DE2149523A1 - Tungsten carbide catalyst - for the synthesis of ammonia from its elements - Google Patents

Abstract

WC catalyst, for the synthesis of NH3 from its elements, is made by dissolving a solid W-cpd, such as H2WO4, and re-precipitating it. After washing and drying pref in vacuo, the ppt. is heated and converted to WO3. This cpd. is mixed well with Zn and reacted with 6N-HCl to yield finely-divided, surface-rich W-powder. The metal is heated in a quartz tube to 800 degrees C, in the presence of CO. The resulting WC has a specific surface area >5m2/g. and high catalytic activity which is not subject to irreversible poisoning reactions.

Description

Ammonia Synthesis Catalyst The invention relates to a Catalyst for the synthesis of ammonia from the elements.

Numerous catalysts for this synthesis are known.

On a large scale, one with activators such as aluminum oxide, doped iron catalyst used. This one has; however, the disadvantage that through Impurities in the reaction gas very easily reduced its catalytic effect will.

Such impurities come in addition to sulfur and phosphorus compounds as irreversible poisons also oxygen, water, carbon monoxide, cabbage endioyid among others as reversible poisons.

It is also known to use elemental tungsten as a catalyst for ammonia synthesis to use. However, this is less catalytically effective than that as well Molybdenum studied its catalytic properties and has no practical properties Gained importance. Part of this may be due to the fact that olefin metal is difficult to find Produce in the form of small grains with a large internal surface for embankments leaves. Such beds are a prerequisite for the construction of a catalyst bed, which is easy to flow through for gaseous reactants.

As experiments have shown, tungsten has been used as a catalyst for ammonia synthesis considerable advantages over the industrially used doped iron catalysts, e.g. greater resistance to sintering requiring less frequent renewal of the catalyst required; lower sensitivity to reversible catalyst poisons such as carbon monoxide, carbon dioxide, water vapor, etc.

However, these advantages could not be achieved because of the difficulties noted above the construction of a gas-permeable catalyst bed cannot be used.

The object of the present invention is to create a catalytic converter for the synthesis of ammonia based on tungsten, which allows that for tungsten metal to use as a catalyst known advantages, but in proportion easily produced in a form suitable for catalyst bed beds is.

According to the invention, this object is achieved in that the catalyst for ammonia synthesis consists of tungsten carbide.

The tungsten carbide used should have a specific surface area of at least 5 m2 / g, since tests have shown that above this value Due to the specific surface area, the tungsten carbide is sufficient for the synthesis of amino acids high MtiVitit.

Due to an approximately similar behavior of tungsten powder and Tungsten carbide powder in ammonia synthesis, the latter according to one of the further Process to be described below is produced, one can assume that also in tungsten carbide, tungsten is the more catalytically active component.

Only one of the starting components should be used in the manufacture of the catalyst as a solid, the others, on the other hand, as gases or as liquids with low levels Boiling point. The starting component, which is used as a solid, becomes in the course of the process of the manufacturing process and again in finely divided, surface-rich form filled in before it is converted to carbide.

The following describes the manufacture of the tungsten carbide catalyst in more detail to be discribed. A table then shows which measurement results with these catalysts compared to iron catalysts in ammonia synthesis have revealed.

Preparation of the tungsten carbide catalyst: Example 1: 50 g on the market available tungstic acid H2W04 are in the heat with vigorous stirring in a Mixture of 100 ml of 25 point ammonium hydroxide solution and 100 ml of distilled water solved. This tungstate solution is then made under strong cold in the cold Stirring with concentrated hydrochloric acid (37 goat HC1) tungstic acid precipitated. To Repeated slurrying with distilled water and decanting becomes the iolflamic acid first dried in the cold in a vacuum.

Non receives the Wolrramsoure as a light yellow to white powder, dm by heating to 200 ° C in air, the entire coordination or. Crystal water is withdrawn, whereby a finely divided tungsten trioxide, WO3, is formed.

This tungsten trioxide is mixed with 150 g zinc powder (grain size C 40 / um) Mixed in a laboratory mixer for 10 minutes. The mixture is placed in an acrylic crucible stamped. The mixture is coated with a 1.5 to 2 cm thick layer of zinc powder as protection against oxidation covered. The mixture is reacted in a diegel oven at approx. 600 ° C, whereupon the crucible is immediately removed from the furnace so that it can be exposed to air quickly cools down. After cooling, the tungsten-zinc oxide regulus with 6N - hydrochloric acid all soluble components removed. As soon as no more gas evolution can be observed is washed a few times with distilled water decanting and that way obtained extremely fine, surface-rich tungsten in the cold in a vacuum dried. The tungsten powder is in a quartz tube at 8000C with carbon monoxide implemented. In the case of this implementation, tungsten carbide is formed, which even turns into pyrophoric Form can arise.

Example 2: 20 g of the tungsten trioxide produced according to Example 1 are exposed to a stream of carbon monoxide in a quartz tube at 7000C for 6 hours, whereby tungsten carbide is formed. After cooling down, the carbon monoxide will be in the Quartz tube displaced by argon and in the argon stream the tungsten carbide with 100 ml of 3N sulfuric acid moistened and then dried.

The tungsten carbide samples made according to the two examples had a specific surface area of 17 to 18 m2 / g.

Aktivittsin0ssunn: To test the catalytic activity of the catalysts 2 g of catalyst are placed on a glass frit in a catalyst chamber, which is in a metal bath and thus heated to higher temperatures can. After you have set the target temperature on the catalytic converter Has, a gas flow consisting of one part nitrogen and three parts is passed through it Hydrogen is passed and the ammonia content of the gas is determined after the catalyst. In addition to the temperature, the flow rate of the gas mixture is also varied.

In the following table, the ammonia yields are given as the tauotient from the resulting ammonia partial pressure and the ammonia equilibrium pressure at the corresponding temperature, the results on the tungsten carbide catalyst being compared with those obtained on an iron catalyst under the same conditions; = NH3 yield on the WC catalyst = NlI3 yield on the Fe catalyst = NH3 partial pressure on the Fe catalyst - NI13 equilibrium pressure at the test temperature Table: Comparison of the amounts of ammonia formed on 2 g of tungsten carbide and 2 g of iron under the same conditions will. Gas pressure 1 at. emp. Current velocity: 'indity WC Fe cl / hxx THI3 Ni13 360 4 0.004 0.60 400 3.5 0.04 0.90 445 3.5 0.1 = * 0.98 500 3.5 0.60 O.9Q 500 2.0 0.90 The table shows that the activity of tungsten carbide becomes comparable to that of iron above about 5000C. The progress becomes clear when one considers the advantages of the tungsten carbide catalyst over the iron catalyst, already mentioned above - low sensitivity to catalyst poisons, greater resistance to sintering; In addition, there is a much shorter activation time, so that ammonia formation begins without delay - considered in this context with the activities together.

Mechanically stable tungsten carbide pellets of any grain size can be produced in a simple manner by using non-powdery starting substances - Tungsten trioxide or tungsten - but pressed moldings of the same substances reacted with carbon monoxide and then diesc to a suitable grain size be ground.

There is evidence that the tungsten carbide catalyst for the ammonia synthesis can be further optimized, so that the results on the iron catalyst can be achieved.

Such optimization is by changing the manufacturing conditions of the tungsten carbide catalyst is possible.

Claims (4)

Expectations 1. Catalyst for ammonia synthesis, characterized in that it is made of tungsten carbide. 2. Catalyst according to claim 1, characterized in that the used Tungsten carbide has a specific surface area of at least 5 in2 / g. 3. A method for preparing the catalyst according to claim 1 or 2, characterized in that only one of the starting components as a solid, the others, however, exist as gases or as liquids with a low boiling point. 4. The method according to claim 3, characterized in that the as Starting component used solid dissolved in the course of the manufacturing process and is precipitated again in finely divided, surface-rich form before the reaction to the carbide.