CN108855183B

CN108855183B - Nitrogen-phosphorus-doped graphene-supported palladium catalyst and preparation method thereof

Info

Publication number: CN108855183B
Application number: CN201810611481.8A
Authority: CN
Inventors: 金燕仙
Original assignee: Taizhou University
Current assignee: Taizhou University
Priority date: 2018-06-14
Filing date: 2018-06-14
Publication date: 2021-05-28
Anticipated expiration: 2038-06-14
Also published as: CN108855183A

Abstract

The invention relates to a nitrogen-phosphorus doped graphene palladium-supported catalyst and a preparation method thereof, belonging to the field of preparation of fuel cell nano electro-catalysts. The nitrogen-phosphorus doped graphene-supported palladium catalyst comprises 3.12 +/-0.2% of P, 7.41 +/-0.5% of N, 2.93 +/-0.5% of Pd, 72.65 +/-5% of C and the balance of O, wherein the above are atomic ratios. According to the method, a nitrogen-phosphorus doped graphene palladium-loaded catalyst, a single-element doped graphene (N-doped and P-doped) palladium-loaded catalyst and an undoped graphene palladium-loaded catalyst are subjected to CV (cyclic voltammetry characteristic curve), LSV and other electrochemical performance characterization, and obtained characterization images are compared to obtain an optimal technical route of nitrogen-phosphorus doped graphene. The catalyst introduces different active sites on the surface of graphene by an element doping surface modification technology, and provides strong conductivity and a carrier effect beneficial to metal Pd attachment.

Description

Nitrogen-phosphorus-doped graphene-supported palladium catalyst and preparation method thereof

Technical Field

The invention relates to a graphene catalyst, in particular to a nitrogen-phosphorus doped graphene palladium-supported catalyst and a preparation method thereof, and belongs to the field of preparation of fuel cell nano electro-catalysts.

Background

With the continuous development and progress of society, environmental pollution and energy crisis become the difficult problems that must be broken through for the progress of human society. The fuel cell rapidly occupies the sight of people because of the advantages of cleanness, safety, low noise, diversified fuels and high conversion efficiency (40-70%), and selects an excellent system, so that a high-performance catalyst becomes the main direction of research of experts and scholars at present.

Pt-based catalysts are considered the best catalysts in methanol fuel cells, but Pt is costly, resource limited, and susceptible to CO poisoning, thereby limiting the large-scale use of Pt-based catalysts. Pd is considered as a better substitute, and has the advantages of low cost, better CO poisoning resistance and the like, so that Pd is a hot spot of research in recent years. The catalyst performance is largely influenced by the support material. Generally, the ideal electrocatalyst support material should have high specific surface area, high conductivity, proper pore structure, excellent corrosion resistance and proper surface functional groups^[3-6]. As a novel two-dimensional carbon material, the graphene material has excellent electron conductivity and super-large specific surface area (theoretical specific surface area of 2620 m)²·g^-1) And special mechanical, quantum and electrical properties, which makes it have very good application prospect in the aspect of catalyst carrier^[7,8]. However, in the preparation of the current graphene supported Pd catalyst, one method is to adopt a micro-mechanical exfoliation Highly Oriented Pyrolytic Graphite (HOPG) method and chemical vapor depositionGraphene prepared by a product method (CVD) or the like is used as a carrier. The graphene has a complete crystal structure, excellent electronic conductivity and mechanical properties, and is not easy to dope metal particles because the surface of the graphene is not active. The other method is to synthesize Graphene Oxide (GO) by liquid phase oxidation through a Hummers method, and then prepare a graphene supported palladium catalyst by chemical one-step reduction, which is also a synthesis method adopted more at present. However, the preparation method has the problems that the surface of the graphene contains a large amount of oxygen species, so that the conductivity of the graphene cannot reach an ideal value. The surface of the graphene can be modified by doping the heteroatom, so that the electronic property of the surface of the graphene can be modulated. For example, the B-N doped graphene can modulate the spin density and charge distribution of C atoms by doping N atoms, and the participation of B atoms enhances the asymmetric spin density and also promotes the electron mobility in the planar structure of graphene, i.e., enhances the conductivity of graphene.

Disclosure of Invention

The invention aims to provide a nitrogen-phosphorus-doped graphene palladium-loaded catalyst (Pd/N-P-G and Pd/P-N-G) with excellent electrocatalytic performance, wherein different active sites are introduced to the surface of graphene by an element-doped surface modification technology, so that strong conductivity and a carrier effect favorable for attaching metal Pd are provided.

The technical scheme adopted by the invention for solving the technical problems is as follows:

the nitrogen-phosphorus doped graphene-supported palladium catalyst comprises 3.12 +/-0.2% of P, 7.41 +/-0.5% of N, 2.93 +/-0.5% of Pd, 72.65 +/-5% of C and the balance of O, wherein the above are atomic ratios.

Taking Pd/N-P-G as an example, the catalyst is prepared by the following method: preparing graphene oxide by using a Hummers synthesis method, adding a dicyandiamide solution into a high-pressure reaction kettle, filtering and drying, roasting at 400-550 ℃ under the protection of argon gas, and performing high-temperature pyrolysis; adding a phosphoric acid solution into the high-pressure reaction kettle, carrying out suction filtration, drying, roasting at the temperature of 500-650 ℃ under the protection of argon gas, and carrying out high-temperature pyrolysis to obtain nitrogen-phosphorus doped graphene; and then loading Pd metal by adopting a potassium borohydride method to obtain the nitrogen-phosphorus doped graphene palladium-loaded catalyst. The method is characterized in that graphene oxide is prepared by a Hummers synthesis method, dicyanodiamide and phosphoric acid are respectively used as an N source and a P source, and then the graphene oxide is immersed in a high-pressure kettle and subjected to inert protection high-temperature pyrolysis for doping. And loading Pd metal on the product by a potassium borohydride method to obtain the nitrogen-phosphorus doped graphene palladium-loaded catalyst.

A preparation method of the nitrogen-phosphorus doped graphene supported palladium catalyst comprises the following steps: preparing graphene oxide by using a Hummers synthesis method, dipping dicyanodiamine as an N source in an autoclave, carrying out high-temperature pyrolysis at 400-550 ℃ under inert protection for doping, dipping phosphoric acid as a P source in the autoclave, and carrying out high-temperature pyrolysis at 500-650 ℃ under inert protection for doping; and finally, loading Pd metal on the catalyst by a potassium borohydride method to obtain the nitrogen-phosphorus doped graphene palladium-loaded catalyst.

Preferably, the method comprises the steps of:

preparation of N-GO

Carrying out ultrasonic treatment on dicyanodiamine solution with the mass concentration of 20-50% and graphene oxide together by using an ultrasonic crusher for 30-40min until the mixture is uniform and is in an ink shape, transferring the obtained solution into a high-pressure reaction kettle with a polytetrafluoroethylene lining, reacting for 6-8h at the temperature of 150 ℃ and 250 ℃, naturally cooling, and carrying out suction filtration and drying; transferring the obtained solid to a quartz boat, placing the quartz boat in an energy-saving tube furnace, introducing argon gas into the furnace, roasting the quartz boat at 550 ℃ and carrying out high-temperature pyrolysis for 2 to 4 hours to obtain N-GO;

preparation of N-P-GO

Mixing N-GO with a phosphoric acid solution with the mass concentration of 10-50%, uniformly performing ultrasonic treatment, transferring the mixture to a high-pressure reaction kettle, reacting at the temperature of 130-;

preparation of Pd/N-P-G

Adding a proper amount of deionized water into the N-P-GO prepared in the last step, performing ultrasonic treatment for 15-20min to obtain a uniform ink shape, and adding PdCl₂A solution; after the ultrasonic treatment is finished, the pH value is adjusted to be alkalescent by ammonia water solution, and KBH is added dropwise under stirring₄Condensing and refluxing the solution at 50-55 deg.C for 4-6h, filtering while hot, washing, oven drying at 70 + -5 deg.C, and grinding to obtain Pd/vesselN-P-G catalyst.

Preferably, the mass concentration of the dicyanodiamine solution is 28-32%, and the mass concentration of the phosphoric acid is 28-32%.

A preparation method of the nitrogen-phosphorus doped graphene supported palladium catalyst comprises the following steps: preparing graphene oxide by using a Hummers synthesis method, firstly taking phosphoric acid as a P source, soaking in an autoclave and carrying out high-temperature pyrolysis at the temperature of 500-; dipping dicyanodiamine as an N source in an autoclave and carrying out high-temperature pyrolysis at 400-550 ℃ under the protection of inertia for doping; and finally, loading Pd metal on the catalyst by a potassium borohydride method to obtain the nitrogen-phosphorus doped graphene palladium-loaded catalyst.

Preferably, the method comprises the steps of:

preparation of P-GO

Mixing graphene oxide with a phosphoric acid solution with the mass concentration of 10-50%, performing ultrasonic treatment to obtain an ink shape, pouring the ink shape into a high-pressure reaction kettle, reacting for 5-6h in a drying box at the temperature of 130-;

preparation of P-N-GO

Ultrasonically treating dicyanodiamine solution with the mass concentration of 20-50% and P-GO to be in an ink shape by using an ultrasonic crusher, transferring the obtained mixture into a high-pressure reaction kettle with a polytetrafluoroethylene lining, reacting at the temperature of 150 ℃ and 250 ℃ for 6-8h, naturally cooling, performing suction filtration and drying, transferring the obtained solid to a quartz boat, placing the quartz boat in an energy-saving tube furnace, introducing argon gas into the energy-saving tube furnace, roasting at the temperature of 400 ℃ and 550 ℃ for performing high-temperature pyrolysis for 2-4h, and obtaining P-N-GO;

preparation of Pd/P-N-G

Adding a proper amount of deionized water into the P-N-GO prepared in the last step, performing ultrasonic treatment for 15-20min to obtain a uniform ink shape, and adding PdCl₂A solution; after the ultrasonic treatment is finished, the pH value is adjusted to be alkalescent by ammonia water solution, and KBH is added dropwise under stirring₄And carrying out condensation reflux reaction on the solution at 50-55 ℃ for 4-6h, filtering while the solution is hot, washing, drying at 70 +/-5 ℃, and grinding to obtain the Pd/P-N-G catalyst.

According to the method, a nitrogen-phosphorus doped graphene palladium-loaded catalyst, a single-element doped graphene (N-doped and P-doped) palladium-loaded catalyst and an undoped graphene palladium-loaded catalyst are subjected to CV (cyclic voltammetry characteristic curve), LSV and other electrochemical performance characterization, and obtained characterization images are compared to obtain an optimal technical route of nitrogen-phosphorus doped graphene. The advantages and disadvantages of the electrocatalysis performance of the five catalysts can be read from a comparison image, wherein the nitrogen-phosphorus doped graphene supported palladium catalyst (Pd/N-P-G) has the optimal performance, and finally, a characterization image of the catalyst in a pure potassium hydroxide (KOH) system and a characterization image in an N CH system are compared₃The characterization images of the solution system with OH, n KOH and 1:1 are compared, and the electrocatalytic performance of the catalyst is better under the latter system.

Drawings

FIG. 1 is a diagram of an experimental setup for electrochemical performance characterization of a catalyst;

FIG. 2 shows 5 catalyst electrodes at 1mol/L KOH +1mol/L CH₃The scanning speed of the cyclic voltammetry curve in the electrolyte solution of OH is 50 mV/s;

FIG. 3 shows 5 electrodes at 1mol/L KOH +1mol/L CH₃Linear sweep voltammetry in OH electrolyte solution at a sweep rate of 2 mV/s;

FIG. 4 is a spectrum of cyclic voltammetry curves of 5 catalyst electrodes in 1mol/L KOH electrolyte, with a scan rate of 20 mV/s;

FIG. 5 shows 5 catalyst electrodes at-0.2V potentiostat at 1mol/L KOH +1mol/L CH₃Polarization current curve in electrolyte solution of OH;

FIG. 6 is an XRD pattern for 5 catalysts;

FIG. 7 is TEM images of transmission electron micrographs of (a) Pd/N-P-G, (b) Pd/P-N-G, (c) Pd/P-G and (d) Pd/G catalyst;

FIG. 8 is an XPS plot of (a) Pd/N-P-G and Pd/P-G catalysts, (b) N1s peak plots for Pd/N-P-G catalysts, (c), (d), (e) catalyst P2P peak plots for Pd/P-G, Pd/N-P-G and Pd/P-N-G, respectively, (f), (G) catalyst Pd3d peak plots for Pd/N-P-G and Pd/P-N-G, respectively, and (h) Pd peak comparison plots for 4 catalysts.

Detailed Description

The technical solution of the present invention will be further specifically described below by way of specific examples. It is to be understood that the practice of the invention is not limited to the following examples, and that any variations and/or modifications may be made thereto without departing from the scope of the invention.

In the present invention, all parts and percentages are by weight, unless otherwise specified, and the equipment and materials used are commercially available or commonly used in the art. The methods in the following examples are conventional in the art unless otherwise specified.

The main apparatus is as follows: CHI760D electrochemical analyzer, D8 advanced X-ray diffraction instrument, Japanese JEOL-2010 transmission electron microscope, ESCALB 250Xi photoelectron spectrometer.

Example (b):

synthesis of Pd/N-P-G catalyst

Preparation of 1N-GO

21.429g of dicyanodiamide is weighed, 50mL of deionized water is added for dissolution, 200mg of graphene oxide is added, and ultrasonic treatment is carried out for 30min by an ultrasonic crusher until the mixture is uniformly mixed to form ink. Transferring the obtained solution into a high-pressure reaction kettle with a polytetrafluoroethylene lining, reacting at 180 ℃ for 6-8h, naturally cooling, and performing reduced pressure suction filtration and drying. Transferring the obtained solid to a quartz boat, placing the quartz boat in an energy-saving tube furnace, introducing argon gas into the furnace, roasting the quartz boat at 550 ℃, and performing high-temperature pyrolysis for 2-4h to obtain the N-GO.

Preparation of 2N-P-GO

Weighing 100mg of N-GO, mixing with 50mL of phosphoric acid solution (mass fraction is 30%), carrying out uniform ultrasonic treatment, transferring to a high-pressure reaction kettle, reacting at 150 ℃ for 5-6 hours, taking out, carrying out reduced pressure suction filtration, transferring the obtained solid to a quartz boat, placing in an energy-saving tube furnace, roasting at 500-650 ℃ under the protection of argon gas for 2-4 hours, and cooling after the reaction is finished to obtain the N-P-GO carrier.

Preparation of 3Pd/N-P-G

Adding 50mL of deionized water into the 80mg of N-P-GO prepared in the last step, performing ultrasonic treatment for 15min to obtain a uniform ink, and adding 2.82mL of 0.05mol/L PdCl₂A solution; after the ultrasound is finishedAdjusting pH to weak alkalinity with aqueous ammonia, and adding 120mL of 2.0mg/mL KBH dropwise under stirring₄And (3) carrying out condensation reflux reaction on the solution at 50 ℃ for 4-6h, filtering while the solution is hot, washing, drying at 70 ℃, and grinding to obtain the Pd/N-P-G catalyst.

Synthesis of di, Pd/P-N-G catalyst

Preparation of 1P-GO

200mg of graphene oxide and 50mL of phosphoric acid solution (30%) are weighed, mixed and sonicated until an ink is formed. Pouring the mixture into a high-pressure reaction kettle, reacting for 5-6h in a drying box at 150 ℃, cooling, decompressing, filtering, drying, transferring the obtained solid to a quartz boat, placing the quartz boat in an energy-saving tube furnace, and roasting for 2-4h at 650 ℃ under the protection of argon to obtain the P-GO.

Preparation of 2P-N-GO

21.429g of dicyanodiamide is weighed, 50mL of deionized water is added for dissolution, 100mg of P-GO is added, and the mixture is subjected to ultrasonic treatment by an ultrasonic crusher to form an ink. Transferring the obtained mixture into a high-pressure reaction kettle with a polytetrafluoroethylene lining, reacting at 180 ℃ for 6-8h, naturally cooling, and performing suction filtration and drying. Transferring the obtained solid to a quartz boat, placing the quartz boat in an energy-saving tube furnace, introducing argon gas into the furnace, roasting the quartz boat at 550 ℃, and performing high-temperature pyrolysis for 2 to 4 hours to obtain the P-N-GO.

Preparation of 3Pd/P-N-G

Adding 50mL of deionized water into the 80mg of P-N-GO prepared in the last step, performing ultrasonic treatment for 15min to obtain a uniform ink, and adding 2.82mL of 0.05mol/L PdCl₂A solution; after the sonication was completed, the pH was adjusted to slightly alkaline with aqueous ammonia, and 120mL of 2.0mg/mL KBH was added dropwise with stirring₄And (3) carrying out condensation reflux reaction on the solution at 50 ℃ for 4-6h, filtering while the solution is hot, washing, drying at 70 ℃, and grinding to obtain the Pd/P-N-G catalyst.

Characterization of the catalyst

1X-ray diffraction (XRD)

X-ray diffraction (XRD) test: the XRD measurements were carried out on a Bruker AXS D8Advance type X-ray diffractometer (CuK α, λ ═ 0.154056nm), with a working voltage of 40kV, a current of 40mA, a scanning speed of 2 °/min and a step width of 0.04 °.

2 preparation of the electrodes

5mg of Pd/N-P-G catalyst is weighed by an analytical balance and placed in a centrifugeTransferring Nafion solution and absolute ethyl alcohol solution (1:2) into a centrifuge tube by using a sample injection needle, ultrasonically treating the tube to form ink by using an ultrasonic cleaning machine, and transferring 3 mu L of the solution into a ground and dried electrode (the electrode material is Al-plated electrode material)₂O₃The diameter of the polished glassy carbon is 4 mm. ) The working electrode used in the examples is the catalyst dried.

3 Assembly of the Experimental device

A three-electrode system (shown in figure 1) is adopted, wherein 1 is a platinum sheet, 2 is a working electrode, 3 is a saturated calomel electrode, and 4 is a glass diaphragm. Reference electrode: a saturated calomel electrode for determining the working electrode potential; auxiliary electrode: a platinum electrode for conducting electrical current. The electrocatalytic performance of the catalyst on formic acid oxidation was studied using CV, LSV and IT. The CV can observe the current peaks of catalytic oxidation of the catalyst (the scanning range is between-0.2V and 0.8V, and the scanning rate is 50mV/s and 20 mV/s); LSV may determine the initial oxidation potential of the catalyst; IT can measure the change of the electrocatalytic oxidation current of formic acid with time (constant potential is 0.4V). Electrolyte is 1mol/L CH₃OH +1mol/L KOH (hereinafter, both are simply referred to as an electrolyte). And introducing nitrogen into the electrolyte for 20min to remove dissolved oxygen in the electrolyte, so as to perform electrochemical performance test.

4 electrochemical Performance test analysis

4.1 catalyst at 1mol/L CH₃Electrochemical Properties of OH +1mol/L KOH

Fig. 2 is the electrocatalytic oxidation performance of different carrier-supported Pd nanoparticle composites on methanol. In the forward scan, the oxidation peak appearing around-0.2V (vs. sce) is derived from oxidation of methanol; in the reverse scan, the oxidation peak occurring near-0.4V (vs. sce) can be assigned to secondary oxidation of the carbon-containing intermediate. The electrocatalytic activity of the catalyst on the methanol oxidation reaction is reflected by the size of the oxidation peak current density in the forward scanning process, and the higher the oxidation peak current density is, the stronger the electrocatalytic activity on the methanol oxidation is. It can be seen from the figure that the peak current densities of the catalysts Pd/N-P-G and Pd/P-N-G are 121.1mA cm respectively^-2And 92.6mA · cm^-2The ratio of Pd/P-G (70.5mA cm)^-2)、Pd/N-G(42.6mA·cm^-2) And Pd/G(24.3mA·cm^-2) The corresponding value of (A) is high, and the catalyst shows better catalytic performance for methanol. The result shows that the performance of the catalyst loaded with Pd and prepared by N, P double doping is obviously superior to that of a single P-doped or N-doped electro-catalytic catalyst, and the N-P co-doped graphene carrier has a better promotion effect on the performance of the catalyst loaded with Pd.

FIG. 3 shows that the concentration of CH is 1mol/L₃The linear sweep voltammogram of Pd/P-N-G, Pd/N-P-G, Pd/P-G, Pd/N-G, Pd/G in OH + KOH solution at a sweep rate of 2 mV/s. The figures clearly show that: each catalyst generates an oxidation peak at a voltage of about-0.02V, the graphene palladium-on-graphene catalyst doped with P or N has a higher current density than the graphene palladium-on-graphene catalyst not doped with P or N, and the double-element doped graphene palladium-on-graphene catalyst has a higher current density than the single-element doped graphene palladium-on-graphene catalyst, wherein the doping sequence Pd/N-P-G has better performance than Pd/P-N-G. This shows that the Pd/N-P-G catalyst has better performance for methanol oxidation catalytic reaction in alkaline medium.

4.2 electrochemical Performance of the catalyst in a 1mol/L KOH System

In order to study the electrochemical performance of the catalyst, the modified glassy carbon electrode was tested on the electrocatalytic oxidation of methanol, and different catalysts were compared. FIG. 4 shows Pd/N-P-G, Pd/P-N-G, Pd/P-G, Pd/N-G and Pd/G at 1 mol. L^-1Cyclic voltammogram in KOH solution. As can be seen from the figure, there is a broad peak at about-0.7 to-0.4V (vs. SCE) during the positive sweep, corresponding to the desorption peak of hydrogen; during the retrace, the reduction peak at-0.45V corresponds to the reduction of Pd (II) to Pd (0), and the shoulder that appears after a further negative shift is derived from the hydrogen adsorption peak. The hydrogen absorption/desorption peak areas can be used to qualitatively evaluate the electrochemically active surface area (ECSA) of the catalyst, and it is obvious from the figure that the hydrogen absorption/desorption peak areas are in order of magnitude: Pd/N-P-G>Pd/P-N-G≈Pd/P-G>Pd/N-G>Pd/G. This indicates that Pd is more uniformly dispersed on the Pd/N-P-G catalyst at the same loading, which exhibits the greatest specific surface area of activity.

4.3 catalyst stability

The stability of the electrode catalyst is one of the key factors in the application of the fuel cell. The stability test results of Pd/N-P-G, Pd/P-N-G, Pd/P-G, Pd/N-G and Pd/G on the electrocatalytic oxidation reaction of methanol are shown in FIG. 5, and it can be seen from the figure that the catalytic current density of 5 catalysts is rapidly attenuated in the initial stage, because methanol is continuously oxidized on the surface of the catalyst, and the generated reaction intermediate products (CO, CHO, etc.) are accumulated and adsorbed on the surface thereof, causing catalyst poisoning, reducing effective catalytic active sites, thereby preventing further oxidation of methanol, so that the current density is continuously reduced. Eventually reaching a steady state over time. From the process of attenuation, the current density of Pd/N-P-G is always higher than the corresponding value of Pd/N-P-G before 1750s, and after 1750s, the current density and the current density are consistent. The current densities of Pd/N-P-G and Pd/N-P-G are always higher than the corresponding values of Pd/P-G, Pd/N-G and Pd/G catalysts in the test period of 3600 s. At 3600s, the current densities of Pd/N-P-G and Pd/N-P-G are both 11.9mA cm^-2Pd/P-G (2.45mA · cm), respectively^-2)、Pd/N-G(1.22mA·cm^-2) And Pd/G (0.85mA cm)^-2) 4.9 times, 9.8 times and 14 times. Thus, the N/P doping promotes the stability of the catalyst to be improved, and the Pd/N-P-G has the best stability.

5 characterization of the catalyst

5.1 XRD Pattern analysis

Part of the catalyst was selected for XRD characterization and the results are shown in fig. 6. As can be seen from the figure, the two samples, Pd/P-G and Pd/N-P-G, exhibited broadened (002) diffraction peaks of graphite C around 2 θ ═ 25.8 ° (d (002) ═ 0.344nm) and 25.3 ° (d (002) ═ 0.352nm), and on the one hand, exhibited the structural characteristics of multilayer graphene; on the other hand, compared with Pd/P-G, Pd/N-P-G has larger (002) crystal face spacing, which shows that the graphene sheet layer is less agglomerated after double doping by N, P, thereby being capable of providing larger available specific surface area and being beneficial to the loading and dispersion of the catalyst active component. Meanwhile, diffraction peaks of the two catalysts at 40.0 degrees, 46.4 degrees, 68.0 degrees and 82.0 degrees are respectively diffraction peaks of (111), (200), (220) and (311) crystal face characteristics of Pd, and the Pd particles are shown to be of a face-centered cubic crystal structure (JCPDS No. 46-1043). In addition, it can be seen from the intensity of the diffraction peak that the supported Pd metal particles have good crystallinity. According to the diffraction peak corresponding to the crystal face of Pd (111), the average particle size of Pd can be calculated by using the Scherrer formula d of 0.89 lambda/beta cos theta (wherein lambda is the wavelength of X-rays, beta is the half-peak width (expressed by radian) corresponding to the diffraction peak, and theta is the angle corresponding to the crystal face), and the average particle sizes of Pd in the Pd/P-G, Pd/N-P-G and Pd/P-N-G catalysts are respectively 13.6nm, 7.6nm and 9.7 nm.

5.2 TEM Pattern analysis

FIG. 7 is TEM images of Pd/N-P-G, Pd/P-N-G, Pd/P-G and Pd/G. In all four samples, the graphene can be observed to be in a disordered, transparent and folded lamellar structure, and the size and distribution condition of the Pd particles can also be seen. The Pd particles on the Pd/G catalyst are uniformly distributed and spherical, the particle size is 5-20nm, and a serious agglomeration phenomenon exists. The agglomeration of the Pd particles on the Pd/P-G catalyst is slightly improved. In contrast, the Pd/N-P-G, Pd/P-N-G catalyst has relatively more uniform and finer Pd particle distribution, and the particle size is about 5-10 nm. The doping of N and P is favorable for the dispersion of Pd metal particles on the surface of graphene.

5.3 XPS Spectroscopy

XPS is an effective surface analysis means, and can qualitatively analyze information such as elements and molecular structures of a sample and semi-quantitatively analyze the composition of elements. XPS results for both Pd/N-P-G and Pd/N-P-G catalysts are shown in FIG. 8, and specific elemental analysis fitting results are shown in Table 1. Where FIG. 8(a) is a full spectrum of two samples, it can be seen that both show signal peaks at binding energies of 134.0,286.0,340.0 and 530.0eV, which correspond to characteristic peaks for P2P, C1 s, Pd3d and O1 s, respectively, and that the Pd/N-P-G sample also shows a signal for N1s at a binding energy of 400.0 eV. These indicate that P or N, P was successfully doped into the structure of the carbon material. The elemental analysis result shows that the doping amount of P is increased from 1.81 percent (Pd/P-G) to 3.12 percent (Pd/N-P-G) of atomic fraction, which indicates that the graphene is beneficial to subsequent P doping after N doping.

The N1s line fit of Pd/N-P-G is shown in FIG. 8(b), and it was found by analysis that the sample contained four types of N, pyridine type N (N1,398.4eV), pyrrole type N (N2,399.7eV), graphite type N (N3,400.8eV) and oxide type N (N4,402.8eV), in amounts of 28.38%, 36.64%, 24.08% and 10.90%, respectively.

Peak-splitting fitting of XPS spectra for P2P from Pd/P-G, Pd/N-P-G and Pd/P-N-G catalysts revealed that P2P from Pd/P-G (FIG. 8(C)) could be separated into two peaks, corresponding to a P-C bond (133.19eV) and a P-O bond (133.98eV), with atomic percentages for the two species being 38.09: 61.91. The atomic percent of P-O bonds (134.54eV) in the Pd/N-P-G and Pd/P-N-G (fig. 8(d) and 8(e)) samples decreased to 35.5% and 53.5% and the atomic percent of P-C bonds (133.16eV) increased to 64.5% and 46.5%, indicating that bi-doping of graphene with N, P significantly increased the proportion of P-C phase compared to single P-doped graphene, on the one hand because N is an electron rich atom and N is more electronegative (3.04) than C (2.55), which, after N-doping of graphene, would cause adjacent C atoms to impart electropositivity. P is an electron-rich atom, but the electronegativity (2.19) of P is smaller than that of C, and P can serve as an electron donor, so that the uniform distribution of electrons is facilitated when P doping is carried out on the basis of N doping. This distribution of electrons facilitates conjugation of the large pi bond of the C atom of the skeleton, and makes it possible to exert a synergistic effect of N, P.

The Pd3d XPS spectra of the Pd/N-P-G and Pd/P-N-G catalysts are shown in FIGS. 8(f) and 8 (G). The XPS spectral line of Pd3d can obtain three pairs of peaks through peak separation treatment, and the three pairs of peaks respectively correspond to Pd⁰PdO and PdCl₂. Pd in Pd/N-P-G sample⁰(335.53, 340.75eV), PdO (335.96, 341.3eV), and PdCl₂The content percentages of (337.84, 343.33eV) were 57.0%, 17.4%, and 25.6%, respectively. The percentage of the Pd/P-N-G catalyst is 38.83 percent, 40.33 percent and 20.84 percent respectively.

By comparing Pd3d XPS spectra of Pd/G, Pd/P-G, Pd/N-P-G and Pd/P-N-G catalysts, see FIG. 8 (h). The two largest signal peaks for Pd3d for Pd/G occur at binding energies of 335.9 and 340.95eV, while both the Pd/P-G, Pd/N-P-G and Pd/P-N-G catalysts are biased toward 335.6 and 340.9eV, which are lower in binding energy. In addition, in combination with Table 1, it can be found that the content of metallic Pd and Pd in Pd/G²⁺The contents of Pd in the samples are respectively 6.42 percent and 63.03 percent, and the contents of metal Pd in the Pd/P-G samples and the Pd/P-N-G samples are respectively increased to34.68% and 38.83%, while the metallic Pd content of the Pd/N-P-G catalyst is as high as 57.0%. This shows that the electronic structure of graphene is significantly affected by doping the graphene carrier. The specific expression is that not only the 3d binding energy of Pd is reduced, but also the content of metal Pd in the sample is increased. In addition, we can see that the Pd/N-P-G sample has the highest metal Pd content and the lowest Pd²⁺The contents indicate that N and P have synergistic effect, thereby increasing the content of metal Pd in the sample, namely indicating that the double-doped Pd/N-P-G and Pd/P-N-G samples have more active sites than the single-doped Pd/P-G samples.

TABLE 1 comparison of XPS peak fits for Pd/G, Pd/P-G and Pd/N-P-G catalysts

Conclusion

According to the invention, 5 catalysts of Pd/N-P-G, Pd/P-N-G, Pd/P-G, Pd/N-G and Pd/G are synthesized and electrochemically characterized, and the results show that the catalytic activity and stability of the catalysts are sequentially as follows: Pd/N-P-G > Pd/P-N-G > Pd/P-G > Pd/N-G > Pd/G. Therefore, the conclusion is drawn that the catalytic performance of the palladium-loaded catalyst is greatly improved after the graphene is doped with N and P. The double-element doping is better than the single-element doping graphene Pd-loaded catalyst in catalytic activity and stability. In addition, the doping sequence can affect the catalytic performance of the catalyst, and the catalytic performance of the first N and then P doped graphene supported Pd catalyst is obviously superior to that of the first P and then N doped graphene supported Pd catalyst.

The results show that compared with three catalysts, namely Pd/P-G, Pd/G and Pd/N-G, the Pd/N-P-G and Pd/N-P-G have better catalytic activity and electrochemical stability, and the Pd/N-P-G catalyst has the most excellent performance. This is due to: 1. the electronic structure of the surface of graphene is obviously changed after the graphene is codoped by N and P, the formation and the stability of a large N-shaped bond of a framework C are facilitated, the riveting of Pd on the surface of the framework C is facilitated, and the agglomeration of Pd nanoparticles is hindered, so that the content of metal Pd in the catalyst is improved, and active sites are increased; 2. the reduction of the 3d binding energy of Pd is beneficial to the removal of reaction intermediate products generated in the catalytic oxidation process of methanol, thereby exposing more active sites and finally improving the catalytic activity and stability of the catalyst.

The above-described embodiments are only preferred embodiments of the present invention, and are not intended to limit the present invention in any way, and other variations and modifications may be made without departing from the spirit of the invention as set forth in the claims.

Claims

1. A preparation method of a nitrogen-phosphorus doped graphene supported palladium catalyst is characterized by comprising the following steps: preparing graphene oxide by using a Hummers synthesis method, dipping dicyanodiamine as an N source in an autoclave, carrying out high-temperature pyrolysis at 400-550 ℃ under inert protection for doping, dipping phosphoric acid as a P source in the autoclave, and carrying out high-temperature pyrolysis at 500-650 ℃ under inert protection for doping; finally, loading Pd metal on the catalyst by a potassium borohydride method to obtain a nitrogen-phosphorus doped graphene palladium-loaded catalyst; the catalyst contains P3.12 + -0.2%, N7.41 + -0.5%, Pd 2.93 + -0.5%, C72.65 + -5%, and the rest is O, which are atom ratios.

2. The preparation method of the nitrogen-phosphorus-doped graphene-supported palladium catalyst according to claim 1, characterized by comprising the following steps:

preparation of N-GO

Carrying out ultrasonic treatment on dicyanodiamine solution with the mass concentration of 20-50% and graphene oxide together by using an ultrasonic crusher for 20-40 min until the mixture is uniform and is in an ink shape, transferring the obtained solution into a high-pressure reaction kettle with a polytetrafluoroethylene lining, reacting for 6-8h at the temperature of 150 ℃ and 250 ℃, naturally cooling, and carrying out suction filtration and drying; transferring the obtained solid to a quartz boat, placing the quartz boat in an energy-saving tube furnace, introducing argon gas into the furnace, roasting the quartz boat at 550 ℃ and carrying out high-temperature pyrolysis for 2 to 4 hours to obtain N-GO;

preparation of N-P-GO

preparation of Pd/N-P-G

Adding a proper amount of deionized water into the N-P-GO prepared in the last step, performing ultrasonic treatment for 15-20min to obtain a uniform ink shape, and adding PdCl₂A solution; after the ultrasonic treatment is finished, the pH value is adjusted to be alkalescent by ammonia water solution, and KBH is added dropwise under stirring₄And carrying out condensation reflux reaction on the solution at 50-55 ℃ for 4-6h, filtering while the solution is hot, washing, drying at 70 +/-5 ℃, and grinding to obtain the Pd/N-P-G catalyst.

3. The preparation method of the nitrogen-phosphorus-doped graphene-supported palladium catalyst according to claim 2, characterized by comprising the following steps: the mass concentration of the dicyanodiamine solution is 28-32%, and the mass concentration of the phosphoric acid is 28-32%.