The adsorption and oxidation mechanism of formaldehyde (HCHO) with time and temperature was systematically investigated by in situ FT-IR and mass spectroscopy methods on pure MCM-41 and Ag/MCM-41 catalysts pretreated with different atmospheres (H-2 or O-2). HCHO-TPSR experiments showed that the Ag catalyst pretreated with O-2 was highly active for HCHO oxidation compared with that pretreated with H-2. And it was found by in situ FT-IR that the DOM, formate and CO species were the main reaction intermediates for HCHO oxidation on Ag/MCM-41. Importantly, the formation of the formate was not only derived from the HCHO dissociation adsorption by consuming hydroxy group, but also from the disproportionation of DOM active ad-species on Ag/MCM-41 catalysts. It was our first observation of the existence of adsorbed CO on the active silver structure by HCHO-TPSR. The formation of the formate and CO on the surface of the catalyst were both promoted after the Ag/MCM-41 was pretreated with O-2 due to the formation of subsurface oxygen species and the stronger interaction of silver with the support. The quantitative analysis of the formate formation and the calculation of reaction rates for HCHO and CO oxidation suggested that the amount of formed formate and the reaction rates for HCHO and CO oxidation showed the positive linear relationship. An possible reaction scheme for HCHO oxidation on Ag/MCM-41 catalyst was also given in this manuscript. (c) 2013 Elsevier B.V. All rights reserved.