Single Molecule Imaging of Oxygenation of Cobalt Octaethylporphyrin at the Solution/Solid Interface: Thermodynamics from Microscopy

摘要

For the first time, the pressure and temperature dependence of a chemical reaction at the solid/solution interface is studied by scanning tunneling microscopy (STM), and thermodynamic data are derived. In particular, the STM is used to study the reversible binding of O_2 with cobalt(Ⅱ) octaethylporphyrin (CoOEP) supported on highly oriented pyrolytic graphite (HOPG) at the phenyloctane/CoOEP/ HOPG interface. The adsorption is shown to follow the Langmuir isotherm with P_(1/2)~(298K) = 3200 Torr. Over the temperature range of 10-40 ℃, it was found that ΔH_P = -68 ± 10 kJ/mol and ΔS_P = -297 ± 30 J/(mol K). The enthalpy and entropy changes are slightly larger than expected based on solution-phase reactions, and possible origins of these differences are discussed. The big surprise here is the presence of any O_2 binding at room temperature, since CoOEP is not expected to bind O_2 in fluid solution. The stability of the bound oxygen is attributed to charge donation from the graphite substrate to the cobalt, thereby stabilizing the polarized Co-O_2 bonding. We report the surface unit cell for CoOEP on HOPG in phenyloctane at 25 ℃ to be A = (1.46 ± 0.l)n nm, B = (1.36 ± 0.l)m nm, and α = 54 ± 3°, where n and m are unknown nonzero non-negative integers.