Unraveling the Effects of Reducing and Oxidizing Pretreatments and Humidity on the Surface Chemistry of the Ru/CeO2 Catalyst during Propane Oxidation

Abstract

This work investigates the surface chemistry of the Ru/CeO₂ catalyst under varying pretreatment conditions and during the oxidation of propane, focusing on both dry and humid environments. Our results show that the Ru/CeO₂ catalyst calcined in O₂ at 500 °C initiates propane oxidation at 200 °C, achieves high conversion rates above 400 °C, and demonstrates almost no change in activity in the presence of water vapor across the entire studied temperature range of 200–500 °C. Prereduction of the oxidized Ru/CeO₂ catalyst in H₂ significantly enhances its activity, though this enhancement diminishes at higher temperatures. Adding water to the reaction mixture boosts the low-temperature activity of the prereduced catalyst but decreases it at 300–400 °C. Several ex-situ analytical techniques in combination with the in-situ NAP-XPS analysis reveal that while exposed to oxygen, Ru nanoparticles on the ceria surface oxidize to form RuO₂ below 200 °C and volatile RuO_x (x > 2) at higher temperatures. The presence of water vapor in the reaction mixture leads to the transformation of RuO₂ into ruthenium hydroxide at 200 °C, which, in turn, facilitates propane oxidation. At higher temperatures, the water does not have much influence on the oxidation state of Ru but slightly inhibits its evaporation from the surface. It is also demonstrated that Ru in the Ru/CeO₂ catalyst exists predominantly in the Ruⁿ⁺ (n > 4) oxidation states at typical VOC oxidation temperatures rather than the expected Ru⁴⁺ state.