CN113145122B - Composite metal oxide catalyst, preparation method thereof and application thereof in catalyzing direct decomposition of nitrogen oxide - Google Patents
Composite metal oxide catalyst, preparation method thereof and application thereof in catalyzing direct decomposition of nitrogen oxide Download PDFInfo
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Abstract
The invention discloses a composite metal oxide catalyst, a preparation method thereof and application thereof in catalyzing direct decomposition of nitrogen oxide, wherein the catalyst is MgO-loaded K 2 NiF 4 The preparation method of the metal oxide is a one-step method, and the citric acid nitrate method is used for preparing K 2 NiF 4 And adding magnesium nitrate in the process of forming the metal oxide precursor solution, presintering and roasting to obtain the composite metal oxide catalyst. The invention has the characteristics of high yield, low raw material price and no reducing agent consumption, and solves the problem of the prior K for catalyzing the direct decomposition of oxynitride 2 NiF 4 The low reaction activity of the metal oxide catalyst simplifies the preparation process of the supported catalyst.
Description
Technical Field
The invention belongs to the technical field of waste gas treatment, and particularly relates to a method for catalyzing Nitrogen Oxide (NO) x ) The preparation and application of the composite oxide catalyst which is directly decomposed into nitrogen and oxygen can be used for eliminating and reducing emission of nitrogen oxides in waste gas of power plants and automobiles.
Background
Nitrogen Oxides (NO) x ) Mainly NO and NO 2 More than 95% of the components are NO, and the main sources are tail gas of thermal power plants, boilers and automobiles. Nitrogen Oxides (NO) x ) The acid rain is an important factor for forming acid rain, is one of important precursors for forming ozone and photochemical pollution, and is also a main reason for forming ultrafine particles (PM 2.5). The emission of nitrogen oxides rapidly increases along with the rapid increase of energy consumption and motor vehicle reserves in China, and particularly, after 2012, large-area haze pollution frequently appears in the whole country, so that the problem of nitrogen oxide pollution is more and more emphasized. NO, the most widely studied NO in recent years x The purification technology mainly comprises the following steps: direct NO decomposition technique, NO x (Storage)Reduction technique (NSR), NO x Adsorption technology, selective catalytic reduction technology (NH) using ammonia as reducing agent 3 -SCR), etc. The NO direct decomposition process does not need to add a reducing agent additionally, and the product is nitrogen (N) which is pollution-free to the atmosphere 2 ) And oxygen (O) 2 ) Therefore, this technique is considered to be the most desirable NO removal technique. Direct decomposition of NO to N 2 And O 2 It is completely feasible in the thermodynamic range, but the activation energy of the reaction is up to 364kJ/mol, so the research on the decomposition of NO is essentially a question of researching the kinetics, namely, finding a suitable catalyst to reduce the activation energy of the reaction is the key of the NO direct decomposition technology.
At present, the catalyst systems used for the direct decomposition of NO are mainly: noble metals, molecular sieves, oxides, and the like. Noble metal catalysts were the first to be investigated, mainly comprising A1 2 O 3 、ZrO 2 And Pt and Pd on equal supports. Noble metal catalysts are expensive, have poor low temperature activity, have severe oxygen inhibition phenomena, are prone to sulfur poisoning, and the like, making them difficult to apply. The Cu-ZSM-5 type molecular sieve catalyst is used for catalyzing the direct decomposition of NO, has wide research and low reaction temperature, has the conversion rate of about 60 percent at 500 ℃, has high research value when being used as the NO low-temperature decomposition catalyst, but also has a plurality of defects in practical application, the airspeed has great influence on the catalytic activity, the phenomenon of oxygen inhibition exists, and the air velocity has great influence on water vapor and SO 2 Is very sensitive and is not suitable for industrial application. The metal oxide also has a certain promotion effect on the decomposition of NO, particularly the transition metal oxide has more remarkable promotion effect, the reaction mechanism of the metal oxide is consistent with that of a noble metal catalyst, and O 2 The desorption capacity at the active site is also a key factor in determining the activity of the metal oxide. The perovskite and perovskite-like metal oxide is a catalyst which is widely researched at present and solves NO pollution, and shows higher NO catalytic activity, better thermal stability and selectivity in a temperature range of 400-850 ℃. The documents Zhu J, zhuao Z, xiao D, et al.application of cyclic voltammetry in heterologous catalysis, NO decomposition and reduction [ J]Among electrochemical Communications,2005,7 (1): 58-61, zhu Junjiang et al, studied K 2 NiF 4 Form LaSrCoO 4 The direct catalytic decomposition reaction activity of the catalyst powder is that the NO conversion rate is only 20% at 850 ℃. Thus, laSrCoO 4 The effect of the catalyst to directly catalyze the decomposition of NO is not ideal. In addition, laSrCoO subjected to high-temperature roasting 4 The specific surface area of the powder is only 2.3m 2 The adsorption of the catalyst to the gas is greatly limited.
The supported catalyst is a hot spot of current research, and the carrier not only can increase the specific surface area of the composite catalyst, but also can generate certain influence on the reaction activity through the interaction between the carrier and a supported object. ZrO is the most studied support at present 2 、Al 2 O 3 Etc., mgO is less studied as a carrier. MgO has higher thermal stability, and the specific surface area of the MgO is still kept at 10m after being calcined at the high temperature of 1400 DEG C 2 Left and right, and Al treated the same 2 O 3 And ZrO 2 Has a surface area of only 1m 2 The range of the concentration is about/g. The preparation of supported catalysts is complicated by the literature Ladavos A K, pomonis P J, structure and catalytic activity of metals La-Ni-O supported on aluminum and zirconia [ J].Applied Catalysis B:Environmental,1993,2(1):27-47，La 2 NiO 4 /ZrO 2 The catalyst is prepared by an impregnation method, and active components need to be impregnated on a roasted oxide carrier for roasting for a plurality of times in equal volume, so that the process is complex, and the energy consumption is high.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provides a composite metal oxide catalyst, a preparation method thereof and application thereof in catalyzing direct decomposition of nitrogen oxide. The technical scheme of the invention uses a citrate nitrate method to prepare the supported catalyst in one step, synthesizes the catalyst for efficiently eliminating nitrogen oxides within the temperature range of 500-850 ℃, and obviously improves K 2 NiF 4 The NO of the metal oxide catalyst is directly decomposed and catalytically active, so that the use amount of expensive metal nitrate is saved, and the preparation process of the supported catalyst is simplified.
The technical purpose of the invention is realized by the following technical scheme.
A composite metal oxide catalyst, wholly magnesium oxide-supported K 2 NiF 4 Structured complex metal oxides (i.e. containing magnesium oxide and K) 2 NiF 4 Structural composite metal oxide two phase), K 2 NiF 4 The structural composite metal oxide has a loading of 5-95 wt%, preferably 10-30 wt%, and a LaAB composition x B′ 1-x O 4 MgO, in which La and A form K, B and B 'form Ni, A is Ba, sr or Ca, B is Co, fe, ni, cu or Mn, B' is Co, fe, ni, cu or Mn, x is greater than zero and less than one, preferably x = 0.2-0.8.
Further, A is Sr, B is Co, B' is Fe, and x is 0.2.
A composite metal oxide catalyst, the whole body is magnesium oxide loaded K 2 NiF 4 Composite oxide of structural composite metal oxide and Ni (i.e., containing magnesium oxide, K) 2 NiF 4 Three phases of composite oxides of structural composite metal oxides and Ni), K 2 NiF 4 The co-supporting amount of the structural composite metal oxide and the composite oxide of Ni is 5 to 95wt%, preferably 10 to 30wt%, K 2 NiF 4 The composition of the structural composite metal oxide is LaAB x B′ 1-x O 4 -MgO, wherein La and a form K, B and B 'form Ni, a is Ba, sr or Ca, B is Co, fe, ni, cu or Mn, B' is Co, fe, ni, cu or Mn, x is greater than zero and less than one, preferably x = 0.2-0.8; the composite oxide of Ni is composed of a composite oxide of B and B', and when prepared, is according to KNiF 3 Feeding to obtain K 2 NiF 4 A structural composite metal oxide and a composite oxide of Ni.
Further, A is Sr, B is Co, B' is Fe, and x is 0.2.
The preparation method of the catalyst comprises the following steps:
step 1, uniformly dispersing EDTA in deionized water, heating in a water bath, and adding ammonia water to completely dissolve the EDTA;
in step 1, the temperature of the water bath is 60 to 80 ℃, preferably 65 to 75 ℃.
in step 2, magnetic stirring is adopted for uniform dispersion, and the stirring speed is 300-500 revolutions per minute.
In step 2, the ammonia concentration is 20 to 25wt%, and the pH is adjusted to 7 to 8.
In the step 2, after adding the magnesium nitrate and the citric acid, uniformly dispersing by using magnetic stirring, wherein the stirring time is 1-2 hours, and the stirring speed is 300-500 revolutions per minute.
in step 3, the temperature of the water bath is 70 to 90 ℃, preferably 80 to 90 ℃.
in step 4, presintering is carried out by using an air atmosphere, the presintering temperature is 250-350 ℃, and the presintering time is 3-4 hours.
And 5, roasting the powder obtained in the step 4 to obtain the required catalyst, wherein the roasting atmosphere is air, the roasting temperature is 700-900 ℃, and the roasting time is 4-8 hours.
In the step 5, the roasting temperature is 750-850 ℃ and the roasting time is 6-8 hours.
The load of the supported catalyst in the present invention is calculated as K 2 NiF 4 (i.e. A) 2 BO 4 ) The mass of the structural material/the total mass of the composite catalyst.
The catalyst of the present invention is used in the direct NO decomposing catalysis, such as 40-60 mesh catalyst, reaction gas mixture of 2000ppm NO and N 2 For balance gas, total gas flow was 20mL/min, W/F =1.5g · s · mL -1 The temperature was 850 ℃.
Compared with the prior art, the invention has the advantages thatOvercomes the defects of the prior art, and the prepared catalyst solves the problem of pure K 2 NiF 4 (i.e. A) 2 BO 4 ) The catalyst of the structural metal oxide has low activity in a high-temperature area and can not effectively eliminate nitrogen oxides. When the catalyst is prepared, the loading capacity of the active component is reduced, the usage amount of the metal nitrate is reduced, and the production cost is reduced. The one-step method for preparing the supported catalyst replaces the original impregnation method to prepare the supported catalyst, thereby simplifying the process flow, saving the production time and cost and being suitable for large-scale industrial production.
Drawings
FIG. 1 is an SEM photograph of a catalyst prepared according to the present invention.
FIG. 2 is an EDS picture of the catalyst prepared by the present invention.
Figure 3 is an XRD spectrum diagram of the prepared catalyst of the present invention.
FIG. 4 is a graph of oxygen-TPD measurements for catalysts made according to the present invention.
FIG. 5 is a graph of the NO-TPD test for the catalyst prepared according to the present invention.
FIG. 6 is a graph of the N2 yield of the catalytic NO direct decomposition reaction of the catalyst prepared according to the present invention.
FIG. 7 shows N at 850 ℃ under different oxygen partial pressures for catalysts prepared according to the present invention 2 Yield profile.
Detailed Description
The technical scheme of the invention is further explained by combining specific examples. The stirring is carried out by magnetic stirring, the stirring speed is 500 revolutions per minute, and the ammonia water concentration is 25wt% of ammonia water solution.
Example 1
(1) According to 10wt.% La 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3 /MgO (i.e., la) 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3 mass/La 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3 And magnesium oxide) of the nitrate, namely 0.3295g of lanthanum nitrate hexahydrate, 0.1611g of strontium nitrate, 0.3544g of cobalt nitrate hexahydrate and ferric nitrate nonahydrate0.1231g and 19.26g of magnesium nitrate, and dissolving the mixture in 100ml of deionized water;
(2) Dissolving 26.97g of EDTA in 100ml of deionized water, heating the solution in a water bath at 65 ℃, and adding ammonia water to completely dissolve the EDTA;
(3) Mixing the nitrate solution in the step (1) with the solution in the step (2), stirring for half an hour, adding 20.23g of citric acid monohydrate into the solution, stirring for 1 hour, and adjusting the pH value to 8 by using ammonia water;
(4) Stirring the mixed solution in 80 deg.C water bath, evaporating to remove water to obtain gel, placing the gel on a heating plate, gradually heating, and maintaining at 350 deg.C for 4 hr to obtain black powder without spark;
(5) After grinding and sieving, the catalyst A is calcined in a muffle furnace (air atmosphere) at 850 ℃ for 6h to form the required catalyst A.
Example 2
(1) According to 10wt.% LaSrCo 0.8 Fe 0.2 O 4 Weighing nitrates of 0.4139g of lanthanum nitrate hexahydrate, 0.2023g of strontium nitrate, 0.2226g of cobalt nitrate hexahydrate, 0.0772g of ferric nitrate nonahydrate and 19.26g of magnesium nitrate according to MgO components, and dissolving the nitrates in 100ml of deionized water;
(2) Dissolving 27.31g of EDTA in 100ml of deionized water, heating the solution in a water bath at 65 ℃, and adding ammonia water to completely dissolve the EDTA;
(3) Mixing the nitrate solution in the step (1) with the solution in the step (2), stirring for half an hour, adding 20.48g of citric acid monohydrate into the solution, stirring for 1 hour, and adjusting the pH value to 8 by using ammonia water;
(4) Stirring the mixed solution in 80 deg.C water bath, evaporating to remove water to obtain gel, placing the gel on a heating plate, gradually heating, and maintaining at 350 deg.C for 4 hr to obtain black powder without spark;
(5) After grinding and sieving, the catalyst B is calcined in a muffle furnace (air atmosphere) at 850 ℃ for 6h to form the required catalyst B.
Comparative example 1
(1) According to LaSrCo 0.8 Fe 0.2 O 4 The stoichiometric ratio of the elements in the formula (I) is 0.49671g lanthanum nitrate hexahydrate, 2.4278g strontium nitrate, 2.6708g cobalt nitrate hexahydrate and 0 ferric nitrate nonahydrate.9269g was dissolved in 100ml deionized water;
(2) Dissolving 12.09g of EDTA in 100ml of deionized water, heating in a water bath at 65 ℃, and adding ammonia water to completely dissolve the EDTA;
(3) Mixing the nitrate solution in the step (1) with the solution in the step (2), stirring for half an hour, adding 9.06g of citric acid monohydrate into the solution, stirring for 1 hour, and adjusting the pH value to 8 by using ammonia water;
(4) Stirring the mixed solution in 80 deg.C water bath, evaporating to remove water to obtain gel, placing the gel on a heating plate, gradually heating, and maintaining at 350 deg.C for 4 hr to obtain black powder without spark;
(5) After grinding and sieving, the catalyst C is calcined in a muffle furnace (air atmosphere) at 850 ℃ for 6h to form the required catalyst C.
Comparative example 2
(1) According to LaSrCoO 4 The stoichiometric ratio of the elements in the method is that 0.9295g of lanthanum nitrate hexahydrate, 0.4543g of strontium nitrate hexahydrate and 0.6247g of cobalt nitrate hexahydrate are dissolved in 100ml of deionized water;
(2) Dissolving 2.25g of EDTA in 100ml of deionized water, heating the solution in water bath at 65 ℃, and adding a proper amount of ammonia water to completely dissolve the EDTA;
(3) Mixing the nitrate solution in the step (1) with the solution in the step (2), stirring for half an hour, adding 1.69g of citric acid monohydrate into the solution, stirring for 1 hour, and adjusting the pH value to 8 by using ammonia water;
(4) Stirring the mixed solution in 80 ℃ water bath, evaporating water to dryness to form gel, placing the gel on a heating plate, gradually heating, and keeping at 350 ℃ for about 4 hours until black powder is formed without sparks;
(5) After grinding and sieving, the catalyst was calcined in a muffle furnace (air atmosphere) at 850 ℃ for 6h to form the desired catalyst D.
Comparative example 3
(1) 32.05g of magnesium nitrate hexahydrate is dissolved in 100ml of deionized water;
(2) Dissolving 43.83g of EDTA in 100ml of deionized water, heating the solution in water bath at 65 ℃, and adding a proper amount of ammonia water to completely dissolve the EDTA;
(3) Mixing the nitrate solution in the step (1) with the solution in the step (2), stirring for half an hour, adding 32.87g of citric acid monohydrate into the solution, stirring for 1 hour, and adjusting the pH value to 8 by using ammonia water;
(4) Stirring the mixed solution in 80 ℃ water bath, evaporating water to dryness to form gel, placing the gel on a heating plate, gradually heating, and keeping at 350 ℃ for about 4 hours until black powder is formed without sparks;
(5) After grinding and sieving, the catalyst E is calcined in a muffle furnace (air atmosphere) at 850 ℃ for 6h to form the desired catalyst E.
The prepared catalyst is characterized, and SEM images of the catalyst are shown in figure 1, (a) is pure MgO prepared by a citric acid complexation method, and (b) is K 2 NiF 4 Type metal oxide LaSrCo 0.8 Fe 0.2 O 4 (in the production method of the present invention, magnesium nitrate was not added), (c) was 1 wt% LaSrCo 0.8 Fe 0.2 O 4 A unit of a combination of/MgO, (d) 10wt% of La-Sr-Co-Fe/MgO (prepared according to the method of the present invention for the 10wt% of La-Sr-Co-Fe/MgO formulation). MgO and LaSrCo 0.8 Fe 0.2 O 4 Are all uniformly-stacked spherical, the particle diameter of MgO is 180nm relatively 0.8 Fe 0.2 O 4 Has a particle diameter of 120nm and, after loading, 10% LaSrCo 0.8 Fe 0.2 O 4 The particle diameter of/MgO was significantly reduced, only 70nm,10% of La-Sr-Co-Fe/MgO also reduced to 80nm (the above particle diameters were calculated by imageJ software). The SEM data above show that the addition of MgO is relative to K 2 NiF 4 The metal oxide has obvious dispersing effect, and the diameter of the particle is reduced, so that the sintering phenomenon in the roasting process of catalyst preparation is obviously reduced, the dispersion degree of the particle is increased, more reaction active sites are exposed, and the active effect on the improvement of the reaction activity is achieved. The SEMmaping picture of the invention is shown in figure 2, all elements are uniformly distributed, which is one of the advantages of the one-pot method, and compared with the traditional dipping method, the SEMmaping picture has better uniformity and is more beneficial to reaction.
Figure 3 is an XRD pattern of each catalyst. XRD of catalyst A and catalyst B showed two phases, mgO and LaSr (Co/Fe) O 4 The components of catalyst C, D are LaSrCo 0.8 Fe 0.2 O 4 、LaSrCoO 4 。LaSrCo 0.8 Fe 0.2 O 4 Compared with LaSrCoO 4 The XRD pattern of (A) is shifted to a small angle direction, which indicates that Fe is successfully doped into LaSrCoO 4 The crystal lattice (according to the Bragg formula, elements with large atomic radius are doped, the XRD pattern moves towards a small angle direction, and the atomic radius of Fe is larger than that of Co); 10% of LaSrCo 0.8 Fe 0.2 O 4 The XRD pattern of the/MgO contains LaSrCo 0.8 Fe 0.2 O 4 And two phases of MgO, in which LaSrCo 0.8 Fe 0.2 O 4 The phase peak shape of (a) is weaker because of its lower content (10 wt%); but 10% of La 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3 XRD pattern of/MgO and 10% LaSrCo 0.8 Fe 0.2 O 4 The same as for MgO, the description is based on 10% of La 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3 The MgO is fed, and the sample prepared by the one-pot method forms LaSrCo 0.8 Fe 0.2 O 4 (K 2 NiF 4 Type metal oxide) and MgO, not La 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3 (perovskite-type metal oxide ABO 3) and MgO. This is due to the strong interaction between the two, the form of metal oxide that is more stable on MgO being K 2 NiF 4 Rather than ABO 3 Even according to ABO 3 The feed was carried out in proportions such that, at a loading of 10%, the metal oxide formed was still K 2 NiF 4 And (3) a metal oxide. And, when the composition is calculated, the excess Co (Fe) Ox is formed on the surface of the metal oxide, the content of La is 10% 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3 The actual composition of the catalyst fed with MgO is LaSrCo containing Co (Fe) Ox 0.8 Fe 0.2 O 4 PerMgO, hereinafter for convenience, the sample is expressed as 10% La-Sr-Co-Fe/MgO.
The following table shows the BET test results for the catalysts prepared according to the invention.
The specific surface area (LaSrCo) of the composite catalyst is remarkably increased by adding MgO 0.8 Fe 0.2 O 4 7.9→10％LaSrCo 0.8 Fe 0.2 O 4 /MgO34.4、LaSrCo 0.8 Fe 0.2 O47.9→10％La-Sr-Co-Fe/MgO29.5)，10％LaSrCo 0.8 Fe 0.2 O 4 MgO pore volume and pore diameter and pure LaSrCo 0.8 Fe 0.2 O 4 Compared with the prior art, the method has the advantages that the increase of the particle gaps is proved, the effect of increasing the dispersion degree of the composite catalyst particles is also proved by the MgO, and the increase of the specific surface area is favorable for the adsorption of gas and the direct decomposition reaction of NO. 10% of the pore diameter of La-Sr-Co-Fe/MgO with 10% 0.8 Fe 0.2 O 4 The ratio of/MgO is greatly reduced, which shows that Co (Fe) Ox exists in LaSrCo 0.8 Fe 0.2 O 4 And the gaps between the MgO and the porous ceramic material are blocked to a certain extent, so that the pore volume of the pore is reduced. 10% reduction of the pore size and pore volume of La-Sr-Co-Fe/MgO indicates that Co (Fe) O x But the presence of Co (Fe) Ox was not observed in the XRD pattern because of Co (Fe) O x High dispersion and too little amount.
Factors influencing the direct decomposition activity of NO expand around oxygen vacancies, the oxygen vacancies are the active sites of the direct decomposition reaction of NO, in addition, because oxygen is contained in the products of the direct decomposition reaction of NO, the Removal of the oxygen from the active center of the catalyst inevitably influences the reaction rate, the better the mobility of lattice oxygen is, the more favorable the Removal of the oxygen is, the more favorable the increase of the reaction activity [ ZhuJ, thomas A. Chem Informim Abstract: perovskite-Type Mixed oxygen Catalytic Material for NO Removal [ J]Chem In form,2010,41 (3): 225-233 ]. As shown in FIG. 4, the catalyst 10% LaSrCo 0.8 Fe 0.2 O 4 MgO and LaSrCo 0.8 Fe 0.2 O 4 Compared with the prior art, the addition of the magnesium oxide obviously increases the number of oxygen vacancies of the composite catalyst (the desorption peak of beta oxygen at 400-600 ℃ is increased, the beta oxygen is oxygen desorbed from the oxygen vacancies, and the number of the beta oxygen is increasedRepresenting the content of oxygen vacancies in the catalyst), increases the mobility of lattice oxygen (the desorption peak of gamma oxygen is increased at 600-800 ℃, and the oxygen of gamma oxygen is removed from the lattice), and is favorable for the direct decomposition reaction of NO. 10% of La-Sr-Co-Fe/MgO the number of oxygen vacancies was not as large as 10% 0.8 Fe 0.2 O 4 Although there is much MgO, the lattice oxygen has a strong mobility, which is also advantageous for the reaction.
The research on the adsorption and activation of NO molecules on the perovskite type composite catalyst by utilizing NO-TPD has important significance for disclosing the catalytic property of NO reduction on the catalyst. During temperature programmed desorption (NO-TPD), adsorbed NO molecules may react and be converted to other adsorbates such as nitrates, nitrosos [ Chen J, shen M, wang X, et al]Catalysis Communications,2013,37 (Complete): 105-108). Adsorption of NO by perovskites and related oxides is generally related to surface area and oxygen vacancies under anoxic conditions. The first NO desorption peak is the desorption of NO molecules adsorbed on the catalyst, and molecular adsorbed NO is less strongly adsorbed and will be desorbed at low temperature. The second NO desorption peak is that NO is adsorbed on the surface of the catalyst to form nitrate or nitrite which is decomposed at high temperature to release NO. As shown in FIG. 5, 10% LaSrCo 0.8 Fe 0.2 O 4 MgO and 10% of La-Sr-Co-Fe/MgO the NO adsorption amount ratio LaSrCo 0.8 Fe 0.2 O 4 And MgO, mainly due to the increase in the number of oxygen vacancies and the increase in the specific surface area, which is similar to O in the above 2 TPD and BET results are in agreement. The NO adsorption is enhanced, and the direct decomposition reaction of NO is extremely favorable. 10% of LaSrCo 0.8 Fe 0.2 O 4 MgO equivalent to 10% of the total NO adsorbed amount of La-Sr-Co-Fe/MgO, but there was a significant difference in the amount of adsorption of the two peaks, the catalyst 10% of LaSrCo was prepared according to the present invention 0.8 Fe 0.2 O 4 The first NO desorption peak of MgO is relatively large, 10% by weight, the second NO desorption peak of La-Sr-Co-Fe/MgO, which is decomposition of nitrate or nitrite formed after reaction of NO with perovskite, is relatively large under high temperature conditions, 10% by weightThe NO adsorbed by La-Sr-Co-Fe/MgO forms more nitrates or nitroso groups, i.e., the surface is more oxidized, 10% of La-Sr-Co-Fe/MgO ratio 10% LaSrCo 0.8 Fe 0.2 O 4 The main reason why the reactivity of MgO is high.
The prepared catalyst is tabletted, crushed and screened to obtain 40-60 mesh particles for later use. In the examples and comparative examples, the catalyst mass was 0.5g (activity test was performed using 0.5g of a tableted catalyst), the concentration of NO was 2000ppm, he equilibrium, the inlet mixed gas flow rate was 20mL/min, and W/F =1.5g · s · mL -1 Detecting the outlet N by a gas chromatography TCD detector 2 Content, yield calculations were performed (i.e. online measurement of the N2 content produced by the reaction using gas chromatography). The activity test range is 500-850 ℃. As shown in FIG. 6, the reactivity of each catalyst increases with the increase of temperature, and since the calcination temperature of the catalyst is 850 ℃, the highest test temperature is 850 ℃, and an excessively high reaction temperature is not suitable for industrial production. 10% of LaSrCo 0.8 Fe 0.2 O 4 MgO ratio LaSrCo 0.8 Fe 0.2 O 4 The reaction activity of (A) is more than one time higher; 10% La-Sr-Co-Fe/MgO samples had higher catalytic activity for direct decomposition of NO, depending on the presence of Co (Fe) Ox. Compared with pure LaSrCoO 4 、LaSrCo 0.8 Fe 0.2 O 4 MgO-supported K 2 NiF 4 Structure (i.e. A) 2 BO 4 ) The metal oxide catalysts have higher direct NO decomposition activity, which indicates that MgO is used as a carrier and can obviously promote the direct NO decomposition activity.
Oxygen is one of reaction products, has a strong inhibiting effect on the direct decomposition reaction of NO, and can be adsorbed on oxygen vacancies, so that the active center of the reaction is difficult to regenerate, and finally, the activity of the catalyst is reduced and even inactivated. La0.7Ba in the literature 0.3 Mn 0.8 In 0.2 O 3 At 1073k, NO; W/F =3gcatscm -3 Under the condition of no oxygen, N 2 The yield was around 62%, the activity dropped sharply by 20% after the oxygen partial pressure reached 1%, by 40% at 5% [ Ishihara T, ando M, sada K, et al direct composition of NO into N2, and O2La(Ba)Mn(In)O 3,perovskite oxide[J].Journal of Catalysis,2003,220(1):104-114】；Ba 0.8 La 0.2 Mn 0.8 Mg 0.2 O 3 At 1123K, NO =1%, he: balance, W/F =3.0gscm -3 Under the condition of no oxygen, N 2 The yield was about 78%, the activity decreased sharply by 30% after the oxygen partial pressure reached 1%, and by 40% at the oxygen partial pressure of 5% [ Iwakuni H, shinmou Y, yano H, et al direct composition of NO into N2 and O2 on BaMnO3-based Perovskite oxides [ J]Applied Catalysis B Environmental,2007,74 (3-4): 299-306. As shown in FIG. 7, N at 850 deg.C under different oxygen partial pressures 2 The yield curve shows that the catalyst prepared by the invention has excellent oxidation resistance of 10 percent of La-Sr-Co-Fe/MgO, the yield is reduced by less than 2 percent at 850 ℃ under the condition of 1 percent of oxygen partial pressure, the yield is reduced by less than 10 percent even after the oxygen partial pressure reaches 5 percent, and the yield is maintained at about 54 percent, thereby having certain industrial application prospect.
The catalyst can be prepared by adjusting the process parameters according to the content of the invention, and the performance of the catalyst is basically consistent with that of the invention. The invention has been described in an illustrative manner, and it is to be understood that any simple variations, modifications or other equivalent changes which can be made by one skilled in the art without departing from the spirit of the invention fall within the scope of the invention.
Claims (6)
1. A composite metal oxide catalyst characterized in that the entirety is magnesium oxide-supported K 2 NiF 4 Structural composite metal oxides, K 2 NiF 4 The loading capacity of the structural composite metal oxide is 10wt%, and the composition is LaAB x B′ 1-x O 4 MgO, in which La and A form K, B and B 'form Ni, A is Sr, B is Co, B' is Fe and x is 0.8, according to the following steps:
step 1, uniformly dispersing EDTA in deionized water, heating in a water bath, and adding ammonia water to completely dissolve the EDTA, wherein the temperature of the water bath is 60-80 ℃;
step 2, according to 10wt.% La 0.5 Sr 0.5 Co 0.8 Fe 0.2 O 3 Weighing corresponding nitrate and uniformly dispersing the nitrate in the aqueous solution of the EDTA obtained in the step 1 in the composition of MgO, adding citric acid and uniformly dispersing, and adjusting the pH to 6-8 by using ammonia water;
step 3, evaporating the water of the mixed solution obtained in the step 2 by using a water bath to obtain gel, wherein the temperature of the water bath is 70-90 ℃;
step 4, placing the gel obtained in the step 3 on a heating plate for presintering to obtain black powder, wherein the presintering temperature is 200-400 ℃, and the presintering time is 1-5 hours;
and 5, roasting the powder obtained in the step 4 to obtain the required catalyst, wherein the roasting atmosphere is air, the roasting temperature is 700-900 ℃, and the roasting time is 4-8 hours.
2. The composite metal oxide catalyst according to claim 1, wherein in step 2, magnetic stirring is used for uniform dispersion, and the stirring speed is 300-500 rpm; the concentration of ammonia water is 20-25 wt%, and the pH is adjusted to 7-8; after adding citric acid, uniformly dispersing by using magnetic stirring, wherein the stirring time is 1-2 hours, and the stirring speed is 300-500 revolutions per minute.
3. The composite metal oxide catalyst according to claim 1, wherein in step 4, the calcination is performed using an air atmosphere, the calcination temperature is 250 to 350 ℃, and the calcination time is 3 to 4 hours.
4. The composite metal oxide catalyst as claimed in claim 1, wherein in the step 5, the calcination temperature is 750 to 850 ℃ and the calcination time is 6 to 8 hours.
5. Use of the composite metal oxide catalyst of claim 1 in the catalysis of direct decomposition of NO.
6. The complex metal oxygen of claim 5The application of compound catalyst in direct NO decomposition and catalysis features that 40-60 mesh catalyst is used and the reaction gas mixture is 2000ppm NO and N 2 For balance gas, total gas flow was 20mL/min, W/F =1.5g · s · mL -1 The temperature was 850 ℃.
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