Atomic-Scale Studies of Heterogeneous Catalysis, Substrate-Mediated Interactions and Interface Structures
Open Access
- Graduate Program:
- Chemistry
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- October 01, 2010
- Committee Members:
- Paul S Weiss, Dissertation Advisor
- Paul S Weiss, Committee Chair
- Thomas E Mallouk, Committee Member
- Benjamin James Lear, Committee Member
- Seong H Kim, Committee Member
- Michael V Pishko, Committee Member
- Keywords:
- substrate-mediated interactions
- low temperature
- STM
- catalysis
- spectroscopy
- Abstract:
- The direct observation and measurement of the chemical and physical properties of nanoscale structures on noble metal surfaces have been accomplished utilizing an ultrastable low temperature (4 K) extreme high vacuum (XHV) scanning tunneling microscope (STM). The ultrastability of the STM enables the measurement of single molecules with great precision. These high-resolution microscopic and spectroscopic studies are essential in developing benchmarks for the design of novel catalytic systems, understanding the communicative behavior of adsorbates through substrate-mediated interactions (SMIs), and probing the buried interface structures of monolayers. Toward understanding heterogeneous catalysis at the atomic scale, studies of the subsurface hydride (SSH) of Pd, the key material and reactant hypothesized to be critical both to hydrogenation reactions and metal embrittlement, are presented. The SSH species has been created directly for use as a chemical reagent, in preparation for tunneling electron-induced reactions that can be monitored by recording the discrete spectroscopic signatures of intermediates and products isolated at 4 K. Additionally, Pd surface-bound atomic hydrogen and deuterium (as well as vacancies at higher coverages) exhibit diffusion even at low temperature, offering insight into the behavior of these species prior to reaction. Because molecular thiophene (and its heterocyclic analogues) poses a significant challenge to desulfurization due to its stabilizing ƒÎ electrons, it is an ideal candidate for investigating the mechanism of hydrodesulfurization (HDS). The chemical structure and adsorption of thiophene on Pd{111} and Pd{110} are presented. Through topographic measurements and differential conductance (<I>d</I>I/<I>d</I>V) imaging, we show a dramatic difference between the adsorption of thiophene on Pd{111} and that on Pd{110}. The observation of the electronic structure of thiophene on Pd reveals that the surface state may play a role in molecular adsorption via SMIs, however, the surface state dispersion for Pd suggests that mediation may come from bulk-state or image-state electrons. Further, these techniques will be used both to manipulate single adsorbates into proximity of SSH and to induce reaction in situ. We have studied the SSH of Pd{111} and the subsurface deuteride (SSD) of Pd{110} toward understanding chemical reactions with these novel reactants. Deposition of deuterium into the bulk and interstitial subsurface sites of Pd{110} causes a (1 ~ 2) surface reconstruction. We have found that while SSH causes unidirectional lattice distortion for Pd{111}, SSD formation results in the growth of facets along the substrate direction, reverting to the (1 ~ 1) phase. Single-molecule vibrational spectroscopy of thiophene adsorbates on both flat and faceted regions allude to distinctly different chemical identities, suggesting that we are observing the reaction intermediates of the HDS of thiophene. Additionally, the controversy of STM image interpretation of self-assembled monolayers (SAMs) has been resolved using a technique that images both the chemically bound head-groups and exposed tail-groups in bi-component alkanethiolate (ALK) SAMs on Au{111} simultaneously, with molecular resolution. The polar tilt and azimuthal angles of single molecules within monolayers have been measured and demonstrate that ordered domains with different superstructures also have varied buried sulfur head-group structures.