(618a) Structure and Activity of Alumina-Supported VOx-TiO2 Catalysts

Supported vanadium oxide materials have been extensively studied for alkane oxidative dehydrogenation (ODH) reactions due to their high activity and selectivity. The activity of these catalysts has been found to be dependent on the nature of the support. The variety of surface VO_x species, distribution of vanadium-oxygen bonds as well as surface acidity of the support can affect the selectivity in alkane ODH reactions.¹ However, Carrero et al. in their review of the kinetics of propane ODH postulated that the selectivity-conversion trends remain unchanged regardless of the support material, implying that the selectivity of VO_x on a given support cannot be improved.² An alternative approach to enhance the activity and selectivity of supported VO_x catalysts is to modify the surface of the support with a third metal oxide prior to the deposition of vanadium oxide.^3-4

Here, we study the interaction of VO_x species with amorphous polymeric domains of TiO₂ deposited on an alumina support and compare these materials to VO_x on bulk alumina and titania. The deposition of VO_x and TiO₂ domains by atomic layer deposition (ALD) enables precise control over the amount as well as the order of deposition of the two metal oxides. The surface density of VO_x was kept constant at approximately 2 V/nm², whereas varying amounts of TiO₂ were deposited. Cyclohexane ODH reaction studies show similar catalytic behavior of VO_x/Al₂O₃ and VO_x supported on Al₂O₃ modified with sub-monolayer TiO₂ domains. The activity of VO_x/TiO₂/Al₂O₃ materials increases with increasing TiO₂ coverage. The catalytic activity is also enhanced when the order of deposition is reversed and a single ALD cycle of TiO₂ follows the deposition of VO_x on Al₂O₃. The variations in catalytic activity are consistent with the reducibility of the surface VO_x species: more easily reducible catalysts exhibit increased activity. UV Raman spectroscopy reveals a shift of the V=O stretching mode to a higher frequency upon the deposition of TiO₂ domains. The origin of this shift is further explored with DFT calculations aimed at understanding the atomic and electronic structure of V=O as a function of the varying distribution of V-O-Al and V-O-Ti bonds. Additionally, in-situ reduction experiments are performed with UV Raman to identify the VO_x surface structures present in these TiO₂-modified materials.

1. Dinse, A.; Frank, B.; Hess, C.; Habel, D.; Schomäcker, R. J. Mol. Catal. A: Chem. 2008, 289, 28.
2. Carrero, C.; Schlögl, R.; Wachs, I.; Schomaecker, R. ACS Catal. 2014, 4 (10), 3357.
3. Dai, H.; Bell, A. T.; Iglesia, E. J. Catal. 2004, 221, 491.
4. Hamilton, N.; Wolfram, T.; Müller, G. T.; Hävecker, M.; Kröhnert, J.; Carrero, C.; Schomäcker, R.; Trunschke, A.; Schlögl, R. Catal. Sci. Technol. 2012, 2, 1346.