Influence of NOx on the activity of Pd/θ-Al2O3 catalyst for methane oxidation: Alleviation of transient deactivation

Alumina supported Pd catalyst(Pd/Al2O3) is active for complete oxidation of methane, while often suffers transient deactivation during the cold down process. Herein, heating and cooling cycle tests between 200 and 900 °C and isothermal experiments at 650 °C were conducted to investigate the influence of NOxon transient deactivation of Pd/θ-Al2O3catalyst during the methane oxidation. It was found that the co-fed of NO alleviated transient deactivation in the cooling ramp from 800 to 500 °C, which was resulted from the in situ formation of NO2during the process of methane oxidation. Over the Pd/θ-Al2O3, thermogravimetric analysis and O2temperature programmed oxidation measurements confirmed that transient deactivation was due to the decomposition of Pd O particles and the hysteresis of Pd reoxidation, while the metal Pd entities were less active for methane oxidation than the Pd O ones. CO pulse chemisorption and scanning transmission electron microscopy characterizations rule out the NO2effect on Pd size change. Powder X-ray diffraction and X-ray photoelectron spectroscopy characterizations were used to obtain palladium status of Pd/θ-Al2O3before and after reactions, indicating that in lean conditions at 650 °C, the presence of NO2increases the content of active Pd O on the catalyst surface, thus benefits methane oxidation. Homogeneous reaction between CH 4, O2, and NOxmay be partially responsible for the alleviation above 650 °C. The interesting research of alleviation in transient deactivation by NOx, the components co-existing in exhausts, are of great significance for the application.