Several MnO2 catalysts,
promoted with Pb2+ ions and supported on a wash-coated
monolith (WMon), briefly, xPbyMn-WMon
(x = 0, 0.5,
1.0, 1.5, 2, and 2.5 and y = 8 wt %), were prepared.
The presence of Pb2+ affects the manganese oxidation state,
crystalline phase, thermal resistance, metal dispersion, and catalytic
performance. According to XPS spectra, XRD patterns and HRTEM images,
manganese was dispersed on the monolith surface as Mn3+ and Mn4+ species in both α and β crystalline
phases. The ratios of Mn4+/Mn3+ states and α/β phases were highly enhanced, and the desired PbxMn8O16 phase (coronadite)
was formed. Concentrations of the defect oxygen (Mn–O–H)
and oxygen vacancies, which improve the catalyst reducibility and
the MnO2 reduction temperature, were also increased. Further,
based on the H2 chemisorption analysis, the Pb2+ template would increase the manganese dispersion and the reaction
sites. Meanwhile, the average MnO2 crystallite size was
decreased from 13.26 to 8.15 nm. The optimum catalyst 1.5Pb8Mn-WMon
exhibited an activity 149% more than the manganese-only catalyst in
decomposition of H2O2. Evaluation of catalyst
stability in the presence of Pb2+ after 10 recycles showed
only a 6.8% decrease. The catalytic reaction was evaluated based on
different criteria.
Hydrogen peroxide catalytic activation holds great promise in the treatment of persistent pollutants. In this study, the novel Mn-Acacair/Al, Mn-Acacarg/Al and Mn-BTCarg/Al catalysts, supported on Al2O3, were applied for rapid hydrogen peroxide activation and azithromycin antibiotic removal. The catalysts were prepared by the calcination-hydrothermal method under air or argon atmosphere. The characterization confirmed that the modification of manganese with acetylacetonate and benzene-1,3,5-tricarboxylic acid (H3BTC) O-donor ligands highly improves the catalyst porosity, amorphousity, and abundance of coordinately unsaturated sites, which facilitate the generation of reactive oxygen species. The hydrogen peroxide activation and azithromycin removal reached 98.4% and 99.3% after 40 min using the Mn-BTCarg/Al catalyst with incredible stability and reusability. Only a 5.2% decrease in activity and less than 2% manganese releasing in solutions were detected after five regeneration cycles under the optimum operating conditions. The removal intermediates were identified by LC-MS/MS analysis, and the pathways were proposed. The hydroxylation and decarboxylation reactions play a key role in the degradation reaction.
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