Aging effects on commercial lean NOx trap catalysts

Licentiatavhandling, 2018

As a countermeasure to global warming, vehicles with low carbon emissions are being investigated by promoting lean-burn engines and high combustion efficiencies. However, one limitation to the increasing use of these engines is the need to develop corresponding catalytic systems for controlling the nitrogen oxide (NOx) emissions in their lean exhaust. Lean NOx trap (LNT) represents a simple and cost-efficient technology for the abatement of NOx emissions from lean-burn gasoline and diesel engines. It consists of trapping NOx emissions from lean exhaust and then, before NOx slip becomes significant, the NOx stored is released and reduced to N2 under rich exhaust conditions. Although LNTs have been commercialized for some applications, the durability of LNT catalysts still remains problematic; sulfur poisoning and thermal aging are the major causes of deactivation.

In this work, commercial LNT catalysts were aged and characterized in order to elucidate the effect of aging on their performance and to establish a correlation with an accelerated aging method. A vehicle-aged catalyst was driven in the vehicle chassis dynamometer for 100 000 km, while a rapid-aged catalyst was treated at 800 °C for a period of time equivalent to a driving distance of 160 000 km. Engine dynamometer studies were performed with the purpose of testing the NOx storage and reduction performance of the LNT samples. This testing campaign was followed by running some vehicle emission cycles in the vehicle chassis dynamometer with a 2.0 l Volvo diesel vehicle with the aim of studying the catalysts’ performance under real driving conditions and monitoring the gradual deterioration of the vehicle-aged catalyst during aging. Afterwards, detailed characterization studies were carried out on a flow reactor with small cylindrical cores extracted from commercial LNT catalysts. Physicochemical characterization techniques were also employed in this investigation.

The catalyst evaluation revealed that aging resulted in a significant deterioration of NOx storage and reduction functions as a consequence of precious metal sintering, loss of surface area of the NOx storage and support compounds, phase transitions of the adsorber material, and a large accumulation of poison species, especially at the catalyst inlet. Among the aged samples examined, the middle (lengthwise) vehicle-aged sample showed the highest catalytic activity since it was exposed to both chemical poisoning and thermal degradation to a lesser extent. It was found that the oven-aged method proposed in this study is definitely a good approach to mimic the long-term catalytic activity in which the thermal stress applied is able to replicate the NO oxidation and NOx storage activity of a 100 000 km vehicle-aged catalyst.