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Title:Characterization of Alumina-Supported Molybdenum-Iridium Clusters (x-Ray, Spectroscopy)
Author(s):Uchiyama, Winston Sei
Department / Program:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Chemistry, Inorganic
Abstract:Heteronuclear organometallic compounds have recently gained attention as precursors in heterogeneous catalysis. They have the potential of controlling surface metal particle size and stoichiometry. This may allow one to more efficiently study the role of components in heterogenous catalysts and perhaps effectively create important catalytic systems.
The mixed-metal compounds ((eta)('5)-C(,5)H(,5))MoIr(,3)(CO)(,11) and ((eta)('5)-C(,5)H(,5))(,2)Mo(,2)Ir(,2)(CO)(,10) are supported on partially dehydroxylated (gamma)-alumina and decomposed by heating to 500(DEGREES)C in a flow of hydrogen. They are characterized and compared with various mixtures of alumina-supported Ir(,4)(CO)(,12) and {((eta)('5)-C(,5)H(,5))Mo(CO)(,3)}(,2) treated in the same manner. Characterization includes reactivity and spectroscopy.
Reactivity of surface metal atoms is monitored by chemisorption, reactive thermal desorption and catalysis. The chemisorption measurements indicate that the metals are well dispersed on the alumina surface. Significantly, iridium-based hydrogenolysis of n-butane results in MoIr(,3)/Al(,2)O(,3) showing enhanced activity (5-10 times) over Ir(,4)/Al(,2)O(,3) and {Ir(,4) + Mo(,2)}/Al(,2)O(,3), while selectivity towards ethane remains at (TURN)73%. In contrast, Mo(,2)Ir(,2)/Al(,2)O(,3) exhibits a change in ethane selectivity to (TURN)54%. All supported stoichiomertic mixtures have the same activity and selectivity as Ir(,4)/Al(,2)O(,3).
Direct observation of surface metal clusters is carried out by x-ray absorption spectroscopy of the molybdenum K-edge. The x-ray absorption near edge structure (XANES) provides information related to the chemical nature (oxidation state and coordination sphere) of the absorbing atom. The extended x-ray absorption fine structure (EXAFS) is used to determine the identity, radial distances and coordination number of atoms surrounding the absorbing atom.
Results from this work indicates that the mixed-metal cluster precursors provide unique metal-metal interactions not obtainable through stoichiometric mixtures. The molybdenum and iridium in the stoichiometric mixtures remain segregated throughout analyses. The initial tetrahedral metal core does not, however, appear to remain intact on the alumina surface.
Issue Date:1984
Description:233 p.
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1984.
Other Identifier(s):(UMI)AAI8502327
Date Available in IDEALS:2014-12-15
Date Deposited:1984

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