Environmental Engineering Reference
In-Depth Information
anion adsorption reactions on the bare Pt sites are directly proportional. From the
relation between the charge magnitudes, it is possible to obtain the ratio between m
and n (usually referred to as the stoichiometry of the adatom reaction), as discussed
in the following paragraphs.
The adatom coverage is given by
q
Ad
nq
Pt(hkl)
u
¼
(7
:
6)
where q
Ad
is the charge density involved in the adatom oxidation process and q
Pt(hkl)
is
the charge density corresponding to the transfer of one electron per Pt atom on the
surface [241 mC/cm
2
for Pt(111) and 209 mC/cm
2
for Pt(100)].
The charge density corresponding to hydrogen adsorption on the free Pt sites is
given by
q
H
¼
q
u
¼
0
(1
mu)
(7
:
7)
H
where q
u
¼
H
is the maximum hydrogen charge density attained in the absence of the
adatom. From (7.6) and (7.7), it is possible to eliminate u, obtaining the following
relation between hydrogen and adatom charge densities:
q
u
¼
0
H
m
n
q
H
¼
q
u
¼
0
q
Pt(hkl)
q
Ad
(7
:
8)
H
Then, the maximum adatom charge density corresponding to full blockage of hydro-
gen adsorption is
q
Ad
¼
n
m
q
Pt(hkl)
(7
:
9)
Since in many cases it is not possible to unambiguously separate hydrogen from anion
adsorption processes, it is convenient to write another equation similar to (7.7) for the
charge density corresponding to anion adsorption on the free Pt sites:
q
An
¼
q
u
¼
0
An
(1
mu)
(7
:
10)
At the time when most of the studies on the characterization of adatom-modified
Pt surfaces were done, it was customary to integrate the total Pt voltammetric
charge, corresponding to the sum of the hydrogen and anion adsorption processes
on the free Pt sites [Clavilier et al., 1989a, 1990a; Evans and Attard, 1993; Feliu
et al., 1991, 1993a, b; G ´ mez et al., 1992]:
¼
q
u
¼
0
Pt
1
m
q
u
¼
0
Pt
q
Pt(hkl)
q
Ad
q
Ad
nq
Pt(hkl)
m
n
q
Pt
¼
q
H
þ
q
An
¼
q
u
¼
0
þ
q
u
¼
0
An
(7
:
11)
H
In this equation, it has been assumed that the adatom exerts the same blockage on
hydrogen and anion adsorption (i.e., the stoichiometric number m is the same for
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