Large Outer Surface Research Articles

Metal−Organic Frameworks: Why Make Them Small?

Recent studies have demonstrated that metal−organic frameworks (MOFs) can be miniaturized down to the submicroscale and even nanoscale to create small MOFs. Herein, the basic concepts and recent progresses concerning the core question of “MOFs, why make them small?” are briefly introduced from three perspectives: those of size, shape, and processability. Small sizes endow MOFs with large outer surface area/volume ratios, leading to important differences in their characteristics (e.g., outer surface energy, number of defects, etc.) relative to their bulk scale counterparts and thus, opening the door to size‐dependent properties such as flexibility or catalytic performance. Downsizing MOFs also enables shaping of them into unusual shapes. Among the most appealing morphologies are 2D nanosheets, whose structure can favor certain porosity‐related applications. The controlled miniaturization of MOF particles also favors their integration onto surfaces and into devices as well as their colloidal stability, which in turn translates to the ability to use readily processable MOF‐based liquids in countless processes, such as to drive the assembly of sophisticated meso‐ and macroscopic architectures, gels, monoliths, and porous liquids. It is believed that ongoing research to shrink MOFs via nanotechnology, as has been done with other classes of materials, will continue to yield novel MOF‐related materials that exhibit unprecedented structural and functional properties.

Calculation of structure-borne sound in a\xa0direct drive wind turbine

In this publication, the methods will be presented that are deployed to formulate a multi-physical system model of a direct drive wind turbine in order to calculate structure borne sound. The model includes excitation effect as well as sound radiating behaviour. The mechanical structure as a medium partner between excitation and radiation will be formulated through a multi-body simulation model in the time domain. In the multi-body simulation model, all relevant drivetrain components are considered with their structural eigenmodes in the frequency range of interest. The electromagnetic forces of the multi-pole ring generator are calculated and introduced into the mechanical structure at each stator tooth, rotor pole and various axial positions individually. Similarly, the modelling of the bearings is investigated for a range of available methods. Sound emission is evaluated at the large outer surface structures like tower, blades and nacelle cover. To minimize computational effort, the surface accelerations are not calculated for each surface node, instead a modal approach is used. Through a combination of mode shapes with mode participation factors of the respective structures, the surface accelerations can be regained during a post-processing step. Those results are used as input for airborne sound calculations. Nevertheless, the high number of modal and spatial degrees of freedom results in high computing costs.

Fabrication of ultra-thin carbon nanofibers by centrifuged-electrospinning for application in high-rate supercapacitors

The novel technique of centrifuged-electrospinning is employed to fabricate immiscible polyacrylonitrile (PAN)/polymethyl methacrylate (PMMA) polymer fibers, followed by carbonization to form ultra-thin carbon nanofibers (UT-CNF) with 28 ± 11 nm diameters. An additional centrifugal force provides a strong stretching force to stretch the dispersed droplets (PAN) into ultra-thin nanofibers, as confirmed by electron microscopy. This structure presents good electrochemical properties compared to electrospun carbon nanofibers with 126 ± 16 nm diameters. Electrochemical impedance spectroscopy analysis shows enhanced efficient surface areas, which accumulate ions more quickly, resulting in a decrease in the charge distribution and ion diffusion resistance because the reduction in diameter provides a short pore length and large outer surface. Applied to a supercapacitor, galvanostatic charge/discharge analysis gives a maximum specific capacitance of 243 F/g at 1 A/g and capacitance retention of 77.1% at a charge/discharge rate of 100 A/g for UT-CNF. This result is significantly higher than that of traditional electrospun carbon nanofibers.

(Invited) Ionic Liquid Mixture Electrolytes to Increase the Performance of Electrochemical Capacitors

While known for a higher power density than batteries, electrochemical capacitors are limited by their energy density for some energy storage applications. Since the energy density of the device is proportional to the square of operating potential window, an effective way to increase energy density of the device is by increasing the window. Ionic liquid electrolytes are beneficial for this reason since they can theoretically operate at up to 6 V, though experimentally, the value is between 3-4 V, depending on the properties of electrode materials. By employing a novel electrochemical technique to study symmetric supercapacitors with carbon electrodes and ionic liquid electrolytes, we have found one of the possible reasons for the observed smaller operating potential window of ionic liquids in practical applications. We observed that even for a symmetric device, the different properties of the cation and anion results in an asymmetric performance of the two electrodes. However, a solution to this asymmetry is found by mixing two ionic liquids with the same cation, which ultimately results in a higher operating potential window of the device and therefore a higher energy density.1 Onion-like carbon (OLC) was used as the electrode material due to its large (>500 m2/g) outer surface available for ion adsorption and double layer formation, and no internal porosity that could restrict the movement of ions.2 However, electrode materials can also be made from high surface area carbons that contain a network of pores (e.g., activated or carbide-derived carbon) in which ions are adsorbed during charging. Though they boast a large operating potential window, ionic liquids are known to contain large and bulky ions. This can make it difficult to use an ionic liquid on a porous carbon with a range of pore sizes, some being smaller than the ion size, even though the specific surface area of the electrode material measured by gas adsorption is high and therefore the energy storage capability is more impressive. It has also been shown that the capacitance of porous electrodes is maximized when the ion size and pore size are equal.3 In this case, the ion is small enough to fit inside the pores while still large enough to take advantage of the surface area within the pore walls. While materials with a large outer surface allow a more accessible surface for adsorbing large ions, their capacitance is limited by their specific surface area. By designing an electrolyte based on mixed ionic liquids, we can match the ions in the mixture to the multiple pore sizes of the electrode material. Recently, it was found that the capacitance of porous carbon could be increased with the presence of multiple ions in the electrolyte mixture. Imidazolium-based ionic liquids are chosen based on their variable alkyl chain length which alters the cation size and bis(trifluoromethylsulfonyl)- imide (TFSI) is chosen as the anion for all ionic liquids. Different porous carbons with varying pore size distributions are used to illustrate the effect of ionic liquid mixture electrolytes. Finally, it has been shown previously that ionic liquid mixture electrolytes can be used to increase the operating temperature window of a supercapacitor to -50 °C – 100 °C.4 Ionic liquid mixtures have therefore been shown to improve the potential window, the capacitive performance, and the temperature window of supercapacitors, becoming an important area of study in the field of energy storage. 1. Van Aken, K. L., Beidaghi, M. & Gogotsi, Y. Formulation of Ionic-Liquid Electrolyte to Expand the Voltage Window of Supercapacitors. Angew. Chemie 127, 4888–4891 (2015). 2. McDonough, J. K. et al. Influence of the structure of carbon onions on their electrochemical performance in supercapacitor electrodes. Carbon N. Y. 50, 3298–3309 (2012). 3. Lin, R. et al. Solvent effect on the ion adsorption from ionic liquid electrolyte into sub-nanometer carbon pores. Electrochim. Acta 54, 7025–7032 (2009). 4. Lin, R. et al. Capacitive Energy Storage from -50 to 100 C Using an Ionic Liquid Electrolyte. J. Phys. Chem. Lett. 2, 2396–2401 (2011). Figure 1

Exclusion of small terminase mediated DNA threading models for genome packaging in bacteriophage T4.

Tailed bacteriophages and herpes viruses use powerful molecular machines to package their genomes. The packaging machine consists of three components: portal, motor (large terminase; TerL) and regulator (small terminase; TerS). Portal, a dodecamer, and motor, a pentamer, form two concentric rings at the special five-fold vertex of the icosahedral capsid. Powered by ATPase, the motor ratchets DNA into the capsid through the portal channel. TerS is essential for packaging, particularly for genome recognition, but its mechanism is unknown and controversial. Structures of gear-shaped TerS rings inspired models that invoke DNA threading through the central channel. Here, we report that mutations of basic residues that line phage T4 TerS (gp16) channel do not disrupt DNA binding. Even deletion of the entire channel helix retained DNA binding and produced progeny phage in vivo. On the other hand, large oligomers of TerS (11-mers/12-mers), but not small oligomers (trimers to hexamers), bind DNA. These results suggest that TerS oligomerization creates a large outer surface, which, but not the interior of the channel, is critical for function, probably to wrap viral genome around the ring during packaging initiation. Hence, models involving TerS-mediated DNA threading may be excluded as an essential mechanism for viral genome packaging.

“Ideal” bifunctional catalysis over Pt-acid zeolites

The conditions of “ideal” bifunctional catalysis were specified on the example of two reactions involved in major refining and petrochemical processes, the hydroisomerisation of n-alkanes (n-Cx) and of ethylbenzene (EB) over PtHzeolites. In both cases, the balance between the hydrogenating and acid functions (CPt/CH+) was shown to affect positively the activity (actually the turnover frequency of the acid sites), the stability and the selectivity of the bifunctional catalysts. Optimal catalytic characteristics were obtained for CPt/CH+ values high enough so that the acid catalysed steps be rate-limiting, provided however, an additional condition is satisfied: low number of acid sites between two Pt sites in order that only one skeletal transformation of n-Cx intermediates could occur or distance between zeolite protonic sites sufficient for limiting the demanding reaction of ethylbenzene disproportionation. The location of the active acid sites which determines their access by the olefinic intermediates plays also an essential role; the highest selectivity values were obtained with sites located at the micropore mouth of the zeolites, after however blockage by carbonaceous deposits of the accessibility to the non-selective inner sites. The only limitation of the corresponding catalysts is their low activity related to the low concentration of zeolite outer sites, which should be easily overcome by preparing zeolite samples with larger outer surface. The generalisation of this analysis to other reactions should facilitate the design of the “ideal” bifunctional catalysts, opening the route to cleaner commercial processes.

Electro-photocatalytic degradation of acid orange II using a novel TiO 2/ACF photoanode

A novel photoanode was prepared by immobilizing TiO 2 film onto activated carbon fibers (TiO 2/ACF) using liquid phase deposition (LPD) to study the electro-photocatalytic (EPC) degradation of organic compounds exemplified by an azo-dye, namely, Acid Orange II (AOII). Results demonstrated that by applying a 0.5 V bias (vs. SCE) across the TiO 2/ACF electrode, the AOII degradation rate was increased significantly compared to that of photocatalytic (PC) oxidation. The application of an electric field promotes the separation of photogenerated electrons and holes as confirmed by electrochemical impedance spectroscopy (EIS) measurements. The structural and surface morphology of the TiO 2/ACF electrode was characterized by field emission scanning electron microscopy (FE-SEM) and X-ray diffraction (XRD). SEM images showed that TiO 2 was deposited on almost every carbon fiber with an average thickness of about 200 nm with the inner space between neighboring fibers being maintained unfilled. The morphological features of the photo-anode facilitated the passage of solution as well as UV light through the felt-form electrode and created a three-dimensional environment favorable to EPC oxidation. Both the large outer surface area of the 3D electrode and the good organic adsorption capacity of the ACF support promoted high contact efficiency between AOII and TiO 2 surface. Anatase was the major crystalline TiO 2 deposited. UV–vis spectrophotometry, TOC (total organic carbon) analysis, and HPLC technique were used to monitor the concentration change of AOII and intermediates as to gain insight into the EPC degradation of AOII using the TiO 2/ACF electrode.

Synthesis, characterization, and catalysis of UTM-1: an MTF-type zeolite composed of the same building unit as MFI-type zeolites

Synthesis conditions for an MTF-type zeolite UTM-1 have been studied and this material has been characterized in detail. UTM-1 is crystallized in the presence of hexamethyleneimine (HMI) under the synthesis conditions similar to those of MCM-22 but from mother gels with high SiO 2/Al 2O 3 ratio at a narrow range of synthesis temperatures. The crystal growth of UTM-1 proceeds quite rapidly after a long induction period in contrast to the gradual crystal growth of MCM-22. Through the optimization of synthesis conditions, pure silica and Ti-incorporated materials with the MTF structure can be easily synthesized in the hydroxide media without employing any co-template other than HMI. UTM-1 is proved to have strong acidity comparable to that of ZSM-5 having a similar SiO 2/Al 2O 3 ratio, presumably reflecting their structural similarity. UTM-1 has large outer surface area so as to show high catalytic activities in reactions occurring at the outer surface of the catalyst, whereas its small pore aperture restricts the diffusivity of reactant molecules into the pore system.

Synthesis Conditions and Catalytic Properties of Highly Acidic Zeolite, UTM-1

Abstract Preferable synthesis conditions for zeolite UTM-1 are studied; UTM-1 is crystallized under similar conditions to those of MCM-22, although it prefers high SiO2/Al2O3 ratio in mother gel. UTM-1, composed of the same basic building unit as ZSM-5, shows strong acidity comparable to that of ZSM-5. Although small aperture of UTM-1 actually restricts the catalytic activities, its large outer surface area enables the high catalytic activity in reactions occurring at the outer surface of catalyst.

The use of MCM-22 as catalyst for the Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam

A variety of catalysts have been applied to the Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam. Experiments with MFI structures yielded good results showing the importance of weak acidic sites, a large outer surface and pore structure accessible only through 10 MR channels. Recent reports on the discovery and characterisation of the MCM-22 catalyst with its large outer surface and special structure recommended this material for the use in the Beckmann rearrangement. In the present study the synthesis and brief characterisation of MCM-22 was carried out followed by the first in depth analysis of its applicability for the Beckmann rearrangement of cyclohexanone oxime to ε-caprolactam. The performance of the material was compared to one of the established catalysts, the [B]-MFI, showing several drawbacks like lower yields and poor mechanical stability.