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The reversible phase transitions in phase-change memory devices can switch on the order of nanoseconds, suggesting a close structural resemblance between the amorphous and crystalline phases. Despite this, the link between crystalline and amorphous tellurides is not fully understood nor quantified. Here we use in-situ high-temperature x-ray absorption spectroscopy (XAS) and theoretical calculations to quantify the amorphous structure of bulk and nanoscale GeTe. Based on XAS experiments, we develop a theoretical model of the amorphous GeTe structure, consisting of a disordered fcc-type Te sublattice and randomly arranged chains of Ge atoms in a tetrahedral coordination. Strikingly, our intuitive and scalable model provides an accurate description of the structural dynamics in phase-change memory materials, observed experimentally. Specifically, we present a detailed crystallization mechanism through the formation of an intermediate, partially stable 'ideal glass' state and demonstrate differences between bulk and nanoscale GeTe leading to size-dependent crystallization temperature., (© 2024. The Author(s).)

Supported, bimetallic catalysts have shown great promise for the selective hydrogenation of CO 2 to methanol. In this study, we decipher the catalytically active structure of Ni-Ga-based catalysts. To this end, model Ni-Ga-based catalysts, with varying Ni:Ga ratios, were prepared by a surface organometallic chemistry approach. In situ differential pair distribution function (d-PDF) analysis revealed that catalyst activation in H 2 leads to the formation of nanoparticles based on a Ni-Ga face-centered cubic (fcc) alloy along with a small quantity of GaO x . Structure refinements of the d-PDF data enabled us to determine the amount of both alloyed Ga and GaO x species. In situ X-ray absorption spectroscopy experiments confirmed the presence of alloyed Ga and GaO x and indicated that alloying with Ga affects the electronic structure of metallic Ni (viz., Ni δ- ). Both the Ni:Ga ratio in the alloy and the quantity of GaO x are found to minimize methanation and to determine the methanol formation rate and the resulting methanol selectivity. The highest formation rate and methanol selectivity are found for a Ni-Ga alloy having a Ni:Ga ratio of ∼75:25 along with a small quantity of oxidized Ga species (0.14 mol Ni -1 ). Furthermore, operando infrared spectroscopy experiments indicate that GaO x species play a role in the stabilization of formate surface intermediates, which are subsequently further hydrogenated to methoxy species and ultimately to methanol. Notably, operando XAS shows that alloying between Ni and Ga is maintained under reaction conditions and is key to attaining a high methanol selectivity (by minimizing CO and CH 4 formation), while oxidized Ga species enhance the methanol formation rate., Competing Interests: The authors declare no competing financial interest., (© 2024 The Authors. Published by American Chemical Society.)

The development of effective CO 2 sorbents is vital to achieving net-zero CO 2 emission targets. MgO promoted with molten salts is an emerging class of CO 2 sorbents. However, the structural features that govern their performance remain elusive. Using in situ time-resolved powder x-ray diffraction, we follow the structural dynamics of a model NaNO 3 -promoted, MgO-based CO 2 sorbent. During the first few cycles of CO 2 capture and release, the sorbent deactivates owing to an increase in the sizes of the MgO crystallites, reducing in turn the abundance of available nucleation points, i.e., MgO surface defects, for MgCO 3 growth. After the third cycle, the sorbent shows a continuous reactivation, which is linked to the in situ formation of Na 2 Mg(CO 3 ) 2 crystallites that act effectively as seeds for MgCO 3 nucleation and growth. Na 2 Mg(CO 3 ) 2 forms due to the partial decomposition of NaNO 3 during regeneration at T ≥ 450°C followed by carbonation in CO 2 .

MgO-based CO 2 sorbents promoted with molten alkali metal nitrates (e.g., NaNO 3 ) have emerged as promising materials for CO 2 capture and storage technologies due to their low cost and high theoretical CO 2 uptake capacities. Yet, the mechanism by which molten alkali metal nitrates promote the carbonation of MgO (CO 2 capture reaction) remains debated and poorly understood. Here, we utilize 18 O isotope labeling experiments to provide new insights into the carbonation mechanism of NaNO 3 -promoted MgO sorbents, a system in which the promoter is molten under operation conditions and hence inherently challenging to characterize. To conduct the 18 O isotope labeling experiments, we report a facile and large-scale synthesis procedure to obtain labeled MgO with a high 18 O isotope content. We use Raman spectroscopy and in situ thermogravimetric analysis in combination with mass spectrometry to track the 18 O label in the solid (MgCO 3 ), molten (NaNO 3 ), and gas (CO 2 ) phases during the CO 2 capture (carbonation) and regeneration (decarbonation) reactions. We discovered a rapid oxygen exchange between CO 2 and MgO through the reversible formation of surface carbonates, independent of the presence of the promoter NaNO 3 . On the other hand, no oxygen exchange was observed between NaNO 3 and CO 2 or NaNO 3 and MgO. Combining the results of the 18 O labeling experiments, with insights gained from atomistic calculations, we propose a carbonation mechanism that, in the first stage, proceeds through a fast, surface-limited carbonation of MgO. These surface carbonates are subsequently dissolved as [Mg 2+ ···CO 3 2- ] ionic pairs in the molten NaNO 3 promoter. Upon reaching the solubility limit, MgCO 3 crystallizes at the MgO/NaNO 3 interface., Competing Interests: The authors declare no competing financial interest., (© 2022 The Authors. Published by American Chemical Society.)

Gallia-alumina (Ga,Al) 2 O 3( x  :  y ) spinel-type solid solution nanoparticle catalysts for propane dehydrogenation (PDH) were prepared with four nominal Ga : Al atomic ratios (1 : 6, 1 : 3, 3 : 1, 1 : 0) using a colloidal synthesis approach. The structure, coordination environment and distribution of Ga and Al sites in these materials were investigated by X-ray diffraction, X-ray absorption spectroscopy (Ga K-edge) as well as 27 Al and 71 Ga solid state nuclear magnetic resonance. The surface acidity (Lewis or Brønsted) was probed using infrared spectroscopy with pyridine and 2,6-dimethylpyridine probe molecules, complemented by element-specific insights (Ga or Al) from dynamic nuclear polarization surface enhanced cross-polarization magic angle spinning 15 N{ 27 Al} and 15 N{ 71 Ga} J coupling mediated heteronuclear multiple quantum correlation NMR experiments using 15 N-labelled pyridine as a probe molecule. The latter approach provides unique insights into the nature and relative strength of the surface acid sites as it allows to distinguish contributions from Al and Ga sites to the overall surface acidity of mixed (Ga,Al) 2 O 3 oxides. Notably, we demonstrate that (Ga,Al) 2 O 3 catalysts with a high Al content show a greater relative abundance of four-coordinated Ga sites and a greater relative fraction of weak/medium Ga-based surface Lewis acid sites, which correlates with superior propene selectivity, Ga-based activity, and stability in PDH (due to lower coking). In contrast, (Ga,Al) 2 O 3 catalysts with a lower Al content feature a higher fraction of six-coordinated Ga sites, as well as more abundant Ga-based strong surface Lewis acid sites, which deactivate through coking. Overall, the results show that the relative abundance and strength of Ga-based surface Lewis acid sites can be tuned by optimizing the bulk Ga : Al atomic ratio, thus providing an effective measure for a rational control of the catalyst performance., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

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 CT x of the MXene family. Surface termination groups T x (O, OH, and F) are reductively de-functionalized in Mo 2 CT 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 CT 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 CT 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., (© 2021. The Author(s).)

The addition of molten alkali metal salts drastically accelerates the kinetics of CO 2 capture by MgO through the formation of MgCO 3 However, the growth mechanism, the nature of MgCO 3 formation, and the exact role of the molten alkali metal salts on the CO 2 capture process remain elusive, holding back the development of more-effective MgO-based CO 2 sorbents. Here, we unveil the growth mechanism of MgCO 3 under practically relevant conditions using a well-defined, yet representative, model system that is a MgO(100) single crystal coated with NaNO 3 The model system is interrogated by in situ X-ray reflectometry coupled with grazing incidence X-ray diffraction, scanning electron microscopy, and high-resolution transmission electron microscopy. When bare MgO(100) is exposed to a flow of CO 2 , a noncrystalline surface carbonate layer of ca. 7-Å thickness forms. In contrast, when MgO(100) is coated with NaNO 3 , MgCO 3 crystals nucleate and grow. These crystals have a preferential orientation with respect to the MgO(100) substrate, and form at the interface between MgO(100) and the molten NaNO 3 MgCO 3 grows epitaxially with respect to MgO(100), and the lattice mismatch between MgCO 3 and MgO is relaxed through lattice misfit dislocations. Pyramid-shaped pits on the surface of MgO, in proximity to and below the MgCO 3 crystals, point to the etching of surface MgO, providing dissolved [Mg 2+ …O 2- ] ionic pairs for MgCO 3 growth. Our studies highlight the importance of combining X-rays and electron microscopy techniques to provide atomic to micrometer scale insight into the changes occurring at complex interfaces under reactive conditions., Competing Interests: The authors declare no competing interest.

Background: Recent research shows AKI increases the risk of incident CKD. We hypothesized that perioperative AKI may confer increased risk of subsequent CKD compared to nonperioperative AKI., Methods: A MEDLINE search was performed for "AKI, CKD, chronic renal insufficiency, surgery, and perioperative" and related terms yielded 5209 articles. One thousand sixty-five relevant studies were reviewed. One thousand six were excluded because they were review, animal, or pediatric studies. Fifty-nine studies underwent full manuscript review by two independent evaluators. Seventeen met all inclusion criteria and underwent analysis. Two-by-two tables were constructed from AKI +/- and CKD +/- data. The R package metafor was employed to determine odds ratio (OR), and a random-effects model was used to calculate weighted ORs. Leave-1-out, funnel analysis, and structured analysis were used to estimate effects of study heterogeneity and bias., Results: Nonperioperative studies included studies of oncology, percutaneous coronary intervention, and myocardial infarction patients. Perioperative studies comprised patients from cardiac surgery, vascular surgery, and burns. There was significant heterogeneity, but risk of bias was overall assessed as low. The OR for AKI versus non-AKI patients developing CKD in all studies was 4.31 (95% CI 3.01-6.17; p < 0.01). Nonperioperative subjects demonstrated OR 3.32 for developing CKD compared to non-AKI patients (95% CI 2.06-5.34; p < 0.01) while perioperative patients demonstrated OR 5.20 (95% CI 3.12-8.66; p < 0.01) for the same event., Conclusions: We conclude that studies conducted in perioperative and nonperioperative patient populations suggest similar risk of development of CKD after AKI.

The two-dimensional morphology of molybdenum oxycarbide (2D-Mo 2 CO x ) nanosheets dispersed on silica is found vital for imparting high stability and catalytic activity in the dry reforming of methane. Here we report that owing to the maximized metal utilization, the specific activity of 2D-Mo 2 CO x /SiO 2 exceeds that of other Mo 2 C catalysts by ca. 3 orders of magnitude. 2D-Mo 2 CO x is activated by CO 2 , yielding a surface oxygen coverage that is optimal for its catalytic performance and a Mo oxidation state of ca. +4. According to ab initio calculations, the DRM proceeds on Mo sites of the oxycarbide nanosheet with an oxygen coverage of 0.67 monolayer. Methane activation is the rate-limiting step, while the activation of CO 2 and the C-O coupling to form CO are low energy steps. The deactivation of 2D-Mo 2 CO x /SiO 2 under DRM conditions can be avoided by tuning the contact time, thereby preventing unfavourable oxygen surface coverages.

Adequate cerebral perfusion is necessary to maintain consciousness in upright humans. Following maximal anaerobic exercise, cerebral perfusion can become compromised and result in syncope. It is unknown whether post-exercise reductions in cerebral perfusion can lead to visual-cognitive deficits prior to the onset of syncope, which would be of concern for emergency workers and warfighters, where critical decision making and intense physical activity are combined. Therefore, the purpose of this experiment was to determine if reductions in cerebral blood velocity, induced by maximal anaerobic exercise and head-up tilt, result in visual-cognitive deficits prior to the onset of syncope. Nineteen sedentary to recreationally active volunteers completed a symptom-limited 60° head-up tilt for 16 min before and up to 16 min after a 60 sec Wingate test. Blood velocity of the middle cerebral artery was measured using transcranial Doppler ultrasound and a visual decision-reaction time test was assessed, with independent analysis of peripheral and central visual field responses. Cerebral blood velocity was 12.7 ± 4.0% lower (mean ± SE; P < 0.05) after exercise compared to pre-exercise. This was associated with a 63 ± 29% increase (P < 0.05) in error rate for responses to cues provided to the peripheral visual field, without affecting central visual field error rates (P = 0.46) or decision-reaction times for either visual field. These data suggest that the reduction in cerebral blood velocity following maximal anaerobic exercise contributes to visual-cognitive deficits in the peripheral visual field without an apparent affect to the central visual field., (© 2016 The Authors. Physiological Reports published by Wiley Periodicals, Inc. on behalf of the American Physiological Society and The Physiological Society.)

Sustained postexercise vasodilation, which may be mediated at both a neural and vascular level, is seen in previously active skeletal muscle vascular beds following both large and small muscle-mass exercise. Blunted sympathetic vascular transduction and a downward resetting of the arterial baroreflex contribute to this vasodilation after cycling (large muscle-mass exercise), but it is unknown if these responses also contribute to sustained vasodilation following small muscle-mass exercise. This study aimed to determine if baroreflex sensitivity is altered, the baroreflex is reset, or if sympathetic vascular transduction is blunted following small muscle-mass exercise. Eleven healthy, college-aged subjects (five males, six females) completed one-leg dynamic knee-extension exercise for 1 h at 60% of peak power output. While cardiovagal baroreflex sensitivity was increased ~23% postexercise relative to preexercise (P < 0.05), vascular and integrated baroreflex sensitivity were not altered following exercise (P = 0.31 and P = 0.48). The baroreflex did not exhibit resetting (P > 0.69), and there was no evidence of changes in vascular transduction following exercise (P = 0.73). In conclusion, and in contrast to large muscle-mass exercise, it appears that small muscle-mass exercise produces a sustained postexercise vasodilation that is largely independent of central changes in the baroreflex., (© 2015 The Authors. Physiological Reports published by Wiley Periodicals, Inc. on behalf of the American Physiological Society and The Physiological Society.)