A method for preparing highly photocatalytically active g-C3N4 / biochar from molten salt pyrolysis of sewage sludge and its application

Abstract

This invention discloses a method and application for preparing highly photocatalytically active g-C3N4 / biochar from wastewater sludge via molten salt pyrolysis, belonging to the fields of energy catalysis and environmental remediation technology. The method for preparing g-C3N4 / biochar from wastewater sludge via molten salt pyrolysis includes the following steps: drying the wastewater sludge to obtain sludge powder; mixing the sludge powder with melamine to obtain a mixed material precursor, then mixing it with a molten salt medium and pyrolyzing it to obtain the g-C3N4 / biochar. This invention uses wastewater sludge as a carbon source to combine biochar with g-C3N4, which increases the specific surface area of ​​the material, promotes electron transfer, and thus improves photocatalytic activity. The photocatalytic material prepared by this invention has a narrow band gap and a large visible light absorption range, exhibiting excellent photocatalytic activity. Simultaneously, using wastewater sludge as a carbon source achieves the rational utilization of waste.

Description

Technical Field

[0001] This invention relates to the fields of energy catalysis and environmental remediation technology, and in particular to a method and application for preparing highly photocatalytically active g-C3N4 / biochar from molten salt pyrolysis of sewage sludge. Background Technology

[0002] Energy crisis and environmental pollution are two major challenges urgently needing solutions in today's society, both requiring sustained efforts from researchers to develop new solutions. Among these, photocatalysis technology, with its broad application prospects, lack of secondary pollution, and environmentally friendly characteristics, provides solid scientific support for the sustainable use of energy and the restoration of the ecological environment. Within this technology, the development and preparation of novel, highly efficient photocatalytic materials is a key research focus.

[0003] Compared with photocatalytic materials such as titanium dioxide and zinc oxide, graphitic carbon nitride (g-C3N4), as a polymer semiconductor material, possesses high thermal and chemical stability, good electronic conductivity, excellent photoelectric properties, and a suitable redox potential, making it highly valuable for research. However, the layered structure of pure g-C3N4 results in a small specific surface area and also suffers from several defects, including easy recombination of photogenerated electrons and holes, a limited number of reactive sites, and a tendency to aggregate. Modification strategies such as constructing heterojunctions, morphology control, and heteroatom doping can mitigate these defects and improve the photocatalytic activity of g-C3N4 to some extent, but further improvement remains possible.

[0004] Combining carbonaceous materials with g-C3N4 is one research direction. Biochar, as a carbon material, has a large specific surface area and good photoelectric properties. Combining biochar with g-C3N4 can improve photocatalytic activity to a certain extent. Chinese invention patent CN 107376972A uses withered magnolia petals as a carbon source to prepare a biochar-modified carbon nitride composite photocatalyst, which can effectively degrade dimercaptobenzothiazole in environmental wastewater. Chinese invention patent CN 109304203A uses soybean stalks as a carbon source to prepare a biochar-supported graphitic carbon nitride composite photocatalyst, which can effectively degrade petroleum hydrocarbons in soil. However, although these existing g-C3N4 / biochar composite materials have made some progress, they still have problems such as complex synthesis steps, long preparation cycle, high cost, and the need for further improvement in catalytic performance, which cannot fully meet the performance requirements. Summary of the Invention

[0005] The purpose of this invention is to provide a method and application for preparing highly photocatalytically active g-C3N4 / biochar by molten salt pyrolysis of sewage sludge, thereby solving the aforementioned problems in the background art. This invention uses sewage sludge as a carbon source to composite biochar with g-C3N4, which increases the specific surface area of ​​the material, promotes electron transfer, and thus enhances photocatalytic activity. The photocatalytic material prepared by this invention has a narrow band gap and a large visible light absorption range, exhibiting excellent photocatalytic activity. Simultaneously, by using sewage sludge as a carbon source, it achieves the rational utilization of waste.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] One of the technical solutions of this invention is to provide a method for preparing g-C3N4 / biochar from molten salt pyrolysis wastewater sludge, comprising the following steps:

[0008] The sewage sludge is dried to obtain sludge powder;

[0009] The sludge powder and melamine are mixed to obtain a mixed material precursor, which is then mixed with a molten salt medium and subjected to pyrolysis to obtain the g-C3N4 / biochar.

[0010] Preferably, the drying process involves drying the sewage sludge at 80°C to a constant weight, and then passing it through a 200-mesh sieve.

[0011] Preferably, the pyrolysis treatment further includes the steps of washing with water and drying; the drying is performed at 80°C for 24 hours.

[0012] Preferably, the iron content in the sludge powder is 1-5%, and the aluminum content is 10-15%.

[0013] Preferably, the mass ratio of the sludge powder to melamine is 1-10:1.

[0014] Preferably, the mass ratio of the molten salt medium to the mixed material precursor is 0.5-5:1.

[0015] Preferably, the melting point or eutectic point of the molten salt medium is ≤810℃; the molten salt medium includes one or more of carbonates, chlorides, nitrates and sulfates.

[0016] When the molten salt medium is a single-component salt, its melting point should not exceed 810°C; when the molten salt medium is a mixture of multiple salts, the eutectic point of the mixed salts should not exceed 810°C. More preferably, the molten salt medium is a chemically stable salt that does not decompose in the range of 400-800°C.

[0017] Preferably, the carbonate includes one or more of Li₂CO₃, Na₂CO₃, K₂CO₃, and CaCO₃; the chloride salt includes one or more of LiCl, NaCl, KCl, CaCl₂, and MgCl₂; the nitrate includes one or more of LiNO₃, NaNO₃, KNO₃, and Ca(NO₃)₂; and the sulfate includes two or more of Li₂SO₄, Na₂SO₄, K₂SO₄, and CaSO₄.

[0018] Preferably, the pyrolysis treatment is carried out under a protective atmosphere, with the temperature increased to 400-800°C at a rate of 10°C / min, and then held at that temperature for 2 hours.

[0019] Preferably, the protective atmosphere is a nitrogen atmosphere.

[0020] The second technical solution of the present invention provides a g-C3N4 / biochar prepared according to the above method.

[0021] The third technical solution of the present invention provides an application of the above-mentioned g-C3N4 / biochar in the field of photocatalytic degradation of organic dyes.

[0022] Preferably, the organic dye is Rhodamine B (RhB).

[0023] The fourth technical solution of the present invention provides an application of the above-mentioned g-C3N4 / biochar in the field of photocatalytic decomposition for hydrogen production.

[0024] The beneficial technical effects of the present invention are as follows:

[0025] (1) The preparation process of the present invention is simple, low in cost and has an ideal yield.

[0026] (2) This invention uses sewage sludge as a carbon source to combine biochar with g-C3N4, which can increase the specific surface area of ​​the material, promote electron transfer, and thus improve photocatalytic activity.

[0027] (3) Due to the addition of flocculants in sewage treatment plants, the sewage sludge is rich in iron. At the same time, the sewage sludge contains solid particles such as mud and sand mixed in with domestic sewage or industrial wastewater, which makes the sewage sludge rich in aluminum. The doping of iron and aluminum is intended to expand the photoresponse range of g-C3N4 and thus improve its photocatalytic performance.

[0028] (4) The molten salt in this invention can provide a unique liquid-phase catalytic reaction environment. The use of molten salt pyrolysis is conducive to the development of pores in biochar, which promotes the formation of a large number of interconnected pores, thereby providing more active sites for photocatalytic reactions.

[0029] (5) Li in molten salt + Na + K+ Ca 2+ Doping with metal ions can further modulate the band gap of g-C3N4 / biochar, which is more conducive to improving photocatalytic activity, thereby increasing hydrogen production efficiency and the degradation rate of pollutants.

[0030] (6) The photocatalytic material prepared by this invention has a narrow band gap and a large visible light absorption range, and has excellent photocatalytic activity. At the same time, it uses sewage sludge as a carbon source, realizing the rational utilization of waste. Attached Figure Description

[0031] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0032] Figure 1 The images show the XRD patterns of the products of Comparative Examples 1-3 and Example 3. Among them, (a) is Comparative Example 1, (b) is Comparative Example 2, (c) is Comparative Example 3, and (d) is Example 3.

[0033] Figure 2 The UV-Vis diffuse reflectance spectra of the products of Comparative Examples 1-3 and Example 3 are shown.

[0034] Figure 3 The graph shows the photocatalytic hydrogen production performance of the products of Comparative Examples 1-3 and Example 3.

[0035] Figure 4 The graph shows the photocatalytic degradation performance of RhB by the products of Comparative Examples 1-3 and Example 3. Detailed Implementation

[0036] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention. It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the present invention.

[0037] Furthermore, regarding the numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Any stated value or intermediate value within a stated range, as well as each smaller range between any other stated value or intermediate value within said range, are also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0038] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar to or equivalent to those described herein may be used in the implementation or testing of this invention.

[0039] The terms “comprising,” “including,” “having,” “containing,” etc., used in this invention are all open-ended terms, meaning that they include but are not limited to.

[0040] The sewage sludge used in the following embodiments and comparative examples of the present invention is the residual sludge from the Huangxiaohe Sewage Treatment Plant; the iron content in the sludge powder is 4.2%, and the aluminum content is 11.5%.

[0041] Unless otherwise specified, "room temperature" in this invention refers to 10-30°C.

[0042] All raw materials used in the following embodiments and comparative examples of the present invention are commercially available products.

[0043] Example 1

[0044] A method for preparing g-C3N4 / biochar from molten salt pyrolysis of sewage sludge, comprising the following steps:

[0045] (1) Preparation of hybrid material precursors:

[0046] Sewage sludge was dried at 80℃ to constant weight, and then sieved through a 200-mesh sieve to obtain sludge powder; the sludge powder was mixed with melamine at a mass ratio of 10:1 to obtain a mixed material precursor.

[0047] (2) Preparation of g-C3N4 biochar photocatalytic material:

[0048] Li2CO3, Na2CO3, and K2CO3 were mixed in a molar ratio of 50.5:27.5:22 to prepare a molten salt medium. The molten salt medium and the mixed material precursor were weighed at a mass ratio of 0.5:1. The molten salt medium and the mixed material precursor were added to a crucible and wrapped with aluminum foil. The crucible was placed in a tube furnace and heated to 700℃ at 10℃ / min under a nitrogen atmosphere. The temperature was maintained at 700℃ for 2 hours, then allowed to cool naturally, washed with water, and dried at 80℃ for 24 hours to obtain g-C3N4 / biochar.

[0049] Example 2

[0050] A method for preparing g-C3N4 / biochar from molten salt pyrolysis of sewage sludge, comprising the following steps:

[0051] (1) Preparation of hybrid material precursors:

[0052] Sewage sludge was dried at 80℃ to constant weight, and then sieved through a 200-mesh sieve to obtain sludge powder; the sludge powder was mixed with melamine at a mass ratio of 10:1 to obtain a mixed material precursor.

[0053] (2) Preparation of g-C3N4 biochar photocatalytic material:

[0054] Li2CO3, Na2CO3, and K2CO3 were mixed in a molar ratio of 50.5:27.5:22 to prepare a molten salt medium. The molten salt medium and the mixed material precursor were weighed at a mass ratio of 0.5:1. The molten salt medium and the mixed material precursor were added to a crucible and wrapped with aluminum foil. The crucible was placed in a tube furnace and heated to 400℃ at 10℃ / min under a nitrogen atmosphere. The temperature was maintained at 400℃ for 2 hours, then allowed to cool naturally, washed with water, and dried at 80℃ for 24 hours to obtain g-C3N4 / biochar.

[0055] Example 3

[0056] A method for preparing g-C3N4 / biochar from molten salt pyrolysis of sewage sludge, comprising the following steps:

[0057] (1) Preparation of hybrid material precursors:

[0058] Sewage sludge was dried at 80℃ to constant weight, and then sieved through a 200-mesh sieve to obtain sludge powder; the sludge powder was mixed with melamine at a mass ratio of 10:1 to obtain a mixed material precursor.

[0059] (2) Preparation of g-C3N4 biochar photocatalytic material:

[0060] Li2CO3, Na2CO3, and K2CO3 were mixed in a molar ratio of 50.5:27.5:22 to prepare a molten salt medium. The molten salt medium and the mixed material precursor were weighed at a mass ratio of 5:1. The molten salt medium and the mixed material precursor were added to a crucible and wrapped with aluminum foil. The crucible was placed in a tube furnace and heated to 700℃ at 10℃ / min under a nitrogen atmosphere. The temperature was maintained at 700℃ for 2 hours, then allowed to cool naturally, washed with water, and dried at 80℃ for 24 hours to obtain g-C3N4 / biochar.

[0061] Example 4

[0062] A method for preparing g-C3N4 / biochar from molten salt pyrolysis of sewage sludge, comprising the following steps:

[0063] (1) Preparation of hybrid material precursors:

[0064] Sewage sludge was dried at 80℃ to constant weight, and then sieved through a 200-mesh sieve to obtain sludge powder; the sludge powder was mixed with melamine at a mass ratio of 10:1 to obtain a mixed material precursor.

[0065] (2) Preparation of g-C3N4 biochar photocatalytic material:

[0066] LiCl, NaCl, and CaCl2 were mixed in a molar ratio of 47.5:15:37.5 to prepare a molten salt medium. The molten salt medium and the mixed material precursor were weighed at a mass ratio of 5:1. The molten salt medium and the mixed material precursor were added to a crucible and wrapped with aluminum foil. The crucible was placed in a tube furnace and heated to 700℃ at a rate of 10℃ / min under a nitrogen atmosphere. The temperature was maintained at 700℃ for 2 hours, then allowed to cool naturally, washed with water, and dried at 80℃ for 24 hours to obtain g-C3N4 / biochar.

[0067] Example 5

[0068] A method for preparing g-C3N4 / biochar from molten salt pyrolysis of sewage sludge, comprising the following steps:

[0069] (1) Preparation of hybrid material precursors:

[0070] Sewage sludge was dried at 80℃ to constant weight, and then sieved through a 200-mesh sieve to obtain sludge powder; the sludge powder was mixed with melamine at a mass ratio of 1:1 to obtain a mixed material precursor.

[0071] (2) Preparation of g-C3N4 biochar photocatalytic material:

[0072] Li2CO3, Na2CO3, and K2CO3 were mixed in a molar ratio of 50.5:27.5:22 to prepare a molten salt medium. The molten salt medium and the mixed material precursor were weighed at a mass ratio of 5:1. The molten salt medium and the mixed material precursor were added to a crucible and wrapped with aluminum foil. The crucible was placed in a tube furnace and heated to 700℃ at 10℃ / min under a nitrogen atmosphere. The temperature was maintained at 700℃ for 2 hours, then allowed to cool naturally, washed with water, and dried at 80℃ for 24 hours to obtain g-C3N4 / biochar.

[0073] Comparative Example 1

[0074] A method for preparing g-C3N4 photocatalytic material, comprising the following steps:

[0075] Weigh 5g of melamine and add it to a 30mL crucible. Cover the crucible with a lid and wrap it with aluminum foil. Place the crucible in a muffle furnace and heat it from room temperature to 700℃ at a heating rate of 10℃ / min. Hold the temperature at 700℃ for 2 hours. After cooling to room temperature with the furnace, pure g-C3N4 photocatalytic material can be obtained.

[0076] Comparative Example 2 (molten salt medium omitted)

[0077] A method for preparing g-C3N4 / biochar photocatalytic material, comprising the following steps:

[0078] (1) Preparation of hybrid material precursors:

[0079] Sewage sludge was dried at 80℃ to constant weight, and then sieved through a 200-mesh sieve to obtain sludge powder; the sludge powder was mixed with melamine at a mass ratio of 10:1 to obtain a mixed material precursor.

[0080] (2) Preparation of g-C3N4 / biochar photocatalytic material:

[0081] The mixed material precursor was added to a crucible and wrapped with aluminum foil. The crucible was placed in a tube furnace and heated to 700°C at 10°C / min under a nitrogen atmosphere. The temperature was then maintained at 700°C for 2 hours, followed by natural cooling, washing with water, and drying at 80°C for 24 hours to obtain g-C3N4 / biochar photocatalytic material.

[0082] Comparative Example 3 (replacing sewage sludge with peanut shells)

[0083] A method for preparing g-C3N4 / biochar photocatalytic material by molten salt pyrolysis of peanut shells, comprising the following steps:

[0084] (1) Preparation of hybrid material precursors:

[0085] The purchased peanut shells were washed with water and dried at 80℃ to constant weight. After being crushed, they were sieved through a 200-mesh sieve to obtain peanut shell powder. The peanut shell powder was then mixed with melamine at a mass ratio of 10:1 to obtain a mixed material precursor.

[0086] (2) Preparation of g-C3N4 / biochar photocatalytic material:

[0087] Li2CO3, Na2CO3, and K2CO3 were mixed in a molar ratio of 50.5:27.5:22 to prepare a molten salt medium. The molten salt medium and the mixed material precursor were weighed at a mass ratio of 5:1. The molten salt medium and the mixed material precursor were added to a crucible and wrapped with aluminum foil. The crucible was placed in a tube furnace and heated to 700℃ at 10℃ / min under a nitrogen atmosphere. The temperature was maintained at 700℃ for 2 hours, then allowed to cool naturally, washed with water, and dried at 80℃ for 24 hours to obtain g-C3N4 / biochar.

[0088] Effect verification

[0089] 1. Figure 1The images show the XRD patterns of the products of Comparative Examples 1-3 and Example 3. Among them, (a) is Comparative Example 1, (b) is Comparative Example 2, (c) is Comparative Example 3, and (d) is Example 3.

[0090] Depend on Figure 1 As can be seen, the peak at 22.2° in the g-C3N4 / biochar prepared under nitrogen atmosphere (Comparative Example 2), g-C3N4 / biochar prepared by molten salt pyrolysis of peanut shells (Comparative Example 3), and g-C3N4 / biochar prepared by molten salt pyrolysis of sewage sludge (Example 3) may be a characteristic peak of amorphous carbon, indicating that biochar has been incorporated into g-C3N4; all samples have a strong peak at 27.5°, which is consistent with the diffraction angle of pure graphitic carbon nitride, indicating that the prepared photocatalyst has the same crystal form as pure graphitic carbon nitride, proving that the biochar modification process of the present invention has not damaged its basic structure.

[0091] 2. Figure 2 The UV-Vis diffuse reflectance spectra of the products of Comparative Examples 1-3 and Example 3 are shown.

[0092] Depend on Figure 2 As can be seen, the absorption boundary of pure g-C3N4 (Comparative Example 1) can be calculated to be 474 nm using the intercept method, with a corresponding band gap of 2.62 eV. The absorption boundary of g-C3N4 / biochar prepared under nitrogen atmosphere (Comparative Example 2) extends to the visible light region of about 498 nm, with a corresponding band gap of 2.49 eV, indicating that biochar modification enhances the catalyst's response range to visible light, but it is still relatively small. The absorption boundary of g-C3N4 / biochar prepared by molten salt pyrolysis of peanut shells (Comparative Example 3) extends to the visible light region of about 510 nm, with a corresponding band gap of 2.43 eV. The absorption boundary of g-C3N4 / biochar prepared by molten salt pyrolysis of sewage sludge (Example 3) further extends to about 523 nm, with a corresponding band gap of 2.37 eV, indicating that the use of molten salt pyrolysis to prepare g-C3N4 / biochar can further regulate the band gap of the composite material and expand its photoresponse range.

[0093] 3. The photocatalytic hydrogen production performance was evaluated using the following method: Pt was used as a co-catalyst, and triethanolamine as a sacrificial agent, in a photocatalytic water splitting hydrogen production system. 50 mg of the prepared photocatalyst was added to a solution of 90.0 mL deionized water and 10 mL triethanolamine, followed by the addition of H₂PtCl₆·6H₂O (3% by mass of Pt). A vacuum was applied before the test, and cooling water was kept on throughout the test to maintain the reaction system at room temperature. The light source (300W xenon lamp, filter λ>420 nm) was turned on to conduct the photocatalytic reaction. The hydrogen produced was measured using online gas chromatography. The test results are as follows: Figure 3 As shown.

[0094] Figure 3 The graph shows the photocatalytic hydrogen production performance of the products of Comparative Examples 1-3 and Example 3.

[0095] Depend on Figure 3 As can be seen, under visible light, the rate of hydrogen production from water splitting of the g-C3N4 / biochar photocatalyst prepared by molten salt pyrolysis reaches 416 μmol / h / g, which is higher than the hydrogen production rate of Comparative Examples 1, 2, and 3. This indicates that the g-C3N4 / biochar photocatalyst prepared by molten salt pyrolysis of the present invention has higher visible light activity for hydrogen production from water splitting.

[0096] 4. The photocatalytic degradation performance of RhB was evaluated. The specific test method was as follows: 50 mg of the prepared photocatalyst was added to 50 mL of a prepared RhB solution (RhB concentration of 10 mg / L), and the reaction was carried out in the dark for 30 minutes to allow the photocatalyst to reach adsorption-desorption equilibrium with RhB in the solution. Then, the solution was placed under a light source (300W xenon lamp, filter λ>420 nm) and magnetically stirred to carry out the photocatalytic reaction. The absorbance of the test solution at 552 nm (the wavelength of the maximum characteristic peak of the RhB solution) was measured using a UV-Vis spectrophotometer to detect the change in RhB concentration. The test results are as follows: Figure 4 As shown.

[0097] Figure 4 The graph shows the photocatalytic degradation performance of RhB by the products of Comparative Examples 1-3 and Example 3.

[0098] Depend on Figure 4 It is evident that, under visible light, the g-C3N4 / biochar photocatalytic material prepared by molten salt pyrolysis can completely degrade RhB solution within 80 min, with degradation efficiency higher than that of pure g-C3N4 (Comparative Example 1), g-C3N4 / biochar prepared under nitrogen atmosphere (Comparative Example 2), and g-C3N4 / biochar prepared by molten salt pyrolysis of peanut shells (Comparative Example 3).

[0099] The embodiments described above are merely preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Various modifications and improvements made by those skilled in the art to the technical solutions of the present invention without departing from the spirit of the present invention should fall within the protection scope defined by the claims of the present invention.

Claims

1. A method for preparing g-C3N4 / biochar from molten salt pyrolysis of sewage sludge, characterized in that, Includes the following steps: The sewage sludge is dried to obtain sludge powder; The sludge powder and melamine were mixed to obtain a mixed material precursor, which was then mixed with a molten salt medium and subjected to pyrolysis to obtain the g-C3N4 / biochar. The iron content in the sludge powder is 1-5%, and the aluminum content is 10-15%.

2. The method according to claim 1, characterized in that, The mass ratio of the sludge powder to melamine is 1-10:

1.

3. The method according to claim 1, characterized in that, The mass ratio of the molten salt medium to the mixed material precursor is 0.5-5:

1.

4. The method according to claim 1, characterized in that, The melting point or eutectic point of the molten salt medium is ≤810℃; the molten salt medium includes one or more of carbonates, chlorides, nitrates and sulfates.

5. The method according to claim 4, characterized in that, The carbonate includes one or more of Li₂CO₃, Na₂CO₃, K₂CO₃, and CaCO₃; the chloride salt includes one or more of LiCl, NaCl, KCl, CaCl₂, and MgCl₂; the nitrate includes one or more of LiNO₃, NaNO₃, KNO₃, and Ca(NO₃)₂; and the sulfate includes two or more of Li₂SO₄, Na₂SO₄, K₂SO₄, and CaSO₄.

6. The method according to claim 1, characterized in that, The pyrolysis treatment involves heating the temperature to 400-800℃ at a rate of 10℃ / min under a protective atmosphere, and then holding the temperature for 2 hours.

7. A g-C3N4 / biochar prepared by the method according to any one of claims 1-6.

8. The application of the g-C3N4 / biochar of claim 7 in the field of photocatalytic degradation of organic dyes.

9. The application of the g-C3N4 / biochar according to claim 7 in the field of photocatalytic decomposition for hydrogen production.

Images