape similar to hemispherical caps with a density of 5.4 × 1012 particles/cm2 until ∼1.6 ML Au coverage, above which the Au particles become thicker than hemispherical caps. At 300 K, the initial heat of adsorption of Au onto CeO1.95(111) is 259 kJ/mol, which is 37 kJ/mol lower than that on CeO1.80(111). This indicates stronger binding of Au to oxygen vacancies. On both surfaces, the Au heats of adsorption increase slowly with coverage, approaching the bulk heat of sublimation of Au­(solid) (368 kJ/mol) by ∼2 ML (3.2 nm in diameter on CeO1.95(111) and 2.4 nm on CeO1.80(111)). The heat of adsorption remains higher on the reduced surface than on the oxidized surface at all particle sizes. At 100 K, the initial heat of Au adsorption onto CeO1.95(111) is 209 kJ/mol (50 kJ/mol lower than at 300 K), which is due to a higher fraction of Au atoms adsorbing to terraces rather than at step sites. The adhesion energy of Au­(solid) to CeO1.95(111) at 300 K was found to be 2.53 J/m2 for 3.6 nm diameter particles and 2.83 J/m2 onto CeO1.80(111) for 2.5 nm diameter particles. This further indicates that Au particles bind more strongly to surfaces with a larger fraction of oxygen vacancies.