Platinum oxidation and sulphur deactivation in NO_x storage catalysts

摘要

Flow reactor experiments and X-ray photoelectron spectroscopy (XPS) measurements were used to investigate the importance of platinum oxide formation on Pt BaO A1_2O_3 NO_x storage catalysts during reactions conditions.The reaction studied was NO(g)+1/2 O_2(g)<=> NO_2(g).During NO_2 exposure of the catalyst the NO_2 dissociation rate decreased during the reaction.This activity decrease with time was also studied with XPS and it was found to be due to platinum oxide formation.The influence of sulphur exposure conditions on the performance of the NO_x storage catalysts was studied by exposing the samples to lean and or rich gas mixtures,simulating the conditions in a mixed lean application,containing SO_2.The main results show that all samples are sensitive to sulphur and that the deactivation proceeds faster when SO_2 is present in the feed under rich conditions than under lean or continuous SO_2 exposure.Additionally,the influence of the noble metals present in the catalysts was investigated regarding sulphur sensitivity and it was found that a combination of platinum and rhodium seems to be preferable to retain high performance of the catalyst under SO_2 exposure and subsequent regeneration.Finally,the behaviour of micro-fabricated model NO_x storage catalysts was studied as a function of temperature and gas composition with area-resolved XPS.These model catalysts consisted of a thin film of Pt deposited on one-half of a BaCO_3 pellet.It was found that the combination of SO_2 and O_2 resulted in migration of Pt on the BaCO_3 support up to one mm away from the Pt/BaCO_3 interface.