Irreversible Catalyst Deactivation Mechanisms of PdO/γ-Al2O3 Catalysts for Lean Methane Oxidation

PdO/γ-Al2O3 catalysts suffer from gradual and irreversible catalyst deactivation under lean CH4 oxidation conditions, especially in a wet feed. Time-resolved CO chemisorption DRIFTS measurements are conducted systematically on a series of PdO/γ-Al2O3 catalysts to probe the surface reactivity of PdO nanoparticles after various in situ pretreatments. At 80 °C, CO barely adsorbs on fully oxidized PdO surfaces but interacts with coordinatively unsaturated Pd sites, causing gradual reduction of the PdO surfaces. This results in the formation of characteristic IR bands on various metallic Pd0 sites. By monitoring and comparing the formation kinetics of these IR bands on samples before and after CH4 oxidation, we theorize that the irreversible catalyst deactivation during CH4 oxidation is caused by PdO surface reconstruction, in which coordinatively unsaturated Pd sites gradually become fully coordinated by oxygen. Effectively, the surface reconstruction leads to the formation of a passivation layer on the PdO nanoparticles, which hinders their ability in activating CH4, and hence the subsequent oxidation reaction. Temperature-programmed reduction with CO as the reductant (CO-TPR) reveals that the passivation layer formed during CH4 oxidation is significant enough to increase the reduction temperature of PdO nanoparticles of the 3.0% PdO/γ-Al2O3 samples, although such an effect is less obvious for the 0.4% PdO/γ-Al2O3 samples. On the other hand, it is also discovered that the passivation layer is not completely inert. Under certain reaction conditions, with some being relatively mild, such as low-temperature CO oxidation in a net lean atmosphere and in the presence of H2O, the passivation layer can undergo structure change which results in regeneration or even activation of CH4 oxidation activity of an already deactivated catalyst. Additionally, it is discovered that the fully coordinated Pd–O surface is a metastable phase under CH4 oxidation conditions. In the presence of H2O and at ambient temperatures, surfaces with coordinatively unsaturated Pd sites are thermodynamically more favorable.