groups are gradually lost, as is the catalyst’s ability as the
hydrogen bond donor. Repeated UV irradiations restore the
surface hydroxyl groups and catalytic activity in each cycle. Later
UV irradiation may also remove organic residues on the catalyst
surface by photo-oxidation, thus restoring the catalyst activity.
However, low catalytic ability from fresh TiO catalyst calcined
2
at 500 1C but stored in the dark suggests that coking may not be
the major deactivation factor.
2 3
We observed similar results from SnO and WO materials in
eqn (3) (See ESIw). The wettability can be switched between
21
2
22
and WO
3
hydrophobic and super hydrophilic on SnO
by
UV irradiation. Thus the mechanism of their catalytic activity
switches can also be explained by Scheme 1. Other metal oxides,
Fe O and Fe(OH) , did not show an appreciable activity
2
3
3
2
Fig. 3 The yields of eqn (4) using TiO catalysts from UV irradiation
difference in hydrogen bond catalysis by dark storage or UV
or non UV irradiation in 8 batches of the reaction.
irradiation (See ESIw) for eqn (3). Unlike TiO , SnO and WO ,
2
2
3
suggesting that UV irradiation will restore the super hydro-
Fe O and Fe(OH)3 are not known for their light induced
2
3
2
philicity and surface hydroxyl groups on TiO catalyst. There-
wettability changes. We are exploring other semi-conducting
metal oxides for their behaviours in catalysis with UV irradiation.
We gratefully thank the National Natural Science
Foundation of China (NSFC 50725207, 20821003 and
fore, the concentration of the surface hydroxyl groups can be
regulated by UV irradiation or use in the dark. We believe that
such changes in the concentration of surface hydroxyl groups
provide a switch mechanism for the catalytic activity of the
2
0873156),
Ministry
of
Science
and Technology
TiO catalyst in the above reactions.
2
(
MOST2007CB936400 and 2009CB930400) and the Chinese
We tested the activity switch in the aldol reaction (eqn (4)).
Academy of Sciences for financial support.
The experiments started with UV irradiated TiO catalyst, after
2
one batch of reaction the catalyst was collected and washed and
then directly tested in another batch of the same reaction.
Afterward, the catalyst was collected, UV irradiated for 1 h
and tested again. Such a cycle was repeated 4 times. As shown in
Fig. 3, in 8 batches of reactions, the yields of eqn (4) are constant
at about 60% for the UV irradiated catalyst, and about 20% for
the non UV irradiated catalyst. Apparently, the UV irradiation
of the catalyst will switch on the activity; while during the
reaction in dark, the activity is gradually switched off.
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catalysis. We propose the following mechanism in Scheme 1 to
7
8
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Scheme 1 Mechanism of photo-induced catalytic activity switch.
4
740 | Chem. Commun., 2009, 4738–4740
This journal is ꢀc The Royal Society of Chemistry 2009