Surface sticking coefficient

Here, if Z is expressed in moles of collisions per square centimeter per second, r is in moles per square centimeter. We assume the condensation coefficient to be unity, that is, that all molecules that hit the surface stick to it. At very low Q values, F as given by Eq. XVII-3 is of the order expected just on the basis that the gas phase continues uniformly up to the surface so that the net surface concentration (e.g., F2 in Eq. XI-24) is essentially zero. This is the situation... 

The probability for sticking is known as the sticking coefficient, S. Usually,. S decreases with coverage. Thus, the sticking coefficient at zero coverage, the so-called initial sticking coefficient,. S q, reflects the interaction of a molecule with the bare surface. 

L exposure would produce 1 ML of adsorbates if the sticking coefficient were unity. Note that a quantitative calculation of the exposure per surface atom depends on the molecular weight of the gas molecules and on the actual density of surface atoms, but the approximations inlierent in the definition of tire Langmuir are often inconsequential. 

The addition of potassium to Fe single crystals also enliances the activity for ammonia synthesis. Figure A3.10.19 shows the effect of surface potassium concentration on the N2 sticking coefficient. There is nearly a 300-fold increase in the sticking coefficient as the potassium concentration reaches -1.5 x 10 K atoms cm ... 

Very recently, considerable effort has been devoted to the simulation of the oscillatory behavior which has been observed experimentally in various surface reactions. So far, the most studied reaction is the catalytic oxidation of carbon monoxide, where it is well known that oscillations are coupled to reversible reconstructions of the surface via structure-sensitive sticking coefficients of the reactants. A careful evaluation of the simulation results is necessary in order to ensure that oscillations remain in the thermodynamic limit. The roles of surface diffusion of the reactants versus direct adsorption from the gas phase, at the onset of selforganization and synchronized behavior, is a topic which merits further investigation. 

It shows that sticking is proportional to the availability of empty sites (because there are no lateral interactions in the adsorbate), and the sticking probabilities, Sy and S, are weighted by the fraction of the adsorbate-free surface that is reconstructed or not. This can obviously introduce a substantial temperature dependence in the 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. 

If we now assume that this surface at temperature T is in equilibrium with a gas then the adsorption rate equals the desorption rate. Since the atoms/molecules are physisorbed in a weak adsorption potential there are no barriers and the sticking coefficient (the probability that a molecule adsorbs) is unity. This is not entirely consistent since there is an entropic barrier to direct adsorption on a specific site from the gas phase. Nevertheless, a lower sticking probability does not change the overall characteristics of the model. Hence, at equilibrium we have... 

The rate of adsorption of a gas on a surface is determined by the rate of collision between the gas and the surface and by the sticking coefficient ... 

Measuring the uptake of a gas by a surface as a function of the dose to which the surface is exposed is the most straightforward way to determine a sticking coefficient. In such experiments, great care should be taken to ensure that gas and surface are in thermal equilibrium. In addition, we need to determine the coverage, either by surface sensitive methods (XPS, AES, IR) or by thermal desorption and ensure that adsorption is not accompanied by desorption. 

Frequently, adsorption proceeds via a mobile precursor, in which the adsorbate diffuses over the surface in a physisorbed state before finding a free site. In such cases the rate of adsorption and the sticking coefficient are constant until a relatively high coverage is reached, after which the sticking probability declines rapidly. If the precursor resides only on empty surface sites it is called an intrinsic precursor, while if it exits on already occupied sites it is called extrinsic. Here we simply note such effects, without further discussion. 

Having estimated the sticking coefficient of nitrogen on the Fe(lll) surface above, we now consider the desorption of nitrogen, for which the kinetic parameters are readily derived from a TPD experiment. Combining adsorption and desorption enables us to calculate the equilibrium constant of dissociative nitrogen adsorption from... 

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. 

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