Tunnel structure effect of manganese oxides in complete oxidation of formaldehyde

Three manganese oxides with different square tunnel sizes, pyrolusite, cryptomelane and todorokite, were prepared, and characterized by X-ray powder diffraction, specific surface area measurement, temperature programmed reduction by H2, high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy. The results of catalytic tests in complete oxidation of formaldehyde revealed that cryptomelane had much higher activity than pyrolusite or todorokite. The characterization results suggested that the catalytic activity was primarily predominated by the tunnel structures of manganese oxides rather than specific surface area, degree of crystallinity, reducibility and average oxidation state of manganese. The effective tunnel diameter of cryptomelane similar to the dynamic diameter of formaldehyde molecule was proposed to be a major factor to give rise to the high catalytic activity.