Hydroperoxyl-bicarbonate mechanism for low-temperature CO oxidation by PdO/CeOx/γ-Al2O3 mesoporous nanocatalysts

Abstract

Although substantial progress has been made in CO oxidation over supported noble metal catalysts, the development of an efficient Pd catalyst for complete CO oxidation below 150 °C still remains a challenge. In this study, a PdO/CeO_x/γ-Al₂O₃ catalyst synthesized by wetness and vortex methods showed excellent activity with a T₅₀ of 80–90 °C and T₁₀₀ of 120 °C. The catalyst had a large surface area of 150 m² g⁻¹ and a pore diameter of 12 nm, which are characteristic of mesoporous materials. Pd(101), CeO₂(111) and γ-Al₂O₃ were identified by X-ray diffraction, high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy. HRTEM images showed PdO(101) nanoparticles (1–5 nm) dispersed over the mesoporous support. The high activity and stability of the fresh and spent catalysts may be due to the dispersion of PdO nanoparticles, the synergistic effect of support–support interactions, and the presence of defective oxygen and adsorbed water on the surface of catalysts and dual supports generating reactive oxygen species. Some of the unique features and advances reported in this study include the identification of PdO(101) as the active center for CO and O₂ activation, hydroperoxyl and bicarbonate intermediates, reduction of CO, mass transfer limitations (MTLs) at lower temperatures even when conversions are increasing, and high intrinsic activity of the catalyst overpowering MTLs. This study shows the potential of an efficient and stable PdO/CeOx/γ-Al₂O₃ catalyst for low-temperature CO oxidation under lean and dry conditions and the scope for further research on oxygen and CO activation.