Surface immobilized catalysts are highly promising candidates
for
a range of energy conversion reactions, and atomistic mechanistic
understanding is essential for their rational design. Cobalt tetraphenylporphyrin
(CoTPP) nonspecifically adsorbed on a graphitic surface has been shown
to undergo concerted proton-coupled electron transfer (PCET) in aqueous
solution. Herein, density functional theory calculations on both cluster
and periodic models representing π-stacked interactions or axial
ligation to a surface oxygenate are performed. As the electrode surface
is charged due to applied potential, the adsorbed molecule experiences
the electrical polarization of the interface and nearly the same electrostatic
potential as the electrode, regardless of the adsorption mode. PCET
occurs by electron abstraction from the surface to the CoTPP concerted
with protonation to form a cobalt hydride, thereby circumventing Co(II/I)
redox. Specifically, the Co(II) d-state localized orbital interacts
with a proton from solution and an electron from the delocalized graphitic
band states to produce a Co(III)–H bonding orbital below the
Fermi level, corresponding to redistribution of electrons from the
band states to the bonding states. These insights have broad implications
for electrocatalysis by chemically modified electrodes and surface
immobilized catalysts.