Influence of silver on the catalytic properties of the cryptomelane and Ag-hollandite types manganese oxides OMS-2 in the low-temperature CO oxidation

Abstract

Manganese oxide octahedral molecular sieves (OMS) are important materials in environmental chemistry, electrochemistry, and heterogeneous catalysis. Cryptomelane and Ag-hollandite type manganese oxides (OMS-2) were synthesized by microwave-reflux and hydrothermal methods, respectively. In this current study, silver doping of cryptomelane and Ag-hollandite was performed using both UV irradiation and KBH4 reduction methods. The formation process, particle size, crystallite size, crystal structure, and properties of these nanomaterials were characterized by powder X-ray diffraction, field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM), and nitrogen sorption. Studies by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) showed that the produced OMS-2 have a nanofiber structure. The produced catalysts showed high activity, as high as 100% in the low-temperature CO oxidation at 140°C. The catalysts doped with silver by KBH4 reduction method showed higher activities than other doping methods and template catalysts. In longtime stability tests, 80% CO conversion can be maintained for 26h at 120°C. The high activities, and stabilities of the Ag/K-OMS-2-KBH and Ag/Ag-OMS-2-KBH were attributed to the stable presence of Ag0 and Ag+ species and the unique morphologies of the cryptomelane and Ag-hollandite nanofibers. CO oxidation is believed to follow the Mars–van Krevelen mechanism via the Ag+−O2−−Mn4+↔Ag0−Mn3++O2 redox reaction.