The promoting role of Ag in Ni-CeO2 catalyzed CH4-CO2 dry reforming reaction

• Ag-Ni surface alloy catalyst supported on CeO2 is coke-resistant for the dry reforming reaction of methane and carbon dioxide.
• Ag binds on under coordinated step sites on Ni surface to inhibit the nucleation and growth of graphic and whisker carbon.
• 0.3 mol.% Ag promoter is enough to block Ni surface steps for Ni/CeO2, excessive Ag does not cause the elevation of activation energy.

The catalytic performance of Ag promoted Ni/CeO2 catalyst has been investigated in methane and carbon dioxide dry reforming reaction. The temperature-dependent catalytic activity and time-on-stream catalytic performance at 760 °C under different feeding conditions have been explored. Pristine Ni/CeO2 is not stable as a result of coke deposition and surface reconstruction. In addition, co-feeding of H2 can adversely influence the long-term stability of Ni/CeO2. Ag severely reduces the intrinsic catalytic activity of Ni/CeO2 catalyst, while enhances the long-term stability by diminishing coke deposition. Ag is applicable as a promoter under various reaction conditions, from N2 diluted feedstock to H2 co-fed and carrier-gas free feedstock. Kinetic studies show that Ag elevates the activation energy from 91 to ca. 140 kJ/mol by a loading of 0.3 mol.%, excessive Ag loading does not affect activation energy. The role of Ag is to block step sites on Ni surface that is associated with carbon nucleation and growth, and to promote gasification of coke formed. Besides, Ag alters the type of coke formed over Ni surface from recalcitrant whisker/encapsulating carbon to easily gasifiable amorphous carbonaceous species.

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