Optimization of the conditions in the molten salt hydrate assisted synthesis method of TiO2
Creators: name: Huang, Chenfeng
Publisher: name: No Publisher Supplied
Year: 2020

description: With the current transition in energy usage and concern for the environment, a primary goal is to find renewable and environmentally friendly catalysts with high activity and efficacy. To achieve this, advanced catalytic properties, such as high surface area, high acidity and low band gap energy, are of the utmost importance for heterogeneous catalysts. TiO2 is one promising metal dioxides with good mechanical structure, prevalent in both Lewis and Bronsted acid sites, low band gap energy, non-toxicity and cost effectiveness. Synthesis of TiO2 is itself not particularly novel. Sol-gel, hydrothermal, physical vapor deposition, and microwave methods have been widely studied for the production of titania. However, synthesis of TiO2 via molten salt hydrates (MSHs) have limited exposure in the open literature. In MSH, ion-water interaction is optimized and water-water interactions are minimized; by changing ratios of water/salt and salt/precursor, modification of the structure, morphology and chemical properties of synthesized materials can be achieved. Therefore, the project was undertaken in order to understand how the MSH system affects crystalline growth and to synthesize TiO2 with desirable catalytic properties. In this work, Titanium Isopropoxide (TTIP) was utilized as the precursor, using a MSH-assisted sol-gel method to synthesize TiO2 with varying properties by changing the ratio of water/LiBr and LiBr/TTIP. In order to understand the effect of the MSH system, XRD, N2-BET, Raman, SEM, UV-vis and TPD-MS techniques were applied to characterize these samples. Likewise, TiO2 phase transformation was observed to investigate thermal stability. Samples were calcined at different temperatures ranging from 400 to 600°C to observe phase transformation in TiO2 using Raman; XRD was further used to quantify preponderance of anatase, brookite and rutile phases and to identify phase transition. Since TiO2 serves as a semiconductor and photocatalyst, electronic spectroscopy was taken for all samples to measure the band gap energy via the Kubelka-Munk equation. To gain further insight into the potential of TiO2 as a material for acid-catalyzed reaction, identification and quantification of Lewis and Bronsted acid sites was conducted by pyridine and 2,6-dimethylpyridine TPD-MS. The results show that all samples exhibit anatase and brookite phases. Utilizing Raman and XRD methods, it was found that brookite changes to anatase, which further transforms to rutile. Band gap energy of MSH TiO2 was measured to be much lower than that of commercial anatase, brookite or even rutile. Moreover, a unique decreasing trend of band gap energy with increasing salt/TTIP ratio was observed. Based on results from TPD-MS, MSH TiO2 is comprised of both Lewis and Bronsted acid sites, though these sites vary in quantity and strength. SEM images show MSH TiO2 have flake-like morphology, and the thickness of these flakes decreases slightly with increasing ratios of salt/TTIP. BET surface area of some samples reaches to 200 m2/g, which is much higher than that of commercial anatase. These promising properties highlight the potential of using MSH TiO2 in future research and industrial applications, such as photocatalysis and alkylation reactions.