WO2022189689A1

WO2022189689A1 - Method for obtaining h2, co and/or synthesis gas, using perovskita-type materials and use of said materials

Info

Publication number: WO2022189689A1
Application number: PCT/ES2022/070134
Authority: WO
Inventors: María ORFILA DEL HOYO; María LINARES SERRANO; Raúl Molina Gil; Ángel Javier MARUGAN AGUADO; Juan Ángel BOTAS ECHEVARRÍA; Raúl SANZ MARTÍN; María Teresa AZCONDO SÁNCHEZ; Ulises Julio AMADOR ELIZONDO
Original assignee: Universidad Rey Juan Carlos; Fundación Universitaria San Pablo-Ceu
Priority date: 2021-03-08
Filing date: 2022-03-08
Publication date: 2022-09-15
Also published as: ES2926393A1; ES2926393B2

Abstract

Disclosed is a method for obtaining H2, CO and/or synthesis gas, using perovskite-type materials, and the use of said materials. The present invention relates to a method for obtaining H2, CO and/or synthesis gas by means of thermochemical cycles based on perovskite-type materials, more specifically by reacting perovskite-type metallic oxides of formula (I): A1-xBxM1-yNyO3-z with steam and/or CO2. The perovskite-type metallic oxides of the invention displayed more activity than other perovskite-type metallic oxides found in the prior art and operate at a lower temperature. In addition, they also display very good cyclability, i.e., good production of H2, CO and/or synthesis gas is obtained during the cycles. The invention also relates to the use of the oxides of formula (I) to produce H2, CO and/or synthesis gas.

Description

METHOD OF OBTAINING H ₂ . CO AND/OR SYNTHESIS GAS USING PEROVSKITE TYPE MATERIALS AND USE OF THESE MATERIALS

DESCRIPTION

FIELD OF THE INVENTION

The present invention belongs to the technical field of new materials for producing fuels, and more specifically to a method for obtaining hydrogen (H2), carbon monoxide (CO) and/or synthesis gas by means of thermochemical cycles based on perovskite-type materials. The invention also relates to the use of these perovskite-type materials for the production of H2, CO or synthesis gas.

BACKGROUND OF THE INVENTION - Production of H2

Hydrogen production at an industrial level is carried out through steam reforming, partial oxidation and autothermal reforming of hydrocarbons, gasification of coal and heavy hydrocarbons (96% of production) and electrolysis of water (4 % of production), so that the main raw materials are hydrocarbons and water (1). In the case of processes that use hydrocarbons, it is necessary to carry out a subsequent purification stage of the H2 obtained, while in the case of electrolysis, it is not (2).

The processes that start from hydrocarbons are highly endothermic, require high pressures and the cost of production depends largely on the cost of the raw materials. In addition, the hydrogen obtained must be purified and with this method the dependence on fossil fuels is not eliminated. Regarding electrolysis, this process requires a lot of electrical energy and is much more inefficient and expensive than hydrocarbon-based technologies (2).

Therefore, traditional hydrogen production processes have serious drawbacks associated with them. For this reason, those processes that use water and biomass as raw material and renewable energy sources, especially solar energy, are gaining importance. Among these processes, the photochemical processes (3-4), the electrochemical processes (5), and the thermochemical processes (6). However, the efficiency of the conversion of solar energy to fuel (H2) is very low (7).

An alternative process to those traditionally used for the production of hydrogen and more efficient than those indicated above is the rupture of the water molecule through a series of endothermic and exothermic reactions, using solar energy as an energy source.

These processes are known as thermochemical cycles. Many multi-stage thermochemical cycles have been investigated (8), but two-stage thermochemical cycles based on metal oxides are the most attractive due to their simplicity and potential application on an industrial scale (9). In these cycles, the metal oxide is thermally reduced using concentrated solar energy as a source of energy to reach the required temperature. In a second stage, the reduced metal oxide reacts with H2O to produce H2, with CO2 to produce CO or with a mixture of H2O and CO2 to obtain H2 and CO (synthesis gas), recovering the initial metal oxide (10) .

Two-stage thermochemical cycles can be divided into thermochemical cycles based on volatile metal oxides and non-volatile metal oxides.

The cycles based on volatile metal oxides are so named because during the reduction stage the reduced material is in the gas phase, like those based on the ZnO/Zn and CdO/Cd pairs (11). The problem with these cycles is the operating temperatures (1500-2000°C) and the need to quench or rapidly cool the reduction products to prevent their recombination (12).

Regarding thermochemical cycles based on non-volatile metal oxides, these in turn are divided into two groups, stoichiometric and non-stoichiometric metal oxides.

Among the two-stage thermochemical cycles based on stoichiometric metal oxides, the one based on iron oxide (FesCL/FeO) stands out. This cycle is interesting because the oxides used are not corrosive or dangerous. The problem with this cycle is that the reduction reaction requires temperatures of 2000°C, while oxidation takes place at 600°C, which implies low efficiency due to the high temperature required for the reduction stage and the high temperature difference (8,13).

In addition to the problems indicated above, the thermochemical cycles based on the FesCU/FeO redox couple have another disadvantage, which is the deactivation of the active material due to its sintering due to the high operating temperatures (14).

For these reasons, instead of working with pure FesCU, we work with ferrite-type materials in which Fe atoms have been partially replaced by atoms of other metals such as Mn, Cu, Ni or Co, obtaining materials of type (Fei. _x Me _x )3C>4 capable of being reduced at lower temperatures. For example, the Nio ferrite _. 5M no _. 5Fe2C>4 is reduced from 1100°C, and subsequently decomposes water at temperatures of 800°C (13,15).

Another example is the MeFe ₂ 0 ₄ ferrites (Me=Cu, Ni and Mn) that can be reduced at 1200°C and reoxidized in the presence of water at 800°C (16). However, despite the fact that the use of ferrites notoriously reduces operating temperatures, sintering processes are still important and hydrogen production is low (0.37%) (8,12). One solution to sintering problems was to support the materials with ZrÜ2, AI ₂ O ₃ , Hf0 ₂ or T1O2, but H ₂ production remains low (12).

For this reason, the study of thermochemical cycles based on non-stoichiometric oxides arose. These oxides have labile oxygen atoms in their structure thanks to which they are capable of undergoing high degrees of reduction at moderate temperatures. Furthermore, they have favorable oxidation thermodynamics, are stable, and exhibit rapid reduction and oxidation kinetics (17). Cerium oxide and perovskite-type materials are the most studied non-stoichiometric oxides.

The thermochemical cycle of cerium oxide for the production of H2 consists of a first stage in which cerium oxide IV (CeC>2) is reduced to Ce2C>3 at temperatures above 2000°C (18), to be subsequently reoxidized with water at temperatures of 400°C (19). However, the reduction temperature is still very high, and very different from the oxidation temperature. For this reason, the investigations focused on carrying out the partial reduction of CeC>2 up to CeC>2-6 at temperatures of 1500°C, to subsequently produce H2 by reaction with water (20). This material has been tested for many successive cycles demonstrating its stability (21,22). On the other hand, to further reduce the reduction temperature, some authors have partially replaced the cerium atoms in the oxide with other metals (21,23,24), obtaining materials with oxygen defects, which favors the reduction, contributing to an increase. of hydrogen production during the oxidation step.

The other type of non-stoichiometric oxides studied for thermochemical cycles are perovskite-type materials (17,25,26), which can have very different compositions for a wide range of oxygen non-stoichiometry. In the case of this type of materials, in general, the reduction takes place at temperatures between 1000 and 1400°C, while the oxidation takes place at temperatures between 600 and 800°C (17,25, 27). These materials have lower hydrogen yields than the other oxides discussed in this section.

- Synthesis gas production

For the production of synthesis gas there are several routes that use solar, photochemical/photobiological, electrochemical and thermochemical energy. The first route uses the photons of solar radiation to carry out different chemical and biological reactions (28). In the electrochemical route, the electrical energy obtained through photovoltaic panels is used to carry out the rupture of the H2O and CO2 contained in mixtures of these gases (29).

The thermochemical route uses solar energy to carry out different chemical reactions for the transformation of raw materials of fossil origin, H2O or CO2 into H2 and CO. These chemical reactions can be steam reforming of natural gas (30), gasification of carbon compounds such as coal or biomass (31), or based on the concept of hydrogen production through thermochemical cycles, the production of synthesis gas by simultaneously decomposing water and carbon dioxide in the oxidation stage.

Fewer materials have been studied to obtain synthesis gas by thermochemical cycles than in the case of hydrogen production. In this case, the most studied have been cerium oxide (32) and perovskite-type materials (27). The limitations associated with the production of synthesis gas through thermochemical cycles based on cerium oxide or perovskite-type materials are similar to those described for the production of H ₂ : high operating temperatures in the case of cerium oxide, and low H ₂ yields for the case of perovskite-type materials.

In summary, the known processes for the generation of H ₂ , CO and synthesis gas through thermochemical cycles have serious associated problems due to the high reduction temperatures, the lack of stability of the materials at those temperatures and/or the low production of _H2 or CO.

Therefore, there is a need for new materials in the H ₂ , CO and synthesis gas generation sector in order to produce these fuels efficiently, compatible with current solar technology and over time.

DESCRIPTION OF THE INVENTION

The present invention solves the existing problems in the state of the art through a method of obtaining hydrogen (H ₂ ), carbon monoxide (CO) and/or synthesis gas through thermochemical cycles based on perovskite-type materials, more specifically through the reaction of perovskite-type metal oxides of formula Ai- _c B _c Mi- _g N _g q _3-z (I) with water vapor and/or CO ₂ .

The perovskite-type metal oxides of the present invention showed superior activity to that of other perovskite-type oxides found in the state of the art and operate at lower temperatures. In addition, most of the perovskite-type metal oxides of the present invention have very good cyclability, that is, a good production of H ₂ , CO and/or synthesis gas is obtained throughout the cycles.

The term "synthesis gas" in the present invention refers to a mixture of H ₂ and CO.

The term "thermochemical cycle" in the present invention refers to a process that comprises a plurality of chemical reactions in which at least one of them is the reduction of the perovskite-type metal oxide, and at least one of them is the re-oxidation of this one, for the production of H ₂ , CO and/or synthesis gas. the stage of Reduction is an endothermic reaction while the H2, CO and/or synthesis gas production step is exothermic. The heat required in the endothermic stage can be provided by any convenient means: electrical resistors, fuel combustion, radiation (including solar energy), or any other means suitable for producing the reaction.

The term "perovskite-type metal oxide" in the present invention refers to a chemical compound with the generic formula A _IX B _X M _IY N _Y 0 _3-z, referred to in the present invention as Formula (I), where A represents a element of the lanthanide, alkali or alkaline earth series (according to the IUPAC Periodic Table of Elements), B represents an element of the alkali or alkaline earth series, M represents Fe or Co, N represents a transition metal or a element of the groups of block p of the Periodic Table, x and y has a value between 0 and 1, and z has a value between 0 and 0.75, so that the oxygen content of perovskite-type oxides can vary between a maximum value of 3 and a minimum of 2.25 per formula.

The M and N type atoms are located in the center of a more or less regular polyhedron of oxygen atoms, polyhedrons that can be octahedrons, square-based pyramids or flat-square environments depending on the oxygen content and the nature of the element M. For their part, type A and B atoms occupy generally irregular environments with coordination of neighboring oxygen atoms ranging from 12 to 6. The three-dimensional arrangement of type A (and B) and M (and N) atoms and oxygens can give rise to highly regular and symmetric (cubic) structures as well as highly distorted structures with low symmetry (monoclinic and triclinic symmetries).

Likewise, when there are unoccupied oxygen positions (vacancies), they can be totally disordered or present different degrees of order, giving rise to what is called superstructure. Regardless of the greater or lesser symmetry, the greater or lesser number of oxygen vacancies and their greater or lesser degree of order and the more or less complex chemical composition, all these compounds are considered compounds with a perovskite-type structure and are thus refer to in the present invention (33).

In a first aspect, the present invention provides a method for obtaining H2, CO or synthesis gas, comprising the following steps: i) heating a perovskite-type metal oxide of formula (I)

Al-xBxMl-yNy0 ₃ - _Z (I) where A represents an element of the lanthanide, alkaline or alkaline-earth series;

B represents an element of the alkali or alkaline earth series;

M represents Fe or Co;

N represents a transition metal or an element selected from the groups of the p block of the Periodic Table; x and y have a value between 0 and 1 ; and z has a value between 0 and 0.75; for the thermal reduction of said metal oxide in a current of air or an inert gas; and ii) reacting the reduced metal oxide obtained from step i) with a gaseous stream comprising steam and/or CO2 to obtain H2, CO or synthesis gas and re-oxidation of the metal oxide to recover the oxide metal of formula (I), in which steps i) and ii) are carried out at a temperature between 300 and 800°C and a pressure between 0.1 MPa and 20 MPa.

Preferably, in the perovskite-type metal oxide of formula (I): A represents La or Nd; B represents Ca or Sr; M represents Fe or Co; N represents Fe, Co, Ti, Mo, Sb or Al; x and y have a value between 0 and 1 and z has a value between 0 and 0.75.

Preferably, the gas stream of step i) is air.

Additionally, the gas stream of step ii) comprises air or an inert gas, such as N2 or Ar, but not limited to these.

In a particular embodiment, the method of obtaining the present invention comprises a first stage in which a metal oxide of the perovskite type of formula (I) is heated, as described above, and a second stage of re-oxidation of the metal oxide obtained in the previous stage and production of H2 by reaction with a current of air or inert gas (for example, N2 or Ar) saturated in water. The method of this embodiment is characterized by an operation temperature ranging from 300 to 800°C and a working pressure in the range of 0.1 MPa to 20 MPa.

The gas stream fed into the reduction stage (first stage) is preferably air, although an inert gas can be used, preferably N2 or Ar (but not limited to these) with a sufficient flow rate to entrain the oxygen produced during the reduction stage. while the stream fed into the oxidation stage (second stage) is air or any other inert carrier gas (for example N2 or Ar) saturated in steam at 50-120°C or a steam stream.

In another particular embodiment, the method of obtaining the present invention comprises a first stage in which a metal oxide of the perovskite type of formula (I) is heated, as described above, and a second stage of re-oxidation of the metal oxide obtained in the previous stage and production of CO by reaction with a current of air or inert gas mixed with CO2 in a sufficient proportion for the reaction to take place.

The method of this particular embodiment is characterized by an operating temperature ranging between 300 and 800°C and a working pressure in the range of 0.1 MPa to 20 MPa.

The gas stream fed into the reduction stage (first stage) is preferably air, although an inert gas can be used, preferably N2 or Ar (but not limited to these) with a sufficient flow rate to entrain the oxygen produced during the reduction stage. reduction, while the stream fed to the oxidation stage (second stage) is CO2 mixed with air or any other inert carrier gas (for example N2 or Ar) in a volume percentage of CO2 in the range 10-100%, preferably between 30 and 100%.

In another particular embodiment, the method of obtaining the present invention comprises a first stage in which a metal oxide of the perovskite type of formula (I) is heated, as described above, and a second stage of re-oxidation of the metal oxide obtained in the previous stage and production of synthesis gas by reaction with a stream of inert gas (N2 or Ar) or air mixed with water vapor and CO2 in a sufficient proportion for the reaction to take place.

The gas stream fed into the reduction stage (first stage) is preferably air, although an inert gas can be used, preferably N2 or Ar (but not limited to these) with a sufficient flow rate to entrain the oxygen produced during the reduction stage. reduction, while the stream fed to the oxidation stage (second stage) is a mixture of N2, Ar or air with CO2 in a volume percentage of CO2 in the range 10-100%, preferably between 30 and 100%. %, saturated in water vapor at 50-120°C or a mixture of a stream of water vapor and CO2 with a volume percentage of CO2 of 10-90%.

In a second aspect, the present invention describes the use of a perovskite-type metal oxide of formula (I)

B represents an element of the alkali or alkaline earth series;

M represents Fe or Co;

N represents a transition metal or an element selected from the groups of the p block of the Periodic Table; x and y have a value between 0 and 1 ; and z has a value between 0 and 0.75, for the production of H2, CO or synthesis gas (CO+H2).

Preferably, in the perovskite-type metal oxide of formula (I): A represents La or Nd; B represents Ca or Sr; M represents Fe or Co; N represents Fe, Co, Ti, Mo, Sb or Al; x and y have a value between 0 and 1 ; and z has a value between 0 and 0.75. DETAILED DESCRIPTION OF THE INVENTION

Example 1. Evaluation of the material Ndi/3Sr _2/3 Co03- in the production of CO

The perovskite-type metal oxide with composition Ndi _/ 3Sr2 _/ 3CoC>3- _z was synthesized through different synthesis methods, which are known in the state of the art and could be used without difficulty by any expert in the field. These methods have been described without prejudice to the fact that it can be synthesized with other methods, leading to obtaining samples with identical characteristics in relation to the use that is protected in this present invention.

The methods chosen to carry out this example were the following: conventional high-temperature ceramic method, Pechini sol-gel method, solution combustion and mechano-chemical synthesis. a) Preparation by the ceramic method

The following stoichiometric amounts of metal nitrates were used: 0.7235 g of Nd(N0 ₃ ) ₃ 6H ₂ 0, 1.4552 g of Co(N0 ₃ ) ₂ 6H ₂ 0 and 0.4921 g of Sr (N0 ₃ ) to obtain 1 g of perovskite oxide with composition Ndi _/ 3Sr _2/ 3CoC>3- _z .

The mixture of reagents was ground and homogenized. This mixture was subjected to a first heat treatment at 800°C for 12 hours in an air oven. After grinding and homogenizing the mixture of precursors, a pellet was formed and heated at 1100°C for 72 hours with intermediate grinding every 24 hours. The resulting sample was characterized by X-ray diffraction, obtaining a highly crystalline pure material corresponding to a highly symmetrical perovskite, with lattice parameters a=3.89±3 Á and cubic symmetry Pm3m. b) Preparation by the Pechini sol-gel method

The following stoichiometric amounts of metal nitrates were used: 0.7235 g of Nd(N0 ₃ ) ₃ 6H ₂ 0, 1.4552 g of Co(N0 ₃ ) ₂ 6H ₂ 0 and 0.4921 g of Sr (N0 ₃ ) ₂ to obtain 1 g of perovskite oxide with composition Ndi _/ 3Sr _2/ 3CoC>3- _z .

The reagent mixture was dissolved in 50 mL of distilled water at room temperature. This solution was heated to around 80°C with stirring on a plate heater and citric acid was added in a stoichiometric ratio of 1 metal:3 citric acid, in this case 2.7089 g.

Maintaining heating and stirring, 1/10 of the initial volume of ethylene glycol (2 ml_) was added and polymerization with the formation of the sol occurred. The solid mixture was allowed to cool and was ground to produce a powder which was calcined at 800°C to remove organic matter and produce an inorganic ash. Said ash was ground to homogenize it and treated at 900°C for 12 hours. The resulting sample was characterized by X-ray diffraction, obtaining a highly crystalline pure material corresponding to a highly symmetrical perovskite, with lattice parameters a=3.87±3 Á and cubic symmetry Pm3m. c) Preparation by the spray-pyrolysis-assisted combustion method in solution

The following stoichiometric quantities of metal nitrates were used: 3.6175 g of Nd(N0 ₃ ) ₃ -6H ₂ 0, 7.2760 g of Co(N0 ₃ ) ₂ -6H ₂ 0 and 2.4605 g of Sr(N0 ₃ ) ₂ to obtain 5 g of perovskite oxide with composition Ndi _/3 Sr _2/3 Co0 _3-z .

The reagent mixture was dissolved in 30 mL of distilled water at room temperature. Subsequently, 4.5735 g of glycine were dissolved in said solution.

This solution was used in a spray-pyrolysis system (consisting of a liquid projection system in the form of fine drops, like a nebulizer, which is projected onto a hot surface, 450°C in this case) that promotes the combustion reaction by thermal activation.

The powder produced in the pyrolysis was treated at 900°C for 4 hours to produce the perovskite of composition Ndi _/3 Sr _2/3 Co0 _3-z . The resulting sample was characterized by X-ray diffraction, obtaining a highly crystalline pure material corresponding to a highly symmetrical perovskite, with lattice parameters a=3.89±3 Á and Pm3m symmetry. d) Preparation by the mechano-chemical method

The following stoichiometric quantities of reagents were used: 1.3180 g of Nd ₂ 0 ₃ , 1.8740 g of Sr0 ₂ and 2.1605 g of CoOOH to obtain 5 g of perovskite oxide with composition Ndi _/3 Sr _2/3 Co0 _{3 .z} . The mixture of the reagents was placed in a 50 mL volume zirconia grinding container with 5 mm diameter balls of the same material with a total mass of 50 g. Milling was carried out for 12 hours at 500 rpm.

The resulting sample was characterized by X-ray diffraction, obtaining a highly crystalline pure material corresponding to a highly symmetrical perovskite, with lattice parameters a=3.9±1 Á and cubic symmetry Pm3m.

In order to evaluate the CO production, the Ndi _/ 3Sr2 _/ 3CoC>3- _z sample, obtained by the conventional ceramic method, was used as perovskite-type metal oxide in the method of the present invention.

50 mg of the perovskite type metal oxide Ndi _/ 3Sr2 _/ 3CoC>3- _z were introduced into a 150 pL alumina crucible in the thermobalance. Next, the sample was heated with a ramp of 10°C/min until reaching 800°C. This temperature was kept constant for the rest of the test.

During the initial heating, an air current of 100 mL/min was introduced into the thermobalance to entrain the O2 produced during the reduction stage. After completing the heating, the air stream fed to the thermobalance was replaced by a stream of pure CO2 to carry out the production of CO and the recovery of the initial perovskite.

This oxidation step was maintained for 90 min. After this time, the introduction of CO2 was stopped and air was re-introduced to carry out the reduction again. This stage lasted approximately 70 min. These changes in the gas current were carried out periodically until 16 cycles were carried out. The mass variations recorded in the thermobalance correspond to the oxygen lost or gained by the material depending on whether it is in the reduction or oxidation stage. The results of this example are shown in Table 1.

Ndi _/ 3Sr2 _/ 3CoC>3- _z has a stable behavior against cycles with a steady state CO production of 3038 pmol/g _material · cycle, much higher than that of other perovskite-type materials. Furthermore, the reduction has been carried out at much lower temperatures (see Table 1). Example 2. Evaluation of the material Ndi/3Sr _2/3 Co03-z in the production of H2

In this example, the production of H2 was evaluated using the Ndi _/ 3Sr2 _/ 3CoC>3- _z sample, obtained by the conventional ceramic method described in example 1, as perovskite-type metal oxide in the method of the present invention. 1 g of the perovskite-type metal oxide Ndi _/ 3Sr2 _/ 3CoC>3- _z was introduced into a crucible of Pt/Rh 90/10 in an oven. To carry out the reduction step, the temperature was increased to 700°C with a heating ramp of 10°C/min. During this stage of thermal reduction of the material, the furnace was fed with an air current of 50 NL/h, to transport the O2 produced in this stage to a gas analyzer. Once the temperature of 700°C was reached and the reduction finished, the air stream was saturated with water at 80°C to feed it to the reactor and carry out the perovskite oxidation stage and H2 production.

In the first cycle, a production of H2 of _898.7 pmol/g material is obtained, which decreases slightly when cycling to reach a stationary value of 497.9 pmol/g _material - cycle, which is higher than that of many materials of the state of the art that work at much higher temperatures (see Table 2).

Example 3. Evaluation of the material Lai/3Sr _2/3 Co03-z in the production of CO

The perovskite-type metal oxide with composition Lai _/ 3Sr2 _/ 3CoC>3- _z was synthesized through different synthesis methods, which are known in the state of the art and could be used without difficulty by any person skilled in the art. These methods have been described without prejudice to the fact that it can be synthesized with other methods, leading to obtaining samples with identical characteristics in relation to the use that is protected in this present invention.

The methods chosen to carry out this example were the following: conventional high temperature ceramic method, Pechini sol-gel method and solid state combustion. a) Preparation by the ceramic method The following stoichiometric amounts of metal nitrates were used: 0.6786 g of La(N0 ₃ ) ₃ -6H ₂ 0, 1.3822 g of Co(N0 ₃ ) ₂ -6H ₂ 0 and 0.6633 g Sr(N0 ₃ ) ₂ to obtain 1 g of the perovskite oxide of composition Lai _/3 Sr _2/3 Co0 _3-z .

The mixture of reagents was ground and homogenized. This mixture was subjected to a first heat treatment at 800°C for 12 hours in an air oven. After grinding and homogenizing the mixture of precursors, a pellet was formed and heated at 1100°C for 72 hours with intermediate grinding every 24 hours.

The resulting sample was characterized by X-ray diffraction, obtaining a highly crystalline pure material corresponding to a highly symmetrical perovskite, with lattice parameters a=3.89±3 Á and cubic symmetry Pm3m. b) Preparation by the Pechini sol-gel method

The following stoichiometric amounts of metal nitrates were used: 0.6786 g of La(N0 ₃ ) ₃ -6H ₂ 0, 1.3822 g of Co(N0 ₃ ) ₂ -6H ₂ 0 and 0.6633 g Sr(N0 ₃ ) ₂ to obtain 1 g of the perovskite oxide of composition Lai _/3 Sr _2/3 Co0 _3-z .

The reagent mixture was dissolved in 50 mL of distilled water at room temperature. This solution was heated to around 80°C with stirring on a hot plate and citric acid was added in a stoichiometric ratio of 1 metal:3 citric acid, in this case 2.7665 g.

Maintaining heating and stirring, 1/10 of the initial volume of ethylene glycol (2 mL) was added and polymerization occurred with formation of the sol. The solid mixture was allowed to cool and was ground to produce a powder which was calcined at 800°C to remove organic matter and produce an inorganic ash. Said ash was ground to homogenize it and treated at 1100°C for 12 hours.

The resulting sample was characterized by X-ray diffraction, obtaining a highly crystalline pure material corresponding to a highly symmetrical perovskite, with lattice parameters a=3.83±3 Á and cubic symmetry Pm3m. c) Preparation by the solid state combustion method The following stoichiometric amounts of metal nitrates were used: 0.6786 g of La(N0 ₃ ) ₃ -6H ₂ 0, 1.3822 g of Co(N0 ₃ ) ₂ -6H ₂ 0 and 0.6633 g Sr(N0 ₃ ) ₂ to obtain 1 g of the perovskite oxide of composition Lai _/3 Sr _2/3 Co0 _3-z .

The reagent mixture was dissolved in 10 mL of distilled water at room temperature. Subsequently, 1.5365 g of glycine were dissolved in said solution.

This solution was dehydrated at 60°C under vacuum for 24 hours, obtaining a solid resin that was ground. Later, it was shaped into a cylinder using a die and a hand press.

Subsequently, the ignition of the tablet was induced by contact with a flame from a lighter or gas torch. The dust produced in the combustion was treated at 1100°C for 12 hours.

The resulting sample was characterized by X-ray diffraction, obtaining a highly crystalline pure material corresponding to a highly symmetrical perovskite, with lattice parameters a=3.84±3 Á and cubic symmetry Pm3m.

In order to evaluate the production of CO, the Lai _/3 Sr _2/3 Co0 _3-z sample, obtained by the conventional ceramic method, was used as perovskite-type metal oxide in the method of the present invention.

50 mg of the perovskite-type metal oxide Lai _/3 Sr _2/3 Co0 _3.z were introduced into a 150 pL alumina crucible in the thermobalance. Next, the sample was heated with a ramp of 10°C/min until reaching 800°C. This temperature was kept constant for the rest of the test.

During the initial heating, an air current of 100 mL/min was introduced into the thermobalance to entrain the 0 ₂ produced during the reduction stage. After heating, the air stream fed to the thermobalance was replaced by a stream of pure C0 ₂ to carry out the production of CO and the recovery of the initial perovskite.

This oxidation step was maintained for 90 min. After this time, the introduction of C0 ₂ was stopped and air was re-introduced to carry out the reduction again. This stage lasted approximately 70 min. These changes in the gas stream performed periodically until performing 4 cycles. The mass variations recorded in the thermobalance correspond to the oxygen lost or gained by the material depending on whether it is in the reduction or oxidation stage. The results of this test are shown in Table 1.

The Lai _/ 3Sr2 _/ 3CoC>3- _z does not have a stable behavior against cycles with a production of CO in the first cycle of 500 pmol/g _material . However, the reduction has been carried out at much lower temperatures (see Table 1).

Example 4. Evaluation of the material Lai/3Sr _2/3 Co03- ₇ in the production of H2

In this example, the production of H2 was evaluated using the Lai _/ 3Sr2 _/ 3CoC>3- _z sample, obtained by the conventional ceramic method described in example 3, as perovskite-type metal oxide in the method of the present invention.

1 g of the perovskite-type metal oxide Lai _/ 3Sr2 _/ 3CoC>3- _z was placed in a crucible of Pt/Rh 90/10 in an oven. To carry out the reduction step, the temperature was increased to 800°C with a heating ramp of 10°C/min. During this stage of thermal reduction of the material, the furnace was fed with a stream of N2 at 50 NL/h, to transport the O2 produced in this stage to a gas analyzer. Once the temperature of 800°C was reached and the reduction was complete, the N2 stream was saturated with water at 80°C to feed it to the reactor and carry out the perovskite oxidation stage and H2 production.

The H2 production of the first cycle is higher than that of the rest, in this case a H2 production of _225.1 pmol/g material is obtained, because the first cycle is always considered a material stabilization stage. During the rest of the cycles, the production of H2 is stable, reaching a steady state value of 133.7 _prnol /g material - cycle, similar to that found in the literature for other perovskite-type materials (see Table 2), but leading The process is carried out in isotherm at 800°C, which implies working temperatures much lower than those listed in Table 2 (1250-1400°C).

Example 5. Evaluation of the material Lai _/ 3Sr2 _/ 3Coo _. 69Ugly _. 3iC>3- _z in H2 production

The perovskite-type metal oxide of composition Lai _/ 3Sr2 _/ 3Coo _. 69Ugly _. 3iC>3- _z was synthesized through different synthesis methods, which are known in the state of the art. art and any expert in the field could use them without difficulty. These methods have been described without prejudice to the fact that it can be synthesized with other methods, leading to obtaining samples with identical characteristics in relation to the use that is protected in this present invention.

The methods chosen to carry out this example were the following: conventional high temperature ceramic method, Pechini sol-gel method and solid state combustion. a) Preparation by the ceramic method

The following stoichiometric quantities of metal nitrates were used: 1.3634 g of La(N0 ₃ ) ₃ -6H ₂ 0, 1.9160 g of Co(N0 ₃ ) ₂ -6H ₂ 0, 1.1950 g of Fe(N0 ₃ ) ₂ -9H ₂ 0 and 1.3327 g of Sr(N0 ₃ ) ₂ to obtain 2 g of perovskite oxide of composition

Lai/ ₃ Sr _2/3 Coo.69Ugly. _3i 0 ₃ -z.

The mixture of reagents was ground and homogenized. Subsequently, it was subjected to a first heat treatment at 900°C for 12 hours in an air oven. After grinding and homogenizing the mixture of precursors, a pellet was formed and heated at 1350°C for 48 hours with intermediate grinding every 24 hours.

The resulting sample was characterized by X-ray diffraction, obtaining a highly crystalline pure material corresponding to a highly symmetrical perovskite, with lattice parameters a=3.84±3 Á and cubic symmetry Pm3m. b) Preparation by the Pechini sol-gel method

Lai/ ₃ Sr _2/3 Coo.69Ugly. _3i 0 ₃ -z.

The reagent mixture was dissolved in 50 mL of distilled water at room temperature. This solution was heated to around 80°C with stirring on a hot plate and citric acid was added in a stoichiometric ratio of 1 metal:3 citric acid, in this case 11 g. Maintaining heating and stirring, 1/10 of the initial volume of ethylene glycol (4 ml_) was added and polymerization occurred with the formation of the sol. The solid mixture was allowed to cool and was ground to produce a powder which was calcined at 800°C to remove organic matter and produce an inorganic ash. Said ash was ground to homogenize it and treated at 1200°C for 24 hours.

The resulting sample was characterized by X-ray diffraction, obtaining a highly crystalline pure material corresponding to a highly symmetrical perovskite, with lattice parameters a=3.84±3 Á and cubic symmetry Pm3m. c) Preparation by the solid state combustion method

Lai/ ₃ Sr _2/3 Coo.69Ugly. _3i 0 ₃ -z.

The reagent mixture was dissolved in 10 mL of distilled water at room temperature. Subsequently, 3.3087 g of glycine were dissolved in said solution.

The pellet was then ignited by contact with a flame from a gas burner or torch. The dust produced in the combustion was treated at 1100°C for 12 hours.

In order to evaluate the production of H ₂ , the Lai _/3 Sr _2/3 Coo _.69 Feo _.3i 0 _3-z sample, obtained by the Pechini sol-gel method, was used as a perovskite-type metal oxide in the method of the present invention. 1 g of the perovskite-type metal oxide Lai _/ 3Sr2 _/ 3Coo was introduced into a furnace _. 69Ugly _. 3iC>3- _z using a Pt/Rh 90/10 crucible. To carry out the reduction step, the temperature was increased to 800°C with a heating ramp of 10°C/min. During this stage of thermal reduction of the material, the furnace was fed with a stream of N2 at 50 NL/h, to transport the O2 produced in this stage to a gas analyzer. Once the temperature of 800°C was reached and the reduction was complete, the N2 stream was saturated with water at 80°C to feed it to the reactor and carry out the perovskite oxidation stage and H2 production.

The H2 production of the first cycle is higher than that of the rest (as explained above), obtaining in this case 514.8 pmol/g _material . During the rest of the cycles, the production of H2 is stable around 435.0 pmol/g _mater¡ai · cycle, this production being higher than that found in the literature for other perovskite-type materials (see Table 2), and also the process is carried out isothermally at 800°C, which implies working temperatures much lower than those listed in Table 2 (1250-1400°C).

Example 6. Evaluation of the SrFeC>3-z material in the production of H2

The perovskite-type metal oxide with composition SrFeC>3- _z was synthesized through different synthesis methods, which are known in the state of the art and could be used without difficulty by any expert in the field. These methods have been described without prejudice to the fact that it can be synthesized with other methods, leading to obtaining samples with identical characteristics in relation to the use that is protected in this present invention.

The methods chosen to carry out this example were the following: conventional high-temperature ceramic method, co-precipitation, Pechini sol-gel method and microwave-assisted solution combustion. a) Preparation by the ceramic method

The following stoichiometric amounts were started: 0.4170 g Fe2C>3 and 0.7677 g of Sr(CC>3) to obtain 1 g of perovskite oxide with composition SrFeC>3- _z. The mixture of reagents was ground and homogenized. Subsequently, it was subjected to a first heat treatment at 900°C for 12 hours in an air oven. After grinding and homogenizing the mixture of precursors, a pellet was formed and heated at 1300°C for 48 hours, after which the oven was turned off and the sample was allowed to cool slowly.

The resulting sample was characterized by X-ray diffraction, obtaining a highly crystalline pure material. In this case, a material with a structure derived from tetragonal symmetry perovskite was obtained.

The structure of the compound obtained depends on the oxygen content, so that it can present different perovskite-type structures depending on the stoichiometry in oxygen; thus, for 0£z<0.125 a high symmetry perovskite is obtained (cubic with lattice parameters a=3.85±2 Á and Pm3m symmetry), for 0.125£z<0.250 the order of the oxygen vacancies produces a tetragonal perovskite (symmetry 14/mmm and parameters a=10.93(2) Á and c=7.70(2) Á), for 0.250£z<0.50 the order of the oxygen vacancies produces an orthorhombic perovskite (Cmmm symmetry and parameters a=10.97(2) Á, b=7.70(2) Á and c=5.47(2) Á), z=0.5 marks the compositional limit for which a compound derived from the Perovskite with orthorhombic symmetry (Ima2) and lattice parameters z=15.78(2) Á, b=5.57(2) Á and c=5.47(2) Á, which is called brownmillerite.

In any case, whatever the oxygen content and the structure obtained in the synthesis, at the operating temperatures of the method of the present invention (equal to or greater than 300°C), the SrFeC> _3-z compound has a structure brownmillerite in the reducing stage and a highly symmetric cubic structure in the oxidizing stage. b) Preparation by the precipitation method

The following stoichiometric amounts of metal nitrates were used: 2.1008 g of Fe(NC> ₃ ) ₃ -9H ₂ 0 and 1.1005 g of Sr(NC> 3) 2 to obtain 1 g of perovskite oxide SrFeC>3-z composition. The reagent mixture was dissolved in 50 mL of distilled water at room temperature and a 0.5 M Na2(CC>3) solution was added until reaching a pH between 12 and 14, producing the quantitative coprecipitation of Fe(OH)3 and SrCC>3.

Subsequently, the precipitate was filtered and the solid obtained was washed repeatedly with distilled water, dried, ground and treated at 1100°C for 12 hours. The resulting sample was characterized by X-ray diffraction, obtaining a mixture of perovskites with tetragonal (66%) and orthorhombic (33%) symmetry (see section a)). c) Preparation by the Pechini sol-gel method

The following stoichiometric amounts of metal nitrates were used: 2.1008 g of Fe(NC> ₃ ) ₃ -9H ₂ 0 and 1.1005 g of Sr(NC> 3) 2 to obtain 1 g of perovskite oxide SrFeC>3-z composition.

The reagent mixture was dissolved in 50 mL of distilled water at room temperature. This solution was heated to around 80°C with stirring on a hot plate and citric acid was added in a stoichiometric ratio of 1 metal:3 citric acid, in this case 2.9971 g.

Maintaining heating and stirring, 1/10 of the initial volume of ethylene glycol (2 mL) was added and polymerization occurred with formation of the sol. The solid mixture was allowed to cool and was ground to produce a powder which was calcined at 800°C for 12 hours to remove organic matter and produce an inorganic ash. Said ash was ground to homogenize it and treated at 1000°C for 12 hours.

The resulting sample was characterized by X-ray diffraction, obtaining a highly crystalline pure material that corresponds to a structure derived from perovskite with tetragonal symmetry. d) Preparation by the microwave-assisted solution combustion method

The following stoichiometric amounts of metal nitrates were used: 10.5040 g of Fe(N0 ₃ ) ₃ -9FÍ ₂ 0 and 5.5025 g of Sr(NC>3)2 to obtain 5 g of perovskite oxide with composition SrFeC>3-z. The reagent mixture was dissolved in 50 mL of distilled water at room temperature. Subsequently, 5.4217 g of glycine were dissolved in said solution.

The powder obtained from the combustion induced by treatment in a microwave oven was treated at 1100°C for 12 hours to produce the perovskite of composition SrFeC> _3-z. The resulting sample was characterized by X-ray diffraction, obtaining a highly crystalline pure material that corresponds to a structure derived from orthorhombic symmetry perovskite. e) Preparation by the spray-pyrolysis-assisted combustion method in solution

The following stoichiometric quantities of metal nitrates were used: 10.5555 g of Fe(NC> ₃ ) ₃ -9H ₂ 0 and 5.6254 g of Sr(NC> 3) 2 to obtain 5 g of the perovskite oxide of SrFeC>3-z composition.

The reagent mixture was dissolved in 50 mL of distilled water at room temperature. Subsequently, 5.4454 g of glycine were dissolved in said solution.

This solution was used in a spray-pyrolysis system that promotes the combustion reaction by thermal activation. The powder obtained from the combustion was treated at 1100°C for 12 hours to produce the perovskite with composition SrFeC> _3-z. The resulting sample was characterized by X-ray diffraction, obtaining a highly crystalline pure material that corresponds to a structure derived from orthorhombic symmetry perovskite.

In order to evaluate the production of H ₂ , the SrFeC> _3-z sample, obtained by the Pechini sol-gel method, was used as perovskite-type metal oxide in the method of the present invention.

1 g of the perovskite-type metal oxide SrFeC>3- _z was introduced into a furnace using a Pt/Rh 90/10 crucible. To carry out the reduction step, the temperature was increased to 800°C with a heating ramp of 10°C/min. During this stage of thermal reduction of the material, the furnace was fed with a stream of N2 at 50 NL/h, to transport the O2 produced in this stage to a gas analyzer. Once the temperature of 800°C was reached and the reduction was complete, the N2 stream was saturated with water at 80°C to feed it to the reactor and carry out the perovskite oxidation stage and H2 production. The production of H2 is remarkable from the first cycle, 718.2 pmol/g material and remains stable, _714.1 pmol/g _material · cycle, so that this material does not seem to need a stabilization step. Its steady-state production is similar to that achieved from the beginning, being higher than that found in the literature for most of the perovskite-type state-of-the-art materials (see Table 2), but carrying out the process in isotherm at 800°C, which represents a notable decrease in the working temperature with respect to those listed in Table 2 (1250-1400°C).

Example 7. Evaluation of the material SrFeC> 3- _z in the production of CO In this example, the production of CO was evaluated using the sample of SrFeC> _3-z , obtained by the Pechini sol-gel method described in example 6, as perovskite-type metal oxide in the method of the present invention.

50 mg of the perovskite-type metal oxide SrFeC>3- _z were introduced into a 150 pl_ alumina crucible in the thermobalance. Next, the sample was heated with a ramp of 10°C/min until reaching 800°C. This temperature was kept constant for the rest of the test.

This oxidation step was maintained for 90 min. After this time, the introduction of CO2 was stopped and air was re-introduced to carry out the reduction again. This stage lasted approximately 70 min. These changes in the gas current were carried out periodically until 16 cycles were carried out. The mass variations recorded in the thermobalance correspond to the oxygen lost or gained by the material depending on whether it is in the reduction or oxidation stage. The results of this example are shown in Table 1. This material does not present a stable behavior against cycles. Although in the first cycle it presents a high production of CO, 1262 prnol/g _matemi , as the cycles go by a modification of the redox process is observed.

Example 8. Evaluation of the material SrFeo9oMoo io0 ₃ - in the production of H2 The metal oxide type perovskite of composition SrFeo _. 9oMoo _.i oC>3- _z was synthesized through different synthesis methods, which are known in the state of the art and could be used without difficulty by any person skilled in the art. These methods have been described without prejudice to the fact that it can be synthesized with other methods, leading to obtaining samples with identical characteristics in relation to the use that is protected in this present invention.

The methods chosen to carry out this example were the following: conventional high-temperature ceramic method, Pechini sol-gel method and spray-pyrolysis assisted solution combustion. a) Preparation by the ceramic method The following stoichiometric amounts were started from: 0.3677 g Fe2C>3 _, 0.0737 g of M0O3 and 0.7553 g of Sr(CC>3) to obtain 1 g of the perovskite oxide of composition SrFeo.9oMoo.ioC>3-z.

The mixture of reagents was ground and homogenized. Subsequently, it was subjected to a first heat treatment at 900°C for 12 hours in an air oven. After grinding and homogenizing the mixture of precursors, a pellet was formed and heated at 1350°C for 48 hours, after which the oven was turned off and the sample was allowed to cool slowly.

The resulting sample was characterized by X-ray diffraction, obtaining a highly crystalline pure material corresponding to a highly symmetrical perovskite, with lattice parameters a=3.88±2 Á and cubic symmetry Pm3m. b) Preparation by the Pechini sol-gel method The following stoichiometric quantities of metal nitrates were used: 3.7207 g of Fe(N0 ₃ ) ₃ -9H ₂ 0, 2.1586 g of Sr(N0 ₃ ) ₂ and 0.1800 g of (NH ₄ ) ₆ Mo ₇ 0 ₂₄ -4H ₂ 0 to obtain 2 g of the perovskite oxide of composition SrFeo _.9 oMoo _.i o0 _3-z.

The reagent mixture was dissolved in 50 mL of distilled water at room temperature. This solution was heated to around 80°C with stirring on a hot plate and citric acid was added in a stoichiometric ratio of 1 metal:3 citric acid, in this case 5.8788 g.

The resulting sample was characterized by X-ray diffraction, obtaining a highly crystalline pure material corresponding to a highly symmetrical perovskite, with lattice parameters a=3.90±1 Á and cubic symmetry Pm3m. c) Preparation by the spray-pyrolysis-assisted combustion method in solution

The following stoichiometric quantities of metal nitrates were used: 9.3001 g of Fe(N0 ₃ ) ₃ -9H ₂ 0, 5.3966 g of Sr(N0 ₃ ) ₂ and 0.4590 g of (NH ₄ ) ₆ Mo ₇ 0 ₂₄ ·4H ₂ 0 to obtain 5 g of perovskite oxide with composition SrFeo _.9 oMoo _.i o0 _3-z .

The reagent mixture was dissolved in 50 mL of distilled water at room temperature. Subsequently, 8.0115 g of glycine were dissolved in said solution.

This solution was used in a spray-pyrolysis system that promotes the combustion reaction by thermal activation. The powder obtained from the combustion was treated at 1000°C for 8 hours to produce the perovskite with composition SrFeo _.9 oMoo _.i o0 _3-z.

The resulting sample was characterized by X-ray diffraction, obtaining a highly crystalline pure material corresponding to a highly symmetrical perovskite, with lattice parameters a=3.889±5 Á and cubic symmetry Pm3m. In order to evaluate the H2 production, the SrFeo sample was used _. 9oMoo _.i oC>3- _z , obtained by the ceramic method, as perovskite-type metal oxide in the method of the present invention.

1 g of the perovskite-type metal oxide SrFeo was introduced into a furnace _. 9oMoo _.i oC>3- _z using a Pt/Rh 90/10 crucible. To carry out the reduction step, the temperature was increased to 800°C with a heating ramp of 10°C/min. During this stage of thermal reduction of the material, the furnace was fed with a stream of N2 at 50 NL/h, to transport the O2 produced in this stage to a gas analyzer. Once the temperature of 800°C was reached and the reduction was complete, the N2 stream was saturated with water at 80°C to feed it to the reactor and carry out the perovskite oxidation stage and H2 production. The results of this test are shown in Table 2.

The production of H2 is high from the first cycle, 882.7 prnol/g _matemi , and grows slightly until reaching a steady-state production of 1176.7 prnol/g _matemi -cycle, which is much higher than that found in the literature for the most state-of-the-art materials of the perovskite type (see Table 2), but carrying out the process in isotherm at 800°C, which implies a notable decrease in the working temperature with respect to those collected in Table 2 ( 1250-1400°C).

Example 9. Evaluation of the material SrFeo9oMoo io0 ₃ - in the production of CO In this example the production of CO was evaluated using the SrFeo sample _. 9oMoo _.i oC>3- _z , obtained by the ceramic method described in example 8, as perovskite-type metal oxide in the method of the present invention.

50 mg of the perovskite-type metal oxide SrFeo were introduced _. 9oMoo _.i oC>3- _z in a 150 pl_ alumina crucible in the thermobalance. Next, the sample was heated with a ramp of 10°C/min until reaching 800°C. This temperature was kept constant for the rest of the test.

During the initial heating, an air current of 100 mL/min was introduced into the thermobalance to entrain the O2 produced during the reduction stage. After completion of heating, the air stream fed to the thermobalance was replaced by a stream of pure CO2 to carry out the production of CO and the recovery of the initial perovskite.

This oxidation step was maintained for 90 min. After this time, the introduction of CO2 was stopped and air was re-introduced to carry out the reduction again. This stage lasted approximately 70 min. These changes in the gas current were carried out periodically until 16 cycles were carried out. The mass variations recorded in the thermobalance correspond to the oxygen lost or gained by the material depending on whether it is in the reduction or oxidation stage. The results of this test are shown in Table 1. This material has stable behavior against cycles with a steady state CO production of 988 pmol/g _material , comparable to that of other perovskite-type materials. Furthermore, in this case, the reduction is carried out at much lower temperatures (see Table 1). Example 10. Evaluation of the material _{SrFeo9oT¡oio03-7} in the production of H2

The perovskite-type metal oxide of composition SrFeo _. 9oTio _.i oC>3- _z was synthesized through different synthesis methods, which are known in the state of the art and could be used without difficulty by any person skilled in the art. These methods have been described without prejudice to the fact that it can be synthesized with other methods, leading to obtaining samples with identical characteristics in relation to the use that is protected in this present invention.

The methods chosen to carry out this example were the following: conventional high-temperature ceramic method, Pechini sol-gel method and microwave-assisted solution combustion. a) Preparation by the ceramic method

We started from the following stoichiometric amounts: 1.9071 g of Fe(NC> ₃ ) ₃ -9H ₂ 0, 0.0419 g T1O2 (anatase) and 1.1100 g of Sr(NC> 3) 2 to obtain 1 g of the perovskite oxide of composition SrFeo.9oTio.ioC>3-z. The mixture of reagents was ground and homogenized. Subsequently, it was subjected to a first heat treatment at 900°C for 12 hours in an air oven. After grinding and homogenizing the mixture of precursors, a pellet was formed and heated at 1300°C for 48 hours, after which the oven was turned off and the sample was allowed to cool slowly.

The resulting sample was characterized by X-ray diffraction, obtaining a highly crystalline pure material corresponding to a highly symmetrical perovskite, with lattice parameters a=3.88±2 Á and cubic symmetry Pm3m. b) Preparation by the Pechini sol-gel method

We started from the following stoichiometric quantities: 1.9071 g of Fe(N0 ₃ ) ₃ -9H ₂ 0, 0.1784 g (NH ₄ ) ₈ [Tί ₄ (0 ₆ H ₄ 0 ₇ )(0 ₂ )] ·8H ₂ 0 and 1.1105 g of Sr(N0 ₃ ) ₂ to obtain 1 g of perovskite oxide with composition SrFeo _.9 oTio _.i o0 _3-z .

The reagent mixture was dissolved in 50 mL of distilled water at room temperature. This solution was heated to around 80°C with stirring on a hot plate and citric acid was added in a stoichiometric ratio of 1 metal:3 citric acid, in this case 6.0461 g.

Maintaining heating and stirring, 1/10 of the initial volume of ethylene glycol (2 mL) was added and polymerization occurred with formation of the sol. The solid mixture was allowed to cool and was ground to produce a powder which was calcined at 800°C for 12 hours to remove organic matter and produce an inorganic ash. Said ash was ground to homogenize it and treated at 1000°C for 24 hours.

The resulting sample was characterized by X-ray diffraction, obtaining a highly crystalline pure material corresponding to a highly symmetrical perovskite, with lattice parameters a=3.88±2 Á and cubic symmetry Pm3m. c) Preparation by the microwave-assisted solution combustion method

We started from the following stoichiometric quantities: 1.9071 g of Fe(N0 ₃ ) ₃ -9H ₂ 0, 0.1784 g (NH ₄ ) ₈ [Tί ₄ (0 ₆ H ₄ 0 ₇ )(0 ₂ )] ·8H ₂ 0 and 1.1105 g of Sr(N0 ₃ ) ₂ to obtain 1 g of perovskite oxide with composition SrFeo _.9 oTio _.i o0 _3-z . The reagent mixture was dissolved in 10 mL of distilled water at room temperature. Subsequently, 1.9431 g of glycine were dissolved in said solution.

The powder obtained from the combustion induced by treatment in a microwave oven was treated at 1000°C for 12 hours to produce the perovskite of composition SrFeo _. 9oThio _.i oC>3- _z. The resulting sample was characterized by X-ray diffraction, obtaining a highly crystalline pure material corresponding to a highly symmetrical perovskite, with lattice parameters a=3.88±3 Á and cubic symmetry Pm3m.

In order to evaluate the H2 production, the SrFeo sample was used _. 9oTio _.i oC>3- _z , obtained by the ceramic method, as perovskite-type metal oxide in the method of the present invention.

1 g of the perovskite-type metal oxide SrFeo was introduced into a furnace _. 9oThio _.i oC>3- _z using a Pt/Rh 90/10 crucible. To carry out the reduction step, the temperature was increased to 800°C with a heating ramp of 10°C/min. During this stage of thermal reduction of the material, the furnace was fed with a stream of N2 at 50 NL/h, to transport the O2 produced in this stage to a gas analyzer. Once the temperature of 800°C was reached and the reduction was complete, the N2 stream was saturated with water at 80°C to feed it to the reactor and carry out the perovskite oxidation stage and H2 production. The results of this test are shown in Table 2.

The production of H2 is notable from the first cycle, 568.8 prnol/g _matemi , and grows slightly until reaching a steady-state production of 719.6 prnol/g _matemi -cycle, which is higher than that found in the literature for most of the state-of-the-art materials of the perovskite type (see Table 2), but carrying out the process in isotherm at 800°C, which implies a notable decrease in the working temperature with respect to those collected in Table 2 (1250 -1400°C).

Example 11. Evaluation of the material SrFeo9oTio io03- in the production of CO

In this example, CO production was evaluated using the SrFeo sample _. 9oTio _.i oC>3- _z , obtained by the ceramic method described in example 10, as perovskite-type metal oxide in the method of the present invention. 50 mg of the perovskite-type metal oxide SrFeo were introduced _. 9oTio _.i oC>3- _z in a 150 pl_ alumina crucible in the thermobalance. Next, the sample was heated with a ramp of 10°C/min until reaching 800°C. This temperature was kept constant for the rest of the test. During the initial heating, an air current of 100 mL/min was introduced into the thermobalance to entrain the O2 produced during the reduction stage. After completing the heating, the air stream fed to the thermobalance was replaced by a stream of pure CO2 to carry out the production of CO and the recovery of the initial perovskite. This oxidation step was maintained for 90 min. After this time, the introduction of CO2 was stopped and air was re-introduced to carry out the reduction again. This stage lasted approximately 70 min. These changes in the gas current were carried out periodically until 16 cycles were carried out. The mass variations recorded in the thermobalance correspond to the oxygen lost or gained by the material depending on whether it is in the reduction or oxidation stage. The results of this test are shown in Table 1.

This material has stable behavior against cycles with a steady state CO production of 1262 pmol/g _material , higher than that of other perovskite-type materials. Furthermore, in this case, the reduction is carried out at much lower temperatures (see Table 1).

Table 1. Comparison of the production of CO by thermochemical cycles of oxides with perovskite structure as active material described in the state of the art (27) compared to metal oxides of perovskite type of the present invention.

* Data corresponding to the first cycle, materials not stable against the skies.

Table 2. Comparison of the production of H2 by thermochemical cycles of oxides with perovskite structure as active material described in the state of the art (27) compared to metal oxides of perovskite type of the present invention.

BIBLIOGRAPHIC REFERENCES

(1) F. Yilmaz, M. Tolga Balta, “Energy and exergy analyzes of hydrogen production step in boron based thermochemical cycle for hydrogen production,” International Journal of Hydrogen Energy, 42(4), (2017) 2485-2491.

(2) P. Nikolaidis and A. Poullikkas, “A comparative overview of hydrogen production processes,” Renewable Sustainable. Energy Reviews, 67, (2017) 597-611.

(3) K. Ladomenou, M. Natali, E. lengo, G. Charalampidis, F. Scandola, A.G. Coutsolelos, “Photochemical hydrogen generation with porphyrin-based systems” Coordination Chemistry Reviews, 304-305, (2015) 38-54.

(4) S. Rai, A. Ikram. S. Sahai, S. Dass, R. Shrivastav, V.R. Satsangi, “CNT based photoelectrodes for PEC generation of hydrogen: A review” International Journal of Hydrogen Energy, 42 (7), (2017) 3994^006.

(5) R. Bhattacharyya, “Photovoltaic solar energy conversion for hydrogen production by alkaline water electrolysis: Conceptual design and analysis,” Energy Conversion and Management, 133, (2017) 1-13.

(6) N. Koumi, “An overview of hydrogen gas production from solar energy” Renewable and Sustainable Energy Review, 16 (9), (2012) 6782-6792.

(7) Z. Wang, “Comparison of thermochemical, electrolytic, photoelectrolytic and photochemical solar-to-hydrogen production technologies,” International Journal of Hydrogen Energy, 37 (21), (2012) 16287-16301.

(8) R. Fernández-Saavedra, "Bibliographic review on the production of solar hydrogen by thermochemical cycles" CIEMAT Technical Reports, (2007) 1-16.

(9) D. Yadav, R. Banerjee, “A review of solar thermochemical processes,” Renewable and Sustainable Energy Reviews, 54 (2016) 497-532.

(10) A. Steinfeld, “Solar thermochemical production of hydrogen - A review” Solar Energy, 78 (5), (2005) 603-615.

(11) C. Perkins, A. Weimer, “Likely near-term solar-thermal water splitting technologies”, International Journal of Hydrogen Energy, 29(15), (2004) 1587-1599.

(12) M. Roeb, M. Neises, N. Monnerie, F. Cali, H. Simón, C. Sattler, M. Schmücker, R. Pitz-Paal, “Materials-related aspects of thermochemical water and carbon dioxide splitting: A review”, Materials, 5 (11), (2012) 2015-2054.

(13) C. Sattler, M. Roeb, “Thermochemical Cycles”, Fuels, (2009) 384-393.

(14) F. Fresno, T. Yoshida, N. Gokon, R. Fernández-Saavedra, T. Kodama, “Comparative study of the activity of nickel ferrites for solar hydrogen production by two-step thermochemical cycles” International Journal of Hydrogen Energy, 36 (16) (2010) 8503-8510.

(15) Y. Tamaura, A. Steinfeld, P. Kuhn, K. Ehrensberger, “Production of solar hydrogen by a novel, 2-step, water-splitting thermochemical cycle” Energy, 20 (4) (1995) 325-330 .

(16) S.B. Han, T.B. Kang, O.S. Joo, K.D. Jung, “Water splitting for hydrogen production with ferrites,” Solar Energy, 81 (5) (2007) 623-628.

(17) J.R. Scheffe, D. Weibel, A. Steinfeld, “Lanthanum-Strontium-Manganese Perovskites as Redox Materials for Solar Thermochemical Splitting of H2O and CO2”, Energy and Fuels, 27 (8) (2013) 4250-4257.

(18) N.R. Rhodes, M. M. Bobek, K.M. Alien, D.W. Hahn, “Investigated of long-term reactive stability of ceria for use in solar thermochemical cycles”, Energy, 89 (2015) 924-931.

(19) S. Abanades, G. Flamant, “Thermochemical hydrogen production from a two-step solar-driven water-splitting cycle base on cerium oxides”, Solar Energy, 80 (12) (2006) 1611-1623.

(20) J.R. Scheffe, A. Steinfeld, “Thermodynamic analysis of cerium-based oxides for solar thermochemical fuel production”, Energy and Fuels, 26 (3) (2012) 1928-1936.

(21) N. Gokon, T. Suda and T. Kodama, “Thermochemical reactivity of 5-15 mol% Fe, Co, Ni, Mn-doped cerium oxides in two-step water-splitting cycle for solar hydrogen production”, Thermochimica Minutes, 617 (2015) 179-190.

(22) D. Marxer, P. Furler, J. Scheffe, H. Geerlings, C. Falter, V. Batteiger, A. Sizmann, A. Steinfeld, “Demonstration of the entire production Chain to renewable kerosene via solar thermochemical splitting of H2O and CO2” Energy and Fuels, 29 (5) (2015) 3241-3250.

(23) A. Le Gal, S. Abanades, “Catalytic investigation of cera-zirconia solid Solutions for solar hydrogen production”, International Journal of Hydrogen Energy, 36 (8) (2011) 4739-4748.

(24) A. Le Gal, S. Abanades, “Dopant incorporation in Ceria for Enhanced Water-Splitting Activity during Solar Thermochemical Hydrogen Generation”, Journal of Physical Chemistry C, 116 (2012) 13516-13523.

(25) M. Orfila, M. Linares, R. Molina, JA Botas, R. Sanz and J. Marugán, “Perovskite materials for hydrogen production by thermochemical water splitting”, International Journal of Hydrogen Energy, 41 (2016) 19329- 19338. (26) MT Azcondo, M. Orfila, J. Marugán, R. Sanz, A. Muñoz-Noval, E. Salas, C. Ritter, F. Garcia-Alvarado, and U. Amador, “Novel Perovskite Materials for Ther al Water Splitting at Moderate Temperature” ChemSusChem, 12 (2019) 4029-4037.

(27) A. Haussler, S. Abanades, J. Jouannaux, A. Julbe, “Non-stoichio etric redox active perovskite materials for solar thermochemical fuel production: a review”

Catalysts, 8 (2018), 611-632.

(28) A. Meier and C. Sattler, “Solar fuels fro concentrated sunlight” SolarPACESJEA SolarPACES i ple enting agree ent, (2009).

(29) G. Centi and S. Perathoner, “Towards solar fuels fro water and CO2” ChemSusChem, 3 (2010) 195-208.

(30) D. Hirsch, M. Epstein, A. Steinfeld, “The solar thermal decarbonization of natural gas,” International Journal of Hydrogen Energy, 26 (2001) 1023-1033.

(31) M. Flechsenhar and C. Sasse, “Solar gasification of biomass using oil shale and coal as candidate materials,” Energy, 20 (1995) 803-810. (32) W.C. Chueh, C. Falter, M. Abbott, D. Scipio, P. Fuerler, S.M. Haile, A. Steinfeld,

“High-Flux Solar-Driven Thermochemical Dissociation of CO2 and H2O using nonstoichiometric ceria”, Science, 330 (2010) 1797-1801.

(33) R.H. Mitchell, Perovskites modern and ancient. Almaz Press, Thunder Bay, Ontario, (2002).

Claims

1. Method for obtaining H2, CO or synthesis gas, comprising the following steps: i) heating a perovskite-type metal oxide of formula (I)

B represents an element of the alkali or alkaline earth series;

M represents Fe or Co;

N represents a transition metal or an element selected from the groups of the p block of the Periodic Table; x and y have a value between 0 and 1 ; and z has a value between 0 and 0.75; for thermal reduction of said metal oxide in a gaseous stream that is air or an inert gas; and ii) reacting the reduced metal oxide obtained from step i) with a gaseous stream comprising steam and/or CO2 to obtain H2, CO or synthesis gas and re-oxidation of the metal oxide to recover the oxide metallic of formula (I); in which steps i) and ii) are carried out at a temperature between 300 and 800°C and a pressure between 0.1 MPa and 20 MPa.

2. Production method according to claim 1, wherein in the metal oxide of the perovskite type of formula (I)

Al- _X B _x Ml-yNy03- _Z (I)

A represents La or Nd;

B represents Ca or Sr;

M represents Fe or Co;

N represents Fe, Co, Ti, Mo, Sb or Al; x and y have a value between 0 and 1 ; and z has a value between 0 and 0.75.

3. Production method according to claim 1 or 2, in which the gaseous current of step i) is air.

4. Production method according to any of claims 1 to 3, in which the gaseous stream of step ii) additionally comprises air or an inert gas.

5. Production method according to any of claims 1 to 4, wherein the amount of CO2 in the gas stream of stage ii) is 10-100% by volume of the stream.

6. Production method according to any of claims 1 to 4, wherein the amount of CO2 in the gas stream of stage ii) is 30-100% by volume of the stream.

7. Production method according to any of claims 1 to 4, wherein the gaseous stream of stage ii) comprises a mixture of a stream of steam and CO2 with a volume percentage of CO2 of 10-90% .

8. Use of a perovskite-type metal oxide of formula (I)

B represents an element of the alkali or alkaline earth series;

M represents Fe or Co;

N represents a transition metal or an element selected from the groups of the p block of the Periodic Table; x and y have a value between 0 and 1 ; and z has a value between 0 and 0.75, for the production of H2, CO or synthesis gas.

9. Use according to claim 8, wherein in the perovskite-type metal oxide of formula (I)

Al-xBxMl-yNy0 ₃ - _Z (I)

A represents La or Nd;

B represents Ca or Sr;

M represents Fe or Co;