Sour water–gas shift reaction over Pt/CeO 2 catalysts

Abstract

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 H 2S present, which is a typical level in feed streams for WGS reactors in IGCC power plants. The effect of H 2S 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 = 7500 l kg −1 h −1 in an H 2S-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 H 2S addition as the Pt 2+ fraction grew from ca. 40% to 50% and the Ce oxidation states remained similar too. The bimodal pore structure of the mesoporous CeO 2 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 H 2S and subsequent formation of surface sulfates and thus responsible for lowering the WGS activity of the ceria surface in the presence of H 2S. 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.