Chemistry Reference
In-Depth Information
for such conversions and the high costs associated with the use of some of
the most expensive noble metals as these catalysts.
Electrocatalytic reactions at surfaces are very sensitive to the nanoscale
and atomic-level structure of the heterogeneous interface at which they take
place.
2
Due to the large surface to volume ratio and unique active binding
sites present on the surface, metal nanoparticles (NPs) differ significantly
from their bulk counterparts and have been considered to be excellent al-
ternatives for advanced electrocatalysis. Currently, NPs based on Pt, Ru, Ir
and their alloys are the universal choice as catalysts to improve reaction
kinetics and to reduce reaction over-potentials. However, the-state-of-the-art
NP electrocatalysts developed for advanced electrochemical devices such as
fuel cells and metal-air batteries are still far from their predicted catalytic
potentials, leaving much room for rational design of NP catalysts towards
electrocatalytic activity enhancement.
3
In particular, it has been well docu-
mented that the size, composition, and structure of metal NPs are important
factors in determining their catalysis. In this chapter, we summarize our
recent efforts in controllable synthesis of monodisperse metal NPs for
electrocatalytic reactions, including ORR, formic acid oxidation (FAOR), and
selective CO
2
reduction. We intend to use these high quality NPs as models
to illustrate the important controls achieved in NP catalysis and to under-
stand structure-activity correlations. We believe such studies are important
for rational design and optimization of metallic NPs as commercially viable
catalysts for alternative energy applications.
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9.2 Synthesis of Monodisperse Metal NPs
Being capable of producing monodisperse NPs (standard deviation in
diameter
o
10%) is the first step to study NP properties inherent to nano-
scale dimensions and to distinguish them from those associated with
structural heterogeneities observed in large particles or polydisperse NPs.
4
For
example, the color sharpness of an optical device based on semiconductive
NPsisstronglyNP-uniformitydependent,
5
and the magnetic orientation and
narrow magnetization transitions from NP to NP in a magnetic NP array is
critical for the next-generation multi-terabit magnetic recording applications.
6
This section outlines the general synthetic strategies applied in solution phase
chemistry to prepare monodisperse metal NPs.
.
9.2.1 General Concept on NP Formation
Currently, NPs are often prepared from the high temperature organic phase
reaction. This synthetic method has become highly ecient for the fabri-
cation of metallic NPs with well-defined dimensions.
4,7,8
Specifically, it
allows for the fine control over a number of reaction variables including
reactant, solvent, surfactant, reaction temperature. Compared to the re-
action performed in an aqueous phase solution, organic phase synthesis can
be carried out in a much wider temperature range (from below 0 1C to over
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