Direct measurements of chemical bonding at solid surfaces using a unique calorimetric method: Towards understanding surface chemistry in energy technologies.

摘要

Measuring the heat released when gas phase species adsorb onto surfaces provides essential information about the energies of surface species and the reactions they undergo. Here, heats of adsorption of technologically-interesting surface species were measured using a unique microcalorimetric technique in ultrahigh vacuum. Specifically, systems were studied which are relevant to understanding and improving transition metal catalysts and organic electronics. Metal adsorption energies were measured which elucidate metal-to-oxide and metal-to-polymer interfacial binding, and molecule adsorption energies were measured to understand how catalyst structure influences the energies of adsorbed reaction intermediates.;Oxide-supported metal nanoparticles form the basis for many industrial catalysts. Nanoparticle activity, selectivity and resistance to sintering can depend strongly on particle size, oxide support, and defects on the oxide. To investigate the dependence of catalytic properties on oxide surface defects, defects were introduced on MgO(100) and CeO2(111), and their affect on the adsorption energy of metal atoms and the energy of supported nanoparticles was measured. These measurements help to explain why transition metal catalysts sinter more slowly and maintain smaller particles when supported on CeO 2 compared to other oxides, and how surface defects influence nanoparticle formation and film growth on oxides. The effect of nanoparticle size on the adsorption energy of CO on different-sized Pd nanoparticles on Fe3O 4(111) was measured, providing the first direct evidence that the heat of adsorption of CO decreases with decreasing Pd nanoparticle size. Knowledge of the direction and magnitude of particle size effects is necessary for improving existing catalysts and designing new ones.;The metal/polymer interface is important because it impacts charge injection, extraction, and transport in organic electronics. Large-scale energy production using polymer photovoltaics is currently unfeasible due in part to their low efficiency and short lifetimes. Polymer degradation at the interface with the metal electrode is believed to impact device efficiency and lifetime. Calcium adsorption on poly(3-hexylthiophene) was investigated because it is one of the most efficient electrode/polymer combinations. The results were striking: calcium diffused nanometers into the polymer and reacted with the polymer backbone. A method to suppress diffusion was demonstrated, which may lead to improved devices.