Irreversible Catalyst\nDeactivation Mechanisms of PdO/γ-Al<sub>2</sub>O<sub>3</sub> Catalysts for Lean Methane Oxidation

PdO/γ-Al<sub>2</sub>O<sub>3</sub> catalysts suffer\nfrom gradual\nand irreversible catalyst deactivation under lean CH<sub>4</sub> oxidation\nconditions, especially in a wet feed. Time-resolved CO chemisorption\nDRIFTS measurements are conducted systematically on a series of PdO/γ-Al<sub>2</sub>O<sub>3</sub> catalysts to probe the surface reactivity of\nPdO nanoparticles after various <i>in situ</i> pretreatments.\nAt 80 °C, CO barely adsorbs on fully oxidized PdO surfaces but\ninteracts with coordinatively unsaturated Pd sites, causing gradual\nreduction of the PdO surfaces. This results in the formation of characteristic\nIR bands on various metallic Pd<sup>0</sup> sites. By monitoring and\ncomparing the formation kinetics of these IR bands on samples before\nand after CH<sub>4</sub> oxidation, we theorize that the irreversible\ncatalyst deactivation during CH<sub>4</sub> oxidation is caused by\nPdO surface reconstruction, in which coordinatively unsaturated Pd\nsites gradually become fully coordinated by oxygen. Effectively, the\nsurface reconstruction leads to the formation of a passivation layer\non the PdO nanoparticles, which hinders their ability in activating\nCH<sub>4</sub>, and hence the subsequent oxidation reaction. Temperature-programmed\nreduction with CO as the reductant (CO-TPR) reveals that the passivation\nlayer formed during CH<sub>4</sub> oxidation is significant enough\nto increase the reduction temperature of PdO nanoparticles of the\n3.0% PdO/γ-Al<sub>2</sub>O<sub>3</sub> samples, although such\nan effect is less obvious for the 0.4% PdO/γ-Al<sub>2</sub>O<sub>3</sub> samples. On the other hand, it is also discovered that the\npassivation layer is not completely inert. Under certain reaction\nconditions, with some being relatively mild, such as low-temperature\nCO oxidation in a net lean atmosphere and in the presence of H<sub>2</sub>O, the passivation layer can undergo structure change which\nresults in regeneration or even activation of CH<sub>4</sub> oxidation\nactivity of an already deactivated catalyst. Additionally, it is discovered\nthat the fully coordinated Pd–O surface is a metastable phase\nunder CH<sub>4</sub> oxidation conditions. In the presence of H<sub>2</sub>O and at ambient temperatures, surfaces with coordinatively\nunsaturated Pd sites are thermodynamically more favorable.