Surface acidic sites strengthened core-shell HC@MnO2 for enhanced gaseous ammonia adsorption

Abstract

As a common gaseous pollutant in atmospheric environment, ammonia (NH₃) not only contributes to the formation of haze, but also disturb the nitrogen balance in ecosystem through atmospheric nitrogen deposition. Therefore, the control of NH₃ emission has important environmental significance. Adsorption is the most commonly used technology for NH₃ purification in practice, and efficient adsorbents are the key to adsorption method. Herein, a core-shell structured HC@MnO₂ adsorbent was constructed by in-situ growth of layered δ-MnO₂ on hydrochar (HC) surface, and its surface acidic sites were further strengthened. The enhancement of surface acidic sites significantly improved the adsorption performance of HC@MnO₂ for NH₃, reaching 34.49 mg NH₃/g, which was superior to commercial carbon-based materials (whose adsorption capacity was 8.47 times that of Coal-based activated carbon, 14.25 times that of Coconut shell activated carbon, and 12.77 times that of Bamboo charcoal). Moreover, the operating parameters and adsorption kinetics were detailly investigated. The adsorption of HC@MnO₂ on NH₃ was in accordance with pseudo-second-order adsorption kinetics model. Large surface area of core-shell structure and abundant surface acidic sites of δ-MnO₂ are the decisive reasons for the excellent adsorption performance of HC@MnO₂. Importantly, the enhancement of surface stronger Brønsted acidic sites is the key to improve NH₃ adsorption performance of HC@MnO₂. Finally, the thermal regeneration and recycling performance of HC@MnO₂-H were also investigated. This study provides a suggestive for further research on low-cost composite materials with excellent NH₃ adsorption performance.

Keywords: Acidic sites; Adsorption mechanism; Hydrochar; Manganese oxide.