Thermal Regeneration of Manganese Supported on Activated Carbons Treated by HNO3 for Desulfurization

Abstract

Manganese supported on activated carbons treated by HNO3 (Mn/NAC) was prepared using an excessive impregnation method and calcined at 650 °C, and the deactivation and recovery factors of Mn/NAC for desulfurization were investigated. The results showed that fresh catalyst calcined at 650 °C has breakthrough sulfur capacity of 141.8 mg/g and breakthrough time of 300 min and that the catalysts thermally regenerated at different temperatures under N2 atmosphere exhibit different removal capacity of SO2. After the catalysts undergo the first thermal regeneration at 650 °C, the catalysts have breakthrough sulfur capacity of 144.9 mg/g and breakthrough time of 299 min. These values are close to those of the fresh catalysts, suggesting that active sites can be recovered almost completely. In the following cycles, the SO2 removal capacity of the regenerated catalysts gradually decreases, indicating that active sites reduce gradually. The fresh catalyst has 710 m2/g specific surface area and 0.404 cm3/g total pore volume with 0.262 cm3/g micropore volume; after desulfurization, the specific surface area and micropore pore volume of the sample decrease to 612 m2/g and 0.220 cm3/g, respectively. The regenerated catalysts at different temperatures have different texture, but the first regenerated catalysts at 650 °C still has an 800 m2/g specific surface area and 0.448 cm3/g total pore volume with 0.291 cm3/g micropore volume. These values decrease with the increase of the number of regeneration cycles. Both sulfates and manganese oxides such as MnO and Mn3O4 are detected in the regenerated catalysts, and with the increase of the number of regeneration cycles, average crystalline size of MnO increase from 29.8 to 40.3 nm, indicating that sulfates are partially decomposed in N2 atmosphere and reduced by neighboring C atoms. After desulfurization, the relative content of C═O and C–O decrease while that of O═C–O is almost unchanged, indicating that C═O and C–O play a role in the desulfurization reaction. Thermal regeneration can recover C═O and change its relative content, while the unreduced sulfates increase with the increase of the number of regeneration cycles and accumulate in the catalysts, leading to a gradual decrease of SO2 removal capacity.