First Principles Analysis of Ethylene Oligomerization on Single-site Ga3+ Catalysts Supported on Amorphous Silica

Amorphous, single site, silica-supported main group metal catalysts have recently been found to promote olefin oligomerization with high activity at moderate temperatures and pressures (~250°C and 1 atm). Herein, we explore the molecular-level relationship between active site structures and the associated oligomerization mechanisms by developing amorphous, silica-supported Ga3+ models from periodic, first-principles calculations. Representative Ga3+ sites, including three- and four-coordinated geometries, are tested for multiple ethylene oligomerization pathways. We show that the three-coordinated Ga3+ site promotes oligomerization through a facile initiation process that generates a Ga-alkyl intermediate, followed by a Ga-alkyl-centered Cossee-Arlman mechanism. The strained geometry of a three-coordinated site enables a favorable free energy landscape with a kinetically accessible ethylene insertion transition state (1.7 eV) and a previously unreported β-hydride transfer step (1.0 eV) to terminate further C-C bond formation. This result, in turn, suggests that Ga3+ does not favor polymerization chemistry, while microkinetic modeling confirms that ethylene insertion is the rate-determining step. The study demonstrates promising flexibility of main group ions for hydrocarbon transformations and, more generally, highlights the importance of the local geometry of metal ions on amorphous oxides in determining catalytic properties.

Cr-Based Sputtered Decorative Coatings for Automotive Industry

The present work aims to study the impact of O and N addition on Cr-sputtered coatings on plastic (polycarbonate, PC) used in automobile parts, as a promisor alternative for auto part metallization, while eliminating the usage of toxic hexavalent chromium. The coatings were deposited using DC magnetron sputtering from a single pure Cr target in a reactive atmosphere (N2 and/or O2). The deposition of the coatings was performed maintaining the total pressure constant and close to 1 Pa by tuning Ar pressure while reactive gases were added. The target current density was kept at JW = 20 mA·cm−2. Structural characterization revealed a mixture of α-Cr, δ-Cr, β-Cr2N, and CrN crystalline structures as well as amorphous oxides. The coating hardness ranged from 9 GPa for the CrON coating to 15 GPa for the CrN coating. All deposited coatings showed a particularly good interface adhesion; adjusting the amount of O and N made it possible to tune the optical properties of the Cr-based coatings as desired. The promising results open future industrialization of sputtered Cr-based coatings for automotive industries.

Iron Redistribution Upon Thermokarst Processes in the Yedoma Domain

Ice-rich permafrost has been subject to abrupt thaw and thermokarst formation in the past and is vulnerable to current global warming. The ice-rich permafrost domain includes Yedoma sediments that have never thawed since deposition during the late Pleistocene and Alas sediments that were formed by previous thermokarst processes during the Lateglacial and Holocene warming. Permafrost thaw unlocks organic carbon (OC) and minerals from these deposits and exposes OC to mineralization. A portion of the OC can be associated with iron (Fe), a redox-sensitive element acting as a trap for OC. Post-depositional thaw processes may have induced changes in redox conditions in these deposits and thereby affected Fe distribution and interactions between OC and Fe, with knock-on effects on the role that Fe plays in mediating present day OC mineralization. To test this hypothesis, we measured Fe concentrations and proportion of Fe oxides and Fe complexed with OC in unthawed Yedoma and previously thawed Alas deposits. Total Fe concentrations were determined on 1,292 sediment samples from the Yedoma domain using portable X-ray fluorescence; these concentrations were corrected for trueness using a calibration based on a subset of 144 samples measured by inductively coupled plasma optical emission spectrometry after alkaline fusion (R2 = 0.95). The total Fe concentration is stable with depth in Yedoma deposits, but we observe a depletion or accumulation of total Fe in Alas deposits, which experienced previous thaw and/or flooding events. Selective Fe extractions targeting reactive forms of Fe on unthawed and previously thawed deposits highlight that about 25% of the total Fe is present as reactive species, either as crystalline or amorphous oxides, or complexed with OC, with no significant difference in proportions of reactive Fe between Yedoma and Alas deposits. These results suggest that redox driven processes during past thermokarst formation impact the present-day distribution of total Fe, and thereby the total amount of reactive Fe in Alas versus Yedoma deposits. This study highlights that ongoing thermokarst lake formation and drainage dynamics in the Arctic influences reactive Fe distribution and thereby interactions between Fe and OC, OC mineralization rates, and greenhouse gas emissions.

Transition between bipolar and abnormal bipolar resistive switching in amorphous oxides with a mobility edge

Resistive switching is an important phenomenon for future memory devices such as resistance random access memories or neuronal networks. While there are different types of resistive switching, such as filament or interface switching, this work focuses on bulk switching in amorphous, binary oxides. Bulk switching was found experimentally in different oxides, for example in amorphous gallium oxide. The forms of the observed current–voltage curves differ, however, fundamentally. Even within the same material, both abnormal bipolar and normal bipolar resistive switching were found. Here, we use a new drift–diffusion model to theoretically investigate bulk switching in amorphous oxides where the electronic conductivity can be described by Mott’s concept of a mobility edge. We show not only that a strong, non-linear dependence of the electronic conductivity on the oxygen content is necessary for bulk switching but also that changing the geometry of the memristive device causes the transition between abnormal and normal bipolar switching.

Soil Organic Carbon Stabilization: Influence of Tillage on Mineralogical and Chemical Parameters

The interaction of organic carbon (OC) with clay minerals and amorphous iron and aluminum oxides, especially in the finest soil fractions (<20 μm), represents a good method for its stabilization, and different tillage practices can improve or reduce the persistence of OC in soils. This study investigates the effects of conventional (CT) and no (NT) tillage and soil depth (0–30, 30–60, and 60–90 cm) on the soil organic carbon (SOC) in four soil size fractions and its interactions with clay minerals and amorphous oxides. To identify the mineralogical composition of the four soil size fractions isolated from each soil, the X-ray powder diffraction (XRPD) technique was used with near infrared (NIR) spectroscopy, while the X-ray fluorescence (XRF) technique was used to determine the chemical composition of soil fractions. The higher OC content found in the finest soil fraction is related to its higher content of clay minerals and amorphous oxides. The SOC content is similar among CT and NT treatments as well as the mineralogical composition and the amount of amorphous oxides, suggesting that more than ten years of different tillage did not influence those parameters.

Effect of Soil Washing Using Oxalic Acid on Arsenic Speciation and Bioaccessibility in Soils

Objectives : Soil washing process has been widely applied for remediation of contaminated soil with arsenic and heavy metals in Korea. The application of soil washing could change physical and chemical properties of soils and metal speciation in soil, which could affect the risk to the environment and human health. Thus, it is necessary to evaluate metal and arsenic speciation and their mobility in soil after soil remediation in order to evaluate effectiveness of soil remediation process and manage soil quality effectively. The purpose of this study is to evaluate the risk of arsenic in soil after remediation of arsenic contaminated soil via soil washing.Methods : Arsenic contaminated soil collected at the abandoned mine site was washing with oxalic acid. The arsenic contaminated soil was divided into 2,000-500 µm, 500-250 µm, 250-150 µm, 150-75 µm, 75-38 µm, < 38 µm particle size fractions. After soil washing for each soil particle size fraction, arsenic speciation via sequential extraction and bioaccessibility in the soils were evaluated. Results and Discussion : Generally, arsenic and metal concentrations were higher in the soil fractions with smaller particle sizes. But high arsenic concentration was observed at the large particle size fractions (>250 µm), which might be due to the presence of mineral phases containing arsenic such as arsenolite or pyrite in the large particle size fraction soils. Sequential extraction showed that arsenic in mine soils was majorly present as associated with amorphous oxides. After soil washing with oxalic acid, arsenic in soils associated with amorphous oxides was greatly decreased, whereas the arsenic fraction associated sulfide and organic matter was increased. Soil washing decreased the bioaccessible arsenic concentration (mg/kg) in soil, but increased the bioaccessibility (%) depending on the soil characteristics. Conclusions : Soil washing changed arsenic species in soils, which affected mobility and risk of arsenic in soil.