Supported gold and gold–palladium nanoparticles were found to be effective catalysts for the selective oxidation of glycerol and benzyl alcohol. The properties and stabilities of catalysts are often sensitive to factors such as the dimensions, shape, and composition of the metal nanoparticles. Although colloidal methods provide an easy and quick way to synthesize supported metal catalysts, they typically involve the use of polymers such as polyvinyl alcohol and polyvinylpyrrolidone as steric stabilizers, which can sometimes be detrimental in subsequent catalytic reactions. Herein, we report the synthesis of supported gold and gold–palladium nanoparticles without the addition of stabilizing polymers. The catalysts prepared with and without the addition of polymers performed very similarly in the selective oxidation of glycerol and benzyl alcohol, which suggests that polymers are not essential to make active catalysts for these reactions. Thus, this new stabilizer‐free method provides a facile and highly effective way of circumventing the inherent problems of polymer stabilizers in the preparation of gold and gold–palladium catalysts.

Au and AuPd nanoparticles supported on MgO and Al2O3 were employed for the selective aqueous phase oxidation of glycerol under basic conditions. Catalysts were prepared by sol‐immobilization without the addition of a stabilizing agent such as polyvinyl alcohol (PVA), which is generally added to stabilize the noble metal sol prior to immobilization. The obtained materials prepared with and without stabilizing agent were active for glycerol oxidation and showed similar catalytic performances—implying that the stabilizing polymer is not required to obtain active materials. Depending on the support used, it was possible to tailor the selectivity towards the desired oxidation products by using catalysts prepared with or without stabilizing agent. PVA‐free Au/γ‐Al2O3 exhibited a remarkably high selectivity towards tartronic acid (40 % at 97 % conversion), which was not observed for Au/γ‐Al2O3 prepared with PVA (27 % at isoconversion). Selective glycerol oxidation performed under base‐free conditions over AuPd/MgO catalysts also corroborated the previous results that the presence of a stabilizing polymer is not required to prepare active catalysts by sol‐immobilization. Thus, a facile way to circumvent the inherent drawbacks encountered by the use of polymer stabilizers during catalyst preparation is presented herein. Experimental results suggest that the presence of the polymer stabilizers can affect the reaction pathways and control selectivity.

The oxidation of glycerol represents both a viable route to catalytic upgrading of biomass and has become a model reaction for catalytic polyol oxidation. Gold and gold-palladium nanoparticle catalysts prepared by colloidal methods involving polymer additives have been extensively studied. However, the effect of residual polymer at the catalyst surface on reaction pathways has not been decoupled from particle size effects. We show that when using catalysts prepared without polymer stabilisers the addition of either polyvinyl alcohol or polyvinylpyrrolidone to the reaction changes the reaction rate and results in a change in reaction selectivity. We conclude that the polymer additive has a significant effect on the reaction pathway and that these systems should be considered as a metal surface-polymer interface catalytic systems and properties should not be rationalised solely based on nanoparticle size.

AuPd nanoparticles supported on P25 TiO2 (AuPd/TiO2) were prepared by a facile sol-immobilisation method and investigated for surface plasmon-assisted glycerol oxidation under base-free conditions.

Au nanoparticles supported on P25 TiO2 (Au/TiO2) were prepared by a facile sol-immobilisation method and investigated for the surface plasmon-assisted glycerol oxidation under base-free conditions. The Au/TiO2 samples were characterized by UV–vis spectroscopy and transmission electron microscopy. Catalysts were prepared using polyvinyl alcohol as stabiliser as well as in the absence of polymer stabiliser. Both the conversion and the reaction selectivity are affected by the plasmon-assisted oxidation and there is an interplay between the presence of the stabiliser and the Au nanoparticle size. Graphic Abstract

The Cover picture illustrates the preparation of a colloidal Au catalyst for alcohol oxidation.In their Communication, L. Abis et al. demonstrate that polymer stabilizers are not needed to prepare active catalysts and correlate these results with the presence/absence of the stabilizing agent and nanoparticle size. Our results show that the absence of stabilizers leads to equally active catalysts and might affect the selectivity to certain products, suggesting different active site availability. More information can be found in the Communication by L. Abis et al. on page 2914 in Issue 15, 2017 (DOI: 10.1002/cctc.201700483).

Precious metal nanoparticles have important applications in catalysis. In order to maximize the atomic efficiency and therefore reduce the usage of valuable resources, the particle size, morphology and their interaction with the support need to be carefully controlled and optimized. In the case of monometallic Au and Pt catalysts prepared via wet chemical routes, advanced electron microscopy has allowed us to obtain an unprecedented and more complete view of the complex dispersion of metal entities present, including nanoparticles, sub-nm clusters, and atomically dispersed species. With this new information, a better understanding of the catalytically active sites/species can be established.[1] By correlating catalytic performance data with the nanostructures generated by various catalyst preparation methods the most active species can be identified and specifically targeted for synthesis, thus significantly improving the atomic efficiency.Making nanoalloys is another way of improving atomic efficiency for precious metal catalysts. In some cases, an earth abundant metal is combined with the precious metal, in an effort to maximize the exposure of the latter on the particle surface. Quite often, a synergistic effect can be observed, meaning that the nanoalloy performs better than either of its individual components working in isolation. Fabricating nanoalloy catalysts is however, considerably more difficult than it sounds because the dispersion of two metals needs to be controlled simultaneously. Using Pd-based bimetallic systems [2] as an example, we will demonstrate how advanced electron microscopy, which provides atomic level structural and chemical analysis, can help us to achieve such a goal. With the help of electron microscopy, metal-support interactions can be identified and in some cases manipulated to improve the performance complex bimetallic catalyst systems. [3] Finally, we will highlight our recent attempts to investigate the structure of nanoalloy colloids at early stages of their formation, in an effort to understand how different synthesis conditions (e.g. different reducing agents, different stabilizing ligands) can affect the size and composition distribution of the resultant particles. A combination of in-situ spectroscopies and electron microscopy has been used to monitor the nucleation and growth of such colloidal metal nanoparticles. We will share our findings on the quenching the embryonic colloidal solutions using plunge freezing techniques. We will also use high surface area support materials (e.g. carbon black) to immobilize the particles at later stages of the growth and study the composition distribution of the two metals. These will bring insights into the particle formation mechanisms and lead us to a better control of size, morphology and composition of the bimetallic nanoparticles [4].