Chlorine-doped catalysts Ni(Clx)/ZEO(2) (x=0.0-0.6) were prepared for selective methanation of CO in H-2-rich gas (CO-SMET). Effects of chlorine ions were studied by means of X-ray diffraction (XRD), temperature programmed reduction (TPR) and adsorption measurements. It is found that chlorine ions could promote Ni crystallite growth and suppress CO2 adsorption. As a result, a deep removal of CO to below 10 ppm was achieved with a high selectivity of more than 50% in a wide reaction temperature range. And, at the reaction temperature of 220 degrees C, the selectivity of CO methanation kept at 100% with CO outlet concentration below 10 ppm in the 100 h durability test over the catalyst Ni(Cl-0.1)/ZrO2. However, the selectivity decreased with reaction time at the higher reaction temperatures of 240 degrees C and 280 degrees C. Both X-ray fluorescence spectroscopy (XRF) analysis and ion chromatography analysis proved that chlorine ions evolved in the catalytic reaction, which then allowed for more CO2 to be adsorbed on the catalyst and hydrogenated into methane. This finally resulted in the decrease of the selectivity of CO methanation. (C) 2017 Elsevier B.V. All rights reserved.