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

Abstract

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.

Ab Initio Multiscale Process Modeling of Ethane, Propane and Butane Dehydrogenation Reactions: A Review

“…Although there have been several studies of ethane dehydrogenation over Pt and Pt-based alloys employing DFT [23][24][25][26][27], they remain confied to monoscale modeling. Coupling energetics with kinetics has proved to be a much more challenging task, which most research omits.…”

Structural trends in the dehydrogenation selectivity of palladium alloys

“…Theoretical studies of dehydrogenation reactions, using density functional theory, have focused mainly on Pt and Pt alloys. [18][19][20][21][22][23][24][25][26] Early mechanistic studies on Pt 3 Sn alloys 18,27 showed that a simple thermodynamic selectivity descriptor (dened as the difference between the propylene desorption and the activation energy barrier of the rst deep dehydrogenation reaction) correlates with the observed higher selectivity of Pt-Sn alloys as compared to pure Pt. This descriptor has been used to computationally estimate olen selectivities for many alloy of Pt.…”

“…This descriptor has been used to computationally estimate olen selectivities for many alloy of Pt. 21,22,26 In addition, recent work with microkinetic model analysis by Saerens et al 23 on Pt (111) has shown that, along with the propylene dehydrogenation step, C-C bond breaking of propyne, which is a deeply dehydrogenated derivative of propylene, is also one of the relevant steps for byproduct formation and thereby adversely affects the selectivity of propylene production. Together, these studies suggest that both the simple selectivity descriptor, comparing desorption and further dehydrogenation of propylene, and C-C bond breaking of deeply dehydrogenated species can be important for prediction of selectivity trends across alloys.…”

First principles micro-kinetic model of catalytic non-oxidative dehydrogenation of ethane over close-packed metallic facets

“…[2] Hook and Celik calculated the trends in the free energy landscape of the dehydrogenation intermediates on platinum alloy FCC(111) facets and showed that a single reactivity parameter can predict selectivity enhancement relative to platinum while varying the ligand. [18] Xu et al presented a screening study of a broader set of alloys using the difference between the C 2 H 4 desorption transition state energy and the CH 3 CH 2 dehydrogenation transition state energy as a descriptor for selectivity. [51] Contrary to Xu et al, we will show that the desorption transition state is not a relevant descriptor in low conversion conditions, but that the different CH 3 CH 2 dehydrogenation transition states are.…”