[en] Highlights: • A convenient method to tune the surface composition of bimetallic catalysts. • Establish a correlation between the surface composition and catalytic activities. • The COOH_ad intermediate of CO oxidation reaction was detected by in-situ FTIRS. • New insights into the CO electro-oxidation reaction is proposed. Improving the CO tolerance of Pt-based catalysts is very meaningful for the application in proton exchange membrane fuel cells and direct alcohol fuel cells. The behavior of Pt-based bimetallic catalysts is depended on their phase and surface composition. In this work, we tuned the surface composition of Pt₃Co nanocatalysts by heat treatment under different atmosphere. The results of atomic-resolution HAADF-STEM, XRD, XPS and electrochemical characterization demonstrated that the surface composition of Pt₃Co catalysts with Co-increased, Intermetallic and Pt-increased were obtained by metal segregation approach. Due to the differences in the surface atomic distribution and alloying extent, the nanocatalysts show different CO poisoning tolerance in the order of Co-increased>Intermetallic>Pt-increased. CO stripping voltammetry and in-situ Fourier transform infrared spectroscopy (FTIRS) were used together to investigate the origin of varied CO poisoning tolerance on three Pt₃Co catalysts. The results illustrated that electronic effect plays a major role in weakening CO adsorption on Pt₃Co nanocatalysts and thus promoting CO oxidation to form COOH_ad intermediate consistent with Langmuir-Hinselwood mechanism. Oxophilic effect promotes the oxidation of COOH_ad intermediate into the final products CO₂/CO₃^2-. This work provides a new insight into tuning phase and surface composition of catalysts thus enhancing CO tolerance of Pt-based bimetallic catalysts.