Isotopic Studies of Sn-Cr Binary Oxide Catalysts for Methane Total Oxidation

A series of Sn-Cr binary oxide catalysts were prepared by a co-current co-precipitation method and tested for methane total oxidation. The binary oxide catalysts have much higher surface areas and catalytic activities for methane oxidation than pure SnO₂. CrO_x/SnO₂ with a Cr/Sn atomic ratio of 3:7 displays the highest activity. Selected samples were subjected to temperature-programmed ¹⁸O isotope-exchange measurements. Both complete and partial heteromolecular ¹⁸O isotope exchange, as well as oxygen release, was observed for all catalysts. Reaction between CH₄ and ¹⁸O₂ under static conditions was performed to investigate the reaction mechanism and it was found that the total oxidation of methane over Sn-Cr binary oxide catalysts occurs via a redox cycle with the chromium ion in a high oxidation state as the active center. Oxygen mobility of the catalyst plays an important role in the total oxidation of methane, but too high a mobility leads to very high oxygen release and a reduction of the surface reoxidability. This causes a decrease in the catalytic oxidation activity.