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ABSTRACT: We report a surface spectroscopy investigation of metathesis reactions among propene, ethene, 1-butene, 1,3-butadiene, norbornene, and cyclopentylidene on a planar polycrystalline β-Mo2C surface. Surface cyclopentylidene was prepared by exposure of the clean metal carbide to cyclopentanone. Propagator species, surface methylidene, ethylidene, propylidene, propylidiene, and ring-opened norbornene were detected using reflectance absorbance infrared spectroscopy. High-resolution X-ray photoelectron data were obtained for the interaction between propene and surface cyclopentylidene. Vibrational and thermal desorption measurements as a function of the number of exposure cycles were carried out for the reaction with propene. Turnover to yield butene was detected. The carbonyl bond dissociative adsorption of propanal and acrolein was used to prepare propylidene and propylidiene to further identify the metathesis propagator species. Given that molybdenum carbide displays metallic conductance, olefin metathesis at the chemisorption bond may find use in the design of devices in which good electrical contact between an electrode and an organic phase is desired. More generally, it offers the opportunity to use routine surface science techniques to study olefin metathesis on extended surfaces.
07/2009;
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ABSTRACT: Surface alkylidenes can be formed on β-Mo2C through the selective carbonyl bond scission of ketones and aldehydes. Spectroscopic studies show that alkylidene groups remain intact on the carbide surface to above 900 K under ultrahigh vacuum conditions. A rationalization for the anomalously high thermal stability is presented on the basis of surface analysis studies. It is shown that the relatively high reactivity of the clean carbide surface permits both carbonyl bond scission and other less selective processes involving CC and CH bond cleavage. The combined decomposition channels lead to the deposition of excess carbon and high-coordination oxygen, resulting in an inert surface on which alkylidenes are thermally stable. Removal of surface carbon, through CO desorption and carbon diffusion, occurs at high temperatures leading to a newly reactive surface. Alkylidenes trapped at low temperatures on the intrinsically passivated surface can survive until the clean surface is partially restored at high temperatures.
04/2007;
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Kwang Taeg Rim, Mohamed Siaj,
Shengxiong Xiao,
Matthew Myers,
Vincent D Carpentier,
Li Liu,
Chaochin Su,
Michael L Steigerwald,
Mark S Hybertsen,
Peter H McBreen,
George W Flynn,
Colin Nuckolls
Angewandte Chemie International Edition 02/2007; 46(41):7891-5. · 13.45 Impact Factor
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ABSTRACT: A combination of surface spectroscopy and chemical reactivity studies was used to show that alkylidene groups, formed on β-Mo2C through the dissociative adsorption of ketones, are stable under ultrahigh vacuum conditions to anomalously high temperatures. Results are presented for cyclobutylidene, cyclopentylidene, and cyclohexylidene. Reflectance absorbance infrared spectroscopy in the CH2 stretching region, high-resolution X-ray photoemission measurements, labeled oxygen insertion, and metathesis reactions were used to detect surface alkylidenes at or above 900 K. The anomalous thermal stability of alkylidenes on the carbide surface shows that carbenes may be present on the carbide phase of Mo/ZSM-5 catalysts during the methane dehydroaromatization reaction. The latter reaction is typically carried out at approximately 970 K.
01/2007;
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ABSTRACT: Atomic nitrogen and oxygen were deposited on beta-Mo(2)C through dissociative adsorption of NO. Reflectance absorbance infrared spectroscopy (RAIRS), thermal desorption, and synchrotron X-ray photoelectron spectroscopy (XPS) measurements were used to investigate the interplay between atomic nitrogen, carbon, and oxygen in the 400-1250 K region. The combination of the high resolution and high surface sensitivity offered by the synchrotron XPS technique was used to show that atomic nitrogen displaces interstitial carbon onto the carbide surface. Thermal desorption measurements show that the burnoff of the displaced carbon occurs at approximately 890 K. The incorporation of nitrogen into interstitial sites inhibits oxygen dissolution into the bulk. RAIRS spectroscopy was used to identify surface oxo, terminal oxygen, species formed from O(2) and NO on beta-Mo(2)C.
The Journal of Physical Chemistry B 09/2005; 109(32):15376-82. · 3.70 Impact Factor
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ABSTRACT: The known range of chemisorption bonds forms the toolbox for the design of electrical contacts in molecular electronics devices. Double-bond contacts to technologically relevant materials would be attractive for a number of reasons. They are truly single-site, bonding to a single surface atom. They obviate the need for a thiol linkage, and they may be amenable to further modification through olefin-metathesis methodologies. We report olefin-metathesis methods for establishing, varying, and growing thermally stable double-bond contacts to molybdenum carbide, a conducting material.
Science 08/2005; 309(5734):588-90. · 31.20 Impact Factor
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ABSTRACT: The dissociative adsorption of acetaldehyde on beta-Mo2C was studied using reflection absorption infrared spectroscopy. In contrast to what is observed for all metals previously studied, acetaldehyde undergoes selective carbonyl bond scission on the carbide surface. By comparison to calculated spectra, the surface product is identified as an oxo-ethylidene species. The study thus provides the first extended-range infrared spectrum of a propene metathesis initiator or propagator alkylidene. Aldehydes may be formed in the presence of olefins during the induction period of supported metal oxide olefin metathesis catalysts. Hence, the observed dissociative chemisorption of acetaldehyde suggests a possible answer to the question of how initiator sites are formed in heterogeneous olefin metathesis. This question has never been satisfactorily answered. In the proposed mechanism, aldehydes formed during the induction period subsequently react with the catalyst surface to generate alkylidene sites.
Journal of the American Chemical Society 09/2004; 126(31):9514-5. · 9.91 Impact Factor
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Journal of Organometallic Chemistry 691:5497-5504. · 2.38 Impact Factor
