Lead poisoning and regeneration of Mn-Ce/TiO₂ catalysts for NH₃-SCR of NO_x at low temperature

The effect of lead on the catalytic performance of Mn-Ce/TiO₂ catalysts in the selective catalytic reduction (SCR) of NO_x with ammonia at low temperature was investigated; with the help of nitrogen sorption, XRD, FT-IR spectroscopy, H₂-TPR and NH₃-TPD characterization, the causes of lead poisoning and acid regeneration were clarified. The results indicate that the doping of Pb in Mn-Ce/TiO₂ leads to a significant decrease of the low-temperature SCR activity; with a Pb loading of 11%, the conversion of NO over Mn-Ce/TiO₂ at 180 °C decreases from original 100% on the fresh catalyst to 44% on the Pd-poisoned catalyst. The presence of Pb may reduce the content of active Mn⁴⁺ and Ce³⁺ species on the Mn-Ce/TiO₂ catalyst, which suppresses the redox cycle of Mn⁴⁺ + Ce³⁺ ↔ Mn³⁺ + Ce⁴⁺; moreover, the decrease of surface acidity on the Mn-Ce/TiO₂ catalyst by the doping of Pb is also disadvantageous to the adsorption and activation of NH₃. The Pd-poisoned Mn-Ce/TiO₂ can be regenerated by nitric acid treatment; after the regeneration, the catalytic activity of Mn-Ce/TiO₂ in NH₃-SCR of NO is almost completely recovered and even exceeds that of the fresh catalyst at 80–150 °C. The nitric acid treatment can restore the redox capacity of Mn-Ce/TiO₂, increase the surface area, and create new acid sites, which contribute to recovery of the activity of Pb-poisoned Mn-Ce/TiO₂ catalyst in NH₃-SCR.