Nature and Distribution of Cu and Pd Species in CuPd/TiO2-Na Bimetallic Catalysts for Glycerol Hydrodeoxygenation

We demonstrated recently that CuPd/TiO2-Na bimetallic catalysts synthesized by sequential wet impregnation are active, selective, and stable for the hydrodeoxygenation (HDO) of glycerol into propylene glycol at low H-2 pressure. The present study reports on the nature and distribution of Cu and Pd surface species in CuPd/TiO2-Na bimetallic catalysts using different scanning transmission electron microscopy techniques that supply cluster-specific alloying details. In particular, we used atomic-resolution Z-contrast imaging, X-ray energy-dispersive spectroscopy, and electron energy-loss spectroscopy. We also include X-ray photoelectron spectroscopy results. Our analysis shows that the metallic nanoparticles adopt mainly five different structures according to how the Cu and Pd atoms coordinate among themselves: a homogeneous CuPd alloy structure (45-61%), a Cu shell/CuPd core (15-23%), a smaller number of particles formed by Cu on the surface and Pd in the nucleus (10-17%), and there are also nanoparticles formed only by Pd (4-7%) or by Cu (8-13%). We determined that there is a inhomogeneous distribution of Cu and Pd in the bimetallic nanoparticles, with Cu being predominant on the surface (between 76 and 90% of the total area analyzed for each particle). Most bimetallic nanoparticles have sizes below 6 nm, the Pd monometallic nanoparticles are in the 2-4 nm range, whereas the monometallic Cu nanoparticles are larger than 8 nm. Bimetallic nanoparticles with sizes smaller than 6-7 nm are fundamentally made up of Cu-0-Pd-0 and Cu1+-Pd-0. The nanoparticles with sizes greater than 7 nm consist of Cu2+ and Cu2+-Pd2+. Our obtained results also help describe reports about the activation of HDO by Pd-Cu in the absence of H-2, an effect apparently not observed with other bimetallic systems.