No abstract

Au–Pt alloy nanoparticles deposited on Mg(OH)2 (see STEM‐HAADF image) show high activity in the selective oxidation of polyols using molecular oxygen as oxidant at mild and base‐free conditions.

We report a convenient excess anion modification and post-reduction step to the impregnation method which permits the reproducible preparation of supported bimetallic AuPd nanoparticles having a tight particle size distribution comparable to that found for sol-immobilization materials but without the complication of ligands adsorbed on the particle surface. The advantageous features of the modified impregnation materials compared to those made by conventional impregnation include a smaller average particle size, an optimized random alloy composition, and improved compositional uniformity from particle-to-particle resulting in higher activity and stability compared to the catalysts prepared using both conventional impregnation and sol immobilization methods. Detailed STEM combined with EDX analyses of individual particles have revealed that an increase in anion concentration increases the gold content of individual particles in the resultant catalyst, thus providing a method to control/tune the composition of the nanoalloy particles. The improved activity and stability characteristics of these new catalysts are demonstrated using (i) the direct synthesis of hydrogen peroxide and (ii) the solvent-free aerobic oxidation of benzyl alcohol as case studies.

Tuning the catalytic activities of supported metal nanoparticles can be achieved by manipulating their structural properties using an appropriate design and synthesis strategy. Each step in a catalyst synthesis method can potentially play an important role in preparing the most efficient catalyst for a particular chemical reaction. Here we report the careful manipulation of the post-synthetic heat treatment procedure, together with control over the the amount of metal loading, to prepare a highly efficient 0.2 wt.% Pt/TiO2 catalyst for the chemoselective hydrogenation of 3nitrostyrene. We found that for Pt/TiO2 catalysts with 0.2 and 0.5wt.% loading levels, reduction alone at 450 °C induces the coverage of Pt nanoparticles by TiOx through a strong metal support interaction which is detrimental for their catalytic activities. However, this surface coverage can be avoided by combining a calcination treatment at 450 °C with a subsequent reduction treatment at 450 °C allowing us to prepare a exceptionally active Pt/TiO2 catalyst with the optimum Pt distribution. Detailed characterisation of these catalysts has revealed that the peripheral sites at the Pt metal/TiO2 support interface are the most likely active sites for this hydrogenation reaction.

In the solvent-free oxidation of benzyl alcohol to benzaldehyde using supported gold-palladium nanoparticles as catalysts, two pathways have been identified as the sources of the principal product, benzaldehyde. One is the direct catalytic oxidation of benzyl alcohol to benzaldehyde by O(2), whereas the second is the disproportionation of two molecules of benzyl alcohol to give equal amounts of benzaldehyde and toluene. Herein we report that by changing the metal oxide used to support the metal-nanoparticles catalyst from titania or niobium oxide to magnesium oxide or zinc oxide, it is possible to switch off the disproportionation reaction and thereby completely stop the toluene formation. It has been observed that the presence of O(2) increases the turnover number of this disproportionation reaction as compared to reactions in a helium atmosphere, implying that there are two catalytic pathways leading to toluene.

The use of bio-renewable resources, such as glycerol, a by-product from bio-diesel manufacture, can provide a viable way to make valuable products using greener technology. In particular, glycerol can be reduced to give 1,2-propanediol that can then be selectively oxidised to lactate, which has immense potential as a monomer for the synthesis of biodegradable polymers. We show that gold-palladium alloy catalysts can be very effective for the selective oxidation of 1,2-propanediol to lactate. Two supports, TiO 2 and carbon, and two preparation methods, wet impregnation and sol-immobilisation, are contrasted. The addition of palladium to gold significantly enhances the activity and retains the high selectivity to lactate using O 2 as oxidant (we observe 96% lactate selectivity at 94% conversion). Use of hydrogen peroxide is also possible but lower activities are observed as a result of the reaction conditions, but in this case no marked enhancement is observed on addition of palladium to gold. Comparison of the activity for C 3 alcohols shows that the reactivity decreases in the order: glycerol > 1,2-propanediol > 1,3-propanediol ~1-propanol > 2-propanol. The use of a sol-immobilisation preparation method as compared to impregnation leads to alloy catalysts with the highest activity for lactate formation from the oxidation of 1,2-propanediol; the origins of these activity trends are discussed.

No abstract

Lignin depolymerisation has received considerable attention recently due to the pressing need to find sustainable alternatives to fossil fuel feedstock to produce chemicals and fuels.