Characterization of Pd-CeOx Interaction on α-Al2O3 Support

CARLOS E. GIGOLA; M. SERGIO MORENO; IGNACIO O. COSTILLA; MIGUEL D. SÁNCHEZ

Bariloche

Conferencia; 13th Internatinal Conference on Solid Films and Surfaces; 2006

The addition of small quantities of  CeO2 (0.3-0.5%) to a Pd(1%)/α-Al2O3, as catalyst promoter, was investigated by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), infrared spectroscopy (FTIR), and transmission electron microscopy (TEM and HRTEM). The activity and selectivity of this catalyst, used in the reforming of CH4 with CO2 [1], were in good agreement with that of other supported metal catalysts (Ni, Pt) modified with different promoters [2]. Previous studies [3] have shown the beneficial effect of 20 wt% of CeOx in the activity and stability of a Pd(2 wt%)/γ-Al2O3 catalyst, demonstrating a remarkable reduction of carbonaceous deposits. On the basis of this study we have recently shown [4] that the aggregate of a 2.5 wt% of Ce to a Pd(1wt%)/α-Al2O3 catalyst also eliminates the deposition of carbon, with a slight increase in the CO/H2 ratio. The XPS results showed that CeOx promotes the partial oxidation of Pd to PdO even after in-situ reduction treatment at 773K in H2. Both, XRD and HRTEM, indicate that the CeOx forms small crystallites around the Pd particles. FTIR spectra of adsorbed CO demonstrate that Ce is not deposited on the Pd particles. 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 give evidence 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 and XRD results. The crystallographic analysis realized from HRTEM images show that Ce presents different oxidations states between Ce+4 and Ce+3 in correspondence with the results obtained from XPS spectra of the Ce 3d region. From a catalytic point of view, the overall results stand for a redox promoter mechanism of CeOx during the reforming of CH4 with CO2.