Role of yttrium loading in the physico-chemical properties and soot combustion activity of ceria and ceria–zirconia catalysts

Abstract

Ce 1− x Y x O 2 and Ce 0.85− x Zr 0.15Y x O 2 mixed oxides have been prepared by 1000 °C-nitrates calcination to ensure thermally stable catalysts. The physico-chemical properties of the mixed oxides have been studied by N 2 adsorption at −196 °C, XPS, XRD, Raman spectroscopy and H 2-TPR, and the catalytic activity for soot oxidation in air has been studied by TG in the loose and tight contact modes. Yttrium is accumulated at the surface of Ce 1− x Y x O 2 and Ce 0.85− x Zr 0.15Y x O 2, and this accumulation is more pronounced for the former formulation than for the latter, because the deformation of the lattice due to zirconium doping favours yttrium incorporation. Yttrium and zirconium exhibit opposite effects on the surface concentration of cerium; while zirconium promotes the formation of cerium-rich surfaces, yttrium hinders the accumulation of cerium on the surface. For experiments in tight contact between soot and catalyst, all the Ce 1− x Y x O 2 catalysts are more active than bare CeO 2, and Ce 0.99Y 0.01O 2 is the most active catalyst. The benefit of yttrium doping in catalytic activity of ceria can be related to two facts: (i) the Y 3+ surface enrichment hinders crystallite growth; (ii) the surface segregation of Y 3+ promotes oxygen vacancies creation. High yttrium loading ( x = 0.12) is less effective than low dosage ( x = 0.01) because yttrium is mainly accumulated at the surface of the particles and hinders the participation of cerium in the soot oxidation reaction, which is the active component. For the mixed oxides with formulation Ce 0.85− x Zr 0.15Y x O 2 (operating in tight contact) the effect of zirconium on the catalytic activity prevails with respect to that of yttrium. For experiments in loose contact between soot and catalyst, the catalytic activity depends on their BET surface area, and the catalysts Ce 0.85− x Zr 0.15Y x O 2 (BET = 10–13 m 2/g) are more active than the catalysts Ce 1− x Y x O 2 (BET = 2–3 m 2/g). In the loose contact mode, the yttrium doping and loading have a minor or null affect on the activity, and the stabilising effect of the BET area due to zirconium doping prevails.