Clues for the Molecular-Level Understanding of Electrocatalysis on Single-Crystal Platinum Surfaces Modified by p-Block Adatoms - Fuel Cell Catalysis: A Surface Science Approach - page 217

redox state of the adatom can take place during the collapse of the double layer in
UHV. The question could be solved with careful measurements of the work function
changes after emersion, since it is expected that the work function of the emersed
electrode should be proportional to the emersion potential, while variations from
this trend would indicate that the electrode no longer behaves as ideally polarizable
[Stuve et al., 1995].

7.3 ENERGETICS OF THE SURFACE REDOX PROCESS

There are conceptual similarities between the linear sweep voltammetry and the
temperature programmed desorption (TPD) experiments. The latter are commonly
performed in gas phase studies of adsorption to determine binding energies of
adsorbed species. In this regard, the variation of the electrode potential in the voltam-
metric experiment can be understood as the imposition of an increasing driving force
towards desorption, analogous to the effect of the temperature increase in TPD exper-
iments. Hence, the potential at which desorption takes place contains energetic infor-
mation about the adsorption/desorption process. In the case of the redox processes
exhibited by irreversibly adsorbed adatoms, the reaction driven by the potential is
the adsorption/desorption of oxygenated species, and the peak potential must then
be related to the energy of adsorption of oxygen or hydroxyl species on the
adatom-modified surface. In some particular cases, however, the adatom oxidation
also involves the desorption of the oxidized adatom species. In this case, the peak
potential value will also be influenced by the interaction energy of the adatom with
the surface.
Applying the common equations for the thermodynamics of reversible cells, it is
possible to extract energetic parameters for the adatom redox reaction. This approach
requires the measurement of voltammograms at different temperatures. If we consider
that the adatom oxidation reaction involves the formation of the hydroxide, we can
write the following equation for the overall cell reaction:

Pt-M þ nH 2 O ! Pt-M(OH) n þ 2 nH 2

(7 : 14)

Then, the equilibrium potential ( peak potential) is related to DG 0 for this reaction by:

DG 0 ¼ nFE

(7 : 15)

(Since the reaction in the working electrode is an oxidation when the overall reaction is
(7.14), the cell potential in (7.15) is defined as E ¼ E work - E ref ¼ E anode 2 E cathode
and the sign in this equation is opposite to that obtained with the more common con-
vention that defines the cell potential as E ¼ E cathode - E anode .) From the temperature
variation of the cell potential, the following equation can be written for the entropy of
the overall reaction:

@ E
@ T

DS 0 ¼ nF

(7 : 16)

u