(625b) Impregnation of Koac on PdAu/SiO2 Causes Pd-Acetate Formation and Metal Restructuring

Potassium-promoted, silica-supported PdAu is catalytically active for the gas-phase acetoxylation of ethylene with acetic acid to form vinyl acetate monomer (VAM). Potassium, which is incorporated into the catalyst via wet impregnation of its salt solution, is known to improve long-term activity and selectivity for VAM. This common catalyst preparation step is generally assumed to have no effect on catalyst structure. However, in this work, we report evidence to the contrary. We initially synthesized a silica-supported PdAu (PdAu/SiO₂, 8 wt% Pd, 4 wt% Au) model catalyst containing Pd-rich PdAu alloy and pure Au phases. A Design of Experiments (DOE) approach was implemented to impregnate the as-synthesized PdAu/SiO₂ catalyst with varying loadings of potassium acetate (0, 5, and 10 wt% K) using wet impregnation solutions with varying concentrations of acetic acid (0, 50, and 100 vol% AcOH in water). Samples were then characterized to detect changes in metal nanoparticle structure (XRD, XPS, and XAS) and surface species formation (DRIFTS). Impregnation with KOAc aqueous solution revealed no XRD-detectible changes to the bimetal structure, whereas DRIFTS indicated the presence of Pd₃(OAc)₆ species, which led to up to 2% Pd loss after washing with water. Carrying out the impregnation step with an AcOH-only solution caused significant enlargement of the pure Au grain size and generated a smaller amount of Pd₃(OAc)₆. During co-impregnation of AcOH and KOAc, grain sizes were enlarged slightly, and substantial amounts of K₂Pd₂(OAc)₆ and Pd₃(OAc)₆ were detected by DRIFTS and correlated to up to 32% Pd loss after washing. Multivariate analysis confirmed the individual effects from KOAc and AcOH as well as their interactions were statistically significant. Ultimately, these results indicate metal leaching and restructuring can occur during the wet impregnation process, suggesting care should be taken during this common catalyst preparation procedure.