About active sites and interfaces in catalysis
Prof Jeroen van Bokhoven
Catalysts often consist of nano-sized catalyst particles on an support. It has been recognized since long that the role of the support is not only to stabilize the catalytically active particles of particular size, shape and composition, but that they may actively participate in the catalytic reaction. Specific reaction steps may occur completely on the support or at the metal-support interface. In other cases, the support may be responsible for the communication between different catalytic entities by stabilizing and facilitating transport of species over its surface, a process called spillover.
In my talk, I will describe some of our contributions in this field. Ceria is an often-used catalyst support and catalytic material because of its ability to reversibly store oxygen. Thus, in oxidation reactions, oxygen activation may occur on ceria and the reaction at the interface of metal and support. The role of Ce3+ is often invoked to explain catalytic activity, however, a quantitative understanding of its role is lacking. Using transient X-ray emission and absorption spectroscopy, we were able to distinguish between Ce3+ that participates in a catalytic cycle, oxidation of carbon monoxide over Pt/CeO2, and Ce3+ that does not contribute to catalytic conversion and is thus a spectator.i The reaction takes place at the metal-support interface and the rate of reaction correlates to the rate of ceria reduction, not the oxygen storage capacity.
Spillover of hydrogen over reducible and non-reducible supports is often suggested to be responsible for a catalytic action at a distance. To quantify the phenomenon of hydrogen spillover, we designed a supported metal catalyst, which contains platinum and iron oxide particles with a distance that is controlled to a nanometer. The further development of single-particle spectroscopy enabled visualization of the phenomenon of hydrogen spillover.ii As expected, hydrogen atoms can freely move over a titania surface as electron-proton pair. On alumina, the situation is more complex and a gradient of hydrogen coverage away from the platinum particle is observed. The strong competition of water with the same adsorption sites, make the occurrence of hydrogen spillover over an alumina surface much less likely.
i. Catalytically Active and Spectator Ce3+ in Ceria-Supported Metal Catalysts, Kopelent R., van Bokhoven, J. A., Szlachetko J., Edebeli J., Paun C., Nachtegaal M., Safonova O. V. Angew. Chem. Int. Ed. 54, 8728 (2015)
ii. Catalyst support effects on hydrogen spillover, Karim, W., Spreafico, C., Kleibert, A., Gobrecht, J., VandeVondele, J., Ekinci, Y., van Bokhoven, J.A. Nature 541, 68 (2017).