Study On Preparation Of Pyrochlore Tungsten Oxide From Sodium Tungstate Solution

Pyrochlore-type WO3(H2W2O7) has recently found its wide applications in many different areas such as photoelectron-chemistry, functional material and sensing due to its special structure with3-dimensional tunnels and metastability. In addition, H2W2O7can be used as intermediate product for the industrial production of WO3. The hydrothermal method that has overwhelming superiorities than others also has its own sets of flaws such as high cost, long reaction time and low quality product. Therefore, how to intensify the preparation process of H2W2O7to obtain high quality product with low cost has become an urgent problem.In this study, we report on the preparation of H2W2O7directly from sodium tungstate solution. We tried to prepare H2W2O7by new methods firstly and focused mainly on the effects of hydrothermal conditions on the preparation process. XRD, TGA-DSC, and SEM were used to characterize the products. The sodium content in the H2W2O7was determined by chemical analysis and most sodium could be removed by ion exchange method. The main research results are listed as follows.(1)The seeded precipitation and salting-out precipitation methods are not appropriate to prepare H2W2O7but salting-out precipitation method suggests a new way to prepare Na2WO4and Na2W2O7.(2)Under hydrothermal conditions, the addition of oxalic acid has its optimum value; when C(Na2WO4)=150g/L,T=126℃,t=11h, adding oxalic acid65g/L, the reaction ratio reaches its peak value91%. Additive C plays a part in reaction ratio and seeds of H2W2O7could accelerate reaction rate. The W7O246-ions may be the precusor in precipitating H2W2O7, but the Fe2+ions seem to change the precipitation mechanism.(3)The TGA-DSC curve shows a continuous water loss from ambient to450℃, and the H2W2O7remains stable under340℃. The SEM pictures of the H2W2O7exhibit cubic morphology which can be changed by organic surfactant (4)Sodium is an inevitable impurity with its content varying from3.3%to4.9%, and ion exchange is a valid way to remove sodium content to about 0.3%.