Redispersion of supported RuO2 by reduction with organic compounds
Résumé
Background and motivation. Oxidation-reduction of supported metal nanoparticles is commonly reported as a method for redispersing supported metal nanoparticles. While multiple molecules can be used to reduce supported metal oxide nanoparticles, mainly H2 is reported [1]. Yet replacing the H2 with other organic molecules could significantly impact the redispersion phenomena as it is well-known that metal nanoparticles' configuration is affected by molecular adsorption. In this view, we report the reduction of RuO2/SiO2 into Ru/SiO2 through the oxidation of 6 organic compounds and H2. The products obtained from the oxidation of the reducing agents by RuO2, the energy released during the reaction, and the impact of the reduction process on the resulting Ru nanoparticles were studied, providing a comprehensive view of the reduction process. The hydrogenation of furfural was used as a model reaction to understand the impact of the reduction process on the catalytic performance of the material. Materials and methods. RuO2/SiO2 catalyst (2.9 wt.% Ru) with RuO2 nanoparticle size of 16.2 ± 6.0 nm was used. Temperature-programmed reduction of RuO2/SiO2 (500 mg), dried at 400°C under Ar, was performed in the presence of methanol, ethanol, isopropanol, acetone, heptane, cyclohexane, and H2 (50 mL/min - 5% in Ar) between 100 and 375 °C. The products from the reaction were monitored and quantified by online MS. The enthalpy of reaction (ΔHreaction) was quantified by measuring the difference between the bed temperature and a control reference (pure Ar) and using the enthalpy of reduction of Pt/Al2O3 with H2 as the calibration standard. The parent and reduced catalysts were characterized by XRD, XPS, and HR-TEM. Furfural hydrogenation was performed at 80°C (T < Treduction (H2)) for 1 h in the liquid phase under 6 bar of H2 and using H2O as a solvent. Results and discussion. According to HR-TEM and XPS, RuO2 was reduced by all compounds with the reduction temperature ranging from 159 °C with H2 to 332°C with heptane. The products and ΔHreaction showed two reaction pathways: (i) conversion of RuO2 to metallic Ru due to oxidative dehydrogenation and/or oxidation of the reducing agents, and (ii) dehydrogenation of the organic molecules when enough metallic Ru is available. The energy released during reduction was substantially lower with organic molecules (27-85 kJ/molRuO2), as opposed to H2 (156 kJ/molRuO2). In addition, smaller Ru nanoparticles resulted from the reduction of organic molecules (4.7-6.7 nm) instead of H2 (11.9 nm). This observation was attributed to a redispersion phenomenon, which was not observed when using H2. The occurrence of small nanoparticle clusters was correlated with the slower reaction kinetics. All catalysts reduced with organic compounds displayed a two-fold increase in activity compared to that reduced by H2 (Table 1). References [1] Morgan, K.; Goguet, A; Hardacre, C, ACS Catalysis 2015, 5, 3430-3445.