(82f) Rare Earth Sulfides as High-Temperature Regenerable Sulfur Adsorbents and Catalysts for the Water-Gas Shift Reaction

The ever increasing global energy demand, scarcity of resources, and the need for cleaner and greener power generation drive much of the advanced energy research of our time. The ultimate goal is the transition from the fossil fuel economy to the hydrogen economy in the shortest possible time. Fuel cell technologies have been proposed but due to the lack of infrastructure for hydrogen distribution and storage, there is great interest in using liquid fuels (such as diesel and jet fuels) as hydrogen sources. Removal of H2S from the fuel gas is necessary in the above energy generation systems to meet the rigorous environmental regulations for sulfur emissions and prevent fouling of the anode materials in the fuel cell.

Single or mixed lanthanide (La, Pr) oxides have been used for non-regenerative high temperature desulfurization because of their excellent sulfidation equilibria, which allows sub-ppmv H2S concentration levels to be achieved at temperatures as high as 800oC. The sulfur capacity for 1ppmv breakthrough of H2S exceeds 50 mg S/gsorbent and in some cases we have found it to be ~70 mg S/gsorbent. Regenerative desulfurization is also possible with these sorbents, if one uses just the sorbent surface capacity [1]. Accordingly, adsorbed H2S on the oxysulfide surface is purged during regeneration and recovered downstream. This avoids adverse structural issues and limited regenerability typical of bulk sulfide sorbent regeneration.

Lanthanide oxides, especially ceria, are used not only as sorbent materials for high temperature removal of H2S, but also as the primary support for Pt, Pd, Cu, Ni, and Au for low- and high-temperature redox reactions, such as the WGS reaction. The sulfur capacity of presulfided samples, along with their activity for the reverse water-gas shift reaction, present a new potential of these materials as sulfur-resistant catalysts for the high temperature water-gas shift applications.

References

1. Flytzani-Stephanopoulos, M., M. Sakbodin, and Z. Wang, Regenerative adsorption and removal of H2S from hot fuel gas streams by rare earth oxides. Science, 2006. 312(5779): p. 1508-1510.