We also probed the OH binding energy (OHBE) of the catalysts by CO stripping experiments (Supplementary Figure ). It is proposed that a lower CO oxidation potential indicates a stronger OHBE . For carbon-supported catalysts, the CO oxidation peak potential of Ru/C is smaller than Pt/C, in agreement with previous studies, indicating a reverse trend for their OHBE . Whereas Pt/pN-C has a similar CO oxidation potential to Pt/C, Ru/pN-C has a CO oxidation peak potential much higher than that of Ru/C (Supplementary Figure ). This result indicates a weakening of OHBE by the pN-C support to Ru sites. Although the peak shift in CO stripping experiments might also be due to varying binding strength of CO to the catalyst surfaces , our correlation of CO oxidation potential with OHBE is consistent with the charge transfer from Ru to pN-C that decreases its binding energies with adsorbates.