The Role of Low-Coordinated Sites on the Adsorption of Glycerol on

Article pubs.acs.org/JPCC

The Role of Low-Coordinated Sites on the Adsorption of Glycerol on Defected Ptn/Pt(111) Substrates: A Density Functional Investigation within the D3 van der Waals Correction Rafael C. Amaral, Polina Tereshchuk, Yohanna Seminovski, and Juarez L. F. Da Silva* São Carlos Institute of Chemistry, University of São Paulo, PO Box 780, 13560-970 São Carlos, SP, Brazil S Supporting Information *

ABSTRACT: Several experimental studies have been reported for the conversion of glycerol on transition metal surfaces; however, only few studies have addressed the role of surface defects in the glycerol−substrate interactions. Here, we report ab initio calculations based on density functional theory within the D3 van der Waals (vdW) correction to investigate the adsorption properties of glycerol on flat and defected Pt(111) substrates, namely, (i) flat surface, (ii) dispersed adatoms, (iii) linear-type defect, (iv) island-type defect, and (v) vacancy-type defect. In the lowest and higher energy configurations, glycerol binds to the flat and defected Pt(111) substrates via one or two hydroxyl groups, in which the anionic O atom binds to cationic Pt site with the O−H bond nearly parallel to the Pt−Pt bonds in several cases, which indicates a contribution of the HO−Pt interaction to the adsorption energy. The PBE adsorption energy is stronger on the low-coordinated sites of single Pt adatoms and six-adatom triangular defects, which can be explained by the shift of the center of gravity of the occupied d-states toward the Fermi level due to the reduction in the coordination adsorption site. The addition of the D3 vdW correction changes the preference of the adsorption sites, in particular, glycerol binds with a stronger adsorption energy on the six-adatom linear and triangle defects due the binding of multiple hydroxyls (the terminal and central one) to the substrate, which is expected due the attractive nature of the vdW correction. On the basis of Bader analysis, we obtained an effective charge transfer from glycerol to the Pt substrates, which helps to explain the work function reduction of the substrates upon glycerol adsorption along with the polarization of glycerol. These results indicate that permanent-induced dipole forces play an important role in the glycerol−substrate binding mechanism.

I. INTRODUCTION

Thus, periodic DFT calculations within semilocal functional have obtained important insights into the mechanisms of glycerol adsorption and decomposition on TM surfaces.8,9,14−20 For example, it was found that glycerol binds preferentially on top sites of clean TM surfaces by the terminal oxygen lone pair,14,18−20 though Chen et al.16 found that the central hydroxyl has the major contribution to the binding energy. Furthermore, DFT calculations combined with semiempirical methods have shown that dehydrogenation of glycerol precedes the breaking of C−C bonds on Pt(111),15,16 whereas the C−O bond scission can be favored by changing the TM catalyst.18 Although these studies14−18,20 addressed the interaction of glycerol with clean surfaces, in real catalytic systems, the particles commonly present complex structures that exhibit high concentrations of defects on the surface, such as steps, kink sites, and vacancies. The low-coordinated sites are thought to be the catalytic active sites as the reactants bind much stronger to them due to the lower four-electron repulsion between occupied orbitals on the molecule and filled states of

Crude glycerol generated as a byproduct from biodiesel production has become an economic−environmental problem in Brazil, e.g., about 10 kg of glycerol is generated for every 100 kg of biodiesel.1,2 Thus, several possibilities have been explored to obtain higher value products from glycerol (C3H8O3) such as propanediols,3 syngas (mixture of H2 and CO),4,5 hydrogen (H2),6−10 etc. The formation of the sp3 hybridization among the C s- and p-states contributes to the strong binding energy among the C−C and C−O bonds, and hence, those products are usually obtained at high temperature processes with complex reaction pathways;3−10 thus, catalysts are essential to lower the energetic costs and to improve the selectivity toward the desired products. Transition metal (TM) particles supported on oxide surfaces have been studied extensively as catalysts for glycerol conversion.5−7,10−13 However, the atomistic understanding of the glycerol−TM interactions cannot be easily extracted from experiments at real conditions,3,8,9 while several questions remain open, such as what is the interaction mechanism between glycerol and TM surfaces and what is the role of TM surface defects on the adsorption properties of glycerol. © XXXX American Chemical Society

Received: December 5, 2016 Revised: January 20, 2017 Published: January 23, 2017 A

DOI: 10.1021/acs.jpcc.6b12238 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C the surface.21 For example, low-coordinated sites on stepped surfaces of Pt were related with the enhancement of the catalytic activity toward both methanol oxidation22−24 and oxygen reduction.25−27 Therefore, the theoretical modeling of defected surfaces is of utmost importance to improve our understanding on the role of low-coordinated sites in real particles. Only few studies have addressed theoretically the effect of surface structure in the adsorption properties of glycerol on TM surfaces.8,9,19,28 Previously, Tereshchuk et al.19 investigated the adsorption properties of glycerol on the Pt(100), Pt(110), and Pt(111) and found that glycerol binds stronger to the more open Pt(110) surface. Recently, Garcia et al.28 showed that the selectivity in glycerol electro-oxidation on Pt changes with the orientation of the surface due to different bond modes of glycerol intermediate in the Pt(111) and Pt(100) surfaces. The electro-oxidation of methanol29 and ethanol30 on Pt was also found to be sensitive to the surface structure. For all those cases,19,28−30 the differences in the coordination of Pt atoms between the surfaces are considered to play a major role on their interaction with the molecules. Nonetheless, lowcoordinated sites might also hamper reactions, like those of triangular island defects on Pt(100), which were suggested to overbind glycerol and increase its electro-oxidation onset.8,9 Though these findings provide important insights into how the surface structure affects its interaction with glycerol, our understanding on glycerol adsorption on defected surfaces is still far from being complete. Furthermore, most of aforementioned studies8,9,14−18 employed semilocal functional which are known for not properly describe the long-range electron correlations that are fundamental for weakly bound system such as glycerol on TM surfaces. Thus, the inclusion of van der Waals (vdW) corrections is essential to improve the long-range interaction that controls physisorption. Currently there are various approaches in use to deal with dispersion corrections; among them, the two most commons are (i) the DFT-D3 method, which consists of an atom pairwise sum over C6R−6 potentials,31 and (ii) the nonlocal dispersion density functional, which explicit account the dispersion interactions.32 A recent survey on the performance of several density based vdW correction on the adsorption properties of glycerol can be found elsewhere.20 In this study, we report a DFT investigation of the glycerol adsorption properties on clean and defected Pt(111) substrates employing vdW corrections. The DFT+D3 framework proposed by Grimme31 has been used in this work since it yields similar or better results when compared to other more computationally expensive vdW correction methods.33,34 We explored the behavior of glycerol on four different defected substrates: (i) dispersed adatoms, (ii) linear-type defect, (iii) island-type defect, and (iv) vacancy-type defect. Here we show how structural and electronic properties of Pt(111) substrates vary with the defected surface and the effect of vdW correction over these properties. We also present the putative lowest energy configuration for the adsorbate/substrate systems, where glycerol can bind in different modes according to the surface structure of the substrate. Both PBE and PBE+D3 yield similar results, e.g., the same preferred adsorption sites and number of −OH bonding to the substrate, although a significant improvement of glycerol adsorption parameters was found by the inclusion of vdW corrections. Here we also discuss the electronic aspects of the problem by means of

density of states, work function change, and Bader charge analysis.

II. THEORETICAL APPROACH AND COMPUTATIONAL DETAILS Total Energy Calculations. Our total energy calculations were based on spin-polarized DFT35,36 within the generalized gradient approximation37 proposed by Perdew−Burke−Ernzerhof38 (PBE) for the exchange-correlation energy functional. To improve the description of the long-range correlation effects, we employed the vdW correction proposed by Grimme,31 namely, the D3 framework, which includes two-body, E(2), and three-body, E(3), energies. The DFT+D3 total energy as , is obtained by the sum of proposed by Grimme,31 EDFT+D3 tot the self-consistent plain DFT total energy, EDFT tot , and the vdW (2) (3) energy correction, EvdW = E + E , as follows: energy DFT + D3 DFT vdW Etot = Etot + Eenergy

(1)

The E(2) and E(3) terms are given by the equations E(2) =

∑∑ AB n = 6,8

E(3) =

sn

CnAB n fd, n (rAB) rAB

ABC ∑ fd,(3) ( rABC ̅ )E ABC

(2)

(3)

where sn is a scaling factor that depends on the selected exchange-correlation functional (e.g., s6 = 1.00 and s8 = 0.72 for the PBE functional), CAB represent the averaged nth-order n dispersion coefficients for each AB pair and were computed from time-dependent DFT, rAB is the atomic distance between the A and B atoms, fd,n is a damping function employed to avoid near-singularities for small rAB distances, and EABC is the nonadditive triple dipole dispersion term. Implementation details are reported elsewhere,41,42 where it has been discussed that the three-body energy yields an almost negligible contribution to the vdW energy, and hence, it has not been included in the Vienna ab initio simulation package (VASP) implementation. The Kohn−Sham equations in the DFT-PBE+D3 framework were solved using the all-electron projected augmented wave (PAW) method,43,44 as implemented VASP (version 5.4.1),39,40 and using the PAW projectors provided within VASP to describe the electron−ion interactions,45 whereas the valence electrons are treated within the scalar relativistic approximation.46,47 To obtain the equilibrium volumes of bulk Pt in the face-centered cubic (fcc) structure,48,49 we minimized the stress tensor using a plane-wave cutoff energy of 589 eV, while a cutoff energy of 466 eV was employed for all the remaining total energy calculations. The integration of the Brillouin zone for the surface calculations was performed using a 3 × 3 × 1 kpoint mesh for total energy calculations and 6 × 6 × 1 for density of states and work function calculations. To model the Gly−Ptn/Pt(111) systems, we employed the repeated slab geometry using a (4 × 4) surface unit cell, 5 Pt layers, 21 Å for the vacuum region, and the respective equilibrium aPBE or aPBE+D3 lattice constant.50 Glycerol was 0 0 adsorbed on only one side of the slab, and hence, dipole correction51 was applied, which is essential to obtain a correct description of the work function. For all surface calculations, the bottom layer of the slab was kept frozen in their respective relaxed clean surface positions, while all the remaining atoms in the unit cell were allowed to relax. For glycerol in the gas phase, B


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The Journal of Physical Chemistry C

Figure 1. Top view of the clean and defected Pt(111) substrates using a (4 × 4) surface unit cell, where the top (1-fold), bridge (2-fold), hcp (3fold), and fcc (3-fold) adsorption sites are indicated. The Pt adatoms are indicated in dark gray. The effective Bader charges (in e) are also indicated on each atom. The PtT6 /Pt(111) structure is 77 meV higher in energy than the PtL6 /Pt(111) structure.

Figure 2. Lowest energy PBE and PBE+D3 configurations obtained for glycerol adsorption on the Pt(111), Pt/Pt(111), PtL6 /Pt(111), PtT6 /Pt(111), and Pt13/Pt(111) substrates. The adsorption energies, Ead, are given below each configuration, while higher energy isomers are reported in the Supporting Information.

Gly/Ptn/Pt(111) Configurations. Surface reconstructions of the clean Pt(111) substrate are not expected to occur up to 1329 K;54,55 however, Pt electrodes employed in the electrooxidation of glycerol are not free of surface defects;56−58 e.g., a wide range of low-coordinated Pt sites are present on the topmost surface layers. Furthermore, recent experimental studies have designed defected Pt substrates to study the role of defected Pt sites on the electro-oxidation of glycerol;8,9,59,60

which has been studied previously using high-level quantum chemistry calculations,52,53 we employed a cubic box with 20 Å and only the Γ-point for the Brillouin zone integration as there is no dispersion in the electronic states within the first Brillouin zone. For all calculations, we obtained the equilibrium geometries once the atomic forces were smaller than 0.025 eV/Å on every atom and using a total energy convergence of 10−5 eV. C


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Table 1. Energetic, Structural, and Electronic Properties of the Ideal Clean Pt(111) Surface and Ptn/Pt(111) Substrates, Namely, Single-Adatom Defect, Pt/Pt(111), Six-Adatom Linear Defect, PtL6 /Pt(111), Six-Adatom Triangular Island Defect, PtT6 / Pt(111), and Vacancy Defect, Pt13/Pt(111)a PBE substrates

EPt ad

Pt/Pt(111) PtL6 /Pt(111) PtT6 /Pt(111) Pt13/Pt(111) Pt(111)

−4.52 −5.06 −5.05 −5.43

PBE+D3

ECN

dad av

Φ

3.74 6.15 6.38 8.08 8.98

2.57 2.69 2.67 2.78 2.81

4.95 5.31 5.23 5.57 5.69

ad

ΔΦ

εad d

EPt ad

−0.74 −0.38 −0.46 −0.12

−1.88 −2.29 −2.42 −2.39 −2.43

−5.12 −5.66 −5.67 −6.16

ad

ECN

ad

3.67 6.14 6.32 8.07 8.97

dad av

Φ

ΔΦad

εad d

2.57 2.68 2.66 2.76 2.78

5.30 5.29 5.21 5.57 5.69

−0.39 −0.40 −0.48 −0.12

−1.89 −2.33 −2.44 −2.48 −2.54

a

ad Adsorption energy per adatom, EPt ad (in eV), adatoms average effective coordination number (ECN ) in number of nearest neighbors (NNN), ad adatoms weighted average bond lengths, dav (in Å), substrate work function, Φ (in eV), work function change of the Pt(111) surface due to the adatoms adsorption, ΔΦad (in eV), and average d-band center of the occupied d-states for the Pt adatoms, εad d (in eV). For Pt(111), the results are obtained for the topmost surface layer.

while the lowest energy configurations are shown in Figure 2 and will be discussed below.

however, our atomistic understanding of those experiments is still incomplete, as discussed in the Introduction. Therefore, to obtain an improved understanding of the interaction of glycerol with defected Pt(111) substrates, we designed four defected Pt substrates employing a Pt(111)-(4 × 4) surface unit cell and three Pt coverages, namely, 1 , 6 , and

III. RESULTS AND DISCUSSION Defected Pt(111) Substrates. The lattice constant a0 = 3.97 Å (PBE) is in good agreement with the experimental result, 3.92 Å,48 i.e., larger by 1.27%, and due to the attractive nature of the vdW correction, a0 is reduced toward 3.92 Å employing PBE+D3, i.e., it yields the experimental result. Using

16 16

13 . 16

For each coverage, we selected several Pt-adatom

configurations, which are shown in the Supporting Information along with their relative total energies. Thus, on the basis of our total energy DFT-PBE calculations, we selected the following Pt substrates: flat Pt(111) surface for reference, single-adatom defect, Pt/Pt(111), six-adatom linear defect, PtL6 /Pt(111), sixadatom triangular island defect, PtT6 /Pt(111), and vacancy defect, Pt13/Pt(111), which are shown in Figure 1. Although one may think that single Pt adatoms are not stable enough to exist at experimental conditions, single Pt atoms anchored on oxide surfaces were reported as active sites in the oxidation of CO61 and water-gas shift (WGS) reaction.62 In the context of glycerol decomposition (C3H8O3 → 4H2 + 3CO), both reactions are important since CO, a well-known poisoning agent, is consumed to produce CO2. Thus, the selected substrates provide a wide range of adsorption Pt sites, i.e., from lower to higher coordination, which is essential to understand the role of defects on the adsorption properties of glycerol8,9 and further reactions.61,62 For 6 coverage substrates, we

4.0 × 2 a

0 = 11.09 Å for the (4 × 4) aPBE+D3 , we obtained a = 0 2.0 surface unit cell, and hence, the interaction between glycerol and its images is nearly negligible, i.e., less than 0.13 meV. To characterize the selected defected Pt(111) substrates (Figure 1) and the differences between the PBE and PBE+D3 results, we calculated the adsorption energy per adatom, EPt ad, the average effective coordination number (ECN) in number of nearest neighbors (NNN) of the adatoms,64,65 average weighted adatom bond lengths, dad av , substrate work function, Φ, work function change due to the adatoms adsorption, ΔΦad, and the average center of gravity of the occupied d-states for the adatoms, εad d . Furthermore, we calculated also the Bader charge for every atom in the slab. For the Pt(111) substrate, those properties were calculated for the topmost surface layer. All the results are shown in Figure 1 and summarized in Table 1. For 1 , the Pt adatom binds to Pt(111) with an adsorption

16

16

selected the two lowest energy configurations since both could be observed under experimental conditions. To identify the adsorption sites and glycerol orientation on the selected Pt substrates, we employed results from our previous calculations, which were based on first-principles molecular dynamic simulations.8,9,19 For example, a wide range of glycerol snapshots were selected and further optimized by the conjugate gradient (CG) algorithm as implemented in VASP, and hence, a wide range of glycerol configurations is available. Therefore, we selected the most important glycerol conformation and used them on the defected Pt substrates to build a wide range configurations considering different adsorption sites, e.g., on-top, fcc, hcp, and adatom sites, and glycerol orientations. Furthermore, we considered also glycerol adsorbed structures reported in previous studies for Gly/ TM(111).16,18,63 All the Gly/Ptn/Pt(111) configurations were optimized using the CG algorithm using the PBE functional, while the putative lowest energy PBE configurations were reoptimized using the PBE+D3 functional. All optimized configurations are shown in the Supporting Information,

energy of −4.52 eV in the fcc site, which is the expected site due to the stacking of the Pt layers along the [111] direction, i.e., ...ABCABC..., and in agreement with previous results,66 while the on-top and bridge sites are saddle points. For 6 , we 16

obtained a slight preference for the linear defect instead of the triangular island defect by 77 meV per unit cell (i.e., about 15 meV/adatom), which can be explained by attractive nature of the lateral forces felt by the Pt adatoms that connect the linear defect with its images. For 13 , the putative lowest energy 16

configuration is based on triangular hollow, which is 216 meV lower in energy than a three-Pt-atom linear defect. We found that the adsorption energy per adatom increases from −4.52 eV ( 1 ), −5.06 eV (linear defect, 6 ), and −5.43 eV ( 13 ), which 16

16

16

can be explained by an increasing in the coverage of the Pt adatoms (supported by our ECNad results) and by attractive nature of the lateral interactions; i.e., the lateral interactions contribute to increase the adsorption energy per adatom for increased coverage configurations. D


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For example, using PBE+D3, glycerol carbon chain binds closer to the surface when compared to the respective PBE result (Figure 2), which might play a role in the breaking of C−C and C−H bonding, an essential step for syngas production.4,5,57 This behavior can be attributed to the attractive nature of the vdW correction, which significantly enhances the attraction in weakly bound systems. The higher energy configurations in both clean and defected surfaces are characterized by the glycerol binding via the central anionic O atom or by the terminal one while oriented nearly orthogonal to the surface, and therefore the remaining atoms are farther away from the surface, which decreases their interaction with the Pt atoms. For particular higher energy isomers, glycerol binds entirely over the defects, which rises the system energy due to repulsive interaction caused by the slightly negative charge of Pt adatoms. Equilibrium Parameters. To characterize the Gly−Pt systems, we selected five geometric parameters as defined in Figure 3, and the results are summarized in Table 2. For all

For Pt(111), we obtained a tiny expansion for the topmost interlayer spacing by 0.89% (PBE) and 1.91% (PBE+D3), which is consistent with previous DFT calculations.19,50,66−69 As expected, the average bond length is shorter for a single Pt adatom (e.g., 2.57 Å) than the Pt−Pt distance in Pt(111) due to the lower adatom coordination, e.g., from ECNad = 3.74 NNN (PBE) to 3.67 NNN (PBE+D3). The adatom average bond length increases by increasing the adatom coverage, and dad av nearly reaches the Pt(111) limit at the highest Pt coverage (e.g., 13 ). For most substrates, the vdW correction decreases dad av 16 by about 0.01 Å, which is a negligible contribution. For Pt(111), the effective Bader charge is −0.05 e (anionic) for the topmost surface atoms, which is expected due to the charge flow toward the vacuum region, and the presence of the Pt adatoms slightly affects the Bader charge, e.g., few atoms turn slightly cationic; however, there is no clear trend on the sign of the effective Bader charges from our analysis. We obtained a decreasing in the substrate work function upon the Pt adsorption, which can be explained by the increased electron density corrugation,67,70,71 which is largest for a single Pt adatom. As expected, Φ increases toward the Pt(111) by increasing the coverage. From density of states analyze, the center of gravity of the occupied d-states is closer to the Fermi level for lower coordinated adatoms, while it shifts further away by increasing the coverage and, naturally, moves toward the Pt(111) result. Glycerol Adsorption on Defected Pt(111) Substrates. Lowest Energy Configurations. The lowest energy PBE and PBE+D3 structures obtained for glycerol adsorption on the Pt(111), Pt/Pt(111), PtL6 /Pt(111), PtT6 /Pt(111), and Pt13/ Pt(111) substrates are shown in Figure 2, while higher energy structures are reported in the Supporting Information. Glycerol has three C atoms, which form a frame with an angle of 113.03° (in gas phase) that does not change (significantly) upon adsorption; e.g., the CCC angle in Gly/ Pt(111) system is 116.65°. The high stability of the CCC angle can be explained by the tetrahedral sp3 hybridization of the C orbitals that binds to the C, O, and H atoms. Each C atom binds to a hydroxyl group, in which the H atoms are free to rotate around the anionic O atoms. Using both PBE and PBE +D3 functional, we obtained that glycerol binds via one of the hydroxyl groups to the Pt(111) surface, where the anionic O atom binds on the on-top Pt site with the O−H nearly parallel to the surface. Glycerol is nearly parallel to the surface, and hence, additional atoms are also closer to the surface, e.g., the hydrogen atom that binds to the middle C atom is closer to the Pt surface; however, the nearby hydroxyl group is not pointing toward the surface. The third hydroxyl group is pointing toward the surface, in which the H atom is closer to the surface than the anionic O atom. For all the defected Pt(111) substrates, glycerol binds also via the anionic O atom nearly to the on-top Pt site of the lowcoordinated sites; i.e., there is a general trend in the binding mechanism of glycerol to Pt substrates, which includes ideal and defected surfaces. Notice that glycerol binds with two hydroxyl groups (anionic O atom) to the substrates which have multiple low-coordinated sites as for the 6 coverage cases. 16 Although glycerol binds via the low-coordinated Pt atoms, we notice an energetic preference of at least 262 meV when the carbon chain atoms interact with the terrace region instead of the adatoms (see Figures S7 and S9). The vdW correction promoted substantial changes in the adsorbed configurations.

Figure 3. Geometric parameters employed to characterize the adsorption of glycerol on the Pt(111) substrates, namely, the shortest distances between the O, C, HO, and HC atoms to the Pt atoms, dO−Pt, dC−Pt, dHO−Pt, and dHC−Pt, respectively, and the relative orientation of the glycerol with respect to the surface, which is characterized by the angle between the surface normal and the line that interconnect the edge carbons atoms, αCC⊥.

substrates, the shortest PBE O−Pt distance, dO−Pt, ranges from 2.17 Å for Gly/Pt/Pt(111) up to 2.37 Å for Gly/Pt(111), which indicates a stronger binding to the low-coordinated sites. Based on the atomic radius of the O (0.60 Å) and Pt (1.35 Å) species,72 which sum up about 2.0 Å, there is a direct bonding between the anionic O and Pt atoms, which is not the case for the C−Pt bond; e.g., the C−Pt distance, dC−Pt, ranges from 3.17 to 4.25 Å. However, for the cases of HO−Pt and HC−Pt, the distances range from 2.02 to 3.33 Å and from 2.51 to 3.91 Å, respectively, indicating a strong interaction of the hydrogens with the substrates. The vdW correction affects the dO−Pt distance only slightly, i.e., the changes in the O−Pt distances are from 0.02 Å up to 0.04 Å; however, there are significant changes in the HC−Pt and HO−Pt distances, which can be explained by the closer approach of glycerol to the surface. Thus, we can conclude that the attractive nature of the vdW correction and the closer approach of glycerol to the substrates should both play a crucial role in the enhancement of the adsorption energy by the vdW correction, as shown in the next section. E


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The Journal of Physical Chemistry C Table 2. Structural Parameters for Glycerol Adsorption on Defected Pt(111) Substratesa PBE Gly/Pt(111) Gly/Pt/Pt(111) Gly/PtL6 /Pt(111) Gly/PtT6 /Pt(111) Gly/Pt13/Pt(111)

PBE+D3

dC−Pt

dO−Pt

dHC−Pt

dHO−Pt

αCC⊥

dC−Pt

dO−Pt

dHC−Pt

dHO−Pt

αCC⊥

3.17 3.99 3.21 3.23 4.25

2.37 2.17 2.31 2.28 2.19

2.02 3.33 3.12 2.69 3.15

3.91 2.51 2.62 2.64 3.17

78.49 82.38 88.21 86.25 81.61

3.07 3.23 3.13 3.26 3.96

2.33 2.20 2.29 2.24 2.17

1.91 2.22 2.66 2.24 2.87

3.64 2.85 2.63 2.67 2.94

81.03 75.24 77.55 79.70 82.88

a Equilibrium bond lengths, dC−Pt, dHC−Pt, dHO−Pt, and dO−Pt, in Å, and angle of an imaginary straight line between the terminal carbon atoms with respect to the surface normal, αCC⊥, in degrees.

Table 3. Adsorption Energy, Ead (in eV), Work Function Change Due to Glycerol Adsorption, ΔΦad (in eV), and the Effective Total Charge of the Adsorbed Glycerol Molecule, ΔQGly (in Elementary Charge) PBE Gly/Pt(111) Gly/Pt/Pt(111) Gly/PtL6 /Pt(111) Gly/PtT6 /Pt(111) Gly/Pt13/Pt(111)

PBE+D3

Ead

ΔΦ

ΔQGly

Ead

ΔΦ

ΔQGly

−0.44 −0.97 −0.85 −0.97 −0.79

−0.66 −0.28 −1.33 −0.67 −0.24

0.16 0.11 0.19 0.16 0.10

−1.36 −1.92 −2.08 −2.06 −1.85

−0.72 −0.94 −0.69 −0.56 −0.26

0.18 0.16 0.15 0.14 0.09

Adsorption Energy. To characterize the strength of the Gly−Pt interaction, which can help to identify the role of the low-coordinated Pt sites, we calculated the adsorption energy, Ead, using the equation Gly/Pt Pt Gly Ead = Etot − Etot − Etot

configurations (Figure 2). We found that this enhancement is dependent on the atomic configuration of the system, e.g., surface structure and the adsorption configuration of the glycerol, and ranges between 0.92 and 1.23 eV, for Gly/Pt(111) and Gly/PtL6 /Pt(111) structures, respectively. Our results are consistent with previous studies.69,73,74 Density of States. To investigate the role of the electronic states in the Gly−Pt interactions, we calculated the local density of states (LDOS) for the PBE and PBE+D3 lowest energy configurations, which are shown in Figure 4. We averaged the LDOS by species, namely, the C, O, and H atoms, while for Pt atoms the LDOS were averaged separately for atoms in the first layer, Ptfirst, and the adlayer, Ptad. The LDOS results for glycerol in gas phase, shown in the topmost panel of Figure 4, are discussed in the Supporting Information. From the second panel, which shows the differences among the LDOS between the clean and defected Pt(111) surfaces, we notice that the substrate with a single adatom presents, as expected, a larger number of occupied dstates at higher energies and, thus, closer to the Fermi level. As the number of adatoms in the surface increases, a higher number of d-states moves to lower energy levels, which shift the εd value further away from the Fermi level, as previously discussed. Concerning the adsorbed systems, it can be seen that glycerol states are shifted downward in energy with respect to the Fermi level by about −1.5 eV for Gly/Pt13/Pt(111) to 3.0 eV for the 6 Pt coverage due to the lower potential at the

(4)

where EGly/Pt is the total energy of glycerol on the Pt(111) tot substrates, EPt tot is the total energy of the Pt(111) substrates, and EGly tot is the total energy of glycerol in the gas phase. The PBE and PBE+D3 results are shown in Figure 2 and Table 3. We obtained an adsorption energy of −0.44 eV for Gly/ Pt(111) using PBE, i.e., a weak interaction, which is consistent with previous results, e.g., −0.46 eV using DFT-PW91,15,18 −0.46 eV using DFT-PBE,16 and −0.46 eV using DFT-PBE.19 The adsorption energy is substantially enhanced by the presence of low-coordinated Pt sites, e.g., −0.97 eV (PBE) for Gly/Pt/Pt(111), where the anionic O atom binds to a single Pt adatom, and it correlates well with the shortest PBE O−Pt distance, dO−Pt, for the respective system. As discussed below, the enhancement of the adsorption energy can be explained by the displacement of the center of gravity of the occupied dstates toward the Fermi level due to the low coordination of the Pt adatoms. Although glycerol binds stronger to the Pt/Pt(111) system, the Pt adatom remains at the fcc site upon glycerol adsorption, which can be explained by the larger magnitude of the Pt adatom adsorption energy compared with glycerol adsorption. For example, the Pt adatom adsorption energy on Pt(111) is −4.52 eV, while the adsorption energy of glycerol on the Pt adatom is only −0.97 eV. Hence, the glycerol−adatom interaction is not strong enough to affect the adsorption site preference of Pt on Pt(111). The enhancement of the adsorption energy is observed also for the remaining substrates, but not in the same magnitude. Furthermore, it can be seen that the adsorption energy decreases from −0.97 eV (1-fold) toward the −0.44 eV (9-fold) value by increasing the Pt adatom coverage. As expected from previous studies, the vdW correction increases the adsorption energy for our putative lowest energy

16

surface relative to the vacuum.75 We notice a broadening of the glycerol high-energy states upon adsorption which may imply delocalization of the O p-states caused by its coupling to the substrate states. A small depopulation of the O p-states is indicated by a tail that extends above the Fermi energy, which suggests a surface-induced polarization of the glycerol. Both PBE and PBE+D3 functionals yield almost the same LDOS for the gas phase glycerol and clean substrates, although slightly changing εd of the latter. However, for the adsorbed systems, we notice a larger broadening of the glycerol highenergy states; i.e., C, H, and O atoms increase the interaction F


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calculated the work function change upon glycerol adsorption, ΔΦ ΔΦ = ΦGly/Ptn /Pt(111) − ΦPt n /Pt(111)

(5)

where ΦGly/Ptn/Pt(111) and ΦPtn/Pt(111) are the work functions of the Gly/Ptn/Pt(111) systems and the clean Ptn/Pt(111) substrates, respectively. The results are shown in Table 3. The adsorption of glycerol on Pt substrates reduce the work function by 0.24 eV for Pt13/Pt(111) up to 1.33 eV for PtL6 / Pt(111), which in general indicates charge transfer to surface and/or dipole formation at the region between the surface and adsorbate. The vdW correction affects slightly the work function change for the clean surface, as we found previously;19,74 however, it does not hold for the defected surfaces, in which ΔΦ can change up to 0.64 eV due the structural changes induced by the vdW correction. To estimate the magnitude of the electron density redistribution among the chemical species, we performed the Bader charge analysis, which can be used to calculate the effective charge on every atom, namely, Qeff = Zval − QBader. Zval is the number of valence electrons (1, 4, 6, and 16 e for H, C, O, and Pt, respectively), and QBader is the Bader charge. Because of significant differences, we separate the hydrogens into two groups, the C-bound ones, HC, and the O-bound ones, HO. As expected, the effective charge on the glycerol atoms in gas phase is mostly determined by the electronegativity difference of its components, e.g., 2.55 for C, 3.44 for O, and 2.20 for H.78 Hence, the most electronegative element, O, has a mean effective charge of −1.11 e, mostly displaced from the C (QCeff = O

0.49 e) and the HO (QHeff = 0.60 e) atoms, whereas the HC have a mean effective charge of 0.02 e. Using Bader analysis, we obtained an effective charge transfer from glycerol to the substrate of 0.10 e to 0.19 e (PBE) and 0.09 e to 0.18 e (PBE+D3), which can explain the reduction of the work function upon the glycerol adsorption for all Pt substrates. To provide further insights, we plot the work function change versus the magnitude of the charge transferred from glycerol to the Pt substrates (Figure 5). Our results show a clear correlation between both physical parameters, which indicates that the work function decreases almost linearly with the increase of charge transferred from glycerol to the surface.

Figure 4. Local density of states (LDOS) for glycerol in gas-phase, clean, and defected Pt(111) substrates and for glycerol adsorption on Pt substrates. For the C, O, and H atoms, we performed an average of the LDOS over all chemical species, while we separated the Pt LDOS into two groups, namely, the Pt adatoms LDOS and the LDOS of the Pt atoms located in the topmost surface layer. The Fermi energy for all systems are indicated by the vertical dashed lines.

with the surface, once the vdW correction is included, which is expected as the vdW correction contributes to move the molecule closer to the surface (Table 2). The d-band model suggests that the d-band center of gravity, εd, correlates with the magnitude of the adsorption energy of molecules on TM surfaces.76,77 To check the validity of the dband model for weak interacting systems such as glycerol adsorption on flat and defected Pt(111) substrates, we plotted the adsorption energy versus the center of gravity of the Pt substrates (Figure S10). Our results do not show a clear linear relation between these properties, which suggest that Gly−Pt interaction is not driven by a strong hybridization among molecule and substrate states. Work Function and Charge Transfer. To better understand the electron density rearrangements in the Gly−Pt systems, we

Figure 5. Work function change, ΔΦ, versus charge transfer obtained by Bader analysis from glycerol to the substrate upon glycerol adsorption on Pt(111) substrates. The PBE and PBE+D3 results are in black and red, respectively. G


Article

The Journal of Physical Chemistry C When comparing the charge distribution of glycerol in gas phase with the adsorbed state, we notice a negligible change in the effective charges of O and HO atoms, while C atoms gain up C to 0.08 e and QH decreases by 0.02 e up to 0.04 e. Thus, we can conclude that the charge transferred to the substrate is mostly due the HC atoms. To obtain further understanding, we calculated separately the effective charge for the platinum atoms that directly interacts with glycerol oxygen, PtO, which were found to be positively charged (from 0.09 e to 0.15 e), while the remaining Pt atoms of the surface retain an anionic charge. This indicates that the anionic O atom locally induces a polarization in the substrates surface and forms an interaction of permanentinduced dipole type, which might play an even more important role in the 6 coverage systems that present two polarized Pt 16 sites.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Ph +55 (16) 3373 6641; Fax +55 (16) 3373 9952 (J.L.F.D.S.). ORCID

Juarez L. F. Da Silva: 0000-0003-0645-8760 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. The authors acknowledge the National Laboratory for Scientific Computing (LNCC/MCTI, Brazil) for providing HPC resources of the SDumont supercomputer, which have contributed to the research results reported within this paper. Authors thank also the Laboratory of Advanced Scientific Computing (University of São Paulo) and the Department of Information Technology - Campus São Carlos, for hosting our cluster.

IV. SUMMARY We investigated, by means of DFT calculations employing PBE functional with D3 dispersion correction,31 the adsorption properties of glycerol on defected Pt(111) surfaces, namely, ideal clean Pt(111) surface for reference, single-adatom defect, Pt/Pt(111), six-adatom linear defect, PtL6 /Pt(111), six-adatom triangular island defect, PtT6 /Pt(111), and vacancy defect, Pt13/ Pt(111). We found that the energetic, structural, and electronic properties of the flat and defected Pt(111) substrates can be rationalized in terms of surface coverage; i.e., these properties converge to the Pt(111) results at the limit of full coverage. We found that the arrangement of low-coordinated Pt sites on Pt(111) surface plays a major role on the glycerol adsorption properties. For example, glycerol was found to bind stronger on low-coordinated single-adatom and sixadatom triangular defects, as shown by the higher adsorption energy and shorter O−Pt distance. This correlates with the position of the d-band center of gravity toward the Fermi level as the coordination of the adatom decreases. Nevertheless, the addition of vdW correction changes this trend and yields the 6 16 coverage substrates, six-adatom linear and triangle defects, as the most strong interactions, since glycerol binds by two hydroxyls on these cases, while one-adatom defect is by 0.16 eV higher in energy. This finding can be related to the coordination number concept applied to find the dispersion coefficients in D3 correction. Besides that, PBE+D3 significantly improves glycerol adsorption energy and decreases its distance from the substrates when compared to PBE results. Results show that there is a reduction of the work function upon the adsorption of the glycerol on the surface, which seems to correlate almost linearly with the charge transferred from glycerol to the substrates, although polarization effects might also contribute to decrease the work function. Our Bader analysis indicates that glycerol loses charge through the Cbound hydrogens, while a very small rearrangement of charge was found for the remaining molecule atoms. As the anionic oxygen of glycerol binds to the surface, the adsorption site becomes positively charged, suggesting that permanent-induced dipole forces might play an important role in the glycerol− substrate interaction mechanism.

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Extra results for the geometric and electronic properties (PDF)

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REFERENCES

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b12238. H


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