Interface site factor

This approach of using 2D and 3D monodisperse nanoparticles in catalytic reaction studies ushers in a new era that will permit the identification of the molecular and structural features of selectivity [4,9]. Metal particle size, nanoparticle surface-structure, oxide-metal interface sites, selective site blocking, and hydrogen pressure have been implicated as important factors influencing reaction selectivity. We believe additional molecular ingredients of selectivity will be uncovered by coupling the synthesis of monodisperse nanoparticles with simultaneous studies of catalytic reaction selectivity as a function of the structural properties of these model nanoparticle catalyst systems. 

This chapter does not intend to indicate that these observations are of general validity for the SCR-HC reaction and catalysts. Indeed, the results show that still further effort is necessary to understand the surface chemistry of this process. There are several factors affecting NOx removal efficiency with hydrocarbons, such as dispersion, coordination, and local electronic states of the metal cations. The oxide matrix surrounding the active sites is not inert, but plays both an indirect role (through electronic influence on the active centres) and a direct role (role of interface sites, surface acido-base centres — both Brpnsted and Lewis sites — and oxygen vacancies). The role of the oxide matrix shows several analogies with the role of the zeolite matrix in metal cation containing zeolites for SCR-HC [61]. 

Other factors to account for topography with regard to valley or hillside sites should include possible inversion and failure to disperse pollutants. Temperature inversion occurs when the temperature at a certain layer of the atmosphere stays constant, or even increases with height, as opposed to decreasing with height, which is the norm for the lower atmosphere. Inversions may occur on still, clear nights when the earth and adjacent air cools more rapidly than the free atmosphere. They may also occur when a layer of high turbulence causes rapid vertical convection so that the top of the turbulent layer may be cooler than the next layer above it at the interface. 

There are several factors through which anions can influence the pathway and O2 reduction kinetics. The main factors are competition with O2 for surface sites changes in the activity coefficients of the reactants, intermediates, and transition states and the acidity and dielectric properties of the electrolyte side of the interface [Adzic, 1998]. For example, perfluoro acids have higher O2 solubility and lower adsorbability than... 

---