Study On Preparation And Support Effect Of Only-Palladium Close Coupled Catalysts

A series of Pd/Co_xZr_1-xO₂ , Pd/Mn_xZr_1-xO₂ , Pd/Sn_xZr_1-xO₂ , Pd/SnO2 , Pd/ZrO2 and Pd/SnO2 -Al2O3 catalysts with Pd loading of 0.5 wt.% were prepared by impregnation method. MOx -ZrO2 (M=Co, Mn, Sn) binary oxides with different atom ratios and pure SnO2 , ZrO2 were prepared by urea-hydrolysis method. SnO2 -Al 2 O3 support was prepared by deposition-precipitation method. These catalysts were tested for the low-temperature C3 H8 oxidation, and compared with Pd/Al 2 O3 catalyst. The support effect and the calcination temperature effect on the activity of the catalysts were investigated. The results show that Pd/Sn0 .4 Zr0.6O2 exhibits the best oxidation performance and stability. Afterwards, the effects of preparation conditions on the activity of Pd/Sn0.4Zr0.6O2 were studied. It reveals that the support prepared by urea- hydrolysis method demonstrates good thermal stability and makes the supported Pd species disperse morderately, leading to the best oxidation activity. Compared with PdCl2 as precursor, the catalyst prepared by Pd(NO3 )2 displays better activity, possibly due to the poisoning of active sites by Cl-. Increasing Pd loading from 0.5 to 4.0 wt.% does not bring out significant differences between the activities of the catalysts.Additionally, X-ray diffraction and Raman Spectroscopy were employed for phase identification of these supported Pd catalysts. H2 -TPR and O2-TPD techniques, accompanying with X-ray photoelectron spectroscopy and in situ diffuse reflectance FT-IR spectroscopy, were used to investigate the support effect and effect of preparation conditions on redox behavior, as well as surface physiochemical properties of PdOx species. The results were further correlated to the catalytic performance of these catalysts. It shows that PdOx crystallites of Pd/Sn0.4Zr0.6O2 and Pd/Sn can be reduced even at room temperature, and the Pd in Pd/Sn0.4Zr0.6O2 has the lowest oxidation state, which implies the presence of the largest amount of oxygen vacancies (Pd) in this catalyst, resulting in the highest redox efficiency of Pd-PdO site pairs during oxidation process. Besides, intensity of IR band, attributable to CO bridged species on Pd/Sn0.4Zr0.6O2, is strongest and such species are relatively stable, making PdOx crystallites favorable for successive C3H8 dissociative adsorption. Whereas, PdOx species on Pd/SnO2 -Al2O3 and Pd/Al 2 O3 catalysts are reduced in relatively higher temperature and tend to form relatively strong CO liner species, indicating the higher dispersion of such species and the lower catalytic activity. With respect to preparation conditions, calcination temperature and Pd loading could hardly affect the reduction behaviors of PdOx on Pd/Sn0.4Zr0.6O2 ; Using PdCl2 as precursor leads to the disappearance of oxygen desorption peak, the elevation of PdOx reduction temperature and the increase of the intensity of CO adsorption peak, which suggests the formation of highly dispersed PdOx, and it could make the catalytic activity decline. Increasing Pd loading to 4.0 wt.% could slightly increases the oxidation activity of the catalyst, and the co-presence of PdO and Pd0 phase can be detected due to the aggregation of Pd species. In the present work, the mechanism of C3 H8 oxidation is discussed and the related key factors are analyzed, which could be used as reference for optimizing the support materials of only-palladium close coupled catalysts.