Hydrogen is a promising energy vector in many industries such as: metallurgical industry, thermal treatments, petrochemical industry, food industry, electrical energy production, electronic industry, mechanical industry because of the zero environmental impact of the combustion products (only water). Main sources of hydrogen are: fossil (reforming of petroleum, reforming of natural gas, carbon gasification), nuclear thermochemical and renewable processes (electrolysis from eolic and photovoltaic electrical energy, solar thermochemical processes, biomass gasification). Steam reforming of hydrocarbons is the dominant technology but today an interesting renewable source for hydrogen production is water splitting. The theoretical energy to split water to produce H2 and O2 (e.g. under solar light) is: H2O + hν → H2 + ½ O2 ΔG = 237 kJ mol-1 Energy for water splitting can be supplied by various processes such as: electrolysis, photochemical, photocatalytic, thermal decomposition. Photocatalytic water-splitting technology using nano-sized TiO2 has great potential for low-cost, environmentally friendly solar-hydrogen production to support the future hydrogen economy (Fujishima and Honda, 1972). Presently, the solar-to-hydrogen energy conversion efficiency is too low for the technology to be economically sound. The main barriers are the rapid recombination of photo-generated electron/hole pairs as well as the backward reactions and the poor activation of TiO2 by visible light (only 3-5% of photons present in the solar light can photoactivate it). In response to these deficiencies some investigators studied the effects of addition of sacrificial reagents and carbonate salts to prohibit rapid recombination of electron/hole pairs and backward reactions. Other research focused on the enhancement of photocatalysis by modification of TiO2 by means of metal loading, metal ion doping, dye sensitization (O'Reganand Grätzel, 1991), composite semiconductor, anion doping and metal ion-implantation. Metal ion-implantation and dye sensitization are very effective methods to extend the activating spectrum to the visible range. Therefore, they play an important role in the development of efficient photocatalytic hydrogen production (Ni et al. 2007). The metal ion implanted TiO2 could efficiently work as a photocatalyst under visible light irradiation. Some field tests under solar light irradiation clearly revealed that the Cr or V ions implanted TiO2 samples showed 2-3 times higher photocatalytic reactivity than the un-implanted TiO2. Instead, a visible light responsive TiO2 thin film photocatalyst, developed by a single process using a radio frequency magnetron sputtering (RF-MS) deposition method, showed high photocatalytic reactivity for various reactions such as reduction of NOx, degradation of organic compounds, and splitting of H2O under visible and/or solar light irradiations (Takeuchi et al. 2012). Cation or anion-doped metal oxides or metal oxynitride were used to prepare visible light-responsive TiO2 thin films by a radio frequency magnetron sputtering method that were applied for the separate evolution of H2 and O2 from water under visible or solar light irradiation (Matsuoka et al. 2007). A photochemical water splitting system, composed by two reactors divided by a proton conducting membrane in which photocatalytic half-reactions of water reduction and water oxidation took place, was proposed by Zamfirescu et al. Complex molecular devices based on ruthenium-(bipyridine)(3)(2+) photosensitizers were dissolved in both reactors, which generate electrons or holes when exposed to high energy photonic radiation, and acted as catalysts for water splitting. These molecular devices for water reduction have a unique property to enhance the existence time of photoelectrons, such that the likelihood of generated electron pairs to produce a molecule of hydrogen is increased (Zamfirescu et al. 2011).Thermochemical cycles using water as raw material and nuclear/renewable energies as sources of energy is believed to be a safe, stable and sustainable route of hydrogen production. Amongst the well-studied thermochemical cycles, the sulfur-iodine (S-I) cycle is capable of achieving an energy efficiency of 50%, making it one of the most efficient cycles among all water-splitting processes (Kar et al. 2012). The S-I cycle is characterized by three basic reactions as shown below: 1. I2 + SO2 + 2H2O -> 2HI + H2SO4 (120°C) 2. 2H2SO4 -> 2SO2 + 2H2O + O2 (830°C) 3. 2HI ---> I2 + H2 (450°C) In order to overcome the low efficiency due to the poor equilibrium decomposition of HI in the third reaction, ongoing research is dedicated toward development of a hydrogen-permselective membrane reactor. Proper identification of suitable membranes (e.g. asymmetric silica membrane) and introduction of membrane reactor is proposed to improve the efficiency of the overall cycle and make hydrogen production more economical. The challenges are associated toward development of a membrane reactor which can be applied in highly corrosive environment like HI under a high temperature of about 500°C (Kar et al. 2012). Perovskites are investigated as potential redox catalyst materials for the thermochemical production of hydrogen where water is dissociated giving rise to the production of pure hydrogen during the oxidation step. The oxidation and reduction steps can be combined in a membrane reactor constructed from dense perovskite membranes towards a continuous and isothermal operation. At steady state and 900°C, 25 ±7 cm3(STP)H2 m-2 min-1 is produced in purified state (Nalbandian et al. 2009). In a perovskite hollow-fiber membrane the oxygen permeation flux increased 3.5 times more than that of the blank membrane when feeding reactive CH4 onto the permeation side of the membrane. The membrane was used as reactor to shift the equilibrium of thermal water dissociation for hydrogen production because it allows the selective removal of the produced oxygen from the water dissociation system. It was found that the hydrogen production rate increased from 0.7 to 2.1 mLH2 min-1 cm-2 at 950°C after depositing a perovskite-Pd porous layer onto the perovskite membrane (Jiang et al. 2010). References Fujishima A, Honda K (1972) Electrochemical photolysis of water at a semiconductor electrode. Nature 238:37-38 Jiang HQ, Liang FY, Czuprat O, Efimov K, Feldhoff A, Schirrmeister S, Schiestel T, Wang HH, Caro J(2010) Hydrogen production by water dissociation in surface-modified BaCoxFeyZr1-x-yO3-delta hollow-fiber membrane reactor with improved oxygen permeation. Chemistry-A European J 16:7898-7903 Kar S, Binclal RC, Prabhakar S, Tewari PK (2012) The application of membrane reactor technology in hydrogen production using S-I thermochemical process: A roadmap. Int J Hydrogen Energy 37:3612-3620 Matsuoka M, Kitano M, Takeuchi M, Tsujimaru K, Anpo M, Thomas JM(2007) Photocatalysis for new energy production - Recent advances in photocatalytic water splitting reactions for hydrogen production. Catal. Today 122:51-61 Nalbandian L, Evdou A, Zaspalis V (2009) La(1-x)Sr(x)MO(3) (M = Mn, Fe) perovskites as materialsfor thermochemical hydrogen production in conventional and membrane reactors. Int J Hydrogen Energy 34:7162-7172 Ni M, Leung MKH, Leung DYC, Sumathy K (2007)A review and recent developments in photocatalytic water-splitting using TiO2 for hydrogen production. Renew. & Sust. Energy Rev. 11: 401-425 O'Regan B, Grätzel M (1991) A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films. Nature 353(6346): 737–740. Takeuchi M, Matsuoka M, Anpo M (2012) Ion engineering techniques for the preparation of the highly effective TiO2 photocatalysts operating under visible light irradiation. Res Chem Intermed 38:1261-1277 Zamfirescu C, Dincer I, Naterer GF(2011) Analysis of a photochemical water splitting reactor with supramolecular catalysts and a proton exchange membrane. Int J Hydrogen Energy 36:11273-11281
Hydrogen production by water splitting
MOLINARI, Raffaele
2013-01-01
Abstract
Hydrogen is a promising energy vector in many industries such as: metallurgical industry, thermal treatments, petrochemical industry, food industry, electrical energy production, electronic industry, mechanical industry because of the zero environmental impact of the combustion products (only water). Main sources of hydrogen are: fossil (reforming of petroleum, reforming of natural gas, carbon gasification), nuclear thermochemical and renewable processes (electrolysis from eolic and photovoltaic electrical energy, solar thermochemical processes, biomass gasification). Steam reforming of hydrocarbons is the dominant technology but today an interesting renewable source for hydrogen production is water splitting. The theoretical energy to split water to produce H2 and O2 (e.g. under solar light) is: H2O + hν → H2 + ½ O2 ΔG = 237 kJ mol-1 Energy for water splitting can be supplied by various processes such as: electrolysis, photochemical, photocatalytic, thermal decomposition. Photocatalytic water-splitting technology using nano-sized TiO2 has great potential for low-cost, environmentally friendly solar-hydrogen production to support the future hydrogen economy (Fujishima and Honda, 1972). Presently, the solar-to-hydrogen energy conversion efficiency is too low for the technology to be economically sound. The main barriers are the rapid recombination of photo-generated electron/hole pairs as well as the backward reactions and the poor activation of TiO2 by visible light (only 3-5% of photons present in the solar light can photoactivate it). In response to these deficiencies some investigators studied the effects of addition of sacrificial reagents and carbonate salts to prohibit rapid recombination of electron/hole pairs and backward reactions. Other research focused on the enhancement of photocatalysis by modification of TiO2 by means of metal loading, metal ion doping, dye sensitization (O'Reganand Grätzel, 1991), composite semiconductor, anion doping and metal ion-implantation. Metal ion-implantation and dye sensitization are very effective methods to extend the activating spectrum to the visible range. Therefore, they play an important role in the development of efficient photocatalytic hydrogen production (Ni et al. 2007). The metal ion implanted TiO2 could efficiently work as a photocatalyst under visible light irradiation. Some field tests under solar light irradiation clearly revealed that the Cr or V ions implanted TiO2 samples showed 2-3 times higher photocatalytic reactivity than the un-implanted TiO2. Instead, a visible light responsive TiO2 thin film photocatalyst, developed by a single process using a radio frequency magnetron sputtering (RF-MS) deposition method, showed high photocatalytic reactivity for various reactions such as reduction of NOx, degradation of organic compounds, and splitting of H2O under visible and/or solar light irradiations (Takeuchi et al. 2012). Cation or anion-doped metal oxides or metal oxynitride were used to prepare visible light-responsive TiO2 thin films by a radio frequency magnetron sputtering method that were applied for the separate evolution of H2 and O2 from water under visible or solar light irradiation (Matsuoka et al. 2007). A photochemical water splitting system, composed by two reactors divided by a proton conducting membrane in which photocatalytic half-reactions of water reduction and water oxidation took place, was proposed by Zamfirescu et al. Complex molecular devices based on ruthenium-(bipyridine)(3)(2+) photosensitizers were dissolved in both reactors, which generate electrons or holes when exposed to high energy photonic radiation, and acted as catalysts for water splitting. These molecular devices for water reduction have a unique property to enhance the existence time of photoelectrons, such that the likelihood of generated electron pairs to produce a molecule of hydrogen is increased (Zamfirescu et al. 2011).Thermochemical cycles using water as raw material and nuclear/renewable energies as sources of energy is believed to be a safe, stable and sustainable route of hydrogen production. Amongst the well-studied thermochemical cycles, the sulfur-iodine (S-I) cycle is capable of achieving an energy efficiency of 50%, making it one of the most efficient cycles among all water-splitting processes (Kar et al. 2012). The S-I cycle is characterized by three basic reactions as shown below: 1. I2 + SO2 + 2H2O -> 2HI + H2SO4 (120°C) 2. 2H2SO4 -> 2SO2 + 2H2O + O2 (830°C) 3. 2HI ---> I2 + H2 (450°C) In order to overcome the low efficiency due to the poor equilibrium decomposition of HI in the third reaction, ongoing research is dedicated toward development of a hydrogen-permselective membrane reactor. Proper identification of suitable membranes (e.g. asymmetric silica membrane) and introduction of membrane reactor is proposed to improve the efficiency of the overall cycle and make hydrogen production more economical. The challenges are associated toward development of a membrane reactor which can be applied in highly corrosive environment like HI under a high temperature of about 500°C (Kar et al. 2012). Perovskites are investigated as potential redox catalyst materials for the thermochemical production of hydrogen where water is dissociated giving rise to the production of pure hydrogen during the oxidation step. The oxidation and reduction steps can be combined in a membrane reactor constructed from dense perovskite membranes towards a continuous and isothermal operation. At steady state and 900°C, 25 ±7 cm3(STP)H2 m-2 min-1 is produced in purified state (Nalbandian et al. 2009). In a perovskite hollow-fiber membrane the oxygen permeation flux increased 3.5 times more than that of the blank membrane when feeding reactive CH4 onto the permeation side of the membrane. The membrane was used as reactor to shift the equilibrium of thermal water dissociation for hydrogen production because it allows the selective removal of the produced oxygen from the water dissociation system. It was found that the hydrogen production rate increased from 0.7 to 2.1 mLH2 min-1 cm-2 at 950°C after depositing a perovskite-Pd porous layer onto the perovskite membrane (Jiang et al. 2010). References Fujishima A, Honda K (1972) Electrochemical photolysis of water at a semiconductor electrode. Nature 238:37-38 Jiang HQ, Liang FY, Czuprat O, Efimov K, Feldhoff A, Schirrmeister S, Schiestel T, Wang HH, Caro J(2010) Hydrogen production by water dissociation in surface-modified BaCoxFeyZr1-x-yO3-delta hollow-fiber membrane reactor with improved oxygen permeation. Chemistry-A European J 16:7898-7903 Kar S, Binclal RC, Prabhakar S, Tewari PK (2012) The application of membrane reactor technology in hydrogen production using S-I thermochemical process: A roadmap. Int J Hydrogen Energy 37:3612-3620 Matsuoka M, Kitano M, Takeuchi M, Tsujimaru K, Anpo M, Thomas JM(2007) Photocatalysis for new energy production - Recent advances in photocatalytic water splitting reactions for hydrogen production. Catal. Today 122:51-61 Nalbandian L, Evdou A, Zaspalis V (2009) La(1-x)Sr(x)MO(3) (M = Mn, Fe) perovskites as materialsfor thermochemical hydrogen production in conventional and membrane reactors. Int J Hydrogen Energy 34:7162-7172 Ni M, Leung MKH, Leung DYC, Sumathy K (2007)A review and recent developments in photocatalytic water-splitting using TiO2 for hydrogen production. Renew. & Sust. Energy Rev. 11: 401-425 O'Regan B, Grätzel M (1991) A low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films. Nature 353(6346): 737–740. Takeuchi M, Matsuoka M, Anpo M (2012) Ion engineering techniques for the preparation of the highly effective TiO2 photocatalysts operating under visible light irradiation. Res Chem Intermed 38:1261-1277 Zamfirescu C, Dincer I, Naterer GF(2011) Analysis of a photochemical water splitting reactor with supramolecular catalysts and a proton exchange membrane. Int J Hydrogen Energy 36:11273-11281I documenti in IRIS sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
