Structural phase transitions and lean NO removal activity of copper-modified alumina

Copper-modified alumina catalysts, designed for NO removal under lean-burn engine conditions, have been investigated from the viewpoint of the structural phase transition and thermal stability. The structural changes of crystalline components heat-treated at temperatures from 500 to 1100°C were characterized by the neutron diffraction (ND) method. In the as-prepared materials, powder-diffraction patterns revealed a mixture of crystalline γ-Al2O3 and CuO and electron spin resonance (ESR) data showed well-dispersed Cu2+ cations coordinated by O atoms in an open-octahedron geometry. ND measurements confirmed the elimination of the CuO phase above 800°C and suggested the stabilization of a γ′-phase of alumina by 10 mol% CuO-doping at 900–1000°C. This CuO-alumina catalyst which was subjected to heat treatment at 900°C in air showed a 20% lean de-NOx removal efficiency in a test using a model exhaust gas mixture of space velocity = 100,000 h−1.