The effect of ceria content on the properties of Pd/CeO2/Al2O3 catalysts for steam reforming of methane

摘要

The effect of CeO2 loading on the surface properties and catalytic behaviors of CeO2-Al2O3-supported Pd catalysts was studied in the process of steam reforming of methane. The catalysts were characterized by S-BET, X-ray diffraction (XRD), temperature-programmed reduction (TPR), UV-vis diffuse reflectance spectroscopy (DRS) and Fourier transform infrared spectroscopy (FTIR). The XRD measurements indicated that palladium particles on the surface of fresh and reduced catalysts are well dispersed. TPR experiments revealed a heterogeneous distribution of PdO species over CeO2-Al2O3 supports; one fraction of large particles, reducible at room temperature, another fraction interacting With CeO2 and Al2O3, reducible at higher temperatures of 347 and 423 K, respectively. The PdO species reducible at room temperature showed lower CO adsorption relative to the PdO species reducible at high temperature. In contrast to Pd/Al2O3, the FTIR results revealed that CeO2-containing catalyst with CeO2 loading >= 12 wt.% show lower ratio (LF/HF) between the intensity of the CO bands in the bridging mode at low frequency (LF) and the linear mode at high frequency (HF). This ratio was constant with increasing the temperature of reduction. The FTIR spectra and the measurement of I'd dispersion suggested that Pd surface becomes partially covered with ceria at all temperature of reduction and with increasing ceria loading in Pd/CeO2-Al2O3 catalysts. Although the PdO/Al2O3 showed higher I'd dispersion compared to that of CeO2-containing catalysts, the addition of ceria resulted in an increase of the turnover rate and specific rate to steam reforming of methane. The CH4 turnover rate of Pd/CeO2M2O3 catalysts with ceria loading >= 12 wt.% was around four orders of magnitude higher compared to that of Pd/Al2O3 catalyst. The increase of the activity of the catalysts was attributed to various effects of CeO2 such as: (i) change of superficial Pd structure with blocking of Pd sites; (ii) the jumping of oxygen (O-*) from ceria to Pd surface, which can decrease the carbon formation on Pd surface. Considering that these effects of CeO2 are opposite to changes of the reaction rate, the increase of specific reaction rate with enhancing the ceria loading suggests that net effect results in the increase of the accessibility of CH4 to metal active sites. (c) 2006 Elsevier B.V. All rights reserved.