CN102302930A - Transition metal doped cerium and titanium compound oxide catalyst for selective catalytic reduction of nitric oxide by ammonia - Google Patents

Abstract

The invention relates to a transition metal-doped cerium-titanium composite oxide catalyst for the selective catalytic reduction of nitrogen oxides by ammonia and a preparation method thereof. The catalyst in the invention is a transition metal (iron, tungsten, molybdenum) doped cerium-titanium composite oxide catalyst. The preparation method of the catalyst is the co-precipitation method, that is, the required cerium salt, titanium salt and the salt corresponding to any one or more transition metals in iron, tungsten and molybdenum are prepared into a mixed solution, and ammonia water or sodium hydroxide or carbonic acid One of sodium, ammonium bicarbonate or urea is used as a precipitating agent, continuously stirred at 50-150°C for 3-15 hours, and then suction filtered, washed, dried and roasted. The raw materials used in the present invention are non-toxic and harmless, and the preparation method is simple and easy. The prepared catalyst has the characteristics of high catalytic activity, excellent selectivity of _N2 generation, wide operating temperature window, and can adapt to high space velocity reaction conditions. The mobile source represented by vehicle exhaust and the fixed source nitrogen oxide catalytic purification device represented by the flue gas of coal-fired power plant.

Description

Transition metal-doped cerium-titanium composite oxide catalysts for the selective catalytic reduction of nitrogen oxides by ammonia

technical field

The invention is applied in the technical field of environmental catalysis, and relates to a transition metal (tungsten, molybdenum, iron) doped cerium-titanium composite oxide catalyst used for catalytic purification of nitrogen oxides in stationary source flue gas and diesel vehicle exhaust.

Background technique

At present, the NH ₃ selective catalytic reduction of NO _x (NH ₃ -SCR) catalysts for industrial applications are mainly WO ₃ or MoO ₃ doped V ₂ O ₅ /TiO ₂ systems. This catalyst system has been widely used in stationary source flue gas denitrification, and has also been used in the catalytic elimination of NO _x from diesel vehicle exhaust. However, this catalyst system still has some problems, such as: it contains toxic substance V, if it falls off during use, it has great biological toxicity when it enters the environment; the operating temperature window is narrow, and a large amount of greenhouse gas _N2 is generated at high temperature O: easy to catalyze the conversion of SO ₂ in flue gas into SO ₃ .

In 2001, Smirniotis et al. (PG, Smirniotis, D A, Pena.B S Uphade, Low-temperature selectivecatalytic reduction (SCR) of NO with NH ₃ by using Mn, Cr, and Cu oxides supported on HombikatTiO ₂ .Angewandte Chemie International Edition, 40 (2001) 2479-2482.) reported Mn/TiO ₂ catalysts with excellent low-temperature NH ₃ -SCR activity in "Angew.Chem.Int.Ed.", which attracted the attention of relevant scholars on Mn-based oxide catalysts. Related research reports are increasing rapidly. The main application target of Mn-based oxide catalysts is the denitrification of stationary source flue gas after desulfurization and dust removal units, and a few researchers have tried to use it for the catalytic purification of NO _x from diesel vehicle exhaust. Although this type of catalyst has excellent activity at low temperature, its ability to resist water and sulfur is poor, its activity at medium and high temperature is low, and its by-product N ₂ O is more, making it impossible to be practically applied. In recent years, transition metal-exchanged molecular sieve catalysts (especially Fe molecular sieves and Cu molecular sieve catalysts) have attracted extensive attention in the field of diesel vehicle exhaust purification due to their good medium-high temperature SCR activity and high space velocity resistance. However, the low low temperature activity, poor hydrothermal stability and hydrocarbon poisoning effect limit its large-scale application in the purification of NO _x from diesel vehicle exhaust. Therefore, it is necessary to develop a new type of non-vanadium-based catalyst system with high NH ₃ -SCR activity, wide operating temperature window, high space velocity environment, high stability, wide temperature window, high space velocity environment, non-toxic and harmless for stationary source smoke Gas denitrification and elimination of NO _x in diesel vehicle exhaust are still the research hotspots in the field of environmental catalysis.

Contents of the invention

In order to solve the shortcomings of the existing NH ₃ -SCR catalyst system, such as poor low-temperature activity, low high-temperature N ₂ selectivity, narrow temperature window, and sensitivity to reaction space velocity, the present invention provides a new type of transition metal (iron, tungsten, Molybdenum) doped cerium-titanium composite oxide catalyst and preparation method thereof can be used as NH ₃ -SCR catalyst for stationary source flue gas denitrification and diesel exhaust _NOx purification.

The preparation method of the catalyst in the present invention is a co-precipitation method, that is, the salt corresponding to the required metal oxide is formulated into a mixed solution, wherein the cerium salt is at least one of cerous chloride or cerium nitrate or cerium ammonium nitrate or cerium sulfate , the titanium salt is at least one of titanium tetrachloride, titanium sulfate or tetrabutyl titanate, and the salts corresponding to the doped transition metals tungsten, molybdenum and iron are ammonium tungstate, ammonium molybdate and iron nitrate, respectively. In terms of the molar ratio of metal elements, the ratio of cerium to titanium is 0.1-1.0, and the ratio of doped transition metal (at least one of tungsten, molybdenum, and iron) to titanium is 0.1-0.5, and ammonia water or sodium hydroxide Or one of sodium carbonate, ammonium bicarbonate or urea is used as a precipitating agent. Stir continuously for 3-15 hours at a temperature of 50-150°C, then perform suction filtration and washing, and put the filter cake in an oven at 80-120°C Dry overnight, and finally roast in air at 400-600°C for 3-8 hours in a muffle furnace to obtain the finished catalyst.

The present invention has the following advantages:

(1) Non-toxic components are used, which will not cause harm to human health and the ecological environment;

(2) The operating temperature window is wide (especially with good low-temperature activity), which is suitable for the application environment where the temperature of motor vehicle exhaust varies greatly; in the aspect of stationary source flue gas denitrification, it is expected to become a substitute for vanadium-based catalysts;

(3) It is not sensitive to the reaction space velocity, especially suitable for the high space velocity of motor vehicle exhaust purification. When it is applied to fixed source flue gas denitrification, it can greatly reduce the amount of catalyst used, reduce costs and save space;

(4) It has very excellent _N2 generation selectivity.

Detailed ways

In order to illustrate the present invention more clearly, the following examples are cited, but they do not limit the scope of the present invention in any way.

[Example 1-3]

Use cerium nitrate hexahydrate, ammonium tungstate and titanium sulfate as cerium salt, tungsten salt and titanium salt respectively, prepare a solution with a Ce/W/Ti molar ratio of 0.2/0.1/1.0 and mix well, add excess urea to the solution solution, and continuously stirred at 90°C for 12 hours, then suction filtered and washed, put the filter cake in an oven and dried overnight at 100°C, and finally roasted in the air at 500°C in a muffle furnace for 5 hours to obtain a powder catalyst. The prepared catalyst was pressed into tablets, crushed, sieved, and 40-60 meshes were taken for later use, which was called catalyst A. With other conditions unchanged, ammonium tungstate was changed to ammonium molybdate and iron nitrate to prepare catalysts B and C respectively.

[Example 4-6]

Use cerium nitrate hexahydrate, ammonium tungstate and titanium sulfate as cerium salt, tungsten salt and titanium salt respectively, prepare a solution with a Ce/W/Ti molar ratio of 0.2/0.2/1.0 and mix well, add excess urea to the solution Solution, and continuously stirred at 90°C for 12h, then suction filtered and washed, put the filter cake in an oven and dried overnight at 100°C, and finally roasted in the air at 500°C in a muffle furnace for 5h to obtain a powder catalyst. The prepared catalyst was pressed into tablets, crushed, sieved, and 40-60 meshes were taken for later use, called catalyst D. Catalysts E and F were prepared respectively by changing the Ce/W/Ti molar ratio to 0.2/0.3/1.0 and 0.2/0.5/1.0 while other conditions remained unchanged.

[Example 7]

Using the catalysts A, B, C, D, E and F prepared in Examples 1-6, the reaction activity of NH ₃ selective catalytic reduction of NO _x was investigated in a self-made miniature fixed-bed reactor. The usage amount of the catalyst is 0.12ml, the composition of the reaction gas mixture is: [NO]=[NH ₃ ]=500ppm, [O ₂ ]=5%, N ₂ is used as balance gas, the total gas flow rate is 500ml/min, and the space velocity is 250,000h ^-1 , and the reaction temperature is 150-450°C. NO and NH ₃ as well as by-products N ₂ O and NO ₂ were measured by infrared gas cell. The reaction results are shown in Table 1.

Table 1 Catalyst activity evaluation results

Catalyst D with the best activity can achieve more than 100% _NOx conversion rate in the temperature range of 250-450°C under the condition of 250,000h ^-1 space velocity and 100% N ₂ selectivity.

[Embodiment 8]

Using catalyst D, the effect of reaction space velocity on catalyst activity was investigated in a self-made miniature fixed-bed reactor. The usage amount of the catalyst is 0.3ml, 0.12ml, 0.06ml respectively, the composition of the reaction mixture gas is: [NO]=[NH ₃ ]=500ppm, [O ₂ ]=5%, N ₂ is used as balance gas, the total gas flow rate 500ml/min, the corresponding space velocities are 100,000h ^-1 , 250,000h ^-1 , 500,000h ^-1 , and the reaction temperature is 150-450°C. NO and NH ₃ as well as by-products N ₂ O and NO ₂ were measured by infrared gas cell. The reaction results are shown in Table 2.

Table 2 Effect of reaction space velocity on the activity of catalyst D

Catalyst D can achieve a _NOx conversion of more than 95% in the temperature range of 200-450°C under the condition of a space velocity of 100,000h ^-1 , and the selectivity of _N2 formation is 100%, showing a very Wide operating temperature window; even under the condition of high space velocity of 500,000h ^-1 , Catalyst D can still achieve a _NOx conversion of more than 95% in the temperature range of 275-450℃, and the _N2 generation selectivity is 100 %, indicating that the catalyst has very excellent anti-high space velocity reaction performance.

[Example 9]

Using catalyst D, the effects of water vapor and _CO2 on the catalyst activity were investigated in a self-made miniature fixed-bed reactor. The amount of catalyst used is 0.12ml, and the composition of the reaction mixture is: [NO]=[NH ₃ ]=500ppm, [O ₂ ]=5%, [H ₂ O]=5%, [CO ₂ ]=5% , N ₂ as the balance gas, the total gas flow rate is 500ml/min, the space velocity is 250,000h ^-1 , and the reaction temperature is 150-450°C. NO and NH ₃ as well as by-products N ₂ O and NO ₂ were measured by infrared gas cell. The reaction results are shown in Table 3.

Table 2 Effect of H ₂ O and CO ₂ on the activity of catalyst D

H ₂ O and CO ₂ have a greater impact on the low-temperature activity of catalyst D, but have no effect on the high-temperature activity of the catalyst. Even if 5% H ₂ O and 5% CO ₂ are fed into the reaction atmosphere, Catalyst D can still achieve 100% _NOx conversion in the temperature range of 250-450°C, and the N ₂ generation selectivity is 100 %, indicating that the catalyst has excellent resistance to H ₂ O and CO ₂ poisoning.

【Example 10】

Using catalyst D, the influence of _SO2 on the catalyst activity was investigated in a self-made miniature fixed-bed reactor. The consumption amount of catalyst is 0.12ml, and the composition of reaction gas mixture is: [NO]=[NH ₃ ]=500ppm, [O ₂ ]=5%, [SO ₂ ]=100ppm, N ₂ is used as balance gas, and space velocity is 250,000h ^-1 , the reaction temperature is 300°C. NO and NH ₃ and the by-products N ₂ O and NO ₂ were all measured by infrared gas cell. During the 12 hours under investigation, the NO _x conversion rate remained stable between 98.5% and 100%, and there was no obvious downward trend. It can be seen that the catalyst has a good ability to resist SO ₂ poisoning.

Claims (5)

1. A transition metal-doped cerium-titanium composite oxide for ammonia selective catalytic reduction of nitrogen oxides for diesel vehicle exhaust purification or stationary source flue gas denitrification. 2. The catalyst according to claim 1, characterized in that the precursor of cerium used in the preparation process is at least one of cerous chloride or cerium nitrate or cerium ammonium nitrate or cerium sulfate, and the precursor of titanium is tetrachloride At least one of titanium oxide or titanium sulfate or tetrabutyl titanate, the doped transition metal is one or more of tungsten, molybdenum, and iron, and the precursors corresponding to each transition metal are tungstic acid ammonium, ammonium molybdate, and ferric nitrate. 3. The catalyst according to claim 1, characterized in that, based on the molar ratio of metal elements, the ratio of cerium to titanium is 0.1-1.0, and the ratio of doped transition metal to titanium is 0.1-0.5. 4. The catalyst according to claim 1, characterized in that in the preparation process, one of ammonia water, sodium hydroxide, sodium carbonate, ammonium bicarbonate, or urea is used as a precipitating agent, and the mixture is continuously stirred at a temperature of 50-150°C 3 to 15 hours, then perform suction filtration and washing, put the obtained filter cake into an oven for drying at 80 to 120°C, and finally roast in air at 400 to 800°C in a muffle furnace for 3 to 8 hours. 5. The catalyst according to claim 1, characterized in that the catalyst is coated in the form of a coating on the wall surface of the flow channel of the honeycomb structure made of ceramics or metal, or the catalyst is made into a spherical or plate shape.