Author : Joseph David Kistler
Release : 2014
Genre :
Kind : eBook
Book Rating : 764/5 ( reviews)
Book Synopsis Supported Mono- and Bimetallic Complexes and Clusters by : Joseph David Kistler
Download or read book Supported Mono- and Bimetallic Complexes and Clusters written by Joseph David Kistler. This book was released on 2014. Available in PDF, EPUB and Kindle. Book excerpt: Site-isolated solid supported metal catalysts are important in industry and technology due to the cost efficiency to make and to recover and reuse them. These types of materials have catalytic properties similar to molecular complexes in solution while being easy to separate in heterogeneous catalytic reactions. The goal of this work was to synthesize supported metal complex catalysts while maintaining uniform catalytic sites. The syntheses were performed using precise glovebox and Schlenk techniques to achieve these highly uniform structures. These materials were then used to understand the relationship between structure of a catalytic site and the activity of the catalyst. This fundamental understanding of catalysts is important in advancing the field of catalysis. The structure of the catalysts were characterized using infrared (IR), extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) spectroscopies along with high angle annular dark field- scanning transmission electron microscopy (HAADF-STEM), with the HAADF-STEM work carried out by colleagues in other research groups. The catalytic activity of the catalysts was examined with gas chromatography (GC) and mass spectrometry (MS). The samples characterized in this work include complexes and clusters of second and third row transition metals supported on highly crystalline metal oxides. Specifically, there is a large focus in this work on supported rhodium complexes prepared from the organometallic precursor, Rh(C2H4)2(C5H7O2) and a pre-calcined magnesium oxide (MgO). This specific catalyst is important as not only is it active for olefin hydrogenation at mild temperatures but also there are reports of a unique surface mediated synthesis of uniform rhodium dimers, which are ideal for catalytic comparison of structures with different nuclearities. Reactivities of the MgO-supported rhodium complexes and dimers for carbon monoxide oxidation were investigated with the results showing the dimers were significantly more active for the reaction at 353 K. The stability of the dimers was tested in different reactive conditions with the results showing that under conditions with excess oxygen, the dimers are less stable and less active than under conditions with excess carbon monoxide.A bimetallic catalyst was synthesized on MgO incorporating rhodium and osmium using Rh(C2H4)2 (acac) and Os3(CO)12 as precursors. A unique synthesis method was developed to create a site-isolated segregated bimetallic catalyst with the osmium and rhodium sites acting independently of each other for ethylene hydrogenation at 298 K. The metals remained structurally segregated and catalytically independent even following reduction in H2 at 393 K. Zeolites, another class of highly crystalline supports, were studied to gain information on the support effects in catalysts. The analogous rhodium complexes as were synthesized on the MgO were synthesized on zeolite HY. These catalysts were tested to determine structural and catalytic stability under hydrogen, a reducing gas, and CO, a catalyst poison, with the results showing that, as compared to the complexes on zeolite HY, MgO-supported rhodium complexes form more uniform stable clusters under H2 and develop unique catalytic properties, selectivity for partial hydrogenation of dienes, when exposed to CO. Another zeolite, KLTL, was studied with supported platinum complexes synthesized from the salt precursor, Pt(NH3)4(NO3)2. This catalyst was oxidized at 633K to form supported single-atom platinum complexes. Both the as-prepared Pt(NH3)4 and oxidized PtOx complexes were analyzed structurally and studied as catalysts for CO oxidation. The oxidized platinum complexes proved to have significantly higher activity for CO oxidation at 423 K. Furthermore, HAADF-STEM was used to directly identify the locations of the platinum atoms in the pores of the zeolite before and after oxidative treatment, providing a method of ex-situ tracking of supported metal atoms.
