Adsorption of C2H4 on Stepped Cu (410) Surface: A Combined TPD

Article pubs.acs.org/JPCC

Adsorption of C2H4 on Stepped Cu(410) Surface: A Combined TPD, FTIR, and DFT Study Takamasa Makino,† Michio Okada,*,† and Anton Kokalj*,‡ †

Department of Chemistry, Graduate School of Science, Osaka University, 1-1 Machikaneyama-cho, Toyonaka, Osaka 560-0043, Japan ‡ Department of Physical and Organic Chemistry, Jožef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia S Supporting Information *

ABSTRACT: Adsorption of ethylene (C2H4) on stepped Cu(410) surface was investigated with infrared reflection−absorption spectroscopy, temperature-programmed desorption, and density functional theory calculations. At 93 K, ethylene adsorbs molecularly on Cu(410) and forms three different π-bonded species, all of which desorb below 230 K. Even though this finding straightforwardly suggests that the three species correspond to ethylene adsorbed at the step-edge and at the two different terrace atoms, a more detailed analysis reveals this is not the case. Instead, the origin of the three species stems from the interplay between (i) strong preference of ethylene to adsorb at the stepedge, (ii) significant intermolecular repulsion between the ethylene adsorbed at the adjacent step-edge sites, and (iii) the random adsorption on the terrace sites. Recent experimental studies have suggested that ethylene is dehydrogenated on Cu(410) due to the open step-edges (Kravchuk et al. J. Phys. Chem. C 2009, 113, 20881). The present data confirm the occurrence of dehydrogenation but not the formation of a di-σ-bonded ethylene. single crystal surfaces, for example, Cu(210),6 in order to discuss the effect of step sites on reactivity. The interaction between ethylene and transition metal surfaces has attracted interest since the earliest days of surface science. Ethylene on a metal surface is a basic model system to study the surface reactions of olefins. The interaction is also important in catalytic reactions such as hydrogenation,7 dehydrogenation,8−10 epoxidation,11 and so on. These reactions involve a wide variety of intermediate species derived from ethylene, such as π-bonded or di-σ-bonded ethylene, ethylidyne, vinylidene, and so on. Moreover, many studies on the synthesis of two-dimensional graphene by chemical vapor deposition of ethylene have also been done because of graphene’s unique physical properties.12−14 There are a number of studies on ethylene bonded to flat Cu surfaces and it has been shown that the molecule only weakly interacts with flat Cu surfaces and retains its π character;15−19,21,22 other bonding modes of ethylene have not been reported. On Cu(100) and Cu(111), π-bonded ethylene adsorbs with its molecular plane parallel to the surface and no di-σ bonding occurs.15,16 A theoretical study on ethylene bonded to Cu(111) attributed the absence of di-σ-bonded ethylene to the energy barrier between the two species.20 On Cu(110), the CC axis of ethylene appears significantly tilted around 100 K, according to the investigations based on infrared

1. INTRODUCTION Structural defects on a solid surface have specific character of electronic structure due to their low coordination and symmetry and, as a result, different chemical properties. The presence of defects is one of the dominant gaps between ideally controlled surfaces and real catalysts for practical use. Thus, it is important to characterize each type of defects in detail, in order to control more complicated systems. A single crystal surface cut along a high Miller-index face is a good model candidate to gain basic understanding of the properties of defects due to its own well-defined structure of defects and flat terraces.1 Steps on surfaces are very common structural defect, however, despite of extensive studies, the correlation between reactivity and properties of steps is still an open question. For example, Pt(5̅57) surface is less reactive than Pt(1̅12) for hydrogenolysis of cyclohexane to yield n-hexane,2 while methane is dissociated on Pd(679) more efficiently than on Pd(311).3 As for the hydrogenolysis of cyclohexane, a surface with narrower terrace has better reactivity, although the methane dissociation is catalyzed better by a surface with wider terrace. Studies of C2H4 adsorption on Ag(210) and Ag(410), based on supersonic molecular beam technique, have demonstrated that the sticking probability, measured with the King and Wells technique, is increased on stepped surfaces compared to flat surface, while the achievable maximum coverage is rather independent of the density of steps.4,5 In this paper, we present the results of the experimental and computational investigation on adsorption of ethylene on Cu(410) and compare them to those obtained on other Cu © 2014 American Chemical Society

Received: September 11, 2014 Revised: October 27, 2014 Published: October 31, 2014 27436

dx.doi.org/10.1021/jp509228v | J. Phys. Chem. C 2014, 118, 27436−27448

The Journal of Physical Chemistry C

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reflection−absorption spectroscopy (IRAS).17,18 In contrast, a combined photoemission and photoelectron diffraction study revealed that at 115 K ethylene adsorbs on Cu(110) with its CC axis parallel to the surface.21 The same molecular orientation was reported also by scanning tunneling microscopy (STM) at 4 K, that is, the molecule was found to bond to the short bridge site on the close-packed row of Cu(110).22 It has already been demonstrated that ethylene is not reactive on flat Cu surfaces. However, Kravchuk et al. has recently reported the formation of di-σ-bonded ethylene and even dehydrogenation of ethylene occurring on Cu(410) due to the open steps on the surface.23−25 Current report reveals that a structure of regular steps on a solid surface can induce a drastic change in reactivity. This suggests that it may be possible to modify or control the reactivity of a Cu surface with step sites. To examine the differences between two types of stepped Cu surfaces, Cu(410) and Cu(210),6 we investigated ethylene adsorbed on Cu(410), which consists of regular steps and narrow terraces (see Figure 1), with IRAS, temperatureprogrammed desorption (TPD), and density-functional-theory (DFT) calculations.

whose tangent is 1/4, that is, 14°. The nominal density of stepedge sites of the clean Cu(410) face is half of that of Cu(210). The IRAS spectra were recorded using a FTIR spectrometer (JASCO, FTIR 6100) with a liquid-nitrogen cooled HgCdTe (MCT) detector. The spectral range lies between 800 and 3500 cm−1, although a stable operation is difficult below 950 cm−1. Spectral resolution was set to be 4 cm−1 and typically 512 or 1024 scans were collected in the present experiments. The incident IR beam was p-polarized with a wire grid polarizer and focused through a BaF2 window onto the sample surface at an incident angle of 80° off normal. A dried air generator (Parker Balston, 7562JA) supplied CO2-free dry air to the optical paths and the interferometer. The TPD measurements were carried out with a QMS enclosed by a shield in order to exclude the effect of gases desorbing from the sample holder and/or the chamber during the heating process. To maximize the signal of desorbing molecules during TPD, the sampling orifice of the QMS housing was positioned within 1 mm away from the sample. The sample was cooled down with liquid nitrogen and was exposed to high purity C2H4 or CO (Taiyo Nippon Sanso Co. Inc.) by backfilling the chamber. 2.2. Computational. DFT calculations were performed using the generalized gradient approximation (GGA) of Perdew−Burke−Ernzerhof (PBE),26 as implemented in the PWscf code contained in the Quantum ESPRESSO distribution,27 while molecular graphics were produced by the XCRYSDEN28 graphical package. We used the pseudopotential method with ultrasoft pseudopotentials.29,30 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). Brillouin-zone (BZ) integrations were performed with specialpoint technique31 and Fermi-surface effects were treated by the smearing technique of Methfessel and Paxton32 using a smearing parameter of 0.03 Ry. The Cu(410) surface was modeled by 16 (410) layers (this thickness is compatible with the Cu(100) slab composed of four (100) layers). The bottom four layers were constrained to the bulk positions and the in-plane lattice spacing was fixed to the calculated equilibrium Cu bulk lattice parameter of 3.67 Å, while all other degrees of freedom were relaxed. Molecules were adsorbed on one side of the slab and the thickness of the vacuum region, the distance between the top of the admolecule and the adjacent slab, was set to about 10 Å. The adsorption was modeled using the (2 × 1), (3 × 1), and (4 × 1) supercells and the 3 × 3 × 1, 2 × 3 × 1, and 2 × 3 × 1 uniformly shifted kpoint meshes, respectively. 2.2.1. Energy Equations. The average adsorption energy of ethylene is calculated as

Figure 1. Geometry of the Cu(410) surface (top and side views); the Cu(410)−(1 × 1) unit cell is indicated with yellow parallelogram. Four distinct surface Cu atoms are labeled as S, T1, T2, and B; S ≡ step-edge atom; T1 and T2 ≡ two different terrace atoms; and B ≡ atom below the step-edge. The step-edge atoms are colored somewhat brighter to enhance the visual perception of the surface geometry.

2. TECHNICAL DETAILS 2.1. Experimental Section. The experiments were performed in an ultrahigh-vacuum (UHV) chamber pumped to a base pressure below 4 × 10−8 Pa with a turbo-molecular pump and a titanium-sublimation pump. A schematic view of the apparatus was already reported in ref 6. The chamber is equipped with a Fourier-transform infrared (FTIR) spectrometer, a quadrupole mass spectrometer (QMS) and a low-energy electron diffraction (LEED) optics. A mechanically polished single crystal surface (15 mm × 15 mm × 1.5 mm, Surface Preparation Laboratory) was used in the present study. The clean surface of Cu(410) was prepared by several cycles of Ne ion bombardment (2 keV, ∼3 μA cm−2, 30 min) and subsequent annealing above 670 K for 20 min. The sample cleanliness was monitored by Auger electron spectroscopy (AES) and LEED. A sharp LEED pattern corresponding to Cu(410) was observed after the cleaning process, indicating that no reconstruction occurred on the Cu(410) surface. The Cu(410) surface consists of (110) steps and three atoms wide (100) terraces (see Figure 1). The (100) terrace-plane is rotated away from the (410) plane by an angle

Eads =

1 [E(C2H4)n /slab − (Eslab + nEC2H4)] n

(1)

where E(C2H4)n/slab, Eslab, and EC2H4 stand for total energies of adsorption system with n adsorbed molecules per supercell, clean slab, and isolated ethylene molecule, respectively. Even if the adsorption is nonactivated (barrier-less), the average adsorption energy is a rather poor approximation for desorption energy, in particular, for the purpose of TPD interpretation; namely, the average adsorption energy would represent the desorption energy only if all molecules would desorb simultaneously. The desorption energies can be 27437



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approximated by using a sufficiently large supercell and removing one molecule per supercell at a time, that is, Edes ≈ (E(C2H4)n−1 /slab + EC2H4) − E(C2H4)n /slab

(2)

The adsorption energy of ethylene on CO preadsorbed surface is calculated as C 2 H4 Eads = EC2H4 + (CO)m /slab − (E(CO)m /slab + EC2H4)

(3)

where m is the number of adsorbed CO molecules per supercell. 2.2.2. Ab Initio Atomistic Thermodynamics. Thermodynamic stability of adsorption structures that differ in surface coverage is evaluated by means of adsorption surface free energy (γads) as a function of adsorbate chemical potential (μ). To a first approximation the γads can be related to average adsorption energy and μ via the relation: n γads ≈ (Eads − μ) (4) A where A is the area of supercell and n is the number of adsorbed molecules per supercell. The important point of eq 4 is that γads is a linear function of μ with the slope being proportional to the negative of surface coverage, −n/A. This implies that the larger is the coverage the steeper is the slope of the corresponding γads line. Thermodynamically the most stable structure at given μ is the one with the smallest γads value. If the surface is in ambient of a gas phase, the μ is a function of temperature, T, and partial pressure, p. Within the ideal-gas approximation: μ(T , p) = μ(T , p0 ) + kT ln(p /p0 )

(5)

where p0 stands for a standard pressure and k is a Boltzmann constant. Equation 5 implies that low values of μ correspond to high T and low p (vice versa for the high values of μ). 2.3. Structure Naming Convention and Surface Coverage. Cu(410) consists of four different surface Cu atoms (Figure 1): S ≡ step-edge atom; T1 and T2 ≡ two distinct terrace atoms; and B ≡ atom below the step-edge. Standalone adsorption structures will be labeled as s-T, where s stands for site, which is either top, short bridge, long bridge, or hollow, whereas T stands for the type of Cu atom the molecule bonds to, T ≡ S, T1, T2, or B; the term “standalone” means that adsorbed ethylene has no nearest-neighbor molecules. For example, the “top-S” indicates a molecule bonded on top of a step-edge atom, whereas the “short bridge-S,T1” indicates a molecule bonded onto a bridge site that consists of step-edge S and terrace T1 atoms. High-coverage adsorption structures will be labeled as (m/N)T, where the fraction m/N indicates that m out of N sites of T type are occupied; the omission of m/N implies that every T site is occupied. For example, the “1/2S” implies a structure with every second S site occupied, whereas the composite “1/2S + 1/4T1” label stands for a structure with every second S and every fourth T1 sites occupied. Surface coverage, Θ, specified in fraction of ML (monolayer), is defined as the inverse of the number of surface Cu atoms per adsorbed molecule.

Figure 2. (a) Typical TPD spectra of C2H4 adsorbed on Cu(410) at 93 K. The heating rate was 2.0 K/s. (b) Uptake curve for C2H4 determined from TPD peak area; the decomposition to individual components corresponding to β (squares) and α (triangles) peaks is also shown.

equals 1.33 × 10−4 Pa·s. The heating rate was 2.0 K/s and the signal at m/e = 27 (C2H3+) was monitored by QMS; note that this signal exclusively originates from ethylene. Each TPD spectrum was measured after the exposure of the freshly prepared Cu(410) to C2H4. Three desorption peaks, labeled as α1, α2, and β, were observed and their peak temperatures (TD) are 124, 154, and 196 K for the C2H4-saturated surface, respectively. Only the β peak was observed for the exposure of 0.15 L (TD = 201 K). The feature was slightly asymmetric and became saturated for the exposure of 0.5 L (TD = 196 K). The asymmetric shape and nonobvious peak shifts are typical for the desorption profile of first order kinetics.33 During the evolution of the β peak, the α2 peak appears as a shoulder and increases in intensity with increasing the exposure up to 5 L. The α1 peak develops at exposures greater than 1 L as a low temperature shoulder of the α2 peak. Figure 2b shows the uptake curve determined from TPD spectra (see Figure 2a). The uptake increases linearly for the exposures smaller than 0.5 L and almost saturates at 1 L. No significant change in LEED pattern

3. RESULTS 3.1. Experimental Results. 3.1.1. TPD and IRAS of C2H4 on Cu(410). Figure 2a shows the exposure dependent TPD spectra of ethylene adsorbed on clean Cu(410) at 93 K. Exposures are reported in units of Langmuirs (L) where 1.0 L 27438



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Figure 3. (a) Exposure dependence of IRAS spectra of adsorbed C2H4 on Cu(410) at 93 K. Exposure dependence of IRAS (b) peak position and (c) fwhm for the C2H4 adsorbed on Cu(410) (full marks) at 93 K; open marks correspond to the data on Cu(210), taken from ref 6.

Table 1. Vibrational Frequencies of Ethylene on Cu Surfaces (cm−1)a Cu(110)

CH2 wag. CH2 sciss. CC str. CH2 str. a

Cu(410) 93 K IRAS 5.0 L

Cu(410) 145 K HREELS 1.0 L23

Cu(210) 90 K IRAS 1.0 L6

100 K IRAS 1.0 L17

910, 921 1289 1551

935 1290 1569 3018,3134

898 1284 1543

1261 1522

110 K IRAS 0.1 L18

Cu(111) 91 K IRAS 1.0 L16

Cu(100) 80 K HREELS 8.0 L15

906 1275

910 1285 1535 3075

903 1290 1560 2999

ethylene gas49 943 1342 1623 2989− 3103

Vibrational patterns are abbreviated as follows: wag. = wagging, sciss. = scissors, and str. = stretching.

Figure 3a shows IRAS spectra of ethylene adsorbed on Cu(410) at 93 K. The IR bands were assigned to vibrational modes of ethylene based on the previous reports.15−19,23,37 The band which shifted down from 931 to 921 cm−1 is assigned to the CH2 wagging (out-of plane) mode, and the bands at 1551 and 1289 cm−1 are assigned to the CC stretching mode and the CH2 (symmetric) scissors mode, respectively (schematic models of these modes are displayed at the top of Figure 3a). The additional CH2 wagging mode was observed at 910 cm−1 for the exposures above 1 L. Although some features due to the CH2 stretching mode were expected to appear, no feature was evident around 3000 cm−1; presumably its absorbance was below the detection limit (the background noise around 3000 cm−1 is ca. 5 × 10−5 in absorbance units). The assignments are summarized in Table 1. At 0.15 L, the CH2 wagging band appeared and became evident as ethylene dose increased. The two IR bands of the CC stretching and CH2 scissors modes were also observed with very small intensity at 0.15 L and their intensity increased at higher exposures.

was observed. Only the background intensity becomes larger as exposure increases. We calibrate the adsorbate coverage in Figure 2b with TPD peak area by tentatively assuming that the β-state C2H4 saturates and occupies every second on-top site of the step-edge (1/2S) at the exposure of 0.5 L. We separate the β component by utilizing the peak shape (single component) of the TPD spectrum at the low coverage. The corresponding surface coverage is 0.125 ML (1 ML corresponds to 1.53 × 1015 molecules cm−2). The desorption activation energy, Edes(Θ), of the β-state C2H4 on Cu(410) can be estimated from the TPD spectra using the inversion-optimization method34 (see the Supporting Information) assuming the Polanyi−Wigner rate equation.33,35 The so-obtained desorption activation energy of ethylene on Cu(410) is 55 ± 5 kJ/mol at the zero-coverage limit and the corresponding best fit prefactor is 1013.2±1.3 s−1. This value is nearly the same as the DFT calculated value of 50 kJ/mol. One would obtain a value of 51 kJ/mol for the Edes of β state by applying the simple first-order Redhead formula36 with a preexponential factor of 1013 s−1. 27439



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the CC stretching mode of di-σ-bonded ethylene, was not observed at all. On Cu(410) this feature was reported at 1123 cm−1 at 193 K;23 note that this temperature corresponds to the β peak in TPD spectra of Figure 2a. During the heating process of the β state, a di-σ-bonded ethylene may be nevertheless formed in a small amount that is below our detection limit. 3.1.2. Coadsorption of CO and C2H4. The coadsorption of CO and C2H4 was studied using two different procedures, that is: (1) ethylene was dosed on CO preadsorbed surface and (2) CO was dosed on C2H4 preadsorbed surface. Figure 5 shows

The IRAS results in Figure 3a show that ethylene is not strongly perturbed by the surface and retains its π character. The CC stretching band shows a relatively small shift with respect to the free molecule, so it is obvious that the interaction between Cu(410) and ethylene is weak and does not induce a rehybridization from sp2 to sp3. To evaluate the hybridization of ethylene on transition metal surfaces, the π−σ parameter is a good benchmark.10 This parameter can be easily calculated from the frequency of the CC stretching and CH2 scissors modes. It is zero for free ethylene in the gas phase and unity for C2H4Br2. The π−σ parameter of ethylene on Cu(410) at 0.3 L is 0.22, whereas on Cu(210) at 0.5 L it is 0.24.6 Note in passing that π−σ parameter of ethylene on Pd(100) and Ru(100) is 0.78 and 0.74, respectively, where ethylene is dehydrogenated.10,38 The position and width (full-width-half-maximum, fwhm) of the features are plotted in Figure 3b and c, respectively, in comparison with the data of Cu(210).6 On Cu(410) the peaks for the CC stretching and CH2 scissors modes red-shifted slightly, by less than 5 cm−1. In contrast, the frequency of the CH2 wagging mode shifted down by 10 cm−1 and an additional peak appeared at 910 cm−1 above 1 L. The red shifts are smaller on Cu(410) than on Cu(210). On Cu(410), the fwhm of the CC stretching and CH2 scissors modes does not show any clear dependence on the C2H4 coverage. On the other hand, at higher exposures, the fwhm of the CC stretching and CH2 scissors modes increases significantly on Cu(210).6 The fwhm of the CH2 scissors band is 10.2 cm−1 at 0.5 L and 20 cm−1 above 1.0 L. Similarly, the width of the CC stretching mode increases from 9.1 cm−1 at 0.5 L to 14.4 cm−1 above 1.0 L. In contrast, the fwhm of the CH2 wagging mode increases only slightly on Cu(410), while the fwhm on Cu(210) fluctuates between 8.7 and 21 cm−1 up to 1.0 L, and decreases to 9.2 cm−1 at exposures greater than 1.0 L. The increase of fwhm of the CH2 wagging mode on Cu(410) is due to the overlap of the two peaks. Figure 4 shows the IRAS spectra before and after the annealing up to 164 K of 5.0 L ethylene dosed on Cu(410) at 93 K, suggesting that the β peak in TPD corresponds to the πbonded ethylene. The peak around 1150 cm−1, which is due to

Figure 5. (a) TPD spectra for the 6.0 L exposure of C2H4 on the 0.3 L CO predosed Cu(410). The signal of m/e = 28 is reduced to 0.52 times to view the CO signal clearly. The peak at 210 K is due to CO desorption. (b) The corresponding IRAS spectra at each step of coadsorption: first step of CO exposure (top) and second step of C2H4 exposure (bottom).

the TPD and the corresponding IRAS for the CO predosed surface. In particular, the sample was exposed to 6 L C2H4 after the pre-exposure of 0.3 L CO. In the first step of 0.3 L CO dose, CO occupies most of the step-edge on-top sites.39 Thus, from the TPD spectrum of the coadsorption, it might be intuitively anticipated that CO is located at the step-edge and C2H4 is accommodated on the terrace. However, IRAS demonstrates that this is not the case. After the first step of 0.3 L CO dose, the sharp absorption peak corresponding to the

Figure 4. IRAS spectra for the 5.0 L exposure of ethylene on Cu(410) before (bottom curve) and after the annealing to 164 K (top curve). 27440



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C2H4 is accommodated on the terrace. However, IRAS demonstrates that this is not the case. Namely, only two CO peaks were observed at 2045 and 2030 cm−1, which correspond to CO adsorbed at the terrace (see the preceding paragraph), whereas the peak at 2072 cm−1 corresponding to the step-edge CO interacting with the neighboring C2H4 was not observed in this case. IRAS shows only π-bonded ethylene and the wagging mode is not affected by the coadsorption. The observed wagging mode may be largely attributed to the step-edge C2H4. The postadsorption of CO cannot replace the step-edge C2H4 in the adsorption process. After the desorption of C2H4, CO moves to the step-edge sites before its eventual desorption. Figures 5 and 6, therefore, clearly demonstrate that the coadsorption states at 94 K depend on the order of the dose of coadsorbed molecules, even though the similar TPD spectra were observed in both cases. 3.1.3. Dehydrogenation Reactions of C2H4. It was reported previously that carbonaceous residue due to dehydrogenation of ethylene remains on Cu(410) after the desorption of C2H4, and the residue deactivates step-edge sites on Cu(410).23 We also observed the carbonaceous residue in the Auger spectra after a cycle of dosing the ethylene and annealing the sample (Figure 7a). We repeated the dose-annealing cycles: 0.36 L C2H4 dose (β state) at 96 K and annealing to ∼450 K. After five cycles we measured the C AES spectrum that is shown by the top curve in Figure 7a. During each cycle, we measured the TPD spectra shown in Figure 7b. The TPD spectra in Figure 7b do not change so dramatically, as shown in refs 23 and 25, although we observed the carbonaceous residue in the Auger spectra. This discrepancy may be caused by our lower heating rate. Auger measurements were also performed (see Figure 7a) for the CO and C 2 H 4 coadsorbed on Cu(410); the coadsorption system was prepared in a similar way to that in Figure 5 (i.e., C2H4 was dosed on CO preadsorbed surface). We repeated the dose-annealing cycles: 7.2 L C2H4 dose at 96 K on the 1 L CO pre-exposed surface and then annealing to ∼450 K. A total of 1 L CO exposure can cover the whole stepedge and also some part of the terrace. After the first and fifth cycle we measured the C AES spectra that are shown by bottom and middle curves in Figure 7a, respectively, whereas the TPD spectra were measured during each cycle (Figure 7c). 3.2. Computational Results. 3.2.1. Stability of C2H4 Adsorption Sites at Low Coverage. The stability of various adsorption sites of ethylene at low coverage, as predicted by DFT calculations, is shown in Figure 8; here the term low coverage implies only that molecules are not adsorbed on nearneighboring sites (ethylene has no permanent dipole moment thus displaying rather weak lateral interactions at larger intermolecular distances). DFT predicts the following: (i) ethylene prefers to adsorb on top sites, bridge sites are inferior, whereas hollow sites are unstable; notice that bonding to topsite (bridge-site) is tantamount to π-bonded (di-σ-bonded) ethylene; (ii) the stability of top sites follow the S > T1 ≈ T2 trend, whereas B top site is unstable; (iii) the helicopter rotation of ethylene at the top site is relatively free (the rotation barrier is on the order of 10 meV). Due to superior stability of top sites, further discussion will exclusively focus on topbonded ethylene. 3.2.2. Stability of Various C2H4 Adsorption Structures as a Function of Coverage. As shown in Figure 8, ethylene favors the S site by about 0.2 eV over the T1 and T2 sites. In principle, molecules can occupy every second S site, which corresponds to coverage of 1/8 ML. There is no significant change in

on-top CO on the step-edge Cu atom was observed at 2101 cm−1. After the coadsorption of C2H4, three absorption peaks for the CO stretching mode were observed at 2072, 2043, and 2031 cm−1. The peaks at 2043 and 2031 cm−1 may correspond to the terrace CO39 because these very broad vibrational peaks were observed at high CO coverage, while the peak at 2072 cm−1 may be attributed to the step-edge CO interacting with the nearest C2H4. After the coadsorption, some step-edge CO moves to the terrace as result of the replacement with C2H4. IRAS shows only π-bonded ethylene and the wagging mode shifts to the higher frequency due to the interaction between the coadsorbed molecules. Figure 6 shows the TPD and the corresponding IRAS for the C2H4 predosed surface. In particular, the sample was exposed to 0.3 L CO after the pre-exposure of 1 L C2H4. In the first step of the exposure of 1 L C2H4, ethylene occupies the step-edge ontop sites and probably some terrace sites. Similarly to the previous case of Figure 5, it can be again intuitively anticipated from the TPD spectrum that CO is located at the step-edge and

Figure 6. (a) TPD spectra for the 0.3 L exposure of CO on the 1 L C2H4 predosed Cu(410). The signal of m/e = 28 is reduced to 0.50 times to view the CO signal clearly. The peak at 210 K is due to CO desorption. (b) The corresponding IRAS spectra at each step of coadsorption: first step of C2H4 exposure (top) and second step of CO exposure (bottom). 27441



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Figure 7. (a) Carbon KLL AES spectra after the 5 cycles of dose-annealing of 0.36 L C2H4 exposed on Cu(410) at 96 K (top curve), 5 cycles of dose-annealing of 7.2 L C2H4 exposed on 1 L CO predosed Cu(410) (middle curve), and 1 cycle of dose-annealing of 7.2 L C2H4 exposed on 1 L CO predosed Cu(410) (bottom curve). (b) TPD spectra measured during each cycle of repeated the dose-annealing: 0.36 L C2H4 dose (β state) at 96 K and annealing to ∼450 K. After 5 cycles, we measured the C AES spectra, corresponding to the top AES spectrum of (a). Inset in (b) shows the dependence of each cycle on the coverage. (c) TPD spectra measured during each cycle of repeated dose-annealing cycles: 7.2 L C2H4 dose at 96 K on the 1 L CO pre-exposed surface and then annealing to ∼450 K. After the first and fifth cycle, we measure the C AES spectra, corresponding to the bottom and middle AES spectrum of (a), respectively. The plain lines and lines with points correspond to m/e = 28 and 27, respectively. The signal of m/e = 28 (full line) is reduced to 0.73 times to view the CO signal clearly.

Figure 8. Stability of various standalone adsorption modes of C2H4 @ Cu(410), as given by DFT calculations.

are shown along with the average adsorption energy per molecule. Because the average adsorption energies are not a good measure of thermodynamic stability when structures differ in surface coverage, the stability is evaluated by means of adsorption surface free energy, which is plotted as a function of chemical potential of ethylene (μ C 2 H 4 ) in Figure 10.

adsorption energy if less than every second S site is occupied, but significant lateral Pauli repulsion appears for coverages larger than 1/8 ML. According to DFT results, the ethylene does not prefer to adsorb at the terrace, but continues to further occupy the step-edge even after it is approximately half occupied. Thus, small patches of step-edge begin to appear where every S site is occupied. Note that all among the tested high-coverage structures where ethylene is adsorbed partially on step-edge and partially on terrace are less stable than structures where ethylene is adsorbed exclusively on step-edge. This is evidenced by Figure 9, where several selected structures of C2H4 @ Cu(410) at various coverages, Θ ∈ [1/8, 3/8] ML,

Thermodynamically the most stable structure at given μC2H4 is the one with the smallest adsorption surface free energy. This implies that at very low values of chemical potential of ethylene (μC2H4 < −0.54 eV), the clean surface is the most stable, but as the μC2H4 increases, first the low coverage 1/3S structure and 27442



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Figure 9. Most stable identified structures of C2H4 @ Cu(410) at various coverages according to DFT calculations (for 2/8 ML also the secondmost stable structure is shown). Thermodynamically viable structures are marked by green check-marks, whereas nonviable structures are marked by red ×-marks. The average adsorption energies (normalized per C2H4 molecule) are also reported; beware that average |Eads| is not equal to desorption energy (cf. eqs 1 and 2).

Figure 10. DFT calculated adsorption surface free energy (γads) of C2H4 @ Cu(410) as a function of chemical potential of C2H4 (μC2H4). Thermodynamically the most stable structure at given range of μC2H4 is the one with the smallest γads value; the stablest C2H4 @ Cu(410) structures are marked by color labels and the pertinent μC2H4 ranges are indicated by color stripes.

Figure 11. Several DFT optimized structures of coadsorbed C2H4 and CO on Cu(410). The coverage of CO ranges from 4 to 2 molecules per 8 surface Cu atoms of the Cu(410)−(2 × 1) surface supercell, whereas the coverage of ethylene is 1/8 ML. Ethylene does not adsorb if the coverage of CO is greater than about 1/2 ML. The adsorption energies of ethylene, as calculated by eq 3, are also reported.

neglected in above treatment, should relatively favor the mixed (N-m)/NS + m/NT structures over the pure S structure. 3.2.3. Coadsorption of CO and C2H4. Before addressing the coadsorption of CO and C2H4, let us first briefly summarize the adsorption features of CO @ Cu(410). According to current DFT calculations, its adsorption energy is −0.90, −0.86, and −0.83 eV for S, T1 and T2 top-sites, respectively. CO thus favors the step-edge over the terrace sites, but the preference is remarkably small. This small preference seems to be a known feature of CO on copper surfaces.40 CO is also known as a species that displays very small surface-diffusion barrier, which is a further indication of the small site-preference. A widely known deficiency of DFT-GGA functionals, including the currently used PBE, with respect to adsorption of CO on transition metal surfaces is that they favor the adsorption into

then the 1/2S and 3/4S structures are the stablest. Finally, at μC2H4 > −0.13 eV the fully occupied step-edge structure becomes the stablest. Figure 10, therefore, reveals that only structures with the ethylene adsorbed at the step-edge are thermodynamically stable. Note that only a few discrete coverages are considered in Figure 10; there can be many intermediate situations, but the point is that one passes from low coverage to high-coverage S-only structures as the μC2H4 increases. Although structures that involve ethylene adsorbed at terrace sites are not predicted to be thermodynamically stable under the whole viable range of μC2H4, it should be noted that the energy difference between the S and 1/2S + 1/2T structures at 2/8 ML coverage is very small, only 0.03 eV/ molecule; moreover the configurational entropy, which is 27443



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the high-coordination hollow sites, while CO actually favors the low-coordination top sites; they also substantially overestimate the CO−surface bond strengths (e.g., see ref 41). This is also true in the current case: not only is the CO bonding predicted to be too strong, but also the hollow sites are slightly favored over the top sites, by 0.05 eV. However, the step-edge over the terrace preference is similar for both types of sites. For this reason, the focus is devoted to adsorption of CO onto top sites as to be more in line with experiments. A further characteristic of CO adsorption on Cu(410) is that it prefers to form high-coverage structures on Cu(410)42 and it has been also established experimentally that its sticking coefficient is rather high.39 As for the coadsorption, DFT calculations reveal that ethylene can adsorb on preadsorbed CO surface only if the coverage of CO is smaller than about 1/2 ML. Several relevant C2H4+CO @ Cu(410) structures are shown in Figure 11, where the coverage of CO ranges from 4/8 to 2/8 ML and the coverage of C2H4 is 1/8 ML. The adsorption energies of C2H4, as calculated by eq 3, are also reported. Because the coverage of C2H4 is relatively small, the Eads ≈ −Edes relation holds. This figure clearly reveals that structures with C2H4 adsorbed at the step-edge are significantly more stable than those with ethylene adsorbed at the terrace, which is not surprising given that DFT results of separate C2H4 @ Cu(410) and CO @ Cu(410) systems clearly reveal that ethylene displays much larger preference for the step-edge than the CO. Figure 11 also reveals that the CO, if present at sufficient coverage, can considerably diminish the desorption energy of ethylene, which can in turn explain the absence of the β peak of ethylene in TPD spectra of Figures 5 and 6.

Figure 12. Schematic of desorption energetics of C2H4 @ Cu(410) according to DFT calculations. Circles represent the step-edge adsorption sites; solid-circle ≡ occupied site, void-circle ≡ empty site. The desorption energy equals to ΔE = Ebefore‑adsorption − Eafter‑adsorption only if there is no extra barrier (this is true for all but the topmost desorption event); Ebefore‑adsorption and Eafter‑adsorption is the total energy of the system before and after the desorption of pertinent molecule, respectively. Desorption events, which are likely observed experimentally, are marked by green check-marks; the topmost event marked by red question mark is (probably) not observed in current experiments.

completely occupied step-edge is remarkably small, only 0.2 eV, as given by a series of DFT constrained relaxation calculations, where ethylene was desorbed stepwise from the surface. Actually, the net energy difference (ΔE = Ebefore‑desorption − Eafter‑desorption) is even smaller, only 0.09 eV, but there is an additional barrier as indicated by the inset of Figure 12. This extra barrier emerges from the ethylene−surface attraction and the lateral intermolecular repulsion between adjacent molecules. In the course of desorption, first the ethylene−surface bond is broken and only when the ethylene is sufficiently removed from the surface can the two neighboring molecules relax laterally (i.e., they displace by about 0.4 Å toward the just formed vacant site). The desorption temperature corresponding to 0.2 eV is below 100 K; hence, this does not correspond to the low T peak of TPD. Moreover, the step-edge wholly decorated by ethylene is probably not experimentally achievable, because the reverse process, i.e., the adsorption into a single void site within a long patch of decorated stepedge is activated and, correspondingly, the sticking probability would be minutely small. Also, from the thermodynamic point of view, the range of chemical potential of ethylene, where the wholly decorated step-edge is thermodynamically stable, is relatively narrow (see Figure 10). Consequently, it is much more realistic that instead of fully decorated step-edge, short decorated patches would form (e.g., doubles, triples, quadruples) that are separated by at least one void site. The desorption energy from the midst of such a short patch is much larger, 0.35 eV, because in the short patch the molecules can slightly relax toward the void S sites. Next possibility is desorption of ethylene that has only one S nearest-neighbor; the corresponding Edes is 0.43 eV. The last possibility is desorption of standalone ethylene, Edes = 0.52 eV. The corresponding desorption temperatures would be (assuming β = 2 K/s and ν = 1013 s−1): 0.35 eV → 132 K, 0.43 eV → 162 K, and 0.52 eV → 198 K, which is in rather good agreement with experimental peak temperatures of 124, 154, and 196 K.

4. DISCUSSIONS 4.1. Assignment of TPD Peaks of Ethylene to Its Adsorption States. The TPD spectra shown in Figure 2a suggest that three different configurations of ethylene are present on Cu(410). The TPD peak at 196 K corresponds to the ethylene adsorbed on the most stable site. The asymmetric desorption profile above 0.3 L indicates that the second stablest species desorbs at around 160 K. Higher exposure to ethylene resulted in an additional species and its TPD peak maximum was around 124 K. Given that the possibility of the multilayer formation is excluded for α peaks (vide infra), these findings seem to suggest that the β state corresponds to the on-top adsorption on the step-edge Cu atoms, and α1 and α2 states correspond to the on-top adsorption on the terrace Cu atoms. But this cannot be completely right for at least two reasons: (i) in the TPD spectra of the coadsorbed CO and C2H4 on Cu(410) (Figures 5a and 6a) the β peak is missing, although both IRAS and DFT demonstrate that C2H4 is adsorbed at the step-edge also in this case; (ii) substantial occupation of terrace sites by ethylene is not supported by DFT, because such structures are not predicted to be thermodynamically stable. This reasoning seems to suggest that α peaks may originate (also) from the step-edge. A reflection on this issue reveals that the origin of the three peaks boils down to the interplay between strong step-edge preference and Pauli repulsion between the ethylene adsorbed at the adjacent step-edge sites. This is basically a one-dimensional (1D) adsorption system and the ethylene can either have two, one, or zero nearest-neighbor molecules at the step-edge. A simple schematic of possible desorption events along with the associated desorption energetics is presented in Figure 12. The energy required to remove a single ethylene from 27444



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Although these peak temperatures are rather compatible with experiment, there is a problem, because the number of molecules surrounded by left and right nearest-neighbors would be too small if only short decorated patches form during adsorption, and consequently, the relative area of the α1 peak should be much smaller than observed experimentally. Along this line of reasoning, also the relative area of β peak should be larger than experimentally observed; namely, in this step-edge only scenario, the area of the β peak should be about as large as the area of α1 and α2 peaks together. This shortcoming is alleviated by the step-edge + random terrace site scenario. Namely, if the steering into the step-edge sites is not too strong during the adsorption, then some molecules would randomly land on the terrace and adsorb there. Note that the temperature of α1 peak is compatible with ethylene adsorbed at the top sites on the terrace. DFT calculations may suggest that the diffusion barrier is too large for ethylene to be mobile at the adsorption temperature of 93 K (see ref 43). It is also reasonable that the height of the terrace → step-edge diffusion barrier depends on the coverage, that is, for standalone adsorbed ethylene it is likely smaller than for ethylene that have some other neighboring molecules adsorbed at the step-edge. This would explain the absence of α1 peak for low ethylene exposures, because in this case the ethylene would diffuse from terrace to step-edge before desorbing. However, for larger exposures the amount of ethylene on the surface is large enough to prevent the diffusion of ethylene from terrace to step-edge sites as the temperature is raised during the TPD. In this scenario, the α plateau is due to combination of randomly adsorbed ethylene at terrace top sites (α1) and ethylene adsorbed at step-edge and forming decorated patches (α1 is compatible with desorption of ethylene that was surrounded by two nearest-neighbors, whereas α2 appears to correspond to desorption of ethylene with only one nearest-neighbor). Another consequence of the above considerations is that the coverage as tentatively estimated in Figure 2b is somewhat too large, because this figure is based on the assumption that the saturated β peak corresponds to every second occupied stepedge top site. However, if instead of fully decorated step-edge only short decorated patches form then a very simple reasoning reveals that saturated β peak corresponds to less than every second occupied step-edge top site. Let us for the sake of illustration consider two simple examples: (i) step-edge decorated by ethylene doubles separated by a single void (the corresponding 1D unit-cell pattern is •-•-x-, where • = occupied site and x = empty site) and (ii) step-edge decorated by ethylene doubles and triples separated by a single void (the 1D unit-cell pattern is •-•-x-•-•-•-x-). These two examples lead to the following standalone ethylene configurations: (i) •-x-x- and (ii) •-x-x-•-x-•-x-, which have the 1/3 and 3/7 occupied step-edge, corresponding to 1/12 = 0.083 ML and 3/ 28 = 0.107 ML coverage, respectively, both of which are smaller than the tentatively assumed 0.125 ML. In Figure 2b the largest estimated coverage of ethylene is 0.43 ML, however, on the basis of geometrical considerations (i.e., van der Waals size of ethylene molecule) the largest viable monolayer coverage of ethylene should be likely smaller than 3/8 = 0.375 ML, see Figure 13. Thermodynamic consideration based on DFT calculated adsorption surface free energy (Figure 10) reveals that the coverage of 3/8 ML is indeed too large and is not thermodynamically viable. If the coverage of 0.43 ML is rescaled by a correction factor as given by the above two simple models, then the value of 0.29 and 0.37 ML is obtained for

Figure 13. Cu(410) covered by 3/8 ML of ethylene forming the S + 1/2T2 structure. The atomic spheres are drawn with corresponding van der Waals radii. Note that ethylene molecules already overlap and the remaining voids are far too small to accommodate further ethylene molecules. Red dashed lines indicate the position of step-edges (i.e., center of step-edge Cu atoms).

model (i) and (ii), respectively (the corresponding correction factors are 0.083/0.125 and 0.107/0.125). 4.2. Discussion of Ethylene Vibrational Features. From the exposure dependence of IRAS in Figure 3a, the features of IRAS spectra can be related to each desorption peak. At the exposure of 0.15 L (Θ = 0.05 ML), the peak due to the CH2 wagging mode at 931 cm−1 was observed more strongly than the other two modes. The TPD spectrum in Figure 2a shows that only one species was present at 0.15 L. Hence, the IRAS spectrum of this β state is characterized by the strong peak of the CH2 wagging mode and the other weak IRAS features. In a similar way, we relate the CC stretching band and the CH2 scissors band, which became more evident at the exposure of 0.5 L, to the appearance of the α2 peak. Also the broadening of the CH2 wagging mode that emerged at the exposure of 1 L is related to the α1 peak appearance. As shown in Figure 3b and c, the CH2 wagging band evolved differently from the peaks of the two other modes. In addition, when the surface was precovered with 0.3 L of carbon monoxide, the three peaks of C2H4 were clearly detectable (Figure 5b). When ethylene adsorbs with its molecular plane parallel to the surface, the CH2 wagging mode is the only vibrational mode whose transition dipole is arranged along a surface normal. Since IRAS is much more sensitive to a perpendicular vibrational mode (due to “surface-selection rule”), the spectral feature of the CH2 wagging mode is expected to be more evident than the others. In fact, absorbance of the CH2 wagging mode is about 10 times larger than the other modes listed in Table 1 on Cu(111) surface.16 On the other hand, it has been reported that the IRAS features of the CC stretching and CH2 scissors modes are comparable in intensity to that of the CH2 wagging mode when ethylene adsorbs on Cu(110).17,18 Furthermore, only the CC stretching band and the CH2 scissors band are observed on Cu(110) exposed to a larger amount of ethylene. Our IRAS spectra are rather similar to the case of ethylene on Cu(110), although all three modes were comparable in intensity at higher exposures on Cu(410) and also Cu(210). One possible explanation for the large absorbance due to the CC stretching and CH2 scissors modes is that ethylene is tilted with respect to the surface.44,45 On Cu(410), the effect appears enhanced at higher coverages where ethylene adsorbed on the terrace is parallel to the local (100) rather than the global (410) plane (see also section 4.3). Alternatively the C 27445



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for the “parallel” the CC axis seems constrained parallel to the step-edge direction). On the other hand, at larger coverage the helicopter orientation of ethylene is fixed due to lateral intermolecular interactions and only the “perpendicular” S structure is viable. In the case of ethylene on Cu(110) at high coverage, only CC stretching and CH2 scissors modes were observed in IRAS for the adsorption of ethylene on the on-top sites.17,18 Thus, the geometry of S bonded ethylene at higher coverage may be similar to that on Cu(110). The TPD feature around 154 K (α2) and 124 K (α1) is related to the on-top site at the step-edge and on the terrace; such assignment is not inconsistent with the DFT calculations (cf. Figures 8 and 9). These peaks do not correspond to multilayer formation, since no CH2 asymmetric scissors band around 1430 cm−1 was observed in this work. Furthermore, no strong enhancement of the wagging mode was observed even with increasing C2H4 coverage. These modes are IR active for free ethylene in the gas phase, and it has been demonstrated that the corresponding IRAS bands become very strong if the multilayer of ethylene is formed on a Cu surface.37 The TPD and IRAS results for the coadsorption of C2H4 and CO, shown in Figures 5 and 6, also support the lack of multilayer formation. The frequency of the CH2 wagging mode shifted down by 10 cm−1 and additional peak appeared at 910 cm−1 above 1 L. The latter peak may correspond to C2H4 adsorbed on the terrace, whereas the down-shift may be caused by the lateral interactions between adsorbed molecules at larger coverage. Dehydrogenation of C2H4 on Cu(410)23−25 is confirmed by the results presented in Figure 7, because carbonaceous residue is detected on the surface after annealing to ∼450 K. In contrast, dehydrogenation of C2H4 was not observed on Cu(210) in the similar measurements.6 On the Cu(410) the lateral intermolecular interactions across the step-edges are weaker than on Cu(210), as suggested from the frequency shift with coverage in Figure 3b, but the lateral interactions driven by the step-edge decoration effects are stronger on Cu(410), because, according to DFT calculations, the step-edge decoration is not favored on Cu(210). The reactivity toward ethylene is lowered by increasing the density of open steps, indicating that not only the step-edge atoms but also the adsorption sites on terraces are of importance for the dehydrogenation of C2H4. Note, however, that it is not appropriate to discuss the reasons for dehydrogenation solely on the adsorption characteristics of ethylene, because the reaction involves both ethylene as reactant and vinyl as product. Surface-catalyzed dehydrogenations are typical reactions with late (product-like) transition states that well obey the Bronsted−Evans−Polanyi (BEP) linear relationship.48 This implies that adsorption specifics of products are likely more important in such cases. Thus, the fact that dehydrogenation occurs on Cu(410) and not on Cu(210) might be not only due to ethylene but also due to differences of vinyl on the two surfaces. This inference is supported by preliminary DFT calculations, which indeed indicate that vinyl bonds more strongly to Cu(410) than to Cu(210). It should be noted that the desorption activation energy of C2H4 on Cu(210) was estimated to 34 kJ/mol at the zerocoverage limit by the inversion-optimization method and the Polanyi−Wigner rate equation (see Supporting Information),6 whereas the first-order Redhead formula36 with a preexponential factor of 1013 Hz gives the value of 38 kJ/mol for the Edes of the β state. This value appears closer to the

C axis may remain parallel to the surface and the interaction between ethylene and the surface causes the enhancement. An example of the latter can be found in recent experimental study of ethylene adsorption on cold-deposited Cu film,37 suggesting that several Raman bands of ethylene can appear in IRAS spectra due to defect sites on such a surface. On Cu(110), as mentioned above, several studies have already demonstrated that ethylene adsorbs with its CC axis parallel to the surface,21,22,46 even though strong CC stretching and CH2 scissors bands are observed in IRAS studies.17,18 It has also been demonstrated that roughness of Cu surface can increase the intensity of the CC stretching and CH2 scissors bands.37 The Raman-active CH2 scissors and CC stretching modes appear, which are assigned to adsorption at surface enhanced Raman scattering (SERS) active sites. The IR excitation mechanism by transient electron transfer to the adsorbate π* state can deliver a discrete vibrational band of a Raman-active vibration only under certain circumstances. By analogy, we may assume that the CC axis of ethylene on Cu(410) is parallel to the global (410) surface and the adsorption on the SERS-active sites enhances the absorbance of the CC stretching and CH2 scissors modes. 4.3. Adsorption States and Dehydrogenation of Ethylene. It is difficult to determine the exact location of each species from experimental data alone, however, it is plausible that π-bonded ethylene occupies the step-edge sites on Cu(410) surface, since adsorption onto step-edge is generally favored on stepped surfaces. DFT calculations clearly demonstrate that ethylene adsorbs preferably at the step-edge sites on Cu(410) (cf. Figure 8). Another DFT study of ethylene adsorption on rough Cu(111) has also shown that the adsorption energy of ethylene on step-edge is larger by 0.35− 0.38 eV (34−37 kJ/mol) than that of ethylene on flat (111) face.47 CO adsorbs at on-top site of step-edge atoms on Cu(410) and all on-top sites are occupied at Θ = 0.25 ML.39 In the present study, the features of ethylene appearing in IRAS can be assigned partially to the step-edge ethylene, even if the stepedge is predecorated by the CO (see Figure 5). We therefore attribute the dominant features appearing below 0.3 L in Figure 3a as well as the β peak at 196 K in Figure 2a to ethylene adsorbed to on-top sites at the step-edge. Such assignment is strongly supported by the DFT calculations (cf. Figures 8 and 9). The same site assignment was also reported by previous studies.23,24 On this site the CC stretching and the CH2 scissors modes are weaker than the wagging mode at low coverages. On the other hand, the observed relatively clear CC stretching and CH2 scissors modes at higher coverages can be assigned to the S type structures in Figure 9 and, possibly, to the terrace ethylene. The lateral interactions between the molecules on the step-edge may alter the adsorption geometry which can modify the FTIR spectrum. According to DFT calculations, ethylene can helicopter rotate almost without any penalty at lower coverage, e.g., the top-S structure in Figure 8 is drawn with CC axis perpendicular to step-edge direction, but the structure with CC axis parallel to the step-edge is of the same stability at low coverage. It should be noted, however, that it seems reasonable that the CC stretching and CH2 scissors intensity of the “parallel” orientation is smaller than that of the “perpendicular”, because the molecular plane of the “perpendicular” can more easily deviate from the (410) plane (i.e., its CC axis may lean toward either (110)-step-edge or (100)-terrace plane, whereas 27446



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activation energy for the desorption from flat Cu(100) surface than to the activation energy for the desorption from Cu(410) (55 kJ/mol). It is therefore reasonable that on Cu(210) the desorption rate is too large and ethylene cannot reach the reaction channel for dehydrogenation in a detectable amount. This is another factor, in addition to the above arguments related to vinyl, why dehydrogenation was observed only on Cu(410). It was suggested that ethylene adsorbed onto the long-bridge (LB) site at the step-edge of Cu(410) is a metastable species which is readily dehydrogenated to form carbonaceous residue.23,24 However, even after the desorption of the coadsorbed CO and C2H4, where the step-edge CO might be intuitively expected to prevent the C2H4 to adsorb on the stepedge sites, we could detect the similar amount of carbonaceous residue due to dehydrogenation as for only C2H4 doseannealing treatment (see Figure 7a). Even if C2H4 is dosed to the CO precovered surface, it still replaces the step-edge CO and can react at the step-edge as shown in Figures 5 and 7. Dissociation of ethylene leading to hydrogen and carbonaceous species on Cu(410) was detected by HREELS below 190 K.23 This may imply that the C2H4 adsorbed on the step-edge sites contributes dominantly to the dehydrogenation reaction.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge MEXT for a Grant-in-Aid for Scientific Research (Nos. 26248006, 25620013, 20350005, 22655005, and 17550011). This work was also financially supported by The Sumitomo Foundation and the Murata Science Foundation. M.O. was also supported financially by Shin-Etsu Chemical Co., Ltd., Japan. A.K. acknowledges financial support from Slovenian Research Agency (Grant No. P2-0148). We are greatly thankful to Prof. Mario Rocca, Prof. Luca Vattuone, and Mr. Tetsuya Inukai for their valuable discussions and suggestions.

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(1) Vattuone, L.; Savio, L.; Rocca, M. Bridging the Structure Gap: Chemistry of Nanostructured Surfaces at Well-Defined Defects. Surf. Sci. Rep. 2008, 63, 101−168. (2) Blakely, D. W.; Somorjai, G. A. The Dehydrogenation and Hydrogenolysis of Cyclohexane and Cyclohexene on Stepped (High Miller Index) Platinum Surfaces. J. Catal. 1976, 42, 181−196. (3) Klier, K.; Hess, J. S.; Herman, R. G. Structure Sensitivity of Methane Dissociation on Palladium Single Crystal Surfaces. J. Chem. Phys. 1997, 107, 4033−4043. (4) Vattuone, L.; Savio, L.; Rocca, M. Coverage Dependence of the Dynamics of Ethylene Adsorption on Ag(210). J. Phys.: Condens. Matter 2004, 16, S2929−S2936. (5) Savio, L.; Vattuone, L.; Rocca, M. Coverage Dependence of the Sticking Probability of Ethylene on Ag(410). Surf. Sci. 2005, 587, 110− 120. (6) Yamazaki, D.; Okada, M.; Franco, F. C., Jr.; Kasai, T. Ethylene Adsorption on Regularly Stepped Copper Surface: C2H4 on Cu(210). Surf. Sci. 2011, 605, 934−940. (7) Godbey, D.; Zaera, F.; Yeates, R.; Somorjai, G. A. Hydrogenation of Chemisorbed Ethylene on Clean, Hydrogen, and Ethylidyne Covered Platinum (111) Crystal Surfaces. Surf. Sci. 1986, 167, 150− 166. (8) Creighton, J. R.; White, J. M. A SIMS Study of the Dehydrogenation of Ethylene on Pt(111). Surf. Sci. 1983, 129, 327− 335. (9) Bao, S.; Hofmann, Ph.; Schindler, K.-M.; Fritzsche, V.; Bradshaw, A. M.; Woodruff, D. P.; Casado, C.; Asensio, M. C. The Local Geometry of Reactant and Product in a Surface Reaction: The Dehydrogenation of Adsorbed Ethylene on Ni(111). Surf. Sci. 1995, 323, 19−29. (10) Stuve, E. M.; Madix, R. J. Bonding and Dehydrogenation of Ethylene on Palladium Metal. Vibrational Spectra and TemperatureProgrammed Reaction Studies on Pd(100). J. Phys. Chem. 1985, 89, 105−112. (11) Campbell, C. T.; Paffett, M. T. Model Studies of Ethylene Epoxidation Catalyzed by the Ag(110) Surface. Surf. Sci. 1984, 139, 396−416. (12) Cazzanelli, E.; Caruso, T.; Castriota, M.; Marino, A. R.; Politano, A.; Chiarello, G.; Giarola, M.; Mariotto, G. Spectroscopic Characterization of Graphene Films Grown on Pt(111) Surface by Chemical Vapor Deposition of Ethylene. J. Raman Spectrosc. 2013, 44, 1393−1397. (13) Dong, G. C.; van Baarle, D. W.; Rost, M. J.; Frenken, J. W. M. Graphene Formation on Metal Surfaces Investigated by In Situ Scanning Tunneling Microscopy. New. J. Phys. 2012, 14, 053033. (14) Robinson, Z. R.; Ong, E. W.; Mowll, T. R.; Tyagi, P.; Gaskill, D. K.; Geisler, H.; Ventrice, C. A. Influence of Chemisorbed Oxygen on the Growth of Graphene on Cu(100) by Chemical Vapor Deposition. J. Phys. Chem. C 2013, 117, 23919−23927.

5. CONCLUSIONS We reported the results of the combined experimental and computational study on adsorption of ethylene on Cu(410) and compared them to those obtained on other Cu single crystal surfaces. At 93 K ethylene adsorbs molecularly on Cu(410) and TPD and IRAS reveal three types of π-bonded ethylene on the surface, which desorb at 124, 154, and 196 K. The possibility of the multilayer formation was excluded for the low temperature peak. These results may suggest that the hightemperature and the two low-temperature peaks correspond to the ethylene adsorbed at the step-edge and at the two different terrace atoms, respectively. However, further analysis reveals that the actual situation is not as straightforward, because in the CO + C2H4 coadsorption experiments the high-temperature peak of C2H4 is not observed, yet the pertinent IRAS and DFT results clearly reveal that C2H4 is nevertheless adsorbed (also) at the step-edge sites; according to DFT ethylene also displays much larger preference for the step-edge than the CO. These results therefore imply that the step-edge bonded ethylene also contributes, at least partially, to the low temperature peaks. On the basis of a careful analysis of DFT and experimental results we attribute the origin of the three species to the interplay between (i) strong preference of ethylene to adsorb at the stepedge, (ii) significant intermolecular repulsion between the ethylene adsorbed at the adjacent step-edge sites, and (iii) the random adsorption on the terrace sites. Our experimental results further indicate that step-edge sites contribute dominantly to the dehydrogenation of C2H4 on Cu(410).

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ASSOCIATED CONTENT

S Supporting Information *

TPD analysis for C2H4 on Cu(410) and Cu(210). This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

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(15) Nyberg, C.; Tengstål, C. G.; Andersson, S.; Holmes, M. W. Vibrational Excitations and Structure of C2H4 Adsorbed on Cu(100). Chem. Phys. Lett. 1982, 87, 87−91. (16) McCash, E. M. 1989 C R BURCH PRIZE Surfaces and Vibrations. Vacuum 1990, 40, 423−427. (17) Jenks, C. J.; Bent, B. E.; Bernstein, N.; Zaera, F. Evidence for an Unusual Coordination on Cu(110). Surf. Sci. Lett. 1992, 277, L89− L94. (18) Kubota, J.; Kondo, J. N.; Domen, K.; Hirose, C. IRAS Studies of Adsorbed Ethene (C2H4) on Clean and Oxygen-Covered Cu(110) Surfaces. J. Phys. Chem. 1994, 98, 7653−7656. (19) Linke, R.; Becker, C.; Pelster, Th.; Tanemura, M.; Wandelt, K. Adsorption of Ethene on Cu(111). Surf. Sci. 1997, 377−379, 655− 658. (20) Witko, M.; Hermann, K. Different Adsorbate Binding Mechanisms of Hydrocarbons: Theoretical Studies for Cu(111)− C2H2 and Cu(111)−C2H4. Appl. Catal., A 1998, 172, 85−95. (21) Schaff, O.; Stampfl, A. P. J; Hofmann, Ph.; Bao, S.; Schindler, K.M.; Bradshaw, A. M.; Davis, R.; Woodruff, D. P.; Fritzsche, V. Ethene Adsorbed on Cu(110): A Combined Photoemission and Photoelectron Diffraction Study. Surf. Sci. 1995, 343, 201−210. (22) Buisset, J.; Rust, H.-P.; Schweizer, E. K.; Cramer, L.; Bradshaw, A. M. Adsorbate Site Determination with the Scanning Tunneling Microscope: C2H4 on Cu{110}. Phys. Rev. B 1996, 54, 10373−10376. (23) Kravchuk, T.; Venugopal, V.; Vattuone, L.; Burkholder, L.; Tysoe, W. T.; Smerieri, M.; Rocca, M. Ethene Adsorption and Decomposition on the Cu(410) Surface. J. Phys. Chem. C 2009, 113, 20881−20889. (24) Venugopal, V.; Vattuone, L.; Kravchuk, T.; Smerieri, M.; Savio, L.; Jupille, J.; Rocca, M. Dynamics of Ethene Adsorption on Clean and C-Contaminated Cu(410). J. Phys. Chem. C 2009, 113, 20875−20880. (25) Kravchuk, T.; Vattuone, L.; Burkholder, L.; Tysoe, W. T.; Rocca, M. Ethylene Decomposition at Undercoordinated Sites on Cu(410). J. Am. Chem. Soc. 2008, 130, 12552−12553. (26) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (27) Giannozzi, P.; Baroni, S.; Bonini, N.; Calandra, M.; Car, R.; Cavazzoni, C.; Ceresoli, D.; Chiarotti, G. L.; Cococcioni, M.; Dabo, I.; et al. QUANTUM ESPRESSO: A Modular and Open-Source Software Project for Quantum Simulations of Materials. J. Phys.: Condens. Matter 2009, 21, 395502. Code available from http://www.quantum-espresso. org/. (28) Kokalj, A. XCrySDenA New Program For Displaying Crystalline Structures and Electron Densities. J. Mol. Graph. Model. 1999, 17, 176−179. Code available from http://www.xcrysden.org/. (29) Vanderbilt, D. Soft Self-Consistent Pseudopotentials in a Generalized Eigenvalue Formalism. Phys. Rev. B 1990, 41, 7892−7895. (30) Ultrasoft pseudopotentials for H, C, and Cu were taken from the Quantum Espresso Pseudopotential Download Page: http://www. quantum-espresso.org/pseudopotentials/, 2014. Files: H.pbe-rrkjus.UPF, C.pbe-rrkjus.UPF, and Cu.pbe-d-rrkjus.UPF. For oxygen the ultrasoft pseudopotential is taken from ref 50. (31) Monkhorst, H. J.; Pack, J. D. Special Points for Brillouin-Zone Integrations. Phys. Rev. B 1976, 13, 5188−5192. (32) Methfessel, M.; Paxton, A. T. High-Precision Sampling for Brillouin-Zone Integration in Metals. Phys. Rev. B 1989, 40, 3616− 3621. (33) Parker, D. H.; Jones, M. E.; Koel, B. E. Determination of the Reaction Order and Activation Energy for Desorption Kinetics Using TPD Spectra: Application to D2 Desorption from Ag(111). Surf. Sci. 1990, 233, 65−74. (34) Tait, S. L.; Dohnálek, Z.; Campbell, C. T.; Kay, B. D. n-Alkanes on Pt(111) and on C(0001)/Pt(111): Chain Length Dependence of Kinetic Desorption Parameters. J. Chem. Phys. 2006, 125, 234308. (35) King, D. A. Thermal Desorption from Metal Surfaces: A Review. Surf. Sci. 1975, 47, 384−402. (36) Redhead, P. A. Thermal Desorption of Gases. Vacuum 1962, 12, 203−211.

(37) Priebe, A.; Pucci, A.; Otto, A. Infrared Reflection−Absorption Spectra of C2H4 and C2H6 on Cu: Effect of Surface Roughness. J. Phys. Chem. B 2006, 110, 1673−1679. (38) Parlett, P. M.; Chesters, M. A. The Surface Reactions of Ethene on Ru(001) Studied by Reflection−Absorption Infrared Spectroscopy. Surf. Sci. 1996, 357−358, 791−795. (39) Makino, T.; Okada, M. CO Adsorption on Regularly Stepped Cu(410) Surface. Surf. Sci. 2014, 628, 36−40. (40) Vollmer, S.; Witte, G.; Wöll, C. Determination of Site Specific Adsorption Energies of CO on Copper. Catal. Lett. 2001, 77, 97−101. (41) Mason, S. E.; Grinberg, I.; Rappe, A. M. First-Principles Extrapolation Method for Accurate CO Adsorption Energies on Metal Surfaces. Phys. Rev. B 2004, 69, 161401. (42) Kokalj, A.; Makino, T.; Okada, M. Manuscript in preparation. (43) Diffusion from the T2 to T1 site involves the top-T2 → short bridge-T2,T1 → top-T1 path, whereas the diffusion from T1 to S site involves the top-T1 → short bridge-T1,S → top-S path. The short bridge-T1,T2 and -S,T1 sites are by about 0.2 and 0.1 eV less stable than the terrace top sites, respectively (Figure 8). However, the diffusion from top to bridge sites, which are minima on the potentialenergy-surface, involves passing through a transition-state (TS) structure. If the energy of the TS is even marginally above the short-bridge-T1,T2 structure, then the diffusion would be kinetically hindered at 93 K. (44) Pearce, H. A.; Sheppard, N. Possible Importance of a “Metal− Surface Selection Rule” in the Interpretation of the Infrared Spectra of Molecules Adsorbed on Particulate Metals; Infrared Spectra From Ethylene Chemisorbed on Silica-Supported Metal Catalysts. Surf. Sci. 1976, 59, 205−217. (45) Syomin, D.; Koel, B. E. IRAS Studies of the Orientation of Acetone Molecules in Monolayer and Multilayer Films on Au(111) Surfaces. Surf. Sci. 2002, 498, 53−60. (46) Itoh, K.; Kiyohara, T.; Shinohara, H.; Ohe, C.; Kawamura, Y.; Nakai, H. DFT Calculation Analysis of the Infrared Spectra of Ethylene Adsorbed on Cu(110), Pd(110), and Ag(110). J. Phys. Chem. B 2002, 106, 10714−10721. (47) Skibbe, O.; Vogel, D.; Binder, M.; Pucci, A.; Kravchuk, T.; Vattuone, L.; Venugopal, V.; Kokalj, A.; Rocca, M. Ethene Stabilization on Cu(111) by Surface Roughness. J. Chem. Phys. 2009, 131, 024701. (48) Michaelides, A.; Liu, Z.-P.; Zhang, C. J.; Alavi, A.; King, D. A.; Hu, P. Identification of General Linear Relationships between Activation Energies and Enthalpy Changes for Dissociation Reactions at Surfaces. J. Am. Chem. Soc. 2003, 125, 3704−3705. (49) Willian, K. R.; Ewing, G. E. Infrared Spectra and Structure of Ethene on NaCl(100). J. Phys. Chem. 1995, 99, 2186−2193. (50) Dal Corso, A. Density-Functional Perturbation Theory with Ultrasoft Pseudopotentials. Phys. Rev. B 2001, 64, 235118.

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Adsorption of C2H4 on Stepped Cu (410) Surface: A Combined TPD

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