Au / mo n model catalyst and preparation method thereof

Abstract

The application belongs to the technical field of catalysts, and particularly relates to an Au / MoN model catalyst and a preparation method thereof. The Au / MoN model catalyst is successfully prepared, and the interface between the metal and the nitride is constructed. SEM, EDX-mapping, AFM and the like characterizations prove that the Au can be highly dispersed on the MoN carrier, and the in-situ Ar + XPS characterization proves that the Au and the MoN carrier are in a phase separation state, and are mainly segregated on the surface of the MoN, so as to form the interface with the MoN. The in-situ XPS proves that the charge transfer process exists at the interface between the Au and the MoN, so that the Au is positively charged, and the dispersion-aggregation state of the Au structure can be controlled through the alternative treatment of nitridation-oxidation. These findings have important significance for understanding the SMSI effect of the metal catalyst supported by the nitride, and lay a foundation for developing the cheap catalyst with high catalytic performance.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of catalysts, and particularly relates to an Au / MoN model catalyst and a preparation method thereof. BACKGROUND

[0002] Supported metal catalysts have been widely used in catalytic reactions for the preparation of various high-value-added chemicals and exhibit good catalytic performance. At present, related fields have prepared catalysts with oxides, carbides and nitrides and other materials as carriers and have significant performance. In view of the exploration of catalysts, researchers have found that the structure of the catalyst plays a crucial role in the efficient activation and directional conversion of the reaction. The size, morphology, coordination environment of the active metal and the use of different carriers can significantly change the conversion rate of the catalytic reaction and the selectivity of the product. In addition, the carrier not only plays a role in dispersing the metal, but also participates in the catalytic reaction or interacts with the active metal in some cases, thereby affecting the catalytic performance. This is also an important manifestation of the strong metal-support interaction (SMSI). The classic SMSI effect has a strong regulating effect on the adsorption behavior of surface active molecules of the catalyst, and this effect can be used to realize the directional regulation of product selectivity and catalytic stability at high temperature. Therefore, understanding the relationship between the structure and performance of the catalyst and preparing a catalyst with a suitable structure are the keys to realizing efficient conversion.

[0003] Because transition metal nitrides and transition metal carbides exhibit good catalytic activity in catalytic reactions, some of which have performance close to or better than traditional noble metal catalysts, they are called "quasi-platinum catalysts". At present, transition metal nitrides and transition metal carbides can not only be used as catalysts alone, but also be used as catalyst carriers to load active metals for catalytic reactions.

[0004] For example, the strong interaction between the metal and the molybdenum nitride carrier makes the active metal in a highly dispersed state on the molybdenum nitride surface, which has excellent anti-sintering performance. Lin et al. (Lin L, Yu Q, Peng M, et al., Atomically dispersed Ni / alpha-MoC catalyst for hydrogen production from methanol / water [J]. Journal of American Chemical Society, 2020, 143 (1), 309-317.) found that the atomically dispersed Ni metal catalyst loaded on alpha-MoC exhibited excellent hydrogen production activity in the methanol / water reforming reaction. In addition, the Ni-C xThe synergistic effect between α-MoC and transition metals generates an active interface structure for water dissociation and methanol activation. It is worth noting that transition metal carbides and nitrides can also regulate the morphology of the supported metal as another important feature of strong metal-support interaction. For example, Esposito et al. (Esposito D V, Hunt S T, Kimmel Y C, et al., A new class of electrocatalysts for hydrogen production from water electrolysis: metal monolayers supported on low cost transition metal carbides [J]. Journal of American Chemistry Society, 2012, 134 (6), 3025-3033.) found that metal Pt can be deposited on the surface of WC (W2C) in a monolayer dispersion. Therefore, it is of great significance to understand the interface interaction between the active metal and the support for the design and synthesis of new high-efficiency catalysts.

[0005] In the early stage, gold has always been regarded as a precious commodity. Due to the poor chemical adsorption ability of gold to reaction molecules compared with platinum group metals, its catalytic activity has not been recognized. Haruta et al. (Date M, Haruta M J, et al., Moisture effect on CO oxidation over Au / TiO2 catalyst[J]. Journal of Catalysis, 2001, 201 (2), 221-224.) found that highly dispersed Au nanoparticles showed surprising activity in the CO oxidation reaction, and the nano-Au catalyst began to be widely concerned. Based on the noble metal-like electronic structure and electronic structure of transition metal carbides and transition metal nitrides, relevant studies have shown that they can specifically anchor Au through metal-metal bonds, thereby maintaining the metal properties of Au. At present, Au has been proved to be able to achieve high dispersion in transition metal carbide catalysts and exhibit excellent catalytic activity. According to Yao et al. (Yao S, Zhang X, Zhou W, et al., Atomic-layered Au clusters on α-MoC as catalysts for the low-temperature water-gas shift reaction[J]. Science, 2017, 357 (6349), 389-393.), two-dimensional layered gold clusters supported by α-MoC can exhibit surprising catalytic activity for the water-gas shift reaction under reaction conditions below 423 K. In addition, the inventors of the present application believe that the thin-layer structure of Au dispersed on α-MoC is more active than that of Au nanoparticles. However, there has been controversy about the existence form and distribution state of active Au species. Correctly identifying the existence form and distribution state of active Au species is conducive to understanding the catalytic reaction mechanism and laying a foundation for designing efficient catalysts.

[0006] Based on this, the Au / MoN model catalyst with regular surface and two-dimensional characteristics is successfully prepared, and the construction of the metal and nitride interface is realized. SUMMARY

[0007] The purpose of the present application is to provide an Au / MoN model catalyst and a preparation method thereof to solve the existing problems.

[0008] The present application is realized by the following technical solutions:

[0009] A preparation method of an Au / MoN model catalyst, comprising the following steps:

[0010] S1, dissolving Na2MoO4 and HAuCl4·3H2O to form a mixed solution as a precursor;

[0011] S2, coating the precursor of step S1 on the surface of an alumina single crystal substrate;

[0012] S3, placing the alumina single crystal substrate coated with the precursor in a tube furnace, using high-purity Ar as the carrier gas, controlling the carrier gas flow rate to be 100 sccm, and stepwise heating to the growth temperature;

[0013] S4, when the temperature reaches the growth temperature, introducing NH3 as a nitrogen source, controlling the NH3 flow rate to be 10 sccm, and performing a chemical vapor deposition reaction under normal pressure conditions, with a reaction time of 2 h;

[0014] S5, after the reaction is completed, cooling to room temperature in an Ar atmosphere to obtain an ultra-thin epitaxial Au / MoN model catalyst.

[0015] Further, in step (1), the concentration of HAuCl4·3H2O in the mixed solution is adjusted to control the loading amount of Au.

[0016] Further, in step S1, when dissolving Na2MoO4 and HAuCl4·3H2O to form a mixed solution, a composite dispersant is added in an amount of 1-2% of the total mass of the mixed solution;

[0017] The preparation of the composite dispersant includes the following steps:

[0018] (1) Dissolve trisodium citrate powder in deionized water according to a mass ratio of 1:10 to prepare a 10% trisodium citrate solution, then place the solution in an ultrasonic cleaning instrument, set the power to 300 W and the temperature to 60-65°C, and treat for 2-3 h, while introducing oxygen at a flow rate of 5-6 sccm, to obtain pretreated trisodium citrate;

[0019] (2) Add 3-aminopropyltriethoxysilane to the pretreated trisodium citrate according to a volume ratio of 5:1, and stir at 400-500 rpm for 1-2 h to obtain a composite dispersant.

[0020] The addition of the composite dispersant promotes the uniform dispersion of metal ions in the solution, avoids the agglomeration of the precursor caused by excessive local concentration, and significantly improves the stability of the mixed solution, providing a more uniform precursor source for the subsequent coating step.

[0021] Further, in step S2, the coating method is spin coating, and the specific operation is as follows: drop the precursor onto the surface of the alumina single crystal substrate through a dropper, and then use a spin coater to spin coat the mixed solution on the substrate surface.

[0022] Further, the purity of the high-purity Ar in step S3 is not less than 99.999%.

[0023] Further, the step of gradually increasing the temperature to the growth temperature in step S3 is specifically: first increasing the temperature to 500 DEG C at a temperature increasing rate of 8-10 DEG C / min, and then increasing the temperature to 950 DEG C at a temperature increasing rate of 4-6 DEG C / min.

[0024] The step of gradually increasing the temperature can reduce the thermal stress difference between the substrate and the precursor, avoid the substrate cracking or the precursor decomposition uneven phenomenon caused by the temperature rising too fast, and ensure the subsequent nitridation reaction under more stable conditions.

[0025] Further, the growth temperature in step S3 is 950 DEG C.

[0026] Further, the step of reaching the growth temperature in step S4 is specifically: after the temperature in the tube furnace is increased to 950 DEG C and stabilized, NH3 is introduced.

[0027] Further, the Ar flow rate is maintained at 100 sccm during the step of decreasing the temperature in Ar atmosphere in step S5.

[0028] Compared with the prior art, the present application has the following advantages:

[0029] 1. The Au / MoN model catalyst with regular surface and two-dimensional characteristics is successfully prepared based on the gas-liquid-solid growth mechanism, and the interface between the metal and the nitride is constructed. It is confirmed that Au is segregated on the MoN surface to realize the phase separation process at high temperature, and Au is highly dispersed on the MoN surface. In addition, there is a charge transfer process between Au and MoN to form a strong interaction. This discovery is of great significance for understanding the SMSI effect of the nitride-supported metal catalyst, and lays a foundation for developing cheap catalysts with high catalytic performance.

[0030] 2. The present application confirms that Au can be dispersed on the surface of MoN and the SMSI effect between Au and the nitride carrier through various characterization methods, which deepens the understanding of the SMSI effect. The main conclusions are as follows:

[0031] (1) The Au / MoN model structure with regular surface, two-dimensional characteristics and horizontal size of about 500 nm can be prepared on the surface of alumina single crystal by using the mixed solution of HAuCl4·3H2O and Na2MoO4 as the precursor and through the method of atmospheric pressure chemical vapor deposition.

[0032] (2) SEM (scanning electron microscope), EDX-mapping (energy dispersive X-ray mapping), AFM (atomic force microscope) and XRD (X-ray diffraction) and the like characterization confirmed that Au can be highly dispersed on the MoN carrier, in-situ Ar ion etching XPS (X-ray photoelectron spectroscopy) characterization confirmed that Au and MoN carrier is in a phase separation state, mainly segregating on the surface of MoN, thereby forming an interface with MoN.

[0033] (3) In-situ XPS proves that there is a charge transfer process at the interface of Au and MoN, which makes Au positive, and the dispersion-aggregation state of Au structure can be controlled by the alternating treatment of nitridation-oxidation. These characteristics show that there is a strong interface effect between Au and MoN, which is the embodiment of SMSI effect. BRIEF DESCRIPTION OF DRAWINGS

[0034] Figure 1 a is a growth schematic diagram of Au / MoN model structure;

[0035] Figure 1 b to Figure 1 c is a SEM image of Au / MoN model structure;

[0036] Figure 1 d is an AFM image of Au / MoN model structure;

[0037] Figure 1 e is an EDX-mapping image of surface element Mo of Au / MoN model structure;

[0038] Figure 1 f is an EDX-mapping image of surface element N of Au / MoN model structure;

[0039] Figure 1 g is an EDX-mapping image of surface element Au of Au / MoN model structure;

[0040] Figure 2 a is an XPS spectrum of Mo 3d of Au / MoN model structure;

[0041] Figure 2 b is an XPS spectrum of N 1s + Mo 3p of Au / MoN model structure;

[0042] Figure 2 c is an XPS spectrum of Au 4f of Au / MoN model structure;

[0043] Figure 2 d is an XRD spectrum of Au / MoN model structure;

[0044] Figure 3HRTEM image of Au / MoN model structure;

[0045] Figure 4 a is a SEM image of Au / MoN model structure; Figure 4 b is a SEM image of Au / Na2MoO4model structure;

[0046] Figure 4 c is an EDX-mapping image of surface element Mo of Au / Na2MoO4model structure;

[0047] Figure 4 d is an EDX-mapping image of surface element Au of Au / Na2MoO4model structure;

[0048] Figure 5 a is a SEM image of Au / MoN model structure; Figure 5 b is an in-situ Ar-ion etching XPS spectrum of Au film on ZrO2surface;

[0049] Figure 5 c is an AFM image of Au film on ZrO2surface;

[0050] Figure 5 d is the thickness of Au film on ZrO2surface;

[0051] Figure 6 In-situ Ar-ion etching XPS spectrum of Au / MoN model structure;

[0052] Figure 7 a is a SEM image of CoMoN x model structure;

[0053] Figure 7 b is an EDX-mapping image of surface element Mo of CoMoN x model structure;

[0054] Figure 7 c is an EDX-mapping image of surface element N of CoMoN x model structure;

[0055] Figure 7 d is an EDX-mapping image of surface element Co of CoMoN x model structure;

[0056] Figure 7 e is an XRD spectrum of CoMoN x model structure;

[0057] Figure 8 In-situ Ar-ion etching XPS spectrum of CoMoN x model structure;

[0058] Figure 9 a to Figure 9 b is the SEM image of the Au / MoN model structure after oxidation;

[0059] Figure 9 c to Figure 9 d is the EDX-mapping image of the surface elements (Mo, Au) of the Au / MoN model structure after oxidation;

[0060] Figure 10 a is the Raman spectrum of the Au / MoN model structure after oxidation;

[0061] Figure 10 b is the XRD spectrum of the Au / MoN model structure after oxidation;

[0062] Figure 10 c to Figure 10 d is the XPS spectrum of the Au / MoN model structure after oxidation;

[0063] Figure 11 is the in-situ Ar-ion etching XPS spectrum of the Au / MoN model structure after oxidation;

[0064] Figure 12 is the in-situ XPS spectrum of the Au / MoN model structure during pre-reduction to oxidation process;

[0065] Figure 13 is the schematic diagram of SMSI effect between Au overlayer and nitride support. DETAILED DESCRIPTION

[0066] In order to further explain the present application, the following specific examples are combined to illustrate the present application.

[0067] Example 1

[0068] Construction of Au / MoN model structure

[0069] The Au / MoN model catalyst is prepared on the surface of an alumina single crystal by using the method of atmospheric pressure chemical vapor deposition based on the gas-liquid-solid growth mechanism. Specifically, 2 mL of Na2MoO4 with a concentration of 150 mg / mL and 2 mL of HAuCl4·3H2O with a concentration of 5 mg / mL are taken as the same volume to form a mixed solution, 1.5% of the total mass of the mixed solution is added as a composite dispersant when the mixed solution is formed, as a precursor, and the MoN is loaded with different amounts of Au by changing the concentration of HAuCl4·3H2O; the preparation of the composite dispersant comprises the following steps: (1) according to the mass ratio of 1:10, the trisodium citrate powder is dissolved in deionized water to configure a 10% trisodium citrate solution, then the solution is placed in an ultrasonic cleaning instrument, the power is set to 300 W and the temperature is set to 60℃, and the treatment is 2.5 h, and oxygen is introduced at the same time, the flow rate of oxygen is 6 sccm, and the pretreated trisodium citrate is obtained after completion; (2) according to the volume ratio of 5:1, 3-aminopropyltriethoxysilane is added to the above-mentioned pretreated trisodium citrate, and the composite dispersant is obtained by stirring at 400 rpm for 1.5 h; 0.2 mL of the precursor is sucked by a dropper and dropped onto the surface of the alumina single crystal, and the mixed solution is spin-coated onto the surface of the substrate by using a spin coater; then the spin-coated sample is placed in a tube furnace, the growth temperature is 950℃, the carrier gas is high-purity Ar (the purity is not less than 99.999%), the flow rate is 100 sccm, the temperature is first raised to 500℃ at a rate of 9℃ / min, and then raised to 950℃ at a rate of 5℃ / min. Figure 1 (a) is a schematic diagram of the growth of the Au / MoN model catalyst, when the reaction temperature rises to the target temperature, NH3 is introduced to provide a nitrogen source, the flow rate is 10 sccm, the chemical vapor deposition reaction is carried out under atmospheric pressure conditions, the reaction time is 2 h, when the reaction is completed, the sample is cooled in an Ar atmosphere, and in the process of cooling in the Ar atmosphere, the Ar flow rate is maintained at 100 sccm, and when the temperature drops to room temperature, the Au / MoN model structure is successfully prepared.

[0070] The SEM image is taken by a JSM-7800F field emission scanning electron microscope, and the operating voltage is 5 kV. The energy dispersive X-ray spectrometer equipped with the device can perform element distribution characterization under the condition of 10 kV. Figure 1 As shown in (b), the SEM image shows that the surface of the substrate alumina single crystal presents a triangular and hexagonal nanosheet structure with regular surface and consistent orientation, but there are a small amount of cluster or particle structures due to the non-uniformity during spin coating. The lateral size of the triangular nanosheet structure is about 500 nm. In addition, no agglomeration or particle structure is found on the surface of the triangular nanosheet structure Figure 1(c), which indicates that the distribution of the relevant structure in the structure is uniform, but there are a small number of defects. At the same time, the morphology and thickness of the Au / MoN nanosheet were characterized by AFM on the alumina single crystal. The AFM test was completed in an argon-filled glove box by using a Cypher ES type atomic force microscope (Asylum Research brand, USA Oxford Instruments Co., Ltd.). As shown in Figure 1 (d), the AFM results show that the structure surface of the truncated hexagonal nanosheet is regular, and there is no agglomeration and particle structure, which is consistent with the structure of SEM. In addition, the thickness of the single nanosheet is about 40 nm. The EDX-mapping characterization of the triangular nanosheet structure, Figure 1 (e-g) shows that Mo, N and Au elements are uniformly distributed in the triangular structure nanosheet, which indicates that the two-dimensional Au / MoN model structure with a regular surface can be prepared on the substrate surface by using a mixed solution of Na2MoO4 and HAuCl4·3H2O as a precursor by chemical vapor deposition method. In addition, the above results also preliminarily indicate that Au is in a highly dispersed state in the MoN structure, not in an agglomeration and particle structure.

[0071] In order to further confirm the composition of the triangular nanosheet structure on the alumina single crystal surface, the chemical composition and chemical state of the structure were analyzed in depth by XPS. The XPS test was carried out on a PHOIBOS-100 type X-ray photoelectron spectrometer (SPECS Company) using non-monochromatic Al-Kα X-ray as the excitation source. As shown in Figure 2 (a-b), the binding energy position of N 1s is 397.8 eV, Mo 3d 5 / 2 and the binding energy of Mo 3d 3 / 2 is 228.8 and 231.9 eV, respectively, indicating the interaction between Mo and N, i.e. the prepared structure belongs to MoN, which is consistent with the phenomenon reported by the applicant before. It is worth noting that, by peak fitting processing of Mo 3d and N 1s + Mo 3p signals, the MoN structure has an oxidation state. As shown in Figure 2 (c), the Au 4f results show that there is Au element in the nanosheet structure, and according to the binding energy, it is judged that the chemical state of the Au belongs to Au 0 .

[0072] In order to further confirm the phase of MoN and the dispersion degree of Au, the Au / MoN / alumina single crystal model structure was characterized by XRD. The XRD test uses a SmartLab type XRD (produced by Rigaku Company, Japan), equipped with an 8 kW X-ray source, and the scanning rate is 5° / min. All scanning data are normalized by the highest peak intensity. As shown in Figure 2(d) shows that the results show that MoN exists in two phases of δ and γ, mainly in δ (002), δ (004), γ (111) and γ (222). It is worth noting that in the XRD spectrum of the Au / MoN model structure, no signal peak of Au is found, which indicates that the content of Au is below the detection limit range or in a highly dispersed state. Combined with the results of EDX-mapping of the Au / MoN model structure, it is considered that Au is in a highly dispersed state in the MoN structure.

[0073] The present application further verifies the phase of the Au / MoN model structure and the interface formed between Au and MoN by HRTEM. The high-resolution transmission electron microscope (HRTEM) image is taken by JEM-2100 microscope with a working voltage of 200 kV. Since the mode used by TEM is bright field mode, the dark area is the heavy element area and the light area is the light element area. As shown in Figure 3 (a) The light and dark degree of the HRTEM image indicates that the dark area may be the Au element (blue line box), and the light area is MoN (yellow line box). The lattice constant of the light area indicates that the atomic structure of the prepared two-dimensional crystal exhibits a hexagonal phase, with an in-plane lattice constant of 0.27 nm ( Figure 3 (b)), which is consistent with the lattice corresponding to δ (002). At the same time, there is a small amount of amorphous structure on the surface of MoN, which may be caused by the formation of an oxide layer on the surface of MoN, which matches the results of XPS. As shown in Figure 3 (c) shows that the in-plane lattice constant of the dark area is 0.20 nm, which corresponds to the (200) lattice of Au. This fully illustrates that there is an interface between Au and MoN, and since no signal of Au is found in XRD, it is speculated that Au exists in the MoN structure in a layered structure.

[0074] Figure 3 Middle: (b) lattice fringes of MoN region; (c) lattice fringes of Au region.

[0075] In order to further verify that Au is in a highly dispersed state in the MoN structure, a comparative experiment is carried out. In the experiment, the mixed precursor of Na2MoO4 and HAuCl4·3H2O is placed in a chemical vapor deposition tube furnace, heated to 950 ℃ by high-purity Ar, and kept for 2 h. After it is cooled to room temperature, the surface morphology and element distribution are characterized by SEM. As shown in Figure 4 (a) shows that the surface of the alumina single crystal substrate presents irregular morphology, three-dimensional phase, and rough structure. At the same time, Figure 4(b) The area marked by the red box indicates the presence of bright-colored aggregated granular structures on the surface and boundaries of the irregularly shaped structure. Characterization of this structure using EDX-mapping revealed that the bright-colored granular structures are Au, while the other regions are Mo (…). Figure 4 (cd)). The above results verify that Au is highly dispersed on the MoN support, while Au is an aggregated particulate structure on the Na2MoO4 support.

[0076] Example 2

[0077] In-depth analysis of the Au / MoN model structure

[0078] Currently, the distribution of active metals in nitrides remains controversial, and no systematic studies on this issue have been published. To better identify the distribution of Au in MoN model catalysts, this invention utilizes Ar ion-etched XPS to perform depth profiling on the surface of Au / MoN nanosheets. The XPS depth profiling analysis was performed using a PHI 5000 XPS system with Ar ion sputtering etching. Furthermore, to determine the etching rate of Au by Ar ions at the same etching power, an in-situ etching study was first conducted on the Au film deposited on the ZrO2 surface. In this process, an etching power of 3 keV was used, and the etched area was 3 x 3 mm. 2 The etching time for each cycle is 30 seconds.

[0079] like Figure 5 As shown in (a), the signal intensity of Au did not change significantly before the 33rd etching cycle, indicating that its average thickness was greater than the detection depth of XPS before the 33rd etching cycle. After the 33rd etching cycle, the signal intensity of Au decreased with the increase of the number of etching cycles. When the number of etching cycles reached 45, the amplitude of the change in Au signal began to decrease, and when the number of etching cycles reached 60, the signal intensity of Au remained basically unchanged. Figure 5 (b) indicates that Au begins to stabilize. It's worth noting that because the magnetron sputtered Au film has a small particle structure, even after most of the Au has been etched, a small number of Au particles remain unetched, resulting in a small amount of Au signal. For example... Figure 5 As shown in (c), the Au film on the ZrO2 surface was characterized by AFM, and the results showed that the surface of the Au film was relatively uniform. Furthermore, the thickness of the Au film was approximately 65 nm. Figure 5 (d) Calculations show that when the etching power is 3 KeV and the etching time per cycle is 30 s, the etching rate for the Au film is approximately 2 nm / min.

[0080] Further, the present application utilizes the same etching power to perform in-situ Ar ion etching XPS characterization on the Au / MoN model structure. As shown in Figure 6 (a), the Au 4f signal intensity does not change significantly after etching for 1 min, which is due to the contamination of the Au / MoN surface, i.e. the impurity carbon on the surface of Au / MoN and its oxide layer are etched within 1 min. It is worth noting that the signal of Au 4f begins to gradually decrease with the increase of etching time. When etching for about 4 min, the signal intensity of Au changes in a decreasing trend. When etching continues to 8 min, the signal of Au is basically disappeared. It is considered that within the first 4 min, the Au in the MoN structure and the residual Au on the surface of the substrate are etched, while after 4 min, the Au in the MoN structure is completely etched, and the residual Au signal is derived from the surface of the substrate. Since the Au on the surface of the substrate exists in the form of agglomerated particles, its signal is weak, but the thickness is thick, so it is difficult to be completely etched. Therefore, according to the etching rate of the Au film on the surface of ZrO2, after removing the surface oxide layer and contamination, it is preliminarily speculated that the thickness of the Au layer in the Au / MoN model structure is about 5 nm. In addition, the signal change of Mo 3d and Mo 3p + N 1s shows that with the increase of etching time, the signal is enhanced (b-c). This is because the layered Au on the surface of MoN is etched, exposing more MoN surface, thereby enhancing the XPS of MoN. In summary, it is considered that Au is mainly segregated on the surface of MoN to form an Au-MoN interface. Figure 6

[0081] In order to further confirm that Au is mainly segregated on the surface of MoN, Co metal is used instead of Au to combine with MoN to prove the above conclusion. At present, there are related research reports that Co can form a Co6Mo6C2 structure with a crystal phase by combining with Mo2C, and the carbide forms a covalent bond with Co, which can effectively improve the stability of the loaded Co. Therefore, it is considered that Co can form a uniform CoMoN x structure in MoN. A mixed solution of CoCl2 and Na2MoO4 with the same concentration as the HAuCl4 solution is used as a precursor, and the same chemical vapor deposition method is used to prepare a CoMoN x model structure. As shown in Figure 7 (a), the surface of the alumina single crystal is uniformly covered with a nano-sheet structure with consistent orientation, relatively regular surface and two-dimensional characteristics. The nano-sheet structure exists in the form of triangles and hexagons, and the average lateral size is about 1 μm. The hexagonal nano-sheet structure is characterized by EDX-mapping, Figure 7 ​(bd) indicates that Mo, N, and Co are uniformly distributed within the hexagonal nanosheet structure. However, EDX-mapping results for Co show a low Co content, which may be because Co is not primarily distributed on the MoN surface but rather relatively uniformly throughout the MoN structure. Furthermore, the phase composition of the prepared CoMoNx structure was characterized using XRD. Figure 7 As shown in (e), the XRD results are consistent with the reported results of related studies. Therefore, it is believed that CoMoN with a uniform crystal structure can be successfully prepared using CoCl2 and Na2MoO4. x .

[0082] Similarly, this invention also relates to CoMoN x The model structure was characterized by Ar ion etching using XPS, with an etching power of 3 keV. Figure 8 (a) shows the change in Co 2p signal intensity with etching time. It was found that after 1 min of etching, the Co 2p signal was stronger than the initial signal. Similarly, this is due to the influence of CoMoN... x The carbon contamination and oxide layer on the surface of the model structure, as the surface impurities are etched away, cause CoMoN... x The intrinsic structure is gradually exposed, ultimately leading to an enhancement of the Co signal. Notably, after 1 min of etching, the Co signal intensity did not change significantly with increasing etching time. Furthermore, the signals of Mo 3d and Mo 3p + N 1s only showed signs of surface oxide layer etching; no change in signal intensity was observed. Figure 8 (bc)). In summary, Co can form a covalently bonded crystal structure with MoN, and compared to Au, Co is more uniformly distributed throughout the MoN structure. These structures further confirm that in the Au / MoN model structure, Au is mainly distributed on the surface of MoN to achieve phase separation.

[0083] Example 3

[0084] Transformation of Au / MoN model structure in oxygen atmosphere

[0085] To further investigate the changes in Au structure caused by variations in the support, the Au / MoN model catalyst was heated to 350 °C in an oxygen atmosphere and held for 1 h to obtain Au / MoO3 samples. The Au / MoO3 samples were characterized using SEM. Figure 9 As shown in (a), the area marked by the red box represents the bright-colored granular structure on the surface of the triangular nanosheets, presumably composed of Au. Furthermore, a more pronounced bright-colored granular structure was observed on the surface of thick hexagonal nanosheets. Figure 9(b). Meanwhile, the structure of MoN nanosheet was changed obviously after oxidation, from regular surface with two-dimensional plane structure to rough surface with non-two-dimensional sheet structure and reduced crystallinity. In order to confirm the composition of the bright particle structure, EDX-mapping was used to characterize the nanosheet after oxidation. Figure 9 (c-d) further confirmed that the bright particle structure belongs to Au element. The above results combined with the existence form of Au in Au / MoN model structure showed the process of Au particle "dispersion" and "agglomeration" again, which well corresponds to the carrier treatment process of nitridation-oxidation, which is related to the weak interaction between Au and MoO3, and the strong interaction between Au and MoN.

[0086] In order to further confirm the agglomeration state of Au, the model structure of Au / MoN after oxidation was characterized by XRD. As shown in Figure 10 (b), the XRD spectrum showed that Au / MoN appeared signals at 2 Theta of 38.2°, 44.4° and 81.7° after oxidation, which belonged to (111), (200), (222) peaks of Au. This showed that highly dispersed Au formed highly crystalline agglomerates during oxidation. This is consistent with the conclusions of SEM and EDX-mapping. In addition, the signal peaks of MoO3 appeared in XRD, such as 27.3° and 25.7°, which appeared after the oxidation of Au / MoN model structure. This showed that Au / MoN would form MoO3 after being oxidized by oxygen. However, it is worth noting that even if Au / MoN is oxidized to form MoO3, there are still strong signals of MoN in XRD. It is speculated that this is because MoN is a dense structure, and its oxidation layer is located on the surface, and the bulk structure is still MoN. Further, the surface structure was characterized by Raman spectroscopy. Raman spectroscopy test used LabRAM HR 800 type spectrometer, and used 532 nm laser light source. The signal peaks of 820 cm -1 and 995 cm -1 in Raman spectrum also confirmed that MoN surface was oxidized to MoO3. Figure 10 (a). Meanwhile, Figure 10 The results of XPS in (c) also confirmed that within the detection depth of XPS, the surface of MoN had completely changed to MoO3, and there was no signal of MoN. Moreover, the signal of Mo 3d after oxidation was stronger than that before oxidation. It is worth noting that the signal intensity of Au in ex situ XPS changed obviously after oxidation. As shown in Figure 10(d) as shown, the signal intensity of Au was found to decrease dramatically after the support MoN was oxidized. By calculation, the signal intensity of Au after oxidation was only 10% of the initial. The above results combined with the change of Mo 3d XPS signal intensity also confirmed that the highly dispersed Au supported on MoN support surface formed an aggregated state after the support was oxidized, which further illustrated that Au had a strong interaction with MoN and a weak interaction with oxidized molybdenum.

[0087] The present application carried out Ar ion etching XPS study on the model structure of Au / MoN after oxidation, but in the process, due to the low content of Au, the XPS signal intensity is poor, in order to more clearly show the change of Au in MoO3 structure, the loading amount of 5% Au is used for testing. Figure 11 (a) is the signal intensity of Au 4f with the change of etching time. Au 4f is etched for 1 min, and its signal intensity is enhanced, which is related to the etching of surface impurities and contamination. With the increase of etching time, the signal intensity of Au is not found to change significantly. The signal of Mo 3d and Mo 3p shows that the composition of the surface is MoO3, and there is no MoN. With the increase of etching, it is found that Mo 3d begins to appear at a lower binding energy position, and N 1s signal is gradually displayed Figure 11 (b-c). Continue to etch, the signal of MoN gradually increases. This is consistent with the results of XRD in Figure 10 (b), that is, after the oxidation of Au / MoN by oxygen, the signal of MoN still exists. MoN is a dense two-dimensional structure, and its oxidation process occurs on the surface layer. With the formation of the oxide on the surface layer, the surface oxidation layer will hinder the deep oxidation of MoN at the same temperature. In summary, Au exists in the form of aggregated particles on the MoO3 support, and its thickness is greater than the detection depth of XPS, therefore, the signal of Au does not change in the etching process, which further confirms that the dispersed Au of the model structure of Au / MoN is aggregated after oxidation in SEM.

[0088] Example 4

[0089] Charge transfer process between Au and MoN support

[0090] Au nanoparticles can interact strongly with many oxide supports, often exhibiting strong charge transfer at the metal-oxide interface after high-temperature reduction treatment. Current research indicates that Au and carbide supports exhibit strong interactions after pretreatment. The above results have confirmed that Au nanoparticles can be highly dispersed on the surface of MoN, and a reversible dispersion-aggregation process occurs during subsequent oxidation. Therefore, it is speculated that similar strong interactions exist between Au and nitrides, enabling highly dispersed and stable Au nanostructures.

[0091] To further confirm the generalized SMSI effect between Au and nitrides, the electronic interactions at the Au / MoN interface were investigated using quasi-in-situ near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS). The NAP-XPS experiment was performed on a SPECSEnviroESCA spectrometer equipped with monochromatic Al Kα X-rays as the excitation source. Figure 12 As shown in (a), in the initial Au / MoN model structure, the binding energy of Au 4f is located at 84.0 eV, which corresponds to the metallic state of Au. Then, the sample was heated to 500 °C in a N2 / H2 (1:3) mixed atmosphere for a pre-activation process, the purpose of which was to remove the oxide layer on the MoN surface. Furthermore, the decreased signal intensity at the high binding energies of Mo 3d and Mo 3p also confirmed that most of the surface oxide layer was removed. Figure 12 (bc)). It was found that the signal intensity of Au increased after the Au / MoN model catalyst was pre-activated, which is due to the removal of the surface oxide layer and a small amount of impurities. Furthermore, it is noteworthy that the binding energy of Au 4f shifted by 0.3 eV towards a higher binding energy direction after pre-activation. Therefore, it is believed that Au transferred some of its charge to the support, thereby forming a strong interaction.

[0092] Following this, oxygen was introduced into the pre-activated Au / MoN model structure and heated to 400 °C for oxidation treatment. It was found that during this process, the signal intensity of Au decreased dramatically, which is consistent with... Figure 10(d) The ex situ XPS results are highly consistent. This indicates that the Au / MoN model structure is oxidized and the highly dispersed Au nanoparticles are aggregated. In addition, the binding energy of Au 4f shifts to the low binding energy, which is 84.0 eV, the same as the binding energy of the initial state of Au. The signals of Mo 3d and Mo 3p also indicate that the surface layer of MoN is completely oxidized to the oxidation state. In addition, the signal intensity of Mo 3d and Mo 3p after oxidation increases, which also confirms that the dispersed Au on the surface of MoN is aggregated to expose more surface of the oxidation layer. The above results indicate that the charge transfer effect disappears after the oxidation treatment, indicating that there is a weak charge transfer effect in the Au / MoO3 system. In summary, the reversible charge transfer between Au and nitride is an important manifestation of the SMSI effect between the two.

[0093] Example 5

[0094] Summary of the strong interaction between Au and MoN support

[0095] In summary, the influence of the molybdenum nitride support on the morphology of Au is obvious. The structural transformation in the Au / MoN catalyst can be illustrated by the schematic diagram of Figure 13 , which illustrates the process of highly dispersed Au coverage layer and strong charge transfer from Au to nitride and oxide. By using a mixed solution of HAuCl4 and NaMoO4 as a precursor, Au / MoN structure can be formed after nitridation. Due to the structural difference between Au and MoN, it cannot form a solid solution structure, and therefore, it is believed that Au and MoN form an interfacial structure in a phase-separated form. In addition, also in an Ar atmosphere, the above-mentioned precursor is heated, and it is found that Au and Na2MoO4 are also in a state of immiscibility, and therefore, they also belong to phase separation. In addition, at high temperatures, MoN tends to form a dense two-dimensional planar structure. During growth, in order to more easily form a phase-separated state, Au tends to segregate on the surface of the MoN structure. Based on the results of in situ XPS characterization, it is confirmed that there is a charge transfer process between Au and MoN to form a strong interaction. It is precisely because of the existence of strong interaction that the gold coverage layer exists in the form of an ultrathin layer and high dispersion on the surface of the MoN support, thereby forming an Au-MoN interface. In addition, with the change of the surface structure of the molybdenum-based catalyst, the morphology and state of gold will also change. When MoN is oxidized, Au is transformed from a two-dimensional layered structure to a three-dimensional cluster structure. This phenomenon indicates that the interface interaction between Au and MoO3 is weak, and Au cannot continue to maintain a layered dispersion and aggregation.

[0096] The Au / MoN model structure with regular surface and two-dimensional structure is prepared by using the atmospheric pressure chemical vapor deposition method. In addition, we confirm that Au can be dispersed on the surface of MoN and the SMSI effect between Au and nitride carrier by various characterization methods, which deepens the understanding of SMSI effect. The main conclusions are as follows:

[0097] (1) The Au / MoN model structure with regular surface, two-dimensional characteristics and horizontal size of about 500 nm is prepared on the surface of alumina single crystal by using the mixed solution of HAuCl4·3H2O and Na2MoO4 as precursor and atmospheric pressure chemical vapor deposition method.

[0098] (2) The characterization of SEM, EDX-mapping, AFM and XRD confirms that Au can be highly dispersed on the MoN carrier, and the in-situ Ar ion etching XPS characterization confirms that Au and MoN carrier are in a phase separation state, mainly segregating on the surface of MoN, thereby forming an interface with MoN.

[0099] (3) The in-situ XPS proves that the charge transfer process at the interface of Au and MoN makes Au positive, and the dispersion-aggregation state of Au structure can be controlled by the alternating treatment of nitriding-oxidation, which shows that there is a strong interface effect between Au and MoN, which is the embodiment of SMSI effect.

[0100] The above specific embodiments further illustrate the purpose, technical solutions and beneficial effects of the present application. It should be understood that the above description is only a specific embodiment of the present application and is not used to limit the protection scope of the present application. Any modification, equivalent replacement, improvement, etc. within the spirit and principles of the present application should be included in the protection scope of the present application.

Claims

1. A method for preparing an Au / MoN model catalyst, characterized in that, Includes the following steps: S1. Dissolve Na2MoO4 and HAuCl4·3H2O to form a mixed solution, which serves as the precursor; S2. Coat the precursor described in step S1 onto the surface of an alumina single crystal substrate; S3. Place the alumina single crystal substrate coated with the precursor in a tube furnace, use high-purity Ar as the carrier gas, control the carrier gas flow rate to 100 sccm, and gradually increase the temperature to the growth temperature. S4. When the temperature reaches the growth temperature, NH3 is introduced as a nitrogen source, and the flow rate of NH3 is controlled at 10 sccm. Chemical vapor deposition reaction is carried out under normal pressure for 2 h. S5. After the reaction is complete, the mixture is cooled to room temperature in an Ar atmosphere to obtain an ultrathin epitaxial Au / MoN model catalyst.

2. The method for preparing an Au / MoN model catalyst according to claim 1, characterized in that, In step (1), the loading of Au is controlled by adjusting the concentration of HAuCl4·3H2O in the mixed solution.

3. The method for preparing an Au / MoN model catalyst according to claim 1, characterized in that, In step S1, when Na2MoO4 and HAuCl4·3H2O dissolve to form a mixed solution, 1-2% of the total mass of the mixed solution is added as a composite dispersant. The preparation of the composite dispersant includes the following steps: (1) Dissolve trisodium citrate powder in deionized water at a mass ratio of 1:10 to prepare a 10% trisodium citrate solution. Then place the solution in an ultrasonic cleaner, set the power to 300 W, the temperature to 60~65℃, and treat for 2~3 h. At the same time, oxygen is introduced, and the oxygen flow rate is 5~6 sccm. After completion, pretreated trisodium citrate is obtained. (2) Add 3-aminopropyltriethoxysilane to the pretreated trisodium citrate at a volume ratio of 5:1 and stir at 400~500 rpm for 1~2 h to obtain a composite dispersant.

4. The method for preparing an Au / MoN model catalyst according to claim 1, characterized in that, The coating method described in step S2 is spin coating. The specific operation is as follows: the precursor is drawn up by a dropper and dropped onto the surface of the alumina single crystal substrate, and then the mixed solution is spin coated onto the substrate surface using a spin coater.

5. The method for preparing an Au / MoN model catalyst according to claim 1, characterized in that, The purity of the high-purity Ar mentioned in step S3 is not less than 99.999%.

6. The method for preparing an Au / MoN model catalyst according to claim 1, characterized in that, The stepwise heating to the growth temperature described in step S3 is as follows: first, the temperature is increased to 500℃ at a heating rate of 8~10℃ / min, and then increased to 950℃ at a heating rate of 4~6℃ / min.

7. The method for preparing an Au / MoN model catalyst according to claim 1, characterized in that, The growth temperature described in step S3 is 950℃.

8. The method for preparing an Au / MoN model catalyst according to claim 1, characterized in that, Step S4, when the temperature reaches the growth temperature, specifically involves: after the temperature inside the tubular furnace rises to 950°C and stabilizes, NH3 is introduced.

9. The method for preparing an Au / MoN model catalyst according to claim 1, characterized in that, During the cooling process in the Ar atmosphere described in step S5, the Ar gas flow rate is maintained at 100 sccm.

10. The Au / MoN model catalyst prepared by the method according to any one of claims 1 to 9.

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