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Research Interests in Surface Chemistry and Heterogeneous CatalysisIf you are interested in providing funding for the following research projects, please visit our benefunder page:
Mechanisms of Surface Hydrogenation ReactionsOur group conducts fundamental studies of the mechanisms of chemical reactions that take place on the surfaces of transition metals. These studies are motivated by a desire to gain a more detailed understanding of the surface chemical reactions that occur in heterogeneous catalysis. Spectroscopic methods are used to identify important surface intermediates and to measure the kinetics of surface chemical reactions. The technique of reflection absorption infrared spectroscopy (RAIRS) 1 has proven to be particularly effective in identifying molecules on surfaces and in distinguishing between adsorbates with subtle differences in structure. We have worked to steadily improve the experimental capabilities of RAIRS 2 and to understand various physical phenomena that influence the spectra. Key insights into surface chemistry can be gained through measurements of reaction kinetics. With its high resolution, RAIRS is exquisitely sensitive to different surface chemical species and by recording spectra as a function of time the rates at which reactants are transformed into products can be measured. Unlike other measurements of surface kinetics that rely on the rate of adsorption from the gas phase of a reactant onto a surface or the rate of desorption of a product from the surface, RAIRS can be used to measure rates while reactants and products remain on the surface. We have used this approach to study the combination of H and N atoms to form NH,3 the dissociation of adsorbed HCN into H and CN,4 and the rate of formation of CNH2 from CN and H,5 all on the Pt(111) surface. Recently we have studied the N + H → NH reaction on the Ru(0001) surface and have found clear evidence that quantum mechanical tunneling plays an important role in this simple hydrogenation reaction.6 Our work on using RAIRS to study the kinetics of surface hydrogenation reactions continues. There is much current interest within the surface chemistry and catalysis communities in studies carried out under ambient pressure conditions. We have constructed an apparatus that allows single crystal surfaces to be prepared and characterized by standard ultrahigh vacuum techniques and then to be transferred to a high pressure cell for in situ characterization with RAIRS while reactions take place in the presence of gases at pressures up to one atmosphere. We have recently used this apparatus to measure the hydrogenation of acetylene. The method relies on the polarization dependence of the infrared radiation to observe only gas phase species or both gas and surface species. Using this method, we were able to simultaneously monitor the surface and gas phase species as C2H2(g) was first converted to C2H4(g) and then to C2H6(g).7 We will continue to use this approach in our studies of surface catalysis. In a new area of research, we are performing experiments on the mechanisms of hydrogenation reactions on two bimetallic surfaces, which other researchers have recently shown to have promising properties for high selectivity. The first surface is an example of a single atom alloy (SAA) and consists of isolated Pd atoms on a Cu(111) surface. Molecular hydrogen dissociates at the Pd sites to produce surface hydrogen atoms that then spillover to Cu sites. The resulting surface promotes low temperature selective hydrogenation reactions. The second surface consists of a pseudomorphic monolayer of Pt on Ru(0001), which binds hydrogen in a way that should also favor selectivity. The surfaces will be prepared under ultra high vacuum conditions according to well documented procedures. The reactions to be studied on these surfaces include the selective hydrogenation of acrolein to vinyl alcohol and of acetylene to ethylene. In each case, the objective is to obtain information on the identity of reaction intermediates and the activation barriers of elementary steps of the overall hydrogenation mechanism. Catalytic Pathways for Heteroatom Removal from Heavy Fossil FuelsA major challenge in utilizing newer petroleum sources including shale oil and tar sands is the need to remove heteroatoms such as sulfur and nitrogen in order to meet ever more stringent environmental standards. The development of new and better catalysts and catalytic processes is essential to meeting this challenge. For this reason, a basic understanding of the surface chemistry associated with the pathways involved in the reactions of key prototypical molecules over well-characterized model catalyts is needed. We have recently begun a new project to test various hypotheses related to the catalytic pathways underlying the oxidative desulfurization of dibenzothiophene, 4-methyl dibenzothiophene, and 4,6-dibenzothiophene over V2O5 thin films grown on a Pd(111) single crystal surface. These molecules represent some of the most difficult organosulfur compounds to remove from heavy fossil fuels using traditional hydro-desulfurization catalyts. The reactions will be studied with the surface sensitive techniques of X-ray photoelectron spectroscopy (XPS), temperature programmed reaction spectroscopy (TPRS), and RAIRS. The XPS experiments will be performed both in our laboratory at UIC using laboratory X-ray sources and at the Advanced Photon Source at nearby Argonne National Laboratory where a newly installed system yields exceptionally high resolution and sensitivity. Among the hypotheses to be tested are that these compounds can be oxidized on the surface to the corresponding sulfones via sulfoxide intermediates and that the reactivity will be lower for the methyl-containing compounds because of a weaker interaction with the surface due to steric hindrance. Surface Structure and Chemistry of the Boron-Rich SolidsThe broad class of materials known as the boron-rich solids constitute a unique and fascinating group of compounds with many current and potential applications.8 They include the metal borides of stoichiometry MBn, where n = 2, 4, 6, 12; solids based on B12 icosahedral units such as boron carbide, B12O2, B12P2, B12As2; as well as compounds of highly unusual stoichiometries such as YB66. Common to all of these solids are extended networks of covalently bonded boron atoms. The bonding within the boron networks is generally described as electron deficient to convey the fact that they possess more bonding orbitals than can be filled by boron electrons. The unusual properties of these compounds make them uniquely well-suited to certain applications, such as the use of LaB6 and CeB6 as thermionic emitters 9, the use of YB66 as an X-ray monochromator,10 and the use of ZrB2(0001) substrates for the epitaxial growth of GaN thin films.11 Thin films of diborides, such as HfB2, have recently been shown to be highly conformal and to possess other attractive properties for various applications.12 Our group has published numerous studies on the surface structure and chemistry of single crystal surfaces of various boron-rich solids including LaB6(100)13-14, YB66(100)15-17, HfB2(0001)18-19, TaB2(0001),20 and ZrB2(0001).21 We recently reviewed surface studies of metal hexaborides.22 Although we are not currently active in this area, work may resume if a new funding source can be found.
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