Glycerol Adsorption on Platinum Surfaces: A Density Functional

Article pubs.acs.org/JPCC

Glycerol Adsorption on Platinum Surfaces: A Density Functional Theory Investigation with van der Waals Corrections Polina Tereshchuk,† Anderson S. Chaves,‡ and Juarez L. F. Da Silva*,† †

Instituto de Química de São Carlos and ‡Instituto de Física de São Carlos, Universidade de São Paulo, São Carlos, Sao Paolo13560-970, Brazil S Supporting Information *

ABSTRACT: Glycerol has been suggested as an additional energy source for hydrogen production for fuel cell applications, and several experimental studies have been reported on glycerol conversion on transition-metal surfaces; however, our atomistic understanding of glycerol−metal interactions is still far from satisfactory. In this work, we will report a theoretical investigation of the adsorption properties of glycerol on the Pt(110), Pt(100), and Pt(111) surfaces based on density functional theory (DFT) calculations with van der Waals (vdW) corrections to the DFT total energy. In the lowest energy configurations, which differ by millielectron volts, glycerol is almost parallel to the Pt surfaces and interacts with the Pt surfaces via one of the hydroxyl groups near the on-top Pt sites, which strongly affects the orientation of glycerol relative to the surface. Our results and analyses indicate that the adsorbate structure is among the high energy configurations of glycerol in gas phase. The vdW correction enhances the adsorption energy and hence decreases the equilibrium glycerol−metal distance; in particular, it affects mainly the adsorbate structure for glycerol on Pt(110), leading to the binding of two edge glycerol hydroxyl groups to the Pt(110) surface. Moreover, the vdW correction enhances the magnitude of the glycerol adsorption energy more on Pt(111) and less on Pt(110) surfaces; however, without changing the relative order of the adsorption energies, that is, glycerol binds more strongly on Pt(110) and more weakly on Pt(111) surface. We found large work function changes (0.7−1.0 eV) upon glycerol adsorption and negligible changes in the Bader charges of the H, C, O, and Pt atoms, and hence, polarization effects play a crucial role in the interaction mechanism.

I. INTRODUCTION Glycerol (C3H8O3) has been suggested as an additional energy source to produce hydrogen (H2) for fuel cell applications,1−3 beyond the wide range of applications in the chemical industry.4−6 Glycerol can be produced from oils and fats using several processes, and is also a byproduct of biodiesel,7 whose worldwide production has been increased in the last ten years. Steam reforming has been employed as a possible approach to produce H2 from glycerol (ideal reaction: C3H8O3(g) + 3H2O(g) → 7H2(g) + 3CO2(g)), which includes dehydrogenation, cleavage of C−C and C−O bonds, and hence, its success has a strong dependence on the selected catalysts, namely, transition metals (TMs) supported on oxides, which affect the formation of desirable products. Thus, a wide range of TMs supported on oxides has been studied to improve the conversion of glycerol and H2 selectivity, namely, Ir,8 Ni,8,9 Co,8 Pt10 on CeO2; Pt,1,10,11 Ni,1 Pd,1 Ru,1 Rh1 on Al2O3; Pt10 and Ni9 on MgO; and Pt/ ZrO2,10 Ni/TiO2,9 Ni/La2O3,12 and Ru/Y2O3.13 It has been determined that Ir/CeO2 yields a complete glycerol conversion and hydrogen selectivity of about 85% at 400 °C,8 while hydrogen selectivity of 90% was obtained using Ru/Y2O3 at 600 °C.13 Furthermore, several studies have indicated that the deposition of Re, or Ni or 3d TM monolayers on Pt(111) enhances H2 production2,14,15 compared with Pt(111), which © XXXX American Chemical Society

can be attributed to the role played by the strain induced on the 3d TM monolayers and the reactivity of the low-coordinated TM sites. Despite the large number of experimental studies,1,2,8−15 to our knowledge, only few theoretical studies have been reported so far for glycerol adsorption and reactions on TM surfaces.16−20 Most of these theoretical studies have addressed the decomposition of glycerol on the Pt(111) surface using density functional theory (DFT) calculations combined with chemistry models to understand the thermochemistry of glycerol intermediates and the kinetics of bond breaking, in particular, C−C and C−O. Furthermore, few adsorption results were reported for glycerol on Pd(111), Rh(111), Cu(111), and Ni(111).16−20 For example, it was determined that the TM surfaces exhibit decreasing activity and selectivity from Pt to Cu (i.e., Pt > Pd > Rh > Ni > Cu) for the C−C scission,20 which could be expected based on previous results obtained for alcohol systems. It was found that glycerol interacts with the TM surface mainly via its terminal hydroxyl group,16−20 with a binding energy of −0.46 eV on Pt(111) and about 2.21 Å for Received: March 25, 2014 Revised: June 19, 2014

A

dx.doi.org/10.1021/jp502969s | J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

Article (2) where Edisp + E(3). The E(2) and E(3) terms are two- and tot = E three-body energies and can be obtained from the following equations:

the glycerol−Pt(111) distances. Thus, it is similar to the adsorption mechanism obtained for ethanol.21−25 As mentioned above, the glycerol−Pt(111) interaction is weak, and hence, long-range van der Waals (vdW) interactions might play an important role, which was verified for ethanol and water adsorption on close-packed TM surfaces.23−25 For example, the addition of vdW corrections to plain DFT total energy calculations can contribute to increase the adsorption energy by 1−3 times, which affects the adsorbate structure of the molecular systems, and its distance to the solid surface. For example, for the particular case of ethanol on 3d, 4d, and 5d TM close-packed surfaces, DFT calculations lead to the perpendicular orientation of C−C ethanol bond with respect to the surface, while the vdW corrected DFT calculations yield an almost parallel orientation.23,25 However, the role of vdW correction to the glycerol adsorption on TM surfaces is unclear due to the large size and the role played by the three hydroxyl groups. Furthermore, there is no theoretical investigation reported for the interaction of glycerol with open surfaces such as (110) and (100). Thus, we can conclude that our understanding of the interaction of glycerol with TM surfaces is far from satisfactory and is an important step to improve our understanding of experimental results. In this work, we report a theoretical investigation of the adsorption properties of glycerol on the Pt(110), Pt(100), and Pt(111) surfaces using DFT calculations with vdW corrections. In the lowest energy DFT glycerol/Pt(hkl) configurations, glycerol is almost parallel to the Pt surfaces and interacts with the Pt surfaces via one of the hydroxyl groups, which binds near the on-top Pt sites. The interaction of glycerol OH groups with Pt surfaces affects the atomic structure of glycerol compared with the gas phase, in particular, the orientation of the OH groups with respect to the C−C bonds; however, the changes in the C−C, C−H, O−H bond lengths and CCC angles are very tiny. Our results and analyses indicate that the adsorbate glycerol structure is among the high energy configurations of glycerol in gas phase. The vdW correction enhances the adsorption energy, as expected, and hence decreases the equilibrium glycerol−metal distance, which affects mainly the adsorbate structure for glycerol on Pt(110) due to the stronger interaction with the Pt(110) surface, and two hydroxyl groups bind to the Pt(110) surface instead of only one. We found a large work function change (0.7−1.0 eV) upon glycerol adsorption and negligible changes in the Bader charges of the H, C, O, and Pt atoms, and hence, polarization effects might play a crucial role in the interaction mechanism.

E(2) =

AB n = 6,8

E(3) =

sn

CnAB n fd, n (rAB) rAB

ABC ∑ fd,(3) ( rABC ̅ )E ABC

(2)

(3)

Here the CAB n dispersion coefficients of the nth order for each AB pairs were computed from first-principles calculations for designed systems in gas phase. rAB indicates the atomic distance between the A and B atoms, sn is a scaling factor that depends on the selected xc functional (i.e., s6 = 1.00 and s8 = 0.72 for PBE), fd,n is a damping function employed to prevent nearsingularities for small rAB distances, and EABC is the nonadditive dispersion term.27,28 To solve the DFT framework equations, we employed the all-electron projected augmented wave (PAW) method,29,30 as implemented in the Vienna Ab Initio Simulation Package31,32 (VASP), using the PAW projectors to describe the electron− ion interactions.33 For glycerol/Pt(hkl), we employed a planewave cutoff energy of 400 eV, while a cutoff energy of 461 eV was used for stress tensor Pt bulk calculations. For the integration of the Brillouin zone (BZ) of the surface calculations, we used a 4 × 4 × 1 k-point mesh for total energy calculations and 8 × 8 × 1 k-point mesh for density of states and work function calculations. The equilibrium geometries were obtained when the atomic forces were smaller than 0.010 eV/Å on each atom, using a total energy convergence of 10−6 eV. B. Surface Modeling and Glycerol Adsorption Configurations. The unreconstructed Pt(110), Pt(100), and Pt(111) surfaces were modeled using the repeated slab geometry with a vacuum region of 20 Å and employing (2 × 3), (3 × 3), and (3 × 3) surface unit cells, respectively. To improve consistency among the different surface calculations, we employed slabs with similar thickness, D(hkl),34 that is, D(110) = 8a0/√8 = 11.26 Å, D(100) = 6a0/√4 = 11.94 Å, and D(111) = 5a0/√3 = 11.49 Å using aDFT = 3.98 Å (i.e., 8, 6, and 5 layers 0 for (110), (100), and (111), respectively). We considered the adsorption of one glycerol molecule per surface unit cell and only on one side of the slab, and hence, dipole correction was employed, which is crucial to obtain correct work function changes upon molecular adsorption. In our calculations, only the bottom layer of the slab was kept frozen in the relaxed clean surface positions. To identify the adsorption sites and glycerol orientation on the Pt surfaces, we performed first-principles molecular dynamics (MD) simulations using the Nose algorithm with a cutoff energy of 300 eV for about 20 ps (time step of 1 fs) starting from the initial temperature of 300 K up to a final temperature of nearly 0 K. In these calculations, only the topmost surface layers and glycerol were relaxed. Along the simulations, we selected several snapshots (about 15 for each system), which were optimized using the conjugated gradient algorithm. Special care was taken in the selection of the configurations to avoid similar initial model structures. For the vdW corrections, we selected several representative configurations, which was once again optimized using the DFT-PBE +D3 framework.

II. THEORETICAL APPROACH AND COMPUTATIONAL DETAILS A. Total Energy Calculations. Our calculations were based on DFT with the generalized gradient approximation proposed by Perdew−Burke−Ernzerhof26 (PBE) to the exchangecorrelation (xc) energy functional. To improve the description of the nonlocal long-range vdW interactions between adsorbate molecules and metal surfaces, we employed the vdW correction proposed by S. Grimme (DFT+D3),27,28 in which the DFT +D3 total energy, EDFT+D3 , is obtained by the sum of the selftot DFT , and the dispersion consistent DFT total energy, Etot disp correction, Etot DFT + D3 DFT disp Etot = Etot + Etot

∑∑

(1) B



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far from the experimental findings. The changes in the work function and d-band center due to the vdW corrections are less than 0.10 eV, and hence, within the error bar in our calculations. Thus, our analyses indicate that the benefits of DFT+D3 for the clean Pt surface properties are unclear, since it improves the results only for particular cases. Therefore, in this work, all calculations for glycerol/Pt(hkl) were performed using only the DFT-PBE equilibrium lattice constant, and the differences between DFT and DFT+D3 results are only due to the vdW correction and its indirect effects on the atomic structures. B. Glycerol in Gas Phase. Glycerol has three flexible hydroxyl groups, and the formation of intramolecular hydrogen bonds45 contributes to a substantial increase in the number of isomers (e.g., 126 isomers have been reported).46 Thus, over the years, several theoretical45−52 and experimental47,49,51,53,54 studies have investigated glycerol conformations. In this work, we selected 10 glycerol configurations from literature46 and 10 from MD simulations following the same procedure outlined in section B in Theoretical Approach and Computational Details Section. All those initial configurations were optimized, and four representative isomers are shown in Figure 1, while the

III. RESULTS A. Bulk and Clean Pt Surfaces. We obtained for the bulk lattice constant a0 = 3.98 Å (DFT) and 3.93 Å (DFT+D3), is while the experimental result is 3.92 Å.41 Thus, aDFT+D3 0 DFT 0.26% larger than aexp. by 1.26%, which is 0 and smaller than a0 expected as DFT-PBE commonly overestimates42,43 a0, and the vdW correction is an attractive interaction, which contributes to decrease the lattice constants. Thus, DFT+D3 improves the agreement with experimental results for a0.41 To check the performance of DFT+D3 for Pt(hkl), we calculated several clean surface properties, namely, the work function, Φ, topmost interlayer relaxations, Δd12, and the d-band center of the occupied d-states for the topmost 2 layers, ε1d and ε2d. Δd(hkl) 12 = (hkl) (hkl) (100) = a0/√8, d(100) = a0/√4, and (d(hkl) 12 − d0 )/d0 , where d0 0 d(111) = a0/√3. Due to the difference between the aDFT and 0 0 DFT+D3 and the lack of systematic investigations of the a0 performance of the DFT+D3 functional for metal surfaces, we performed the clean Pt surface DFT+D3 calculations using two lattice constants, namely, aDFT and aDFT+D3 . The results are 0 0 summarized in Table 1. Table 1. Clean Pt(hkl) Surface Propertiesa (110)

Φ

Δd12

ε1d

ε2d

DFT DFT+D3 DFT+D3* Exp. (100)

5.28 5.33 5.21 5.35b Φ

−15.91 −11.63 −16.36 −19.48c Δd12

−2.41 −2.44 −2.45

−2.95 −3.09 −3.01

ε1d

ε2d

DFT DFT+D3 DFT+D3* Exp. (111)

5.69 5.69 5.67 5.82e Φ

−2.28 −0.53 −3.55 +0.5d Δd12

−2.32 −2.40 −2.35

−3.05 −3.19 −3.15

ε1d

ε2d

DFT DFT+D3 DFT+D3* Exp.

5.69 5.69 5.65 5.70g

+0.86 +1.95 −0.66 +1.5f

−2.45 −2.57 −2.51

−2.99 −3.16 −3.11

a Work function, Φ (in eV), topmost interlayer relaxations, Δd12 (in %), and center of gravity of the occupied d-bands for the first, ε1d (in eV), and second, ε2d (in eV), topmost layers. DFT+D3* indicates the DFT+D3 results using the aDFT lattice constant for the slab. bField electron microscopy.35 cX-ray diffraction.36 dRutherford backscattering.37 ePhotoelectron spectroscopy.38 fMedium energy ion scattering.39 gPhotoelectric method.40

Figure 1. Four lowest energy DFT configurations of glycerol in gas phase. The relative energies are indicated below every configuration.

most important structural parameters are summarized in the Supporting Information. In the selected glycerol isomers, the C−C, C−O, C−H, and O−H bond lengths differ by less than 0.01 Å, while the OCC and HOC angles differ by 1−5°; however, the largest changes, as suggested in previous studies,45 are in their torsion angles, which are used to indicate the relative orientation of the hydroxyl groups (e.g., the torsion CCCO and HOCC angles spread from 5° to 100°). Thus, we expect the interaction of the hydroxyl groups with the Pt surfaces to play a critical role in the structural conformation of glycerol on Pt surfaces. The van der Waals correction does not affect those structural parameters or even change the relative stability of the isomers. Our lowest energy configuration for glycerol corresponds to the lowest energy isomer obtained by high level quantum chemistry calculations,45,46 which indicates that DFT provides a correct description of glycerol in the gas phase. C. Lowest Energy Configurations for Glycerol on Pt(110), Pt(100), and Pt(111) Surfaces. Following the steps outlined above, we optimized several configurations for glycerol adsorption on platinum (110), (100), and (111) surfaces. The

As expected from previous studies,36,44 we obtained a larger interlayer contraction for (110), −15.91% (DFT), than for (100), −2.28% (DFT), while there is an expansion of +0.86% (DFT) for (111).42 Our DFT-PBE results are close to the experimental results taking into account the experimental deviations.35,37−40 In contrast to the Pt bulk, the vdW correction could not improve the interlayer relaxations. For example, using the aDFT+D3 parameter for the slab, we obtained 0 a contraction of −11.63% for (110) (DFT+D3) instead of −15.91% using DFT-PBE, while the experimental result is −19.48%.36 Thus, the vdW correction decreases the magnitude of the contraction for the (110) surface, and similar behavior can be observed for (100), while it enhances the expansion for (111). However, using aDFT for the slab, we obtained Δd12 = 0 −16.36% for (110) surface (DFT+D3), which is closer to the DFT-PBE result and in better agreement with the experimental result, while for (100) and (111) surfaces the results are much C



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Figure 2. Two lowest energy configurations for glycerol on the Pt(110), Pt(100), and Pt(111) surfaces obtained by DFT and DFT+D3 functionals. The relative total energies are given with respect to the lowest energy configuration.

Table 2. Adsorption Energy of Glycerol (Gly) Molecule and Gly-Layer on the Pt(hkl) Surfaces, Emolec and Elayer ad ad (eV), ⊥ ⊥ ⊥ Perpendicular Distances of the Closest C, O, And H atoms to the Pt(hkl) Surfaces, dC−Pt, dO−Pt, and dH−Pt (in Å), Angle of the C−C Bond with Respect to the Surface Normal, αCC⊥ (in deg) and Glycerol CCC Angle (in deg) Gly DFT Emolec ad Elayer ad ⊥ dC−Pt d⊥O−Pt d⊥H−Pt αCC⊥ C−C C−O C−H O−H CCC

1.53 1.44 1.11 0.98 112.92

Gly/Pt(110)

Gly/Pt(100)

Gly/Pt(111)

DFT+D3

DFT

DFT+D3

DFT

DFT+D3

DFT

DFT+D3

1.53 1.44 1.10 0.98 112.87

−0.71 −0.89 2.95 2.22 1.96 69.02 1.52 1.43 1.11 0.99 112.97

−1.71 −1.81 2.57 2.20 1.42 88.95 1.52 1.44 1.11 0.99 115.83

−0.44 −0.54 3.12 2.45 2.00 79.46 1.52 1.44 1.11 0.98 116.09

−1.31 −1.47 3.00 2.39 1.87 82.31 1.52 1.43 1.11 0.98 116.56

−0.36 −0.47 3.16 2.51 2.04 77.13 1.52 1.43 1.11 0.98 116.83

−1.29 −1.45 2.99 2.39 1.86 78.89 1.52 1.43 1.11 0.98 117.41

among the Gly/Pt isomers are very small (i.e., few meV), however, we would like to point out substantial differences in the orientation of the hydroxyl groups, which can be explained by their great flexibility compared with the glycerol frame formed by the C atoms.

two lowest energy model structures for each surface obtained by DFT and DFT+D3 functionals are shown in Figure 2, while the most important geometric parameters are summarized in Table 2. Few extra high energy model structures are given in the Supporting Information. The total energy differences D



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small energy difference (0.08 eV) between Gly−Pt(100) and Gly−Pt(111) (e.g., using DFT, Emolec = −0.71, −0.44, and ad −0.36 eV for glycerol on (110), (100), and (111), respectively). The vdW correction increases the adsorption energy; however, the magnitude depends on the surface orientation, which is consistent with previous calculations for ethanol and water adsorption on close-packed TM surfaces.23,24 For example, α = /EDFT = 2.41, 2.98, and 3.58, for (110), (100), and EDFT+D3 ad ad (111), respectively. Then, the vdW correction is larger for Gly/ Pt(111); however, the changes in the glycerol orientation on Pt(hkl) are pronounced for Gly/Pt(110); see Figure 2. Furthermore, we calculated the interaction energy between Gly−Pt(hkl) using eq 4; however, instead of the glycerol gas phase total energy, Emolec tot , we used the total energy of the layer formed by the glycerol molecules in the same surface unit cell layer used for the Gly/Pt(hkl) calculations, which yields Ead summarized in Table 2. The magnitude of the adlayer adsorption energy is larger than the magnitude for the molecule adsorption energy since it includes the intermolecular Gly−Gly interactions. From the results for Emolec and Elayer ad ad , we estimated the Gly−Gly interaction as 179, 98, and 111 meV for (110), (100), and (111) using the DFT results, and 95, 152, and 154 meV using DFT+D3 results. Thus, these results are almost the same as the Gly−Gly interactions in the glycerol layer without the substrate (i.e., 178, 96, and 109 meV for (110), (100), and (111), using DFT, and 94, 151, and 153 meV for (110), (100), and (111), using DFT+D3), which indicates that the effects induced by the Pt surfaces on the glycerol molecule do not affect the Gly−Gly interactions, in particular, due to the large distance between the glycerol molecules and the Pt surfaces. To investigate the effect of the Pt surfaces on the atomic structure of glycerol, we calculated the total energy of the frozen glycerol molecule without the Pt substrate and using a cubic box. The energy differences compared with the lowest energy glycerol isomer (gas phase) are 0.17, 0.14, and 0.12 eV (0.40, 0.15, and 0.18 eV using DFT+D3) for (110), (100), and (111), respectively. Once the frozen configuration was optimized, the energy difference was reduced to 60−92 meV (DFT) and 4−77 meV (DFT+D3), which has similar magnitude to the energy difference among several glycerol isomers. Thus, from this analysis, we can conclude that glycerol configurations on the Pt surfaces are high energy glycerol gas phase isomers. Our results are consistent with previous results.20 For example, Liu et al.20 obtained an adsorption energy of −0.46 eV for Gly/Pt(111), while we obtained −0.36 (i.e., a difference of 0.10 eV). They employed a p(4 × 4) surface unit cell with 3 layers in the slab, while we employed a (3 × 3) surface unit cell. For the two closest O atoms, they obtained vertical O distances to the Pt(111) surface of 2.21 and 3.36 Å, and the closest H is at 1.69 Å above the surface, while our results 2.51, 3.66, and 2.04 Å, respectively. Thus, the equilibrium parameters indicate important differences in the structure. E. Density of States. To obtain a better understanding of the electronic properties, we calculated the total density of states for the energy window from −30 to 10 eV for the lowest energy configurations, which was decomposed into the local density of states (LDOS), namely, the s-, p-, and d-states for every chemical species. The DFT-PBE and DFT-PBE+D3 results are shown in Figure 3 for the energy window from −10 to 2 eV. There are several C, O, H, and Pt atoms in the Gly/ Pt(hkl) systems, and hence, we averaged the LDOS for every particular state. For example, the C p-state shown in Figure 3 is

Glycerol binds to the Pt surfaces by the hydroxyl group near or on the on-top Pt sites, which is in agreement with a recent study.20 The O−H bond, which takes part in the Gly−Pt binding, is nearly parallel to the surface, which is similar to the binding of ethanol and water with TM surfaces.22,23,55 As indicated by the geometric parameters, there are important differences among the Gly/Pt systems, which cannot be easily seen in Figure 2. For example, the vertical distances of the closest C, O, and H atoms to the surface, d⊥C−Pt, d⊥O−Pt, and d⊥H−Pt, respectively, are smaller for Gly/Pt(110) and larger for Gly/Pt(111), which is related with the stronger and weaker binding of glycerol to the (110) and (111) surfaces, respectively. The addition of the vdW correction decreases the distances between the molecule and the Pt surfaces, as expected, particularly for Gly/Pt(110), and, the changes are larger for C and H atoms closer to the surface and smaller for d⊥O−Pt. For DFT and DFT+D3, H is the atom that gets closer to the surface, which can be explained by its smaller atomic size. Furthermore, DFT calculations yield glycerol less parallel to the Pt(110) surface compared with Gly/Pt(111), which can be verified by the angle between glycerol axis, defined by the line passing through the two edge C atoms, and the surface normal, αCC⊥. We found that αCC⊥ = 69.02°, 79.46°, and 77.13° for (110), (100), and (111), respectively (αCC⊥ = 90° implies parallel orientation). However, the vdW correction increases αCC⊥ slightly for (100) and (111) surfaces (αCC⊥ = 82.31 and 78.89°), while glycerol is almost parallel on the (110) surface (i.e., αCC⊥ = 88.95°). Thus, for Gly/Pt(110), two hydroxyl groups bind to the Pt(110) surface. We observed changes of 0.010−0.03 Å in the bond lengths (C−C, C−O, C−H, O−H) upon adsorption compared with the lowest energy glycerol configuration in gas phase, while the CCC, OCC, and HOC angles change up to 4°. As expected from the gas phase discussion, most of the differences among the isomers occur in the dihedral angles; in particular, the CCCO angle increases by about 100° for all Gly/Pt systems. Furthermore, the H2O2C2C3 angle increases by about 103° on Pt(110) and by only 21−24° on Pt(100) and Pt(111) (see the table in the Supporting Information). The structural differences compared with the high energy isomers are mainly on the orientation of the hydroxyl groups, and due to the small magnitude of the relative total energies (few meV). Our findings indicate that the glycerol frame formed by the C−C, C−O, and O−H bonds and CCC angles is only slightly affected by the interaction with the Pt surfaces; however, the orientation of the hydroxyl groups depends on the Pt surface and isomers. Due to the large number of isomers in gas phase, the number of possible Gly/Pt configurations with a small energy difference is very large, and at room temperature, all those configurations might exist, and hence may play a crucial role in the interaction with TM surfaces and reactions. D. Adsorption Energy. To characterize the magnitude of the Gly−Pt(hkl) binding, we calculated the adsorption energy, Emolec ad molec Gly /Pt(hkl) Pt(hkl) Gly Ead = Etot − Etot − Etot

(4)

where EGly/Pt(hkl) , EPt(hkl) , and EGly tot tot tot are the total energies of Gly/ Pt(hkl), clean Pt(hkl) surface, and glycerol in gas phase. The Emolec results are summarized in Table 2. ad According to the surface atom coordination, the largest binding energy is obtained for Gly/Pt(110), while there is a E



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states, in particular, of the O states, while the C states are almost not affected by the Gly−Pt interaction, which can be explained by the large perpendicular distance of the C atoms to the surface (e.g., d⊥C−Pt = 2.95−3.16 Å for the closest C atom), while the O atoms approach closer to the surface (e.g., d⊥O−Pt = 2.22−2.51 Å for the closest O atom). Furthermore, it can be seen that a small tail of the O p-states extends slightly above the Fermi level, which might indicate that a polarization of glycerol takes place. Furthermore, from the calculations of the d-band center for Pt(hkl) and Gly/Pt(hkl), we found a shift in the dband center of the topmost surface layer upon glycerol adsorption of 0.07, 0.03, and 0.04 eV for (110), (100), and (111), respectively, which indicates a weak hybridization among the glycerol and Pt states; however, it is important to mention that this small change is in the limit of accuracy of our analyses based on LDOS (i.e., LDOS depends on the selection of the atomic radius for every chemical species). It can be seen in Figure 3 that the LDOS obtained by DFTPBE and DFT-PBE+D3 calculations are very similar, in particular, for glycerol in gas phase and the clean Pt(hkl) surfaces, which is expected as the vdW correction does not affect the glycerol structure in gas phase, and the clean Pt(hkl) surfaces calculated using the DFT lattice constant. However, those conclusions do not hold for Gly/Pt(110), in which there are clear differences in the LDOS among the two functionals. As mentioned above, DFT+D3 decreases substantially the Gly− Pt(110) distance, and hence, it increases the broadening of the electronic states and moves the states down, in particular, for the O atoms closer to the surface. In contrast, only slight differences are observed for glycerol on Pt(111) and Pt(100). F. Work Function and Bader Charges. To obtain a better understanding of the electron density rearrangements, we calculated the work function changes, ΔΦ = ΦGly/Pt(hkl) − ΦPt(hkl), where ΦGly/Pt(hkl) is the work function of the Gly/ Pt(hkl) systems and ΦPt(hkl) is the work function of the clean Pt surfaces (Table 1). The ΔΦ results are summarized in Table 3.

Figure 3. Local density of states (LDOS) for glycerol in gas phase (top panel), topmost surface layer for the clean Pt(hkl) surfaces and for Gly/Pt(hkl). The vertical dashed line indicates the Fermi energy for Pt(hkl) and Gly/Pt(hkl) and the highest occupied molecular orbital for glycerol. The clean Pt(hkl) LDOS are shifted from zero for better visualization.

the average over all the p-states from all the C atoms. The same procedure was repeated for all chemical species and states, except for the Pt atoms, in which the averages were performed only for the Pt atoms in the first layer, Pt1, and second layer, Pt2, respectively. As expected, for glycerol in gas phase, there is a strong hybridization among the s (C, O, H) and p (C, O) states. The energy states from −20 to −19 eV are mainly derived from the O 2s states, while the states from −13 to −9 eV below the highest occupied molecular orbital (HOMO) is mainly derived from C 2s states; however, all those states have contribution from the remaining states. The states between −9 and 0 eV are derived from the C 2p, O 2p, and H 1s states; however, the high energy states have a larger contribution from the O 2p states, and in particular, HOMO is dominated by O p states. Thus, the O p states play a crucial role in the interaction, in particular, the pz-states. In the panel below the glycerol LDOS, we can observe the LDOS differences among the clean Pt(hkl) surfaces. For example, the center of gravity of the occupied Pt states (dominated by dstates) are 2.41, 2.32, and 2.45 eV below the Fermi level for (110), (100), and (111), respectively, and hence, based on the d-band model,56 we would expect the larger and smaller adsorption energies for glycerol on (100) and (111), which is not supported by our calculations. Thus, it indicates that the Gly−Pt interaction is not dominated by a strong hybridization among glycerol and Pt states. The electronic states of glycerol are affected upon adsorption on Pt(hkl). For example, the glycerol states mainly derived from the O 2s and C 2s states do not increase the broadening; however, they are shifted downward by almost 3 eV with respect to the Fermi level due to the interaction with the surface potential. The most important electronic states for the Gly-Pt binding energy are ranging from −10 to 0 eV (Figure 3). We can see clearly a broadening of the glycerol electronic

Table 3. Work Function Change, ΔΦ (in eV), and Average Effective Charge, Q (in e), for the C, O, H, and Pt Atoms for Glycerol in Gas Phase and Glycerol/Pt(hkl)a Gly/Pt(hkl) ΔΦ ΔΦDFT+D3 QC QO C QH O QH PtO Q QPt

Gly gas phase

(110)

(100)

(111)

+0.42 −1.11 +0.06 +0.59

−0.76 −1.12 +0.42 −1.09 +0.07 +0.60 +0.12 −0.07

−0.84 −0.85 +0.42 −1.09 +0.08 +0.60 +0.10 −0.08

−0.93 −0.99 +0.42 −1.09 +0.07 +0.60 +0.08 −0.07

DFT

a

HC and HO indicates H atoms with binding with C and O atoms, respectively, PtO presents the Pt atom directly below the O atom nearest to the surface, while Pt are the remaining Pt atoms in the first layer.

As obtained for ethanol/Pt(111),23,24 glycerol adsorption on Pt surfaces reduces the work function from −0.76 eV for (110) to −0.93 eV for (111); however, it does not imply a stronger adsorption energy for Gly/Pt(111). For example, the magnitude of the adsorption energy is larger for Gly/Pt(110), −0.71 eV (DFT), while it is smaller for Gly/Pt(111), −0.36 eV (DFT). When DFT+D3 is employed, ΔΦ decreases by 0.01− F



Article

the gas phase configurations, several configurations might be reached at different temperatures and electrochemical potentials, as well as in chemical reactions. As expected from previous studies, DFT-PBE+D3 enhances the glycerol adsorption energy on Pt surfaces by a large factor, which is not the same for all surfaces (larger for (111) and smaller for (110)); however, it does not change the relative order of the adsorption energies. That is, glycerol binds more strongly on Pt(110) and weakly on Pt(111), while the magnitude of the glycerol adsorption energy on Pt(100) is in the middle. Furthermore, the vdW correction decreases the perpendicular distance of the C, O, and H atoms to the surfaces compared with DFT results, which is expected due to the enhancement of the adsorption energy. Although the Gly− Pt(hkl) binding is stronger for Gly/Pt(110) (i.e., −0.71 (DFT) and −1.71 eV (DFT+D3)), we found only slight changes in the glycerol bond lengths and angles; however, as mentioned above, the torsion angles are strongly affected. Our calculations of the work function change, local density of states, and Bader charges indicate a classical case of physisorption,59−61 in which there is a large change in the clean surface work function upon glycerol adsorption, tiny changes in the bond lengths and surface relaxations, and no charge transfer between both systems (supported by Bader charge). Thus, polarization effects on glycerol and topmost surface layers might play a crucial role in the interaction mechanism, which also helps to explain the great flexibility of the hydroxyl groups.

0.06 eV for glycerol on (100) and (111); however, DFT+D3 increases ΔΦ by 0.36 eV for Gly/Pt(110), which can be explained by the structural distortions induced by the vdW correction. Thus, our results indicate a strong electron density reorganization in the separated systems (polarization effects) or a charge transfer between glycerol and the Pt surface based on the electronegativity difference between the O (3.44) and Pt (2.28) atoms.57 To investigate a possible charge transfer or polarization effects, we calculated the Bader charge, QBader, on every atom in the system using a high density Fast Fourier Transform (FFT) grid in VASP, which is necessary to obtain accurate Bader charges.58 Thus, using the Bader charge and the number of valence electrons, we calculated the average effective charge, Q = Zval − QBader, on every chemical species, where Zval = 4, 6, 1, and 10 e for C, O, H, and Pt atoms, respectively. The results are summarized in Table 3. In the gas phase, as expected due to the electronegativity differences (2.55 for C, 3.44 for O, and 2.20 for H),57 we obtained an effective negative charge on the O atoms of −1.11 e, while there is an effective positive charge of +0.42 e on the C atoms. The hydrogen atoms are separated into C two groups, namely, C−H (HC) and O−H (HO), and QH = O

+0.06 e and QH = +0.59 e. The glycerol adsorption on the Pt surfaces changes only slightly the effective charge on the C, O, and H atoms (i.e., changes on the order of 0.01−0.02 e). Thus, our results and analyses indicate a negligible charge transfer between glycerol and Pt surfaces; however, it is not zero. We found a small relative difference in the effective charge on the Pt atoms in the first layer. For example, the Pt atom directly below the O atoms has an effective positive charge from +0.08 e for (111) to +0.12 e for (110), while the remaining Pt atoms have an effective charge of about −0.07 e for all surfaces. There are two explanations, namely, tiny charge transfer between the systems or a charge transfer between the Pt atoms in the first layer. Therefore, we can conclude that reduction of the clean surface work function upon glycerol adsorption is mainly due to the polarization effects on both systems, which is similar to the adsorption of rare-gas atoms on TM surfaces.59,60 The vdW correction does not change this picture (i.e., the effective charges are nearly the same for both functionals).

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ASSOCIATED CONTENT

S Supporting Information *

Further atomic configurations for glycerol adsorption on the Pt surfaces. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Authors thank the São Paulo Research Foundation (FAPESP), National Council for Scientific and Technological Development (CNPq), and Coordination for Improvement of Higher Level Education (CAPES) for the financial support. Authors thank also the Research Computing Support Group (Rice University) for the computing time provided in the Blue Gene/P supercomputer and the Laboratory of Advanced Scientific Computing (University of São Paulo), and the Department of Information Technology - Campus São Carlos for the infrastructure provided to our computer cluster.

IV. CONCLUSIONS We carried out ab initio DFT calculations with and without dispersion correction employing the PBE functional to study the adsorption properties of glycerol on the Pt(110), Pt(100), and Pt(111) surfaces. It has been known that glycerol in gas phase has a large number of identified isomers (e.g., 126) with close energy,46 which differ mostly by the relative orientations of their OH groups (torsion angles).45−54 Our results and analyses indicate that those isomers play a crucial role in the glycerol adsorbate structures. We found that the glycerol molecule in the lowest and higher energy Gly/Pt(hkl) configurations binds to the Pt surfaces via hydroxyl group near the on-top Pt sites. That is, although the configurations are different, the group interacting with the Pt surface is preserved, which is similar to the adsorption of ethanol on close-packed TM surfaces.21−25 Our results and analyses suggest that glycerol adsorbate structures are among the higher energy isomers in gas phase; hence, the Gly−Pt(hkl) interactions help the glycerol molecule reach high-energy gas phase isomers. Thus, due to the small energy difference among

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Glycerol Adsorption on Platinum Surfaces: A Density Functional

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