AVS 60th International Symposium and Exhibition
    Surface Science Tuesday Sessions
       Session SS-TuA

Paper SS-TuA8
Calorimetric Measurement of Adsorption and Adhesion Energies of Cu on Pt(111)

Tuesday, October 29, 2013, 4:20 pm, Room 201 A

Session: Metals and Alloys: Structure, Reactivity & Catalysis
Presenter: S.L. Hemmingson, University of Washington
Authors: S.L. Hemmingson, University of Washington
T.E. James, University of Washington
J.R.V. Sellers, University of Washington
C.T. Campbell, University of Washington
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Bimetallic catalysts are an important category of industrial catalysts that exhibit unique electronic and chemical properties from their parent metals. Pt-based bimetallic catalysts are the most commonly studied and well-characterized bimetallic systems, with many providing enhanced selectivity, activity, and stability for industrially significant reactions such as hydrogenation/dehydrogenation, CO oxidation, and the water-gas shift reaction. The modifications in the catalytic properties of Pt bimetallics are most significant when the admetal coverage is less than a single monolayer. Yet little is known about the strength of metal-metal bonds in some bimetallic catalysts because indirect energy measurement techniques such as temperature programmed desorption cannot be employed. This study employs a novel ultrahigh vacuum Single Crystal Adsorption Calorimeter (SCAC) to directly measure the adsorption energy of Cu overlayers on Pt(111) at 300 K. A pulsed and collimated Cu atomic beam is generated using an electron beam-heated source and deposited onto a Pt(111) single crystal surface. The measured heats of adsorption as a function of total Cu coverage were interpreted with respect to the known layer-by-layer growth mechanism of Cu overlayers on Pt(111). Two distinct Cu coverage regimes of heat of adsorption have been observed. From 0 to just above one monolayer (ML), the heat decreases from 357.4 kJ/mol to 334.5 kJ/mol where Cu grows pseudomorphically on Pt(111). The high initial heat of adsorption indicates a strong Cu-Pt interaction for isolated Cu atoms which are stronger than the Cu-Cu metallic bond. As Cu coverage increases the heats decrease due to repulsive interactions between neighboring Cu atoms or lattice strain buildup. From 1.3 and 3 ML, where Cu grows layer-by-layer with the Cu(111) lattice spacing, the heats of adsorption increases slowly to the bulk heat of sublimation of Cu, 337.4 kJ/mol, due to lattice mismatch between the first Cu monolayer and the subsequent monolayers. By 3 ML no further lattice strain is felt by additional Cu. This represents the first ever direct measurements of late transition metal bimetallic adsorption energies.