Towards Atomically Precise Supported Catalysts from Monolayer-Protected Clusters: The Critical Role of the Support.

Controlling the size and uniformity of metal clusters with atomic precision is essential for fine-tuning their catalytic properties, however for clusters deposited on supports, such control is challenging. Here, by combining X-ray absorption spectroscopy and density functional theory calculations, it is shown that supports play a crucial role in the evolution of monolayer-protected clusters into catalysts. Based on the acidic nature of the support, cluster-support interactions lead either to fragmentation of the cluster into isolated Au-ligand species or ligand-free metallic Au0 clusters. On Lewis acidic supports that bind metals strongly, the latter transformation occurs while preserving the original size of the metal cluster, as demonstrated for various Aun sizes. These findings underline the role of the support in the design of supported catalysts and represent an important step toward the synthesis of atomically precise supported nanomaterials with tailored physico-chemical properties.

Colloidal Au Catalyst Preparation: Selective Removal of Polyvinylpyrrolidone from Active Au Sites.

Colloids with controlled dimensions, morphology and composition can be used to obtain supported metal catalysts with desired characteristics. Yet, removal of capping agents which block active metal sites in such catalysts can be challenging: mild methods often result in incomplete removal of capping agents, whereas harsher methods can cause change in particle size or morphology or cause metal segregation for bimetallic particles. Here we provide evidence that polyvinylpyrrolidone used as a stabilizing agent for gold colloids is present both on the metal and the support surface after colloid deposition on the TiO2 support. The polymer adsorbed on Au sites blocks their catalytic activity if it cannot be desorbed/decomposed under reaction conditions. Polyvinylpyrrolidone can be removed completely from the active gold surface of Au/TiO2 without particle growth using a number of mild treatment methods described in this work, despite only partial removal (≈45 %) of the stabilizer from the bulk of Au/TiO2 can be achieved. The remaining >50 % of polyvinylpyrrolidone resides exclusively on the TiO2 support and has no effect on the optical properties and catalytic activity of gold nanoparticles. The treated catalysts demonstrate catalytic activity and selectivity similar to those of a catalyst prepared by impregnation. These findings are important for further advancing the preparation of well-defined supported catalysts using metal colloids.

Carbon Support Surface Effects in the Gold-Catalyzed Oxidation of 5-Hydroxymethylfurfural.

Oxidation of 5-hydroxymethylfurfural into 2,5-furandicarboxylic acid is an important transformation for the production of bio-based polymers. Carbon-supported gold catalysts hold great promise for this transformation. Here we demonstrate that the activity, selectivity, and stability of the carbon-supported gold nanoparticles in the oxidation of 5-hydroxymethylfurfural strongly depend on the surface properties of the carbon support. Gold nanoparticles supported on basic carbon materials with a low density of functional groups demonstrate higher activity in 5-hydroxymethylfurfural oxidation (TOFAu up to 1195 h-1), higher selectivity to 2,5-furandicarboxylic acid, and better stability in comparison to gold nanoparticles supported on carbon materials with acidic surface groups. Surface groups of basic carbon supports that are positively charged under the reaction conditions result in a higher adsorption and local concentration of hydroxyl ions, which act as cocatalysts for gold and enhance gold-catalyzed dehydrogenation. Negatively charged surface groups of acidic carbons repel hydroxyls and the intermediate monoacid anions, which leads to lower reaction rates and a high selectivity toward 2,5-hydroxymethylfurancarboxylic acid. Understanding the role of support surface charge and local hydroxyl anion concentration provides a basis for the rational design of the optimal carbon support surface chemistry for highly active, selective, and stable catalysts for the oxidation of 5-hydroxymethylfurfural and related reactions.

Chemically synthesised atomically precise gold clusters deposited and activated on titania. Part II.

Synchrotron XPS was used to investigate a series of chemically synthesised, atomically precise gold clusters Au(n)(PPh3)y (n = 8, 9 and 101, y depending on the cluster size) immobilized on anatase (titania) nanoparticles. Effects of post-deposition treatments were investigated by comparison of untreated samples with analogues that have been heat treated at 200 °C in O2, or in O2 followed by H2 atmosphere. XPS data shows that the phosphine ligands are oxidised upon heat treatment in O2. From the position of the Au 4f(7/2) peak it can be concluded that the clusters partially agglomerate immediately upon deposition. Heating in oxygen, and subsequently in hydrogen, leads to further agglomeration of the gold clusters. It is found that the pre-treatment plays a crucial role in the removal of ligands and agglomeration of the clusters.