Chemical Composition Analysis On Niobium Hydroxide Synthesized By Hydrofluoric Acid Method And Its Thermal Decomposition Kinetics

Amorphous niobium hydroxide would be effective as the catalystfor reactions in which water participates as a reactant or product. In thispaper, niobium hydroxide was synthesized by hydrofluoric acid method.After drying naturally in the air, product was divided evenly into fiveparts then heated at65°C,75°C,85°C,95°C and105°C for12h,respectively, to obtain the final samples for experiment and marked asNH-65、NH-75、NH-85、NH-95、NH-105, in order. The samples werecharacterized by XRD, Fourier transform infared spectroscopy,thermogravimetry and Raman spectroscopy in order to determine thecomposition and structure. Then combined ammonia contentdetermined by improved distillation-acid-base titrations method andniobium pentoxide content measured by TG, molecular formula forniobium hydroxide synthesized by hydrofluoric acid method in thetemperature range of65~105°C can be achieved by calculation. Finally,the thermal decomposition kinetics of niobium hydroxide was studied.(1) FTIR analysis shows that there exists NH4+in niobiumhydroxide, while the Nb-OH structure does not exist; TG analysisshows that water in niobium hydroxide can not be classified asconstitution water, which also means that the Nb-OH structure does notexist in niobium hydroxide molecule. Based on FTIR and TG analysis,(NH4)xNbO2.5+0.5x·yH2O can be the reasonable molecular formula forniobium hydroxide synthesized by hydrofluoric acid method.(2)Ammonia content of NH-65、NH-75、NH-85、NH-95、NH-105 determined by improved distillation-acid-base titrations method is4.59%、4.57%、4.49%、4.36%、4.19%, respectively. Combined with theniobium pentoxide content of the five samples measured by TG (themeasurements are80.55%,82.24%,83.61%,85.25%and85.84%, inorder)，molecular formula for niobium hydroxide synthesized byhydrofluoric acid method in the temperature range of65~105°C can bewritten as (NH4)0.4NbO2.7·yH2O(0.7≤y≤1.1). It can be seen thatammonia content has little change as the increase of temperature, whilewater content trends to decrease.(3) The thermal decomposition of niobium hydroxide which is(NH4)0.38NbO2.69·0.67H2O from30°C to600°C can be divided intotwo stages. The first mass loss of niobium hydroxide is due to theexpulsion of water, which occurs from30°C to107°C and the weightloss is about5.24%. The apparent activation energy andpre-exponential constant of the first stage are76.46kJ·mol-1and30.25s-1, respectively. The reaction mechanism of this stage can be classifiedas f(α)=1/2(1-α)3. The second mass loss of niobium hydroxide isrelated to the expulsion of water and ammonia, which occurs from108°C to500°C and the weight loss is about8.70%. The apparentactivation energy and pre-exponential constant of the first stage are100.67kJ·mol-1and23.97s-1, respectively. The most probable kineticmodel function is f(α)=1/4(1-α)[-ln(1-α)]-3.