Catalyst for purifying exhaust gas

Abstract: A catalyst for purifying an exhaust gas, which comprises a first component having the activity for reducing nitrogen oxides with ammonia and a second component having at least one of the activity for forming nitrogen oxide from ammonia and the activity for forming carbon dioxide from carbon monoxide. The first component is formed of a composition containing oxide of at least one member selected from titanium, vanadium, tungsten and molybdenum and the second component is formed of a composition containing a salt of a noble metal selected from platinum, palladium and rhodium or any one of these noble metals preliminarily supported on a porous material selected from zeolite, alumina and silica. A method of purifying an exhaust gas containing NOx, CO and leak NH₃ by the catalyst.

Description: The present invention relates to a catalyst for purifying an exhaust gas and a method of purifying an exhaust gas with said catalyst. In particular, it relates to a catalyst suitable for removing nitrogen oxides (NOx) contained in an exhaust gas and an exhaust gas containing ammonia (NH₃) or carbon monoxide (CO), and a method of purifying an exhaust gas with said catalyst. NOx in flue gases from power plants, various other plants and automobiles is a substance to cause photochemical smog and acid rain. Mainly in power plants, a method of removing NOx in flue-gas by selective catalytic reduction using NH₃ as a reducing agent is widely used as an effective NOx removal method. As a catalyst, there is used a titanium oxide (TiO₂)-based catalyst containing vanadium (V), molybdenum (Mo) or tungsten (W) as an active component. In particular, a titanium oxide-based catalyst containing vanadium as one of active components is a mainstream NOx removal catalyst nowadays, since not only has it high activity but also it is less deteriorated by impurities contained in an exhaust gas and can be used even at a low temperature (JP-A-50-128681). To cope with an increase in a power demand in recent years, particularly with an increase in a power demand in summer times, an increasing number of gas turbines or co-generation systems using gas turbines are constructed mainly around centers of cities. These facilities are often constructed around areas with a concentration of population, and the nitrogen oxides (NOx) exhaust control is an overall amount control. It is therefore desired to keep the amount of NOx to be exhausted from the facilities at the lowest level possible. For this reason, a study is under way on a method in which a NOx removal apparatus is operated at a high NOx removal rate by increasing the amount of a packed catalyst and increasing the amount of ammonia to be charged as a reducing agent. With the above high-rate NOx removal being desired, it is now essential to decrease the amount of unreacted ammonia not used for a NOx removal reaction (hereinafter referred to as leak ammonia) to a level equal to or below the NOx level. For decreasing the amount of leak ammonia, a study is being made to place an ammonia-decomposing catalyst in a downstream to a NOx removal catalyst (JP-A-2 -191527). This catalyst for decomposing leak ammonia has been also studied, and there is known a catalyst containing, as an active component, copper (Cu) or iron (Fe) which has excellent activity to oxidize ammonia (JP-A-52-43767). In addition to the above NOx and NH₃, it is also desired to control CO contained in a combustion exhaust gas to a low level. Gas turbines in operation in U.S.A. have been already widely employing a method in which a platinum (Pt)-based catalyst is placed in the fore-stream to the NOx removal apparatus to remove CO contained in an exhaust gas by oxidizing it. Further, it is desired to remove not only an exhaust gas emitted from a combustion device but also CO and NH₃ emitted from various plants until their amounts are decreased to the lowest level possible. It is therefore a social issue to develop a catalyst and a process which can remove NOx, NH₃ and CO at the same time. The above prior art techniques for the removal of NOx and CO in an exhaust gas require a plurality of layers of packed catalysts, and involve a problem that their processes are complicated. For example, for a NOx removal apparatus in which the amount of a packed NOx removal catalyst is increased for operating it at a high NOx removal ratio and the amount of ammonia as a reducing agent is increased, there is a problem in that the amount of leak ammonia increases. That is, as is shown by solid lines in Fig. 5, which shows behaviors of NOx concentration and leak ammonia concentration in a catalyst layer outlet when a NOx removal apparatus is operated with changing the NH₃/NOx ratio, a high NOx removal ratio can be obtained when the apparatus is operated at a NH₃/NOx ratio of nearly 1, while the amount of leak ammonia sharply increases when the NH₃/NOx ratio reaches nearly 1. In this case, since the NOx removal apparatus has no capability of decomposing CO, no CO in an exhaust gas is decomposed, as is shown by a solid line in Fig. 5. When there is used an apparatus having two catalyst layers shown in Fig.6, one for decreasing leak ammonia, in which a conventional NOx removal catalyst is placed in a fore-stream portion and a conventional ammonia-decomposing catalyst is placed in a downstream portion, it is true that the amount of leak ammonia decreases as compared with a case shown in Fig. 5 when the NH₃/NOx ratio is increased, but a side reaction takes place to form NO under decomposition of NH₃ by oxidation shown in the following reaction scheme (1), and no high NOx removal rate can be obtained, as is shown by a solid line in Fig. 7. NH₃ + 5/4O₂ ---> NO + 3/2H₂O (1) As described above, in merely employing the above method in which the amount of a packed NOx removal catalyst is increased and an ammonia-decomposing catalyst is placed in a downstream thereto for preventing the leak of unreacted ammonia, many catalysts for decomposing NH₃ produce NOx as a byproduct and are liable to decrease the NOx removal ratio. It is to low levels due to the amount of NH₃ and gas conditions. On the other hand, in the apparatus for the removal of CO and NOx in which a catalyst for oxidizing CO is placed in a forestream to a NOx removal catalyst, a CO oxidation reactor is required to be placed apart from a NOx removal reactor in order to avoid the oxidation of NH₃ with the catalyst for oxidizing CO. Moreover, since the optimum reaction temperature differs between a CO oxidation reaction and a NOx removal reaction, the system is complicated and expensive. EP-A- 0 208 434 discloses a process for removing nitrogen oxides and carbon monoxides simultaneously from a gas including nitrogen oxides, carbon monoxide and oxygen which is characterized in that the gas contacted with a catalyst in the presence of steam and ammonia at a temperature of 150-160°C to remove the nitrogen oxides by the reaction with ammonia and to remove carbon monoxide by the reaction with steam, said catalyst comprising titanium as a main component in the form of oxide, at least one metal selected from the group consisting of molybdenum, tungsten, vanadium, cerium, nickel, cobalt and manganese as the second component in the form of oxide and at least one metal selected from the group consisting of platinum, palladium, rhodium and ruthenium as the third component. EP-A- 0 174 495 discloses a catalyst for purifying diesel engine exhaust gas, said catalyst comprising a refractory three-dimensional structure having a gas filter function, a porous inorganic carrier supported on it, and (a) vanadium oxide and (b) at least one metal selected from the group consisting of platinum, rhodium and palladium supported on the carrier, the amount of component (a) being in the range of 0.2 to 40.0 g as V₂O₅ per liter of said structure and the amount of component (b) being in the range of 0.1 to 4.0 g as metal per liter of said structure. EP-A- 0 070 736 discloses an exhaust gas catalyst comprising a support having palladium and tungsten deposited thereon. GB-A- 2 006 038 discloses a catalyst system comprising a support and a catalyst material including platinum and/or palladium carried on the support characterised in that the support also carries molybdenum in an amount of at least 0.5% by weight based on the weight of the support, and not less than 7 times the weight of the platinum and/or palladium. The present invention has been made in view of the above social background, and it is an object of the present invention to provide a novel catalyst having high activity for reactions to remove NOx, CO and NH₃ so that the following highly functional apparatus for purifying an exhaust gas can be realized; (i) an apparatus which hardly causes the leak of NH₃, (ii) an apparatus which can remove both CO and NOx, and (iii) an apparatus which can convert NH3 to harmless nitrogen. The above object of the present invention is achieved by a the following basic constitution. That is, the present invention is first directed to a catalyst for purifying an exhaust gas, a catalyst for purifying an exhaust gas, which comprises a first component having the activity for reducing nitrogen oxides with ammonia and a second component having the activity for forming nitrogen oxide from ammonia and/or the activity for forming carbon dioxide from carbon monoxide, wherein the first component is an oxide of any combination of titanium-tungsten, titanium-tungsten-molybdenum, titanium-vanadium, titanium-tungsten-vanadium, titanium-molybdenum and titanium-molybdenum-vanadium, and the second component is a platinum, palladium or rhodium compound preliminarily supported on a porous material selected from zeolite, alumina and silica, the content of the noble metal in the catalyst being greater than zero but not greater than 1,000 ppm and the ratio of the second component to the first component being 1/99 to 10/90. The above catalyst can be used as a NOx removal catalyst for not only catalytically reducing nitrogen oxides but also decomposing ammonia which has been charged as a reducing agent but has not reacted. More specifically, the above catalyst can be obtained by mixing the following two components in such amounts that the noble metal concentration is 1,000 ppm or less, adding water to the resultant mixture, kneading the mixture, shaping the mixture to a plate-like, honeycomb-like or particulate form by conventional method and calcining the shaped mixture at a predetermined temperature. (A) The first component is a composition having NOx removal activity and containing an oxide of any combination of Ti-V, Ti-Mo, Ti-W, Ti-V-W, Ti-Mo-V or Ti-W-Mo or zeolite such as mordenite carrying copper (Cu) or iron (Fe). (B) The second component is a composition having the function of forming NOx by oxidizing NH₃ with oxygen, and containing a platinum, palladium or rhodium compound preliminarily supported on zeolite, porous silica or porous alumina by ion exchange and impregnation. The second component/first component mixing ratio by weight is preferably 1/99 to 10/90. Further, the object of the present invention is also achieved by the following constitution. That is, the present invention is directed to a method of purifying an exhaust gas, which uses the above catalyst as a catalyst for decomposing nitrogen oxides or nitrogen oxides and carbon monoxide in an exhaust gas and for decomposing ammonia in an unreacted state from ammonia injected into the exhaust gas as an agent for reducing the nitrogen oxides, or by a method of purifying an exhaust gas, which comprises placing the above catalyst in a downstream to a NOx removal catalyst for removing, by catalytic reducing, nitrogen oxides in an exhaust gas containing nitrogen oxides or nitrogen oxides and carbon monoxide and further containing ammonia injected into the exhaust gas as an agent for reducing the nitrogen oxides, to decompose and remove unreacted ammonia or unreacted ammonia and carbon monoxide in the exhaust gas. Fig. 1 schematically shows pores of a catalyst according to the present invention, each pore having a structure in which a NOx removal catalyst component (first component) 1 forms a macropore and a porous material such as zeolite, etc., forms micropores from place to place in the macropore. A noble metal-containing component (second component) 2 is in a state in which it is held in the micropores. In the above structure, ammonia which is easily adsorbed on a NOx removal catalyst component is selectively adsorbed on the NOx removal catalyst component 1 in the inlet to the macropore, and used up since it reacts with NOx being diffused as shown in Fig. 2. Due to this, ammonia does not reach the noble metal-containing component (second component) 2 having high diffusion resistance and being in the micropores, and exhibits a high NOx removal ratio like a general NOx removal catalyst. On the other hand, when adsorbed ammonia is no more used due to a decrease in NOx, the ammonia is diffused into the micropores to reach the second component 2. Further, when an exhaust gas contains ammonia alone, oxidation reactions of the above scheme (1) and the following scheme (2) proceed. NH₃ + 5/4O₂ ---> NO + 3/2H₂O (1) NO + NH₃ + 1/4O₂ ---> N₂ + 3/2H₂O (2) That is, NO generated according to the scheme (1) is diffused from the micropores to the macropore or goes out of the catalyst shown in Fig.3, and at this step, the NO collides with ammonia adsorbed on the inner surface of the macropore to be reduced into nitrogen by a NO removal reaction according to the scheme (2). As described above, the catalyst of the present invention neither forms NO nor decreases the NOx removal ratio when the above catalyst is used in a NOx removal apparatus using NH₃ as a reducing agent. Further, another big characteristic feature of the catalyst of the present invention is that the amount of leak ammonia from the catalyst layer can be decreased even when an excess amount of NH₃ is added. Further, the catalyst of the present invention can efficiently decompose NH₃ into N₂ by oxidation when it is used for purifying an ammonia-containing exhaust gas, and serves to realize a variety of apparatus for preventing offensive odors. Moreover, since CO contained in an exhaust gas is, by its nature, easily diffused into catalyst pores, it rapidly comes in contact with an oxidizing catalyst component as shown in Fig. 4 to be oxidized into CO₂ according to the scheme (3). CO + 1/2O₂ ---> CO₂ (3) Therefore, the catalyst of the present invention has another big characteristic feature in that it can carry out the purification of an exhaust gas by oxidizing and decomposing CO contained in the exhaust gas in addition to the NOx removal. As described above, the catalyst according to the present invention is a novel three-way functional catalyst which works as an ordinary NOx removal catalyst when NOx is present, which can decompose ammonia into nitrogen by a combination of the activity of the noble metal-containing component (second component) 2 for catalytically oxidizing ammonia and the activity of the NOx removal component (first component) 1 when NOx is used up to leave ammonia, and further which can also remove CO at the same time. Therefore, when the catalyst of the present invention is used alone, the amount of leak ammonia, which constitutes a prior art problem since it increases as shown by a solid line in Fig. 5 when the NH₃/NO ratio is increased, can be kept at a very low value as shown by a dotted line in Fig. 5, and at the same time, CO can be removed. Further, when the catalyst 3 of the present invention is placed in a downstream of an exhaust gas 5 in a two-layered reactor shown in Fig. 6 (with a usual NOx removal catalyst 4 in a forestream therein) to oxidize and decompose leak ammonia, no NOx is generated. Therefore, a decrease in the NOx removal ratio, which is a problem of a conventional catalyst for decomposing ammonia as shown by a solid line in Fig. 7, no longer occurs, and a high NOx removal ratio as shown by a dotted line in Fig. 7 can be obtained. Further, leak ammonia can be decreased. In a prior art technique, a catalyst for decomposing ammonia is required as much as a NOx removal catalyst, while the catalyst of the present invention serves to decrease the amount of a NOx removal catalyst since it works as a NOx removal catalyst when NOx is present, and as a result, the catalyst amount in total can be appreciably decreased. As described above, the catalyst of the present invention makes it possible to constitute a system which maintains a high NOx removal ratio and generates little leak ammonia when used alone or placed in a down stream to a NOx removal catalyst. Further, when used for purifying a gas containing of NH₃ (e.g., an exhaust gas from a dizao copying machine), the catalyst of the present invention can efficiently decompose NH₃ into N₂. The catalyst of the present invention has its characteristic feature in the above catalyst constitution, and it can be naturally produced by any preparation method if such a constitution can be realized. A catalyst which works excellently can be produced by the following method. Concerning the catalyst components, the first component is selected from a variety of the above-described ones, and an excellent catalyst can be produced by the following method. Among the above-described ones, the use, as a catalyst component, of an oxide catalyst formed of any element combination of Ti-V, Ti-V-Mo and Ti-W-V etc. can produce a favorable result. This oxide catalyst can be obtained by adding a salt including oxoacid salt of vanadium, molybdenum or tungsten to a slurry of hydrous titanium oxides such as metatitanic acid, etc., converting the resultant mixture to a paste while evaporating water with a heating kneader, drying the paste, calcining the resultant product at a temperature between 400°C and 700°C and optionally milling it. The second component may be added by a method in which a soluble salt of the above noble metal is dissolved in water and the resultant solution is preliminarily allowed to be held in micropores of a porous material such as zeolite, silica or alumina by ion-exchange or kneading and then the porous material holding the second component is added to the first component. The zeolite used as a carrier for the second component is selected from a hydrogen-substituted type, sodium type and calcium type of zeolite selected from mordenite, clinoptilite, erionite and Y-type zeolite. The silica and alumina as a carrier are selected from those having a surface area of 100 m²/g to 500 m²/g, obtained by calcining hydrous oxides at a low temperature. The particle size of these is approximately 1 to 10 µm. These may be milled before use to such an extent that the structure of zeolite, etc., is not destroyed. The carrier is immersed in an aqueous solution containing a noble metal in the form of a chloride, a nitrate or an ammine complex for ion exchange, or evaporated to dryness together with an aqueous solution to obtain a powder supporting 0.01 wt% to 0.1 wt% of the noble metal, and the powder is used as a second component. The so-obtained first and second components are mixed in a second component/first component mixing ratio in weight of 10/90 to 1/99. Then, known additives for shapability improvement such as water, an inorganic binder, a shaping modifier and an inorganic fiber are added, and the resultant mixture is kneaded with a kneader to obtain a paste-like catalyst. The paste-like catalyst is shaped into a plate-like catalyst by applying it to a net-like substrate of an inorganic fiber or a metal plate whose surface is roughened by flame spraying, or shaped in the form of a column or honeycomb with an extruder. The second component/first component mixing ratio is particularly important. When this ratio is greater than the above upper limit, NOx is generated and the NOx removal ratio decreases. When it is smaller than the above lower limit, the ammonia decomposition rate cannot be increased. In particular, a favorable result can be obtained when any one of zeolite, silica and alumina etc. which have high capability to support a noble metal is used to decrease the second component/first component mixing ratio within the above range and when the amount of a supported noble metal based on the total catalyst amount is in the range which is greater than 0 ppm but not greater than 1,000 ppm, preferably in the range of from 10 to 100 ppm. In this case, as schematically shown in Fig. 1, micropores formed of the second component 2 are sparsely present in each macropore formed of the first component 1, and NH₃ is selectively adsorbed on the first component and easily used for a NOx removal reaction. Further, in addition to an economical effect that the catalyst unit price can be decreased due to a decrease in the noble metal amount, there is also an effect that the NOx removal reaction and the ammonia oxidation can be easily separated depending upon the presence or absence of NO. Furthermore, when the catalyst of the present invention is used alone, it is preferred to decrease the second component/first component mixing ratio within the specified range, whereby a favorable result can be obtained. When it is used in a two-layered reactor as shown in Fig. 6, it is preferred to increase the second component/first component mixing ratio within the specified range and increase the noble metal content, whereby a favorable result can be obtained. Fig. 1 is a schematic cross-sectional view of a catalyst of the present invention to show its characteristic features. Fig. 2 is a schematic view of a catalyst of the present invention to show its activities. Fig. 3 is a schematic view of a catalyst of the present invention to show its activities. Fig. 4 is a schematic view of a catalyst of the present invention to show its activities. Fig. 5 shows behaviors of NOx and ammonia in the outlet of a NOx removal apparatus using a conventional catalyst and a catalyst of the present invention. Fig. 6 shows a constitution using a NOx removal catalyst and an ammonia-decomposing catalyst as two layers. Fig. 7 shows problems to be caused when a NOx removal catalyst and an ammonia-decomposing catalyst are used as two layers. Fig. 8 shows comparisons of CO-decomposing performance, NOx removal performance and amount of unreacted NH₃ among the catalysts prepared in Example 1 and Comparative Examples 1 and 2. Fig. 9 shows comparisons of NOx removal performance, amount of unreacted NH₃ among the catalysts prepared in Example 1 and Comparative Examples 1 and 2. Due to the sole use of the catalyst of the present Invention the leak of unreacted ammonia can be decreased to the lowest level possible when a NOx removal apparatus is operated at a high NH₃/NOx ratio. Further, when carbon monoxide is contained in an exhaust gas, the carbon monoxide can be removed by decomposing it. When the catalyst of the present invention is used to decompose unreacted ammonia (leak ammonia) by placing it in a downstream to other NOx removal catalyst having high activity, the leak of unreacted ammonia which would be caused by an unbalanced ammonia amount can be prevented, and CO harmful to a human body can be also removed. Therefore, there can be realized a multi-functional exhaust gas treatment process desired for NOx removal apparatus to be constructed around centers of cities. Further, the catalyst of the present invention works as a NOx removal catalyst when NOx is present and it works as a catalyst for decomposing ammonia when NOx is absent. Moreover, when the catalyst of the present invention decomposes ammonia, NOx is hardly generated. Therefore, the catalyst of the present invention also has a characteristic feature in that the amount of the catalyst can be decreased to a great extent as compared with a case using two layers of a NOx removal catalyst and a conventional ammonia-decomposing catalyst. The present invention will be explained further in detail by reference to Examples. Example 1 3.59 Kilograms of ammonium paratungstate [(NH₄)₁₀H₁₀ · W₁₂O₄₆ · H₂O] and 1.29 kg of ammonium metavanadate were added to 67 kg of a metatitanic acid slurry (TiO₂ content: 30 wt%, SO₄ content: 8 wt%), and while water was evaporated, the resultant mixture was kneaded with a heating kneader to obtain a paste having a water content of 36 %. The paste was extruded in the form of 3 mm⊘ columns, granulated, dried with a fluidized dryer, and then calcined in atmosphere at 550°C for 2 hours. The resultant granules were milled with a hammer mill such that the content of at least 60 % of the powder had a particle size of 1 µm to give a NOx removal catalyst powder as a first component. This powder had a V/W/Ti composition of 4/5/91 (atomic ratio). On the other hand, 500 g of H type mordenite having an Si/Al atomic ratio of about 21 and an average particle size of about 10 µm was added to a solution of 0.665 g of chloroplatinic acid (H₂[PtCl₆]·6H₂O) in 1 liter of water, and the resultant mixture was subjected to evaporation to dryness over a sand bath to allow the mordenite to support Pt. The resultant product was dried at 180°C for 2 hours and calcined in atmosphere at 500°C for 2 hours to prepare 0.05 wt% Pt-mordenite as a second component. Separately, net-like fabrics each prepared by plain-weaving twist yarns each consisting of 1,400 E glass fibers having a fiber dimeter of 9 µm in a texture coarseness of 10 yarns/0,0254 m (10 yarns/inch) were impregnated with a slurry containing 40 % of titania, 20 % of silica sol and 1 % of polyvinyl alcohol, and dried at 150°C to obtain catalyst substrates having stiffness. 5.3 Kilograms of a silica alumina-based inorganic fiber and 17 kg of water were added to 20 kg of the first component and 408 g of the second component, and the resultant mixture was kneaded with a kneader to obtain a catalyst paste. This catalyst paste was placed between the two catalyst substrates above, and the resultant laminate was allowed to pass a press roll to attach the catalyst to the inter-stitch areas and surface of the substrate under pressure, whereby a plate-like catalyst having a thickness of about 1 mm was obtained. The catalyst was dried at 180°C for 2 hours, and calcined in atmosphere at 500°C for 2 hours. The catalyst had a second component/first component ratio of 2/98 and a Pt content of 10 ppm when the catalyst substrate and the inorganic fiber here excluded. Comparative Example 1 A catalyst was prepared in the same manner as in Example 1 except that no second component was added. Comparative Example 2 A catalyst was prepared in the same manner as in Example 1 except that the first component was replaced with titania produced by a chlorine method (trade name: CR50, supplied by Ishihara Sangyo Kabushiki Kaisha). Test Example 1 The catalysts obtained in Example 1, Comparative Example 1 and Comparative Example 2 were respectively cut to sheets having a width of 20 mm and a length of 100 mm. Three sheets taken from each catalyst were packed in a reactor at intervals of 3 mm, and measured for a NOx removal ratio, an unreacted ammonia amount in a reactor outlet and a CO oxidation ratio when the ammonia amount was varied under the conditions shown in Table 1. Fig. 8 shows the results. Component Content NO 200 ppm NH₃ varied CO 200 ppm O₂ 10 % H₂O 6 % CO₂ 6 % SO₂ 2 ppm gas amount 180 liters/h temperature 350°C area velocity (AV) 17 m/h As shown in Fig. 8, the catalyst of Example 1 showed a high CO oxidation ratio and yet showed a NOx removal ratio equivalent to that of the catalyst formed of the NOx removal component alone, prepared in Comparative Example 1, when the ammonia amount was increased to increase the NH₃/NO ratio. Further, when the catalyst of Example 1 was used, the ammonia concentration in the reactor outlet was as low as one digit number in ppm. In contrast, when the catalyst of Comparative Example 1 was used, CO was hardly oxidized, and an increasingly high concentration of ammonia was detected in the reactor outlet as the NH₃/NO ratio increased. On the other hand, when the titania-containing catalyst of Comparative Example 2, which contained the second component but had no NOx removal activity, was used, the CO oxidation ratio was high, and the NH₃ concentration in the reactor outlet was low. However, it showed negative values of the NOx removal ratio. In this case, the CO decomposition ratio was high as shown in Fig. 8. As is clear from the above results, the catalyst according to this Example of the present invention is an excellent catalyst due to synergistic effects of the first component and the second component, which shows a high NOx removal ratio equivalent to that of a usual NOx removal catalyst when the NH₃/NO ratio is low, which can decrease residual ammonia without generating NOx when NOx is used up by a NOx removal reaction and which can simultaneously decompose and remove CO. Examples 2 and 3 A second component was prepared in the same manner as in Example 1 except that the H type mordenite was replaced with fine silica particles (trade name: Mycon-F, supplied by Tomita Pharmaceutical Kabushiki Kaisha) (Example 2) or with γ-alumina particles (Sumitomo Chemical Kabushiki Kaisha) (Example 3). Then, catalysts were prepared in the same manner as in Example 1 in a second component/first component mixing ratio of 2/98 except that the second component was replaced with the above-prepared second components. Example 4 A catalyst was prepared in the same manner as in Example 2 except that the second component was replaced with 833 ml of a chloroplatinic acid aqueous solution (Pt concentration: 1.2mg/ml). Example 5 A catalyst was prepared in the same manner as in Example 2 except that the ammonium paratungstate used in the preparation of the first component was replaced with ammonium paramolybdate [(NH₄)₆ · Mo₇O₂₄ · 4H₂O]. Examples 6 and 7 A mordenite supporting palladium or rhodium in an amount of 0.05 wt% was prepared in the same manner as in Example 2 except that the chloroplatinic acid was replaced with a nitric acid solution of palladium nitrate [Pd(NO₃)₃] (Example 6) or with a nitric acid solution of rhodium nitrate [Rh(NO₃)₃] (Example 7). Catalysts were prepared by respectively adding the above-prepared products to the first component prepared in the same manner as in the preparation of Pt-mordenite. Examples 8 - 10 Catalysts were prepared in the same manner as in Example 2 except that the second component/first component ratio was changed from 2/98 to 0.5/99.5, 1/9 and 2/8. Examples 11 - 13 A second component was prepared in the same manner as in Example 2 except that the amount of chloroplatinic acid was changed to 2.66 g. Then catalysts were prepared in the same manner as in Example 2 except that the second component/first component ratio was changed as in Examples 8 to 10. Comparative Example 3 A catalyst was prepared in the same manner as in Example 5 except that no second component was added. Comparative Examples 4 - 7 Catalysts were prepared in the same manner as in Examples 2, 3, 6 and 7 except that each of the first components was replaced with the same titania as that used in Comparative Example 3. Test Example 2 The catalysts prepared in Examples 1 to 13 and Comparative Examples 1 to 7 were measured for NOx removal ratios, unreacted ammonia decomposition ratios and CO removal ratios under the conditions shown in Table 2. Tables 3 and 4 show the results. The unreacted ammonia decomposition ratio was determined by the following equation. Ammonia decomposition ratio (%) = {(NH₃ concentration in inlet - concentration of NH₃ used in reaction - NH₃ concentration in outlet) / (NH₃ concentration in inlet - concentration of NH3 used in NOx removal reaction)} × 100 Component Content NO 200 ppm NH₃ 280 ppm CO 200 ppm O₂ 10 % H₂O 6 % CO₂ 6 % SO₂ 2 ppm gas amount 180 liters/h temperature 350°C area velocity (AV) 17 m/h Catalyst Second component First component (atomic ratio) Mixing ratio (weight ratio) Nobel metal content (ppm) Activities (%) NOx removal ratio NH₃ decomposition ratio CO oxidation ratio Ex.1 0.05%Pt-mordenite Ti/W/V =91/5/4 2/98 10 96 94 92 Ex.2 0.05%Pt-silica Ti/W/V =91/5/4 2/98 10 93 98 97 Ex.3 0.05%Pt-γ-alumina Ti/W/V =91/5/4 2/98 10 88 42 68 Ex.4 chloroplatinic acid Ti/W/V =91/5/4 - 50 96 65 18 Ex.5 0.05%Pt-silica Ti/Mo/V =91/5/4 2/98 10 94 91 95 Ex.6 0.05%Pd-silica Ti/W/V =91/5/4 2/98 10 97 74 72 Ex.7 0.05%Rh-silica Ti/W/V =91/5/4 2/98 10 96 67 54 Ex.8 0.05%Pt-silica Ti/W/V =91/5/4 0.5/99.5 2.5 98 96 88 Ex.9 0.05%Pt-silica Ti/W/V =91/5/4 1/9 50 91 99 99 Ex.10 0.05%Pt-silica Ti/W/V =91/5/4 2/8 100 88 99 99 Catalyst Second component First component (atomic ratio) Mixing ratio (weight ratio) Nobel metal content (ppm) Activities (%) NOx removal ratio NH₃ decomposition ratio CO oxidation ratio Ex.11 0.2%Pt-silica Ti/W/V =91/5/4 0.5/99.5 10 98 96 95 Ex.12 0.2%Pt-silica Ti/W/V =91/5/4 1/9 200 68 99 99 Ex.13 0.2%Pt-silica Ti/W/V =91/5/4 2/8 400 41 99 99 CEx.1 - Ti/W/V =91/5/4 - 0 98 3 3 CEx.2 0.05%Pt-silica inert TiO₂ - 10 -52 98 98 CEx.3 - Ti/M o /V =91/5/4 - 0 96 2 1.8 CEx.4 0.05%Pt-silica inert TiO₂ 2/98 10 -55 99 98 CEx.5 0.05%Pt-γ-alumina inert TiO₂ 2/98 10 -38 79 77 CEx.6 0.05%Pd-silica inert TiO₂ 2/98 10 -22 77 61 CEx.7 0.05%Rh-silica inert TiO₂ 2/98 10 13 64 32 Tables 3 and 4 clearly show that the catalysts obtained in Examples of the present invention showed high NOx removal ratios, high ammonia decomposition ratios and CO oxidation ratios as compared with the catalysts obtained in Comparative Examples and that the catalysts according the present invention were excellent catalysts which had the capability of NOx removal and CO oxidation while preventing the leak of unreacted ammonia. Test Example 3 The catalysts obtained in Examples 1 to 13 and Comparative Examples 1 to 7 were measured for ammonia decomposition ratios in the absence of NO, and NOx concentrations were measured, under the conditions shown in Table 2 except that the NO concentration was changed to 0 ppm. Tables 5 and 6 show the results. The ammonia decomposition ratio was determined by the following equation. Ammonia decomposition ratio (%) = {(NH₃ concentration in reactor inlet - NH₃ concentration in reactor outlet) / NH₃ concentration in reactor inlet} × 100 Catalyst Second component First component (atomic ratio) Mixing ratio (weight ratio) Nobel metal content (ppm) NH₃ decomposition ratio (%) Generated NOx (ppm) Ex.1 0.05%Pt-mordenite Ti/W/V =91/5/4 2/98 10 97 0.5 Ex.2 0.05%Pt-silica Ti/W/V =91/5/4 2/98 10 99 0.1 Ex.3 0.05%Pt-γ-alumina Ti/W/V =91/5/4 2/98 10 63 3.8 Ex.4 chloroplatinic acid Ti/W/V =91/5/4 - 50 65 2.0 Ex.5 0.05%Pt-silica Ti/Mo/V =91/5/4 2/98 10 91 0.3 Ex.6 0.05%Pd-silica Ti/W/V =91/5/4 2/98 10 74 0.8 Ex.7 0.05%Rh-silica Ti/W/V =91/5/4 2/98 10 67 0.7 Ex.8 0.05%Pt-silica Ti/W/V =91/5/4 0.5/99.5 2.5 98 0.2 Ex.9 0.05%Pt-silica Ti/W/V =91/5/4 1/9 50 99 1.1 Ex.10 0.05%Pt-silica Ti/W/V =91/5/4 2/8 100 99 2.3 Catalyst Second component First component (atomic ratio) Mixing ratio (weight ratio) Nobel metal content (ppm) NH₃ decomposition ratio (%) Generated NOx (ppm) Ex.11 0.02%Pt-silica Ti/W/V =91/5/4 0.5/99.5 10 96 0.9 Ex.12 0.02 %Pt-silica Ti/W/V =91/5/4 1/9 200 99 2.8 Ex.13 0.02%Pt-silica Ti/W/V =91/5/4 2/8 400 99 5.0 CEx.1 - Ti/W/V =91/5/4 - 0 3 0.6 CEx.2 0.05%Pt-mordenite inert TiO₂ - 10 98 120 CEx.3 - Ti/W/V =91/5/4 - 0 2 1.8 CEx.4 0.05%Pt-silica inert TiO₂ 2/98 10 99 135 CEx.5 0.05%Pt-γ-alumina inert TiO₂ 2/98 10 79 103 CEx.6 0.05%Pd-silica inert TiO₂ 2/98 10 77 125 CEx.7 0.05%Rh-silica inert TiO₂ 2/98 10 64 48 Tables 5 and 6 clearly shows that the catalysts obtained in Examples of the present invention showed high ammonia decomposition ratios as compared with the catalysts obtained in Comparative Examples, and were excellent catalysts free from NOx generation caused by ammonia decomposition. Test Example 4 The catalysts obtained in Example 1, Comparative Example 1 and Comparative Example 2 were respectively cut to sheets having a width of 20 mm and a length of 100 mm. Three sheets taken from each catalyst were packed in a reactor at intervals of 3 mm, and measured for a NOx removal ratio, an unreacted ammonia amount in a reactor outlet and a CO oxidation ratio when the ammonia amount was varied under the conditions shown in Table 7. Differing from the gas treated in Test Examples 1 and 2, the gas to be treated in this Test Example 4 contained no CO. Component Content NO 200 ppm NH₃ varied O₂ 10 % H₂O 6 % CO₂ 6 % SO₂ 2 ppm gas amount 180 liters/h temperature 350°C area velocity (AV) 17 m/h Fig. 9 shows the results. As shown in Fig. 9, when the catalyst of Example 1 was used, the NOx removal ratio was equivalent to that of the catalyst formed of the NOx removal component alone, prepared in Comparative Example 1, when the ammonia amount was increased to increase the NH₃/NO ratio, and yet the ammonia concentration in the reactor outlet was as low as one-digit value in ppm. In contrast, when the catalyst of Comparative Example 1 was used, an increasingly high concentration of ammonia was detected in the reactor outlet as the NH₃/NO ratio increased. On the other hand, when the catalyst of Comparative Example 2, which contained the second component but contained titania having no NOx removal activity, was used, the NH₃ concentration in the reactor outlet was low, and yet it generated a large amount of NOx and showed negative values of the NOx removal ratio. As is clear from the above results, the catalyst according to Example 1 of the present invention is an excellent catalyst due to synergistic effects of the first component and the second component, which shows a high NOx removal ratio equivalent to that of a usual NOx removal catalyst when the NH₃/NO ratio is low, and which can decrease residual ammonia without generating NOx when NOx is used up by a NOx removal reaction. Examples 14 and 15 A second component was prepared in the same manner as in Example 1 except that the H type mordenite was replaced with fine silica particles (trade name: Mycon-F, supplied by Tomita Pharmaceutical Kabushiki Kaisha) (Example 14) or with γ-alumina particles (Sumitomo Chemical Kabushiki Kaisha) (Example 15). Then, catalysts were prepared in the same manner as in Example 1 in a second component/first component mixing ratio of 1/9 except that the second component was replaced with the above-prepared second components. Examples 16 A catalyst was prepared in the same manner as in Example 1 except that the ammonium paratungstate used in the preparation of the first component was replaced with ammonium paramolybdate [(NH₄)₆·Mo₇O₂₄·4H₂O]. Examples 17 and 18 A mordenite supporting palladium or rhodium in an amount of 0.05 wt% was prepared in the same manner as in Example 1 except that the chloroplatinic acid was replaced with a nitric acid solution of palladium nitrate [Pd(NO₃)₃] (Example 17) or with a nitric acid solution of rhodium nitrate [Rh(NO₃)₃] (Example 18). Catalysts were prepared by respectively adding the above-prepared products to the first component prepared in the same manner as in the preparation of Pt-mordenite. Examples 19 - 21 Catalysts were prepared in the same manner as in Example 1 except that the second component/first component ratio was changed from 2/98 to 0.5/99.5 (Example 19), 1/9 (Example 20) and 2/8 (Example 21). Examples 22 - 24 A second component was prepared in the same manner as in Example 1 except that the amount of chloroplatinic acid was changed to 2.66 g. Then catalysts were prepared in the same manner as in Example 1 except that the second component/first component ratio was changed as in Examples 19 to 21. Test Example 5 The catalysts prepared in Examples 1, 4 and 14 to 24 and Comparative Examples 1, 3 and 8 to 12 were measured for NOx removal ratios, unreacted ammonia decomposition ratios and CO removal ratios while the ammonia concentration was set at a constant rate of 280 ppm under the conditions shown in Table 7. Tables 8 and 9 show the results. The unreacted ammonia decomposition ratio was determined by the following equation. Ammonia decomposition ratio (%) = {(NH₃ concentration in outlet) / (NH₃ concentration in inlet - concentration of NH3 used in NOx removal reaction)} × 100 Catalyst Second component First component (atomic ratio) Mixing ratio (weight ratio) Nobel metal content (ppm) Activities (%) NOx removal ratio NH₃ decomposition ratio Ex.1 0.05%Pt-mordenite Ti/W/V =91/5/4 2/98 10 96 94 Ex.4 chloroplatinic acid Ti/W/V =91/5/4 - 50 96 65 Ex.14 0.05%Pt-silica Ti/W/V =91/5/4 1/9 50 85 99 Ex.15 0.05%Pt-γ-alumina Ti/W/V =91/5/4 1/9 50 83 81 Ex.16 0.05%Pt-mordenite Ti/Mo/V =91/5/4 2/98 10 94 91 Ex.17 0.05%Pd-mordenite Ti/W/V =91/5/4 2/98 10 97 74 Ex.18 0.05%Rh-mordenite Ti/W/V =91/5/4 2/98 10 96 66 Ex.19 0.05%Pt-mordenite Ti/W/V =91/5/4 0.5/99.5 2.5 98 48 Ex.20 0.05%Pt-mordenite Ti/W/V =91/5/4 1/9 50 91 98 Ex.21 0.05%Pt-mordenite Ti/W/V 91/5/4 2/8 100 88 99 Catalyst Second component First component (atomic ratio) Mixing ratio (weight ratio) Nobel metal content (ppm) Activities (%) NOx removal ratio NH₃ decomposition ratio Ex.22 0.2%Pt-mordenite Ti/W/V =91/5/4 0.5/99.5 10 97 93 Ex.23 0.2%Pt-mordenite Ti/W/V =91/5/4 1/9 200 72 99 Ex.24 0.2%Pt-mordenite Ti/W/V =91/5/4 2/8 400 41 99 CEx.1 - Ti/W/V =91/5/4 - 0 98 3 CEx.2 0.05%Pt-mordenite inert TiO₂ - 10 -52 98 CEx.3 - Ti/Mo/V =91/5/4 - 0 96 2 CEx.4 0.05%Pt-silica inert TiO₂ 1/9 50 -64 99 CEx.5 0.05%Pt-γ-alumina inert TiO₂ 1/9 50 -29 76 CEx.6 0.05%Pd-mordenite inert TiO₂ 2/98 10 -22 77 CEx.7 0.05%Rh-morde inert TiO₂ nite 2/98 10 13 64 Tables 8 and 9 clearly show that the catalyst according to Examples of the present invention showed high NOx removal ratios and high unreacted ammonia decomposition ratios as compared with the catalysts of Comparative Examples even when gas to be treated contained no CO, and that the catalyst according to the present invention is an excellent catalyst which can prevent the leak of unreacted ammonia.

Claims: A catalyst for purifying an exhaust gas, which comprises a first component having the activity for reducing nitrogen oxides with ammonia and a second component having the activity for forming nitrogen oxide from ammonia and/or the activity for forming carbon dioxide from carbon monoxide, wherein the first component is an oxide of any combination of titanium-tungsten, titanium-tungsten-molybdenum, titanium-vanadium, titanium-tungsten-vanadium, titanium-molybdenum and titanium-molybdenum-vanadium, and the second component is a platinum, palladium or rhodium compound preliminarily supported on a porous material selected from zeolite, alumina and silica, the content of the noble metal in the catalyst being greater than zero but not greater than 1,000 ppm and the ratio of the second component to the first component being 1/99 to 10/90. A process for producing a catalyst recited in claim 1 for purifying an exhaust gas, which comprises adding a salt of at least one member selected from tungsten, tungsten-molybdenum, vanadium, tungsten-vanadium, molybdenum and molybdenum-vanadium to a slurry of hydrous titanium oxides, converting the resultant mixture to a paste while evaporating water with a heating kneader, drying the paste, calcining the resultant product at a temperature between 400°C and 700°C with a powder carrying a noble metal selected from platinum, palladium or rhodium, obtained by immersing a porous material selected from zeolite, silica and alumina in an aqueous solution of chloride, nitrate or amine complex of platinum, palladium or rhodium for ion exchange or by subjecting a mixture of the porous material with the aqueous solution to evaporation to dryness. A method of purifying an exhaust gas containing nitrogen oxides, which is characterized by comprising: adding ammonia to an exhaust gas; contacting the exhaust gas with the catalyst recited in claim 1 to decompose nitrogen oxides and unreacted ammonia. A method of purifiying an exhaust gas in accordance with claim 3 containing nitrogen oxides and carbon monoxide, which is characterized by comprising: adding ammonia to an exhaust gas; contacting the exhaust gas with the catalyst recited in claim 1 to decompose nitrogen oxides and carbon monoxide and unreacted ammonia. A method of purifying an exhaust gas in accordance with claim 3 containing nitrogen oxides, which is characterized by comprising: injecting ammonia into the exhaust gas as an agent for reducing the nitrogen oxides; contacting the exhaust gas with a reducing catalyst for catalytic reduction of the nitrogen oxides in the presence of ammonia and, subsequently, with a second catalyst as recited in claim 1 to decompose unreacted ammonia in the exhaust gas. A method of purifying an exhaust gas in accordance with claim 3 containing nitrogen oxides and carbon monoxide, which is characterized by comprising: injecting ammonia into the exhaust gas as an agent for reducing the nitrogen oxides; contacting the exhaust gas with a reducing catalyst for catalytic reduction of the nitrogen oxides in the presence of ammonia and, subsequently, with a second catalyst as recited in claim 1 to decompose unreacted ammonia and carbon monoxide in the exhaust gas.

Inventors: HAMADAA IKUHISA, KATO YASUYOSHI, KONISHI KUNIHIKO, MORITA ISATO, NAKAJIMA FUMITO, HAMADAA, IKUHISA, KATO, YASUYOSHI, KONISHI, KUNIHIKO, MORITA, ISATO, NAKAJIMA, FUMITO
Applicants: BABCOCK HITACHI KK, BABCOCK-HITACHI KABUSHIKI KAISHA
Application year: 1992
Main classification: B01J23
IPC codes: B01J37, B01D53, B01J23