The effect of CeO 2 structure on the activity of supported Pd catalysts used for methane steam reforming

Abstract

Palladium (Pd) supported on CeO 2-promoted γ-Al 2O 3 with various CeO 2 (ceria) crystallinities, were used as catalysts in the methane steam reforming reaction. X-ray diffraction (XRD) analysis, FTIR spectroscopy of adsorbed CO, and X-ray photoelectron spectroscopy (XPS) were employed to characterize the samples in terms of Pd and CeO 2 structure and dispersion on the γ-Al 2O 3 support. These results were correlated with the observed catalytic activity and deactivation process. Arrhenius plots at steady-state conditions are presented as a function of CeO 2 structure. Pd is present on the oxidized CeO 2-promoted catalysts as Pd 0, Pd + and Pd 2+, at ratios strongly dependent on CeO 2 structure. XRD measurements indicated that Pd is well dispersed (particles <2 nm) on crystalline CeO 2 and is agglomerated as large clusters (particles in 10–20 nm range) on amorphous CeO 2. FTIR spectra of adsorbed CO revealed that after pre-treatment under H 2 or in the presence of amorphous CeO 2, partial encapsulation of Pd particles occurs. CeO 2 structure influences the CH 4 steam reforming reaction rates. Crystalline CeO 2 and dispersed Pd favor high reaction rates (low activation energy). The presence of CeO 2 as a promoter conferred high catalytic activity to the alumina-supported Pd catalysts. The catalytic activity is significantly lower on Pd/γ-Al 2O 3 or on amorphous (reduced) CeO 2/Al 2O 3 catalysts. The reaction rates are two orders of magnitude higher on Pd/CeO 2/γ-Al 2O 3 than on Pd/γ-Al 2O 3, which is attributed to a catalytic synergism between Pd and CeO 2. The low rates on the reduced Pd/CeO 2/Al 2O 3 catalysts can be correlated with the loss of Pd sites through encapsulation or particle agglomeration, a process found mostly irreversible after catalyst regeneration.