Zeolite yttrium- and magnesium oxide-supported molecular metal complexes and clusters: Synthesis, characterization, and catalytic properties

This dissertation is a report of synthesis of mononuclear iridium and gold complexes on zeolites and metal oxides, their characterization, and their testing as catalysts, and an investigation of the conditions for their interconversion to extremely small supported metal clusters.;Time-resolved EXAFS spectra recorded as the zeolite-supported iridium complexes were treated in flowing hydrogen as the temperature was increased to 353 K show that the Ir-Ir coordination number increased from 0 to 3, indicating the formation of Ir4 clusters. IR results showing the formation of ligands requiring the presence of more than one Ir atoms confirmed the formation of clusters. The isosbestic points present in the X-ray absorption near edge structure (XANES) clearly indicate a change from one uniform structure to another. When the flow was switched to ethene only, the data indicated break-up of the clusters into mononuclear complexes. Elucidation of such changes was only possible because of the high degree of uniformity of the catalyst. These findings were applied to tune the structure of the zeolite-supported molecular iridium catalysts as the catalyst was functioning in ethene hydrogenation. This work is the first demonstration of tuning the structure of a solid catalyst at the molecular scale as it is functioning for a reaction.;Iridium complexes were also synthesized on MgO powder by adsorption of Ir(C2H4)2(C5H7O 2); images determined by aberration-corrected scanning transmission electron microscopy show individual Ir atoms, demonstrating that the supported complexes were site-isolated for the first time.;The same approach was applied to illustrate the site-isolation of mononuclear gold complexes on MgO by the aberration-corrected STEM. The metal oxide- and zeolite-supported mononuclear gold complexes, on the basis of spectroscopic data, have been inferred to be catalytically active for alkene hydrogenation and for CO oxidation. However, direct evidence of the site-isolation of the Au atoms in the catalysts in the absence of gold clusters or particles had been missing. The STEM images reported here are the first evidence of uniquely site-isolated gold complexes on a high-area support---in the absence of gold clusters or particles.;Highly dealuminated Y zeolite-supported mononuclear iridium complexes with reactive ethene ligands were synthesized precisely with a high degree of uniformity by the reaction of Ir(C2H4)2(C 5H7O2) with the zeolite. The resultant structure and its treatment in helium, CO, ethylene, and H2 were investigated with infrared (IR) and extended X-ray absorption fine structure (EXAFS) spectroscopies. The IR spectra show that Ir(C2H4)2(C 5H7O2) reacted readily with surface OH groups of the zeolite, leading to the removal of C5H7O 2 ligands and the formation of supported mononuclear iridium complexes; extended X-ray absorption fine structure (EXAFS) data indicate the presence of site-isolated mononuclear iridium complexes on the zeolite, confirmed by the lack of detectable Ir-Ir contributions, with each Ir atom bonded (on average) to 4 C atoms (two ethene ligands) (at a distance of 2.10 A) and 2 zeolite O atoms (2.15 A). The supported iridium-ethene complex catalyzed ethene hydrogenation at atmospheric pressure and 294 K. Probing the sample with CO demonstrated the exceptional uniformity of the zeolite-supported iridium complexes, evidenced by formation of sharp uco bands by the exchange of CO with ethene ligands. The iridium ethene complex on the crystalline zeolite support is inferred to be one of the most nearly uniform supported metal complex catalysts.;Starting from these site-isolated mononuclear gold complexes the conditions (318 K with the sample in helium) for preparation of size-controlled clusters of only 2 to 6 Au atoms each on high-area MgO was demonstrated. This had been the major challenge in the identification of the active species in supported gold catalysts. The STEM images presented here are the first of such small nanoclusters in the absence of larger gold clusters on a high-area support. Treatment at higher temperatures (373 K) in flowing helium resulted in the formation of gold clusters with diameters of 0.58 +/- 0.15 nm (containing roughly 10 Au atoms), again in the absence of larger nanoparticles. Upon exposure of the supported nanoclusters to the electron beam, they underwent aggregation to gold clusters approximately 1 nm in average diameter, as shown in consecutive STEM images. (Abstract shortened by UMI.)...