Abstract
In the present work, a series of different materials was investigated in order to enhance the understanding of the role of modern lean NOx trap (LNT) components on the sulfur poisoning and regeneration characteristics. Nine different types of model catalysts were prepared, which mainly consisted of three compounds: (i) Al2O3, (ii) Mg/Al2O3, and (iii) Mg/Ce/Al2O3 mixed with Pt, Pd, and Pt-Pd. A micro flow reactor and a diffuse reflectance infrared Fourier transform spectrometer (DRIFTS) were employed in order to investigate the evolution and stability of the species formed during SO2 poisoning. The results showed that the addition of palladium and magnesium into the LNT formulation can be beneficial for the catalyst desulfation due mainly to the ability to release the sulfur trapped at relatively low temperatures. This was especially evident for Pd/Mg/Al2O3 model catalyst, which demonstrated an efficient LNT desulfation with low H2 consumption. In contrast, the addition of ceria was found to increase the formation of bulk sulfate species during SO2 poisoning, which requires higher temperatures for the sulfur removal. The noble metal nature was also observed to play an important role on the SOx storage and release properties. Monometallic Pd-based catalysts exhibited the formation of surface palladium sulfate species during SO2 exposure, whereas Pt-Pd bimetallic formulations presented higher stability of the sulfur species formed compared to the corresponding Pt- and Pd-monometallic samples.
Highlights
Stringent exhaust gas emission and fuel economy regulations, such as the European RealDriving Emission (RDE) and US Tier 3, have encouraged the development of advanced lean burn gasoline and diesel engines for their improved fuel economy and reduced greenhouse gas emissions.The implementation of these engines are, constrained by the need of catalytic systems in their lean exhaust to control the nitrogen oxide (NOx ) emissions
In order to investigate the stability of the formed nitrates, NO2 -temperature programmed desorption experiments were conducted over all samples according to the experimental procedure described earlier
Mg-Al2O3 samples show a similar Temperature Programmed Desorption (TPD)-profile with an additional peak located the TPD step is slightly lower than the total NOx adsorbed during NO2 adsorption and Ar flushing around 380 °C, ascribed to the decomposition of NOx stored on magnesium sites [26]
Summary
Stringent exhaust gas emission and fuel economy regulations, such as the European Real. Lean NOx trap is an effective NOx reduction strategy able to attain high NOx conversions over a wide range of temperatures without the need of additional on-board reductants [1,2] It can be used in combination with SCR catalysts to achieve low NOx emissions. Sulfates formed on the NOx storage sites have higher thermal stability than nitrates and nitrites and, their decomposition needs very high temperatures and alternating rich/lean exhaust mixtures [8,9]. The sulfur tolerance of modern LNT catalysts should be improved by replacing the NOx trapping material with a component with lower affinity towards sulfation, and with enhanced desorption of sulfur species during regeneration. Nine different types of model catalysts were prepared, which mainly consisted of three compounds: (i) Al2 O3 , (ii) Mg/Al2 O3 , and (iii) Mg/Ce/Al2 O3 mixed with Pt, Pd, and Pt-Pd
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