Tunnel structured manganese dioxides for the gaseous ammonia adsorption and its regeneration performance

Ammonia, as a common gaseous pollutant in both indoor and outdoor air, seriously deteriorates the atmospheric environment and endangers human health. Especially, ammonia makes a significant contribution to the formation of secondary aerosols as the precursor of haze. Therefore, there is a strong practical demand to control the emission of ammonia and realize its cost-effective removal. Herein, different tunnel structured (beta-, alpha-, gamma-, delta-, tau-) MnO2 were evaluated for the adsorption of gaseous ammonia at ambient temperature. alpha-MnO2 exhibited the best adsorption performance, reaching 17.76 mg NH3/g, whose adsorption capacity was 25 to 50 times higher than that of unmodified commercially available carbon-based materials (Bamboo charcoal (0.47 mg NH3/g), Coconut shell activated carbon (0.35 mg NH3/g) and Coal-based activated carbon (0.66 mg NH3/g)). Moreover, the effects of space velocity, ammonia concentration, relative humidity, and operating temperature on its adsorption performance were investigated. Characterization and theoretical calculations confirmed that the unique tunnel structure and the abundance of surface acidic sites are the key factors for high adsorption performance. Since alpha-MnO2 has a well-developed tunnel structure that can match the molecular size of ammonia, which is conducive to the physical adsorption of ammonia. Besides, alpha-MnO2 has relatively more and stronger surface acid sites, especially Bronsted acid sites, which can significantly enhance the chemical adsorption of alkaline gas ammonia. In addition, alpha-MnO2 can be regenerated and recycled at a relatively low temperature (125 degrees C) without any rinse treatment. The excellent adsorption capacity of the carrier-free alpha-MnO2 at high humidity and low-temperature regenerability make it an attractive adsorbent for the purification of ammonia.