NH3 Regeneration of Sulfur Poisoned Pt/Al2O3
Sulfur poisoning is a ubiquitous challenge in diesel emission control. The commercial Ammonia Slip Catalyst (ASC) is composed of an NH3 oxidation function (Pt/Al2O3) and a selective catalytic NOx reduction function (Cu/zeolite). This report focuses on the impact of sulfur in the form of SOx (x = 2, 3) on the performance of a low loading Pt/Al2O3 (0.1 wt.% Pt/Al2O3) monolith catalyst characteristic of the oxidation function of the ASC. Sulfur that accumulates in the form of sulfates and bisulfates significantly decreases the Pt/Al2O3 catalytic oxidation activity. However, during the ensuing temperature ramp mimicking engine warmup the presence of NH3 serves to regenerate the S-poisoned catalyst. A mechanism involving sulfate formation in the form of Al2(SO4)3 at low temperature, followed by reaction with NH3 forming (NH4)2SO4 and its subsequent decomposition at elevated temperature is postulated to explain the restoration of the ASC activity. A combination of light-off experiments, temperature-programmed reaction, and DRIFTS measurements corroborate the mechanism. NH3 reacts with some of the aluminum sulfate formed during sulfur-aging process, forming ammonium sulfate. Under a temperature ramp, the ammonium sulfate decomposes, releasing NH3 and SO2, leading to the recovery of the catalytic activity. Since NH3 regeneration method works pretty well for the low loading Pt/Al2O3 catalyst, it is of interest to see if this method is applicable to high loading Pt/Al2O3 catalyst. The impact of SOx (x = 2, 3) on the activity of high loading Pt/Al2O3 is examined for the oxidations of sulfur dioxide (SO2), propylene (C3H6) and nitric oxide (NO). Flow reactor studies of the high loading Pt/Al2O3 before and after sulfation are conducted. The effectiveness of NH3 as a regenerant to remove accumulated sulfur species at low to moderate temperature is investigated. A combination of steady state reaction, temperature programmed desorption (TPD) and temperature programmed reaction (TPR), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) are conducted to characterize the poisoning and regeneration processes. Mechanistic explanations are proposed consistent with DRIFTS-identified surface species. The findings suggest a potential mitigation strategy for restoring the activity of Pt/Al2O3 in diesel emission control.