First Principles Micro-kinetic Model of Catalytic Non-oxidative Dehydrogenation of Ethane over Close-packed Metallic Facets

Catalytic dehydrogenation of light alkanes may offer more efficient routes to selectively producing light olefins, which are some of the most important chemical building blocks in the industry, in terms of scale. We present a descriptor based micro-kinetic model of the trends in selectivity and activity of the non-oxidative dehydrogenation of ethane over the close-packed metal facets and through varied thermodynamic conditions. Our model predicts and explains the experimentally observed promotion effect on turnover rate from co-feeding hydrogen as an effect of the shifting equilibria in steady state. Our model predicts platinum group metals to have the most intrinsically active close-packed facets for ethane dehydrogenation and adding small amounts of tin to platinum is predicted to promote selectivity through a purely electronic effect. The trend in selectivity is understood through linear scaling relations and it is due to the re-action mechanism predicted by the model, which shows the path to ethene going through ethane dehydrogenation to ethyl, CH3CH2*, then to ethene. The non-selective pathway to methane and deeply dehydrogenated species is predicted to go through dehydrogenation via CH3CH*. This implies that the desorption step of ethene is not the limiting step for selectivity and that geometric effects that stabilize CH2CH2* compared to CH3CH* are desirable properties of a better catalyst. Removing reactive bridge and 3-fold sites facilitates this, which may be achievable by sufficient concentrations of tin in platinum. The included model code furthermore provides a base for easy tuning and reproduction and for expanding the study to other thermodynamic conditions, other facets, alloys or the reaction network to longer hydrocarbons or to oxidative pathways.