(717b) Bimetallic-Doped ZnO Sorbents for Simultaneous H2S Removal and COS Hydrolysis From Reformate Streams

Removal of both H2S and COS from reformate streams is critical for maintaining the activity of the fuel processing catalysts for the fuel cell applications. The objective of our work is developing the sorbents for an efficient, cost-effective and scalable removal of H2S and COS over the broad temperature range, without significant activity loss upon multiple regeneration cycles, and understanding the mechanism of sulfur sorption by the metal oxide-promoted ZnO-based sorbents. The bimetallic-promoted sorbents FexMnyZnO (1-x-y) supported on SiO2 and Al2O3 (M, N = Mn, Fe, Ni, Mg, Cu and 0≤x,y≤1) prepared by impregnation/calcination were studied in packed bed, with model reformate gases (1 vol% H2S, 33% CO/CO2 in H2, H2O), at room temperature to 400 C. The un-promoted ZnO/SiO¬2 sorbent shows 5-12 fold increase in H2S saturation capacity vs. commercial ZnO extrudates. The promoted FexMnyZnO(1-x-y)/SiO2 sorbents show over 90% of theoretical metal oxide utilization at room temperature. Removal of COS is difficult since COS is inactive due to its neutrality; we have studied the equilibrium COS concentrations in the presence of ZnO based sorbents and at variable temperatures and chemical composition of the reformate. At temperatures < 250 C, COS formation is effectively inhibited, but at temperatures above 250 C, significant amount of COS is formed in presence of CO2/CO and H2S. A novel layered bed approach was adopted with layer 1) performing COS hydrolysis over the supported Al2O3, followed by a layer of 2) performing high efficiency H2S removal over bimetallic-promoted supported ZnO sorbent. The role of Mn and Fe promoter cations in the ?calcined' and ?sulfided' forms of the FexMnyZnO (1-x-y)/SiO2 sorbent has been studied by the in-situ ESR, temperature dependent XPS and in-situ UV-Vis Diffuse Reflectance Spectroscopy. Based on the results, the mechanism of H2S adsorption on the promoted ZnO sorbent has been proposed, where the surface of ZnO nanocrystal is enriched in Fe ions/clusters and the ?bulk? of the nanocrystal primarily contains Mn ions/clusters.