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aniline and oxidation of 2-propanol to acetone occurred with The following is an expected working mechanism for reduction
high stoichiometry as shown in eqn (1); in other words, of nitrobenzene to aniline in 2-propanol suspensions of
2-propanol was used only for reduction of nitrobenzene. We Au/TiO2–Ag under irradiation of visible light. Four processes
also confirmed that further irradiation to the reaction mixture would occur: (1) the incident photons are absorbed by Au
did not alter the amount of aniline (Fig. 2(a)). From these particles through their SPR excitation,4,5 (2) electrons are
results, we can point out two excellent aspects of this reaction injected from the Au particles into the conduction band of
system as well as stoichiometry. The first is that only 2-propanol TiO2, (3) the resultant electron-deficient Au particles oxidize
was oxidized even in the presence of a reduced product, i.e., 2-propanol to acetone and return to their original metallic
aniline. In our previous study,5f we examined intermolecular state, and (4) electrons in the conduction band of TiO2 transfer
competitive oxidation of benzyl alcohol and aniline to benzalde- to the reduction site, i.e., the Ag nanoparticles as co-catalysts.
hyde and nitrobenzene, respectively, in aqueous suspensions of
In summary, a plasmonic Au/TiO2 photocatalyst was suc-
Au/CeO2 under irradiation by green light from an LED and we cessfully functionalized with an Ag co-catalyst by combination
found that the oxidation on Au nanoparticles prioritized the of the PH method (Ag) and the CPH method (Au). In the
hydroxyl group of alcohol over the amino group. The second photocatalytic reduction of nitrobenzene to aniline along with
excellent aspect is that no other reactions, i.e., re-oxidation of oxidation of 2-propanol to acetone under irradiation of visible
aniline, fruitless decomposition of acetone and 2-propanol, and light, Au/TiO2–Ag was more active than Ag-free Au/TiO2 and
reduction of TiO2, occurred even by excessive photoirradiation aniline was almost quantitatively formed with a stoichiometric
after complete consumption of nitrobenzene as shown in Fig. 2(a). amount of acetone.
To clarify the effects of the Ag co-catalyst on this reaction, the
This work was partly supported by a Grant-in-Aid for Scientific
Ag-free Au/TiO2 sample shown in Fig. 1(c) was also used under Research (No. 23560935) from the Ministry of Education, Culture,
the same conditions, and the results are shown in Fig. 2(b). The Sports, Science, and Technology (MEXT) of Japan.
Ag-free Au/TiO2 sample exhibited rates of nitrobenzene consump-
Notes and references
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c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 2551--2553 2553