inorganic chemistry

(Ben Green) #1

is very important to predict the possibility for driving a
photocatalytic reaction. On the other hand, the gap between
the CB bottom and VB top, a band gap, determines the photo-
absorption spectrum, that is, wavelength range to be absorbed by
a photocatalyst. For ordinary simple and mixed metal oxides, it
has been reported in the 1980s that change in the metal induces a
shift of the CB bottom position, while the VB top position is
unchangedbecausetheVBismainlycomposedofoxygen2patomic
orbitals commonly contained in metal oxides ( 42 ). This means that
narrowing the band gap of a metal-oxide photocatalyst shifts the
CB bottom position to more anodic, that is, decreasing the ability
of reduction by e–. In most applications of photocatalysis, molecu-
lar oxygen is reduced by e–, and its standard electrode potential
of one-electronreductionofoxygen togivesuperoxideanionradical
(O 2 d–) lies just below the CB bottom of anatase titania, absorbing
only ultraviolet light. A shift of the absorption range of titania
to visible means a shift of the CB bottom below the potential
for one-electron reduction of oxygen. The reason for negligible
photocatalytic activity of tungsten(VI) oxide (tungstena), absorb-
ing visible light of wavelength up to ca. 470 nm, for oxidative
decomposition of organic and inorganic compounds in air is
accounted for by the lower CB bottom position. Thus, ordinary
metal oxides cannot be a photocatalyst being active for oxidative
decomposition under visible-light irradiation ( 43 ).
Recently, it was reported that loading small amount of plati-
num onto tungsten(VI) oxide enhances the visible-light
photocatalytic activity significantly and this is caused by the cat-
alytic action of platinum to induce multiple-electron transfer to
oxygen ( 44 ). Reactions of two and four-electron transfer processes
are as follows (potential in parentheses is standard electrode
potential versus standard hydrogen electrode at pH 0).


O 2 þ2Hþþ2e!H 2 O 2 ðÞ 0 :695V; ð 8 Þ

O 2 þ4Hþþ4e!2H 2 O1ðÞ:23V: ð 9 Þ

The standard electrode potentials are far more anodic than that of
one-electron transfer process,–0.284 V (SHE) and the visible-light
photocatalytic activity of platinum-loaded tungsten(VI) oxide could
be interpreted by enhanced multiple-electron transfer process by
deposited platinum( 45 ), since it is well known that platinum and
the other noble metals catalyze such multiple-electron transfer pro-
cesses. Similar phenomena, cocatalyst promoted visible-light
photocatalytic activity, have been reported with palladium ( 46 )
and copper oxide ( 47 ). Thus, change of reaction process seems
beneficial to realize visible-light photocatalytic activity.


420 B. OHTANI

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