Surface spectroscopic studies of mono- and bimetallic model catalysts

Abstract

This dissertation is focused on understanding heterogeneous bimetallic catalysts using model catalyst systems, such as Pd-Au/Mo(110), Pd/Au(111) and Pd/Au(100). Monometallic and bimetallic model catalysts, composed of Pd and Au, were prepared by physical vapor deposition (PVD) onto well-characterized metal substrates. Subsequent characterization was performed using an arsenal of modern surface science tools: low energy ion scattering spectroscopy (LEISS), infrared absorption spectroscopy (IRAS), temperature programmed desorption (TPD), and x-ray photoelectron spectroscopy (XPS). Electronic, morphological, and chemical properties of the prepared model catalysts were compared to those observed from monometallic single-crystal model catalysts such as Cu(100), Pd(111), Au(100), and Au(111). Between 700 K and 1000 K, formation of stable alloy surfaces of Pd-Au mixtures on Mo(110) was accompanied by substantial enrichment in Au. Annealing a 1:1 Pd-Au mixture at 800 K yields a Pd0.2Au0.8 surface alloy; the concentration of isolated Pd sites in this surface alloy can be systematically controlled by a judicious selection of initial bulk Pd-Au concentration. Pd-Au catalysts supported on Au(111) and Au(100) substrates generated a surface ensemble of Pd monomers surrounded by Au after annealing the systems at or above 550 K. To test the activity and selectivity of the prepared bimetallic model catalysts, the formation rate of vinyl acetate monomer (VAM) was examined. More significant enhancement of VAM formation rate was observed for bimetallic catalysts supported on Au(100) compared to those on Au(111). A critical surface ensemble composed two non-contiguous Pd monomers was proposed for the VAM synthesis. Oxygen plays a critical role in the efficiency of the synthetic route. Structure-reactivity correlations were established based on the suggested elementary reactions leading to the oxidative coupling of ethylene and acetic acid to form VAM.