Adsorption of Dibenzothiophene and Fluorene on TiO2(110) and

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Adsorption of Dibenzothiophene and Fluorene on TiO(110) and Supported Ag clusters 2

Elizabeth Rene Webster, Aileen Park, Miranda B. Stratton, Victoria C. Park, Amber M. Mosier, Ryan S. Shine, and Lauren Benz Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef401581x • Publication Date (Web): 24 Sep 2013 Downloaded from http://pubs.acs.org on September 26, 2013

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Adsorption of Dibenzothiophene and Fluorene on TiO2(110) and Supported Ag clusters Elizabeth R. Webster, Aileen Park, Miranda B. Stratton, Victoria C. Park, Amber M. Mosier, Ryan S. Shine, and Lauren Benz* Department of Chemistry and Biochemistry, University of San Diego, CA 92110

ABSTRACT. New adsorbent materials, many of which rely on supported Ag nanoclusters or exchanged Ag+ ions, have recently been employed in petroleum processing in order to further reduce sulfur content in fuels following catalytic hydrodesulfurization (HDS). HDS refractory species include aromatic heterocycles such as dibenzothiophene (DBT) and its methylated derivatives. Herein, we report a fundamental study of the adsorption of two structurally analogous petroleum-relevant molecules: DBT and fluorene, on a clean TiO2(110) surface and one with supported Ag nanoclusters. Using thermal desorption spectroscopy, we determined the desorption activation energies to be 106 ± 2 kJ/mol and 103 ± 2 kJ/mol for DBT and fluorene, respectively. The similar desorption activation energies imply that the interaction of DBT on TiO2 is not strongly dependent on the S atom (which fluorene lacks). When adsorbed on supported Ag nanoparticles, both desorption activation energies shifted to 111 ± 2 kJ/mol, suggesting a non-selective binding enhancement which likely involves the π electron systems. After heating the Ag/TiO2(110) surface to 650 K to force agglomeration of the particles, no

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enhancement in binding was observed for either molecule, suggesting that cluster size is critical for the observed enhancement. These results point to the importance of metal particle size in addition to oxidation state in commercially employed sorbents. 1. INTRODUCTION The presence of sulfur-containing molecules in petroleum is of environmental concern since the combustion of such species in downstream fractions leads to harmful sulfur dioxide which can cause acid rain, as well as sulfate aerosols which impact local smog conditions and the global climate.1 Organosulfur contaminants also give rise to a number of industrial problems including the corrosion of processing equipment and catalyst poisoning.2 Natural petroleum deposits contain a significant amount of sulfur, typically in the range of 0.05% - 6% by mass (~103 ppm).2 The types of molecules which contain sulfur can been determined analytically from vacuum residues formed during the distillation process,3 and grouped by structure type and reactivity.4 Common core sulfur-containing structures include thiols, thioethers, and thiophenes of varying size and complexity. Current removal methods rely predominantly on hydrodesulfurization catalysts (HDS) to bind to sulfur and chemically remove it from its core structure.5 This method is effective for many of the core structures, but complex aromatic heterocyclic sulfur compounds such as dibenzothiophene and its methylated derivatives are challenging to remove, requiring higher temperatures and higher pressures of hydrogen in HDS to bring sulfur levels down to an acceptable range. Current limits in the US for ultra-low sulfur (ULS) gasoline and ULS diesel are 30 ppm and 15 ppm, respectively, and refinery processes frequently need to be adjusted to match this demand. Increasingly popular fuel cell applications have practically zero tolerance


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for sulfur, requiring less than 1 ppm sulfur to function properly.6 As a result, alternative sulfur removal methods capable of further reducing sulfur content are under development. A number of new sulfur removal methods are based on the adsorptive binding of refractory compounds by employing a sorbent following catalytic HDS. When thinking of appropriate sorbents, one can imagine targeting the sulfur atom and/or the aromatic system of the refractory sulfur species as potential binding sites on the organosulfur species. A variety of approaches along these lines can be taken, including the use of intermolecular forces such as van der Waals or charge attractions,7,8 π-complexation,9 and reactive chemisorption.10-13 An example of the use of van der Waals forces is the well-known use of activated carbon (AC) as a sorbent. Activated carbon is often employed due to its ability to bind via dispersive forces over its high surface area, and when prepared with silver nanoparticles binding becomes more selective for sulfur-containing aromatics.10 Another approach which uses oxidized AC demonstrates higher adsorption capacity in comparison to unoxidized AC.7,8 It is proposed that the sulfur moiety interacts with oxygen species on the surface and possibly other groups, enhancing the traditional role of the pore structure and demonstrating the interesting combination of physical and chemical forces at play. Ion-exchanged zeolites (Ag-Y, Cu(I)-Y, Ni(II)-Y, and Ni(II)-X) have demonstrated success as π-complexing agents, providing a bond which is stronger than a van der Waals attractive force, yet weak enough to be broken by heating and/or decreasing pressure.9 The πcomplexation process involves σ-bonding between the cation s-orbitals and π system of the thiophenic compound, as well as π-back-bonding from the metal d orbitals to anti-bonding orbitals of the S aromatic rings. A copper-exchanged zeolite was determined to have the best


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adsorption capacity when used with a guard bed. These π-complexing agents have also proven to be selective for thiophenic compounds over carbon-based aromatic molecules. Finally, in reactive chemisorption, catalyst materials typically poisoned by sulfur compounds often make good sorbent candidates. ConocoPhillips, for example, has developed the “S Zorb” process which is reportedly capable of removing refractory species including 4,6dimethyldibenzothiophene.12,13 The process uses propietary metal oxide and silica supported bimetallic promoters. A number of reports have also recently appeared in the literature on oxidesupported Ag for the adsorptive removal of benzothiophenes,14,15 as well as extensive surface characterization of some of these sorbents.16-19 In these reports, Ag was supported on both titania alone,16-19 and a more complex series of sorbents including TiOx-Al2O3 and TiOxSiO2.14,15 The addition of alumina and silica were reported to improve uptake due to a high surface area relative to titania. The active sites were determined to be oxygen-containing sites (either associatively or dissociatively bound oxygen) on both Ti and Ag, leading to a sulfone-like association between the sulfur atom of dibenzothiophene (DBT) and these oxygen species. Such oxygen species are present on the mixture of nanodispersed TiO2 (> 80% anatase) and Ag particles. The active Ag in this case is reported to be in the +1 state based on electron spin resonance (ESR) measurements of a trapped NO redox probe. In these experiments a pressed pellet containing the mixture was used and prewetted with a DBT-octane mixture prior to experiments to simulate fuel before introduction into a vacuum chamber for experimentation. In addition to the above informative experiments using pressed powders, zeolites, etc., one can also employ a well-defined model single crystalline substrate alone or as a support for active species to study adsorption processes. In this way, the entire complexity range can be studied from single crystals, to powders, to small and eventually large-scale reactors to form a


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complete picture. A handful of experiments reported in the literature on smaller organosulfur compounds and single crystalline substrates provide a fundamental understanding of the interactions of such compounds with a variety of oxides20-27 and oxide-supported Ni clusters.28 A number of studies are also reported involving small sulfur-containing molecules and single crystalline metals.29-32 No studies have been reported, to the best of our knowledge, involving the larger refractory DBT and single crystalline substrates (oxides or metals), perhaps due to the experimental challenges involved in working with DBT, a solid, and ultrahigh vacuum. Such studies can help further elucidate the nature of the adsorbant/refractory molecule interaction, particularly in light of the current employment of adsorptive removal in addition to HDS. Herein, we use thermal desorption and X-ray photoelectron spectroscopy (XPS) to study the interaction of DBT with a TiO2(110) surface and TiO2(110)-supported Ag clusters. We also examined the adsorption of the structurally analogous fluorene molecule, which lacks the sulfur moiety, in order to probe the roles of the π-system and the sulfur atom. 2. EXPERIMENTAL SECTION The experiments described were performed in an ultrahigh vacuum (UHV) chamber with a base pressure of ~1 × 10-10 torr, and typical operating pressures between 0.5 – 1 × 10-9 torr. The chamber is equipped with a quadrupole mass spectrometer for thermal desorption studies (300 amu range, Hiden Analytical, HAL 301/3F), an ion bombardment gun for in-situ cleaning (RBD 04-165), an X-ray photoelectron spectrometer with a cylindrical mirror analyzer for surface analysis (PHI 15-255G), and a directed doser consisting of a leak valve and a ¼” stainless steel tube, in front of which the sample was positioned during dosing (~1 cm). The TiO2(110) sample (Princeton Scientific, 1 cm square × 1 mm wide, polished both sides) was mounted to an equally sized tantalum back plate using thin tantalum wires. The sample mount


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was in contact with a liquid nitrogen reservoir, and a type-k thermocouple was glued into a hole in edge of the crystal with UHV-compatible ceramic glue (Ceramabond 503). In this way the sample could be cooled to 90 K, and heated to 900 K using a tungsten wire positioned ~2 mm behind the sample. Following installation, the sample was treated with multiple (> 100 total) cycles of argon ion bombardment (1 – 2 kV, 10 – 15 minutes, 2 μA, PAr = 2 × 10-5 torr) and annealing (800 – 850 K, 10 – 15 minutes). The color of the TiO2 crystal was deep blue-gray during these studies, indicative of a fairly reduced state (refer to Supporting Information, S1A, for quantification of relative amounts of Ti ions). Dibenzothiophene (Alfa Aesar, 98%) and fluorene (Alfa Aesar, 98+%) were used as received, and installed in a glass tube mounted to a stainless steel dosing line. It was necessary to heat the entire line to 60 – 70°C for fluorene experiments, and 80 – 90°C for dibenzothiophene experiments in order to achieve a sufficient vapor pressure for dosing, consistent with the reported relative vapor pressures.33,34 In order to remove trapped water and other ambient gases, the warm solids and dosing line were pumped directly with a turbo pump for several cycles of ~5 - 10 minutes prior to use, and fragmentation patterns were checked upon leaking the gases into the chamber, as well as in the condensed multilayer state. Due to the challenging nature of quantifying molecular coverages on TiO2,35 we report relative coverages here based on integrated peak areas in the desorption spectra, setting one monolayer (1.00 ML) equal to the total peak area at a coverage just prior to the growth of the multilayer peak. Ag deposition was accomplished using an evaporative doser (McAllister, EVAP-A) filled with Ag shot (Alfa Aesar, 99.9999%) which was degassed prior to use by slow heating to 900°C with the shutter in the closed position. The deposition time was 1 minute, following which XPS was used to


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determine surface coverage relative to Ti, resulting in a coverage of ~10% Ag relative to Ti (see S2). X-ray photoemission data was collected using a Mg Kα X-ray source (hν = 1253.6 eV, maximum resolution of 1.1 eV) at 90 K. Binding energies were calibrated using a half-copper (2p3/2, 932.4 eV) half-gold (4f7/2, 83.8 eV) substrate to give a wide range of peak separation and an accurate binding energy scale. This led to a Ti 2p3/2 photoelectron peak at 458.4 eV on the annealed surface, consistent with previous reports.25 XPS spectra were fit with CasaXPS software following subtraction of a Shirley background using combined Gaussian-Lorentzian functions. 3. RESULTS AND DISCUSSION 3.1 Dibenzothiophene and Fluorene on TiO2(110) The desorption of dibenzothiophene was followed as a function of exposure using mass spectrometry, the result of which is shown in Figure 1. Two main desorption peaks are visible at 255 K and 400 K, corresponding to multilayer and surface-bound DBT, respectively. In addition, a weaker shoulder is visible at ~300K, possibly caused by a change in the orientation of DBT relative to the surface. It has been observed that for thiophene on TiO2(110), the molecule is flat at low coverage, interacting with the surface through the conjugated pi system.26,27 At higher coverages, however, weaker binding via the S atom becomes competitive, a phenomenon also observed for thiophene on metal surfaces.36,37 We assign the observed desorption at 400 K here to DBT adsorbed flat or nearly flat on the surface, with little interaction with the S atom. Further evidence for this type of adsorption is given in the comparison to fluorene desorption below. The lack of interaction of S with the surface is consistent with the known difficulty in


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removing sulfur from DBT during HDS. That is, pi conjugation increases the stability of the molecule and decreases basicity of the S moiety making it more difficult to bind and remove S specifically. As the coverage increases, like thiophene, the packing on the surface changes such that the barrier to desorption from the surface decreases. Assuming first order desorption in the limit of low coverage and a preexponential factor of 1013 s-1, one can calculate an approximate desorption activation energy of 106 ± 2 kJ/mol (error from peak temperature selection).38 This selected value of preexponential factor is typically adopted in the study of other organics on TiO2(110),25,39 however, it can vary significantly system to system. More importantly, this factor is likely the same for both molecules studied here given their structural similarities, and therefore serves the purpose of comparison. This energy is high relative to smaller sulfur-containing molecules,27 and a value of ~100 kJ/mol is sometimes used as a rough borderline between physisorption and chemisorption. We note, however, that the activation energy of desorption is only comparable to the adsorption energy in non-activated desorption processes, common in the case of the adsorption of unreactive molecules on clean single-crystalline surfaces.40 We make this assumption throughout the discussion below. In the case of DBT, the desorption activation energy is likely the result of a physical interaction between the large pi system and the surface cationic Ti sites. Finally, a complete scan of the 300 amu range of the spectrophotometer in 10 amu increments following multilayer exposures revealed no desorption features created by reaction of DBT.


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Figure 1. Thermal desorption spectra of dibenzothiophene, during which the parent ion, m/z = 184, was monitored. XPS data were also collected after dosing a multilayer (6 ML) of DBT on the TiO2(110) surface at 200 K and subsequently heating to temperatures corresponding to specific states in the thermal desorption, as shown in Figure 2. The C 1s region shows a clear peak at 284.2 eV, which we assign to C in DBT, in good agreement with the spectrum of neat DBT as expected.18 Upon heating to 300 K, the multilayer desorbs (Figure 1) and only surface-bound DBT remains. This results in a drop in intensity by a factor of 3, yet the peak position remains at 284.2 eV. Further heating to 650 K almost completely removes DBT from the surface. The small amount remaining is estimated to be ~10% of a monolayer by area integration in comparison to a monolayer of DBT following correction for a small amount of adventitious C present on the surface for these experiments. This residual C is likely due to reaction with defect sites on the surface such as step edges, oxygen vacancies, and possibly near-surface interstitial Ti3+ species known to migrate to the surface and react with some adsorbates.41-43


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Correspondingly, a small peak at 163.9 eV is visible in the S 2p region following exposure to a multilayer of DBT. The ratio of S:C can be calculated and is 0.075, close to the ratio of 0.083 expected from the 1:12 ratio of S:C in DBT. This peak also decreases in intensity by a factor of ~3 upon heating to 300 K, and its position does not change significantly. The lack of shift in the binding energies of the C 1s and S 2p peaks is consistent with the pi-type interaction proposed here for DBT and TiO2(110), as is also the case for thiophene adsorption on TiO2(110). Upon heating to 650 K the S 2p peak decreased to baseline levels. We also monitored the Ti 2p region before and after deposition of DBT (see S1B), and no measurable change in the peak positions or shapes was observed. This is consistent with molecular adsorption and desorption. The adsorption/desorption was completely reversible with no subsequent C or S build-up on the surface for multiple exposure and heating cycles, aside from the 10% decomposition noted above which saturated and did not grow further.

Figure 2. X-ray photoelectron spectra of (A) the C 1s and (B) the S 2p regions following a multilayer exposure of DBT on TiO2(110) at 200 K with subsequent heating to the indicated temperatures.


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In order to further explore the binding of DBT to TiO2 we selected an analogous molecule, fluorene, to test for comparison purposes to probe the effect, if any, of the presence of the S atom in DBT, as fluorene has a carbon atom in its place. We therefore completed the same set of thermal desorption and photoelectron studies with fluorene on a similarly prepared TiO2(110) surface. The desorption spectra as a function of coverage are shown in Figure 3. Similar to DBT, two major peaks were visible, a multilayer feature at 250 K and a surfacerelated peak at 390 K, the latter corresponding to a desorption activation energy of 103 ± 2 kJ/mol, calculated as discussed above. The relative desorption temperatures of the multilayers for fluorene and DBT (250 K and 255 K, respectively) are consistent with their reported relative heats of sublimation as expected (~85 kJ/mol and 94 kJ/mol, respectively).33,34 Likewise, the surface-related desorption features for fluorene and DBT followed the same trend, with that of fluorene (390 K at 0.26 ML) appearing at a slightly lower temperature than that of DBT (400 K at 0.18 ML). Overall the thermal desorption spectra appear quite similar, indicating that the effect of the S atom is minimal. The fluorene desorption spectra lacked the weak shoulder present at ~300 K for DBT, causing the multilayer to appear sharper overall. As this shoulder was attributed to a shift at higher coverages to competitive binding via the S atom, the fact that it is missing is consistent with the fact that the S atom is absent in fluorene.


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Figure 3. Thermal desorption spectra of fluorene, during which the parent ion, m/z = 166, was monitored. We also employed XPS to examine the C 1s region following exposure of the TiO2(110) surface to a multilayer dose of fluorene. The spectra collected as seen in Figure 4 were nearly identical to that of the C 1s region of DBT. Carbon appears at ~284 eV following exposure to a multilayer dose (3.5 ML), and remains at that binding energy, decreasing in intensity by a factor of 3 when heated to 310 K. At 310 K only a monolayer is present on the surface. The peak decreases to ~10% of the monolayer coverage upon further heating to 650 K, and likewise we assign this small amount of residual C to a minor amount of decomposition of fluorene over defect sites.


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Figure 4. X-ray photoelectron spectra of the C 1s region following a multilayer exposure of fluorene on TiO2(110) at 90 K with subsequent heating to the indicated temperatures. 3.2 Dibenzothiophene and Fluorene on Ag/TiO2(110) In order to probe the interaction of supported Ag clusters with DBT and fluorene we evaporatively deposited Ag onto the surface in situ. An XPS spectrum of the resulting surface with Ag is given in the Supporting Information (S2). Area analysis of this spectrum indicates an atomic percentage of approximately 10% Ag relative to Ti. Luo et al. found that under similar deposition conditions the Ag clusters grow following a 3-D Volmer-Weber growth mode, and that for the coverages reported here, clusters are expected to be approximately 3 nm in diameter and 1 nm high.44 Furthermore, in the work of Luo et al. no strong chemical interaction occurred between the Ag clusters and substrate.


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Figure 5. Thermal desorption of (A) dibenzothiophene and (B) fluorene both with (dotted traces) and without (solid traces) the presence of Ag nanoparticles on the TiO2(110) surface. In both cases, the peaks shift to higher desorption temperature on the Ag-decorated surface. The thermal desorption spectra of dibenzothiophene and fluorene are shown in Figure 5A and 5B with and without the presence of Ag nanoparticles following an exposure of ~0.25 ML DBT and fluorene. In 5A the presence of Ag resulted in a measurable shift in the peak desorption temperature of DBT by approximately 20 K from 400 K to 420 K. This indicates an enhancement in the desorption activation energy to 111 kJ/mol. Interestingly, the enhancement was only visible on the first desorption, and any subsequent exposure of the surface to DBT or fluorene resulted in spectra very similar to that on the pure TiO2(110) surface. The surface has to be re-prepared in order to observe the enhancement again. Ag clusters deposited at low temperature are reported to sinter upon heating.44 Furthermore, it has been proposed that a transition in the electronic properties of the clusters occurs as they grow in size, and that the character is less metallic at smaller sizes with the appearance of a band gap in scanning tunneling spectroscopy studies.44,45 The enhancement in binding to DBT therefore likely depends on the electronic nature of the cluster, with enhanced binding for clusters which are less metallic in nature. We also compared this enhanced desorption activation energy to that of fluorene, which


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lacks the sulfur moiety, but is also aromatic in nature. Interestingly, the shift was present for fluorene as well, with a shift in the maximum desorption temperature from approximately 390 K to 420 K, indicating an enhancement in the desorption activation energy from 103 kJ/mol to 111 kJ/mol, a slightly greater enhancement than that of DBT. Small Ag clusters supported on TiO2(110) therefore interact in a non-sulfur-selective manner with both DBT and fluorene, with the enhancement possibly attributed to an interaction with the aromatic systems of these molecules. We also scanned for possible reaction products following DBT adsorption by monitoring biphenyl and benzenethiol, but did not observe the formation of these by reaction with the Ag/TiO2 surface. Some understanding of the above results can be gained by comparison to earlier work on zeolites. In the work on Ag+ exchanged zeolites, both modeled and tested experimentally by Yang et al.,9 π complexation was proposed to enhance selective binding to thiophene over benzene. Donation of electron density from the π orbital of thiophene to the vacant s-orbital of Ag, and back-donation from the Ag d-orbitals to the π* orbital of thiophene was predicted from a natural bond order analysis. For benzene, the latter back-donation was dominant, and the overall adsorption bond energy was lower by about 4 kJ/mol. It may be that the outermost atoms of the Ag nanoclusters studied here are able to complex to DBT, but in a manner in which the backdonation process is favored due to the additional delocalization of electron density on S within the larger π system, making the enhancement similar to that of fluorene. Another possibility is that an enhanced van der Waals interaction occurs between both molecules and the Ag clusters. The enhanced interactions observed in the thermal desorption experiments of fluorene/Ag/TiO2 and DBT/Ag/TiO2 did not result in any measureable changes in the C 1s (for either molecule) or S 2p (for DBT) binding energies in XPS experiments following exposure of


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the molecule of interest to the Ag/TiO2 surface and heating, as the data in both cases was nearly identical to that of Figures 2 and 4, respectively (see S3 and S4). In previous experiments with a more reactive molecule, thiophenol, on reduced TiO2(110), a peak appeared at 162 eV coincident with thiolate formation and subsequent desulfurization upon heating, which was not observed here.27 This data supports the observation that no chemical reaction occurred, in agreement with the thermal desorption data. We also investigated the Ag 3d region (Figure 6) and did not observe any significant changes in binding energy resulting from the deposition of 0.25 ML DBT at 200 K , however, the binding energy of Ag is fairly insensitive to oxidation state changes (the binding energies of Ag2S, AgO and Ag are all within 1 eV of each other, which is close to the resolution limit of our instrument).46


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Figure 6. X-ray photoelectron spectra of the Ag 3d region of (A) a clean TiO2 surface covered with 0.10 ML Ag (B) the same surface following adsorption of 0.25 ML DBT at 200 K. Aside from a small amount of peak attenuation caused by the overlayer of DBT, no differences were observed in binding energy or peak shape. 4. CONCLUSIONS In summary, we have found that DBT and fluorene bind to a TiO2(110) surface in a similar manner with similar desorption activation energies (106 kJ/ mol and 103 kJ/mol, respectively), interacting predominantly via the pi system. Ag nanoparticles enhance the adsorption of both DBT and fluorene on a single-crystalline TiO2(110) surface. This is somewhat surprising, as one might expect that Ag clusters would bind DBT more strongly than fluorene given the presence of the S atom, based on hard-soft acid base theory47 and the known affinity of Ag for S. However, it seems based on the comparison of results found here and reports in the literature mentioned above, that in order to be selective for adsorption of large refractory S-containing molecules, Ag must be oxidized. The size of the nanoparticles may, however, influence adsorption by enhancing affinity for the pi system. Future experiments on oxidized Ag surfaces and clusters could help isolate the individual (but potentially synergistic) effects of cluster size and Ag oxidation state. AUTHOR INFORMATION Corresponding Author *Tel.: 619-260-4117. Fax: 619-260-2211. E-mail: [email protected]. Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT Acknowledgments are made to the Donors of the American Chemical Society Petroleum Research Fund for partial support of this research (#50385-UNI5). Additional financial support was provided by the University of San Diego and the Henry Luce Foundation’s Clare Boothe Luce Program. ASSOCIATED CONTENT Supporting Information. Additional XPS data are included in the Supporting Information section. This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES (1) Stern, D. I. Chemosphere 2005, 58, 163-175. (2) Speight, J. G., The Chemistry and Technology of Petroleum. Marcel Dekker: New York, 1999. (3) Muller, H.; Andersson, J. T.; Schrader, W. Anal. Chem. 2005, 77, 2536-2543. (4) Choudhary, T. V. Indus. Eng. Chem. Res. 2007, 46, 8363-8370. (5) Satterfield, C. N., Heterogeneous Catalysis in Industrial Practice. McGraw-Hill: New York, 1991. (6) Babich, I. V.; Moulijn, J. A. Fuel 2003, 82, 607-631. (7) Jiang, Z. X.; Liu, Y.; Sun, X. P.; Tian, F. P.; Sun, F. X.; Liang, C. H.; You, W. S.; Han, C. R.; Li, C. Langmuir 2003, 19, 731-736. (8) Ania, C. O.; Bandosz, T. J. Langmuir 2005, 21, 7752-7759. (9) Yang, R. T.; Hernandez-Maldonado, A. J.; Yang, F. H. Science 2003, 301, 79-81. (10) Seredych, M.; Bandosz, T. J. Energy Fuels 2009, 23, 3737-3744. (11) Ma, X.; Lu, S.; Song, C. Catal. Today 2002, 77, 107-116. (12) Khare, G. P. Desulfurization Process and Novel Bimetallic Sorbent Systems for Same. U.S. Patent 6,274,533, 2001. (13) Khare, G. P. Process for the Production of a Sulfur Sorbent. U.S. Patent 6,184,176, February 6, 2001. (14) Hussain, A. H. M. S.; Tatarchuk, B. J. Fuel 2013, 107, 465-473. (15) Nair, S.; Tatarchuk, B. J. Adsorption 2011, 17, 663-673.


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(16) Samokhvalov, A.; Duin, E. C.; Nair, S.; Bowman, M.; Davis, Z.; Tatarchuk, B. J. J. Phys. Chem. C 2010, 114, 4075-4085. (17) Samokhvalov, A.; Duin, E. C.; Nair, S.; Tatarchuk, B. J. Surf. Interface Anal. 2010, 42, 1476-1482. (18) Samokhvalov, A.; Duin, E. C.; Nair, S.; Tatarchuk, B. J. App. Surf. Sci. 2011, 257, 32263232. (19) Samokhvalov, A.; Nair, S.; Duin, E. C.; Tatarchuk, B. J. App. Surf. Sci. 2010, 256, 36473652. (20) Casarin, M.; Favero, G.; Glisenti, A.; Granozzi, G.; Maccato, C.; Tabacchi, G.; Vittadini, A. J. Chem. Soc. Farad. Trans. 1996, 92, 3247-3258. (21) Dvorak, J.; Jirsak, T.; Rodriguez, J. A. Surf. Sci. 2001, 479, 155-168. (22) Halevi, B.; Vohs, J. M. J. Phys. Chem. B 2005, 109, 23976-23982. (23) Halevi, B.; Vohs, J. M. Catal. Lett. 2006, 111, 1-4. (24) Halevi, B.; Vohs, J. M. Surf. Sci. 2008, 602, 198-204. (25) Liu, G.; Rodriguez, J. A.; Chang, Z.; Hrbek, J.; Gonzalez, L. J. Phys. Chem. B 2002, 106, 9883-9891. (26) Liu, G.; Rodriguez, J. A.; Hrbek, J.; Long, B. T.; Chen, D. A. J. Mol. Catal. A 2003, 202, 215-227. (27) Benz, L.; Park, A.; Corey, J. R.; Mezher, M. P.; Park, V. C. Langmuir 2012, 28, 1020910216. (28) Ozturk, O.; Park, J. B.; Black, T. J.; Rodriguez, J. A.; Hrbek, J.; Chen, D. A. Surf. Sci. 2008, 602, 3077-3088. (29) Friend, C. M.; Chen, D. A. Polyhedron 1997, 16, 3165-3175. (30) Rodriguez, J. A.; Dvorak, J.; Jirsak, T. Surf. Sci. 2000, 457, L413-L420. (31) Jaffey, D. M.; Madix, R. J. J. Am. Chem. Soc. 1994, 116, 3020-3027. (32) Jaffey, D. M.; Madix, R. J. Surf. Sci. 1991, 258, 359-375. (33) Ruzicka, K.; Mokbel, I.; Majer, V.; Ruzicka, V.; Jose, J.; Zabransky, M. Fluid Phase Eq. 1998, 148, 107-137. (34) Verevkin, S. P. Fluid Phase Eq. 2004, 225, 145-152. (35) Brinkley, D.; Engel, T. J. Phys. Chem. B 2000, 104, 9836-9841. (36) Milligan, P. K.; Murphy, B.; Lennon, D.; Cowie, B. C. C.; Kadodwala, M. J. Phys. Chem. B 2001, 105, 140-148. (37) Vaterlein, P.; Schmelzer, M.; Taborski, J.; Krause, T.; Viczian, F.; Bassler, M.; Fink, R.; Umbach, E.; Wurth, W. Surf. Sci. 2000, 452, 20-32. (38) Redhead, P. A. Vacuum 1962, 12, 202-211. (39) Henderson, M. A. J. Phys. Chem. B 2004, 108, 18932-18941. (40) King, D. A. Surface Sciences 1975, 47, 384-402. (41) Benz, L.; Haubrich, J.; Jensen, S. C.; Friend, C. M. ACS Nano 2011, 5, 834-843. (42) Diebold, U. Surf. Sci. Rep. 2003, 48, 53-229. (43) Lira, E.; Wendt, S.; Huo, P. P.; Hansen, J. O.; Streber, R.; Porsgaard, S.; Wei, Y. Y.; Bechstein, R.; Laegsgaard, E.; Besenbacher, F. J. Am. Chem. Soc. 2011, 133, 6529-6532. (44) Luo, K.; St Clair, T. P.; Lai, X.; Goodman, D. W. J. Phys. Chem. B 2000, 104, 3050-3057. (45) Lai, X.; St Clair, T. P.; Valden, M.; Goodman, D. W. Prog. Surf. Sci. 1998, 59, 25-52. (46) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F.; Muilenberg, G. E., Handbook of X-ray Photoelectron Spectroscopy. Perkin-Elmer Corporation: Eden Praire, MN, 1979. (47) Pearson, R. G. J. Am. Chem. Soc. 1963, 85, 3533-3539.


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Thermal desorption spectra of fluorene, during which the parent ion, m/z = 166, was monitored. 84x102mm (600 x 600 DPI)


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X-ray photoelectron spectra of the C 1s region following a multilayer exposure of fluorene on TiO2(110) at 90 K with subsequent heating to the indicated temperatures. 81x95mm (600 x 600 DPI)


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Thermal desorption of (A) dibenzothiophene and (B) fluorene both with (dotted traces) and without (solid traces) the presence of Ag nanoparticles on the TiO2(110) surface. 53x33mm (600 x 600 DPI)


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Adsorption of Dibenzothiophene and Fluorene on TiO2(110) and

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