Page 201 - Multifunctional Photocatalytic Materials for Energy

Metal-based semiconductor 9

nanomaterials for

photocatalysis

†
Laura Clarizia*, Danilo Russo*, Ilaria Di Somma , Roberto Andreozzi*,
Raffaele Marotta*
* University of Naples Federico II, Naples, Italy, Italian National Research Council
†
(IRC-CNR), Naples, Italy

9.1 Introduction

Sunlight is considered a source of energy for long-term sustainability [1]. Among
the different techniques to convert and store sunlight energy as fuel, solar production
of hydrogen as a green energy vector is particularly interesting [2]. Today hydrogen
is generally produced as a constituent of syngas from fossil fuels through steam re-
forming and water gas shift processes [3]. Other minor technologies for the produc-
tion of hydrogen include chemical, electrochemical, biological, and thermal processes
[4–7]. With regard to solar energy, hydrogen production can be achieved by means
of thermo-chemical [8], photoelectrochemical, and photochemical processes [9]. In
particular, photochemical processes require the adoption of catalysts able to absorb
the solar radiation in order to promote electrons to the higher energy levels needed for
hydrogen production. Photocatalytic processes for hydrogen production in aqueous
solutions are water catalytic photosplitting [10] and catalytic photoreforming [11].
Water catalytic photosplitting consists of water’s decomposition into hydrogen
and oxygen. Such decomposition is achieved under near-ambient conditions through
the combined use of radiation and catalysts. In catalytic photoreforming, which may
be considered as an intermediate process between water photocatalytic splitting and
photocatalytic oxidation, selected organics (called sacrificial agents) are oxidized to
generate hydrogen without water dissociation.

9.2 Thermodynamics and kinetics of the water splitting
process

Water splitting via
H O ® H + 05 O
.
2 l () 2 2
o
−1
o
−1
is a highly endergonic reaction (ΔG = 237 kJ mol , ΔH = 286 kJ mol ,
−1
o
−1
ΔS = 163 J K mol ). Because the change in Gibbs free energy of the water splitting
reaction is zero for temperatures higher than 1800 K, the equilibrium yields of the
Multifunctional Photocatalytic Materials for Energy. https://doi.org/10.1016/B978-0-08-101977-1.00010-7
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