Preparation and Characterization of PT-RU Bimetallic Catalysts Using Electroless Deposition Methods and Mechanistic Study of RE and CS Promoters for AG-Based, High Selectivity Ethylene Oxide Catalysts

Pt-Ru bimetallic catalysts are widely used for direct methanol fuel cells, biomass upgrading and hydrocarbon refining and offer unique properties compared to Pt or Ru monometallic catalysts due to ensemble effects, electronic effects, and/or bifunctional effects. To achieve better performance, strong metal-metal interactions and true bimetallic surface are needed. Electroless deposition (ED) methods are used in our laboratory to synthesize such bimetallic catalysts. In this study, two series of Ru@Pt/C (Pt deposited on Ru surfaces) and Pt@Ru/C (Ru deposited on Pt surfaces) catalysts have been synthesized. Characterization data from temperature programmed reduction (TPR), selective chemisorption, X-ray photoelectron spectroscopy (XPS), and scanning transmission electron microscopy (STEM) are presented to confirm formation of Pt-Ru bimetallic surfaces with strong metal-metal interactions. The second part of this study describes the roles of high-valent Re oxyanions and alkali metal promoters for high selectivity ethylene epoxidation catalysts. The world-wide production of ethylene oxide (EO) currently exceeds 25 Mt/yr, placing the synthesis of this oxygenated organic chemical high on the list of the world’s most commonly produced chemicals and the highest volume chemical produced by catalytic oxidation. Ag, Cs, Re are the main components for current generation EO catalysts. Even though the first patent describing Re-promoted EO catalysts appeared in 1984 only three peer-reviewed publications had appeared in the literature (prior to our work) addressing the mechanism of Re and co-promoters for improved EO selectivity. We have prepared extensive series of Cs-Ag, Re-Ag, Cs-Re-Ag and Cs-Re-Mo-Ag catalysts, all supported on a commercial α-Al2O3 carrier; they have been evaluated for ethylene epoxidation at industrially-relevant conditions (high pressure and 200-1000 hrs on-line) to determine the mechanism of EO selectivity enhancement from high-valent Re oxyanions and other co-promoters. Analyses by XPS and SEM suggest the origin of both Re and Cs promotions are electronic. A reaction scheme detailing the mechanism of EO formation over Re-modified, Cs-promoted Ag catalysts is presented