Hydrothermal Aging of Pd/LTA Monolithic Catalyst for Complete CH4 Oxidation

Abstract

Palladium-based catalysts are known to provide high CH4 oxidation activity. One drawback for these materials is that they often lose activity in the presence of water vapor due to the formation of surface hydroxyls. It is however possible to improve the water vapor tolerance by using zeolites as support material. In this study, we have investigated Pd supported on thermally stable LTA zeolite with high framework Si/Al ratio (Si/Al = ~44) for CH4 oxidation and the effect of hydrothermal aging at temperatures up to 900 °C. High and stable CH4 oxidation activity in the presence of water vapor was observed for Pd/LTA after hydrothermal aging at temperatures ≤ 700 °C. However, aging at temperatures of 800–900 °C resulted in catalyst deactivation. This deactivation was not a result of structural collapse of the LTA zeolite as the LTA zeolite only showed minor changes in surface area, pore volume, and X-ray diffraction pattern after 900 °C aging. We suggest that the deactivation was caused by extensive formation of ion-exchanged Pd2+ together with Pd sintering. These two types of Pd species appear to have lower CH4 oxidation activity and to be more sensitive to water deactivation compared to the well dispersed Pd particles observed on the LTA support prior to the hydrothermal aging. By contrast, Pd/Al2O3 was generally sensitive to water vapor no matter of the aging temperature. Although the aging caused extensive Pd sintering in Pd/Al2O3, only minor deterioration of the CH4 oxidation activity was seen. The results herein presented show that Pd/LTA is a promising CH4 oxidation catalyst, however Pd rearrangement at high temperatures (≥800 °C) is one remaining challenge.

Recent Advances in Complete Methane Oxidation using Zeolite‐Supported Metal Nanoparticle Catalysts

“…[56] As previously mentioned, hydrothermal sintering becomes relevant at the temperature where the typical issues with hydroxyl formation decrease. [47] However, it may be difficult to distinguish the effects of the two types of deactivation at intermediate temperatures from activity measurements alone. In this case, transmission electron microscopy (TEM) may be used to resolve the cause of deactivation.…”

“…temperatures. [28,47] Huang et al [48] highlighted the consequences of not accounting for transient water effects in light-off experiments. Essentially, the authors showed that running some types of catalytic experiments without the addition of water will result in artificially high activity since the reactor diluent may adsorb the water produced by the reaction.…”

“…Generally, the two types of direct water deactivation are more problematic below 450 °C. Hydrothermal sintering becomes a severe threat to catalyst stability at higher temperatures [28,47] . Huang et al [48] .…”

“…After washing, drying, and calcination, the final material experienced significant sintering even under dry reaction conditions. Friberg et al [18,47] . investigated the effect of Si/Al and prepared a series of catalysts by impregnation and calcination in air.…”

Cobalt-Induced PdO Formation in Low-Loading Pd/BEA Catalysts for CH4 Oxidation

“…Pd species in zeolites can be present as either PdO nanoclusters or atomically dispersed ionic Pd, with PdO nanoclusters believed to be more active than isolated ionic Pd toward the CH 4 oxidation reaction. , Recently, Khivantsev et al showed that the state of Pd within the zeolite structure depends on the Pd loading . In Pd/SSZ-13 (Si/Al = 6) with Pd loading lower than 2 wt %, the majority of Pd was present in its ionic state.…”