Deactivation Aspects of Methane Oxidation Catalysts Based on Palladium and ZSM-5

Abstract

Catalytic methane oxidation is used in exhaust gas after treatment to reduce methane emissions in stationary combustion processes as well as in natural gas vehicles. Pd/zeolite catalysts provide higher activity than the commonly used Pd/Al2O3, but suffer from rapid deactivation under reaction conditions and in the presence of steam. This work presents a detailed study of deactivation of 1 wt.% Pd/H-ZSM-5 catalyst in reaction conditions in dry and wet feed including characterization of both the active phase and the support. Initially well-dispersed PdO particles are not stable and sinter immediately after the exposure to the reaction mixture even in dry feed and stabilize at lower dispersion level. Despite the reasonable methane oxidation activity exhibited by the large particles, the initial PdO particles could potentially provide a 40 o C lower methane oxidation temperature. In the presence of water in the feed at moderate temperature (450°C), support degradation governs the deactivation of the catalyst, which is evidenced by a significant decrease in BET surface area and generation of extra-framework aluminium species. These results suggest that active and stable catalysts could be obtained by careful choice and design of the support material with the aim to guarantee high dispersion of PdO.

Stable complete methane oxidation over palladium based zeolite catalysts

“…While being advantageous for the synthesis in oxidizing conditions, the increased acidity of the zeolite could enhance the mobility of palladium over the support under reaction conditions, which include methane and large amounts of steam in the feed, eventually resulting in particle growth 13 . In addition, dealumination degrades the zeolite crystallinity and leads to the formation of acidic extra-framework aluminum species, which further promote palladium sintering 13 , 28 . Provided that the active palladium phase can be kept stable at high dispersion, the use of low-silica zeolite thus provides a unique opportunity to achieve high loading and high dispersion of the metal.…”

Water Inhibition in Methane Oxidation over Alumina Supported Palladium Catalysts

“…Not only is slipped methane the most difficult hydrocarbon to render environmentally harmless by catalytic oxidative aftertreatment, − which must be used to reach accepted vehicle emission limits. The combustion exhausts contain high levels of water (up to 13%) that strongly inhibit the desired catalytic functions, making catalytic emission control far from trivial. , Even for the most active palladium-based catalysts applied hitherto, water inhibition is detrimental for temperatures as high as 600 °C. − Understanding the mechanistic reason(s) behind water inhibition is thus the key, which if progressed will pave the way for new methane oxidation catalyst formulations with improved water tolerance. To respond to this need, we have conducted a detailed operando vibrational study following a bottom-up research approach as to identify how water interacts with the individual components (PdO and Al 2 O 3 ) of an alumina supported palladium (Pd/Al 2 O 3 ) catalyst and how it inhibits the lean methane oxidation over Pd/Al 2 O 3 model catalysts.…”

Methane emissions from natural gas vehicles in China

“…However, heavy-duty NG buses and trucks produced in China are equipped with lean-burn (LB) engines and oxidation catalysts (OC) 2 . The exhaust temperature of LB engines is usually lower than the ideal temperature for OC (450 °C) to effectively remove CH 4 2 , 24 . The mean value of fuel-specific EFs found in previous studies for LB engines with OC is six times higher than that of stoichiometric (SM) engines equipped with a three-way catalyst (TWC, Supplementary Table 2 lists CH 4 EFs for different technologies) 25 .…”