Vinyl Acetate Synthesis over Model Pd−Sn Bimetallic Catalysts

Abstract

Pd−Sn bimetallic model catalysts were prepared as alloy films on a Rh(100) substrate via physical vapor deposition. The surface composition, structure, and chemisorption properties were studied by low energy ion scattering spectroscopy (LEIS), low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), infrared reflection adsorption spectroscopy (IRAS), and temperature programmed desorption (TPD). The ordered surface alloy of c(2 × 2) was formed after annealing the Pd−Sn mixtures to 700 K as evidenced by LEED and LEIS showing a 50% surface concentration of Pd. This ordered surface arrangement was further confirmed by IRAS, LEED, and TPD studies using CO as a probe molecule, in which the surface Pd atoms are completely isolated by Sn atoms. The surface Pd composition was determined to be 0.5 monolayers (ML). The catalytic properties of this Pd−Sn surface were tested with respect to vinyl acetate (VA) synthesis by ethylene acetoxylation that showed a maximum in the VA formation rate at a surface Pd coverage of 0.5 ML, i.e., a c(2 × 2) surface arrangement. This is consistent with our previous proposal that a pair of suitably spaced, isolated Pd monomers is the more efficient site for VA synthesis.