Non-steady activity during methane combustion over Pd/Al2O3 and the influences of Pt and CeO2 additives

Methane combustion over Pd/Al2O3 catalysts with and without added Pt and CeO2 in both oxygen-rich and methane-rich mixtures at temperatures in the range 250-520 degrees C has been investigated using a temperature-programmed reaction procedure with on-line gas analysis (FTIR). During the temperature loop under oxygen-rich conditions, there was an appreciable hysteresis in the activity of unmodified Pd/Al2O3, which was greatly enhanced over Pd-Pt/Al2O3. Over both catalysts the hysteresis was reversed under slightly methane-rich atmospheres, and as temperature was reduced, a sudden collapse or fluctuations in activity were shown respectively over Pd-Pt/Al2O3 and Pd/Al2O3. Such non-steady behaviour was almost eliminated over Pd/Al2O3-CeO2. Under a very narrow range of conditions and over a Pd/Al2O3 packed bed, oscillation of methane combustion was observed. (C) 1999 Elsevier Science B.V. All rights reserved.