(638e) A Combined HAADF STEM and Density Functional Theory Study of Tantalum and Niobium Location in the Mo-V-Te-Ta(Nb)-O M1 Phase

The mixed metal Mo-V-Nb(Ta)-Te-O oxides have attracted
significant interest of the catalysis community due to their unique ability to
selectively (amm)oxidize propane directly into
important industrial chemicals, such as acrylonitrile and acrylic acid^1,2. The active and selective catalyst
contains two major crystalline phases, M1 and M2^2,3. The M1 phase (Figure 1) is primarily responsible for the
activity and selectivity in (amm)oxidation
of propane, whereas the M2 phase is unable to activate propane and instead may
assist the M1 phase in the (amm)oxidation of the
propylene intermediate.  However,
the knowledge of the bulk and surface structure and catalytic roles of each
metal ion in these mixed metal oxides is still limited due to the structural
and compositional complexity of this system and the limitations of experimental
techniques to distinguish Nb from Mo in the M1 phase⁴. Hence, the location of Nb in M1 phase crystal framework and therefore its role
played in the chemistry of the propane (amm)oxidation over the catalyst remain unsolved.

In this study we investigate the
position of Ta and Nb in Mo-V-Te-Ta-O
(Ta-M1) and Mo-V-Te-Nb-O M1
(Nb-M1) phases, respectively.  The location
of Ta in the bulk Ta-M1 phase generated by hydrothermal synthesis is first
studied using high-angle annular dark field (HAADF) scanning transmission
electron microscopy (STEM) because Ta can be easily distinguished from Mo due
its much greater atomic weight and yet is chemically similar to Nb.  Density functional theory (DFT)
calculations are performed to determine the energy of Ta in the different cation sites using cluster models of the M1 phase
consisting of several truncated ab planes^5,6.
The results of the DFT calculations are in good agreement with the Ta location
determined by HAADF STEM to be overwhelmingly the pentagonal bipyramidal site 9 (S9) in the bulk M1 structure.  The same computational approach is then
applied to the Nb-M1 phase and the results indicate that Nb
likewise prefers S9.  However, in
the surface ab
plane our DFT results indicate that Ta prefers S10 and S11 while Nb prefers S9. 
The possible consequences of the different surface locations of Nb and Ta on the activity and selectivity of the M1 phase toward
propane (amm)oxidation will
be discussed.

Figure 1.
The crystalline structure of Mo-V-Te-Nb/Ta-O M1 phase; Nb is
presumably in S9 site as Ta.

References:

(1)      Grasselli, R. K. Top. Catal. 2002, 21,
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(2)      Korovchenko, P.; Shiju, N. R.;
Dozier, a. K.; Graham, U. M.; Guerrero-P¨¦rez, M. O.; Guliants,
V. V. Top. Catal. 2008, 50, 43-51.

(3)      Desanto, P.; Buttrey, D.; Grasselli, R. K.; Lugmair, C. G.;
Volpe, A. F.; Toby, B. H.; Vogt, T. Z. Kristallogr.
2004, 219, 152-165.

(4)     DeSanto, P.; Buttrey, D. J.; Grasselli, R. K.; Pyrz, W. D.; Lugmair, C. G.; Volpe, A. F.; Vogt, T.; Toby, B. H. Top.
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(6)      Muthukumar, K.; Yu, J.; Xu, Y.; Guliants, V. V. Topics in Catalysis 2011, 54,
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