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Dispersive Bonding Explains Chemical Attraction:
Quinone on Onion-like Carbon Surfaces

Scientific Achievements: 

A combination of in situ spectroscopy, neutron scattering and computational modeling have been combined to explain the unexpected stability of 9,10-phenanthrenequinone observed in electrochemical measurements.

Significance and Impact:

Detailed description of bonding and diffusion along with measurements of electrochemical processes in this model redox system will provide a basis for understanding of proton coupled electron transfer (PCET) reactions that are crucially important in many energy related reactions

 Research Details:

•  PQ is bonded  to OLC in a parallel orientation through π interactions

•  Changes of the torsional and out-of-plane ring deformation modes of the adsorbed molecule are signaled by red-shifts in the lowest frequency modes

•  Dispersive forces lead to attraction and adsorption energy near 1 eV as indicated by DFT-D calculationz

Comparison of inelastic neutron scattering with computed low energy modes of PQ in bulk and ad-sorbed state demonstrate dispersive parallel bonding

Additional Description:

Research in the Fluid Interface Reactions, Structures and Transport (FIRST) Center is aimed at learning how reactions occur at fluid solid interfaces. Many electrochemical and photochemical reactions of energy relevance (e.g water splitting, carbon dioxide reduction, oxygen reduction) undergo stepwise or coupled electron and proton transfers, the key reactions in energy conversion. To learn how these reactions occur, researchers at FIRST Center have been studying electron/proton transfer reactions on a model redox couple, the quinone-hydroquinone conversion on carbon surfaces. Of key importance is understanding the surface bonding and interaction of the adsorbed quinone molecules at a fluid solid interface. In this work we have used inelastic neutron scattering and Raman, closely linked to density functional theory (DFT) to probe the orientation and bonding of adsorbed 9,10-phenanthrenequinone (PQ) on onion-like carbon (OLC). Electrochemical measurements demonstrated relatively strong bonding of PQ to the OLC as indicated by persistent and reversible features in the cyclic voltammetry. Spectra of bulk solid and adsorbed PQ were obtained by Inelastic Neutron Scattering (INS) and Raman spectroscopy, and the bands were compared with vibrational energies calculated from DFT. At energy losses (frequencies) above 400 cm-1, no band shifts in INS or Raman spectra were observed between bulk solid and adsorbed PQ. However, adsorption of PQ resulted in shifts in the lowest frequency modes (< 400 cm-1), compared to crystalline PQ, which could only be identified by INS. DFT calculations also provided adsorption energies and from these and comparison of computed and experimental spectra it is determined that the molecule adsorbs parallel to the onion-like carbon surface through p-p stacking interaction.

Credit:
This work was published by D. M. Anjos, A. Kolesnikov, Z. Wu, Y. Cai, M. Neurock, G. M. Brown and S. H. Overbury, Carbon 2013, 52, 150.
This work is supported as part of the Fluid Interface Reactions, Structures and Transport (FIRST) Center an Energy Frontier Research Center funded by the U.S. Department of Energy, Office eof Science and Office of Basic Energy Sciences.

Surface Chemistry and Catalysis R&D Projects

Provided by Oak Ridge National Laboratory's Chemical Sciences Division
Rev:   February 2014