Magnetic properties of Sn- and Mn-incorporated Co2TiO4 from single-step calcination

This report deals with the rapid synthesis of cobalt titanate spinels (Co₂TiO₄ (CTO), Co₂Sn_0.50Ti_0.50O₄ (CSTO), and Co₂Mn_0.50Ti_0.50O₄ (CMTO) by the single-step calcination of hydroxide precursors and their extensive characterization. The cubic unit cell expanded and contracted when Ti in CTO was partially replaced with Sn and Mn, respectively. The Raman spectra confirmed the cubic spinel structure and showed a systematic shift with the inclusion of tin and manganese. The broadening of Raman bands suggested cation disorder. The absorbance spectra of CTO and CSTO proved the existence of cobalt in the +2 and +3 states with optical bandgaps of 0.6 and 1.1 eV, respectively. X-ray photoelectron spectroscopic (XPS) analysis confirmed the presence of cobalt in both the +2 and +3 states with titanium in the +3 state in CTO. While cobalt and titanium existed in the +2 and +3 states in CMTO, mixed-valence (+3 (minor) and +4 (major amounts) states) was deduced for manganese. All these samples showed an exchange-bias effect and a long-range ferrimagnetic ordering with T_N of 50 (CTO), 46 (CSTO), and 54 K (CMTO). These transitions have been independently verified from heat capacity measurements performed at zero fields and applied fields of 500 and 1000 Oe. The compensation of magnetic moments from the tetrahedral and octahedral units of the spinel structure was not observed in any of these samples, perhaps because of the asymmetric distribution of cations among the available crystallographic sites.

The Influence of High-Energy Faceted TiO2 Supports on Co and Co-Ru Catalysts for Dry Methane Reforming

The reforming of methane from biogas has been proposed as a promising method of CO2 utilization.  Co-based catalysts are promising candidates for dry methane reforming. However, the main constraints limiting the large-scale use of Co-based catalysts are deactivation through carbon deposition (coking) and sintering due to weak metal-support interaction. We studied the structure-function properties and catalytic behavior of Co/TiO2 and Co-Ru/TiO2 catalysts using two different types of TiO2 supports, commercial TiO2 and faceted non-stoichiometric rutile TiO2 crystals (TiO2*). The Co and Ru metal particles were deposited on TiO2 supports using a wet-impregnation method with the percentage weight loading of Co and Ru of 5% and 0.5%, respectively. The materials were characterized using SEM, STEM-HAADF, XRD, XPS and BET. The catalytic performance was studied using the CH4:CO2 ratio of 3:2 to mimic the methane-rich biogas composition. Our results indicate that the addition of Ru to Co catalysts supported on TiO2* reduces carbon deposition and influences oxygen mobility. Co and Co-Ru catalysts supported on TiO2* has superior activity with the highest conversion of CO2 and CH4 of 34.7% and 23.5%, respectively. Despite the improved performance, the Co-Ru/TiO2* catalyst has limited stability due to the proliferation of nanoparticle growth and TiOx layers on the surface of the nanoparticles indicating the prevalence of the strong-metal support interaction.

Magnetic compensation and critical behavior in spinel Co2TiO4

We report studies of the complex magnetic ordering, pressure-induced magnetic properties, large exchange bias (EB), spin-glass (SG), critical behavior, and electron spin resonance (ESR) in spinel Co2TiO4. The magnetic compensation behavior occurs in the vicinity of the compensation temperature Tcomp ∼ 32.5 K (defined as the susceptibility χZFC = χFC = 0), which can be attributed to the behavior that the magnetization of two bulk sublattices balances each other completely. The nature of this unusual case is demonstrated by the spin direction upon applied field and A-B sublattice (site) coupling. Specifically, the values of exchange integrals JAA and JBB play a crucial role at lower and higher temperatures, respectively. It is prominent that intrinsic coercivity Hcj increases by 168% at a pressure of 1000 MPa, from which we illustrate the antiferromagnetic (AFM) transition based on magnetic hysteresis loops M(H) and temperature dependent magnetization M(T) curves. The SG behavior of Co2TiO4 is confirmed by a series of reliable measurements and fitting parameters (τ0, zv), and a large EB field is also found through the asymmetry in the M(H) curve. Besides, the critical behavior of Co2TiO4 is studied initially in our present work, and the critical exponents (β, γ, and δ) indicate long-range ferromagnetic (FM) coupling accompanied by a short-range interaction in Co2TiO4.

Neutron diffraction evidence for local spin canting, weak Jahn-Teller distortion, and magnetic compensation in Ti1-x Mn x Co2O4 spinel

A systematic study using neutron diffraction and magnetic susceptibility is reported on Mn substituted ferrimagnetic inverse spinel Ti_1-x Mn _x Co₂O₄ in the temperature interval 1.6 K [Formula: see text] T [Formula: see text] 300 K. Our neutron diffraction study reveals cooperative distortions of the TO₆ octahedra in the Ti_1-x Mn _x Co₂O₄ system for all the Jahn-Teller active ions T  =  Mn³⁺ , Ti³⁺ and Co³⁺ , having the electronic configurations 3d ¹, 3d ⁴ and 3d ⁶, respectively which are confirmed by the x-ray photoelectron spectroscopy. Two specific compositions (x  =  0.2 and 0.4) have been chosen in this study because these two systems show unique features such as; (i) noncollinear Yafet-Kittel type magnetic ordering, and (ii) weak tetragonal distortion with c/a  <  1, in which the apical bond length d _c (T _B -O) is longer than the equatorial bond length d _ab (T _B -O) due to the splitting of the e _g level of Mn³⁺ ions into [Formula: see text] and [Formula: see text]. For the composition x  =  0.4, the distortion in the T _B O₆ octahedra is stronger as compared to x  =  0.2 because of the higher content of trivalent Mn. Ferrimagnetic ordering in Ti_0.6Mn_0.4Co₂O₄ and Ti_0.8Mn_0.2Co₂O₄ sets in at 110.3 and 78.2 K, respectively due to the presence of unequal magnetic moments of cations, where Ti³⁺ , Mn³⁺ , and Co³⁺ occupy the octahedral, whereas, Co²⁺ sits in the tetrahedral site. For both compounds an additional weak antiferromagnetic component could be observed lying perpendicular to the ferrimagnetic component. The analysis of static and dynamic magnetic susceptibilities combined with the heat-capacity data reveals a magnetic compensation phenomenon (MCP) at T _COMP  =  25.4 K in Ti_0.8Mn_0.2Co₂O₄ and a reentrant spin-glass behaviour in Ti_0.6Mn_0.4Co₂O₄ with a freezing temperature of  ∼110.1 K. The MCP in this compound is characterized by sign reversal of magnetization and bipolar exchange bias effect below T _COMP with its magnitude depending on the direction of external magnetic field and the cooling protocol.

Co2TiO4/Reduced Graphene Oxide Nanohybrids for Electrochemical Sensing Applications

For the first time, the synthesis, characterization, and analytical application for hydrogen peroxide quantification of the hybrid materials of Co2TiO4 (CTO) and reduced graphene oxide (RGO) is reported, using in situ (CTO/RGO) and ex situ (CTO+RGO) preparations. This synthesis for obtaining nanostructured CTO is based on a one-step hydrothermal synthesis, with new precursors and low temperatures. The morphology, structure, and composition of the synthesized materials were examined using scanning electron microscopy, X-ray diffraction (XRD), neutron powder diffraction (NPD), and X-ray photoelectron spectroscopy (XPS). Rietveld refinements using neutron diffraction data were conducted to determine the cation distributions in CTO. Hybrid materials were also characterized by Brunauer-Emmett-Teller adsorption isotherms, Scanning Electron microscopy, and scanning electrochemical microscopy. From an analytical point of view, we evaluated the electrochemical reduction of hydrogen peroxide on glassy carbon electrodes modified with hybrid materials. The analytical detection of hydrogen peroxide using CTO/RGO showed 11 and 5 times greater sensitivity in the detection of hydrogen peroxide compared with that of pristine CTO and RGO, respectively, and a two-fold increase compared with that of the RGO+CTO modified electrode. These results demonstrate that there is a synergistic effect between CTO and RGO that is more significant when the hybrid is synthetized through in situ methodology.

Multishelled Transition Metal-Based Microspheres: Synthesis and Applications for Batteries and Supercapacitors

With the rapid growth of material innovations, multishelled hollow nanostructures are of tremendous interest due to their unique structural features and attractive physicochemical properties. Continued effort has been made in the geometric manipulation, composition complexity, and construction diversity of this material, expanding its applications. Energy storage technology has benefited from the large surface area, short transport path, and excellent buffering ability of the nanostructures. In this work, the general synthesis of multishelled hollow structures, especially with architecture versatility, is summarized. A wealth of attractive properties is also discussed for a wide area of potential applications based on energy storage systems, including Li-ion/Na-ion batteries, supercapacitors, and Li-S batteries. Finally, the emerging challenges and outlook for multishelled hollow structures are mentioned.

Spinel Cobalt Titanium Binary Oxide as an All-Non-Precious Water Oxidation Electrocatalyst in Acid

Replacing precious water oxidation electrocatalysts used in proton exchange membrane (PEM) water electrolyzers with the nonprecious and abundant electrocatalysts is still a poorly addressed issue in the field of hydrogen generation in acidic medium through water electrolysis. Herein we report such an all-nonprecious binary spinel metal oxide the "cobalt titanate" (Co₂TiO₄) as an efficient alternate to expensive IrO₂ and RuO₂ for PEM water electrolyzer. The synthesized Co₂TiO₄ octahedral nanocrystals of size 50 to 210 nm showed excellent oxygen evolution reaction (OER) activity in 0.5 M H₂SO₄, which was comparable to IrO₂ and better than spinel Co₃O₄ when examined under identical experimental conditions. Overpotential of just 513 mV was sufficient enough to drive a kinetic current density of 10 mA cm^-2, which is a significant figure of merit as far as acidic water oxidation electrocatalysis is concerned.

Catalytic oxidation of dibromomethane over Ti-modified Co3O4 catalysts: Structure, activity and mechanism

Ti-modified Co₃O₄ catalysts with various Co/Ti ratios were synthesized using the co-precipitation method and were used in catalytic oxidation of dibromomethane (CH₂Br₂), which was selected as the model molecule for brominated volatile organic compounds (BVOCs). Addition of Ti distorted the crystal structure and led to the formation of a Co-O-Ti solid solution. Co₄Ti₁ (Co/Ti molar ratio was 4) achieved higher catalytic activity with a T₉₀ (the temperature needed for 90% conversion) of approximately 245°C for CH₂Br₂ oxidation and higher selectivity to CO₂ at a low temperature than the other investigated catalysts. In addition, Co₄Ti₁ was stable for at least 30h at 500ppm CH₂Br₂, 0 or 2vol% H₂O, 0 or 500ppm p-xylene (PX), and 10% O₂ at a gas hourly space velocity of 60,000h^-1. The final products were CO_x, Br₂, and HBr, without the formation of other Br-containing organic byproducts. The high catalytic activity was attributed to the high Co³⁺/Co²⁺ ratio and high surface acidity. Additionally, the synergistic effect of Co and Ti made it superior for CH₂Br₂ oxidation. Furthermore, based on the analysis of products and in situ DRIFTs studies, a receivable reaction mechanism for CH₂Br₂ oxidation over Ti-modified Co₃O₄ catalysts was proposed.