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Production of\'renewable Methane\'has attracted renewed research interest as a fundamental probe reac-tion and process for CO2 utilization through potential use in C1 fuel production and even for future space exploration technologies.CO2 methanation is a structure sensitive reaction on Ni/CeO2 catalysts.To pre-cisely elucidate the size effect of the Ni metal center on the CO2 methanation performance,we prepared 2％Ni/CeO2 catalysts with pre-synthesized uniform Ni particles (2,4 and 8 nm) on a high surface area CeO2 support.Transmission electron microscopy (TEM) and ambient pressure X-ray photo spectroscopy(AP-XPS) characterization have confirmed that the catalyst structure and chemical state was uniform and stable under reaction conditions.The 8 nm sized catalyst showed superior methanation selectivity over the 4 and 2 nm counterparts,and the methanation activity in term of TOF is 10 times and 70 times higher than for the 4 and 2 nm counterparts,respectively.The DRIFTS studies revealed that the larger Ni (8 nm particles) over CeO2 efficiently facilitated the hydrogenation of the surface formate intermediates,which is proposed as the rate determining step accounting for the excellent CO2 methanation performance.