Effects of textural parameters and noble metal loading on the catalytic activity of cryptomelane-type manganese oxides for formaldehyde oxidation

Nanoporous manganese oxides were synthesized by a sol–gel method with maleic acid as an organic reducing agent. Noble metals (Pt and Ag) were then loaded by an impregnation method. The as-prepared materials are tunneled manganese oxides, cryptomelane (K-OMS-2). Further characterization was performed using X-ray diffraction (XRD), transmission electron microscopy (TEM) and N₂ adsorption/desorption techniques. The structure and textural properties are heavily dependent on reaction conditions. Reaction solutions using low temperature produced well-crystallized nanorods, whereas solutions at high temperature yielded poorly-crystallized nanoparticles. Formaldehyde oxidation was carried out to evaluate the catalytic activity of these materials. The cryptomelane nanoparticles that possess higher specific surface area and more open pores showed better catalytic performance than the cryptomelane nanorods. Effects of Pt and Ag on the cryptomelane catalysts indicate that not only noble metal types, but also the existence form of metals on the catalyst surface greatly affects the catalytic activity. Pt/K-OMS-2 consisting of ca. 2 nm Pt nanoparticles presents higher activity than Ag/K-OMS-2, but lower activity than the corresponding cryptomelane materials without metal loading, due to the cover of Pt on the active sites of manganese oxides.