Reaction Mechanisms on Metal Surfaces

Our 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 RAIRS2 and to understand various physical phenomena that influence the spectra.

In recent years we have focused on the surface chemistry associated with carbon-nitrogen bonds. This chemistry is related to industrially important catalytic processes such as the synthesis of HCN from ammonia and methane over Pt catalysts3 and the hydrogenation of adiponitrile to 1,6-diaminohexane.4 We have shown than CN can be formed on Pt(111) through the reaction of surface carbon and nitrogen atoms 5. In the course of this work, we have explored the preparation and hydrogenation properties of N atoms on the Pt(111) surface.6,7 We have carried-out a RAIRS study of the kinetics of NH formation on Pt(111) and have compared the experimentally determined activation energies to ones calculated using density functional theory.8 The theoretical part of the work was carried out through a collaboration with Professor Randall J. Meyer, of the UIC Chemical Engineering Department. Other recent work includes the characterization of C2 molecules on Pt(111)9 and the thermal decomposition chemistry of acetylene on Pt(111).10

A surface chemical reaction that we characterized with RAIRS several years ago is the N-protonation of methylisocyanide to produce a stable methylaminocarbyne molecule.11,12 Recently, through a collaboration with the Surface Chemistry Group at RIKEN in Wakoshi, Japan, this reaction was studied at the single-molecule level using a low temperature scanning tunneling microscope. We were able to observe individual molecules of methylisocyanide before and after their hydrogenation to methylaminocarbyne. A pulse from the STM tip led to the deprotonation of the methylaminocarbyne to produce the starting molecule, methylisocyanide. Our work thus demonstrating a reversible chemical cycle for this system was published in Science.13 This achievement has implications for molecular electronics, and was highlighted in C&E News14, and was even mentioned in the Chicago Tribune on July 16, 2007. Our work in this area is currently funded by the National Science Foundation.

Spectroscopic Studies of Boron-Based Hydrogen Storage Materials

          Research into alternative energy sources is vitally important to achieve a smooth transition to a post-petroleum world.  One compelling vision is to replace gasoline internal combustion engines with automobiles powered by hydrogen fuel cells.  A major challenge that must be met before that goal can be reached is the on-board storage of hydrogen at sufficient densities to achieve a performance comparable to today’s automobiles.  Complex chemical hydrides are seen as one of the most promising ways of storing hydrogen at the required high densities and only hydrides of elements of low atomic number are likely to meet the challenge. Boron is second only to carbon in the rich variety of compounds formed with hydrogen, and boron hydrides and related compounds are being intensely studied as potential hydrogen storage materials.  Among the materials being explored are boranes and carboranes such as decaborane (B10H14) and 1,2-dicarbadodecaborane (C2B10H12), borohydrides such as LiBH4 and Ca(BH4)2 as well as compounds containing boron, nitrogen, and hydrogen, such as ammonia borane (H3N:BH3), which has an impressive hydrogen weight percentage of 19.0.  There is a great deal of interest in the use of catalysts to promote the release and uptake of hydrogen in these compounds.  Our group is using surface science methods such as RAIRS to understand the catalytic dehydrogenation chemistry of compounds such as B10H14 and C2B10H12 on transition metal surfaces.  Our work in this area is currently funded by a grant from the U. S. Department of Energy. 

 Surface Processes in the Epitaxial Growth of Cadmium Telluride on Silicon Surfaces

 A better understanding of surface processes underlying the epitaxial growth of one semiconductor on another is of crucial importance for optimizing the properties of the materials obtained.  Our group has been using surface science techniques, including scanning tunneling microscopy, to study the atomic scale processes that underlie the growth of II-VI semiconductors such as ZnTe, CdTe, and HgCdTe on the rarely studied high-index silicon substrates, such as Si(112).  This work is motivated by the important technological challenge of integrating the best infrared detector materials with large, high quality silicon wafers.  The UIC Microphysics Laboratory pioneered the successful growth of II-VI compounds on silicon substrates by molecular beam epitaxy (MBE) and has long been the leading academic research center in this field.  However, further advances in the quality and efficiency of the growth process is needed and it is widely recognized that a better understanding of the atomic structure of the substrates and of the atomic scale processes involved are urgently needed.  In the MBE studies it has been found empirically that a monolayer of arsenic deposited onto the Si(112) surface improves the quality of the subsequently deposited HgCdTe layer.  However, the underlying reasons for this are not understood.  Our group has used STM to characterize the way an arsenic layer alters the atomic-scale structure of the Si(112) surface15-17.  The insights thus gained should lead to improved procedures for the growth of HgCdTe on Si(112).  More importantly, the accomplishments to date demonstrate the value of an interdisciplinary effort combining basic studies of the atomic structure of surfaces with more technologically oriented efforts to grow better HgCdTe thin films for infrared focal plane array applications.  Work in this area has been funded by the Army Research Office but is currently not supported.

Surface Structure and Chemistry of the Boron-Rich Solids

          The broad class of materials known as the boron-rich solids constitute a unique and fascinating group of compounds with many current and potential applications.18  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 19 or the use of YB66 as an X-ray monochromator.20  The transition metal diborides share many physical properties with metal carbides and nitrides, such as very high hardness, very high melting points, and high metallic electrical conductivity.  In fact, many of the diborides are better electrical conductors than the parent metal. Thin films of diborides, such as HfB2, have recently been shown to be highly conformal and to possess other attractive properties for various applications.21  Our group has published numerous studies on the surface structure and chemistry of single crystal surfaces of various boron-rich solids including LaB6(100)22,23, YB66(100)24-26, HfB2(0001)27,28, and TaB2(0001).29  Although we are not currently active in this area, work may resume if a new funding source can be found. 

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