Got Coke? Self-Limiting Poisoning Makes an Ultra Stable and Selective Sub-Nano Cluster Catalyst

Abstract

Supported sub-nano clusters hold great promise as economical and highly active catalysts. However, they tend to deactivate rapidly by poisoning and sintering, impeding their widespread use. We find that self-limiting poisoning can stabilize and promote cluster catalysis, that is, poisoning is not always detrimental but can sometimes be exploited. Specifically, Pt–Ge alloy clusters supported on alumina undergo slow and self-limiting coking (carbon deposition) under conditions of thermal dehydrogenation, modifying the cluster framework and electronic properties but preserving the Pt sites required for strong ethylene binding. For the case of Pt4Ge/alumina, theory shows a number of thermally populated isomers, one of which catalyzes carbon deposition. Because the clusters are fluxional at high temperatures, this isomer acts as a gateway, slowly converting all clusters to Pt4GeC2. The surprising result is that Pt4GeC2 is highly catalytically active and selective against further coking, that is, coking produces functional, stable catalytic clusters. Ge and C2 have synergistic electronic effects, leading to efficient and highly selective catalytic dehydrogenation that stops at alkenes and improving stability. Thus, under reaction conditions, the clusters develop into a robust catalyst, suggesting an approach to practicable cluster catalysis.

Promoter–Poison Partnership Protects Platinum Performance in Coked Cluster Catalysts

“…The prediction of Ge as a superior dopant via DFT was based on MgO as the support, which was substantiated by experiments on Al 2 O 3 , also showing that the role of dopant can be generalized across support. 7,28 Surface-supported clusters are fluxional, isomerizing rapidly under the high temperatures of real reactions such that metastable structures are thermally populated. 29 Higher energy isomers often show greater catalytic activity than the global minimum isomer.…”

“…29 Higher energy isomers often show greater catalytic activity than the global minimum isomer. 28,30,31 If the kinetic barriers for isomerization are low, then the isomer populations will be determined by a Boltzmann distribution. 32 If a cluster were to have high isomerization barriers then the ensemble would not thermally equilibrate, and macroscopic properties would be determined by the reduced set of accessible isomers, which could be useful if the prevalent isomers have superior activity.…”

“…With supporting experimental evidence from ISS and TPD data, we observe that Ge mitigates the deactivating effect of C toward ethane dehydrogenation and improves the selectivity toward ethylene desorption. In order to best match the previous experimental results, 28 which are performed under UHV surface science conditions, we have performed the DFT calculations without additional coverage of H 2 , which additionally best matches conditions for the endothermic cooling of supersonic jets, a potential application for ultraselective direct dehydrogenation catalysts in the absence of any added H 2 . While H 2 is added to the chemical feed in industrial ethane dehydrogenation to improve selectivity, here we explore changes in selectivity independent of this factor and presume conditions where added H 2 would not be present.…”

“…For the adsorbate binding, we explored C− H activation without additional coverage of H 2 in order to best match the surface science conditions of the experiment, where D 2 is observed to readily desorb from the clusters during TPD cycles. 28 The in-house codes PGOPT and GOCIA were used to run the global optimizations.…”

“…The relative intensities in He + ion scattering spectroscopy (ISS) were summarized from the series of ISS previous results for Pt 4 /alumina and Pt 4 Ge/ alumina. 28 The C atoms are mostly 3-coordinate, and there are two motifs of carbon binding; either the C 2 unit has been split or it remains intact. The Pt atoms are on the edges of the clusters, so incorporation of C into the Pt 4 cluster does not block Pt sites from interacting with gas-phase molecules.…”