Heterogeneous ammonia decomposition

Spectroscopic Monitoring of the Heterogeneous Catalytic Decomposition of Gaseous Ammonia 256... 

The stoichiometry of decomposition of [Ni(NH3)4](NCS)2 was dependent on the method of salt preparation [1126]. Ammonia was lost in three successive steps (—NH3, —NH3, —2 NH3) from the solution-prepared salt, but the first intermediate could not be isolated from the similar reaction of material prepared by heterogenous reaction. The difference in behaviour was ascribed to differences in perfection of the crystallites resulting from the alternative preparative methods. 

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. 

Following filtration at low temperature to remove small amounts of sodium amide and disodium acetylenediolate, and removal of the ammonia, a thermally unstable deep red solid could be obtained. However, this material decomposed quickly above 0°C to give a heterogeneous brown-black solid which contained only small amounts of [V(CO)6]. The nature of the bulk of the decomposition material remains an interesting problem (vide infra). Treatment of the deep red solution with various electrophiles [Eq. (17)] gave... 

M. Grunzel [ The Chemical Physics of Solid Surfaces and Heterogeneous Catalysis, Synthesis and Decomposition of Ammonia vol.4, Ed. D.A. King (Elsevier Scientific Publishing Company, Amsterdam, 1982)]... 

The facts are readily explained on the theory that decomposition of the ammonium amalgam occurs very slowly as an homogeneous reaction at temperatures below zero and that a rapid, heterogeneous reaction takes place at the surface of little droplets of liquid ammonia which are dispersed throughout the mercury. The surface tension of the mercury is sufficient to cause liquefaction of the first ammonia which is liberated. The autocatalytic effect occurs when the reaction has had time to build up these droplets of liquid ammonia. 

It was found that this heterogeneous reaction can be stopped by the addition of small amounts of lithium. It is likely that lithium with its higher electrode potential is able to prevent the self-ionization of liquid ammonia, and the heterogeneous reaction between ammonium in solution and liquid ammonia is inhibited. When this heterogeneous reaction is prevented, the decomposition follows a perfectly satisfactory second-order reaction, and the temperature must be raised up to the range from zero to 20° in order to obtain a measurable reaction rate. The reaction is satisfactorily explained on the assumption that ammonium dissociates in mercury giving ammonium ions and free electrons NH4—>NH4+ -be . The older view that the ammonium exists as a free radical, NH4, seems less likely. Such a radical would be unstable and it would not be expected to have such a long life. Ammonium ions, however, are more stable. 

There are very few cases of reactions of zero total order, and so these equations do not have wide application. Most of the known cases involve heterogeneous reactions occurring on surfaces, such as the decomposition of nitrous oxide gas on hot platinum wire, 2N2O 2N2 + O2, and the de-e.omposition of ammonia gas on a hot platinum wire, 2NH3 N2 + 3H2. The explanation seems to be that the reaction occurs only at the surface of the catalyst, and if the surface becomes saturated at a given gas or liquid concentration, further increase in the concentration in these phases cannot further change the surface concentration and so, beyond this point, the reaction seems to proceed at a rate independent of concentration in the gas phase. 

Table 4 shows that the activity of the catalysts varied in the sequence Pt > Ag > Fe > Cu. The rate constante for the formation (kj) and the decomposition (kj) of ammonia were calculated using eq. 15. In the presence of catalysts k, exceeded k2, sometimes by an order of magnitude, but without catalysts kj was significantly lower than k2. This indicated that the displacement of the steady state was due to different effects of the catalyst on the rates of formation and decomposition of ammonia in a plasma. The linear relation (Fig. 19) between catalyst activity and the electron work function 9 was adduced as evidence in support of a heterogeneous ionic mechanism (Sect. 4.1.1.4). 

Comparatively, ammonia has a residence time of 5 to 10 days in the atmosphere, depending on the location and weather conditions. The main fraction that evaporates into the air can undergo a variety of reactions, such as (1) decomposition, thermal, and photochemical reactions, (2) aqueous phase reactions that produce aerosols, and (3) heterogeneous reactions with soot particles. 

Kinetics of Ammonia Synthesis and Decomposition on Heterogeneous Catalysts... 

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