Credit: © 2009 ACS

Nanocatalysts can be thought of as a bridge between heterogeneous and homogeneous systems, combining some of the advantages of both. Much research has been carried out to try and understand the effects that particle size has on catalytic performance; it is known to be important in the conversion of CO and H2 into hydrocarbons — known as the Fischer–Tropsch (FT) reaction.

Cobalt is a well-studied FT catalyst and previous research has shown that, although particle size has no effect on those that are between approximately 10 and 200 nm, smaller particles show slower performance and a higher selectivity for producing methane rather than longer hydrocarbons. Now Krijn de Jong and colleagues from Utrecht University, in collaboration with the Norwegian University of Science and Technology, have revealed1 the origin of these phenomena.

Using a technique to measure the surface coverage and residence time of the important reaction species CO, OHx and CHx, they observed that the reduction in activity for smaller particle size is related to CHx covering less of the surface, and being there longer. This was caused by more CO molecules binding irreversibly to and blocking the reactive corner and edge sites, which increase in fraction as the nanoparticles become smaller. Smaller particles therefore have a proportionally higher surface coverage of hydrogen, which explains why they have an increased selectivity for methane.