Pt-based
bimetallic nanoframes have been demonstrated to have high
activity for a number of electrocatalytic reactions. Their morphology,
crystal facets, and compositions are important factors that regulate
their catalytic activities. Herein, we synthesized a series of Pt-surfaced
PtNi dodecahedral nanoframes with variable Pt/Ni ratios. The nanoframes
were prepared by oxidative etching of presynthesized PtNi rhombic
dodecahedron nanoparticles. The Pt ratio in the PtNi nanoframes have
been tuned from 28% to 65% by changing the duration of oxidative etching.
In terms of catalytic performance, the PtNi nanoframes display a volcano-type
behavior in specific oxygen
reduction reaction (ORR) activity as a function of Pt ratio with a
maximum ORR specific activity of 1.9 mA cm–2 observed
with 47% Pt. The mass activity of the particles ranges from 0.72 to
0.92 A mg–1, which significantly exceeds the mass
activity of 0.19 A mg–1 measured for commercial
Pt NP/C. Density functional theory calculations reveal that the Pt
ratio underneath the Pt skin in the nanoframes affects the binding
energy of oxygenate species and thus the ORR activity. The trend of
OH binding energy versus PtNi composition from the computational results
qualitatively agrees with the trend of ORR activity from the experiments.
A template-directed, sol–gel
synthesis is utilized to produce
crystalline RuO2 nanowires. Crystalline nanowires with
a diameter of 128 ± 15 nm were synthesized after treating the
nanowires at 600 °C in air. Analysis of these nanowires by X-ray
powder diffraction revealed the major crystalline phase to be tetragonal
RuO2 with a small quantity of metallic ruthenium present.
Further analysis of the nanowire structures by high-resolution transmission
electron microscopy reveals that they are polycrystalline and are
composed of interconnected, highly crystalline, nanoparticles having
an average size of ∼25 nm. Uniform 3 nm Pt nanoparticles were
dispersed on the surface of RuO2 nanowires using an ambient,
solution-based technique yielding a hybrid catalyst for methanol oxidation.
Linear sweep voltammograms (LSVs) and chronoamperometry performed
in the presence of methanol in an acidic electrolyte revealed a significant
enhancement in the onset potential, mass activity, and long-term stability
compared with analogous Pt nanoparticles supported on commercially
available Vulcan XC-72R carbon nanoparticles. Formic acid oxidation
LSVs and CO stripping voltammetry revealed that the RuO2-supported Pt nanoparticles exhibit significantly higher CO tolerance,
which leads to higher catalytic stability over a period of several
hours. X-ray photoelectron spectroscopy results suggest that crystalline
RuO2 leads to less-significant oxidation of the Pt surface
relative to more widely studied hydrous RuO2 supports,
thereby increasing catalytic performance.
The electrochemical oxidation of small organic molecules (SOMs) such as methanol and glucose is a critical process and has relevant applications in fuel cells and sensors. A key challenge in SOM oxidation is the poisoning of the surface by carbon monoxide (CO) and other partially oxidized intermediates, which is attributed to the presence of Pt−Pt pair sites. A promising pathway for overcoming this challenge is to develop catalysts that selectively oxidize SOMs via "direct" pathways that do not form CO as a primary intermediate. In this report, we utilize an ambient, template-based approach to prepare PtAu alloy nanowires with tunable compositions. X-ray photoelectron spectroscopy measurements reveal that the surface composition matches that of the bulk composition after synthesis. Monte Carlo method simulations of the surface structure of PtAu alloys with varying coverage of oxygen adsorbates and varying degrees of oxygen adsorption strength reveal that oxygen adsorption under electrochemical conditions enriches the surface with Pt and a large fraction of Pt−Pt sites remain on the surface even with the Au content of up to 50%. Electrochemical properties and the catalytic performance measurements of the PtAu nanowires for the oxidation of methanol and glucose reveal that the mechanistic pathways that produce CO are suppressed by the addition of relatively small quantities of Au (∼10%), and CO formation can be completely suppressed by 50% Au. The suppression of CO formation with small quantities of Au suggests that the presence of Pt−Au pair sites may be more important in determining the mechanism of SOM oxidation rather than Pt−Pt pair site density.
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