Pd / L-C3N5 photocatalyst as well as preparation method and application thereof

Abstract

The invention provides a Pd / L-C3N5 photocatalyst as well as a preparation method and application thereof, and belongs to the technical field of photocatalysts. The preparation method of the Pd / L-C3N5 photocatalyst comprises the following steps: uniformly mixing 3-amino-1, 2, 4-triazole and lithium chloride, and calcining to obtain an L-C3N5 nanosheet; and dipping the L-C3N5 nanosheet in a palladium salt solution, and then reacting under the action of a reducing agent to obtain the Pd / L-C3N5 photocatalyst. The Pd / L-C3N5 catalyst obtained through a molten salt method has a stable layered structure, and the preparation method is simple and easy to implement and beneficial to industrial production. The catalyst obtained by the preparation method can efficiently catalyze formic acid to produce hydrogen, and the time for catalyzing formic acid to produce hydrogen is only 1.5 min; and the catalyst has excellent cycle stability, and can still catalyze formic acid to be completely decomposed to produce hydrogen after five cycles.

Description

Technical Field

[0001] The present invention relates to the technical field of photocatalysts, and particularly to a Pd / L-C 3 N 5 photocatalyst and its preparation method and application. Background Art

[0002] In recent years, the rapid growth of industrialization and population has triggered a series of serious energy and environmental problems. On the one hand, the current energy supply mainly relies on fossil fuels, and after combustion, a large amount of carbon dioxide is emitted into the atmosphere, causing climate change and sea-level rise. On the other hand, the use of organic solvents, dyes, antibiotics, and other toxic substances in industry and daily life has had a significant negative impact on the surrounding environment. Therefore, sustainable development has become a key element in the development of humanity today, guiding researchers in the global academic and industrial communities to strive to find advanced practical technologies.

[0003] Hydrogen is a renewable resource. However, the effective storage and release of hydrogen are one of the biggest problems hindering the use of hydrogen energy. Formic acid (FA) has the advantages of (a) high hydrogen content (4.4 wt%), (b) non-toxic at room temperature, (c) simple synthesis, and (d) convenient storage and transportation, and is considered an excellent hydrogen carrier. Efficiently catalyzing the decomposition of formic acid to produce hydrogen under mild conditions is the key to its practical application, and developing highly active and stable catalysts is the core challenge in this field.

[0004] Currently, the research on formic acid decomposition hydrogen production catalysts mainly focuses on noble metal (such as Pd, Au, Pt, etc.)-based materials. Among them, Pd-based catalysts have received extensive attention due to their excellent dehydrogenation activity and selectivity. However, traditional supported Pd catalysts still have the following problems: (1) The specific surface area of the support is limited, resulting in poor dispersion of Pd nanoparticles and insufficient exposure of active sites; (2) The electronic interaction between the support and Pd is weak, making it difficult to optimize the adsorption energy of reaction intermediates; (3) In the photocatalytic system, the light absorption ability of the support is insufficient and the recombination rate of photogenerated carriers is high, limiting the improvement of the photo-thermal synergistic catalytic efficiency. Summary of the Invention

[0005] In view of this, the purpose of the present invention is to provide a Pd / L-C 3 N 5 photocatalyst and its preparation method and application.

[0006] To achieve the above purpose, the present invention provides the following technical solutions:

[0007] One of the technical solutions of the present invention, a preparation method of a Pd / L-C 3 N 5 photocatalyst, comprising the following steps:

[0008] Mix 3-amino-1,2,4-triazole (C 2 H 4 N 4 ) and lithium chloride (LiCl) evenly and then calcine to obtain L-C 3 N 5 nanosheets;

[0009] Immerse the obtained L-C 3 N 5 nanosheets in a palladium salt solution, and then react under the action of a reducing agent to obtain the Pd / L-C 3 N 5 photocatalyst.

[0010] In the present invention, the mass ratio of the 3-amino-1,2,4-triazole to the lithium chloride is 1:5.

[0011] In the present invention, the method of mixing evenly is grinding.

[0012] In the present invention, the calcination procedure is: heat up to 500 °C at a rate of 5 °C / min and keep warm for 180 min. After the calcination is completed, it also includes the steps of washing and drying the obtained product in sequence.

[0013] In the present invention, the ratio of the L-C 3 N 5 nanosheets to Pd in the palladium salt solution is 0.2 g:(0.05 - 0.15) mmol (preferably 0.2 g:0.05 mmol, 0.2 g:0.1 mmol or 0.2 g:0.15 mmol).

[0014] In the present invention, the palladium salt solution is a K 2 PdCl 4 solution.

[0015] In the present invention, the impregnation time is 24 h and the temperature is room temperature.

[0016] In the present invention, the reducing agent is sodium borohydride; the mass ratio of the sodium borohydride to the L-C 3 N 5 nanosheets is 0.03:0.2.

[0017] In the present invention, when reacting under the action of a reducing agent, the reaction temperature is -3 °C and the time is 5 h; after the reaction is completed, it also includes collecting the product and washing and drying the obtained product.

[0018] The second technical solution of the present invention is a Pd / L-C 3 N 5 photocatalyst prepared by the above preparation method.

[0019] The third technical solution of the present invention is an application of the above-mentioned Pd / L-C 3 N 5 photocatalyst in the hydrogen production from formic acid.

[0020] In the method for producing hydrogen from formic acid (FA) by using the Pd / L-C 3 N 5 photocatalyst, under light illumination conditions, when the temperature of the reaction system is 25 °C to 45 °C, the concentration of sodium formate (SF) is 1.5 to 4.5 mmol, and the catalyst dosage is 25 to 100 mg, hydrogen is produced from formic acid.

[0021] The present invention discloses the following technical effects:

[0022] The Pd / L-C 3 N 5 catalyst obtained by the molten salt method of the present invention has a stable layered structure, and this preparation method is simple and easy to implement, which is beneficial to industrial production.

[0023] The catalyst prepared by the present invention can efficiently catalyze the hydrogen production from formic acid. The time required for catalyzing the hydrogen production from formic acid is only 1.5 min; and it has excellent cycle stability. After 5 cycles, it can still catalyze the complete decomposition of formic acid to produce hydrogen. Description of the Drawings

[0024] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required to be used in the embodiments. Obviously, the drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained based on these drawings.

[0025] Figure 1 Among them, (a) and (b) are SEM images of Pd / L-C 3 N 5 at different magnification factors in Example 1, and (c)-(e) are TEM images of the Pd / L-C 3 N 5 catalyst at different magnification factors in Example 1, (f) is a lattice fringe calculation diagram, and (g) is the HAADF-STEM image of the Pd / L-C 3 N 5 catalyst and the corresponding mapping diagrams of elements C, N, and Pd.

[0026] Figure 2 Among them, (a) is L-C 3 N 5 , Pd / L-C 3 N 5 and Pd / C in Example 13 N 5 XRD patterns of (b) L-C 3 N 5 , Pd / L-C 3 N 5 and Pd / C 3 N 5 FT-IR spectra of

[0027] Figure 3 For Pd / L-C in Example 1 3 N 5 and Pd / C 3 N 5 XPS survey spectra (a), XPS spectra of Pd / L-C 3 N 5 and Pd / C 3 N 5 in the (b) C 1s region, (c) N 1s region, (d) Pd 3d region

[0028] Figure 4 For Pd / L-C in Example 1 3 N 5 and Pd / C 3 N 5 (a) UV-visible absorption spectra, (b) band gap energy, (c) XPS valence band spectra, (d) band structure schematic diagrams of

[0029] Figure 5 At selected frequencies of 1000 Hz, 2000 Hz, and 3000 Hz, (a) Mott-Schottky plots of Pd / L-C in Example 1 3 N 5 and (b) Mott-Schottky plots of Pd / C in Example 1 3 N 5

[0030] Figure 6 For Pd / L-C in Example 1 3 N 5 and Pd / C 3 N 5 (a) Photoluminescence spectra, (b) time-resolved PL decay spectra, (c) EIS Nyquist plots, and (d) transient photocurrent response spectra of

[0031] Figure 7 In the effect verification, (a) relationship between time and volume of hydrogen production by catalytic decomposition of FA with different metal amounts under visible light irradiation and (b) corresponding TOF values of FA decomposition

[0032] Figure 8 ​For effect verification, (a) Relationship diagram of time and volume of hydrogen production by different salt templates catalyzing the decomposition of FA under the conditions of 30 °C with light; (b) Comparison of TOF values corresponding to the catalysts.

[0033] Figure 9 For effect verification, (a) Pd / L-C 3 N 5 Relationship diagram of time and volume of hydrogen production by catalyzing the decomposition of FA with and without light; (b) Corresponding TOF values of FA decomposition.

[0034] Figure 10 For effect verification, (a) Pd / L-C under light 3 N 5 Relationship between gas production and reaction time of catalyzing the FA decomposition reaction at different temperatures; (b) Arrhenius curve of corresponding lnk and 1000 / T.

[0035] Figure 11 For effect verification, (a) Under visible light at 30 °C, different amounts of Pd / L-C 3 N 5 Relationship diagram of gas volume generated by catalyzing the decomposition of FA and time; (b) Logarithmic relationship diagram of gas generation rate and catalyst dosage.

[0036] Figure 12 For effect verification, under the conditions of 30 °C with light, different concentrations of SF solution on Pd / L-C 3 N 5 Rate curve of dehydrogenation of FA catalyzed by Pd / L-C.

[0037] Figure 13 For effect verification, Pd / L-C 3 N 5 Isotope labeling experiment of formic acid dehydrogenation on Pd / L-C catalyst, where (a) Variation curve of gas volume generated using different isotope-labeled reagents with time, (b) Kinetic isotope effect (KIE) data.

[0038] Figure 14 For effect verification, Pd / L-C 3 N 5 Cyclic performance diagram of catalyzing FA dehydrogenation reaction under light conditions.

[0039] Figure 15 For effect verification, Pd / L-C 3 N 5 Full XRD pattern after cyclic reaction.

[0040] Figure 16 For effect verification, (a)-(d) Pd / L-C 3 N 5XPS comparison spectra after cyclic reaction. Detailed implementation mode

[0041] The various exemplary implementation modes 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, characteristics, and implementation schemes of the present invention.

[0042] It should be understood that the terms described in the present invention are only for describing specific implementation modes and are not used to limit the present invention. In addition, for the numerical ranges in the present invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Any intermediate value within any stated value or stated range, as well as each smaller range between any other stated value or intermediate value within the stated range, is also included in the present invention. The upper and lower limits of these smaller ranges can be independently included or excluded from the range.

[0043] Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the present invention pertains. Although the present invention only describes preferred methods and materials, any methods and materials similar or equivalent to those described herein can also be used in the implementation or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials related to the documents. In case of conflict with any incorporated document, the content of this specification shall prevail.

[0044] Without departing from the scope or spirit of the present invention, various improvements and changes can be made to the specific implementation modes of the present invention specification, which are obvious to those skilled in the art. Other implementation modes obtained from the present invention specification are obvious to those skilled in the art. The present invention specification and examples are only exemplary.

[0045] Regarding "comprising", "including", "having", "containing", etc. used herein, they are all open-ended terms, meaning including but not limited to.

[0046] The relevant experiments and methods involved in the present invention are as follows:

[0047] 1. Characterization method:

[0048] (1) Transmission electron microscope (TEM): The microscopic morphology analysis of the sample was carried out with the JEM-2100F field emission transmission electron microscope of JEOL Ltd.

[0049] Field emission transmission electron microscope.

[0050] (2) Scanning Electron Microscope (SEM): A Czech TESCAN MIRA LMS field emission scanning electron microscope (SEM) was used to characterize and analyze the catalyst samples. In the sample pretreatment stage, the powder samples were fixed on the sample stage by conductive adhesive, effectively ensuring the conductivity of the samples and the stability of observation.

[0051] (3) Powder X-ray Diffraction Pattern (XRD): A Rigaku SmartLab SE powder diffractometer was used for phase identification, equipped with a copper target (CuKα, ), and the scanning rate was 1°·min -1 .

[0052] (4) X-ray Photoelectron Spectroscopy (XPS): The surface element chemical states of the samples were analyzed by a Thermo Fisher K-Alpha small area electron spectrometer.

[0053] (5) Ultraviolet-Visible Diffuse Reflectance Absorption Spectroscopy (UV-vis DRS): The light absorption characteristics of the samples were measured using a Shimadzu UV-3600 spectrophotometer, and data were collected in the diffuse reflectance mode.

[0054] (6) Photoluminescence Spectroscopy (PL): Carrier recombination information was obtained using an Edinburgh FLS1000 fluorescence spectrometer, and the tests included time-resolved fluorescence decay curves and steady-state emission spectra.

[0055] (7) Photoelectric Performance Test: A CHI660E electrochemical workstation was used to test the photoelectrochemical properties. For the transient photocurrent response, the test light source was a xenon lamp >400 nm, and the light illumination time interval was 20 s (light on for 20 s, light off for 20 s), and more than 4 cycles of tests were carried out; the test frequency range of electrochemical impedance (EIS) was 0.1 Hz - 100 KHz, and the electrolyte was 2.5 mmol·L -1 potassium ferricyanide solution; for the Mott-Schottky test, 0.5 M Na 2 SO 4 solution was used as the base solution, and the potential range was -1 to 1 V, with multi-frequency scanning from 1000 - 3000 Hz.

[0056] 2. Hydrogen Production Experiment by Formic Acid Decomposition:

[0057] The dosage of the catalyst used in the catalytic reaction of the present invention is: Pd / L-C 3 N 5: 50 mg; The reactor is a specially designed photocatalytic reactor. The reaction temperature is controlled by a low-temperature constant temperature bath. The mixed solution of HCOOH and HCOONa is injected into the photoreactor with a medical syringe, and the photocatalytic decomposition reaction of formic acid starts. The reaction rate of hydrogen production by formic acid decomposition is calculated according to the recorded gas volume and reaction time. In this experiment, the influence of a single variable on the hydrogen production rate of formic acid decomposition is explored by the method of controlling variables.

[0058] 3. Catalyst cyclic stability test

[0059] Based on the experimental data, catalysts with better performance are selected and their cyclic stability tests for catalytic reactions are carried out. The reaction conditions remain unchanged. After completing one catalytic reaction, the reacted catalyst is centrifuged, washed with water, dried, and ground to obtain the recovered catalyst. The first experimental operation is repeated repeatedly, and the cyclic stability of the catalyst is tested by comparing the reaction rates of the recovered catalyst for hydrogen production by formic acid decomposition.

[0060] 4. Catalyst performance evaluation parameters

[0061] (1) Activation energy (E a )

[0062] The activation energy reflects the minimum energy threshold required for reactant molecules to transform into the activated state, and its value directly reflects the difficulty of chemical reactions. The E a in this article is calculated from the Arrhenius equation (1):

[0063]

[0064] This equation reveals the internal relationship between the rate constant k and temperature. A is the pre-exponential factor, R is the molar gas constant, and T is the thermodynamic temperature. From the formula, when the activation energy decreases, the reaction energy barrier decreases, and molecules are more likely to cross the energy barrier, thus significantly enhancing the reaction kinetic rate.

[0065] (2) Turnover frequency (TOF)

[0066] TOF is a key parameter characterizing the amount of product converted per unit mass or mole of the catalyst per unit time, and its value can directly reflect the catalytic performance of the catalyst. The TOF value in this article is calculated according to Equation (2):

[0067]

[0068] In the formula, n H2 is the total number of moles of H 2 released. n metal corresponds to the total number of moles of the metal component in the catalyst, and t is the reaction time.

[0069] The technical solutions described in the present invention are all conventional solutions in the art unless otherwise specified. The reagents or raw materials used are all purchased from commercial channels or are publicly available unless otherwise specified.

[0070] To better understand the present invention, the content of the present invention will be further clarified below in conjunction with embodiments. However, the content of the present invention is not limited to the following embodiments.

[0071] Example 1

[0072] (1) Preparation of L-C 3 N 5

[0073] Weigh 2.00 g of C 2 H 4 N 4 and 10.00 g of LiCl in a mortar, quickly grind them until they are evenly mixed, then spread them evenly in a quartz boat, place it in a muffle furnace for roasting, raise the temperature to 500 °C at a rate of 5 °C / min in static air, keep it warm for 180 min, and the program ends. After cooling to room temperature, collect the yellow flaky solid, put it into a 500 mL beaker, add 500 mL of deionized water, place it on a magnetic stirrer and stir vigorously for 24 h. After that, carry out suction filtration, then put the solid product into a 500 mL beaker for secondary water washing for 24 h. After that, carry out suction filtration, put the obtained solid product into a blast drying oven to dry, grind and weigh it, and then seal it for storage for later use.

[0074] C 3 N 5 The preparation method is the same as the above steps, except that the addition of LiCl and the water washing with deionized water are omitted, and the yellow product (C 3 N 5 ) obtained by roasting is directly ground, weighed, and sealed for storage.

[0075] (2) Preparation of Pd / L-C 3 N 5

[0076] Weigh 0.20 g of the carrier and put it at the bottom of the nested cup, then add K 2 PdCl 4 solution (in the added K 2 PdCl 4 solution, K 2 PdCl 4The molar amount of () is 0.15 mmol), a magnetic stir bar was added, the cup was sealed with plastic wrap, and the nested cup was placed on a magnetic stirrer and stirred at room temperature for 24 h; then a low-temperature constant temperature bath was connected, and a reducing agent (0.03 g sodium borohydride dissolved in 4 mL deionized water) was added at -3 °C, and the mixture was stirred at a constant temperature for 5 h. After the reduction was completed, the mixture in the nested cup was centrifuged in a high-speed centrifuge to remove the supernatant, and the obtained solid product was dried in a blast drying oven for 12 h. After thorough grinding, a black powder catalyst was obtained (when the carrier is L-C 3 N 5 When, the obtained catalyst is Pd / L-C 3 N 5 ; when the carrier is C 3 N 5 When, the obtained catalyst is Pd / C 3 N 5 ).

[0077] Example 2

[0078] The difference from Example 1 is only that the added K 2 PdCl 4 In the solution, the molar amount of K 2 PdCl 4 was adjusted from 0.15 mmol to 0.05 mmol and 0.1 mmol, and other steps and parameters were the same as those in Example 1. The obtained Pd / L-C 3 N 5 is denoted as Pd / L-C 3 N 5 -x, where x represents the molar amount of K 2 PdCl 4 .

[0079] Example 3

[0080] The difference from Example 1 is only that LiCl was adjusted to KCl, NaCl, and LiF, and other steps and parameters were the same as those in Example 1. The obtained Pd / L-C 3 N 5 are respectively denoted as Pd / L-C 3 N 5 -KCl, Pd / L-C 3 N 5 -NaCl, Pd / L-C 3 N 5 -LiF.

[0081] Characterization:

[0082] The morphologies, XRD, etc. of the catalysts obtained with different raw material ratios are similar. Taking the catalyst obtained in Example 1 as an example, it was characterized.

[0083] AsFigure 1 In (a)-(b) are the catalyst morphologies at 500 nm and 100 nm under scanning electron microscopy. It can be seen that the catalyst exhibits an obvious layered structure. This hierarchical porous structure is beneficial to exposing more active sites, significantly improving the dispersion and surface utilization rate of metallic Pd. Figure 1 In (c)-(e) are the catalyst images at 500 nm and 100 nm under transmission electron microscopy. Figure 1 In (f), the lattice fringe spacing of Pd measured by high-resolution TEM characterization is 0.224 nm, corresponding to the (111) crystal plane of Pd, further confirming the successful loading of Pd nanoparticles in the catalyst. Figure 1 In (g) is the high-angle annular dark-field transmission electron microscopy (HAADF-STEM) image and the elemental mapping images of C, N, and Pd of Pd / L-C 3 N 5 It can be clearly observed from the figure that each element is uniformly distributed in the catalyst, which further confirms the uniform loading of Pd in the catalyst.

[0084] Figure 2 In (a) is the XRD pattern of the catalyst L-C 3 N 5 、Pd / L-C 3 N 5 、Pd / C 3 N 5 . It can be observed from the figure that all three samples have a common diffraction peak at 2θ = 12° and 27°, corresponding to the (100) and (002) crystal planes of carbon nitride. Observing the XRD patterns of Pd / L-C 3 N 5 、Pd / C 3 N 5 of the two catalysts, it is found that diffraction peaks appear at 2θ = 40.12°, 46.67° and 68.12°, which correspond to the (111), (200) and (220) crystal planes of Pd (PDF#46-1043). Based on the TEM characterization results, it can be confirmed that metallic Pd nanoparticles have been successfully loaded on the surface of the support material.

[0085] The chemical bond characteristics of C 3 N 5 composed of triazole and two triazine groups were studied by Fourier transform infrared spectroscopy (FT-IR). The catalyst shows sharp peaks at 810 and 891 cm-1, corresponding to the condensed C-N heterocycles of the triazine moiety, while the peaks in the range of 1200-1700 cm-1 (including the peaks at 1458 and 1635 cm-1) are attributed to the stretching vibration modes of the triazine C-N heterocycles. It is worth noting that for triazole-based C 3 N 5Show significant peaks at 740 and 775 cm-1, revealing the presence of N-N heterocyclic bonds in the triazole moiety. In addition, C 3 N 5 The new peak appearing at 2180 cm-1 is attributed to the cyano group (-C≡N) transformed from the terminal -C-NH in the melon structural unit. 2 All samples show broad band peaks in the range of 3200 - 3400 cm-1, which is attributed to the water molecules in the material and the terminal amino groups in the CN framework.

[0086] Figure 3 (a) in is the full XPS spectra of Pd / L-C 3 N 5 and Pd / C 3 N 5 By XPS characterization, characteristic signals of C, N, O, and Pd are successfully detected in the samples. Among them, the elements C, N, and Pd mainly originate from the catalyst itself, while the small amount of O signal may be due to the adsorption of carbon dioxide or water on the material surface. Pd / L-C 3 N 5 and Pd / C 3 N 5 The C1s XPS spectra of Figure 3 (b) in have a sharp peak at 288.0 eV, corresponding to the N 2 -C=N bond, and the characteristic peak at 284.8 eV corresponds to the C-C bond, which is related to the introduced triazole group. Its introduction increases the number of N atoms, thereby enhancing the electron absorption ability of C atoms. Figure 3 (c) in is the N1s XPS spectra of Pd / L-C 3 N 5 and Pd / C 3 N 5 The characteristic peaks at 398.7 eV and 400.2 eV correspond to the C-N=C bond and the N-H bond, respectively. Figure 3 (d) in is the Pd 3d XPS spectra of Pd / L-C 3 N 5 and Pd / C 3 N 5 Due to spin-orbit coupling, this orbital will split into two peaks, namely Pd 0 and Pd 2+ For Pd / L-C 3 N 5 the Pd peaks can be divided into Pd 0 (335.4 eV and 340.8 eV) and Pd 2+ (337.4 eV and 343.1 eV), while for Pd / C 3 N5 , whose Pd peaks can be divided into Pd 0 (335.1 eV and 340.7 eV) and Pd 2+ (337.3 eV and 343.0 eV). These results indicate that the binding energy (BE) of Pd 2+ has shifted by 0.1 eV towards a higher binding energy value, and the BE of Pd 0 has shifted by 0.1 - 0.3 eV towards a higher binding energy value. The occurrence of this phenomenon can be attributed to the fact that the introduction of LiCl may cause a charge transfer effect in metallic Pd, and it further confirms the strong interaction between the metal and the support in the catalyst. This interaction enhances the catalytic performance of the catalyst for the decomposition of formic acid to produce hydrogen.

[0087] As Figure 4 shown in (a) of 3 N 5 , both samples exhibit significant optical absorption responses in the range of 200 - 600 nm. Further analysis of the curves according to the Kubelka - Munk function of the optical absorption edge energy conversion calculates that the band gaps of Pd / L - C 3 N 5 and Pd / C Figure 4 are 1.85 eV and 2.49 eV respectively ((b) of 3 N 5 ). The Pd / L - C Figure 4 catalyst has the smallest band gap. This result indicates that the introduction of the salt template LiCl effectively reduces the band gap of the catalyst, lowers the excitation energy required for electron transition, and is beneficial to improving the generation efficiency of photo - generated carriers. Further analysis by X - ray photoelectron spectroscopy valence band spectrum (VB - XPS) ((c) of 3 N 5 ) determines that the valence band positions of the two samples are 1.03 eV (Pd / L - C 3 N 5 ) and 1.48 eV (Pd / C Figure 4 ). The energy band position diagram drawn by combining the band gap data ((d) of 3 N 5 ) shows that compared with Pd / C 3 N 5 , the bottom of the conduction band of Pd / L - C 3 N 5It has a stronger ability to reduce photo-generated electrons. This enhanced reduction ability is mainly attributed to the introduction of the salt template, which improves the utilization rate of carriers, enabling more photo-generated electrons to participate in the reduction reaction. Secondly, the salt template effectively inhibits the recombination process of electron-hole pairs, thereby prolonging the lifetime of photo-generated carriers. These synergistic effects ultimately promote a significant improvement in the efficiency of the photothermal catalytic reaction, providing new ideas for the development of efficient photocatalysts.

[0088] As Figure 5 shown, the curve slopes of Pd / L-C 3 N 5 and Pd / C 3 N 5 are both positive, so it is determined that both catalysts are n-type semiconductors. During the experiment, the reference electrode selected was the Ag / AgCl electrode, and the obtained conduction band potentials were -1.02 V (Pd / L-C 3 N 5 ) and -1.21 V (Pd / C 3 N 5 ). According to the Nernst equation E fb = E Ag / AgCl + E 0 Ag / AgCl the position of E fb relative to the normal hydrogen electrode (NHE) can be calculated. Among them, the value of E 0 Ag / AgCl at room temperature is about 0.197 V. After calculation, it can be known that the E 3 N 5 of Pd / L-C fb is -0.82 V vs NHE, and the Efb of Pd / C 3 N 5 is -1.01 V vs NHE, which means that the introduction of LiCl may make the catalyst bandgap smaller and more conducive to electron transport, which is consistent with the bandgap size data obtained from UV characterization. In photocatalysis, a more positive flat band potential may mean a higher conduction band edge, enhancing the reduction ability of the catalyst.

[0089] As Figure 6 shown in (a) are the photoluminescence (PL) spectra of Pd / L-C 3 N 5 and Pd / C 3 N 5 two catalysts. Comparing the two, the PL intensity of Pd / L-C 3 N 5 is the weakest, indicating that the recombination of photo-generated electron-hole pairs is reduced and the charge separation efficiency is improved. Time-resolved PL decay spectra ( Figure 6In (b), it reflects the survival time of photo-generated electron-hole pairs, Pd / L-C 3 N 5 has an average lifetime of 0.62 ns, and Pd / C 3 N 5 has an average lifetime of 0.75 ns. Comparing the two, Pd / L-C 3 N 5 has a slightly shorter average lifetime, which means that the recombination rate of electron-hole pairs is faster, and it also indicates that the migration efficiency of carriers is higher, resulting in faster participation in the reaction. Combining with the M-S test results, the more positive flat-band potential makes the conduction-band electrons more likely to migrate to the catalyst surface to participate in the reaction, shortening the excited-state lifetime. The arc radius in the electrochemical impedance spectroscopy (EIS) is usually related to the charge transfer resistance (Rct). As Figure 6 shown in (c), Pd / L-C 3 N 5 has the smallest arc radius, indicating lower charge transfer resistance and higher migration and transfer frequency of charges in the catalyst, which is consistent with its shorter lifetime and weaker PL intensity. The transient photocurrent response spectrum is usually directly related to the generation, separation, and migration efficiency of photo-generated carriers, and the response intensity is directly related to the effective charge separation efficiency. As Figure 6 shown in (d), Pd / L-C 3 N 5 has a larger and stable transient photocurrent response, which indicates that more photo-generated carriers in Pd / L-C 3 N 5 participate in the reaction, which can effectively accelerate charge separation and transfer.

[0090] Effect verification:

[0091] Weigh 50 mg of the catalyst and pour it obliquely into the bottom of the photocatalytic reactor. Add a magnetic stir bar, seal the reactor top cover by the liquid seal method, then place the reactor on a magnetic stirrer, connect a low-temperature constant temperature bath, set the reaction temperature. After the temperature is stable, turn on the xenon lamp to make the light source center focus on the center of the photoreactor. Check the airtightness of the reactor. Inject a 2 mL mixed solution of HCOOH (1.5 mmol) and HCOONa (4.5 mmol) preheated at the same temperature into the reactor with a syringe, and the photocatalytic formic acid decomposition reaction starts. Start recording the reaction time from the first bubble discharge, record the time every 5 mL of gas is produced until no bubbles are generated, and then stop timing.

[0092] In this experiment, the influence of a single variable on the hydrogen production rate of ammonia borane decomposition was explored by the method of controlling variables. By changing the amount of Pd metal loaded on the catalyst (0.05 mmol, 0.10 mmol, 0.15 mmol), the concentration of sodium formate (0.00 mmol, 1.50 mmol, 3.00 mmol, 4.50 mmol), the amount of catalyst (25 mg, 50 mg, 75 mg, 100 mg), the reaction temperature (25 °C, 30 °C, 35 °C, 40 °C, 45 °C), different salt templates (LiCl, KCl, NaCl, LiF), different solvents (HCOOD, DCOOH, D 2 O), and the presence or absence of light, the influence of each factor on the hydrogen production rate of the catalyst-catalyzed formic acid decomposition was explored.

[0093] 1. Influence of different Pd loadings on the performance of the catalyst

[0094] The influence of different metal loadings (0.05 mmol, 0.10 mmol, 0.15 mmol, i.e., Example 1 and Example 2) on the process of photocatalytic hydrogen production from formic acid by the catalyst was explored. Under the condition that the reaction temperature was 25 °C, a xenon lamp was used as the light source to evaluate the ability of the catalyst loaded with different amounts of metal to catalyze the decomposition of formic acid to produce hydrogen. Figure 7 (In the figure, n metal = 0.05 mmol, n metal = 0.1 mmol, n metal = 0.15 mmol respectively represent Pd / L-C 3 N 5 -x-0.05, Pd / L-C 3 N 5 -0.1, Pd / L-C 3 N 5 ). (a) in it shows the reaction rates of different catalysts for the dehydrogenation of formic acid decomposition, Figure 7 (b) in it is the graph of the TOF values of the catalytic reactions corresponding to the catalysts with different amounts of metal. It can be seen from the figure that when 0.15 mmol of Pd metal is loaded, the catalytic activity of the catalyst is the best, and the corresponding TOF value also reaches the maximum value, which is 2295.91 h -1 .

[0095] 2. Influence of different salt template agents on the performance of the catalyst

[0096] The influence of the salt template on the performance of the catalyst for the decomposition of formic acid to produce hydrogen was explored. Under the condition of 30 °C with light, a blank control group was set up respectively (without adding a salt template, that is, Pd / C in Example 1 3 N 5), and LiCl was replaced with KCl, NaCl, and LiF as salt templates for comparative analysis (i.e., Example 1 and Example 3). Experimental data show that the introduction of the salt template significantly improves the hydrogen evolution efficiency of the catalytic system, and its effect has obvious ion-dependent characteristics. As Figure 8 (In the figure, LiCl, KCl, NaCl, LiF, None represent Pd / L-C 3 N 5 、Pd / L-C 3 N 5 -KCl, Pd / L-C 3 N 5 -NaCl, Pd / L-C 3 N 5 -LiF, Pd / C 3 N 5 ) shows that the catalyst with LiCl as the template exhibits the most excellent catalytic activity. The decomposition reaction time of formic acid is only 1.5 min, and the corresponding TOF value is as high as 2295.91 h -1 . NaCl is the second (reaction time is 10.1 min), followed by KCl (time is 11.6 min). The catalytic activity of LiF in the salt template is the worst (reaction time is 47.3 min, TOF is only 98.6 h -1 ). When no salt template is added, the synthesized catalyst Pd / C 3 N 5 has poor hydrogen production efficiency for the decomposition of formic acid (reaction time is 24.5 min, TOF is only 218.66 h -1 ). In summary, the salt template affects the exposure of catalytic active sites by regulating the dispersion of carbon nitride and the formation process of nanostructures. Among them, the special role of LiCl may be related to the strong polarization ability of Li + and the coordination effect of Cl - , which provides an important theoretical basis for the design of efficient formic acid decomposition catalysts.

[0097] 3. Influence of the presence or absence of light on the catalytic activity of the catalyst for formic acid decomposition to produce hydrogen

[0098] Controlled the temperature at 30 °C, and tested the catalytic activity of the catalyst under light and dark conditions respectively to investigate the influence mechanism of the photoinduced effect on the hydrogen production performance of Pd / L-C 3 N 5 for formic acid decomposition. As Figure 9 shown in (a)-(b), Pd / L-C 3 N 5 has unique photosensitive properties under light conditions, and its TOF value is as high as 2295.91 h -1, higher than the TOF under non-light conditions (1785.71 h -1 ), with an increase of 28.6%. Combining Figure 4 with the optoelectronic test characterization analysis, this result is attributed to the photon excitation generated by the xenon lamp, which promotes the directional migration of the conduction band electrons of the catalyst to the Pd active center, effectively weakening the breaking energy barrier of the C-H bond within the formic acid molecule; the relatively narrow bandgap also accelerates the electron transition process from the valence band to the conduction band; the efficient separation of electron-hole pairs under light illumination also effectively inhibits the carrier recombination, thereby enhancing the charge diffusion rate.

[0099] 4. Influence of the catalyst on the catalytic activity of formic acid decomposition to hydrogen at different temperatures

[0100] To systematically evaluate the kinetic characteristics of the Pd / L-C 3 N 5 photocatalytic formic acid decomposition to hydrogen system, the catalytic reaction was carried out using the catalyst at different temperatures (25 °C, 30 °C, 35 °C, 40 °C, and 45 °C). The experimental results are as shown in Figure 10 (a). The rate of the Pd / L-C 3 N 5 catalyst for the photocatalytic formic acid decomposition to hydrogen reaction under light illumination conditions increases with the increase of the reaction temperature. When the reaction temperature is increased from 25 °C to 30 °C, the time for the catalytic reaction to complete is reduced from 3.12 min to 1.52 min, and the rate is significantly increased by 105%. However, when the temperature is further increased to 35 - 45 °C, the reaction time basically remains between 1.43 - 1.52 min, indicating that continuous heating fails to bring an obvious reaction acceleration effect. This may be due to factors such as catalyst deactivation or mass transfer limitation caused by high temperature. Based on the above kinetic behavior, 30 °C is determined as the optimal reaction temperature, at which the catalyst exhibits the best balance of activity and stability.

[0101] Based on the Arrhenius theory of chemical reaction kinetics, the reaction rate constant k follows the Arrhenius equation with temperature. Plotting with the reciprocal of time as the abscissa and the slope of the catalyst-catalyzed FA decomposition to hydrogen rate curve at different temperatures as the ordinate, it is calculated that under light illumination conditions, the activation energy for the Pd / L-C 3 N 5 catalyst to catalyze the decomposition of FA to hydrogen is 37.8 kJ·mol -1 ( Figure 10 (b)). This activation energy value is significantly lower than that of traditional noble metal catalysts, which may stem from the electronic synergy effect between the L-C 3 N 5 support and Pd nanoparticles, as well as the photocatalytic synergy effect reducing the reaction energy barrier, which provides an important theoretical basis for the design of an efficient photothermal synergistic catalytic system.

[0102] 5. Influence of Catalyst Dosage on the Reaction Rate of Hydrogen Production by Formic Acid Decomposition

[0103] Under the conditions of a reaction temperature of 30 °C and light illumination, the influence law of different amounts of Pd / L - C 3 N 5 catalysts (25 mg, 50 mg, 75 mg, 100 mg) on the reaction kinetics of FA decomposition to produce hydrogen was investigated. As shown in Figure 11 (a) below, when the amount of catalyst increased from 25 mg to 50 mg, the time for the catalytic reaction to complete decreased directly from 7.42 min to 1.52 min, with a decrease of 79.5%, indicating that the catalyst dosage has a significant effect on the promotion of the reaction rate. When the catalyst dosage was further increased to 75 mg (1.43 min) and 100 mg (1.18 min), compared with that of 50 mg, the reaction time was only shortened by 5.9% and 22.4%. Therefore, when the catalyst dosage exceeds 50 mg, the increase in the reaction rate significantly decreases. This may be because the coverage rate of the catalyst active sites tends to be saturated, and the excessive catalyst may cause an agglomeration effect, thereby reducing the utilization rate of the specific surface area. To further analyze the kinetic relationship between the catalyst dosage and the reaction rate of hydrogen production by formic acid decomposition, the logarithmic values of the two were taken for linear fitting to obtain Figure 11 (b) below. The slope of the obtained fitting curve was 0.92, indicating that the catalyst dosage can be regarded as first - order reaction kinetics for FA decomposition to produce hydrogen. This further shows that the reaction rate of FA decomposition to produce hydrogen depends to a large extent on the catalyst dosage.

[0104] 5. Catalytic Performance of the Catalyst for Hydrogen Production by Formic Acid Decomposition at Different Sodium Formate Concentrations

[0105] To investigate the influence of different sodium formate concentrations (0 mmol, 1.50 mmol, 3.00 mmol, 4.50 mmol) on the activity of the Pd / L - C 3 N 5 catalyst in the catalytic reaction, the hydrogen production rate curves of formic acid decomposition corresponding to each concentration were obtained under the conditions of 30 °C and light illumination. The experimental results are shown in Figure 12 as follows. As the sodium formate concentration increased, the reaction rate increased significantly. When no sodium formate was added to the reaction system, it took 27.1 min to complete the hydrogen evolution process. After introducing sodium formate, the reaction kinetics was significantly improved. In the sodium formate solutions of 1.50, 3.00, and 4.5 mmol, the reaction time was shortened to 5.62, 3.75, and 1.52 min, respectively. Compared with the blank system without sodium formate, the acceleration was 4.8 times, 7.2 times, and 17.8 times. This may be because the formate ions present in the sodium formate molecules optimize the surface charge distribution of the catalyst through a cooperative adsorption effect, thereby enhancing the separation efficiency of carriers.

[0106] 6. Influence of Different Substrates and Solvents on the Hydrogen Production Rate of Formic Acid Decomposition Catalyzed by the Catalyst

[0107] In the study of the reaction mechanism of formic acid dehydrogenation, the kinetic isotope effect (KIE) was used to reveal the hydrogen transfer behavior of the key steps in the reaction pathway. The Pd / L-C 3 N 5 catalyst was used, and different substrates (including HCOOH, formic acid-d acid (DCOOH), formic acid-d (HCOOD), formic acid-d 2 (DCOOD)) and solvents (including H 2 O and D 2 O) were used respectively. Since the α-H (carboxyl hydrogen) in the formic acid molecule can rapidly undergo isotope exchange with heavy water at room temperature, while the exchange rate of methyl hydrogen can be ignored under the experimental conditions, the reaction contributions of different hydrogen sites can be effectively distinguished through the isotope labeling strategy. The kinetic test results show ([[]] Figure 13 in (a)), when the solvent for the catalytic reaction is HCOOD + D 2 O, the dehydrogenation reaction rate is significantly better than that of DCOOD + D 2 O. This phenomenon indicates that the cleavage of α-H may be the rate-determining step of the reaction. By calculating the KIE values, as shown in Figure 13 (b), the first set of KIE values (28.50) indicates that when both the substrate and the solvent are fully H-substituted, the reaction rate is 28.5 times faster than that of the fully D-substituted system. This may be related to the bond dissociation energies of C-H and C-D bonds. The second set of KIE values (2.68) reflects the solvent isotope effect, indicating that D 2 O has a certain inhibitory effect on the reaction. The fourth set of KIE values (10.64) reveals the synergistic isotope effect between the substrate and the solvent, which may result from the synergistic effect of solvation and substrate activation. By comparing the KIE values, the reaction mechanism involving the cleavage of α-H in the rate-determining step was further verified.

[0108] 7. Recycling Stability of the Catalyst

[0109] To test the recycling stability of the Pd / L-C 3 N 5 catalyst, under the condition of light irradiation at 30 °C, 50 mg of the catalyst was used for multiple catalytic decompositions of formic acid to produce hydrogen and the data was recorded as shown in Figure 14 . Under the condition of light irradiation, after 5 cyclic tests, the reaction time for the catalyst to catalyze the decomposition of formic acid to produce hydrogen increased, but it could still catalyze the complete decomposition of formic acid to produce hydrogen, indicating that the recycling stability of this catalyst is good.

[0110] For the Pd / L-C 3 N 5The catalyst was characterized by XRD, and the characterization results were compared with the catalyst that did not participate in the reaction to determine whether the crystal structure of the catalyst changed before and after the reaction. The characterization results are as Figure 15 shown. It can be observed that whether before or after cycling, the peak intensities of the catalyst on the Pd(111), Pd(220), and Pd(200) crystal planes hardly changed, indicating that the Pd nanoparticles did not sinter or agglomerate, and the crystal structure remained stable. The peak intensities of the catalyst at (100) and (002) decreased. The (100) crystal plane of L-C 3 N 5 represents the interlayer stacking structure, and the (002) crystal plane reflects the periodicity of the conjugated aromatic layer, indicating that the intermolecular hydrogen bonds or π-π stacking interactions weakened, resulting in changes in the layered structure, reducing the exposure of active sites, and decreasing the cyclic stability of the catalyst.

[0111] The Pd / L-C 3 N 5 catalyst after the cyclic test was characterized by XPS, and the characterization results were compared with the catalyst that did not participate in the reaction. The results are as Figure 16 shown in (a)-(d). In the XPS spectra before and after cycling, obvious characteristic signals of each element are still present, and the peaks corresponding to each element hardly show obvious displacement, indicating that the surface chemical composition, chemical state, and electronic structure of the catalyst after the catalytic cyclic test hardly changed. Combining the analysis with the XRD after cycling, the collapse of the layered structure covered the active sites, making them unable to contact the reactants, but the surface element composition and valence state remained stable. This is also the reason why the catalyst can still catalyze the complete decomposition of formic acid to hydrogen although its catalytic activity has decreased.

[0112] In summary, (1) The Pd / L-C 3 N 5 catalyst obtained by the molten salt method has a stable layered structure. Under the condition of light at 30 °C, the carrier obtained by calcining 3-amino-1,2,4-triazole and lithium chloride in a ratio of 1:5, and the catalyst prepared by loading 0.15 mmol of Pd has the best catalytic performance, and the corresponding TOF is 2295.92 h -1 , which is 1.29 times that in the absence of light (1785.71 h -1 ).

[0113] (2) The reaction rate curves of the Pd / L-C 3 N 5 catalyst for the decomposition of formic acid to hydrogen at different temperatures show that the reaction rate increases with the increase of the reaction temperature, and the activation energy of the reaction is 37.8 kJ / mol.

[0114] (3) The kinetic isotope effect (KIE) was used to explore Pd / L-C3 N 5 Catalytic mechanism of formic acid dehydrogenation to hydrogen. Deuterated formic acids (DCOOH, HCOOD, DCOOD) were used in combination with H 2 2 2 2 2 2 2 2. The kinetic test results showed that the dehydrogenation rate was significantly higher when the solvent for the catalytic reaction was HCOOD + D

[0115] (4) From the cyclic test of hydrogen production by formic acid decomposition, it can be seen that after 5 cycles, the reaction time of Pd / L-C 3 N 5 catalyst for formic acid decomposition to hydrogen was prolonged, but it could still catalyze the complete decomposition of formic acid to hydrogen, indicating that the catalyst had good cyclic stability.

[0116] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and modifications can be made, and these improvements and modifications should also be regarded as the protection scope of the present invention.

Claims

1. A method for preparing a Pd / L-C3N5 photocatalyst, characterized in that: The following steps are involved: 3-amino-1,2,4-triazole and lithium chloride are uniformly mixed and then calcined to obtain L-C3N5 nanosheets; The L-C3N5 nanosheets are immersed in a palladium salt solution, and then reacted under the action of a reducing agent to obtain the Pd / L-C3N5 photocatalyst.

2. The preparation method according to claim 1, characterized in that: The mass ratio of the 3-amino-1,2,4-triazole to lithium chloride is 1:

5.

3. The preparation method according to claim 1, characterized in that: The method of uniform mixing is grinding.

4. The preparation method according to claim 1, characterized in that: The calcination procedure is: heating to 500° C. at a rate of 5° C. / min and keeping the temperature for 180 min.

5. The preparation method according to claim 1, characterized in that: The ratio of the L-C3N5 nanosheets to Pd in ​​the palladium salt solution is 0.2 g: (0.05-0.15) mmol.

6. The preparation method according to claim 1, characterized in that: The immersion time is 24 hours and the temperature is room temperature.

7. The preparation method according to claim 1, characterized in that: The reducing agent is sodium borohydride; the mass ratio of the sodium borohydride to the L-C3N5 nanosheets is 0.03:0.

2.

8. The preparation method according to claim 1, characterized in that: When the reaction is carried out under the action of a reducing agent, the reaction temperature is -3°C and the reaction time is 5 hours.

9. A Pd / L-C3N5 photocatalyst prepared by the preparation method according to any one of claims 1 to 8.

10. Use of the Pd / L-C3N5 photocatalyst according to claim 9 in catalyzing hydrogen production from formic acid.

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