| Popis: |
Thermalageing of the commercial selective catalytic reduction catalysts used inScania’s trucks was investigated using catalyst characterization techniques.Catalyst samples were oven-aged at 550 °C for up to 990 hours and investigatedwith nitrogen adsorption, oxygen chemisorption, X-ray fluorescence, X-raydiffraction, X-ray photoelectron spectroscopy and temperature-programmeddesorption of ammonia. The two latter methods are new to Scania and wereevaluated in depth. Furthermore, field-aged samples, which had had theircatalytic performance tested in another study, were investigated, in an attemptto find some link between characterization results and catalytic activity. Theinvestigation of oven-aged samples yielded information about the timescales ofcarrier sintering and sintering of catalytically active material, showing theformer to be much slower than the latter. It was also noted that the rate withwhich the catalyst’s ability to store ammonia decreases during thermal ageingwas similar to the rate of sintering of catalytically active material,suggesting that the loss of ammonia storage capability due to thermal ageing isrelated to sintering of the catalytically active material. X-ray photoelectronspectroscopy revealed that the fraction of vanadium in the outermost surfacelayer of the catalysts had increased during ageing. At the same time, thischaracterization technique appeared to have a low repeatability, possibly dueto the investigated catalyst having a high surface inhomogeneity. Whether ornot 500 ppm of NOx was present in the ageing atmosphere did notappear to affect the deactivation of the catalyst. Finally, no clear link couldbe found between characterization results and catalytic activity for field-agedsamples. |
