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J. Phys. Chem. C 2009, 113, 18730–18739
Attraction-Repulsion Mechanism for Carbon Monoxide Adsorption on Platinum and Platinum-Ruthenium Alloys Nicholas Dimakis,*,† Matthew Cowan,† Gehard Hanson,‡ and Eugene S. Smotkin§ 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 ReceiVed: April 21, 2009; ReVised Manuscript ReceiVed: August 21, 2009
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. 1. Introduction 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 * To whom correspondence should be addressed. E-mail:
[email protected]. † University of Texas-Pan American. ‡ University of Texas-Austin. § Northeastern University.
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 back-donation 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 Fo¨hlisch 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 1π˜ , d˜π, and 2π˜ * orbitals. In the π-σ model description, the d˜π 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 Fo¨hlisch et al.,31 to the unperturbed orbitals of the π˜ 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
10.1021/jp9036809 CCC: $40.75 2009 American Chemical Society Published on Web 10/02/2009
Attraction-Repulsion Mechanism for CO Adsorption molecule. In the π-σ model framework, the weakening of the adsorbate internal C-O bond (COads internal bond) is attributed to higher 1π˜ 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 d˜π 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 4σ˜ , 5σ˜ , and d˜σ orbitals, where the d˜σ 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 σ˜ orbitals rather than the extent of σ donation to the substrate.31 The σ˜ system (via electron density redistribution) dampens the effect of 1π˜ 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 σ˜ 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 Gajdosˇ et al.,34 as well as extended Hu¨ckel 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 4σ˜ and 1π˜ 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 4σ˜ to as high as the occupied part of the 2π˜ * 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**++
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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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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
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.
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 Fre`hard 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 Monkhorst-Pack grid.70 The Fermi energy and the density matrix are evaluated on a denser grid of 24 × 24 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.
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 4σ˜ , 5σ˜ , 1π˜ , d˜π, d˜σ, the occupied part of the 2π˜ *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.
Attraction-Repulsion Mechanism for CO Adsorption
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TABLE 2: CO Contributions to the Adsorbate 4σ˜ , 5σ˜ , 1π˜ , d˜π, d˜σ, the Occupied Part of the 2π˜ * Orbitals, and the Ptc s, p, and d orbitals for Pt26, Pt22Ru4, Pt26CO, and Pt22Ru4CO Clusters and Corresponding Slabsa cluster molecule/atom
orbital
CO
4σ˜ 5σ˜ (d˜σ) 1π˜ (d˜π) 2π˜ * 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 4σ˜ , 5σ˜ , 1π˜ , d˜π, d˜σ, and the Occupied Part of the 2π˜ * Orbitals Per C and O Atom for Clusters and Corresponding Slabsa clusters
slabs
CO contribution
C
O
C
O
4σ˜ 5σ˜ (d˜σ) 1π˜ (d˜π) 2π˜ *
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.
σ˜ System. The σ˜ system consists of the 4σ˜ , 5σ˜ , and d˜σ orbitals, with the 4σ˜ and d˜σ having C-O antibonding character. However, special treatment is necessary for the 5σ˜ orbital.31,28 The effect of 4σ˜ orbital was first described by Hu et al.78 for COads on Pd (110). The CO contribution to the σ˜ 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 4σ˜ 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 4σ˜ 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 4σ˜ and 5σ˜ 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 4σ˜ and 5σ˜ 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 4σ˜ orbital by only 0.04 e. Moreover, the 4σ˜ 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 5σ˜ C-Pt bonding and d˜σ C-Pt antibonding orbitals. The 5σ˜ orbital shifts to much lower energies (∆ε5σ = 4 eV) with respect to the 4σ˜ 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 5σ˜ becomes nonbonding between carbon and oxygen. This is probably due to the 5σ˜ 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 4σ˜ case, results in C-Pt bonding. Concomitantly, the dz2-band is depleted, primarily due to the
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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 d˜σ and 2π˜ * 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 d˜σ and 2π˜ * orbitals.
corresponding orbital depletion on the Ptc atom. This effect indicates a shift of the d˜σ 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 d˜σ extends beyond the Fermi level, thus verifying that d˜σ 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 d˜σ orbital, since the next available σ-type orbital (i.e., the 6σ˜ * orbital) has been experimentally observed at about 20 eV above the 2π*.30 This observation on d˜σ 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 d˜σ orbital minimizes the C-Pt bonding caused by the 4σ˜ and 5σ˜ orbitals (Table 2). The
resultant σ˜ 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 d˜σ orbital.30 Fo¨hlisch et al.31 state that the strengthening of the COads internal bond cannot be ascribed solely to changes in the 4σ˜ and 5σ˜ 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.
Attraction-Repulsion Mechanism for CO Adsorption
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 4σ˜ and 5σ˜ orbitals verifies the 4σ, 5σfdz2,sp donation mechanism.
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, Fo¨hlisch et al., using constrained space orbital variation calculation on CO/Ni and CO/Cu clusters, observed that the inclusion of 4σ˜ , 5σ˜ , and d˜σ orbital in the calculations reduces the extent of Pauli repulsion. This provides a compelling argument that if Pauli repulsion is excluded from the σ˜ system description, the net effect of the remaining σ˜ orbitals is bonding to the metal. π˜ System. The π˜ system consists of the 1π˜ , d˜π, and the occupied part of the 2π˜ *. The d˜π in this work, differs from the original π-σ model d˜π 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 2π˜ * is in line with the π-σ model, where a “tail” of the 2π˜ * is contained in the dispersed d˜π-band in the energy region just below the Fermi level, and with other past reports.32,34 The CO contribution to the π˜ 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 1π˜ is C-Pt bonding and d˜π 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 1π˜ orbital in the DOS spectrum is slightly lower than its corresponding position for the free CO molecule (∆ε1π = 0.16 eV). The d˜π 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 d˜π orbital via dxz,yzfd˜π. The 1π CO contribution to the adsorbate 1π˜ is diminished, thus weakening the COads internal bond. This effect is accompanied by increased charge in the CO region of the d˜π orbital, which strengthens the COads internal bond. The weakening the COads internal bond, due to the 1π depletion, is maximized by the observed increased 1π˜ polarization toward carbon, with respect to free CO.27,28 The resultant electron charge in the CO region of the 1π˜ and d˜π orbitals is less than the 1π population of the free CO. Therefore, the above-mentioned 1π˜ and d˜π population changes weaken the COads internal bond. Finally, the 2π* CO MO broadens, mixes with the substrate
J. Phys. Chem. C, Vol. 113, No. 43, 2009 18735 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 π˜ system weakens the COads internal bond, and as in the σ˜ 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 4σ˜ and 5σ˜ 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 Hu¨ckel 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 d˜σ 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 CO region of the 2π˜ * via dxz,yzf2π* (Figure 2), with approximately 0.5 e for the cluster and the periodic calculations. The 1πfdxz,yzfd˜π transfer sequence causes only a minor dxz,yzband population increase (
