Transient, isothermal photocatalytic oxidation (PCO) was combined with
isotope labeling and temperature-programmed desorption and oxidation
to directly identify reaction pathways and intermediates for the room-
temperature PCO and ethanol on TiO2. The intermediates identified are
acetaldehyde, acetic acid (acetate), formaldehyde, and formic acid (fo
rmate). The alpha-carbons of ethanol, acetaldehyde, and acetic acid we
re labeled with C-13 so that the reaction pathway of each carbon could
be followed. For each molecule, the alpha-carbon preferentially oxidi
zed to CO2 as the two-carbon species were sequentially oxidized. Ethan
ol forms acetaldehyde, which either desorbs or oxidizes through at lea
st two parallel pathways, only one of which involves acetic acid. Part
of the ethanol reacts on the surface through the pathway: acetaldehyd
e --> acetic acid --> CO2 + formaldehyde --> formic acid --> CO2. The
remaining ethanol oxidizes more slowly through a pathway that does not
contain acetic acid as an intermediate: acetaldehyde --> formic acids
+ formaldehyde --> formic acid --> CO2. The oxidation of ethanol to a
cetaldehyde is not the rate-determining step. The oxidations of formal
dehyde to formic acid, and formic acid to CO2, occur at about the same
rate, which is faster than acetic acid oxidation. Acetaldehyde oxidiz
es to form intermediates at approximately the same rate as they are ox
idized. The presence of acetaldehyde on the surface, however, decrease
s the reactivity of other intermediates, suggesting that increasing th
e rate of acetaldehyde oxidation would increase the overall rate of CO
2 production. (C) 1998 Academic Press.