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Using Aberration-Corrected Scanning Transmission Electron Microscopy (STEM) to Explore the Effect of the Addition/exclusion of Elements to/from the Orthorhombic Mo-V-Te-Nb-O M1' Framework

William D. Pyrz1, Douglas A. Blom2, N. Raveendran Shiju3, Vadim V. Guliants3, Thomas Vogt4, and Douglas J. Buttrey1. (1) Chemical Engineering, University of Delaware, 150 Academy Street, Newark, DE 19716, (2) NanoCenter and Electron Microscopy Center, University of South Carolina, 715 Sumter Street, Columbia, SC 29208, (3) Department of Chemical and Materials Engineering, University of Cincinnati, Cincinnati, OH 45221-0012, (4) NanoCenter and Department of Chemistry and Biochemistry, University of South Carolina, 631 Sumter Street, Columbia, SC 29208

Selective oxidation catalysis is crucial to society, resulting in about 25% of all important organic chemicals and intermediates used to make consumer and industrial products [1]. Current processes used to produce high-demand C3 derivatives, namely acrylic acid and acrylonitrile, require the use of multi-component bismuth molybdates and the starting material propene [1-2]. Significant cost savings exist by replacing the expensive propene with propane as a feedstock. A top candidate for this replacement is based on the multiphase MoVTeNbO complex oxide system [1-2]. The best MoVTeNbO catalysts with respect to selectivity and activity are two-phase mixtures comprised of an orthorhombic network bronze phase (M1) and a hexagonal tungsten bronze (HTB)-type phase (M2) [1-2]. Structural models for M1 and M2 currently exist based on simultaneous Rietveld refinement of high-resolution synchrotron X-ray and neutron powder diffraction data [2]. In the present study, we use aberration-corrected STEM methods to directly image the M1 structural framework, and systematically study, atom column-by-atom column, the structural and composition changes that occur within the M1 phase as the combination of atoms within the M1 framework are varied.

By using the simple Z2 relationship for scattering contrast in a high-angle annular dark field image (HAADF), structural models that include estimates for the atomic coordinates and site occupancies can be developed, and subsequently compared to each other and the previously reported structural models for the M1 phase [2-4]. Using this technique, we have recently developed models for the four-component MoVTeNbO catalyst prepared using both slurry and hydrothermal methods [5-6]. These models were shown to be largely consistent with the previously published models, and for some atomic sites, slight variations to the elemental occupancies were suggested [5-6]. In this work, we present structural models for several additional M1 formulations and discuss the possible role that each element plays upon inclusion into the M1 framework.

[1] R. K. Grasselli, Top. Catal. 21 (2002) 79.

[2] P. DeSanto et al., Z. Kristogr. 219 (2004) 152.

[3] P. DeSanto et al., Top. Catal. 38 (2006) 31.

[4] H. Murayama et al., Appl. Catal. A 318 (2007) 137.

[5] W. D. Pyrz et al., Angew. Chem., Int. Ed. 47 (2008), 2788.

[6] W. D. Pyrz et al., J. Phys. Chem. C in press.