Electronic and Geometric effects in Pd-based alloy catalysts 

Tanmayi Bathena ^a, Truc Phung ^a, Scott R. Svadlenak ^a, Yu Liu ^b, Lars C. Grabow^b,c, and Konstantinos A. Goulas ^a*

 ^a School of Chemical, Biological, and Environmental Engineering, Oregon State University, Corvallis, Oregon 97331, United States

^b William A. Brookshire Department of Chemical and Biomolecular Engineering, University of Houston, Houston, Texas 77204-4004, United States

^c Texas Center for Superconductivity at the University of Houston (TcSUH), Houston, Texas 77204, United States

Alloying of metals in heterogeneous catalysis is a ubiquitous method to improve catalytic performance. In this work, we assess the ligand (electronic) and ensemble (geometric) effects associated with palladium-based bimetallic catalysts. Here, we investigate Pd_xCu_y alloy in the context of oxygenate upgrading of biofuels. 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. Temperature-programmed reduction and chemisorption experiments are conducted to study the preparation-temperature-induced reactivity, dispersion, and activation trends of the catalysts respectively. Most importantly, data obtained from operando EXAFS and XANES evinced variations in the surface composition of the catalysts. The data for Pd_x-Cu_y/SiO₂ shows a clear dominance of ensemble effects over the improved selectivity in the kinetics. More significantly, the electronic effects of alloying are observed in the selectivity of active sites to the desired (de)hydrogenation reactions.  We further attempt to generalize the results of the PdCu alloying to other Pd-based systems. We show that Pd-Zn alloying results in charge transfer from Pd to Zn and similarly promotes desirable C-H bond activation over C-C bond scission reactions while alloying of Pd and Ag results in the suppression of activity for both reactions. We attribute this absence of activity to the low-lying Ag d-band center and the relatively limited promotion of Ag by Pd.

Keywords: Bimetallic catalysis, electronic effects, palladium