Zero coverage, measurements

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] ... 

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...

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.

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. 

Here, AH(A-B) is the partial molar net adsorption enthalpy associated with the transformation of 1 mol of the pure metal A in its standard state into the state of zero coverage on the surface of the electrode material B, ASVjbr is the difference in the vibrational entropies in the above states, n is the number of electrons involved in the electrode process, F the Faraday constant, and Am the surface of 1 mol of A as a mono layer on the electrode metal B [70]. For the calculation of the thermodynamic functions in (12), a number of models were used in [70] and calculations were performed for Ni-, Cu-, Pd-, Ag-, Pt-, and Au-electrodes and the micro components Hg, Tl, Pb, Bi, and Po, confirming the decisive influence of the choice of the electrode material on the deposition potential. For Pd and Pt, particularly large, positive values of E5o% were calculated, larger than the standard electrode potentials tabulated for these elements. This makes these electrode materials the prime choice for practical applications. An application of the same model to the superheavy elements still needs to be done, but one can anticipate that the preference for Pd and Pt will persist. The latter are metals in which, due to the formation of the metallic bond, almost or completely filled d orbitals are broken up, such that these metals tend in an extreme way towards the formation of intermetallic compounds with sp-metals. The perspective is to make use of the Pd or Pt in form of a tape on which the tracer activities are electrodeposited and the deposition zone is subsequently stepped between pairs of Si detectors for a-spectroscopy and SF measurements. 

Fig. 58. Ixical density of states on the clean metal surface at the Fermi level and (extrapolated) IR stretch frequency of CO adsorbed on the same surfaces. For samples of Pl/zeolite with different zeolite acidities, the clean metal-surface Ef LDOS was found from curves as in 1 ig. 57. Next, CO was adsorbed on these same samples, and the stretch frequency was measured as a function of coverage. The value plotted here is an extrapolation to zero coverage. This plot indicates that the C-O intramolecular bond after chemisorption will be weaker if the initial clean-surface LDOS is higher.

Zero Coverage. Inverse gas chromatography has been used successfully In the past decade for studying the surface properties of solids by adsorption of vapour at a gas-solid Interface. Unlike conventional adsorption techniques, IGC allows the measurement of adsorption data down to low vapour concentrations where the surface coverage approaches zero, adsorbate-adsorbate Interactions are negligible, and thermodynamic functions depend on only adsorbate-adsorbent Interactions. 

Zero coverage. In order to eliminate physically adsorbed species, fibers were cleaned by heating at 160°C In a N2 (Linde, ultra high purity, with C02 content less than 1 ppm) atmosphere until constant retention volumes were obtained (100 to 120 h). Using finite concentration IGC and n-alkanes as sorbates, the surface area of these fibers was determined to be 0.40 m g"1 and 0.59 m g 1 for T-300 and P-55, respectively. The n-alkanes octane to trldecane (analytical grade) were obtained from Polyscience Corporation (Quantklt). Retention data were measured with a Hewlett-... 

Zero Coverage. The peaks at Infinite dilution were slightly skewed (skew ratio 0.8), with virtually no dependence of retention volume on Injection size. Instead of the peak maximum method, retention volumes were measured by the method proposed by Conder and Young (32). To ensure that the adsorption of n-alkanes on carbon fibers was taking place under equilibrium conditions, the flow rate was varied In the range 20 to 32 cm3 min-1. The net retention volumes were essentially Independent of flow rate. 

It is to be noted that the considerations presented above demonstrate the application of the gas chromatographic version of the molecular probes method, proposed by authors in [46]. This method can be used to obtain reliable estimates of the size and shape of micropores on a chromatographic adsorbent surface. The fact that these estimates agree with the results of adsorption chromatographic measurements at non-zero coverage (Figure 2) supports the conclusion regarding the applicability of the method to the study of surface-porous adsorbents. 

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