Why Gold Nanoparticles Strangely Act as Catalyst and Could Purify Production of Plastics

Counter-intuitively, gold nanoparticles on titanium were found to function as a very useful catalyst in chemical reactions, and until now it was a mystery how the passive and stable precious metal could work in this way. Theoretical and experimental researchers at the Ruhr-Universität Bochum, Germany, looked in detail on what happens on the gold surface during the chemical reaction and reported their findings in the international edition of the journal "Angewandte Chemie".

Using catalysts made of gold particles, methanol to formaldehyde -- the reaction that is the starting point for the synthesis of many everyday plastics -- could be produced without the environmentally hazardous waste generated in conventional methods. 

"That nanoparticles of gold actually selectively transform methanol into formaldehyde is remarkable", says Prof. Dr. Martin Muhler of the Laboratory of Industrial Chemistry at the RUB. "As a stable precious metal, gold should not really be suitable as a catalyst." However, gold particles of a few nanometres in size, anchored to a titanium dioxide surface, fulfil their purpose. You only need oxygen to set the reaction in motion, and the only waste product is water. How this is achieved is examined by Muhler's team together with the groups of Prof. Dr. Dominik Marx of the Chair of Theoretical Chemistry and Dr. Yuemin Wang of the Department of Physical Chemistry I.

The chemists identified the active site of the catalyst, i.e. the point at which the oxygen and methanol bind and are converted to water and formaldehyde. Elaborate calculations by Dr. Matteo Farnesi Camellone showed that oxygen binds at the interface between titanium dioxide and gold particles. Since titanium dioxide is a semiconductor, and thus electrically conductive, a charge exchange between oxygen, gold particles and titanium dioxide is possible here. Oxygen vacancies in the titanium dioxide further favour this charge transfer. Electrons transitionally transfer from the catalyst to the oxygen molecule. This allows the methanol to bind to the gold particles. In several further reaction steps, formaldehyde and water form. The solid, which consists of gold and titanium dioxide, is in the same state at the end of the reaction cycle as at the beginning, and is thus not consumed.

The RUB team clarified the individual reaction steps in detail. The researchers used computer simulations, so-called density functional calculations, and various spectroscopic techniques. "Through an intensive cooperation between theory and experiment, we have been able to qualitatively and quantitatively explore the active site and the entire reaction mechanism of this complex catalyst", explained Prof. Marx.

Reference:

M. Farnesi Camellone, J. Zhao, L. Jin, Y. Wang, M. Muhler, D. Marx (2013): Molecular understanding of reactivity and selectivity for methanol oxidation at the Au/TiO2 interface, Angewandte Chemie International Edition, DOI: 10.1002/anie.201301868