Role of the residual Na+ ions on the dispersion of WOx species on titania nanotubes by in situ thermo-Raman study

Titania nanotubes synthesized by alkali hydrothermal method were used to support 5–20 wt.% of WOx by wet impregnation method. Samples were characterized by in situ thermo-Raman spectroscopy, electronic microscopy and X-ray diffraction. Initially and below 400 °C, WOx species have octahedral coordination as isolated species. After annealing at 500 °C, the nanotubular support collapsed and transformed into anatase. This event released Na+ ions that reacted with surface WOx species converting into sub-nanometric sodium tungstate (Na2WO4) particles, homogeneously dispersed on the support. At higher temperature, that is at 700 °C and 800 °C, Na2WO4 nanoparticles were detected on samples with W content ≤10 wt.%, whereas a mixture of Na2WO4 and Na2W2 O7 was obtained on samples with W content above 15 wt.%. A very thin layer of sodium tungstate covered the titania (anatase or rutile, depending on W loading) support after annealing at 800 °C yielding core–shell particles, with titania phase in the core and Na2WO4 and/or Na2W2O7 on the shell.