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

• Published in: Journal of catalysis Vol. 301; pp. 38 - 45
• Main Authors: Dreher, Marian, Johnson, Benjamin, Peterson, Andrew A., Nachtegaal, Maarten, Wambach, Jörg, Vogel, Frédéric
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier Inc 01.05.2013; Elsevier; Elsevier BV
• Subjects: adsorption; biochemical compounds; biomass; Catalysis; Catalysts; catalytic activity; Chemistry; DFT; ethanol; Exact sciences and technology; EXAFS; gases; General and physical chemistry; hydrogen; In situ; Isotope labeling; Methanation; Methane; methane production; methanol; particle size; Poisoning; renewable energy sources; Ruthenium; SCWG; Sulfur; Supercritical water gasification; Supported metal catalyst; temperature; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry; X-ray absorption spectroscopy; Methanation; Ruthenium; DFT; SCWG; In situ; Poisoning; Supercritical water gasification; Supported metal catalyst; EXAFS; Isotope labeling; Water; Gaseous fuel; Reforming; Biomass; Supported catalyst; Gasification; Sulfur; Platinoid; Isotopes; Catalyst poisoning; Catalytic reaction; Transition metal; Supercritical state; Carbon; EXAFS spectrometry; Heterogeneous catalysis; Density functional
• ISSN: 0021-9517; 1090-2694
• DOI: 10.1016/j.jcat.2013.01.018

The mechanisms of the methanation reaction and sulfur poisoning of Ru/C were studied via operando EXAFS in supercritical water, combined with isotope labeling and DFT calculations. The results show that the Ru surface is only partially covered by sulfur under these conditions, leading to a change in the abundance of CHx surface adsorbates and thus to a change in catalyst reactivity and selectivity. [Display omitted] ► First structural analysis of S–Ru/C in supercritical water (SCW) via in situ EXAFS. ► Sulfur surface coverage of 40% determined by in situ EXAFS. ► Gasification in supercritical D2O leads to isotope labeling of surface adsorbates. ► Adsorbed sulfur changes abundance of CHx surface species on Ru/C. ► Adsorbed sulfur changes the selectivity of the methanation reaction. 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.