Adsorption and Oxidation of n-Butane on the Stoichiometric RuO2

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Adsorption and Oxidation of n-Butane on the Stoichiometric RuO(110) Surface Tao Li, Rahul Rai, Zhu Liang, Minkyu Kim, Aravind R. Asthagiri, and Jason F Weaver J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b02316 • Publication Date (Web): 12 Apr 2016 Downloaded from http://pubs.acs.org on April 18, 2016

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The Journal of Physical Chemistry

Adsorption and Oxidation of n-Butane on the Stoichiometric RuO2(110) Surface Tao Li1, Rahul Rai1, Zhu Liang1, Minkyu Kim2, 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

*To whom correspondence should be addressed, [email protected] Tel. 352-392-0869, Fax. 352-392-9513 1 ACS Paragon Plus Environment


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Abstract We investigated the adsorption and oxidation of n-butane on a stoichiometric RuO2(110) surface using temperature-programmed reaction spectroscopy (TPRS) and density functional theory (DFT) calculations. At low coverage, molecularly adsorbed n-butane achieves a binding energy of ~100 kJ/mol on RuO2(110), consistent with a strongly-bound σ-complex that forms through dative bonding interactions between the n-butane molecule and coordinatively unsaturated (cus) Ru atoms. We find that a fraction of the n-butane reacts with the RuO2 surface during TPRS to produce CO, CO2 and H2O that desorb above ~400 K, and present evidence that adsorbed σ-complexes serve as precursors to the initial C-H bond cleavage and ultimately the oxidation of n-butane on RuO2(110). From measurements of the product yields as a function of surface temperature, we estimate that the initial reaction probability of n-butane on RuO2(110) decreases from 9% to ~4% with increasing surface temperature from 280 K to 300 K, and show that this temperature dependence is accurately described by a precursor-mediated mechanism. From kinetic analysis of the data, we estimate a negative, apparent activation energy of -35.1 kJ/mol for n-butane dissociation on RuO2(110) and an apparent reaction pre-factor of 6 × 10-8. The low value of the apparent reaction pre-factor suggests that motions of the adsorbed n-butane precursor are highly restricted on the RuO2(110) surface. DFT calculations confirm that n-butane forms strongly-bound σ-complexes on RuO2(110) and predicts that C-H bond cleavage is strongly favored energetically. The n-butane binding energies and energy barrier for C-H bond cleavage predicted by DFT agree quantitatively with our experimental estimate.

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Introduction Advancing the fundamental understanding of alkane activation on oxide surfaces is important for improving the catalytic processing of alkanes in energy conversion and environmental applications as well as designing new approaches for efficiently transforming low molecularweight alkanes to value-added products. Model surface science studies can be challenging, however, because alkanes interact only weakly with many metal oxides and consequently desorb rather than dissociating during temperature programmed reaction spectroscopy (TPRS) experiments. The PdO(101) surface has been an exceptional case as this surface readily promotes the C-H bond cleavage of molecularly-adsorbed n-alkanes (> C2) at temperatures as low as 200 K under ultrahigh vacuum (UHV) conditions.1-4 A key finding from this prior work is that alkane C-H bond activation occurs on PdO(101) by a mechanism in which adsorbed alkane σcomplexes serve as precursors to initial C-H bond dissociation. We have shown that σ-complex formation strengthens the binding of alkanes on PdO(101) in addition to weakening the Pdcoordinated C-H bonds, with these effects acting together to render the PdO(101) surface highly active toward alkane C-H bond cleavage. While experimental and computational evidence of adsorbed alkane σ-complexes has been reported only for the PdO(101) surface, recent density functional theory (DFT) calculations predict that the formation and facile C-H bond activation of alkane σ-complexes should also occur on RuO2 and IrO2 surfaces.1, 5 In support of this prediction, Erlekam et al.6 have reported TPD results showing that methane and ethane desorb from the RuO2(110) surface at temperatures that are considerably higher than typical peak temperatures for these alkanes adsorbed on close-packed metal surfaces.7 Density functional theory (DFT) calculations indeed predict that strongly-bound methane σ-complexes form on RuO2(110) through dative 3 ACS Paragon Plus Environment


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interactions with the Rucus atoms,1 analogous to the binding of alkanes on PdO(101). Those calculations also predict that the energy barrier for C-H bond cleavage is higher than that for desorption of the methane σ-complex, and thus suggest that the methane complexes will react negligibly with RuO2(110) during TPD, consistent with previous experimental findings.6 Taken together, these prior results demonstrate that alkane σ-complexes form on RuO2(110), and that such complexes will dissociate during TPRS experiments if the alkane chain length is sufficiently long to cause the barrier for dissociation to fall below that for molecular desorption. In the present study, we show that n-butane forms strongly-bound σ-complexes on the stoichiometric RuO2(110) surface, and that a fraction of these complexes undergo C-H bond cleavage and ultimately oxidation during TPRS. We report kinetic parameters which show that the C-H bond cleavage of n-butane complexes on RuO2(110) is strongly favored energetically but strongly disfavored entropically. Our results suggest that the σ-complex mechanism is a common pathway for alkane activation on late transition-metal oxide surfaces which expose pairs of coordinatively unsaturated (cus) metal and oxygen atoms.

Experimental Details Details of the three-level UHV chamber utilized in the present study have been reported previously.8-9 Briefly, the Ru(0001) crystal employed in this study is a circular disk (9 mm × 1 mm) that is attached to a liquid-nitrogen cooled, copper sample holder by 0.016” W wires that are secured to the edge of crystal. A type K thermocouple was spot-welded to the backside of the crystal for temperature measurements. Resistive heating, controlled using a PID controller that varies the output of a programmable DC power supply, supports linearly ramping from 80 K to 1500 K or maintaining the sample temperature. Sample cleaning consisted of cycles of Ar+ 4 ACS Paragon Plus Environment

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sputtering (2000 eV, 10 mA) at 1000 K, followed by annealing at 1500 K for several minutes. We considered the Ru(0001) sample to be clean when we obtained sharp low energy electron diffraction (LEED) patterns consistent with the Ru(0001) surface, and did not detect CO production during flash desorption after adsorbing oxygen. The inductively coupled RF plasma source (Oxford Scientific Instruments) is housed in a two-stage differentially pumped chamber attached to the UHV chamber and was utilized to partially dissociate O2 (BOC gases 99.999%) to generate atomic oxygen beams for this study. Details about the beam system can be found in previous publications.8-9 To produce a stoichiometrically-terminated RuO2(110) film (“s-RuO2(110)”), we exposed a Ru(0001) sample held at 750 K to an ~76 ML dose of gaseous oxygen atoms supplied by the atomic oxygen beam, where we define 1 ML as equal to the Ru(0001) surface atom density of 1.57 × 1015 cm-2. To ensure uniform impingement of the beam across the sample, we positioned the sample at a 45° angle from the beam axis and at a distance of about 50 mm from the end of the final collimating aperture. We find that this procedure generates a conformal RuO2(110) film that has a stoichiometric surface termination, contains ~18 ML of oxygen atoms and is about 6 nm thick. We present O2 TPD spectra and LEED images obtained from the oxidized Ru(0001) surface and the conformal s-RuO2(110) film in the Supporting Information (SI). Our findings are consistent with extensive prior studies showing that a RuO2(110) layer develops preferentially during the oxidation of Ru(0001).10-11 The rutile RuO2(110) surface is characterized by a rectangular unit cell with dimensions of (3.12 × 6.38 Å) with the corresponding lattice vectors aligned along the [100] and [110] crystallographic directions, respectively. The stoichiometric termination of RuO2(110) consists of rows of cus-Ru atoms (Rucus) separated by rows of bridging-O atoms (Obr) that run parallel to



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the [100] direction. The Rucus and Obr atoms each lack a bonding partner compared with the bulk, and thus expose single coordination vacancies toward the vacuum. From the RuO2(110) unit cell, one finds that the areal densities of Rucus atoms and Obr atoms are each equal to 0.32 ML. Throughout this paper, we designate absolute surface coverages obtained on RuO2(110) in units of ML where 1 ML is equal to the surface atom density of Ru(0001). A Rucus atom coverage of 0.32 ML thus means that the absolute density of Rucus atoms on RuO2(110) is equal to 32% of the Ru atom density on Ru(0001). We use this coverage scale to facilitate comparisons between the RuO2(110) and Ru(0001) surfaces and also to establish consistency with the coverages reported in our prior studies with PdO(101). Lastly, in some cases, we use the symbol “ot” to describe atoms or molecules that bind “on-top” of the Rucus atoms, such as Oot atoms. We studied the reactivity of n-C4H10 (Airgas, 99.99%), and n-C4D10 (Aldrich, 98 atom % D) on the s-RuO2(110) surface using TPRS. We delivered n-butane to the sample from a calibrated beam doser at an incident flux of approximately 0.0045 ML s-1 (n-C4H10) and 0.0041 ML s-1 (nC4D10) with the sample-to-doser distance set to about 15 mm to ensure uniform impingement of n-butane across the sample surface. We collected TPRS spectra after n-butane exposures by positioning the sample in front of a shielded mass spectrometer at a distance of about 8 mm and then heating at a constant rate of 1 K s-1 until the sample temperature reached 800 K. Initially, we monitored a wide range of desorbing species to identify the main products that are generated from reactions of n-butane on s-RuO2(110). We specifically conducted TPRS experiments in which we monitored each mass between 1 and 100 amu using a short dwell time and a heating rate of 0.5 K s-1. These experiments reveal that the only species desorbing from the n-butaneexposed s-RuO2(110) sample are n-butane, water, CO and CO2. We find that the stoichiometric surface termination can be fully restored after our TPRS experiments by exposing the surface to


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5 L of O2 at 750 K supplied through a tube doser.10 We confirmed that our surface restoration approach is effective by performing TPRS experiments with CO to titrate bridging O-atoms (see SI). This restoration approach is effective because the final temperature reached during the TPRS measurements lies below that at which the RuO2(110) layer begins to thermally decompose. We estimate n-butane coverages by scaling the integrated desorption spectra of mass 43 amu by an integrated TPD spectrum collected from a monolayer of n-butane adsorbed on Ru(0001) at 110 K and assuming that the monolayer saturates at 0.20 ML on Ru(0001). This coverage scaling assumes that n-butane monolayers achieve the same saturation coverages on the Ru(0001) and Pt(111) surfaces since the value of 0.20 ML corresponds to the saturation coverage of an nbutane monolayer on Pt(111) as determined previously from calibrated molecular beam experiments.12 This assumption is reasonable because prior studies show that physically adsorbed alkanes experience similar binding energies on close-packed surfaces of transition metals,7 and we have further found that n-butane monolayers saturate at the same coverages on Pt(111) and Pd(111).4,

13

In fact, the absolute saturation coverages of propane and n-butane

monolayers are about 0.20 ML on the PdO(101) and TiO2(110) surfaces as well,2, 4,

14

even

though the structures of these oxide surfaces are significantly different from close-packed metal surfaces. These prior results suggest that intermolecular interactions play a dominant role in determining the packing densities in alkane monolayers, and that the saturation coverages are nearly independent of the molecule-surface interaction.15 We estimate the n-butane incident flux at the sample surface by fitting the initial portion of the n-butane uptake curve on Ru(0001) with a linear function, and assuming an initial trapping probability of unity. This assumption is based on a previous study in which the trapping probability of n-butane molecules with thermal kinetic energies (~2.5 kJ/mol) is close to unity on Pt(111).16



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We estimate atomic oxygen coverages by scaling integrated O2 TPD spectra with those obtained after exposing s-RuO2(110) at 300 K to a saturation dose of O2 and assuming that the O2 exposure generates an atomic oxygen coverage of 0.275 ML, which is equal to 86% of Rucus coverage.10, 17 To estimate CO desorption yields, we scaled integrated CO desorption spectra by an integrated TPD spectrum collected from a saturation coverage of CO on Ru(0001) prepared at 300 K and assuming that the saturation coverage is 0.68 ML.18 To estimate CO2 desorption yields, we scaled integrated CO2 desorption spectra by an integrated TPD spectrum collected from a saturated monolayer of CO on Ru(0001) and using a relative sensitivity factor relating the CO and CO2 signal intensities measured with the mass spectrometer at an electron energy of 100 eV (Hiden Analytical). To estimate H2O desorption yields, we conducted TPRS experiments of H2 on RuO2(110) covered initially with a saturation amount of Oot atoms (0.275 ML), and monitored the desorption yields of H2O and the O2 resulting from the recombination of Oot atoms. In these experiments, the adsorbed hydrogen is completely converted to water during TPRS by reacting with Oot atoms and the H2O yield is thus equal to the decrease in the Oot recombination yield relative to its value on the Oot-saturated surface.19

Computational Details All plane wave DFT calculations were performed using the projector augmented wave pseudopotentials20 provided in the Vienna ab initio simulation package (VASP).21-22 The Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional23 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.24 We find that this method provides accurate estimates of the adsorption energies of n-alkanes on PdO(101) in comparison with TPD-derived values.25-26 We 8 ACS Paragon Plus Environment

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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 0.03 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. In the present study, we define the binding energy, Eb, of an adsorbed n-C4H10 molecule on the surface using the expression,

Eb = (EC 4 H 10 + Esurf ) − EC 4 H 10 / surf ,

(1)

where EC4H10/surf is the energy of the initial state containing the adsorbed C4H10 molecule, Esurf is the energy of the bare surface, and EC4H10 is the energy of an isolated C4H10 molecule in the gas phase. All reported binding energies are corrected for zero-point vibrational energy. From Eq. (1), a large positive value for the binding energy indicates a high stability of the adsorbed C4H10 molecule under consideration. All of our calculations were performed for a single n-butane molecule adsorbed within the 1×4 surface model of RuO2(110), and corresponds to an n-butane coverage equal to 25% of the Rucus density or equivalently 0.08 ML of n-butane. This coverage is close to that determined experimentally for the uncompressed layer of n-butane adsorbed on the Rucus sites (see below). We have also performed selected calculations using 1×8 and 2×4 unit cells, and find that the n-butane binding energies are within 2 kJ/mol of our results using the 1 × 4 unit cell. This comparison indicates that interactions between the periodic images of adsorbed n-butane in the 1×4 unit cell make only a minor contribution (~2%) to the computed binding energies for the n-butane coverage considered.



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Results and Discussion Adsorption of n-butane on s-RuO2(110) as a function of coverage Figure 1a shows n-butane TPD spectra obtained as a function of the initial n-butane coverage prepared on a s-RuO2(110) film at 80 K. Initially, n-butane desorbs in a peak (α) at 251 K that broadens and shifts to 244 K as the n-butane coverage increases to ~0.06 ML. A small peak (γ1) at ~140 K also appears during the initial uptake, and likely corresponds to butane molecules that become kinetically trapped in a weakly-bound state before the more strongly-bound state(s) saturates. As the n-butane coverage increases above ~0.06 ML, a broad feature (β) develops between about 150 and 200 K, and reaches saturation once the total butane coverage increases above 0.10 ML. Thereafter, we observe sharp TPD peaks developing at 120 and 140 K (γ2 and γ1) up to a coverage of ~0.20 ML. The peak (M) at 100 K arises from butane desorption from a multilayer. Below, we present evidence that the α state arises from n-butane σ-complexes adsorbed along the Rucus rows in an uncompressed layer, while the β state corresponds to the desorption of n-butane from a compressed layer of σ-complexes.


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(a) n-Butane TPD n-C4H10 + s-RuO2(110) Ts = 80 K

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n-Butane uptake n-C4H10 + s-RuO2(110) Ts = 80 K

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Figure 1. (a) n-Butane TPD spectra obtained from s-RuO2(110) as a function of the initial n-butane coverage prepared at 80 K. (b) Desorption yield from the α state (red) and the total desorption yield (black) of n-butane as a function of the n-butane exposure.

The uptake plots shown in Figure 1b reveal that n-butane preferentially adsorbs into the α states during initial adsorption, with the α desorption yield saturating at a value of 0.054 ML and starting to plateau once the butane exposure reaches about 0.14 ML. As an estimate, we assume that the coverage in the α state is equal to the amount of n-butane that desorbs above 210 K 11 ACS Paragon Plus Environment


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during TPD, where this temperature coincides with a pronounced inflection in the TPD traces obtained for n-butane coverages greater than 0.06 ML (Figure 1a). At saturation of the n-butane monolayer (~0.20 ML), the desorption yield from the α state plus the yield of reacted n-butane (0.016 ML as discussed below) is equal to 22% of the Rucus density (~0.07 ML). Thus, the nbutane complexes appear to reach saturation at a coverage close to one n-butane molecule per four Rucus atoms when the layer is uncompressed. Prior studies report similar saturation coverages for uncompressed layers of n-butane adsorbed on the cus-metal rows of PdO(101)4 and TiO2(110)14 (~0.08 ML vs. 0.11 ML), where the latter is isostructural with the s-RuO2(110) surface under study. These considerations support the conclusion that the α TPD feature originates from n-butane σ-complexes adsorbed on the Rucus rows of RuO2(110). The TPD spectra that we observe for n-butane adsorbed on s-RuO2(110) are similar to those reported recently for n-butane on TiO2(110),14 indicating common adsorption behavior of nbutane on these isostructural surfaces. In a recent paper, Chen et al. 14 show that n-butane initially adsorbs on the cus-metal atom rows of TiO2(110), giving rise to desorption features analogous to the α and β features that we observe here for butane on RuO2(110). As noted by those authors, the broad β feature is consistent with a compressed layer of n-butane on the cus-metal atom rows, the formation of which is certain to cause n-butane molecules to adopt less favorable binding configurations compared with those which form at low coverage. The work of Chen et al. 14 further suggests that the sharp TPD peaks at ~120 and 140 K correspond to n-butane molecules that interact mainly with the Obr atoms of RuO2(110). The general evolution of the TPD spectra suggests that n-butane adsorbs into similar types of configurations on both RuO2(110) and TiO2(110). However, the present work shows that n-butane achieves significantly higher binding energies on the cus-metal sites of RuO2(110) compared with TiO2(110) (~100 vs. 65 kJ/mol).


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The high desorption temperature of n-butane in the α state is a clear indication that n-butane forms a strongly-bound σ-complex on RuO2(110) by datively bonding with Rucus atoms. For the peak temperature of 251 K observed at low coverage, we estimate a binding energy of ~100 kJ/mol for n-butane adsorbed in the α state on RuO2(110). This binding energy appears to be the highest reported to date for n-butane adsorbed on a solid surface. For comparison, we find that physically adsorbed n-butane on Ru(0001) desorbs in a peak at 156 K during TPD (see SI), corresponding to a binding energy of about 47 kJ/mol. The binding energy is significantly enhanced (by ~53 kJ/mol) for n-butane adsorbed on RuO2(110) compared with Ru(0001), thus supporting the conclusion of σ-complex formation on the oxide. Notably, we used a maximum value of the desorption pre-factor (1.7 × 1020 s-1) to estimate the binding energy of the n-butane σ-complex at low coverage on RuO2(110), where we used formulas reported by Tait et al.27 to compute the maximum pre-factor at the peak desorption temperature observed at low n-butane coverage (251 K). This upper limit appears to be appropriate in the present case since the large corrugation of RuO2(110) is likely to hinder inplane motion of the adsorbed σ-complex. We present additional evidence below that supports the assertion that the desorption pre-factor is close to the upper limit, and plan a future submission detailing a more thorough analysis of alkane TPD spectra obtained from RuO2(110). The strong binding of n-butane on RuO2(110) is similar to that observed previously for alkane σ-complexes on PdO(101),1, 11 for which we have shown that the dative interaction with PdO(101) activates the Pd-coordinated C-H bonds, and thereby causes a large fraction (> 60%) of the n-butane complexes to undergo C-H bond cleavage and complete oxidation during TPD.



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TPRS of CO, CO2 and H2O products from n-butane oxidation on RuO2(110) Our experiments reveal that H2O, CO and CO2 desorb during TPRS and thus provide evidence that a fraction of the adsorbed n-butane molecules are oxidized by reaction with the RuO2(110) surface. Figure 2a shows H2O, CO and CO2 TPRS spectra obtained after adsorbing 0.22 ML of n-butane on s-RuO2(110) at 80 K. The majority of the oxidized products desorb in broad features between ~425 K and 750 K, while small amounts of CO2 and H2O also desorb in peaks at 310 K and 390 K, respectively. Based on prior studies,10-11,

28

as well as our own

experiments (see SI), we attribute the small CO2 peak at 310 K to the oxidation of CO that adsorbed in small quantities (< 0.01 ML) from the UHV background. Similarly, the H2O peak at 390 K arises from H2O adsorbed on Rucus sites (see SI),29-30 and likely results from H2O that adsorbed from the background. In contrast, however, the CO and CO2 features above ~400 K are consistent with reaction products that result from the oxidation of adsorbed n-butane, and the broad H2O desorption feature near 600 K is attributable to the recombination of HObr groups.10-11, 30-31

We assert that the OHbr groups result from both n-butane dehydrogenation as well as the

adsorption of small quantities of H2 (< 0.03 ML) from the UHV background.


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(a)

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Figure 2. (a) TPRS traces of CO2, CO and H2O obtained from a saturated n-butane monolayer on sRuO2(110) prepared at 80 K. (b) TPRS product yields as a function of the n-butane exposure. The yield of reacted n-butane is equal to the CO + CO2 yield divided by four to account for reaction stoichiometry. The blue dashed lines are linear fits showing a break in linearity of the reacted n-butane yield vs. exposure relation at the exposure (0.14 ML) at which the strongly-bound α state saturates.

Figure 2b shows the evolution of product yields (H2O, CO and CO2) that desorb above 400 K as a function of the n-butane exposure. The figure also includes estimates of the amount of nbutane which reacts during TPRS, referred to as the reaction yield. We define the n-butane



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reaction yield as the CO + CO2 yield divided by four, where the factor of four accounts for the stoichiometry of n-butane oxidation to either CO or CO2. Our definition assumes that all of the CO and CO2 that desorbs above 400 K originates from the oxidation of adsorbed n-butane. We find that the H2O desorption yield increases from ~0.06 to 0.085 ML with increasing butane exposure, and that the CO and CO2 yields above 400 K are similar in value, each increasing from about 0.01 to 0.035 ML with increasing butane exposure over the range studied. Notably, the total yield of oxidized products at saturation of the n-butane monolayer corresponds to the removal of 0.19 ML of O-atoms from the RuO2(110) surface. This value is equal to ~58% of the nominal coverage of Obr atoms on RuO2(110) (0.32 ML), and is thus a significant quantity. The relationships between the product yields and the n-butane exposure suggest that a fraction of the reaction products, particularly after low exposures, originate from background H2 adsorption and reaction of n-butane at defect sites within the RuO2 layer. The H2O yield above 400 K exhibits an initial jump to 0.06 ML after only a 0.014 ML n-butane exposure and subsequently increases more slowly with increasing butane exposure. This behavior suggests that a portion of the HObr groups that recombine to produce the H2O originate from background H2 adsorption. Support for this interpretation may be found by examining how the H2O to CO + CO2 yield ratio evolves with the n-butane exposure. We find that this ratio decreases steadily from 2.6 to 1.3 with increasing n-butane exposure, where the value observed at the highest butane exposure agrees well with the yield ratio of 1.25 determined from the stoichiometry of nbutane oxidation to CO and CO2. The evolution of the H2O and CO + CO2 yields thus suggests that a considerable portion of the high temperature H2O results from the oxidation of background H2, with this portion decreasing as butane molecules occupy an increasing fraction of the Rucus sites.


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The yield vs. exposure relations for CO and CO2 also exhibit a sharp increase at low butane exposure, and then increase more slowly with increasing exposure. Evaluation of the data suggests that this behavior originates from an initially high reactivity of n-butane on defects sites within the RuO2 layer, rather than the oxidation of a background gas on the surface, followed by slower reaction of n-butane σ-complexes on the RuO2(110) structure. Support for this conclusion comes firstly from observations that exposure of the RuO2(110) surface to the vacuum background produces negligible CO and CO2 desorption above 400 K during TPRS. Furthermore, as elaborated below, we observe negligible quantities of CO and CO2 desorbing above 400 K during TPRS experiments with n-C4D10. These results demonstrate a lack of reactivity in the absence of n-C4H10 and thus indicate that the oxidation of n-butane is responsible for the initial CO + CO2 products (~0.02 ML) that we observe. From reaction stoichiometry, we estimate that ~0.006 ML of n-butane reacts with the surface to produce the initial increase in product yields. The sharp increase in product yield at low exposures is indicative of facile reaction on a minority defect site.32 We assert that n-butane dissociates with high probability on a particular type of defect site(s), causing the product yield to rise sharply during the initial n-butane exposure. The adsorbed products rapidly deactivate these sites and thereafter n-butane dissociation occurs more slowly. Below, we present additional evidence that the slower reactivity observed above an nbutane exposure of ~0.01 ML corresponds to n-butane oxidation on the RuO2(110) structure. From the present data, we are unable to identify the type of defect site that appears to be reactive toward n-butane at low coverages, though it is worth noting that preliminary data provides evidence that neither bridging oxygen vacancies nor large O-covered metallic Ru domains (see SI) are responsible for the high initial reactivity.



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Our analysis reveals that ~0.065 ML of CO + CO2 desorbs above 400 K at saturation of the n-butane monolayer, which would correspond to the oxidation of ~0.016 ML of n-butane. Since we attribute ~0.006 ML of the reacted n-butane to activation at defects, we estimate that about 0.01 ML of n-butane dissociates at sites within the ideal RuO2(110) structure. Assuming that the n-butane which oxidizes originates from the α state, a yield of 0.01 ML is equal to an appreciable fraction (~14%) of the n-butane that was initially adsorbed in the α state (0.07 ML). For comparison, we have previously reported that ~64% of the n-butane σ-complexes (~0.05 ML) dissociates on PdO(101) during TPRS.4, 13 This comparison generally supports the conclusion that the high temperature CO and CO2 products observed in the present study result from the oxidation of n-butane σ-complexes on RuO2(110), and further suggests that n-butane oxidation is more facile on PdO(101) compared with RuO2(110). Finally, we find that the n-butane reaction yield as a function of the butane exposure exhibits an abrupt decrease in slope as the butane exposure increases above about 0.14 ML. This change in slope is evident upon visual inspection, particularly in a magnified view, and we also note that separate linear fits below and above an exposure of 0.14 ML yield R2 values greater than 0.99, while a single linear fit is less accurate for the entire curve, yielding an R2 value of 0.94. The break in linearity coincides with the exposure at which the desorption yield of butane in the α state saturates (Figure 1b), and thus reveals a correlation between the amount of butane which populates in the α state and the amount of butane which oxidizes during TPRS. We have reported a similar, though more pronounced, correlation for the oxidation of propane and n-butane on PdO(101) and shown that this correlation exists because the α desorption feature corresponds to adsorbed σ-complexes that serve as precursors for alkane C-H bond cleavage on PdO(101).2, 4 Thus, the correlation between the reaction yield and the yield of n-butane desorbing in the α


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feature is consistent with the idea that the CO and CO2 products that evolve above 400 K result from reaction of n-butane σ-complexes on the RuO2(110) surface.

Adsorption of n-C4D10 on s-RuO2(110) and site blocking with H2O We investigated the adsorption of n-C4D10 on s-RuO2(110) using TPRS (Figure 3a) and obtain additional evidence that n-butane dissociates on s-RuO2(110) via C-H bond cleavage and that adsorbed σ-complexes serve as precursors for reaction. Figure 3a compares n-butane and CO2 TPRS traces obtained after generating ~0.20 ML of n-C4D10 vs. n-C4H10 on s-RuO2(110) at 80 K. The CO traces exhibit similar behavior as the CO2 traces and thus are omitted from the figure. Our TPRS experiments with n-C4D10 on RuO2(110) reveal negligible quantities of CO and CO2 desorbing above 400 K as well as HDO and D2O (not shown), and thus demonstrate that kinetic isotope effects completely suppress the conversion of n-C4D10 to CO and CO2. This behavior supports the conclusion that n-butane initially dissociates on RuO2(110) via C-H bond cleavage. We also find that the desorption yield in the α state is higher for n-C4D10 than n-C4H10 by about 0.014 ML, where this amount is nearly equal to the amount of n-C4H10 that reacts (0.016 ML) during the TPRS experiment. This comparison shows that the yields of CO and CO2 desorbing above 400 K are inversely correlated with the amount of n-butane that desorbs in the α state, and thus further supports the conclusion that n-butane σ-complexes on RuO2(110) serve as precursors for C-H bond activation and ultimately oxidation to CO and CO2.



(a) TPRS Butane+s-RuO2(110) Ts=80K Desorption Rate (a.u.)

n-C4H10 n-C4D10

CO2 (×1.7)

α

Butane 100

200

300

400

500

600

700

800

Temperature (K)

(b) TPRS n-Butane + RuO2(110) Ts= 80 K

CO2(×5) Desorption Rate (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 38

s-RuO2(110) H2Ocus pre-saturated

β 100

200

α

n-C4H10 300

400

500

600

700

800

Temperature (K)

Figure 3. (a) TPRS traces of n-butane and CO2 obtained after preparing saturated monolayers of nC4H10 (red) vs n-C4D10 (green) on s-RuO2(110) at 80 K. (b) TPRS traces of n-C4H10 and CO2 obtained after preparing 0.19 ML of n-C4H10 on s-RuO2(110) (red) vs. RuO2(110) that was initially covered with 100% H2O on the Rucus sites (blue).

We also investigated the active sites for n-butane activation on s-RuO2(110) by performing site blocking experiments with adsorbed water. Similar site-blocking experiments have shown


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that alkane σ-complexes bind on the Pdcus sites of PdO(101).33 Previous studies show that H2O preferentially adsorbs on the Rucus sites of RuO2(110), and thereafter binds more weakly on Obr sites,29, 34-35 with desorption from these states producing distinct TPD peaks at 390 K and 200 K, respectively (see also the SI). We thus exposed the RuO2(110) surface to water at 300 K to selectively pre-adsorb water on the Rucus sites, while minimizing water adsorption on Obr sites. Figure 3b shows a comparison of CO2 and n-butane TPRS traces obtained after adsorbing ~0.20 ML of n-C4H10 on initially clean RuO2(110) vs. a RuO2(110) surface on which ~100% of the Rucus sites was pre-covered with H2O. Our results show that pre-saturating the Rucus sites with H2O completely suppresses n-butane desorption in the α and β TPD features, and also eliminates the production of CO2 above 400 K. The appearance of an intense n-butane TPD peak at 120 K suggests that the pre-adsorbed water forces the n-butane molecules to adopt weakly-binding configurations along the Obr rows. The water site-blocking experiments provide evidence that the α and β TPD features originate from n-butane molecules that bind to Rucus sites of RuO2(110). Furthermore, the experiments confirm a correlation between the yield of n-butane desorbing in the α (and β) peaks and the yield of COx that desorbs above 400 K, and thus further support the conclusion that adsorbed σ-complexes on the Rucus rows serve as precursors to n-butane dissociation and oxidation on RuO2(110).

Kinetic analysis of n-butane dissociation: Precursor-mediated mechanism We performed measurements to characterize the kinetics governing n-butane dissociation on RuO2(110) and further test the idea that reaction occurs by a precursor-mediated mechanism. We specifically evaluated experimental estimates of the n-butane reaction probability obtained as a function of the surface temperature using a kinetic model for the precursor-mediated dissociation



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of n-butane. In this model, the dissociative chemisorption of an alkane from a molecularlyadsorbed precursor state is represented by the following kinetic scheme,2, 7, 13, 36

RH(g) ⇌ RH(ad) → R(ad) + H(ad)

where RH represents an alkane molecule, is the probability for molecular adsorption, F is the incident flux of gaseous RH at the surface, and kd and kr are rate coefficients for desorption and dissociation (“reaction”) via C-H bond cleavage of the molecularly-adsorbed RH species. The model defines a single rate coefficient for reaction, kr, and thus neglects differences in the kinetics of primary vs. secondary C-H bond cleavage. Prior work demonstrates that this approach provides an accurate representation of the initial dissociation kinetics of alkanes on solid surfaces.2,

7, 13, 36

We estimate that the molecular adsorption probability for the n-butane σ-

complex lies between 40% and 60% , where we obtained this estimate by evaluating initial slopes of n-butane coverage vs. exposure relations determined from TPD. One may obtain the following expression for the dissociative chemisorption probability in the limit of zero coverage ( ) by applying the steady-state approximation to the rate of formation of molecularly-adsorbed alkanes;

=

+

One may then represent this equation in the following Arrhenius construction by assuming that the Arrhenius equation describes the temperature dependence of each rate coefficient;


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ln (

( − ) − 1) = ln ( ) −

where and represent the pre-factor and activation energy for reaction j, and is the surface temperature. Thus, if n-butane dissociates on RuO2(110) by a precursor-mediated mechanism, then a plot of ln (

!"

− 1) vs.

#

%$provide values for the apparent pre-factor (

will be linear and the Arrhenius construction will

& &'

) and activation energy − for initial C-H

bond cleavage. To obtain estimates of the n-butane dissociation probability, we measured the CO and CO2 TPRS yields obtained as a function of the n-butane exposure at several fixed surface temperatures between 280 and 300 K. We selected 280 K as the lower limit because this temperature lies just above the trailing edge of the n-butane α TPD peak, and is thus sufficiently high to minimize the accumulation of molecularly-adsorbed n-butane. We selected 300 K as the upper limit to minimize the loss of oxygen from the surface and possible changes in surface reactivity during the n-butane exposures. Because molecularly adsorbed n-butane accumulates negligibly above 280 K, the CO and CO2 yields are equal to the amount of n-butane which irreversibly dissociates on the surface during the n-butane exposures. Figure 4a shows n-butane reaction yields measured as a function of the n-butane exposure at several surface temperatures, where we conducted short n-butane exposures to maintain low coverages of the reaction products. Each isotherm of the reaction yield vs. exposure is wellapproximated as linear, with the slopes decreasing with increasing surface temperature. The linear behavior is expected because the probability for dissociative chemisorption is approximately independent of the adsorbate coverage at low coverage. In this limit, the coverage



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of dissociated n-butane [] is given by the equation, [] = ( where the n-butane exposure is equal to the product of the exposure time ( and the incident flux which estimate as 4.5 × 10-3 ML/s. The slope of the initial portion of an isotherm is thus equal to the initial dissociation probability of n-butane on RuO2(110) at the surface temperature at which the exposure was conducted. Notably, each of the yield vs. exposure lines exhibits a non-zero intercept of ~0.006 ML. We attribute this intercept to n-butane dissociation at defect sites, as discussed above, and assume that the adsorbed products render the defects inactive once the exposure reaches ~0.02 ML. Figure 4b shows the initial dissociation probability, as determined from the slopes of the isotherms, plotted as a function of the surface temperature. We estimate that the initial dissociation probability decreases from about 9% to 3.6% with increasing surface temperature from 280 to 300 K. The decrease in dissociation probability with increasing temperature is indeed characteristic of a facile precursor-mediated mechanism for n-butane C-H bond cleavage on RuO2(110) wherein the activation energy for dissociation is less than that for desorption of the molecularly adsorbed precursor. The good linear fit of ln (

!"

− 1) vs.

#

%$(Figure 4c) further

supports the conclusion that n-butane dissociation on RuO2(110) occurs by a precursor-mediated mechanism. For a molecular adsorption probability of 50%, we estimate that the apparent prefactor and activation energy for n-butane dissociation on RuO2(110) are equal to and − = -35.1 kJ/mol.


& &'

= 6.0 × 10-8

Page 25 of 38

(a) 0.020

Reaction yield n-C4H10 + s-RuO2(110) 280K

0.016

S0 = 9.0%

[R] (ML)

285K 0.012

290K 295K 300K

0.008

S0 = 3.6% 0.004

0.000 0.00

0.02

0.04

0.06

0.08

0.10

0.12

Butane Exposure (ML)

(b) 0.12

(c) 3.2 n-C4H10+ s-RuO2(110)

3.0

0.10

Arrhenius Plot n-C4H10 + s-RuO2(110)

Ed-Er = 35.1 kJ/mol 7

νd/νr = 1.6 × 10 ξ = 0.50

2.8 2.6

0.08

2.4

ln(ξ/S0 - 1)

Initial dissociation probability

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.06

0.04

2.2 2.0 1.8 1.6

0.02

1.4 1.2

0.00 275

280

285

290

295

300

1.0

305

0.00330

Temperature (K)

0.00335

0.00340

0.00345

0.00350

0.00355

0.00360

-1

1/Ts (K )

Figure 4. (a) Yields of reacted butane on s-RuO2(110) as a function of the n-butane exposure for adsorption at surface temperatures between 280 and 300 K. The slopes of the yield vs. exposure relations are equal to the n-butane dissociation probability at each temperature. (b) Initial dissociation probability of n-butane on s-RuO2(110) as a function of the surface temperature from 280 K to 300 K. The curve shows as a function of the surface temperature determined from the kinetic analysis discussed in the text. (c) Arrhenius construction derived from the precursor-mediated kinetic model using the values of determined from the isotherms shown in (a) and = 0.50.



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Page 26 of 38

Discussion of kinetic parameters for n-butane dissociation on RuO2(110) The estimated kinetic parameters indicate that the initial C-H bond cleavage of n-butane on RuO2(110) is strongly favored energetically but strongly disfavored entropically. Recall that we estimate a binding energy of ~100 kJ/mol for n-butane σ-complexes on RuO2(110) at low coverage. Using this value, we estimate an activation energy of = 64.9 kJ/mol for C-H bond cleavage of the adsorbed n-butane complex on RuO2(110) in the zero coverage limit. This value agrees well with estimates of the activation energy for the initial C-H bond cleavage of n-butane complexes on PdO(101). In that case, the n-butane binding energy is ~87 kJ/mol at low coverage,4, 26 and measurements of the initial dissociation probability yield a value of − = -23.2 kJ/mol for the apparent activation energy for dissociation, thus resulting in an estimate of = 64 kJ/mol for the activation energy for dissociation.13 This comparison suggests that the more negative apparent activation energy for n-butane dissociation on RuO2(110) vs. PdO(101) results mainly from stronger binding of the n-butane σ-complexes on the RuO2(110) surface. The apparent pre-factor for dissociation is unusually small and acts to offset the favorable apparent activation energy for reaction. Within the context of transition state theory, the apparent pre-factor for reaction is proportional to the difference in entropy between the transition structure for C-H bond cleavage and the transition structure for molecular desorption, the latter of which may be taken as a gas-phase n-butane molecule minus one translational degree of freedom. The apparent pre-factor for the dissociative chemisorption of an alkane is usually less than unity because a gas-phase alkane, even with only two free translations, has a higher entropy than the


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adsorbed transition structure for C-H bond cleavage. Previously measured values of / lie between about 10-2 and 10-4 for small alkanes (C1 to C4).7, 37 For example, we have reported apparent reaction pre-factors of 3 × 10-4 and 8 × 10-4 for the precursor-mediated dissociation of propane and n-butane on PdO(101), respectively.2, 13 These values are about 104 times higher than that estimated here for n-butane dissociation on RuO2(110). The remarkably small value of the apparent pre-factor for n-butane dissociation on RuO2(110) suggests that motions of the adsorbed states are highly restricted for this system. Estimates of the pre-factors for desorption and reaction may be obtained using transition state theory formulas and estimates of the partition functions for the transition structures (SI). The main challenge in computing these partition functions arises from uncertainty in modeling the frustrated motions of the adsorbed species since these motions can transform from localized vibrations to nearly free motions over a temperature range at which desorption and reaction occur at appreciable rates.38-39 In general, differences in the frustrated motions between the adsorbed alkane and the transition structure for C-H bond cleavage are small so one can accurately estimate the reaction pre-factor using normal mode frequencies computed from DFT. In other words, the contributions of the frustrated motions cancel out to a large extent when calculating the reaction pre-factor. The entropy of activation is usually negative for the dissociation of an adsorbed alkane, and the reaction pre-factors have values near 1012 s-1 at room temperature i.e., less than kBT/h. In contrast, desorption pre-factors are highly sensitive to the description of the frustrated adsorbate motions because these motions only determine the entropy of the initial adsorbed state. We have previously shown that an apparent pre-factor / near 10-4, in good agreement with the experimental estimate, can be obtained from DFT results for propane dissociation on



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PdO(101) only if two of the frustrated motions of the adsorbed molecule are modeled as free motions and the remaining motions are modeled as harmonic vibrations.25 The apparent prefactor for reaction in that case decreases to 10-6 if all of the adsorbate motions are modeled as harmonic vibrations. This finding suggests that the frustrated motions of adsorbed propane complexes on PdO(101) are only weakly hindered at temperatures (> 250 K) at which desorption and reaction occur at appreciable rates. Such behavior is consistent with a more general discovery made by Campbell and Sellers38,

40

showing that the entropies of many adsorbed

molecules lie closer to values estimated for an ideal 2D gas than for a 2D lattice gas at temperatures for which the desorption rate is above ~10-3 ML/s. Those authors demonstrate that the entropies of adsorbed and gaseous states are linearly correlated for many molecule-surface combinations, with the entropy of the adsorbed species equal to roughly 2/3 the entropy of the gas-phase species.38, 40 Using the Campbell-Sellers (CS) correlation, we estimate a pre-factor of about 2 × 1016 s-1 for n-butane desorption at 290 K. The ratio of a typical reaction pre-factor (1012 s-1) and the CS desorption pre-factor suggests an apparent reaction pre-factor near 10-4 for n-butane dissociation on RuO2(110). This value of the apparent reaction pre-factor lies in the range of previously reported values for alkane dissociation, but is much higher than the value estimated from our kinetic analysis for n-butane/RuO2(110). Assuming a typical value of the reaction pre-factor, then our estimated apparent reaction pre-factor of 6 × 10-8 suggests that the pre-factor for nbutane desorption from RuO2(110) is equal to about 1019 s-1. This value of the desorption prefactor is about ten times lower than the upper limit that one obtains by assuming that each of the adsorbate motions is a harmonic vibration with a partition function equal to unity.27 An implication is that the in-plane motions of the n-butane σ-complex on RuO2(110) remain


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strongly hindered at the temperatures (~250 to 300 K) at which these species desorb and react readily. As noted by Campbell and Sellers,38 the entropies of adsorbed species could be much lower than that predicted by the CS correlation if the molecule-surface potential is highly corrugated. The RuO2(110) surface indeed presents a large geometric corrugation between the Rucus and Obr rows that is certain to restrict adsorbate motion. Furthermore, sizeable energy barriers may need to be overcome for n-butane to diffuse along the Rucus row since such motion necessarily disrupts relatively strong CHx-Rucus dative bonds. We thus conclude that the very low value of the apparent reaction pre-factor results from a low entropy of the n-butane σ-complex on RuO2(110), and consequently a significant entropic driving force for the complex to desorb rather than react. This entropic effect is quite significant in reducing the net dissociation probability below the values that one would predict using typical values of the apparent reaction pre-factor.

DFT predictions of n-butane adsorption and activation on RuO2(110) We performed DFT-D3 calculations to identify preferred configurations of n-butane on the RuO2(110) surface and confirm the formation and facile C-H bond activation of n-butane σcomplexes. The DFT calculations predict that n-butane can form a strongly-bound σ-complex on RuO2(110), and identify three molecular configurations that achieve significantly higher binding energies than others considered (Figure 5). In each configuration, the n-butane molecule aligns along the Rucus row and forms a H-Ru η1 dative bond at two different CHx groups, resulting in two H-Ru dative bonds per molecule. The predicted binding energies for these configurations range from about 83 to 93 kJ/mol, with the higher value in good agreement with our estimates



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from the n-butane TPD data. As mentioned in the Computational Details, our computations consider an n-butane coverage equal to 25% of the Rucus density which is close to the saturation coverage of n-butane in the α state as determined from TPRS. At this coverage, we find that the n-butane molecules, aligned along the Rucus rows, are separated by roughly two lattice spacings (~6.2 Å), and interact only weakly with one another. Our results thus suggest that n-butane molecules in the α state experience weak interactions, and that the computed binding energies at 25% coverage are representative of effectively isolated n-butane σ-complexes adsorbed on RuO2(110). The preferred configurations that we predict are analogous to those identified previously for n-butane σ-complexes on the PdO(101) surface.4,

13, 26

In the energetically preferred

configuration ( = 93 kJ/mol), the n-butane molecule adopts a trans-conformation with the carbon backbone aligned parallel to the Rucus row and the molecular plane oriented nearly perpendicularly from the surface plane. Following our earlier work, we designate the preferred configuration as the p,s-η1 (st) configuration, where “p” and “s” indicate that a η1 H-Ru dative bond forms at both a “primary” C-H bond and a “secondary” C-H bond of the molecule. The abbreviation “st” refers to the “staggered” geometry of the CHx groups when the adsorbed molecule is viewed within the surface plane and along a direction perpendicular to the Rucus row. The p,s-η1 (st) configuration can transform to the p,s-η1 (fl) configuration, where “fl” refers to “flat-lying”, through a rotation of the molecular plane about an axis that is approximately parallel to the Rucus row. The p,s-η1 (fl) configuration maintains two H-Ru dative bonds, but has a lower binding energy ( = 83 kJ/mol) than the p,s-η1 (st) configuration. We also identified a favorable configuration in which the n-butane molecule adopts the gauche conformation and


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forms a H-Ru dative bond at each CH3 group, resulting in the 2p-η1 configuration. The binding energy of the 2p-η1 configuration is only 5 kJ/mol lower than that of the preferred p,s-η1 (st) configuration. Overall, our DFT calculations demonstrate that n-butane can form strongly-bound σ-complexes on the RuO2(110) surface, with binding energies that agree well with our experimental estimates. While the p,s-η1 (st) configuration represents the energetically preferred geometry, the close-lying values of the binding energies suggest that the n-butane complexes could be present in multiple configurations on RuO2(110) at low coverage. a)

b)

p,s-η1 (st) 92.6 kJ/mol

c)

p,s-η1 (fl) 83.0 kJ/mol

2p-η1 87.8 kJ/mol

Figure 5. Preferred configurations of n-butane σ-complexes on RuO2(110) as predicted by DFT-D3 calculations. The designation of the molecular geometry and the computed binding energy is given under the image of each complex.



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Figure 6 shows an energy diagram for the favored pathway for primary C-H bond cleavage of the p,s-η1 (st) n-butane complex on RuO2(110) as determined using DFT-D3 calculations. The diagram includes molecular images of the initial, transition and final states for the reaction pathway, and the energies are corrected for zero-point vibrational energy. We focus only on primary C-H bond cleavage because prior studies show that primary C-H bond cleavage is the preferred pathway for the dissociation of an p,s-η1 (st) n-butane complex on PdO(101), with the energy barrier predicted to be about 5 kJ/mol lower for primary vs. secondary C-H bond cleavage.13 The dissociation pathway involves cleavage of the Ru-coordinated primary C-H bond via H-atom transfer to a neighboring Obr atom, thus affording a 1-butyl group and a HObr species. The calculations predict that the n-butane complex achieves a binding energy of 92.6 kJ/mol while the energy barrier for C-H bond cleavage of the complex is only 55 kJ/mol. The resulting apparent reaction barrier is thus equal to -37.6 kJ/mol according to DFT-D3, and agrees well with the value determined from our kinetic analysis of experimental data (-35.1 kJ/mol). This comparison provides strong support for our conclusion that n-butane forms a strongly-bound σcomplex on RuO2(110) and that C-H bond cleavage of the complex is strongly favored energetically.


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80 40

Binding energy (kJ/mol)

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0

Er - Ed = -37.6 -40 -80 -120

-92.6

-100.3

-160 -200

Figure 6. Energy diagram for C-H bond cleavage of the favored η1 n-butane complex on s-RuO2(110) as determined by dispersion-corrected DFT.

We computed pre-factors for desorption and reaction of the p,s-η1 (st) n-butane complex using transition state theory formulas with normal mode frequencies determined from our DFTD3 results. We list the computed normal mode frequencies for each structure and describe the pre-factor calculations in the SI. By treating each adsorbate motion as a harmonic vibration, we calculate desorption and reaction pre-factors of = 1.4 × 1018 s-1 and = 1.1 × 1012 s-1 for the n-butane complex on RuO2(110) at 290 K. The corresponding apparent reaction pre-factor is / = 7.8 × 10-7 and is higher than the experimentally-estimated value by slightly more than a factor of 10. Additional calculations and analysis are needed to fully assess the source of discrepancy between the computed desorption pre-factor and that estimated from the experimental data. Overall, the agreement between the experimental and computationally derived estimates of the apparent reaction pre-factor is quite good, and supports the conclusion that the motions of the adsorbed n-butane σ-complexes are highly restricted on the RuO2(110) surface, 33 ACS Paragon Plus Environment


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thus resulting in a small value of the apparent reaction pre-factor and an entropic limitation to reaction.

Summary We investigated the molecular adsorption and dissociation of n-butane on a s-RuO2(110) film using TPRS and DFT calculations. Our TPD data shows that n-butane adsorbs into a stronglybound molecular state on RuO2(110) that is consistent with an adsorbed σ-complex. We also find that a fraction of the strongly-bound n-butane reacts with the RuO2 surface during TPRS to produce CO, CO2 and H2O that desorbs above ~400 K. The measured variation of the product yields with n-butane exposure, the low reactivity of n-C4D10 and site blocking experiments with water all provide evidence that adsorbed σ-complexes act as precursors to the initial C-H bond cleavage of n-butane on s-RuO2(110). DFT calculations confirm that n-butane molecules can form strongly-bound σ-complexes along the Rucus rows of RuO2(110) by adopting configurations in which two CHx-Ru dative bonds form per molecule. From kinetic measurements, we estimate an apparent activation energy of -35.1 kJ/mol and an apparent pre-factor of 6 × 10-8 for the C-H bond activation of n-butane complexes on RuO2(110). The apparent activation energy for dissociation agrees quantitatively with the value predicted by DFT. The unusually small apparent pre-factor for reaction suggests that motions of the active n-butane complex are highly restricted on RuO2(110) due to strong corrugation in the molecule-surface interaction.

Acknowledgements


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

Supporting Information LEED patterns and O2 TPD spectra following the oxidation of Ru(0001) by atomic oxygen and formation of a RuO2(110) film; CO and CO2 TPRS spectra for an as-prepared s-RuO2(110) surface and following the generation and restoration of heavily-reduced RuO2(110); n-butane TPD spectra obtained from n-butane monolayers prepared on Ru(0001) and s-RuO2(110) at 80 K; n-butane, H2O and CO2 TPRS spectra obtained as a function of the initial H2O coverage prepared on s-RuO2(110); comparison of n-butane and CO2 TPRS spectra for n-butane adsorbed on RuO2(110) vs. Ru(0001) with 0.50 ML of adsorbed O-atoms; equations used to calculate desorption and reaction pre-factors of n-butane on RuO2(110) and table of normal mode frequencies determined from DFT-D3 results for the initial and transition states.

References

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ToC graphic


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Adsorption and Oxidation of n-Butane on the Stoichiometric RuO2

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