Heterogeneous catalysts consisting of transition metal nanoparticles dispersed across oxide supports are ubiquitous in industrial-scale chemistry and energy technology. Despite their importance, our fundamental understanding of the physical and chemical properties that make these systems catalytically effective is still incomplete. This investigation uses a novel ultrahigh vacuum single-crystal adsorption calorimeter to study the adsorption and adhesion energies of Au atoms and clusters on single-crystal MgO supports as models for real catalyst systems in order to better understand catalyst stability and its relation to deactivation rates through nanoparticle sintering. Because the adhesion energy of Au nanoparticles on MgO(100) has previously been studied using other techniques such as TEM, this investigation also provides the first opportunity to directly compare adhesion energies acquired by SCAC with those acquired by other techniques. Thin films of MgO(100) (~6nm) were grown on Mo(100). Au was vapor-deposited on these films, and the morphology of Au was investigated using x-ray photoelectron spectroscopy and ion scattering spectroscopy at 100 K and 300 K. The average particle density for Au on MgO(100) was found to be nearly an order of magnitude smaller than for Au on CeO2-x(111), which supports previous observations that Au binds weakly to MgO(100) terraces and has a strong tendency to coarsen. The heat of adsorption of Au as a function of particle size was found to increase until it reaches the Au bulk heat of sublimation (ΔHsub = 368 kJ/mol) for particles &gt;6.0 nm in diameter. However, the initial heat of adsorption at low Au coverage was higher than for Au on CeO2-x(111), which was surprising. We speculate that this is due to large particles forming quickly, resulting in a greater contribution from Au-Au bonds (rather than Au-oxide bonds) when compared to Au on CeO2-x(111). Studying this system at 100 K reveals a much lower initial heat of adsorption at low Au coverage and a higher average particle density, indicating the formation of more (weaker) metal-oxide bonds. These results are compared to previous work done in our group examining Ag, Cu, and Pb on MgO(100), as well as to Ag, Cu, and Au on CeO2-x(111). Combining these results with our existing data for group-11 transition metals on CeO2-x(111) and MgO(100) reveals periodic trends in particle-oxide interfacial bonding energies.

Adsorption and adhesion energetics of Au on MgO(100) studied by single crystal adsorption calorimetry: Comparison to Cu and Ag on MgO(100) and Au on CeO2-x(111)

251st National Meeting (2016)

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