Structure and reactivity in the selective oxidation of methane to formaldehyde of low-loaded FeOx/SiO2 catalysts

The structure of low-loaded (0.09-0.73 Fe wt%) FeOx/SiO2 catalysts prepared by adsorption-precipitation of Fe2+ precursor on silica (AP) has been assessed by DR UV-Vis and Mössbauer techniques in comparison with counterpart incipient wetness (IW) systems. Spectroscopic findings indicate the speciation of the active phase into isolated Fe3+ species, oligomeric 2-d FeOx patches, and 3-d Fe2O3 nanoparticles, characterised by decreasing interaction strength with silica carrier. The reduction pattern of the above surface structures has been probed by temperature-programmed reduction (TPR) measurements, and a deconvolution analysis of spectra allowed the concentration of the various surface species to be highlighted. The catalytic activity in the selective oxidation of CH4 to HCHO with oxygen (MPO) has been systematically evaluated by batch (BR) and continuous-flow (CF) measurements in the range of 400-750 oC. Direct relationships among Fe dispersion and specific atomic rate of CH4 conversion (FeCH4, s-1) and HCHO formation (FeHCHO, s-1) confirm the superior catalytic performance of AP systems. After a poor reactivity of "isolated species" and the unselective behaviour of 3-d Fe2O3 particles, 2-d oligomeric patches feature the best catalytic pattern, owing to an optimum Fe-O bond strength. Sintering and surface reconstructuring phenomena affect the catalyst stability during the MPO reaction.