Steps in CH₄ oxidation on Pt and Rh surfaces: High‐temperature reactor simulations

TLDR

The study proposes a complete energy diagram for methane oxidation at low coverages, justified by strong agreement between the model and both low‑ and atmospheric‑pressure reactor simulations. A 19‑elementary‑step model simulating the direct oxidation of CH₄ to H₂ and CO over Pt‑ and Rh‑coated alumina foam monoliths at high temperatures was used, with reaction parameters drawn from literature or prior fits. The model reproduces low‑pressure CH₄ oxidation rates and product selectivities, explains Pt–Rh differences through distinct surface steps, and predicts that under CH₄‑rich, high‑temperature conditions H₂ and CO are the primary products via a pyrolysis mechanism.

Abstract

Abstract The direct oxidation of CH 4 to H 2 and CO in O 2 and in air at high temperatures over alumina foam monoliths coated with high loadings of Pt and Rh has been simulated using a 19‐elementary‐step model of adsorption, desorption and surface reaction steps with reaction parameters from the literature or from fits to previous experiments. The surface reaction model for Pt is in good agreement with previously reported low‐pressure(0.1 to 1 torr) reactor measurements of CH 4 oxidation rates at temperatures from 600 to 1,500 K and of OH radical desorption during CH 4 oxidation at 1,300 to 1,600 K over polycrystalline Pt foils. The model predictions for both catalysts are also consistent with product selectivities observed over monolithic catalysts in an atmospheric‐pressure laboratory‐scale reactor, and the differences between Pt and Rh can be explained by comparing individual reaction steps on these surfaces. Because of the good agreement between the model and both low‐and atmospheric‐pressure reactor simulations, a complete energy diagram for methane oxidation at low coverages is proposed. The model results show that under CH 4 rich conditions at high temperatures, H 2 and CO are primary products of the direct oxidation of methane via a pyrolysis mechanism.