Four-Way Catalysts (FWCs) are needed for the emission control of stoichiometric natural gas vehicles (NGVs). Platinum group metal (PGM)-based catalysts augmented with mixed-metal oxides can effectively oxidize CH4 in addition to CO and NO under modulated feed conditions [1]. The ability to accurately model catalyst activity is paramount for the further development and optimization of catalysts. The first step in FWC modeling is the development of a reliable CH4 oxidation model for Pt-Pd/Al2O3 catalysts.
Figure 1a shows the results of a tuned three-reaction scheme consisting of complete oxidation, partial oxidation, and the water gas shift (WGS) to kinetics data collected for a Pt-Pd/Al2O3 monolith under isothermal conditions. The data was collected for a gas feed containing CH4, O2, and N2 at a GHSV 112.5k hr-1 over a catalyst supplied by CDTI. The oxidation kinetics were adapted from the model proposed by Chin et al. [2], and the WGS was modeled as an equilibrium-based reaction. Kinetic parameters were estimated through a simple PFR simulation using a Trust Region Reflective method. A complicating feature was uncovered during the experiments in the form of rate multiplicity, as shown in Fig. 1b.
Current research is focused on incorporating the kinetic model into a low-dimensional, dual-layer monolith reactor model that accounts for heat and mass transfer in the washcoat and fluid phases. Model predictions show good agreement over a wide range of O2 feed concentrations and feed temperatures.
[1] Kang, S.B. K. Karinshak, P.W. Chen, S. Golden, and M. P. Harold, “Coupled Methane and NOx Conversion on Pt+Pd/Al2O3 Monolith: Conversion Enhancement Through Feed Modulation and Mn0.5Fe2.5O4 Spinel Addition,†Catal. Today, 360, 284-293 (2021).
[2] Y.H. Chin, C. Buda, M. Neurock, E. Iglesia, Reactivity of chemisorbed oxygen atoms and their catalytic consequences during CH4-O2 catalysis on supported Pt clusters, J. Am. Chem. Soc. (2011). doi:10.1021/ja202411v.
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