Gas-phase oxidation of 1-butene using nanoscale TiO2 photocatalysts

The effects of preparation methods, humidity, and calcination temperatures on the behavior of nanoscale TiO2 photocatalysts were investigated in this study. Application of these photocatalysts in the gas-phase decomposition o f l-butene demonstrated that the nanoscale catalysts, prepared by the acid- assisted sol-gel technique, showed higher photocatalytic reactivity than co mmercially available Degussa P-25 TiO2, while lower photoactivity was obtai ned on TiO2 catalysts prepared without adding acid to titanium isopropoxide . Our experimental results revealed that the oxidation rates of I-butene de creased exponentially with increasing water concentrations in the flowing s tream. However, a trace amount of water vapor was indispensable in maintain ing the stability of the catalysts. Proper calcination temperature (300 deg rees C) could promote the resistance of catalysts against the poisoning eff ects of humidity. Higher calcination temperature (400 degrees C) unfavorabl y lowered the photoactivity due to phase transformations occurring at such a temperature. Amorphous and rutile-typed TiO2 showed less photocatalytic r eactivity. XRD patterns and BET measurements indicated that moderate surfac e areas (from 100 to 160 m(2)/g), appropriate crystallite sizes (5 similar to 6 nm), and crystallinity of anatase were beneficial to the photoactivity of TiO2 catalysts. In situ FTIR studies indicated that catalyst surfaces c ontained large amounts of chemisorbed mater and hydroxyl groups, which are considered to be active sites in photocatalytic reactions. The accumulation of carbonate species on active sites resulted in the deactivation of TiO2 catalysts under dry conditions. Quantum size effects were thought to be res ponsible for the high photoactivity achieved on the nanoscale TiO2 catalyst s prepared by sol-gel methods. (C) 1999 Academic Press.