Effect of CeO2 on Supported Pd Catalyst in the SCR of NO: A DRIFT Study

The activity of Pd/Al₂O₃ and Pd/Al₂O₃–CeO₂ samples has been tested in the selective catalytic reduction of NO by propene. It is found that the activity of Pd/Al₂O₃ decreases with calcination temperature, while the activity of Pd/Al₂O₃–CeO₂ increases abnormally with increasing calcination temperature. Surface-area measurement shows both samples suffer a linear decrease in their surface area, so it is reasonable to attribute the activity enhancement to the effect of CeO₂. The adsorption behavior and state of surface-active sites have been characterized by diffuse reflectance FTIR spectroscopy using CO and NO as probes and the effect of CeO₂ has been revealed. The CeO₂ component increases and stabilizes the dispersion of surface Pd species to prevent it from aggregating at high temperature. CeO₂ may also act as a buffer during the redox cycle of Pd, lengthen the period of Pd redox procedure and render Pd a property of inertia in its redox process, thus increasing the activity of the Pd/Al₂O₃–CeO₂ sample. The essential feature of both effects is the strong interaction between Pd and CeO₂. The intensity of interaction increases linearly with calcination temperature and so does the sample activity.