(460a) Molecular Catalysis On Surfaces: Metal Complexes and Metal Clusters Characterized During Operation

The typical industrial catalyst is a highly nonuniform set of nano-structures dispersed on a porous, nonuniform support. In contrast, some supported catalysts, such as those used for olefin polymerization, are much more nearly uniform, being essentially molecular in character and offering the prospective advantages of molecular catalysts in solution, such as high selectivity. Our goals were to gain a deeper understanding of the class of oxide- and zeolite-supported metal catalysts with virtual molecular properties. A key to meeting the goal of structural uniformity is precise synthesis of the supported species, by the methods of organometallic chemistry extended to surfaces. Keys to understanding of the structures and properties of these materials are characterization by complementary spectroscopic and microscopic techniques, even, when possible, with the samples in reactive atmospheres and functioning as catalysts. The characterization methods include vibrational, NMR, and X-ray absorption spectroscopies; high-resolution transmission electron microscopy (even with atomic resolution); and density functional theory (DFT). The characterization results determine metal nuclearities, bonding of metals to supports, and identification of intermediates bonded to the metals.

Results are presented for mononuclear complexes and clusters of rhodium, osmium, and iridium on oxides and zeolites. For example, complexes of iridium bonded to ultrastable Y zeolite were prepared from the precursor Ir(C2H4)2(acac) [acac is C5H7O2], giving supported Ir(C2H4)2 complexes with each Ir atom bonded to two oxygen atoms of the zeolite. These complexes were converted reversibly into clusters approximated as Ir4, and the changes were followed in real time by XANES, EXAFS, and IR spectroscopies; the data indicate changes in the ligand spheres of the metal during the cluster formation and breakup, including evidence of changes in the metal?support interface, and they indicate how to tune catalytic structure and properties by choice of the reaction conditions.

Characterization of the supported clusters by EXAFS spectroscopy and DFT indicates metal?support-oxygen bonding and the presence of cations of the metal at the metal?support interface, helping to stabilize the dispersion of the metals. Supported molecular catalysts are an emerging class of materials that is expected to offer new reactivities and catalytic properties. Some of the lessons that are emerging from understanding of their structure and bonding appear to pertain to supported catalysts generally.