The effect of separation distance on hydrogen spillover in Os promoted Co@HCS catalysts†

Abstract

Spillover effects from an easily reduced metal (Os) to a less easily reduced metal (Co) were investigated by use of hollow carbon spheres (HCSs) of different shell thicknesses, x, (x = 16 nm, 28 nm and 51 nm). The Co (10%) was loaded inside the HCS and the Os (1%) outside three HCS supports (Os/Co@HCSx). Temperature programmed reduction, together with BET and XRD studies, were used to monitor the effects of the Os on the Co as a function of HCS shell thickness. When no Os was present on the outside of the HCSs, the effect of H₂ diffusion on the two Co reduction peaks was determined. Comparison with the Os containing Co@HCSs catalysts indicated that the two Co reduction peaks were influenced differently by the HCS shell thickness. Spillover of hydrogen could be observed at distances of ca. >100 nm, as shown by the shift of the first Co reduction peak (Co₃O₄ to CoO) while that of the second reduction peak (CoO to Co) was only observed at distances up to ca. 50 nm. The disordered carbon material is proposed to be responsible for the H transfer reaction between Os and Co. The Os/(Co@HCSx) catalysts were tested for Fischer–Tropsch (FT) activity and the data indicated a drop in the FT activity with shell thickness. This suggests that HCSs require an optimum thickness (to provide stability, good porosity and auto-reduction behaviour) to generate high FT activity/selectivity, with spillover effects aiding the reaction.