TiO2 Catalysts: The Role of Titania

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Ethanol partial oxidation over VOx/TiO2 catalysts: the role of titania surface oxygen on the vanadia reoxidation in the Mars-van Krevelen mechanism Dongmin Yun, Yong Wang, and Jose Efrain Herrera ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b03327 • Publication Date (Web): 02 Apr 2018 Downloaded from http://pubs.acs.org on April 2, 2018

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ACS Catalysis

Ethanol Partial Oxidation Over VOx/TiO2 Catalysts: The Role Of Titania Surface Oxygen On The Vanadia Reoxidation In The Mars-Van Krevelen Mechanism.

Dongmin Yun1, Yong Wang1,2 and José E. Herrera3,†

1

The Gene and Linda Voiland School of Chemical Engineering and Bioengineering, Washington State

University, Pullman, WA, USA, 99164 2

Institute for Integrated Catalysis, Pacific Northwest National Laboratory, Richland, WA, USA, 99352

3

Department of Chemical and Biochemical Engineering, Western University London, Ontario, Canada

N6A 5B9

†

Corresponding author

José E. Herrera Associate Professor Department of Chemical and Biochemical Engineering Western University Tel: (519) 661-2111 ext. 81262 E-mail: [email protected]

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Abstract The mechanism for the reoxidation step in the Mars-van Krevelen mechanism for ethanol partial oxidation over vanadia anchored on titanium oxide is examined. Kinetic parameters such as ethanol heat of adsorption, the activation energy for the rate-limiting step (alpha hydrogen abstraction on the adsorbed ethoxide) were obtained while the energetics of the catalyst reoxidation step were explored. A comparison of the parameters obtained from kinetic analysis and the apparent activation energies reported in the literature indicated that a kinetic model that incorporates a catalyst reoxidation step where molecular oxygen adsorbs into a titania vacancy accurately predicted the kinetic parameters. In contrast, a model where molecular oxygen directly adsorbs on the reduced vanadia resulted in an underestimation of the ethanol heat of adsorption and activation energy for the alpha hydrogen abstraction step. A computational analysis was implemented to elucidate a mechanistic pathway for reduced vanadia that incorporates oxygen adsorption on a titania vacancy. The results indicated that the vanadia reoxidation step involves surface oxygen migration from the titania surface to the reduced vanadia center. The quantification of oxygen uptake by the reduced catalyst validates the premise of this assumption: titania vacancies are created during ethanol partial oxidation and are active sites for oxygen adsorption.

Keywords: supported vanadium oxide catalyst, reoxidation, kinetic, density functional Theory (DFT), titania supported vanadia, VOx/TiO2, lattice oxygen.


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1.

Introduction

The electronic and molecular structure of supported vanadia on titania (TiO2) has been extensively investigated, with the aim to understand its superior catalytic activity for the oxidative dehydrogenation (ODH) of light alkanes and the partial oxidation of light alcohols1–10. These body of research indicates that dispersed and even isolated monomeric vanadia species are present at low vanadium loadings while both dispersed and aggregated vanadia coexist at higher coverages11,12. However, there still exists disagreement in terms of the identification of the most active catalytic species. This seems to arise from the use of different reaction conditions used for the probe reactions. For instance, some researchers have reported that the apparent steady state turnover frequency (TOF) values for propane ODH, usually conducted at relative high temperatures (773 K), are independent on vanadia coverage13. By contrast, for the case of light alcohol partial oxidation carried at lower temperatures (473 K), a correlation between vanadia domain size and activity has been reported14,15. While some groups report that under certain conditions the highest ethanol partial oxidation reaction rate will be observed at intermediate vanadia loadings, our previous work suggests the presence of catalytically inactive large vanadia clusters acting as spectators during ethanol partial oxidation16.

For the case of light alcohol oxidation process over titania supported vanadium catalysts, a Mars-van Krevelen-type redox cycle7,17–19 has been proposed, which involves sequential oxidation and reduction of the catalytically active species. The dominant pathways of this process can be divided into three sequential steps: i) equilibrated and dissociative adsorption of alcohol on the vanadia nanocluster8,18, ii) irreversible hydrogen transfer from the adsorbed alkoxy species to an adjacent oxygen (rate limiting)8,20, and iii) VOx reoxidation step by lattice oxygen. This analysis is followed by a set of assumptions including an excess of oxygen supply21. As a result, a detailed reaction mechanism, identification of the rate limiting step, and values for the adsorption equilibrium constant, adsorption enthalpies and apparent rate constants for alcohols alcohol partial oxidation over supported vanadia active sites can be found in the previous literature 22. However, those for the reoxidation step have not been elucidated.


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Despite of a lack of a detailed kinetic mechanism for the vanadia reoxidation step, previous research has reported thermodynamic values associated with this step. Vohs and collaborators23 calorimetrically measured the heat of reoxidation of vanadia/titania catalysts partially reduced by methanol at different vanadia coverages. They reported a heat of reoxidation of the reduced vanadia catalyst (heat of oxygen adsorption on the reduced vanadia), of -120 ± 5 kJ·mol-1 per O atom regardless of vanadia coverage in the catalyst. Two different sites have been proposed for the vanadia reoxidation step, depending on where gaseous oxygen dissociates. One proposed site is a surface oxygen vacancy associated with the reduced vanadium atom in the vanadia lattice, as suggested by shifts in the position of the IR bands of the adsorbed species during temperature programmed oxidation and desorption experiments24. In this case, the reoxidation is assumed to proceed via dissociative adsorption of molecular oxygen at a vacancy site associated with a reduced vanadia atom to form a peroxide-type species. One of the O atoms in this species occupies the vacancy and then the remaining O atom sequentially migrates through the surface in close proximity until it is adsorbed in an adjacent reduced vanadia site8,9. The other proposed site is related to titania lattice oxygen. Comparison of 17O2 isotope exchanges rates between vanadia free titania and monolayer vanadia supported titania samples25 indicated that the oxygen exchange rate decreased as the vanadia coverage in the catalyst increased, suggesting that formation larger vanadia clusters prevented molecular oxygen from approaching the active sites responsible for oxygen adsorption. The implication of these results is that oxygen adsorption takes place on titania. However, a detailed mechanism through this proposal has yet to be established.

In this contribution, we are attempting to close this literature gap, by calculating true heat of adsorption, the activation energy for the rate-limiting step, and an activation energy value for the reoxidation step all integrated in a complete mechanistic picture for ethanol partial oxidation over titania supported vanadia catalysts that rigorously includes the catalyst reoxidation step. Both aforementioned proposals for a catalyst reoxidation pathway were considered to derive a general kinetic expression for partial oxidation.


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The experimentally calculated heat of adsorption and the activation energies are compared with theoretical values obtained using density functional theory.

2.

Methodology

2.1

Catalyst Preparation and characterization

Nano-powdered TiO2 were purchased from Sigma Aldrich (99.7%, 21 nm). Prior to VOx impregnation, the pristine TiO2 powder was calcined at 400oC in flowing of dry air (Praxair, zero grade, 0.83 cm3·s-1) for 2hrs to remove impurities. Raman and infrared spectra was obtained on the support as a means of quality control (supporting information section) VOx impregnation over these supports was carried by incipient wetness impregnation, using aqueous vanadium pentoxide (V2O5, Sigma Aldrich, ACS grade) dissolved in 1M oxalic acid (H2C2O4, Sigma Aldrich, ACS grade) containing the desired amount of vanadium to achieve the desired vanadium loading. After impregnation, the catalysts were dried in the oven at 363 K overnight, and further calcined at 673 K for 2h in flowing dry air (Praxair, zero grade, 0.83 cm3·s-1). In situ Raman spectroscopy was used to confirm the degree of dispersion of the vanadia phase on the catalyst surface. The detailed experimental procedure has been reported elsewhere15 and is also briefly explained in the supporting information section (Fig. S10).

2.2

Catalytic testing and titration of vanadia active sites

Before catalytic testing all samples were pressed into wafers, crushed and sieved to a pellet size between 120-250 µm. Steady-state partial oxidation of ethanol experiments were conducted in a continuous flow fixed bed quartz glass micro reactor (5 mm I.D.) oriented vertically in an electrically heated furnace at atmospheric pressure equipped with a digital temperature controller (Watlow series 97 and 96). Catalyst samples were supported on a quartz frit equipped with K-type thermocouples placed at the vertical center of the catalyst bed on both sides. The catalyst was pretreated using a 0.16 K·sec-1 at temperature ramp to reaction temperature (433, 453, and 473 K) and held at this temperature for 30 min under a O2/He mixture


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(5% O2, Praxair, UHP, 2.68 cm3·s-1) before exposing catalysts to the reactants. After this pretreatment step liquid absolute ethanol (Brampton, Ontario) was evaporated into the flowing 5%O2/He influent stream at 393 K using a liquid syringe pump (KDS scientific) with the flow adjusted to give the desired ethanol partial pressure (0.5 – 11 kPa). All gas transfer lines before and after the liquid injection port were kept above 393 K to prevent condensation of reactants and products. Ethanol conversion rates on the VOx/TiO2 catalysts were measured at 473 K. The flow of 5%O2/He were adjusted by a mass flow controller (MKS instruments) during these experiments. The rates of ethanol partial oxidation are reported as total V-atom turnover rates [moles of ethanol converted per total V atom (∗ ) per second]. Chemical species in the feed and reactor effluent stream were measured using an online gas chromatograph (Shimadzu gas chromatograph, GC-2014) with a capillary column (HP-1, 30 m × 0.53 mm, 1.0 µm thickness) connected to a flame ionization detector. For the identification of the observed GC peaks, the effluent stream was diverted to the mass spectrometer detector (Agilent, 5975C).

2.3

Oxygen uptake

Oxygen chemisorption was measured in a continuous flow fixed bed quartz glass micro reactor (5 mm I.D.), with its end being connected to a four-way gas splitter, diverting to 1) a mass spectrometer (Agilent, 5975C), 2) a thermal conductivity detector (TCD, referenced to He), 3) a flame ionization detector (FID), and 4) a metering valve. The metering valve was used to adjust the inner pressure of the mass spectrometer intake port. The entire system was purged in helium overnight to get rid of any residue gases in the system, such as hydrocarbons, water, O2 and N2. This was confirmed by checking the reference signals on the TCD and FID detectors. Prior to adsorption measurements the catalysts were pretreated using a 0.16 K sec-1 at temperature ramp to 473 K and held at this temperature for 30 min. Instead of a pretreatment with continuous oxygen flow, O2 pulses were sent to the catalyst bed at 473 K (He/5%O2, Praxair, UHP, pulse size 100µL, a total of 30 pulses delivered for 1.0 hr.) before exposing catalysts to the reactants. After this pretreatment step the catalysts were reduced by continuous feed of gas phase ethanol (absolute, Brampton, Ontario) for 1 hour (1 kPa ethanol, in He), delivered using an injection port and a


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liquid syringe pump (KDS scientific) at 423K. After reduction, the system was purged at the same temperature until the TCD and FID signals turned back to reference values. All gas transfer lines were kept above 393 K to prevent condensation of reactants and products. After this step O2 pulses (He/5% O2, Praxair, UHP, 100µL) were sent at 473 K, the effluent gas was analyzed by mass spectroscopy (m/z = 32). The adsorption isotherm measured at 473 K was analyzed to quantitatively determine the O2 uptake. In these experiments, a set of vanadia/titania (VOx/TiO2) catalysts with different vanadia coverages were tested.

2.4

Computational Details

Density functional theory calculations were performed within the generalized gradient approximation (GGA) and the periodic plan-wave approach, using the Perdew-Burke-Ernzerhof26 (PBE) exchangecorrelation functional and Vanderbilit ultra-soft pseudo-potentials27. The DFT+U correction is applied to both titanium and vanadium metal atoms in the support. We use U=2.3 eV on Ti and 2.0 eV on V d-states based on the suggestions made in the literature28 Plan-wave basis set cutoffs for the kinetic and density cutoff were 50 and 500 Ry, respectively ensuring convergence. The Plane-Wave Self-Consistent Field (PWscf) code of the Quantum-Espresso package29, running on SHARCNET, was used to obtain geometric optimizations and to calculate total energy. The k-point sampling of the Brillounin zone was limited to gamma. The Broyden-Fletcher-Goldfarb-Shanno (BFGS) algorithm was used for geometry optimization, with threshold values of 0.092 eV/Å and 6.8 × 10-4 eV for residual forces and energy variation, respectively. Anatase (101) surface was chosen for simulation since this surface is the most abundant among the exposed anatase facets. Prior to geometry optimization of hydroxylated anatase (101) surfaces, a clean anatase (101) surface was modeled with a periodically repeated slab. A 1 × 3 surface cell containing 72 atoms, with corresponding surface area of 10.24 × 11.36 Å2, was modeled with a vacuum of 10 Å. Gas-phase molecules were simulated in a 15 Å cubic box, which is large enough to ignore interaction between the gas molecules. The potential energy profiles of the coordinate reactions were calculated by nudged elastic band (NEB) method as implemented in Quantum Espresso. The convergence


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threshold value for NEB approach was set to 0.01 eV/Å. Ball-stick models for selected steps were visualized using VESTA30.


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3. Results and discussion 3.1

Rate equation for ethanol partial oxidation with a reoxidation term

The mechanistic pathways for ethanol partial oxidation to acetaldehyde have been proposed in previous studies7,8,31. Scheme 1 shows a plausible sequence of simplified lumped steps for the conversion of ethanol to acetaldehyde via a Mars-van Krevelen redox cycle with two distinct possibilities for the catalyst reoxidation step. In one case the vacancy in the vanadia lattice is filled using molecular oxygen (3-1 in Scheme 1) and the other through the creation of a vacancy in the titania lattice (3-2, Scheme 1). Before the redox cycle starts, the VOx/TiO2 system, which encompasses both an oxidized V+5 redox site and surrounding titania surface, is present and denoted as (O*). The first step is the quasi-equilibrated dissociative adsorption of CH3CH2OH to form an adsorbed ethoxylate (O*-OCH2CH3) at the oxidized V+5 redox site (O*) (step 1 in Scheme 1). The next step involves successive surface reactions where H atom abstraction from the alpha carbon atom takes places. This step is known to be irreversible, leading to the generation of adsorbed acetaldehyde and the reduction of the vanadia site (*v)

7,8,32

. Then hydroxyl

recombination takes place leading to the formation of adsorbed water; this specific surface step is assumed to be fast

18,33

. The sequential desorption of acetaldehyde and water results in the generation of

an oxygen vacancy. Finally, as stated above, regeneration of the original oxidized redox site is required (*v → O*) and can take place either by 1) the sequential irreversible dissociation of O2 onto two adjacent reduced vanadium sites (*v + O2 → O* + Om, and *v + Om → O*,) and/or 2) via the migration of a lattice oxygen atom (OL) from the titania support, which in turn is generated by chemisorption of O2 onto surface

oxygen vacant sites in titania ( + ↔ and *v+ OL → O* + , where is an oxygen vacancy in titania). We need to point out that these two scenarios are different; the first one describes the formation of a vacancy in a vanadium cluster (*v) which is filled directly by molecular oxygen, the second

one refers to the annihilation of a titania vacancy ( + ↔ ) which is regenerated after oxygen is transferred to the reduced vanadia center (*v). Redox cycles including adsorption (step 1), surface reaction (step 2) and reoxidation (either step 3-1 or 3-2) are denoted as route 1 (reoxidation of vanadia


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through direct dissociative chemisorption of molecular oxygen over the vanadia cluster) and route 2 (reoxidation of vanadia through migration of a lattice oxygen atom from titania) in the discussion that follows. Assuming that hydroxyl recombination and desorption are very fast and thus kinetically-irrelevant, as previously proposed18,33, a pseudo-steady-state analysis using the mechanistic steps proposed above for route 1 and route 2 lead to rate equations of the form of eq(1) and eq(2), respectively (derivation details can be found in the supporting information):

r = ∗

, ,

, , 1 + , + 2,

O*

r = ∗

O*-OCH2CH3

*v

, ,

, , 1 + , + , [ ]!.#

O*

O*-OCH2CH3

eq(1)

eq(2)

*v

Each term in the denominators corresponds the surface coverage ratio of a specific surface species present: an active site on oxidized vanadia (O*), an active site on oxidized vanadia with ethanol adsorbed (O*CH2CH3) and a reduced active site (*v). Kads,1 and Kads,2 are the equilibrium constants for the reversible ethanol adsorption step (CH3CH2OH + O* → *O-CH2CH3), krls,1 and krls,2 is the rate constant for the ratelimiting H-abstraction step, kox,1 is the rate constant for the reoxidation step (*v + O2 → O* + Om) in route 1; kox,2 is the rate constant for the reoxidation step (*v+ OL → O* + ) in route 2. is the equilibrium constant for molecular oxygen adsorbing on a vacancy in titania ( ). [ ] is the surface concentration of vacancies in the titania lattice. As mentioned above, in most kinetic studies reported in the literature for alcohol oxidation over vanadia18,32,34, the concentration of reduced vanadia species (*v) was assumed to be negligible (*v


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L+∗ − + − +B@ L ). This assumption is valid since, as seen in Fig.2, the estimated pseudo rate constant, , [ ], increases as temperature increases. As shown in the Arrhenius plot in Fig.3, the DE

apparent positive slope in the reoxidation step for the route 2 indicates the value of +, + ∆(B@ →∗ which reflects the summation of the kinetic barriers for the overall reoxidation process is +47.63 kJ·mol-1.

‡ This result validates our conclusion: K+ .,, −+∗@ K > L+∗ − + − +B@ L . The comparison of the

values obtained for the reoxidation step suggests that oxygen migration from titania lattice to the reduced ‡ vanadia center (K+ .,, −+∗@ K) requires more energy than the formation of an oxygen defect from a

titania surface, which the reverse reaction of a titania vacancy annihilation process (L+B@ − +∗ − + L=L+∗ ∗

− + − +B@ L).

We can drive the consequences of the inequalities described in the paragraph above further using experimentally measurable values to get an approximated value for the energy barrier of the reoxidation step (+, ). The value for the heat of oxygen adsorption over titania in the presence of vanadia ( +

→ ∗), which corresponds to (+∗ − + − +B@ ), has been measured using calorimetry, resulting

in a differential heat of -120 kJ·mol-1 per 0.5O2, regardless of vanadia coverage

23,36

. Moreover, based on

our kinetic results, the measured overall energy requirement for the reduced vanadia reoxidation step DE

‡ through titania lattice oxygen migration: +, + ∆(B@ →∗ = *+ .,, −+∗@ , + M+∗ − + − +B@ N is

47.63 kJ·mol-1. The two experimentally measured values: the heat of oxygen adsorption over titania in the presence of vanadia (-120 kJ·mol-1,

23

) and the overall energy requirement for the reoxidation step

calculated based in our kinetic data (47.63 kJ·mol-1), thus provide a boundary for the energy barrier ‡ (+ .,, −+∗@ ) in route 2. This energy barrier has to be lower than the value obtained by subtracting the

heat of oxygen adsorption (-120 kJ·mol-1) over titania from our calculated value for route 2’s overall reoxidation energy (47.63 kJ·mol-1) resulting in a value of ~167 kJ·mol-1.


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3.4

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Theoretical approach

The proposed lumped steps and the associated energetics for ethanol partial oxidation over vanadium oxide supported on titania are presented in Scheme1 and 2 respectively. The theoretical calculation is divided into two main sections: the vanadia reduction step and the reduced vanadia reoxidation step. 3.4.1 The dissociative adsorption of ethanol We explored the energetics of the kinetically-relevant pathways for ethanol partial oxidation using periodic density functional theory (DFT) calculations to evaluate plausible intermediate molecular structures. In this exercise, we also aimed at rationalizing the reoxidation route 2, specifically trying to identify which oxygen atom in the titania lattice is more likely to get involved in the catalytic cycle. For this evaluation, a monomeric vanadia moiety (VO3, 3 oxygen atoms in VOx) anchored on a (101) anatase surface was used. This system selected has been comprehensively studied independently 22,37,38 by several groups. An alternative titania surface model saturated with hydroxyl was also investigated, with the aim to simulate more realistic conditions, but no significant energy differences were observed (data not shown). To examine the most energetically stable adsorbed CH3CH2OH structure, we calculated the adsorption enthalpy (∆+ ), a key parameter in predicting the geometric coordination of the system. The ∆+ value was calculated by subtracting the sum of the total energy optimized CH3CH2OH free gas molecule ( + ) and the clean anatase surface (Esurf) from the total energy of the system representing the adsorbed ethanol molecule on the anatase slab (Esys):

∆+ = +O − +PE −

+ . A negative Eads value indicates that molecule adsorption is exothermic and thus the adsorbate bearing surface system is thermodynamically stable. When ethanol adsorbs, the most stable configuration results from a V-O-Ti moiety reacting with the ethanol hydroxyl to generate V-O-CH2CH3 and a surface OH species (Fig.S1, supporting information). We looked at the differences in adsorption energy when CH3CH2OH reacts with either Ti-O2c-H (O2c – twofold coordinated oxygen) or Ti-O3c-H (O3c, threefold coordinated oxygen). We also evaluated the cases where the oxygen atom of the resulting V-O-CH2CH3


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and surface OH groups (OH being either O2c or O3c) interact with each other creating a hydrogen bond (Fig.S1, NI and I stand for “non-interaction” and “interaction”). The results suggest the processes involving the V-O3c-Ti bonds (path O3c-NI, in Fig.S1 -31.4, and path O3c-I in Fig.S1, -46.2 kJ·mol-1) are more thermodynamically favorable than those taking place over the V-O2cTi ones (O2c-NI, -11.9 and O2c-I, -14.9 kJ·mol-1) as the computed heat of adsorption values indicate. In addition, those configurations that result in the formation of hydrogen bond (path O3c-I) tend to be more thermodynamically favorable than those where hydrogen bond formation does not take place. This could be attributed to the extra electron density transferred from oxygen in V-O-Ti to hydrogen in OH, concomitant to hydrogen bond formation, which results in additional stabilization of the adsorbed ethoxylate species. Fig.4 displays the most energetically favorable pathway for the dissociative adsorption of CH3CH2OH obtained by our calculations. Ethanol dissociatively adsorbs at the V-O3c-Ti bridging bond, forming an ethoxylate (V-O-CH2CH3) and a hydroxyl (O3cH) group attached to the titania support. The length of the hydrogen bond formed between the newly formed titania hydroxyl and the oxygen atom in the ethoxylate is 1.70 Å (Fig.5A). The computed value obtained for the heat of adsorption which is an energy gap between (1) and (2) stages depicted in Scheme 2 (∆( = -46.2 kJ·mol-1) is comparably analogous to our experimentally calculated value (-42.97 kJ·mol-1, Table 1) attained when route 2 was assumed for the catalyst reoxidation step. These two values are also in agreement with a previously theoretically determined value (-49 kJ·mol-1) reported by Shapovalov et al. for adsorption of methanol on an identically modelled system 22, and the value of ~-48 kJ·mol-1, obtained by Beck et al. 7. 3.4.2 The rate-limiting step, H-abstraction We attempted to calculate the activation energy for the H-abstraction step, denoted as to E in scheme 2. Two possible pathways were considered. On the first case the alpha H transfers to an oxygen atom directly bonded to the titania surface. On the second case, the alpha H from the carbon atom in the adsorbed ethoxylate transfers to an oxygen atom directly bonded to vanadium. The two cases describe two different scenarios, on the first case the adsorbed molecule undergoes intermolecular transfer while


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the second describes an intramolecular H transfer. For the analysis of both scenarios the energetically most stable adsorbed ethoxylate configuration (Fig.4), was used as starting point for the H-abstraction process depicted in Fig.5A.

The calculated energetic requirements together with selected chemical structures depicting the process along the reaction coordinate are shown in Fig.S3 and Fig.5, respectively. For the case of the intermolecular H transfer scenario, where titania oxygen atoms are involved in the hydrogen abstraction step, the results indicate that rotation around the O-C axis of the of the V-O-C bond is needed. This step requires very little energy (~15 kJ/mol) and does not contribute to the energy barrier required to achieve the transition state (Fig. S3, C‡). This rotation takes place preserving the hydrogen bond created during the initial adsorption step (arrow in Fig.S3, A). Following the rotation step, the energy slightly increases after this hydrogen bond is elongated and simultaneous generation of a new hydrogen bond between the alpha-H and the H-accepting O atom in titania lattice (Fig.S3, C‡) takes place. Energetically driven by the newly formed hydrogen bond, the H-abstraction step ends with the formation of new hydroxyl group (O3cH) in the titania surface (Fig.S3, D). For this hydrogen transfer mechanism, the energy requirement to reach the activation barrier is 15.0 kJ·mol-1 (energy difference between systems A and C‡, Fig.S3) and the resulting heat of reaction for the H transfer process is -319 kJ·mol-1 (energy difference between system A and D, Fig.S3). The low activation energy value indicates that under conditions where bare surface oxygen atoms from the titania are available to participate in the reaction, acetaldehyde formation would be almost spontaneous. However, in practice this scenario is extremely unlikely since most of the titania surface is saturated with surface hydroxyl groups under typical ethanol partial oxidation conditions39.

The other possible scenario (direct H-transfer to the vanadia center) is depicted in Fig.5 and Fig.S4. In this scenario, two possibilities for the initial configuration of the adsorbed ethoxylate are considered. These paths describe alpha-hydrogen transfer to an O atom bonded to vanadium. In these cases, instead of the O-C bond rotation observed in the previous scenario, bending on the V-O-C moiety takes place as first


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step. Our calculations indicate that this bending requires a small amount of energy (10 kJ/mol). This step takes place while the original hydrogen bond present in the adsorbed ethoxylate is preserved. However, for this scenario the calculated activation energies were found to be 56.6 and 64.9 kJ·mol-1 for the initial adsorbed ethoxylate configurations depicted in Fig.5A and Fig.S4A as starting states, respectively. For both cases, at the transition state, the structure of the system contains a four membered H··O-V-O ring (Fig.5B‡ and Fig.S4B‡. The existence of the four-membered ring and the calculated activation energy (56.6 - 64.9 kJ·mol-1) strongly suggests that the H transfer step toward the terminal O bonded to vanadium atom is responsible for H abstraction. This calculated energy value is in good agreement with the experimentally obtained value (66.8 kJ kJ·mol-1) obtained in section 3.2. Previously reported apparent activation energy values for alcohol partial oxidation over supported vanadia are: 133 kJ·mol-1

32

for

methanol and 67 kJ·mol-1 for ethanol 40. These values have also been reported as independent of vanadia loading32. Therefore, the consistence between the experimentally obtained activation energy for ethanol partial oxidation reported in section 3.2 (66.8 kJ·mol-1) and the theoretical value we obtained indicates that lattice oxygen in the titania support do not play a role in the H abstraction step and that the oxygen atom in the terminal V-O in the partial oxidation reaction is the key site responsible for H abstraction. Indeed, the energy difference (8.3 kJ·mol-1) between the two scenarios depicted in Fig.5A and Fig.S4A could be originated from the additional stabilization caused by the formation of a hydrogen bond between the oxygen atom in the V-O-C moiety and the H in the titania hydroxyl group that takes place in the first scenario (Fig.5). Lastly, the large energy barriers (~330 kJ·mol-1) of the reverse reaction for the Habstraction step indicate that the H atoms on the surface and H atoms in the ethoxide species do not participate in the catalytic cycle.

3.4.3 Reoxidation where the lattice O in titania is involved Throughout this study, we have proposed that the catalyst reoxidation pathway involves the surface diffusion of oxygen from titania to the reduced vanadium-bearing center. This scenario is plausible, since the presence of Ti3+ under reductive conditions has been demonstrated through the use of electron


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paramagnetic resonance,41 17O isotopic exchange coupled with magic angle spinning NMR25. To gain a deeper insight on this phenomena, we simulated two possible oxygen diffusion scenarios: one involves the surface threefold oxygen moiety, bridging a vanadium and two titanium atoms (depicted as grey lines in Scheme 2). That oxygen migrates to the vacant terminal oxygen vanadyl position, with the oxygen vacancy migration process occurring through subsurface oxygen atoms migrating along the 1Q01Q direction (depicted in blue color in Fig. S2a). The second scenario (depicted as black lines in Scheme 2) involves a twofold oxygen moiety, bridging a vanadia with a titanium atom, migrating to the vacant terminal oxygen vanadyl position, and a surface oxygen vacancy migration process along the 01Q0 direction (highlighted in green in Fig.S2b). To evaluate the energetics of the first scenario (oxidation through subsurface oxygen vacancy migration), we first built a model for a reduced vanadia center, shown in Fig.6A. The reduced vanadia model was built by removing the oxygen atom in the vanadyl group of the pentavalent vanadia model presented in section 3.4.2. (no vanadyl functional group in VOx). We optimized several vanadia slab models (Fig.S7). Validity of the selected model was verified by calculating the energy for the reverse process of oxygen

DE

defect formation: -43 kJ·mol-1 (RSTUTVWU XYZYUTY ↔ [WU\]WU XYZYUTY + ,) denoted as ∆(∗@ →∗ As indicated in Table 1, the oxygen defect formation energy for vanadia supported on TiO2 has been reported to be 44 kJ·mol-1. Following this calculation, the reoxidation step was modeled using the two aforementioned oxygen diffusion processes. Selected stages along the reoxidation pathway involving subsurface titania lattice oxygen are shown in Fig.6 together with the corresponding energy profile for the process referenced to the reduced vanadia model. This reoxidation route consists of two main stages. The first stage encompasses the migration of threefold oxygen (colored with purple in Fig.6) from the titania surface resulting in the formation of a terminal vanadyl oxygen and an oxygen vacancy in titania (from A to C through transition state B‡ in Fig.6), The second stage is where the sequential migration of the resulting vacancy to the inner titania lattice (from C to F in Fig.6) takes place. The calculated activation energy for the first stage (A to C) was


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ACS Catalysis

found to be 202 kJ·mol-1 (B‡ in Fig.6). It should be noted that, as discussed in section 3.3, we anticipated that the activation energy for the reoxidation process to be at the very most ~167 kJ·mol-1, thus the calculated activation energy of 202 kJ·mol-1 exceeds this limit. In the second stage (sequential oxygen atom migration in the titania lattice), the position of the vacancy site in titania lattice significantly affects the energy of each configuration shown in in Fig. 6. For example, if the vacancy site is located directly underneath the titanium atom close to vanadia (Ov in configuration C in Fig.6), the energy for that particular configuration is much higher than the case where the vacancy is further away from vanadia (see Ov in configuration E and F). At the same time, along the reaction coordinate, the transition state in the first vacancy filling in the titania lattice (D‡ in Fig.6) was found to have the highest energy demand all stages we depicted in Fig.6. Thus, the energy difference between configuration A and D‡ in Fig. 6 reflects the activation energy of the reoxidation step. In the other words, the minimum required energy to reach the transition state starting from reduced vanadia is ~253 kJ·mol-1 (energy difference between A and D‡ in Fig.6) in the lumped reoxidation process. Unfortunately, since we used a with (4 ×1) supercell slab model as proxy for the titania support, we were unable to continue the simulation to the final the vacancy filling stage process as the supercell model cannot properly capture the energetics of vacancy annihilation by gas phase oxygen on bulk titania. However, it is reasonable to assume that in the case of bulk titania the vacancy will continue to migrate further away from the vanadium atom until oxygen adsorption takes place on the vacancy. The energetics of this process would be then regulated by the properties of the bulk titania surface. We also simulated the second scenario: vanadia reoxidation process through titania surface oxygen migration. In this case, one of the three bridging oxygen atoms in the V-O-Ti reduced vanadia moiety transfers to a terminal vanadyl oxygen, and then the filling of the resulting vacancy takes place through a sequential migration of oxygen atoms in the titania surface. Selected stages along this reoxidation pathway are shown in the Fig.7A-E. In this scenario, an oxygen attached to vanadium (colored in purple in Fig.7A) migrates to the vacant terminal vanadyl oxygen position (Fig.7C) through transition state B‡


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(Fig.7). We computed the activation energy of this first step of the reoxidation route and obtained a value of 122 kJ·mol-1. Comparing the energy difference between initial stage (A) and transition state (B‡) in Fig.6 and 7, the obtained value in Fig.7 is much lower than one obtained for the analogous step when subsurface oxygen vacancies are involved in vanadia reoxidation (202 kJ·mol-1, Fig. 6). It should be noted that for the next stage of the vacancy migration process a model structure (depicted Fig.S5a) could not be optimized. Instead, the optimization process yielded a different structure with the oxygen atom in question on a different position (structure depicted in Fig.S5b) this configuration is identical to the one depicted in Fig.7E. This occurs because both threefold surface oxygen atoms in the titania slab directly under the vanadium atom in the (010) direction (depicted in dark green and light blue in structures C and E) migrate simultaneously, leading to a single transition state (D‡) in Fig.7. This simultaneous surface oxygen migration results in the formation of an oxygen vacancy (Fig.7E) positioned further away from the vanadia center. We found that the activation energy for the oxygen vacancy migration process (D‡) was ~130 kJ·mol-1, which satisfies the boundary value set in Section 3.3 for the activation energy of the reoxidation step. When we modeled the first case, (reduced vanadia oxidation through titania subsurface oxygen vacancy migration) we were unable to capture titania oxygen vacancy filling in bulk titania. In contrast, for the second scenario (vanadia reoxidation process through titania surface oxygen migration) we are able to complete the vacancy filling process by calculating the oxygen addition (vacancy annihilation) energy using the model structures depicted in Fig.7E representing vanadia/titania structure with vacancy in titania lattice and Fig.4(left) representing a fully oxidized vanadia/titania structure. This strategy assumes that the vacancy generated on the titania surface generated by oxygen migration toward vanadia reacts with gas phase oxygen gas. The resulting enthalpy of oxygen addition (titania vacancy annihilation, route 4 → DE

5 in Scheme 2), denoted as ∆(B@ →∗ is -123 kJ·mol-1, which is similar to the previously reported experimental value for this process (close to -120 kJ·mol-1 per 0.5 O2)23.


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ACS Catalysis

Recall that the activation energy of reduced vanadia reoxidation with titania subsurface oxygens was calculated to be ~253 kJ while the analogous process with titania surface oxygens to be 130 kJ·mol-1. The former value is higher than the boundary energy computed in section 3.3, while the latter meets the established criteria. Therefore, the comparison of energy requirements for the two different reoxidation routes (subsurface vs. surface) led us to conclude that the reoxidation process with surface oxygen in titania lattice is more energetically plausible than one with subsurface oxygens.

3.5. The dependence of O2 uptake on vanadia loading A key hypothesis of this study, elaborated later in the following sections, is that an oxygen vacancy on titania can potentially play a in the oxygen adsorption step that completes the Mar-van Krevelen cycle for ethanol partial oxidation over VOx/TiO2 catalysts. To start exploring this possibility, we carried oxygen uptake experiments to quantify the amount of oxygen uptake by reduced vanadia. Previous studies have carried oxygen uptake experiments on reduced vanadium oxide, in these cases vanadia was reduced using molecular hydrogen. After H2 reduction, oxygen uptake measurements were carried to evaluate the ratio of reducible vanadia present on the catalyst42,43. The catalysts were reduced by H2 at a sufficiently high temperature to assure complete reduction of the vanadia moieties accessible by hydrogen. Previous contributions indicate that the reducibility of the vanadia moieties anchored on titania does depend on the geometry of the vanadia structure3,15 , and that for the case of ethanol partial oxidation, not all vanadia is switching between redox states during catalysis16. Therefore, H2 pretreatment as proxy to the first step in ethanol partial oxidation where reduced vanadia forms, can potentially generate results that are not representative of the actual amounts of reducible vanadia active for ethanol partial oxidation, moreover taking into consideration the higher surface diffusivity of hydrogen which could result on it accessing vanadium site that are not available to the larger ethanol molecule. Thus, in our case, to avoid misestimation of the fraction of reducible vanadia under reaction conditions, the fully oxidized vanadia catalyst was pretreated using CH3CH2OH at 473K instead of molecular hydrogen. This pretreatment was selected to mimic reaction conditions, and has been successfully used in the past to estimate the amount


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of ethanol reduced to acetaldehyde during temperature programmed reaction16. Following this step oxygen pulses were introduced and oxygen uptake by the catalyst was measured. Figure S6 shows the O2 adsorption isotherm measured at 473K for a series of VOx/TiO2 catalysts with different vanadia loadings (0.4, 0.8, 1.6, 2.3, 3.6, 5.1 and 7.2 wt%). The calculated molecular oxygen uptake per vanadium atom (oxygen uptake density) in the catalyst is plotted in Fig. 8 as a function of vanadia loading in the catalyst. Previous studies have reported values for similar catalytic systems gathered using a hydrogen pretreatment43. However, to the best of our knowledge, the range of low vanadia coverages has not been tested in previous research. Our results indicate that as vanadia coverage increases, the total oxygen adsorption capacity increases (Fig.S6) while the oxygen uptake density (O2 adsorbed per V atom) decreased (Fig.8). This decrease follows a well-defined linear trend in the 1.6-7.2% wt. vanadia loading range. The extent of the decrease is however dependent on the vanadia loading. The oxygen uptake density at very low vanadia coverages ( 1.6wt%). The trends obtained at these higher coverages is similar to the trend previously reported43. If the oxygen vacancy created in the reduced vanadia is mainly responsible for oxygen adsorption, the oxygen uptake density (O2/V) value should remain constant as the vanadia coverage increases, since higher vanadia loadings would result in larger amount of oxygen vacancy sites generated after pretreatment. Moreover, if only the edge sites of large vanadia clusters were catalytically active, a steep slope in the oxygen uptake density (O2/V) values as a function of vanadia content at high vanadia coverages would not be observed. However, the data presented in Figure 8 for the region with V2O5 loadings above 1.5wt. % do display a very gentle slope, suggesting that inaccessible vanadium sites are indeed being created at higher vanadia loadings. What is particularly interesting is that in the region where constant values in O2/V values should be observed (vanadium loadings below 1wt.%) where only dispersed and polymerized vanadia is dominantly


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present and three dimensional vanadia structure formation is restricted4,15, there is a sharp decline in oxygen uptake density as a function of vanadia coverage. This could be interpreted in terms of a blocking of titania sites that can generate oxygen vacancies along the (010) direction. In other words, as the size of vanadia cluster grows along the (010) direction (vanadia cluster oligomerization), oxygen atoms, which could become vacancies under reductive environment, get covered by the vanadia cluster, and hence are unable to participate in the Mars Van Krevelen cycle.

To gain a more quantitative insight on these phenomena, we calculated the total concentration of oxygen vacancies created per total oxygen in each catalyst using the following equation: 0^,_`a Bbc

(%) =

c efghij ×mkcn × k

Bbc

× 100

eq.(8)

where Op,6qr are the number of moles of total oxygen vacancies created after a complete reduction treatment, > the total stoichiometric molar content of oxygen in TiO2, smn is the molar content of V atoms in the catalyst and

P

TiO2 Catalysts: The Role of Titania

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