Iron-ammonia catalysts adsorption isotherms

Emmett P H and Brunauer S 1937 The use of low temperature van der Waals adsorption isotherms in determining the surface area of iron synthetic ammonia catalysts J. Am. Chem. See. 59 1553-64... 

The log—log plot of the adsorption isotherm, which can possibly be correlated to the pressure-dependency of the catalytic reaction rate, is very flat. The adsorption of ethylene on nickel increases only by 10% for an increase of the equilibrium pressure by a factor of 10, although the surface is still far from being covered by a monolayer. The work of Laidler et al. (3), who studied the ammonia-deuterium exchange reaction on a promoted iron catalyst by means of the microwave method, also throws doubt on the zero-order kinetics with respect to observations made by Farkas (4). 

Fia. 6. Comparison of the isotherms for total carbon monoxide adsorption at —183° and for the physical adsorption on about 45 g. of pure iron synthetic ammonia catalyst (No. 973) and on a similar quantity of a doubly promoted iron catalyst (No. 931) (39). 

Figure 1. Adsorption isotherms for a pure iron synthetic ammonia catalyst for various gases near their boiling points. Curve lA is for physical plus chemical adsorption of CO. Curve IB is for physical adsorption occurring at -1830C after the evacuation of the samples at -78 C for an hour. The solid symbols are for desorption. (Reproduced from Ref. 22. Copyright 1937, American Chemical Society.)...

In 1935 Brunauer and Emmett [121] carried out the first successful attempt to determine - by means of isotherm adsorption of six different gases - the surface area of an iron synthetic ammonia catalyst. Later [122], in 1937, these authors determined the surface area of two different silica gels measuring adsorption isotherms of seven different gases.. In the above mentioned works the surface area was determined by extrapolating the middle linear sections of experimental isotherms to zero pressure in order to obtain the amount of gas required to cover the adsorbent surface with a monomolecular layer. On condition that the monomolecular layer was in a close-packed stage, the surface area was then evaluated from the monolayer adsorbed amount. Brunauer and Emmett [121,122] also proposed to determine the monolayer adsorption amount from the so-called point B of the experimental isotherm. It was assumed that this point corresponds to the inflection point and can be obtained from the beginning of the linear section of adsorption isotherms. 

It can be seen from the above-mentioned discussion that the Temkin s theory of catalytic reaction kinetics on heterogeneous surfaces is confirmed not only by overall reaction kinetic data of ammonia synthesis, isotherms and adsorptive rates of nitrogen on iron catalysts, more importantly, perhaps, but also some very useful generalization results derived from this theory. For instance, Temkin s equation is obtained based on two steps or simplified two steps mechanism. So, it can apply to any kind of catalytic reactions. The problem is Are some simplifications required by reasonable formation of two step mechanism, and can the assumption of rate determining step be made for non-uniform surface The answer of Temkin s theory is positive, and especially Temkin s theory of catalytic reaction kinetics on non-uniform plays an important role in solving the selection and improvement of catalysts. 

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