(111f) Oxygenate Reactions over PdCu/PdAg Catalyst: Role of Synergistic Effects

Catalytic processes that involve C-C bond forming and chain elongation reactions such as Guerbet and aldol condensation are essential for upgrading biomass-derived oxygenates to fuel-range hydrocarbons. In this context, bimetallic catalysis has garnered extensive attention due to their tunable properties that result in selective C-H and C-O bond activation. In this study, we investigate and compare the activity of Pd_xCu_y/SiO₂ and Pd_xAg_y/SiO₂ catalysts in tandem hydrogenation-aldol reaction. We measure the rates of decarbonylation and hydrogenation of butyraldehyde, the reactive intermediate for Guerbet condensation, and correlate them with the properties of the catalysts via a range of characterization efforts. The selectivity of Pd_xCu_y/SiO₂ catalysts towards C-H bond formation increases with increasing Cu fraction, pointing to the geometric effect of Cu on the selectivity. In contrast, the turnover frequency peaks at a 3:1 Cu:Pd ratio, pointing to the promotional effect of Pd on Cu. Temperature-programmed reduction experiments indicate that alloying takes place, while chemisorption experiments show that surface Pd is replaced by Cu. Consistent with this, EXAFS data for Pd_xCu_y/SiO₂ shows a segregation of Cu to the surface. The charge transfer from Cu to Pd is observed in Pd LIII XANES data; this modifies the electronic properties of the metals, thereby improving the chemical reactivity of metallic surfaces. On the other hand, in Pd_xAg_y/SiO₂, Ag atoms significantly segregate to the surface, but do not show any charge transfer with Pd, negating any promotional effect. Hence PdCu is more reactive than PdAg. We use the implications of these findings to provide valuable design principles for oxygenate catalysis.