Impact of Vanadium Loading and Thermal Aging on the Surface Properties of Titania-Supported Vanadium Oxide
Alexander Nellessen, Andreas Schaefer, Anna Martinelli, Agnes Raj, Andrew D. Newman, Per‐Anders Carlsson
Abstract
High Resolution Image Download MS PowerPoint Slide A series of VO x /TiO 2 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 NH 3 and NO. The fresh catalyst with 0.5 wt % V hosts monomeric VO x as the majority species. The fresh catalyst with 1.5 wt % V contains both mono- and polymeric VO x, whereof the latter becomes the majority species after aging. Polymeric VO x is the main species in the fresh 2 wt % V catalyst; however, upon aging, V 2 O 5 is detected. The fresh 4 wt % V catalyst contains polymeric VO x and V 2 O 5 species, whereas the fresh 8 wt % V catalyst has mostly crystalline V 2 O 5 . Upon aging, both of these catalysts show crystalline V 2 O 5 as the majority species. Further, all catalysts expose V 4+ . Adsorption of NH 3 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 NO 2 changes with the increased vanadium favoring bridge-bound nitrates on crystalline V 2 O 5 . Vanadia appears to promote the morphological changes and phase transitions of titania. The NO x conversion during standard SCR conditions was measured in a chemical flow reactor, showing that high V loadings are beneficial for the low-temperature NO x 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 NO x conversion after aging. Normalization by V loading, V surface density, specific surface area, and ammonia uptake suggests polymeric VO x to be the most active species and that SSA and ammonia uptake are less important design parameters for stationary conditions.