Ru/CeO2 catalysts for supercritical water gasification of glycerol: stability, activity, and coke hydrothermal oxidation

Abstract

Cerium oxide (CeO₂) was investigated as a support for Ru nanoparticles in the continuous supercritical water gasification (SCWG) of glycerol as a model biomass compound. The stability of pure CeO₂ under reductive SCW conditions was first assessed in both batch and continuous modes. In batch, CeO₂ readily underwent phase transformation to CeCO₃OH, implying reduction of Ce⁴⁺ to Ce³⁺, leading to severe loss of surface area. The formation of CeCO₃OH was completely prevented in continuous flow due to the flushing of CO₂ and reductive compounds before cooling down to a temperature thermodynamically favouring CeCO₃OH. Although pristine CeO₂ showed low activity in SCWG, thermal treatment strongly influenced product distribution, with CO disappearing in favour of C₃H₈ after a reductive pretreatment. When Ru nanoparticles were supported on thermally stabilized CeO₂ (calcined at 700–900 °C, 0.11–1.00 wt% Ru), the catalysts displayed high efficiency in SCWG of glycerol, yielding CO₂, CH₄, and H₂ as the main products. These Ru/CeO₂ catalysts achieved apparent conversion rates up to three times higher than a commercial 5 wt% Ru/AC benchmark, while maintaining similar selectivity trends. Both Ru/CeO₂ and carbon-based catalysts exhibited similar decreasing activity trends under operation. Post-reaction analysis revealed some carbon deposition on Ru/CeO₂, likely the main cause of the observed loss of activity, which could be effectively removed either by thermal oxidation in air at 300 °C or by in situ hydrothermal oxidation with H₂O₂ at 110 °C. Overall, Ru/CeO₂ emerges as a durable and regenerable alternative to Ru/carbon for SCWG applications.