Density Functional Investigation of the Adsorption of EthanolâWater

Article pubs.acs.org/JPCC

Density Functional Investigation of the Adsorption of Ethanol−Water Mixture on the Pt(111) Surface Polina Tereshchuk†,§ and Juarez L. F. Da Silva*,† †

Instituto de Química de São Carlos, Universidade de São Paulo, 13560-970 São Carlos, SP, Brazil Institute of Nuclear Physics of Uzbekistan ASRUz, Ulugbek, Tashkent 100214, Uzbekistan

§

ABSTRACT: Steam reforming of ethanol−water mixture is a promising renewable route to obtain hydrogen; however, our atomistic understanding of the interaction of ethanol−water mixture with transition-metal surfaces is far from satisfactory. In this work, we report a density functional theory investigation of the adsorption properties of the ethanol− water mixture on the Pt(111) surface employing semilocal exchange-correlation functional within nonlocal van der Waals corrections. From our calculations and analysis, we found that water molecules are located near the surface instead of ethanol, which is in contrast with our initial expectation based on the large magnitude of the adsorption energy of ethanol on Pt(111) compared with water/Pt(111). We found that the formation of hydrogen bonds among ethanol−ethanol, water−water, and ethanol−water molecules plays an important role in the adsorbate structure of the ethanol−water mixture on Pt(111), in particular, due to the enhancement of the binding energy of the hydrogen bonds induced by the interaction with the Pt(111) surface. We found that the van der Waals correction does not strongly affect the adsorbate structures of the ethanol−water mixture; however, as expected, it enhances the adsorption energy, which is coverage-dependent. Furthermore, we also report results and analysis for the adsorption of ethanol and water molecules on the Pt(111).

I. INTRODUCTION Ethanol, with its high energy density, has been considered as an attractive renewable and CO2-neutral energy resource for the production of hydrogen (H2) for transportation and stationary power fuel cell applications,1−3 where the chemical energy can be converted into electrical energy. Ethanol can be produced from biomass such as sugar cane and corn fermentation,4−6 and biotechnology studies have indicated the possibility to obtain ethanol from low-cost vegetation such as crop wastes.7 Among several processes to convert ethanol into H2,8,9 the steam reforming reaction, which combines ethanol (C2H5OH) and water (H2O), that is, C2H5OH + 3H2O → 2CO2 + 6H2 (endothermic reaction, ΔHR = 173.4 kJ/mol at 300 K),9,10 has been widely studied as a promising route to obtain H2.9−12 For example, Duan and Senkan,10 using combinatorial methods have found among 840 distinct materials that Pt particles supported on TiO2 and CeO2 yield the highest ethanol conversion, that is, ∼90%, and H2 selectivity of ∼30% at 573.15 K; however, our understanding of the ethanol−water reactions is far from complete due to the large number of intermediates.1,9 For example, several studies have reported the formation of ethylene, methane, acetaldehyde, acetone, and so on.3,9,11 In contrast with the large number of adsorption and reaction studies of ethanol and water on transition-metal (TM) surfaces (see refs 13−18 and references therein), there are only a few © XXXX American Chemical Society

studies of the adsorption of ethanol−water mixture on TM surfaces.19−21 For example, using classical molecular dynamics (MD) simulation within force-fields implemented in DL_POLY, Kholmurodov et al.19 studied the interaction of the ethanol−water mixture with Pt(111). From the analysis of the radial distribution function (RDF) profiles, they identified an adsorption layer from ∼2 to 5 Å, which is dominated by ethanol molecules, while a second less dense layer composed mainly of water molecules was identified from ∼5 to 8 Å. For larger distances from the surface, the RDF indicates that the surface structure does not affect the ethanol−water mixture, and it behaves like an ethanol−water liquid,19 which is expected. Although the work reported by Kholmurodov et al.19 improved our understanding of the ethanol+water/Pt(111) system, the results are based on classical force field, which have difficulties to describe liquid−metal interfaces. For example, electron density redistribution in the interface affects the molecular interactions among the ethanol and water molecules. Furthermore, we would like to mention that even firstprinciples calculations have to address several problems, for example, the formation of hydrogen bonds among the Received: April 4, 2013 Revised: July 23, 2013

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dx.doi.org/10.1021/jp403352u | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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We found an equilibrium lattice constant of 3.98 (PBE) and 3.93 Å (PBE+D3), 18 which are consistent with the experimental result of 3.92 Å,42 that is, deviations of about 0.3 to 1.5%, and, in particular, the PBE+D3 result is closer to the experimental value. Furthermore, it is in a good agreement with previous theoretical calculations.43,44 The adsorption of ethanol, water, and ethanol−water mixture on Pt(111) was modeled using the repeated slab geometry and employing a (3 × 3) surface unit cell, which was employed in our previous calculations for the adsorption of low-coverage ethanol and water on 12 close-packed TM surfaces.18 We used four layers in the slab separated by a vacuum region of ∼21 Å, which is required for the present study due to the ethanol− water mixture adsorption on Pt(111). Because of the weak interaction between the molecular systems such as ethanol and water with TM surfaces,13,18 only the topmost surface layer is slightly affected by the interaction. Thus, four layers in the slab are enough to provide an accurate description of the problems addressed in this work. The molecules were adsorbed on only one side of the slab, and hence we employed the dipole correction to obtain a correct description of work function changes.45 For the Brillouin zone integration, we employed a 4 × 4 × 1 k-point Γ-centered mesh, while for electronic structure calculations, for example, densities of states (DOS), we used 8 × 8 × 1 k-point mesh. For all optimizations, equilibrium geometries with a total energy convergence of 10−4 eV were obtained when the atomic forces were smaller than 0.025 eV/Å on each atom; see the Appendix. Within this setup, the accuracy of the adsorption energy is less than ±10 meV. A. Atomic Configurations. An important problem in the study of adsorbate systems on surfaces is the search of the lowest energy configurations, which is very complicated for systems such as the ethanol−water mixture on Pt(111). Thus, to identify reliable atomic configurations for ethanol, water, and ethanol−water mixture on Pt(111), we employed the following steps: (i) Different initial configurations for nH2O/Pt(111), n = 1, 3, 6, were generated by the adsorption of water molecules on-top Pt sites or near of the on-top sites. The selection of the initial adsorption sites is supported by previous studies, which found that water molecules preferentially bind on the on-top or near the on-top sites.13,17,18,46 For the particular case of 6H2O/Pt(111), which has been previously studied,47−49 our selected configurations include hexagon-, pentagon-, and quadrangle-like structures with different water orientations relative to the surface. (ii) Several atomic configurations were selected for nC2H5O/ Pt(111) with n = 1, 2, which takes into account parallel and perpendicular orientations of the C−C bonds18 and the formation of hydrogen bonds.50 For example, previous PBE studies for low coverage found that the C−C bond is almost perpendicular to the Pt(111) surface,18,50,51 while PBE+D3 yields that the C−C bond is almost parallel to the Pt(111) surface.18 Thus, it is the case in which vdW corrections play an important role beyond increasing the magnitude of the adsorption energy.31,37 (iii) For the most complex systems, that is, n(C2H5OH +3H2O)/Pt(111) with n = 1 and 2, we employed two complementary strategies: (a) ethanol molecules were

molecular species, the description of the asymptotic behavior of the long-range nonlocal van der Waals (vdW) interactions, and the search of atomic structure models for ethanol+water/ Pt(111). Therefore, because of the limited number of studies, our understanding of the interaction of ethanol−water mixture with TM surfaces is far from satisfactory. In this work, we performed an ab initio investigation based on density functional theory (DFT) of the interaction of the ethanol−water mixture with the Pt(111) surface, which is an important step to obtain a deep atomistic understanding of the ethanol+water−Pt(111) interface. Furthermore, we studied also the interaction of ethanol and water molecules with Pt(111), which were used as reference data for comparison.

II. THEORETICAL APPROACH AND COMPUTATIONAL DETAILS Our spin-polarized calculations are based on DFT within the generalized gradient approximation22 (GGA) proposed by Perdew−Burke−Ernzerhof23 (PBE) to the exchange-correlation (xc) energy functional. First-principles calculations have obtained that PBE yields good results for the energy strengths and bond lengths of hydrogen bonds compared with coupledcluster calculations,24 which is important to describe ethanol− water mixture on Pt(111). Local and semilocal xc functional does not provide the correct asymptotic behavior for the longrange nonlocal vdW interactions,25 which affects the interaction of molecular systems with surfaces.18,26−33 To improve the description of the PBE functional, we employed the vdW correction proposed by S. Grimme (DFT+D3),34,35 in which a correction is added to the selected xc functional, and it has been employed in several studies.18,31,36,37 In the DFT+D3 framework,34,35 the total energy, EDFT+D3, contains the self-consistent DFT total energy, EDFT, and the vdW correction, Edisp, that is E DFT + D3 = E DFT + Edisp (1) where Edisp = E(2) + E(3). The E(2) and E(3) terms are two- and three-body energies: E(2) =

∑ ∑ AB n = 6,8,10, ···

E(3) =

sn

CnAB n fd , n (rAB) rAB

ABC ∑ fd ,(3) ( rABC ̅ )E ABC

(2)

(3)

Here CAB n is the dispersion coefficient of the nth order for each pair AB computed from first-principles calculations for molecular systems.34,35 rAB is the distance between the A and B atoms, sn is a scaling factor depending on the selected xc functional, fd,n is the damping function used to prevent nearsingularities for small rAB distances, and EABC is the nonadditive dispersion term. Further details can be found in refs 34 and 35. 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) method,38,39 as implemented in the Vienna ab initio simulation package (VASP).40,41 The total energy calculations were performed using a plane-wave cutoff energy of 400 eV, while a higher cutoff energy (461 eV) was used to calculate the equilibrium lattice constant for bulk Pt in the face-centered cubic structure employing the minimization of atomic forces and stress tensor. B



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adsorbed on the mH2O/Pt(111) configurations (m = 3, 6) at different heights and orientations and (b) firstprinciples simulated annealing (SA) calculations were performed for selected n(ethanol+3water)/Pt(111) configurations with a cutoff energy of 400 eV for ∼20 ps from an initial temperature of 300 to 0 K. Several snapshots (∼15 for each system) were selected to be used for optimization. All atomic configurations were optimized using the conjugated gradient algorithm, as implemented in VASP. Among all structures obtained, the lowest-energy configurations and their close isomers in the case of ethanol−water mixture on Pt(111) were selected for discussion. Although we calculated only a finite number of configurations, the selected steps provide a set of representative configurations, which can be studied to obtain a better understanding of the interaction of ethanol−water mixture on the Pt(111) surface.

oxygen atom with a vertical O distance to the surface of 2.39 Å, which is consistent with previous DFT calculations.18,50,51 As obtained in our previous study,18 PBE yields an almost perpendicular orientation for C−C, that is, βCC⊥ = 17°, while PBE+D3 yields an almost parallel orientation for C−C (βCC⊥ = 81°). Thus, the vdW correction moves the C atoms closer to the surface, which might play a role in the breaking of the C−C bonding in ethanol. For 2C2H5OH/Pt(111), we found that both PBE and PBE +D3 yield similar adsorbate structures, which is in contrast with the results obtained for 1ethanol/Pt(111). The dependence of the ethanol orientation as a function of the coverage can be explained by the formation of the hydrogen bonds, which increases the O vertical distance for the second ethanol. For example, for the first ethanol molecule, d⊥Pt−O = 2.27 Å, while the second ethanol has a O vertical distance of 3.24 Å, that is, ∼1.0 Å higher. Furthermore, the formation of the hydrogen bond decreases the H vertical distance for the OH group. For example, d⊥Pt−H decreases from 2.57 Å (αOH⊥ = 107°) for the first ethanol to 2.42 Å (αOH⊥ = 34°) for the second ethanol molecule. The two molecules have different orientations, which can be seen from the C−C orientation, that is, βCC⊥ = 12 ° and 78°. Thus, it is in contrast with the results obtained for 2ethanol/Rh(111), in which the two C−C bonds are almost perpendicular to the surface.15 2. Water Adsorption on Pt(111). The lowest-energy configurations for nH2O/Pt(111) are shown in Figure 3, while the geometric parameters are summarized in Table 1. A water monomer binds to the surface via an oxygen atom nearly on the on-top site with d⊥Pt−O = 2.47 Å and αHO⊥ = 95°; that is, water is nearly parallel to the surface, which is in a good agreement with previous DFT calculations.13,17,18,46 Although an upward orientation of the HOH plane over the surface maximizes the water-dipole surface-image dipole interactions,52 in which electrostatic interactions play an important role, it has been suggested that the parallel geometry is favored by covalent interactions.13 This suggestion can be considered to be controversial due to the small magnitude of the adsorption energy of water on Pt(111). The lowest energy structure for a water trimmer, 3H2O/ Pt(111), has an oxygen atom located nearly on the on-top site (O1 in Figure 3) with an almost parallel orientation of the HOH plane, that is, αHO⊥ = 105°. We found a vertical distance of 2.22 Å, which is ∼0.25 Å shorter than for a monomer. The remaining two molecules are downright oriented, following each other, lying at the bridge sites with a larger O vertical distances, 3.24 and 3.35 Å, and hence, those O atoms interact weakly with the surface. In contrast, one of the H atoms is closer to the surface. The formation of hydrogen bonds can be clearly seen from Figure 3, which leads to the long vertical distance of the second and third O atoms to the surface. Our lowest-energy structure differs from previous DFT calculation using a (√3 × √3)R30° unit cell,17 which reported that the flat water is between two downward oriented water molecules. Using (3 × 3) unit cell, we found that this configuration is 202 meV higher in energy than our reported lowest energy structure; that is, the coverage affects the orientation of the water molecules on the surface. For 6H2O/Pt(111), the water molecules form an array of hexagonal motifs with the O atoms nearly on the on-top sites, which is consistent with previous studies.46,47,53,54 Three water molecules are parallel to the surface, while the remaining three molecules are (O−H) perpendicular to the surface. The two

III. RESULTS A. Adsorbate Structures. For the analysis of the adsorbate structures, we selected a set of geometric parameters (Figure 1)

Figure 1. Schematic figure with the definition of the most important geometric parameters for ethanol and water molecules on Pt(111). Vertical distance of the O atoms to the surface, d⊥Pt−O; atomic distance between O atoms, dO−O; relative orientation of the O−H and C−C bonds to the Pt(111) surface normal, αHO⊥ and βCC⊥, respectively; and angles between the HOH (water) or the CCO atoms (ethanol), γ.

that provide a good description of the most important structure features. For example, the vertical O distance to the surface, d⊥Pt−O, was selected as the ethanol and water bound to the surface through the O atoms.13,18 The angles of the O−H (ethanol and water) and C−C (ethanol) bonds relative to the surface normal, αHO⊥ and βCC⊥, can be used to indicate the orientation of the ethanol and water molecules relative to the surface. For water, αHO⊥ = 0° (90°) implies a perpendicular (parallel) orientation. Furthermore, we also calculated the distance between the O atoms of the nearest neighbor molecules, dO−O, which can used to estimate the bond lengths of the hydrogen bonds from the difference between the dO−O and dO−H parameters. 1. Ethanol Adsorption on Pt(111). The lowest-energy configurations for ethanol adsorption on Pt(111) are shown in Figure 2, while the geometrical parameters are summarized in Table 1. For C2H5OH/Pt(111), both PBE and PBE+D3 yield that ethanol binds nearly on the on-top sites through the C



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Figure 2. Atomic structures of C2H5OH/Pt(111) and 2C2H5OH/Pt(111) in the (3 × 3) surface unit cell.

Table 1. Adsorption Structural Parameters for Ethanol and Water on the Pt(111) Surfacea PBE (no vdW correction) C2H5OH/Pt(111) 2C2H5OH/Pt(111) H2O/Pt(111) 3H2O/Pt(111)

6H2O/Pt(111)

oxygen sites

d⊥Pt−O (Å)

dO−H (Å)

top top top top top1 bridge2 bridge3 top2 top5 top1 top3 top4 top6

2.39 2.27 3.24 2.47 2.22 3.24 3.35 2.33 2.85 3.01 3.27 3.28 3.30

0.98 1.01 0.99 0.98 1.01 0.99 0.98 1.01 0.99 0.99 1.00 1.00 1.00

dO−O (Å) 2.68 2.68 2.58 2.58−2.72 2.72 2.67−2.87 2.80−2.89 2.67−2.87 2.88−2.97 2.80−2.87 2.86−2.97

PBE+D3 (with vdW correction) αHO⊥

βCC⊥

d⊥Pt−O (Å)

dO−H (Å)

89 107 34 95 105 46 18 108 97 101 13 3 9

17 12 78

2.39 2.23 3.14 2.43 2.21 3.12 3.19 2.34 2.52 2.90 3.18 3.18 3.17

0.98 1.02 1.00 0.98 1.01 0.99 0.98 1.01 0.99 0.99 0.99 0.99 0.99

dO−O (Å) 2.65 2.65 2.57 2.57−2.71 2.71 2.67−2.80 2.87−2.89 2.67−2.86 2.88−2.89 2.80−2.88 2.86−2.87

αHO⊥

βCC⊥

85 107 33 94 104 45 19 107 102 102 12 5 10

81 13 76

All parameters are defined in Figure 1, and the sites are numbered as in Figure 3 for water/Pt(111). All angles parameters (αOH⊥ and βCC⊥) are given in degrees.

a

by hydrogen bonds with the dO−O distance from 2.58 (water− water) to 2.97 Å (ethanol−water), which are consistent with experimental results for liquid water and ice (2.76 to 2.83 Å).55,56 For ethanol+3water/Pt(111), the atomic configuration of the water molecules is similar to the lowest energy configuration for 3water/Pt(111), which is supported by the results obtained for d⊥Pt−O and αHO⊥. The deviations in the geometric parameters are due to the presence of ethanol on Pt(111), which affects the water configuration due to the formation of the hydrogen bond between the ethanol−water molecules. In the lowest energy configuration the ethanol is nearly perpendicular to the surface, while in the higher energy configuration (75 meV higher), ethanol is nearly parallel to the surface with a βCC⊥ = 82°. For the D configuration in Figure 4 (113 meV higher), ethanol is nearly perpendicular to the surface. Our results show a stronger adsorption energy for ethanol/ Pt(111) than for water/Pt(111), which is supported by PBE and PBE+D3 results for a wide-range close-packed TM surfaces.18 Thus, we would expect that ethanol could approach

orientations do not form an alternate orientation sequence in the hexagonal motif, which can be seen in Figure 3. For the flat water, d⊥Pt−O = 2.33 to 3.01 Å, while the remaining molecules have vertical distances from 3.27 to 3.30 Å. We found that a hexagonal motif with alternate water orientations between flat and perpendicular47 is 162 meV higher in energy than our nonalternating motif, which provides an indication of the magnitude of the energy involved in the water reorientations. 3. Ethanol−Water Mixture on Pt(111). We found that PBE and PBE+D3 yield similar results for the adsorbate structures of ethanol−water mixture on Pt(111), and hence only the four lowest energy PBE structures are shown in Figure 4, which are nearly degenerated. For example, the relative energy difference among the selected configurations is in the range from 5 to 115 meV, and the configurations mainly differ on the orientation of the ethanol and water molecules, which is expected. The most important geometric parameters for both PBE and PBE+D3 are summarized in Table 2 for the lowest energy configuration. For both compositions, we found that ethanol−water mixtures form structures in which the ethanol and water molecules are linked D



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Figure 3. Lowest energy PBE configurations for H2O, 3H2O, and 6H2O water molecules on the Pt(111) surface in the (3 × 3) surface unit cell.

Figure 4. Lowest energy configurations for ethanol−water mixture on the Pt(111) surface using a (3 × 3) unit cell surface.

E



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60

34

39

γ

111 33 47 30 116 5 40 71 67 69 61 60

All angles (ΑOh⊥, ΒCc⊥, and γ) are given in degrees. a

(2C2H5OH + 6H2O)/Pt(111)

βCC⊥ αHO⊥

2.59−2.75 2.75−2.83 2.59−2.69 2.69−2.83 2.66−2.68 2.62−2.74 2.57−2.71 2.66−2.85 2.62−2.85 2.57−2.86 2.66−2.85 2.71−2.80 1.02 0.99 1.00 0.99 1.01 1.01 1.01 1.00 1.01 1.00 0.99 1.01

dO−O (Å) dO−H (Å)

2.16 3.20 3.31 3.42 2.21 3.11 3.84 3.87 4.04 5.09 5.22 5.25 water ethanol water water water water water water ethanol water water ethanol

(Å) molecule

(C2H5OH + 3H2O)/Pt(111)

PBE (no vdW corrections)

59

38

γ βCC⊥

109 33 44 31 115 6 38 72 68 72 60 60 2.52−2.58 2.72−2.82 2.58−2.69 2.69−2.82 2.62 2.60−2.72 2.56−2.69 2.62−2.85 2.60−2.83 2.56−2.85 2.65−2.83 2.68−2.79

αHO⊥ dO−O (Å)

1.02 0.99 1.00 0.99 1.01 1.01 1.01 1.00 1.01 1.00 0.99 1.01 2.15 3.03 3.15 3.25 2.17 3.04 3.60 3.73 3.82 4.82 5.01 5.00

dO−H (Å) (Å)

107 109 105 104 104 105 109 110 112 103 107 109

PBE+D3 (with vdW corrections)

d⊥Pt−O d⊥Pt−O

Table 2. Adsorption Structural Parameters as Defined in Figure 1 for the Adsorption of Ethanol−Water Mixture on the Pt(111) Surfacea

35

108 108 105 104 104 104 109 110 112 104 107 109

closer to the surface than water; however, we found that water (O atom) is closer to the surface, d⊥Pt−O = 2.16 Å, than ethanol (O atom), d⊥Pt−O = 3.20 Å, while the two remaining water molecules are far from the surface. To verify the present result, we calculated the ethanol+1water/Pt(111) system in the (3 × 3) surface unit cell, and the same result was confirmed; that is, water gets closer to the surface than ethanol upon the formation of the hydrogen bond between the ethanol and water. In principle, we would expect that a second molecule (water or ethanol) could bind directly to the surface through an oxygen atom as the coverage increases in the (3 × 3) surface unit cell; however, it was not observed in our calculations. For example, for the lowest energy configuration of (2C2H5OH +6H2O)/Pt(111), we found that only one molecule (water) has a vertical distance, d⊥Pt−O, of 2.21 Å, while all remaining molecules have d⊥Pt−O > 3.11 Å. As seen in Figure 4, the formation of a water layer occurs near to the surface, while the ethanol molecules are above this layer and hence far away from the surface, for example, d⊥Pt−O = 4.04 and 5.25 Å for ethanol. We found that similar trend can be also verified for the highenergy configurations (B−D) shown in Figure 4. The orientation of the O−H bonds in the ethanol and water molecules follows an interesting pattern for high coverages. For example, αHO⊥ = 116, 5, and 40° for the nearest three molecules to the surface; however, it is nearly αHO⊥ ≈ 65° for the remaining ethanol and water molecules. Our results indicate that the O−H bond in the ethanol and water is nearly parallel to the surface due to the direct interaction with the Pt(111) surface, which is weaker for molecules away from the surface, and hence, it might play an important role in the hydrogenation of the ethanol. We found that PBE+D3 slightly affects the ethanol−water structure on Pt(111), which is supported by geometric parameters summarized in Table 2. In contrast with isolated ethanol on Pt(111), in which PBE+D3 changes the orientation of the C−C bond, the angles αHO⊥ and βCC⊥ are about the same for PBE and PBE+D3, that is, deviations from 1 to 4°. The largest change can be observed for the vertical O distance, d⊥Pt−O, in particular, for the longest distances, which reduced up to 0.25 Å upon the vdW correction. Thus, the vdW correction moves the molecules closer to the surface compared with the PBE, which is known to yield longer bond lengths compared with experimental results.57 Because of the shift of the O positions, the hydrogen bond lengths (O−O distance) are also slightly affected; however, it does not change the observed trends. Our results show that ethanol and water molecules in the mixture on Pt(111) change slightly the geometric parameters compared with gas phase. For example, in general, the O−H bond length increases from 0.98 (ethanol) and 0.97 Å (water) in gas phase to about 0.99 to 1.02 Å, which is expected due to the formation of the hydrogen bonds and the interaction with the surface. Similar changes can also be noticed for the internal angles, that is, γCCO and γHOH. In contrast, we found that the adsorption of ethanol+water on Pt(111) strongly affects the magnitude of the hydrogen bond length. For example, in the gas phase, we obtained that the hydrogen bond length for ethanol−ethanol, ethanol−water, and water−water is ∼1.91 Å (PBE), which changes for values from 1.56 (most of the bonds) to 1.91 Å (few bonds). Thus, it indicates an enhancement of the binding energy of the hydrogen bond upon the adsorption on the Pt(111) surface, which helps to explain the role of the F



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Table 3. Adsorption Energy of the Molecular Systems on Pt(111), Ead, Adsorption Energy of the Adsorbate Layer on Pt(111), adlayer Eadlayer , Binding Energy of the Hydrogen Bonds on the Surface, EPt , and ad H , and in the Adlayer Structure without Substrate, EH a the Work Function Change, ΔΦ PBE (no vdW correction) C2H5OH/Pt(111) 2C2H5OH/Pt(111) H2O/Pt(111) 3H2O/Pt(111) 6H2O/Pt(111) (C2H5OH + 3H2O)/Pt(111) (2C2H5OH + 6H2O)/Pt(111) a

Ead

Eadlayer ad

−0.23 −0.75 −0.20 −1.21 −3.04 −1.89 −4.10

−0.23 −0.54 −0.20 −0.77 −0.88 −1.09 −0.75

EPt H

PBE+D3 (with vdW correction) Eadlayer H

−0.29

−0.21

−0.31 −0.37 −0.35 −0.35

−0.22 −0.43 −0.26 −0.48

ΔΦ

Ead

Eadlayer ad

−1.36 −1.41 −0.68 −0.55 −0.46 −0.85 −1.35

−0.81 −1.86 −0.44 −1.90 −4.54 −3.02 −5.90

−0.81 −1.57 −0.44 −1.40 −2.12 −2.04 −1.78

EPt H −0.24 −0.29 −0.38 −0.30 −0.23

Eadlayer H −1.21 −0.29 −0.65 −0.25 −0.48 −0.33 −0.59

ΔΦ −1.37 −0.40 −0.66 −0.76 −1.31

All quantities are given in electronvolts.

obtained from previous calculations for ethanol and water adsorption on TM surfaces;18 however, the relative trends among the systems remain the same. We would like to point out that the adsorption energy ratios, EPBE+D3 /EPBE ad ad , have different values; for example, the ratio is 3.52, 2.20, 1.60, and 1.44 for ethanol, water, ethanol+3water, and 2ethanol+6water on Pt(111). C. Hydrogen Binding Energies. To obtain a better understanding of the hydrogen bonds, we calculated the magnitude of the binding energy of the hydrogen bonds for the molecular systems on Pt(111), EPt H , and in the adlayer structure . The EPt without the substrate, Eadlayer H H can be calculated as the is difference between the adsorption energies, Ead, while Eadlayer H defined as the difference between the adlayer total energies and the total energies of the isolated molecular systems. Both quantities are given per hydrogen bond; that is, for a system of three water molecules, there are two hydrogen bonds. The results are summarized in Table 3. For all systems on Pt(111), we obtained that the binding energies of the hydrogen bonds spread from −0.29 to −0.37 eV for PBE, while the magnitude of the hydrogen bond in the adlayer structures spreads from −0.21 to −0.48 eV. Thus, for particular systems, we found an enhancement of the binding energy of the hydrogen bond on the Pt(111) surface, which is consistent with previous results;46 however, it does not hold for high coverage systems, in particular, for 6water/Pt(111) and 2ethanol+6water/Pt(111). Thus, the enhancement of the binding energy can explain the significant reduction, ∼0.30 Å, of the bond length of the hydrogen bond for the molecules on the Pt(111) compared with gas phase. The increase in the binding energy can be explained by the electron density redistribution in the interface region, in particular, on the oxygen and hydrogen atoms. We found that the vdW correction slightly decreases the magnitude of the hydrogen bond, however, PBE+D3 increases the binding energy of the adlayer to the surface. D. Work Function Change. The adsorption of atoms or molecules on surfaces promotes changes in the substrate work function (ΔΦ = Φmolecule/Pt(111) − ΦPt(111)), which helps to characterize the electron density rearrangement on the surface. For Pt(111), we found ΦPt(111) = 5.72 (PBE) and 5.69 eV (PBE +D3), which is consistent with previous PBE results, for example, 5.69,43 and 5.76 eV,58 while the experimental results are spread in the range 5.70 to 6.40 eV.59−61 Our results are summarized in Table 3. As expected, because of the high electronegativity of the O atoms compared with the Pt atoms, there is a charge rearrangement of the electron density within the molecules-

hydrogen bond for the ethanol−water layer on the Pt(111) surface. B. Adsorption Energy. One of the key parameters that helps to characterize the adsorption of molecular species on surfaces is the magnitude of the binding strength. In this work, we calculate the adsorption energy of the molecular species to the surface, Ead, and the adsorption energy of the adsorbate , which are defined by the following layer on the surface, Eadlayer ad equations n molecules/Pt(111) mol − i Pt(111) Ead = Etot − (∑ Etot + Etot ) i=1

layer molecules/Pt(111) adlayer Pt(111) Ead = Etot − (Etot + Etot )

(4) (5)

where Emolecules/Pt(111) and EPt(111) are the total energies of the tot tot molecules/Pt(111) and clean Pt(111) surface systems, mol−i respectively. Etot is the total energy of the gas-phase molecules calculated using a cubic box with size of 20 Å, while Eadlayer is the total energy of the adsorbate layer without tot the substrate, however, using frozen atomic positions obtained from the molecules/Pt(111) system. n is the number of molecules on the surface. For ethanol/Pt(111), we found Ead = −0.23 eV, which increases to −0.75 eV for two ethanol, and hence there is an attractive interaction among the two molecules. For water/ Pt(111), we found an adsorption energy of −0.20 eV, which is lower than previous results, that is, −0.30 to −0.35 eV.13,17,46 The deviations can be explained by the different unit cell sizes and xc functional employed in their calculations. For three and six water molecules on Pt(111), we found Ead = −0.77 and −0.88 eV, respectively; that is, the adsorption energy increases substantially from one water to three water molecules; however, the same ratio is not obtained for six water molecules. Thus, it indicates that a repulsive interaction starts to build in among the molecules due to the size of the unit cell for six water molecules. For ethanol+water/Pt(111), we found Ead = −1.89 and −4.10 eV for n = 1, 2, respectively, while the adsorption energy of the adlayer is −1.09 and −0.75 eV. It decreases by increasing the number of molecules, which is not expected; however, a fast inspection of the atomic structure provides important insights. For example, an increased number of molecules did not imply an increased number of molecules in direct contact with the surface, and hence the adsorption energy of the adlayer decreases. The PBE+D3 functional yields larger adsorption energies (Ead, Eadlayer ) compared with PBE results, which was also ad G



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Pt(111) interface, which reduces the work function of the Pt(111) surface and yields a negative value for ΔΦ for all studied systems. Thus, effectively, there is a charge transfer from the substrate to the adsorbates; however, it is important to mention that polarization of the adsorbate species can also contribute to reduce the work function of the Pt(111) surface, which was observed for Xe/Pt(111).26,27 For 2ethanol/Pt(111), we obtained ΔΦ = −1.41 eV, which is larger in absolute value than for 1ethanol/Pt(111), −1.36 eV, that is, ΔΦ increases. However, the same does not hold for water/Pt(111). For example, the absolute value of ΔΦ decreases for an increased number of water molecules, that is, −0.68, −0.55, and −0.46 eV for one, three, and six water molecules on Pt(111), respectively, which is consistent with previous results.46 Thus, it indicates an effective decrease in the charge transfer from the substrate to the adlayer, which can be explained by the direct binding of only one water molecule to the Pt(111) surface. For ethanol−water mixture on Pt(111), we expect to obtain a work function change between the values obtained for ethanol and water on Pt(111), which is in fact the case for 1ethanol+3water, while for 2ethanol+6water the work function change is nearly the same as that for 1ethanol/ Pt(111). Thus, in contrast with water, the work function change increases for increased coverages. The vdW correction slightly affects the work function changes, that is,

Density Functional Investigation of the Adsorption of EthanolâWater

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