Surface reactions dissociation

If transport through a membrane involving both surface reaction (dissociation) and diffusion was limited by surface reactions, then n = 1. If transport was diffusion-limited, then n = 0.5. Intermediate values of n (0.5proton transport membranes was modeled with the same form of the equations used to model diffusion membranes, Eqns. 3 and 4. Values of k H2 and n were determined from Eqn. 3. The concentration of hydrogen on the permeate-side was insignificant relative to the concentration on the retentate-side. Therefore was equated to... 

Surface reactions dissociative, associative and transfer reactions selectivity. 

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. 

Surface science has tlirived in recent years primarily because of its success at providing answers to frmdamental questions. One objective of such studies is to elucidate the basic mechanisms that control surface reactions. For example, a goal could be to detennine if CO dissociation occurs prior to oxidation over Pt catalysts. A second objective is then to extrapolate this microscopic view of surface reactions to the... 

The CO oxidation occurring in automobile exhaust converters is one of the best understood catalytic reactions, taking place on Pt surfaces by dissociative chemisoriDtion of to give O atoms and chemisoriDtion of CO, which reacts with chemisorbed O to give CO, which is immediately released into the gas phase. Details are evident from STM observations focused on the reaction between adsorbed O and adsorbed CO [12]. 

The oxidation of HC and CO must proceed in balance with the reduction of NO by CO, HC, or H2. For the NO removal reaction, a reductant is required. First NO is adsorbed on the catalyst surface and dissociates forming N2 which leaves the surface, but the O atoms remain. CO is required to remove the O atoms to complete the reaction cycle (53). 

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 and E refer to the desorption, dissociation, decomposition or other surface reactions by which the reactant or reactants represented by M are converted into products. If [M] is constant within the temperature interval studied, then the values of A and E measured refer to this process. Alternatively, if the effective magnitude of [M] varies with temperature, the apparent Arrhenius parameters do not specifically refer to the product evolution step. This is demonstrated quantitatively by the following example [36]. When E = 100 kJmole-1 andA [M] = 3.2 X 1030 molecules sec-1, then rate coefficients at 400 and 500 K are 2.4 X 1017 and 1.0 X 1020 molecules sec-1, respectively. If, however, E is again 100 kJ mole-1 and A [M] varies between 3.2 X 1030 molecules sec-1 at 500 K and z X 3.2 X 1030 molecules sec-1 at 400 K, the measured values of A and E vary significantly, as shown in Fig. 7, when z ranges from 10-3 to 103. Thus, the measured value of E is not necessarily identifiable with the rate-limiting step if a concentration of a participant is temperature-dependent. This... 

X = CO2R or CN). Theoretical calculation at B3LYP/6-31G //HF/STO-3G level showed that the Si-H bond dissociation energies of H-Si(l 11) and (MesSifsSi-H are very similar, which further justifies the use of the well-established radical-based reactivity of (MesSifsSiH as a model for surface reactions. 

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... 

The barrier for dissociation at the surface is much lower than that for dissociation in the gas phase, which for N2 would require about 9.8 eV (945 kJ mol ) and would lead to two free N radicals. The surface reaction offers an energetically much more... 

Next we will adopt a kinetic scheme and see if it describes the data of Fig. 7.16. Several treatments of HDS kinetics are available in the literature. Here we use a simplified scheme in which thiophene (T) exclusively adsorbs on sulfur vacancies, denoted by A, and H2 adsorbs dissociatively on all the sites (indicated by ) to form butadiene (B) and H2S in a rate-determining surface reaction (we ignore the kineti-cally insignificant hydrogenation steps of butadiene) ... 

Steady state and non steady state kinetic measurements suggest that methane carbon dioxide reforming proceeds in sequential steps combining dissociation and surface reaction of methane and CO2 During admission of pulses of methane on the supported Pt catalysts and on the oxide supports, methane decomposes into hydrogen and surface carbon The amount of CH, converted per pulse decreases drastically after the third pulse (this corresponds to about 2-3 molecules of CH< converted per Pt atom) indicating that the reaction stops when Pt is covered with (reactive) carbon CO2 is also concluded to dissociate under reaction conditions generating CO and adsorbed... 

Nienhuis [189] has used a fitting procedure for the seven most sensitive elementary parameters (reactions SiH4 -t- SiH2 and Si2H6 -I- SiHi, dissociation branching ratio of SiH4, surface reaction coefficient and sticking probability of SiHa, and diffusion coefficients of SiH and H). In order to reduce the discrep-... 

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 ... 

Gomez-Sainero et al. (11) reported X-ray photoelectron spectroscopy results on their Pd/C catalysts prepared by an incipient wetness method. XPS showed that Pd° (metallic) and Pdn+ (electron-deficient) species are present on the catalyst surface and the properties depend on the reduction temperature and nature of the palladium precursor. With this understanding of the dual sites nature of Pd, it is believed that organic species S and A are chemisorbed on to Pdn+ (SI) and H2 is chemisorbed dissociatively on to Pd°(S2) in a noncompetitive manner. In the catalytic cycle, quasi-equilibrium ( ) was assumed for adsorption of reactants, SM and hydrogen in liquid phase and the product A (12). Applying Horiuti s concept of rate determining step (13,14), the surface reaction between the adsorbed SM on site SI and adsorbed hydrogen on S2 is the key step in the rate equation. 

Transfer hydrogenolysis of benzyl acetate was studied on Pd/C at room temperature using different formate salts.244 Hydrogen-donating abilities were found to depend on the counterion K+ > NH4 + > Na+ > Li+ > H+. Formate ion is the active species in this reaction. Adsorption of the formate ion on the Pd metal surface leads to dissociative chemisorption resulting in the formation of PdH- and C02. The kinetic isotope effect proves that the dissociative chemisorption of formate is the rate-limiting step. The adsorption and the surface reaction of benzyl acetate occurs very rapidly. 

Extension of the analysis developed above to cases where the stoichiometry of the surface reaction differs from that considered is relatively simple and straightforward. An interesting case is that where the overall conversion rate is limited by adsorption of a species that dissociates on adsorption. Consider a reaction whose stoichiometry can be represented by... 

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