Single-Molecule Study on the Catalytic Role of Co–O2 Binding in ORR by In Situ ECSTM

We report here the in situ electrochemical scanning tunneling microscopy (ECSTM) study of O2 adsorption and reduction on cobalt porphyrin (CoOEP) and phthalocyanine (CoPc). Electrochemical measurements reveal higher ORR activity of CoOEP than CoPc. Self-assembled CoOEP and CoPc monolayers are fabricated on Au(111) substrates and molecularly resolved by ECSTM. The direct molecular imaging demonstrates that O2 adsorbs on CoII sites and barely on CoIII sites. The CoOEP–O2 and CoPc–O2 complexes exhibit high contrast in ECSTM images due to O2 binding. The surface coverage of CoOEP–O2 is ca. 1.4 times higher than that of CoPc–O2. The dynamics of O2 adsorption is measured quantitatively by fitting data from potential step experiments to obtain rate constants for O2 adsorption (kad) and dissociation (kdi) on Co centers. kad and kdi are higher on CoOEP than on CoPc, indicating higher dynamics of O2 binding and lower barriers to Co–O dissociation on CoOEP.