Desorption Kinetics of Carbon Dioxide from a Graphene Covered Pt

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Desorption Kinetics of Carbon Dioxide from a Graphene Covered Pt(111) Surface R. Scott Smith, and Bruce D. Kay J. Phys. Chem. A, Just Accepted Manuscript • Publication Date (Web): 26 Mar 2019 Downloaded from http://pubs.acs.org on March 26, 2019

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The Journal of Physical Chemistry

Desorption Kinetics of Carbon Dioxide from a Graphene Covered Pt(111) Surface R. Scott Smith* and Bruce D. Kay* Physical and Computational Sciences Richland, Washington 99352

Directorate, Pacific Northwest National Laboratory,

Abstract The interaction of carbon dioxide (CO2) with a graphene covered Pt(111) surface was investigated using temperature programmed desorption (TPD) and reflection absorption infrared spectroscopy (RAIRS). The TPD spectra show monolayer and multilayer desorption peaks, however, the multilayer peak is not well-separated from the monolayer peak.

The TPD spectra for

submonolayer and multilayer coverages align on separate common leading edges. This alignment is a signature of zero-order desorption kinetics. The RAIRS spectra for submonolayer coverages have a relatively sharp peak at ~2350 cm-1 which is assigned to the 3 asymmetric stretch. The peak is observed at the onset of CO2 adsorption and the area of the peak increases linearly with coverage. This suggests that CO2 does not lie flat on the surface but instead has a component of its bond axis perpendicular to the graphene surface.

*Corresponding Authors R. Scott Smith, Pacific Northwest National Laboratory, (509) 371-6156, [email protected] Bruce D. Kay, Pacific Northwest National Laboratory, (509) 371-6143, [email protected]

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I. Introduction The interaction of carbon dioxide with surfaces has long been of interest in many areas of research, in particular catalysis, and there are several review articles on the subject.1-5 More recently, there has been a renewed interest in the interactions of CO2 with graphene and modified graphene for applications in photocatalysis, adsorbent materials, sensors, and separations.6-16 Adsorbate-graphene interactions are also important as models to understand weak molecular interactions17-20 and to astrophysicists interested in adsorbate adsorption on astrophysical bodies composed of carbon.21 In all of the above-mentioned research areas the geometry of the adsorbate on the surface will affect its interaction energy and possibly its chemical behavior. 6-16 For example, the adsorbate geometry can affect the electrical response and selectivity of graphene based sensors.15 In weakly bound systems small changes in geometry can have large effects on the interaction energies.17-20 The effects of geometry are also relevant for other small molecules. For example, the reactivity of acetonitrile on Pt(111) will depend on if the molecule interacts with the surface with the N end of the molecule perpendicular to or if the C-N bond is parallel to the surface.22 In this paper we focus on the interactions of CO2 with a pristine (non-modified) graphene covered Pt(111) surface. Molecular beam dosing, temperature programmed desorption (TPD), and reflection absorption infrared spectroscopy (RAIRS) are employed in the investigation. The TPD technique is used to determine the binding energy and RAIRS is used to determine the CO2 geometry. The TPD results show that the CO2 monolayer desorption kinetics are zero-order. Analysis of the desorption rate data shows that CO2 is weakly bound (physisorbed) to the graphene layer with a binding energy of 26.1  2 kJ/mol. The RAIRS results suggest that CO2

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does not lie parallel to the surface but is instead oriented perpendicular to or tilted away from the graphene surface.

II. Experiment The TPD and RAIRS experiments were conducted in an ultra-high vacuum system (UHV) (base pressure < 1  10-10 torr) which has been described in detail elsewhere.23,24 Within the chamber, a 1 cm diameter, 1 mm thick Pt(111) single crystal was connected to a closed-cycle helium cryostat which cools the crystal to a base temperature of ~20 K. The sample was resistively heated through two tantalum leads spot-welded on the back and the temperature measured using a K-type thermocouple spot-welded to the back of the sample. The temperature was measured with a precision of better than 0.01 K and was calibrated to an absolute accuracy of better than ± 2 K using the desorption rate curves of various substances including Ar, Kr, and H2O multilayers.25 The graphene layer was grown by heating the Pt(111) surface to 1100 K and then exposing the surface to a molecular beam of decane. Details on growth and characterization of the graphene layer are published elsewhere.26 Carbon dioxide was deposited using a quasi-effusive molecular beam collimated by four stages of differential pumping at normal incidence and at 25 K. The beam flux was 0.33 ML/s where one ML was defined as the quantity of CO2 needed to saturate the monolayer desorption peak (see discussion below). We estimate 1 ML to correspond to ~7.7  1014 CO2 molecules/cm2 using the 2/3 root of the CO2 number density. TPD spectra were acquired using a linear heating rate of 1 K/s and an Extrel quadrapole mass spectrometer in a line-of-sight configuration. RAIRS spectra were obtained prior to desorption using a Bruker Vertex 70 Fourier transform infrared spectrometer with a beam that was incident on the sample at a glancing angle of 84  1

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from normal. The spectra were obtained by coadding 600 scans with 4 cm-1 resolution. The detector was a liquid nitrogen cooled mercury−cadmium−telluride (MCT) narrow band detector.

III. Results A. Desorption of Carbon Dioxide from Graphene covered Pt(111) Figure 1 displays the TPD spectra for CO2 films deposited at 25 K on a graphene covered Pt(111) substrate and heated at 1.0 K/s. The desorption spectra for CO2 coverages from 0.07 to 1.0 ML (blue curves) are displayed in Fig. 1a. The leading edges of the desorption spectra align on a common curve and the desorption peaks shift to higher temperature with increasing coverage. These two features are signatures of zero-order desorption kinetics.27-32 A monolayer coverage, ML, is defined as the maximum dose where the TPD initial leading edge is aligned on the same curve as the TPD spectra in Figure 1a. For doses greater than 1 ML, the initial TPD leading edges are aligned on a separate curve (see below). In general, zero-order kinetics indicates that the rate of a given process is independent of the amount of the species of interest. In this case, it means that the desorption rate is independent of the coverage. While it may seem counterintuitive for submonolayer coverages, zero-order kinetics can occur if isolated adsorbates diffusing on the surface interact in equilibrium with two-dimensional islands.27,33,34 The two-phase coexistence of islands and individual adsorbates in equilibrium establishes the chemical potential of the system which sets the vapor pressure of the system and hence the desorption rate. In this situation the desorption rate will be independent of coverage and depend only on temperature. We have previously observed zero-order desorption kinetics for many adsorbates on graphene including water, methanol, ethanol, Ar, Kr, Xe, N2, O2, CO, methane, ethane, propane, and cyclohexane.29-32 However,

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zero-order desorption kinetics for submonolayer coverages requires the formation of two-dimensional islands. In cases where the adsorbate lateral interactions are weak or repulsive, first-order desorption kinetics from graphene are observed.32 Figure 1b displays the CO2 desorption spectra for both the first layer (blue) and the second layer (1.2 to 1.7 ML, red). For the second layer coverages, the leading edges of the TPD spectra are initially aligned on a common new curve having a higher desorption rate but then eventually the desorption rate decreases and aligns with the monolayer desorption rate curve. This is consistent with the initial desorption being from adsorbates in the second layer and eventually as the coverage decreases, desorption is from molecules in the first layer. This occurs because the firstand second-layer desorption peaks are not well-separated which means that the difference in the binding energies of the first- and second-layer adsorbates is small. Also note that the second layer peak shifts to higher temperature beyond the saturated monolayer peak. Normally one would expect the second layer peak to be at a lower temperature than the monolayer peak. However, because the peaks are not well separated, desorption of the monolayer is delayed until the second layer is completely desorbed which results in the monolayer peak being shifted to higher temperature. The desorption spectra for higher coverages (2.3 to 10 ML, green) are displayed in Figure 1c. In this case the leading edges of the TPD spectra are aligned on the same curve as the second layer desorption spectra indicating that the second- and multi-layer binding energies are essentially identical. Arrhenius plots of the CO2 TPD spectra are displayed in Figure 2. All of the TPD spectra from Figure 1 are plotted in Figure 2a. The desorption rates for the monolayer TPD spectra (blue curves) align onto a single straight line (up to the desorption peak). The desorption rates for the second layer (red curves) and multilayer (green) TPD spectra are initially aligned on a single 5



straight line but at higher temperatures the rate decreases onto the monolayer desorption rate line. The temperature where the rate decreases to the monolayer desorption rate, increases with the initial coverage of CO2. As discussed above, this is because the first- and second-layer binding energies are close in energy leading to overlapping desorption peaks. For clarity, Figure 2b displays only the 1, 5, 6.6, and 10 ML TPD spectra. The dashed lines are Arrhenius fits to the leading edges of the monolayer and multilayer desorption spectra. The 1 ML fit yielded an activation energy of 26.1  2 kJ/mol and a prefactor of 5  1015  1 ML s-1 and the multilayer fit yielded an activation energy of 25.7  2 kJ/mol and a prefactor of 5  1015  1 ML s-1. The binding energy for the monolayer is slightly larger than that of the multilayer, however the difference is smaller than the estimated error. The main point is that the CO2 interaction energy with the graphene covered Pt(111) substrate is nearly the same as the CO2-CO2 interaction energy in the multilayer. The reported CO2 sublimation energy of 27.2  4 kJ/mol (obtained measurements from 70 to 103 K) is in good agreement with our results.35 B. Infrared Spectra of Carbon Dioxide from Graphene covered Pt(111) Figure 3 displays the RAIRS spectra for a series of CO2 coverages on graphene covered Pt(111). All CO2 coverages were deposited at 25 K and then heated to 65 K and cooled back to 25 K where the RAIRS spectra were obtained. No significant CO2 desorption occurred during the temperature anneal. The reason for annealing the films prior to obtaining the RAIRS spectra was to probe the structure of the film at or near the temperature of desorption. A comparison of the RAIRS spectra for the annealed films with the as-deposited films (not shown here) does show some sharpening of the peaks but the overall behavior of the RAIRS spectra with coverage was

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the same. Above 40 K multilayer CO2 is known to crystallize.36 We show only the RAIRS spectra for temperature annealed films here. Carbon dioxide has two infrared active vibrational modes, the asymmetric stretch, 3, and the doubly degenerate bend, 2. The symmetric stretch, 1, is infrared inactive. The bend frequency is between 640 to 680 cm-1.36-38 Our narrow band MCT detector loses sensitivity below ~750 cm-1 and therefore the 2 bending vibration could not be observed. For that reason, we only plot the RAIRS spectra from 2300 to 2450 cm-1. Figure 3a displays the RAIRS for monolayer coverages of CO2 (0.07 to 1 ML, blue curves). The spectra are from the corresponding films displayed in Figure 1 prior to desorption. A single feature centered at 2350 cm-1 is observed and is assigned to the asymmetric stretch, 3, of CO2.36,37,39 The peak is observable at the lowest coverage (0.07 ML) and increases in intensity with increasing coverage. Figure 3b displays the RAIRS spectra for second layer (1.2 to 1.7 ML, red curves) and monolayer (blue curves) CO2 coverages. Adsorption of CO2 into the second layer results in the appearance of a second peak at ~2378 cm-1 which also increases in intensity with increasing coverage. The peak is blue shifted by about 28 cm-1 from the one observed for monolayer coverages at 2350 cm-1. A blue shift typically means that the intermolecular interactions of the vibrating species have decreased. This suggests that monolayer CO2 molecules have a slightly stronger interaction with the graphene substrate than second layer CO2 molecules have with monolayer CO2 molecules. This is consistent with the TPD results which yielded a slightly higher binding energy for monolayer CO2 than that for the second layer and multilayer coverages. Figure 3c displays the RAIRS spectra for multilayer (2.3 to 10.0 ML, green curves) along with the spectra for monolayer and second layer coverages. The multilayer peak is initially at the same frequency as the second layer peak but blue shifts slightly (2 to 3cm-1) with

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increasing coverage. The shift in the peak with coverage has been previously observed and has been attributed to ordering of the CO2 molecules into a crystalline-like structure.36 Figure 4 displays plots of the integrated intensities for the peaks at 2350 cm-1 and 2378 cm-1 versus coverage from the RAIRS spectra displayed in Figure 3. For the 2350 cm-1 peak the area was obtained by integrating from 2300 to 2364 cm-1 and for the 2378 cm-1 peak the area was obtained by integrating from 2364 cm-1 to 2450 cm-1. Figure 4a displays the 2350 cm-1 (blue filled circles) and 2378 cm-1 (red filled circles) peak areas for coverages up to 2 ML. The solid black curve is the total integrated intensity from 2300 to 2450 cm-1. The plot shows that the 2350 cm-1 peak intensity increases linearly up to ~1.0 ML and then plateaus to a near constant level. The non-zero peak intensity even at the lowest coverage confirms that CO2 is infrared active from the onset of deposition. In contrast the 2378 cm-1 peak is not observed for CO2 coverages up to ~1 ML. Above 1 ML the 2378 cm-1 peak intensity begins to increase linearly with coverage. The slope of the increase of the 2378 cm-1 peak are above 1 ML is about two times greater than the slope of the increase of the 2350 cm-1 peak intensity from 0 to 1 ML. These differences in slope reflect band strength changes that can occur due to changes in the intermolecular environment or the orientation of the CO2 molecule relative to the surface.37 Figure 4b displays the 2350 cm-1 and 2378 cm-1 peak areas for coverages up to 10 ML. Above 1 ML the 2350 cm-1 peak area remains relatively constant whereas the 2378 cm-1 peak area increases with coverage albeit in a sub-linear manner above 3 ML.

IV. Discussion The TPD experiments in Figure 1 and Figure 2 clearly show that monolayer and submonolayer coverages of CO2 desorb from a graphene covered Pt(111) with zero-order desorption kinetics.

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The desorption kinetics for second layer and greater coverages are also zero-order and their desorption rates all align onto a common leading edge indicating that the second and subsequent layer binding energies are not distinguishable. Arrhenius analysis of the TPD spectra yield binding energies of 26.1  2 kJ/mol for the monolayer and 25.7  2 kJ/mol for the multilayer with both having a prefactor of 5  1015  1 ML s-1. These results are consistent with previously published binding energies of 24  2 kJ for the monolayer and 24  2 kJ for the multilayer and a prefactor of 6  1014  1 ML s-1 for CO2 desorption from graphite (HOPG).28 That work also reported zero-order desorption kinetics for submonolayer coverages of CO2 on graphite. Two more recent papers have studied the interactions of CO2 with graphene covered substrates.40,41 For graphene covered Ru(0001) films, no CO2 adsorption was observed at 85 K (base temperature of their system) which is consistent with our results which show the desorption of submonolayer coverages begins below 85 K.40 For graphene grown on a SiC(0001) substrate, the authors report first-order desorption kinetics for monolayer and submonolayer coverages and zero-order desorption for multilayer coverages.41 The binding energy (26.4  1.5 kJ/mol) and desorption prefactor (6.2  1014  1 ) obtained in that work are consistent with our results. However, they report first-order desorption kinetics for coverages below 0.2 ML (using our definition of a ML) and that multilayer desorption begins before the saturation of the monolayer based on the onset of zero-order desorption. We and others have previously observed first-order desorption kinetics for submonolayer coverages of CO2 from oxide substrates TiO2, Mg2SiO4(011), Fe3O4.42-44 However in those systems, the CO2 interaction energy with the ionic surfaces is typically stronger (>40 kJ/mol compared to ~26 kJ/mol on graphene). It is interesting to note that in the graphene on SiC(0001) work, a transition from first-order kinetics at low coverages to zero-order desorption at higher coverages is observed when the coverage is below 9



~0.2 ML. It could be that desorption at low coverages is from defect sites and the desorption at higher coverages is from the graphene monolayer. The onset of zero-order desorption kinetics does not necessarily indicate that the desorption is from the multilayer and is the hallmark of coexistence between two-dimensional condensed (islands) and isolated adsorbate phases in the submonolayer coverage regime as we have discussed above. Further work is needed to resolve the subtle differences in the desorption kinetics of CO2 from graphene on SiC(0001)41 and from graphene on Pt(111). The infrared data from Figure 3 and Figure 4 provide some insight into the structure of the CO2 on the graphene substrate. For monolayer coverages, a single peak is observed and assigned to the asymmetric stretch, 3. For second and multilayer coverages, a second peak is observed that is blue-shifted from the monolayer. The appearance of the second infrared peak only after the coverage exceeds 1 ML supports the idea of layer-by-layer growth which is consistent with the TPD filling curves in Figure 1. The observation of the two separate peaks is consistent with the monolayer interacting with the graphene layer and the second layer interacting with the CO2 monolayer. Thus, the second layer infrared red spectra are beginning to be more representative of the bulk. This is consistent with the shift of the second peak to 2378 cm-1 which corresponds to the longitudinal optical (LO) phonon mode coupled to the 3 CO2 asymmetric stretch in crystalline CO2.36,38 The transverse optical (TO) phonon mode (2342 cm-1) which dominates the bulk crystalline CO2 absorption spectrum in transmission at normal incidence is not observed in reflection because it is polarized parallel to the graphene surface. The monolayer asymmetric stretch peak at 2350 cm-1 is observable even at the lowest coverages (0.07 ML). The observation of this vibration indicates that the CO2 molecule does not lie flat

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and parallel to the graphene substrate. Instead it must have a component of its molecular axis aligned perpendicular to the graphene substrate.45 The selection rules for adsorbates on metals state that only light polarized perpendicular to the surface, p-polarized, can interact with the adsorbate. Thus, only vibrations having a component of their transition dipole perpendicular to the surface can be observed. The linear increase in peak intensity with coverage indicates that the CO2 molecules are aligned, or are at least partially tilted, towards the graphene surface normal. The alignment of CO2 tilted to the normal of the graphene substrate is contrary to several theoretical studies that calculate the minimum energy configuration to be with CO2 parallel to the graphene surface.6,7,11,15,41,46 Although one report calculates the difference in the parallel and perpendicular configurations to be less than ~5 kJ/mol.46 One difference between the experimental and the theory papers is that the theoretical graphene substrate is represented by a single, free standing layer of graphite whereas our experiments and others are performed on a graphene layer grown on a substrate. The underlying substrate could affect the interaction energy and adsorbate configuration. Experimental studies on metals show that CO2 typically lies parallel to the surface.1,2 However for many metals determination of the exact surface geometry of CO2 is complicated by charge transfer that creates a CO2- which will have different bonding properties than neutral CO2.1 Our results show that CO2 is weakly bound to graphene which suggests very little charge transfer is occurring. For CO2 on TiO2(110) there is both experimental and theoretical evidence that monolayer CO2 is not parallel to the surface but is instead tilted ~45° away from the surface.47 Alignment of the CO2 molecular axis directly along the surface normal would be contradictory to our desorption results. If the CO2 molecules were all completely perpendicular to the surface, significant quadrupole repulsions would likely limit

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the formation of two-dimensional islands that are needed for submonolayer zero-order desorption kinetics.32 A more likely possible structure would be for the CO2 to be tilted away from the surface in alternating pattern where nearest neighbors are tilted in opposite directions to minimize the repulsive interactions. This has been seen for CO2 on TiO2(110).47 Our arguments for the submonolayer structure of CO2 are largely based on the infrared selection rules for adsorbates on metal substrates. There are interesting cases where molecules (e.g. N2, O2) lying with their molecular axis parallel to the metal surface are infrared active due to induced dipoles from surface interactions.48-50 Additional experimental evidence using other techniques is needed to confirm our proposed structure. The desorption and infrared results presented here provide a quantitative reference for future theoretical and experimental studies on both pristine and modified graphene surfaces. For theorists these results provide data to benchmark the calculations of weak interactions (CO2 with graphene) that are important in many areas and are difficult to quantify. For experimentalists these results provide a reference for CO2 behavior on pristine graphene in order to compare with its behavior on modified graphene surfaces.

Acknowledgement This work was supported by the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences. The research was performed using EMSL, a national scientific user facility sponsored by DOE’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory, which is operated by Battelle operated for the DOE under Contract No. DE-AC0576RL01830. 12


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Methane, Ethane, and Propane from Graphene and Amorphous Solid Water Surfaces. J. Phys. Chem. B 2016, 120, 1979-1987. (31)

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Band Strengths of H2O, CO and CO2 in Laboratory Simulations of Astrophysical Ice Mixtures. Astron. Astrophys. 1995, 296, 810-818.

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Koitaya, T.; Mukai, K.; Yoshimoto, S.; Suemitsu, M. et al. Adsorption of CO2 on Graphene: A Combined Tpd, Xps, and Vdw-Df Study. J. Phys. Chem. C 2017, 121, 2807-2814. (42)

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Displacement Kinetics of H2O and CO2 on TiO2(110). J. Phys. Chem. B 2014, 118, 8054-8061. (43)

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Figure Captions: Figure 1: TPD spectra for CO2 deposited on a graphene covered Pt(111) substrate at 25 K and heated at 1.0 K/s. (a) A plot of the CO2 TPD spectra for monolayer coverages of 0.07, 0.13, 0.20, 0.27, 0.33, 0.47, 0.60, 0.73, 0.87, and 1.0 ML (blue curves). (b) A plot of the CO2 TPD spectra for second layer coverages of 1.2, 1.3, 1.5, and 1.7 ML (red curves) along with those for the monolayer (blue curves) coverages. (c) A plot of the CO2 TPD spectra for multilayer coverages of 2.3, 3.3, 5.0, 6.7, 8.3, and 10.0 ML (green curves) along with those for the monolayer (blue curves) and second layer (red curves) coverages. Figure 2: (a) Arrhenius plot of the monolayer (blue curves), second layer (red curves), and multilayer (green curves) CO2 TPD spectra displayed in Figure 1. (b) Arrhenius plot of the 1 ML (blue curve) and 5, 6.7, and 10 ML (green curves) CO2 TPD spectra. The dashed lines (black lines) are Arrhenius fits to the monolayer and 5 ML TPD spectrum. The 1 ML fit yielded an activation energy of 26.1  2 kJ/mol and a prefactor of 5  1015  1 ML s-1 and the 5 ML fit yielded an activation energy of 25.7  2 kJ/mol and a prefactor of 5  1015  1 ML s-1. Fits to the other multilayer curves (6.7 and 10 ML) yielded activation energies within these error estimates.

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Figure 3: RAIRS spectra for a series of CO2 coverages on graphene covered Pt(111). All CO2 coverages were deposited at 25 K and then heated to 65 K and cooled back to 25 K where the RAIRS spectra were obtained. Displayed is the CO2 asymmetric stretch, 3, region from 2300 to 2450 cm-1. (a) The RAIRS spectra for CO2 coverages of 0.07, 0.13, 0.20, 0.27, 0.33, 0.47, 0.60, 0.73, 0.87, and 1.0 ML (blue curves). (b) The RAIRS spectra for CO2 coverages of 1.2, 1.3, 1.5, and 1.7 ML (red curves) along with those for the monolayer (blue curves) coverages. (b) The RAIRS spectra for CO2 coverages of multilayer coverages of 2.3, 3.3, 5.0, 6.7, 8.3, and 10.0 ML (green curves) along with those for the monolayer (blue curves) and second layer (red curves) coverages. Figure 4: Plots of the peak areas versus coverage from the RAIRS spectra displayed in Figure 3. (a) Plot of the 2350 cm-1 (filled blue circles) and the 2378 cm-1 (filled red circles) peak areas for CO2 coverages up to 2 ML. The 2350 cm-1 peak area was obtained by integrating from 2300 to 2364 cm-1 and the 2378 cm-1 peak area was obtained by integrating from 2364 cm-1 to 2450 cm-1. The total area of both peaks (black curve) was obtained by integrating from 2300 to 2450 cm-1. (b) Plot of the 2350 cm-1 (filled blue circles) and the 2378 cm-1 (filled red circles) peak areas for CO2 coverages up to 10 ML.

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Figure 1 0.40 0.35 0.30 0.25

a) CO2 Coverage 0.07 to 1 ML

0.20 0.15 0.10 0.05 0.00

Desorption Rate (ML/s)

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


0.40 0.30

b) CO2 Coverage 0.07 to 1 ML 1.2 to 1.7 ML

0.20 0.10 0.00 2.0 1.5 1.0

c) CO2 Coverage 0.07 to 1 ML 1.2 to 1.7 ML 2.3 to 10 ML

0.5 0.0 70

75

80

85

Temperature (K)


90

95

Figure 2

CO2 Desorption Rate (ML/s)

90 0

10

Page 22 of 25

Temperature (K) 80

a)

70

CO2 Coverage 0.07 to 1 ML 1.2 to 1.7 ML 2.3 to 10 ML

10-1 10-2 10-3 10-4

11

90 CO2 Desorption Rate (ML/s)

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

10

12

13 1000/T(K)

14

Temperature (K) 80

b)

15

70

CO2 Coverage 1 ML 5, 6.7, 10 ML

10-1 10-2 10-3 10-4

11

12

13 1000/T(K)


14

15

Page 23 of 25

Figure 3

CO2 Coverage 0.07 to 1 ML

2350 cm-1

a)

0.002

Absorbance

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


CO2 Coverage 0.07 to 1 ML 2378 cm-1 2350 cm-1 1.2 to 1.7 ML

b)

0.002

CO2 Coverage 0.07 to 1 ML 1.2 to 1.7 ML 2.3 to 10 ML

c)

0.010

2450

2400 2350 -1) Frequency (cm ACS Paragon Plus Environment

2300

Figure 4

Infrared Peak Integrals

0.20

Page 24 of 25

a)

Total Area -1 0.15 2350 cm peak 2378 cm-1 peak 0.10

0.05

0.00 0.0 0.70

Infrared Peak Integrals

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


0.5

1.0 Coverage (ML)

1.5

2.0

b)

0.60 Total Area

2350 cm-1 peak 0.50 2378 cm-1 peak 0.40 0.30 0.20 0.10 0.00 0.0

2.0

4.0 6.0 8.0 Coverage (ML) ACS Paragon Plus Environment

10.0

12.0

Page 25 of 25 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


TOC Graphic

First Layer CO2

2400

2nd Layer CO2

2300

Graphene

2400

2300

Graphene


Desorption Kinetics of Carbon Dioxide from a Graphene Covered Pt

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