More Like this

1. Basal Plane Hydrogen Evolution Activity from Mixed Metal Nitride MXenes Measured by Scanning Electrochemical Microscopy

   https://doi.org/10.1002/adfm.202001136  

   Djire, Abdoulaye ; Wang, Xiang ; Xiao, Chuanxiao ; Nwamba, O. Charles ; Mirkin, Michael V. ; Neale, Nathan R. ( May 2020 , Advanced Functional Materials)

   2D early transition metal carbide and nitride MXenes have intriguing properties for electrochemical energy storage and electrocatalysis. These properties can be manipulated by modifying the basal plane chemistry. Here, mixed transition metal nitride MXenes, M‐Ti₄N₃T_x(M = V, Cr, Mo, or Mn; T_x= O and/or OH), are developed by modifying pristine exfoliated Ti₄N₃T_xMXene with V, Cr, Mo, and Mn salts using a simple solution‐based method. The resulting mixed transition metal nitride MXenes contain 6–51% metal loading (cf. Ti) that exhibit rich electrochemistry including highly tunable hydrogen evolution reaction (HER) electrocatalytic activity in a 0.5mH₂SO₄electrolyte as follows: V‐Ti₄N₃T_x> Cr‐Ti₄N₃T_x> Mo‐Ti₄N₃T_x> Mn‐Ti₄N₃T_x> pristine Ti₄N₃T_xwith overpotentials as low as 330 mV at −10 mA cm⁻²with a charge‐transfer resistance of 70 Ω. Scanning electrochemical microscopy (SECM) reveals the electrochemical activity of individual MXene flakes. The SECM data corroborate the bulk HER activity trend for M‐Ti₄N₃T_xas well as provide the first experimental evidence that HER results from catalysis on the MXene basal plane. These electrocatalytic results demonstrate a new pathway to tune the electrochemical properties of MXenes for water splitting and related electrochemical applications.

    

   more » « less
2. Oxidative Amide Coupling from Functionally Diverse Alcohols and Amines Using Aerobic Copper/Nitroxyl Catalysis

   https://doi.org/10.1002/anie.201906130  

   Piszel, Paige E. ; Vasilopoulos, Aristidis ; Stahl, Shannon S. ( July 2019 , Angewandte Chemie International Edition)

   The aerobic Cu/ABNO catalyzed oxidative coupling of alcohols and amines is highlighted in the synthesis of amide bonds in diverse drug‐like molecules (ABNO=9‐azabicyclo[3.3.1]nonaneN‐oxyl). The robust method leverages the privileged reactivity of alcohols bearing electronegative hetero‐ atoms (O, F, N, Cl) in the β‐position. The reaction tolerates over 20 unique functional groups and is demonstrated on a 15 mmol scale under air. Steric constraints of the catalyst allow for chemoselective amidation of primary amines in the presence of secondary amines. All catalyst components are commercially available, and the reaction proceeds under mild conditions with retention of stereocenters in both reaction partners, while producing only water as a by‐product.

    

   more » « less
3. Oxidative Amide Coupling from Functionally Diverse Alcohols and Amines Using Aerobic Copper/Nitroxyl Catalysis

   https://doi.org/10.1002/ange.201906130  

   Piszel, Paige E. ; Vasilopoulos, Aristidis ; Stahl, Shannon S. ( July 2019 , Angewandte Chemie)

   The aerobic Cu/ABNO catalyzed oxidative coupling of alcohols and amines is highlighted in the synthesis of amide bonds in diverse drug‐like molecules (ABNO=9‐azabicyclo[3.3.1]nonaneN‐oxyl). The robust method leverages the privileged reactivity of alcohols bearing electronegative hetero‐ atoms (O, F, N, Cl) in the β‐position. The reaction tolerates over 20 unique functional groups and is demonstrated on a 15 mmol scale under air. Steric constraints of the catalyst allow for chemoselective amidation of primary amines in the presence of secondary amines. All catalyst components are commercially available, and the reaction proceeds under mild conditions with retention of stereocenters in both reaction partners, while producing only water as a by‐product.

    

   more » « less
4. Valences of Ti, Cr, and V in Apollo 17 high‐Ti and very low‐Ti basalts and implications for their formation

   https://doi.org/10.1111/maps.13123  

   Simon, Steven B. ; Sutton, Stephen R. ( June 2018 , Meteoritics & Planetary Science)

   To assess the variability of redox states among mare basalt source regions, investigation of the valence of Ti, Cr, and V and the coordination environment of Ti in pyroxene and olivine in lunar rocks viaXANES(X‐ray absorption near‐edge structure) spectroscopy has been extended to Apollo 17 basalts: two high‐Ti (70017 and 74275) hand samples, and three very low‐Ti (70006,371, 70007,289B, and 70007,296) basalt fragments from the Apollo 17 deep drill core. Valences of Ti in pyroxene of both suites range from 3.6 to 4, or from 40% to 0% Ti³⁺, averaging 15–20% Ti³⁺. Assuming Ti³⁺is more compatible in pyroxene than Ti⁴⁺, then even lower Ti³⁺proportions are indicated for the parental melts. TheVLTpyroxene exhibits a slightly wider range of V valences (2.57–2.96) than the high‐Ti pyroxene (2.65–2.86) and a much wider range of Cr valences (2.32–2.80 versus 2.68–2.86); Cr is generally reduced inVLTpyroxene compared to high‐Ti pyroxene. Valences of Ti and Cr inVLTpyroxene become less reduced with increasing FeO contents, possibly indicating change in oxygen fugacity during crystallization. Olivine in all samples has very low (<20%) proportions of Ti³⁺, with no Ti³⁺and higher proportions of Ti in tetrahedral coordination in theVLTs than in the high‐Ti basalts. Olivine in 74275, including that in a dunite clast, has much higher proportions of Cr²⁺than the pyroxene in that sample, consistent with previous studies indicating that the olivine grains in this sample are xenocrysts and possibly indicating oxidation just prior to pyroxene crystallization. Results for this sample, theVLTs, and previously studied Apollo 14 and 15 basalts all indicate that mare magmas were in reducing environments at depth, as recorded in early crystallization products, and that later, presumably shallower environments, were relatively oxidizing; single, characteristicfO₂s of formation cannot be assigned to these samples. A process likely to account for this feature seen in multiple samples is loss by degassing of a reducing, H‐rich vapor (probably H₂) during ascent and/or eruption, causing oxidation of the residual melt, recorded in relatively late‐crystallized pyroxene.

    

   more » « less
5. Accurate analyses of key petrogenetic minor and trace elements in olivine by electron microprobe

   https://doi.org/10.1016/j.chemgeo.2022.121199  

   Jiang, Peng ; Perfit, Michael ; Foster, David A. ; Trucco, Andres ( December 2022 , Chemical Geology)

   Abundances of minor and trace elements in olivine are increasingly used as petrogenetic indicators for mantle source lithologies, mantle metasomatism history, mantle potential temperatures, and magmatic differentiation. As it is common for olivine to be complexly zoned on a fine-scale, high precision analytical methods for EPMA (electron microprobe microanalyzer, or Electron Microprobe) trace element analysis under high spatial resolution have been developed. However, previous studies have focused more on analytical precision with fewer efforts in examining the accuracy of the data. In this study, we used the Cameca SXFive field emission (FE) EPMA to fully evaluate the effects of beam settings, background offsets and background regression models, and primary calibration standards on the data accuracy of 10 key petrogenetic elements (Na, Al, P, Ca, Ti, Cr, Mn, Co, Ni, and Zn) using MongOlSh11–2 olivine as a reference material. Our results indicate that high voltage, high beam current and long counting time not only improve data precision, but also improve data accuracy, especially on elements with low P/B (peak/background) ratios such as Zn and Cr. Importantly, careful background offsets and background regression models need to be obtained via high resolution WDS relative scans or step scans on each target element. Special care needs to be paid to Co element analysis to avoid or correct for peak interference of Fe Kβ. Among 10 minor and trace elements, exponential background regression models need to be applied to Al, Mn, and Ti elements, whereas other elements require linear background regression. Furthermore, to avoid Al and Zn surface contamination due to alumina polishing or brass presence, ultrasonic cleaning between each intermediate polishing steps and plasma cleaning immediately prior to EPMA experiments is highly recommended. Micro-inclusions such as chromite and spinel in olivine or adjacent Ca-rich phases need to be avoided to minimize primary or secondary fluorescence-related contamination on Al, Cr, or Ca. As a volatile element, Na element needs to be analyzed first with appropriate counting time to minimize the Na loss under high beam conditions. It needs mentioning that major elements (Mg, Fe, and Si) are best analyzed using MongOlSh11–2 or San Carlos olivine as primary standards for calibrations, which can yield more accurate data for both major elements and trace elements because of the improved matrix- corrections. Using our recommended analytical protocols, we have successfully discriminated “depleted” mantle olivine cores from an EMORB in northern East Pacific Rise (EPR) via Ca, Ti, Ni, Co, and Mn abundances. Our olivine data from Siqueiros Transform and the nearby 8◦20′ N seamounts also help reveal a metasomatized peridotite mantle beneath the northern EPR. Overall, the protocols proposed in this study can serve as a guide for accurate EPMA olivine trace element analyses, which potentially contributes to the efforts of fostering a comparable olivine database worldwide. 

   more » « less