Environmental Engineering Reference
In-Depth Information
of quasi-melt [Ajayan and Marks, 1988], and this has been indeed confirmed exper-
imentally [Henry, 2003].
Relaxation of a metal nanoparticle to its equilibrium shape requires substantial acti-
vation energy, and thus may not be reached on the time scale of catalytic reactions.
This is especially the case for catalysts obtained by low temperature preparation
approaches (e.g., colloidal or micellar) and used in model electrochemical cells in
the temperature range between 20 and 80 8C or in low temperature fuel cells operating
below about 130 8C. Various approaches to obtain preferentially oriented cubic,
pyramidal, elongated, etc. particles have been developed [Burda et al., 2005;
El-Deab et al., 2005], and exploration of their electrocatalytic properties [Solla-
Gullon et al., 2006; Hernandez et al., 2005] is a fascinating area of research that
adds to the understanding of the role of sites of different geometry in (electro)catalysis.
So far, we have discussed the shapes of nanocrystals in contact with vacuum.
Immersion of the particles in gas or liquid environments may result in substantial
changes. Indeed, it is known that adsorption decreases the surface energy [Herring,
1952]. Since the adsorption energy usually depends on the surface crystallography,
the equilibrium particle shape may be altered upon adsorption. The influence of
adsorption on the shapes and lattice parameters of metal nanoparticles has been pre-
dicted theoretically and confirmed experimentally (see the references cited in Henry
[2003]). Adsorption/desorption occurring during heterogeneous catalytic reactions
may thus result in dynamic changes of particle shapes, as predicted by Monte Carlo
simulations [Zhdanov, 2002; Kovalyov et al., 2003, 2008]. In electrochemical sys-
tems, a potential drop develops at the interface between the catalytic nanoparticles
and the electrolyte. This may also induce changes in particle shape. Experimental
data concerning the particle shapes determined in situ during electrochemical oper-
ation are rare. Mukerjee and McBreen performed in situ extended X-ray absorption
fine structure (EXAFS) studies of carbon-supported Pt particles with the average
diameters ranging from 3 to 9 nm in 1 M HClO
4
[Mukerjee and McBreen, 1998].
Pt - Pt apparent CNs at 0.0 V (with respect to a reversible hydrogen electrode, RHE)
were in excellent agreement with those calculated for a cubo-octahedral shape.
However, an increase of the electrode potential to 0.54 V vs. RHE resulted in a
substantial drop in Pt - Pt CNs, which led Mukerjee and McBreen to conclude that
there was a change in particle morphology from a cubo-octahedral to a plane raft-
like configuration. Further in situ studies would be desirable for establishing the
influence of the interfacial potential drop and the adsorbates on the shapes of metal
nanoparticles.
In fuel cell electrocatalysis, high metal loadings per unit area of support are nor-
mally utilized in order to minimize the thickness of the catalytic layers [Vielstich
et al., 2003]. This leads to yet another complication: particle coalescence and
formation of complex structures where nanometer-sized particles contact each other
through grain boundaries. This is illustrated in Fig. 15.1, which shows Pd (a) and
Pt-Ru (b) particles supported on model carbon supports of the Sibunit family.
One can see that the Pd catalyst comprises isolated 2 - 5 nm particles consisting
of single grains, while in high loading Pt-Ru catalysts, individual grains merge
together to form complex structures with a high density of grain boundary regions.
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