Editing Adsorption (section)

==Portal site mediated adsorption==
Portal site mediated adsorption is a model for site-selective activated gas adsorption in metallic catalytic systems that 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).<ref>{{cite journal |last1=Brandt |first1=Robert K. |last2=Hughes |first2=M.R. |last3=Bourget |first3=L.P. |last4=Truszkowska |first4=K. |last5=Greenler |first5=Robert G. |title=The interpretation of CO adsorbed on Pt/SiO2 of two different particle-size distributions |journal=Surface Science |date=April 1993 |volume=286 |issue=1–2 |pages=15–25 |doi=10.1016/0039-6028(93)90552-U |bibcode=1993SurSc.286...15B }}</ref> A similar, but independent model was developed by King and co-workers<ref>{{cite book |doi=10.1016/s0167-2991(97)80418-1 |chapter=The effects of alkali promoters on the dynamics of hydrogen chemisorption and syngas reaction kinetics on Ru/SiO2 surfaces |title=Dynamics of Surfaces and Reaction Kinetics in Heterogeneous Catalysis, Proceedings of the International Symposium |series=Studies in Surface Science and Catalysis |year=1997 |last1=Uner |first1=D.O. |last2=Savargoankar |first2=N. |last3=Pruski |first3=M. |last4=King |first4=T.S. |volume=109 |pages=315–324 |isbn=9780444826091 }}</ref><ref>{{cite journal |last1=Narayan |first1=R.L |last2=King |first2=T.S |title=Hydrogen adsorption states on silica-supported Ru–Ag and Ru–Cu bimetallic catalysts investigated via microcalorimetry |journal=Thermochimica Acta |date=March 1998 |volume=312 |issue=1–2 |pages=105–114 |doi=10.1016/S0040-6031(97)00444-9 |bibcode=1998TcAc..312..105N }}</ref><ref>{{cite journal |last1=VanderWiel |first1=David P. |last2=Pruski |first2=Marek |last3=King |first3=Terry S. |title=A Kinetic Study on the Adsorption and Reaction of Hydrogen over Silica-Supported Ruthenium and Silver–Ruthenium Catalysts during the Hydrogenation of Carbon Monoxide |journal=Journal of Catalysis |date=November 1999 |volume=188 |issue=1 |pages=186–202 |doi=10.1006/jcat.1999.2646 |url=https://lib.dr.iastate.edu/rtd/12532 }}</ref> 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 process|Fischer–Tropsch synthesis]]).<ref>{{cite journal |last1=Uner |first1=D. O. |title=A Sensible Mechanism of Alkali Promotion in Fischer−Tropsch Synthesis: Adsorbate Mobilities |journal=Industrial & Engineering Chemistry Research |date=1 June 1998 |volume=37 |issue=6 |pages=2239–2245 |doi=10.1021/ie970696d }}</ref> Zupanc ''et al.'' (2002) subsequently confirmed the same model for hydrogen adsorption on magnesia-supported caesium-ruthenium bimetallic catalysts.<ref>{{cite journal |last1=Zupanc |first1=C. |last2=Hornung |first2=A. |last3=Hinrichsen |first3=O. |last4=Muhler |first4=M. |title=The Interaction of Hydrogen with Ru/MgO Catalysts |journal=Journal of Catalysis |date=July 2002 |volume=209 |issue=2 |pages=501–514 |doi=10.1006/jcat.2002.3647 }}</ref> Trens ''et al.'' (2009) have similarly described CO surface diffusion on carbon-supported Pt particles of varying morphology.<ref>{{cite journal |last1=Trens |first1=Philippe |last2=Durand |first2=Robert |last3=Coq |first3=Bernard |last4=Coutanceau |first4=Christophe |last5=Rousseau |first5=Séverine |last6=Lamy |first6=Claude |title=Poisoning of Pt/C catalysts by CO and its consequences over the kinetics of hydrogen chemisorption |journal=Applied Catalysis B: Environmental |date=November 2009 |volume=92 |issue=3–4 |pages=280–284 |doi=10.1016/j.apcatb.2009.08.004 |bibcode=2009AppCB..92..280T }}</ref>