Surface lifetime

For a molecule characterised by a AH value of 40 k.I mol 1 and undergoing facile surface diffusion, i.e. a A/ dir value close to zero, then each molecule will visit, during its surface lifetime (10 r s), approximately 107 surface sites. Since the surface concentration a is given by a = NtSUIf, then for a AH value of 40 kJ mol-1 and zsurf= 10-6 s at 295 K, the value of a is 109 molecules cm-2. These model calculations are illustrative but it is obvious that no conventional spectroscopic method is available that could monitor molecules present at a concentration 10-6 monolayers. These molecules may, however, contribute, if highly reactive, to the mechanism of a heterogeneously catalysed reaction we shall return to this important concept in discussing the role of transient states in catalytic reactions. 

In the gas phase, the reaction of O- with NH3 and hydrocarbons occurs with a collision frequency close to unity.43 Steady-state conditions for both NH3(s) and C5- ) were assumed and the transient electrophilic species O 5- the oxidant, the oxide 02 (a) species poisoning the reaction.44 The estimate of the surface lifetime of the 0 (s) species was 10 8 s under the reaction conditions of 298 K and low pressure ( 10 r Torr). The kinetic model used was subsequently examined more quantitatively by computer modelling the kinetics and solving the relevant differential equations describing the above... 

The rotational energy distributions by the REMPI spectra of desorbed NO from the hep hollow species on Pt(l 1 1), which is induced by 2.3-6.4 e V laser irradiation, are represented by a Boltzmann distribution such as in Fig. 16a. It is impossible to understand that molecules desorbed by a non-thermal process reach thermal equilibrium in the rotational energy distribution during the very short residence time in the excited state for chemisorbed species on metal and semiconductor surfaces (lifetime = 10 16-10-14 s [62, 72, 73]). Besides, no rotational freedom exists in the chemisorbed state and the desorption is... 

The strong adsorption of the intermediates/products increases their surface lifetime and decreases the surface availability of oxygen sites for anhydride synthesis. Both aspects concur in the decrease in selectivity to anhydrides on deaefivated surfaces. 

It is shown below that a number of molecular species are important in the interaction of oxygen with platinum surfaces, and these are precursors to the dissociative state. The interception of these adsorbed states in the electrochemical process are important in determining the catalytic mechanism of electroreduction, and we also show below that surface defects play an important role in their reactivity and surface lifetimes. 

Breaking of Aggregates and Agglomerates (Deagghmeration) 387 The surface lifetime can be calculated from,... 

The presence of a large fraction of adsorbed species on the catalyst surface thus not only decreases the number of available active sites for alkane selective oxidation, but also decreases their specific turnover number, thus increasing the surface lifetime of adsorbed intermediates on the PVO deactivated surface. This increases the probability of parallel unselective reactions with a decrease in the selectivity to anhydrides. 

The design of a maximum bubble pressure method for high bubble formation frequencies must address three main problems the measurement of bubble pressure, the measurement of bubble formation frequency, and the estimation of surface lifetime and effective surface age. 

To separate the surface lifetime from the total time interval between subsequent bubbles an approximation of the dead time according to geometric parameters of capillary and bubble volume was derived Fainerman Lylyk (1982) and Fainerman (1990). A substantial improvement for the exact determination of surface lifetime and its calculation was carried out by Fainerman (1992) who defined a critical point in the experimental curve in co-ordinates "pressure-gas flow rate". This point corresponds to a change in the flow regime from individual bubble formation to a gas jet regime. The calculation of the so-called effective age the surface (effective adsorption time) from the bubble surface lifetime was discussed by different authors ... 

For the practical use a criterion is needed to decide whether it is justified that such a kinetic equation is applied. This criterion must ensure that the kinetic constants are independent of the parameters of the adsorption process, mainly the surfactant concentration and the monolayer coverage. Experimental data for various surfactants show that for surface lifetimes shorter than 20 ms the reduced desorption rate constant k , = kj /T is nearly constant and of the order of 100 s [16]. This important result allows to define a simple criterion for a non-diffusional adsorption mechanism by comparing the characteristic times of diffusion and adsorption kinetics according to the model of Eq. (4.15). The condition for mixed or kinetic controlled... 

We can summarise that the problem of diffusion-controlled adsorption kinetics of reorientable surfactant molecules is formulated and solved even for the case when the reorientation process within the surface layer requires some time. It was shown that this non-instantaneous reorientation can result in either acceleration or deceleration of the surface tension decrease, depending on the adsorption characteristics of the different molecular states, and on the actual surface lifetime [227] faster (for medium n values) or slower (large n values) decrease of y is caused by an oversaturation of the surface layer by the state possessing maximum molar area. 

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