Adsorption and adhesion energies of metal films and nanoparticles studied by adsorption calorimetry: Understanding catalytic systems.

摘要

Metal nanoparticles dispersed across solid surfaces form the basis of many important technologies such as heterogeneous catalysts, electrocatalysts, chemical sensors, microelectronics, and fuel cells. Understanding energetics of chemical bonding between the metal and oxide in these systems is important for the development of more efficient devices. First, in Chapter 2, this dissertations discusses a new, ultrahigh vacuum single crystal adsorption calorimeter which is used to directly measure metal adsorption and adhesion energies to model catalytic surfaces from 77-350 K. Some of the key instrumental improvements over previous designs include the capability of real-time metal atom flux monitoring and a decreased thermal radiation contribution to the heat signal. Next, in chapter 3, an improved data analysis method to determine average particle size and number density from low energy ion scattering spectroscopy (LEIS) measurements of nanoparticles that grow with the shape of hemispherical caps is discussed and validated. A correction is applied for the case when nanoparticles cause substrate shadowing due to source ion incident and detection angles being non-normal to the surface. The model was demonstrated for Cu growth on slightly reduced CeO2(111) where it improved the fit ~3-fold.;In Chapters 4 and 5, the adsorption energy and growth morphology of vapor deposited copper atoms onto slightly reduced CeO2(111) was measured at 100 and 300 K. Copper was determined to grow as three-dimensional particles with preferential adsorption to stoichiometric ceria sites, opposite of what has been observed for other metals such as Ag, Au and Pt on ceria. An important result was the measurement of copper atom chemical potentials starting from single copper atoms up to large nanoparticles which provides unique insight into the increased reactivity of the small aggregates and their propensity to sinter. In Chapter 6, gold adsorption energies onto slightly reduced ceria was also measured. Like copper, gold grows as hemispherical caps on ceria, but with a smaller number density for a given temperature and extent of ceria reduction. Gold also adsorbs more strongly to reduced ceria sites than to stoichiometric sites. The adhesion energy between copper, silver, and gold nanoparticles and slightly reduced ceria was compared to previous adhesion energy trends discovered by our group. Adhesion energy of metals onto well-defined oxides adhere more strongly to ceria than MgO, and scales with the adsorbed metal's heat of sublimation minus the heat of formation of the its most stable oxide, providing a method to predict adhesion energies of metals to oxides. Lastly, in Chapter 7, the adsorption and adhesion energy of 2D copper overlayers on Pt(111) was measured by calorimetry. The adsorption energy of copper atoms in each layer was used to explain the thermodynamic driving force of copper to form the quasi-pseudomorphic layer-by-layer structure.;These studies provide new insights into interfacial chemical bonding and provide important benchmarks to test new density functional theory calculations. The results will aid in the rational design of more efficient catalysts. Future aims and conclusions of this work are presented in Chapter 8.