CoCr2-xO4-delta type catalyst for catalytic combustion of low-concentration gas and preparation method of CoCr2-xO4-delta type catalyst

Abstract

The invention belongs to the technical field of gas catalytic materials, and discloses a CoCr < 2-x > O < 4-delta > type catalyst for low-concentration gas catalytic combustion and a preparation method thereof.The CoCr < 2-x > O < 4-delta > type catalyst is prepared by synthesizing an AB2O4 type spinel CoCr < 2 > O < 4 > precursor through a sol-gel method and then conducting surface etching on CoCr < 2 > O < 4 > through dilute nitric acid; the prepared CoCr < 2-x > O < 4-delta > type catalyst has a higher Co < 2 + > / Co < 3 + > ratio, a lower Cr < 3 + > / Cr < 6 + > ratio, more active oxygen vacancies and more excellent oxidation-reduction performance. According to the present invention, the conversion rate of 90% can be achieved when the methane concentration is 1% and the temperature is 396 DEG C, and the catalyst has good catalytic performance when the methane concentration is less than 1%, has excellent high-temperature stability, is the low-concentration methane catalytic combustion material with excellent performance, and has wide application prospects.

Description

Technical Field

[0001] The present invention relates to the technical field of gas catalytic materials, and in particular to a CoCr catalyst for catalytic combustion of low-concentration gas and a preparation method thereof. 2-x O 4-δ type catalyst and its preparation method. Background Art

[0002] Methane is the main component of gas and accounts for a large proportion in greenhouse gases. It is considered the second-largest greenhouse gas after carbon dioxide (CO 2 ), and its greenhouse effect is more than 20 times that of CO 2 . The C-H bond of methane is very stable and is usually difficult to activate. Direct combustion requires high-temperature conditions and is prone to generating secondary pollutants such as NOx. In contrast, catalytic combustion, as a more efficient treatment method, has the advantages of low operating temperature, high energy utilization rate, and recoverable reaction heat. Therefore, developing highly active, stable, and inexpensive catalysts is the key to realizing the application of gas catalytic combustion technology.

[0003] At present, methane combustion catalysts mainly include several types such as noble metal catalysts, hexaaluminate catalysts, perovskite-type catalysts, and spinel metal catalysts. Among them, spinel metal catalysts have lower costs and good availability, show relatively high catalytic activity under specific reaction conditions, and have good application prospects in methane catalytic oxidation.

[0004] For spinel oxides, the metal cations occupying the octahedral voids (the Cr site in the CoCr 2 O 4 molecular formula) are more exposed on the surface, and the Cr-site cations occupying the octahedral coordination in the spinel oxide may also interact with the Co-site cations occupying the tetrahedral coordination, which has an important impact on the properties of the spinel oxide, thereby affecting its behavior in the complete oxidation of methane. Currently, the CoCr 2 O 4 synthesized and improved by existing technologies still have low activity, poor stability in methane catalysis, and cannot achieve complete catalytic oxidation at low temperatures when the methane concentration is below 1%. Summary of the Invention

[0005] The present invention aims to provide a CoCr 2-x O 4-δ type catalyst for catalytic combustion of low-concentration gas and a preparation method thereof, so as to solve the problems of low activity, poor stability of methane combustion catalysts, and inability to achieve complete catalytic oxidation at low temperatures when the methane concentration is below 1% in the background art. In the present invention, after the spinel CoCr 2 O 4 catalyst is etched with dilute nitric acid, the catalyst exhibits a higher Co 2+ / Co 3+ Ratio, lower Cr 3+ / Cr 6+ , more active oxygen vacancies and better redox performance, and the catalyst also exhibits excellent stability.

[0006] To achieve the above object, the present invention provides the following technical solutions:

[0007] A CoCr 2-x O 4-δ type catalyst preparation method for low-concentration gas catalytic combustion, comprising the following steps:

[0008] S1. Synthesize AB 2 O 4 type spinel CoCr 2 O 4 precursor by sol-gel method;

[0009] S2. Calcinate the prepared CoCr 2 O 4 precursor, and etch the surface of the catalyst with an acid solution after calcination to obtain a CoCr 2-x O 4-δ type catalyst;

[0010] In step S1, the process of preparing CoCr 2 O 4 precursor by sol-gel method is as follows:

[0011] S11. Disperse ethylenediaminetetraacetic acid in water, add ammonia water to dissolve it, and then add citric acid to mix evenly;

[0012] S12. Mix and disperse Co(NO 3 ) 2 ·6H 2 O and Cr(NO 3 ) 3 ·9H 2 O in an aqueous solution to obtain a salt solution;

[0013] S13. Slowly dropwise add the salt solution in step S12 to the mixed solution in step S11, adjust the pH value with ammonia water, form a gel after heating and stirring, and finally dry to obtain CoCr 2 O 4 precursor.

[0014] Furthermore, in step S1, the stoichiometric ratio of citric acid, ethylenediaminetetraacetic acid and the two metal ions is 2:1:1, and the stoichiometric ratio of the two metal ions is Co:Cr = 1:2.

[0015] Further, in step S13, the pH value is adjusted to the range of 7±0.5 with ammonia water, and drying is carried out in a forced-air drying oven at a drying temperature of 200°C and a drying time of 5 h.

[0016] Further, in step S2, the calcination temperature is 450 - 650°C and the heat preservation time is 3 - 5 h.

[0017] Further, in step S2, the acid solution is dilute nitric acid, the mass fraction of dilute nitric acid is 3 - 8%, and the pickling time is 1 - 3 h.

[0018] A CoCr 2-x O 4-δ type catalyst for low-concentration gas catalytic combustion, in which the ratio of Co to Cr in the catalyst is greater than 0.5; the ratio of Co to O is greater than 0.25. This catalyst is used as a catalyst in the process of low-concentration methane catalytic combustion.

[0019] The beneficial effect of the technical solution is that the CoCr 2-x O 4-δ type catalyst prepared by the present invention is used as a catalyst in the process of low-concentration methane catalytic combustion. Among them, when the volume concentration of methane is 1%, the CoCr 2-x O 4-δ type catalyst reaches 50% conversion at 343°C and 90% conversion at 396°C, and shows excellent stability. It still has good catalytic performance when the volume concentration of methane is less than 1%.

[0020] In the present invention, after the spinel CoCr 2 O 4 catalyst is etched with dilute nitric acid, the catalyst shows a higher Co 2+ / Co 3+ ratio, a lower Cr 3+ / Cr 6+ ratio, more active oxygen vacancies and better redox performance, and the catalyst also shows very excellent stability.

[0021] The catalyst prepared by the present invention is a non-noble metal catalyst with good methane catalytic effect. It is an excellent low-concentration methane catalytic combustion material and has broad application prospects. Description of the Drawings

[0022] Figure 1 It is the X-ray powder diffraction pattern of the precursor CoCr 2 O 4 at different calcination temperatures in Example 1 of the present invention;

[0023] Figure 2 It is the precursor CoCr 2 O4 X-ray powder diffraction patterns at different calcination times;

[0024] Figure 3 For CoCr with the optimal calcination temperature and calcination time in Example 3 of the present invention 2 O 4 X-ray powder diffraction patterns etched with dilute nitric acid of different mass fractions;

[0025] Figure 4 For CoCr with the optimal calcination temperature and calcination time in Example 3 of the present invention 2 O 4 Partial enlarged view of the X-ray powder diffraction pattern etched with dilute nitric acid of different mass fractions;

[0026] Figure 5 For CoCr with the optimal calcination temperature and calcination time of the present invention 2 O 4 Field emission scanning electron microscope images;

[0027] Figure 6 For CoCr of the present invention 2-x O 4-δ Field emission scanning electron microscope images;

[0028] Figure 7 For CoCr with the optimal calcination temperature and calcination time of the present invention 2 O 4 and CoCr 2-x O 4-δ Co 2 of P X-ray photoelectron spectroscopy spectra;

[0029] Figure 8 For CoCr with the optimal calcination temperature and calcination time of the present invention 2 O 4 and CoCr 2-x O 4-δ Cr 2 of P X-ray photoelectron spectroscopy spectra;

[0030] Figure 9 For CoCr prepared in Example 1 of the present invention 2 O 4 1% methane catalytic conversion rate graph of the catalyst;

[0031] Figure 10 For CoCr prepared in Example 2 of the present invention 2 O 4 1% methane catalytic conversion rate graph of the catalyst;

[0032] Figure 11 For CoCr prepared in Example 3 of the present invention 2-x O4-δ 1% methane catalytic conversion rate diagram of type catalyst;

[0033] Figure 12 CoCr prepared in Example 3 of the present invention 2-x O 4-δ Diagram of methane catalytic conversion rate below 1% of type catalyst;

[0034] Figure 13 CoCr prepared in Example 3 of the present invention 2-x O 4-δ Test results of methane catalytic combustion stability of type catalyst. Specific implementation mode

[0035] The present invention will be further described in detail below with reference to the drawings and embodiments:

[0036] Example 1

[0037] Prepare CoCr 2 O 4 type catalyst by different calcination temperatures:

[0038] Disperse 5.8448 g of ethylenediaminetetraacetic acid (EDTA) in water, add NH 3 ·H 2 O to dissolve it, and then add 8.4056 g of citric acid (CA) to mix evenly: Disperse 1.9402 g of Co(NO 3 ) 2 ·6H 2 O, 5.3354 g of Cr(NO 3 ) 3 ·9H 2 O in an aqueous solution to obtain a salt solution, and then slowly add the salt solution drop by drop to the mixed solution of EDTA and CA, and adjust the pH value to about 7 with NH 3 ·H 2 O. After heating and stirring for a period of time, a gel is formed, which is dried in a forced-air drying oven at 200 °C for 5 h to obtain CoCr 2 O 4 precursor. Calcinate the CoCr 2 O 4 precursor in a muffle furnace at different temperatures for 4 h, and the calcination temperatures are 450, 550, and 650 °C respectively. Finally, CoCr 2 O 4 type catalyst is obtained.

[0039] Example 2

[0040] Prepare CoCr 2 O 4 type catalyst by different calcination times:

[0041] Disperse 5.8448 g of ethylenediaminetetraacetic acid (EDTA) in water, add NH 3 ·H 2 O to dissolve it, and then add 8.4056 g of citric acid (CA) to mix evenly: Disperse 1.9402 g of Co(NO 3 ) 2 ·6H 2 O, 5.3354 g of Cr(NO 3 ) 3 ·9H 2 O in an aqueous solution to obtain a salt solution, and then slowly add the salt solution dropwise to the mixed solution of EDTA and CA, and adjust the pH value to about 7 with NH 3 ·H 2 O. After heating and stirring for a period of time, a gel is formed, and it is dried in a forced-air drying oven at 200 °C for 5 h to obtain a CoCr 2 O 4 precursor. Calcinate the CoCr 2 O 4 precursor in a muffle furnace at 450 °C for different times, and the calcination times are set to 3, 4, and 5 h respectively. Finally, a CoCr 2 O 4 type catalyst is obtained.

[0042] Example 3

[0043] Prepare CoCr 2-x O 4-δ type catalyst by etching with dilute nitric acid solutions of different mass fractions:

[0044] Prepare dilute nitric acid solutions with mass fractions of 3%, 5%, and 8%. Add 1 g of the catalyst to the dilute nitric acid solutions of 3%, 5%, and 8% and stir on a constant-temperature magnetic stirrer for 1.5 h. Subsequently, wash the above solutions containing dilute nitric acid alternately with absolute ethanol and deionized water in a circulating water vacuum pump until neutral. Finally, dry the obtained samples in an oven at 85 °C for 3 h. Finally, a CoCr 2-x O 4-δ type catalyst is obtained.

[0045] Testing experiments

[0046] (1) Conduct XRD tests on Examples 1-3.

[0047] As Figure 1 and Figure 2 shown, they are the X-ray powder diffraction patterns at different calcination temperatures in Example 1 and Example 2. From Figure 1 and Figure 2 it can be seen that the prepared materials all exhibit spinel CoCr2 O 4 The structure (conforming to the standard card), without other miscellaneous peaks, indicates that the prepared sample is a pure cubic spinel structure and different calcination temperatures and times will not change the spinel structure.

[0048] Figure 3 It is the X-ray powder diffraction pattern after etching with dilute nitric acid of different mass fractions in Example 3. Figure 4 It is Figure 3 The partial enlarged view of Figure 3 , Figure 4 It can be seen that after etching with dilute nitric acid of different concentrations, the catalyst CoCr 2-x O 4-δ still maintains a good spinel structure and no new impurity phases are generated, indicating that etching with dilute nitric acid will not damage the crystal structure of the catalyst. At the same time, it can be seen from the partial enlarged view that the diffraction peaks of the acid-etched catalyst shift to a higher angle, indicating that after acid etching, the catalyst CoCr 2-x O 4-δ has lattice contraction, resulting in crystal defects.

[0049] (2) Perform SEM tests on the optimal CoCr 2 O 4 in Examples 1 and 2 and CoCr 2-x O 4-δ in Example 3.

[0050] As Figure 5 shown is the field emission scanning electron micrograph of CoCr 2 O 4 at the optimal calcination temperature and time; from Figure 5 it can be seen that the CoCr 2 O 4 catalyst without dilute nitric acid treatment shows a dense surface, indicating that the spinel CoCr 2 O 4 catalyst has a smaller specific surface area and larger catalyst particles at this time, and these are all factors leading to low catalyst activity. As Figure 6 shown is the field emission scanning electron micrograph of the spinel CoCr 2-x O 4-δ after etching with dilute nitric acid in Example 3. It can be seen from the figure that the catalyst particles are further reduced, the surface becomes slightly rough, and the surface becomes more porous. It may be because after acid etching, the crystal surface defects and lattice defects increase. At the same time, these smaller crystal particles are conducive to the migration of lattice oxygen, resulting in more oxygen defect sites and thus being more likely to participate in the oxidation reaction.

[0051] (3) For the optimal CoCr 2 O4 and CoCr in Example 3 2-x O 4-δ was subjected to XPS testing.

[0052] As Figure 7 and Figure 8 shown are the X-ray photoelectron spectroscopy diagrams of the Co 2 2 O 4 and CoCr 2-x O 4-δ type catalysts for the Co and Cr 2 P and Cr 2 P orbits. It can be seen from Figure 7 that the CoCr2-xO4-δ type catalyst after acid etching has a higher Co 2+ / Co 3+ ratio. The higher content of Co 2+ indicates that there are more oxygen vacancies in the catalyst, and further indicates that there are more surface defects on the catalyst surface. It can be seen from Figure 8 that after acid etching, the CoCr 2-x O 4-δ type catalyst has a lower Cr 3+ / Cr 6+ , probably due to the formation of more Cr 6+ . The relative increase in the content of Cr 6+ is more conducive to the catalytic reaction of methane at low temperatures.

[0053] (4) The catalysts prepared in Examples 1-3 were tested for the catalytic combustion conversion rate of 1% methane at different temperatures.

[0054] The test process was as follows: The catalytic activity of methane was carried out in a quartz reactor with a diameter of 8 mm. Specifically, the catalyst and quartz sand were granulated separately (40-60 mesh). 0.2 g of the catalyst and 0.4 g of quartz sand were mixed and placed in the middle of the reactor, and the samples were fixed with quartz wool at both ends. Subsequently, 1 Vol.% CH4 and 99 Vol.% air were introduced into the mass flowmeter and mixed evenly, and the total gas flow rate was controlled at 100 ml·min-1, corresponding to a reaction space velocity of 30000 ml·g-1h-1. After the mixed gas was introduced from the upper end of the reactor, before the catalytic reaction, the catalyst was activated with the reaction gas at 200 °C for 30 minutes. Then, the temperature was programmed from 200 °C to 650 °C at a heating rate of 10 °C / min, and the tail gas signal at different temperature conditions was detected by a gas detector at the lower end of the reactor. The activity data were collected, and one temperature point was measured every 50 °C.

[0055] The test results are as Figure 9 , Figure 10 and Figure 11 shown. It can be seen from Figure 11It can be seen that the CoCr prepared in Example 3 2-x O 4-δ type catalyst has the highest conversion rate, reaching 50% conversion at 343 °C and 90% conversion at 386 °C. Compared with the CoCr 2 O 4 type catalyst, the CoCr 2-x O 4-δ type catalyst has achieved a greater improvement in conversion rate at low temperatures.

[0056] (5) The catalyst CoCr prepared in Example 3 2-x O 4-δ was tested for the catalytic combustion conversion rate of methane below 1% at different temperatures.

[0057] The test results are as Figure 12 shown. From Figure 12 it can be seen that the CoCr prepared in Example 3 2-x O 4-δ type catalyst has little effect on the catalytic activity at lower methane concentrations and still maintains a high catalytic activity, indicating that the catalyst CoCr 2-x O 4-δ is also applicable to the catalytic oxidation of methane concentrations lower than 1%.

[0058] (6) The CoCr prepared in Example 3 2-x O 4-δ type catalyst was tested for stability.

[0059] The test process was as follows: When conducting the catalyst stability test, after starting to heat up, the reaction temperature was controlled at 500 °C, and the reaction space velocity was stabilized at 30000 ml·g -1 h -1 . A mixed gas was introduced, and the change in the catalytic activity of the sample within 100 h was tested.

[0060] The test results are as Figure 13 shown. From Figure 13 it can be seen that the conversion rate of the CoCr prepared in Example 3 2-x O 4-δ type catalyst at 500 °C did not decrease significantly with time. Its conversion rate after 100 h of use was 94%, and the conversion rate did not decrease significantly, indicating that the catalyst prepared by the present invention exhibits excellent stability under high-temperature conditions.

[0061] Through the above experiments, it is shown that a CoCr for catalytic combustion of low-concentration methane proposed by the present invention 2-x O 4-δ type catalyst and its preparation method obtained CoCr through the surface etching of the spinel structure 2-x O 4-δA catalyst, which has a higher Co 2+ / Co 3+ ratio, a lower Cr 3+ / Cr 6+ , more active oxygen vacancies and better redox performance. When the volume concentration of methane is less than 1% and the temperature is 396 °C, a conversion rate of 90% is achieved. Moreover, the catalyst also exhibits excellent high stability. It is an excellent catalytic combustion material for low-concentration methane, with broad application prospects.

[0062] The above are only embodiments of the present invention. Specific technical solutions or common knowledge such as characteristics known in the art are not described in detail herein. It should be noted that for those skilled in the art, without departing from the technical solution of the present invention, several modifications and improvements can be made, which should also be regarded as the protection scope of the present invention, and these will not affect the implementation effect of the present invention and the practicality of the patent. The protection scope required by this application should be based on the content of its claims, and the specific implementation manners described in the specification can be used to interpret the content of the claims.

Claims

1. A method for preparing a CoCr2-xO4-δ type catalyst for catalytic combustion of low-concentration gas, characterized in that: The following steps are involved: S1. Synthesize AB2O4 type spinel CoCr2O4 precursor by sol-gel method; S2, calcining the prepared CoCr2O4 precursor, and after calcination, etching the catalyst surface with a dilute nitric acid solution to obtain a CoCr2-xO4-δ type catalyst; The process of preparing CoCr2O4 precursor by sol-gel method in step S1 is as follows: S11, dispersing ethylenediaminetetraacetic acid in water, adding ammonia water to dissolve it, and then adding citric acid to mix well; S12, mixing and dispersing Co(NO3)2·6H2O and Cr(NO3)3·9H2O in an aqueous solution to obtain a salt solution; S13, slowly adding the salt solution of step S12 drop by drop into the mixed solution of step S11, adjusting the pH value with ammonia water, forming a gel after heating and stirring, and finally drying to obtain a CoCr2O4 precursor.

2. The method for preparing a CoCr2-xO4-δ type catalyst for catalytic combustion of low-concentration gas according to claim 1, characterized in that: In the step S1, the stoichiometric ratio of citric acid, ethylenediaminetetraacetic acid and the two metal ions is 2:1:1, and the stoichiometric ratio of the two metal ions is Co:Cr=1:

2.

3. The method for preparing a CoCr2-xO4-δ type catalyst for catalytic combustion of low-concentration gas according to claim 1, characterized in that: In step S13, the pH value is adjusted to the range of 7±0.5 with ammonia water, and the mixture is dried in a blast drying oven at a temperature of 200° C. for 5 hours.

4. The method for preparing a CoCr2-xO4-δ type catalyst for catalytic combustion of low-concentration gas according to claim 1, characterized in that: In the step S2, the calcination temperature is 450-650° C., and the holding time is 3-5 hours.

5. The method for preparing a CoCr2-xO4-δ type catalyst for catalytic combustion of low-concentration gas according to claim 1, characterized in that: In step S2, the mass fraction of dilute nitric acid is 3-8%, and the pickling time is 1-3 hours.

6. A CoCr2-xO4-δ type catalyst for catalytic combustion of low concentration gas prepared according to claim 1, characterized in that: The ratio of Co to Cr in the catalyst is greater than 0.5; the ratio of Co to O is greater than 0.

25.

7. A CoCr2-xO4-δ type catalyst for catalytic combustion of low concentration gas prepared according to claim 1, characterized in that: The catalyst is applied to the catalyst in the catalytic combustion process of low-concentration methane.

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