Page 214 - Multifunctional Photocatalytic Materials for Energy

Metal-based semiconductor nanomaterials for photocatalysis 199

conventional systems constituted by a single photocatalyst or metal-semiconductor
system, because the proper combination of materials reduces the energy required for
water dissociation.
Among the nanostructured photocatalytic materials previously reported, the higher
values of quantum yields of hydrogen generation are reached for Ba-doped Sr 2 Nb 2 O 7
(50%, alkaline pure water, UV light) [86] and NiO/NaTaO 3 (56%, pure water UV
light) [87]. However, these results have a limited value for practical hydrogen pro-
ductions because, as stated, UV light accounts for only about 5% of terrestrial so-
lar radiation energy. On the other hand, the maximum quantum yields (<3%) with
visible-light-driven photocatalysts are still far from the value (>10%) indicated as the
starting point for commercial applications [88,89].
In some cases, the dopant metal limits the efficiency of these materials because
it does not favor the migration of charge carriers either in the catalyst bulk or on the
surface because of the generation of trapping centers for photogenerated electrons and
holes [17].

9.5 Catalytic photoreforming

In catalytic water photosplitting, the back reaction of hydrogen and oxygen to regen-
erate water remains thermodynamically and kinetically favored. In order to prevent a
H 2 /O 2 recombination, it is possible to perform the photocatalytic process under an in-
ert atmosphere using an organic sacrificial agent (reductant agent or hole scavenger).
Catalytic photoreforming [90] can be considered as an intermediate process between
photocatalytic water splitting and photocatalytic oxidation of organic substances. To
this purpose, the most commonly used sacrificial organic species are short chain alco-
hols (i.e., methanol, ethanol, and glycerol) and carboxylic acids (e.g., formic acid and
oxalic acid), and carbohydrates such as glucose [11]. Substrates derived from biomass
can also be used as sacrificial agents [11,91]. It is important to stress that similar
biomass-derived substrates are often constituents of food and paper industry sewage
[92,93]. Consequently, catalytic photoreforming bears relevance on the promotion of
cleaner technologies for wastewater treatment and production of an energy carrier
with high added value.
Sacrificial organic species are supposed to have oxidation potential values lower
than the corresponding value for water (1.23 V NHE at pH = 0).
In catalytic photoreforming, photogenerated holes oxidize the sacrificial agent in-
stead of water molecules with the production of protons that are reduced to hydrogen
by photogenerated electrons. Note that oxygen gas is not generated in the medium
during photocatalytic reforming.
Generally, alcoholic substances are adsorbed on the catalyst surface both in undis-
sociated structures and in forming alcoxy species [94,95]. Moreover, hydrogen pro-
duction rates increase along with the increasing number of HO groups in the molecule
[96]: polyols, such as glycerol and glucose, show higher activity during the photo-
catalytic reforming. This result has been ascribed to the role of hydroxyl groups that
promote the substrate adsorption on the active sites of the photocatalyst. A possible