In situ time-resolved X-ray absorption
fine structure spectra of Pt/C, Pt3Co/C, and Pt3Ni/C cathode electrocatalysts in membrane electrode assemblies (catalyst
loading: 0.5 mgmetal cm–2) were successfully
measured every 100 ms for a voltage cycling process between 0.4 and
1.0 V. Systematic analysis of in situ time-resolved X-ray absorption
near-edge structure and extended X-ray absorption fine structure spectra
in the molecular scale revealed the structural kinetics of the Pt
and Pt3M (M = Co, Ni) bimetallic cathode catalysts under
polymer electrolyte fuel cell operating conditions, and the rate constants
of Pt charging, Pt–O bond formation/breaking, and Pt–Pt
bond breaking/re-formation relevant to the fuel cell performances
were successfully determined. The addition of the 3d transition metals
to Pt reduced the Pt oxidation state and significantly enhanced the
reaction rates of Pt discharging, Pt–O bond breaking, and Pt–Pt
bond re-forming in the reductive process from 1.0 to 0.4 V.
The degradation of Pt electrocatalysts
in membrane electrode assemblies
(MEAs) of polymer electrolyte fuel cells under working conditions
is a serious problem for their practical use. Here we report the kinetics
and mechanism of redox reactions at the surfaces of Pt/C and Pt3Co/C cathode electrocatalysts during catalyst degradation
processes by an accelerated durability test (ADT) studied by operando
time-resolved X-ray absorption fine structure (XAFS) spectroscopy.
Systematic analysis of a series of Pt LIII-edge time-resolved
XAFS spectra measured every 100 ms at different degradation stages
revealed changes in the kinetics of Pt redox reactions on Pt/C and
Pt3Co/C cathode electrocatalysts. In the case of Pt/C,
as the number of ADT cycles increased, structural changes for Pt redox
reactions (charging, surface, and subsurface oxidation) became less
sensitive because of the agglomeration of catalyst particles. It was
found that their rate constants were almost constant independent of
the agglomeration of the Pt electrocatalyst. On the other hand, in
the case of Pt3Co/C, the rate constants of the redox reactions
of the cathode electrocatalyst gradually reduced as the number of
ADT cycles increased. The differences in the kinetics for the redox
processes would be differences in the degradation mechanism of these
cathode electrocatalysts.
Decarbonylation-promoted Ru nanoparticle formation from Ru3(CO)12 on a basic K-doped Al2O3 surface was investigated by in situ FT-IR and in situ XAFS. Supported Ru3(CO)12 clusters on K-doped Al2O3 were converted stepwise to Ru nanoparticles, which catalyzed the selective hydrogenation of nitriles to the corresponding primary amines via initial decarbonylation, the nucleation of the Ru cluster core, and the growth of metallic Ru nanoparticles on the surface. As a result, small Ru nanoparticles, with an average diameter of less than 2 nm, were formed on the support and acted as efficient catalysts for nitrile hydrogenation at 343 K under hydrogen at atmospheric pressure. The structure and catalytic performance of Ru catalysts depended strongly on the type of oxide support, and the K-doped Al2O3 support acted as a good oxide for the selective nitrile hydrogenation without basic additives like ammonia. The activation of nitriles on the modelled Ru catalyst was also investigated by DFT calculations, and the adsorption structure of a nitrene-like intermediate, which was favourable for high primary amine selectivity, was the most stable structure on Ru compared with other intermediate structures.
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