Spatiotemporal insights into forced dynamic reactor operation for fast light-off of Pd-based methane oxidation catalysts

Abstract

Forced dynamic reactor operation by means of short reducing pulses (SRPs) during otherwise lean operation was investigated as a strategy for enhancing the catalytic activity of a monolithic Pd/CeO₂ methane oxidation catalyst. Compared to static reactor operation under lean conditions, the application of SRPs during light-off experiments enables light-off at significantly lower temperatures both in dry and humid gas streams. In the presence of 10 vol% H₂O, full CH₄ conversion was achieved at 420 °C in dynamic operation, whereas in static lean conditions only 40% CH₄ conversion were achieved at 600 °C. In addition, the results suggest that SRP operation of Pd/CeO₂ is a feasible strategy to overcome long-term deactivation during hydrothermal aging. Combining in situ spatial profiling (SpaciPro) experiments with ex situ X-ray photoelectron spectroscopy (XPS) reveals that dynamic operation results in the formation of two reaction zones in the catalyst sample, with a highly active rear zone that exhibits a PdO/Pd mixed phase with highly active PdO sites that do not suffer from water inhibition due to the continuous removal of hydroxyl groups during the short rich phases. Kinetic activity tests in realistic gas mixtures demonstrate that forced dynamic reactor operation enhances the catalytic activity over the entire temperature window relevant for exhausts from lean-operated natural gas engines.