Metal sticking coefficient

For alkali modified noble and sp-metals (e.g. Cu, Al, Ag and Au), where the CO adsorption bond is rather weak, due to negligible backdonation of electronic density from the metal, the presence of an alkali metal has a weaker effect on CO adsorption. A promotional effect in CO adsorption (increase in the initial sticking coefficient and strengthening of the chemisorptive CO bond) has been observed for K- or Cs-modified Cu surfaces as well as for the CO-K(or Na)/Al(100) system.6,43 In the latter system dissociative adsorption of CO is induced in the presence of alkali species.43... 

This backdonation of electron density from the metal surface also results in an unusually low N-N streching frequency in the a-N2 state compared to the one in the y-N2 state, i.e. 1415 cm 1 and 2100 cm"1, respectively, for Fe(l 11)68. Thus the propensity for dissociation of the a-N2 state is comparatively higher and this state is considered as a precursor for dissociation. Because of the weak adsorption of the y-state both the corresponding adsorption rate and saturation coverage for molecular nitrogen are strongly dependent on the adsorption temperature. At room temperature on most transition metals the initial sticking coefficient does not exceed 10 3. 

The sticking coefficient of H2 on a metal has been determined through an adsorption experiment. The metal surface is assumed to have No = 1.5 x 10 sites m and each adsorption site is assumed to be occupied by one hydrogen atom when the surface is saturated. The experiment was performed by exposing the surface to a known pressure of hydrogen over a well-defined period of time (dosis) and then sequentially determining how much was adsorbed by, for example, TPD. All adsorption experiments where performed at such low temperatures that desorption could be neglected. 

Research on cathode materials focuses on reduction of the high chemical activity of the lower WF metals (e.g., Ca/Al), the increase of the chemical stability, and improvement of the sticking coefficient of the interlayer materials (e.g., LiF/Al). 

Eor inert SAMs such as n-aUcanethiolates/Au, alkaline earth and alkali metal deposition on inert SAMs tends to exhibit low sticking coefficients of the nascent metal atoms due to quite weak interactions with the -CH3 terminus sometimes <10 of the impinging metal atoms stick to the surface while the rest scatter off the smface [23, 58]. Bammel and co-workers observed quite slow penetration of Na through this inert SAM [59]. In the case of Mg and Ca depositions on n-aUcanethiolate SAMs it was observed that while Mg does not react it does undergo continuous penetration thorough the SAM. In contrast, Ca does react to some extent resulting in calcium carbide species formation [56, 57]. In the case of K on an n-aUcanethiolate SAM the results are more complicated. For example, at 10 K atoms per SAM molecule, it has been reported that half of the deposited metal penetrates to the SAM/Au interface while the remainder is claimed to remain embedded within the SAM matrix [60], though such space is not available theoretically in a dense SAM. 

As usual, the rate of dissociative adsorption (e.g. of 02 on various metals [92, 95, 99, 100]) rapidly decreases with increasing surface coverage. As a rule, this is attributed to the fact that dissociative adsorption requires two unoccupied cells, i.e. the sticking coefficient must be S(9) = S(60) Po (0). If a solid surface adsorbs only molecules A, in the quasi-chemical approximation we will have the set of equations... 

Adsorption-desorption of CO. CO adsorption is monomolecular. On all the Pt metals except Ir it proceeds through the pre-adsorbed ("precursor ) state [17, 93], The activation energy is practically zero and the initial sticking coefficient is high (0.5-1.0). Oxygen does not inhibit CO adsorption [55, 94], The sticking coefficient is weakly dependent on the surface concentration of CO. During the adsorption on Ru and Ir, surface carbides can form. 

We have seen in Sect. 2.2.3(b) that, when the rate of adsorption of molecules becomes comparable with the rate of desorption of atoms, we expect failure of the half-order rate law and, eventually, first-order behaviour will be observed. All the metals investigated by Brennan and Fletcher [5, 7] exhibited this transition. Figure 11(a) illustrates the behaviour using the example of the H2— Pt system at 1750 K. According to eqn. (51), the limiting probability of atomisation is the molecular sticking coefficient, s2, values of which are listed in Table 5. 

Only a few kinetic studies of the rate of sulfur adsorption on metals have been made. They reveal that rates of adsorption of H2S on metals are generally very rapid, the high sticking probability suggesting no barrier to adsorption and dissociation until saturation is approached. In the case of Pt and Cu (83, 92), two adsorption regimes are observed (1) at 0 < 0.25-0.3, the adsorption of sulfur occurs with a high sticking coefficient ( 1.0) and... 

Since preadsorbed sulfur generally blocks the adsorption of other molecules, it would only be logical to expect that it would also prevent the adsorption of H2S or S2. Previously discussed studies of sticking coefficients (73, 83, 92, 99, 101) and H2S adsorption on metals (57, 106, 112-115) provide evidence that the sticking coefficient and heat of adsorption for H2S or S2 decrease with increasing coverage. Thus, rates and strengths of sulfur adsorption on sulfur-saturated metal surfaces are clearly lower than those on a clean metal surface. 

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