The effect of a Cr2O3-addition on the phase transformation and catalytic properties of γ-Al2O3 in treatment of lean-burn exhausts

The solid-state thermal behaviour of γ-Al2O3 doped with 10 mol% Cr (oxide) was studied with respect to phase-transition behaviour and the co-ordination of the dopant Cr cations. A series of transformations: γ-Al2O3 → δ-Al2O3 → θ-Al2O3 → α-Al2O3 was observed for Cr2O3-doped alumina samples between 500 °C and 1100 °C. Rapid grain growth occurred at temperatures close to 1100 °C. The electron spin resonance (ESR) spectra for the sample heat-treated at 500 °C corresponded to the resonance of β-Cr3+ in an amorphous Cr oxide impregnated onto the γ- and δ-alumina support. The change of ESR spectrum indicated the existence of Cr3+, suggesting the formation of a solid solution with the same structures as δ-Al2O3 and/or θ-Al2O3 at 800–1000 °C. The evaluation of catalytic activities for model exhaust was performed under lean-burn (an excess of oxygen) condition of air/fuel ratio A/F = 18 and space velocity SV = 100 000 h–1 . The modified Al2O3 catalyst heat-treated at 1000 °C in air showed removal conversion of 100% for hydrocarbon (C3H6), 92% for CO and 5% for NO at 550 °C. Present results suggest that Cr-modification to Al2O3 leads to catalytic improvement with good thermal durability.