Durable Catalysts for Complete Methane Oxidation

Replacing heavy fuel oil with liquefied natural gas in marine engines can potentially provide significant CO₂ savings due to the higher hydrogen to carbon ratio of natural gas. For many existing engines, the change can be done only minor modifications. One challenge remains to realize the potential green house gas savings: Methane slip. This longstanding issue with large engines revolves around the stability of the CH₄ molecule, resulting in significant amounts of unburned  methane being emitted to the atmosphere. The high global warming potential (GWP) of methane means that the CO₂ savings are nullified by just a few percent of methane slip. To solve this challenge in the already highly optimized engines, a  catalyst can be used to convert the last CH₄ to CO₂. 

Methane oxidation catalysts have been in development for several decades and are essentially still faced with the same set of challenges as 40 years ago, namely three different types of deactivation. The first part of this dissertation aims to thoroughly present the progress in the field of methane oxidation and attempts to prevent catalyst deactivation. Arguably the most interesting leap forward has been the adoption of zeolite-supported catalysts which promises to solve both hydrothermal sintering and direct water-induced deactivation by offering nanoparticle encapsulation in a hydrophobic microporous structure. This topic is so novel and interesting that it was decided to cover it in a review article, presented here as Paper 1.

The first experimental project was an attempt to build on top of the promising work by others on zeolite-supported methane oxidation catalysts. A series of Pd/MOx@S-1 catalysts (M = Ce, Ba) was synthesized and tested in a purpose-built catalytic test setup. None of the synthesized materials exhibited a catalytic performance or durability superior to the state-of-the-art Pd/Al₂O₃ catalyst and the work remains unpublished for that reason.

In the second project, the influence of the zeolite counter-ion on SO₂ tolerance was explored through a series of ion-exchanged materials. A Pd/H-CHA catalyst (CHA = zeolite with CHA framework) showed impressive stability when exposed to SO₂ for more than 200 hours. When the protons in the zeolite structure were exchanged to alkali metal ions, the catalyst quickly deactivated and irreversibly lost most of its methane oxidation activity. This result is remarkable since a lot of literature on water-induced deactivation points towards H-form zeolites being inferior to their alkali-exchanged counterparts due to the difference in hydrophobicity. This contradiction points out the importance of testing under realistic conditions when developing catalysts prone to multiple deactivation pathways. The results of the project were written into a manuscript, presented here as Paper 2.

The third and final experimental project describes our view on water-induced deactivation of palladium-based methane oxidation catalysts. The unique observation that deactivation requires methane to be converted at water-saturated sites opens up multiple new angles to improve the understanding of the problem. The dependence of methane concentration, water concentration, temperature, and pressure was determined through a series of catalytic tests. Methane temperature  programmed reduction (CH₄-TPR) experiments were used to describe the deactivated PdO phase, showing that the deactivation is a bulk phenomenon. The reduction temperature of deactivated PdO increased steadily with deactivation time. Finally, a complete mechanism and model for water-induced deactivation is proposed based on the experimental data. The project is presented here as Paper 3, supplemented by some additional analysis and illustrations for the thesis. 

Throughout the PhD project and this dissertation, realistic testing conditions is a recurring theme, as it should increasingly be viewed as a necessity for catalyst development. The challenges around designing a durable catalyst for complete methane oxidation are so numerous and interconnected that insisting on solving one isolated problem at a time will likely slow down progress significantly.