Investigation of the Deactivation of a NO_x-Reducing Hydrocarbon-Selective Catalytic Reduction (HC-SCR) Catalyst by Thermogravimetric Analysis: Effect of the Fuel and Prototype Catalyst

摘要

Diesel engines, fuels, and after treatment systems have to be optimized together to meet the targets imposed in the engine and vehicle emissions regulations, especially for paniculate matter (PM) and nitrogen oxides (NO_x). Hydrocarbon-selective catalytic reduction (HC-SCR) over Ag/Al_2O_3 catalysts is an attractive, cost-effective choice for reducing NO_x, especially in the presence of hydrogen, which can be produced on-board in a fuel reformer. However, at low temperatures, the Ag/Al_2O_3-SCR catalyst activity decays rapidly, indicating a lime period of activity loss. In this work, the catalyst deactivation process has been studied using a thermogravimetric analyzer (TGA). The effect of the space velocity, hydrogen addition, and the engine exhaust from the engine operation on gas-to-liquid (GTL). rapeseed methyl ester (RME), and ultra-low sulfur diescl (ULSD) fuels at different operating modes was investigated. In addition, the presence of a prototype oxidation catalyst located in-between the engine out and the SCR catalyst was studied, and its effect on the SCR performance was addressed. Results from the TGA confirm that the accumulation of species on the catalyst is more accentuated at low load. Fueling the engine with GTL fuel can reduce the deposition of poisoning species (i.e., carbonitrates and soot) on the catalyst surface in the less favorable conditions (low temperature and low load). The incorporation of the prototype catalyst in front of the SCR catalyst, although it improved the NO_x reduction reaction over the Ag/Al_2O_3 catalyst, did not significantly alter the deposition of possible poisoning species on the catalyst surface. Similarly, the hydrogen addition in the exhaust gas upstream from the SCR catalyst improved the NO_x reduction without significantly affecting the deposition of such species on the catalyst.