Deactivation of SCR catalysts - Impact of sulfur and the use of biofuels
In a near future, limits on CO2 emissions from vehicles will be introduced, which requires development of more fuel-efficient engines and most likely a transition towards the use of more biofuels. With the implementation of biofuels several issues could arise, one being the lack of fuel standards for these new type of fuels, leading to higher concentrations of catalyst poisons compared to conventional fossil fuels. This work specifically focuses on catalyst poisoning originating from biofuels and is based on two papers.
The aim of the work presented in paper I is to study the influence of SO2 on the low-temperature performance of a Cu-SSZ-13 SCR (selective catalytic reduction) catalyst. In particular the sulfur exposure temperature and the influence of the NO2/NOx ratio are considered, and two different regeneration temperatures are investigated. The results show that the temperature at which the Cu-SSZ-13 catalyst is exposed to SO2 is a critical parameter. The lowest exposure temperature (220°C) resulted in the most pronounced deactivation, while the highest exposure temperature (400°C) caused the lowest degree of deactivation of the catalyst. It was also shown that the exposure to SO2 resulted in decreased N2O selectivity. Engine-aging of the Cu-SSZ-13 catalyst resulted in decreased SCR activity and increased selectivity towards N2O formation, which most likely is caused by impurities from the fuel and engine-oil.
In paper II, the influence of the fuel on the functionality of a commercial vanadia-based SCR catalyst after extended field-operation is investigated. The NH3-SCR activity, NH3-oxidation activity, NH3 adsorption capacity, specific surface area and surface composition were measured before and after field-operation in two heavy-duty Euro V vehicles fuelled with fatty acid methyl ester (FAME) and hydrotreated vegetable oil (HVO), respectively. For the catalyst samples taken from the vehicle fuelled with FAME, the NH3-SCR activity, NH3-oxidation activity and NH3 adsorption capacity were significantly lower compared to the fresh sample and the samples taken from the vehicle fuelled with HVO. This is likely due to accumulation of catalyst poisons that originates from the FAME fuel that cause blocking of the active sites on the vanadia-based catalyst.
The studies of single poison compounds in lab-scale experiments are important for the understanding of catalyst deactivation mechanisms, however, there are many more parameters that dictates the deactivation in a vehicle. This can be seen from the engine-aged samples in both paper I and II where a single poison cannot fully explain the observed deactivation.