Method for producing hydrogen from water vapor by contacting water vapor and a catalytically active mixed metal oxide and a device

Abstract

The invention relates to a method for producing hydrogen from water vapor by contacting water vapor and a catalytically acting mixed metal oxide and to a device for this purpose.

Description

The invention relates to a method and a device for producing hydrogen from water vapor by contacting water vapor with at least one catalytically acting mixed metal oxide. Electrochemical and thermal processes are known for the production of hydrogen. Above 2000 °C, it is possible to split water into its components, oxygen and water, purely thermally. The splitting can be carried out at lower temperatures through the use of catalysts. The thermochemical hydrogen production concept, based on metal oxides and a two-step reaction, is well-known in this context. CeO2, for example, is a metal oxide commonly used in the prior art. The two-step reaction can be described based on the following equations (1) and (2): Equation (1) corresponds to a first stage in which a metal oxide, acting as an oxygen storage component, is at least partially thermally reduced. A metal oxide in a reduced state is obtained. This first stage takes place at high operating temperatures, particularly at 1600 °C or higher, creating oxygen vacancies in the metal oxide. The reaction according to equation (1) in the first stage is endothermic. Equation (2) describes the second stage, in which the oxygen vacancies are filled by reaction with water, releasing hydrogen. An oxidized metal oxide is obtained. The reaction according to equation (2) in the second stage is exothermic. This is typically carried out at operating temperatures above 600 °C. The first and second stages can be carried out cyclically and / or continuously in a single process. The essential element is that there is a change between the operating temperatures of the first and second stages. For a conventional, state-of-the-art process, the difference between the operating temperatures is at least 150 °C, but is typically much greater. Known devices from the prior art for producing hydrogen from steam are solar furnaces. A special collector and reactor geometry makes it possible to spatially separate the second stage, with its lower operating temperature, from the first stage, with its higher operating temperature, thus creating warmer and colder zones within the solar furnace. These furnaces feature fluidized bed reactors in which catalytically active metal oxide particles are circulated with the process gases. By controlling and / or regulating the flow through the warmer zones, the duration of the thermal reduction in the first stage can be adjusted according to equation (1). Subsequently, water is added to the reaction mixture to initiate hydrogen production in the colder zone of the solar furnace.In the state of the art, CeO2 has become established as a metal oxide because it combines a high oxygen storage density with fast oxygen diffusion kinetics. It is now an object of the invention to provide an improved method and an improved device for the energy-efficient production of hydrogen from water vapor. The problem is solved by providing a process for producing hydrogen from water vapor by contacting water vapor and a catalytically active mixed metal oxide, the process comprising the following steps: a) providing at least one catalytically active mixed metal oxide having an octahedral lattice structure distorted by the Jahn-Teller effect, wherein the octahedral lattice sites comprise titanium atoms and / or aluminum atoms, and wherein the at least one mixed metal oxide comprises at least one redox-active element, and contacting the mixed metal oxide with a carrier gas stream to supply water vapor, b) splitting water vapor into hydrogen and oxygen by absorbing oxygen at and / or into the at least one mixed metal oxide, c) regenerating the at least one mixed metal oxide by releasing oxygen.wherein step b) and step c) are performed alternately at an operating temperature and the change between step b) and step c) is controlled by a change in the concentration of water vapor in the carrier gas stream and / or by a change in the operating temperature. Preferred embodiments of the method according to the invention are specified in claims 2 to 11. According to a preferred embodiment, step a) is followed immediately by one or more of the step sequences “b) and c)” and / or “c) and b)”. Between and / or after steps a), b) and c), in particular b) and c), an optional further step may be inserted, for example, for cleaning the apparatus in which the process is carried out, for cleaning and / or replacing the provided mixed metal oxide, and / or for analyzing the materials and / or material flows used. The problem is further solved by providing a device for producing hydrogen from steam, in particular for carrying out the process according to any one of claims 1 to 11, comprising: i) at least one reactor vessel comprising at least one catalytically active mixed metal oxide having an octahedral lattice structure distorted by the Jahn-Teller effect, wherein the octahedral lattice sites comprise titanium atoms and / or aluminum atoms and wherein the at least one mixed metal oxide comprises at least one redox-active element, ii) at least one inlet of a carrier gas stream with which steam can be supplied, iii) at least one outlet with which hydrogen can be discharged in the carrier gas stream, iv) at least one control unit for controlling the concentration of steam in the carrier gas stream. Preferred embodiments of the device according to the invention are specified in claims 13 to 15. The present process enables the production of hydrogen from steam, requiring lower temperatures in contrast to previous metal oxide-catalyzed production methods. Surprisingly, the special doping of the metal oxides used as catalysts allows the transition between steps b) and c) to be controlled by changing the steam concentration in the carrier gas stream. Thus, the process can be operated continuously at a constant temperature if required, with fluctuations in the operating temperature having only a minor impact. Since no significant temperature differences need to be maintained, the process can be powered by process heat from another, for example, industrial, process.This allows for the effective use of constant thermal energy, which would otherwise be released into the environment as unused waste heat. This improves the energy efficiency of the industrial process, as well as the process for producing hydrogen from steam. According to a variant of the method according to the invention, it is also possible to control and / or regulate the production of hydrogen from steam using the present method alternatively or cumulatively by changing the operating temperature. In step a), at least one mixed metal oxide is provided. This at least one mixed metal oxide comprises at least one redox-active element and acts as an oxygen storage component. The at least one mixed metal oxide is catalytically active and has an octahedral lattice structure, with the octahedral lattice sites occupied by titanium and / or aluminum atoms. Aluminum has the advantage of stabilizing the lattice structure. Titanium, in addition to this stabilizing effect, also exhibits redox activity. The lattice structure of the at least one mixed metal oxide is distorted by the Jahn-Teller effect. In particular, the distortion of the octahedral lattice structure of the at least one mixed metal oxide influences the electron distribution, especially the electron distribution of the cations arranged in the lattice structure, as well as the atomic arrangement in the lattice structure. The Jahn-Teller effect describes the phenomenon that, particularly in octahedral complexes involving elements with half-filled ε₀ orbitals in octahedral coordination, a reduction in symmetry occurs. This reduction arises primarily from a distortion of the octahedral complex's geometry along a spatial direction. This symmetry reduction leads to an increase in the stability of the complex. Specifically, the distortion of the octahedral lattice structure affects the electron distribution, particularly the electron distribution of the cations within the lattice, as well as the atomic arrangement within the lattice. Preferably, this distortion destabilizes at least one mixed metal oxide, and in particular, its lattice structure. Distortion of the octahedral lattice structure due to the Jahn-Teller effect generates the advantageous effect that the regeneration of the at least one mischmetal oxide, particularly according to equation (1), can be carried out at significantly lower temperatures compared to metal oxides exhibiting an undistorted lattice structure. It is assumed that the aforementioned effect is due to the electron configuration present in the at least one mischmetal oxide, especially to half-occupied e^(θ) orbitals. Additionally, oxygen diffusion in the at least one mischmetal oxide is accelerated, thereby increasing the kinetics of the reaction according to equation (1), i.e., the reduction of the at least one mischmetal oxide. The stabilization of the at least one mischmetal oxide by the aluminum can counteract the destabilization of the at least one mischmetal oxide by the distortion.It is particularly advantageous that, although the distortion allows the regeneration of at least one mixed metal oxide with the release of oxygen in step c) to be carried out at lower temperatures, the lattice structure does not collapse, especially due to the stabilization by aluminium. It is possible that at least one of the mixed metal oxides is a mixed metal oxide with the general formula (I) and / or the general formula (II): M1M2xAl2-xO4(I), where 0 ≤ x ≤ 2, preferably 0 ≤ x < 2, further preferably 0.1 ≤ x ≤ 1.5, and / or M1M2yTi1-yO3(II), where 0 ≤ y ≤ 1, 0 ≤ y < 1, further preferably 0.1 ≤ x ≤ 0.9. In formula (I) and / or formula (II), M1 represents a divalent and / or trivalent cation and M2 represents a trivalent and / or tetravalent cation. The values ​​for x and y are selected from the set of real numbers. M1, M2, as well as Al and / or Ti, are encompassed by M in equation (1). The at least one mixed metal oxide can have a lattice structure type selected from the group consisting of spinel, perovskite, pyrochlore, and mixtures thereof. In particular, formula (I) and formula (II) describe the oxidized state of the at least one mixed metal oxide. It is possible that the Jahn-Teller effect is induced by at least one doping element selected from the group consisting of manganese, chromium, copper, and mixtures thereof. The at least one dopant element is preferably encompassed by M2 in formula (I) and / or formula (II). Preferably, the at least one mixed metal oxide is provided doped at least twice, more preferably three times, and more preferably four times with the dopant element. It is possible that the at least one redox-active element is selected from transition metals and / or lanthanides. Preferably, the at least one redox-active element is selected from the group consisting of vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, zirconium, cerium, titanium, rhenium, tungsten, preferably manganese, iron, titanium, vanadium, further preferably iron, vanadium, and mixtures thereof. The at least one redox-active element is preferably comprised of M1 and / or M2 in formula (I) and / or formula (II). The oxygen storage capacity is integrated into the at least one mixed metal oxide by the aforementioned redox-active elements. The at least one mixed metal oxide can be in the oxidized and / or reduced state. The at least one mixed metal oxide is preferably provided in a form selected from the group consisting of powder, pellet, granules, extrudate, tablet, additively manufactured structure, and combinations thereof. In a preferred embodiment, the at least one mixed metal oxide is provided in the form of additively manufactured structures and / or extrudate. The amount of the at least one mixed metal oxide provided depends in particular on the volumetric flow rates used and the dimensions of the reactor and / or dosage form and the dimensions in which the at least one mixed metal oxide is provided. It is advantageous to determine the amount of the at least one mixed metal oxide to be provided separately for each reactor dimension. Alternatively or additionally, it is possible that the at least one mixed metal oxide is arranged in the form of a coating in the at least one reactor vessel and / or in at least one reactor section. Furthermore, it is possible that the at least one mixed metal oxide is arranged in the form of a coating on an extrudate and / or an additively manufactured structure, on packing materials, and / or on elements of the at least one reactor vessel. The coating can be applied, in particular, by a method selected from the group consisting of dip coating, spray coating, powder coating, chemical vapor deposition, physical vapor deposition, and combinations thereof. A coating is particularly advantageous if the at least one reactor vessel and / or the at least one reactor section has, at least in an area where the coating is applied, a mean diameter, in particular a pipe diameter, of less than 20 mm, preferably less than 10 mm, more preferably less than 5 mm, and even more preferably less than 3 mm. Preferably, the pipe diameter, in particular the pipe diameter of a tube of a tube bundle reactor, has a mean diameter selected from a range of 0.1 mm to 20 mm, preferably from 1 mm to 10 mm, and more preferably from 3 mm to 5 mm.The at least one mixed metal oxide can be provided in the oxidized or reduced state in step a). If the at least one mixed metal oxide is provided in the oxidized state, step c) of regeneration is preferably carried out after step a). If the at least one mixed metal oxide is provided in the reduced state, step b) of steam cracking into hydrogen is preferably carried out after step a). In a preferred embodiment, the at least one mixed metal oxide has the general formula (I) and / or (II), wherein the Jahn-Teller effect is induced by a dopant selected from the group consisting of manganese, chromium, copper, and mixtures thereof, wherein the redox-active element is selected from transition metals and / or lanthanides, preferably from the group consisting of vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, zirconium, cerium, titanium, rhenium, tungsten, more preferably manganese, iron, titanium, vanadium, and even more preferably iron, vanadium, and mixtures thereof, and wherein the at least one mixed metal oxide is provided in a form selected from the group consisting of powder, pellet, granules, extrudate, tablet, additively manufactured structure, and combinations thereof, and / or as a coating.In a preferred embodiment, the mixed metal oxide is provided with a particle diameter, preferably sieve-equivalent, from a range of 63 µm to 1000 µm, preferably from 100 µm to 700 µm, and more preferably from 250 µm to 400 µm. The particle diameter is preferably determined using a sieve tower and analytical sieves, in particular according to a method known in the prior art. Furthermore, in step a), the at least one mixed metal oxide is brought into contact with the carrier gas stream at an operating temperature, preferably a first and / or second operating temperature. Water vapor can be supplied to the at least one mixed metal oxide via the carrier gas stream. It is possible that the supplied carrier gas stream comprises an inert carrier gas. In particular, the carrier gas stream comprises argon, nitrogen, helium, and / or neon. According to a preferred embodiment, nitrogen and / or argon, more preferably nitrogen, is used as the carrier gas. It is possible that the carrier gas stream is supplied and / or provided at the beginning of step a) at a temperature selected from the range of 100 °C to 1500 °C, preferably from 200 °C to 1000 °C, and more preferably from 400 °C to 700 °C. In an alternative embodiment, it is possible that the carrier gas stream is not preheated, but is supplied and / or provided at room temperature, for example at a temperature of 20 °C to 25 °C. In step b), the carrier gas stream contains water vapor. This is split into hydrogen and oxygen in step b). Oxygen is absorbed by and / or into the at least one mixed metal oxide. After step b), the at least one mixed metal oxide is at least partially in an oxidized state. It is possible that in step b) a water vapor concentration of greater than 0 vol.-% is set in the carrier gas stream up to the saturation concentration of the respective operating temperature, preferably the first operating temperature. According to a preferred embodiment, the carrier gas stream in step b) has a water vapor concentration selected from a range of 50% to 100%, preferably 60% to 99%, more preferably 70% to 95%, of the saturation concentration of water vapor in the respective carrier gas at the respective operating temperature. Preferably, the carrier gas stream in step b) has a water vapor concentration selected from a range of 0.1 vol.% to 70 vol.%, more preferably from 1 vol.% to 50 vol.%, and even more preferably from 5 vol.% to 40 vol.%. Step b) is carried out particularly at an operating temperature, preferably a first operating temperature. It is possible that the splitting of water vapor into hydrogen and oxygen, preferably in step b), takes place from an operating temperature of 400 °C, more preferably 500 °C, more preferably 600 °C, and even more preferably 700 °C. In particular, the splitting of water vapor into hydrogen takes place, preferably in step b), at an operating temperature selected from a range of 400 °C to 1500 °C, preferably from 400 °C to 1300 °C, more preferably from 410 °C to 1200 °C, and even more preferably from 420 °C to 700 °C. The duration of step b) is defined in particular by the oxygen storage capacity of the at least one mixed metal oxide, the geometry and / or the filling level of the at least one reactor vessel, and / or the operating conditions, such as pressure, temperature, and / or water vapor concentration. For example, increasing the surface area provided by the chosen geometry leads to faster hydrogen formation. Increasing the filling level by providing a larger quantity of the at least one mixed metal oxide increases the amount of hydrogen that can be obtained and / or allows step b) to be carried out over a longer period. Experiments have shown that providing a large surface area, a high oxygen storage capacity, and a high filling level is preferable. The duration of step b) is preferably determined empirically and / or by concentration measurements of hydrogen, water vapor, carrier gas and / or oxygen in the at least one outlet in the carrier gas stream and is preferably adjusted periodically by means of the at least one control unit. The concentration measurements are preferably carried out using a sensor, more preferably a lambda sensor, and / or a measuring point. In a preferred embodiment, in step b) the carrier gas stream has a water vapor concentration selected from a range of 0.1 vol.% to 70 vol.%, more preferably from 1 vol.% to 50 vol.%, and even more preferably from 5 vol.% to 40 vol.%, and the operating temperature is selected from a range of 350 °C to 1500 °C, more preferably from 400 °C to 1300 °C, more preferably from 410 °C to 1200 °C, and even more preferably from 420 °C to 700 °C. In step c), the at least one mixed metal oxide is regenerated by releasing oxygen. In step c), particularly according to equation (1), the at least one mixed metal oxide is at least partially reduced. This process creates oxygen vacancies in the at least one mixed metal oxide. After step c), the at least one mixed metal oxide is at least partially in the reduced state. The release of oxygen is endothermic. It is possible for the regeneration of the at least one mixed metal oxide to take place at an operating temperature of 400 °C, preferably 500 °C, and more preferably 600 °C. It is also possible for the regeneration of the at least one mixed metal oxide to take place at an operating temperature selected from a range of 400 °C to 1500 °C, preferably 410 °C to 1400 °C, more preferably 500 °C to 1300 °C, even more preferably 600 °C to 1200 °C, and more preferably 700 °C to 1000 °C. This enables a particularly energy-saving process. According to a preferred embodiment, the carrier gas stream in step c) has a water vapor concentration that is selected from a range of 0% to 60%, preferably 0% to 50%, more preferably 0% to 30%, of the saturation concentration of water vapor in the respective carrier gas at the respective operating temperature. Preferably, the carrier gas stream in step c) has a water vapor concentration selected from a range of 0 vol.% to 5 vol.%, more preferably from 0 vol.% to 1 vol.%, and even more preferably from 0 vol.% to 0.1 vol.%. In a preferred embodiment of step c), the carrier gas stream has a water vapor concentration selected from a range of 0 vol.% to 5 vol.%, more preferably from 0 vol.% to 1 vol.%, and is carried out at an operating temperature selected from a range of 400 °C to 1500 °C, more preferably from 500 °C to 1300 °C, more preferably from 600 °C to 1200 °C, and more preferably from 700 °C to 1000 °C. In step c), the at least one mixed metal oxide is preferably contacted with a carrier gas stream with or without water vapor. It is possible that the concentration of water vapor in the carrier gas stream in step b) is higher than the concentration of water vapor in step c). Preferably, in step b), the carrier gas stream comprises a water vapor concentration that is higher than the water vapor concentration in step c) by a value selected from a range of 0.1 vol.% to 70 vol.%, preferably from 1 vol.% to 50 vol.%, and more preferably from 5 vol.% to 40 vol.%. Preferably, the concentration of water vapor in the carrier gas stream is controlled and / or regulated to 0 vol.% at the beginning of step c). Preferably, the carrier gas stream contains no water vapor at step c) and therefore preferably consists of carrier gas. Step c) is performed at an operating temperature, particularly a second operating temperature. Preferably, the difference, and especially the magnitude of the difference, between steps b) and c) is less than 250 °C, more preferably less than 200 °C, and more preferably less than 150 °C. It is possible that the difference, and especially the magnitude of the difference, between steps b) and c) in the operating temperature is selected from a range of 0 °C to 250 °C, more preferably from 10 °C to 200 °C, and more preferably from 50 °C to 150 °C. Preferably, step c) is performed at a higher operating temperature than step b). It is preferred that changes in the operating temperature are not actively controlled, but rather that changes in the operating temperature result from temperature changes in the immediate environment. Therefore, it is preferred that no control unit is present to control and / or regulate changes in the operating temperature. In one embodiment, it is preferred that the operating temperature, preferably the first operating temperature, at step b) corresponds to the operating temperature, preferably the second operating temperature, at step c). It is preferred that step c) and / or step c) are carried out essentially at atmospheric pressure, preferably 1 bar absolute. Alternatively or additionally, it is possible that a pressure lower than that applied in step b) is used in step c). Changing the applied pressure between step b) and step c) positively influences the hydrogen production and the regeneration of the at least one mixed metal oxide, in particular reducing the duration of step c). Preferably, the applied pressure in step c) is 1 bar absolute or less, preferably selected from a range of 0.1 bar absolute to 1 bar absolute, and more preferably from 0.5 bar absolute to 0.9 bar absolute. It is possible that the applied pressure in step b) is 1 bar absolute or more, preferably selected from a range of 1 bar absolute to 5 bar absolute, and more preferably from 1 bar absolute to 3 bar absolute. It is preferred that, in step b) and / or step c), the required energy input is obtained using process heat from another process. This other process may, in particular, be an industrial process. In other words, it is possible that the energy required to carry out step b) and step c) is taken from the process heat, especially the waste heat, of another process. The operating temperature of the process, in particular the first and / or second operating temperature, can preferably be predetermined by the further process. It is preferred that changes in the process heat occur during the further process. Furthermore, it is possible that a change in the operating temperature, in particular between the first and second operating temperature, during a change between step b) and step c), is not actively controlled, but results from changes in the further process. An industrial process is understood to be a process for the production or further processing of intermediate and final products and / or the generation of energy and chemicals and / or the incineration or disposal of materials and waste. For example, an industrial process can be an incinerator and / or a synthesis reactor. Alternatively or additionally to water vapor, the carrier gas stream can include carbon dioxide. This makes it possible to produce carbon monoxide with at least one mixed metal oxide and carbon dioxide. The control and / or regulation of the carbon dioxide concentration can be analogous to the control and / or regulation of the water vapor concentration. It is particularly advantageous if water vapor and carbon dioxide are included in the carrier gas stream and brought into contact with at least one mixed metal oxide. This yields synthesis gas, a mixture of hydrogen and carbon monoxide. The device for producing hydrogen from steam comprises at least one reactor vessel. Within this reactor vessel is arranged at least one mixed metal oxide, which has an octahedral lattice structure and features titanium and / or aluminum atoms at the octahedral lattice sites. The mixed metal oxide is catalytically active and contains at least one redox-active element. The octahedral lattice structure is distorted by the Jahn-Teller effect. Furthermore, the device has at least one inlet for a carrier gas stream. Through this at least one inlet, steam can be supplied to the at least one reactor vessel and / or the at least one mixed metal oxide. Furthermore, the device has at least one outlet. Hydrogen, carrier gas and / or steam can be discharged from the at least one reactor vessel via this at least one outlet. The device can include at least one carburetor. Water is added to the carrier gas stream as steam via the carburetor. Preferably, the carburetor is selected from the group consisting of evaporators, saturators, carburetors, and combinations thereof. Furthermore, the device comprises at least one control unit. This control unit can be used to control and / or regulate the concentration, in particular the volume concentration, of water vapor in the carrier gas stream. It is also possible for the applied pressure to be controlled and / or regulated by this control unit. In particular, the device comprises at least one separate control unit for each reactor vessel. It is possible that at least one control unit and / or other optional control units are included in a gas management system. By controlling and / or regulating the carrier flow and / or the volume concentration of water vapor in the carrier flow by means of at least one control unit, preferably the gas management system, it is possible to control and / or regulate the change between steps b) and c) and / or the duration of these steps. In particular, the device includes sensors and / or measuring points for determining pressure and / or gas volume concentrations in the carrier gas stream in the at least one supply line, in the at least one outlet, and / or in the at least one reactor vessel, preferably in at least one reactor section. The sensors and / or measuring points acquire a measurement result that is transmitted to and processed by the at least one control unit. A suitable sensor can be a lambda sensor. Preferably, the gas volume concentrations of at least one of the following gases are measured: water vapor, hydrogen, and / or oxygen, and / or optionally carbon dioxide and / or carbon monoxide. It is possible that the device, in particular the at least one reactor vessel, is designed as a tubular reactor or as a reaction vessel. It is possible that the reactor vessel is spherical, cuboid, prismatic, or cylindrical. The at least one reactor vessel can be designed as a tubular reactor or be enclosed by a tube bundle reactor. Particularly in the case of a tubular reactor, it is advantageous if the inner surface of the tubular reactor or a tube bundle is coated with the at least one mixed metal oxide. Preferably, the at least one reactor vessel, especially if it is designed as a tubular reactor or enclosed by a tube bundle reactor, has a mean diameter selected from a range of 0.1 mm to 20 mm, preferably from 1 mm to 10 mm, and more preferably from 3 mm to 5 mm. Alternatively or additionally, the at least one reactor vessel may be designed as a reaction vessel, preferably a fluidized bed reactor. Preferably, the at least one reactor vessel, particularly when designed as a reaction vessel, preferably a fluidized bed reactor, has a mean diameter selected from a range of 0.1 cm to 200 cm, preferably from 1 cm to 100 cm, and more preferably from 10 cm to 50 cm. The device may comprise at least two, preferably at least three, reactor vessels. More preferably, the device comprises at least two, preferably at least three, tubular reactors and / or at least two, preferably at least three, reaction vessels. The at least two, preferably at least three, tubular reactors may be configured as a tube bundle reactor. It is possible that each reactor vessel has a separate inlet, a separate outlet and / or a separate control unit. The device according to the invention is in particular designed in such a way that it is suitable for sequential hydrogen production and / or for continuous hydrogen production. In sequential hydrogen production, the apparatus, in particular the at least one reactor vessel, is operated such that the process alternates between step b) of hydrogen production and step c) of regenerating the at least one mixed metal oxide. Spatially, step b) and step c) are not performed simultaneously in the at least one reactor vessel. Therefore, either step b) or step c), or a further optional step, is performed in the at least one reactor vessel. The optional step could, for example, be a step for cleaning the apparatus in which the process is carried out, for cleaning and / or replacing the provided at least one mixed metal oxide, and / or for analyzing the materials and / or material streams used. The process for sequential hydrogen production preferably comprises two time stages, in particular a first and a second time stage. Specifically, hydrogen is produced in the first time stage, and the at least one mixed metal oxide is reduced in the second time stage. In the first time stage, which can be assigned to step b), the at least one reactor vessel, in which the at least one mixed metal oxide is arranged in the reduced state, is purged with the carrier gas stream, which contains water vapor. At the end of the first time stage, the at least one mixed metal oxide is preferably at least partially in the oxidized state.In the second time period, which can be assigned to step c), the at least one reactor vessel, in which the at least one mixed metal oxide is at least partially arranged in the oxidized state, is purged with the carrier gas stream, whereby the water vapor concentration is lower compared to the first time period. At the end of the second time period, the at least one mixed metal oxide is preferably at least partially in the reduced state. In sequential hydrogen production, the change of time periods is triggered not in particular by the spatial arrangement of the at least one mixed metal oxide, but preferably by changing the composition of the carrier gas stream in the at least one reactor vessel. In a continuous hydrogen production system, the at least one reactor vessel has at least two reactor sections, preferably at least one first and at least one second reactor section. The at least two reactor sections are preferably arranged side by side, and more preferably parallel to each other, wherein each reactor section has at least one inlet and at least one outlet for the carrier gas stream, and wherein the carrier gas stream can preferably be controlled separately by means of the at least one control unit. The device can have further reactor sections in addition to the first and second reactor sections. In the at least two reactor sections, the at least one mixed metal oxide is preferably present in different states.Preferably, in the at least two reactor sections, the at least one mixed metal oxide is present alternately in the oxidized or reduced state. For example, the at least one mixed metal oxide is present in the oxidized or reduced state in the at least one first reactor section, while in the at least second reactor section it is present in the other state. The reactor vessel can be configured as a tube bundle reactor, with the reactor sections being configured as tubular reactors. In continuous hydrogen production, the device, in particular the at least one reactor vessel, is operated such that, spatially speaking, the process is switched between step b) of hydrogen production and step c) of the regeneration of the at least one mixed metal oxide, preferably in at least two reactor sections. It is therefore preferred that step b) is carried out in at least one reactor section while step c) is carried out simultaneously in at least one other reactor section. It is possible that the device, in particular the at least one reactor vessel, is arranged within and / or adjacent to a further device for carrying out a further process, or is encompassed by it, with independent material flows present. The further process can, in particular, be an industrial process, and the further device can also be an industrial device. In particular, the reactor vessel is arranged completely within the further device. This arrangement offers the advantage that the process heat released by the further process, for example in the form of waste heat, can be used directly. Since independent material flows are present, the respective reactants and products of the process according to the invention and of the further process do not come into contact with each other.It is therefore possible to arrange the device according to the invention, in particular the at least one reactor vessel, within and / or next to another device that can be assigned to a different technical field. The at least one inlet, the at least one outlet, the at least one control unit, and / or optionally the gas management system preferably exhibit high temperature resistance and / or high leak tightness. In particular, the at least one inlet, the at least one outlet, the at least one control unit, and / or optionally the gas management system exhibit temperature resistance to the first and second operating temperatures and the temperatures of the further process, so that no material fatigue occurs during the execution of the process according to the invention. In particular, the device is classified in zones according to DIN EN 60079-10-1:2009-10 (“Explosive atmospheres - Part 10-1: Classification of zones - Gas explosion hazard zones (IEC 60079-10-1:2008); German version EN 60079-10-1:2009”, issue date: 2009-10) of the risk of flammability and / or explosion tendency.In particular, the at least one inlet, the at least one outlet, the at least one control unit, and / or optionally the gas management system exhibit sufficient tightness against hydrogen so that no unintentionally escaping hydrogen is measurable. Preferably, the at least one inlet, the at least one outlet, the at least one control unit, and / or optionally the gas management system have a tightness class of L0.01 and / or a specific leakage rate of ≤0.01 mg / (s·m) for a test medium. The test medium can be, for example, helium, hydrogen, or methane. The leakage class is preferably determined according to a method described in DIN EN 1591-1:2014-04 (“Flanges and their joints – Rules for the design of flanged joints with round flanges and gaskets – Part 1: Calculation; German version EN 1591-1:2013”, Date: 2014-04). It is possible that the device, in particular the at least one reactor vessel, includes elements that have and / or form an internal heat exchange surface and / or an external heat exchange surface. The internal heat exchange surface can be provided and / or enlarged by elements having a shape selected from the group consisting of packings, sieve trays, intermediate trays, struts, ribs, honeycomb structures, bulges, depressions, bosses, and combinations thereof. The elements can be arranged longitudinally and / or transversely within the reactor vessel, in a grid, in a pattern, and / or randomly. In a preferred embodiment, the reactor vessel is a tubular reactor, with the elements arranged longitudinally and / or transversely within the reactor vessel. In a particularly preferred embodiment, the internal heat exchange surface is formed by elements in the form of honeycomb structures, which are arranged, in particular, longitudinally within the reactor vessel, which is preferably a tubular reactor. It is possible that the elements providing the internal heat exchange surface are coated with at least one mixed metal oxide. The coating can be arranged as described above. The internal heat exchange surface improves heat transfer between the at least one mixed metal oxide and the carrier gas stream. Furthermore, the shape of the elements comprising and / or forming the internal heat exchange surface enables improved flow through the at least one reactor vessel and improved stability of the reactor vessel. The internal heat exchange surface, formed in particular by elements in the form of honeycomb structures arranged longitudinally within the reactor vessel, allows for high surface density and thermal efficiency to establish a high heat flow, high mechanical stability, and low back pressure from the flowing gas.In a preferred embodiment, the reactor vessel has a relative surface density, compared to a reactor vessel without additional elements, that is selected from a range of 0.05 to 0.5, preferably from 0.1 to 0.2. The relative surface density is a relative quantity and dimensionless. The relative surface density of a reactor vessel with a heat exchange surface provided or increased by elements, particularly in the form of honeycomb structures, is compared to a reactor vessel provided without such elements. When using elements in the form of honeycomb structures, the relative surface density can be calculated using the following equation (3). In equation (3) ρ corresponds to the relative surface density, I to the side length of a honeycomb in meters and t to the wall thickness of a honeycomb in meters. The external heat exchange surface may have elements that have and / or form a shape selected from the group consisting of ribs, fins, branch structures and combinations thereof. The external heat exchange surface enables improved heat transfer from outside the reactor vessel into the reactor vessel. Of course, the above-mentioned material characteristics can also be applied equivalently in a process, or the above-mentioned process characteristics can be applied in the device. The invention is explained below using an exemplary embodiment. The illustrated embodiment is not to be understood as limiting and can be varied by the features described above. Example implementation: For the production of hydrogen from steam on a laboratory scale, a steel tube from a steel tube bundle reactor was provided as a reactor vessel. The steel pipe had an average inner diameter of 1 cm and a length of 20 cm. The operating temperature of the steel pipe was controlled by a heating sleeve containing a thermocouple and a control unit. The reactor vessel had an inlet and an outlet for the carrier gas stream. The hydrogen concentration in the outlet could be measured and processed using a lambda sensor. The measurement result was output in volume percent. The composition of the carrier gas stream was controlled and / or regulated by a control unit. 2.25 g of a mixed metal oxide in granular form with a particle diameter, preferably sieve-equivalent, of 250 µm to 400 µm, were provided according to step a) and arranged in the reactor vessel. The particle diameter, preferably sieve-equivalent, of the mixed metal oxide was determined using a sieve tower and analytical sieves (Retsch), in particular according to a method known in the prior art. The mixed metal oxide was provided as spinel and, in its oxidized state, exhibited the composition FeMn0.375Al1.625O4. The redox-active element function was provided by iron and manganese, with the Jahn-Teller effect being initiated by the manganese doping. The mixed metal oxide was regenerated, as described in step c), in a carrier gas stream with nitrogen as the inert carrier gas at a constant operating temperature of 500 °C in the reactor vessel and an applied pressure of 1 bar absolute. The carrier stream contained 100 vol% nitrogen and 0 vol% water vapor in the feed line. The regeneration step was carried out for 30 minutes. Subsequently, step b) of splitting water vapor into hydrogen and oxygen was carried out. For this purpose, the composition of the carrier gas stream was changed. The composition of the carrier gas stream in the feed line was adjusted to 90 vol% nitrogen and 10 vol% water vapor. The process continued to be carried out at a constant operating temperature of 500 °C in the reactor vessel and an applied pressure of 1 bar absolute. Step b) of hydrogen production was carried out for 30 minutes. Subsequently, the water vapor concentration in the carrier gas stream was again adjusted to 0 vol% water vapor, thereby proceeding to a further step c) of the regeneration of the mixed metal oxide. As described above, a step of regeneration of the mixed metal oxide took place, followed by a further step of hydrogen production. QUOTES INCLUDED IN THE DESCRIPTION This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited non-patent literature DIN EN 60079-10-1:2009-10

[0079] IEC 60079-10-1:2008

[0079] EN 60079-10-1:2009

[0079] DIN EN 1591-1:2014-04

[0079] EN 1591-1:2013

[0079]

Claims

A process for producing hydrogen from water vapor by contacting water vapor and a catalytically active mixed metal oxide, characterized in that the process comprises the following steps: a) providing at least one catalytically active mixed metal oxide having an octahedral lattice structure distorted by the Jahn-Teller effect, wherein the octahedral lattice sites comprise titanium atoms and / or aluminum atoms and wherein the mixed metal oxide comprises at least one redox-active element, and contacting the mixed metal oxide with a carrier gas stream to supply water vapor, b) splitting water vapor into hydrogen and oxygen by absorbing oxygen at and / or into the mixed metal oxide, c) regenerating the mixed metal oxide by releasing oxygen.wherein step b) and step c) are performed alternately at an operating temperature and the change between step b) and step c) is controlled by a change in the concentration of water vapor in the carrier gas stream and / or by a change in the operating temperature. Method according to claim 1, characterized in that the concentration of water vapor in the carrier gas stream at step b) is greater than the concentration of water vapor at step c). Method according to claim 1 or 2, characterized in that the carrier gas stream does not contain water vapor in step c). Method according to one of the preceding claims, characterized in that the difference in operating temperature at step b) and at step c) is less than 250 °C. Method according to one of the preceding claims, characterized in that in step c) a pressure is applied which is less than a pressure applied in step b). A method according to one of the preceding claims, characterized in that the at least one mixed metal oxide is a mixed metal oxide with the general formula (I) or the general formula (II): M1M2xAl2-xO4(I), wherein 0≤x≤2, M1M2yTi1-yO3(II), wherein 0 ≤ y ≤ 1, where: M1 represents a divalent and / or trivalent cation and M2 represents a trivalent and / or tetravalent cation. Method according to one of the preceding claims, characterized in that the Jahn-Teller effect is induced by at least one dopant selected from the group consisting of manganese, chromium, copper, and mixtures thereof. Method according to one of the preceding claims, characterized in that the at least one redox-active element is selected from transition metals and / or lanthanides, preferably from the group consisting of vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, zirconium, cerium, titanium, rhenium, tungsten, and mixtures thereof. Method according to one of the preceding claims, characterized in that the energy input required in step b) and / or step c) is carried out using process heat from a further method. Method according to one of the preceding claims, characterized in that the splitting of water vapor into hydrogen and oxygen takes place from a temperature of 400 °C. Method according to one of the preceding claims, characterized in that the regeneration of the at least one mixed metal oxide takes place from a temperature of 400 °C. Apparatus for producing hydrogen from steam comprising: i) at least one reactor vessel comprising at least one catalytically active mixed metal oxide having an octahedral lattice structure distorted by the Jahn-Teller effect, wherein the octahedral lattice sites comprise titanium atoms and / or aluminum atoms and wherein the mixed metal oxide comprises at least one redox-active element, ii) at least one supply of a carrier gas stream by which steam can be supplied, iii) at least one outlet by which hydrogen can be discharged in the carrier gas stream, iv) at least one control unit for controlling the concentration of steam in the carrier gas stream. Device according to claim 12, characterized in that the device is designed as a tubular reactor or as a reaction vessel. Device according to claim 12 or 13 characterized in that the device comprises at least two tubular reactors and / or at least two reaction vessels. Device according to one of claims 12 to 14 characterized in that the device is arranged within and / or next to a further device for carrying out a further method or is encompassed by it, wherein independent material flows are present.