Low temperature NH3 regeneration of a sulfur poisoned Pt/Al2O3 monolith catalyst

Abstract

Sulfur poisoning is a ubiquitous challenge in diesel emission control. This study examines the potential for in situ, reactive regeneration of a SO_x (x = 2, 3) poisoned, low loading Pt/Al₂O₃ (0.1 wt%) washcoated monolith, representative of the catalytic oxidation function of the ammonia slip catalyst (ASC). A combination of light-off, temperature-programmed reaction, and DRIFTS experiments are conducted to characterize and understand NH₃ regeneration of the poisoned Pt catalyst. Systematic variation in the temperature ramp rate and reactive feed composition (NH₃, O₂, SO₂) for fresh and poisoned catalysts reveals significant deactivation from sulfur exposure as well as the viability of NH₃ as a regenerant. DRIFTS measurements show the accumulation of surface ammonium sulfates and bisulfates during exposure to NH₃ at 200 °C. During an ensuing temperature ramp mimicking engine warmup, the presence of NH₃ serves to regenerate the S-poisoned catalyst in a unique way. A mechanism is proposed that explains the restoration of the poisoned catalytic activity. Al₂(SO₄)₃ formed during SO_x exposure is converted to (NH₄)₂(SO₄) in the presence of NH₃ and decomposes by 350 °C during a subsequent temperature ramp, releasing NH₃ and SO₂. During a cofeed of SO₂ and NH₃ on the fresh catalyst only ammonium sulfate or bisulfate is detected. The findings suggest that NH₃ is a formidable reagent to enable conversion of thermodynamically stable aluminum sulfate to less stable ammonium sulfate, offering a potentially practical method for low temperature in situ regeneration of Pt-containing catalysts in the diesel emission control system.