Coatomer Research Articles

Controlling the coordination environment of Co atoms derived from Co/ZIF-8 for boosting photocatalytic H2 evolution of CdS

Regulating the coordination environment of metal–Nx species by replacing N with low electronegativity atoms is an approach of tuning the electrocatalytic performance of metal-based sites. However, such effects on the enhancement of photocatalytic H2 evolution over semiconductors are not discussed yet. Herein, we designed and prepared Co-based cocatalysts with controlled coordination environment via calcination Co/ZIF-8 loaded with triphenylphosphine followed by a sulfurization treatment. It was then used as cocatalyst to modify CdS. The effects of the coordination environment of Co atoms on the H2 evolution activity of CdS were discussed. The obtained Co was co-stabilized by N, P, and S atoms and embedded in graphitic carbon (denoted as Co-NxPS/C). Experimental results indicated that the Co-NxPS/C exhibited high activity in enhancing H2 evolution of CdS with a value of 1260 μmol after 5 h irradiation. The simultaneous replacement of N with P and S atoms in N-stabilized Co embedded in carbon could enhance light harvesting, accelerate the transfer of photogenerated electrons from CdS to carbon with increased electrons accumulation ability and conductivity, improve charge separation efficiency, and enhance proton reduction kinetics. It is believed that the results of this study could promote the development of other high performance MOF-derived atomically dispersed cocatalysts to increase photocatalytic H2 evolution.

An oxygen-deficient cobalt-manganese oxide nanowire doped with P designed for high performance asymmetric supercapacitor

Transition metal oxides (TMOs) have poor electronic conductivity and insufficient electrochemical active sites, which prevent them from being widely applied in high-performance supercapacitors. Herein, a phosphorus-doped cobalt-manganese oxide nanowires electrode with rich oxygen vacancies (P-Co 2 MnO 4-x ) is successfully synthesized by two simple methods (hydrothermal and calcination). The oxygen vacancies formed in high temperature sintering increase the conductivity of the original electrode materials. In addition, trace doping of P atoms in Co 2 MnO 4-x can improve the redox kinetics and provide more electrochemical active centers for electrode materials. As a result, the optimized P-Co 2 MnO 4-x nanowires electrode delivers an ultrahigh mass specific capacity of 838 F g − 1 at 1 A g − 1 , and a super cycling stability (80.3% capacitance retention after 10,000 cycles). A hybrid supercapacitor assembling with P-Co 2 MnO 4-x electrode exhibits a supernal energy density of 25.18 Wh kg −1 along with a remarkable power energy of 800.07 W kg −1 . The introduction of double defects into binary cobalt-manganese oxide is shown to be resultful in improving the electrochemical performance of materials in this study. Moreover, this research also provides a novel way of thinking for the further study of defect engineering.

МАТЕМАТИЧНА МОДЕЛЬ ДИФУЗІЙНИХ ПРОЦЕСІВ ПІД ЧАС ЕЛЕКТРОННОКАТАЛІТИЧНОЇ ПЕРЕРОБКИ СО2 В ОРГАНІЧНІ СПОЛУКИ

Сучасне суспільство викидає в навколишнє атмосферне повітря велику кількість різноманітних хімічних сполук, серед яких – СО2, якого викидаються мільйони тонн. Одним із напрямів його переробки є електроннокаталітична переробка СО2 в органічні сполуки. Важливою складовою цього методу є дифузійні процеси, які через нетривалий час існування частини сполук найкраще досліджувати за допомогою математичної моделі. Тому було складено і вирішено математичну модель дифузійних процесів на поверхні каталізатора при електроннокаталітичній переробці СО2. В результаті моделювання отримано залежності коефіцієнта дифузії, швидкості дрейфу частинок, загального об’ємного коефіцієнта масовіддачі, а також швидкості масовіддачі.

Avoiding Sabatier’s conflict in bifunctional heterogeneous catalysts for the WGS reaction

Summary A new strategy for designing efficient bifunctional heterogeneous catalysts for the water-gas-shift (WGS) reaction (CO + H2O → CO2 + H2) is presented. We avoid conflicting tasks that require ∗OH or ∗O from dissociated water to adsorb on and then desorb from the substrate of a catalyst. Instead, the CO on metal directly obtains OH from water that is connected by weak hydrogen bonds to the substrate. The task of the substrate is to efficiently channel ∗H on its surface and release them as H2 gas. Experimental and theoretical results show that bifunctional catalysts with weakly reactive substrates have significantly higher CO conversion rates compared with highly reactive substrates that follow either the LH-associative or LH-redox process.

Flexible Co(OH)2/NiOxHy@Ni hybrid electrodes for high energy density supercapacitors

It is a great challenge to develop a new kind of electrode materials simultaneously possessing the high energy density as well as good flexibility for high performance wearable supercapacitors. Herein, a flexible bimetallic electrode with novel interconnected hierarchically porous network, i.e. ultrathin Co(OH)2 nanopetals deposited on nanoporous nickel oxide/hydroxide@nickel shell@core structure (Co(OH)2/np-NiOxHy@Ni), is synthesized through dealloying and electrodeposition. The hierarchical porous structure provides high effective surface area to reserve electrolytes and facilitates ions diffusion, which significantly improves the reaction kinetics of the active materials. Owing to the hierarchical porous structure and synergetic effect of Co(OH)2 and NiOxHy electroactive materials, the Co(OH)2/np-NiOxHy@Ni hybrid electrode delivers wider voltage window (–0.1~0.6 V) and higher specific capacitance (1421.1F cm−3 at 0.5 A cm−3) than the single transition-metal electrodes. Density functional theory (DFT) further proves that the d-orbit of Co atom in Co(OH)2 and the p-orbit of O atom in NiO forms a strong hybridization, which endows the newly developed Co(OH)2/np-NiOxHy@Ni electrode an improved electrochemical performance. Furthermore, the flexible all-solid-state symmetric supercapacitors (SCs) is assembled using Co(OH)2/np-NiOxHy@Ni hybrid electrode. The SCs deliver a high energy density of 59.2 mWh cm−3, which far excels those of commercial supercapacitors (less than10 mWh cm−3). Moreover, the watch could run for more than 45 min while the SCs is subjected with different deformations, demonstrating a fascinating application prospects in the wearable fields.

Adsorption mechanism of the environmentally friendly insulating gas C5F10O and its main decomposition products on a Cu (1 1 1) surface

The perfluoroketone C5F10O, not only has good environmental compatibility, but also has excellent insulation properties and has the potential to replace SF6 in medium and low voltage switchgear. This paper focuses on the compatibility of C5F10O and its main decomposition products with the copper material inside the equipment and specifically constructs several adsorption models of C5F10O and its main decomposition products on a Cu (1 1 1) surface. The adsorption mechanism was studied from a system structure, electron density distribution and density of states perspective. The results showed that C5F10O mainly adsorbed chemically on the Cu surface through carbonyl oxygen atoms. Its maximum adsorption energy is −24.48 kcal mol−1, with electrons transferring from the copper surface to the adsorbed oxygen atom; and the adsorption energies of the main breakdown products CF4, C2F6, C3F8, C3F6, CF2O, C2F4O and CO2 with the surface are all less than −9.56 kcal mol−1. There is no obvious electron transfer between these decomposition gases and the copper surface, and thus, they belong to physical adsorption; while the adsorption energies of C3F6O, C2F4 and CO in the top, bridge and Hcc sites are respectively −9.661 kcal mol−1, −14.651 kcal mol−1 and −25.489 kcal mol−1, electrons on the copper surface transfer to the oxygen atom of C3F6O and carbon atoms of C2F4 and CO, forming stable chemical bonds, which belong to chemical adsorption. Through further analysis of the electron density of states, it is found that C5F10O and C3F6O bonded to the copper surface mainly by p orbitals on carbonyl oxygen atom and the pseudogaps are 3.2 eV and 2.5 eV. C2F4 and CO bonded to the copper surface by sp hybrid orbitals of carbon atoms, with a pseudogap of about 4 eV. Based on the above analysis, it can be seen that C5F10O and its main decomposition products C3F6O, C2F4 and CO have poor compatibility with metal copper.

Influence atoms of Co, Ni, Cu on the catalytic activity of small Pt clasters: First principles calculations

Based on the calculations from the first principles, we obtained the distributions of valence electron densities and electronic energy spectra for small Ptn clusters (where n = 1-5 atoms). According to the results of calculations, it is determined that the inclusion of oxygen atoms or atoms of other kinds in small Ptn clusters, as a rule, affect the catalytic activity of research systems. It is established that during doping of small platinum clusters by atoms of 3-d transition metals (Cu, Ni, Co), the electronic structure of the cluster and the band gap change. This in turn helps to increase the catalytic activity of platinum.

First-principles study of CO adsorption on Os atom doped anatase TiO2 (1 0 1) surface

In this paper, the adsorption of CO on the three anatase TiO2 (101) surface systems of intrinsic TiO2, an oxygen vacancy modification (VO/TiO2), and co-modified by an oxygen vacancy and a doped noble metal atom Os (Os/TiO2) are studied by first-principles. The results show that the adsorption performance of CO on intrinsic TiO2 is fine; especially the adsorption performance is greatly improved after adding defects. The adsorption energies of the TiO2 system, the VO/TiO2 system, and the Os/TiO2 system are −0.552 eV, −1.591 eV, and −3.971 eV, respectively. By analyzing the adsorption energy (ΔEads), bond length, charge distribution, density of states (DOS) and charge differential density (CDD) of the three systems, it is found that their adsorption mechanism and adsorption methods are different. The intrinsic TiO2 system is physical adsorption, and the carbon atom in CO has a strong interaction with Ti5C, but they cannot form bonds. The VO/TiO2 system is chemical adsorption, and the carbon atom of CO exerts a strong interaction with Ti2, resulting in a C-Ti2 bond. The Os/TiO2 system is chemical adsorption, and the strong interaction between carbon atom of CO and OS atom causes the formation of C-Os bond.

Alkaline Earth Metals Activate N2 and CO in Cubic Complexes Just Like Transition Metals: A Conceptual Density Functional Theory and Energy Decomposition Analysis Study.

Following the recent discovery of stable octa‐coordinated alkaline earth metals with N2 and CO, the role of group II metals in the catalytic reduction of these ligands by means of density functional theory (DFT) calculations and conceptual DFT‐based reactivity indices is investigated. Cubic group IV and octahedral group VI transition metal complexes as well as the free ligands are computed for reference. The outer and most accessible atoms of N2 and CO become much more nucleophilic and electrophilic in all complexes, relevant for N2 fixation, as probed by the Fukui function and local softness. Within one row of the periodic table, the alkaline earth complexes often show the strongest activation. On the contrary, the electrostatic character is found to be virtually unaffected by complexation. Trends in the soft frontier orbital and hard electrostatic character are in agreement with calculated proton affinities and energy decomposition analyses of the protonated structures, demonstrating the dominance of the soft (HOMO–LUMO) orbital interactions.

Characterization of Acid and Basic Sites on Zirconia Surfaces and Nanoparticles by Adsorbed Probe Molecules: A Theoretical Study

Acid and basic sites on monoclinic and tetragonal zirconia were investigated at the DFT level by computing IR and NMR properties of adsorbed probe molecules. Regular and stepped ZrO2 surfaces as well as stoichiometric zirconia nanoparticles have been considered. Acidity and basicity were probed by the adsorption of carbon monoxide and pyrrole, respectively. CO adsorption shows a positive shift of the C–O stretching frequency in IR spectra while the C atom of CO is shielded and 13C chemical shifts moves to higher field as a function of the strength of the acid site. For the study of basic sites we used a pyrrole molecule, but the interaction between the pyrrole ring and the surface leads to adsorption modes that cannot be used to titrate the surface basicity. On the other hand, at high coverage the molecule assumes an upright position and the formation of a hydrogen bond of the pyrrole NH group with the oxygen atoms of the surfaces provides a proxy of the basic properties of these sites. In particular, we focus on changes of the N–H IR frequency, 1H, 15N, and 17O NMR chemical shifts and their correlations with the surface basicity. Among the correlations found, that between the N–H stretching frequency of adsorbed pyrrole and the 17O NMR chemical shift of the O ion where the molecule is bound show a nice linear correlation. These two properties can provide useful information about the basic character of various O sites on the surface of zirconia.Graphic

Microscopic Distribution of Chemical Constituents in the Interlayer Space of OTAC Intercalated Montmorillonite Complex:Molecular Simulation Study

AbstractThis work employs molecular simulation to study the microscopic distribution of chemical constituents in the interlayer of octadecyl trimethyl ammonium chloride (OTAC) intercalated montmorillonite (MMT) complex. The results show that in the anhydrous MMT interlayers, the atoms of Co, Ho and No (C, H and N in OTAC) change from double‐layer to uneven multi‐layer distribution as the charge density of MMT increases, Ot (O in tetrahedron sheets) and No coordinate at the distance of 4.5 Å, the position of Co and Ho are closer to the siloxane surfaces than that of No, and the diffusivity of them decreases with the increase of charge density. In the hydrous MMT interlayers, the atoms of Co, Ho and No distribute in multiple layers along Z‐axis, the diffusivity of No is higher than that in the absence of water, but as for Co and Ho, only when charge density of MMT is low, the diffusivity of them is higher than that in the absence of water, and OTAC exhibit obvious three‐four‐five layers and six inclined layers arrangement between MMT layers with the increase of charge density.

Single Atom Nucleation of Cobalt over Graphene Oxide: Theory and Experimental Study.

Nucleation and growth mechanism of Co electrodeposition over graphene oxide (GO) was studied using cyclic voltammetry (CV) and chronoamperometry techniques. The studies were performed over an aqueous electrolyte containing 10 mM CoSO4·7H2O maintained at an initial pH of 2.5. CV studies established that the deposition mechanism was diffusion-controlled and irreversible. Chronoamperometry studies revealed the presence of three concurrent processes: an initial adsorption current, which indicated adatom layer formation, 3D nucleation and growth of Co islands over GO, and hydrogen evolution over the deposited Co nanoclusters. It was observed that the nucleation rate increased with increasing the overpotential (η) for deposition (from 2.71 × 104 cm-2 s-1 at η = 0.35 V to 3.62 × 106 cm-2 s-1 at η = 0.90 V). Application of the classical theory of nucleation over the chronoamperometry results suggested that the free energy of formation of the critical nucleus was lower than room temperature thermal energy. This indicated that the nucleation and growth process was not activation-controlled but rather a kinetically controlled process. Application of Milchev's atomistic theory revealed that every single atom of Co deposited over the GO sheet was a supercritical nucleus that could grow into a cluster irreversibly.

Electronic Properties of the Interface Between Metallic Doped Zigzag Graphene and Pristine Graphene Nanoribbons

The electronic properties of the interface between metallic doped zigzag graphene nanoribbons and perfect graphene nanoribbons are investigated. The density functional based tight-binding approach and a non-equilibrium Green function method are employed for our calculations. The atoms of Ni, Co, and Fe are used as doping atoms. The graphs of current–voltage, density of states, electron density, transmission spectrum, and rectification ratio are obtained. The results show that the bond lengths around the atoms of Ni, Co, and Fe are increased. Moreover, it is observed that for the graphene-based nanodevice the curves of current–voltage are linear and symmetric when no impurity exists and with the effect of impurities the curves are non-linear and asymmetric. While Ni, Co, and Fe impurities are applied into the systems we found that the maximum electron densities are located around the impurities of Ni and the current is decreased. The density of states and transmission spectrum are also examined for different systems. It is found that for certain amount of energies some resonances occur for the current, and the atoms at the edge of nanoribbon are mostly responsible for the transfer of the electrons. The obtained results can be of interest for the construction of nanoelectronic devices and can have practical applications.

First-Principles Study of 3<i>D</i> Transition Metal Doped Single-Layer Graphene

The electronic structure and magnetic properties of C atoms in Co, Ni-substituted graphene single-layers were studied by first-principles calculation method based on density functional theory. The study found that the pure graphene single-layer is an insulator, does not have magnetism, and we found that the doping of Co and Ni atoms alone does not make the system magnetic. Both Co and Ni atoms are capable of generating impurity levels in the graphene single-layer system. The impurity level of Co atom doping is 0.75 eV below the Fermi level, and the impurity level of Ni atom doping is 0.4 eV above the Fermi level. Studies on the coupling doping of Co and Ni atoms show that two different distance Co atoms or Ni atoms in the graphene single-layer are not always ferromagnetically coupled, and a stable magnetic ground state cannot be obtained. It can produce different magnetic ground states by controlling different doping distances, thus we provide one new way to control the spin properties.

Mechanism of Cu dopant in transition metal oxide Mn1.56Co(0.96-x)Ni0.48CuxO4+δ (0 ≤ x ≤ 0.2) thin films

To identify the substitution of Cu element for cations in transition metal oxide AB2O4 structure, we propose to investigate the structural, optical and magnetic properties of Cu-doped Mn1.56Co(0.96-x)Ni0.48CuxO4+δ films with 0 ≤ x ≤ 0.2. The X-ray diffraction of Mn1.56Co(0.96-x)Ni0.48CuxO4+δ films indicates good crystallization and cubic spinel structure. Raman spectra show two peaks of F2g and A1g vibration modes, and the peak position changes with Cu dopant. The transition temperatures TP1, TP2 shift towards lower temperature by Cu doping, and the saturation magnetization MS decreases monotonously. The results show that the substitution of Cu ions does not vary the crystal structure but changes the ratio of Mn3+/Mn4+. Our proposed way overcomes the difficulties to directly observe the atoms of Mn, Co, Ni, and Cu with closed sizes in microstructure, and it is instructive to further understand the conduction mechanism of doped manganese cobalt nickel oxides in theory.

Investigating the chemisorption of CO and CO2 on Al- and Cu-doped ZnO nanowires by density-functional calculations

The chemisorption of CO and CO2 on zinc oxide triangular nanowires, doped with Al and Cu atoms, was studied using density functional calculations. Adsorption energies for CO on doped nanowires are higher than those for the pristine nanowire. In particular, configurations forming CAl bonds have double the adsorption energies when compared to those in the pristine nanowire. A lower adsorption energy is found when both the CO atoms are bonded to the Al-doped ZnO nanowire. For the Cu-doped nanowire, double-bond configurations have the highest adsorption energies. CO2 forms tri-dentate structures in the pristine nanowires. In Al-doped nanowires, we observed mono- and bi-dentate modes of adsorption, and only one configuration showed enhancement in the adsorption energy. The CO2 prefers to bond to a Zn atom in the Cu-doped nanowire. Reductions in the band gap energy and shifts in the stretching frequency due to adsorption are also discussed.

Structures of selected transition metal complexes with 9-(2-hydroxyethyl)adenine: Potentiometric complexation and DFT studies

The dissociation constants of ligand 9-(2-hydroxyethyl)adenine (9-HOEtAde) and the molecular property stability constant (Kf1) of ML complex has been potentiometrically determined with different media and temperatures. Fe(III) and Cu(II) form very stable ML complexes with Kf1 11.02 and 9.17 respectively in 75% Ethanol-water medium at 25 °C, while Kf1 with Mn(II), Co(II), Ni(II) and Zn(II) range from 6.25 to 6.47. The formation of ML complexes is spontaneous and exothermic.Complexes of Co(II), Zn(II) and Cd(II) with 9-HOEtAde were synthesized and characterized by FTIR, NMR, thermal analysis and elemental analysis. The molecular structural property of Co(II) and Zn(II) complexes were studied by X-ray diffraction. Co(II) atoms in Co(acac)2(9-HOEtAde-N7)2.2H2O (1) displayed octahedral CoO4N2 coordination with four oxygen atoms of acetylacetonate ligands occupying the equatorial plane and two N7 atoms of 9-HOEtAde ligand in the axial plane. While Zn(9-HOEtAde-N7)2Cl2 (2) has a tetrahedral structure with π-π stacking. H-bonding in complexes (1) and (2) results in formation of chains and 3D-supramolecular array.CAM-B3LYP/6-31G(D)/LanL2DZ level of theory predicted the properties of tetrahedral Zn(9-HOEtAde-N7)2Cl2 (2) and the optimized structure was computed by DFT.

Reaction mechanisms for reduction of CO2 to CO on monolayer MoS2

Since the reduction of CO2 to fuels by consuming over-generated electricity, which can establish artificial carbon cycle and energy storage at the same time, extensive studies have been devoted to developing suitable catalysts for CO2 conversion in materials science. Recently, MoS2, a typical member of transition metal dichalcogenides, has been widely investigated for its high activity and low energy cost to catalyze CO2 reduction. In this work, we simulate the microscopic dynamic process of the CO2 reduction process in the framework of density functional theory (DFT). Our results reveal that Mo exposed edges of MoS2 are inclined to adsorb CO2 molecule and tend to catalytically reduce CO2 to CO. CO2 molecule is activated by two neighboring Mo atoms and the CO double bond reconstructs in the adsorption process. The first proton/electron (H+ + e−) reaction taking place at MoS2 edges undergoes a different pathway from that on transition metal catalyst, contributing to the product selectivity towards CO. Finally, we demonstrate that desorption of CO from MoS2 edges is in virtue of unique diffusion process for adsorbed CO atoms.

Dynamic Simulation and Experimental Study of Magnesia Formed Between Magnesium Vapor and CO Under Vacuum

We report the behavior of Mg vapor and CO by dynamic simulation and an experimental study under vacuum. The results of the density functional theory indicated that the interaction of a single CO molecule with the Mg(100) surface was only physical adsorption. During the interaction of two CO molecules with the Mg(100) surface, a strong C-C bond was formed between C1 and C2, and C1-O1 and C2-O2 were stretched to different levels. Three CO atoms combined to form C-C-C and released O atoms from the Mg(100) surface. The phase and composition of the condensate were examined by means of scanning electron microscopy. It was found that magnesium vapor condensation with CO produced MgO and carbonaceous aggregates under vacuum. The formation mechanism of MgO in condensed magnesium vapor is explained as the formation of a carbon chain that drives the whole reaction.

Initial quantum levels of captured muons in CO, CO2, and COS

The role of valence electrons for the muon capture process by molecules is experimentally investigated with the aid of cascade calculations. Low-momentum muons are introduced to gas targets of CO, CO2, and COS below atmospheric pressure. The initial states of captured muons are determined from the measured muonic X-ray structure of the Lyman and Balmer series. We propose that the lone pair electrons in the carbon atom of CO significantly contribute to the capture of a muon with large angular momenta.