A Rhodium-Based Methane Oxidation Catalyst with High Tolerance to H₂O and SO₂

Increased use of natural gas as a fuel in efficient, lean-burning engines could offer environmental advantages, but this necessitates a catalytic oxidation of methane escaping from the engine. The challenge for the catalytic oxidation of CH₄ is that the catalyst must operate in the exhaust, which contains H₂O (5-15 vol %) and SO₂ (∼1 ppm), and both components cause a severe inhibition of the catalyst. Here, we report that a 2 wt % Rh/ZSM-5 catalyst offers great promise by showing that high methane conversion can be reached at practically achievable conditions and high space velocities also in the presence of H₂O and SO₂. Rh-based catalysts, which are in the form of Rh₂O₃ under reaction conditions, become superior to the state-of-the-art Pd-based catalysts in an atmosphere with both H₂O and SO₂. Although both H₂O and SO₂ inhibit Pd and Rh catalysts, water is found to have a destabilizing effect on rhodium sulfate that enables a partial decomposition of the sulfate below 400 °C. We propose that this partly alleviates the sulfur poisoning of Rh catalysts; wherefore, the combined inhibition from H₂O + SO₂ is weaker for Rh than that for Pd.