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.

The adsorption of gold vapor onto MgO(100) films grown on Mo(100) was studied at 300 and 100 K using single crystal adsorption calorimetry (SCAC). The Au particle morphology was investigated using He + low-energy ion scattering spectroscopy (LEIS) and X-ray photoelectron spectroscopy (XPS). The LEIS data combined with particle shape measurements from the literature (Benedetti, S.; Myrach, P.; di Bona, A.; Valeri, S.; Nilius, N.; Freund, H.-J. Phys. Rev. B 2011, 83 (12), 125423) reveal that, at both 300 and 100 K, Au grows as 2D islands with bilayer thickness (∼0.41 nm) up to a diameter of ∼7 nm. At higher coverage, the islands thicken with little increase in diameter. The island densities are 3.0 × 10 11 and 5.4 × 10 11 per cm 2 at 300 and 100 K, respectively. The initial sticking probability of Au is 0.90 at 300 K and 0.95 at 100 K. The surface residence time of the Au atoms that do not stick is <10 ms, implying that gold monomers bind to MgO(100) weakly (<69 kJ/mol). The adsorption energies indicate that Au particles of the same size bind more strongly to MgO(100) when grown at 300 K than at 100 K, which we attribute to Au binding to step edges or defects at 300 K, but at perfect MgO(100) terraces at 100 K (because Au diffusion is too slow to find defects). The adsorption energy of Au onto ∼30-atom Au clusters is 285 kJ/mol at 300 K, ∼68 kJ/mol higher than at 100 K, attributed to the difference between particles on defects versus terraces. Similarly, the adhesion energy of Au nanoparticles to MgO(100) extracted from the adsorption energies at 300 K is much higher (1.8 J/m 2 for ∼7 nm particles at defects) than at 100 K (0.3 J/m 2 for ∼7 nm particles at terraces). This 100 K adhesion energy is close to that estimated from electron-microscopy shape measurements of Au particles at terraces on MgO(100) (0.45−0.67 J/m 2 ). The heat of Au adsorption and Au chemical potential change by >100 kJ/mol as gold's 2D island size increases from 0.7 to 7 nm diameter, implying dramatic changes in catalytic activity and sintering rates with 2D diameter. This is the first experimental measurement of any metal adsorption energy on any oxide as a function of island diameter when making 2D islands, as well as the first direct comparison of any adhesion energy found from calorimetric adsorption energies to that from particle shape analysis.

Bimetallic catalysts are an important class of heterogeneous catalysts with catalytic properties distinct from either of their bulk metal constituents. The structural, electronic, chemisorptive, and catalytic properties of bimetallic surfaces have been widely studied. Surface reactivity often correlates with adsorption energy of one metal on a singlecrystal surface of the other as measured using temperatureprogrammed desorption (TPD). However, TPD only works for systems where the metals are immiscible. For bimetallic systems that form an alloy or intermetallic compound, TPD generally fails because the adsorbed metal penetrates into the bulk upon heating. The metal-on-metal adsorption energy is unmeasured for all but one such system previously but often calculated because these adlayers often have interesting catalytic properties. We report here calorimetric measurements of the adsorption energy versus coverage of an adlayer of one metal on another for such a bimetallic system, where the metals prefer to alloy: Au on Pt(111). This bimetallic combination is important in catalysis and electrocatalysis. The first monolayer (ML) of Au grows pseudomorphically with the Pt(111) surface at 300 K, with an average heat of adsorption of 389, ∼21 kJ/mol greater than the bulk heat of sublimation of Au. The heat increases with coverage by ∼11 kJ/mol in the first 0.03 ML and then by another ∼2 kJ/mol up to a maximum of 395 kJ/mol at 0.7 ML, and it then decreases to near the bulk heat of sublimation (368 kJ/mol) at 1 ML. The increase in heat is attributed to the increase in size of the two-dimensional Au islands that nucleate at a very low coverage and their corresponding increase in the average number of Au−Au nearest neighbor bonds. The highcoverage decrease in heat is attributed to the buildup of strain associated with the 4% Au/Pt lattice mismatch. The second and possibly third layers of Au show similar but much smaller oscillations in heat around 370 kJ/mol, attributed to the same two effects.