JPS6134859B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Application of the Invention] The present invention relates to a method for producing a metal catalyst used in a nuclear power plant to recombine oxygen and hydrogen present in off-gas and return them to water.

[Background of the invention]

Conventionally, a wide variety of catalysts have been used in many fields such as the chemical industry and nuclear power generation for removing impurities in exhaust gas and product gas, or for manufacturing products. These catalysts are required to be inexpensive, have high catalytic performance, and have high mechanical strength, and catalyst development is proceeding primarily to solve these problems.

However, since the performance of a catalyst is essentially determined by the catalyst metal, in order to improve the performance it is necessary to increase the surface area of the catalyst carrier and make the catalyst metal uniform on the carrier. For this reason, so-called ceramic catalysts have conventionally been used in which ceramics such as alumina are used as catalyst carriers, and catalytic metals are supported on the carriers. However, with this type of combined catalyst, the catalyst wears out due to vibrations during filling or use, and the generated catalyst powder scatters within the system, which often causes local reactions and clogs in equipment such as valves. There is a high risk of trouble occurring,
Furthermore, the catalyst itself had the disadvantage that its performance deteriorated due to wear. As an alternative catalyst, a metal catalyst is used in which a highly wear-resistant metal is used as a carrier and a catalytic metal is supported on the carrier. Although metal catalysts do not cause wear, they generally have low performance as catalysts and have the disadvantage that the catalyst layer is quite thick. In order to eliminate and improve these drawbacks, a metal catalyst manufactured by a metal catalyst manufacturing process as shown in FIG. 1 has been proposed. In the figure, 1 and 10 represent a metal carrier and a metal catalyst, respectively, 2 is an acid treatment process, 3 is an alkali treatment process, 4 is an oxidation process, 5 is a calcination process, 6 is a catalyst metal salt support process, and 7 is a catalyst The metal salt decomposition step, 8 is a water washing step, and 9 is a drying step. A palladium catalyst will be described as an example.
A nickel-aluminum alloy carrier is used as the metal carrier 1, and in the acid treatment step 2, the nickel on the surface is dissolved to expose the aluminum. Next, after converting the exposed aluminum into aluminum hydroxide in alkali treatment step 3, oxidation step 4
and is converted into γ-alumina in the calcination step 5. Next, in a catalyst metal salt supporting step 6, palladium chloride is supported on the alumina by immersing it in an aqueous palladium chloride solution. Thereafter, in a catalyst metal salt supporting step 7, the palladium chloride is heated to about 600°C to decompose it into metal palladium, and then decomposition products such as chlorine are washed with water in a water washing step 8, and dried in a drying step 9. A palladium catalyst is obtained as the metal catalyst 10.

FIG. 2 schematically shows a cross section near the surface of the metal catalyst 10 prepared using such a manufacturing process. In the figure, 11 and 12 are nickel and aluminum forming the metal carrier 1, respectively;
13 is alumina produced from aluminum exposed on the surface of nickel 11, and 14 is palladium. That is, palladium 1 with catalytic activity
4 is alumina 13 exposed on the surface of nickel 11
The catalytic reaction takes place here.

However, the metal catalyst 10 having such a structure
Since the palladium 14 is mainly supported on the alumina 13 exposed on the surface, the entire catalyst surface is not utilized, and no significant improvement in catalytic activity can be expected. In addition, since a part of the catalytic metal is in direct contact with the metal carrier, the temperature of the catalytic metal decreases significantly, and in low-temperature atmospheric gas,
The disadvantage was that the heat of reaction was not retained in the catalyst metal, resulting in low catalytic activity. In addition, the manufacturing process for such catalysts is a complex one that alternately combines in-liquid and in-air treatments.
Quality control has been difficult because the production time is long and the performance of the catalyst obtained as a product varies depending on the conditions of these steps.

[Purpose of the invention]

An object of the present invention is to produce a metal catalyst with excellent catalytic activity suitable for use in an off-gas system oxyhydrogen recombiner in a nuclear power plant.

[Summary of the invention]

A feature of the present invention is that in a method for producing a metal catalyst that has a catalyst metal on a metal carrier and is used in an off-gas oxyhydrogen combiner in a nuclear power plant, the metal carrier is completely coated. A heat insulating material layer is formed by plating at least one metal selected from the group consisting of chromium and its alloys, and then a catalytic metal that is active in a metallic state is plated on the metal constituting the heat insulating material. Then, the heat insulating material and the catalytic metal in a metallic state are simultaneously oxidized without heat treatment, and then the catalytic metal is reduced.

[Embodiments of the invention]

When observing the state of various carrier metals plated with palladium using a scanning electron microscope,
When palladium is plated on the surface of a metal support that tends to generate stable oxides such as chromium or chromium alloy,
Palladium adheres to the surface of the metal carrier in a spherically dispersed state. Furthermore, it has been found that when the amount of palladium plating is increased, spherical palladium adheres and grows three-dimensionally on the spherical palladium, increasing the surface area of the palladium. The present invention was made based on this result.

That is, the present invention reduces the temperature of the catalyst metal by coating the entire surface of the metal support with a heat insulating material that is an oxide of chromium or chromium alloy as an intermediate layer between the metal support and the catalyst metal. By preventing this and improving low-temperature activity, and by coating the heat insulating material and catalytic metal using a plating method,
This makes it possible to simplify the manufacturing process.

Examples of the present invention will be described below.

FIG. 3 shows a process for producing a metal catalyst according to an embodiment of the present invention. 21 and 30 represent a metal carrier and a metal catalyst, respectively, 22 is a heat insulating material plating process, 22 is a catalytic metal plating process,
23 indicates an oxidation process, and 24 indicates a reduction process.
FIG. 4 shows a cross section of the surface of the metal catalyst obtained by such a manufacturing process, where 31 is the heat insulating material coated on the metal carrier 21, and 32 is the heat insulating material 3.
1 is a catalytic metal coated on top of the catalytic metal.

Here, a metal with good adhesion to the heat insulating material 31 is selected as the metal carrier 21, and its shape may be plate-like, net-like, porous, etc. depending on the intended use of the catalyst. Since the heat insulating material 31 prevents heat conduction from the catalyst metal 32 to the metal carrier 21,
Chromium or chromium alloys that produce oxides with low thermal conductivity are effective. As the catalyst metal, conventionally used metals are used depending on the purpose of the catalyst. For example, palladium is used for the removal of oxyhydrogen gas from nuclear reactor off-gas systems, and platinum is selected for the removal of acetylene from flue gases in the acetylene industry.

Next, as an example, a method for producing a palladium metal catalyst for oxyhydrogen recombination for removing oxyhydrogen gas in nuclear reactor off-gas will be described based on FIG. Nickel is used as the metal carrier 21, and chromium is coated as the heat insulating material 31 in the heat insulating material plating step 22. In the plating step 22 of the heat insulating material 31, the metal carrier 21 made of nickel is immersed in a plating solution that is a mixed solution of chromic acid and sulfuric acid, and a current of about 25
This is done by applying current at A/dm ² . At this time, the thickness of chromium plating on the surface of the metal carrier 21 is 1
Adjust the plating conditions so that the thickness is ~10μ. In the next catalytic metal plating step 23, palladium is used as the catalytic metal 32. When plating palladium, the plating current density is relatively high in order to reduce the particle size of the deposited palladium. To give an example, chromium plating is carried out in a plating bath with a concentration composition of 3.7 g/dipalladium chloride, 100 g/dibasic sodium phosphate, 20 g/dibasic ammonium phosphate, and 2.5 g/benzoic acid. A metal carrier 21 made of nickel is inserted as a cathode, and the current is applied to about
Apply current at 0.3A/dm ² to deposit palladium to an average thickness of 1~
Plate 10μ. The palladium plated on the heat insulating material (chromium) 31 is heated at approximately 500°C in a nitrogen gas atmosphere with an oxygen concentration of 20 vol% in the next oxidation step 24.
oxidation treatment by heating. This treatment converts chromium and palladium into chromium oxide (Cr ₂ O ₃ ).
and converted to palladium oxide. Next reduction step 2
In step 5, this is heated to 150° C. or higher in nitrogen gas with a hydrogen concentration of 100 vol% to carry out a ring treatment. Through this treatment, only palladium oxide is reduced to palladium. As shown in FIG. 4, in the palladium metal catalyst produced in this way, palladium having catalytic activity is transferred to nickel as the metal catalyst 21 via chromium oxide as the heat insulating material 31 produced by oxidation in the oxidation step 24. The metal catalyst 30 is supported on the entire surface of the metal catalyst 30.

The hydrogen conversion ratio (inlet hydrogen concentration/outlet hydrogen concentration) of this palladium metal catalyst is compared with that of a conventional metal catalyst as shown in FIG. The horizontal axis of the figure shows the catalyst layer gas temperature (° C.), and the vertical axis shows the hydrogen conversion ratio, where A shows the measurement results for the present invention and B shows the measurement results for the conventional metal catalyst. As is clear from this figure, the catalytic activity in case A is higher than in case B, and it can be seen that a high hydrogen conversion ratio can be obtained even at low temperatures. As shown in FIG. 4, the entire surface of the nickel metal carrier 21 is effectively utilized, and the palladium catalyst metal 32 is not in direct contact with the nickel metal carrier 21; This is because the speed at which the generated reaction heat moves to the metal carrier 21 is slow. Due to the increase in palladium surface area due to the spherical three-dimensional attachment of palladium to the surface of chromium,
Hydrogen conversion ratio improves. Furthermore, as is clear from the comparison of the catalyst manufacturing processes in FIGS. 1 and 3, the manufacturing process of the embodiment is significantly simplified compared to the conventional one, and the manufacturing time and manufacturing cost can be significantly reduced. Furthermore, although the quality of a catalyst generally depends on variations in the conditions of each manufacturing process, the manufacturing process of the example is simplified, so there is less manufacturing unevenness, and the quality of the catalyst can be improved.

Chromium may be plated onto the metal carrier surface by hot melt plating instead of electroplating.

A chromium alloy such as a Ni-Cr alloy (10% Cr) may be used instead of chromium as a heat insulating material. Ni-
The metal catalyst when using Cr alloy (Cr10%) is
It can be manufactured using a process similar to that described above.

Further, as another example, a case of a palladium oxide metal catalyst that is effective as a catalyst for automobile exhaust gas will be described. In this case, iron is used as the metal carrier 21 and chromium is used as the heat insulating material 31.
In the heat insulating material plating step 22, chromium was plated on the metal carrier to a thickness of 1 to 10 microns by electroplating as in the previous example. This chromium-plated metal carrier 21 is subjected to the next catalytic metal plating step 2.
3, palladium is applied to the surface to a thickness of 1 to 10
μ-paste. The plating conditions may be the same as those in the previous example, but in this case, activation of the catalyst metal 32 in the ring formation step 25 is not required, so
Increase the current density to make palladium crystals smaller. The plating bath in this case contained palladium chloride, 6 g/ammonium, and 10 g/ammonium chloride, and the pH was adjusted to 0.1 to 0.5 with hydrochloric acid. After plating palladium, in an oxidation step 24, palladium and chromium are oxidized by heating to about 500° C. in nitrogen gas containing about 20 vol% oxygen. Through this treatment, chromium becomes chromium oxide, and the oxidized palladium oxide is supported on iron, which is the metal carrier 21, through this. In the metal catalyst thus produced, palladium oxide can exhibit catalytic activity for oxidation removal of carbon monoxide or hydrocarbons in exhaust gas, so the subsequent reduction step 25 is not necessary. This catalyst also functions in the same manner as in the above-mentioned embodiments, has the same effects, and can exhibit the effects of improving catalytic activity and simplifying the manufacturing process.

To summarize the effects of the metal catalysts of the above examples, they have an activity approximately 100 times that of conventional nickel sponge metal catalysts and approximately three times that of ceramic catalysts in the relatively low temperature range of 100 degrees Celsius or less, and have an activity of 400 degrees ~500℃
A metal catalyst that does not deteriorate even at high temperatures is obtained and is suitable for oxyhydrogen recombination of nuclear reactor off-gas. Also,
This metal catalyst can be used not only for gas phase-gas phase reactions, but also for water-gas phase reactions.
It is also effective as a reaction catalyst between liquid phase and gas phase, such as isotope exchange reaction between hydrogen gas.

〔Effect of the invention〕

In the method of the present invention, the catalytic metal is plated on the chromium layer and then subjected to oxidation and reduction treatment, so that the intermediate layer of chromium oxide is clearly separated and the catalyst becomes metallic. It is possible to prevent a decrease in performance due to oxidation of chromium and a decrease in activity as a catalyst poison due to diffusion of chromium into the catalyst. Therefore, it is effective over a wide temperature range and has the effect of significantly improving catalyst activity.

Furthermore, since the metal catalyst produced by the method of the present invention is used in an off-gas system oxyhydrogen recombiner in a nuclear power plant, its operating temperature is 400°C or less, and the reduced catalyst metal will not be oxidized. do not have. Therefore, according to the present invention, a catalyst metal with high performance that is active in a metallic state (much better than a catalyst metal that is active in an oxidized state) can be used for a long period of time while maintaining its high performance. There is.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the manufacturing process of a conventional metal catalyst, Fig. 2 is a schematic cross-sectional view of the surface portion of a conventional metal catalyst, and Fig. 3 is an explanatory diagram showing the manufacturing process of a metal catalyst according to an embodiment of the present invention. , FIG. 4 is a cross-sectional view of the surface of an example of a metal catalyst obtained by the method of the present invention, and FIG. 5 is a relationship between catalyst layer gas temperature and hydrogen conversion ratio of an example of a metal catalyst obtained by the method of the present invention. FIG. 2 is a characteristic diagram showing a comparison between the conventional metal catalyst and a conventional metal catalyst. 21...Metal carrier, 30...Metal catalyst, 31...
...Insulating material, 32...Catalytic metal, 22...Insulating material plating step, 23...Catalytic metal plating step, 24...
...oxidation step, 25...reduction step.

Claims (1)

[Claims] 1. In a method for producing a metal catalyst that has a catalyst metal on a metal carrier and is used in an off-gas oxyhydrogen combiner in a nuclear power plant, chromium and its alloys are used to completely coat the metal carrier. A heat insulating material layer is formed by plating at least one metal selected from the group consisting of: a catalytic metal that is active in a metallic state is then plated on the metal constituting the heat insulating material; A method for producing a metal catalyst, characterized in that the heat insulating material and the catalyst metal are simultaneously oxidized without heat treatment, and then reduced.