Dynamics of Quinone on Onion-like Carbon Surfaces
Significance and Impact:
Proton coupled electron transfer (PCET) reactions of surface adsorbates play an important role in energy conversion processes. It is important to understand how the adsorbate diffusivity influences the PCET kinetics.
FIRST Center is studying 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 dynamics of the adsorbed quinone molecules at a fluid solid interface.
In this work we have used quasielastic neutron scattering (QENS) to study the dynamics of phenanthrenequinone (PQ) on the surface of onion-like carbon (OLC), or so-called carbon onions, as a function of surface coverage and temperature. For both the high- and low-coverage samples, we observed two diffusion processes. On the high-coverage surface, the slow diffusion process is of long-range translational character, whereas the fast diffusion process is spatially localized on the length scale of ∼4.7 Å. On the low-coverage surface, both diffusion processes are spatially localized; on the same length scale of ∼4.7 Å for the fast diffusion and a somewhat larger length scale for the slow diffusion. Arrhenius temperature dependence is observed except for the long-range diffusion on the highcoverage surface. We attribute the fast diffusion process to the generic localized in-cage dynamics of PQ molecules, and the slow diffusion process to the long-range translational dynamics of PQ molecules, which, depending on the coverage, may be either spatially restricted or long-range. On the low-coverage surface, uniform surface coverage is not attained, and the PQ molecules experience the effect of spatial constraints on their long-range translational dynamics. Unexpectedly, the dynamics of PQ molecules on OLC as a function of temperature and surface coverage bears qualitative resemblance to the dynamics of water molecules on oxide surfaces, including practically temperature-independent residence times for the low-coverage surface. The dynamics features that we observed may be universal across different classes of surface adsorbates.
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