Catalysis in supercritical water: Pathway of the methanation reaction and sulfur poisoning over a Ru/C catalyst during the reforming of biomolecules

Abstract

In the development of new processes that provide “green energy”, supercritical water (SCW) has emerged as a powerful reaction medium to convert biomass into combustible gases such as hydrogen or methane. Due to typical SCW catalytic process conditions (400°C, 25MPa), in situ characterization of materials and catalysts used in selective biomass conversion is difficult, and accordingly, there is limited knowledge about catalyst structure and reaction pathways under these conditions. Particularly, catalyst-poisoning mechanism by sulfur, a major obstacle in catalytic biomass conversion, needs to be understood in order to design sulfur-resistant catalysts and regeneration procedures. We followed the dynamic structural changes of a Ru catalyst during the conversion of biomass model compounds (methanol and ethanol) to methane in supercritical water in a continuous flow reactor. In situ X-ray absorption spectroscopy (XAS) showed that the catalyst is being activated by the organic compounds at low temperature without a change in particle size over 8h of operation. Combining XAS with isotope labeling and electronic structure calculations, we demonstrated that sulfur poisoning proceeds via irreversible adsorption of S2− with a surface coverage of about 40% instead of bulk sulfidation. The adsorption of sulfur significantly changes the nature and abundance of hydrocarbon adsorbates – the precursors for methane formation – on the catalyst’s surface. This affects both the activity and selectivity of the catalyst for the methanation reaction. These results provide an incentive for designing sulfur-resistant catalysts or effective regeneration procedures.