Sequential high temperature reduction, low temperature hydrolysis for the regeneration of sulfated NO_x trap catalysts

We describe a new method that minimizes irreversible Pt sintering during the desulfation of sulfated Pt/BaO/Al₂O₃ lean NO_x trap (LNT) catalysts. While it is known that the addition of H₂O to H₂ promotes desulfation, we find that the significant and irreversible Pt sintering arising from the presence of water is unavoidable. Control of precious metal sintering is considered to be one of the critical issues in the development of durable LNT catalysts. The new method described here is a sequential desulfation process: the first step is to reduce the sulfates with hydrogen only at higher temperatures to form BaS, followed by a treatment of the thus reduced sample with water at low to moderate temperatures to convert BaS to BaO and H₂S. The data showed that Pt sintering was significantly inhibited due to the absence of H₂O during the desulfation at high temperatures, and also demonstrates the similar NO_x uptake with the desulfated sample cooperatively with H₂ and H₂O. This study clearly revealed both positive and negative roles for water in desulfation processes and these factors must be considered when optimizing LNT operation.