Upon photoexcitation, the electron produced in the semiconductor migrates to the surface and
reduces water to H 2 with the aid of Pt metal. La-doped Ta particles loaded with a NiO co-catalyst
exhibit the highest efficiency for H 2 and O 2 generation using UV light, with a quantum yield of
56% for sustained periods (>400 h) (Kato et al. 2003). The obvious difficulty in this case is the
need for UV light. H 2 and O 2 are produced under visible light at efficiencies in excess of 30%
with narrow-band-gap semiconductors but, thus far, a sacrificial reagent is required if a single
component is used (Kudo et al. 2004).
Current efforts to improve the efficiency of direct water photolysis focus on band engineering by
doping (Tang et al. 2004) or the formation of solid solutions (Kudo et al. 2004). Nitride,
oxynitride, and oxysulfide semiconductors, which have band gaps in the visible region of the
spectrum, have been studied with sacrificial electron donors and acceptors (Kasahara et al. 2002;
Ishikawa et al. 2004). Improved O 2 evolution catalysts are needed to achieve overall water
splitting with dye-sensitized semiconductor particles. The most active and stable O 2 evolution
catalyst studied to date is colloidal IrO 2 (Morris et al. 2004). Molecular O 2 -evolving catalysts
based on Ru and Mn (Limberg et al. 2001) are also interesting as components of
microheterogeneous water-splitting systems.
Photoreduction in Porous Materials. Porous materials such as zeolites and molecular sieves,
usually made of silica, have been employed as the catalyst support with more success than in the
semiconductor systems. In these cases, isolated metal centers such as Ti or Zr are substituted into
the silica framework or anchored on the pore surface.
When loaded with CO 2 and water and irradiated with UV
light, CH 3 OH and CH 4 are produced (Matsuoka and Anpo
2003). Recent mechanistic studies indicate that CO 2
splitting to CO is the primary single-photon step, with
water acting as the electron source. Reduction of CO 2 to
CO is accomplished with longer-wavelength light by
charge transfer to Zr from a second metal center (Lin and
Frei 2005). This heterogeneous approach is amenable to
exploration of other metals such as Ru and Co, which are
known to activate CO 2 from studies in homogeneous
media. By exploiting the compartmentalized nature of
nanoporous supports and developing methods for the
precise arrangement of the functionalities, such binuclear
sites offer opportunities for exploring CO 2 reduction by
water to a carbon-based fuel by means of visible-light,
multi-photon schemes (Figure 13) (Hirose et al. 2003).
Figure 13 Zr and Cu redox sites on a
nanoporous inorganic support carry out
the stepwise conversion of CO 2 and
water via CO to fuels using visible light
in a solvent-free system.