Surface Science Meets Catalysis Research: Iron oxide films for in-situ model catalysis

An in-depth model catalysis study on a complex system is reviewed. Both unpromoted and potassium-promoted iron oxide model catalysts films of single crystalline quality are prepared and characterized in ultrahigh vacuum (UHV) using surface science methods. In order to bridge the pressure and material gap for the catalytic dehydrogenation of ethylbenzene to styrene in presence of steam, this reaction is studied at reactive gas pressures between 10-6 and 36 mbar. The samples are transferred under vacuum into an stagnation point micro-flow reactor where the reaction is studied, followed by post-reaction characterization in UHV. Clean hematite Fe2O3 is an excellent catalyst but deactivates quickly by reduction and by coking. Addition of H2O limits reduction to the oxidation state of magnetite Fe3O4 and counteracts coking. Both deactivation mechanisms can be avoided by addition of some O2 to the feed. Potassium has basically the same functions as O2. It does not seem to be involved in the catalytic dehydrogenation step but rather to block active sites if its concentration is high. Long-term deactivation occurs mainly by potassium removal in form of volatile KOH. Regeneration by “steaming” in pure H2O accelerates this process while ethylbenzene in the feed stabilizes potassium. This is ascribed to the formation of non-volatile K2CO3 which is an intermediate in potassium catalysed coke removal. The addition of O2 instead of K-promotion may be an alternative reaction route

A new method to position and functionalize metal-organic framework crystals

With controlled nanometre-sized pores and surface areas of thousands of square metres per gram, metal-organic frameworks (MOFs) may have an integral role in future catalysis, filtration and sensing applications. In general, for MOF-based device fabrication, well-organized or patterned MOF growth is required, and thus conventional synthetic routes are not suitable. Moreover, to expand their applicability, the introduction of additional functionality into MOFs is desirable. Here, we explore the use of nanostructured poly-hydrate zinc phosphate (α-hopeite) microparticles as nucleation seeds for MOFs that simultaneously address all these issues. Affording spatial control of nucleation and significantly accelerating MOF growth, these α-hopeite microparticles are found to act as nucleation agents both in solution and on solid surfaces. In addition, the introduction of functional nanoparticles (metallic, semiconducting, polymeric) into these nucleating seeds translates directly to the fabrication of functional MOFs suitable for molecular size-selective applications

Molecular decoding using luminescence from an entangled porous framework

Chemosensors detect a single target molecule from among several molecules, but cannot differentiate targets from one another. In this study, we report a molecular decoding strategy in which a single host domain accommodates a class of molecules and distinguishes between them with a corresponding readout. We synthesized the decoding host by embedding naphthalenediimide into the scaffold of an entangled porous framework that exhibited structural dynamics due to the dislocation of two chemically non-interconnected frameworks. An intense turn-on emission was observed on incorporation of a class of aromatic compounds, and the resulting luminescent colour was dependent on the chemical substituent of the aromatic guest. This unprecedented chemoresponsive, multicolour luminescence originates from an enhanced naphthalenediimide–aromatic guest interaction because of the induced-fit structural transformation of the entangled framework. We demonstrate that the cooperative structural transition in mesoscopic crystal domains results in a nonlinear sensor response to the guest concentration

Understanding the adsorption process in ZIF-8 using high pressure crystallography and computational modelling

Understanding host–guest interactions and structural changes within porous materials is crucial for enhancing gas storage properties. Here, the authors combine cryogenic loading of gases with high pressure crystallography and computational techniques to obtain atomistic detail of adsorption-induced structural and energetic changes in ZIF-8