DE2027472B2 - Process for the production of a fuel catalyst from tungsten carbide for electrochemical fuel cells - Google Patents

Description

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 96 176; BE-PS 7 42 234; Nature 224 [1969J 1299-1300), but the current densities that can be achieved with them are so low that that an economical use of these substances does not appear worthwhile. When using the fuels hydrogen (H ₂ ), formaldehyde (HCHO) and formic acid (HCOOH) in fuel cells with sulfuric acid as the electrolyte, at a potential of 250 mV and a temperature of 70 ° C z. B. achieved the following current densities:

H ₂ 100 mA / g HCHO 40 mA / 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 again subjected to a sintering process at temperatures of 600-700 ⁰ C. All of these processes must lead to relatively inactive substances, since a large part of the active centers on the surface is deactivated at the temperatures used and are therefore no longer available for catalysis.It is therefore necessary to re-establish electrodes obtained in this way before use to be activated, for example by anodic or cathodic current load, whereby unreacted metallic components or oxide layers are released from the electrode and thus new active centers are created.

The object of the present invention is to provide electrodes for the electrochemical conversion of cheaper fuels (e.g. formaldehyde or hydrogen) create that work so profitably that their use in 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

jo to the gaseous end product carbon dioxide CO ₂ , which escapes from the electrolyte, takes place in two stages:

HCHO + H ₂ O-HCOOH + 2H ⁺ + 2e "(1)

HCOOH- ^ CO ₂ + 2H ⁺ 2e "

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

HCHO + H, O- »CO, + 4H ⁺

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

sn is 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.

■ n The preparation of the electrodes according to the invention is carried out under conditions that have a large specific surface (> 1 mVg) as well as to generate a large number of reactive points and, above all, to get fortune. In particular, all processes

bo 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

b5 solved during the manufacturing process and in fine-grained, surface-rich form precipitated again and then the special one still to be described Subject to conditions. 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 moistened with distilled water or with an electrolyte-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 is used as the starting material for the further processing described in the examples: 50 g of commercially available tungstic acid H ₂ WO ₄ are mixed in the heat with vigorous stirring 100 ml of 25% ammonium hydroxide solution and 100 ml of distilled water are then dissolved from this tungstate solution in the cold while stirring vigorously with concentrated hydrochloric acid (37% HCl) dried in vacuo to give the tungstic acid as a pale yellow to white powder, which is removed by heating at 200 ⁰ C in air, the entire coordination or crystal water, whereby a fine-particle tungsten trioxide, WO3, formed

1st example

10 g of the tungsten trioxide prepared according to the above procedure are reacted with carbon monoxide, CO, in a quartz tube at 700 ° C. During this reaction, which is complete after 4-5 hours, tungsten carbide is formed, which can even result in pyrophoric preparations. To measure the hydrogen activity 100-150 mg tungsten carbide powder can be slurried in a graphite felt and these are in an electric i ^r> denhalter clamped for measuring the activity against liquid and / or soluble in the electrolyte fuel μιτι 0.966 g WC grain fraction <60 and 0.034 g Ammonium carbamate of the same grain size as a pore former is intimately mixed 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-forming agent is removed with warm water and the electrode is then clamped into a gold wire to conduct electricity

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

2nd example

50 g of the tungsten trioxide produced according to the above instructions are mixed with 150 g of zinc powder (grain size <40 μm) in a laboratory mixer for 10 minutes. This mixture is tamped into a porcelain crucible is made to react in a crucible furnace at approx. 600 ^° C., after which the crucible is immediately taken out of the furnace so that it cools down quickly in the air Soluble constituents removed As soon as no more gas evolution can be observed, it is washed a few times with distilled water and the resulting extremely finely divided, 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 wol Frame carbide preparations are created.

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

3rd example

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

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 H ₂ SO ₄ and 6 m HCHO, is used. The activity measurement takes place again as above in the half-cell arrangement.

The results of these four examples are compiled in the following table, the Activities refer to a potential of 250mV at a temperature of 70 ° C:

example Manufacturing H ₂ -AlCt. HCHO act. HCOOH act. BET upper
area (like) (like) (like) (mVg) 1 WO ₃ + CO 300 174 18th 18th 2 WO ₃ + Zn
W + CO 320 153 24 15th 3 WO ₃ + CO
WC + H ₂ O 380 298 54 17th 4th WO ₃ + CO
WC + (H ₂ SO ₄
+ HCHO) 410 307 100 20th

Claims (2)

Patent claims: 1. Process for the production of a fuel catalyst from tungsten carbide for direct production 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, characterized by the following process steps: a) Precipitation of tungstic acid hydrate from an alkaline tungstate 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) Implementation of tungsten oxide or one produced therefrom by reaction with zinc Tungsten powder, the tungsten oxide mixed with zinc powder, this mixture at 600 ° C brought to reaction and after cooling the zinc oxide from the resulting mixture is dissolved out with hydrochloric acid, with carbon monoxide at temperatures of at most 700 ° C until tungsten carbide is completely formed. 2. The method according to claim 1, characterized in that the catalyst or one thereof in on known way produced electrode in a final 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