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.