Kinetic and thermodynamic parameters of the decomposition of chromium chromate in different gas atmospheres (original) (raw)
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Interconversion of CrO2 formed by decomposition of chromium(III) nitrate nonahydrate
Journal of Solid State Chemistry, 1995
The products of the decomposition of chromium(II1) nitrate nonahydrate (CNN) and interconversions occurring in the system CrO,-CrOOH have been investigated using thermal analysis, mass spectrometry, X-ray powder diffraction, and magnetic resonance. The studies indicate that changes in the conditions of CNN calcination, especially in the temperature, greatly influence the ratio of Cr(IV) to Cr(II1) oxides in the products. CrO, formed during slow heating to ca. 350°C is very stable and its complete transformation into Cr,Oj requires a temperature higher than 600°C. The rapid crystallization of the amorphous products of CNN calcination, occurring at ca. 38O"C, leads to the decomposition of all intermediates during this strongly exothermal reaction, finally resulting in Cr,O,. The interconvertability of the components of the redox cycle CrO,-CrOOH was proven experimentally. Due to its unique characteristics, this redox cycle is suggested for the difficult identification of CrO, in amorphous chromium-oxygen systems. o 1~s Academic Press, Inc.
TG-DTA-MS of chromium(III) formate
Thermochimica Acta, 2000
The thermal decomposition of chromium(III) formate pentahydrate, Cr 3 (OH) 2 (HCO 2) 7 Á5H 2 O, in helium atmosphere and 20% O 2 in helium atmosphere has been successfully studied by means of TG-DTA-MS, i.e., thermogravimetry-differential thermal analysis (TG-DTA) coupled with evolved gas analysis (EGA) using mass spectrometry (MS). The TG-DTA-MS is useful to interpret the complicated successive reactions and to determine the mechanism of the thermal decomposition. The thermal process in He atmosphere proceeded by four steps, while in 20% O 2 ±He atmosphere it proceeded by three steps. The decomposition scheme of Cr 3 (OH) 2 (HCO 2) 7 Á5H 2 O in He atmosphere was proposed by the following decomposition mechanism via formation of three intermediates:
Paralinear Oxidation of Chromium in O2 + H2O Environment at 600–700 °C
Oxidation of Metals, 2008
The oxidation of chromium in dry O 2 and in O 2 + 10%H 2 O at 600 and 700°C is studied. Scale morphology is investigated by several methods, including scanning electron microscopy (SEM) of cross sections prepared by focussed ion beam milling (FIB). In O 2 + H 2 O at 600 and 700°C, chromium forms a duplex scale consisting of an inner barrier oxide and a discontinuous outer oxide made up of bladeshaped crystals. Thermogravimetric (TG) measurements show that water vapour influences chromium oxidation by causing vaporization of the protective oxide, resulting in paralinear oxidation kinetics. An extension of the original treatment by Tedmon is deduced, which allows for the determination of the evaporation rate constant k s and the parabolic oxidation rate constant k d from TG data acquired during short exposures. The results show that k d is the same in dry O 2 and in O 2 + 10%H 2 O. Equivalently, the transport properties of chromia are the same in the two environments. The equilibrium constant of CrO 2 (OH) 2 formation from chromia is reported. The activation enthalpy of the vaporization reaction is determined.
Combustion and Flame, 1998
A comprehensive thermochemical analysis of the equilibrium gaseous product composition resulting from burning chromium, hydrocarbon, and chlorohydrocarbon polymers in air at 101 kPa is presented. Calculations have been made over the range ϭ 0.25-2.50 at adiabatic flame temperatures from 600 to 2100 K. The main chromium-containing species formed are CrO 3 , CrO 2 , CrO 2 Cl, and CrO 2 (OH). The yield of Cr(6ϩ) derivatives reaches a maximum of 77% at 1650 K and ϭ 0.25, mainly in the form of CrO 3 (g). The yield of CrO, CrOCl, CrOCl 2 , CrO 2 Cl 2 , CrOOH, and CrO 2 (OH) 2 does not exceed 0.1% of the initial chromium content, and that of other chromium-containing species is less than 10 Ϫ2 mol%. A reaction scheme is constructed on the basis of the data obtained. It includes reactions of the secondary oxidants CO 2 , H 2 O, HCl, OH, and O, as well as the reducing agents CO, H 2 , and H.
The physical chemistry of thermal decomposition of South African chromite minerals
Metallurgical and Materials Transactions B, 2005
The decomposition of natural spinels strongly depends upon the chemical potential imposed in the forms of temperature, pressure, and pH difference in aqueous media (e.g., during natural weathering). In this investigation, we studied the thermal decomposition behavior of South African chromite ores in order to relate the influence of oxygen potential with the likely product phases formed. The decomposition is also a generic step in the understanding of the formation of sodium chromate during soda-ash roasting and the reduction of chromite ores for ferrochrome alloy making. The phase equilibria in South African chromite minerals were investigated by isochronal thermal analysis and isothermal heat treatment of chromite mineral in air, argon, and Ar-5 pct H 2 atmospheres over a temperature range from 473 to 1473 K. The effects of the oxygen partial pressure and temperature on the phase constituents of the heat-treated product are discussed by referring to the results of X-ray diffraction (XRD), scanning electron microscopy with energy-dispersive X-ray detector (EDX), and electron probe microanalysis (EPMA). The structure of phases formed and the morphology of phase-separated regions in chromite appear to be strongly dependent on the oxygen partial pressure. The mechanism of the decomposition of complex spinel phases is described under the influence of oxygen partial pressure and temperature.
Effect of temperature on synthesis of chromium oxide
Acta Crystallographica Section A Foundations and Advances, 2017
Our synthesis of Chromium oxide nanoparticle involves hydrothermal method using Chromium anhydride and anhydrous alcohol as the raw materials.Variation of temperature in particle size of nanoparticle is observed by increasing and decreasing the temperature.Various characterization method including XRD,TEM and SEM were used for characterizing the synthesised product.It was found that with increase in temperature good particle size of Chromium oxide is formed.As the process is simple and easy it can be used in various applications such as catalysis,colorants any many more.
Phase relations in the system (chromium+rhodium+oxygen) and thermodynamic properties of CrRhO3
The Journal of Chemical Thermodynamics, 2009
Phase relations in the system (chromium+rhodium+oxygen) at T=1273K have been determined by examination of equilibrated samples by optical and scanning electron microscopy, powder X-ray diffraction (XRD), and energy dispersive spectroscopy (EDS). Only one ternary oxide, CrRhO3 with rhombohedral structure (R3¯, a=0.5031, and c=1.3767nm) has been identified. Alloys and the intermetallics along the (chromium+rhodium) binary were in equilibrium with Cr2O3. The thermodynamic properties of the CrRhO3 have been determined in the temperature range (900 to 1300)K by using a solid-state electrochemical cell incorporating calcia-stabilized zirconia as the electrolyte. For the reaction,1/2Cr2O3(solid)+1/2Rh2O3(solid)→CrRhO3(solid),ΔG∘±140/(J·mol-1)=-31967+5.418(T/K),where Cr2O3 has the corundum structure and Rh2O3 has the orthorhombic structure. Thermodynamic properties of CrRhO3 at T=298.15K have been evaluated. The compound decomposes on heating to a mixture of Cr2O3-rich sesquioxide solid solution, Rh, and O2. The calculated decomposition temperatures are T=1567±5K in pure O2 and T=1470±5K in air at a total pressure p∘=0.1MPa. The temperature-composition phase diagrams for the system (chromium+rhodium+oxygen) at different partial pressures of oxygen and an oxygen potential diagram at T=1273K are calculated from the thermodynamic information.
Advanced Materials Research, 2012
Little information has been known on the initial – stage oxidation if there is water vapor involved. Cr samples of 10 mm × 10 mm × 1 mm were isothermally oxidized in dry and wet environment respectively for 86.4 ks. Compact and even surface of Cr2O3 was formed on samples oxidized in dry environment. However, the ability to form compact and even Cr2O3 was retarded in wet environment. XRD analysis on all samples shows that Cr2O3 can be formed in dry and wet environment. The IR transmission spectra for samples oxidized in dry environment, were consisting of more intense peak while samples oxidized in wet environment has more relaxed peak. Moreover the peak of samples oxidized in dry tends to be narrower, while samples oxidized in wet tend to have broader peak.