Colloidal chemistry holds promise to prepare uniform and size-controllable pre-catalysts; however, it remains a challenge to unveil the atomic-level transition from pre-catalysts to active catalytic surfaces under the reaction conditions to enable the mechanistic design of catalysts. Here, we report an ambient-pressure X-ray photoelectron spectroscopy study, coupled with in situ environmental transmission electron microscopy, infrared spectroscopy, and theoretical calculations, to elucidate the surface catalytic sites of colloidal Ni nanoparticles for CO₂ hydrogenation. We show that Ni nanoparticles with phosphine ligands exhibit a distinct surface evolution compared with amine-capped ones, owing to the diffusion of P under oxidative (air) or reductive (CO₂ + H₂) gaseous environments at elevated temperatures. The resulting NiP_x surface leads to a substantially improved selectivity for CO production, in contrast to the metallic Ni, which favors CH₄. The further elimination of surface metallic Ni sites by designing multi-step P incorporation achieves unit selectivity of CO in high-rate CO₂ hydrogenation.