Environmental Engineering Reference
In-Depth Information
Finally, the enthalpy of the reaction can be extracted from
@
(E
=
T)
@
(1
=
T)
DH
0
¼
nF
(7
:
17)
u
It is interesting to compare these thermodynamic magnitudes with the bulk formation
of the hydroxide, given by:
M
þ
2
H
2
þ
2
O
2
!
M(OH)
n
(7
:
18)
Clearly, (7.14) and (7.18) are related by the reaction of water formation:
H
2
þ
2
O
2
!
H
2
O
(7
:
19)
Consequently, it is convenient to convert the thermodynamic results obtained in the
electrochemical media, corresponding to the reaction (7.14), to those corresponding
to the analogous reaction (7.18). This can easily be done by adding the tabulated
values of the energetic parameters of water formation: (DG
H
2
O
¼
237
:
1kJ
=
mol,
DH
H
2
O
¼
285
:
8kJ
=
mol, and DS
H
2
O
¼
163
:
3J
=
mol K) [Lide and Frederikse, 1998].
It is worth mentioning that, in some cases, the adatom oxidation reaction may
involve the formation of oxide species. In particular, the oxidation of electronegative
adatoms, such as Te and Se, involves the interchange of four electrons, giving rise to
Te(IV) and Se(IV) species. The formation of tetravalent hydroxide species seems very
unlikely, both for steric reasons and because of the strong polarizing character of the
cation, and hence the following equation for the overall cell reaction can be proposed:
Pt-M
þ
2
H
2
O
!
Pt-MO
n
=
2
þ
2
H
2
(7
:
20)
In this case, the values of the thermodynamic magnitudes can be also determined
from (7.15) - (7.17), but these results should be compared with the bulk formation
of the oxide:
M
þ
4
O
2
!
MO
n
=
2
(7
:
21)
Unfortunately, the above thermodynamic approach has only been followed for
two systems, namely As-Pt(111) and Bi-Pt(111) [Blais et al., 2001, 2002].
Table 7.3 summarizes the main results. Thermodynamic data about the bulk formation
of As(OH)
3
and Bi(OH)
2
are not available for comparison. The only data available is
the standard enthalpy for the bulk formation of Bi(OH)
3
(DH
f
¼
711
:
3kJ
=
mol).
Considering that the standard enthalpies of formation of various oxides, sulfides,
and halides of As, Sb, and Bi are very close to each other, the similarity between
this value and the standard enthalpy for the surface formation of As(OH)
3
,
DH
f
¼
680 + 20 kJ
=
mol, was taken as an indication that, indeed, the As redox pro-
cess on Pt(111) involves the formation of hydroxide species [Blais et al., 2001]. For
comparison with DH
f
for the surface formation of Bi(OH)
2
, the strategy followed
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