Pressure dependence and mechanism of Mn promotion of silica-supported Co catalyst in the Fischer-Tropsch reaction.

[Display omitted] • Effect Mn promotion Co-catalyzed Fischer-Tropsch reaction strongly pressure dependent. • MnO dispersed on Co metal particles provides additional sites for CO dissociation. • Mn raises selectivity at low pressure and promotes CO dissociation not chain growth. • At low pressure, Mn enhances activity and C 5+ selectivity. • At high pressure, Mn enhances activity but lowers C 5+ selectivity. The mechanism of Mn promotion of a silica-supported Co catalyst in the Fischer-Tropsch reaction has been studied at varying pressures up to 20 bar. IR spectroscopy in combination with DFT calculations suggest adsorbed CO is activated by reaction with an oxygen vacancy in the MnO, which covers the Co surface. This leads to a higher activity, higher CH x coverage and thus higher C 5+ and lower CH 4 selectivity. Increasing the pressure magnifies the selectivity differences. However, above around 4 bar, the effect of Mn on the selectivities is reversed and the C 5+ selectivity is decreased by Mn addition. This is tentatively attributed to Mn promoting the C-O bond dissociation but not the chain growth. Formed monomers have to migrate to stepped sites for chain growth on the Co surface. Whilst this is migration is not impeded by co-adsorbates at low pressure, migration could be hindered by especially the high CO coverage at high pressure. [ABSTRACT FROM AUTHOR]