Nondissociated 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). 

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. 

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. 

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. 

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. 

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. 

Results of Calculations of Nondissociative Adsorption of Simple Molecules on Trigonal Aluminum LASs ... 

For most metals, two general types of low-temperature spectra have been reliably identified and have been assigned to two distinct nondissociative adsorption modes the di-c-adsorption mode(I) and the ir-adsorption mode(II), see Fig. 5. 

Rate constants and equilibrium constants should be checked for thermodynamic consistency if at all possible. For example, the heat of adsorption derived from the temperature dependence of should be negative since adsorption reactions are almost always exothermic. Likewise, the entropy change A5ads for nondissociative adsorption must be negative since every gas phase molecule loses translational entropy upon adsorption. In fact, AS < S (where Sg is the gas phase entropy) must also be satisfied because a molecule caimot lose more entropy than it originally possessed in the gas phase. A proposed kinetic sequence that produces adsorption rate constants and/or equilibrium constants that do not satisfy these basic principles should be either discarded or considered very suspiciously. 

Molecularly adsorbed oxygen results from nondissociative adsorption and is more weakly held. 

One of the first comprehensive kinetic analyses over Au/TS-1 concerned the oxidation of H2 to form water [65]. DPT calculations [63,65] showed the RDS to be the formation of adsorbed H2O2 which ultimately decomposes to form water [65]. In the presence of propylene, the generated H2O2 is expected to perform the epoxidation, therefore similarities between the production of PO and water can likely be drawn. Traditional kinetic analysis [65] produced a power rate law for water production of rn o = h exp[-(37.1 1.1 kj mol- )/RT][H2]° 0.02jqj0.17 0.02 Development of a series of elementary steps [Eqs. (11.1-11.5)] capable of reproducing the observed experimental orders and consistent with DPT calculations proved to require two active sites one capable of nondissociative adsorption of O2 and dissociative adsorption of H2 and a second available for only dissociative adsorption of H2 [65]. The resulting rate expression [65] is presented as Eq. (11.6). 

Y.-H. Kim, Y. Zhao, A. Williamson, M.J. Heben and S.B. Zhang, Nondissociative adsorption of H2 molecules in light-element-doped fullerenes . Physical Review Letters, 96, 016102 (2006). 

Kim Y H, Zhao Y E, Williamson A, Heben M J and Zhang S B (2006), Nondissociative adsorption of H-2 molecules in light-element-doped fnUerenes, Phys. Rev. Letters, 96, 016102. 

Where is the active site for hydrogen (H), S is the active site for the aromatic compound (E), EtCH is ethylcyclohexane, and j is equal to unity for nondissociative adsorption or 2 for dissociative. For competitive adsorption, S is equal to X, where X is the number of sites, where ethylbenzene is adsorbed. Taking into account steady-stated conditions... 

Only in rare cases each particle striking the surface will become adsorbed, but only a fraction s, called the sticking coefficient. Generally, s will decrease with increasing coverage from its initial value Sq in the simplest (Langmuir) case of nondissociative adsorption as s = Sq(1 — S). This is the simplest case that assumes that whenever a particle strikes an empty site it will be adsorbed with probability Sq, otherwise it is reflected and the adsorbates are randomly distributed on the surface. 

Activated adsorption is primarily found with dissociative adsorption as can be rationalized on the basis of Fig. 1.4. Adsorption in the molecular state, A2,ad (he., trapping), is sometimes denoted as "intrinsic" precursor from where the activation barrier for dissociation has to be surmounted. Usually at least two neighboring free adsorption sites are required for this process, so for random distribution of the adsorbates in the Langmuir picture the sticking coefficient is expected to vary with coverage as s =Sq(1 —5). Again, as with nondissociative adsorption, such a situation is found only in exceptional cases since usually various complications (such as the influence of defects or the need for more than two adjacent vacant sites, etc.) come into play. 

FIGURES.6. STM data from (nondissociative) adsorption of O2 on a Ag(l 10) surface at 65K, demonstrating the operation of the hot adparticle mechanism [29]. (a) STM image after adsorption of about 2% of a monolayer, (b) Model with adsorbed O2 molecules (black) and Ag atoms from the first (white) and second (gray) layers. 

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