Sour water-gas shift reaction over Pt/CeO2 catalysts

Ceria-supported catalysts with 0.38-2.0 wt.% Pt have been studied under high-temperature water-gas shift (WGS) conditions with and without 20 ppm H2S present, which is a typical level in feed streams for WGS reactors in IGCC power plants. The effect of H2S on CO conversion decreased with increasing temperature in the range 573-723 K and also with increasing Pt content. A 1 wt.% Pt catalyst exhibited excellent stability during a 300 h test at 673 K and GHSV = 75001 kg(-1) h(-1) in an H2S-charged feed with steam-to-CO ratio 2 as CO conversion remained stable around 73%. This was still close to the corresponding thermodynamic limit of 78.8%, which was reached under sulfur-free conditions. The Pt ionization degree in WGS-tested catalysts increased only slightly with the H2S addition as the Pt2+ fraction grew from ca. 40% to 50% and the Ce oxidation states remained similar too. The bimodal pore structure of the mesoporous CeO2 was also retained after WGS experiments although the effective surface area had declined by nearly 20% following the 300 h test in sour syngas. This and the concomitant decline of surface OH groups were likely caused by adsorption of H2S and subsequent formation of surface sulfates and thus responsible for lowering the WGS activity of the ceria surface in the presence of H2S. We propose that Pt centers can mitigate this effect through hydrogen spillover in their vicinity thus maintaining a high WGS activity of these catalysts in sour syngas. (C) 2011 Elsevier B.V. All rights reserved.