For chemical reactions, the iron oxide cycle (Fe3O4/FeO) is the original two-step thermochemical cycle proposed for use for hydrogen production. [1] It is based on the reduction and subsequent oxidation of iron ions, particularly the reduction and oxidation between Fe3+ and Fe2+. The ferrites, or iron oxide, begins in the form of a spinel and depending on the reaction conditions, dopant metals and support material forms either Wüstites or different spinels.
The thermochemical two-step water splitting process uses two redox steps. The steps of solar hydrogen production by iron based two-step cycle are:
Where M can by any number of metals, often Fe itself, Co, Ni, Mn, Zn or mixtures thereof.
The endothermic reduction step (1) is carried out at high temperatures greater than 1400 °C, though the "Hercynite cycle" is capable of temperatures as low as 1200 °C. The oxidative water splitting step (2) occurs at a lower ~1000 °C temperature which produces the original ferrite material in addition to hydrogen gas. The temperature level is realized by using geothermal heat from magma [2] or a solar power tower and a set of heliostats to collect the solar thermal energy.
Like the traditional iron oxide cycle, the hercynite is based on the oxidation and reduction of iron atoms. However unlike the traditional cycle, the ferrite material reacts with a second metal oxide, aluminum oxide, rather than simply decomposing. The reactions take place via the following two reactions:
The reduction step of the hercynite reaction takes place at temperature ~ 200 °C lower than the traditional water splitting cycle (1200 °C). [3] This leads to lower radiation losses, which scale as temperature to the fourth power.
The advantages of the ferrite cycles are: they have lower reduction temperatures than other 2-step systems, no metallic gasses are produced, high specific H2 production capacity, non-toxicity of the elements used and abundance of the constituent elements.
The disadvantages of the ferrite cycles are: similar reduction and melting temperature of the spinels (except for the hercynite cycle as aluminates have very high melting temperatures), and slow rates of the oxidation, or water splitting, reaction.
The Haber process, also called the Haber–Bosch process, is the main industrial procedure for the production of ammonia. It converts atmospheric nitrogen (N2) to ammonia (NH3) by a reaction with hydrogen (H2) using a finely divided iron metal catalyst:
In chemistry, a half reaction is either the oxidation or reduction reaction component of a redox reaction. A half reaction is obtained by considering the change in oxidation states of individual substances involved in the redox reaction. Often, the concept of half reactions is used to describe what occurs in an electrochemical cell, such as a Galvanic cell battery. Half reactions can be written to describe both the metal undergoing oxidation and the metal undergoing reduction.
Iron(II,III) oxide, or black iron oxide, is the chemical compound with formula Fe3O4. It occurs in nature as the mineral magnetite. It is one of a number of iron oxides, the others being iron(II) oxide (FeO), which is rare, and iron(III) oxide (Fe2O3) which also occurs naturally as the mineral hematite. It contains both Fe2+ and Fe3+ ions and is sometimes formulated as FeO ∙ Fe2O3. This iron oxide is encountered in the laboratory as a black powder. It exhibits permanent magnetism and is ferrimagnetic, but is sometimes incorrectly described as ferromagnetic. Its most extensive use is as a black pigment (see: Mars Black). For this purpose, it is synthesized rather than being extracted from the naturally occurring mineral as the particle size and shape can be varied by the method of production.
Water splitting is the chemical reaction in which water is broken down into oxygen and hydrogen:
The sulfur–iodine cycle is a three-step thermochemical cycle used to produce hydrogen.
A ferrite is one of a family of iron oxide-containing magnetic ceramic materials. They are ferrimagnetic, meaning they are attracted by magnetic fields and can be magnetized to become permanent magnets. Unlike many ferromagnetic materials, most ferrites are not electrically conductive, making them useful in applications like magnetic cores for transformers to suppress eddy currents.
The Q cycle describes a series of sequential oxidation and reduction of the lipophilic electron carrier Coenzyme Q (CoQ) between the ubiquinol and ubiquinone forms. These reactions can result in the net movement of protons across a lipid bilayer.
Iron shows the characteristic chemical properties of the transition metals, namely the ability to form variable oxidation states differing by steps of one and a very large coordination and organometallic chemistry: indeed, it was the discovery of an iron compound, ferrocene, that revolutionalized the latter field in the 1950s. Iron is sometimes considered as a prototype for the entire block of transition metals, due to its abundance and the immense role it has played in the technological progress of humanity. Its 26 electrons are arranged in the configuration [Ar]3d64s2, of which the 3d and 4s electrons are relatively close in energy, and thus it can lose a variable number of electrons and there is no clear point where further ionization becomes unprofitable.
The copper–chlorine cycle is a four-step thermochemical cycle for the production of hydrogen. The Cu–Cl cycle is a hybrid process that employs both thermochemical and electrolysis steps. It has a maximum temperature requirement of about 530 degrees Celsius.
The hybrid sulfur cycle (HyS) is a two-step water-splitting process intended to be used for hydrogen production. Based on sulfur oxidation and reduction, it is classified as a hybrid thermochemical cycle because it uses an electrochemical reaction for one of the two steps. The remaining thermochemical step is shared with the sulfur-iodine cycle.
For chemical reactions, the zinc–zinc oxide cycle or Zn–ZnO cycle is a two step thermochemical cycle based on zinc and zinc oxide for hydrogen production with a typical efficiency around 40%.
Cerium(III) oxide, also known as cerium oxide, cerium trioxide, cerium sesquioxide, cerous oxide or dicerium trioxide, is an oxide of the rare-earth metal cerium. It has chemical formula Ce2O3 and is gold-yellow in color.
Thermochemical cycles combine solely heat sources (thermo) with chemical reactions to split water into its hydrogen and oxygen components. The term cycle is used because aside of water, hydrogen and oxygen, the chemical compounds used in these processes are continuously recycled.
The Schikorr reaction formally describes the conversion of the iron(II) hydroxide (Fe(OH)2) into iron(II,III) oxide (Fe3O4). This transformation reaction was first studied by Gerhard Schikorr. The global reaction follows:
Cuprospinel is a mineral. Cuprospinel is an inverse spinel with the chemical formula CuFe2O4, where copper substitutes some of the iron cations in the structure. Its structure is similar to that of magnetite, Fe3O4, yet with slightly different chemical and physical properties due to the presence of copper.
Iron(II) selenate (ferrous selenate) is an inorganic compound with the formula FeSeO4. It has anhydrous and several hydrate forms. The pentahydrate has the structure, [Fe(H2O)4]SeO4•H2O, isomorphous to the corresponding iron(II) sulfate. Heptahydrate is also known, in form of unstable green crystalline solid.
Water oxidation catalysis (WOC) is the acceleration (catalysis) of the conversion of water into oxygen and protons:
Iron(II) perchlorate is the inorganic compound with the formula Fe(ClO4)2·6H2O. A green, water-soluble solid, it is produced by the reaction of iron metal with dilute perchloric acid followed by evaporation of the solution:
A ceria based thermochemical cycle is a type of two-step thermochemical cycle that uses as oxygen carrier cerium oxides for synthetic fuel production such as hydrogen or syngas. These cycles are able to obtain either hydrogen from the splitting of water molecules, or also syngas, which is a mixture of hydrogen and carbon monoxide, by also splitting carbon dioxide molecules alongside water molecules. These type of thermochemical cycles are mainly studied for concentrated solar applications.
