Alkali metal added self-grown pd-based reverse water gas shift catalyst, synthesis method and application

Abstract

The application discloses an alkali metal added self-grown Pd-based reverse water gas shift catalyst, a synthesis method and application, and the Pd / gamma-Al2O3 active component catalyst and Cs / gamma-Al2O3 auxiliary catalyst are prepared through a separate impregnation method, and then the two products are physically mixed in a powder form, the Cs electronic regulation and structure modification are combined, the Cs added Pd-based reverse water gas shift catalyst with excellent performance, stability and controllability is prepared, the methanation side reaction is inhibited through the Cs auxiliary agent, the CO desorption is promoted, the competitive adsorption among different active components is avoided through separate impregnation, the precise independent regulation of the loadings of the components is realized, the synergistic effect of the Pd active sites and the Cs auxiliary agent is realized through physical mixing, the method is convenient to operate, the active component of the obtained composite catalyst is uniformly dispersed and has low impurity content, the final RWGS catalyst has high CO selectivity and high CO2 conversion rate, the low-temperature catalytic activity is significantly improved, the RWGS catalyst is convenient to prepare into a modular structure, and has good resource utilization effect and industrial application prospect.

Description

Technical Field

[0001] This invention belongs to the field of water gas catalyst preparation technology, specifically relating to a self-growing Pd-based reverse water gas catalyst with alkali metal addition, its synthesis method, and its application. Background Technology

[0002] With rapid industrial development, the concentration of carbon dioxide (CO2) in the atmosphere has continued to rise, posing a serious threat to the global ecological environment and the sustainable development of human society. Achieving carbon peaking and carbon neutrality ("dual carbon") has become a global consensus, and developing efficient CO2 conversion and resource utilization technologies is one of the key paths to achieving this goal.

[0003] Among numerous CO2 conversion technologies, the reverse water-gas shift (RWGS) reaction has attracted significant attention due to its substantial application advantages. This reaction utilizes hydrogen (H2) produced from renewable energy sources to efficiently convert CO2 into carbon monoxide (CO). The generated CO is not only an important intermediate for the synthesis of fuels and high-value-added chemicals such as olefins and alcohols, but also serves as a core feedstock for downstream processes such as Fischer-Tropsch synthesis and methanol synthesis, effectively realizing the high-value utilization of CO2 and possessing significant industrial application value. The core technical bottleneck of the RWGS reaction lies in the fact that the CO2 molecule has extremely strong thermodynamic stability due to the high C=O bond energy, resulting in a high activation energy barrier for the reaction. Simultaneously, this reaction is endothermic, requiring high temperatures to drive the forward reaction. However, high-temperature environments can trigger the methanation side reaction of excessive CO2 hydrogenation, generating the competing product methane (CH4), significantly reducing the CO selectivity of the RWGS reaction and hindering its industrial application. Therefore, designing a catalyst that can balance high CO selectivity and high CO2 conversion rate is crucial to overcoming the technical bottleneck of the RWGS reaction and promoting its industrialization.

[0004] The essence of the RWGS reaction lies in the breaking and activation of the C=O bond in the CO2 molecule, the dissociation and activation of H2, and the desorption of the product CO. Supported metal catalysts, due to the synergistic effect between their metal active sites and oxide supports, can effectively enhance the CO2 hydrogenation performance, making them a current research hotspot for RWGS catalysts. Typical active metals in this class of catalysts include single or multi-component metals such as palladium (Pd), platinum (Pt), ruthenium (Ru), rhodium (Rh), and copper (Cu). Among them, the noble metal Pd, due to its unique 4d electron shell structure and tunable valence state, possesses excellent H2 dissociation ability and can enhance catalytic activity by providing abundant active sites, showing good application potential in the RWGS reaction. However, Pd nanoparticles, due to their small particle size and high specific surface area, exhibit thermodynamic instability and are prone to aggregation, limiting their full catalytic performance and practical application in the RWGS reaction. γ-alumina (γ-Al2O3) is a commonly used catalyst support. It has a high specific surface area, well-developed pore structure and tunable Lewis acidity. It can form a Pd-O-Al covalent anchoring structure with Pd. Its surface hydroxyl groups and defect sites can donate electrons to Pd to build a local electron enrichment layer, which can effectively reduce the surface energy of Pd nanoparticles and inhibit their atomic migration. This locks Pd in ​​a highly dispersed state and achieves a simultaneous increase in the number of active sites and thermal stability. Therefore, it is often used to support Pd to prepare Pd / γ-Al2O3 catalysts.

[0005] However, traditional Pd / γ-Al2O3 catalysts still have significant technical drawbacks: CO molecules have a strong affinity for Pd metal surfaces, readily adsorbing strongly onto Pd nanoparticles, and the adsorbed CO is easily further hydrogenated by surface-active hydrogen (*H); simultaneously, the efficient hydrogen dissociation capability of Pd metal sites synergistically interacts with the Lewis acid sites of the γ-Al2O3 support, stabilizing the CHx intermediate and leading to excessive CO hydrogenation to CH4 rather than desorption, thus inhibiting the RWGS reaction pathway and significantly reducing CO selectivity, failing to meet the performance requirements of industrial applications. Therefore, improving the desorption capacity of CO on Pd metal surfaces has become a key breakthrough for achieving high selectivity in the RWGS reaction. Currently, the promoting effects of alkali metals (lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs)) in various hydrogenation reactions have been extensively studied. Studies have shown that the addition of alkali metal promoters mainly works in two ways: on the one hand, it can increase the basicity of the catalyst surface and promote the adsorption and activation of CO2; on the other hand, it can act as an electron donor to modify the electronic properties of the metal active sites, regulate the adsorption behavior of reactants and the formation and transformation of subsequent intermediates, thereby promoting CO desorption, guiding the reaction pathway toward RWGS, and improving reaction selectivity.

[0006] Currently, common preparation methods for supported metal catalysts mainly include wet impregnation, precipitation, sol-gel, and ion exchange methods. Among these, wet impregnation is one of the most widely used methods for supporting catalyst preparation due to its convenience and controllable cost. It can be further divided into single impregnation and co-impregnation. Co-impregnation involves simultaneously impregnating multiple active components onto the surface of a support. While this method is simpler, it has significant drawbacks: multiple active components compete for adsorption on the support surface, leading to uneven dispersion, agglomeration, and difficulty in forming stable active sites. Furthermore, it is impossible to precisely control the loading and dispersion state of a single active component, thus affecting the synergistic effect between the support and the active components, as well as among the active components themselves, ultimately reducing the catalytic performance and stability of the catalyst. Summary of the Invention

[0007] This invention addresses the problems of low CO selectivity, easy agglomeration of Pd nanoparticles, uneven dispersion of active components, difficulty in precise control of loading, and weak synergistic effect of components in the preparation of such modified catalysts by co-impregnation method. It provides a process for preparing Pd / γ-Al2O3+Cs / γ-Al2O3 powder physical mixed composite catalyst by "separate impregnation-powder physical mixing".

[0008] Among various alkali metals, Cs exhibits the lowest electronegativity and the strongest electron-donating ability and promoting effect, making it a promising catalyst doping agent for RWGS reactions. Based on this, to address the technical shortcomings of existing Pd / γ-Al2O3 catalysts, such as low CO selectivity and poor low-temperature activity, this invention utilizes the excellent promoting effect of Cs to modify and optimize catalytic performance. This is an effective way to overcome the technical bottlenecks of RWGS reactions and is also the core research direction of this invention.

[0009] This invention prepares Pd / γ-Al2O3 active component catalyst and Cs / γ-Al2O3 promoter catalyst separately by impregnation, and then physically mixes the two products in powder form. Combining the electronic regulation and structural modification effects of Cs, a high-performance, stable and controllable Cs-added Pd-based reverse water-gas shift catalyst is prepared. The Cs promoter inhibits the methanation side reaction and promotes CO desorption. Separate impregnation avoids competitive adsorption between different active components and achieves precise and independent control of the loading of each component. Physical mixing achieves the synergistic effect of Pd active sites and Cs promoter. The method is convenient to operate, and the resulting composite catalyst has uniform dispersion of active components and low impurity content. The final RWGS catalyst has both high CO selectivity and high CO2 conversion rate, significantly improved low-temperature catalytic activity, and is easy to prepare into a modular structure. It has good resource utilization effect and industrial application prospects.

[0010] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: A method for synthesizing a self-growing Pd-based countercurrent gas catalyst with alkali metal addition, comprising the following steps: (1) Weigh palladium nitrate hydrate (Pd(NO3)2•2H2O) according to the proportion and dissolve it in deionized water. Stir at room temperature until completely dissolved to prepare a Pd precursor solution. Then add a preset amount of γ-Al2O3 support to the solution, stir thoroughly at room temperature, and continue stirring at the set temperature until the system is dry. Place the obtained solid in a high-temperature resistant container and transfer it to a calcination device. Heat to the calcination temperature according to the set program, keep it warm for a certain time, and collect the product after it cools naturally to room temperature. Pd / γ-Al2O3 catalyst powder with Pd loading within the preset range is obtained. (2) Weigh out cesium carbonate (Cs2CO3) according to the proportion and dissolve it in deionized water. Stir at room temperature until completely dissolved to prepare a Cs precursor solution. Then add a preset amount of γ-Al2O3 support to the solution, stir thoroughly at room temperature, and continue stirring at the set temperature until the system is dry. Place the obtained solid in a high-temperature resistant container and transfer it to a calcination device. Heat to the calcination temperature according to the set program, keep it at the temperature for a certain time, and collect the product after it cools naturally to room temperature. Cs / γ-Al2O3 catalyst powder with Cs loading within the preset range is obtained. (3) The Pd / γ-Al2O3 catalyst powder prepared in step (1) and the Cs / γ-Al2O3 catalyst powder prepared in step (2) are placed in a mixing and grinding equipment according to a preset mass ratio. They are first physically mixed and then fully ground until they are uniform and fine. After pressing and sieving, the target Pd / γ-Al2O3+Cs / γ-Al2O3 composite catalyst, namely the alkali metal-added self-growing Pd-based countercurrent gas catalyst, is obtained.

[0011] As a preferred technical solution of the present invention, in step (1), the amount of Pd(NO3)2•2H2O is determined based on the target Pd loading of the Pd / γ-Al2O3 catalyst, and the Pd loading range is 0.5~5wt%.

[0012] As a preferred technical solution of the present invention, in step (2), the amount of Cs2CO3 is determined based on the target Cs loading of the Cs / γ-Al2O3 catalyst, and the Cs loading range is 5~20wt%.

[0013] As a preferred technical solution of the present invention, in steps (1) and (2), the ambient temperature of the room temperature stirring is 15~30℃, the stirring speed is 300~500r, and the stirring time is 20~60min, to ensure that the precursor is completely dissolved and the carrier and the precursor are fully contacted and adsorbed.

[0014] As a preferred technical solution of the present invention, in steps (1) and (2), the temperature of continuous stirring until dry is 50~70°C, industrial-grade magnetic stirring is adopted, the stirring speed is 300~500r, until the water in the system is completely evaporated, and the resulting solid is in the form of loose powder.

[0015] As a preferred technical solution of the present invention, in steps (1) and (2), the calcination equipment is an industrial-grade muffle furnace or a continuous calcination furnace, the calcination process is carried out in an air atmosphere, the heating rate is 4~8℃ / min, the calcination temperature is 600~700℃, the calcination time is 3~5h, and the furnace is naturally cooled to room temperature after calcination.

[0016] As a preferred technical solution of the present invention, in step (1), the amount of γ-Al2O3 support is determined according to the target Pd loading, to ensure that the Pd precursor is fully adsorbed by the support and to achieve uniform loading of Pd on the support surface; in step (2), the amount of γ-Al2O3 support is determined according to the target Cs loading, to ensure that the Cs precursor is fully adsorbed by the support and to achieve uniform loading of Cs on the support surface.

[0017] As a preferred technical solution of the present invention, in step (3), the mass ratio of the Pd / γ-Al2O3 catalyst to the Cs / γ-Al2O3 catalyst is 1:2 to 2:1, and the physical mixing and grinding are carried out for 5 to 15 minutes to ensure that the two catalysts are mixed evenly and to achieve full contact between the Pd active site and the Cs promoter.

[0018] As a preferred technical solution of the present invention, in step (3), after tableting and sieving, 60~120 mesh particles are selected to obtain the target Pd / γ-Al2O3+Cs / γ-Al2O3 composite catalyst.

[0019] The present invention also provides an alkali metal-added self-growing Pd-based countercurrent gas catalyst prepared by the above-described synthesis method.

[0020] This invention also provides the application of the alkali metal-added self-growing Pd-based reverse water gas catalyst in a reverse water gas shift (RWGS) reaction. The catalyst is tableted and sieved to select particles of 60-120 mesh. 50 mg of catalyst is weighed and packed into a quartz tube lined with quartz wool. Before the reaction, the catalyst is pretreated in a reducing atmosphere, specifically a 10% (v / v) H2 / Ar mixture (flow rate 60 mL / min) at 600°C for 30 min. After pretreatment, a reaction mixture (CO2 / H2 volume ratio 1:4, total flow rate 60 mL / min) is introduced; the reaction temperature range is set to 400-500°C, and the mass hourly space velocity (WHSV) used in the reaction is 72,000 mL / g / h. Application results show that the alkali metal-modified self-growing Pd-based catalyst of the present invention can simultaneously achieve high CO2 conversion and excellent CO selectivity in the reverse water-gas shift reaction, effectively suppressing side reactions such as methanation, and exhibiting good catalytic activity and target product selectivity.

[0021] The present invention has the following beneficial effects: 1. Achieving precise optimization of the performance of Pd-based RWGS catalysts. Conventional Pd / γ-Al2O3 catalysts suffer from defects such as low CO selectivity, poor low-temperature activity, and easy Pd agglomeration. This invention prepares Pd / γ-Al2O3 and Cs / γ-Al2O3 by impregnation separately, and then physically processes the two products in powder form to achieve the synergistic effect of Pd active sites and Cs promoter. Cs plays a dual role as an electronic promoter and structural promoter, effectively suppressing methanation side reactions and promoting CO desorption, while avoiding competitive adsorption between Pd and Cs. This solves the problems of uneven dispersion of active components and easy Pd agglomeration, significantly improving the catalytic activity, selectivity, and stability of the catalyst, and breaking through the technical bottleneck of RWGS reaction.

[0022] 2. The preparation process of the Pd / γ-Al2O3+Cs / γ-Al2O3 powder physical mixing composite catalyst is simple and safe. Pd / γ-Al2O3 and Cs / γ-Al2O3 are prepared separately by impregnation method. The operation method is simple, and the loading of Pd and Cs can be precisely controlled. The product composition is simple and controllable. The subsequent physical mixing step is convenient and does not require a complex reaction process. The entire preparation process does not involve high temperature and high pressure operation, which makes it safe and operable. The reaction conditions are easy to control and easy to realize large-scale industrial production.

[0023] 3. The preparation cost of the Pd / γ-Al2O3+Cs / γ-Al2O3 powder physical mixing composite catalyst is low. Compared with other high-performance RWGS catalysts on the market, this invention uses conventional γ-Al2O3 as the support, Pd as the active component, and Cs as the promoter. The raw materials are widely available and the cost is controllable. At the same time, it adopts a simple single impregnation method and physical mixing process, which does not require complex equipment and cumbersome steps, which can significantly reduce the cost of catalyst preparation. Moreover, the resulting composite catalyst has excellent performance, significant high added value characteristics, and obvious gains. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1 XRD patterns of different catalysts; Figure 2 Carbon dioxide conversion rates of the catalysts in the examples and comparative examples at different temperatures; Figure 3 The carbon monoxide selectivity of the catalysts in the examples and comparative examples was measured at different temperatures. Detailed Implementation

[0026] The present invention will be further described below with reference to specific embodiments. It should be understood that the following embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Those skilled in the art can make some non-essential improvements and adjustments based on the above-described invention.

[0027] Example 1

[0028] This invention provides a method for preparing alkali metal-added self-growing Pd-based countercurrent gas catalysts using separate impregnation-powder physical mixing, the specific steps of which are as follows: (1) A fixed amount of 25.03 mg of palladium nitrate hydrate (Pd(NO3)2•2H2O) was weighed and dissolved in 10 mL of deionized water. The solution was prepared by stirring at room temperature for 30 min. Then, 990 mg of γ-Al2O3 was added to the solution and stirred at room temperature for 30 min. The solution was then stirred at 60 °C until dry. The resulting solid was transferred to a muffle furnace and heated to 650 °C at a heating rate of 6 °C / min. The solid was calcined for 4 h. After cooling to room temperature, the sample was collected to obtain a Pd(1) / γ-Al2O3 catalyst with a Pd loading of 1 wt%.

[0029] (2) Weigh a fixed amount of 196.12 mg of cesium carbonate (Cs2CO3) and dissolve it in 10 mL of deionized water. Stir at room temperature for 30 min to prepare a solution. Then add 840 mg of γ-Al2O3 to the solution and stir at room temperature for 30 min. Then stir continuously at 60 °C until dry. Transfer the obtained solid to a muffle furnace and heat it to 650 °C at a heating rate of 6 °C / min. Calcine for 4 h. After cooling to room temperature, collect the sample to obtain Cs(16) / γ-Al2O3 with a Cs loading of 16 wt%.

[0030] (3) The Pd(1) / γ-Al2O3 catalyst powder and Cs(16) / γ-Al2O3 catalyst powder prepared in the above steps are placed in a mortar in a mass ratio of 1:1, physically mixed, and then ground thoroughly until uniform and fine. After pressing and sieving, 60~120 mesh particles are selected to obtain the composite catalyst of the target Pd(1) / γ-Al2O3+Cs(16) / γ-Al2O3 powder mixture.

[0031] Example 2

[0032] This embodiment provides a method for preparing alkali metal-added self-growing Pd-based countercurrent gas catalysts using separate impregnation-powder physical mixing. The specific steps are as follows: (1) A fixed amount of 12.52 mg of palladium nitrate hydrate (Pd(NO3)2•2H2O) was weighed and dissolved in 10 mL of deionized water. The solution was prepared by stirring at room temperature for 20 min. Then, 995 mg of γ-Al2O3 was added to the solution and stirred at room temperature for 20 min. The solution was then stirred at 50 °C until dry. The resulting solid was transferred to a muffle furnace and heated to 600 °C at a heating rate of 4 °C / min. The solid was calcined for 3 h. After cooling to room temperature, the sample was collected to obtain a Pd(0.5) / γ-Al2O3 catalyst with a Pd loading of 0.5 wt%.

[0033] (2) A fixed amount of 61.29 mg of cesium carbonate (Cs2CO3) was weighed and dissolved in 10 mL of deionized water. The solution was prepared by stirring at room temperature for 20 min. Then, 950 mg of γ-Al2O3 was added to the solution and stirred at room temperature for 20 min. The solution was then stirred at 50 °C until dry. The resulting solid was transferred to a muffle furnace and heated to 600 °C at a heating rate of 4 °C / min. The solid was calcined for 3 h. After cooling to room temperature, the sample was collected to obtain Cs(5) / γ-Al2O3 with a Cs loading of 5 wt%.

[0034] (3) The Pd(0.5) / γ-Al2O3 catalyst powder and Cs(5) / γ-Al2O3 catalyst powder prepared in the above steps are placed in a mortar in a mass ratio of 1:2, physically mixed, and then ground thoroughly until uniform and fine. After pressing and sieving, 60~120 mesh particles are selected to obtain the composite catalyst of the target Pd(0.5) / γ-Al2O3+Cs(5) / γ-Al2O3 powder mixture.

[0035] Example 3

[0036] This embodiment provides a method for preparing alkali metal-added self-growing Pd-based countercurrent gas catalysts using separate impregnation-powder physical mixing. The specific steps are as follows: (1) A fixed amount of 50.07 mg of palladium nitrate hydrate (Pd(NO3)2•2H2O) was weighed and dissolved in 10 mL of deionized water. The solution was prepared by stirring at room temperature for 40 min. Then, 980 mg of γ-Al2O3 was added to the solution and stirred at room temperature for 40 min. The solution was then stirred at 60 °C until dry. The resulting solid was transferred to a muffle furnace and heated to 650 °C at a heating rate of 5 °C / min. The solid was calcined for 4 h. After cooling to room temperature, the sample was collected to obtain a Pd(2) / γ-Al2O3 catalyst with a Pd loading of 2 wt%.

[0037] (2) Weigh a fixed amount of 122.57 mg of cesium carbonate (Cs2CO3) and dissolve it in 10 mL of deionized water. Stir at room temperature for 40 min to prepare a solution. Then add 900 mg of γ-Al2O3 to the solution and stir at room temperature for 40 min. Then stir continuously at 60 °C until dry. Transfer the obtained solid to a muffle furnace and heat it to 650 °C at a heating rate of 5 °C / min. Calcine for 4 h. After cooling to room temperature, collect the sample to obtain Cs(10) / γ-Al2O3 with a Cs loading of 10 wt%.

[0038] (3) The Pd(2) / γ-Al2O3 catalyst powder and Cs(10) / γ-Al2O3 catalyst powder prepared in the above steps are placed in a mortar at a mass ratio of 2:1, physically mixed, and then ground thoroughly until uniform and fine. After pressing and sieving, 60~120 mesh particles are selected to obtain the composite catalyst of the target Pd(2) / γ-Al2O3+Cs(10) / γ-Al2O3 powder mixture.

[0039] Example 4

[0040] This embodiment provides a method for preparing alkali metal-added self-growing Pd-based countercurrent gas catalysts using separate impregnation-powder physical mixing. The specific steps are as follows: (1) A fixed amount of 100.14 mg of palladium nitrate hydrate (Pd(NO3)2•2H2O) was weighed and dissolved in 10 mL of deionized water. The solution was prepared by stirring at room temperature for 50 min. Then, 960 mg of γ-Al2O3 was added to the solution and stirred at room temperature for 50 min. The solution was then stirred at 65 °C until dry. The resulting solid was transferred to a muffle furnace and heated to 700 °C at a heating rate of 7 °C / min. The solid was calcined for 4.5 h. After cooling to room temperature, the sample was collected to obtain a Pd(4) / γ-Al2O3 catalyst with a Pd loading of 4 wt%.

[0041] (2) A fixed amount of 220.63 mg of cesium carbonate (Cs2CO3) was weighed and dissolved in 10 mL of deionized water. The solution was prepared by stirring at room temperature for 50 min. Then, 820 mg of γ-Al2O3 was added to the solution and stirred at room temperature for 50 min. The solution was then stirred at 65 °C until dry. The resulting solid was transferred to a muffle furnace and heated to 700 °C at a heating rate of 7 °C / min. The solid was calcined for 4.5 h. After cooling to room temperature, the sample was collected to obtain Cs(18) / γ-Al2O3 with a Cs loading of 18 wt%.

[0042] (3) The Pd(4) / γ-Al2O3 catalyst powder and Cs(18) / γ-Al2O3 catalyst powder prepared in the above steps are placed in a mortar at a mass ratio of 1:2, physically mixed, and then ground thoroughly until uniform and fine. After pressing and sieving, 60~120 mesh particles are selected to obtain the composite catalyst of the target Pd(4) / γ-Al2O3+Cs(18) / γ-Al2O3 powder mixture.

[0043] Example 5

[0044] This embodiment provides a method for preparing alkali metal-added self-growing Pd-based countercurrent gas catalysts using separate impregnation-powder physical mixing. The specific steps are as follows: (1) Weigh a fixed amount of 125.17 mg of palladium nitrate hydrate (Pd(NO3)2•2H2O) and dissolve it in 10 mL of deionized water. Stir at room temperature for 60 min to prepare a solution. Then add 950 mg of γ-Al2O3 to the solution, stir at room temperature for 60 min, and then continue stirring at 70 °C until dry. Transfer the obtained solid to a muffle furnace and heat it to 700 °C at a heating rate of 8 °C / min. Calcine for 5 h. After cooling to room temperature, collect the sample to obtain a Pd(5) / γ-Al2O3 catalyst with a Pd loading of 5 wt%.

[0045] (2) Weigh a fixed amount of 245.15 mg of cesium carbonate (Cs2CO3) and dissolve it in 10 mL of deionized water. Stir at room temperature for 60 min to prepare a solution. Then add 800 mg of γ-Al2O3 to the solution and stir at room temperature for 60 min. Then stir continuously at 70 °C until dry. Transfer the obtained solid to a muffle furnace and heat it to 700 °C at a heating rate of 8 °C / min. Calcine for 5 h. After cooling to room temperature, collect the sample to obtain Cs(20) / γ-Al2O3 with a Cs loading of 20 wt%.

[0046] (3) The Pd(5) / γ-Al2O3 catalyst powder and Cs(20) / γ-Al2O3 catalyst powder prepared in the above steps are placed in a mortar at a mass ratio of 2:1, physically mixed, and then ground thoroughly until uniform and fine. After pressing and sieving, 60~120 mesh particles are selected to obtain the composite catalyst of the target Pd(5) / γ-Al2O3+Cs(20) / γ-Al2O3 powder mixture.

[0047] Comparative Example 1 This invention provides a co-impregnation method for preparing a self-growing Pd-based countercurrent gas catalyst with alkali metal addition. The specific steps are as follows: A fixed amount of 25.03 mg of palladium nitrate hydrate (Pd(NO3)2•2H2O) and 196.12 mg of cesium carbonate (Cs2CO3) were weighed and dissolved in 20 mL of deionized water. The solution was prepared by stirring at room temperature for 30 min. Then, 830 mg of γ-Al2O3 was added to the solution, and the mixture was stirred at room temperature for 30 min. The mixture was then stirred continuously at 60 °C until dry. The resulting solid was transferred to a muffle furnace and heated to 650 °C at a heating rate of 6 °C / min. The solid was calcined for 4 h. After cooling to room temperature, the sample was collected. After tableting and sieving, particles of 60-120 mesh were selected to obtain the Pd(1)-Cs(16) / γ-Al2O3 catalyst with a Pd loading of 1 wt% and a Cs loading of 16 wt%.

[0048] Comparative Example 2 This invention provides a method for preparing alkali metal-added, self-growing Pd-based countercurrent gas catalysts through separate impregnation and particle physical mixing. The specific steps are as follows: (1) A fixed amount of 25.03 mg of palladium nitrate hydrate (Pd(NO3)2•2H2O) was weighed and dissolved in 10 mL of deionized water. The solution was prepared by stirring at room temperature for 30 min. Then, 990 mg of γ-Al2O3 was added to the solution and stirred at room temperature for 30 min. The solution was then stirred at 60 °C until dry. The resulting solid was transferred to a muffle furnace and heated to 650 °C at a heating rate of 6 °C / min. The solid was calcined for 4 h. After cooling to room temperature, the sample was collected to obtain a Pd(1) / γ-Al2O3 catalyst with a Pd loading of 1 wt%.

[0049] (2) Weigh a fixed amount of 196.12 mg of cesium carbonate (Cs2CO3) and dissolve it in 10 mL of deionized water. Stir at room temperature for 30 min to prepare a solution. Then add 840 mg of γ-Al2O3 to the solution and stir at room temperature for 30 min. Then stir continuously at 60 °C until dry. Transfer the obtained solid to a muffle furnace and heat it to 650 °C at a heating rate of 6 °C / min. Calcine for 4 h. After cooling to room temperature, collect the sample to obtain Cs(16) / γ-Al2O3 with a Cs loading of 16 wt%.

[0050] (3) The Pd(1) / γ-Al2O3 catalyst powder and Cs(16) / γ-Al2O3 catalyst powder prepared in the above steps are placed in different mortars and ground thoroughly until they are uniform and fine. They are then pressed into tablets and sieved. Particles of 60~120 mesh are selected and physically mixed in a mass ratio of 1:1 to obtain a composite catalyst of Pd(1) / γ-Al2O3+Cs(16) / γ-Al2O3 particles.

[0051] Comparative Example 3 This invention provides a single impregnation method for preparing a self-growing Pd-based countercurrent gas catalyst, the specific steps of which are as follows: A fixed amount of 25.03 mg of palladium nitrate hydrate (Pd(NO3)2•2H2O) was weighed and dissolved in 10 mL of deionized water. The solution was prepared by stirring at room temperature for 30 min. Then, 990 mg of γ-Al2O3 was added to the solution, and the mixture was stirred at room temperature for 30 min. The mixture was then stirred continuously at 60 °C until dry. The resulting solid was transferred to a muffle furnace and heated to 650 °C at a heating rate of 6 °C / min. The mixture was calcined for 4 h. After cooling to room temperature, the sample was collected to obtain a Pd(1) / γ-Al2O3 catalyst with a Pd loading of 1 wt%.

[0052] Comparative Example 4 This invention provides a single impregnation method for preparing a self-growing Cs-based countercurrent gas catalyst, the specific steps of which are as follows: A fixed amount of 196.12 mg of cesium carbonate (Cs₂CO₃) was weighed and dissolved in 10 mL of deionized water. The solution was prepared by stirring at room temperature for 30 min. Then, 840 mg of γ-Al₂O₃ was added to the solution, and the mixture was stirred at room temperature for 30 min. The mixture was then stirred continuously at 60 °C until dry. The resulting solid was transferred to a muffle furnace and heated to 650 °C at a rate of 6 °C / min. The furnace was calcined for 4 h. After cooling to room temperature, the sample was collected to obtain Cs(16) / γ-Al₂O₃ with a Cs loading of 16 wt%.

[0053] The crystal structures of Pd and Cs in the catalyst were characterized using XRD. Figure 1 As shown, the distinct diffraction peaks at 2θ of 32.8°, 36.8°, 39.6°, 45.7°, 60.3°, and 67.1° are attributed to characteristic diffraction peaks of γ-Al2O3 (PDF#00-047-1308). The diffraction peaks at 2θ of 33.5°, 33.9°, and 60.1° are attributed to the (002), (101), and (103) crystal planes of PdO, respectively (PDF#04-007-4016). Notably, no diffraction signals containing Cs crystal phases were detected in the XRD patterns of both Pd(1)-Cs(16) / γ-Al2O3 and Cs(16) / γ-Al2O3 catalysts, indicating that Cs species may exist in an amorphous form on the catalyst surface.

[0054] Application examples 50 mg of catalyst sample (Example 1 and Comparative Examples 1-4) was weighed and placed in a quartz tube lined with silica wool. Before the reaction, the catalyst was pretreated in a reducing atmosphere by reduction at 600 °C for 30 min in a 10% (v / v) H₂ / Ar mixture (flow rate 60 mL / min). After pretreatment, a reaction mixture (CO₂ / H₂ volume ratio 1:4, total flow rate 60 mL / min) was introduced; the reaction temperature range was set to 400-500 °C, and the mass hourly space velocity (WHSV) used was 72,000 mL / g / h. The results are shown in Tables 1 and 2.

[0055] Table 1. Carbon dioxide conversion rates of the catalysts in the examples and comparative examples at different temperatures.

[0056] Table 2. Carbon monoxide selectivity of the catalysts in the examples and comparative examples at different temperatures.

[0057] In this invention, Pd(1) / γ-Al2O3 and Cs(16) / γ-Al2O3 were compounded by two methods: particle physical mixing (Comparative Example 2) and powder physical mixing (Example 1). The effects of different physical mixing methods and different impregnation preparation methods on the CO2 conversion rate and CO selectivity of the catalyst were systematically investigated.

[0058] The results showed that, within the reaction temperature range of 400–500℃, the Pd(1) / γ-Al2O3 catalyst prepared by the single impregnation method (Comparative Example 3) exhibited high catalytic activity, but its CO selectivity was poor. The Pd(1)-Cs(16) / γ-Al2O3 catalyst prepared by the co-impregnation method (Comparative Example 1) and the Cs(16) / γ-Al2O3 catalyst prepared by the single impregnation method (Comparative Example 4) had excellent CO selectivity, but their catalytic activity was significantly lower. The CO2 conversion rate of the particulate physical mixture sample (Comparative Example 2) was higher than that of the powder physical mixture sample (Example 1), but its CO selectivity was even worse.

[0059] The above results indicate that particle mixing reduces the interfacial contact between Pd and Cs active sites, inhibiting hydrogen overflow from Pd to Cs sites and thus promoting methanation side reactions, leading to decreased CO selectivity. In contrast, powder physical mixing effectively improves the contact state and hydrogen overflow efficiency between active components, achieving superior CO2 conversion performance while maintaining high CO selectivity. In summary, the powder physical mixing strategy adopted in this invention can balance high catalytic activity and high CO selectivity, with significantly better overall catalytic performance than particle mixing and co-impregnation methods, demonstrating clear technical advantages.

[0060] This invention proposes a separate impregnation method to prepare Pd / γ-Al2O3 and Cs / γ-Al2O3 catalysts, respectively. The two products are then physically mixed in powder form, pressed into tablets, and sieved to obtain a composite catalyst consisting of the target Pd / γ-Al2O3 + Cs / γ-Al2O3 powder mixture. Its core advantages are as follows: 1. It effectively avoids competitive adsorption between different active components (Pd, Cs), allowing each active component to be uniformly dispersed on the γ-Al2O3 support surface through adsorption, reducing active component aggregation, and facilitating the formation of more stable active sites, thus enhancing catalytic activity; 2. It can achieve the desired adsorption properties for each active component. The precise and independent control of the loading of active components allows for flexible adjustment of the loading ratio of Pd and Cs according to catalytic performance requirements, optimizing the synergistic effect between the two and thus specifically addressing the low CO selectivity of traditional Pd / γ-Al2O3 catalysts; 3. Individual impregnation can achieve the combination of active components and support in steps, better preserving the original high specific surface area and well-developed pore structure advantages of the γ-Al2O3 support, further improving the stability of the catalyst; 4. The process is simple and convenient to operate, requiring no complex equipment, and has a low preparation cost. Compared with the co-impregnation method, it is easier to achieve large-scale industrial production and can effectively ensure the repeatability and stability of catalyst performance.

[0061] The foregoing has shown and described the basic principles and main features of the present invention, as well as its advantages. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A method for synthesizing a self-growing Pd-based countercurrent gas catalyst with alkali metal addition, characterized in that... Includes the following steps: (1) Weigh palladium nitrate hydrate (Pd(NO3)2•2H2O) according to the proportion and dissolve it in deionized water. Stir at room temperature until completely dissolved to prepare a Pd precursor solution. Then add a preset amount of γ-Al2O3 support to the solution, stir thoroughly at room temperature, and continue stirring at the set temperature until the system is dry. Place the obtained solid in a high-temperature resistant container and transfer it to a calcination device. Heat to the calcination temperature according to the set program, keep it warm for a certain time, and collect the product after it cools naturally to room temperature. Pd / γ-Al2O3 catalyst powder with Pd loading within the preset range is obtained. (2) Weigh out cesium carbonate (Cs2CO3) according to the proportion and dissolve it in deionized water. Stir at room temperature until completely dissolved to prepare a Cs precursor solution. Then add a preset amount of γ-Al2O3 support to the solution, stir thoroughly at room temperature, and continue stirring at the set temperature until the system is dry. Place the obtained solid in a high-temperature resistant container and transfer it to a calcination device. Heat to the calcination temperature according to the set program, keep it at the temperature for a certain time, and collect the product after it cools naturally to room temperature. Cs / γ-Al2O3 catalyst powder with Cs loading within the preset range is obtained. (3) The Pd / γ-Al2O3 catalyst powder prepared in step (1) and the Cs / γ-Al2O3 catalyst powder prepared in step (2) are placed in a mixing and grinding equipment according to a preset mass ratio. They are first physically mixed and then fully ground until they are uniform and fine. After pressing and sieving, the target Pd / γ-Al2O3+Cs / γ-Al2O3 composite catalyst, namely the alkali metal-added self-growing Pd-based countercurrent gas catalyst, is obtained.

2. The method for synthesizing the alkali metal-added self-growing Pd-based countercurrent gas catalyst according to claim 1, characterized in that, In step (1), the amount of Pd(NO3)2•2H2O is determined based on the target Pd loading of the Pd / γ-Al2O3 catalyst, and the Pd loading range is 0.5~5wt%.

3. The method for synthesizing the alkali metal-added self-growing Pd-based countercurrent gas catalyst according to claim 1, characterized in that, In step (2), the amount of Cs2CO3 is determined based on the target Cs loading of the Cs / γ-Al2O3 catalyst, and the Cs loading range is 5~20wt%.

4. The method for synthesizing the alkali metal-added self-growing Pd-based countercurrent gas catalyst according to claim 1, characterized in that, In steps (1) and (2), the ambient temperature for room temperature stirring is 15~30℃, the stirring speed is 300~500r, and the stirring time is 20~60min, to ensure that the precursor is completely dissolved and the carrier and precursor are fully contacted and adsorbed.

5. The method for synthesizing a self-growing Pd-based countercurrent gas catalyst with alkali metal addition according to claim 1, characterized in that, In steps (1) and (2), the temperature for continuous stirring until dry is 50~70℃, and an industrial-grade magnetic stirring method is used with a stirring speed of 300~500r until the water in the system is completely evaporated and the resulting solid is in the form of loose powder.

6. The method for synthesizing the alkali metal-added self-growing Pd-based countercurrent gas catalyst according to claim 1, characterized in that, In steps (1) and (2), the calcination equipment is an industrial-grade muffle furnace or a continuous calcination furnace. The calcination process is carried out in an air atmosphere, with a heating rate of 4~8℃ / min, a calcination temperature of 600~700℃, and a calcination time of 3~5h. After calcination, the furnace is naturally cooled to room temperature.

7. The method for synthesizing the alkali metal-added self-growing Pd-based countercurrent gas catalyst according to claim 1, characterized in that, In step (1), the amount of γ-Al2O3 support is determined according to the target Pd loading to ensure that the Pd precursor is fully adsorbed by the support and to achieve uniform loading of Pd on the support surface; in step (2), the amount of γ-Al2O3 support is determined according to the target Cs loading to ensure that the Cs precursor is fully adsorbed by the support and to achieve uniform loading of Cs on the support surface.

8. The method for synthesizing the alkali metal-added self-growing Pd-based countercurrent gas catalyst according to claim 1, characterized in that, In step (3), the mass ratio of the Pd / γ-Al2O3 catalyst powder to the Cs / γ-Al2O3 catalyst powder is 1:2 to 2:1, the grinding time is 5 to 15 min, and 60 to 120 mesh particles are selected to obtain the target Pd / γ-Al2O3+Cs / γ-Al2O3 composite catalyst.

9. An alkali metal-added self-growing Pd-based countercurrent gas catalyst prepared by the synthesis method according to any one of claims 1-8.

10. The application of the alkali metal-added self-growing Pd-based reverse water gas catalyst according to claim 9 in the reverse water gas shift (RWGS) reaction.

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