Hydrothermal Stabilization by Lanthanum of Mixed Metal Oxides and Noble Metal Catalysts for Volatile Organic Compound Removal

Catalysts based on manganese or copper oxides, mixed with platinum or palladium, supported on alumina alone or doped with lanthanum were characterized by BET, XRD, SEM, HRTEM, TPR, and XPS and tested for the oxidation of a mixture of naphthalene, CO, and CH4 in air, CO2, and water. The catalysts were also aged at 900°C for 300 h in 10% steam. La was found in a dispersed phase in the alumina washcoat both before and after aging. Fresh manganese oxide catalysts, calcined at 800°C, contained mostly Mn2O3, while on fresh copper samples, surface Cu2+ species were prevalent. It was found that Mn and Cu were better dispersed onto the alumina washcoat in the presence of La and that La increased the saturation of copper into alumina. The reducibility by H2 of copper was also lowered by La. After aging, only CuO appeared on the unpromoted Cu-based catalysts, whereas, in the presence of La, the surface Cu2+ species were still present. The presence of active phases, particularly Cu and Pd, increased the formation of α-Al2O3 and the particle size of the alumina washcoat after aging. More than 60% of CuO reacted with alumina to form the spinel compound, CuAl2O4, whereas in presence of La, no spinel was formed. The addition of La onto fresh manganese-based catalysts led to an enhancement of the oxidation of CH4. The CuO–Pd/La–Al2O3 catalyst was found to be the most active catalyst for the oxidation of CO, C10H8, and CH4 before and after aging.