Frequency dependence coverage

Contrary to the findings for water, the relative change of the absolute values of Ti and of its frequency dependence is much more pronounced for n-heptane, which possesses a high affinity for the surface coke. One of the steps in the dispersion function can be interpreted as being related to the onset of full coverage of the surface. Because the maximum coke content corresponds to an average layer... 

The general expression (10.3) guides development of impedance models from proposed reaction sequences. The reaction mechanisms considered here include reactions dependent only on potential, reactions dependent on both potential and mass transfer, coupled reactions dependent on both potential and surface coverage, and coupled reactions dependent on potential, surface coverage, and mass transfer. The proposed reaction sequence has a major influence on the frequency dependence of the interfacial Faradaic impedance described in Qiapter 9. 

Turnover frequencies depend on the local gas-phase species concentrations above the catalyst surface, the temperature, and the surface coverage. The surface reaction rate is specific to the catalyst formulation, such that there is a unique rate expression for every catalyst which depends not only on the active material but also on the support, the washcoat type and structure, and the preparation method. Different catalyst surface structures generally vary in their reaction pathways and their corresponding kinetic expressions. 

Figure 2.15. Dependence of the initial heats of CO adsorption. AH 0, on the alkali coverage, as estimated from the CO TPD spectra at very low CO coverages assuming invariable frequency factor45,46 (a) and on the corresponding work function change AO45,46 (b). Reprinted with permission from Elsevier Science.

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. 

In the limit of small pressure perturbations, any kinetic equation modeling the response of a catalyst surface can be reduced to first order. Following Yasuda s derivation C, the system can be described by a set of functions which describe the dependence of pressure, coverage amplitude, and phase on T, P, and frequency. After a mass balance, the equations can be separated Into real and Imaginary terms to yield a real response function, RRF, and an Imaginary response function, IRF ... 

Loffreda D, Simon D, Sautet P. 1999. Dependence of stretching frequency on surface coverage and adsorbate-adsorbate interactions A density-functional theory approach of CO on Pd (111). Surf Sci 425 68-80. 

---