278624 A Combined HAADF STEM and Density Functional Theory Study of Tantalum and Niobium Location in the Mo-V-Te-Ta(Nb)-O M1 Phase

Thursday, November 1, 2012: 9:45 AM
321 (Convention Center )
Junjun Yu1, Jungwon Woo1, Albina Borisevich2, Ye Xu3 and Vadim V Guliants1, (1)School of Energy, Environment, Biological and Medical Engineering, University of Cincinnati, Cincinnati, OH, (2)Materials Science & Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN, (3)Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, TN

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 acid1,2. The active and selective catalyst contains two major crystalline phases, M1 and M22,3. 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 planes5,6. 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, 79-88.

(2)      Korovchenko, P.; Shiju, N. R.; Dozier, a. K.; Graham, U. M.; Guerrero-P¨Śrez, 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. Catal. 2006, 38, 31-40.

(5)      Govindasamy, A.; Muthukumar, K.; Yu, J.; Xu, Y.; Guliants, V. V. The Journal of Physical Chemistry C 2010, 114, 4544-4549.

(6)      Muthukumar, K.; Yu, J.; Xu, Y.; Guliants, V. V. Topics in Catalysis 2011, 54, 605-613.


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