Cobalt-chromium composite oxide catalyst and preparation method thereof

Abstract

The invention relates to the technical field of toluene catalytic combustion, and discloses a cobalt-chromium composite oxide catalyst and a preparation method thereof.The catalyst is composed of oxides of cobalt and chromium, and the molar ratio of Co to Cr is (0.5-2.0): 1; the catalyst is prepared by the method comprising the following steps: preparing cobalt salt and chromium salt into a mixed solution according to the molar ratio, adding a complexing agent, heating and evaporating to form sol, gel and xerogel, and finally calcining at 500-600 DEG C, the Co-Cr composite oxide is successfully prepared through a sol-gel method, and a remarkable synergistic catalytic effect is generated between Co and Cr; the low-temperature catalytic activity of the catalyst is obviously superior to that of a single CoO or CrO catalyst and is also superior to that of composite catalysts with other molar ratios.

Description

Technical Field

[0001] This invention relates to the field of toluene catalytic combustion technology, specifically to a cobalt-chromium composite oxide catalyst and its preparation method. Background Technology

[0002] Volatile organic compounds (VOCs) are one of the major sources of air pollution, posing a serious threat to human health and the ecological environment. Toluene, as a typical VOC, is widely present in the exhaust gases emitted by industries such as chemical, printing, and coating. Its efficient purification and treatment has become a research hotspot in the field of environmental catalysis. Catalytic combustion technology is one of the most effective techniques for completely oxidizing VOCs into CO2 and H2O at relatively low temperatures. The core of this technology lies in developing highly active, stable, and low-cost catalysts. Currently, commonly used catalysts mainly include noble metal catalysts and transition metal oxide catalysts. While noble metal catalysts (such as Pt and Pd-based catalysts) possess excellent low-temperature catalytic activity, their scarcity, high price, and susceptibility to deactivation due to sintering or halogen / sulfide poisoning greatly limit their large-scale industrial application. In contrast, transition metal oxide catalysts exhibit greater application potential due to their lower cost, wider availability, and good thermal stability and resistance to poisoning. Among numerous transition metal oxides, cobalt oxide (Co3O4) and chromium oxide (Cr2O3) have been widely studied for catalytic combustion reactions due to their variable valence states and good redox properties. However, single cobalt oxide or chromium oxide suffers from problems such as insufficient activity, high ignition temperature, and easy sintering at high temperatures. Constructing composite oxides and utilizing the synergistic effect between metals is an effective strategy to improve their catalytic performance. There have been some studies on cobalt-chromium composites in the existing technology, but their catalytic performance, especially their activity for the catalytic combustion of toluene, still needs to be improved. Summary of the Invention

[0003] The purpose of this invention is to provide a cobalt-chromium composite oxide catalyst and its preparation method, which has the advantages of high catalytic activity, low ignition temperature and low cost.

[0004] The above-mentioned technical objective of the present invention is achieved through the following technical solution: a cobalt-chromium composite oxide catalyst, wherein the catalyst is composed of oxides of cobalt and chromium, and the molar ratio of Co to Cr is (0.5-2.0):1; the catalyst is prepared by a method including the following steps: preparing a mixed solution of cobalt salt and chromium salt according to the molar ratio, adding a complexing agent, evaporating by heating to form a sol, gel, and dry gel, and finally calcining at 500-600℃.

[0005] A further setting of the present invention is that the molar ratio of Co to Cr is (1.0-1.5):1.

[0006] A further setting of the present invention is that the molar ratio of Co to Cr is 1.25:1.

[0007] A further feature of the present invention is that the calcination temperature of the catalyst is 550°C.

[0008] A further feature of the present invention is that the catalyst has characteristic diffraction peaks at 2θ = 31°, 36°, 44°, 58°, and 64° in the X-ray diffraction pattern.

[0009] A method for preparing a cobalt-chromium composite oxide catalyst includes the following steps: S1: Weigh out cobalt salt and chromium salt according to the Co / Cr molar ratio of 0.5-2.0, and prepare a mixed metal salt solution; S2: Add citric acid monohydrate as a complexing agent to the mixed salt solution, wherein the total molar ratio of citric acid monohydrate to metal ions is (1.5-2.5):1; S3: The solution obtained in step S2 is heated and evaporated at a constant temperature of 50-70℃ to form a sol, and then evaporated until a dry gel is formed; S4: After grinding the dry gel into powder, the temperature is raised to 500-600℃ in air and calcined for 1-4 hours to obtain the catalyst.

[0010] A further provision of the present invention is that, in step S1, the cobalt salt is Co(NO3)3·6H2O and the chromium salt is Cr(NO3)3·9H2O.

[0011] A further setting of the present invention is that in step S3, the temperature of the constant temperature heating is 60°C, and the process of evaporation to form a sol takes 3-5 hours.

[0012] A further setting of the present invention is: in step S4, the programmed heating rate is 1-10℃ / min, and the calcination temperature is 550℃. In summary, the present invention has the following beneficial effects: 1. Co-Cr composite oxides were successfully prepared via the sol-gel method, exhibiting a significant synergistic catalytic effect between Co and Cr. Experimental results showed that the catalyst under optimal conditions (Co / Cr = 1.25, calcined at 550℃) significantly improved the ignition temperature (T0) of toluene during catalytic combustion. 50 The temperature can be as low as approximately 100°C, with a complete conversion temperature (T). 90 It can reach temperatures below 240℃, and its low-temperature catalytic activity is significantly better than that of single Co3O4 or Cr2O3 catalysts, as well as composite catalysts with other molar ratios.

[0013] 2. Using non-precious metals as active components, the raw material cost is much lower than that of precious metal catalysts such as platinum and palladium, which is more conducive to large-scale industrial applications; the sol-gel preparation process used is simple, has low equipment requirements, and is easy to repeat and scale up production.

[0014] 3. By precisely controlling the Co / Cr molar ratio, the microstructure and texture properties of the catalyst were optimized. Studies have shown that when the Co / Cr molar ratio is 1.25, the catalyst may form a phase structure that is more conducive to catalytic reaction while maintaining a suitable specific surface area, thereby exposing more active sites. Attached Figure Description

[0015] Figure 1 The images show the SEM images of Co / Cr ratios of 0.5 and 2 in this invention. Figure 2 The XRD patterns of the catalysts of this invention are shown below. Figure 3 The O 1s spectrum of the Co1Cr1Ox-500℃ (a) catalyst of this invention; Figure 4 The O 1s spectrum of the Co1.25Cr1Ox-500℃ (b) catalyst of this invention; Figure 5 The O 1s spectrum of the Co1Cr1Ox-550℃(c) catalyst of this invention; Figure 6 The Co 2p spectrum of the Co1Cr1Ox-500℃ (a) catalyst of this invention is shown below. Figure 7 The Co 2p spectrum of the Co1.25Cr1Ox-500℃ (b) catalyst of the present invention is shown. Figure 8 This is the Co 2p spectrum of the catalyst of this invention; Figure 9 The Cr 2p spectrum of the Co1Cr1Ox-500℃ (a) catalyst of this invention is shown. Figure 10 The Cr 2p spectrum of the Co1.25Cr1Ox-500℃ (b) catalyst of this invention; Figure 11 The Cr 2p spectrum of the Co1Cr1Ox-550℃(c) catalyst of this invention; Figure 12 The above is the H2-TPR spectrum of the CoCrOx catalyst of this invention; Figure 13 The images show the activity curves of the catalysts in this invention. Detailed Implementation

[0016] The present invention will be further described below with reference to the accompanying drawings in the embodiments of the present invention.

[0017] Example 1: Preparation of a catalyst with a Co / Cr molar ratio of 0.5 (Group A) Raw material preparation: Accurately weigh Co(NO3)3·6H2O and Cr(NO3)3·9H2O, control the molar ratio of Co to Cr to be 0.5:1, and dissolve them together in deionized water to prepare 100 mL of mixed salt solution with a total metal ion concentration of 1 mol / L.

[0018] Complexation: Add citric acid monohydrate to the above mixed solution, the amount of which is twice the total molar amount of metal ions in the solution, and stir magnetically for 30 minutes at room temperature to ensure thorough mixing and complexation.

[0019] Formation of sol and gel: The above mixed solution was transferred to a constant temperature water bath and slowly evaporated at 60°C; after about 4 hours, the solution transformed into a free-flowing, light green, transparent sol; after continuing to evaporate at 60°C for about 9 hours, the sol further transformed into a viscous, green, transparent gel.

[0020] Drying: The resulting gel was further evaporated at 60°C until it was completely solidified to form a dry gel; the dry gel was then placed in a drying oven and dried at 120°C for 6 hours.

[0021] Calcination: The dried block material is ground into a fine powder, and the powder is placed in a muffle furnace. Under static air atmosphere, the temperature is increased from room temperature to 500℃ at a heating rate of 5℃ / min, and then calcined at this temperature for 2 hours. After natural cooling to room temperature, a composite oxide catalyst with a Co / Cr molar ratio of 0.5 is obtained, denoted as A.

[0022] Example 2: Preparation of a catalyst with a Co / Cr molar ratio of 1.0 (Group B) The steps are the same as in Example 1, except that the weighing amounts of Co(NO3)3·6H2O and Cr(NO3)3·9H2O in the first step are changed, and the molar ratio of Co to Cr is controlled to be 1.0:1; the catalyst obtained by calcination at 500°C is denoted as B.

[0023] Example 3: Preparation of a catalyst (Group C) with a Co / Cr molar ratio of 1.25 The steps are the same as in Example 1, except that the weighing amounts of Co(NO3)3·6H2O and Cr(NO3)3·9H2O in the first step are changed, and the molar ratio of Co to Cr is controlled to be 1.25:1; the catalyst obtained by calcination at 500°C is denoted as C.

[0024] Example 4: Preparation of a catalyst (Group D) with a Co / Cr molar ratio of 1.5 The steps are the same as in Example 1, except that the weighing amounts of Co(NO3)3·6H2O and Cr(NO3)3·9H2O in the first step are changed, and the molar ratio of Co to Cr is controlled to be 1.5:1; the catalyst obtained by calcination at 500°C is denoted as D.

[0025] Example 5: Preparation of a catalyst (Group E) with a Co / Cr molar ratio of 2.0 The steps are the same as in Example 1, except that the weighing amounts of Co(NO3)3·6H2O and Cr(NO3)3·9H2O in the first step are changed, and the molar ratio of Co to Cr is controlled to be 2.0:1; the catalyst obtained by calcination at 500°C is denoted as E.

[0026] Example 6: Preparation of catalysts at different calcination temperatures (Groups B1 and B2) The same preparation steps as in Example 2 were used until a dried powder was obtained. The powder was then divided into two portions and placed in a muffle furnace, respectively. The temperature was increased to 550°C and 600°C at a heating rate of 5°C / min, and then calcined at these temperatures for 2 hours. The resulting catalysts were designated as B1 (550°C) and B2 (600°C), respectively.

[0027] Comparative Example 1: Commercial Co3O4 Catalyst In comparison, a commercially available pure-phase Co3O4 catalyst (analytical grade) was selected and subjected to subsequent catalytic performance tests without any treatment, in order to compare its performance with that of the CoCr composite oxide catalyst of the present invention.

[0028] Comparative Example 2: Commercial Cr2O3 Catalyst In comparison, a commercially available pure-phase Cr2O3 catalyst (analytical grade) was selected and subjected to subsequent catalytic performance tests without any treatment, for performance comparison with the CoCr composite oxide catalyst of the present invention.

[0029] Catalyst performance testing The catalysts prepared in the above embodiments and comparative examples were tableted, crushed, and sieved, and particles of 40-60 mesh were selected for activity evaluation.

[0030] Test apparatus: Fixed-bed reactor (8 mm inner diameter).

[0031] Catalyst loading: 100 mg.

[0032] Reaction gases: Toluene concentration 1000 ppm, equilibrium gas is N2.

[0033] Airspeed (GHSV): 40000 h⁻¹.

[0034] Detection method: The concentration of toluene in the reactor outlet gas was analyzed online using a gas chromatograph equipped with a flame ionization detector (FID).

[0035] Evaluation index: Toluene conversion rate. The ignition temperature (T) of the catalyst was measured using a programmed temperature increase reaction. 50 The temperature at which the conversion rate reaches 50% and the temperature at which the conversion is complete (T) 90 The activity of the catalyst is evaluated by measuring the temperature at which the conversion rate reaches 90%.

[0036] Test Results and Analysis Table 1 lists the performance test results of some catalysts. Table 1 Table 2 shows the specific surface area test results. From groups A to E, it can be seen that as the molar percentage of Co increases, the specific surface area first decreases and then increases, but overall shows a downward trend. Comparing groups B, B1, and B2, it can be seen that group B, with a lower calcination temperature of 500 ℃, has a larger specific surface area. The characteristics of this type of catalyst mean that it can exhibit more active sites, thus providing more reaction interfaces for the catalytic process. However, excessively high calcination temperatures may lead to catalyst sintering, resulting in a smaller specific surface area.

[0037] Table 2 Table 3 Surface elemental analysis of the two catalyst groups Table 3 Depend on Figure 2 It can be seen that the characteristic diffraction peaks of this series of catalysts are maintained at 2θ=31°, 36°, 44°, 58°, and 64°. With increasing molar proportion of Co, the diffraction peaks show almost no shift, but the intensities differ, with group B (Co / Cr=1, calcination temperature 500 ℃) exhibiting the highest peak intensity. Comparing groups B, B1, and B2, it is evident that the higher the calcination temperature, the greater the diffraction peak intensity. Further investigation of the diffraction patterns reveals some additional non-primary peaks. These impurity peaks are likely due to the partial formation of secondary crystalline phases such as CoO and CrO during calcination. The formation of these secondary crystalline phases weakens the intensity of the main diffraction peaks to some extent. Comparative analysis shows that this weakening effect decreases accordingly when the molar proportion of Co decreases, indicating a negative correlation between the molar proportion of Co and the degree of weakening of the diffraction peak intensity.

[0038] Oxygen species on the catalyst surface generally exist in the form of chemisorbed oxygen (Oads) and lattice oxygen (Olatt). Oads contain active oxygen species such as O22-, O-, and -OH groups, which have higher mobility than lattice oxygen and exhibit high oxidation activity in reactions; for example... Figure 3 As shown, in the Co1Cr1Ox-500 ℃ group, Olatt and Oads eluted at binding energies of 531-534 eV and 530-532 eV, respectively; as Figure 4 As shown, in the Co1.25Cr1Ox-500 ℃ group, Olatt and Oads eluted at binding energies of 630-632 eV and 629-631 eV, respectively; Figure 5 As shown, the Olatt and Oads of the Co1.25Cr1Ox-550 ℃ group eluted at binding energies of 630-632 eV and 631-632 eV, respectively.

[0039] like Figure 6 , 7 As shown in Figure 8, the Co²⁺ and Co³⁺ ions of the three catalysts all eluted at binding energies of 795-798 V and 780-783 eV, respectively, indicating that the mole fraction of Co²⁺ / (Co²⁺++Co³⁺) remained almost constant. Figure 9 , 10 As shown in Figure 11, the three groups of Cr2+ and Cr3+ all exhibited peaks at binding energies of 586-588 eV and 576-578 eV, respectively, and the molar fraction of Cr2+ / (Cr2++Cr3+) remained almost constant. Therefore, it can be considered that the changes in the valence states of Co and Cr under different cobalt-chromium molar ratios at the same calcination temperature are not the main factors affecting the catalyst activity, and the changes in the valence states of Co and Cr under different calcination temperatures at the same cobalt-chromium molar ratio are also not the main factors affecting the catalyst activity.

[0040] Figure 12 The figures show H2-TPR spectra for different Co / Cr molar ratios. As can be seen, when the Co / Cr molar ratios are 0.5, 1, and 1.25, the low-temperature reduction peaks are concentrated around 200 °C, and the high-temperature reduction peaks are concentrated around 600 °C. When the Co / Cr molar ratios are 1.5 and 2, the low peaks are around 350 °C, and the high peaks are around 500 °C. The relative changes in the low peaks may be due to the valence state change of Co, while the changes in the high peaks may be due to the valence state change of Cr. This change in the valence state of the Co group ensures that the catalyst is electroneutrally neutral, which is beneficial to the oxidation of toluene. The figures also show that as the Co / Cr molar ratio increases, the low peaks shift later and the high peaks shift earlier, resulting in varying degrees of overlap of the reduction peaks between 400 °C and 500 °C.

[0041] Figure 13The figure shows the catalytic activity of the CoCrOx catalyst in the catalytic combustion of toluene. It clearly depicts the catalytic activity of this series of catalysts, exhibiting an overall "S"-shaped curve pattern. Notably, all catalysts achieve conversion rates above 90% at temperatures below 250℃, strongly demonstrating their excellent catalytic activity for toluene. Further analysis of catalysts in groups A to E reveals a complex trend of initial increase, decrease, and then increase again with increasing Co molar ratio. Group C (Co / Cr = 1.25, calcination temperature 500℃) exhibits the highest overall activity. When x = 0.4, group C (Co / Cr = 1.25, calcination temperature 500℃) shows the best catalytic effect, with an ignition temperature of 100℃ and a complete conversion temperature of 240℃. Comparing the activities of groups B, B1, and B2, when Co / Cr = 1, the catalyst in group B1 with a calcination temperature of 550℃ shows the highest catalytic activity. In summary, it can be predicted that the catalyst exhibits the highest catalytic activity for toluene when Co / Cr = 1.25 and the calcination temperature is 550 °C.

[0042] The above description is only a preferred embodiment of the present invention. Therefore, all equivalent changes or modifications made to the structure, features and principles described in the claims of this patent application are included in the scope of this patent application.

Claims

1. A cobalt-chromium composite oxide catalyst, characterized in that, The catalyst is composed of oxides of cobalt and chromium, with a molar ratio of Co to Cr of (0.5-2.0):

1. The catalyst is prepared by a method including the following steps: preparing a mixed solution of cobalt salt and chromium salt according to the molar ratio, adding a complexing agent, evaporating by heating to form a sol, gel, and dry gel, and finally calcining at 500-600℃.

2. The cobalt-chromium composite oxide catalyst according to claim 1, characterized in that, The molar ratio of Co to Cr is (1.0-1.5):

1.

3. The cobalt-chromium composite oxide catalyst according to claim 2, characterized in that, The molar ratio of Co to Cr is 1.25:

1.

4. The cobalt-chromium composite oxide catalyst according to claim 1, characterized in that, The catalyst is calcined at 550°C.

5. The cobalt-chromium composite oxide catalyst according to claim 1, characterized in that, The catalyst exhibits characteristic diffraction peaks at 2θ = 31°, 36°, 44°, 58°, and 64° in its X-ray diffraction pattern.

6. A method for preparing a cobalt-chromium composite oxide catalyst, characterized in that, Includes the following steps: S1: Weigh out cobalt salt and chromium salt according to the Co / Cr molar ratio of 0.5-2.0, and prepare a mixed metal salt solution; S2: Add citric acid monohydrate as a complexing agent to the mixed salt solution, wherein the total molar ratio of citric acid monohydrate to metal ions is (1.5-2.5):1; S3: The solution obtained in step S2 is heated and evaporated at a constant temperature of 50-70℃ to form a sol, and then evaporated until a dry gel is formed; S4: After grinding the dry gel into powder, the temperature is raised to 500-600℃ in air and calcined for 1-4 hours to obtain the catalyst.

7. The method for preparing the cobalt-chromium composite oxide catalyst according to claim 6, characterized in that, In step S1, the cobalt salt is Co(NO3)3·6H2O, and the chromium salt is Cr(NO3)3·9H2O.

8. The method for preparing the cobalt-chromium composite oxide catalyst according to claim 6, characterized in that, In step S3, the constant temperature heating temperature is 60°C, and the evaporation process to form a sol takes 3-5 hours.

9. The method for preparing the cobalt-chromium composite oxide catalyst according to claim 6, characterized in that, In step S4, the rate of temperature increase is 1-10℃ / min, and the calcination temperature is 550℃.

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