Desulafation Dynamics of NOx Storage Catalysts 2002-01-2886
The dynamics of the desulfation of a Ba-containing and a K-containing NOx storage catalyst have been investigated. When both catalysts were desulfated using a temperature ramp in exhaust that simulated gasoline exhaust with a 13:1 A/F, the maximum desulfation rate for the Ba-containing catalyst was seen at 620°C, while the maximum for the K-containing catalyst was at 760°C. This is consistent with the widely known fact that K2SO4 is more stable than BaSO4.
The BaSO4 decomposed when either hydrogen or water was in the feed, but not when both were absent. The decomposition, therefore, requires hydrogen to be present and the water can provide sufficient hydrogen for the decomposition via the water-gas shift reaction. With either water or hydrogen in the uncycled feed, the primary sulfur compound formed from the decomposition was H2S for both the Ba and K-containing catalysts.
Cycling the feed increased the amount of sulfur released as SO2 from the Ba-containing catalyst, but was not as effective in removing sulfur as continuous rich operation. Cycling the feed to the K-containing catalyst greatly reduced the desulfation of the catalyst without significantly increasing the sulfur release as SO2.
Thermodynamic calculations of the desulfation reactions with CO or H2 are consistent with the experimental results. The sulfates (K2SO4, BaSO4, and Ce2(SO4)3) are far less stable in H2 than in CO and their decomposition to form H2S is clearly favored over their reactions with CO to form SO2. Among these sulfates, Ce2(SO4)3 is the least stable and is the only one that decomposes in the absence of hydrogen.