PEF viability. Among the most valuable, nanoengineering has allowed the development of new nanomaterials as alternative cathodes, anodes, and catalysts. This
chapter reviews the characteristics and applications of these materials, mainly
prepared as: (i) cathodes to enhance the H 2 O 2 electrogeneration and/or to promote
heterogeneous Fenton’s reaction, (ii) anodes to favor the electrocatalytic and
photoelectrocatalytic production of
• OH, and (iii) Fe-based catalysts to extend the
pH range. Nanomaterials for hybrid treatments involving EF or PEF in combination
with adsorption are described as well.
11.2 Nanomaterials as Cathodes
Many nanomaterials have been utilized to enhance the H 2 O 2 production at the
cathode of an electrolytic cell from the two-electron reduction of O 2 gas by reaction
in Eq. 11.1. In some cases, they can simultaneously act as catalysts that favor the
heterogeneous Fenton’s reaction, mimicking the conventional Fenton’s reaction
(Eq. 11.2) with
•
OH generation (Brillas et al. 2009; Oturan and Aaron 2014; Sirés
et al. 2014; Martínez-Huitle et al. 2015; Moreira et al. 2017). The use of these
modified cathodes is described in this section.
O 2 g
ð Þ þ 2H
þ
þ 2e
À
! H 2 O 2
ð11:1Þ
Fe
2þ
þ H 2 O 2 ! Fe
3þ
þ
• OH þ OH
À
ð11:2Þ
11.2.1 Carbon-Based Nanomaterials
Carbonaceous materials are appealing targets to be used as catalysts for H 2 O 2
electrogeneration since they are abundant, cheap, durable, and show good faradaic
efficiency in the reaction in Eq. 11.1 (Čolić et al. 2018). A recent theoretical study by
Chai et al. (2017) has reported that the reduction of O 2 molecules takes place by
approaching the hydrogen sites on the carbon surface to form the hydroperoxide ion
(HO 2
À ), whose subsequent protonation yields H 2 O 2 . The two-electron oxygen
reduction reaction (ORR) is then very effective in acidic media, which agrees with
the optimum pH of conventional Fenton’s reaction (Eq. 11.2), whereas in alkaline
medium, the four-electron ORR to form H 2 O prevails. The use of carbon-based
nanomaterials allows increasing the electroactive surface area of the cathode, and
consequently higher O 2 mass transport rates are achieved.
Carbon-based nanomaterials may exhibit outstanding electronic, photonic,
electrocatalytic, chemical, and mechanical properties, depending on their nanoscale
structure. Two main groups of materials can be distinguished: nanosized and
nanostructured carbons (Khataee and Hasanzadeh 2017). The former is
11 The Use of Nanomaterials in Electro-Fenton and Photoelectro-Fenton Processes
259
chapter reviews the characteristics and applications of these materials, mainly
prepared as: (i) cathodes to enhance the H 2 O 2 electrogeneration and/or to promote
heterogeneous Fenton’s reaction, (ii) anodes to favor the electrocatalytic and
photoelectrocatalytic production of
• OH, and (iii) Fe-based catalysts to extend the
pH range. Nanomaterials for hybrid treatments involving EF or PEF in combination
with adsorption are described as well.
11.2 Nanomaterials as Cathodes
Many nanomaterials have been utilized to enhance the H 2 O 2 production at the
cathode of an electrolytic cell from the two-electron reduction of O 2 gas by reaction
in Eq. 11.1. In some cases, they can simultaneously act as catalysts that favor the
heterogeneous Fenton’s reaction, mimicking the conventional Fenton’s reaction
(Eq. 11.2) with
•
OH generation (Brillas et al. 2009; Oturan and Aaron 2014; Sirés
et al. 2014; Martínez-Huitle et al. 2015; Moreira et al. 2017). The use of these
modified cathodes is described in this section.
O 2 g
ð Þ þ 2H
þ
þ 2e
À
! H 2 O 2
ð11:1Þ
Fe
2þ
þ H 2 O 2 ! Fe
3þ
þ
• OH þ OH
À
ð11:2Þ
11.2.1 Carbon-Based Nanomaterials
Carbonaceous materials are appealing targets to be used as catalysts for H 2 O 2
electrogeneration since they are abundant, cheap, durable, and show good faradaic
efficiency in the reaction in Eq. 11.1 (Čolić et al. 2018). A recent theoretical study by
Chai et al. (2017) has reported that the reduction of O 2 molecules takes place by
approaching the hydrogen sites on the carbon surface to form the hydroperoxide ion
(HO 2
À ), whose subsequent protonation yields H 2 O 2 . The two-electron oxygen
reduction reaction (ORR) is then very effective in acidic media, which agrees with
the optimum pH of conventional Fenton’s reaction (Eq. 11.2), whereas in alkaline
medium, the four-electron ORR to form H 2 O prevails. The use of carbon-based
nanomaterials allows increasing the electroactive surface area of the cathode, and
consequently higher O 2 mass transport rates are achieved.
Carbon-based nanomaterials may exhibit outstanding electronic, photonic,
electrocatalytic, chemical, and mechanical properties, depending on their nanoscale
structure. Two main groups of materials can be distinguished: nanosized and
nanostructured carbons (Khataee and Hasanzadeh 2017). The former is
11 The Use of Nanomaterials in Electro-Fenton and Photoelectro-Fenton Processes
259