The kinetics of the partial oxidation of methane to formaldehyde MPO on a '' precipitated'' silica catalyst in the 500 ᎐ 800ЊC range was in®estigated. The influence of temperature and reaction mixture composition on the density of reduced sites of the catalyst under steady state was e®aluated by in situ reaction temperature oxygen chemisorption measurements. The kinetics of the MPO on silica is pseudo first and zero-order on P and P , respecti®ely, while the density of reduced sites depends C H o 4 2 upon the square root of the P rP ratio. The rate-determining step is the acti®ation C H O 4 2 ( y 1 ) of C ᎐ H bond of CH molecules E s 142 kJ ؒ mol , while oxygen replenishment 4 r e d ( y 1 ) is a nonacti®ated reaction step E f 20 kJ ؒ mol . A Langmuir-Hinshelwood kinetic o x model, accounting for a two-site-dissociati®e acti®ation mechanism of both CH and 4 O , thoroughly describes the steady state of the silica surface and the reaction kinetics 2 of the MPO in the 500 ᎐ 800ЊC range. CH CH 4 4 . Ž . P , 0.1 and P P , 0.025᎐0.40; P 0.2 , respectively. O O O C H 2 2 2 4 After pretreatment, the catalyst sample was flushed in the Ž . Ž He carrier flow F s F and then O pulses V He HerCH rO 2 O 4 2 2 . s12.9 nmol were injected until saturation. O uptakes were 2 quantified by a TCD connected to a DP 700 Data Processor