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HAADF-STEM Study of Mo/V Distributions in Mo-V-Te-Ta-O M1 Phases and Their Correlations with Surface Reactivity

The MoVTeTaO M1 phases were prepared by conventional hydrothermal (HT) and microwave-assisted HT synthesis methods (MW) employing two different Ta precursors, Ta ethoxide and a custom-made Ta oxalate complex. The profile intensity analysis of the HAADF-STEM image of M1 phases oriented along [hk0] directions from the surface to bulk region of HAADF-STEM images indicated that the chemical composition of surface ab planes is very similar to their composition in the bulk. The HAADF-STEM image analysis showed that synthesis methods have a significant impact on the Mo/V distribution in the MoVTeTaO M1 phases and their reactivity in propane ammoxidation. Enhanced acrylonitrile (ACN) yield and 1st order irreversible reaction rate constants for propane consumption, normalized to the estimated surface ab plane areas, correlated with increased V content in the proposed catalytic center (S2-S4-S4-S7-S7). These observations lend further support to the idea that multiple VOx sites present in the surface ab planes may be responsible for the activity and selectivity of the M1 phase in propane ammoxidation.


Introduction
The detailed atomic-scale information about the surface structure and chemical composition of crystalline mixed metal oxides that are currently investigated for the direct propane ammoxidation to acrylonitrile (ACN) is critical for understanding the fundamental surface structure-reactivity relationships and further improving these catalytic materials for this practical application. However, the characterization of topmost surface in such polycrystalline mixed metal oxides, as the M1 phase, remains a long-standing challenge in materials science of catalytic materials because the vast majority of spectroscopic and structural tools are bulk techniques [1][2][3][4]. In recent years, High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy (HAADF-STEM) possessing sub-angstrom resolution emerged as a promising tool to study the structure and compositions of crystalline multicomponent metal oxides [5][6][7]. Pyrz et al. [8] analyzed the HAADF-STEM images of the M1 phase and determined the local chemical composition of the ab planes of this phase proposed as the location of the active centers for propane ammoxidation. However, the local chemical compositions of the ab planes of the MoVTeTa M1 phase determined in previous HAADF-STEM studies were essentially bulk compositions based on average intensities of entire atomic columns corresponding to a large number of ab planes [8,9]. Therefore, the first major objective of this study was to explore the potential of HAADF-STEM to analyze the composition of the topmost ab planes of the M1 phase. This profile HAADF-STEM study probed the intensities of atomic columns, which were entirely located within the surface ab planes by investigating the [hk0] crystal orientations of the M1 phase.
Moreover, this profile HAADF-STEM technique enabled examining the variation of the ab plane composition from the topmost surface to the bulk. This composition analysis was accomplished by applying the same HAADF-STEM methodology that was used previously for the bulk characterization of the M1 phase oriented down the [001] crystal axis.
The second major objective of this study was to examine the location, concentration and catalytic role of Nb (and Ta) in propane ammoxidation to acrylonitrile. Although the presence of Nb is known to significantly improve the activity and selectivity of the M1 phase in propane (amm)oxidation [10], its location in the M1 phase could not be directly established by X-ray and neutron diffraction methods due to similar scattering properties of Nb and Mo centers. Instead, Pyrz et al. [11] investigated the Ta-substituted M1 phase by the HAADF-STEM and determined that Ta was located in so-called pentagonal bipyramidal site 9. It was further suggested that Nb being chemically similar to Ta is also located in site 9. Based upon this structural model of the M1 phase, Grasselli et al. [12][13][14] proposed a hypothetical pathway of propane ammoxidation over the ab planes of the M1 phase. According to this pathway, Nb cations, located in site 9 and surrounded by five Mo cations, spatially isolate adjacent V-containing active sites from one another thereby improving the selectivity of propane ammoxidation to acrylonitrile ( Figure 1). However, the M1-Ta catalyst prepared by a rapid slurry evaporation method [11] also revealed some unusual microstructural features which were expected to be detrimental for the selectivity to acrylonitrile according to the site isolation model [15]. For example, the HAADF-STEM images of this catalyst indicated much lower Ta occupancy in site 9 than that in the structural model based on a combination of synchrotron X-ray powder diffraction (S-XPD) and neutron powder diffraction (NPD) methods [16]. Moreover, the Ta segregation from the surface to the bulk was observed in a previous HAADF-STEM study of the M1-Ta catalyst [11].
However, the Ta, V, and Mo locations and partial site occupancies in hydrothermally synthesized M1 phases have not been yet investigated despite the fact that hydrothermal synthesis offers improved control over the nucleation and growth of polycrystalline mixed metal oxides [17][18][19].  [24] In this study, the synthesis of well-defined MoVTeTaO M1 phase catalysts was explored employing approaches aimed at enhancing the Ta occupancy of site 9 and minimizing its surface segregation. Well-defined MoVTeTaO M1 phase catalysts were prepared by conventional hydrothermal (HT) and microwave-assisted HT synthesis methods (MW) employing two different Ta precursors, Ta ethoxide and a custom-made Ta oxalate complex (denoted as "P"
Ammonium paramolybdate (Alfa Aesar, 81-83% as MoO 3 , 5.35 g) and telluric acid (Fluka, 99%, 1.16 g) were dissolved under stirring in 20 ml of distilled water for 30 min. A second solution was prepared by dissolving vanadyl (Ⅳ) sulfate (Alfa Aesar, 99.9%, 2.37 g) in 10 ml of distilled water and then a third solution was prepared by dissolving tantalum(V) ethoxide (99%, Alfa Aesar) or tantalum oxalate complex [20] in 10 ml of distilled water. The Ta oxalate complex was used as an alternative Ta precursor in the synthesis of MoVTeTaO M1 phase catalysts, which is more hydrolytically stable than the conventional Ta ethoxide source. The Ta oxalate complex was synthesized starting with Ta 2 O 5 using the basic flux method. Ta 2 O 5 was fused with KOH (Ta 2 O 5 :KOH = 1:10) in an alumina crucible at 723K for 5 h. The solid product was dissolved in distilled water, and the resulting solution was filtered. Glacial acetic acid was added to the transparent solution until a pH value below 3. The white precipitate was filtered, washed thoroughly to remove the remaining acetic acid, and dissolved in oxalic acid solution (Ta:oxalic acid = 1:20) at 333K. The Ta content in the Ta oxalate complex was determined by the ICP analysis and further checked by thermogravimetric analysis (TGA) in air. These characterization results were in good agreement with those reported in the literature [20]. The second solution for the M1 phase synthesis containing vanadium was added to the first solution of molybdenum and tellurium sources, and the resulting solution was stirred for 5 min. The third solution containing a tantalum source was finally added and the resulting yellow-green slurry was stirred for 10 min.
The slurry was transferred into the Teflon inner tube of a stainless steel autoclave. The autoclave was sealed and heated at 448K for 48 h. After the hydrothermal reaction, the dark blue powder obtained was filtered, washed with distilled water (200 ml) and dried overnight at 353K. The dry catalyst precursors from HT method were calcined under ultra-high purity nitrogen flow (50 ml/min) at 873 for 2 h prior to catalytic studies.
In the case of the microwave-assisted synthesis (MW), the same slurry was prepared and transferred to a 100 ml capacity cylindrical Teflon vessel, sealed and then kept at 448K for 2 h in a microwave-accelerated Mars 5 reaction system (CEM, USA) operated at maximum power (400 W) [18]. After the microwave treatment, the dark blue powder was filtered, washed with distilled water (200 ml), and dried overnight at 353K. The dry catalyst precursor was calcined under ultra-high purity nitrogen flow (50 ml/min) at 873 for 2 h prior to catalytic studies.
In order to obtain pure M1 phases, the M2 phase impurity initially present in all calcined as-synthesized catalysts was selectively removed by a hydrogen peroxide treatment. The dissolution of the M2 phase was carried out by stirring the calcined as-synthesized catalysts in an aqueous 30% hydrogen peroxide solution at room temperature for 3 h [21]. The resulting suspension was filtered, washed with distilled water (200 ml), and dried overnight at 353K.

Powder XRD characterization
After the calcination at 873K for 2 h, the catalysts were thoroughly ground with a mortar and pestle for 10 min. Powder X-ray diffraction was recorded using a Siemens D500 diffractometer with Cu Kα radiation (tube voltage: 45 kV, tube current: 40 mA).

Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS)
The SEM and EDS characterization was conducted in the Advanced Materials Characterization Center (AMCC) at the University of Cincinnati. SEM (FEI/Philips XL 30 FEG ESEM) was equipped with the energy dispersive X-ray analyzer from EDAX and had a resolution of 3.5 nm at 30 kV.

Specimen preparation for STEM
The specimens employed in STEM studies were prepared as follows. The MoVTeNb(Ta)O M1 phase catalysts prepared by different synthesis approaches were thoroughly ground by a mortar and pestle for 5 min. The ground M1 samples were embedded into a resin and sectioned by a microtome into ca. 50 nm slices [22] in order to study the [001] zone axis orientation of the M1 phase. These specimens were then deposited onto a lacey-carbon supported Cu grid.
High-resolution STEM imaging was performed using an FEI-Titan 60/300 microscope equipped with a CEOS aberration corrector on the 300kV illumination system in the Advanced Microscopy Division of the Oak Ridge National Laboratory (ORNL). Z-contrast images in this work were acquired at 300 kV with a probe size of ~0.6 Å, a convergence angle of 30 mrad, and inner collection angle of 60 mrad. The acquisition time used to collect these images was 6 secs/1024*1024 pixels. All specimens were plasma-cleaned using Fischione plasma cleaner by electron beam irradiation to prevent specimen contamination during the STEM operation.

Propane Ammoxidation Reaction
The catalytic behavior of the MoVTeTa(Nb) M1 phase catalysts prepared by different synthesis methods was tested in propane ammoxidation using a fixed bed micro-reactor equipped with an on-line GC under steady-state conditions at atmospheric pressure and 380 -460 °C. The calcined catalysts were ground with a mortar and pestle for 5 min, diluted with quartz sand, loaded into the micro-reactor, and then heated to a desired temperature in flowing feed gas. The feed was composed of C 3 H 8 :NH 3 :O 2 :He in the molar ratio of 5.7:8.7:17.1:68.4 (the total flow rate of 26.3 mL•min -1 ). The reactants and products were analyzed by an on-line GC system (Shimadzu 14 A) equipped with flame ionization and thermal conductivity detectors. The total carbon balances agreed within ±5%.

STEM image analysis
In HAADF-STEM image analysis, the center-of-mass fit determined the positions and intensities of atomic columns. In order to interpret the intensity quantitatively in terms of elemental occupancies, the following assumptions were made: (1) the thickness of each unit cell was constant; (2) the background intensity was constant throughout each unit cell; and (3) the atomic column intensity was proportional to the square of the atomic number following the Rutherford scattering relationship [23]. Based on these assumptions, the intensities of atomic columns were computed using Digital Microscopy (DM) scripts. The obtained atomic column intensities were normalized to the average intensity of the 100% Mo sites S5, S6, S8, S10, and S11 [24].  [12,16]. It is noteworthy to mention that M1-Ta HT(P), made using the Ta oxalate complex as an alternative Ta source, was synthesized for the first time in this study. The Ta

SEM images and elemental composition of MoVTeTaO M1 phase catalysts
The representative SEM images of the MoVTeTaO M1 catalysts are shown in Figure 2 of Supporting Information. All SEM images showed typical rod-like particle morphology of M1 phases regardless of the synthesis methods employed. It is important to note that the morphology of the MoVTeTaO M1 phases of this study was different from those of the previously reported MoVTeTa(Nb)O catalysts prepared by slurry evaporation (SE) (Figure 2f of Supporting Information) [16,25]. This result is not surprising because the SE method is known to result in rather poor control over nucleation and growth of the desired M1 phase, and often lead to the formation of M2 and other impurity phases [18]. As shown above, the SEM images of hydrothermal MoVTeTaO M1 phases, i.e., HT (0.09, 0.12, 0.15), MW, and HT(P), showed a much better defined crystal morphology and typical rod-like crystal habit of the M1 phase ( Figure 2 of Supporting Information). These observations indicated that the synthesis method has a profound impact on the crystal morphology of the MoVTeTaO M1 phase catalysts. Moreover, they suggested that HT and MW synthesis methods yield well-defined M1 phases that are better suited for establishing accurate relationships between the metal distributions in the ab planes of the M1 phase and its catalytic performance in propane ammoxidation.

Bulk characteristics of MoVTeTaO M1 catalysts
All MoVTeTaO M1 catalysts were characterized by SEM/EDS with respect to their bulk chemical compositions and BET surface areas ( Table 1). The bulk elemental composition of all catalysts differed slightly depending on the synthesis methods. It is generally observed that the bulk Ta/Mo molar ratio of all catalysts determined by SEM/EDS is much higher than that in the synthesis gel. The results of previous studies [11, 16,24] indicated that Ta is exclusively located in site 9 (S9). If we assume that Ta fully occupies S9 and is not present anywhere else in the M1 lattice, while the remaining sites are occupied by 70% Mo and 30% V, then the Ta/Mo ratio would be ca. 0.14. Since the observed Ta/Mo ratios (Table 1) are markedly higher than 0.14, the higher molar ratio of Ta/Mo suggested that the excess Ta is likely present outside the M1 phase lattice. This observation is supported by the results of DeSanto et al. [16], who detected a Ta 2 O 5 impurity in the M1-Ta sample prepared by slurry evaporation in a combined synchrotron (S-XPD) and neutron power diffraction (NPD) study of this phase. Moreover, this observation also suggested that not all Mo was incorporated into the M1 phase during synthesis.  [8,9,11] were in good agreement with the full structural model of the M1 phase based on the S-XPD and NPD refinement [16]. Therefore, we further applied the HAADF-STEM to studies of the M1-Ta catalysts given the atomic-scale imaging capability and elemental composition information offered by this method. Firstly, the Ta segregation observed in these previous studies was addressed by employing catalysts made by the HT and MW methods, which allow better control over the formation of desired M1 phase than the slurry evaporation method used previously [11]. Furthermore, we employed specimens of constant thickness (~ 50 nm) for HAADF-STEM imaging made by sectioning the embedded samples to eliminate the impact of thickness variation on the atomic column intensities.
Representative HAADF-STEM images of M1-Ta HT(0.12) which were recently reported by our group [24] were shown in Figure 3 of Supporting Information. The HAADF-STEM images showed that the pentagonal sites (S9), located at the center of circles visible in Figure 3a of Supporting Information, were much brighter than other metal sites. The significant brightness contrast between S9 and all other metal lattice sites, including those occupied by Mo, clearly indicated the location of heavy Ta atoms in S9. There were no appreciable contrast differences between S9 column located in the middle of the M1 crystal ( Figure 3b of Supporting Information) and its surface region (Figure 3c of Supporting Information). These findings confirmed that Ta is uniformly distributed throughout the M1-Ta HT catalyst. Uniform Ta distribution in M1-Ta HT suggested that the Ta segregation observed in the previous M1-Ta SE is likely due to mass-transfer limitations during rapid slurry evaporation synthesis [16].
Density functional theory (DFT) was further employed to probe the energies of Ta and Nb cations located in several crystallographic sites of the M1 phase [24]. These spin-polarized periodic DFT calculations were performed in the generalized gradient approximation (GGA-PBE) [26] using the Vienna Ab initio Simulation Package (VASP) [27][28][29][30]. Three truncated ab planes of the M1 phase structure were employed as cluster models for the DFT calculation. The Ta located at site 9 was interchanged one by one with other metal sites present in the M1 phase and the energies of cluster models resulting from the Ta interchange were calculated from DFT by applying statistical thermodynamics considering both the internal energy and entropy contributions to the free energy of the Ta distribution for the ab planes. The same computational approaches were applied to Nb in the cluster models of M1 phase catalyst. The predicted probabilities of finding Ta and Nb in S9, 10, and 11 determined by DFT calculations combined with methods of statistical thermodynamics are shown in Figure 3 of Supporting Information.
The probability of finding Ta and Nb in S9 is predicted to be nearly 100% confirming that Ta (and Nb) is predominantly located in S9 in good agreement with the results of HAADF-STEM image analysis of the M1-Ta phase.
A comparison of relative intensities of metal lattice sites for the MoVTeTa(Nb)O M1 catalysts prepared by different synthesis methods is shown in Figure 4 of Supporting Information.
The relative intensity for M1-Nb HT is also shown for comparison between the M1-Nb and M1-Ta systems. The normalization was done in reference to the average intensities of S5, S6, S8, S10, and S11 which were assumed to be 100% Mo (Z=42). This assumption was based on the results of earlier X-ray, neutron diffraction, and HAADF-STEM studies [11, 31,32], which reported S5, S6, S8, S10, and S11 to be 100% Mo sites. S5, S6, S8, S10, and S11 indeed showed constant intensities throughout all unit cells examined in this study. The higher intensities (greater than 1) of S9 atomic columns relative to 100% Mo sites suggested the presence of a heavier atom, i.e., Ta (Z=73), while the relative intensities less than 1 for S1, S2, S3, S4 and S7 indicated the presence of a lighter metal atom, i.e., V (Z=23), in agreement with results of the previous synchrotron X-ray diffraction and HAADF-STEM studies [11,16]. Therefore, the analysis of normalized intensities of atomic columns in the HAADF STEM images of M1-Ta catalysts employed in this study strongly indicated that Ta is predominantly located in site 9. It is noteworthy to observe significant variation of relative intensity of Ta in S9 as well as V in S1, S2, S3, S4, and S7 among M1-Ta catalysts shown in Figure 4 of Supporting Information. These findings suggested that different synthesis approaches have a marked impact on the Ta and V occupancies in the ab planes of M1 phase, which is discussed in greater detail below.

Intensity line profile of HAADF-STEM images of M1-Ta HT (0.12) oriented down [hk0]
A  [35]. In yet another study, the projection along the [001] crystal direction of AlAs/GaAs system grown on the top of a GaAs substrate was analyzed in high-resolution HAADF images obtained from super STEM [36]. The HAADF intensity profiles of three GaAs dumbbells were used to estimate the average Al and Ga content due to the Z-contrast nature of HAADF images, i.e., Al (Z = 13) and Ga (Z = 31). The line profile of HAADF-STEM images was also used to determine atomic scale structures of Pt monolayer electro-catalysts in combination with other characterization techniques, such as energy-dispersive X-ray spectrometry (EDS), electron energy-loss spectroscopy (EELS), and in situ extended X-ray absorption fine structure (EXAFS) [37].   (Table 1 of Supporting Information). The metal occupancies reported in Table 1 of Supporting Information were calculated based on the two assumptions stated in section 2.6.
Three different M1 phase particles for each catalyst were chosen for analysis and two images for each particle were examined to determine the metal column intensities. The atomic column intensities in four unit cells in each image were thoroughly analyzed. The collected data for each catalyst were statistically analyzed and the atomic column intensities for each crystallographic site in all catalysts varied less than 10 % for the same site in different unit cells in the same M1 particle and different M1 phase particles. This finding indicates that the metal occupancies presented in present study are representative for the entire catalysts as a bulk material. The chemical composition of the M1-Ta prepared by slurry evaporation reported in a previous study [18] is also shown for comparison [16]. In a previous S-XPD study [16] [16]. This observation is very important because site 4 is an integral part of the hypothetical catalytic center (S2-S4-S4-S7-S7) for propane ammoxidation proposed by Grasselli et al. [12][13][14]. The impact of the V partial occupancy in site 4 on the catalytic performance in propane ammoxidation is discussed in the next section. Lastly, the variation of metal occupancies observed in M1 catalysts suggested that the partial occupancies of constituent metal species are strongly influenced by the choice of synthesis methods.  [41]. They reported these dissociation constants to be in the range of ~ 8.0×10 -12 to 3.0×10 -19 . Therefore, the Ta oxalate precursor employed in the M1-Ta HT(P) synthesis is expected to provide fewer free hydroxylated Ta species as compared to Ta ethoxide that are incorporated into the M1 phase lattice during synthesis.

Correlations between Mo/V distributions and Ta content in synthesis
The Mo, V, and Ta sites occupancies of the MoVTeTaO M1 phase catalysts prepared by the hydrothermal method using different synthesis Ta concentrations were determined by the systematic analysis of atomic column intensities from the HAADF-STEM images (   Table 2. As reported earlier, propylene is the major product at low propane conversion, suggesting that propylene is the primary intermediate during propane ammoxidation, which is converted further to acrylonitrile, and then to unselective products, i.e., acetonitrile and CO x , as propane conversion increases [4]. The propylene selectivity behavior found in this study is also in agreement with previous observations [4] where propylene selectivity was found to be  Table 2 is further discussed below on the basis of metal occupancies in ab planes of M1 phase catalysts. The relationships between the Ta occupancy in site 9 of the M1 phase and the selectivity toward acrylonitrile were investigated on the basis of the site isolation model, defined as "the spatial separation of active sites from each other on the surface of a heterogeneous catalyst" by Grasselli et al. [15]. According to this site isolation model, the presence of redox-inactive Ta as opposed to possibly redox-active, but unselective Mo at site 9 (S9) was proposed to play the role of isolating each hypothetical catalytic center comprised of one S2, two S4, two S7, and two S12 thereby improving the selectivity to acrylonitrile during propane ammoxidation.
The maximum selectivity to acrylonitrile observed during the propane ammoxidation reaction under a limited set of experimental conditions was plotted against the Ta S9 occupancy in MoVTeTaO M1 phase catalysts in Figure 6 of Supporting Information. The M1-Ta HT(0.12) showed the highest observed selectivity to acrylonitrile (~ 77 mol. %) among all MoVTeTaO M1 phase catalysts ( Another possible explanation is that there might be no relationship between the Ta occupancy in S2>S3>S7>S4≈S1 [43]. In summary, these findings suggested that Mo cations were displaced from S9 with increasing Ta content in the synthesis medium and competed with V cations for S1, S2, S3, S4, and S7. Next we examined the correlations between the acrylonitrile selectivity and Mo/V ratios on the basis of the probability model.

Validation of probability model for MoVTeTaO M1 phase catalysts for propane ammoxidation
The catalytic performance of MoVTeTaO M1 phase catalysts examined in this study was discussed above in section 3.5.1 ( Table 2). In this section, we explore the correlations between suggested that the ab planes of M1 phase contain the active and selective sites for propane ammoxidation [42,[44][45][46]. Our recent study of a MoVTeNbO M1 phase catalyst that selectively exposed ab planes demonstrated that the ab planes of M1 phase are responsible for high activity and selectivity in propane ammoxidation [47]. Furthermore, Grasselli et al. [13,14,25,42] developed a probability model where the Mo/V distribution in the proposed catalytic center present in the bulk ab planes of the M1 phase and consisting of sites 2, 4, 7, and 11 (S2-S4-S4-S7-S7), correlated with the selectivity to acrylonitrile. They advanced a hypothesis that one V 5+ cation present in the proposed catalytic center activates propane selectively, two V 5+ cations lead to waste products, while the absence of V 5+ cations in the proposed catalytic center corresponds to the lack of catalytic activity. Based on these assumptions, they estimated that the MoVTeNbO M1 phase catalyst contained 44% of active and selective sites for propane ammoxidation, 46% of inactive sites, and 10% of waste-forming sites. Using these estimates, they predicted the maximum acrylonitrile selectivity to be 81 mol. % (i.e., 44/(44+10) × 100%), which is somewhat higher than the maximum selectivity to acrylonitrile observed under a limited range of experimental conditions (72 mol. %) [42]. Therefore, this probability model suggested that the catalytic activity and selectivity for propane ammoxidation to acrylonitrile could be correlated with the Mo/V distribution at the proposed catalytic center.
In previous studies, the Mo Our approach to determine the elemental composition of surface ab planes was introduced in order to address the limitations of powder diffraction methods [16,32,48,49] and further advance the HAADF-STEM methodology for the M1 catalytic system [8,9,11,50]. As  Tables 6 and 7 of Supporting Information, respectively.
The probability, P, was calculated as shown below: Probability of zero V cations in the catalytic center: where Mo2, Mo4, and Mo7 are the partial occupancies in S2, S4, and S7, respectively (Table 6 of Supporting Information).
Probability of a single V cation in the catalytic center: where V2, V4, and V7 are the partial occupancies in S2, S4, and S7, respectively ( Table 6 of Supporting Information).
Probability of two V cations in the catalytic center: Probability of three V cations in the catalytic center: Probability of four V cations in the catalytic center: Probability of five V cations in the catalytic center: According to the data shown in Table 8

New probability models to predict propane ammoxidation behavior
A recent study of Li et al. [49] indicated that in addition to sites 4 and 7 in the catalytic center, site 3 also contains V 5+ , suggesting that propane can also be activated over site 3.
Therefore, the first new model, Model 1, was proposed on the basis of the Grasselli et al. [13,14,25,42] probability model where S3 was included in the catalytic center. Moreover, the Li et al. study [49] also indicated that S2 contained only reduced V 4+ , which was considered to be inactive in propane ammoxidation [42,54,55] and, therefore, S2 was excluded from our new  (Table 9 of Supporting Information). According to our assumptions stated above, cases 1 and 5 shown in Table 9 of Supporting Information were considered to be only selective for propylene, while cases 2 and 3 were regarded to be selective for both propylene and acrylonitrile.
The sums of probabilities for cases 1 and 5, assumed to form only propylene, and for   [13,14,25,42], were also included in Table 10 Table 2. Also, the predicted maximum selectivities to acrylonitrile for MoVTeTaO M1 phase catalysts shown in Table 10 of Supporting Information are considerably lower than the experimentally observed maximum selectivities reported in Table 2.
The combined yield of propylene and acrylonitrile was further plotted in Figure 10 Table 2. The Arrhenius plots of the first-order irreversible reaction rate constant, k", for MoVTeTaO M1 phase catalysts were linear supporting the conclusion that the rate of propane consumption is the 1 st order reaction (Figure 11 of Supporting Information).
The activation energies of propane consumption over MoVTeTaO M1 phase catalysts from the Arrhenius plots (Figure 11 of Supporting Information) are in the range of 112~158 (kJ·•mol -1 ).
These activation energies are in good agreement with the activation energy (131 kJ·•mol -1 ) of MoVTeNbO M1 phase reported in the previous study where propane ammoxidation was also assumed to be a first-order reaction [4].
The propane consumption rate constants, k", for all catalysts are plotted against the total V content in the catalytic center (S2-S4-S4-S7-S7) of M1 phase in Figure 12 of Supporting Information, which showed a lack of correlation. It may be explained by the fact that k" values have been normalized to the total BET surface area of the M1 catalysts, whereas the catalytic activity in propane ammoxidation is thought to be associated with the presence of only surface ab planes. This view is supported by the results of our recent study of a MoVTeNbO M1 phase selectively exposing ab planes which provided additional evidence that the ab planes may be responsible for its high activity and selectivity in propane ammoxidation [47]. The MoVTeTaO M1 phase catalysts employed in this study showed rod-shaped crystal morphologies with different extent of exposure of surface ab planes (Figure 2 of Supporting Information). In order to account for these differences, the reaction rate constants, k", were normalized to the surface ab plane areas of these M1 phases estimated by measuring the lengths and diameters of statistically representative numbers (N=30) of M1 phase crystals in respective SEM images of each catalyst (Table 11 of Supporting Information). The ab plane surface areas were estimated based on three assumptions: all MoVTeTaO catalysts in this study were (1) pure M1 phase, (2) 100% crystalline, and possessed a perfect cylindrical shape.
It is worth mentioning that the length of M1 crystals decreased with the Ta synthesis concentration for M1-Ta HT(0.09), M1-Ta HT(0.12), and M1-Ta HT(0.15). This observation is in good agreement with previous findings that Nb 5+ (and oxalate) ions suppressed crystal growth of the MoVTeNbO M1 phase [58]. The estimated surface ab plane areas were used to calculate the new reaction rate constant, k" ab . Those k" ab were plotted against the total V content in the catalytic center (S2-S4-S4-S7-S7) of MoVTeTaO M1 phase catalysts in Figure 4. The reaction rate constant, k" ab , was found to increase with the total V content in the catalytic center (S2-S4-S4-S7-S7) of M1 phase catalysts (Figure 4), with one exception of M1-Ta HT(P), which is discussed further below. These observations are consistent with the results of recent studies that showed a linear relationship between the initial rate of propane conversion and the vanadium content of MoVTeNbO M1 phase in propane oxidation to acrylic acid [51,52].
In summary, the new probability model 2 was relatively successful in establishing a correlation between the yield of acrylonitrile and the total V content in the proposed catalytic center of M1 phase ( Figure 3) and a correlation between the reaction rate constant, k" ab , and the total V content in the proposed catalytic center of M1 phase (Figure 4). These findings are also consistent with the conclusions of the probability model proposed by Grasselli et al. [13,14,25,42] and other studies suggesting the importance of V content in the M1 phase for propane activation during propane ammoxidation [51,52,59].
Although the new probability Model 2 established a correlation between the catalytic behavior and the total V content in the catalytic center of the MoVTeTaO M1 phase catalysts of this study, it did not probe the importance of multiple vanadium sites for propane ammoxidation, which were previously suggested to be more efficient in propane activation [60][61][62]. Indeed, a previous experimental study of propane oxidation to acrylic acid combined with chemical probe chemisorption and low energy ion scattering (LEIS) conducted by our group indicated that multiple VO x sites present in M1 phases may be more efficient in selective propane oxidation than isolated VO x species [60]. Therefore, another probability model, Model 3, was explored in order to improve the predictive capability for all MoVTeTaO M1 phase catalysts employed in this study. Since Model 2 established that the reactivity in propane ammoxidation correlated with the total V content in the catalytic center, Model 3 was proposed to test the hypothesis that the activity and selectivity in propane ammoxidation were correlated with presence of more than one V in the catalytic center (S2-S4-S4-S7-S7) of M1 phase.
In Model 3, the catalytic reactivity in propane ammoxidation was correlated with: (1) P(V=2), the probability of two V in the catalytic center (case 1); (2) P(V=2)+P(V=3), the combined probability of two V and three V in the catalytic center (case 2); and (3) P(V=2)+P(V=3)+P(V=4), the combined probability of two V, three V, and four V in the catalytic center (case 3). Although the probability of five V in the catalytic center is theoretically possible, the P(V=5) was significantly below 0.1 for all MoVTeTaO M1 phase catalysts and, therefore, was not included in this study.
The probabilities of P(V>2) in the catalytic center (S2-S4-S4-S7-S7) shown in Table 3 were calculated according to equations shown in section 3.5.2.2. The probabilities of V for case 3 for all MoVTeTaO M1 phases were plotted against the yield of acrylonitrile in Figure 5. As shown in Figure 5, the yield of acrylonitrile increased with the probability of V>2 in the catalytic center except M1-Ta HT(P), which is discussed further below.  phase catalysts in Figure 13 of Supporting Information. Similar to Model 2, no correlations between the propane consumption rate constants, k", and probabilities of V>2 in the catalytic center are observed. Therefore, the reaction rate constants, k" ab , based on the geometric estimates of surface ab plane areas (Table 11 of Supporting Information) were plotted against the probabilities of V>2 in the catalytic center ( Figure 6), which showed similar increasing trends of k" ab with the probabilities of V>2 in the catalytic center to those for Model 2 shown in Figure 4.
The observed correlations between k" ab and the probabilities of V>2 in S2-S4-S4-S7-S7 lend further support to our previous findings [60][61][62] that multiple VO x redox sites present in the surface ab planes of the M1 phase may be more active in propane (amm)oxidation. However, the new probability models were unable to explain the catalytic behavior of M1-Ta HT(P), which was an outlier according to Model 2 and Model 3. These observations may be understood in terms of the detrimental impact of oxalate anion on the oxidation state of Te in the synthesis gel. In a recent study, Ivars et al. [58,63] prepared the MoVTeNbO M1 phases at different oxalate/Mo molar ratios in the synthesis gel and found that the most selective catalysts for propane oxidation possessed the oxalate/Mo ratios of 0.4~0.6. They proposed that the oxalate anion was responsible for Te 6+ reduction to Te 4+ and suggested that controling the oxalate concentration in the synthesis gel is critical to obtaining highly active and selective M1 phases for propane oxidation. Therefore, in light of these earlier findings [58,63], we speculate that the low reactivity of M1-Ta HT(P) in propane ammoxidation may be explained by excessive Te 6+ reduction to Te 4+ due to the presence of oxalate during synthesis.

Conclusions
In this study, the MoVTeTaO M1 phase catalysts were successfully synthesized by  Tables   Table S1. Metal site occupancies of the M1-Ta HT(0.12), MW, and HT(P) catalysts calculated from HAADF-STEM images. Table S2. Metal occupancies of the M1-Ta HT(0.09), HT(0.12), and HT(0.15) catalysts calculated from HAADF-STEM images.     Table S11. Geometric estimates of surface ab plane areas based on the analysis of M1 phase crystals in SEM images.   68.9 *Theoretical selectivity to acrylonitrile is defined as 100% x P(1V)/(1-P(0V)), where P(1V) and P(0V) are probabilities of finding a single and zero V cations, respectively, in S2-S4-S4-S7-S7; ‡ experimental maximum selectivity to acrylonitrile.   Table S9)]. It is assumed that all propylene, formed at the selective centers to ACN, is converted to ACN. 3.3 l mean : the mean of length of particles d mean : the mean of diameter of ab planes; R: the radius of ab planes; C.ab.S: calculated ab planes surface area (m 2 /g).             The 1 st order irreversible reaction rate constant [7] was calculated as follows:

Legends for Figures
A → products, for any constant ε A -r" A = - k"τ" = -(1+ε A ) ln (1-X A )-ε A X A where S is catalyst surface; τ" is the catalyst area-time time; ε A is the fractional change in volume of the system for complete conversion of reactant A; X A is a fraction of A converted.