Bimetallic catalysts are an important class of heterogeneous catalysts with catalytic properties distinct from either of their bulk metal constituents. The structural, electronic, chemisorption and catalytic properties of bimetallic surfaces have been widely studied, and many measurements have been made of the adsorption energies of one metal adsorbed on a single-crystal surface of another using temperature programmed adsorption (TPD) to measure its desorption energy.  However, TPD only works for systems where the metals are immiscible.  For bimetallic systems which form an alloy or intermetallic compound, TPD generally fails, since the adsorbed metal penetrates into the bulk upon heating, to make that alloy or compound, before it desorbs. For such systems, the metal-on-metal adsorption energy is unknown from experiments, but has often been calculated since these adlayers often have interesting properties.  We report here the first experimental measurement of the adsorption energy of an adlayer of one metal on another for any bimetallic system where the metals like to alloy. Two such systems that are particularly interesting catalytically are Au and Cu allows with Pt.  Employing a novel Single Crystal Adsorption Calorimeter technique, the adsorption energies of Au and Cu atoms onto Pt(111) were measured as a function of coverage at 300 K. The heat of adsorption as a function of Au adlayer thickness on Pt(111) is compared to the established layer-by-layer growth mechanism. The first monolayer (ML) of Au grows pseudomophically with the Pt(111) surface, with a heat of 395 kJ/mol, nearly 30 kJ/mol greater than the bulk heat of sublimation of Au (368 kJ/mol). Further layer growth continues with a slightly elevated heat until ~ 3ML where Au assumes its ideal Au(111) lattice spacing and the bulk heat of sublimation is reached. The initial heat of Cu adsorption onto Pt(111) is also 30 kJ/mol larger than the bulk heat of Cu sublimation (337.4 kJ/mol) however, while the first ML growth is also pseudomorphic with the Pt(111) surface, the heat rapidly decreases due to the effects of island-island repulsions.

Energetics of Au and Cu adsorption and film growth on Pt(111) measured by single-crystal adsorption calorimetry

251st National Meeting (2016)

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