Surface-selective direct 17^{17}O DNP NMR of CeO2_{2} nanoparticles

Surface-selective direct $^{17}$O DNP has been demonstrated for the first time on CeO$_{2}$ nanoparticles, for which the first three layers can be distinguished with high selectivity. Polarisation build-up curves show that the polarisation of the (sub-)surface sites builds up faster than the bulk, accounting for the remarkable surface selectivity.We are grateful for financial support by the Oppenheimer Foundation (M. A. H.), the Cambridge Commonwealth Trusts (D. M. H.), the National Natural Science Foundation of China (NSFC) (21573103 and 21661130149) and the Royal Society Newton Fund (L. P.). The DNP experiments were performed at the DNP MAS NMR Facility at the University of Nottingham, with thanks to the EPSRC for funding of pilot studies (EP/L022524/1)

Microscopic study of TiF3 as hydrogen storage catalyst for MgH2

To understand the catalytic influence of TiF3 on the de- and rehydrogenation of ball milled MgH2–TiF3, a detailed study has been performed of the different crystallographic phases of the de- and rehydrogenated materials. Rietveld-refined XRD analysis of the dehydrogenated material indicates the presence of a rutile, nonstoichiometric MgF2–x phase and a hexagonal Mg1–yTiyFz phase. After rehydrogenation three different fluorine environments were found by 19F NMR, which can be attributed to MgF2–x, MgF2–xHx, and MgF2–x dispersed with MgO nanodomains. The remarkable MgF2–xHx phase, which is apparently formed from the nonstoichiometric MgF2–x phase in the dehydrogenated material, may indicate a diffusion pathway of hydrogen within Mg/MgH2–x. Furthermore, the previously found nonstoichiometric MgH2–x phase appears to be metastable as is observed by XRD analysis after long-term storage. TEM with EDX measurements performed for both dehydrogenated and rehydrogenated samples provide information that the nanoparticles are highly crystalline and that the catalyst is homogenously distributed over the sample at a 100 nm length scale. More locally, the fluorine-containing phase is observed mainly at the surface of the Mg and MgH2 particles and the Ti-containing species are present throughout the particles, likely as TiH2 inclusions in the Mg structure and/or as Mg–Ti–F compound. Additionally, an about 2–5.5 nm MgO layer covers the Mg aggregates. A model of the hydrogen uptake and sample morphology is proposed

Surface modification of oxidic nanoparticles using 3-methacryloxypropyltrimethoxysilane

Tin oxide, antimony-doped tin oxide (ATO), and silica nanosized particles in aqueous dispersion were reacted with various amounts of 3-methacryloxypropyltrimethoxysilane (MPS). The kinetics were followed by 29Si NMR and the products were analyzed by FTIR and 29Si NMR. The kinetic experiments on ATO and silica revealed that the hydrolysis is the rate-determining step in these reactions. The reaction of MPS with the particles is favored over the homocondensation of MPS. Quantitative analysis using FTIR revealed that the amount of MPS grafted onto the tin oxide and silica particles is limited to the amount needed to fill one monolayer. For ATO the maximum amount of grafted MPS was only 50–70% of the amount that is needed for a closed monolayer. The MPS molecules are for the most part oriented parallel to the oxide surface, and a hydrogen bond between the MPS-carbonyl and the oxide is formed.status: publishe

Thermoplastic elastomers with cross-polymerizable hard blocks

Abstract onl

Unraveling the Reaction Mechanisms of SiO Anodes for Li-Ion Batteries by Combining in Situ ⁷Li and ex Situ ⁷Li/²⁹Si Solid-State NMR Spectroscopy

Silicon monoxide is a promising alternative anode material due to its much higher capacity than graphite, and improved cyclability over other Si anodes. An in-depth analysis of the lithium silicide (LixSi) phases that form during lithiation/delithiation of SiO is presented here and the results are compared with pure-Si anodes. A series of anode materials is first prepared by heating amorphous silicon monoxide (a-SiO) at different temperatures, X-ray diffraction and 29Si NMR analysis revealing that they comprise small Si domains that are surrounded by amorphous SiO2, the domain size and crystallinity growing with heat treatment. In and ex situ 7Li and 29Si solid-state NMR combined with detailed electrochemical analysis reveals that a characteristic metallic LixSi phase is formed on lithiating a-SiO with a relatively high Li concentration of x = 3.4-3.5, which is formed/decomposed through a continuous structural evolution involving amorphous phases differing in their degree of Si-Si connectivity. This structural evolution differs from that of pure-Si electrodes where the end member, crystalline Li15Si4, is formed/decomposed through a two-phase reaction. The reaction pathway of SiO depends, however, on the size of the ordered Si domains within the pristine material. When crystalline domains of >3 nm within a SiO2 matrix are present, a phase resembling Li15Si4 forms, albeit at a higher overpotential. The continuous formation/decomposition of amorphous LixSi phases without the hysteresis and phase change associated with the formation of c-Li15Si4, along with a partially electrochemically active SiO2/lithium silicate buffer layer, are paramount for the good cyclability of a-SiO.

Iron clusters occluded in zeolite ZSM5 micropores

The effect of high temerature calcination and steaming on iron-contg. zeolite was studied. Iron was introduced by sublimation of FeCl3. FTIR measurements clearly show the disappearance of Broensted acid sites upon such treatments. This is attributed to a reaction between small occluded Fe oxide clusters obtained after initial low temp. calcination and the zeolite protons to give cationic Fe species compensating the neg. zeolite charge. High temp. calcination and esp. steaming considerably increase the catalyst activity in nitrous oxide decompn. The two treated catalysts exhibit a higher apparent activation energy than the original sample. This high apparent activation energy is compensated by a high pre-exponential factor. In Fe/ZSM5 the activity mainly derives from small Fe oxide clusters, while upon treatment more active cationic species are generated. The kinetic parameters point to a stronger Fe-O bond for the latter case

Characterization of Ga/HZSM-5 and Ga/HMOR synthesized by chemical vapor deposition of trimethylgallium

Chemical vapor deposition (CVD) of trimethylgallium (TMG) has been studied as a method to disperse extraframework Ga in acidic ZSM-5 and mordenite zeolite. Various samples were extensively characterized by ICP, XPS, NMR, and FTIR. Silylation with tetramethyldisilazane is explored as a method for deactivating the external zeolite surface. The deposition of TMG in silylated ZSM-5 results in a gallium-to-aluminum ratio close to unity, which indicates a homogeneous metal distribution in the micropore space. However, pore blockage in the one-dimensional channels of mordenite results in a inhomogeneous distribution and a low Ga loading. Upon exposure to moistened air, the adsorbed methylgallium species decompose and alkoxy groups are formed. Subsequent oxidation or reduction leads to the complete removal of methyl groups. The reductive route is the preferred one resulting in a better dispersion of Ga, since oxidation of the methyl groups leads to water formation and hydrolysis of cationic Ga species.status: publishe

On the synthesis of highly acidic nanolayered ZSM-5

Nanolayered ZSM-5 zeolites were synthesized at high framework Al content. Crystallization of ZSM-5 zeolite synthesis gel at Si/Al ratio of 20 at 423 K using C22H45–N+(CH3)2–C6H12–N+(CH3)2–R (R = hexyl or propyl) as structure-directing agents resulted in amorphous product. When the hydrothermal synthesis was carried out at 443 K instead of 423 K, crystalline nanolayered ZSM-5 zeolites were obtained. By replacing the hexyl end group by propyl, the crystallization rate increased due to increased occupancy of the intersections of the MFI framework by quaternary ammonium centers. Optimized hydrothermal synthesis at Si/Al = 20 (443 K, NaF addition) with this surfactant resulted in highly crystalline nanolayered ZSM-5 zeolite. Not all Al atoms are built into the zeolite framework, which limits the overall Brønsted acidity. The catalytic performance in n-heptane hydroisomerization of the various zeolites trends well with the Brønsted acid site density. The product distribution during n-heptane hydroisomerization points to an increased rate of product desorption (higher isomers’ selectivities and increased selectivity of dibranched isomers). The nanolayered ZSM-5 zeolites outperform bulk ZSM-5 zeolite in the methanol-to-hydrocarbons reaction in terms of the methanol conversion capacity

Relaxivity of liposomal paramagnetic MRI contrast agents

Paramagnetic liposomes, spherical particles formed by a lipid bilayer, are able to accommodate a high payload of Gd-containing lipid and therefore can serve as a highly potent magnetic resonance imaging contrast agent. In this paper the relaxation properties of paramagnetic liposomes were studied as a function of composition, temperature and magnetic field strength. The pegylated liposomes with a diameter of approximately 100 nm were designed for favorable pharmacokinetic properties in vivo. The proton relaxivity, i.e. the T1 relaxation rate per mmol of Gd(III) ions, of liposomes with unsaturated DOPC phospholipids was higher than those with saturated DSPC lipids. Addition of cholesterol was essential to obtain monodisperse liposomes and led to a further, although smaller, increase of the relaxivity. Nuclear magnetic relaxation dispersion measurements showed that the relaxivity was limited by water exchange. These results show that these paramagnetic liposomes are very effective contrast agents, making them excellent candidates for many applications in magnetic resonance imaging.status: publishe

Network density and diene conversion in peroxide-cured gumstock EPDM rubbers : a solid-state NMR study

Peroxide-cross-linked EPM and EPDM rubbers have been investigated with magic-angle spinning 1H NMR spectroscopy and static 1H NMR relaxometry. The results yield a consistent nanoscale picture of the chemical and physical network properties in terms of chemical cross-links formed via macro-radical combination, chemical cross-links formed via addition to the double bonds, and physical cross-links resulting from chain entanglements