(42a) CH4 Conversion By Steam Reforming during Oxidation over Pt + Pd/Al2O3 Monolith Catalysts: Kinetic Model Development

Four-way catalysts are needed for stoichiometric natural gas vehicles (NGVs) emission control. Platinum group metal (PGM) based catalysts under lean/rich feed modulation and the addition of spinel oxides (AB₂O₄) to reduce the requisite PGM loading effectively convert CH₄ in addition to CO, NOx and other hydrocarbons [1]. To further improve these catalysts we have developed a kinetic model for PGM + Mn_0.5Fe_2.5O₄ spinel monolith catalysts that accounts for coupled conversion of CH₄ by oxidation and steam reforming (MSR) along with water gas shift (WGS) in a near-stoichiometric feed. The kinetic model has been incorporated into a dual-layer monolith reactor model for optimizing catalyst formulation and the operation of stoichiometric NGVs emission.

A Langmuir-Hinshelwood type rate expression was used to model the MSR and WGS reactions. The MSR + WGS kinetic model output and experimental data are compared in Figure 1(a). The decrease in CH₄ conversion with increasing CH₄ feed concentration is caused by the faster formation of the inhibiting products CO and H₂. To couple the MSR and WGS kinetics with CH₄ oxidation reaction and incorporate into the monolith reactor model, we adapted the CH₄ oxidation scheme from Chin et al. [2]. Figure 1(b) compares the preliminary results of a tuned, three global reaction model (oxidation, MSR and WGS) to collected flow reactor data. As the O₂ concentration increases, the CH₄ consumption rate reaches a maximum for a given temperature before rapidly decreasing, due to the rate inhibition by O₂. Our reactor model containing the kinetics shows good agreement for CH₄ conversion over a wide range of O₂ feed concentrations and feed temperatures.

References

[1] S. B. Kang, K. Karinshak, P. W. Chen, S. Golden, M. P. Harold, Catalysis Today, 360 (2021) 284-293.

[2] Y. H. Chin, C. Buda, M. Neurock, E. Iglesia, J. Am. Chem. Soc., 133 (2011) 15958.