Kinetics and Mechanism of Methanol Conversion over Anatase Titania

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Kinetics and Mechanism of Methanol Conversion over Anatase Titania Nanoshapes Guo Shiou Foo, Guoxiang Hu, Zachary D. Hood, Meijun Li, De-en Jiang, and Zili Wu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b01456 • Publication Date (Web): 05 Jul 2017 Downloaded from http://pubs.acs.org on July 6, 2017

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Kinetics and Mechanism of Methanol Conversion over Anatase Titania Nanoshapes Guo Shiou Foo,† Guoxiang Hu,‡ Zachary D. Hood,§ Meijun Li,† De-en Jiang,‡ Zili Wu†*

† Chemical Sciences Division and Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ‡ Department of Chemistry, University of California, Riverside, California 92521, United States § School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332, United States *Email: [email protected]

Notice: This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC0500OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

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Abstract: The kinetics and mechanism of methanol dehydration, redox and oxidative coupling were investigated at 300 °C under dilute oxygen concentration over anatase TiO2 nanoplates and truncated bi-pyramidal nanocrystals in order to understand the surface structure effect of TiO2. The two TiO2 nanoshapes displayed both (001) and (101) facets, with a higher fraction of (001) facet exposed on the nanoplates while truncated bi-pyramidal nanocrystals were dominated by (101) facet. Kinetic study using in situ titration with ammonia shows that the active sites for methanol dehydration are acidic and non-equivalent compared to redox and oxidative coupling. In situ FTIR spectroscopy reveals that adsorbed methoxy is the dominant surface species for all reactions while the observed methanol dimer is found to be a spectator species through isotopic methanol exchange, supporting the dissociative mechanism for methanol dehydration via surface methoxy over TiO2 surfaces. Density functional theory calculations show that the formation of dimethyl ether involves the C-H bond dissociation of an adsorbed methoxy, followed by coupling with another surface methoxy on the fivefold coordinated Ti cations on the (101) surface, similar to the mechanism reported on (001) surface. Kinetic isotope effects are observed for dimethyl ether, formaldehyde, and methyl formate in the presence of deuterated methanol (CD3OH and CD3OD), confirming that the cleavage of the C-H bond is the rate-limiting step for the formation of these products. Comparison between estimated kinetic parameters for methanol dehydration over various TiO2 nanocrystals suggest that (001) has a higher dehydration reactivity compared to (101), but the surface density of active sites could be limited by the presence of residue fluorine atoms originating from the synthesis. The (001) surface of TiO2 is also more active than the (101) surface in redox and oxidative coupling of methanol, which is due to the reactive surface oxygen on (001) compared to the (101) surface.


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Keywords: TiO2, surface structure, dehydration, redox, oxidative coupling, dimethyl ether, formaldehyde, methyl formate


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1. Introduction The conversion of methanol is paramount in C1 chemistry, as it can undergo dehydration to form dimethyl ether (DME),1 redox to form formaldehyde,2 and oxidative coupling to form methyl formate.3 These products are vastly important in different chemical industries. DME can be used as a replacement for fossil fuels such as liquefied petroleum gas (LPG) and diesel.4-5 Furthermore, it is a key intermediate in the production of olefins and aromatics over zeolite catalysts.6-11 Both formaldehyde and methyl formate are used in the agriculture and fragrance industry.12-13 In addition, methyl formate is an intermediate in the manufacturing of household products. Most of the research on the conversion of methanol has been carried out using a variety of catalysts, such as γ-Al2O3,14-15 zeolites,1, 16-17 and mixed metal oxides.18-19 However, there are few studies that utilize titanium dioxide (TiO2) compared to other metal oxides.20 The advantages of TiO2 over other catalysts include low toxicity, high chemical and thermal stability, and its natural earth abundance.21 The three polymorphs of TiO2 are rutile, brookite and anatase. Although the rutile polymorph is the most thermodynamically stable phase, it has been shown that anatase is catalytically more active.22 Unfortunately, brookite is often found as a mixture with the other polymorphs. Studies detailing the conversion of methanol over TiO2 has focused on using singlecrystal model surfaces for adsorption and reaction.23 Among the various polymorphs and facets, theoretical studies suggested that the anatase TiO2 (001) surface has the highest reactivity.24-25 In addition, it was reported that the anatase TiO2 (001) facet undergoes reconstruction to form an energetically favorable two-domain (1×4) surface.26-27 However, most anatase TiO2 crystals are


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dominated by the thermodynamically stable (101) facet.22, 24, 28 As shown in Figure 1, the surface atomic arrangement and coordination status of the Ti and O sites are different on the (001) and (101) surfaces. On the (101) surface, the twofold-coordinated oxygen anions are bound to the fivefold-coordinated Ti4+ cations, while the Ti4+ cations on the ridge and terrace of the reconstructed (001) surface are fourfold and fivefold-coordinated, respectively. The surface oxygen anions on (001) are all twofold-coordinated. In an effort to synthesize anatase TiO2 nanocrystals with the highly reactive surface, Yang et al. reported the use of hydrofluoric acid to stabilize the formation of the (001) facet.29

Figure 1. Front view of the structure of anatase TiO2 (101) and (001) - (1×4) surface. Color code: red, O; grey: Ti. Bennett et al. performed temperature-programmed desorption of methanol on truncated bi-pyramidal nanocrystals to examine the effect of crystallite size and shape of anatase TiO2 on the reactivity.30-31 The nanocrystals were mainly dominated by (101) compared to (001). It was concluded that methanol only adsorbs molecularly on the fivefold-coordinated Ti cations of the (101) surface, while the minority (001) surface is responsible for the formation of DME. This result is in contradiction with reports from the literature, as it has been shown that methanol can 5 ACS Paragon Plus Environment

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adsorb dissociatively on anatase TiO2 (101) surfaces.32 Recently, Xiong et al. studied the dehydration of methanol on anatase TiO2 (001) single crystals.33 With the aid of theoretical calculations, the authors demonstrated that the formation of DME proceeds through the coupling of methoxy species on the fourfold-coordinated Ti4+ sites. However, there is still a lack of understanding in the reaction mechanism on the dehydration of methanol over anatase TiO2 (101) surface. Furthermore, TiO2 is often used as a support for the redox and oxidative coupling of methanol over noble metals and reducible metal oxides,34-35 and there are limited studies that examine the redox ability of various anatase TiO2 facets. To the best of our knowledge, there are no reports in the literature that compare the kinetics of methanol conversion between the two anatase TiO2 (101) and (001) facets on nanocrystalline powders, which are relevant for practical applications. In this work, we used anatase TiO2 nanoplates and truncated bi-pyramidal nanocrystals to investigate the dehydration and oxidation of methanol over (001) and (101) surfaces. TiO2 nanoplates have a higher fraction of (001) facets, while the TiO2 truncated bi-pyramidal nanocrystals are mainly dominated by (101) facets.36 Through the characterization of the active sites, along with kinetic, spectroscopy and theoretical analysis, we found that the formation of DME can occur through the dissociative pathway on the (101) facet, but at a lower rate compared to (001). In addition, the mechanistic framework for the redox and oxidative coupling reactions of methanol over the titania nanocrystals was also illustrated. 2. Experimental Section Synthesis. The synthesis of TiO2 nanocrystals follows a report in literature.36 Aqueous TiCl4 solution was prepared by adding 6.6 mL of TiCl4 dropwise to 50 ml of aqueous HCl


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solution (0.43 M) under vigorous stirring in an ice bath. The resulting aqueous TiCl4 solution was then added dropwise to a 5 wt.% aqueous NH3⋅H2O solution under vigorous stirring. After a light blue Ti(OH)4 (precursor) precipitate was formed, 10 mL of 4 wt.% aqueous NH3⋅H2O solution was added to adjust the pH between 6 and 7. After 2 h of ageing at room temperature, the suspension was centrifuged. The Ti(OH)4 precipitate was washed twice with water and once with ethanol. To synthesize TiO2 truncated bi-pyramidal nanocrystals, 2 g of fresh Ti(OH)4 precursor was dispersed in 30 mL of 50 vol.% water/isoproponal. The suspension was transferred to a 50 mL Teflon-lined autoclave, and was heated to 180 °C for 24 h. After the reaction, the resulting product was centrifuged and washed thrice with deionized water, and once with ethanol. The sample was calcined at 300 °C for 4 h in air and named Octahedra_300C. TiO2 nanoplates were prepared using titanium(IV) butoxide as the precursor. Titanium(IV) butoxide (5 mL) was added to 20 mL ethanol dropwise under stirring. Subsequently, 0.5 mL of 47 wt.% aqueous HF was slowly added after stirring for 15 min. The suspension was further stirred for 30 min before transferring to a 50 ml Teflon-lined autoclave and was heated to 180 °C for 24 h. The product was centrifuged after the reaction and washed thrice with ethanol. The product was calcined at 300 °C or 500 °C for 4 h in air, and the samples were named Disks_300C and Disks_500C, respectively. Na+ exchanged Disks_300C was synthesized to block Brønsted acid sites on the surface. Briefly, 100 mg of Disks_300C was stirred in 200 ml of a NaCl solution (1 M). The solution was maintained at 60 °C and the pH was adjusted between 11.0-11.5 by periodically adding different volumes of a 0.05 M NaOH solution. After 24 h, the sample was centrifuged and washed five


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times with deionized water. The sample was then dried in an oven at 100 °C for 8 h and named as Na+/Disks_300C. To investigate the effect of fluorine content on Brønsted acidity, the impregnation of fluoride on Disks_500C was performed based on a reported study.37 Briefly, 100 mg of Disks_500C was stirred in 100 ml of deionized water. Next, 60 wt.% of ammonium fluoride (60 mg) was added into the suspension and stirred for 8 h. Subsequently, the suspension was dried in an oven at 100 °C for 12 h. The resulting sample was calcined at 500 °C for 4 h in air and named as F-/Disks_500C. Characterization. Nitrogen physisorption was performed on a Micromeritics Gemini 2375 Surface Area and Pore Size Analyzer at -196 °C. Each catalyst was evacuated for 1 h prior to measurement. The BET method38 was used to calculate surface areas. X-ray diffraction (XRD) patterns were collected using a PANalytical X’Pert Pro system with Cu Κα radiation. Diffractograms were obtained at incident angles for 2θ = 7‒90°. A Hitachi HT7700 microscope (Hitachi, Tokyo, Japan) was used to collect all transmission electron microscopy (TEM) images at an operating voltage of 120 kV. Each sample was sonicated in ethanol, drop casted onto a carbon-coated copper grid, and dried under ambient conditions prior to TEM analysis. X-ray photoelectron spectroscopy (XPS) measurements were collected on a Thermo Scientific K-Alpha spectrometer. Each spectrum was collected over 30 scans at an operating pressure under 3.0 x 10-7 Pa with a spot size of 400 µm using a Al-Kα microfused monochromatized source (1486.6 eV) with a resolution of 0.1 eV. Elemental compositions were determined for F, Ti, and O using the Avantage Data System, a software package available through Thermo Scientific.


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To determine the strength and concentration of adsorption sites, the chemisorption of NH3, methanol, and oxygen were performed using a Micromeritics 3Flex Characterization Analyzer coupled with a Setaram Sensys Evo DSC microcalorimeter.39 Each sample was loaded into one side of a custom-made quartz bi-tube,40 and the tube was inserted into the aluminum block of the microcalorimeter. The tube opening was connected to the chemisorption port of the 3Flex instrument for degassing and dosing. Each sample was evacuated and heated at 300 °C for 1 h, dosed with 500 mmHg of O2 and evacuated for 30 min each for two rounds, and then cooled down to 150 °C (NH3 adsorption) or 30 °C (methanol or oxygen adsorption). For oxygen chemisorption, 500 mmHg of H2 was dosed instead of O2 during pretreatment. The temperature was held for 1 h to reach thermal equilibrium. Subsequently, the sample was exposed to doses of NH3, methanol, or oxygen. After the final target pressure was reached, the sample was evacuated for 1 h at the same temperature and dosed with the same adsorbate again. The concentration of adsorption sites was determined by the amount of irreversibly adsorbed probe molecules. A control experiment using an empty bi-tube showed negligible amount of irreversible adsorption of the probe molecule. Steady State Kinetic Measurements. Reactivity experiments for the conversion of methanol were performed in an Altamira Instruments system (AMI-200). Each catalyst (0.025 g) was sieved (177 – 250 µm), mixed with silicon carbide (0.200 g), and placed inside a quartz utube. The catalysts bed was held in place by placing quartz wool at both ends of the bed. Methanol (99.8%, Sigma Aldrich) was fed into the reactor using a Chemyx Nexus 3000 syringe pump with 30 ml/min of 5% O2/He. All of the lines were heated to 100 °C to prevent condensation. Experiments were performed at 260-300 °C and under differential conditions (< 10% conversion). The catalyst was pretreated at 300 °C (Disks_300C and Octahedra_300C) or 9 ACS Paragon Plus Environment

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500 °C (Disks_500C) for 1 h under 30 ml/min of 5 % O2/He (ultra-high purity, Air Liquide) before methanol was introduced into the reactor at various feed rates. For in situ titration experiments using NH3 or CO2, 2% NH3/He or CO2 (99.98%) was fed into the reactor at various flow rates with 5% O2/He, while ensuring the total gas flow and molar ratio of methanol to oxygen are constant. The products were analyzed and quantified using a Buck Scientific Model 910 gas chromatograph equipped with a Restek 4% carbowax-20M column, a flame ionization detector and a methanizer. The Weisz-Prater criterion was applied to ensure that all reactivity experiments were performed under conditions free of internal mass transfer limitations (see Supporting Information). FTIR Spectroscopy. To determine the nature of the acid sites, pyridine and ammonia adsorption followed by FTIR spectroscopy was performed on a Thermo Nicolet Nexus 670 FTIR spectrometer equipped with an MCT detector. Each spectrum was recorded with 32 scans at a resolution of 4 cm-1. Each sample was loaded into a porous ceramic cup, and the cup was inserted into a Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) cell (HC-900, Pike Technologies). The sample was pretreated at 300 or 500 °C for 1 h under 30 mL/min of 5% O2/He and cooled down to 150 (pyridine) or 300 °C (ammonia). The gas was switched to 30 mL/min of He and a background spectrum was collected. For pyridine adsorption, He was bubbled through a saturator filled with liquid pyridine at 25 °C. A pulse of pyridine saturated gas (sample loop 0.5 mL) was introduced into the sample cell. After purging with 30 mL/min of He for 15 min, a spectrum was collected. For ammonia adsorption, a pulse of 2% NH3/He (0.5 mL sample loop) was introduced into the DRIFTS cell and purged with 30 mL/min of He for 10 min before a spectrum was collected.


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To observe the type of surface species present during the reaction, the conversion of methanol was performed in the DRIFTS cell under reaction conditions similar to the AMI-200 instrument. Each sample was pretreated at 300 or 500 °C for 1 h in 30 mL/min of 5% O2/He and a background spectrum was collected at 300 °C. Subsequently, methanol was fed into the DRIFTS cell using a Chemyx Nexus 3000 syringe pump for 1 h at various flow rates. At the end of the experiment, the DRIFTS cell was purged with 30 mL/min of 5% O2/He for 1 h. For in situ titration experiments using NH3, 2% NH3/He was introduced into the DRIFTS cell at 5 mL/min for 1 h while methanol was flowing through the cell. The total gas flow rate and molar ratio of methanol to oxygen are constant prior to the introduction of NH3. For the isotopic methanol exchange experiment over Disks_300C, the sample was initially exposed to natural abundant methanol flow for 1 h with 30 mL/min of 5% O2/He at 300 °C. Subsequently, methanol was stopped and the sample was purged for 1 h under the same carrier gas. Deuterated methanol (CD3OD) was then introduced into the cell at the same feed rate for 1 h and changes were monitored. Computational methods. Density functional theory (DFT) calculations were performed using the Vienna ab initio simulation package (VASP).41 The ion-electron interaction was described with the projector augmented wave (PAW) method.42 Electron exchange-correlation was represented by the functional of Perdew, Burke and Ernzerhof (PBE) of generalized gradient approximation (GGA).43 A cutoff energy of 400 eV was used for the plane-wave basis set. The anatase TiO2(101) and TiO2(001) surfaces were modeled with four TiO2 layers. Reconstruction of the anatase TiO2(001) suggested by Lazzeri and Selloni was used.44 A vacuum layer of 15 Å along the z-direction was employed. The Brillouin zone was sampled by (3×3×1) MonkhorstPack k-point mesh. The top two TiO2 layers of the slab were allowed to relax together with the 11 ACS Paragon Plus Environment

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adsorbates and the convergence threshold for structural optimization was set to be 0.025 eV/Å in force. The van der Waals interactions were included using the DFT-D3 method.45 The adsorption energies of methanol on TiO2 surfaces were calculated by = ( − − )/, where , , and represent the total energy of the methanol-slab system, the total energy of the slab, and the energy of one gas phase methanol molecule, respectively. A negative value of suggests favorable absorption. The climbingimage nudged elastic band (CI-NEB) method implemented in VASP was used to determine the energy barriers.46 The transition states were obtained by relaxing the force below 0.05 eV/Å. 3. Results 3.1 Morphology and surface properties of TiO2 nanocrystals Table 1 shows the surface area of the different titania nanocrystals obtained from nitrogen physisorption. Octahedra_300C displayed the highest surface area, followed by Disks_300C and Disks_500C. Table 1. Surface area of TiO2 nanocrystals and surface density of sites accessible by ammonia, methanol and oxygen. Catalyst Disks_300C Disks_500C Octahedra_300C a: Negligible amount

Surface Area (m2/g) 60 36 101

Ammonia (µmol/m2) 2.7 2.4 3.0

Methanol (µmol/m2) 5.8 5.6 7.4

Oxygen (µmol/m2) 0.09 N.Aa N.Aa

Figure 2A shows the XRD patterns of Disks_300C, Disks_500C, and Octahedra_300C nanocrystals, all displaying the anatase phase of titania (JCPDS File No. 21-1272).47-48 Figure 2B-2D shows TEM images of the TiO2 nanocrystals along with the schematic drawing of each 12 ACS Paragon Plus Environment

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sample, which is obtained from simulation of the X-ray diffraction data using the same samples.49 All of them are terminated with both (001) and (101) facets. On the basis of the simulated shapes, the percentage of the (001) facet is estimated to be 39, 14, and 10% on Disks_300C, Disks_500C, and Octahedra_300C, respectively, according to our recent work of facet analysis of TiO2 nanocrystals.49 Furthermore, the average crystallite size for each TiO2 catalyst is summarized in Table S1.

Figure 2. (A) XRD patterns of the different anatase TiO2 nanocrystals, TEM images of (B) Disks_300C, (C), Disks_500C, (D) Octahedra_300C. Inset: schematic drawings of TiO2 nanocrystals.


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To quantify the fluorine content after the synthetic processes, the surfaces of the TiO2 nanocrystals were analyzed via XPS (Figure 3). The spectra showed a peak around 684.9 ± 0.2 eV, which is consistent with fluorine bound to the surface and does not indicate the presence of fluorine doped into the TiO2 lattice.29, 50-52 Disks_300C displayed the highest fluorine content, while this value decreased significantly after calcination at 500 °C (Disks_500C). The fluorine content of F-/Disks_500C increased to 3.67 % after impregnation with ammonium fluoride, while much smaller amount of F remained after Na+ exchange with Disks_300C, which has negligible Na content. Octahedra_300C has a negligible amount of fluorine content, consistent with the synthesis procedure where no F source was used.

684.9

Catalyst

F content (%)

Disks_300C

8.31

-

Counts (s-1)

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F /Disks_500C

3.67

Na+/Disks_300C

1.31

Disks_500C

0.85

Disks_300C F-/Disks_500C Na+/Disks_300C Disks_500C

680

685 690 Binding Energy (eV)

695

Figure 3. XPS spectra of the F 1s peak for TiO2 nanocrystals. Inset: atomic percentage of F content. Figure 4 shows the FTIR spectra for the adsorption of pyridine on the TiO2 nanocrystals to determine the nature of acid sites present. The peaks observed at 1445 cm-1 and 1605-1610 cm-1 are assigned to pyridine coordinated to Lewis acid sites, which are exposed 14 ACS Paragon Plus Environment

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undercoordinated surface Ti cations.53-54 The peak at 1540 cm-1 is only observed for Disks_300C, indicating the presence of Brønsted acid sites. NaOH treatment of Disks_300C eliminated these acid sites, which seems to indicate that Brønsted acid sites are associated with the presence of F on TiO2. However, the addition of F- to TiO2 Disks_500C did not result in new acid sites with Brønsted acidic nature. More discussion on the Brønsted acid sites can be found in the Supporting Information.

1445

1610 1540 Absorbance (a.u.)

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Disks_300C

1605

Disks_500C Octahedra_300C Na+/Disks_300C F-/Disks_500C 1600

1500 Wavenumbers (cm-1)

1400

Figure 4. FTIR spectra of pyridine adsorption on TiO2 nanocrystals at 150 °C. To quantify the strength and surface density of adsorption sites and oxygen vacancies, the adsorption of ammonia, methanol, and oxygen was performed on the TiO2 nanocrystals using a microcalorimeter. Figure 5 shows the heat of adsorption of ammonia and methanol, and the heat of adsorption of oxygen is shown in Figure S1. The initial heats of adsorption of ammonia on the TiO2 nanocrystals are approximately 110 kJ/mol. Disks_300C and Disks_500C have a similar distribution of acid sites, while Octahedra_300C have slightly weaker acid sites at low surface coverage (< 2 µmol/m2). For the adsorption of methanol on the TiO2 nanocrystals, the 15 ACS Paragon Plus Environment

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initial heat of adsorption is approximately 102 kJ/mol regardless of the TiO2 surfaces, and the distribution of sites is also similar up to a surface coverage of 6 µmol/m2. The differential heats of oxygen adsorption on the H2 pretreated samples are significantly higher compared to ammonia and methanol, which indicates the dissociation and chemisorption of oxygen and the reoxidation of TiO2.

Heat of Adsorption (kJ/mol)

140

(A)

120 100 80 60 40

Disks_300C Disks_500C Octahedra_300C

20 0 0

1

2

3

4

5

6

2

Surface Coverage (µmol/m ) 120 Heat of Adsorption (kJ/mol)

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(B)

100 80 60 40 Disks_300C Disks_500C Octahedra_300C

20 0 0

2

4

6

8

10

2

Surface Coverage (µmol/m )

Figure 5. Heat of adsorption of (A) ammonia and (B) methanol on TiO2 nanocrystals.


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The surface density of sites that are irreversibly adsorbed by ammonia, methanol and oxygen are shown in Table 1. The density of acid sites on the three TiO2 nanocrystals is close, between 2.4 to 3.0 µmol/g. The maximum surface density of sites accessible by methanol adsorption on Octahedra_300C is over 20% higher compared to Disks_300C and Disks_500C. The H2 pretreated Disks_300C has the highest surface density of sites accessible by oxygen, while it is negligible for Disks_500C and Octahedra_300C. This result indicates that TiO2 with a higher fraction of (001) facets is more prone to H2 reduction to generate O-vacancies compared to the samples with a lower fraction of (001) facets. 3.2 Steady state kinetic measurement Methanol conversion was performed over the three TiO2 nanocrystals in the presence of dilute oxygen. Figure 6 shows the synthesis rates (normalized by the surface area of TiO2) of dimethyl ether (dehydration pathway), formaldehyde (redox pathway), and methyl formate (oxidative coupling pathway) as a function of methanol pressure. For the formation of dimethyl ether, Octahedra_300C consistently displayed the lowest synthesis rate compared to Disks_300C and Disks_500C. The synthesis rate of formaldehyde and methyl formate is 1~2 orders of magnitude lower compared to dimethyl ether, and it decreased as the ratio of oxygen to methanol decreased to less than 1 (methanol pressures > 5 kPa). No observable amount of formaldehyde and methyl formate is formed from the conversion of methanol over Disks_500C. In the absence of oxygen, only dimethyl ether is observed for all TiO2 nanocrystals. Overall, dehydration is the dominant pathway for methanol conversion over TiO2 surfaces.


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Methyl Formate Synthesis Rate Formaldehye Synthesis Rate Dimethyl Ether Synthesis Rate (µmol/m2.min) (µmol/m2.min) (µmol/m2.min)

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(A)

3.00

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Disks_300C

Disks_500C

2.00

Octahedra_300C 1.00

0.20

(B)

Disks_300C Octahedra_300C

(C)

Disks_300C Octahedra_300C

0.15 0.10 0.05 0.00

0.09

0.06

0.03

0

2

4

6

8

10

Methanol Pressure (kPa)

Figure 6. (A) Dimethyl ether, (B) formaldehyde, and (C) methyl formate synthesis rate from the conversion of methanol over TiO2 nanocrystals. Solid lines for dimethyl ether represent model fits from equation 1. Reaction conditions: 300 °C, PMeOH = 1.01-9.27 kPa, 30 ml/min 5% O2/He, 25 mg catalyst.


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Table 2 shows the apparent activation energy for the formation of dimethyl ether (dehydration), formaldehyde (redox), and methyl formate (oxidative coupling). For the same reaction, the apparent activation energy is similar between various TiO2 nanocrystals. Dehydration reaction displayed the highest apparent activation energy, followed by redox and oxidative coupling. In the absence of oxygen, a similar apparent activation energy of 110 kJ/mol for dimethyl ether is observed for all three TiO2 nanocrystals. Table 2. Apparent activation energies for the conversion of methanol over TiO2 nanocrystals at 260-300 °C, 30 mL/min 5% O2/He, 25 mg catalyst and 3.92 kPa methanol. Apparent Activation Energy (kJ/mol) Catalyst Dehydration Redox Oxidative Coupling Disks_300C 115 88 77 Disks_500C 109 Octahedra_300C 106 94 85 Experimental error: ± 10 kJ/mol In situ titration experiments were performed over Disks_300C and Octahedra_300C to assess sites requirement for the formation of dimethyl ether, formaldehyde and methyl formate. Ammonia was used to probe acidic sites, while CO2 was used to probe basic sites. Upon introduction of 0.25 kPa of ammonia, the formation rate of dimethyl ether rapidly decreased to a different extent between Disks_300C (81%) and Octahedra_300C (52%), while the formation of formaldehyde and methyl formate was mostly blocked (Figure 7A and 7B). The rates of the products are restored to a certain extent once the flow of ammonia was stopped. The effects of in situ titration using CO2 are shown in Figure S2A and S2B. The rates of dimethyl ether, formaldehyde and methyl formate are not affected in the presence of 11 and 23 kPa of CO2. The adsorption of ammonia on Disks_300C and Octahedra_300C was performed in the DRIFTS cell to determine the nature of acid sites present at the reaction temperature of 300 °C (Figure S3). 19 ACS Paragon Plus Environment

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Brønsted acid sites are observed on Disks_300C, while Lewis acid sites are present for both catalysts, which is in good agreement with pyridine adsorption.

Normalized Synthesis Rate

(A)

1.2 1.0 0.8 0.6

PNH3

PNH3

PNH3

0 kPa

0.25 kPa

0 kPa

0.4 DME FH MF

0.2 0.0 0

30

60

90

120 150 180 210

Time (min)

(B) Normalized Synthesis Rate

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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1.2 1.0 0.8 0.6

PNH3 PNH3

0 kPa

0.4

0.25 kPa

PNH3 0 kPa

DME FH MF

0.2 0.0 0

30

60

90 120 150 180 210 Time (min)

Figure 7. Transient formation rates of dimethyl ether (DME), formaldehyde (FH) and methyl formate (MF) from the conversion of methanol over Disks_300C (A), and Octahedra_300C (B) during co-feeding and removal of 2% NH3/He. Rates are normalized to their initial steady state values. Reaction conditions: 300 °C, 25 mg catalyst, PMeOH = 3.92 kPa, total gas flow rate 40 ml/min.


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3.3 Mechanism of DME formation and kinetic isotope effects Since DME is the dominant product (selectivity > 82%) for both TiO2 nanoshapes and under both oxygen-present and oxygen-free conditions, we focused on the dehydration mechanism of methanol to form DME with density functional theory (DFT). We study only the anatase TiO2 (101) surface, the dominant facet on the Octahedra_300C nanocrystal, since DFT study of the mechanism on the anatase (001) surface was recently reported.33 In addition, the key processes for the bimolecular dehydration of methanol on anatase (001) is calculated using the same methods in this work for comparison (Table S2). Since the difference in energetics is reasonably small, it is concluded that the calculations on anatase (001) surface performed by Xiong et al.33 can be used for comparison with the anatase (101) surface. Figure 8 shows the calculated energy profile for the adsorption and dehydration of methanol on the fivefoldcoordinated Ti cations (Ti5c). Initially, two methanol molecules adsorb on separate Ti5c cations with an adsorption energy of 1.09 eV (105 kJ/mol) per methanol molecule. Then, each methanol molecule dissociates, with a barrier of 0.56 eV for the first O-H breaking (TS1 in Figure 8) and 0.52 eV for the second O-H breaking (TS2 in Figure 8), leading to adsorbed CH3O on Ti5c and H on surface O; see the structure of state (1) in Figure 8. Subsequently, one of the methoxy species undergoes C-H dissociation, and couples with the second adsorbed methoxy to form CH3OCH2O; the structures of the transition state, TS3, and the final state (2) are shown in Figure 8. The calculated energy barrier is the highest (1.46 eV or 140 kJ/mol) among the elemental steps, in reasonable agreement with our experimental value of 106 ± 10 kJ/mol for DME formation on Octahedra_300C (Table 2). The CH3OCH2O intermediate then undergoes a series of facile surface reactions to form DME and water, state (3) in Figure 8 from which DME


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desorbs with an uphill energy of 1.00 eV and then water desorbs with an uphill energy of 1.05 eV.

Figure 8. Calculated DFT energy profile of methanol dehydration at Ti5c sites on anatase TiO2 (101) to form dimethyl ether. The structures of the initial (1), transition (TS3), and final (2) states for the coupling of adsorbed CH3O on fivefold-coordinated Ti cations of anatase TiO2 (101) to CH3OCH2O are shown in the bottom. Color code: red, O; grey: Ti; green, C; white, H. Since the DFT profile shows that the C-H breaking of adsorbed CH3O is the rate-limiting step in DME formation, we verified this prediction experimentally by investigating the kinetic isotope effects (KIE). For both fully (CD3OD) and partially deuterated (CD3OH) methanol, KIE were observed for the formation of dimethyl ether, formaldehyde and methyl formate (Table 3). The KIE between CD3OH and CD3OD are similar for the formation of each product, indicating that O-H cleavage is kinetically irrelevant in the formation of these products, in agreement with 22 ACS Paragon Plus Environment

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our DFT results in Figure 8. The largest primary isotope effect was observed for the formation of methyl formate, followed by formaldehyde and dimethyl ether. The higher rate of DME formation for CH3OH vs CD3OH confirms our DFT-calculated mechanism on anatase (101) and the previous DFT work on (001)33 that the C-H scission is the rate-limiting step. Table 3. Measured kinetic isotope effects for the conversion of methanol over TiO2 nanocrystals at 300 °C, 30 ml/min 5% O2/He, 25 mg catalyst, and 3.92 kPa reactant. , , and represents rate of formation using CH3OH, CD3OH, and CD3OD, respectively.

Catalyst

Dimethyl Ethera / /

Disks_300C 1.16 Disks_500C 1.20 Octahedra_300C 1.19 a: Experimental error: ± 0.05 b: Experimental error: ± 0.20

Formaldehydeb / /

1.24 1.31 1.25

1.43 1.55

1.44 1.60

Methyl Formateb / / 1.70 1.68

1.73 1.72

3.4 In Situ FTIR spectroscopy on the Conversion of Methanol over TiO2 Nanocrystals To understand the surface chemistry, the conversion of methanol over TiO2 nanocrystals was monitored by DRIFTS under reaction conditions. Under various methanol partial pressures for Disks_300C (Figure 9A), peaks observed at 1437 and 1152 cm-1 are assigned as the δCH3 and νC-O vibration of methoxy species, while the sharp and intense peaks at 2926 and 2828 cm-1 are due to its asymmetric and symmetric νCH3 vibrations, respectively.20, 55 The negative peaks around 3655 cm-1 can be attributed to the perturbation of surface hydroxyl groups (νOH), while the peak at 3326 cm-1 is tentatively assigned as the νOH vibration of methanol dimer. Further evidence for its assignment to methanol dimer is shown in Figure S4. The peak at 3326 cm-1 is downshifted to 2481 cm-1 in the presence of deuterated methanol, which is consistent with previous reports.56 After purging Disks_300C under 5% O2/He for 1 h at 300 °C, two prominent 23 ACS Paragon Plus Environment

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peaks appeared at 1566 and 1378 cm-1, which are assigned as the asymmetric and symmetric νCOO- vibration of formate (Figure 9A).57-58 The separation in the frequency between νasymCOO- and νsymCOO- of 188 cm-1 suggests that formate binds to the surface via a bidentate configuration.59-60 The peak at 1038 cm-1 is likely due to the νC-O vibration of methanol dimer (as evidenced by the 3326 cm-1 peak) that is still present on the surface.

5% O2/He

Absorbance (a.u.)

Purge

1566

2926 2828

(A) 1378

3326

1038

3655 9.27 kPa MeOH

1437

1152

1.0 kPa MeOH

2830

5% O2/He

(B)

2928

Purge

Absorbance (a.u.)

1589

1368

9.2 kPa MeOH

3668

1164 1464 1.0 kPa MeOH

5% O2/He

2929 2831

1566

1378

Purge

(C)

1.0 kPa 3670 MeOH 1463

Absorbance (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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1160

9.2 kPa MeOH

4000

3500

3000 1500 Wavenumber (cm-1)


1000

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Figure 9. FTIR spectra of methanol conversion over (A) Disks_300C, (B) Disks_500C and (C) Octahedra_300C at 300 °C and PMeOH = 1.01-9.27 kPa. During the conversion of methanol over Disks_500C (Figure 9B), methoxy surface species are observed based on the presence of major peaks at 2928 (νasymCH3), 2830 (νsymCH3), 1464 (δCH3) and 1164 cm-1 (νC-O). The peak corresponding to methanol dimer (3326 cm-1) was not observed. After purging under 5% O2/He for 1 h, bidentate formate species was also observed at 1589 (νasymCOO-) and 1368 cm-1 (νsymCOO-). In the case of Octahedra_300C (Figure 9C), peaks observed at 2929 (νasymCH3), 2832 (νsymCH3), 1463 (δCH3) and 1160 cm-1 (νC-O) are also due to the presence of surface methoxy species. As the partial pressure of methanol increased, the intensity of the peaks due to bidentate formate (1566 and 1378 cm-1) decreased. The intensity of the two peaks were restored after purging in the presence of 5% O2/He. To probe the site origin and reactivity of the methanol dimer species, in situ ammonia titration and isotopic methanol exchange were performed in the DRIFTS cell (see detailed results in Supporting Information). The adsorbed methanol dimer on Disks_300C was subsequently found to be only a spectator specie, i.e., it is not significantly involved in any reaction. 4. Discussion 4.1 Dehydration of methanol over anatase TiO2 (101) and (001) facets Descriptors relating to reactivity and mechanistic pathways for the dehydration of methanol have been extensively studied using solid Brønsted acid catalysts.16,

61-62

However,

there is limited literature on the reactivity of methanol on different facets of anatase TiO2,


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especially on the (101) facet. Wu et al. studied the adsorption of methanol on powdered TiO2 using FTIR spectroscopy and reported that methanol adsorbs dissociatively via a monodentate and bidentate configuration on surface Ti4+ cations.63 On the anatase (101) surface, the adsorption of methanol was studied using temperature-programmed desorption (TPD) and first principles calculations.32,

64

The studies reported that methanol adsorbs undissociatively on a

clean (101) surface, unless surface hydroxyls or defect sites are present. Nevertheless, the kinetics and reaction mechanism of methanol conversion over the (101) facet of TiO2 were not investigated. Surface sites required for methanol dehydration can be inferred from in situ titration and FTIR spectroscopy. Titration experiments with NH3 revealed that the formation rate of dimethyl ether is inhibited to a certain degree for both Disks_300C and Octahedra_300C (Figure 7), while in situ FTIR spectroscopy (Figure S3) showed that NH3 adsorbs on both Brønsted and Lewis acid sites of Disks_300C, and only Lewis sites for Octahedra_300C under reaction conditions. Since the formation of methanol dimer on Brønsted acid sites is shown to be only a spectator species (see Supporting Information), the results suggest that the Lewis acidic sites are involved in the synthesis of dimethyl ether. The synthesis rate of dimethyl ether is not affected in the presence of CO2 up to 23 kPa (Figure S2A and S2B). This result is likely due to the fact that CO2 is not a strong titrant to block basic sites at the reaction temperature.65-67 The involvement of surface basic O sites is expected for the dehydration reaction from the spectroscopy observation of methoxy intermediate and DFT calculations where hydrogen abstraction via surface O sites is involved. Similar kinetic isotope effects were observed in the presence of partially and fully deuterated methanol (CD3OH and CD3OD) in the dehydration reaction (Table 3). This result 26 ACS Paragon Plus Environment

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indicates that the cleavage of C-H bond is a rate-limiting step compared to the dissociation of the O-H bond, which is in good agreement with FTIR spectroscopy and DFT calculations. On the basis of in situ FTIR spectroscopy, methoxy species were observed on all of the titania nanocrystals under reaction conditions (Figure 9), suggesting that O-H dissociation is facile with a low calculated energy barrier of ~ 0.56 eV (Figure 8) and surface methoxy is a reaction intermediate on all TiO2 surfaces. DFT calculations show that the dehydration of methanol over anatase TiO2 (101) involves the formation of methoxy species on the fivefold-coordinated Ti cations (Figure 8). One of the adsorbed methoxy undergoes C-H dissociation with the highest energy barrier of 1.46 eV (140 kJ/mol) among the reaction steps, and couples with another adsorbed methoxy to eventually form dimethyl ether and water, indicating that the cleavage of the C-H bond is the rate-limiting step for dehydration. Furthermore, the apparent activation energy of 106 kJ/mol for Octahedra_300C, which is mainly dominated by the (101) facet, is close to the calculated reaction barrier of 140 kJ/mol. This calculated reaction pathway is similar to methanol dehydration on the reconstructed anatase TiO2 (001)-(1×4) surface, where one of the methoxy adsorbed on a fourfold coordinated Ti cation undergoes dehydrogenation and couples with another adsorbed methoxy to form dimethyl ether and water.33 As such, the KIE of methanol dehydration is lower compared to redox and oxidative coupling (Table 3), as only one of the two adsorbed methoxy undergoes C-H bond cleavage to form DME in contrast to multiple C-H bonds cleavage to form formaldehyde and methyl formate. Although the calculated adsorption energy (160 kJ/mol) and dehydration barrier (210 kJ/mol) on the (001) facet is higher compared to (101),33 the experimental value for the activation energy (99 kJ/mol) from the TPD study is close to our experimental value obtained for the TiO2 nanocrystals with the highest percentage of the (001) facet. Considering that the 27 ACS Paragon Plus Environment

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dehydration of methanol over the three titania nanocrystals with different ratio of (001) and (101) facets is approximately 110 kJ/mol, it is thus concluded that the apparent activation barrier for methanol dehydration is not significantly different on the two TiO2 surfaces. The adsorption energy of methanol is also similar on the two surfaces, as the initial adsorption energy of methanol on the three TiO2 nanocrystals is around 102 kJ/mol from the microcalorimetry measurements (Figure 5B). We conclude that the reaction mechanism for methanol dehydration over the two TiO2 surfaces follows similar dissociative pathway as shown in the DFT study of the (101) (Figure 8) and (001) facet.33 Kim et al. studied the adsorption and temperature-programmed desorption of methanol on a single crystal rutile TiO2 (001) surface.68 It was reported that DME is formed from two methoxides coordinated to fourfold coordinated Ti cations. Even though the polymorph of TiO2 is different, it is worth noting that the site requirement for the formation of DME is similar between rutile and anatase (001) surfaces. Since the surface mechanism for the dehydration of methanol on both (101) and (001) only consist of surface methoxy species, i.e. the dissociative pathway, the synthesis of dimethyl ether can be modeled by the rate expression presented in eq 1.

=

( ( !" #$%&' )$%&'

(1)

(*#$%&' )$%&' )(

where is the synthesis rate of dimethyl ether, +* represents the intrinsic rate constant for the coupling of two adsorbed methoxy species, ,- is the equilibrium constant for the adsorption of methanol on Ti cations, and .- is the gas phase methanol pressure. The values of the kinetic parameters for Disks_300C, Disks_500, and Octahedra_300C at 300 °C were estimated 28 ACS Paragon Plus Environment

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using the data presented in Figure 6A and are shown in Table 4. A parity plot comparing the measured dimethyl ether formation rate and those obtained from eq 1 is shown in Figure 10. The data points are clustered along the diagonal, indicating that the proposed model describes the observed rate accurately. This result further supports the dissociative pathway for DME formation via the methoxy intermediate over the three TiO2 nanocrystals.

Table 4. Estimated kinetic parameters of dimethyl ether formation over TiO2 nanocrystals at 300 °C using kinetic data from Figure 6 and model presented in eqs 1, with estimated percentage of (001) facet (P001) based on polydisperse model.49 Catalyst

/0 (µmol/m2.min)

1234 (kPa-1)

P001 (%)


3.95 ± 0.41 2.83 ± 0.17 1.72 ± 0.05

0.77 ± 0.19 2.03 ± 0.48 1.67 ± 0.16

39 14 10

Predicted DME Synthesis Rate (µmol/m2.min)

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3

2

1

0 0

1 2 3 Observed DME Synthesis Rate (µmol/m2.min)


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Figure 10. Parity plot for the synthesis rate of dimethyl ether derived from kinetic data and eq 1. Reaction conditions: 300 °C, PMeOH = 1.01-9.27 kPa, 30 ml/min 5% O2/He, 25 mg catalyst. The estimated percentage of the (001) facet for each nanocrystal is also shown in Table 4 for comparison, which is based on a polydisperse model calculated in the literature for the same TiO2 nanocrystals.49 A general trend is found between the percentage of the (001) facet and the rate constant k1, i.e., the higher the percentage of (001) facet, the larger the rate constant. Thus, the anatase (001) facet is intrinsically more reactive for methanol dehydration compared to (101). The percentage of (001) facets on Disk_500C is approximately 40% higher compared to Octahedra_300C. This difference resulted in a higher rate and equilibrium constant of about 60%, indicating that the anatase TiO2 (001) facet exhibits a higher reactivity compared to the (101) surface. However, even though Disks_300C displayed the highest percentage of (001) facets (2.8 times of Disks_500C) and the surface density of adsorption sites accessible by ammonia and methanol between Disks_300C and Disks_500C is similar (Table 1), the intrinsic rate constant only increased by 37%, while the equilibrium constant decreased. Although a simple linear relationship for k1 is not expected between the (001) and (101) facets, the decrease in KMeOH is likely due to the presence of fluorine atoms that inhibit or modify some of the surface sites on the (001) facet of Disks_300C, as XPS analysis revealed 8.3% of fluorine atoms present on the surface (Figure 3). Yang et al. reported the use of fluorine to stabilize the formation of (001) facet in the synthesis of anatase TiO2 nanoplates,29 which was also used in the synthesis of Disk_300C. Detailed surface analysis in the report revealed that fluorine atoms bind to surface Ti and O atoms, which promotes the growth of (001) facet during synthesis. As such, the presence of residual fluorine on Disk_300C could inhibit methanol from assessing the active sites under the reaction conditions. 30 ACS Paragon Plus Environment

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The difference in catalytic activity between the (101) and (001) facet is dependent on the electronic and molecular structure of the surface. Gong et al. showed that the electronic density of states on the valence band of the (001) surface has higher energies compared to the (101) surface in the presence of methanol interaction.69 The upper edge of the valence band on (001), which is mainly responsible for its catalytic activity, is formed by the density of states from the twofold-coordinated oxygen anions. This result is in good agreement with Xiong et al, who reported the facile breaking of Ti4c-O2c bond for all of the dissociative adsorption configurations of methanol on the Ti4c sites, followed by the rate-limiting dehydrogenation step on one of the two adsorbed methoxy to form DME.33 However, the breaking of the Ti-O bond is not observed on the (101) facet in our study. This is due to the high tensile stress of the surface bridging oxygen atom on the (001) facet,70 which likely leads to a higher catalytic activity of methanol dehydration compared to the (101) surface. Furthermore, it was reported that the anatase (101) surface has defects such as step edges or vacancies,71 which could contribute to the overall reactivity of the (101) surface. 4.3 Redox and oxidative coupling reactions over TiO2 nanocrystals Highly reducible metal oxide catalysts, such as MoO3 and V2O5, are often dispersed on a metal oxide support (Al2O3, TiO2, SiO2, etc.) and used in the oxidation of methanol.34, 57, 72 From these systems, oxides supported on TiO2 have been widely reported due to its high catalytic activity. However, there are limited studies that have decoupled the catalytic oxidation effect between the active phase and the support, as TiO2 is also reducible.73-74 Furthermore, the anatase surface is highly heterogeneous, resulting in different reactivity of surface Ti cations.75 As such, it is important to understand the surface structure effect of anatase TiO2 on the redox and oxidative coupling activity of methanol. 31 ACS Paragon Plus Environment

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Under dilute oxygen concentrations, the synthesis rate of formaldehyde is consistently higher compared to methyl formate, and the rates of both products decreased with increasing methanol concentration (Figure 6). In addition, the formation of formaldehyde and methyl formate are only observed in the presence of oxygen. These results suggest that oxygen is required for these reactions to occur. Furthermore, various studies in the literature have reported that formaldehyde is an intermediate in the production of methyl formate in the oxidative coupling of methanol.76-77 Previous studies in the literature have reported the formation DME, formaldehyde and methane in the temperature-programmed desorption of methanol on powder anatase TiO2 sample in the absence of oxygen.78 This discrepancy is likely due to the presence of a highly heterogeneous surface on the TiO2 samples in the previous studies, in contrast to the current study of TiO2 nanocrystals with well-defined surfaces. In addition, the presence of F atoms on the surface may not be involved in the inhibition of these products, as there is no observable amount of formaldehyde from the conversion of methanol over Octahedra_300C. Although methoxy is also the intermediate in the oxidation of methanol over TiO2 surfaces, in situ titration experiments with NH3 showed that the Lewis acidic sites involved in the oxidation of methanol is not equivalent compared to the sites required for dehydration (Figure 7). We hypothesize that the active Lewis sites involved in the oxidation of methanol are associated with more reactive oxygen sites on TiO2 surface. Oxygen chemisorption results revealed that the H2 pretreated Disk_300C hold a higher concentration of oxygen vacancies compared to the other two nanocrystals (Table 1), indicating that the lattice oxygen on the (001) surface is more reactive than that on (101) facets. It has been shown that the oxidation of methanol over oxide surfaces is generally a redox process,57, 32 ACS Paragon Plus Environment

77, 79

i.e. oxygen vacancies are

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involved in the redox and oxidative coupling of methanol. As such, it is conceivable that under reaction conditions, the surface of Disks_300C is more prone to oxygen vacancy formation than Octahedra_300C. Since oxygen vacancies on reducible metal oxides are Lewis acidic in nature,80-81 the creation of such Lewis sites on the (001) surface would result in a higher reaction rate compared to Octahedra_300C, which is supported by the rate measurement in Figure 6. The results in this study indicate that the anatase TiO2 (001) surface has a higher rate than the (101) surface for both dehydration, redox and oxidative coupling reactions, which is consistent with a report in the literature.31 The apparent activation energy for the formation of formaldehyde is approximately 90 kJ/mol (Table 2), which is lower compared to 142 kJ/mol from the temperature-programmed desorption of methanol on anatase TiO2 (001) single crystal in the absence of oxygen.33 This result provides further evidence that a different site is involved in redox compared to dehydrogenation of methanol. Similar kinetic isotope effects were observed for partially and fully deuterated methanol for both redox and oxidative coupling reactions (Table 3), indicating that the cleavage of the CH bond is the rate determining step. In situ FTIR spectroscopy showed the formation of formate surface species after purging the titania nanocrystals under dilute oxygen (Figure 9), suggesting that surface methoxy is eventually oxidized to formate. With this information, a reaction pathway for the redox and oxidative coupling of methanol is proposed and shown in Scheme 1. Initially, methanol is adsorbed onto an unsaturated surface Ti cation and undergoes dissociation to form methoxy surface species. Subsequently, redox reaction proceeds with a surface oxygen atom to form formaldehyde and water. In a different pathway, formaldehyde adsorbs onto the surface and oxidizes to a formate surface species. Next, these species couple with a surface 33 ACS Paragon Plus Environment

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methoxy to form methyl formate and water. The oxygen vacancy is regenerated by the adsorption and dissociation of molecular oxygen.

Oxidative Coupling

Regeneration

RDS

RDS

Redox Scheme 1. Proposed reaction pathway for the redox and oxidative coupling of methanol over TiO2 nanocrystals. RDS: rate determining step. 5. Conclusion The conversion of methanol was performed over anatase TiO2 nanoplates and truncated bi-pyramidal nanocrystals at 300 °C in the presence of dilute oxygen, with the nanoplates exhibiting a higher fraction of (001) facet while the truncated bi-pyramidal nanocrystal dominated with (101). The formation rate of dimethyl ether, formaldehyde, and methyl formate are obtained over a range of methanol partial pressures (1.01-9.2 kPa) for dehydration, redox, and oxidative coupling, respectively. Only dimethyl ether is observed as a product in the absence of oxygen, indicating that oxygen is required in the formation of formaldehyde and methyl


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formate. DFT calculations suggest that methanol dehydration occurs on the Ti5c sites of (101) via dehydrogenation and coupling of adsorbed methoxy species, which is similar to the dehydration pathway on the Ti4c sites of (001) reported in literature. This is consistent the similar activation barrier measured for methanol dehydration over TiO2 (001) and (101) facets. Kinetic isotope effects confirm the DFT mechanism that C-H bond cleavage is the rate-limiting step for all products. In situ titration experiments with NH3 showed that the acidic sites for dehydration is not equivalent to redox and oxidative coupling. On the basis of FTIR spectroscopy, adsorbed methoxy is the dominant and reactive surface species under reaction condition, while the formation of methanol dimer on Brønsted acid sites was found to be a spectator species through isotopic methanol exchange. It is proposed that a surface reactive oxygen atom is required for both redox and oxidative coupling reactions. The results in this study indicate that the anatase TiO2 (001) surface is intrinsically more reactive than the (101) surface for both dehydration, redox and oxidative coupling reactions of methanol. This is attributed to the high tensile stress of the surface bridging oxygen atom on (001), which causes the breaking of the Ti4c-O2c bond upon dissociative adsorption of methanol. However, the presence of residue fluorine atoms from the synthesis of nanoplates bound to the surface Ti cations could inhibit the overall reactivity of the (001) surface.

ASSOCIATED CONTENT Supporting Information. Weisz-Prater Criterion test, crystallite size of anatase TiO2 nanocrystals, differential heat of oxygen adsorption, co-feeding of CO2 in the conversion of methanol, adsorption of ammonia followed by FTIR spectroscopy, FTIR spectra of natural


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abundance and deuterated methanol over Disks_300C, comparison of DFT calculations, in situ titration of NH3 over Disks_300C monitored by DRIFTS, FTIR spectra of methanol conversion over various TiO2 nanocrystals, isotopic methanol switching experiment over Disks_300C monitored by DRIFTS. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *Email: [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT This research is sponsored by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Chemical Sciences, Geosciences, and Biosciences Division. Part of the work including XRD, FTIR spectroscopy, and kinetic measurement were conducted at the Center for Nanophase Materials Sciences, which is a DOE Office of Science User Facility. ZDH gratefully acknowledges a graduate fellowship from the National Science Foundation under Grant No. DGE-1148903 and the Georgia Tech-ORNL Fellowship. This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. REFERENCES


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Kinetics and Mechanism of Methanol Conversion over Anatase Titania

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