Often in catalysis, the active material comprises very small (i.e. nano-size) particles dispersed over the outside and throughout the pores of a metal oxide. In some cases, the metal oxide is relatively inert and just has the role of holding the metal particles in place. In other cases, the metal oxide is more reactive, and can act as a source or reservoir of chemical species that become directly involved in the catalytic reaction. The common feature is that the catalytically-active sites are on the metal particles, while the metal oxide has a secondary role, and hence is described as the support. 

Ceria (cerium dioxide) is an example of a reactive support. Its function is generally assumed to be that of supplying reactive oxygen under certain conditions where O2 molecules are not available in the reactant feed. 

Paul Collier and colleagues at Johnson Matthey Technology Centre and the University of Oxford, UK have shown that it is possible to reverse the metal-support relationship. By covering precious-metal particles in ceria, Collier was able to control the ease with which the metal oxide gives up its oxygen to a reactant gas. This control mechanism shows a dependence on a fundamental physical property of the metal. This property is known as the work function and is a measure of the ease of electron transfer to and from the metal. 

Quite simply, the better the metal is at accepting electrons, the more readily the ceria gives up its oxygen. 'This is exciting because previously people had exclusively thought that precious metals function in an indirect chemical way by activating gas phase chemical reducing agents that subsequently help ceria to give up its oxygen,' says Collier. 'Now we know that there is another way that precious metals can encourage ceria to give up its oxygen and that is a direct electronic one'. Collier hopes that scientists will be able to use what they have learnt about the importance of electronic promotion to improve the performance of ceria redox catalysts. 

Kathleen Too