Adsorption Kinetics and Dynamics of CO2 on Ru (0001) Supported

Article pubs.acs.org/JPCC

Adsorption Kinetics and Dynamics of CO2 on Ru(0001) Supported Graphene Oxide Nilushni Sivapragasam, Mindika T. Nayakasinghe, and Uwe Burghaus* Department of Chemistry and Biochemistry, North Dakota State University, Fargo, North Dakota 58108, United States S Supporting Information *

ABSTRACT: Adsorption kinetics and dynamics of CO2 on Ru(0001), graphene grown on Ru, and graphene oxide (GO) on Ru were studied. Graphene and GO were made in ultrahigh vacuum by benzene decomposition and subsequent atomic oxygen adsorption, respectively. The samples were characterized by AES (Auger electron spectroscopy) and XPS (X-ray photoelectron spectroscopy). As determined by TDS (thermal desorption spectroscopy), CO2 physisorbs molecularly at ∼85 K on Ru and GO, but not on graphene. Binding energies amount to ∼26 kJ/mol and were slightly enhanced on GO as compared with Ru. Similarly, adsorption probabilities, as determined by molecular beam scattering, are larger on GO than on Ru.

to synthesize graphene/graphene oxide flakes in bulk quantities, also enabling applications in industrial catalysis.18 Therefore, the literature is dominated by synthetic (wet-)chemistry projects and chemical engineering type works on graphene and GO powders.17,19,20 Unfortunately, the inhomogeneity of powders makes it difficult to extract molecular level, mechanistic insights in the underlying elementary reaction steps. The first step in any surface reaction (considered by surface chemists by itself as a reaction) is the adsorption of the gas-phase species, as studied here. 1.2. Literature about CO2 Adsorption. A rather simple model system for eventual metal-free catalysis that promises gaining detailed insights is CO2 adsorption on GO. CO2 surface chemistry was reviewed in refs 21−25. Accordingly, on most metallic systems, CO2 physisorbs molecularly, but can form carbonates in coadsorption with oxygen. CO2 dissociation was only clearly evident on very reactive metals such as nickel. For example, the interaction of CO2 on Ru(0001), which is used here as a support to grow GO, has been studied with infrared spectroscopy and thermal desorption spectroscopy (TDS): CO2 physisorbed molecularly at about 100 K, but no detailed TDS data were reported.26 CO2 is more reactive on metal oxides where carbonates can directly be formed. The interaction with graphitic systems at UHV is weak and requires ultralow temperatures.27,28 However, CO2 adsorption on multilayer graphene powders at high pressure was reported.29 The literature on CO2 interactions with various other nanomaterials is extensive.21−25 However, the morphology and cleanliness of many of these materials is not well

1. INTRODUCTION 1.1. Motivation. Carbon materials are inexhaustible and as such sustainable, in contrast to precious metals. Therefore, (precious) metal-free heterogeneous catalysis using functionalized carbon is an intriguing alternative to traditional chemical synthesis.1−9 The concept of metal-free catalysis dates back many years,1−7 but was so far mostly explored in complex reactions with liquid phase reactants. For example, C60 has been reported to catalyze the hydrogenation of nitrobenzene.9 However, heterogeneous gas-phase reactions are preferred for industrial processes due to easier separation and less waste formation.10,11 In addition, gas-phase processes usually allow for gaining easier detailed mechanistic insights than reactions with liquids. Besides sustainability, another advantage of carbon materials is their functionalization,12 resulting potentially in highly selective catalysts, reduced side reactions, less waste products, and increased energy efficiency (green chemistry). A scientifically interesting allotrope of carbon is graphene.13−15 Among the many functionalizations of carbon already known today, graphene oxide (GO) is probably the most obvious choice and technically rather simple to realize. During oxidation, the sp2-hybridized carbon of graphene changes to an sp3 in order to accommodate the oxygen.16 DFT studies have shown that the oxidation of graphene can form various oxygen-containing functional groups: epoxide, hydroxyl, and carboxyl.8 Both the aromaticity and the reactive oxygen-containing functional groups in GO can act toward making GO a reactive material. For example, GO was used to catalyze the oxidation of alcohols in the liquid phase.17 (See, e.g., Tables 1 and 2 in ref 8 for further examples and references.) However, as far as we know, surface reactions on graphene oxide have not been studied at ultrahigh vacuum (UHV) using surface science techniques. It is relatively simple © XXXX American Chemical Society

Received: September 21, 2016 Revised: November 10, 2016 Published: November 14, 2016 A

DOI: 10.1021/acs.jpcc.6b09573 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

to ∼85 K.41 The reading of the thermocouple has been calibrated (±5 K) in situ by thermal desorption spectroscopy (TDS) measurements of condensed alkanes (see Figure S1). Binding energies, Ed, were determined from the TDS peak positions (with uncertainty of ±0.5 kJ/mol) using a Redhead42 analysis and a pre-exponential of 1 × 1013/s. The heating rate amounts to 1.6 K/s. The exposures for CO2 TDS, χ, are given in seconds exposure by a He-CO2 beam. Oxygen was dosed by backfilling the vacuum chamber at 1 × 10−6 mbar with exposures given in Langmuir (1 L = 1 × 10−6 Torr). 2.2. Sample Preparation. 2.2.1. Graphene Formation. Briefly, the graphene sample was grown on a UHV cleaned Ru(0001) support by applying several C6D6 adsorption/ desorption cycles to thermally decompose deuterated benzene (see ref 15 for more details). The residual D2 TDS signal can be inspected to determine the defect density in the crystalline graphene layer and amounts to below 1% for the samples used here. (A typical example is shown in Figure S2.) The graphene/ Ru(0001) samples were further characterized by AES and XPS; for typical examples, see Figures S3 and S4 as well as ref 15 for a more detailed discussion. (Note that C XPS and C AES lines unfortunately overlap with Ru signals.15) 2.2.2. Graphene Oxide (GO) Formation. As the next step, graphene oxide (short GO or GO/Ru) was synthesized by dosing oxygen at 300 K on the graphene sample using a hot Wfilament to dissociate partially the oxygen. Afterward, CO2 adsorption kinetics and dynamics were studied on the soprepared GO samples (see next sections; Figure S5 summarizes the project structure). Because graphene oxide was (to the best of our knowledge) not successfully made before on ruthenium, in the following, we provide a more detailed discussion and characterization. Figure 1A depicts AES spectra of graphene/Ru(0001) together with AES data of GO/Ru. As evident from AES, graphene does not show an oxygen AES signal, but graphene oxide does. (The inset shows only the O-AES line and the background signal.) Similarly, Figure 1B shows XPS data for the starting structure, graphene/Ru(0001), and graphene oxide. Again, only Ru, C, and oxygen lines are evident, excluding an uptake of impurities. (For example, tungsten AES/XPS lines were below the detection limit.) Figure 2 depicts the evolution of the O-AES signal with oxygen exposure. Results from different preparations, samples 1−4, which all follow the same trend, are depicted. The O-AES signal initially increases rapidly, but levels out already at exposures of 150−200 L. For a few experiments, oxygen-18 isotope was used in order to label the oxygen in graphene oxide.43 The C-AES intensity remained approximately unchanged while forming GO. Perhaps amazing are the relatively small O2 exposures required to saturate the surface. However, O atoms will likely have an adsorption probability close to one. The hot W-filament doser may have an efficiency of 20%. Thus, 150 L O2 exposure roughly corresponds to “30 L O atoms”, which is more than enough to saturate a surface. Similar experimental conditions and procedures are reported for GO formation on SiC(0001) where 600 L O2 and a W-filament doser were used.44 From the oxygen-to-carbon AES ratio, we roughly estimate an oxygen coverage of 38−50% for the saturated GO surface (see the Supporting Information for details). (As usual in surface science, a fully saturated surface is assigned coverage of 1 ML (one monolayer).) Similarly, our C-to-O XPS ratio, at saturation, amounts to ∼4−5, which corresponds to ∼34%

characterized. In addition, most studies have been conducted at high-pressure conditions or in the liquid phase. For example, CO2 adsorption was studied on carbon nanotubes.28,30−32 CO2 physisorbs with extremely small binding energies as determined by measuring adsorption isotherms at high pressure.28 Regarding that literature information, one may expect weak CO2 physisorption on GO. However, one could hypothesize that the oxygen functionalities enhance GO’s reactivity and may even allow for the formation of carbonates. Thus, GO may be catalyzing the adsorption of CO2 on otherwise rather inert graphene or it may at least enhance CO2 adsorption as compared with graphene. Adsorption of CO2 on GO or surface reactions with CO2 on GO have, to the best of our knowledge, not been studied so far. Note also that molecular beam scattering experiments (adsorption dynamics) on any graphitic system are rare27,33−35 as well as graphene oxide was apparently not made before on a ruthenium support. 1.3. Reaction and Stabilization of Other Small Molecules on GO. Narayanan et al.36 describe a related and encouraging high-pressure study on an apparently thick layer of supported commercial graphene oxide flakes. On that system, CO oxidation (as a gas-surface reaction) was apparently seen successfully at high-pressure conditions.36 In a DFT study, an enhancement of NOx and N2O2 adsorption on hydroxyl and carbonyl covered GO, compared with graphene, was theoretically predicted. Here, hydrogen bond formation and even weak covalent bonds (by hybridization of NOx and GO bonds) were invoked.37 Similar theoretical results were reported about ammonia (NH3) adsorption on GO.38 In this study, GO was nanofabricated on Ru(0001) in UHV (ultrahigh vacuum) and characterized by AES (Auger electron spectroscopy), XPS (X-ray photoelectron spectroscopy), and TDS (thermal desorption spectroscopy). Subsequently, CO2 adsorption of GO/Ru was studied by the aforementioned methods as well as molecular beam scattering. Although CO2 does not adsorb on graphene (at ∼85 K), it physisorbs on GO/ Ru; i.e., binding energies on GO/Ru are larger than those for graphene/Ru. Similarly, an enhancement in adsorption probabilities is evident as compared with clean Ru(0001).

2. EXPERIMENTAL PROCEDURES 2.1. Experimental Setup. The measurements have been conducted in a home-built, triply differentially pumped molecular beam scattering apparatus. The supersonic beam is attached to a scattering chamber equipped with mass spectrometers, a CMA (cylindrical mirror analyzer) based Auger system (AES), and CMA XPS (X-ray photoelectron spectrometer) as well as the common surface science tools; for details, see ref 39. Unfortunately, the energy resolution and count rates of the XPS system (PerkinElmer from ∼1970s/ RBD) do not allow for a very detailed XPS analysis. The impact energy, Ei, of CO2 (3% of CO2 in He) could be varied within 0.7−1.25 eV by a variation of the nozzle temperature within 300−625 K. (The impact energy was calculated as described in ref 39, which is consistent with measured impact energies using a TOF (time-of-flight) system.) Adsorption probabilities were measured by means of the King and Wells technique.40 The uncertainty in the initial adsorption probabilities, S0, amounts to ±0.05. The beam was directed perpendicular toward the surface. The lowest adsorption temperature reached with liquid N2 cooling (lN2) and He gas bubbled through the Dewar amounts B


Article


due to defects in the graphene layer can be ruled out since the defect densities of the very same graphene layers were below 1% (see Figure S2). Note also that the C-AES signals do not change while forming GO. In addition, Ru-oxide formation requires higher temperatures and larger exposures.45,46 That the oxygen exposure by itself causes a large defect density in the graphene layer also can be ruled out as follows. Oxygen intercalation was reported in an STM (scanning tunneling microscopy) study about oxygen adsorption on graphene/Ru.47 In that case, however, large exposures of oxygen were required at much greater temperatures (>500 K).47 Similarly, graphene etching by oxygen was reported for graphene grown on Ru and Ir supports, but again at much greater temperatures (>980 K) as those used in this study (300 K).48 In most prior studies, molecular oxygen was dosed onto graphene samples; atomic oxygen will be more reactive. However, even with atomic oxygen, apparently nearly pure hyperthermal (highly energetic, 5 eV) atomic oxygen is required to actually etch graphite and then at a rather low removal rate of 0.045 C atoms per impinging O atom (0.045 = 1/22 at 300 K with 5 eV O).49 A simple W-filament doser just generates thermal oxygen (∼30 meV). At the measuring conditions used here, we have no indication for defect formation or oxygen intercalation while forming graphene oxide on Ru(0001). Both effects also would lead to difficulties in reproducing results since the sample morphology would change over time. Our samples were stable for at least several days of experiments. Annealing GO samples in UHV (or even CO) at 500 K did not change the O-XPS (or O-AES) signals or O-to-C ratios (see Figures S6−S8). Thus, the oxygen functionalities are thermally stable at least up to 500 K. Unfortunately, an analysis of the exact chemical state of the oxygen forming GO would require, e.g., XPS of much greater energy resolution than available to us. Note, however, that a large variety of oxygen species are mostly reported for wetchemistry preparations of GO.50,51 UHV fabricated GO is likely dominated by epoxy - like oxygen.44,52,53 (Partial pressure of water in UHV is small, reducing, e.g., hydroxyl formation.) Here, a maximum of one epoxy-oxygen per graphene unit cell (2:1 stoichiometry of C:O, i.e., 1 ML oxygen) is expected.

Figure 1. (A) Auger electron spectra (AES) of the three systems considered. The inset shows the O-AES region (electron energy = 2 keV, modulation 2 V). (B) X-ray photoelectron spectra (XPS) of the three systems considered. The inset shows the O-AES region (pass energy = 200 eV; Mg Kα line at 1253.6 eV).

3. PRESENTATION AND DISCUSSION OF THE RESULTS 3.1. CO2 Adsorption Kinetics on Ru(0001), Graphene/ Ru, and GO/Ru. Figure 3 depicts sets of CO2 TDS curves for Ru(0001), graphene/Ru, and GO/Ru collected for different initial CO2 exposures at a surface temperature of ∼85 K. Data for small exposures are given as insets. The temperature and intensity scales are identical for these three graphs. No significant adsorption/desorption of CO2 was seen for clean graphene (Figure 3A), as expected,27,28 which translates to a desorption energy smaller than about 25 kJ/mol (assuming molecular adsorption and a first-order pre-exponential of 1 × 1013/s). The condensation temperature of CO2 at UHV is relatively high for a small molecule, but it still amounts to ∼79 K (ref 54), which is just below the lowest adsorption temperature we could reach. Thus, for none of the experiments included here was the temperature low enough to efficiently condense CO2. (That fact makes the data interpretation and adsorption probability measurements (see next sections) actually simpler.) CO2 desorbs from the clean Ru(0001) support (Figure 3B) within 90−120 K, which is consistent with an early study.26 For

Figure 2. O-AES peak height vs oxygen exposure time (for different samples) recorded while fabricating graphene oxide (GO). Exposures are given in Langmuir.

oxygen on GO (see the Supporting Information for details). Since we report here about a “coadsorption study” with CO2, an even larger oxygen coverage is not really desirable since a fully “oxygen terminated” graphene surface may hinder the coadsorption. 2.2.3. Excluding Side Effects of GO Formation. That the OAES signal just originates from oxygen adsorbed on Ru(0001) C


Article


is seen up to 750 K. Thus, again, we can rule out accidental formation of Ru-oxides, which would result in very different CO2-TDS data. On the basis of these results, we rule out CO2 dissociation and oxygen−CO2 surface reactions. Symmetric desorption traces would be expected for a first-order adsorption/ desorption process. However, for molecular (first-order) adsorption, traditionally two TDS peaks would be expected, which are commonly assigned to molecular adsorption in the monolayer range and condensation.56,57 As already discussed, efficient multilayer condensation of CO2 would simply require significantly lower adsorption temperatures as available with our liquid nitrogen cooling system. Therefore, only one TDS peak is seen. Could there be an alternative explanation? On some surfaces, the adsorption energy to the surface is comparable to adsorbate−adsorbate interactions,58 which also can result in the detection of only one TDS peaks. In that case, monolayer adsorption and condensation cannot easily be distinguished. For these systems, however, due to zeroth-order like condensation kinetics, the TDS peak shifts to larger desorption temperatures with increasing exposure and, importantly, the TDS peak does not saturate,58 in contrast to the TDS curves seen here (Figure 3). Adsorption on hydrophobic surfaces13 or Stranski−Krastanov-like adsorption processes can also result in only one TDS peak. However, also, here the TDS peak intensity would continue to increase with increasing exposure. In any case, the simplest explanation of all features seen here is to assign the TDS data to molecular adsorption of CO2 (i.e., physisorption). The peak shifts are indicative of weak initial lateral interactions. The TDS data for Ru and GO are fairly similar (Figure 3B,C). However, recall that CO2 does (at 85 K) not adsorb at all on graphene (Figure 3A). Therefore, the functionalization of graphene as graphene oxide indeed enhances significantly the reactivity of graphene, even for adsorption of a small molecule. What is a possible mechanism for the reactivity enhancement? Our XPS/AES data do not provide evidence for carbonate formation or other surface reactions. That CO2 does not adsorb on plain graphene would suggest that ruthenium is simply poisoned by carbon, an effect often seen in catalysis and described as site blocking. It would also suggest that interactions of the support are not efficiently transmitted through the graphene layer. Indeed, it was proposed in theoretical studies that only 30% of van der Waals interactions are transmitted through graphene.59 If that transmission is enhanced for GO has not been considered theoretically, yet. The oxygen functionalization will certainly disturb the electronic properties of graphene significantly. Therefore, either an electrostatic stabilization of CO2 by the oxygen functionalities could be present (via van der Waals interactions) and/or van der Waals interactions (polarization) of the Ru support affect GO surface properties and CO2 adsorption (see also section 1.3). 3.2. Kinetics Parameters for CO2 on Ru(0001) and GO/ Ru. The TDS peak temperatures (Figure 3) were converted to binding energies, Ed, using the Redhead equation.42,57 Molecular first-order adsorption/desorption with a preexponential of 1 × 1013/s was assumed. CO2 coverages, Θ, were obtained by integrating the TDS peaks and assigning a coverage of 1 ML CO2 to saturation of GO with CO2 (i.e., for large CO2 exposures). The results for Ru(0001) and GO/Ru are depicted in Figure 4. Note that, also, the kinetics of CO2 on

Figure 3. CO2 TDS curves as a function of exposure to a seeded CO2 beam for (A) graphene/Ru(0001), (B) Ru(0001), and (C) GO/ Ru(0001). The insets show data at small exposures. Temperature and intensity scales are the same in all panels. Exposures are given in seconds using a CO2/He molecular beam.

GO/Ru (Figure 3C), the lowest TDS peak temperature is merely greater by 5 K than for Ru. With increasing exposure, initially the TDS peaks shift by ∼15 K to lower temperatures. Only one fairly symmetric TDS peak is seen which saturates above about 40 s CO2 exposure. Multimass TDS did not reveal signals besides those expected for gaseous CO2; GO samples made using 18O isotope did not indicate isotope scrambling in CO2. AES recorded before and after TDS experiments did not show any change (Figure S9). Note that literature CO2 TDS data for RuO2(110) look very different than any of our TDS data. For example,55 for RuO2(110), four TDS peaks were seen at 93, 175, 188, and 315 K. In our TDS data, on the Ru support and GO/Ru, only one peak is present up to 200 and 250 K, respectively (the greatest temperatures we used), and for G/Ru, no desorption structure D


Article


Figure 4. CO2 coverage dependent CO2 binding energy, Ed, on Ru(0001) and GO, as indicated, obtained from the TDS peak positions in Figure 3 using Redhead analysis (pre-exponential = 1 × 1013/s, heating rate 1.6 K/s). (Lines are shown as a guide for the eye.)

Ru(0001) was not reported quantitatively before. Overall, the binding energies are small, but typical for the physisorption of CO2 on graphitic systems.22 Ed decreases slightly with Θ due to initial lateral interactions corresponding to the small TDS peak shifts seen. A trend of Ed being larger on GO than on Ru is evident, but the enhancement amounts to only a few percent. 3.3. Adsorption Dynamics of CO2 on Ru(0001) and GO/Ru − Adsorption Transients. Typical adsorption transients of CO2 adsorption on graphene and GO, at the lowest attainable temperature but just above CO2 condensation, are depicted in Figure 5A. (Data for Ru(0001) are not explicitly included in the graph since they look qualitatively similar to those for GO.) The transients were determined by means of the so-called King and Wells technique40 using a molecular beam scattering system.60 Briefly, plotted is the background CO2 signal (or CO2 partial pressure) detected by a mass spectrometer as a function of CO2 exposure time, t. The gas started to adsorb on the surfaces at t = 0 by removing a beam flag. The CO2 trace is labeled such that 1 − S vs t is displayed, with S denoting the adsorption probability. The steplike transient for graphene/Ru (dotted line, Figure 5A) indicates that CO2 does not adsorb, which is in agreement with the TDS data (Figure 3A) already discussed. The signal increases for t > 0 s quickly to a saturation level since the gas load in the UHV chamber increases when the beam flag is opened and the CO2 beam enters the scattering chamber. In contrast, for CO2 adsorbing on GO (solid line in Figure 5A), the transient consists of an initial steplike increase, followed by a slow decrease (up to ∼40 s), and finally an increase of the signal to the saturation level for t > 70 s. This transient, as compared to the steplike curve for graphene, indicates that indeed CO2 adsorbs on GO (again, consistent with the TDS data; see Figure 3B). The initial adsorption probability can directly be read from the graph at t = 0 and amounts for this example to S0 = 0.22 (or 1 − S = 0.78 as shown). At t = 0, the transient describes the adsorption behavior in the limit of zero CO2 coverage on the surface. With increasing exposure time and coverage, the dip in the transient corresponds to an adsorption probability of Smax ∼ 0.4 (at t = 40 s). Finally, the surface saturates at t ∼ 70 s, which corresponds to the maximum CO2 coverage reached; that surface concentration is typically assigned to 1 ML of CO2.60

Figure 5. (A) Typical CO2 adsorption transients for graphene (at 84 K) and GO (at 88 K). (B) Coverage dependent adsorption probability for GO, as obtained by integrating the transient shown in panel A (CO2/He beam, impact energy Ei = 0.69 eV).

(Note that also the TDS peaks saturate at about the same exposure; see Figure 3C.) Data for Ru follow the same trend, including a dip in the transients, except that S0 is for Ru at the detection limit (S0 ∼ 0.05−0.03). The adsorption temperature is too large for forming condensed CO2 films, i.e., described is the molecular adsorption probability within the monolayer coverage range. (Therefore, formally S ∼ 0 is reached at the saturation level.) The shape of the S vs t curve is indicative of strong precursor effects; specifically, so-called adsorbateassisted adsorption is evident.22,61−63 The curve shape is easier to discuss when integrating the transient to obtain the coverage, Θ, dependent adsorption probability, in short denoted as S(Θ) (see Figure 5B). Figure 5B depicts the integrated transient for GO; i.e., S(Θ) is shown. Starting at S0, the adsorption probability increases with coverage until the surface saturates and S drops naturally to zero. Smax is reached at Θ ∼ 0.6 ML. Similar transients have been seen many times for other adsorbate systems (see, e.g., Table 1 in ref 61) and may be considered as a kind of autocatalytic adsorption or better as an adsorbate-assisted adsorption. S increases with Θ; i.e., the already adsorbed CO2 molecules assist the adsorption of the newly arriving species. This curve shape also is consistent with extrinsic precursor states (see Monte Carlo simulations in, e.g., refs 62−69); i.e., CO2 is initially trapped above adsorption sites which are already blocked by CO2 (or the oxygen on GO). The trapped CO2 diffuse along the precursor state until they reach an empty adsorption site and adsorb. Thus, the enhancement in adsorption (increase is S with Θ) is caused by a combination of two effects: precursor states with a rather long lifetime at low temperatures and adsorbate-assisted adsorption. E


Article


the kinetic energy that needs to be transferred to the surface for adsorption to occur increases and the interaction time of CO2 with the surface decreases with Ei. Therefore, the decrease in S0 reflects the decrease in energy transfer efficiency when Ei becomes larger. We did detect a significant enhancement in S0 for GO as compared with graphene (S0: 0.05 → 0.22 at 0.69 eV). Therefore, it appears unlikely that transmission of van der Waals interactions or substrate polarizations affect the adsorption dynamics of CO2 on GO. On the other hand, the oxygen on graphene will increase the corrugation of the surface, which usually enhances adsorption due to more efficient scrambling of perpendicular and parallel momentum components of the projectile.70 We also plotted the maximum, Smax, observed in the adsorption probabilities that correspond to the dip seen in the adsorption transients (see Figure 5B). Also, Smax decreases with Ei, for the same reasons just outlined. The deviation of S0 and Smax reflects the enhancement of energy transfer due to adsorbate-assisted adsorption. Interestingly, for GO, Smax is by a factor of ∼2 larger than S0, independent of Ei (within the considered energy range). Thus, the relative enhancement of energy transfer is independent of Ei. This implies also that the initial slope of S vs Θ curves is independent of Ei, an effect which was also seen for other very different systems (e.g., CO adsorption on ZnO, ref 65; CO2 on Cr(110), ref 71); i.e., it seems to be intrinsic to the adsorbate-assisted adsorption rather than the substrate. 3.5. Ethylene, CO, and Hydrogen Adsorption on GO/ Ru. We also dosed ethylene, CO, and atomic hydrogen on graphene oxide/Ru(0001), but could not detect any adsorption down to surface temperatures of ∼85 K. Therefore, assuming a pre-exponential of 1 × 1013/s for a first-order process, the desorption energies of these species on GO/Ru at UHV must be smaller than 25 kJ/mol. Annealing GO in ethylene at elevated temperature (e.g., 600 K, 1 × 10−8 mbar) did not remove oxygen from the surface (see Figure S6), similarly to atomic hydrogen exposure (Figure S7), or CO exposure (100 L ∼ 35 ML, 500 K, Figure S8). Therefore, e.g., ethylene epoxidation on GO/Ru at UHV conditions or CO oxidation seems not to proceed with detectable rates. From the exposures, we can roughly estimate that the reaction probability72 to remove, e.g., CO, must be ΔOxygen lower than CO flux ≈ 3 × 10−4 , assuming a detection limit of AES of 0.01 ML (1% ML).

The simplest explanation of the latter effect is the perfect mass match of adsorbed CO2 and impinging CO2, which causes the enhancement in S. The kinetic energy of the gas-phase species needs to be dissipated (or transferred) to the surface in order to adsorb. The better the mass match, the more efficient are energy transfer processes and the larger is the adsorption probability. Therefore, energy transfer between CO2 and CO2 is more efficient than between CO2 and GO. Hence, S(Θ) > S0 until saturation is reached. The larger the CO2 coverage, the more likely there will be an impact of CO2 on already adsorbed CO2. Therefore, S increases with CO2 coverage, which generates the quasi autocatalytic or self-accelerating effect. The initial adsorption probabilities are very small, in particular for Ru(0001). Thus, it seems that the adsorbate-assisted adsorption effect is to a large extent responsible to detect CO2 adsorption at all. 3.4. Energy Dependent Adsorption Probability. Figure 6A depicts the initial adsorption probability for GO as a

4. SUMMARY AND CONCLUSIONS The adsorption of CO2 was studied on Ru(0001), graphene grown on Ru, and graphene oxide (GO) on Ru using kinetics techniques and molecular beam scattering (dynamics). The samples were made in ultrahigh vacuum and spectroscopically characterized. CO2 physisorbs molecularly at ∼85 K on Ru and GO. However, adsorption on graphene was below the detection limit. At low concentrations, binding energies of CO2 amount to about ∼26 kJ/mol and were generally slightly larger on GO as compared with Ru. These binding energies are typical for CO2 physisorption on less reactive surfaces.22,23 Similarly, adsorption probabilities, as determined by molecular beam scattering, show a trend of enhancement for GO/Ru vs Ru. Initial adsorption probabilities on GO are, however, with S0 = 0.22 (at 85 K, Ei = 0.69 eV) quite small and

Figure 6. A) Initial adsorption probability, S0, of CO2 as a function of CO2 impact energy, Ei, for GO (CO2/He beam, adsorption temperature 85 K). B) Maximum adsorption probability corresponding to the dip in the adsorption transients for GO and Ru(0001) (Lines are shown as a guide for the eye.).

function of CO2 impact energy, Ei, as extracted from transients similar to these shown in Figure 5. S0(Ei) data for Ru(0001) are at the detection limit (∼0.05) of the King and Wells method. S0 for GO decreases with Ei, as expected for molecular and nondissociative adsorption. For an activated process or dissociation, S0 would be expected to increase with increasing Ei. For molecular adsorption, however, with increasing Ei, also F


Article


(11) Ertl, G., Knoezinger, H., Weitkamp, J., Eds. Handbook of Heterogeneous Catalysis; VCH: Weinheim, 1997. (12) Dreyer, D. R.; Park, S.; Bielawski, C. W.; Ruoff, R. S. The Chemistry of Graphene Oxide. Chem. Soc. Rev. 2010, 39, 228−240. (13) Chakradhar, A.; Burghaus, U. Adsorption of Water on Graphene/Ru(0001)an Experimental Ultra-high Vacuum Study. Chem. Commun. 2014, 50, 7698−7701. (14) Chakradhar, A.; Sivapragasam, N.; Nayakasinghe, M. T.; Burghaus, U. Support Effects in the Adsorption of Water on CVD Graphene: an Ultra-high Vacuum Adsorption Study. Chem. Commun. 2015, 51, 11463−11466. (15) Chakradhar, A.; Trettel, K. M.; Burghaus, U. Benzene Adsorption on Ru(0001) and Graphene/Ru(0001)  How to Synthesize Epitaxial Graphene without STM or LEED? Chem. Phys. Lett. 2013, 590, 146−152. (16) Vinogradov, N. A.; Schulte, K.; Ng, M. L.; Mikkelsen, A.; Lundgren, E.; Mårtensson, N.; Preobrajenski, A. B. Impact of Atomic Oxygen on the Structure of Graphene Formed on Ir(111) and Pt(111). J. Phys. Chem. C 2011, 115, 9568−9577. (17) Dreyer, D. R.; Jia, H. P.; Bielawski, C. W. Graphene Oxide: A Convenient Carbocatalyst for Facilitating Oxidation and Hydration Reactions. Angew. Chem. 2010, 122, 6965−6968. (18) www.graphene-supermarket.com (accessed 2016). (19) Zhang, J.; Liu, X.; Blume, R.; Zhang, A.; Schloegl, R.; Su, D. S. The Chemistry of Graphene Oxide. Science 2008, 322, 73−77. (20) Li, B.; Xu, Z. Graphene Oxide Catalysts. J. Am. Chem. Soc. 2009, 131, 16380−16385. (21) Freund, H. J.; Roberts, M. W. Surface Chemistry of Carbon Dioxide. Surf. Sci. Rep. 1996, 25, 225−299. (22) Burghaus, U. Surface Chemistry of CO2 - Adsorption of Carbon Dioxide on Clean Surfaces at Ultrahigh Vacuum. Prog. Surf. Sci. 2014, 89, 161−182. (23) Burghaus, U. Surface Science Perspective of Carbon Dioxide Chemistry − Adsorption Kinetics and Dynamics of CO2 on Selected Model Surfaces. Catal. Today 2009, 148, 212−225. (24) Burghaus, U. Gas-carbon Nanotubes Interactions: A Review of Ultra-high Vacuum Surface Science Studies on CNTs. In Carbon Nanotubes: New Research; Nova Science: New York, 2009. (25) Burghaus, U. In New and Future Developments in Catalysis: Activation of Carbon Dioxide; Suib, S. L., Ed.; Elsevier: Amsterdam, 2013; Chapter 3. (26) Hoffmann, F. M.; Weisel, M. D.; Paul, J. The Activation of CO2 by Potassium-promoted Ru(001) I. FT-IRAS and TDMS Study of Oxalate and Carbonate Intermediates. Surf. Sci. 1994, 316, 277−284. (27) Kadossov, E.; Burghaus, U. Adsorption Dynamics and Kinetics of CO2 on Fe/FeOx Nanoclusters Supported on HOPG. Surf. Interface Anal. 2008, 40, 893−899. (28) Cinke, M.; Li, J.; Bauschlicher, C. W.; Ricca, A.; Meyyappan, M. CO2 Adsorption in Single-walled Carbon Nanotubes. Chem. Phys. Lett. 2003, 376, 761−767. (29) Ghosh, A.; Subrahmanyam, K. S.; Krishna, K. S.; Datta, S.; Govindaraj, A.; Pati, S. K.; Rao, C. N. R. Uptake of H2 and CO2 by Graphene. J. Phys. Chem. C 2008, 112, 15704−15709. (30) Zhao, J.; Buldum, A.; Han, J.; Lu, J. P. Gas Molecule Adsorption in Carbon Nanotubes and Nanotube Bundles. Nanotechnology 2002, 13, 195−203. (31) Yim, W. L.; Byl, O.; Yates, J. T.; Johnson, J. K. Vibrational Behavior of Adsorbed CO2 on Single-walled Carbon Nanotubes. J. Chem. Phys. 2004, 120, 5377−5387. (32) Matranga, C.; Bockrath, B. Controlled Confinement and Release of Gases in Single-Walled Carbon Nanotube Bundles. J. Phys. Chem. B 2005, 109, 9209−9212. (33) Politano, A. Probing Growth Dynamics of Graphene/Ru(0001) and the Effects of Air Exposure by Means of Helium Atom Scattering. Surf. Sci. 2015, 634, 44−51. (34) Shichibe, H.; Satake, Y.; Watanabe, K.; Kinjyo, A.; Kunihara, A.; Yamada, Y.; Sasaki, M.; Hayes, W. W.; Manson, J. R. Probing Interlayer Interactions Between Graphene and Metal Substrates by

were even smaller for Ru. Interestingly, S increases with CO2 coverage by about a factor of 2, an effect typically described as adsorbate-assisted adsorption. In principle, CO2 could be electrostatically stabilized on GO via van der Waals interactions induced by the oxygen functionalities, and/or by modification of graphene’s electronic properties when forming GO, and/or by polarization (van der Waals interactions) of the substrate transmitted through GO. Given the fact that GO is more reactive than Ru and graphene, large effects of the Ru substrate appear unlikely. Similarly, enhancements of the adsorption dynamics will likely be caused by increased corrugation of GO compared with graphene. In summary, although no chemical reactions such as carbonate formation were obvious, the adsorption on GO was significantly enhanced compared with graphene or even compared with ruthenium.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b09573. Additional data, experimental setup, and further synthesis details (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: 701.231.8831. https:// www.ndsu.edu/chemistry/; www.uweburghaus.us. ORCID

Uwe Burghaus: 0000-0002-6349-5608 Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS The Donors of the American Chemical Society and Petroleum Research Fund are acknowledged for partial financial support. REFERENCES

(1) Su, D. S.; Zhang, J.; Frank, B.; Thomas, A.; Wang, X.; Paraknowitsch, J.; Schlögl, R. Metal-Free Heterogeneous Catalysis for Sustainable Chemistry. ChemSusChem 2010, 3, 169−180. (2) Schreiner, P. R. Metal-free Organocatalysis through Explicit Hydrogen Bonding Interactions. Chem. Soc. Rev. 2003, 32, 289−296. (3) Zhang, J.; Su, D.; Zhang, A.; Wang, D.; Schlögl, R.; Hébert, C. Nanocarbon as Robust Catalyst: Mechanistic Insight into CarbonMediated Catalysis. Angew. Chem., Int. Ed. 2007, 46, 7319−7323. (4) Wang, Y.; Li, H.; Yao, J.; Wang, X.; Antonietti, M. Synthesis of Boron doped Polymeric Carbon Nitride Solids and their use as Metalfree Catalysts for Aliphatic C−H bond Oxidation. Chem. Sci. 2011, 2, 446−450. (5) Sun, X.; Wang, R.; Su, D. Research Progress in Metal-free Carbon-based Catalysts. Chin. J. Catal. 2013, 34, 508−523. (6) Qi, W.; Su, D. Metal-Free Carbon Catalysts for Oxidative Dehydrogenation Reactions. ACS Catal. 2014, 4, 3212−3218. (7) Zhang, M.; Dai, L. Carbon Nanomaterials as Metal-free Catalysts in next Generation Fuel Cells. Nano Energy 2012, 1, 514−517. (8) Su, C.; Loh, K. P. Carbocatalysts: Graphene Oxide and Its Derivatives. Acc. Chem. Res. 2013, 46, 2275. (9) Li, B.; Xu, Z. A Nonmetal Catalyst for Molecular Hydrogen Activation with Comparable Catalytic Hydrogenation Capability to Noble Metal Catalyst. J. Am. Chem. Soc. 2009, 131, 16380−16391. (10) Thomas, J. M.; Thomas, W. J. Principles and Practice of Heterogeneous Catalysis; VCH: Weinheim, 2005. G


Article

The Journal of Physical Chemistry C Supersonic Rare-gas Atom Scattering. Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 91, 155403. (35) Kadossov, E.; Goering, J.; Burghaus, U. Adsorption of Linear and Branched Alkanes on HOPG: a Molecular Beam Scattering Study. Surf. Sci. 2007, 601, 3421−3425. (36) Narayanan, B.; Weeks, S. L.; Jariwala, B. N.; Macco, B.; Weber, J. W.; Rathi, S. J.; van de Sanden, M. C. M.; Sutter, P.; Agarwal, S.; Ciobanu, C. V. Carbon Monoxide-induced Reduction and Healing of Graphene oxide. J. Vac. Sci. Technol., A 2013, 31, 040601−9. (37) Tang, S.; Cao, Z. Adsorption of Nitrogen Oxides on Graphene and Graphene Oxides: Insights from Density Functional Calculations. J. Chem. Phys. 2011, 134, 044710−21. (38) Tang, S.; Cao, Z. Adsorption and Dissociation of Ammonia on Graphene Oxides: A First-Principles Study. J. Phys. Chem. C 2012, 116, 8778−8791. (39) Wang, J.; Burghaus, U. Adsorption Dynamics of CO2 on ZnZnO: a Molecular Beam Study. J. Chem. Phys. 2005, 122, 044705−12. (40) King, D. A.; Wells, M. G. Molecular Beam Investigation of Adsorption Kinetics on Bulk Metal Targets: Nitrogen on Tungsten. Surf. Sci. 1972, 29, 454−458. (41) Yates, J. T. Experimental Innovations in Surface Science: A Guide to Practical Laboratory Methods and Instruments; Springer: New York, 1998. (42) Redhead, P. A. Desorption of Gases. Vacuum 1962, 12, 203− 209. (43) Ojifinni, R. A.; Gong, J.; Froemming, N. S.; Flaherty, D. W.; Pan, M.; Henkelman, G.; Mullins, C. B. Carbonate Formation and Decomposition on Atomic Oxygen Precovered Au(111). J. Am. Chem. Soc. 2008, 130, 11250−11261. (44) Hossain, Z.; Johns, J. E.; Bevan, K. H.; Karmel, H. J.; Liang, Y. T.; Yoshimoto, S.; Mukai, K.; Koitaya, T.; Yoshinobu, J.; Kawai, M.; Lear, A. M.; Kesmodel, L. L.; Tait, S. L.; Hersam, M. C.; et al. Chemically Homogeneous and Thermally Reversible Oxidation of Epitaxial Graphene. Nat. Chem. 2012, 4, 305−309. (45) Hrbek, J.; van Campen, D. G.; Malik, I. J. The Early Stages of Ruthenium Oxidation. J. Vac. Sci. Technol., A 1995, 13, 1409−1412. (46) Böttcher, A.; Niehus, H. Oxygen Adsorbed on Oxidized Ru(0001). Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 60, 14396− 14400. (47) Zhang, H.; Fu, Q.; Cui, Y.; Tan, D.; Bao, X. Growth Mechanism of Graphene on Ru(0001) and O2 Adsorption on the Graphene/ Ru(0001) Surface. J. Phys. Chem. C 2009, 113, 8296−8300. (48) Starodub, E.; Bartelt, N. C.; McCarty, K. F. Oxidation of Graphene on Metals. J. Phys. Chem. C 2010, 114, 5134−5144. (49) Nicholson, K. T.; Minton, T. K.; Sibener, S. J. Spatially Anisotropic Etching of Graphite by Hyperthermal Atomic Oxygen. J. Phys. Chem. B 2005, 109, 8476−8480. (50) Hummers, W. S.; Offeman, R. E. Preparation of Graphitic Oxide. J. Am. Chem. Soc. 1958, 80, 1339−1345. (51) Szabo, T.; Berkesi, O.; Forgo, P.; Josepovits, K.; Sanakis, Y.; Petridis, X. D.; Dekany, I. Evolution of Surface Functional Groups in a Series of Progressively Oxidized Graphite Oxides. Chem. Mater. 2006, 18, 2740−2751. (52) Böttcher, S.; Vita, H.; Horn, K. Photon-induced Oxidation of Graphene/Ir(111) by SO2 Adsorption. Surf. Sci. 2015, 641, 305−312. (53) Böttcher, S.; Vita, H.; Horn, K. Reversible Photon-induced Oxidation of Graphene by NO2 Adsorption. Surf. Sci. 2014, 621, 117− 123. (54) Camplin, J. P.; Clowes, S. K.; Cook, J. C.; McCash, E. M. A Study of Adsorbate Vibrations at Cryogenic Temperatures. Surf. Rev. Lett. 1997, 04, 1365−1373. (55) Wang, Y.; Lafosse, A.; Jacobi, K. Adsorption and Reaction of CO2 on the RuO2(110) Surface. J. Phys. Chem. B 2002, 106, 5476− 5483. (56) Somorjai, G. A. Introduction to Surface Chemistry and Catalysis; Wiley: NewYork, 1999. (57) Christmann, K. Introduction to Surface Physical Chemistry; Steinkopff Verlag: Darmstadt, Germany, 1991.

(58) Funk, S.; Hokkanen, B.; Wang, J.; Burghaus, U.; Bozzolo, G. H.; Garces, J. E. Adsorption Dynamics of CO2 on Cu(110): a Molecular Beam Study. Surf. Sci. 2006, 600, 583−591. (59) Shih, C. J.; Strano, M. S.; Blankschtein, D. Wetting Translucency of Graphene. Nat. Mater. 2013, 12, 866. (60) Wang, J.; Burghaus, U. Adsorption of CO on the Copperprecovered ZnO(0001) Surface: a Molecular-beam Scattering Study. J. Chem. Phys. 2005, 123, 184716−184732. (61) Burghaus, U. Adsorption Dynamics in The Case of Preadsorbates: Measuring Methods and Precursor Models. Trends Phys. Chem. 2001, 8, 21−27. (62) Stephan, J.; Burghaus, U. Effect of Defects and Preadsorbates on Adsorption Probabilities: a Monte Carlo Simulation Point of View. Surf. Sci. 2002, 507−510, 736−745. (63) Fichthorn, K. A.; Weinberg, W. H. Theoretical Foundations of Dynamical Monte Carlo Simulations. J. Chem. Phys. 1991, 95, 1090− 1099. (64) Burghaus, U.; Vattuone, L.; Kobal, I. In Proceedings of the 9th International Symposium on Heterogeneous Catalysis, Varna, Bulgaria, 2000; Institute of Catalysis, Bulgarian Academy of Sciences: Sofia, Bulgaria, 2000. (65) Stephan, J.; Burghaus, U. Kinetic Monte Carlo Simulation of Adsorption Probabilities: the Auto-catalytic Adsorption Phenomenon. J. Vac. Sci. Technol., A 2003, 21, 1284−1288. (66) Burghaus, U. Monte Carlo Simulation of the Adsorbate Assisted Adsorption in the Case of CO/ZnO. Surf. Rev. Lett. 2001, 8, 353−361. (67) Burghaus, U. Monte Carlo Simulations of Adsorption Probabilities: the Case of CO/disordered-Cu(110) and CO/OIr(110). Surf. Rev. Lett. 2003, 10, 7−14. (68) Kadossov, E.; Burghaus, U. Multi-site Kinetic Monte Carlo Simulations of Thermal Desorption Spectroscopy Data. Surf. Sci. 2009, 603, 2494−2499. (69) Burghaus, U.; Stephan, J.; Rogowska, J. M.; Vattuone, L. A Practical Guide to Monte Carlo Simulations And Classical Molecular Dynamics Simulation - An Example Book; Nova Science: New York, 2006. (70) Darling, G. R.; Holloway, S. The Dissociation of Diatomic Molecules at Surfaces. Rep. Prog. Phys. 1995, 58, 1595−1602. (71) Funk, S.; Nurkic, T.; Hokkanen, B.; Burghaus, U. CO 2 Adsorption on Cr(110) and Cr2O3(0001)/Cr(110). Appl. Surf. Sci. 2007, 253, 7108−7113. (72) Burghaus, U.; Vattuone, L.; Gambardella, P.; Rocca, M. HREELS study of CO oxidation on Ag(100) by O2 or O. Surf. Sci. 1997, 374, 1−6.

H


Adsorption Kinetics and Dynamics of CO2 on Ru (0001) Supported

Recommend Documents