Methane is the second largest greenhouse gas contributor to climate change. It accounts for about 10% CO2e of anthropogenic greenhouse gas emissions in the US. As the global warming potential of methane is about 28-36 times greater than that of CO2 over a 100-year scale, complete oxidation of methane to CO2 from certain emission sources, such as natural gas powered engine exhaust or coal mine ventilation air, would reduce its environmental impact by more than 96%. However, owing to its strong C-H bonding energy (435 kJ/mol) and symmetrical configuration, oxidation of methane typically requires reaction temperatures above 500C, even with the most active Pd-based catalysts. For the state-of-the-art commercial catalysts with Pd supported on alumina, in addition to high operating temperatures, high Pd loadings (above 5%) are also necessary to overcome the water inhibition effect in order to achieve near 100% methane conversion efficiency. All these severely limit the wide adoption of catalytic solutions for methane abatement. Recently, zeolites, especially those with high silicon to aluminum ratios or siliceous zeolites, have been reported to be superior to alumina supports in promoting Pd for methane oxidation. In the feed that contains water vapor, zeolite supported Pd catalysts showed noticeably lower light-off temperature, and, more importantly, substantially stable steady state methane conversion over time compared to alumina supported Pd catalysts. The hydrophobic nature of the zeolite supports with high Si/Al ratios is generally believed to play a major role in minimizing water interactions with Pd sites, hence reducing the water inhibition effects. When Pd is supported on hydrophobic high surface area amorphous SiO2 supports, however, the catalysts show very poor methane activity. Clearly, zeolite framework structures play another major role although this is poorly understood. In this presentation, roles of zeolite framework structures, silicon to aluminum ratio and acid sites of the zeolite support, and Pd location and particle size distribution in the zeolite support in influencing methane oxidation activity will be reviewed. Discussions will be focusing on the nature of active Pd sites and the associate anchoring sites of the zeolite supports.

Zeolite supported Pd catalysts for complete methane oxidation

ACS Spring 2022

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