Avelino Corma and colleagues of the Universidad Politécnica de Valencia have shown that gold nanoparticles on titanium dioxide or cerium dioxide can catalyse the reactions necessary to produce azobenzenes. And this is with above 98% efficiency under mild reaction conditions. The technique could be used to make cheaper, safer dyes for use in pigments, food additives and drugs, say the researchers.

The key to the technique is to substitute stoichiometric processes in the reactions with catalytic ones, explains Corma. "In this way you can greatly reduce the formation of sub-products," he told nanotechweb.org. "This is exactly what we have done."

The researchers found that gold nanoparticles are able to stabilise TiO2 vacancies, activate aniline reactant molecules and activate oxygen, which also acts a reactant. Aniline can form a radical cation by giving one electron either to a vacancy on the TiO2 support or to cationic gold atoms. The reaction is then initiated. The oxygen is activated either on TiO2 or CeO2 vacancies and/or on gold nanoclusters.

"The advantage of this method is that it avoids using lead tetracetate or stoichiometric amounts of nitrites," said Corma. "Moreover, we can carry out a 'one-pot' process starting from the nitroaromatics (the chemicals needed to produce azobenzenes) instead of the anilines, as is done today. This avoids a reaction step for preparing the aniline from the nitroaromatics."

Corma says that the work could provide an environmentally friendly route to make industrial amounts of azo-compounds for dye manufacture. "We now need to do more work to better understand the mechanism and the exact nature of the catalyst active sites and improve yields for these asymmetric azo-materials."

The work was published in Science.