Study Of Nanostructured Au/MO_x-CeO₂ Catalysts Used For The Preferential CO Oxidation In H₂-rich Gases

A series of mesoporous CeO₂ and MOX-CeO₂ (M=Fe, Mn, Ni, Co) mixed oxide supports were synthesized by co-precipitation, the corresponding gold-supported catalysts were prepared by deposition-precipitation. The effect of the transition metals and the ultrasonic pretreatment on the catalyst structure and catalytic performance for preferential oxidation of CO (PROX), were systematically investigated.The structure of the catalysts was characterized carefully by techniques including N2 adsorption-desorption (BET), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XRD), high resolution transmission electron microscopy (HR-TEM) and energy dispersive X-ray spectrum (EDX). The results of BET and XRD indicate that all these catalysts exhibit uniform mesoporous structure. The introduction of 3d-transition metals M almost doubles the specific surface area and pore volume, meanwhile, the CeO₂ dispersion is increased, which contributes to the interaction between Au and support. The results of XPS reveal that the gold is present predominantly in metallic state, as well as small amounts of Au+ and Au3+ species. Compared with Au foil, the interaction between Au and support modified the electronic structure of Au, as indicated by its decreased binding energy. The activity measurements show that the gold particle size, and its interaction with the support, determines the catalytic activity and selectivity. The smaller particle size corresponds to the stronger interaction, and the lower binding energy, which results in the improved activity and selectivity.The ultrasonic pretreatment promotes the CO conversion in the temperature range from 80 to 120 oC over Au/MnOX-CeO₂ catalyst. At 120 oC, the conversion is increased by 20%. More importantly, the selectivity is increased to a great extent. In the temperature range from 80 to 100 oC, the selectivity is increased by 30%. BET results indicate that ultrasonic pretreatment increases the specific surface area and pore volume of the Au/MnOX-CeO₂ catalysts, which is favorable to the gold deposition. TEM results show that the gold particle size decreases, after ultrasonic pretreatment, from 5¹0 nm to about 5 nm. The results of XPS and TPR suggest that the increased gold dispersion strengthens the interaction between Au and the support. As a result, the activity and selectivity is greatly improved by ultrasonic pretreatment.