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1. Encapsulation of tricopper cluster in a synthetic cryptand enables facile redox processes from Cu ^I Cu ^I Cu ^I to Cu ^II Cu ^II Cu ^II states

   https://doi.org/10.1039/D0SC05441K  

   Zhang, Weiyao ; Moore, Curtis E. ; Zhang, Shiyu ( March 2021 , Chemical Science)

   One-pot reaction of tris(2-aminoethyl)amine (TREN), [Cu I (MeCN) 4 ]PF 6 , and paraformaldehyde affords a mixed-valent [ TREN4 Cu II Cu I Cu I (μ 3 -OH)](PF 6 ) 3 complex. The macrocyclic azacryptand TREN4 contains four TREN motifs, three of which provide a bowl-shape binding pocket for the [Cu 3 (μ 3 -OH)] 3+ core. The fourth TREN caps on top of the tricopper cluster to form a cryptand, imposing conformational constraints and preventing solvent interaction. Contrasting the limited redox capability of synthetic tricopper complexes reported so far, [ TREN4 Cu II Cu I Cu I (μ 3 -OH)](PF 6 ) 3 exhibits several reversible single-electron redox events. The distinct electrochemical behaviors of [ TREN4 Cu II Cu I Cu I (μ 3 -OH)](PF 6 ) 3 and its solvent-exposed analog [ TREN3 Cu II Cu II Cu II (μ 3 -O)](PF 6 ) 4 suggest that isolation of tricopper core in a cryptand enables facile electron transfer, allowing potential application of synthetic tricopper complexes as redox catalysts. Indeed, the fully reduced [ TREN4 Cu I Cu I Cu I (μ 3 -OH)](PF 6 ) 2 can reduce O 2 under acidic conditions. The geometric constraints provided bymore »the cryptand are reminiscent of Nature's multicopper oxidases (MCOs). For the first time, a synthetic tricopper cluster was isolated and fully characterized at Cu I Cu I Cu I ( 4a ), Cu II Cu I Cu I ( 4b ), and Cu II Cu II Cu I ( 4c ) states, providing structural and spectroscopic models for many intermediates in MCOs. Fast electron transfer rates (10 5 to 10 6 M −1 s −1 ) were observed for both Cu I Cu I Cu I /Cu II Cu I Cu I and Cu II Cu I Cu I /Cu II Cu II Cu I redox couples, approaching the rapid electron transfer rates of copper sites in MCO.« less
2. Probing active sites for carbon oxides hydrogenation on Cu/TiO2 using infrared spectroscopy

   https://doi.org/10.1038/s42004-022-00650-2  

   Shaaban, Ehab ; Li, Gonghu ( December 2022 , Communications Chemistry)

   Abstract The valorization of carbon oxides on metal/metal oxide catalysts has been extensively investigated because of its ecological and economical relevance. However, the ambiguity surrounding the active sites in such catalysts hampers their rational development. Here, in situ infrared spectroscopy in combination with isotope labeling revealed that CO molecules adsorbed on Ti 3+ and Cu + interfacial sites in Cu/TiO 2 gave two disparate carbonyl peaks. Monitoring each of these peaks under various conditions enabled tracking the adsorption of CO, CO 2 , H 2, and H 2 O molecules on the surface. At room temperature, CO was initially adsorbed on the oxygen vacancies to produce a high frequency CO peak, Ti 3+ −CO. Competitive adsorption of water molecules on the oxygen vacancies eventually promoted CO migration to copper sites to produce a low-frequency CO peak. In comparison, the presence of gaseous CO 2 inhibits such migration by competitive adsorption on the copper sites. At temperatures necessary to drive CO 2 and CO hydrogenation reactions, oxygen vacancies can still bind CO molecules, and H 2 spilled-over from copper also competed for adsorption on such sites. Our spectroscopic observations demonstrate the existence of bifunctional active sites in which the metal sitesmore »catalyze CO 2 dissociation whereas oxygen vacancies bind and activate CO molecules.« less
3. Computational screening of chemically active metal center in coordinated dipyridyl tetrazine network

   https://doi.org/10.1088/1361-648X/acb8f3  

   Din, Naseem Ud ; Le, Duy ; Rahman, Talat S. ( February 2023 , Journal of Physics: Condensed Matter)

   Creation, stabilization, characterization, and control of single transition metal (TM) atoms may lead to significant advancement of the next-generation catalyst. Metal organic network (MON) in which single TM atoms are coordinated and separated by organic ligands is a promising class of material that may serve as a single atom catalyst. Our density functional theory-based calculations of MONs in which dipyridyl tetrazine (DPTZ) ligands coordinate with a TM atom to form linear chains leads to two types of geometries of the chains. Those with V, Cr, Mo, Fe, Co, Pt, or Pd atoms at the coordination center are planar while those with Au, Ag, Cu, or Ni are non-planar. The formation energies of the chains are high (∼2.0–7.9 eV), suggesting that these MON can be stabilized. Moreover, the calculated adsorption energies of CO and O₂on the metal atom at center of the chains with the planar configuration lie in the range 1.0–3.0 eV for V, Cr, Mo, Fe, and Co at the coordination center, paving the way for future studies of CO oxidation on TM-DPTZ chains with the above five atoms at the coordination center.
4. Stability of adsorption of Mg and Na on sulfur-functionalized MXenes

   https://doi.org/10.1039/d1cp03433b  

   Chaney, G. ; Çakır, D. ; Peeters, F. M. ; Ataca, C. ( November 2021 , Physical Chemistry Chemical Physics)

   Two-dimensional materials composed of transition metal carbides and nitrides (MXenes) are poised to revolutionize energy conversion and storage. In this work, we used density functional theory (DFT) to investigate the adsorption of Mg and Na adatoms on five M 2 CS 2 monolayers (where M = Mo, Nb, Ti, V, and Zr) for battery applications. We assessed the stability of the adatom ( i.e. Na and Mg)-monolayer systems by calculating adsorption and formation energies, as well as voltages as a function of surface coverage. For instance, we found that Mo 2 CS 2 cannot support a full layer of Na nor even a single Mg atom. Na and Mg exhibit the strongest binding on Zr 2 CS 2 , followed by Ti 2 CS 2 , Nb 2 CS 2 and V 2 CS 2 . Using the nudged elastic band method (NEB), we computed promising diffusion barriers for both dilute and nearly full ion surface coverage cases. In the dilute ion adsorption case, a single Mg and Na atom on Ti 2 CS 2 experience ∼0.47 eV and ∼0.10 eV diffusion barriers between the lowest energy sites, respectively. For a nearly full surface coverage, a Na ion moving onmore »Ti 2 CS 2 experiences a ∼0.33 eV energy barrier, implying a concentration-dependent diffusion barrier. Our molecular dynamics results indicate that the three (one) layers (layer) of the Mg (Na) ion on both surfaces of Ti 2 CS 2 remain stable at T = 300 K. While, according to voltage calculations, Zr 2 CS 2 can store Na up to three atomic layers, our MD simulations predict that the outermost layers detach from the Zr 2 CS 2 monolayer due to the weak interaction between Na ions and the monolayer. This suggests that MD simulations are essential to confirm the stability of an ion-electrode system – an insight that is mostly absent in previous studies.« less
5. Temperature dependence of collisional broadening and shift for the Kr 4 p ⁶ S 01→5 p [3/2] ₂ electronic transition

   https://doi.org/10.1364/AO.380102  

   Sahoo, Abinash ; Zelenak, Dominic ; Narayanaswamy, Venkateswaran ( February 2020 , Applied Optics)

   Temperature scaling of collisional broadening parameters for krypton (absorber)$4p^6{S}_0^1\to <\#comment/>5p[3/2{]}_2$electronic transition centered at 107.3 nm in the presence of major combustion species (perturber) is investigated. The absorption spectrum in the vicinity of the transition is obtained from the fluorescence due to the two-photon excitation scan of krypton. Krypton was added in small amounts to major combustion species such as${\mathrm{C}\mathrm{H}}_4$,${\mathrm{C}\mathrm{O}}_2$,${\mathrm{N}}_2$, and air, which then heated to elevated temperatures when flowed through a set of heated coils. In a separate experimental campaign, laminar premixed flat flame product mixtures of methane combustion were employed to extend the investigations to higher temperature ranges relevant to combustion. Collisional full width half maximum (FWHM) ($w_C$) and shift (${\mathrm{\delta <\#comment/>}}_C$) were computed from the absorption spectrum by synthetically fitting Voigt profiles to the excitation scans, and their corresponding temperature scaling was determined by fitting power-law temperature dependencies to the$w_C$and${\mathrm{\delta <\#comment/>}}_C$data for each perturber species. The temperature exponents of$w_C$and${\mathrm{\delta <\#comment/>}}_C$for all considered combustion species (perturbers) were$-<\#comment/>0.73$and$-<\#comment/>0.6$, respectively. Whereas the temperature exponents of$w_C$are closer tomore »the value ($-<\#comment/>0.7$) predicted by the dispersive interaction collision theory, the corresponding exponents of${\mathrm{\delta <\#comment/>}}_C$are in between the dispersive interaction theory and the kinetic theory of hard-sphere collisions. Comparison with existing literature on broadening parameters of NO, OH, and CO laser-induced fluorescence spectra reveal interesting contributions from non-dispersive interactions on the temperature exponent.

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