Development of a ReaxFF Reactive Force Field for Fe/Cr/O/S and Application to Oxidation of Butane over a Pyrite-Covered Cr₂O₃ Catalyst

We developed a ReaxFF force field for Fe/Cr/O/S, which is parametrized against data from quantum mechanical (QM) calculations. Using this force field, we studied the Cr-oxide catalyzed oxidation reaction of butane at 1600 K. Our simulation results demonstrate that the active oxygen species on the oxide surface play an important role in the conversion of butane. Dehydrogenation of butane, which is found to be catalyzed by oxygen species on the oxide surface, initiates the reaction and generates butane radicals and surface OH groups. The radical intermediates are associated with the oxygen atoms to form C-O bonds or make double bonds when neighboring carbon atoms are dehydrogenated, forming light alkenes. On the clean Cr-oxide, the major oxidation product is CH₂O. The presence of iron pyrite (FeS₂), a common inorganic component in coal-derived fuels and a major slagging component, on Cr-oxide accelerates the complete oxidation of butane forming CO₂ and CO. Surface reconstruction by iron pyrite is probably responsible for the change of the catalytic behavior. Reoxidation of the reduced oxide surface can occur through removal of surface H₂O and adsorption of gaseous molecular oxygen at the vacancy sites on the clean Cr-oxide. On the other hand, on the modified Cr-oxide, it is found that a considerable amount of SOH molecules are released from the surface. These results can provide the detailed mechanisms for the catalytic oxidation of alkane and product distributions in Cr-oxide catalyst and give, for the first time, atomistic-scale insight in the complex surface chemistry of these catalysts under realistic operating conditions.