Rate determining step during surface

Independently of this, chronoabsorptiometric measurements by Genies et al. have proved that PPy films grow in timer linear to t and not to j/t. In the opinion of the authors this implies that the rate-determining step during film growth is a radical ion coupling and not the diffusion of the uncharged monomer towards the electrode surface. The attested phenomenon that PPy polymerizes... 

The various TPR peaks may correspond to different active sites. One hypothesis assumed cyclization over metallic and complex (Section II,B,4) platinum sites (62e) the participation of various crystallographic sites (Section V,A) cannot be excluded either. Alternatively, the peaks may represent three different rate determining steps of stepwise aromatization such as cyclization, dehydrogenation, and trans-cis isomerization. If the corresponding peak also appears in the thermodesorption spectrum of benzene, it may be assumed that the slow step is the addition of hydrogen to one or more type of deeply dissociated surface species which may equally be formed from adsorbed benzene itself (62f) or during aromatization of various -Cg hydrocarbons. Figure 11 in Section V,A shows the character of such a species of hydrocarbon. 

Mass transport processes are involved in the overall reaction. In these processes the substances consumed or formed during the electrode reaction are transported from the bulk solution to the interphase (electrode surface) and from the interphase to the bulk solution. This mass transport takes place by diffusion. Pure diffusion overpotential t]A occurs if the mass transport is the slowest process among the partial processes involved in the overall electrode reaction. In this case diffusion is the rate-determining step. 

Most of the redox centers in a polymer film cannot rapidly come into direct contact with the electrode surface. The widely accepted mechanism proposed for electron transport is one in which the electroactive sites become oxidized or reduced by a succession of electron-transfer self-exchange reactions between neighboring redox sites [22]. However, control of the overall rate is a more complex problem. To maintain electroneutrality within the film, a flow of counterions and associated solvent is necessary during electron transport. There is also motion of the polymer chains and the attached redox centers which provides an additional diffusive process for transport. The rate-determining step in the electron site-site hopping is still in question and is likely to be different in different materials. 

A simple topochemical model for the growth of NiO islands on the Ni surface during the reaction of oxygen with a Ni(lll) crystal is clearly described by Holloway and Hudson [112]. They considered three cases in which the rate-determining step is, respectively (a) oxygen sorption from the gas phase (surface diffusion is fast), (b) surface diffusion of oxygen, and (c) oxygen insertion over the island boundary. 

Van Ho and Harriott have postulated a different mechanism involving intermediate adsorbed carbon atoms.188 This is based on their observation that a larger fraction of the surface of the catalyst was covered by adsorbed carbon monoxide during reaction. The rate determining step was assumed to... 

Further research (22-24) has shown that butene oxidation can produce many selective reaction products (furan, acetaldehyde, and methyl vinyl ketone), which are not detected during butane oxidation. It cannot be assumed that the oxidation of butane and of the unsaturated reactants proceed along the same pathway. The kinetics data must be viewed with this point in mind, although butane activation is widely accepted to be the rate-determining step. The intermediates are capable of desorbing from the surface (as observed in the TAP investigations), but they do not, indicating that the further reactions occur more readily than desorption. 

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