Modification of Ni/Al2O3 catalyst with Pd nanoparticles for selective phenylacetylene semihydrodenation

NiPd/Al2O3 (4.9 wt% of Ni–0.1 wt% of Pd), Ni/Al2O3 (5 wt% of Ni) and Pd/Al2O3 (0.1 wt% of Pd) catalysts were tested in the liquid-phase semihydrogenation of phenylacetylene in batch- and flow-type reactors. Catalysts were characterized by temperature-programmed reduction, high resolution transmission electron microscopy, and X-ray photoelectron spectroscopy. Differences in the formation of palladium particles during deposition of metals on the surface of the alumina were revealed. Relatively large Pd particles observed in Pd/Al2O3 transformed to β-PdHx phase in a hydrogen atmosphere. When Pd and Ni are co-supported, no monometallic palladium particles were detected. Pd was apparently atomically distributed in the close vicinity of some NiO particles. As a result, there were two types of NiO particles in NiPd/Al2O3: NiO particles isolated from Pd species and those in close contact with atomically dispersed palladium. This distribution of metals provided the formation of activated hydrogen, which, in turn, led to an increase in the phenylacetylene conversion compared to monometallic Ni/Al2O3, while maintaining high selectivity of styrene formation. The close interaction of metals and their co-action in the hydrogenation of phenylacetylene was confirmed by the kinetic data obtained in a batch-type reactor. These data showed the difference in the reaction mechanisms on Ni/Al2O3 and NiPd/Al2O3.