Selectivity tuning over monometallic and bimetallic dehydrogenation catalysts: effects of support and particle size

Abstract

The efficacy of tandem dehydrogenation–condensation catalysts for the upgrade of bio-derived intermediates is largely determined by their relative (de-)hydrogenation and decarbonylation activity. Here, the effects of support and particle size of heterogeneous PdCu alloy catalysts on (de-)hydrogenation and decarbonlylation reactions were investigated using kinetic measurements, X-ray absorption spectroscopy and density functional theory (DFT). The chemical mismatch of Cu²⁺ with Ti⁴⁺ and Ca²⁺ prevents the substitution of Cu into the lattice of TiO₂ or hydroxyapatite supports, and facilitates its alloying with Pd, resulting in improved selectivity for hydrogenation–dehydrogenation reactions compared to decarbonylation reactions. Based on kinetic measurements of butyraldehyde reactions over Pd and PdCu/SiO₂ model catalysts, decarbonylation activity is attributed to the presence of Pd surface ensembles, while (de-)hydrogenation reactions are catalyzed by PdCu sites on the surface. This is consistent with selectivity and CO coverage trends with increasing conversion, and DFT-based microkinetic modeling. Selectivity control can also be achieved using the PdCu nanocluster size. Smaller nanoparticles favor the C–CO bond scission step of the decarbonylation reaction, due to the stronger binding of CO and alkyl species to sites of lower coordination. CO-induced segregation of reactive Pd atoms to under-coordinated step/edge sites also amplifies the geometric effect on the catalytic behavior.