Ethanol and Water Adsorption on Close-Packed 3d ... - ACS Publications

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Ethanol and Water Adsorption on Close-Packed 3d, 4d, and 5d Transition-Metal Surfaces: A Density Functional Theory Investigation with van der Waals Correction Polina Tereshchuk†,‡ and Juarez L. F. Da Silva*,§,† †

Instituto de Física de São Carlos, Universidade de São Paulo, Caixa Postal 369, 13560-970, São Carlos, SP, Brazil Institute of Nuclear Physics of Uzbekistan AS RUz, Ulugbek, Tashkent 100214, Uzbekistan § Instituto de Química de São Carlos, Universidade de São Paulo, Caixa Postal 780, 13560-970, São Carlos, SP, Brazil ‡

ABSTRACT: Nowadays, there is a great interest in the economic success of direct-ethanol fuel cells; however, our atomistic understanding of the designing of stable and low-cost catalysts for the steam reforming of ethanol is still far from satisfactory, in particular due to the large number of undesirable intermediates. In this study, we will report a first-principles investigation of the adsorption properties of ethanol and water at low coverage on close-packed transition-metal (TM) surfaces, namely, Fe(110), Co(0001), Ni(111), Cu(111), Ru(0001), Rh(111), Pd(111), Ag(111), Os(0001), Ir(111), Pt(111), and Au(111), employing density functional theory (DFT) calculations. We employed the generalized gradient approximation with the formulation proposed by Perdew, Burke, and Erzenholf (PBE) to the exchange-correlation functional and the empirical correction proposed by S. Grimme (DFT+D3) for the van der Waals correction. We found that both adsorbates binds preferentially near or on the on-top sites of the TM surfaces through the O atoms. The PBE adsorption energies of ethanol and water decreases almost linearly with the increased occupation of the 4d and 5d d-band, while there is a deviation for the 3d systems. The van der Waals correction affects the linear behavior and increases the adsorption energy for both adsorbates, which is expected as the van der Waals energy due to the correlation effects is strongly underestimated by DFT-PBE for weak interacting systems. The geometric parameters for water/TM are not affected by the van der Waals correction, i.e., both DFT and DFT+D3 yield an almost parallel orientation for water on the TM surfaces; however, DFT+D3 changes drastically the ethanol orientation. For example, DFT yields an almost perpendicular orientation of the C−C bond to the TM surface, while the C−C bond is almost parallel to the surface using DFT +D3 for all systems, except for ethanol/Fe(110). Thus, the van der Waals correction decreases the distance of the C atoms to the TM surfaces, which might contribute to break the C−C bond. The work function decreases upon the adsorption of ethanol and water, and both follow the same trends, however, with different magnitude (larger for ethanol/TM) due to the weak binding of water to the surface. The electron density increases mainly in the region between the topmost layer and the adsorbates, which explains the reduction of the substrate work function.

I. INTRODUCTION

CH4 + CO + H2), ethanol dehydrogenation to acetaldehyde (C2H5OH → C2H4O + H2), ethanol decomposition into acetone (2C2H5OH → CH3COCH3 + CO + 3H2), coking from decomposition of methane (CH4 → 2H2 + C). Several reaction paths produce CO, and hence, the water-gas shift (WGS) reaction has been used to convert CO in CO2 and H2 through a reaction with steam (CO + H2O → CO2 + H2).8 Experimental studies have indicated that the specific reaction route in the steam reforming of ethanol depends strongly on the selected catalysts,3,4,8 which have motivated a large number of studies in the search to obtain catalysts that minimize the formation of undesirable intermediates and maximize the

The adsorption and reactions of alcohols such as methanol (CH3OH), ethanol (C2H5OH), and propanol (C3H7OH) on transition-metal (TM) surfaces have attracted great interest in the recent years, as alcohols, in particular ethanol, can be considered as one of the most important renewable resources for hydrogen (H2) production for fuel cell applications.1−6 Steam reforming of ethanol (C2H5OH + 3H2O → 2CO2 + 6H2) has been considered as a promising process to produce H2, which can be attributed to the catalytic role of water in ethanol reactions.7 However, several experimental studies have indicated that various pathways can occur in the steam reforming process of ethanol.4,8,9 For example, the following intermediate reactions have been reported in refs 4 and 8: ethanol dehydration to ethylene (C2H5OH → C2H4 + H2O), ethanol decomposition or cracking to methane (C2H5OH → © 2012 American Chemical Society

Received: September 6, 2012 Revised: November 1, 2012 Published: November 2, 2012 24695

dx.doi.org/10.1021/jp308870d | J. Phys. Chem. C 2012, 116, 24695−24705

The Journal of Physical Chemistry C

Article

der Waals correction in the semilocal exchange-correlation (xc) energy functional currently employed in DFT calculations for all the TM surfaces, which is an important step to improve the theoretical description of those systems. Reported DFT calculations showed that ethanol binds to the TM surfaces through the O atoms, which is similar to the adsorption of water on TM surfaces.55−60 Thus, for comparison, we studied the adsorption of water on all those mentioned TM surfaces, which is also important as water molecules are part of the steam reforming of ethanol.

conversion of ethanol and production of H2. Oxide catalysts such as alumina (Al2O3) and vanadia (V2O3)9 yield about 100% conversion of ethanol, however, with a small production of H2 due to the formation of undesirable intermediates. Further studies have been reported for ceria (CeO2)10 and titanium dioxide (TiO2).11 To our knowledge, most of the experimental studies have been performed for metal/oxides catalysts, such as Co/Al2O3,12 Co/ZnO,13 Co/SiO2,14 Pd/CeO2,10 Rh/MgO,2,4 Rh/CeO2,15 Rh/Al2O3,16 Rh/Y2O3,17 Rh/ZnO2,4 Pt/CeO2,15 Pt/Rh/SnO2,5 etc. (see refs 4 and 8). Surface science techniques such as high-resolution photoemission spectroscopy (XPS), high-resolution electron energy loss spectroscopy (HREELS), temperature-programmed desorption (TPD), infrared reflection−absorption spectroscopy (IRAS), and photoelectron spectroscopies, as well as electrochemistry analysis,18−21 have been employed to obtain a better understanding of the success or failure of model TM catalysts using well-designed flat and stepped TM surfaces. For example, several studies have been reported for Co(0001),22 Ni(111),23 Rh(111), 24−27 Rh(553), 27 Pd(111), 28−30 Pd(110), 31 Ag(110),32 and Pt(111).33−38 Furthermore, first-principles density functional theory (DFT) calculations have been widely employed to study ethanol adsorption, decomposition, and synthesis on several substrates, e.g., ice,39 activated carbon model,40 Si(100),41 3Ni/α-Al2O3(0001),42 2Ru/ZrO2,43 2Rh/ γ-Al2O3(110),44 Rh/CeO2,45 Rh(111),27,46,47 Pd(111),48 and Pt(111).49−52 Those studies have greatly contributed to improve our understanding of ethanol adsorption and decomposition. For example, experimental studies have suggested that the activation of the O−H bond scission in the ethanol molecule on Ni(111) requires an almost parallel orientation of the O−H bond to the surface, which is expected to increase the interaction with the TM surface, and hence, it induces a deuterium kinetic isotope effect for the ethanol decomposition.23 DFT calculations have obtained that ethanol binds preferentially on the top sites on Rh(111)47 and Pt(111)51 surfaces thought the O atom, with the C−C bond nearly perpendicular to the surface and a weak adsorption energy, e.g., −500 meV for ethanol/Rh(111) and about −280 meV for Pt(111).49 It was reported a red-shift of the vibration frequency of the OH group for ethanol/Rh(111), which has a strong dependence on the coverage.47 Therefore, few facts can be summarized: (i) The adsorption energy of ethanol on TM surfaces is on the order of hundreds of meV,47,49,51 and hence, its magnitude indicates a binding mechanism between true chemisorption and physisorption interactions.53,54 Thus, the van der Waals interactions, which are not well described by plain DFT, might play an important role in the magnitude of the adsorption energies as well as in the adsorbed structures. (ii) The adsorption properties of ethanol have been investigated on few substrate TM surfaces, while typical molecules such as water,55−60 CO,61,62 NO,63,64 etc., have been studied on a wide range of substrates, which have greatly improve the understanding of those systems. To improve our understanding of the adsorption properties of ethanol on TM surfaces, in this work, we will report a firstprinciples DFT investigation of the adsorption properties of ethanol on 12 close-packed TM surfaces, namely, Fe(110), Co(0001), Ni(111), Cu(111), Ru(0001), Rh(111), Pd(111), Ag(111), Os(0001), Ir(111), Pt(111), and Au(111), using a (3 × 3) unit cell or similar unit cell size such as (2 × 3) for Fe(110). Furthermore, we will investigate the role of the van

II. THEORETICAL APPROACH AND COMPUTATIONAL DETAILS Our spin-polarized calculations are based on DFT65,66 within the generalized gradient approximation67 using the formulation proposed by Perdew, Burke, and Erzenholf68 (PBE) to the xc energy functional. To improve the description of the van der Waals interactions, which is well-known to play an important role in weak adsorbed systems,54,69 we employed the empirical van der Waals correction proposed by S. Grimme (DFT +D3),70 which is a recent improvement over the DFT+D2 framework.71 In this approach the total energy, EDFT+D3, is obtained by the sum of the self-consistent DFT total energy, EDFT, with the van der Waals correction, Edisp, i.e. E DFT + D3 = E DFT + Edisp (1) Edisp is the sum of the two- and three-body energies, i.e., Edisp = E(2) + E(3), where E(2) =

∑

∑

AB

n = 6,8,10,...

sn

CnAB n fd , n (rAB) rAB

(2)

All the atom pairs are taken into account by the first sum, and CAB n are the dispersion coefficients of the nth order for each pairs AB computed from first principles calculations for molecular systems.70 rAB is the distances between the A and B atoms, while sn is the scaling factor, which depends on the xc functional, and fd,n is the damping function to avoid nearsingularities for small distances. The three-body term is given by E(3) =

ABC ∑ fd ,(3) ( rABC ̅ )E ABC

(3)

ABC

where E is the nonadditive dispersion term. Further details can be found elsewhere.70 From now, DFT-PBE and DFT-PBE +D3 will be called shortly by PBE and PBE+D3, respectively. To solve the Kohn−Sham equations, we employed the allelectron projected augmented wave (PAW) method72,73 as implemented in the Vienna Ab-initio Simulation Package (VASP).74,75 The total energy calculations were performed using a plane-wave cutoff energy of 400 eV for all systems, while higher cutoff energies were employed to obtain the equilibrium lattice constants of the bulk systems in the facecentered cubic (fcc) structure for Ni, Cu, Rh, Pd, Ag, Ir, Pt, and Au, hexagonal close-packed (hcp) structure for Co, Ru, and Os, and the body-centered cubic (bcc) structure for Fe, by minimizing the stress tensor and atomic forces. The theoretical lattice constants are required to the surface calculations.76 The ethanol and water adsorption on closed-packed TM surfaces were modeled using the repeated slab geometry employing a (3 × 3) surface unit cell for the fcc(111) and hcp(0001) surfaces, while for the bcc(110) surface, we employed a (2 × 3) unit cell with similar surface area. Thus, 24696



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Table 1. Equilibrium Lattice Constants (in Å)a

the adsorbates (water and ethanol) are well separated (low coverage), i.e., from 5.0 to 7.0 Å, which depends on the ethanol orientation. Four layers slab were considered for all calculations, with a vacuum region of about 21 Å, which is enough to provide an accurate description of the ethanol/TM and water/TM systems due to the weak interaction between the molecular systems and the TM surfaces. We relaxed the topmost three surface layers, while the slab bottom layer was frozen along of the geometric optimizations; however, we would like to point out that only the topmost surface layer are slightly affected by the molecule−substrate interaction. Furthermore, we would like to point out that four layers slab has been employed in several calculations, including the chemisorption of NO molecules on the Pt(111) surface.63,64,77 The ethanol and water were adsorbed only on one side of the slab, and hence, dipole correction was employed in our calculations, which is very important to obtain correct work function changes upon ethanol/water adsorption. We found that the adsorption energies are only slightly affected by the dipole correction. For the surface Brillouin zone integration, we employed a 4 × 4 × 1 k-point mesh for all calculations. For all optimizations, the equilibrium geometries of the ethanol/TM and water/TM are obtained when the atomic forces are smaller than 0.010 eV/Å on each atom and employing a total energy convergence of 10−6 eV. The electronic structure calculations, i.e., density of states (DOS), were performed using 8 × 8 × 1 kpoint mesh for all studied systems. The adsorption of water molecules at low coverage on closepacked TM surfaces has been widely studied;55−60 it has been established that the O atom binds preferentially near to the ontop site of the TM atoms, and the plan formed by the water molecule is almost parallel to the surface. To improve the identification of the adsorption sites and orientation of ethanol on the TM surfaces, we performed first-principles simulated annealing (SA) calculations using a cuttoff energy of 300 eV for about 20 ps with an initial temperature of 300 K and final temperature of about 0 K. Along the SA calculations, several snapshots (about 15 for each system) were selected, which were used for standard PBE optimizations using the conjugated gradient algorithm as implemented in VASP. Similar calculations were also performed for water adsorption on the TM surfaces. Finally, several PBE molecule/TM configurations were selected for conjugated gradient optimization using the PBE +D3 functional, which takes into account the van der Waals corrections. Thus, the search for the lowest energy configurations for the ethanol/TM and water/TM systems were carefully performed.

PBE Fe Co

bcc hcp

Ni Cu Ru

fcc fcc hcp

Rh Pd Ag Os

fcc fcc fcc hcp

Ir Pt Au

fcc fcc fcc

a0 a0 c0 a0 a0 a0 c0 a0 a0 a0 a0 c0 a0 a0 a0

2.84 2.50 4.03 3.52 3.63 2.73 4.31 3.85 3.96 4.16 2.76 4.35 3.88 3.98 4.16

PBE+D3

(−1.05) (−0.40) (−0.98) (0.00) (+0.55) (+0.74) (+0.70) (+1.32) (+1.80) (+1.71) (+0.73) (+0.69) (+1.04) (+1.53) (+1.96)

2.81 2.47 4.00 3.48 3.57 2.71 4.28 3.81 3.90 4.09 2.74 4.33 3.84 3.93 4.12

(−2.09) (−1.59) (−1.72) (−1.14) (−1.11) (0.00) (0.00) (+0.26) (+0.26) (0.00) (0.00) (+0.23) (+0.00) (+0.26) (+0.98)

a

The numbers in parentheses are the relative errors in percentage compared with experimental results.78

wave (FP-LAPW) calculations employing the PBE functional,79 i.e., an average deviation of about 0.2%. b. Clean Surface Properties. DFT results for the surface energy, interlayer relaxations, and work functions have been widely reported for compact TM surfaces,76,80−85 and hence in this study, we will report only the work function, Φ, which is required for our analysis of the work function change upon the ethanol and water adsorption. The work function, Φ, which measures the minimum energy required to remove an electron from a solid surface, was calculated as the energy difference between the electrostatic potential energy far from the surface and the Fermi energy, which is commonly employed in DFT studies.76 The Φ results are summarized in Table 2 along with Table 2. Work Function (in eV) of Clean Close-Packed TM Surfaces Fe(110) Co(0001) Ni(111) Cu(111) Ru(0001) Rh(111) Pd(111) Ag(111) Os(0001) Ir(111) Pt(111) Au(111)

III. RESULTS A. Bulk, Clean Surface, and Isolated Molecules. a. Equilibrium Lattice Parameters. Our results for the lattice constants (a0, c0) of the ground-state bulk structures are summarized in Table 1 along with the relative error in percentage compared with the experimental results.78 PBE overestimates the lattice constants for all systems by less than 2.0%, except for Fe and Co, for which PBE yields smaller lattice constants than the experimental results.78 For all systems, the equilibrium PBE+D3 lattice constants are smaller than the PBE results by about 0.5−1.5%. Thus, for the 4d and 5d systems, PBE+D3 improves slightly the lattice constants compared with the experimental results,78 while the relative error increases for the 3d systems. Our PBE results are in excellent agreement with recent all-electron full-potential linearized augmented plane

PBE

PBE+D3

4.82 4.93 5.02 4.76 5.00 5.11 5.25 4.45 5.37 5.42 5.73 5.13

4.80 4.92 5.01 4.77 5.00 5.11 5.23 4.40 5.36 5.41 5.70 5.06

theory

expt

4.73a 5.4b 5.35,c 5.42d 4.94,d 4.98c 5.03e 5.23f 5.42g 4.46,e 4.44h

e

5.76 5.15e

5.55,d 5.6c 4.74c,d 5.76c 5.70c 5.31c

a

Reference 80; DFT-GGA result. bReference 81; DFT-PW91 result. Reference 88; experiment. dReference 86; experiment. eReference 82; DFT-GGA result. fReference 83; DFT-GGA result. gReference 84; DFT-GGA result. hReference 85; DFT-GGA result.

c

previous DFT and experimental results. There is no significant differences between the PBE and PBE+D3 results, i.e., differences from 0.00 to 0.07 eV, which is expected as the empirical van der Waals correction employed in this work does not affect the electronic states; i.e., the electronic states are affected only indirectly by the changes in the atomic structure. Except for Cu(111), Ag(111), and Au(111), Φ increases by 24697



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about 0.10−0.36 eV from left to right in the periodic table, i.e., Fe(110) → Ni(111), Ru(0001) → Pd(111), Os(0001) → Pt(111), and from 3d to 5d systems (Φ3d < Φ4d < Φ5d). For Cu, Ag, and Au, we obtained that ΦAu(111) > ΦCu(111) > ΦAg(111). Our Φ results are in a good agreement with previous theoretical and experimental results. 76,80−83,85−88 For Ru(0001), Pd(111), Ag(111), Pt(111), and Au(111), the work function error is less than 1% compared with previous PBE results,82,84,85 while for other systems the deviation is about 1.9−3.2%,80,83 except for Co(0001), where we found a deviation of about 9.5%.81 In general, our results are smaller than the experimental values.86,88 The smallest deviation (about 0.5%) is obtained for Pt(111),88 while for the remaining systems the deviation is about 3.5−6.6%,86,88 which have been attributed to the large error bar among the experimental results.76 c. Isolated Ethanol and Water Molecules. For isolated ethanol and water molecules, the calculated PBE and PBE+D3 structures are almost identical, i.e., differences of about ±0.001 Å and 0.01° in the geometric parameters, while the PBE and PBE+D3 total energies differ by less than 10−4 eV for water and 10−2 eV for ethanol, and hence, the van der Waals contribution to the binding energy of ethanol and water is negligible. Thus, only the PBE ground-state structures of isolated ethanol and water are shown in Figure 1 along with the bond lengths and Figure 2. Lowest energy PBE structures, top view (a) and side view (b), for ethanol adsorption on close-packed TM surfaces, namely, Fe(110), Co(0001), Ni(111), Cu(111), Ru(0001), Rh(111), Pd(111), Ag(111), Os(0001), Ir(111), Pt(111), and Au(111).

Figure 1. Atomic structure of ethanol (a) and water (b) in gas phase calculated with the PBE functional. The bond lengths, in Å, and angles, in degrees, are indicated.

angles. For water, we found a bond length of 0.97 Å for O−H and an angle of 104.5° for HOH, which are in excellent agreement with experimental results (bond lengths of 0.96 Å for O−H and HOH angle of 104.5°) obtained by the spectroscopic technique.89,90 For ethanol, the C−O and C−C bond lengths are 1.44 Å (1.43 Å) and 1.52 Å (1.51 Å), while O−H and C−H are 0.97 Å (0.97 Å), which is the same as for O−H in water. The numbers in parentheses are experimental results.89 For C−H, as expected, the bond lengths depend on the C atom, i.e., 1.11 Å for CH2 and 1.10 Å for CH3. The most important angles, COH, CCO, and CCH, are 108.5°, 107.9°, and 110.6°, respectively. Our results are in good agreement with experimental results,89 i.e., deviations of less than 1.0%. Similar deviations are found compared with previous first-principles DFT calculations.7,39,40,91−93 B. Ethanol and Water Adsorption on Close-Packed TM Surfaces. 1. Adsorbate Structures. The lowest energy ethanol/TM configurations are shown in Figures 2 (PBE) and 3 (PBE+D3), while for the water/TM systems, only the PBE lowest energy structures are shown in Figure 4, as the PBE +D3 functional yields almost the same geometric parameters for water/TM. The most important geometrical parameters that were employed to characterize the ethanol and water adsorption on the TM surfaces are defined in Figure 5, and the

Figure 3. Lowest energy PBE+D3 structures, top view (a) and side view (b), for ethanol adsorption on close-packed TM surfaces, namely, Fe(110), Co(0001), Ni(111), Cu(111), Ru(0001), Rh(111), Pd(111), Ag(111), Os(0001), Ir(111), Pt(111), and Au(111).

results are summarized in Table 3 for all systems and functionals. Both molecules binds to the TM surfaces using the O atoms, which are located near or on the on-top sites, which is consistent with previous DFT calculations for ethanol/ Rh(111)47 and water/TM.57,60,94 For ethanol/TM (water/ 24698



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TM), the Δtop parameter, which measures the lateral displacement of the O atoms to the on-top sites, is 0.13−0.29 Å (0.00− 0.27 Å) using PBE, while it increases using the PBE+D3 functional (Table 3). Basically, we found that the van der Waals correction contributes to increase the preference for the hollow sites on the studied compact TM surfaces, which can be explained as follows. The Lennard-Jones interatomic pair potential favors high-coordinated adsorption sites (hollow) for rare-gas atoms on TM surfaces, which is not supported by DFT calculations for rare-gas atoms adsorbed on compact TM surfaces.54,69 As the van der Waals correction proposed by S. Grimme (PBE+D3)70 does not take into account electron densities redistribution, it might favors a preference for hollow sites as the empirical Lennard-Jones potential does for atoms on close-packed surfaces. For ethanol/TM, using the PBE functional, we found that dO−TM = 2.14−2.39 Å for all systems, except for Ag and Au, for which we found larger O−TM bond lengths, i.e., 2.69 and 2.77 Å, respectively. The PBE+D3 functional leads to decrease slightly the O−TM bond lengths (0.00−0.07 Å), and the largest change occurs for ethanol/Au(111), which is expected due to the largest enhancement of the adsorption energy by the van der Waals correction (see section below). The results for the O−TM bond lengths for water/TM follow the same trends as for ethanol/TM. The main difference between the PBE and PBE+D3 functionals occurs for the orientation of ethanol on the TM surfaces. For PBE, α = 66.49°−76.62° for all studied systems; i.e., the C−C bond is almost perpendicular to the TM surfaces, while for PBE+D3, α = 6.25°−11.35° for all systems. We found only one exception for the case of ethanol/Fe(110), for which α = 68.90°. For ethanol/Fe(110), the nearly parallel and perpendicular configurations differ by about 120 meV; i.e., both configurations might be reached at room temperature, and hence, it might play an important role in the breaking of the C−C bonds. Thus, our results indicate that van der Waals corrections might play an crucial role in the adsorbated structure of ethanol on TM surfaces. We found that the water molecule is nearly parallel to the TM surfaces, and the tilt angle between TM surfaces and HOH plane, β, ranges from 1.6° to 16.3° for PBE (Table 3). Michaelides et al. suggested that the almost planar orientation of water on the TM surfaces is due to the predominance of the

Figure 4. Lowest energy PBE structures, top view (a) and side view (b), for water adsorption on close-packed TM surfaces, namely, Fe(110), Co(0001), Ni(111), Cu(111), Ru(0001), Rh(111), Pd(111), Ag(111), Os(0001), Ir(111), Pt(111), and Au(111).

Figure 5. Definition of the most important geometric parameters of ethanol and water adsorbed on the TM surfaces.

Table 3. Adsorption Structural Parameters of Ethanol and Water on TM Surfaces (in Å) Calculated with the PBE and PBE+D3 Functionals ethanol adsorption on TM surfaces PBE Fe(110) Co(0001) Ni(111) Cu(111) Ru(0001) Rh(111) Pd(111) Ag(111) Os(0001) Ir(111) Pt(111) Au(111)

water adsorption on TM surfaces

PBE+D3

PBE

PBE+D3

O−TM

α

Δtop

O−TM

α

Δtop

O−TM

β

Δtop

O−TM

β

Δtop

2.15 2.22 2.14 2.32 2.29 2.28 2.39 2.69 2.29 2.33 2.39 2.77

67.18 75.78 70.65 72.71 73.45 74.24 66.49 76.62 68.13 72.85 72.71 72.67

0.18 0.13 0.22 0.25 0.17 0.17 0.29 0.21 0.18 0.17 0.23 0.24

2.12 2.17 2.17 2.32 2.29 2.28 2.36 2.66 2.26 2.33 2.39 2.70

68.90 8.54 10.56 8.98 10.27 9.48 6.25 10.25 7.96 11.35 8.91 7.73

0.20 0.17 0.28 0.32 0.24 0.26 0.31 0.53 0.11 0.20 0.29 0.40

2.21 2.25 2.18 2.37 2.33 2.31 2.41 2.69 2.30 2.36 2.47 2.82

15.43 11.24 16.33 8.56 5.74 7.88 4.33 4.51 10.48 7.90 4.61 1.64

0.00 0.00 0.06 0.07 0.00 0.08 0.19 0.27 0.00 0.08 0.13 0.22

2.19 2.23 2.17 2.31 2.30 2.30 2.39 2.68 2.29 2.33 2.43 2.73

11.87 7.25 5.65 3.36 8.90 6.93 1.63 0.36 9.35 7.50 2.86 4.47

0.08 0.08 0.14 0.23 0.06 0.10 0.35 0.56 0.06 0.09 0.24 0.52

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covalent interactions over the electrostatics.57 We found that the van der Waals correction decreases slightly this angle compared with PBE calculations, which might be a consequence of the displacement of the O atoms from the on-top sites toward the hollow sites. To compare our results with experiments, we measured the angle between ethanol O−H bond and the TM surface. Experimental results using the IRAS, XPS, and TPD techniques have suggested that the ethanol O−H bond is almost parallel to the Ni(111) surface,23 which favors a strong interaction of the OH group with the TM surfaces. We found a parallel orientation of O−H bond (0.2°−12.8°) for all systems for both PBE and PBE+D3 functionals, which support the experimental findings. In the case that similar experimental information is available for the orientation of the C−C bond, it could be used to indicate the orientation of the ethanol on the surface, which is an important information. The ethanol and water geometries revealed little distortions upon the adsorption, which is consistent with previous results.47,55,57,59,60 We found that water O−H bonds are extended by a maximum of 0.01 Å, and the HOH angle increases by about 2° for PBE and PBE+D3 compared with water in gas phase (0.9726 Å and 104.5°). The ethanol C−C, C−O, C−H, and O−H bonds are slightly expanded by maximum of 0.03 Å for both PBE and PBE+D3, while C−C bond lengths are shortened by about 0.01 Å for PBE+D3. The CCO angle increases in average by 3°−6° compared with the gas-phase value (107.9°). Those small changes is a consequence of the weak interaction between the molecules and the surfaces. 2. Adsorption Energies. The adsorption energy, which measures the strength of the interaction of the molecular systems to the TM surface, can be calculated by the equation molecule/TM molecule clean surface Ead = Etot − (Etot + Etot )

Table 4. Adsorption Energy (in meV) of Ethanol and Water on Close-Packed TM Surfaces Calculated with the PBE and PBE+D3 Functionals; αr = EPBE+D3 /EPBE ad ad ethanol/TM Fe(110) Co(0001) Ni(111) Cu(111) Ru(0001) Rh(111) Pd(111) Ag(111) Os(0001) Ir(111) Pt(111) Au(111)

water/TM

PBE

PBE+D3

αr

PBE

PBE+D3

αr

−378 −267 −290 −174 −431 −365 −266 −136 −492 −332 −237 −115

−690 −763 −766 −667 −828 −788 −721 −480 −945 −797 −771 −536

1.83 2.86 2.64 3.83 1.92 2.16 2.71 3.53 1.92 2.40 3.25 4.66

−343 −274 −285 −175 −418 −348 −254 −138 −469 −305 −215 −109

−543 −503 −525 −427 −624 −554 −464 −314 −721 −539 −454 −323

1.58 1.84 1.84 2.44 1.49 1.59 1.83 2.28 1.54 1.77 2.11 2.96

The adsorption energies of ethanol and water have similar trends, in particular using the PBE functional, as both molecules binds to the compact TM surfaces using the OH group with similar geometric parameters for Δtot and dO−TM. For example, for the 4d and 5d TM surfaces, the PBE absolute value of the adsorption energy of ethanol and water decreases almost linearly with the increased occupation of the d-band; however, for the 3d TM surfaces, there is a large deviation from the linear behavior observed for the 4d and 5d TM surfaces as ethanol/water have about the same adsorption energy on the Co(0001) and Ni(111) surfaces. The results for water/TM are consistent with previous DFT calculations57 taken into account differences in the unit cell size, which can affect the magnitude of the adsorption energy. Ag Au For ethanol and water, we found that −ECu ad > −Ead > Ead , Ni Pd Pt −Ead > −Ead > −Ead; however, this trend changes for Co, Rh, Ir, and Fe, Ru, and Os (Figure 6). This trend changes, in particular, due to the adsorption energy on the Co(0001) and Fe(110) surfaces, which can be explained by the strong ferromagnetic interactions present on those surfaces, which can affect the interaction mechanism of ethanol and water to the surfaces. The van der Waals correction increases substantially the magnitude of the adsorption energy; however, the enhancement factor is not the same for all the systems (Table 4). For example, the adsorption energy ratio between the PBE+D3 and PBE results, αr = EPBE+D3 /EPBE ad ad , increases with the occupation of the d-band for all systems; i.e., the magnitude of the van der Waals correction is larger for weak adsorbed systems such as ethanol and water on Cu(111), Ag(111), and Au(111). For all the studied systems, we found that αr is larger for ethanol/TM, which can be explained by the large size of the ethanol compared with water, and hence, the van der Waals correction yields a larger effect on the ethanol adsorption energy. It can be seen in Figure 6 that PBE+D3 changes slightly the linear dependence of the adsorption energy for the 4d and 5d ethanol/TM and water/TM systems, while PBE+D3 strongly affects the adsorption energy of the 3d TM surfaces such as the adsorption energy of ethanol/Co(0001) is larger than for ethanol/Fe(110), i.e., the opposite that obtained by the PBE functional. We found that the PBE (PBE+D3) adsorption energies of ethanol/TM are 1.08−1.24 (1.25−1.60) larger than for water/ TM, which is consistent with spectroscopic studies of

(4)

surface where Emolecule/TM , Emolecule , and Eclean are the total energies tot tot tot of the molecule/TM system, isolated molecule, and TM clean surface, respectively. The ethanol and water adsorption energies are shown in Figure 6 and summarized in Table 4.

Figure 6. Adsorption energies of ethanol and water on the closepacked TM surfaces, namely, Fe(110), Co(0001), Ni(111), Cu(111), Ru(0001), Rh(111), Pd(111), Ag(111), Os(0001), Ir(111), Pt(111), and Au(111), calculated with the PBE and PBE+D3 functionals. 24700



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Figure 7. PBE local density of states of the C, O, H, TMb, and TM* atoms for ethanol adsorption on close-packed TM surfaces, namely, Fe(110), Co(0001), Ni(111), Cu(111), Ru(0001), Rh(111), Pd(111), Ag(111), Os(0001), Ir(111), Pt(111), and Au(111). TMb indicates transition-metal atoms binded directly to the O atoms in the ethanol, while TM* indicates the remaining eight transition metal atoms in the topmost surface layer.

Figure 8. PBE+D3 local density of states of the C, O, H, TMb, and TM* atoms for ethanol adsorption on close-packed TM surfaces, namely, Fe(110), Co(0001), Ni(111), Cu(111), Ru(0001), Rh(111), Pd(111), Ag(111), Os(0001), Ir(111), Pt(111), and Au(111). TMb and TM* indicate transition-metal atoms binded directly to the O atoms in the ethanol and the remaining eight transition metal atoms in the topmost surface layer, respectively.

adsorbated n-alcohols and water on the Cu(111) and Pt(111) surfaces.95 For water adsorption on TM surfaces, which has been widely studied,57−60 we found an excellent agreement with previous DFT calculations. For example, the adsorption energies calculated by Michaelides et al.57 range from 130 to 420 meV for Au(111) and Rh(111), respectively, while our results for the same elements are 115 and 365 meV, respectively; however, it should take into account that the unit cell are different, i.e., (2 × 2) in ref 57 and (3 × 3) in this study. 3. Density of States. To obtain a better understanding of the electronic properties of the ethanol/TM and water/TM

systems, we calculated the local densities of states (LDOS) of the C, O, H, and TM atoms in the lowest energy adsorbed structures. For water/TM, we report only the results obtained with the PBE functional as the PBE+D3 results are about the same, i.e., very similar structures, while for ethanol/TM, we report the PBE and PBE+D3 results. The results are shown in the Figures 7, 8, and 9. For the particular case of the TM atoms, we separated the TM atoms in the topmost surface layer into two groups, i.e., TM atom that bind directly to the O atoms, TMb, and the LDOS of the TM atoms that do not take part in the O−TM bonding, TM*. For the C, H, and TM* atoms, the 24701



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Figure 9. PBE local density of states of the O, H, TMb, and TM* atoms for water adsorption on close-packed TM surfaces, namely, Fe(110), Co(0001), Ni(111), Cu(111), Ru(0001), Rh(111), Pd(111), Ag(111), Os(0001), Ir(111), Pt(111), and Au(111). TM* and TMb are the eight transition metal atoms of the topmost surface layer and transition-metal atoms binded directly to the O atoms in the ethanol, respectively.

PBE+D3 functional yields a structure with the C−C bonds almost parallel to the surface, and hence, it affects the electronic structure of the ethanol/TM systems. We found that the van der Waals correction increases the broadening of the C p-states, in particular, of the C pz-states, and a tail of the C pz-states extends also above the Fermi level. For water/TM, the van der Waals correction changes slightly the electronic structure as the PBE+D3 functional almost does not change the atomic structure of the water/TM systems. It has been suggested that the center of gravity of the occupied d-band, Cdg , with respect to the Fermi level can be correlated with the magnitude of the adsorption energy of molecular species on TM surfaces.96,97 Thus, to improve our analysis of the LDOS, we calculated Cdg for the lowest energy ethanol/TM and water/TM configurations, and the results are summarized in Figure 10. Our results do not show a linear relation between the center of gravity and the magnitude of the adsorption energy, which can be explained by the weak chemisorption character of the interaction of ethanol and water with TM surfaces. It can be seen that the center of gravity for the TMb and TM* atoms follows the same trend, which is expected as the ethanol and water does not affect strongly the TM surfaces. 4. Work Function Changes. The work function change, ΔΦ, has been widely employed as an analysis tool to identify trends in the electron density rearrangement upon the adsorption of molecular species on surfaces.54,69 We calculated ΔΦ as

LDOS were averaged over all the C, H, and TM* atoms, respectively. For the ethanol/TM systems, we found that the O states derived from the O 2s-states are 21−23 eV below the Fermi level, and the energy distance with respect to the Fermi level increases slightly with the depopulation of the d-band, i.e., from Ag(111) to Ru(0001) for the 4d system, which is expected due to the different attractive potential generated to the molecular electronic states. We found small differences in the position of the O 2s-states between the 3d, 4d, and 5d systems, in particular, for the Cu, Ag, and Au surfaces. Similar results are obtained for the water/TM systems; i.e., the O s-states derived from the 2s-states follow a similar trend as in ethanol/TM, which is expected as most of the properties show similar trends as a function of the occupation of the d-band. The main contribution of the C 2s-states locate at about 14−15 eV below the Fermi level and follows the same trend as for the O 2sstates. The C and O p-states (px- and py-states) derived from the pstates are located slightly below and within the bottom of the dband, while the C and O pz-states are located at about the middle of the TM d-band. There is a clear broadening of the C and O pz-states; in particular, it is larger for the O pz-states, which is expected due to the closer distance to the TM topmost surface layer. A tail of the O pz-states extends above the Fermi level, which indicates a slightly depopulation of the O pz-states that contributes to enhance the polarization of the O atom and, hence, the interaction of the molecule to the TM surface. The ethanol adsorption on the TM surfaces does not affect strongly the atomic structure of the topmost surface layer, and hence, the differences in the LDOS of the TMb and TM* atoms can be attributed to the interaction of ethanol with the TMb atoms. Our analysis shows tiny differences in the LDOS of the TMb and TM* d-states, in particular, most of the changes are located near to the Fermi level, which is expected as those states are easier affected by the interaction with the adsorbate. We found that the van der Waals correction affects the lowest energy atomic structure of the ethanol/TM systems, i.e., the

ΔΦ = Φmolecule/TM − ΦTM surface

(5)

where Φmolecule/TM is the work function of the molecule/TM systems and ΦTM surface is the work function of the clean surface. The PBE and PBE+D3 results are summarized in Table 5 for all studied systems. As expected, ΔΦ < 0 for all studied systems, which indicates an effective charge transfer from the TM surfaces to the region above the surface; i.e., the electron density increases at the region between the surface and adsorbate and on the adsorbate due to the small polarization of the molecules. 24702



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Figure 11. Work function change upon the adsorption of ethanol and water on the close-packed TM surfaces, namely, Fe(110), Co(0001), Ni(111), Cu(111), Ru(0001), Rh(111), Pd(111), Ag(111), Os(0001), Ir(111), Pt(111), and Au(111), calculated with the PBE and PBE+D3 functionals. Figure 10. Center of gravity of the occupied d-states of the ethanol/ TM and water/TM systems (TM = Fe(110), Co(0001), Ni(111), Cu(111), Ru(0001), Rh(111), Pd(111), Ag(111), Os(0001), Ir(111), Pt(111), and Au(111)) calculated with the PBE and PBE+D3 functionals. The zero energy indicates the Fermi energy. TM* and TMb correspond to the transition metal atoms of the topmost surface layer that do not take part in binding and transition-metal atoms binded directly to the O atoms in the ethanol, respectively.

binds to the surface by the OH group, the interaction shows noticeable differences and depends on the electron density redistribution on the substrate and molecule. Moreover, nearly parallel orientation of ethanol and water on TM surfaces obtained by PBE+D3 calculations leads to the slight reduction of the magnitude of ΔΦ obtained by PBE, except for a few cases. Our results are consistent with previous DFT studies for ethanol/Rh(111),47 that is, −1.28 eV for the same unit cell, and water/TM (such as Ru, Rh, Pd, Ag, and Au),59 where there are small differences in the work function of the systems in comparison with our results that can be explained by smaller size of unit cell (p(2 × 2)) for their studied systems.

Table 5. Work Function Change (in eV) of Ethanol and Water on Close-Packed TM Surfaces Calculated with the PBE and PBE+D3 Functionals ethanol/TM Fe(110) Co(0001) Ni(111) Cu(111) Ru(0001) Rh(111) Pd(111) Ag(111) Os(0001) Ir(111) Pt(111) Au(111)

water/TM

PBE

PBE+D3

PBE

PBE+D3

−1.26 −1.53 −1.56 −1.23 −1.39 −1.35 −1.08 −0.79 −1.56 −1.48 −1.36 −0.87

−1.24 −1.58 −1.33 −1.04 −1.25 −1.22 −1.04 −0.59 −1.62 −1.35 −1.21 −0.66

−0.72 −0.86 −0.89 −0.67 −0.74 −0.73 −0.57 −0.30 −0.89 −0.82 −0.69 −0.30

−0.63 −0.99 −0.73 −0.49 −0.68 −0.70 −0.48 −0.16 −0.85 −0.81 −0.64 −0.14

IV. CONCLUSION In this study, we performed PBE calculations without and with the empirical van der Waals correction proposed by S. Grimme70 (PBE+D3) to investigate the adsorption trends and the role of van der Waals interactions in the adsorption properties of ethanol and water on 12 close-packed TM surfaces, namely, the Fe(110), Co(0001), Ni(111), Cu(111), Ru(0001), Rh(111), Pd(111), Ag(111), Os(0001), Ir(111), Pt(111), and Au(111) surfaces, which are the most important TM systems employed in catalysis. We found that both ethanol and water bind preferentially near or on the on-top sites of the TM surfaces through the O atoms, and the OH group is almost parallel to the surface for all TM surfaces. The parallel displacement of the O atoms with respect to the on-top site, Δtop, is 0.0−0.29 Å using PBE, while it increases using the PBE+D3 functional (Table 3). Thus,the van der Waals correction contributes to increase the preference for the hollow sites on the studied compact TM surfaces, which might be a consequence of the empirical van der Waals correction as the PBE+D3 correction does not take into account electron density redistribution directly. The adsorption energies of ethanol and water on the studied TM surfaces have similar trends. For example, for the 4d and 5d TM surfaces, the PBE absolute value of the adsorption energy

We found that the magnitude of the work function change for the 4d and 5d systems decreases as a function of d-band occupation upon the adsorption of ethanol and water (Figure 11). The same trend was not observed for the 3d systems, e.g., for the case of Fe(110), where ΔΦ is smaller. We explain that by the difference in the atomic structure of the TM surfaces, i.e., Fe(110) compared with fcc(111) surfaces. It can be noticed that the reduction of the work function of TM surfaces is substantial for ethanol adsorption, which is about 1.5−2 times larger than for water/TM systems. Thus, this result points out that even that both ethanol and water 24703



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decreases almost linearly with the occupation of the d-band; however, for the 3d TM surfaces, there is a large deviation from the linear behavior as the adsorption energy has about the same value for ethanol/water on the Co(0001) and Ni(111) surfaces. The empirical van der Waals correction increases substantially the magnitude of the adsorption energy, which is expected, however, the enhancement factor is not the same for all the systems (Table 4), which can be explained as the Cij6 coefficients are different for different species. For example, the adsorption energy ratio, αr = EPBE+D3 /EPBE ad ad , increases with the occupation of the d-band for all systems; i.e., the magnitude of the van der Waals correction is larger for weak adsorbed systems such as ethanol and water on Cu(111), Ag(111), and Au(111). We found that αr is larger for ethanol/TM, which can be explained by the larger size of the ethanol molecule compared with water. The geometric parameters for water/TM are not strongly affected by the van der Waals correction; i.e., both PBE and PBE+D3 functionals yield an almost parallel orientation for water on the studied TM surfaces. However, the van der Waals correction changes drastically the ethanol orientation on the TM surfaces. For example, PBE yields an almost perpendicular orientation of the C−C bond to the TM surface, while the C− C bond is almost parallel to the surface using PBE+D3 functional for all systems, except for ethanol/Fe(110). We would like to point out that an almost parallel orientation contributes to enhance the interaction of ethanol with the surface, and hence, it affects all the adsorption properties. We found that the work function decreases upon the adsorption of ethanol and water, and both follow the same trends, however, with different magnitude (larger for ethanol/ TM) due to the weak binding of water to the surface. The electron density increases mainly in the region between the topmost layer and the adsorbates.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; dasilva_juarez@yahoo. com. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work is supported by the São Paulo Research Foundation (FAPESP). The authors thank Prof. Dr. Stefan Grimme and Jonas Möellmann for providing the D3 subroutines to be added in VASP.

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REFERENCES

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Ethanol and Water Adsorption on Close-Packed 3d ... - ACS Publications

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