Surface reactants

A kinetics or reaction model must take into account the various individual processes involved in the overall process. We picture the reaction itself taking place on solid B surface somewhere within the particle, but to arrive at the surface, reactant A must make its way from the bulk-gas phase to the interior of the particle. This suggests the possibility of gas-phase resistances similar to those in a catalyst particle (Figure 8.9) external mass-transfer resistance in the vicinity of the exterior surface of the particle, and interior diffusion resistance through pores of both product formed and unreacted reactant. The situation is illustrated in Figure 9.1 for an isothermal spherical particle of radius A at a particular instant of time, in terms of the general case and two extreme cases. These extreme cases form the bases for relatively simple models, with corresponding concentration profiles for A and B. 

Surface Reactant Method of decomposition Type of spectrum r CH3 as CH3 s 25CH3 as 5CH3 as SCH3s pCH3 rCM Reference... 

Parameters in Compensation Equation Calculated by Making Appropriate Allowances for the Occurrence of Temperature-Dependent Changes in Concentration of Surface Reactants ... 

Over time, original surface reactants will eventually become depleted and secondary emissions of by-products should be reduced. Many studies have shown that ozone uptake on indoor surfaces tends to decrease with continued exposure, a phenomenon known as aging (Morrison and NazarofF, 2000 Morrison et al., 1998 Reiss et al, 1995a Sabersky, Sinema and Shair, 1973 Simmons and Colbeck, 1990). Further, there is evidence that secondary emission rates also decrease with time. Morrison and Nazaroff (2002) showed that secondary aldehydes on new carpet fibers, in a fixed-bed reactor, could be depleted in a day however, the reactivity of whole carpet was not substantially decreased over the relatively short time periods studied. Wang and Morrison (2006) showed that carpet in older homes... 

When the reaction proceeds at the catalyst surface, reactants are transferred from the gas phase to the catalyst surface. This is because the concentration of the reactant in the vicinity of the surface is less than that in the gas phase far from the surface. The diffusion takes place due to the concentration gradient according to Fick s first law. The rate of mass-transfer j through a unit area per unit time to the direction of the x-axis perpendicular to the catalyst surface is given by ... 

When diffusion is fast relative to surface kinetics, - 0, rj -> 1, and ravg = rsurface. Under these conditions, all the pore area is accessible and effective for reaction. When —> oo, that is, when diffusion is slow relative to kinetics, the reaction occurs exclusively at the particle external surface reactant gas does not penetrate into the pores. 

Figure 9.2 Top view of a simplified substitution/elimination surface. Reactants are in top center.

Hydrogen is generated by thermal decomposition or hydrolysis. For thermal decomposition some doped-catalytic reagents are applied to reduce the decomposition temperature to practical levels. Hydrolysis uses catalysts to generate hydrogen under ambient conditions. [See Chapter 6.5 for thermal decomposition and Chapter 6.8 for hydrolysis]. In these reactions, the aid of surface reactants is needed to liberate hydrogen at the gas-solid or liquid-solid interfaces. 

UHV provides for an ultra-clean environment in which the chemistry of a surface and the surface reactants, as well as surface reaction kinetics, are relatively controllable. Surfaces can be held in or reconditioned to a pristine state if needed, and the identity and purity of reactants can be assured. Reactions can be started and stopped (by controlling reactant concentrations in vacuum) to allow for the systematic study of reaction progress. 

Being distributed over the cell surface, reactants are transported through the GDL/backing layers to the catalyst layers, where the electrochemical reactions occur (Figure 1.4). The main mode of reactant transport through the GDL is usually diffusion due to the concentration gradient. 

RRDE voltammetry is designed to provide inherent analysis of the reaction selectivity instead of relying on complex calculations and data plotting techniques such as K-L plot. With electrode rotation, the electrolyte will flow the same way as in RDE experiment. Thus, while the reaction is occruring on the catalyst surface, reactants will continuously be swept away by hydrodynamic flow. This flow will traverse along the electrode surface, passing over the ring electrode. 

P. Cordoba-Torres, K. Bar-Eli, and V. Fairen, Non-diffusive spatial segregation of surface reactants in corrosion simulations, /. Electroanal. Chem 571 189-200 (2004). 

The rate expressions for surface processes thus take the same form as well-known expressions for chemical reactions in gas phase or in solution. Whereas the activities of gas- or liquid-phase reactants are expressed as pressures or concentrations, the activities of surface reactants are expressed in terms of the fractional coverages of adsorbates and of free sites. For surface processes, we can define reaction fractions by taking the product of the activities of the left-hand side of a reaction equation and dividing it by the right-hand side. For example, with the adsorption-desorption reaction given by... 

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