Abstract
Early transitional metal carbides are promising catalysts for hydrogenation of CO
2
. Here, a two-dimensional (2D) multilayered 2D-Mo
2
C material is prepared from Mo
2
C
T
x
of the MXene ...family. Surface termination groups
T
x
(O, OH, and F) are reductively de-functionalized in Mo
2
C
T
x
(500 °C, pure H
2
) avoiding the formation of a 3D carbide structure. CO
2
hydrogenation studies show that the activity and product selectivity (CO, CH
4
, C
2
–C
5
alkanes, methanol, and dimethyl ether) of Mo
2
C
T
x
and 2D-Mo
2
C are controlled by the surface coverage of
T
x
groups that are tunable by the H
2
pretreatment conditions. 2D-Mo
2
C contains no
T
x
groups and outperforms Mo
2
C
T
x
, β-Mo
2
C, or the industrial Cu-ZnO-Al
2
O
3
catalyst in CO
2
hydrogenation (evaluated by CO weight time yield at 430 °C and 1 bar). We show that the lack of surface termination groups drives the selectivity and activity of Mo-terminated carbidic surfaces in CO
2
hydrogenation.
Understanding the links between chemical composition, nano-structure and the dynamic properties of silicate melts and glasses is fundamental to both Earth and Materials Sciences. Central to this is ...whether the distribution of mobile metallic ions is random or not. In silicate systems, such as window glass, it is well-established that the short-range structure is not random but metal ions cluster, forming percolation channels through a partly broken network of corner-sharing SiO
tetrahedra. In alumino-silicate glasses and melts, extensively used in industry and representing most of the Earth magmas, metal ions compensate the electrical charge deficit of AlO
tetrahedra, but until now clustering has not been confirmed. Here we report how major changes in melt viscosity, together with glass Raman and Nuclear Magnetic Resonance measurements and Molecular Dynamics simulations, demonstrate that metal ions nano-segregate into percolation channels, making this a universal phenomenon of oxide glasses and melts. Furthermore, we can explain how, in both single and mixed alkali compositions, metal ion clustering and percolation radically affect melt mobility, central to understanding industrial and geological processes.
Because of their importance in both the geosciences and the glass-making industry, alkali aluminosilicate melts have been the focal point of many past studies, but despite progress many problems ...remain unresolved, such as the complex behaviour of the thermodynamic properties of aluminium-rich alkali silicate melts. This paper presents a study of Na2O–Al2O3–SiO2 glasses and melts, containing 75mol% SiO2 and different Al/(Al+Na) ratios. Their structure has been investigated by using Raman spectroscopy, as well as, 23Na, 27Al and 29Si 1D MAS NMR spectroscopy. Results confirm the role change of Na+ cations from network modifier to charge compensator in the presence of Al3+ ions. In addition, polymerization increases with increase of the Al/(Al+Na) ratio. These structural changes explain the observed variations in the viscosity of these melts. The viscosity data in turn allow us to calculate the configurational entropy of melts at the glass transition temperature the Sconf(Tg). The variations of the Sconf(Tg) are strongly nonlinear, with sharp increases and decreases depending on the Al/(Al+Na) ratio. More importantly, a strong increase of the Sconf(Tg) is observed when a few Al2O3 is added to sodium silicate melt. A strong decrease is observed after crossing the tectosilicate join, when Al/(Al+Na)>0.5 and when Al3+ ions are present in fivefold coordination, Al5, in the glass. Furthermore, in situ27Al NMR spectra of the peraluminous melt show a clear increase of the Al5 concentration with increasing temperature. When considered in combination with melt fragility and heat capacity, our data demonstrate that Al5 is clearly a transient unit at high temperature in highly polymerized tectosilicate and peraluminous melts. However, when present in glasses, Al5 increases the stability of the aluminosilicate network, hence the Tg of glasses. This could be explained by the ability of Al5 to carry threefold coordinated oxygen atoms in its first coordination shell, as observed in minerals. Localisation of threefold coordinated oxygen atoms on Al5 implies an increase of the medium-range order in the glass, an hypothesis that is in agreement with the low Sconf(Tg) of peraluminous glasses.
The origin of the glass transition is still an open debate, especially for the new class of glasses, formed from metal‐organic compounds. High‐temperature in situ 2H Nuclear Magnetic Resonance (NMR) ...experiments are performed on deuterated samples of ZIF‐62 (Zn(C3H4N2)2‐x(C7H6N2)x, with x = 0.25 and x = 0.05), the prototypical metal–organic framework glass former. Using lineshape analysis, frequencies and angular amplitudes of oscillations of the imidazolate ring during heating up to the melt progressively increasing from ≈10 to 150 MHz, and from ≈5° to 25° are found. This behavior is compositionally dependent and points to the origin of the glass transition lying in organic linker movement, in a similar vein to that witnessed in some organics and contrary to the purely inorganic‐based view of Metal–Organic Framework (MOF) glasses taken to date. This experimental approach shows the potential to elucidate the melting and/or decomposition process for a wide range of MOFs.
In situ high‐temperature deuterium solid‐state Nuclear Magnetic Resonance spectroscopy reveals the origin of the glass transition and melting mechanisms of Metal–Organic Frameworks. Polymer‐like softening of the network at frequencies in the 100 MHz range and steric hindrance between ligands are the key phenomenon in Zeolitic Imidazolate Framework's melt and super‐cooled liquid.
Opiate addiction develops as a chronic relapsing disorder upon drug recreational use or following misuse of analgesic prescription. Mu opioid (MOP) receptors are the primary molecular target of ...opiates but increasing evidence support in vivo functional heteromerization with the delta opioid (DOP) receptor, which may be part of the neurobiological processes underlying opiate addiction. Here, we used double knock-in mice co-expressing fluorescent versions of the MOP and DOP receptors to examine the impact of chronic morphine administration on the distribution of neurons co-expressing the two receptors. Our data show that MOP/DOP neuronal co-expression is broader in morphine-dependent mice and is detected in novel brain areas located in circuits related to drug reward, motor activity, visceral control and emotional processing underlying withdrawal. After four weeks of abstinence, MOP/DOP neuronal co-expression is still detectable in a large number of these brain areas except in the motor circuit. Importantly, chronic morphine administration increased the proportion of MOP/DOP neurons in the brainstem of morphine-dependent and abstinent mice. These findings establish persistent changes in the abstinent state that may modulate relapse and opiate-induced hyperalgesia and also point to the therapeutic potential of MOP/DOP targeting.
This article is part of the Special Issue entitled ‘Receptor heteromers and their allosteric receptor-receptor interactions’.
Display omitted
•Mu-delta neuronal co-expression is broadened in morphine-dependent mice.•Most changes in mu-delta neuronal distribution remain in morphine abstinent mice.•Mu-delta neuronal distribution is enlarged in the forebrain.•The proportion of mu-delta neurons is increased in the hindbrain.•Mu-delta neurons are involved in somatic and emotional processing.
Y3Al5O12 (YAG) is a widely used phosphor host. Its optical properties are controlled by chemical substitution at its YO8 or AlO6/AlO4 sublattices, with emission wavelengths defined by rare‐earth and ...transition‐metal dopants that have been explored extensively. Nonstoichiometric compositions Y3+xAl5‐xO12 (x ≠ 0) may offer a route to new emission wavelengths by distributing dopants over two or more sublattices simultaneously, producing new local coordination environments for the activator ions. However, YAG typically behaves as a line phase, and such compositions are therefore challenging to synthesize. Here, a series of highly nonstoichiometric Y3+xAl5‐xO12 with 0 ≤ x ≤ 0.40 is reported, corresponding to ≤20% of the AlO6 sublattice substituted by Y3+, synthesized by advanced melt‐quenching techniques. This impacts the up‐conversion luminescence of Yb3+/Er3+‐doped systems, whose yellow‐green emission differs from the red‐orange emission of their stoichiometric counterparts. In contrast, the YAG:Ce3+ system has a different structural response to nonstoichiometry and its down‐conversion emission is only weakly affected. Analogous highly nonstoichiometric systems should be obtainable for a range of garnet materials, demonstrated here by the synthesis of Gd3.2Al4.8O12 and Gd3.2Ga4.8O12. This opens pathways to property tuning by control of host stoichiometry, and the prospect of improved performance or new applications for garnet‐type materials.
A family of highly nonstoichiometric Y3Al5O12 (YAG) ceramics is isolated by advanced melt‐quenching methods. In these materials, rare‐earth dopants can populate two crystallographic sublattices, providing a mechanism for luminescence color tuning that is not available to conventional stoichiometric YAGs. The concept is generalized to other garnet ceramics including GAG and GGG, opening new avenues for exploration in this important materials class.
Gallium‐rich heavy metal oxide glasses have become highly attractive optical materials since they exhibit a wide transparency window spanning from the ultraviolet ∼270 nm up to the mid‐infrared (IR) ...region ∼6 μm making them promising for a future integration in optical fiber devices. Nonetheless, in most composition, surface crystallization is a key limiting factor for optical fiber drawing using the classical preform‐to‐fiber method. Herein, taking advantage of structural information from vibrational spectroscopies (Raman/IR) and 71Ga Solid‐State Nuclear Magnetic Resonance, we describe the key role of lanthanum and yttrium rare—earth elements on the glass structure and their impact on the capability to draw those new glass compositions into optical fibers. This approach emphasizes that yttrium ions as compared with lanthanum ones favor the glass disorder, increasing significantly the fraction of GaO5 units with respect to GaO4. That, combined with thermal analysis and examination of the crystallization behaviors, highlights that Y2O3 prevents the glass devitrification during the glass shaping. The smaller yttrium radius is believed to be the key physical parameter preventing the precipitation of the BayGa5‐yGey+1La3‐yO14 (y = 0, 1, 2, 3) langasite‐type crystal phase. This study remains particularly relevant and opens up the way for the development of highly robust power scaled fiber devices operating from the visible up to the challenging mid‐IR domain.
The development of thermally driven water‐sorption‐based technologies relies on high‐performing water vapor adsorbents. Here, polymorphism in Al–metal–organic frameworks is disclosed as a new ...strategy to tune the hydrophilicity of MOFs. This involves the formation of MOFs built from chains of either trans‐ or cis‐ µ‐OH‐connected corner‐sharing AlO4(OH)2 octahedra. Specifically, Al(OH)(muc) or MIP‐211, is made of trans, trans‐muconate linkers, and cis‐µ‐OH‐connected corner‐sharing AlO4(OH)2 octahedra giving a 3D network with sinusoidal channels. The polymorph MIL‐53‐muc has a tiny change in the chain structure that results in a shift of the step position of the water isotherm from P/P0 ≈ 0.5 in MIL‐53‐muc, to P/P0 ≈ 0.3 in MIP‐211. Solid‐state NMR and Grand Canonical Monte Carlo reveal that the adsorption occurs initially between two hydroxyl groups of the chains, favored by the cis‐positioning in MIP‐211, resulting in a more hydrophilic behavior. Finally, theoretical evaluations show that MIP‐211 would allow achieving a coefficient of performance for cooling (COPc) of 0.63 with an ultralow driving temperature of 60 °C, outperforming benchmark sorbents for small temperature lifts. Combined with its high stability, easy regeneration, huge water uptake capacity, green synthesis, MIP‐211 is among the best adsorbents for adsorption‐driven air conditioning and water harvesting from the air.
The cis‐ or trans‐µ‐OH‐connectivity in the corner‐sharing chains of AlO6 octahedra in water stable microporous aluminum‐based metal–organic frameworks leads to two polymorphs, which solely differ in the straight or helical‐shape of the chains. This polymorphism is associated with a drastic change in their water sorption behavior in a view of tuning the heat allocation and water harvesting performances.
We report the study of high-temperature melts (1600-2300 °C) and related glasses in the SrO-Al2O3-SiO2 phase diagram considering three series: (i) depolymerized (SrO/Al2O3 = 3); (ii) fully ...polymerized (SrO/Al2O3 = 1); and (iii) per-aluminous (SrO/Al2O3 < 1). By considering the results from high-temperature 27Al NMR and high-temperature neutron diffraction, we demonstrate that the structure of the polymerized melts is controlled by a close-to-random distribution of Al and Si in the tetrahedral sites, while the depolymerized melts show smaller rings with a possible loss of non-bridging oxygens on AlO4 units during cooling for high-silica compositions. A few five-fold coordinated VAl sites are present in all compositions, except per-aluminous ones where high amounts of high-coordinated aluminium are found in glasses and melts with complex temperature dependence. In high-temperature melts, strontium has a coordination number of 8 or less, i.e. less than in the corresponding glasses. The dynamics of high-temperature melts were studied from 27Al NMR relaxation and compared to macroscopic shear viscosity data. These methods provide correlation times in close agreement. At very high temperatures, the NMR correlation times can be related to the oxygen self-diffusion coefficient, and we show a decrease of the latter with increasing Si/(Al + Si) ratios for polymerized melts with no compositional dependence for depolymerized ones. The dominant parameter controlling the temperature dependence of the aluminum environment of all melts is the distribution of Al-(OSi)p(OAl)(4-p) units.
The study of the provenance of dolomitic marble artefacts has become relevant since it was discovered that quarries of this marble other than that of Cape-Vathy located on the island of Thasos have ...been exploited since Antiquity. To improve our knowledge about the provenance of materials and the extent of their dispersion, multiple archaeometric studies were performed in the past including isotope analyses, petrography, cathodoluminescence, and elemental analyses. In the present work, solid-state nuclear magnetic resonance (NMR) spectroscopy has been added to this panel of techniques. NMR allows the characterization of the material at a molecular level by looking at different nuclei: carbon, magnesium, and calcium. Statistical analysis of the data collected on both quarry samples and archaeologic items was also implemented and clearly demonstrates the efficiency of a holistic approach for provenance elucidation. Finally, the first 25Mg NMR tests have shown the potential of this technique to discriminate between dolomitic marbles of different provenance. The results are discussed in terms of their historical meaning and illustrate the exploitation of sources of dolomitic marbles other than the Greek Thasos source.
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