Thermokinetic study of Fischer-Tropsch synthesis on Fe_2Cu_1 and FeCu surfaces with comparison to Fe(110) and Cu(111) catalysts by the UBI-QEP method

摘要

The purpose of this study is to predict the activation barriers and enthalpy for elementary steps in the process of Fischer-Tropsch (F-T) on the surfaces of Fe(110), Cu(111) and Fe/Cu alloys catalyst using ‘‘Unity Bond Index-Quadratic Exponential Potential’’ method aimed at predicting the activity and selectivity on the basis of energy criteria. The elementary steps, such as dissociation of CO, hydrogenation of carbidic carbon, C-C chain growth by insertion of CH_2 versus CO into the metal-alkyl bonds, and chain termination, which lead to hydrocarbons (alkanes versus α-olefins) or oxygenates are discussed in detail. The results show that metallic Fe(110) is necessary to produce the carbidic carbon from CO dissociation, but the synthesis of hydrocarbons and oxygenates can effectively proceed on Cu(111) surface. For optimum performance of F-T synthesis catalyst, these conflicting properties must be optimized. In this regard, we studied Fe/ Cu alloy catalyst. On all the catalyst surfaces, the energetically preferred path to initiate the alkyl chain growth is via insertion of a CH_(2,s) group into the carbon-metal bond of a CH_(3,s) group. On FeCu catalyst surface, the activation barrier for termination of alkyl chain growth by b-elimination of hydrogen is found to be lower than that for α-addition of hydrogen and consequently for this catalyst, olefins are expected to form more readily than paraffins. The results of the model for a single metal surface are in agreement with the experimental data.