Characterization of Pd–CeOx interaction on α-Al2O3 support

The Pd–Ce interaction was studied over CeO2 (0.3–2.5 wt.%)–Pd (1 wt.%)/α-Al2O3 catalysts used in the reforming reaction of CH4 with CO2. The samples were characterized by using high resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS). The activity and selectivity behavior was in good agreement with that of other supported metal catalysts (Ni and Pd) modified with different promoters. The preliminary results of HRTEM would indicate that the CeOx forms small crystallites around the Pd particle. The XPS analysis for the regions of Ce 3d and Pd 3d, gives an account of Ce being present mostly as Ce3+ and a high binding energy for Pd 3d5/2 (335.3 eV), an evidence of Pd–Ce chemical interaction. The Pd/Al XPS intensity ratios vs. the Pd average particle size, determined by TEM, show an excellent correlation for fresh and used catalyst. These results indicate that the diminution of the Pd/Al ratios was due to Pd sintering. Consequently, the small amounts of CeOx species do not cover the Pd particle, in agreement with the HRTEM results. The overall results stand for the promoter action mechanism of the CeOx for the reforming reaction with CO2.