(346j) DFT Insights into Propane Dehydrogenation Mechanisms over Iron Carbide

Catalytic propane dehydrogenation (PDH) has emerged as an important strategy for meeting the increasing demand for propylene caused by the shale gas revolution. The activity of Fe₃C toward PDH has been demonstrated in the literature, yet the underlying mechanism is largely unknown (e.g., Tan et al., ACS Catal. 6, 5673, 2016). In this work, we used density functional theory (DFT) to better understand why the PDH reaction is highly selective on Fe₃C surfaces. The stability of various Fe₃C surface terminations were identified as a function of the reaction environment using ab initio thermodynamics. These phases then served as our surface models for investigating kinetic barriers during the PDH reaction, where the climbing image nudged elastic band (cNEB) method was used to obtain the kinetic barriers of each reaction step. By comparing the propylene desorption barrier with the barrier for further dehydrogenation, we find that carbon-rich surfaces show much higher selectivity compared to iron-rich surfaces for propylene production over competing cracking reactions. The propylene desorption barrier was found to be critical to the overall selectivity, which is dependent on the adsorption strength of propylene. We applied a d-band center analyses of surface iron atoms and crystal orbital Hamilton population (COHP) analyses of the bonds between propylene and the surface, which demonstrates that the high selectivity of carbon-rich surfaces originates from the disruption of surface Fe ensembles caused by the spacing effect of surface carbon.