A detailed insight into the preparation of nanocrystalline TiO2 powders in supercritical carbon dioxide

Abstract

This work reports detailed investigations for the preparation of nanostructured titania powders by a solvent-free sol-gel derived process, operated in supercritical CO2 (SC-CO2) at high pressures (10 -30 MPa) and large range of temperatures (373 K -823 K).Depending on the processing temperature, the reaction between Ti(OiPr)4 and water performed in a single supercritical phase led to the formation of either amorphous (Ti(OH)4 -Titanium hydroxide) or crystalline (TiO2 -Titania or Titanium dioxide) nanostructured particles. Crystalline (anatase) mesoporous powders with high specific surface area were obtained directly in CO2 solvent under supercritical conditions at temperatures as low as 523 K. The effect of hydrodynamic key process parameters such as stirring and water injection rate on both powder morphology and aggregation degree was also investigated in details. The optimized TiO2 anatase powders exhibited attractive photocatalytic activity, with high potential for the degradation of water pollutants.

Investigation of the surface properties and microstructure of TiO2 sorbents prepared in supercritical CO2 for the treatment of Sr2+ contaminated effluents

“…As detailed in a previous article [14] the decrease of SC-CO 2 density when temperature increases, and the associated increase of self-diffusion coefficients for nanoparticles and/or reacting species, favor reaction kinetics and crystallite growth. Crystallite growth seems to be heterogeneous at 350 °C, certainly due to multi-step precipitation during the temperature rise step in the reactor [16], yielding a wide (bimodal) particle size distribution for #P350 with maxima at ~ 15 and 35 nm.…”

“…TiO 2 powders were synthesized in SC-CO 2 using a batch reactor (described in Hertz et al [14]), consisting of a 0.5 L vessel which can be heated up to 350 °C and can withstand operating pressures up to ~ 30 MPa. The TIP sol-gel precursor (50 mL of isopropanol + 10 mL of TIP + 5 mL of nitric acid) was poured into the reactor, the reactor was closed and CO 2 was injected up to a pressure of ~ 5 MPa.…”

“…The intensity of the C=O band from carbonyl and carboxylic acid groups is inversely related to the synthesis temperature. The small number of C-O (Ti-C-O) bonds observed in all the samples may come from the partial degradation of organic reagents (uncomplete oxidation [14]) during synthesis in SC-CO 2 . There is also evidence of surface defects: the O/Ti atomic ratio ( Table 2) is slightly higher than 2 (the value expected for anatase TiO 2 ) in the TiO 2 lattice (indicating the presence of Ti 3+ instead of Ti 2+ ), while increasing the synthesis temperature leads to a decrease in the proportion of stoichiometric defects such as oxygen vacancies ([TiO 2−x ]/[TiO 2 ]) .…”

“…A second weight loss event, corresponding to the dehydration and dehydrogenation of carboxylic acid, is observed for the #P150 and #P250 samples between 150 and 280 °C. The final weight loss between 280 and 400 °C was attributed to the removal of chemically bonded hydroxyl groups and to the complete degradation of carbon species (formed by uncomplete oxidation of isopropanol and TIP during SC-CO 2 synthesis [14]).…”

“…Synthesizing nanostructured anatase TiO 2 with a high surface area and a well-defined mesoporous network would thus be a key advance for sorption-based decontamination processes.The TiO 2 sorbents investigated here were synthesized using supercritical carbon dioxide (SC-CO 2 ) as the reaction solvent, which has been shown to offer several advantages for the preparation and processing of inorganic nanostructured materials (nanopowders, thin films, impregnation of catalysts…) [12,13]. The unique diffusion properties and solvent power of SC-CO 2 allow mesoporous anatase powders with a high specific area to be prepared at low temperatures without any surfactant [14]. The density of the CO 2 is known to be a crucial parameter in controlling the particle size and morphology of metal oxides synthesized in SC-CO 2 [15].…”