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J. Phys. Chem. C XXXX, xxx, 000
1
2
A
Attraction-Repulsion Mechanism for Carbon Monoxide Adsorption on Platinum and
Platinum-Ruthenium Alloys
3
Nicholas Dimakis,*,† Matthew Cowan,† Gehard Hanson,‡ and Eugene S. Smotkin§
4
5
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Department of Physics and Geology, UniVersity of Texas-Pan American, Edinburg, Texas, Department of
Mechanical Engineering, UniVersity of Texas-Austin, Austin, Texas, and Department of Chemistry,
Northeastern UniVersity, Boston, Massachusetts
7
ReceiVed: April 21, 2009; ReVised Manuscript ReceiVed: August 21, 2009
8
Cluster and periodic density functional theory (DFT) of carbon monoxide adsorbed atop on Pt (COads) show
that ruthenium alloying weakens both the COads internal and C-Pt bonds and reduces the COads adsorption
energy. A new theoretical model based on the π-attraction σ-repulsion is used to explain the above results.
This model correlates (1) Mulliken population, (2) density-of-states analysis of the COads orbitals, (3) the
individual interaction of these orbitals with the metal lattice bands, and (4) their polarizations within the
COads molecule. In this study, the σ interaction has both attractive and repulsive components via electron
donation to the metal bands and Pauli repulsion, respectively. Cluster DFT shows that the overall weakening
of the COads internal bond upon alloying is due to the dominance of reduced σ donation to the metal (which
weakens the COads internal bond) over increased π bonding between the carbon and oxygen. However, periodic
DFT calculations show that both the σ donation and the COads internal π bonding are simultaneously reduced.
The C-Pt bond weakening upon alloying is primarily due to increased exchange repulsion between the
adsorbate and the substrate. The adsorbing Pt atom sp/dz2 orbitals population increase upon alloying for both
calculations.
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1. Introduction
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Platinum fcc alloys serve as core structures for direct
methanol fuel cell (DMFC) anode catalysts.1-14 During methanol
oxidation, tenaciously adsorbed CO (COads)15,16 blocks Pt sites
required for oxidative adsorption of methanol.17 The enthalpy
of adsorption (Eads) can be tuned by variation of the alloy
composition (compositional tuning) or the electrode potential
(Stark tuning).18 Such variations affect the interactions between
the CO molecular orbitals (MOs) and the metal substrate bands.
Thus, changes in the electronic structure of mixed-metal
catalysts can be monitored by polarization modulated infrared
reflection adsorption spectroscopy (PM-IRAS) of COads.18
It has been shown, both computationally19 and experimentally,18 that both the CO stretching frequencies (νCO) and the
Eads decrease as the alloy Ru mole fraction (XRu) is increased.
To complicate the picture further, Hammer et al.20 ascribes the
reduced Eads to the lowering of the d-band center energy with
increased XRu. The simultaneous reduction of the Pt d-band
center energy and the νCO are difficult to reconcile with increased
back-donation to the 2π* CO MO. Dimakis et al.19 reconciled
these observations with the band dispersion mechanism based
on density functional theory (DFT) and FEFF21 calculations on
a library of COads-Pt(Ru) clusters. Although the Pt d-band
center is lowered with increased XRu, the dispersion of the d-band
(via hybridization of the substrate bands with the CO MOs) is
asymmetric and energetically top heavy, enhancing the Fermi
level electron density, where the LUMOs of the CO orbitals
reside. The band dispersion theory explains how asymmetric
broadening of the Pt d-band center toward the Fermi level
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* To whom correspondence should be addressed. E-mail:dimakis@utpa.edu.
†
University of Texas-Pan American.
‡
University of Texas-Austin.
§
Northeastern University.
10.1021/jp9036809 CCC: $40.75
enhances 2π* back-donation (ergo reduces the νCO), even though
the d-band center energy is reduced. Another correlation with
Eads reduction is the calculated elongation of the C-Pt bond.
Dimakis et al. attributed this to the electrostatic and Pauli
repulsion.22 FEFF simulation spectra on Pt LIII-edge of PtRu
alloys, used to advance the band dispersion theory, were in
agreement with experimental XANES spectra showing increased
d-band vacancies upon alloying.23,24
Traditionally, the reduction of the νCO of the free CO relative
to COads on metals is explained by the “Blyholder model”,25
which considers only the frontier 5σ and 2π* CO MOs as 5σ
donation and backdonation from substrate metal d-band to the
2π* MO. However, in the original Blyholder paper,26 the entire
adsorbate π-system was considered, while the 5σ MO was
assumed unchanged between the free and the adsorbed CO.
Recently Nilsson et al.,27,28 Bennich et al.,29 and later Föhlisch
et al.30,31 proposed an alternative explanation for molecular
adsorption on metal surfaces by using X-ray emission spectroscopy (XES) to measure the electronic structure of N2 and
CO molecules adsorbed on Ni(100) and Cu (100) surfaces,
respectively. The XES data, complemented by quantum mechanical calculations,31 suggest an adsorbate-metal π bonding
and σ repulsion (π-σ model) scheme, in which both effects
increase with the number of coordinated metal atoms.30
The π-σ model describes adsorbate-metal π bonding as an
effect of three hybrid CO-metal tilde-type orbitals including the
1π̃, d̃π, and 2π̃* orbitals. In the π-σ model description, the d̃π
is a hybrid of the unperturbed 1π and 2π* CO MOs mixed with
the metal dxz,yz orbitals. The first-order perturbation theory,
applied by Föhlisch et al.,31 to the unperturbed orbitals of the π̃
system (the CO 1π, 2π*, and the substrate dxz,yz orbitals)
accounts for charge exchange between the CO orbitals and the
substrate bands. The application of second-order perturbation
theory accounts for electron density polarization within the CO
 XXXX American Chemical Society
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J. Phys. Chem. C, Vol. xxx, No. xx, XXXX
molecule. In the π-σ model framework, the weakening of the
adsorbate internal C-O bond (COads internal bond) is attributed
to higher 1π̃ polarization toward carbon with respect to free
CO, due to mixing of the 1π and 2π*.27,28 The π-σ model does
not assume direct back-donation from the substrate metal bands
to the unperturbed 2π* as does the “Blyholder model”. However,
the CO contribution to the d̃π orbital consists of 1π and 2π*
MOs and thus “indirect” dxz,yzf2π* back-donation is implied.
The σ repulsion in the π-σ model is ascribed to the effects
of 4σ̃, 5σ̃, and d̃σ orbitals, where the d̃σ is a hybrid of the
unperturbed 5σ CO MO mixed with the metal dz2 orbitals. This
repulsion is primarily due to electron density redistribution in
the CO region of the σ̃ orbitals rather than the extent of σ
donation to the substrate.31 The σ̃ system (via electron density
redistribution) dampens the effect of 1π̃ polarization discussed
above. In contrast to the repulsive nature of the σ system in the
original π-σ model, Kresse et al.32 using the ab initio DFT
VASP program33 for COads on Pt(111), discuss the importance
of the 5σfdz2 donation mechanism for driving the CO toward
the atop site. This work further elucidates the effect of the σ̃
system on CO adsorption.
This paper revisits our band dispersion theory and defines a
new theoretical approach in light of the π-σ model for COads
on pure Pt and PtRu alloys. Cluster and periodic DFT methods
are used to correlate relative shifts of the Pt sp- and d-band
centers, charge exchange between the CO MOs and the substrate
bands, and polarizations within the CO molecule, with the
weakening of the COads internal and C-Pt bonds upon alloying.
The periodic DFT calculations by Gajdoš et al.,34 as well as
extended Hückel theory calculations by Li et al.,35 suggest that
the populations of the 4σ and 1π CO MOs that contribute to
the formation of the adsorbate 4σ̃ and 1π̃ orbitals, respectively,
are diminished with respect to the corresponding populations
of the free CO MOs. Therefore, we consider all CO contributions to the adsorbate orbitals from energies as low as the 4σ̃
to as high as the occupied part of the 2π̃*in this study.
2. Models and Computational Methods.
Cluster Models. Pt (100) is modeled as a three-layer
(13)(12)(1) Pt26 cluster with a lattice parameter of 3.924 Å;
where applicable, a single CO molecule was adsorbed atop (θCO
) 1/12) (Figure 1a). The alloy clusters are constructed by
substituting four Pt atoms with Ru atoms at nearest neighbor
sites on the CO adsorbing face without changing the lattice
parameter; Ru atoms are only located at the top layer (surface
layer). It has been reported19 that these clusters are adequate
for modeling the effect of single atop CO chemisorption on Pt
and PtRu alloy surfaces. Unrestricted DFT36-38 (UDFT) under
the hybrid X3LYP39,40 functional is used to determine C-O
fragment optimal geometries, orbital populations, and C-O and
C-Pt stretching frequencies (νCO and νCPt). The X3LYP is an
extension to the previously employed B3LYP41 functional
providing more accurate heats of formation. Although UDFT
might suffer from spin contamination,42 it is the recommended
choice for clusters of high-spin multiplicity configuration, such
typical of this study.43 A triple-ζ basis is used on all atoms of
the cluster calculations; Pt and Ru heavy atom wave functions
are described by the LACV3P**++ basis set. This includes
valence and outermost core electrons, polarization,44 and diffuse45 basis set functions (denoted by “**” and “++”, respectively). The 5s25p65d96s1 and 4s24p64d75s1 “valence”46 configuration are used for Pt and Ru, respectively, while the remaining
core electrons are treated with effective core potentials (ECP).47
The ECP accounts for mass-velocity and Darwin relativistic
corrections. For carbon and oxygen, the all-electron 6-311G**++
Dimakis et al.
Figure 1. (a) Pt22Ru4CO cluster and (b) the unit cell of the
corresponding periodic slab.
Pople basis set is used.42 The selection of the triple-ζ basis set
minimizes48 the basis set superposition error (BSSE).49 DFT
calculations on clusters are performed using Jaguar 6.5,50 which
incorporates the pseudospectral method51-56 to calculate most
of the fundamental time-consuming integrals with the same
accuracy as the fully analytical DFT codes. For each cluster,
the ground-state multiplicity is iteratively determined by
calculating the SCF energy for various spin multiplicity
values.19,57 The spin-optimized cluster is then geometrically
optimized by letting the C and O atoms relax, while all other
atoms remain locked to the original positions. In contrast to
our previous work,19 the Pt atom, on which the CO is adsorbed
(Ptc), is not allowed to move during the geometry optimization
process. This precludes gross cluster relaxation, which would
not be characteristic of the periodic lattice structure we aim to
model. Computing the νCO for the COads using the partial Hessian
approach for C, O, and Ptc conserves CPU time by avoiding
the unnecessary calculation of cluster Pt-Pt normal mode
vibrations. Pt and Ru electron populations are calculated using
the Natural Bond Order (NBO)58 program, which is incorporated
into Jaguar. NBO calculates, among others, atomic electron
populations (per angular momentum). It must be cautioned that
NBO is not free of artifacts associated with the populations of
the cluster edge atoms (this statement also applies to Mulliken59
population analysis).60 These artifacts do not occur with the
periodic DFT methods. Density-of-states (DOS) for C, O, and
Pt/Ru atoms are calculated using the AOMIX program.61,62
AOMIX processes output files from a variety of quantum
mechanical packages and generates DOS spectra in terms of
constituent chemical fragments. The cluster calculation Fermi
levels are the lowest of the quantity (EHOMO + ELUMO)/2 for
either R or β electrons.
Periodic Slab Models. A three-layer periodic slab is used
to model Pt (100). The atop CO is generated as a c(4 × 4)
overlayer (Figure 1b). Consistent with the cluster calculations,
the CO coverage is low (θCO ) 1/8) in order to eliminate
possible CO-CO interactions that would affect C-O, C-Pt
distances, and corresponding vibrational frequencies. For pure
Pt, a five-layer slab (5L) is also examined for consistency with
5L
3L
- EFermi
= 0.01 eV, thus no
the three-layer (3L) slab: EFermi
variation is observed on the Fermi level due to the presence of
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Attraction-Repulsion Mechanism for CO Adsorption
J. Phys. Chem. C, Vol. xxx, No. xx, XXXX C
TABLE 1: Calculated C-O and C-Pt Distances and Corresponding Stretching Frequencies νCO, νCPt; HOMO/LUMO, and
Fermi Levels; Eads Adsorption Energies of the DFT Geometrically Optimized Pt26, Pt26CO and Pt26Ru4, Pt26Ru4CO Clusters and
Corresponding Periodic Slab Modelsa
system
cluster
property
dC-O (Å)
νCO (cm-1)
dC-Pt (Å)
νCPt (cm-1)
HOMO
LUMO
EFermi (eV)
Eads(eV)
Pt26
-5.21
-4.70
-4.95
Pt26CO
1.140
2260
1.845
563
-5.30
-4.36
-4.83
-1.90
Pt22Ru4
slab
Pt22Ru4CO
-5.16
-4.50
-4.94
1.144
2221
1.850
549
-5.06
-4.75
-5.05
-1.53
Pt
-5.69
-5.31
-5.55
PtCO
1.139
2135b
1.874
656b
-5.54
-5.50
-5.52
-1.81
PtRu
190
191
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234
additional layers. Pt and Ru atoms are fixed in the Pt crystallographic positions during geometry optimization. Relaxation
effects will be examined in the future. Periodic DFT calculations
on Pt and PtRu slabs with and without the c(4 × 4) - CO
overlayer are performed using the CRYSTAL0663 program,
which employs Gaussian type function basis sets centered at
the atoms. Additionally, CRYSTAL06 has the capability of
normal-mode frequency estimation at the Gamma point (k )
0).64 X3LYP functional is not available by CRYSTAL06;
instead, a “modified” version of the hybrid B3LYP functional
is employed that uses the VWN565 correlation functional. Similar
to the cluster calculations, the innermost orbitals of the Pt and
Ru heavy atoms are described by ECP.66 Effective valence basis
sets for Pt and Ru atoms employed here are as follows; Pt atoms
are described by the optimized-for-crystalline calculations basis
set of [4s4p2d],67 Ru, C, and O atoms are optimized from
corresponding atomic basis sets used at molecular calculations.
For Ru atoms the basis set of [7s5p3d2fg]68 is contracted to
[4s3p2d] by dropping functions with exponents less than 0.1
and concurrently removing f and g functions from the original
basis set. The latter basis set for the Ru atoms is preferred over
the smaller basis set of [2s2p2d] by Frèhard and Sautet69 that
proved to be inadequate for accurate calculations of COads on
PtRu surfaces. For carbon and oxygen atoms, the original
6-311++G** basis set, described as [5s4p1d], is contracted to
[4s3p1d] for either element. Brillouin zone integrations are
performed on a 12 × 12 × 1 Monkhorst-Pack grid.70 The Fermi
energy and the density matrix are evaluated on a denser grid of
24 × 24 × 1 points (Gilat grid).71,72 Pt and Ru electron
populations are calculated using Mulliken population analysis.
The Fermi level is directly calculated by CRYSTAL06; HOMOs
and LUMOs are obtained by band calculation of the corresponding slab. Due to CPU time restrictions CRYSTAL06
frequency calculations were performed on a two-layer slab using
the C-O fragment optimized geometry of the corresponding
three layer counterpart. Dimakis et al.,19 showed that the effect
of a third layer addition on the Pt-CO cluster on the νCO value
was minimal. We believe that this observation extends to the
νCPt calculations as well. This approximation applies to both
the pure Pt and the alloy slab. Thus, any errors on the νCO and
νCPt should be systematic.
3. Results and Discussion
3.1. CO Adsorbed on Pure Pt. 3.1.1. C-O, C-Pt Optimal
Geometries, Vibrational Frequencies, and Eads Adsorption
Energies. DFT calculated C-O, C-Pt distances, corresponding
stretching frequencies νCO, νCPt, HOMO, LUMO energy levels,
1.142
2064b
1.879
467b
-5.29
-4.99
-5.28
-1.52
-5.36
-5.15
-5.25
a
For slab calculations HOMO refers to the top of the valence band and LUMO to the bottom of the conduction band.
calculations using three layer slab optimized geometry.
189
PtRuCO
b
Two-layer slab
Fermi levels, and Eads of clusters and slabs are summarized in
Table 1. Upon CO adsorption on pure Pt, the C-O interatomic
distance is increased, accompanied by νCO downshift (with
respect to free CO). These observations indicate weakening of
the COads internal bond. DFT optimized C-O and C-Pt
distances are within the range of the experimentally observed
distances for COads on pure Pt surface, reported at 1.15 and 1.85
Å, respectively.73 Additionally, for the Pt-CO systems of this
work, the calculated νCO and νCPt (Table 1) are within the range
of the experimentally observed values of 2093 and 467 cm-1,
respectively (low CO coverage).74 Calculated frequencies are
systematically overestimated by DFT. Appropriate scaling
factors can be applied for quantitative comparison with experimental observations.75 However, such parameters are still not
known for the functional/basis set pair employed here.
The increase of the Pt-CO Fermi level with respect to clean
Pt indicates that the CO molecule is an electron donor to the Pt
lattice. The calculated Eads for the Pt26CO cluster is lower by
about 0.2 eV with respect to our previous results, which was
obtained with a smaller basis set. However, Eads values for COads
on pure Pt reported here (as well as values reported in our
previous work) are within the range of the latest experimental
value of 1.89 ( 0.20 eV reported by Yeo et al.76 for pure Pt at
(111) face and low CO coverage.
3.1.1. CO MOs Hybridization with sp/d-Bands of the Pure
Pt Crystal. When CO is adsorbed on the Pt(100) surface, the
4σ, 5σ, 1π, and 2π* CO MOs are lowered in energy and mixed
with the Pt sp- and d-bands of the crystal forming corresponding
hybrid tilde-type orbitals.77 Although 1σ, 2σ, and 3σ CO MOs
do not substantially participate in tilde-type bond formation (i.e.,
chemisorption), they are associated with Pauli repulsion between
the CO molecule and Pt. Concomitantly, electron density
polarization is observed within the COads molecule. The CO
contribution to the adsorbate 4σ̃, 5σ̃, 1π̃, d̃π, d̃σ, the occupied
part of the 2π̃*orbitals, and the sp and d orbital populations of
the Ptc for COads on pure Pt and PtRu alloys are summarized in
Table 2. Additionally, electron charge differences of the abovementioned CO contributions to the adsorbate orbitals for COads
on the PtRu alloy with respect to corresponding adsorption on
pure Pt are summarized in Table 3. The effects of the CO
chemisorption are localized in proximity of the Ptc. In the case
of the Pt26CO cluster calculations, only Pt atoms of the upper
two layers are affected by CO adsorption, whereas for the c(4
× 4) - CO/Pt periodic slab only Ptc is involved in the process.
An interaction diagram for the CO chemisorption on pure Pt is
shown in Figure 2.
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236 T1
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240
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245
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249
250
251
252
253
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255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271 T2
272
273
274 T3
275
276
277
278
279
280 F4
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D
J. Phys. Chem. C, Vol. xxx, No. xx, XXXX
Dimakis et al.
TABLE 2: CO Contributions to the Adsorbate 4σ̃, 5σ̃, 1π̃, d̃π, d̃σ, the Occupied Part of the 2π̃* Orbitals, and the Ptc s, p, and d
orbitals for Pt26, Pt22Ru4, Pt26CO, and Pt22Ru4CO Clusters and Corresponding Slabsa
cluster
molecule/atom
orbital
CO
4σ̃
5σ̃(d̃σ)
1π̃(d̃π)
2π̃*
6s
6p
5dz2
5dxz†
5dxy‡
5d
Ptc
Pt26
Pt26CO
0.77
0.03
1.91
1.91
1.83
9.39
1.53
1.53(0.29)
3.58(0.33)
0.49
0.80
0.04
1.60
1.86
1.94
9.20
slab
Pt22Ru4
Pt22Ru4CO
0.89
0.03
1.91
1.87
1.88
9.41
1.58
1.58(0.36)
3.59(0.38)
0.51
0.83
0.05
1.67
1.82
1.95
9.20
Pt
Pt-CO
0.83
0.34
1.88
1.77
1.72
8.84
1.69
1.54(0.37)
3.68(0.28)
0.59
0.83
0.54
1.46
1.81
1.83
8.74
PtRu
PtRuCO
0.96
0.54
1.91
1.71
1.75
8.82
1.71
1.56(0.34)
3.67(0.23)
0.66
0.90
0.65
1.54
1.76
1.86
8.87
a
CO contributions are calculated by DOS spectrum (Figure 3 and 4) integration at appropriate energy ranges. The Pt populations are
obtained by the NBO program and Mulliken population analysis for cluster and periodic calculations, respectively. The NBO calculations
include populations from Rydberg states. Average values (5dxz + 5dyz)/2,† (5dxy + 5dx2-y2)/2‡ are assumed for Pt atoms.
TABLE 3: Electron Charge Differences for COads on PtRu Alloy with Respect to COads on Pure Pt for the CO Contribution to
the Adsorbate 4σ̃, 5σ̃, 1π̃, d̃π, d̃σ, and the Occupied Part of the 2π̃* Orbitals Per C and O Atom for Clusters and Corresponding
Slabsa
clusters
slabs
CO
contribution
C
O
C
O
4σ̃
5σ̃(d̃σ)
1π̃(d̃π)
2π̃*
0.027
0.022(0.028)
-0.038(0.042)
-0.022
0.025
0.033(0.061)
0.049(0.010)
0.061
0.025
0.019(-0.022)
0.024(-0.013)
0.026
-0.009
0.013(-0.009)
0.015(-0.040)
0.030
a
These values are calculated by integration of CO DOS spectrum (Figures 3 and 4) after it has been factor decomposed into C and O atomic
contributions.
Figure 2. Orbital interaction and electron transfer schematic during
adsorption of free CO upon clean Pt. The hybridization of CO MOs
with the Pt bands yields tilde-orbitals (center manifold). Numerical
values represent electrons transferred based on NBO calculations
(bracketed values based on periodic DFT Mulliken population analysis).
Dashed rectangle illustrates the upshift (in energy) of the sp-band of
the Me-CO periodic slab surface. Lines are associated to electron
transfers as follows: solid lines to substrate dz2-, dotted lines to sp-,
and dashed lines to dxz,yz-bands.
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282
283
284
285
286
287
288
289
290
291
σ̃ System. The σ̃ system consists of the 4σ̃, 5σ̃, and d̃σ orbitals,
with the 4σ̃ and d̃σ having C-O antibonding character. However,
special treatment is necessary for the 5σ̃ orbital.31,28 The effect
of 4σ̃ orbital was first described by Hu et al.78 for COads on Pd
(110). The CO contribution to the σ̃ DOS spectrum for the
Pt26CO cluster and the c(4 × 4) - CO/Ptslab is shown in the
upper graphs of Figures 3 and 4, respectively. The above DOS
is factor decomposed into contributions from the carbon and
the oxygen atomic orbitals. Upon CO adsorption on pure Pt,
the 4σ̃ orbital (C-Pt bonding) is shifted toward lower energies
4σ
4σ
- εPt-CO
= 1 eV). Moreover, the 4σ CO
(∆ε4σ ≡ εCO
contribution to the adsorbate 4σ̃ orbital is diminished, with
respect to the corresponding population of the 4σ free CO MO.
This effect strengthens the COads internal bond and is mainly
due to electron donation to the metal substrate via 4σfdz2.
Weaker electron transfer is also observed toward the substrate
sp-band. Figure 5 shows the lower energy part of the substrate
dz2 - and sp-band DOS spectrum for the Pt-CO systems. The
existence of peaks for the above bands, that are coincident in
energy with the 4σ̃ and 5σ̃ peaks, is indicative of electron
donation from these σ-type orbitals to the substrate bands. More
specifically, by integrating the dz2- and sp-band DOS spectrum
in the above-mentioned energy regions, we obtain the number
of electrons transferred from the CO region of the 4σ̃ and 5σ̃
orbitals toward the dz2- and sp-bands. For example, the cluster
calculations show 0.31 and 0.12 e transferred from the 4σ to
the dz2- and sp-bands, respectively. The overall transfer to the
substrate bands differs from the population reduction of the 4σ
MO in the adsorbate 4σ̃ orbital by only 0.04 e. Moreover, the
4σ̃ polarizes toward carbon, thus diminishing the strengthening
of the COads internal bond, which is caused by the 4σ depletion
(Figures 3 and 4, upper graphs). Additionally, the 5σ CO MO
mixes with the substrate dz2-band, broadens and splits into 5σ̃
C-Pt bonding and d̃σ C-Pt antibonding orbitals. The 5σ̃ orbital
shifts to much lower energies (∆ε5σ = 4 eV) with respect to
the 4σ̃ orbital shift. For the free CO, the 5σ CO MO is slightly
C-O antibonding between the carbon 2s and oxygen 2p orbitals.
However, Crystal Orbital Overlap Population (COOP) calculations on the Pt26CO cluster show that 5σ̃ becomes nonbonding
between carbon and oxygen. This is probably due to the 5σ̃
polarization toward oxygen (Figures 3 and 4) that reduces the
carbon 2s population.31 Therefore, upon CO adsorption, the 5σ
depletion strengthens the COads internal bond. However, this
depletion, as in the 4σ̃ case, results in C-Pt bonding. Concomitantly, the dz2-band is depleted, primarily due to the
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J. Phys. Chem. C, Vol. xxx, No. xx, XXXX E
Figure 3. σ and π CO DOS calculated by the AOMIX program for Pt26CO and Pt22Ru4CO clusters. CO DOS (thick black solid line) is factor
decomposed into contributions from C (thin blue solid line) and O (red dashed line) atomic orbitals. Arrows indicate approximate energy regions
for the occupied part of the d̃σ and 2π̃* orbitals.
Figure 4. σ and π CO DOS calculated by the CRYSTAL06 program for PtCO and PtRuCO slabs. CO DOS (thick black solid line) is factor
decomposed into contributions from C (thin blue solid line) and O (red dashed line) atomic orbitals. Arrows indicate approximate energy regions
for the occupied part of the d̃σ and 2π̃* orbitals.
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corresponding orbital depletion on the Ptc atom. This effect
indicates a shift of the d̃σ above the Fermi level, with the higher
the dz2-band depletion the stronger the C-Pt bond. The inserts
of the upper graphs of Figures 3 and 4 show that d̃σ extends
beyond the Fermi level, thus verifying that d̃σ is partially
occupied. For example, the presence of a peak in the σ CO
DOS at about 1 eV above the Fermi level belongs to the d̃σ
orbital, since the next available σ-type orbital (i.e., the 6σ̃*
orbital) has been experimentally observed at about 20 eV above
the 2π*.30 This observation on d̃σ orbital is consistent with
inverse photoemission spectra measurements for CO adsorbed
on Ni, Pd, and Pt surfaces by Rangelov et al.79 The C-Pt
antibonding effect caused by the d̃σ orbital minimizes the C-Pt
bonding caused by the 4σ̃ and 5σ̃ orbitals (Table 2). The
resultant σ̃ system effect, as defined in this work, strengthens
the COads internal bond and results in C-Pt bonding. The latter
observation seems to be in disagreement with the π-σ model,
which states that the overall σ effect is repulsive. In the π-σ
model, the effect of σ repulsion is based on the absence of the
σ donation effect and a full occupied d̃σ orbital.30 Föhlisch et
al.31 state that the strengthening of the COads internal bond cannot
be ascribed solely to changes in the 4σ̃ and 5σ̃ polarization.
They claimed that the observed reduction of the carbon 2s
population is not due to σ donation since the populations of
both the σ and dz2 also reduced. We will show that if adsorption
is considered in a stepwise fashion, σ donation concomitant with
a reduction of the dz2 population is possible.
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J. Phys. Chem. C, Vol. xxx, No. xx, XXXX
Figure 5. Lower energy part of the Pt-CO DOS spectrum, for the
cluster (left) and periodic calculations (right), obtained by AOMIX and
CRYSTAL06 programs, respectively. The existence of peaks in the Pt
dz2- and sp-band DOS spectrum in the energy regions of the 4σ̃ and 5σ̃
orbitals verifies the 4σ, 5σfdz2,sp donation mechanism.
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In this work, we do not consider the effect of Pauli repulsion
between the “inner” σ-type CO MOs (i.e., 1σ, 2σ, and 3σ) and
the substrate’s core orbitals. Moreover, Föhlisch et al., using
constrained space orbital variation calculation on CO/Ni and
CO/Cu clusters, observed that the inclusion of 4σ̃, 5σ̃, and d̃σ
orbital in the calculations reduces the extent of Pauli repulsion.
This provides a compelling argument that if Pauli repulsion is
excluded from the σ̃ system description, the net effect of the
remaining σ̃ orbitals is bonding to the metal.
π̃ System. The π̃ system consists of the 1π̃, d̃π, and the
occupied part of the 2π̃*. The d̃π in this work, differs from the
original π-σ model d̃π since it does not contain contributions
from the unperturbed 2π* CO MO. The effect of the latter MO
is considered separately. Partial population of the 2π̃* is in line
with the π-σ model, where a “tail” of the 2π̃* is contained in
the dispersed d̃π-band in the energy region just below the Fermi
level, and with other past reports.32,34 The CO contribution to
the π̃ DOS spectrum is shown in the lower graphs of Figures 3
and 4. Hybrid orbitals are formed by mixing of the unperturbed
π CO MOs with the substrate dxz,yz-bands. The first two orbitals
are C-O bonding, whereas the last orbital is C-O antibonding.
Moreover, COOP calculations show that 1π̃ is C-Pt bonding
and d̃π is C-Pt antibonding. These observations are in agreement
with theoretical calculations of Aizawa and Tsuneyuki80 for CO/
Pt(111). The energy position of the 1π̃ orbital in the DOS
spectrum is slightly lower than its corresponding position for
the free CO molecule (∆ε1π = 0.16 eV). The d̃π orbital is due
to an oxygen lone pair and appears in the region of [-8, -6]
eV with respect to the Fermi level for the cluster and the periodic
calculations; charge is transferred to the CO region of the d̃π
orbital via dxz,yzfd̃π. The 1π CO contribution to the adsorbate
1π̃ is diminished, thus weakening the COads internal bond. This
effect is accompanied by increased charge in the CO region of
the d̃π orbital, which strengthens the COads internal bond. The
weakening the COads internal bond, due to the 1π depletion, is
maximized by the observed increased 1π̃ polarization toward
carbon, with respect to free CO.27,28 The resultant electron charge
in the CO region of the 1π̃ and d̃π orbitals is less than the 1π
population of the free CO. Therefore, the above-mentioned 1π̃
and d̃π population changes weaken the COads internal bond.
Finally, the 2π* CO MO broadens, mixes with the substrate
Dimakis et al.
dxz,yz-band, and is shifted below the Fermi level, thus partially
populated. This effect further weakens the COads internal bond,
and results in C-Pt bonding. The higher population recorded
by periodic DFT on the resultant 1π depletion and 2π*
population is due to an overemphasis of the DFT functional on
the 2π* back-donation.35 The overall π̃ system weakens the
COads internal bond, and as in the σ̃ case, also results in C-Pt
bonding.
σ/π Exchanges with the Substrate sp/d-Bands. The resultant
number of electrons transferred from the CO regions of the 4σ̃
and 5σ̃ orbitals toward the Pt sp- and dz2-bands are 0.94 and
0.77 e for the cluster and the periodic calculations, respectively
(Figure 2). These transfers are larger than the reported ones by
Li et al.35 using extended Hückel theory on CO/Ni(100) (i.e.,
∼0.5 e), mainly due to their observed reduced 4σ donation to
the substrate Ni crystal, with respect to our observations on CO/
Pt(100). The dz2-band is depopulated due to the band’s partial
shift above the Fermi level (Table 2), accompanied by electron
transfer from the band toward the CO region of the d̃σ orbital.
The above-mentioned effects (i.e., σ-donation and dz2 depletion)
imply an instantaneous increase of the resultant dz2- and spband population by 0.97 and 0.83 e for the cluster and the
periodic calculations, respectively. These quantities are transferred to the dxz,yz-band via the sp-band (sp- and d-band overlap
in energy). This process could explain how σ donation and
overall dz2 reduction are possible. Moreover, charge is transferred
to the 2π* CO region of the 2π̃* via dxz,yzf2π* (Figure 2),
with approximately 0.5 e for the cluster and the periodic
calculations. The 1πfdxz,yzfd̃π transfer sequence causes only
a minor dxz,yz-band population increase (<0.1 e). For the Pt26CO
cluster, the remaining 0.57 e is equally transferred toward the
dx2-y2,xy and the sp orbitals of the Ptc as excess electron
population of the transition dxz,xyfsp + dx2-y2,xy (0.17 e) and
the nearest neighbor Pt atoms as dfsp within this group of
atoms (0.15 e). The increased Ptc sp orbital population with
respect to the orbital population of the clean surface counterpart
has been ascribed to exchange repulsion and Pauli effect, which
prevents the adsorbate from coming closer to the substrate
surface.22 The remaining electron population (∼0.25 e), is
mainly due to the presence of low occupancy Rydberg states
in the NBO population analysis, such as the Pt 6d, 7s, and 7p.
Moreover the presence of diffuse basis set functions,49 and
numerical integration errors account for any remaining population mismatch. For the c(4 × 4) - CO/Pt slab, the remaining
dxz,yzelectron population is transferred primarily to the dxz,yz,
dx2-y2,xy, and sp orbitals of the Ptc, coupled with weaker electron
transfers toward the sp- and d-bands of the second layer atoms.
The number of electrons deposited on the Ptc sp orbital via the
above-mentioned electron transfer sequence in the periodic
calculations is much larger than the corresponding deposition
observed in the cluster calculations. Additionally, increased
charge is observed in the CO region of the 4σ̃ orbital by the
periodic calculations with respect to the cluster calculations
(Table 2). These observations are indicative of increased σ
repulsion (via reduced 4σfdz2 donation) of the adsorbate for
the periodic calculations with respect to the corresponding
cluster calculations; it is verified by the longer C-Pt bond length
for CO adsorbed on the pure Pt slab with respect to adsorption
on the Pt26 cluster. This effect has been previously reported.81
Center-of-Bands. The cluster calculations indicate that the
Pt d-band center of the Pt26CO cluster is downshifted by 0.13
eV with respect to the d-band center of the clean surface. This
downshift in energy is systematically applied to the substrate
dz2-, dxz,yz-, and dx2-y2,xy-bands. Lowering the metal substrate
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Attraction-Repulsion Mechanism for CO Adsorption
Figure 6. DOS spectrum for clean Pt and PtRu alloy clusters (upper
graph) and periodic slabs (lower graph). Arrows denote major electron
transfers to and from the Pt and Ru bands (orbitals for isolated Ru
atoms) as Pt is alloyed with Ru atoms. Thick horizontal bars represent
the Pt-band center-of-masses. The horizontal dashed line is the Fermi
level.
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d-band center of the Me-CO system does not always translate
to increased d-band population upon CO adsorption on the
substrate surface. For example, the downshift of the dz2-band
center for the Pt26CO cluster is due to electron population
reduction in the energy region just below the Fermi level, thus
depleting the Ptc dz2. The Pt d-band center of the c(4 × 4) CO/Pt slab is slightly upshifted by 0.03 eV with respect to the
corresponding band center of the clean surface. Since adsorption
is a surface phenomenon, we may also focus our attention to
the behavior of the periodic slab surface layer. In this case, the
center of the surface d-band is slightly downshifted by 0.02 eV
in the same trend with cluster calculations. The Pt sp-band center
of the Me-CO cluster is downshifted in energy with respect to
the clean surface counterpart. However, the opposite trend
appears for periodic slabs. Either shift may be attributed to the
number of electrons transferred to the substrate sp-band via
σfspdonation. The sp-band center shifts are substantial in
cluster calculations, but negligible in the periodic calculations.
3.2. Pt Alloying with Ru Atoms. Electron Transfers. The
effect of alloying Pt with Ru atoms in the Pt crystal is
summarized in Figure 6. Upon alloying Pt with Ru atoms on
the substrate surface layer (Figure 1), the Ru electronic
configuration becomes s0.59d7.13 and s0.40p0.13d6.84 for the cluster
J. Phys. Chem. C, Vol. xxx, No. xx, XXXX G
and the periodic calculations, respectively (d7s1 for isolated Ru
atoms). Alloying causes substantial depletion of the Ru sp-band
by 0.41 and 0.47 e/atom for the cluster and the periodic
calculations, respectively. Concomitantly, the Ru d-band is
increased by 0.13 e/atom for the cluster calculations and reduced
by 0.16 e/atom for the periodic calculations. For clusters, the
electron depletion of the Ru sp-band is predominantly due to
following electron transfer processes: the Ru spf(Ptc, Pt1L*,Pt2L)
sp and the Ru spfRu d, Pt1L* d transfer (PtnL are the nth layer
Pt atoms and Pt1L* are first layer Pt atoms excluding Ptc and its
nearest neighbors). The latter process is responsible for the
increase of the Ru d-band population (Figure 6). Other minor
electron transfers are also observed between the Pt sp- and
d-bands within Pt atoms of the same layer. For periodic slabs,
we cannot identify the individual effects, which cause the Ru
sp- and d-electron reductions. The resultant electron reduction
of the combined Ru sp/d-band is mainly due toRu sp/df(Ptc,
Pt1L*,Pt2L) sp electron transfer. Additionally, a secondary transfer
Pt3L spfPt2L+3L d is also observed. For either system alloying
Pt with Ru atoms increases the overall Pt sp and d populations.
Moreover, the individual Ptc sp population increases, indicating
increased repulsion of the adsorbate via σ repulsion.
Center-of-Bands. Electron transfers, caused by Pt alloying
with Ru atoms result in small upshifts of the Pt d-band center
by 0.06 eV for the cluster calculations and larger downshifts
by 0.2 eV for the periodic calculations. The latter shift is in
agreement with our previous report, where local DOS was
obtained using the FEFF882 program. The Pt d-band shifts for
the cluster and the periodic calculations systematically apply
to all of their corresponding Pt d-band components, the bulk
and the surface layers of the periodic calculations. On the basis
of the d-band center argument by Hammer et al.,20 the downshift
in energy of the alloy Pt d-band center indicates reduced 2π*
involvement (less overlap of the d-band with the 2π*), thus
νCO increase contrary to experimental and computational
observations. However, cluster and periodic calculations indicate
reduced νCO irrespective of the Pt d-band center shift. For finite
clusters the Pt sp-band center remains unaffected upon alloying,
whereas for the periodic slabs the Pt sp-band center of the
surface layer is substantially upshifted by 0.43 eV. As was
mentioned before, upshifts/downshifts of band-centers do not
always correspond to electron population reductions/increases.
It was found by integrating the Pt sp-DOS spectrum for Pt and
PtRu slabs, that Pt sp-electron population per atom is increased
upon alloying, contrary to what is expected from the upshift of
the Pt sp-band center.
3.3. CO Adsorbed on PtRu Alloy. 3.3.1. C-O, C-Pt
Optimal Geometries, Vibrational Frequencies, and Eads Adsorption Energies. CO adsorption on the PtRu alloy increases
both C-O and C-Pt distances, when compared to corresponding distances for COads on pure Pt. Concomitantly, as Pt is
alloyed with Ru atoms, the νCO and νCPt stretching frequencies
are both downshifted indicating weakening of the corresponding
bonds. The downshift of the Fermi level of the PtRu-CO system
with respect to corresponding clean surface indicates that the
CO molecule is an electron acceptor to the alloy lattice. The
calculated Eads is lowered due to alloying of Pt with Ru atoms
in agreement with our previous report and other past observations.83
3.3.2. CO MOs Hybridization with Pt sp/d-Bands of the
PtRu Crystal. σ̃ System. As CO is adsorbed on the PtRu alloy
surface, the 4σ̃ and 5σ̃ orbitals are further downshifted in energy
for both cluster and periodic calculations, with respect to
adsorption on pure Pt. However, the d̃σ center of the PtRu-CO
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J. Phys. Chem. C, Vol. xxx, No. xx, XXXX
system is upshifted for the cluster calculations and downshifted
for the periodic calculations. Upon alloying, the cluster and the
periodic calculations show increased 4σ CO contribution to the
adsorbate 4σ̃ orbital, which is indicative of reduced 4σ donation
to the metal (Table 2). This effect weakens both the COads and
the C-Pt bonds, when compared to corresponding adsorption
on pure Pt. For the c(4 × 4) - CO/PtRu periodic slab, the 4σ̃
polarization toward carbon is strongly enhanced, thus increasing
the orbital’s C-O antibonding character, whereas it remains
almost unchanged in the corresponding cluster case. In general,
CO MO polarizations upon alloying do not follow the same
trends for the cluster and the periodic calculations (Table 3).
The effect of the 4σ̃ polarization in the periodic slab case
maximizes the COads internal bond weakening, which is caused
by the increased charge in the CO region of the 4σ̃ orbital upon
alloying. However, this effect diminishes the weakening of the
C-Pt bond. COOP calculations on the Pt22Ru4CO cluster show
that 5σ̃ is slightly C-O antibonding, with concomitant charge
increase on the carbon and oxygen atoms. The charge increase
on carbon is solely due to an increase of the atom’s 2s orbital,
which is responsible for the 5σ̃ C-O antibonding character.
These statements also apply to the c(4 × 4) - CO/PtRu slab
case. For the Pt26Ru4CO cluster, increased charge (upon
alloying) in the CO region of the 5σ̃ orbital, coupled with
increased polarization of the orbital toward the oxygen atom,
weakens the C-Pt bond. However, for the c(4 × 4) - CO/
PtRu slab case, the 5σ̃ polarization toward the oxygen atom is
reduced. This is verified by observing the charge density
differences on the 5σ̃ orbital due to alloying (Table 3). More
specifically, upon alloying, the carbon contribution to the 5σ̃
orbital is larger for the cluster with respect to the periodic
calculations (0.22 and 0.19 e for the cluster and the periodic
calculations, respectively). The higher population on the carbon
2s orbital is indicative of increased repulsion between the
adsorbate and the metal substrate. Therefore, changes on the
5σ̃ polarization alone result in a stronger C-Pt bond for COads
on PtRu slabs with respect to COads on corresponding clusters.
The cluster calculations show increased charge in the CO region
of the σ̃ system upon alloying. However, the periodic calculations show that the above charge remains almost unchanged.
In both calculations, alloying Pt with Ru further weakens the
COads internal bond, due the 5σ̃ bonding-type change from C-O
nonbonding in the PtCO to antibonding in the PtRuCO.
Moreover, the prior depletion of the Pt dz2-band for Pt-CO
systems is reduced for both the cluster and the periodic
calculations, indicating more σ-dz2 antibonding states, thus
increasing C-Pt σ repulsion (via reduced σ donation). This
repulsion is also enhanced by the additional increase in the Ptc
sp orbital population. Therefore, upon alloying, changes on the
σ̃ system weaken both the COads internal and the C-Pt bonds.
π̃ System. As CO is adsorbed on the PtRu alloy, the 1π̃ system
is further downshifted in energy, with the 1π̃ downshift to be
more pronounced for the periodic calculations. The CO contribution to the adsorbate 1π̃ orbital remains mostly unaffected
upon alloying, for the cluster and the periodic calculations. The
cluster calculations indicate increased charge of the CO
contribution to the d̃π with respect to corresponding adsorption
on pure Pt, due to the orbital’s broadening in energy (Figures
3 and 4). However, the periodic calculations show an opposite
trend. Similarly, the cluster calculations show reduction of the
1π̃ polarization toward carbon, whereas the opposite trend
appears for the periodic calculations. Therefore, upon alloying,
the π-σ model (for the π system alone) will predict stronger
COads internal bond for the cluster calculations and weaker for
Dimakis et al.
the periodic calculations. Concomitantly, for the cluster and the
periodic calculations, the 2π̃* broadens and increases its electron
population, thus indicating further weakening of the COads
internal bond. The observed 2π̃* broadening is consistent with
past observations by Dimakis et al. (band dispersion theory).19
Upon alloying, for cluster calculations, the 2π̃* orbital is
polarized more toward oxygen, thus diminishing the weakening
of the COads internal bond and reducing the C-Pt bond
strengthening, which are caused by the increased charge in the
orbital’s CO region. The increased 2π̃* polarization toward
oxygen is verified by the reduction of the 2π̃* charge in the
carbon atom region by 0.022 e. This reduction alone is indicative
of COads internal bond strengthening (i.e., reduction of the 2π̃*
C-O antibonding character) and C-Pt π bond weakening. The
periodic calculations do not show electron density redistribution
for the 2π̃* upon alloying. The cluster calculations of the π̃
system, upon alloying, indicate strengthening the COads internal
bond and weakening the C-Pt bond. However, the periodic
calculations show an opposite trend.
Center-of-Bands. The Pt d-band center of the Pt22Ru4CO alloy
cluster is upshifted upon alloying; however, the opposite trend
appears for the periodic calculations. In either case, the Pt dz2band center shifts along with the corresponding d-band center
shift. In the cluster calculations, upon alloying, the upshift of
the Pt dz2-band center accompanied by downshift of the 4σ̃ and
5σ̃ orbitals is indicative of reduced mixing of these orbitals with
the metal Pt dz2-band. This effect explains the increased Ptc dz2
population for COads on the alloy cluster with respect to pure
Pt. The above statement also applies to the periodic calculations,
where the downshift in energy of the 4σ̃ and 5σ̃ orbitals is larger
than the downshift of the Pt dz2-band center.
C-O and C-Pt Bonds. For cluster and periodic calculations,
a competition between σ̃ and π̃ orbitals further weakens the
COads internal bond for CO adsorbed on the alloy PtRu surface
with respect to corresponding adsorption on pure Pt. More
specifically, for the cluster calculations, the CO contribution to
the adsorbate σ̃ system increases, thus weakening the COads
internal bond. This effect compensates the corresponding bond
strengthening, which is caused by the increased CO contribution
to the adsorbate π-bonding orbitals (1π̃ and d̃π orbitals).
However, for the periodic calculations, the reduction of the
above charge coupled with increased electron population of the
C-O antibonding 2π̃* orbital, finally weakens the COads internal
bond. The “Blyholder model”, due to concomitant increase of
the 5σ and the 2π* populations for COads on the PtRu alloy
surface, will predict C-Pt bond strengthening upon alloying,
contrary to the observed νCPt downshift. In the cluster calculations, the effect of the C-Pt bonding orbitals (i.e., CO
contributions to the adsorbate 4σ̃, 5σ̃, 1π̃, and 2π̃*orbitals)
counterbalances the effect of the C-Pt antibonding orbitals (i.e.,
CO contributions to the adsorbate d̃σ and d̃π orbitals). One might
expect this as a reason for the invariance of the C-Pt bond
strength upon alloying Pt with Ru atoms. However, it is the
increase of the Ptc sp/dz2 electron populations (Table 2), which
when combined with increased charge of the overall σ CO
contribution to the adsorbate σ̃ results in an exchange
repulsion.19,83 The situation is different for CO adsorbed on the
PtRu surface calculated with periodic DFT. The reduced charge
of the CO contribution to the d̃π orbital is coupled with the
increased charge in the CO region of the 1π̃ and 2π̃* orbitals,
thus causing small C-Pt contraction. However, the C-Pt
π-bonding effect is compensated by the substantial increase of
the Ptc sp/dz2 population (Table 2), thus finally repelling the CO
molecule as verified by the C-Pt elongation and Eads reduction.
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Attraction-Repulsion Mechanism for CO Adsorption
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4. Conclusion
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Molecular cluster and periodic slab calculations show that
as CO is adsorbed on pure Pt the COads internal bond is
weakened, as evidenced by calculated νCO reductions and bond
elongation. The weakening of the COads internal bond is
correlated with changes in the electron populations and density
polarizations of all CO MO contributions to the adsorbate
orbitals from energies as low as the 4σ̃ to as high as the occupied
part of the 2π̃*. Upon CO adsorption on pure Pt, the overall σ̃
system strengthens the COads internal bond due to 4σfdz2
donation, whereas 5σ̃ appears as C-O nonbonding due to the
5σ̃ polarization toward the oxygen atom. The above-mentioned
effect is mainly countered by the increased charge in the CO
region of the 2π̃*, thus finally weakening the COads internal
bond. Our results on the σ̃ system differ from ones presented
by the π-σ model, where the σ̃ system is assumed repulsive.
In this work, both the σ̃ and π̃ systems each contributes to the
formation of a stable C-Pt bond. The σ̃ system of this work
does not include Pauli repulsion between the “inner” σ-type
CO MOs (which do not substantially participate in the chemisorption) and the substrate’s core orbitals. As Pt is alloyed with
Ru atoms, the Ptc sp/dz2 orbital populations increase via electron
transfer from the Ru and other Pt atoms of the substrate lattice.
When CO is adsorbed over the alloy PtRu surface, both the
COads internal and C-Pt bonds weaken with respect to adsorption on pure Pt, as evidenced by reductions of the νCO, νCPt,
Eads, and corresponding bond elongations. The cluster calculations show that the further weakening of the COads internal bond
due to alloying, is attributed to the domination of reduced σ
donation over increased π bonding between the carbon and
oxygen. However, the periodic calculations show that both the
σ donation and the COads internal π bonding are simultaneously
reduced. Reduced σ donation increases the C-Pt σ repulsion
between the carbon atom and the substrate. For the π̃ system,
there is an inverse relationship between the COads internal bond
and the C-Pt bond strength. This observation is in agreement
with the C-Pt π attraction of the original π-σ model. For the
cluster calculations, the C-Pt bond strength is reduced due to
increased Ptc sp/dz2 population and the overall σ CO contribution
to the adsorbate σ̃. However, for periodic calculations, only the
above-mentioned electron populations of the Ptc are increased,
whereas the resultant σ CO electron population remains
unchanged upon alloying. In either case, this effect leads to
exchange repulsion, thus preventing the CO from coming closer
to the metal surface. Work is now in progress to show that the
methodology presented here will explain COads and C-Pt bond
strengths at higher CO coverages.
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Acknowledgment. Thanks are due to thorough insightful
comments from the reviewers that resulted in a step improvement in the manuscript. Funding for ESS was provided by ARO
Contract No. W911NF-08-C-0037. Calculations were performed
using the High Performance Computing Cluster at the University
of Texas-Pan American.
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References and Notes
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