Effect of Methanol on the Lewis Acidity of Rutile TiO2 Nanoparticles

pubs.acs.org/Langmuir © 2010 American Chemical Society

Effect of Methanol on the Lewis Acidity of Rutile TiO2 Nanoparticles Probed through Vibrational Spectroscopy of Coadsorbed CO Dimitar A. Panayotov,† Steven Burrows,† Mihail Mihaylov,‡ Konstantin Hadjiivanov,‡ Brian M. Tissue,† and John R. Morris*,† †

Department of Chemistry, Virginia Tech, Blacksburg, Virginia 24061, and ‡Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences, Sofia 1113, Bulgaria Received September 3, 2009. Revised Manuscript Received April 27, 2010

Infrared spectroscopy of adsorbed CO has been used to characterize the effect of adsorbed methanol on the Lewis acidity of 4 nm rutile TiO2 nanoparticles. Measurements of CO absorbance and vibrational frequency have revealed that CO adsorbs primarily at one class of Lewis acid sites on clean TiO2 particles, where evidence for lateral interactions between neighboring molecules suggests dense coverage occurs near saturation. The response of the CO infrared intensities and frequencies to methanol exposure has shown that methanol uptake occurs primarily at the Lewis acid sites and through hydrogen bonding to surface OH groups. These surface sites appear to be responsible for driving both molecular and dissociative adsorption of methanol on the titania. Most importantly, these studies have revealed that the parent methanol and associated methoxy products lower the Lewis acidity of neighboring sites on TiO2 nanoparticles.

1. Introduction Nanoparticulate TiO2 is widely studied for potential use as a reactive sorbent in the abatement of deleterious organic species,1,2 including chemical warfare agents (CWAs). A number of recent studies have demonstrated that TiO2 is an effective sorbent for dimethyl methylphosphonate (DMMP),3-7 the most common mimic for the CWA, Sarin. The high activity of TiO2 toward the uptake and decomposition of DMMP appears to be due, in part, to the presence of strong Lewis acid sites on the surface of the particles.4,7 The electron-rich phosphoryl oxygen of DMMP has been shown to react with titania by donating a lone pair of electrons to coordinatively unsaturated surface sites (CUS) Tinþ (n=4, 3).4,5 However, there are few fundamental studies that have directly explored the response of unsaturated titania surface sites to the adsorption of DMMP or its decomposition products, including methanol, carbon monoxide, and carbonyl-containing species. Here, we present a detailed study into the uptake of methanol on well-characterized rutile TiO2 nanoparticles to provide new insight into the surface sites responsible for dissociative adsorption and how the binding of this molecule affects the Lewis acidity of the titania surface. *Corresponding author: Ph 540-231-2472; e-mail [email protected].

(1) Linsebigler, A. L.; Lu, G. Q.; Yates, J. T. Chem. Rev. 1995, 95, 735. (2) Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W. Chem. Rev. 1995, 95, 69. (3) Aurian-Blajeni, B.; Boucher, M. M. Langmuir 1989, 5, 170. (4) Rusu, C. N.; Yates, J. T. J. Phys. Chem. B 2000, 104, 12292. (5) Kim, C. S.; Lad, R. J.; Tripp, C. P. Sens. Actuators, B 2001, 76, 442. (6) Trubitsyn, D. A.; Vorontsov, A. V. J. Phys. Chem. B 2005, 109, 21884. (7) Panayotov, D. A.; Morris, J. R. Langmuir 2009, 25, 3652. (8) Suda, Y.; Morimoto, T.; Nagao, M. Langmuir 1987, 3, 99. (9) Hussein, G. A. M.; Sheppard, N.; Zaki, M. I.; Fahim, R. B. J. Chem. Soc., Faraday Trans. 1991, 87, 2655. (10) Henderson, M. A.; Otero-Tapia, S.; Castro, M. E. Faraday Discuss. 1999, 114, 313. (11) Wu, W. C.; Chuang, C. C.; Lin, J. L. J. Phys. Chem. B 2000, 104, 8719. (12) Herman, G. S.; Dohnalek, Z.; Ruzycki, N.; Diebold, U. J. Phys. Chem. B 2003, 107, 2788. (13) Farfan-Arribas, E.; Madix, R. J. Surf. Sci. 2003, 544, 241. (14) Onda, K.; Li, B.; Zhao, J.; Petek, H. Surf. Sci. 2005, 593, 32. (15) Pang, C. L.; Lindsay, R.; Thornton, G. Chem. Soc. Rev. 2008, 37, 2328.

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Despite the lack of information about how the Lewis acidity of particles changes in response to methanol adsorption, several experimental8-17 and theoretical18-22 studies have focused on other aspects of methanol-TiO2 surface chemistry. Most relevant to the work presented below, methanol has been used as a probe molecule to characterize anion vacancies on TiO2.8 In addition, methoxy species, formed via dissociative adsorption of methanol, have been shown to bond via its oxygen atom to one, two, or more metal cations.23 Other studies24 have explored how methanol uptake differs on the surface of anatase and rutile crystal modifications of TiO2. Highly acidic sites on the surface of anatase are suggested to be responsible for molecular adsorption of methanol.24 In contrast, methanol uptake may be driven primarily by dissociative adsorption on rutile to produce surface-adsorbed methoxy groups.24 Notwithstanding, other researchers10,13,25,18-21 have demonstrated that both molecular and dissociative forms of methanol exist on many different types of TiO2. The work described below furthers the overall understanding of methanol chemistry on titania nanoparticles by using the probe molecule CO to reveal how the Lewis acidity of the particles changes upon methanol binding. The vibrational frequency of adsorbed carbon monoxide, a soft Lewis base, is highly sensitive to the electronic properties of the site to which it is bound. Therefore, infrared spectroscopy can be used to provide insight into the electron density of titania surface sites.26 Previous studies with CO adsorption (16) Taylor, E. A.; Griffin, G. L. J. Phys. Chem. 1988, 92, 477. (17) Wang, L.-Q.; Ferris, K. F.; Winokur, J. P.; Shultz, A. N.; Baer, D. R.; Engelhard, M. H. J. Vac. Sci. Technol., A 1998, 16, 3034. (18) S
anchez de Armas, R.; Oviedo, J.; San Miguel, M. A.; Sanz, J. F. J. Phys. Chem. C 2007, 111, 10023. (19) Bates, S. P.; Gillan, M. J.; Kresse, G. J. Phys. Chem. B 1998, 102, 2017. (20) Tilocca, A.; Selloni, A. J. Phys. Chem. B 2004, 108, 19314. (21) Oviedo, J.; S
anchez-de-Armas, R.; San Miguel, M. A.; Sanz, J. F. J. Phys. Chem. C 2008, 112, 17737. (22) Kieu, L.; Boyd, P.; Idriss, H. J. Mol. Catal. A: Chem. 2002, 188, 153. (23) Badri, A.; Binet, C.; Lavalley, J.-C. J. Chem. Soc., Faraday Trans. 1997, 93, 1159. (24) Ramis, G.; Busca, G.; Lorenzelli, V. J. Chem. Soc., Faraday Trans. 1 1987, 83, 1591. (25) Farfan-Arribas, E.; Madix, R. J. J. Phys. Chem. B 2002, 106, 10680. (26) Bolis, V.; Fubini, B.; Garrone, E.; Morterra, C.; Ugliengo, P. J. Chem. Soc., Faraday Trans. 1992, 88, 391.

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revealed the structural heterogeneity of anatase TiO2 by showing that at least three different classes of Ti4þ Lewis acid sites exist on the surface.26-34 The highest energy CO binding sites arise from metal carbonyl complexes to produce an infrared adsorption band at 2208-2205 cm-1. These sites are associated with defects or nontypical crystal planes and are usually present at low concentration.33,34 The most abundant sites appear to be due to the formation of metal carbonyl complexes that produce an infrared absorption band near 2190 cm-1. This CO band shifts toward lower frequency with increasing CO coverage, reaching 2175 cm-1 at saturation.33,34 Finally, research has shown that a CO band near 2165 cm-1 is indicative of the existence of a third site characterized as a weak Ti4þ Lewis acid.33,34 In contrast to the body of work on anatase, studies of CO adsorption on rutile surfaces are less common. Of the few investigations that exist, a computational study suggests rutile Ti4þ Lewis acid sites from the (110) plane of titania are slightly more electrophilic than the anatase (101) acid sites.35 Several other authors reported one type of Lewis acid sites on rutile, with the corresponding metal carbonyl band observed in the 21952178 cm-1 region.26,36-39 At ambient temperature, the band undergoes a red-shift by 2-13 cm-1 with increasing CO coverage.26,36At low temperatures, the adsorption of CO on “oxidized” rutile leads to the formation of a band at 2192-2178 cm-1, which red-shifts by 4-10 cm-1 with increasing coverage.40,37,39 In contrast, others have observed no CO adsorption at room temperature and only a negligible metal carbonyl band around 2170 cm-1 at low temperature.41 Overall, these studies highlight the complexity of TiO2 surface chemistry and demonstrate the utility of CO as a probe molecule for characterizing the electronic nature of various surface sites. In the work described below, we employ infrared spectroscopy of adsorbed CO to characterize the Lewis acidity of 4 nm rutile TiO2 particles and to map how the acidity changes upon uptake of methanol. For this work, the particles were synthesized by using a one-step laser evaporation and condensation method to provide clean particles free from solvent or other potential contaminants, and the uptake studies were performed in a clean vacuum cell. The results from this work demonstrate the importance of surface OH groups for driving molecular adsorption, and highly active Lewis acid sites, which appear to affect dissociative adsorption to form surface methoxy groups. Interestingly, the methanol adsorbates are found to have a significant effect on the Lewis acidity of adjacent surface sites on the TiO2 nanoparticles. These studies are aimed at providing a more complete understanding of how surface adsorbates affect the character of Lewis acid sites, i.e., coordinatively unsaturated (Ti4þ) surface sites, which provides new insight into this important aspect of titania surface chemistry. (27) Primet, M.; Pichat, P.; Mathieu, M.-V. J. Phys. Chem. 1971, 75, 1221. (28) Busca, G.; Saussey, H.; Saur, O.; Lavalley, J. C.; Lorenzelli, V. Appl. Catal. 1985, 14, 245. (29) Morterra, C. J. Chem. Soc., Faraday Trans. 1 1988, 84, 1617. (30) Hadjiivanov, K.; Davydov, A.; Klissurski, D. Kinet. Katal. 1988, 29, 161. (31) Hadjiivanov, K.; Klissurski, D.; Kantcheva, M.; Davydov, A. J. Chem. Soc., Faraday Trans. 1991, 87, 907. (32) Hadjiivanov, K.; Saur, O.; Lamotte, J.; Lavalley, J. C. Z. Phys. Chem. (Muenchen, Ger.) 1994, 187, 245. (33) Hadjiivanov, K.; Lamotte, J.; Lavalley, J.-C. Langmuir 1997, 13, 3374. (34) Hadjiivanov, K. I.; Klissurski, D. G. Chem. Soc. Rev. 1996, 25, 61. (35) Scaranto, J.; Giorgianni, S. J. Mol. Struct. (THEOCHEM) 2008, 858, 72. (36) Yates, D. J. C. J. Phys. Chem. 1961, 65, 746. (37) Zaki, M. I.; Kn€ozinger, H. Spectrochim. Acta, Part A 1987, 43, 1455. (38) Waqif, M.; Bachelier, J.; Saur, O.; Lavalley, J.-C. J. Mol. Catal. 1992, 72, 127. (39) Hadjiivanov, K. Appl. Surf. Sci. 1998, 135, 331. (40) Ferretto, L.; Glisenti, A. Chem. Mater. 2003, 15, 1181. (41) Su, W.; Zhang, J.; Feng, Z.; Chen, T.; Ying, P.; Li, C. J. Phys. Chem. C 2008, 112, 7710.

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2. Experimental Section 2.1. Material Preparation. The method for synthesizing the rutile TiO2 nanoparticles has been described previously.42-44 Briefly, a 50 W CO2 laser is focused onto a metal oxide target to produce a vapor of material in a buffer gas. Nanoparticles condense due to cooling by the buffer gas and a plume of material deposits onto a collection substrate. The resulting particle size depends on the specific material, the buffer gas pressure, and the target-to-substrate distance. The resulting size is reproducible for constant conditions. In this study, we use a vacuum chamber that can by evacuated to a background pressure of 0.05 Torr and then backfilled by O2 to pressures of 1-500 Torr. The target was a commercially available sintered pellet of reduced TiO2 (CERAC Inc.), and the collection substrate was a stainless steel hemisphere 4.4 cm from the target. 2.2. Material Characterization. One hour of laser vaporization produced a flaky film of approximately 10-15 mg of nanoparticles, which scraped easily from the collection substrate. The nanoparticulate TiO2 was stored in a laboratory desiccator, and there was no sign of moisture uptake before material characterization. Morphology and particle size characterization were accomplished with transmission electron microscopy and microscale powder X-ray diffraction. The transmission electron microscopy (TEM) images were taken with a Zeiss 10CA instrument with a high-resolution imaging system (AMT Advantage GR/HR-B CCD). Approximately 1 mg of material was dispersed in 5 mL of HPLC grade ethyl alcohol, and this suspension was sonicated for 30 min in an ultrasonic water bath. Immediately following ultrasonic dispersal, one drop of the alcohol suspension was applied to a Formvar/Cu mesh electron microscopy grid (Electron Microscopy Sciences), and the ethyl alcohol was allowed to evaporate. Each sample was examined by TEM at magnifications of 50000-200000, and the images were stored in electronic format. Each electron micrograph was subsequently retrieved with image processing software (ImageJ, National Institutes of Health), and 50-100 nanoparticle diameters were measured manually. The average particle diameter was calculated from measurements from several micrographs. High-resolution electron micrographs were obtained with a FEI Titan 300 scanning transmission electron microscope (STEM). Samples were dispersed as described above and deposited on a lacey carbon support, and images were obtained at an operating voltage of 200 kV. Powder X-ray diffraction characterization was performed on a Gemini Ultra series diffractometer (Oxford Diffraction, Inc.). Nanoparticle samples were loaded and sealed into 0.3 mm o.d. X-ray diffraction glass capillary sample tubes (Charles Supper Co.), which were subsequently mounted on the diffractometer goniometer stage. Diffraction patterns were collected at ambient temperature with Cu KR radiation at 1.54 A˚ from 5° to 88° 2θ. Pattern collection and processing was performed with CrysAlis Pro software (Oxford Diffraction, Inc.). 2.3. FTIR Spectroscopy. The nanoparticulate TiO2 powder was pressed as a 0.7 cm diameter spot on a tungsten grid of 80% transmission with a hydraulic press. Ni support clamps hold the grid containing the sample in a stainless steel ultrahigh-vacuum transmission IR cell described previously.45,46 The IR cell is equipped with KBr spectroscopic windows sealed by differentially pumped Viton O-ring seals. The cell is connected to a stainless steel ultrahigh vacuum system pumped down to 1  10-8 Torr background pressure by both an ion pump and a turbomolecular pump. Gas pressure in the system is measured with a capacitance (42) Tissue, B. M. Chem. Mater. 1998, 10, 2837. (43) Gordon, W. O.; Tissue, B. M.; Morris, J. R. J. Phys. Chem. C 2007, 111, 3233. (44) Gordon, W.; Morris, J.; Tissue, B. J. Mater. Sci. 2009, 44, 4286. (45) Basu, P.; Ballinger, T. H.; Yates, J. T. Rev. Sci. Instrum. 1988, 59, 1321. (46) Mawhinney, D. B.; Rossin, J. A.; Gerhart, K.; Yates, J. T. Langmuir 1999, 15, 4617.

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Figure 1. Transmission electron microscopy (TEM) images of (A) the 4 nm rutile TiO2 sample and (B) Degussa P25 titanium oxide. (C) Particle size distribution of as-prepared rutile TiO2 sample made from measurement of 88 particles diameters. Fitting a histogram of the measurements to a log-normal distribution gives the same result. (D) High-resolution transmission electron microscopy (HRTEM) image of the 4 nm rutile TiO2 sample. manometer (Baratron, 0.001-1000 Torr). The TiO2 sample temperature is maintained constant to (1 K by feedback control using a type K thermocouple welded to the top center of the tungsten grid. Transmission IR spectra were acquired in an N2-purged Matteson Research Series I FTIR spectrometer at 4 cm-1 spectral resolution using a liquid-N2 cooled HgCdTe IR detector. Usually 200 or 1000 scans in the mid-IR region (4000-500 cm-1) were acquired for each FTIR spectrum. The FTIR spectra of rutile samples presented in absorption mode (A) are obtained after subtraction of the background spectrum of the tungsten grid measured on the empty upper spot of the grid. The difference FTIR spectra (ΔA vs. wavenumber) are used to follow the changes in the IR spectrum of rutile sample during adsorption of CO and methanol. The difference in the IR absorbance (ΔA) was obtained by subtraction of the initial sample spectrum from each spectrum obtained during the exposure of sample to an adsorbate. TiO2 samples were heated in vacuum (residual pressure of 1  10-8 Torr) to 673 K to remove adsorbed water, H-bonded Ti-OH species, and traces of carbonate-like species prior to adsorption experiments. Traces of hydrocarbon impurities adsorbed during sample preparation were removed by O2 treatment and evacuation at the same temperature, 673 K. To avoid readsorption of desorbing gases and water vapors onto the TiO2 sample, the reentrant Dewar of the IR cell46 was kept at liquid N2 temperature or at 200 K by using a dry ice-acetone mixture. Before adsorption of probe molecules, the samples denoted as “treated-in-vacuum” were treated at 675 K in vacuum for 1 h and cooled to room temperature. Oxidation of the vacuum-treated samples at 673 K by 20 Torr of O2 at the same temperature for 30 min and then evacuation at 473 K for 10 min produce the “oxidized samples”.

3. Results and Discussion We have used infrared spectroscopic measurements to track the adsorption and decomposition pathways for methanol on the surface of nanoparticulate TiO2, with the goal of understanding the fundamental uptake mechanisms and how the adsorbed parent and product moieties affect the properties of the particles. These goals are accomplished by using FTIR to follow the dynamics of the methanol-titania reaction, while simultaneously characterizing the population and strength of Lewis acid sites on the surface by monitoring the intensity and the vibrational frequency of the IR modes of adsorbed CO probe molecules. 3.1. Initial Sample Characterization. Figure 1A shows a TEM image of an as-prepared TiO2 sample synthesized by the 8108 DOI: 10.1021/la100861n

Figure 2. Powder X-ray diffraction patterns of the Degussa P25 titanium oxide sample (top), the 4 nm rutile TiO2 sample (middle), and an empty glass capillary sample tube (bottom). The diffraction patterns are offset vertically for clarity.

inert-gas condensation method in 1 Torr of O2.42,43,47 A TEM image of a commercial Degussa P25 TiO2 sample is presented in Figure 1B for comparison. The two micrographs are shown on the same scale. Figure 1A reveals the typical open, networked morphology of gas-phase condensed nanoparticles.42,43,47 The nanoparticles are difficult to disperse completely, and some interparticle material appears to bridge neighboring particles. Particle diameter measurements from five TEM micrographs indicate that the particles have a mean diameter of 4.1 ( 0.6 nm (Figure 1C). Figure 1D shows the high-resolution transmission electron micrograph (HRTEM) of the same TiO2 sample. The image reveals that the particles are crystalline and confirms the typical particle diameter of ∼4 nm. The powder XRD patterns of commercial Degussa P2548 TiO2 (top) and the as-synthesized TiO2 sample (middle) are presented in Figure 2. Peaks characteristic of the rutile modification of titania are the dominant features of the XRD data for the gas-phase condensed nanoparticles. The broad diffraction peak at ∼25° and the nonzero baseline are attributed to SiO2 of the glass capillary, as verified with a blank diffraction pattern from an empty capillary (see Figure 2). Although the particles are predominantly rutile, (47) Tissue, B. M.; Yuan, H. B. J. Solid State Chem. 2003, 171, 12. (48) In Highly Dispersed Metallic Oxides Produced by Aerosil Process. Degussa Technical Bulletin Pigments; Degussa AG: Frankfurt, Germany, 1990; Vol. 56, p 13.

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Figure 3. Adsorption of CO on clean rutile surface. Panel A: transmission FTIR spectra; (a) clean nanoparticulate rutile TiO2 sample under vacuum (1  10-6 Torr) at 295 K. The as-prepared sample was heated in vacuum from 295 to 675 K at a rate 12 K/min; treated at 675 K for 3 h in vacuum; oxidized in 20 Torr of O2 at 675 K for 30 min; and finally evacuated at 473 K for 10 min; (b) oxidized sample obtained in (a) exposed to 20 Torr of CO at 295 K for 20 min. Inset: ν(OH) region of the FTIR spectra (a) and (b). Panel B: difference spectra; oxidized sample exposed to decreasing pressure of CO at 295 K for 20 min: (a) 20, (b) 15, (c) 10, (d) 5, (e) 2.5, (f) 1, (g) 0.5, (h) 0.1, and (i) 0.01 Torr; (j) dynamic vacuum ∼10-6 Torr; (k) after 10 min evacuation ∼10-6 Torr. Panel C: difference spectra; oxidized sample exposed to decreasing pressure of CO at 165 K for 20 min: (a-j) the same corresponding CO pressures as in panel B; (k) after 30 min evacuation ∼10-6 Torr. Inset: ν(OH) region of the difference spectra (a)-(k).

the presence of a trace amount of anatase cannot be excluded given the small reflection peaks at 25° and 48°. Figure 3A (spectrum a) shows the FTIR spectrum of the assynthesized 4 nm rutile sample following vacuum treatment and oxidation at 675 K. The spectrum indicates that the sample is highly dehydroxylated, and the IR bands at 3718, 3694, and 3673 cm-1 (Figure 3A inset, spectrum a) are attributed to trace amounts of isolated OH groups.40,49 However, our vacuum system contains some background water vapor (P < 1  10-6 Torr), which reacts on the surface to produce a very small amount of associated OH groups. The concentration of adsorbed hydrocarbons is below the detection limit of our spectrometer, which we estimate to be less than 1% of a monolayer. In the low-wavenumber region, weak bands are observed at 1610, 1590-50, 1470, 1360, and 1260 cm-1. The first band is due to traces of adsorbed water; the next three bands are most likely due to a small amount of residual carboxylate-carbonate species.23,39,50-52 The band at 1260 cm-1 is attributed to occluded nitrogen impurities (nitro species). Extensive additional experiments verified that this band is not affected by adsorption of a variety of molecules. 3.2. Adsorption of CO on Clean Rutile TiO2. Carbon monoxide adsorption has been used extensively to probe the surface sites of metal and metal oxide materials. The utility of CO stems from its inert character, small size, and significant absorptivity in the infrared region of the electromagnetic spectrum. Most importantly, the IR spectrum for CO is highly sensitive to interfacial bonding environment, strength, and mechanism.35,53 For the work presented here, we employed IR spectroscopy of adsorbed CO to probe the nature of Lewis acid sites on nanoparticulate TiO2. It is well established that coordinatively unsaturated cations on metal oxides, such as CUS Ti4þ on TiO2, can generate strong electrostatic fields that give rise to large attractive forces for CO (49) Contescu, C.; Popa, V. T.; Schwarz, J. A. J. Colloid Interface Sci. 1996, 180, 149. (50) Binet, C.; Daturi, M.; Lavalley, J.-C. Catal. Today 1999, 50, 207. (51) Martra, G. Appl. Catal., A 2000, 200, 275. (52) Venkov, T.; Fajerwerg, K.; Delannoy, L.; Klimev, H.; Hadjiivanov, K.; Louis, C. Appl. Catal., A 2006, 301, 106. (53) Hadjiivanov, K. I.; Vayssilov, G. N. Adv. Catal. 2002, 47, 307. (54) Casarin, M.; Maccato, C.; Vittadini, A. J. Phys. Chem. B 1998, 102, 10745. (55) Noguera, C.; Finocchi, F.; Goniakowski, J. J. Phys.: Condens. Matter 2004, S2509.

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adsorption due to both electrostatic and polarization effects.53-56 Binding to cationic surface sites stabilizes the 5σ level of CO (centered primarily on the carbon atom) to reduce the antibonding character and increase the overall bond order of the molecule.57 Bonding via the carbon atom to form Ti-CO is preferential as compared to the Ti-OC configuration.56,57 The relatively strong Ti-CO bond (the direct result of adsorption at Lewis acid sites) results in a blue shift of the ν(CO) stretching frequency relative to the gas-phase molecule.26,35,53-56,58 Thus, the infrared spectrum of adsorbed CO can be employed as a sensitive marker of Lewis acid strength, as well as concentration, on nanoparticulate TiO2. Spectrum b in Figure 3A shows the FTIR spectrum of the clean particles upon exposure to 20 Torr of CO at 295 K. The most prominent feature of this spectrum is an intense band at 2193 cm-1, attributed to the fundamental ν(CO) stretching mode.53 This mode is significantly blue-shifted, by 50 cm-1 (with respect to gas-phase CO), indicating the existence of strong Lewis acid sites on the 4 nm rutile nanoparticles. The 50 cm-1 blue-shift strongly suggests that the molecules are adsorbed at CUS Ti4þ cationic sites, as previously observed for both anatase26-34 and rutile26,36-41 TiO2. The difference spectra of Figure 3B,C, which show how the ν(CO) intensity and frequency respond as the coverage decreases, provide insight into the strength and the mechanism of the CO-titania bond formation. The spectrum labeled a in Figure 3B, recorded at the initial pressure of 20 Torr of CO at 295 K, shows a symmetric band with a maximum at 2193 cm-1, a fwhm =16 cm-1, and a very weak shoulder at 2208 cm-1. As the pressure of CO decreased from 20 to 10-6 Torr, a systematic blue shift of the primary ν(CO) frequency from 2193 to 2200 cm-1, with accompanying decrease of the fwhm from 16 to 10 cm-1, was observed. The shoulder at 2208 cm-1 diminished before the primary band, indicating that it corresponds to a weakly bound species. The frequency shift of the ν(CO) band with increasing CO coverage is caused by either dynamic or static lateral interactions between the adsorbed molecules.26,33,53,57 The dynamic effect is due to coupling between the vibrating dipoles of adjacent adsorbates (56) Sorescu, D. C.; Yates, J. T. J. Phys. Chem. B 1998, 102, 4556. (57) Yang, Z.; Wu, R.; Zhang, Q.; Goodman, D. W. Phys. Rev. B 2001, 63, 045419. (58) Garrone, E.; Bolis, V.; Fubini, B.; Morterra, C. Langmuir 1989, 5, 892.

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Figure 4. Methanol adsorption on rutile nanoparticles. Panels A-C: difference spectra; (a) clean rutile sample at 295 K; (b)-(d) incrementally increased exposure (0.1 Torr equilibrium pressure) to methanol followed by evacuation at 295 K.

that results in a shift to higher vibrational frequencies.26,33,53,57,59,60 The static shift may have two origins: direct electrostatic interactions between the molecules and indirect interactions transmitted through the surface, which work to lower the vibrational frequencies of adsorbates. The static shift has been shown to have the most significant effect on CO vibrational frequencies when adsorbed on metal oxides at high coverages.26,33,53,57,59,60 Specifically, recent studies employing 12CO diluted with 13CO on TiO2 established that the dynamic blue-shift for 12CO is þ4 cm-1, whereas the static redshift of the ν(12CO) band is -17 cm-1.33 Therefore, the red-shift for CO adsorbed on TiO2 (Figure 3B) is most likely due to static interactions induced at high coverage. The spectra shown in Figure 3C, obtained at a surface temperature of 165 K, compliments the room temperature data by revealing the adsorption characteristics of more weakly bound CO. At this temperature, near saturation, CO readily adsorbs to Ti4þ Lewis acid sites that span a larger range of binding energies; see spectrum a in Figure 3C (20 Torr of CO).26,28,33,39,53,58-60 The spectrum consists of a nearly symmetric main band with maximum at 2183 cm-1 and fwhm=21 cm-1. As the CO pressure is decreased from 20 to 10-6 Torr, the position of the band at 2183 cm-1 blueshifts to 2198 cm-1 and its fwhm decreases from 21 to 14 cm-1. A weak band at ∼2141 cm-1 also appears, and its intensity simultaneously decreases with the intensity of a band at 3540 cm-1 at decreasing CO pressure, as shown in the inset of Figure 3C. This correlation indicates that the 2141 cm-1 band is due to CO adsorbed on surface hydroxyl groups:33,39,53 as CO is desorbed from these sites, the CO-HO associated hydroxyl groups are converted to free OH groups and produce a rise of ν(OH)free modes at ∼3720 cm-1 and at 3690-3670 cm-1. Finally, the weak shoulder at 2208 cm-1, which is present in the room temperature and 165 K spectra, is the first feature to diminish upon sample evacuation, indicating that it arises from a weakly bound physisorbed COcontaining species. The data presented in Figure 3 for room temperature and lowtemperature CO adsorption suggest the presence of one class of coordinatively unsaturated Ti4þ sites that span a range of binding energies. The ratio of the areas of the main peak (2193 and 2183 cm-1) at the two surface temperatures suggests that, at equilibrium coverage and CO pressure of 20 Torr, only ca. 36% of the Ti4þ sites capable of adsorbing CO at 165 K are occupied at room temperature. In the following sections, the characteristic frequencies (59) Pacchioni, G.; Cogliandro, G.; Bagus, P. S. Int. J. Quantum Chem. 1992, 42, 1115. (60) Pacchioni, G.; Ferrari, A. M.; Bagus, P. S. Surf. Sci. 1996, 350, 159.

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for adsorbed CO are used to probe how the titania particles respond to the adsorption and decomposition of the small polar molecule, methanol. 3.3. Methanol Uptake, Decomposition, and Effect on the Lewis Acidity of Rutile TiO2. The transmission FTIR experimental setup has been employed to study how the uptake and decomposition of methanol affects the properties of nanoparticulate TiO2. Specifically, we have used infrared spectroscopy to track methanol product formation during exposure and then employed CO adsorption as an in situ probe of how the acidity of nanoparticulate TiO2 responded to the presence of preadsorbed methanol and associated products. As highlighted above, the vibrational frequency of adsorbed CO on TiO2 is sensitive to the Lewis acidity of surface sites. In addition, the intensity of CO infrared bands are directly proportional to the CO coverage and, hence, the number of Lewis acid sites on a surface. Our studies into the fundamental dynamics of methanol adsorption on TiO2 were performed under two complementary exposure regimes. First, we monitored infrared spectra of the rutile particles as methanol coverage increased (section 3.3.1) to track the dynamics of methanol uptake on clean particles. These experiments were augmented by a second series of studies carried out on methanol-covered particles (section 3.3.2). In this series, we used postadsorption of CO to characterize the response of surface acidity to a uniform layer of adsorbed methanol. 3.3.1. Molecular and Dissociative Adsorption of Methanol. The data from the first set of experiments are shown in Figure 4, where panels A, B, and C exhibit different regions of the same spectra. The highly dehydroxylated, oxidized rutile TiO2 sample (see Figure 4, spectrum a) was exposed for 5 min to 0.1 Torr of methanol and then evacuated to 1  10-6 Torr, where we recorded spectrum b of Figure 4. This procedure was repeated until no further uptake could be observed. Spectrum d of Figure 4 presents the infrared data for the TiO2 sample after it was fully saturated with methanol, and no further spectral changes were observed. Our results reveal that the adsorption of methanol on the clean 4 nm rutile particles leads to both molecular and dissociative uptake, in agreement with the results of previous experimental8,10 and theoretical18,20,21 studies. The spectra reported in Figure 4A,B show bands that are associated with surface OH groups and CHx stretching vibrations, while in the low-wavenumber region of the spectra (Figure 4C), new bands assigned to CHx deformations and O-C stretching modes are observed. As methanol is adsorbed, the IR signal associated with isolated OH groups at 3727, 3718, and 3692 cm-1 disappear, while the band at 3674 cm-1 appears to be more persistent and diminishes Langmuir 2010, 26(11), 8106–8112

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Figure 5. Adsorption of CO on methanol precovered surfaces;decreasing methanol coverage. Panel A: transmission FTIR spectra. Panel B: difference spectra. Spectra obtained at 295 K after saturation of sample with 0.15 Torr of methanol (a) and after evacuation at elevated temperatures: (b) 356, (c) 397, (d) 436, (e) 476, (f) 516, (g) 573, and (h) 629 K; (i) 675 K for 5 min; (j) 675 K for 30 min. After each heat treatment the rutile sample was cooled to 295 K and subjected to 80 Torr CO exposure for 20 min. Panel C: plot of the ν(Ti-CO) frequency vs CO coverage as determined from the integrated area of CO band. The data are taken from panel B.

only at the highest methanol coverage (Figure 4A,B, spectrum d). As the bands associated with isolated OH groups disappear, a new broad band at 3500-2700 cm-1 (Figure 4B) due to the vibrations of hydrogen-bonded O-H groups emerges. Typically, this band is assigned to hydrogen-bonded molecular methanol on surface OH groups of the oxide.8,16,24,61 Additional evidence for molecularly adsorbed methanol is provided by the asymmetric ν(CH3)a and the symmetric ν(CH3)s modes appearing at 2951 and 2849 cm-1 (Figure 4B, spectrum b), respectively.11,16,61-63 The adjacent bands at 2927 and 2827 cm-1 are assigned to the corresponding ν(CH3)a and ν(CH3)s modes of adsorbed methoxy groups that result from dissociatively adsorbed methanol.8,11,16,23,62,63 All of these bands slightly red-shift (4-5 cm-1) as methanol coverage increases (Figure 4B, spectrum d), likely due to repulsive adsorbateadsorbate interactions.64 The intermediate band at 2885 cm-1 is due to the overtone of the δ(CH3) deformation mode in Fermi resonance with ν(CH3) vibrations.23,65,66 In addition to the C-H and O-H stretching regions, the changes in the low-frequency modes (Figure 4C) provide additional information about the bonding of methanol and methoxy species to the surface. The band at 1153 cm-1 is attributed to the rocking mode F(CH3) of an adsorbed methyl group, and molecular adsorption is evidenced by the ν(OC) mode at 1050 cm-1. The pairs of bands at 1440 cm-1 [δ(CH3)] and 1109 cm-1 [ν(OC)]; at 1460 cm-1 [δ(CH3)] and ∼1060 cm-1 [ν(OC)]; and at 1460 cm-1 [δ(CH3)] and 1020 cm-1 [ν(OC)] indicate the presence of mono, double, and triply bonded methoxy groups, respectively.11,23,62 The small band at 1628 cm-1 (Figure 4C, spectra a-c) is due to the δ(HOH) mode of water, a product of methanol decomposition. However, this band diminishes upon increased methanol exposure (Figure 4C, spectrum d), suggesting that the water may further react with the TiO2 to produce associated surface OH groups observed as a broad band at 35002700 cm-1. The data from this set of experiments clearly show that, on small rutile nanoparticles, methanol adsorbs at room temperature in both molecular and dissociative forms throughout the whole (61) (62) (63) (64) (65) 1441. (66)

Wang, C.-y.; Groenzin, H.; Shultz, M. J. Langmuir 2003, 19, 7330. Yamakata, A.; Ishibashi, T.; Onishi, H. J. Phys. Chem. B 2002, 106, 9122. Chuang, C.-C.; Chen, C.-C.; Lin, J.-L. J. Phys. Chem. B 1999, 103, 2439. Somorjai, G. A. J. Phys. Chem. 1990, 94, 1013. Binet, C.; Jadi, A.; Lavalley, J. C. J. Chim. Phys., Phys. Chim. Biol. 1992, 89, Chabal, Y. J. Surf. Sci. Rep. 1988, 8, 211.

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range of coverage from initial uptake on clean particles to the point where they are saturated with parent and product species. The fact that saturation is achieved slowly at repeated equal doses of methanol implies that the diffusion of methanol through the tightly pressed rutile particles is a relatively slow process. 3.3.2. Effect of Methanol Adsorption on Lewis Acidity. Following saturation of the particles by methanol (spectrum d in Figure 4), the surface was exposed to a relatively high pressure of the probe molecule, CO. Spectrum a of Figure 5A,B shows that, while methanol coverage is at a maximum, virtually no CO adsorbs to the surface. It appears that the adsorbed methanol and its associated products fully block the Ti4þ sites that are responsible for CO adsorption on the clean particles. This observation is consistent with the expected competition for the Ti4þ Lewis acid sites between two molecules of significantly different Lewis basicity. That is, the Ti4þ-CO interaction occurs through the 5σ orbital, located primarily on the carbon atom, which is a weak σ-donor ligand relative to stronger bases such as ammonia, water, and methanol.33,39,53 Therefore, CO is incapable of displacing methanol or any of its more strongly bound products, and we do not observe surface-bound CO until some fraction of the methanol is thermally driven from the surface. Previous reports on thermal desorption of methanol from TiO2 surfaces show the existence of three different regimes: molecular desorption, recombinative desorption of methanol from methoxy and hydroxyl constituents, and decomposition of methoxy adsorbates. Work performed by Kim et al. suggests that parent methanol desorption occurs at ∼350 K and recombinative desorption occurs over the thermal range of ∼390 K up to 570 K for anatase TiO2.67 Similar results have been reported for Degussa P25 particles.11,16 Henderson et al. reported that methanol desorption from defective rutile TiO2(110) surfaces occurs at temperatures near 380 K.10 Madix and co-workers observed recombinative desorption of methanol from methoxide and hydroxide adsorbates on TiO2 at temperatures as low as 320 K.13 However, several groups have shown that strongly bound molecular methanol begins to thermally decompose near 420 K63 to 500 K.16 These previous results are in agreement with the data presented in Figure 5A, which show evidence for thermal desorption and decomposition of methanol over the range of 356-675 K. (67) Kim, K. S.; Barteau, M. A.; Farneth, W. E. Langmuir 1988, 4, 533.

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Figure 5A reveals the dynamics of methanol desorption over the thermal range from 365 to 573 K. Upon heating the methanolsaturated surface, we observe that the coverage of hydrogenbonded methanol species (bands at ∼3370-3260 and 2947 cm-1) decreases (Figure 5A, spectra b-g), while the concentration of strongly bound methoxy species (band at 2923 cm-1) increases. Above 573 K (Figure 5A, spectra h-j), the concentration of methoxy adsorbates systematically decreases and, at 673 K, the rutile particles are nearly free from methanol-derived species (spectrum j).11 However, a small fraction of methoxy adsorbates remain on the TiO2 surface, even at 723 K (not shown), as observed in previous studies.16 Spectrum b of Figure 5B shows that a small number of surface sites are liberated for CO adsorption upon annealing the sample to 365 K. However, the vibrational frequency of CO on the open surface sites is significantly red-shifted from where it appears on the clean particles (Figure 4). The 14 cm-1 red shift in the ν(CO) mode indicates that the adsorbed methanol and its associated products induce a significant decrease in the Lewis acidity of the particles.33,39 In addition, as hydrogen-bonded methanol species desorb from the surface, more CO is adsorbed (Figure 5B, spectra b-g), and the CO band maximum shifts continually toward higher frequency. This change in frequency suggests that the liberation of methanol constituents from the surface leads to an increase in the acidity of the surface sites that are responsible for binding CO. After heating to 573 K, when the majority of the hydrogen-bonded methanol is removed from the surface (Figure 5A,B, spectrum g), the CO coverage is ∼40% of that on the clean surface. However, the CO band frequency, of 2181 cm-1, remains 10 cm-1 below that observed for the methanol-free surface. The original intensity and the position of the CO band are fully recovered only after liberation of nearly all of the methoxy species from the surface by evacuation at 675 K for 30 min (Figure 5B, spectrum j). Figure 5C shows that the blue-shift of the CO frequency correlates linearly with the intensity of the CO band. This result implies that the acidity of each CO adsorption site, as indicated by the band position, is directly proportional to the coverage of methanol/methoxy adsorbates, as indicated by the band intensity. The above results agree with previously reported data for coadsorption of ammonia and CO on anatase33 and rutile39 powders at low temperature (100 K). These studies demonstrated that the presence of preadsorbed ammonia affects the surface acidity, as evidenced by a red-shift of the ν(CO) band from 2181 to 2173 cm-1.39 On a partially covered surface, a single ammonia molecule appears to block two CO adsorption sites, which suggests a strong effect of ammonia adsorption on the electrophilicity of neighboring Lewis acid sites.31,39 Two types of closely related chemical effects may be responsible for the decreased interaction energy of CO with the surface Ti cations. Specifically, the red shift in the ν(CO) frequency in the presence of an adsorbate on a neighboring site may be due to induction68,69 and relaxation.19,35,56,69 The first effect is associated with electronic density changes at a given site induced by adsorption at a neighboring site.68,69 The latter effect is associated with the displacement of a surface atom upward upon binding to an adsorbate and the relaxation of the neighboring atoms that have no adsorbate as the surface atoms rearrange upon adlayer formation.19,35,56,69 Relaxation of surface atoms have been predicted to occur for methanol adsorption on TiO2(110).19 The (68) Ferrari, A. M.; Ugliengo, P.; Garrone, E. J. Chem. Phys. 1996, 105, 4129. (69) Zecchina, A.; Scarano, D.; Bordiga, S.; Ricchiardi, G.; Spoto, G.; Geobaldo, F. Catal. Today 1996, 27, 403. (70) Borse, P. H.; Kankate, L. S.; Dassenoy, F.; Vogel, W.; Urban, J.; Kulkarni, S. K. J. Mater. Sci.: Mater. Electron. 2002, 13, 553.

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Ti5-Ob-Ti5 atoms in the (110) direction of methanol-covered titania appear to alternate from highly strained (at adsorption sites) to relaxed (neighboring sites with no adsorbate).19 The relaxed sites appear less acidic than the strained methanol-occupied sites.19 Hence, the interaction energy of CO with a surface cation located adjacent to an adsorbed methanol fragment decreases as the Ti(5f) site relaxes. These effects are reflected by a smaller blue-shift in the CO frequency on the methanol-covered surface (34 cm-1) than on the clean surface (48 cm-1).33,39 The reduced Lewis acidity of the titania surface upon methanol adsorption for the clean TiO2 particles studied here may be a general result that affects the overall chemistry of titania nanoparticles. In fact, the surface chemistry of titania is known to depend strongly on the method of preparation, which may be due to the presence of adsorbates that shift the acidity of coordinatively unsaturated surface sites. Future studies will investigate this effect further by probing other adsorbates to search for correlations between the basicity of the gas phase species and the resulting change in the Lewis acidity of the surface.

4. Conclusion Infrared spectroscopy of adsorbed CO on the surface of 4 nm rutile TiO2 particles has been employed to investigate how the Lewis acidity of coordinatievely unsaturated surface sites responds to the adsorption of methanol, a common product of chemical warfare agent chemistry on titania. The FTIR spectra of adsorbed CO show the presence of one class of Lewis acid Ti4þ adsorption sites on the nanoparticulate rutile surface, unlike the multiple types of sites often observed on anatase surfaces. In addition, we find that the CO molecules appear to be adsorbed on associated sites, i.e., on regular crystal planes where a CO molecule experiences lateral interactions59 with neighboring CO molecules. Moreover, the frequency of the metal-carbonyl band on methanol-free titania is blue-shifted by 58 cm-1 relative to the gas phase, which is very high for a typical sample of TiO2. The dramatic difference in the vibrational frequency of adsorbed CO suggests that the Lewis acidity of the 4 nm particles used here is greater than that of previously studied particles. The adsorption of methanol on clean 4 nm particles is governed by both molecular and dissociative pathways that occur primarily at Lewis acid sites and through hydrogen bonding to surface OH groups. Most interestingly, the vibrational mode for CO reduces significantly, by 34 cm-1, on the methanol-covered surface, relative to the clean surface. This effect is attributed to a weakening of the Lewis acidity of the available surface sites due to the close proximity of surrounding methanol molecules. A linear correlation with coverage suggests that this effect is due to changes in the inherent acidity of the Lewis acid sites rather than due to lateral electrostatic interactions between adsorbates. The net result is that the Lewis acidity of coordinatively unsaturated surface sites on titania depends linearly on methanol coverage. Future studies will explore whether the effects of surface acidity on adsorption of small molecules is general by investigating a variety of particulate sizes and adsorbates. Acknowledgment. We are grateful to Carla Slebodnick for assistance in acquiring the powder diffraction data and to Kathy Lowe (Vet School) and John McIntosh (ICTAS HR-TEM) for help in acquiring the electron micrographs. This work is supported by the Army Research Office, W911NF-04-1-0195, and the Defense Threat Reduction Agency, W911NF-06-1-0111. M.M. and K.H. acknowledge the financial support of the Bulgarian Scientific Foundation (Grants DO 02-82 and DO 02-290). Langmuir 2010, 26(11), 8106–8112