Size-Dependent Adsorption and Adhesion Energetics of Ag Nanoparticles on Graphene Films on Ni(111) by Calorimetry

Interest in the use of carbon supports for late transition metal nanopartide catalysts has expanded rapidly due to the increasing importance of electrocatalysts for dean energy and environmental technologies and the use and storage of renewable electricity. Compared to oxide supports, almost nothing is known about the effect of metal nanoparticle size on the energies of the metal atoms within carbon-supported nanopartides, yet these energies are crucial for understanding their surface reactivity and sintering kinetics. Here, the growth morphology and adsorption energetics of vapor-deposited Ag onto dean graphene/Ni(111) surfaces have been studied using a combination of single-crystal adsorption calorimetry (SCAC) and He+ low-energy ion scattering (LEIS). The differential heat of Ag adsorption is 207 kJ/mol for making similar to 30 atom Ag particles on graphene terraces at 100 K and 16 kJ/mol higher for making similar to 9 atom Ag clusters at defect sites at the same temperature. The heat of adsorption increases rapidly with Ag coverage as 3D Ag nanopartides nucleate and grow in size, asymptotically reaching within 5 kJ/mol of the bulk Ag sublimation enthalpy (285 kJ/mol) by 2 ML. The heats of adsorption and Ag nanoparticle densities from LEIS (similar to 10(16)/m(2)) were combined to provide the Ag/graphene adhesion energy (E-adh = 1.8 J/m(2) in the large-partide limit) and the Ag chemical potential (mu) versus effective particle diameter (D). The Ag chemical potential was well-fitted by mu(D) = (3 gamma(v/M) - E-adh)(1 + (1.5 nm)/D)(2V(m)/D), where gamma(v)(/M) is the surface energy of bulk Ag and V-m is its molar volume. The same equation is known to fit similar data for late transition metals on clean surfaces of metal oxide single crystals. The adhesion energy of Ag measured here on graphene falls within the wide range measured for Ag on those oxide surfaces and is almost as large as on the oxide that binds Ag particles most strongly, namely CeO2(111), which is well-known to be very effective at resisting catalyst deactivation by metal sintering. These results imply that carbon supports will be effective at resisting sintering and that Ag particles smaller than 6 nm on graphene will bind small adsorbed reaction intermediates more weakly than supports with weaker adhesion to Ag, like MgO(100).