Predicting Selectivity for Ethane Dehydrogenation and Coke Formation Pathways over Model Pt–M Surface Alloys with ab Initio and Scaling Methods

5 years ago

Predicting Selectivity for Ethane Dehydrogenation and Coke Formation Pathways over Model Pt–M Surface Alloys with ab Initio and Scaling Methods

Alec Hook, Fuat E. Celik

The effects of alloying platinum with transition and post-transition metals on the kinetics and thermodynamics of dehydrogenation and coke formation pathways during light alkane dehydrogenation have been studied using density functional theory. Supported Pt catalysts are known to be active for light alkane dehydrogenation, but the high temperatures required by these endothermic reactions leads to significant coke formation and deactivation. A limited set of Pt alloys have been investigated experimentally previously, with decreases coke formation and deactivation. Using periodic density functional theory, we have investigated a wider range of Pt-alloy compositions, including metals from groups 7–15, to better understand the reduction in surface carbon formation and enhanced selectivity during ethane dehydrogenation. The post-transition metal alloys show the greatest ability to decrease the binding energy of carbonaceous species. At low alloy coverage (¹/₄ ML), these elements affect binding energies primarily through electronic effects, leaving binding geometries unaltered. A scaling relation was developed between simple CH_x species and the barriers for ethene reactions to predict selectivity for ethene desorption vs dehydrogenation. At higher alloy coverage (¹/₂ ML), geometric effects play an important role in surface–adsorbate interactions. There is no global optimum alloying element, rather the best alloy depends on the alloy coverage. PtPb and PtSb show the most promise at low and high alloy coverages, respectively. However, this work predicts that PtSn is the prevailing industrial catalyst because it shows good performance for both alloy coverages, an important property when synthetic control over precise alloy ratio in each metal particle is difficult or impractical to attain.

Publisher URL: http://dx.doi.org/10.1021/acs.jpcc.7b03789

DOI: 10.1021/acs.jpcc.7b03789