ARGONNE NATIONAL LABORATORY

CHEMICAL SCIENCES AND ENGINEERING DIVISION MONDAY MORNING SEMINAR SERIES

 

SPEAKER:         Bruce C. Gates   

                             University of California, Davis

 

TITLE:                 Metal Clusters on Supports:  Synthesis, Structural Characterization and Catalytic Properties

 

TIME:                  11:00 am

                             (Refreshments will be available starting at 10:45 am in front lobby)

 

DATE:                 November 26, 2007

 

PLACE:               Building 200, Auditorium

 

HOST:                 Stefan Vajda

         

Abstract:  The typical industrial catalyst is a highly non-uniform set of nano-structures dispersed on a porous, non-uniform 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.  We have investigated mononuclear complexes and clusters of metals of groups 6, 7, and 8.  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; 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 complexes of rhodium, iridium, and of gold on oxides and zeolites and for clusters of rhenium, rhodium, iridium, osmium, and gold on these same supports.  For example, complexes of rhodium bonded to ultrastable Y zeolite were prepared from the precursor Rh(C₂H₄)₂(acac) [acac is C₅H₇O₂], giving supported Rh(C₂H₄)₂ complexes with each Rh atom bonded to two oxygen atoms of the zeolite.  The ethylene ligands were characterized by IR, EXAFS, and ¹³C NMR spectroscopies, and the NMR data demonstrate a uniformity of the supported species nearly matching that of the precursor in the crystalline state. 

Supported gold complexes on zeolite NaY and on La₂O₃ were formed from Au(CH₃)₂(acac); these catalyze CO oxidation at room temperature, and the La₂O₃-supported Au^III complexes are almost as active as the most highly active supported gold catalysts for this reaction.  Investigation of supported gold catalysts containing mixtures of zerovalent and cationic gold (characterized by XANES, XPS, Mössbauer, and EXAFS spectroscopies, as well as TEM) shows that the catalytic activity for CO oxidation increases with the content of cationic gold.  These results, taken together, cast doubt on the prevailing attributions of catalytic activity of dispersed gold to unique size-dependent properties of gold nanoclusters.   

Highly dispersed metal catalysts containing supported clusters of only several metal atoms each, exemplified by Ir₄, Ir₆, and Rh₆, were prepared by removal of CO ligands from supported precursors, for example, Ir₄(CO)₁₂ and Ir₆(CO)₁₆, and Rh₆(CO)₁₆.  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. 

We have used transient spectroscopic methods to investigate the formation of supported metal clusters from metal complexes and their breakup in reactive gases; the most valuable of these methods are EXAFS and XANES (carried out at the APS) and IR spectroscopy.  The results provide insights into the catalytic reaction intermediates and the stabilities of the catalytic species as they depend on the reaction atmosphere.

These 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.