Control of Hg⁰ and NO from coal-combustion flue gases using MnO_x-CeO_x/mesoporous SiO₂ from waste rice husk

MnO_x- and MnO_x-CeO_x-impregnated SiO₂ particles were developed from waste rice husk via a hydrothermal synthesis for removal of Hg⁰ and NO_x. MnO_x of 5 or 20 wt% associated with 5 or 10 wt% CeO_x was impregnated into resource-recovery mesoporous SiO₂. The presence of MnO_x and CeO_x reduced the specific surface area (S_BET) and pore volume of the SiO₂ particles. Nevertheless, MnO_x(5%)-CeO_x(5%)/SiO₂ possessed the largest S_BET (123 m² g⁻¹) among the impregnated samples. Electron microscopy images show that the metal-oxide-impregnated SiO₂ was plate-shaped with sizes of 3–10 μm. Mn⁴⁺/Mn³⁺ and Ce⁴⁺/Ce³⁺ were the major valence states in the MnO_x-CeO_x/SiO₂ samples; the ratio of Mn⁴⁺/Mn³⁺ increased as CeO_x was present. The presence of both amorphous/highly dispersed and crystalline MnO_x was confirmed by X-ray diffraction and X-ray photoelectron spectroscopy. Removal of Hg⁰ and NO was achieved by both MnO_x/SiO₂ and MnO_x-CeO_x/SiO₂ at between 150 and 350 °C. MnO_x-CeO_x/SiO₂ showed greater Hg and low-temperature NO removal potentials than MnO_x/SiO₂, with MnO_x(20%)-CeO_x(10%)/SiO₂ having Hg⁰ removal efficiency of 96% at 150 °C and nearly 100% reduction of NO under a broad operation window of 100–300 °C. These results indicate that resource recovery of waste rice husk for producing SiO₂ adsorbent/catalyst for Hg⁰ and NO_x removal is successful.