Examining the effect of manganese distribution on alcohol production in CoMn/TiO2 FTS catalysts

Abstract

Mn-doped Co₃O₄ supported on TiO₂ is a well-known Fischer–Tropsch Synthesis (FTS) catalyst. It has been shown that when the Mn doping exceeds 3 wt%, CO conversion drops and the product selectivity to alcohols and olefins increases dramatically. Here we examine the effect of the preparation method to determine how the proximity of the Mn in the as-prepared catalyst affects FTS performance. Three preparation procedures were examined: preparation of Mn doped Co(Mn)₃O₄ mixed oxides, surface doping of Co₃O₄ with Mn₃O₄ and a physical mixture of the two spinels. Characterisation studies including XRD, XPS and STEM-EDS, of the as-synthesised materials confirmed the successful preparation of spinel materials with crystallite sizes ∼20 nm. Surface enrichment of Mn on Co₃O₄ was seen in the as-prepared surface doped samples but not in the mixed oxide ones. STEM EDS studies revealed that after reduction Mn oxide had migrated to the surface in the mixed oxide samples similar to the surface doped samples. Subsequently, similar CO conversion and product selectivity was observed in both types of sample. However, unlike the surface doped and mixed oxide catalysts, the physically mixed oxide samples did not yield alcohols and olefins, although enhanced CO conversion was observed for the 3% physical mix. The results highlight the prevalence and importance of the effects of surface Mn doping on the Co speciation which leads to enhanced alcohol/olefin selectivity.