FI68978C - HYDROFOB CATALYTIC FOER RECOMBINATORER - Google Patents

Abstract

1. Catalyst mat for recombination of hydrogen and oxygen, especially recombination of the gases formed during the operation of electric batteries, produced by impregnating a carrier material of polytetrafluoroethylene and activated charcoal with a catalytically active material and rolling, characterised in that a first mat is rolled from a polytetrafluoroethylene activated charcoal mixture, that the mat is impregnated with the aqueous solution of a salt forming the basis of a catalytically active metal, that the mat is then rendered hydrophobic and is sintered, and that a second activated charcoal mat, free from catalytically active material, is then brought into close mechanical contact with this mat.

Description

68978

The invention relates to a catalyst mat for the recombination of hydrogen and oxygen, in particular gases generated by the use of electric batteries, which mat is prepared by impregnating a support material of polytetrafluoroethylene and activated carbon with a catalytically active material and by rolling.

The low-maintenance use of modern batteries is made possible mainly by the effect of efficient recombiners compared to the hydrogen and oxygen gases generated during use and discharge. Usually, the recombiners are placed above the vent opening of the battery case cover and conveniently immediately into the closure plugs.

For the proper functioning of the recombinant alone, it is not important to select a suitable active ingredient, in this case usually palladium, but also to ensure the long-term activity of this material. For this reason, the quality of the catalyst support is also crucial. It must give the gas the largest possible surface area, i.e. it must be very porous and, on the other hand, it must be able to withstand the humidification caused by the condensing liquid. One natural requirement for a catalyst support is resistance to chemical corrosion.

Both mineral fibrous materials such as asbestos and activated carbon can be used as catalyst supports. In this case, special measures are required to impart hydrophobic properties to the catalyst support. Thus, it is known from GB-PS 1 471 307 to separate metallic Pd in a very finely divided form from activated carbon impregnated with a solution of PdCl 2 by the action of a hydrazine solution and to mix polytetrafluoroethylene powder in addition to the activated carbon thus prepared.

2,68978

Experience has shown that the persistence of hydrophobization can often otherwise be desired when the reduction of the palladium salt takes place in a known manner, usually in a medium containing a wetting agent, which wetting agent must be washed off thoroughly before further processing of the activated carbon. In this way, the wetting is effected even before the actual use of the catalyst. In general, the appropriate use of a powdered, at least finely divided, material is always bound to the containers and is therefore not advantageous for installation.

It is therefore an object of the present invention to provide a catalyst and a process for its preparation by which said drawbacks can be eliminated to a great extent in a product.

According to the invention, this object is achieved in that a first mat is rolled from the polytetrafluoroethylene-active mixture, that the mat is impregnated with an aqueous solution of a salt based on a catalytically active metal, that the mat is subsequently made water-resistant and sintered, and that a second activated carbon mat free of catalytically active material has been brought into close mechanical contact with this mat.

The catalyst according to the invention is a flexible double film formed by a compacted PTFE-activated carbon mixture, the proportion of both components being able to vary between 40:60% by weight and 10:90% by weight, but preferably about 25:75% by weight.

The content of active ingredient is 0.1 to 5% by weight, preferably 0.3 to 1% by weight.

According to the invention, the second activated carbon mat combined with the actual catalyst mat is free of catalytically active ingredients and yet can be impregnated with copper.

II

3,68978 with oxide (CuO). Usually, this second mat has the same composition (activated carbon, PTFE, post-hydrophobized) as the first catalytically active mat. It protects the catalyst against lead-toxic antimony hydrogen (SbH3>) contained in lead-acid gases. The second carbon mat simultaneously acts as a diffusion barrier when the recombinant plug is overloaded in the presence of too much gas, which can reduce the recombination rate to less than 80%.

It has also been found that rolled catalyst supports on an activated carbon support alone do not guarantee sufficient reliability, as they glow after a short contact time with explosive gases, decompose as a result and thus can also give rise to an explosion. This danger is reduced by a second activated carbon mat rolled on top of the first activated carbon mat.

The production method according to the invention, which will be explained below, has a decisive effect on the quality of the catalyst mats and their advantageous use.

10 to 40% by weight * of PTFE powder, the average grain size of which can vary within wide limits from 10 to 100, is subjected to a mixing process with 90 to 60% by weight of activated carbon, in which the particles are comminuted by strong mechanical impact forces. The mixture is then vibrated and then rolled. The vibration treatment counteracts the tendency of the mixture to agglomerate and thus enables good rollability. This vibration treatment can be performed, for example, on a vibrating table. It is also possible to allow the mixture to pass through a vibrating table to a rolling mill.

The obtained rolling strip is then drawn through a 1 to 20% palladium chloride solution with a residence time of at least 1 minute, followed by drying with hot air.

4,68978

An essential step for the next invention is the post-hydrophobization of the surface of the rolling strip with a PTFE suspension of about 10-30%, either by dipping or spraying. In this case, the exposure time must be less than 10 seconds. A short time prevents the PTFE suspension from penetrating too strongly into the catalyst support and the formation of a layer that is too thick on the surface, otherwise there is a risk that the layer will tear when dried and thus become ineffective.

However, proper dosing of PTFE can also occur by adjusting the corresponding dispersion concentration.

Thereafter, the slightly dried and no longer sticky molding strip can be dried with hot air or infrared heating11a. then the rolling strip must have a smooth, closed, milky-white PTFE layer.

The final step in the preparation of the catalyst mat is activation, which takes place in a process such as sintering. Here is the formula N2

PdCl2 ____ ^ Pd + Cl2

360 ° C

according to palladium chloride forms metallic palladium in fine form. The fine distribution of palladium chloride over a large surface area of activated carbon means the most advantageous additional condition for this distribution process.

The activation takes place, for example, in a tube furnace at a temperature of 330 to 430 ° C, in which case the residence time can be 3 to 30 minutes, depending on the current temperature. By decomposing PdCl2, the surface PTFE layer caused by hydrophobization is torn.

A reducing furnace atmosphere is not necessary to activate the catalyst mat or rolling strip. The above decomposition reaction also takes place in air, but preferably in a stream of 5,68978 nitrogen. This nitrogen flow must be adjusted so that the resulting gaseous decomposition results, such as Cl 2, C 2 F 4, or the decomposition results of the wetting agents that came with the hydrophobization of the PTFE dispersion, can be eliminated.

The pre-sintered and activated roll strip can now be brought into close mechanical contact with another, palladium-free roll strip. This can be done, for example, by rolling or riveting.

Figure 1 shows a catalyst according to the invention assembled from the actual catalyst mat 1 and the activated mat mat 2, which is not activated with palladium but may be impregnated with CuO 5.

The catalyst prepared according to the invention has good mechanical strength and retains its hydrophobicity in addition to the good catalyst effect for a long time. Its surface should be black-gloss and free of any visible cracks. The catalyst according to the invention can be used in various ways as recombinants for batteries, namely in all installations in which hydrogen is generated as a by-product, for example in fuel cell installations for burning hydrogen blown with inert gases, for example in submarine battery compartments, EVD installations or small installations. toothbrushes. It can also be used to remove radiolysis gases in nuclear power plants or to generate heat from hydrogen, oxygen and air.

Finally, hydrogen from hydrogenation and reduction processes can be oxidized by a catalyst, thus counteracting the potential risk of explosion due to gas accumulations. In said processes, the heat of reaction can be used for preheating.

Claims (9)

1. Catalyst mat for recombining hydrogen and oxygen, especially when using gas accumulator-producing gases prepared by impregnating a carrier of polytetrafluoroethylene and activated carbon with a catalytically active substance, and by rolling, characterized by a polytetrafluoroethylene mixture is rolled a first mat, that the mat is impregnated with an aqueous solution of a site based on a catalytically active metal, that the mat is subsequently made waterproof and sintered, and that in close mechanical contact with this mat, a second active carbon mat is released from catalytically active substance. Catalyst according to claim 1, characterized in that the polytetrafluoroethylene reactive carbon blend ratio is between 40:60 wt% and 10:90 wt%, most preferably 25:75 wt%. Catalyst according to claims 1 and 2, characterized in that the first mat contains 0.1-5% by weight, most preferably 0.3-1% by weight palladium. Catalyst according to claim 3, characterized in that the second catalytically inactive carbon mat is impregnated with copper oxide. Process for preparing a catalyst according to any one of claims 1-4, characterized in that the polytetrafluoroethylene activated carbon mixture is rolled into a mat, that the mat is saturated with palladium chloride solution and after intermediate drying is wetted with an aqueous polytetrafluoroethylene dispersion. after renewed intermediate drying, sintering is subjected to inert or reducing furnace air, that a second mat is produced without PdCl2 saturation and that both mats are brought into close mechanical contact. Il