Atmospheric- and Low-Level Methane Abatement via an Earth-Abundant Catalyst

Abstract

Climate action scenarios that limit changes in global temperature to less than 1.5 °C require methane controls, yet there are no abatement technologies effective for the treatment of low-level methane. Here, we describe the use of a biomimetic copper zeolite capable of converting atmospheric- and low-level methane at relatively low temperatures (e.g., 200-300 °C) in simulated air. Depending on the duty cycle, 40%, over 60%, or complete conversion could be achieved (via a two-step process at 450 °C activation and 200 °C reaction or a short and long activation under isothermal 310 °C conditions, respectively). Improved performance at longer activation was attributed to active site evolution, as determined by X-ray diffraction. The conversion rate increased over a range of methane concentrations (0.00019-2%), indicating the potential to abate methane from any sub-flammable stream. Finally, the uncompromised catalyst turnover for 300 h in simulated air illustrates the promise of using low-cost, earth-abundant materials to mitigate methane and slow the pace of climate change.

Figure 1

Catalyst activation as a function of gas composition, temperature, duration, and repetition illustrates that activation can be achieved under ambient-to-moderate conditions with reuse potential. The carbon conversion efficiency during the reaction steps is shown for all activation trials, which were conducted for 30 min at 450 °C and with a freshly prepared catalyst unless otherwise noted and in 100% (black) or 20% oxygen (red) in helium mixtures. All methane conversion reactions were conducted at 200 °C in 2 ppmv methane; note that 200 °C is not the maximum conversion temperature but enables one to see a spread in the behavior as a function of the activation temperature. The effect of (a) activation temperature, (b) activation duration, and (c) multiple reactivations is shown.

Figure 2

Powder XRD pattern of copper-exchanged ammonium zeolite powder (mordenite) after activations varying from 0 to 240 min in duration (unexchanged, 0, 30, 240 min from bottom to top).

Figure 3

Methane conversion efficiency as a function of reaction temperature and operation mode. (a) In a two-step, activation followed by the conversion approach, both activation temperature and reaction conversion temperature influence methane removal. All activation steps were carried out for 30 min in 20% O₂. (b) In a continuous reaction approach, 30 min activation in methane-free gas mixtures (20% O₂) was followed by a 30 min conversion reaction in the presence of atmospheric levels of methane (2 ppmv) to simulate isothermal operations. Each data point was generated with a freshly prepared catalyst and represents the mean of at least 21 measurements.

Figure 4

Methane conversion rate increases with the input methane concentration over a range of sub-flarable levels. Methane conversion was tested from 2 ppmv to 2% v/v methane in the presence of 20% oxygen in isothermal operation at 310 °C (30 min initial activation in methane-free gas; asterisks), and following 30 min (filled red symbol) and 60 min (open symbol) activations (450 °C) and reaction (200 °C) in 20% oxygen. Each data point collected with the freshly prepared catalyst represents at least 20 methane conversion measurements.

Figure 5

Long-term activity of the catalyst. Low-level methane (2 ppmv methane in 20% oxygen) was catalytically reacted over 300 h under continuous, isothermal operation at 310 °C (asterisks, following 8 h activation) and a traditional two-step process (red circles; 450 °C, 30 min activation followed by 200 °C continuous reaction).