| Ammonium niobium oxalate is a white crystalline solid, which used as catalyst, iron oxidizer, water-soluble compound of dielectric material addition agent. In recent years, ammonium niobium oxalate is used as a catalyst in many aspects, such as, environmental protection, catalytic activity, using cycle and so on, which has the big superiority and the applications are widespread.In this dissertation, the pure ammonium niobium oxalate was obtained with oxalic acid, niobium acid, ammonium oxalate. According to a certain molar ratio to prepare compounded solution, then proceeded to filtrate, crystallize, re-crystallize, dry and so on.Ammonium niobium oxalate was synthesized and characterized by element analysis, X radial diffraction analysis, infrared spectra analysis and thermogravimetric analysis, which made clear the structure and compose. The molecular formula of ammonium niobium oxalate was NH4·[NbO(C2O4)2(H2O)2]·3H2O. Thermal decomposition process of ammonium niobium oxalate was investigated by thermogravimetric analysis and the infrared spectra analysis of ammonium niobium oxalate degraded in different temperatures. It showed that the thermal decomposition process occurred in three steps.The weight loss percentage of the first step (30200 ?C) was 22.68 %, and the dehydrated process of ammonium niobium oxalate, the weight loss mainly caused by crystal water, and the crystal water number was five. The weight loss percentage of the second step (200350 ?C) was 40.96 %, and the step was the major decomposition process of ammonium niobium oxalate. It was the degradation process of NH4+ and oxalate ion. The weight loss percentage of the third step (550630 C) was 4.15 %, in which carbon dioxide was lost. The thermal decomposition residue of ammonium niobium oxalate was niobium pentoxide.As far as we known, the study on the kinetics of thermal decomposition of ammonium niobium oxalate has not yet been reported. The thermal decomposition kinetics of ammonium niobium oxalate was studied in different heating rate (1, 2, 3, 5, 10 o C/min) in air atmosphere. It was divided in three steps:(1) The activation energy of the three steps thermal decomposition of ammonium niobium oxalate were calculated by FWO, KAS, Iteration and Friedman method;(2) Degradation mechanism was deduced by Coats-Redfern integral equation and Achar-Bridly-Sharp differential equation;(3) The results obtained by above steps were used in determination pre-exponential factor.According to calculation results, the Eαvalues were almost identical through the iterative of ln[β/H(x)] vs. 1/T and ln[β/h(x)T2] vs. 1/T,so the Eαvalues obtained from iteration method were more reliable. The value of activation energy was the average of the four methods values. The activation energy of three steps thermal decomposition were 88.97 kJ·mol-1,134.60 kJ·mol-1,206.23 kJ·mol-1. 41 kinds of kinetic models corresponding to G (α)and f (α)are introduced to equation: In the first step the 15 function was the most likely mechanism, that is G (α)=[-ln(1-α)]3/4 and f (α)=(3/4)(1-α)[-ln(1-α)]1/4, the kinetic parameter lnA was 2.55 s-1; In the second step the 19 function was the most likely mechanism, that is G (α)=ln(1-α)3 and f (α)=(1/3)(1-α)[-ln(1-α)]-2, the kinetic parameter lnA was 2.85 s-1. In this study, the third-step thermal decomposition was due to the degradation process of the absorption of carbon dioxide which surfaced ammonium niobium oxalate. Therefore the exactly mechanism can not be deduced by above two methods.The gained data in this paper for the thermal stability and thermal decomposition kinetics mechanism of ammonium niobium oxalate has provided theoretical basis. The obtained conclusion has provided the foundation basis for ammonium niobium oxalate in the relevant aspects application. |
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