The energies of silver (Ag) atoms in Ag nanoparticles supported on different cerium and magnesium oxide surfaces, determined from previous calorimetric measurements of metal adsorption energies, were analyzed with respect to particle size. Their stability was found to increase with particle size below 5000 atoms per particle. Silver nanoparticles of any given size below 1000 atoms had much higher stability (30 to 70 kilojoules per mole of silver atoms) on reduced CeO2(111) than on MgO(100). This effect is the result of the very large adhesion energy (approximately 2.3 joules per square meter) of Ag nanoparticles to reduced CeO2(111), which we found to be a result of strong bonding to both defects and CeO2(111) terraces, apparently localized by lattice strain. These results explain the unusual sinter resistance of late transition metal catalysts when supported on ceria.

The adsorption energies and growth morphology of silver on reduced CeO 2 (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 CeO 2-x (111), with x ) ∼0.1, were grown on a Pt(111) singlecrystal. The AES and ISS signals varied with Ag coverage in a manner indicative of 3D Ag particle growth, with a density of ∼4 × 10 12 particles/cm 2 . 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 CeO 1.9 (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 CeO 2 (111) films between 1-4 nm thick at 300 K and at all Ag coverages.

We present calorimetric measurements of the effect of cluster size on the adsorption enthalpy of carbon monoxide on Pd nanoclusters sized from 120 to 4900 Pd atoms per particle, which were grown in situ on a well-ordered Fe 3 O 4 / Pt͑111͒ film. A substantial decrease in the initial heat of adsorption amounting to about 20-40 kJ mol −1 was observed on the smallest Pd nanoparticles as compared to the larger Pd clusters and the extended Pd͑111͒ single-crystal surface. We discuss this effect in terms of the size-dependent properties of the Pd nanoparticles. DOI: 10.1103/PhysRevB.81.241416 PACS number͑s͒: 68.43.Ϫh, 82.65.ϩr This Rapid Communication addresses the question: how does the heat of chemisorption of a molecule change when comparing a metal single-crystal surface with a supported metal nanoparticle, and how does it depend on particle size? This is an exceptionally important question that lies at the very heart of understanding particle size effects in catalysis. 1 The energetics of interaction of gaseous molecules, particularly carbon monoxide, with well-defined metal nanoparticles were previously addressed indirectly in nonisothermal temperature-programed desorption ͑TPD͒ experiments 2 and in isothermal modulated molecular-beam studies, 3 where the adsorption energies were obtained by modeling the desorption process and analyzing the lifetimes of the adsorbate on the surface. However, these indirect methods did not provide a clear trend in the changes in the adsorption energy with the particles size: whereas the TPD studies found a decrease in the adsorption energy by about 10 kJ mol −1 on the 2.5 nm sized Pd particles as compared to the extended single-crystal surfaces, the kinetic model used for analysis of the molecular-beam experiments predicted a pronounced increase in the adsorption energy by about 35 kJ mol −1 on the particles smaller than 1.5 nm.A strategy to overcome the shortcomings of such indirect methods based on the model assumptions is direct calorimetric measurement of adsorption enthalpies. Recently, two types of calorimeters were developed that allow direct adsorption energy measurements on single-crystal surfaces. 4,5 However, many phenomena inherent to dispersed supported catalysts and technically relevant nanoparticle-based materials cannot be addressed on such simplified model systems since they do not reproduce some properties of realistic surfaces such as different particles sizes or interactions between nanoparticles and their support material. Only a limited amount of calorimetric data is available today on dispersed supported metal powder catalysts, 6,7 suffering, however, from a high degree of inhomogeneity in metal particle size distribution and low support homogeneity. A strategy to surmount this shortcoming was the development of welldefined, single-crystal-based model surfaces, consisting of metal nanoparticles deposited on flat thin oxide films. 8 The structural properties of these model systems such as particle size and shape can be controlled and characterized in grea...

The adsorption of Ca on poly(3-hexylthiophene) (P3HT) has been studied by adsorption microcalorimetry, atomic beam/surface scattering, X-ray photoelectron spectroscopy (XPS), low-energy He(+) ion scattering spectroscopy (LEIS), and first-principles calculations. The sticking probability of Ca on P3HT is initially 0.35 and increases to almost unity by 5 ML. A very high initial heat of adsorption in the first 0.02 ML (625-500 kJ/mol) is attributed to the reaction of Ca with defect sites or residual contamination. Between 0.1 and 0.5 ML, there is a high and nearly constant heat of adsorption of 405 kJ/mol, which we ascribe to Ca reacting with subsurface sulfur atoms from the thiophene rings of the polymer. This is supported by the absence of LEIS signal for Ca and the shift of the S 2p XPS binding energy by -2.8 eV for reacted S atoms. The heat of adsorption decreases above 0.6 ML coverage, reaching the sublimation enthalpy of Ca, 178 kJ/mol, by 4 ML. This is attributed to the formation of Ca nanoparticles and eventually a continuous solid Ca film, on top of the polymer. LEIS and XPS measurements, which show only a slow increase of the signals related to solid Ca, support this model. Incoming Ca atoms are subject to a kinetic competition between diffusing into the polymer to react with subsurface thiophene units versus forming or adding to three-dimensional Ca clusters on the surface. At approximately 1.6 ML Ca coverage, Ca atoms have similar probabilities for either process, with the former dominating at lower coverage. Ultimately about 1.6 ML of Ca (1.2 x 10(15) atoms/cm(2)) reacts with S atoms, corresponding to a reacted depth of approximately 3 nm, or nearly five monomer-unit layers. Density-functional theory calculations confirm that the heat of reaction and the shift of the S 2p signal are consistent with Ca abstracting S from the thiophene rings to form small CaS clusters.