Photocatalytic oxidation and decomposition of acetic acid on titanium silicalite

Transient reaction of adsorbed monolayers of acetic acid was used to charac terize the photocatalytic properties of titanium silicalite zeolites (TS-1) . The TS-1 zeolites having Si/Ti ratios of 5, 12.5, and 50 are effective ca talysts at room temperature for both photocatalytic oxidation (PCO) and dec omposition (PCD) of acetic acid. The rates of PCO are higher than the rates of PCD for each catalyst. Acetic acid oxidized photocatalytically in 0.2% O-2 form gasphase CO2 and CH4 and adsorbed H2O on the TS-1 catalysts, where as no CH4 formed on Degussa P25 TiO2. Isotope labeling showed that, on both TiO2 and TS-1 catalysts, the alpha -carbon formed CO2 whereas the beta -ca rbon formed CH4 and CO2. The rates of oxidation of the two carbons have dif ferent dependencies on UV intensity. The catalysts with higher Si/Ti ratios adsorbed significantly more acetic acid, and the PCO rates per gram of tit anium are highest on the TS-1 catalyst with the lowest Ti content, apparent ly because a larger fraction of the Ti atoms are surface atoms on this cata lyst. During PCD in an inert atmosphere, CO2, CH4, and C2H6 formed on TiO2 and on the catalyst with a Si/Ti ratio of 5, but C2H6 was not detected on t he other catalysts. The CO2/CH4 selectivity during PCD increased with incre asing Si/Ti ratio. The first step in PCO and PCD on TS-1 catalysts appears to be similar and involves formation of a CH3 radical.