Monitoring by in situ NAP-XPS of active sites for CO2 methanation on a Ni/CeO2 catalyst

Ni/CeO2 catalysts are very active and selective for total hydrogenation of CO2 to methane, but the nature of the active sites is still unclear. The surface of a Ni/CeO2 catalyst has been monitored under CO2 methanation conditions by Near Ambient Pressure-XPS (NAP-XPS) using synchrotron radiation, and has been concluded that the species involved in the redox processes taking place during the CO2 methanation mechanism are the Ni2+-CeO2/Ni-0 and Ce4+/Ce3+ pairs. In addition, a small fraction of nickel is present on the catalyst surface forming NiO and Ni2+-carbonates/hydroxyls (around 20% of the total surface nickel), but these species do not participate in the redox processes of the methanation mechanism. Under CO2 methanation conditions the H-2 reduction rate of the Ni2+-CeO2/Ni-0 and Ce4+/Ce3+ couples is much faster than their CO2 reoxidation rate (2 times faster, at least, at 300 degrees C), but a certain proportion of nickel always remains oxidized under reaction conditions. The high activity of Ni/CeO2 catalysts for CO2 methanation is tentatively attributed to the simultaneous presence of Ni2+-CeO2 and Ni-0 active sites where CO2 and H-2 are expected to be efficiently dissociated, respectively.

Automated summary

The active sites of Ni/CeO2 catalysts for CO2 methanation are identified as Ni2+-CeO2 and Ni-0. The H-2 reduction rate of Ni2+-CeO2/Ni-0 and Ce4+/Ce3+ couples is much faster than their CO2 reoxidation rate, indicating the high activity of the catalyst. The presence of oxidized nickel under reaction conditions also suggests the simultaneous presence of Ni2+-CeO2 and Ni-0 active sites.