Temperature dependence of the 2-butene hydrogenation over supported Pd nanoparticles and Pd(111)

The activity and induction times for 2-butene hydrogenation have been investigated over a Pd(1 1 1) single crystal surface and model Pd nanoparticles supported on Fe3O4/Pt(1 1 1) by isothermal pulsed molecular beam experiments, in the temperature range of 220-340 K. C-modification of supported Pd particles induced persistent hydrogenation activity at low temperatures (220-260 K). C-modification of the Pd(1 1 1) surface, in contrast, did not result in significant reactivity changes. At low temperatures (220-260 K), hydrogenation activity was only maintained over the C-modified Pd particles, while at temperatures (>= 280 K) persistent hydrogenation was observed over all Pd catalysts at comparable rates. Two principal reaction mechanisms are discussed that could be responsible for the observed hydrogenation activity at different Pd surfaces. We show that on Pd nanoparticles, the reaction mechanism involving subsurface hydrogen species plays an important role under all investigated conditions. This subsurface-related reaction pathway relies on an effective replenishment of the subsurface hydrogen reservoir, which is affected by the presence of strongly adsorbed hydrocarbon species that are formed in the induction period. We discuss the correlation between the induction times and the hydrogenation activity of different Pd surfaces. (c) 2013 Elsevier B.V. All rights reserved.