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

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.