J. Phys. Chem. C 2008, 112, 1019-1027
1019
Activated Adsorption of Ethylene on Atomic-Oxygen-Covered Ag(100) and Ag(210): Formation of an Oxametallacycle Anton Kokalj,*,†,‡ Paola Gava,‡ Stefano de Gironcoli,‡ and Stefano Baroni‡ Jozˇef Stefan Institute, SI-1000 Ljubljana, SloVenia, and INFM-CNR DEMOCRITOS Theory@Elettra group and Scuola Internazionale Superiore di Studi AVanzati (SISSA), I-34014 Trieste, Italy ReceiVed: June 20, 2007; In Final Form: October 8, 2007
The activated adsorption of ethylene on atomic-oxygen-covered Ag(100) surface and on an open-type step edge thereon was studied using density-functional-theory. On perfect Ag(100), such adsorption results in the formation of an oxametallacycle (OMC), with an activation energy slightly larger than 0.3 eV. We find that this activation energy is only weakly dependent on the coverage of on-surface oxygen (for Θ e1/2 ML), whereas the OMC-surface interaction is substantially reduced at high oxygen coverage. Three types of OMCs have been identified on the (100) surface, which display similar stability, and the transformation between them is facile with activation energies below 0.1 eV. We find that the presence of subsurface oxygen reduces the activation energy for OMC formation and substantially increases the OMC-surface interaction. The reactivity of the step edge toward the OMC formation strongly depends on the local coverage of oxygen. Our calculations indicate that the relative stability of the OMC intermediate in ethylene epoxidation reaction is strongly affected by the coverage and configuration of chemisorbed oxygen.
1. Introduction The interaction of ethylene (C2H4) with Ag surfaces has been studied extensively, mainly because silver displays unique catalytic properties in ethylene epoxidation reaction.1 As for the adsorption of ethylene on transition metal surfaces, two different modes of molecularly adsorbed species have been characterized: a π-bonded olefinic and di-σ metallacyclic.2 We considered, by means of density-functional-theory (DFT) calculations, the two adsorption modes of ethylene on oxygenated Ag(100) surface shown in Figure 1. The first is a weakly bound π-bonded ethylene that has been described in our previous work.3 Here we describe the second, more strongly bound, ethylene adsorption mode. This is an ethylene oxametallacycle (OMC), where one of the ethylene C atoms binds to an O atom while the other binds to surface Ag atom(s). The formation of the OMC is, however, an activated process in which ethylene undergoes an sp2 to sp3 rehybridization. The OMC has been recently shown to be an intermediate in selective formation of ethylene epoxide4-9 as well as in the formation of acetaldehyde that leads to total combustion.6,10 As concerns the silver surfaces, the oxametallacycles have been characterized on Ag(111),5,7,11,12 on Ag(110),13-15 on Ag1.83O model16 of p(4 × 4)-O/Ag(111) surface oxide12,17 (very recently this Ag1.83O model has been shown to be incompatible with new findings18,19), and on the new “Ag6” model18,19 of p(4 × 4)-O/Ag(111).20 To the best of our knowledge the oxametallacycles have not yet been characterized on the Ag(100) low Miller index surface. In this paper we characterize the ethylene OMC on Ag(100) and on the step edge defect thereon. In particular, we choose the Ag(210) surface as a model for the step edge. We notice here that the step edges of this kind are * Corresponding author. Address: Jozˇef Stefan Institute, Jamova 39, SI1000 Ljubljana, Slovenia. Tel: +386-1-477-35-23. Fax: +386-1-477-3822. E-mail:
[email protected]. URL: http://www-k3.ijs.si/kokalj/. † Joz ˇ ef Stefan Institute. ‡ INFM-CNR DEMOCRITOS and SISSA.
Figure 1. Two distinct DFT predicted ethylene adsorption modes on oxygenated Ag(100): weakly π-bonded ethylene (left) and ethylene oxametallacycle (right). The C, H, and O atoms are colored in yellow, cyan, and red, respectively, whereas Ag atoms are gray. In this paper, only the oxametallacycle is discussed, whereas the π-bonded adsorption mode was discussed in our previous work.3
exposed in the missing-row (2x2 × x2) reconstruction of Ag(100) induced by oxygen chemisorption at low temperature.21 The selectivity of a given catalyst is usually described in terms of structural (ensemble) and electronic (ligand) effects. An fcc (100) surface has a 4-fold symmetry, and it is expected to show different ensemble effects from the corresponding 3-fold (111) and 2-fold symmetric (110) surfaces. As for the electronic effects, the surface atoms of the fcc (100) surface are 8-fold coordinated, an intermediate between the 9- and 7-fold coordination of (111) and (110), respectively. For this reason, it is worth to investigate how different symmetry of the surface, in the current case the (100), affects energetic and structural properties of OMC. 2. Computational Method Our calculations were performed in the framework of DFT using the generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE).22 We used the pseudo-potential method with ultra-soft pseudo-potentials.23,24 Kohn-Sham orbitals were expanded in a plane-wave basis set up to a kinetic energy cutoff of 27 Ry (216 Ry for the charge-density cutoff).
10.1021/jp0747961 CCC: $40.75 © 2008 American Chemical Society Published on Web 01/08/2008
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Brillouin-zone (BZ) integrations have been performed with the Gaussian-smearing25 special-point technique,26 using a smearing parameter of 0.03 Ry. All calculations have been done using the PWscf code from the Quantum ESPRESSO distribution,27 whereas molecular graphics were produced by the XCRYSDEN28 graphical package. The Ag(100) was modeled by periodic slabs consisting of five (100) layers, whereas 14 (210) layers were used for Ag(210). The latter corresponds to seven-layer thick (100) terraces, which are rotated and merged so as to form the Ag(210). Ethylene (oxametallacycle) molecules and oxygen atoms were adsorbed on both sides of the slab. The thickness of the vacuum region, i.e., the distance between facing ad-molecules, was set to about 8 Å. BZ integrations were performed using a (4 × 4) and (3 × 5) uniformly shifted k-mesh29 for the (2 × 2) and (2 × 1) surface supercells of Ag(100) and Ag(210), respectively. Unless specified otherwise, the calculations have been performed using these two supercells, which represent 1/4 ML coverage of OMC. For a few calculations on Ag(100) also (4 × 4) surface supercell was usedsrepresenting 1/16 ML coverage of OMCs with a (2 × 2) k-mesh sampling. The in-plane lattice spacing was fixed at the calculated equilibrium bulk lattice parameter of 4.16 Å, whereas all other structural parameters have been optimized so as to minimize the total energy of the system. Transition states have been located as the configurations of largest energy along the minimum-energy paths (MEP) connecting the reactants with the products. MEPs have been identified using the climbing-image nudged-elastic band (CI-NEB) method,30,31 whereas the precise location of the TS therein has been validated by verifying that the magnitude of the atomic forces was below a threshold of 50 meV/Å. The adsorption energy of OMC is calculated with respect to chemisorbed oxygen and ethylene in the gas-phase
EEth ads ) EOMC/Ag - [EEth + EO/Ag]
(1)
where EOMC/Ag, EEth, and EO/Ag are total energies of the adsorbate system, gas-phase ethylene, and oxygen-silver substrate, respectively. The adsorbed OMC molecules will be labeled as OxCy, where x and y indicate the sites to which the O and C atoms of OMC bind. In particular, x and y are b, h, and t, which stand for bridge, hollow, and top sites, respectively. Thus, for example, ObCt designates an OMC with its O and C atoms bonded to bridge and top sites, respectively. 3. Results In Figure 2, we display the identified OMC structures on the Ag(100) surface. The adsorption energies and structural parameters are reported as well. In the two most stable OMCs, ObCb and ObCt, the O atom is located on a bridge site, whereas the C atom is located on an adjacent bridge site in the ObCb (Figure 2a) and on a top site in the ObCt (Figure 2b). Adsorption energies at 1/4 ML for the ObCb and ObCt are -0.35 and -0.32 eV, respectively. The OhCt with the O atom located close to the hollow site is less stable, EEth ads ) -0.25 eV (Figure 2c). In this structure, oxygen is displaced in the [001] direction by 0.34 Å from the hollow site. The ethylene fragment within an OMC can be seen as a di-σ bonded ethylene with the first C atom linked to surface Ag atom(s) and with the second one bound to oxygen. Ethylene undergoes an sp2 to sp3 rehybridization during the formation of OMC, which is evident from a reduction of the trans H-C-
Figure 2. Identified OMC structures on Ag(100) surface. Bond distances (Å) and adsorption energies at 1/4 ML coverage, calculated with respect to ethylene in the gas-phase, are also reported.
C-H dihedral angle and an elongation of the C-C bond. The former reduces from 180° (gas-phase ethylene) to 124° and 116° for the OhCt and ObCb, respectively; this angle would be 120° for an eclipsed ethane. The C-C distance is elongated from 1.33 to 1.52 Å for ObCb, a distance characteristic for a single C-C bond (our GGA calculated C-C distance for ethane is 1.54 Å). Because the carbon atoms of OMC are sp3 hybridized, it is not surprising that they are located much closer to the surface than in π-bonded ethylene. In particular, the Ag-C distances are in the range of 2.2-2.4 Å for the OMC structures and 2.6-2.9 Å for the π-bonded ethylene at perfect Ag(100).32,33 The reported EEth ads of ≈ -0.3 eV for these OMC structures are comparable to Eads of π-bonded ethylene on defect sites (Eads ) -0.32 eV at an adatom-defect, whereas at the Ag(410) stepedge site, it is -0.25 eV; see refs 33 and 34). Despite these similar adsorption energies, the OMC is more strongly bound to surface than the π-bonded ethylene discussed in our previous works.3,33 This is evident from Figure 3 which displays the charge density difference, i.e., the difference between the density of the adsorption system and its constituents (ethylene molecule and substrate). This figure distinctly shows that charge redistribution is much larger for the OMC than in π-bonded ethylene, which is a clear indication of the enhanced chemical bonding in OMC. The reason for the small adsorption energies of OMC, in spite of the strong molecule-surface interaction evident from Figure 3b, is due to an interplay between bond-breaking and bondmaking. Indeed, the formation of the OMC from the chemisorbed oxygen and gas-phase ethylene can be decomposed into the following steps: (i) break of the chemisorbed O(a)-surface bond, (ii) the formation of the C-O bond and the transformation of the CdC double bond into a C-C single bond, and (iii) the formation of the OMC-surface bonds, which include one C-metal and one O-metal bond. Contribution from step (ii) is roughly independent of the substrate and can be considered as a constant. Therefore, the OMC’s EEth ads is mainly characterized by the interplay of bondbreaking (i) and bond-making (iii) contributions. Due to the reasons described above, the adsorption energy calculated with respect to gas-phase ethylene is not an appropriate measure of the OMC-surface interaction. For this reason, we define a removal energy, Erem, as the energy required to remove the OMC from the surface to a given reference state in the gas-phase. For the reference state, we choose the ethylene epoxide (EO). In this process, the OMC-surface bonds are
Formation of an Oxametallacycle
J. Phys. Chem. C, Vol. 112, No. 4, 2008 1021
Figure 3. Charge density difference, ∆n(r), for (a) π-bonded ethylene at an adatom defect and (b) ObCb oxametallacycle on Ag(100). ∆n(r) ) nEth/Sub(r) - [nSub(r) + nEth(r)], where Eth/Sub, Sub, and Eth stand for the adsorption system, the substrate [Ag/Ag(100) in (a) and O/Ag(100) in (b)], and the ethylene molecule, respectively. Contours are drawn in linear scale from -0.005 to 0.005 e/a30, with the increment of 0.001 e/a30. The blue color represents the electron deficit regions, whereas the electron excess regions are colored in red (i.e., charge flows from blue to red regions). For OMC (panel b), the isosurfaces at (0.01 e/a30 are also shown on the right. EO TABLE 1: Calculated Adsorption Energies of OMC with Respect to Gas-Phase Ethylene (EEth ads ) and Ethylene-Epoxide (Eads), as Eth EO Defined by eqs 1 and 2, Their Difference ∆E ) Eads - Eads, and the Chemisorption Energy of the O(a) That Was Embedded omc a into OMC (EO chem) Calculated by eq 7
system
ΘOMC (ML)
Θextra O (ML)
ObCb@Ag(100)-(4 × 4) ObCt@Ag(100)-(4 × 4) OhCt@Ag(100)-(4 × 4) ObCb@Ag(100)-(2 × 2) ObCt@Ag(100)-(2 × 2) OhCt@Ag(100)-(2 × 2) ObCb + O hollow@Ag(100)-(2 × 2) ObCt + O hollow@Ag(100)-(2 × 2) OhCt + O hollow@Ag(100)-(2 × 2) OhCt + O + O@Ag(100)-(2 × 2)
1/16
0
1/4
0
1/4
1/4
1/4
1/2
ObCt@Ag(210)-(2 × 1) ObCt + O@Ag(210)-(2 × 1) OACt + O@Ag(210)-(2 × 1) OACt + O + O@Ag(210)-(2 × 1)
1/4 1/4
0 1/4
1/4
1/2
a
ΘOMC is the coverage of OMC, whereas
Θextra O
EEO ads (eV)
∆E (eV)
Oomc Echem (eV)
shown in Figure
-0.25 -0.24 -0.18 -0.35 -0.32 -0.25 +0.00 -0.07 +0.10 -0.44
+0.07 +0.08 +0.14 +0.05 +0.08 +0.15 +0.66 +0.59 +0.76 -0.32
-0.32 -0.32 -0.32 -0.40 -0.40 -0.40 -0.66 -0.66 -0.66 -0.12
-0.87 -0.87 -0.87 -0.79 -0.79 -0.79 -0.53 -0.53 -0.53 -1.07
2a 2b 2c 5a 5b 5c 7a
-0.62 -0.04 +0.10 -0.64
-0.11 +0.23 +0.37 -0.29
-0.51 -0.27 -0.27 -0.35
-0.68 -0.92 -0.92 -0.84
6a 6b 6c 7b
is the coverage of remaining O(a) after the OMC formation.
broken, and a new C-O bond is formed within the EO molecule, the latter being a constant contribution. The removal energy so defined is therefore the opposite of the adsorption energy of OMC with respect to EO in the gas-phase, EEO ads
EEO ads ) - Erem ) EOMC/Ag - [EAg + EEO]
(2)
where EAg and EEO are total energies of silver surface and EO in the gas-phase, respectively. EO The difference between the OMC’s EEth ads and Eads is related to the chemisorption energy of O(a) that is embedded into OMC. By subtracting eqs 1 and 2, we obtain EO EEth ads - Eads ) [EEO - EEth] + [EAg - EO/Ag]
(3)
The enthalphy of the overall ethylene-epoxidation reaction, Eth(g) + (1/2)O2(g) f EO(g), may be written as
1 ∆HEO r ) EEO - EEth - EO2 2
(4)
and the chemisorption energy of oxygen is given by
1 omc EOchem ) EO/Ag - EAg - EO2 2
EEth ads (eV)
(5)
where the EO2 is the total energy of gas-phase oxygen molecule. omc is the chemisorption energy of We emphasize that the EOchem the specific O atom that will be embedded into the OMC; therefore, we use the suffix “omc”. Note that the labels O/Ag and Ag should be more precisely written as [nO]/Ag and [(n 1)O]/Ag, respectively, where n is the number of O atoms per supercell. By adding and subtracting the (1/2)EO2 to first and second term in eq 3, we obtain EO EO Oomc EEth ads - Eads ) ∆Hr - Echem
(6)
The experimental value for the enthalpy of ethylene epoxidation is -1.08 eV,35 whereas our GGA calculated value is -1.19 eV. By plugging into eq 6 the GGA value, we therefore obtain (in eV units) EO Oomc EEth ads - Eads ) -1.19 - Echem
(7)
This has to be kept in mind when comparing OMC’s EEO ads and EEth ads . Table 1 summarizes adsorption energies of OMC with respect EO to ethylene (EEth ads ) and EO (Eads) molecules in the gas-phase, as EO defined by eqs 1 and 2, their difference EEth ads - Eads, and the Oomc Echem as derived from eq 7 for all OMC configurations
1022 J. Phys. Chem. C, Vol. 112, No. 4, 2008 SCHEME 1: Two Identified Paths Leading to Formation of OMC on Ag(100)a
Kokalj et al. SCHEME 2: Transformations between the Identified OMC Forms on Ag(100) with Corresponding Activation Energies, E*a
a
The two transition states are shown in Figure 4. The activation, E*, and adsorption energies, EEth ads , are calculated with respect to ethylene in the gas-phase.
a The direction of transformations is indicated by arrows, which always points from less stable to more stable OMC.
Figure 4. Two identified transition states, TS1 and TS2, for the formation of OMCs on the Ag(100) surface. The activation energies, E*, are calculated with respect to ethylene in the gas-phase.
considered in this paper. As for the OMC on the clean Ag(100) surface, the magnitude of the EEth ads is reduced by ∼0.1 eV when going from the 1/4 to 1/16 ML OMC’s coverage. On the other hand, the EEO ads is almost not affected when passing from 1/4 to 1/16 ML. This is so, because the EEth ads depends on the magnitude of the oxygen chemisorption energy, which decreases with coverage (due to lateral electrostatic repulsion); therefore, destabilization of O(a) in the OMC formation costs more at 1/16 ML than at 1/4 ML coverage. The formation of an OMC is activated. We have calculated reaction barriers for the formation of all three identified OMCs. The activation energy, E*, for the formation of the most stable ObCb oxametallacycle is 0.48 eV (the corresponding transition state is labeled as TS1), whereas the E* for the formation of ObCt and OhCt is only 0.33 eV, and their formation proceeds via the transition state labeled TS2. The process of an OMC formation is schematically shown in Scheme 1, and the corresponding transition states, TS1 and TS2, are shown in Figure 4. The reported activation energies are calculated with respect to ethylene in the gas-phase. In particular, the initial state for these reaction paths calculations is an ethylene molecule located approximately 4 Å above the surface. The activation energy for the TS1 is significantly higher than that for the TS2. The formation of OMC will, therefore, follow the path through TS2, and various OMC structures will then be formed by transformation between them according to Scheme 2: calculated activation energies are lower than 0.1 eV (and less than 0.2 eV in all directions). 3.1. Oxametallacycles at High Coverage of Oxygen. We also investigated how sensitive the formation of an OMC with respect to the coverage of atomic-oxygen on the surface is. We found that at high on-surface oxygen coverage (ΘO ) 1/2 ML) the barrier for the OMC formation via the TS2-like transition state is 0.36 eV, hence only marginally larger by 0.03 eV than that reported in Scheme 1. Nonetheless, the resulting OMC structures are substantially less stable, due to destabilizing effect
Figure 5. OMC structures on O/Ag(100) surface at high coverage of on-surface oxygen. The label’s suffix “+Oh” indicates that in addition to an OMC also an atomic oxygen is chemisorbed in the hollow site per (2 × 2) unit-cell.
of nearby oxygen atoms, which reduce the OMC-surface interaction by about 0.5 eV (compare EEO ads values in Table 1). The OMC structures at high oxygen coverage are shown in Figure 5, which also reports the corresponding adsorption energies and structural parameters. At high O coverage, the ObCt becomes the most stable, and it is the only one with an exothermic EEth ads adsorption energy. The formation of ObCb is athermic, whereas for OhCt is endothermic. 3.2. Role of Defects. 3.2.1. Oxametallacycles at Step Edge of Ag(210). Figure 6a shows an OMC at the step edge of Ag(210) that is analogous to the ObCt identified on flat Ag(100), but its formation is substantially more exothermic, EEth ads ) -0.62 eV, than on the flat surface, and the calculated activation energy for its formation is smaller, 0.14 eV. Also the OMCsurface interaction is stronger compared to corresponding cases on Ag(100), as witnessed by EEO ads of -0.11 eV. At an oxygen coverage of 1/2 ML, step edges of Ag(210) are fully decorated by O atoms,29 and we identified two forms of OMC, ObCt and OACt, which are shown in panels b and c of Figure 6. The letter A appearing in OACt stands for an oxygen site above the step edge [see Figure 1 of ref 36 for our naming of sites on Ag(210)]. Surprisingly, under these circumstances,
Formation of an Oxametallacycle
Figure 6. OMC structures at step edge of Ag(210). (a) OMC at clean step edge. (b,c) Two identified OMC forms at oxygen-decorated step edge. The label’s suffix “+OA” indicates that in addition to an OMC also an atomic oxygen is chemisorbed at the step edge per (2 × 1) unit-cell. The dashed lines indicate the position of step edge.
the two EEth ads energies are similar to those on Ag(100). The activation energy for the formation of OACt is 0.46 eV, whereas the formation of the other OMC, labeled ObCt, proceeds indirectly through the OACt form. The activation energy is thus substantially larger than that on the flat Ag(100). This, to some extent unexpected finding, is due to peculiar behavior of atomic oxygen at the step edge. As found in our previous work,29 O atoms prefer to decorate the step edge by forming -O-AgO-Ag- chains (we refer to these chains as oxygen decorated step edges). This is somewhat counterintuitive, because one would expect the negatively charged O atoms to be as far apart as possible to reduce the electrostatic repulsion. The reason for such a behavior is the good alignment of O and Ag atoms along the step edge; hence, these chains have the structure of electrostatically stable ‚‚‚ + - + - ‚‚‚ strings. Therefore, when the step edge is fully decorated by oxygen, ethylene has to perturb a particularly stable oxygen configuration during the OMC formation; therefore, the activation energy is large in this case, even larger than that on Ag(100). This peculiar behavior of oxygen at the step edge has yet another effect on the formation of OMC. If we envisage an experiment in which the oxygen is preadsorbed on the Ag(210) surface, then even if the nominal coverage of the preadsorbed oxygen is small the O atoms would prefer to cluster so as to form the stable -O-Ag-O-Ag- chains of limited length, unless the temperature of the surface is low enough to suppress the oxygen diffusion. For this reason, the number of “isolated” oxygen atoms at the step edge would be fairly small. The ethylene would therefore have at disposal only the O atoms within the -O-Ag-O-Ag- chains to react with, which makes the OMC structures shown in panels b and c of Figure 6 more representative for the step edge. Another possibility would be that an OMC is formed at the end of the limited length -OAg-O-Ag- chain. 3.2.2. Missing-Row Reconstructed (2x2 × x2)-Ag(100). The adsorption of oxygen on Ag(100) can induce the so-called (2x2 × x2) missing-row reconstruction at low temperatures.21 In such a reconstructed surface, there are two missing-row step edges per unit cell (i.e., two edges per missing-row), and these edges are of the same type as the step edge of Ag(210) and have the same kind of oxygen decoration as discussed above for Ag(210). The structure of the missing-row edge is therefore
J. Phys. Chem. C, Vol. 112, No. 4, 2008 1023
Figure 7. OMC structures in the presence of on-surface + subsurface oxygen on (a) Ag(100) and (b) Ag(210). The subsurface oxygen is located in the octahedral site beneath the C atom of OMC as indicated by arrows; in (a) the subsurface oxygen is behind the Ag atom near the arrow. In (b) the dashed line indicates the position of step edge.
locally very similar to that of the oxygen decorated step edge of Ag(210). For this reason, we expect that the above-discussed OMCs (Figure 6, panels b and c) can serve as a model for OMCs that could form on the missing-row reconstructed (2x2 × x2)Ag(100). 3.3. Role of Subsurface Oxygen. In our previous works,3,37 we found that the adsorption energy of π-bonded C2H4 may increase considerably when oxygen is adsorbed into subsurface sites. Our results indicate that the increased reactivity of surface Ag atoms is due to their decreased coordination due to the push out effect of oxygen underneath.3 Similarly, here we find that such push out Ag atoms are more reactive also toward an OMC formation. As found in our previous works,3,36 oxygen atoms prefer to occupy a fraction of subsurface sites, when the coverage exceeds 1/2 ML. In these configurations, O atoms occupy on-surface hollow and the subsurface interstitial octahedral sites. Here we use the O/Ag(100)-(2 × 2) and O/Ag(210)-(2 × 1) supercell models with a nominal 3/4 ML coverage of oxygen. For Ag(100), 1/2 ML of oxygen is adsorbed in the surface hollow sites, and 1/4 ML is adsorbed into subsurface octahedral sites, whereas for Ag(210), 1/2 ML of oxygen is adsorbed at the step edge, and 1/4 ML is adsorbed into subsurface octahedral sites just below the step edge. The identified OMC structure on these substrates is shown in Figure 7 and labeled as OhCt + Ohollow + Osub@Ag(100)-(2 × 2) and OACt + OA + Osub@Ag(210)(2 × 1) in Table 1. Such O/Ag models substantially reduce the energy barrier for the OMC formation on the flat Ag(100) (E* )0.17 eV) as well as at the step edge of Ag(210) (E* ) 0.22 eV), and considerably increase the exothermicity of the OMC adsorption energies, EEth ads ) -0.44 and -0.64 eV at the (100) facet and at the step edge, respectively. Subsurface oxygen also increases substantially the OMC-surface interaction: compare the corresponding EEO ads values in Table 1 to those without subsurface oxygen. 4. Discussion 4.1. Formation of Oxametallacycle. As described in our previous DFT works,3,32,37 the π-bonded ethylene binds rather weakly to both clean and oxygen pre-covered Ag(100), the
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TABLE 2: Activation Energies for OMC Formation on Ag(100) and Ag(210) as a Function of Oxygen Coverage Calculated with Respect to Gas-Phase Ethylenea surface
oxygen-coverage
E* (eV)
Ag(100)
low high high + subsurface-oxygen low high high + subsurface-oxygen
0.33 0.36 0.17 0.14 0.46 0.22
Ag(210)
a The coverage of on-surface oxygen before the OMC formation is 1/4 ML (low) and 1/2 ML (high), whereas the coverage of subsurface oxygen is 1/4 ML.
adsorption energy being around 0.1 eV. The desorption barrier of the π-bonded ethylene is thus much smaller than the barrier for the OMC formation, and under the UHV (ultrahigh-vacuum) conditions, the π-adsorbed ethylene would desorb before reacting with oxygen. However, recently Stegelmann9 pointed out that, even though the ethylene desorbs at very low temperature under UHV, it may have a significant coverage at the high-pressure typically used in industrial heterogeneous catalysis. These arguments are compatible with the fact that high pressure of ethylene is needed to initiate the reaction.38,39 Due to the small adsorption energy of ethylene, larger olefin molecules, such as styrene8 and norbornene,40 are usually used in the epoxidation studies performed under UHV conditions,41 because larger molecules bind more to the surface due to stronger van der Waals interactions, which contribute roughly 0.04, 0.02, and 0.11 eV per C-H, C-C, and CdC bond, respectively.42 Although we modeled the formation of OMC from the gasphase ethylene, we do not imply the OMC formation to be of Eley-Rideal-type. The calculated equilibrium height of the π-adsorbed ethylene molecule is about 3 Å above the Ag(100),3 which is slightly higher than the height of the ethylene fragment in the transition-state structure for the OMC formation, 2.7 Å (see Figure 4b). Therefore, the transition state can be accessed either from the gas-phase or from adsorbed state of ethylene. Although the E* for the OMC formation from the π-adsorbed ethylene would be larger than that from gas-phase ethylene by the amount of ethylene adsorption energy, this may be compensated by a prefactor, which is typically many orders of magnitude larger for the Langmuir-Hinshelwood than for the Eley-Rideal mechanism.43,44 4.1.1. Dependence of ActiVation Energy on Oxygen CoVerage. The activation energies for the OMC formation on Ag(100) show a weak dependence on the on-surface oxygen coverage, as seen from Table 2. On the basis of the structural analysis of several transition states, we realized that the activation energy depends on two effects: the penalty to displace the O(a) from the optimum site, and the strength of the forming C-metal bond. Indeed, these effects are similar to those governing the EEth ads as described in the Results section, but they are less developed, because the transition state is early: compared to gas-phase ethylene the carbon-carbon distance is increased merely by ≈0.05 Å and hydrogen atoms are slightly up-shifted (e.g., see Figures 4 and 8). On Ag(100) at high coverage, the O(a) binds less to the surface, so it will be easier to incorporate it into the OMC. However, at the same time, large coverage of oxygen will also reduce the strength of the forming C-metal bond due to bonding competition. So the two effects are opposite, and therefore, the dependence of the activation energy on the coverage of oxygen is weak.
Figure 8. Comparison of TS structures for the formation of OMCs on high oxygen covered Ag(100) surface in the absence (a) and presence (b) of subsurface oxygen. (a) is the TS for the formation of OMCs shown in Figure 5, whereas (b) is the TS for the formation of OMC shown in Figure 7a. Note that in (b) the C-metal bond is ≈ 0.5 Å shorter compared to that in (a).
Figure 9. Correlation between the activation energy for OMC formation and its adsorption energy, EEth ads . Blue line represents the Brønsted-Evans-Polanyi (BEP) fit: E* ) a + bEEth ads . The rms error is 0.06 eV, whereas the largest error is 0.08 eV.
These arguments also explain why on Ag(210) the dependence of E* on the coverage of on-surface oxygen is much larger. It is due to “reversed” behavior of O(a) at the stepedge: the high coverage oxygen step-edge decoration is more stable than the low coverage configurations.36 Therefore, on the decorated step-edge, it is costly to incorporate O(a) into OMC, and at the same time, the forming C-metal bond will be weak due to a bonding competition with the nearby oxygen. On the other hand, at low coverage, the O(a) binds less and the forming C-metal bond is stronger. So the two effects are cumulative on Ag(210), causing a larger dependence of E* on the coverage of on-surface oxygen. This also explains why the variation of EEth ads is much larger on Ag(210) compared to that on Ag(100) (see Table 1). The presence of subsurface oxygen reduce the E* substantially on both surfaces (Table 2). It is known that subsurface oxygen enhances the adsorbate-surface bonding3 and reduces the dissociation barriers.45 Due to this increased reactivity, the C-metal bond length is reduced by about 0.5 Å at the TS structure (see Figure 8). The increased C-metal interaction at the TS stabilizes it, making the activation energy smaller. In Figure 9, we show the Brønsted-Evans-Polanyi (BEP) Eth relation between the E* and EEth ads , i.e., E* ) a + bEads . The
Formation of an Oxametallacycle slope of BEP line is only 0.42 indicating weaker dependence of E* on the coverage and configuration of oxygen than EEth ads . 4.2. Structural Characteristics of Oxametallacycles. The oxametallacycles presented in this paper can be classified, with respect to the sites the O and the C atoms bind to, into ObCb, ObCt, and OhCt forms; the OACt at the step edge can be seen as a variant of the OhCt form. In their recent work, Bocquet and Loffreda17 discussed the structure of OMCs in terms of staggered or eclipsed conformers with respect to the orientation of the two adjacent CH2 groups. As for the OMCs discussed in this work, the ObCb and OhCt forms have the two CH2 groups eclipsed, whereas the ObCt is an example of staggered OMC. Usually, the number of metal atoms involved in the oxametallacycle ring is also discussed. In particular, Mavrikakis et al.13 introduced an OME and OMME notation, which stands for oxygen-metal-...-ethylene, and the number of M characters indicates the number of surface metal atoms involved in the oxametallacycle ring. The majority of OMC structures presented in this work belong to the OMME class. The only OME-type oxametallacycles discussed here are those identified in the presence of the subsurface oxygen shown in Figure 7. Also the OACt bonded to the step edge (Figure 6c) is an OME, but it is less stable than the ObCt OMME shown in Figure 6b. 4.3. Comparison with Other Low-Miller Index Ag Surfaces. On Ag(111), only an OMME-type ObCt form has been reported in a periodic slab model DFT study.12 As for the bond lengths, they are similar on the two surfaces provided that the same form (ObCt) is compared: dO-Ag and dC-Ag about 2.2 Å, dO-C ≈ 1.4 Å, and dC-C ≈ 1.5 Å. Actually, Barteau et al.11 identified also an OME-type OhCt form obtained on a cluster model of Ag(111), which they predicted to be about 0.2 eV less stable than the OMME. Barteau et al.15 also identified an OMME-and OME-type OMCs on a cluster model of Ag(110) surface, which according to our notation can be labeled as OtCt and ObCt, respectively. The OME-type OMC was predicted to be about 0.4 eV less stable than the OMME. Both OMCs are bonded along the Ag-atomic ridge of the (110) surface extending along the [11h0] direction. At low O coverage, our calculated activation energy for the formation of OMC on Ag(100) from gas-phase ethylene is 0.33 eV, which is similar to a value of 0.32 eV obtained on Ag(111) by Bocquet et al.12 However, the formation of OMC from ethylene is more exothermic on Ag(111) than on Ag(100). At 12 a coverage of 1/16 ML, the EEth ads on Ag(111) is -0.47 eV to be compared with our value of -0.25 eV on Ag(100). This is due to chemisorbed O(a), which is about 0.4 eV46 more strongly bound to Ag(100) than to Ag(111). Therefore, its incorporation into OMC requires larger energy penalty on Ag(100). On the other hand, the OMC-surface interaction is slightly stronger on Ag(100) as expected because open surface are more reactive 12 (the EEO ads values at 1/16 ML are +0.07 and +0.15 eV on Ag(100) and Ag(111), respectively). 4.4. Energetics of Oxametallacycle as an Intermediate in the Expoxidation of Ethylene. A minimal-sequence of elementary steps during the ethylene epoxidation reaction would include the following: (i) dissociative adsorption of oxygen, (ii) reaction of ethylene with chemisorbed atomic oxygen to form the OMC, and (iii) the transformation of OMC to ethylene epoxide, which eventually desorbs from the surface. For a more elaborate list of elementary steps see, for example, the work of Stegelmann.9 As for the dissociation of oxygen, it has been shown on the basis of BEP relation that for reactions, where the dissociative adsorption is the rate-limiting step, the optimum dissociative
J. Phys. Chem. C, Vol. 112, No. 4, 2008 1025 chemisorption energy is adsorbate independent and should be in range between -0.8 and -1.4 eV/molecule.47 Although the value of EOchem on a given metal depends on the oxygen coverage and surface plane, in general, the magnitude of EOchem on transition metals decreases from left to right and from top to bottom in the periodic table.48 Among all of the transition and IB noble metals, Ag and Pd are the closest to the above range for EOchem.47 All other metals are too reactive (display too exothermic EOchem), with the exception of gold, where EOchem is endothermic.47,49 Although the use of thermodynamic and BEP arguments alone may be questionable in discussing kinetic favored reactions, ethylene epoxidation being the notable example,50 we argue that some useful observations can still be drawn on the basis of BEP relation. As for the step (i), it is known that dissociative adsorption obey the BEP remarkably well,47,51 whereas according to our calculations also steps (ii) and (iii) follow the BEP relation to some extent53 (Figure 9 shows the BEP fit for the step (ii)). On Ag surfaces, the magnitude of oxygen chemisorption energy3,46,52 is smaller than the magnitude of the enthalpy of the overall ethylene epoxidation reaction (our GGA calculated ∆Hr for C2H4 + 1/2O f EO is -1.19 eV), thus making the overall energetics of steps (ii) + (iii) exothermic. However an individual step may be endothermic, therefore we analyze the two steps separately. In step (ii) ethylene reacts with chemisorbed oxygen to form OMC, and the enthalpy of this step will be mainly given by two opposite effects (the cost to break the chemisorbed O-surface bonds and the gain in forming the OMC’s C- and O-surfaces bonds; other contributions are roughly constant). For example, a very reactive surface will bind oxygen strongly, but at the same time will also make strong OMC-surface bonds. For this reason, the magnitude of the enthalpy of reaction step (ii) will likely not be large due to mutual cancellation of the two opposite effects, irrespective of the reactivity of metal. On the other hand, in step (iii), the OMC-surface bonds have to be broken, a new C-O bond is formed in the EO (this is a constant energy contribution), and the product EO binds weakly to the surface.13 Therefore, the more reactive the metal surface is, the more it will cost to form the EO from the OMC, as has been demonstrated by Mavrikakis et al.13 They showed that on late transition metals the stability of OMC with respect to EO indeed follows the trend Ru > Rh > Pd > Ag; on reactive surfaces such as Rh and Ru, the step (iii) may be endothermic by over 2 eV.13 In Figure 10, we display energy profiles for the overall ethylene epoxidation reaction for various cases on Ag(100) and Ag(210). Because the adsorption energies of ethylene and EO are small on silver,3,12,54 we consider in this figure only the relative stabilities of 1/2O2(g), O(a), OMC(a), and EO(g) [actually 1/2O2(g) and O(a) stand for 1/2O2(g) + Eth(g) and O(a) + Eth(g), respectively]. Depending on the coverage and configuration of oxygen, the energy profiles vary substantially. For comparison, the energy profiles for low oxygen covered Ag(111) and AgO model of the p(4 × 4)-O/Ag(111) surface oxide are also shown in Figure 10 by red and black solid lines, respectively (these profiles were derived from the results of Bocquet et al.12,17). Note that the two profiles fall in the range spanned by various cases on Ag(100), indicating the importance of oxygen coverage. All the cases, but those involving subsurface oxygen and low oxygen covered step-edge defect show the “preferred thermodynamic” order of stability: 1/2O2(g) < O(a) < OMC(a) < EO(g), demonstrating why silver is a good catalyst for epoxidation of ethylene: it binds not only oxygen but also OMC to an adequate amount.
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Figure 10. Relative energies of the stationary points in the ethylene-epoxidation reaction for various cases on Ag(100) (left panel) and Ag(210) (right panel). The zero-energy is chosen as the energy of gas-phase ethylene and 1/2O2(g). For comparison the profiles derived from the results of Bocquet et al.12,17 on Ag(111)-(4 × 4) and Ag1.83O-(4 × 4) are presented by red and black solid lines in left panel, respectively. Left panel: profiles labeled as Oh + Osub + Ag(100)-2 × 2, Oh + Ag(100)-2 × 2, and Ag(100)-2 × 2 correspond to OMCs shown in Figures 7a, 5b, and 2a, respectively. Right panel: profiles labeled as OA + Osub + Ag(210)-2 × 1, OA + Ag(210)-2 × 1, and Ag(210)-2 × 1 correspond to OMC’s shown in Figures 7b, 6b, and 6a, respectively.
As for the energy profiles on Ag(100) involving only onsurface oxygen, the exothermicity of steps (i) and (ii) reduces with oxygen coverage and that of step (iii) increases. However, the presence of subsurface oxygen completely changes the picture: in this case, steps (i) and (ii) are very exothermic, and step (iii) is endothermic. On the basis of BEP relation, we may therefore anticipate a large variation of activation energies for the three elementary steps depending on the coverage and configuration of oxygen. This is compatible with results of kinetic studies,5,9,55 which demonstrate that the rate-determining step is governed by the reaction conditions used. A comparison of energy profiles shown in Figure 10 suggests that the presence of subsurface oxygen would increase the activation energy for the step (iii) (transformation of OMC to EO). Therefore, this step would not take place on sites in the vicinity of subsurface oxygen, which is at variance to a common belief that subsurface oxygen is required to form EO.38,56 However, recently Stegelmann et al.9 pointed out that subsurface oxygen is not important under steady-state conditions and may only serve as an oxygen reservoir in transient experiments. 5. Conclusions In this paper, we have used DFT-GGA calculations to characterize the oxametallacycles that form upon ethylene activated adsorption on atomic-oxygen-covered Ag(100) and Ag(210). Three forms of oxametallacycles have been identified on the (100) facet, named as ObCb, ObCt, and OhCt (Figure 2). They belong to the so-called OMME-type, which involves two surface metal atoms in the oxametallacycle ring. The transformation between the various oxametallacyle forms is predicted to be facile due to small corresponding activation energies. On perfect Ag(100), the activation energy for the formation of OMC from the gas-phase ethylene is about 0.3 eV and is only weakly dependent on the coverage of oxygen. On the other hand, at the step edge of Ag(210), the activation energy strongly depends on the coverage of oxygen. At low coverage, the barrier is low, 0.14 eV. However, the step edge prefers to be fully
decorated by oxygen, and the corresponding barrier for the OMC formation is higher than on Ag(100), because in this case ethylene has to perturb a particularly stable oxygen configuration during the oxametallacycle formation. Our calculations indicate that the presence of nearby subsurface oxygen substantially reduces the activation energy for the formation of oxametallacycle and stabilizes it to a large extent. This is due to an increased reactivity of the surface Ag atoms, because of their decreased coordination due to the sizable push out effect of the oxygen underneath. In this case, the oxametallacycles involve only one metal atom in the oxametallacycle ring; hence, they belong to the so-called OME oxametallacycles. The reasons why silver is a good catalyst for ethylene epoxidation have also been discussed, and they stem from the ability of silver to bind atomic oxygen and oxametallacycle to an adequate amount. Finally, we anticipate that the energetics of the intermediate steps in ethylene epoxidation reaction is strongly affected by the coverage and configuration of chemisorbed oxygen. Acknowledgment. This work has been supported in part by the Italian INFM through Iniziativa trasversale calcolo parallelo and by the Slovenian Research Agency (Grant No. P2-0148). References and Notes (1) Barteau, M. A.; Madix, R. J. The Surface Reactivity of Silver: Oxidation Reactions. In The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis; King, D. A., Woodruff, D. P., Eds.; Elsevier: Amsterdam, 1982; Vol. 4. (2) Zaera, F. Chem. ReV. 1995, 95, 2651. (3) Kokalj, A.; Dal Corso, A.; de Gironcoli, S.; Baroni, S. J. Phys. Chem. B 2006, 110, 367-376. (4) Medlin, J. W.; Barteau, M. A.; Vohs, J. M. J. Mol. Catal. A 2000, 163, 129. (5) Linic, S.; Barteau, M. A. J. Catal. 2003, 214, 200-212. (6) Linic, S.; Barteau, M. A. J. Am. Chem. Soc. 2003, 125, 4034. (7) Linic, S.; Piao, H.; Adib, K.; Barteau, M. A. Angew. Chem., Int. Ed. 2004, 43, 2918. (8) Klust, A.; Madix, R. J. Surf. Sci. 2006, 600, 2025.
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