Consecutive adsorption mechanisms

Fig. 18. Three-dimensional kinetic dependence for the "consecutive adsorption mechanism. 

These values can be correlated with the heat of adsorption of hydrogen on the catalytic metal since the oxidation mechanism, apart from diffusion and mass transport limitations, is controlled by an adsorption step in a two consecutive step mechanism ... 

This is an adsorption mechanism (we call it a "parallel adsorption mechanism, since the adsorption of reactants follows two independent routes in what follows we will consider one more adsorption mechanism that is "consecutive ). 

Let us examine one more simple three-step mechanism whose steady-state characteristics are also of the hysteresis type. In what follows we will show that their type differs considerably from the previous one. It is the mechanism including steps of "consecutive adsorption one gas-phase substance is adsorbed on unoccupied sites and is then joined by a second gaseous substance, whereupon the two intermediates interact. In the general form this... 

A comparative analysis of steady-state characteristics for "consecutive and "parallel three-step adsorption mechanisms with two independent intermediates shows that, to interpret the multiplicity of steady states, the... 

The interaction of H2S, organic sulfides, and other sulfur compounds may involve a number of consecutive steps including reversible molecular adsorption of the sulfur compound, its dissociation, reorientation or reconstruction of the metal surface, formation of a 2-D surface sulfide, and at still higher H2S/H2 ratios, formation of a three-dimensional (3-D) (bulk) metal sulfide. Kinetic information about these processes may generally be helpful in elucidating the adsorption mechanism. Unfortunately, such quantitative kinetic information is not adequately available, with one exception, formation of bulk sulfides (9, 96). 

As discussed earlier, such mechanisms involving two adsorbed H species with different bond energies and sites probably apply to all transition metals. It is likely that the sites for the HER adsorbed intermediate (OP HajJ are the weakly bonded on-top sites. But on oxidizable metals, H electroadsorption in the hollow strongly bonded sites followed by absorption is possible only at potentials below the limit of desorption of the oxygen species, calculated for Ni and Fe to be dose to 0V(rhed [28]. It is admitted that on iron and ferrous alloys the HER and the HAR share a common H ds intermediate and that the surface-bulk transfer step is in quasiequilibrium [8] (see Section 2.2.2.2.S). The mechanism of H absorption into the bulk of transition metals at cathodic overpotential must then involve a sequence of two consecutive adsorption steps ... 

In general, three basic kinds of sorption mechanisms for trace elements in geologic aqueous systems can be distinguished (56). Due to non-specific forces of attraction between sorbent and the solute, a physical adsorption may occur. This sorption mechanism results in the binding of species from the solution in several consecutive layers on exposed solid surfaces. This would be a rapid non-selec-tive and reversible process, fairly independent of nuclide concentration and only little dependent on ion exchange capacity of the solid. 

We finish this section with an example of a dehydrogenation reaction on a metal, which is instructive as it does not rely on equilibrium adsorption, unlike most other examples given in this book. The dehydrogenation of methylcydohexane, CfiHiiCHs, to toluene, QH5CH3, on platinum is well described by a reaction mechanism consisting of consecutive steps in the forward direction only ... 

The kinetics of the ammoxidation of xylenes over a vanadium catalyst and mixed vanadium catalysts were studied. The reaction rate data obtained were correlated with the parallel consecutive reaction scheme by the rate equations based upon the Langmuir-Hinshelwood mechanism where the adsorption of xylenes was strong. The reaction rates of each path are remarkably affected by the kind of xylene and catalyst. The results of the physical measurement of catalysts indicated that the activity and the selectivity of reaction were affected by the nature and the distribution of metal ions and oxygen ion on catalyst surface. 

If these conditions are not satisfied, some process will be involved to prevent accumulation of the intermediates at the interface. Two possibilities are at hand, viz. transport by diffusion into the solution or adsorption at the electrode surface. In the literature, one can find general theories for such mechanisms and theories focussed to a specific electrode reaction, e.g. the hydrogen evolution reaction [125], the reduction of oxygen [126] and the anodic dissolution of metals like iron and nickel [94]. In this work, we will confine ourselves to outline the principles of the subject, treating only the example of two consecutive charge transfer processes O + n e = Z and Z 4- n2e — R. 

A general scheme of hydrogenation of 1,3-butadiene, the rake mechanism, is given in Scheme 7. Two kinds of selectivity for the intermediate butenes can be defined. Selective formation of butenes can be expected if > 4 (mechanistic selectivity). On the other hand, if k /k- > kj,/k, adsorption of the diene prevents the readsorption of butenes and hence the consecutive hydrogen addition cannot take place (thermodynamic... 

Dautzenberg et al. (3) have determined the kinetics of the Fischer-Tropsch synthesis with ruthenium catalysts. The authors showed, that because the synthesis can be described by a consecutive mechanism, the non steady state behaviour of the catalyst can give information about the kinetics of the process. On ruthenium they found that not only the overall rate of hydrocarbon production per active site is small, but also that the rate constant of propagation is low. Hence, Dautzenberg et al. find that the low activity of Fischer-Tropsch catalysts is due to the low intrinsic activity of their sites. On the other hand, Rautavuoma (4) states that the low activity of cobalt catalysts is due to a small amount of active sites, the amount being much smaller than the number of adsorption sites measured. 

However, the reason of the appearance of negative impedance is always a chemical/electrochemical process. In most cases the blocking (inactivation) of the electrode (metal) surface is the pivotal (autoinhibition) step in the mechanism behind the emergence of the oscillating behavior. The blocking can be a consequence of adsorption of ions or molecules, chemisorption of molecular fragments, deposition of metals, salts or other compounds, formation of oxide layer etc. In all cases several coupled, consecutive, and simultaneous processes occur. The oscillating behavior appears only at a certain set of parameters (concentrations of the electro-chemically active species, the nature and the concen-... 

These fuel cell results, completed by the different spectroscopic and chromatographic results, allowed us to propose a detailed reaction mechanism of ethanol oxidation, involving parallel and consecutive oxidation reactions, on Pt-based electrodes, where the key role of the adsorption steps was underlined. 

As evident from Fig. 8.4, an increase in the selectivity has been observed in IL/ scCOj biphasic systems media (>99.5%) with respect to scCO assayed alone (95%). These results could be explained by the use of water-immiscible ILs which have a specific ability to reduce water activity in the enzyme microenvironment. The synthetic activity of the immobilized lipase in IL/scCO biphasic systems is lower than that in scCO assayed alone. Similar results were found by Mori et al. [40] in IL/ hexane biphasic systems. These authors reported that the enzymatic membranes prepared by simple adsorption of CaLB onto the surface were more reactive than membranes prepared with ILs. As can be observed in Fig. 8.4, the initial reaction rate in the assayed IL/scCO biphasic systems increased in the following sequence [bdimim ][PF ]<[bmim ][PFg ]<[bmim ][NTfj ]<[omim ] [PF ], which was practically in agreement with flie activity sequence reported by these authors using free Candida antarctica lipase B in homogeneous ionic liquid systems ([bmim ] [PF ]<[bdmim+][PFg ]<[bmim+][NTfj ]<[omim ][PF ]), with the exception of [bmim [PF ] and [bdimim+][PFg ]. These results were explained taking into account that biotransformation occurs within the ionic liquid phase, so substrates have to be transported from scCOj to the ionic liquid phase. The mechanism of substrate transport between the ionic liquid and the supercritical carbon dioxide could be by three consecutive steps diffusion of the substrates through the diffusion... 

Fig. 6-9. Experimental results of Cu in soil solution for McLaren soil vs. time during adsorption and desorption for initial concent rat ion C = 50 and 100 mg L. The solid and dashed curves are predictions based on parameters from adsorption data using the multireaction and second-order models given in Tallies b -I anil b V tespcclively. The irreversible mechanism was accounted for based on consecutive reactions...

Obviously, the assumption of non-dissociative adsorption as well as that of the reaction being first order in both reactants are gross simplifications, since both methane and oxygen will dissociate prior to the reaction, and the further surface reaction mechanism will involve several consecutive and parallel reaction steps. Nevertheless, the model should allow for a qualitative test of the assumption that site competition between methane and oxygen account for the observed trend in ignition temperatures. 

Kinetic measurements of the rate of transport of a labelled compound in a tissue depend on the specification of the time and the site of entry of the labelled compound into the system. Ideally, one would like to inject the labelled compound directly into the system under study. However, the local application of the labelled substances introduces a serious risk of disturbing both the timing of precursor adsorption into the system and the rate of incorporation. There may be no way of knowing whether the true physiological circumstances are preserved. Furthermore, the local application of the labelled compound does not enhance the specificity of its incorporation in the polypeptide, as opposed to other uptake mechanisms . Since the measurement of isotope accumulation requires that the animal be sacrificed, it is not possible to take consecutive samples from the same animal as a function of time. The kinetic measurements must therefore represent a picture of the mean behaviour of the isotope in a population of animals . 

The methoxy species formed by the dissociation of methanol can react with a second methanol molecule to give dimethyl ether and a proton. This is termed the alkoxy intermediate for the consecutive direct reaction mechanism. Alternative reaction paths exist which can be described as associative reaction mechanisms in which the product of an association reaction is formed without the formation of an alkoxy intermediate species . For two coadsorbed methanol molecules, the most stable adsorption mode does... 

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