Study On The Highly Active Nano-Gold Catalysts Used For CO Preferential Oxidation In H₂-Rich Stream

A series of MnO_x-CeO₂ （Mn/Ce=1:1） mixed oxide supports were synthesized by co-precipitation, and the corresponding gold-supported catalysts were prepared by deposition-precipitation （DP）. As a novel and rather convenient method, ultrasonic pretreatment was employed for the preparation of nanostructured Au/MnO_x-CeO₂ catalysts which were used for CO preferential oxidation. The effect of ultrasonic pretreatment on the structures and catalytic performance of the Au/MnO_x-CeO₂ was first investigated. The effects of preparation conditions such as the synthesis pH （7.0–11.0）, Au loading （0.5–5.0 wt.%）, calcination temperature （150–350℃） and ultrasonic time （0–30 min） on the PROX performance of these catalysts were systematically investigated. After optimization, the Pt was introduced to Au/MnO_x-CeO₂, and its influence on the PROX performance of the catalysts was also studied.Ultrasonic pretreatment of MnO_x-CeO₂ promotes the performance of Au/MnO_x-CeO₂, with CO conversion increased by 24 % at 120℃. Meanwhile, the selectivity of oxygen to CO₂ is promoted in the whole temperature range, especially in 80-120℃, the selectivity is increased by 15-21%. For the samples prepared by ultrasonic-assisted DP, the Au（1.0）/MnO_x-CeO₂-10.0-250-5 catalyst prepared at pH=10.0 with 1.0 wt% Au loading, calcined at 250℃and ultrasonically pretreated for 5 min exhibits the best catalytic performance, giving not only the highest CO conversion of 90.9% but also the highest oxygen to CO₂ selectivity during the normal operating temperature range （80–120℃） for the fuel cell; meanwhile, the stability of catalytic activity for this catalyst was investigated at 120℃, showing that the CO conversion and oxygen to CO₂ selectivity can be well maintained for 100 h, only a very slight decrease is observed. The introduction of Pt further improves the PROX performance to some extent, especially for CO conversion, which can reach 100% at 100℃.The structure and morphology of the catalysts were characterized carefully by techniques including N2 adsorption-desorption （BET）, X-ray diffraction （XRD）, high resolution transmission electron microscopy （HR-TEM）, scanning electron microscope （SEM）, X-ray photoelectron spectra （XRS）, UV-visible diffuse reflectance spectra （UV-vis DRS） and temperature-programmed reduction by H₂ （H₂-TPR）. The HR-TEM, XRD and UV-vis DRS results reveal that ultrasonic pretreatment mainly changes the morphology of the flake-like support with rather flat surface into aggregation of small particles, leading to a higher dispersion of gold nanoparticles with smaller size （<5nm）. The results of XRD, HR-TEM and XPS indicate that the ultrasonic-assisted Au（1.0）/MnO_x-CeO₂-10.0-250-5 catalyst possesses the highest dispersion of Au species and the largest amount of surface adsorbed oxygen species, which facilitates CO oxidation. The XPS and H₂-TPR results reveal that the strong interaction between Au species and the supports in the catalyst Au（1.0）/MnO_x-CeO₂-10.0-250-5 decreases its capability for H₂ dissociation, effectively inhibiting the hydrogen spillover, as a result, the selectivity of oxygen to CO₂ is remarkably increased. For the bimetallic catalyst with Pt introduction, the results of XRD, SEM and H₂-TPR indicate that after introduction of Pt, a new phase has formed, which change the morphology and reducibility of the catalyst, leading to better PROX performance for the bimetallic catalyst.