Ultrahigh Vacuum Studies of Surface Reactions on the Pd(111) Surface

Surface science focuses on understanding the complex interactions that occur at the interfaces of materials. Various experimental techniques, like RAIRS, TPD, LEED, and XPS, are used to study the surface’s structures and reactivity. In recent years, with advancement in computing power, the synergy between surface science and computational chemistry has been very pivotal. Computational methods, generally DFT, are used frequently to interpret experimental data and to understand the adsorption and surface reactivity in the catalytic environment. The ideal condition to study the model catalyst is under ultrahigh vacuum and low temperatures. All the work reported in this thesis has been performed by incorporating experimental and computational methods. The interaction of acetylene with the Pd(111) surface has been studied with RAIRS and TPD. The motivation for this work is longstanding discrepancies in the literature regarding the identity of the stable surface species that forms at room temperature from acetylene. Some reports indicate vinylidene, CCH2, while other reports identify the intermediate as ethylidyne, CCH3. The results provide definitive evidence that acetylene first isomerizes to vinylidene, which then is readily hydrogenated to ethylidyne. Identification of vinylidene relies on a good match between the experimental spectra and simulated spectra based on DFT calculations for vinylidene and ethylidyne bound to a Pd19 cluster model of the Pd(111) surface. The next project discusses the interaction of 1-propanol with the Ag(111) surface. Knowledge of the structure of molecules adsorbed on metal surfaces is crucial for understanding the mechanisms of surface chemical reactions. For small molecules that exist in only one structural form in the gas phase, specifying their structure on surfaces usually consists of identifying the adsorption site and orientation of one or two molecular axes with respect to the surface normal. For example, CO usually adsorbs at either atop, two-fold bridge, or three-fold hollow sites with the CO axis perpendicular to the surface. For 1-propanol, CH3CH2CH2OH, low rotational barriers about the two C–C bonds and the C–O bond leads to five unique conformers with very similar energies such that at room temperature the gas phase consists of an equilibrium mixture of conformers. The relative stability of five different conformers of 1-propanol adsorbed on the Ag(111) surface was determined with density functional theory calculations. The calculations were also used to simulate reflection absorption infrared spectra (RAIRS) for comparison to experimental spectra. The experimental RAIR spectra were obtained by adsorbing multilayers of 1-propanol at 90 K and then annealing the surface to successively higher temperatures to desorb the multilayer and produce the most stable 1-propanol monolayer structure on Ag(111). The multilayer spectrum features a broad O-H stretch characteristic of hydrogen bonding between the molecules as well as broad and complex peaks in the C–H stretch and C–H deformation region. After annealing to 180 K, the O–H stretch peak disappears, and the remaining peaks are unusually sharp. Comparison of the experimental and simulated spectra indicates that 1-propanol adsorbs as only one of the five conformers. The final work discusses the adsorption and decomposition of Zr(BH4)4 on Pd(111). Zr(BH4)4 is a volatile compound that has been widely used as a single-source precursor to grow carbon-free thin films of zirconium diboride by chemical vapor deposition (CVD). Palladium was chosen as the substrate as it is a good dehydrogenation catalyst. Zirconium borohydride, Zr(BH4)4, molecularly adsorbs on Pd(111) at 90 K to yield an infrared spectrum nearly identical to that of solid Zr(BH4)4. The molecule remains intact after annealing the surface to 150 K, but further annealing to 200 K leads to decomposition to form a surface intermediate that retains terminal and bridging B-H bonds. While XPS results are inconclusive as to whether annealing a condensed Zr(BH4)4 layer to higher temperatures leads to formation of a ZrB2 layer, exposure of the Pd(111) surface at 773 K does produce a zirconium diboride film.