VOL.2. No. 1
Twe Drn~crSYN~HESE OF AMMONIA
285
THE DIRECT SYNTHESIS OF AMMONIA: A LECTURE TABLE EXPERIMENT ALFRED
T. LARSON, FIXEDNITROGENRESEARCHLABORATORY, UNITED STATES DEPARTMENT OF AGRICULTURE, WASHINGTON, D. C.
A number of requests have been made for an apparatus which will demonstrate the catalytic formation of ammonia from its elements and which a t the same time can he employed either upon the lecture table or as a laboratory exercise in advanced inorganic chemistry. An apparatus suitable for such purposes is described in this note. The essential features of the apparatus are shown in Fig. 1. A and B are flowmeters' by means of which hydrogen and nitrogen may be mixed in such proportions that the gases which enter the deoxidizer C will be approximately in the ratio of three volumes of hydrogen to one volume of nitrogen. The sketch indicates mercury or water-sealed traps placed in the gas supply line. When the gases are withdrawn from high pressure cylinders these traps facilitate the control of the gas flow and prevent "blowing" of the meters. The deoxidizer consists of reduced granular copper oxide, maintained a t a temperature of approximately 300-400°C. by means of the vapors of a high boiling liquid. The vapors of boiling diphenylamine have been successfully employed for this purpose. Boiling sulfur may also be used, although i t produces a higher temperature than is necessary. As the gases leave the deoxidizer they pass through the driers, D and Dr. D is filled with 6-10 mesh soda lime free from dust, while D' contains fused potassium hydroxide. The purified gases then come in contact with the A discussion of details of construction and calibration of flowmeters is given by Benton, Jour. Ind. E q . C h m . , 11, 623 (1919).
ammonia catalyst contained in the tube E. This tube has a capacity of about 10 cc. and is shown with a preheating coil which need not be employed if the flow of gas is restricted to 100-200 cc. per minute. The ammonia catalyst is heated in the vapors of boiling sulf~tr. The entire apparatus is constructed of pyrex glass and all connections are welded. Mounted upon a suitable support, it may be made an attractive permanent piece of lecture table equipment. The success of the ammonia synthesis depends upon the purity of the nitrogen and hydrogen gases, and the activity of the ammonia catalyst.
I t is important, therefore, that the suggestions given below for the preparation of the purifying reagents and the catalysts be closely followed. Preparation of the Deoxidizer Any granular copper oxide, for example, such as is commonly used in organic analysis, makes a fairly satisfactory material from which to prepare the copper deoxidizer. A better material is made by precipitation, the method being practically the same as that described below for the manufacture of the ammonia catalyst.= The oxide is reduced in place by a stream of hydrogen at about 250-350° C. As water is one of the products of this reaction, the process should be completed before attaching the driers and the ammonia catalyst tube. "arson, Newton and Hawkins, Chnn. & Met. Eng., 26, 495 (1922).
VOL.2, NO. 4
Tng DIRECTSYNTHESIS OP Arn~Om.4
287
Preparation of Drying Agents C. P. soda lime should be employed. Since this usually contains water purposely added in order to make it a better absorbent for gases such as carbon dioxide, it is advisable to give i t a preliminary drying. This is readily accomplished by placing a tube of the material in a sulfur vapor bath and drawing dry air through it for about an hour. A convenient method for handling fused potassium hydroxide consists in coating it on steel wool such as is sold for scouring kitchen ware. This is accomplished by dipping small rolls or balls of steel wool in the fused alkali contained in an iron dish or crucible. The operation is repeated until a fairly thick coating is obtained. The coated steel wool is cooled in a desiccator in which it is kept until needed. After use, the drying properties of this material may be restored by again dipping the steel wool in the fused hydroxide. Preparation of the Catalyst The catalyst is essentially a mass of porous iron which is made from an oxide of iron by reduction in a stream of hydrogen or of the mixture of nitrogen and hydrogen used in the synthesis. A satisfactory oxide can be prepared as follows: 1000 grams of femc nitrate (c. P.) are dissolved in 16 liters of water and allowed to stand for 24 hours. It is then warmed to 40' C. and poured slowly with constant stirring into 2.5 liters of 5.5% ammonia, which has also been warmed to 40" C. If the final mixture is not slightly alkaline, enough dilute ammonia is added to make i t so. The precipitate is allowed to settle and is washed by decantation until the washings are practically free from ammonium nitrate. It is then transferred to a Biichner funnel and largely freed from water. The gelatinous mass is now placed in a mortar and kneaded until a very thin paste has been produced. This paste is transferred again to a Biichner funnel, and as much water as possible is removed by suction. The filter cake is broken into small lumps and placed in a drying oven, where it is dried for 24 hours a t 7C-80' C., and again for 24 hours a t 120130' C. It is then crushed and screened to 8-14 mesh. This preparation makes an excellent experiment for students in advanced inorganicchemistry. The iron oxide is placed in the tube E and heated to the temperature of boiling sulfur. The mixture of nitrogen and hydrogen is then allpwed to flow over the oxide. As reduction of the oxide nears completion, ammonia formation will begin. The amount of ammonia formed will slowly increase, finally reaching a maximum value which under favorable conditions will, of course, be the equilibrium amount possible a t one atmosphere pressure and the temperature of boiling sulfur. This is about 0.2570 ammonia. Half of this amount is easily obtainable and is sufficient for demonstration purposes.
Alternative Methods Apparatus. Where only a temporary piece of equipment is desired, the ingenuity of the experimenter will suggest many modifications of the apparatus descrihed. For instance, there may he substituted for the flowmeters any device which will furnish the nitrogen and hydrogen in approximately the ratio of 1: 3. The drying units may he constructedfrom ail glass wash bottles, or from calcium chloride jars. Split type electric furnaces are sometimes used in place of the vapor baths. The use of rubber connections should be avoided if possible. If rubber connections must he employed these should be as short as possible. All rubber parts should heheatedfor a time in dilute sodium hydroxide and then carefully dried. The Catalyst. Some natural magnetites are sufficiently free from catalyst poisons such as sulfur and phosphorus to give satisfactory results when substituted for the precipitated iron oxide described. A satisfactory oxide may also be made by burning an iron rod in an oxy-hydrogen flame. The oxide drippings which collect during the cutting of iron with the oxyacetylene or oxy-hydrogen torch are equally satisfactory and may he used if available. A sample of prepared catalyst sufficiently large for demonstration purposes will be furnished to any edt~cationalinstitution, if a request he sent to the Director of the Fixed Nitrogen Research Laboratory. Sources of Hydrogen and Nitrogen. In some localities, hydrogen and nitrogen are most conveniently obtained compressed in cylinders. The usual laboratory methods for preparing these gases may be employed, however. When these methods are used, care must be taken that no gaseous poisons are introduced which are not removed by the purifying system descrihed. A convenient method for preparing a mixture of nitrogen and hydrogen in proper proportions consists in decomposing ammonia gas.3 If pure ammonia gas be passed over a mass of iron catalyst heated to 450-550' C. the ammonia will be largely decomposed into its elements. If the residual ammonia be removed by scrubber^"^ and the remaining gases carefully dried, these will be present in just the right proportions for the synthesis operation. It is evident to anyone familiar with the ammonia reaction that this "cracking" process combined with the synthesis process offers an opportunity for discussing such features as exothermicity, equilibrium and volume change. Larson, Newton and Hawkins, Chem. & Met. Eng., 26, 493 (1922). dilute sulfuric acid is allowed to trickle.
' Tall towers filled with beads or glass pearls, over which
