JPS62234547A - Catalyst for purifying exhaust gas - Google Patents

Abstract

PURPOSE:To obtain the title catalyst for purifying exhaust gas having excellent performance in simultaneously removing hydrocarbons, carbon monoxide, and nitrogen oxides at low temp. after endurance traveling by allowing tin oxide to coexist in a ternary catalyst consisting of platinum and/or palladium, rhodium, and cerium oxide. CONSTITUTION:A refractory inorg. oxide deposited with at least one kind between platinum and palladium and rhodium is mixed with specified amts. of CeO2 and SnO2, dilute nitric acid is added, and the mixture is crushed on a wet basis to obtain a slurry. The slurry is deposited on a monolithic honeycomb carrier to obtain a finished catalyst. From 0.1-10g total of the noble metals and rhodium, 1-150g CeO2, 0.1-100g SnO2, and 20-200g refractory inorg. oxide are deposited per 1l of the catalyst. In addition, activated alumina is preferably used as the refractory inorg. oxide.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a catalyst for purifying exhaust gas. Specifically, the present invention efficiently removes hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NOx), which are harmful components contained in exhaust gas from internal combustion engines such as automobiles, at low temperatures. The present invention relates to an exhaust gas purifying catalyst that is removed in the following manner.

[Conventional technology and problems]

Many catalysts have been proposed for purifying exhaust gas emitted from internal combustion engines such as automobiles.
Three-way catalysts that simultaneously remove nitrogen oxides (hereinafter referred to as NOx) and nitrogen oxides (hereinafter referred to as NOx) have become mainstream. In addition, a three-way catalyst with excellent performance in purifying HC, Co, and NOx at a low source is strongly desired under conditions where exhaust gas temperature is decreasing due to fuel efficiency improvements in automobiles in recent years.

Conventional three-way catalysts contain platinum (Pt) as a catalytically active species.
Precious metals such as , palladium (Pd), and rhodium (Rh) are mainly used, and rare earth metals such as cerium (Ce) are also used to promote the catalytic action of these precious metals.
Catalysts were commonly used in combination with However, with these conventional three-way catalysts, H
The purification performance of C, Co and NOx was not necessarily satisfactory.

[Means for solving problems]

As a result of intensive research, the present inventors have discovered that the conventional three-way catalyst described above can be improved by making tin oxide coexist with a three-way catalyst containing at least one kind of noble metals such as Pt and Pd, and Rh and Ce as active components. It was discovered that the purification performance of HC, Co, and NOx at low temperatures after endurance running, which was an issue in the industry, was dramatically improved, and the present invention was completed.

The catalyst of the present invention exhibits HC at low temperatures even after running for a long time.
The reason for the excellent purification performance of Co, Co and NOx is thought to be that the added tin oxide promotes the transfer of electrons to the noble metal, which is the main active species. Conventionally, exhaust gas using a combination of noble metal and tin oxide As a catalyst for gas purification, an oxidation catalyst using noble metals and tin oxide, tungsten oxide, titanium oxide, or nickel molybdate is disclosed in JP-A-56-141838, but it is difficult to prevent poisoning by lead, etc. This is a proposal for a durable catalyst, and it is quite clear that tin oxide has improved purification performance at low temperatures as in the present invention.

On the other hand, in Japanese Patent Application Laid-open No. 175546/1983.

A heat-resistant oxidation catalyst in which PT is supported on tin oxide calcined at 600°C or higher has been disclosed, but this is a proposal for an oxidation catalyst with excellent heat resistance that uses only PT as an active ingredient, and there is no mention of NOx purification performance. This is different from the three-way catalyst disclosed in the present invention, which also has excellent NOx purification performance. Furthermore, in the present invention, it is not necessary to support pt on tin oxide.

As described above, the noble metal Ce disclosed in the present invention,
It was previously completely unpredictable that a catalyst with a combination of Sn would exhibit excellent HC, Co, and NOx purification performance at low temperatures after endurance running. By supporting transition metal oxides such as Cr), manganese (Mn), iron (Fe), cobalt (Co), and nickel (N1), the effect of electron transfer to and from noble metals by tin oxide can be well controlled. It has also been found that the effect of tin can be made even more pronounced.

[Structure of the invention] The present invention is specified as follows.

(1) Hydrocarbons and monoxide in exhaust gas supported on a honeycomb carrier having a monolithic structure containing a catalytically active material consisting of at least one noble metal such as platinum and palladium, rhodium, cerium oxide, tin oxide, and a refractory inorganic oxide. An exhaust gas purifying catalyst characterized by simultaneously purifying and reducing carbon and nitrogen oxides at low temperatures.

(2) per 1 liter of catalyst, the total amount of at least one noble metal of platinum and palladium and rhodium is 0.1 to 10 Il;
The catalyst according to (1) above, wherein the cerium oxide is supported in an amount of 1 to 1,501 l as CeO□, 0.1 to 100.1 as SnO2, and the refractory inorganic oxide is supported in a range of 20 to 200 l.

(3) The catalyst according to (1) or (2) above, wherein the refractory inorganic oxide is activated alumina.

(4) An integrated structure of a catalytically active material consisting of at least one noble metal such as platinum and palladium, an oxide of at least one element among rhodium, cerium oxide, and a transition metal, tin oxide, and a refractory inorganic oxide. An exhaust gas purifying catalyst characterized in that it simultaneously purifies and reduces hydrocarbons, carbon monoxide, and nitrogen oxides in exhaust gas supported on a honeycomb carrier at a low temperature.

(5) The total amount of at least one noble metal such as platinum and para-zoum and rhodium is 0.1 to 10.9 per liter of catalyst.
．． Cerium oxide is 1 to 150I as CeO□, transition metal oxide is 0.1 to 10% by weight relative to tin oxide, tin oxide is 0.1 to 100N as SnO2, and refractory inorganic oxide is 20 to 100N. The catalyst according to (4) above, characterized in that a range of 200I is supported.

(6) The catalyst according to (4) or (5) above, wherein the refractory inorganic oxide is activated alumina.

(7) The transition metal used with tin oxide is chromium.

The catalyst according to (4) or (5) above, which is at least one of manganese, iron, cobalt and nickel.

The refractory inorganic oxide powder used in the present invention includes alumina, silica, titania, and zirconia.

Examples include alumina-silica, alumina-titania, alumina-zirconia, silica-titania, silica-zirconia, titania-zirconia, and alumina madanesia powder, and use of alumina powder, particularly activated alumina powder, is preferred. Activated alumina has a specific surface area of 50 to 1
Activated alumina of 80 m /I is preferred, and its crystal forms can be gamma, delta, theta, chi, kappa, and eta. Another lantern.

Rare earth elements such as cerium and neodymium or calcium,
0.1 of at least one alkaline earth element such as barium
Activated alumina loaded at ~30% by weight can also be used.

The pt source and Pd source used in the present invention include chloroplatinic acid, dinitrodiammine platinum, platinum sulfite complex salt,
Preferred examples include platinum tetramine chloride, radium nitrate, radium chloride, radium sulfite complex salts, and palladium tetramine chloride. Also, Rh
Preferred sources include rhodium nitrate, rhodium chloride, rhodium sulfate, rhodium sulfite complex, and rhodium ammine complex. The amount of these noble metals supported is
The total of pt, pd, and Rh is 0.1 to 1
A range of 09 is preferred.

The Ce source used in the present invention is CeO2 in the catalyst.
The starting material is not particularly limited as long as it can exist as a starting material. For example, commercially available CeO□, cerium carbonate, cerium hydroxide, cerium oxalate, etc. can be used. Further, a solution of a cerium salt, such as a cerium nitrate solution, may be impregnated and supported on the above-mentioned refractory inorganic oxide. The amount of cerium oxide supported is 1 to 150 S, preferably 10 to 100 J as CeO2 per liter of catalyst.

The tin oxide used in the present invention is not particularly limited and may be commercially available tin oxide, but tin salts such as Sn SO4,
Sn(SO4)2.5nC22,SnCZ,s
Tin oxide obtained by calcining and decomposing tin oxide, or tin oxide obtained by calcining a coprecipitate of tin salt and aqueous ammonia may be used. Furthermore, the above-described refractory inorganic oxide may be impregnated with a solution of tin salt, such as a S n CL4 solution, and then fired, and used as tin oxide.

The amount of tin oxide supported is from 0.1 to 100 g, preferably from 1 to 50.9 g per liter of catalyst.

Tin oxide appears to be introduced into the catalyst in the form of tin dioxide (S nO2), which promotes the transfer of electrons to the noble metal, which is the main active species, in the reaction atmosphere, improving catalyst performance at low temperatures. However, by combining tin oxide with a transition metal oxide in advance and controlling the rate of electron transfer by tin oxide, the catalyst according to the present invention exhibits even better catalytic performance. Transition metals bonded and supported on tin oxide include Cr r Mn and Fe t Co.
, N+ gives good results. Supporting of transition metals is carried out by impregnating tin oxide with a solution of each water-soluble salt and drying and baking. is remarkable.

The honeycomb carrier having an integral structure used in the present invention is
Any material that is normally called a ceramic honeycomb carrier may be used, especially cordierite, mullite, α-alumina, zirconia, titania, titanium phosphate, aluminum titanate, tallite, spodumene, alumino-silicate, magnesium silicate, etc. Honeycomb carriers are preferred, and among them, cordierite carriers are preferred for use in internal combustion engines. Others, stainless steel or Fe −
A monolithic structure made of a heat-resistant metal with oxidation resistance, such as a Cr-A1 alloy, is also used. The shape of the gas passage port (cell shape) may be hexagonal, square, triangular, or corrugated, and the cell density (number of cells/unit cross-sectional area) is 150 to 600 cells/ Square inches are sufficient and give good results.

The catalyst according to the present invention comprises the above-mentioned refractory inorganic oxide,
At least one noble metal of Pt, Pd, Rh, Ce,
It is produced using tin oxide or tin oxide bonded with a transition metal oxide, for example, by the following method.

(1) An aqueous solution of a water-soluble salt of at least one noble metal such as Pt or Pd and an aqueous solution of a water-soluble salt of Rh are mixed and impregnated and supported on activated alumina. This noble metal supported alumina, CeO
A slurry is prepared by mixing a predetermined amount of tin oxide with S no 2 or transition metal Z-carrying bond, adding dilute nitric acid water, and wet-pulverizing the mixture in a ball mill. A finished catalyst is prepared by supporting the slurry on a honeycomb carrier having an integral structure.

(2) Mix a predetermined amount of activated alumina, CeO□, and tin oxide or tin oxide that supports a transition metal, add dilute nitric acid water, and wet-mill it in a ball mill. P
A completed catalyst is prepared by supporting at least one noble metal such as T, Pd, and Rh.

However, in the method for producing a catalyst according to the present invention, the catalytic active layer contains at least one noble metal such as Pt or Pd and Rh.

It goes without saying that the manufacturing method is not limited to the above method as long as it is made of CeO7, tin oxide, and a refractory inorganic oxide.

〔Example〕

EXAMPLES Hereinafter, the present invention will be explained in more detail with reference to Examples, but it goes without saying that the present invention is not limited only to these Examples.

Example 1 A catalyst was prepared using a commercially available cordierite monolith carrier (manufactured by Nippon Insulators Co., Ltd.). The monolith carrier used had a cylindrical cross section with an outer diameter of 33 mm and a length of 76 mL and a volume of about 65 rnl, with a cross section of about 400 gas flow cells per square inch.

Nitric acid aqueous solution of dinitro diammine platinum (Pt content:
1 o o g/l) 12.5 ml and rhodium nitrate aqueous solution (Rh content: 50, ! i'/l) 5 ml
An aqueous solution diluted with 150 ml of pure water and a surface area of 120 m
279 activated alumina powder 120y was thoroughly mixed for 30 minutes, dried at 150° C. for 3 hours, and then calcined in air at 400° C. for 2 hours to prepare a pt- and Rh-containing alumina powder.

Stannous sulfate (SnSO4: Wako Pure Chemical Industries, Ltd.) was calcined in air at 800°C for 5 hours, and 30 g of tin oxide, 75 g of cerium oxide powder, and the alumina powder containing pt and Rh were combined. 2 in a ball mill with dilute nitric acid water
A slurry for coating was prepared by pulverizing at 0 o'clock. The monolithic carrier was immersed in this coating slurry for one minute, and then taken out from the slurry and the excess slurry in the cells was blown with pressure and II air to remove clogging from all cells. The catalyst was then dried at 150°C for 3 hours and then calcined in air at 400°C for 2 hours to obtain a finished catalyst.

This catalyst contains 1 liter of sialium 80I, cerium oxide (
(as CeO2) 50g, tin oxide (as SnO2)
209, Pt O, 83,! i'1Rh Q, 17I was supported.

Example 2 Palladium nitrate aqueous solution (pa content: 100 g/l) 12
, 5 ml and rhodium nitrate aqueous solution (Rh content: 50
Fi/l) 5TILl diluted with 150ml of pure water and the same activated alumina powder 1 used in Example 1.
2ON was thoroughly mixed for 30 minutes, dried at 150°C for 3 hours, and then calcined in air at 400°C for 2 hours to form Pd and R.
An h-containing alumina powder was prepared.

A finished catalyst was obtained in the same manner as in Example 1 using this Pd- and Rh-containing aluminum powder, the same tin oxide 30F used in Example 1, and cerium oxide 75F.

This catalyst contained 5oII of sialumina, 50.9% of cerium oxide (as CeO2) per liter. Tin oxide (as SnO2) 20g, PdO, 83g, RhO, 17,9
was carried.

Example 3 Nitric acid aqueous solution of dinitro diammine platinum (pt content:
100.9/l') 8.9rll, Halacium nitrate water soluble i (Pd content: 1001/l) 3.6rn
l, and rhodium nitrate aqueous solution (Rh content: 509/l!
) 5rn/ with 150rnl of pure water to make an aqueous solution,
The same activated alumina powder 120I used in Example 1 was
Pt, Pd, and Rh-containing alumina powder was prepared by thoroughly mixing for 0 min, drying at 150 °C for 3 h, and then calcining at 400 °C for 2 h in air.

This Pt + Pd and Rh-containing alumina, 30 g of the same tin oxide used in Example 1, and 75 g of cerium oxide
! ! A finished catalyst was obtained in the same manner as in Example 1 using:

This catalyst contains alumina 80I per liter, serif oxide A (
(as CsO□) 50I, tin oxide (as SnO2)
20.9. Pt0.59g, PdO, 24N, Rh
O, 17, and 9 were supported.

Example 4 A finished catalyst was obtained in the same manner as in Example 1, except that 7.5II of tin oxide was used.

This catalyst contains 80I alumina and cerium oxide (per liter).
(as CeO2) 50J tin oxide (as SnO2)
5E, Pt 0.83.9, and Rh 0.17F were supported.

Example 5 In Example 1, a method similar to Example 1 was used except that commercially available tin oxide (5n02: manufactured by Kanto Kagaku Co., Ltd.) was used instead of tin oxide obtained by baking stannous sulfate. , a finished catalyst was obtained.

This catalyst contains 80 g of alumina per 1°, serif oxide A
50 g (as CeO), 20 g of tin oxide (as 5 nOz), 0.83 g of Pt, and Rh 0.17 F were supported.

Example 6 Stannic chloride (5nCt4.x H20 manufactured by Wako Pure Chemical Industries, Ltd.) 34-1li' (15g as 5n02
) dissolved in 150 ml of pure water and 135 g of the same activated alumina powder used in Example 1 were thoroughly mixed for 30 minutes, dried at 150°C for 3 hours, and then calcined in air at 800°C for 5 hours. Thus, tin oxide-containing alumina powder was prepared.

An aqueous solution prepared by diluting 12.5 rnl of a dinitro diammine platinum nitric acid aqueous solution (Pt content: 1001/l), 5 rrtl of a rhodium nitrate aqueous solution (Rh content: 501/l) with 150 tnl of pure water, and the SnO□-containing alumina powder. Mix thoroughly for 30 minutes and incubate at 150°C for 30 minutes.
After drying for an hour, it was calcined in air at 400°C for 2 hours to prepare an alumina powder containing tin oxide, PT, and Rh.

This tin oxide, Pt and Rh containing alumina powder,
Cerium oxide powder 75I! A finished catalyst was obtained in the same manner as in Example 1 using:

This catalyst contains 90g of alumina and cerium oxide (per liter).
(as CeO2) 50,9. Tin oxide (as SnO2) 10 g, Pt0.83. I Rh0.17. ! 7 was carried.

Example 7 An aqueous solution in which 17.9 g of nitrate OA [Cr(NO3)3' 9H20, Wako Pure Chemical Industries, Ltd.] was dissolved in 30 rnl of pure water and the same tin oxide 30I used in Example 1
i was thoroughly mixed for 30 minutes, dried at 150° C. for 3 hours, and then calcined in air at 500° C. for 1 hour to prepare tin oxide containing chromium corresponding to 5% by weight of chromium oxide.

A finished catalyst was obtained in the same manner as in Example 1, except that the chromium-containing tin oxide was used instead of tin oxide.

This catalyst contained 5 ol I of sialumina and 50.9 mol of cerium oxide (as CeO2) per liter. Tin oxide (as 5nOz) 20I, chromium oxide (as Cr203) 1 &, P
t0.83g, Rh O, 17,9 was supported.

Example 8 In Example 7, nitric acid-r7 was used instead of chromium nitrate.
Gun [Mn(NO3)2.6H2o, Wako Pure Chemical Industries, Ltd.] 6.1. ! A finished catalyst was obtained in the same manner as in Example 7, except that Example 9 was used.

This catalyst contains 80g of alumina and cerium oxide A per liter.
(as CeO2) 50.9, soot oxide (as SnO2) 20I, manganese oxide (as Mn0z) 1! i,
0.83.9SRh0.17g of Pt was supported.

Example 9 In Example 7, iron nitrate [:
Pa(No3)3-9H20, Wako Pure Chemical Industries, Ltd.]
A finished catalyst was obtained in the same manner as in Example 7 except that 7.6 g was used.

This catalyst contains 1j of sialumina soy, cerium oxide (
as CeO□) 50.1 tin oxide (as SnO2)
201 iron oxide (as Fe 205) III, PtO
, 83,9, Rh 0.17 g was supported.

Example 10 In Example 7, cobalt nitrate [co(Np3)2.6H20, Wako Pure Chemical Industries, Ltd.] was used instead of chromium nitrate. 5.8. ! A finished catalyst was obtained in the same manner as in Example 7 except that i+ was used.

This catalyst contains 80g of alumina and cerium oxide (per liter).
(as CeO2) 50g, tin oxide (as SnO2)
201 cobalt oxide (coo toshite) IJg, Pt 0
．． 83. li' and Rh0.17.9 were supported.

Example 1l Same as Example 7 except that nickel nitrate/1/[Ni(NO3)2-6H20, Wako Pure Chemical Industries, Ltd.] 5.8II was used in place of chromium nitrate. The completed catalyst was obtained using a method.

This catalyst contains alumina soy and cerium oxide (per liter).
(as CeO2) 50.9. Tin oxide (as SnO2) 20g, nickel oxide/l/ (as NjO) 1.9
, PtO, 83II, Rho, i'7y were supported.

In order to demonstrate that the catalyst of the present invention has excellent purification performance for HC, Co, and NOx at low temperatures, a catalyst that does not contain tin oxide as shown in the following comparative example was prepared.

Comparative Example 1 A finished catalyst was obtained in the same manner as in Example 1, except that tin oxide was not used.

This catalyst contains 80g of alumina and cerium oxide (per liter).
CI! As 02 > 50.9. Pt 0.83.
9. Rh0.171 was supported.

Comparative Example 2 Example 2 except that tin oxide is not used in Example 2.
A completed catalyst was obtained in a similar manner.

This catalyst contains 8o1l of alumina and cerium oxide A per 1l.
(as CeO2) 50.9. Pd0.83.9. R
hO,17,9 was supported.

Comparative Example 3 A finished catalyst was obtained in the same manner as in Example 3, except that tin oxide was not used.

This catalyst contains 80g of alumina and cerium oxide (per liter).
(as CeO2) 50&, Pt0.59g, PdO,2
4g, Rh 0.17,! 9 was carried.

〔Effect of the invention〕

The catalyst performance of the catalysts from Example 1 to Example 1l and the catalysts from Comparative Example 1 to Comparative Example 3 at low temperatures after engine endurance running was investigated.

Endurance running was carried out using a commercially available electronically controlled engine engine (8 cylinders, 4400cc) with each catalyst filled in a multi-converter and connected to the engine's exhaust system. The operating conditions of the engine were steady operation (2800r, p, m,
, -220mHg) for 60 seconds and deceleration (fuel cut) operation for 6 seconds, the converter temperature during steady operation was 750℃, and the durability was 100 hours.The catalyst performance at low temperature is The multi-converter filled with the catalyst after the above endurance running was connected to the exhaust system of a commercially available electronically controlled engine (4 cylinders, 1800 cc), and evaluated under the conditions shown below.

The engine was operated under constant conditions with an average air-fuel ratio of 14°6 (2200r
, p, m, #-350 [Hg], and at this time, an I Hz sine wave type signal was introduced from an external oscillator to the control unit of Ning Shin, and the air-fuel ratio was adjusted to 10.5 A.
A'.

It was vibrated with IH2. Under these constant operating conditions, the catalyst inlet exhaust gas temperature is continuously changed from 200°C to 450°C by a heat exchanger installed before the catalytic converter in the exhaust system, and the catalyst inlet gas composition at that time and the outlet gas The composition of each catalyst was analyzed to determine the purification rate of HC# Co and NOx. The obtained HC, Co, and NOx purification rates versus catalyst inlet temperature are plotted on a graph, and the catalyst inlet temperatures at which the purification rates are 50% and 80% (T58° and T8o, respectively) are determined to determine the catalyst's purification performance at low temperatures. was used as the evaluation standard.

Next, Table 1 shows the results obtained by the above evaluation method.

As is clear from Table 1, the catalysts of Examples 1 to 6 according to the present invention have higher HC,
It can be seen that activity is exhibited from a low temperature of about 40° C. at 50% and 80% conversion rates of Co and NOx. Furthermore, the 1 liter catalyst from Example 7 using tin oxide supporting a transition metal had even better purification performance for HC, Co, and NOx at low temperatures. These results show that tin oxide and metallurgy.

It is clear that tin oxide supporting a transfer metal dramatically improves catalyst performance at low temperatures after endurance running.

As described above, the present invention includes at least one noble metal such as platinum or palladium, and rhodium or ceria.

An exhaust gas purification catalyst in which a refractory inorganic oxide and tin oxide or a transition metal-supported tin oxide is supported on a Noronicum carrier having an integral structure has a high performance in purifying I(C, Co, and NOx) at low temperatures after endurance running. The catalyst is extremely superior to conventional catalysts, and is effective in purifying exhaust gas not only for conventional cars but also for fuel-efficient cars and small cars where exhaust gas temperature tends to drop.

Patent applicant: Agent of Nippon Shokubai Chemical Co., Ltd.
Tsuyoshi Yamaguchi, 1L, procedural correction
(4) (Voluntary) May Zq, 1986

Claims (7)

[Claims] (1) At least one noble metal of platinum and palladium;
Hydrocarbons, carbon monoxide, and nitrogen oxides in exhaust gas are simultaneously purified and reduced at low temperatures by supporting a catalytically active material consisting of rhodium, cerium oxide, tin oxide, and refractory inorganic oxides on a honeycomb carrier with an integral structure. An exhaust gas purification catalyst characterized by: (2) Per 1 liter of catalyst, the total amount of at least one noble metal such as platinum and palladium and rhodium is 0.1 to 10 g, cerium oxide is 1 to 150 g as CeO_2, tin oxide is 0.1 to 100 g as SnO_2, fire resistance The catalyst according to claim 1, wherein the inorganic oxide is supported in an amount of 20 to 200 g. (3) The catalyst according to claim (1) or (2), wherein the refractory inorganic oxide is activated alumina. (4) at least one noble metal of platinum and palladium;
At least one of rhodium, cerium oxide, and transition metals
Hydrocarbons, carbon monoxide, and nitrogen oxides in exhaust gas are simultaneously purified at low temperatures by supporting a catalytically active material consisting of oxides of certain elements, tin oxide, and refractory inorganic oxides on a honeycomb carrier with an integral structure. A catalyst for purifying exhaust gas that is characterized by reducing exhaust gas. (5) Per 1 liter of catalyst, the total amount of at least one noble metal such as platinum and palladium and rhodium is 0.1 to 10 g, cerium oxide is 1 to 150 g as CeO_2, and the transition metal is 0.0 g to tin oxide. 1 to 10% by weight,
The catalyst according to claim (4), wherein the tin oxide is supported in an amount of 0.1 to 100 g as SnO_2 and the refractory inorganic oxide is supported in an amount of 20 to 200 g. (6) The catalyst according to claim (4) or (5), wherein the refractory inorganic oxide is activated alumina. (7) The catalyst according to claim (4) or (5), wherein the transition metal used together with tin oxide is at least one of chromium, manganese, iron, cobalt, and nickel.