The realization of a hydrogen economy would be facilitated by the discovery of a water-splitting electrocatalyst that is efficient, stable under operating conditions, and composed of earth-abundant ...elements. Density functional theory simulations within a simple thermodynamic model of the more difficult half-reaction, the anodic oxygen evolution reaction (OER), with a single-walled carbon nanotube as a model catalyst, show that the presence of 0.3–1% nitrogen reduces the required OER overpotential significantly compared to the pristine nanotube. We performed an extensive exploration of systems and active sites with various nitrogen functionalities (graphitic, pyridinic, or pyrrolic) obtained by introducing nitrogen and simple lattice defects (atomic substitutions, vacancies, or Stone–Wales rotations). A number of nitrogen functionalities (graphitic, oxidized pyridinic, and Stone–Wales pyrrolic nitrogen systems) yielded similar low overpotentials near the top of the OER volcano predicted by the scaling relation, which was seen to be closely observed by these systems. The OER mechanism considered was the four-step single-site water nucleophilic attack mechanism. In the active systems, the second or third step, the formation of attached oxo or peroxo moieties, was the potential-determining step of the reaction. The nanotube radius and chirality effects were examined by considering OER in the limit of large radius by studying the analogous graphene-based model systems. They exhibited trends similar to those of the nanotube-based systems but often with reduced reactivity due to weaker attachment of the OER intermediate moieties.
Volcanic activity occurring in tropical moist atmospheres can promote deep convection and trigger volcanic thunderstorms. These phenomena, however, are rarely observed to last continuously for more ...than a day and so insights into the dynamics, microphysics and electrification processes are limited. Here we present a multidisciplinary study on an extreme case, where volcanically-triggered deep convection lasted for six days. We show that this unprecedented event was caused and sustained by phreatomagmatic activity at Anak Krakatau volcano, Indonesia during 22-28 December 2018. Our modelling suggests an ice mass flow rate of ~5 × 10
kg/s for the initial explosive eruption associated with a flank collapse. Following the flank collapse, a deep convective cloud column formed over the volcano and acted as a 'volcanic freezer' containing ~3 × 10
kg of ice on average with maxima reaching ~10
kg. Our satellite analyses reveal that the convective anvil cloud, reaching 16-18 km above sea level, was ice-rich and ash-poor. Cloud-top temperatures hovered around -80 °C and ice particles produced in the anvil were notably small (effective radii ~20 µm). Our analyses indicate that vigorous updrafts (>50 m/s) and prodigious ice production explain the impressive number of lightning flashes (~100,000) recorded near the volcano from 22 to 28 December 2018. Our results, together with the unique dataset we have compiled, show that lightning flash rates were strongly correlated (R = 0.77) with satellite-derived plume heights for this event.
We propose a theory for the drying of liquid droplets of surfactant solutions. We show that the added surfactant hinders droplet receding and facilitates droplet spreading, causing a complex behavior ...of the contact line of an evaporating droplet: the contact line first recedes, then advances, and finally recedes again. We also show that the surfactant can change the deposition pattern from mountain-like to volcano-like and then to coffee-ring-like. Specially, when the contact line motion undergoes a clear receding–advancing transition, a two-ring pattern is formed. The mechanism of the two-ring formation is different from the stick–slip mechanism proposed previously and may be tested experimentally.
Open conduit basaltic volcanoes can be potentially hazardous as the eruptive activity may turn suddenly from a steady state to highly explosive. Unexpected changes in explosion intensity are ...recurrent at Stromboli volcano, where major explosions and large-scale paroxysms sometimes break off the ordinary, Strombolian activity with little or no warning. Two powerful paroxysmal eruptions took place at Stromboli volcano during the summer 2019, causing widespread fires, consistent damages across the island, injuries and one fatality. Prediction of similar events is really challenging for the modern volcanology, though models propaedeutic to early-warning monitoring systems are not properly assessed yet in many volcanoes worldwide. Here, we present a multi-parametric study that combines petrological and geophysical data to investigate processes generating the two paroxysms. The time information derived by Li enrichments in plagioclase crystals correlates with tilt time series derived by seismometers installed on the island, highlighting the dominant role of shallow conduit processes in triggering the 2019 paroxysmal activity. Our dataset conceives a mechanism of gas slug formation and fast upward migration that finally triggered the eruptions in very limited times. The proposed model questions our capability to forecast such kind of paroxysms in times that are rapid enough to allow mitigation of the associated risk.
Abstract
Buoyant magmas abundant in exsolved volatiles (bubbles) drive the rapid upward-propagation of feeder dikes from magma chambers. The consequence of a feeder dike reaching the surface can ...result in an explosive volcanic eruption depending, partly, on the retention of volatiles. Therefore, timely detection of the vesicularity and overpressure of the magma during feeder dike ascent is critical for the prediction of the explosivity of any future eruption. In this study, we evaluated the explosivity of eruptions based on field investigations of the erupted products and the overpressure of magma in the conduit based on the dimensions of exposed feeder dikes. We found a positive correlation between the explosivity of eruptions and the magma overpressure generated in the conduit during recent fissure eruptions of Miyakejima volcano. Because the buoyancy of low-density magma produces positive overpressure at the dike’s top, feeder dikes with highly-vesiculated magmas possess high amounts of overpressure. An enlargement of the opening width of a feeder dike by magmatic overpressure results in a higher flux of vesiculated magma, which causes vigorous explosive activity. Our results suggest the possibility of forecasting the explosivity of an impending eruption if the width (or opening) of an ascending feeder dike is monitored in real-time through measurements of ground deformation and seismicity induced by the dike.
For over two decades, the OpenGL API provided users with the means for implementing versatile, feature-rich, and portable real-time graphics applications. Consequently, it has been widely adopted by ...practitioners and educators alike and is deeply ingrained in many curricula that teach real-time graphics for higher education. Over the years, the architecture of graphics processing units (GPUs) incrementally diverged from OpenGL’s conceptual design. The more recently introduced Vulkan API provides a more modern, fine-grained approach for interfacing with the GPU, which allows a high level of controllability and, thereby, deep insights into the inner workings of modern GPUs. This property makes the Vulkan API especially well suitable for teaching graphics programming in university education, where fundamental knowledge shall be conveyed. Hence, it stands to reason that educators who have their students’ best interests at heart should provide them with corresponding lecture material. However, Vulkan is notoriously verbose and rather challenging for first-time users, thus transitioning to this new API bears a considerable risk of failing to achieve expected teaching goals. In this paper, we document our experiences after teaching Vulkan in both introductory and advanced graphics courses side-by-side with conventional OpenGL. A collection of surveys enables us to draw conclusions about perceived workload, difficulty, and students’ acceptance of either approach. In doing so, we identify suitable conditions and recommendations for teaching Vulkan to both undergraduate and graduate students.
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•How to transition introductory graphics courses from OpenGL to Vulkan.•Programming framework suitable for introductory graphics courses.•Student questionnaire evaluations about the transition to Vulkan.•Vulkan usage in advances graphics courses.•Programming framework suitable for teaching in advanced graphics courses and research.
We report preparation and electrochemical properties of molybdenum nitride MoxN (x from 1 to 2) and Mo thin films prepared by the magnetron sputtering. We adjusted amount of nitrogen during reactive ...sputtering, to achieve different ratios of Mo:N in the films. We could prepare well-defined and extremely smooth films. We determined hydrogen evolution reaction (HER) activity and other kinetics parameters such as Tafel slopes and exchange current densities. Corrosion stability at anodic potentials was also evaluated from onset potentials of dissolution of prepared films. We found that MoxN thin film HER activity dependence on nitrogen content in the film exhibits volcano-like diagram. From XRD measurements we identified several phases present in the films from pure Mo to β-Mo2N and δ-MoN. We find that β-Mo2N phase has the highest HER activity. Stability of studied nitrides at anodic potentials increases with increasing content of nitrogen. Mo films sputtered at high substrate temperatures exhibit higher HER activity than MoxN systems. In addition, with increasing substrate temperature during sputtering, HER activity of pure Mo thin films increases.
Linear free energy scaling relationships (LFESRs) and volcano plots are routinely used to assess the performance of heterogeneous electrocatalysts and have only recently been concretely exploited in ...homogeneous catalysis. These tools efficiently compare and provide a global evaluation of catalyst performance while highlighting the limitations for a given reaction. In the framework of solid-state water oxidation, a minimal overpotential of 0.4 eV has been predicted on the basis of LFESRs. Considering the very different nature of homogeneous catalysts compared to solid-state systems, the validity of scaling relationships determined for the former cannot be assumed. To evaluate the global limitations of molecular O2 evolution catalysts, LFESRs are established for all key intermediates for different metal (Mn, Co, Ru, Rh, Ir) and ligand (corrole and perfluoro-porphyrin) combinations assuming a mononuclear mechanism that proceeds through *–OH, *O, and *–OOH intermediates. Our computations indicate that the LFESRs strongly depend on the choice of density functional. Using GMC-QDPT2 as a benchmark, strong scaling relationships between all intermediates are observed, but the relationships between *–OH and *O significantly differ from those found in solid-state systems. Consequently, the shape of the molecular volcano plot changes drastically from its solid-state counterpart and shows a broad plateau at the top where the overpotential is nearly independent of the choice of catalyst. This plateau renders the performance of molecular catalysts extremely robust, but inhibits improvements by proceeding through alternative reaction mechanisms.
Herein, an effective top‐down etching route is presented to in situ fabricate CuO/CeO2 nanohybrids on the surface of Cu2O microcube templates. This method has well taken into account the factors both ...in thermodynamics and in kinetics, including surface structural nanocrystallization, construction of mesopores, formation of stable core@shell structures, and strengthened synergistic effects, in order to realize the structural design and hence greatly improve catalytic performance caused by surface nanocrystallization of Cu2O cubes. After etched by aid of ammonia and Ce3+ ions the final products are in a well‐defined spiny yolk@shell structures, in which the unetched part of Cu2O cubes serves as the core and the shell is composed by the CuO nanothorns encapsulated by CeO2 nanoparticles. Systematical characterizations including scanning electron microscopy, transmission electron microscopy, X‐ray diffraction, X‐ray photoelectron spectroscopy, H2‐temperature programmed reduction, N2 sorption, firmly disclose the relationship between the catalytic properties and the structures of samples. By simply tuning the usage amount of ammonia and Ce3+ ions, the samples show a typical volcano curve in the model reaction of catalytic CO oxidation. Sample CuO@CeO2‐0.05 exhibits the optimal catalytic activity and stability. It is believed that this top‐down strategy has shown promising future to design high‐performance catalysts for the practical need of application.
An effective top‐down route is presented to in situ fabricate CuO/CeO2 nanohybrids on the surface of Cu2O microcube templates. After etched by aid of ammonia and Ce3+ ions the products are in well‐defined spiny yolk@shell structures, which show excellent catalytic performance on catalytic CO oxidation.
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