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. This work explores 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. The model proposed herein 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.

Deposited PtGe Clusters as Active and Durable Catalysts for CO Oxidation**

“…Apparently, this concentration dependence is very reaction‐specific. Our results suggest that Pt 4 Ge 3 composition has the most adequate reactivity for CO oxidation, in contrast to dehydrogenation reactions where Pt 4 Ge outperforms Pt 4 Ge 3 [33] …”

“…Before moving to the discussion of the CO reaction mechanisms we first study the adsorption of the reactants, viz ., CO and O 2 , on Pt 4 Ge n /MgO with n=0–4. These surface‐mounted nanoclusters have been recently characterized and are predicted to be synthetically accessible under Ge‐rich conditions [33] . For each composition, various conformers would coexist since they are energetically competitive, except for Pt 4 Ge in which a single isomer, the global minimum (GM), dominates the population.…”

“…For each composition, various conformers would coexist since they are energetically competitive, except for Pt 4 Ge in which a single isomer, the global minimum (GM), dominates the population. Note that, while all the bimetallic clusters have a closed‐shell electronic structure, Pt 4 /MgO possesses unpaired electrons in most cases [33] . Because isomers can exhibit different reactivity, CO and O 2 were adsorbed on the most populated conformers of each Pt 4 Ge n composition.…”

“…Germanium has been demonstrated to be a promising promoter for Pt. In the past, it has been used to boost the performance of Pt catalysts in a variety of reactions such as naphtha reforming [27–29] and alkane dehydrogenation [30–36] . Similarly, Pt−Ge catalysts were shown to exhibit an enhanced performance for the methanol electrooxidation [37] .…”

“…Previously, we showed that alloying small‐size Pt clusters with Ge significantly diminishes the CO poisoning, as the interaction between Pt and CO is weakened upon alloying [42,43] . Furthermore, the composition in such bimetallic catalysts might have a great influence in their physico‐chemical properties [33,43] …”