By the late 1980s, electrochemical nuclear magnetic
resonance spectroscopy (EC-NMR) was introduced as a new
technique for electrochemical surface science. EC-NMR is a
powerful local probe that combines solid-state NMR with
electrochemistry and provides an electronic level
description of electrochemical interfaces based on the
Fermi level local densities of states (Ef-LDOS). 13C and
195Pt NMR form an ideal pair of non-invasive microscopic
probes which can be used to investigate both sides of the
electrochemical interface to understand nanoparticle
electrode surface modifications (particle size effects,
adsorption/deposition, electrode potential change, etc).
CO molecules adsorbed on Pt nanoparticle electrode surfaces
become metallic as evidenced by the 13C Knight shift and
Korringa relaxation behavior. Thus, the adsorbate on a
platinum electrode belongs to the metal part of the
platinum-solution interface. A layer model analysis can
adequately describe the 195Pt NMR spectrum of nanoscale
electrode materials. The shifts of the surface and
sub-surface peaks of 195Pt NMR spectra correlate well with
the electronegativity of various adsorbates, while the
Knight shift of the adsorbate varies linearly with the
Ef-LDOS of the clean metal surface. Further, a
spatially-resolved oscillation in the s-like Ef -LDOS was
observed via 195Pt NMR of a carbon-supported Pt catalyst
sample. The data indicate that much of the observed
broadening of the bulk-like peak in 195Pt NMR spectra of
such systems can be attributed to spatial variations of the
Ds(Ef). 195Pt NMR of commercial Pt-Ru alloy nanoparticles
has indicated that the surface of these catalysts are
enriched with Pt atoms.
The electrode potential dependence of the 13C NMR spectra
of CO adsorbed on Pt and Pd nanoparticles provide direct
evidence for electric field induced alterations in the Ef
-LDOS. In relation to fuel cell catalysis, EC-NMR
investigations of Pt nanoparticles decorated with Ru show
that there exist two different kinds of CO populations
having markedly different electronic properties. COs
adsorbed on the Ru phase of Pt/Ru electrodes undergo faster
diffusion and have a reduced Ef -LDOS, indicating that the
CO-metal interaction is weakened due to the presence of Ru.
The enhanced catalytic activity of Pt/Ru electrodes shows a
direct correlation with the electronic alterations revealed
by EC-NMR studies. These results suggest that prospects for
further studies are bright and indeed the method will have
broader applications to improve our understanding of the
electronic structure of electrochemical interfaces.
We have expanded our EC-NMR project to the study of new
catalyst systems like Ru-Se (77Se NMR) and to new materials
(carbon "nanohorns") that have potential applications in
the field of fuel cell catalysis.