Chapter 8 127 Bi-Functionalized Hybrid Materials as Novel Adsorbents for Heavy Metal
7. Summary conclusion
Corrosion inhibitors play an important role in metal protection engineering and the national economic construction. Judging from the current anticorrosion of equipment and other industrial fields, the use of corrosion inhibitors is an effective and economical anticorrosion method. The research on the theory, testing technol- ogy, and calculation method of inhibitor has made certain progress, which has promoted the development and application of new corrosion inhibitors.
In the future, the mechanism of the corrosion inhibitor and the relationship between the molecular structure of the corrosion inhibitor and the corrosion inhibi- tion effect should be further developed. More complete and specific environmental performance assessment methods need to be proposed; neural networks, density functional theory, and other computer science, quantum chemistry, and other research methods should be better applied to the development and evaluation of new corrosion inhibitors, and the new high-efficiency corrosion inhibitors should be designed and synthesized. In addition, promote green chemistry and research on the corrosion inhibitors with low-cost and non-polluting chromium-free, zinc-free, low- phosphorus, and even phosphorus-free which is the promising development direc- tion of water treatment corrosion inhibitor. The corrosion inhibitors can be made from natural raw materials, thereby expanding the application range. Furthermore, researchers should pay more attention to the development of multifunctional corrosion inhibitors used in different working conditions. It should focus on the development of copolymer corrosion inhibitors and other multi-purpose corrosion inhibitors that have the properties of scale inhibition, biocidal, and biodegradability.
© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/
by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Water Chemistry
It is possible that the effective corrosion inhibitor for industrial circulating cooling water may be not suitable for the treatment of oil field sewage, which
containing large concentration of Cl−. The typical oil fields in China, such as Shengli, Zhongyuan, Jianghan, Dagang, and Huabei Oilfield, are with the NaCI up to around 200,000 mg/L in the sewage, which also include CO2, H2S, dissolved oxygen, oil stain and miscellaneous, etc. CT2-7 corrosion inhibitor (the main component is organic amine) is mixed with HEDP and 1227, which is the promising reagent for oil field.
6. Results and discussion
Corrosion inhibitors are classified into inorganic corrosion inhibitors and organic corrosion inhibitors according to their composition. Inorganic corrosion inhibitors passivate the metal on the surface of the anode by its inorganic anion or prevent ions from the anode portion of the metal surface from entering the solu- tion, thereby inhibiting corrosion. The organic corrosion inhibitor mainly forms a precipitation film by the reaction between a reactive group on the organic molecule and a metal ion generated during the etching process and suppresses the electro- chemical processes of the anode and the cathode. They have good adsorption to
Main chemical composition Acid-base
concentration (%) Temperature range (°C)
Pyridine slag quaternary ammonium salt 15–20 HCl 70–90
Coal tar pyridine residue extract 15–20 HCl 80–120
Coal tar pyridine residue extract adding dyeing agent 15–28 HCl 90–180 Pyridine hydrochloride residue and surfactant 15–20 HCl 80–120
Ketoamine aldehyde condensate 15–28 HCl 90–150
Imidazoline 15 HCl 90
Imidazoline plus formaldehyde 15–28 HCl 80–150
Pyridine hydrochloride slag + alkyl sulfonate 15–28 HCl 90
MBT, thiourea, OP, and other pyridine residues 7 HCl + 3 HF 30–70
Benzyl quaternary ammonium salt 15–28 HCl 90–190
Pyridine derivatives, formaldehyde, etc. 15–28 HCl 90–120
Cyclohexanone aniline 15–28 HCl 90–180
Ketoamine condensate 15–28 HCl 90–150
Acetophenone aniline, etc. 15–28 HCl 90–160
Pyridine quaternary ammonium salt, etc. 12 HCl + 6 HF 120
Quinoline quaternary ammonium salt, etc. 12 HCl + 6 HF 150
Alkylpyridine quaternary ammonium salt and other
components 15–28 HCl 196
Ketoamine aldehyde condensate, alkynol, etc. 15–28 HCl 90–130
Imidazoline derivative 15 HCl 90–100
Imidazoline 15 HCl 90
Pyridine quaternary ammonium salt, etc. 15–28 HCl 90–120
Table 17.
Acidification inhibitors commonly used in oil fields and their application conditions.
Formulation of Corrosion Inhibitors
DOI: http://dx.doi.org/10.5772/intechopen.88533
Author details
Yun Chen* and Wenzhong Yang
School of Chemistry and Molecular Engineering, Nanjing Tech University, Nanjing, China
*Address all correspondence to: [email protected]
the metal surface in the corrosive medium. Many corrosion inhibitors containing heteroatoms rely on functional groups to adsorb on the metal surface. The nitrogen atoms in the corrosion inhibitor become cations after quaternization and are easily adsorbed by the negatively charged metal surface to form a monomolecular protec- tive film. The charge distribution and interfacial properties of the metal surface tend to stabilize the energy state of the metal surface. The process can increase the activation energy of the corrosion reaction, slow down the corrosion rate, and greatly inhibit the discharge of hydrogen ion, inhibit the cathode reaction, and effectively improve the corrosion inhibition efficiency of the corrosion inhibitor.
7. Summary conclusion
Corrosion inhibitors play an important role in metal protection engineering and the national economic construction. Judging from the current anticorrosion of equipment and other industrial fields, the use of corrosion inhibitors is an effective and economical anticorrosion method. The research on the theory, testing technol- ogy, and calculation method of inhibitor has made certain progress, which has promoted the development and application of new corrosion inhibitors.
In the future, the mechanism of the corrosion inhibitor and the relationship between the molecular structure of the corrosion inhibitor and the corrosion inhibi- tion effect should be further developed. More complete and specific environmental performance assessment methods need to be proposed; neural networks, density functional theory, and other computer science, quantum chemistry, and other research methods should be better applied to the development and evaluation of new corrosion inhibitors, and the new high-efficiency corrosion inhibitors should be designed and synthesized. In addition, promote green chemistry and research on the corrosion inhibitors with low-cost and non-polluting chromium-free, zinc-free, low- phosphorus, and even phosphorus-free which is the promising development direc- tion of water treatment corrosion inhibitor. The corrosion inhibitors can be made from natural raw materials, thereby expanding the application range. Furthermore, researchers should pay more attention to the development of multifunctional corrosion inhibitors used in different working conditions. It should focus on the development of copolymer corrosion inhibitors and other multi-purpose corrosion inhibitors that have the properties of scale inhibition, biocidal, and biodegradability.
© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/
by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
Water Chemistry
[1] Satpati AK, Ravindran PV.
Electrochemical study of the inhibition of corrosion of stainless steel by 1,2,3-benzotriazole in acidic media.
Materials Chemistry and Physics.
2008;109:352-359
[2] Behpour M, Ghoreishi SM, Salavati-Niasari M, Ebrahimi B.
Evaluating two new synthesized S-N Schiff bases on the corrosion of copper in 15% hydrochloric acid.
Materials Chemistry and Physics.
2008;107:153-157
[3] Bentiss F, Bouanis A, Mernari B, Traisnel M, Vezin H, Lagrenee M.
Understanding the adsorption of 4H-1,2,4-triazole derivatives on mild steel surface in molar hydrochloric acid. Applied Surface Science.
2007;253:3696-3704
[4] Tetsuo H. Kurita Handbook of Water Treatment. Second English ed. Japan:
Kurita Water Industries Ltd; 1999
[5] Growcock FB, Lopp VR. The inhibition of steel corrosion in hydrochloric-acid with 3-phenyl- 2-propyn-1-ol. Corrosion Science.
1988;28:397-410 References
Section 3
Water Splitting
Water Chemistry
[1] Satpati AK, Ravindran PV.
Electrochemical study of the inhibition of corrosion of stainless steel by 1,2,3-benzotriazole in acidic media.
Materials Chemistry and Physics.
2008;109:352-359
[2] Behpour M, Ghoreishi SM, Salavati-Niasari M, Ebrahimi B.
Evaluating two new synthesized S-N Schiff bases on the corrosion of copper in 15% hydrochloric acid.
Materials Chemistry and Physics.
2008;107:153-157
[3] Bentiss F, Bouanis A, Mernari B, Traisnel M, Vezin H, Lagrenee M.
Understanding the adsorption of 4H-1,2,4-triazole derivatives on mild steel surface in molar hydrochloric acid. Applied Surface Science.
2007;253:3696-3704
[4] Tetsuo H. Kurita Handbook of Water Treatment. Second English ed. Japan:
Kurita Water Industries Ltd; 1999
[5] Growcock FB, Lopp VR. The inhibition of steel corrosion in hydrochloric-acid with 3-phenyl- 2-propyn-1-ol. Corrosion Science.
1988;28:397-410 References
Section 3
Water Splitting
Chapter 10
On the Limits of Photocatalytic Water Splitting
Bahar Ipek and Deniz Uner
Abstract
The major drawbacks on the limited H2and O2evolution activities of one-step photocatalytic water splitting systems are given here with the emphasis on charge recombination, back-oxidation reactions, and mass transfer limitations. Suppres- sion of these unwanted phenomena is shown to be possible with the usage of small crystal-sized photocatalysts with low defect concentrations, presence of phase junctions, selection of co-catalyst that would be active for H2evolution but inactive for O2reduction, coating of the co-catalyst or the whole photocatalyst with
selectively permeable nanolayers, and usage of photocatalytic systems with high solid–liquid and liquid–gas surface areas. The mass transfer limitations are shown to be important especially in the liquid–gas interfaces for agitated and suspended systems with estimated H2transfer rates in the range of200–8000μmol/h.
Keywords:hydrogen production, photocatalyst, water splitting, mass transfer, back-oxidation
1. Introduction
Hydrogen gas is one of the best alternatives to fossil fuels since it has a high gravimetric energy density (142 MJ/kg) and it produces zero carbon upon combus- tion. Hydrogen is also used as a major reactant in environmentally important reac- tions such as carbon dioxide hydrogenation to methanol [1] or ammonia production (Haber-Bosch reaction) [2]. For hydrogen to be used as acleanenergy source, its production via renewable ways is of great importance. It is conventionally produced via steam reforming of methane and fossil fuels (energy intensive,
ΔH0rxn= 206 kJ/mol, 700–1100°C [3]) and coal gasification, which results in sig- nificant amounts of carbon dioxide production. The renewable ways for carbon-free production include biological sources (microalgae and cyanobacteria) and electrol- ysis of water using wind energy and photovoltaic cells as electricity generation sources. In addition to the mentioned renewable ways, photocatalytic water split- ting/oxidation is a promising alternative, in which solar energy is used as the driving force to split water molecules to hydrogen and oxygen on the surface of a catalyst. This renewable production method of hydrogen is advantageous over other renewable methods due to the free source of energy and lower cost of the
photocatalysts when compared to that of photovoltaic cells or wind turbines.
Solar-driven catalytic (photocatalytic) reactions are considered to be of fundamen- tal importance to the catalysis community since the solar energy is inexhaustible;
i.e., the solar energy absorbed by the lands and oceans on an hourly basis (432 EJ/h
Chapter 10
On the Limits of Photocatalytic Water Splitting
Bahar Ipek and Deniz Uner
Abstract
The major drawbacks on the limited H2and O2evolution activities of one-step photocatalytic water splitting systems are given here with the emphasis on charge recombination, back-oxidation reactions, and mass transfer limitations. Suppres- sion of these unwanted phenomena is shown to be possible with the usage of small crystal-sized photocatalysts with low defect concentrations, presence of phase junctions, selection of co-catalyst that would be active for H2evolution but inactive for O2reduction, coating of the co-catalyst or the whole photocatalyst with
selectively permeable nanolayers, and usage of photocatalytic systems with high solid–liquid and liquid–gas surface areas. The mass transfer limitations are shown to be important especially in the liquid–gas interfaces for agitated and suspended systems with estimated H2transfer rates in the range of200–8000μmol/h.
Keywords:hydrogen production, photocatalyst, water splitting, mass transfer, back-oxidation
1. Introduction
Hydrogen gas is one of the best alternatives to fossil fuels since it has a high gravimetric energy density (142 MJ/kg) and it produces zero carbon upon combus- tion. Hydrogen is also used as a major reactant in environmentally important reac- tions such as carbon dioxide hydrogenation to methanol [1] or ammonia production (Haber-Bosch reaction) [2]. For hydrogen to be used as acleanenergy source, its production via renewable ways is of great importance. It is conventionally produced via steam reforming of methane and fossil fuels (energy intensive,
ΔH0rxn= 206 kJ/mol, 700–1100°C [3]) and coal gasification, which results in sig- nificant amounts of carbon dioxide production. The renewable ways for carbon-free production include biological sources (microalgae and cyanobacteria) and electrol- ysis of water using wind energy and photovoltaic cells as electricity generation sources. In addition to the mentioned renewable ways, photocatalytic water split- ting/oxidation is a promising alternative, in which solar energy is used as the driving force to split water molecules to hydrogen and oxygen on the surface of a catalyst. This renewable production method of hydrogen is advantageous over other renewable methods due to the free source of energy and lower cost of the
photocatalysts when compared to that of photovoltaic cells or wind turbines.
Solar-driven catalytic (photocatalytic) reactions are considered to be of fundamen- tal importance to the catalysis community since the solar energy is inexhaustible;
i.e., the solar energy absorbed by the lands and oceans on an hourly basis (432 EJ/h
or 120,000 TW [4]) is comparable to the Earth’s yearly energy consumption (reaching 575 EJ/year or 18 TW in 2017). However, the solar-to-hydrogen energy conversion efficiency value for photocatalytic water splitting systems is much lower (targeted to be 10%, currently reaching 1% [5]) than that of photovoltaic-assisted electrolysis (reaching 30% [6]) due to the major drawbacks in the one-step photocatalytic water splitting systems. Herein, we firstly introduce photocatalytic water splitting systems and give the major developments in materials such as visible light utilization and corresponding H2and O2production activity values (in
Section 2). Then in Section 3, we discuss the causes of the low efficiencies in photocatalytic water splitting systems and the recent approaches in preventing energy efficiency-lowering factors such as inefficient visible light utilization, charge recombination, back-oxidation reactions, and mass transfer limitations.
2. Photocatalysis and water splitting
The first report on water splitting via harvesting photon energy is authored by Fujishima and Honda using a photoelectrochemical cell with a TiO2photoelectrode [7]. Following this first report suggesting the oxidation of water molecule via photo- generated holes on TiO2surface with the aid of small electrical voltage,
photocatalytic water splitting on powder photocatalyst particles is demonstrated by other authors in the late twentieth century [8–15]. Metal-loaded semiconductors (such as Pt/TiO2) are described as“short-circuited photoelectrochemical cells”that provide both the oxidizing centers and the reduction centers on the same catalyst (seeFigure 1) [16].
Photocatalytic reactions are initiated by absorption of light having an energy higher than (or equal to) the bandgap of the photocatalysts that consist of semi- conductor materials. This bandgap energy should be larger than 1.23 V for overall water oxidation reaction, for which the maximum of the valence band and the minimum of the conduction band should be located at proper potentials for the oxygen and hydrogen evolution reactions to occur. To illustrate, the minimum of
Figure 1.
Schematic representation of photocatalytic water splitting on metal-loaded semiconductor particle systems:
(1) light absorption and charge excitation from valence band to conduction band, (2) transfer of the photo- generated electrons and holes to the catalyst surface, (3) surface redox reactions, and (4) charge recombination.
Water Chemistry
the conduction band energy level should be located at a more negative potential than 0 V vs. NHE, at pH = 0 for H2evolution (Eq. (1)), and the maximum of the valence band should be at a more positive potential than 1.23 V vs. NHE at pH = 0 for oxygen evolution reaction (Eq. (2)):
4Hþþ4e�!2H2 (1)
2H2Oþ4 hþ!4HþþO2 (2)
Following the light absorption, photoexcited electrons are transferred to the conduction band, while a positively charged charge carrier (hole) is generated at the valence band. These charge carriers are then transferred to the catalyst surface (step 2 inFigure 1) to be utilized in surface redox reactions, unless they
recombine in the bulk or on the surface (step 4). Ultimately, electrons and holes reduce/oxidize the adsorbed species on the catalyst surface (step 3), the
products of which should then be desorbed from the surface to complete the overall process.
2.1 Semiconductors
TiO2, having a large bandgap (anatase: 3.2 eV), is the most commonly used photocatalyst due to its photostability, nontoxicity, and high activity (upon UV radiationλ<387 nm). Following the report on water oxidation reaction [7], various photochemical reaction activities of TiO2such as carbon dioxide reduction with H2O [17–19], alkene and alkyne hydrogenation [20, 21], CH3Cl oxidation [22], 1- octanol degradation [23], phenol degradation [24], surfactant degradation [25], and more have been reported. Detailed reviews on TiO2-based materials and
photocatalytic performances can be found in literature [26–28].
As photostable and active TiO2is, UV light requirement to activate the large bandgap of TiO2motivated research for visible light active semiconductors as well as bandgap engineering for TiO2such as nonmetal ion doping (N [29], C [30], F [31], S [32]). Substitution of lattice oxygen atoms by these anions is reported to shift the valence band level upward and narrow the bandgap to as low as 2.25 eV
(�550 nm) with 16.5% N doping [33].
Similar to TiO2, oxides of other transition metals with d0(such as Ti4+, Zr4+, Nb5+, Ta5+, and W6+[34, 35]) and d10electronic configurations (such as Ga3+, In3+, Ge4+, Sn4+, and Sb5[36–38]) are shown to possess large bandgap energies (>3 eV) due to the maximum valence band levels consisting O2porbitals located near 3 V (vs. NHE at pH = 0). These d0and d10metal oxide catalysts are reported to show remarkable one-step photocatalytic water splitting activity under UV light irradia- tion [39] reaching 71% quantum yield with photocatalysts such as Al-doped SrTiO3
[40] or Zn-doped Ga2O3[41]. The H2and O2evolution activity under UV radiation and the apparent quantum yields of some of these materials are given inTable 1.
The apparent quantum yield is defined as the number of reacted electrons and holes divided by the number of incident photons on the photocatalysts.Table 1is not intended to cover the whole range of particulate catalysts in literature but rather to give a selection of examples. A wider selection of d0and d10metal oxide particulate catalysts’one-step water oxidation activity and apparent quantum yields can be found in the works of Kudo et al., Chen et al., and Domen et al. [39, 42, 43].
The most remarkable upgrades in the apparent quantum yields are achieved by material engineering such as (i) doping the metal oxides/perovskites with cations having lower valences, (ii) decreasing the crystal sizes to submicron levels, and (iii) loading with H2/O2evolution co-catalysts.
On the Limits of Photocatalytic Water Splitting DOI: http://dx.doi.org/10.5772/intechopen.89235
or 120,000 TW [4]) is comparable to the Earth’s yearly energy consumption (reaching 575 EJ/year or 18 TW in 2017). However, the solar-to-hydrogen energy conversion efficiency value for photocatalytic water splitting systems is much lower (targeted to be 10%, currently reaching 1% [5]) than that of photovoltaic-assisted electrolysis (reaching 30% [6]) due to the major drawbacks in the one-step photocatalytic water splitting systems. Herein, we firstly introduce photocatalytic water splitting systems and give the major developments in materials such as visible light utilization and corresponding H2and O2production activity values (in Section 2). Then in Section 3, we discuss the causes of the low efficiencies in photocatalytic water splitting systems and the recent approaches in preventing energy efficiency-lowering factors such as inefficient visible light utilization, charge recombination, back-oxidation reactions, and mass transfer limitations.
2. Photocatalysis and water splitting
The first report on water splitting via harvesting photon energy is authored by Fujishima and Honda using a photoelectrochemical cell with a TiO2photoelectrode [7]. Following this first report suggesting the oxidation of water molecule via photo- generated holes on TiO2surface with the aid of small electrical voltage,
photocatalytic water splitting on powder photocatalyst particles is demonstrated by other authors in the late twentieth century [8–15]. Metal-loaded semiconductors (such as Pt/TiO2) are described as“short-circuited photoelectrochemical cells”that provide both the oxidizing centers and the reduction centers on the same catalyst (seeFigure 1) [16].
Photocatalytic reactions are initiated by absorption of light having an energy higher than (or equal to) the bandgap of the photocatalysts that consist of semi- conductor materials. This bandgap energy should be larger than 1.23 V for overall water oxidation reaction, for which the maximum of the valence band and the minimum of the conduction band should be located at proper potentials for the oxygen and hydrogen evolution reactions to occur. To illustrate, the minimum of
Figure 1.
Schematic representation of photocatalytic water splitting on metal-loaded semiconductor particle systems:
(1) light absorption and charge excitation from valence band to conduction band, (2) transfer of the photo- generated electrons and holes to the catalyst surface, (3) surface redox reactions, and (4) charge recombination.
Water Chemistry
the conduction band energy level should be located at a more negative potential than 0 V vs. NHE, at pH = 0 for H2evolution (Eq. (1)), and the maximum of the valence band should be at a more positive potential than 1.23 V vs. NHE at pH = 0 for oxygen evolution reaction (Eq. (2)):
4Hþþ4e�!2H2 (1)
2H2Oþ4 hþ!4HþþO2 (2)
Following the light absorption, photoexcited electrons are transferred to the conduction band, while a positively charged charge carrier (hole) is generated at the valence band. These charge carriers are then transferred to the catalyst surface (step 2 inFigure 1) to be utilized in surface redox reactions, unless they
recombine in the bulk or on the surface (step 4). Ultimately, electrons and holes reduce/oxidize the adsorbed species on the catalyst surface (step 3), the
products of which should then be desorbed from the surface to complete the overall process.
2.1 Semiconductors
TiO2, having a large bandgap (anatase: 3.2 eV), is the most commonly used photocatalyst due to its photostability, nontoxicity, and high activity (upon UV radiationλ<387 nm). Following the report on water oxidation reaction [7], various photochemical reaction activities of TiO2such as carbon dioxide reduction with H2O [17–19], alkene and alkyne hydrogenation [20, 21], CH3Cl oxidation [22], 1- octanol degradation [23], phenol degradation [24], surfactant degradation [25], and more have been reported. Detailed reviews on TiO2-based materials and
photocatalytic performances can be found in literature [26–28].
As photostable and active TiO2is, UV light requirement to activate the large bandgap of TiO2motivated research for visible light active semiconductors as well as bandgap engineering for TiO2such as nonmetal ion doping (N [29], C [30], F [31], S [32]). Substitution of lattice oxygen atoms by these anions is reported to shift the valence band level upward and narrow the bandgap to as low as 2.25 eV
(�550 nm) with 16.5% N doping [33].
Similar to TiO2, oxides of other transition metals with d0(such as Ti4+, Zr4+, Nb5+, Ta5+, and W6+[34, 35]) and d10electronic configurations (such as Ga3+, In3+, Ge4+, Sn4+, and Sb5[36–38]) are shown to possess large bandgap energies (>3 eV) due to the maximum valence band levels consisting O2porbitals located near 3 V (vs. NHE at pH = 0). These d0and d10metal oxide catalysts are reported to show remarkable one-step photocatalytic water splitting activity under UV light irradia- tion [39] reaching 71% quantum yield with photocatalysts such as Al-doped SrTiO3
[40] or Zn-doped Ga2O3[41]. The H2and O2evolution activity under UV radiation and the apparent quantum yields of some of these materials are given inTable 1.
The apparent quantum yield is defined as the number of reacted electrons and holes divided by the number of incident photons on the photocatalysts.Table 1is not intended to cover the whole range of particulate catalysts in literature but rather to give a selection of examples. A wider selection of d0and d10metal oxide particulate catalysts’one-step water oxidation activity and apparent quantum yields can be found in the works of Kudo et al., Chen et al., and Domen et al. [39, 42, 43].
The most remarkable upgrades in the apparent quantum yields are achieved by material engineering such as (i) doping the metal oxides/perovskites with cations having lower valences, (ii) decreasing the crystal sizes to submicron levels, and (iii) loading with H2/O2evolution co-catalysts.
On the Limits of Photocatalytic Water Splitting DOI: http://dx.doi.org/10.5772/intechopen.89235