JPH01315340A - Production of catalyst for purifying exhaust gas - Google Patents

Abstract

PURPOSE:To increase ability to purify exhaust gas contg. hydrocarbon, CO and NOx by supporting a catalytically active material contg. Pt and/or Pd, Rh, CeO2 and a fireproof inorg. oxide on a honeycomb carrier and by further supporting SnO2. CONSTITUTION:A catalytically active material contg. Pt and/or Pd, Rh, CeO2 and a fireproof inorg. oxide is supported on a honeycomb carrier having a monolithic structure by coating. The carrier is then dipped in an aq. soln. of a water soluble tin compd. and dried and/or calcined to further support SnO2 and a catalyst for purifying exhaust gas is produced. The pref. total amt. of Pt and/or Pd and Rh supported is 0.1-10g per 1l catalyst and 1-150g CeO2, 0.01-50g SnO2 and 20-200g fireproof inorg. oxide are preferably supported.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a catalyst for purifying exhaust gas. To be more specific, the present invention simultaneously removes hydrocarbons (HC), oxidized aphrodisiacs (Co), and nitrogen oxides (NOx), which are harmful components contained in exhaust gas from internal combustion engines such as automobiles.
The present invention relates to a catalyst for purifying exhaust gas that efficiently removes exhaust gas at low temperatures.

[Conventional technology and problems]

Conventionally, many catalysts for purifying exhaust gas emitted from internal combustion engines such as automobiles have been proposed.
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, in a situation where exhaust gas temperature is decreasing due to fuel efficiency improvements in automobiles in recent years, there is a strong demand for three-way catalysts that have excellent purification performance for HC, CO, and NOX at low temperatures.

Conventional three-way catalysts contain platinum (Pt) as a catalytically active species.
, palladium (Pd), rhodium (Rh), etc. are mainly used, and rare earth metals, especially 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 ClC0 and NOX was not necessarily satisfactory.

Furthermore, as an exhaust gas purifying catalyst using a noble metal and tin oxide, Japanese Patent Application Laid-Open No. 141838/1983 discloses an oxidation catalyst using a negative metal and tin oxide, tungsten oxide, titanium oxide, or nickel molybdate. However, this proposal is for a catalyst that is resistant to poisoning by lead, etc., and there is no knowledge that tin oxide has improved purification performance at low temperatures as in the present invention.

In addition, in Japanese Patent Application Laid-open No. 60-175546, 600°C
Although a heat-resistant oxidation catalyst in which PT is supported on calcined tin oxide is disclosed above, this is a proposal for an oxidation catalyst with excellent heat resistance that uses only PT as an active ingredient, and NOx purification performance is not mentioned at all. No, the present invention discloses NO.
This is different from a three-way catalyst, which also has excellent X purification performance.

Furthermore, JP-A No. 62-234547 discloses a method for simultaneously purifying HC, Go, and NOx supported on a honeycomb carrier having an integral structure with a catalytically active material consisting of a noble metal, tin oxide, an inorganic refractory oxide, and cerium oxide at a low temperature. There is a proposal for a method of preparing an exhaust gas purification catalyst that reduces the amount of exhaust gas. This method is a catalyst preparation method that uses tin oxide as a starting material, mixes it with a refractory inorganic material supporting a precious metal, and cerium oxide, and supports the mixture on a honeycomb carrier. HC at low temperature after running
As a three-way catalyst required to purify lGo and NOx, it was not necessarily satisfactory.

Therefore, an object of the present invention is to improve the drawbacks of conventional three-way catalysts containing tin oxide.

[Means for solving problems]

As a result of intensive research to achieve the above object, the inventors of the present invention found that
When producing a three-way catalyst containing at least one metal selected from the group consisting of Pt and Pd, Rh, cerium oxide and tin oxide as active ingredients, the above noble metal and cerium oxide are supported in a film on a honeycomb carrier. The inventors have found that the catalyst obtained by immersing the catalyst in an aqueous solution of a water-soluble tin compound, drying and/or calcining the catalyst has excellent ternary properties at low temperatures.

That is, in the present invention, since a water-soluble tin compound is used, tin oxide exists in a more highly dispersed state in the catalyst supporting layer than in a case where tin oxide is used as a starting material. Therefore, the interaction between the noble metal and tin oxide is promoted, and even when a small amount of tin is used, the low-temperature purification performance is further improved compared to the conventional three-way catalyst.

Furthermore, the present inventors have discovered that water-soluble tin compounds as well as water-soluble chromium (Cr), manganese (Mn), and iron (Fe
), cobalt (CO), and nickel (Ni).
They discovered a three-way catalyst with excellent purification performance for x, and completed the present invention.

The invention is thus specified as follows.

(1) A catalytically active material containing at least one noble metal from the group consisting of platinum and palladium, rhodium, cerium oxide, and a refractory inorganic oxide is supported on a henicum carrier having an integral structure, and a water-soluble A method for producing an exhaust gas purifying catalyst, characterized in that tin oxide is supported on the coating by immersing it in a water-soluble tin compound and firing it.

(2) The total amount of at least one metal of platinum and palladium and rhodium per liter of catalyst is 0.1 to 1och, a
The method according to (1), wherein the cerium t oxide is supported in an amount of 1 to 150 g as CeO2, the tin oxide is supported in an amount of 0.01 to 50 g as SnO2, and the refractory inorganic oxide is supported in an amount of 20 to 200 q.

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

(4) From the group consisting of water-soluble transition metal compounds in which a catalytically active substance consisting of at least one noble metal of platinum and palladium, and rhodium, cerium oxide, and a refractory inorganic oxide is supported on a honeycomb carrier having an integral structure. It is immersed in an aqueous solution of a compound of at least one selected element and a water-soluble tin compound and then calcined to form tin oxide and transition gold jII! 1. A method for producing an exhaust gas purifying catalyst, characterized in that a chloride compound is supported on the coating.

(5) The total amount of at least one metal of platinum and palladium and rhodium per liter of catalyst is 0.1 to IOQ, and cerium oxide is 1 to 150q1 as CeO2, and 0.1 to 10% by weight of transition metal as oxide relative to tin oxide. %, the tin oxide is supported in the range of 0.01 to 50a as 3n02, and the refractory inorganic oxide is supported in the range of 20 to 2009.

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

(1) The method according to (4) or (5), wherein the transition metal used together with the tin compound is at least one selected from the group consisting of chromium, manganese, iron, cobalt, and nickel.

The refractory inorganic oxide powder used in the present invention includes aluminum, steel, silica, titania, zirconia, and alumina.
Silica, alumina-titania, alumina-zirconia,
Examples include silica-titania, silica-zirconia, titania-zirconia, and alumina-magnesia powder, among which it is preferable to use alumina powder, particularly active alumina powder. As the activated alumina, activated alumina having a specific surface area of 50 to 180 m2/a is preferable, and its crystal form can be gamma, delta, theta, chi, kappa, or eta. In addition, at least one of rare earth elements such as lanthanum, cerium, neodymium, or alkaline earth elements such as calcium, barium, etc.
Activated alumina loaded with 0.1 to 30% by weight of seeds can also be used.

The Ptm and Pd sources used in the present invention include chloroplatinic acid, dinitrodiammine platinum, platinum sulfite complex salt,
Preferred examples include platinum tetramine chloride, palladium nitrate, palladium chloride, palladium sulfite complex salt, and palladium tetramine chloride. Further, as the Rh source, rhodium nitrate, rhodium chloride, rhodium sulfate, rhodium sulfite complex salt, Odium ammine complex salt, etc. are preferable. The amount of these precious metals supported is 1J of catalyst! The total amount of Pt, Pd, and Rh is preferably in the range of 0.1 to 10 g.

CeWA used in the present invention includes CeO in the catalyst.
The starting material is not particularly limited as long as it can exist as 2. For example, commercially available CeO2, cerium carbonate, cerium hydroxide, cerium silicate, etc. can be used.

In the case of a cerium salt solution, for example, a cerium nitrate solution may be impregnated and supported on the above-mentioned refractory inorganic oxide. The amount of cerium oxide supported is 1J of catalyst! Hit Ce
O2 is used in an amount of 1 to 150 g, preferably 10 to 100 g.

The tin compound used in the present invention is not particularly limited as long as it is water-soluble, and tin salts such as commercially available 3nC+2, Sn
C+4.5nOC+3, SnSO4,3n (SO4
)2 etc. may be used. The amount of tin compound supported is SnO
2 is used in a range of 0.01 to 50 g per liter of catalyst. The tin compound exists in the catalyst in the form of SnO2 after calcination, and promotes the transfer of electrons to the noble metal, which is the main active species, in the reaction atmosphere, thereby improving the catalyst performance at low temperatures.

Further, by using an aqueous solution containing a water-soluble tin compound and a water-soluble transition metal compound and controlling the rate of electron transfer in tin oxide after calcination, the catalyst according to the present invention exhibits even better performance. The transition metals are preferably chromium, manganese, iron, cobalt, and nickel. Water-soluble salts of each of the transition metals are used, and the supported amount of the transition metals is 0.5% of tin oxide as the transition metal oxide. The above effects are significant when the content is in the range of 1 to 10% by weight.

The honeycomb carrier having an integral structure used in the present invention is
Any material that is usually called a ceramic honeycomb carrier may be used, especially cordierite, mullite, α-alumina, zirconia, titania, titanium phosphate, aluminum titanate, petalite, subodumene, alumino-silicate, magnesium silicate, etc. Honeycomb carriers are preferred, and cordierite carriers are particularly preferred for use in internal combustion engines. Other items other than ceramics,
For example, stainless steel or Fe-0r-Aj! A unitary structure made of a heat-resistant metal with oxidation resistance, such as an alloy, is also used. The shape of the gas flow port (cell shape) may be hexagonal, square, triangular, or corrugated, and the cell density (number of cells/unit cross-sectional area) may be 150 to 600 cells/unit. Square inches are sufficient and give good results.

The catalyst according to the present invention comprises the above-mentioned refractory inorganic oxide P.
It is produced, for example, by the following method using at least one metal selected from the group consisting of t1Pd, Rh, Ce, and 3n, and a transition metal if necessary.

An aqueous solution of a water-soluble salt of at least one metal selected from the group consisting of Pt and Pd and an aqueous solution of a water-soluble salt of Rh are mixed, impregnated into activated alumina, dried and/or fired, and supported. The obtained precious metal-supported alumina and CeO
A slurry is prepared by mixing a predetermined amount of 2, adding dilute nitric acid water, and wet-pulverizing the mixture in a ball mill. A honeycomb carrier having an integral structure is immersed in the slurry, the excess slurry is blown off, and after drying and/or firing, it is soaked in an aqueous solution of a water-soluble tin compound and, if necessary, a water-soluble transition metal compound. The carrier carrying the slurry is immersed, dried and/or calcined, and tin oxide is supported on the coating to form a finished catalyst.

However, it goes without saying that the method for producing a catalyst according to the present invention is not limited to the above production method.

〔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 monolithic carrier used was
The cross section had a cylindrical shape with an outer diameter of 331lIIIφ and a length of 76WIL, having about 400 gas flow cells per square inch, and a volume of about 65d.

Nitric acid aqueous solution of dinitro diammine platinum (Pt content: 1
0C1/jri12.5Id and rhodium nitrate aqueous solution (Rh
Content: 50Q/Jl) 51+1l7 with 150d of pure water
The aqueous solution diluted with
After drying at 0° C. for 3 hours, it was calcined in air at 400° C. for 2 hours to prepare a Pt- and Rh-containing alumina powder. The pt and Rh containing alumina powder and cerium oxide powder 7
5 g was milled with dilute nitric acid water in a ball mill for 20 hours to prepare a coating slurry. 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 away with compressed air to remove clogging from all cells. The mixture was then dried at 150°C for 3 hours and then calcined in air at 400°C for 2 hours to obtain an integrated product coated with the catalyst material. Next, the integrated product was immersed in an aqueous solution (SnC14:175a/j) of stannic chloride (SnCI4: Wako Pure Chemical Industries, Ltd.) for one minute, and then removed from the aqueous solution and the excess aqueous solution in the cell was blown off with compressed air. Then, after drying at 150°C for 3 hours, it was dried at 400°C in air.
A completed catalyst was obtained by calcining at ℃ for 2 hours.

This catalyst contains 90Q alumina, 50oC cerium oxide (as CeO2), and 1L tin dioxide (1L).
As SnO2>IC1, PtO, 83Q, Rh0.1
70 was carried.

Example 2 Palladium nitrate aqueous solution (Pd containing ffi: 100a/1
) 12.5ml Nitrogen 1[]ium water IM (contains Rh)
: An aqueous solution of 50C1 #5d diluted with 150d of pure water and 135g 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 400°C for 2 hours to prepare PD and Rh-containing alumina powder. Using this alumina powder containing Pd and Rh, a completed catalyst was obtained in the same manner as in Example 1.

This catalyst has a volume of 1 lj! Hit alumina 90o, IQ
Cerium oxide (as CeO2) 50 (J1 tin oxide (S)
(as nO2) 10g, PdO, 83G, Rh0.17
0 was carried.

Example 3 Nitric acid aqueous solution of dinitrodiammine platinum (Pt matrix: 10
0q/j 8.9m, palladium nitrate aqueous solution (Pd content: 100a/j 3.6d and rhodium nitrate aqueous solution (Rh
Content: 50Q/Jri! An aqueous solution of M diluted with 150% 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 fired in air at 400°C for 2 hours to form Pt, Pd and R.
An h-containing alumina powder was prepared. Using this alumina containing Pt, Pd, and Rh, a finished catalyst was obtained in the same manner as in Example 1.

This catalyst has a capacity of 1A1J! 90g of alumina, cerium oxide (as CeO2), 50a1 tin oxide (Sn
as O2) 10(IJlPtO,59Q1PdO,2
4(J, Rh0.17q was supported.

Example 4 In Example 1, an aqueous solution of stannic chloride (SncI,
A: A finished catalyst was obtained in the same manner as in Example 1 except that 87.5Q/j was used.

This catalyst contains 90Q alumina, 5 oa of cerium oxide (as CeO2), and tin oxide (S) per liter of volume.
(as nO2) 5C1, pto, a3a, Rh0.17
q was supported.

Example 5 In Example 1, an aqueous solution of stannic chloride (SncIa
A finished catalyst was obtained in the same manner as in Example 1 except that 1.76Q/J) was used.

This catalyst has a volume of 1j! per alumina 90q1 cerium oxide (as CeO2) 50g, tin oxide (SnO
2)0. IQ, Pt0.83Q.

Rh0.17Q was supported.

Example 6 In Example 1, an aqueous solution of stannous sulfate (SnSOa: Wako Pure Chemical Industries, Ltd.) was used instead of stannic chloride.
A finished catalyst was obtained in the same manner as in Example 1 except that nSOa: 72.3Q/1) was used.

This catalyst contains 90q1 alumina/cerium oxide (as CeO2) and 50q1' tin PI (Sn
O+>50. PtO, 83Q, and Rh0.17G were supported.

Example 7 In Example 1, a mixed aqueous solution (SnC+4: 1) of stannic chloride and chromium nitrate (Cr (NO3>3 ・9H20; Wako Pure Chemical Industries, Ltd.)) was used instead of stannic chloride.
75 (7/J!, Cr(NO3)3:15.9g/J)
A catalyst was obtained in the same manner as in Example 1 except that .

This catalyst contains 900 g of alumina, cerium oxide (>50q as CeO2, and tin oxide (SnO2) per liter of volume.
) 1C1, Mlf chromium (as Cr203)
0.5q, PtO, 83q, and Rh0.17g were supported.

Example 8 In Example 1, a mixed aqueous solution (SnCl:
175g/l Mn (NO3)2: 10.4g/,
A finished catalyst was obtained in the same manner as in Example 1 except that e) was used.

This catalyst contains 90a of alumina per liter of volume! ! Cerium oxide (CeO2 and T) 50 (J, tin oxide (Sn)
as O2) 10a, II(t, manganese (M n 0
2) 0.5a, Pt0.83g, Rh0.17a
was carried.

Example 9 In Example 1, a mixed aqueous solution of stannic chloride and iron nitrate (Fe (NO3) s 9H20: Wako Pure Chemical Industries, Ltd.) (SnC1a: 17) was used in place of stannic chloride in Example 1.
5g/j! , Fe (NO3)3: 15.4Q/j
A finished catalyst was obtained in the same manner as in Example 1, except that the catalyst was used.

This catalyst contains 90q of alumina, 50 (7) cerium oxide (as CeO2), and 70 (7) tin oxide (S) per 81i17 of the volume.
(as nO2) 10g, iron oxide (as Fe203)
0.5 Q, PtO, 83G, and Rh0.17G were supported.

Example 10 In Example 1, a mixed aqueous solution (SnCl:
175Q/1, (Co(NO3)2: 12.4g
A finished catalyst was obtained in the same manner as in Example 1 except that /j was used.

This catalyst contains 90q of alumina, 50g of cerium oxide (as CeO2), and tin oxide (SnO2) per liter of volume.
2) 10g, cobalt oxide (as Coo) 0.
5o1Pt0.83Q, Rho.

17q was carried.

Example 1l In Example 1, a mixed aqueous solution (SnC14: 17
5Q/fl, (N i (NO3) 2 :12.
A finished catalyst was obtained in the same manner as in Example 1 except that 4q/J) was used.

This catalyst has a volume of 1a1j! per alumina 90q, cerium oxide (as CeO2) 50 (1, tin oxide (S)
IOg (as nO2), nickel oxide (as NiO)
0.5 g, Pt 0.83 g, and Rh 0°170 were supported.

Comparative Example 1 A finished catalyst was obtained in the same manner as in Example 1 except that stannic chloride was not used.

This catalyst contains 90 g of alumina, 50 g of cerium oxide (as CeO2), PtO, 83Q, R
h0.170 was supported.

Comparative Example 2 A finished catalyst was obtained in the same manner as in Example 2, except that stannic chloride was not used.

This catalyst has a volume of 1j! Hit alumina 90Q.

Cerium oxide (as CeO2) 50g, PdO, 83
q, Rh0.17Q was supported.

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

This catalyst contains 90 g of alumina per 1 Jl of volume, 50 Q of cerium oxide (as CeO2), PtO, 59a,
Pd 0.24 g and Rh 0.170 were supported.

Comparative Example 4 Nitric acid aqueous solution of dinitro diammine platinum (containing Pt:
100a/1) 12.5ae and rhodium nitrate aqueous solution (Rh content: 50Q/j! >5m in pure water 150d
The aqueous solution diluted with
After drying for an hour, it was baked in the air at 400℃ for 2 hours and
and Rh-containing alumina powder were prepared.

The Pt and Rh containing alumina powder and commercially available tin oxide (
15 g of SnO+ (manufactured by Kanto Kagaku Co., Ltd.) and 75 g of cerium oxide were milled together with dilute nitric acid water in a ball mill for 20 hours to prepare a coating slurry. 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 out with compressed 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 has a volume of 1 lj! Each contains 90g of alumina, 50g of cerium oxide (as CeO2), and tin oxide (SnO2).
2) 109, Pt0.83g, and Rh0.17G were supported.

Comparative Example 5 Commercially available tin oxide (SnO2; used in Comparative Example 4)
A catalyst was obtained in the same manner as in Comparative Example 4, except that 0.150 (manufactured by Kanto Kagaku Co., Ltd.) was used.

This catalyst contains 90 g of alumina, 50 g of cerium oxide (as CeO2), and tin oxide (SnO2) per liter of volume.
0.1q), 0.83g of Pt, and 0.17q of Rh were supported.

〔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 5 at low temperatures after engine endurance running was investigated.

Endurance running was conducted using a commercially available electronically controlled engine (8 cylinders 4
A multi-converter was filled with each catalyst and connected to the exhaust system of the engine. The operating conditions of the engine were steady operation (280 Or, p, m, -
220 #l1lIHO) for 60 seconds and deceleration (fuel cut) operation for 6 seconds, the converter temperature during steady operation was 750° C., and the durability was 100 hours.

The catalyst performance at low temperatures was evaluated under the conditions shown below by connecting a multi-converter filled with the catalyst after the above endurance running to the exhaust system of a commercially available electronically controlled engine (4-cylinder 1800a).

The engine was operated under constant conditions with an average air-fuel ratio of 14.6 (2200r
, p, m, , -35041H (1), and at this time, the IH2 sine wave signal was introduced from an external oscillator to the engine control unit to adjust the air-fuel ratio by ±0.5 A/
F, vibrated at 1 Hz. 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 and outlet gas The composition of each catalyst was analyzed to determine the purification rate of HC, Go, and NOX. The obtained purification rate of HClC0 and NOx versus catalyst inlet temperature is plotted on a graph, and the catalyst inlet water temperature (T50' ``80, respectively) at which the purification rate is 50% and 80% is determined to determine the purification performance of the catalyst at low temperature. was used as the evaluation standard.

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

Table 1 Catalytic activity after endurance running As is clear from Table 1, the catalysts of Examples 1 to 6 according to the present invention contained 50% of HC, Go, and NOx, compared to the reacting Comparative Examples 1 to 5. 80%, dropping rate temperature about 45℃
It can be seen that it shows activity at low temperatures. It can also be seen that the catalyst according to the invention has a significantly lower tin content and at least a high performance. Furthermore, the 1 liter catalyst from Example 7 containing tin oxide and transition metal had even better HClGo and NOx purification performance at low temperatures. From these results, it is clear that the catalytic performance of the catalyst according to the present invention at low temperatures after endurance running is dramatically improved.

Claims (7)

[Claims] (1) A catalytically active material containing at least one metal selected from the group consisting of platinum and palladium, as well as rhodium, cerium oxide, and a refractory inorganic oxide is coated and supported on a honeycomb carrier having an integral structure, and then water 1. A method for producing a catalyst for exhaust gas purification, characterized in that the catalyst is immersed in an aqueous solution of a soluble tin compound, dried and/or calcined, and tin oxide is supported on the coating. (2) The total amount of at least one metal selected from the group consisting of platinum and palladium and rhodium per liter of catalyst is 0.1 to 10 g, and cerium oxide is 1 to 10 g as CeO_2.
150g, tin oxide is 0.01-50 as SnO_2
2. The method according to claim 1, wherein the amount of the refractory inorganic oxide and the refractory inorganic oxide is supported in a range of 20 to 200 g. (3) The method according to claim (1) or (2), wherein the refractory inorganic oxide is activated alumina. (4) A catalytically active material containing at least one metal selected from the group consisting of platinum and palladium, as well as rhodium, cerium oxide, and a refractory inorganic oxide is coated and supported on a honeycomb carrier having an integral structure, and then water It is immersed in an aqueous solution of a compound of at least one element selected from the group consisting of soluble transition metal compounds and a water-soluble tin compound, dried and/or fired, and the tin oxide and transition metal oxide are supported on the coating. A method for producing an exhaust gas purifying catalyst, characterized in that: (5) Per 1 liter of catalyst, the total amount of at least one metal selected from the group consisting of platinum and palladium and rhodium is 0.1 to 10 g, and the amount of cerium oxide is 1 as CeO_2.
~150g, 0 for tin oxide as transition metal oxide
．． 1 to 10% by weight, tin oxide is 0.0 as SnO_2
1 to 50g and 20 to 200g of refractory inorganic oxide
Claim (4) characterized in that:
Method described. (6) The method according to claim (4) or (5), wherein the refractory inorganic oxide is activated alumina. (7) A claim characterized in that the transition metal used together with the tin compound is at least one selected from the group consisting of chromium, manganese, iron, cobalt and nickel (
4) or the method described in (5).