O-17 Solid-State NMR Study on the Size Dependence of Oxygen Activation over Silver Catalysts

The capabilities of silver catalysts with different particle sizes for oxygen activation were systematically studied by O-17 solid-state NMR. The observation of O-17 signal at around 0 ppm for silver catalysts indicates that silver activates O-17(2) molecules and that subsequent formation of active O-17 species leads to the oxygen exchange with the mesoporous SiO2 supports. The initial appearances of the O-17 signals at different temperatures for three silver catalysts show that oxygen activation strongly depends on the size of Ag particle. Their ability of activating oxygen molecules is in the order Ag-3/MCM-41 > Ag-5/SBA-15 > Ag-10/SBA-15. Ag-3/MCM-41 is able to activate oxygen molecules and facilitates the oxygen exchange with the support even at room temperature. Although the NMR signal of active O-17 species on silver catalysts is not observed directly, the approach described in this work can be used to indirectly determine the conditions on which the active oxygen species can be produced during catalytic oxidation reactions. The silver catalysts can also be used for O-17 enrichment of the oxide-based materials under mild conditions