Nondissociative adsorption

To derive an explicit expression of the rate of desorption we restrict ourselves to nondissociative adsorption, listing references to other systems— such as multicomponent and multilayer adsorbates with and without precursors—for which such a treatment has been given, later. We look at a situation where the gas phase pressure of a molecular species, P, is different from its value, P, which maintains an adsorbate at coverage 6. There is then an excess flux to re-establish equilibrium between gas phase and adsorbate so that we can write [7-10]... 

Even if the peak behavior fits well for a given apparent desorption order, the real kinetic situation may be a different one. As a rate controlling step in a second-order desorption, random recombination of two particles is assumed most frequently. However, should the desorption proceed via a nonrandom recombination of neighboring particle pairs into an ordered structure, the resulting apparent first-order desorption kinetics is claimed to be possible (36). The term pseudo-first-order kinetics is used in this instance. Vice versa, second-order kinetics of desorption can appear for a nondissociative adsorption, if the existence of a dimer complex is necessary before the actual desorption step can take place (99). A possibility of switching between the apparent second-order and first-order kinetics by changing the surface coverage has also been claimed (60, 99, 100). 

Quantum chemical calculations have recently been extended to In.891 Adsorption has been found to be nondissociative and the metal-water interaction has been proposed to be in the sequence Hg < Ag(100) < In < Cu(100). Compared with the data in Tables 27 and 28, it appears that the positions of In and Ag(100) are exchanged. 

The reaction coordinate that describes the adsorption process is the vibration between the atom and the surface. Strictly speaking, the adsorbed atom has three vibrational modes, one perpendicular to the surface, corresponding to the reaction coordinate, and two parallel to the surface. Usually the latter two vibrations - also called frustrated translational modes - are very soft, meaning that k T hv. Associative (nondissociative) adsorption furthermore usually occurs without an energy barrier, and we will therefore assume that A = 0. Hence we can now write the transition state expression for the rate of direct adsorption of an atom via this transition state, applying the same method as used above for the indirect adsorption. 

Adsorption of water is thought to occur mainly at steps and defects and is very common on polycrystalline surfaces, and hence the metal oxides are frequently covered with hydroxyl groups. On prolonged exposure, hydroxide formation may proceed into the bulk of the solid in certain cases as with very basic oxides such as BaO. The adsorption of water may either be a dissociative or nondissociative process and has been investigated on surfaces such as MgO, CaO, TiOz, and SrTi03.16 These studies illustrate the fact that water molecules react dissociatively with defect sites at very low water-vapor pressures (< 10 9 torr) and then with terrace sites at water-vapor pressures that exceed a threshold pressure. Hydroxyl groups will be further discussed in the context of Bronsted acids and Lewis bases. 

The nondissociative adsorption of methanol also decreased the work function, indicating that the molecule adsorbed with the negative end of... 

These reactions clearly illustrate that the adsorbed oxygen atom is a very strong base. It can abstract hydrogen atoms from molecules of low acidity (as measured in aqueous solution) and can attack carbon nucleophilically, as shown in reactions (8), (9), (10), and (11). In addition to this striking property, the presence of atomic oxygen induces nondissociative adsorption as exemplified in reactions (11) and (12). In these reactions adsorption of molecular species up to ten times the number of oxygen adatoms is induced by the intermediate formed in the primary reaction with the oxygen. 

As was suggested in a previous paper dthe steady state etching of solid material by exposure to gas phase particles with or without a plasma is usually described by the following sequence of steps (1) nondissociative adsorption of gas phase species at the surface of the solid being etched (2) dissociation of this absorbed gas (i.e., dissociative chemisorption) (3) reaction between adsorbed radicals and the solid surface to form an adsorbed product molecule, e.g., SiF fads) (4) desorption of the product molecule into the gas phase and (5) the removal of nonreactive residue (e.g., carbon) from the surface. 

The model followed single-site, nondissociative, Langmuir-Hinshelwood poisoning. This resulted in the same adsorption coefficients for deactivation and start-of-cycle kinetics. 

Reaction rates for the start-of-cycle reforming system are described by pseudo-monomolecular rates of change of the 13 kinetic lumps. That is, the rates of change of the lumps are represented by first-order mass action kinetics with the same adsorption isotherm applicable to each reaction step. Following the same format as Eq. (4), steady-state material balances for the hydrocarbon lumps are derived for a plug-flow, fixed bed catalytic reformer. A nondissociation, Langmuir-Hinshelwood adsorption model is employed. Steady-state material balances written over a differential fractional catalyst volume dv are the following ... 

The literature of the vibrational spectra of adsorbed alkynes (acetylene and alkyl-substituted acetylenes) is very much in favor of single-crystal studies, with fewer reported investigations of adsorption on oxide-supported metal catalysts. Fewer studies still have been made of the particulate metals under the more advantageous experimental conditions for spectral interpretation, namely, at low temperatures and on alumina as the support. (The latter has a wide transmittance range down to ca. 1100 cm-1.) A similar number of different single-crystal metal surfaces have been studied for ethyne as for ethene adsorption. We shall review in more detail the low-temperature work which usually leads to HCCH nondissociatively adsorbed surface structures. Only salient features will be discussed for higher temperature ethyne adsorption that often leads to dissociative chemisorption. Many of the latter species are those already identified in Part I from the decomposition of adsorbed ethene. 

To date, only propyne (methylacetylene) and but-2-yne (dimethylacety-lene) seem to have been studied as adsorbates. Nondissociative adsorption at low temperature is supported by the experimental results in all cases. We first discuss the results obtained from but-2-yne, as the adsorbed species are likely to be more symmetrical and hence, with the use of the MSSR, more effective for structure elucidation. 

By analogy with the species found from the adsorption of ethene on these surfaces, it would be anticipated that nondissociatively adsorbed cyclohexene would be di-tr-bonded to Pt and -bonded to Pd, Cu, and Ag. In agreement with these expectations, the VEEL spectra of the species on Pd(110) and Ag(110) are closely similar to each other and quite different from that of the species on Pt(lll). The vCH RAIR spectrum of the Cu(100) species has also been interpreted in terms of a 7r-complex. The VEEL spectra of the species on the (110) faces exhibit all the features expected for a jr-bonded species approximately parallel to the metal surface, i.e., showing rCH modes at wavenumbers above and below 3000 cm-1 ( = CH and CH2, respectively), an absorption at ca. 1630 cm1 (vC = C) [Ag (110)], and very strong bands between 680 and 660 cm-1 (yCH = CH) (see 270). 

With respect to the atomic arrangements of the surfaces, the adsorption of cyclohexane occurs very similarly on (111) and (110) planes, in the former case as a nondissociative complex of symmetry C3v as is often the case, the results on the (100) face [of Pt(100)] are qualitatively different. On Ni the (111) face is less reactive for cyclohexane dehydrogenation than the stepped and kinked [5(111) X (110)] plane. 

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. 

Within the frameworks of the lattice gas model it is reasonable to classify the elementary processes by the number of sites m, which a given process occurs on, i.e., one- and two-site cases. In the first case the changing parameter is the occupancy state of one site. The processes such as these include isomerization associated with changes in the internal degrees of freedom of the adspecies (ZA- ZB, i.e., transition of the adspecies from state A to state B), adsorption-desorption of the atoms and nondissociating molecules (A + Z- ZA), reaction according to the collision mechanism (A + ZB ->ZD + C, Eley-Rideal s-type mechanism). It should be remembered that ZA, Z and A denote adspecies A, empty lattice site and species A in the gaseous phase, respectively. 

If a mineral surface becomes hydrophobic as a result of adsorption of a hydrolytic product of the collector, i.e. a nondissociated component, the modified Langmuir isotherm gets the form... 

The mechanism of interaction of an anionic surfactant with a mineral surface occupied with PDIH+ and OH- is confirmed by the position of IP in relation to the optimum of mineral flotability. If IP of a mineral occurs at a lower pH than the flotability optimum the adsorption of the nondissociated component of the surfactant is negligible. If the condition... 

As far as phenomenological modeling is concerned, an excellent review of earlier thermodynamic approaches to chemisorption and surface reactivity was given by Benziger (156), who also developed some general thermodynamic criteria for dissociative versus nondissociative adsorption of diatomic and polyatomic molecules on transition metal surfaces (137, 156). In particular, for quantitative estimates of QA, A = C, N, or O, Benziger (156) used the heats of formation of bulk metal carbides, nitrides, and oxides. The BOC-MP approach is different, however, not only analytically but also in making direct use of experimental values of QA. 

The nondissociative adsorption of diatomic molecules (CO, N2, NO, and H2) on the surface ions of oxides and halides is accompanied by distinct perturbations of the vibrational spectra. This statement is documented in detail for CO in this review. At this stage of the discussion, it is sufficient to mention the following points. 

Important oxidation processes involving the metal centers can occur at the surfaces of transition metal oxides (e.g, -Cr203) upon dissociative oxygen adsorption (Scheme 3). In some cases the (nondissociative) adsorption of oxygen can lead to the formation of superoxide ( )2 or peroxide 02 species with simultaneous oxidation of surface metal cation centers. 

The surface chemical effects of interest do not go as far as those induced in (extensively) modified carbon electrodes [248], e.g., by pyrolyzed phthalocyanines or macrocycles [249-255], by anthraquinone or its derivatives [126,247,256-259], or by aryl groups [125], or those of stable and efficient sonoelectrocatalysts by modifying GC electrodes with 9,10-phenanthraquinone or 1,2-naphthoquinone [260], Instead, it is explored here whether and how a seemingly simple but crucial issue has been addressed or resolved what makes 02 adsorption in ORR nondissociative The isotopic labeling evidence for this experimental fact has been presented half a century ago [261], and it has not been challenged [262], The implication, based on the equally noncontrover-sial literature that 02 chemisorption on carbons (even at room temperature) is dissociative, is summarized below ... 

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