Hydrogen spillover in the Fischer–Tropsch synthesis: An analysis of platinum as a promoter for cobalt–alumina catalysts

Abstract

Hydrogen spillover has been invoked to explain the functioning of platinum as a promoter in cobalt-based Fischer–Tropsch catalysts. In this study, the operation of Pt was investigated using a model hybrid catalyst (i.e. mixture of Pt/Al2O3+Co/Al2O3) which allowed for decoupling of hydrogen spillover effects from those requiring direct Pt–Co contact. Pt improved the reducibility of the hybrid catalyst despite physical separation of Pt from Co.TPR, TGA, in situ XRD and quasi in situ XPS confirmed that supported Co3O4 reduced via formation of CoO as a stable intermediate. Pt had only a slight effect on Co3O4→CoO reduction, but greatly improved the CoO→Co0 reduction which was severely hindered by interaction with the alumina support. The catalysing effect of Pt on the reduction was attributed to H2 dissociation and its subsequent spillover occurring more readily than direct H2 activation by the cobalt oxides. During the Fischer–Tropsch reaction, the role of spillover hydrogen was again invoked to explain the higher TOF and enhanced selectivity towards CH4 and paraffins. Spillover from Pt was proposed to induce a hydrogen-rich microenvironment that resulted in a ‘cleansing’ effect on the catalyst surface and an increase in the selectivity of hydrogenated products.