Supported Pd−Au alloy catalysts were developed for the highly efficient and selective hydrosilylation of α,β-unsaturated ketones and alkynes. The Pd/Au atomic ratio of the Pd−Au alloy and the supporting material affected the catalytic activity, and supported Pd−Au alloy nanoparticles with a low Pd/Au atomic ratio functioned as highly active heterogeneous catalysts under mild reaction conditions. Structural characterization of supported Pd−Au alloy catalysts by X-ray diffraction, X-ray absorption spectroscopy (XAS), and transmission electron microscopy revealed the formation of random Pd−Au alloy nanoparticles with a uniform size of around 3 nm on the support. Furthermore, XAS and X-ray photoelectron spectroscopy elucidated the charge transfer from Pd to Au and the formation of isolated single Pd atoms in random Pd−Au alloys with a low Pd/Au ratio, which enabled efficient hydrosilylation of a variety of substrates under mild reaction conditions.

Controlled nucleation in a micromixer and further crystal growth were used to synthesize Ni(2,6-ndc)dabco (2,6-ndc - 2,6-naphthalenedicarboxylate, dabco - 1,4-diazabicyclo[2.2.2]octane), also termed DUT-8(Ni) (DUT = Dresden University of Technology), with narrow particle size distribution in a range of a few nm to several μm. The crystal size was found to significantly affect the switching characteristics, in particular the gate opening pressure in nitrogen adsorption isotherms at 77 K for this highly porous and flexible network. Below a critical size of about 500 nm, a type Ia isotherm typical of rigid MOFs is observed, while above approximately 1000 nm a pronounced gating behaviour is detected, starting at p/p = 0.2. With increasing crystal size this transition gate becomes steeper indicating a more uniform distribution of activation energies within the crystal ensemble. At an intermediate size (500-1000 nm), the DUT-8(Ni) crystals close during activation but cannot be reopened by nitrogen at 77 K possibly indicating monodomain switching.

Switchable metal-organic frameworks (MOFs) standt out for potential applications in energy storage, separation, sensing, and catalysis. The understanding of MOF switchability mechanisms has progressed significantly over the past two decades. Nanostructuring is essential for the integration of such materials into thin films, hierarchical composites, and membranes and for biological applications. However, downsizing below critical dimensions causes dramatic changes in the dynamic behavior and responsiveness towards external stimuli. We discuss the most important experimental findings and derive general guidelines and hypotheses of relevance for the impact of crystal size on switchability. Understanding nanostructure thermodynamics and implications for the tailoring of dynamic porous systems requires an interdisciplinary approach, advanced physical characterization techniques, and new modeling strategies to cover a wider range of time and length scales. MOF Switchability and FunctionalityMOFs are among the most porous systems, with great potential in diverse applications such as energy storage, separation, air purification, and many more [1,2]. An outstanding feature, compared with other, traditional porous materials, is their ability to transform (switch) between different phases with well-defined crystalline structures triggered by external stimuli, often by guest inclusion [3,4]. The latter leads, in some cases, to important improvements in gas separation or storage due to ultrahigh selectivity [5] or high deliverable capacity [6]. The terms 'flexible MOF', 'soft porous crystals', and 'switchable MOF' are used synonymously [7,8]. An important aspect indicated by the term 'switchability' is the stepwise (first order) character of the structural transition between two phases (bistability). Recently, even multistable systems have emerged, leading to remarkable recognition effects [9,10]. These phase transitions induced by external stimuli (e.g., changes in temperature, external pressure, gas pressure, vapor pressure, or electromagnetic radiation) are associated with a latent heat of transformation, L, governing the energetics of the bulk phase transition. An activation barrier (E A or ΔG ǂ ) governs the kinetics and potential windows for metastable states. Nucleation (for nucleation theory see Glossary and see supplemental information online for additional Glossary terms) of the new solid phase is an important characteristic of the solid-solid phase transition. Nevertheless, the fluid kinetics of adsorption and, potentially, fluid nucleation kinetics may also play a role. HighlightsSwitchable metal-organic frameworks (MOFs) conquer advanced applications in energy storage, sensing, gas separation, catalysis, and biomedicine. They benefit from unique adsorption characteristics and responsive behavior leading to anomalously high separation selectivity and deliverable storage capacity.Nanostructuring provides an effective means to tailor the responsive behavior of switchable MOFs. The characteristic switching pressure d...