(212f) Flame Synthesis and High Pressure High Temperature Annealing of Anatase TiO2 for Increased Photocatalytic Activity

Titanium dioxide (TiO₂) is of particular interest in photocatalysis due to its photoactivity under ultraviolet illumination and its stability under catalytic reaction conditions. As most of incident solar radiation is visible light, many researchers have sought a visible-light active photocatalyst by modifying TiO₂ in order to reduce the band gap energy (BGE). It is known that the lattice structure, particle size, dopant concentration, and supported metal nanoparticles affect the BGE of TiO₂. Through low pressure flame spray pyrolysis, carbon doping, high pressure high temperature annealing, and supporting of metals we have successfully decreased the BGE and increased the photocatalytic activity of anatase phase TiO₂.

High pressure high temperature annealing (HPHT) was conducted at 200 °C in a Parr reactor under 250 psig of hydrogen or nitrogen. We have observed that, compared to the parent sample, HPHT increases particle size, decreases the BGE, causes a slight darkening of color, and increases the absorbance throughout the UV and visible spectrum. The increase in UV and visible light absorbance indicates that HPHT samples will have a greater photocatalytic activity than un-modified anatase. This was seen via an increase in hydrogen production during methane steam reforming (MSR). Hydrogen yield increased from a negligible amount over anatase to 1.05 % and 1.12 % under UV and visible light irradiation MSR over HA-Anatase at 700 ºC.

It was found that supporting copper on anatase, 1%Cu(O)/Anatase, decreased the band gap from 3.22 eV to 3.13 eV. By combining the two modification methods, HPHT and metal supporting, we aim to decrease the BGE further and therefore increase the photocatalytic activity under UV and visible light irradiation. An order of operations variation was performed by annealing in hydrogen either before or after metals were supported and the sample calcined. All variations of HPHT and copper supporting yielded BGE lower than unmodified anatase, with HPHT hydrogen annealed copper supported on anatase, HA-1%Cu(O)/Anatase, having the smallest BGE of 3.07 eV. EPR analysis indicates that HPHT annealing under hydrogen or nitrogen increases Ti³⁺ centers in the samples. Since it has been reported that oxygen vacancies play an important role in absorption and active sites, we hypothesize that the HPHT samples have a higher photocatalytic activity than the unmodified parent sample partially due to the effect of the Ti³⁺ centers.

A third area of interest has been the low pressure flame synthesis of carbon doped TiO₂. Various compositions and flow rates of the premixed gases are introduced to the low pressure synthesis chamber and ignited to produce a flat flame. The low pressure flame spray pyrolysis method produces various polymorphs of TiO₂ which show promise in photocatalytic conversion of methanol at moderately low temperatures. Flame temperature profiles and carbon loading are predicted via computational analysis using a SPIN code to ensure that the flame of the temperature is comparable across samples made from different pre mixed gases. Photocatalytic activity of the carbon doped samples is determined via methanol oxidation reactions under UV illumination.