Investigation of Complex Iron Surface Catalytic Chemistry Using the ReaxFF Reactive Force Field Method CHENYU ZOU 1,2 and ADRI VAN DUIN 1 1.—Department of Mechanical Engineering, Pennsylvania State University, University Park, PA 16802, USA. 2.—e-mail: cvz5048@psu.edu 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 undissoci- ated CO molecules absorbed on the surface of the catalyst as described in the oxygenate mechanism. This process leads to the generation of surface absorbed CH X – 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 CH 2 – radicals, which is an important intermediate species in the syn- thesis of methane as well as the chain initiation. Results from the C–C coupling simulation suggested the preference of coupling between CH– and CH 2 – groups, which agrees with the alkenyl scheme of the carbene mecha- nism. 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 service- ability of the ReaxFF method for studying the complex heterogeneous cata- lytic 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. INTRODUCTION Fischer–Tropsch (FT) synthesis 1 is an important industrial process that converts a mixture of hydrogen and carbon monoxide into varieties of hydrocarbons. Its effectiveness of producing sulfur- free fuels as well as organic chemicals of industrial importance has attracted the attention of the petroleum industry. Although widely used for nearly a century, 2 it suffers from lacking a detailed and precise reaction mechanism. Experimental observations demonstrate that the process has a strong dependence upon the choice of the catalysts in terms of the product selectivity. 3,4 Iron-based catalysts regenerate H 2 from H 2 O through a water– gas–shift (WGS) reaction and hence allow the use of feed gas with low H 2 content. In the meantime, iron favors oxygenated hydrocarbons and suffers from the accumulation of surface carbide, which intends to deactivate the catalysts. In contrast, nickel-based catalysts avoid the formation of inactive surface carbon species but suffer from relatively high price and high methane selectivity. 3 The complexity of the reaction mechanism has led to a larger number of experimental measure- ments 5–15 identifying possible reaction intermedi- ates and chemical kinetics modeling. 16–20 Key observations concerning the methanation process from early experimental studies confirmed that methylidyne (CH–), methylene (CH 2 –), and methyl (CH 3 –) groups are all intermediates during a JOM, Vol. 64, No. 12, 2012 DOI: 10.1007/s11837-012-0463-5 © 2012 TMS 1426 (Published online October 16, 2012)