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On the Active Oxygen in Bulk MoO3 during the Anaerobic Dehydrogenation of Methanol Matthew G. O’Brien,† Andrew M. Beale,† Simon D. M. Jacques,‡ Thomas Buslaps,§ Veijo Honkimaki,§ and Bert M. Weckhuysen*,† Inorganic Chemistry and Catalysis Group, Debye Institute for NanoMaterials Science, Utrecht UniVersity, Sorbonnelaan 16, 3584 CA Utrecht, The Netherlands, Department of Chemistry, UniVersity College London, 20 Gordon Street, London WC1H 0AJ, Department of Crystallography, Industrial Materials Group, Birkbeck College, UniVersity of London, London WC1E 7HX, U.K., and The European Synchrotron Facility, 6 Rue Jules Horowitz, BP220, 38043, Grenoble, CEDEX 9, France ReceiVed: NoVember 27, 2008; ReVised Manuscript ReceiVed: January 16, 2009
The oxidation of methanol under anaerobic reaction conditions over MoO3 has been studied using an in situ approach, combining ultraviolet-visible (UV-vis), Raman, wide-angle X-ray scattering (WAXS), and online mass spectroscopy (MS) techniques. Comparison of the UV-vis and MS data reveals that during the initial stages of the reaction methanol is chemisorbed onto the oxide’s surface, primarily at defect sites. Reaction then begins, producing formaldehyde, dimethyl ether, and water. At low temperatures, CO and MoO2 are also produced, as reoxidation of the reactive sites cannot occur rapidly enough to avoid additional reduction. After the initial heating, continued reduction of the bulk oxide by Mars-Van Krevelen oxygen transfer to the active surface sites is observed as a change in the total Raman intensity. Most significantly, after 125 min of reaction, bulk MoO2 is observed and here a Rietveld analysis of the MoO3 WAXS data indicates qualitatively that one of the three unique oxygen environments (O1) becomes more active than the others. This is confirmed by changes in the Raman data, indicating that the Mo-O1 bond to this oxygen is broken more quickly. This work has therefore identified the oxygen most likely to be transferred through the bulk of the oxide during the Mars-Van Krevelen oxygen transfer. However, a comparison with previous works and our MS and UV-vis data indicates no particular relationship between the bonding in the bulk oxide and its surface reactivity. Therefore, although O1 is abstracted and transferred through the bulk, it may be replacing other oxygen atoms (i.e., O2 or O3) at the oxide surface. This work then also demonstrates that to fully understand a parent oxide we cannot rely on a bulk view of the entire system, but must obtain separate details about both the surface sites (responsible for selectivity) and the bulk sites (that maintain catalytic activity by oxygen transfer), particularly under oxygen-free conditions. Introduction Heterogeneous molybdenum oxide-based catalysts are commonly employed for selective oxidation. They are used in many industrial chemical processes, including, for example, the production of acrolein from propene (R,β,γ-BiMoOx), the conversion of methanol to formaldehyde [Fe2(MoO4)3/MoO3], and propane oxidation (MoTeVNbO).1-3 They have therefore attracted much research interest in an attempt to understand the key properties that make them so appropriate for the catalytic “job in hand”. Such knowledge will ultimately allow the development of more efficient materials, bringing the “designer” catalyst a step closer to reality.4 However, mixed molybdate materials are often structurally complex, containing various components in different coordination and oxidation states. Consequently, a detailed analysis of each component’s role in the catalytic process can be difficult to evaluate. It is often more convenient to examine the catalyst’s “parent” oxides, where a more detailed insight into these materials can then be transferred to the more structurally complex multicomponent system.5 In this regard, the partial oxidation activity of R-MoO3 has been well studied.6-10 * Author to whom correspondence should be addressed. Fax: 00 31 (0) 30-251-1027. E-mail:
[email protected]. † Utrecht University. ‡ University College London and University of London. § The European Synchrotron Facility.
One of the important steps in understanding (and therefore improving) an oxidation catalyst is a detailed knowledge of the interaction and conversion of reactants on the active sites at the surface. Previous analysis of R-MoO3 has revealed that the basal (010), side (100), and apical (001 + 101) faces are primarily exposed, although the exact ratios depend on the sample preparation.10,11 These faces can then react with methanol via both Mars-Van Krevelen and Eley-Rideal mechanisms to produce formaldehyde and side products, such as dimethyl ether (DME), depending on the exact sample preparation and operational conditions.6,10,12 Generally, methanol oxidation on R-MoO3 is believed to proceed by the formation of methoxy and hydroxyl intermediates which then decompose to produce formaldehyde and two surface hydroxyl groups.13-15 These groups then form water via oxygen extraction from the catalyst in a Mars-Van Krevelen-type manner.16 It is widely accepted that the bulk MdO (M ) metal) bond length and strength play a significant role in the activity and selectivity of these materials.5,17 For example, if the bond is too strong, the catalyst remains unreactive, while if it is too weak, overoxidation occurs.5 However, more recently, a comparison between the activity and bond distance of a number of catalyst materials revealed no support for such a relationship. This is partly due to the fact that there is often very little correlation between bonding at surface and in the bulk.18
10.1021/jp810428e CCC: $40.75 2009 American Chemical Society Published on Web 03/04/2009
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Anaerobic Oxidation of Methanol over MoO3 While the reactions at catalyst surface sites are very important for understanding the formation mechanisms, a second and equally essential step requires a knowledge of site regeneration, as this can determine the life of the catalyst. Numerous 18O2 experiments indicate that in many oxide systems a Mars-Van Krevelen mechanism occurs.5,19,20 Under aerobic conditions, the oxygen removed from the framework in this process can be regenerated from the gas stream. However, under anaerobic or oxygen poor conditions, for reactivity to continue past initial surface reduction some oxygen must be continually supplied from the bulk.5,13,21 Therefore, to fully understand surface site regeneration (and the ability of the catalyst to maintain activity), a detailed knowledge of the oxygen transfer in the bulk is also often required. In this paper, we use a multitechnique in situ approach22-24 to probe MoO3 during methanol to formaldehyde conversion under anaerobic conditions. This approach combines congruent measurements from WAXS, Raman, UV-vis, and online MS to observe the oxide in a complementary manner, revealing more detailed changes within the material, particularly within the bulk. It will be shown that with such a setup it is possible to identify the most active oxygen within the bulk oxide (i.e., the oxygen most likely to be transferred to the surface during the Mars-Van Krevelen transfer) and demonstrate that there is indeed little correlation between the bulk and surface environments of such materials.18 Experimental Section For all experiments, MoO3 (Merck >99% purity) was used as received. The experimental setup utilized was designed specifically to enable larger, more operando-like vessels (quartz, 4 mm internal, 6 mm external diameter) and is based on an “open” architecture with the reactor heated by two “minisensor” heat guns (max temp ) 750 °C) rather than the closed nature of the geometry associated with a furnace. This open architecture allows for many more techniques to be focused on the sample in a single experiment without seriously compromising data quality. For these experiments, MoO3 was loaded into reactor vessels to a depth of 15 mm, and data were measured from a single point located approximately 3 mm from the top of the sample bed. For all experiments, the heat guns were ramped at a rate of 5 °C/min to 540 °C, which (after calibration) results in a true ramp rate of 3.23 °C/min to 350 °C in the sample. For the anaerobic methanol experiment, the sample was heated from 26 to 350 °C under a continuous flow of methanol/ He supplied via a gas bubbler and mass flow controller at 50 mL/min. On the basis of the vapor pressure of methanol at 21 °C, methanol saturation in the gas stream at room temperature was estimated to be 15%.25 Under these conditions, neither intercalate “bronze”-type materials nor high-temperature suboxides were observed to be forming.26-28 Comparative WAXS studies were also performed on samples under flowing air and water vapor, while Raman was also collected on the water vapor experiment. WAXS data were collected on the ID15B beam line at the European synchrotron radiation facility (ESRF), Grenoble, France, operating at 6 GeV with a peak current of 200 mA. The high flux and energies available on this beam line allow high-quality diffraction to be collected through the relatively thick sample cell.29 An energy of 88.85 KeV (0.13955 Å) was selected and this, coupled with an appropriately positioned CCD fast X-ray detector, enabled a recordable data range of 0.26700°-9.9968° 2θ (point-to-point resolution of 0.0032° 2θ), suitable to measure all the major Bragg reflections of MoO3.
Data were collected for a period of 8 s/pattern with a gap of 8 min between each collection. The two-dimensional diffraction data were integrated over the full range of the detector using the Datasqueeze software suite, resulting in high-resolution onedimensional scans.30 The profiling software Fityk was used for initial examination of the data and full profiling was then performed using the Fullprof suite of programs.31-33 Initial parameters were based on those given in the work of Negishi et al., and to retain a good fit, a two-phase fitting procedure was employed upon appearance of MoO2 using parameters based on those of Magneli (Supporting Information).34,35 Prior to the full Rietveld analysis, accurate unit cell parameters were obtained using Le Bail fitting procedures. The Bond_str plugin to Fullprof was then used to calculate bond distances. UV-vis spectra were recorded in diffuse reflectance mode from 280 to 1100 nm using an AVASPEC fiber optical AVSSD2000 spectrometer (Avantes) and a 400 µm fiber optical cable. A typical collection time of 50 ms/spectrum was employed, of which 80 patterns were collected and summed together to improve the signal-to-noise ratio, resulting in a resolution of 4 s/spectra. Due to a lack of signal quality, d-d transitions for Mo4/5+ were not determined. Band profiling of the entire spectra was therefore performed using Gaussian functions and a least-squares fitting routine in the Thermo Galactic Grams AI v. 7.0 software. Initial band positions/widths were determined on the basis of previously reported values for intervalence charge transfer transitions for Mo6+.36,37 Raman data were collected using a Kaiser Optical Systems RXN spectrometer possessing a diode laser operating at 532 nm and a maximum output power of 70 mW. A 5.5 in. objective lens was used to focus a laser beam to a spot size of 200 µm and to collect the backscattered radiation via a CCD camera. Each Raman spectrum was collected for a period of 5 s, with a time gap of 8 min between each spectrum. Least squares peak profiling was performed using Gaussian functions in the Thermo Galactic Grams AI v. 7.0 software. Results Here we first detail the results from each of the techniques, namely, WAXS, Raman, UV-vis, and MS, and discuss the initial state of the MoO3 unit cell. Furthermore, any changes that can be attributed to effects such as sample heating, rather than methanol reactivity, are also discussed. Figure 1 displays the temperature/time-resolved 3D surface plots for the methanol experiment from each of the techniques used in the setup. Figure 1a shows a portion of the WAXS data. The peaks at 1.133°, 2.090°, 2.269°, 2.310°, 2.447°, and 2.635° 2θ (after the initial peak shift during heating) are indexed to the (101), (400), (201), (210), and (301) reflections of R-MoO3, respectively.34 After 125 min at reaction temperature a peak at 2.339° 2θ indexed to the (11-1)/(011) peak of MoO2 is observed.35 Figure 1b contains the UV-vis data and is characterized by a strong ligand-to-metal charge transfer (LMCT) band at 360 nm. Although clearer in the 1D-plot (Figure 5), two weaker bands are also initially observed at 650 and 850 nm, while at the end of the experiment another band at 435 nm is observed. From previous works these can be assigned to the following environments: 360 nm, d0 groundstate octahedral Mo6+-O6; 650 nm, undercoordinated Mo5+-O5; 850 nm, reduced Mo5+-O6; and 435 nm, d2 groundstate octahedral Mo4+-O6.36,37 For convenience, these are represented, with the molybdenum atom charge, in the standard molecular formula as “Mo6+O3”, “Mo5+O2.5”, “Mo5+O3”, and “Mo4+O2” environments, respectively. Finally, Figure 1c plots
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Figure 1. Three-dimensional temperature/time plots from each of the three techniques used to monitor the reaction of methanol over MoO3. The WAXS data (a) indicates the major reflections for MoO3 and MoO2 (indicated with *), in the UV-vis data (b) the two strongest bands are clearly observed, and in the Raman data (c) a number of peaks due to Mo-O bonds are indicated.
the in situ Raman data in which several strong bands are clearly observed. These are characterized as (from left to right) 285/ 295 cm-1 (MdO wagging), 335 cm-1 (Mo-O-Mo bend), 366/ 376/380 cm-1 (Mo-O-Mo scissor), 479 cm-1 (weak, Mo-O-Mo asymmetric stretch), 664 cm-1 (Mo-O-Mo symmetric stretch), 820 cm-1 (Mo-O-Mo asymmetric stretch), and 994 cm-1 (MdO asymmetric stretch).37 Due to the presence of a holographic notch filter, peaks at lower wavenumbers could not be observed. From these initial data a series of more detailed measurements were performed as discussed in the Experimental Section. WAXS. In Figure 2 the results of a Rietveld analysis of the WAXS data at the beginning (a) and end (b) of the experiment are given. In the latter, parameters for reduced MoO2 were used
to ensure a good fit of the data (Supporting Information).34,35 The small variation in the difference plots along with the conventional Rietveld parameters Rp ) 8.23, Rwp ) 9.38, Rexp ) 8.08 and χ2 ) 1.35 (a) and Rp ) 9.10, Rwp ) 10.5, Rexp ) 9.32, and χ2 ) 1.26 (b) indicate good fits for this in situ experimental setup and time resolution. From these data it is then possible to measure changes in the oxide structure such as unit cell size, bond distances, and, as demonstrated in Figure 2c, changes in phase composition. Here we observe that (as observed in the initial 3D WAXS plot) MoO2 begins to form after ∼125 min at reaction temperature and constitutes ∼35% of the sample at the end of the experiment. Table 1 details the calculated unit cell, atom positions, and bond lengths for the initial MoO3 data set (refined in the Pnma
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Figure 2. Rietveld fits of the WAXS data before (a) and at the end (b) of the methanol reaction. The experimental (exp) and simulated (sim) patterns are shown, along with the difference between these (diff). By then performing Rietveld on all data, a comparison of the percentage of each crystalline oxide can be calculated (c). From the initial fit the structure of the MoO3 can be deduced (d) and consists of five unique oxygen atoms highlighted and labeled (around a central molybdenum atom of a 1 × 2 × 2 supercell representation).
TABLE 1: Various Unit Cell Parameters Abstracted from the Rietveld Analysis of Our Initial MoO3 Data Compared with That Given by Negishi et al. and Leisegang et al. parameter MoO3 at 22 °C Negishi et al. at 27 °C Leisegang et al.a A 13.83714(64) B 3.68726(16) C 3.95278(17) Mo(x) 0.10179(6) Mo(z) 0.08513(18) O1(x) 0.43427(37) O1(z) 0.4984(13) O2(x) 0.08818(40) O2(z) 0.5273(12) O3(x) 0.2220(38) O3(z) 0.0429(15) Mo-O1b 2.341(6) Mo-O1a 1.9406(17) Mo-O2b 2.213(5) Mo-O2a 1.758(5) Mo-O3 1.672(6) volume 201.68(2)
13.8649(1) 3.69756(2) 3.96290(3) 0.10163(3) 0.08461(8) 0.4370(1) 0.4963(5) 0.0869(1) 0.5216(4) 0.2188(1) 0.0376(5) 2.305(1) 1.956(1) 2.241(1) 1.744(1) 1.635(1) 203.17
13.8621(9) 3.6970(2) 3.9614(4) 0.1001(4) 0.077(2) 0.4302(21) 0.490 (11) 0.0911(23) 0.473(11) 0.2263(23) 0.044(13) 2.370(30) 1.927(30) 2.396(47) 1.574(47) 1.755(32) 203.01(2)
a In this paper, the unit cell is in the nonstandard Pbnm space group and has been transcribed here to Pnma.
space group), along with those from previous experiments.34,38,39 The molybdenum can be described as being in either a 4- or 6-fold coordination depending on the measurement technique, although the bond angles around the molybdenum atom indicate that the coordination state is a mixture between the two.37,40,41 Each unit cell contains four formula units of R-MoO3 with each molybdenum atom surrounded by six oxygen atoms. As shown in Figure 2d and Table 1, this then results in three unique oxygen environments (O1, O2, and O3) and five different Mo-O bond lengths [Mo-O1a (two identical bonds), Mo-O1b, Mo-O2a,
Mo-O2b, and Mo-O3]. Two oxygen atoms (O1 and O2) are bound to two molybdenum atoms, forming two-dimensional networks along the b and c directions, respectively. The third oxygen (O3) is bound to a single Mo atom and has a significantly shorter bond (1.635 Å) with a strong covalent character.34 This atom resides within van der Waals gaps creating a channel-like system running along the a axis. The shift in MoO3 peak positions during the initial heating of the sample (as observed in Figure 1a) is a result of anisotropic thermal expansion of the unit cell.34 This is observed in the calculated unit cell parameters (Supporting Information) as an expansion along the a and c axis and a contraction along the b axis. Interestingly, however, we observe that there is actually an initial expansion of the b axis, observed under all experimental conditions (methanol, water, and air). Although not observed previously (possibly due to lower temperature/time resolutions and ex situ studies), this appears characteristic of this material (Supporting Information).31 Additionally, we note that there is no change in the full width at half-maximum (fwhm) of the MoO3 peaks (beyond the resolution of the instrument) during reaction, indicating (from a Scherrer analysis) no variation in particle size. From the calculated atom positions, the Mo-O bond distances in MoO3 were determined and in Figure 3 we plot selected distances and oxygen thermal parameters. Here we note that, due to the difference in scattering power between molybdenum and oxygen, the precision of the oxygen positions is not so high. However, the measurements still highlight qualitative changes in the trends in both bond length and thermal parameters. During the temperature ramp, significant variations in Mo-O bond lengths (Figure 3 left) are observed. However, as similar changes
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Figure 3. Changes in the Mo-O1a and Mo-O3 bond distance (left) and the corresponding thermal parameter for O1 and O3 (right) as a function of temperature/time during the methanol reaction.
are observed for the water measurement (Supporting Information), these changes are most likely due to the heating of the sample and subsequent variation of the unit cell.31 At the same time, an expected increase in the thermal parameters of these atoms is also observed (Figure 3 right). We do note a slight deviation in the behavior of the Mo-O2 bond distances between the methanol and water experiment (Supporting Information), and although its cause remains unknown, it can be explained by a slight dislocation of the oxygen along the c axis. On becoming isothermal, changes in both bond lengths and thermal parameters stop until the appearance of MoO2. At this point, the Mo-O1a bond length and O1 thermal parameter (Figure 3 top) both increase, while a small decrease in the Mo-O1b bond distance is noted (Supporting Information). At the same time, the Mo-O3 bond distance decreases; however, this is not accompanied by a change in the O3 thermal parameter. Raman. From the initial Raman data the peaks at 994, 820, and 664 cm-1 were fitted. These were chosen as they are welldefined and have previously been proposed to correspond to the three distinct oxygens of the MoO3 structure (Mo-O3, Mo-O2, and Mo-O1, respectively) and correlate well with those of the shortest bond distances for Mo-O (1.68, 1.75, and 1.94 Å).17,42 To provide a more quantitative intensity measurement, the data were corrected for laser power decay and darkening of the oxide from the diffuse reflectance of the UV-vis data as described previously (Supporting Information).43,44 The first plot in Figure 4 represents the intensity of the 820 cm-1 band throughout the reaction. This initially increases until ∼340 °C before decreasing slowly until the peak can no longer be clearly identified. Such changes are observed for all bands in the Raman but are not observed in the water experiment. Accompanying this, the position of all peaks shift to lower wavenumbers (∼2-4 cm-1), apart from the peak at 820 cm-1. This is consistent with a weakening and thermal activation of the Mo-O3 and Mo-O1 bonds as the temperature increases.10,17 To measure comparative rates of change, in Figure 4 we give the intensity ratios for the Raman bands relative to each other. During initial heating, the intensity of these ratios varies significantly; however, as with the WAXS, this is also observed in the water experiment (Supporting Information). Therefore, these changes are due to either a heat effect or a nonspecific interaction of species (water or methanol) on the surface and
Figure 4. Analysis of the total Raman intensity for the 820 cm-1 peak (Mo-O2) and the ratios of the peak intensities at 664 cm-1:994 cm-1 (Mo-O1:Mo-O3), 820 cm-1:994 cm-1 (Mo-O2:Mo-O3), and 664 cm-1:820 cm-1 (Mo-O1:Mo-O2).
not specifically methanol reactivity. After the isothermal situation is reached, no further changes are observed until the point MoO2 is observed in the WAXS (∼125 min at reaction temperature). At this point, the intensity of the 664 cm-1 band decreases rapidly compared to both the 820 and 994 cm-1 bands (decreases in Mo-O1:Mo-O3 and Mo-O1:Mo-O2), while in contrast the 994 band is now decreasing slightly more rapidly than the 820 band (resulting increase in ratio Mo-O2:Mo-O3). By comparison, there are no changes in the water experiment during this period, indicating that these changes are related specifically to methanol reactivity and reduction of the oxide. UV-Vis. Figure 5 shows an initial and final plot of the measured in situ UV-vis data. From this the bands at 360, 435, 650, and 850 nm (as assigned above) are clearly observed, although we note that the 435 nm band is only present in the
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Figure 5. UV-vis spectra recorded at the beginning of the reaction (a) and after 280 min at the reaction temperature (b). The negative spike in the spectrum is due to some interference from the 532 nm Raman laser.
final plot. In Figure 6 we profile the area of each of these bands throughout the experiment in a manner similar to previous works.37 We observe several changes that occur earlier in the reaction than seen in either the Raman or WAXS. The band due to Mo6+O3 undergoes an initial general decrease and then remains constant when the isothermal situation is reached. It then remains unchanged until ∼130 min at the reaction temperature, at which point it rapidly decreases and then stabilizes again. We note, however, the sudden decrease at this point is observed in all of the bands visible at this stage and is a result of an overall variation in signal intensity. Therefore, it is most likely due to some sample movement and not specific changes in the oxide. The Mo4+O2 band is observed only after ∼185 °C, where it increases rapidly and then stabilizes throughout the rest of the reaction. The Mo5+O2.5 undercoordinated Mo band is weak; however, it is clearly present in the initial data, decreasing as the reaction continues and disappearing completely at ∼185 °C. During this same period, the Mo5+O3 band initially increases until ∼185 °C and then begins to decrease rapidly until 350 °C. It then stabilizes until ∼80 min and then continues to decrease very slightly throughout the remainder of the reaction. MS. Online MS was used to monitor the reactivity throughout the experiment. In Figure 7 the traces for the key reactants/ products methanol (m/z 31), formaldehyde (m/z 30, corrected for the small contribution of methanol), dimethyl ether (DME) (m/z 46), CO (m/z 28), and water (m/z 18) are given, respectively. Due to being typically produced in low amounts, masses m/z ) 60 (methyl formate) and 76 (dimethoxymethane) were not monitored. The amount of methanol flowing through the oxide initially increases prior to formaldehyde production (indicating retention in the catalyst) and reaches equilibrium at ∼165 °C as formaldehyde production begins. The flow rate then drops rapidly and then slowly increases again after 350 °C and throughout the remainder of the experiment. Formaldehyde generally “mirrors” methanol consumption, starting at ∼165 °C, as methanol transport drops, and then increases rapidly until about 20 min after reaching the isothermal situation. It then stabilizes and begins to slowly decrease later in the reaction. The side product (DME) is observed forming slightly before formaldehyde (∼145 °C). Interestingly, production initially peaks at ∼308 °C, before rapidly decreasing until 350 °C. It then decreases more slowly throughout the rest of the reaction. CO is observed from higher temperatures than both formaldehyde and DME (∼235 °C), although like DME it also peaks (but just after 350 °C) and then decreases continually throughout
Figure 6. UV-vis band areas for the 360 nm (Mo6+O3) band (a), 435 nm (Mo4+O2) band (b), 650 nm (Mo5+O2.5) band (c), and 850 nm (Mo5+O3) band (d) as a function of temperature/time throughout the methanol reaction.
Figure 7. The MS traces for methanol, formaldehyde, DME, CO, and water measured as a function of temperature/time throughout the methanol reaction.
the reaction, even after bulk MoO2 is detected. Finally, water forms at the same time as formaldehyde and increases in production in a similar manner, reaching a peak around 20 min at isothermal. It then decreases throughout the remainder of the reaction. Discussion Initially we observe that, under the anaerobic conditions employed in this study, the production of formaldehyde and water continues after the initial reactivity and throughout the experiment. This indicates that bulk oxygen must be supplied to the oxide’s surface to regenerate the active sites in a Mars-Van Krevelen-type manner. This will then result in a reduction within the bulk of the catalyst, as confirmed by the “bell-shaped” intensity of the corrected Raman data throughout
4896 J. Phys. Chem. C, Vol. 113, No. 12, 2009 the experiment caused by a variation in the reciprocal and readsorption efficiency of the signal as the stoichiometry of Mo-Ox decreases.37 During the initial stages of the reaction, significant changes are observed only in the UV-vis and MS data. Here, the initially low methanol flow indicates sorption on the oxide surface prior to reaction, while the decrease in Mo5+O2.5 and increase Mo5+O3 sites indicate that this occurs at undercoordinated defect vacancy sites.12,15,45 This is not surprising, as such sites have been previously shown to be important for the reactivity of oxidation catalysts.41,46 However, a decrease in the Mo6+O3 sites is also observed, although this species does not completely disappear, suggesting that while methanol is primarily located on undercoordinated sites some is chemisorbed to the surface, forming hydroxyl groups and additional Mo5+O3 sites. As expected, the onset of reactivity results in the liberation of methoxy species from the Mo5+O3 sites. However, the rapid increase in Mo4+O2 (localized MoO2-like materials not observed in the WAXS) and the peak in CO production suggest that these sites are not regenerated but undergo further reduction.13 Indeed, the observation of strong CO production after formaldehyde is very similar to previous TPD results where too little oxygen in the system results in secondary reduction of the active sites with additional chemisorbed methanol.13 Interestingly, however, Mo4+O2 formation and CO production are not maintained, indicating that, after this initial period, sufficient oxygen becomes available at the active sites. This can be explained by considering that, as reactivity begins, methoxy concentration is very high. Therefore, the further reduction of the active sites by methoxy species before regeneration through oxygen transport is very likely. However, as the reaction proceeds, the number of surface methoxy species is reduced (decrease in the number of Mo5+O3 sites), allowing oxygen transfer to occur before further reduction. This eventually results in the steadystate between methoxy adsorption and formaldehyde production observed after 350 °C with the stabilization of the number of Mo6+O3 and Mo5+O3 sites. The initially high methoxy concentration is also supported by the complementary observation of a peak in DME production, as the large amount of methoxy chemisorption results in a correspondingly high number of hydroxyl groups that act as Brønsted acid sites known to favor DME production.6,47 There are therefore two types of reduction occurring in the system under anaerobic conditions. A “surface” reduction due to interaction of species in the gas flow with the oxide’s surface and a “deep” reduction due to Mars-Van Krevelen oxygen transfer from the bulk. Clearly, as the deep reduction is not interacting with any surface species, it will not produce CO, although it should form additional Mo4+O2. However, this is not observed in the UV-vis data, while the WAXS technique only observes it later in the reaction. The lack of change in the UV-vis data could indicate that the probe is focused at a point on the oxide where deep reduction is not occurring. However, we note that the UV-vis is sensitive to the initial changes, while the WAXS and Raman are not. This then suggests that, in this setup, the UV-vis is more sensitive to changes that occur near the catalyst surface. The lack of initial change in the WAXS may be partially due to well-known lack of sensitivity of the technique to small concentrations of crystalline material (
