Oxidation State Of Chromium Research Articles

Preparation and characterization of chromium oxide thin films: The response of morphology, crystal structure, and electrical properties to oxygen pressure under magnetron sputtering

This study focuses on Cr2O3 prepared via reactive magnetron sputtering, a direct deposition technique that significantly impacts the microstructure and optoelectronic properties of the films. Reactive sputtering alters grain size, density, and surface morphology, which in turn affects the structural order and stoichiometry. By adjusting the ratio of process gases, particularly the oxygen partial pressure, the stoichiometry of the deposited films was controlled. This is crucial for managing the oxidation state of chromium and the concentration of free electrons. An optimal oxygen partial pressure of 75 % was identified, substantially reducing oxygen interstitial defects in the CrOx materials and enhancing conductivity by nearly two orders of magnitude. Additionally, increasing the oxygen partial pressure helped integrate more oxygen atoms into the lattice, transitioning the electronic structure from a metallic state in CrO2 to a semiconductive state in CrO3. This fine-tuning of oxygen doping not only adjusts carrier concentrations but also optimizes the photoelectric properties of the materials, achieving a tailored high band gap of 3.34 eV. This study highlights the potential of reactive magnetron sputtering to customize semiconductor materials through oxygen doping, offering a novel approach to enhance the versatility and application range of chromium oxide materials in advanced technological applications.

Effect of annealing temperature on the properties and the chromium oxidation state of ZnAl1.94Cr0.06O4 nanoparticles prepared by coprecipitation method

Nanostructured materials doped with Cr+3 have potential applications. However, it has been reported that heat treatment can modify the oxidation state of Cr+3, resulting in the formation of toxic Cr+6 in host matrices such as TiO2, Al2O3, SiO2, ZrO2, and Fe(OH)3. The possibility of this occurrence in nanostructured ZnAl2O4:Cr+3 has not been previously considered. In this study, we report the effect of annealing temperatures within the range of 600–1300 °C on the structure, morphology, optical properties, and, more specifically, the chromium oxidation state of ZnAl1.94Cr0.06O4 nanoparticles obtained through the coprecipitation method. The obtention of single-phase systems with a crystal size ranging from 5.25 nm to 142.75 nm was determined by X-ray diffraction and Rietveld refinement. At 800 °C, a more significant oxidation of Cr+3 was observed, evident in the increased absorption intensity of UV–vis bands associated with Cr+6 and a reduction in magnetic susceptibility at ambient temperature. At 1200 °C, a complete reduction of Cr+6 occurred, leading to the appearance of bands related to Cr+3 in the UV–vis absorption spectra and increased magnetic susceptibility; the results were confirmed by X-ray photoelectron spectroscopy. The outcomes of this study offer new insights into the thermal stability of chromium in ZnAl2O4 nanoparticles.

Structure and magnetic properties of a La0.75Sr0.25Cr0.90O[formula omitted] single crystal

We have successfully grown large and good-quality single crystals of the La0.75Sr0.25Cr0.90O3−δ compound using the floating-zone method with laser diodes. We investigated the crystal quality, crystallography, chemical composition, magnetic properties and the oxidation state of Cr in the grown single crystals by employing a combination of techniques, including X-ray Laue and powder diffraction, scanning electron microscopy, magnetization measurements, X-ray photoelectron spectroscopy and light absorption. The La0.75Sr0.25Cr0.90O3−δ single crystal exhibits a single-phase composition, crystallizing in a trigonal structure with the space group R3̄c at room temperature. The chemical composition was determined as La0.75Sr0.25Cr0.90O3−δ, indicating a significant chromium deficiency. Upon warming, we observed five distinctive characteristic temperatures, namely T1=21.50(1) K, T2=34.98(1) K, T3=117.94(1) K, T4=155.01(1) K, and TN=271.80(1) K, revealing five distinct magnetic anomalies. Our magnetization study allows us to explore the nature of these anomalies. Remarkably, the oxidation state of chromium in the single-crystal La0.75Sr0.25Cr0.90O3−δ, characterized by a band gap of 1.630(8) eV, is exclusively attributed to Cr3＋ ions, making a departure from the findings of previous studies on polycrystalline materials.

Low-Cost Material Derived from Coconut Husk for the Removal of Chromium(VI) from Water Environment

A low-cost bio-material was derived from biomass agricultural by-product (coconut husk) and applying it in water treatment, makes both economic and environmental sense. Hexavalent chromium has been confirmed to be the most hazardous oxidation state of chromium and is potentially carcinogenic; therefore, it must be removed from the industrial effluent pipe before being discharged. The presents work focus on the fabrication of biochar from coconut husk and using for removing of chromium hexanvalent from aquatic environment. The Batchwise study and a chromatographic operation were used to exame the chromium(VI) reduction/adsorption performance. The typical results indicated that the removal of chromium(VI) was achieved in acidic region via adsorption and reduction processes. The kinetic study followed pseudo-second-order model (R2 ≈ 1.0), the results revealed that biochar at 800oC (BCH-8) system was occurred 6.2 times faster than that of raw coconut husk (BCH-raw). The diffusion-chemisorption kinetic model indicated that the BCH-raw system was only adsorption, but BCH-8 was both adsorption and reduction processes at pH 3.0. The removal of chromium(VI) by biochar based coconut husk was also achieved by chromatographic operation.

Machine Learning Algorithms for Intelligent Decision Recognition and Quantification of Cr(III) in Chromium Speciation.

Cr(III) is a common oxidation state of chromium, and its presence in the environment can occur naturally or as a result of human activities, such as industrial processes, mining, and waste disposal. This article explores the application of machine learning algorithms for the intelligent decision recognition and quantification of Cr(III) in chromium speciation. Three different machine learning models, namely, the Decision Tree (DT) model, the PCA-SVM (Principal Component Analysis-Support Vector Machine) model, and the LDA (Linear Discriminant Analysis) model, were employed and evaluated for accurate and efficient classification of chromium concentrations based on their fluorescence responses. Furthermore, stepwise multiple linear regression analysis was utilized to achieve a more precise quantification of trivalent chromium concentrations through fluorescence visualization. The results demonstrate the potential of machine learning algorithms in accurately detecting and quantifying Cr(III) in chromium speciation with implications for environmental and industrial applications in chromium detection and quantification. The findings from this research pave the way for further exploration and implementation of these models in real-world scenarios, offering valuable insights into various environmental and industrial contexts.

Tissue Accumulation, Cytotoxicity, Oxidative Stress, and Immunotoxicity in African Catfish, Clarias gariepinus Exposed to Sublethal Concentrations of Hexavalent Chromium.

Hexavalent chromium (Cr6+) is one of the stable oxidation states of chromium that has been reported to elicit various toxic effects in aquatic organisms. However, the mechanisms of Cr6+ toxicity are still poorly understood. Thus, the present study investigated the tissue accumulation, cytotoxic, oxidative stress, and immunotoxic effects of Cr6+ in juvenile Clarias gariepinus. The fish were exposed to waterborne Cr6+ concentrations (0, 0.42, 0.84, and 1.68 mg/L) for 28 days, after which they were sacrificed and various organs were harvested for the determination of Cr6+ levels. Other parameters that were indicators of oxidative stress, cytotoxicity, and immunotoxicity were measured. Cr6+ accumulated more in the kidney and liver of the exposed fish, especially at the highest concentration. The levels of lipid peroxidation and DNA fragmentation increased significantly in the exposed fish. The activities of superoxide dismutase and lactate dehydrogenase increased significantly in exposed fish compared to the control. The total white blood cells, lymphocytes, and neutrophils counts were significantly higher in the exposed fish compared to the control fish. The respiratory burst activity decreased significantly in the exposed fish while the myeloperoxidase content did not differ significantly. There were upregulations of TNF-α and HSP 70 while CYP II and MHC 2 were downregulated in the exposed fish. Also, exposure to Cr6+ resulted in various histopathological alterations in the architecture of the head kidney. The results indicate concentration-dependent toxic effects of Cr6+ in C. gariepinus. The study reveals the potentials of Cr6+ to accumulate in the different tissues of fish and caused cytotoxic, oxidative stress, and immunotoxic effects in the exposed fish.

X-ray absorption spectroscopy study of the electronic and structural properties of hexavalent chromium adsorbed by volcanic ashes

X-ray absorption spectroscopy (XAS) was used to characterize the volcanic ash (VA) materials as a low-cost and natural environmental adsorbent for the sorption of Cr (VI). The batch adsorption method was employed to synthesize the sample of chromium (VI) sorbed by VA. The results showed that the Cr-VA material is in the chromium oxidation state (III) and can be described as a composition of Cr2O3 (47.9%), FeCr2O4 (45.2%), and Na2Cr2O7 (6.9%). The promotion of the 1s Cr core electron implies the electronic transitions 1s to 3d(t2g) and 1s to 3d(eg). The first and second shells in the Cr-VA material were coordinated with the oxygen and chromium atoms in a distorted geometrical arrangement. The adsorption mechanism was associated with the chemical reduction of Cr (VI) to Cr (III) via an ion-exchange process. The XAS technique significantly contributes to the characterization of environmental VA, which can be proposed as an economical and alternative adsorbent for the sorption of Cr from contaminated wastewater.

Analysis of Chromate-based Primers for Protection of Aluminium Alloys on Historical Aircraft

ABSTRACT Immediately following the use of aluminium alloys, particularly Duralumin, in the construction of aircraft, protective coatings with anti-corrosion properties were developed. Among the numerous solutions to prevent corrosion, the most widely employed materials were organic primers containing chromates. This paper reports the study of the corrosion inhibitive compounds (chromates) used by aircraft manufacturers during World War II. More specifically, we have identified the nature of the compounds and assessed their current efficiency. Our analyses on samples collected from three different wrecks (French and German) excavated on terrestrial sites reveal that the primer contained either zinc tetroxychromate or lead chromate. Using XANES to study the chromium oxidation states elucidated the different phenomena taking place during the aging of the paint primer. We found that the reduction of Cr(VI) to Cr(III) species occurred either within the primer or at the interface of the primer with the Al alloy. This is believed to be due to alteration of the paint layer and its long exposure to the natural environment. However, even after 80 years, Cr(VI) is still present in the primers of these archaeological parts. Although a little altered, the chromate compound can still provide a source of inhibitor, thus maintaining the protection of the alloys.

Charge analysis in (RE)CrO4 scheelites by combined Raman spectroscopy and computer simulations

The quest for structure-property relationships in scheelite-type (RE)CrO4 compounds (where RE is a rare earth element) is a difficult task due to the number of exceptions found in RE empirical trends and the uncommon Cr(V) oxidation state. In this work, we experimentally and computationally analyse how the stretching vibrational frequencies ν1(Ag) and ν3(Eg) associated with the [CrO4] tetrahedral units evolve in the (RE)CrO4 crystal family (RE ​= ​Nd, Gd, Dy, Ho, and Lu). Since previously reported Cr–O distances and volume changes along with the RE series are not sufficiently accurate to explain the monotonic decrease observed for the ν1(Ag) and ν3(Eg) frequencies, a deeper analysis was performed involving the well-known fact that the bond strength (force constant) decreases as the interatomic distance increases. Our results demonstrates that structural and spectroscopic parameters can be reconciled with classical solid state chemistry ideas when charge effects are considered. This analysis provides a new method for predicting chromium oxidation states from Raman spectroscopy that can be generalised to the study of other crystal families.

Electrochemical behavior of chromium carbide MXene modified electrodes: Hydrazine sensing

Electrochemical characteristics of few-layered two dimensional chromium carbide MXene (FL-Cr2CTx) modified electrode towards the detection of hydrazine (HZ) is reported for the first time in this paper. The modified electrode was fabricated using binder-free and mechanically immobilized FL-Cr2CTx onto the surface of paraffin impregnated graphite electrode. Electrochemical behavior of modified electrode was investigated by cyclic voltammetric studies and the effect of supporting electrolytes, scan rate and stability was also examined. The modified electrode exhibited multiple quasi-reversible peaks with three reduction peaks at − 0.39 V, − 0.88 V and − 1.20 V and two oxidation peaks at − 0.56 V and 0 V respectively, corresponding to surface oxy and hydroxyl groups in FL-Cr2CTx and due to the variable oxidation states of chromium as well. The electrode was utilized for electrochemical sensing of HZ, which exhibited a high sensitivity of 1.810 ± 0.013 µA µM−1 with a detection limit of 6.60 × 10−7 M (at S/N = 3). The developed sensor was further used for quantitatively analyzing HZ liquid propellant spiked in water, as a part of real sample analysis and the results were found satisfactory. The plausible mechanism suggests that the surface adsorption/desorption redox process of Cr2COx/Cr2COxH2 is responsible for the detection of HZ.

Effect of Chromium Exposure on the Antioxidant Enzymes Activity and Haematological Indices in Labeo rohita

Chromium is one of the most common elements in the earth’s crust and is used in more than 50 different industries. There are three oxidation states in case of Chromium viz., Cr (II), Cr (III), Cr (VI). Among which Cr (II) is most unstable. Cr (III) and Cr (VI) are the stable oxidation state of Chromium in the environment. Being one of the commonly used metals Chromium and its particulates enter the aquatic medium through effluents discharged from different industries like textiles, tanneries, electroplating workshops, ore mining, dyeing, printing-photographic and medical industries. The discharge from these industries pollutes the waters and affects the biota. Chromium is known to cause various health effects. The health hazards associated with exposure to chromium are dependent on its oxidation state. The Hexavalent form is toxic than trivalent form. The hematological alterations produced on exposure to sub-lethal concentration (1/10th of LC50/96 hrs) of chromium were investigated in fresh water fish, Labeo rohita. Labeo rohita common name (Rohu) is a species of fish of the carp family, found in rivers of South Asia. This fish is commonly found in Pakistan, India, Nepal, Bangladesh, Burma, Thailand, China, Kampuchea and Sri Lanka. The rich source of high-quality protein filled with vitamins and omega-3 fatty acids encourage the human being to uptake fish as a major food source. Significant decrease in haematological indices RBC, Hb, PCV, MCH, and MCHC and WBC and MCV values were significantly increased. The decrease in hematological parameters clearly indicates that the exposed fishes have become anemic due to heavy metal exposure.

CrOx/SiO2 mesoporous catalysts prepared using beta-cyclodextrin as a template and their catalytic properties in propane oxidative dehydrogenation in the presence of carbon dioxide

This work is aimed at the preparation and investigation of CrOx/SiO2 catalysts of oxidative propane dehydrogenation in the presence of carbon dioxide. The mesoporous silicas used as carriers were prepared via a sol-gel method using a beta-cyclodextrin and urea mixture as a template. The amounts of water added during Si(OEt)4 hydrolysis and chromium oxide loading were varied. The obtained catalysts were characterised via nitrogen low-temperature adsorption-desorption, XRD, SAXS, SEM with EDX, TEM, XPS, UV–Vis diffuse reflectance spectroscopy and the catalytic tests.The obtained mesoporous silicas have monomodal narrow pore size distributions with the maxima at 7, 9, 10 nm increasing when the amounts of water used during the synthesis increase, and large surface areas of 680, 460, 410 m2g-1, respectively. All the samples do not contain micropores. Chromium oxidation states +6 and + 3 were found according to XPS and UV–Vis diffuse reflectance spectroscopy data. The catalytic tests revealed that there is a correlation between the catalytic activity and synthesis conditions. The catalytic activity in the reaction of oxidative propane dehydrogenation in the presence of CO2 decreases in the series with a pore diameter of 9 > 10 > 7 nm. The most active and selective sample showed the selectivity to propylene up to 80% while the conversion was 20%.

Chromium(VI) Removal from Water by Lanthanum Hybrid Modified Activated Carbon Produced from Coconut Shells.

Cr(VI) is considered to be the most hazardous and toxic oxidation state of chromium and hence the development of effective removal technologies, able to provide water with Cr(VI) below the drinking water limits (US EPA 100 μg/L, European Commission 50 μg/L, which will be reduced to 25 by 2036) is a very important issue in water treatment. This study aimed at examining the performance of activated carbon produced from coconut shells, modified by lanthanum chloride, for Cr(VI) removal from waters. The structure of the formed material (COC-AC-La) was characterized by the application of BET, FTIR and SEM techniques. The effect of the adsorbent’s dosage, pH value, contact time, initial Cr(VI) concentration and water matrix was examined with respect to Cr(VI) removal. The results indicated that the maximum Cr(VI) removal was observed at pH 5; 4 h contact time and 0.2 g/L of adsorbent’s dosage was adequate to reduce Cr(VI) from 100 μg/L to below 25 μg/L. Freundlich isotherm and pseudo-second order kinetic models fitted the experimental data sufficiently. The maximum adsorption capacity achieved was 6.3 μg/g at pH 5. At this pH value, the removal percentage of Cr(VI) reached 95% for an initial Cr(VI) concertation of 30 μg/L. At pH 7 the corresponding efficiency was roughly 60%, resulting in residual Cr(VI) concentrations below the anticipated drinking water limit of 25 μg/L of total chromium, when the initial Cr(VI) concentration was 50 μg/L. Consecutive adsorption and regeneration studies were conducted using 0.01 M of NaOH as an eluent to evaluate the reusability of the adsorbents, Results showed 20% decrease of adsorption capacity after 5 regeneration cycles of operation.

Oxidation State and Local Structure of Chromium Ions in LaOCl.

LaOCl doped with 0–10 mol% Cr was synthesized by thermal decomposition of chlorides. X-ray diffraction (XRD) analysis revealed that incorporation of chromium results in a decrease of the lattice parameter a and a simultaneous increase of the lattice parameter c. The local structure of chromium ions was studied with X-ray photoelectron (XPS), X-ray absorption (XANES), multifrequency electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) spectroscopy techniques. It was determined that synthesis in oxidizing atmosphere promotes the incorporation of chromium ions predominantly in the 5+ oxidation state. Changes of chromium oxidation state and local environment occur after a subsequent treatment in reducing atmosphere. Spin-Hamiltonian (SH) parameters for a Cr5+ and two types of Cr3+ centers in LaOCl were determined from the EPR spectra simulations.

The effects of temperature and pressure on the oxidation state of chromium in silicate melts

The oxidation state of Cr, Cr2+/ΣCr (where ΣCr = Cr2+ + Cr3+ = 0.35 wt%), in Fe-free silicate glasses quenched from melts equilibrated as a function of pressure to 3.5 GPa at 1500 °C, and as a function of temperature to 1500 °C at atmospheric pressure, and at oxygen fugacities (fO2, in log units relative to the quartz-fayalite-magnetite (QFM) buffer) between ∆QFM = − 1 and − 2, was determined by XANES spectroscopy. Increasing temperature stabilises Cr2+ and increasing pressure stabilises Cr3+. A general expression for Cr3+/Cr2+ in silicate melts was derived: log(Cr3+/Cr2+) = 1/4(∆QFM + 8.58 − 25,050/T + 940P/T − 0.02P) + 9770/T − 7.69 + 6.22Λ + (900P − 172P2)/T, where P is pressure in GPa, T is temperature in K, and Λ is the optical basicity of the composition. This equation reproduces 213 Cr2+/ΣCr values reported here and in the literature with an average ΔCr2+/ΣCr of 0.02. A MORB melt at 1400 °C and QFM is predicted to have Cr2+/ΣCr ~ 0.35 at the surface but Cr2+/ΣCr ~ 0 at a depth of ~ 60 km. Although Cr2+ is an important oxidation state in silicate melts it is not preserved at low temperatures due to an electron exchange reaction with Fe3+.

The Effects of Oxidation States and Spin States of Chromium Interaction with <i>Sargassum Sp</i>.: A Spectroscopic and Density Functional Theoretical Study

The study of various oxidation states of chromium with Sargassum sp. is of particular interest since hexavalent chromium is reduced to trivalent chromium in an aqueous solution. In this study, a systematic density functional theory (DFT) calculations were performed to study the interactions of transition metal chromium ion with different oxidation states and spin states with the Sargassum sp. decorated with carboxylate (acetate) at the wB97XD/6-311++ G(d,p) level of theory. The structures and binding energies of chromium metal-carboxylate complexes at various oxidation states and spin states in gas phase were examined. The coordination strength of Cr(VI) with the acetate ligand was predominantly the strongest compared to the other oxidation states. Vibrational frequency analysis, for the homoleptic monomers of tris [CrIII(AC)3]0 and [CrVI(AC)3]3+ complexes, illustrate good harmony with the experimental and theoretical calculated frequencies. Using the time-dependent DFT (TD-DFT) at the level of CAM-B3LYP/6-311++G(d,p), the vertical excitation energies were obtained. The stabilization energies derived using the second order perturbation theory, Eij(2), of NBO analysis confirmed the greater charge transfer for the observed trends in the metal binding. The calculated binding energies (ΔE) and interactions energies SEij(2) favor the formation of [CrVI(AC)3]3+ complexes. The findings of this study identify efficient electronic factors as major contributors to the metal binding affinities, with promising possibilities for the design of metal-ligand complexes and sensing of the metal ions.

Ab initio study of Pr1-xSrxCrO3-δ cubic perovskites: Solid oxide fuel cells applications

Perovskite-type materials have been proposed to produce solid oxide fuel cells (SOFCs). The cathode is one layer that makes up the SOFCs, and figure as important player in order to catalyze the reduction of O2. As of the Density Functional Theory, this work presents a theorical study of oxygen vacancy (VO) effects on perovskites Pr1-xSrxCrO3-δ (x ​= ​0, 0.25 and 1; δ ​= ​0.042), in oxygen-poor and oxygen-rich growth atmospheres, for several charge states (VOq). Oxygen vacancy has a fundamental role to optimize the catalysis process in SOFCs cathode. From the formation energy (Ef) calculation, VO+1 is the most stable state in metallic SrCrO2.958, which indicates its donor character. For PrCrO2.958, oxygen vacancy is a negative-U center, with transition energy between charge states (VO2+/VO0) at 0.38 ​eV above valence band top. Among the three distinct structures of Pr0.75Sr0.25CrO2.958, namely, C1, C2 and C3, VO1− is energetically most favorable in C3, which gives its acceptor character. Relative to other systems, SrCrO2.958 has the lowest VO formation energy. Through magnetic moment study, oxidation states of chromium are Cr4+ (d2) in SrCrO3-δ, and Cr3+ (d3) in PrCrO3-δ and Pr0.75Sr0.25CrO3-δ. Furthermore, praseodymium Pr3+ (f2) is present in PrCrO3-δ and Pr0.75Sr0.25CrO3-δ.

Investigation of Isobutane Dehydrogenation on CrOx/MCM-41 Catalyst

The syntheses of MCM-41 (Mobil Composition of Matter No. 41) supported chromium oxide cat-alysts at different chromium concentrations (4–10 % by mass) were carried out hydrothermally. The aim of this study was to determine the effect of chromium concentration in the catalyst structure on the chro-mate types and chromium oxidation states, as well as the activity of the catalyst in the isobutane dehydro-genation reaction. Inactive α-Cr2O3 crystals for isobutane dehydrogenation were shown to increase in the catalyst structure as the chromium loading increased. The highest amount of Cr6+ on the catalyst surface was detected in the catalyst (H4-MCM-41) with 4 % chromium by mass. Catalytic tests (T = 600 °C, P = atmospheric pressure, WHSV = 26 h–1) were performed under fixed bed reactor conditions. The high-est isobutane conversion (~60 %) and selectivity (~80 %) were observed on the H4-MCM-41 catalyst, which had the highest amount of Cr6+ and monochromate structures. Catalyst deactivation was not due to coke deposition but, rather, was caused by the formation of inactive α-Cr2O3 crystal structures.

CaNa[Cr(OH)6] – A Layered Hydroxochromate(III) with Ordered Brucite Structure and its Thermal Decomposition

Green single‐crystals of the hydroxochromate(III) CaNa[Cr(OH)6] were grown under highly alkaline hydrothermal conditions at about 200 °C. The starting materials Ca(NO3)2·6H2O and Cr(NO3)3·9H2O were reacted in a mixture of water and sodium hydroxide with the molar ratio of 2.8:1. CaNa[Cr(OH)6] crystallizes in the non‐centrosymmetric trigonal space group R3 with the lattice parameters a = 583.86(2) pm and c = 1428.73(6) pm [T = 100(1) K]. Characteristically, the crystals are reverse‐obverse as well as inversion twins. The crystal structure is a stack of uncharged metal hydroxide layers, which can be regarded as a cation‐ordered rhombohedral variant of the Mg(OH)2 (brucite) structure type. The oxidation state of chromium(III) and its coordination by hydroxide groups was confirmed by UV/Vis and IR spectroscopy, respectively. Temperature‐dependent magnetic measurements revealed paramagnetic behavior with an effective moment of 3.82 μB per chromium atom. The thermal decomposition of CaNa[Cr(OH)6] takes place at about 225 °C, where the fast elimination of 1.5 equivalents of water is followed by the oxidation of chromium(III) to chromium(VI). Upon further heating to 1000 °C and 1200 °C, the intermediate decomposition products CaCrO4 and Na2CrO4 transform into the oxochromates(V) Ca5(CrO4)3O0.5 and Ca3(CrO4)2, respectively.

Low-Temperature Catalytic CO Oxidation Over Non-Noble, Efficient Chromia in Reduced Graphene Oxide and Graphene Oxide Nanocomposites

Herein, bare chromia nanoparticles (Cr2O3 NPs) and chromia supported on reduced graphene oxide (rGO) and graphene oxide (GO) hybrids were synthesized, followed by characterization by means of FESEM, Raman spectroscopy, TGA, XRD, TEM/HRTEM, XPS and N2 sorptiometry. The investigated bare Cr2O3 and the hybrids (Cr2O3/rGO and Cr2O3/GO) were employed as catalysts for low-temperature CO oxidation. Compared with the other catalysts, the results revealed efficient catalytic activity using Cr2O3/GO, which was attributed to its higher surface area together with the mixed oxidation state of chromium (Cr3+ and Cr>3+). These are important oxidation sites that facilitate the electron mobility essential for CO oxidation. Moreover, the presence of carbon vacancy defects and functional groups facilitate the stabilizing of Cr2O3 NPs on its surface, forming a thermally stable hybrid material, which assists the CO oxidation process. The Cr2O3/GO hybrid is a promising low-cost and efficient catalyst for CO oxidation at low temperatures. The higher activity of graphene oxide supported Cr2O3 NPs can provide an efficient and cost-effective solution to a prominent environmental problem.