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Sulfur poisoning mechanism of steam reforming catalysts: an X-ray absorption near edge structure (XANES) spectroscopic study.

The present XANES study aims at elucidating the roles of carbon deposits and metal sulfides in the catalyst deactivation in steam reforming reactions with the presence of sulfur. CeO(2)-Al(2)O(3)-supported Ni and Rh-based catalysts were tested in steam reforming of liquid hydrocarbon fuel containing 350 ppm sulfur for H(2) production at 800 degrees C. The Rh catalyst demonstrated much better sulfur tolerance than the Ni catalyst. XANES revealed that there are various sulfur species (metal sulfide, sulfonate, sulfate and organic sulfide) on the used Ni and Rh catalysts. Metal sulfide and organic sulfide are the dominant sulfur species on the Ni catalyst whereas sulfonate and sulfate predominate on the Rh catalyst. Meanwhile organic sulfide and sulfate are also observed on the support alone. Furthermore, there are more carbon deposits formed in the presence of sulfur on both catalysts. More carboxyl groups occur on the carbon deposits formed on the same catalyst when there is no sulfur in the fuel. From correlation analysis of the amounts of nickel sulfide and carbon deposits along with the relative catalytic activity loss, we conclude that sulfur causes the initial deactivation of the Ni catalyst by metal sulfide formation in the first few hours while build-up of carbon deposits contributes mainly to the subsequent deactivation.