Sulfur durability of NOx storage and reduction catalyst with supports of TiO2, ZrO2 and ZrO2-TiO2 mixed oxides

This research focuses on investigating the influence of the various compositions of TiO2 and ZrO2 on the NOx removal ability over a sulfur-treated NSR catalyst. On the NSR catalyst, potassium was loaded as the NOx storage material and platinum as the precious metal, and the ZrO2 content was varied from 0 to 100 wt%. A relatively high NOx removal ability above 500 °C was obtained with the 60 to 80 wt% ZrO2 content, and the maximum value was 70 wt%, while the TiO2-rich supports were superior below 400 °C when compared to the ZrO2-rich supports. K/Pt/ZrO2 had a poor NOx removal activity over the entire temperature range. The analysis of the sulfur-aged catalysts with the supports of 70 wt%ZrO2-30 wt%TiO2, pure TiO2, and ZrO2 indicated that the TiO2 support presented a higher resistance to potassium sulfate-formation, while the ZrO2 support suppressed the solid phase reaction with potassium. The catalyst with 70 wt%ZrO2-30 wt%TiO2 retained the highest amount of remaining potassium, which was neither the formed-sulfate nor the solid-phase-reacted potassium. The sulfur-deactivation of the potassium sites could increase the activity of the metallic platinum, and a suitable combination of metallic platinum with the adequate potassium sites lead to a higher NOx removal activity for the TiO2-rich catalysts at low temperatures. In the case of the K/Pt/ZrO2 catalyst, almost all the potassium changed into sulfate, which caused a poor de-NOx ability over the entire temperature range. The support's acidity is an important factor regarding the sulfur tolerance of the NSR catalyst. The ZrO2-TiO2 catalyst containing 70 wt% ZrO2 was verified to have the highest acid amount among the sample supports, and was supposed to be the best support against sulfur-poisoning. In addition, this support contained 60 mol% ZrO2, and favorably suppressed the solid phase reaction with potassium. These properties of the ZrO2-TiO2 support containing 60 mol% ZrO2 balanced the sulfur tolerance and thermal resistance, and led to its highest NOx purification ability at high temperatures following the sulfur-aging treatment.