DE2027472C3 - Process for the production of a fuel catalyst from tungsten carbide for electrochemical fuel cells - Google Patents

Description

The object of the present invention is to make electrodes for the electrochemical conversion cheaper To create fuels (e.g. formaldehyde or hydrogen) which are so profitable that they can be used for electrochemical processes makes economic sense.

Another object of the present invention is to provide electrodes which, in addition to formaldehyde, also Implement formic acid to a greater extent. This goal results from the following consideration: The electrochemical Oxidation of formaldehyde in acidic solution to the gaseous end product carbon dioxide CO2, that escapes from the electrolyte takes place in two stages:

The invention relates to a method for producing a fuel catalyst from tungsten carbide for the direct generation of electrical energy through the oxidation of hydrogen, formaldehyde or Formic acid in electrochemical fuel cells, by carburizing tungsten or tungsten oxide by means of carbon monoxide.

Catalysts made of tungsten carbide for the conversion of cheap organic fuels and / or hydrogen are known (DE-OS 14 96176; BE-PS 7 42 234; Nature 224 [1969], 1299-1300), but the current densities that can be achieved with them are so low that an economic use of these substances not the fuels, hydrogen (H _2), formaldehyde (HCHO) and formic acid (HCOOH) in fuel cells with sulfuric acid as an electrolyte seems worthwhile in use, at a potential of 250 mV and a temperature of 70 ° C e.g. . B. achieved the following current densities:

HCHO + H ₂ O-HCOOH

> HCOOH + 2H ⁺ + 2e "(1) -> CO ₂ + 2H ⁺ 2e ~ (2)

The aim is to achieve the greatest possible conversion according to the summation of equations (1) and (2) resulting equation (3):

HCHO + H ₂ O-CO ₂ + 4H ⁺ + Ac ' , (3)

A major reason for these relatively low current densities is from hard material technology because this conversion results in the greatest current yield, but also prevents the Electrolyte enriched in formic acid and must be replaced. A parallelism in the change the activities of the substances used here against formaldehyde and formic acid is not of given in advance, as is the case with formaldehyde and hydrogen.

The stated objects are achieved according to the invention by a method having the characterizing features Features of the main claim.

The preparation of the electrodes according to the invention is carried out under conditions which have a large specific surface (> 1 mVg) as well as a large number of reactive sites to generate and above all to get fortune. In particular, all processes are carried out at the lowest possible temperature (not above 700 ° C) and / or carried out within the shortest possible time. Only one of the starting components is a solid the others, on the other hand, are in the form of gases or liquids with a low boiling point; the solid becomes dissolved during the manufacturing process and precipitated again in a fine-grained, surface-rich form and then subjected to the special conditions to be described. As solids can

Metal powder, precipitated metal acids, metal oxides, metal oxide hydrates or metal hydrides can be used.

In order to achieve a high formic acid activity, the electrode can be used in a final process step be moistened with distilled water or with an electrolyte j fuel mixture before using comes into contact with the oxygen in the air

In the following it will first be described how the fine-grained, surface-rich solid is produced, which serves as the starting material for the further processing described in the examples ι ο: 50 g of commercially available tungstic acid H ₂ WO ₄ are heated with vigorous stirring in a Mixture of 100 ml of 25% ammonium hydroxide solution and 100 ml of distilled water is then dissolved from this tungstate solution in the cold while stirring vigorously with concentrated hydrochloric acid (37% HCl) Cold dried in vacuo The tungstic acid is obtained as a pale yellow to white powder, from which ^{the entire coordination or crystal water is removed by heating to 200 °} C. in air, a finely divided tungsten trioxide, WO ₃ , being formed

1st example

10 g of the Wolframtrioxids prepared by the above procedure are wetting in a quartz tube at 700 ⁰ C with carbon monoxide, CO, is reacted With this implementation jo, after 4 - 5 hours completed, forms tungsten carbide, in which even pyrophoric preparations may arise. To measure the hydrogen activity, 100-150 mg of tungsten carbide powder are slurried in a graphite felt and this is clamped in an electrode holder Grain size intimately mixed as a pore former and pressed in a mold with a pressure of 18 t / cm ² to form an electrode 15 mm in diameter and 0.5 mm thick. The pore former is dissolved out with warm water and the electrode is then clamped into a gold wire to conduct electricity. 4th

The activity is measured in the half-cell arrangement customary for this purpose.

2nd example

50 g of the = 0 tungsten trioxide produced according to the above procedure are mixed with 150 g of zinc powder (grain size <40μΐη) in a laboratory mixer for 10 minutes.This mixture is tamped into a porcelain crucible The mixture is reacted in a crucible furnace at approx. 600 ^° C., whereupon the crucible is immediately removed from the furnace so that it cools quickly in the air. After cooling, the tungsten-zinc oxide regulus with 6N hydrochloric acid becomes all soluble Components removed As soon as no more gas evolution can be observed, it is washed a few times with distilled water and the resulting extremely fine, surface-rich tungsten is dried in the cold in a vacuum. 10 g of this tungsten powder are reacted with carbon monoxide ^{in a quartz tube at 700 ° C.} which is described in Example 1 for the tungsten trioxide. Here, too, pyrophoric Wo lfram carbide preparations are produced.

The electrodes for the activity measurements are again produced as in Example 1.

3rd example

1.82 g of the tungsten trioxide prepared according to the above procedure are pressed in a synthetic resin press die at a pressure of 1 t / cm ² to give a tablet 15 mm in diameter and 3 mm thick. This is then placed in a quartz tube at 700 ^° C. for 6 hours in a stream of carbon monoxide exposed, whereby tungsten carbide is formed. After cooling, the carbon monoxide in the quartz tube is displaced by argon and the electrode is moistened with 10 ml of distilled water in the argon stream

The activity towards formaldehyde and formic acid is again determined in the usual way Half-cell arrangement.

4th example

This example corresponds exactly to the 3rd example until the carbon monoxide is replaced by argon. For moisture penetration, however, not distilled water, but 10 ml electrolyte-fuel mixture, consisting of 3 η H2SO4 and 6 m HCHO, used The activity measurement is again carried out as above in the Half-cell arrangement.

The results of these four examples are compiled in the following table, the Activities to a potential of 25OmV to one Get a temperature of 70 ° C:

Example production

H ₂ -AlCt. (like)

HCHO act. HCOOH act. BET upper

area

(like)

(like)

(m ² / g)

1 WO ₃ + CO 300 174 18th 2 WO ₃ + Zn
W + CO 320 153 24 3 WO ₃ + CO
WC + H ₂ O 380 298 54 4th WO ₃ + CO
WC + (H ₂ SO ₄
+ HCHO) 410 307 100

18th
15th

17th
20th

The highest activities achieved so far with the electrodes according to the invention (in mA / g at a potential of 250 mV and a temperature of 70 ^° C.) are at the following values:

H ₂₆₀₀

HCHO 780

HCOOH 180

The advantage of the invention is that the electrodes according to the invention are inexpensive, acid-resistant Represent Sioffe, with which activities can be achieved with respect to cheap fuels that have a use make such electrodes appear economically reasonable in electrochemical systems.

Claims (2)

Patent claims: 1. Process for the manufacture of a fuel catalytic converter made of tungsten carbide for the direct generation of electrical energy through the oxidation of Hydrogen, formaldehyde or formic acid in electrochemical fuel cells, through carburization of tungsten or tungsten oxide by means of carbon monoxide, characterized by the following procedural steps: a) Precipitation of tungstic acid hydrate from an alkaline WoUYamat solution using concentrated Hydrochloric acid in the cold, b) Removal of adhering water and adhering hydrochloric acid from the tungstic acid hydrate with the formation of tungstic acid in the cold by means of a vacuum, c) heating the tungstic acid to 200 ° C. in air to remove the entire coordination or Water of crystallization with formation of tungsten trioxide WO3, d) Reaction of the tungsten oxide or a tungsten powder produced therefrom by reaction with zinc, the tungsten oxide being mixed with zinc powder, this mixture being ^{reacted at 600 °} C. and, after cooling, the zinc oxide being dissolved out of the resulting mixture with hydrochloric acid, with carbon monoxide at temperatures from a maximum of 700 ° C until the complete formation of tungsten carbide. 2. The method according to claim 1, characterized in that the catalyst or one thereof in on known way produced electrode in a last 'process step with distilled Water or an electrolyte-fuel mixture is moistened before the catalyst or electrode comes into contact with the oxygen in the air. H ₂ 100 mA / g HCHO 40mA / g HCOOH 1 mA / g known process technology used in the manufacture of the electrodes to look for, according to which z. As metal and carbon are sintered together at temperatures of at least 600 to 700 ° C or the electrodes by a gas phase reaction and after deposition from the gas phase on porous bodies _t again to a sintering process at temperatures of 600-700 ° C are unteiworfen. All these methods have led to relatively inactive materials, since the temperatures employed a large proportion of the active sites is desakiiviert on the surface and is not more to the _y available thus for catalysis It is therefore necessary electrodes obtained in the ^ e way ahead to reactivate the use, for example by anodic or cathodic current load, whereby unreacted metallic components or oxide layers are loosened from the electrode and thus new active centers are created.