Balance of Nanostructure and Bimetallic Interactions in Pt Model Fuel Cell Catalysts: An in Situ XAS and DFT Study Page: 4 of 18
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Journal of the American Chemical Society
2.4. Computational methods. All HERFD XAS calcu-
lations were carried out using the FEFF 8.4 program, which
employs a full multiple-scattering formalism.35 By using the
"NOHOLE" card, potentials and phase shifts were calculated
assuming complete screening of the core-hole, resulting in
better agreement with experimental white-line intensities. This
is fully consistent with previously reported FEFF results on
transition metal L2 and L3 edges. 1036-39 The line-sharpening
effect observed in HERFD was modeled by reducing the theo-
retical lifetime broadening by 1.75 eV using the
"EXCHANGE" card. This value was determined by compar-
ing the computed XANES spectrum of a clean Pt/Rh(1 11)
surface with an experimental spectrum at a potential corre-
sponding to the double-layer region, namely E=+0.4 V. More
detailed information about the model structures and input pa-
rameters for the FEFF8 calculations are provided in the Sup-
porting Information.
2.5 DFT calculations. All DFT calculations were per-
formed using GPAW4 '41 within the ASE environment using
the RPBE functional for exchange and correlation.43 The
standard PAW setups and double-zeta polarized basis set pro-
vided with GPAW were used.44 We used a grid spacing of
0.18 A, and a Brillouin zone sampling of 2x2 k-points along
the periodic directions. O adsorption energies were calculated
on small Pt clusters supported on the (111) fcc surface of Rh.
The cluster adheres to the surface such that the Pt atoms all
coincide with substrate lattice sites. The lattice constant was
optimized with the Rh substrate using a separate DFT calcula-
tion. O binding energies were calculated on each fcc hollow
site on the (111) plane of the Pt cluster. No geometry optimi-
zation has been used. The O atom was always kept at a dis-
tance of 2.07 A from the neighboring Pt atoms. The unit cell
for the fcc surface of Rh contains 8 by 8 atoms in an ortho-
rhombic cell and has 5 layers. 5A of vacuum is added be-
tween the non-periodic cell boundary and the atom closest to
that boundary.
3. RESULTS AND DISCUSSION
It is well-known that the fabrication of well-defined metal
monolayers can be challenging, especially in the case of Pt
which in general, due to its high surface energy,45 is likely to
tend towards a Volmer-Weber growth mode rather than the
desired fully two-dimensional growth of one monolayer.
However, on substrates with higher surface energies than Pt,
such as Rh, Ru or Ir,45 one would expect a growth mode of
either the Frank-van der Merwe or Stranski-Krastanov type to
be favored, i.e. at least up to a coverage of 1 ML, Pt would
grow in a single 2D layer. The latter has been confirmed for
the growth of Pt under UHV conditions on Rh(111)25 and
Ru(0001).4 While a well-defined 2D Pt monolayer was suc-
cessfully prepared in ultrahigh vacuum (UHV) by Pt vapor
deposition, we discovered that 3D island growth occurs when
an electrochemical preparation is chosen, which consists of the
redox displacement of an underpotential deposited (upd) Cu
monolayer.2447
The morphology of the deposited Pt layers on Rh(111) was
determined for both UHV and electrochemically prepared
samples using in situ Pt L3 extended x-ray absorption fine
structure (EXAFS), recorded at potentials close to hydrogen
evolution. The Fourier transformed EXAFS magnitudes for
both samples are shown in Figure la and b. Least-square fit-
ting with Pt-Pt and Pt-Rh nearest-neighbor coordinationshells gives coordination numbers (Table 1) that can be used
to determine the film morphology. For the vapor deposited
sample, we found very good agreement with a pseudomorphic
Pt layer of 1 ML thickness which uniformly covers the Rh
surface. In contrast, the significantly smaller Pt-Rh coordina-
tion number in the redox-displacement sample indicates the
three-dimensional nature of the deposit where only -50% of
the Pt atoms are in direct contact with the Rh substrate. Since
the Pt-Pt coordination number, at the same time, is also signif-
icantly below values that would be expected for a uniform
bilayer or multilayers, there must be a large number of under-
coordinated Pt atoms. The observed coordination numbers can
be explained with a model structure consisting of three-
dimensional islands. After a detailed consideration of various
island model structures (Supporting Information) we find
agreement with the EXAFS results for 3D Pt/Rh(111) within
experimental error bars for islands of which the most range
from two to four layers thickness and -1 to -4 nm lateral
width. This range of island widths is in very good agreement
with the sizes of Pt islands that can be seen in an in situ STM
measurement47 of Pt/Rh(111), where the same redox-
displacement technique was employed.
Table 1: In situ EXAFS fitting results
Pt-Pt Pt-Rh R factor
2D Pt/Rh(111)a 0.0282
N 6.5+0.8 3.2+0.7
R(A) 2.72+0.02 2.72+0.02
62(A2) 0.005 0.005
3D Pt/Rh(111)b 0.0238
N 7.3+0.7 1.4+0.5
R(A) 2.74+0.02 2.68+0.05
62(A2) 0.005 0.005
aData range: k=3.0-9.8 A.
bData range: k=3.0-9.5 A-.
The near-edge region (XANES) of Pt L3 spectra shows a
characteristic absorption maximum ("white-line") due to
2p-5d transitions and thus provides a probe of the unoccu-
pied part of the Pt 5d band.41'49 The information about the en-
ergy distribution of unoccupied d states is limited by the Pt 2p
core hole lifetime broadening, but significantly sharpened
spectral features can be obtained in the High Energy Resolu-
tion Fluorescence Detection (HERFD) mode36'50 which we
used in our experiment. The HERFD technique, together with
the use of well-defined single-crystal samples and sufficiently
large model structures in the multiple-scattering computations,
eliminates uncertainties in the interpretation of in situ XAS, in
particular at high electrochemical potentials where contradic-
tory models of Pt-O interactions have been proposed. 11-20ACS Paragon Plus Environment
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Friebel, Daniel; Viswanathan, Venkatasubramanian; Miller, Daniel James; Anniyev, Toyli; Ogasawara, Hirohito; Larsen, Ask Hjorth et al. Balance of Nanostructure and Bimetallic Interactions in Pt Model Fuel Cell Catalysts: An in Situ XAS and DFT Study, article, May 31, 2012; United States. (https://digital.library.unt.edu/ark:/67531/metadc831407/m1/4/: accessed April 18, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.