Enhanced ozonation of pollutants by MgO nanoclusters/sewage sludge-derived hierarchical porous carbon: experimental and theoretical study

Abstract

Highly reactive nanoclusters of metal oxides are extremely difficult to synthesize due to their thermodynamic instability. For the first time, MgO nanoclusters supported on sewage sludge-derived hierarchical porous carbon (MgO nanoclusters/SC) were successfully prepared by grafting 1.31 ± 0.03 mmol g⁻¹ acidic groups on the SC, then dynamically controlling the chemisorption of Mg²⁺ on the modified SC within 20 s, and finally transforming the fixed Mg²⁺ into MgO nanoclusters with a size of less than 2.4 nm via thermal treatment. Moreover, the cluster size could be facilely tailored by controlling the dynamic adsorption time and initial aqueous Mg²⁺ concentration. The resulting five MgO nanoclusters/SC-20 s samples with 0.43–1.12% (mass percentage) Mg have a turnover frequency of 0.038 towards the catalytic ozonation of 4-chlorophenol, much higher than those of the bigger nanoclusters and nanocrystals (i.e. MgO/SC-20 s with 1.43% Mg, MgO/SC-3 min with 0.85–2.0% Mg). The pseudo-first-order reaction constant of MgO nanoclusters/SC with 1.12% Mg is 0.0126 min⁻¹, 6.0 and 4.1 times that of SC and bulky MgO/SC, respectively. The presence of MgO nanoclusters/SC also enhanced the removal of cephalexin, florfenicol and sulfamethoxazole by 54.3%, 45.5%, and 57.4%, respectively. The high activity can be ascribed to the rich and highly under-coordinated Mg in the smaller MgO nanoclusters and the unblocked hierarchical pores of the SC support. Finally, DFT calculations revealed at the atomic level that the under-coordinated Mg in MgO nanoclusters could directly or indirectly activate ozone through the protonated hydroxyl or just hydroxyl bonded to the under-coordinated Mg. Except for (MgO)₂, the smaller the nanocluster, the higher the activity. This work shed a light on the design and preparation of highly reactive nanoclusters of metal oxide catalysts for catalytic ozonation of refractory organic pollutants.