Study of Thermal Behavior of Cr(no 3 ) 3 ⋅9h 2 O in N2 Atmosphere Using Tga

Using a technique of thermogravimetry and under N2 gas atmosphere from 25 to 600°C, the thermal behavior of Cr(NO 3) 3 ⋅9H 2 O was studied. The received products were investigated by XRD. Depending on the conditions of reaction (temperature, heating rate, mass of sample, surrounding atmosphere) thermal decomposition leads to chromium oxides Cr2O3, different size and porosity or specific area of the forming grains. Calculations related to mass loss were also reported.


INTRODUCTION
Due to their chemical and physical characteristics, chromium compounds are used in a wide variety of industries all over the world [1,2,3,4,5,6] .One of the most important characteristics of the chromium concerns its reactions to acids and bases as well as to air, a process known as oxidation.Some are the end product of a desired process but most of them are undesired byproducts known as disturb compounds [7,8,9] .
Chromite is one of the most significant compounds used in steel industries [ 8,9,10].The morphology of these compounds was also studied [11].Earlier works showed that two or three step reduction pathway of unsupported CrO3 is or may be accompanied by simultaneous decomposition of intermediate CrOx phases (Cr3O8 and Cr5O12), which are eventually transformed into crystalline Cr2O3 at about 500 °C [12,13] of magnitude higher than the parabolic oxidation constant.Other Experiments demonstrated that the presence of nitrogen in the substrate is always a precursor to breakdown of the oxide layer and does not result from diffusion through the Cr2O3 Layer [14] .The present research intended to investigate the thermal properties of chromium nitrate under different temperature conditions.TG/DSC equipment and its temperature was raised from 25 to 600°C, with a heating rate of 10°C min -1 , under N2 gas atmosphere.TG curve of this sample are represented in Fig. 1.

X-ray powder diffraction
The product from TG experiment was prepared for X-ray and it was exposed to CuK  1 radiation for 2 h. Figure 2 shows the XRD diagram of the compound Cr2O3 (end product).

Thermal investigations of Cr(NO3)3
The TG curve of thermal decomposition of Cr(NO3)3 in N2 atmosphere is shown in Fig. 1.The curve shows the mass loss (vertical axis) of Cr(NO3)3 in N2, while horizontal axis shows temperature increase.
So one can better differentiate between the steps of the thermal decomposition.The results indicate that thermal decomposition consists of three steps in the temperature range of 25-600°C, these results are summarized in table 1.
The first step takes place between 48-120°C.This phenomenon is the decomposition of the starting compound and loss of H2O.Spectroscopic quantitative analysis in this study also corroborates this finding.The computed stoichiometry of decomposition products are in good agreement with experimental results (quantitative and percent decrease in mass).A heating rate of 10°C min -1 was chosen to determine more information on what is taking place in this temperature range.The experiment was accomplished in the N2 atmosphere with a constant gas flow of 30 ml. min -1 .
As can be inferred from the results of the Fig. 1, the first phase of (decomposition) pyrolysis reaction of Cr(NO3)3.9H2Ooccurs in the range of 48-120°C.Concerning the case of Cr(NO3)3, the separation of H2O was also observed.Investigation of TG curve N2 atmospheres for this step indicates the presence of the process i.e. loss of H2O.By subtracting the experimentally determined quantity of materials (by spectrometric quantitative analysis) from the entire amount of mass loss (21.13%).
Second step of decomposition (120.18-274.35°C) The experiment results show that the product lost about 55.75% of its mass within the range 120-274°C.The experiment was accomplished in the N2 atmosphere with a constant gas flow of 30 ml. min -1 .The X-ray analysis supplied an amorph XRD diagram.The evaluation of the results as well as spectrometric analysis confirms the brutto formula: CrO2.6.
Third step of decomposition (274.35-475.15°C) The experiment results show that the product lost about 5.59% of its mass within the range 274-475°C.The X-ray analysis supplied a XRD diagram identical to Cr2O3 (fig.2).The evaluation of the results as well as spectrometric analysis confirms the brutto formula: Cr2O3.