Environmental Engineering Reference
In-Depth Information
15.6 SUMMARY AND OUTLOOK
We have presented a selected survey of the literature devoted to the investigation of
PSEs in fuel cell-related electrochemical reactions. This review has been almost
entirely devoted to the discussion of Pt/C, with some occasional allusions to other
materials. This largely reflects today's situation in electrocatalysis at supported
metal nanoparticles, and makes generalizations difficult, if not impossible. In order
to further understanding of PSEs, it would be desirable to extend the range of
metals and supports investigated. Studies are now underway in Hayden's group,
utilizing Au and other metals supported on oxide-based (e.g., TiO
2
) supports
Guerin et al. [2006a, b]; Hayden et al. [2007] (see Chapter 16). Such investigations
are imperative for better understanding of PSEs and particle - support interactions.
Despite some controversies, most workers in this field admit the existence of PSEs
in the particle interval from 1 to 5 nm. Most often, so-called “negative” PSEs are
observed, with SA decreasing with decreasing particle size. This has been documented
for the ORR and the MOR and for CO monolayer electro-oxidation. Meanwhile, for
the FOR and the HOR, the activity increases with decreasing particle size, exhibiting a
so-called “positive” PSE. As underlined in this chapter, up to now PSEs have been
investigated in the scalable size range. They scale with inverse particle diameter,
suggesting that they are related to properties dependent on the surface-to-volume
ratio rather than to nonscalable quantum size effects. In order to account for PSEs,
the following reasons have been invoked:
†
An “ensemble” effect related to the variation of the number of sites of specific
configuration with particle size. Ensemble effects have been, for example,
suggested to account for the decrease in H
UPD
coverage, and the SA in the MOR.
†
A surface crystallography effect, which is, in fact, closely related to the “ensem-
ble” effect.
†
Lower average CNs related to the increase in the contribution of edge and corner
sites, concomitant with the d-band shift and an increase in the proportion of
d-band vacancies, resulting in changes in adsorbate bond strength (stronger
OH/O and CO bonding to Pt nanoparticles), and also changes in the type of
adsorption sites (cf. atop CO on Pt nanoparticles versus partitioning among
atop, bridge, and three-fold hollow site bonded CO on Pt(111)).
†
Changes in electronic properties, such as Fermi level shifts and changes in the
DOS, which still have to be confirmed experimentally. For example, Yano and
co-workers used
195
Pt NMR to probe possible changes in electronic properties
induced by particle size [Yano et al., 2006]. They concluded that the surface
Knight shifts as well as E
F
-LDOS of surface Pt atoms showed no noticeable
size dependence in the particle size range from 4.8 down to 1.6 nm; hence,
electronic properties are independent of size in this interval.
†
Changes in the scaling of chemical potential with 1/d [Nagaev, 1991, 1992;
Parmon, 2007; Savinova, 2006]. This could account for the similar slopes of
the SA versus 1/d plots for different electrochemical reactions.
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