Influence of Gd and Pr doping on the properties of ceria: texture, structure, redox behaviour and reactivity in CH4/H2O reactions in the presence of H2S

Abstract

Ceria materials doped by Gd and/or Pr were prepared by a precipitation route and calcined in air at 500 and 900 °C. The effects of doping and thermal treatment on the materials' surfaces were studied by X-ray photoelectron and Raman spectroscopies. The redox properties were investigated using temperature-programmed reduction experiments in H₂ and CH₄ followed by temperature-programmed oxidation in O₂. The catalytic properties of these materials in CH₄/H₂O reaction in the absence/presence of H₂S were evaluated at 750 °C in a large excess of CH₄ with respect to H₂O. The materials were classified by order of reactivity as follows: CPO > CGPO > CeO₂ > CGO, which can be primarily related to the surface area variation. Comparing the activity per m² of various samples indicates that the catalytic activity is correlated with the Ce³⁺ concentration at the surface as measured by XPS. Gd-doped and undoped samples exhibit the highest surface Ce³⁺ concentration as well as the highest activity in SMR per m², while Pr-doped samples are the least active (activity per m²) with the lowest surface Ce³⁺ concentrations. H₂S has a sharp promoting effect on the catalytic behavior of ceria-based materials, improving the production of H₂ by almost one order of magnitude.