Carbon Monoxide Adsorption Coverage Study on Platinum and

Article pubs.acs.org/JPCC

Carbon Monoxide Adsorption Coverage Study on Platinum and Ruthenium Surfaces Nicholas Dimakis,*,† Nestor E. Navarro,‡ Thomas Mion,†,¶ and Eugene S. Smotkin§ †

Department of Physics and Geology, University of Texas-Pan American, Edinburg, Texas 78541, United States Department of Chemistry, University of Texas-Pan American, Edinburg, Texas 78541, United States § Department of Chemistry and Chemical Biology, Northeastern University, Boston, Massachusetts 02115, United States ‡

ABSTRACT: Periodic density functional theory calculations elucidate carbon monoxide coverage effects on platinum and ruthenium surfaces. As expected the CO stretching frequencies increase with coverage. Unexpectedly, overlap population calculations show that increased stretching frequencies may not always correspond to stronger bonds. A theoretical framework is established based on a modified π-attraction σ-repulsion scheme. This phenomenological model directly relates the internal adsorbate bond strength to the net change of the carbon 2s and 2pxy contributions to the π- and σcomponents, respectively. The variation of the metal−surface bond is examined by using the charges, polarizations, and electron densities of the adsorbate CO orbitals. For the systems studied here, the traditional frontier orbital model of the 5σ-donation/2π*-backdonation with the metal substrate bands is not always sufficient to explain the relative C−O and C−Metal bonds strengths.

1. INTRODUCTION An understanding of how adsorbates interact with metal surfaces is important to heterogeneous catalysis, electrochemistry, corrosion, and molecular electronics, not only with respect to the adsorbate as a reactant in a device or reaction, but also as a probe of the metal substrate surface structure and function.1,2 A detailed atomistic understanding of bonding to the surface will enhance the level of spatial and electronic structural information that can be obtained by the study of probe adsorbates. Carbon monoxide adsorbed (COads) on metals has attracted broad scientific interest.3−13 In particular, CO absorbance on Pt and Pt-based Ru alloys is of significant importance to electrochemists since these substrates serve as anode catalysts for direct methanol fuel cells.14−27 As CO is adsorbed on the substrate surface, the CO molecular orbitals mix with the substrate bands, thus weakening the adsorbate C−O bond (COads internal bond), while forming a C−Metal bond. The downshift of the C−O stretching frequency (νCO) relative to free CO is generally interpreted as COads internal bond weakening, and is monitored by modulated infrared absorption spectroscopy (PM-IRAS) for COads on platinum (Pt),28−30 ruthenium (Ru),31 and Pt-based alloys.23,32 Moreover, the PMIRAS results have been complemented by corresponding density functional theory (DFT) calculations for COads on Pt-group metals33 and PtRu,34−36 PtOs, and PtRuOs alloys.37,38 Phenomenological models have correlated the weakening of the COads internal bond strength with changes in the COads orbitals charges and polarizations and the substrate d-band center-of-mass. The frontier orbital model of Bagus and Pacchioni5 ascribes the weakening of the COads internal bond to the interaction of the 5σ and 2π* CO frontier molecular © 2014 American Chemical Society

orbitals with the substrate bands as 5σ-donation/2π*-backdonation. This model, also known as the “Blyholder model”,3 originally considered only the entire adsorbate π-system (i.e., the 5σ CO molecular orbital was assumed unaffected by adsorption). Hammer et al.39 used the frontier orbital model to correlate CO adsorption energies (Eads) with substrate d-band center-of-mass for COads on pure metals and alloys: The d-band center downshift is indicative of less overlap of the 2π* CO molecular orbital with the substrate d-band, thus leading to a stronger COads internal bond. The frontier model successfully explains CO absorption on pure metals (e.g., COads on Ni, Fe, Cr, and Ti4 and on Pt40), but fails to explain changes on the COads internal bond strength and the C−Pt bond strength as Pt was alloyed with Os atoms.38 Nilsson et al.,7,12 Bennich et al.,8 and Föhlisch et al.9−11 challenged the frontier orbital model by employing hybrid adsorbate−substrate orbitals as a π-attraction and σ-repulsion (π−σ) model to describe surface adsorption. The π-attraction is ascribed to three tilde-type orbitals/bands: the 1π̃ orbital and the d π̃ - and 2π̃*-bands. In the original π−σ model, the 1π̃ orbital is bonding to the surface, whereas the 2π̃*-band is antibonding to the surface and is located above the Fermi level. The d π̃ -band is a hybrid of the 1π and 2π* CO molecular orbitals with the substrate dxz,yx-bands. The d π̃ -band region close to the Fermi level is bonding to the surface, whereas that energetically more distant from the Fermi level is antibonding. The original π−σ model does not assume direct back-donation to the 2π* molecular orbital from the substrate surface: The πReceived: February 17, 2014 Revised: May 7, 2014 Published: May 8, 2014 11711

dx.doi.org/10.1021/jp501709q | J. Phys. Chem. C 2014, 118, 11711−11722

The Journal of Physical Chemistry C

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Table 1. Effect of Increased Charge in the CO Region of the Tilde-Type Orbital/Band (first-order effect) and Polarization Toward Carbon (second-order effect) for the Extended π−σ Modela effect COads orbital/band

C−O

C−Metal

4σ̃

antibonding

bonding

5σ̃ d σ̃

substrate dependent antibonding

bonding antibonding

1π̃

bonding

bonding

d π̃

bonding

antibonding

2π̃*

antibonding

bonding

CO charge ↑

polarization (toward C)

C−O antibonding ↑ C−Metal bonding ↓ C−Metal bonding ↑ C−O antibonding ↑ C−Metal antibonding ↑ C−O bonding ↑ C−O bonding ↓ C−Metal bonding ↓ C−Metal bonding ↑ C−O bonding ↑ C−O bonding ↓ C−Metal antibonding ↑ C−O antibonding ↑ C−Metal bonding ↑

a

Up (↑) and down arrows (↓) indicate increases and decreases, respectively. The 5σ̃ orbital is slightly C−O bonding, antibonding, or nonbonding (i.e., the C−O bonding type is substrate dependent).

marginal between strong and weak.41 In this work, we develop relationships between the carbon atomic contributions to the adsorbate σ̃ and π̃ orbitals/bands and the strength of the COads bond for CO/Pt and CO/Ru versus CO coverages up to 0.5 ML (Steckel et al. using periodic density functional theory calculations for CO/Pt(111) reported CO saturation coverage of about 0.67 ML45).

attraction weakens the COads internal bond due to increased 1π̃ polarization toward carbon relative to the polarization of the 1π molecular orbital of the free CO.12 In the original π−σ model, the σ-repulsion is ascribed to the 4σ̃ and 5σ̃ orbitals and the d σ̃ -band. The d σ̃ -band is a hybrid of the 5σ CO molecular orbital with the substrate dz2-band. The 4σ̃ and 5σ̃ are bonding to the surface, whereas the d σ̃ -band is antibonding. Föhlisch et al.10 observed charge decrease in the CO regions of the 4σ̃ and 5σ̃ orbitals concomitant with decrease in the substrate dz2-band population, which was interpreted as an absence of σ-type charge donation to the surface, in contrast to the frontier orbital model. Therefore, in the original π−σ model, σ-repulsion strengthens the COads internal bond primarily due to a dramatic charge redistribution within the 4σ̃ and 5σ̃ orbitals (vide infra). Our past work36 on COads on Pt and PtRu challenged the sole repulsive character of the σ̃-system of the original π−σ model: If charge transferred between the adsorbate and the substrate is considered in a stepwise fashion, σ-donation concomitant with substrate dz2-band population decrease is possible. More specifically, as CO is adsorbed on the substrate, charge is initially transferred from the CO regions of the 4σ̃ and 5σ̃ orbitals to the substrate sp- and dz2-bands, the latter being pushed above the Fermi level, and thus partially depopulated. The dz2-band charge is then moved to the dxz,yz-band through the overlapping in energy sp-band, and then to the CO region of the 2π̃*-band, thus weakening the COads internal bond. This extension of the original π−σ model (extended π−σ model) with the inclusion of the attractive σ components (i.e., σ donation) is used in this work to analyze COads on Pt and Ru at various CO coverages. Moreover, in the extended π−σ model, the d π̃ -band is C−O bonding and C−Metal antibonding36−38 (vide infra). Table 1 summarizes the effects of CO charge and polarization variations on the extended π−σ model tilde-type orbitals/bands for the COads internal bond and the C−Metal bond. CO adsorption coverage studies on metal and alloys have been reported.41−47 Ueba41 examined the different behavior for CO adsorbed strongly and weakly on surfaces: For the former case, the COads internal bond strengthens as CO coverage increases, whereas for the latter case this effect was flat or mildly in the opposite direction. The CO molecule adsorbs strongly on Ru, whereas its adsorption on Pt is considered

2. MODELS AND COMPUTATIONAL METHODS The Pt and Ru substrates are modeled by three-layer periodic slabs as Pt(100) and Ru(0001), respectively. The COads are placed atop as c(4 × 4), (2 × 2), and c(2 × 2) overlayers to the Pt(100), and as (3 × 3), (2 × 2), and c(2 × 2) overlayers to the Ru(0001) (Figure 1). The adsorbing metal (Metalc) and the nearest neighbor substrate atoms (Metalo) are also shown in Figure 1. These arrangements correspond to CO coverages of 0.125−0.111, 0.25, and 0.5 ML. Our configurations on 0.25− 0.5 ML coverages do not contain bridge-bonded adsorption sites. Although this work focused on atop configuration the methodology it is also applicable to other adsorptions sites. Similar to our prior work,36,37 the Pt and Ru substrate atoms are fixed in the corresponding crystallographic positions during geometry optimization. Relaxation effects will be examined in a future work. Periodic DFT calculations with and without the CO overlayers were performed with use of the CRYSTAL0948 program, which employs Gaussian-type function basis sets centered at the atoms. The normal mode spectrum is obtained by CRYSTAL09 at the Gamma point (k = 0).49 The “modified” version of the hybrid B3LYP functional was employed, which consists of the same exact and exchange functionals as the original B3LYP functional,50 while replacing the VWN correlation functional with the VWN551 functional. The innermost orbitals of the Pt and Ru atoms are described by effective core pseudopotentials.52 The effective valence basis sets for these atoms are as follows: Pt atoms are described by the [4s4p2d] basis set, whereas Ru atoms are described by the [4s3p2d] basis set. For carbon and oxygen atoms, the original 6-311++G** basis set53 described as [5s4p1d] is contracted to [4s3p2d] for each element. All basis sets are optimized-forcrystalline calculations.54 Brillouin zone integrations are performed on a 12 × 12 Monkhorst−Pack grid.55 The Fermi energy and the density matrix are evaluated on a denser grid of 11712



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grid with 75 radial and 974 angular points was used as the integration grid (XLGRID keyword). Moreover, both the first and second derivatives of energy were calculated numerically, which improves energy gradient accuracy for conducting systems. The Pt and Ru atomic orbital populations were calculated by using Mulliken population analysis.59 Crystal orbital overlap populations (COOP) for a selected pair of atoms60 and the Fermi level were directly calculated by CRYSTAL09. The νCO and νC−Metal calculations were performed in the C−O fragment optimized geometry of the three-layer slab without omitting the third layer, as previously reported (CO molecules freely move during the optimization process).36 Charges in the CO region of the adsorbate−substrate orbitals were obtained by integrating the corresponding partial density-of-states (DOS) spectrum within appropriate energy regions. Charge density plots were obtained by using the XcrySDen graphical package.61 The CO Eads values were calculated with and without the basis set superposition error (BSSE).62 The BSSE arises due to the finite size of the basis sets used for the Eads calculations and was treated by using the counterpoise correction.63 The counterpoise correction minimizes the BSSE by including “ghost” atoms (i.e., massless atoms) in the fragment SCF energy calculations of the adsorbate−substrate structure.

Figure 1. Models for CO adsorption on Pt(100) (left column) and Ru(0001) (right column). CO overlayers, shown by blue-line rectangles, are as follows: (a) c(4 × 4), (b) 2 × 2, (c) c(2 × 2), (d) 3 × 2, (e) (2 × 2), and (f) c(2 × 2). Metalc atoms have an atop COads molecule. For CO/Pt, Metal0 atoms are green, whereas for CO/ Ru all non-Metalc top surface atoms are Metal0 atoms.

3. RESULTS AND DISCUSSION 3.1. The Effect of CO Coverage on C−O and C−Metal Bond Strengths for CO/Pt and CO/Ru. Table 2 shows the C−O and C−Metal DFT fragment optimal geometries, the corresponding stretching frequencies νCO and νC−Metal, Eads (with and without the BSSE correction), and Fermi energies for COads on Pt and Ru under various CO coverages. As CO coverage varies, the inverse relationships between the C−O and νCO and the C−Metal and νC−Metal hold for CO/Ru and fail for CO/Pt (Table 2). The inverse relationship between the bond length and the corresponding frequency is commonly known as Badger’s rule.64−66 In our past work on COads on Pt and PtOs alloys we observed the failure of the Badger’s rule between C− Pt and the corresponding stretching frequency.37 This effect was attributed to DFT inability to accurately resolve distances at orders of 10−3 Å (with the currently used functional and basis set), and was assumed that for COads on Pt and PtOs alloys stretching frequencies variations, which are calculated as second derivatives of the potential energy surface relative to

24 × 24 points (Gilat grid).56,57 A Gaussian smearing of 0.005 hartree for the Fermi surface was used. SCF energy convergence was achieved by employing Anderson quadratic mixing,58 coupled with additional mixing of the occupied with the virtual orbitals. The SCF threshold energy between geometry optimization steps was set between 10−9 hartrees (low CO coverages) and 10−10−10−11 hartrees (0.24−0.5 ML CO coverages): these values are much smaller than the default value of 10−7 hartrees. The root-mean-square value for geometry optimizations was set to the default value of 0.0012 Å. Postgeometry optimization calculations were performed to ensure stability of the final structure (FINALRUN keyword set to 4). Tighter truncation criteria were used for the bielectronic integrals. For example, the overlap thresholds for exchange and Coulomb integrals was set to 10−8 (TOLINTEG keyword set to 8 8 8 8 16, whereas the default value is 6 6 6 6 12). A pruned

Table 2. C−O and C−Metal DFT Optimized Distances, Corresponding Vibrational Frequencies νCO and νC−Metal, Eads, and Fermi Energy for COads on Pt and Ru at Various CO Coverages metal

supercell

CO coverage (ML)

C−O (Å) (νCO, cm−1)

C−Me (Å) (νC−Metal, cm−1)

EadsBSSE‑corrected (eV) (Eadsuncorrected, eV)

EFermi (eV)

Pt

c(4 × 4)

0.125 0.25

c(2 × 2)

0.50

(3 × 3)

0.111

(2 × 2)

0.25

c(2 × 2)

0.50

1.877 (441) 1.881 (441) 1.872 (449) 1.887 (439) 1.895 (431) 1.908 (402)

−1.38 (−1.48) −1.40 (−1.51) −1.28 (−1.39) −1.56 (−2.26) −1.49 (−2.44) −1.44 (−1.71)

−5.481

(2 × 2)

1.138 (2132) 1.138 (2141) 1.139 (2156) 1.155 (2003) 1.150 (2059) 1.143 (2072)

Ru

11713

−5.479 −5.411 −3.081 −3.645 −4.036



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Table 3. COOP between C and O, O and Metalc, and C and Metalc,o, for COads on Pt and Ru at Various CO Coveragesa CO/Pt

a

CO/Ru

atom pair

0.125 MLb

0.25 MLb

0.5 MLb

0.111 MLb

0.25 MLb

0.5 MLb

C−O O−Metalc C−Metalc C−Metalo

0.695 −0.029 0.048 0.009

0.690 −0.029 0.051 0.010

0.682 −0.027 0.052 0.011

0.470 −0.012 0.181 0.027

0.503 −0.012 0.181 0.025

0.531 −0.010 0.158 0.022

Metalc,o are shown on Figure 1. bCO coverage.

Yoshinobu and Kawai71 using infrared reflection absorption spectroscopy and reported νCO at 2095 cm−1. The relative strength of the C−Metal bond is measured by changes on νC−Metal, Eads, and C−Metalc,o COOP. Although it is generally expected that |Eads| varies along with νC−Metal, lack of correlation between Eads and νC−Metal was reported by Koper et al. between CO/Ru and CO/PtRu72 at 1/3 ML CO coverage and by our past work on CO/Pt24Os4 and CO/Pt22Ru2Os2 clusters.38 This effect is attributed to the fact that νC−Metal and Eads are derived from a local and global property of the potential energy surface, respectively, and thus may not always be correlated. At low CO coverages, the νC−Metal is essentially unchanged for CO/Pt relative to CO/Ru. However, at the same low coverages, the BSSE-corrected Eads appears more negative for CO/Ru relative to CO/Pt (i.e., stronger adsorption, Table 2). Table 3 shows that the C−Metalc COOP is substantially larger for COads on Ru relative to Pt (e.g., 0.181 and 0.048 for CO/Ru and CO/Pt, respectively, at low coverages), which is indicative of a stronger C−Ru bond relative to the C−Pt bond, in agreement with past reports.41 This is due to the much greater overlap between the Ru d-band and the orbitals of the carbon atom relative to corresponding Pt d-band overlap (vide infra). Here, the slightly longer C−Ru distance relative to C−Pt distance (by 0.01 Å) could be attributed to steric effects between the adsorbate and the substrate. Loffreda et al.73 in their report on NO adsorbed on Rh and Pd surfaces observed similar contradictions between COOP and N−Metal bond lengths. Our trends on C−Metal distances and νC−Metal are in agreement with past observations by Koper et al.33,72 Table 3 reveals secondary bonding between carbon and Metalo for CO/ Ru, in agreement with past reports,74 whereas this interaction is much less for CO/Pt. Non-BSSE corrected Eads for CO/Pt at 0.125 ML coverage is more positive relative to our past reports (ref 37, Eads at −1.81 eV). This is due to the higher quality basis sets used here for the carbon atom and to the current version of the CRYSTAL software. Indeed, this value is close to the one reported by Steckel et al.45 (−1.55 eV) using five-layer Pt(111) substrates under periodic DFT. CO/Pt and CO/Ru at 0.25 and 0.5 ML Coverages. For both CO/Pt and CO/Ru, calculated νCO increase along with CO coverage (Table 2), in agreement with past reports. More specifically, Chung-Chang and Weaver42 reported an upshift of the νCO per increased CO coverage for CO/Pt under electrochemical environment and ultrahigh vacuum. Later, Liu et al.32 reported a systematic upshift of the νCO under 50%, 1%, and 0.1% CO coverages at various potentials. DFT CO coverage calculations for Pt clusters44 and slabs45,46 show similar trends for the νCO. The effect of the CO coverage for adsorption on the Ru(001) substrate was studied by Pfnür et al.75 using infrared reflection−absorption spectroscopy, where νCO increased as a function of CO coverage.

distance, more accurately represent bond strength variations, when compared to corresponding bond length changes. However, in this work, the COOP between two atoms is also used as a measurement of the corresponding bond strength, where positive and negative COOP values indicate bonding and antibonding patterns, respectively. For primarily covalent bonds, Haslingerova67 presented a relationship between bond energy and COOP as Ek =

ΔH COOPk ∑i COOPi

(1)

where Ek is the kth bond energy, ΔH is the heat of atomization, COOPi is the COOP overlap for the ith bond, and the summation runs over all bonds for the structure. For ionic and dipole-type long-range electrostatic interactions bond strengths cannot be associated with COOPs, since for these bonds the overlap between atoms is minimal irrespective of the bond strength. However, Sung and Hoffman used COOP as a measurement of the C−O and C−Metal bond strengths for COads on Ti, Cr, Fe, Co, and Ni.4 Table 3 shows the COOP between O and C, C and Metalc, C and Metalo, and O and Metalc. DFT calculated frequencies are generally overestimated relative to experimentally measured counterparts.68 For example, Liu et al.,32 using coveragedependent infrared absorption spectroscopy for linearly bound COads on Pt and Ru surfaces, reported νCO values of 2080 and 2001 cm−1 for CO/Pt and CO/Ru, respectively (at 0.5 ML coverage). The measurements by Liu et al. are smaller by no more than about 3.5% relative to the corresponding values reported in Table 2. BSSE-corrected Eads energies are less negative relative to the corresponding uncorrected values, in agreement with past reports.62,69 Moreover, the BSSE appears more significant for CO/Ru relative to CO/Pt, and is attributed to the size of the Ru basis set used. While for cluster calculations a higher basis set may be used at the cost of increased CPU demand, in periodic calculations this is not always possible. The Ru basis set was kept at this size to avoid linear dependencies, which arise when small exponents are used on Gaussian-type basis sets. CO/Pt and CO/Ru at Low Coverages. Table 2 shows that the νCO calculated value for CO/Pt at 0.125 ML coverage is larger by 129 cm−1 relative to CO/Ru at 0.111 ML coverage. These results are in agreement with corresponding C−O COOP calculations, which show a decrease in the COads internal bond strength for CO/Ru relative to CO/Pt. Similar observations appear at 0.25 and 0.5 ML coverages (Table 3). Our calculated νCO values are in agreement with prior experimental observations. More specifically, Thomas et al.70 studied CO/Ru at 0.07 ML using electron loss spectroscopy and reported νCO values at ranges of 1980−2080 cm−1. The CO absorption on Pt at 0.1 ML coverage has been studied by 11714



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Figure 2. CO molecule and its constituent carbon and oxygen atoms DOS spectra for CO/Pt in the arrangements of Figure 1a,c,e calculated by CRYSTAL09. Vertical dashed lines are Fermi levels.

Figure 3. CO molecule and its constituent carbon and oxygen atoms DOS spectra for CO/Ru in the arrangements of Figure 1b,d,f calculated by CRYSTAL09. Vertical dashed lines are Fermi levels.

The upshift of the νCO due to increased CO coverage should not always be interpreted as COads internal bond strengthening.

The C−O COOP shows a striking difference between CO/Pt and CO/Ru: although in both cases the νCO increases along 11715



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Table 4. CO Contributions to the Adsorbate 4σ̃, 5σ̃, and 1π̃ Orbitals and d π̃ , d σ̃ , 2π̃ *-Bands, and the Pt[Ru]c-s, -p, and -d Orbital Populations for CO/Pt and CO/Ru at Various CO Coverages CO/Pt molecule/atom COads

Ptc/Ruc

a

CO/Ru

orbital/band

0.125 MLa

0.25 MLa

0.5 MLa

0.111 MLa

0.25 MLa

0.5 MLa

4σ̃ 5σ̃ d σ̃ 1π̃ d π̃ 2π̃* s p d

1.761 1.581 0.364 3.634 0.189 0.596 0.78

1.763 1.563 0.375 3.653 0.196 0.592 0.77

1.760 1.572 0.461 3.681 0.226 0.581 0.76 0.53

1.858 1.498 0.325 3.792 0.112 0.902

1.845 1.503 0.334 3.756 0.126 0.845

1.829 1.541 0.318 3.900 0.122 0.769 0.64

0.51 8.67

0.66 0.40 6.80

CO coverage.

which in turn diminishes the orbital’s dative bond with the substrate. This statement is verified by observing the charge variations in the CO region of the 1π̃ orbital as coverage varies. For example, the downshift of the CO/Pt-1π̃ orbital as CO coverage increases to 0.25 ML is accompanied by increased charge in the CO region of this orbital, which is indicative of descreased C−Pt bonding. Table 4 shows that the overall Metalc-d population remains unaffected by the CO coverage. CO Adsorption on Pt and Ru at Low CO Coverages. The effect the CO adsorption on Pt has been examined previously.36−38 Figure 4 shows the CO/Pt and CO/Ru 4σ̃

with CO coverage, the COads internal bond weakens for CO/Pt and strengthens for CO/Ru. The lack of correlation between νCO and νC−Metal and corresponding COOPs could be attributed to the mechanical coupling among the COads molecules and with the internal coordinates of the substrate atoms. Webber et al.76 verified mechanical coupling for Nafion normal modes. Table 2 shows that, in general, as CO coverage increases |Eads| decreases, in agreement with prior reports.77 However, a minimal increase of the |Eads| is observed for CO/ Pt, when CO coverage increases from 0.125 to 0.25 ML. A similar small increase of the CO |Eads| has been observed by Stampfl and Scheffler73 for CO and O co-absorption on Ru surfaces and was attributed to lateral interactions of the CO with the substrate atoms. For CO/Ru, increased CO coverage reduces the adsorbate−substrate interaction, as verified by decreases in νC−Ru and |Eads| (Table 2), and in the combined C−Ruc and C−Ruo COOPs (Table 3). However, contrary to the CO/Ru case, the C−Ptc,o COOP increases along with CO coverage, which is indicative of a stronger C−Pt bond. This statement partially agrees with changes in the νC−Pt, which is constant up to 0.25 ML coverages and increases at higher coverages. Table 3 shows higher C−Metalc,o COOP for CO/Ru relative to CO/Pt at all CO coverages examined here. Variations on the Fermi energy for the adsorbate−substrate conformations are minimal for CO/Pt as CO coverage varies (Table 2). However, for CO/Ru the Fermi energy monotonically decreases as CO coverage increases. This effect is attributed to the much smaller size of the ruthenium atom relative to platinum. 3.2. The Extended π−σ Model for CO/Pt and CO/Ru at Low CO Coverages. Figures 2 and 3 show the DOS spectra for the CO molecule and its constituent carbon and oxygen atoms for CO/Pt and CO/Ru, respectively, in the arrangements of Figure 1. Table 4 shows the contributions to the adsorbate σ̃ and π̃ orbitals and the adsorbing metal Metalc-s, -p, and -d orbital populations for CO/Pt and CO/Ru at various CO coverages. At 0.5 ML CO coverage, the CO contributions to the 4σ̃, 5σ̃, and 1π̃ orbitals broaden relative to lower coverages for the same systems (Figures 2 and 3) as an effect of enhanced CO−CO interaction at this coverage. As CO coverage increases up to 0.25 ML, the CO/Pt-1π̃ orbital is downshifted by 0.01 eV relative to the Fermi level, whereas the exact opposite is observed for the CO/Ru-1π̃ orbital (upshift by 0.11 eV). Due to the location of the 1π̃ orbital relative to the substrate d-band, the downshift of the 1π̃ orbital is indicative of decreased overlap of this orbital with the substrate d-band,

Figure 4. Charge density contour plots for CO/Pt and CO/Ru at low CO coverages. Values represent charges in electrons at the oxygen and the carbon atoms regions of the CO molecule. Arrows denote polarization directions within the COads molecule. Ten contour lines are drawn in a linear scale from −0.1 to 0.1 e/a03.

and 5σ̃ charge density contour plots at low CO coverages (0.125 and 0.111 ML for CO/Pt and CO/Ru, respectively). At these coverages, the 4σ̃ and 5σ̃ charge distributions, within the CO, are starkly different between the two systems: the CO/Pt4σ̃ and 5σ̃ are polarized toward carbon and oxygen, respectively. In the case of CO/Ru, these orbitals polarizations are reversed, thus resembling the polarization directions of the free CO. These polarization differences alone underpin a weaker COads internal bond on CO/Ru relative to CO/Pt due to changes in the σ̃-system.10 The 4σ̃ orbital is bonding between carbon 2s and oxygen 2pz and antibonding between oxygen 2s and carbon 2s. Increased carbon 2s contribution to the 4σ̃ and 5σ̃ orbitals enhances the orbitals’ antibonding character.9 The CO/Ru-4σ̃ polarization toward oxygen, which is accompanied by descreased carbon and increased oxygen contributions to the 4σ̃ orbital, reduces 11716



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charge contribution to these orbitals: here, this value is approximately constant between the two systems. The last of the σ̃-system components is the d σ̃ -band, mostly of metallic character (d σ̃ -band is C−Metal antibonding). Figures 2 and 3 reveal that the oxygen contribution to the d σ̃ -band is minimal. The charge in the carbon region of the CO/Ru-d σ̃ -band is decreased relative to CO/Pt-d σ̃ (Table 4), which is indicative of a stronger C−Ru bond relative to the C− Pt bond. Overall, for CO/Ru relative to CO/Pt and at low CO coverages, the combined charge in the CO region of the 4σ̃ orbital and the d σ̃ -band increases by 0.06 e, which translates to a weaker COads internal bond, while changes in the 5σ̃ orbital augment this weakening. The weaker COads internal bond, for CO/Ru relative to CO/Pt, due to changes in the σ̃-system is in agreement with the original π−σ model, which only considers intra-adsorbate atom polarizations. However, the original π−σ model also predicts a weaker C−Ru bond relative to the C−Pt bond due to increased σ-repulsion. Therefore, for the systems of this work, the inclusion of the charge donation to the substrate is of substantial importance for correctly predicting the relative strength of the C−Metal bond. Similarly, for the frontier orbital model the increased depopulation of the 5σ CO molecular orbital for CO/Ru relative to CO/Pt is indicative of a stronger COads internal bond, in contrast to our predictions. The π̃- and σ̃-system must be juxtaposed to complete the bonding scenario. We recall that the π̃-system consists of the 1π̃ orbital and the d π̃ - and 2π̃*-bands. The d π̃ -band, in this work and in our previous reports,36−38 differs from the d π̃ in the original π−σ model, which does not contain contributions from the unperturbed 2π* CO molecular orbital. For low CO coverages, the charge in the CO region of the 1π̃ orbital is increased for CO/Ru relative to CO/Pt (by about 0.16 e, Table 4), thus strengthening the COads internal bond and weakening the C−Metal bond. This increased charge is distributed equally between the oxygen and the carbon regions of the orbital (i.e., no change in the orbital polarization). The d π̃ -band is mostly an oxygen lone pair with minor charge located at the carbon atom (Figures 2 and 3). For CO/Pt and at low CO coverage, the d π̃ -band of the extended π−σ model is C−O bonding and C−Pt antibonding.36−38 For CO/Ru, charge in the CO region of the d π̃ -band is decreased (by about 0.07 e, Table 4) mostly due to charge decrease in the oxygen region of the band, thus weakening the COads internal bond and strengthening the C− Metal bond relative to CO/Pt. The latter statement is consistent with the substantially decreased O−Ruc destabilizing interaction relative to O−Ptc (Table 3). Finally, we examine changes in the 2π̃*-band between the two systems. At the low CO coverages discussed here, substantially increased charge is observed in the CO region of the 2π̃*-band of the CO/Ru relative to CO/Pt (about 0.31 e, Table 4), mostly due to an increase in the carbon region of the orbital. This charge increase is concomitant with the Ru d-band upshift relative to the Pt d-band (Figure 5), in agreement with the d-band center argument of Hammer et al.6 Additionally, it explains the larger COOP C−Ru value relative to C−Pt (Table 3). Changes in the 2π̃*-band, as described here, weaken the COads internal bond and more than offset the strengthening caused by changes in the 1π̃ orbital. These results are in agreement with both the frontier orbital model and the original π−σ model. For the COads internal bond, the effects of the π̃system more than offset the effects of the σ̃-system leading to an overall weaker COads internal bond for CO/Ru relative to

the orbital’s antibonding character relative to CO/Pt-4σ̃. Therefore, changes in the 4σ̃ polarization alone for CO/Ru4σ̃ relative CO/Pt-4σ̃ strengthen the COads internal bond and weaken the C−Metal bond (Table 1). Table 4 reveals that at low CO coverages, the overall 4σ̃ orbital charge, in the orbital’s CO region, is larger for CO/Ru relative to CO/Pt: this observation is indicative of decreased 4σ → dz2 charge donation to the Ru substrate relative to Pt, which leads to the weakening of both the COads internal bond and the C−Metal bond between the two systems. Figure 5 shows the DOS spectra of

Figure 5. The D-band DOS spectra for CO/Pt, CO/Ru, and corresponding clean substrates. Arrows denote d-band centers for clean substrates for whole bands (i.e., including d-band regions above the Fermi level). The vertical dashed line is the Fermi level.

the substrate d-bands for Pt and Ru, with and without an adsorbed CO (low CO coverage). The presence of peaks at the locations of the 4σ̃ and 5σ̃ orbitals (Figure 5, insert) in the substrate d-band DOS spectra for CO/Pt and CO/Ru supports the 4σ, 5σ → dz2 donation mechanism, since these peaks are absent from the same spectra of the corresponding clean substrate. At low CO coverages, the polarization 4σ̃ toward oxygen augments the weaker C−Ru bond relative to C−Pt bond caused by the decreased charge donation to the substrate. The same polarization minimizes the weakening of the COads internal bond caused by the decrease of the 4σ donation effect relative to CO/Pt. The decrease of the C−Ru bond relative to the C−Pt bond due to changes in the 4σ̃ orbital is evidenced by observing the corresponding contour plots, which show smaller overlap between carbon and the substrate for CO/Ru-4σ̃ relative to CO/Pt-4σ̃ (Figure 4, top). For low CO coverage, the CO/Pt-5σ̃ is a C−O nonbonding orbital.36,38 However, at 0.111 ML coverage, the CO/Ru-5σ̃ polarization toward carbon changes the 5σ̃ orbital from C−O nonbonding in the CO/Pt to weakly antibonding in the CO/ Ru. This is explained by decomposing the 5σ̃ orbital into its C− O bonding and antibonding components: the CO/Ru- 5σ̃ orbital is bonding between carbon 2pz and oxygen 2pz and antibonding between carbon 2s and the oxygen 2pz. For CO/ Ru relative to CO/Pt, the increased carbon 2s contribution to the 5σ̃ verifies the weakly C−O antibonding character of the CO/Ru-5σ̃. Moreover, the contour plots show that changes in the 5σ̃ orbital lead to a stronger C−Ru bond relative to the C− Pt bond (Figure 5). The combined effect of the 4σ̃ and 5σ̃ orbitals to the C−Metal bond could be indicated by the net CO 11717



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Figure 6. Contour plots of charge density differences for CO/Pt and CO/Ru at adsorbate coverages of 0.25 ML relative to 0.125 ML (CO/Pt; left column), 0.25 ML relative to 0.111 ML (CO/Ru; left column), and 0.5 ML relative to 0.25 ML (CO/Pt and CO/Ru; right column) within appropriate energy regions. Values represent charge differences in electrons at the oxygen and the carbon atoms regions of the CO molecule per tilde-orbital. Blue areas denote change-deficient regions, while red areas denote charge-surplus regions. (i.e., charge flows from the blue areas to the red areas). Ten contour lines are drawn in a linear scale from −0.01 to 0.01 e/a03.

For CO/Pt, increased CO coverage to 0.25 ML reduces the 4σ̃ polarization toward carbon, without affecting the overall charge at the orbital’s CO region (Figure 6, upper left, and Table 3). The decreased 4σ̃ polarization toward carbon translates to a decrease of the carbon 2s contribution, thus decreasing the 4σ̃ antibonding character. The same polarization reduces C−Pt bonding in agreement with the corresponding contour plot blue region between carbon and Pt, which is indicative of charge deficiency between the two atoms (Figure 6, upper left). The increased CO coverage to 0.25 ML reduces the charge in the CO region of the 5σ̃ orbital and its polarization toward oxygen (Figure 6). As discussed in the last section, changes in the 5σ̃ orbital have minimal impact on the COads internal bond due to the orbital’s C−O nonbonding character for CO/Pt. However, these same changes in the 5σ̃ orbital strengthen the C−Pt bond. For example, the observed charge decrease in the CO region of the 5σ̃ orbital is indicative of increased charge donation to the substrate, which enhances C−Pt bonding. However, the corresponding contour plot (Figure 6) shows no change in the region between carbon and Pt, which is indicative

CO/Pt, whereas both systems contribute to a stronger C−Ru bond relative to the C−Pt bond. In this work, the weakening of the COads internal bond is ascribed to the sum of the carbon 2s contribution to the σ̃system (σ̃ polarization) and the carbon 2pxy contribution to the π̃-system (π̃ polarization). At low coverages, this factor is increased by about 0.41 e for CO/Ru relative to CO/Pt, which corresponds to COads bond weakening, in agreement with the above discussion. CO Adsorption on Pt at 0.25 and 0.5 ML Coverages. Figure 6 shows the contour plots of the charge density differences (per tilde-type orbital and overall) as CO coverage is increased up to 0.5 ML for CO/Pt and CO/Ru. The d π̃ - and 2π̃*-bands contour plots also contain information about the d σ̃ -band, since the d σ̃ -band appears in the energy regions of the other two bands (Figures 2 and 3). The contour plots of Figure 6 allow the examination of the spatial distribution of the orbitals’ charge densities, thus elucidating their contribution to the COads internal bond and the C−Metal bond strengths, as CO coverage varies. 11718



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0.25 ML, whereas it is slightly reduced to about 0.99 e at 0.50 ML coverage. The final C−Pt bond slightly strengthens due to the decreased s-Ptc atomic population as CO coverage increases, in agreement with C−Ptc COOP results (Table 3). As the CO coverage increases from 0.25 to 0.5 ML, the 4σ̃ polarization increases toward carbon, which is in contrast to what is observed when coverage increases from 0.125 to 0.25 ML (Figure 6). Interestingly, for CO/Pt, no trends are drawn between the CO coverage and the 4σ̃ and 5σ̃ charges (in the orbitals’ CO regions) and polarizations (Table 4 and Figure 6). However, the carbon 2s contribution to the σ̃-system increases along with the CO coverage, thus weakening the COads internal bond. Some trends are drawn between the π̃-orbitals/bands and the CO coverage. For example, the charge in the CO regions of the 1π̃ orbital and the d π̃ -band increases along with CO coverage. Similarly to the σ̃-system, the π̃-system also weakens the COads internal bond as coverage increases, and is verified by the increased carbon 2pxy contribution to the π̃-system. The strengthening of the C−Pt bond due to changes in the 4σ̃, 5σ̃, and 1π̃ orbitals is verified by the red regions between carbon and Pt for all of these orbitals (Figure 6). Figure 7 shows that for CO/Pt the frontier orbital model fails to explain the weakening of the COads bond as CO coverage increases. It is noteworthy that for CO/Pt the static COads dipole moment remains constant at 0.127 D for CO coverages up to 0.25 ML, and thereafter slightly decreases to 0.126 D at 0.5 ML coverage. This suggests that (for CO/Pt and at the adsorbate arrangements of this work) dipole−dipole interactions are minimal for CO coverages up to 0.25 ML. This statement also applies to CO/Ru (vide infra). CO Adsorption on Ru at 0.25 and 0.5 ML Coverages. The effect of CO coverage on the tilde-type orbitals is starkly different between CO/Pt and CO/Ru. The contour maps in Figure 6, related to CO/Ru, appear slightly distorted due to minor tilts of the COads on the Ru surface under the various coverages of Figure 1. We first examine the effect of the CO coverage on the σ̃system. We recall that for CO/Ru at low CO coverage (i.e., 0.111 ML), the 4σ̃ and 5σ̃ orbitals’ polarizations resemble those of the free CO (i.e., 4σ̃ and 5σ̃ polarize toward oxygen and carbon, respectively). These polarizations are decreased along with increased CO coverage (Figure 6), thus strengthening both the CO ads internal bond and the C−Ru bond. Furthermore, these bonds are strengthened by the decreased charge in the CO region of the 4σ̃ orbital as coverage increases. Table 4 shows that, for CO/Ru, charge variations in the CO regions of the 4σ̃ and 5σ̃ orbitals, due to coverage effects, vary inversely to each other. Therefore, the charge decrease in the CO region of the 5σ̃ orbital as coverage increases minimizes the strengthening of the C−Ru bond due to changes in the 4σ̃ orbital. However, contrary to the CO/Pt case, no trend is drawn here between the charge in the CO region of the d σ̃ -band and the CO coverage. In contrast to the CO/Pt case, for CO/Ru as CO coverage increases, the σ̃-system strengthens the COads internal bond, as verified by its decreased carbon 2s contribution to the σ̃-system (1.27 e, 1.26 e, and 1.22 e, for CO coverages of 0.111, 0.25, and 0.5 ML, respectively). The contour plots of Figure 6 show that for CO/Ru all tildetype orbitals/bands contribute to changes in the C−Ru bond as coverage varies. For the σ̃-system, the C−Ru bond strength is indicated by the overall charge transferred from the CO regions of 4σ̃ and 5σ̃ bonding to the surface orbitals to the substrate subtracting the charge in the CO region of the d σ̃ -band

of a constant C−Pt bond. These contradictory statements are resolved by observing that tilde-type orbitals, which are adjacent in the energy spectrum, have parts in overlapping energy regions (e.g., 5σ̃ and 1π̃, Figure 2). Indeed, the corresponding contour plot for the 1π̃ orbital shows a red region in the area between carbon and Pt, which is indicative of C−Pt bond strengthening as a result of the combined effect of the 5σ̃ and 1π̃ orbitals. Therefore, in this case, changes in the C−Pt bond due to the σ̃-system variations as CO coverage increases cannot be separated from the π̃-system variations. For CO adsorption on Pt, the CO contribution to the d σ̃ -band increases along with the CO coverage, thus weakening both the COads internal bond and the C−Pt bond. As CO coverage is increased to 0.25 ML, the weakening of the COads internal bond caused by changes in the d σ̃ -band more than offsets the strengthening caused by the above-discussed changes in the 4σ̃ polarization, in agreement with the slightly increased overall carbon 2s contribution to the σ̃-system (by 6 × 10−4 e). We now examine the effect of the increased CO coverage on the π̃-system. The charge in the oxygen region of the 1π̃ orbital is increased as CO coverage increases to 0.25 ML, thus strengthening the COads internal bond. However, in this case, the increased charge in the carbon regions of the d π̃ - and 2π̃*bands weakens the COads bond: this weakening more than offsets the strengthening of the COads internal bond caused by the changes in the 1π̃ orbital. This statement is verified by the increased carbon 2pxy contribution to the π̃-system (by 0.013 e). Figure 7 shows the sum of the carbon 2s and 2pxy

Figure 7. Left axis: C−O COOP relative to CO coverage for CO/Pt and CO/Ru. Right axis: overall negative charge of the CO contributions to the adsorbate 5σ̃ orbital and the occupied part of the 2π̃*-band and the negative sum of the carbon 2s and carbon 2pxy contributions to the σ̃- and π̃-systems, respectively.

contributions to the σ̃- and π̃-systems, respectively, under various CO coverages, for COads on Pt and Ru: a trend is drawn between this value and the C−O COOP as CO coverage increases. The effect of the increased CO coverage on the C−Pt bond will be analyzed by using the contour plots of Figure 6. These plots show that the combined effects of the d π̃ -, 2π̃*-, and d σ̃ -bands on the C−Pt bond are minimal as CO coverage varies. Therefore, C−Pt bond strength variations due to changes in the CO coverage are mostly related to changes in the 4σ̃, 5σ̃, and 1π̃ orbitals. These orbitals are bonding to the substrate; thus the total amount of charge transferred to the substrate from the CO regions of the 4σ̃, 5σ̃, and 1π̃ orbitals could be used as an indicator for the C−Pt bond strength. Here, this value is constant at 1.02 e, for CO coverages up to 11719



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(antibonding to the surface). These values are 0.33 e, 0.32 e, and 0.31 e for CO coverages of 0.111, 0.25, and 0.5 ML, respectively. Therefore, the C−Ru bond slightly weakens as CO coverage increases, due to changes in the σ̃-system. Finally, we examine the effect of the π̃-system on CO/Ru as coverage increases. The carbon 2pxy contribution to the π̃system varies as 1.75 e, 1.68 e, and 1.69 e for CO coverages of 0.111, 0.25, and 0.5 ML, respectively. Therefore, for CO/Ru, no trend is drawn between the carbon 2pxy contribution to the π̃-system and the CO coverage. Overall, for CO/Ru, the COads internal bond and the C−Ru bond weaken along with increased CO coverage (Figure 7). For the π̃-system, the C−Ru bond strength is indicated by the overall charge transferred between the CO regions of the 1π̃ orbital and 2π̃*-band and the substrate (C−Ru bonding) subtracting the charge in the CO region of the d π̃ -band (C−Ru antibonding): this value is decreasing along with increasing CO coverage. In the case of CO/Ru, the static COads dipole moment remains constant at 0.119 D for CO coverages up to 0.25 ML, and thereafter increases to 0.123 D at 0.5 ML coverage. This is opposite to the CO/Pt case, where the static COads dipole moment was decreased when the CO coverage increased to 0.5 ML. Moreover, for CO/Ru, the frontier orbital model correctly predicts the strength of the COads internal bond as CO coverage increases. The different behavior of COads on Pt and Ru is more understood by observing that the Ru substrate promotes strong CO adsorption, whereas the adsorption on Pt is weaker. The CO adsorption on Ru is governed by changes in the 2π̃*-band, with the charge in the orbital’s carbon region being decreased as CO coverage increases. For CO/Ru, the both carbon and oxygen are negatively charged: the overall adsorbate charge is decreased as coverage increases. To the contrary, the same adsorption on Pt is governed by both σ̃ and π̃ variations as described in this work: for CO/Pt, the tilde-orbital polarizations of the σ̃- and π̃-systems play an important role in the chemisorption process. As CO is adsorbed on Pt, the COads becomes C+O−, whereas increased CO coverage causes charge to be transferred to the carbon atom. Our results are consistent with prior work by Ueba.41

strengthening of the C−Metal bond has been correlated with the charge transferred between the CO and the substrate. The COads internal bond appears weaker on CO/Ru relative to CO/Pt irrespective of the CO coverage. This is attributed to the substantially increased back-donation to the CO region of the 2π̃*-band for CO/Ru relative to CO/Pt, and to the starkly different 4σ̃ and 5σ̃ polarization directions between the two systems. The former effect is in agreement with the increased dvacancies and the upshift of the d-band center for Ru relative to Pt. The latter effect is indicative of the minor role of the σ̃system on the CO/Ru. The weaker COads internal bond for CO/Ru relative to CO/Pt is in agreement with the observed decreased O−Metalc repulsion for CO/Ru relative to CO/Pt. The CO molecule is adsorbed more strongly on Ru relative to the Pt surface. However, the C−Ruc bond lengths appear longer relative to C−Ptc, which is indicative of a weaker C−Ruc bond. These contradictory results are reconciled by observing that lateral interactions between the CO molecule and the substrate are more pronounced for adsorption on Ru relative to Pt.

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AUTHOR INFORMATION

Corresponding Author

*Tel: (956) 665-8761. E-mail: [email protected]. Present Address ¶

T.M.: Department of Physics, Boston College, Boston, MA.

Notes

The authors declare no competing financial interest.

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REFERENCES

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4. CONCLUSION Periodic DFT calculations on CO adsorbed on Pt and Ru substrates were used to elucidate changes in the COads internal bond and the C−Metal bond as coverage varies. Our calculated νCO values increase along with increased CO coverage, in agreement with prior experimental and calculated measurements for both CO/Pt and CO/Ru. The upshift of the adsorbate stretching frequency was previously interpreted as strengthening of the internal adsorbate bond. However, our C− O COOP calculations show that this is not always the case: for CO/Pt, the COads internal bond weakens as coverage increases, while the exact opposite is observed for the CO/Ru case. These bond strength changes are described quantitatively by the sum of the carbon 2s and 2pxy contributions to the σ- and πcomponents, respectively. For CO/Pt, the frontier orbital model fails to predict the decreased COads bond strength as CO coverage increases; however, the same model correctly predicts the strengthening of the COads bond for the CO/Ru upon increased coverage. Similarly, the C−Metalc,o COOPs indicate that as coverage increases the C−Metal bond strengthens for CO/Pt, while it weakens for CO/Ru. In this work, the 11720



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Carbon Monoxide Adsorption Coverage Study on Platinum and

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