Parallel adsorption 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 ). 

Since the same mechanisms are involved, aggregation times are generally similar to adsorption times. As a consequence, ffoc growth rates typically parallel adsorption rates or, for continuous polymer addition, the rate of polymer addition. Furthermore, ffoc growth continues only so long as fresh polymer is... 

The interaction of N2 with transition metals is quite complex. The dissociation is generally very exothermic, with many molecular adsorption wells, both oriented normal and parallel to the surface and at different sites on the surface existing prior to dissociation. Most of these, however, are only metastable. Both vertically adsorbed (y+) and parallel adsorption states (y) have been observed in vibrational spectroscopy for N2 adsorbed on W(100), and the parallel states are the ones known to ultimately dissociate [335]. The dissociation of N2 on W(100) has been well studied by molecular beam techniques [336-339] and these studies exemplify the complexity of the interaction. S(Et. 0n Ts) for this system [339] in Figure 3.36 (a) is interpreted as evidence for two distinct dissociation mechanisms a precursor-mediated one at low E and Ts and a direct activated process at higher These results are similar to those of Figure 3.35 for 02/ Pt(lll), except that there is no Ts... 

For the infrared spectra there is, of course, no impact mechanism available for exciting additional features from not completely symmetrical modes, and none of the in-plane ca. 1395 (pi3), 1275 (v9), or 1147 cm 1 (vU) or vv) modes would be allowed if the MSSR applied strictly to parallel adsorption on (111) or (100) facets. They would, however, all become allowed on a Cs site, such as would arise from adsorption on twofold bridges. The infrared spectra of alternative monosubstituted or ortho-disubstituted benzenes (the most likely dissociatively adsorbed species) would give rise to two additional strong bands between 1400 and 1620 cm-1, and so the observed spectrum is again seen to be consistent with nondissociative adsorption. 

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 rate of contaminant adsorption onto activated carixm particles is controlled by two parallel diffusion mechanisms of pore and surface diffusion, which operate in different manners and extents depending upon adsorption temperature and adsorbate concentration. The present study showed that two mechanisms are separated successfully using a stepwise linearization technique incorporated with adsorption diffusion model. Surface and pore diffiisivities were obtained based on kinetic data in two types of adsorbers and isothermal data attained from batch bottle technique. Furthermore, intraparticle diffiisivities onto activated carbon particles were estimated by traditional breakthrough curve method and final results were compared with those obtained by more rigorous stepwise linearization technique. 

SCHEME 2.5 Wroblowa s oxygen electroreduction mechanism with electrochemical rate constants k3 forthe direct reduction to water, k2 and k 2 for the parallel electrochemical reduction and oxidation of adsorbed hydrogen peroxide, k3 for the electrochemical decomposition of hydrogen peroxide to water, k4 for the chemical catalytic decomposition of hydrogen peroxide to oxygen, and ks and fc 5 for the parallel adsorption/desorption of the adsorbed hydrogen peroxide. The species with the super index and ° are located at the interface and in the bulk of the solution, respectively. The sub index ads refers to the adsorbed state, and the term dif symbolizes the diffusion to the bulk of the solution and vice versa. 

Various laboratory-scale projects have been carried out to estimate the sorption affinity and adsorption mechanisms of MTBE on different porous materials. Most of the results have been obtained from batch tests, the adsorbents used varying from natural soils [63] to natural and synthetic resins [60, 62,64] and zeohtes [58,59,65-69]. Parallel studies were performed by most authors to compare the adsorption potential of the above-mentioned adsorbents with GAC. Table 6 shows the adsorption of MTBE on zeolites and resins, where represents the maximiun adsorption capacity at equilibrium, q is the initial concentration and Ce the equihbrium concentration [61]. 

The mechanism of aromatic hydrogenation is still the subject of extensive research. Most of the work presented describes hydrogenation as proceeding through parallel adsorption of the aromatic ring on the metallic surface, by interaction of the r-electrons of the aromatic ring with empty <7-orbitals of the metal... 

In nonlinear adsorption systems where parallel diffusion mechanisms hold, the mass balance equation given in eq. (9.2-lb) is still valid. The difference is in the functional relationship between the concentrations of the two phases, that is the local adsorption isotherm. In general, this relationship can take any form that can describe well equilibrium data. Adsorption isotherm such as Langmuir, Unilan, Toth, Sips can be used. In this section we present the mathematical model for a general isotherm and then perform simulations with a Langmuir isotherm as it is adequate to show the effect of isotherm nonlinearity on the dynamics behaviour. The adsorption isotherm takes the following functional form ... 

This chapter has presented a number of adsorption models for homogeneous particles where parallel diffusion mechanism is operating. This type of mechanism is applicable to solids such as activated carbon. We will present in the next chapter a number of models for zeolite type solids where a bimodal diffusion mechanism is operating. 

Llewellyn and Maurin (2005) demonstrated that gas (nitrogen and argon) adsorption microcalorimetry can be used a powerful technique for depth examination of the surface state of adsorbents and a minute following of adsorption mechanisms such as phase changes and transitions. The use of this technique in parallel to the measurements (and appropriate analysis) of the adsorption isotherms of the same gases, and DSC and/or NMR cryoporometry measurements can provide deeper insight into the interfacial phenomena over a broad temperature range. 

The adsorption of higher -alkanols from their dilute solutions in n. heptane" exhibited a pronounced step indicating a strong cooperative adsorption mechanism leading to a closely-packed monolayer of alkanol molecules oriented with their chain axis oriented parallel to the graphite basal planes. This unusual adsorption behavior in dilute nonaqueous solutions was attributed to an order-disorder transition of the alkanols adsorbed in the pores at the liquid-carbon interface. Prakash," while studying the adsorption isotherms of two cationic pesticides, diquat and paraquat from aqueous... 

MCM-41 is the well known model mesoporous material containing extended hexagonal arrays of parallel tubular pores separated by amorphous silica (Fig. 1.). This is a common description, on the heise of most studies to interpret adsorption phenomenon results. However, the structural and chemical parameters of the host porous material have a crucial influence on the adsorption mechanism. Actually each synthesis produces MCM-41 sample with its own silica wall and porosity. Our MCM-41 (19 A < 0 < 40 A) samples have been extensively characterized by sorption isotherm, microcalorimetry and neutron diffraction measurements... 

Adsorption onto the membrane does contribute to the removal of noxious compounds, such as interleukin-1 and interleukin-6 (lL-1, lL-6), tumor neaosis factor (TNF), peptides and 62-microglobuhn [71], As a result of the restricted surface area of dialyzers, however, adsorption capacity will rapidly be saturated. Acceptable rates of net removal by adsorption can only be achieved if the surface area is drastically increased by specifically designed devices [75, 76] or blood purification systems combining filtration and adsorption mechanisms in two devices, either arranged in parallel or in series. 

In either equation, /c is given by Eq. (16-84) for parallel pore and surface diffusion or by Eq. (16-85) for a bidispersed particle. For nearly linear isotherms (0.7 < R < 1.5), the same linear addition of resistance can be used as a good approximation to predict the adsorption behavior of packed beds, since solutions for all mechanisms are nearly identical. With a highly favorable isotherm (R 0), however, the rate at each point is controlled by the resistance that is locally greater, and the principle of additivity of resistances breaks down. For approximate calculations with intermediate values of R, an overall transport parameter for use with the LDF approximation can be calculated from the following relationship for sohd diffusion and film resistance in series... 

Even if it is assumed that the reaction is ionic, Occam s Razor would lead to the conclusion that the system is too complex and that the effort to keep it ionic is too great. It is difficult to undersand why step 8c is slow and why a simple uncharged complex would not be equally reasonable. We prefer a mechanism in which the carbon monoxide molecule is adsorbed parallel to the surface and in which the oxygen orbitals as well as the carbon orbitals of C=0 bond electrons interact with the metal. It seems reasonable that hydrogenolysis occurs exclusively only because the oxygen is held in some way while the two bonds are broken and it finally desorbs as water. The most attractive picture would be (a) adsorption of CO and H2 with both atoms on the surface... 

Tswett s initial column liquid chromatography method was developed, tested, and applied in two parallel modes, liquid-solid adsorption and liquid-liquid partition. Adsorption ehromatography, based on a purely physical principle of adsorption, eonsiderably outperformed its partition counterpart with mechanically coated stationary phases to become the most important liquid chromatographic method. This remains true today in thin-layer chromatography (TLC), for which silica gel is by far the major stationary phase. In column chromatography, however, reversed-phase liquid ehromatography using chemically bonded stationary phases is the most popular method. 

Ensemble or Third-Body Ejfects. These effects concern the selective blockage of a particular adsorption site by adatom deposition. This can be advantageous when the reaction mechanism contains parallel paths that can be affected differently by blocking particular sites. In some cases, the undesired reaction needs more than one free adjacent site (ensemble), and can be inhibited by blocking particular sites without decreasing the reactivity of the surface for the catalyzed reaction. 

Montanari el al., for example, studied a Co—H-MFI sample through FT-IR spectroscopy of in situ adsorption and coadsorption of probe molecules [o-toluonitrile (oTN), CO and NO] and CH4-SCR process tests under IR operando conditions. The oTN adsorption and the oTN and NO coadsorption showed that both Co2+ and Co3+ species are present on the catalyst surface. Co3+ species are located inside the zeolitic channels while Co2+ ions are distributed both at the external and at the internal surfaces. The operando study showed the activity of Co3+ sites in the reaction. The existence of three parallel reactions, CH4-SCR, CH4 total oxidation and NO to NOz oxidation, was also confirmed. Isocyanate species and nitrate-like species appear to be intermediates of CH4-SCR and NO oxidation, respectively. A mechanism for CH4-SCR has been proposed. On the contrary, Co2+ substitutional sites, very evident and predominant in the catalyst, which are very hardly reducible, seemed not to play a key role in the SCR process [173],... 

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