Pd/Zeolite Catalysts: Clean Exhaust in Natural Gas Vehicles

Through the loading of various second metals and altering zeolitic properties, the hydrothermal stability and methane oxidation performance are boosted in Pd/zeolite catalysts. This research project aims to meet the low temperature (less than 400°C) and steam-containing exhaust of natural gas vehicles.

Fig. 1 Graphical abstract showing the wide variety of tools available when using zeolites as supports in facing the challenges of methane oxidation in natural gas vehicles.

Overview

Natural gas vehicles are essential in the lengthy transition from traditional gasoline and diesel engines to renewable alternatives. Natural gas, composed primarily of methane, burns cleaner and produces H₂O and CO₂. The carbon footprint is further reduced when using biogenic sources, making natural gas a renewable resource. The challenge in using natural gas vehicles (NGVs) is unburnt methane in the exhaust which is 25 times more potent than CO₂ in global warming potential. The unburnt methane must be eliminated in exhaust containing conditions that include low temperature (less than 400 °C) and large amounts of steam (Figure 1).

The Kyriakidou research group is focused on studying palladium (Pd) ion-exchanged zeolite catalysts for the total abatement of methane in NGVs. Pd/Al₂O₃ is conventionally used but suffers from instability in the presence of steam. Zeolites are a promising alternative with high surface area and the ability to alter hydrophobicity and acidity by varying Si/Al ratio and loading of second metals. The group has lowered the temperature required for methane oxidation in steam-containing simulated exhaust by loading co-cations on high Si/Al small pore zeolites.

Computational work is also performed through the exploration of potential active sites on Pd-based zeolites using density functional theory (DFT). Theoretical work aims to identify possible active sites that may exist within Pd-based zeolites on the molecular level, and to determine how active they are towards methane oxidation. Activity is evaluated based on the energies and activation barriers of chemical reactions that occur during methane oxidation such as the dissociative adsorption of methane (Figure 2). These energies and activation barriers are used to construct a microkinetic model to recreate light-off curves that can be compared to experimentally derived light-off curves.