Dissociative O2 adsorption

For example, in the case of dissociative O2 adsorption in solid-state Pt/YSZ electrodes,... 

The model also correctly predicts that the maximum in the NOx conversion is coincident with 100% C3H6 conversion being reached. This can be understood as follows. Oxygen is deposited onto the Pt surface from dissociative O2 adsorption and from NO dissociation. This oxygen is rapidly removed from the Pt surface by reaction with the C3H6-derived species present. The competition between NO dissociation and O2 adsorption determines the NOx... 

The catalytic activity for CO oxidation on the unreconstructed (1x1) phase is considerably higher than on the hex phase, which has been traced back to the very low sticking probability for dissociative O2 adsorption on the hex phase ( 10 ) compared with the (1x1) phase (> 0.2)7 49,22 imbihl and Ertl73-24 were the first to associate this phase transition directly with the existence, under specified... 

O2 (760 Torr = 1 atm) under differential reactor conditions, an apparent activation energy of 29 kcal mole for CO2 formation was observed. Near 900 K, selectivity to CO2, rather than CO, was about 75% or higher, and the reaction orders from a power rate law are given in Table 1. Propose a L-H-type model for CO2 formation with a sequence of elementary steps that results in a derived rate expression consistent with these results. It can be assumed that only the adsorbed reactants and products need be included in the site balance, and dissociative O2 adsorption occurs. Under low-conversion conditions, the surface concentrations of the products can be ignored, so what is the form of the rate equation Fitting this latter equation to the data produced the optimized rate parameters listed in Table 2, where k is the lumped apparent rate constant. Evaluate them to determine if they are consistent and state why. 

Dissociative O2 Adsorption. When oxygen is dosed to the surface by dissociation of 02(g), we define the 02(g) chemieal potential reference state as ... 

Figure 7 HREEL spectra recorded after O2 adsorption at different Ex on Ag(41 0) at T = 105 K. The supersonic molecular beam hits the surface at 0 = 31°, i.e. normally to the step heights, in all experiments. Only dissociative adsorption is observed at the lowest E as witnessed by the vibration at 40 meV. An additional adatom species (hoy = 32 meV) forms at slightly larger Ex and admolecules are stable for Ex above 0.30 eV.

Although the mechanism is similar to the mechanistic steps presented in reference [65], Taylor et al. [55] used enhanced H2 dissociation in the presence of O2, consisfenf wifh bofh previous experimenfal observation [91] and DFT calculations [63,64], Moreover, the rate determining step (Eq. 11.12) is unique because the fractional propylene order required the inclusion of adsorbed propylene in fhe RDS [55]. This mechanism and ifs associated rate expression (Eq. 11.15) were the simplest means of reproducing fhe observed reaction orders (n = 0.18, m = 0.14). It does imply a relation between H2 and O2 orders m + 1/2, 2rti), however. The addition of a third active site for dissociative H2 adsorption [65] would provide independent control over all three reaction orders. 

The relation in eq. (4) is nicely reflected in the abilities of metals to dissociate O2, NO and CO (the dissociation energies being around 500, 630 and 1100 kJ mol-1, respectively). Since the dissociation energy of the oxygen molecular bond is low it is dissociated by all close packed surfaces of metals, including Ag, whereas CO is not easily dissociated on Rh(lll). On the other hand, Rh(lll) will readily dissociate NO, whereas Pt(111) will not (Root et al., 1983). Of course, there is an interplay between thermodynamics and kinetics here, but nevertheless there is a correlation between ease of dissociation and thermodynamic stability. Generally, the weaker the adsorption heal, the higher the barrier to dissociative adsorption. 

It can be the case that both adsorption channels are important for a particular system. Examples of this are given here for O2 adsorption on Ag and Cu and for N2 dissociation on Fe. In these cases we can generalise and say that the precursor mediated route tends to dominate at low substrate and gas temperatures, while direct activated adsorption dominates at high gas temperatures. Furthermore, in all these cases, molecular chemisorbed states of adsorption can exist which complicate the pathway of adsorption. A one dimensional potential energy profile is shown in fig. 8 for the case of O2 adsorption on Ag taken from the work of Dean and Bowker (1988/89, 1989) and of Campbell (1985), although this is likely to be a general representation for this type of adsorption system with other adsorbate/metal combinations. 

Some authors favoured the CO oxidation mechanism over Ce02 via an adsorbed superoxide species (Sass et al. 1986, Tarasov et al. 1989). Superoxide and peroxide species can be considered as intermediates formed during O2 adsorption/dissociation according to the following scheme (Zhang and Klabunde 1992) ... 

Dissociative adsorption requires several sites on the catalyst surface. For a diatomic molecule (H2, O2) adsorption occurs on two adjacent sites... 

We assume dissociative adsorption for CH4. O2 and Ho. For methane adsorption, complete dissociation into C and 4 H is furthermore assumed, since no experimental data is available for individual dissociation steps. While this enters the species balance on the surface, the kinetic rate law for dissociative methane adsorption is formulated with... 

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