Photocatalytic decomposition of 2-propanol in air by mechanical mixtures of TiO2 crystalline particles and silicalite adsorbent: The complete conversion of organic molecules strongly adsorbed within zeolitic channels

Fundamental photocatalytic behaviors were investigated for mechanical mixtures of TiO2 crystalline particles (P25) and MFI type zeolite (silicalite) in the decomposition reaction of 2-propanol vapor in air for the first time. Mechanical mixing enables reliable comparisons to be made between photocatalysts because the contents of TiO2 and the adsorbent can be widely varied (51 times in this study) while keeping the particle size and crystallinity of TiO2 unchanged. That is, the use of mechanical mixture highlights the behavior of molecules adsorbed in the microporous crystals, keeping the TiO2 unchanged. In the case of the mixed photocatalysts, the initial 2-propanol concentration in the gas phase was significantly reduced because of adsorption into the zeolite. After photo-irradiation started, 2-propanol was decomposed to CO2 with no (or trace amount of) acetone detected in the gas phase. The analysis of final amount of CO2 formed by the decomposition demonstrated that just by the mechanical mixing of TiO2 and zeolite, the TiO2 photocatalyst decomposed completely the reactant and intermediate molecules strongly adsorbed into the zeolite. On the other hand, in reference experiments in which TiO2 and zeolite were not mixed and were separately placed in a photoreactor, the organic compounds strongly adsorbed in the zeolite could not be decomposed to CO2 by the photocatalyst. It is notable that the CO2 formation rates for the mixed photocatalysts were mostly constant for those comprising 40 wt% or larger amounts of zeolite, while being slower than for pure TiO2. The rate-determining step was discussed based on these data. The present study showed that the mixed photocatalyst could remove organic vapors by adsorption in the dark and decompose completely to CO2 at moderate reaction rates under photo-irradiation with minimized evolution of intermediate molecules into the gas phase.