Controlled synthesis of PVP-based carbon-supported Ru nanoparticles: synthesis approaches, characterization, capping agent removal and catalytic behavior†
Polyvinylpyrrolidone (PVP) stabilized Ru nanoparticles (NPs) with a mean size of ca. 2.4 nm were synthesized and immobilized on mesoporous Sibunit, macroporous carbon nanofibers of platelet structure (CNF-Pl) and micro-/mesoporous TiC carbide-derived carbon (CDC), providing 1.7–2.9 wt% Ru/C catalysts with a mean Ru size of 2.1–2.7 nm. The presence of PVP on the catalyst surface drastically diminished the activity in structure-sensitive hydrogenation of galactose to galactitol. Different PVP removal and support modification methods were tested to elucidate the effect of support hydrophilicity/hydrophobicity, preliminary support functionalization, and additional Ru NPs washing prior to immobilization as well as degree of PVP removal on catalytic behavior. For characterization of the carbon supports and synthesized catalysts TEM, XPS, XRD, XRF, water adsorption/desorption experiments and N2 physisorption were applied. Carbon supports were functionalized before Ru NPs immobilization by treatment with 5 wt% HNO3, conc. HNO3, Ar (700 °C), air (350 °C), and H2 (700 °C) without visible support microstructure alterations. Several protocols for PVP removal were applied such as solvothermal (with water and acetic acid aqueous solutions at 220 °C) and thermal treatment (in air, argon, hydrogen or nitrogen at different temperatures) not resulting in noticeable changes in Ru NPs size. The thermal treatment in air at 180 °C followed by reduction at 250 °C was found to be more effective in the case of Sibunit and CNF-Pl, while solvothermal treatment in water at 220 °C (PN2 25 bar) improved significantly the activity of TiC-CDC-based catalysts. Untreated Sibunit and CNF-Pl carbon supports provided higher activity in galactose hydrogenation with Ru/Sibunit exhibiting the best catalytic activity and being also the most hydrophilic according to water sorption isotherms. The catalytic activity of untreated micro-/mesoporous Ru/TiC-CDC increased noticeably depending on the support pretreatment in the order: untreated < 5% HNO3 < conc. HNO3. Additional TiC-CDC support functionalization was proposed to be required because of a relatively low amount of oxygen-containing groups on the surface compared to Sibunit and CNF.