The effect gas composition during thermal aging on the dispersion and NO oxidation activity over Pt/Al2O3 catalysts

Authors: Xavier Auvray, Torben Pingel, Eva Olsson, Louise Olsson

Abstract:
The aging of a model 1 wt.% Pt/Al₂O₃ catalyst was performed stepwise under different reactive atmosphere to study the evolution of metal dispersion and NO oxidation activity. After each aging step the dispersion was evaluated by CO chemisorptions and the activity ofthe catalystfor NO oxidation was measured using 500 ppm NO and 8% O₂ diluted in Ar. After a degreening step at 500^◦C, aging was performed at 600, 700, 800 and 900 ^◦C. Five wash-coated cordierite monoliths were aged in Ar, 10% O₂, 1% H₂, 30 ppm SO₂ and 30 ppm SO₂ + 10% O₂, respectively. The general trend showed a linear decrease in dispersion when increasing the aging temperature for the lower aging temperatures and for the highest ones the dispersion levels off. When the platinum dispersion decreased the NO oxidation activity increased, due to that the reaction is structure sensitive. H2 seemed to hinder sintering at low aging temperature. Interestingly, after aging in 10% oxygen at 600^◦C the NO oxidation activity was significantly higher compared to the Ar aged sample, although the dispersions were similar. Aging in oxygen at higher temperatures resulted in a decrease of dispersion and a slightly decreasing NO oxidation activity. Moreover lower dispersion limit was reached with oxygen aging. Aging in SO₂ provoked a severe dispersion drop atlow aging temperature meanwhile the activity increased only moderately. However, activity kept increasing with further treatments at higher temperature. The combination of O₂ and SO₂ enabled to decrease rapidly the dispersion and to greatly enhance the catalytic NO oxidation activity after the first aging step at only 600 ^◦C. The best overall conversion was obtained for the catalyst treated with this mixture after aging at 800^◦C. A 22-h aging at 250^◦C in a mixture containing 500 ppm NO, 10% O₂ and 30 ppm SO₂ led to a significant decrease of Pt dispersion, which shows the ability of SO₂ to promote platinum sintering already 250 ◦C. The low temperature sintering was confirmed with STEM measurements. Several larger particles were observed, but also many small particles remained. Thus the SO₂ + O₂ induced low temperature sintering results in a large variation of particle sizes. This treatment resulted in an increase of the maximum NO conversion (after reduction of the sample) from 45% to 76%. The different aging experiments show that it is beneficial to add SO₂ during aging and the reason is the increased particle size, but also a clear chemical effect was observed.

Keywords:
NO oxidation
NO₂
Pt/Al₂O₃
DOC
Diesel oxidation catalyst
Aging
Deactivation
SO₂
Flow reactor
TEM

Published in: Applied catalysis B: Environmental  (Volume 129, Pages 1-614, 17 January 2013)

Publisher: Elsevier  

ISSN Information: 0926-3373

The effect gas composition during thermal aging on the dispersion and NO oxidation activity over Pt/Al2O3 catalysts

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