GB1568594A - Catalyst composition for use in reduction of nitrogen oxides - Google Patents

Description

(54) CATALYST COMPOSITION FOR USE IN REDUCTION OF NITROGEN OXIDES (71) We, MITSUI PETRO CHEMICAL INDUSTRIES LTD., a Japanese Body Corporate of 2-5, 3-chome, Kaumigaseki, Chiyoda-ku, Tokyo, Japan, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to a process for the catalytic vapor phase reduction of nitrogen oxides to nitrogen and water, and to an improved catalyst composition which retains high catalytic activity stably over long periods of time, has superior strength that can withstand a destructive force exerted during its handling and use, and can catalyze the reduction of nitrogen oxides contained in waste and exhaust gases to hramless nitrogen and water with marked industrial advantage.

The catalyst composition of the invention retains its high catalytic activity and superior resistance to poisoning for long periods of time in the reduction of nitrogen oxides. It has moldability into any desired shape. The molded catalyst composition has high physical strenth sufficient to withstand the physical destructive force exerted on it during its handling and transportation, and its use in various operations such as charging into a reducing zone or reducing treatment. The catalyst composition also permits the decrease of pressure drop and maintains a low pressure drop over long periods. Moreover, the low cost of the catalyst composition adds to the industrial advantages of the process of this invention.

Nitrogen oxides (mainly denoting NO and NO2; generically called "nitrogen oxides" in the present application) are contained in waste and exhaust gases of various sources, such as boiler exhausts and motor vehicle exhausts. They are toxic to humans, and are considered to be substances which will cause air pollution in the form of, say, photochemical smog, affecting not only the workers of industrial establishments but also the general public. This is a serious social problem, and needs to be solved by exploiting effective measures for removing nitrogen oxides from various exhaust and waste gases or converting them to harmless compounds.

A number of methods have heretofore been suggested in an attempt to meet this need. These prior methods can be divided roughly into a wet absorbing process by which nitrogen oxides are removed by causing them to be absorbed by, for example, an alkali aqueous solution or an aqueous solution of a bisulfite, and a dry catalytic reduction process by which nitrogen oxides are reduced using a catalyst.

Of these methods, a gas-phase catalytic reduction method using ammonia is currently considered to be most advantageous for commercial operations.

The present invention relates to a process for the vapor phase catalytic reduction of nitrogen oxides by this type of process, and to a catalyst composition used for it.

Reduction of nitrogen oxides by this type of process may be performed by contacting an exhaust gas containing the nitrogen oxides with an ammonia gas at elevated temperatures in the presence of a catalyst, thereby to reduce the nitrogen oxides to harmless nitrogen and water, as schematically shown below.

6NO + 4NH3 < 5N2 + 6H2O 6NO2 + 8NH3 < 7N2 + 12H2O The reducing method using ammonia is called a selective reducing method because nitrogen oxides react with ammonia selectively to some extent even in the presence of oxygen. It has the advantage that the consumption of ammonia by oxygen present in exhaust gases can be inhibited.

Known catalysts for use in the vaporphase catalytic reduction of nitrogen oxides of the above type include a catalyst comprising vanadium oxide or molybdenum oxide supported on alumina or silica gel (U.S. Patent 3,279,884), a catalyst comprising an oxide of a transition metal (excepting noble metals), such as copper oxide, iron oxide, chromium oxide, cobalt oxide or nickel oxide, supported on alumina of a high surface area (Japanese Laid-Open Patent Publication No. 75464/74), and a catalyst comprising an oxide of a transition metal (excepting noble metals), such as vanadium oxide or cerium oxide, supported on titanium oxide (Japanese Laid-Open Patent Publications Nos. 51996/75 and 65467/75).

The catalyst comprising a transition metal oxide (excepting noble metal oxides), such as vanadium oxide, molybdenum oxide, copper oxide or iron oxide, supported on an ordinary carrier such as alumina or silica gel exhibits high catalytic activity in the early stage, but has the defect that it cannot retain its activity stably over long periods of time because it is poisoned by sulfur oxides (mainly denoting SO2 and SO,; generically called "sulphur oxides" in the present application) which are normally contained in nitrogen oxide-containing exhaust gases.

The catalyst comprising a transition metal oxide (excepting noble metal oxides), such as vanadium oxide or cerium oxide, supported on titanium oxide has good catalytic activity and good resistance to poisoning by sulfur oxides. On the other hand, it has poor moldability into the desired shape, and poor physical strength with susceptibility to destruction by a physical destructive force exerted during its handling or use. Because of the low physical stength, this catalyst is subject to various troubles during use for long periods, and the restriction on its shape mades it difficult to reduce pressure drop and to maintain a low pressure drop for long periods of time.

The present invention provides a catalyst composition for use in the vapor phase reduction of nitrogen oxides with ammonia, said catalyst composition comprising, as non-noble metal compound, an oxide or sulfate of a metal of Group IB, VIB, VB, VIIB or VIII of Mendelejeff's Periodic Table (short period) or of cerium, supported on a shaped carrier comprising a titanium oxide, a clay mineral having an average particle size of 0.1 to 100 microns and an inorganic fibrous material, the amount of the inorganic fibrous material being 0.5 to 25 0 by weight based on the total weight of the titanium oxide and clay mineral.

The catalyst composition of the invention retains high catalytic activity and superior resitance to poisoning in the reduction of nitrogen oxides over long periods of time.

The catalyst composition can be molded into various desired shapes, and has satisfactory physical strength sufficient for withstanding physical destruction during its handling or use. When used in the catalytic vapor-phase reduction of nitrogen oxides, the catalyst composition of the invention has the ability to permit the decrease of pressure drop during a reducing operation, maintains a low-pressure drop over long periods of time, has an increased activity per unit weight of catalyst, prevents the masking of catalyst surface and blockage of the catalyst layer due to dust present in waste gases to be treated and provides for continued stability of treatment.

The term "non-noble metal" as used herein denotes metals of the specified Groups excepting noble metals.

The term "a titanium oxide" used herein is a term which denotes hydrous titanium oxides, such as orthotitanic acid or metatitanic acid, obtained by hydrolyzing titanium salts such as titanium chloride or titanium sulfate, and titanium oxides obtained by drying these hydrous titanium oxides at room temperature to l,000 C. It includes oxides of titanium such as titanium (II) oxide (titanium monoxide, TiO), titanium (III) oxide (titanium sesquioxide, Ti203), titanium (IV) oxide (titanium dioxide, TiO2), ortho-titanic acid (a-titanic acid), and meta-titanic acid (p-titanic acid), and mixtures of these.

Examples of the clay mineral used in the present invention are montmorillonite-type clay minerals such as montmorillonite, bentonite, acid terra alba, activated clay or fuller's earth; kaolin-type clay minerals such as German kaolinite (occurring in west Germany), kibushi-clay (occurring in Aichi, Japan), Gaerome clay (occurring in Aichi, Japan), Georgia kaolin, and kaolinite; halloysite-type clay minerals such as halloysite or hydrated halloysite; pyrophyllite-type clay minerals such as agalmatolite or pyrophyllite; sericite-type clay minerals such as Murakami clay (occurring in Niigata, Japan); and mixtures of these clay minerals in optional combinations.

The good moldability and physical strength of the catalyst composition are results of the clay mineral having an average particle size of 0.1 to 100 microns.

The non-noble metal compound used in this invention is an oxide or sulfate of a metal selected from metals of Groups IB, VIB, VB, VIIB and VIII of Mendelejeff's Periodic Table (short period) and cerium.

Oxides of metals selected from copper, vanadium, chromium, molybdenum, tungsten, manganese, iron and cerium are especially preferred. These compounds can be used either alone or as mixtures of two or more.

The oxidation number of the metal oxide is not restricted in particular, and oxides of any oxidation number are feasible. Specific examples of suitable non-noble transition metal compounds are vanadium pentoxide (V203), vanadium dioxide (VO2), vanadium trioxide (V2O3), cupric oxide (CuO), ferric oxide (Fe2O3), ferrosoferric oxide (Fe3O4), molybdenum trioxide (MoO3), tungsten dioxide (WO2), and tungsten trioxide (WO3).

A preferred catalyst composition of this invention comprises 0.1 to 20% by weight, preferably 0.5 to 15% by weight, of the nonnoble transition metal compound, 1 to 60% by weight, preferably 5 to 20% by weight, of the clay mineral, and 20 to 90% by weight, preferably 70 to 90% by weight, of the titanium oxide, all proportions being based on the weight of the catalyst composition.

The shaped carrier of the catalyst composition of this invention further includes an inorganic fibrous material, optionally mixed with a silica hydrogel or silica sol. The amount of this additional carrier component is 0.5 to 25% by weight based on the total weight of the titanium oxide and the clay mineral.

Preferred proportions of the ingredients of the catalyst composition of this invention are, for example, as follows: Titanium oxide 60 to 95% by weight Clay mineral 5 to 30% by weight Additional carrier component 3 to 10% by weight Non-noble metal compound 3 to 15% by weight The inorganic fibrous material as the other carrier component may be inorganic fibrous materials of natural origin and artificial or synthetic origin. Examples of the inorganic fibrous material are glass wool, glass fibers, rock wool, kaowool, asbestos, and mixtures of at least two of these. Preferably, the inorganic fibrous material has a fiber length of 0.01 to 200 mm and a fiber diameter of not more than about 1 mm. It is especially preferred that the fiber length be at least about 10 times the fiber diameter.

The silica hydrogel or silica sol which may also be present as an additional carrier component mixed with the inorganic fibrous material is preferably in the form of fine particles having an average particle size of not more than 10 my, for example, 0.1 to 10 m,u, in order to increase the thermal stability of the catalyst.

The silica hydrogel and silica sol, as used in the invention, contain at least 50% by weight of a silica component, and may embrace a hydrogel or sol of silica-alumina or silica-magnesia containing at least 50% by weight of a silica component.

The catalyst composition of this invention preferably is in the form of hollow cylinders.

The catalyst composition of this invention is especially suitable for fabrication into a hollow cylindrical shape because it can be molded into any desired shape and has superior physical strength. The previously suggested catalysts consisting of a nonnoble metal compound supported on a titanium oxide carrier cannot be fabricated into such a shape because of their poor moldability and low physical strength, and catalysts feasible for practical purposes cannot be obtained.

The cross-sectional shape of the hollow cylindrical carrier of the catalyst composition of this invention is preferably circular or nearly circular. Generally, the catalyst pellets have such a shape as will permit minimization of the areas of contact between the pellets when they are packed in a reducing zone with their axes of cylinder aligned in one direction, namely such a shape as will allow sufficient spaces among the cylindrical pellets, and for example, they may have a polygonal (e.g. heptagonal or more) cross-sectional shape. It should be understood that these shapes are also included within the term "cylindrical" as referred to in the present application.

It is especially preferred that the pellets of the catalyst composition of this invention molded into a hollow cylindrical shape especially preferably have an inside diameter of 5 to 40 mm, especially 15 to 40 mm, and an outside diameter-to-inside diameter ratio of from 1.2 to 1.6. The height of each cylinder is preferably at least about 100 mm, for example, 100 to 3,000, more preferably 200 to 1,000 mm.

With conventional catalyst compositions used to reduce nitrogen oxides with ammonia in the vapor phase, attempts are made to minimize the particle diameter of the catalyst pellets in order to increase the apparent surface area of the catalyst composition per unit weight thereof. Thus, these catalyst compositions are generally used in the form of small spheres, tablets or cylindrical pellets each having a diameter and a height of 5 to 10 mm. When a catalyst of such a shape is used as packed in a fixed bed reactor, dust normally contained in a fairly large amount in nitrogen oxidecontaining waste gases tends to cause the masking of the catalyst surface or the blockage of the catalyst-packed layer. In order to avoid these disadvantages, it has been the previous practice to modify the reactor itself by using complicated and expensive means in designing and production. In contrast, the present invention can afford a catalyst composition having superior catalytic activity and being of a cylindrical shape as a result of marked improvement in moldability and physical strength. The aforesaid masking or blocking troubles can be easily avoided just by packing the catalyst composition into a reactor in such a manner that the axes of the hollow cylindrical pellets are aligned in the direction of gas flow. There is no need to subject the reactor to the complicated and expensive modification.

The catalyst composition of this invention in the form of hollow cylindrical pellets of the suitable sizes described above is especially preferred because it conveniently avoids the aforementioned troubles, and exhibits superior catalytic activity and resistance to poisoning over long periods of time in addition to having a good efficiency of contact with gases and high physical strength, and permitting the decrease of pressure drop.

The catalyst composition of this invention can be prepared, for example, by mixing an aqueous solution of a water-soluble salt of a non-noble transition metal, such as nitrates, carbonates, oxalates or ammonium salts, the titanium oxide, and the clay mineral with or without the additional carrier component described hereinabove, shaping the mixture into the desired shape (for example, by extrusion molding), drying the molded mixture, and calcining it. Alternatively, it can be prepared by first preparing a shaped carrier by kneading the titanium oxide and clay mineral with or without the additional carrier component in the presence of a small amount of water, molding the mixture into the desired shape (for example, by extrusion molding), drying the molded mixture and calcining it; impregnating the shaped carrier with an aqueous solution of the water-soluble salt of the transition metal; drying it at 20 to 1000C. for 10 to 50 hours; and calcining the dried product.

Calcination can be performed, for example, at a temperature of at least 100"C., usually 300 to 700"C. In the catalyst preparing method described, a dissolving or mixing aid such as monoethanolamine may be added at the time of kneading the individual components of the catalyst or starting materials for these components.

The shape of the catalyst can be a sphere, tablet, cylinder, ring, or honeycomb.

Preferably, the catalyst composition, as stated hereinabove, is in the form of hollow cylindrical pellets having a fairly large size.

According to the present invention, there can be provided a process for the catalytic vapor-phase reduction of nitrogen oxides contained in a waste gas, which comprises contacting said waste gas with a catalyst composition of the invention in the presence of ammonia gas at a temperature of 200 to 6000C.

The catalytic vapor-phase reduction of the nitrogen oxide-containing waste gas can be performed by introducing a gaseous mixture of it with 0.5 to 5 moles, preferably 1 to 2 moles, per stoichiometrically required mole of the nitrogen oxides to be removed through a reaction zone packed with the catalyst composition. Alternatively, this can be performed by introducing the nitrogen oxide-containing waste gas and ammonia simultaneously from separate openings through the catalyst layer. Any of moving bed, fluidized bed and fixed bed can be used in the reaction, but the use of a fixed bedsystem is preferred.

The reaction temperature somewhat differs according to the proportions of the ingredients of the catalyst composition. The suitable reaction temperature is generally 200 to 6000C., preferably 250 to 4500 C.

When the reaction is carried out at a temperature exceeding 600"C., part of the ammonia used as a reducing agent tends to react with oxygen present in the exhaust gas to form nitrogen oxides. On the other hand, if the reaction temperature is lower than 200"C., there is a tendency to the formation of ammonium nitrate or ammonium nitrite, and in the presence of SO2, sulfates or sulfites tend to be formed, or NO and NO2 cannot be reduced to nitrogen and nitrogen suboxide (N2O) is formed as a by-product.

Hence, temperatures outside the specified range are undesirable.

The reaction time, which is expressed in terms of space velocity, differs according to the proportions of the catalyst ingredients.

Usually, the space velocity is 1,000 to 100,000 Nm3, preferably 5,000 to 30,000 Nm3, per hour per m3 of catalyst.

The reaction pressure may be atmospheric, reduced or elevated pressures.

In a preferred embodiment of the present invention in which hollow cylindrical catalyst pellets are used, it is preferred to arrange the catalyst pellets regularly in a catalytic reaction zone so that the axes of the hollow cylindrical pellets correspond substantially with the direction of the flow of the feed gas in the reaction zone.

Specifically, the catalyst pellets may be arranged in a square shape or diamond shape. In this manner of arrangement, it is desirable to stack the catalyst pellets coaxially to a suitable thickness. As a result, not only the inside surfaces of the hollow portions of the catalyst pellets, but also their outside surfaces can be effectively utilized.

The packing of the hollow cylindrical catalyst pellets can be performed by a close contact method in which they are arranged so that their outside surfaces contact one another partly, or by a space packing method in which they are arranged with suitable spaces among them to avoid contact. The latter method of arrangement can be practised, for example, by extending wires within a reactor to support the catalyst pellets properly; or by supporting them by any other suitable supports.

According to the process of this invention, a high ratio of nitrogen oxide removal can be maintained for very long periods of time even when reducing nitrogen oxides in a waste gas, such as a boiler exhaust gas, consisting of 100 to 1,000 ppm of nitrogen oxides (mainly NO), 200 to 2,000 ppm of sulfur oxides (mainly SO2) 1 to 8% by volume of oxygen, 5 to 20% by volume of CO2, and 5 to 20% by volume of steam in the presence of ammonia. In the treatment of waste gases containing large quantities of dusts, for example, boiler exhaust gases, the use of hollow cylindrical catalysts of this invention can advantageously avoid masking or blocking of the catalyst layer, and the catalysts can be used over long periods of time. Hence, the present invention is of great commercial .significance.

The following examples illustrate the present invention in greater detail.

The mechanical strength of the catalyst is expressed in terms of pressure resistant strength which is measured by the following method. The catalyst is compressionmolded (compression ratio 24) into cylindrical pellets each having a diameter of 5 mm and a height of 3.8 mm. The destruction strength of each of the cylindrical pellets is measured by a Kiyatype hardness tester. When the catalyst is in the form of hollow cylindrical pellets, they are cut so that the length of each pellet in the direction of its hollow axis equals the outside diameter of its cross-section, and the destruction strength of each hollow cylindrical catalyst pellets in its diameterical direction is measured by a Kiya-type hardness tester.

Example 1.

A 100-liter stainless steel kneader equipped with a steam jacket was charged with 60 kg of meta-titanic acid containing 30 O,/o of titanium oxide, and with stirring, the pH of the material was adjusted to 8.5 with a 15 % aqueous solution of ammonia.

Separately, 2.75 kg of ammonium metavanadate was dissolved in 25 kg of hot water to form a solution having a concentration of about 10 %.

The neutralized meta-titanic acid was stirred, and mixed with ammonium metavanadate. Then, 3.75 kg of kaolin having a particle size of 0.1 to 2 microns (occurring in Shipaha) and 1.25 kg of glass fibers (Nittobo Glass Fiber Chopped Strands, CS-3E-221) were added, and sufficiently mixed with the solution. The resulting slurry was heated, masticated, concentrated to a water content suitable for fabrication and then molded by an extruder into pellets having a diameter of 5.0 mm and a length of 5.0 to 6.0 mm. The pellets were dried overnight at 1100C, and calcined in the air at 400"C for 3 hours. The catalyst so prepared was packed in an amount of 2 liters into a reaction tube having an inside diameter of 10 cm and a height of 50 cm, and a mixture consisting of 300 ppm of ammonia and a gas simulating a typical boiler exhaust gas, i.e. a gas consisting 300 ppm of NO, 300 ppm of SO2, 15 % by volume of CO2, 12 % by volume of H2O, and the remainder being N2, was passed through the catalyst layer at a temperature of 350"C and a space velocity of 30,000 hr-'. The NO removal ratio at this time is shown in Table 1. The NO removal ratio was also determined after continuing this reaction for 500 hours, and the results are also shown in Table 1.

The pressure resistant strength of the catalyst and its NO removal ratio were determined, and the results are shown in Table 1.

TABLE 1

Example 1 Active ingredient Type V2O5 % by weight 8 TiO2 (wt. %) 72 Catalyst Type Kaolin occurring ingredients Clay in Shipaha Clay Particle size (,L) 0.1-2 % by weight 15 Glass fibers (% by weight) 5 Pressure resistant strength (kg) 35.4 Initial 90 NO removal ratio (So) After 500 hours 90 Examples 2 and 3 and Comparative Examples 1 and 2.

A 100-liter stainless steel kneader equipped with a steam jacket was charged with 60 kg of meta-titanic acid containing 30 % of titanium oxide, and with stirring, the pH of the material was adjusted to 8.5 with a - 15 % aqueous solution of ammonia.

Separately, 2.75 kg of ammonium metavanadate was dissolved in 25 kg of hot water to form a solution having a concentration of about 10 %.

The neutralized meta-titanic acid was stirred and mixed with ammonium metavanadate. Then, 3.75 kg of kaolin occurring in Shipada and having a particle diameter of 0.1 to 2 microns and 1.25 kg of glass fibers (Glass fiber chopped strands, CS-3E-221; a product of Nittobo, Japan) were added to the mixture, and they were thoroughly mixed. The slurry was heated, masticated, and concentrated to water content suitable for molding. The mixture was then molded by an extruder into hollow cylindrical pellets each having an outside diameter of 33.0 mm, an inside diameter of 23.0 mm and a height of 300 mm. The strength of this catalyst was measured by the method described hereinabove, and its strength in the diametrical direction is shown in Table 2.

Eighty-one cylindrical catalyst pellets obtained were packed into a reactor in a square shape in intimate contact with one another, and stacked concentrically to a height of 2.7 m (9 layers). A gaseous mixture of a boiler exhaust gas composed of 230 ppm of NO, 800 ppm of SO2, 12% by volume of CO2, 10% by volume of H2O and the remainder being N2 and containing 150 mg/Nm3 dry of dust with 230 ppm of ammonia was passed into the reactor at a space velocity of 5,000 hr-1 and a temperature of 350 C.

The NO removal, the liquid velocity (L.V.) and the pressure drop at this time are shown in Table 2.

For comparison, a V2Os-TiO2-glass fiber catalyst was prepared by the same method as in the preparation of catalyst in Example 3 except that the clay was not used. The catalyst was molded into hollow cylindrical pellets each having an inside diameter of 23 mm, an outside diameter of 33 mm, and a height of 300 mm. These cylindrical pellets were packed closely in a reactor in the same way as in Example 3, and quite the same reaction as in Example 3 was performed.

The strength of the hollow cylindrical catalyst, the NO removal ratio, the liquid velocity (L.V,), and the pressure drop are shown in Table 2.

TABLE 2

Example Catalyst ingredients Strength NO removal ratio Pressure drop (E.x.) or (% by weight) of (%) (mm Aq) Comparative catalyst Example (diameter) L.V. S.V.

Glass After After (CE.) V2O5 TiO2 Clay (kg) (m/sec.) (hr-1) Initial Initial fiber 500 hours 500 hours Ex. 2 8 72 15 5 10 86 5,000 92 92 80 80 CE. 1 9 81 0 10 4 86 5,000 92 92 80 80 Ex. 3 8 77 15 0 6 86 5,000 92 92 80 80 CE. 2 8 92 0 0 1 86 5,000 91 89 80 100 Examples 4 and 5 The procedure of Example 2 was repeated except that the composition of the catalyst was changed as shown in Table 3.

The results obtained are shown in Table 3.

Example 6.

A catalyst was preapared in the same way as in Example 2 except that a hydrogel containing silica with a size of 2.51 m was further added as an additional carrier component. Using this catalyst, the same reaction as in Example 2 was performed except that the reaction temperature was changed to 420 C. The results are shown in Table 3.

TABLE 3

Composition of the catalyst (% by weight) NO removal ratio (%) Type and Clay mineral Additional carrier component amount Amount Strength (wt. %) of of After of active TiO2 Type and amount Size Type and amount Size catalyst 500 Example ingredient (wt. %) (wt. %) ( ) (wt. %) (m ) (kg) Initial hours 4 V2O5 (8) 72 Kaolin occurring 0.1-2 Asbestos - 9.5 91 91 in <S

Claims (12)

1. WHAT WE CLAIM IS:1. A catalyst composition for use in the vapor phase reduction of nitrogen oxides with ammonia, said catalyst composition comprising, as non-noble compound, an oxide or sulfate of a metal of Group IB, VIB, VB, VIIB or VIII of Mendelejeff's Periodic Table (short period) or of cerium, supported on a shaped carrier comprising a titanium oxide, a clay mineral having an average particle size of 0.1 to 100 microns and an inorganic fibrous material, the amount of the inorganic fibrous material being 0.5 to 25 % by weight based on the total weight of the titanium oxide and clay mineral.
2. 2. A catalyst composition according to claim 1, wherein the inorganic fibrous material is glass wool, glass fiber, rock wool, kaowool or asbestos.
3. 3. A catalyst composition according to claim 1 or 2, wherein the shaped carrier also contains a silica hydrogel or silica sol having an average particle size of 0.1 to 10 m,u.
4. 4. A catalyst composition according to any of the preceding claims, wherein the non-noble metal compound is an oxide of copper, vanadium, chromium, molybdenum, tungsten, manganese, iron or cerium.
5. 5. A catalyst composition according to any one of claims 1 to 4 wherein the clay mineral is montmorillonite, kaolin, halloysite, pyrophyllite or sericite.
6. 6. A catalyst composition according to any one of the preceding claims which contains 0.1 to 20% by weight of the nonnoble metal compound, 1 to 60% by weight of the clay mineral, and 20 to 90% by weight of the titanium oxide, all based on the weight of the catalyst composition.
7. 7. A catalyst composition according to any one of the preceding claims which is in the form of hollow cylindrical pellets.
8. 8. A catalyst composition according to claim 7 wherein each hollow cylindrical pellet has an inside diameter of 5 to 40 mm, and the ration of its outside diameter to its inside diameter is from 1.2 to 1.6.
9. 9. A catalyst composition according to claim l substantially as described in any one of the Examples.
10. 10. A process for the catalytic vapor phase reduction of nitrogen oxides in a waste gas, which comprises contacting the waste gas containing nitrogen oxides with a catalyst composition as claimed in any one of the preceding claims in the presence of ammonia gas at a temperature of 200 to 6000C.
11. 11. A process according to claim 10 wherein the waste gas is a boiler exhaust gas.
12. 12. A process according to claim 10 substantially as described in any one of the Examples.