90
potential cycles required to deposit a full monolayer of Pt was determined by comparing the rate of increase in measured surface area with the expected values based
on the core particles’ average diameter. Estimates of Pt loading obtained from averaged Cu UPD stripping charges measured before and after depositing a monolayer
yielded a Pt:Pd mass ratio within 15% of the values obtained from measurements of
inductively coupled plasma atomic emission spectroscopy.
7.3.3 Characterization of Pt Monolayer Electrocatalysts
Structural, morphologic, electronic characterization of electrocatalysts can be carried out using various in situ and ex situ techniques. There are only a few techniques
that can be applied in an electrochemical cell, that are in situ, which can provide
atomic level information and molecular specificity. There is a larger number of techniques requiring ultra-high vacuum (UHV) for their application, which can provide
high-resolution, atomic, and molecular-level information. The electrode has to be
taken out of the cell, transferred in the UHV chamber, characterized using UHV
techniques (ex situ), and then transferred back into the cell to run the reaction.
Ex situ techniques had an important role 30 years ago in establishing accurate
structure of surfaces and surface adlayers and identifying adsorbates when adequate
surface in situ techniques were not available. The use of transfer system to bring the
crystal from the UHV chamber after proper characterization into the electrochemical cell to run the reaction and bring it back to the UHV to determine a structural
change, if any, and identify the reaction products and intermediates was used with
some success. The position of some researches that observation of the same structure in electrochemical cell and in UHV means that we can freely use UHV structures to interpret data in the cell is wrong. That procedure only proves that the
transfer is reversible. When the potential is removed we lose control of the surface
state. Nevertheless, the use of new high resolution electron microscopies and related
techniques plays a big role in surface electrochemistry providing structural information with details that we did not think about couple years ago.
The morphology and structure of Pt ML electrocatalysts were characterized by
high-angle annular dark-field (HAADF) imaging using scanning transmission electron microscopy (STEM). Fig. 7.8 shows a typical HAADF-STEM Z-contrast
image of Pt ML /Pd/C nanoparticle, bright shell, and a relatively darker core, suggesting the formation of a core-shell structure, that is a Pt (Z = 78) shell on a Pd (Z = 46)
nanoparticle core [9]. Figure 7.8 b,c illustrates the line profile analysis by STEM/
energy-dispersive X-ray spectrometry (EDS), showing the distribution of Pt and Pd
component in a single nanoparticle. The analysis of Pt and Pd distribution exhibits
the following three marked features: (1) Pt atoms fully cover the Pd nanoparticle
surface; (2) the Pt intensity is fairly constant along the center of Pd nanoparticle;
and (3) at both the edges of Pd nanoparticle, the Pt intensity is approximately doubled compared with that at the center. These features demonstrate the formation of
a core (Pd) – shell (Pt) structure. The elemental distribution of the catalysts can also
7 Platinum Monolayer Electrocatalysts
potential cycles required to deposit a full monolayer of Pt was determined by comparing the rate of increase in measured surface area with the expected values based
on the core particles’ average diameter. Estimates of Pt loading obtained from averaged Cu UPD stripping charges measured before and after depositing a monolayer
yielded a Pt:Pd mass ratio within 15% of the values obtained from measurements of
inductively coupled plasma atomic emission spectroscopy.
7.3.3 Characterization of Pt Monolayer Electrocatalysts
Structural, morphologic, electronic characterization of electrocatalysts can be carried out using various in situ and ex situ techniques. There are only a few techniques
that can be applied in an electrochemical cell, that are in situ, which can provide
atomic level information and molecular specificity. There is a larger number of techniques requiring ultra-high vacuum (UHV) for their application, which can provide
high-resolution, atomic, and molecular-level information. The electrode has to be
taken out of the cell, transferred in the UHV chamber, characterized using UHV
techniques (ex situ), and then transferred back into the cell to run the reaction.
Ex situ techniques had an important role 30 years ago in establishing accurate
structure of surfaces and surface adlayers and identifying adsorbates when adequate
surface in situ techniques were not available. The use of transfer system to bring the
crystal from the UHV chamber after proper characterization into the electrochemical cell to run the reaction and bring it back to the UHV to determine a structural
change, if any, and identify the reaction products and intermediates was used with
some success. The position of some researches that observation of the same structure in electrochemical cell and in UHV means that we can freely use UHV structures to interpret data in the cell is wrong. That procedure only proves that the
transfer is reversible. When the potential is removed we lose control of the surface
state. Nevertheless, the use of new high resolution electron microscopies and related
techniques plays a big role in surface electrochemistry providing structural information with details that we did not think about couple years ago.
The morphology and structure of Pt ML electrocatalysts were characterized by
high-angle annular dark-field (HAADF) imaging using scanning transmission electron microscopy (STEM). Fig. 7.8 shows a typical HAADF-STEM Z-contrast
image of Pt ML /Pd/C nanoparticle, bright shell, and a relatively darker core, suggesting the formation of a core-shell structure, that is a Pt (Z = 78) shell on a Pd (Z = 46)
nanoparticle core [9]. Figure 7.8 b,c illustrates the line profile analysis by STEM/
energy-dispersive X-ray spectrometry (EDS), showing the distribution of Pt and Pd
component in a single nanoparticle. The analysis of Pt and Pd distribution exhibits
the following three marked features: (1) Pt atoms fully cover the Pd nanoparticle
surface; (2) the Pt intensity is fairly constant along the center of Pd nanoparticle;
and (3) at both the edges of Pd nanoparticle, the Pt intensity is approximately doubled compared with that at the center. These features demonstrate the formation of
a core (Pd) – shell (Pt) structure. The elemental distribution of the catalysts can also
7 Platinum Monolayer Electrocatalysts