Atomic‐Scale Determination of Active Facets on the MoVTeNb Oxide M1 Phase and Their Intrinsic Catalytic Activity for Ethane Oxidative Dehydrogenation

Abstract

Aberration-corrected high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) has been used to image the basal {001} plane of the catalytically relevant M1 phase in MoVTeNb complex oxides. Facets {010}, {120}, and {210} are identified as the most frequent lateral termination planes of the crystals. Combination of STEM with He ion microscopy (HIM) images, Rietveld analysis, and kinetic tests reveals that the activation of ethane is correlated to the availability of facets {001}, {120}, and {210} at the surface of M1 crystals. The lateral facets {120} and {210} expose crystalline positions related to the typical active centers described for propane oxidation. Conversely, the low activity of the facet {010} is attributed to its configuration, consisting of only stable M 6 O 21 units connected by a single octahedron. Thus, we quantitatively demonstrated that differences in catalytic activity among M1 samples of equal chemical composition depend primarily on the morphology of the particles, which determines the predominant terminating facets.Ethene and propene are important building blocks in the chemical industry. The availability of shale gas, as well as the interest in adding value to previously under-utilized carbon feedstocks, makes the production of ethene and propene by oxidative dehydrogenation (ODH) of the corresponding alkanes one of the most attractive alternatives to the current industrial practice of steam cracking. [1] Among the existing catalysts, Mo-V-Te-Nb mixed oxides have shown high selectivity and activity in ODH of ethane to ethene, [2] propane to propene and selective (amm)oxidation to acrylic acid [3] and acrylonitrile. [3b, 4] It is generally agreed that the crystalline phase M1 (Scheme 1) is responsible for the outstanding catalytic activity and selectivity of MoVTeNbOx, being the only phase able to catalyze the initial homolytic hydrogen abstraction from alkanes. [3a, 5] Therefore, the crystallographic structure and composition of this phase has been extensively studied in the past decade. [6,7] M1 particles crystallize in shapes elongated in z direction, [8] but showing a broad range of morphologies ranging from particles with a flattened profile (Figure 1 A) to needle-like round rods (Figure 1 B) as shown by He ion microscopy (HIM). The ability of the M1 phase to activate alkanes has generally been ascribed to its basal [001] plane. [9] Celaya et al. suggested that not only the basal planes but also the lateral surface of M1 particles show activity in the reaction of propane to acrylic acid. [10] Later it was proposed that half-pipe hexagonal or heptagonal channels could expose active sites as well along the lateral of the particles. [11] However, apart from these studies, little attention has been devoted to the crystal termination of M1 and the nature of the lateral surface, even though, due to the elongated morphology of M1 particles, it can indeed account to up to more than 80 % of the total surface. [9a] In this work, we explore quantitatively the relati...

Study of super-efficient defective MoVSbO catalysts used for ethane oxidative dehydrogenation by HAADF-STEM and of their thermal evolution by environmental electron microscopy and tomography

“…22 Recently, a new method of synthesis of MoVTeNbO catalysts was described and allowed the production of M1 phase nanorods with a particular morphology characterized by the development of {120} and {210} lateral facets exposing the typical active centers described earlier in the [001] planes for propane oxidation. 23,24 This work confirmed the hypothesis from Celaya Sanfiz et al and Zhang et al, 25,26 who proposed that not only the basal planes but also the lateral surface of M1 particles may exhibit catalytic activity. It also disclosed that it was possible to evaluate the activity of the sites located on the lateral facets and that they displayed an activity equal to or greater than that of the sites located in the 001 planes.…”

“…Nevertheless, the surface distribution has a slightly higher frequency of containing no active sites. 23 Indeed, the relative proportion of (010) plane was very low for the new MoVTeNbO catalysts. 23 This tend to confirm that the origin of the high activity of the new MoVSbO catalysts was linked to the presence of a high number of catalytic active sites that are related to the exposure of the {120}, {210}, {110}, and {130} planes.…”

“…23 Indeed, the relative proportion of (010) plane was very low for the new MoVTeNbO catalysts. 23 This tend to confirm that the origin of the high activity of the new MoVSbO catalysts was linked to the presence of a high number of catalytic active sites that are related to the exposure of the {120}, {210}, {110}, and {130} planes. It can also be linked to the presence of the new planes {130} and {110} identified although their frequency was low.…”

“…Nanorods facets frequency (in %) calculated for the synthesized MoVSbO catalyst and the counterpart values for the same facet-types reported in the literature for the best MoVTeNbO catalyst23 …”

“…For the MoVSbO catalysts, it was equal to 279 mol g cat s −1 , a value that was higher than the one computed for the catalyst MoVTeNbO, that is 234 mol g cat s −1 . 23 These two catalysts have a comparable frequency value for the different basal and lateral planes; however, a larger frequency of the inactive (010) planes is observed for MoVSbO catalyst. It is proposed the textural changes and the nanorods fragmentation not observed in MoVTeNbO catalysts, have a very positive contribution by exposing the more active basal planes in MoVSbO catalyst, making it slightly more active.…”