Methane Oxidation Catalysts

W. Addy Majewski, Hannu Jääskeläinen

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Abstract: Palladium-based catalysts are typically selected for the control of methane emissions from lean burn gas engines. While Pd is the most active CH4 oxidation catalyst, high catalyst temperatures are still required to oxidize methane and other short chain hydrocarbons. Pd catalysts are also sensitive to sulfur poisoning and can be deactivated even at trace levels of sulfur in the exhaust gas. The deactivation by sulfur can be reversed by thermal regeneration, preferably under reducing conditions.

Catalytic Oxidation of Methane

Reactions and Catalysts

The oxidation of methane over an oxidation catalyst is described by the equation:

CH4 + 2O2 = CO2 + 2H2O(1)

Since oxygen is needed, methane oxidation catalysts (MOC) can only be used to control CH4 emissions from lean-burn gas engines. In stoichiometric (sometimes referred to as “rich burn”) gas engines, methane can be controlled with a three-way catalyst (TWC).

Methane is a highly stable compound—high activation energy is required to break the C-H bond and to oxidize the molecule. Therefore, a higher catalyst temperature is necessary for the oxidation of methane, compared to longer chain hydrocarbons. To achieve high CH4 conversions under realistic conditions, methane oxidation catalysts must be operated at temperatures of around 500°C. While some laboratory studies reported CH4 catalyst light-offs (T50) of less than 350°C [3666], such results are limited to catalysts of very high precious metal loadings, tested without or with very low exposure to sulfur.

Pd-based catalysts are generally considered to be most active to oxidize methane and other short chain hydrocarbons, Figure 1 [3651]. However, in addition to the high catalyst temperature, other challenges in the catalytic oxidation of methane include:

[SVG image]
Figure 1. Methane oxidation efficiency for different precious metals versus temperature

CH4 1000 ppm, O2 10%, H2O 10%, SV = 40,000 1/h.

Other precious metals considered for methane oxidation are platinum and rhodium. Platinum-based catalysts are of interest for lean-burn applications due to their low sensitivity to water and sulfur containing compounds. Platinum does not form stable hydroxides or sulfates. Sulfur can even promote methane oxidation over platinum supported catalysts. However, the low temperature activity of platinum-based catalysts for methane oxidation is poor under lean conditions, Figure 1 [3649][3651]. Oxidation catalysts based on Rh have also been tested. Rhodium has light-off characteristics between those for Pd and Pt. Its sensitivity to sulfur poisoning also falls between that for Pd and Pt [3650].