Nickel-silver catalyst for hydrogen production by methane dry reforming and method for hydrogen production by dry reforming

Abstract

The invention relates to the technical field of hydrogen production by catalytic cracking of methane, in particular to a nickel-silver catalyst for hydrogen production by dry reforming of methane and a method for hydrogen production by dry reforming. The nickel-silver catalyst for hydrogen production by methane dry reforming is a mixture of NiO and Ag, the molar ratio of Ni to Ag is (0.73-1.51): 1, the grain size of NiO is 8.7-16.8 nm, the surface area of the nickel-silver catalyst is 3-20 m < 2 > / g, and the catalyst has better methane and carbon dioxide conversion rate and higher hydrogen / carbon monoxide ratio at a medium temperature (650-750 DEG C), and can be applied to hydrogen production by methane dry reforming. The method is of great significance to energy conservation and cost reduction. The method for producing hydrogen through methane dry reforming is low in reaction temperature, high in methane and carbon dioxide conversion rate and high in hydrogen / carbon monoxide ratio.

Description

Technical Field

[0001] The present invention relates to the technical field of hydrogen production by catalytic cracking of methane, and particularly relates to a nickel-silver catalyst for dry reforming of methane to produce hydrogen and a method for dry reforming of methane to produce hydrogen. Background Art

[0002] With the growth of global energy demand and the increasing severity of environmental pollution problems, it has become particularly important to search for clean and efficient energy conversion technologies. Dry Reforming of Methane (DRM), as a technology for converting methane and carbon dioxide into syngas (mainly containing hydrogen and carbon monoxide), has received extensive attention because it can simultaneously achieve hydrogen production and greenhouse gas emission reduction.

[0003] In the DRM reaction, the choice of catalyst is crucial because it directly affects the reaction efficiency and product selectivity. Currently, commonly used catalysts include nickel-based catalysts, but due to their tendency to coke and sinter at high temperatures, problems such as a decrease in activity and stability are likely to occur. Existing research has shown that a bimetallic catalyst formed by combining nickel and silver can provide more advantages than a single-metal Ni catalyst and effectively improve the problems of decreased activity and stability. However, a common problem with existing nickel-silver bimetallic catalysts is that the optimum catalytic reaction temperature is too high (usually around 800 °C). Such a high catalytic reaction temperature requires a large amount of energy consumption during the reaction, is not conducive to energy conservation, and the requirements for equipment are also relatively harsh, resulting in an increase in costs. Summary of the Invention

[0004] Based on this, the present invention provides a new nickel-silver catalyst for dry reforming of methane to produce hydrogen. This catalyst has better methane and carbon dioxide conversion rates and a higher hydrogen / carbon monoxide ratio at medium temperatures (650 - 750 °C), which is of great significance for energy conservation and cost reduction.

[0005] The present invention also provides a method for dry reforming of methane to produce hydrogen. Using this method for dry reforming of methane to produce hydrogen, the reaction temperature is low, and the methane and carbon dioxide conversion rates are high, with a high hydrogen / carbon monoxide ratio.

[0006] The present invention realizes the above technical objectives through the following technical solutions:

[0007] The present invention provides a nickel-silver catalyst for dry reforming of methane to produce hydrogen, which at least includes a NiO, Ag mixture with NiO as the dominant phase. Among them, the molar ratio of Ni to Ag is (0.73 - 1.51):1, the grain size of the NiO is 8.7 - 16.8 nm, and the surface area of the nickel-silver catalyst is 3 - 20 m 2 / g.

[0008] The nickel-silver catalyst for methane dry reforming to produce hydrogen as described above further includes a promoter, the promoter being silicon carbide, and the mass ratio of the mixture of NiO and Ag to the promoter is (1-3):1.

[0009] For the nickel-silver catalyst for methane dry reforming to produce hydrogen as described above, the mixture of NiO and Ag is obtained by co-precipitating nickel salt and silver salt, and the resulting precipitate is washed, dried, ground, and then calcined in an air atmosphere at a temperature above 600°C for 3-7 hours. The nickel salt and silver salt are in terms of nickel ions and silver ions, and the molar ratio of the two is (1-2):1.

[0010] For the nickel-silver catalyst for methane dry reforming to produce hydrogen as described above, the precipitate is formed by precipitating a solution containing nickel salt and silver salt under the condition of pH 9-11.

[0011] For the nickel-silver catalyst for methane dry reforming to produce hydrogen as described above, the washing is carried out using ethanol and distilled water.

[0012] For the nickel-silver catalyst for methane dry reforming to produce hydrogen as described above, the temperature during drying is 60-120°C.

[0013] For the nickel-silver catalyst for methane dry reforming to produce hydrogen as described above, the nickel salt is selected from one or more of nickel nitrate hexahydrate, nickel chloride, and nickel sulfate; the silver salt is selected from one or two of silver nitrate and silver sulfate.

[0014] For the nickel-silver catalyst for methane dry reforming to produce hydrogen as described above, the nickel salt and silver salt are in terms of nickel ions and silver ions, and the ratio of the two is (0.8-1.2):1.

[0015] The present invention also provides a method for methane dry reforming to produce hydrogen, including the following steps:

[0016] Contact the above nickel-silver catalyst with a mixed gas containing methane, carbon dioxide, and argon, and raise the temperature to 650-750°C for reaction. The volume ratio of methane, carbon dioxide, and argon in the mixed gas is (15-25):(15-25):(40-70).

[0017] For the method for methane dry reforming to produce hydrogen as described above, before the nickel-silver catalyst contacts with the mixed gas, it further includes a step of mixing evenly with silicon carbide powder.

[0018] For the method for methane dry reforming to produce hydrogen as described above, the heating rate is 3-8°C / min.

[0019] The nickel-silver catalyst provided by the present invention for hydrogen production by dry reforming of methane can be prepared only by coprecipitation and calcination. The preparation process is simple and the cost is low. Moreover, the catalyst has better methane and carbon dioxide conversion rates and a higher hydrogen / carbon monoxide ratio at medium temperatures, making it easier to meet the energy-saving requirements in industrial applications and reducing the requirements for reaction equipment at the same time.

[0020] The hydrogen production method by dry reforming of methane provided by the present invention conducts catalytic reaction at a medium temperature of 650 - 750 °C, and has better methane and carbon dioxide conversion rates and a higher hydrogen / carbon monoxide ratio, with low energy consumption. Description of the Drawings

[0021] Figure 1 TG-DSC curve of the 2NA1000 catalyst in Example 1;

[0022] Figure 2 X-ray diffraction patterns of some catalysts prepared in Example 1;

[0023] Figure 3 Rietveld refinement results of the catalyst 1NA600 prepared in Example 1;

[0024] Figure 4 SEM pictures of some catalysts prepared in Example 1, where a is the SEM image of the catalyst 1NA600, b is the SEM image of 1.5NA600, and c is the SEM image of 2NA600;

[0025] Figure 5 XPS spectra of Ag species of the catalysts 1NA600, 1.5NA600, and 2NA600 prepared in Example 1;

[0026] Figure 6 XPS spectra of Ni species of the catalysts 1NA600, 1.5NA600, and 2NA600 prepared in Example 1;

[0027] Figure 7 H2-TPR curves of the catalysts 1NA600, 1.5NA600, and 2NA600 prepared in Example 1;

[0028] Figure 8 XRD patterns of the catalysts 1NA600, 1.5NA600, and 2NA600 prepared in Example 1 after being reduced at 400 °C for 1 hour;

[0029] Figure 9Conversion results of CH4 and CO2 obtained by the catalysts 1NA600, 1.5NA600, and 2NA600 prepared in Example 1 at 600 - 850 °C, where a is the conversion result of CH4 and b is the conversion result of CO2;

[0030] Figure 10 H2 / CO ratio and H2 selectivity results obtained by the catalysts 1NA600, 1.5NA600, and 2NA600 prepared in Example 1 at 600 - 850 °C, where a is the H2 / CO ratio and b is the H2 selectivity result;

[0031] Figure 11 Conversion results of CH4 and CO2 obtained by the catalysts 1NA600, 1.5NA600, and 2NA600 prepared in Example 1 at 650 °C, where a is the conversion result of CH4 and b is the conversion result of CO2;

[0032] Figure 12 H2 / CO ratio and H2 selectivity results obtained by the catalysts 1NA600, 1.5NA600, and 2NA600 prepared in Example 1 at 650 °C, where a is the H2 / CO ratio result and b is the H2 selectivity result;

[0033] Figure 13 XRD patterns of the catalysts 1NA600, 1.5NA600, and 2NA600 prepared in Example 1 after the catalytic reaction at 650 °C;

[0034] Figure 14 Raman spectra of the catalysts 1NA600, 1.5NA600, and 2NA600 prepared in Example 1 after the catalytic reaction at 650 °C. Detailed implementation manners

[0035] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0036] The present invention provides a nickel - silver catalyst for hydrogen production by dry reforming of methane. The nickel - silver catalyst is a mixture of NiO and Ag with NiO as the dominant phase. Among them, the molar ratio of Ni to Ag is (0.73 - 1.51):1, the grain size of the NiO is 8.7 - 16.8 nm, and the surface area of the nickel - silver catalyst is 3 - 20 m 2 / g.

[0037] The nickel-silver catalyst for methane dry reforming to produce hydrogen has better methane and carbon dioxide conversion rates and a higher hydrogen / carbon monoxide ratio at medium temperatures (650-750 °C). The catalytic reaction temperature is much lower than that of existing similar catalysts, which is of great significance for energy conservation and cost reduction.

[0038] Furthermore, the nickel-silver catalyst also includes a promoter, the promoter is silicon carbide, and the mass ratio of the mixture of NiO and Ag to the promoter is (1-3):1.

[0039] Specifically, the mass ratio of the mixture of NiO and Ag to the promoter is 1:1, 1.5:1, 2:1, 2.5:1, 3:1, and the range between any two of the above ratios.

[0040] In one embodiment of the present invention, the above-mentioned nickel-silver catalyst for methane dry reforming to produce hydrogen is obtained by co-precipitating nickel salt and silver salt, and then the obtained precipitate is washed, dried, ground, and calcined in an air atmosphere at a temperature above 600 °C for 3-7 h. The nickel salt and silver salt are based on nickel ions and silver ions, and the molar ratio of the two is (1-2):1.

[0041] Specifically, the calcination temperature can be 600 °C, 650 °C, 700 °C, 750 °C, 800 °C, 850 °C, 900 °C, 950 °C, 1000 °C, and the range between any two of the above values.

[0042] The precipitate mentioned above is the precipitate generated when the solution containing nickel salt and silver salt is at a pH of 9-11. That is, the co-precipitation of nickel salt and silver salt means that the solution containing nickel salt and silver salt is precipitated at a pH of 9-11. The method of adjusting the pH can be carried out with conventional alkaline solutions, such as sodium hydroxide, potassium hydroxide solutions, etc.

[0043] The washing of the above-mentioned precipitate is carried out according to the conventional washing method in the art. In some specific embodiments of the present invention, the washing of the above-mentioned precipitate refers to the process of suspending and centrifuging the precipitate successively with ethanol and distilled water.

[0044] The washed precipitate is subjected to a drying treatment. The method of the drying treatment is not limited. In some specific embodiments of the present invention, the drying is to dry the washed precipitate at a temperature of 60-120 °C.

[0045] In the present invention, the nickel salt and silver salt can be selected from soluble salts, and are not specifically limited. For example, the nickel salt can be selected from one or more of nickel(II) nitrate hexahydrate, nickel chloride, nickel sulfate, and the silver salt can be selected from one or two of silver nitrate and silver sulfate.

[0046] Through the characterization of the above catalysts, it is known that the crystal grains, specific surface areas, etc. of the catalysts obtained with different ratios of nickel salts and silver salts are all different. For example, the specific surface area of the 1NA600 catalyst is 20 m 2 / g, and the specific surface areas of the 2NA600 and 1.5NA600 catalysts are 3 m 2 / g and 5 m 2 / g respectively. This also makes the performance of the catalysts prepared with different ratios of nickel and silver contents still have certain differences. The inventor's research found that the 1NA600 catalyst has the best performance at 650 °C. Among them, the selectivity to hydrogen reaches 71%, the hydrogen / carbon monoxide ratio reaches 0.7, and at the same time, its methane and carbon dioxide conversion rates are also the highest at this temperature. That is, when the nickel salt and silver salt are calculated based on nickel ions and silver ions, the ratio of the two is (0.8 - 1.2):1, the performance of the obtained catalyst is better.

[0047] It should be noted that: 1NA600 in this article represents the product after calcination at 600 °C of the precipitate with a Ni / Ag molar ratio of 1, 1.5NA600 represents the product after calcination at 600 °C of the precipitate with a Ni / Ag molar ratio of 1.5, 2NA600 represents the product after calcination at 600 °C of the precipitate with a Ni / Ag molar ratio of 2, 1NA700 represents the product after calcination at 700 °C of the precipitate with a Ni / Ag molar ratio of 1, 1.5NA700 represents the product after calcination at 700 °C of the precipitate with a Ni / Ag molar ratio of 1.5, 2NA700 represents the product after calcination at 700 °C of the precipitate with a Ni / Ag molar ratio of 2, 1NA1000 represents the product after calcination at 1000 °C of the precipitate with a Ni / Ag molar ratio of 1, 1.5NA1000 represents the product after calcination at 1000 °C of the precipitate with a Ni / Ag molar ratio of 1.5, 2NA1000 represents the product after calcination at 1000 °C of the precipitate with a Ni / Ag molar ratio of 2, 1NA1200 represents the product after calcination at 1200 °C of the precipitate with a Ni / Ag molar ratio of 1, 1.5NA1200 represents the product after calcination at 1200 °C of the precipitate with a Ni / Ag molar ratio of 1.5, 2NA1200 represents the product after calcination at 1200 °C of the precipitate with a Ni / Ag molar ratio of 2. Among them, the Ni / Ag molar ratio refers to the ratio of the raw materials used in the preparation.

[0048] The present invention also provides a method for hydrogen production by dry reforming of methane, comprising the following steps:

[0049] Contact the above nickel-silver catalyst with a mixed gas containing methane, carbon dioxide, and argon, and raise the temperature to 650 - 750 °C for reaction. The volume ratio of methane, carbon dioxide, and argon in the mixed gas is (15 - 25):(15 - 25):(40 - 70).

[0050] In some specific embodiments of the present invention, before the nickel-silver catalyst contacts with the mixed gas, it further includes a step of uniformly mixing with silicon carbide powder.

[0051] In the present invention, during the catalytic reaction, the flow rate of the mixed gas is not particularly limited and can be carried out according to the conventional operations in the art. For example, in some specific embodiments, the flow rate of the mixed gas is 80 - 120 mL / min.

[0052] In the present invention, during the catalytic reaction, the heating rate is not particularly limited and can be carried out according to the conventional operations in the art. For example, in some specific embodiments, the heating rate is 3 - 8 °C / min.

[0053] The following specifically describes the nickel-silver catalyst, its preparation method and application according to the present invention with reference to specific examples. The detection and analysis methods involved in the following examples are as follows:

[0054] Thermogravimetric analysis differential scanning calorimetry (TG-DSC) was carried out on a TA Instruments SDT Q600. The sample was heated from 25 °C to 1000 °C at a rate of 5 °C / min, and the analysis was carried out under a total flow rate of 100 mL / min (20 v / v% O2 in helium). A Pfeiffer Omnistar mass spectrometer (Germany) monitored the gases released during the analysis process. X-ray powder diffraction (XRD) was carried out using a Bruker D8 (Bruker AXS D8 Advance) diffractometer. The diffractometer adopted a Bragg-Brentano geometry and used Cu Kα radiation Equipped with a LynxEye detector, at room temperature, patterns in the range of 2θ = 10–80° were collected with a step size of 0.02° and a counting time of 96 seconds per step. EVA (version 6.1.0.4 Bruker AXS GmbH, Karlsruhe, Germany) software was used for phase identification, and the unit cell parameters were optimized by Rietveld refinement using Jana2020 crystallography software.

[0055] Elemental analysis was carried out by inductively coupled plasma-optical emission spectrometry (ICP-OES) 720-ES ICP-OES (Agilent, USA), which has an axial view and simultaneous CCD detection. The quantitative determination of metal content was based on the analysis of certified standard solutions. ICP ExpertTM software (version 2.0.4) provided the concentration of metals in the sample, allowing the estimation of the weight percentages of Ag and Ni. All analyses were performed 40 minutes after the spectrometer was turned on to achieve a stable plasma and constant and reproducible sample introduction. Sample preparation was carried out by dissolving 10 mg of the dried and ground sample catalyst in 4 mL of concentrated HNO3. All sample solutions were ultrasonically heated at 80 °C for 2 hours. Then, the solution was diluted to 20 mL (diluted with ultrapure water) and then analyzed by ICP-OES.

[0056] The specific surface area was calculated according to the Brunauer-Emmett-Teller (B.E.T) from the N2 adsorption-desorption isotherm measured on a high-precision flow method specific surface area analyzer (FlowSorb III Micromeritics) produced by Micromeritics, Norcross, Georgia, USA. The morphology of the catalyst was examined using a scanning electron microscope (SEM) (S-3400N, Hitachi, Japan). Energy-dispersive X-ray spectroscopy (EDX) (Thermo SCIENTIFIC UltraDry) was used to obtain the energy-dispersive X-ray spectrum.

[0057] X-ray photoelectron spectroscopy (XPS) was performed on an Escalab 220XL spectrometer. Using a monochromatic Al Kα X-ray source, the electron energy was measured in the constant analyzer energy mode. The pass energy was 100 eV for the survey spectrum and 40 eV for the single-element spectrum. All XPS binding energies were referenced to the C1s core level at 285 eV. The angle between the incident X-ray and the analyzer was 58°, and the photoelectrons were collected perpendicular to the sample surface. The spectra were analyzed using CasaXPS software (version 2.3.25).

[0058] The reducible species present in the catalyst were analyzed by temperature-programmed reduction. Hydrogen temperature-programmed reduction (H2-TPR) was measured on a fully automated temperature-programmed chemisorption analyzer (Micromeritics AutoChem II 2920 instrument), which was equipped with a thermal conductivity detector (TCD) to monitor H2 consumption. After calibrating H2 on the TCD, the sample was sealed in a U-shaped quartz tube reactor and pretreated in an argon atmosphere to remove surface impurities. Then, it was heated from 25 °C to 1000 °C in a 5% v / v H2 / Ar flow at a rate of 5 °C / min. Laser-Raman spectra from 200 to 1500 cm -1 were recorded at room temperature using an FT-Raman spectrometer (Dilor XY Raman, France) with an excitation wavelength of 647.1 nm, a laser power of 3 mW, and a spectral resolution of 0.5 cm -1 .

[0059] Example 1

[0060] Preparation of Ni-Ag bimetallic catalyst: Nickel(II) nitrate hexahydrate (Ni(NO3)2·6H2O, ≥97%) and silver nitrate (AgNO3, ≥98%) were separately dissolved in distilled water. Then the two solutions were mixed and stirred evenly according to the molar ratios of Ni and Ag being 1, 1.5, and 2 respectively. After adding the precipitant NaOH (2M) until the pH reached 10, the precipitate was collected and washed repeatedly with distilled water and ethanol. After drying at 100 °C and grinding, it was calcined in a constant air flow at 600 °C, 700 °C, 1000 °C, and 1200 °C (with a heating rate of 5 °C / min) for 4 h. The resulting powder was the nickel-silver catalyst, denoted as 1NA600 (representing the product of the precipitate with a Ni / Ag molar ratio of 1 calcined at 600 °C), 1.5NA600 (representing the product of the precipitate with a Ni / Ag molar ratio of 1.5 calcined at 600 °C), 2NA600 (representing the product of the precipitate with a Ni / Ag molar ratio of 2 calcined at 600 °C), 1NA700 (representing the product of the precipitate with a Ni / Ag molar ratio of 1 calcined at 700 °C), 1.5NA700 (representing the product of the precipitate with a Ni / Ag molar ratio of 1.5 calcined at 700 °C), 2NA700 (representing the product of the precipitate with a Ni / Ag molar ratio of 2 calcined at 700 °C), 1NA1000 (representing the product of the precipitate with a Ni / Ag molar ratio of 1 calcined at 1000 °C), 1.5NA1000 (representing the product of the precipitate with a Ni / Ag molar ratio of 1.5 calcined at 1000 °C), 2NA1000 (representing the product of the precipitate with a Ni / Ag molar ratio of 2 calcined at 1000 °C), 1NA1200 (representing the product of the precipitate with a Ni / Ag molar ratio of 1 calcined at 1200 °C), 1.5NA1200 (representing the product of the precipitate with a Ni / Ag molar ratio of 1.5 calcined at 1200 °C), 2NA1200 (representing the product of the precipitate with a Ni / Ag molar ratio of 2 calcined at 1200 °C).

[0061] (1) The thermal stabilities of the above catalysts were evaluated using thermogravimetric analysis - differential scanning calorimetry (TG-DSC), mass spectrometry (MS), and X-ray diffraction (XRD), TG-DSC test techniques.

[0062] The TG-DSC test results of the 2AN1000 nickel-silver catalyst obtained in the temperature range from ambient temperature (25 °C) to 1000 °C are as Figure 1 shown Figure 1 where the black represents the mass change curve and the red represents the heat change curve. From Figure 1It can be seen that there are three important loss steps before 600 °C. The sample has a heat flow peak at 256 °C (endothermic phenomenon) and 950 °C (heat flow peak not attributed), and water and carbon dioxide are determined to be the species causing weight loss. Mass spectrometry confirmed the presence of water (m / z = 18) and CO2 (m / z = 44). The first step corresponds to the removal of physically adsorbed water from the material surface, while the second step involves the release of interlayer water and carbon dioxide from the catalyst structure. As the temperature continues to rise, no additional weight loss or significant changes are observed, and the surface material structure has reached a relatively stable state.

[0063] The XRD analysis results obtained for the above-mentioned partial samples are as Figure 2 shown. It can be Figure 2 seen that regardless of the calcination temperature, the main phases obtained after calcination are metallic silver ((PDF file number 89-3722) and nickel oxide (PDF file number 47-1049). The composition of the catalyst remains stable after different high-temperature calcinations. From TG-DSC, no weight loss is observed above 600 °C. Except for the peaks corresponding to silver and nickel oxide, no other diffraction peaks are observed in the XRD analysis even at 1200 °C. These findings indicate that the composition of the above-mentioned catalyst remains stable at high temperatures. Considering that the DRM reaction is carried out in the temperature range of 600 to 850 °C, 600 °C is selected as the calcination temperature to maintain the structure and texture of the catalyst.

[0064] (2) ICP-OES and EDX spectral analyses were carried out on the stoichiometry of the above-mentioned catalyst, and the results are shown in Table 1 below.

[0065] Table 1

[0066]

[0067] As can be seen from Table 1 above, EDX analysis confirmed the presence of silver and nickel elements. ICP-OES spectra showed that the measured amounts of all analyzed species were slightly lower than the nominal amounts used in the preparation. The low amount of Ni species in all cases can be explained by the relatively low calcination temperature (600 °C / 4 h) or incomplete precipitation of Ni(OH)2 during the preparation synthesis. In contrast, for all three catalysts, the overall EDX analysis revealed a Ni / Ag mass ratio higher than the theoretical value, indicating a relatively high concentration of nickel in the outer layer of the catalyst.

[0068] (3) The structural characterization (XRD) of the above-mentioned partial catalysts is as Figure 2 shown. Figure 2The presence of NiO and Ag materials is shown, with peaks located at 2θ = 37.24°, 43.27°, 62.87°, 75.41° and 79.40°, corresponding to the (111), (200), (220), (311) and (222) crystal planes of the cubic NiO phase (PDF 47-1049), while the peaks at 38.12°, 44.307°, 64.45° and 77.41° belong to the (111), (200), (220) and (311) crystal planes of metallic, cubic Ag (PDF 89-3722).

[0069] To obtain more information about the relative proportions of the phases present in the catalyst, Rietveld refinement was performed on three samples annealed at 600 °C. Rietveld refinement (i.e., XRD refinement) was carried out using the JANA 2020 program to determine the lattice parameters, grain size (using the fundamental parameters method), and the relative amounts of each crystalline phase. These parameters are given in Table 1, and the results for Ni / Ag = 1 can be seen in Figure 3 . The grain size of NiO varied between 8.7 and 16.8 nm, indicating differences between the samples. The results also showed that as the Ni / Ag ratio increased, the grain size of NiO also increased. For all catalysts, NiO was identified as the dominant phase, which is consistent with the results of EDX analysis. The lattice parameters (a) of the detected NiO and Ag phases were similar for all three catalysts. The relatively small grain size of NiO observed in the 1NA600 sample corresponded to its larger surface area (20 m 2 / g, Table 1).

[0070] (4) Textural characterization of the above catalysts. The specific surface area of the catalyst powder was determined by applying the Brunauer–Emmett–Teller (BET) method, i.e., N2 adsorption–desorption analysis, and the results can be seen in Table 1. The morphology of the synthesized materials was evaluated using scanning electron microscopy (SEM). The BET data showed that the specific surface area decreased from 20 m 2 / g for the 1NA600 sample to 3 m 2 / g and 5 m 2 / g for the 2NA600 and 1.5NA600 samples, respectively. These results are consistent with the results of the Rietveld refinement analysis. As the surface area increased, the size of the NiO grains decreased proportionally, and this decrease in BET surface area corresponded to an increase in the Ni / Ag ratio. Figure 4 SEM images of some of the catalysts are shown, revealing significant changes in the particle shape. The contrast in the SEM images is due to the presence of the NiO and Ag phases. For the 1NA600 sample, Figure 4The microscopic image in a shows a surface composed of Ag and NiO nanoparticles, which aggregate to form clusters of different sizes and are randomly dispersed. The formation of relatively large spheres can be attributed to the aggregation tendency of NiO nanoparticles, which are antiferromagnetic in nature. Regarding the 1.5NA600 sample, Figure 4 The SEM microscopic image in b shows that the powder coexists with aggregates of nano-scale particles and larger particle aggregates. Significant differences in particle shape can be observed, and aggregation is more obvious compared to the 1NA600 sample. For the 2NA600 sample, the surface morphology of NiO shows the random growth of nano-cubes of different sizes, as Figure 4 shown in c, forming smaller and larger nano-cubes. Figure 4 The aggregation shown in c is more than that of the 1NA600 sample but less than that of the 1.5NA600 sample. In summary, it is obvious that Ni loading significantly affects the morphology of the catalyst.

[0071] XPS measurements were carried out on the surface chemical properties of the 1NA600, 1.5NA600, and 2NA600 catalysts, and the results are as Figure 5 、 6 shown. From Figure 5 、 6 , it can be seen that regardless of the amount used in the synthesis process, the binding energies of the two species (Ag and Ni) actually do not change. The decomposition of the XPS spectrum does not show any components, and the recorded binding energy values clearly indicate that both Ag and Ni are in a unique oxidation state (only 1 oxidation state for each species), thus explaining the formation of a single phase corresponding to each species (i.e., each species forms a single phase) as observed by XRD analysis. The binding energy of the Ag 3d photoelectron peak is located at 368.8 eV (1NA600), 368.7 eV (1.5NA600), and 368.5 eV (2NA600), while the binding energy of Ni 2p3 / 2 is located at 854 eV (1NA600), 854.1 eV (1.5NA600), and 853.7 eV (2NA600). These results are in good agreement with the face-centered cubic (fcc) metallic silver structure obtained by XRD. For the nickel species, the binding energy of Ni(II) is found near 854.9 eV for NiO, accompanied by a shoulder peak of about 1.2 eV, which fully confirms that Ni(II+) comes from the NiO phase rather than other phases of Ni-containing species that may form during the synthesis process (consistent with the XRD data). As observed by EDX analysis, the Ni / Ag ratio is also higher for the three samples, indicating that the surfaces of the 1NA600, 1.5NA600, and 2NA600 samples are enriched with Ni(II) species. This surface behavior is consistent with the large amount of NiO estimated by Rietveld refinement.

[0072] Example 2

[0073] At atmospheric pressure, the catalytic performance was tested using a fixed-bed quartz flow reactor placed in a programmable furnace. Before loading the catalyst (200 mg) into the reactor, it was thoroughly mixed with silicon carbide (SiC) powder (100 mg). A mixture of methane, carbon dioxide, and argon (CH4:CO2:Ar = 20:20:60) was introduced into the reactor at a total flow rate of 100 mL / min, and the reaction was initiated by heating from room temperature to the set temperature (400 - 850 °C) at a rate of 5 °C / min. An on-line mass spectrometer was used to analyze all the exhaust gases during the reaction.

[0074] (1) The reducibility of the fresh catalyst was studied by temperature-programmed reduction (TPR) with hydrogen, and the catalyst was analyzed by X-ray diffraction (XRD) after reduction. The variation of hydrogen consumption with temperature in different system studies is as Figure 7 shown. For the three samples, a slight difference in hydrogen consumption (about 5 - 9 mmol / g) was observed. Specifically, the amount of hydrogen consumed per gram of 1NA600 was 6.4 mmol, the amount of hydrogen consumed per gram of 1.5NA600 was 5.5 mmol, and the amount of hydrogen consumed per gram of 2NA600 was 8.6 mmol. The TPR curves of the three catalysts showed a similar pattern, with two main hydrogen consumption peaks demonstrated between 100 - 400 °C. The first peak could be attributed to the reduction of surface nickel species, as surface species tend to be reduced at lower temperatures than those in the bulk. The second peak was related to the reduction of surface and / or surface-bulk NiO, representing the Ni reduction mechanism. The peak at 350 - 500 °C could be attributed to the reduction of NiO. The TPR curves indicated that the reduction of nickel was a two-step process, and the intensity of the second peak was higher than that of the first peak. With the increase in the Ni / Ag content, especially in the 1NA600 and 2NA600 samples, the TPR peak position shifted slightly to higher temperatures, and the peak area increased, indicating that the presence of silver decreased the reduction temperature of nickel.

[0075] After in-situ reduction of the catalyst at 400 °C for 1 hour, the reduced catalyst was analyzed by XRD, and the results are as Figure 8 shown. Figure 8Two or three different phases are shown, depending on the concentration. Different peaks are identified as those of metallic Ag (PDF 89-3722) at 2θ = 38.12° (111), 44.307° (200), 64.45° (220), and 77.41° (311); those of metallic Ni (PDF 04-0850) at 2θ = 44.5° (111), 51.8° (200), and 76.4° (220); and those of NiO (PDF 47-1049) at 2θ = 37.24° (111), 43.27° (200), 62.87° (220), and 75.41° (311), for both the 1.5NA600 and 2NA600 samples. In the case of the 1NA600 sample, no NiO peaks were observed, indicating that NiO had been completely reduced to metallic Ni during the reduction process. In contrast, the presence of NiO peaks observed in the diffraction patterns of the 1.5NA600 and 2NA600 samples suggests that NiO was not completely reduced to Ni during the H2 treatment, probably due to the higher NiO content in the samples, as observed by EDX analysis (Table 2). The grain size of the metallic species increased after reduction (Table 2); for the low ratio formulation (Ni / Ag = 1), this increase was very significant.

[0076] Table 2

[0077]

[0078]

[0079] (2) Catalytic activity and temperature tests. The test results of the catalyst activity in the range of 600 - 850 °C are as Figure 9 、 10 shown. Figure 9 、 10Shows the curves of activity, methane and carbon dioxide conversion rates, hydrogen selectivity and hydrogen / carbon monoxide ratio as a function of temperature. Among them, the methane and carbon dioxide conversion rates of the three catalysts showed similar trends. The 1NA600 catalyst reached the highest methane and carbon dioxide conversion rates at 650 °C, and the 1.5NA600 and 2NA600 catalysts showed similar conversion behaviors. For the 1NA600 catalyst, the activation of carbon dioxide started at 650 °C, while for the 1.5NA600 and 2NA600 catalysts, it started at 700 °C and 750 °C respectively. There were differences in the hydrogen / carbon monoxide ratios of the three catalysts. At 650 °C, the hydrogen / carbon monoxide ratio of 1NA600 reached a maximum value of 0.7. In contrast, the values of 1.5NA600 and 2NA600 were 0.1 and 0.4 respectively. Regarding selectivity, 2NA600 showed the highest selectivity, and 2NA600 reached a maximum selectivity of 85% at 600 °C. At 650 °C and 850 °C, the hydrogen selectivity of 1NA600 reached the highest values, 71% and 73% respectively. It may be mainly due to the side reactions (i) CO + 3H2 → CH4 + H2O, (ii) CO2 + 4H2 → CH4 + 2H2O and / or (iii) RWGS contribution, resulting in activity loss. The participation of the reactions CO + 3H2 → CH4 + H2O and CO2 + 4H2 → CH4 + 2H2O can be excluded because of thermodynamic reasons (if T < 500 °C, ΔG of the two reactions < 0, and at 800 °C, ΔG(i) ~ +55.41 Kj / mol and ΔG(ii) ~ +55.98 Kj / mol). The contribution of the RWGS reaction seems to be the most likely (ΔG ~ 0.5 Kj / mol), resulting in the re-oxidation of the active sites (Ni°) by the H2O vapor formed during the catalytic test.

[0080] At 650 °C, the 1NA600 had the highest methane and carbon dioxide conversion rates, and the hydrogen selectivity also reached 71%. The activity of 1NA600 was higher than that of 1.5NA600 and 2NA600, which may be attributed to its higher specific surface area and smaller NiO grain size. 1NA600 and 2NA600 showed lower carbon loss, while 1.5NA600 had no carbon loss.

[0081] (3) Aging test. To evaluate the stability of the synthesized catalyst over time, we conducted a series of 4-hour time-feed flow tests at 650 °C, and the results are as Figure 11 、 12 shown. Among them, the isothermal test was to heat the catalyst sample to the reaction temperature (650 °C) in an inert gas and then expose it to the reactants for 4 hours under the same conditions. From Figure 11 、 12It can be seen that among these catalysts, 1NA600 exhibits a higher conversion rate than 1.5NA600 and 2NA600, but the conversion rate of the reactants gradually decreases over time. The conversion rates of the reactants for 1.5NA600 and 2NA600 remain relatively low and stable throughout the study period (up to 240 minutes). The CH4 conversion rates of the 1.5NA600 and 2NA600 catalysts are similar. The 1NA600 catalyst starts showing signs of deactivation from the 12th minute, which may be attributed to sintering. However, some residual activity is still observed after 150 minutes, and the results of the isothermal activity test at 650 °C are consistent with those of the temperature-rise activity test conducted at the same temperature. Different from the temperature-rise test at 650 °C showing no CO2 conversion, 2NA600 shows a low CO2 conversion rate at the 50th minute. The H2 / CO ratio of 1NA600 initially reaches a maximum value of 0.5 but gradually decreases and stabilizes at around 0.3 after the 100th minute. For 2NA600, after an unexpected decline, the H2 / CO ratio gradually increases, approaching a value of 0.6. In the case of 1.5NA600, it reaches a maximum value at the 30th minute and remains stable. Although the H2 selectivity of 1NA600 tends to decrease over time, it starts to increase again from the 130th minute. On the other hand, for 2NA600 and 1.5NA600, it continues to increase over time.

[0082] It is noteworthy that the CH4 conversion rates of the 1.5NA600 and 2NA600 catalysts are higher than the CO2 conversion rates, which may be attributed to methane combustion occurring due to a lower H2 / CO ratio. This combustion is related to the reduction of the catalyst, and the released oxygen is responsible for the oxidation of methane. The results of the three catalysts at the 20-minute mark are consistent with those of the temperature-rise catalytic test conducted at 650 °C.

[0083] The catalysts were analyzed using XRD and Raman spectroscopy after the isothermal test, and the results are as Figure 13 、 14 shown. As Figure 13 can be seen, for the lines corresponding to the cubic structure of silver (PDF 89-3722), its peaks are located at 2θ = 38.12° (111), 44.307° (200), and 64.45° (220), and for the metallic nickel phase (PDF 04-0850), its peaks are located at 2θ = 44.5° (111), 51.8° (200), and 76.4° (220). In addition, the characteristic peaks of SiC are also observed. The Raman spectrum ( Figure 14 ) mainly shows the lines of SiC at 760, 780, and 960 cm -1 , and there is no line at 1500 cm -1The line at [location] indicates no deposited carbon. Post-reaction analysis indicates that deactivation may be mainly due to sintering phenomena. After the reaction, the grain size of Ni metal species underwent a volcanic change (Table 2); the 1NA600 sample showed the lowest grain size value of Ni metal species.

[0084] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present invention, rather than limiting it; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A nickel-silver catalyst for hydrogen production by dry reforming of methane, characterized in that, Comprising at least a mixture of NiO and Ag, wherein the molar ratio of Ni to Ag is (0.73 - 1.51):1, the grain size of the NiO is 8.7 - 16.8 nm, and the surface area of the nickel-silver catalyst is 3 - 20 m2 / g.

2. The nickel-silver catalyst for methane dry reforming to produce hydrogen according to claim 1, characterized in that, It further comprises a promoter, the promoter being silicon carbide, and the mass ratio of the mixture of NiO and Ag to the promoter is (1 - 3):

1.

3. The nickel-silver catalyst for methane dry reforming to produce hydrogen according to claim 1, characterized in that, The mixture of NiO and Ag is obtained by co-precipitating nickel salt and silver salt, and the resulting precipitate is washed, dried, ground, and then calcined in an air atmosphere at a temperature above 600°C for 3 - 7 h. The nickel salt and silver salt are in terms of nickel ions and silver ions, and the molar ratio of the two is (1 - 2):

1.

4. The nickel-silver catalyst for methane dry reforming to produce hydrogen according to claim 3, wherein, The precipitate is produced by a solution containing nickel salt and silver salt under the condition of pH 9 - 11.

5. The nickel-silver catalyst for hydrogen production by dry reforming of methane according to claim 3 or 4, characterized in that, The washing is carried out using ethanol and distilled water.

6. The nickel-silver catalyst for methane dry reforming to produce hydrogen according to claim 3 or 4, characterized in that, The temperature during drying is 60 - 120°C.

7. The nickel-silver catalyst for methane dry reforming to produce hydrogen according to claim 3 or 4, characterized in that, The nickel salt is selected from one or more of nickel nitrate hexahydrate, nickel chloride, and nickel sulfate; The silver salt is selected from one or two of silver nitrate and silver sulfate.

8. The nickel-silver catalyst for methane dry reforming to produce hydrogen according to claim 3 or 4, characterized in that, The nickel salt and silver salt are in terms of nickel ions and silver ions, and the ratio of the two is (0.8 - 1.2):

1.

9. A method for hydrogen production by dry reforming of methane, characterized in that, Comprising the following steps: Contacting the nickel-silver catalyst according to any one of claims 1 - 8 with a mixed gas containing methane, carbon dioxide, and argon, and heating to 650 - 750°C for reaction. The volume ratio of methane, carbon dioxide, and argon in the mixed gas is (15 - 25):(15 - 25):(40 - 70).

10. The method for hydrogen production by dry reforming of methane according to claim 9, characterized in that, Before the nickel-silver catalyst contacts with the mixed gas, it further comprises a step of mixing evenly with a promoter.

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