Understanding the Binding of Aromatic ... - ACS Publications

Subscriber access provided by Bethel University

C: Surfaces, Interfaces, Porous Materials, and Catalysis 2

Understanding the Binding of Aromatic Hydrocarbons on Rutile TiO(110) Long Chen, Shengjie Zhang, Rudradatt Randy Persaud, R. Scott Smith, Bruce D. Kay, David A Dixon, and Zdenek Dohnalek J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b03355 • Publication Date (Web): 07 Jun 2019 Downloaded from http://pubs.acs.org on June 7, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 26 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

The Journal of Physical Chemistry

Understanding the Binding of Aromatic Hydrocarbons on Rutile TiO2(110) Long Chen,a Shengjie Zhang,b Rudradatt R. Persaud,b R. Scott Smith,a Bruce D. Kay,a,*David Dixon,b,* and Zdenek Dohnáleka,c,* a

Physical and Computational Sciences Directorate and Institute for Integrated Catalysis, Pacific

Northwest National Laboratory, P.O. Box 999, Richland, WA, 99352, USA, b

Department of Chemistry, University of Alabama, Shelby Hall, Box 870336, Tuscaloosa, Alabama

35487, USA, c

Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman,

WA 99163, USA Corresponding authors: ZD: [email protected], DD: [email protected], and BDK: [email protected] Abstract The adsorption of cyclohexane, benzene, and alkyl-substituted benzene derivatives is studied on rutile TiO2(110) by a combination of molecular beam dosing, temperature programmed desorption, and density functional theory (DFT). An inversion analysis is used to extract the coverage dependent desorption energies from TiO2(110). The values of the suitable prefactors are derived from simple statistical mechanical models assuming different limits in the adsorbate mobility on the surface. The prefactor values determined using the vibrational frequencies from DFT calculations corroborate this analysis and show that the adsorbates are mobile in 1- or 2-dimensions on corrugated TiO2(110) surface. The adsorption of benzene derivatives is found to be dominated by the dative Lewis acid-base interactions of the π system with the surface Ti ions. While the desorption energy generally increases with increasing the length and the number of substituents, the difference between the desorption energies decreases as the number and length of substituents is increased. This is a consequence of the destabilization of the optimum bonding configuration of the benzene ring and the alkyl groups with their increasing length and number. The absolute saturation coverages of uncompressed layers correspond approximately to one molecule per three Ti5c sites and decrease slightly with increasing molecule size in a good agreement with van der Waals sizes of the molecules.

1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 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 2 of 26

1. Introduction Hydrocarbons are the backbone of the chemical process industry.1 In this context, understanding their adsorption and dissociation on well-defined model surfaces is critical as it represents the first step in their catalytic conversion to value-added products.2-5 Further, accurate kinetic parameters governing the adsorption and desorption can serve as a reference for benchmarking theoretical methods and for the studies of more complex systems.

5-9

Hence, the interactions of hydrocarbons with well-defined single crystal

metal and metal oxide surfaces have been extensively studied. 2, 5, 9-11 Despite the broad interest, fundamental data about the adsorption of hydrocarbons on oxides is mostly missing. In a recent publication,12 we reported the adsorption and desorption of a series of small aliphatic hydrocarbons (C1–C4) on a prototypical model oxide surface, rutile TiO2(110) that is also the subject of this study. We found that desorption energies increase from n-alkanes to 1-alkenes and 1-alkynes of the same chain length. This trend was explained by the dative bonding of the π system in 1-alkenes and 1-alkynes to the five-fold coordinated titanium sites (Ti5c) of TiO2(110). Nonetheless, as the hydrocarbon chain length is increased from C2 to C4, the differences between desorption energies decrease, likely due to the less favorable adsorption configurations that C=C and C≡C segments and/or their side chains can adopt. Based on the flux-calibrated molecular beam13 and the saturation of the Ti5c-related TPD features, we also determined the absolute saturation coverages for each hydrocarbon on Ti5c sites.12 Recently, the adsorption energies were also determined for small alkanes on the isostructural RuO2(110), and IrO2(110) surfaces.14-16 High adsorption energies were observed for C1-C4 hydrocarbons on RuO2(110) and were interpreted as being the result of the formation of strongly-bound s-complexes with dative bonding to Rucus atoms (cus = coordinatively unsaturated).14 On IrO2(110), strikingly, s-complexes of even the smallest alkanes (methane and ethane) bind so strongly that they undergo a complete decomposition and combust to CO, CO2, and H2O.15-16 In this report, we extend our adsorption/desorption studies on TiO2(110) to a series of alkylsubstituted benzene derivatives. Additionally, cyclohexane is studied as a reference cyclic hydrocarbon without a π system. Molecular beam dosing and temperature-programmed desorption (TPD) are used to measure sticking coefficients and desorption kinetics, respectively, for all molecules. An inversion analysis is used to obtain the coverage dependent desorption energies desorption from Ti5c rows.

5, 12, 17-18

The

prefactor values are determined using simple statistical mechanical models assuming different-limits in the adsorbate mobility on the surface. The prefactor values were also obtained using the vibrational frequencies from the density functional theory (DFT) calculations of large cluster models and corroborate that the adsorbates are mobile in 1- or 2-dimensions on the surface. We find that the aromatic hydrocarbons are more tightly bound than cyclohexane, primarily due to the dative Lewis acid-base bonding of the π system of the benzene ring to the undercoordinated Ti5c sites. The desorption energy increases with increasing the length and number of substituents on the benzene ring. However, the difference between the desorption energies decreases as the length and number of substituents on the phenyl ring is increased. The absolute 2


Page 3 of 26 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


saturation coverages decrease slightly with increasing number and length of alkyl chains, but in all studied cases, the coverage corresponds to approximately one molecule per two Ti5c sites.

2. Methods 2.1 Experimental Details All experiments were performed in a UHV molecular beam surface scattering apparatus with a base pressure of < 1 × 10-10 Torr which has been described in more detail in our previous publications. 12

19 20

The synthetic rutile TiO2(110)-1×1 single crystal (Princeton Scientific, 10 × 10 × 1 mm3) was mounted on a Ta sample holder which was attached to a closed-cycle He cryostat for cooling to ~60 K and for resistive heating to > 900 K. The temperature was measured by a chromel-alumel (type K) thermocouple glued with a high-temperature ceramic adhesive (Aremco) to the edge of the crystal. The initial surface cleaning procedures have been described in our early studies.12 19 20 To maintain the surface cleanliness and order, a brief, 4 min Ne+ ion sputtering at 300 K followed by 10 min of annealing at 870 K was used on a daily basis. Reproducibility of the surface structure was further confirmed using H2O TPD.21 The population of bridge bonded oxygen vacancies (VO’s) on the surface was determined to be ~5% based on the ratio of water recombination desorption peak at ~500 K to the monolayer desorption peak at ~270 K. 21 22 The adsorbates studied were cyclohexane and alkyl substituted benzenes (benzene, methylbenzene, ethylbenzene, 1,2-, 1,3-, and 1,4-dimethylbenzenes, and 1,3,5-trimethylbenzene). All chemicals were purchased from Sigma-Aldrich with research grade purity (> 98%). Prior to use, all chemicals were purified using three freeze-pump-thaw cycles. A flux-calibrated effusive molecular beam of neat gas was used for dosing at normal incidence with respect to the sample surface plane.13 During adsorption, the sample was held at ~60 K, a temperature well below the onset of multilayer desorption of all adsorbates. At this temperature, the sticking coefficients were determined by the beam reflection technique of King and Wells to be close to unity for all studied molecules.23 Figure S1 in the Supporting Information (SI) shows a representative sticking coefficient measurement using an effusive beam of benzene at a normal angle of incidence. Absolute coverages for all adsorbates were defined in monolayers (ML) relative to the number of Ti5c sites (1 ML ≡ 5.2 × 1014 cm−2) on TiO2(110).13, 24 All TPD spectra were acquired using a quadrupole mass spectrometer (UTI 100C) in a line-of-sight geometry at a constant heating rate of 1 K/s. The reaction or dissociation of adsorbates on TiO2(110) can be excluded, as confirmed by XPS and by the excellent reproducibility of the TPD spectra upon repeated doses. 2.2. Computational Details 3



Page 4 of 26

The density functional theory calculations were carried out for a (TiO2)40 cluster model of the surface. The dangling O atoms on the edges of the cluster were terminated by adding an H atom to form OH groups. There were no Ti=O bonds present. A cluster model was chosen because of the need to calculate vibrational frequencies to be used in the prefactor calculations. Because of the large number of low-lying frequencies and the coupling of the 6 modes describing the interaction of the organic with the cluster model of the surface with lattice modes, analytic calculation of the second derivatives was chosen to obtain improved values. The geometry optimizations were done at the B97D/DZVP2 level25-27 using NWChem.28 The D correction is the D3 of Grimme29 and was used to account for van der Waals interactions of the organic molecule with the cluster model of the TiO2 surface. The harmonic vibrational frequencies were calculated analytically using Gaussian-16.30 The geometry and molecular (Kohn-Sham) orbital analysis were done with the AGUI interface from Semichem. Mulliken charges are reported to provide qualitative insights into the adsorption-induced changes in the Ti5c charges.

3. Results and Discussion 3.1 Coverage dependent TPD spectra To understand how the aromatic ring affects the binding energies, we first compare TPD spectra of cyclohexane and benzene. Figure 1 displays the coverage dependent TPD spectra of benzene and cyclohexane on TiO2(110). Only a representative subset of the measured spectra are plotted for both adsorbates. The absolute coverages listed in the figure were determined using the calibrated beam fluxes,13 measured sticking coefficients, and dose times, and are referenced relative to the number of Ti5c sites. For cyclohexane (Figure 1a), the coverage dependent TPD spectra develop in a similar fashion and exhibit analogous features to those of small n-alkanes (C1 – C4) on TiO2(110) described in our earlier study.12 Specifically, at low cyclohexane coverages, a single desorption peak is observed at ~190 K. This peak increases in intensity with coverage and saturates at ~0.4 ML (blue trace, Figure 1a), while the peak maximum remains constant at ~190 K. Further coverage increases above 0.5 ML, initially result in the development of a shoulder (0.51 ML, red trace) on the low-temperature side of the 190 K peak and subsequently (> 0.71 ML) lead to the development of a low-temperature peak at ~144 K. By analogy with n-alkanes and other adsorbates (e.g. H2O, CO2, CO, N2 and O2) studied previously on TiO2(110),13, 17, 31-32 we assign the ~190 K peak (labeled as Ti5c) to the desorption of cyclohexane from the Ti5c sites and the ~144 K peak (labeled Ob) to the desorption from Ob sites. The plateau between the two peaks corresponds to the compressed layer of cyclohexane on Ti5c sites. This compression leads to less favorable binding configurations and is mirrored by a shift of the peak leading edge to lower temperature without further growth of the peak intensity.13, 33 The compression continues until the chemical potential of the adsorbed cyclohexane on Ti5c rows reach that of the molecules in the second layer adsorbed on Ob 4


Page 5 of 26 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


rows.33 As the coverage is increased above 0.9 ML, a multilayer desorption peak (labeled as M) which continuously increases in intensity with increasing dose, develops on the leading edge of the peak from the Ob sites. In addition to the three main desorption peaks, an additional small desorption peak above 230 K develops at high coverages. Similar to n-alkanes adsorption on TiO2(110),12 this peak is determined to be an artifact due to minor adsorption on the sample holder. The integrated TPD peak areas are plotted as a function of cyclohexane coverage in the inset of Figure 1a. The integrals clearly show that the TPD peak area increases linearly with coverage and passes through the origin of the plot. The observed linear relationship is consistent with the absence of any irreversible reactions during the TPD experiments. The coverage dependent TPD spectra of benzene on TiO2(110) are shown in Figure 1b. A similar coverage dependent progression as for cyclohexane is observed with benzene initially populating the Ti5c sites (~210 K), followed by Ob sites (150 K) and then multilayers (onset at 140 K). Similar to 1-alkynes adsorption on TiO2(110),12 the TPD peak from the Ob sites cannot be isolated from the multilayer desorption peaks as the coverage is increased above the saturation of the Ti5c sites. This is likely a consequence of higher peak temperatures for desorption from benzene multilayers (lower vapor pressure) as compared to cyclohexane. Similar to cyclohexane and small aliphatic hydrocarbons (C1-C4),12 specific VO-related TPD features are not observed from benzene on TiO2(110). Further, no irreversible reaction is observed during the TPD experiments, as evidenced by the linear relationship between the TPD peak areas and benzene coverages (inset in Figure 1b). In contrast to cyclohexane, two notable differences can be observed for benzene on TiO2(110). First, the desorption temperature for benzene from Ti5c sites is at least 15 K higher than that of cyclohexane, indicating benzene is more tightly bound. As we show below, the higher binding energies for benzene is due to dative Lewis acid-base bonding of the benzene π system with the coordinatively-unsaturated Ti5c sites. Second, benzene desorption peak from Ti5c sites shifts significantly towards lower temperature with increasing benzene coverage on the surface. This shift is likely due to the decrease in the optimum alignment of the benzene π system with the underlying Ti5c sites as the coverage is increased. In contrast, the cyclohexane desorption peak from Ti5c sites is essentially independent of cyclohexane coverage.

5



Page 6 of 26

Figure 1. Coverage-dependent TPD spectra for (a) cyclohexane (m/z = 56 amu), and (b) benzene (m/z = 78 amu) on TiO2(110). All TPD spectra were acquired using their most intense mass fragments (listed in the parentheses) and a constant ramp rate of 1 K/s. The assignments of TPD peaks from titanium (Ti5c) sites, bridge-bonded oxygen (Ob) sites, and multilayers (M) are indicated in each panel. The red traces correspond to the saturation coverage of compressed layers on Ti5c rows (before the appearance of the TPD features from the Ob sites). The blue traces correspond to the coverages of uncompressed layers as represented by the saturation of TPD peaks from Ti5c sites. The insets show the TPD area vs. the coverage, θ, of cyclohexane and benzene, respectively. The absolute coverages listed on the right side of each panel were determined using the calibrated beam fluxes, measured sticking coefficients, and the dose times and are referenced relative to the number of Ti5c sites (1 ML ≡ 5.2 × 1014 cm−2).

6


Page 7 of 26 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


Figure 2. Coverage-dependent TPD spectra of (a) benzene, (b) methylbenzene, (c) ethylbenzene, 7



Page 8 of 26

(d) 1,4-dimethylbenzene, and (e) 1,3,5-trimethylbenzene (on TiO2(110). All TPD spectra were acquired using their most intense mass fragments and a constant ramp rate of 1 K/s. The assignments of the TPD peaks from titanium (Ti5c) sites, bridge-bonded oxygen (Ob) sites, and multilayers (M) are indicated in each panel. The red traces correspond to the saturation coverage of compressed aromatic hydrocarbon layers on Ti5c rows (before the appearance of the TPD features from the Ob sites). The blue traces correspond to the coverages of uncompressed layers as represented by the saturation of TPD peaks from Ti5c sites. The absolute coverages listed in the parentheses were determined using the calibrated beam fluxes, measured sticking coefficients, and the dose times and are referenced relative to the number of Ti5c sites (1 ML ≡ 5.2 × 1014 cm−2) To further understand how the adsorption and binding changes with the size of the aromatics, various alkyl substituted benzene molecules were studied. The coverage dependent TPD spectra of methylbenzene (Figure 2b), ethylbenzene (Figure 2c), 1,4-dimethylbenzene (Figure 2d), and 1,3,5-trimethylbenzene (Figure 2e) are shown in Figure 2. (Other dimethylbenzene isomers (1,2- and 1,3-dimethylbenzene) were also studied and can be found in Figure S2 in the SI. To facilitate the comparison, the coverage dependent TPD spectra of benzene are also included (Figure 2a). Generally, a similar coverage dependent progression as for benzene is observed for all substituted aromatic hydrocarbons. No distinct features can be correlated with the presence of VO's on the surface. Initially, the Ti5c sites are populated and the peak maximum shifts toward lower temperatures with increasing dose. As already stated, this is likely a consequence of less favorable adsorption configurations on the underlying Ti5c sites at higher coverages. Moreover, as the number and length of substituents on the benzene ring increase, the TPD peak maxima from Ti5c sites shift to higher temperatures. Specifically, the TPD peak temperatures for the molecules desorbing from the Ti5c sites increase from ~205 K for benzene to 232, 245, 255 and 260 K for methylbenzene, ethylbenzene, 1,4dimethylbenzene, and 1,3,5-trimethylbenzene, respectively. This increase is a consequence of increasing van der Waals (vdW) interactions with increasing molecular weight as observed previously for small aliphatic hydrocarbons on TiO2(110) and many metal and metal oxide surfaces.5, 10, 12, 18, 34 Similar to benzene, the TPD peak from the Ob sites cannot be clearly isolated from the multilayer desorption peaks. No reaction is observed during the TPD experiments, as evidenced by the linear relationship between TPD peak areas and the coverages of different aromatic hydrocarbons (data not shown).

3.2 Coverage dependent desorption energies To extract the kinetic parameters for desorption, we use the TPD inversion analysis described in detail in previous studies.12, 17, 32 Briefly, the desorption rate in TPD spectra follows the Polanyi−Wigner equation, dθ/dt = ν(θ,T)·θn·exp(−Ed(θ)/RT), where θ is the adsorbate coverage, t is the time, ν is the prefactor of desorption, n is the desorption order, Ed(θ) is the activation energy of desorption, R is the gas constant, and T is the temperature. T and t are related by dT/dt = β, where β is the heating rate. For the analysis, we assume 8


Page 9 of 26 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


first-order desorption kinetics (n = 1) and a constant value of the prefactor. The coverage dependent desorption energy is given by Ed(θ) = −RT ln((−dθ/dt)/vθ). This expression is solved for TPD spectra at saturation coverage (e.g., for the red spectra in Figures 1 and 2) and the results of the analysis are plotted in Figure 3. Since the values of v cannot be accurately determined from the analysis of the desorption spectra,17 we select three physically significant prefactors that correspond to different mobilities of the adsorbates on the surface at the desorption temperature. The detailed procedure for the calculation of the desorption prefactors is described in the SI. Note that for the prefactors described here, we assume that there are no significant electronic or intramolecular vibrational excitations and therefore those partition functions are all equal to one and thus cancel for the transition and adsorbed states. Briefly, the minimum prefactor, ν2T+1R, is calculated by assuming the adsorbate is a two-dimensional translational gas and is freely rotating about the surface normal.6, 12, 32 This corresponds to the minimum entropy change going from the surface to the gas phase. The maximum prefactor, vMax, is calculated by assuming the adsorbate is completely immobile on the surface.12, 32 This corresponds to the maximum entropy change going from the surface to the gas phase. Considering the corrugated surface structure of TiO2(110), we also calculate an intermediate prefactor, v1T+0R, that lies between v2T+1R and vMax, by assuming the adsorbate behaves as a non-rotating onedimensional translational gas with mobility only along the Ti5c rows.12, 32 The values of the prefactors were calculated for the Ti5c peak temperatures of each adsorbate. Inversions using these three prefactors, v2T+1R, v1T+0R, and vMax, yield the lowest, medium, and the highest possible desorption energy coverage dependencies, respectively. The calculated prefactor values are listed in Table 1 and the desorption energy dependencies are displayed in Figure 3. We have also determined Ed(θ) dependencies (dash-dot-dotted green line, figure 3) using the prefactors from DFT calculations, vtheory. There, we calculated the molecular harmonic vibrational frequencies as the second-derivatives of the energy at the minimum (see Section 3.3 for details) on the (TiO2)40 cluster. It is further instructive to compare the prefactors from our simple models and from the DFT with the prefactors obtained using a procedure put forward in a recent review article by Campbell and Sellers.6 There, they used the literature values of desorption entropies for a number of adsorbates on oxide surfaces to show that the adsorbates maintain ∼2/3 of the entropy of the gas-phase species. They utilized this correlation to derive a general formula (Eqn. 16, Ref. 6) for the determination of prefactors from the adsorbate gas phase entropies. We have calculated the prefactors using this formula and plotted the desorption energies for cyclohexane and benzene in Figure 3 (green dotted lines). The obtained desorption energy dependencies lay within limits defined by ν2D+1R and ν1D+0R models and are very close to the values obtained using the prefactors obtained from theory (green dashed lines). For comparison, we further include the Ed(θ) dependencies (dotted brown lines) obtained using the prototypical (here unrealistic) prefactor of 1013 s-1 that is often used in simple analyses such as Redhead analysis.35 9



Page 10 of 26

Figure 3. Coverage-dependent desorption energy, Ed(θ), for (a) cyclohexane and (b) benzene on TiO2(110). The desorption energy dependencies were obtained using the inversion analysis of the high coverage experimental TPD spectra (~1.2 ML). First order desorption and constant prefactors, ν, were used in the analysis. Different prefactors were calculated and used in the analysis: immobile adsorbate (dash-dotted line), a 1-dimensional translation gas in the absence of rotational motion (solid), a 2-dimensional translation gas with in-plane rotations (dashed), the DFT calculation for the (TiO2)40 cluster model (green dash-dot-dotted), the Campbell and Sellers procedure (green dotted),6 and the typical value of 1013 s-1 (brown dotted). See the text and SI for details. The arrows with Ti5c, Ob, and M symbols indicate the plateau regions that correspond to the desorption energies from the Ti5c, Ob, and multilayers, respectively. The inset in Figure 2b shows the linear regions (dashed red lines) on the Ed(θ) curve (for 1-dimensional translational gas) in the Ti5c, Ob, and the compression region (transition region between Ti5c and Ob). Figure 3 shows the results of the inversion analysis yielding Ed(θ) dependencies using the abovedescribed prefactors for cyclohexane (Figure 3a) and benzene (Figure 3b). Similar general Ed(θ) line shapes can be observed for all dependencies both for cyclohexane and benzene. At very low coverages (< 0.1 ML), a sharp decrease of Ed is observed for both molecules. Based on our previous studies, this region reflects the coverage range where molecules are desorbing from the higher energy sites such as VO's despite the absence of distinct desorption features in the TPDs (Figure 1). At higher coverages (> 0.1 ML), relatively flat regions corresponding to the desorption from Ti5c sites, Ob sites, and multilayers are observed (marked Ti5c, Ob, and M in Figure 3). The Ti5c regions correspond to the coverages where the well-defined TPD peaks from the Ti5c sites develop and increase in intensity. As the coverage is increased to θpeak, where the peak intensities maximize (blue TPD spectra, Figure 1), the energy starts to decrease sharply due to the development of compressed layers.33 There, molecules cannot adopt their most favorable configurations on the Ti5c sites (discussed above), and their binding energy collectively decreases. This compression continues 10


Page 11 of 26 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


until the saturation coverages, θsat, (red TPD spectra, Figure 1) are reached and the second layer starts to develop. Table 1. The physically reasonable prefactors calculated by assuming the adsorbates behave as 2dimensional translational gas with in-plane rotations, or 1-dimensional translational non-rotational gas, or are immobilized on the surface (see also Table S2 in the SI). ν [s-1]

Adsorbate 2D translational, 1D rotational gas

1D translational, non-rotational gas

Immobile gas

Cyclohexane

3.6 × 1015

1.7 × 1018

6.4 × 1019

Benzene

2.8 × 1015

7.1 × 1017

2.7 × 1019

Methylbenzene

5.6 × 1015

1.4 × 1019

6.7 × 1020

Ethylbenzene

1.8 × 1016

6.4 × 1019

3.5 × 1021

1,2-Dimethylbenzene

9.6 × 1015

3.4 × 1019

1.9 × 1021

1,3-Dimethylbenzene

9.7 × 1015

3.5 × 1019

1.9 × 1021

1,4-Dimethylbenzene

1.0 × 1016

1.8 × 1019

9.8 × 1020

1,3,5-Trimethylbenzene

9.9 × 1015

2.4 × 1019

1.5 × 1021

The Ed vs. θ dependencies can be further used to determine the energies in the limit of zero coverage by linear extrapolation of the desorption energies from the Ti5c sites to zero coverage.5, 18 Further, the coverages, that correspond to the saturation of uncompressed, θpeak, (blue spectra in Figures 1 and 2) and compressed layers on Ti5c, θsat, (red spectra in Figures 1 and 2), can be determined from the intercepts of the linear extrapolations of the Ed(θ) in the uncompressed and compressed Ti5c regions, and compressed Ti5c region and Ob/multilayer regions, respectively, as illustrated in the inset of Figure 3b. Using the Ed vs. θ dependencies, we can compare the desorption energies of cyclohexane and benzene at specific characteristic coverages (0, θpeak, and θsat). For the discussion, we pick the dependencies obtained for v1T+R0 prefactors that correspond to the adsorbates freely moving along the Ti5c rows. The desorption energies at the highest measured coverages of 1ML yield a value of 51 kJ/mol for cyclohexane, which is only slightly (~6 %) lower than that of 54 kJ/mol for benzene (see Figure 3). These values are approaching the values of sublimation energies in the multilayer range and are in a good agreement with the prior TPD measurements on graphene on Pt(111) of 49 and 53 kJ/mol, respectively.36 This can be contrasted with the Ed-0 values (see Table 2) from the Ti5c sites at zero coverage limit where cyclohexane 11



Page 12 of 26

(72 kJ/mol) is somewhat less-tightly bound (~13 %) than benzene (83 kJ/mol). As revealed by electronic structure calculation (see below), the stronger interactions of benzene with the TiO2 surface are a result of dative Lewis acid-base bonding interactions of the benzene π system with the coordinatively-unsaturated Ti5c sites. Interestingly, the value of Ed in Figure 3 drops rapidly with increasing coverage for benzene, while it is nearly independent of the coverage for cyclohexane. This results in practically identical desorption energies, Ed-peak and Ed-sat, for cyclohexane and benzene of 68 and 72 kJ/mol at θpeak and 54 and 54 kJ/mol at θsat, respectively. As already mentioned, we speculate that the continual decrease of Ed for benzene is likely due to the decrease in the optimum alignment of the benzene π system with the underlying Ti5c sites with increasing coverage.

Figure 4. Coverage-dependent desorption energies, Ed(θ), for benzene (black), methylbenzene (red), ethylbenzene (green), 1,4-dimethylbenzene (blue), and 1,3,5-trimethylbenzene (cyan) on TiO2(110). The Ed(θ) dependencies were obtained using the inversion analysis of the ~1.2 ML TPD spectra assuming first order desorption and a constant prefactor, ν1T+0R (adsorbate moving freely along the Ti5c rows but not rotating). The arrows with Ti5c and Ob or M symbols approximately indicate the plateau regions that correspond to desorption energies from the Ti5c and Ob or multilayers, respectively. Analogous plots using ν2T+1R and vMax are shown in Figure S3 in the SI. A comparison for 1,2-, 1,3-, and 1,4-dimethylbenzene is shown in Figure S4, in the SI. To further evaluate the effects of side chain length and number on the Ed, we perform the inversion analysis using v1T+0R for the alkyl substituted benzenes. We expect that v1T+0R is a physically reasonable approximation describing the available degrees of freedom of the adsorbates on Ti5c sites, considering the 12


Page 13 of 26 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


high topographic and electronic corrugation of potential energy surfaces on TiO2(110). As we show below, this model is also in sound agreement with the results of the theoretical calculations and serves as a good guide when the theory is not available. Table 2. Desorption energies (Ed-0, Ed-peak, Ed-sat) of cyclohexane, benzene, and various substituted benzene molecules from Ti5c sites at characteristic coverages (0, θpeak, θsat). All values were determined using the inversion analysis with v1T+0R prefactors (molecules behaving as 1-dimensional gas on the Ti5c rows) and procedures described for the inset of Figure 3b. The θvdW values are obtained assuming the maximum packing of the molecules on the Ti5c rows using their vdW lengths as described in the text. Adsorbate

Ed-0

θpeak

[kJ/mol]

Ed-peak

θsat

Ed-sat

θvdW

[kJ/mol]

[kJ/mol]

Cyclohexane

72

0.37

68

0.52

54

0.40

Benzene

83

0.37

70

0.56

54

0.40

Methylbenzene

98

0.34

85

0.54

61

0.36

Ethylbenzene

106

0.34

93

0.60

67

0.34

1,2-Dimethylbenzene

109

0.33

96

0.52

71

0.36

1,3-Dimethylbenzene

110

0.32

95

0.51

69

0.36

1,4-Dimethylbenzene

108

0.33

95

0.48

69

0.33

1,3,5-Trimethylbenzene

112

0.33

98

0.51

74

0.36

Figure 4 shows the inversion analysis results for different alkyl substituted benzenes using v1T+0R values (see Table 1). Analogous plots using ν2T+1R and vMax are shown in Figure S3 in the SI. The Ed(θ) dependencies for all the molecules exhibit rather similar lineshapes exhibiting regions that were already described with Figure 3. The systematic shift of the dependencies to higher desorption energies is seen with both increasing the length and number of alkyl side chains. However, in contrast with our prior results for small

aliphatic hydrocarbons on TiO2(110),12 the Ed increase with each additional CH3/CH2 segment is not constant but rather decreases. This can be clearly illustrated in the alkylbenzene sequence (benzene – black, methylbenzene – red, ethylbenzene – green) where the spacing between the Ed(θ) dependencies from benzene to methylbenzene is larger than that from methylbenzene to ethylbenzene. Similarly, the Ed values do not increase linearly with the increasing number of methyl substituents (methylbenzene - red, 1,4dimethylbenzene - blue, 1,3,5-trimethylbenzene – cyan). Since the positioning of the molecules on the Ti5c row is primarily determined by the strongest interaction with the benzene ring, it is likely, that this behavior 13



Page 14 of 26

is a consequence of suboptimal alignment of the CH3/CH2 segments with the underlying Ti5c sites. It is interesting to note that ethylbenzene (green) and 1,4-dimethylbenzene (blue) energies are very similar as one would ideally expect for different isomers. Similarly, almost identical Ed(θ) dependencies can be seen for 1,2-, 1,3-, 1,4-dimethylbenzene (Figure S4 in the SI) indicating that the relative positions of the methyl groups do not appreciably affect the dimethylbenzene binding.

Figure 5. The desorption energies of alkyl substituted benzenes (filled symbols) and cyclohexane (empty symbols) in the limit of zero coverage, Ed-0, (circles), at coverages corresponding to the saturation of TPD peaks on Ti5c, Ed-peak, (triangles), and the saturation of compressed layers on Ti5c, Ed-sat, (squares). The solid lines connect symbols for benzene and benzene substituted with n methyl groups in the 1, 1,3, and 1,3,5 positions. The symbols for various chemical structures are colorcoded as indicated. Some symbols are slightly offset along the n axis from the integer values to minimize their overlap. The desorption energies were obtained from the inversion analysis using v1T+0R prefactors and analysis described with the inset of Figure 3b. The zero-point corrected energies, ΔH(0 K), from the theoretical calculations for the most stable binding configurations are displayed as diamonds (also see Table 3).

14


Page 15 of 26 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


Figure 6. Optimized structures of C6H6 and C6H12 on the (TiO2)40 cluster that is used to model TiO2(110). Two adsorbed configurations, labeled (a) and (b), that are very close in energy are observed. In the schematics, the red balls are the O ions; gray, the Ti ions; light blue, the Ti5c ions representing the row on TiO2(110); black, the carbon atoms of the adsorbate; and orange, the hydrogen atoms of the adsorbate. To better understand the observed trends for substituted benzene molecules, we tabulate their 15



Page 16 of 26

desorption energies, Ed-0, Ed-peak, and Ed-sat at characteristic coverages of 0, θpeak, and θsat in Table 2. The tabulated values are obtained using the inversion analysis for the v1T+0R prefactor (1-dimensional translational gas along the Ti5c rows) and procedures described with the inset of Figure 3. The desorption energy and coverage dependencies as a function of the number of CH3/CH2 segments are further displayed graphically in Figures 5 and 6. Similar trends can be observed for all desorption energy dependencies obtained at different characteristic coverages (Ed-0, Ed-peak, Ed-sat determined at 0, θpeak, and θsat, respectively). The lines connect symbols for methyl-substituted benzenes with the number of methyl groups, n, in the 1, 3, and 5 positions increasing from 0 to 3. Additional Ed’s for n = 2, correspond to 1,2-, 1,4-dimethylbenzene, and ethylbenzene. In all cases, their energies are very close to those of 1,3-dimethylbenzene indicating that the position of the alkyl side groups plays only a minor role in determining their values. The symbols for various chemical structures are color-coded as indicated. The black lines for methyl substituted benzenes show that the Ed-0 and Ed-peak energies do not exhibit a linear increase with increasing number n of CH3-/-CH2segments. In contrast, Ed-sat dependence appears to be linear. This is likely because Ed-0 and Ed-peak are controlled by the interactions of the molecules with the Ti5c ions. As already discussed, these vary depending on the alignment of the alkyl moieties on different molecules, providing a plausible explanation for the nonlinear dependence. In contrast, the Ed-sat on fully saturated surfaces is obtained when the chemical potential of the first layer is equal to that of the second layer and therefore reflects binding energies of the molecules to themselves. In this case, the molecular weight is expected to play a dominant role. This argument is further supported by the comparison of benzene and cyclohexane binding in the multilayer regime where they are essentially identical to the desorption energies determined in prior TPD measurements on graphene.36

3.3 Computationally-determined adsorption energies and desorption prefactors To model the interactions of studied molecules with a TiO2(110), we examined their adsorption on a (TiO2)40 cluster model of the surface described above as shown in Figure 6. The use of a cluster model not only allows us to determine the zero-point corrected energies, ΔH(0 K), but also lets us calculate the analytic second derivatives of the energy at the minimum with respect to the coordinates and determine the molecular harmonic vibrational frequencies. These frequencies are subsequently used to predict the prefactors for desorption, νtheory. The theoretical prefactor, νtheory, is computed using transition state theory with the calculated harmonic vibrational modes. Clearly, this approximation has its limitations when the adsorbate is only slightly hindered on the surface at the desorption temperature. There, the reduced barrier height and attendant anharmonicities in the potential energy landscape may become important and affect the prefactor values. The details of this procedure are presented in the SI. The determined vtheory and ΔH(0 K) values for 16


Page 17 of 26 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


the subset of the molecules studied are given in Table 3, and the corresponding low energy adsorption structures for cluster-C6H6 and cluster-C6H12 are displayed in Figure 6. Methylbenzene and 1,2-, 1,3-, and 1,4-dimethylbenzenes bind similarly to benzene, and their low energy structures are shown in the SI. Before discussing the adsorption configurations and the factors that control binding, we continue our analysis of desorption energies. The calculated values of ΔH(0 K) are summarized in Table 3. Additional benchmarks results comparing the binding of C2H2 and C2H4 on the same cluster to previous experimental results12 are given in the SI, and shown good agreement between the calculated values and experiment. Two distinct adsorption structures that are close in energy are predicted for benzene and its substituted analogs (see Figure 6a and b). The structures that are not the lowest energy configurations are highlighted by gray in Table 3. The lowest energy configurations are graphically compared to the experimental results in Figure 5 (diamonds). Generally, an excellent agreement can be seen with the Ed-0 (circles, also Tables 2 and 3) values determined using v1T+0R. Table 3. Theoretically-determined prefactors, vtheory, and corresponding zero point-corrected energies, ΔH(0 K), for two adsorbed configurations (described in the text) on (TiO2)40. The energy configurations that are higher in energy are grayed out. The desorption energies at 0 ML coverage limit determined from the experimental data using vtheory (for the low energy configurations) are also listed. The values of v1T+0R ν and Ed-01T+0R are provided for the comparison. Adsorbate Cyclohexane Benzene Methylbenzene 1,2-Dimethylbenzene 1,3-Dimethylbenzene 1,4-Dimethylbenzene

Structure (a) vtheory ΔH(0 K) [s-1] [kJ/mol] 1016

1.9 × 1.2 × 1017 1.2 × 1017 9.5 × 1019 9.4 × 1018 1.9 × 1020

69.9 84.6 106.5 115.5 115.9 124.8

Structure (b) vtheory ΔH(0 K) [s-1] [kJ/mol] 1016

2.8 × 1.1 × 1017 4.4 × 1018 1.9 × 1020 5.0 × 1018 -

67.7 88.7 99.9 113.3 114.3 -

ν

ν

Ed-0theory

Ed-01T+0R

[kJ/mol] 64 (a) 79 (b) 98 (a) 112 (a) 106 (a) 114 (a)

[kJ/mol] 72 83 98 109 110 108

v1T+0R [s-1] 1.7 × 1018 7.1 × 1017 1.7 × 1018 3.4 × 1019 3.5 × 1019 1.8 × 1019

The prefactors, vtheory, that were calculated from the vibrational frequencies of the adsorbates on the (TiO2)40 cluster (see the SI for details) allow us to calculate the desorption energies in the same fashion as described in Section 3.2. The results of the inversion analysis for cyclohexane and benzene are shown in Figure 3 (green dash-dot-dotted lines) and can be compared with the inversion analysis using ν2D, ν1D, and vMax. For cyclohexane (Figure 3a), the Ed(θ) dependence for vtheory (green dash-dot-dotted line) follows closely that for ν2D (black dashed line) rather than for ν1D (black solid line) while for benzene (Figure 3b) it falls in between the ν2D and ν1D dependencies showing that both 2- and 1-dimensional gas represent suitable ν approximations. The resulting Ed-0theory values obtained from Ed(θ) dependences for vtheory are summarized in Table 3. It is reasonable to assume that for weakly-bound adsorbates such as those studied here, the desorption process is barrierless. Therefore, the desorption and adsorption energies are identical, which allows us to directly compare desorption energies from the experiment with theoretically-determined energies. Excellent agreement with ΔH(0 K) values can be seen, validating this approach. For comparison, the desorption energies determined assuming a prefactor of 1013 s-1 are also shown in Figure 3. This choice of prefactor severely underestimates the desorption energies due to neglecting the large increase in entropy the adsorbed molecule experiences upon desorption.

17



Page 18 of 26

3.4 Adsorption configurations As illustrated in Figure 6, the molecules line up in the channels created by the rows of Ti5c and Ob ions with Ti-Ti distances averaging 3.42 Å in the y direction (across the rows) and 2.94 Å in the z direction (along the rows) with corresponding O-O distances of 2.93 and 2.57 Å, respectively. The cyclohexane interactions with the surface show three C-H bonds lined up over in-plane O atoms and the center of the ring offset from being over a Ti5c atom. There are 5 short (C)H-O interactions with one at 2.52 Å, two at 2.61 Å, and two at 2.64 Å for structure (a). The shortest C-O interaction is at 3.20 Å and the shortest C-Ti5c interaction is at 3.77 Å, both longer than what is found for benzene. For structure (b), the center of the ring is lined up over a Ti5c atom. Thus, the dominant interactions for cyclohexane are for H-O interactions, which correspond to weak hydrogen bonds between the positively charged H in a C-H bond and the negatively charged O in the TiO2 lattice. For benzene, there are two structures with a difference in binding energy of less than 4 kJ/mol. Structure (a) has the center of the benzene ring lined over an in-plane oxygen, and there are two short CTi5c distances for two C atoms adjacent to each other that we label ortho interactions. Structure (b) has the center of the ring lined up over a Ti5c site with short C-Ti5c distances for three C atoms that are separated by one carbon, so it is like two meta interactions. In the lowest energy structure (b), the Ti5c-C distances range from 3.27 to 3.76 Å, and the C-O distances range from 2.99 to 3.74 Å. The three short Ti5c-C distances are 3.27, 3.39 and 3.40 Å. The (C)H-Ti5c distances range from 3.21 - 3.27 Å and the (C)H-O distances range from 2.66 Å to 3.13 Å. The corresponding van der Waals interaction distances are C-Ti5c = 3.9 Å, C-O = 3.1 Å, H-O = 2.6 Å , and H-Ti = 3.4 Å. Thus the Ti5c-C distances are well within the sum of the van der Waals radii for C-Ti5c, are slightly smaller than the sum for the C-O and H-Ti5c distances, and are larger than the H-O sum. Similar values are calculated for structure (a) for benzene. In this case, the two shortest Ti5c-C distances are 3.20 and 3.28 Å. In contrast with cyclohexane, for benzene, the dominant interactions are for C-Ti and C-O interactions. Substitution of a methyl group on benzene also led to two stable structures (see Table 3 and the SI). Structure (a) has the CH3 group aligned along the rows, and the center of the ring becomes more offset from being over an O atom than for benzene. Structure (b) for toluene is like structure (b) for benzene with the ring slightly offset being over a Ti5c atom, and the molecule now rotates so that the CH3 group is away from being purely in the channel. Addition of a second methyl group at the ortho position to toluene structures (a) and (b) leads to little change in the molecular orientation. There is a larger tilt away from the surface for the ortho structure as compared to the monomethyl derivative. The addition of a methyl group in the 2position does not change configuration (a) of toluene in any substantial way, but the structure (b) no longer has a methyl group oriented along the channel. As expected, substitution at the 4- position has both CH3 groups aligned along the channel and the center of the ring is displaced from being over an O just as in benzene and toluene. 18


Page 19 of 26 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


The results in Tables S16-21, SI show that the shortest interaction distance between a C and Ti5c decreases on average as methyl groups are substituted on benzene starting from an average of 3.24 Å for the two benzene orientations to 3.20 Å for one methyl group to 3.12 Å for two methyl groups. Cyclohexane does not fit this trend. For the benzene compounds, the C-O interaction shows only small variations from 2.95 Å to 3.01 Å. The C-O interaction is also much large for cyclohexane. The shortest H-Ti distance varies from 2.99 Å to 3.13 Å for the benzenes and is much shorter for cyclohexane averaging about 2.70 Å for the two conformers. The shortest H-O distances show significant variations. The largest of the shortest distances is for benzene with an average of 2.68 Å for the two conformers. There are larger variations between the shortest H-O distances for the (a) and (b) structures for methylbenzene and 1,2dimethylbenzene. The shortest H-O distances for the (a) structures of the methyl substituted benzenes range between 2.22 Å to 2.26 Å consistent with moderate hydrogen bonding between a positively charged H on a C-H bond and a negatively charged O form the lattice. The H-O interactions for cyclohexane average about 2.50 Å and are consistently longer than those for the methyl substituted benzenes. Thus, the stronger bonding for the more substituted benzenes correlates with larger polarizability, shorter Ti5c-C distances, and shorter H-O distances. One possibility to consider is that the binding energies would correlate with molecular polarizabilities. The polarizabilities of the organic ring compounds are given in Table S5, SI. This is clearly not the case for cyclohexane and benzene as benzene is less polarizable than cyclohexane and the benzene binding energy is larger than that of cyclohexane. This is consistent with the fact that the bonding of cyclohexane to the surface proceeds by a different mode than bondng of the benzenes. Comparing benzene with methylbenzene, one finds that the polarizability increases by 21% and the binding energy by 19% so this is consistent with the simple polarizability hypothesis. This is consistent with the fact that the bonding of benzene and methylbenzene to the surface are by similar processes. The dimethyl benzenes are predicted to have polarizabilities that are essentially the same and 42-43% larger than that of benzene. The interaction energies for the 1,2- and 1,3-isomers are only 30% larger than that of benzene whereas the interaction energy for the 1,4-isomer is 39% larger than of benzene. Again, the polarizability proportionality argument does not work here consistently. However, it does show that benzene, methylbenzene, and 1,4 dimethylbenzene exhibit a consistent and similar type of bonding. The two other dimethylbenzenes follow the same pattern, but the bonding is partially disrupted by steric effects due to the location of the second methyl group. The atomic charges show some interesting behavior (Tables S8-S13, SI). The carbons become more positive when bonded to the cluster. There is a larger transfer of charge from benzene (0.23 e for both (a) and (b) configurations). For the adsorbed structure (a), the two atoms closest to the Ti5c show the most charge transfer. The CH units in gas phase benzene molecule have zero charge, and for the (a) structure, the two that are closest to the Ti5c in the complex have positive charges of 0.17 and 0.09 e. There is a negative charge polarization at C6, the other C atom adjacent to C1 (see Figure S5, SI for numbering of C atoms in 19



Page 20 of 26

adsorption configurations of different molecules). For the (b) structure, the situation is completely different with two of the closest atoms not exhibiting much charge transfer and C3 having the largest negative charge. The C atoms that are not that close exhibit positive charges showing that there is a loss of charge from them, possibly to the C atoms bonded to the surface. For gas phase methylbenzene, there is already a nonuniform charge distribution in the ring due to the CH3- group. Just as for the adsorbed benzene structure (a), the two closest atoms to the Ti5c in structure (a) of toluene have lost negative charge towards the surface. There is a net transfer of charge of 0.30 e to the surface for structure (a) but only a transfer of 0.25 e for (b) for toluene. Again the (b) structure is more complicated as the closest atom to a Ti5c (C4) gains a negative charge. Substitution of a second CH3- group to form the 1,2-dimethylbenzene leads to a transfer of 0.32 e to the surface for the (a) conformation and 0.28 e for the (b) structure. As in benzene and methylbenzene, there is significant negative charge transfer from the two closet C atoms to the Ti5c for (a) and (b) exhibits different behavior again. The 1,3dimethylbenzene (a) conformer behaves like the 1,2-dimethylbenzene one with 0.33 e transferred to the surface and the (b) conformer has a slightly lower charge transfer of 0.30 e. The atoms where charge transfer occurs follow the patterns noted above. For the 1,4- dimethylbenzene isomer, the charge transfer is again comparable, 0.32 e, and the largest charge transfer is for the closest atom. The charge transfer from the benzenes to the surface is consistent with the formation of dative Lewis acid-base interactions between the benzenes and the surface. Even though cyclohexane has no π bonds, it still transfers 0.20 e to the surface for structure (2) and 0.22 for conformer (b). The surface induces an alternation of charge with the three CH2 groups closest to the surface becoming positive and the other three becoming negative. There is also alternation of charge for the (b) conformer, but only C1 and C3 are closer to the surface. Even though C2 is closer to the Ti5c than C5, C2 is negative, and C5 is positive. The transfer of charge from the cyclohexane to the surface is due to the formation of hydrogen bond interactions and polarizability as there is no π electron cloud to transfer charge to the surface as there is in the benzenes. The bare TiO2 cluster has a band gap that is 0.3 to 0.5 eV below the experimental values for rutile of 3.0 eV and anatase of 3.2 eV (see SI). The HOMO of the aromatic organic is always less negative than that of the bare cluster. The addition of an aromatic organic to the TiO2 cluster leads to a stabilization of the organic HOMO and a decrease in the band gap of the system as the HOMO is located on the organic compound and the LUMO, which is not much changed by the organic, is on the metal oxide. The stabilization of the HOMO in the complex can be up to ~40% of the stabilization energy. Again, this is consistent with charge transfer from the organic to the cluster in the formation of the dative Lewis acidbase bond from the pi cloud of the benzenes to the positively charged Ti5c. The interaction of C6H12 with the cluster is different. The HOMO energy is not dependent on binding to the cluster, and the decrease in the band gap relative to the cluster is just due to lowering the ionization potential of the organic from that of the cluster. 20


Page 21 of 26 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


Figure 7. Coverages determined from the TPD spectra that correspond to the saturation of the uncompressed (θpeak, triangles) layers and compressed (θsat, squares) adsorbates on Ti5c. The coverages (θvdW, hexagons) obtained from the vdW lengths of the molecules. The solid and dotted lines connect θpeak and θvdW values, respectively, for benzene and benzene substituted with n methyl groups in the 1, 1,3, and 1,3,5 positions. For the details of the symbol coding and analysis see Figure 5 caption. 3.5 Saturation coverages As described above (inset, Figure 3b), the characteristic coverages for cyclohexane, benzene and its alkyl substituted derivatives can be determined from their Ed vs θ curves. Figure 7 summarizes the resulting θpeak (triangles) and θsat (squares) values. The θpeak coverages correspond to the highest coverages where steric interactions between adjacent molecules are small. The values of θpeak for all of the molecules lie in a relatively narrow range of 0.33 – 0.37 ML. On average, this coverage range corresponds to one molecule adsorbed per every three Ti5c sites. These values can be compared with what can be expect based on the vdW lengths of the molecules. The values are also similar to those of linear C2-C4 alkanes and alkenes that were studied previously.12 Longitudinal packing along the Ti5c rows is likely preferred since the 6.3 Å width of the Ti5c trough restricts other packing scenarios in which the molecules are arranged crosswise. To estimate the lengths, we determine the distance between the most distant hydrogen atoms in the molecules and add two hydrogen vdW radii (total of 2.4 Å).37 Between the shortest (benzene) and longest (ethylbenzene) molecules, we obtain vdW lengths ranging from 8.17 to 8.76 Å, respectively. The coverages obtained from the vdW length of the molecules, θvdW, are listed in Table 2 and graphically displayed in Figure 7 (hexagons). They closely match the experimentally determined θpeak coverages supporting the arguments put forward above. The saturation coverages on the Ti5c sites, θsat, are also similar for all the molecules but have significantly higher values ranging from 0.48 for 1,4-dimethylbenzene to 0.60 for ethylbenzene. These 21



Page 22 of 26

values indicate that on average one molecule can be adsorbed per every two Ti5c sites. Clearly, these high coverages require a significant departure from the optimum binding configurations thereby resulting in a significant decrease in the desorption energies (see Table 2).

4. Conclusions The adsorption and desorption of cyclohexane, benzene and its alkyl-substituted derivatives have been studied on TiO2(110) using a combination of molecular beam dosing, TPD, and DFT. The results show that benzene is more tightly bound on Ti5c sites than cyclohexane, due to the dative Lewis acid-base bonding of the π system of the benzene ring to the undercoordinated positively charged Ti5c sites with a total charge transfer of up to 0.3 electrons. Two close-energy binding configurations are predicted, one with two nearestneighbor C atoms on the benzene ring bound on top of two neighboring Ti5c atoms and the other with short C-Ti5c distances for three C atoms that are separated by one carbon. The calculations also show a significant stabilization of the π HOMO in the benzenes on complexation to the surface consistent with the predicted charge transfer and the formation of a dative Lewis acid-base interaction between the π orbitals of the benzenes and the positively charged Ti5c with a vacant site. Inversion analysis of the TPD spectra is carried out using prefactors derived from simple statistical mechanics models assuming different mobilities on the surface as well as from the transition state analysis using the frequencies from the DFT calculations. The agreement between the calculated prefactors using simple statistical mechanical models and the transition state theory using the DFT calculated frequencies is very good. The comparison shows that upon desorption, the adsorbates can be viewed as 1- or 2-dimensional translational gas on the surface. This result provides support for the use of the statistical models for other adsorbates and surfaces where such DFT calculations are not feasible. The results further demonstrate that on the Ti5c sites, the desorption energies for benzene drop rapidly with increasing coverage. This is likely a consequence of the decrease in the optimum alignment of benzene with the underlying Ti5c sites with increasing coverage. In contrast, the desorption energies for cyclohexane on the Ti5c sites are nearly independent of coverage. This likely results from the flexibility of the cyclohexane ring, enabling it to optimize its adsorption configuration along the underlying Ti5c row as the coverage increases as the cyclohexane interaction is dominated by H from C-H bonds forming hydrogen bonds with lattice O atoms. The influence of substituents on the phenyl ring of the aromatic hydrocarbons was also studied. Both electronic structure theory and experiment reveal that at low coverage limit, the desorption energies increase with increasing the number of CH3/CH2 segments. However, the difference between the desorption energies decreases as the substituent length or number is increased. This is a consequence of the destabilization of the optimum bonding configuration of the benzene ring and the alkyl groups as their length and number increases. Through quantitative analysis of the TPD spectra, we have also determined the absolute saturation coverages for cyclohexane, and a series of alkyl substituted benzenes. The maximum coverages 22


Page 23 of 26 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


of uncompressed layers of cyclohexane, benzene, and alkyl-substituted benzene derivatives are found to be approximately one molecule per every three Ti5c sites and decrease slightly with increasing molecule size in a good agreement with van der Waals sizes of the molecules. Further coverage increase leads to the compression of adsorbed molecules and maximum coverages of approximately one molecule per every two Ti5c sites.

Acknowledgments This work was supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences and Biosciences. DAD also thanks the Robert Ramsay Chair Fund of The University of Alabama for support. A part of this work was performed in EMSL, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory (PNNL). PNNL is a multiprogram national laboratory operated for DOE by Battelle.

Supporting Information References 1, 28, 30, and 31 with the complete list of authors; sticking coefficient measurements; TPD spectra and coverage dependent desorption energies for dimethylbenzenes, procedures for calculating the desorption prefactors using various models; coverage dependent desorption energies determined using ν2Dgas and vMax; calculated molecular polarizabilities; Mulliken charges of carbon atoms; shortest adsorbatesurface bond lengths; theoretical results for ethyne and ethene; adsorbate frontier orbital energies and images; coordinates of calculated low energy configurations. This information is available free of charge via the Internet at http://pubs.acs.org

References 1. Arakawa, H.; Aresta, M.; Armor, J. N.; Barteau, M. A.; Beckman, E. J.; Bell, A. T.; Bercaw, J. E.; Creutz, C.; Dinjus, E.; Dixon, D. A., et al., Catalysis research of relevance to carbon management: Progress, challenges, and opportunities. Chem. Rev. 2001, 101, 953-996. 2. Weaver, J. F.; Carlsson, A. F.; Madix, R. J., The adsorption and reaction of low molecular weight alkanes on metallic single crystal surfaces. Surf. Sci. Rep. 2003, 50, 107-199. 3. Juurlink, L. B. F.; Killelea, D. R.; Utz, A. L., State-resolved probes of methane dissociation dynamics. Prog. Surf. Sci. 2009, 84, 69-134. 4. Zaera, F., An Organometallic guide to the chemistry of hydrocarbon moieties on transition metal surfaces. Chem. Rev. 1995, 95, 2651-2693. 23



Page 24 of 26

5. Tait, S. L.; Dohnalek, Z.; Campbell, C. T.; Kay, B. D., n-Alkanes on MgO(100). I. Coverage-dependent desorption kinetics of n-butane. J. Chem. Phys. 2005, 122, 164707. 6. Campbell, C. T.; Sellers, J. R. V., Enthalpies and entropies of adsorption on well-defined oxide surfaces: Experimental measurements. Chem. Rev. 2013, 113, 4106-4135. 7. Kubas, A.; Berger, D.; Oberhofer, H.; Maganas, D.; Reuter, K.; Neese, F., Surface adsorption energetics studied with "Gold Standard" wave-function-based Ab Initio Methods: Small-molecule binding to TiO2(110). J. Phys. Chem. Lett. 2016, 7, 4207-4212. 8. Boese, A. D.; Sauer, J., Accurate adsorption energies for small molecules on oxide surfaces: CH4/MgO(001) and C2H6/MgO(001). J. Comput. Chem. 2016, 37, 2374-2385. 9. Jones, G.; Studt, F.; Abild-Pedersen, F.; Nørskov, J. K.; Bligaard, T., Scaling relationships for adsorption energies of C2 hydrocarbons on transition metal surfaces. Chem. Eng. Sci. 2011, 66, 6318-6323. 10. Tait, S. L.; Dohnalek, Z.; Campbell, C. T.; Kay, B. D., N-alkanes on Pt(111) and on C(0001)/Pt(111): Chain length dependence of kinetic desorption parameters. J. Chem. Phys. 2006, 125, 234308. 11. Weaver, J. F.; Hakanoglu, C.; Antony, A.; Asthagiri, A., Alkane activation on crystalline metal oxide surfaces. Chem. Soc. Rev. 2014, 43, 7536-7547. 12. Chen, L.; Smith, R. S.; Kay, B. D.; Dohnálek, Z., Adsorption of small hydrocarbons on rutile TiO2(110). Surf. Sci. 2016, 650, 83-92. 13. Li, Z.; Smith, R. S.; Kay, B. D.; Dohnalek, Z., Determination of absolute coverages for small aliphatic alcohols on TiO2(110). J. Phys. Chem. C 2011, 115, 22534-22539. 14. Li, T.; Kim, M.; Rai, R.; Liang, Z.; Asthagiri, A.; Weaver, J. F., Adsorption of alkanes on stoichiometric and oxygen-rich RuO2(110). Phys. Chem. Chem. Phys. 2016, 18, 22647-22660. 15. Liang, Z.; Li, T.; Kim, M.; Asthagiri, A.; Weaver, J. F., Low-temperature activation of methane on the IrO2(110) surface. Science 2017, 356, 299-303. 16. Bian, Y.; Kim, M.; Li, T.; Asthagiri, A.; Weaver, J. F., Facile dehydrogenation of ethane on the IrO2(110) surface. J. Am. Chem. Soc. 2018, 140, 2665-2672. 17. Dohnalek, Z.; Kim, J.; Bondarchuk, O.; White, J. M.; Kay, B. D., Physisorption of N2, O2, and CO on fully oxidized TiO2(110). J. Phys. Chem. B 2006, 110, 6229-6235. 18. Tait, S. L.; Dohnalek, Z.; Campbell, C. T.; Kay, B. D., n-Alkanes on MgO(100). II. Chain length dependence of kinetic desorption parameters for small n-alkanes. J. Chem. Phys. 2005, 122, 164708. 19. Chen, L.; Li, Z.; Smith, R. S.; Kay, B. D.; Dohnálek, Z., Conversion of 1,2-propylene glycol on rutile TiO2(110). J. Phys. Chem. C 2014, 118, 15339-15347. 20. Chen, L.; Li, Z.; Smith, R. S.; Kay, B. D.; Dohnalek, Z., Conversion of 1,3-propylene glycol on rutile TiO2(110). J. Phys. Chem. C 2014, 118, 23181-23188. 21. Henderson, M. A., The interaction of water with solid surfaces: Fundamental aspects revisited. Surf. Sci. Rep. 2002, 46, 1-308. 22. Henderson, M. A., Structural sensitivity in the dissociation of water on TiO2 single-crystal surfaces. Langmuir 1996, 12, 5093-5098. 23. King, D. A.; Wells, M. G., Molecular beam investigation of adsorption kinetics on bulk metal targets: Nitrogen on tungsten. Surf. Sci. 1972, 29, 454. 24


Page 25 of 26 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


24. Dohnalek, Z.; Lyubinetsky, I.; Rousseau, R., Thermally-driven processes on rutile TiO2(110)-(1 x 1): A direct view at the atomic scale. Prog. Surf. Sci. 2010, 85, 161-205. 25. Godbout, N.; Salahub, D. R.; Andzelm, J.; Wimmer, E., Optimization of Gaussian-type basis sets for local spin density functional calculations. Part I. Boron through neon, optimization technique and validation. Can. J. Chem. 1992, 70, 560-571. 26. Sosa, C.; Andzelm, J.; Elkin, B. C.; Wimmer, E.; Dobbs, K. D.; Dixon, D. A., A local density functional study of the structure and vibrational frequencies of molecular transition-metal compounds. J. Phys. Chem. 1992, 96, 6630-6636. 27. Becke, A. D., Density-functional thermochemistry. V. Systematic optimization of exchangecorrelation functionals. J. Chem. Phys. 1997, 107, 8554-8560. 28. Valiev, M.; Bylaska, E. J.; Govind, N.; Kowalski, K.; Straatsma, T. P.; Van Dam, H. J. J.; Wang, D.; Nieplocha, J.; Apra, E.; Windus, T. L., et al., NWChem: A comprehensive and scalable open-source solution for large scale molecular simulations. Comput. Phys. Commun. 2010, 181, 1477-1489. 29. Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H., A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu. J. Chem. Phys. 2010, 132, 154104. 30. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Petersson, G. A.; Nakatsuji, H., et al. Gaussian 16 Rev. B.01, Wallingford, CT, 2016. 31. Lin, X.; Yoon, Y.; Petrik, N. G.; Li, Z.; Wang, Z.-T.; Glezakou, V.-A.; Kay, B. D.; Lyubinetsky, I.; Kimmel, G. A.; Rousseau, R., et al., Structure and Dynamics of CO2 on Rutile TiO2(110)-1x1. J. Phys. Chem. C 2012, 116, 26322-26334. 32. Smith, R. S.; Li, Z.; Chen, L.; Dohnalek, Z.; Kay, B. D., Adsorption, Desorption, and Displacement Kinetics of H2O and CO2 on TiO2(110). J. Phys. Chem. B 2014, 118, 8054-8061. 33. Kimmel, G. A.; Persson, M.; Dohnalek, Z.; Kay, B. D., Temperature independent physisorption kinetics and adsorbate layer compression for Ar adsorbed on Pt(111). J. Chem. Phys. 2003, 119, 6776-6783. 34. Öberg, H.; Nestsiarenka, Y.; Matsuda, A.; Gladh, J.; Hansson, T.; Pettersson, L. G. M.; Öström, H., Adsorption and Cyclotrimerization Kinetics of C2H2 at a Cu(110) Surface. J. Phys. Chem. C 2012, 116, 9550-9560. 35. Redhead, P. A., Thermal desorption of gases. Vacuum 1962, 12, 203. 36. Smith, R. S.; Kay, B. D., Desorption kinetics of benzene and cyclohexane from a graphene surface. J. Phys. Chem. B 2018, 122, 587-594. 37. Batsanov, S. S., Van der Waals radii of elements. Inorg. Mater. 2001, 37, 871-885.

25



Page 26 of 26

TOC Graphics

26


Understanding the Binding of Aromatic ... - ACS Publications

Recommend Documents