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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*,† †

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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 ‡ Université de Lyon, INSA Lyon, MATEIS, UMR CNRS 5510, 7, avenue Jean-Capelle, 69621 Villeurbanne Cedex, France ABSTRACT: In situ environmental STEM-(HA)ADF has been used to characterize a light alkane 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 reactions: i.e., 350 °C under 1 mbar of a 30/15/55 O2/C2H6/N2 gas mixture. They further demonstrate that {Te−Ox} chains present in the hexagonal channels of the structure participate in the redox process of the catalyst and constitute a preferential pathway for the reoxidation 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 toward the atomic columns between the two hexagonal channels. This process is proposed to involve the reduction of tellurium only under the strongest reducing conditions. The study also shows a constant ending of the [001] zone by {Mo(Mo)5} structural units and the bulk catalytic sites generally proposed should not appear on the facets of this type of rod. KEYWORDS: ETEM, in situ STEM-HAADF, MoVTeNbO oxide catalysts, selective oxidation, ammoxidation, alkanes, ethylene, M1 phase

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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 in its oxidation power inhibits the overoxidation of acrylic acid, acrylonitrile, or ethylene.4−6 When Nb is not present in the phase, the M′ position is occupied by Mo.7 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.8 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 this technique has already been used with success to gain chemical and structural information on various M1 phases.2,4,9−16 The technique turned out to be an unsurpassed way to determine the structure and metal distributions on the crystallographic sites,2,4,9−14 understand

eterogeneous mild oxidation catalysts generally exhibit a complex structure and composition designed to address the multiple different reaction steps of a Mars−van Krevelen type mechanism.1 Crystalline MoVTeNbO mixed oxides are good examples of such catalysts with an active phase called M1 able to perform all the steps by itself to give selective oxidation products while avoiding the overoxidation 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 11 corner-sharing MO6 (M = Mo, V) octahedra, forming pentagonal {(M′)Mo5} columns aligned in the [001] direction, connected to each other by one or several cornersharing octahedra, forming hexagonal and heptagonal channels (Figure 1).2 The hexagonal channels are partially occupied by oxygen and Te atoms forming {Te−Ox} more or less continuous chains depending upon the Te content and redox conditions.3 The heptagonal channels are generally empty, although some intercalated tellurium cations and oxygen anions may occupy these channels. All of the 11 distinct crystallographic sites of the octahedral network are occupied by V or Mo cations, and the degree of occupation of the sites by these two cations appears to vary from one to the other and with the total composition of the phase.3 Niobium atoms preferentially © 2016 American Chemical Society

Received: April 19, 2016 Revised: June 8, 2016 Published: June 13, 2016 4775

DOI: 10.1021/acscatal.6b01114 ACS Catal. 2016, 6, 4775−4781

Research Article

ACS Catalysis

(Table 1). The chemical composition of the phase was that predicted 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.6 Typical STEM-(HA)ADF images of the M1 phase at room temperature under vacuum (step A) are given in Figure 2. They

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.

how interfaces with other phases of the system form,14,15 or evidence compositional or sublattice ordering.16,17 Unlike previous studies, the novelty of the present approach was to use environmental transmission electron microscopy (ETEM) to image the MoVTeO M1 phase under various atmospheres. The characterization studies performed in situ or under operando conditions clearly demonstrate that such types of characterizations were mandatory to understand the compositional and redox dynamics of the complex M1 phase.18−21 Today environmental TEM (ETEM) is on the way to transforming our vision of working catalysts since it allows us to tackle chemical processes at gas−solid interfaces in situ with atomic scale resolution (see representative reviews21−24). 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 reported that (001) basal plane micrographs were useful to evidence disorder at high temperature.17

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RESULTS AND DISCUSSION Catalyst Characterization under Vacuum at Room Temperature. We have adopted a hydrothermal method to synthesize a pure M1 phase (see the Experimental Section for details). The characterization of the phase shows conventional unit cell parameters and mean crystal size for an M1 phase

Figure 2. STEM imaging of the MoVTeO M1 phase under high vacuum at room temperature: (a) low-magnification image; (b) highresolution STEM-(HA)ADF image showing the [001] projection of the M1 phase with a scaled unit cell model superimposed.

Table 1. Physicochemical Properties of the Studied M1 Phase with Its Catalytic Properties Obtained at 385 °C with C2H6/O2/ (Ne+He) = 30/20/50 Feedstock unit cell composition

selectivity (%)

Mo

V

Te

SSA (m2 s−1)

unit cell param (nm)

ethane conversn (μmol s−1 m−2)

C2H6

COx

33.2

6.8

3.2

14.70 ± 0.04

a = 2.1134(2) b = 2.6658(2) c = 0.40146(3)

0.710

95

5

4776

DOI: 10.1021/acscatal.6b01114 ACS Catal. 2016, 6, 4775−4781

Research Article

ACS Catalysis

phase, and thus the observed {Mo(Mo)5} structural unit decoration is characteristic of the M1 phase alone. Moreover, and in contrast to the case of the M2 phase, this decoration appears to be 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 Figure 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.25,26 The present observations thus exclude the possible presence of such structurally 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 discarded. It should be recalled that surface analyses of M1 phases have reported the presence of excess Te and in certain cases of V.19,27 Such clusters could constitute active sites, as proposed earlier or evidenced on other supports.27,28 A recent study showed that the pentameric sites could be exposed in [120] and [210] lateral facets.29 We did not observe 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 postsynthesis treatment. Finally, because we focused on imaging in the [001] zone axis, no conclusion regarding the termination of the (001) surface can be drawn. Study under Various Environmental (Gas and Temperature) Conditions. HRTEM imaging of the catalyst have been recorded under the various successive conditions given in Table 2.

are 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.14 The high-temperature bronze-like solid framework of the M1 phase consisting of corner-sharing MO6 (M = Mo, V) octahedra are clearly evidenced. Although the Te atoms have a greater atomic number in comparison to Mo and V, the TeOx columns in the hexagonal channels appeared less bright since the Te content was low and the site occupation in the columns did not surpass 40%. On the other hand, the variations in contrast on the basis of the differences in atomic numbers of Mo and V did not allow visual detection of 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 Figure 2A, gave the respective averaged contents for the cationic elements in the phase (atomic percentages): Mo, 33.2; V, 6.8; 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. 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 edges with five surrounding MoO 6 octahedra, as shown in Figure 3. These columns are linked to

Table 2. Conditions Used for the ETEM Studya conditions

atmosphere

temp (°C)

A B C D E F

high vacuum Air high vacuum O2/C2H6/N2 30/15/55 high vacuum after reduction by C2H6 air after reduction by C2H6

20 20 350 350 350 350

High vacuum corresponds to about 7 × 10−7 to 5 × 10−6 mbar and other conditions to 1 mbar of total pressure.

a

The general observation of the M1 phase did not show tremendous structure change under 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.17 To illustrate this result, images of the structure, recorded at 350 °C under the catalytic reaction gas mixture, are shown in Figure 4. 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} chain columns (Figure 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 the maximum after background subtraction (Figure 5). For each set of recording conditions up to 20 of these line scans have been summed and the resulting profiles have been analyzed using a least-squares fitting approach.30 The results obtained are

Figure 3. High-resolution STEM-(HA)ADF image showing the [001] projection of a MoVTeO M1 phase with the scaled model unit cell superimposed to highlight the termination of the lateral edge of the needle-shaped particles.

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 they contain Ta, whereas it does not contain similar columns in its basic structure.15 It was proposed that the formation of these decorating columns around the M2 phase particles were thermodynamically and kinetically preferred versus self-nucleation of other phases. Such a phenomenon should facilitate coherent intergrowths between the M1 and M2 phases when both phases are present.15 It has to be recalled that the preparation of the studied sample did not involve the selective dissolution of the M2 4777

DOI: 10.1021/acscatal.6b01114 ACS Catal. 2016, 6, 4775−4781

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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).

Figure 6. Fitted line scan profiles between heptagonal pores across the site sequence S3−S12−S2−S12−S3 and normalized against the maximum after background subtraction. The profiles have been recorded in images taken under the successive observation conditions as indicated (Table 2).

Table 3. Results of the Least-Squares Fitting with Gaussian Functions of the Average Line Scan between Heptagonal Pores across 20 Site Sequences S3−S12−S2−S12−S3 from Images Recorded under the Successive Observation Conditionsa Figure 5. High-resolution STEM-(HA)ADF images recorded at 350 °C (left, A) under a catalytic reaction gas mixture (step D) and (right, B) under high vacuum (step E) showing the apparent disappearance of Te peaks. The studied line scans between heptagonal pores across the site sequence S3−S12−S2−S12−S3 is marked with a dash line.

step S12a−S2 (nm) A B C D E F

presented in Figure 6 and Table 2. 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 measurement of the displacement observed in peak positions with a high accuracy to of about 0.032 nm. In all of the fits, S12 peaks corresponding to {Te−Ox} chain 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. 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 from an eye inspection of the micrographs such as Figure 5B is actually not evidencing a departure of Te by vaporization. Interestingly the profiles recorded under catalytic reaction conditions always showed the S12 peaks. In this case there is also a displacement of the S12 peaks but without any important increase of the fwhm of the peaks.

0.30 0.30 0.27 0.28 0.27 0.31

S12b−S2 (nm)

A(S12a)/ A(S12b)

A(S12)/ A(S2)

0.30 0.28 0.27 0.27 0.26 0.29

1.45 1.52 1.64 1.35 1.56 1.53

0.45 0.50 0.47 0.40 0.50 0.47

a

All profiles were background subtracted and normalized against the 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) is their respective peak intensity ratio; A(S12)/A(S2) is the ratio between the mean intensity of the S12 peak intensities and the intensity of the S2 peak situated in between.

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 trigonalpyramidal groups with one lone pair of electrons (E).2,3 Two of the oxygen anions bound to the tellurium cations belong to the M6O6 (M = V, Mo) window, whereas the third or fourth anion is in the channels, forming with Te more or less defective infinite chains. Similar features were observed for the Sbcontaining M1 phase.31 Calculation of the total stoichiometry in the case of Te containing the M1 phase showed a partial occupation of the Te sites in the channels but the presence of almost only TeO4E entities (Figure 7).3,32 4778

DOI: 10.1021/acscatal.6b01114 ACS Catal. 2016, 6, 4775−4781

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ACS Catalysis

of the oxide anions from one site to an unoccupied or defective structure, by retracting with very low energy consumption.3 An analysis of the full width at half-maximum of the peaks (fwhm), which represents the thermal displacement of the atoms from their sites, showed that at 350 °C in air or under catalytic reaction conditions its increase is very small in comparison 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 disorder is observed. However, under very reducing conditions (steps D and F in Table 3) the full width at halfmaximum of the peaks increased (0.15 ± 0.03 nm). The difference in behavior between oxidizing or catalytic conditions and reducing conditions at the same temperature indicates that this effect does not have a simple thermal origin, such as an increase in the thermal vibration amplitude classically described by the Debye−Waller factor. This may thus arise from the formation of oxygen vacancies in the octahedral network that destabilized the cations on their sites. We may postulate 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.18 It is worth mentioning that, in contrast to the other sites, the fwhm of the S2 sites remained the same, whatever the temperature and redox atmosphere. This could be explained by the coming together of the tellurium cations under reducing conditions that may contribute to stabilization of the S2 sites. Another interesting feature can be drawn from the ETEM observations: while the relative intensities of the two S3 peaks in each profile always remain comparable, the relative areas of the two S12 neighboring atomic columns differ but with a constant A(S12a)/A(S12b) ratio 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 (Figure 8). In Figure 8, the red bars correspond to profiles for which the most intense S12 site (marked with a circle) is located toward the top of the image and the green bars are profiles for which the most intense S12 site is located toward the bottom of the figure. It is possible to see that the two types of profiles are randomly distributed in the crystallite. The origin of the systematic difference in intensity of the two S12 sites associated in the same profile (pentamer) is as yet unknown. 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.

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.

Under reductive conditions, labile oxygens are removed from the channels and TeO3E and TeO2 entities should be formed. Under less reducing conditions (catalytic reaction conditions, step D in Table 3) Te4+ is not reduced but TeO3E should be the major entities (Figure 7). The charge balance is assumed by the reduction of V and Mo. In that respect, operando XPS measurements have shown that Te remained 4+ under catalytic reaction conditions.20 The lone pairs of electrons of Te4+ cations that are located between two labile oxygen anions should move toward 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 center in the channels, toward 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 toward S2: this is confirmed by a measurement of the distance, which decreases from 0.30 to 0.28 nm. Under the most reducing conditions (steps C and E in Table 3), Te4+ is reduced to Te2+ and TeO2 entities are formed (Figure 7). As a matter of fact, previous studies by XANES and XPS spectroscopy on the redox dynamics of Te4+ in the M1 phase had shown that, upon reduction by ethane or propane, both elements could be partially reduced to Te2+.33 At the same time the departure of a large amount of the labile oxygens in the channels led to an increase in their mobility, which in its turn led to an increase in the fwhm of the corresponding peaks 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 Te0.34 The latter species are volatile and can be lost by vaporization. In this respect, a departure of tellurium has recently been observed after a long time under 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 channel direction.35 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.18,33 Indeed, the labile oxygens form a reservoir that distributes the reduction of the solid over the bulk, thus avoiding a restructuring of the surface when the reduction is important, which could be detrimental to the catalytic properties.33,36 Interestingly, the mobility of oxygen in the channels should be promoted by the presence of the lone pair of electrons that facilitate the motion

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CONCLUSION In this work an environmental transmission electron microscope (ETEM) has been used for the first time to characterize an MoVTeO M1 phase under 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 4779

DOI: 10.1021/acscatal.6b01114 ACS Catal. 2016, 6, 4775−4781

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ACS Catalysis

do not yet understand the origin of this difference, we may 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 particles 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 structurally 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. However, this result is in agreement with a recent work that pointed out the possible presence of active sites on other lateral [120] and [210] facets. It is important to note that the present study does not provide any new information on the M1 (001) termination surfaces supposed to contain the active sites, which remain to be characterized in more detail. 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 obtained by ETEM, how dynamic processes can be induced by changes in the gas environment, and how oxidation catalysts can be analyzed under operating conditions even though the gas pressure (1 mbar) is lower by far than the ambient pressure under which they are generally working.

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 positions of the more intense S12 peaks are marked with circles; the two types of profiles on the basis of their relative orientations are shown in green and red, respectively.

room and elevated temperature. Several important results have been obtained, as follows. (1) 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 a gas reaction mixture at a pressure of 1 mbar. There is in particular no sublattice disordering of the corner-sharing octahedra forming the catalytic sites between the rigid edge-sharing pentagonalbipyramidal {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 under conditions concerning catalytic reaction gas mixtures. This increase is due to an increase in 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. (2) The role of hexagonal channels as oxygen reservoirs and the involvement of {Te−Ox} chains in the catalytic redox bulk process have been evidenced. Upon reduction and under 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 toward the central S2 sites. This means that under reaction conditions the reduction is not limited to the surface but also involves hexagonal channels that should be a preferential pathway for the reoxidation of the catalytically active sites in a way similar to that of bismuth columns in bismuth molybdate structures. The removal or diffusion of labile oxygens in the chains does not require Te4+ to be reduced, and charge balance may be assigned to other elements such as Mo and V. Under 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 partial volatilization of Te at the surface of the solid has been shown to occur during reduction in other studies. (3) 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

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EXPERIMENTAL SECTION The MoVTeO M1 phase has been prepared by reacting at 175 °C for 24 h ammonium and sodium salts of {Mo132-Te} keplerate-type polyoxometalate and vanadyl sulfate under hydrothermal conditions.8 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 1 s counting time per step of 0.02° and analyzed by the 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 with 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.33 An FEI TITAN ETEM G2 80−300 kV instrument equipped with an objective Cs aberration corrector has been used for the in situ environmental observations of the catalyst. The microscope was also equipped with an energy-dispersive Xray (EDX) analyzer (SDD X-Max 80 mm2 from Oxford Instruments) used for local elemental chemical analysis. The powdered sample was embedded in a resin that was then cut using an ultramicrotome and deposited on a grid placed in a Gatan furnace-type holder in Inconel compatible with the direct observation of catalytic reactions at elevated temperatures and under a controlled atmosphere.37 Since the temperature was 4780

DOI: 10.1021/acscatal.6b01114 ACS Catal. 2016, 6, 4775−4781

Research Article

ACS Catalysis

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only measured with a thermocouple placed around the furnace of the sample holder, a slight discrepancy may exist between the imposed (measured) and real temperatures. During the ETEM study of the solid sample in the catalytic gas phase mixture, a homemade 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 reactants and products was checked by analyzing the effluent exiting the microscope chamber by mass spectroscopy (Pfeiffer). The samples were studied by ETEM at 20 and 350 °C successively under various atmospheres. The successive conditions used are shown in Table 2. In addition to 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 in nitrogen with a relative O2/C2H6/N2 ratio of 30/15/55 corresponding to that currently used to test this type of catalyst in a laboratory testing apparatus between 320 and 400 °C.33 In all cases, when the sample was exposed to gas, the 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 collection angles in the range 45−200 mrad. According to the relatively low value of the inner cutoff angle, all STEM images were labeled as STEM-(HA)ADF images, since they do not strictly correspond to high-angle ADF conditions.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for J. M. M. Millet: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Thanks are due to the CLYM (Consortium Lyon-St-Etienne de Microscopie; www.clym.fr) for their 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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REFERENCES

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DOI: 10.1021/acscatal.6b01114 ACS Catal. 2016, 6, 4775−4781