A mechanistic basis for the effects of Mn loading on C2+ oxygenates synthesis directly from syngas over Rh–MnOx/SiO2 catalysts

• The intrinsic kinetics of CO hydrogenation over Rh–MnOx catalysts was established.
• The Ea for CH4 increased rapidly, but changed little for ethanol when adding MnOx.
• CO dissociation is enhanced by tilted CO adsorption on Rh–MnOx interface.
• Linearly adsorbed CO on Rhδ+ sites promoted CO insertion over Rh–MnOx catalysts.

Kinetics of the synthesis of C2+ oxygenates directly from syngas was investigated over a series of Rh–MnOx/SiO2 catalysts (atomic ratio of Mn/Rh=0, 0.5 and 5.0). It was evidenced that the activation energies for C2+ oxygenates changed little when introducing MnOx, while the apparent activation energies for hydrocarbons, especially methane, increased remarkably. Kinetic parameters indicate that the formation of hydrocarbons can be suppressed in the presence of MnOx. In the meantime, the reaction orders with respect to different products were also altered with the variation of MnOx loadings. In accordance with in situ DRIFTS, we found that two new active sites on Rh surfaces resulting from the Rh–MnOx interaction might be responsible for the enhancement of reaction rates and the selectivity to C2+ oxygenates. However, the over-loading of MnOx leads to a decrease in the number of active sites.

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