Surface reactions dissociative adsorption

P6.04.23. SURFACE REACTION OR ADSORPTION, WITH OR WITHOUT DISSOCIATION. 

Baraldi and co-workers [52] have described a wealth of dynamic XPS studies on surface reactions, including adsorption, dissociation, desorption, and even catalytic reactions, such as the epoxidation of alkenes [60], and the reduction of NO by H2 and CO [61]. 

In what follows, we briefly review our density functional theory (DFT) calculations that investigate the mechanism of CO electrooxidation reaction. A commonly accepted mechanism for CO electrooxidation is the Langmuir-Hinshelwood mechanism, where oxygen-containing species, OH, formed on the Pt surface reacts with adsorbed CO to form CO2 [16, 34, 35]. In this model, OH is formed on the electrode surface by dissociative adsorption H2O. [26, 35] OH adsorption is assumed to be reversible and is given by the following two reactions ... 

Good examples that illustrate direct photon adsorption for an adsorbate are the photodissociation of Mo(Co)g on Cu(lll) and Ag(lll). Here, the wavelength dependence of photodissociation is nearly the same as that in the gas phase. Direct absorption can also mediate charge-transfer surface reactions. Dissociative electron attachment of adsorbates is an important process in surface chemistry induced by laser excitation this will be the topic of Section 27.2. 

Reaction A2 -t B R -I- S, with A2 dissociated upon adsorption and with surface reaction rate controlling ... 

In a recent paper [11] this approach has been generalized to deal with reactions at surfaces, notably dissociation of molecules. A lattice gas model is employed for homonuclear molecules with both atoms and molecules present on the surface, also accounting for lateral interactions between all species. In a series of model calculations equilibrium properties, such as heats of adsorption, are discussed, and the role of dissociation disequilibrium on the time evolution of an adsorbate during temperature-programmed desorption is examined. This approach is adaptable to more complicated systems, provided the individual species remain in local equilibrium, allowing of course for dissociation and reaction disequilibria. 

A classical example of promotion is the use of alkalis (K) on Fe for the ammonia synthesis reaction. Coadsorbed potassium (in the form of K20) significantly enhances the dissociative adsorption of N2 on the Fe surface, which is the crucial and rate limiting step for the ammonia synthesis5 (Fig. 2.1). 

In this figure, the activation energies of N2 dissociation are compared for the different reaction centers the (111) surface structure ofan fee crystal and a stepped surface. Activation energies with respect to the energy of the gas-phase molecule are related to the adsorption energies of the N atoms. As often found for bond activating surface reactions, a value of a close to 1 is obtained. It implies that the electronic interactions between the surface and the reactant in the transition state and product state are similar. The bond strength of the chemical bond... 

For W(100)/O, Benninghoven et al. (14) made some conclusions concerning the different stages of the reaction based on the behavior of the O ", W+, WO+, O, WOj and WOj SIMS intensities as a function of exposure. They concluded that atdissociative adsorption occurred and was characterized by W+ and O emission. Between 1 and lOL, WOj emission was observed and considered to be representative of a "monomolecular W-O structure on the surface. Above lOL, WO emission was observed and it was suggested that 3D oxidation was occurring. 

Methane reforming with carbon dioxide proceeds in a complex sequence of reaction steps involving the dissociative adsorption/reaction of methane and COj at metal sites. Hydrogen is generated during methane dissociation In the second set of reactions CO2 dissociates into CO and adsorbed oxygen. The reaction between the surface bound carbon (from methane dissociation) and the adsorbed oxygen (from CO2 dissociation ) yields carbon monoxide. A stable catalyst can only be achieved if the two sets of reactions are balanced. 

Therefore, we arrive at the same conclusion for the mechanism of COad oxidation in the lower potential regime as for Pt-free Ru(OOOl), postulating that at potentials E < 0.55 V, only strongly bound OHad/Oad species are present in the mixed COad + OHad/Oad adlayer, which are not reactive towards CO2 formation, while for E > 0.55 V, additional, weakly adsorbed OHad/Oad species are formed, which can react with the (likewise destabilized) COad- Similar to COad oxidation on a Ru(OOOl) surface, the reaction starts by dissociative adsorption of H2O on the Ru(OOOl) surface (no shift in the onset potential). In this case, however, the Pt islands can accelerate the reaction by accepting the Hupd resulting from a homolytic dissociation process. Thus, we tentatively propose a mechanism for CO oxidation at potentials between the reaction onset up to the bending point (see also Lin et al. [1999]), which is... 

Figure 3.3. Schematic representation of the adsorption, surface diffusion, and surface reaction steps identified by surface-science experiments on model supported-palladium catalysts [28]. Important conclusions from this work include the preferential dissociation of NO at the edges and defects of the Pd particles, the limited mobility of the resulting Nads and Oads species at low temperatures, and the enhancement in NO dissociation promoted by strongly-bonded nitrogen atoms in the vicinity of edge and defect sites at high adsorbate coverages. (Figure provided by Professor Libuda and reproduced with permission from the American Chemical Society, Copyright 2004).

Figure 3.10. Schematic representation of the elementary steps used in microkinetic simulations of the reduction of NO on supported metal particles [23]. The mechanism represented here incorporates adsorption and desorption steps, surface reactions such as NO dimerization and dissociation and N2, N20 and C02 formation, surface oxidation, and mobility of adsorbates. (Figure provided by Professor Libuda and reproduced with permission from Elsevier, Copyright 2005).

Equations 16 and 17 imply that 02 adsorption is not dissociative, which is coherent with the kinetic data. However, 02 should be dissociated in further steps of the surface reaction. On ceria, new sites for 02 activation are created at the metal/support interface or in the vicinity of metal particles. As CO and 02 do not compete with the same sites, the rate equation becomes ... 

One of the most significant recent insights in surface chemical dynamics is the idea that the principle of detailed balance may be used to infer the properties of a dissociative adsorption reaction from measurements on an associative desorption reaction.51,52 This means, for example, that the observation of vibrationally-excited desorption products is an indicator that the dissociative adsorption reaction must be vibrationally activated, or vice versa the observation of vibrationally-cold desorption products indicates little vibrational promotion of dissociative adsorption. In this spirit, it is... 

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