Heterogeneous catalysis involves chemical reactions that take place at the solid-gas or solid-liquid interface. An example is the catalytic converter in an automobile in which toxic gases in the engine exhaust are transformed into harmless substances. The key reactions take place at the surface of the metal particles that constitute the active component of the catalyst. Hundreds of different catalytic reactions are of great importance to problems of pollution control and chemical manufacture. Physical chemists throughout the world are actively seeking to establish the mechanistic details of surface reactions of relevance to heterogeneous catalysis as one step towards producing better and more efficient catalysts. REU students will have the opportunity to work in a laboratory equipped with a variety of advanced surface science instruments on a project designed to gain a better understanding of chemical reactions that take place on solid surfaces. The projects will be in one of the two following areas.
Spectroscopic Characterization of Surface Intermediates
In order to establish the mechanisms of catalytic reactions, techniques are needed to identify and characterize molecular intermediates that form on the surfaces of catalytic metals. For example, platinum, palladium and rhodium are the metals most commonly used in automotive catalysts. This area of research focuses on the use of reflection absorption infrared spectroscopy to measure the IR spectrum of molecular species present on metal surfaces at coverages of one monolayer or less. From the measured spectra, the identity and structure of the surface species are determined.An important tool used in interpreting the spectra is density functional theory, which allows theoretical IR spectra to be calculated for possible intermediates. As one example, RAIRS has been used to identify the surface intermediates involved in the reaction of NH3 with O2 on a Pt(111) surface. The oxidation of ammonia over platinum catalysts is a key step in the industrial production of nitric acid. The spectra show that the reaction occurs by way of an NH3-O2 complex and that the NH diatomic molecule is an important and stable intermediate in the overall reaction. The same methodology can be used to study a wide range of important reactions catalyzed by metal surfaces.
Reversible Dehydrogenation of Boron Nano-Clusters
Many people believe that hydrogen will someday replace gasoline as the fuel used in automobiles. Cars powered by hydrogen emit no pollution or greenhouse gases as the only product of the oxidation is harmless water vapor. Although hydrogen is not a primary energy source, because it requires more energy to produce than is released when it is oxidized, it is an attractive way to store energy. If renewable energy such as solar or wind power is used to generate hydrogen from water, then a sustainable energy source would be available for transportation that will not contribute to global warming. A key challenge in using hydrogen to power automobiles is finding a method to store it at high densities. One of the most promising methods of storing hydrogen is in the form of complex hydrides. The goal is to chemically bind hydrogen to another element in a reversible fashion in a compound with a hydrogen content of at least 10 % by weight. A variety of boron-hydrogen compounds known as boranes based on three-dimensional boron cage structures exist that may prove useful in this respect. If the appropriate catalyst can be found, it should be possible to dehydrogenate certain boranes while preserving the basic boron-cage structure. If so, it should be possible to rehydrogenate the cage to reform the original compound. This project will involve basic surface science studies on the mechanism of hydrogenation and dehydrogenation reactions of various boranes. Hydrogenation reactions of other boron nanostructures will also be explored.
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