Pt:Ge ratio as a lever of activity and selectivity control of supported PtGe clusters in thermal dehydrogenation

Abstract

Ethylene is a key molecule in the chemical industry and it can be obtained through the catalytic dehydrogenation of ethane. Pt-based catalysts show high performance toward alkane dehydrogenation, but suffer from coke formation and sintering that deactivate the catalyst. Ge was recently discovered to be a promising alloying element that suppresses deactivation of Pt while preserving its catalytic activity toward alkane dehydrogenation. In this work we explore the effect of the Ge content in supported PtGe cluster alloys, on the activity toward ethane dehydrogenation, selectivity against deeper dehydrogenation and coking, and sintering resistance. Our model is a tetrameric Pt cluster supported on magnesia, with varying amounts of added Ge. The phase diagram for these clusters was computed using global optimization at the density functional theory level, and under the paradigm of a statistical ensemble of many states populated by clusters at catalytic temperatures. The phase diagram shows that various Ge contents should be synthetically accessible, with Pt4Ge/MgO and Pt4Ge4 /MgO being the most likely phases. The subsequent adsorption and mechanistic studies show that the clusters with the 1:4 Ge to Pt ratio (Pt4Ge/MgO) feature the largest resistance to sintering and best selectivity in the ethane dehydrogenation toward ethylene. Clusters without Ge are too active and easily coke, whereas clusters with higher Ge content start losing the catalytic activity toward ethane dehydrogenation. Thus, Ge concentration is a lever of control of Pt cluster stability and selectivity, and of cluster catalyst design. The effect of the Ge concentration on the cluster properties is explained on the basis of the electronic structure.