Zero-coverage

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. 

Determination of the equilibrium spreading pressure generally requires measurement and integration of the adsorption isotherm for the adhesive vapors on the adherend from zero coverage to saturation, in accord with the Gibbs adsorption equation [20] ... 

The sticking coefficient at zero coverage, Sq T), contains the dynamic information about the energy transfer from the adsorbing particle to the sohd which gives rise to its temperature dependence, for instance, an exponential Boltzmann factor for activated adsorption. 

Figure 6.41. Reactivity of a pseudomorfic overlayer of Ni deposited on Ru(OOOl) for the dissociative adsorption of methane. At zero coverage the measurements reveal the sticking of methane on pure Ru. When nickel atoms are deposited on the surface, the dissociation...

The reader is left to make this trivial conversion. Please note that the slope of the uptake curve at zero coverage equals So(T), and that the above derivation implicitly assumes that the adsorbates do not interact, which is seldom the case. Hence, sticking coefRcients in the limit of zero coverage are the most meaningful quantity. 

Table 10.4 lists the rate parameters for the elementary steps of the CO + NO reaction in the limit of zero coverage. Parameters such as those listed in Tab. 10.4 form the highly desirable input for modeling overall reaction mechanisms. In addition, elementary rate parameters can be compared to calculations on the basis of the theories outlined in Chapters 3 and 6. In this way the kinetic parameters of elementary reaction steps provide, through spectroscopy and computational chemistry, a link between the intramolecular properties of adsorbed reactants and their reactivity Statistical thermodynamics furnishes the theoretical framework to describe how equilibrium constants and reaction rate constants depend on the partition functions of vibration and rotation. Thus, spectroscopy studies of adsorbed reactants and intermediates provide the input for computing equilibrium constants, while calculations on the transition states of reaction pathways, starting from structurally, electronically and vibrationally well-characterized ground states, enable the prediction of kinetic parameters. 

Figure 2.14 Simulated TDS spectra and the results of a number of different analysis procedures for determining the activation energy of desorption. The solid line represents the input for the simulations. Note that only the complete analysis [16] and the leading edge procedure of Habenschaden and Kiippers [29] give reliable results. The Chan-Aris-Weinberg curves [28] extrapolate to the correct activation energies at zero coverage (from de Jong and Niemantsverdriet [31]).

Figure 7. Comparison of (a, solid) electrochemical and (b, dashed) UHV measurements of the H, coverage/potentiaI differential versus potential on Pt(lll).1.) cathodic sweep (25 mV/s) voltammogram in 0.3 M HF from Ref. 20, constant double layer capacity subtracted, b.) dB/d(A ) versus A plot derived from A versus B plot of Ref. 26. Potential scales aligned at zero coverage. Areas under curves correspond to a.) 0.67 and b.) 0.73 M per surface Pt atom.

It is likely that the decreases observed can be rationalized in terms of two contributions. Changes in surface optical properties resulting from modification by the foreign metal have been shown to decrease the electromagnetic enhancement contribution to SERS. However, for the case of Pb UPD on Ag, this effect has been shown to account for only ca. 40% of the decrease in going from zero coverage to one monolayer.(14) Moreover, this model does not account for the relatively rapid decrease in intensity observed with the deposition of small (i.e., less than 20% of a monolayer) amounts of Pb on the Ag surface. 

Figure 10.21 Heats of adsorption for CO on (A) 0.5 ML Au/Ti02, (B) 0.25 ML Au/Ti02, and (C) 0.125 ML Au/Ti02. For comparison, the heat of adsorption of CO on bulk gold at the zero coverage limit is 10.9 kcal/mol. (Reprinted from Meier, D.C. and Goodman, D.W., J. Am. Chem. Soc., 126, 1892-1899, 2004. Copyright 2004. With permission from American Chemical Society.)...

The appearance of the optical absorption bands (Q and B) has a clear threshold at a low non-zero coverage, implying that the electronic structure of the first adsorbed molecules is different from that of the bulk ones. Thus, a clear distinction between molecules directly bonded to the aluminium substrate d- < 0.3 run) and molecules not directly bonded to the substrate d- >0.3 nm) can be made. In the latter case the electronic structure, as revealed by EELS, is identical to that of bulk CuPc, while in the former case modification of the electronic structure prevents transitions toward the LUMO orbital. Above 1.0 nm the Q and B band intensities saturate. The optical transitions are inhibited for molecules directly bonded to the alumiiuum substrate... 

The heat of evaporation of Na from W was measured for coverages of 0 to 10 monolayers of Na, and Eev p varied from 32 kcal./mole at zero coverage to a minimum value of 17 kcal./mole at = 0.75. 

In an adsorption process involving ionic or covalent bonding, the adsorption heats of principal interest are — A//o, the heat of adsorption at zero coverage and S( — AH), the decrease in the heat of adsorption with coverage. It is in connection with the latter that the role of the work func-... 

From simulation studies [23] it arises that the isosteric heat of adsorption increases from zero coverage up to 0 2/3. This increase is mainly due to attractive interactions between neighboring methane. This attraction favours very much the adsorption of pair of methane molecules as small clusters (dimers). The structure of this quasi-one-dimensional phase is essentially determined by the local minima in the gas-solid potential. 

Fig. 27. Variation of the sticking coefficient at zero coverage, S0, with temperature for oxygen adsorption on Pd(lll) (101).

A similar analysis was performed for 0/Rh(l 11) where in the limit of zero coverage i>d = 2.5 x 10-3 cm2 sec-1 and d = 56 2 kcal/mol were derived (146). The latter value is similar to the desorption energies determined by thermal desorption spectroscopy for Pd(lll) [55 kcal/mol (130)] and for Ir(lll) [65 kcal/mol (133)]. A somewhat higher value (80 kcal/mol) was reported for Ru(0001) (148), which probably accounts for the smaller reactivity of this metal in the CO oxidation reaction. Isosteric heats of adsorption were only performed with Pd(100) [60 kcal/mol at medium coverages (756)] and with Pd(110) (2). In the latter case the adsorption energy was found to vary between 80 and 48 kcal/mol with increasing coverage which is similar to the TDS data derived for Ir(l 10) (124). 

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