The influence of residual sodium on the formation and reductive decomposition of hexagonal tungsten oxide

摘要: The metastable hexagonal form of tungsten trioxide (h-WO3) has recently been attracting much interest [1]. synthesis routes h-WO3 involve the dehydration tungstic oxide hydrates, WO3 · nH2O (n= 0.3 [2, 3]; n= 0.9 [4]) or oxidation ammonium bronze [5–7], ion-exchanging templating ions from [8]. Regarding role and effect possible residual chemical impurities, considerable uncertainty remained. In this laboratory, we have prepared oxide–hydrate precursor, 1/3H2O by route Na2WO4 2H2O→ H2WO4 ·H2O → 9, 10]. According to a previous report, formation requires presence sodium in reaction system, resulting incorporation phase [11]. This work focuses on kinetic influence transformation/dehydration into hydrogen reduction elemental tungsten. experiments (of varying contents) were carried out either at isothermal conditions (300 ◦C, 15, 30 90 min, N2–O2 (80–20%) atmosphere) thermal gravimetry equipment (Mettler, Registrierender Vakuum-Thermoanalyzer, H2 (5N) ambient) where oxygen loss during was also followed. X-ray powder patterns recorded (at room temperature Guinier focusing camera using CuKα radiation, λ = 0.154051 nm) after each heat treatment analysis. positions reflection lines film determined computer-controlled densitometric analysis [12]. Lattice parameters obtained least-squares refinement d values 23–26 reflections range 13◦ < 22 30◦. calculated cell for (JCPDS card 33-1387) samples with [Na]= 163– 3420 ppm are collected Table I. Morphology grain size studied scanning electron microscopy (SEM; Jeol 25). Transparent transmission (TEM; Philips CM 20, operating 200 kV) depositing dispersed crystallites onto holder, ion milling [14, 15]. TEM investigations reveal streaks diffraction pattern (Fig. 1) showing stacking faults lattice. Second phases, precipitates preferred sites allocation (Na+, Na2O) other than channel (as suggested [16]) not found; all contents, however, examined. Sodium concentration found time needed dehydration/phase transformation as is demonstrated XRD spectra thermogravimetric (TG) plots. Details isothermally heated times show “shift” 0 4 orthorhombic position 2 peak 2). shift, indicating samples, observed be faster sample 3400 that 160 1050 ppm. intermediate temporary new peaks appeared, which cannot identified among peaks. TG curves plotted Fig. 3a. accordance observations shown 2, content solid rate dehydration. dehydrates minimal velocity. velocity 1000 increases increasing content. Samples levels sodium, 495 740 ppm, exhibit greater