Adsorption reactants

In studies of electrochemical photocurrents obtained at molecular semiconductor thin films (PcZn, (CN)sPcZn, TPyTAPZn, MePTCDI) and their dependence on the concentration of the reactant in the electrolyte (O2, ethylthiolate (RS ), hydroquinone/benzoquinone (HQ/BQ), Fe(CN)g, a sat- 

The time dependence of photocurrents on the scale of seconds to minutes showed a decay which was caused by changes in the surface coverage of the reactant as a result of the electrochemical reaction (term kfC /k in Equation 10.3b) and a monoexponential decrease was expected within the Langmuir model  

Monoexponential decay functions were found to fit the transients in the light- induced reduction of O2 at electrodes of PeZn showing the validity of the model . Qualitatively similar transients were also observed, e.g., in the light-induced oxidation of RS but its detailed analysis showed that a monoexponential 

This is another complicating phenomenon, the possible occurrence of which is sometimes ignored. The essential problem to be solved is that a species, e.g. the species O, diffuses to or from the electrode surface because of two processes occurring simultaneously, viz. a charge transfer reaction 0 + ne = R and an adsorption O = Oads. The flux of O equals the sum of the rates of both processes 

The phenomenon was observed for the first time by Brdicka [13] in the reduction of riboflavin and methylene blue studied by d.c. polaro-graphy. It did not receive much attention until it was rediscovered by several workers studying systems such as Tl+/Tl(Hg), Pb2+/Pb(Hg), and the more obvious system Co(III)—Co(II)—tris-ethylenediamine using impedance measurements [14—16]. The confusion that arose initially in the literature around this subject in the end led to the correct theoretical description [15, 17, 18], the general treatment remaining very complex [18,19]. 

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The adoption of new hypotheses for the reactants adsorption, the removal of all the empiric laws and parameters, and a reevaluation of the temperature effect on product distribution have allowed us to obtain significant improvements with respect to our previous work,10 in terms of both fitting ability and model consistency. 

Holub, K. and van Leeuwen, H. P. (1984). Influence of reactant adsorption on limiting currents in normal pulse polarography. 2. Theory for the stationary, spherical electrode, J. Electroanal. Chem., 162, 55-65. 

Carbon monoxide oxidation is a relatively simple reaction, and generally its structurally insensitive nature makes it an ideal model of heterogeneous catalytic reactions. Each of the important mechanistic steps of this reaction, such as reactant adsorption and desorption, surface reaction, and desorption of products, has been studied extensively using modem surface-science techniques.17 The structure insensitivity of this reaction is illustrated in Figure 10.4. Here, carbon dioxide turnover frequencies over Rh(l 11) and Rh(100) surfaces are compared with supported Rh catalysts.3 As with CO hydrogenation on nickel, it is readily apparent that, not only does the choice of surface plane matters, but also the size of the active species.18-21 Studies of this system also indicated that, under the reaction conditions of Figure 10.4, the rhodium surface was covered with CO. This means that the reaction is limited by the desorption of carbon monoxide and the adsorption of oxygen. 

Bimolecular surface reactions reactants adsorption, 29 111-112 with single reactant, 29 108-109 1,1 -Binaphthyl, dehydrocyclization, 28 318 Binary oxides, 32 119 Binding energy, 32 160-162 chemisorbed sulfur, 37 281 hydrogen, sulfur effect, 37 295-296 shift, Pd, 37 62-64 ZnO/SiOj, 37 21-22 Binor-S, see Norbomadiene Biological systems, hydrogen in, activation of, 11 301... 

The enhancement of SWV net peak current caused by the reactant adsorption on the working electrode surface was utilized for detection of chloride, bromide and iodide induced adsorption of bismuth(III), cadmium(II) and lead(II) ions on mercury electrodes [236-243]. An example is shown in Fig. 3.13. The SWV net peak currents of lead(II) ions in bromide media are enhanced in the range of bromide concentrations in which the nentral complex PbBr2 is formed in the solntion [239]. If the simple electrode reaction is electrochemically reversible, the net peak cnnent is independent of the composition of supporting electrolyte. So, its enhancement is an indication that one of the complex species is adsorbed at the electrode snrface. 

Steam is invariably present in a real exhaust gas of motor vehieles in relatively high concentration due to the fuel combustion. The influence of water vapor on catalytic performances should not be ignored when dealing with the aim to develop a practical TWCs. Cu/ZSM-5 catalysts once were regarded as suitable substitutes to precious metal catalysts for NO elimination[78], nevertheless, they are susceptible to hydrothermal dealumination leading to a permanent loss of activity[79], Perovskites have a higher hydrothermal stability than zeolites[35]. Although perovskites were expected to be potential autocatalysts in the presence of water[80], few reports related to the influence of water on the reactants adsorption, the perovskite physicochemical properties, and the catalytic performance in NO-SCR were previously documented. The H2O deactivation mechanism is also far from well established. 

Effect of Steam on Reactants Adsorption over LaFeo.8Cuo.2O3... 

Enhanced reaction rates have also been observed in some instances of decreased polymer solubility with conversion when the precipitating polymer absorbs the small-molecule reactant. Adsorption results in an increased concentration of the reactant at the actual polymerization site (the polymer coils). Similar effects are observed if a catalyst is adsorbed onto the precipitating polymer [Beresniewicz, 1959]. 

The electrochemical behavior of Cd-oxine complexes was analyzed by square-wave stripping voltammetry [80] from the mechanistic point of view, applying the theoretical model developed previously [81-83]. Influence of ligand and reactant adsorption, and the ligand concentration on the Cd-oxine electroreduction were also examined [84] using SWV. Typical curves recorded and calculated for several Egw are shown in Fig. 5. 

Lovric M (2002) Theory of square-wave voltammetry of a reversible redox reaction complicated by the reactant adsorption. Electroanalysis 14 405-414. 

In the case of reactant adsorption (see Sect. 1.1.3), the charging current is given by an expression similar to eqn. (7) (with TA replaced by T0 in the example discussed) and it is to be noted that T0 plays a role in determining both the faradaic currrent density, jF, and the charging current density, c. 

In spite of its advantages and the simplicity of its performance, chrono-coulometry is seldom used for studying charge transfer kinetics. In fact, the method is much more popular because of its suitability for the study of reactant adsorption at the initial potential, which is manifested as an extra time-independent amount of faradaic charge involved in the potential step. This will be considered in more detail later. 

Only those mechanisms that involve unstable intermediates are considered here, i.e. it is assumed that all intermediates are present at the interface in a concentration much lower than the sum of the interfacial concentrations cD and cR. This means that no account need be taken of the diffusion or adsorption of the intermediate species. Further, it is supposed that complications like reactant adsorption or coupled homogeneous reactions are absent or unimportant, so that O and R are involved only in the charge-transfer process and are transported only by diffusion. The complications ignored here will be considered in the Sects. 5—7. 

As this chapter is primarily dedicated to the study of electrode kinetics, we wish to deal only briefly with the fundamental consequences of reactant adsorption for the methodology of the relaxation techniques, again confined to the potential step and the impedance methods. In addition, we will review briefly the potentialities of these methods with regard to the study of adsorption itself in the case of the reversible electrode reaction. 

The latter function needs special attention as it contains the mean surface concentrations c 0 and cR corresponding to the d.c. potential control. In the absence of reactant adsorption, one would have ip —Dq2Cq + Dr2cr, but in the presence of reactant adsorption, this expression should be corrected by a term containing T0 [143]. 

Another important factor which can significantly affect the shape and position of the voltammetric wave, and hence rate constants, is adsorption of product or reactant on the electrode surface. For a linear adsorption isotherm, if the reactant is adsorbed, a reduction wave will be shifted towards more negative potentials and if the product is adsorbed, towards more positive potentials [160], Non-linear adsorption isotherms give rise to pre-waves (product adsorption) and post-waves (reactant adsorption), a phenomenon which was first discussed by Brdidka at the DME and since then by many authors [161, 162]. At the RDE,... 

If reactant adsorption can be described by the above equations it is to be expected (59) that groups of related reactions will exhibit compensation behavior. 

Carbon dioxide is adsorbed quite strongly. The reactant adsorption strengths are estimated to decrease in the order C02 > CO > H2. 

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