Mechanistic Investigations of Heterogeneously Catalyzed Steam Gasification of Coke Precursors

Atomic modeling was conducted to mechanistically investigate the CsOH steam gasification catalyst coating that has been successfully demonstrated to eliminate coke deposits during high temperature fuel pyrolysis. This effective coke mitigation was interpreted from the atomic modeling results to be due to the multiple functionalities of the CsOH coating for blocking the underlying metal surface from catalyzing coke formation, preventing deposition of coke-forming precursors and products, and catalyzing the oxidation of coke precursors in the presence of water. The discovery that the CsOH(010) surface was only predicted to strongly interact with hydrocarbon radicals that bombard surfaces during hydrocarbon pyrolysis led to the creation of novel reaction mechanisms for the effective steam gasification of radical coke precursors. The CsOH(010) surface was predicted to locally rearrange and form vacancies to facilitate the decomposition and oxidation of adsorbed hydrocarbon radicals. Two CsOH(010) heterogeneously catalyzed steam gasification reaction mechanisms were proposed involving hydrocarbon radical and H₂O coreactants for the decomposition and oxidation of a methyl adsorbate. The first “H vacancy mechanism” oxidized a methyl radical through a bimolecular reaction with H₂O. In the second “Cs insertion mechanism,” the adsorbed methyl radical was oxidized directly by reduction of the CsOH surface. The resulting OH vacancy was refilled by H₂O dissociation, in order to restore the surface reaction site. This latter mechanism was more energetically downhill overall and had a modest rate-limiting energy barrier that could be easily overcome during fuel pyrolysis at high temperatures. These mechanisms are consistent with the experimentally observed stability of the CsOH coating, which functions as a true catalyst that is not consumed or dissolved over time. Observations of the CsOH coating behavior over a range of temperatures supported the hypothesis that effective coke mitigation functionality is the result of a dynamic balance between steam gasification of coke precursors arriving at the surface and the removal of already accumulated deposits.