The Promotional Effect of Gold in Catalysis by Palladium-Gold

Abstract

Acetoxylation of ethylene to vinyl acetate (VA) was used to investigate the mechanism of the promotional effect of gold (Au) in a palladium (Pd)-Au alloy catalyst. The enhanced rates of VA formation for low Pd coverages relative to high Pd coverages on Au single-crystal surfaces demonstrate that the critical reaction site for VA synthesis consists of two noncontiguous, suitably spaced, Pd monomers. The role of Au is to isolate single Pd sites that facilitate the coupling of critical surface species to product, while inhibiting the formation of undesirable reaction by-products.

Catalysis on singly dispersed bimetallic sites

“…Atoms of a catalytic site arrange into a configuration that essentially offers a specific electronic structure for the catalytic cycle. Bimetallic nanoparticle catalysts play significant roles in chemical and energy transformations since the second metal (A) could tune catalytic performance of a monometallic catalyst (M) through ligand effect, geometric effect, bi-functional effect or lattice strain effect [2][3][4][5] (Fig. 1a,b).…”

Promotional Effects of Au in Pd-Au Catalysts for Vinyl Acetate Synthesis

“…In heterogeneous catalysis, for example, the addition of a second metal component can greatly enhance the catalytic performance, particularly with respect to activity and selectivity [2][3][4][5]. A thorough understanding of the reaction mechanism over bimetallic surfaces at the atomic level can aid in understanding reaction mechanisms and in the optimization of catalytic performance.…”

“…Reaction conditions: the VA synthesis was carried out at 453 K, with acetic acid, ethylene, and O 2 pressures of 5.33×10 2 , 1.07×10 3 , and 2.66×10 2 Pa, respectively. The total reaction time was 3 h. (111) surfaces at 100 K showing the presence (300 K anneal) and absence (600 K anneal) of contiguous Pd sites [3]. The Pd/Au(100) and Pd/Au(111) surfaces were prepared by depositing 4 ML of Pd at 100 K, then annealing to 300 and 600 K, respectively, for 10 min.…”

“…The silicon doser consisted of a high-purity silicon sliver wrapped with a W-wire and resistively heated. The details of the SiO 2 film preparation have been described previously [37,38].The VA synthesis experiments were carried out in a combined elevated-pressure reactor-UHV XPS chamber [3,39]. After preparation and characterization in the UHV chamber, the Pd/Au(100) sample was transferred in-vacuo into the reaction chamber through a double-stage, differentially pumped Teflon sliding seal.…”

“…The TPD apparatus utilizes a line-of-sight quadruple mass spectroscopy (QMS) with a differentially pumped aperture and a linear heating ramp of 5 K/s. These experimental systems have been described in detail elsewhere [3,34,35].The Mo(112) single crystal was cleaned by oxidation (1200 K, 6.67×10 5 Pa O 2 ) and vacuum annealing (2200 K) cycles until no carbon and oxygen was detected by AES and a sharp (1×1) LEED pattern was observed. Pd and Au were evaporated from dosers made of high-purity Pd and Au wires (99.99%) wrapped around Ta filaments.…”