Multipollutant Control of Hg0/NO from Coal-Combustion Flue Gases Using MnOx-CeOx/Mesoporous SiO2 from Waste Rice Husk

Mercury (Hg) and NOx emissions from coal-fired power plants (CFPPs) have both aroused greatest concern globally due to the high toxicity and bioaccumulability of Hg and the formation of photocatalytic smog and secondary PM2.5 from NOx, respectively. A marked amount of waste rice husk produced in Taiwan also needs to be treated or resource-recovered through a more cost-benefit approach. In this study, MnOx and MnOx-CeOx impregnated SiO2 particles were developed from waste rice husk via a hydrothermal synthesis for simultaneous removal of Hg and NOx. MnOx of 5 and 20 wt% associated with 5 and 10 wt% CeOx was impregnated into the resource-recovery mesoporous SiO2. The presence of MnOx and MnOx-CeOx was shown to decrease the BET surface area and pore volume of the SiO2 particles. It was observed that MnOx(5%)-CeOx(5%)/SiO2 catalyst possessed the largest specific surface area (SBET = 123 m2/g) among the MnOx-impregnated samples. SEM and TEM images indicated that the metal oxide-impregnated SiO2 catalysts had plate-shaped particles with sizes of 3-10 μm. Mn4+/Mn3+ and Ce4+/Ce3+ were the major valence states in the MnOx-CeOx/SiO2 samples. However, the ratio of Mn4+/Mn3+ increased after adding CeOx. The presence of both amorphous/highly dispersed and crystalline MnOx was confirmed by the XRD and XPS analyses. Multipollutant control of the Hg0 and NO was achieved by both MnOx/SiO2 and MnOx-CeOx/SiO2 at between 150 and 350°C under the tested gas conditions. Notably, MnOx-CeOx/SiO2 showed greater low-temperature deNOx and deHg potentials than MnOx/SiO2 sample, with MnOx(20%)-CeOx(10%)/SiO2 having nearly 100% reduction of NO under a broad operation window (100-300°C) and removal efficiency of Hg of 96% at 150°C. Langmuir-Hinshelwood model can successfully explain the deHg by MnOx and MnOx-CeOx/SiO2 catalysts. These results indicate that resource recovery of waste rice husk for producing SiO2 adsorbent/catalyst for Hg0/NOx removal is successful.