Supported Gold Nanoparticles from Quantum Dot to Mesoscopic Size Scale:  Effect of Electronic and Structural Properties on Catalytic Hydrogenation of Conjugated Functional Groups

Abstract

Titania- and zirconia-supported gold particles of 1−5 nm size, prepared by various routes of synthesis, were employed in the partial hydrogenation of acrolein. In-depth characterization of their structural and electronic properties 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 characteristics of the catalysts, as mean particle size, size distribution, and dispersion, distinctly depend on the synthesis applied and the oxide support used whereby the highest gold dispersion (D Au = 0.78, Au/TiO2) results from a modified sol−gel technique. 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. Besides the influence of the surface geometry on the adsorption mode of the α,β-unsaturated aldehyde, the marked structure sensitivity of the catalytic properties with decreasing particle size is attributed to the electron-donating character of paramagnetic F-centers forming electron-rich gold particles as active sites. The effect of structural and electronic properties due to the quantum size effect of sufficiently small gold particles on the partial hydrogenation is demonstrated.

In situ ESR study of gold supported on NaY zeolite

“…[16] Considering the density of gold (ρ Au = 19.3 kg m −3 , M Au = 197 × 10 −5 kg mol −1 ) and assuming a spherical shape, δ = 3.82 × 10 −47 J could be obtained. And then taking υ F = 10 6 m s −1 , γ e = 1.76 × 10 11 s −1 T −1 , E F = 8.8 × 10 −19 J, and hν = 6.3 × 10 −24 J, the relation between the particle diameter and the peak-to-peak line width becomes:…”

“…2), and one relatively sharp single peak g = 2.009. The latter signal is tentatively attributed to defects of the support or to gold species, [16,17] which needs further evidence. When the gold loading is increased to 0.4%, a set of new splitting signals (signal II) is observed; moreover, its intensity increases with the gold concentration, and the simulated spectrum is given in Fig.…”

Gold Stabilized by Nanostructured Ceria Supports: Nature of the Active Sites and Catalytic Performance

“…For the WGS reaction, isolated gold cations have been claimed as the active sites [12], although more recently there appears to be consensus that metallic gold cluster are the active sites [13,14]. A similar debate is apparent about the active sites in alkene hydrogenation [11,[15][16][17].…”

Activation of dihydrogen on supported and unsupported silver catalysts

“…In general, metals and alloys that interact weakly with H 2 seem to have a higher selectivity towards the unsaturated alcohol in the hydrogenation of ,-unsaturated aldehydes. For example, supported gold catalysts interact weakly with H 2 [72,73] compared to conventional hydrogenation catalysts such platinum or palladium and exhibit high selectivities towards allylic alcohols [74][75][76]. Modification of Rh/SiO 2 by tin decreases the heat of H 2 adsorption compared to the monometallic Rh/SiO 2 catalyst [77][78][79] whereas the intramolecular selectivity to transcrotylalcohol in the gas phase hydrogenation of crotonaldehyde strongly increases with the Sn/(Rh+Sn) ratio (up to x = 0.6).…”