Adsorption and Oxidation of Ethylene on the Stoichiometric and O

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Adsorption and Oxidation of Ethylene on the Stoichiometric and O-Rich RuO(110) Surfaces 2

Zhu Liang, Minkyu Kim, Tao Li, Rahul Rai, Aravind R. Asthagiri, and Jason F. Weaver J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b06865 • Publication Date (Web): 01 Sep 2017 Downloaded from http://pubs.acs.org on September 4, 2017

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The Journal of Physical Chemistry

Adsorption and Oxidation of Ethylene on the Stoichiometric and O-rich RuO2(110) Surfaces Zhu Liang1,†, Minkyu Kim2,†, Tao Li1, Rahul Rai1, Aravind Asthagiri2, Jason F. Weaver1*

1 2

Department of Chemical Engineering, University of Florida, Gainesville, FL 32611, USA

William G. Lowrie Chemical & Biomolecular Engineering, The Ohio State University, Columbus, OH 43210, USA

†

Zhu Liang and Minkyu Kim contributed equally to this work.

*To whom correspondence should be addressed, [email protected] Tel. 352-392-0869, Fax. 352-392-9513

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Abstract We investigated the adsorption and oxidation of ethylene (C2H4) on stoichiometric and oxygen-rich RuO2(110) surfaces using temperature-programmed reaction spectroscopy (TPRS) and density functional theory (DFT) calculations. We find that C2H4 binds strongly on the coordinatively-unsaturated (cus) Ru sites of RuO2(110), and desorbs in a peak at ~315 K during TPRS. According to DFT, C2H4 initially adsorbs in a π-bonded configuration on atop Rucus sites but converts to a more stable di-σ species of the form C2H4O, prior to desorbing or reacting. Our TPRS results show that the stoichiometric RuO2(110) surface exhibits limited reactivity toward C2H4, whereas the O-rich surface is highly active toward promoting the extensive oxidation of C2H4. We find that the absolute yield of reacted C2H4 increases to a maximum with increasing initial coverage of on-top O-atoms (Oot) on RuO2(110), and show that this behavior is accurately described by a model that assumes unit reaction probability of C2H4 molecules adsorbed at RucusOot surface pairs. Our DFT calculations predict that the C-H bond cleavage of adsorbed C2H4 is energetically prohibitive on stoichiometric RuO2(110) relative to desorption, but that Obr-RucusOot surface ensembles provide facile reaction pathways on O-rich RuO2(110), wherein C-H bond cleavage of adsorbed C2H4 is strongly preferred over desorption.


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Introduction Understanding the adsorption and reaction of small hydrocarbons on transition-metal oxides is important for improving the catalytic properties of oxides and developing new catalysts such as hybrid or doped metal-oxides. Late transition-metal oxides can be highly active catalysts for the oxidation of hydrocarbons and other compounds, provided that the surfaces expose pairs of coordinatively unsaturated (cus) metal and oxygen atoms. For example, we have previously shown that alkanes undergo C-H bond cleavage on the PdO(101), RuO2(110) and IrO2(110) surfaces by a mechanism in which the alkanes adsorb strongly on cus-metal atoms and transfer a H-atom to a neighboring cus-O atom.1-3 These prior studies have focused mainly on the activation of alkanes on stoichiometrically-terminated oxide surfaces which expose only one type of cus-oxygen species that is present in the same concentration as the cus-metal atoms. This oxygen species is referred to as a bridging oxygen atom (Obr) on the rutile RuO2(110) and IrO2(110) surfaces because the O-atom bonds with two underlying metal atoms of the oxide lattice. Oxygen adsorption can also produce a second type of reactive oxygen species that bonds on-top of a cus-metal atom, referred to as an on-top oxygen atom (Oot). Investigators have demonstrated that both the Obr and Oot atoms of RuO2(110) are chemically reactive, but that the Oot atoms tend to be more active as H-atom acceptors.1, 4 Systematic investigations to clarify the reactivity of Obr and Oot atoms toward hydrocarbon activation and oxidation are important for developing a molecular-level understanding and description of the catalytic properties of late transition-metal oxides, using, for example, kinetic Monte Carlo simulations. Prior studies reveal a dramatic difference in the ability of stoichiometric (s-) vs. Oot-rich RuO2(110) surfaces to promote the complete oxidation of ethylene.5-6 The s-RuO2(110) surface binds ethylene strongly but exhibits limited reactivity. The addition of Oot atoms to the RuO2(110)



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surface significantly enhances reactivity, resulting in the complete oxidation of a large fraction of the adsorbed ethylene.6 The marked difference in reactivity suggests that Oot atoms promote a reaction step that lies early along the pathway for complete ethylene oxidation, such as initial CH or C-C bond cleavage, whereas Obr atoms are much less effective in initiating reaction and C2H4 mainly desorbs as a result. Details about the reaction mechanism are needed to test this idea, and clarify the factors that are responsible for the large difference in activity of stoichiometric vs. Oot-rich RuO2(110) toward C2H4 oxidation. Such findings may have general implications for understanding the role of Obr vs. Oot atoms in hydrocarbon oxidation on RuO2(110) and potentially other metal-oxide surfaces. In the present study, we investigated the adsorption and oxidation of C2H4 on stoichiometric and O-rich RuO2(110) surfaces. We show that the initial C-H bond cleavage of C2H4 occurs by low-energy pathways at Obr-Rucus-Oot ensembles, but that this reaction is energetically prohibited on the s-RuO2(110) surface. Our results suggest that the large difference in activity of the s- and O-rich RuO2(110) surfaces toward complete oxidation of C2H4 originates entirely from the differences in surface activity toward the initial C-H bond cleavage of C2H4.

Experimental Details The experiments were performed in an ultrahigh vacuum (UHV) chamber with a base pressure of 2 × 10−10 Torr. Details of the chamber have been described previously.7 Briefly the chamber is equipped with an inductively coupled RF plasma source (Oxford Scientific Instruments) for generating atomic oxygen beams, a low energy electron diffraction (LEED) optics (SPECS) for surface order measurements, and a quadrupole mass spectrometer (QMS)


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(Hiden HAL 201) for temperature-programmed desorption (TPD) and reaction spectroscopy (TPRS). The Ru(0001) single crystal (dia. 9 mm × 1 mm) was attached to two 0.016” tungsten wires mounted on an LN2 cooled sample holder. A type K thermocouple was used for temperature measurements. The crystal was cleaned by cycles of Ar+ sputtering (2 keV) at 1000 K and annealing to 1500 K. The surface cleanness was verified with LEED and CO TPRS. With the latter method, CO TPD spectra were collected after exposing O2 to the surface. The absence of a recombinative CO desorption peak indicates the absence of surface carbon. Research grade ethylene (C2H4, 99.99%, Airgas), ultra-high purity oxygen (O2, 99.999%, BOC gases) and oxygen-18 (18O2, 99.999%, Aldrich) were used as received without further purification and the purity of gases was checked by QMS. The stoichiometrically-terminated RuO2(110) film, denoted as s-RuO2(110), was prepared by exposing the Ru(0001) crystal to an atomic oxygen beam at 750 K. As shown previously, with a total exposure of ~76 MLRu(0001) of O atoms (where 1 MLRu(0001) is defined as the surface density of Ru(0001), i.e., 1.57 × 1015 atoms/cm-2), we are able to grow a 4.5 nm thick s-RuO2(110) film that contains ~18 MLRu(0001) of oxygen atoms.8 The as-prepared s-RuO2(110) film was then exposed to 5 Langmuir (L) of O2 at 750 K in order to fill Obr vacancies on the surface and used for further experiments with ethylene. The surface structure of s-RuO2(110) is analogous to the well-known rutile TiO2(110) surface, and contains alternating ruthenium and oxygen rows along the [001] direction (Figure 1a).9 Two types of ruthenium atoms, six-fold and five-fold coordinated Ru, are present on the surface. The five-fold coordinated ruthenium atoms are known as Rucus (coordinatively unsaturated) with one dangling bond perpendicular to the surface. The s-RuO2(110) surface exposes both three-fold and two-fold coordinated O-atoms, with the later known as bridging oxygen atoms, Obr (Figure



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1a). Dissociative adsorption of O2 occurs readily on RuO2(110) and can generate a third type of oxygen atom that adsorbs on top of the Rucus atoms, which we denote as on-top oxygen atoms, Oot (Figure 1b). Previous studies demonstrate that Oot atoms recombinatively desorb from RuO2(110) to produce an O2 TPD peak between 400 and 500 K while the Obr atoms evolve when the entire s-RuO2(110) film thermally decomposes to produce an O2 TPD peak near 1040 K.10-11 This difference in stability has also been confirmed with DFT calculations,12 and facilitates the controlled generation of Oot atoms on RuO2(110). Specifically, we prepared different Oot precoverages by first exposing the s-RuO2(110) surface to 5 L of O2 at 300 K to saturate the surface with Oot atoms (~0.86 ML), followed by heating the O-rich surface to specific temperatures to desorb the desired amount of Oot atoms, where the Oot coverage is determined from integration of the O2 TPD trace. As shown previously, this approach provides excellent control and reproducibility of the initial Oot coverage up to ~0.86 ML for experiments with O-rich RuO2(110).11 The Rucus, Obr, and Oot atoms are chemically active due to their coordinative undersaturation and thus are our main focus in present study.


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Figure 1: Model representation of (a) stoichiometric RuO2(110) and (b) O-rich RuO2(110) surfaces. Bridging (Obr) and on-top (Oot) O-atoms are indicated.

Given that the RuO2(110) unit cell lies along the [001] and [11ത0]directions with lattice constants of 3.12 and 6.38 Å, respectively, one can determine that the surface density of RuO2(110) is 0.32 MLRu(0001). Since surface adsorbates bind strongly on the Rucus sites of RuO2(110), we specify adsorbate coverages in units of ML, where 1 ML is equal to the surface density of Rucus atoms. One RuO2(110) unit cell contains one Rucus and one Obr atom so the surface density of Rucus and Obr atoms are each equal to 1 ML. The maximum coverage of Oot atoms that is achieved in this study is 0.86 ML (~86% of Rucus). This can be understood by the fact that Oot atom diffusion along the Rucus rows has a high energy barrier.10,

13

As the Oot

coverage increases, stranded Rucus sites are formed because of the kinetic limitation to Oot-atom diffusion. The generation of Oot atoms ceases when only single unoccupied Rucus sites remain on the surface, since O2 dissociation requires two adjacent empty Rucus sites. This behavior is 7 ACS Paragon Plus Environment


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analogous to the well-known model of random sequential adsorption of dimers on onedimensional chains, where the maximum coverage is ~0.86 ML.14-16 Ethylene was introduced to the sample via a calibrated beam doser with the sample-to-doser distance set at 15 mm. After an ethylene exposure, TPRS spectra were collected with the sample positioned in front of the shielded QMS and the temperature increased at a constant rate of 1 K/s. After each ethylene TPRS experiment, the RuO2(110) surface was restored by exposing to 5 L of O2(g) at 750 K through a tube doser, which replenishes the Obr vacancies created during reactions. We used the following methods to quantify the absolute product yields from TPD spectra. The CO and H2 yields were estimated by integrating the CO and H2 TPRS spectra and comparing with a known standard for saturation coverage on Ru(0001). The standard CO TPD spectrum was measured by exposing Ru(0001) to CO at 300 K, and assuming a saturation coverage of 0.66 MLRu(0001).17-18 The H2 standard was obtained by exposing Ru(0001) to a saturation amount of H2(g) at 173 K and then annealing to 600 K, which gives a known coverage of 1 MLRu(0001) of H atoms.19-20 The O2 yields were estimated similarly, where the standard spectra were obtained by saturating s-RuO2(110) with O2 at 300 K, which has a saturation coverage of 0.86 ML (86% of Rucus site) in terms of O atoms.21 To estimate H2O yields, we used the stoichiometry of the reaction H2 + Oot → H2O (Oot: H2O = 1:1) where the amount of reacted Oot is quantified as the decrease in recombinative Oot-atom desorption from an Oot-saturated RuO2(110) surface after reacting with H2. For species for which standards are unavailable, such as C2H4 and CO2, the product yields were estimated using a relative sensitivity factor that relates the measured intensities of the target products to a standard with known saturation coverage. For example, the measured CO2 spectrum was first corrected with respect to CO using their relative sensitivity


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factors and then the area under the corrected spectrum was compared to the CO standard to obtain an estimate of CO2 yield. The C2H4 desorption yields were estimated in a similar manner as that for CO2.

Computational Details All plane wave DFT calculations were performed using the projector augmented wave pseudopotentials22 provided in the Vienna ab initio simulation package (VASP).23-24 The Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional25 was used with a plane wave expansion cutoff of 400 eV. Dispersion interactions are modeled using the DFT-D3 method developed by Grimme et al.26 We find that this method provides accurate estimates of the adsorption energies of n-alkanes on PdO(101)2 and RuO2(110)11 in comparison with TPDderived values. We employed four layers to model the RuO2(110) film, resulting in an ~12 Å thick slab. The bottom two layers are fixed, but all other lattice atoms are allowed to relax during the calculations until the forces are less than 0.05 eV/Å. A vacuum spacing of ~25 Å was included, which is sufficient to reduce the periodic interaction in the surface normal direction. In terms of system size, a 1 × 4 unit cell with a corresponding 4 × 2 × 1 Monkhorst-Pack k-point mesh is used. We employed the same slab size in our calculations of C2H4 adsorbed on stoichiometric and Oot-covered RuO2(110). In the present study, we define the binding energy, ‫ܧ‬௕ , of an adsorbed C2H4 molecule on the surface using the expression, ‫ܧ‬௕ = ൫‫ܧ‬஼మ ுర + ‫ܧ‬௦௨௥௙ ൯ − ‫ܧ‬஼మ ுర /௦௨௥௙


(1)


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where ‫ܧ‬஼మ ுర /௦௨௥௙ is the energy of the state containing the adsorbed C2H4 molecule, ‫ܧ‬௦௨௥௙ is the energy of the bare surface, and ‫ܧ‬஼మ ுర is the energy of an isolated C2H4 molecule in the gas phase. All reported binding energies are corrected for zero-point vibrational energy. From the equation above, a large positive value for the binding energy indicates a high stability of the adsorbed C2H4 molecule under consideration. We evaluated the barriers for C-H bond cleavage of adsorbed C2H4 on the RuO2(110) surface using the climbing nudged elastic band (cNEB) method.27 For all the pathways discussed in the paper, the full set of NEB images can be found in the Supporting Information (SI). Unless otherwise noted, our DFT calculations were performed for a single C2H4 molecule adsorbed within the 1 × 4 surface model of RuO2(110), and corresponds to a C2H4 coverage equal to 25% of the Rucus density.

Results and Discussion Adsorption of ethylene on s-RuO2(110) as a function of ethylene coverage Figure 2a shows C2H4 TPD spectra obtained after generating different C2H4 coverages on sRuO2(110) at 88 K. On the stoichiometric surface ethylene desorbs in a peak at 320 K at low coverages, which broadens and shifts to 315 K as the coverage increases. Following the peak labeling in previous studies of C2H4 on RuO2(110) by Paulus et al.,5-6 we denote this peak as the γ state. Later we show that the γ state, consistent with a previous study, arises from C2H4 adsorbed on Rucus sites and that only a small quantity (~0.06 ML) undergoes oxidation during TPRS. Before the γ state saturates, a small peak at 100 K appears and a broad region between 110 K and 260 K develops with increasing coverage. The broad region, denoted as the β state, is consistent with a compressed layer of C2H4 that is commonly observed for small hydrocarbons


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on rutile (110) oxide surfaces.11, 28 Lastly, we attribute the TPD peak at ~100 K (α peak) to C2H4 adsorbed on Obr sites. We find that the α-state coverage saturates at ~0.06 ML on s-RuO2(110) for adsorption at 88 K and is only weakly influenced by pre-adsorbed Oot atoms (see below), consistent with adsorption onto Obr rather than Rucus atoms. Our assignment of the α TPD peak differs from the previous report by Paulus et al.,6 who assigned the α peak to a multilayer state, but agrees with the assignment of a similar peak observed in TPD spectra obtained from C2H4 on the TiO2(110) surface.28 We estimate that the saturation coverage of γ-C2H4 species on s-RuO2(110) is equal to ~0.38 ML, where 0.32 ML desorbs and ~0.06 ML oxidizes to CO, CO2 and H2O at higher temperature. The saturation coverage corresponds to just over one C2H4 molecule per three Rucus atoms in the uncompressed layer. We also find that the sum of the γ and β states that desorb from the surface plus the amount of reacted ethylene, which accounts for the total amount of ethylene adsorbed on Rucus sites, saturates at ~0.60 ML (Figure 2b). We equate the β state coverage with the yield of C2H4 that desorbs in the broad region from ~110 to 250 K. These coverages agree well with saturation coverages of ethylene adsorbed on the TiO2(110) surface, where the uncompressed ethylene layer saturates when 38% of the Ticus sites are occupied and the total amount of ethylene adsorbed on Ticus saturates at 57%.28 Below we show that our estimate of the ethylene saturation coverage on s-RuO2(110) from TPD spectra also agrees reasonably well with DFT predictions.



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total C2H4 on Rucus

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Figure 2. a) C2H4 TPD spectra as a function of the C2H4 coverage obtained after adsorbing ethylene at 88 K on sRuO2(110). b) TPRS yields of molecularly desorbed C2H4 in different states (α, β, γ) and the total C2H4 coverage on Rucus sites (reacted + β + γ) as a function of the ethylene exposure. c) Coverage-dependent desorption energy, Ed(θ), of C2H4 on RuO2(110). The two arrows indicate coverages where the γ and β states saturate and the dashed line indicates the binding energy predicted by DFT-D3 for low C2H4 coverage on s-RuO2(110). We performed an inversion analysis of the TPD spectra shown in (a) with a high coverage of C2H4 (0.73 ML) to obtain the Ed(θ) curve. See the text for details. d) TPRS yields of reacted (CO + CO2), molecularly desorbed (β + γ), and total amount of C2H4 (on Rucus sites) as a function of the ethylene exposure.

Figure 2c shows the coverage-dependent desorption energy obtained by inversion analysis2930

of the TPD data with the rates modeled using the Polanyi-Wigner equation, −

E (θ ) dθ = ν (θ , T )θ n exp( − d ) dt RT


(2)

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where the desorption rate ( − d θ ) is determined by the temperature (T), the instantaneous dt

coverage (θ), the kinetic desorption order (n), the activation energy of desorption (Ed), and the pre-factor of desorption (ν). Assuming first-order desorption for C2H4 (which is consistent with the TPD spectra shown in Fig. 2a), a coverage and temperature independent prefactor, and taking the heating rate as dT/dt = 1 K/s, we obtain the coverage-dependent desorption energy as a function of temperature (T) and coverage (θ) from the following equation,

E d (θ ) = − RT ln( −

1 dθ ) νθ dT

(3)

An upper (νmax) and lower (νmin) bound of the pre-factor (ν) can be estimated by the model developed by Tait et al.,29-31 which has also been utilized in a prior study of alkene desorption from TiO2(110).28 The coverage-dependent desorption energy (Ed) is shown in Fig. 2c, where the black and red curves correspond to Ed(θ) obtained using νmax = 2.2 ×1017 s−1 and νmin = 3.2 ×1013 s−1, respectively. Our analysis predicts that the binding energy of C2H4 in the γ state on sRuO2(110) has lower and upper bounds of 83.5 and 104.1 kJ/mol. As elaborated below, our DFT-D3 calculations predict that C2H4 adsorbed in its most stable configuration on s-RuO2(110) achieves a binding energy of 122.5 kJ/mol (dashed line in Fig. 2c), which is about 17% higher than the upper limit that we estimate from the TPD data and indeed closer to the upper limit than the lower limit. This comparison suggests that the actual pre-factor for C2H4 desorption from sRuO2(110) is close to the maximum value (νmax) computed from the model of Tait et al.30-31 A large pre-factor (νmax) for desorption is physically reasonable in this case, considering that the high corrugation of RuO2(110) is likely to restrict the motions of adsorbed C2H4. Similar findings have been reported for alkanes on RuO2(110)11 and TiO2(110)28, where the desorption pre-factors for small hydrocarbons on a corrugated surface are close to the upper limit (νmax). 13 ACS Paragon Plus Environment


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Despite the relatively strong-binding (large desorption energy), only a small fraction (~10%) of the adsorbed C2H4 undergoes oxidation on the stoichiometric RuO2(110) surface (Fig. 2d). We attribute this limited reactivity to a high activation barrier for the initial C-H bond cleavage on a stoichiometric surface, as discussed below in the section of DFT results. The reaction yield also tracks the C2H4 desorption yield in the γ-TPD peak with increasing C2H4 coverage - both yields initially increase and plateau at nearly the same C2H4 coverages, after which the C2H4 desorption yields in the α and β features begin to increase sharply. This behavior suggests that only C2H4 adsorbed in the uncompressed layer (γ-state) reacts appreciably during TPRS.

Adsorption of ethylene on O-rich RuO2(110) as a function of Oot coverage Figure 3a shows C2H4 TPD spectra obtained as a function of the initial Oot coverage on RuO2(110) after saturating the O-rich surfaces with C2H4 at 88 K. The TPD spectra reveal that the γ peak diminishes sharply while the α peak intensifies with increasing Oot coverage. The β desorption feature diminishes with increasing Oot coverage as well, though to a lesser extent than the γ peak. The γ peak temperature also gradually shifts toward lower temperatures from 315 K to 290 K, whereas the desorption temperature of the α peak is unaffected by co-adsorbed Oot atoms.


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Temperature (K)

Figure 3. a) C2H4 TPD spectra obtained for saturated C2H4 layers prepared at 90 K as a function of the initial Oot coverage on RuO2(110). b) C2H4 desorption yields and total coverage in the γ-state (reacted + γ-desorbed) as a function of the initial Oot coverage on RuO2(110).

Figure 3b shows the desorption yields from the α(Obr), β(Rucus), and γ(Rucus) TPD features and the total quantity of C2H4 initially adsorbed in the γ-state as a function of the initial Oot coverage. We define the total γ-C2H4 coverage as the C2H4 desorption yield in the γ TPD peak plus the total amount of C2H4 that reacts. This definition assumes that only C2H4 adsorbed in the uncompressed layer (γ state) on Rucus sites reacts, and suggests that reaction initiates as compressed configurations of C2H4 depopulate during TPRS. The desorption yield of the γ state decreases nearly linearly with increasing Oot coverage, while the α desorption yield increases slightly. The β desorption yield decreases only above an Oot coverage of ~0.20 ML. At first glance the decrease in the γ-desorption yield appears to be consistent with a site-blocking effect wherein Oot atoms occupy the Rucus sites needed for C2H4 to populate the γ-state. However, we find that the reaction yield (the amount of C2H4 that undergoes oxidation) increases as the Oot coverage increases to ~0.40 ML. We discuss the reactivity of C2H4 on the O-rich RuO2(110) surface below. Assuming that only the γ-state of C2H4 contributes to the reaction yield, we



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estimate that the total amount of γ-C2H4 that adsorbs on Rucus sites remains approximately constant at 0.38 ML as the Oot coverage increases to ~0.40 ML, and decreases thereafter (Figure 3b). This behavior suggests that C2H4 molecules achieve higher local coverages on the O-rich RuO2(110) surface compared with s-RuO2(110) such that the total coverage initially remains constant with increasing Oot coverage, even though the coverage of available Rucus atoms decreases linearly. Below, we present evidence that enhanced local-coverages arise from C2H4 adsorbed within short chains of consecutive Rucus sites, where the Rucus chains are bounded on each end by an Oot atom.

DFT predictions of ethylene adsorption on s-RuO2(110) and O-RuO2(110) Figure 4 shows the favored configurations of C2H4 adsorbed on stoichiometric and O-rich RuO2(110) surfaces that we identified using DFT-D3 calculations. We find that an C2H4 molecule can adopt both π-bonded and σ-bonded configurations on the RuO2(110) surfaces, denoted hereafter as π- and di-σ-C2H4. In the most stable π-bonded configuration, the C2H4 molecule coordinates with a single Rucus atom and adopts a flat-lying geometry with the C=C bond aligned perpendicularly to the Rucus row. The geometry of the π-C2H4 species is identical on the stoichiometric and O-rich RuO2(110) surfaces (Fig. 4a and b), with the corresponding binding energies equal to 98.4 and 96.5 kJ/mol, respectively. The similarity in binding energy and structure demonstrates that neighboring Oot atoms have only a weak influence on the π-C2H4 species. For comparison, we have recently reported that C2H6 binds on s-RuO2(110) in a flatlying 2η1 configuration wherein the C-C bond axis aligns along the Rucus row and is nearly centered between adjacent Rucus atoms (bridge site).11 The configuration of the 2η1 C2H6 complex is generally analogous to that of a di-σ-C2H4 species that would bind to the surface via 16 ACS Paragon Plus Environment

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two C-Rucus σ-bonds, i.e., as Rucus-CH2-CH2-Rucus. However, our calculations predict that a diσ-C2H4 species that bonds only to Rucus atoms is unstable on s-RuO2(110) – in all attempts to generate such a configuration, the C2H4 molecule reverted to a π-bonded configuration during relaxation. Consistent with our findings, Heard et al. predict that C2H4 preferentially bonds in a π-configuration rather than a di-σ-configuration on the metallic Ru(0001) surface as well.32



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(a)

(c) 00

π-C2H4 on s-RuO2(110)

di-σ C2H4 on s-RuO2(110)

(C2H4-Rucus) 98.4 kJ/mol

(Rucus-CH2-CH2-Obr) 122.5 kJ/mol

(b)

(d)

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π-C2H4 on O-RuO2(110)

di-σ C2H4 on O-RuO2(110)

di-σ C2H4 on O-RuO2(110)

(C2H4-Rucus) 96.5 kJ/mol

(Rucus-CH2-CH2-Oot) 150.5 kJ/mol

(Rucus-CH2-CH2-Obr) 125.4 kJ/mol

Figure 4. Preferred configurations of C2H4 adsorbed on (a,c) stoichiometric and (b,d,e) O-rich surfaces of RuO2(110) as predicted by DFT-D3 calculations. The designation of the molecular geometry and the computed adsorption energy is given under the image of each configuration.


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Our DFT-D3 calculations predict that adsorbed ethylene becomes more stable on RuO2(110) by forming di-σ-C2H4 species that bond with a Rucus and an O atom of the surface. The di-σC2H4 species on s-RuO2(110) forms an η2(C,O)CH2CH2O complex by bonding with a Rucus and an Obr atom and achieves a binding energy of 122.5 kJ/mol (Fig. 4c). Figures 4d and 4e show the two configurations predicted for the η2(C,O)CH2CH2O complex on O-RuO2(110), one interacting with an Obr atom and the other with an Oot atom. The predicted binding energy of the di-σ-C2H4 species that binds to an Oot atom (Rucus-CH2-CH2-Oot) is 150.5 kJ/mol, and is 25.1 kJ/mol higher than the binding energy of the species that bonds with an Obr atom (Rucus-CH2CH2-Obr). The η2(C,O)CH2CH2O complex formed on RuO2(110) is similar to the oxometallacycle (OMC) species that has been identified as the intermediate on Ag surfaces that leads to the formation of ethylene oxide.33 Our predictions agree well with prior HREELS results showing that ethylene initially adsorbs in a π-bonded configuration on RuO2(110) at 85 K and transforms to σ-bonded ethylene at 260 K and 200 K on s- and O-rich RuO2(110), respectively. The conversion from π- to σ-bonded configurations occurs at temperatures lower than the main TPD peak temperature for C2H4 desorption from RuO2(110). We thus attribute the γ-C2H4 TPD peak at ~320 K to the desorption of di-σ-bonded η2(C,O)CH2CH2O complexes which form as the surface is heated during TPD. Our DFT-D3 calculations predict an activation barrier of 42.5 kJ/mol for the π to σ conversion of C2H4 on the stoichiometric RuO2(110) surface and a barrier of ~35 kJ/mol on the O-rich surface. These barriers are smaller than the desorption activation energy of the π-C2H4 species (~97 kJ/mol), and would thus be overcome at temperatures lower than those at which π-C2H4 species would desorb at appreciable rates, in good agreement with prior HREELS results5-6 and our assignment of the γ-C2H4 TPD peak to di-σ-C2H4 species.



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Our DFT calculations suggest that an uncompressed layer of C2H4 on s-RuO2(110) would saturate at a coverage of 0.50 ML. More specifically, the calculations predict that C2H4 molecules can adsorb on alternating Rucus atoms while maintaining a strong interaction with the s-RuO2(110) surface, but that configurations with neighboring C2H4 molecules are less stable (see SI). In contrast, our TPD data suggests that the amount of C2H4 adsorbed on Rucus rows saturates at 0.38 ML for the uncompressed layer (γ phase) and at 0.60 ML for the compressed layer (γ + β phase). The lower coverage that we observe in the uncompressed layer from TPD compared with the DFT results may arise, in part, from dynamic effects. We performed ab initio molecular dynamics (AIMD) simulations to study the in-plane rotational motion (“helicopter” rotation) of the π-C2H4 species adsorbed on s- and O-rich RuO2(110) surfaces. These simulations show that the π-C2H4 species readily execute helicoptering motion on s-RuO2(110) at low coverage, and demonstrate that neighbor and next-nearest neighbor π-C2H4 molecules hinder the rotational motion. A possibility is that the adsorbed π-C2H4 species tend to maintain a separation of three lattice constants along the Rucus rows, corresponding to a C2H4 coverage of 0.33 ML, to avoid the dynamic repulsion that results when these species rotate nearly-freely in the surface plane. The AIMD simulations further show that neighboring Oot atoms also hinder the helicopter rotation of π-C2H4 species adsorbed on stranded Rucus sites of O-rich RuO2(110). However, a key difference is that the in-plane rotation of an C2H4 molecule destabilizes nearby C2H4 molecules on s-RuO2(110), whereas the rotational motion of an C2H4 molecule has a negligible effect on the stability of the more strongly-bound Oot atoms. As a result, C2H4 molecules can readily adsorb adjacent to Oot atoms, and may thus be able to achieve higher coverages in an “unperturbed” state on O-rich RuO2(110) compared with s-RuO2(110).


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Oxidation of ethylene on O-RuO2(110) as a function of Oot coverage Figures 5a and b show TPRS spectra obtained after adsorbing saturation amounts of C2H4 on the s-RuO2(110) surface vs. an O-rich RuO2(110) surface with an initial Oot-coverage of 0.70 ML. Most of the C2H4 on s-RuO2(110) desorbs below 400 K, but a small quantity reacts with the surface and produces CO2, CO, and H2O that desorbs above 400 K. Adding Oot atoms to the surface significantly enhances the reactivity of RuO2(110) toward ethylene oxidation. As seen in Figure 5b, oxidation of C2H4 on the O-rich surface produces an intense CO2 TPRS peak at 500 K as well as a smaller CO peak at the same temperature. The concomitant decrease in the quantity of C2H4 that desorbs in the γ-peak demonstrates that a large fraction of C2H4 adsorbed on the Rucus sites undergo extensive oxidation during TPRS.



6

6

TPRS C2H4 + O-RuO2(110) (70% Oot)

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TPRS C2H4 + s-RuO2(110) (0% Oot)

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total γ-C2H4 on Rucus

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0.2

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Figure 5. TPRS spectra of C2H4, H2O, H2, CO and CO2 obtained from s-RuO2(110) with a) 0%, b) 70% Oot precoverage. c) Solid symbols: Total γ-C2H4 coverage (black) and reacted C2H4 yield (red) as a function of the initial Oot coverage on RuO2(110). Dashed lines represent model simulations discussed in the text. d) Fractional γ-C2H4 reaction yield, defined as the reacted amount of C2H4 divided by the initial adsorbed amount of γ-C2H4, as a function of the initial Oot coverage.

The H2O TPRS feature is also different from the O-rich surfaces compared to the stoichiometric surface. On s-RuO2(110), H2O desorbs in a broad region between 450 and 750 K (Fig. 5a), whereas H2O desorbs from the O-rich surface in three distinct peaks centered at ~385, 490, and 575 K (Fig. 5b). Based on previous studies,21 we attribute the H2O TPRS peak at 385 K to the desorption-limited evolution of cus-H2O, meaning that the cus-H2O species form during TPRS at temperatures below that for H2O desorption from RuO2(110). Hydrogen transfer from 22 ACS Paragon Plus Environment

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C2H4-derived fragments or HObr groups to Oot atoms could produce cus-H2O species below ~385 K.34 The H2O peak at 490 K overlaps the CO and CO2 peaks and is consistent with the reactionlimited production of water as C2H4-derived fragments rapidly oxidize and COx species evolve.35 Prior studies demonstrate that the broad H2O peak from 450 to 750 K arises from the recombination of HObr groups.1 We also observe a small H2 TPRS peak from the O-rich surface that evolves at the same temperatures as the CO and CO2 products. Figure 5c shows how the reacted yield of the γ-C2H4 species evolves with the initial Oot coverage, and includes the total γ-C2H4 coverage for comparison. The yield of reacted C2H4 species increases to a maximum value of ~0.25 ML as the Oot coverage increases to ~0.50 ML and decreases thereafter, with the maximum corresponding to about a fourfold increase in the absolute reaction yield relative to ethylene oxidation on the s-RuO2(110) surface. The proportion of adsorbed γ-C2H4 that is oxidized also increases dramatically from 10% to 84% as the Oot coverage increases to saturation (Fig. 5d). These results demonstrate that Oot atoms impart the RuO2(110) surface with high activity toward C2H4 oxidation, whereas the stoichiometric surface exhibits only low reactivity. The large difference in reactivity between the stoichiometric and Orich surfaces suggests that Oot atoms promote a reaction step(s) that occurs early in the pathway for the deep oxidation of C2H4 on RuO2(110) and decides the fate of the adsorbed C2H4 species between desorption and oxidation. Below, we discuss DFT calculations which predict that Oot atoms provide low energy pathways for the initial C-H bond cleavage of ethylene on RuO2(110).

Model of C2H4 adsorption and reaction yields: Distribution of Rucus-Oot pair sites



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Our results demonstrate that high reactivity toward C2H4 oxidation occurs when the RuO2(110) surface initially exposes both Oot atoms and unoccupied Rucus sites. This behavior is consistent with the DFT results that we present below which predict that Rucus-Oot pairs are highly active in promoting C2H4 dissociation on O-rich RuO2(110). One may thus expect that the reacted yield of C2H4 scales with the quantity of Rucus-Oot pairs. To test this idea, we developed a model that explicitly considers C2H4 adsorption and reaction at Rucus-Oot pairs and within the interior of chains of consecutive, unoccupied Rucus atoms of varying length. Our model describes the distribution of Oot atoms on the Rucus rows using the random sequential adsorption (RSA) model for dimers (O2) adsorbing on a one-dimensional lattice.15-16 The RSA model assumes that each adsorbed dimer occupies a pair of adjacent sites and that the dimers adsorb randomly and are immobile on the surface. We expect that the RSA model provides a reasonable description of the distribution of Oot atoms resulting from O2 dissociation on s-RuO2(110) because the Oot atoms encounter a high diffusion barrier10 along the Rucus rows and should thus tend to remain close to their initial positions at moderate temperature. The RSA model describes the Oot-atom distribution in terms of the probabilities of finding chains containing k consecutive, unoccupied Rucus sites, denoted as k-mers. Such distributions of vacant Rucus sites have been observed by STM on an O-rich RuO2(110) surface.36 We provide details of the RSA analysis in the SI. Briefly, we define ܸ௞ as the probability of finding a chain containing exactly k consecutive vacant sites, with each end being occupied by an Oot atom, i.e., ܸ௞ is the void probability.16 We also define ‫ܧ‬ଵ as the probability of finding a single vacant site irrespective of the occupation of adjacent sites, and note that ‫ܧ‬ଵ = 1 - θ where θ is the Oot-atom coverage. Using the RSA model we obtain the following equation for ܸ௞ as a function of ‫ܧ‬ଵ , ܸ௞ = ‫ܧ‬ଵ (‫ ݔ‬௞ିଵ − 2‫ ݔ‬௞ + ‫ ݔ‬௞ାଵ )


(4)

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where, ‫ = ݔ‬lnඥ‫ܧ‬ଵ + 1

(5)

The probability of finding a Rucus‒Oot pair is equal to twice the fraction of vacant k-mers (k ≥ 2) plus the fraction of vacant monomers, where the factor of two accounts for the fact that each kmer is bounded by a Rucus‒Oot pair at each end. The RSA model predicts that the fraction of Rucus‒Oot pairs increases to a maximum value of about 0.29 at an Oot coverage of 0.54 ML and decreases thereafter (see SI), in qualitative agreement with the evolution of the C2H4 reaction yield with the Oot coverage (Fig. 5c). Our model considers the adsorption and reaction of C2H4 molecules adsorbed at Rucus‒Oot pairs and at Rucus sites located in the interior of Rucus k-mers, which we denote as end vs. interior C2H4 species. We assume that each monomer and dimer accommodates one end-C2H4 species, corresponding to local C2H4 coverages of 1.0 and 0.50 ML, respectively, and that k-mers with k ≥ 3 each accommodate two end C2H4 species. We further set the number of C2H4 species adsorbed on interior sites of a k-mer equal to f*(k – 2) where f = 0.38 ML and corresponds to the saturation coverage of C2H4 in the uncompressed layer on s-RuO2(110) (Fig. 2b). This formulation assigns an occupation probability of 0.38 to the interior sites of each k-mer. According to the model, the local coverages decrease toward a limiting value of 0.38 ML with increasing k. A comparison of the black curve in Figure 5c with the experimental data demonstrates that our model accurately reproduces the total γ-C2H4 coverage on RuO2(110) as a function of the Oot coverage. The model predicts that the total coverage changes only weakly with increasing Oot coverage to ~0.40 ML because the fraction of C2H4 molecules on end sites initially increases with the Oot coverage while the fraction on interior sites decreases (Fig. S9b). Above 0.50 ML, both populations decrease with increasing Oot coverage. 25 ACS Paragon Plus Environment


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The red curve shown in Figure 5c represents our simulation of the reaction yield as a function of the Oot coverage using the RSA-based model. We modeled the reaction yield by defining two reaction probabilities corresponding to reaction of C2H4 end-species (Rend) and C2H4 interior species in k ≥ 3 chains (Rint). We set the reaction probability for interior species (Rint = 0.17) equal to the fractional yield of γ-C2H4 that reacts during TPRS when the layer is initially saturated on the s-RuO2(110) surface. We optimized the reaction probability of the end-species and find that a value of Rend = 0.99 provides the best fit to the data. The good agreement between the model and experimental data supports our assertion as well as DFT results that Rucus-Oot pairs are responsible for the high reactivity of the O-rich RuO2(110) surface toward ethylene oxidation and that the fraction of Rucus‒Oot pairs determines the amount of C2H4 that oxidizes.

DFT predictions of ethylene activation on stoichiometric and O-rich RuO2(110) Figures 6a and b show energy diagrams computed using DFT-D3 for the dissociation of C2H4 on s-RuO2(110) via two different pathways. Pathway (a) involves direct C-H bond cleavage of the π-C2H4 species, whereas in pathway (b) π-C2H4 first transforms to di-σ C2H4, and then undergoes C-H bond cleavage. Direct C-H bond cleavage of the π-C2H4 species on s-RuO2(110) occurs by H-atom transfer to an Obr atom, and the pathway features an energy barrier of 68.5 kJ/mol (Fig. 6a). Direct C-H bond cleavage is energetically favored over desorption of the πC2H4 species (68.5 vs. 98.4 kJ/mol). However, the reverse barrier is nearly identical to the C-H bond cleavage barrier (70.5 vs. 68.5 kJ/mol), so regeneration of the π-C2H4 species should occur at a similar rate as C-H bond cleavage. Furthermore, transformation from the π- to di-σ C2H4 species features a barrier of 42.5 kJ/mol (Fig. 6b), with a higher reverse barrier of 66.6 kJ/mol the barrier for the π- to di-σ conversion is ~25 kJ/mol lower than the barrier for direct C-H bond 26 ACS Paragon Plus Environment

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cleavage. These results suggest that conversion of π-C2H4 species to the di-σ C2H4 species is strongly preferred over direct C-H bond cleavage of the π-C2H4 species both kinetically and thermodynamically. Our calculations also predict that the di-σ C2H4 species on s-RuO2(110) must overcome an energy barrier of 140.9 kJ/mol to undergo C-H bond cleavage (Figure 6b). Since the binding energy of the di-σ C2H4 species relative to the gas-phase is 122.5 kJ/mol, we conclude that desorption of the di-σ C2H4 species is favored over C-H bond cleavage on sRuO2(110). The DFT results thus suggest that the limited ability of the s-RuO2(110) surface to activate a C-H bond is responsible for the low activity of s-RuO2(110) towards C2H4 oxidation.

a) Direct C-H bond cleavage of π-C2H4 species on s-RuO2(110)



0

C2H4(g) -29.9

Binding energy (kJ/mol)

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C2H3 + HObr

π-C2H4 -200

b) Conversion of π-C2H4 to di-σ C2H4 on s-RuO2(110), followed by C-H bond cleavage

Figure 6. Energy diagrams for C-H bond cleavage of C2H4 on s-RuO2(110). (a) Direct C-H bond cleavage of πC2H4 species. (b) Conversion of π-C2H4 to di-σ C2H4Obr species followed by C-H bond cleavage. The Ru and O atoms are represented as cyan and red spheres, respectively.


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In contrast to the low reactivity found for the s-RuO2(110) surface, we identified two reaction pathways in which C-H bond cleavage of C2H4 is strongly favored over desorption on the O-rich RuO2(110) surface (Figure 7a and b). In pathway (a), the adsorbed π-C2H4 species first converts to a di-σ C2H4 species (OMC intermediate) by bonding with an Obr atom, and the resulting C2H4Obr species subsequently undergoes C-H bond cleavage via H-atom transfer to a neighboring Oot atom (Fig. 7a). Formation of the di-σ C2H4Obr species requires surmounting an energy barrier of only 33.8 kJ/mol and is highly favored over desorption. The calculations also predict that the C-H bond cleavage reaction, represented as Oot + C2H4Obr → HOot + C2H3Obr, is exothermic by 53.1 kJ/mol and features a barrier of only 13.5 kJ/mol. In pathway (b), the π-C2H4 species converts to a di-σ C2H4 species bonded to an Oot atom, and subsequently a neighboring Obr atom acts as a H-atom acceptor during C-H bond cleavage via the reaction, Obr + C2H4Oot → HObr + C2H3Oot. According to DFT-D3, the π- to di-σ-C2H4 conversion involves an energy barrier of 35.7 kJ/mol, which is similar to pathway (a), and the C-H bond cleavage reaction is exothermic by 149.5 kJ/mol and subject to a moderate barrier of 51.1 kJ/mol (Fig. 7b). The predicted energetics indicate that C-H bond cleavage is highly favored over desorption for C2H4 species adsorbed at Rucus-Oot pairs of O-rich RuO2(110). The apparent reaction barriers, defined as the difference between the barrier for reaction and desorption, are equal to -99.3 and -111.9 kJ/mol for the di-σ-C2H4Obr and C2H4Oot species, respectively. These apparent barriers suggest that every adsorbed π-C2H4 molecule that can access an Oot atom will undergo C-H bond cleavage rather than desorbing, in excellent agreement with our model simulations (Fig. 5c). In contrast, the energetics predicted by DFT suggest that C-H bond cleavage is suppressed relative to C2H4 desorption on the s-RuO2(110) surface. These trends in the C-H bond cleavage activity are consistent with our experimental findings that s-RuO2(110) exhibits low



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activity toward the complete oxidation of C2H4 whereas O-rich surfaces are highly active. An implication is that the large difference in the activity of the s- and O-rich RuO2(110) surfaces toward complete oxidation of C2H4 originates entirely from the differences in surface activity toward the initial C-H bond cleavage of C2H4. The calculations also identify an Obr-Rucus-Oot ensemble as the active structure for initial C-H bond cleavage of C2H4 on RuO2(110). The predicted C-H bond cleavage pathways generate distinct adsorbed intermediates with bonding characteristics that are consistent with previous HREELS characterization of C2H4 oxidation on O-rich RuO2(110).6 The C2H3Obr product generated in pathway (a) is characterized by a C-O single bond and effectively a Ru=C double bond as represented by Rucus=CH-CH2-Obr. The C2H3Oot product generated in pathway (b) forms a C=O double bond and a Ru-C single bond as represented by Rucus-CH2-CH=Oot. Formation of the C=O double bond makes the C2H3Oot species more stable than the C2H3Obr species, by ~122 kJ/mol. The HREELS study by Paulus et al. demonstrates that adsorbed intermediates with C-O single and/or C=O double bonds form after heating a C2H4 layer on O-rich RuO2(110) to 260 and 350 K.6 This observation is generally consistent with our prediction that facile C-H bond cleavage can occur through pathways leading to a mixture of C2H3Obr and C2H3Oot species. Comparison with prior computational results shows that C-H bond cleavage of the di-σ-C2H3Oot species is also favored over other possible reactions, including ring closure and a 1,2 H-atom shift to produce ethylene oxide or acetaldehyde, respectively.13 Although we did not explore additional reaction steps with DFT, we expect that the C2H3O intermediates preferentially undergo further dehydrogenation and deep oxidation to CO2 and H2O on O-rich RuO2(110).


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b) C2H4(g) 0

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-100

-60.8 -99.3 -96.5

-150

-150.5

-200

π-C2H4 -250

di-σ-C2H4Oot -300

-300.1

C2H3Oot + HObr

Figure 7. Energy diagrams for C-H bond cleavage pathways of C2H4 on O-rich RuO2(110) resulting in the formation of (a) HOot + C2H3Obr and (b) HObr + C2H3Oot species. The Ru and O atoms are represented as cyan and red spheres, respectively.



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Ethylene oxidation on 18O-RuO2(110) To further understand the role of Oot atoms during C2H4 oxidation, we investigated the reaction of C2H4 on 18Oot pre-covered surfaces where the coverage of 18Oot atoms was varied in the same manner as described in the experimental section. Figure 8a shows the total TPRS yields of CO2, CO, H2O, and H2 as a function of the initial

18

Oot coverage. The CO2 yield exhibits a

sharp jump from an Oot-free surface to a surface covered by only 10% Oot atoms and thereafter increases monotonically as the Oot coverage increases to 0.70 ML. The CO yield also initially increases but then decreases as the Oot coverage increases beyond 0.40 ML. The sharp decrease in the CO yield causes the total C2H4 reaction yield to pass through a maximum as a function of the Oot coverage (Fig. 5c), even though the CO2 yield continues to increase. The H2O yield exhibits similar behavior as that of CO2, but appears to reach a maximum at an Oot coverage of 0.50 ML. We also observed H2 production only on the O-rich surface and find that the H2 yield increases slowly with increasing Oot coverage to 0.70 ML. All of the product yields decrease abruptly as the Oot coverage increases above 0.70 ML.


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0.6

0.6

a

TPRS yields 18 C2H4 + O-RuO2(110)

0.5

H2O

0.4

CO2

0.3


b



0.5

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total H2O

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0.3 bridge hydroxyl 0.2 rxn-limited

CO 0.1

0.1 cus-H2O

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d 0.5

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16

C O O 18

C O2 0.1

0.4 16

H2 O 0.3

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H2 O

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Figure 8. a) Total TPRS yields of CO2, CO, H2O, and H2 as a function of the initial 18Oot coverage obtained after adsorbing C2H4 to saturation on 18Oot pre-covered RuO2(110). b) Total H2O TPRS yield and yields of H2O desorbing via recombination of HObr groups, reaction-limited H2O and desorption-limited cus-H2O as a function of the initial 18Oot coverage. c) TPRS yields of C18O2 (blue), C18O16O (green), and C16O2 (red) and d) TPRS yields of H216O (red) and H218O (blue) as a function of the initial 18Oot coverage.

The oxidation of C2H4 on O-rich RuO2(110) produces H2O in three distinct TPRS peaks that we attribute to the desorption of cus-H2O, reaction-limited H2O evolution and recombination of HObr groups (Fig. 5b). Figure 8b shows how the TPRS yields of these H2O peaks evolve with the Oot coverage. The recombination of HObr groups accounts for more than 50% of the total H2O production and the yield remains constant at ~0.22 ML for Oot coverages between 0.10 and 0.70



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ML. In contrast, the yield of reaction-limited H2O increases to a maximum of ~0.15 ML at an Oot coverage of 0.50 ML and decreases slowly thereafter. The cus-H2O yield increases only moderately as the Oot coverage increases, and contributes less than 10% of the total H2O production. These results demonstrate that the formation of cus-H2O at low temperature plays only a minimal role in promoting ethylene oxidation on O-rich RuO2(110). An implication is that the removal of hydrogen from the surface and the resulting regeneration of reactive O-atoms is not a primary reason that Oot-atoms enhance ethylene oxidation on RuO2(110). This conclusion is consistent with our DFT predictions that the initial C-H bond cleavage of C2H4 is energetically unfavorable relative to desorption on s-RuO2(110) but that Oot atoms provide low-energy pathways for C-H bond cleavage. Figures 8c and d show how the yields of evolve with increasing

18

18

O and

16

O containing CO2 and H2O products

Oot coverage. The data reveals that about 85% of the

incorporated in the CO2 and CO products for

18

Oot atoms are

18

Oot coverages up to 0.70 ML, and that only a

small fraction desorbs as H218O. This partitioning of

18

O atoms between the COx and H2O

products suggests that Oot atoms mainly participate in reaction steps involving C-O bond formation. We estimate that the number of C-18O bonds in the COx products is equal to or slightly greater than the coverage of Rucus-Oot pairs at all initial

18

Oot coverages, possibly

suggesting a tendency for C-O bonds to form with the Oot atoms present at the ends of Rucus kmers. Lastly, it is worth mentioning that the C18O16O and H2 yields exhibit similar slopes as a function of the initial

18

Oot coverage. This similarity as well as the overlap of the CO2 and H2

TPRS peaks (Fig. 5b) may indicate that a bidentate

18

Oot-CH2-16Obr surface intermediate forms

after C-C bond cleavage, and liberates H2 and C18O16O products via a reaction-limited process. Overall, the isotope experiments demonstrate that most of the on-top oxygen atoms are


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incorporated into the carbon-containing products, and thus that the Oot atoms mainly participate in C-O bond formation during C2H4 oxidation on O-rich RuO2(110).

Summary We studied the adsorption and oxidation of C2H4 on stoichiometric and oxygen-rich RuO2(110) surfaces using TPRS and DFT calculations. We find that C2H4 preferentially adsorbs onto the Rucus sites of s-RuO2(110) surface and binds relatively strongly up to a saturation coverage of ~0.38 ML, followed by the development of a compressed layer to 0.60 ML. The sRuO2(110) surface exhibits low reactivity toward C2H4 as only about 17% of the strongly-bound species oxidizes during TPRS. In contrast, our results show that Oot atoms strongly promote the oxidation of C2H4 on RuO2(110), with reaction producing CO2, CO, H2O and H2 that desorb above 400 K. We find that the yield of reacted C2H4 increases to a maximum at an Oot coverage of ~0.50 ML, reaching a value that is four times greater than reaction yield obtained on the sRuO2(110) surface. We show that the evolution of the C2H4 reaction yield as a function of the initial Oot coverage is accurately described by an RSA-based model that assumes unit reaction probability for C2H4 molecules adsorbed at Rucus-Oot pairs. Our DFT calculations predict that C2H4 initially binds in a π-configuration on top of a Rucus atom and that the π-C2H4 species reacts readily with a surface O-atom to produce more stable, di-σ-bonded species of the form C2H4O, prior to desorbing or further reacting. The C2H4 TPD peak at ~315 K is consistent with desorption of di-σ-C2H4O species. The DFT calculations predict that the di-σ-C2H4Oot species is more stable than the di-σ-C2H4Obr species by ~25 kJ/mol on O-rich RuO2(110), though the activation barriers governing their formation from the π-C2H4 species are similar in value at ~40 kJ/mol. Our DFT calculations also predict that C-H bond 35 ACS Paragon Plus Environment


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cleavage is strongly preferred (by ~100 kJ/mol) over desorption for C2H4O species formed at Rucus-Oot pairs, whereas C2H4 desorption is favored over C-H bond cleavage on s-RuO2(110). We identified two favorable pathways for C-H bond cleavage of C2H4 at a Rucus-Oot pair, one producing a C2H3Oot species and a HObr group and the other producing a C2H3Obr species and a HOot group. The DFT calculations support the conclusion that Rucus-Oot pairs are the active sites for C2H4 oxidation on RuO2(110), and suggest that the significantly higher reactivity of O-rich vs. stoichiometric RuO2(110) arises from facile pathways for initial C-H bond cleavage of C2H4 at Rucus-Oot surface pairs.

Supporting Information Coverage dependence of C2H4 binding energy on s-RuO2(110) from DFT; Helicopter mode of C2H4 on s- and O-rich RuO2(110); Molecular images used in NEB calculations; Distribution of vacant Rucus k-mers as a function of Oot coverage.

Acknowledgements We gratefully acknowledge SABIC for financial support of this work. We also acknowledge the Ohio Supercomputing Center for providing computational resources.

References 1. 2. 3. 4. 5.

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ToC graphic C2H4

X

CO2 + H2O

s-RuO2(110)

O-rich RuO2(110)

CO2 + H2O


Adsorption and Oxidation of Ethylene on the Stoichiometric and O

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