Platinum cluster size has a significant influence on the activity, selectivity, and stability as well as the reaction mechanism during propane dehydrogenation (PDH). Wellcontrolled platinum catalysts of different cluster sizes are prepared by a seed growth method and supported on calcined hydrotalcite. The Pt catalysts show strong structure-sensitive behavior both in the C−H bond activation of propane and in the C−C bond activation to yield ethylene, methane, and coke. The Pt clusters of small cluster sizes, with (211) dominating on the surface, have a lower dehydrogenation energy barrier and thus higher activity. However, large Pt clusters with Pt(111) dominating result in a weakened binding strength of propylene and an increased energy barrier for the activation of C−H bonds in propylene, which leads to higher selectivity toward propylene by lowering the possibility of deep dehydrogenation. Kinetic analysis illustrates that the reaction order in hydrogen decreases and activation energy increases with an increasing Pt cluster size. Combined with density functional theory calculations and isotope effect experiments, it gives strong evidence of the change in reaction mechanism with Pt cluster size. It suggests that on small Pt clusters that are mostly surrounded by undercoordinated surface sites, the first C−H bond activation is likely to be the rate-determining step, while the second C−H bond activation is kinetically relevant on large Pt particles with terrace sites dominating.
The structure–performance relationship is a critical fundamental issue in heterogeneous catalysis, and the size-dependent structure sensitivity of catalytic reactions has long been researched in catalysis. Yet it remains elusive for most of the reactions in a full-size range, from a single atom and subnanometer clusters to nanoparticles. Herein, we report complete size dependence of Pt catalysts used in propane dehydrogenation in terms of activity, selectivity, and stability due to coke formation. The turnover frequency (TOF) of the atomically dispersed Pt/Al2O3 catalyst was approximately 3-fold and 7-fold higher than the subnanometer-sized clusters and the nanoparticles, respectively. A canyon- shaped size dependence of the propene selectivity was observed with a bottom at about 2 nm of Pt particle size. The subnanometer-sized clusters have opposite size dependence of the propene selectivity compared to nanoparticles. Both atomically dispersed Pt and large Pt nanoparticles possess high propene selectivity. The atomically dispersed platinum centers with a positive charge dramatically enhanced the activity, weakened propylene adsorption, and prevented its deep dehydrogenation. Besides, the absence of multiple Pt–Pt sites effectively inhibited undesired side reactions (e.g., C–C cracking), thus improved propylene selectivity and stability. This work demonstrates the promising application of a supported atomically dispersed Pt catalyst for highly selective dehydrogenation of propane.
Atomic regulation of metal catalysts has emerged as an intriguing yet challenging strategy to boost product selectivity. Here, we report a density functional theory‐guided atomic design strategy for the fabrication of a NiGa intermetallic catalyst with completely isolated Ni sites to optimize acetylene semi‐hydrogenation processes. Such Ni sites show not only preferential acetylene π‐adsorption, but also enhanced ethylene desorption. The characteristics of the Ni sites are confirmed by multiple characterization techniques, including aberration‐corrected high‐resolution scanning transmission electron microscopy and X‐ray absorption spectrometry measurements. The superior performance is also confirmed experimentally against a Ni5Ga3 intermetallic catalyst with partially isolated Ni sites and against a Ni catalyst with multi‐atomic ensemble Ni sites. Accordingly, the NiGa intermetallic catalyst with the completely isolated Ni sites shows significantly enhanced selectivity to ethylene and suppressed coke formation.
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