The Ru-additive or Ru-free Ni-based catalysts were applied in the methanation of CO in hydrogen-rich gas. In situ CO chemisorbed infrared spectroscopy exhibited that the defect sites formed on the metallic Ni-0 particles dominated the reaction activity. As compared with the monometallic Ni-P catalyst, the addition of Ru increased the defect sites. This Ni-Ru synergistic effect depended on the particle structure. The NiRu-P catalyst with bimetallic Ni-Ru particles formed the defect sites 3 fold more than NiRu-NP catalyst whose Ru separated from Ni-0 particles. Consequently, the methanation activity decreased in the order of NiRu-P > NiRu-NP > Ni-P catalyst. (C) 2016 Elsevier B.V. All rights reserved.