Pressure virtual

Molecular depolymerization at low temperatures (270-370°C) and under virtually pressure-free (<0.1bar) conditions. 

The three seminal ideas in this early work of Temkin are quite general. The first is that adsorption of nitrogen is rate determining. The second is the virtual pressure or fugacity of adsorbed nitrogen, a concept of great importance to the understanding of catalytic cycles at the steady state. The third idea is the kinetic description of the catalytic surface as a nonuniform one. The last was systematized later by Temkin s school, both in theory and in application, to a... 

For a reaction of such complexity as methanation (or FTS) an exact kinetic theory is actually out of the question. One has to introduce one or more approximations. The usual assumption made is that one reaction step is rate determining (r.d.s.) and other steps are in equilibrium or steady state. Adsorption equilibria are described by Langmuir formulas (Langmuir-Hinshelwood, Hougen-Watson approach) [15] and the approach is sometimes made simpler by using so-called virtual pressures [16] (cf. Chapter 3). 

The concept of virtual pressure is however very useful whenever an adsorption- desorption step in a catalytic cycle at the steady state is not at equilibrium. 

According to the discussion above, we can define Ca also from the normal isotherm equation where the virtual pressure, P, is used in place of the aetual partial pressure Pa-... 

In these equations, p T) is the chemical standard potential of the component i, p is a standard pressure, tt is the spreading pressure of the adsoibed solution, and /i is an activity coefficient. The virtual pressure p ( ) (not the vapor pressure) depends on the spreading pressure and is the pressure of the component / which has the same spreading pressure either in the pure state or in the gaseous mixture. The equality q = g in the case of thermodynamic equilibrium leads to Raoult s law of adsorption... 

In the catalytic synthesis or decomposition of aimnonia, adsorbed N is, by assmnption, in equilibrium with gaseous NH3 and H2. By contrast, gaseous N2 is not in equilibrium with N. The virtual pressure of N2 can be understood from Fig. 2.29. 

The dashed arrows represent virtual equihbrium (case (b)). Since both systems (b) and (c) are equivalent, the equilibrium constant for case (c) can be used to define the virtual pressure of N2 ... 

Prom this relation, it is easy to calculate the virtual pressure of N2 during ammonia synthesis (Table 2.13). 

Here Pm, the pressure of nitrogen atoms in the gas, is a virtual pressure and serves as an auxiliary quantity only. 

Even larger virtual pressure effects (up to about 30 kbar) are observed when bulkier molecules, such as metal phthalocyanines, are synthesized in-situ within zeolite Y cages [3,16]. Fig. 4 represents an encaged Co-phthalocyanine complex within a faujasite framework supercage, experiencing distortions which are reflected in the electronic vibration transitions of the macrocycle. 

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