Coverage by adsorbates

An interesting field of application is the protection of tantalum against hydrogen embrittlement by electrical connection to platinum metals. The reduction in hydrogen overvoltage and the shift of the free corrosion potential to more positive values apparently leads to a reduced coverage by adsorbed hydrogen and thereby lower absorption [43] (see Sections 2.1 and 2.3.4). 

Here Pe and pH are the partial pressures of ethane and hydrogen, respectively, and the parameter a is equal to (6 — x)/2. This analysis was subsequently generalized to include cases in which equilibrium is not established between adsorbed C2H and gas phase ethane (16). Provided that surface coverage by adsorbed species is low, and that equilibrium is maintained between the surface species C2H5 and C2Hx, and H2 in the gas phase, a kinetic analysis leads to the rate expression... 

The spectra in Figure 2.44(b) show the dependence of the EMIRS response on the amplitude of the potential modulation. These were reported to indicate a decrease in coverage by adsorbed species on entering the region of sustained methanol oxidation, as would be expected. 

Isotopic Tracing with l NO. To trace out the dynamics of nitrogen incorporation into the products under conditions where the surface coverage by adsorbed NO remains constant, experiments... 

As will be discussed in Sect. 8, the actual density number of active sites on the electrode for a particular reaction may also depend on the degree of dispersion of catalyst or coverage by adsorbate. The geometric area of the electrode may differ from the true electroactive area, which may also change with electrode potential. 

The simplest treatment of this problem considers that, for a given potential, the electrode reaction rate coefficient is a linear function of the coverage by adsorbate. The overall electrode reaction rate coefficient is thus expressed as a weighted linear combination of the rate coefficients at the covered sites, fex, and at the adsorbate free surface, kQ... 

Finally, we note that the two steps for isobutane activation are kineti-cally significant under the high-temperature reaction conditions. The kinetic model was used to calculate the predicted surface coverage under reaction conditions. Approximately 96% of the surface sites are free under the low-temperature reaction conditions, whereas more than 99% of the sites are free at the higher temperatures. The most abundant surface species are predicted to be adsorbed C3—C. The most abundant heavy species are predicted to be Cg—Cq. The fractional surface coverage by adsorbed Q0—CJ2 is very low (e.g., 10-8), consistent with our decision to neglect surface species heavier than C 2. 

The SECM can be used in the feedback mode to probe lateral mass-charge transfer [79-83]. The theory of SECM feedback surveyed in Section IV.A.2 assumes that the substrate surface is uniformly reactive. When lateral mass and/ or charge transfer occurs on the substrate surface, or within a thin film, the surface reactivity of the substrate becomes non-uniform and the SECM feedback theory must be modified. Unwin and Bard [79] developed the theory for adsorption-desorption of a redox species at the substrate that allowed for surface diffusion of the adsorbate. They introduced a new approach, the scanning electrochemical microscope induced desorption (SECMID), as a way to probe surface diffusion. The set of differential equations for the diffusion problem comprise Eqs. (8a,b), and Eq. (26), which relates the redox concentration at the substrate surface and the surface coverage by adsorbed species... 

What is the dependence of the surface coverage by adsorbate A on the particle size What are the K particle sizes that allow this depen dence to be expected ... 

The coverage by adsorbed hydrogen atoms must be very low, since none was ever detected, even at the highest overpotentials measured. This also rules out atom-atom recombination as the fast second step, since the rate of the reaction... 

The potential dependence of the coverage by adsorbed intermediates should, as a rule, be discussed in terms of the combined adsorption isotherm. The importance of the use of this isotherm grows with the size of the adsorbed species. Thus, the effect is small and may be negligible when we consider small species, occupying only one site on the surface (e.g., and ). It becomes predominant when n is larger than 2 or 3. 

In fact, such a plateau on the open-circuit-decay plot is a good indication of significant coverage by adsorbed intermediates. 

Next we illustrate how electrode reactions differ fundamentally from regular heterogeneous reactions on account of the involvement of electron charge transfer, a process that can be directly modulated in its rate in an instrumen-tally controlled way (by means of a potentiostat and/or an on-line computer). Because of this possibility, the extent of coverage by adsorbed intermediates and the surface electron density of the electrode can also be correspondingly modulated in an experimentally determinable way through measurement of the interfacial double-layer capacitance (7). 

These four methods are complementary in that they all involve, in one way or another, a modulation of the kinetics and course of the reaction in time. The resulting response behavior is then analyzable in terms of (a) rate equations for various steps (104, 108) and (b) potential dependences of coverages by adsorbed intermediates in those steps. The methods have their analogs in temperature- and pressure-step methods (T-jump or P-jump techniques of Eigen) used in the study of the kinetics of fast homogeneous reactions. In fact a T-jump method has recently been developed for the study of electrochemical reactions by Feldberg (109). 

The analysis of the situation is then quite complex but was worked out by Gilroy et al. (124) and applied to change of coverage by adsorbed oxygen species in the anodic O2 evolution reaction at nickel oxide. A more recent development of their analysis has been given by Lasia (125). 

XV. Determination of Coverage by Adsorbed H in Hydrogen Evolution Reaction at Transition Metals... 

Until recently, surprisingly little work had been done experimentally on the important aspect of coverage by adsorbed H in the kinetic and catalytic behavior of the cathodic H2 evolution reaction. Theoretically, the relation between potential dependence of coverage, 0 , of the H intermediate [see Eqs. (65) and (81)] and the mechanism and kinetics of the HER had been treated extensively, but experimentally evaluated On data to which kinetic behavior could be related remained mostly lacking until recently. It is obviously a very important aspect of electrocatalysis behavior that should be experimentally determined. 

Fig. 19. Integration of versus potential relations for H adsorption at Ni and the Ni-Mo composites giving the change of coverage by adsorbed H in the HER with increasing overpotential. (From Ref 75.)...

Rao et al. (433) also reported that a linear relationship exists between d-band vacancy and the coverage by adsorbed oxygen in an 02-saturated solution. Burke et al. (413) and Llopis and Vazquez (435) showed that the greater the d-band vacancy in the metal, the lower is the potential at which anodic film initiation is observed. The work of Arikado et al. (357, 407)... 

The agreement between the results obtained either by direct measurements at 200° or indirect calculations for oxygen chemisorbed at 250° is considered as a proof of the accuracy of the thermochemical cycles which were used for the indirect calculations (49). As the temperature is increased, the maximum surface coverage by adsorbed oxygen... 

The surface coverage by adsorbed oxygen is smaller than the coverage by adsorbed carbon monoxide [which is partially transformed by oxygen into C08"(ads) ions]. Moreover, interaction between adsorbed oxygen ions and carbon monoxide yields adsorbed carbon dioxide. 

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