Controllable Synthesis of Metal Nanoparticles for Electrocatalytic Activity Enhancement - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 242

HCOOH and to inhibit the dehydration reaction that leads to the formation
of CO. In durability test, the fct-Fe 43 Pt 37 Au 20 NPs are found to be more stable
than the fcc counterparts, suggesting that the fct structure does favor the
durability enhancement of FePtAu NPs for FAOR.

d n 9 r 4 n g | 5

9.4.3 Metal NPs as Catalysts for Electrochemical Reduction
of CO 2
The monodisperse metal NPs prepared from the organic solution phase
reaction allows for detailed studies of NP catalysts not only for fuel cell re-
actions, but many other reactions as well. One such reaction that has
stimulated great interest recently is the catalytic reduction of CO 2 into other
active carbon forms to reduce the problems caused by the accelerated con-
sumption of fossil fuels and the resultant over-production of CO 2 . 64 Among
many different approaches developed thus far for CO 2 reactivation, elec-
trochemical reduction of CO 2 is considered a potentially 'clean' method as
the reduction proceeds at the expense of a sustainable supply of electric
energy. 65 Theoretically, CO 2 can be reduced to different kinds of
hydrocarbons such as carbon monoxide, formic acid, methane or other
hydrocarbons at potentials around þ 0.2 to 0.2 V (vs. RHE). Experi-
mentally, however, very negative potentials must be applied to initiate CO 2
reduction. 66
Recent studies have shown that nanocatalyst can be used to enhance
electrochemical reduction of CO 2 , as demonstrated in the reduction of CO 2
to CO by Ag NPs in ionic liquid solution, 67 and by Au nanostructured sur-
face. 68 Au NPs are also active for CO 2 reduction and their activity is NP size
dependent. In the study, monodisperse polycrystalline Au NPs were first
prepared via the organic phase reduction of HAuCl 4 . 4 nm and 6 nm Au NPs
(Figure 9.13a) were formed by fast injection of borane tert-butylamine
complex. Au NPs of sizes 8 nm and 10 nm were synthesized by seed-
mediated growth of Au over the 6 nm Au seeding NPs. 69 These NPs were first
deposited on the carbon support and activated by thermal annealing at
180 1C in air. Electrochemical analyses show that CO 2 is reduced CO on the
Au NP surface and the 8 nm Au NPs are the most selective catalyst with the
reduction faraday eciency (FE) reaching 90% at 0.67 V (Figure 9.13b),
while the 4 nm NPs have the highest mass activity due to their smaller size
(Figure 9.13c). The high selectivity of the 8 nm Au NPs arises from the
dominant edge sites present on their surface due to the polycrystalline na-
ture (with an average crystal domain diameter of 4 nm) of the NPs. This is
supported by the DFT calculations that the edge sites on the Au NP surface
facilitate CO 2 reduction to CO while the corner sites are active for proton
reduction to hydrogen. When the Au crystallite have the diameter greater
than 4 nm, the edge sites become dominant over corner sites on the NP
surface (Figure 9.13d). These studies suggest that metal NPs, when prepared
with the size and structure control, can be optimized for electrochemical
reduction of CO 2 .

.