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Silica, titania, zirconia and alumina supported gold particles of 1–6 nm size, prepared by various synthetic routes (sol–gel technique, deposition–precipitation, metal organic-chemical vapor deposition, impregnation, dip-coating) were employed in the selective hydrogenation of acrolein, crotonaldehyde and 1,3-butadiene and in the low-temperature oxidation of carbon monoxide. In-depth characterization of their structural and electronic properties by transmission electron microscopy (TEM), electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS) was aimed at disclosing the nature of the active sites controlling the hydrogenation and oxidation reactions. The structural characteristics such as mean particle size, size distribution and dispersion depend both on the synthetic method employed and the nature of the support. For extremely small gold particles on titania and zirconia (1.1 and 1.4 nm mean size), conduction electron spin resonance of the metal and paramagnetic F-centers (trapped electrons in oxygen vacancies) of the support were observed. The marked structure-sensitivity observed for hydrogenation properties with decreasing particle size may be attributed to structural and electronic properties due to the quantum-size effect of sufficiently small gold particles. Furthermore, the adaptability of gold particles in coatings is demonstrated for a microchannel reactor. |
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