For

chemical reaction A chemical reaction is a process that leads to the chemistry, chemical transformation of one set of chemical substances to another. When chemical reactions occur, the atoms are rearranged and the reaction is accompanied by an Gibbs free energy, ...

s, the iron oxide cycle (Fe₃O₄/FeO) is the original two-step

thermochemical cycle In chemistry, 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 c ...

proposed for use for

hydrogen production Hydrogen gas is produced by several industrial methods. Nearly all of the world's current supply of hydrogen is created from fossil fuels. Article in press. Most hydrogen is ''gray hydrogen'' made through steam methane reforming. In this process, ...

. It is based on the reduction and subsequent oxidation of

iron Iron is a chemical element; it has symbol Fe () and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, forming much of Earth's o ...

ions, particularly the reduction and oxidation between Fe³⁺ and Fe²⁺. The

ferrites Ferrite may refer to: * Ferrite (iron), one of the allotropes of iron that is stable at room temperature and pressure, α-Fe * Ferrite (magnet), a ferromagnetic ceramic material See also * * Ferrite bead, a component placed on the end of a data c ...

, or iron oxide, begins in the form of a

spinel Spinel () is the magnesium/aluminium member of the larger spinel group of minerals. It has the formula in the cubic crystal system. Its name comes from the Latin word , a diminutive form of ''spine,'' in reference to its pointed crystals. Prop ...

and depending on the reaction conditions, dopant metals and support material forms either

Wüstite Wüstite ( Fe O, sometimes also written as Fe0.95O) is a mineral form of mostly iron(II) oxide found with meteorites and native iron. It has a grey colour with a greenish tint in reflected light. Wüstite crystallizes in the isometric-hexoc ...

s or different spinels.

Process description

The thermochemical two-step

water splitting Water splitting is the chemical reaction in which water is broken down into oxygen and hydrogen: Efficient and economical water splitting would be a technological breakthrough that could underpin a hydrogen economy. A version of water splitti ...

process uses two

redox Redox ( , , reduction–oxidation or oxidation–reduction) is a type of chemical reaction in which the oxidation states of the reactants change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is t ...

steps. The steps of solar hydrogen production by iron based two-step cycle are: :

\begin
\ce &\ce
\\
\ce & \ce
\end

Where M can by any number of metals, often Fe itself, Co, Ni, Mn, Zn or

mixtures In chemistry, a mixture is a material made up of two or more different chemical substances which can be separated by physical method. It is an impure substance made up of 2 or more elements or compounds mechanically mixed together in any proporti ...

thereof. The endothermic reduction step (1) is carried out at high temperatures greater than , though the " Hercynite cycle" is capable of temperatures as low as . The oxidative water splitting step (2) occurs at a lower ~ temperature which produces the original ferrite material in addition to hydrogen gas. The temperature level is realized by using geothermal heat from

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma (sometimes colloquially but incorrectly referred to as ''lava'') is found beneath the surface of the Earth, and evidence of magmatism has also ...

or a solar power tower and a set of

heliostat A heliostat () is a device that reflects sunlight toward a target, turning to compensate for the Sun's apparent motion. The reflector is usually a plane mirror. The target may be a physical object, distant from the heliostat, or a direct ...

s to collect the

solar thermal energy Solar thermal energy (STE) is a form of energy and a technology for harnessing solar energy to generate thermal energy for use in Industrial sector, industry, and in the residential and commercial sectors. Solar thermal collectors are classified ...

Hercynite cycle

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 Aluminium oxide (or aluminium(III) oxide) is a chemical compound of aluminium and oxygen with the chemical formula . It is the most commonly occurring of several aluminium oxides, and specifically identified as aluminium oxide. It is commonly ...

, rather than simply decomposing. The reactions take place via the following two reactions: :

\begin

\ce &\ce
\\
\ce &\ce

\end

The reduction step of the hercynite reaction takes place at temperature ~ lower than the traditional water splitting cycle (). This leads to lower radiation losses, which scale as temperature to the fourth power.

Advantages and disadvantages

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

, reaction.

References

External links

Solar hydrogen from iron oxide based thermochemical cycles
{{DEFAULTSORT:Iron Oxide Cycle Chemical reactions Hydrogen production