(683g) Selectivity and Activity of Fe3c As a Propane Dehydrogenation Catalyst

There is a rising gap between propylene demand and production caused by the industrial shift to lighter hydrocarbon feedstocks. Catalytic propane dehydrogenation (PDH) has the potential to fill this gap. Commercialized PDH catalysts are based on Pt or Cr, which are expensive and toxic, respectively. Experimental work has demonstrated that various earth-abundant and environment-friendly metals, such as iron, can form in situ carbide phases exhibiting good activity and high selectivity for PDH (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 Fe and Fe₃C surface phases (P. Wang, T. P. Senftle. Phys. Chem. Chem. Phys. 23, 1401, 2021; P. Wang, T. P. Senftle. AIChE J. e17454, 2021). First we use ab initio thermodynamics to identify stable Fe₃C surface terminations as a function of the reaction environment. The comparison between the kinetic barriers for propylene desorption and for further dehydrogenation (i.e., leading to coke formation and catalyst deactivation) show that carbon-rich surfaces have much higher selectivity for propylene production over competing cracking reactions. Further investigation into the electronic structure of adsorbed coke precursors indicates that the high selectivity of carbon-rich surfaces is the result of the disruption of surface Fe ensembles caused by the spacing effect of surface carbon. As a result, propylene does not bind strongly on the surface and readily desorbs before further dehydrogenation. This insight provides a model for designing multi-component catalysts with binding sites that are optimized for PDH.