Global adsorption probability

Figure 9 Adsorption process of NO on Pd particles supported on MgO(l 00). (a) Global adsorption probability as a function of surface temperature and for various particle sizes (from Ref. [89]). (b) Schematic representation of die elementary processes in die molecular adsorption of NO on supported Pd particles (1) quasi-elastic redection on die bare support, (2) physisorption-diffusion-desorption from the bare support, (3) direct chemisorption on die Pd particles, (4) NO chemisorption on the Pd particles via a precursor physisorbed state on die bare support. Xs is die mean diffusion length of die NO molecules on the support and p is die width of die collection zone around die Pd particles.

The global adsorption probability has been directly measured for CO and NO on Pd/MgO(100) model catalysts [61,62]. Figures.13 presents such measurements for NO ]62]. 

Fig. 3.12. Principle of the measurement of the global adsorption probability in a molecular beam experiment. The desorption pulse is composed by a fast component ( S fast) corresponding to molecules reflected or desorbing from the substrate and a slow component (5siow) corresponding to (chemisorbed) molecules desorbing from the metal clusters. The relative amplitude of the slow component gives the global adsorption probability...

Fig. 3.13. Variation with temperature of the global adsorption probability of NO on three different Pd/MgO(100) model catalysts (from [62])...

We see the global adsorption to increase when the temperature decreases. At high temperature it is equal to Ac, the fraction of the substrate covered by the metal clusters. In that case, the width of the capture zone, p, is zero and only molecules directly impinging on the clusters are chemisorbed and Og is equal to nR n. When the temperature decreases, the mean diffusion length, Xg, grows and p increases. Finally, the global adsorption probability reaches saturation when the capture zones overlap. The maximum value of the adsorption probability is given by ... 

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