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Local microenvironment tuning induces switching between electrochemical CO₂ reduction pathways
No full textGas diffusion layers (GDL) have become a critical component in electrochemical CO₂ reduction (CO₂R) systems because they can enable high current densities needed for industrially relevant productivity. Besides this function, it is often assumed that the choice of catalyst and electrolyte play much more important roles than the GDL in influencing the observed product selectivity. Here, we show that tuning of the GDL pore size can be used to control the local microenvironment of the catalyst and hence, effect significant changes in catalytic outcomes. This concept is demonstrated using sputtered Ag films on hydrophobic PTFE substrates with 6 different pore sizes. Although Ag is known to be a predominantly CO generating catalyst, we find that smaller pore sizes favor the generation of formate up to a faradaic efficiency of 43%. Combined experimental and simulation results show that this is due to the influence of the pore size on CO₂ mass transport, which alters the local pH at the electrode, resulting in reaction pathway switching between CO and formate. Our results highlight the importance of the local microenvironment as an experimental knob that can be rationally tuned for controlling product selectivity: a key consideration in the design of CO₂R systems
Laser Absorption Sensor Targeting Potassium for Hypersonic Velocity, Temperature, and Enthalpy Measurements
No full textA laser absorption-based sensor for hypersonic gas flows was developed, targeting the D1 spectroscopic transition of atomic potassium near 770 nm. The sensor applies rapid-scanning tunable diode laser absorption spectroscopy to measure velocity from the Doppler shift and to infer temperature from the hyperfine-split transition lineshape. This sensor measured velocities and temperatures across three distinct conditions and six shots in the Hypervelocity Expansion Tube at the California Institute of Technology. Velocity and temperature were sampled at 5 μs intervals, and temperature measurements were validated with a supplementary laser absorption-based sensor targeting carbon dioxide transitions near 4.2 μm. Measured velocities across the three conditions ranged from 3.3 to 4.4 km/s, and measured temperatures ranged from 900 to 1600 K. The combined measurements were used to infer the freestream specific total enthalpy, which ranged from 7 to 10 MJ/kg. Because atomic potassium naturally forms in the test gas of many hypersonic impulse facilities, similar sensors may be widely applicable to facility characterization
Interchange reconnection as the source of the fast solar wind within coronal holes
No full textThe fast solar wind that fills the heliosphere originates from deep within regions of open magnetic field on the Sun called ‘coronal holes’. The energy source responsible for accelerating the plasma is widely debated; however, there is evidence that it is ultimately magnetic in nature, with candidate mechanisms including wave heating and interchange reconnection. The coronal magnetic field near the solar surface is structured on scales associated with ‘supergranulation’ convection cells, whereby descending flows create intense fields. The energy density in these ‘network’ magnetic field bundles is a candidate energy source for the wind. Here we report measurements of fast solar wind streams from the Parker Solar Probe (PSP) spacecraft6 that provide strong evidence for the interchange reconnection mechanism. We show that the supergranulation structure at the coronal base remains imprinted in the near-Sun solar wind, resulting in asymmetric patches of magnetic ‘switchbacks’7,8 and bursty wind streams with power-law-like energetic ion spectra to beyond 100 keV. Computer simulations of interchange reconnection support key features of the observations, including the ion spectra. Important characteristics of interchange reconnection in the low corona are inferred from the data, including that the reconnection is collisionless and that the energy release rate is sufficient to power the fast wind. In this scenario, magnetic reconnection is continuous and the wind is driven by both the resulting plasma pressure and the radial Alfvénic flow bursts
Two-dimensional infrared-Raman spectroscopy as a probe of water’s tetrahedrality
No full textTwo-dimensional spectroscopic techniques combining terahertz (THz), infrared (IR), and visible pulses offer a wealth of information about coupling among vibrational modes in molecular liquids, thus providing a promising probe of their local structure. However, the capabilities of these spectroscopies are still largely unexplored due to experimental limitations and inherently weak nonlinear signals. Here, through a combination of equilibrium-nonequilibrium molecular dynamics (MD) and a tailored spectrum decomposition scheme, we identify a relationship between the tetrahedral order of liquid water and its two-dimensional IR-IR-Raman (IIR) spectrum. The structure-spectrum relationship can explain the temperature dependence of the spectral features corresponding to the anharmonic coupling between low-frequency intermolecular and high-frequency intramolecular vibrational modes of water. In light of these results, we propose new experiments and discuss the implications for the study of tetrahedrality of liquid water
No thick carbon dioxide atmosphere on the rocky exoplanet TRAPPIST-1 c
No full textSeven rocky planets orbit the nearby dwarf star TRAPPIST-1, providing a unique opportunity to search for atmospheres on small planets outside the Solar System. Thanks to the recent launch of the James Webb Space Telescope (JWST), possible atmospheric constituents such as carbon dioxide (CO₂) are now detectable. Recent JWST observations of the innermost planet TRAPPIST-1 b showed that it is most probably a bare rock without any CO₂ in its atmosphere. Here we report the detection of thermal emission from the dayside of TRAPPIST-1 c with the Mid-Infrared Instrument (MIRI) on JWST at 15 µm. We measure a planet-to-star flux ratio of f_p/f⁎ = 421 ± 94 parts per million (ppm), which corresponds to an inferred dayside brightness temperature of 380 ± 31 K. This high dayside temperature disfavours a thick, CO₂-rich atmosphere on the planet. The data rule out cloud-free O₂/CO₂ mixtures with surface pressures ranging from 10 bar (with 10 ppm CO₂) to 0.1 bar (pure CO₂). A Venus-analogue atmosphere with sulfuric acid clouds is also disfavoured at 2.6σ confidence. Thinner atmospheres or bare-rock surfaces are consistent with our measured planet-to-star flux ratio. The absence of a thick, CO₂-rich atmosphere on TRAPPIST-1 c suggests a relatively volatile-poor formation history, with less than 9.5^(+7.5)_(−2.3) Earth oceans of water. If all planets in the system formed in the same way, this would indicate a limited reservoir of volatiles for the potentially habitable planets in the system
The ENCODE Uniform Analysis Pipelines
No full textThe Encyclopedia of DNA elements (ENCODE) project is a collaborative effort to create a comprehensive catalog of functional elements in the human genome. The current database comprises more than 19000 functional genomics experiments across more than 1000 cell lines and tissues using a wide array of experimental techniques to study the chromatin structure, regulatory and transcriptional landscape of theHomo sapiensandMus musculusgenomes. All experimental data, metadata, and associated computational analyses created by the ENCODE consortium are submitted to the Data Coordination Center (DCC) for validation, tracking, storage, and distribution to community resources and the scientific community. The ENCODE project has engineered and distributed uniform processing pipelines in order to promote data provenance and reproducibility as well as allow interoperability between genomic resources and other consortia. All data files, reference genome versions, software versions, and parameters used by the pipelines are captured and availableviathe ENCODE Portal. The pipeline code, developed using Docker and Workflow Description Language (WDL;https://openwdl.org/) is publicly available in GitHub, with images available on Dockerhub (https://hub.docker.com), enabling access to a diverse range of biomedical researchers. ENCODE pipelines maintained and used by the DCC can be installed to run on personal computers, local HPC clusters, or in cloud computing environmentsviaCromwell. Access to the pipelines and dataviathe cloud allows small labs the ability to use the data or software without access to institutional compute clusters. Standardization of the computational methodologies for analysis and quality control leads to comparable results from different ENCODE collections - a prerequisite for successful integrative analyses.
Database URL: https://www.encodeproject.org
Accuracy and precision of ESI-Orbitrap-IRMS observations of hours to tens of hours via reservoir injection
No full textOrbitrap isotope ratio mass spectrometry (Orbitrap-IRMS) has recently been applied to high-precision, natural-abundance isotope ratio measurements of a diverse range of compounds, including amino acids, oxyanions, fatty acids, and metals. These measurements can characterize many isotope ratios simultaneously at high (≈1.0‰) precision. In a successful experiment, observed precision will track the shot-noise limit and be limited by experimental time. Some isotope ratios, for example those involving ¹⁷O in organic compounds or multiply-substituted (‘clumped’) isotopologues, require experimental times of hours to tens of hours to achieve desired precision, while current sample introduction techniques focus on observations on the order of seconds to tens of minutes. In this study, we characterize Orbitrap-IRMS performance for three long duration measurements (individual acquisitions ≥1 h and as long as 24 h) using an automated reservoir injection system coupled to a Q Exactive HF Orbitrap with an electrospray ionization (ESI) source. First, we characterize long-term intra-measurement stability through a 24-h long measurement of acetone. We report the following isotope ratios and precisions (as acquisition errors, errors on the observed ratio within this measurement (σAE)): ¹³C/¹²C (σ_(AE) = 0.07‰), ¹⁷O/¹⁶O (σ_(AE) = 1.1‰), ¹⁸O/¹⁶O (σ_(AE) = 0.3‰), and ¹³C¹³C/¹²C (σ_(AE) = 0.65‰). The σ_(AE) of each tracks the shot noise limit throughout and is limited by the challenging conditions (high resolution and low numbers of ions per scan) required for ¹⁷O/¹⁶O measurement in the presence of ¹³C via Orbitrap. Second, we characterize inter-measurement stability via a sequence of seven 75-min analyses of perchlorate. We observe the following ratios and acquisition errors: ³⁷Cl/³⁵Cl (σ_(AE) = 0.09‰); ¹⁷O/¹⁶O (σ_(AE) = 1.6‰); ¹⁸O/¹⁶O (σ_(AE) = 0.7‰), ³⁷Cl¹⁷O/³⁵Cl¹⁶O (σ_(AE) = 2.7‰), and ³⁷Cl¹⁸O/³⁵Cl¹⁶O (σ_(AE) = 1.2‰). However, we find that inter-measurement drift between acquisitions limits our accuracy and precision for standardized measurements (i.e., error on reported δ values) to ≈1‰ for the ³⁷Cl/³⁵Cl measurement. Hence, the benefits of low σ_(AE) may not be fully realized. Third, we demonstrate accuracy via sample/standard comparisons of a methionine sample with ¹³C enrichment of ≈20‰ relative to a known standard. Using a sequence of seven 60-min analyses, we recover the following isotope ratios and standardized precisions (i.e., error on reported δ values, denoted propagated acquisition errors, σ_(PAE)): ³³S/³²S (σ_(PAE) = 1.0‰), ³⁴S/³²S (σ_(PAE) = 0.7‰), ¹⁵N/¹⁴N (σ_(PAE) = 2.1‰), ²H/¹H (σ_(PAE) = 3.2‰),¹³C/¹²C (σ_(PAE) = 0.4‰), ¹⁸O/¹⁶O (σ_(PAE) = 1.6‰), & ¹³C¹³C/¹²C (σ_(PAE) = 2.8‰) with confirmation of accurate results for the known ¹³C/¹²C and ¹³C¹³C/¹²C enrichments. Together, our results demonstrate the viability of Orbitrap-IRMS for long duration measurements of diverse sample types via an automated reservoir injection system. Inter-measurement stability remains a challenge; we expect our methods to be most applicable to extended measurements of hard-to-observe properties, such as ¹⁷O in organics and clumped isotopologues
Photogenerated Ni(I)-Bipyridine Halide Complexes: Structure-Function Relationships for Competitive C(sp²)-Cl Oxidative Addition and Dimerization Reactivity Pathways
No full textWe report the facile photochemical generation of a library of Ni(I)–bpy halide complexes (Ni(I)(ᴿbpy)X (R = t-Bu, H, MeOOC; X = Cl, Br, I) and benchmark their relative reactivity toward competitive oxidative addition and off-cycle dimerization pathways. Structure–function relationships between the ligand set and reactivity are developed, with particular emphasis on rationalizing previously uncharacterized ligand-controlled reactivity toward high energy and challenging C(sp²)–Cl bonds. Through a dual Hammett and computational analysis, the mechanism of the formal oxidative addition is found to proceed through an S_(N)Ar-type pathway, consisting of a nucleophilic two-electron transfer between the Ni(I) 3d(z²) orbital and the C_(aryl)–Cl σ* orbital, which contrasts the mechanism previously observed for activation of weaker C(sp²)–Br/I bonds. The bpy substituent provides a strong influence on reactivity, ultimately determining whether oxidative addition or dimerization even occurs. Here, we elucidate the origin of this substituent influence as arising from perturbations to the effective nuclear charge (Z_(eff)) of the Ni(I) center. Electron donation to the metal decreases Z_(eff), which leads to a significant destabilization of the entire 3d orbital manifold. Decreasing the 3d(z²) electron binding energies leads to a powerful two-electron donor to activate strong C(sp²)–Cl bonds. These changes also prove to have an analogous effect on dimerization, with decreases in Z_(eff) leading to more rapid dimerization. Ligand-induced modulation of Z_(eff) and the 3d(z²) orbital energy is thus a tunable target by which the reactivity of Ni(I) complexes can be altered, providing a direct route to stimulate reactivity with even stronger C–X bonds and potentially unveiling new ways to accomplish Ni-mediated photocatalytic cycles
Detecting Exoplanets Closer to Stars with Moderate Spectral Resolution Integral-field Spectroscopy
No full textWhile radial velocity surveys have demonstrated that the population of gas giants peaks around 3 au, the most recent high-contrast imaging surveys have only been sensitive to planets beyond ∼10 au. Sensitivity at small angular separations from stars is currently limited by the variability of the point-spread function. We demonstrate how moderate-resolution integral-field spectrographs can detect planets at smaller separations (≲ 0.3") by detecting the distinct spectral signature of planets compared to the host star. Using OSIRIS (R ≈ 4000) at the W.M. Keck Observatory, we present the results of a planet search via this methodology around 20 young targets in the Ophiuchus and Taurus star-forming regions. We show that OSIRIS can outperform high-contrast coronagraphic instruments equipped with extreme adaptive optics and non-redundant masking in the 0.05"–0.3" regime. As a proof of concept, we present the 34σ detection of a high-contrast M dwarf companion at ≈0.1" with flux ratio of ≈ 0.92% around the field F2 star HD 148352. We developed an open-source Python package, breads, for the analysis of moderate-resolution integral-field spectroscopy data in which the planet and the host star signal are jointly modeled. The diffracted starlight continuum is forward-modeled using a spline model, which removes the need for prior high-pass filtering or continuum normalization. The code allows for analytic marginalization of linear hyperparameters, which simplifies the posterior sampling of other parameters (e.g., radial velocity, effective temperature). This technique could prove very powerful when applied to integral-field spectrographs such as NIRSpec on the JWST and other upcoming first-light instruments on the future Extremely Large Telescopes
A pair degree condition for Hamiltonian cycles in 3-uniform hypergraphs
No full textWe prove a new sufficient pair degree condition for tight Hamiltonian cycles in 3-uniform hypergraphs that (asymptotically) improves the best known pair degree condition due to Rödl, Ruciński, and Szemerédi. For graphs, Chvátal characterised all those sequences of integers for which every pointwise larger (or equal) degree sequence guarantees the existence of a Hamiltonian cycle. A step towards Chvátal’s theorem was taken by Pósa, who improved on Dirac’s tight minimum degree condition for Hamiltonian cycles by showing that a certain weaker condition on the degree sequence of a graph already yields a Hamiltonian cycle.
In this work, we take a similar step towards a full characterisation of all pair degree matrices that ensure the existence of tight Hamiltonian cycles in 3-uniform hypergraphs by proving a 3-uniform analogue of Pósa’s result. In particular, our result strengthens the asymptotic version of the result by Rödl, Ruciński, and Szemerédi