Hydrogen production method

Abstract

Hydrogen production method for carrying out, in alternating sequence inside a single reactor, hydrogen production through oxidation of metallic iron by means of water and regeneration of the metallic iron through reduction of iron oxide by means of a reducing compound.

Description

[0001] “HYDROGEN PRODUCTION METHOD”

[0002] FIELD OF THE INVENTION

[0003] The present invention concerns a method for producing essentially pure hydrogen, for example to power fuel cells in order to generate energy.

[0004] BACKGROUND OF THE INVENTION

[0005] Many methods for producing molecular hydrogen are known. In particular, the steam-iron process exploits the oxidation of metallic iron by water vapor in order to obtain iron oxide and essentially pure hydrogen, advantageously not mixed with other gases such as carbon monoxide or carbon dioxide. Pure hydrogen is particularly required to power fuel cells, such as polymer membrane fuel cells (PEMFCs) for example, which require a carbon monoxide content in the hydrogen that generally powers them of less than 10 ppm (parts per million).

[0006] The oxidation steps of the metallic iron, or hydrogen production steps, are generally alternated with reduction steps, or iron regeneration steps, during which the iron oxide is treated with reducing agents, such as carbon monoxide, other fuels or fuel mixtures, to once again obtain metallic or partly reduced iron.

[0007] A disadvantage of the steam-iron process is the high temperatures it requires, of the order of 600°C - 900°C, which make it energy-intensive. Furthermore, since iron can assume different oxidation states, iron oxide actually occurs in three forms: ferrous oxide, or iron(II) oxide, or wustite, FeO, ferrous-ferric oxide, or iron(II) and iron(III) oxide, or magnetite, FesCL, and ferric oxide, or iron(III) oxide, or hematite, Fe20s.

[0008] Therefore, both the iron’s reduction reaction and oxidation reaction can involve a mixture of four of the iron’s oxidation states, and it is generally difficult to completely oxidize the iron in the oxidation step, or to completely reduce the iron in the reduction step.

[0009] Document EP 3 194 331 Bl describes processes for the synthesis of hydrogen gas (H2) in a reactor under mild hydrothermal conditions, at a temperature comprised between 120° C and 240° C and a pressure below 70 bar. The processes comprise putting a compound containing metallic iron (Fe°) or Fe(II) in contact with an aqueous composition comprising carbonate ions; and allowing the metallic iron or the compound containing Fe(II) to react with water, thereby obtaining magnetite (FesC ) and hydrogen gas. This document focuses only on the oxidation of iron to magnetite. Nothing is mentioned regarding the reduction step or any regeneration of the work material. This document then indicates the use of a gaseous catalyst (CO2) as a promoter or catalyst of the iron oxidation reaction. This document also indicates the presence of an electrolyte in the water, with the purpose of combining with the carbon dioxide (CO2) introduced in order to form carbonate ions.

[0010] Document US 5 840 270 A discloses an FUO-iron reaction method, comprising the reaction of H2O and iron material to form hydrogen in situ, on board an electric vehicle, in the presence of a catalyst comprising a dissolved alkali metal hydroxide that enhances the FUO-iron reaction. This document essentially describes the production of hydrogen by making iron react with H2O in the presence of potassium hydroxide (KOH). This document also does not provide any regeneration of the work material. In this document, in the context of road transport applications, a tubular reactor is therefore described in which to make this reaction occur on board the vehicle, for the use of hydrogen as a fuel for road transport.

[0011] Document EP 0 637 291 Bl describes a method for generating hydrogen, on board a vehicle, by making H2O come into contact with ground or crushed iron, the iron being ground or crushed in situ to increase its activity and used in a reaction with the H2O within one operating cycle after the grinding or crushing.

[0012] The method comprises the steps of:

[0013] (a) supplying iron as a fluidized bed of iron particles and a source of H2O in combination with a hydrogen-air fuel cell;

[0014] (b) generating hydrogen for the hydrogen-air fuel cell by making the iron react with the H2O at a maximum temperature of 450°C;

[0015] (c) grinding or crushing the iron into pellet form to produce active iron particles, the reactivity of which is increased in order to generate hydrogen in step (b) at a rate of at least two percent per minute (2% / min) at the maximum temperature of 450°C, the grinding or crushing being performed in situ within one operating cycle of the hydrogen-air fuel cell.

[0016] This document therefore discloses a manufactured article made of activated and compacted iron metal powders which, at about 450°C and in contact with water, release hydrogen from the latter. This document therefore discloses a system for the production of hydrogen in situ, on board a vehicle. In the context of road transport applications, this document provides that water is made to drip onto the manufactured article on board the vehicle in order to release the hydrogen on-board for motor propulsion. Also in this document there is no mention at all of the regeneration of the work material.

[0017] It is therefore evident that the known state of the art does not consider carrying out, in any way, a step of regenerating the work material once it has oxidized (or is spent) for the purposes of a possible new usage cycle. In the absence of this aspect, it can be assumed, in the known background documents, that the oxidized material, whatever it may be, is disposed of in the context of “disposable” situations, or that it is regenerated through metallurgical means, that is, by retracing the path used for the production of the metal itself.

[0018] There is therefore the need to perfect a hydrogen production method that can overcome at least one of the disadvantages of the state of the art. To do this, it is necessary to resolve the technical problem of producing essentially pure hydrogen in an energy-efficient manner.

[0019] In particular, one purpose of the present invention is to perfect a hydrogen production method that is less energy-intensive than the steam-iron method of the state of the art. Another purpose of the present invention is to perfect a hydrogen production method wherein the hydrogen production and iron regeneration steps are optimized.

[0020] The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages.

[0021] SUMMARY OF THE INVENTION

[0022] The present invention is set forth and characterized in the independent claim. The dependent claims describe other characteristics of the present invention or variants to the main inventive idea. In accordance with the above purposes and to resolve the technical problem described above in a new and original way, also achieving considerable advantages compared to the state of the prior art, a hydrogen production method according to the present invention carries out, in alternating sequence inside a single reactor, hydrogen production through oxidation of metallic iron by means of water and regeneration of the metallic iron through reduction of iron oxide by means of a reducing compound.

[0023] According to one aspect of the invention, the hydrogen production occurs at a temperature up to 50°C, in particular up to 30°C, lower than the critical temperature of water, in the presence of water in the liquid state for the purposes of the oxidation of the metallic iron, of an inert electrolyte and of an iron oxidation promoter. Moreover, the regeneration of the iron occurs by adding the reducing compound into the reactor. An “inert electrolyte” is understood as a chemical species dissolved in water that allows electrical conduction, is stable at least in the temperature range used and does not react significantly with the other substances present in the reactor.

[0024] An “iron oxidation promoter” is understood as a chemical species that promotes, or makes more energetically advantageous, one or more iron oxidation reactions. Doing so achieves at least the advantage of operating at temperatures lower than

[0025] 374°C (critical temperature of water), much lower than the typical temperatures of an iron-steam process according to the state of the art, which are in the order of 600°C - 900°C, achieving significant energy savings.

[0026] The above mentioned regeneration of the iron can occur at least in part in the presence of at least one iron reduction promoter at a temperature up to 50°C, in particular up to 30°C, lower than the critical temperature of water, in the presence of water in the liquid state. An “iron reduction promoter” is understood as a chemical species that promotes, or makes more energetically advantageous, one or more iron oxides reduction reactions. DESCRIPTION OF SOME EMBODIMENTS

[0027] We will now refer in detail to the possible embodiments of the invention. The function of the phraseology and terminology used in the present disclosure is to provide a non-limiting example of the invention itself, since the scope of protection is defined by the claims. It is understood that elements and characteristics of one embodiment can be conveniently combined or incorporated into other embodiments without further clarifications.

[0028] The embodiments described here concern a hydrogen production method that carries out, in alternating sequence inside a single reactor, hydrogen production through oxidation of metallic iron by means of water and regeneration of the iron through reduction of iron oxide by means of a reducing compound.

[0029] In the production of hydrogen, the main reaction is a reaction between metallic iron and water, which leads to the formation of magnetite and molecular hydrogen:

[0030] Other reactions are also possible, some advantageous, for example a further oxidation of the magnetite to hematite Fe2O3 with development of additional hydrogen, others disadvantageous, such as reduction reactions that are reverse to the iron oxidation reactions just described. The hydrogen produced is essentially pure, in the sense that it has a carbon monoxide content of less than 10 ppm and is therefore suitable to power polymer membrane fuel cells (PEMFCs).

[0031] The production of hydrogen can provide to make available metallic iron and water inside the reactor. The metallic iron can be in the form of a filing, or a powder, or a micro- or nano-structured substrate, in order to increase the contact surface with water, promoting the reactions with the water. The temperature and pressure conditions inside the reactor can be controlled in a known manner.

[0032] The temperature and pressure conditions inside the reactor can be chosen and maintained in such a way that the water contained in it is in the liquid state. In other words, the temperature inside the reactor is lower than the critical point temperature, or critical temperature of water, that is, 374°C, and the pressure inside the reactor is equal to or higher than the vapor pressure of the water at the temperature inside the reactor.

[0033] The hydrogen production according to the present invention occurs at a temperature up to 50°C lower, in particular up to 30°C lower, than the critical temperature of water, in the presence of water in the liquid state for the purposes of the oxidation of the metallic iron, of an inert electrolyte and of an iron oxidation promoter.

[0034] Preferably, the hydrogen production occurs at a temperature between 320°C and 373°C, in particular between 350°C and 373°C, and at a pressure equal to or higher than the vapor pressure of the water corresponding to this temperature.

[0035] These temperature conditions are advantageously much lower than the typical temperatures of an iron-steam process according to the state of the art, which are of the order of 600°C - 900°C, and thus allow for significant energy savings.

[0036] The oxidation of iron at these relatively low temperatures can be promoted by introducing an inert electrolyte and an iron oxidation promoter into the reactor.

[0037] An “inert electrolyte” is understood as a chemical species dissolved in water that allows electrical conduction, is stable at least in the temperature range used and does not react significantly with the other substances present in the reactor. In this way, the inert electrolyte promotes conduction in the reactor, facilitating the oxidation-reduction reactions that occur inside it without triggering other reactions concurrent to the reactions of interest. The inert electrolyte can be inert with respect to the above mentioned oxidation and reduction of iron. The inert electrolyte can be chosen from a group comprising NaCl (sodium chloride), KC1 (potassium chloride), KNO3 (potassium nitrate), Na2SC>4 (sodium sulfate), K2SO4 (potassium sulfate), NaOH (sodium hydroxide) and KOH (potassium hydroxide). Preferably, the inert electrolyte is NaCl. An “iron oxidation promoter” is understood as a chemical species that promotes, or makes more energetically advantageous, the aforementioned iron oxidation reactions. A good iron oxidation promoter lowers the iron’s extraction level (workfunction), which is the minimum energy required to extract one electron from the surface of the material. The extraction level is linked to the Fermi level because, in general, materials with low extraction level values have higher Fermi levels. The iron oxidation promoter can lower the iron’s extraction level and / or raise its Fermi level, promoting the transfer of electrons by the iron, that is, its oxidation.

[0038] The iron oxidation promoter allows the hydrogen production through iron oxidation by means of water to occur at temperatures lower than the critical temperature of water.

[0039] The iron oxidation promoter can be an element of group 10 of the periodic table, or an alloy thereof. The oxidation promoter can be chosen from a group comprising nickel, palladium, platinum, silver and alloys thereof, preferably being nickel or Raney nickel, or palladium or silver. In some embodiments, a weight ratio of water in the liquid state to metallic iron made available at the start of the hydrogen production is between 5:1 and 22:1.

[0040] In some embodiments, a weight ratio of oxidation promoter to metallic iron made available at the start of the hydrogen production is between 1 :15 and 1 :6. After having made available metallic iron, water, inert electrolyte and oxidation promoter inside the reactor, the method can provide to pressurize and heat the contents of the reactor so as to reach a temperature comprised between 320°C and 373°C, in particular between 350°C and 373°C, and a pressure equal to or higher than the vapor pressure of the water corresponding to that temperature. This increase in temperature triggers the oxidation of iron by the water inside the reactor, and therefore the production of hydrogen.

[0041] In principle, the iron oxidation reactions can continue until an equilibrium is reached or until the total oxidation of the iron initially made available. In practice, it is necessary to find a duration for the production that allows for a reasonable hydrogen yield without making the residual iron oxides in the reactor too difficult to regenerate.

[0042] In fact, as the oxidation reactions continue, grains of ferrous material are formed in the reactor comprising fully oxidized iron, that is, hematite Fe2Ch mostly on the surface and, deeper down, partly oxidized iron, such as magnetite FesCfi, and / or non-oxidized iron (metallic iron). The longer the production lasts, the more the water’s oxygen can react with more internal layers of the ferrous material; on the other hand, the greater the oxidation of the iron and the more difficult this will be to regenerate. The Applicant has verified, by way of example, that a good yield can be achieved starting from water and metallic iron in a ratio of 11 : 1 by weight, with an amount of oxidation promoter of about 10% by weight with respect to the iron, making the production last for about two hours: starting from about 535 - 540 mmol (millimoles) of iron, about 50 - 55 mmol of molecular hydrogen can be obtained.

[0043] During production, it can be provided to agitate the water inside the reactor, so as to promote contact between water and iron and the release of the hydrogen gas produced. The hydrogen produced can accumulate in a head-space of the reactor, above the water. In this way, advantageously, the liquid water also acts as a physical barrier between the hydrogen produced and the iron oxides, preventing the hydrogen produced from being able to again reduce the iron of the oxides, competing with the production. The agitation has to be such as not to cause the incorporation of the hydrogen into the water. In some embodiments, the hydrogen production method is of the batch type: after a predetermined reaction time has elapsed, production is interrupted allowing the reactor contents to cool. The hydrogen produced is then removed from the reactor, for example by tapping. The hydrogen removal occurs at a temperature lower that the 320°C - 373°C, in particular 350 - 373°C of the production, such as to prevent the pressure drop from causing the evaporation of the water in the reactor. This prevents tapping hydrogen mixed with water vapor, keeping the hydrogen essentially pure.

[0044] In some embodiments, the hydrogen can instead be removed from the reactor continuously, without cooling the reactor, taking care to inject an inert gas, such as nitrogen, into the reactor so as to keep the pressure in the reactor equal to or greater than the vapor pressure of the water.

[0045] The subsequent regeneration of the iron occurs by adding a reducing compound into the reactor. The reducing compound can be a reducing gas, or a mixture of reducing gases. For example, the reducing compound can consist of, or comprise, carbon monoxide or an unsaturated light hydrocarbon such as ethene, propene, butene or pentene.

[0046] The ferrous materials remaining in the reactor following the hydrogen production are substantially hematite Fe Ch and magnetite FeaO4. The reduction of these compounds can occur in a cascade: first the hematite is reduced to magnetite, then the magnetite is reduced to elemental iron.

[0047] In the event the reducing compound consists of, or comprises, carbon monoxide, the reduction of the hematite can occur according to the equation 3 Fe2Ch + CO 2 FesO4 + CO2, while the reduction of the magnetite can occur according to the equation FesO4 + 4 CO - 3 Fe + 4 CO2. Other reactions are also possible: for example, the hematite can be reduced directly to elemental iron.

[0048] In some embodiments, the iron regeneration occurs at a temperature between 320°C and 373°C, in particular between 350°C and 373°C, and at a pressure equal to or higher than the vapor pressure of the water corresponding to this temperature. The iron regeneration can occur at least in part in the presence of at least one iron reduction promoter.

[0049] An “iron reduction promoter” is understood as a chemical species that promotes, or makes more energetically advantageous, the aforementioned iron oxides reduction reactions. A good iron reduction promoter raises the extraction level and / or lowers the Fermi level of the iron in the oxide, promoting the acquisition of electrons by the iron, that is, its reduction.

[0050] The iron reduction promoter can be an element having lower electrochemical potential than the iron to be reduced.

[0051] For example, the iron reduction promoter can be chosen from a group comprising copper, lead, chromium, zinc, manganese, aluminum, magnesium, palladium, silver and alloys thereof, preferably being palladium, silver or copper.

[0052] The iron reduction promoter can allow the iron regeneration to occur at lower temperatures, for example at a temperature up to 50°C lower, in particular up to 30°C lower, than the critical temperature of water, in the presence of water in the liquid state.

[0053] In a first part of the iron regeneration, the main reaction can be a reduction of the hematite. In some embodiments, the at least one iron reduction promoter is a first iron reduction promoter having a lower electrochemical potential than the electrochemical potential associated with the transition from iron(III) to iron(II), for example copper. This first reduction promoter can be used at least in the first part of the iron regeneration as above. In a second part of the iron regeneration, the main reaction can be a reduction of the magnetite.

[0054] In some embodiments, a second iron reduction promoter with an electrochemical potential lower than the electrochemical potential associated with the transition from iron(II) to metallic iron, for example zinc, aluminum or magnesium, can be used at least in the second part of the iron regeneration as above.

[0055] In some embodiments, the second part of the iron regeneration does not provide to use any iron reduction promoter whatsoever.

[0056] In some embodiments of the method described here, a first step is therefore provided in which a production or generation of hydrogen from water is carried out. These embodiments provide an improvement of the oxidation of Fe to FesO4 (magnetite) through pressurized liquid water, for example at 33O°C using Pd or Ag. The oxidation of metallic iron (Fe) to magnetite (Fe3O4) is a key reaction in various industrial and environmental processes. Pressurized liquid water at high temperatures can serve as an effective medium for this transformation. The efficiency of this process can be further improved by introducing metals that are less electronegative than iron, such as palladium (Pd) or silver (Ag). These metals, when in contact with iron, establish galvanic effects that promote the oxidation of Fe by water. The mechanisms, feasibility, and implications of using Pd and Ag to accelerate Fe oxidation under these conditions are described below.

[0057] A first aspect is the mechanism of improvement by means of galvanic effect, in which there is provided:

[0058] • Formation of galvanic couples:

[0059] - Iron (Fe) and palladium (Pd) form a galvanic couple when in direct contact. Pd has a more positive standard reduction potential (Pd2+ / Pd=+0.99 V) than iron (Fe2+ / Fe=-0.44 V) - This difference in potential creates a flow of electrons from Fe (anode) to Pd

[0060] (cathode), promoting the oxidation of Fe to FesC

[0061] • Oxidation reaction (anode - Fe):

[0062] - On the surface of the Fe, the oxidation proceeds as:

[0063] Fe — > Fe2++ 2e“ • Reduction reaction (cathode - Pd):

[0064] - The electrons transferred to the Pd improve water reduction:

[0065] 2H2O + 2e“ — > H2+ 2OH“

[0066] • Overall reaction:

[0067] Combining the anodic and cathodic reactions in the presence of water and oxygen results in:

[0068] 3Fe + 4H2O — > FesO4+ 4H2

[0069] A second aspect is the considerations regarding Gibbs free energy, reported below.

[0070] • Free energy for the Fe oxidation: The reaction for the oxidation of Fe to FesO4is:

[0071] 3Fe + 4H2O — FesO4+ 4H2

[0072] The change in Gibbs free energy (AG) for this reaction at a temperature of 330°C and a pressure of 20-25 MPa can be calculated using the standard Gibbs free energy values (AG°) of reactants and products. In standard conditions:

[0073] - FesO^ -1015.4 kJ / mol

[0074] - H2O(1): -237.1 kJ / mol

[0075] - H2(g)(g): 0 kJ / mol - Fe: 0 kJ / mol

[0076] The AG° is:

[0077] AG°=[(- 1015 ,4)+4(0)]-[3(0)+4(-237.1 )]~-58.0 kJ / mol

[0078] A negative AG° indicates that the reaction is thermodynamically favorable.

[0079] • Effect of Pd: The palladium acts to lower the activation energy, rather than directly contributing to the thermodynamics of the Fe oxidation. The galvanic effect improves the reaction kinetics by providing a direct path for the electron transfer.

[0080] A third aspect is the stability of the Pd at 330°C and high pressure (e.g. 20-25 MPa), as reported below. • Thermodynamic stability:

[0081] Palladium is thermodynamically stable under reducing conditions and does not readily oxidize at 330°C in the presence of water. However, in the presence of oxygen or under highly oxidizing conditions, Pd can form oxides (PdO), which can affect its catalytic properties. • Chemical interaction with water:

[0082] At 330°C and 20-25 MPa, water is in a subcritical state, with high density and reactivity. Pd remains stable in this environment, promoting the electron transfer without significant degradation.

[0083] • Interaction with hydrogen: Pd has a high affinity for hydrogen and can absorb it to form palladium hydrides (PdH), which could affect its surface properties. Although this absorption does not significantly hamper its role as a galvanic promoter, prolonged exposure to hydrogen may require monitoring.

[0084] A fourth aspect is the overall impact of Pd, as reported below. • Kinetic improvement:

[0085] The galvanic effect provided by Pd accelerates the reaction kinetics by improving the flow of electrons and promoting the splitting of water at the cathode.

[0086] • Energy efficiency: Lower activation energy reduces the energy required for the oxidation process.

[0087] • Potential challenges with Pd:

[0088] - PdO formation under certain conditions could reduce efficacy.

[0089] - The hydrogen absorption in Pd could alter its surface characteristics over time. The proposed method of using, for example, palladium or silver to improve the oxidation of Fe to FesCL in pressurized liquid water at 330°C offers a new approach for improving oxidation efficiency. The use of palladium to improve the oxidation of Fe to FesCL in pressurized liquid water is feasible both thermodynamically and also kinetically. The reaction is guided by the intrinsic thermodynamics of Fe oxidation, and is accelerated by the galvanic and catalytic properties of Pd or Ag. This process not only accelerates Fe conversion, but also allows to recover the hydrogen, offering potential applications in energy storage and sustainable chemical processes.

[0090] In some embodiments of the method described here, a second step is also provided in which a regeneration of the iron oxides is carried out using reducing gas. These embodiments provide an improvement of the reduction of FesCL to Fe through CO using a galvanic couple in pressurized liquid water.

[0091] The reduction of iron oxides (FesO^ to metallic iron (Fe) is a critical process in metallurgical and chemical industries. While carbon monoxide (CO) is a widely used reducing agent, the efficiency of this process can be improved by introducing a second metal, such as zinc, aluminum, chromium, magnesium, calcium, or other metals that are more electronegative than iron, which form galvanic couples with Fe3O4. This approach exploits electrochemical principles to lower the activation energy of the reduction reaction. However, those metals such as zinc are consumed and do not fully achieve the desired results. In order to improve the reduction of FesCL to Fe through CO in the presence of pressurized liquid water at 330°C without using a sacrificial anode, and on the contrary using only CO as reducing agent, another metal can be considered, with properties that differ from zinc. The ideal metal should: - Promote the galvanic coupling effect, improving the reduction kinetics.

[0092] - Resist complete oxidation, maintaining long-term stability.

[0093] - Operate effectively within the specific high temperature, high pressure and water containing environment. On the basis of these criteria, the Applicant has identified noble metals such as silver (Ag) and palladium (Pd) as potential candidates.

[0094] Advantages of Ag and Pd: i) Partial oxidation but high stability:

[0095] - Unlike Zn, Ag and Pd are less prone to complete oxidation under these conditions.

[0096] - Ag and Pd can promote localized electron transfer by forming a galvanic couple with Fe3O4, without being completely consumed. ii) Catalytic properties:

[0097] - Pd catalyzes the reactions correlated to CO, potentially improving the reaction between CO and FesO4.

[0098] - Ag has a high electrical conductivity, which can support the transfer of electrons between FesO4 and CO. iii) Resistance to corrosion in water:

[0099] - Both metals are resistant to corrosion in pressurized liquid water at 330°C, maintaining their structural integrity for prolonged periods.

[0100] Another aspect is the galvanic effect:

[0101] - Pd and Ag also have the potential to form a galvanic couple with FesO4 in the presence of CO. In this case, Pd or Ag can act as an electron well, promoting the transfer of electrons to Fe2O4 in order to reduce it to Fe:

[0102] Fe3O4+ 4CO + 8e“ 3Fe + 4CO2

[0103] The mechanism provides the following:

[0104] - CO acts as a reducing agent, donating electrons to Pd or Ag.

[0105] - Pd or Ag transfer electrons to Fe3O4, reducing it to Fe.

[0106] - CO oxidizes to CO2.

[0107] - Pd or Ag remain in their metallic state and are not consumed, acting as electrochemical catalysts.

[0108] Thermodynamics is promoted:

[0109] - The reduction of Fe3O4 by CO is thermodynamically favorable at 33O°C (AG°<0).

[0110] - The presence of Pd or Ag can lower the reaction activation energy, increasing the reaction rate.

[0111] Pd and Ag therefore act as electronic mediators. An electronic mediator is a material or element that promotes the transfer of electrons between two reactant species without being consumed in the reaction. Pd and Ag are suited to this role thanks to:

[0112] - High electrical conductivity, allowing for efficient electron transport.

[0113] - Intermediate redox potentials, which allow them to alternatively accept and donate electrons depending on the reaction conditions.

[0114] - Stability under reaction conditions, maintaining their metallic state and avoiding complete oxidation or dissolution.

[0115] In these reactions, Pd or Ag form a localized electrochemical couple with Fe or FesCU, accelerating the redox process without altering the net reaction.

[0116] It is important to note a fundamental difference in the role between oxidation and reduction:

[0117] • In oxidation (from Fe to Fe3O4):

[0118] - Pd and Ag enhance the process by promoting the oxidation of Fe by means of galvanic coupling.

[0119] - They act as cathodes, driving the splitting reaction of the water into hydrogen gas.

[0120] • In reduction (from Fe3O4 to Fe):

[0121] - Pd and Ag promote the reduction by acting as electron mediators, transferring electrons from CO to Fe3O4.

[0122] - They act catalytically and are not consumed.

[0123] Below are some additional considerations on the Water Gas Shift (WGS) reaction.

[0124] The WGS occurs within the range of 250-300°C in the presence of iron oxides (FeOx):

[0125] 4CO + 4H2O-> 4CO2+ 4H2

[0126] Therefore, hydrogen together with CO also reduces magnetite to metallic iron:

[0127] Therefore, the presence of CO can induce the conversion of water into hydrogen (by means of the WGS) which is finally also used to reduce the Fe3O4 mediated by the Pd.

[0128] Some practical implications of the above can be:

[0129] • Dual functionality:

[0130] Pd and Ag can theoretically improve both oxidation as well as reduction reactions. Their role depends on the dominant driving force in the system:

[0131] - In the presence of water and Fe, they promote oxidation.

[0132] - In the presence of CO and Fe3O4, they promote reduction.

[0133] • Stability:

[0134] Both metals remain stable in hot, pressurized water at 33O°C, making them suitable for an extended use.

[0135] • Reaction optimization:

[0136] Careful control of the reaction environment is critical to guarantee that the desired reaction prevails (oxidation or reduction):

[0137] - In the oxidation scenario, a water-rich environment and the absence of CO favor the oxidation of Fe.

[0138] - In the reduction scenario, a CO-rich environment and the controlled removal of CO2 favor the reduction of FesO4. Pd and Ag act as electron mediators promoting an efficient transfer of electrons between the reactants. Their high reduction potentials and the stable metallic state allow them to improve both the oxidation of Fe by water and also the reduction of FesC by CO, depending on the environmental conditions. This dual functionality derives from their ability to alternately accept and donate electrons, catalyzing redox reactions without being consumed.

[0139] EXPERIMENTAL SECTION

[0140] The Applicant has found experimental evidence that reproduces the method described here, which includes the first step of oxidation of Fe to produce H2 and the second step of regeneration that provides the reduction of the oxide to metallic Fe. In particular, fig. 1 is a graph that shows the trend of hydrogen generation with respect to reaction time, both with the presence of Pd (Pd, solid line) and also without (no Pd, dashed line), at 330°C in pressurized liquid water with the addition of sodium chloride (NaCl) to imitate seawater (in seawater, in general the average concentration of dissolved salts is 35 g / L and NaCl is present on average between 70% and 80% by weight of the total dissolved salts). Up to 120 minutes of reaction time, the evolution of hydrogen is similar in both conditions. This is mainly due to the fact that the reaction environment remains almost neutral during this period. Iron undergoes oxidation by water and Pd forms unstable oxides (PdO). However, as the hydrogen content gradually accumulates in the reactor, the reaction atmosphere becomes increasingly reducing. The available hydrogen reacts quickly with the PdO, reducing it to Pd and preventing a further reoxidation thereof.

[0141] Once reduced, the Pd is “free” to exert its catalytic effect, significantly improving the oxidation of Fe to iron oxides (FeOx), accompanied by the generation of hydrogen. Within 180-200 minutes, the reaction environment becomes highly reducing, with abundant hydrogen that counteracts the oxidation of iron and instead promotes its reduction. In these conditions, the Pd passes to the second step of the method, acting as an electron mediator. This allows the Pd to promote the reduction of the iron oxides into metallic iron by using hydrogen as a reducing agent.

[0142] In these experiments, the hydrogen is not removed from the reactor. As a result, at 240 minutes, the Pd continues to express its activity by further reducing the iron oxides. The lower amount of hydrogen available after 240 minutes in the presence of Pd confirms the above assumptions.

[0143] In industrial applications, hydrogen is typically removed as the process gradually progresses, shifting the balance to favor further hydrogen generation. Once the available iron is fully oxidized, CO is introduced, and the resulting CO2 is removed to improve the reduction of the iron oxides to metallic iron, with Pd playing a key role in promoting this reaction.

[0144] It is clear that modifications and / or additions of parts may be made to the hydrogen production method as described heretofore, without thereby departing from the field and scope of the present invention, as defined by the claims.

[0145] It is also clear that, although the present invention has been described with reference to some specific examples, a person of skill in the art will be able to achieve other equivalent forms of hydrogen production method, having the characteristics as set forth in the claims and hence all coming within the field of protection defined thereby.

[0146] In the following claims, the sole purpose of the references in brackets is to facilitate their reading and they must not be considered as restrictive factors with regard to the field of protection defined by the claims.

Claims

CLAIMS1. Hydrogen production method which carries out, in alternating sequence inside a single reactor, hydrogen production through oxidation of metallic iron by means of water and regeneration of said metallic iron through reduction of iron oxide, characterized in that said hydrogen production occurs at a temperature up to 50°C lower than the critical temperature of water, in the presence of water in the liquid state for the purposes of the oxidation of the metallic iron, of an inert electrolyte and of an oxidation promoter of said iron, and in that said regeneration of said iron occurs by adding a reducing compound into said reactor, wherein said oxidation promoter of said iron is chosen from a group comprising nickel, palladium, platinum, silver and their alloys, and wherein said regeneration of said iron occurs at least in part in the presence of at least one reduction promoter of said iron at a temperature up to 50°C lower than the critical temperature of water, in the presence of water in the liquid state.

2. Method as in claim 1 , characterized in that said electrolyte is inert with respect to said oxidation and reduction of the iron, and is chosen from a group comprising NaCl, KC1, KNO3, Na2SO4, K2SO4, NaOH and KOH.

3. Method as in any claim hereinbefore, characterized in that said reducing compound is a reducing gas, or a mixture of reducing gasses.

4. Method as in any claim hereinbefore, characterized in that said regeneration of said iron occurs at a temperature between 320°C and 373 °C, and at a pressure equal to or higher than the vapor pressure of the water corresponding to said temperature.

5. Method as in any claim hereinbefore, characterized in that said at least one reduction promoter of said iron is chosen from a group comprising copper, lead, chromium, zinc, manganese, aluminum, magnesium, palladium, silver and their alloys.

6. Method as in any claim hereinbefore, characterized in that said reduction promoter of said iron has an electrochemical potential lower than the electrochemical potential associated with the transition from iron (III) to iron (II) and is used in a first part of said regeneration of said iron, wherein the main reaction is a hematite reduction.

7. Method as in any claim hereinbefore, characterized in that a weight ratiobetween said water in the liquid state and said metallic iron made available at the beginning of said hydrogen production is between 5:1 and 22:1.

8. Method as in any claim hereinbefore, characterized in that a weight ratio between said oxidation promoter and said metallic iron made available at the beginning of said hydrogen production is between 1:15 and 1 :6.

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