(451h) Spatiotemporal Insights during Dynamic Reactor Operation for Fast Light-Off and Enhanced Low-Temperature Methane Oxidation over Pd-Based Catalysts

Palladium-based catalysts that are commonly applied for methane oxidation under lean conditions suffer from water inhibition and commonly lose their initially high activity over time. The origin of inhibition and deactivation has been traced back to the accumulation of hydroxyl groups on the catalyst surface that block active PdO surface sites and sintering of the noble metal particles, respectively. To overcome these drawbacks, optimization of the catalyst formulation and dynamic reactor operation, i.e. short reducing pulses (SRP, absence of oxygen), has been suggested for establishing a more active Pd/PdO-mixed-phase on the catalyst.

In this context, Pd/Al₂O₃ and Pd/CeO₂ catalysts coated onto a monolithic honeycomb structure with a total loading of 100 g/ft³ were subject to extensive activity tests under static and dynamic conditions. Dynamic operation by applying SRP did not only stabilize the catalytic activity in presence of water in the long run, but shifted the light-off substantially towards lower temperatures (Fig. 1a). Moreover, spatially resolved gas phase species concentration and temperature profiles along a single catalyst channel uncovered the formation of different zones along the catalyst channel (Fig. 1b) induced by SRP: A comparably low catalytic activity was found in the front zone, corresponding to a high amount of less active metallic Pd, whereas the rear part of the channel (approx. after 14 mm) is highly active for CH₄ conversion, which is attributed to the predominant presence of highly active PdO. A high support-related oxygen storage capacity helps to preserve the high activity and mitigates sintering-induced deactivation.

Our present work demonstrates how short reducing pulses (SRP) help to overcome the otherwise pronounced water inhibition effect on methane oxidation catalysts. Spatial profiling provides novel information that can serve as a blueprint for designing dynamic reactor operation procedures that maximize the activity of catalytic converters.