The mechanism by which single metal atoms and small, zeolite-encapsulated metal particles are stabilized against migration and growth is not currently well understood. In this work, we employ an unbiased density functional global optimization strategy to identify the locations and energetic barriers for migration pathways between sites for platinum (Pt) confined within the microporous volume of a purely silicious zeolite with Linde type A topology and its aluminosilicate and borosilicate variants. We observe an impressive stabilization of single Pt atoms caused by a hitherto unreported binding mode, in which the six rings in the framework are broken, leading to trapped, highly accessible metal centers. In addition, heteroatom substituents are found to significantly enhance the incorporation of Pt via an unexpected insertion into framework SiO–H bonds. Migration of Pt is hindered by high barriers, which are predicted to vary significantly with Si:X (X = Al and B) ratios. It is proposed that an optimal Si:X ratio exists for a given zeolite topology, in which the barriers will reach the maximum value. The energetic preference for Pt clustering (via Ostwald ripening) remains but is significantly reduced with respect to isolated clusters because of the strong interactions between Pt atoms and the framework. Our findings suggest a means to control noble-metal particle sintering, despite a thermodynamic driving force toward Pt clustering. This work provides an explanation for the surprisingly high degree of kinetic stability of ultrasmall, encapsulated metal particles observed experiment.

Ultrathin layers of oxides deposited on atomically flat metal surfaces have been shown to significantly influence the electronic structure of the underlying metal, which in turn alters the catalytic performance. Upscaling of the specifically designed architectures as required for technical utilization of the effect has yet not been achieved. Here, we apply liquid crystalline phases of fluorohectorite nanosheets to fabricate such architectures in bulk. Synthetic sodium fluorohectorite, a layered silicate, when immersed into water spontaneously and repulsively swells to produce nematic suspensions of individual negatively charged nanosheets separated to more than 60 nm, while retaining parallel orientation. Into these galleries oppositely charged palladium nanoparticles were intercalated whereupon the galleries collapse. Individual and separated Pd nanoparticles were thus captured and sandwiched between nanosheets. As suggested by the model systems, the resulting catalyst performed better in the oxidation of carbon monoxide than the same Pd nanoparticles supported on external surfaces of hectorite or on a conventional Al2O3 support. XPS confirmed a shift of Pd 3d electrons to higher energies upon coverage of Pd nanoparticles with nanosheets to which we attribute the improved catalytic performance. DFT calculations showed increasing positive charge on Pd weakened CO adsorption and this way damped CO poisoning.

Durch die Abscheidung von ultradünnen Oxidschichten auf atomar‐flachen Metalloberflächen konnte die elektronische Struktur des Metalls und hierdurch dessen katalytische Aktivität beeinflusst werden. Die Skalierung dieser Architekturen für eine technische Nutzbarkeit war bisher aber kaum möglich. Durch die Verwendung einer flüssigkristallinen Phase aus Fluorhectorit‐Nanoschichten, können wir solche Architekturen in skalierbarem Maßstab imitieren. Synthetischer Natriumfluorhectorit (NaHec) quillt spontan und repulsiv in Wasser zu einer nematischen flüssigkristallinen Phase aus individuellen Nanoschichten. Diese tragen eine permanente negative Schichtladung, sodass selbst bei einer Separation von über 60 nm eine parallele Anordnung der Schichten behalten wird. Zwischen diesen Nanoschichten können Palladium‐Nanopartikel mit entgegengesetzter Ladung eingelagert werden, wodurch die nematische Phase kollabiert und separierte Nanopartikel zwischen den Schichten fixiert werden. Die Aktivität zur CO‐Oxidation des so entstandenen Katalysators war höher als z. B. die der gleichen Nanopartikel auf konventionellem Al2O3 oder der externen Oberfläche von NaHec. Durch Röntgenphotoelektronenspektroskopie konnte eine Verschiebung der Pd‐3d‐Elektronen zu höheren Bindungsenergien beobachtet werden, womit die erhöhte Aktivität erklärt werden kann. Berechnungen zeigten, dass mit erhöhter positiver Ladung des Pd die Adsorptionsstärke von CO erniedrigt und damit auch die Vergiftung durch CO vermindert wird.

The encapsulation of noble metal atoms into zeolites is a promising route to generate controlled size distributions of stable metal catalysts. Pinning of single metal atoms to particular binding sites...

The encapsulation of noble metals into zeolites is a promising route to generate controlled size distributions of stable metal catalysts. Pinning of single metal atoms to particular binding sites represents the optimal atom-efficiency and is a desirous outcome, despite the propensity of metal clusters to sinter. Currently, sintering resistance of noble metals in siliceous frameworks is incompletely understood, while the role of influencing factors such as adsorbates and exchange of metal type, have not been ascertained. Here, we investigate the binding and migration pathways of atomic Pt and Au in the siliceous zeolite framework LTA, via density functional global structure optimisation and kinetic Monte Carlo simulations. We show that strong binding of Pt atoms to the framework severely hinders migration, while Au diffuses freely through the pore. Reducing agents CO and H2 change the preferred binding site of Pt, induce volatility, reduce migration barriers and thereby promote particle growth.This work provides an atomistic picture of single atom kinetics inside high-silica zeolites, which represent a fundamental basis for understanding nano-catalyst deactivation.