Comprehensive evolved gas analysis of amorphous precursors for S-doped titania by in situ TG–FTIR and TG/DTA–MS: Part 1. Precursor from thiourea and titanium(IV)-isopropoxide

Decomposition of an amorphous precursor for S-doped titania (TiO2) nanopowders, prepared by controlled sol–gel hydrolysis–condensation of titanium(IV) tetraisopropoxide and thiourea in aqueous isopropanol, has been studied up to 800 °C in flowing air by simultaneous thermogravimetric and differential thermal analysis coupled online with quadrupole mass spectrometer (TG/DTA–MS) and FTIR spectrometric gas cell (TG–FTIR) for analysis of gases and their evolution dynamics (by evolved gas analysis, EGA) in order to explore and model thermal annealing processes of fabrication techniques leading to S:TiO2 photocatalysts with photocatalytic activities under visible light. sor sample prepared with thiourea, releases first water endothermically from room temperature to 190 °C, carbonyl sulfide (COS) from 120 to 280 °C in two stages, and ammonia (NH3) from 170 to 450 °C in three steps. Evolution of CO2, sulfur dioxide (SO2), and dinitrogen oxide, as of oxidation products, (N2O), as of oxidation products, occurs between 150 and 220, 180 and 330, 280 and 450 °C, respectively. Exothermic DTA peaks at 170, 220, and 310 °C correspond to the maximum rate of CO2, SO2, and N2O evolution, respectively. The fourth exothermic heat effect at 430 °C is probably due to the simultaneous burning out of residual carbonaceous-species, solid-state oxidation of residual sulfureous and/or sulfurous species, and transformation of amorphous titania into anatase. Titanium-oxo-sulfate formation at 500 °C has been confirmed by FTIR. Furthermore, evolution of sulfur dioxide reoccurs again in a definite step between 550 and 800 °C, indicating the significant extent of previous sulfate formation processes. Anatase, which formed also in the exothermic peak at 430 °C, keeps its structure, no rutile formation is detected below or at 800 °C by XRD.