Page 215 - Multifunctional Photocatalytic Materials for Energy

200 Multifunctional Photocatalytic Materials for Energy

mechanism of photocatalytic reforming of alcohols considers the oxidation of the
molecules through an alpha-hydrogen abstraction by photogenerated hole forming
·
RCH OH radical, which is converted to aldehyde [97]:
h +

R — CH OH® R — CHOH + H +
2
+
h

R — CHOH® R —CHO + H +
The hydrated form of aldehyde can be further oxidized to the respective carboxylic
acid. Longer chain alcohols form alkanes as undesired by-products together with hy-
drogen and carbon dioxide.
Short chain carboxylic (i.e., formic acid) and dicarboxylic acids (i.e., oxalic acid)
are mineralized to carbon dioxide and water:

h +
·
R — COOH® R — CO O + H +
·
R —CO O ® CO + R ·
2
Proton ions generated during the oxidation process are reduced to hydrogen by
photoelectrons. The mechanism of carbohydrate photoreforming is more complex
and not sufficiently elucidated. Some efforts have been made to define the chemical
pathway of glucose oxidation [98,99]. It would proceed via bridge adsorption on the
surface of the catalyst through one or more hydroxyl groups [100]. Several exam-
ples of photocatalytic reforming of other organic substances, such as carboxylic acids
[101,102], aldehydes [103,104], azodyes [105], and cellulose [106] are also reported.
Additional information on different classes of sacrificial agents were recently reported
by Puga [11].
Most of the systems used for photocatalytic reforming include the same materials
reported for photocatalytic water splitting. Noble metals, despite their high cost, are
extensively studied as co-catalysts in photoreforming. In general, it appears that plati-
num is the most effective metallic element, followed by palladium and gold [107,108].
In order to reduce the cost of photocatalytic materials, some earth abundant coinage
metals, such as nickel [109], cobalt [110], and copper have been used as co-catalysts.
In particular, Cu-based metal oxides have been developed and tested in the presence of
different sacrificial agents [22,111], and also in aqueous matrices with excess nitrate
[112] and chloride [113] ions.
Some reviews report numerous heterojunction-based nanostructured materials used
for photocatalytic reforming of different organic species [11,23,41].
As for water photosplitting, all these materials are manufactured following differ-
ent preparation methods, such as photoelectrochemical deposition, wet impregnation,
photodeposition, sol-gel, solvothermal and precipitation and chemical vapor deposi-
tion, thermal and hydrothermal, and so on [76].
Generally, under the same experimental conditions, hydrogen production rates more
than two orders of magnitude higher than water photosplitting have been recorded
4
[114]. The highest activities have been achieved over metal/niobates (>4.3⋅10 μmol