Synthesis And Properties Of Niobium Tungstate By Hydro/Solvothermal Process

Pyrochlore structure compounds have attracted widespread attention because oftheir unique features. Pyrochlore is a kind of oxide minerals with an ideal formula ofA2B2O7.This structure is built of two corner-sharing BO6octahedra and aninterpenetrating A2O’ chain forming a three-dimensional network. A wide variety ofmixed metal oxides adopt this structure depending on the choice of metal cations Aand B. For defect pyrochlores, because the A and O’ atom of the chain are notessential in the stabilization of the ideal structure, vacancies may occur and leading tothe defect. The defect pyrochlores may possess interesting properties, such asion-exchange, electrical conductivity and magnetic properties.Up to now, the most of pyrochlore compouds are prepared by convetional solidstate reaction. Here, we systematically studied the hydrothermal/solvothermalsynthesis of niobium tungstates with defect pyrochlore structure, and their propertiessuch as ion exchange, photocatalytic and luminescence are studied.The major results are as follows:1. The defect pyrochlore KNbWO6H2O is obtained by a two-step solvothermalprocess. We have studied the influence of reaction conditions on the synthesis ofKNbWO6·H2O with pyrochlore structure, such as the proportion of octanol and water,pH value, reaction temperature and time. The ion exchange and photocatalyticperformance of KNbWO6·H2O are studied. The as-obtained KNbWO6H2O exhibitshigh capacity for the removal of toxic Pb2+in aqueous solution. To the best of ourknowledge, this is the first report on the ion exchanging property of defect pyrochlore with high selectivity. The maximum removal capacity is86.95mg/g at optimized pH5.0. High selectivity to Pb2+is observed in aqueous solution which coexisting with ofMn2+, Cd2+and Co2+. The kinetics of ion exchange process could be expressed by thepseudo-second-order rate kinetic model. KNbWO6·H2O with pyrochlore structure hasgood photocatalytic property for the degradation of organic compounds. Thephotocatalytic degradation performances of organic light Rhodamine B andmethylene are investigated. The highest decolorization ratio98%and97%areobserved for Rhodamine B and methylene blue, respectively.2. Hydrothermal/solvothermal synthesis and properties of the ANbWO6·H2O(A=Pb2+, Na+, NH4+). ANbWO6·H2O（A=Pb2+, Na+, NH4+）with defect pyrochlorestructure are prepared by using ion exchange method. NaNbWO6·H2O with defectpyrochlore can also be prepared by direct hydrothermal method by using NaOHinstead of KOH. The morphology of NaNbWO6·H2O obtained from KNbWO6·H2Oby ion exchanging method is different from that of NaNbWO6·H2O directly obtainedin NaOH system.3． Hydrothermal/solvothermal synthesis and luminescence properties of theKNbWO6·H2O:xLn(Ln=Eu3+, Tb3+, Dy3+). KNbWO6H2O is an ideal ion exchangingmaterial for trivalent metal ion. KNbWO6H2O:xEu3+is prepared by ion exchangingin hydrothermal system. Three K+ions of KNbWO6H2O are exchanged by one Eu3+ion and KNbWO6H2O:xEu3+keeps its defect pyrochlore structure. The amount ofdoping Eu3+ion in defect pyrochlore structure can be adjusted by changing thehydrothermal conditions. Thermal analysis results indicate that Eu3+ion get into thechannel of defect pyrochlore structure in the form of aqueous ion [Eu(H2O)n]3+.The water of [Eu(H2O)n]3+loss at temperature of450C, which is much higher thanthat of KNbWO6H2O. The hydration of Eu3+ion in the pyrochlore structure hassignificant influence on the luminescence properties of KNbWO6H2O:xEu3+.10times increase of the emission intensity is observed for the as-preparedKNbWO6H2O:xEu3+annealed at450C for2h. The KNbWO6H2O with defectpyrochlore structure can be used as efficient luminescent host. KNbWO6·H2O:xDy3+sample emit intense yellow light under the excitation of388nm. KNbWO6·H2O:xTb3+show typical luminescence properties attributed to Tb3+andemit green light under379nm UV excitation.