Subnanometer-sized Pt/Sn alloy cluster catalysts for the dehydrogenation of linear alkanes

Abstract

The reaction pathways for the dehydrogenation of ethane, propane, and butane, over Pt are analyzed using density functional theory (DFT). Pt nanoparticles are represented by a tetrahedral Pt 4 cluster. The objectives of this work were to establish which step is rate limiting and which one controls the selectivity for forming alkenes as opposed to causing further dehydrogenation of adsorbed alkenes to produce precursors responsible for catalyst deactivation due to coking. Further objectives of this work are to identify the role of adsorbed hydrogen, derived from H 2 fed together with the alkane, on the reaction pathway, and the role of replacing one of the four Pt atoms by a Sn atom. A comparison of Gibbs free energies shows that in all cases the rate-determining step is cleavage of a C-H bond upon alkane adsorption. The selectivity to alkene formation versus precursors to coking is dictated by the relative magnitudes of the activation energies for alkene desorption and dehydrogenation of the adsorbed alkene. The presence of an adsorbed H atom on the cluster facilitates alkene desorption relative to dehydrogenation of the adsorbed alkene. Substitution of a Sn atom in the cluster to produce a Pt 3 Sn cluster leads to a downward shift of the potential energy surface for the reaction and causes an increase of the activity of the catalyst as suggested by recent experiments due to the lower net activation barrier for the rate limiting step. However, the introduction of Sn does not alter the relative activation barriers for gas-phase alkene formation versus loss of hydrogen from the adsorbed alkene, the process leading to the formation of coke precursors.

Effect of Direct Reduction Treatment on Pt–Sn/Al2O3 Catalyst for Propane Dehydrogenation

“…The Pt catalyst used in the PDH process is often impregnated with Sn as a co-catalyst [7][8][9][10][11]. Because the Pt-alkene interaction is stronger than the Pt-alkane interaction, unwanted side reactions such as hydrogenolysis and isomerization often occur [12].…”

Effects of composition and metal particle size on ethane dehydrogenation over PtxSn100−x/Mg(Al)O (70⩽x⩽100)

“…The addition of a second metal, such as tin, indium, or gallium, to platinum has been found to be effective in increasing ethene selectivity and suppressing coke formation [6,[8][9][10][11]. Both geometric and electronic effects of the second metals have been proposed to explain their roles in modifying the catalyst surface and changing the surface chemistry involved in ethane dehydrogenation [12][13][14][15][16][17][18][19].…”

Recent computational studies on transition‐metal carbon–hydrogen bond activation of alkanes

“…While this aspect has been extensively investigated in terms of both experimental and theoretical methods, one concern is the issue of low alkene selectivity with respect to further dehydrogenation leading to the coking problem that the Pt catalysts suffered from in industry [17a,b] . To this end, Hauser et al reported that, with tetrahedral Pt 4 clusters, alkene desorption can be favored over further dehydrogenation of the adsorbed alkene when either H is absorbed on the cluster or substituting a Pt atom in the cluster with a Sn atom . This work provides good agreement with the superior selectivity and high atom efficiency when alloying Pt with Sn or adding hydrogen to the alkane feed…”