Catalysts contribute their own oxygen for reactions
A catalyst: normally a substance that speeds up a chemical reaction, without adding molecules to this reaction. Scientists from the group of Prof. Marc Koper of Leiden University and Prof. Yang Shao-Horn of MIT now discovered that in specific reactions that need oxygen, catalysts called metal oxides contribute oxygen themselves. Publication 9 January in Nature Chemistry.
Reason for effectiveness
Chemical reactions that release molecular oxygen O2 in the presence of a catalyst are a crucial part of chemical energy storage, a storage method interesting for renewable energy. For these reactions, metal oxides in particular suitable catalysts. This new research shows that oxygen atoms that are normally stuck in the crystal lattice – a grid-like arrangement of the atoms – of the catalyst, with this type of catalyst are released for the reaction. ‘Although it was already known that some metal oxides display this behaviour, our research is the first to establish a link between the chemical properties of the metal oxide and this specific behaviour’, says Professor of Catalysis and surface chemistry Marc Koper, one of the Leiden co-authors of the article. This knowledge could help researchers to develop more efficient catalysts for chemical energy storage.
Active versus inactive
A catalyst has an active site: the surface of the catalyst where the chemical reactions take place. The general consensus amongst scientists is that in oxygen-evolution reactions the bulk of the catalyst is inactive. An international team including former PhD candidate Oscar Diaz-Morales and Marc Koper thought of a way to determine the contribution that might be coming from the bulk of the catalyst.
Diaz-Morales used ‘labeled’ oxygen-18, a heavier variant of oxygen which hardly exists in nature. First, the researchers made the catalyst with almost exclusively oxygen-18, after which they executed the reaction in ‘regular’ oxygen-16-containing water. In this way, they could determine under which circumstances the metal oxide contributed oxygen to form O2.
New mechanisms proposed
An unexpected finding was that both varying the chemical composition of the metal oxide as varying the acidity had a great impact on the speed of the reaction and the way in which the reaction took place. The effects of the acidity and the contribution of oxygen by the catalyst ‘cannot be explained by the traditional mechanism’, Diaz Morales says. ‘We have proposed different mechanisms to account for these effects, which requires further experimental and computational studies.