In Situ Environmental STEM Study of the MoVTe Oxide M1 Phase

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In situ environmental STEM study of the MoVTe oxide M1 phase catalysts for ethane oxidative dehydrogenation Mimoun Aouine, Thierry Epicier, and Jean-Marc M Millet ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01114 • Publication Date (Web): 13 Jun 2016 Downloaded from http://pubs.acs.org on June 20, 2016

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ACS Catalysis

In situ environmental STEM study of the MoVTe oxide M1 phase catalysts for ethane oxidative dehydrogenation Mimoun Aouine1, Thierry Epicier1,2, Jean-Marc M. Millet1* 1

Institut de Recherches sur la Catalyse et l’Environnement de Lyon, IRCELYON,

UMR5256 CNRS-Université Claude Bernard, Lyon I, 2 avenue A. Einstein, 69626 Villeurbanne Cedex, France. 2

Université de Lyon, INSA Lyon, MATEIS, UMR CNRS 5510, 7, avenue Jean-Capelle, 69621 Villeurbanne Cedex, France.

* Corresponding author: [email protected]

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ABSTRACT

In situ environmental STEM-(HA)ADF has been used to characterize a light alkanes mild oxidation catalyst corresponding to a MoVTeO M1 phase. The results obtained show that there is almost no structure disordering under reaction conditions closed to ethane oxidative dehydrogenation catalytic reaction i.e. 350°C under 1 mbar of 30/15/55= O2/C2H6/N2 gas mixture. They further demonstrate that {Te-Ox} chains present in the

hexagonal channels of the structure participate to the redox process of the catalyst and constitute a preferential pathway for the re-oxidation of the surface catalytic active sites. Upon reduction it is proposed that labile oxygens are removed from the chains leading to a high content of Te4+O3E and Te2+O2 species and a displacement of the tellurium position towards the atomic columns in between the two hexagonal channels. This process is proposed to involve the reduction of tellurium only in the strongest reducing conditions. The study also shows a constant ending of the [001] zone by {Mo(Mo)5} structural unit and the bulk catalytic sites generally proposed should not appear on this type of rod’s facets.

KEYWORDS: ETEM, In situ STEM-HAADF, MoVTeNbO oxide catalysts, selective oxidation, Ammoxidation, Alkanes, Ethylene, M1 phase.


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Heterogeneous mild oxidation catalysts exhibit generally a complex structure and composition designed to address the multiple different reaction steps of a Mars and van Krevelen type mechanism. Crystalline MoVTeNbO mixed oxides are good examples of such catalysts with an active phase called M1 able to perform alone all the steps to selective oxidation products while avoiding the over-oxidation to COx. This phase, which is very efficient for selective oxidation of ethane to ethylene, propane to acrylic acid, and, in the presence of ammonia, propane ammoxidation to acrylonitrile, is a solid solution containing four metallic elements among which three of them may be present with different valence state. The M1 phase has an orthorhombic bronze-type structure built up with eleven cornersharing MO6 (M = Mo or V) octahedra forming pentagonal {(M’)Mo5} columns aligned in the [001] direction, connected to each other by one or several corner-sharing octahedral, forming hexagonal and heptagonal channels (Fig. 1).1 The hexagonal channels are partially occupied by oxygen and Te atoms forming {Te-Ox} chains more or less continue depending upon the Te content and redox conditions.2 The heptagonal channels are generally empty although some intercalated tellurium cations and oxygen anions may occupy these channels. All the eleven distinct crystallographic sites of the octahedral network are occupied by V or Mo cations, and the sites’ degree of occupation by these two cations appears to vary from one to the other and with the total composition of the phase.2 Niobium atoms, preferentially occupy the central cationic position M’ in {(M’)Mo5} columns (S9 crystallographic site). They are not essential for the activity of the M1 phase but their presence is beneficial to the selectivity: they decrease the surface acidity of the M1 phase, which promotes unselective oxidation of propene; the decrease of its oxidation


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power inhibits the over-oxidation of acrylic acid, acrylonitrile or ethylene.3-5 When Nb is not present in the phase, The M’ position is occupied by Mo.6

Figure 1: Schematic representation of the structure of the M1 phase with the indexed crystallographic sites and the pentameric clusters proposed as active sites. For the purpose of clarity, the oxygen atoms, bound to Te cations (in yellow) in hexagonal channels, are not shown. We have recently developed a new hydrothermal method of preparation of catalysts containing Mo, V and Te that allowed synthesizing M1 phase pure samples with controllable composition and we have undertaken a deep characterization of samples obtained using this method by various physical techniques.7 In this study we use HAADF (High Angle Annular Dark Field) imaging in the STEM (Scanning Transmission Electron Microscopy) mode to characterize the synthesized phases at the atomic-level. The excellent spatial resolution and image contrast provided by the technique has already been used with success to gain chemical and structural information on various M1 phases.1,3,8-15 The technique turned out to be an unsurpassed way to determine the structure and metal distributions on the crystallographic sites,1,3,8-13 understand how interfaces with other phases of the system form,13,14 or evidence compositional or sublattice ordering.15,16 Unlike previous studies, the novelty of the present approach was to use Environmental Transmission Electron Microscope (ETEM) for imaging the MoVTeO M1 phase in various atmospheres. The characterization studies performed in situ or in operando conditions clearly demonstrate that such type of characterizations were mandatory to understand the compositional and redox dynamics of the complex M1 phase.17-20 Today environmental


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TEM (ETEM) is on the way to transform our vision of working catalyst since it alloys to tackle chemical processes at gas–solid interfaces in situ with atomic-scale resolution (see representative reviews.20-23). In the case of the M1 phase, two main goals were challenging: (i) provide insight into the dynamic changes at atomic resolution taking place during redox reactions and (ii) investigate the stability of the structure under various atmospheres. For that purpose imaging was essentially performed in the [001] viewing direction of the M1 particles since it was previously reporting than (001) basal planes micrographs were useful to evidence disordering at high temperature.16

Results and discussion Catalysts characterization under vacuum at room temperature We have adopted a hydrothermal method to synthesize a pure M1 phase (see Experimental Section for details). The characterization of the phase show conventional unit cell parameters and mean crystal size for a M1 phase (Table 1). The chemical composition of the phase was that predictable from the relative stoichiometry of the starting products. Moreover its catalytic activity in ethane oxidative dehydrogenation corresponded well to that of an efficient M1 phase with the chosen V content; its selectivity to ethylene was also very high although not optimal as expected because of the absence of Nb.5

Table 1: Physico-chemical properties of the studied M1 phase with its catalytic properties obtained at 385°C with C2H6/O2/(Ne+He)=30/20/50 feedstock.


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Unit cell composition

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SSA

Unit cell parameters

Ethane conversion

(nm)

(µmol.s-1.m-2)

Mo

V

Te

(m2.s-1)

33.2

6.8

3.2

14.70±0.04

a= 2.1134(2)

Selectivity (%) C2H6

0.710

95

COx

5

b= 2.6658(2) c= 0.40146(3)

Typical STEM-(HA)ADF images of the M1 phase at room temperature under vacuum (Step A) is reported in Fig. 2. It is in agreement with all previous observations of the literature and shows large ordered regions within the crystallites with only some defects corresponding to intergrowth of M1 and a trigonal phase as previously described.13 The high temperature bronze-like solid framework of the M1 phase consisting of cornersharing MO6 (M = Mo, V) octahedral clearly evidenced. Although the Te atoms have larger atomic number than Mo and V, the TeOx columns in the hexagonal channels appeared the less bright since the Te content was low and the sites occupation in the columns did not overpass 40%. On another hand, the variations in contrast based on the differences in atomic numbers of Mo and V did not allow detecting visually the columns rich and poor in vanadium. Empty or almost empty heptagonal channels were clearly evidenced in the images with only very weak intensity found in few heptagonal channels. EDX analyses of large areas, such as shown in Fig. 2a), gave the respective averaged contents for the cationic elements in the phase, which were in atomic percentage Mo: 33.2, V: 6.8 and Te 1.5. These contents were in relatively good agreement with those calculated from chemical analysis (Table 1) except for the Te content, which was lower.


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Figure 2: STEM imaging of the MoVTeO M1phase under high vacuum at room temperature. a): low mag image; b): High-resolution STEM-(HA)ADF image showing the [001] projection of the M1 phase with a scaled unit-cell model superimposed. A remarkable feature deduced from STEM-(HA)ADF images concerns the edge of all M1 crystallites when viewed in the [001] zone axis: the side facets of the M1 phase were systematically constituted of {(Mo)Mo5} columns composed with the staking of units made up of a central MoO7 pentagonal bipyramid, sharing edge with 5 surrounding MoO6 octahedra as shown in Fig. 3. These columns are linked to each other by one octahedron (S3 site) in the bulk and these chains are systematically decorating the particle surfaces. Interestingly the same observation had been made concerning the M2 phase, another phase commonly encountered in active catalysts in particular when it containing Ta whereas it does not contain similar columns in its basic structure.14 It was proposed that the formation of these decorating columns around the M2 phase particles were thermodynamic and kinetic preferred versus self-nucleation of other phases. Such phenomenon should facilitate coherent intergrowths between the M1 and M2 phases when both phases are present.14

Figure 3: High-resolution STEM-(HA)ADF image showing the [001] projection of a MoVTeO M1 phase with the a scaled model unit-cell superimposed to highlight the termination of the lateral edge of the needle-shaped particles. It has to be recalled that the preparation of the studied sample did not involve the selective dissolution of the M2 phase and thus the observed {Mo(Mo)5} structural units decoration is characteristic of the M1 phase alone. Moreover, and contrarily to the case of


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the M2 phase, this decoration appears continuous without defect and with only one type of arrangement. Previous studies have proposed that the active sites corresponded to pentameric clusters circled in blue in Fig. 1 and formed by mixed Mo/V sites S2, S4, and S7 bordered by the two sites S12 occupied by tellurium in the neighboring hexagonal channels.24,25 The present observations exclude thus the possible presence of such structural active sites on the [001] zone facets. The presence of isolated atomic clusters that would contain Te and V on the facets cannot however be discard. It should be recalled that surface analyses of M1 phases have reported the presence of excess of Te and in certain cases of V.18,26 Such clusters could constitute active sites as earlier proposed or evidenced on other supports.26,27 A recent study showed that the pentameric sites could be exposed in [120] and [210] lateral facets.28 We did not observed this type of faceting in our samples and if it exists, it should be limited. The presence of such facets may be strongly dependent upon the morphology of the M1 phase deriving from their synthesis and post-synthesis treatment. Finally, because we focused on imaging in the [001] zone axis no conclusion onto the termination of the (001) surface can be drawn Study in various environmental (gas and temperature) conditions HRTEM imaging of the catalyst have been recorded under the various successive conditions listed in Table 2.

Table 2: list of the conditions used for the ETEM study. High vacuum corresponds to about 7 10-7 to 5 10-6 mbar and other conditions to 1 mbar of total pressure.


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Conditions

Atmosphere

Temperature (°C)

A

High vacuum

20

B

Air

20

C

High vacuum

350

D

O2/C2H6/N2 = 30/15/55

350

E

High vacuum after reduction by C2H6

350

F

Air after reduction by C2H6

350

The general observation of the M1 phase did not show tremendous structure change in any of the studied conditions and in particular no extended sublattice disordering of the corner-sharing octahedra forming the catalytic sites (pentameric clusters) was visible at high temperature as reported before.16 To illustrate this result, images of the structure, recorded at 350°C under the catalytic reaction gas mixture, are shown in Fig. 4.

Figure 4: High-resolution HAADF STEM image of the MoVTeO M1 phase, recorded at 350°C and under 1 mbar of C2H6/O2/(Ne+He)=30/20/50. If the octahedral network appeared scarcely affected by the temperature and the nature of the reducing gaseous atmosphere, a change in contrast was observed in the hexagonal channels with, when the conditions are the most reducing (C and E), the almost total disappearance of the peaks corresponding to the {Te-Ox} chains columns (Fig. 5). In order to analyze this feature more quantitatively, intensity profiles between heptagonal pores across the site sequence S3-S12-S2-S12-S3 have been drawn and normalized against maximum after background subtraction (Fig. 5). For each recording conditions up to 20 of these line scans have been summed and the resulting profiles have been analyzed using a least square fitting approach.29 The results obtained are presented in Fig. 6 and Table 2.


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Good matches of the profiles were obtained when each of the five intensity peaks corresponding to the S3-S12-S2-S12-S3 sites were fitted with a Gaussian. The quality of the Gaussian fits authorizes to measure the displacement observed in peak positions with a high accuracy to of about 0.032 nm. In all the fits, S12 peaks corresponding to {Te-Ox} chains columns were needed and the apparent disappearance of the columns can be explained by both the increase of the full width half maximum (FWHM) of the S3 and S12 site peaks along the line scan and the displacement of the S12 peaks closer to the S2 central peaks.

Figure 5: High-resolution STEM-(HA)ADF images recorded at 350°C under catalytic reaction gas mixture (step D) left and under high vacuum (step E) right showing the apparent disappearance of Te peaks. The studied line scans between heptagonal pores across site sequence S3-S12-S2-S12-S3 is marked with a dash line. Figure 6: Fitted Line scan profiles between heptagonal pores across site sequence S3-S12S2-S12-S3, and normalized against maximum after background subtraction. The profiles have been recorded in images taken in the successive observation conditions as indicated (Table 2). It is remarkable that the S12/S2 intensity ratios present almost the same values in all steps (Table 3). This demonstrates that the apparent vanishing of the S12 peaks as deduced form an eye inspection of the micrographs such as Fig. 5b is actually not evidencing a departure of Te by vaporization. Interestingly in the profiles recorded under catalytic reaction conditions always showed the S12 peaks. In this case there is well a displacement of the S12 peaks but without any important increase of the FWHM of the peaks.


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Table 3: Results of the least square fitting with Gaussian functions of the average line scan between heptagonal pores across 20 site sequences S3-S12-S2-S12-S3 from images recorded in the successive observation conditions. All profiles were background subtracted and normalized again maximum. S12a-S2 and S12b-S2 are the measured distances between the two sites S12 (a and b) and the site S2 situated in between and A(S12a) / A(S12b) are their respective peak intensities ratio; A(S12)/A(S2) are the ratio between the mean intensity of the S12 peaks intensities and the intensity of the S2 peak situated in between. Step

S12a-S2

S12b-S2

A(S12a) / A(S12b)

A(S12)/A(S2)

(nm)

(nm)

A

0.30

0.30

1.45

0.45

B

0.30

0.28

1.52

0.50

C

0.27

0.27

1.64

0.47

D

0.28

0.27

1.35

0.40

E

0.27

0.26

1.56

0.50

F

0.31

0.29

1.53

0.47

At that point and to interpret these data, it is important to recall some structural features concerning the M1 phase. The tellurite entities present in the hexagonal channels of M1 and M2 phases correspond to TeO4E and TeO3E trigonal pyramid groups with one lone pair of electrons (E).1,2 Two of the oxygen anions bound to the tellurium cations belong to the M6O6 (M=V, Mo) window, whereas the third or fourth one is in the channels, forming with Te infinite chains more or less defective. Similar features were observed for Sb containing M1 phase.30 Calculation of the total stoichiometry in the case of Te containing


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M1 phase showed a partial occupation of the Te sites in the channels but the presence of almost only TeO4E entities (Fig. 7).2,31

Figure 7: Schematic representation of the Te (yellow) - O (red) entities present in the hexagonal channels under various atmospheres; E stands for the lone pair electrons. In reductive conditions, labile oxygens are removed from the channels and TeO3E and TeO2 entities should be formed. In the less reducing conditions (catalytic reaction conditions, step D) Te4+ is not reduced but TeO3E should be the majority entities (Fig. 7). The charge balance is assumed by the reduction of V and Mo. With that respect, operando XPS measurements have shown that Te remained 4+ in catalytic reaction conditions.19 The lone pairs of electrons of Te4+ cations that are located in between two labile oxygen anions should move towards the oxygen vacancy but since the spatial extension of the lone pair approximately corresponds to the volume of an oxide anion, repulsion takes place, thus moving the Te cation equilibrium position, which was already off centered in the channels, towards the border of the M6O6 window. Consequently the departure of oxygen anions bound to the tellurium cation results in a displacement of the Te cations in S12 sites towards S2: this is confirmed by the measurement of the distance which decreases from 0.30 nm to 0.28 nm. In the most reducing conditions (steps C and E), Te4+ are reduced to Te2+ and TeO2 entities are formed (Fig. 7). As a matter of fact previous studies by XANES and XPS spectroscopy on redox dynamics of Te4+ in M1 phase had shown that upon reduction by ethane or propane, both elements could be partially reduced to Te2+.32 In the same time the departure of a large amount of the labile oxygens in the channels lead to an increase of


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their mobility, which in its turn leads to an increase of the corresponding peaks FWHM and the distribution of the peak intensity in the channels and a decrease of the S12 to S2 distance from 0.30 to 0.26 nm. It is known that Te2+ is not stable and can disproportionate to Te4+ and Te0 or be easily further reduced to Te0.33 The later species are volatile and can be lost by vaporization. In this respect, a departure of tellurium has recently been observed after long time in strongly reducing catalytic reaction conditions, preferentially at the extremities of the particles of the M1 phase, across the (001) planes, which are perpendicular to the hexagonal channels direction.34 Analysis of the solid deposited at the reactor outlet showed that it consisted of Te0. These observations support well the role of tellurium in the formation of an oxygen reservoir in the hexagonal channels of the structure that had been postulated before.17,32 Indeed the labile oxygens form a reservoir that distributes the reduction of the solid over the bulk thus avoiding a restructuration of the surface when the reduction is important, which could be detrimental to the catalytic properties.35 Interestingly the mobility of oxygen in the channels should be promoted by the presence of the lone pair of electrons that facilitate the motion of the oxide anions from one site to an unoccupied structural or defective one, by retracting with very low energy consumption.2 The analysis of the full width at half maximum of the peaks (FWHM) which represents the thermal displacement the atoms off their sites showed that at 350°C under air or in catalytic reaction conditions, its increase is very small as compared to the room temperature case (0.11±0.02 to 0.12±0.02 nm). This shows well that in the catalytic reaction no important disordering is observed. However under very reducing conditions (steps D and F) the full width at half maximum of the peaks increased (0.15±0.03 nm). The difference in behavior between oxidizing or catalytic conditions and reducing conditions at


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the same temperature indicates that this effect does not have a simple thermal origin, like an increase of the thermal vibration amplitude classically described by the Debye-Waller factor. This may thus arise from the formation of oxygen vacancies in the octahedra network that destabilized the cations on their sites. We may postulated that at 753°C under a similar atmosphere it would increase even more. This would explain the results obtained by Blom et al. at this temperature, showing FWHM values reaching 0.18-0.20 nm, together with the observation of a strong disordering of the structure.16 It is worth mentioning that contrarily to the others sites, the full width half maximum (FWHM) of the S2 sites remained the same whatever the temperature and redox atmosphere. This could be explained by the coming closer together of the tellurium cations in reducing conditions that may contribute to stabilize the S2 sites. Another interesting feature can be drawn out form the ETEM observations: while the relative intensity of the two S3 peaks in each profile remains always comparable, the relative areas of the two S12 neighboring atomic columns differ but with a constant ratio A(S12a) / A(S12b) equal to 1.5±0.1 as reported in Table 2. This strongly suggests that the

Te occupation of the two neighboring hexagonal channels is constantly not equal. In order to check whether a periodicity exists in the respective arrangement of the two types of such sites in the structure, we have analyzed all the profiles in the same region of a sample (Fig. 8). In the image, the red bars correspond to profiles for which the most intense S12 site (marked with a circle) is located towards the top of the image and the green bars are profiles for which the most intense S12 site is located towards the bottom of the figure. It is possible to see that the two types of profile are randomly distributed in the crystallite. The


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origin of the systematic difference in intensity of the two S12 sites associated in the same profile (pentamer) is unknown yet.

Figure 8: High-resolution STEM-(HA)ADF image showing the [001] projection of the MoVTeO M1 phase with the recorded S3-S12-S2-S12-S3 profiles. The position of the more intense S12 peaks is marked with a circle; the two types of profiles based on their relative orientation are colored in green and red respectively. It might be possible that the respective occupation of the Te sites is correlated to the V and Mo distributions over the surrounding sites. If the same feature is observed on other M1 phases it should be taken into consideration for further studies to model the M1 phase structure or even conceive active sites with given charge distributions over the cations constituting these sites.

Conclusion In this work an environmental transmission electron microscope (ETEM) has been used for the first time to characterize a MoVTeO M1 phase in various atmospheres. The results obtained by an extensive STEM-(HA)ADF study open up intriguing new possibilities to evidence and visualize structure modifications of the catalyst in the presence of reactive gases at room and elevated temperature. Several important results have been obtained: - We have been able to show that there was definitively no strong disordering of the structure at the catalytic reaction temperature (350°C) and under gas reaction mixture at a pressure of 1 mbar. There is in particular no sublattice disordering of the cornersharing octahedra forming the catalytic sites in between the rigid edge-sharing


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pentagonal bipyramidal {Nb(Mo5)} columns. The STEM micrographs exhibit a slight increase of the widths of the dots imaging atomic columns at 350°C under vacuum and after reduction by C2H6, but not in conditions concerning catalytic reaction gas mixtures. This increase is due to the increase of the relative displacement of the atoms on their sites, probably resulting from a high content of oxygen vacancies in or close to the corresponding atomic columns. - The role of hexagonal channels as an oxygen reservoir and the involvement of {TeOx} chains in the catalytic redox bulk process have been evidenced. Upon reduction and in the catalytic reaction conditions the M1 phase is reduced and labile oxygens are removed from the chains, leading to a high content of TeO3E species and a displacement of the tellurium positions towards the central S2 sites. This means that in reaction conditions the reduction is not limited to the surface but also involves the hexagonal channels that should be preferential pathway for the re-oxidation of the catalytic active sites in a similar way that the bismuth columns are in bismuth molybdate structures. The removal or diffusion of labile oxygens in the chains does not need for Te4+ to be reduced and charge balance may be assigned to other elements like Mo or V. In strongly reducing conditions, all labile oxygens are removed and tellurium should be reduced to Te2+ leading to a high degree of freedom of displacement. This is the reason why partial volatilization of Te at the surface of the solid has been shown to occur during reduction in other studies. - We have been able to detect a constant and significant difference in the Te occupancies of the two neighboring S12 sites that was invisible in powder X-ray diffraction. Although we do not understand yet the origin of this difference, we may


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postulate that it could be linked to the distribution of Mo and V on the adjacent sites. From a fundamental point of view this observation confirms that a simple average crystallographic model may be insufficient to describe the inhomogeneity of a working oxide catalyst. Finally all STEM images of particle viewed along the [001] zone axis have revealed the systematic presence of {Mo(Mo)5} terminating structural units linked to each other by one octahedron (S3 site) at the surface of the M1 crystallites. This leads to the conclusion

that the structural active sites commonly proposed are not exposed on these facets. This finding does not completely discard the possible formation of isolated amorphous cationic clusters on these facets, which could then still have some activity. This result is in agreement with a recent work that however pointed out to the possible presence of active sites on other lateral [120] and [210] facets. It is important to point out that the present study does not bring any new information on the M1 (001) termination surfaces supposed to contain the active sites, and which remain to be characterized in more details. This study brings new insights into the atomic structure of the M1 phase investigated in operando at the catalytic reaction temperature. It constitutes a typical example demonstrating how atomic-scale information can be by ETEM, about dynamic processes induced by changes in the gas environment, and how oxidation catalysts can be analyzed in operating conditions even though the gas pressure (1 mbar) is by far lower than the ambient pressure in which they are generally working.

Experimental


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The MoVTeO M1 phase has been prepared by reacting at 175°C and for 24h ammonium and sodium salt of {Mo132-Te} keplerate-type polyoxometallate and vanadyl sulfate in hydrothermal conditions.7 The final solid was washed with water and dried with ether. The phase purity and crystallinity was checked by X-ray diffraction. The powder pattern was collected on a Brüker D5005 diffractometer using Cu-Kα radiation, between 5 and 80° (2θ) with 1s counting time per step of 0.02° and analyzed by Rietveld method using TOPAS 3.0 software. The metal contents of the solids were determined by atomic absorption (ICP) in Argon plasma with a SPECTROLAME-ICP spectrometer. The solids (10 mg) were solubilized in an aqueous solution (100 ml) containing HF (5 ml) H2SO4 (5 ml) and HNO3 (5 ml) at 200°C under stirring for 6 h. The solution was vaporized in plasma to measure the emission intensity of the radiation characteristic of the elements. The solid appeared to be constituted by a pure M1 phase, well crystallized with the expected chemical composition. The efficiency of the catalyst has been checked using a testing device and the method described previously.32 A FEI TITAN ETEM G2 80-300 KV equipped with an objective Cs aberration corrector has been used for the in-situ environmental observation of the catalyst. The microscope was also equipped with an energy-dispersive X-ray (EDX) analyzer (SDD X-Max 80mm2 from Oxford InstrumentsTM) used for local elemental chemical analysis. The powdered sample was embedded in a resin and cut using an ultramicrotome and deposited on a grid placed into a Gatan

TM

furnace-type holder in Inconel compatible with the direct

observation of catalytic reactions at elevated temperatures and under a controlled atmosphere.35 Since the temperature was only measured with a thermocouple placed around the furnace of the sample holder a slight discrepancy may exist between the


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imposed (measured) and real temperatures. During the ETEM study of the solid sample under the catalytic gas phase mixture, a home-made system of several Brooks mass flow controllers and a Valco two-position switching valve was used to produce the gas mixtures and the presence of the different reactant and products was checked by analyzing effluent exiting the microscope chamber by mass spectroscopy (Pfeiffer). The samples were studied in ETEM at 20 and 350°C successively under various atmospheres. The list of successive conditions used is shown on Table 2. Besides vacuum (residual pressure in the range 7 10-7 to 5 10-6 mbar) and air the catalyst has been exposed to a mixture of ethane and oxygen diluted into nitrogen with a relative ratio O2/C2H6/N2=30/15/55 corresponding to that currently used to test this type of catalyst into a laboratory testing apparatus between 320 and 400°C.32 In all cases when the sample was exposed to gas a total pressure was accurately stabilized to 1 mbar. Most of the STEM images were acquired on a dark field detector with a camera length of 160 mm corresponding to a collection angles in the range 45 - 200 mrad. According to the relatively low value of the inner cut-off angle, all STEM images will be labeled as STEM-(HA)ADF images, since they do not strictly correspond to High Angle ADF conditions.

Acknowledgments Thanks are due to the CLYM (Consortium Lyon - St-Etienne de Microscopie, www.clym.fr) for its guidance in the Ly-EtTEM (Lyon Environmental tomographic TEM) project, which was financially supported by the CNRS, the Région Rhône-Alpes, the ‘GrandLyon’ and the French Ministry of Research and Higher Education.


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GRAPHICAL ABSTRACT


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Figure 1: Schematic representation of the structure of the M1 phase with the indexed crystallographic sites and the pentameric clusters proposed as active sites. For the purpose of clarity, the oxygen atoms, bound to Te cations (in yellow) in hexagonal channels, are not shown. 81x80mm (300 x 300 DPI)


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Figure 4: High-resolution HAADF STEM image of the MoVTeO M1 phase, recorded at 350°C and under 1 mbar of C2H6/O2/(Ne+He)=30/20/50. 84x84mm (300 x 300 DPI)


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Figure 7: Schematic representation of the Te (yellow) - O (red) entities present in the hexagonal channels under various atmospheres; E stands for the lone pair electrons. 55x37mm (300 x 300 DPI)


In Situ Environmental STEM Study of the MoVTe Oxide M1 Phase

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