Dual-site dissociation

Under concentration control, the reversible hydrogen electrode exhibits Nemstian reversibility. This provides for a potential shift of 29.75 mV at room temperature, which translates to a shift of 46.8 mV at 200 °C for each decade of change in hydrogen concentration. Under fuel-cell operating conditions with highly dispersed electrocatalysts, it is possible to approach the kinetic rate determined by the dual-site dissociation of the hydrogen molecule, viz. ... 

The basic problem can be stated as follows minimize an objective function fjx), where x is a vector variable comprising the parameters to be estimated, andXx) is the sum of squares to be minimized. For instance, in the oxidation of carbon monoxide, already mentioned in Chapter 4 and discussed in more detail in Chapter 11 (and in [Asprey, 1997]), the following Langmuir-Hinshelwood Dual Site Dissociative Adsorption Model is used to fit experimental data ... 

Single-site surface coverage by CO and methanol is described by a classic Langmuir isotherm, whereas H2 requires modification for dual-site dissociative adsorption. Hence,... 

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. 

Sachtler [195] proposed a dual-site mechanism in which the hydrogen is dissociated on the Ni surface and then migrates to the substrate that is coordinated to the adsorbed dimeric nickel tartrate species. In their model, adsorption of modifier and reactants takes place on different surface atoms in contrast to Klabunovskii s proposal. Adsorbed modifier and reactant are presumed to interact through hydrogen bonding (Scheme 14.5). The unique orientation of adsorbed modifier molecules leads to a sterically favored adsorbed reactant configuration to achieve this bonding. 

Recently, Praharso et al also developed a Langmuir-Hinshelwood type of kinetic model for the SR kinetics of i-Cg over a Ni-based catalyst. In their model, it was assumed that both the hydrocarbon and steam dissociatively chemisorb on two different dual sites on the catalyst surface. The bimolecular surface reaction between dissociated adsorbed species was proposed as the ratedetermining step. The following generalized rate expression was proposed ... 

The equation reflects dual site reversible adsorption. Methanol and formaldehyde compete for sites, while oxygen is dissociatively adsorbed on different sites. At a not-too-low oxygen pressure (>0.01 atm) the coverage of the oxygen sites is complete and the equation reduces to... 

Sachtler proposes a "dual site" mechanism where the hydrogen is dissociated on the Ni surface and then migrates to the substrate which is coordinated to the adsorbed nickel-tartrate complex. In this context it is of interest that the well known Sharpless epoxidation probably takes place on a dimeric tartrate complex of Ti. Sachtler suggests that both the anion and the cation have a function which varies according to the conditions used. It is not clear whether the spillover mechanism is also proposed for the reaction in solution [55]. 

In many reactions it has been demonstrated that more than one site is involved in the catalytic process. This is particularly often the case for dissociation reactions. The same procedure as depicted above for a single site model can be used for the derivation of the rate expression for a dual site model, but the result is somewhat different. This is exemplified for the following dissociation reaction A 2B, which is thought to proceed according to the three step sequence ... 

Whereas the rate-determining step for hydrogen molecule oxidation now is recognized69,70 to be the dissociative chemisorption of the hydrogen molecule on dual sites at the platinum surface, the rate of this step is so high that in most electrochemical environments platinum electrocatalysts are almost always operating under diffusion control. 

Carbon monoxide is produced from fuel processing of hydrocarbons, e.g. reforming, and is strongly adsorbed as a surface metal carbonyl, inhibiting the dual-site hydrogen molecule dissociation step. 

Analogously, limiting cases can be distinguished for the dual-site model, in which the order of A can even become negative (see for example Eqn. (3.27)). This is common for dissociation reactions. 

Pignet and Schmidt have fitted a Langmuir-Hinshelwood model for non-competitive dual-site adsorption and with dissociative oxygen adsorption ... 

Epoxidation of butadiene occurs by addition of dissociatively-adsorbed oxygen to one of the localized C=C bonds to form epoxybutene. The addition of oxygen across the terminal carbon atoms does not occur to any measurable extent. The direct participation of molecular oxygen can be ruled out based both on selectivity arguments as well as the kinetic model for the reaction. The kinetics imply a dual site mechanism. One site, which is unpromoted, serves as the site for butadiene adsorption, while the second site, which is promoted, functions as the site for dissociative oxygen adsorption and epoxybutene formation. 

The first step is to develop equations for all possible controlling steps. They will include, among many possibilities, dissociation of one or more of the reactants, the common single-site mechanism for reactant decomposition, the dual-site mechanism where two sites are involved in the decomposition, and half-site mechanism where two molecules are adsorbed on a single site. Having formulated sufficient models (usually 15 to 20 are even excess of 100), various experimental and sophisticated statistical methods are available for selecting the most probable model, as described in many texts (e.g., Froment and Bischoff, 1990). A comprehensive stepwise procedure (not discussed here) is suggested by Doraiswamy (2001). 

For a dual site mechanism with dissociative adsorption of hydrogen. 

Dual site formalism can be easily extended to other cases, e.g. for dissociative adsorption. 

Elucidating the function of distal Cub in O2 reduction by HCOs has recently been the major focus of biomimetic studies of HCOs ". Cub has long been assumed to facilitate 0-0 bond heterolysis, or stabilize peroxo-level intermediates against dissociation, by forming a bridged Feaj-ZO j-Cug unit upon O2 binding Such a situation would imply that a bimetallic catalytic site is required for efficient O2 reduction. This would be comparable to the dual site mechanism of O2 reduction at Pt , which is supported by some recent results . However, no peroxo-level intermediates of any kind have ever been observed during O2 reduction by HCOs and it remains uncertain whether an intermediate where... 

The two cobalt centers in dicobalt cofacial diporphyrins have been reported to act in concert in order to achieve four-electrrMi reduction of oxygen [51,52], One of the sites reportedly functions as a Lewis acid to stabilize the intermediate(s) in the cavity, ensuring that it does not dissociate before completion of the reaction [53], To corroborate the dual-site postulate and specificity of the reaction site, a parallel type of mechanism involving both the two- and four-electron reduction was observed when one of the Co(III) centers in C02FTF4 was replaced by A1(III) [53]. A simplified scheme of the proposed mechanism of oxygen by dicobalt cofacial porphyrins is shown in Fig. 7.10 [48]. 

Sidik and Anderson further studied the O2 electroreduction when bonded to Pt dual sites [45]. A Pt dimmer, Pt2, with the bulk distance of 2.775 A, was used to provide one- and twofold bonding sites for coordinating O2,0, OOH, and OH. The results suggest that the O2 on dual sites does not dissociate before the first electron transfer and the product for this step, OOH, easily dissociates with a small 0.06 eV activation barrier to form O and OH. The first electron transfer step has the highest activation barrier (0.60 eV at 1.23 V), which is close to the experimental value, and is predicted to be the rate-determining step. 

---