CATALYSIS AND CATALYSTS 201 



that pressure in a mixture of gases tends to facilitate the forma- 

 tion of that substance which occupies the smallest volume, in 

 this case the ammonia, as shown by the equation, 



N 2 +3H 2 =2NH 3 . 



The introduction of a catalyst in the form of an active metal 

 greatly increases the yield of ammonia, and when the ammonia 

 formed is withdrawn from the mixture as quickly as possible, by 

 absorption by an acid or otherwise, the action is still further 

 promoted. 



It is only recently that proposals to use the nitrogen of the 

 air for the manufacture of ammonia have taken a practical 

 shape. 



Experiments carried on during the last seven years, under the 

 direction of Professor Haber of Karlsruhe, with the support of 

 the Badische Colour Company at Ludwigshafen, have resulted 

 in the production of ammonia on an industrial scale. 



The arrangements are understood to be somewhat as follows : 

 A mixture of nitrogen with three times its volume of hydrogen 

 gas is passed under pressure of about 150 atmospheres into a tube 

 which contains the catalysing substance, and which is maintained 

 by means of an electric coil at a temperature between 500 and 

 700 C. To collect the ammonia thus produced the mixed gases 

 return through a coil surrounded by liquid air where the ammonia 

 condenses. 1 Now as the nitrogen and hydrogen which are 

 returned along with a fresh supply of the mixed gases to the 

 contact chamber are at a low temperature they have to be 

 warmed by passing through a heat exchanging coil. Here they 

 receive heat from the mixture of gases as they leave the tube 

 containing the catalyst. 



The most active catalyst appears to be metallic osmium, but 

 in view of its costliness and the very limited supply of this rare 

 metal, many other materials have been tried, and if patents are 

 to be regarded as any indication of what is going on pure iron 

 appears to be the favourite. 



The nitrogen required in the process is obtained by fractional 

 evaporation of liquid air to be explained in a later chapter, and 

 thus the process is made continuous. Whether Haber's process 

 is destined to take a permanent place among chemical industries 



1 The temperature must not be much below -75 or the ammonia may 

 freeze and stop up the pipes. 



