Fig. 8. (a) CO oxidation light-off curves of different CuO catalysts prepared by UAHP at 90 °C for 2 h under different aging times of 0, 2, 24 and 72 h and calcination time of 2–4 h in ambient air at 400 °C. The feed mixture consisted of 4% CO, 20% O₂ and balance He and was passed over the catalyst bed (50 mg) at a flow rate of 60 cm³ min⁻¹ (72 000 cm³ g⁻¹ h⁻¹ WHSV), (b) CO oxidation long–term stability of CuO(A_{24 h}–C_{4 h}) catalyst under continuous stream for 48 h at ∼150 ± 5 °C and (c and d) CO oxidation light-off curves of (c) CuO(A_{2 h}–C_{2 h}) and (d) CuO(A_{24 h}–C_{4 h}) catalysts showing the effect of changing the residing time on the CO conversion.