Adsorption of Monocyclic Aromatics on Transition Metal Surfaces

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Adsorption of Monocyclic Aromatics on Transition Metal Surfaces: Insight into Variation of Binding Strength from First-Principles Xin Jia, and Wei An J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06321 • Publication Date (Web): 31 Aug 2018 Downloaded from http://pubs.acs.org on August 31, 2018

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The Journal of Physical Chemistry

Adsorption of Monocyclic Aromatics on Transition Metal Surfaces: Insight into Variation of Binding Strength from First-Principles Xin Jia, Wei An* College of Chemistry and Chemical Engineering, Shanghai University of Engineering Science, Songjiang District, Shanghai 201620, China *E-mail: [email protected] ABSTRACT: Understanding the adsorption of monocyclic aromatics on transition metal surfaces is of great interest to both fundamental and applied research. Herein, using density functional theory, we report a systematic study on the binding mechanism of four monocyclic aromatic compounds (benzene, toluene, phenol, m-cresol) on 3d metal surfaces [Fe (110), Co (111), Ni (111),Cu(111)], 4d and 5d noble metal surfaces [Ru (0001), Rh (111), Pd (111), Pt (111)]. Our results show that van der Waals (vdW) corrections to the calculated adsorption energies can be remarkably sensitive to the relative molecular polarizability of aromatics and the calculated adsorption energies using optB88-vdW functional agree well with the experimental results. The role of functional groups at phenyl ring is less significant in enhancing adsorption strength compared to phenyl ring itself which contributes most to the electronic interactions with the surface metal atoms. We have analyzed the origin of both electronic and geometric effects on the variation of binding strength of monocyclic aromatics adsorbed on metal surfaces. By incorporating the coupling of five states of gas-phase benzene to the d-states of metals, our model-predicted adsorption energies agree reasonably well with the calculated results using GGA-PBE functional. Simulated scanning tunneling microscopy images have provided the atom-resolved aromatics/metal surface morphology and the visual support for differentiating σ- and π-type bindings.

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1. INTRODUCTION The adsorption of aromatics on transition metal surfaces is the starting point to a wide range of applications including the tuning of a self-assembled monolayer,1-2 organic-based devices (e.g., organic light-emitting diodes, organic photovoltaic cells and organic field effect transistors)3-5 and catalytic upgrading of lignin-derived bio-oils (e.g., hydrodeoxygenation of phenolics).6-9 Over the years, tremendous effort has been aimed at fundamental understanding of the interaction mechanism of atomic species and small molecules on metal surfaces.10-13 In particular, the conceptual geometric (ensemble) effects and electronic (ligand) effects are evoked to gain mechanistic insights into the observed catalytic reactivity furthering the rational catalyst design. At the molecular scale, the former effects refer to the variations in the catalytic properties of an ensemble of atoms in the surface as the number and configuration of the ensemble sites change, while the latter effects describe those as the local electronic structure of the surface ensemble sites changes. For tuning the two effects, multiple techniques have been employed such as alloying,14-17 formation of strained overlayers,18-19 and core-shell nanostructures20-22 to enhance the catalytic performance in various reactions. However, it is difficult to fully separate the two effects due to their intertwined nature of one affecting the other. To address this issue, several approaches have been adopted from theoretical perspective. Liu and Nørskov23 distinguished the geometric and electronic effects that determine the adsorption properties of Au/Pd (111) bimetallic alloy surfaces by varying the Au content in the surface. As such, the electronic effect on the adsorption of CO, O, N can be well explained by the change of d-band center of surface Pd, while the geometric effect was described by a simple linear interpolation model in which the adsorption energy at a mixed site is the appropriate average of the properties of the constituents. They found that the contribution of the ligand effects to the adsorption energy is considerably less than that of the ensemble effects for Au/Pd (111) bimetallic alloy surfaces Alternatively, Mavrikakis et al.,24 introduced artificial compressive/tensile stress into Ru lattice, and Kitchin et al.,25 constructed a monolayer of one metal (Ni, Pd, or Pt) deposited on the surface of other metals forming a bimetallic surface.25 They showed that the broadening/narrowing of the width of the surface d band result in the lowering/raising of d-band center, which consequently decrease/increase the surface reactivity towards dissociative adsorption of CO24 or H225.


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Several small molecules, small unsaturated fragments and atomic species have been commonly employed to theoretically probe a surface property.24-32 For example, the trend of CO adsorption strength on a series of metal surfaces [including Ni(111), Cu(111), Ru(0001), Pd(111), Ag(111), Pt(111), Au(111), Cu3Pt(111)] can be understood using a simple two-level model describing the coupling of the CO 5σ and 2π* states to the metal d valence states.29 The adsorption strength of CO2 is controlled by the d-band center of the metal surfaces and also affected by the charge transfer from the metal surfaces to the chemisorbed CO2.30 For adsorption of small unsaturated fragments such as CHx (x = 0, 1, 2, 3), NHx (x = 0, 1, 2), OHx (x = 0, 1), and SHx (x = 0, 1) species on a range of close-packed and stepped transition-metal surfaces including fcc(111), fcc(100), hcp(0001), and bcc(110), and the stepped fcc(211) and bcc(210), the adsorption energy of any of the molecular species considered scales approximately with the adsorption energy of the central C, N, O, or S atom with the scaling constant depending only on the value of x.33 Methoxy (CH3O) adsorption on eight metal surfaces were also reported including group IB metals of Au(111), Ag(111), and Cu(111) with filled d electrons and group VIII metals of Pt(111), Pd(111), Ni(111), Rh(111), and Fe(100) with unfilled but more than half-filled d electrons.34 The results showed that the binding energies were linearly correlated to the d-band center for VIII metals and to the size of coupling matrix element of adsorbate sp-states to metal d-states for IB metals. One legitimate question can be: are these findings still applied to the large closed-shell molecules such as monocyclic aromatics? In fact, the π-conjugated aromatic molecules, particularly benzene,35-50 and even mono- and polysubstituted aromatic oxygenates of phenolics (e.g., phenol, anisole, guaiacol)51-54 or halogenated benzene,55 have also been employed to study the important role of aromaticity and van der Waals (vdW) interactions in the binding mechanism on metal surfaces including weakly bound systems (e.g., Cu, Ag and Au) and strongly bound systems (e.g., Rh, Pd, Ir and Pt); however, their binding strengths calculated using GGA-type functionals were severely underestimated with respect to the experimentally measured values. This leads to the employment of the nonlocal van der Waals correlation functionals44, 50, 54-56 (e.g., PBE+vdWsurf, optB88-vdW, optB86b-vdW, vdWDF, vdW-DF2, BEEF-vdW, PBE-dDsC) or other schemes (e.g., random phase approximation)48 for describing the contributions of vdW-dispersion interactions in chemisorbed systems. In this work, using density functional theory (DFT) methods, we investigate the adsorption of four monocyclic aromatic molecules of benzene, toluene, phenol and m-cresol (Figure 1) on eight ACS Paragon Plus Environment


transition metal surfaces including 3d metals [Fe (110), Co (111), Ni (111),Cu(111)], 4d and 5d noble metals [Ru (0001), Rh (111), Pd (111), Pt (111)] focusing on understanding the binding mechanism. In particular, the roles of the vdW-dispersion and the different functional groups at phenyl ring were examined by employing three vdW-corrected functionals (DFT-D3, optB86b-vdW and optB88-vdW) in computations and by simulating scanning tunneling microscopy (STM). Several descriptors were evoked for describing the electronic effects including d-band center (εd), d-band width (Wd), coupling matrix element (Vsd2) of adsorbate sp-states to metal d-states, transferred charge (∆q), and work function (W) and for describing the geometric effects including geometry match (r1/R1 and r2/R2), distortion energy ( ) and interaction energy ( ).

2. COMPUTATIONAL METHODS All calculations were performed using the spin-polarized periodic DFT as implemented in the Vienna ab-initio simulation package (VASP).56-57 GGA-PBE functional58 and three vdW-inclusive techniques, namely, DFT-D3,59 optB86b-vdW and optB88-vdW60-61 were employed for differentiating the contributions of dispersion interactions. The projector augmented wave (PAW) method62-63 for describing the core−valence interactions and a plane-wave basis set with a kinetic energy cutoff of 400 eV were used. The four-layer close-packed p(4×4) slabs of Fe (110), Co (111), Ni (111), Cu (111), Ru (0001), Rh (111), Pd (111), Pt (111) with a 14 Å of vacuum region in z-direction were used to accommodate the monocyclic aromatic molecules of benzene, toluene, phenol and m-cresol, corresponding to 0.375ML of coverage in terms of six C atoms of phenyl ring. The size of chosen supercell is sufficient to avoid the interactions between the imaging cells. The adsorbate and the top two-layer metal atoms were allowed to relax while the bottom two layers were held fixed at their bulk positions (Lattice constant: Fe: 2.831 Å, Co: 3.544 Å, Ni: 3.524 Å, Cu: 3.615 Å for 3d–metals, Ru: 2.706 Å, Rh: 3.804 Å, Pd: 3.891 Å for 4d–metals, and Pt: 3.924 Å for 5d– metal). The Brillouin zone in reciprocal space was sampled by Γ-centered Monkhorst−Pack scheme64 with 3×3×1 and 5×5×1 k-points grids for geometry optimization and electronic structure calculations, respectively. The conjugate gradient algorithm was used in ionic optimization, and the convergence threshold was set to 10-4 eV in electronic relaxation and 0.02 eV/Å in Hellmann-Feynman force on each atom. The 1st order Methfessel-Paxton smearing of kBT = 0.1 eV was employed to speed up the


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convergence. All reported energies are extrapolated to kBT = 0 eV. The convergence tests using 10-7 eV show that the calculated adsorption energies of benzene are nearly identical with those using 10-4 eV (Table S1). The adsorption energy of monocyclic aromatic molecules on metal surfaces is defined as the binding energy between the optimized systems:

Eads = E( ads / slab ) − E( slab ) − E( gas − phase )

(1)

Where E(ads/slab), E(slab), and E(gas-phase) are the total energies of adsorbate and slab, clean slab, and gas-phase adsorbate, respectively. Negative values imply favorable adsorption. The distortion energy is defined as the difference between the energy of the isolated aromatic in its optimized gas-phase geometry (or clean slab) and the energy of the isolated aromatic distorted as in the adsorption state (or adsorbed slab): distorted geometry

Edist ( gas) = E molecule

distorted geometry

Edist ( slab) = E slab

gas phase

− E molecule

clean

− E slab

(2)

The interaction energy (Eint) is defined as the binding energy between the distorted systems: ∗ ∗ + Eslab ) Eint = E( ads / slab ) − ( Eads

(3)

Where * denotes the distorted geometry due to adsorption, with which the single-point energy calculation is performed to obtain E*. Negative values imply favorable interaction. The d-band center (εd) is defined as the average energy of entire d-band: ∞

εd

∫ = ∫

−∞ ∞

ρ EdE

−∞

(4)

ρ dE

The d-band width is defined as the average energy of entire d-band:

wd =

∫

∞

ρ E 2 dE

−∞ ∞

∫

−∞

(5)

ρ dE

Where ρ(E) is the density of electronic states at the given energy of E. The work function of a given metal surface is defined as: W=Evac − EF

(6)

Where Evac is the average electrostatic potential energy at the center of the vacuum region, and EF is the Fermi energy of the system.



3. RESULTS AND DISCUSSION 3.1. van der Waals Interactions (A)

(B)

Figure 1. (A) Schematic representation of the structure of (a) benzene, (b) toluene, (c) phenol and (d) m-cresol. The C atoms of phenyl ring are labeled for the reference of STM images shown in section 3.6. and Figure S6. (B) Optimized structures of m-cresol adsorbed on metal surfaces. Upper panel: side view; Lower panel: top view. Using benzene (C6H6), toluene [C6H5(CH3)], phenol [C6H5(OH)] and m-cresol [C6H5(OH)(CH3)] as probe molecules (Figure 1A), we evaluated the contribution of vdW interactions to their calculated


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adsorption energy on a variety of close-packed metal surfaces including 3d metals [Fe (110), Co (111), Ni (111)], 4d and 5d noble metals [Ru (0001), Rh (111), Pd (111), Pt (111)], and coinage metal [Cu(111)]. The monocyclic aromatic compounds adsorb on metal surfaces most likely across the two threefold hollow sites (fcc and hcp) in a bridge position (bri30)65 with the phenyl ring parallel to rather than perpendicular to the surface (Figure 1B). In particular, methyl (-CH3) is at σ-position, while hydroxyl (-OH) is at π-position, following the nomenclature of Réocreux et al.51 Such a adsorption configuration has been observed to be most stable for benzene and phenolic compounds (e.g., guaiacol, anisole, cresol and phenol) on close-packed metal surfaces including Ni(111),7, 9, 52 Fe(110),9, 66-67 Fe(100),41 Pt(111),49, 51, 54, 65, 67-71 Pd(111),66, 69-70 Rh(111),72 Ru(0001).67, 73 Thereafter, all calculated results in this study are based on such a bri30 adsorption configuration, providing an additional benefit of eliminating the influence of alternative adsorption configurations on varied metal surfaces.39, 74-75 It is known that vdW interactions are ubiquitous in nature and DFT-calculated adsorption energy (Eads) using GGA-type functionals without including the vdW-corrections is severely underestimated with respect to the experimental measurements.44, 76-77 Our results show that the calculated Eads of benzene using varied functionals are increased as optB86b-vdW > DFT-D3 > optB88-vdW > PBE on all metal surfaces studied (Figure 2a and Table S1). This enhanced binding strength can be regarded as the contribution from vdW interactions which are accounted for by vdw-inclusive functionals. Note that there is ∼0.1eV of fluctuation that can be observed from using optB86b-vdW and optB88-vdW, which actually belong to the ‘opt’ functionals family where the exchange functionals were optimized for the correlation part. The similar trend of calculated Eads of toluene, phenol, and m-cresol were also observed on 3d-, 4d- and 5d-metal surfaces studied [except Cu (111)] using optB86b-vdW, DFT-D3, optB88-vdW with respect to PBE (Figure S1, Table S2, Table S3), indicating vdW interactions of monocyclic aromatic compounds with metal surfaces are dominated primarily by phenyl ring. It appears that optB86b-vdW and DFT-D3 results tend to overestimate vdW-dispersion interactions while optB88-vdW results however quantitatively agree well with experimental adsorption energies (Table S1). Similar findings including the deviation of coinage metal surfaces were also reported by Yildirim et al.,40 Gautier et al.,50 Carrasco et al.,37 and Liu et al.35 Thereafter, we use only optB88-vdW results (labelled as ) for representing the

DFT-calculated adsorption energies with vdW-interactions included in the follow-up sections unless ACS Paragon Plus Environment


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specified.

(a)

(b)

Figure 2. (a) DFT-calculated adsorption energies (Eads) of benzene on metal surfaces using different functionals. Those of phenolics are shown in Figure S2. (b) Linear correlation of vdW-dispersion corrections (∆) and relative molecular polarizability (∆α) of monocyclic aromatics, where ∆E =

− , and ∆α = αaromatics − αbenzene, where αaromatics and αbenzene correspond to experimental

polarizability78 of toluene (-CH3), phenol (-OH), m-cresol [(-CH3)+(-OH)] and benzene (-H).

3.2. Role of functional groups of phenyl ring in adsorption Table 1. Comparison of adsorption energy ( ) for toluene, phenol and m-cresol on various metal surfaces using benzene as a reference. Δ (eV)a

(eV)

Fe (110) Co (111) Ni (111) Cu (111) Ru (0001) Rh (111) Pd (111) Pt (111)

benzene (-H)

toluene (-CH3)

phenol (-OH)

m-cresol/sumb [(-CH3)+(-OH)]

-1.95 -1.49 -1.66 (-1.79)c -0.77 (-0.68)d -2.28 -2.26 (-2.27)d -2.12 (-1.91)d -1.80 (-1.84)d

-0.09 -0.05 -0.06 -0.14 -0.01 -0.04 -0.06 -0.11

-0.01 0.01 0.01e -0.12 -0.10 -0.07 -0.02 -0.07e

-0.10/-0.10 -0.03/-0.04 -0.06/-0.05 -0.26/-0.26 -0.11/-0.11 -0.13/-0.11 -0.09/-0.08 -0.18/-0.18

a

= (aromatics) − (benzene), where aromatics = toluene, phenol, m-cresol having functional group of CH3, OH, and CH3+OH at phenyl ring, respectively.


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b

Summation of contributions from CH3 + OH using Δ (toluene) and Δ (phenol).

c,d

Data in parenthesis were retrieved from ref.35,

37

Experimental Eads = -2.16eV on

Ni(111) and -2.60 eV on Pt(111).48 e Experimental Eads = -1.82 eV on Ni(111) and -1.81 eV on Pt(111).55 In Table 1, one can see that the calculated adsorption energies ( ) of benzene on metal

surfaces increase approximately in the order of coinage metal < 3d metals < 4d and 5d noble metals, i.e., -0.77 eV/Cu (111), -1.95 eV/Fe (110), -1.49 eV/Co (111), -1.66 eV/Ni (111), -2.28 eV/Ru (0001), -2.26 eV/Rh (111), -2.12 eV/Pd (111), -1.80 eV/Pt (111), which are in good agreement with the previous results using also optB88-vdW functional.37 The small functional groups of OH (phenol), CH3 (toluene), and CH3 + OH (m-cresol) have measurable impact on calculated vdW-corrections (∆E), in the order of -H (benzene) < -OH (phenol) < -CH3 (toluene) < -CH3 + -OH (m-cresol). This can be explained by the increase in their relative molecular polarizability of ∆α (i.e., benzene: 0.0 a.u, phenol: 9.9 a.u, toluene: 15.2 a.u, m-cresol: 25.1 a.u).78 As shown in Figure 2b and Figure S1, a linear correlation of vdW-dispersion corrections (∆E) and relative molecular polarizability (∆α) of monocyclic aromatics can be observed regardless of metal surfaces, where ∆E is increased as Cu (111) < Pd (111) < Ru (0001) < Ni (111) < Fe (110) < Rh (111) < Pt (111) < Co (111) for any of the adsorbates (i.e., benzene, phenol, toluene, and m-cresol). The similar linear correlation51 was also obtained for vdW-dispersion energy as a function of molecular weight of aromatic compounds. The underlying origin is that the polarizability is proportional to the molecular weight according to the Clausius−Mossotti relation. Nonetheless, the role of functional groups at phenyl ring is less significant in enhancing adsorption strength compared to phenyl ring itself, i.e., -0.14 eV from methyl (toluene), -0.12 eV from hydroxyl (phenol), and -0.26 eV from methyl and hydroxyl (m-cresol) with respect to H (benzene). That is to say, the effect of small-size functional groups on vdW-corrections is negligible for those chemisorbed aromatics. This is evidenced by the similarity of three-dimensional isosurfaces of charge density difference (∆ρ) for adsorbed benzene, toluene, phenol and m-cresol (Figure 3), where the major electronic interactions occur between phenyl ring and surface metal atoms [e.g., Ni (111) and Pt(111)]. Our assessment was supported by recent calorimetry study on benzene and phenol adsorbed on Ni (111) and Pt(111) that exhibits almost identical adsorption behavior.48, 55



(A)

(B)

(C)

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(D)

Figure 3. Isosurface of charge density difference (∆ρ) for adsorbed (A) benzene, (B) toluene, (C) phenol and (D) m-cresol on Pt (111) and Ni (111) surfaces. Both top and side views are displayed. The isosurface level was set at 0.004 e/bohr3. Yellow: charge accumulation; Cyan: charge depletion.. The charge density difference is defined as ∆ρ = ρ ( slab + ads ) − ρ ( slab ) − ρ ( ads ) . 3.3. d-band center (εd) and coupling matrix element (Vsd2)

(a)

(b)


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(c)

(d)

Figure 4. (a) Linear correlation of d-band width (Wd) and d-band center (εd), (b) Linear correlation of adsorption energy ( ) and d-band center (εd), (c) top and (d) side view of plane fitting of −

(εd,Vsd2) for adsorbed benzene on metal surfaces, where Vsd2 is coupling matrix element of adsorbate sp-states to metal d-states. Those of other aromatics (toluene, phenol, m-cresol) are shown in Figure S3. Table 2. PBE-calculated d-band center (εd), adsorption energy ( ), and other parameters that are used for model predictions. All energies are in eV.

Metal

εd

f

Ed-hyb

Erepl

Fe (110) -0.90(-0.92)a 0.70 -1.76 3.54 Co (111) -1.24 (-1.17) 0.80 -1.00 3.18 Ni (111) -1.31(-1.29) 0.90 -0.93 2.93 Cu (111) -2.38(-2.67) 1.00 -0.35 2.67 Ru (0001) -1.45(-1.41) 0.70 -2.51 4.06 Rh (111) -1.72(-1.73) 0.80 -1.85 3.71 Pd (111) -1.77(-1.83) 0.90 -1.42 3.33 Pt (111) -2.21(-2.25) 0.90 -1.58 3.93 a Data in parenthesis are retrieved from reference.11 b See S1 in Supporting Information. c

Ed

Espb

Emodel

Vsd2c

1.78 2.18 2.00 2.32 1.55 1.87 1.88 2.35

-2.90 -2.90 -2.90 -2.90 -3.10 -3.10 -3.10 -3.30

-1.12 -0.72 -0.90 -0.58 -1.55 -1.23 -1.22 -0.95

-1.17 -0.64 -0.88 -0.08 -1.53 -1.45 -1.25 -0.96

1.59 1.34 1.16 1.00 3.87 3.32 2.78 3.90

The coupling matrix elements of adsorbate sp-states to metal d-states (Vsd2) [normalized to 1.0 for

Cu (111)] were retrieved from reference.11 It has been well established that d-band center (εd) of metal surfaces

14, 79-81

can be used as a

good descriptor for surface reactivity in terms of adsorption strength of adsorbates. As shown in



Figure 4a, for d-band of constant filling, as the d-band width (Wd) is changed, the average energy of the d band characterized by εd must change accordingly in order to conserve both the d-band filling and the total number of d states, leading to the linear correlation between the d-band width and the d-band center, i.e., as the d band becomes wider/narrower, εd moves down/up in energy. Our results agree well with those of previous studies.11, 14-15, 25-26 Here, we focus on elucidating the underlying binding mechanism of monocyclic aromatics on transition metal surfaces as a result of the electronic effect. As can be seen from Figure 4b, there exist two linear relations between adsorption energy ( ) and d-band center (εd) of surface metal atoms, i.e., one for 3d metals [Fe (110), Co (111), Ni

(111), Cu (111)], and the other for 4d and 5d noble metals [Ru (0001), Rh (111), Pd (111), Pt (111)]. Moreover, the two independent linear trends both follow the prediction of d-band model regarding the surface reactivity, in which as the d-band center shifts down relative to Fermi level, the adsorption energy reduces regardless of the d-band filled or not. However, the additional factors besides d-band center dictate that the line of 3d metals is on top of 4d and 5d noble metals for adsorbed monocyclic aromatics. In details, the adsorption energy of an adsorbate on metal surface can be divided into two parts electronically: (i) the coupling of adsorbate sp-states to metal surface s-states (in terms of Esp, Table 2), which usually contributes to the largest part of the binding strength and involves considerable hybridization and charge transfer,33 and (ii) the coupling of adsorbate sp-states to metal surface d-states (Ed), which usually contributes less to the binding strength and differentiates the chemical reactivity of metal surfaces.79-80 The former coupling is essentially little different for transition and noble metals and leads to the down-shift and broadening (i.e., energy resonance) of the sp-states of binding C atoms of phenyl ring (Figure 5), while the latter coupling essentially determines the variation or the trend of chemisorption energy due to the discrepancy of detailed electronic structure of metal surfaces, which can be characterized mainly by d-band center (Figure 4a). In principle, the coupling to the d-band (in terms of Ed, Table 2) can be further divided into two parts (i) Covalent attraction or hybridization energy (Ed-hyb) owing to the stabilization of the sp-d orbital hybridization and (ii) Pauli repulsion energy (Erepl)27,

29

arising from the energy cost of the sp-d orbital

orthogonalization, where Ed-hyb is proportional to V2/|∆ε| (V2: coupling matrix element proportional to Vsd2, which is coupling matrix element of adsorbate sp-states to metal d-states, ∆ε: energy difference between adsorbate sp-states and metal surface d-states), and Erepl scales with SV (S: overlap integral ACS Paragon Plus Environment

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roughly proportional to V2) (see Supporting Information for details). Accordingly, as the overlap becomes stronger, both repulsion and hybridization become larger. For ingredients that determine the magnitude of Ed, there exist two categories of Vsd2 for 3d metals and 4d-and-5d noble metals, respectively (Table 2). That is, Vsd2 is 1.00/Cu (111), 1.16/Ni (111), 1.34/Co (111), 1.59/Fe (110), against 3.90/Pt (111), 2.78/Pd (111), 3.32/Rh (111), 3.87/Ru (0001), resulting in the enhancement of both repulsion (Erepl) and hybridization (Ed-hyb) from d electrons of 4d-and 5d- noble metals generally larger than those from 3d metals, which ends up with a positive Ed (Eq. 7) on both metals with Ed (3d) > Ed (4d-and-5d) (Table 2). This explains the line of 3d metals sits on top of 4d and 5d noble metals (i.e., weakening of the binding strength) for the adsorption of four monocyclic aromatics at similar εd values (Figure 4b). Accordingly, adsorption energy should be described more accurately in terms of both the coupling matrix element Vsd2 and the surface d-band center (εd),31 as shown in Figure 4c and 4d. We note here that the discrepancy of the vdW-dispersion corrections (∆) using optB88-vdW functional is ∼ 0.1eV for the same adsorbate on the varied metal surfaces with respect to PBE-calculated results (Figure 2b, Table S3), making the trend of using (optB88-vdW

results) invariant with those of results for interpreting binding mechanism (Figure S4).



Figure 5. Projected density of states (PDOS) for sp2-orbitals of C atoms of benzene (BZ) in gas-phase and in adsorbed states. Dashed line: Fermi level.


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(a)

(b)

Figure 6. Model predictions for adsorption energy (Emodel) of benzene on TM surfaces (3d, 4d, 5d)

against DFT-calculated results ( and ). (a) Five-states coupling, (b) HOMO-LUMO

coupling. Data of Cu(111) was not included in linear regression. We take a further step by modeling adsorption energy (Emodel) of benzene on TM surfaces (3d,

4d, 5d) to compare with DFT-calculated results of and which represent adsorption

energy without and with vdW-corrections included, respectively. Unlike modeling adsorption of small molecules, small unsaturated fragments and atomic species such as CO,29 OH,28 O,26 monocyclic aromatics interact with the metal surface through phenyl ring in a much complex manner. Based on the previous models,26, 28-29 we consider the coupling of five states of gas-phase benzene36, 38, 41

namely, e2u[π*(C-C)], 1e1g[π(C-C)], 1a2u[π(C-C)], 2e1u[σ(C-H)], 2a1g[σ(C-H)], to the d-states of

metal surfaces (Figure 5 and Figure S5a). These five states are selected based on the photoemission spectra that 1e1g[π(C-C)], 1a2u[π(C-C)], 2e1u[σ(C-H)], and 2a1g[σ(C-H)] orbitals of adsorbed benzene on Al(111) are dominant while 1b2u[σ(C-C)], 1b1u[σ(C-H)], and 1e2g[σ(C-C)] orbitals are heavily suppressed.82 Despite that the orbitals of benzene may contribute to a varied degree to the binding states of adsorbate, the five states chosen are predicted to contribute significantly to the binding energy of benzene.38, 41 The equations used for predicting adsorption energy (Emodel) are expressed as: Emodel = Esp + Ed = Esp + (Ed-hyb + Erepl)


(7)


Page 16 of 29

      Vπ 2 Vπ 2 Vπ 2 Ed ≈ −4  f + fSπ Vπ  − 4 (1 − f ) + (1 + f ) Sπ Vπ  − 4 (1 − f ) + (1 + f )Sπ Vπ  ε d − ε 2u ε d − ε1g  ε d − ε 2u          Vσ 2 Vσ 2 −2 (1 − f ) + (1 + f ) Sσ Vσ  − 2 (1 − f ) + (1 + f ) Sσ Vσ  ε d − ε1u ε d − ε 1g    

(8) The first term in each bracket describes covalent attraction or hybridization energy (Ed-hyb) owing to the stabilization of the sp-d orbital hybridization, where the sp orbitals correspond to e2u(π*), 1e1g(π), 1a2u(π), 2e1u(σ), 2a1g(σ) in a sequence of lowering energy relative to Fermi level (Figure S5b), while the second term in each bracket describes the Pauli repulsion (Erepl) arising from the energy cost of the sp-d orbital orthogonalization. Accordingly, we obtain:   Vπ 2  Vπ 2 E d − hyb ≈ − 4  f  − 4  (1 − f ) ε d − ε 1g   ε d − ε 2u 

    Vπ 2  Vσ 2  Vσ 2  − 4  (1 − f )  − 2  (1 − f )  − 2  (1 − f ) ε d − ε 2u  ε d − ε 1u  ε d − ε 1g    

  

(9)

Erepl ≈−4[ fSπVπ ] −4[ (1+ f )SπVπ ] − 4[(1+ f )SπVπ ] −2[ (1+ f )SσVσ ] − 2[ (1+ f )SσVσ ] (10) Where we use ε2u = +2.19 eV, ε1g = -3.42 eV, ε2u = -5.01 eV, ε1u = -6.72 eV and ε1g = -9.33 eV (relative to Fermi level) of the adsorbed states on Al (111) for the renormalized orbitals of e2u(π*), 1e1g(π), 1a2u(π), 2e1u(σ), 2a1g(σ), respectively (Figure S5b). Note that this approach can mimic the interaction of benzene with the sp-band of transition metal surfaces owing to no d-electrons that Al has.28-29 See more details in Supporting Information. As shown in Figure 6(a), our model-predicted adsorption energies agree reasonably well with PBE-calculated adsorption energies resulting in a nearly linear diagonal line with ∼0.18eV of underestimation,

which

is

however

greatly

underestimated

by

∼0.88eV

relative

to

optB88-vdW-calculated adsorption energies, suggesting the vdW-dispersion contributions to adsorption energies are not captured by our model. We also evaluated the effect of reducing the number

of

coupling

states

of

benzene

by

including

only

HOMO-1e1g[π(C-C)]

and

LUMO-e2u[π*(C-C)] as for model prediction of CO,29 OH,28 O26 adsorption. As can be seen from


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Figure 6(b), the linearity is compromised with substantial deviation from the diagonal line, making the model-predicted adsorption energies severely overestimated. 3.4. Work Function (∆W) and transferred charge (∆q)

Figure 7. (a) Linear correlation of adsorption energy( of benzene and total charge transferred

from metal substrate to six binding C atoms of phenyl ring (∆q); (b) Linear correlation of ∆q as a function of εd and ∆W, where ∆W is work function difference defined as ∆W= W(gas-phase benzene)

- W(slab). Data point of Cu (open triangle) was removed from the linear fitting. To gain more insight, we invoke Bader charge analysis83 and work function for analyzing the binding mechanism of monocyclic aromatics on metal surfaces. As shown in Figure 7a, there exist two linear relations between adsorption energy ( ) and total charge transferred from metal substrate to six binding C atoms of phenyl ring (∆q), similar to those of vs εd in Figure 4a. The

adsorption energy increases with the amount of charge transferred. In particular, benzene (as well as

other aromatics) is physisorbed on Cu (111) ( =-0.08 eV, = -0.77 eV) as a result of nearly

zero (-0.02 e-) charge transferred (Table S4) and its adsorption energy is attributed almost entirely from vdW-dispersion contributions. To explore the origin of charge transferred, we draw the plot of ∆q as a function of εd and ∆W, where ∆W is the work function difference between gas-phase benzene and metal surfaces (Table S5). As shown in Figure 7b, the amount of transferred charge increases proportionally with the lifted d-band center (εd) towards Fermi level, pointing to the origin of metal surface reactivity essentially associated with the capability of donating electron. This is reinforced by that the amount of charge transferred increases proportionally with the lowered work function of metal surfaces with respect to that of gas-phase benzene [Figure 7b], where the exception is Cu (111)



Page 18 of 29

which has the largest d-band filling of 1.0 (Table 2), a characteristic electronic feature of coinage metals. As can be seen from Figure 5, the e2u(π*) states of Csp of benzene on Cu(111) remain discrete as an unoccupied states unlike the rest of the metal surfaces on which e2u(π*) states are partially populated downshifted below Fermi level, in line with nearly no charge transferred from Cu(111) and its physisorption nature. However, a good linear correlation of (∆q vs ∆W) was observed for highly active species of O, OH, OOH chemisorbed on Cu(111),84 suggesting the chemical reactivity of adsorbate also plays a nonnegligible role in its binding mechanism besides metals’ intrinsic properties.27 3.5. Geometry match (r1/R1 and r2/R2), distortion energy ( ) and interaction energy ( )

(a)

(b)

(c) Figure 8. (a) Schematics of geometric match between phenyl ring (r1 and r2) and rhombus-shaped adsorption site (R1 and R2) at bri30 adsorption configuration, where r1/R1 and r2/R2 have impact on C-M-C bidentate π-type binding and C-M monodentate σ-type binding, respectively; (b) Linear


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correlation between adsorption energy ( of monocyclic aromatics and r1/R1; (c) Linear correlation between adsorption energy ( of monocyclic aromatics and r2/R2. Data points of Fe

(hollow) were removed from the linear fitting. Geometric (ensemble) effect also plays an important role in determining the variation of adsorption energy as the number and/or geometry of the ensemble sites changes.11, 17, 23, 32 Such an effect also exists in form of variation of site geometry where the differed metal surfaces are compared despite the fact that their electronic effect is inseparable which is intrinsically associated with the chemical nature of metals. For eight most close-packed metal surfaces studied (which have almost identical adsorption sites), the variation in adsorption energy is also associated with the geometric effect. We uncovered that the geometry match (r1/R1 and r2/R2) between phenyl ring (r1 and r2) and rhombus-shaped adsorption site (R1 and R2) of M are the factors that have impact on C-M-C bidentate π-type binding and C-M monodentate σ-type binding, respectively (Figure 8a). The adsorption energy ( of monocyclic aromatics is increased with the ratio of r1/R1 and r2/R2, i.e.,

degree of match is highest towards unity (Figure 8b and 8c), which follows coincidentally the same linear pattern and the ordering of the metal surfaces as displayed by − εd (Figure 4b) and − ∆q (Figure 7a), implying that the geometric effect arising from the varied lattice constant of

metals is likely to be secondary while electronic effect is primary in determining adsorption energy. In comparison, varies with r2/R2 more dramatically than r1/R1 for 3d metals and for 4d and 5d

noble metals, implying that C-M monodentate σ-type binding is more pronounced than C-M-C bidentate π-type binding in contributing to the adsorption strength of monocyclic aromatics on metal surfaces (see also Section 3.6.).

(a)

(b)



(c)

Page 20 of 29

(d)

Figure 9. (a) Linear correlation of total distortion energy [ (total)] of toluene, phenol, m-cresol with respect to (total) of benzene, where (total) = (gas) + (slab); (b) Linear correlation of interaction energy ( ) of benzene as a function of (total), where = − (total); (c) Linear correlation of adsorption energy of benzene on metal surfaces as a function of interaction energy ( ); (d) Variation of adsorption energy ( of monocyclic aromatics with distortion energy (total).

Two indicators of distortion energy and interaction energy have been widely used to dissect the calculated adsorption energy and to derive the characteristics of adsorption.39, 85

41, 49, 51, 55, 65,

Technically, the adsorption energy can be decomposed into three separate terms: Eads = (gas)

+ (slab) + Eint (Section 2), where the former two terms are accounted for the energy cost (i.e.,

destabilizing) due to geometry distortion of adsorbate and surface, and the latter term is accounted for the energy gain (i.e., stabilizing) arising from the electronic interaction between adsorbate and surface. In comparison, (gas) of benzene (0.93eV~1.58 eV) is overwhelmingly dominant over (slab) (0.14eV~0.30 eV) (Table S6), indicating that unlike atomic species or small molecules,

monocyclic aromatics must overcome the major obstacle of geometric distortion or frustration of phenyl ring for effective adsorption. In particular, the small-size functional groups at phenyl ring make little difference in calculated distortion energies relative to benzene (Figure 9a). Interestingly, the negative interaction energy is closely associated with the positive distortion energy, i.e., the more positive the distortion energy is (or the more distorted the aromatic geometry is), the more negative the interaction energy is (Figure 9b). Furthermore, the more negative interaction energy leads to the stronger adsorption strength (Figure 9c). However, there is no clear linearity between the calculated adsorption energy ( and the distortion energy (total) as illustrated in Figure 9d,


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consistent with the previous studies.39, 49

3.6. Simulated Scanning Tunneling Microscopy (STM)

Figure 10. (A) Simulated STM images of benzene adsorbed on metal surfaces with constant height mode at -2.00V sample bias, where the four-angle stars point to the brightest spots corresponding to C atoms having the shortest binding distance with surface. Four supercells of 0.375ML adsorbed benzene are displayed in figures; (B) Optimized gas-phase benzene structure with the same orientation as those of adsorbates in (A); (C) Isosurface of charge density (ρ = 0.2 e/bohr3) for gas-phase benzene.

Table 3. PBE-calculated binding distance (d) of C-M bonds for adsorbed benzene, where the shortest d are listed in the first column and the numbers in subscript correspond to labeled number of C atoms of phenyl ring in Figure 10 Catalysts

ԁ1/ԁ4 (Å)

ԁ2/ԁ3/ԁ5/ԁ6 (Å)

Fe (110)

2.04/2.04

2.16/2.15/2.19/2.20

Co (111)

2.11/2.11

2.16/2.12/2.15/2.13

Ni (111)

2.07/2.07

2.11/2.08/2.11/2.08



Ru (0001)

2.28/2.24

2.31/2.31/2.32/2.29

Rh (111)

2.20/2.19

2.21/2.20/2.20/2.20

Pd (111)

2.21/2.20

2.24/2.24/2.24/2.23

Pt (111)

2.17/2.16

2.19/2.22/2.20/2.21

Cu (111)

3.32/3.31/3.33/3.37/3.31

STM86-87 is a highly sensitive technique for probing both electronic and geometric structure of a surface, providing information on atom-resolved structures of surface (or interface) and adsorbate geometry.88-89 For π-conjugated aromatic molecules adsorbed on metal surfaces, numerous STM studies have been reported over the years including benzene adsorbed on Rh(111),72, 90 Pt(111),91 Ni(110) and Cu(110),92-93 Si(100).94 Our DFT-simulated STM images of benzene adsorbed on eight close-packed metal surfaces studied are displayed in Figure 10. One can clearly see the similar pattern of six-member ring of adsorbed benzenes, which exhibit only the subtle variation in spot brightness (equivalent to magnitude of electron density near Fermi level) depending on the metals. There are in particular two spots (four-angle stars) that are much brighter than others corresponding to the two C atoms of aromatic ring (C1 and C4) forming monodentate σ-type binding with the shortest binding distance (Table 3), i.e., in close proximity to surface metal atoms, while those of less brighter spots correspond to bidentate π-type binding of four C atoms [(C2=C3 and C5=C6)]. This reveals that metal surface acting as an electron reservoir stimulates the perturbation of electron density of adsorbed benzene by transferring partial charge from metal atoms to six binding C atoms. Among which, those having the shortest C-M binding distance are affected to the largest degree therefore inducing richer electron density around their nuclei, resulting in their higher brightness in STM images compared to other binding C atoms. The only exception is Cu(111) where the physiosorbed benzene is unaffected resulting in the identical brightness for six equivalent phenyl C atoms. For validity, we note here that the features of our simulated STM images show excellent match with those of low-coverage adsorbed benzene observed in previous experiment/simulation.72 For comparison, the brighter spot can be identified as O atom of adsorbed phenol on metal surfaces, suggesting the negatively charged O atom with the intrinsically rich electron density around O nucleus is competitive with those of binding C atoms of phenyl ring regardless of metals (Figure S6). Still, the only exception is STM image of phenol adsorbed on Cu(111), which reflects its physisorption nature.

4. CONCLUSIONS


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Using DFT calculations, we systematically investigated the binding mechanism of four monocyclic aromatic compounds (benzene, toluene, phenol, m-cresol) on 3d metal surfaces [Fe (110), Co (111), Ni (111),Cu(111)], 4d and 5d noble metal surfaces [Ru (0001), Rh (111), Pd (111), Pt (111)]. Our results show that calculated Eads of four aromatics using varied functionals are increased as optB86b-vdW > DFT-D3 > optB88-vdW > PBE on all metal surfaces studied [except on Cu (111)], among which optB88-vdW results quantitatively agree well with experimental adsorption energies. A linear correlation of vdW-corrections and relative molecular polarizability of monocyclic aromatics has been observed regardless of metal surfaces. It is found that the role of small-size functional groups at phenyl ring is less significant in enhancing adsorption strength compared to phenyl ring itself which contributes most to the electronic interactions with the surface metal atoms. For electronic effects, we have identified two linear relations between adsorption energy and d-band center of surface metal atoms, i.e., one for 3d metals and the other for 4d and 5d noble metals. The same linear pattern is also observed for adsorption energy as a function of total charge transferred from metal substrate to six binding C atoms of phenyl ring. By incorporating the coupling of five states of gas-phase benzene to the metal d-states, our model-predicted adsorption energies agree reasonably well with the DFT-calculated results using GGA-PBE functional. For geometric effect, we have uncovered that the geometry match between phenyl ring and rhombus-shaped adsorption site of M are the factors that have impact on C-M-C bidentate π-type binding and C-M monodentate σ-type binding, in which the latter may be more pronounced than the former in contributing to the adsorption strength. Unlike atomic species or small molecules, monocyclic aromatics must overcome the major obstacle of geometric distortion or frustration of phenyl ring for effective adsorption. Moreover, the negative interaction energy is closely associated with the positive distortion energy. Finally, our simulated STM images have provided the atom-resolved aromatics/metal surface morphology and the visual support for differentiating σ- and π-type bindings.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acscatal.xxxxxxx. Computational details, reversible reduction potentials, diffusion barriers, DFT-optimized



structures and free energy profiles.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21673137), the Science and Technology Commission of Shanghai Municipality (16ZR1413900). The DFT calculations were performed on TianHe-1(A) at the National Supercomputer Center in Tianjin, China, and using resources of the Center for Functional Nanomaterials, which is a U.S. DOE Office of Science Facility, and the Scientific Data and Computing Center, a component of the Computational Science Initiative, at Brookhaven National Laboratory under Contract No. DE-SC0012704

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Adsorption of Monocyclic Aromatics on Transition Metal Surfaces

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