Energetics of Au Adsorption and Film Growth on Pt(111) by Single-Crystal Adsorption Calorimetry

Abstract

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 single-crystal surface of the other as measured using temperature-programmed 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 high-coverage 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.