Pd-Au single-crystal surfaces: Segregation properties and catalytic activity in the selective hydrogenation of 1,3-butadiene

Pd70Au30 (1 1 1) and (1 1 0) surfaces have been characterized by low energy electron diffraction, Auger electron spectroscopy and low energy ion scattering spectroscopy. The reactivity of Pd-Au(1 1 1) towards the gas phase hydrogenation of 1,3-butadiene has been investigated in a static reactor, and compared to those of Pd-Au(1 1 0), Pd(1 1 1) and Au(1 1 1). Both Pd-Au surfaces are unreconstructed and exhibit strong surface segregation of gold. The gold concentrations in the top layer of Pd70Au30 (1 1 1) and (1 1 0) are found equal to 75 at.% and 85 at.%, respectively. The hydrogenation of butadiene has been performed at room temperature and 5 Torr in a large excess of hydrogen. While Au(1 1 1) is inactive, Pd-Au(1 1 1) is initially less active than Pd-Au(1 1 0) and Pd(1 1 1) for the formation of butenes. However, a spectacular increase in the reaction rate is observed upon evacuation of the reacted gases and introduction of a fresh reactant mixture. Unlike Pd(1 1 1), Pd-Au(1 1 1) is weakly active for the conversion of butenes to butane. Its subsequently higher selectivity towards butenes could originate from an increased desorption of butenes from the Pd-Au surface. Moreover, a substantial absorption of hydrogen in the course of the reaction is evidenced, and its possible impact on the surface properties is discussed.