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Methanol Oxidation to Formate on ALD-Prepared VOx/θ-Al2O3 Catalysts: A Mechanistic Study Weiqiang Wu,† Kunlun Ding,‡ Jian Liu,‡ Tasha Drake,†,‡ Peter Stair,†,‡ and Eric Weitz*,†,‡ †

Institute for Catalysis in Energy Processes and Center for Catalysis and Surface Science and ‡Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States S Supporting Information *

ABSTRACT: Well-defined supported VOx/θ-Al2O3 catalysts were prepared by atomic layer deposition (ALD) with vanadium coverages of 0.48, 1.20, and 3.40 wt %. In-situ Raman and UV−vis diffuse reflectance spectroscopy confirm that the monovanadate, VO4, is the predominant vanadium species at low loadings (0.48 and 1.20 wt %), while polyvanadate VO4 is the predominant vanadium species for the 3.40 wt % VOx/θ-Al2O3 catalyst. In-situ FTIR spectroscopy of methanol oxidation to formate, in the absence of gasphase oxygen, on the 0.48 wt % VOx/θ-Al2O3, identified two different formates. A comparison of the frequencies for the formates adsorbed on just V2 O 5 and on just θ-Al 2 O 3 demonstrates that one of these formates is located on aluminum sites of VOx/θ-Al2O3 while the other is located on vanadium sites. The oxidation state of vanadium for the VOx/ θ-Al2O3 catalyst was determined by XPS after different reaction times. On the basis of the time dependence of the formate absorptions and the change in the oxidation state of vanadium in VOx/θ-Al2O3, a mechanism is proposed for methanol oxidation and we discuss the role of the alumina support in the mechanism.

1. INTRODUCTION Supported vanadia is an active catalyst that is widely used for selective oxidation reactions.1−10 For example, VOx/TiO2 catalysts have been reported to oxidize methanol to a number of products including formaldehyde, dimethyl ether, formate, dimethoxymethane, and formic acid.9−12 A fundamental and important question for systems in which a reaction can yield a multiplicity of products is how a catalyst affects the pathways and/or rate-determining step to alter the dominant product(s) formed. Thus, it is of both fundamental and practical significance to investigate the mechanism of oxidation reactions on supported vanadia catalysts under conditions that favor different products.1,2,13−15 The chemical composition of supported vanadia species prepared by traditional wet chemical methods has been extensively characterized by different spectroscopic techniques including IR,7,8,16−18 Raman,18−22 UV−vis diffuse reflectance spectroscopy (UV−vis DRS),5,21,23 X-ray absorption finestructure spectroscopy (XANES/EXAFS),24,25 X-ray photoelectron spectroscopy (XPS), and solid state NMR.26,27 It has been convincingly demonstrated that the isolated mono-VO4 unit is the main form of vanadium oxide when the concentration of vanadia is low. Polymeric-VO4 units are formed as the vanadia concentration increases. Once the coverage of vanadia approaches a monolayer, aggregation to form VOx nanoparticles can occur. As reported by Kim et al.,20 the surface vanadium species, such as mono-VO4 and © XXXX American Chemical Society

polymeric-VO4, which are formed below a monolayer coverage of VOx, are reactive, while crystalline VOx nanoparticles that form above a monolayer coverage are less reactive for methanol oxidation and lead to a decrease in the number of exposed catalytically active vanadia surface sites. In addition to the effect of the structure of vanadia on reactivity and selectivity, the support can also affect reactivity and selectivity. For example, silica-supported vanadium pentoxides exhibit high selectivity for methanol oxidation to formaldehyde at temperatures near 300−400 °C,28−31 whereas monolayer vanadia catalysts supported on TiO2 show high selectivity toward methyl formate and dimethoxymethane at lower temperatures (100−200 °C).11,32−36 At higher temperatures CO2 is also observed as a product with vanadia catalysts.11 The oxidation of methanol to formaldehyde has been the focus of a number of studies.28−31 Most of these have focused on vanadia supported on ceria, titania, and silica. The mechanisms for the two-electron (2e−) oxidation of methanol to formaldehyde are reported to differ on reducible versus nonreducible supports.37−40 In addition, the large majority of experimental studies have been performed in the presence of oxygen. When oxygen is present it is possible for a Mars−Van Krevlen- like mechanism to oxidize V4+ to V5+, and the two Received: July 28, 2017 Revised: October 19, 2017

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formation of CO2, which is typically produced with oxygen as the oxidizer, is not observed. Under the anaerobic conditions used in the present experiments formate is the major product. The vanadyl oxygen is incorporated into the formate moiety and the change in products under anaerobic versus aerobic conditions can be viewed as resulting from the choice of oxidant. Finally, it is noted that under anaerobic conditions the oxidation of methanol is effectively stoichiometric. However, addition of oxygen can regenerate the supported vanadia and allow for additional reaction cycles.

electrons lost in the oxidation of methanol to formaldehyde can be donated to a single vanadium atom that has cycled through a V4+ oxidation state.17,27,41−43 In addition, reoxidation of vanadia under atmospheric conditions regenerates the V5+ oxidation state, and if this step is rapid enough it could preclude direct observation of a change in the oxidation state of vanadium during the reaction. This study reports on a study of methanol oxidation over well-defined VOx/θ-Al2O3 prepared by atomic layer deposition (ALD). ALD affords exceptional control over the amount of catalyst deposited on a support, which can affect the chemical nature of the catalytic material.6,44,45 The ALD method of preparation provides a catalyst that is typically much more homogeneous with regard to the active catalytic species than catalysts made by more traditional methods, such as wet impregnation. In the present work, the selective oxidation of methanol to formate is studied in the absence of oxygen. The focus of this work is on the mechanism for methanol oxidation when the oxidant is isolated vanadia moieties. The four-electron (4e −) oxidation of methanol to formate requires the participation of at least two vanadium atoms since the stable oxidation states of vanadium are V3+, V4+, and V5+. Gas-phase oxygen, which could regenerate a higher oxidation state, is excluded, and the maximum number of electrons a given vanadium site can accept without reoxidation is then two. We note that even though gas-phase oxygen is not added, an oxidation reaction takes place with the incorporation of vanadyl oxygen into the formate product. As alluded to above, with added oxygen, species other than formate can be major products. Thus, studies of reactions under different sets of conditions can highlight different reaction pathways and products. The absence of oxygen may also allow for the direct observation of lower oxidation states of vanadium which are too rapidly oxidized in the presence of atmospheric oxygen to be directly observed under aerobic conditions. The observation of lower oxidation states of vanadium can provide direct evidence for proposed reaction steps. Anaerobic conditions can also reveal reaction steps which could take place efficiently and are effectively precluded by rapid reoxidation of lower oxidation states of vanadium. Such steps can potentially be taken advantage of in chemical processes that involve “looping”, where a metal oxide oxidizer is sequentially reduced in the absence of oxygen and then reoxidized with added oxygen.46 Finally, it is noted that both the support and the nature of the oxidant can affect the products and the reaction mechanism. In this study, we (i) use in-situ FTIR spectroscopy to identify the surface species produced by methanol oxidation, (ii) characterize two different formate species, one of which forms on alumina sites and one of which forms on vanadium sites, and (iii) determine the vanadium oxidation state for VOx/θ-Al2O3 after methanol oxidation by XPS which is supplemented by monitoring the shift in the frequency of a CO probe molecule by IR spectroscopy. These data provide evidence that under the reaction conditions the dominant process for the 4e− oxidation of methanol to formate involves the production of two vanadium atoms in the V3+ oxidation state. On the basis of these data a molecular level mechanism for formate formation on VOx/θ-Al2O3 is proposed, which explicitly incorporates the role of the alumina support. In this mechanism both the initial methoxy and the formate products are mobile and can move between the alumina support and the vanadium redox sites. This mechanism also involves a mobile intermediate with carbon in the zero oxidation state. Further, significant

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. One-cycle, 2-cycle, and 10-cycle VOx/θ-Al2O3 (1c, 2c, 10c VOx/θ-Al2O3) catalysts were prepared via atomic layer deposition (ALD), which uses cycles of self-limiting reactions between gas-phase precursor molecules and a substrate to produce uniform films in a layer by layer fashion.6,44,45,47,48 VOx ALD was performed using a viscous flow reactor described previously.49−53 Prior to the ALD process, the θ-Al2O3 support was calcined at 600 °C in air for 10 h to remove any carbon contamination. For each ALD sample, ∼0.6 g of θ-Al2O3 powder was loaded into a fixed bed powder holder. The VOx was deposited at 150 °C with alternating exposure to vanadium oxytriisopropoxide (VOTP, Sigma-Aldrich, 99.98%) with a dose time of 120 s and Millipore water with a dose time of 50 s and a purge time of 240 s. The VOTP and water reservoirs were kept at 45 °C and room temperature, respectively. The inlet lines were heated to 100 °C to prevent VOTP condensation. The dose time necessary to achieve saturation coverage was determined using direct weight gain measurements with an analytical microbalance. θ-Al2O3 was obtained from Johnson Matthey (London, UK) with a surface area of ∼125 m2 g−1. The θ-Al2O3 particles were crushed and sieved with a particle size range of 60−80 mesh. Ultra-high-purity (99.999%) nitrogen was used as the carrier gas for the VOTP precursor at a flow rate of 240 mL min−1 and a pressure of ∼1.2 Torr. All ALD-prepared VOx/θ-Al2O3 samples were calcined at 650 °C for 6 h in the air before use. The vanadium loading was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES) (at the IMSERC facility at Northwestern University) to be 0.48, 1.20, and 3.40 wt % for 1c VOx/θ-Al2O3, 2c VOx/θ-Al2O3, and 10c VOx/θ-Al2O3, respectively. The composition of the VOx/θAl2O3 samples was characterized by in-situ UV Raman spectroscopy and UV−vis DRS. In addition, V2O5 powders used in current experiments was purchased from Sigma-Aldrich with 99% purity. 2.2. In-Situ UV Raman Spectroscopy. In-situ UV Raman spectra of the VOx/θ-Al2O3 samples were collected after calcination (under dehydrated conditions) in a home-built fluidized bed reactor using a UV Raman instrument built at Northwestern University.54−57 The 244 nm light is generated by a Lexel 95 SHG laser equipped with an intracavity nonlinear BBO crystal that frequency doubles 488 nm light into the midultraviolet region. The laser power was controlled to be between 3 and 5 mW as measured at the sample. The VOx/θ-Al2O3 samples were first heated in the fluidized bed reactor in flowing O2 (100 mL min−1) at 350 °C for 1 h to remove adsorbed moisture. Oxygen is present to prevent reduction of vanadium during calcination. In-situ Raman spectra were then recorded at room temperature in flowing O2 (100 mL min−1) for 40 min by averaging 4 scans. The band centers in the Raman spectra were determined using the B

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The Journal of Physical Chemistry C program PeakFit v4.11.21,58 The Raman shift was calibrated using cyclohexane as a liquid standard and Zr(SO4)2 as a solid standard. A quadratic fit of the observed to the actual Raman shift of the standards was employed in the calibration.21,58 2.3. UV−vis DRS. UV−vis DRS spectra were recorded at room temperature on a PerkinElmer LAMBDA 1050 spectrophotometer equipped with a diffuse-reflectance attachment (Keck Biophysics facility at Northwestern University). Before taking UV−vis DRS spectra, the VOx/θ-Al2O3 samples were calcined in O2 for 2 h to remove residual water and then transferred into a desiccator. These powder samples were then loaded in a glovebox into a sample holder sealed with a quartz window to a sample thickness greater than 1 mm and were then removed from the glovebox and rapidly mounted in the spectrometer. The UV−vis DRS spectra were transformed into Kubelka−Munk units, F(R∞) = (1 − R∞)2/2R∞, where R∞ is the experimentally measured reflectivity coefficient of infinitely thick samples. The edge energy (Eg) for transitions was determined from the intercept of the straight line that characterizes the low-energy rise in a plot of [F(R∞)hν]2 against the photon energy, hν.59 2.4. In-Situ FTIR Spectroscopy. In-situ FTIR spectra were acquired using a Nicolet 6700 infrared spectrometer equipped with a liquid nitrogen-cooled MCT-B (mercury−cadmium− telluride) detector which allowed for measurement of infrared spectra from ∼1000 to 4000 cm−1. The lower frequency limit was determined by the CaF2 cell windows. The samples were mounted in a custom-fabricated IR cell designed for transmission mode studies of highly scattering powder samples that are supported on a tungsten gird (∼2 cm2) that can be uniformly heated.60−62 All FTIR spectra were obtained by averaging 128 scans at 4 cm−1 resolution and referenced to a background which was a spectrum of the samples after they were pressed into the wire grid and after calcination. The procedure prior to IR measurements involved sequentially sonicating the tungsten grid in isopropanol/ ethanol and deionized water and blowing it dry with nitrogen. The VOx/θ-Al2O3 samples were then pressed into the tungsten grid which supports the samples. Excess sample powder on the front surface of the grid was carefully removed with a blade. The weight of the sample, which is typically around 20 mg, is calculated from the difference between the weight of the tungsten grid before and after sample loading. Finally, a K-type chromel−alumel thermocouple was spot welded to the center of the grid. The sample was then mounted in the IR cell which is connected to a turbo pumped gas manifold with a base pressure of ∼1 × 10−5 Torr. The sample was then heated in the cell at 350 °C for 2 h to remove moisture. After this pretreatment, methanol vapor (Sigma-Aldrich, >99.9%) was admitted to the cell for 1h, with the pressure monitored with a capacitance manometer, and allowed to adsorb on the sample which was held at 110 °C. This was followed by pumping under vacuum for 0.5 h to remove physisorbed methanol. The samples were then heated to 200 °C, and the in-situ FTIR spectra were recorded as a function of time at 200 °C. 2.5. X-ray Photoelectron Spectroscopy (XPS). XPS measurements were performed on a Thermo Scientific ESCALAB 250Xi instrument (Thermo Scientific) equipped with an Al Kα X-ray radiation source (hν = 1486.6 eV). For the reference measurements, the sample was calcined at 300 °C under O2 for 1 h to remove the water residue in the sample. The sample was then reduced under H2 at 350 °C for 3 h. XPS measurements of the catalyst samples were performed after 0.5,

1, and 3 h reaction times. For these measurements, the same quantity of a given batch of 1c VOx/θ-Al2O3 was allowed to react with methanol for the indicated time and then cooled to room temperature and transferred to the XPS apparatus where 64 spectra were acquired and averaged. The binding energies were calibrated using the C 1s peak at 284.8 eV as a reference.

3. RESULTS AND DISCUSSION 3.1. In-Situ UV Raman Spectroscopy. In-situ UV Raman spectroscopy has the ability to selectively detect the monovanadate (VO4) and polyvanadate moieties as well as VOx nanoparticles.20,58 Figure 1 shows the in-situ UV Raman

Figure 1. In-situ Raman spectra of dehydrated ALD-prepared VOx/θAl2O3 taken at room temperature.

spectra of VOx/θ-Al2O3 samples. The 1c VOx/θ-Al2O3 (0.48 wt %) exhibits two primary peaks, at 1015 and 910 cm−1, which are due to VO and V−O−Al vibrations of the monovanadate species, respectively.21,58 The other two peaks in this spectrum, at approximately 840 and 746 cm−1, are due to the θ-Al2O3 support. The 2c VOx/θ-Al2O3 (1.20 wt %.) and 10c VOx/θAl2O3 (3.40 wt %) exhibit similar but shifted Raman peaks for the VO vibration, i.e., 1021 cm−1 for 2c VOx/θ-Al2O3 (1.20 wt %) and 1026 cm−1 for 10c VOx/θ-Al2O3 (3.40 wt %). Each of these samples also has a peak near 910 cm−1 due to the V− O−Al vibration. As previously reported, the shift of the Raman bands with increasing loading is due to the formation of polyvanadate species.58 However, VOx nanoparticles are undetectable in these samples as evidenced by the absence of a Raman peak around 994 cm−1, which has been ascribed to the VO vibration in VOx nanoparticles.58 3.2. UV−vis DRS Measurements. As reported in the literature,23,5 the fraction of monovanadate,VO4, in the ALD samples can be estimated from UV−vis DRS based on a linear relationship between the UV−vis DRS edge energy, Eg (eV), and Xm, the fraction of monovanadate in the sample: Eg = 3.02 + 0.53X m

On the basis of the UV−vis DRS spectra shown in Figure 2, Eg (eV) for the three sample is determined as 3.58, 3.41, and 3.09 eV. The fraction of monovanadate in the samples is then calculated to be ∼100%, ∼75%, and ∼13%, respectively. This is consistent with the conclusion from in-situ Raman measurements that monovanadate (VO4) is the exclusive vanadia C

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study focuses on an investigation of the 1c VOx/θ-Al2O3 sample where the only observed species is the monovanadate. 3.3.1. In-Situ FTIR Spectroscopy. 3.3.1.1. Identification of Two Formates. As shown in Figure 3, there are multiple absorption bands observed between 1100 and 3700 cm−1 when the 1c VOx/θ-Al2O3 catalyst is exposed to methanol. The absorption band at 1180 cm−1 is assigned to the C−O stretch of methoxy.8,10,36 In addition, as seen in the inset in Figure 3, there are two absorption bands located at 2943 and 2827 cm−1 which are assigned to the C−H stretch and the overtone of the C−H deformation of methoxy, respectively.10,63,64 The absorption bands centered near 1600, 1390, and 1375 cm−1 have been assigned to the O−C−O− stretches of the formate moiety with the latter two being symmetric stretching absorptions of two different formate species.10,36,64 As shown in Figure 4, the absorption band centered near 1600 cm−1 can

Figure 2. UV−vis DRS spectra of dehydrated ALD-prepared VOx/θAl2O3 taken at room temperature.

species in 1c VOx/θ-Al2O3, while polyvanadate is the predominant vanadia species in 10c VOx/θ-Al2O3. 3.3. Methanol Oxidation to Formate over VOx/θ-Al2O3. Methanol oxidation on supported metal oxides has received considerable attention. For example, Busca et al. studied the formation of formate and methyl formate on titania-supported vanadia.36 The catalysts were prepared by wet impregnation with a much higher vanadia loading than the ALD-prepared catalysts used in this study, which would be expected to lead to different surface moieties and thus potentially different surface reaction sites than in the current study. Additionally, their study was done under atmospheric pressure, and the oxidation state of vanadium was not directly interrogated.36 Though mechanisms for the formation of dimethyl ether and formaldehyde from methanol over vanadia have been the subject of both theoretical and experimental investigations,9,10,27,39 there are still questions about the details of the molecular level mechanism for the formation of formate on a supported vanadia catalyst. In this study, methanol oxidation was monitored by in-situ FTIR spectroscopy on a VOx/θ-Al2O3 catalyst in the absence of added oxygen. We note that methanol oxidation over 1c, 2c, and 10c VOx/θ-Al2O3 all yielded absorptions assignable to two different formates. Since the 1c VOx/θ-Al2O3 sample contains essentially 100% monovanadate, the results for this sample could be interpreted considering only the catalytic activity of the monovanadate. Thus, the present

Figure 4. Deconvolution of the in-situ FTIR spectra generated after adsorption of methanol on 1c VOx/θ-Al2O3 (0.48 wt %) at 200 °C taken after 2 h reaction time.

be deconvoluted into two absorptions with peaks at 1620 and 1590 cm−1. These absorptions are attributed to the asymmetric stretch of the O−C−O− moiety of the two different formate species. Although corresponding absorptions of formates have been reported before,10,36,63,64 it does not appear that it has been previously recognized that, at least in some cases, two distinct formates were being observed. The two different formates observed in this study are denoted as type I with

Figure 3. In-situ FTIR spectra of methanol oxidation on 1c VOx/θ-Al2O3 (0.48 wt %) as a function of time at 200 °C. Arrows indicate the direction of the change in absorptions. D

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The Journal of Physical Chemistry C absorptions at 1620 and 1390 cm−1 and type II with absorptions at 1590 and 1375 cm−1. The next paragraph discusses how it is possible to distinguish between and assign these two formates to specific entities. There are basically two types of active sites on the surface of the 1c VOx/θ-Al2O3 catalyst. There are vanadium sites, which are redox active, as well as acid sites on the alumina. Thus, it is plausible that the difference between the two formates is that one resides on vanadium sites and the other on aluminum sites. To investigate this hypothesis further, gaseous formic acid was allowed to adsorb on θ-Al2O3 or on V2O5 powder (SigmaAldrich, >99%), each of which had been calcined in situ under O2 at 350 °C for 1 h. This led to the formation of adsorbed formic acid and surface-bound formate. As summarized in Table 1 and shown in Figure S1, formate on θ-Al2O3 exhibits

Figure 5. (a) Plot of the integrals of the absorptions at 1375 (vanadium site) and 1390 cm−1 (aluminum site) for formate formed on 1c VOx/θ-Al2O3 (0.48 wt %) at 200 °C. (b) Relative yield of formate on vanadium and aluminum sites (I1375 cm‑1/I1390 cm‑1) based on the data in a.

Table 1. Assignments of the Vibrational Modes of Formate on θ-Al2O3, V2O5, and 1c VOx/θ-Al2O3(0.48 wt %) θ-Al2O3 V2O5 VOx/θ-Al2O3

νas(COO−) (cm−1)

νs(COO−) (cm−1)

1624 1587 1620 1590

1392 1373 1390 1375

gas-phase and physisorbed methanol are pumped out of the cell and the sample is heated. At 110 °C some methoxy absorptions are visible, and at 200 °C significant methoxy absorptions are evident at 2943 and 2827 cm−1. OH absorptions in the 3500 cm−1 region grow in on the same time scale as the methoxy absorptions, consistent with the formation of methoxy by loss of an H from the methanol OH group. These H atoms react with surface OHs to form surface-bound water absorbing in the 3500 cm−1 region. Though methoxy absorptions are observed on the vanadia catalyst even at 110 °C and grow as the temperature is increased to 200 °C, very little, if any, formate formation is observed at 110 °C. Thus, under these conditions formation of methoxy is not the rate-limiting step in formate formation. There are also absorptions in the OH stretching region between 3600 and 3800 cm−1. As seen in Figure S4, there is a depletion of an absorption in the OH region near 3700 cm−1, which is a result of the loss of surface OH65 and the concomitant growth of an absorption in the OH absorption region for water.65 This is consistent with the H atom being lost when methanol chemisorbs reacting with a surface OH to form water. 3.3.1.2. Vanadium Oxidation State. As seen in Figure 6a, a “negative” peak develops around 2035 cm−1 on exposure of the vandia catalyst to methanol. This “negative” peak is seen even at 110 °C, and there is a modest increase in the magnitude of the peak as the temperature is increased to 200 °C. This peak is assigned to a decrease in the intensity of the overtone of the vanadyl VO stretch as a result of a change in the oxidation state of the vanadium in the V5+O bond on the 1c VOx/θAl2O3 catalyst.10 This change in oxidation state appears as a “negative absorption” since, as with all of our spectra, the spectrum in Figure 6a is referenced to a background taken prior to exposure to methanol. This background spectrum contains absorptions of unreduced V5+O moieties. Since the spectrum in Figure 6a is produced by dividing a single-beam spectrum taken after reaction by the single-beam background spectrum, a decrease in the absorbance of a species in the spectrum taken after reaction relative to its concentration in the background will appear as a negative going feature. As discussed below, it is demonstrated that the dominant vanadium oxidation state of +5 before reaction changes to +3 during the reaction. However,

two absorptions, 1624 (νasOCO‑) and 1392 cm−1 (νsOCO‑), which are very close to absorptions assigned to type I formates (1620 and 1390 cm−1). As also shown in Figure S1, formate on V2O5 has two absorptions, 1590 (νasOCO‑) and 1375 cm−1 (νsOCO‑), which are effectively the same frequencies as observed for type II formate. These measurements provide strong evidence that the two sets of formate absorptions observed in the reaction of methanol on 1c VOx/θ-Al2O3 result from formate adsorbed on vanadia sites and formate adsorbed on sites on alumina. Previous studies of the formation of formates were typically part of studies of the oxidation of methanol on vanadium (redox sites). Even though absorptions for these two types of formates were reported, it does not appear it was recognized that there were two distinct formates and that one formate interacts with the redox active oxide and the other resides on the support (presumably on the acid sites).8,11 The two formates were monitored via the absorptions at 1375 (νsOCO‑ of formate on vanadium site) and 1390 cm−1 (νsOCO‑ of formate on aluminum site) as a function of reaction time and temperature.62 As shown in Figure 5a, at 200 °C the amount of formate on both the vanadium and the aluminum sites increases almost linearly with reaction time over the first 3 h of reaction. However, the rate of increase for the formate on vanadium sites is faster than the rate of increase of the absorption for the formates on the aluminum sites. As shown in Figure S2, for a sample of 1c VOx/θ-Al2O3, the rate of production of formate approaches an asymptote at longer reaction times. Similarly, the ratio of the intensities of the two types of formates also approaches an asymptote. This latter behavior is what would be expected if the two formates were approaching an equilibrium between the species on vanadium and those on aluminum sites, though the data does not require that an equilibrium is being approached. Figure S3 shows a spectrum of a sample of the same θ-Al2O3 used as a support for the ALD-prepared vanadia samples that has been exposed to gas-phase methanol. The first trace is taken on admission of gas-phase methanol and shows absorptions characteristic of that species. Subsequent traces are taken after E

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Figure 7. Deconvolution of the V 2p3/2 signal for after the methanol oxidation reaction has proceeded for 3 h at 200 °C on 1c VOx/θ-Al2O3 (0.48 wt %).

points from the deconvolution of the XPS spectra are plotted in Figure 8 and show that, under these reaction conditions, after 3 Figure 6. (a) Time dependence of the in-situ FTIR spectra of the overtone of the vanadyl VO stretch during methanol oxidation on 1c VOx/θ-Al2O3 (0.48 wt %) at 200 °C. (b) In-situ FTIR spectra of CO adsorbed on 1c VOx/θ-Al2O3 (0.48 wt %) after the methanol oxidation reaction has been run for 3 h at 200 °C. For both a and b, after reaction the sample was cooled to room temperature and exposed to 10 Torr of CO for ∼20 min and then evacuated, and an FTIR spectrum was recorded.

a change in the oxidation state of +2 is not required for the development of a negative peak in the VO stretch region. The change in the oxidation state of vanadium simply must be sufficient that the frequency of the VO stretch changes. This would be expected to happen even with a partial oxidation state change, which is what is anticipated to take place on adsorption of a methoxy on a V5+ site. The fact that a new peak due to a V−O bond with a bond order less than 2 does not appear suggests that there may be slightly different changes in oxidation states at different vanadium sites with the possibility of a slightly different frequency at each site which would lead to overlap of a set of small absorptions that could easily be unresolvable from the background. XPS measurements provide quantitative information about the oxidation state of vanadium on the surface of the catalyst. In the present experiment, XPS spectra of 1c VOx/θ-Al2O3 have been recorded under the following conditions: (i) oxidation at 350 °C with O2 for 3 h; (ii) reduction at 350 °C with H2 for 3 h; (iii) oxidation of methanol at 200 °C for different reaction times. The binding energies of V5+, V4+, and V3+ have been reported to be 517.6, 516.6, and 515.7 eV, respectively.66−68 As shown in Figure S5, only V5+ with a binding energy at 517.8 eV was observed in the 1c VOx/θ-Al2O3 after oxidation with O2, while a mixture of V5+, V4+ (516.6 eV), and V3+(515.7 eV) was observed for the 1c VOx/θ-Al2O3 that was reduced as a result of reaction. The XPS spectra of 1c VOx/θ-Al2O3 after methanol oxidation at 200 °C have been recorded at 0.5, 1, and 3 h. As shown in Figure 7, after a 3 h reaction time the XPS spectrum demonstrates that the oxidation state of vanadium for the 1c VOx/θ-Al2O3 sample is a mixture of +5, + 4, and +3. Data

Figure 8. Relative intensity of V5+, V4+, and V3+ obtained from a deconvolution of vanadium XPS data for methanol oxidation on 1c VOx/θ-Al2O3(0.48 wt %) as a function of the reaction time.

h of reaction V5+ is mainly reduced to V3+. In addition to the XPS measurements, CO adsorption on the VOx/θ-Al2O3 has also been used to identify the oxidation state of vanadium after reaction.69,70 As shown in Figure 6b, CO(ads) exhibits an absorption at 2178 cm−1 which has been previously assigned to CO interacting with a V3+ site.69,71 This observation is consistent with the XPS data which indicates that after ∼3 h V3+ is the dominant vanadium species on the surface. It was also investigated whether CO could be used as a probe for the V4+ oxidation state. This was achieved by reducing the vanadia sample with hydrogen at 400 °C, evacuating the cell, cooling to room temperature, and then adding CO. When CO was first added a relatively broad peak was observed where CO bound to V4+ has been reported.71 This absorption is shown in Figure S6 where the V4+ in this figure is generated as a result of oxidation of methanol on VOx/θ-Al2O3. When the cell was then pumped on to remove excess CO, the peak due to CO interacting with V4+ narrowed and decreased in intensity. Continued pumping F

DOI: 10.1021/acs.jpcc.7b07498 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Scheme 1. Proposed Mechanism for Methanol Oxidation to Formate on 1c VOx/θ-Al2O3(0.48 wt %).

presence of oxygen V 3+ is not formed in significant concentrations and that vanadium moieties are reduced by one electron, but are then rapidly oxidized back to V5+. As also discussed, the only identifiable vanadia species on the 1c VOx/θ-Al2O3 is the monovanadate. If the minimum number of vanadium atoms is involved in the oxidation process, an intermediate would be expected to form from methoxy as a result of a 2e− reduction of one vanadium atom and would have a carbon atom in the zero oxidation state (C(0)). The most efficient reaction path for this intermediate would be to interact with another V5+ to complete oxidation of the intermediate to formate. On the basis of the required chemistry this intermediate must have a carbon atom, an O atom, and two H atoms. Interaction with a second vanadium atom would have to involve either mobility of the intermediate or reaction at proximate pairs of monovanadate moieties. For the loading that is present for 1c VOx/θ-Al2O3, the number of monovanadate pairs that would be expected to form, based on statistics, is much smaller than the number of observed formates. Pairs could form as a result of vanadium mobility, but polyvanadate species were not observed in the Raman spectra of the lowloading samples, which were taken after calcination. Therefore, the formation of a significant number of proximate pairs seems very unlikely since they would have to approach each other closely enough to interact with the same methoxy-derived intermediate but not interact enough to form polyvanadate moieties. Thus, it seems more plausible that the critical intermediate in the reaction, which would contain C(0), is mobile. In principle, the mobility of the C(0) intermediate in Scheme 1 could be a result of a desorption step producing a gas-phase formaldehyde-like moiety with subsequent adsorption on an aluminum site, with this process continuing until the

over a period of about 1 h led to a continued reduction in the amplitude of the peak until it was no longer observable. This behavior indicates the CO−V4+ interaction is sufficiently weak at room temperature that the moiety dissociates with time. Thus, our conclusion was that this interaction could not be used as a quantitative probe of the amount of V4+ that was present in a given sample. The relatively rapid loss in intensity for CO bound to V4+ on exposure of the system to vacuum is in contrast to what was observed for CO interacting with V3+, where no significant change in intensity of the CO absorption was observed on the time scale of at least 0.5 h when the system was pumped on. 3.3.2. Mechanisms of Methanol Oxidation to Formate over VOx/θ-Al2O3. The first step in methanol oxidation involves the adsorption of gas-phase methanol. Adsorption of methanol is observed on VOx/θ-Al2O3 as well as on samples containing only θ-Al2O3 and only V2O5 powder samples. The oxidation of methanol to formate involves a transfer of 4e− in which the methanol carbon goes from an oxidation state of −2 to a formate carbon with an oxidation state of +2. The only reducible metal is vanadium. As shown by the XPS data, vanadium is present as V5+ in the vanadyl moieties prior to reaction. Vanadium also has stable V3+ and V4+ oxidation states. As previously mentioned, V4+ has been reported to be the primary species formed on reduction during methanol oxidation reactions that are conducted in the presence of oxygen.27,38,43 Under such circumstances, reoxidation of V4+ to V5+ by oxygen is reported to be rapid. Vining et al. demonstrated that reoxidation of V4+ to regenerate V5+ is sufficiently rapidly that this rapidly regenerated V5+ can accept the necessary electrons for oxidation of methanol in preference to further reduction of V4+.27 Thus, as opposed to what is observed in the absence of oxygen, it is reported that in the G

DOI: 10.1021/acs.jpcc.7b07498 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C intermediate reached an aluminum site with a proximate V5+ moiety or adsorption directly on a V5+ site. It could then interact with and be oxidized by that V5+ site. To test the plausibility of a mechanism involving desorption/adsorption of an intermediate, reactions were run where methanol was allowed to adsorb on the catalyst while the methanol pressure was maintained at ∼8 Torr for 0.5 h. The cell was then opened to the vacuum system and left open while the catalyst was heated to reaction temperature. The reaction was monitored for 2 h under these conditions, and the percent of methanol converted in this time was comparable to what was observed when the reaction was run with the cell valved off from the vacuum system. These results eliminate the possibility of desorption/adsorption steps as the source of mobility of the intermediate, at least at 200 °C. On the basis of available data the mechanism shown in Scheme 1 is proposed for the formation of formate. We emphasize that the proposed mechanism is not intended to be a unique depiction of what is taking place at a molecular level during the reaction. Rather, it is proposed as a plausible mechanism that is consistent with the experimental data. For simplicity, Scheme 1 shows methanol being adsorbed on a vanadium site as a methoxy. However, since the data indicate that the surface of the catalyst is extensively covered with methoxy, the majority of the methoxy that is oxidized is expected to also come from methanol initially adsorbed on an aluminum acid site which migrates to a vanadium site. As alluded to below, it is proposed that an intermediate that has undergone a 2e− oxidation migrates to another V5+ to be further oxidized. Since most V5+ sites will have adsorbed methoxy, the migration of methoxy from the alumina to the V5+ must be reversible so that an oxidized intermediate can interact with a vanadium site that can accept two additional electrons. In step 2 in Scheme 1, an H atom is transferred from the methoxy to one of the O atoms in the vanadyl species along with the concomitant generation of a V4+ site. The CH2O− moiety thus formed could then displace an H from an OH and form a hydroxymethyl radical, containing a C(0) carbon, bound to the O. Calculations for gas-phase species indicate that the hydroxymethyl radical is almost isoenergetic with its methoxy isomer.72 The subsequent breaking of the bond between the vanadium atom and the oxygen further reduces vanadium to a V3+ species. This intermediate, which is a CH2O-like moiety, could desorb after formation. Though these results rule out significant desorption to produce formaldehyde at 200 °C, higher temperatures would be expected to favor desorption relative to mobility since the former process would be expected to have a larger pre-exponential factor. It is noted that formaldehyde is the major product in the reaction of methanol on silica-supported vanadia at 300−400 °C, and less formate formation is observed on silica (not shown) than on alumina at experimental reaction temperatures.28,29,37 In such a situation, the desorption of the intermediate as formaldehyde would be expected to dominate further reaction of the intermediate to yield formate. An explanation for these observations, though not a unique one, is that the proposed hydroxymethyl radical intermediate is not as mobile on silica as it is on alumina.8,27,38 This could be due to differences in the acidity of the support.73,74 Further oxidation of this intermediate must take place at another vanadium site, with the oxygen for the second oxidation process coming from a vanadyl moiety. In the proposed scheme, the mobility of the intermediate takes place

as a result of an H from an alumina OH being displaced by the hydroxymethyl radical which then bonds to the oxygen of a hydroxyl group on alumina. This process can be repeated as the hydroxymethyl radical moves to another OH site and the H from that site occupies the vacated surface Al−O site. A schematic of this mechanism for mobility of the hydroxymethyl intermediate is shown in Scheme S1. Further oxidation of the intermediate is shown in step 5 of Scheme 1, where the intermediate interacts with another vanadium site which is reduced to V4+. Following this reduction, steps 6 and 7 result in the reduction of a second vanadium, from +4 to +3, and the formation of a formate and an OH. When formate is formed on the vanadium site, the formate can transfer to sites on the alumina support until a steady state (step 8) or possibly an equilibrium is reached between formates on vanadia and formates on alumina. The buildup of formate on the support and the role the support plays in the mobility of various species indicates that the aluminum support is not a spectator; rather it plays a role in the reaction mechanism. The time dependence of the formation of formate is shown in Figures S2 and S7, where it is apparent that it tracks the loss of methoxy. This behavior has implications for the kinetics of the proposed mechanism. In previous studies, the rate-limiting step in the oxidation of methanol to formaldehyde was proposed to be the formation of the CH2O-like intermediate, which is formed by the first hydrogen migration from methoxy, which is accompanied by the reduction of V5+ to V4+ (as in step 2 in Scheme 1).7,39 The second hydrogen transfer leading to the formation of formaldehyde has been calculated to be more energetically favorable than the first hydrogen transfer for vanadia on silica.39 As stated, formaldehyde is not observed in these experiments either in the gas phase or on the surface. In the mechanism in Scheme 1 a hydroxymethyl radical intermediate is proposed, with its carbon atom in the zero oxidation state, and is formed from CH2O−, which is a possible intermediate that could, on desorption, yield formaldehyde. Thus, observations suggest that on alumina at 200 °C when the hydroxymethyl radical is formed from the CH2O− intermediate it rapidly migrates and interacts with another vanadyl site and moves further along the reaction pathway to formate.37,39,40 The key steps in the mechanism proposed are summarized in Scheme 1. Methoxy + V 5 + ↔ CH 2O−(Intermediate) + V 4 +

(2)

CH 2O− + V 4 + ↔ − CH 2OH(hydroxymethyl) + V3 + (3)

CH 2OH(hydroxymethyl) + V 5 + ↔ CH 2O2−(Intermediate) + V 4 +

(5)

CH 2O2 (Intermediate) + V 4 + ↔ HCO2−(Formate) + V3 + (7)

The data in Figure S7, which shows that the loss of methoxy and the growth of formate track each other, suggests that the overall reaction is controlled by step 3. After formation, a CH2O− intermediate reacts to form hydroxymethyl, and this species rapidly progresses to formate. This hypothesis is consistent with the discussion above about the competing pathways for formate versus formaldehyde, where once the hydroxymethyl intermediate is formed, subsequent steps that lead to formation of formate are rapid. As discussed below, the proposed mechanism is also consistent with the observations of H

DOI: 10.1021/acs.jpcc.7b07498 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C Scheme 2. Proposed Mechanism for Methanol Oxidation to Formaldehyde on 1c VOx/θ-Al2O3 (0.48 wt %)

ultimate reduction of V5+ to V3+, though each methoxy and its subsequent reaction products would have to access more than two vanadium moieties. If more than two vanadium moieties are involved in the overall oxidation process then an additional mobile intermediate is needed for each step that accesses another (third or fourth) vanadium moiety. Thus, in this respect the mechanism presented in Scheme 1 adheres to Occam’s Razor, and we feel it is more plausible than a more complex mechanism involving additional vanadium surface sites. It is also a priori conceivable that there are a small number of proximate pairs of vanadium atoms that are extremely reactive for formate formation. The yield of formate per vanadium atom for 1c versus 2c ALD-prepared samples can be used to provide some insights into this issue since the number of polymeric vanadium species increases for the 2c samples versus the 1c sample. The formate/V atom ratio is lower for 2c VOx/θ-Al2O3 (0.17) than that for 1c VOx/θ-Al2O3 (0.28), suggesting that interacting vanadium atoms (polyvanadate), which are more prevalent as the number of ALD cycles is increased, are less active for methanol oxidation than the monovanadate, thus making this scenario less plausible. The question can now be asked: Why at higher vanadia coverages and/or at higher temperature is formaldehyde formed on VOx/θ-Al2O3 in preference to formate? As discussed, the coverage of vanadia affects the nature of the surface vanadia species which, in turn, affects the product distribution. The nature of the vanadia species can then affect the operative reaction pathways. A mechanism, shown in Scheme 2, that could be operative at higher vanadia loadings is proposed. This mechanism involves proximate pairs of vanadia sites. In this mechanism, a hydrogen transfer reaction can take place from the initially bound methoxy to a neighboring vanadia leading to the formation of a formaldehyde-like moiety bound to a V5+ site. This species can then desorb as formaldehyde, leaving behind two vanadium atoms in the +4 oxidation state. In the presence of oxygen, these sites would be rapidly reoxidized to V5+. This pathway appears to be consistent with Busca’s schematic mechanism for methanol oxidation at high loadings of vanadia.36 It should be noted that such a pathway is not operative in the absence of proximate pairs of vanadium atoms since a vanadium site proximate to the site on which the formaldehyde precursor is bound supplies one electron for the two-electron oxidation process. Scheme 2 is consistent with the conclusion that only V4+ sites are needed for methanol oxidation at higher coverages, where these sites can be rapidly reoxidized in the presence of oxygen at temperatures above ∼300 °C.

the time dependence of vanadium oxidation states monitored via XPS. The data shown in Figure 8, obtained by deconvolution of the V 2p3/2 XPS spectra, allow us to comment on the time dependence of the ratios of the vanadium oxidation states. Over the first half hour, the signals due to V3+ and V4+ rise while the signal due to V5+ falls. Between 0.5 and 3 h the signal due to V3+ continues to grow while those for V5+ and V4+ fall at approximately the same rate. The behavior of these data is compatible with an equilibrium between V5+ and V4+ that is established by approximately 0.5 h of reaction time. This could be the equilibrium suggested in eq 2 above. Since V3+ continues to grow as V4+ and V5+ are depleted, it is clear that either V3+ is not in equilibrium with V5+ and V4+ or equilibrium has not been established. Thus, combining the proposed mechanism with the data presented herein we conclude that since the reactions are run in the absence of added oxygen the reaction of methoxy does not progress beyond a one-electron transfer until V4+ is reduced to V3+. Once the 2e− reduction of V5+ takes place and the hydroxymethyl radical forms, the rest of the reduction process is rapid compared to the rate of reduction of V5+ to produce the initially formed V4+ species. Again, this proposed behavior is consistent with the observation that the rate of loss of methoxy mirrors the rate of formation of formate. The data in Figure S2 are also consistent with the number of vanadyl moieties being the limiting factor in the conversion of methoxy to formate. Note that the reaction is approaching an asymptote for conversion of methoxy and formation of formate on the time scale of 8 h. This behavior is consistent with the reaction terminating as the V5+ sites are reduced to V3+. To test this hypothesis, the loss in methoxy is estimated assuming that the absorbance of the C−H stretch in methoxy is comparable to that for gas-phase methanol.75 On the basis of this assumption, the loss in the methoxy absorbance corresponds to oxidation of ∼6 × 1017 adsorbed methoxy moieties. The total number of vanadium atoms in the sample has been determined using ICP as ∼3 × 1017 atoms. Thus, the number of methoxy species that are oxidized is within an order of magnitude of one-half the number of vanadium atoms on the surface. This estimate also implies that a significant fraction of the V5+ sites are reactive. This conclusion is consistent with a result obtained by Nair and Bertsch in which the number of active redox sites on some supported metal oxides was quantified by the isothermal anaerobic titration for the oxidation of ethanol to acetaldehyde.76 The fraction of active redox sites per vanadium atom was determined to be ∼0.7 per metal atom for vandia on alumina, which is within the limits of the approximations used to estimate the number of active sites for the sample and the reaction of interest in this work. The current data do not allow us to exclude a mechanism in which more than two vanadium moieties are involved in methanol oxidation. Such a mechanism could still lead to the

4. CONCLUSION One cycle (0.48 wt %), 2 cycle (1.20 wt %), and 10 cycle (3.40 wt %) VOx/θ-Al2O3 catalysts were synthesized using the atomic layer deposition (ALD) method and characterized by in-situ I

DOI: 10.1021/acs.jpcc.7b07498 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C ORCID

Raman and UV−vis DRS spectroscopy. Monovanadate, VO4, is the dominant surface species at a loading of 0.48 and 1.20 wt %, while the polyvanadate species is the main species at a loading of 3.40 wt %. This study focuses on the oxidation of methanol to formate using 1 cVOx/θ-Al2O3 samples. The initial step in this reaction is the interaction of a methoxy species with a vanadium site. These data imply that methoxy is mobile and can interact with both vanadium and aluminum binding sites. In-situ FTIR spectroscopy allows us to identify two different formates that are located on vanadium or aluminum sites, respectively, and are mobile species that can move between vanadium moieties and sites on the alumina. The oxidation of methoxy to formate is a 4e− process. As shown in reactions 2−7, a mechanism is proposed for this reaction for low vanadium coverage under anaerobic conditions. This mechanism is proposed to have a rate-limiting step of the reduction of V4+ to V3+ and involves the formation of a mobile hydroxymethyl radical intermediate with a carbon with an oxidation state of zero. A mechanism is proposed for the mobility of this radical which allows this intermediate to access another V atom to complete the 4e− oxidation process. Thus, the two vanadium atoms that change from V5+ to V3+ do not have to be proximate to each other. It is proposed that the hydroxymethyl radical is formed at one vanadium site, is mobile on the surface of the alumina support, and can undergo a further 2e− oxidation at another vanadium site. While it is possible that there are proximate pairs of vanadium atoms that could be involved in both the oxidation to a carbon zero species and the subsequent oxidation of this intermediate to formate, we discuss why it is more plausible that the critical intermediate, proposed to be a hydroxymethyl radical, is mobile. The observed kinetics and the XPS data are consistent with an equilibrium being established between V5+ and V4+. Once V4+ is reduced to V3+ the data are consistent with a rapid subsequent reaction of the hydroxymethyl radical to yield formate. The initial equilibrium between V5+ and V4+ controls the breaking of the VO bond in Scheme 1. Formate is eventually formed by the addition of oxygen from the surface vanadyl moieties. Thus, each formate formed is accompanied by reduction of two vanadium sites from +5 to +3. This change in oxidation state from V5+ to V3+ during the oxidation of methanol is confirmed by XPS and CO adsorption experiments. The V5+ sites can be regenerated on exposure to the gas-phase oxygen at suitable temperatures.

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Jian Liu: 0000-0002-9684-339X Eric Weitz: 0000-0002-7447-4060 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy (Award No. DE-FG02-03-ER15457).

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b07498. Infrared spectra for adsorbed formic acid; time dependence of methoxy and formate absorptions; IR spectra of methanol on θ-alumina; OH and C−H stretch regions of a sample undergoing methanol oxidation; spectra of CO when used as a probe of V oxidation state; deconvolution of XPS data; IR spectra of the methoxy region after methanol oxidation; additional information on the proposed mechanisms for migration of a C(0) intermediate (PDF)

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The Journal of Physical Chemistry C

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DOI: 10.1021/acs.jpcc.7b07498 J. Phys. Chem. C XXXX, XXX, XXX−XXX