Tailoring Gold Nanoparticle Characteristics and the Impact on Aqueous-Phase Oxidation of Glycerol

Poly(vinyl alcohol) (PVA)-stabilized Au nanoparticles (NPs) were synthesized by colloidal methods in which temperature variations (−75 to 75 °C) and mixed H2O/EtOH solvent ratios (0, 50, and 100 vol/vol) were used. The resulting Au NPs were immobilized on TiO2 (P25), and their catalytic performance was investigated for the liquid phase oxidation of glycerol. For each unique solvent system, there was a systematic increase in the average Au particle diameter as the temperature of the colloidal preparation increased. Generation of the Au NPs in H2O at 1 °C resulted in a high observed activity compared with current Au/TiO2 catalysts (turnover frequency = 915 h–1). Interestingly, Au catalysts with similar average particle sizes but prepared under different conditions had contrasting catalytic performance. For the most active catalyst, aberration-corrected high angle annular dark field scanning transmission electron microscopy analysis identified the presence of isolated Au clusters (from 1 to 5 atoms) for the first time using a modified colloidal method, which was supported by experimental and computational CO adsorption studies. It is proposed that the variations in the populations of these species, in combination with other solvent/PVA effects, is responsible for the contrasting catalytic properties

Supported Gold Nanoparticles for Furfural Valorization in the Future Bio-based Industry

Gold nanoparticles prepared by sol immobilization using PVA as protecting agent were supported over different oxides (CeO2, ZrO2 and TiO2). The activity of the gold catalysts was tested in the oxidative esterification of furfural by an efficient and sustainable process. The samples supported on ceria and zirconia (AuCe and AuZr) have shown complete conversion and selectivity under mild reaction conditions. The catalytic performances are better than those obtained with the reference commercial AuTiWGC. The DR UV–Vis spectroscopic results indicate that gold nanoparticles that give rise to similar plasmonic band, as in the case of AuCe and AuZr, are very active and highly selective. The extent of crystallinity of the gold particles seems to have no influence on the activity of the sol immobilized catalysts. The null selectivity of the catalyst supported on titania is probably due to the presence of residual sulphate groups. The effect of the oxygen pressure on the furfural conversion is almost negligible for all the samples and, despite of the oxygen pressure lowering, it is possible to obtain always high conversion and selectivity. The selectivity of the process is unaffected by using air instead of oxygen, even at very low pressures. On the contrary, the presence of PVA seems to influence the conversion when working in air at low pressures, due to a dilution effect of the oxidant atmosphere. This is confirmed by the results obtained for a sample synthesized by deposition precipitation of gold on zirconia, where the high metal dispersion and the absence of the protecting agent allowed to reach high conversion even in low air pressure