Abatement of methane in engine exhaust through the development of a novel catalytic combustion system

<p dir="ltr">Lean-burn natural gas engines have faced scrutiny due to the extent of methane emissions escaping from their exhaust. Methane, released from natural gas engine exhaust, is particularly concerning because it has a much higher global warming potential than carbon dioxide (CO2). The implementation of catalyst combustion mitigation technology has proven to be an effective approach to reduce methane emissions, most notably with the use of palladium metal as a catalyst, which has demonstrated high activity and durability. Additionally, lean, humid methane exhaust streams in ventilation air methane (VAM) have been successfully treated using a palladium catalyst supported on zeolite TS-1. However, it is important to note that VAM and natural gas engine exhaust (NGEE) have significant differences, especially in their inherent water concentration, posing challenges in terms of catalytic stability under high-water concentrations. Several catalysts were studied, especially focussing of their stability under lean methane conditions with varying water concentrations in order to simulate the conditions in the prevalent exhaust emission streams of VAM and NGEE. The key findings include the effect of the catalyst support under these conditions, the change in reaction mechanism and kinetics and overall intrinsic catalytic activity, the impact of increasing the hydrophobicity of the catalyst's support and active metal, and the role of enhanced oxygen mobility through additives to the catalyst structure. Various supports were studied under VAM and NGEE conditions to examine the role of acidity under varying water concentrations. Supports (Al2O3, TS-1, and two ZSM-5 samples) with differing silicon-to-alumina ratios (SiO2:Al2O3 = 23 & 50) were examined under a simulated humid exhaust stream. A Pd/TS-1 catalyst was found to be active under both NGEE and VAM conditions. This is attributed to the basic properties of the TS-1 support and titanium's ability to anchor the palladium sites, leading to a higher level of metal dispersion compared to other supports. The alumina-supported catalyst also exhibited higher level of palladium dispersion among the other two supports. The higher silicon concentration in the ZSM-5 support displayed the greatest stability under VAM conditions, likely due to its reduced alumina content. Both ZSM-5 catalysts displayed no stability under NGEE conditions. Despite the superior performance of Pd/TS-1 catalyst, some catalyst deactivation was observed under NGEE conditions, although not under VAM conditions, indicating that the differences in the two conditions must factor into the deactivation mechanisms. Sudden shifts in VAM and NGEE conditions indicated that the Pd/TS-1 catalyst maintains an equilibrium relationship between activity and water concentration. Further kinetic studies were conducted on a Pd/TS-1 catalyst to better understand the catalyst under differing conditions of NGEE and VAM. It was found that coke formation was not the cause of deactivation under high water concentrations. Under differential conditions, it was confirmed that the reaction order for oxygen at 10 vol% water is zero, supported by literature findings at lower water concentrations. This suggests that the reaction order does not change with water concentration. The same was observed for methane, which also maintained a reaction order of one, indicating that methane does not compete with water for active sites. According to the reaction mechanism, a balance exists between the hydroxyl concentration and the methane concentration on the catalyst surface, specifically, methane adsorbs preferentially on non-hydroxylated palladium species. Water was identified as the primary reason for the decreased activity transitioning from VAM to NGEE, with a reaction order of -1. Variations in water concentration led to a significant increase in activation energy between 0-3%; however, the activation energy remained constant at higher concentrations over Pd/TS-1. The decrease in methane adsorption enthalpy at higher water concentrations is suggested to be the main reason for changes in activation energy with water concentration. Furthermore, a secondary route for deactivation was noted, indicating the formation of hydroxyls on the zeolite surface. A study was conducted on the effect of altering the preparation methods of a Pd/TS-1 catalyst. A series of palladium catalysts were prepared with a different precursor and an increased hydrophobicity. It was found that greater hydrophobicity affects the support ability to anchor the active metals, resulting in reduced overall dispersion. Under VAM conditions, the hydrophobic catalyst demonstrated greater stability than the non-hydrophobic catalyst during long-term experiments lasting 1,000 hours. Adjusting the water concentration during the experiments highlighted the effectiveness of each catalyst under humid conditions. The performance of the alternative precursor and hydrophobic catalysts showed vast improvement under higher water concentrations, whereas the conventional nitrate precursor Pd/TS-1 catalyst performed poorly. The non-hydrophobic catalysts exhibited similar activation energies, while the hydrophobic catalyst had a much higher activation energy but showed no difference under humid conditions. The addition of mixed metal oxides was tested under NGEE conditions. Various concentrations of cobalt and cerium oxides were prepared on a Pd/TS-1 catalyst. Cerium was found to be uniformly dispersed among the support material, whereas cobalt tended to agglomerate into larger clusters. This resulted in cerium displaying superior performance compared to cobalt on TS-1 catalysts, attributed to its higher oxygen mobility. Increasing the loadings of cobalt and cerium led to enhanced stability under NGEE conditions. On alumina catalysts, cobalt showed greater activity and stability than cerium, likely due to cerium's high dispersion among the support, which reduced the overall available sites in the nonmicroporous structure. In contrast, cobalt performed well because of the high thermal stability of alumina and the availability of sites for palladium anchoring. </p>