Hydrogen sticking probability

So far only aqueous solutions have been considered however, mixtures of HF and ethanol or methanol are quite common, because this addition reduces the surface tension and thereby the sticking probability of hydrogen bubbles. While substantial quantities of ethanol or methanol are needed to reduce the surface tension, cationic or anionic surfactants fulfill the same purpose in concentrations as low as 0.01 M [So3, Chl6]. 

The results of these studies are tabulated in Table IV. This table lists the surfaces studied, the intermediates observed, the products, the adsorption temperature (T ds). the initial sticking probability of HCOOH (Sq), the peak temperature for product evolution (7J,), and the activation energy (E i) and the preexponential factor (v) determined by methods discussed earlier (see Section I,D). Data for HCOOD is given where available in order to distinguish the two hydrogens in the acid. 

The decrease of the sticking probability is typical for atomic or molecular adsorption where the molecule adsorbs non-dissociatively. Consequently, it was assumed that the hydrogen molecules do not directly dissociate on Pd(l 0 0). They are rather first trapped in a molecular precursor from which they then dissociate [25, 44], and it is the trapping probability into the precursor state that determines the dependence of the sticking probability on the kinetic energy. 

In Fig. 5, additionally the calculated and measured vibrational temperatures [50] are plotted. In contrast to the rotational cooling, there is vibrational heating indicating that there should be enhanced dissociation for vibrating hydrogen molecules on Pd(l 00). Vibrationally enhanced dissociation has been known for years in the gas phase dynamics community [53]. Usually it is associated with strongly curved reaction paths in activated systems [4]. However, the most favorable path towards dissociative adsorption in the system H2/Pd(l 0 0) is purely attractive and has a rather small curvature (see Fig. 2a). Therefore one would not expect any substantial influence of the vibrational state of H2 on the sticking probability. 

For a Langmuir-Hinshelwood reaction in which both reactants are thermally equilibrated on the surface, reaction is initiated by thermal activation of the adsorbate. This thermal sampling can be turned to advantage by invoking detailed balance to equate the rates of adsorption and desorption for a surface at equilibrium [66, 67]. This allows us to relate the final state distributions measured for desorption to the detailed sticking probability for each product quantum state (v, J). This approach has been applied very successfully to hydrogen adsorption/desorption [4, 68] but its use has not been widely explored for reactions of heavier molecules. 

Figure 1 Hydrogen dissociation on Cu(l 1 0) [12]. The initial dissociative sticking probability Sq is measured as a function of incident translational energy E. Measurements are made by increasing die nozzle temperature Ta ( ), and by seeding at constant nozzle temperature (Tn = 1100 K) ( ). The latter measurement provides a measure of die translational energy onset (ca. 125 meV)...

Figure 26 Sq for D2 ( ) on Pt(5 3 3) as a function of E at T = 300 K and i = 0° [63]. The dashed line represents the contribution of direct sticking on the (111) terraces of Pt(5 3 3). Subtraction of this function from the data ( ) provides an estimate of the (1 0 0) steps contribution ( ). A polynomial fit (dotted line) is included to guide the eye. The solid curve is a hard cube calculation of the sticking probability of phy si sorbed hydrogen on platinum.

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