Interfacial Electronic Structure of CO Absorbed on a Platinum Electrode in Electrochemical Environment Probed by Double Resonance Sum Frequency Generation Spectroscopy

Absorbed CO on Pt surface has attracted much interest among scientists in surface science, catalysis, and electrochemistry. Information about oxidation mechanisms of CO on Pt, which depends on geometric and electronic structures of CO/Pt interface, is important to design and develop new catalysts for various reactions including H₂-O₂ and direct methanol fuel cells. Small amount of CO present in H₂ produced from fossil fuel readily adsorbs on Pt and inhibits H₂ oxidation. Adsorbed CO is also formed when Pt is in contact with methanol solution and inhibits the methanol oxidation reaction. Compared to extensive study on CO/Pt system under ultrahigh vacuum (UHV), interfacial electronic structure of CO/Pt interface in electrochemical environment is less understood due to the difficulty in determining it in solution. Accordingly, it is very important to investigate interfacial electronic structure of CO/Pt under electrochemical condition. Because of the presence of solution, it is not possible to use techniques with electron probes, which are most powerful in UHV. Double resonance sum frequency generation (DR-SFG) spectroscopy, which is a highly surface-sensitive nonlinear optical technique with the tunable frequencies of both incident fundamental lights, i.e., visible and IR, can provide information of surface molecular and electronic structures, which is not obtained by traditional surface vibrational technique. Here, we employed DR-SFG technique to probe potential-dependent electronic structure of CO/Pt interface in methanol solution. SFG signal due to C-O stretching was observed and was dependent on frequency of visible light as a result of the energetic coupling of electronic transition with the input visible photon and/or output SF photon, i.e., double resonance effect. Furthermore, resonant visible energy linearly depended on electrode potentials. Thus, the electronic resonance could be explained by the transition from Fermi energy, E_f, of Pt to the unoccupied sigma anti-bonding state of CO.