Dehydrogenation of cyclohexane on lanthanide–nickel bimetallic catalysts (Eu–Ni and Yb–Ni)

Abstract

The Eu–Ni and Yb–Ni (Ln–Ni) bimetallic catalysts, prepared by the reaction of nickel powders with lanthanides dissolved in liquid ammonia, have been characterised by the dehydrogenation of cyclohexane combined with isotopic exchange between cyclohexane and deuterium. Their catalytic behaviour varied markedly with different levels of lanthanide content. First, the relative rates of cyclohexane dehydrogenation decreased when the loading of lanthanide metals on the Ni surface was low and subsequently increased with increasing loading. Isotopic exchange reactions exhibited a similar dependence of activity on lanthanide content. An analysis of the initial decrease in the rates of reaction suggested a decrease in catalytically active nickel sites due to coating with lanthanide metals. The surface is gradually covered with the lanthanide metals and simulataneously certain interactions occur to produce newly active centres in the region of high loading, resulting in acceleration of the dehydrogenation and isotope-exchange reaction. The presence of lanthanide metals on Ni enhanced the capacity of this surface to dissociate the C—H bond. Multiple exchange for the reactions between cyclohexane and deuterium tended to decrease with increasing loading of lanthanides. Furthermore, the effects of a lanthanide metal overlayer on the reaction kinetics over an Ni metal surface were discussed.