Monatomic defect catalyst for CO oxidation and preparation method and application thereof

Abstract

This invention relates to a single-atom defect catalyst for CO oxidation, its preparation method, and its application. The single-atom defect catalyst comprises a defective TiO2 support and an active component supported on the defective TiO2 support. The active component comprises a noble metal single atom. The defective TiO2 support has defect sites, which are defect structures caused by oxygen vacancies and titanium vacancies. The active component is located at the defect sites on the defective TiO2 support. The active component exists in the single-atom defect catalyst in a coordination structure with a coordination number of 5. The single-atom defect catalyst provided by this invention utilizes the defective structure of the catalyst support as a site for loading the active component. The noble metal active component is uniformly dispersed in the form of single atoms on the defect sites of the support, reducing the noble metal content, achieving high atom utilization, and exhibiting high CO catalytic oxidation activity.

Description

Technical Field

[0001] This invention belongs to the field of catalysis technology, and particularly relates to a single-atom defect catalyst for CO oxidation, its preparation method, and its application. Background Technology

[0002] Incomplete combustion of fossil fuels leads to excessive CO emissions into the atmosphere, causing a series of negative consequences for air quality and human health. Catalytic oxidation is one of the most important technologies for CO emission control. Supported noble metal catalysts, due to their excellent activity and stability at lower reaction temperatures, have been widely used. The high cost and scarcity of noble metals have prompted the search for solutions that significantly reduce their content while maintaining excellent catalytic performance. Research has found that improving metal dispersion is a feasible method; catalysts with higher metal dispersion often exhibit higher catalytic activity. Defect sites can enhance the interaction between noble metals and the support, allowing noble metal elements to exist as single atoms on the support. Therefore, highly dispersed single-atom defect catalysts with defect sites show promising application prospects.

[0003] Regarding the preparation of single-atom catalysts, relevant patents have been reported in China. CN117414820A discloses an ultra-low-load, highly dispersed, single-atom Pt-supported TiO2 catalyst and its preparation method. This method achieves ultra-low-load, highly dispersed, single-atom Pt loading by optimizing the type of Pt precursor, light intensity, and light exposure time to control the Pt reduction and deposition rate. Compared with other preparation methods, the Pt / TiO2 photocatalyst preparation method provided by this patent significantly reduces the noble metal loading while exhibiting excellent catalytic activity, and the Pt single-atom promoter demonstrates excellent stability. However, the catalyst described in this patent is mainly intended for applications in catalytic photocatalytic water splitting for hydrogen production; its effect on CO oxidation remains unclear. Regarding the research on defect catalysts related to TiO2, CN115245823B discloses a method for preparing a defective mixed-crystal TiO2 catalyst for indoor air purification. The method involves obtaining a mixed-crystal TiO2 with a specific ratio of anatase and rutile through high-temperature calcination and controlled calcination conditions. Then, the mixed-crystal TiO2 is subjected to defect treatment in a reducing atmosphere to form oxygen vacancies. By controlling the defect treatment conditions, the oxygen vacancy content on the surface of the mixed-crystal TiO2 can be effectively regulated. Finally, an active metal is loaded. Although the catalyst exhibits good CO oxidation performance, the loaded active metal content is high (>5 wt.wt%), making it difficult for the active metal to exist in single-atom form.

[0004] Existing technologies suffer from drawbacks such as complex catalyst support preparation methods, high catalyst manufacturing costs due to the high proportion of precious metal active components in the catalyst, and the tendency of precious metals to agglomerate and disperse unevenly on the support. Therefore, how to provide a catalyst with a simple preparation method, low content of precious metal active components, uniform dispersion of precious metals, and the ability to achieve efficient catalytic oxidation of CO has become an urgent problem to be solved. Summary of the Invention

[0005] To address the aforementioned technical problems, the present invention aims to provide a single-atom defect catalyst for CO oxidation, its preparation method, and its application. The single-atom defect catalyst provided by the present invention has oxygen and titanium vacancies on the catalyst support, which can serve as sites for loading the active component. The noble metal active component is uniformly dispersed on the defect sites of the support in the form of single atoms, resulting in high atomic utilization and reduced noble metal usage. Furthermore, the single-atom defect catalyst exhibits high catalytic oxidation activity.

[0006] To achieve this objective, the present invention adopts the following technical solution:

[0007] In a first aspect, the present invention provides a single-atom defect catalyst for CO oxidation, the single-atom defect catalyst comprising a defective TiO2 support and an active component supported on the defective TiO2 support, the active component comprising a noble metal single atom; the defective TiO2 support having defect sites, the defect sites being defect structures caused by oxygen vacancies and titanium vacancies; the active component being located at the defect sites of the defective TiO2 support.

[0008] The single-atom defect catalyst for CO oxidation provided by this invention has a catalyst support with defect structures that can serve as loading sites for active components. The noble metal active components are uniformly dispersed on the defect sites of the support in the form of single atoms, which reduces the content of noble metals, has high atom utilization, and has high catalytic oxidation activity.

[0009] The following are preferred technical solutions of the present invention, but are not intended to limit the technical solutions provided by the present invention. The technical objectives and beneficial effects of the present invention can be better achieved and realized through the following preferred technical solutions.

[0010] Preferably, the active component exists in the single-atom defect catalyst in a coordination structure, and the coordination number of the coordination structure is 5.

[0011] In this invention, the active component in the single-atom defect catalyst exists in a coordination structure with a coordination number of 5, unlike the conventional catalyst support where the active component has a 4-coordinate structure. Conventional catalysts exhibit CO oxidation following the Langmuir-Hinshelwood (LH) mechanism, where CO and O2 are simultaneously adsorbed on the 4-coordinate active component. However, the strong adsorption of CO by the active component hinders the adsorption and activation of O2, thus limiting the catalyst's reactivity. In this invention, the active component, in a single-atom form, chemically coordinates with the defect sites in the defect catalyst. This not only enhances the binding force between the active component and the catalyst support, preventing detachment during use, but also modulates the CO adsorption performance of the active component, strengthening the catalyst support's oxygen adsorption and activation capabilities. This allows the active component to better exert its catalytic activity and improves catalytic efficiency.

[0012] Preferably, based on a total mass of 100 wt% of the single-atom defect catalyst, the content of the active component in the single-atom defect catalyst is 0.02-0.5 wt%, for example, it can be 0.02 wt%, 0.03 wt%, 0.04 wt%, 0.05 wt%, 0.06 wt%, 0.1 wt%, 0.15 wt%, 0.2 wt%, 0.25 wt%, 0.3 wt%, 0.35 wt%, 0.4 wt%, 0.45 wt%, or 0.5 wt%, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable, preferably 0.05-0.2 wt%.

[0013] In this invention, the content of the active component in the single-atom defect catalyst is controlled to be 0.02-0.5 wt%. Since the active component is dispersed in the defective TiO2 support in the form of single atoms, the amount of noble metal active component is greatly reduced. The minimum noble metal loading can reach 0.02 wt%, which can achieve good catalytic activity and reduce the production cost of defect catalysts. When the content of the active component in the single-atom defect catalyst is greater than 0.5 wt%, it will lead to the agglomeration of noble metal elements, which is not conducive to the active component playing a role, reduces the atom utilization rate of the catalyst, and also leads to a significant increase in the cost of the catalyst, limiting the widespread application of the catalyst.

[0014] Preferably, the noble metal single atom includes any one or a combination of at least two of Pt single atoms, Pd single atoms, or Au single atoms, with Pt single atoms being the most preferred. Typical but non-limiting combinations include combinations of Pt single atoms and Pd single atoms, combinations of Pd single atoms and Au single atoms, combinations of Pt single atoms and Au single atoms, and combinations of Pt single atoms, Pd single atoms, and Au single atoms.

[0015] In single-atom defect catalysts, the active components are dispersed on the defect support in the form of single atoms, which improves the atom utilization rate and reduces the content of noble metal active components, thereby significantly reducing the production cost of the catalyst. In addition, this invention preferably uses noble metal Pt as the active component, which has higher activity for CO catalytic oxidation and better stability in complex flue gas atmospheres compared to other noble metals.

[0016] In a second aspect, the present invention provides a method for preparing a single-atom defect catalyst for CO oxidation as described in the first aspect, the method comprising the following steps:

[0017] (1) Titanium precursor, silicon precursor and water are mixed and hydrolyzed to obtain a first mixture. The first mixture is subjected to a first rotary evaporation, a first drying and a first calcination to obtain a titanium-silicon composite.

[0018] (2) The titanium-silicon composite obtained in step (1) is mixed with alkaline solution for etching. After solid-liquid separation, washing and second drying, a defective TiO2 support with defect sites is obtained.

[0019] (3) The mixed noble metal source is impregnated with the defective TiO2 support, and then subjected to a second rotary evaporation, a third drying and a second calcination to obtain the single-atom defect catalyst.

[0020] This invention prepares titanium-silicon composites by co-hydrolysis of titanium and silicon precursors without adding other solvents. Compared with organic solvents such as ethanol, water is a faster hydrolyzing agent, which is more conducive to the formation of Ti-O-Si composite structures between titanium and silicon. This reduces the preparation cost and is more environmentally friendly and pollution-free.

[0021] Furthermore, the present invention further utilizes alkaline chemical etching to remove silicon from the obtained titanium-silicon composite, thereby destroying the Ti-O-Si structure in the titanium-silicon composite and constructing defect sites. These defect sites promote the formation of oxygen vacancies, enhance the catalyst's adsorption and activation performance for oxygen, and optimize the coordination environment of noble metal single atoms, thereby improving the catalyst's CO oxidation activity.

[0022] In this invention, a noble metal is loaded onto a defective TiO2 support using an impregnation method. The defect sites promote electron transfer between the noble metal and the support, forming a multi-coordinated Ti-OM (M is a noble metal) structure, thereby strengthening the interaction between the noble metal and the support. This results in the noble metal element being highly dispersed and existing in a single-atom form on the defect sites of the TiO2 support. The impregnation method used is simple and easy to operate, and the resulting single-atom defect catalyst has high catalytic activity and low production cost.

[0023] This invention creates conditions for subsequent loading of noble metal atoms by pre-preparing a defective TiO2 support, thereby improving the adhesion of the loaded single-atom noble metal to the support and preventing the subsequent detachment of the single-atom noble metal. Without pre-preparing a support with a defective structure, the adhesion of noble metal atoms is weak, and the noble metal is prone to agglomeration during calcination and operation, forming nanoparticles or clusters, resulting in low catalytic activity of the catalyst and low utilization rate of noble metal atoms.

[0024] Preferably, the titanium precursor in step (1) comprises any one or a combination of at least two of tetrabutyl titanate, ethyl titanate, or propyl titanate, preferably tetrabutyl titanate, wherein typical but non-limiting combinations include a combination of tetrabutyl titanate and ethyl titanate, a combination of ethyl titanate and propyl titanate, a combination of tetrabutyl titanate and propyl titanate, or a combination of tetrabutyl titanate, ethyl titanate, and propyl titanate.

[0025] Preferably, the silicon precursor comprises any one or a combination of at least two of diethyl silicate, triethyl silicate, or tetraethyl silicate, preferably tetraethyl silicate. Typical but non-limiting combinations include combinations of diethyl silicate and triethyl silicate, combinations of triethyl silicate and tetraethyl silicate, combinations of diethyl silicate and tetraethyl silicate, and combinations of diethyl silicate, triethyl silicate, and tetraethyl silicate.

[0026] Preferably, the molar ratio of the titanium precursor to the silicon precursor is 1:(0.1-1), for example, it can be 1:0.1, 1:0.2, 1:0.3, 1:0.4, 1:0.5, 1:0.6, 1:0.7, 1:0.8, 1:0.9 or 1:1, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable, preferably 1:(0.2-0.5).

[0027] In the preparation method of the catalyst for controlling single-atom defects in this invention, the molar ratio of titanium precursor to silicon precursor is 1:(0.1-1). If there is too much titanium precursor, the number of defect sites generated after etching of the composite support will be less. If there is too much silicon precursor, it will hinder the formation of the crystal structure of the support, causing the defect support to become amorphous TiO2.

[0028] Preferably, the molar ratio of the titanium precursor to water is 1:(40-80), for example, it can be 1:40, 1:45, 1:50, 1:55, 1:60, 1:65, 1:70, 1:75 or 1:80, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable, preferably 1:(50-60).

[0029] Preferably, the temperature of the co-hydrolysis is 20-60℃, for example, it can be 20℃, 25℃, 30℃, 35℃, 40℃, 45℃, 50℃, 55℃ or 60℃, but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0030] Preferably, the co-hydrolysis time is 8-12 hours, for example, it can be 8 hours, 9 hours, 10 hours, 11 hours or 12 hours, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0031] Preferably, the pressure of the first rotary evaporation in step (1) is 0.08-0.1 MPa, for example, it can be 0.08 MPa, 0.085 MPa, 0.09 MPa, 0.095 MPa or 0.1 MPa, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0032] In this invention, a method of impregnation is used to load precious metals onto a defect carrier, wherein the pressure of the first rotary evaporation is 0.08-0.1 MPa.

[0033] Preferably, the temperature of the first rotary evaporation is 50-80°C, for example, it can be 50°C, 55°C, 60°C, 65°C, 70°C, 75°C or 80°C, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0034] Preferably, the first rotary evaporation time is 2-4 hours, for example, it can be 2 hours, 2.5 hours, 3 hours, 3.5 hours or 4 hours, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0035] Preferably, the temperature of the first drying is 100-120°C, for example, it can be 100°C, 105°C, 110°C, 115°C or 120°C, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0036] Preferably, the first drying time is 2-4 hours, for example, it can be 2 hours, 2.5 hours, 3 hours, 3.5 hours or 4 hours, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0037] Preferably, the temperature of the first calcination is 400-800℃, for example, it can be 400℃, 450℃, 500℃, 550℃, 600℃, 650℃, 700℃, 750℃ or 800℃, but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0038] Preferably, the first calcination time is 4-6 hours, for example, it can be 4 hours, 4.5 hours, 5 hours, 5.5 hours or 6 hours, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0039] Preferably, the alkaline solution in step (2) includes sodium hydroxide and / or potassium hydroxide.

[0040] Preferably, the concentration of the alkaline solution is 1-4 mol / L, for example, it can be 1 mol / L, 1.5 mol / L, 2 mol / L, 2.5 mol / L, 3 mol / L, 3.5 mol / L or 4 mol / L, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0041] Preferably, the mass ratio of the titanium-silicon composite to the alkaline solution is 1:(20-40), for example, it can be 1:20, 1:22, 1:24, 1:26, 1:28, 1:30, 1:32, 1:34, 1:36, 1:38 or 1:40, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0042] Preferably, the etching temperature is 40-80℃, for example, it can be 40℃, 45℃, 50℃, 55℃, 60℃, 65℃, 70℃, 75℃ or 80℃, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0043] Preferably, the etching time is 10-16 hours, for example, 10 hours, 11 hours, 12 hours, 13 hours, 14 hours, 15 hours or 16 hours, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0044] Preferably, there is no particular limitation on the method of solid-liquid separation; for example, it can be filtration or centrifugation.

[0045] Preferably, the washing endpoint is determined when the pH of the defective TiO2 carrier solution reaches 7-8. For example, the pH can be 7, 7.5, or 8, but is not limited to the listed values; other unlisted values ​​within the range are also applicable.

[0046] Preferably, the temperature of the second drying is 100-120°C, for example, it can be 100°C, 105°C, 110°C, 115°C or 120°C, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0047] Preferably, the second drying time is 2-4 hours, for example, it can be 2 hours, 2.5 hours, 3 hours, 3.5 hours or 4 hours, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0048] Preferably, the impregnation in step (3) is performed using the following steps:

[0049] A defective TiO2 support was mixed with water and heated to obtain a defective TiO2 support solution. A noble metal source was then added to the defective TiO2 support solution for impregnation.

[0050] In this invention, during the impregnation step, the defective TiO2 support is first dispersed by mixing and heating with water. Once the defective TiO2 support is uniformly dispersed in the water, the noble metal source is then added to the defective TiO2 support solution for impregnation. This is different from directly mixing and heating the defective TiO2 support and the noble metal source together. Since the defective TiO2 support is a solid powder, and the noble metal source content is below 0.5 wt%, direct mixing would be difficult to achieve uniform dispersion. Therefore, the step of dispersing before impregnation ensures that the noble metal is dispersed in the defective TiO2 support in a single-atom form, avoiding the aggregation of the noble metal active component, reducing the amount of noble metal source used, and achieving higher catalytic activity.

[0051] Preferably, the precious metal source in step (3) includes any one or at least two of Pt source, Pd source or Au source, wherein typical but non-limiting combinations include the combination of Pt source and Pd source, the combination of Pd source and Au source, the combination of Pt source and Au source, and the combination of Pt source, Pd source and Au source.

[0052] Preferably, the Pt source includes any one or a combination of at least two of platinum nitrate, platinum chloride, or chloroplatinic acid, preferably chloroplatinic acid, wherein typical but non-limiting combinations include a combination of platinum nitrate and platinum chloride, a combination of platinum chloride and chloroplatinic acid, a combination of platinum nitrate and chloroplatinic acid, or a combination of platinum nitrate, platinum chloride, and chloroplatinic acid.

[0053] Preferably, the Pd source comprises any one or a combination of at least two of palladium nitrate, palladium chloride, or palladium chlorate, preferably palladium chlorate, wherein typical but non-limiting combinations include combinations of palladium nitrate and palladium chloride, combinations of palladium chloride and palladium chlorate, combinations of palladium nitrate and palladium chlorate, and combinations of palladium nitrate, palladium chloride, and palladium chlorate.

[0054] Preferably, the Au source comprises any one or a combination of at least two of chloroauric acid, gold chloride, or gold acetate, wherein typical but non-limiting combinations include a combination of chloroauric acid and gold chloride, a combination of gold chloride and gold acetate, a combination of chloroauric acid and gold acetate, or a combination of chloroauric acid, gold chloride, and gold acetate.

[0055] Preferably, the heating temperature is 40-80℃, for example, it can be 40℃, 45℃, 50℃, 55℃, 60℃, 65℃, 70℃, 75℃ or 80℃, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0056] Preferably, the heating time is 1-2 hours, for example, 1 hour, 1.2 hours, 1.5 hours, 1.8 hours or 2 hours, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0057] Preferably, the temperature of the second rotary evaporation is 50-80°C, for example, it can be 50°C, 55°C, 60°C, 65°C, 70°C, 75°C or 80°C, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0058] Preferably, the second rotary evaporation time is 2-4 hours, for example, it can be 2 hours, 2.5 hours, 3 hours, 3.5 hours or 4 hours, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0059] Preferably, the temperature of the third drying is 100-120°C, for example, it can be 100°C, 105°C, 110°C, 115°C or 120°C, but is not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0060] Preferably, the third drying time is 2-4 hours, for example, 2 hours, 2.5 hours, 3 hours, 3.5 hours or 4 hours, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0061] Preferably, the second calcination temperature is 400-800℃, for example, it can be 400℃, 450℃, 500℃, 550℃, 600℃, 650℃, 700℃, 750℃ or 800℃, but is not limited to the listed values, and other unlisted values ​​within the range are also applicable.

[0062] Preferably, the second calcination time is 4-6 hours, for example, 4 hours, 4.5 hours, 5 hours, 5.5 hours or 6 hours, but not limited to the listed values. Other unlisted values ​​within the range are also applicable.

[0063] As a preferred embodiment of the preparation method described in the second aspect of the present invention, the preparation method includes:

[0064] (1) Titanium precursor, silicon precursor and water are mixed in a molar ratio of 1:(0.1-1):(40-80) and hydrolyzed at 20-60℃ for 8-12h to obtain a first mixture. The first mixture is subjected to first rotary evaporation at 0.08-0.1MPa and 50-80℃ for 2-4h, followed by first drying at 100-120℃ for 2-4h and first calcination at 400-800℃ for 4-6h to obtain titanium-silicon composite.

[0065] (2) The titanium-silicon composite obtained in step (1) is mixed with an alkaline solution with a concentration of 1-4 mol / L, etched at 40-80℃ for 10-16 h, and then subjected to solid-liquid separation, washing to pH 7-8 and second drying at 100-120℃ for 2-4 h to obtain a defective TiO2 support with defect sites.

[0066] (3) The defective TiO2 support is mixed with water and heated and stirred at 40-80℃ to obtain a defective TiO2 support solution. The defective TiO2 support solution is mixed with a noble metal source and impregnated. Then, it is subjected to a second rotary evaporation at 0.08-0.1MPa and 50-80℃ for 2-4 hours, followed by a third drying at 100-120℃ for 2-4 hours and a second calcination at 400-800℃ for 4-6 hours to obtain the single-atom defect catalyst.

[0067] Thirdly, the present invention provides the use of a single-atom defect catalyst for CO oxidation as described in the first aspect, said single-atom defect catalyst for CO catalytic oxidation.

[0068] The single-atom defect catalyst prepared by this invention has high atom utilization and good performance in CO catalytic oxidation. It has good application prospects in the field of CO pollutant purification in flue gas.

[0069] The numerical range described in this invention includes not only the point values ​​listed above, but also any point values ​​within the numerical ranges not listed above. Due to space limitations and for the sake of brevity, this invention will not exhaustively list all the specific point values ​​included in the range.

[0070] Compared with the prior art, the present invention has at least the following beneficial effects:

[0071] (1) The catalyst support for CO oxidation provided by the present invention has defect sites that can be used as loading sites for active components. The noble metal active components are uniformly dispersed on the defect sites of the support in the form of single atoms, which reduces the content of noble metals, has high atomic utilization and high catalytic oxidation activity.

[0072] (2) This invention prepares a titanium-silicon composite by co-hydrolysis of titanium and silicon precursors, and then removes silicon by alkaline chemical etching to destroy the Ti-O-Si structure in the titanium-silicon composite, thereby constructing defect sites. The defect sites enhance the interaction between the metal and the support, making the noble metal elements highly dispersed and existing in the form of single atoms. At the same time, the defect sites promote the formation of oxygen vacancies, enhance the adsorption and activation performance of the catalyst for oxygen, and optimize the coordination environment of the noble metal single atoms. The preparation method is mild, simple to operate, and can be mass-produced, with good prospects for industrial application.

[0073] (3) The single-atom defect catalyst prepared by the present invention has high atomic utilization and good catalytic oxidation performance for CO, and has good application in the field of CO pollutant purification in flue gas. Attached Figure Description

[0074] Figure 1 This is a schematic diagram of the preparation process of the Pt single-atom defect catalyst in Example 1 of the present invention;

[0075] Figure 2 These are XRD patterns of the TiO2 supports prepared in Example 1 and Comparative Example 1;

[0076] Figure 3 These are EPR images of the TiO2 supports prepared in Example 1 and Comparative Example 1;

[0077] Figure 4 These are graphs showing the catalytic conversion effects of the catalysts prepared in Example 1 and Comparative Example 1 at different temperatures.

[0078] Figure 5 This is a graph showing the stability test results of the catalyst prepared in Example 1. Detailed Implementation

[0079] The technical solution of the present invention will be further described below with reference to the accompanying drawings and specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be considered as specific limitations thereof.

[0080] In the following examples, unless otherwise specified, all reagents and consumables were purchased from conventional reagent manufacturers in the art; unless otherwise specified, the experimental methods and techniques used are conventional methods and techniques in the art.

[0081] Example 1

[0082] This embodiment provides a single-atom defect catalyst for CO oxidation, the single-atom defect catalyst comprising a defective TiO2 support and a Pt single atom supported on the defective TiO2 support;

[0083] The defective TiO2 support has defect sites, which are defect structures caused by oxygen vacancies and titanium vacancies; the active component is located on the defect sites of the defective TiO2 support; the active component exists in the single-atom defect catalyst in a coordination structure, and the coordination number of the coordination structure is 5.

[0084] Based on a total mass of 100 wt% for the single-atom defect catalyst, the content of the active component in the single-atom defect catalyst is 0.1 wt%.

[0085] The preparation method of the Pt single-atom defect catalyst provided in this embodiment is as follows: Figure 1 Specifically, it includes the following steps:

[0086] (1) Tetrabutyl titanate ( Figure 1 (represented by Ti compounds), diethyl silicate ( Figure 1 The first mixture was obtained by mixing Si compound (represented in the text) and water in a molar ratio of 1:0.5:60 and hydrolyzing it at 40°C for 10 hours. The first mixture was then subjected to rotary evaporation at 0.09 MPa and 65°C for 3 hours, followed by drying at 110°C for 3 hours and calcining at 600°C for 4 hours to obtain a titanium-silicon composite.

[0087] (2) The titanium-silicon composite obtained in step (1) was mixed with a sodium hydroxide solution with a concentration of 2 mol / L at a mass ratio of 1:30. After etching at 60°C for 12 h, the mixture was centrifuged, washed until pH 7, and dried at 110°C for 3 h to obtain a defective TiO2 support with defect sites.

[0088] (3) The defective TiO2 support is mixed with water and heated and stirred at 60°C to obtain a defective TiO2 support solution. The defective TiO2 support solution is mixed with chloroplatinic acid (i.e., Pt source) at a content of 0.1wt%, and then subjected to a second rotary evaporation at 0.09MPa and 65°C for 3h. Then, it is subjected to a third drying at 110°C for 3h and a second calcination at 600°C for 5h to obtain the single-atom defect catalyst.

[0089] The XRD pattern of the prepared defective TiO2 support is shown in the figure. Figure 2 As shown, the defective TiO2 supports are dominated by the characteristic peaks of anatase TiO2, exhibiting better crystal phase stability. No crystal transformation occurs after calcination in air at 500℃ for 4 hours. The crystal strength of the anatase TiO2 in the defective TiO2 supports is significantly weaker. This is because the introduction of silicon during the construction of the defect structure inhibits its formation.

[0090] The EPR image of the prepared defective TiO2 support is shown below. Figure 3As shown, a distinct paramagnetic peak appears at g = 2.003 on the defective TiO2 support. This peak is attributed to oxygen vacancies, proving the existence of defect sites on the support.

[0091] The catalytic conversion effect of the prepared single-atom defect catalyst at different temperatures is as follows: Figure 4 As shown, the CO conversion efficiency reaches over 90% at 200℃;

[0092] Stability test results of the prepared single-atom defect catalyst Figure 5 As shown, no decrease in CO oxidation efficiency was observed after 48 hours, indicating that the Pt single-atom defect catalyst prepared in Example 1 has good stability.

[0093] Example 2

[0094] This embodiment provides a single-atom defect catalyst for CO oxidation, the single-atom defect catalyst comprising a defective TiO2 support and a Pt single atom supported on the defective TiO2 support;

[0095] The defective TiO2 support has defect sites, which are defect structures caused by oxygen vacancies and titanium vacancies; the active component is located on the defect sites of the defective TiO2 support; the active component exists in the single-atom defect catalyst in a coordination structure, and the coordination number of the coordination structure is 5.

[0096] Based on a total mass of 100 wt% for the single-atom defect catalyst, the content of the active component in the single-atom defect catalyst is 0.05 wt%.

[0097] The preparation method of the Pt single-atom defect catalyst provided in this embodiment includes the following steps:

[0098] (1) Ethyl titanate, triethyl silicate and water were mixed in a molar ratio of 1:0.1:40 and hydrolyzed at 20°C for 12 h to obtain a first mixture. The first mixture was subjected to rotary evaporation at 0.08 MPa and 50°C for 4 h, followed by drying at 100°C for 4 h and calcination at 400°C for 6 h to obtain a titanium-silicon composite.

[0099] (2) The titanium-silicon composite obtained in step (1) was mixed with potassium hydroxide at a concentration of 1 mol / L at a mass ratio of 1:40. After etching at 40°C for 16 h, the mixture was centrifuged and washed until pH 8 and dried at 100°C for 4 h to obtain a defective TiO2 support with defect sites.

[0100] (3) The defective TiO2 support is mixed with water and heated and stirred at 40°C to obtain a defective TiO2 support solution. The defective TiO2 support solution is mixed with platinum chloride at a content of 0.05wt% and impregnated. Then, it is subjected to a second rotary evaporation at 0.08MPa and 50°C for 4h, followed by a third drying at 100°C for 4h and a second calcination at 400°C for 6h to obtain the single-atom defect catalyst.

[0101] Example 3

[0102] This embodiment provides a single-atom defect catalyst for CO oxidation, the single-atom defect catalyst comprising a defective TiO2 support and a Pt single atom supported on the defective TiO2 support;

[0103] The defective TiO2 support has defect sites, which are defect structures caused by oxygen vacancies and titanium vacancies; the active component is located on the defect sites of the defective TiO2 support; the active component exists in the single-atom defect catalyst in a coordination structure, and the coordination number of the coordination structure is 5.

[0104] Based on a total mass of 100 wt% for the single-atom defect catalyst, the content of the active component in the single-atom defect catalyst is 0.5 wt%.

[0105] The preparation method of the Pt single-atom defect catalyst provided in this embodiment includes the following steps:

[0106] (1) Propyl titanate, tetraethyl silicate and water are mixed in a molar ratio of 1:1:80 and hydrolyzed at 60°C for 8 hours to obtain a first mixture. The first mixture is subjected to first rotary evaporation at 0.1 MPa and 80°C for 4 hours, and then subjected to first drying at 120°C for 2 hours and first calcination at 800°C for 4 hours to obtain a titanium-silicon composite.

[0107] (2) The titanium-silicon composite obtained in step (1) was mixed with sodium hydroxide at a concentration of 4 mol / L at a mass ratio of 1:20. After etching at 80°C for 10 h, it was centrifuged, washed until pH 7 and dried at 120°C for 2 h to obtain a defective TiO2 support with defect sites.

[0108] (3) The defective TiO2 support is mixed with water and heated and stirred at 80°C to obtain a defective TiO2 support solution. The defective TiO2 support solution is mixed with platinum nitrate at a content of 0.5wt% and impregnated. Then, it is subjected to a second rotary evaporation at 0.1MPa and 80°C for 2h, followed by a third drying at 120°C for 2h and a second calcination at 800°C for 4h to obtain the single-atom defect catalyst.

[0109] Example 4

[0110] This embodiment provides a single-atom defect catalyst for CO oxidation, which differs from Example 1 only in that the molar ratio of tetrabutyl titanate and diethyl silicate in step (1) is 2:0.1 when preparing the Pt single-atom defect catalyst.

[0111] Example 5

[0112] This embodiment provides a single-atom defect catalyst for CO oxidation, which differs from Example 1 only in that the molar ratio of tetrabutyl titanate and diethyl silicate in step (1) is 1:2 when preparing the Pt single-atom defect catalyst.

[0113] Example 6

[0114] This embodiment provides a single-atom defect catalyst for CO oxidation, which differs from Example 1 only in that the concentration of the sodium hydroxide solution in step (2) is 5 mol / L when preparing the Pt single-atom defect catalyst.

[0115] Example 7

[0116] This embodiment provides a single-atom defect catalyst for CO oxidation, which differs from Example 1 only in that the concentration of the sodium hydroxide solution in step (2) is 0.5 mol / L when preparing the Pt single-atom defect catalyst.

[0117] Example 8

[0118] This embodiment provides a Pd single-atom defect catalyst for CO oxidation, which differs from Example 1 only in that, when preparing the Pd single-atom defect catalyst, the chloroplatinic acid in step (3) is replaced with palladium nitrate.

[0119] Example 9

[0120] This embodiment provides an Au single-atom defect catalyst for CO oxidation, which differs from Example 1 only in that, in the preparation of the Au single-atom defect catalyst, the chloroplatinic acid in step (3) is replaced with gold chloride.

[0121] Example 10

[0122] This embodiment provides a catalyst for CO oxidation. The only difference between this catalyst and Example 1 is that, in the preparation of this catalyst, the TiO2 support and chloroplatinic acid in step (3) are added to water and stirred and heated together.

[0123] Comparative Example 1

[0124] This comparative example provides a catalyst for CO oxidation, which differs from Example 1 only in that, when preparing the catalyst, a silicon source is not added in step (1) and the etching in step (2) is not performed at the same time.

[0125] The XRD pattern of the prepared TiO2 support is shown in the figure. Figure 2 As shown, the TiO2 support prepared in Comparative Example 1 exhibits weak rutile TiO2 characteristic peaks, and its stability is not as good as the anatase-type defective TiO2 support prepared in Example 1.

[0126] The EPR image of the prepared TiO2 support is shown below. Figure 3 As shown, no clear paramagnetic peak was found in the TiO2 support at g = 2.003, and its defect content was lower than that of the defective TiO2 support prepared in Example 1.

[0127] The catalytic conversion effect of the prepared catalyst at different temperatures is as follows: Figure 4 As shown, it can only reach over 90% at 260℃, and its catalytic oxidation performance of CO is far inferior to that of Example 1.

[0128] Comparative Example 2

[0129] This comparative example provides a catalyst for CO oxidation, which differs from Example 1 only in that, when preparing this catalyst, steps (1) and (2) are omitted, and the mixing and impregnation step (3) is carried out directly using TiO2 support (CAS No.: 13463-67-7) purchased from Aladdin.

[0130] Comparative Example 3

[0131] This comparative example provides a catalyst for CO oxidation, which differs from Example 1 only in that step (3) is not performed when preparing the catalyst, i.e., the active component Pt is not loaded. Steps (1) and (2) are the same as in Example 1.

[0132] Comparative Example 4

[0133] The catalyst obtained in this comparative example is a defective catalyst, and the catalyst used is the catalyst in Example 1 of CN117680124A.

[0134] In this comparative example, ethanol was used as a solvent in step (1), and acetylacetone, a stabilizer, was also added. The loading process of the active component in step (3) was the same as in Example 1. By comparing Example 1 and Comparative Example 4, it can be seen that the preparation method of the defective TiO2 support of the present invention does not require the addition of other solvent components, which reduces the preparation cost and is more green, environmentally friendly and pollution-free.

[0135] Performance testing

[0136] (1) BET test: The pore diameter, pore volume and specific surface area of ​​the defective TiO2 supports prepared in the above examples and comparative examples were tested using a fully automated physical adsorption instrument based on BET specific surface area. The test conditions were: 100 mg sample, vacuum degassing at 300℃ for 4 hours, N2 adsorption at -196℃, and a relative pressure range of 10. -7 ~1.

[0137] (2) Characterization of catalyst coordination structure: The single-atom defect catalyst of Example 1 was characterized by synchrotron radiation. The EXAFS data showed that the coordination structure of Pt on the single-atom defect catalyst was 5-coordinate, while the coordination structure of Pt on the catalyst with defect structure without prior preparation was 4-coordinate.

[0138] (3) Catalytic conversion performance test: The catalysts prepared in the above examples and comparative examples were used to test the CO oxidation performance. The total gas flow rate was 200 mL / min, the volume fraction of CO was 1%, the volume fraction of O2 was 16%, N2 was used as the carrier gas, the catalyst dosage was 200 mg, and the gas mass hourly space velocity was 60000 h⁻¹. -1 The results obtained with a reaction temperature range of 100–300℃ are shown in Table 1. 90 This is the reaction temperature corresponding to a CO oxidation efficiency of 90%.

[0139] (4) Stability test: The catalysts prepared in the above examples and comparative examples were used to test the CO oxidation performance. The total gas flow rate was 200 mL / min, the volume fraction of CO was 1%, the volume fraction of O2 was 16%, N2 was used as the carrier gas, the catalyst dosage was 200 mg, and the gas hourly space velocity was 60000 h⁻¹. -1 The test temperature was 200℃ and the test time was 48h. The results are shown in Table 1.

[0140] Table 1

[0141]

[0142]

[0143] The test results show that:

[0144] (1) As can be seen from Examples 1-3, the single-atom defect catalyst provided by the present invention has high catalytic activity and stability. The reaction temperature T corresponding to its CO oxidation efficiency of 90% is... 90 At temperatures below 205°C, the catalyst's catalytic activity remains above 60% even after 48 hours at 200°C. This demonstrates that the single-atom defect catalyst prepared in this invention exhibits excellent catalytic oxidation activity and stability.

[0145] (2) By comparing Example 1 with Examples 4-5, it can be seen that by further controlling the molar ratio of titanium precursor to silicon precursor to 1:(0.1-1), the present invention can prepare a defect TiO2 support with a suitable defect structure, which achieves a better catalytic oxidation effect after being loaded with a single atom noble metal. If the content of titanium precursor is too high, the defect sites generated after etching will be insufficient. If the content of titanium precursor is too low, the crystal form of anatase will not be formed, and the catalyst activity will be reduced.

[0146] (3) By comparing Example 1 with Examples 6-7, it can be seen that the present invention further controls the concentration of alkaline solution within the range of 1-4 mol / L. If the concentration of alkaline solution is too high, the pores of the etched TiO2 support will be too large, and the defect sites will be interconnected, which is not conducive to the improvement of specific surface area and the number of active material loading sites will be reduced, ultimately leading to a decrease in the catalytic activity of the single-atom defect catalyst. If the concentration of alkaline solution is too low, it will be difficult to etch away silicon, and only a small number of defect sites will be generated.

[0147] (4) By comparing Example 1 with Examples 8-9, it can be seen that the present invention prepares single-atom defect catalysts for CO oxidation by loading different noble metal single-atom active components on defective TiO2 support. The data results show that when Pd or Au is selected as the noble metal, the catalytic efficiency of the single-atom defect catalyst is not as high as when Pt noble metal is selected as the active component in Example 1.

[0148] (5) By comparing Example 1 and Example 10, it can be seen that if the defective TiO2 support is not heated and dispersed in advance during the active material loading process in step (3), the loaded active components will agglomerate, the active components will be unevenly distributed and easily fall off the support, resulting in a reduction in the service life of the defective catalyst.

[0149] (6) By comparing Example 1 and Comparative Example 1, it can be seen that the Pt single-atom defect catalyst prepared in Example 1 achieves a CO conversion efficiency of over 90% at 195℃, while the Pt catalyst prepared in Comparative Example 1 requires 260℃ to achieve over 90%. This indicates that the CO oxidation performance of the Pt single-atom defect catalyst prepared in Example 1 is significantly better than that of the Pt catalyst prepared in Comparative Example 1. This shows that the present invention prepares a titanium-silicon composite by co-hydrolysis of titanium and silicon precursors, and then uses alkaline chemical etching to remove silicon from the obtained titanium-silicon composite to construct defect sites, thereby enhancing the adsorption and activation performance of the catalyst for oxygen, optimizing the coordination environment of the noble metal single atom, improving the CO oxidation activity of the catalyst, and improving the stability of the catalyst. If the defect sites are not created by etching, the coordination structure of the active component Pt on the prepared catalyst is 4-coordinate, resulting in lower catalytic activity, and the active component is very easy to detach from the support, causing a decrease in the stability and lifespan of the catalyst.

[0150] (7) By comparing Example 1 and Comparative Example 2, it can be seen that when no silicon source is added or no etching is performed during the preparation of the support, the prepared Pt catalyst needs to be 270°C to reach more than 90%. The coordination structure of the active component Pt on the catalyst prepared by this method is 4-coordinate, and its performance is far inferior to that of Example 1. This shows that the defective TiO2 support prepared by alkaline etching of the present invention can disperse the active component in the form of single atoms at the defect sites, thereby reducing the amount of active component and improving the utilization rate of the catalyst.

[0151] (8) By comparing Example 1 with Comparative Example 3, it can be seen that if no active material is loaded, the catalyst prepared has extremely low catalytic activity and has no prospect of use.

[0152] In summary, the single-atom defect catalyst for CO oxidation provided by this invention has defect sites on the catalyst support that can serve as loading sites for the active components. The noble metal active components are uniformly dispersed on the defect sites of the support in the form of single atoms, which reduces the content of noble metals, has high atom utilization, and has high catalytic oxidation activity.

[0153] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method for preparing a single-atom defect catalyst for CO oxidation, characterized in that, The single-atom defect catalyst includes a defective TiO2 support and an active component supported on the defective TiO2 support, wherein the active component includes a noble metal single atom. The defective TiO2 support has defect sites, which are defect structures caused by oxygen vacancies and titanium vacancies; the active component is located on the defect sites of the defective TiO2 support. The preparation method of the single-atom defect catalyst includes the following steps: (1) The titanium precursor, silicon precursor and water are mixed and hydrolyzed to obtain a first mixture. The first mixture is subjected to a first rotary evaporation, a first drying and a first calcination to obtain a titanium-silicon composite. (2) The titanium-silicon composite obtained in step (1) is mixed with alkaline solution for etching. After solid-liquid separation, washing and second drying, a defective TiO2 support with defect sites is obtained. (3) The mixed noble metal source is impregnated with the defective TiO2 support, and then subjected to a second rotary evaporation, a third drying and a second calcination to obtain the single-atom defect catalyst.

2. The preparation method according to claim 1, characterized in that, The active component exists in the single-atom defect catalyst in a coordination structure, and the coordination number of the coordination structure is 5.

3. The preparation method according to claim 1, characterized in that, Based on a total mass of 100wt% for the single-atom defect catalyst, the content of the active component in the single-atom defect catalyst is 0.02-0.5wt%.

4. The preparation method according to claim 1, characterized in that, The noble metal single atom includes any one or a combination of at least two of Pt single atoms, Pd single atoms, or Au single atoms.

5. The preparation method according to claim 4, characterized in that, The noble metal single atom is a Pt single atom.

6. The preparation method according to claim 1, characterized in that, The titanium precursor in step (1) includes any one or a combination of at least two of tetrabutyl titanate, ethyl titanate or propyl titanate.

7. The preparation method according to claim 6, characterized in that, The titanium precursor in step (1) is tetrabutyl titanate.

8. The preparation method according to claim 1, characterized in that, The silicon precursor includes any one or a combination of at least two of diethyl silicate, triethyl silicate, or tetraethyl silicate.

9. The preparation method according to claim 8, characterized in that, The silicon precursor is tetraethyl silicate.

10. The preparation method according to claim 1, characterized in that, The molar ratio of the titanium precursor to the silicon precursor is 1:(0.1-1).

11. The preparation method according to claim 10, characterized in that, The molar ratio of the titanium precursor to the silicon precursor is 1:(0.2-0.5).

12. The preparation method according to claim 1, characterized in that, The molar ratio of the titanium precursor to water is 1:(40-80).

13. The preparation method according to claim 12, characterized in that, The molar ratio of the titanium precursor to water is 1:(50-60).

14. The preparation method according to claim 1, characterized in that, The temperature for co-hydrolysis is 20-60℃.

15. The preparation method according to claim 1, characterized in that, The co-hydrolysis time is 8-12 hours.

16. The preparation method according to claim 3 or 4, characterized in that, Step (1) The pressure of the first rotary evaporator is 0.08-0.1 MPa.

17. The preparation method according to claim 1, characterized in that, The temperature of the first rotary evaporation is 50-80℃.

18. The preparation method according to claim 1, characterized in that, The first rotary evaporation time is 2-4 hours.

19. The preparation method according to claim 1, characterized in that, The temperature for the first drying process is 100-120℃.

20. The preparation method according to claim 1, characterized in that, The first drying time is 2-4 hours.

21. The preparation method according to claim 1, characterized in that, The first calcination temperature is 400-800℃.

22. The preparation method according to claim 1, characterized in that, The first calcination time is 4-6 hours.

23. The preparation method according to claim 1, characterized in that, The alkaline solution in step (2) includes sodium hydroxide and / or potassium hydroxide.

24. The preparation method according to claim 1, characterized in that, The concentration of the alkaline solution is 1-4 mol / L.

25. The preparation method according to claim 1, characterized in that, The mass ratio of the titanium-silicon composite to the alkaline solution is 1:(20-40).

26. The preparation method according to claim 1, characterized in that, The etching temperature is 40-80℃.

27. The preparation method according to claim 1, characterized in that, The etching time is 10-16 hours.

28. The preparation method according to claim 1, characterized in that, The solid-liquid separation includes centrifugation.

29. The preparation method according to claim 1, characterized in that, The washing endpoint is determined when the pH of the defective TiO2 carrier solution reaches 7-8.

30. The preparation method according to claim 1, characterized in that, The second drying temperature is 100-120℃.

31. The preparation method according to claim 1, characterized in that, The second drying time is 2-4 hours.

32. The preparation method according to claim 1, characterized in that, The impregnation process in step (3) is carried out using the following steps: A defective TiO2 support was mixed with water and heated to obtain a defective TiO2 support solution. A noble metal source was then added to the defective TiO2 support solution for impregnation.

33. The preparation method according to claim 1, characterized in that, The precious metal source in step (3) includes any one or a combination of at least two of the following: Pt source, Pd source, or Au source.

34. The preparation method according to claim 33, characterized in that, The Pt source includes any one or a combination of at least two of platinum nitrate, platinum chloride, or chloroplatinic acid.

35. The preparation method according to claim 34, characterized in that, The Pt source is chloroplatinic acid.

36. The preparation method according to claim 33, characterized in that, The Pd source includes any one or a combination of at least two of palladium nitrate, palladium chloride, or palladium chlorate.

37. The preparation method according to claim 36, characterized in that, The Pd source is palladium chlorate.

38. The preparation method according to claim 33, characterized in that, The Au source includes any one or a combination of at least two of chloroauric acid, gold chloride, or gold acetate.

39. The preparation method according to claim 38, characterized in that, The Au source is gold chloride.

40. The preparation method according to claim 32, characterized in that, The heating temperature is 40-80℃.

41. The preparation method according to claim 32, characterized in that, The heating time is 1-2 hours.

42. The preparation method according to claim 1, characterized in that, The temperature of the second rotary evaporation is 50-80℃.

43. The preparation method according to claim 1, characterized in that, The second rotary evaporation time is 2-4 hours.

44. The preparation method according to claim 1, characterized in that, The temperature for the third drying process is 100-120℃.

45. The preparation method according to claim 1, characterized in that, The third drying time is 2-4 hours.

46. ​​The preparation method according to claim 1, characterized in that, The second calcination temperature is 400-800℃.

47. The preparation method according to claim 1, characterized in that, The second calcination time is 4-6 hours.

48. The preparation method according to claim 1, characterized in that, The preparation method of the single-atom defect catalyst includes the following steps: (1) Titanium precursor, silicon precursor and water are mixed in a molar ratio of 1:(0.1-1):(40-80) and hydrolyzed at 20-60℃ for 8-12h to obtain a first mixture. The first mixture is subjected to first rotary evaporation at 0.08-0.1MPa and 50-80℃ for 2-4h, and then subjected to first drying at 100-120℃ for 2-4h and first calcination at 400-800℃ for 4-6h to obtain titanium-silicon composite. (2) The titanium-silicon composite obtained in step (1) is mixed with an alkaline solution with a concentration of 1-4 mol / L, etched at 40-80℃ for 10-16 h, and then subjected to solid-liquid separation, washing until pH 7-8 and second drying at 100-120℃ for 2-4 h to obtain a defective TiO2 support with defect sites. (3) The defective TiO2 support is mixed with water and heated and stirred at 40-80℃ to obtain a defective TiO2 support solution. The defective TiO2 support solution is mixed with a noble metal source and impregnated. Then, a second rotary evaporation is carried out at 0.08-0.1MPa and 50-80℃ for 2-4 hours. Then, a third drying at 100-120℃ for 2-4 hours and a second calcination at 400-800℃ for 4-6 hours are carried out in sequence to obtain the single-atom defect catalyst.

49. The application of a single-atom defect catalyst for CO oxidation prepared by the preparation method according to any one of claims 1-48, characterized in that, The single-atom defect catalyst is used for the catalytic oxidation of CO.

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