Dispersion of MgO on Pt/γ-Al2O3 and the threshold effect in NOx storage

Supported MgO/Pt/γ-Al2O3 catalysts for NOx storage were prepared by impregnation method and characterized by XRD, BET, CO2-chemisorption, and XPS techniques. The results indicated that MgO was dispersed onto the surface of Pt/γ-Al2O3, and the utmost dispersion capacity (i.e. threshold value) was 0.084 g MgO/100 m2 of Pt/γ-Al2O3. NOx storage capacity of catalysts was measured. It increased with increasing MgO loading up to the dispersion threshold, and then decreased at higher MgO loadings. The ‘threshold effect’, a significant change in the catalytic behavior at the dispersion threshold, was found in this catalyst system. This effect indicated that the effective NOx storage component was not the crystalline MgO, but MgO monolayer dispersed on the surface of support.