Reduction of RuO2 nanoparticles supported on silica by organic molecules: a strategy for nanoparticle redispersion

Abstract

High metal surface area is a critical parameter in metal-based supported heterogeneous catalysts. However, supported metal nanoparticle coalescence or sintering is inevitable and a cause for catalyst deactivation. While reversing the sintering process is challenging, it is an essential topic as this would further extend catalyst use and reduce the consumption of critical raw materials often used in catalysts, e.g., noble metals. The cyclic 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, H₂ is the primary reducing agent used when performing redispersion via oxidation/reduction. 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. Herein, we explored organic molecules as reducing agents to reduce silica-supported ruthenium oxide nanoparticles (RuO₂/SiO₂). Six compounds were evaluated: methanol, ethanol, isopropanol, acetone, heptane, and cyclohexane, and the results were compared to the conventional reduction with H₂. The products and the energy released showed two pathways: (i) conversion of RuO₂ to metallic Ru due to oxidative dehydrogenation and 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 mol_RuO2⁻¹), as opposed to H₂ (156 kJ mol_RuO2⁻¹). In addition, smaller Ru nanoparticles resulted from the reduction of organic molecules (4.5–7.0 nm) instead of H₂ (11.9 nm). This observation was attributed to a redispersion phenomenon, which was not observed when using H₂, supported by the existence of clusters of small nanoparticles, which were, in turn, impacted by the kinetics of the reduction reaction. The Ru/SiO₂ catalyst was employed in the furfural hydrogenation reaction as a model reaction, where all catalysts reduced with organic compounds displayed a two-fold increase in activity compared to those reduced with H₂.