Session: 05-04: Methane Emissions Control

Paper Number: 110183

110183 - Intra-Catalyst Methane Oxidation Pathways of Three Way Catalysts and Implications on Nitric Oxide Conversion Profiles for a Natural Gas Vehicle Exhaust Under Lambda Dithering 

The performance of a three-way catalyst (TWC) in natural gas-powered vehicles is enhanced by periodic changes in air-to-fuel ratio (λ-dithering). The reaction networks and sequences inside the catalyst that facilitate such enhanced performance have not been extensively investigated. This work applied intra-catalyst measurements (SpaciMS) to analyze the transient spatiotemporal gas concentrations inside a monolithic TWC to establish relationships between O₂, CH₄, NO, H₂ and CO conversion pathways. In a Pd/Al₂O₃-based commercial TWC, steam reforming and partial oxidation were revealed to be the main CH₄ conversion routes, and the cyclic rich-lean conditions turn these reactions on and off. The rich phase achieves CH₄ and NO conversion, PdO reduction and dynamic oxygen storage capacity (OSC) depletion, while the lean phase replenishes such OSC, oxidizes Pd and removes H₂ and CO. Conversion of NO during rich phase occurs via reaction with H₂, CO, O-vacancies and other surface-bound reducing fragments formed by methane conversion. The length of lean-rich phases impacts the catalyst performance significantly; too short or too long of a rich or lean phase can lower the overall conversion of reactive species. An inhibition of CH₄ conversion was observed during the rich phase possibly due to carbon monoxide-poisoning of active sites. The intra-catalyst measurements revealed that the catalyst consists of three distinct reaction zones and their lengths vary with dithering conditions. Various dithering frequencies, amplitudes, λ-centers, and temperatures were investigated which allowed an understanding of how these parameters affect the reaction zones and catalyst utilization. Lambda dithering creates a dynamic transition between the spatiotemporal catalyst oxidation state and reactant concentration profiles, thereby inducing a back-to-front and front-to-back progression of various Rich-phase and Lean-phase reactions.

While the reference commercial Pd/Al₂O₃-based TWC contains CeZrO_x as the oxygen storage material, another monolith catalyst consisting of PGM as active phase and Mn_0.5Fe_2.5O₄ (spinel structure) as OSM was also investigated in this study. Such spinel materials have seen increasingly more attention as OSM in recent years. Our measurements show that while the reference TWC led to steam reforming and partial oxidation as the dominant routes for CH₄ conversion, the spinel-based catalyst allows a higher extent of total oxidation and diminished reforming or partial oxidation pathways. Lower partial oxidation products shift the NO conversion routes to be dominated by reaction with surface metallic PGM or oxygen vacancies. Understanding the differences in reaction chemistry, network and sequences between these two different types of catalysts will allow more active and durable formulation of natural gas TWC. Moreover, the general understandings from this work can enable adaptive λ-control strategies to optimize the overall TWC performance over a range of vehicle operating conditions.

Presenting Author: Dhruba Jyoti Deka Oak Ridge National Laboratory

Presenting Author Biography: Dhruba Deka is an R&D Associate Staff in the Chemical Process Scale Up Group at Oak Ridge National Laboratory. Dhruba joined ORNL as a Postdoctoral Research Associate in the Applied Catalysis and Emissions Research Group in 2020. His primary research focus is utilization of heterogeneous catalysis and electro-catalysis for environmentally beneficial reactions to achieve decarbonization and pollutant reduction. His interests also include CO2 capture via solvent absorption.

As a part of ORNL, Dhruba has worked on elucidating the real-life degradation chemistry of commercial Cu-SSZ-13 NH3-SCR catalysts. His research also included investigation of reaction network and chemistry inside natural gas three way catalysts under air-fuel dithering conditions. Currently, Dhruba is active in the area of CO2 capture and electro-catalytic conversion.

Dhruba joined ORNL after receiving his PhD in Chemical Engineering from the Ohio State University in May, 2020. His doctoral work involved development of solid oxide electrolysis cell electrodes for applications in CO2/H2O reduction, NH3 production and partial hydrocarbon oxidation.