Adsorption-inhibition model

Electrochemical impedance spectroscopy was used to determine the effect of isomers of 2,5-bis( -pyridyl)-l,3,4-thiadiazole 36 (n 2 or 3) on the corrosion of mild steel in perchloric acid solution <2002MI197>. The inhibition efficiency was structure dependent and the 3-pyridyl gave better inhibition than the 2-pyridyl. X-ray photoelectron spectroscopy helped establish the 3-pyridyl thiadiazoles mode of action toward corrosion. Adsorption of the 3-pyridyl on the mild steel surface in 1M HCIO4 follows the Langmuir adsorption isotherm model and the surface analysis showed corrosion inhibition by the 3-pyridyl derivative is due to the formation of chemisorbed film on the steel surface. 

Kelvin effect, 153 mattress model of adsorption inhibition, 153-154 medical applications, 155 recrystallization- inhibition applications, 155 repeating tripeptide unit, 152 step pinning model of adsorption inhibition, 153-154 structure, 152 subtypes, 152... 

Sharma, S.K., Mudhoo, A., and Khamis, E. 2009a. Adsorption studies, modeling and use of green inhibitors in corrosion inhibition An overview of recent research. The Journal of Corrosion Science and Engineering, 11 1-24. (Available at http //www.jcse.org/view-paper.php vol=l l pap=14.)... 

As will be demonstrated in this chapter, there has been a strong evolution of fundamental studies recently, giving rise to new concepts that will be explored. In tbe present review, we summarize fundamental approaches used to characterize organic molecule adsorption on model metal (and oxidized) surfaces, in relation to their corrosion inhibition properties, by means of atomistic simulations. 

Figure 9-6. Schematic corrosion inhibition model on a metal surface covered with a porous oxide layer (Kdlman, 1994) in neutral aqueous solutions in the presence of oxygen. Adsorption may occur on the metal surface, and/or the inhibitor may be bound to the surface of the oxide layer and/or it may plug the pores.

Langmuir-Hinshelwood kinetics adsorption inhibition, 62 consistency of, 66 rate-controlling step for, 60 two-step model for, 67 Langmuir-Hinshelwood mechanism, 60 Lewis acid, 6 Lewis number definition of, 107 stability as affected by, 266 Local activity factor, 465 Lumping, 81... 

The influence of electronegative additives on the CO hydrogenation reaction corresponds mainly to a reduction in the overall catalyst activity.131 This is shown for example in Fig. 2.42 which compares the steady-state methanation activities of Ni, Co, Fe and Ru catalysts relative to their fresh, unpoisoned activities as a function of gas phase H2S concentration. The distribution of the reaction products is also affected, leading to an increase in the relative amount of higher unsaturated hydrocarbons at the expense of methane formation.6 Model kinetic studies of the effect of sulfur on the methanation reaction on Ni(lOO)132,135 and Ru(OOl)133,134 at near atmospheric pressure attribute this behavior to the inhibition effect of sulfur to the dissociative adsorption rate of hydrogen but also to the drastic decrease in the... 

Adsorption and Film Formation. Inhibition of HC1 corrosion by organic compounds is a complicated multi-step process. Nevertheless, the effect of an inhibitor on corrosion of a metal is often treated mathematically with an equilibrium adsorption model for displacement of water (19,20) ... 

Large concentrations of Fe + develop in the soil solution in the weeks following flooding, often several mM or tens of mM (Figure 4.5). Calculations with chemical equilibrium models show that the ion activity products of pure ferrous hydroxides, carbonates and other minerals are often exceeded 100-fold (Neue and Bloom, 1989). Evidently precipitation of these minerals is inhibited, probably as a result of adsorption of foreign solutes, such as dissolved organic matter and phosphate ions, onto nucleation sites (Section 3.7). However, once a sufficient supersaturation has been reached there is a rapid precipitation of amorphous solid phases, which may later re-order to more crystalline forms. Only a small part of the Fe(II) formed in reduction remains in solution the bulk is sorbed in exchangeable forms or, eventually, precipitated. 

The inhibition-acceleration mechanism. Moffat et al. (37) proposed the inhibition-acceleration mechanism to explain the experimentally observed comer rounding (inversion of curvature. Fig. 19 in Ref. 37) and general shape evolution in superconformal electrodeposition of copper in vias and trenches of nanometer dimensions (37,38). These authors also smdied a three-additive system composed of two inhibitors and one accelerator. They concluded that superconformal deposition and comer rounding may be attributed to competitive adsorption of inhibitor and accelerator. This model is based on the assumption of curvature (in vias and trenches) -enhanced accelerator coverage. 

The influence of Pb + ions on the kinetics of zinc electrodeposition on Zn electrode in acidic sulfate electrolyte was discussed [217] in terms of a reaction model involving hydrogen adsorption and evolution, a multistep mechanism for zinc deposition and the overall reaction for zinc dissolution. The strongly adsorbed Pbads inhibited all the reactions taking place on the zinc electrode. 

The theoretical calculations described have recently been supported by an extraordinary kinetic analysis conducted by Vanrysellberghe and Froment of the HDS of dibenzothiophene (104). That work provides the enthalpies and entropies of adsorption and the equilibrium adsorption constants of H2, H2S, dibenzothiophene, biphenyl, and cyclohexylbenzene under typical HDS conditions for CoMo/A1203 catalysts. This work supports the assumption that there are two different types of catalytic sites, one for direct desulfurization (termed a ) and one for hydrogenation (termed t). Table XIV summarizes the values obtained experimentally for adsorption constants of the various reactants and products, using the Langmuir-Hinshelwood approach. As described in more detail in Section VI, this kinetic model assumes that the reactants compete for adsorption on the active site. This competitive adsorption influences the overall reaction rate in a negative way (inhibition). 

Regarding the kinetics, the oxidation of o-xylene and o-tolualdehyde were compared for catalysts with different V/Ti ratios (Table 36). The ratio between partial and complete oxidation (X for o-xylene and Y for o-tolualdehyde) are influenced similarly, indicating that a change in the catalyst structure influences all the reaction steps. The oxidation of o-tolualdehyde in mixtures with o-xylene revealed that o-tolualdehyde reduces the o-xylene oxidation rate by a factor of about 2. The authors conclude that a redox model is inadequate and that hydrocarbon adsorption cannot be rate-determining. Adsorption of various products should be included, and equations of the Langmuir—Hinshelwood type are proposed. It should be noted that the observed inhibition is not necessarily caused by adsorption competition, but may also stem from different... 

Co-adsorption and mutual interactions between the reactants on the surface form the basis for understanding the microscopic steps of the reaction. Since product formation takes place rather rapidly above room temperature, this information mainly became available from low-temperature studies. As a result, these processes are much more complicated than can be described by a Langmuir-type adsorption model (i.e., simple competition for free adsorption sites) and, moreover, an asymmetric behavior is found which means that pre-adsorbed CO inhibits the adsorption of oxygen, whereas the reverse is not the case. At very low surface concentrations of CO and Oad these will be randomly distributed over the surface as illustrated schematically by Fig. 32a (88). 

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