WO2010063858A1

WO2010063858A1 - Method for obtaining hydrogen

Info

Publication number: WO2010063858A1
Application number: PCT/ES2009/000555
Authority: WO
Inventors: Lluis Soler I Turu; Angélica Maria CANDELA SOTO; Jorge MACANÁS DE BENITO; Maria Muñoz Tapia; Juan Casado Giménez
Original assignee: Universitat Autonoma De Barcelona
Priority date: 2008-12-03
Filing date: 2009-12-02
Publication date: 2010-06-10
Also published as: ES2321793B1; ES2321793A1

Abstract

Method for producing hydrogen. The present invention provides a method for obtaining hydrogen gas (H₂) from the reaction of metallic aluminium (Al) in a basic aqueous medium at pH > 12. The aqueous medium comprises at least one metal salt that acts as reaction catalyst. Therefore, the present invention may be encompassed within the field of chemistry.

Description

PROCEDURE FOR OBTAINING HYDROGEN

FIELD OF THE INVENTION

The present invention provides a process (hereinafter process of the invention) for obtaining hydrogen gas (H ₂ ) from the reaction of metallic aluminum (Al) in a basic aqueous medium, at pH> 12, comprising at least a metal salt that acts as a reaction catalyst. Therefore, the present invention can be encompassed within the field of chemistry.

STATE OF THE TECHNIQUE

WO2006072115 refers to a method of obtaining H ₂ by reacting Al with water in the presence of a catalyst and an initiator. The catalyst used is a water-soluble inorganic salt selected from the group of halides, sulphides, sulfates or nitrates of metals of groups I (alkaline) or II (alkaline earth) of the periodic table. The catalyst used is selected from: sodium chloride, potassium chloride, potassium nitrate, sodium nitrate or combinations thereof, with sodium chloride being preferred in a 1: 1 ratio. In addition, other catalysts that may be employed are alumina, aluminum hydroxide or aluminum oxide, preferably in combination with the salts mentioned above. The pH used in this document has a near neutral value. Specifically on page 6, lines 24-26, this document says that the initial pH varies between 4 and 8, preferably in the range 5-7. Said pH remains close to neutral (4-10) during the rest of the reaction.

WO2007016779 describes a method for the preparation of a metal (including microporous Al) for obtaining H ₂ in its reaction with water; To this end, micropores are introduced by combining the metal particles with an agent that deforms them to produce an intermediate composition. Said agent is selected from citric acid, ice, dry ice, PVA, organic waste, polymers Short chain organic or a water soluble inorganic salt, with NaCl or KCl being preferred. The pH value used in the procedure described in this document varies between 4 and 10 (see page 25: "pH and temperature"). Optionally, additives can be used to improve the reaction with water. The additive optionally used is a metal salt of group I (alkaline) or II (alkaline earth) of the periodic table, with K, Li, Na, Ca, Mg being preferred (see page 26: "additives").

US2007020174 discloses obtaining H ₂ by contacting metallic and water, using reaction promoters and / or reaction promoter precursors. Reaction promoters are chemical compounds that include one or more metal hydroxides selected from groups I (alkaline) or II (alkaline earth) of the periodic table: sodium hydroxide, potassium hydroxide, calcium hydroxide or mixtures thereof . Likewise, the reaction promoter precursors are compounds of metals selected from groups I (alkaline) or II (alkaline earth) of the periodic table which, when reacting with water, generate the reaction promoters indicated above. Thus, the reaction promoter precursors can be, for example: sodium oxide, calcium oxide, calcium hydride, sodium hydride, lithium hydride or mixtures thereof. The pH value selected for this reaction is less than 12, explaining in paragraph [0021] the reason why a pH value greater than this would not be positive to carry out the process.

JP2006321701 describes the obtaining of H ₂ by contacting Al with water at room temperature. The reaction is promoted by the use of catalysts and additives. The additives promote the reaction of Al with the catalyst. The catalyst is preferably mercury and the additive: citric acid, hydrochloric acid, sulfuric acid or other acidic agent.

Document US2003091503 describes a vehicle, whose engine runs on H ₂ as fuel, comprising at least one locomotion system driven by

H ₂ and an operational H ₂ generator that serves as a supply for the locomotion system. The H ₂ generator includes an electrochemical reactor useful for generating H ₂ fuel from water, electrolytes and material containing metals, and a system that supplies said fuel. As provided, for example, in paragraph [0034], the catalyst used is based, at least, on one of the metals or metal oxides belonging to the platinum group and the transition metal group. On the other hand, as can be seen, for example, in paragraph [0051], sodium hydroxide is used herein as an electrolyte.

The present invention relates to a process for obtaining H ₂ from the reaction of metallic Al and water in basic medium, at pH> 12, said reaction being catalyzed by a salt (or mixture of salts) of a metal ( or metals).

Therefore, the present invention differs from WO2006072115 in the following aspects:

• The present invention does not use initiator.

• In the process of the present invention the pH is adjusted to a value> 12. In contrast, the wider range of pH values cited in WO2006072115 is 4-10.

• The salts used as a catalyst in the present invention have been formulated by the combination of the most effective cations and anions that have been selected in the present invention in a particular way.

The present invention differs from WO2007016779 in the following aspects:

• In the present invention it is not necessary that Al be microporous.

The present invention differs from US2007020174 in the following aspects: • In the process of the present invention the pH is adjusted to a value> 12. The pH value selected in US2007020174 is less than 12.

• The compounds used in the present invention as catalysts are neither hydroxides nor metal oxides.

The present invention differs from JP2006321701 in the following aspects:

• The present invention does not use acids.

• None of the salts that act as catalysts in the present invention is a mercury salt.

The present invention differs from US2003091503 in the following aspects:

• The present invention does not contemplate the use of electrolytes. The use made in the present invention of NaOH is to confer basicity (pH> 12), and the experiment with NaOH only acts as a target against which the results of the different catalysts used are compared.

• No reference to the influence of the pH value in the described process has been located in US2003091503, the alkaline pH being an essential factor in the present invention.

The process of the invention, in addition to differentiating itself from the documents located in the state of the art, by the aspects mentioned above, is more effective, as can be seen in the successive tables shown in the description of the present invention where the data of maximum speed and yield in the production of H ₂ for each of the salts used in the present invention.

On the other hand, the present invention demonstrates that the assertion made in several prior art documents (such as in WO2006072115 and WO2007016779) concerning the possibility of using any salt or compound of alkaline or alkaline earth metals as a catalyst for the reaction of obtaining H ₂ could be too wide and far from reality. In the present invention it is evidenced that each of the salts tested has a different, independent and unrelated behavior to the group of the periodic table to which the metal belongs. Each salt has particular physicochemical characteristics that ultimately determine its effectiveness as catalysts. Therefore, it should be emphasized that documents located in the state of the art, despite defining the salt preferably used (in all cases other than those used in the present invention), assert and sow the prejudice regarding Any alkali metal or alkaline earth metal salt can be effectively used as a catalyst for the reaction to obtain H ₂ from metallic Al and water. Said prejudice is dissipated in the present invention since some of the salts tested, such as CaCl ₂ , had inhibitory effects on the reaction of production of H ₂ .

These facts demonstrate that the particularized choice of salts that work as catalysts effectively is not a trivial process and requires a thorough screening process, as has been done in the present invention. Through this selection process, the cations and anions that form the most effective metal salts for the process of the invention and that are different from those used in the prior art were identified.

DESCRIPTION OF THE INVENTION

Brief Description of the Invention

The present invention provides a process for obtaining H ₂ from the reaction of metallic Al in a basic aqueous medium (pH> 12) and at least one metal salt that acts as a catalyst for the reaction. Thus, the technical problem solved by the present invention refers to an alternative, and more effective method, than those located in the state of the art of obtaining H ₂ gas from metallic Al and water.

The effectiveness of the process of the invention is based on the one hand on the pH value used, equal to or greater than 12 (compare the results obtained in the examples at pH 13 and pH 12, described below) and, on the other hand side, in the type of salts specifically selected in the present invention after screening of the anions and cations that act as catalysts for the present reaction of production of H ₂ .

Therefore, in the present invention, H _{2 was} obtained from the reaction of metallic Al and water in basic medium (pH> 12), said reaction being catalyzed by salts formed from cations selected from: Fe ⁺ , Fe ⁺ , Mg ²⁺ , Ag ²⁺ , Na ⁺ , Co ²⁺ , Cu ²⁺ , Zn ²⁺ or NH ₄ ⁺ with anions selected from: F ^" , SO ₄ ^" , PO ₄ ^{3 "} , ClO ₄ ^" , Cl ^" or CO ₃ ^{2 "} . Examples of these salts can be seen in Tables 1, 2, 3 and 5 of the present invention.

It is convenient to clarify that the use that is carried out in the present invention of NaOH is not as a catalyst, but is used to confer basicity to the reaction medium. This fact is demonstrated in Figures 2-5 of the present invention where it can be seen that the blank, against which the results of the different catalysts used are compared, comprises sodium hydroxide. In relation to this fact it is important to note that none of the compounds used as catalysts in the present invention are hydroxides or metal oxides.

The catalyst of the reaction between metallic Al and water can be formed by a single metal salt or by a mixture of metal salts that carry out a synergistic effect making the production reaction of H ₂ more effective than when the salts They act as catalysts independently. Surprisingly, in the present invention it was shown that by using as a catalyst mixtures of salts comprised in Tables 1, 2, 3 and 5 a synergistic effect was achieved that leads to a maximum speed value (ml / min) of production of H ₂ above (even sometimes twice) the maximum production speed value of H ₂ achieved when salts were used as catalysts independently. Examples of these salts can be seen in Table 4.

When the catalyst is formed by a single metal salt, this is a salt formed from cations selected from: Fe ²⁺ , Fe ³⁺ , Mg ²⁺ , Ag ²⁺ , Na ⁺ , Co ²⁺ , Cu ²⁺ , Zn ²⁺ or NH ₄ ⁺ with anions selected from: F ^" , SO ₄ ^2" , PO ₄ ^{3 "} , ClO ₄ ^" , Cl ^" or CO ₃ ^2" . Examples of these salts can be seen in Tables 1, 2, 3 and 5.

When the catalyst is formed by a mixture of salts, said mixture is formed by at least two salts of those present in Tables 1, 2, 3 and 5. Examples of these salt mixtures can be seen in Table 4.

Description of the figures

In the process of the invention illustrated in Figures 2-5, NaOH acts as a target against which the result obtained after the use as a catalyst of the salts mentioned in each figure is compared. In each of Figures 3-5, several consecutive experiments were carried out, where fixed amounts of 1, Og of Al were sequentially added to the same basic aqueous medium with catalyst under study.

Figure 1. Scheme of experimental setup.

1. Reactor.

2. Thermostatic bath.

3. Thermometer

4. Reagent input.

5. Silicone tube. 6. Full glass of water at room temperature. 7. Inverted burette filled with water. Figure 2. Several typical curves (volume H ₂ vs time) of production of H ₂ from 0.2 g of Al in aqueous medium with 0.1 M NaOH and different metal salts acting as catalysts for the reaction are shown. The production of H ₂ took place rapidly at the beginning of the reaction but slows down as Al is consumed, with the white curve showing the lowest hydrogen production. The maximum speeds were calculated from the slope obtained in the regression line of the initial points of the different hydrogen production curves.

Figure 3. Hydrogen production curves for consecutive experiments using Fe ₂ (SO ₄ ) ₃ as catalyst in the aqueous medium.

Figure 4. Hydrogen production curves for consecutive experiments using MgCl ₂ as catalyst in the aqueous medium.

Figure 5. Hydrogen production curves for consecutive experiments using NaF as a catalyst in the aqueous medium.

Figure 6. The figure shows the thermal self-sufficiency of the process of the invention. As it can be seen from 40 minutes the temperature rises to 75 ⁰ C, to obtain a production rate of hydrogen lOOml / min to 100% without heating the reactor externally.

Detailed description of the invention

Description of the method of the invention

As discussed above, the present invention provides an alternative, cost-effective and efficient method for obtaining H ₂ gas. For this, metallic Al was combined in aqueous medium (Al and water are the reactants of the H ₂ production reaction). The aqueous medium comprises NaOH in a concentration suitable to adjust the pH of the reaction to a value> 12 and a salt metal, or a mixture of said salts, which acts as a catalyst for the reaction between metallic Al and water.

The examples demonstrate the effectiveness of the salts used in the invention as catalysts for the production of H ₂ causing significant increases in the speed and / or the yield of hydrogen production. These effects can be observed by comparing the maximum speeds with respect to the maximum speed of the experiment without catalysts (white) and can also be observed by comparing the times at which 50% of the reaction yield is reached.

The combination of all these reagents allowed to obtain H ₂ with high production speeds and yields of 100% in most of the experiments performed. In parallel, aluminum hydroxide was also obtained as a byproduct of the reaction.

Quantification of the speed and yield of the reaction

To quantify the speed and production yield of H ₂ , the experimental assembly depicted schematically in Figure 1 was used. The reagents were added to a 100 ml Pyrex glass reactor containing 75 ml of the aqueous working medium. The reactor was heated in a thermostated water bath to maintain a constant temperature of 75 ° C. The production reactions of H ₂ started at the moment when the solid Al came into contact with the aqueous medium comprising NaOH and a metal salt, or a mixture of said salts, which acted as the reaction catalyst. The salts could be in solution or forming a suspension in the aqueous medium. Said aqueous medium was adjusted to pH> 12 before starting the experiments, to favor the corrosion of Al. Once the corrosion of Al started, the hydrogen produced left the reactor through a silicone tube 40 cm long and 8 mm internal diameter, passed through a water bath at room temperature to condense the water vapor present in the emanated gas, and finally collected in an inverted burette to measure the resulting H ₂ volume. Depending on the type of experiment, different amounts of Al powder were used. In the experiments carried out in order to evaluate the effect on the production of H ₂ of the different chemical additives considered, 0.2g of powdered aluminum (<44 μm diameter, 99.7% purity) were used. On the other hand, successive additions of 1.Og of the same aluminum powder were made in all consecutive experiments performed to study the efficiency of the process.

Cation Effect

From the results shown in Table 1 and 2, it can be concluded that the presence of cations such as Fe ²⁺ , Fe ³⁺ , Mg ²⁺ , Ag ²⁺ , Na ⁺ , Co ²⁺ , Cu ²⁺ , Zn ^{2 +} or NH ₄ ⁺ can cause significant increases in the corrosion rate of aluminum and, consequently, in the production rate of H ₂ . These effects can be observed by comparing the values of the maximum velocities in the presence of the different metal salts with respect to the maximum velocity of the experiment without salts (white). Furthermore, said effect can be observed by comparing the times at which 50% of the reaction yield is reached.

For the specific case of Fe ³⁺ , the effect of this cation is demonstrated by comparing the experiments where FeCl ₃ or NaCl is added, obtaining maximum speeds of 284 ml H ₂ ZnUn and 226 ml H ₂ / min respectively (see tables 1 and 2). Comparing the experiments Na ₂ SO ₄ with respect to Fe ₂ (SO ₄ ) ₃ it can also be observed that the presence of the Fe ³⁺ cation and / or sulfate anion causes a significant increase in the corrosion rate of aluminum and, consequently, of the production speed of H ₂ . This positive effect on the production of H ₂ can be observed by comparing the maximum speeds of both experiments with respect to the target and can also be observed by comparing the times at which 50% of the reaction yield is reached. In all cases where it is used

Fe ³⁺ as a reaction promoter, the yield achieved is 100%. In contrast, in the cases of Fe ²⁺ or Ag ⁺ , a synergistic effect of the SO ₄ ^" anion cannot be ruled out, since in most experiments where sulfate salts are used (except for CuSO ₄ and ZnSO ₄ ) the maximum production rates of H ₂ improve the results obtained using NaOH O ₅ IM without adding any salt.

However, from the experimental results shown in Table 1 it is also observed that cations such as Ni ²⁺ , Pb ²⁺ or Ca ²⁺ can act as inhibitors of the H ₂ production process. This fact confirms the importance of the particularized choice of anion and cation that will eventually form the salt that acts as a catalyst.

Comparing the experimental results obtained for MgCl ₂ and CaCl ₂ , and both with the blank, (see Table 1), it can be affirmed that the presence of the Mg ²⁺ cation causes a significant increase in the corrosion rate of aluminum and, consequently, of the production speed of H ₂ . The comparison of the maximum production rates of H ₂ between these three experiments clearly shows that the use of the Ca cation inhibits the production of H ₂ , obtaining a maximum speed even lower than that of the blank.

Effect of the anions

From the experimental data presented in Table 2 it can be stated that the presence of anions such as F ^" , SO ₄ ^" , PO ₄ ^" , ClO ₄ ^" , Cl ^" or CO ₃ ^" causes significant increases in the production speed of H ₂ . As in the previous case, these effects can be observed by comparing the maximum speed of the experiment without additives (white) with respect to the maximum speeds obtained using anions in the aqueous medium and can also be observed by comparing the times at which 50% is reached of yield of the production reaction of H ₂ .

For the specific case of anion F ^" , a comparison of the experiments using NaCl and NaF, and both with the blank, (see Table 2) shows that the presence of anion F ^" in the working solution causes a significant increase in speed of corrosion of aluminum and its positive effect on the production of H ₂ is far superior to the experiment in the presence of Cl ^" . However, a comparison of the results obtained with NaBr and with the blank (see Table 2) demonstrates that the use of Br ^" anion inhibits the production of H _2. Thus, it can be concluded that each halide has a different effect on the production of H ₂ due to its different physicochemical properties.

Process efficiency in consecutive experiments

The present invention also demonstrates the efficiency of the process of the invention in consecutive stages of production of H ₂ . Table 3 shows the experimental results obtained in a series of experiments carried out with the objective of evaluating the efficiency of the process using catalysts during consecutive H ₂ production experiments.

As can be seen in Table 3, and in Figures 3, 4 and 5, the efficiency of the process in the presence of catalysts is maintained during the consecutive stages of production of H ₂ , improving in all cases the production speed of H ₂ reached in the experiment in the absence of catalysts (0.1M NaOH white).

Also noteworthy are the low times at which 100% of the yield of the H ₂ production reaction is achieved, if compared with the final time required by the blank to reach a 100% yield of more than 8 hours and half.

Autothermal process efficiency

On the other hand, the process of the present invention proved to be autothermal, which makes it possible to dispense with the external heating of the reactor where the H ₂ production reaction is carried out, obtaining 100% yields. Specifically, the reactor was added l, 000g Al, 0,457g NaOH and 0577g Fe ₂ (SO ₄₎ ₃ -9H ₂ O and then water (about 1 drop / second) was added at 25 ⁰ C at a rate of 3ml / min , for 1.5 min, using a compensated pressure funnel coupled to the reactor. This experimental procedure allowed to obtain an initial solution of NaOH of very high concentration that in contact with the aluminum causes the initiation of the exothermic reaction of hydrogen production, heating the solution contained in the reactor. By varying the flow of water added to the reactor it is possible to regulate the reaction temperature, preventing the working solution from exceeding 100 ⁰ C. In Figure 6 it can be seen how from the 40th minute the temperature rises to 75 ° C , obtaining a hydrogen production rate of 100ml / min to reach 100% without the need to heat the reactor externally.

Synergistic effect of the ionic salts combination

Furthermore, in the present invention it was shown that the combination of metal salts of Tables 1 and 2 gave rise to a synergistic catalytic effect, greater than the effect achieved when the salts were used as catalysts but independently.

From the results shown in Table 4, it can be concluded that the combined presence of Fe ³⁺ cation and sulfate and phosphate anions causes a very important increase in the corrosion rate of aluminum and, consequently, in the production speed of hydrogen. This unexpected positive effect manages to double the maximum speeds obtained in the experiments carried out with the Fe ₂ (SO ₄ ) ₃ and Na ₃ PO ₄ salts separately (see Tables 1 and 2).

Although in a smaller amount, the results obtained by combining Fe ³⁺ and sulfate and perchlorate anions also improve the results obtained in experiments using the different salts separately.

The examples set forth below are intended to illustrate the invention without limiting the scope thereof.

EXAMPLES

Example 1. Catalytic effect of non-sodium salts. The pH of the reaction was adjusted to a value of 13.0 by the addition of NaOH, the salt concentrations were 10 ^"2 M (except ZnSO ₄ , which was 0.1 M), the temperature was 75 ° C and 0.2 g Al powder was added to the reactor. The results obtained are shown in Table 1. The effects Catalysts of MgCl ₂ and Fe ₂ (SO ₄ ) ₃ are shown in Figure 2.

Table 1

Example 2. Catalytic effect of sodium salts. The reaction pH was adjusted to a value of 13,0 by adding NaOH, the concentrations of the salts were 10 ^"2 M, the temperature was 75 ⁰ C and 0.2 g Al powder was added in The reactor The results obtained are shown in Table 2. The catalytic effect of NaF is shown in Figure 2.

Table 2

Example 3. Efficiency of the process in consecutive stages. The first stage of all experiments was performed by adjusting the pH to a value of 13.0 by NaOH. All salt concentrations were 10 ² M. The temperature was 75 ° C and 1.0 g Al powder was added in the reactor for each of the steps performed. The results are seen in Table 3 and in Figures 3, 4 and 5.

Table 3

Example 4. Synergistic catalyst effect carried out by mixtures of salts. All experiments were performed by adjusting the pH to a value of 13.0 by NaOH. All salt concentrations were 10 ^"2 M. The temperature was 75 ⁰ C and 0.2 g of Al powder was added in the reactor. The results are shown in Table 4.

Table 4

Example 5. Effect of salts at pH = 12. In this example the experiments were performed by adjusting the pH to 12.0 by NaOH. All salt concentrations were 10 ^"2 M. The temperature was 75 ⁰ C and 0.2 g Al powder was added in the reactor. The results are shown in Table 5.

Table 5

Claims

1. Method for obtaining hydrogen gas by contacting metallic aluminum, water and at least one catalytic salt, characterized in that the reaction medium is adjusted to pH> 12.

2. Method according to claim 1, characterized in that the catalyst used is a metal salt formed from cations selected from: Fe ²⁺ , Fe ³⁺ , Mg ²⁺ , Ag ²⁺ , Na ⁺ , Co ²⁺ , Cu ²⁺ , Zn ²⁺ or NH ₄ ⁺ .

3. Method according to claim 1, characterized in that the catalyst used is a metal salt formed from anions selected from: F, SO ₄ ² -, PO ₄ ^{3 "} , ClO ₄ ^" , Cl ^" or CO ₃ ² -.

4. Method according to claim 1, characterized in that the catalyst used is a salt selected from: MgCl ₂ , Fe (NH ₄ ) (SO ₄ ) ₂ , Fe ₂ (SO ₄ ) ₃ , FeCl ₃ , (NH ₄ ) ₂ Fe (SO ₄ ) ₂ , Ag ₂ SO ₄ , Co (NO ₃ ) ₂ , K ₂ CO ₃ , CuSO ₄ , ZnSO ₄ , NaF, Na ₂ SO ₄ , Na ₃ PO ₄ or NaClO ₄ .

5. Method according to claim 1, characterized in that the catalyst used is formed by a mixture of at least two salts formed from the combination of cations selected from: Fe ²⁺ , Fe ³⁺ , Mg ²⁺ , Ag ²⁺ , Na ⁺ , Co ²⁺ , Cu ²⁺ , Zn ²⁺ or NH ₄ ⁺ with anions selected from: F ^" , SO ₄ ^2" , PO ₄ ^{3 "} , ClO ₄ -, Cl ^" or CO ₃ -.

Method according to claim 1, characterized in that the catalyst is formed by a mixture of salts selected from: FeNH ₄ (SO ₄ ) ₂ + NaClO ₄ or Fe ₂ (SO ₄ ) ₃ + Na ₃ PO ₄ .

7. Method according to claim 1, characterized in that the reaction for obtaining hydrogen is autothermal and can be initiated without an external heat input.

8. Catalyst comprising a mixture of at least two salts selected from: MgCl ₂ , Fe (NH ₄ ) (SO ₄ ) ₂ , Fe ₂ (SO ₄ ) ₃ , FeCl ₃ , (NH ₄ ) ₂ Fe (SO ₄ ) ₂ , Ag ₂ SO ₄ , Co (NO ₃ ) ₂ , K ₂ CO ₃ , CuSO ₄ , ZnSO _4, NaF, Na ₂ SO ₄ , Na ₃ PO ₄ or NaClO ₄ .

9. Catalyst according to claim 8, characterized in that the salt mixture is selected from: FeNH ₄ (SO ₄ ) ₂ + NaClO ₄ or Fe ₂ (SO ₄ ) ₃ + Na ₃ PO ₄ .

10. Use of the catalyst of claims 4, 8 and / or 9 for obtaining hydrogen gas from metallic aluminum and water in a reaction medium at pH> 12.