Polyoxometalate-stabilized ruthenium nanoparticles : the polyoxometalate-metal interface in multifunctional catalysis

Abstract

Polyoxometalates (POMs) are metal-oxide polynuclear anions. The interface between a metal (often in the form of nanoparticles) and a metal oxide is important in heterogeneous catalysis and has been the subject of intense research over years. The stabilization of metal nanoparticles (such as Pd, Pt, Ag, Au, Ir, Rh, Ru) by polyoxometalates has been studied by Weinstock, Papaconstintinou and Kortz. This represents a new type of catalytic material where molecules (e.g. H2) may be activated at the metal nanoparticles while the POM provides redox and Brønsted acid functionality. Metal nanoparticles and POM molecular metal oxides have each been applied in chemical synthesis, electrochemistry and photocatalysis, but by exploiting the interface between metal nanoparticles and POMs, this project aimed to design new nanoscale catalysts that incorporate the features associated with each component. A convenient method of reduction by hydrogen was used to synthesize POMstabilized ruthenium nanoparticles, which were subsequently incorporated into immobilized systems as catalysts. The catalysts were then tested for a range of transformations. The brief history of the development of POMs, the classification and structure of POMs and the function and application of POMs is discussed in Chapter 1. The use of polymer-immobilised ionic liquid phase (PIILP) and graphite-like carbon nitride (C3N4) as supports is introduced based on previous research. In Chapter 2, the preparation of a series of POM stabilized ruthenium nanoparticles in aqueous solution is discussed, wherein the different kinds of POMs (H3PW12O40, H3PMo12O40, H4SiW12O40, K6[P2W18O62] .19H2O and K10[P2W17O61] .20H2O ) were used to prepare Ru0@POM in solution. The result of these showed that the POM anions were adsorbed onto the surface of metal L．Feng (2018) iii nanoparticles due to the POM anions being able to afford negative charges to metal nanoparticles. The performance of the resulting nanoclusters for a range of reactions was assessed. In Fischer-Tropsch synthesis, the hydrogenation of CO to produce hydrocarbons occurred at 150 oC over 18 hrs. According to the results of electrochemical CO2 reduction in acid aqueous solution, Ru0@POM nanoclusters also improved reaction performance compared with pure Ru0 nanoparticles. Ru0 nanoparticles and POMs have been shown to act in tandem for (i) activation of H2 and (ii) creation of strong Brønsted acidity, but these POM-stabilised Ru0 nanoparticles are difficult to recover and recycle. The high solubility of POMs in aqueous solution has limited the application of Ru0@POM nanoclusters; and in Chapter 3 describes attempts to address this problem through the use of watertolerant polymer-immobilised ionic liquid phase (PIILP) supports upon which to immobilize Ru0 /POM nanoparticles, to give Ru0@POM/PIILP bifunctional catalysts for the hydrogenation of trans-cinnamaldehyde, 5- hydroxymethylfurfural and furfural. Ru0@POM/PIILP exhibited excellent catalytic performance in terms of both conversion and selectivity. In Chapter 4, catalysts prepared by adsorbing polyoxometalate-stabilized ruthenium nanoparticles onto carbon nitride are described. The g-C3N4 is a semiconductor with a high surface area and is an attractive support for POMstabilized ruthenium nanoparticles. This Ru0@POM/C3N4 material is a potential redox and photocatalyst for hydrogenolysis of cellobiose to sorbitol, selective hydrogenation of levulinic acid and water splitting. The H2 production efficiency of Ru0@H3PW12O40/C3N4 was 6.44 times higher than that of g-C3N4. The Ru0@H3PW12O40/C3N4 catalyst gave a high sorbitol yield (85%) for hydrogenolysis of cellobiose and was highly selectivite for GVL (almost 100%) in the hydrogenation of levulinic acid. Abstract iv The catalysts described in Chapters 2-4 were characterized by a combination of techniques including NMR, UV-vis, XRD, FT-IR, UV-DRS, TGA, SEM, TEM and XPS analysis. The results obtained in this thesis emphasize the potential of polyoxometalatestabilized ruthenium nanoparticles in new functional composites for catalysis. In the future, more extensive research will expand their use for conversion of biomass and platform chemicals as well as for green electrocatalytic reduction and photocatalysis.