Effect of oxygen pretreatment on the surface catalytic oxidation of HCHO on Ag/MCM-41 catalysts

•O2 pretreatment temperature strongly affects the Ag state and dispersion.•Ag2O clusters diffuse to MCM-41 channels with the O2 treatment temperature.•Ag2O clusters inside of MCM-41 pores accelerate the formation of dioxymethylene.•Smaller sized metallic silver particles improve formation of formate intermediate.•CO2 formation rate from formate species is higher than that from dioxymethylene.

The effect of oxygen pretreatment on HCHO catalytic oxidation of Ag/MCM-41 catalysts has been investigated from 500 °C to 800 °C in this paper. The highest HCHO surface reaction activity is achieved on the silver sample pretreated at 700 °C. The state and the dispersion of silver species on the support are strongly related to the pretreatment temperature. The increase of the O2 pretreatment temperature for Ag/MCM-41 catalysts results in the diffusion of Ag2O clusters from outside to inside of the channels. And the amount of Ag2O clusters inside the channels becomes maximum at 600 °C. Interestingly, it is found that the smaller metallic Ag nanoparticles coexist with large particles when the catalyst is pretreated at 700 °C. It is suggested that Ag2O clusters inside of MCM-41 pores improves the HCHO adsorption capacity and accelerates the formation of HCHO adsorption intermediates, particularly dioxymethylene(DOM) ad-species. And the formed meallic silver particles with smaller size at 700 °C plays an essential effect on the formation of formate intermediate and the enhancement of surface reaction activity. It is also concluded that CO2 formation rate from formate is higher than that from DOM species in HCHO oxidation. The decrease of HCHO surface reaction activity after oxygen pretreatment at 800 °C may be due to the decrease of the Ag2O clusters inside MCM-41 pores and the aggregation of metallic Ag nanoparticles.

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