Catalyst Deactivation Modes of Palladium Oxide on Gamma Alumina Catalysts for Lean Methane Oxidation: Reversible and irreversible modes are identified

Palladium(II) oxide/γ -alumina (PdO/γ-Al₂O₃) catalysts are one of the most active catalytic components for the complete oxidation of methane. Under reaction conditions, especially in a wet feed, the catalysts suffer severe performance degradation. This study establishes a series of testing protocols to systematically investigate the causes of catalyst deactivation under methane oxidation reaction conditions. Four distinct catalyst deactivation modes are identified. Two of the deactivation modes are directly related to water, either from the feed gas or as a part of the reaction products, with one (Mode 2) being attributed to the formation of surface hydroxyl groups and the other (Mode 3) to the competitive adsorption of water on the catalysts. The impact of the two deactivation modes is acute and severe but reversible. In contrast, the other two deactivation modes are gradual and persistent but irreversible. Both modes are induced by methane oxidation reaction, with the impact of a wet feed (Mode 4) being substantially more severe than that of a dry feed (Mode 1). The major cause of the irreversible catalyst deactivation is attributed to surface reconstruction of palladium(II) oxide nanoparticles, which behaves as a passivation layer lowering the number of coordinately unsaturated palladium sites for methane activation. Although the passivation layer is relatively stable against thermal or hydrothermal treatment, it is not completely inert. Formation and partial regeneration of the passivation layer is a highly dynamic process and heavily depends on the reaction temperature: a lower reaction temperature (≤450°C) can lead to quicker catalyst deactivation; but a higher reaction temperature (between 500–550°C) can result in a greater extent of catalyst deactivation.