Supported gold nanoparticles from quantum dot to mesoscopic size scale: Effect of electronic and structural properties on catalytic hydrogenation of conjugated functional groups

Titania- and zirconia-supported gold particles of 1-5 nm site, prepared by various routes of synthesis, were employed in the partial hydrogenation of acrolein. In-depth characterization of their structural and electronic prop erties by electron microscopy, electron paramagnetic resonance, and optical absorption spectroscopy aimed at disclosing the nature of the active sites controlling the hydrogenation of C=O vs C=C bonds. The structural characte ristics of the catalysts, as mean particle size, size distribution, and dis persion, distinctly depend on the synthesis applied and the oxide support u sed whereby the highest gold dispersion (D-Au = 0.78, Au/TiO2) results from a modified sol-gel technique. For extremely small gold particles on titani a and zirconia (1.1 and 1.4 nm mean size), conduction electron spin resonan ce of the metal and paramagnetic F-centers (trapped electrons in oxygen vac ancies) of the support were observed. Besides the influence of the surface geometry on the adsorption mode of the alpha,beta -unsaturated aldehyde, th e marked structure sensitivity of the catalytic properties with decreasing particle size is attributed to the electron-donating character of paramagne tic F-centers forming electron-rich gold particles as active sites. The eff ect of structural and electronic properties due to the quantum size effect of sufficiently small gold particles on the partial hydrogenation is demons trated.