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LostInParadise's avatar

Can you explain the Haber-Bosch nitrogen fixing process?

Asked by LostInParadise (29659points) December 31st, 2008

I came across a book in a bookstore that gives a history of the people who developed the process. I understand that the net result is to break up nitrogen molecules so that the nitrogen atoms can be used for things like the production of fertilizers. (I was always under the impression that petroleum was used to make commercial fertilizer.) The book claims that the development of the Haber-Bosch process is one of the most significant developments of recent times.

What I would like is an overview of what is happening from a chemical point of view. I did a Web search on this and I am still not sure what is going on. I think the process may produce ammonia, but I am not sure. There was also something about the signficance of the use of catalysts.

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2 Answers

Harp's avatar

Here is the most complete description of the reaction that I’ve found. Yes, ammonia (NH3) is the end result. First, hydrogen is derived from natural gas by another process (described in the linked article). The hydrogen is then combined with nitrogen (N2, from air) under great pressure (100–250 atmospheres).

A very small amount of ammonia would result from this combination under pressure alone, but the efficiency of the reaction is increased enormously by the presence of a catalyst in the reaction chamber. Various metals can be used as catalysts for this reaction, but the cheapest and most commonly used is magnetite (an iron oxide) with minor amounts of calcium and aluminum oxides. The catalyst forms intermediate reactions with the nitrogen and hydrogen which allows them to combine with less total energy than they would need to combine directly.

LostInParadise's avatar

Harp, what can I say? Once again you have come through.

Going through the details of the reaction, it interesting to see that some of what is consumed is later produced. Methane is an input but some methane is also created. The same with water. It makes sense when you realize that the objective of the initial stages is to create a large amount of pure hydrogen.

The process reminds me of photosynthesis, which I recently read up on. Photosynthesis is a highly complex multi-step proces, but there are basically two stages. The first, the energy consuming step, splits hydrogen from two molecules of water, causing the oxygen atoms to combine as O2. In the second stage, which can take place without light, the hydrogen combines with CO2 to produce a water molecule and sugar chains of CHOH.

I was wrong about the use of petroleum, but natural gas is also a fuel that is in limited supply.

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