We describe a comparative study of the oxygen reduction reaction on two carbon-supported Pt-based alloy
catalysts in aqueous acidic electrolyte at low temperature. Both alloys have the bulk compositions of 50 and
75 at. % Pt, with the alloying elements being Ni and Co. Comparison is made to a pure Pt catalyst on the
same carbon support, Vulcan XC-72, having the same metal loading (20 wt %) and nominally the same
particle size (4 ± 2 nm). High-resolution electron microscopy was used to determine the size and shape of
the particles as well as the particle size distribution on all catalysts. Electrochemical measurements were
performed using the thin-film rotating ring−disk electrode method in 0.1 M HClO4 at 20−60 °C. Hydrogen
adsorption pseudocapacitance was used to determine the number of Pt surface atoms and to estimate the
surface composition of the alloy catalysts. Kinetic analysis in comparison to pure Pt revealed a small activity
enhancement (per Pt surface atom) of ca. 1.5 for the 25 at. % Ni and Co catalysts, and a more significant
enhancement of a factor of 2−3 for the 50 at. % Co. The 50 at. % Ni catalyst was less active than the Pt
standard and unstable at oxygen electrode potentials. Ring-current collection measurements for peroxide
indicated no significant differences between the Pt−Co catalysts or the 25 at. % Ni catalyst and pure Pt,
while the 50 at. % Ni catalyst had a higher peroxide yield. Together with the observed Tafel slopes and
activation energies, it was concluded that the kinetic enhancement is contained in the preexponential factor
of the conventional transition state theory rate expression. It is, however, not clear why the alloying with Ni
or Co produces this change in the preexponential factor.
We describe an atmospheric pressure nanosampling interface for mass spectrometry based on near-field laser ablation. Pulsed laser radiation is delivered to the sample surface through a near-field optical probe, and the ablation plume is sampled through a capillary orifice and analyzed by standard MS methods. A spatial resolution of less than 200 nm and a sensitivity below 2 amol is demonstrated.
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