(740g) Selectivity and Activity of Iron Carbide for Propane Dehydrogenation

The importance of catalytic propane dehydrogenation (PDH) arises from both the increasing demand for propylene and the predicted decrease in propylene production from steam cracking caused by the shift to lighter hydrocarbon feedstocks due to the shale gas revolution. Current commercialized PDH catalysts are based on Pt or Cr, which are expensive and toxic, respectively. Recent 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. We use ab initio thermodynamics to identify stable Fe₃C surface terminations as a function of the reaction environment, which in turn serve as our surface models for investigating kinetic barriers during the PDH reaction. We find that carbon-rich surfaces show much higher selectivity for propylene production over competing cracking reactions compared to iron-rich surfaces, which is determined by comparing the propylene desorption barrier to the barrier for further dehydrogenation. Furthermore, d-band center analyses of surface iron atoms and crystal orbital Hamilton population (COHP) analyses of the bonds between propylene and the surface demonstrate that the high selectivity of carbon-rich surfaces originates from the disruption of surface Fe ensembles caused by the spacing effect of surface carbon. Iron and iron carbide surfaces show a trade-off relationship between activity and selectivity similar to Pt-based catalysts reported in the literature. The Fe₃C surfaces identified in this work fall in locations along this trendline that are similar to the Pt-based catalysts, confirming their potential as cheaper alternatives for PDH.