Surface Sites of Pd/CeO₂/Al₂O₃ Catalysts in the Partial Oxidation of Propane

Abstract

Propane oxidation was studied on a Pd-supported Al 2 O 3 catalyst promoted by ceria, when this ceria was first grafted as a monolayer over alumina. The reaction proceeded under different feed conditions, and the surface sites were identified by X-ray photoelectron spectroscopy (XPS) in a pretreatment chamber and then compared to temperature-programmed desorption, temperature-programmed surface reaction (TPSR), and catalytic results, in the presence and absence of water. Stable catalyst performance was achieved in experiments after 20 h with time on stream, to get reliable data for further interpretation. This observation is true for all reaction conditions, with reducing, stoichiometric, or excess oxygen content in the reaction feed. The Pd/CeO 2 /Al 2 O 3 catalyst is less active for oxidation at low temperatures but suddenly becomes very active at high temperatures. However, in the case of the latter, large quantities of H 2 were released, which suggests a drastic change in selectivity. TPSR results showed that the presence of CeO 2 affected the oxidation, inhibiting the reaction as well as the re-forming in the first domain. But the combination of both was beneficial in terms of higher H 2 production. XPS results showed that the catalysts containing CeO 2 form the highest oxidation state palladium species, probably PdO 2 (338 eV), after the oxidation of propane. From these results it is suggested that besides the Pd 0 /PdO interface, which are active sites for propane oxidation, the Pd 0 /PdO 2 sites also favor the selective oxidation toward re-forming.

The effect of precursors salts on surface state of Pd/Al2O3 and Pd/CeO2/Al2O3 catalysts

“…Moreover, CeO 2 not only is a source of oxygen but also a stabilizer of the catalyst. It also affects the selectivity for hydrogen formation in the partial oxidation, as reported previously (Guimarães et al 2003).…”

“…On the other hand, the XPS results before and after oxidation in the pre-chamber show a shift of the binding energy of Pd towards 337.2 eV, which is attributed to a higher oxidation state of palladium, according to the literature (Burch et al 1998, Guimarães et al 2003. This shift evidences that metallic Pd and PdO/Pd 0 interfaces coexist, which are the active sites for the oxidation reaction (Burch et al 1998).…”

“…Pd-X or PdCe-X (X means Cl or acac) was used to denote Pd/Al 2 O 3 or Pd/CeO 2 /Al 2 O 3 catalysts prepared using chloride and acetylacetonate precursors (Table I). Both materials were pretreated differently, as reported previously (Guimarães et al 2003). While the Pd/Al 2 O 3 catalyst was submitted to a He flow at 423 K for 30 min, the Pd/CeO 2 /Al 2 O 3 was first calcined with 5%O 2 /He at 673 K for 60 min, followed by He flow at 50 ml/min, at 773 K for 90 min.…”

“…Samples were pretreated with hydrogen and propane oxidation in the presence or absence of water under stoichiometric reaction condition in a pre-chamber. After evacuation and cool down, samples were introduced in the UHV chamber and spectra were obtained as described elsewhere (Guimarães et al 2003). Aiming to identify the oxidation state of palladium and cerium during the oxidation of propane, the samples were submitted separately to 1% C 3 H 8 /He with 5% O 2 /He, and 1% C 3 H 8 /He, 5% O 2 /He with 5% H 2 O, both under stoichiometric conditions (R=5.0), at 150 ml/min and at 673 K for 60 min.…”

“…Moreover, the Cl/Al ratio showed a drastic reduction in residual chlorine. From these results, it was suggested that during the reaction the Pd 0 and PdO could be transformed into PdO 2 , favoring the selective oxidation for reforming and high production of hydrogen (Guimarães et al 2003).…”