An Investigation of the Partitioning of Dissociated H2 and D2 on Activated Nickel Catalysts

Abstract

Heterogeneous nickel catalysts play an important role in numerous processes. In the past, a number of H/D isotopic labeling studies have probed catalytic mechanisms during specific associative and dissociative steps. The current work investigates the adsorption of hydrogen on Ni0 sites, the active surface where H-transfer reactions take place, to determine if preferential partitioning of H or D occurs on the metal surface. This is important for understanding whether differences in surface coverages between H and D could potentially impact KIE analyses. Overall, when H2 and D2 were added competitively to either H-activated NiO or Ni/alumina catalysts, no significant isotopic preference was identified. However, when the catalyst was pretreated with H2 (or D2) prior to flowing the equimolar mixture, a slight isotopic preference for H2 (or D2) was observed. This residual effect suggests that, in both cases, hydrogen exchange with hydroxyl groups on the support occurred. That is, in the case of hydrogen activated NiO, the resulting catalyst is likely a mixture of Ni and residual NiO, the latter presumably containing surface hydroxyl groups. The isotopic preference when H2 (or D2) pretreatment is conducted prior to running the H2/D2 (50–50 %) mixture is enhanced when alumina is used as a support due to greater OH/OD exchange. A similar, but even less pronounced effect, was observed previously with cobalt/alumina catalysts.