Chemisorption nondissociative

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 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 exhibit such an active and selective catalytic effect, the catalyst must be a fairly good hydrogenation catalyst that is able to activate molecular hydrogen. It must also activate carbon monoxide without dissociating it. A nondissociative chemisorption permits the hydrogenation of carbon monoxide to occur on both oxygen and carbon. Considering the formation of surface methoxide in the second mechanism [Eq. (3.43)], a further requirement is that the catalyst not form a too stable metal methoxide. 

Fic. 2. Chemisorption and dissociation of a diatomic AB. (a) The traditional Len-nard-Jones one-dimensional potential diagram E versus R, where R is an ill-defined reaction coordinate, say the AB-surface distance, (b) The conventional two-dimensional potential diagram E versus R(x, y). The reaction coordinates are the A — B distance ( ) and the AB-surface distance (y). The energy minima correspond to the (nondissociated) molecular chemisorbed state DAB + QAB and atomic (dissociated) chemisorbed state gA + gB, the maximum to the transition state (TS) with some finite A — B bond length, (c) The multidimensional BOC potential diagram, similar to (b), but the reaction coordinate is the A — B bond order xAB. The M — AB bond order is conserved to unity (xA + xAB + xB = 1) up to the transition state where 1 > = c > 0 and 8 = 1/2(A ,abj6 + gAB). See text for notations... 

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 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 ... 

Although all three requirements must be satisfied by a good methanol catalyst, there is evidence that activation of carbon monoxide is more difficult than that of hydrogen. It is therefore instructive first to examine the ability of various catalysts to activate CO by nondissociative chemisorption. 

It must be emphasized that nondissociative chemisorption of carbon monoxide, which appears to be a necessary condition for its hydrogenation to methanol, is not a sufficient prerequisite. For example, nickel (29) or gold... 

In general, as I already outlined in a previous section, nondissociative chemisorption is a delicate balance of the very same interactions, which weaken bonds in the adsorbed molecule and in the surface. Dissociative chemisorption and surface reconstruction are just two extremes of the same phenomenon. 

Figure 1.12 Proposed mechanisms for CO2 formation in the carbon-oxygen reaction la Ib Ic, oxygen insertion (dissociative chemisorption) route 2a 2b 2c, nondissociative chemisorption route on the carbene edge sites.

The Langmuir isotherm can also be derived by other methods including statistical mechanics, thermodynamics, and chemical reaction equilibrium. The last approach is especially straightforward and useful, and it is developed as follows. For nondissociative chemisorption, the adsorption step is represented as a reaction, i.e., for an adsorbing gas-phase molecule. A, which adsorbs on a site, ... 

While the isosteric enthalpy of adsorption values reported so far for hydrogen adsorption on porous materials are too low for significant adsorption at ambient temperatures, high values would make hydrogen desorption very difficult. An intermediate weak nondissociative coordination or other weak chemisorption with higher isosteric enthalpy of adsorption values with facile adsorption and desorption characteristics is required for storage applications. 

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