Environmental Engineering Reference
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TABLE 7.4 Summary of Work Function Values [Haas, 1972; Michaelson, 1977;
Trasatti, 1972] and Electronegativities [Lide and Frederikse, 1998] of the Adatoms
Studied in the Present Work. The Corresponding Values for Pt are x 2.2 [Lide and
Frederikse, 1998] and F 5.40 eV [Trasatti, 1972]
S
x¼ 2.58
F 6.2 eV a
Ge [Michaelson, 1977]
As [Haas, 1972]
Se [Michaelson, 1977]
x¼ 2.01
x¼ 2.18
x¼ 2.55
F 5.0 eV
F 4.66 - 4.72 eV
F 5.9 eV
Sn [Trasatti, 1972]
Sb [Trasatti, 1972]
Te [Trasatti, 1972]
x¼ 1.96
x¼ 2.05
x¼ 2.1
F 4.35 eV
F 4.56 eV
F 4.70 eV
Pb [Trasatti, 1972] Bi [Trasatti, 1972]
x¼ 1.8 x¼ 1.9
F 4.18 eV F 4.36 eV
a Work function values calculated from electronegativities [Gordy and Thomas, 1956].
On the other hand, it is also observed that adatom redox processes take place, in
most cases, at higher potential values on a Pt(100) substrate than on a Pt(111) sub-
strate. This behavior is unexpected from the point of view that oxidation of the
Pt(100) substrate takes place at lower potentials than on Pt(111), owing to the more
open surface structure and lower work function of the former.
Finally, it is worth noting that the peak potential value of the adatom redox process
and the enthalpy of formation of the oxide are also correlated with the electronegativity
and work function of the adatom, more electronegative adatoms having higher peak
potential. Electronegativity and work function values of the different adatoms are
summarized in Table 7.4 for comparison purposes.
7.4 STEPPED SURFACES
It has been often stressed that low coordinated atoms (defects, steps, and kink sites)
play an important role in surface chemistry. The existence of dangling bonds makes
steps and kinks especially reactive, favoring the adsorption of intermediate species
on these sites. Moreover, studies of single-crystal surfaces with a complex geometry
have been demonstrated very valuable to link the gap between fundamental studies
of the basal planes [Pt(111), Pt(100), and Pt(110)] and applied studies of nanoparticle
catalysts and polycrystalline materials. In this context, it is relevant to mention results
obtained with adatom-modified Pt stepped surfaces, prior to discussing the effect of
adatom modification on electrocatalysis.
Interestingly, it has been shown that some adatoms can be selectively deposited on
step sites, taking advantage of the enhanced reactivity of these sites. Figure 7.5 shows
the voltammogram of a Pt(775) surface in 0.5 M H 2 SO 4 . The hard sphere model for
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