Desorption of Benzene, 1,3,5-Trifluorobenzene, and

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Desorption of Benzene, 1,3,5-Trifluorobenzene, and Hexafluorobenzene from a Graphene Surface: The Effect of Lateral Interactions on the Desorption Kinetics R. Scott Smith, and Bruce D. Kay J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b00986 • Publication Date (Web): 03 May 2018 Downloaded from http://pubs.acs.org on May 5, 2018

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The Journal of Physical Chemistry Letters

Desorption of Benzene, 1,3,5-Trifluorobenzene, and Hexafluorobenzene from a Graphene Surface: The Effect of Lateral Interactions on the Desorption Kinetics R. Scott Smith* and Bruce D. Kay* Physical and Computational Sciences Richland, Washington 99352

Directorate,

Pacific

Northwest National Laboratory,

*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]

ACS Paragon Plus Environment

The Journal of Physical Chemistry Letters 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

Abstract The desorption of benzene, 1,3,5-trifluorobenzene (TFB), and hexafluorobenzene (HFB) from a graphene covered Pt(111) substrate was investigated using temperature programmed desorption (TPD). All three species have well-resolved monolayer and second layer desorption peaks. The desorption spectra for submonolayer coverages of benzene and hexafluorobenzene are consistent with first-order desorption kinetics.

In contrast, the submonolayer TPD spectra for 1,3,5-

trifluorobenzene align on a common leading-edge which is indicative of zero-order desorption kinetics. The desorption behavior of the three molecules can be correlated with the strength of the quadrupole moments. Calculations (second-order Møller−Plesset perturbation and density functional theory) show that the potential minimum for coplanar TFB dimers is more than a factor of two greater than that for either benzene or HFB dimers. The calculations support the interpretation that benzene and HFB are less likely to form the two-dimensional islands that are needed for submonolayer zero-order desorption kinetics.

TOC Graphic Benzene

First order

Zero order Trifluorobenzene

2


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Understanding the interactions and behavior of adsorbates on graphene substrates is important in a number of areas including catalysis,1-3 material science,4,5 electronics,6,7 and astrophysics.8 The adsorption of aromatic molecules on graphene is of particular interest as model systems to benchmark calculations of van der Waals interactions.9-13 These weak interactions are now believed to play an important role in biological systems.14-21 We have previously measured the desorption kinetics and interaction energies for a number of adsorbates on graphene covered Pt(111). The adsorbates include water, methanol, ethanol, Ar, Kr, Xe, N2, O2, CO, methane, ethane, propane, benzene, and cyclohexane.22-24 For all, except for benzene, the submonolayer desorption spectra are aligned on a common curve which is a signature of the zero-order desorption kinetics. For benzene, the submonolayer desorption kinetics are first-order. Most of the adsorbates listed above are small compared to benzene which might provide an explanation for the different kinetics. However, cyclohexane also exhibits zero-order desorption and therefore size cannot be the explanation for the differences in the desorption kinetics.24 One property of benzene that stands out is its relatively large quadrupole moment (-33.3 × 10−40 C m2)25 which has an absolute magnitude that is more than ten times larger than that of cyclohexane (3.0 × 10−40 C m2).26 In particular, the polar C-H bonds result in benzene having a partial positive charge around the ring of the molecule. This means that lateral interactions between coplanar benzene molecules are likely repulsive, or at least less attractive than for cyclohexane. Lateral adsorbate interactions play a key role in determining the desorption order. For example, submonolayer zero-order desorption kinetics can occur if the adsorbates form two-dimensional islands that are in equilibrium with individual adsorbates diffusing on the surface.27-29 The coexistence of the two phases (islands and individual 3



adsorbates) establishes the chemical potential of the system. If equilibrium is maintained during the desorption process, the chemical potential sets the vapor pressure of the system hence the desorption rate. Thus, the desorption rate will depend only on temperature regardless of the coverage which is the definition of zero-order desorption. In this Letter, we compare the desorption kinetics of benzene, 1,3,5 trifluorobenzene (TFB), and hexafluorobenzene (HFB) to test the hypothesis that the desorption kinetic order is correlated with the quadrupole moment. The three molecules have nearly the same molecular shape (e.g. all are planar and similarly sized) but have quadrupole moments that range from large and negative for benzene (-33.3), to relatively small for TFB (3.31), and to large and positive for HFB (31.7) (all in units of 10−40 C m2).25,30 Thus, these molecules are an ideal set to test the effects of the quadrupole moment on the desorption kinetics. Figure 1 displays the temperature programmed desorption (TPD) spectra for benzene (Figure 1a), TFB (Figure 1b), and HFB (Figure 1c) that were deposited at 25 K on a graphene covered Pt(111) substrate and heated at a rate of 1.0 K/s. All three molecules have well-separated monolayer (blue curves) and second-layer (red curves) features. The desorption between the first and second layer peaks for all of the adsorbates is due to first layer compression and has been described before.28 The desorption is from adsorbates that “squeeze” into the monolayer in order to maximize the interaction with the substrate and this can result in small peaks due to various 2D configurational changes in the monolayer.28 While these 2D structures are interesting, they are not relevant to focus of this paper which is on the submonolayer desorption kinetics for uncompressed coverages. For benzene, Fig 1a, the submonolayer TPD spectra are not aligned on the leading or trailing edges, have asymmetric line shapes, and the peaks are at approximately the same temperature. These features are characteristic of first-order 4


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desorption and this has been observed in prior work.24,31-34 For TFB, Fig 1b, the submonolayer TPD spectra are aligned on the leading edges and have peaks that shift to higher temperature with increasing coverage. These features are characteristic of zero-order desorption. For HFB, Fig 1c, the submonolayer TPD spectra are not aligned on the leading or trailing edges and have asymmetric line shapes. In this case, the peaks shift slightly to lower temperature with increasing coverage. This behavior is consistent with of first-order desorption where there are repulsive interactions between adsorbates. Analysis of the submonolayer TPD spectra for benzene and HFB was done using an inversion analysis procedure that has been described in detail elsewhere and is summarized in the supporting information.23,35-40 The TPD analysis procedures are illustrated in Figure 2. Figure 2a compares the experimental benzene TPD spectra from Figure 1 (open blue circles) and a set of simulated TPD spectra (black lines) for the corresponding coverages. The simulations were performed with an Edes(θ) curve generated using the inverted Polanyi-Wigner equation and a prefactor of 5 × 1015 s-1. The experimental (open red circles) and simulated (black lines) TPD spectra for HFB are displayed in Figure 2c. In this case the Edes(θ) curve was generated using a prefactor of 1 × 1015 s-1. For both benzene and HFB, the simulations are in excellent agreement with the experimental TPD spectra. That is, the inversion of the 1 ML TPD spectrum in each case generates an Edes(θ) curve that accurately describes the desorption behavior for a wide range of initial submonolayer coverages. In addition, the peak temperature dependence with initial coverage (nearly constant for benzene and shifting to lower temperature for HFB) is also captured. The Arrhenius analysis of the TFB submonolayer TPD spectra is displayed in Figure 2b. In this case, the TPD all align on a common straight line (dashed black line). The

5



desorption energy and prefactor are obtained from the slope and intercept of this line, respectively. Simulations of the benzene and HFB submonolayer TPD peak spectra were repeated with different prefactors (not shown) and comparisons with the experimental TPD spectra, analogous to those in Figure 2a and Figure 2c, were made. From these simulations, we estimate the benzene prefactor to be 5 × 1015 ± s-1 and the HFB prefactor to be 1 × 1015 ± 1 s-1. The prefactor for benzene is consistent with previously published values for the desorption of benzene from graphite31,32 and graphene.24 The corresponding Edes(θ) curves for benzene (blue line) and HFB (red line) are plotted in Figure 3. The sloping increase in desorption energy with decreasing coverage for benzene and HFB is an indication of repulsive lateral interactions between adsorbates. Also, shown is TFB desorption energy obtained from the Arrhenius analysis in Figure 2. The prefactor for TFB is 5 × 1019 ± 1 MLs-1. The desorption energies at a coverage of 0.5 ML (vertical dashed line) were 50.3 ± 3 kJ/mol, 73.9 ± 3 kJ/mol, and 59.1 ± 3 kJ/mol for benzene, TFB, and HFB respectively. The benzene desorption energy of 50.3 ± 3 kJ/mol is in good agreement with previously published values of 48 ± 8 kJ/mol31,32 and 54 ± 3 kJ/mol.24 There is a distinct difference in the value of the prefactor for TFB (~1019 MLs-1) and the values for benzene and HFB (~1015 s-1). Note that since the saturation coverages have been defined as 1 ML, the magnitude of the zero-order and first-order prefactors can be compared directly. One can calculate a desorption prefactor using the relationship, ν = (kT/h) q‡/qads, where q‡ and qads are the partition functions for the transition and adsorbed states.36 At the low desorption temperatures here (

Desorption of Benzene, 1,3,5-Trifluorobenzene, and

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