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Controlling the Surface Reactivity of Titania via Electronic Tuning of Self-Assembled Monolayers Lucas D Ellis, Ryan M Trottier, Charles B. Musgrave, Daniel K Schwartz, and J. Will Medlin ACS Catal., Just Accepted Manuscript • Publication Date (Web): 23 Oct 2017 Downloaded from http://pubs.acs.org on October 23, 2017
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Controlling the Surface Reactivity of Titania via Electronic Tuning of Self-Assembled Monolayers Lucas D. Ellis1, Ryan M. Trottier1, Charles B. Musgrave1,2, Daniel K. Schwartz1, J. Will Medlin1* 1 2
Department of Chemical & Biological Engineering, University of Colorado, Boulder, CO 80309 Department of Chemistry & Biochemistry, University of Colorado, Boulder, CO 80309
ABSTRACT: Reactivity of molecular catalysts can be controlled by organic ligands that regulate the steric and electronic properties of catalyst sites. This level of control has generally been unavailable for heterogeneous catalysts. We show that self-assembled monolayers (SAMs) on titania with tunable electronic properties provided fine control over surface reactivity. Controlling the identity of substituents on benzylphosphonic acid SAMs modulated the near-surface electrostatics, enabling regulation of the dehydration activity of 1-propanol and 1-butanol over a wide range, with activities and selectivities of the optimal catalyst far exceeding those of uncoated TiO2. The dipole moment of the adsorbed phosphonate was strongly correlated to the dehydration activity; kinetic measurements and computational modeling indicated that the interfacial electric field altered the transition state structure and energy. Coating catalysts with SAMs having controllable charge distributions may provide a general approach to heterogeneous catalyst design analogous to the variation of ligands in molecular catalysts. KEYWORDS: Dehydration, Titania, Self-Assembled Monolayers, Dipole Moment, Induced Field. 1. INTRODUCTION Although the use of solid catalysts to promote chemical reactions is highly desired due to their ease of separation from the reaction mixture, molecular catalysts have several advantages for precise design of efficient active sites. For example, discrete transition metal complex catalysts can be optimized through design of the ligands surrounding the active metal site1. Ligands can be used to regulate both access of the reactant to the active site and the electronic properties of the site1. Ligand effects that influence the electronic properties of catalytic sites do exist in heterogeneous catalysts with extended surfaces; for example, near subsurface dopants can affect the electronic structure of active sites2. However, optimization via these ligand effects is limited by the ability to prepare properly doped materials. Similarly, control over reactant access is generally achieved with microporous supports, but does not offer the degree of modulation afforded by organic ligands in molecular catalysis. Recently, self-assembled monolayers (SAMs) have been used as heterogeneous catalyst modifiers for supported metal catalysts3–7. Deposition of SAMs, which involves a robust selflimiting process that is highly tolerant to variability of process conditions, provides a way to prepare an ordered arrangement of ligands on solid surfaces, offering the potential for introducing ligand design to heterogeneous catalysis. In fact, similar ligands are already widely used to control the electronic structure of semiconductor surfaces for non-catalytic applications8– 12 . In several cases, SAM modifiers have imparted improved
selectivity for reactions such as selective hydrogenation or deoxygenation3–5,13. These effects have generally been attributed to selective site blocking, which lowers the rate of undesired side reactions, or to non-covalent interactions between the ligands and the reacting species, which bias reactant adsorption or transition state structures toward particular geometries14. Until now, however, there has been no indication that SAM ligands can be rationally tuned to optimize the electronic structure and resulting catalytic properties of the active site. Here, we report the use of ligand design to significantly improve the performance of anatase TiO2 catalysts for the dehydration of alcohols, a fundamental reaction in organic chemistry, and one with a recent resurgence of interest as a key class of reactions for the production of fuels and chemicals from biomass derivatives15. The TiO2 catalysts were modified with various organophosphonate SAMs (Scheme 1). The modified catalysts exhibited improved performance both through restriction of reactant access to non-selective sites and via electronic regulation of the active site for deoxygenation. Importantly, these improvements led to a significantly higher rate of formation of the desired dehydration product (by nearly an order of magnitude), despite the presence of organophosponate ligands that might be expected to block reactive sites. The ability to precisely tune the electronic properties of the organophosphonic acid ligand — in this case, by changing the substituents on tethered benzyl groups (Scheme 1) — provides a new route for engineering high-performance heterogeneous catalysts. Moreover, the results reported here represent the
Scheme 1. Self assembled monolayers (SAMs) used in this study and their calculated dipole moments (free molecule). 4-AmPA = 4aminobenzylphosphonic acid, 2,6-FBPA = 2.6-difluorobenzylphosphonic acid, BPA = benzylphosphonic acid, 4-FBPA = 4fluorobenzylphosphonic acid, 4-BrBPA = 4-bromobenzylphosphonic acid, 3-FBPA = 3-fluorobenzylphosphonic acid, and 2,3,4,5,6pentafluorobenzylphosphonic acid. The sign convention for dipole moment vector is positive: toward the surface and negative: away the surface. ACS Paragon Plus Environment
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control of catalyst selectivity via SAM deposition on a new class of heterogeneous catalysts, metal oxides. 2. EXPERIMENTAL METHODS 2.1 Catalyst Synthesis. Anatase-phase TiO2 powder (≥99% metals basis, ~325 mesh, Sigma Aldrich) was functionalized with: benzylphosphonic acid (≥96.5%, Sigma Aldrich), 2,6difluorobenzylphosphonic acid (unspecified, Sigma Aldrich), 3fluorobenzylphosphonic acid (unspecified, Sigma Aldrich), 4fluorobenzylphosphonic acid (≥98.5%, Sigma Aldrich), 4bromobenzylphosphonic acid (≥96.5%, Sigma Aldrich), 4aminobenzylphosphonic acid (≥95%, Sigma Aldrich), or 2,3,4,5,6pentafluorobenzylphosphonic acid (≥96.5%, Sigma Aldrich), by immersing the catalyst into a 10mM solution of the phosphonic acid in tetrahydrofuran (≥99.9% anhydrous, Sigma Aldrich) and allowed to mix overnight. The solids were then removed from solution via centrifugation and annealed at 120°C for 6 hours. After cooling to room temperature, the powders were extensively rinsed with tetrahydrofuran to remove any physisorbed phosphonic acids. 2.2 Catalyst Characterization. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) was used to characterize the SAM-coated catalysts. Temperature programmed desorption (TPD) was performed in a custom packed bed reactor with a Pfeiffer Prisma QME 200 quadrupole mass spectrometer. Approximately 120 mg of functionalized catalyst was placed in a 0.5” OD quartz tube between two 125 mg sections of quartz wool. With a helium carrier flow rate of 20 sccm, catalysts were pretreated at 120°C for 30 minutes, follow by a temperature ramp of 20°C min-1 up to 620°C. Phosphorous content was measured by inductively coupled plasma – optical emission spectrometry from a modified technique developed by Farrell, Matthes and Mackie16. Samples were dissolved in a 7:3:4 mixture of hydrochloric acid, hydrofluoric acid and nitric acid then digested at 95° C for 2 hours. After cooling, the mixture was diluted with 1.5% (g⋅g-1) boric acid solution and reheated to 95° C for about 15 minutes. Samples were analyzed with an ARL 3410+ inductively coupled optical emission spectrometer (ICP-OES). 2.3 Catalyst Performance. Reactions were performed in a custom packed bed, vapor-phase, reactor. Approximately 60-120mg of catalyst was placed in a 0.5” OD pyrex tube reactor. 1-Propanol (≥99.9%, HPLC grade, Sigma Aldrich), 1-butanol (≥99%, for molecular biology, Sigma Aldrich), 1-butan-d9-ol (99%, Sigma Aldrich), or 4-fluoro1-butanol (≥98.0%, Carbosynth LLC) were delivered to the catalyst by bubbling an inert carrier (helium, ultra high purity, Airgas) through the liquid phase reactant, which was temperature controlled with a water bath. Reactants and products were measured using gas chromatography on an Agilent 7820A with a DB-WAX column (30m x 0.250mm x 0.50µm) and flame-ionization detector (FID). Retention times and peak areas were compared to standard injections to identify products and quantify distributions. In the case of 4-fluoro-1-butanol, the expected products could not be purchased commercially. Instead, the retention time and conversion factor of n-butane and 1-butene were compared with commercially available 4-fluorobutane. The conversion factor of FID area to moles for 4-fluorobutane was scaled using a ratio of the conversion factors for 1-butene and n-butane. We used this scaled conversion factor to provide a conversion of FID-area to moles for 4-fluoro-1-butene. Catalysts deactivated during reaction studies, thus catalyst performance is shown for the same time on stream. Reactions were performed at 250°C, 270°C, 290°C and 310°C. Apparent activation energy studies were performed at differential conversion (< 10%). 2.4 Computational Methods. DFT calculations were performed using periodic plane-wave based calculations with the Vienna ab initio simulation package, VASP.17–20 The generalized gradient approximation (GGA) exchange correlational functional developed by Perdew-Burke-Ernzerhof (PBE) was employed for all systems.21,22
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Dispersion for benzylic SAMs systems was treated using the DFT-D2 correction. To correct for the improper localization of electrons by GGA exchange-correlation functionals a Hubbard U parameter of 3.5 eV was applied to all Ti atoms.23 Core electrons are described by the Projector Augmented-Wave method.24 A plane wave energy cutoff of 520 eV was used. Catalyst surfaces were modeled using a periodic slab with a slab thickness of at least 8 Å, and at least 10 Å of vacuum separating adsorbed species from the periodic images of the slabs. The reacting side of the slab was allowed to fully relax, while the atoms within 2.5 Å of the backside of the slab were constrained to their bulk positions. Simulations were performed on 3x2 or 4x2 supercell expansions of the primitive surface unit cell sampled at the Γ point. All cells were allowed to relax until the maximum force on the atoms was less than 0.02 eV⋅Å-1. Transition states were predicted using a combination of the growing string and dimer methods.25–27
3. RESULTS AND DISCUSSION 3.1 Catalyst Modification and Characterization. Phosphonate self-assembled monolayers (SAMs) have been shown to form on many different materials28,29, including TiO228,30–32. Here anatase TiO2 particles were functionalized with phosphonate monolayers by adsorption of the corresponding phosphonic acid in solution, followed by annealing at 120°C to form a covalently bound layer32,33. We investigated a variety of tail functionalities including alkyl moieties: methyl (MPA), decyl (DPA) and octadecyl (ODPA); and benzylic moieties: benzyl (BPA), 4-aminobenzyl (4-AmBPA), 2,6-difluorobenzyl (2,6-FBPA), 3-fluorobenzyl (3-FBPA), 4-fluorobenzyl (4FBPA), 4-bromobenzyl (4-BrBPA), and 2,3,4,5,6pentafluorobenzyl (2,3,4,5,6-FBPA). After synthesis, these materials were characterized with diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) to identify vibrational modes associated with the tail functionality (supporting information, Figure S1). DRIFTS results demonstrated aromatic C-H stretching (νC-H) in the 3000-3100 cm-1 region as well as methylene stretches (νCH2) in the ~2900 cm-1 region. Depending on the SAM, other characteristic vibrational modes, such as νC=C and νC-F stretches, were evident. No νP-OH vibrations were detected, suggesting that phosphonic acid deposition in the monolayer regime. Inductively coupled plasma – optical emission spectrometry (ICP-OES) was used to determine phosphorous coverage on the catalyst (supporting information, Figure S2). All the benzylic phosphonate SAMs exhibited similar coverages, where molecule grafting densities were determined to be ~3 molecules-nm-2. This coverage agrees with quantum mechanical models (see below) and is similar to previous reports of phoshonate monolayers deposited on anatase TiO231. The phosphonate coatings did not appear to have a large effect on the structure of the TiO2 catalysts; as shown in Table S1, changes in total surface area were negligible. 3.2 Catalytic performance. Functionalization of anatase with phosphonic acids was found to strongly promote catalyst performance for alcohol dehydration. Catalysts were evaluated in a packed bed tubular vapor-phase reactor operated at 250°C. The dominant pathway for 1-propanol on native (unmodified) TiO2 was dehydrogenation to propanal, with a selectivity of 61.6%; dehydration to propene was a minor pathway with a selectivity of 18.5% (Fig. 1a). Dehydration and dehydrogenation of alcohols on TiO2 have previously been reported34–37. Interestingly, no matter the tail functionality, alkyl (supporting information, Figure S3) or benzylic, reactions of
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1-propanol over functionalized TiO2 resulted in high dehydration selectivity (Fig. 1b). The low dehydrogenation selectivity across a range of phosphonic acid modifiers suggested that the phosphonate head group served to selectively poison dehydrogenation. Though the reason for high selectivity to dehydrogenation over dehydration in metal oxides is not well understood38, the mechanism is thought to require a hydride transfer from the alpha-carbon to a nearby hydride acceptor, such as a metal atom39 or hydroxyl35,40. In other words, two sites must be free and in close proximity, a condition that is hypothetically interrupted by the presence of phosphonate adsorbates. Furthermore, modification with SAMs led to increased dehydration rates, suggesting that functionalization of the surface resulted in the formation of sites that were more reactive toward dehydration. Intriguingly, the extent of the rate enhancement was found to be sensitive to the structure of the SAM, as shown by the variations in Fig. 1c. To understand the role of substituents on the organophosphonate ligands, it is particularly useful to focus on the subset of catalysts that were modified with benzylic phosphonic acids having different substitution patterns. Despite relatively subtle differences in the substitution on the ring, significant changes in dehydration rate were observed (Fig. 1c). Moreover, because the coverage of all benzylic-substituted SAMs demonstrated minimal variability (supporting information, Figure S2), the rate differences among these materials could be attributed to the differing molecular characteristics of the various phosphonate ligands. Phosphonate SAMs with different ligands have been used to tune the properties of indium tin oxide and zinc oxide for use in electronic applications8–11. This prior work suggested that the dipole moment of the tail functionality was correlated with the work function of the modified semiconductor materials. We hypothesized that the dehydration activity of the SAMmodified catalysts was related to the electronic structure of the phosphonate ligand. Using DFT calculations, we performed a Bader charge analysis41 of the free phosphonic acids in vacuum, which revealed that the charge distribution around the benzyl ring varied significantly with the substitution pattern (supporting information, Table S2). Changes in the atomic charges of the phosphonic acid functional group, however, were subtle and did not correlate with reaction performance. To capture the differences in charge distribution of the phosphonic acids, we computed molecular dipole moments for the free phosphonic acids (Scheme 1). The phosphonic acid dipole moments were strongly correlated with the dehydration rate of the correspondingly modified catalyst (Fig 2a.), suggesting that the distribution of charge in the modifier played a strong role in catalytic performance. Importantly, the modifiers did not adversely affect catalyst lifetime; the rate of catalyst deactivation was similar for unmodified and SAMmodified catalysts (supporting information, Figure S4). Moreover, the SAMs were found to be stable both under reaction conditions and even at much higher temperatures, suggesting broad applicability of this modification approach (supporting information, Figure S1 & Figure S5). We used DFT calculations to evaluate how phosphonic acid modification influenced the TiO2 surface, using the most stable (101) facet of anatase as a model42. The calculations suggested that a tridentate binding mode, in which the phosphonate was triply coordinated to the surface, was the most stable adsorption configuration (supporting information, Figure S6),
Figure 1. Reaction performance for TiO2 functionalized with phosphonates. Reaction pathway (a.), selectivity (b.) and activity (c.) of 1-propanol dehydration and dehydrogenation at 250°C.
consistent with previous studies of phosphonate ligands on other surfaces11,43 and with NMR and vibrational spectroscopy investigations on TiO233,44,45. We note, however, that binding via monodentate46 and bidentate47 configurations cannot be ruled out entirely, and in fact a temperature and coverage-dependent mixture of multiple adsorption states is possible.47,48
To determine whether binding on the surface significantly changed charge distributions, we calculated the atomic charges and the surface-normal component of the dipole moment for adsorbed benzylic phosphonates on the (101) surface. These phosphonate SAM dipole moments (supporting information, Table S2), like those for the free phosphonic acid molecules, were strongly correlated with the corresponding dehydration rates of 1-propanol. The charges of phosphorous and oxygen atoms in the bonding moiety near the surface (supporting information, Table S2) remained nearly constant and did not correlate with activity trends, suggesting that the surface charges of titanium and oxygen were not sensitive to substituent groups on the benzylic SAMs, and thus did not play a major role in dictating the reactivity differences among the benzylic SAMs. Interestingly, 1-propanol adsorption was calculated to be stabilized at sites adjacent to adsorbed phosphonates for various types of ligands; whereas the propanol adsorption energy was computed to be -88 kJ⋅mol-1 on native TiO2, it was computed to range from -116 kJ⋅mol-1 to -125
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kJ⋅mol-1 when propanol was bound adjacent to phosphonates. The opposite trend was determined for increased surface concentration of 1-propanol on a native surface. Adding 1propanol adjacent to another reactant destabilized the adsorption by 10 kJ⋅mol-1. The stabilization by the presences of the phosphonate suggests the formation of adsorption (and reaction) sites on phosphonate-coated surfaces with modified structure and reactivity, consistent with the observation of large changes in dehydration rates and selectivities across all phosphonates used for surface modification.
Figure 2. Catalyst performance vs the dipole moment of the SAM of free molecule, a) 1-butanol, 1-propanol, and 4-fluoro-1-butanol dehydration rate at 250 °C, b) kinetic isotope effect of 1-butanol (rate constant kH) and 1-butan-D9-ol (rate constant kD).
3.3 Catalytic Mechanism. We performed a series of kinetic studies to better understand the mechanism by which dehydration activity was controlled by the SAMs. Although reaction orders with respect to 1-propanol and the main byproduct of dehydration (water) did not change within experimental error (supporting information, Table S3), the benzylic phosphonic acids caused a large decrease in the dehydration activation energy compared to the unmodified surface (supporting information, Table S3). The decrease in apparent activation barrier for all benzylic SAMs again suggests that the phosphonate modifiers produce sites with enhanced dehydration activity. Moreover, changing the ligands on those sites provides a further level of surface reactivity control. We used DFT to determine whether the presence of a surface dipole layer could account for the variations in rate observed in Fig. 1(a). Primary and secondary alcohols have pre-
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viously been found to proceed through a concerted E2-like mechanism49,50 that requires hydroxyl transfer from the alcohol to the surface and abstraction of H from the beta carbon in the transition state. One recent computational study on TiO2 clusters found that the propanol adsorption energy was -73 kJ/mol, and the dehydration barrier was 163 kJ/mol50; in our own calculations on an extended surface, we find similar values of -88 kJ/mol and 162 kJ/mol, respectively. To approximate the dipole layer, homogeneous electric fields with strengths ranging up to -1.0 V⋅Å-1 were applied normal to a TiO2 anatase (101) surface. Simple calculations of the effective field strength imposed by a homogeneous dipole layer of phosphonates (Table S4) suggest that this approximate range should provide an overall variation in field strength similar to that resulting from changes in phosphonate ligands. Previous studies have shown that adsorbates with significant dipole moments can influence surface chemistry by dipole-dipole interactions that can be understood in terms of a near-surface electric field.51–53 As shown in Figure 3, a negative electric field (towards the surface) reduced the dehydration reaction barrier. As the field increased in strength from -0.25 V⋅Å-1 to -1.0 V⋅Å-1, the computed dehydration reaction barrier decreased by 7.2 kJ⋅mol-1. These results are in line with experimentally measured trends for dehydration rates over benzylic phosphonate-modified TiO2. Specifically, creating a near-surface charge distribution associated with a negative electric field enhanced the rate of dehydration by stabilizing the transition state for the E2 mechanism relative to the reactant state and transition state. One key observation was that application of a more negative electric field lengthened the C-H bond and shortened the OsurfH bond of the dehydration transition state, shifting the transition state toward the products, i.e., “later” in the reaction coordinate. This change in transition state structure is expected for a positively charged proton displaced in the direction of the applied field. To experimentally probe the lateness of the transition state, we measured the primary kinetic isotope effect (PKIE) for dehydration of butanol, following a similar approach to studies utilizing alumina catalysts 49,54,55. Here, we compared the dehydration of 1-butanol (C4H9OH) and 1butan-d9-ol (C4D9OH). As the dipole strength of the tail increased, the PKIE also increased (Fig. 2b). Because previous work has shown that larger PKIEs in the E2 mechanism are associated with later transition states56, these results again suggested that creating a polarized near-surface environment alters the transition state structure consistent with the DFT calculated transition state56. In short, the dipole moment of the phosphonate appears to be correlated with the transition state geometry. We tested the generality of these effects by varying the identity of the reactant in experimental studies. We compared the dehydration activity for 1-propanol with 1-butanol and 4fluoro-1-butanol. Modification with phosphonate SAMs having more negative dipole moments improved the rates for all dehydration reactions. Interestingly, the degree of improvement depended on the reactant structure, with 1-butanol dehydration rates being more sensitive to the choice of SAM than for 1-propanol (supporting information, Table S3). Although the DFT calculations provide a general picture of the relevant surface chemistry, it should be acknowledged that the precise structure the key sites in TiO2-catalyzed alcohol dehydration is still unknown. Prior computational work (as well as the results reported here) have found that the barrier to
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desorption of adsorbed propanol is well below the dehydration barrier50. However, temperature programmed desorption (TPD) experiments show significant high-temperature production of alkene products from adsorbed alcohols37,57,58. This suggests the presence of a population of more strongly-bound alcohols or alcohol-derived intermediates, perhaps at defect sites or oxygen vacancies, which has not been captured by computational models36,57–59. Interestingly, dehydration does not appear to be highly structure sensitive: comparisons of the performance of rutile and anatase have shown similar product distributions.37 The presence of phosphonic acid selfassembled monolayers undoubtedly changes the active site environment, as indicated by the “step change” in dehydration rates compared to the native surface seen in Figure 1. Modification with phosphonates introduces new, P-coordinated O atoms into the surface layer; these atoms can themselves be involved as catalytic sites, and can also influence the reactivity of neighboring Ti centers. Resolving the precise nature of the active site will likely require study on model TiO2 surfaces, which is beyond the scope of the present work.
electrostatic interactions in the near-surface region, i.e., in the vicinity of the catalytic site. Favorable electrostatic interactions can be engineered in a largely predictable way by positioning electron-withdrawing or –donating groups around a tethered benzyl ring. These ligand effects are analogous to ligand effects observed for molecular catalysts, where the ligand can both provide desirable changes to the catalytic properties of a metal center and control over interactions remote from that center that direct reactant binding and conversion60,61. We note that more work is needed to more fully understand the nature of the phosphonate-reactant interaction, which is undoubtedly complex. For example, whereas measured apparent activation energies were found to generally decrease with increasing dipole moments, the SAM with the completely fluorinated benzyl ring was an outlier from that trend (supporting information, Figure S7). The descriptor used here (molecular dipole moment) appears to correlate well with dehydration activity trends, but the detailed mechanism for activity promotion could in fact be related to local reactantSAM interactions that control reaction and transport near the surface. 4. CONCLUSIONS We demonstrated that anatase phase TiO2 can be functionalized with thermally stable phosphonate monolayers, and that these layers allow for tunable activity through variation of the tail functionality. The combined experimental and computational results are consistent with a picture in which control of dehydration activity is due to changes in the near surface electrostatics induced by SAM adsorbates. The use of SAMs with tunable ligand structures provides a new platform for the precise control over catalyst activity and selectivity that may allow the design of solid catalysts to more closely approach the specificities associated with molecular complexes.
ASSOCIATED CONTENT Supporting Information
Figure 3. DFT transition state calculations. a) Dehydration reaction barrier as a function of the applied electric field strength applied to a (101) surface b) transition state bond lengths as a function of an applied electric field, and c) transition state bond orders as a function of applied electric field. The geometry of the d) adsorbed state and e) transition state as a function of the applied field. The sign convention of the applied electric field vector is negative for fields oriented toward the surface.
Tables and figures giving infrared spectroscopy, ICP-OES analysis, reaction performance of alkyl phosphonate functionalized catalysts, time on stream reaction performance, temperature programmed desorption of phosphonate functionalized catalysts, computational analysis of binding modes, apparent activation barriers for dehydration, summary of Bader charge and dipole moments of SAMs and reaction order studies. The Supporting Information is available free of charge on the ACS Publications website.
AUTHOR INFORMATION Corresponding Author *
[email protected] Author Contributions Although the precise structure of the active site remains to be resolved, the results reported here provide a clearer picture of how phosphonate ligands tethered to the surface can be tuned to influence the reaction. We suggest that changing the substituent groups on benzylic phosphonate SAMs affects the reaction chemistry of primary alcohols on TiO2 by tuning the
L.D.E. was responsible for all experimental findings, dipole moment calculations & bader analysis; R.M.T. was responsible for the phosphonate binding configuration study, density of state calculations, and all transition state DFT calculations; L.D.E., R.M.T., C.B.M., D.K.S., J.W.M. wrote the manuscript; and C.B.M., D.K.S., and J.W.M. supervised the project.
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This work was supported by the Department of Energy, Office of Science, Basic Energy Sciences Program, Chemical Sciences, Geosciences and the Biosciences Division, under Grant No. DEFG02-10-ER16206. L.D.E. received partial support from the United States Department of Agriculture, National Institute of Food and Agriculture, predoctoral fellowship program.
Notes The authors declare no competing financial interest.
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ACKNOWLEDGMENT The authors would like to thank Dr. Fred Luiszer for his assistance with ICP-OES measurements, Dr. Svitlana Pylypenko & Michael Dzara for their assistance, and Samantha Millican for her assistance with preliminary density functional calculations.
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ABBREVIATIONS
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SAMs, self-assembled monolayers; TiO2: anatase titania; MPA, methylphosphonic acid; DPA, decylphosphonic acid; ODPA, octadecylphosphonic acid; 4-AmBPA, 4-aminobenzylphosphonic acid; 2,6-FBPA, 2,6-fluorobenzylphosphonic acid; BPA, benzylphosphonic acid; 4-FBPA, 4-fluorobenzylphosphonic acid; 4BrBPA, 4-bromobenzylphosphonic acid; 3-FBPA, 3fluorobenzylphosphonic acid; 2,3,4,5,6-FBPA, 2,3,4,5,6pentafluorobenzylphosphonic acid.
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SYNOPSIS TOC (Word Style “SN_Synopsis_TOC”).
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