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CH₄ oxidation catalysts evaluated in a monolith reactor AlMohamadi, Hamad Hamoud

Abstract

Unburned CH₄ from natural gas vehicle (NGVs) exhausts limits the use of natural gas as a vehicle fuel. CH₄ is a potent greenhouse gas with a high C-H bond strength (~435 kJ mol⁻¹) making it difficult to oxidize in catalytic converters. This study is focused on the assessment of the activity and stability of selected catalysts placed in a monolith reactor with the goal of improving NGV emission control. Firstly, the washcoat formulation was investigated with the activity and stability of PdO/AlOOH/Al₂O₃, PdO/Ce/AlOOH/Al₂O₃ and Pt-PdO/Ce/AlOOH/Al₂O₃ monolith catalysts for CH₄ oxidation in the presence of H₂O, CO, CO₂ and SO₂ reported. Secondly, the effect of adding a washcoat overlayer to improve the performance of the monolith catalyst was investigated. The monolith catalysts were prepared using a cordierite (2MgO.2Al₂O₃.5SiO₂) mini-monolith (400 cells per square inch (CPI), 1 cm diameter x 2.5 cm length; ~52 cells) that was washcoated using a Al₂O₃ suspension combined with boehmite (AlOOH), followed by sequential deposition of Ce and Pd(Pt) by wet impregnation. The initial activity of the mini-monolith catalyst was measured by temperature-programmed CH₄ oxidation (TPO) at a GHSV of 36000 h⁻¹. Time-on-stream (TOS) tests were used to quantify the stability of the catalysts at 425 and 550°C using a feed gas with 10 vol % H₂O. The results showed that the composition of the washcoat plays a major role in the stability of the catalysts, with both AlOOH or CeO₂ enhancing the stability of the PdO/Al₂O₃ catalyst in the presence of H₂O. Moreover, a washcoat overlayer applied to the PdO(Pt/CeO₂)/AlOOH/Al₂O₃ monolith catalysts, is shown to enhance CH₄ oxidation activity and stability at low temperature (< 500°C) in the presence of H₂O and SO₂. Three recent kinetic models of CH₄ oxidation reported in the literature and based on Langmuir-Hinshelwood kinetics have been applied to the data measured herein using the mini-monolith reactor. Data obtained in dry feed gas and with 2 and 5% H₂O in the feed gas were analysed. The results of the models show that adding CeO₂ or the washcoat overlayer decreases H₂O adsorption, which leads to enhanced catalyst activity.

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