Modified Vanadium Oxide Catalysts for the Selective Catalytic Reduction of NOx

Doktorsavhandling, 2025

Combustion processes in diesel engines are expected to play a significant role in the foreseeable future, especially in heavy-duty vehicles. In response to increasingly stringent legislations aimed at reducing emissions of nitrogen oxides (NO_x), considerable efforts have been devoted to developing effective technologies to reduce NO_x emissions. Among them, the selective catalytic reduction (SCR) with ammonia (NH₃) is the most established method for both stationary and mobile sources.
To date, titania-supported vanadium oxides (VO_x/TiO₂) are the most widespread SCR catalyst type in use. However, the application in vehicles presents several challenges due to the dynamic conditions. Both low-temperature activity and high-temperature stability are crucial due to temperature fluctuations in the exhaust gas. Catalyst development has focused on enhancing activity by modification of the active VO_x species and the support.

In this work, the effects of the vanadium loading and catalyst modification were investigated, with a particular emphasis on the behavior after thermal stress, representing a critical factor in the context of mobile source applications. Promising results have been achieved through the modification with oxides of cerium (Ce), niobium (Nb), and antimony (Sb), demonstrating the optimization potential of V-based SCR catalysts. The results showed that the Ce-modification results in an improved low-temperature activity, while the incorporation of Nb contributes to a higher thermal stability. Remarkable catalyst activation was observed after aging upon Sb-modification. The addition of modifiers enhanced the stability by limiting sintering processes of both the TiO₂ and VO_x species. Infrared spectroscopic methods allowed the identification of surface VO_x species, surface hydroxyl groups, and adsorbed surface species of the reactants and products (NH₃, NO, H₂O). Further, the decomposition of the NO adsorption spectra enabled a comparison of the surface species ratio among the samples and provide potential for future in-depth spectroscopic studies. This thesis encourages further exploration of VOx/TiO₂ catalysts to improve fundamental knowledge about catalyst design in the NO_x emission control and beyond other applications.