(617by) Diagnostics of Field-Aged Three-Way Catalyst (TWC) on Stoichiometric Natural Gas Engines

Natural gas is the most attractive alternate fuel for gasoline and diesel due to its competitive price resulting from its abundant availability and lower GHG emissions resulting from its combustion. In stoichiometric natural gas engines, three-way catalysts (TWCs) are widely employed to convert the exhaust pollutants CO, HC (including CH₄) and NO_x to CO₂, N₂ and H₂O. The TWCs contain Platinum Group Metals (PGM) as the active centers for various reactions and ceria-zirconia (CeZrO_x) material provides oxygen storage capacity (OSC), which acts as a buffer for maintaining stoichiometric conditions under broad air to fuel ratios. The activity of PGM and OSC components of TWC degrade upon prolonged real life operation. The degradation could arise due to various reasons such as high temperature hydrothermal exposure of catalysts for prolonged periods and masking/poisoning of the catalytic sites by, for example, chemical contaminates originating from lube oil consumption and/or coolant leakage. Isolating the impact of individual degradation mechanisms on TWC performance is imperative for developing improved catalysts, controls and performance recovery methodologies.

From laboratory hydrothermal aging studies, a linear relationship between NO_x conversions and OSC was shown. Real-world aged TWC catalysts have shown significant decrease in surface area and OSC and can be attributed to hydrothermal aging. In addition, SEM and EDX analysis confirmed the presence of P, Na, K and S containing species concentrated on the inlet portion of the real-world aged TWC and their concentrations rapidly decreased along the axial length of the catalyst. Lower NO_xconversion was observed on the sample having the above-mentioned chemical deposits, e.g. inlet portion, while the performance of the rest of the catalyst unit was minimally impacted. An acid washing methodology was developed to remove the chemical contaminates from the real-world TWC which restored its performance.

In this contribution we describe the i) laboratory evaluation methodologies, ii) real-world aged catalyst characterization, iii) aging mechanisms and iv) quantification and correlation between NO_xconversion and OSC all of which are important to develop TWC and controls.