Semianalytical Treatment of Solvation for Molecular Mechanics and Dynamics

sorption results9 revealed that the iron surface was mostly covered by promoter oxides of AI, Ca, and K. Postreaction XPS results also revealed a C( Is) XPS peak of weak to moderate intensity centered at 284.1-283.7 eV. This binding energy approaches those (ca. 283.5 eV) reported for iron cat bide^.^^*'^ More convincing evidence for carbide formation was obtained from TPHT results collected after reaction studies like those displayed in Figure 1 in which methane was the only product. After reaction at temperatures below 340 OC, only small amounts of reactive carbon could be distinguished with maximum methane desorption rates near 300 OC. However, for higher reaction temperatures, large amounts of methane were produced with a maximum rate just above 400 OC. Since XPS results revealed only small amounts of carbonaceous residue on top of the catalyst surface, this reactive carbon must be associated with carbiding of the catalyst. Consequently, it appears that the active carbon incorporation catalyst is carbided iron. This conclusion is well supported by bulk carbon to iron stoichiometries of 0.1-0.25 estimated from the TPHT peak areas which were adequate to represent 40-60'36 conversion to bulk carbides such as Fe,C or FeSC2. Moreover, preliminary results from studies using bona fide iron carbides have shown similar catalytic b e h a ~ i o r . ~