Abstract
ABSTRACT A series of Rh-based intermetallic compounds supported on silica was prepared and tested in alkene hydrogenation at room temperature. H2 and D2 were used as the hydrogen sources and the kinetic isotope effect (KIE) in hydrogenation was studied. In styrene hydrogenation, the KIE values differed strongly depending on the intermetallic phase, and some intermetallic compounds with Sb and Pb exhibited remarkably high KIE values (>28). An extraordinarily high KIE value of 91, which has never been reported in catalytic reactions at room temperature, was observed particularly for RhPb2/SiO2. RhPb2/SiO2 also showed high KIE values in the hydrogenation of other unsaturated hydrocarbons such as phenylacetylene and cyclohexene. The density functional theory calculation focused on the surface diffusion of hydrogen suggested no contribution of the quantum tunneling effect to the high KIE values observed. A kinetic study revealed that the dissociative adsorption of H2 (D2) was the rate-determining step in the styrene hydrogenation over RhPb2/SiO2. We propose that the large KIE originates from the quantum tunneling occurring at the hydrogen adsorption process with the aid of the specific surface structure of the intermetallic compound and adsorbate alkene.
Highlights
Intermetallic compounds are stoichiometric compounds consisting of two or more metal elements with considerably different electronic characters or atomic radii [1]
The diffractions assigned to the desired intermetallic phase were clearly observed for each catalyst (RhGe [24], RhSn2 [25], RhSb2 [25], RhGa [26], RhPb [27], RhSb [28], RhPb2 [29], RhIn [30], RhBi [31], and Rh-based catalysts (Rh) [26], see Figure S1 for wide range X-ray diffraction (XRD) patterns of RhGa and RhIn), suggesting that the nanoparticles of the Rh-based intermetallic compounds were formed on the SiO2 support
We have prepared a series of Rh-based intermetallic compounds supported on silica and investigated their catalytic properties in the hydrogenation of unsaturated hydrocarbons
Summary
Intermetallic compounds are stoichiometric compounds consisting of two or more metal elements with considerably different electronic characters or atomic radii (apart from each other in the periodic table) [1]. Intermetallic compounds have recently attracted increasing attention as promising inorganic materials for heterogeneous catalysts because of their unique surface characteristics [2,3,4,5,6]. During the course of our attempt to discover further unique catalytic behaviors of intermetallic compounds, we observed that some Rh-based intermetallic compounds show an extraordinarily large kinetic isotope effect (KIE) on alkene hydrogenation (kH/kD = 91). This value is remarkably higher than those typically observed in organic reactions with deuteration [10]. Rather high KIE values (>100) have been observed at cryogenic temperatures because of the quantum tunneling effect, to the best of our knowledge, no catalytic reaction showing such a high KIE value at room temperature has been
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