[1] The size-frequency distributions of rocks >1.5 m diameter fully resolvable in High Resolution Imaging Science Experiment (HiRISE) images of the northern plains follow exponential models developed from lander measurements of smaller rocks and are continuous with rock distributions measured at the landing sites. Dark pixels at the resolution limit of Mars Orbiter Camera thought to be boulders are shown to be mostly dark shadows of clustered smaller rocks in HiRISE images. An automated rock detector algorithm that fits ellipses to shadows and cylinders to the rocks, accurately measured (within 1-2 pixels) rock diameter and height (by comparison to spacecraft of known size) of $10 million rocks over >1500 km 2 of the northern plains. Rock distributions in these counts parallel models for cumulative fractional area covered by 30-90% rocks in dense rock fields around craters, 10-30% rock coverage in less dense rock fields, and 0-10% rock coverage in background terrain away from craters. Above $1.5 m diameter, HiRISE resolves the same population of rocks seen in lander images, and thus size-frequency distributions can be extrapolated along model curves to estimate the number of rocks at smaller diameters. Extrapolating sparse rock distributions in the Phoenix landing ellipse indicate <1% chance of encountering a potentially hazardous rock during landing or that could impede the opening of the solar arrays. Extrapolations further suggest rocks large enough to depress the ground ice table and small enough to be picked up or pushed by the robotic arm should be present within reach for study after landing.
Electrifying ammonia synthesis will be vital to the decarbonization of the chemical industry, as the Haber-Bosch process contributes significantly to global carbon emissions. A lithium-mediated pathway is among the most promising ambient-condition electrochemical ammonia synthesis methods. However, the role of metallic lithium and its passivation layer, the solid electrolyte interphase (SEI), remains unresolved. Here, we apply a multiscale approach that leverages the powerful cryogenic transmission electron microscopy (cryo-TEM) technique to reveal new insights that were previously inaccessible with conventional methods. We discover that the proton donor (e.g. ethanol) governs lithium reactivity toward nitrogen fixation. Without ethanol, the SEI passivates lithium metal, rendering it inactive for ammonia production. Ethanol disrupts this passivation layer, enabling continuous reactivity at the lithium surface. As a result, metallic lithium is consumed via reactions with nitrogen, proton donor, and other electrolyte components. This reactivity across the SEI is vital to device-level performance of lithiummediated ammonia synthesis.
Herein, we describe the use of iterative supervised principal component analysis (ISPCA) in de novo catalyst design. The regioselective synthesis of 2,5-dimethyl-1,3,4-triphenyl-1H-pyrrole (C) via Ti-catalyzed formal [2+2+1] cycloaddition of phenyl propyne and azobenzene was targeted as a proof of principle. The initial reaction conditions led to an unselective mixture of all possible pyrrole regioisomers. ISPCA was conducted on a training set of catalysts, and their performance was regressed against the scores from the top three principal components. Component loadings from this PCA space along with k-means clustering were used to inform the design of new test catalysts. The selectivity of a prospective test set was predicted in silico using the ISPCA model, and only optimal candidates were synthesized and tested experimentally. This data-driven predictive-modeling workflow was iterated, and after only three generations the catalytic selectivity was improved from 0.5 (statistical mixture of products) to over 11 (> 90% C) by incorporating 2,6-dimethyl-4-(pyrrolidin-1-yl)pyridine as a ligand. The successful development of a highly selective catalyst without resorting to long, stochastic screening processes demonstrates the inherent power of ISPCA in de novo catalyst design and should motivate the general use of ISPCA in reaction development. File list (4) download file view on ChemRxiv PCA Manuscript.pdf (2.53 MiB) download file view on ChemRxiv PCA SI.pdf (10.22 MiB) download file view on ChemRxiv PCA4U2.m (43.70 KiB) download file view on ChemRxiv PCAModelingData.xlsx (94.82 KiB)
Multisubstituted pyrroles are commonly found in many bioactive small molecule scaffolds, yet the synthesis of highly- substituted pyrrole cores remains challenging. Herein, we report an efficient catalytic synthesis of 2-heteroatom-substituted...
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