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Catalysis by gold has received considerable attention in recent years. Particles of gold have been reported to be very active in various oxidation reactions. The particle size greatly influences the catalytic activity of gold particles and with decreasing particle size, the activity increases. Moreover, the type of support also affects the catalytic activity. One example of a reaction with gold catalysts is the CO oxidation in presence or absence of hydrogen, which is relevant for the use of hydrogen in fuel cells. Regarding the mechanism, one of the main questions is how oxygen is activated on the catalyst. Jeroen A. van Bokhoven of ETH Zurich's Institute for Chemical and Bio-Engineering and colleagues at the ESRF, Grenoble, and the University of Southampton have now identified a possible reaction mechanism for the oxidation of CO over the gold particles in supported gold catalysts. The research results have been published in "Angewandte Chemie".
The researchers studied gold supported on the nonreducible support Al2O3 and observed a reaction channel that has partially oxidized gold as reaction intermediate. Charge transfer from a reduced gold particle to oxygen activates the oxygen molecule. The researchers propose that reduced gold in small particles has the unique ability to transfer electrons to oxygen. A small fraction of the surface atoms are oxidized and are essential for high catalytic activity for oxidation of CO. The thermodynamic redox behaviour of small gold particles is distinctly different from that of bulk gold, which is inert. The difference likely originates from the different electronic properties of the small gold particles, which contain a large fraction of coordinatively unsaturated atoms with corner and edge positions. The latter have more d-electrons, which are additionally shifted towards the Fermi-level, than atoms in bulk gold. The electronic changes in nano-particles lead to stronger metal-adsorbate bonding and higher reactivity. Exposure of the gold-activated oxygen to CO rapidly forms CO2 and with re-reduction to metallic gold completing the catalytic cycle. Kinetic analysis of the individual reaction steps indicates that reduction is much faster than the re-oxidation and the rate-limiting step is the activation of oxygen on the gold surface.
For their experiments Jeroen A. van Bokhoven and colleagues combined in-situ time-resolved and in-situ high energy-resolution fluorescence detected X-ray spectroscopy. This method is likely to become a valuable tool in determining the structures of catalysts under catalytically relevant conditions. Combining high energy-resolution data with time-resolution and the possibility of in-situ measurement in combination with mass spectrometry at synchrotrons make it a promising tool in determining the structures of catalytically active sites.
Van Bokhoven J.A. et al, Activation of Oxygen on Gold-Alumina Catalysts: In-situ High Energy-Resolution Fluorescence and Time-resolved X-ray Spectroscopy, Angewandte Chemie, early view
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