Portal Site Mediated Adsorption
Portal site mediated adsorption is a model for site-selective activated gas adsorption in metallic catalytic systems which contain a variety of different adsorption sites. In such systems, low-coordination "edge and corner" defect-like sites can exhibit significantly lower adsorption enthalpies than high-coordination (basal plane) sites. As a result, these sites can serve as "portals" for very rapid adsorption to the rest of the surface. The phenomenon relies on the common "spillover" effect (described below), where certain adsorbed species exhibit high mobility on some surfaces. The model explains seemingly inconsistent observations of gas adsorption thermodynamics and kinetics in catalytic systems where surfaces can exist in a range of coordination structures, and it has been successfully applied to bimetallic catalytic systems where synergistic activity is observed.
In contrast to pure spillover, portal site adsorption refers to surface diffusion to adjacent adsorption sites, not to non-adsorptive support surfaces.
The model appears to have been first proposed for carbon monoxide on silica-supported platinum by Brandt et al. (1993). A similar, but independent model was developed by King and co-workers to describe hydrogen adsorption on silica-supported alkali promoted ruthenium, silver-ruthenium and copper-ruthenium bimetallic catalysts. The same group applied the model to CO hydrogenation (Fischer-Tropsch synthesis). Zupanc et al. (2002) subsequently confirmed the same model for hydrogen adsorption on magnesia-supported caesium-ruthenium bimetallic catalysts. Trens et al. (2009) have similarly described CO surface diffusion on carbon-supported Pt particles of varying morphology.
Read more about this topic: Adsorption
Famous quotes containing the words portal, site and/or mediated:
“Some sepulcher, remote, alone,
Against whose portal she hath thrown,
In childhood, many an idle stone—
Some tomb from out whose sounding door
She ne’er shall force an echo more,
Thrilling to think, poor child of sin!
It was the dead who groaned within.”
—Edgar Allan Poe (1809–1849)
“The site of the true bottomless financial pit is the toy store. It’s amazing how much a few pieces of plastic and paper will sell for if the purchasers are parents or grandparent, especially when the manufacturers claim their product improves a child’s intellectual or physical development.”
—Lawrence Kutner (20th century)
“We talk about taking “pleasure in a thing”: but in truth it is pleasure in ourselves, mediated by a thing.”
—Friedrich Nietzsche (1844–1900)