Nickel Particle Size Effects on Cyclohexane Dehydrogenation: A Combined Reaction Kinetics and Surface Science Study

The kinetics of cyclohexane dehydrogenation was studied under near atmospheric pressure (25 Torr) at 600 K on Ni/SiO2/Mo(110) model catalysts prepared under ultrahigh vacuum conditions. The supported Ni model catalysts were characterized by Auger electron spectroscopy, temperature programmed desorption (TPD) and polarization modulation infrared reflection absorption spectroscopy (PM-IRAS). The average Ni particle sizes and particle size distributions were acquired by scanning tunneling microscopy. The kinetics results show that, for Ni particles below 2.5 nm in diameter, the specific rates (turnover frequency of benzene or TOF) increase with a decrease in particle sizes, while for Ni particles larger than 2.5 nm, the TOF is particle size independent. H-2- and CO-TPD experiments as well as CO PM-IRAS spectroscopy confirm the change of Ni particles' surface structures as a function of particle size. The particle size effect on reaction kinetics is explained by the sharp increase in the fraction of low coordinated Ni atoms on the surface of particles below 2.5 nm in diameter.