Regulation of surface acidic sites on birnessite-type MnO2 and its function in the adsorption of gaseous ammonia

Ammonia (NH3) is not only a characteristic malodorous contaminant, but also one of the important precursors in the formation of haze. In engineering practice, the most widely used technology for NH3 purification is adsorption, and the development of adsorbent with excellent NH3 adsorption capacity is the key to adsorption method. Herein, the surface acidic sites of layered birnessite-type MnO2 were regulated for the adsorption of NH3. Results presented that the adsorption capacity of birnessite-type MnO2 reinforced by surface acidic sites could reach 36.1 mg/g, which significantly superior to pristine MnO2 and other commercial carbon materials. In addition, the effects of space velocity, NH3 concentration, adsorption temperature and relative humidity on its adsorption performance were also studied. It was found that the effect of space velocity and relative humidity had no significant effect on the equilibrium NH3 adsorption capacity; while the equilibrium adsorption capacity showed an increasing trend with the rise of NH3 concentration and the decrease of adsorption temperature. Furthermore, the function of surface acidic sites, especially Lewis and Brønsted acidic sites, in the adsorption of birnessite-type MnO2 for NH3 was further revealed. On the one hand, the acidic site strengthening method of acid impregnation can significantly increase the specific surface area of birnessite-type MnO2, which is beneficial for the physical adsorption of NH3. Importantly, this acidic site strengthening method significantly enhances the number and intensity of surface acidic sites of birnessite-type MnO2, especially the Brønsted acidic sites, which greatly promoted the adsorption of alkaline gas NH3. Finally, the cyclic thermal regeneration performance at low temperatures was further studied. This work reports a strategy to improve NH3 adsorption performance by enhancing acidic sites, and further reveals the functional role of surface acidic sites in NH3 adsorption.