Yongsheng Chen, Pennsylvania State University
New Insight into Sulfur Poisoning of Steam Reforming Catalysts by XANES

Abstract

Catalysts for steam reforming of hydrocarbons can deactivate due to carbon deposition and sintering. In addition, most hydrocarbon fuels contain ppm levels of sulfur impurities (collectively called sulfur), and the presence of sulfur can lead to faster and more significant catalyst deactivation (a phenomenon known as sulfur poisoning). It has been believed for decades that sulfur poisons the catalysts through the formation of metal sulfides, however, lacking of direct observations of sulfur species on the deactivated catalysts has been a major drawback. One main reason is that very low levels of sulfur can readily deactivate the catalyst, and consequently there is only very little sulfur remaining on the catalyst after it is recovered from a reaction. Thus, direct measurement of sulfur species on deactivated catalysts has been a challenge to most conventional analytical techniques. 

We used x-ray absorption near edge structure (XANES) spectroscopy to examine sulfur poisoned Ni and Rh catalysts. Four major sulfur species were identified including metal sulfide, organic sulfide (sulfur compound with -C-S-C- bond), sulfonate (-RSO₂O-) and sulfate (SO₄^2-). Sulfur chemistry was very different on the Ni catalyst than the Rh catalyst. Catalyst deactivation due to sulfur poisoning did not correlate with the amount of metal sulfides, instead it had a strong correlation with carbon deposition. Furthermore, carbon species and their surface functional groups were also measured using XANES. The result suggests that sulfur poison suppressed the formation of carboxyl groups on carbon surface thus made carbon species less reactive, as a result more carbon deposited on the catalyst surface. XANES analyses concluded that sulfur not only interacts with the catalyst and forms multiple surface species, which causes some loss in catalyst activity; more importantly, it also affects carbon formation and removal processes, leading to more carbon deposition and therefore significant catalyst deactivation.