Selective hydrogenation of 1,3-butadiene to butenes on ceria-supported Pd, Ni and PdNi catalysts: Combined experimental and DFT outlook

Abstract

The regulation of catalyst activity and selectivity using a reducible support for the industrially relevant hydrogenation of 1,3-butadiene to more valuable butene products was achieved. Supported palladium and nickel–palladium catalysts on ceria were prepared and characterized with hydrogen temperature programmed reduction (H₂-TPR), hydrogen temperature programmed desorption (H₂-TPD), X-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy (HR-TEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), temperature programmed oxidation (TPO), energy dispersive spectroscopy (EDS), and N₂ adsorption–desorption to examine their chemical and physical properties. Pathways guiding the reaction were determined using the density functional theory (DFT). H₂-TPR confirmed that palladium oxide was reduced, and nickel oxide species strongly interacted with the CeO₂ support. The Ce³⁺ concentration determined by XPS showed that all catalysts surface contained the Ce reduced state. The catalysts showed a similar BET surface area, with 4Pd–Ce presenting the lowest value due to particle aggregation, which was confirmed from the EDS mapping analysis. Butadiene conversion consistently increased with temperature (40 °C–120 °C) until full conversion was reached on all the Pd catalysts while the maximum conversion on the 4Ni-Ce catalyst was 88 % at 120 °C. Product distribution revealed that 4 % Pd content directed the products toward butane when 40 °C was exceeded. Constructed mechanisms by DFT calculations featured low reaction barriers for the involved surface hydrogenation steps, and thus, they accounted for the observed low temperature of the surface hydrogenation activity. Selective formation of 1-butene partially stemmed from its relatively weak binding to Ni sites in reference to Pd sites. The mapped-out mechanisms entailed a higher reaction barrier for the formation of 2-butene, in agreement with the experimental observation pertinent to its formation at higher temperatures when compared with that of 1-butene.