Investigation of Complex Iron Surface Catalytic Chemistry Using the ReaxFF Reactive Force Field Method

Abstract

To demonstrate the feasibility of classical reactive dynamics for studying complex surface chemistry, we performed a series of five reactive molecular dynamics simulations addressing the carbon monoxide methanation and the hydrocarbon chain initiation using the ReaxFF reactive force field method. We found that the catalytic surface hydrogenation initiates from the undissociated CO molecules absorbed on the surface of the catalyst as described in the oxygenate mechanism. This process leads to the generation of surface absorbed CHX– groups, which initiates the synthesis of methane and the hydrocarbon chain growth. Direct hydrogenation of the surface carbide was not observed in the simulation. Coordination analysis of the carbon atoms in the system provides possible explanations in that the surface carbon atoms are further stabilized by the surface deformation of the iron catalyst at elevated temperatures. Results from the simulations also indicated that the surface CH– could dissociate into surface carbon atoms or be further hydrogenated into CH2– radicals, which is an important intermediate species in the synthesis of methane as well as the chain initiation. Results from the C–C coupling simulation suggested the preference of coupling between CH– and CH2– groups, which agrees with the alkenyl scheme of the carbene mechanism. The overall results agree with the available experimental observations and quantum mechanics (QM) study. Furthermore, these simulations indicate the possible cooperation among different mechanisms and prove the serviceability of the ReaxFF method for studying the complex heterogeneous catalytic system. These simulations have also allowed us to evaluate the accuracy of the current ReaxFF Fe/C/O/H description, providing crucial information regarding areas where further improvement is required.