A composite catalyst with photocatalytic switching properties and its preparation method and application

Abstract

This invention belongs to the technical field of photoresponsive catalysts and specifically discloses a composite catalyst with photocatalytic switching properties, its preparation method, and its application. The invention involves thermally polycondensing dicyandiamide to produce graphite-phase carbon nitride. Vanadium pentoxide and the graphite-phase carbon nitride are then mixed and reacted to produce M-phase vanadium dioxide-modified graphite-phase carbon nitride, a composite catalyst with photocatalytic switching properties. The composite catalyst prepared by the invention can reduce the presence of vanadium oxide impurities, shorten the preparation process to a single step, and reduce production costs.

Description

Technical Field

[0001] The present invention relates to the technical field of photoresponsive catalysts, and in particular to a composite catalyst with photocatalytic switching properties, a preparation method thereof, and applications thereof. Background Art

[0002] Integrating noble metal nanoparticles with semiconductor photocatalysts can enhance the photocatalytic activity in the visible light region. However, noble metals are expensive, and some nanoparticles have unstable physicochemical properties. They are easily oxidized in complex and variable temperature and pH environments, which in turn affects the performance and lifespan of the composite photocatalyst. However, materials exhibiting surface plasmon resonance (SPR) are not limited to pure metals. Vanadium dioxide (VO2) has attracted widespread attention for its unique property of reversibly transforming from a semiconductor (M phase, space group P21 / c) to a metal (R phase, space group P42 / mnm) at 68°C. This reversible metal-semiconductor transition (MST) of VO2 alters its optical and electrical physicochemical properties. The semiconducting and metallic properties of VO2 can form distinct heterostructures with the photocatalyst, thereby altering the carrier flow in the system. More importantly, the SPR effect exhibited by the metallic R phase of VO2 also contributes to enhanced photocatalytic performance. However, common VO2 preparation and compounding methods are not suitable for large-scale production applications.

[0003] There are many types of vanadium oxides, including V2O3, VO2, V6O 13 , V3O7, V2O5, V5O9 and other 13 types. It is very difficult to accurately locate and prepare VO2 among the complex oxide types without generating other oxides. Even if VO2 is successfully prepared, the different VO2 structures that have been discovered include VO2(A), VO2(B), VO2(P), VO2(D), VO2(M), VO2(R) and other phases. They are typical multi-structure binary compounds, and the differences in structure make the properties of these crystal phases very different. Therefore, it is necessary to regulate the specific crystal form of VO2 to improve the physical and chemical properties of its materials. At present, the crystal form of the product is generally adjusted by controlling factors such as the concentration, temperature, and time of the reactants. The commonly used preparation methods include thermal decomposition, sol-gel method, chemical vapor deposition, reactive sputtering and hydrothermal method, which can only obtain VO2-B phase. If VO2 with reversible phase change properties is to be obtained, tedious additional treatment must be performed. The VO2 prepared by the above method will further undergo multiple steps of hydrothermal, ball milling or heat treatment in the process of forming a composite structure with other substances. The redox reactions occurring in this process will seriously affect the purity of VO2, thereby reducing the use effect and life of the target photocatalytic material.

[0004] Photocatalysts are commonly used in wastewater degradation, hydrogen and oxygen production, CO2 reduction, and organic synthesis reactions. In these industries, the onset and shutdown of photocatalysis is typically controlled by turning off the lights. However, this method consumes a lot of energy. In catalytic systems powered by sunlight, the instability of sunlight also hinders the stable and widespread application of photocatalysis. Furthermore, when photocatalytic agents are used as coatings and in conjunction with other thin-film products, highly oxidizing photocatalytic overproducts can affect the film's service life. Therefore, it is necessary to further improve the efficiency of photocatalysis to ensure that catalysis is completed within the working range and to maintain catalytic inertness during the unneeded phases to protect other materials, reduce the impact on other functional processes, and increase catalytic flexibility.

[0005] Graphitic carbon nitride (g-C3N4), a catalyst composed primarily of carbon, hydrogen, nitrogen, and a small amount of oxygen, is widely studied as a promising photocatalyst due to its ease of preparation, non-toxicity, low cost, and high catalytic performance. However, its catalytic performance remains insufficient for a wide range of applications. Further efforts to increase its degree of polymerization and construct composite structures could unlock its catalytic potential. Summary of the Invention

[0006] In view of this, the present invention provides a composite catalyst with photocatalytic switching properties, a preparation method and application thereof, in order to solve the problems that the existing VO2-M@g-C3N4 preparation steps are complicated and require the use of toxic organic solvents, as well as the problems that the existing photocatalysts have poor catalytic effects, require light or sunlight regulation, and have high energy consumption or instability.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions:

[0008] A method for preparing a composite catalyst having photocatalytic switching properties comprises the following steps:

[0009] 1) subjecting dicyandiamide to thermal polycondensation to obtain graphite-phase carbon nitride;

[0010] 2) mixing vanadium pentoxide with graphite-phase carbon nitride and reacting them to obtain M-phase vanadium dioxide-modified graphite-phase carbon nitride;

[0011] The M-phase vanadium dioxide-modified graphite-phase carbon nitride is a composite catalyst with photocatalytic switching properties.

[0012] Preferably, the thermal polycondensation in step 1) is first maintained at 500-510° C. for 3.5-4 hours, and then heated to 520-530° C. and maintained for 0.4-0.6 hours.

[0013] Preferably, the mass ratio of vanadium pentoxide to graphite carbon nitride in step 2) is 1:9.5-10.5.

[0014] Preferably, the reaction temperature in step 2) is 550-570° C., the reaction time is 1.8-2.2 h, and the reaction pressure is 10-25 Pa.

[0015] Another object of the present invention is to provide a composite catalyst having photocatalytic switching properties prepared by the above preparation method.

[0016] Another object of the present invention is to provide a composite catalyst with photocatalytic switching properties for use in surface coating / film of smart glass, degradation of printing and dyeing wastewater, or photolysis of water to produce hydrogen.

[0017] It can be seen from the above technical solutions that compared with the prior art, the present invention has the following beneficial effects:

[0018] 1. The composite catalyst with photocatalytic switching properties prepared by the present invention is recorded as VO2-M@g-C3N4. The advantages of VO2-M@g-C3N4 preparation include: conventional methods (such as hydrothermal method, sol-gel method, etc.) for preparing VO2 composite system can only obtain a composite material of VO2 B phase and carbon nitride. This process usually requires a long reaction time or the use of toxic organic solvents, and is accompanied by the generation of impurities such as V2O3, V6O 13 The resulting VO2-B@g-C3N4 requires a phase transition treatment at temperatures above 450°C in a high vacuum or inert gas atmosphere to achieve the M-phase VO2 with phase transition properties. This invention precisely controls the mass ratio of g-C3N4 to V2O5, minimizing the presence of vanadium oxide impurities and reducing the vacuum requirement. This shortens the typical two- to three-step composite process to a single step, reducing production costs.

[0019] 2. The present invention uniformly mixes V2O5 with graphite phase carbon nitride and performs heat treatment under a weak vacuum environment at a specific temperature. During the treatment process disclosed by the present invention, carbon nitride can uniformly and gently generate a reducing atmosphere to convert V 5+ Restore to V 4+ , and under the reaction conditions of the present invention, a weak vacuum and low oxygen environment will be continuously maintained, which prevents the generated VO2 from being further reduced to products such as V2O3. Moreover, under the reaction temperature disclosed in the present invention, the most stable phase of VO2 is the R phase. After cooling to room temperature, a single VO2-M can be obtained, thereby ensuring the uniformity, singleness and mildness of VO2 generation.

[0020] 3. The VO2-M@g-C3N4 composite catalytic material prepared in this invention addresses water pollution issues in industry and daily life, such as inorganic and microbial contamination. Major sources include industrial wastewater, the use of pesticides and fertilizers, and oil spills, which lead to the presence of various organic compounds such as benzene, phenols, pesticides, and organic solvents in water. Furthermore, when combined with a phosphorescent material, this composite material can mitigate the unevenness of solar radiation across regions, time of year, and climate, achieving long-term, stable, and efficient catalysis.

[0021] As a surface coating / film for smart glass, it simultaneously provides temperature control and cleaning functions. First, in humid environments (such as rainy and foggy days), the composite photocatalyst film can clean the glass surface. In dry, high-temperature environments, VO2 transforms from a low-temperature semiconductor state to a high-temperature metallic state. Its infrared light resistance also changes from high transmittance at low temperatures to high resistance at high temperatures. The transmittance in the visible light region remains almost unchanged, while the resistance to ultraviolet light remains high. Therefore, by attaching VO2 material to the glass surface, solar heat can be controlled in different seasons, achieving an energy-saving mode of warm in winter and cool in summer.

[0022] For photocatalytic water splitting to produce hydrogen: The production process can be adjusted in stages. When VO2 is metallic, it assists the main catalyst in rapidly and efficiently producing hydrogen, while the M phase inhibits the catalytic effect. At night, a narrow-wavelength cold light source or phosphorescent material that corresponds to the SPR effect of the VO2 metallic phase can be used as a light source for photocatalysis, greatly extending the service life, flexibility, net operating time, and efficiency of the composite photocatalyst. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are merely embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on the provided drawings without paying any creative work.

[0024] Figure 1 XRD pattern (Intensity, Theta) of the VO2-M@g-C3N4 composite catalyst prepared in Example 1 of the present invention;

[0025] Figure 2 XRD patterns (Intensity, Theta) of graphite carbon nitride obtained in Example 1 and Comparative Example 1 of the present invention;

[0026] Figure 3 The photodegradation curves of the graphite carbon nitride obtained in Example 1 and Comparative Example 1 of the present invention undergoing photocatalytic reaction;

[0027] Figure 4 The photodegradation curves of the composite catalyst prepared in Example 1 of the present invention during the photocatalytic reaction at different temperatures. DETAILED DESCRIPTION

[0028] The present invention provides a method for preparing a composite catalyst having photocatalytic switching properties, comprising the following steps:

[0029] 1) subjecting dicyandiamide to thermal polycondensation to obtain graphite-phase carbon nitride;

[0030] 2) Mixing vanadium pentoxide with graphite-phase carbon nitride and reacting them to obtain M-phase vanadium dioxide-modified graphite-phase carbon nitride, which is recorded as VO2-M@g-C3N4;

[0031] The M-phase vanadium dioxide-modified graphite-phase carbon nitride is a composite catalyst with photocatalytic switching properties.

[0032] In the present invention, the thermal polycondensation in step 1) is specifically first maintained at 500-510°C for 3.5-4h, and the holding temperature can be specifically 502°C, 504°C, 505°C, 506°C, and 508°C, and the holding time can be specifically 3.6h, 3.7h, 3.8h, and 3.9h; then the temperature is raised to 520-530°C and maintained for 0.4-0.6h, and the holding temperature can be specifically 522°C, 524°C, 525°C, 526°C, and 528°C, and the holding time can be specifically 0.42h, 0.45h, 0.48h, 0.5h, 0.52h, 0.55h, and 0.58h.

[0033] In the present invention, the temperature difference during the thermal polycondensation process of preparing graphite phase carbon nitride will greatly affect the yield of carbon nitride. In the same time (4 hours as an example), the carbon nitride yield is 67.7% when the thermal polycondensation temperature is 500°C in an air atmosphere, and 26.4% at 520°C; however, the carbon nitride obtained by thermal polycondensation at 520°C has a higher lattice integrity. The present invention chooses to continue heating to 520-530°C and continue to keep warm after calcination at 500-510°C, which can retain a higher yield of about 51.2% while obtaining carbon nitride with a higher degree of polycondensation.

[0034] In the present invention, the mixing in step 2) is preferably performed by first crushing the obtained graphite phase carbon nitride and then mixing it with vanadium pentoxide.

[0035] In the present invention, the mass ratio of vanadium pentoxide to graphite carbon nitride in step 2) is 1:9.5-10.5, preferably 1:9.8-10.2, and more preferably 1:10.

[0036] In the present invention, by accurately controlling the mass ratio of g-C3N4 to V2O5, the appearance of vanadium oxide impurities is reduced, and the vacuum requirement is weakened.

[0037] In the present invention, the reaction temperature in step 2) is 550-570°C, specifically 552°C, 555°C, 558°C, 560°C, 562°C, 565°C, 568°C; the reaction time is 1.8-2.2h, specifically 1.9h, 2h, 2.1h; the reaction pressure is 10-25Pa, specifically 12Pa, 14Pa, 15Pa, 16Pa, 18Pa, 20Pa, 22Pa, 24Pa.

[0038] The present invention also provides a composite catalyst having photocatalytic switching properties prepared by the above preparation method.

[0039] The present invention also provides an application of a composite catalyst with photocatalytic switching properties in surface coating / film of smart glass, degradation of printing and dyeing wastewater, or photolysis of water to produce hydrogen.

[0040] The following is a clear and complete description of the technical solutions in the embodiments of the present invention. Obviously, the embodiments described are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making any creative efforts are within the scope of protection of the present invention.

[0041] Example 1

[0042] 6g of dicyandiamide (DICY, C2H4N4) was placed in a clean porcelain boat and kept evenly distributed. The boat was then placed in a tube furnace and subjected to thermal polycondensation in an air atmosphere. The temperature was increased from room temperature to 500°C at a rate of 10°C / min and held for 4 hours. The temperature was then increased to 520°C and held for 0.5 hours. The sample was then cooled to room temperature in the furnace to obtain a graphite-phase carbon nitride (g-C3N4). The original carbon nitride was a blocky solid. After grinding into a fine and uniform powder, it was washed sequentially with deionized water, anhydrous ethanol, and ethylene glycol as cleaning solvents. The cleaned powder was placed in a vacuum oven and dried at 70°C for 6 hours to obtain graphite-phase carbon nitride (g-C3N4) with a yield of 58.89%.

[0043] 0.12 g of vanadium pentoxide (V2O5) and 1.2 g of g-C3N4 powder were uniformly mechanically mixed (mass ratio V2O5:g-C3N4=1:10) and evenly spread into a porcelain boat. Under a vacuum atmosphere of 10 Pa, the temperature was raised from room temperature to 560°C at a rate of 10°C / min and kept for 2 h. Then, the mixture was cooled to room temperature with the furnace to obtain M-phase VO2-modified g-C3N4 (VO2-M@g-C3N4).

[0044] The obtained VO2-M@g-C3N4 was subjected to XRD detection, and the XRD spectrum was as follows Figure 1 As shown, the characteristic peak of VO2 is consistent with the standard card JCPDS#01-072-0514, which can determine that the material obtained in the present invention is VO2-M.

[0045] Comparative Example 1

[0046] The difference between this comparative example and Example 1 is that the operation of heating the temperature to 520° C. and holding it for 0.5 h during the thermal polycondensation is omitted, thereby obtaining graphite-phase carbon nitride with a yield of 67.7%.

[0047] The g-C3N4 obtained in Example 1 and Comparative Example 1 were subjected to XRD detection (Example 1 was recorded as 500-5204+0.5h, Comparative Example 1 was recorded as 500-4h), and the detection results were as follows: Figure 2 As shown, through Figure 2 It can be seen that the (002) peak representing the order between the graphite phase carbon nitride layers is the highest peak and is also an important indicator for judging the degree of polymerization of g-C3N4. From the PDF standard card (JCPDS#01-087-1526), it can be seen that the peak is located at 26.505°. By comparison, it can be seen that the g-C3N4 polymerized under the conditions of Example 1 has a higher degree of polymerization.

[0048] Then, the graphite phase carbon nitride of Example 1 and Comparative Example 1 was used as catalyst, and a photocatalytic test of Rhodamine B was carried out for 2 hours under 450nm light. The photodegradation curve of the catalytic reaction is shown in FIG. Figure 3 As shown, through Figure 3 It can be seen that the VO2-M@g-C3N4 catalyst obtained in Example 1 of the present invention has higher photocatalytic efficiency.

[0049] Photocatalytic test conditions: The mass of graphite carbon nitride is 35 mg, which is poured into 35 mL of 5×10 -5 mol / L dye (Rhodamine B, RhB) solution. The detection system was placed on a magnetic stirring table with continuous stirring, first stirred for 30 minutes in a dark environment to allow the system to reach adsorption and desorption equilibrium, and then subjected to a 2-hour photodegradation test under 50W 365nm (±5nm) LED cold light irradiation. 3mL of the suspension was taken out every 20 minutes, centrifuged, and the supernatant was taken for absorption spectrum detection using a UV-vis spectrophotometer. The concentration of the dye was determined by monitoring the change in the maximum absorbance intensity of different dye solutions. Using the Beer-Lambert method, the absorbance is proportional to the concentration of the diluted solution, from which the photodegradation percentage can be calculated using the following formula:

[0050]

[0051] Where η represents the degradation rate, C0 is the initial concentration of the RhB solution, and C is the concentration at a given irradiation time. The degradation percentage is plotted as a photodegradation curve. After each test, the sample is returned to the photodegradation system.

[0052] Example 2

[0053] 6g of dicyandiamide (DICY, C2H4N4) was placed in a clean porcelain boat and kept evenly distributed. The boat was then placed in a tube furnace and subjected to thermal polycondensation in an air atmosphere. The temperature was raised from room temperature to 500°C at a rate of 10°C / min and held for 4 hours. The temperature was then raised to 520°C and held for 0.6 hours. The sample was then cooled to room temperature in the furnace to obtain a graphite-phase carbon nitride (g-C3N4). The original carbon nitride was a blocky solid. After grinding into a fine and uniform powder, it was washed sequentially with deionized water, anhydrous ethanol, and ethylene glycol. The cleaned powder was placed in a vacuum oven and dried at 70°C for 6 hours to obtain graphite-phase carbon nitride (g-C3N4) with a yield of 56.16%.

[0054] 0.117 g of vanadium pentoxide (V2O5) and 1.2 g of g-C3N4 powder were uniformly mechanically mixed (mass ratio V2O5:g-C3N4=1:10.2) and evenly spread into a porcelain boat. Under a vacuum atmosphere of 10 Pa, the temperature was raised from room temperature to 570°C at a rate of 10°C / min and kept for 2 h. Then, the mixture was cooled to room temperature with the furnace to obtain M-phase VO2-modified g-C3N4 (VO2-M@g-C3N4).

[0055] Example 3

[0056] 6g of dicyandiamide (DICY, C2H4N4) was placed in a clean porcelain boat and kept evenly distributed. The boat was then placed in a tube furnace and subjected to thermal polycondensation in an air atmosphere. The temperature was raised from room temperature to 508°C at a rate of 10°C / min and held for 4 hours. The temperature was then raised to 520°C and held for 0.45 hours. The sample was then cooled to room temperature in the furnace to obtain a graphite-phase carbon nitride (g-C3N4). The original carbon nitride was a blocky solid. After grinding into a fine and uniform powder, it was washed sequentially with deionized water, anhydrous ethanol, and ethylene glycol. The washed powder was placed in a vacuum oven and dried at 70°C for 6 hours to obtain graphite-phase carbon nitride (g-C3N4) with a yield of 61.2%.

[0057] 0.12 g of vanadium pentoxide (V2O5) and 1.14 g of g-C3N4 powder were uniformly mechanically mixed (mass ratio V2O5:g-C3N4=1:9.5) and evenly spread into a porcelain boat. In a vacuum atmosphere of 20 Pa, the temperature was raised from room temperature to 560°C at a rate of 10°C / min, kept for 2.2 h, and then cooled to room temperature in the furnace to obtain M-phase VO2-modified g-C3N4 (VO2-M@g-C3N4).

[0058] Experimental Example 1

[0059] In order to measure the photocatalytic degradation of RhB aqueous solution with and without visible light irradiation, VO2-M@g-C3N4 prepared in Example 1 was weighed as a photocatalyst and added together with 0.35 g of strontium aluminate long afterglow luminescent powder to 35 mL of a 5×10 -5 mol / L dye solution. The resulting suspension was magnetically stirred in a dark photochemical reaction box for 30 minutes to ensure the adsorption and desorption equilibrium between the dye and the photocatalyst. In order to carry out photocatalytic degradation with long afterglow as the light source, a 365nm (±5nm) LED light source was used for 20 minutes to achieve a stable photocatalytic degradation reaction. At the same time, it will ensure that the long afterglow photoluminescent agent SrAl2O4:Eu 2+ , Dy 3+ It can capture and store excitation / light energy for continuous photocatalytic degradation reactions in a sunless environment. In addition, 3 mL of the suspension was taken out of the reactor, and the supernatant was taken after centrifugation and the concentration of the dye was measured in real time using a UV-vis spectrophotometer. Then the visible light source was turned off, and degradation was carried out for 20 minutes under the action of long afterglow. 3 ml of the suspension was taken out of the reactor, and the dye concentration was detected after centrifugation. After each test, the sample was poured back into the photodegradation system. The above photocatalytic degradation reaction was repeated alternately 3 times for a total of 2 hours. The above experiments were carried out at different temperatures (converted to VO2-R@g-C3N4 at 70°C, recorded as VO2-R@g-C3N4; maintained at VO2-M@g-C3N4 at 25°C, recorded as VO2-M@g-C3N4), and the time-concentration photocatalytic effect diagram was shown in the figure. Figure 4 shown.

[0060] Figure 4 The two data points above are from the same sample (the composite catalyst prepared in Example 1) tested at different temperatures, before and after the VO2 metal (R 68°C)-semiconductor (M 25°C) phase transition, using the same dye under the same illumination conditions. Because the R-phase VO2 in this sample exhibits SPR (surface plasmon resonance) absorption at the phosphorescence wavelength, it promotes photocatalysis, resulting in a 90.2% catalytic effect. However, the M-phase VO2 forms an energy band mismatch with the graphite-phase carbon nitride, inhibiting the catalytic effect and indicating no catalysis. The present invention enables the photocatalytic reaction to be activated or deactivated simply by controlling the temperature.

[0061] When the specific "switch" is on (VO2-R), the catalytic rate is greatly improved within 2 hours. When the "switch" is off (VO2-M), there is almost no photocatalytic effect within 2 hours, and it is possible to selectively promote or inhibit catalysis with high efficiency. Vanadium dioxide (VO2) with metal-semiconductor reversible phase transition properties (MST) exhibits completely different optical and electrical properties in the two phases. Correspondingly, the SPR effect of the VO2 metal R phase and the full light absorption effect of the narrow-bandgap semiconductor M phase will also form two completely different matches with g-C3N4. Therefore, the present invention, starting from the MST theory, introduces VO2 with isomeric properties into g-C3N4, and constructs a metal-carbon nitride or semiconductor-carbon nitride composite structure in situ to obtain a catalytic system with a "photoswitch" effect.

[0062] The various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be referenced to each other.

[0063] The above description of the disclosed embodiments is intended to enable one skilled in the art to implement or use the present invention. Various modifications to these embodiments will be readily apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present invention. Therefore, the present invention is not limited to the embodiments shown herein but is intended to conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing a composite catalyst having photocatalytic switching properties, characterized in that: The steps include: 1) subjecting dicyandiamide to thermal polycondensation to obtain graphite-phase carbon nitride; 2) mixing vanadium pentoxide with graphite-phase carbon nitride and reacting them to obtain M-phase vanadium dioxide-modified graphite-phase carbon nitride; The M-phase vanadium dioxide-modified graphite-phase carbon nitride is a composite catalyst with photocatalytic switching properties; The thermal polycondensation in step 1) is specifically carried out by first maintaining the temperature at 500-510° C. for 3.5-4 hours, then heating to 520-530° C. and maintaining the temperature for 0.4-0.6 hours; The reaction temperature in step 2) is 550-570° C., the reaction time is 1.8-2.2 h, and the reaction pressure is 10-25 Pa.

2. The method for preparing a composite catalyst having photocatalytic switching properties according to claim 1, characterized in that: The mass ratio of vanadium pentoxide to graphite carbon nitride in step 2) is 1:9.5-10.

5.

3. A composite catalyst having photocatalytic switching properties prepared by the preparation method according to any one of claims 1 to 2.

4. Use of the composite catalyst with photocatalytic switching properties according to claim 3 in surface coating / film of smart glass, degradation of printing and dyeing wastewater, or photolysis of water to produce hydrogen.

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