Current Friend Lab Research

-Molecular Interactions with Water on Metal Oxide Surfaces

Ryan Quiller

Reactions of NOx, SO2, O3, and oxygenated hydrocarbons on oxide surfaces in high relative humidity are important in areas such as atmospheric chemistry and environmental catalysis. We are investigating the reactivity of metal oxides such as rutile TiO2 in the presence of water. We hope to gain further insight into the effect of OH and H2O on gas-oxide surface reaction rates, mechanisms, and products. Also, we are studying how these molecular interactions are affected by materials properties of the metal oxide such as terrace widths and defects. Our research involves reflection absorption infrared spectroscopy, temperature programmed reaction spectroscopy, low energy electron diffraction, and Auger electron spectroscopy.

-Modeling the Dynamics of Surfaces Upon Absorption of Atoms and Molecules

Thomas A. Baker and collaboration with co-advisor Efthimios Kaxiras

Surfaces are not perfect static systems. In the past, both experimental and theoretical groups have considered a surface to be static: molecules absorb on top of an unreconstructed surface. However, this picture will not suffice anymore with more and more examples of systems where surface atoms are released from the bulk and incorporated into the absorbed molecules. For example, the absorption of atomic oxygen on Au(111) produces a rough surface with oxygen incorporated in gold adatoms that form islands on the surface:

Understanding how surface atoms are released, their dynamics, and how they interact with absorbed molecules is both fundamentally important to improve the understanding of the interaction between a surface and an absorbed species and practically important so that these systems (which often have unique chemical and physical properties) can be employed in useful applications. Density functional theory (DFT) is being used to first get an understanding of the interaction of absorbed molecules with the surface and their ability to create vacancies:

In the future, molecular dynamics and Monte Carlo will be used to better understand the dynamics of these systems at a larger length scale.

-The Effect of Water on Reactions over Oxide Surfaces

Lauren Benz, Ryan Quiller, and Meaghan Colling

The study of the interaction of metal oxides with atmospheric constituents is an area which has both practical and fundamental significance. Our current work involves the investigation of the effect of water on reactions of volatile organic compounds and other environmentally relevant species such as oxygen and ozone over a model oxide substrate, namely, titanium dioxide. Titanium dioxide is not only a natural oxide present in the earth’s crust, but it is a major ingredient in paints and surface coatings, making it a suitable model for this research. Temperature programmed reaction spectroscopy will be the main tool employed here. Once the model substrate is investigated in a controlled, ultra-high vacuum environment, we plan to explore reactivity of the single crystalline substrate in an ambient pressure, high humidity environment.

-Using Au(111) to add NH to olefins

Xingyi Deng and Thomas A. Baker

Gold-based heterogeneous catalysts have been shown to have a surprising potential as selective catalysts for redox processes. Such catalysts have been investigated for catalytic alcohol oxidation, the direct synthesis of hydrogen peroxide, low temperature CO oxidation, and olefin epoxidation. These systems are of interest because the low temperature at which they operate and their high selectivity have potential for substantial positive impact on the environment and economy. For example, low-temperature CO oxidation using gold has the potential to overcome the cold-startup problem in automotive pollution control, since the Pt- or Pd-based catalyst currently used are inactive below 200C. Hence, a substantial amount of effort has been directed to further improve the performance of heterogeneous gold catalysts and to understand the origin of their catalytic activity.

In spite of the rapidly increasing number of studies of Au catalysis, the entire focus has been on exploring known oxidation reactions. Because of the importance in organic synthesis as building blocks and potential in pharmaceuticals, we have investigated the possibility of using Au to promote aziridine synthesis. Previously, we have shown that a structurally well-defined single crystal surface of gold, Au(111), promotes oxidation reactions that also occur on high-surface area catalysts at higher pressures once oxygen has been adsorbed on the surface (19). This previous work clearly establishes that Au(111) is a good model for understanding molecular-level details of Au-based oxidation catalysis. Hence, reactions that occur on Au(111) should serve as a guide for the type of reactions that may be induced by heterogeneous gold catalysts.

-Formation of Carbon-heteroatom Bonds from Olefins on the Surfaces of Noble Metals

Xiaoying Liu

Olefins are one of the primary starting materials in industrial synthesis and polymer and material sciences, and the fact that they exhibit diverse reactivities toward a set of transformations makes them a class of fundamental reactants. we are interested in the reaction mechanisms of the coupling between olefins and other small building blocks such as O- and N- containing units to form carbon-heteroatom bonds. In our studies, small units, for example O, NH, NR and CR2, are first prepared on the surfaces of noble metals including gold and silver. They are then coupled with olefins to form a series of products as a result of cycloaddition to the C=C double bond and/or nucleophilic attack of the carbon atoms. Two groups of olefins are studied, namely non-allylic and allylic olefins depending on if they have allylic hydrogens. They show different reaction activities as allylic hydrogens can also be activated in these reactions.

Temperature-programmed reaction spectra (TPRS) are used to identify the gaseous reaction products as well as to analyze the reaction kinetics. High-resolution electron energy loss spectroscopy (HREELS), in conjunction with Auger electron spectroscopy (AES) is used to characterize the structures of the reaction intermediates. These studies on metal surfaces provide rich information for understanding the mechanisms of these reactions and predicting the reactivities of the functionalization of olefins.

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-Atomistic Studies of Reactions on Surfaces

Ling Zhou and Robert J. Madix

The overall objective of our research is to understand the effects of spatial organization on the reactivity of adsorbed layers on metal surfaces. This is one of the frontier areas in the emerging field of nanotechnology, as it related directly to the design of heterogeneous catalysts using metal crystals of nanometer dimensions. We are investigating (1) the incorporation of "added" metal atoms to the structures of adsorbates and reaction intermediates on a wide range of catalytic metal surfaces to examine the generality of this surface phenomenon; (2) the effects of complex promoter combinations on the surface structure and reactivity of model silver surfaces chemistry for oxidation; (3) the surface processes involved in two dimensional kinetic explosions. In these studies we coordinate the scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and temperature-programmed reaction spectroscopy (TPRS) measurements.