Electrochemical Atomic-level Controlled Syntheses of Electrocatalysts for the Oxygen Reduction Reaction - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 152

when the displacement step of the SLRR protocol is realized in Pt 41 solution.
In contrast, Figure 6.2B shows that when the displacement is conducted in
Pt 21 solution almost complete Pt ML is accomplished.
Further studies addressed the role of support in determining the activity
of Pt monolayer as catalysts for the oxygen reduction reaction. Several single
crystal surfaces, including Pd(111), Ru(0001), Ir(111), Rh(111), and Au(111)
have been investigated as supports for Pt ML catalysts for the ORR. It is
reported that the ORR activity of Pt ML on Au(111), Ir(111), Pd(111), Rh(111),
and Ru(0001) surfaces showed volcano-type dependence versus the d-band
center of the Pt ML. 24-26 Surprisingly, it was found that Pt ML supported on
Pd(111) single crystal surface possessed better activity toward the ORR than
the best known up to date Pt(111) single crystal surface in perchloric acid
solution. 26 An improved activity of the Pt ML /Pd(111) for the ORR is explained
by geometric and electronic (ligand) effects. 25 The geometric effect accounts
about the degree of compression/tension of the Pt ML as a result of the
atomic size difference between Pt and supporting metal substrate. Thus, due
to the close similarity in the atomic size of Pt and Pd, the Pt ML supported on
Pd(111) surface, is slightly compressed. The compression results in down-
shift of the d-band center of Pt ML, the electronic effect which is beneficial
for the ORR kinetics. 26 According to the DFT calculations, an electronic
interaction (ligand effect) between Pt ML and Pd support results in superior
ORR catalytic activity of Pt ML /Pd(111) due to the reduced OH coverage. 24,25
Significant enhancement of the kinetics of the ORR has also been con-
firmed for Pt ML on Pd carbon supported nanoparticles (NPs) in comparison
to the reaction on Pt(111) and Pt/C nanoparticles. 27 The Pt ML /Pd/C electro-
catalysts are prepared by using the SLRR strategy, where a Cu UPD ML is first
deposited on the Pd NPs surface and then is galvanically displaced by Pt
from [PtCl 4 ] 21 containing solution. It is reported that the four-electron re-
duction mechanism, with a first-charge transfer-rate determining step, is
operative on both Pt ML /Pd(111) and Pt ML /Pd/C surfaces, with a very small
amount of H 2 O 2 detected on the ring electrode in the hydrogen-adsorption
potential region. 27
Initially, the synthesis of the Pt ML elctrocatalysts is carried out on a glassy
carbon electrode with diameter of 5 mm and the quantity of the electro-
catalysts synthesized by one experiment is limited to a few tens of micro-
grams. Later a scale-up synthesis method using a new electrochemical cell
that allows a synthesis of gram quantities of Pt monolayer electrocatalysts has
been developed. 28 The core-shell structure of the Pt ML /Pd/C electrocatalyst,
synthesized by the scaled-up method, has been verified using high-angle an-
nular dark field (HAADF) scanning transmission electron microscopy (STEM)
Z-contrast images, STEM-EELS (electron energy-loss spectroscopy), and STEM/
EDS (energy-dispersive X-ray spectrometry) line profile analysis.
Figure 6.3, shows a HAADF-STEM image of a single carbon supported
Pt ML /Pd nanoparticle, synthesized by the scaled-up method based on the
SLRR of Cu UPD ML by Pt from Pt 21 solution, and a line profile analysis
using STEM-EDS.

d n 9 r 4 n g | 4

.