Studies on the SbOx species of SbOx/SiO2 catalysts for methane-selective oxidation to formaldehyde

Sb2O3/SiO2 and Sb2O5/SiO2 catalysts were prepared and characterized by N2-adsorption, XRD, XPS, UV–vis DRS, UV Raman and H2-TPR and investigated in methane-selective oxidation. No crystalline antimony oxide can be detected on catalysts even when antimony oxides loading reaches 20 wt.%. On Sb2O3/SiO2 catalysts, the oxidation of highly dispersed Sb3+ oxidic entities is more difficult than that of aggregated Sb3+ oxidic entities. The concentration of Sb5+ oxidic entities increases with Sb2O3 loading and about half the amount of Sb3+ oxidic entities can be oxidized to Sb5+ oxidic entities on 20 wt.% Sb2O3/SiO2 catalyst. On Sb2O5/SiO2 catalysts, SbOx species are all Sb5+ oxidic entities and aggregated SbOx species emerge as Sb2O5 loading reaches 5 wt.%. In methane-selective oxidation, highly dispersed SbOx species are more active than aggregated SbOx species and the depression of deep oxidation towards CO2 is more favorable on aggregated SbOx species than on highly dispersed SbOx species regardless of the oxidation state of Sb. Furthermore, highly dispersed Sb3+ oxidic entities are more active than highly dispersed Sb5+ oxidic entities. Such tendency is in line with that, for both Sb2O3/SiO2 and Sb2O5/SiO2 catalysts, the redox performance of aggregated SbOx species are poor compared to that of highly dispersed SbOx species. This results in their decreasing activity but increasing selectivity to CO and formaldehyde with increasing loading of antimony oxides in methane-selective oxidation.