Nanocrystalline Anatase Titania-Supported Vanadia Catalysts: Facet

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Nanocrystalline Anatase Titania Supported Vanadia Catalysts: Facet-dependent Structure of Vanadia Weizhen Li, Feng Gao, Yan Li, Eric Walter, Jun Liu, Charles H. F. Peden, and Yong Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b01486 • Publication Date (Web): 04 Jun 2015 Downloaded from http://pubs.acs.org on June 13, 2015

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The Journal of Physical Chemistry

Nanocrystalline Anatase Titania Supported Vanadia Catalysts: Facet-dependent Structure of Vanadia Wei-Zhen Li1, Feng Gao1*, Yan Li2, Eric D. Walter1, Jun Liu1, Charles H. F. Peden,1 Yong Wang1,2*

1

Institute for Integrated Catalysis, Pacific Northwest National Laboratory, P.O. Box 999, Richland, WA 99352, USA.

2

The Gene & Linda Voiland School of Chemical Engineering and Bioengineering, Washington State University, Pullman, WA 99164, USA.

* Corresponding to: [email protected], [email protected]

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Abstract Titania supported vanadia, a classic heterogeneous catalyst for redox reactions, typically has nonhomogeneous vanadia species on various titania facets, making it challenging not only to determine and quantify each species but also to decouple their catalytic contributions. We prepared truncated tetragonal bipyramidal (TiO2-TTB) and rod-like (TiO2-Rod) anatase titania with only {101} and {001} facets at ratios of about 80:20 and 93:7, respectively, and used them as supports of sub-monolayer vanadia. The structure and redox properties of supported vanadia were determined by XRD, TEM, XPS, EPR, Raman, FTIR and TPR, etc. It was found that vanadia preferentially occupy TiO2 {001} facets and form isolated O=V4+(O-Ti)2 species, and with further increase in vanadia surface coverage, isolated O=V5+(O-Ti)3 and oligomerized O=V5+(O-M)3 (M = Ti or V) species form on TiO2 {101} facets. The discovery on support facet-dependent structure of vanadia on anatase titania is expected to enable the elucidation of structure-function correlations on high surface area TiO2 supported vanadia catalysts.


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1. Introduction A major objective in the field of catalysis is to understand structure-function relationships. Unlike homogenous catalysts with unambiguous molecular structures, most heterogeneous catalysts have high heterogeneity in the structure of active sites, making it difficult not only to quantitatively determine a specific structure but also to assign the corresponding contribution in the catalytic reactivity.1-3 For example, supported vanadia catalysts on high surface area oxides are catalytically active for many redox reactions,4-7 and the active sites are typically thought as VO4 units possessing a single V=O terminal bond and three equivalent or in-equivalent bridging V-Oα,β,γ-Mα,β,γ bonds (M: support cations (denoted as S) or V depending on oligomerizations;

α,β,γ

: atom labels) in a distorted

tetrahedral geometry.8-10 The geometric and electronic structures of the VO4 units are highly sensitive to the identities of M cations and their arrangements, additives or impurities present, and the preparation and pretreatment procedures,11-13 leading to a mixture of multiple in-equivalent VO4 units on practical supports. It thus has attracted growing interest in the synthesis and investigation of simplified model catalysts with well-defined structures. The structures of VO4M3 units can be readily simplified to isolated VO4S3 units by minimizing bridging V-O-V bonds via reducing vanadia loadings. Furthermore, reduction in the types of support facets can greatly reduce the types of assemblages of Sα,β,γ, leaving VO4M3 units uniform on a facet level.14 Thus, synthesis of highly faceted support materials with distinct surfaces is essential to prepare structurally simplified model catalysts. Well-defined oxides can be readily synthesized with the help of organic surfactants and/or inorganic mineral ions. However, removal of the inorganic additives by washing is usually inefficient. Furthermore, burning organic surfactants at elevated temperatures may lead to structural 3 ACS Paragon Plus Environment


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collapse. Taking TiO2 synthesis as an example, needle- and rhombic-shaped anatase TiO2 with mainly low-energy {101} facets, and rod-, tube- and plate-shaped anatase TiO2 with enhanced high-energy {001} facets have been synthesized by solvothermal treatment of titanium precursor using different organic surfactants or heteroatomic mineral ions such as Na+, Cl- or F-, etc.

15-25

However, additive residues may modify the surface structures of oxides thus synthesized and further affect the structures of supported vanadia, and may even provide extra functions during catalytic reactions.26-27 One method for synthesizing TiO2 nanorods has been developed recently, where the use of hydrogen peroxide and ammonia for hydrothermally treating titanium precursors seems to achieve chemical purity since these additives are completely removable with mild calcinations.28-32 In this work, we report the synthesis of high surface area and facet-specific anatase TiO2 with truncated tetragonal bipyramidal (TiO2-TTB) and rod-like (TiO2-Rod) shapes with {101} to {001} ratios of 80:20 and 93:7, respectively, and

use these materials as supports for sub-monolayer

vanadia catalysts. The variation in the facet ratios enables the determination of locations of various vanadia according to their relative abundances. We then identify the vanadia structures which are highly dependent on support facets by a combination of characterization techniques.

We report that

V4+ species appear as O=V4+O2 monomers on titania {001} facets while V5+ species transform from isolated O=V5+O3 to oligomers on {101} facets with the increase of vanadia coverage. 2. Experimental 2.1. Sample preparation The TiO2-Rod support was prepared by slowly adding 10 mL (0.03 mol Ti) of Tetra-N-Butyl Titanate (TNBT) to 300 mL of deionized water under magnetic stirring, followed by stirring the mixture overnight at room temperature. The resulting white precipitate [Ti(OH)4] was centrifugally 4 ACS Paragon Plus Environment

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separated from the solution and washed with deionized water for three times. The obtained titanium hydroxide was dispersed in 200 mL of deionized water. The pH of the suspension was adjusted to 9 by adding ammonium hydroxide solution. The suspension was then converted to an orange transparent solution of peroxotitanium acid after adding 100 mL of hydrogen peroxide (H2O2; Alfa Aesar, 27%). For this peroxotitanium acid solution, the concentration of Ti4+ was 0.1 mol/L and the molar ratio of [H2O2]/[Ti4+] was 25, and the pH was about 2. Each 100 mL of peroxotitanium acid solution was transferred into a 125 mL Teflon lined autoclave and heated at 90 °C for 3 days and then at 120 °C for 2 days and finally at 150 °C for 2 days. After the hydrothermal treatment, the resulting precipitate was centrifugally separated and dried at 110 °C in ambient air overnight. The as-prepared sample was then calcined at 400 °C for 5 h before use. TiO2-TTB preparation followed the same procedure except that no ammonium hydroxide solution was added into the fresh titanium hydroxide suspension. Supported vanadia catalysts were prepared by incipient wetness impregnation of TiO2-Rod and TiO2-TTB with NH4VO3 (99.99%, Alfa Aesar) in saturated oxalic acid aqueous solution at room temperature for 20 h, followed by drying at 110 °C in ambient air overnight and then calcination at 400 °C for 5 h. The final products were donated as mVOx/TiO2-Rod or mVOx/TiO2-TTB, where m is the surface VOx coverage, obtained from the ratio of the nominal vanadia surface density to the vanadia surface density at theoretical monolayer coverage (7 V/nm2). The nominal vanadia surface density was estimated from the vanadia content and the BET surface area. The surface vanadia coverage was deliberatively maintained below 90% for the purpose of exposing all deposited vanadium atoms on the surface. 2.2. Sample characterization 5 ACS Paragon Plus Environment


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Surface areas were measured on a QuantaChrome Autosorb-6 using N2 adsorption isotherms and BET analysis methods. All of the samples were degassed under vacuum at 150 °C for 4 h before the adsorption measurements. X-ray diffraction (XRD) measurement was carried out with a Philips PW3040/00 X’Pert MPD system equipped with a Cu source (λ = 1.5406 Å). Data analysis was accomplished using JADE® (Materials Data, Inc., Livermore, CA) as well as the Powder Diffraction File database (2003 Release, International Center for Diffraction Data, Newtown Square, PA). Transmission electron microscopy was performed on a JEOL JEM 2010 operating at 200 keV with a specified point-to-point resolution of 0.194 nm. TEM specimens were prepared by depositing a suspension of the as-calcined powdered sample on a lacey carbon-coated copper grid. Raman spectra were obtained using the 514.5 nm line of an Argon ion laser (Coherent, Innova 400) and a liquid nitrogen cooled charge-coupled detector (CCD) (Princeton Instruments, Spec 10, 1340 × 400 array). The sample was placed into a Harrick reaction cell and heated to 400 °C for 30 min in a flow of ~20% O2 balanced with helium. Spectra were collected after cooling the samples to 25 °C in the same flow. The excitation laser beam was transmitted to the sample chamber via an optical fiber with a power of 10 mW at the sample. Typical collection time was 20 s and the instrumental resolution was ~2 cm-1. X-ray photoelectron spectra (XPS) were obtained with a Physical Electronics Quantera Scanning X-ray Microprobe with a focused monochromatic Al Kα X-ray (1486.7 eV) source for excitation and a spherical section analyzer. Spectra were calibrated against the Ti2p3/2 line at 458.5 eV. The spectra for V2p3/2 were deconvoluted to peaks centered at 515.8 and 516.8 eV using Shirley type baselines. 6 ACS Paragon Plus Environment

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Electron paramagnetic resonance (EPR) experiments were conducted on a Bruker E580 X-band spectrometer equipped with a SHQE resonator and a continuous flow cryostat. Powder samples (∼10 mg) were contained in 4 mm OD quartz tubes Microwave power was 20 milliwatts, and the frequency was 9.34 GHz. The field was swept by 4900 G in 84 s and modulated at 100 kHz with 5 G amplitude. A time constant of 163 ms was used. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were record using a Bruker Tensor 27 FTIR spectrometer equipped with a Praying MantisTM accessory and a high pressure reaction cell (Harrick Scientific Products Inc.). The samples were in-situ pretreated in 20% O2 balanced with He with a flow rate of 30 mL/min at 400 °C for 30 min and then cooled to 150 °C in He. The spectra of bare samples were acquired using the spectrum of an empty sample holder as the background. Methanol vapor was introduced via flowing He through a bubbler with methanol at room temperature for 10 min. The sample was then flushed with helium for 30 min before measurements. In this case, spectra of the samples before methanol introduction were used as the background. Temperature programmed reduction (TPR) with hydrogen was performed on a Micromeritics AutoChem II 2920 equipped with a thermal conductivity detector. An internal temperature calibration material, RuO2/γ-Al2O3 (RuO2 loading: 1wt.%), which can be reduced at lower temperatures than the VOx species, was used to calibrate the reduction temperatures for different VOx/TiO2 samples. The RuO2/γ-Al2O3 was prepared by incipient wetness impregnation of γ-Al2O3 (Condea, 200 m2/g) with Ruthenium (III) nitrosyl nitrate aqueous solution followed with drying and then calcining at 500 °C for 5 h. In the quartz reactor, the RuO2/γ-Al2O3 and the VOx/TiO2 layers were separated with quartz wool and gases flows through the VOx/TiO2 layer first to minimize 7 ACS Paragon Plus Environment


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artifacts to VOx reduction during the TPR process. The samples were pre-treated in 5% O2 balanced with helium at 400 °C for 30 min and then cooled down to room temperature. The TPR measurements were conducted from room temperate to 1000 °C in a 5% H2/Ar flow of 30 mL/min at a heating rate of 10 °C/min. 3. Results and Discussion Figure 1a shows the nitrogen adsorption-desorption isotherms and the pore size distributions for the TiO2-TTB and TiO2-Rod samples obtained via the BJH method. Their surface areas are 129.3 and 57.5 m2/g, respectively. The TiO2-TTB sample shows a hysteresis loop in a range of 0.65-0.9 P/P0 and a narrow pore size distribution between 3 - 12 nm centered at 8 nm. As for the TiO2-Rod sample, a hysteresis loop above 0.85 P/P0 and a broad pore size distribution between 20 - 120 nm were observed. The characteristics of the hysteresis loops indicate that for TiO2-TTB and TiO2-Rod samples, pores are formed by stacking of small and large particles, respectively.33 The surface areas are largely maintained and the nitrogen adsorption-desorption isotherms remain virtually unchanged after the loading of vanadia. For example, the surface areas for samples with the highest vanadia loadings, i.e., 72%VOx/TiO2-TTB and 86%VOx/TiO2-Rod, are 126.1 and 56.5 m2/g, respectively. Figure 1b displays the XRD patterns for the calcined TiO2 supports and the VOx/TiO2 with the highest vanadia coverages. The patterns for bulk V2O5 are also displayed for comparison. The TiO2-TTB shows the characteristics of a pure anatase phase (PDF#: 21-1272) and their crystallite sizes on the [101] and [001] directions are both estimated to be ~11 nm. The 72%VOx/TiO2-TTB sample shows essentially identical diffractions to the TiO2-TTB support with no diffractions for V2O5 crystallite being observed. The TiO2-Rod also shows the characteristics of a pure anatase phase while their crystallite sizes on the [101] and [001] directions are ~30 and >100 nm, respectively, 8 ACS Paragon Plus Environment

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indicating rod-like shape for the TiO2 crystallites. There are no detectable diffractions for V2O5 crystallite on the 86%VOx/TiO2-Rod sample either. The lack of V2O5 crystallites indicates that the vanadia species are highly dispersed on both supports.

It is noteworthy that for both TiO2-Rod and

TiO2-TTB, the diffractions prior to and after calcination are essentially identical, suggesting that the as-prepared TiO2 samples are very well crystallized, and they display excellent thermal stability during the hydrothermal treatment. Figure 2 shows the TEM images for the as-prepared and calcined TiO2-TTB and TiO2-Rod. Near mono-dispersed truncated tetragonal bipyramidal nanoparticles with crystallite sizes of about 9 - 13 nm are observed for the as-prepared TiO2-TTB sample (Figure 2a), consistent with the equilibrium shape of anatase crystallites predicted by Wulff construction from surface energy calculations (insert in Figure 2a).34 After calcination, the primary particles aggregate to some extent but the crystallite sizes remain unchanged (Figure 2b). This observation is consistent with the results from XRD and from the nitrogen adsorption-desorption isotherms. High-resolution TEM image for the calcined TiO2-TTB sample shows good crystallinity of small crystallites and fringe spacing of 0.35 nm, corresponding to the (101) planes (Figure 2c). Unfortunately, the (001) surface cannot be determined from this HRTEM image since the crystallites are not in the zone orientations. Due to the fact that little change was observed in size and shape of the primary particles prior to and after calcination, the TEM images of as-prepared TiO2-TTB nanoparticles with distinct outlines were used to determine the {001} to {101} facets ratio of according to Wulff construction model. The lengths of A and B (defined in the insert of Fig. 2(a)) for each crystallite were measured and used to estimate the facet ratio using the equation {001}⁄{101} = ⁄(1.73 × ( − .24,35 A total number of two hundred nanoparticles was used to obtain the statistic histogram for the distribution of the 9 ACS Paragon Plus Environment


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percentages of {001} facets in the total surface, which is in a range of 16%-24% and centered at 20% (insert, Fig. 2(b)). Rod-like TiO2 with 15-30 nm in diameter and 80-200 nm in length are commonly seen in the as-prepared TiO2-Rod sample (Figure 2e). Similar images with mono-dispersed nanorods were obtained on the calcined TiO2-Rod sample (Figure 2f). These sizes are in line with estimations made from XRD. Notably, the nanorod particles did not agglomerate during calcination, indicating a loose stack as also reflected from the nitrogen adsorption-desorption isotherms shown in Fig. 1(a). Figure 2g displays analysis of a high-resolution TEM image of a single TiO2 rod (after calcination) with clear crystalline lattice fringes. The growth direction of the nanorod is [001] as determined from the electron diffraction pattern (Figure 2f, insert). The zigzag side surfaces are {101} facets according to the paralleled fringes with spacing of 0.35 nm, and the tip surfaces for the nanorod are {001} facets since the paralleled fringes spacing of 0.48 nm are corresponding to (002) planes. The growth of TiO2-Rod along [001] has been reported via oriented attachment of (001) surfaces between nanoparticles, which is facilitated under weak acidic or basic conditions.32,36 Obviously, the TiO2-Rod has much smaller fractions of exposed {001} facets than that for the TiO2-TTB because of this type of attachment. Since it is not possible to accurately calculate the areas of the zigzag side surfaces, a simplified cylinder model for the TiO2-Rod was used to estimate the {001} fraction for each rod. The ratio of exposed {001} facets to {101} facets thus can be simplified to the area ratio of tip to side surfaces, which can be calculated from the aspect ratio using the equation {001}⁄{101} = ⁄2, where D and L are defined in the insert of Fig. 2(e). As displayed as an insert of Fig. 2(f), the distribution of the percentages of {001} facets in the total surface of TiO2-Rod is in a range of 4%-11% and centered at 7% by measuring a total number of two hundred TiO2 nanorods. It 10 ACS Paragon Plus Environment

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is noteworthy that these percentages of {001} are over estimated since the real rods have much larger side surfaces with the zigzag structure. Figure 2d and 2h display the top-view illustrations of (101) (4x4) and (001) (4x4) surfaces with top two layers of oxygens, which were built by using the CIF with Crystallography Open Database ID of 9008213.37 The different oxygen patterns on (101) and (001) provide various triangular oxygen assemblages which may bridge the V and Ti atoms of isolated O=V(OTi)3 species. Two types of triangular oxygen assembles, Ta (displayed in yellow) and Tb (displayed in magenta), are present on the (101) surface, while only one type of triangular oxygen assemble, Tc (displayed in blue), is present on the (001) surface. Such a difference in the orientation of bridging oxygens on different facets should result in O=V(OTi)3 species with various fine structures. Although TEM is powerful enough to show the local structure with high spatial resolution, at the current stage, this technique is still unable to distinguish the neighboring V and Ti elements and image the O=V(OTi)3 species in detail. Thus, the structures of various vanadia species still need to be determined by other spectroscopic techniques. In this case, the use of two types of TiO2 supports with different facet ratios will benefit the identification of locations of various vanadia species. The oxidation state of vanadium is a key parameter that allows for the determination of the number of binding oxygens. The binding energies of V2p3/2 were measured by XPS and the results are shown in Figure 3. For the VOx/TiO2-TTB samples, only one symmetric XPS peak centered at 515.8 eV was observed on 3% and 10%VOx/TiO2-TTB samples, while a new feature develops at 516.8 eV for samples with higher V loadings. The binding energies of V2p3/2 at 515.8 and 516.8 eV correspond to V4+ and V5+ species, respectively.38-40 Notably, V4+ species appears to saturate at low V coverages while V5+ species grows and dominates with increasing V loading. For the 11 ACS Paragon Plus Environment


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VOx/TiO2-Rod samples, a similar trend in the growth of V2p3/2 XPS peaks was observed. Compared to the VOx/TiO2-TTB samples, the VOx/TiO2-Rod samples appear to have smaller amounts of V4+ species at the same total coverages. EPR spectroscopy is highly sensitive to paramagnetic species having one or more unpaired electron either in the bulk or at the surface of solids. Therefore, it was used to confirm the existence of V4+ species here. Figure 4 shows the EPR spectra of the TiO2 supports and VOx/TiO2 samples. For the TiO2-TTB support, a broad Ti3+ feature is observed at ~3320 G. The VOx/TiO2-TTB samples display roughly equally spaced new features which could be attributed to the hyperfine structure of isolated V4+ species, derived from the interaction of free electrons (3d1) with the magnetic nuclear moment of

51

V(I = 7/2).41 The relative intensities for V4+ to Ti3+ seems unchanged for the

VOx/TiO2-TTB samples with surface vanadia coverages increasing from 3% to 57%.

For the

TiO2-Rod and VOx/TiO2-Rod samples, similarly, little change is observed in the relative intensities for V4+ to Ti3+ with increasing vanadia coverage. The unchanged hyperfine lines indicate that the V4+ species remain as isolated V4+ species in all samples irrespective of vanadia surface coverage. The presence of Ti3+ species in the supports may be responsible to the formation and stabilization of V4+ species under oxidizing conditions. With the verification of V4+ species from EPR, the relative amount of V4+ and V5+ is next quantified from XPS spectra deconvolution, where V4+ and V5+ species are centered at 515.8 and 516.8 eV, respectively. Figure 5 shows the percentages and surface coverages of V4+ and V5+ species on VOx/TiO2-TTB and VOx/TiO2-Rod samples. The percentage of V4+ species decreases gradually from 100% to 54% and to 23% with vanadia surface coverage increasing from 3% to 20% and 72% in VOx/TiO2-TTB samples. Meanwhile, surface coverage for V4+ species increases slowly from 3% 12 ACS Paragon Plus Environment

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to 17% with vanadia coverage increasing from 3% to 72%. On the other hand, the percentage and surface coverage of V5+ increase monotonously. For the VOx/TiO2-Rod samples, the percentage of V4+ species decreases gradually from 92% to 38% and to 11% with vanadia surface coverage increasing from 3% to 20% and 86%. Accordingly, surface coverage for V4+ species increases slowly from 2.7% to 9.8% with vanadia coverage increasing from 3% to 86%. Obviously, the VOx/TiO2-TTB samples have higher percentages of V4+ species than VOx/TiO2-Rod samples at the same vanadia surface coverages. It should be pointed out that the maximum surface coverages of V4+ species are less than the percentages of {001} facets for each series of samples. The quantity of V5+ species in VOx/TiO2-Rod has also been determined by using High-Field and Fast-Spinning 51V MAS NMR which involved no vacuum or reduction treatment of samples.42 This study reveals that a portion of the V-Species is invisible to NMR and the level of such invisibility increases with decreasing V-loading levels, suggesting the existence of paramagnetic V-Species (i.e., V4+) at the surface. The results are well in line with the quantification from XPS, showing the presence of V4+ in our samples under ambient conditions. The quantitative correlations between vanadia species and the support facets suggest that vanadia structures are highly dependent on the support facets. V4+ species have been detected on various support materials below monolayer coverage with oxidizing or reducing treatments, indicating that V4+ species can have stable structures.43-44 V4+ species can form via solid-state interaction between V2O5 and TiO2 in the temperature range of 700-800 K even in the absence of reducing agents.45 A more plausible explanation on the V4+ formation in our work is that Ti3+ acts as the reductant for V5+ in the precursor, resulting in the formation of stable Ti4+-O-V4+ pair.”



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Raman spectroscopy was used to determine the structures of vanadia species. Figure 6 presents the Raman spectra of dehydrated TiO2 supports and VOx/TiO2 samples. Three major Raman bands at 399, 516 and 639 cm-1 are observed for the TiO2-TTB and TiO2-Rod supports, confirming that both materials are the anatase phase of TiO2 (Figure 6a, 6b). Characteristic Raman bands for other titania phases are not observed, which is in line with the XRD results. This anatase phase remains unchanged after the loading of vanadia. A new Raman band at 1026 cm-1 appeared on 3%VOx/TiO2-TTB (Figure 6a). This vibration arises from the terminal V=O bond of isolated surface vanadia species, and is the predominant Raman band when the surface vanadia coverage is below 10%. As the vanadia coverage increases from 20% to 72%, this band shifts gradually from 1026 to 1031 cm-1, demonstrating formation of a new isolated surface vanadia species. Another Raman band at 1002 cm-1 is observed on the VOx/TiO2-TTB when the vanadia coverage reaches 57%. The intensity of this band continues to increase while the position remains unchanged with vanadia coverage increasing to 72%. Raman spectroscopy is much more sensitive to vanadia crystallite than XRD which is not sensitive for the crystallites smaller than 2 nm. No Raman band at 995 cm-1 corresponding to crystalline V2O5 is found on any of these VOx/TiO2-TTB samples, while which indeed is visible when the vanadia coverage reaches 1.5 layers (data not shown), suggesting lack of V2O5 crystallites in the samples with vanadia coverage below one monolayer. The vibration at 1002 cm-1 is tentatively assigned to the symmetric stretching of V=O terminal double bond in oligomerized surface vanadia species. No Raman band corresponding to the stretching mode of V-O-V or V-O-Ti, typically found at 900-950 cm-1 is observed. This is likely due to the small Raman scatter sections. Similar results are observed on VOx/TiO2-Rod samples except the development of the Raman band at 1002 cm-1 occurs at slightly lower coverages (Figure 6b). Overall, both series of 14 ACS Paragon Plus Environment

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samples present two types of isolated vanadia species and only one type of oligomerized vanadia species. The absence of three dimensional vanadia clusters indicates that all the vanadia species adopt 2-dimensional structures on the TiO2 surface so that all the vanadium atoms are accessible for reactants and can be counted as active sites. The locations for the vanadia species were determined by monitoring the depleting of surface hydroxyls of the TiO2 supports with increasing vanadia coverage using DRIFTS. Figure 7 shows the dehydrated DRIFT spectra in the range of 3500-3800 cm-1 for the TiO2 supports and the VOx/TiO2 samples. After dehydrating at 400 ºC in 20% O2/He for 30 min and then cooling to 150 ºC in He, the TiO2-TTB support shows a weak broad peak at 3729 cm-1 and a stronger broad peak in the range of 3700-3600 cm-1 with a shoulder peak at 3687 cm-1 (upper panel, Figure 7a). After loading vanadia, the 10% and 50%VOx/TiO2-TTB samples only display a broad peak in the range 3700-3600 cm-1. Bands at 3715-3730 cm-1 can be assigned to the v(OH) stretching mode of isolated hydroxyls on the anatase surface, 46 while those between 3695-3630 cm-1 are due to the stretching modes of adsorbed water. However, vibrations from different facets of TiO2 are indistinguishable from these measurements. In looking for more sensitive probes, we discovered that methanol titration coupled with DRIFTS allows for distinguishing v(OH) from different facets. Upon adsorption, methanol interacts with surface hydroxyls and these hydroxyls are replaced to various extents. Methanol interacts with isolated hydroxyls dissociatively according the following reaction: Ti-OH + CH3OH = Ti-OCH3 + H2O, while it replaces adsorbed water associatively. Using the spectrum of the sample prior to methanol exposure as background, negative peaks emerge after methanol introduction due to the replacements of hydroxyls and adsorbed water by methanol. As shown in the lower panel of Fig. 7(a), 15 ACS Paragon Plus Environment


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after methanol adsorption followed by purging at 150 ºC, for the TiO2-TTB sample, two negative peaks centered at 3733 and 3712 cm-1 appear in the stretching vibration regime of isolated hydroxyls and three negative peaks centered at 3687, 3674 and 3633 cm-1 appear in the regime of adsorbed water vibrations.46 The fact that methanol adsorption induces splitting of the broad ν(OH) region makes it possible to distinguish the isolated hydroxyls on various facets and to address the locations of vanadia on specific facets of the TiO2 supports. The 10%VOx/TiO2-TTB only presents a weak negative peak at 3712 cm-1, indicating that hydroxyls that give rise to this ν(OH) band are still present while hydroxyl that give rise to the3733 cm-1 band has been completely consumed from vanadia occupation. No negative peaks at 3733 and 3712 cm-1 are observed on 50%VOx/TiO2-TTB, showing the complete depletion of isolated hydroxyls at this rather high vanadia coverage. Meanwhile, the lack of negative peak at 3687 cm-1 and the gradual shift in negative peak positions from 3674 to 3664 and to 3656 cm-1 for 10% and 50%VOx/TiO2-TTB indicate the simultaneous occupations of the TiO2-TTB surface sites by vanadia species. As for the TiO2-Rod support and the VOx/TiO2-Rod samples, broad peaks centered at 3729 and 3673 cm-1 are also evidenced on the TiO2-Rod support while the shoulder peak at 3687 cm-1, which appears on TiO2-TTB support, is invisible; the peak at 3729 cm-1 becomes undetectable and the peak at 3763 cm-1 shifts gradually to 3666 cm-1 with vanadia surface coverage increasing to 50% (Figure 7 b upper panel). With the introduction of methanol at 150 ºC, negative peaks at 3733 and 3712 cm-1 for isolated hydroxyls and those at 3679, 3670 and 3640 cm-1 for adsorbed water appear on the TiO2-Rod. At a vanadia surface coverage of 10%, the negative peak at 3733 cm-1 becomes invisible while the negative peak at 3712 cm-1 remains detectable. For bands associated with adsorbed H2O, the negative peak at 3679 cm-1 disappears but that at 3670 and 3640 cm-1 remain strong. At a higher 16 ACS Paragon Plus Environment

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vanadia coverage of 50%, only a negative peak at 3640 cm-1 remains on the 50%VOx/TiO2-Rod sample. As described above, surface hydroxyls, including two types of isolated hydroxyls and three types of adsorbed water, were detected by FTIR on both TiO2-TTB and TiO2-Rod support materials even after purging in helium at 400 ºC. Their assignments are difficult to determine because of the lack of standard references. However, the likely hydroxyl vibrational frequencies on anatase TiO2 (001) and (101) surfaces have been calculated theoretically: isolated hydroxyls on (001) appear at higher frequencies than those on (101), and the same trend holds for adsorbed water.47 Hence, the IR bands at 3733 and 3712 cm-1 can be reasonably assigned to isolated hydroxyls on TiO2 {001} and {101}, respectively. Similarly, the IR bands at 3687 and 3674 cm-1 or 3679 and 3670 cm-1 can also be assigned to adsorbed water on TiO2-TTB {001} and {101} or TiO2-Rod {001} and {101}, respectively. The frequencies of hydroxyls reflect the interaction strength between adsorbed hydroxyls and titania surface sites: higher frequency indicates a stronger interaction. The adsorbed hydroxyls on these sites can be readily replaced by methanol and vanadia. The disappearance of bands at higher frequencies prior to those of lower frequencies with increasing vanadia coverage implies that vanadia preferentially occupies sites on TiO2 {001}, especially at low vanadia coverages. This is fully expected since the surface energy of {001} facets is much higher than that for {101} facets [15]. A more detailed picture of the structure of TiO2 supported vanadia catalyst thus can be elucidated by combining the above characterization results. TEM images show that the TiO2-TTB has truncated tetragonal bipyramidal morphology with {001} and {101} of ~20% and ~80%, and the TiO2-Rod shows Rod-like shape with {001} and {101} of ~7% and ~93%, respectively. FTIR spectra reveal that the surface structures of {001} and {101} facets are not identical for the TiO2-TTB and 17 ACS Paragon Plus Environment


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TiO2-Rod supports: the sites providing isolated hydroxyls are similar in structure while those adsorbing water are slightly different. Vanadia species preferentially occupy the sites on {001} facets prior to those on {101} facets with increasing of vanadia coverage. XPS and EPR spectra confirm the co-existence of V4+ and V5+ species and further provide their relative ratios for each supported vanadia sample. The dominant vanadia species are V4+ at low vanadia coverages while at high coverages, V5+ dominates. The ratios of V4+ and V5+ species for supported vanadia samples at near monolayer coverage are comparable to the surface ratios of {001} and {101} for both titania supports studied here. Raman spectra demonstrate that V=O terminal bonds exist in both isolated and oligomerized vanadia species. Taken together, vanadia progressively cover titania surface by forming isolated O=V4+(O-Ti)2 on TiO2 {001} facets.

Isolated O=V5+(O-Ti)3 and then

oligomerized O=V5+(O-M)3 (M = Ti or V) species form on TiO2 {101} facets with increasing vanadia surface coverage. The detailed vanadia structures on the titania can be understood from their “footprint” in specific patterns. As shown in Figures 2d and 2h, TiO2 (101) has two types of neighboring triangles sharing one side while TiO2 (001) has only one type of triangle. Thus, both isolated O=V5+(O-Ti)3 and oligomerized O=V5+(O-M)3 (M = Ti or V) species on TiO2 {101} while only isolated O=V4+(O-Ti)2 on TiO2 {001} facets are possible. These fine structures have been theoretically simulated although they have yet to be directly imaged experimentally.14 We understand that different facets of TiO2 supports may have various defects or vacancies which could be additional factors affecting the fine structure of vanadia species. The heterogeneity of vanadia structures on titania is expected to be reflected on their catalytic properties, especially redox properties. Hydrogen temperature programmed reduction (H2-TPR) is a commonly used technique that probes the redox properties of a selective oxidation catalyst. Figure 8 18 ACS Paragon Plus Environment

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displays H2-TPR profiles for the VOx/TiO2 samples and the TiO2 supports. Peak temperatures are calibrated using the reduction peak of 1% RuO2/Al2O3 sample at 88 ºC. All of the VOx/TiO2 samples and TiO2 supports show no reduction below 200 ºC. The TiO2-TTB shows a weak reduction peak at 365 ºC and a much stronger reduction peak at 572 ºC. The 20%VOx/TiO2-TTB shows a broad reduction peak at 382 ºC. For the 50%VOx/TiO2-TTB and 72%VOx/TiO2-TTB, the reduction peaks shift to 425 and 437 ºC, respectively. Notably, the strong reduction peak appears on TiO2-TTB at 572 ºC is no longer observed on the supported vanadia samples, indicating strong electronic interactions between titania and vanadia. The TiO2-Rod presents a weak reduction peak at 340 ºC and a strong one at 667 ºC, showing different redox properties from the TiO2-TTB support. For the 20%VOx/TiO2-Rod sample, there exist one peak at 395 ºC for the reduction of vanadia and another at 667 ºC possibly for the reduction of TiO2-Rod support. For the 50% and 86%VOx/TiO2-Rod samples, the reduction peaks for vanadia shift to 420 and 455 ºC, respectively. It should be noted that the reduction peaks in the 300-500 ºC range may not be entirely attributed to reduction of supported vanadia as the reducibility of the titania support may be substantially altered by vanadia. The variations in redox properties as a function of vanadia coverage for these supported vanadia samples are dependent on individual structures: isolated O=V4+(O-Ti)2 on TiO2 {001} facets are more readily reducible than isolated O=V5+(O-Ti)3 and oligomerized O=V5+(O-M)3 (M = Ti or V) species on TiO2 {101}. However, contributions from the crystallite sizes and morphologies of the support materials to the overall redox properties can be difficult to estimate since samples usually have various fractions of vanadia species with different structures. In selective oxidation of methanol to formaldehyde, variations in the catalytic performance with vanadia coverage and support materials were indeed observed. We show in the present study that catalyst structures and the redox properties 19 ACS Paragon Plus Environment


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for VOx/TiO2 can be better understood by utilizing TiO2 supports with well defined morphologies. The structure-function relationships between these catalysts in methanol oxidation will be presented in the near future. 4. Conclusions The nature of the VOx species as a function of coverage and TiO2 support facets has been established using model TiO2 supports with well-defined structures. With the help from multiple characterization techniques, it is found that vanadia occupy titania surface by first forming isolated O=V4+(O-Ti)2 on TiO2 {001} facets, followed by isolated O=V5+(O-Ti)3 and then oligomerized O=V5+(O-M)3 (M = Ti or V) species on TiO2 {101} facets as vanadia surface coverage increases. Isolated O=V4+(O-Ti)2 species on TiO2 {001} facets

are more readily reduced than isolated

O=V5+(O-Ti)3 and oligomerized O=V5+(O-M)3 (M = Ti or V) species on TiO2 {101}. These new understandings on the structure of model vanadia catalysts and their redox properties are expected to provide better understanding of structure-function relationships in selective oxidation reactions on supported vanadia catalysts.

Present Addresses Dr. Wei-Zhen Li, [email protected] State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023 (China) Acknowledgements This work was supported by U. S. Department of Energy (DOE), Office of Basic Energy Sciences, Division of Chemical Sciences, Biosciences and Geosciences. The research was performed 20 ACS Paragon Plus Environment

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in the Environmental Molecular Sciences Laboratory (EMSL), a national scientific user facility sponsored by the DOE Office of Biological and Environmental Research, and located at Pacific Northwest National Laboratory (PNNL).

PNNL is operated for DOE by Battelle.

References 1. 2.

3. 4. 5.

6. 7. 8. 9. 10. 11. 12.

13. 14. 15.

Schraml, M.; Fluhr, W.; Wokaun, A.; Baiker, A. Vanadia Layers Supported on Titania - an FTIR and Raman Investigation. Ber Bunsen Phys. Chem. 1989, 93, 852-857. Nobbenhuis, M. G.; Baiker, A.; Barnickel, P.; Wokaun, A. Titania-supported Vanadium-oxide Catalysts for the Selective Oxidation of Ortho-xylene to Phthalic-Anhydride - Influence of Vanadia Content on Activity and Surface Species. Appl. Catal. A-Gen. 1992, 85, 157-172. Went, G. T.; Leu, L. J.; Bell, A. T. Quantitative Structural-Analysis of Dispersed Vanadia Species in TiO2(Anatase)-supported V2O5. J. Catal. 1992, 134, 479-491. Deo, G.; Wachs, I. E. Reactivity of Supported Vanadium-oxide Catalysts - the Partial Oxidation of Methanol. J. Catal. 1994, 146, 323-334. Handy, B. E.; Baiker, A.; Schramlmarth, M.; Wokaun, A. Vanadia Supported on TiO2-SiO2 Mixed-oxide Gels - Structure of the Dispersed Phase and Activity for the Selective Catalytic Reduction of NO with NH3. J. Catal. 1992, 133, 1-20. Harding, W. D.; Birkeland, K. E.; Kung, H. H. Selective Oxidation of Butane on Phosphorus-Modified Silica-Supported Vanadia Catalysts. Catal. Lett. 1994, 28, 1-7. Koranne, M. M.; Goodwin, J. G.; Marcelin, G. Partial Oxidation of Methane over Silica-supported and Alumina-supported Vanadia Catalysts. J. Catal. 1994, 148, 388-391. Busca, G. On the Nature of Vanadia Supported on Different Carriers - an FT-IR Study. Mater. Chem. Phys. 1988, 19, 157-165. Dines, T. J.; Rochester, C. H.; Ward, A. M. Raman-Spectroscopic Study of Titania-Supported Vanadia Catalysts. J. Chem. Soc. Faraday. T. 1991, 87, 653-656. Wu, Z. L. Multi-Wavelength Raman Spectroscopy Study of Supported Vanadia Catalysts: Structure Identification and Quantification. Chinese J. Catal. 2014, 35, 1591-1608. Hausinger, G.; Schmelz, H.; Knozinger, H. Effect of the Method of Preparation on the Properties of Titania-Supported Vanadia Catalysts. Appl. Catal. 1988, 39, 267-283. Handy, B. E.; Gorzkowska, I.; Nickl, J.; Baiker, A.; Schramlmarth, M.; Wokaun, A. Vanadia Supported on Titania Prepared by Grafting - Influence of Support Properties on the Structure and Activity of Vanadia for the Selective Catalytic Reduction of NO by NH3. Ber Bunsen Phys. Chem. 1992, 96, 1832-1840. Arena, F.; Frusteri, F.; Parmaliana, A. Structure and Dispersion of Supported-Vanadia Catalysts. Influence of the Oxide Carrier. Appl. Catal. A-Gen. 1999, 176, 189-199. Nair, N. N.; Bredow, T.; Jug, K. Toward an Understanding of the Formation of Vanadia-Titania Catalysts. J. Phys. Chem. B 2005, 109, 12115-12123. Li, X. L.; Peng, Q.; Yi, J. X.; Wang, X.; Li, Y. D. Near Monodisperse TiO2 Nanoparticles and Nanorods. Chem-Eur. J. 2006, 12, 2383-2391. 21 ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

16. 17.

18. 19.

20. 21.

22. 23.

24.

25.

26. 27. 28.

29.

30.

31.

32.

Liu, C. M.; Sun, H.; Yang, S. H. From Nanorods to Atomically Thin Wires of Anatase TiO2: Nonhydrolytic Synthesis and Characterization. Chem-Eur. J. 2010, 16, 4381-4393. Wu, B. H.; Guo, C. Y.; Zheng, N. F.; Xie, Z. X.; Stucky, G. D. Nonaqueous Production of Nanostructured Anatase with High-Energy Facets. J. Am. Chem. Soc. 2008, 130, 17563-17567. Dai, Y. Q.; Cobley, C. M.; Zeng, J.; Sun, Y. M.; Xia, Y. N. Synthesis of Anatase TiO2 Nanocrystals with Exposed {001} Facets. Nano. Lett. 2009, 9, 2455-2459. Sutradhar, N.; Biswas, A.; Pahari, S. K.; Ganguly, B.; Panda, A. B. Fluoride Free Synthesis of Anatase TiO2 Nanocrystals with Exposed Active {001} Facets. Chem. Commun. 2014, 50, 11529-11532. Li, J. M.; Xu, D. S. Tetragonal Faceted-Nanorods of Anatase TiO2 Single Crystals with a Large Percentage of Active {100} Facets. Chem. Commun. 2010, 46, 2301-2303. Shim, H. W.; Lee, D. K.; Cho, I. S.; Hong, K. S.; Kim, D. W. Facile Hydrothermal Synthesis of Porous TiO2 Nanowire Electrodes with High-Rate Capability for Li Ion Batteries. Nanotechnology 2010. 21, 255706. Khan, M. A.; Jung, H. T.; Yang, O. B. Synthesis and Characterization of Ultrahigh Crystalline TiO2 Nanotubes. J. Phys. Chem. B 2006, 110, 6626-6630. Liu, G.; Yang, H. G.; Wang, X. W.; Cheng, L. N.; Lu, H. F.; Wang, L. Z.; Lu, G. Q.; Cheng, H. M. Enhanced Photoactivity of Oxygen-Deficient Anatase TiO2 Sheets with Dominant {001} Facets. J. Phys. Chem. C 2009, 113, 21784-21788. Yang, H. G.; Liu, G.; Qiao, S. Z.; Sun, C. H.; Jin, Y. G.; Smith, S. C.; Zou, J.; Cheng, H. M.; Lu, G. Q. Solvothermal Synthesis and Photoreactivity of Anatase TiO2 Nanosheets with Dominant {001} Facets. J. Am. Chem. Soc. 2009, 131, 4078-4083. Han, X. G.; Kuang, Q.; Jin, M. S.; Xie, Z. X.; Zheng, L. S. Synthesis of Titania Nanosheets with a High Percentage of Exposed (001) Facets and Related Photocatalytic Properties. J. Am. Chem. Soc. 2009, 131, 3152-3153. Deo, G.; Wachs, I. E. Effect of Additives on the Structure and Reactivity of the Surface Vanadium-oxide Phase in V2O5/TiO2 Catalysts. J. Catal. 1994, 146, 335-345. Ramis, G.; Busca, G.; Bregani, F. On the Effect of Dopants and Additives on the State of Surface Vanadyl Centers of Vanadia-Titania Catalysts. Catal. Lett. 1993, 18, 299-303. Ichinose, H.; Taira, M.; Furuta, S.; Katsuki, H. Anatase Sol Prepared from Peroxotitanium Complex Aqueous Solution Containing Niobium or Vanadium. J. Am. Ceram. Soc. 2003, 86, 1605-1608. Zhang, Y.; Wu, L. Z.; Zeng, Q. H.; Zhi, J. F. An Approach for Controllable Synthesis of Different-Phase Titanium Dioxide Nanocomposites with Peroxotitanium Complex as Precursor. J. Phys. Chem. C, 2008, 112, 16457-16462. Seok, S. I.; Vithal, M.; Chang, J. A. Colloidal TiO2 Nanocrystals Prepared from Peroxotitanium Complex Solutions: Phase Evolution from Different Precursors. J. Colloid. Interf. Sci. 2010, 346, 66-71. Murakami, N.; Kurihara, Y.; Tsubota, T.; Ohno, T. Shape-controlled Anatase Titanium(IV) Oxide Particles Prepared by Hydrothermal Treatment of Peroxo Titanic Acid in the Presence of Polyvinyl Alcohol. J. Phys. Chem. C 2009, 113, 3062-3069. Gao, Y.; Luo, H.; Mizusugi, S.; Nagai, M. Surfactant-Free Synthesis of Anatase TiO2 Nanorods in an Aqueous Peroxotitanate Solution. Cryst. Growth Des. 2008, 8, 1804-1807. 22 ACS Paragon Plus Environment

Page 22 of 32

Page 23 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


33.

34. 35.

36.

37. 38.

39.

40. 41. 42.

43.

44. 45.

46. 47.

Groen, J. C.; Peffer, L. A. A.; Perez-Ramirez, J. Pore Size Determination in Modified Microand Mesoporous Materials. Pitfalls and Limitations in Gas Adsorption Data Analysis. Micropor. Mesopor. Mat. 2003, 60, 1-17. Barnard, A. S.; Curtiss, L. A. Prediction of TiO2 Nanoparticle Phase and Shape Transitions Controlled by Surface Chemistry. Nano. Lett. 2005, 5, 1261-1266. Yang, H. G.; Sun, C. H.; Qiao, S. Z.; Zou, J.; Liu, G.; Smith, S. C.; Cheng, H. M.; Lu, G. Q. Anatase TiO2 Single Crystals with a Large Percentage of Reactive Facets. Nature, 2008, 453, 638-U4. Penn, R. L.; Banfield, J. F. Morphology Development and Crystal Growth in Nanocrystalline Aggregates under Hydrothermal Conditions: Insights from Titania. Geochim. Cosmochim. Ac. 1999, 63, 1549-1557. Horn, M.; Schwerdtfeger, C. F.; Meagher, E. P. Refinement of the Structure of Anatase at Several Temperatures. Zeitschrift fur Kristallographie, 1972, 136, 273-281. Nickl, J.; Schlogl, R.; Baiker, A.; Knozinger, H.; Ertl, G. Preparation and Characterization of a Model System for the Study of Monolayers and Multilayers of Vanadia Supported on Titania. Catal. Lett. 1989, 3, 379-387. Fierro, J. L. G.; Gambaro, L. A.; Cooper, T. A.; Kremenic, G. Structure and Activity of Silica-Supported Vanadia Catalysts for the Oxidation of Propylene. Appl. Catal. 1983, 6, 363-378. Sawatzky, G. A.; Post, D. X-Ray Photoelectron and Auger-Spectroscopy Study of Some Vanadium-oxides. Phys. Rev. B 1979, 20, 1546-1555. Aad, E. A.; Aboukais, A. Characterisation by EPR Spectroscopy. Catal. Today 2000, 56, 371-378. Hu, J. Z.; Xu, S. C.; Li, W. Z.; Hu, M. Y.; Deng, X. C.; Dixon, D. A.; Vasiliu, M.; Craciun, R.; Wang, Y.; Bao, X. H.; et al Investigation of the Structure and Active Sites of TiO2 Nano-Rod Supported VOx Catalysts by High Field and Fast Spinning 51V MAS NMR. ACS Catal. 2015, DOI:10.1021/acscatal.1025b00286. Nag, N. K.; Chary, K. V. R.; Rao, B. R.; Subrahmanyam, V. S. Characterization of Supported Vanadium-Oxide Catalysts by Low-Temperature Oxygen-Chemisorption Technique .2. The V2O5/SiO2 System. Appl. Catal. 1987, 31, 73-85. Chiarello, G.; Robba, D.; Demichele, G.; Parmigiani, F. An X-Ray Photoelectron Spectroscopy Study of the Vanadia Titania Catalysts. Appl. Surf. Sci. 1993, 64, 91-96. Centi, G.; Giamello, E.; Pinelli, D.; Trifiro, F. Surface-Structure and Reactivity of V-Ti-O Catalysts Prepared by Solid-State Reaction .1. Formation of a VIV-Interacting Layer. J. Catal. 1991, 130, 220-237. Finnie, K. S.; Cassidy, D. J.; Bartlett, J. R.; Woolfrey, J. L. IR Spectroscopy of Surface Water and Hydroxyl Species on Nanocrystalline TiO2 Films. Langmuir 2001, 17, 816-820. Arrouvel, C.; Digne, M.; Breysse, M.; Toulhoat, H.; Raybaud, P. Effects of Morphology on Surface Hydroxyl Concentration: A DFT Comparison of Anatase-TiO2 and Gamma-Alumina Catalytic Supports. J. Catal. 2004, 222, 152-166.


300 250 200

72%VOx/TiO2-TTB TiO2-Rod

0.5

0.0 0

150 100

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(b)

TiO2-TTB

1.0

Intensity (a.u.)

(a) Volume (mL/g) STP

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

dV(logd) (mL/g)


50

100

Pore diameter (nm)

TiO2-TTB

TiO2-TTB 86%VOx/TiO2-Rod TiO2-Rod

50 0 0.0

V2O5

TiO2-Rod 0.2

0.4

0.6

0.8

1.0

10 20 30 40 50 60 70 80

Releative pressure (P/P0)

o

2θ ( )

Fig. 1. (a) Nitrogen adsorption-desorption isotherms and the pore size distributions (insert) for TiO2-TTB and TiO2-Rod supports; and (b) XRD patterns for TiO2 supports and supported vanadia catalysts. Patterns for bulk V2O5 are also included.


(a)

(b)

(e)

Frenquency (%)

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30

{001}/{101} = 2 2 2 A /(1.73*(B -A ))

(f)

20 10 0 16 18 20 22 24

Frenquency (%)

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30 {001}/{101} = D/2L 20 10 0 3

5

7

9

11

{001}/({101}+{001}) (%)

{001}/({101}+{001}) (%)

(c)

(g)

(d)

(h)

Fig. 2. TEM images for TiO2-TTB (a: as-prepared, b, c: calcined) and TiO2-Rod (e: as-prepared, f, g: calcined) samples; illustrations of the crystallite shapes (insert of a, e), the ratios of {001} facets to total surfaces (insert of b, f) and selected area electron diffraction pattern (insert of g). Also shown are the top-view of (101) (4x4) and (001) (4x4) surfaces with top two layers of oxygens (d, h). The 25 ACS Paragon Plus Environment


oxygen atoms are in red and titanium atoms are in blue. The bright red oxygen atoms are above the surface (displayed in violet); and the dark red oxygen atoms and titanium atoms are underneath. Also displayed are possible oxygen assembles for VOx monomers.

(a) mVO /TiO -TTB x 2

516.8

(b) mVO /TiO -Rod x 2

V2p3/2

V2p3/2 516.8 515.8

515.8

86%

Intensity (a.u.)

72%

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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57% 50% 35%

68% 50% 33% 25%

29%

20%

20%

15%

10%

10%

520

7% 3%

(4X)

3% 518

516

(2X) (4X) (4X)

520

514

Binding Energy (eV)

518

516

514

Binding Energy (eV)

Fig. 3. V2p XPS spectra of mVOx/TiO2-TTB (a) and mVOx/TiO2-Rod (b) samples.


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(a)

(b)

mVOx/TiO2-TTB

mVOx/TiO2-Rod

3+

3+

Ti m = 57%

m = 86%

V

4+

Ti

4+

V

Intensity (a.u.)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


35% 20% 3% TiO2-TTB

50% 15% 3% TiO2-Rod

2800 3000 3200 3400 3600 3800

2800 3000 3200 3400 3600 3800

Magnetic field (G)

Magnetic field (G)

Fig. 4. EPR spectra of the TiO2-TTB support and mVOx/TiO2-TTB catalysts (a), and the TiO2-Rod support and mVOx/TiO2-Rod (b) catalysts.

Solid vertical line indicates the field position of Ti3+,

while the dashed lines show the position of the three high field A┴ transitions of V4+, with a separation of ~70 G that is typical for vanadia on titania. The lower field peaks are not resolved due to obscuration by the Ti3+ and low signal intensity.


40

V

5+

40

4+

V

20

20 V

0 0

4+

0

V

60

60 5+

V

40 20 0 0

40 20

4+

V

V

4+

20 40 60 80 VOx surface coverage (%)

Fig. 5. Percentages and coverages of V4+ and V5+ species on mVOx/TiO2-TTB (a) and

mVOx/TiO2-Rod (b) samples calculated by deconvoluting peaks in the XPS spectra.


0

(%)

80

5+

5+

80

100

VOx/TiO2-Rod

or V

5+

20 40 60 80 VOx surface coverage (%)

(b)

4+

60

100

Page 28 of 32

Coverage of V

60

4+

80

5+

V

5+

80

100

or V (%)

VOx/TiO2-TTB

4+

(a)

Coverage of V

4+

5+

or V (%)

100

Percentage of V

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Percentage of V or V (%)


Page 29 of 32

(b)

m=72%

Intensity (a.u.)

57% 50% 35% 24% 20% 3%

1026

1026

10%

1150

TiO2-TTB 950

750

mVOx/TiO2-Rod

X5 1031 1002

mVOx/TiO2-TTB

X5 1031 1002

(a)

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


550

350

1150

-1

Raman Shift (cm )

m=86% 68% 50% 33% 25% 20% 15% 10% 7% 3% TiO2-Rod

950

750

550

350

-1

Raman Shift (cm )

Fig. 6. Raman spectra of the dehydrated TiO2-TTB support and mVOx/TiO2-TTB (a), and the TiO2-Rod support and mVOx/TiO2-Rod (b) samples.



3729

% m = 50 % m = 10 B TiO 2TT

Bare Sample

CH3OH adsorption

3700

3600

3500

-1

% m = 50 % m = 10 od TiO 2-R

3800

Wavenumber (cm )

% m = 50 % m = 10 od TiO 2-R

Bare Sample

3733 3712

% m = 50 % m = 10 B TiO 2-TT

3733 3712 3800

(b) mVOx/TiO2-Rod Intensity (a.u.)

3729

(a) mVOx/TiO2-TTB Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 32

CH3OH adsorption

3700

3600

3500

-1

Wavenumber (cm )

Fig. 7. FTIR spectra of TiO2-TTB support and mVOx/TiO2-TTB (a) and TiO2-Rod support and

mVOx/TiO2-Rod (b) samples. Background spectrum in top figure is an empty reactor and that in bottom figure is the sample prior to methanol exposure. All spectra were taken at 150 °C. Bare samples were dehydrated at 400 °C for 30 min in flowing He. Methanol (4% in He) was then introduced for 10 min flushed by He for 30 min at 150 °C.


50

50

420

m = 50%

395

TiO2-TTB

200 350 500 650 800

m = 86%

m = 20% 667

572

m = 20%

340

m = 50%

625

455

mVOx/TiO2-Rod

H2 consumption (a. u.)

437 425

m = 72%

382

88

(b)

mVOx/TiO2-TTB

365

(a) H2 consumption (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


88

Page 31 of 32

TiO2-Rod

200 350 500 650 800 o

o

Temperature ( C)

Temperature ( C)

Fig. 8. TPR profiles for TiO2-TTB support and mVOx/TiO2-TTB (a) and TiO2-Rod support and

mVOx/TiO2-Rod (b) samples. TPR data was normalized to 1.2 mg V2O5; reduction of a 1% RuO2/Al2O3 sample was used as an internal standard for temperature calibration.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Table of Contents


Page 32 of 32

Nanocrystalline Anatase Titania-Supported Vanadia Catalysts: Facet

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