On the possibility of AgZSM-5 zeolite being a partial oxidation catalyst for methane

A silver-ion-exchanged HZSM-5 zeolite sample (Ag(H)ZSM-5) evacuated at 573 K exhibited prominent catalytic behavior in the partial oxidation of CH₄ at temperatures above 573 K, exceeding the performance of Ag/SiO₂ṡAl₂O₃ and Ag/SiO₂ catalysts. From the infrared (IR) and X-ray absorption fine structure (XAFS) spectra, as well as the dioxygen adsorption measurement, it was concluded that the simultaneous existence of Ag⁺ ions and small clusters of Ag particles leads to the partial oxidation of methane. Taking the magnitude of the formation enthalpy (per oxygen atom) of Ag₂O (Δ H = 26 kJ / mol) into consideration, we propose the interpretation that the dioxygen activated on small Ag metal clusters formed in ZSM-5 elaborates a surface oxide layer on small Ag clusters and the thus-formed species is simultaneously and easily decomposed at 573 K or above, and the oxygen activated in this way on the Ag metal spills over and can react with methane that has been activated by the Ag⁺ ions exchanged in ZSM-5, resulting in the high catalytic activity of the Ag(H)ZSM-5 sample in the partial oxidation of methane. This interpretation is also well evidenced by XAFS and IR data. It is anticipated that this material has the potential to be a promising catalyst in the conversion of natural gas into higher value-added chemicals and fuels.