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Understanding the reversible and irreversible deactivation of methane oxidation catalysts

Palladium-based methane oxidation catalysts deactivate under water-rich conditions due to sintering, sulfur poisoning, and phase transformations. X-ray absorption spectroscopy at the palladium L₃-edge identified structural changes and dispersion effects that impact catalyst performance. Optimizing supports and reaction conditions could enhance durability, aiding methane emission control in industrial applications.

Methane, a potent greenhouse gas, is increasingly targeted in emission reduction strategies due to its significant impact on global warming. Over a 20-year period, methane has a global warming potential over 80 times higher than carbon dioxide. Consequently, controlling methane emissions has become a high priority, particularly in industries reliant on natural gas engines. Catalytic oxidation, which uses specific catalysts to facilitate methane combustion and convert it into less harmful carbon dioxide, is a promising technology for addressing this challenge. However, methane oxidation catalysts are hindered by rapid deactivation, which reduces their effectiveness and operational lifespan. Understanding the deactivation mechanisms is therefore crucial for improving catalyst durability for more effective emission control.

Deactivation can occur through several different mechanisms. For example, the high temperatures necessary for methane oxidation induce structural changes in palladium supported on alumina (Al2O3), leading to a process known as sintering. During sintering, palladium particles coalesce, reducing the catalytically active surface area and directly weakening the catalyst efficiency. Employing stable, high-surface-area supports has been identified as an effective strategy to mitigate this issue.

Another significant challenge arises from sulfur compounds commonly found in natural gas and exhaust gases. These compounds bind strongly to palladium, forming stable complexes that block active sites on the catalyst’s surface, a process known as sulfur poisoning. This deactivation is irreversible, resulting in permanent loss of catalytic activity. Implementing pre-treatment methods is crucial for extending the catalyst’s operational lifespan in sulfur- containing environments.

The most complex and least understood deactivation mechanism is phase transformation caused by dispersion. Palladium catalysts function optimally in the palladium oxide (PdO) phase, as PdO particles are highly active in oxidizing methane. However, during oxidation in water- rich environments, PdO gradually transitions into a less catalytically active phase as it disperses across the surface of the support. While this phase transformation can be completely and rapidly reversed through reduction of palladium followed by reoxidation, such processes necessitate interruptions to the reaction feed, making them impractical for industrial applications.

By addressing these mechanisms, this work aimed to overcome key obstacles to the commercial viability of methane oxidation catalysts, particularly under realistic operating conditions. Specifically, the study investigated the fate of palladium-based methane oxidation catalysts after prolonged exposure to realistic reaction conditions, including exposure to up to 15% water. Ex-situ X-ray absorption spectroscopy (XAS) at the Pd L3-edge was performed at the ID12 beamline to analyse palladium’s oxidation state and local structure before and after deactivation. Analysis of peak energies and white line intensities (Figure 95) revealed that the oxidation state of Pd remained unchanged during deactivation. However, its reducibility and surface distribution were significantly altered due to phase transformation. This finding, observed only after prolonged deactivation (>24 hours), highlights the sensitivity of the Pd L3-edge to coordination number changes, surpassing the capabilities of the more commonly used K-edge.

Catalyst deactivation as a function of time was also tracked by measuring methane conversion over several hours in a 6x6 experimental grid of varying methane and water partial pressures (Figure 96). This allowed for a concise kinetic analysis of the methane oxidation reaction under realistic conditions. While water coverage on the catalyst surface caused reversible deactivation,

Fig. 95: L3-edge XAS spectra of the fresh, as-prepared catalyst and the deactivated catalyst after 60 hours of wet methane

oxidation (1000 ppm CH4, 10% O2, 10% H2O, N2 balance). The spectra from the first and sixth scans are shown for comparison.