Mnso4 h2o

Four manganese
oxides with different crystalline structures (abbreviated as MnO₂-I, MnO₂-II, MnO₂-III, and MnO₂-IV) were synthesized by a uniform hydrothermal method. In the synthesis
of MnO₂-I, 16 mmol KMnO₄ (Sinopharm, China)
and 6 mmol MnSO₄·H₂O (Sinopharm, China)
were dissolved in 160 mL of deionized water under ultrasonic stirring
for 20 min, and the solution was kept at 140 °C for 12 h in a
200 mL Teflon-lined autoclave. The precipitation was further dried
at 110 °C for 12 h after thorough centrifugation and washing,
and then the obtained powder was annealed at 300 °C for 3 h in
air. The syntheses of MnO₂-II, MnO₂-III, and
MnO₂-IV were similar to that of MnO₂-I except
for the differences of precursors and hydrothermal conditions. Specifically,
MnO₂-II was synthesized with (NH₄)₂S₂O₈ (Sinopharm, 10 mmol) and MnSO₄·H₂O (10 mmol). For MnO₂-III, the solution
with (NH₄)₂S₂O₈ (16 mmol)
and MnSO₄·H₂O (16 mmol) was hydrothermally
reacted at 90 °C for 24 h. Further, MnO₂-IV was synthesized
via the hydrothermal reaction of KMnO₄ (36 mmol) and MnSO₄·H₂O (6 mmol) at 240 °C for 24 h. All
of the reagents were of analytical grade, and the catalysts were screened
with 40–60 mesh before the activity tests.

MA, urea, K₂Cr₂O₇, ascorbic acid, NaOH and HCl were purchased from Aladdin Chemical Co., Ltd (Shanghai, China). Other chemicals, including NaCl, KCl, AgNO₃, CuCl₂·2H₂O, CaCl₂·2H₂O, FeCl₂, MnSO₄·H₂O, BaCl₂, NiCl₂·6H₂O, MgCl₂, Zn(NO₃)₂, FeCl₃·6H₂O, Al(NO₃)₃·9H₂O and CrCl₃·6H₂O, were obtained from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Mung bean seeds were purchased from retail outlets. All the chemical agents were of analytical grade and used as received without further purification. Deionized water was used for preparing the solutions.