Synergy of Contact between ZnO Surface Planes and PdZn Nanostructures: Morphology and Chemical Property Effects in the Intermetallic Sites for Selective 1,3-Butadiene Hydrogenation

In this work, differently shaped ZnO supports were employed to study their effect on PdZn nanostructures. Our goal was to understand the role of ZnO morphology and consequently the major exposed faces (polar (0001), nonpolar (11 (2) over bar0), or nonpolar (10 (1) over bar0) facets) on the structural and electronic properties of the formed PdZn alloys and how it affects their chemical properties in selective 1,3-butadiene hydrogenation. A multitechnique approach that included in situ synchrotron X-ray diffraction (XRD) and in situ X-ray absorption spectroscopy (XAS) measurements combined with in situ diffuse reflectance infrared Fourier transformed spectroscopy (DRIFTS) and mass spectrometry (MS) has been accomplished to understand the formation of the PdZn alloy and the structural and electronic properties. Complementary characterization was obtained by high-resolution transmission electron microscopy (HRTEM), Fourier transform infrared spectroscopy analysis of adsorbed CO (CO-FTIR), and X-ray photoelectron spectroscopy (XPS). The alloying degree and the PdZn nanoparticle shape depend on the contacted ZnO surface. The percentage of Zn in the alloy increases from nonpolar (10 (1) over bar0) to nonpolar (11 (2) over bar0) and to polar (0001) facets. Moreover, zinc modifies low-coordinated surface sites on Pd nanoparticles, increases Pd electron density, and produces elongation of the lattice bonds which play a crucial role in diolefin hydrogenation, but also importantly, the results unravel the sensitivity to the alloy phase structure of this reaction. It seems the PdZn(100) surface constitutes an active and selective catalytic phase for partial hydrogenation of 1,3-butadiene. Synchronous DRIFTS has allowed us to analyze the relative concentrations of surface species during 1,3-butadiene hydrogenation and confirmed the different stabilities of the adsorbed intermediate alkenes depending on the nanoparticle shape and degree of alloying. The differences in selectivities in the 1,3-butadiene hydrogenation reaction have been explained on the basis of of the structural and electronic properties of the formed alloy.