The conversion of methanol (CH4O) to formaldehyde (CH2O or HCHO) is the synthesis reaction which is the starting point for many everyday plastics. Interestingly employing catalysts made from Gold (Au) particles, then CH2O can be produced without the environmentally hazardous waste generated from standard methodologies. Understanding the intricate workings of the Au catalyst was theoretically tested and experimentally researched by Ruhr-Universitat Bochum (RUB) scientists in a co-operation project. The results were published in the international edition of the journal Angewandte Chemie, whereby the researchers provided a detailed report outlining what happens on the Au surface during the chemical catalyst reaction."Gold should not really be suitable as a catalyst"
As stated by Professor Doctor Martin Muhler of the Laboratory of Industrial Chemistry at the RUB - "That nanoparticles of Gold actually selectively transform methanol into formaldehyde is remarkable" and "As a stable precious metal, Gold should not really be suitable as a catalyst." Nonetheless, Au particles which are a few nanometers (nm) in size, anchored onto a titanium dioxide (TiO2) surface, can effectively fulfill this purpose. Only oxygen (O2) is required to set the reaction in motion, with the only waste product being water (H2O). How this is accomplished was investigated by Muhler's team together with the groups of Professor Dr Dominik Marx of the Chair of Theoretical Chemistry and Dr Yuemin Wang of the Department of Physical Chemistry I.
What the chemists found was that the active site of the catalyst - i.e.the point at which O2 and CH4O bind to be converted into H2O and CH2O. Elaborated calculations by Dr Matteo Farnesi Camellone demonstrated that O2 binds at the interface between TiO2 and Au particles. Since traditionally TiO2 is a semiconductor, and,thus, electrically conductive - a charge exchange between O2, Au particles and TiO2 is possible. O2 vacancies in the TiO2 further favour this charge transfer. Effectively electrons transitionally transfer from the catalyst to the O2 molecule, which enables for the CH4O to bind to the Au particles. In further reaction steps - CH2O and H2O form. The catalyst solid consists of Au and TiO2 and it is in the same state at the end of the reaction cycle as at the start, and, therefore, not consumed.
Experimental and theory
In detail the RUB team clarified the new catalyst reaction. The researchers employed computer simulations - known as density functional calculations, and various spectroscopic techniques, namely vibrational spectroscopy (HREELS method) and thermal desorption spectroscopy. In the model calculations - Dr Farnesi quantified the charge exchange taking underway during the catalysis. Thereafter extremely sensitive vibrational spectroscopic measurements by Dr Wang's group clarified the consequences of the charge-transfer in the real system. However, Professor Marx reiterates "Through an intensive co-operation 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." Original article available here
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