The structure sensitivity of CO 2 activation in the presence of H 2 has been identified by ambient-pressure X-ray photoelectron spectroscopy (APXPS) on Ni(111) and Ni(110) surfaces under identical reaction conditions. Based on the APXPS results and computer simulations, we propose that, around room temperature, the hydrogen-assisted activation of CO 2 is the major reaction path on Ni(111), while the redox pathway of CO 2 prevails on Ni(110). With increasing temperature, the two activation pathways are activated in parallel. While the Ni(111) surface is fully reduced to the metallic state at elevated temperatures, two stable Ni oxide species can be observed on Ni(110). Turnover frequency measurements indicate that the low-coordinated sites on Ni(110) promote the activity and selectivity of CO 2 hydrogenation to methane. Our findings provide insights into the role of low-coordinated Ni sites in nanoparticle catalysts for CO 2 methanation.