Preparation method and application of Pd monatomic model catalyst

Abstract

The invention discloses a preparation method and application of a Pd monatomic model catalyst, and relates to the technical field of catalysts, and the key points of the technical scheme are as follows: preparing an ordered CeO2 (111) film on the surface of a Cu (111) single crystal substrate in ultrahigh vacuum through a molecular beam epitaxial growth method; pd and Ce are co-deposited on the surface of a CeO2 (111) film by using an electron beam evaporation source furnace, and the CeO2-loaded Pd monatomic model catalyst is prepared by accurately controlling the deposition rate ratio of Pd to Ce and oxygen partial pressure. The effective preparation method of the CeO2-loaded Pd monatomic model catalyst is developed, the catalyst is applied to a reaction of catalytically reducing NO by CO, meanwhile, the structure and performance of the catalyst are compared with those of a CeO2-loaded Pd and PdO nano-particle model catalyst, the reaction path and the reaction mechanism in the reaction of catalytically reducing NO by CO are further explored, and the catalyst has a good application prospect. The method has important scientific and practical values for promoting the development and industrial application of catalytic science.

Description

Technical Field

[0001] The invention relates to the technical field of catalysts, and more specifically to a preparation method of a Pd single-atom model catalyst and application thereof. Background Art

[0002] Single-atom catalysts are an important research direction in the field of catalysis and have attracted much attention due to their unique activity and selectivity. Single-atom catalysts refer to active metal components fixed on a carrier in the form of isolated atoms, without the presence of metal-metal bonds. Compared with traditional catalysts, single-atom catalysts have significant advantages, including high utilization of active metal components (up to 100%), single active center composition and consistent structure, which helps to improve selectivity and reduce side reactions, while helping to reveal the true reaction mechanism; in addition, single-atom catalysts combine the consistency of active sites in homogeneous catalysis with the easy separation and multiple recycling characteristics of heterogeneous catalysts, building a bridge between homogeneous and heterogeneous catalysis.

[0003] CeO 2 Due to its high dielectric constant, high refractive index, ultraviolet absorption rate and good thermal stability, it is widely used in a variety of optical and electronic devices. In addition, CeO 2 It exhibits good catalytic performance, and the catalyst performance can be improved by increasing the dispersion and oxygen storage capacity of precious metals.

[0004] In practical applications, cerium oxide (CeO 2 )-supported palladium (Pd) catalysts have attracted much attention due to their wide application in catalytic reactions such as automobile exhaust purification, especially Pd single-atom site catalysts that exhibit excellent catalytic performance; however, there is currently no consensus on the nature of active centers in catalytic reactions, the structure-activity relationship of catalysts, and the reaction pathways.

[0005] Therefore, the present invention aims to provide a preparation method of a Pd single-atom model catalyst and its application. By utilizing modern surface science analysis methods combined with ultra-high vacuum in-situ sample preparation technology, the structure and catalytic reaction mechanism of metal / oxide catalysts can be understood at the atomic-molecular level, which is of great significance for the research and development of new catalysts with higher activity and stronger stability. Summary of the invention

[0006] The purpose of the present invention is to provide a method for preparing a Pd single atom model catalyst and its application. 2 The preparation method of supported Pd single atom model catalyst has important scientific and practical value for promoting the development of catalytic science and industrial application.

[0007] The above technical purpose of the present invention is achieved through the following technical scheme: A method for preparing a Pd single atom model catalyst comprises the following steps:

[0008] S1. Using the preset Cu(111) single crystal as the substrate, an ordered stoichiometric CeO was prepared on the surface of the Cu(111) single crystal by molecular beam epitaxy. 2 (111) Thin film;

[0009] S2, using electron beam evaporation source furnace to evaporate Pd metal onto CeO 2 (111) film surface, control the evaporation rate of Pd, prepare Pd nanoparticle surfaces with different coverage, PdO nanoparticle surfaces, and CeO with Pd single atom dispersion 2 Nanoparticle surface;

[0010] S3, prepare CeO 2 The supported Pd single atom, Pd nanoparticle and PdO nanoparticle model catalysts were applied to the model reaction of CO catalytic reduction of NO to study its reaction pathway and mechanism.

[0011] The present invention is further configured as follows: in step S1, an ordered stoichiometric ratio of CeO is prepared. 2 (111) The film process includes:

[0012] The Cu(111) single crystal was treated by cyclic argon etching and annealing until no contaminants such as C and O could be detected by surface XPS, and a clear LEED image was obtained by low-energy electron diffraction.

[0013] In an oxygen atmosphere, Ce metal was deposited on the treated Cu(111) single crystal by physical vapor deposition. The Ce evaporation rate and evaporation time were controlled to prepare 3-4 nm ordered CeO 2 (111)Thin film.

[0014] The present invention is further configured as follows: in step S2, the Pd nanoparticle surface, the PdO nanoparticle surface and the CeO dispersed by Pd single atoms with different coverage are prepared. 2 Nanoparticle surface, PdNPs / CeO 2 (111), PdO NPs / CeO 2 (111) and Pd 1 -CeO 2 NPs / CeO 2 (111) Model catalyst.

[0015] The present invention is further configured as follows: the PdNPs / CeO 2 (111) The preparation process of the model catalyst is as follows:

[0016] The prepared CeO 2 (111) Pd metal was vapor-deposited at room temperature, and the surface covered with Pd nanoparticles of different coverage was prepared by controlling the deposition time to obtain Pd NPs / CeO 2 (111) Model catalyst.

[0017] The present invention is further configured as follows: the PdO NPs / CeO 2 (111) The preparation process of the model catalyst is as follows:

[0018] The prepared CeO 2 (111) Pd metal was vapor deposited in an oxygen atmosphere at room temperature to prepare surfaces covered with PdO nanoparticles of different coverages to obtain PdO NPs / CeO 2 (111) Model catalyst.

[0019] The present invention is further configured as follows: 1 -CeO 2 NPs / CeO 2 (111) The preparation process of the model catalyst is as follows:

[0020] The prepared CeO 2 (111) film was deposited simultaneously by vapor deposition of Pd metal and Ce metal in an oxygen atmosphere at room temperature. 2 (111) The film surface was rapidly heated to 600 K and then cooled back to room temperature to prepare Pd single-atom dispersed CeO 2 The surface covered with nanoparticles gives Pd 1 -CeO 2 NPs / CeO 2 (111) Model catalyst.

[0021] The present invention is further configured as follows: the preparation method is used to prepare CeO 2 Loaded Pd single atom model catalyst.

[0022] In summary, the present invention has the following beneficial effects:

[0023] 1. The single-atom catalyst prepared in the present invention achieves the maximum utilization rate of active metal components, effectively reducing the cost of the catalyst. At the same time, the single-atom catalyst has a unique electronic structure, quantum size effect and maximum atomic utilization, and can show ultra-high intrinsic activity and selectivity;

[0024] 2. The single-atom model catalyst in the present invention is prepared in an ultra-high vacuum, avoiding the acid etching method commonly used in the preparation of traditional single-atom catalysts and reducing pollution in the catalyst preparation process; at the same time, the method of the present invention is simple to operate, has good reproducibility, and is applicable to a variety of metals.

[0025] 3. The present invention precisely regulates the coordination environment and electronic structure of the single-atom catalyst, rationally regulates the metal-support interaction, and simultaneously optimizes the catalytic performance of the single-atom catalyst and improves its stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a schematic flow chart of a method for preparing a Pd single atom model catalyst in an embodiment of the present invention;

[0027] Figure 2 is Pd in ​​the embodiment of the present invention 1 -CeO 2 NPs / CeO 2 (111) Model catalysts and PdO NPs / CeO 2 (111) Schematic diagram comparing the SRPES and STM results of the model catalyst;

[0028] Figure 3 is Pd in ​​the embodiment of the present invention 1 -CeO 2 NPs / CeO 2 (111) IRAS spectrum obtained by in situ infrared characterization of the model catalyst during the CO-catalyzed NO reduction model reaction. DETAILED DESCRIPTION

[0029] The following is combined with Figure 1-3 The present invention is described in further detail.

[0030] Embodiment: A preparation method of a Pd single atom model catalyst and its application, comprising the following steps:

[0031] S1. A Cu(111) single crystal with a diameter of 10 mm and a thickness of 2 mm was used as a substrate. The Cu(111) single crystal was then subjected to cyclic argon etching (1.5 keV energy argon etching for 30 min) and annealing (850 K annealing for 10 min) until no contaminants such as C and O were detected by surface XPS. A very clear LEED image was obtained by low-energy electron diffraction, indicating that the surface of the Cu(111) single crystal had been cleaned.

[0032] Subsequently, in an oxygen atmosphere (P O2 =5×10 -7mbar), Ce (99.9999%, Alfa Aesar) metal was deposited on a clean Cu (111) single crystal substrate surface by physical vapor deposition (PVD) (evaporation was performed using an electron beam evaporation source furnace from Aesar). After the evaporation was completed, the oxygen partial pressure was maintained and the sample was annealed at 850 K for 10 minutes. After the annealing was completed, the oxygen atmosphere was maintained until the sample cooled to room temperature. The evaporation rate of Ce was controlled by and evaporation time (1 hour) to prepare an ordered stoichiometric CeO of 3 to 4 nm. 2 (111)Thin film.

[0033] S2, using Focus electron beam evaporation source furnace to evaporate Pd (purity 99.999%, AlfaAesar) metal onto CeO 2 The evaporation rate of Pd on the (111) film surface was controlled at about 0.01 ML (monolayer) / min. In this embodiment, a single atomic layer of Pd is defined as the surface density of Pd atoms on the Pd (111) surface of the densely packed bulk phase, which is 1.5×10 15 atoms / cm 2 (i.e. 1ML = 1.5 × 10 15 atoms / cm 2 ), Pd NPs / CeO 2 (111) The catalyst was prepared by 2 (111) The film was obtained by vapor deposition of Pd metal at room temperature. By controlling the deposition time, the surface covered with Pd nanoparticles with different coverages was prepared; PdO NPs / CeO 2 (111) The catalyst was prepared by 2 (111) film at room temperature in an oxygen atmosphere (P O2 =5×10 -7 mbar) by vapor deposition of Pd metal to obtain a surface covered with PdO nanoparticles; 1 -CeO 2 NPs / CeO 2 (111) The catalyst was obtained by co-deposition of Pd and Ce in an oxygen atmosphere. O2 =5×10 -7 mbar), in the ordered CeO 2 (111) Pd and Ce (Pd:Ce atomic ratio is about 1:10) are simultaneously vapor-deposited on the film surface. After the room temperature deposition, the surface temperature is rapidly raised to 600 K and then returned to room temperature to obtain CeO with Pd single atom dispersion. 2 Surface Pd covered with nanoparticles1 -CeO 2 NPs / CeO 2 (111).

[0034] like Figure 2 As shown, PdO NPs / CeO 2 (111) and Pd 1 -CeO 2 NPs / CeO 2 (111) SRPES and STM results of the model catalyst (where (a) and (b) are PdO NPs / CeO 2 (111) and Pd 1 -CeO 2 NPs / CeO 2 (111) surface and Pd 3d (hν=450eV) spectra after annealing at different temperatures, (c) PdO NPs / CeO 2 (111) and Pd 1 -CeO 2 NPs / CeO 2 Pd on (111) surface 2+ and Pd 0 The intensity changes with temperature, (d), (e) and (f) are respectively the clean CeO 2 (111) Surface, PdO NPs / CeO 2 (111) Surface and Pd 1 -CeO 2 NPs / CeO 2 (111) STM image of the surface (image size: 50 nm × 50 nm); Figure 2 As shown in a, the Pd 3d spectrum shows that in PdO NPs / CeO 2 (111) There is only one Pd 3d 5 / 2 The characteristic peak, located at 337.6 eV, indicates that only Pd 2+ When the temperature rises to 400K, a new characteristic peak appears at low binding energy (335.7eV), representing metallic Pd; when the temperature rises to 600K, the intensity of the characteristic peak of metallic Pd is significantly enhanced, indicating that with the increase of temperature, part of PdO is rapidly reduced to metallic Pd; and at Pd 1 -CeO 2 NPs / CeO 2 (111) The surface of the model catalyst ( Figure 2 b), only one characteristic peak appears at 337.9 eV, and the valence state of Pd and the intensity of the Pd characteristic peak do not change during annealing to 600 K.2+ It exists on the surface of the catalyst in the form of Pd-O-Ce, and Pd 2+ With CeO 2 The interaction between substrates is strong; Figure 2 c shows PdO NPs / CeO 2 (111) and Pd 1 -CeO 2 NPs / CeO 2 Pd on (111) surface 2+ and Pd 0 The intensity changes with temperature, and it is obvious that in Pd 1 -CeO 2 NPs / CeO 2 Pd on (111) surface 2+ Very stable and does not change with temperature, while PdO NPs / CeO 2 (111) Surface Pd 2+ Very unstable, easily reduced to Pd as the temperature rises 0 ; Figure 2 d, e and f show clean CeO 2 (111) surface, PdO NPs / CeO 2 (111) and Pd 1 -CeO 2 NPs / CeO 2 (111) STM image of the surface, clean CeO 2 (111) Some roughly circular large step surfaces can be observed on the film surface. 2 There are many amorphous nanoparticles on the surface of (111), while on the Pd 1 -CeO 2 NPs / CeO 2 In addition to the large step surface of the cerium oxide film, relatively small cerium oxide nanoparticles can be seen on the surface of (111). Combining the results of XPS and STM, it is concluded that PdO NPs / CeO 2 (111) PdO nanoparticles are randomly dispersed on CeO 2 (111) The surface of the film and the interaction between it and the substrate are weak; 1 -CeO 2 NPs / CeO 2 (111) Surface Pd 2+ It is dispersed in the cerium oxide nanoparticle lattice in the form of unit points and has a strong interaction with the cerium oxide substrate.

[0035] S3 will prepare Pd 1-CeO 2 NPs / CeO 2 (111) The single-atom model catalyst was applied to the catalytic reduction of NO by CO. The intermediates in the reaction were characterized by in situ infrared experiments, and the reaction pathway was then inferred.

[0036] In Pd 1 -CeO 2 NPs / CeO 2 (111) The co-adsorption of CO+NO was studied on the surface. CO+NO gas was gradually exposed on the surface (CO:NO = 1:1). The IRAS spectra of the surface were shown as follows: Figure 3 As shown in Figure 2, we find that as the gas is exposed to the surface, the -1 The infrared vibration peak first appears at Figure 3 a), indicating that CO is preferentially adsorbed on the surface, except for the top-adsorbed CO (corresponding to 2095 cm -1 There are also infrared vibration peaks at 1281 and 1019 cm -1 At the same time, at 2340cm -1 The vibration peak appears at 2 This is because CO reacts with oxygen in Pd-O-Ce to form carbonate, and carbonate is unstable and easily converted to CO 2 , while generating oxygen vacancies (Pd-O v -Ce). According to previous DFT theoretical calculation results, the adsorbed CO and the connected Pd 2+ and Ce 4+ O reacts to form CO 2 With the increase of CO+NO gas exposure, the -1 At the same time, the vibration peaks at 2090, 1281 and 1019 cm -1 The vibration peak at the local amplification Figure 3 b and Figure 3 It is more clearly seen in c. As the reaction proceeds, NO gradually occupies the adsorption sites on the surface and forms NO 2 - (1432cm -1 ), N 2 O 2 2- Species (trans-N 2 O 2 2- :1092cm -1 ;cis-N2 O 2 2- : 1359cm -1 and 1036cm -1 ). A phenomenon worth noting is that as the reaction proceeds, N 2 O 2 2- As the intensity increases, the vibration peak of CO adsorbed on Pd (2095cm -1 ) is gradually weakening, indicating that N 2 O 2 2- Occupies Pd 2+ As the reaction proceeds, trans-N 2 O 2 2- Vibration peak (1092cm -1 ) intensity begins to decrease, accompanied by cis-N 2 O 2 2- Vibration peak (1359cm -1 and 1036cm -1 ) gradually increases in intensity and CO 2 The intensity continues to increase. And, N 2 O 2 2- This trend of species change did not occur when NO was adsorbed alone. 1 -CeO 2 NPs / CeO 2 (111) When adsorbed on the surface, trans-N 2 O 2 2- Vibration peak and cis-N 2 O 2 2- The vibration peaks are always increasing. This is because when NO and CO are co-adsorbed, CO captures the activated oxygen atoms next to Pd and reacts with CO 2 desorbed from the surface in the form of Pd atoms, leaving many oxygen vacancies on the surface. 2 O 2 2- The O atoms in the (ONNO*) species can fill the adjacent oxygen vacancies to further generate cis-N 2 O 2 2 (*ONNO*). In other words, the presence of CO promotes the surface trans-N 2 O 2 2-(ONNO*) species to cis-N 2 O 2 2 (*ONNO*) transformation. Due to cis-N 2 O 2 2- The O in (*ONNO*) fills the O vacancy, so the NO bond is relatively easy to break to generate N 2 Here, the IRAS experiment proves that cis-N 2 O 2 2 (*ONNO*) is converted to N 2 intermediates.

[0037] This specific embodiment is merely an explanation of the present invention and is not a limitation of the present invention. After reading this specification, those skilled in the art may make non-creative modifications to the present embodiment as needed. However, as long as they are within the scope of the claims of the present invention, they are protected by the patent law.

Claims

1. A method for preparing a Pd single atom model catalyst, characterized in that: The following steps are involved: S1. Using a preset Cu(111) single crystal as a substrate, an ordered stoichiometric CeO2(111) thin film is prepared on the surface of the Cu(111) single crystal by a molecular beam epitaxial growth method; S2. Use an electron beam evaporation source furnace to evaporate Pd metal onto the surface of CeO2(111) film, control the evaporation rate of Pd, and prepare Pd nanoparticle surfaces with different coverages; evaporate Pd on the surface of CeO2(111) film in an oxygen atmosphere to obtain a PdO nanoparticle surface; control the evaporation rates of Pd and Ce in an oxygen atmosphere to obtain a CeO2 nanoparticle surface with Pd single atoms dispersed, and compare the structures and stabilities of the obtained Pd single atom catalysts, Pd nanoparticles, and PdO nanoparticle catalysts.

2. The method for preparing a Pd single atom model catalyst according to claim 1, characterized in that: The process of preparing an ordered stoichiometric CeO2 (111) thin film in step S1 includes: The Cu(111) single crystal was treated by cyclic argon etching and annealing until no contaminants such as C and O could be detected by surface XPS, and a clear LEED image was obtained by low-energy electron diffraction. In an oxygen atmosphere, Ce metal is deposited on the treated Cu(111) single crystal by physical vapor deposition, and the evaporation rate and evaporation time of Ce are controlled to prepare a 3-4 nm ordered CeO2(111) film.

3. The method for preparing a Pd single atom model catalyst according to claim 1, characterized in that: In the step S2, Pd nanoparticle surfaces, PdO nanoparticle surfaces and CeO2 nanoparticle surfaces with Pd single atoms dispersed therein with different coverages are prepared to obtain corresponding cerium oxide-supported Pd nanoparticle model catalysts Pd NPs / CeO2(111), cerium oxide-supported PdO nanoparticle model catalysts PdO NPs / CeO2(111) and cerium oxide-supported Pd single atom model catalysts Pd1-CeO2 NPs / CeO2(111), respectively.

4. The method for preparing a Pd single atom model catalyst according to claim 3, characterized in that: The preparation process of the PdNPs / CeO2(111) model catalyst is as follows: Pd metal was vapor-deposited on the prepared CeO2(111) film at room temperature, and a surface covered with Pd nanoparticles with different coverages was prepared by controlling the deposition time to obtain a Pd NPs / CeO2(111) model catalyst.

5. The method for preparing a Pd single atom model catalyst according to claim 3, characterized in that: The preparation process of the PdONPs / CeO2(111) model catalyst is as follows: Pd metal was vapor-deposited on the prepared CeO2(111) film in an oxygen atmosphere at room temperature to prepare a surface covered with PdO nanoparticles with different coverages, thereby obtaining a PdO NPs / CeO2(111) model catalyst.

6. The method for preparing a Pd single atom model catalyst according to claim 3, characterized in that: The preparation process of the Pd1-CeO2 NPs / CeO2(111) model catalyst is as follows: Pd metal and Ce metal are simultaneously vapor deposited on the prepared CeO2(111) film in an oxygen atmosphere at room temperature. After the room temperature deposition, the surface of the CeO2(111) film is rapidly heated to 600K and then returned to room temperature, thereby preparing a surface covered with CeO2 nanoparticles dispersed with Pd single atoms, and obtaining a Pd1-CeO2 NPs / CeO2(111) model catalyst.

7. The method for preparing a Pd single atom model catalyst according to claim 1, characterized in that: The preparation method is used for preparing CeO2-loaded Pd single-atom model catalyst.

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