Impact of Vanadium Loading and Thermal Aging on the Surface Properties of Titania-Supported Vanadium Oxide

A series of VOx/TiO2 model catalysts with 0.5, 1.5, 2.0, 4.0, and 8 wt % V was prepared by incipient wetness impregnation (fresh) and then thermally treated at 580 °C for 100 h in static air (aged). Each catalyst was characterized with nitrogen physisorption, ammonia temperature-programmed desorption, X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy, and in situ diffuse reflectance infrared Fourier transform spectroscopy upon adsorption of NH3 and NO. The fresh catalyst with 0.5 wt % V hosts monomeric VOx as the majority species. The fresh catalyst with 1.5 wt % V contains both mono- and polymeric VOx, whereof the latter becomes the majority species after aging. Polymeric VOx is the main species in the fresh 2 wt % V catalyst; however, upon aging, V2O5 is detected. The fresh 4 wt % V catalyst contains polymeric VOx and V2O5 species, whereas the fresh 8 wt % V catalyst has mostly crystalline V2O5. Upon aging, both of these catalysts show crystalline V2O5 as the majority species. Further, all catalysts expose V4+. Adsorption of NH3 reveals terminal and bridged hydroxyl groups as well as monomeric and polymeric Brønsted sites, which shift with increasing vanadium loading and aging to more bridged hydroxyls and polymeric Brønsted sites. As for the NO adsorption, the relative abundance of surface nitrates and NO2 changes with the increased vanadium favoring bridge-bound nitrates on crystalline V2O5. Vanadia appears to promote the morphological changes and phase transitions of titania. The NOx conversion during standard SCR conditions was measured in a chemical flow reactor, showing that high V loadings are beneficial for the low-temperature NOx conversion at the expense of low selectivity at higher temperatures and low efficiency after aging. On the contrary, catalysts with lower V loadings reveal an improved NOx conversion after aging. Normalization by V loading, V surface density, specific surface area, and ammonia uptake suggests polymeric VOx to be the most active species and that SSA and ammonia uptake are less important design parameters for stationary conditions.