Evidence of Scrambling over Ruthenium-based Catalysts in Supercritical-water Gasification

Catalytic processes that employ Ru catalysts in supercritical water have been shown to be capable of converting organics, such as wood waste, into synthetic natural gas (CH₄) with high efficiencies at relatively moderate temperatures of around 400°C. However, the exact roles of the catalyst and the descriptors that would enable the search for other catalysts with high conversions have not been determined. In the current work, we use electronic structure calculations coupled with batch experiments to understand the interaction of methane (CH₄) and water (H₂O) with a common catalyst material, ruthenium, to understand the final steps of the methanation reaction. The calculations predict that when CH₄ and H₂O react with the Ru surface, the species will undergo rapid scrambling; interchanging most of the hydrogens with the surface before escaping as CH₄ and H₂O once again. We conducted experiments using CH₄ as a feedstock in supercritical D₂O (deuterated water) in the presence of a commercially available carbon-supported Ru catalyst, and found this mechanism to be confirmed: nearly all reacted CH₄ was converted to the fully substituted CD₄ or the 3/4-substituted CHD₃ isotopomers, with less significant production of the 1/4- or 1/2-substituted species CH₃D and CH₂D₂. The experiment was repeated with an in-house impregnated RuO₂-on-carbon catalyst, with similar results. Although other criteria such as the ability to cleave C, C and C, O bonds and resistance to poisoning will also prove important, this study suggests that a characteristic of an effective catalyst for supercritical water gasification to methane is its ability to promote rapid equilibria through scrambling mechanisms.