(165b) Understanding Sulfur-Induced Deactivation in Ru-Based Biomass Methanation Catalysts

Catalytic biomass processes using supercritical water and ruthenium-based catalysts have been demonstrated as a viable source of fuels such as CH₄. A key limitation in scaling up this process is the stability of the catalyst. Biomass contains trace amounts of sulfur which is highly effective in poisoning ruthenium catalysts, and economical large-scale processing will require improved methods to either prevent or regenerate sulfur-poisoned catalysts. We have conducted a density functional theory (DFT) study of sulfur-poisoned ruthenium catalysts used in biomass-SCW methanation reactions in order to get greater insight into the deactivation mechanism. This begins with an examination of the binding energies of sulfur, other common biomass impurities, and methanation reaction intermediates on surfaces with varying levels of sulfur coverage. This enables us to quantify how effective sulfur is in restricting the methanation reaction and whether it is unique among biomass impurities in this regard. Further analysis examines what specific mechanisms contribute to deactivation by sulfur adsorption. Three different mechanisms are proposed: site blocking, in which deactivating particles occupy all catalyst surface sites; coulombic interference, in which their electronic charges repel or attract reaction intermediates; and electronic structure modification, in which their binding to the catalyst surface changes the surface’s electron structure in a way that discourages further binding of other species. The extent to which each of these phenomena contribute to catalyst deactivation, and how these contributions change with increasing sulfur coverage, is explored. Finally, by thoroughly understanding the catalyst deactivation mechanism, we propose and conduct preliminary investigations into modifications that would enable the methanation reaction to proceed with either increased poisoning resistance or more efficient catalyst regeneration.