Ammonia on tungsten

Hinshelwood and Burk [J. Chem. Soc., 127 (1105), 1925] have investigated the decomposition of ammonia on tungsten and platinum... 

The decomposition of ammonia on tungsten is one of the examples most frequently discussed in textbooks of catalysis (2, 3). It is accepted that the reaction is of zero-order in the initial stage, which is interpreted to indicate the catalyst surface to be fully covered by ammonia during the reaction. According to Frankenburger and Hodler 41), on the other hand, a rapid first step of the decomposition is the formation of surface imide, NHj - - metal -> NH-metal -f- Hg, which takes place... 

Measurements of total pressure, torr, at various times, seconds, were made for the decomposition of ammonia on a tungsten wire at 856 C. The rate equation is to be found. 

The rate of decomposition of ammonia on a tungsten catalyst is independent of the concentration of ammonia, since [NHs] = 1. 

Fig. 1 c.—Influence of temperature on the rate of thermal decomposition of ammonia on a tungsten filament. 

Heterogeneous and catalytic reactions also give straight lines over as wide ranges of temperature as can be investigated. For example, in the catalytic decomposition of ammonia on the surface of a tungsten wire the value of A remains constant over the range 904° to 1,129° abs. in a manner which confirms the equation completely. 

Fig. 8.—Rate of decomposition of ammonia on a tungsten wire at 856°, showing approach to theoretical behaviour of a reaction of zero order. The lines on which no experimental points are marked represent the ideal zero-order change. The half-life periods at 50, 100, and 200 mm. initial pressure are in the ratio 1 1-92 3-52. In the ideal case this would be 1 2 0 4 0. In a unimolecular change all would be equal.

In solutions this type of reaction is common with enzymes it is also frequently found in the catalytic hydrogenation of liquids. An example of a nearly zero-order gas reaction is to be found in the catalytic decomposition of ammonia on the surface of tungsten, and also on the surface of molybdenum, f Thus at 856° C., with 200 mm. of ammonia in contact with a heated tungsten wire, 59 mm. were decomposed in 500 seconds, while in 1,000 seconds 112 mm. were decomposed. The falling off from linearity is but slight. This is illustrated by the accompanying figure, where the thicker lines represent the theoretical curves for a reaction of zero-order. 

The decomposition of hydrogen iodide on the surface of gold, and the decomposition of ammonia on the surface of tungsten, nearly conform to the condition. 

The decomposition of ammonia on the surface of platinum takes place at a speed which is inversely proportional to the pressure of the hydrogen present. The combined influence of the two terms E and A produces an apparent heat of activation of more than 100,000 calories. This is in striking contrast with the value of E true, 39,000 calories for the unretarded reaction on the surface of tungsten. The decomposition of ammonia on molybdenum is of zero order, but retarded by nitrogen the value of E according to Burk is 53,200 calories. Kunsman finds 32,000 calories Trans. Faraday Soc., 1922, 17, 621. 

Logan and Kemball ( 5) also studied the decomposition of ammonia on a tungsten evaporated film and observed nitriding to various extent. 

Other experiments on nitrogen fixation have used transition metai compiexes of nitrogen.74 One used a tungsten nitrogen compiex with a ruthenium hydrogen complex at 55°C to produce a 55% yield (based on tungsten) of ammonia.75 Intermetallic compounds of iron and titanium have been used with ruthenium on alumina to make some... 

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