Characterisation and activity of sol-gel-prepared TiO2 photocatalysts modified with Ca, Sr or Ba ion additives

TiO2 (anatase)-based photocatalyst powders containing up to 20 mol% calcium , strontium or barium ions were prepared from alpha-titanic acid by calcini ng gels prepared from triethanolamine-based sols at 600 degrees C. The powd ers were characterised using X-ray diffraction (XRD), scanning electron mic roscopy (SEM), diffuse reflectance ultraviolet spectrophotometry, nitrogen sorption porosimetry and in situ infrared spectroscopy to examine surface a dsorbed species. Compositions containing greater than 15 mol% alkaline eart h ions resulted in largely amorphous materials. The residual anatase showed decreased crystallite sizes and increased crystallographic cell volumes wi th increasing concentration of alkaline earth ions, while the BET surface a reas of the materials increased from around 80 m(2) g(-1) (no additive) to 160 m(2) g(-1) at higher levels of additive. Electronic spectroscopy showed that the band gaps of the materials increased with increasing Ca2+ content , due to the decreasing particle sizes. Under the synthesis conditions used , no other crystalline phase was observed, except rutile (< 1% w.r.t. anata se). The photocatalytic decomposition of oxalic acid was used as a model sy stem to determine the relative influences of the additives on the photocata lytic activities. Titania containing 20 mol% alkaline earth ions showed app roximately double the photocatalytic activity of similarly prepared anatase without additive. Half of the enhancement occurred for titania samples con taining only 2 mol% alkaline earth ions. Comparisons of the physico-chemica l properties of the photocatalysts with oxalic acid decomposition rates wer e carried out to determine the factors influencing the photocatalytic activ ity.