Credit: © 2009 ACS

The catalytic properties of a pure metal surface can be modified by inserting a layer of another metal underneath to create a near-surface bimetallic alloy. Adsorption of a molecule can also alter the surface properties of a metal. Both of these effects can be used to tune catalytic performance.

Now, Ib Chorkendorff and co-workers at the Technical University of Denmark have observed1 an unexpected adsorbate-induced surface rearrangement in a CuPt near-surface catalyst — a promising candidate for the water–gas shift reaction. A CuPt near-surface alloy was exposed to CO at various temperatures and it was found that at lower temperatures the alloy binds CO less well than pure platinum does, but on increasing the temperature at which the alloy is exposed to CO, the binding improves and surpasses that of pure platinum. This observation indicates that the surface undergoes enhancing structural changes at higher temperatures and is confirmed by electron diffraction and spectroscopic studies.

An expected response on reactant adsorption is for the more reactive metal atoms — those with the strongest adsorption energy — to move to the surface. In this case, however, the less reactive copper atoms segregate to the surface. This creates islands of platinum atoms where CO can adsorb and the lateral repulsion between them is reduced.